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Electroluminescence (EL) from natural semiconducting (p-type) diamond
Journal of Luminescence 15 (1977) 405—419 © North-Holland Publishing Company
ELECTROLUMINESCENCE (EL) FROM NATURAL SEMICONDUCTING (p-TYPE) DIAMOND A. LEPEK and A. HALPERIN Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem, Israel Received 5 October 1976
Microscopical examination in this work has shown the EL obtained from a semiconducting diamond using silver-paint electrodes to appear mainly at the negative electrode and only at very small points where the silver grains make contact with the crystal. Complications due to large variations in grain size and shape were eliminated by preparing silver-evaporated electrodes, with dimensions of the order of 1 ~m. The EL brightness was found to increase with the decrease of the electrode size. An expression describing the dependence of the EL on the applied voltage and on the energy of the level to which excitation takes place was fitted to the experimental results. Detailed analysis of the EL suggested that electrons are raised by impact with holes to four discrete levels in the forbidden gap, probably from the level at 0.37 eV above the valence band present in these diamonds. The emission spectrum was found to show three peaks in the spectral range 3600—9000 A, and indicated the existence of another peak beyond 9000 A. All the emission bands seem to result from recombination at one common level, which should be the valence band itself or a level very close to it. Our emission spectrum fitted that given in the literature (ref. [131)for cathodoluminescence of natural semiconducting diamond.
1. Introduction Electroluminescence (EL) in semiconducting (p-type) diamond has been known for a long time [1—5] It occurs mainly in forward polarity and is limited to the vicinity of the negative electrode [1]. Halperin et al. [4] using silver paint electrodes, were able to detect the EL in natural semiconducting diamonds at voltages as low as 1.8 V across the diamond. The emission spectrum was found by these authors to be composed of blue and red bands. The dependence on voltage V of the EL was found to be composed of two segments, each fitting an expression of the form exp(—bV1I2), with b a constant independent of temperature. The EL intensity was found to increase exponentially with temperature in parallel with the increase of concentration of the free holes. The process involved in the EL of semiconducting diamond is still unclear. Wolfe et al. [1] concluded that the EL was due to injection of minority carriers (electrons). .
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Their conclusion was based mainly on the observed polarity. Levinson et al. [6] have considered among other possibilities that of impact ionization. Their experimental results which included also the so-called “electrically excited thermoluminescence”, seemed generally to fit quite well the assumption of impact ionization. They doubted, however, whether the very high fields necessary for excitation by impact to the high lying energy levels involved in EL were in fact formed with only 2 V across the crystal. They have therefore forwarded an idea proposed by Fisher et al. [7] by which the binder in the silver paint may have acted as an insulating layer so that tunneling through such a layer may have excited the EL. In the present work we show that: (a) the mechanism of excitation of EL in semiconducting diamond is the same for various types of electrodes including evapoarted metal, point contact and silver paint electrodes; and (b) the mechanism is field controlled and fields high enough for producing impact excitation really occur in the process. We do not claim that impact excitation is the only process involved in the EL of semiconducting diamond. Our results, however, indicate that under the conditions of our experiments it was the main process in exciting the EL. More than that, assuming this model we were able to obtain interesting information about the structure of the energy levels in the forbidden gap of semiconducting diamond.
2. Experimental All measurements were made on natural type “b semiconducting diamonds and were carried out at room temperature. The crystals were typically about 3 mm in each dimension and had two parallel flat faces. In all cases one electrode had a comparatively large area (a few mm2) and was made by pasting silver paint as a ring along the circumference of one of the flat faces of the diamond, leaving the central area of that face clear for microscopic observations of the electrode on the opposite face, through the diamond. The comparatively large area for this electrode ensured that virtually all the voltage drop occurred at the opposite electrode. Three types of electrodes were used for the opposite face: silver paint, point contact, and small area evaporated silver contacts. The silver paint was Microcircuit SCT-32. The point contacts were polished pins of hard steel, and in some cases just copper wires, pressed to the crystal. The small area silver electrodes were prepared in the following way. A drop of insulating material in solution (such as “Errotex”, used by typists as an erasing material) was left to dry on one of the two flat parallel faces of the diamond. Its concentration was such that a thin insulating film was left. A steel needle was then polished so that its point formed a cone with an angle of about 45°and with a radius of curvature of about 1 pm at the tip. The needle was fixed to an arm of a small balance with its point downwards. The arm was lowered gently to hit the insulating layer on the diamond with a force predetermined by the weight on the balance-arm, thus punching in it a small hole with dimensions depending on the force on the arm. Using a micromanipulator to move the sample enabled us to prepare a
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network of hundreds of holes per square millimeter on the diamond face. The hole diameters ranged between one and a few microns. A thin silver film evaporated onto the mask gave then an array of small-area contacts. The electrical contact was made to the silver film away from the small holes. A good estimate of the area of each hole was obtained by putting the diamond under a projection microscope (before the evaporation of the silver) and measuring the intensity of the light from the light source of the microscope transmitted through the measured hole. An EM! 6256B photomultiplier was used for the measurements. It was mounted at the image plane of the microscope where a variable iris was adjusted to transmit only the light through the individual hole being measured. The intensity of the EL from each silver contact (each “hole”) was measured with the same set-up after the eavporation of the silver and application of a suitable voltage across the diamond. The hole dimensions were also measured on high magnification SEM micrographs which gave virtually the same hole areas. A ramp voltage pulse of about 20 ms at a rate of one pulse per second was used for measuring the EL as a function of voltage. The short pulses were necessary to minimize heating effects and deterioration of the electrodes during a set of experiments. The EL emitted from an array of more then 100 silver evaporated small-hole electrodes was reflected by a high gathering power reflector onto the photocathode of an enhanced S20 RCA C7151W photomultiplier cooled to —25°C.An interference filter with a transmitting band of about 100 A halfwidth was put in front of the photomultiplier. The output from the photomultiplier was fed into a PAR model TDH-9 function-averager and the information stored after averaging over 1000 pulses was recorded on a chart recorder. The measurements were repeated using various interference filters covering the spectral range of 3600—9000 A. The same technique was used with silver paint and point contact electrodes. The spectral distribution of the EL at any desired voltage was deduced from the above measurements. The obtained spectrum was corrected for the transmittance of the various filters and for the depencende of the quantum yield of the photomultiplier on wavelength.
3. Results In preliminary experiments we have compared the EL obtained with silver paint electrodes with that obtained with silver evaporated ones. In both cases the investigated electrodes were nearly 1 mm2 in area, and both experiments were carried out with the same semiconducting diamond (sample Cl). An extreme difference between the two electrodes was observed quite intense EL was obtained with the silver paint electrode while the silver evaporated one gave practically no EL. The silver paint electrode itself and the EL with this electrode have then been examined under the microscope. Fig. 1 a is a micrograph (X 270) of the EL emitted at the negative electrode as obtained by looking at the electrode through the crystal while applying —
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A
B
Fig. 1. Micrographs (X 270) of the EL obtained at the negative silver paint electrode with 30 V across the diamond. (A) exposed for 30 mm soon after the first application of the bias; (B) exposed for 120 mm starting two hours after putting on the field.
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30 V dc across the crystal. The exposure time was 30 mm and no field was applied in this case prior to this exposure. The EL in this case was mainly blue. Its intensity diminished with time of application of the electric field and turned more and more red. Fig. lb is similar to la but was taken 2 h after putting on the field. The exposure in this case was 2 h. The EL is now much weaker and its main intensity comes now from the edges of the electrode, where on the beginning (fig. la) it was comparatively weak. Of interest is the fact that the EL comes from points on the electrodes, presumably single small silver grains with sharp points, typically 1 pm in dimensions. The light from each spot appeared twinkling, especially soon after the first application of the field. The most intense spots were blue and showed strong deterioration of the emission with time. The red spots did not twinkle much and their intensity changed much less with time. No EL was observed under these conditions (30 V across the crystal) on reversing the polarity (making the investigated electrode positive). However, at increased voltages (45 V) there appeared isolated bright points with the total intensity still much below the level obtained in forward bias. The threshold for the reverse-polarity EL decreased somewhat with the deterioration of the forward polarity EL. Fig. 2 shows a SEM micrograph (X 1100) of a silver paint layer. The silver grains
~ Fig. 2. A SliM micrograph ~X 1100) of a silver paint layer showing the silver grains.
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are seen to be irregular in shape and ranging up to about 10 pm in dimensions. The grains are imbedded in the binder and project from it like spikes on the surface. It seems that the EL with silver paint electrodes is emitted at the contact points of the grains on the diamond surface. The EL with the small area silver evaporated electrodes exhibited much higher regularity. Still, each individual small electrode showed some irregularity and quite often it proved to be composed of a few smaller points. An array of small area electrodes of various shapes and sizes was prepared to study the relation between EL and the electrode size. Examples of some of the punched holes taken out from a micrograph of this array (as photographed before the evaporation of the silver) and of the EL obtained from them with 17 V across the crystal are shown in fig. 3, both at the same magnification(X 175). The holes themselves are shown in the upper row and the emitted EL in the lower one. It is seen that as the holes grow larger (advancing from left to right in fig. 3) the EL becomes duller. It is also seen that most of the EL comes from the periphery and is especially bright when the holes exhibit microstructure. The smallest hole in the extreme left of fig. 3a can barely be seen. The smallest dimension of this hole was less than 1 pm, when various effects (mainly misfocusing) caused the light through it to diffuse, and hence its low brightness. The EL from the larger electrodes looked reddish and that from the smaller ones was blue. It is clear from the above that the EL brightness is closely related to the area of the electrodes. In spite of the microstructure in each electrode which disturbed the measuremetn of the effective size of the electrode, we have measured the relation between the electrode area (S) and the average EL brightness (L/S) obtained from each electrode at vairous voltages. The areas S and the EL intensities L were measured using an EMI 6256S photomultiplier (see section 2). No filter was used in the EL measurements. The results are shown in fig. 4 which shows on a log—log scale the brightness as function ofS obtained with 12, 17 and 30 V across the crystal (curves a, b and c respectively). The smallest electrodes were not measured with 30 V be-
A
B Fig. 3. Examples of the dependence of the EL brightness on the electrode size (shown by B and A, respectively; X 175).
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I
________________________________________
100
~
2
101
Electrode area
~
~ 102 2
S
Squa&r
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Fig. 4. Dependence of the EL brightness (L/S) on the electrode area (S). (a) 12 V; (b) 17 V, and (c) 30 V applied across the crystal with the observed electrode negative.
cause they gave highly unstable EL at this voltage. The points on the curves show much scattering which was expected from the irregularity and substructure of the electrodes. However, straight lines can be fitted to the points. Taking curve b we obtain a straight line with a slope of about or L/S x S314 and thus the brightness of the periphery (L/S’/2) decreases with a power of half of the periphery. Although the slopes depend slightly on the applied voltage, the results remain virtually the same for all the curves given in fig. 4. For the examination of the dependence of the EL on voltage we have prepared an array of more then 100 evaporated electrodes of about 4 pm in diameter [8], all of about the same size. The dependence of the EL emitted from this array of electrodes on voltage was found to fit well the expression —~,
L—P where P
112]
(1)
,
1(V—P3)exp[—P2(V—P3)
1 is a parameter depending on the EL intensity, on the light gathering system, on the photomultiplier sensitivity and on the amplification. P2 was found to depend on the photon energy of the emitted EL (as determined by the optical filter used in the measurements). Both P1 and P2 depended on the shape and size of the electrodes. 2 gave in some cases two Plotting ln[L/(V P3)] as a function P3)” portions each describing a straight line butof(V differing in slope. Such plots are shown in fig. 5 for several optical filters. The numerical values of the slopes (given by F 2), and the transmission peaks of the filters (in A units and in electron volts) are indicated on the curves. In virtually all the cases P3 was found to be limited to values be—
—
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A. Lepek et al. /Electroluminescence (EL) from (p-type) diamond 22 20 17
15
13
(v
11
-
10
9
~
V(volts)
1.3)_1/2
Fig. 5. Plots of ln[L/(V — 13)] versus (V — l.3)h/2 obtained with various optical filters (see text). Slope-values and transmission peaks of filters (in A units and in eV) are indicated.
tween 1 and 1.6. These values are small compared to the voltages used in the measurements (mostly between 6 and 22 V). The computer fitting method was therefore insensitive to small variations in P3 and we assumed P3 to be constant. Its average value, 1 .3, was used in all the curves in fig. 5. One can see that the slopes of the curves increase with the increase of the photon energy of the emitted light. Also of interest is the way in which the slopes change: when passing from one curve to the next the lower slope is in many cases preserved in the low voltage part of the curve and only at higher voltages the higher slope takes over. The slopes obtained with the various filters (more than 20 filters covering the wavelength range 3600—9000 A) fell all into one of four groups having the values 7.6 ±0.5; 10.2 ±0.6; 13.5 ±0.6; and 19.6 ±0.6. Fig. 6 gives the slopes obtained with the various filters as a function of the photon energy at the transmission peaks of the filters. To avoid confusion we give in fig. 6 only one slope for each fIlter, namely the slope at about 10 V (see fig. 5). The four groups of slopes appear clearly in fig. 6. We should note that the point obtained at 3.4 eV is less reliable because of the very low intensity of the emission in this range. The results shown in fig. 6 indicate that the EL emission is composed of at least four emission bands differing from each other in the energy required for excitation, a matter which will be discussed further below.
The behavior of silver paint electrodes was more complicated. The EL measured with these electrodes is the result of an integration over many contact points of various shapes and sizes. The electric fields occurring at the smaller contact points (with dimensions of only a fraction of a micron) are therefore much higher compared to those at the larger contacts. To avoid fast deterioration of the electrodes we had to
A. Lepek et al. /Electro!uminescence (EL) from (p-type) diamond
0
~
413
~25;35
Photon Energy of Emitted Light
(eV)
Fig. 6. Dependence of the slopes (see fig. 5) on the photon energy of the peak the filters. The dotted line gives the linear approximation of this dependence.
transmission of
limit our experiments with the silver paint electrodes to considerably lower applied voltages (up to about 10 V). Fig. 7 shows a few curves analogous to those shown in fIg. 5 but obtained with silver paint electrodes. The transmission-peak wavelengths of the filters used are indicated on the curves. The voltages applied are now in the range 2.5—9 V. Two sections can still be distinguished on each curve the high voltage section (above about 6 V) —
having a comparatively higher slope, and the low voltage one with a lower slope.
Even when considering separately just one of the sections not all the curves give
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6
5
(V
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-
3.5
V(volts)
3
2.8
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Fig. 7. Analogous to fig. 5 but for silver paint electrodes.
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straight lines, and it is therefore impossible to assign an accurate slope to each curve. Still, one can see that the slopes generally increase with the increase in the photon energy of the emission, which is qualitatively in agreement with the results obtained with the silver evaporated electrodes (fig. 5). Note that in fig. 7,P3 was chosen to be 1.3, as in fig.5. Some experiments were carried out with pressed metal pin contacts. Observations under the microscope showed each pin contact to exhibit many microscopic EL emitting points similar in behavior to those obtained with silver paint. The spectral distribution of the EL emitted by the array of small evaporated silver electrodes was obtained from the set of experiments with the various filters described above. Fig. 8 shows two spectra obtained choosing voltages of 8.6 and 13.4 V across the crystal, curves a and b, respectively. The curves were corrected for the transmittance of the filters and for the wavelength dependence of the response of the photomultiplier. The spectra show a few bands. The main band appears at about 4500 A (2.8 eV), the next at about 5600 A (2.2 eV) and a third one at 6600 A (1.9 eV). The points at longer wavelengths show considerable scattering which may be attributed to the comparatively low emission at these wavelengths and perhaps mainly to the large and inaccurate correctionsfor the sensitivity of the photomultiplier which drops steeply at this wavelength range. Still, the lower slopes in this range (about 7.6, see fig. 6) indicate that an additional band sets in. Comparing curves a and b in fig. 8 one notes that at the higher voltages (curve b)
Photon Energy
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(eV)
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8.6 volt\~
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.
1
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400
500
600
Wavelength
700
800
900
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Fig. 8. EL emission spectra at 300 K (corrected for transmission of the filters and for spectral response of the photomultiplier). (a) with 8.6 V and (ii) with 13.4 V across the crystal. Curve c was obtained from a and b after elimination of the dependence of excitation on wavelength and voltages (see text).
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the shorter wavelength emission appears enhanced compared to that obtained with the lower voltages (curve a). This effect follows directly from eq. (1) which expresses the dependence of the EL intensity on the voltage V and on the photon energy of the emitted light (through F2). Examination of fig. 6 shows that in spite of the steps on the curve, the dependence ofF2 on the photon energy can be described with fairly good approximation by a straight line. This linear relation is given by the dotted straight line passed through the experimental points in fig. 6. It is preferable to describe the emission spectrum free from the variations in the excitation. To achieve this one has to divide the experimental values for the EL intensity (L) given in curves a and b (fig. 8) by the terms appearing on the right-hand side of eq. (1), except for P1. In other words, we have to plot P1,which includes the effects of the density of states and the transition probabilities, as a function of wavelength. This is given by curve c in fig. 8. In these calculations we have used the values ofF2 given by the dashed straight line in fig. 6. As expected, the values ofF1 obtained from curves a and b now coincide, giving just the same curve c.
4. Discussion The microscopic examinations presented in the present work show clearly that the EL emitted by semiconducting diamond originates from very small contact points at the electrodes. The EL brightness has been shown to be a decreasing function of the dimensions of the contacts and a clear relation has been obtained between the EL intensity per unit length of the periphery of the contact and the length of this periphery. The latter relation fits the observations by which the main EL emission came from still smaller points along the periphery of the contact. We conclude from the above that the EL is field controlled thatItithas occurs at points where work very [6] 5 V/cm,and exist. beenonly stressed in previous high fields, of the order of i0 that some characteristics of the emission induced by electric fields in semiconducting diamond fitted well the impact excitation process. It has only been doubted whether the high electric fields needed for impact excitation over such high energy gaps were indeed formed with only a few volts across the crystal. The present observations settle this point positively in a very clear way. The surface potential barrier ~ in our samples was found to be about 1.1 eV [9— 10] At low voltages, applied in forward polarity, most of the voltage-fall occurs across the depletion layer and acts to flatten the bands near the surface. Further increase in V will cause fields to develop in the bulk of the diamond near the depletion layer. The comparatively large width of the depletion layer rules out the possibility of field emission by tunneling from the electrodes into energy levels in the forbidden gap of the crystal at such low voltages. For impact excitation, holes should be supplied to the high field region. This is easy to achieve in semiconducting (p-type) diamond by voltages applied in forward polarity, when positive holes from the bulk of the crystal will be injected into the .
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high field region. In reverse polarity, however, holes should be supplied from the electrode and have to overcome the surface barrier, which would need higher applied voltages. This fits the observations described in the present work. We shall discuss now the dependence of the EL emission on the voltage applied across the crystal. We assume that for most of the voltages used in the present work we remained in the range of low field excitation. 111 other words, we assume that under our conditions of excitation the average energy gained by the holes between collisions with optical phonons is small compared with the energy needed for excitation, and thus only a small fraction of the accelerated holes will reach the energy needed for excitation. Under these conditions the probability for excitation per unit length will be given by [11] (2)
aAEexp[—e/EX],
where A is a proportionality factor, E is the electric field, e is proportional to the excitation energy, and X is the mean free path for collision with optical phonons. At steady state, the measured EL should then be given by L cx ~(E)v(E)p(E)W(E),
(3)
where v(E) is the drift velocity of the holes which for fields above about l0~V/cm should be proportional to the square root of the field [12] P(E) is the hole concentration in the region near the electrodes, which may be affected by the electric field through high injection, and W(E) is the effective width of the range at which the field is high enough for producing excitations (which should also depend on the field). We assume that all these factors may be entered by just changing the power of E in the exponent and that in front of the exponential factor in eq. (2). Experimentally we measure the voltage V and we have therefore to express L as a function of V. The relation between E and V in the crystal near the potential barrier region is nonlinear and quite complicated. Still, in many cases given in literature the final expression has the form (4) LccV”exp[_aV~m] .
.
We have therefore accepted this form leaving n and m as free parameters to be determined by best fit with the experimental points. Preliminary fitting attempts gave about 1 for n and ~ form. The fit with the experimental points,however, improved considerably introducing V P 3 [see eq. (1)1 instead of V. The introduction of P3 has been interpreted as due to losses mainly due to the surface potential barrier of the diamond. In addition, some voltages losses (though small) may occur at the second large electrode. Previous results obtained with 2] the These same diamonds [4] however, were fitted to carried an expresexperiments, were out sion the paint type Lelectrodes. a exp[—b/V~’ with of silver In addition, they were preliminary in nature, and the effects foP 3 and of the pre-exponential dependence on the voltage have been overlooked. —
.
,
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Wolfe and Woods [1] working with a metal point contact, gave an expression b for the EL of semiconducting diamond;b in their measurements varied between 50 and 100. These authors worked at comparatively high voltages (100— 400 V). It seems that under these conditions the exponential factor in eq. (1) nears saturation, when the pre-exponential factor V F3 should give the main effect. Us,
La V
—
—
ing P2 = 19.6 for the blue emission eq. (1), for the range 200—400 V, indeed gave roughly a straight line which intersected the abscissa at about 100 V. It is worth noting that some curves in fig. 5 in the present work show a tendency of saturation at the higher voltages (near 20 V). It is possible that this indicates the approach of the high field region where the field dependence of ct(E) in eq. (2) given above should change [11]. Some of the curves in fig. 5 gave two straight-line sections, with the lower slope equal to the main slope obtained with filters transmitting at somewhat longer wavelengths. We have thus only four discrete values for P2 (see fig. 6). Comparing eqs. (1) and (2) we see that P2 should be proportional to the excitation energy, and the four slopes therefore show that excitation takes place to four different energy levels in the forbidden gap. The dependence of the slopes on the photon energy of the emitted light, and the nearly linear relation between the two (see the dotted straight line in fig. 6), imply that the various emission bands are obtained by recombination of the excited electrons in the various levels in the forbidden gap with holes in just one energy level. The appearance of more than one slope in the curves in fig. 5 follows from the fact that in a spectral range where two bands overlap the emission due to the transition from the lower level is the main one at low voltages, while that from the higher lying level overtakes at higher voltages. As already stressed above it was essential to have the emission coming from contact points of about the same size and shape, which explains the failure in our attempts to fit the emission obtained with silver paint to eq. (1). With the variety of sizes of the contact points with the silver paint electrodes one gets a wide range of electric fields at each value of the applied voltage and averaging over these does not give the simple relations obtained with the even-sized electrodes. We shall return now to the problem of the energy levels involved in excitation and in emission. Taking the average value of the emission energy (given in the abscissa) for the upper three values ofF2 (see fig. 6) one gets about 1.9, 2.2 and 2.9 eV, which just fits the peak energies of the three bands which appear in the emission spectra given in fig. 8. For the lowest level appearing in fig. 6, comparison is more difficult because our measurements were limited to 9000 A (1.37 eV), while the lowest value ofF2 seems to belong to an emission band at about 1.2 eV reported in the literature [131 It seems that in our measurements we have just reached the high energy tail of this band as indicated both in figs. 6 and 8. The relation between the energy of excitation and that of the emission was obtained by the dotted straight line in fig. 6. There was some freedom in determining this straight line, which introduces some inaccuracy in its interaction with the abscissa. Still, the fact that the intersection occurs at about 0.4 eV indicates that ex.
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rIluton Energy —
3
2.5
400
500
2
600
1.7
700
Wavelenoth
(eV)
1.5 1.4 1.3
800
900
1.2
1000 1100
nm)
Fig. 9. Emission spectrum of a semiconducting diamond (at 50 K) reproduced from fig. 10 of ref. 1131 but on a wavelength scale and corrected for the wavelength dependence of the S — 1
photocathode used in ref. 1131. citation takes place from the known energy level at 0.37 eV above the valcene band [3] ,while the emission is obtained by transition to the valence band itself or to levels very near to it. The emission spectrum obtained in the present work (fig. 8) fits very well that reported by Wight et al. [131 The latter has been obtained by excitation with energetic electrons and should therefore be compared with curve c in fig. 8. In fig. 9 we give for comparison the spectrum from fig. 10 of ref. [13] obtained for sample SA 56B but plotted on a wavelength scale and corrected for the S 1 sensitivity of the photomultiplier used by Wight et al. In spite of the fact that the spectrum in fig. 9 has been obtained at 50 K compared to 300 K in our measurements the fit between the two is quite good. -
Acknowledgement We acknowledge with thanks the helpful discussions with Dr. J. Levinson throughout the work. Thanks are also due to Engineer M. Ronen for help in computer programming.
References [11 R. Wolfe and J. Woods, Phys. Rev.
105 (1957) 921. [2] M. Drake Bell and W.J. Leivo, Phys. Rev. 111(1958)1227. 131 A. Halperin and J. Nahum, J. Phys. Chem. Solids 18 (1961) 297.
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[41 A. Halperin, V. Bar, R. Chen and J. Levinson, Optical and Electrical Properties of Diamond, final scientific report contract AF 61(052) 759 (1967) ch. 6. —
[5] V. Baa, A. Halperin and J. Levinson, Proc. International Conference Lumin. Budapest (1966) p. 1418. [6] J. Levinson, A. Halperin and V. Bar, J. Luminescence 6 (1973) 1. [7] AG. Fisher and HI. Moss, J. Appl. Phys. 34(1963)2112; A.G. Fisher, Symp. on Radiative Recombination, Dunod, Paris (1964) p. 259. [8] These dimensions lor the electrodes were chosen because working in the same voltage range smaller electrodes would be unstable and deteriorate during a set of experiments while larger electrodes would result in a too low light level. [9] A. Lepek, J. Levinson and A. Halperin, Phys. Letters S1A (1975) 345. [10] To be published. [11] A.G. Chynoweth, in: Semiconductors and semimetals, Vol.4. eds. R.K. Willarson and A.C. Beer (Academic Press, New York, 1968). [12] W. Shockley, Bell System Tech. J. 30 (1951) 990; E.A. Konorova and S,A. Shevchenko, Soy. Phys.-Semicond. 1 (1967) 229. 1131 DR. Wight, P.J. Dean, E.C. Lightowlers and C.D. Mobsby, J. Luminescence 4 (1971)169.
ELECTROLUMINESCENCE (EL) FROM NATURAL SEMICONDUCTING (p-TYPE) DIAMOND A. LEPEK and A. HALPERIN Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem, Israel Received 5 October 1976
Microscopical examination in this work has shown the EL obtained from a semiconducting diamond using silver-paint electrodes to appear mainly at the negative electrode and only at very small points where the silver grains make contact with the crystal. Complications due to large variations in grain size and shape were eliminated by preparing silver-evaporated electrodes, with dimensions of the order of 1 ~m. The EL brightness was found to increase with the decrease of the electrode size. An expression describing the dependence of the EL on the applied voltage and on the energy of the level to which excitation takes place was fitted to the experimental results. Detailed analysis of the EL suggested that electrons are raised by impact with holes to four discrete levels in the forbidden gap, probably from the level at 0.37 eV above the valence band present in these diamonds. The emission spectrum was found to show three peaks in the spectral range 3600—9000 A, and indicated the existence of another peak beyond 9000 A. All the emission bands seem to result from recombination at one common level, which should be the valence band itself or a level very close to it. Our emission spectrum fitted that given in the literature (ref. [131)for cathodoluminescence of natural semiconducting diamond.
1. Introduction Electroluminescence (EL) in semiconducting (p-type) diamond has been known for a long time [1—5] It occurs mainly in forward polarity and is limited to the vicinity of the negative electrode [1]. Halperin et al. [4] using silver paint electrodes, were able to detect the EL in natural semiconducting diamonds at voltages as low as 1.8 V across the diamond. The emission spectrum was found by these authors to be composed of blue and red bands. The dependence on voltage V of the EL was found to be composed of two segments, each fitting an expression of the form exp(—bV1I2), with b a constant independent of temperature. The EL intensity was found to increase exponentially with temperature in parallel with the increase of concentration of the free holes. The process involved in the EL of semiconducting diamond is still unclear. Wolfe et al. [1] concluded that the EL was due to injection of minority carriers (electrons). .
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Their conclusion was based mainly on the observed polarity. Levinson et al. [6] have considered among other possibilities that of impact ionization. Their experimental results which included also the so-called “electrically excited thermoluminescence”, seemed generally to fit quite well the assumption of impact ionization. They doubted, however, whether the very high fields necessary for excitation by impact to the high lying energy levels involved in EL were in fact formed with only 2 V across the crystal. They have therefore forwarded an idea proposed by Fisher et al. [7] by which the binder in the silver paint may have acted as an insulating layer so that tunneling through such a layer may have excited the EL. In the present work we show that: (a) the mechanism of excitation of EL in semiconducting diamond is the same for various types of electrodes including evapoarted metal, point contact and silver paint electrodes; and (b) the mechanism is field controlled and fields high enough for producing impact excitation really occur in the process. We do not claim that impact excitation is the only process involved in the EL of semiconducting diamond. Our results, however, indicate that under the conditions of our experiments it was the main process in exciting the EL. More than that, assuming this model we were able to obtain interesting information about the structure of the energy levels in the forbidden gap of semiconducting diamond.
2. Experimental All measurements were made on natural type “b semiconducting diamonds and were carried out at room temperature. The crystals were typically about 3 mm in each dimension and had two parallel flat faces. In all cases one electrode had a comparatively large area (a few mm2) and was made by pasting silver paint as a ring along the circumference of one of the flat faces of the diamond, leaving the central area of that face clear for microscopic observations of the electrode on the opposite face, through the diamond. The comparatively large area for this electrode ensured that virtually all the voltage drop occurred at the opposite electrode. Three types of electrodes were used for the opposite face: silver paint, point contact, and small area evaporated silver contacts. The silver paint was Microcircuit SCT-32. The point contacts were polished pins of hard steel, and in some cases just copper wires, pressed to the crystal. The small area silver electrodes were prepared in the following way. A drop of insulating material in solution (such as “Errotex”, used by typists as an erasing material) was left to dry on one of the two flat parallel faces of the diamond. Its concentration was such that a thin insulating film was left. A steel needle was then polished so that its point formed a cone with an angle of about 45°and with a radius of curvature of about 1 pm at the tip. The needle was fixed to an arm of a small balance with its point downwards. The arm was lowered gently to hit the insulating layer on the diamond with a force predetermined by the weight on the balance-arm, thus punching in it a small hole with dimensions depending on the force on the arm. Using a micromanipulator to move the sample enabled us to prepare a
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network of hundreds of holes per square millimeter on the diamond face. The hole diameters ranged between one and a few microns. A thin silver film evaporated onto the mask gave then an array of small-area contacts. The electrical contact was made to the silver film away from the small holes. A good estimate of the area of each hole was obtained by putting the diamond under a projection microscope (before the evaporation of the silver) and measuring the intensity of the light from the light source of the microscope transmitted through the measured hole. An EM! 6256B photomultiplier was used for the measurements. It was mounted at the image plane of the microscope where a variable iris was adjusted to transmit only the light through the individual hole being measured. The intensity of the EL from each silver contact (each “hole”) was measured with the same set-up after the eavporation of the silver and application of a suitable voltage across the diamond. The hole dimensions were also measured on high magnification SEM micrographs which gave virtually the same hole areas. A ramp voltage pulse of about 20 ms at a rate of one pulse per second was used for measuring the EL as a function of voltage. The short pulses were necessary to minimize heating effects and deterioration of the electrodes during a set of experiments. The EL emitted from an array of more then 100 silver evaporated small-hole electrodes was reflected by a high gathering power reflector onto the photocathode of an enhanced S20 RCA C7151W photomultiplier cooled to —25°C.An interference filter with a transmitting band of about 100 A halfwidth was put in front of the photomultiplier. The output from the photomultiplier was fed into a PAR model TDH-9 function-averager and the information stored after averaging over 1000 pulses was recorded on a chart recorder. The measurements were repeated using various interference filters covering the spectral range of 3600—9000 A. The same technique was used with silver paint and point contact electrodes. The spectral distribution of the EL at any desired voltage was deduced from the above measurements. The obtained spectrum was corrected for the transmittance of the various filters and for the depencende of the quantum yield of the photomultiplier on wavelength.
3. Results In preliminary experiments we have compared the EL obtained with silver paint electrodes with that obtained with silver evaporated ones. In both cases the investigated electrodes were nearly 1 mm2 in area, and both experiments were carried out with the same semiconducting diamond (sample Cl). An extreme difference between the two electrodes was observed quite intense EL was obtained with the silver paint electrode while the silver evaporated one gave practically no EL. The silver paint electrode itself and the EL with this electrode have then been examined under the microscope. Fig. 1 a is a micrograph (X 270) of the EL emitted at the negative electrode as obtained by looking at the electrode through the crystal while applying —
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B
Fig. 1. Micrographs (X 270) of the EL obtained at the negative silver paint electrode with 30 V across the diamond. (A) exposed for 30 mm soon after the first application of the bias; (B) exposed for 120 mm starting two hours after putting on the field.
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30 V dc across the crystal. The exposure time was 30 mm and no field was applied in this case prior to this exposure. The EL in this case was mainly blue. Its intensity diminished with time of application of the electric field and turned more and more red. Fig. lb is similar to la but was taken 2 h after putting on the field. The exposure in this case was 2 h. The EL is now much weaker and its main intensity comes now from the edges of the electrode, where on the beginning (fig. la) it was comparatively weak. Of interest is the fact that the EL comes from points on the electrodes, presumably single small silver grains with sharp points, typically 1 pm in dimensions. The light from each spot appeared twinkling, especially soon after the first application of the field. The most intense spots were blue and showed strong deterioration of the emission with time. The red spots did not twinkle much and their intensity changed much less with time. No EL was observed under these conditions (30 V across the crystal) on reversing the polarity (making the investigated electrode positive). However, at increased voltages (45 V) there appeared isolated bright points with the total intensity still much below the level obtained in forward bias. The threshold for the reverse-polarity EL decreased somewhat with the deterioration of the forward polarity EL. Fig. 2 shows a SEM micrograph (X 1100) of a silver paint layer. The silver grains
~ Fig. 2. A SliM micrograph ~X 1100) of a silver paint layer showing the silver grains.
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are seen to be irregular in shape and ranging up to about 10 pm in dimensions. The grains are imbedded in the binder and project from it like spikes on the surface. It seems that the EL with silver paint electrodes is emitted at the contact points of the grains on the diamond surface. The EL with the small area silver evaporated electrodes exhibited much higher regularity. Still, each individual small electrode showed some irregularity and quite often it proved to be composed of a few smaller points. An array of small area electrodes of various shapes and sizes was prepared to study the relation between EL and the electrode size. Examples of some of the punched holes taken out from a micrograph of this array (as photographed before the evaporation of the silver) and of the EL obtained from them with 17 V across the crystal are shown in fig. 3, both at the same magnification(X 175). The holes themselves are shown in the upper row and the emitted EL in the lower one. It is seen that as the holes grow larger (advancing from left to right in fig. 3) the EL becomes duller. It is also seen that most of the EL comes from the periphery and is especially bright when the holes exhibit microstructure. The smallest hole in the extreme left of fig. 3a can barely be seen. The smallest dimension of this hole was less than 1 pm, when various effects (mainly misfocusing) caused the light through it to diffuse, and hence its low brightness. The EL from the larger electrodes looked reddish and that from the smaller ones was blue. It is clear from the above that the EL brightness is closely related to the area of the electrodes. In spite of the microstructure in each electrode which disturbed the measuremetn of the effective size of the electrode, we have measured the relation between the electrode area (S) and the average EL brightness (L/S) obtained from each electrode at vairous voltages. The areas S and the EL intensities L were measured using an EMI 6256S photomultiplier (see section 2). No filter was used in the EL measurements. The results are shown in fig. 4 which shows on a log—log scale the brightness as function ofS obtained with 12, 17 and 30 V across the crystal (curves a, b and c respectively). The smallest electrodes were not measured with 30 V be-
A
B Fig. 3. Examples of the dependence of the EL brightness on the electrode size (shown by B and A, respectively; X 175).
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I
________________________________________
100
~
2
101
Electrode area
~
~ 102 2
S
Squa&r
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Fig. 4. Dependence of the EL brightness (L/S) on the electrode area (S). (a) 12 V; (b) 17 V, and (c) 30 V applied across the crystal with the observed electrode negative.
cause they gave highly unstable EL at this voltage. The points on the curves show much scattering which was expected from the irregularity and substructure of the electrodes. However, straight lines can be fitted to the points. Taking curve b we obtain a straight line with a slope of about or L/S x S314 and thus the brightness of the periphery (L/S’/2) decreases with a power of half of the periphery. Although the slopes depend slightly on the applied voltage, the results remain virtually the same for all the curves given in fig. 4. For the examination of the dependence of the EL on voltage we have prepared an array of more then 100 evaporated electrodes of about 4 pm in diameter [8], all of about the same size. The dependence of the EL emitted from this array of electrodes on voltage was found to fit well the expression —~,
L—P where P
112]
(1)
,
1(V—P3)exp[—P2(V—P3)
1 is a parameter depending on the EL intensity, on the light gathering system, on the photomultiplier sensitivity and on the amplification. P2 was found to depend on the photon energy of the emitted EL (as determined by the optical filter used in the measurements). Both P1 and P2 depended on the shape and size of the electrodes. 2 gave in some cases two Plotting ln[L/(V P3)] as a function P3)” portions each describing a straight line butof(V differing in slope. Such plots are shown in fig. 5 for several optical filters. The numerical values of the slopes (given by F 2), and the transmission peaks of the filters (in A units and in electron volts) are indicated on the curves. In virtually all the cases P3 was found to be limited to values be—
—
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15
13
(v
11
-
10
9
~
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1.3)_1/2
Fig. 5. Plots of ln[L/(V — 13)] versus (V — l.3)h/2 obtained with various optical filters (see text). Slope-values and transmission peaks of filters (in A units and in eV) are indicated.
tween 1 and 1.6. These values are small compared to the voltages used in the measurements (mostly between 6 and 22 V). The computer fitting method was therefore insensitive to small variations in P3 and we assumed P3 to be constant. Its average value, 1 .3, was used in all the curves in fig. 5. One can see that the slopes of the curves increase with the increase of the photon energy of the emitted light. Also of interest is the way in which the slopes change: when passing from one curve to the next the lower slope is in many cases preserved in the low voltage part of the curve and only at higher voltages the higher slope takes over. The slopes obtained with the various filters (more than 20 filters covering the wavelength range 3600—9000 A) fell all into one of four groups having the values 7.6 ±0.5; 10.2 ±0.6; 13.5 ±0.6; and 19.6 ±0.6. Fig. 6 gives the slopes obtained with the various filters as a function of the photon energy at the transmission peaks of the filters. To avoid confusion we give in fig. 6 only one slope for each fIlter, namely the slope at about 10 V (see fig. 5). The four groups of slopes appear clearly in fig. 6. We should note that the point obtained at 3.4 eV is less reliable because of the very low intensity of the emission in this range. The results shown in fig. 6 indicate that the EL emission is composed of at least four emission bands differing from each other in the energy required for excitation, a matter which will be discussed further below.
The behavior of silver paint electrodes was more complicated. The EL measured with these electrodes is the result of an integration over many contact points of various shapes and sizes. The electric fields occurring at the smaller contact points (with dimensions of only a fraction of a micron) are therefore much higher compared to those at the larger contacts. To avoid fast deterioration of the electrodes we had to
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0
~
413
~25;35
Photon Energy of Emitted Light
(eV)
Fig. 6. Dependence of the slopes (see fig. 5) on the photon energy of the peak the filters. The dotted line gives the linear approximation of this dependence.
transmission of
limit our experiments with the silver paint electrodes to considerably lower applied voltages (up to about 10 V). Fig. 7 shows a few curves analogous to those shown in fIg. 5 but obtained with silver paint electrodes. The transmission-peak wavelengths of the filters used are indicated on the curves. The voltages applied are now in the range 2.5—9 V. Two sections can still be distinguished on each curve the high voltage section (above about 6 V) —
having a comparatively higher slope, and the low voltage one with a lower slope.
Even when considering separately just one of the sections not all the curves give
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Fig. 7. Analogous to fig. 5 but for silver paint electrodes.
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straight lines, and it is therefore impossible to assign an accurate slope to each curve. Still, one can see that the slopes generally increase with the increase in the photon energy of the emission, which is qualitatively in agreement with the results obtained with the silver evaporated electrodes (fig. 5). Note that in fig. 7,P3 was chosen to be 1.3, as in fig.5. Some experiments were carried out with pressed metal pin contacts. Observations under the microscope showed each pin contact to exhibit many microscopic EL emitting points similar in behavior to those obtained with silver paint. The spectral distribution of the EL emitted by the array of small evaporated silver electrodes was obtained from the set of experiments with the various filters described above. Fig. 8 shows two spectra obtained choosing voltages of 8.6 and 13.4 V across the crystal, curves a and b, respectively. The curves were corrected for the transmittance of the filters and for the wavelength dependence of the response of the photomultiplier. The spectra show a few bands. The main band appears at about 4500 A (2.8 eV), the next at about 5600 A (2.2 eV) and a third one at 6600 A (1.9 eV). The points at longer wavelengths show considerable scattering which may be attributed to the comparatively low emission at these wavelengths and perhaps mainly to the large and inaccurate correctionsfor the sensitivity of the photomultiplier which drops steeply at this wavelength range. Still, the lower slopes in this range (about 7.6, see fig. 6) indicate that an additional band sets in. Comparing curves a and b in fig. 8 one notes that at the higher voltages (curve b)
Photon Energy
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Wavelength
700
800
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Fig. 8. EL emission spectra at 300 K (corrected for transmission of the filters and for spectral response of the photomultiplier). (a) with 8.6 V and (ii) with 13.4 V across the crystal. Curve c was obtained from a and b after elimination of the dependence of excitation on wavelength and voltages (see text).
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the shorter wavelength emission appears enhanced compared to that obtained with the lower voltages (curve a). This effect follows directly from eq. (1) which expresses the dependence of the EL intensity on the voltage V and on the photon energy of the emitted light (through F2). Examination of fig. 6 shows that in spite of the steps on the curve, the dependence ofF2 on the photon energy can be described with fairly good approximation by a straight line. This linear relation is given by the dotted straight line passed through the experimental points in fig. 6. It is preferable to describe the emission spectrum free from the variations in the excitation. To achieve this one has to divide the experimental values for the EL intensity (L) given in curves a and b (fig. 8) by the terms appearing on the right-hand side of eq. (1), except for P1. In other words, we have to plot P1,which includes the effects of the density of states and the transition probabilities, as a function of wavelength. This is given by curve c in fig. 8. In these calculations we have used the values ofF2 given by the dashed straight line in fig. 6. As expected, the values ofF1 obtained from curves a and b now coincide, giving just the same curve c.
4. Discussion The microscopic examinations presented in the present work show clearly that the EL emitted by semiconducting diamond originates from very small contact points at the electrodes. The EL brightness has been shown to be a decreasing function of the dimensions of the contacts and a clear relation has been obtained between the EL intensity per unit length of the periphery of the contact and the length of this periphery. The latter relation fits the observations by which the main EL emission came from still smaller points along the periphery of the contact. We conclude from the above that the EL is field controlled thatItithas occurs at points where work very [6] 5 V/cm,and exist. beenonly stressed in previous high fields, of the order of i0 that some characteristics of the emission induced by electric fields in semiconducting diamond fitted well the impact excitation process. It has only been doubted whether the high electric fields needed for impact excitation over such high energy gaps were indeed formed with only a few volts across the crystal. The present observations settle this point positively in a very clear way. The surface potential barrier ~ in our samples was found to be about 1.1 eV [9— 10] At low voltages, applied in forward polarity, most of the voltage-fall occurs across the depletion layer and acts to flatten the bands near the surface. Further increase in V will cause fields to develop in the bulk of the diamond near the depletion layer. The comparatively large width of the depletion layer rules out the possibility of field emission by tunneling from the electrodes into energy levels in the forbidden gap of the crystal at such low voltages. For impact excitation, holes should be supplied to the high field region. This is easy to achieve in semiconducting (p-type) diamond by voltages applied in forward polarity, when positive holes from the bulk of the crystal will be injected into the .
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high field region. In reverse polarity, however, holes should be supplied from the electrode and have to overcome the surface barrier, which would need higher applied voltages. This fits the observations described in the present work. We shall discuss now the dependence of the EL emission on the voltage applied across the crystal. We assume that for most of the voltages used in the present work we remained in the range of low field excitation. 111 other words, we assume that under our conditions of excitation the average energy gained by the holes between collisions with optical phonons is small compared with the energy needed for excitation, and thus only a small fraction of the accelerated holes will reach the energy needed for excitation. Under these conditions the probability for excitation per unit length will be given by [11] (2)
aAEexp[—e/EX],
where A is a proportionality factor, E is the electric field, e is proportional to the excitation energy, and X is the mean free path for collision with optical phonons. At steady state, the measured EL should then be given by L cx ~(E)v(E)p(E)W(E),
(3)
where v(E) is the drift velocity of the holes which for fields above about l0~V/cm should be proportional to the square root of the field [12] P(E) is the hole concentration in the region near the electrodes, which may be affected by the electric field through high injection, and W(E) is the effective width of the range at which the field is high enough for producing excitations (which should also depend on the field). We assume that all these factors may be entered by just changing the power of E in the exponent and that in front of the exponential factor in eq. (2). Experimentally we measure the voltage V and we have therefore to express L as a function of V. The relation between E and V in the crystal near the potential barrier region is nonlinear and quite complicated. Still, in many cases given in literature the final expression has the form (4) LccV”exp[_aV~m] .
.
We have therefore accepted this form leaving n and m as free parameters to be determined by best fit with the experimental points. Preliminary fitting attempts gave about 1 for n and ~ form. The fit with the experimental points,however, improved considerably introducing V P 3 [see eq. (1)1 instead of V. The introduction of P3 has been interpreted as due to losses mainly due to the surface potential barrier of the diamond. In addition, some voltages losses (though small) may occur at the second large electrode. Previous results obtained with 2] the These same diamonds [4] however, were fitted to carried an expresexperiments, were out sion the paint type Lelectrodes. a exp[—b/V~’ with of silver In addition, they were preliminary in nature, and the effects foP 3 and of the pre-exponential dependence on the voltage have been overlooked. —
.
,
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Wolfe and Woods [1] working with a metal point contact, gave an expression b for the EL of semiconducting diamond;b in their measurements varied between 50 and 100. These authors worked at comparatively high voltages (100— 400 V). It seems that under these conditions the exponential factor in eq. (1) nears saturation, when the pre-exponential factor V F3 should give the main effect. Us,
La V
—
—
ing P2 = 19.6 for the blue emission eq. (1), for the range 200—400 V, indeed gave roughly a straight line which intersected the abscissa at about 100 V. It is worth noting that some curves in fig. 5 in the present work show a tendency of saturation at the higher voltages (near 20 V). It is possible that this indicates the approach of the high field region where the field dependence of ct(E) in eq. (2) given above should change [11]. Some of the curves in fig. 5 gave two straight-line sections, with the lower slope equal to the main slope obtained with filters transmitting at somewhat longer wavelengths. We have thus only four discrete values for P2 (see fig. 6). Comparing eqs. (1) and (2) we see that P2 should be proportional to the excitation energy, and the four slopes therefore show that excitation takes place to four different energy levels in the forbidden gap. The dependence of the slopes on the photon energy of the emitted light, and the nearly linear relation between the two (see the dotted straight line in fig. 6), imply that the various emission bands are obtained by recombination of the excited electrons in the various levels in the forbidden gap with holes in just one energy level. The appearance of more than one slope in the curves in fig. 5 follows from the fact that in a spectral range where two bands overlap the emission due to the transition from the lower level is the main one at low voltages, while that from the higher lying level overtakes at higher voltages. As already stressed above it was essential to have the emission coming from contact points of about the same size and shape, which explains the failure in our attempts to fit the emission obtained with silver paint to eq. (1). With the variety of sizes of the contact points with the silver paint electrodes one gets a wide range of electric fields at each value of the applied voltage and averaging over these does not give the simple relations obtained with the even-sized electrodes. We shall return now to the problem of the energy levels involved in excitation and in emission. Taking the average value of the emission energy (given in the abscissa) for the upper three values ofF2 (see fig. 6) one gets about 1.9, 2.2 and 2.9 eV, which just fits the peak energies of the three bands which appear in the emission spectra given in fig. 8. For the lowest level appearing in fig. 6, comparison is more difficult because our measurements were limited to 9000 A (1.37 eV), while the lowest value ofF2 seems to belong to an emission band at about 1.2 eV reported in the literature [131 It seems that in our measurements we have just reached the high energy tail of this band as indicated both in figs. 6 and 8. The relation between the energy of excitation and that of the emission was obtained by the dotted straight line in fig. 6. There was some freedom in determining this straight line, which introduces some inaccuracy in its interaction with the abscissa. Still, the fact that the intersection occurs at about 0.4 eV indicates that ex.
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rIluton Energy —
3
2.5
400
500
2
600
1.7
700
Wavelenoth
(eV)
1.5 1.4 1.3
800
900
1.2
1000 1100
nm)
Fig. 9. Emission spectrum of a semiconducting diamond (at 50 K) reproduced from fig. 10 of ref. 1131 but on a wavelength scale and corrected for the wavelength dependence of the S — 1
photocathode used in ref. 1131. citation takes place from the known energy level at 0.37 eV above the valcene band [3] ,while the emission is obtained by transition to the valence band itself or to levels very near to it. The emission spectrum obtained in the present work (fig. 8) fits very well that reported by Wight et al. [131 The latter has been obtained by excitation with energetic electrons and should therefore be compared with curve c in fig. 8. In fig. 9 we give for comparison the spectrum from fig. 10 of ref. [13] obtained for sample SA 56B but plotted on a wavelength scale and corrected for the S 1 sensitivity of the photomultiplier used by Wight et al. In spite of the fact that the spectrum in fig. 9 has been obtained at 50 K compared to 300 K in our measurements the fit between the two is quite good. -
Acknowledgement We acknowledge with thanks the helpful discussions with Dr. J. Levinson throughout the work. Thanks are also due to Engineer M. Ronen for help in computer programming.
References [11 R. Wolfe and J. Woods, Phys. Rev.
105 (1957) 921. [2] M. Drake Bell and W.J. Leivo, Phys. Rev. 111(1958)1227. 131 A. Halperin and J. Nahum, J. Phys. Chem. Solids 18 (1961) 297.
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[41 A. Halperin, V. Bar, R. Chen and J. Levinson, Optical and Electrical Properties of Diamond, final scientific report contract AF 61(052) 759 (1967) ch. 6. —
[5] V. Baa, A. Halperin and J. Levinson, Proc. International Conference Lumin. Budapest (1966) p. 1418. [6] J. Levinson, A. Halperin and V. Bar, J. Luminescence 6 (1973) 1. [7] AG. Fisher and HI. Moss, J. Appl. Phys. 34(1963)2112; A.G. Fisher, Symp. on Radiative Recombination, Dunod, Paris (1964) p. 259. [8] These dimensions lor the electrodes were chosen because working in the same voltage range smaller electrodes would be unstable and deteriorate during a set of experiments while larger electrodes would result in a too low light level. [9] A. Lepek, J. Levinson and A. Halperin, Phys. Letters S1A (1975) 345. [10] To be published. [11] A.G. Chynoweth, in: Semiconductors and semimetals, Vol.4. eds. R.K. Willarson and A.C. Beer (Academic Press, New York, 1968). [12] W. Shockley, Bell System Tech. J. 30 (1951) 990; E.A. Konorova and S,A. Shevchenko, Soy. Phys.-Semicond. 1 (1967) 229. 1131 DR. Wight, P.J. Dean, E.C. Lightowlers and C.D. Mobsby, J. Luminescence 4 (1971)169.