11. Luminescence

1 1. LUMINESCENCE

1 1 .l. Fundamental Concepts* This chapter provides a synopsis of fundamental concepts of luminescence research. For more detailed information, the reader is referred to the general literature. t’-” In general, any radiation emitted from a substance not in internal thermal equilibrium can be called “luminescence.” In practice, however, this term is usually restricted to emission in or near the visible spectrum “excited” by previous or simultaneous irradiation with photons or particles. Substances emitting luminescence are called “luminophors ” or “phosphors,” the last term being mainly employed for inorganic solid state luminophors. Most phosphors are basically semiconductors, describable in terms of the energy band model with valence and conduction bands, and with localized energy levels in the forbidden region between the bands. The localized levels are associated with impurities or imperfections in the host lattice. Impurities that provide levels which permit radiative transitions are called activators. These levels are generally close to the valence band, and when occupied by electrons can also act as traps for valence band holes. Some impurities provide levels close

t See also Vol. I, Chapter 7.7.

1’. Lenard, F. Schmidt, and R. Tomaschek, in ‘‘ Handbuch der Experimentall’hysik” (W. Wien and F. Harms, eds.), Vol. 23. Akademische Verlagsges., Leipzig, 1928. * N. Riehl, ‘ I Physik und technische Anwendungen der Lumineszenz.” Springer, Berlin, 1941. a F. A. Kroger, “Some Aspects of the Luminescence of Solids.” Elsevier, Amsterdam, 1948. G. R. Fonda and F. Seitz, eds., “Preparation and Characteristics of Solid Luminescent Materials.” Wiley, New York, 1948. P. Pringsheim, “Fluorescence and Phosphorescence.” Interscience, New York, 1949. G. F. J. Garlick, “Luminescent Materials.” Oxford Univ. Press, London and New York, 1949. H. W. Leverenz, “An Introduction to the Luminescence of Solids?’ Wiley, New York, 1950. G. Destriau and H. F. Ivey, Proc. I . R . E. 43, 1911-1940 (1955). F. Matossi, “Elektrolumineszenz und Elektrophotolumineszenz.’’ Vieweg, Braunschweig, Germany, 1956. l o K. Przibram, “Irradiation Coloura and Luminescence.” Pergamon, London, 1956. l 1 G. F. J. Garlick, in “Handbuch der Physik-Encyclopedia of Physics” (S. Fliigge, ed.), Vol. 26. Springer, Berlin, 1958. 1

* Chapters

11.1 to 11.5 are by F. Matossi and S. Nudelman. 293

294

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LUMINESCENCE

to the conduction band, which, if empty, may act as electron traps. Filled electron traps and empty activator levels correspond to the donor and acceptor levels of usual semiconductor terminology. Impurities that produce levels to which radiative transitions are forbidden are called “killers”; they reduce or prevent luminescence. In terms of the energy band model, which provides a simplified picture of the complex behavior of phosphors, typical excitation mechanisms of luminescence involve raising an electron from the filled band or from a filled activator level to the conduction band, or from an activator ground level to some higher activator level. Those electrons that reach the conduction band can return to activator levels or may be trapped. If trapped, they can be released to the conduction band only by absorbing sufficient thermal or other energy. The return of an electron from conduction band to an empty activator level (trapped hole) yields luminescence. There may also occur radiationless transitions to trapped or free holes, in which the energy is transferred to lattice vibrations. The complex consisting of activator impurity, and the surrounding disturbed host lattice, where these transitions take place, is sometimes referred to as a “luminescence center.’’ It may comprise many atoms and may also include t r a p ping impurities associated with activators. Radiation from transitions between the excited and ground states of an activator produces “ fluorescence,” while the delayed return of electrons from traps through the conduction band yields “phosphorescence.” Phosphorescence can be “frozen in” a t low temperatures such that thermal energy is not sufficient for the release of trapped electrons. “ Thermoluminescence ” is the release of the frozen-in phosphorescence by raising the temperature. Thermal release of trapped holes, however, quenches luminescence. Fluorescence and phosphorescence, here defined in terms of a mechanism, are sometimes also defined by the duration of the luminescence afterglow after removal of the excitation source. With this definition, fluorescence would be observed for durations of less than lo-* sec (the order of lifetime of most excited states) and phosphorescence for longer durations. The two kinds of definitions are not entirely equivalent. Delayed emission from metastable activator levels, whose lifetime sec, is therefore not defined uniquely. exceeds With respect to exciting agents, ‘‘ photoluminescence,” which is excited by photons, is distinguished from “ cathodoluminescence,)’ which is excited by particles, in particular by cathode rays. “ Electroluminescence )’ is excited by an electric field that produces and accelerates free electrons within the phosphor; it thus can be considered as a special case of cathodoluminescence. Conduction band electrons produced by photons but accelerated by a field give “ photoelectroluminescence.” The release of

11.1.

295

FUNDAMENTAL CONCEPTS

internal energy by chemical reactions or mechanical treatment leads to chemiluminescence or triboluminescence, respectively. Photoluminescence may be modified by electric fields ( ‘ I electrophotoluminescence”) or by radiation of long wavelengths. These modifications consist of “stimulation” effects with increased light output or “quenching” effects with decreased output (Fig. 1). These effects may be instantaneous or permanent. A special case arises if, during afterglow, stimulation is followed by quenching so that the “light sum” (= time integral of intensity) is the same as it would be without the modifying agent. This effect is often also called stimulation, although sometimes the term “exhaustion” is used.

‘0

1‘

2‘

4

Time

FIG. 1. Modifications of photoluminescence-uv on: tadr;field on: t 1 - t ~ ;a, growth; b, instantaneous stimulation; c, instantaneous quenching; d, permanent quenching; el permanent stimulation; f, “ field-off ” stimulation and recovery; 8, decay, dterglow; h, exhaustion.

Luminescence excited or modified by ac fields contains components periodic in time, superimposed on a steady background. The periodic component is called “brightness wave ” in electroluminescence and (‘ripple ” in electrophotoluminescence. Other concepts, related to spectra, efficiency, decay, glow curves, recombination radiation, are defined in the appropriate chapters or sections. The information obtained from luminescence measurements pertains mainly to the position and state of occupation of energy levels. These can, in general, be obtained from emission and excitation spectra, decay laws, and thermoluminescence. Transition probabilities can be inferred from intensity measurements including stimulation and quenching effects. Field effects are related to motion, interaction, and spatial distribution of electrons. Most of this information is contained in the experimental data in an involved and implicit manner, and only in simple cases is it possible to calcurate a certain numerical value of some property (e.g., trap depth) from a single observed characteristic feature (e.g., peak of thermoluminescence) by a one-to-one relation. There is a close relation-

296

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LUMINESCENCE

ship between luminescence and photoconductivity mechanisms. Therefore, photoconductivity measurements are a valuable complement to luminescence research.

1 1.2. Preparation of Phosphors 1 1.2.1. Powders

The one common characteristic of phosphors ia some deviation from ideal crystal structure either because of structure defects or because of small quantities of impurities. It is not difficult to make luminescent materials, but it is very difficult to make a good phosphor with controlled and reproducible properties. The two most important requirements of phosphor preparation are extremely clean laboratories and extremely pure raw materials. The term ‘(luminescence pure ” describes materials containing impurity concentrations of 0.0001%, compared to “spectroscopically pure ” material whose impurity concentration is 0.001%. The methods of purification include physical processes such as distillation, sublimation, and recrystallization.’ Chemical reactions are also used, generally in connection with some precipitation out of solution. These methods vary, of course, with the material, as does the required purity. After purification, the raw materials consisting generally of a host crystal in powder form, some substance providing the activator element, and often a fusible salt called the “flux,” are mixed to give a uniform blend. The mixture can be obtained in a purely mechanical manner by grinding in a mortar, sieving, or ball milling, or also by mixing the host crystal with solutions of activator salts and drying the mixture by heate2 Occasionally a solution is used followed by a precipitation, which provides good distribution of the activator and flux substances. The flux facilitates the solution and distribution of the activators in the host crystal on firing. It probably acts to provide a charge-compensating co-activator, although the atoms of the flux do not always go into the lattice.s If they do, the flux may also furnish trapping centers. The mixture is heat-treated or “fired.” The variables that determine the nature of the resultant phosphor include the rate of heating and 1 See papers by R. Ward and H. C. Froelich, in “Preparation and Characteristics of Solid Luminescent Materials” (G. R. Fonda and F. Seitz, eds.). Wiley, New York, 1948. 2 See F. A. Kroger, Proc. I . R . E . 48, 1941 (1955). a F. Joliot, Cumpt. rend. 2S8, 1216 (1954).

11.2.

PREPARATION OF PHOSPHORS

297

cooling; time, temperature, and atmosphere of firing; and even the method of milling. Luminescence often reaches a maximum for some particular particle size. Grain sizes of the order of 0.1 p seem to provide the best results. A new method of introducing activators uses neutron bombardment of the host c r y ~ t a lThe . ~ nuclear reaction

+ neutron -+

a0Zn64

260 day8

aoZn6bdroCu66

+ positron

for instance, creates Cu-atoms in the place of Zn-atoms in ZnS or in ZnO, which then is chemically converted into ZnS. The production of radioactive isotopes by neutron bombardment may further be used to determine the impurity contents with very high accuracy. Leverenzs gives the following advice as an “educated guess” for devising new phosphors : Use well-crystallized, colorless, high-melting host crystals of singly valent elements (combinations of elements of the columns 1, 2, 3B, 4A, 5, 6A of the periodic system with those of columns 6B, 7B) ; use multivalent elements for activators; do not use fluxes for phosphors other than sulfides (see also Section 11.2.5). 11.2.2. Single Crystals

Techniques for obtaining single crystal phosphors have been developed only in recent years. Zinc sulfide crystals, for instance, may be obtaineda by heating ZnSpowder in HzS under pressure. Growing the crystals from the gas phase makes it easier, however, to introduce activators.’ A detailed discussion of the manufacture of ZnS single crystals by different methods with controlled crystal structure, chemical composition, and physical properties has been given by Kremheller.* The methods include reactions obtained with zinc metal powder a t 920°C in a gas stream of H2S and Hz, also with ZnS powder at 1200°C in different atmospheres of H2S-H2mixtures or He, and sublimation on a cooled quartz “finger” in a helium stream (Fig. 2). The first method yields crystal sizes of 0.5 X 0.2 X 0.2 mma in four hours, with crystals as large as 10 X 1 X 1 mma being obtainable. The second method produces crystals as large in both rod-like and flat samples, while the last method, which appears to give the best results, gives

* E. Grillot and M. Bauin-Grillot, Brit. J . A p p l . Phys. 6, 895 (1955); J. S. Prener and F. E. Williams, J . Electrochem. Soc. 108, 343 (1956). 6 H. W. Leverenz, “An Introduction to the Luminescence of Solids.” W h y , New York, 1950. D. C. Reynolds and 5. J. Czyeak, Phys. Rev. 79, 543 (1950). W. W. Piper, J . Chem. Phys. 20, 1343 (1952). * A. Kremheller, Syluania Technologist 8, 11 (1955).

298

11. LUMINESCENCE TABLEI. Growth Conditions for ZnS Crystals by Sublimation

Temperature gradient along the crystal Wall temperature a t growth surface Boat temperature Growth habit

40 1050 1200 thin plates

10 1000 1200 rods

6O"C/cm 1020°C 1200°C twins

crystal shapes depending on the growth conditions (see Table I). Hexagonal crystals can be obtained by rapid quenching from 1200"C, while the cubic ZnS is formed if several hours are used in cooling from 1100°C to 800°C. Different ratios of cubic to hexagonal structures can be obtained by annealing a t different temperatures for five hours. I I

0 0 0

0 0 0

I

I

h?(i

9

I 1

O.OO

b

O o 0

I

8

I

I I

FIG. 2. Furnace for crystal growth by sublimation-,

tube, about 80 cm long;

b, dual zone furnace; c, boat for activator material; d, boat for ZnS; e, U-shaped cooled

quartz tube;f, He inlet and outlet; g, window; h, crystals.

Zinc sulfide crystals can be activated with Cu also during the application of Cu-electrodes by evaporation.9 The Cu-atoms are not, however, homogeneously distributed but diffuse into the crystal along straight lines which are probably related to dislocations. I n order to obtain crystals of decomposable compounds from the melt, ovens for high temperatures and pressures may be used.ea 11.2.3. Films

Transparent phosphor coatings have been deposited on glass by chemical reaction in the vapor state.'O Zinc sulfide and cadmium sulfide films activated with As, Cu, Mn, P, and Zn can be obtained by slowly dropping zinc and the activator substance as powders into an electrically heated evaporator, in an HnS atmosphere of a few millimeters Hg. The fYm is formed on a glass plate of 400 to 700°C temperature. The coatings are 0.

G. Diemer, Philips Research Reple. 10, 194 (1955). A. Fischer, 2.Naturforsch. Ma, 105 (1958). F. J. Studer and D. A. Cusano, J. Opt. SOC.Am. 46, 493 (1955); J . phys. radium

17, 742 (1956).

11.2. PBE1PARATION

OF PHOSPHOR8

299

very durable and tightly bound t o the glass. Transparent layers of up to 5 p thickness can be made in one deposit, with thicker layers being possible in several deposits following the polishing of each layer. The thickness of the d m can be judged by interference colors. The brightness of cathodoluminescence is less than that of the usual powdered phosphor screens, mainly because the light output is trapped by multiple reflections. However, such films still are advantageous since they avoid loss of resolution from scattering by the crystallites of a powder. The photoluminescence of such films is also generally weak and in many cases has a different color than the corresponding cathodoluminescence. 1 1.2.4. Electroluminescent Phosphors

The preparation of electroluminescent phosphors is, in principle, not different from that of the normal, photoluminescent phosphors. In general, the electroluminescent powders require a much higher concentration of activator atoms, e.g., lo-* atoms Cu for 1molecule ZnS instead of Cu atoms. This does not mean that such large amounts of impurities actually go into the lattice of the host crystal; it rather appears that the excess metal is concentrated a t grain surfaces establishing a metalsemiconductor contact. The contact provides a barrier layer region in which intense local fields can exist, which are necessary for the excitation of electroluminescence. In support of this idea, Zalm, et al." have obtained electroluminescent phosphors by washing nonelectroluminescent to phosphors with CuSO,. An amount of atoms Cu per atom Zn produces in this case an electroluminescent ZnS phosphor, although a rather inefficient one. Even mechanical mixing of ZnS powder with sharp-edged metal particles produces electroluminescent phosphors, since high fields exist near the edges or points.12 Direct proof of the increase of Cu concentration a t the crystallite surfaces**can be obtained by treating the particles with acids and observing chemically the amount of Cu removed. The maximum Cu concentration for crystallites about l o p thick appears a t approximately 0 . 2 ~ below the surface, with the total thickness of the copper-enriched layer being about 1p. The threshold value of the electric field for excitation of electroluminescence varies with the depth of the acid attack. Successful electroluminescent ZnS phosphors can be produced' by using Cu and Pb as double activators. The procedure consists essentially of mixing the basic ingredients as powders and firing in an Nratmosphere. P. Zalm, G. Diemer, and H. A. Klasens, Philip8 Research Repta. 9, 81 (1954). W. Lehmann, J . Electrochem. Soc. 104, 45 (1957). 18 G. Destriau and H. F. Ivey, Proc. I . R. E . 48, 1911-1940 (1956). 14 H. H. Homer, R. H. Rulon, and K. H. Butler, J . Electrochem. Soc. 100,566 (1953). 11

12

300

11.

LUMINESCENCE

The fired material is then washed in concentrated acetic acid, followed by washings in increasingly diluted acetic acid, and finally with water. The washings remove free ZnO; this increases the electrical resistance and the electroluminescence intensity of the remaining phosphor. The best green phosphors are obtained with 4 X lo-* t o 1.5 X atoms Cu per molecule ZnS, with a final Pb content not less than 2 X lo-" atoms per molecule ZnS. The firing temperature may vary from 750°C to lOOO"C, yielding a cubic ZnS phosphor. Destriau" has produced electroluminescent phosphors with mixtures of ZnS and ZnO powders, omitting any flux. The best results were obtained for a ratio of 75% ZnO to 25% ZnS by weight, adding Cu in the amount of & of the ZnS weight and heating to 120OOC for an hour in an inert atmosphere. Single crystals have been produced by the methods described in Section 11.2.2. Films have been made by the method of Section 11.2.3. This process has proven successful for electroluminescence when the phosphor film was deposited on a glass plate covered by a transparent film of TiOz. The Tiorfilm becomes conducting when the ZnS is deposited on it and serves as one of the electrodes for the application of an electric field.

11.2.5. Classes of Phosphors The most frequently utilized phosphors belong to the following classes or farnilies:l6 ZnS and CdS in various ratios; manganese-activated silicates; manganese-activated group 2 fluorides, e.g., ZnFz; calcium magnesium silicates; ZnO; and lead-activated tungstates. In addition, among the many other classes of phosphors, the following are of particular interest: The alkaline-earth sulfides, called Lenard phosphors, which were the first phosphors investigated extensively; the phosphors with rare earth activators, which show line spectra because of transitions in protected inner shells of the activator atoms; the pure substance or " self-activated " phosphors, which luminesce independent of impurities, including zinc and cadmium sulfides, tungstates, and uranium compounds; alkali halides with F-centers (Section 11.7); a wide variety of organic phosphors. The last three classes are more or less fluorescent (fluors) instead of phosphorescent. For the description of some newly developed or unusual phosphors see Brit. J . A p p l . Phys. 6, supplement No. 4 (1955). Detailed recipes for representative phosphors are given in references 1 and 7 of Chapter 11.1. 15

14

G . Destriau and J. Saddy, J . phys. radium 6, 12 (1945). C. C. A. Hill, Brit. J . Appl. Phya. 6, S 6 (1955).

11.3.

PHOTOLUMINESCENCE AND CATHODOLUMINESCENCE

301

1 1.2.6. Phosphor Symbols

An example of a complete symbolic description of a phosphor according to L e v e r e d would be: ZnS:Cu[Cl1] (O.Ol)[NaC1(2)]8S0"C.

This symbol may be translated as follows: host crystal, ZnS; activator, Cu; doubtful impurity, [CL]; the initial weight of the introduced activator impurity expressed as a percentage of the weight of the host crystal, (0.01) ;the flux used with its initial weight in per cent of the host crystal weight and enclosed by a bracket to indicate doubt as to its existence in the final product, [NaC1(2)]; and the firing temperature, 850OC. This temperature indicates a t the same time that the structure is cubic; when fired at about 12OO0C, the crystals would be hexagonal. Usually, an abbreviated expression is used, e.g., cubic ZnS :Cu for the above example. A symbol such as (Zn,CdS) :(Cu,Pb) indicates a mixture of ZnS and CdS activated by Cu and Pb, while (ZnS,Se) :Cu represents a Cu-activated ZnS-ZnSe mixture.

1 1.3. Photoluminescence and Cathodoluminescence* (General Luminescence Measurements)

While the larger part of this chapter deals with the methods of luminescence measurements, a brief review is included of the various techniques used to obtain modulated light sources. Many of the steady sources of excitation can be found in references 5 and 7 of Chapter 11.1, and they will not be described here. The same can be said of photodetectors, other than to note that in the visible and ultraviolet photocells, phototubes, and photomultipliers are generally used, while in the infrared thermopiles and various semiconducting detectors such as PbS-cells are employed. A variety of photodetectors have been discussed recently.' For the purpose of measuring luminescence properties, phosphor powders are usually deposited on a surface such as transparent glass. Several techniques are used including dusting, wet and dry spraying, evaporation, settling in liquid suspension, and silk screening. Sometimes, the addition of nonluminescent binding material is required. Many of these techniques, particularly those used in the commercial manufacture of phosphorescence screens, are discussed by Sadowsky.a

* See also Vol. 1, Part 7. 1

R. C. Jones, Advance8 in Electronics 6, 1 (1953).

* M. Sadowsky, Trans. Electrochem. SOC.96, 112 (1949).

302

11.

LUMINESCENCE

Although many of the methods to be discussed originated in photoluminescence and cathodoluminescence research, their usefulness is not necessarily restricted to these forms of luminescence. 11.3.1. Modulated Excitation Sources*

Modulated radiation is essential for the measurement of luminescence decay times and also for the improved detection of weak signal levels above background noise (narrow band technique). Modulation by ac source operation is often sufficient, particularly if phase differences between the exciting radiation and luminescence signals are to be measured. If direct observation of a decay process is required, radiation pulses with rise and fall times significantly less than the decay time to be measured will be necessary. These pulses are formed by choppers, which usually consist of a source in combination with some form of mechanical or electrical shutter arrangement. By proper shutter design, pulses as short as fractional microseconds in duration with variable repetition rates can be obtained. Rotating metal disks containing slits, and driven by a synchronous or governor controlled motor provide the simplest periodic shutter. By using various combinations of angular velocity, diameter of disk, and number of slits cut into the disk, one can obtain periodic light pulses from minutes to about 50psec duration. The light source should be imaged on the slits, and the width of the image should be smaller than the distance separating successive slits in order to prevent a dc component of excitation. In a spinning mirror system, the light from a source is reflected by a rotating mirror and made to sweep across the object to be illuminated. The period of illumination is inversely proportional to the rotational speed of the mirror and the distance from the mirror to the object. By varying these parameters, light pulse times as short as lo-@sec have been obtained.s A “light pulse shaper,” using a high speed air turbine to spin the mirror at 5000 rps and a xenon flash lamp as a source4has produced repeatable intense light pulses with controlled durations from lo-’’ to 10-9 sec, and repetition rates of the order of one every ten seconds. The rise time and shape of the pulse are determined by the shape of an exit slit across which the image of the source is swept. Ultrasonic standing waves can be used to sinusoidally modulate a light beam.6 The series of nodes and loops produced every half period +See also Vol. 1, Part 7. * J. H. Webb, J . A p p l . Phys. 26, 1309 (1955). 4 J. B. Gladis, C. S. Jones, and I (.A. Wickersheim, Rev. Sn’. Znstr. 27, 83 (1955). 5 0. Maercks, 2.Physik 109,598 (1938); E. A. Bailey and G. K. Rollefson, J . Chem. Phys. 21, 1316 (1953).

11.3.

PHOTOLUMINESCENCE AND CATHODOLUMINESCENCE

303

act as a transmission diffracting grating, which periodically deviates a part of the light beam. The zeroth order, which is unmodulated, can be eliminated by a simple lens and stop combination.’’ This manner of chopping permits light modulation well into the megacycle range without requiring high voltage signal generators. Birefringence in a liquid upon the application of an electric field as in a Kerr cell can be used to produce a modulated light beam. By placing the cell between crossed Nicol prisms, a light beam can be obtained with the wave shape of the voltage placed across the cell electrodes. The application of the Kerr cell in the ultraviolet region is di5cult since nitrobenzene, which is normally used because of its high Kerr constant, is highly absorbing. A Kerr cell that operates well with almost any liquid’ uses electrodes acting as a transmission line, thereby reducing the capacitance problem. Thus, long path lengths and liquids with lower Kerr constants can be used. Short light pulses can also be obtained by placing a Kerr cell through a delay line in parallel with a spark gap and allowing the light from the breakdown to pass through the cell. The delay line is adjusted to cut off the Kerr cell a prescribed time after breakdown.8 Gaseous discharge tubes providing rapid, intense light sources have recently come into use. They can be actuated by simple resonance circuitry over a wide range of frequencies, or by pulse circuitry to provide single or multiple light flashes.9 Tubes that provide flashes from to sec are now available commercially. Commercial “electron guns ”* are suitable t o excite cathodoluminescence. They can be used either to produce a steady electron flow, modulated..beams with frequencies up to the radiofrequency range, or pulses of different durations. In dealing with cathodoluminescence, it is well to keep in mind the possibility of secondary electron emission. This may cause so-called phosphor sticking, in which the phosphor assumes a lower potential difference to cathode than that corresponding to the acceleration voltage of the electron gun. This can be avoided by evaporating a thin metal film on the phosphorlo or by applying the phosphor onto a conducting glass anode.”

* See Vol. 4. 6

C. F. Ravilious, R. T. Farrar, and S. H. Liebson, J . Opt. SOC.Am. 44,238 (1954). J. W. Beams and H. S. Morton, Jr. , J . Appl. Phys. 22, 523 (1951) ; see also I. A.

Lewis and F. H. Wells, “Millimicrosecond Pulse Techniques.” McGraw-Hill, New York, 1954. 8 F. G. Dunnington, Phys. Rev. 88, 1506 (1931). N. W. Robinson, Philips Tech. Rev. 16, 13 (1954); J. B. B i r h and W. A. Little, Proc. Phys. Soc. (London) A66, 921 (1953). l o J. de Gier, A. C. Kleisma, and J. Peper, Philips Tech. Rev. 16, 26 (1954). 11 L. R. Koller, J . Electrochem. Soc. 108, 214 (1966).

304

11.

LUMINESCENCE

The luminescence excited by electron guns and appropriate phosphors furnishes ultraviolet light sources with a modulated output determined by the signal applied to the grid of the electron gun.I2 With millimicrosecond electronic techniques now well established, the rise times of these lamps are mainly dependent on the phosphor screens used. Inorganic phosphors have provided rise times of the order of lo-' sec, while some organic phosphors make a response of lo-* sec p0ssib1e.l~ In the same manner, X-ray excited phosphors can be used with properly modulated X-ray sources to produce the desired light pulse ~ h a p e . 1 ~ 11J.2. Spectral Measurements

Emission, absorption, and excitation spectra are among the fundamental properties of any luminescent substance. Emission spectra give the luminescence intensity as a function of wavelength while the wavelength of the exciting radiation is kept constant. Excitation spectra determine the luminescence intensity a t a fixed wavelength versus the wavelength of the exciting radiation. In a similar manner, quenching and stimulation spectra can be defined with respect to the influence of a secondary source of radiation such as infrared directed at an already glowing phosphor. In general, stimulation and quenching spectra are related to the emptying of electron and hole traps, respectively. EEsentially, the excitation and emission spectra yield information about the position of activator levels. Excitation occurring in the fundamental absorption band of a lattice relates to transitions from the valence band. Excitation from activator levels corresponds to absorption in the impurity absorption band, which in most cases is a weak band at longer wavelengths than the strong fundamental band. Excitation in regions of strong absorption is more or less restricted t o the surface. This inhomogeneity of excitation may affect the observations, e.g. in electrophotoluminescence effects. Although excitation and absorption are intimately related, the corresponding spectra do not necessarily coincide since energy absorbed need not be re-emitted but may be transformed into heat. A knowledge of absorption spectra is also important for efficiency measurements. Emission and excitation spectra can be measured with a spectroradiometer, while absorption measurements are easily performed with doublebeam spectrophotometers. The techniques for making these measureA. Bril, H. A. Klasens, and P. Z a h , Philips Research tlepts. 8, 393 (1953). W. Hanle, Z. Naturjorsch. fia, 370 (1954). 14 R. K. Swank and W. R. Buck, Rev. Sci. Znslr. 26, 15 (1953); S. H. Liebson, M.E. Bishop, and J. 0. Elliot, Phys. Rev. 80, 907 (1950). 12

I*

11.3.

PHOTOLUMINESCENCE AND CATHODOLUMINESCENCE

305

ments are fundamentally the same as for nonluminescent material.I6 Therefore, many details shall be omitted. Since most luminescence spectra of solids consist of broad spectral bands, the requirements for spectral resolution are less important than in other spectroscopic work. In many cases, absorption filters with broad transmission bands are quite sufficient for separating the relevant spectral regions. Especially useful is the combination of “complementary” filters, such that one filter transmits only radiation within the excitation spectrum but none of the emission spectrum (to be inserted between source and luminophor), and vice versa for the other filter between luminophor and detector. Since, in general, luminescence is very weak, high speed optics is desirable. Quantitative emission spectra from luminescent powders are obtained by comparison with a standard emission source, such as a tungsten lamp with known energy distribution the emission of which is reflected from a perfect white diffuser, such as MgO powder, considered as a standard. Various automatic instruments have been designed to provide these spectra.l6 Rapid scanning spectrographs’’ are useful for spectral observations during decay. Very accurate excitation and emission spectra have been obtained]*by using two monochromators, recording the ratio of emitted and exciting light, either a t a fixed wavelength of the excitation monochromator (emission spectrum) or of the emission monochromator (excitation spectrum). The output of the excitation monochromator can furthermore be adjusted to equal energy throughout the spectrum, which is convenient for efficiency measurements. A rather instructive method for examining luminescence emission and excitation is that of “crossed spectra” (Newton, Stokes). The real spectral image of a continuous excitation source is formed on a phosphor surface. Then only those regions where excitation takes place are found to luminesce. This luminescence is then imaged on the entrance slit of a spectrometer whose plane of operation is perpendicular t o that of the exciting spectrum. This second spectrometer thus shows the spectral content of every luminescent region. A photograph can then be obtained with the exciting wavelength scale in one direction, luminescence emission wavelengths in the perpendicular direction, and the exposure density providing a measure of the luminescence inten~ity.’~ 16See W. E. Forsythe, ed., “Measurement of Radiant Energy.” McGraw-Hill, New York, 1937. 16 V. K. Zworykin, J . Opt. Soc. Am. 29,84 (1939); A. E. Hardy, Trans. Electrochem. Soc. 91, 221 (1947); F. K. Studer, J . Opt. Soc. Am. 38, 407 (1948). l 7 B. W. Bullock and S. Silverman, J . Opt. Soc. Am. 40, 008 (1950). K. H. Butler, J . Electrochem. Soc. 103, 508 (1950). l o F. E. Germann and R. Woodriff, Rev. 815.Znstr. 15, 146 (1944).

306

11.

LUMINESCENCE

Such photographic methods are very convenient for qualitative observations. For quantitative measurements, they are, however, cumbersome and time-consuming, since it would be necessary to employ the techniques of heterochromatic photographic photometry. Many phosphors are available only as powders. Absorption measurements of powders, however, are not very reliable. Since the absorbance of a powder layer thick enough to prevent any transmission is given by A = 1 - R, where R is the reflectance, it is possible to get a measure of the absorbance by measuring the diffuse reflectivity of the powder. For this measurement, the phosphor grain size should be large compared with the wavelength in order to minimize scattering. The reflectance is normally measured by comparison with a “perfectly white diffuse reflector.” Diffuse reflectance measurements are often made with an “integrating sphere.” Essentially, this is a hollow sphere with its inner wall coated with a diffusely reflecting standard surface such as MgO and with a small area (usually in the order of 3% of the total spherical area) covered by the sample. Radiation entering the sphere from a small porthole is either allowed to fall directly on the sample or on some region of the MgO wall. A photodetector measures the brightness of a region of the inner wall that is illuminated only by the isotropic radiation resulting from the diffusely reflecting spherical wall. The ratio of the respective detector readings provides a measure of the diffuse reflectance of the sample.20 11.3.3. EfFiciency

Phosphor efficiency is defined in several ways. Energy efficiency compares the luminescence radiant energy with the corresponding magnitude absorbed by the phosphor. It is, however, often sufficient to determine energy efficiencies relative to the excitation energy incident on the phosphor rather than to the more difficultly determined absorbed quantity. For the luminous efficiency, q L , which takes into account the spectral sensitivity of the human eye, the emitted energy spectrum has t o be weighted by multiplication throughout the spectrum by the luminosity function V X of the eye so that q L = $E,,Vx dX/JEd, dX. This efficiency is measured in units of lumens per watt. Quantum efficiency (qpu) is the ratio of the number of emitted to absorbed quanta. It can be calculated from the energy efficiency ( q e ) by the relation qpu = qeX,/Xoa., where A, and A d r represent average wavelengths of the emission and absorption spectrum and where 7s = $Em dXI$Eabs (11.3.1) 10

H. J. McNicholas, J . Research Natl. Bur. Standards 1, 29 (1928).

11.3.

307

PHOTOLUMINESCENCE AND CATHODOLUMINESCENCE

More accurately,

qqu is given

by ( 11.3.2)

For cathodoluminescence, it is usual to define efficiency as a ratio of the phosphor emission energy to the energy delivered to the phosphor surface by the electron beam, although sometimes the ratio of the numbers of photons to electrons is used. Power or flux ratios are also used to define efficiencies. In principle, any quantitative spectral intensity measurement of luminescent and incident radiation, together with absorption data will yield a determination of the efficiency. However, various precautions have to be taken to present a true result. These include measuring the light emitted in all directions unless the validity of Lambert’s cosine law has been established, exciting the phosphor particle well into its volume to avoid any undue influence of surface effects, and to correct for luminescence radiation absorbed within the powder layer. If scattering losses can be neglected, if the grain size is small compared with the phosphor layer thickness, and if the thickness is sufficient to prevent the excitation radiation from passing through the layer, the true (intrinsic) efficiency (qi) can be calculated from the measured efficiency by qi = 2 ~ & , ( 1 R J , where R, is the reflectance of an infinitely thick layer.a1 The correction is, however, a function of the thickness of the layer. Finally, another correction might be noted, which takes into account the change of solid angle at the transition from the phosphor layer to air. If I is the observed intensity and lo the true intensity, then

+

I0

I cos f#l n (n2 - sin2 4 ) i

=-

(11.3.3)

where n is the refractive index and f#l is the angle between surface normal and direction of observation.22 The following methods to determine efficiencies are of special interest: Bowen28 has introduced a “relative quantum counter” principle to facilitate integration over the emission band. The radiation to be measured is used to excite fluorescence in a solution that is concentrated enough to absorb all pertinent wavelengths, but sufficiently dilute for the quantum yield to be independent of wavelength. The energy distribution of the fluorescent light is then independent of the wavelength of the A. Bril and H. A. Klasens, Philips Tech. Rev. 16, 63 (1953). E. H. Gilmore, G. E. Gibson, and D. 5. McClure, J . Chem. Phys. 28,399 (1955); 20, 829 (1952); A. Shepp, ibid. 26, 579 (1956). ** E. J. Bowen, Proc. Roy. SOC.A M , 349 (1936); E. J. Bowen and J. W. Sawtell, Trans, Faraday Soc. 88, 1425 (1937). 51

3)

308

11.

LUMINESCENCE

exciting light, and its intensity proportional to the number of exciting quanta. The conditions required for the success of this method are satisfied in restricted wavelength intefvals. The region of validity may extend from 2500 to 4400 R. Errors because of nonuniform emission in the different directions can be avoided by using an integrating sphereqZ4 In some cases, it is practicable to determine the efficiency by comparing the luminous output of a phosphor with its calculated outputz6assuming 100% quantum efficiency. This is particularly convenient for measuring fluorescent lamp efficiencies. By comparing lamp efficiencies obtained in this manner with those of the phosphor powders as obtained conventionally, it can be ascertained whether a low efficiency is due to a specific method of lamp manufacture or to the use of a specific phosphor. If the phosphor is a t fault, the efficiencies of phosphor and lamp are approximately equal. Comparison with standard phosphors would be the easiest method of obtaining relative efficiencies. Standard samples of selected representative phosphors are available from the National Bureau of Standards. Several methods and numerical results have recently been reviewed-zb 11.3.4. Growth and Decay The growth and decay of luminescence intensity after the start and after the end of excitation, respectively, offers information about the population and energy distribution of trap levels, in addition to the time constants of luminescence processes. “ Monomolecular” radiative transitions (fluorescence) within a luminescence center (activator) and also nonmonomolecular transitions if controlled by monomolecular ones such as slow thermal trap emptying, produce an exponential decay, I = 10exp( -at). “Bimolecular” recombinations of free electrons and holes would lead to an inverse square law, I = I o / ( l bt)%.The exact decay law depends not only on the nature of the transition processes but also on the energy distribution of the traps. However, there is not yet a satisfactory method of determining trap distributions uniquely from growth or decay without some arbitrary assumptions.26If the decay is exponential, the time constant 7 = 1/a defines the “mean l i e time,’’

+

BrL. 8.Forster and R. Livingston, J . Chem. Phyrr. 20, 1315 (1952). 2sC. W. Jerome, J . Electrochem. SOC.100, 586 (1953); R. N. Thayer and B. T. Barnes, J . Opt. SOC.Am. 29, 131 (1939). J. Tregellaa-Williams, J . Electrochem. Soc. 106, 173, 1958. 26 J. T. Randall and M. H. F. Wilkins, Proc. Roy. SOC. A184,366 (1946); F. Urbach, N. R. Nail, and D. Pearlman, J . Opt. Soc. Am. 39, 675 (1949).

11.3.

PHOTOLUMINESCENCE AND CATHODOLUMINESCENCE

309

while for other decay laws generally the “half-life” is used to characterize the decay, viz., the time a t which I = +Io. Since both the decay time constant and the efficiency are governed by the superposition of radiative and nonradiative processes, a relation between these properties may be expected. From such relations, transition probabilities may be The decay of luminescence can be measured by directly determining the afterglow intensity I as a function of time, or the decrease in time of the “residual light sum,” Lt = I dt, where t is the time after the end of excitation. For exponential decays, LO = IOT.The measurement of this light sum can be made by observing the stimulated extinction or exhaustion under the influence of infrared or heat applied at the time t with a ballistic method (UrbachZ6). Methods of measuring decay proper are described in the following subsections.

1“

P

4

FIQ.3. Slit phosphoroscope--PI phosphor; D1.*, rotating disks with slits.

11.3.4.1. Slow Decay (t > 10-6 Sec). The earliest method of determining decay times uses the phosphoroscope principle (Becquerel). The phosphoroscope consists of two disks mounted on a common rotational shaft and spaced far enough apart for a phosphor to be placed between them (Fig. 3). Each disk has a sector or sectors cut out forming slits to permit excitation radiation to pass through on one side and to allow a measurement of the emission intensity on the other. The two disks are positioned so that the excitation slits lead the viewing slits during common rotation by some prescribed phase angle. The disks act as two shutters, and adjustment of the phase angle permits a determination of luminous intensity at any prescribed time after excitation. The only requirement on the chopping period is that it be large enough to allow the phosphorescence to die out after each periodically repeated excitation. The excitation time must, in general, be long enough for excitation to equilibrium. Advantages of this principle are its simplicity and the possibility of repeated observation which permits the use of integrating detec-

*’

F. A. &tiger, W. Hoogenstraaten, M. Bottema, and T. P. J. Botden, Physica 14, 81 (1948); R. H. Bube, J . Opt. SOC.Am. 88, 681 (1949).

310

11.

LUMINESCENCE

tors such as the photographic plate for the measurement of very weak phosphorescence. A variation of this principle was introduced by R. W. Wood. It consists of placing the phosphor, in the form of an annulus, around the circumference of a disk28 (Fig. 4). The excitation takes place at some position fixed in space. By changing the angular position of the detector with respect to that of the excitation, the phosphorescence is observed at various times after excitation. This method also permits a photographic reproduction of the luminescence pattern, which can be examined with a

FIG.4. Rotating disk phosphoroscope-E, site of excitation (in rest); D, site of detector (in rest) ; P, phosphor (rotating); shading indicates luminescence intensity a t a certain instant.

densitometer t o obtain a decay curve. It also has the advantage of providing direct observation of color changes during the decay process.2e The light emission excited by a light pulse can also be spread out in time by using a rotating mirror.a0 An extremely useful method consists of using a photomultiplier to detect the luminescence intensity, and feeding its output signal after proper amplification to an oscillo~cope.~~~* The vertical component of the oscilloscope trace gives the luminescence intensity, while the horizontal component represents the time scale, which is produced by a linear sweep.

* See also Vol. 2, Section 11.1.3.

J. T. Randall and M. H. F. Wilkins, Proc. Roy. Soc. A184, 347 (1945). R. P. Johnson and W. L. Davis, J . O p t . SOC.Am. 29, 283 (1939). S. J. Wawilow and W. L. Lewschin, Z.Physik 36, 920 (1926). s1 A. Schleede and B. Bartels, Physik. Z. 39, 936 (1938); W. de Groot, Physica 6, 275 (1939). *a

49

11.3.

PHOTOLUMINESCENCE AND CATHODOLUMINESCENCE

311

Oscilloscope traces thus obtained give a direct picture of the decay curve, which can easily be examined for details or be recorded photographically. The light source must be pulsed. The oscilloscope may be triggered to start its sweep at a prescribed time after the end of the excitation pulse. The sweep speed is adjusted so that the entire decay curve will be seen on the scope. If the luminescence decay is exponential, it can be compared with the trace produced by the discharge of a condenser in an RC-circuit using appropriate triggering of the oscilloscope.32By adjusting R and C , the twoatracescan be matched and the time constant calculated from the circuit data. The RC-signal and the luminescence trace can also be mixed in opposition until the signal disappears on the oscilloscope.83Another useful technique34consists of pulsing the photomultiplier power supply, thus avoiding overloading and a t the same time obtaining the zero reference level as part of the oscilloscope trace. For exponentially decaying phosphors, the response of a phosphor to sinusoidal excitation can be used to determine decay times.a6 11.3.4.2. Fast Decay (T < Sec).* The oscilloscopic method is the most desirable since it provides a direct presentation of the decay curve. Since extremely fast components of such a system can now be obtained, it is possible to measure decay times of the order of sec oscilloscopically. The situation has been further improved by the development of coaxial cables suitable for the transmission of such very short pulses and of “color shifters,” which convert ultraviolet luminescence t o a spectral region more closely matching the spectral response of the detectors. Fluorescence lifetimes can also be measured by an indirect method, vie. by the measurement of the phase angle between a sinusoidally modulated excitation source and its luminescence counterpart. If the intensity of fluorescence is I and the intensity of excitation is J, the differential equation governing the rate of emission is given by dI/dt = -CJ

+ CJ

(1 1.3.4)

where C1 and Ci are constants. If the excitation radiation comprises a steady term plus a periodic variation, then I will also show such terms and in addition an exponentially decaying term that can be neglected. The phase shift between the periodic parts of I and J is given by

* See also Vol. 2, Chapter 9.3.

A. van Roggen and R. A. Vroom, J . Sci. Inslr. 82, 180 (1955). J. W. Strange and S. T. Henderson, Proc. Phys. Soc. (Lwdon) 68, 369 (1946). s4K. SkarsvLg, Rev. Sci. Znslr. 26, 397 (1955). 86 D, T. Wilber, J . Opt. SOC. Am, 39, 673 (1949). 52

33

312

11. LUMINESCENCE

tan q5 = w.6.a8 This method has been used to determine luminescence sec with high acc~racy.~’ decay times of It is also possible to calculate the time constant by measuring the fractional modulation of the excitation radiation, F J , and of the luminescence, FI, since 7

=

[ ( F J / F I )~ l]’/~.

( 1 1.3.5)

Instead of comparing the incident and luminescence signals, it is often convenient to measure the luminescence signal relative to the signal from the reflection of the incident radiation by a nonluminescing substance, or relative to a luminescence signal of much faster decay than that to be

-&

Fro. 5. Shorted line circuit-P,

photomultiplier; A , amplifier.

measured. The phase shift as such is usually determined by purely electrical circuit t e ~ h n i q u e s . ~ ~ A slower and less sensitive method, which, however, has the advantage of simple design and easy possibility of changing the modulation fre~ ~ exciting quency, utilizes a ‘ I fluorometer of variable ~ a t h l e n g t h . ”The radiation is modulated by an ultrasonic wave; the emitted radiation reaches the detector after having passed through the same sonic modulator. The phase of the emitted radiation is changed by varying the path length of the exciting radiation between modulator and phosphor, This results in a periodic fluctuation of the radiation at the detector. Comparison with nonluminescent material gives the desired phase shift. A similar fluorometer with constant path length and using stroboscopic illumination of a moving sound wave by the modulated fluorescence or exciting radiation (Maercksae)is also advantageous. *OF. Duschinsky, 2.Phyeik 81, 7 (1933). F. Rohde, 2.Nalurfossch. 8a, 156 (1953); W. Hanle and H. G. Hansen, ibid. Qa, 791 (1954). See A. Schmillen, 2.Phyeik 186, 294 (1953); E. A. Bailey and G. K. Rollefaon, J . Chem. Phya. 21, 1315 (1953). a9 A. Scharmann, 2.Naturforech. l l a , 398 (1956); 0.Maercks, 2.Physik 109, 685 (1938).

11.4.

THERMOLUWNESCENCE-GLOW

313

CURVES

Another indirect method of measuring luminescence decay is by the shorted line technique.40 A voltage pulse from the photomultiplier, responding to the light flash from a scintillation, is propagated along a coaxial cable with a short-circuited side arm of variable length, immediately followed by a germanium diode rectifier (Fig. 5 ) . The output from the coaxial line is fed to an amplifier, whose effective time constant is long compared with the time constant to be measured, and then to a discriminator, which serves to admit only pulses within predetermined voltage intervals. The amplifier integrates the pulses received and produces a signal proportional to the area of the wave shape of the pulse. The decay time is determined by that length of the shorted line that diminishes the original area to (1 - l/e) of its value because of interference effects. Since each point, for a given discriminator level and shorted line length, represents a measurement of many pulses, statistical fluctuations of individual pulses are automatically averaged out. Time constants of about lo-* sec could be determined within 10 to 20% error. “

1 1.4. Thermoluminescence-Glow

Curves

A powerful method for obtaining information about the number of filled traps and their energy distribution is the evaluation of “glow curves.”1 The method consists in exciting a phosphor at low temperatures or exciting it a t normal temperatures followed by immediate cooling. Then, after removal of the exciting source, the temperature is raised at a constant rate, the traps filled by the excitation process are emptied, releasing the frozen-in luminescence. This will happen predominantly at certain temperatures corresponding to the trap depths e; these trap depths can be considered as activation energies. The glow curve is the luminescence versus temperature curve obtained under these circumstances. In the absence of retrapping and for a constant time rate of change of temperature, p = d T / d t , the trap depth is related to the temperature Tm at which the corresponding peak appears in the glow curve by 6

=

kTmIn s

+ kT,,, 1n(kTm2//ae)

(11.4.1)

where s is the attempt-of-escape frequency, which determines the probability of thermal trap emptying, p , by the equation p = s exp(--c/kT). 40

J. 0. Elliot, S. H. Liebson, R. D. Myers, and C. F. Ravilious, Rev. Sci. Znetr. 21,

631 (1950). (See also Vol. 2, Section 9.3.4.) 1

J. T. Randall and M. H. F. W k i n s , Proc. Roy. Soc. A184, 365, 390 (1945).

314

11.

LUMINEBCENCE

The second term in the equation for e is usually much smaller than the first. If there are a number of well separated trapping levels, the glow curve will show a peak for each of these levels. The number of filled traps is proportional to the area under each peak. However, relative numbers of traps of different depths will be correctly obtained from the corresponding peak areas only if the quantum yield of luminescence does not depend on temperature (or trap depth). The use of this method for evaluating trap depths depends on the assumptions of uniform rate of temperature change, separate trap levels, and absence of retrapping. With temperatures above that of liquid air, the usual method of electrically heating an appropriate Dewar vessel is sufficient to obtain uniform rates from 0.5 to 5"C/sec if thermal insulation is good. Small P-values increase the resolution of the glow curve, although extremely low values are to be avoided since thermal leakage may then exist at low temperatures, which may invalidate the condition of " frozen-in luminesshifts with P. cence." T,,, If the peaks are not distinct, it may be mathematically possible t o synthesize the observed glow curve by superimposing assumed component curves. However, even if such a synthesis is successful, its physical significance should not be overestimated. If the luminescence is influenced not only by the thermal emptying of traps but by other processes, in particular by retrapping, the glow curves may become difficult to interpret. In general, retrapping may be neglected as long as the number of empty traps is small compared with the number of luminescence centers. The influence of retrapping may be tested by comparing the relative numbers of electrons in shallow and deep traps as a function of excitation energy.2 If there is appreciable retrapping, it should be expected that a peak corresponding to a deep trap is relatively stronger at low intensities than a t higher ones, since retrapping in deep traps is relatively more important if few traps are filled, i.e. for weak excitation. The dependence on excitation time and the shape of the growth curve taken during excitation immediately after the traps have been emptied by heat treatment are relevant to the question of retrapping.3 Trap levels determined by glow curves need not coincide with those obtained from infrared trap emptying (stimulation) , since the energy transfer processes may be different for the two cases.

* M. H. F. Wilkins and G . F. J. Garlick, Nature 161, 565 (1948).

SR. H. Bube, RCA-Report on Trapping in Solids, Contract N6onr-236, April, 1950. For] critical comments on the glow curve method see further: C. H. Haake, J . Opt. Soc. Amer. 47, 649 (1957); K. W. Boer, S. Oberlilnder, and J. Voigt, 2. Naturforsch. 18a, 544 (1958).

11.5.

ELECTROLUMIN~SCENCE

315

The attempbof-escape frequency is of the order of 109-1010sec-l and may be obtained, a t least in principle, from photoconductivity, phosphorescence decay, and dielectric relaxation measurements. However, the s-values enter into these properties in an involved way, and as yet no clear cut relation exists that will permit a reliable determination of this frequency. Therefore, 8 is usually determined by a trial and error method of curve-fitting. A direct determination of s from the shift of T,,, with B leads to quite different results.4 At low temperatures (60to 80°K),the glow curves show rapid changes of luminescence intensity with temperature, offering the possibility of optically determining small temperature variations in this range.6 In this case, the It is also possible to obtain "electric glow phosphor with its traps filled by ultraviolet excitation is subjected to a uniformly increasing electric field, and the change in conductivity is measured. At certain field strengths, large sudden increases of conductivity occur, which are interpreted as evidence of trap emptying. At very high field strengths, prior to breakdown, luminescence interpreted as electroluminescence appears. Such experiments provide independent evidence for the fundamental processes of electroluminescence and electrophotoluminescence.

1 1.5. Electroluminescence Properties of primary interest in electroluminescence are: (1) the field strength and frequency dependence of the time-average light output; (2) the amplitude and shape of brightness waves when excited by sinusoidal or nonsinusoidal fields; (3) the influence of temperature on these properties; (4)observations on light output of a virgin phosphor, i.e., in the first instants after an electric field is applied to a phosphor whose traps have been completely emptied by heat treatment, infrared radiation, or long rest periods in the dark. Combinations of photoconductors and electroluminescent phosphors in series or in parallel have been designed for image intensifiers, storage light amplifiers, bistable switching devices, and for other applications.' F. A. Kroger, Physica 22, 637 (1956). N. Riehl and H. Ortmann, 2.Naturforsch. Qa, 890 (1955). 6 K. W. Boer and U. Kummel, 2.Naturforsch. Qa, 177 (1964). 1 J. E. Rosenthal, Proc. 1. R . E . 4S, 1882 (1955); B. Kazan and F. H . Nicoll, ibid. p. 1888; E. E. Loebner, ibid. p. 1897; G . Diemer, H. A. Klasena, and J. G. van Santen, Philips Research Rept. 10, 401 (1955).

316

11.

LUMINEBCENCE

In all these devices, a change in the photoresistance by irradiation is used to change the voltage across the phosphor layer and thereby to change the electroluminescence intensity. 11.5.1. Electroluminescence Cells

In order to apply an electric field to phosphors, the phosphor is placed in an “electroluminescence cell.” This cell consists generally of one electrode made of glass coated with a thin transparent conducting film, on which a phosphor powder embedded in a plastic layer is deposited, and an evaporated metal film covering the plastic layer as the other electrode. Essentially the same cell construction applies in the case of luminescent films formed from vapor deposition on a TiOz-covered glass electrode (Section 11.2.3). The powder-plastic layer should be as thin as possible limited by the danger of electric breakdown. Usual cell thicknesses are in the order of 100 1. The conducting film on the glass surface should not have too large a resistance lest there be an appreciable difference of potential and phase between the edge region where the voltage is applied and the opposite edge or the center of the film area. Films of about 200 fl resistance or less for an area of 1 cm2 are satisfactory for most purposes. The use of a plastic as dielectric matrix is advantageous because the phosphor can be embedded in a uniform distribution. It is easily handled and gives reproducible results. These permanent or “solid” cells can be made with a wide variety of dielectrics including araldite (synthetic ethoxyline resin, polymerized by heating at 160’C for about 20 min), plasticized solutions of polystyrene in isobutyl-ketone, Parlodion (nonexplosive purified nitrocellulose), and polyvinylchloride. Ceramics or glasses have also been used, in particular for silicate phosphors; they reduce “fatigue” effects. It is also possible to examine the phosphor powder without any dielectric, or t o use a weakly conducting medium such as tricresyl phosphate.2 An insulating layer such as mica placed between an electrode and the powder is useful, especially if the phosphor powder itself is conducting. For further valuable hints on cell construction see Mattler.* For many purposes, in particular for testing different phosphors, it may be useful to mix the phosphor in a “fluid cell” with liquid dielectrics such as dielectric grade castor oil or silicone oils poured as a thin layer into a container whose bottom is a metal electrode and whose top is a conducting glass electrode. a

P. Zalm, J . phy8. radium 17, 777 (1956); Philips Research Repts. 11, 353 (1956). J. Mattler, J . phys. radium 17, 42 (1956).

1 1.5.

317

ELECTROLUMINESCENCE

It must be realized that the electroluminescence phenomena may be quite different for different kinds of media, mainly because the field applied to the suspended phosphor grains depends on the embedding medium. The weakly conducting medium, for instance, permits the field applied to the grains to be practically identical with that applied to the electrodes, while in insulating media this is prevented by polarization effects. In general, ac electric fields must be used, and the embedding medium is chosen with a dielectric constant as high as possible in order to obtain a high local field strength a t the phosphor grains. It is these local fields, of course, and not the applied field a t the electrodes that determine the electroluminescence effects. Because of the complicated electrical structure of the usual phosphor cells, the effective field cannot be rigorously determined. There are several useful approximations for estimating the electrical properties of the cells. The phase shift between applied and effective field can be approximated4 by tan $ = h / e p w where e is the dielectric constant of the phosphor, p is its resistivity, and w = 2rf is the angular frequency. This expression is valid for homogeneous phosphor grains and absence of diffusion effects. The field at the grain may be estimated for a suspension of spherical particles by6 EP - =

Eo

2Sl

+

361

e2

-

V(S2

- €1)

V

= $ra8N

(11.5.1)

in which Eo is the external field, E p the effective field; €1 and e2 are the dielectric constants of medium and phosphor, respectively; a is the particle radius, and N is their number in unit volume. For the theory of the field distribution within an inhomogeneous phosphor grain containing barrier layers see papers by Zalm2 and by Piper and Williams.' The electrical properties of electroluminescence cells can be described mathematically with the aid of complicated equivalent circuits.' A correct description of this kind would permit estimating the frequency dependence of the effective field a t the phosphor grain. This relation is necessary for a reliable interpretation of the frequency dependence of electroluminescence. In general, it is sufficient to consider a simple circuit of a resistor in series with a parallel arrangement of a condenser and resistor. From such circuitry, it can be shown that the capacitance of the cell should be as small as possible, and the resistance should be as large as possible, in order to avoid undue frequency effects on the effective field. For capacitances as large as 100pf and resistances as low as G. Destriau, Phil. Mag. [7]38, 700 (1947). S. Roberts, J . Opt. SOC.Am. 42, 852 (1952). W. W. Piper and F. E. Williams, Brit. J . Appl. Phys. Suppl. 4, 39 (1954). A. N. Ince and C. W. Oatley, Phil. Mag. [7] 46, 1081 (1965).

318

11.

LUMINESCENCE

lo6 8, these effects would set in around lo4 cycles/sec. The loss factor tan 6 should satisfy the conditions tan 6 < 1 .

1 1 3.2. Crystal Mounts Figure 6 shows a special cell construction for observing electroluminescence from dc to microwave frequencies in single crystals.9 The basic unit is a detachable section of a wave guide, with a capacitance of 1 to 10 ppf. When this unit is detached from the guide, low frequency signal generators can be coupled t o the crystal through a coaxial connector. I

FIG. 6. High-frequency mount for crystals, schematical-1, UHF-connector with teflon insulation; 2, removable rf-connector to small-area electrode; 3, small-area electrode; 4, crystal; 5, large-area grounded electrode around crystal edge; 6, detachable section of a wave guide; 7, heat insulator; 8, beam-splitting (dichroic) mirror; 9, collimating lens; 10, optical filters; 11, photomultiplier chamber; 12, emitted light.

The unit can be fitted with a small removable oven for temperature dependence measurements. In addition, the superposition of radiation is possible by the use of the beam-splitting mirror arrangement. A device for the simultaneous observation of luminescence and current in single crystals is described by Frankl.10 The crystal, with suitable electrodes, is mounted opposite a photomultiplier on a quartz plate that is in contact with a copper block in a bath at constant temperature. This gives good electrical insulation and heat conduction, permitting reliable measurements of temperature effects. * H . A. Klasens, P. Zalm, and G. Diemer, Proc. Intern. Comm. on Illumination, ZzZrieh, 1966. OG. Harman, Rev. Sci. Inslr. 28, 127 (1957). R. D. Frankl, Phys. Rev. 100, 1105 (1956).

11.5.

ELECTROLUMINESCENCE

319

11.5.3. Methods of Observation The operation of the phosphor powder-plastic layer cells requires ac voltages from 200 to 700 volts. For single crystals and films, dc excitation is also possible. High electroluminescence intensities can also be obtained by making the cell a part of a resonance circuit.ll The light output is usually detected by photocells or photomultipliers. Instead of measuring the intensities directly, the threshold of the field at which electroluminescence begins to appear may be utilized as a convenient qualitative method of comparing intensities. The brightness waves can be observed and measured oscillographically, while the timeaverage light output may be measured by feeding the output from the photodetector into an RC averaging (integrating) circuit, using a meter to measure the averaged signal level. The integrated light output or light sum obtained from nonperiodic emission is best observed by ballistic methods.12 Since the details of brightness waves and the frequency dependence of the time-average light output are sensitive to the shape of the electric field pulses or field “waves,” it is advisable to monitor the field oscillographically. At high frequencies and field strengths, precautions should be taken against heating of the phosphor because of dielectric losses. Electroluminescence phenomena can be different (even qualitatively) in different emission bands of the phosphor. Therefore, separate observations for each of the bands is desirable. The separation of the bands can be conveniently done with appropriate filters, care being taken to exclude overlapping spectral regions. If the luminescence is sufficiently intense, the use of a spectrograph is, of course, to be preferred. With the aid of a scanning spectrograph, it is possible to combine oscillographic and spectral observations in one single experiment. ’s This method yields intensity versus wavelength diagrams for several short time intervals within a period of the field. The decay of electroluminescence excited by pulsed electric fields is different from that of photoluminescence decay. Under certain circumstances, it is possible to deduce from the decay rates for two different pulse durations the rate of accumulation of polarization charges.’* In investigating the temperature dependence of electroluminescence, it is essential that the measurements be made in a state of thermal equilibrium. Otherwise, the results resemble glow curves that are modified A. Luyckz, Compt. rend. 240, 2229 (1955). J. F. Waymouth and F. Bitter, Phys. Rev. 96, 941 (1954). 13 E. E. Loebner, Phys. Rev. 92, 846 (1953). 1 4 F. Matossi and S. Nudelman, Phys. Rev. 99, 1100 (1955). 11 12

320

11.

LUMINESCElNCE

by electric fields (“electrothermoluminescence”) rather than the temperature dependence of electroluminescence. For thermal equilibrium, the luminescence versus temperature curves should be the same for increasing and decreasing temperatures, whereas this need not be the case in electrothermoluminescence. The observations are usually made by applying the field anew at each temperature. If the field is maintained throughout the measurements, somewhat different curves are obtained. The reproducibility of the results depends on the cell construction. The temperature dependence of the dielectric constant has to be considered in determining the effective field a t each ternperat~re.~ In many cases, a minimum in the intensity versus temperature curves is obtained, which is related to the trap depth.“ The methods of observing electrophotoluminescence or photoelectroluminescence are not in principle different from electroluminescence methods. However, there are more parameters influencing the effects, such as sequence of, duration of, and resting times between field and radiation ; intensity and absorption of exciting radiation; polarity of dc field pulses with respect to direction of observation and irradiation. 11 5 4 . Surface Effects

The surface of phosphor grains plays an essential role in the electroluminescence phenomena. It provides barrier layers at the contact of semiconducting grain and metal electrode or copper-enriched layers (11.2.4). It also provides boundaries at which polarization charges may accumulate. The problem of the relative importance of surface and bulk regions is not yet solved. Among the observations giving clues t o the interplay of surface and bulk effects, the following may be menti0ned.~,6.’~ Different electrode material may produce different brightness-voltage characteristics. The intensities in subsequent half-periods of the field are often different. A t low field strengths, the emission is stronger when the detector is on the anode side; a t high fields, the stronger emission is observed at the cathode side. This asymmetry is partially due t o self-absorption in the grains, partially to asymmetric cell construction, partially to grain inhomogeneity. Microscopic observation of emission from single grains and H. Gobrecht, D. Hahn, and H. E. Gumlich, Z. Physik 136, 623 (1954). ISP. D. Johnson, W. W. Piper, and F. E. Williams, J . Electrochem. SOC.103, 221 1)

(1956).

17SeeG. Destrieu and H. F. Ivey, PTOC. I . R . E . 43, 1911-1940 (1955); F. Matossi, “Elektrolumineszenz und Elektrophotolumineszenz.” Vieweg, Braunschweig, Germany, 1956.

11.6.

PRODUCTION AND DETECTION OF RECOMBINATION RADIATION

321

dependence of asymmetry on cell thickness, spectral region, field frequency, or superimposed irradiation, can give valuable information. The brightness waves often have primary large peaks approximately in phase with the field and secondary small peaks which are out of phase. These are probably related to polarization and diffusion effects. Interesting results about the behavior of brightness waves are also obtained by superimposing on the sinusoidal field short field pulses at different times within a field period.2 The absence of magnetic field effects on electroluminescence18indicates that the electrons are not appreciably deviated from their accelerated motion along the direction of the electric field. They apparently need only small path-lengths for sufficient energy gain, a situation possible only in high fields near the phosphor surface.

1 1.6. Production and Detection of Recombination Radiation* Radiation which is produced by the recombination of excess holes and electrons has been obtained from a wide variety of semiconductor^.^-^ It has been shown to be of two kinds;6 (1) intrinsic radiation which is due to the recombination of electrons in the conduction band with holes in the valence band; (2) extrinsic radiation which is produced by the recombination of holes or electrons through intermediate states. The energy distribution of the photons of intrinsic radiation permits a determination of the energy gap, while differences between the photon energies associated with the maximum photon emission of intrinsic and extrinsic radiation yield information concerning the energy levels of the intermediate states involved in the recombination process.6a According to the principle of detailed balancing, recombination radiation is produced in any semiconductor which exhibits photoconductivity. In order to obtain detectable amounts of radiation, however, it is neceslsA. N. Ince, Proc. Phys. SOC.(London) B67, 870 (1954). 1 0. V. Lossev, Phil. Mag. [7] 6, 1024 (1928). * K. Lehovec, C. A. Accardo, and E. Jamgochian, Phys. Rev. 88, 603 (1951). 8 J. R. Haynes and H. B. Briggs, Phys. Rev. 86, 647 (1952). 8. R. Braunstein, Phys. Rev. 99, 1892 (1955). C. 1;. Van Doorn and D. DeNobel, Physica 22, 338 (1956). 6 J. R. Haynes and W. C. Westphal, Phys. Rev. 101, 1676 (1956). 6* R. Newman, Phys. Rev. 106, 1715 (1957).

* Chapter

11.6 is by 1. R. Haynes.

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sary to increase the carrier concentration sufficiently above its equilibrium value. This has been done in two ways: (1) by the creation of electron-hole pairs with light; (2) by electrical injection of minority carriers. A schematic diagram of a simple arrangement useds for the production and analysis of recombination radiation is shown in Fig. l(a). Light from a tungsten lamp is passed through a suitable filter and focused on a sample of semiconductor. The filter is chosen* so that it transmits only wavelengths less than the long wave transmission limit of the semiconductor. With this condition the radiation from the tungsten lamp is unable

FIG. 1. Schematic representation of arrangements used to produce recombination radiation by increasing the carrier density: (a) with light of energy greater than the semiconductor forbidden band gap, (b) and (c) with electrical injection.

to enter the spectrometer since it is absorbed in the semiconductor by the creation of electron-hole pairs. The recombination of these carriers, however, produces radiation a t wavelengths near the absorption edge of the semiconductor and this may pass through it into the spectrometer and detector. Recent studies of recombination radiationsa have been made by using an Osram HBO 500 high pressure mercury arc in conjunction with a filter consisting of two Jena KG-1 filter plates 0.5 cm thick separated by 1.5 cm of water. This combination gives greatly increased light intensity and is suitable for use with either germanium or silicon.

* A water cell 10 cm thick is a satisfactory filter for germanium. The addition of 2% CuClt to the water is required for silicon. J. R. Haynes, Phys. Reu. 98, 1866 (1955). J. R. Haynes, M. Lax, and W. F. Flood, 1958 International Conference on Semiconductors, J . Phys. Chem. Solids 8, 392 (1959).

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PRODUCTION AND DETECTION OF RECOMBINATION RADIATION

323

Higher concentrations of minority carriers and larger amounts of radiation may be obtained by the use of electrical injection. Both emitter points and p-n junctions have been used. Usually p-n junctions are to be preferred because of their higher injection efficiency at high current densities. An arrangement used successfully with germanium and silicon is shown in Fig. l(b). The specimens were cut from grown p-n junction single crystals which were optically polished on the large surfaces. The specimen thicknesses used have varied from 13 I.( to 0.5 cm or more. Low resistance electrodes were provided by sandblasting the electrode areas and then electroplating, first with rhodium and finally with gold or copper. When current is passed through the junction in the forward direction indicated, high concentrations of minority carriers may be built up within a diffusion length of each side of the junction. Recombination of these carriers produces recombination radiation that has its origin in a line which may be focused on the entrance slit of a spectrometer. The intensity of the image which can be focused on the spectrometer slit is greatly reduced because of the high index of refraction, n, of most semiconducting materials. This is partly due to the consequent high reff ection loss a t the semiconductor boundary but it is mostly produced by the increased divergence of the rays which emerge a t this point. It is easy to show that the intensity loss due to increased divergence reduces the intensity of the radiation in a direction normal to the semiconductor surface by a factor of l/n2. The choice of source optics deserves careful consideration. Two conditions are required for maximum signal-to-noise ratio: (1) the image focused on the spectrometer slit should completely iill it; (2) the angle of the rays converging to form the image on the slit should be equivalent to the effective aperture of the spectrometer. The use of smaller angles does not utilize the full effective solid angle of the spectrometer while larger angles only serve to increase the scattered light. Since image brightness is a constant with a fixed angle of convergence of rays, the f ratio of the source optics should be chosen to achieve a magnification of the source which is sufficient to completely fill the slit while utilizing the full effective aperture of the spectrometer. An ingenious scheme for using the semiconductor itself to provide optics of large numerical aperture has been described by Aigrain’ and Benoit 3, la Guillaume.8 A schematic drawing of their arrangement is shown in Fig. l(c). An optically polished sphere of semiconductor is ground flat on one side a t the Weierstrass point. An emitter point contact or an alloyed p-n junction is made a t this point. A virtual image of the 8

P. Aigrain, Physica 20, 1010 (1954). C. Benoit A la Guillaume, Compt. rend. 245, 704 (1956).

324

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LUMINESCENCE

radiation source having a magnification of n2 is formed behind the sphere as indicated by the projection of the dotted lines in the figure. The virtual image may then be focused on the spectrometer slit. Thus a Weierstrass sphere may be used to greatly increase the size of the image and so fYl the spectrometer slit. Analysis shows, however, that the image brightness is not increased through its use, and that the brightness loss of l/n2 occurs regardless of the curvature of the semiconductor surface. The choice of detector depends upon the intensity of the radiation produced and the spectral region to be investigated. With all detectors a very considerable increase in signal-to-noise ratio may be obtained by synchronous chopping and detection of the radiation. This may be done with a light chopping wheel and mechanically synchronized contacts. Where electrical injection is used, interruption of the injection current is more desirable, since heating of the semiconductor is reduced by the lowered duty cycle and spurious signals due to ambient radiation are eliminated . In all cases the experimenter should convince himself that the radiation detected is true recombination radiation and not simply heat radiation as modified by Kirchhoff’s law.

1 1J . Color Centers* 11.7.1. Introduction Many inorganic materials develop new absorption bands upon exposure to ionizing radiations, upon heating with an excess of their metallic or The absorbing centers nonmetallic constituents, or upon electrolysis.*~2 responsible for these bands are called “color centers.” The induced absorption bands are usually simple bell-shaped curves which may lie in the ultraviolet, visible, or infrared spectral regions. In the alkali halides, where these phenomena have been studied most extensively, the band that appears earliest and most prominently on exposure to ionizing radiation is called the “F-band,” and the centers responsible for it are called “F-centers” (from the German “Farbzentren ”) . The F-center is believed to consist of an electron trapped a t a halogen ion vacancy. Many other absorption bands both to shorter and longer wavelengths than the F-band may be produced either directly by the above procedures 1 R. W. Pohl, Proc. Phys. Soc. (L&) 49 (extra part), 3 (1937). 2

R. W. Pohl, Physik. Z. 89, 36 (1938).

* Chapter

11.7 is by James H. Schulman and Howard W. Etzel.

11.7.

COLOR CENTERS

325

or by transformations of the initially-formed bands through heat treatment or exposure to light. The various bands in the alkali halides and their corresponding centers have been named in an arbitrary fashion. For example, bands generally lying toward longer wavelengths than the F-band and believed to arise from electron trapping a t crystal imperfections have been designated as F’, R1, Rz, M, N , 0, etc., while those lying generally to shorter wavelengths than the F-band, and believed to arise from the trapping of holes at positive-ion vacancies and similar imperfections have been designated as the Vo, V1, V2, etc., hands. There are bands, known as the a and @ bands, which lie in the ultraviolet and are believed to represent a perturbation of the lattice absorption by negative-ion vacancies and F-centers respectively. Certain absorption bands arise only if the treated crystal contains chemical impurities, and the center responsible involves these impurities. Details concerning the formation of these centers and theoretical models of many of them are discussed in numerous publications.’-7 I n addition to the foregoing bands, which are due to atomically dispersed centers, other absorption bands arise from colloidal dispersions of an excess of one of the crystal constituent^.'^^ The terminology, experimental methods, and theory of color centers have also been applied to analogous coloration phenomena in other inorganic crystalline solids as well as in vitreous materials like the inorganic glasses. Crystal imperfections of thermodynamic, chemical, or mechanical origin, such as vacancies, chemical impurities, or dislocations play an important role in coloration phenomena, which are accordingly very sensitive to the purity and to the mechanical and thermal history of the sample under study. The basic measurement in color center research is the determination of the absorption spectrum, which gives the absorption coefficient as a function of wavelength from the relation ah =

I0 -1 In d I

where ax is the absorption coefficient, for wavelength A, d is the thickness of the specimen, and I/Iois the fraction of light transmitted. The instrua N. F. Mott and R. W. Gurney, “Electronic Process in Ionic Crystals.” Oxford Univ. Press, London and New York, 1940. 4 F. Seita, “The Modern Theory of Solids.” McGraw-Hill, New York, 1940. 6 F. Seitz, Revs. Modern Phys. 18, 384 (1946). 6 F. Seita, Revs. Modern Phys. 26, 7 (1954). 7 K.Praibram, “Irradiation Colours and Luminescence.” Pergamon, London, 1956.

326

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mentation and techniques used in this measurement are discussed in Part lo.* From these experimentally measured optical quantities the product of the concentration of a color center and its oscillator strength has usually been computed by Smakula’s equation4v7(see, however, reference 8) (11.7.1) where N = number of centers/cms; f = oscillator strength for the optical transition producing the absorption; n = refractive index of the crystal for the wavelength a t the maximum of the absorption band; amx = absorption coefficient in cm-l a t the maximum of the absorption band; W = width of the absorption band in electron volts measured where CY is half of its maximum value. For bands well removed from the lattice absorption, n/(n2 2)2 changes very little with wavelength and a simpler relation

+

Nf = Acu,..W

(11.7.2)

may be used with good approximation, where A = 1 X 101s/ev-cm2for all the alkali halides. The optical measurements alone can give the value only of Nf,unless an assumption is made concerning the oscillator strength. It appears that f is near unity for the F-center in the alkali halides. This, of course, is not necessarily the case for other centers in the alkali halides or for color centers in other materials. The experimental determination of N itself is possible in certain circumstances, when the absorption bands can be shown to arise from a single center and where the nature of the center permits it to be counted by chemical9 or other means, such as by the intensity of paramagnetic resonance absorption.

11.7.2. Production of Color Centers 11.7.2.1. Additive Coloration. In this method the material is exposed to the vapor of the metallic or nonmetallic constituent a t appropriate temperatures and pressures, to give one kind of center-either of the trapped electron or the trapped hole type-free of the other type. A stoichiometric excess of the constituent is built into the solid and hence the color centers are not removable without removal of the excess element. The following discussion deals with additive coloring using excess

* See also Vol. 1, Part 7. 8 0

D. L.Dexter, Phys. Rev. 98, 1533 (1955); 101,48 (1956). F. G. Hleinschrod, Ann. Physik [5]27, 97 (1936).

11.7.

COLOR CENTERS

327

metal.*J Coloring with excess nonmetal is carried out in a similar fashion.’t2 The container for the experiment may be a hard glass tube (Pyrex or Supremax) connected to a vacuum pump and to a reservoir of purified metal. A small amount of the metal is vacuum distilled into the vessel, which is then sealed and heated a t the desired temperature. When high temperatures are employed that would either collapse or burst the glass vessel, it can be enclosed in a fused silica tube or metal envelope, with the intervening space filled with inert gas to a partial pressure adjusted to equal that of the added element a t the coloring temperature. The use of fused silica or Vycor to contain the crystal plus alkali metal directly is not advisable, for there appears to be a reaction between the SiOz and the latter which “getters” the metal; however, these containers are suitable for additive coloring with halogen. Alternatively, the solid can be inserted into a perforated stainless steel vessel provided with a closure of the same material. This vessel is then placed in a glass envelope of closely the same size and shape. The apparatus is charged with metal as above and sealed. On heating, the glass collapses around the perforated metal core but remains vacuum tight. In a cruder method of additive coloration sometimes used, the crystal and a lump of the alkali metal are placed together in a steel bomb, which is then capped without exhausting the air, and heated to the required temperature. A fast quenching of the system is desirable, in order t o prevent the precipitation of colloidal centers which often results from slow cooling. At this stage of the coloration process, with sizeable crystals enclosed in fairly massive containers, it is usually impossible to quench fast enough to prevent colloid formation. In these cases it is necessary to perform a subsequent heat treatment, perhaps of a small portion of the crystal, followed by a rapid quench to room temperature or below. This is often done with a bare crystal or a crystal wrapped in a light-tight foil of platinum so that quenching will be facilitated. Since this reheating is done in the absence of an equilibrium pressure of alkali metal vapor, the crystal should be kept a t the high temperature for a minimum length of time or the excess alkali will be distilled out and the color centers will be lost. It is found that the same color centers are formed in a given alkali halide independent of the nature of the alkali metal used in the additive coloring. The various metals will have different vapor pressure versus temperature curves, however, hence the color center concentration attained on additive coloration by the above methods, where both metal and crystal are heated a t the same temperature, will depend on the metal employed. Special vessels can be constructed in which the vapor pressure of the metal and the temperature of the crystal can be controlled

328

11. LUMINESCENCE

independently. At high temperatures crystals of reasonable thickness (-1 cm) can be uniformly colored in a short time, say of the order of hours. At lower temperatures, days to weeks may be required for uniform coloration. The color center concentration as a function of temperature of the crystal, and the pressure of the excess metallic and nonmetallic constib uent are illustrated in Fig. 1 for KBr.le2 It will be noted that (a) the color center concentration is proportional to the concentration of element in the gas phase, and (b) the solubility of the excess metal in the crystal decreases with increasing temperature, while that of the halogen increases with increasing temperature. Thin films of additively colored salts can be prepared by simultaneous evaporation of the compound and an excess of the metal or nonmeLal constituent and condensation of the mixed vapors on a cold substrate.lo 11.7.2.2. Electrolytic Coloration. In this method an electric field is applied to a crystal a t temperatures where electrolytic conduction is appreciable.'' In order to produce only trapped electron centers in the crystal the cathode is a pointed wire which is heated and driven into one end of the crystal. The materials commonly used for the electrode are platinum, molybdenum, or tungsten. The anode in this case is most often a film or foil of platinum which can be applied in a number of ways. A field of about 100 volts/cm, applied with the crystal held about 100°C below its melting point, are reasonable conditions for producing color centers by electrolysis. The furnace usually contains a window to permit observation of the crystal during the coloring. A cloud of color centers appears to emerge from the pointed cathode and migrate towards the anode. Trapped-hole centers can be produced in some of the alkali halides by making the pointed electrode the anode. The color centers formed by electrolysis, like those formed by additive coloration, are due to a stoichiometric excess of one of the elements. Removing the crystal from the furnace after electrolysis usually constitutes enough of a quench to prevent the coagulation of the color centers. 1 1.7.2.3. Coloration by Ionizing Radiation. In this method crystals are exposed to X-rays, gamma-rays, electron beams, or heavy ionizing p a r t i ~ l e s . ~Unlike ~ ~ ~ 7 methods shown in Sections 11.7.2.1 and 11.7.2.2, radiation coloring produces trapped-electron and trapped-hole centers simultaneously, which can interact during the irradiation or subsequent to it. The rate of growth or the efficiency of production of color centers and the saturation value observed, depend in a complicated fashion on 10 W. Buckel and R. Hilsch, 2. Physik 181, 420 (1952). 11

0. Stasiw, Gdttingen Nachr. Gea. Wise. Jahresber. Geschiiiflsjahr Math.-physik.

K1.Fachgruppe ZI, p. 261 (1932).

11.7. COLOR

CENTERS

329

Number of K atoms per cm3 of vapor

(a)

FIQ.1. Solubilities of excess constituents in potassium bromide: (a) K in KBr, and (b) Brz in KBr.

330

11.

LUMINESCENCE

the type of radiation and the dose rate and the temperature, as well as on the purity and mechanical state of the sample. Beryllium-window X-ray tubes operating at 40-50 kvp and 15-25 ma have been most commonly employed for experiments on radiation coloration. Intense colorations can be achieved due to the presence of a large soft X-ray component in the radiation from such a tube. This same fact makes the coloration vary rapidly with depth of penetration, which is a disadvantage for quantitative work in which the optical absorption coefficient is needed. In using such soft polychromatic radiations thin crystals should be employed and irradiation should be applied from both sides to achieve maximum uniformity of coloration. The bands observed upon exposing crystals to ionizing radiation depend on the temperature of the sample during irradiation. Low temperature coloration experiments are of particular interest because of the generation of centers not observed at normal and because of the lower rates of thermal bleaching of the centers formed. For crystals colored at higher temperatures, increased resolution is obtainable if the optical measurements are made a t low temperature. Figure 2 shows a particularly useful arrangement for performing both the irradiation and the optical measurements a t low temperature without an intermediate warm-up of the ~ a m p l e . 1 ~ Colorability by radiation may be considerably enhanced by plastic deformation of a crystal,' incorporation of specific impurities,6*1sand by prior exposure to high energy radiation.I6 In addition to coloring the crystal, high energy radiation causes density changes, measured by the techniques described in Part 4,and ascribed to the creation of positiveand negative-ion vacancies by the radiation itself .I7 Color centers can be formed also by irradiation into the long wavelength tail of the fundamental absorption band of a crystal.I8 The bands produced in this way are usually too weak to be observable to the eye, but can be easily measured by using a spectrophotometer. 11.7.2.4. Special Types of Color Centers. ( a ) Colloidal Centers: Color centers attributed to colloidal aggregates of alkali metal are observed after additive or electrolytic coloration when the crystal is annealed in the R. Casler, P. Pringsheim, and P. Yuster, J . Chem. Phys. 18,887 and 1564 (1950). H. Dorendorf, 2. Physik 120, 317 (1951); see, also, H. Dorendorf and H. Pick, ibid. 128, 166 (1950). 14 W. H. Duerig and I. L. Mador, Rev. Sci. Znstr. 23, 421 (1952). l6 H. W. Etzel, Phys. Rev. 87, 906 (1952). 16 H. W. Etzel, Phys. Ziev. 100, 1643 (1956). 17 I. Estermann, W. J. Leivo, and 0. Stern, Phys. Rev. 76, 627 (1949). A. Smakula, 2. Physik 69, 762 (1930). 1s '3

11.7. COLOR

CENTERS

FIG.2. Low temperature irradiation and absorption cell.

331

332

11.

LUMINESCENCE

vicinity of 400°C. Colloid bands may be distinguished from atomically dispersed centers by ultramicroscopic observation, by the observation of Tyndall scattering, and by the invariance of the halfwidth of the band and the band position with temperature.' The position of these absorption bands depends on the particle size of the colloidal aggregates and is well described by the Mie t h e ~ r y . lIf~ the * ~ ~crystal is raised to a temperature within about 100°C of the melting point and quenched to room temperature or below, the colloid band will vanish and atomically dispersed centers will be regenerated. The colloid band also appears in synthetic alkali halides when the crystals are exposed to very high doses of high energy radiation. ( b ) Centers Associated with Chemical Impurities: Many impurities introduce characteristic absorption bands not present in the pure crystal.21-23When these impure crystals are treated by any of the abovedescribed procedures, new color centers involving the impurities may be formed to the exclusion of, or in addition to, the centers normally formed in the pure crystal.

11.7.3. Coagulation, Transformation and Bleaching of Centers Color centers produced by ionizing or ultraviolet radiation may be removed entirely or transformed into other color centers by exposure to light in their absorption bands. The transformations depend on many factors: the nature of the center, the nature of the host crystal, the temperature, the type of radiation employed to color the crystal originally, and even the density of the color centers. The color center bands in additively or electrolytically colored crystals suffer only transformations to other bands by optical treatment, since the color centers are due to stoichiometric excess of constituent which cannot be removed by this treatment alone. The transformation or bleaching of centers in colored crystals may take place even if the crystals are stored in the dark a t room temperature, or particularly if they are exposed to light in preparation for measurement. Even the light used for transmission measurements may provoke these transformations. In order to minimize such changes, it is desirable, where possible, to make the optical measurements, starting from the longer wavelengths and working towards the shorter wavelengths. Heating to sufficiently high temperatures will completely bleach color M. Savostianova, 2.Physik 64, 262 (1930). G. Mie, Ann. Physik [4] 26, 377 (1908). *l H. Pick, Ann. Physik [5] 36, 73 (1939); 2.Phyeik 114, 127 (1939). z* A. Smakula, 2.Physik 46, 1 (1927). *a J. H. Schulman, J . Phys. Chem. 67, 749 (1953). 19 2"

11.7.

COLOR CENTERS

333

bands produced by ionizing or ultraviolet radiations. Such thermallyinduced bleaching occurs to some extent even at room temperature. Most color centers produced by additive or electrolytic coloration are stable at room temperatures, but a t higher temperature (-400°C) colloid centers may be formed by coagulation of the atomically dispersed centers, and the centers may even be entirely removed by distilling out the stoichiometric excess of coloring element.

11.7.4. Additional Experimental Methods for Study of Color Centers 11.7.4.1. Electrical Properties. (a)Photoconductivity: Currents are sometimes observed to flow in colored crystals under the influence of an electric field when the crystals are illuminated with light lying in certain color center bands.'-l The magnitude of the current observed depends upon the density of color centers in the specimen, the electric field strength, the fraction of the specimen illuminated, and the temperature of the specimen. Care should be taken to prevent photoelectric emission from the electrodes either by masking the light beam or by proper choice of electrode material. Polarization of the sample is usually avoided or reduced by using as low a light level as possible. This requires high electric fields in order to obtain measurable currents. Measurements of the photoconductivity as a function of temperature provide a means for determining whether the color center absorption is due to an excited state of the center or a transition to the ionization continuum. (b) Photoelectric Emission: The emission of electrons is observed when a film of the host lattice, treated to produce color centers, is exposed t o light. In this experiment the film is placed in an evacuated vessel and the currents are detected with an electrometer. The observation of currents when the film is excited with light lying in the fundamental absorption band has important implications with regard to the transfer of energy within the crystal.24 ( c ) Magnetic Properties: Magnetic resonance measurements (described in Part 9) provide a powerful method of obtaining more detailed information on the structure of color centers. For example, in the case of the F-center, these measurements allow the determination of the probability distribution of the electron on the nuclei of the neighboring i0ns.O 1 1.7.4.2. luminescence. Luminescence measurements yield important and useful information concerning the structure of color centers. The emptying of electron and hole traps and the recombination of electrons and holes have been followed by studying glow curves.26Better differen24 $5

E. Taft and L. Apker, Phys. Rev. 79, 964 (1950); 81, 698 (1961); 83, 479 (1951). D. Dutton and R. J. Maurer, Phy8. Rev. 90, 126 (1953).

334

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LUMINESCENCE

tiation between centers whose absorptions overlap strongly has been obtained by examining their excitation spectra. For example, one center may luminesce when irradiated with light it can absorb while the other may not, or the luminescent emission of the two centers may be in different spectral regions.26The excitation spectra can therefore resolve differences not discernable from the absorption spectrum. The bleaching of certain centers on exposure t o light may be accompanied by luminescence in a different spectral region.27Subtle and, as yet, unexplained changes in color centers may occur simply on aging of the colored crystal, which produces no apparent change in the absorption spectrum but produces marked changes in luminescence efficiency.28 Polarization of the luminescence can give important information about the symmetry of the center^.^^^^^ Energy transfer between centers may be observed, such that absorption of light by one center leads to emission from another center.30 The experimental methods used in luminescence are described elsewhere in Part 11. H. W. Etzel and J. H. Schulman, J . Chem. Phys. 22, 1549 (1954). E. Jahoda, Silzber. Akad. Wiss. Wien, Math.-naturw. K1. Abt. ZIa 193, 675 (1926). z8 Th. P. J. Botden, C. 2. van Doom, and Y. Haven, Philips Research Repls. 9, 26

27

469 (1954). 28

39

P. P. Feofilov, Doklady Akad. Nauk S.S.S.R. 92, 743 (1953). J. Lambc and W. D. Compton, Phys. Rev. 106, 684 (1957).