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The decay of luminescence in natural and silver-doped quartz after X-ray excitation
1. Phn Chm. S&s Vol 42. No. 1. pp 61.68. 1984 Printed in Great Bmain.
0022-3697184 Perg~on
$3 Gil i .OO Press Ltd
THE DECAY OF LUMINESCENCE IN NATURAL AND SILVER-DOPED QUARTZ AFTER X-RAY EXCITATION W. HOHENAU,K. SCHWINGENSCHU~ and F. GROSS Institut fiir Experimentalphysik, Universitiit Graz, A-8010 Graz, Austria (Received 23 July 1982;accepted in reuersed
form
30 March 1983)
Abstract-It is well known that X-rays cause luminescence in quartz. This luminescence consists of two broad bands at 2.5 and 3.3 eV. After X-ray excitation natural quartz has an afterglow. The duration of this afterglow and the intensity of the emitted light show a strong dependence on the temperature of the sample. While the intensity is not so reproducible, the duration of the afterglow has a clear temperature- dependence and can be connected with two electron-traps in the customary band model-one with a depth of 8 meV and the other with 260meV-using an Arrhenius-plot, If the quartz crystal has been doped with silver by electr~diffusion, additional luminescence bands at 2.2eV and around 5eV occurs. The latter broad luminescence band consists of a dominant part with a decay-time of 40 psec at 4.8 eV and a weak band at 5.4eV without afterglow, In contrast to natural quartz, the luminescence intensity as well as the decay-time in the silver-doped sample is independent of the temperature in the range 77-300 K. This constant afterglow is associated with partially forbidden transitions in the silver dopant and energy-t~nsfer to the luminescence centres. In the suggested luminescence model, the observed luminescence bands are connected with an energy transition in the silver.
complete ail four tetrahedral bonds, it is generaliy charge-compensated by a monovalent cation (usually H’ or Me’ like Li’, Na’ or Kf) in a nearby interstitial channel. The compensation cation can be moved along this channel and replaced by another cation (in our case silver) during electr~iffusion [7-l 11. The application of X-rays produces a smoky colour in quartz, which can be bleached at higher temperatures. In several ESR investigations, made first in the years 1955-56 [4-61 and then later 138-441the smoky colour was attributed to a trapped hole at the aluminium defect centre (AlOMe). Such a captured hole at the Al centre relaxes the bonding to the monovalent compensation cation, so it is able to diffuse along the lattice channel activated by thermal energy [66,67]. Beside this impurity defect centre, oxygen-vacancy centres and oxygen-silicon-vacancy-centres were investigated in many papers; several may be quoted [17,31,32,45’-533. These defects are able to capture electrons or holes during irradiation. Using ESR, their position in the lattice can be determined exactly. Beside this, the radiation induced coloration and also the thermoluminescence are visible indicators of this fact. The glow curve peak indicate different levels of thermally activated traps and several investigations [23,27,29,54-591 found correlations with impurities. However, these relationships were not so perfect that an impurity analysis could be made from thermoluminescence. It is important, that bleaching of the radiationinduced smoky colour does not corretate with thermoluminescence glow curves and that the luminescence spectra of natural quartz generally are independent of impurities [59,60]. After electrodiffusion, if most of the monovalent interstitial impurities are replaced by Ag+ or Cu+, all the optical properties change [29,26,13,15,61]. During X-ray irradiation of natural quartz, besides the smoky quartz colouring, a strong temperature-dependent
Quartz has trigonal symmetry and the chemical formula SiO,. The silicon atom lies in the centre of a non-exact
tetrahedron with one oxygen at each extremity. Each oxygen atom belongs to two te~ahedrons of this kind, so the structure of quartz can be described by the chemical formula Si04,2. The band structure of an ideal quartz crystal shows a full valence band and an unoccupied conduction band with a gap of 9-12 eV [l-3]. However, there are always a number of defects which cause some local energy levels in the band-gap. Near the conduction band such an energy level forms an electron-trap; energy levels lying near the valence band are called activators. When electrons are excited into the conductionband, a simple luminescence model gives three possibilities: (a) Some electrons reach luminescence centres and fall into activators by emission of photons. This is a direct energy transfer to a few points in the lattice. (b) There may be possible nonradiative transitions to other lattice-defects or complete activators. (c) A portion of the excited electrons fall into traps and remain trapped until these electrons reach the conduction band once again by addition of energy either thermal energy or infrared radiation. Reaching the conduction band, they may fall again into a trap or will arrive at a luminescence centre, as described in (a). The excitation of electrons for luminescence, out of a limited number of traps, implies a decay of luminescence as measured below. On the other hand, thermoluminescence is found if these trapped electrons are activated later by separate addition of thermal energy. Chemical analysis of quartz [16] shows that aluminium is the most important impurity. In natural quartz there are between 30 and 2700Al atoms per million silicon atoms. In the regular lattice the best-known defect consists of a silicon replaced by an aluminium. Since this substitutional aluminium requires an extra cation to 61
62
W. HOHENAU et al.
luminescence intensity can be seen [18,19,20,13]. Also, in preliminary investigations with X-ray-flashes, a temperature-dependent afterglow was found [21] which was associated with trapped electrons activated by thermal energy. After this an apparatus was built, which allowed automatic detection of the duration of the afterglow by a computer [22].
EXPERTS
RESULTS
Natural quartz The luminescence spectra of natural quartz samples during X-ray excitation consist of two broad emissionbands near 3.3 and 2.5 eV. Usually the luminescence band at 2.5 eV is more intense than the band at 3.3eV. Annealing or X-irradiation causes a change in the intensity of the luminescence bands [23]. After cooling the sample to 77 K, the intensity of luminescence increases by a factor of about 500. This temperature-dependent change of the intensity is not clearly reproducible because X-rays are damaging luminescence centres [26] and producing smoky quartz centres of the low-temperature and room-temperature modification [ 14,251. In contrast to this, afterglow is a very reproducible property of quartz and can be measured in our experimental setup, if the duration of luminescence is greater than 2bsec. Figure I(a) shows an oscillogram of the luminescence after X-ray excitation at ambient temperature. In this oscillogram the intensity of luminescence follows closely that of the X-ray pulse applied. Figure lb shows the oscillogram of luminescence of the same sample, but at a temperature of about 77K. One m set after X-ray excitation the quartz crystal is still glowing. In our preliminary paper 1211,the evaluation of the afterglow was made from the oscillograms by measuring the half-life period. In the present paper the luminescence intensity was digitized by a fast analog-to-digital converter 1241and analyzed by a computer. We found that this afterglow is composed of two exponential decay-times with temperature-dependent time constants. In the customary band model, an afterglow can be described by thermal activation of electrons out of a trap. The depth of such a trap can be evaluated using an Arrhenius-plot, if only one electron trap is active in a fixed range of temperature. In the Arrhenius curve of
Fig. l(a). X-axis-5 ps/div. Oscillogram of the luminescence of natural quartz during excitation with X-ray flashes at ambienttemperature. The time dependence of luminescence is the same as that of the X-ray pulse no afterglow can be seen.
Fig. I(b). Oscillogram of the luminescence of the same sample after excitation with X-ray flashes, but cooled with liquid nitrogen (77 K). One millisecond after excitation the crystal is still glowing. X-axis-SO0 &div.
Fig. 2, the slope is a measure of the activation energy of the electron out of the trap, as follows. A=k.$&+tga A is the activation energy, k the Boltzmann constant, T the decay time and T the temperature. If the slope is constant in a fixed range of temperature, only one trap is responsible for the afterglow of luminescence. According to Fig. 2, below 120K electrons coming from a trap with a depth of about 8 mV and reaching the activator by light emission are responsible for the afterglow. The accuracy in the measurement for this low-temperature trap is about t25%, but there is a sample-to-sample variation of 2 meV. Between 170K and 220 K the curve in Fig. 2 shows another range of linearity attributed to an electrontrap with a depth of 260meV. The accuracy in the measurement in this case is ? 5% including sample variations. The time-dependent intensity of luminescence after X-ray excitation can be formulated as follows. 1 = A,l-“‘I + AJ-“‘2
(2)
where I is the intensity, A the amplitude, t the time and T the decay-time. A, and AZ are temperature-dependent variables as shown in Fig. 3. Interpretation of the temperature dependence of the luminescence intensity is in progress. (b) Quartz doped with silver during efectrodiffusion Exchanging lithium, sodium or potassium, the monovalent ch~ge-compensation cations which are thought in most cases to be near aluminium-defects, causes no remarkable change in the luminescence spectra. From this fact we conclude that these interstitials cannot be luminescence centres, but looking at the glow curves of thermoluminescence, they may trap electrons. After doping the crystal partially with silver, the luminescence of natural quartz can be seen only in the silver-free parts. In the visible range of the spectrum, the silver-containing part of the quartz sample shows a very weak luminescence. During X-irradiation at ‘77K, we did not detect an increase in luminescence near to 2.9eV as described in the literature 1651and which is connected to interstitial silver atoms. However, we found a formation of luminescence centres at temperatures above 200K and
The decay of luminescence in natural and silver-doped quartz
63
lo-
1
r ’
I’““““I 300
I
’
1
’
150
200
’
’
’
I
*
l/T
1OOK
Fig. 2. Arrhenius-curve from decay of afterglow in natural quartz after X-ray excitation. The ordinate axis is formed by log of decay-time and the X-axis by the reciprocal temperature.
t Amplitude
: ,
f
;
'
&a l/T t
4
> 210
170 140 120
VT
77 K
Fig. 3. Both observed electron traps are only effective in a small temperature range. The efiiciency is graphically represented by the amplitudes At and AZ.
64
W. HOHENAU et al.
we associate this with a thermal migration of these silver cations to other interstitial places, as has been shown with H’, Li’ and Na’ [66,67]. The fact that the silvercontaining part of a quartz sample after X-irradiation shows a more intense smoky colouration with the same spectral bands in the visible region indicates in our opinion that the silver cation after electrodiffusion lies beside the most important lattice defect, the Al centre. After doping the crystal with silver, not only the absorption and luminescence spectra change, but also the duration of the afterglow. It is noticeable that silverdoped quartz has a more intense luminescence with broad spectral peaks at 2.2 and 4.8eV than natural samples at ambient temperature. Cooling the sample causes an increase of the luminscence during X-ray excitation. Also, a shift of the luminescence peak into the blue region of the visible spectrum can be detected. The luminescence intensity at 4.8eV seems to be nearly independent of temperature, but the afterglow for the visible spectrum as well as for the UV-band gives an additional source of information, Figure 4 shows an oscillogram of the afterglow of silver-doped quartz after X-ray excitation at ambient-temperature and at 77 K. In the oscillograms of Fig. 4 it can be seen that the afterglow in silver-doped quartz consists of two decaytimes, the fast decay at ambient-temperature in Fig. 4(a), as well as the long afterglow at 77 K in Fig. 4(b) which corresponds to the behavior of the natural quartz shown in Fig. 1. On the other hand, in silver-doped quartz there is an afterglow with a decay-time of nearly 55 us which
Fig. 4(a). Decay of luminescencein the visiblespectrum X-axis20 psecjdiv. Oscillogram of the afterglow in a silver-doped sample after X-ray excitation at ambient-temperature,
Fig. 4(b). X-axis-50 silver-doped
psec/div. Os~ilio~ram of the afterglow quartz after X-ray excitation at 77 K.
ln’l
LLS -
tooo-
.
>--0
mev
7
.
I / i
100:
lo-
~
1
’
260
llre,IIr!
300
mev
1
200
,
I
1
,
I
I
Fig. 5. Arrhenius
I
1
I
I
TOOK
150
diagram
of a silver-doped
sample.
’
*‘I*
of
The decay of luminescence in natural and silver-doped quartz
is independent of temperature. This behavior can be understood by considering the mechanism of electrodiffusion, i.e. the monov~ent interstitial cation can only move along the structural channels. Certainly not all such channels are reached during electrodiffusion and so some parts of the quartz stay in the natural form. A portion of aluminium defect centres, however, now are doped with a silver ion and in the luminescence spectra there can be expected luminescence from the natural sample as well as from the silver-doped modi~cation. Contrary to natural quartz, in silver-doped samples the intensity of the additional luminescence bands as well as the decay-times of the afterglow are nearly independent of temperature. The broad band luminescence between 4eV and .56eV, with a maximum of intensity at 4.8eV, has an afterglow with two decay-times, as shown in Fig. 6; both are independent of temperature. Below a wavelength of 225 nanometers no afterglow can be seen, but with increasing wavelength, an afterglow with a decay-time of nearly 40 psec rises in intensity. A graphical attempt to separate the spectral part without afterglow from the whole luminescence band at 4.8 eV is shown in Fig. 7. As described below, the spectrum shown in Fig. 7 cannot be found during continuous X-ray excitation. The quantum-efficiency of the quartz luminescence is about 1000 times below that of commercial phosphors, and so only a small part of the luminescence centres is activated, which is also the reason why no saturation effect in luminescence could be found hitherto. The intensity of luminescence during continuous excitation is not the same as when excited by X-ray flashes. If a comparison is made, the intensity of continuous luminescence corresponds to the area of the oscillograms of afterglow shown. Thus, it can be seen that a luminescence without an afterglow is very weak during a continuous excitation, which is in contrast to a luminescence including afterglow. In our cases, the luminescence band at 4.8 eV is broadened only a little in the blue region of the spectrum which could not be detected. DISCUSSION
As the measurements show, there is no doubt that the luminescence observed is to be connected with the AlMeOZ defect centres in quartz. Natural quartz has a
temperature-dependent afterglow which is illustrated in the band model of Fig. 8 by a bimolecular reaction; X-ray absorption excites an electron from the valence band to the conduction band. Dropping into a trap, the electron stays there until thermal activation returns it to the conduction band, and emitting light falls to an activator at another site in the crystal. As described, the aluminium defect centre may be compensated by one of several monovaIent cations without consequence to the luminescence spectrum. So we presume that the amminium oxygen defect centre itself is the described activator and, consequently, the luminescence centre in natural quartz. The traps observed in natural quartz are not found in silver-doped samples and so they are corretated with the monovalent compensation-cation, i.e. Li’, Na’ and H’, near the aluminium defect centre. In the case of the silver-doped quartz, because of the temperature-independent afterglow, the luminescence seems to be an intramolecular process with partially forbidden transitions of the excited electron. Usually it is difficult to correlate a solid state luminescence spectrum to one of a free atom or ion referred to in tables of spectra. However, in the case of the luminescence bands at 4.8 and 5.4eV in silver-doped quartz, a good correlation can be found with the spectrum of a singleionized silver atom. In contrast to the luminescence in the visible region, the 4.8eV band is detectable quickly after ele~trodiffusion and so this luminescence band is connected to the interstitial silver cation near to the AlO2 defect centre or other non-bonding oxygen. More difficult is the interpretation of the luminescence of silver-doped quartz in the visible region. This luminescence consists of a temperature-independent 2.2eV band and a weak 2.8-2.9eV band with intensity rising during cooling the sample. These latter luminescence bands have the same, also temperature-independent, afterglow. The experimental finding, that these luminescence bands occur only during X-irradiation above 200 K, indicates that cation migration, known from other investigations[66,67], is necessary for the formation of silver luminescence centres. At low temperatures the silver-dependent luminescence near to 2.9eV was connected with interstitial silver atoms [IS, 651. In several papers [62-65] it is shown from ESR investigations that silver atom centres are annealed at
Fig. 6. The abscissa of the oscillograms is calibrated at IOpsecidiv. The intensity of afterglow rises with the wavelength of the emitted light. PCS Vol.45. No. I-E
65
66
W. HOHENAU etal. INTENSITY
3
2
1
C
1
4
Fig. 7. An afterglow-investigation
I
I
5.5
separates
a luminescence band at 5.4 out of the broad-band 4.8 eV.
I
.
TT
I
l-
’
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,
’
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Fig. 8. Illustrated band model for quartz luminescence.
ENERGY
luminescence
x-ray
abs.
(eV)
at
The decay of luminescence in natural and silver-doped quartz
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Fig. 9. temperatures above 200K. This is a contradiction, however, because the broad silver luminescence near to 2.8 eV is easy to detect up to temperature above 400 K! Presuming that, as found in ESR investigations, no silver atom centres are stable at ambient temperature, we can only explain the observed luminescence by a molecular process, because in a silver cation the transition energies of the lower energy levels are much higher than 2.2 or 2.8eV. In our present luminescence model, we suppose that the excited electrons coming from the conduction band are captured by silver cations which are not compensation cations near aluminium defects. The so-formed excited silver atoms are able to emit the observed luminescence light; but interstitial silver atoms are evidently unstable and they lose their valence electrons to the valence band of the regular lattice. Looking at the tables of atomic energy-levels [30] quoted above, transition energies between the second and the first excitation levels f&f-Sp) of silver atoms correlate with the observed luminescence energies. A transition to the ground state of the silver could not be detected hitherto and so we presume that this is a non-radiative transition of the excited electron from the silver atom to the valence band of the lattice. REFEXENCW
1. Trukhin, A. N., Fiz. Khimiya St&la l(5), 466-468(1975). 2. Scheider P. M. and Fowler W. B., Phys. Rev. Letters 36(S), 425-428 (1976). 3. Chelikowsky J. R. and Schliiter M., Phys. Rev. B U(S), 402(1-4029(1976). 4. Griffiths J. H. E., Owen J. and Ward I. M., Defects in cryatailine solids. Bristol Conf., pp.81-87.Phys. Sot. (London, 1955). 5. O‘Brien M. C. M., Proc. Roy. Sot. (Land.) .4231, 404-414 (1955). 6. Cohen A. J., Am. Mineralogist 41, 874-891 (1956).
67
7. Pfenninger H. H. and Laves F., Natutwissenschaften 47, 12, 276 (1960). 8. Wondratschek H., Brnnner G. 0. and Laves F., Noturwissenschaften 47, 12, 275 (1960). 9. Pfenninger H. H., Dissertation Univ. Ziirich (1961). 10. SchGnbacher H., Dissertation Univ. Graz (1966). 11. Ladstitter E., Dissertation Univ. Graz (1969). 12. Gross F., Acta Physica Aust~aca 19,268 (1965). 13. Hohenau W., Acta Physica Aust~aca 39,212-219 (1974). 14. Angerer R., Dissertation Univ. Graz (1971). 15. Trukhin A. N., Etsin S. S. and Shendrik A. V., fzuestiya Akad. Nauk 40(1l), 2329-2333(1976). 16. Bambauer H. U., Schweiz. Mineralog. u. Petrograph. Mitteilungen 41(2), 335-369(1961). 17. Nelson C. M. and Weeks R. A.. J. Am. Ceram. Sot. 43(S). 396-404(1960). 18. Lietz J. and Matheja J., Naturwissenschaften 51, 503 (1964). 19, Frondel C. Am. Min. 30,432 (1945). 20. Fahrenkrog H. H., Dissertation Univ. Hamburg (1966). 21. Hohenau W., Meyer W. and Gross F., Acta Physica Austtiaca 44, 337-347(1976). 22. S~hwingenschuh K., Dissertation Univ. Graz (1977). 23. Lietz J.-and Matheja J., Natu~issenschaften 51,504 (1964). 24. Schwingenschuh and Gross F.. Electronics Letters 14112). 358-359(1978). 25. Angerer R., Gross F. and Koss P., Acta Physica Austriaca 33, 3-7 (1971). 26. Hohenau W. and Gross F., Acta Physica Austriaca 37, 385-390(1973). 27. lchikawa Ti., Japan J. Appt. Phys. 7(3), 220-226(1%8). 28. Schlesinger M., J. Phys. Chem. Solids 26, 1761-1766(1965). 29. Gross F. and Hohenau W., Acfa Physica Ausf~aca 36, 281-285(1972). 30. Moore Ch. E., Atomic Energy levels, Vol. HI, Circular National Bureau of Standards 467. 31. Weeks R. A., J. Appl. Phys. 27, 1376-1381(1956). 32. Weeks R. A., Phys. Rev. 130(2),577-588(1963). 33. Arnold G. W., Phys. Chem. Solids 13, 306320 (1960). 34. Schindler P.. Diss. Univ. Zbrich (19641. 35. Taylor A. i. and Farnell G. W.: Can. J. Phys. 42, 595-607 (1964). 36. Schnadt R. and Schneider J., Phys. Kondens. Materie 11, 19-42 (1970). 37. Schnadt and RBuber A., Solid State Commun. 9, 1.59-161
(1971). 38. Weeks R. A., J. Am. Ceram. Sot. 53(4), 176-179(1970). 39. Mackey J. H. and Boss J. W., .J. Magn. Resonance 3,44-54 ( 1970). 40. Nuttall R. H. D. and Weil J. A., Solid St. Commun. 19, 141-142(1976). 41. Griscom D. L., Proc. 32. Frequ. Contr. Symp., Atlantic-City 30 May 1979,pp. 98-109. 42. Koumvakalis M.. J. Appf. Phys. 51, 5528-5532(1980). 43. Nuttall R. H. D. and Weil J. A., Solid St. Commun. 35, 789-791(1980). 44. Nuttall R. H. D. and Weil J. A., Can. J. Phys. 59, 169d1708, 1886-1892(1981). 45. Castle J. G., Feidmann D. W. and Kiemens P. G., Phys. Reo. l?@(2),577-88 (1963). 46. Jones C. E. and Embree D., J. Appl. Phys. 47(12),5365-5371 (1976). 47. Shendrik A. V. and Yudin D. M., Phys. Status Solidi 85, 343-349(1978). 48. Brower K. L. and Vitko, J. Phys. Status Solidi 95, KL 41-44 (1979). 49. Shendrik A. V. and Yudin D. M., Phys. Status Solidi 95, KL 45-46 (1979). 50. Halliburton L. E. et al.. Solid St. Commu~. 30. 575-579 (1979). 51.
Brower K. L., Phys. Rev. B. 2@(S),1799-1811(1979). 52. Isoya J., Weil J. A. and Halliburton L. E., J. Chem. Phys. Solid 74, 5436-5448(1981). 53. Haberlandt H., Phys. Status Solidi 110, 531-535(1982). 54. Medlin W. L., J. Chem. Phys. 38(5), 1132-1143(1963).
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55. Schlesinger M., Phys. Letters 10(l), 49(50 (1964) 56. Schlesinger M.. J. Phw. Chem. Solids 26. 1761-1766 (1965). 57. Lee H. Y.. Lee B. W. and Jun J. S.. J. Korean Phvs. Sot. 3(2). 54-60 (1970). 58. Lee H. Y., Lee B. W. and Jun J. S., J. Appl. Phys. 42(l), 498-499 (1971). 59. Arnold G. W.. .I. Phys. Chrm. Solids 13, 306-320 (1960). 60. Halperin A. et al., J. Luminescence l(2), 385-397 (1960). 61. Trukhin A. N., Opt. Spectrosc. 40(4), 432-433 (1976). 62. Davis P. H., Huang C. Y. and Weil J. A., J. Phys. Chem. Solid. 39, 897-899 (1978). 63. Davis P. H. and Weil J. A., J. Phys. Chem. Solids 39.775-780 (1978). 64. Solntsev V. P. and Mashkovtsev R. A., Sou. Phys. Solid State 20(3), 471474 (1977). 65. Trukhin A. N. and Shendrik A. V., Phys. Status Sofidi B 98, KL 13-15 (1980). 66. Markes and Halliburton L. E., J. Appl. Phys. 50(12), 81728179 (1979). 67. Martin J. J.. Doherty S. P.. Halliburton L. E., Markes M.. Koumvakalis N. and Sibley W. A., 33rd Annlcctl Symp. Frequ. Confr. 134-147 (1979).
APPENDIX The experfmental setup X-ray flashes were produced by an electrical high-vacuum break-down in a separate part of a hard-vacuum chamber, during the discharge of a high voltage plus capacitor. The high vacuum breakdown was induced by a small pulse discharge between an ignitron electrode and the hollow cathode. Behind the hollow cathode there is an aluminium window protecting the quartz sample from light and electrical fields produced during the discharge. Of course the X-rays coming from the anode could pass this window and reach the quartz sample, which was mounted on a liquid nitrogen-cooled sample holder. Luminescence light, coming from the quartz, was detected by a photomultiplier after passing through a monochromator. With an analog-to-frequency converter, the anode current of the photomultiplier was either digitized and fed to a computer, or displayed on an ocilloscope screen. All parts of the chamber, the transmission line and the capacitor housing were made by ourselves. We used massive steel shielding to protect the outside room against X-rays and stray fields. Usually the anode voltage was 80 kV and the discharge time was 0.8-1.5 wsec.
0022-3697184 Perg~on
$3 Gil i .OO Press Ltd
THE DECAY OF LUMINESCENCE IN NATURAL AND SILVER-DOPED QUARTZ AFTER X-RAY EXCITATION W. HOHENAU,K. SCHWINGENSCHU~ and F. GROSS Institut fiir Experimentalphysik, Universitiit Graz, A-8010 Graz, Austria (Received 23 July 1982;accepted in reuersed
form
30 March 1983)
Abstract-It is well known that X-rays cause luminescence in quartz. This luminescence consists of two broad bands at 2.5 and 3.3 eV. After X-ray excitation natural quartz has an afterglow. The duration of this afterglow and the intensity of the emitted light show a strong dependence on the temperature of the sample. While the intensity is not so reproducible, the duration of the afterglow has a clear temperature- dependence and can be connected with two electron-traps in the customary band model-one with a depth of 8 meV and the other with 260meV-using an Arrhenius-plot, If the quartz crystal has been doped with silver by electr~diffusion, additional luminescence bands at 2.2eV and around 5eV occurs. The latter broad luminescence band consists of a dominant part with a decay-time of 40 psec at 4.8 eV and a weak band at 5.4eV without afterglow, In contrast to natural quartz, the luminescence intensity as well as the decay-time in the silver-doped sample is independent of the temperature in the range 77-300 K. This constant afterglow is associated with partially forbidden transitions in the silver dopant and energy-t~nsfer to the luminescence centres. In the suggested luminescence model, the observed luminescence bands are connected with an energy transition in the silver.
complete ail four tetrahedral bonds, it is generaliy charge-compensated by a monovalent cation (usually H’ or Me’ like Li’, Na’ or Kf) in a nearby interstitial channel. The compensation cation can be moved along this channel and replaced by another cation (in our case silver) during electr~iffusion [7-l 11. The application of X-rays produces a smoky colour in quartz, which can be bleached at higher temperatures. In several ESR investigations, made first in the years 1955-56 [4-61 and then later 138-441the smoky colour was attributed to a trapped hole at the aluminium defect centre (AlOMe). Such a captured hole at the Al centre relaxes the bonding to the monovalent compensation cation, so it is able to diffuse along the lattice channel activated by thermal energy [66,67]. Beside this impurity defect centre, oxygen-vacancy centres and oxygen-silicon-vacancy-centres were investigated in many papers; several may be quoted [17,31,32,45’-533. These defects are able to capture electrons or holes during irradiation. Using ESR, their position in the lattice can be determined exactly. Beside this, the radiation induced coloration and also the thermoluminescence are visible indicators of this fact. The glow curve peak indicate different levels of thermally activated traps and several investigations [23,27,29,54-591 found correlations with impurities. However, these relationships were not so perfect that an impurity analysis could be made from thermoluminescence. It is important, that bleaching of the radiationinduced smoky colour does not corretate with thermoluminescence glow curves and that the luminescence spectra of natural quartz generally are independent of impurities [59,60]. After electrodiffusion, if most of the monovalent interstitial impurities are replaced by Ag+ or Cu+, all the optical properties change [29,26,13,15,61]. During X-ray irradiation of natural quartz, besides the smoky quartz colouring, a strong temperature-dependent
Quartz has trigonal symmetry and the chemical formula SiO,. The silicon atom lies in the centre of a non-exact
tetrahedron with one oxygen at each extremity. Each oxygen atom belongs to two te~ahedrons of this kind, so the structure of quartz can be described by the chemical formula Si04,2. The band structure of an ideal quartz crystal shows a full valence band and an unoccupied conduction band with a gap of 9-12 eV [l-3]. However, there are always a number of defects which cause some local energy levels in the band-gap. Near the conduction band such an energy level forms an electron-trap; energy levels lying near the valence band are called activators. When electrons are excited into the conductionband, a simple luminescence model gives three possibilities: (a) Some electrons reach luminescence centres and fall into activators by emission of photons. This is a direct energy transfer to a few points in the lattice. (b) There may be possible nonradiative transitions to other lattice-defects or complete activators. (c) A portion of the excited electrons fall into traps and remain trapped until these electrons reach the conduction band once again by addition of energy either thermal energy or infrared radiation. Reaching the conduction band, they may fall again into a trap or will arrive at a luminescence centre, as described in (a). The excitation of electrons for luminescence, out of a limited number of traps, implies a decay of luminescence as measured below. On the other hand, thermoluminescence is found if these trapped electrons are activated later by separate addition of thermal energy. Chemical analysis of quartz [16] shows that aluminium is the most important impurity. In natural quartz there are between 30 and 2700Al atoms per million silicon atoms. In the regular lattice the best-known defect consists of a silicon replaced by an aluminium. Since this substitutional aluminium requires an extra cation to 61
62
W. HOHENAU et al.
luminescence intensity can be seen [18,19,20,13]. Also, in preliminary investigations with X-ray-flashes, a temperature-dependent afterglow was found [21] which was associated with trapped electrons activated by thermal energy. After this an apparatus was built, which allowed automatic detection of the duration of the afterglow by a computer [22].
EXPERTS
RESULTS
Natural quartz The luminescence spectra of natural quartz samples during X-ray excitation consist of two broad emissionbands near 3.3 and 2.5 eV. Usually the luminescence band at 2.5 eV is more intense than the band at 3.3eV. Annealing or X-irradiation causes a change in the intensity of the luminescence bands [23]. After cooling the sample to 77 K, the intensity of luminescence increases by a factor of about 500. This temperature-dependent change of the intensity is not clearly reproducible because X-rays are damaging luminescence centres [26] and producing smoky quartz centres of the low-temperature and room-temperature modification [ 14,251. In contrast to this, afterglow is a very reproducible property of quartz and can be measured in our experimental setup, if the duration of luminescence is greater than 2bsec. Figure I(a) shows an oscillogram of the luminescence after X-ray excitation at ambient temperature. In this oscillogram the intensity of luminescence follows closely that of the X-ray pulse applied. Figure lb shows the oscillogram of luminescence of the same sample, but at a temperature of about 77K. One m set after X-ray excitation the quartz crystal is still glowing. In our preliminary paper 1211,the evaluation of the afterglow was made from the oscillograms by measuring the half-life period. In the present paper the luminescence intensity was digitized by a fast analog-to-digital converter 1241and analyzed by a computer. We found that this afterglow is composed of two exponential decay-times with temperature-dependent time constants. In the customary band model, an afterglow can be described by thermal activation of electrons out of a trap. The depth of such a trap can be evaluated using an Arrhenius-plot, if only one electron trap is active in a fixed range of temperature. In the Arrhenius curve of
Fig. l(a). X-axis-5 ps/div. Oscillogram of the luminescence of natural quartz during excitation with X-ray flashes at ambienttemperature. The time dependence of luminescence is the same as that of the X-ray pulse no afterglow can be seen.
Fig. I(b). Oscillogram of the luminescence of the same sample after excitation with X-ray flashes, but cooled with liquid nitrogen (77 K). One millisecond after excitation the crystal is still glowing. X-axis-SO0 &div.
Fig. 2, the slope is a measure of the activation energy of the electron out of the trap, as follows. A=k.$&+tga A is the activation energy, k the Boltzmann constant, T the decay time and T the temperature. If the slope is constant in a fixed range of temperature, only one trap is responsible for the afterglow of luminescence. According to Fig. 2, below 120K electrons coming from a trap with a depth of about 8 mV and reaching the activator by light emission are responsible for the afterglow. The accuracy in the measurement for this low-temperature trap is about t25%, but there is a sample-to-sample variation of 2 meV. Between 170K and 220 K the curve in Fig. 2 shows another range of linearity attributed to an electrontrap with a depth of 260meV. The accuracy in the measurement in this case is ? 5% including sample variations. The time-dependent intensity of luminescence after X-ray excitation can be formulated as follows. 1 = A,l-“‘I + AJ-“‘2
(2)
where I is the intensity, A the amplitude, t the time and T the decay-time. A, and AZ are temperature-dependent variables as shown in Fig. 3. Interpretation of the temperature dependence of the luminescence intensity is in progress. (b) Quartz doped with silver during efectrodiffusion Exchanging lithium, sodium or potassium, the monovalent ch~ge-compensation cations which are thought in most cases to be near aluminium-defects, causes no remarkable change in the luminescence spectra. From this fact we conclude that these interstitials cannot be luminescence centres, but looking at the glow curves of thermoluminescence, they may trap electrons. After doping the crystal partially with silver, the luminescence of natural quartz can be seen only in the silver-free parts. In the visible range of the spectrum, the silver-containing part of the quartz sample shows a very weak luminescence. During X-irradiation at ‘77K, we did not detect an increase in luminescence near to 2.9eV as described in the literature 1651and which is connected to interstitial silver atoms. However, we found a formation of luminescence centres at temperatures above 200K and
The decay of luminescence in natural and silver-doped quartz
63
lo-
1
r ’
I’““““I 300
I
’
1
’
150
200
’
’
’
I
*
l/T
1OOK
Fig. 2. Arrhenius-curve from decay of afterglow in natural quartz after X-ray excitation. The ordinate axis is formed by log of decay-time and the X-axis by the reciprocal temperature.
t Amplitude
: ,
f
;
'
&a l/T t
4
> 210
170 140 120
VT
77 K
Fig. 3. Both observed electron traps are only effective in a small temperature range. The efiiciency is graphically represented by the amplitudes At and AZ.
64
W. HOHENAU et al.
we associate this with a thermal migration of these silver cations to other interstitial places, as has been shown with H’, Li’ and Na’ [66,67]. The fact that the silvercontaining part of a quartz sample after X-irradiation shows a more intense smoky colouration with the same spectral bands in the visible region indicates in our opinion that the silver cation after electrodiffusion lies beside the most important lattice defect, the Al centre. After doping the crystal with silver, not only the absorption and luminescence spectra change, but also the duration of the afterglow. It is noticeable that silverdoped quartz has a more intense luminescence with broad spectral peaks at 2.2 and 4.8eV than natural samples at ambient temperature. Cooling the sample causes an increase of the luminscence during X-ray excitation. Also, a shift of the luminescence peak into the blue region of the visible spectrum can be detected. The luminescence intensity at 4.8eV seems to be nearly independent of temperature, but the afterglow for the visible spectrum as well as for the UV-band gives an additional source of information, Figure 4 shows an oscillogram of the afterglow of silver-doped quartz after X-ray excitation at ambient-temperature and at 77 K. In the oscillograms of Fig. 4 it can be seen that the afterglow in silver-doped quartz consists of two decaytimes, the fast decay at ambient-temperature in Fig. 4(a), as well as the long afterglow at 77 K in Fig. 4(b) which corresponds to the behavior of the natural quartz shown in Fig. 1. On the other hand, in silver-doped quartz there is an afterglow with a decay-time of nearly 55 us which
Fig. 4(a). Decay of luminescencein the visiblespectrum X-axis20 psecjdiv. Oscillogram of the afterglow in a silver-doped sample after X-ray excitation at ambient-temperature,
Fig. 4(b). X-axis-50 silver-doped
psec/div. Os~ilio~ram of the afterglow quartz after X-ray excitation at 77 K.
ln’l
LLS -
tooo-
.
>--0
mev
7
.
I / i
100:
lo-
~
1
’
260
llre,IIr!
300
mev
1
200
,
I
1
,
I
I
Fig. 5. Arrhenius
I
1
I
I
TOOK
150
diagram
of a silver-doped
sample.
’
*‘I*
of
The decay of luminescence in natural and silver-doped quartz
is independent of temperature. This behavior can be understood by considering the mechanism of electrodiffusion, i.e. the monov~ent interstitial cation can only move along the structural channels. Certainly not all such channels are reached during electrodiffusion and so some parts of the quartz stay in the natural form. A portion of aluminium defect centres, however, now are doped with a silver ion and in the luminescence spectra there can be expected luminescence from the natural sample as well as from the silver-doped modi~cation. Contrary to natural quartz, in silver-doped samples the intensity of the additional luminescence bands as well as the decay-times of the afterglow are nearly independent of temperature. The broad band luminescence between 4eV and .56eV, with a maximum of intensity at 4.8eV, has an afterglow with two decay-times, as shown in Fig. 6; both are independent of temperature. Below a wavelength of 225 nanometers no afterglow can be seen, but with increasing wavelength, an afterglow with a decay-time of nearly 40 psec rises in intensity. A graphical attempt to separate the spectral part without afterglow from the whole luminescence band at 4.8 eV is shown in Fig. 7. As described below, the spectrum shown in Fig. 7 cannot be found during continuous X-ray excitation. The quantum-efficiency of the quartz luminescence is about 1000 times below that of commercial phosphors, and so only a small part of the luminescence centres is activated, which is also the reason why no saturation effect in luminescence could be found hitherto. The intensity of luminescence during continuous excitation is not the same as when excited by X-ray flashes. If a comparison is made, the intensity of continuous luminescence corresponds to the area of the oscillograms of afterglow shown. Thus, it can be seen that a luminescence without an afterglow is very weak during a continuous excitation, which is in contrast to a luminescence including afterglow. In our cases, the luminescence band at 4.8 eV is broadened only a little in the blue region of the spectrum which could not be detected. DISCUSSION
As the measurements show, there is no doubt that the luminescence observed is to be connected with the AlMeOZ defect centres in quartz. Natural quartz has a
temperature-dependent afterglow which is illustrated in the band model of Fig. 8 by a bimolecular reaction; X-ray absorption excites an electron from the valence band to the conduction band. Dropping into a trap, the electron stays there until thermal activation returns it to the conduction band, and emitting light falls to an activator at another site in the crystal. As described, the aluminium defect centre may be compensated by one of several monovaIent cations without consequence to the luminescence spectrum. So we presume that the amminium oxygen defect centre itself is the described activator and, consequently, the luminescence centre in natural quartz. The traps observed in natural quartz are not found in silver-doped samples and so they are corretated with the monovalent compensation-cation, i.e. Li’, Na’ and H’, near the aluminium defect centre. In the case of the silver-doped quartz, because of the temperature-independent afterglow, the luminescence seems to be an intramolecular process with partially forbidden transitions of the excited electron. Usually it is difficult to correlate a solid state luminescence spectrum to one of a free atom or ion referred to in tables of spectra. However, in the case of the luminescence bands at 4.8 and 5.4eV in silver-doped quartz, a good correlation can be found with the spectrum of a singleionized silver atom. In contrast to the luminescence in the visible region, the 4.8eV band is detectable quickly after ele~trodiffusion and so this luminescence band is connected to the interstitial silver cation near to the AlO2 defect centre or other non-bonding oxygen. More difficult is the interpretation of the luminescence of silver-doped quartz in the visible region. This luminescence consists of a temperature-independent 2.2eV band and a weak 2.8-2.9eV band with intensity rising during cooling the sample. These latter luminescence bands have the same, also temperature-independent, afterglow. The experimental finding, that these luminescence bands occur only during X-irradiation above 200 K, indicates that cation migration, known from other investigations[66,67], is necessary for the formation of silver luminescence centres. At low temperatures the silver-dependent luminescence near to 2.9eV was connected with interstitial silver atoms [IS, 651. In several papers [62-65] it is shown from ESR investigations that silver atom centres are annealed at
Fig. 6. The abscissa of the oscillograms is calibrated at IOpsecidiv. The intensity of afterglow rises with the wavelength of the emitted light. PCS Vol.45. No. I-E
65
66
W. HOHENAU etal. INTENSITY
3
2
1
C
1
4
Fig. 7. An afterglow-investigation
I
I
5.5
separates
a luminescence band at 5.4 out of the broad-band 4.8 eV.
I
.
TT
I
l-
’
I I
I
I I
I
5
I
,
’
I
?
6d
2.8 eV
2.2 4.8
1 ? 4d
Me
A10
*
I
4.5
I
AL?+
Fig. 8. Illustrated band model for quartz luminescence.
ENERGY
luminescence
x-ray
abs.
(eV)
at
The decay of luminescence in natural and silver-doped quartz
PULSE-CAPACITOR ~ONF - 100~V
COAXIAL TRANS~~ISSIO~~ LINE ~11 TtfPfSTENANODE IGYITRON ELECTRODE !iOLLO',I CATH3DE WIKDOW OF THE C
I I
:19~!9CtiRC)I1AT')'i
NARD VACUUM CtIAMbTR
Fig. 9. temperatures above 200K. This is a contradiction, however, because the broad silver luminescence near to 2.8 eV is easy to detect up to temperature above 400 K! Presuming that, as found in ESR investigations, no silver atom centres are stable at ambient temperature, we can only explain the observed luminescence by a molecular process, because in a silver cation the transition energies of the lower energy levels are much higher than 2.2 or 2.8eV. In our present luminescence model, we suppose that the excited electrons coming from the conduction band are captured by silver cations which are not compensation cations near aluminium defects. The so-formed excited silver atoms are able to emit the observed luminescence light; but interstitial silver atoms are evidently unstable and they lose their valence electrons to the valence band of the regular lattice. Looking at the tables of atomic energy-levels [30] quoted above, transition energies between the second and the first excitation levels f&f-Sp) of silver atoms correlate with the observed luminescence energies. A transition to the ground state of the silver could not be detected hitherto and so we presume that this is a non-radiative transition of the excited electron from the silver atom to the valence band of the lattice. REFEXENCW
1. Trukhin, A. N., Fiz. Khimiya St&la l(5), 466-468(1975). 2. Scheider P. M. and Fowler W. B., Phys. Rev. Letters 36(S), 425-428 (1976). 3. Chelikowsky J. R. and Schliiter M., Phys. Rev. B U(S), 402(1-4029(1976). 4. Griffiths J. H. E., Owen J. and Ward I. M., Defects in cryatailine solids. Bristol Conf., pp.81-87.Phys. Sot. (London, 1955). 5. O‘Brien M. C. M., Proc. Roy. Sot. (Land.) .4231, 404-414 (1955). 6. Cohen A. J., Am. Mineralogist 41, 874-891 (1956).
67
7. Pfenninger H. H. and Laves F., Natutwissenschaften 47, 12, 276 (1960). 8. Wondratschek H., Brnnner G. 0. and Laves F., Noturwissenschaften 47, 12, 275 (1960). 9. Pfenninger H. H., Dissertation Univ. Ziirich (1961). 10. SchGnbacher H., Dissertation Univ. Graz (1966). 11. Ladstitter E., Dissertation Univ. Graz (1969). 12. Gross F., Acta Physica Aust~aca 19,268 (1965). 13. Hohenau W., Acta Physica Aust~aca 39,212-219 (1974). 14. Angerer R., Dissertation Univ. Graz (1971). 15. Trukhin A. N., Etsin S. S. and Shendrik A. V., fzuestiya Akad. Nauk 40(1l), 2329-2333(1976). 16. Bambauer H. U., Schweiz. Mineralog. u. Petrograph. Mitteilungen 41(2), 335-369(1961). 17. Nelson C. M. and Weeks R. A.. J. Am. Ceram. Sot. 43(S). 396-404(1960). 18. Lietz J. and Matheja J., Naturwissenschaften 51, 503 (1964). 19, Frondel C. Am. Min. 30,432 (1945). 20. Fahrenkrog H. H., Dissertation Univ. Hamburg (1966). 21. Hohenau W., Meyer W. and Gross F., Acta Physica Austtiaca 44, 337-347(1976). 22. S~hwingenschuh K., Dissertation Univ. Graz (1977). 23. Lietz J.-and Matheja J., Natu~issenschaften 51,504 (1964). 24. Schwingenschuh and Gross F.. Electronics Letters 14112). 358-359(1978). 25. Angerer R., Gross F. and Koss P., Acta Physica Austriaca 33, 3-7 (1971). 26. Hohenau W. and Gross F., Acta Physica Austriaca 37, 385-390(1973). 27. lchikawa Ti., Japan J. Appt. Phys. 7(3), 220-226(1%8). 28. Schlesinger M., J. Phys. Chem. Solids 26, 1761-1766(1965). 29. Gross F. and Hohenau W., Acfa Physica Ausf~aca 36, 281-285(1972). 30. Moore Ch. E., Atomic Energy levels, Vol. HI, Circular National Bureau of Standards 467. 31. Weeks R. A., J. Appl. Phys. 27, 1376-1381(1956). 32. Weeks R. A., Phys. Rev. 130(2),577-588(1963). 33. Arnold G. W., Phys. Chem. Solids 13, 306320 (1960). 34. Schindler P.. Diss. Univ. Zbrich (19641. 35. Taylor A. i. and Farnell G. W.: Can. J. Phys. 42, 595-607 (1964). 36. Schnadt R. and Schneider J., Phys. Kondens. Materie 11, 19-42 (1970). 37. Schnadt and RBuber A., Solid State Commun. 9, 1.59-161
(1971). 38. Weeks R. A., J. Am. Ceram. Sot. 53(4), 176-179(1970). 39. Mackey J. H. and Boss J. W., .J. Magn. Resonance 3,44-54 ( 1970). 40. Nuttall R. H. D. and Weil J. A., Solid St. Commun. 19, 141-142(1976). 41. Griscom D. L., Proc. 32. Frequ. Contr. Symp., Atlantic-City 30 May 1979,pp. 98-109. 42. Koumvakalis M.. J. Appf. Phys. 51, 5528-5532(1980). 43. Nuttall R. H. D. and Weil J. A., Solid St. Commun. 35, 789-791(1980). 44. Nuttall R. H. D. and Weil J. A., Can. J. Phys. 59, 169d1708, 1886-1892(1981). 45. Castle J. G., Feidmann D. W. and Kiemens P. G., Phys. Reo. l?@(2),577-88 (1963). 46. Jones C. E. and Embree D., J. Appl. Phys. 47(12),5365-5371 (1976). 47. Shendrik A. V. and Yudin D. M., Phys. Status Solidi 85, 343-349(1978). 48. Brower K. L. and Vitko, J. Phys. Status Solidi 95, KL 41-44 (1979). 49. Shendrik A. V. and Yudin D. M., Phys. Status Solidi 95, KL 45-46 (1979). 50. Halliburton L. E. et al.. Solid St. Commu~. 30. 575-579 (1979). 51.
Brower K. L., Phys. Rev. B. 2@(S),1799-1811(1979). 52. Isoya J., Weil J. A. and Halliburton L. E., J. Chem. Phys. Solid 74, 5436-5448(1981). 53. Haberlandt H., Phys. Status Solidi 110, 531-535(1982). 54. Medlin W. L., J. Chem. Phys. 38(5), 1132-1143(1963).
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55. Schlesinger M., Phys. Letters 10(l), 49(50 (1964) 56. Schlesinger M.. J. Phw. Chem. Solids 26. 1761-1766 (1965). 57. Lee H. Y.. Lee B. W. and Jun J. S.. J. Korean Phvs. Sot. 3(2). 54-60 (1970). 58. Lee H. Y., Lee B. W. and Jun J. S., J. Appl. Phys. 42(l), 498-499 (1971). 59. Arnold G. W.. .I. Phys. Chrm. Solids 13, 306-320 (1960). 60. Halperin A. et al., J. Luminescence l(2), 385-397 (1960). 61. Trukhin A. N., Opt. Spectrosc. 40(4), 432-433 (1976). 62. Davis P. H., Huang C. Y. and Weil J. A., J. Phys. Chem. Solid. 39, 897-899 (1978). 63. Davis P. H. and Weil J. A., J. Phys. Chem. Solids 39.775-780 (1978). 64. Solntsev V. P. and Mashkovtsev R. A., Sou. Phys. Solid State 20(3), 471474 (1977). 65. Trukhin A. N. and Shendrik A. V., Phys. Status Sofidi B 98, KL 13-15 (1980). 66. Markes and Halliburton L. E., J. Appl. Phys. 50(12), 81728179 (1979). 67. Martin J. J.. Doherty S. P.. Halliburton L. E., Markes M.. Koumvakalis N. and Sibley W. A., 33rd Annlcctl Symp. Frequ. Confr. 134-147 (1979).
APPENDIX The experfmental setup X-ray flashes were produced by an electrical high-vacuum break-down in a separate part of a hard-vacuum chamber, during the discharge of a high voltage plus capacitor. The high vacuum breakdown was induced by a small pulse discharge between an ignitron electrode and the hollow cathode. Behind the hollow cathode there is an aluminium window protecting the quartz sample from light and electrical fields produced during the discharge. Of course the X-rays coming from the anode could pass this window and reach the quartz sample, which was mounted on a liquid nitrogen-cooled sample holder. Luminescence light, coming from the quartz, was detected by a photomultiplier after passing through a monochromator. With an analog-to-frequency converter, the anode current of the photomultiplier was either digitized and fed to a computer, or displayed on an ocilloscope screen. All parts of the chamber, the transmission line and the capacitor housing were made by ourselves. We used massive steel shielding to protect the outside room against X-rays and stray fields. Usually the anode voltage was 80 kV and the discharge time was 0.8-1.5 wsec.