- Email: [email protected]
Excitation spectra and temperature dependence of luminescence and photoconductivity of diamond
J. Phys. Chem. Solids.
Pergamon
EXCITATION
Press 1962. Vol. 23, pp. 345-358.
Printed in Great Britain.
SPECTRA AND
DEPENDENCE
TEMPERATURE
OF LUMINESCENCE
PHOTOCONDUCTIVITY
AND
OF DIAMOND
J. NAHUM* and A. HALPEIUNt Department
of Physics, The Hebrew University of Jerusalem,
Jerusalem,
Israel
(Receiwed 8 September 1961)
Abstract-Luminescence excitation spectra of type I and intermediate diamonds were measured at 80°K and at room temperature, and were found to show a fine band structure near the absorption The photoconductivity excitation spectrum at 80°K showed inverted peaks edge (210-240 q). (minima) at the wavelengths of the maxima in the luminescence excitation. At room temperature,
the inverted peaks turned into direct peaks (maxima) in the excitation spectra of the photoconductivity. Inverted peaks were also observed in the thermoluminescence excitation spectra at low temperatures. On excitation with wavelengths of 220-240 rnp the photoconductivity was found to rise, and the luminescence to decrease on warming the crystals. On excitation with longer wavelengths, both were found to be temperature independent up to considerably above room temperature. The results are discussed and a model to account for the results is proposed.
IN’I’RODUCTION
THE luminescence of diamond has been investigated by many authors. ROBERTSONet al.(l) were the first to examine extensively the physical properties of diamond. They have classified diamonds into two types differing from each other with regard to practically all their structure sensitive properties. One of the characteristic properties of type I diamonds was the absorption in the U.V. up to about 3000 A, while type II transmitted down to about 2250 A. More recently this classification was found insufficient.@4) Type II diamonds were then further divided(s) into: type IIa showing only a weak continuous absorption tail from the edge at about 2250 A towards longer wavelengths, type IIb distinguished by its semiconducting properties,@) and intermediate diamonds(s) showing only a comparatively weak band structure in the region 2250-2400 A. *Detached from the Israel Atomic Energy Commission. t Present address : The Mellon Institute, Pittsburgh 13, Pa. 345
The relation between this classification and the luminescence still remained obscure. ROBERTSON et al.(l) found that type II diamonds did not luminesce, while type I luminesced strongly, mainly irr the blue. RAM&~) and BAI@) claimed that although pure type II diamonds luminesced very weakly, the luminescence intensity of type I diamonds generally decreased with increasing absorption in the range 2200-3000 A. BAI@) suggested that the luminescence depended on the strength of the band structure at 2250-2400 A. This system of bands is difficult to measure in most diamonds (type I) because of the much stronger absorption in the region 220&3000& This absorption was found(7) to be due to nitrogen impurities present in type I diamonds, and seems to contribute slightly, if at all, to the background luminescence but not to the main luminescence appearing with a band structure in the blue and green regions. In intermediate diamonds, the nitrogen absorption is absent and the band structure near the edge can be measured in transmission, and using very thin specimens this absorption could also be detected in type I diamonds.(W)
346
J. NAHUM
and
If the luminescence depends on the bands at 2250-240011, these bands should appear in the excitation spectrum for the luminescence. That the excitation spectrum indeed reveals a band structure in regions of high absorption, and even within the lattice absorption edge, has been shown pre~ously(lO~ for ZnS. This was done in the present work for diamonds and the band structure revealed was found to fit well the one found in direct measurements. These measurements, besides proving the relation between the band structure and the luminescence systems in diamond, are of advantage compared with the direct absorption measurements also because of the better resolution obtainable in the luminescence measurements. A detailed investigation on the luminescence of diamond was carried out by DYER and ~A~HEWS(~~). These authors, however, used in most of their measurements the 3660 A mercury lines for excitation, and their excitation spectra measurements were confined only to wavelengths higher than 3000 A. Very recently MAKE reported on the luminescence excitation spectra of diamond near the absorption edge. The rne~ur~en~ were, however, confined to room temperature, and so the fine structure in the bands was not fully revealed. The present work, which was carried out independent of ,Ws contains measurements at different temperatures down to liquid air temperature. In addition to the better resolution of the fine structure these measurements also provided valuable information on the temperature dependence of the effects. Type IIb (semiconducting dismonds) did not show any fine structure in the region 2250-2400 A at 80°K. Some properties of type IIb diamonds were reported already in an earlier publication,(ls) and this type was therefore omitted in the present paper. The present paper deals mainly with the luminescence. It was, however, interesting to compare the excitation spectra of the luminescence with those of the photoconductivity and the thermoluminescence. PANT states that “there is a clear inverse correlation between photoconductivPANT based his statement ity and luminescence”. on comparison of the intensities of luminescence
A.
HALPERIN
and phot~ondu~ivi~ in different specimens. It will be shown in the present work that the above statement has a much broader aspect. A detailed description of the photoconductivity and of the thermoluminescence of diamond seems to deserve a separate description and will not be given in the present paper. EXPERIMENTAL Experimental arrangements were similar to those described previously. (13,15)In short, monochromatic light was obtained using a Beckman DU monochrom~or with a Hanovia type 507C, 8OOW Xenon compact arc-lamp. Crystals were mounted in the holder of a metal vacuum cryostat.(15) An E.M.I. 62.5613 photomultiplier having a quartz envelope served as a detector. A nitrobenzene cell O-5 cm thick was put between the crystal and the detector to absorb the U.V. exciting light. This set-up enabled us to obtain luminescence excitation spectra with spectral slit widths of less than 2A throughout the region of 2100-42OOA.. This corresponds to less than 0.006 eV at 2100 A and less than 0.0015 eV at 4200 A. Amplification, recording devices and temperature me~urements were as reported pre~ously.(l5~ Some of the luminescence excitation spectra were taken point by point, keeping the exciting light intensity constant by changing the monochromator slit widths, which were calibrated for this purpose. Later it became evident that details of the fine structure in the excitation spectra could be obtained with higher accuracy by continuous records keeping the monochromator slits constant for comparatively narrow spectral regions. Monochromator slits were changed in such measurements on passing from one spectral region to another in order to secure a proper luminescence intensity in each region (see Fig. 2). The wavelengths of the maxima in the sharp peaks in the excitation spectrum were not affected by the comparatively slow change with wavelength of the exciting light intensity. *Cant should he taken in such measurements to assure that the light source shows a smooth variation of intensity with wavelength. Using an old Xe lamp the excitation spectrum in the U.V. near 2OOOA exhibited a line structure, probably due to W vapors in the light source. These were eliminated on replacing the light source by a new one.
EXCITATION
SPECTRA
AND TEMPERATURE
In the phatoconductivi~ measurements, contacts to the crystal were made with heat resisting silver paint. Currents were measured with a Keithley model 410 micro-microammeter, and recorded on a 20 mV Speedomax recorder. A Cary model 31 vibrating reed electrometer has been used in some of the measurements. The diamonds used were unpolished, and some of them had rough surfaces. All of them were colorless. The distance between the silver paint electrodes varied between 2 and 6 mm according to the size of the specimen. Each diamond was cleaned by immersion in several solvents and in nitric acid. Finally it was washed in distilled water and dried before mounting in the vacuum cryostat.
DEPENDENCE
OF LUMINESCENCE
347
RESULTS (a) LunGzescence excitation spectra near the edge (200-245 rnp) Luminescence excitation spectra at 80”K, as obtained by the point by point method, are given in Fig. 1. Measurements were taken at intervals of only 5 A. The experimental points are not shown in the plotted curves to avoid the indistinctness the many points would necessarily cause. Curves (a) and (c) were obtained with type I diamonds (specimens 1‘3 and Tl respectively), and curve (b) gives the excitation spectrum for an intermediate type diamond (T5). The latter had a comparatively strong band structure in absorption near the edge (22524Omp). Its luminescence can be seen
FIG. 1. Luminescenceexcitationspectranear the absorptionedge at 80°K. Curves (a) and (c) for type I diamonds(T3 and Tl respectively), curve (b) for an intermediate diamond (T.5).
348
J.
I
1
0 210
NAHUM
215
and
A.
I 220
HALPERIN
I 225
I 230
I
235
240
FIG. 2. Luminescence excitation spectrum of T5 taken in each interval with a constant monochromator slit width. Slits were 0.28, 0.17 and 0.1 mm respectively for the three sections in the Figure from left to right.
[Fig. l(b)] to be comparatively strong, and on excitation with wavelengths of 225-240 rnp it is even stronger than that of the type I diamonds [curves (a) and (c)l. Other intermediate diamonds, which showed a weaker absorption system in this region, luminesced much weaker. Type IIa dismonds seem to provide the limiting case of diamonds with experimentally undetectably weak absorption systems near the edge, which explains the very weak luminescence of diamonds of this type.(lz 5,~) Of interest is that one of our examined type I diamonds (4al) luminesced very weakly. It seems that in spite of its strong absorption up to about 300 n-q its system of bands near the edge was very weak, and hence its weak luminescence. The luminescence excitation spectrum as obtained by a continuous record with fixed monochromator slits is given (for T5) in Fig. 2. Slits were kept at 0*1,0*17 and O-28 mm for the regions 234-242, 227-234 and 209-227 respectively. Curves (a), (c) and (e) were taken with the crystal at 80”K, and (b), (d) and (f) at room temperature.
The bands become broadened, shift slightly towards longer wavelengths and their intensity is reduced on warming. At 420°K (not shown in Fig. 2), the whole structure almost disappears. It is convenient to separate the bands into groups at 240,236,230 rnp and the groups of band with shorter wavelengths. The 230 and 236 rnp bands seem to keep the same relative intensities in different specimens (see Fig. 1). This conclusion has already been reached by MALE. On the other hand, the relative intensities of the 240 rnp bands change from one crystal to another, and similarly the intensities of the bands near the edge seem not to keep any relation to that of the other bands. Moreover, the relative intensities of the bands near the absorption edge themselves also change from crystal to crystal, and even in the same crystal they depend on the way of illumination of the diamond. This is shown in Fig. 3. Here curve (a) shows the luminescence excitation spectrum of T3 (type I) at 215-227 q on excitation in a given position, and curve (b) gives the same
EXCITATION
SPECTRA
AND
TEMPERATURE
after turning the diamond so that the opposite face was exposed to the exciting beam. The main maximum appears to shift from about 223 to about 225 mp. It seems to be caused by inhomogeneities within the crystal. Such inhomogeneities may be expected to affect the excitation spectrum near the
I I I t---I I I
I I
I I I
?
\
DEPENDENCE
OF LUMINESCENCE
349
the measuring beam at the rough diamond surfaces made it necessary to use widely open slits in the spectrophotometer (Beckman DK). Better resolved absorption spectra were taken by BAI(~) and RAMANATHAN(*). Table 1 gives the wavelengths of the lines in absorption and those obtained from the luminescence excitation spectra in the present work. The wavelengths in absorption fit well those in the luminescence measurements. It should be noted that the wavelengths in our measurements were taken from the calibrated recorder charts and their accuracy is estimated to be within about 1 A. The temperature dependence of the 236 rnp bands in the luminescence excitation spectrum is given in Fig. 5, where curves (a), (b), (c) and (d) were taken at 80, 200, 295 and 375°K respectively. The energy shift in the peaks was found to be nearly -4 x 10-s eV/deg. The same shift was found in other bands in the system which were isolated enough from the neighboring bands to allow accurate measurements. Thus the shift in the band at 24Omt~ was found to amount to -3.6 x 10-s eV/deg. These values fit well the data on the shifts in the absorption.@? s) (b) The emission spectrum
220
;
5
mP FIG. 3. Effect of changing the illuminated side of the diamond (T3) on its luminescence excitation near the absorption edge.
edge, where the depth of penetration of the exciting light changes rapidly with wavelength. That diamonds may differ in their absorption near the edge is shown in Fig. 4, in which curves A and A’ give the absorption at 80°K of diamonds T4 and H3 respectively. Although both the diamonds are of the intermediate type, their absorption near the edge is not the same. T4 has an absorption peak at about 225 rnp but cuts off below 224 rnp, while H3 does not show the band at 225 rnt~ but cuts off very near to 225 ml_c. To compare the absorption of intermediate diamonds to the luminescence excitation spectrum, the latter is given again in Figure 4, curve L. It is obvious that the peaks in L fit those in A and A’. The fine structure, however, does not appear in the absorption curves, for the high scattering of
The emission spectrum of diamond has been investigated recently in detail by DYER and MATTHEWS, and earlier by MANI( Both found that all luminescing diamonds emitted the blue system which consists of a very sharp band at 415 rnp and a set of broader bands up to nearly 500 q. Some diamonds, mainly of the intermediate type, showed also thegreen system, starting with a sharp band at 503 rnt~ and spreading towards longer wavelengths. In addition, more sharp bands were observed occasionally in some diamonds. The emission spectrum of T5 is shown in Fig. 6. Curves (a), (b) and (c) give the spectra at 80, 295 and 430°K respectively. The 415.2 rnp band has at 80”K, a half-width of only 5 A (less than 0.004 eV). Two other equally sharp bands appear at 490.8 and 496 mp. It is of interest that the width of the sharp lines in type I diamonds was generally larger, probably because of self absorption by the stronger absorption bands at the same wavelengths. It is well known [see, for example, Ref. (ll)] that the blue luminescing diamonds have an absorption band which coincides in wavelength
350
NAHUM
J.
T&e
and
A.
HALPERIN
1. Comparism of wavelengths of reported absorption maxima to those in the luminescence excitation near the edge as obtained in the present work Absorption BAI(S)
Luminescence
(A)
Wavelength
(A)
s.
2388 2359.9 2356.5
2314 2309 2306 2300 2296
V.S. V.S.
2360 2355.9
s.
2309 s. 2306 2301-2295
s.
excitation at 80°K
Present work
~MANATHAN(*)
Wavelength 2405 2399 2396 2395
at 93°K
::::
s.
2294
Wavelength 2405.5 2400 2396 2393 2392 2387 2374 2362 2357 2351 2350 2335 2327 2321 2316 2310 2306 2300 2296 2294 2284 2280
(A)
s.
V.S. V.S.
V.S.
s.
Energy (eV) 5*153 5.165 5.173 5.180 5.182 5.193 5.221 5.248 5.258 5.270 5.272 5.308 5.327 5 a340 5.352 5.366 5.375 5.389 5.398 5.403 5 -427 5.436
2271.4 2267 2260 2252.8
5.468
2254 22.50
V.S. V.S.
5.499 5 *SO9
2242 2231 2219 2209 2203 2198 2165 2110
S. S.
5.528 5.556 5 *586 5.611 5.626 5.639 5.727 5.877
2246 5
8. -
strong
with the 415 q emission band and a set of absorption bands towards shorter wavelengths exhibiting a mirror image of the blue emission system with symmetry (in frequencies) about the 415 mp band. A similar absorption system is related to the green emission system with symmetry about the 503 mp band, which again appears both in emission and absorption. In earlier work, the blue luminescence was usually excited by light
V.S.
-
8.
9.
very strong
within the 415 q absorption system (mostly 3660 d mercury light), and the green emission was found to respond to light absorbed in the 503 mt~ absorption system. It was, therefore, of interest to examine the excitation near the absorption edge separately for the blue and green emission. This was done in the present work, but no differences in the excitation spectra were observed; both the blue and green emission gave just the same
EXCITATION
SPECTRA
AND TEMPERATURE
DEPENDENCE
OF LUMINESCENCE
351
I-
/
),_
I
220
i
I
230
240
i
FIG. 4. Comparison
between the absorption near the edge of intermediate diamonds (curves A and A’ for diamonds T4 and H3, respectively), and the luminescence excitation spectrum (curve L).
structure in their excitation spectra near the edge (Figs. 1 and 2). (c) Luminescence excitation near the edge compared with that in the 415 nyl system Luminescence excitation spectra at 80°K covering the region 230-420 nq.~are shown in Fig. 7. The curves were recorded continuously and replotted with the proper correction for the changes in the intensity* of the exciting light with wavelength. The diamonds were the same as those used in Fig. 1. Again curves (a) and (c) belong to T3 and Tl respectively {both type I) and curve (b) was obtained with T5 (intermediate). T5, which showed the strongest luminescence on excitation near the edge, gives the weakest excitation bands in the 415 nq system. This fits its weak, barely detectable, 415 nq~ system in absorp* No correction was made for the change in photon energy. Thus the number of photons per second used at 230 rnp was nearly half of that used at 420 mp.
tion. The type I diamonds, on the other hand, showed a distinct 415 ~J.Labsorption system. This is shown for Tl in curve A of Fig. 8, while curve L in the same figure gives the luminescence excitation spectrum of the same crystal for comparison. The two curves fit well in the region of the 415 nq~ absorption system, but deviate from each other at shorter wavelengths where the tail of the strong nitrogen absorption starts.
(d) Photoconductivity and luminescence excitation spectra Figure 9 shows the excitation spectrum near the edge for diamond T5. Curve P’ was obtained at 80”K, and curve P at room temperature. P is given on a reduced ordinate scale (by a factor of 2). The luminescence excitation spectrum at 80°K is given for comparison (curve L). There are a few interesting featuresin these curves : (a) on excitation with X < 240 w, the photocurrent rises with rising temperature, in contrast with the decrease
352
J.
1 234
NAHUM
and A. HALPERIN
I 236
238 w
FIG. 5. The effect of temperature on the excitation bands at 236 rnp. (a) 80, (b) 200, (c) 295 and (d) 375°K. in luminescence as described above ; (b) the main maximum in photoconductivity appears at 218 rnp (not shown in the Figure) at 80”K, and moves to 223 rnp at room temperature (curve P); (c) at wavelengths at which the luminescence (curve L) reaches maxima (corresponding to maxima in absorption), the photoconductivity at 80°K (curve P’) shows distinct inverse peaks (minima) ; (d) at room temperature the conductivity peaks appear again as maxima. It follows that warming the crystal from 80 to 300°K changes the peaks in the photoconductivity from inversed peaks to maxima corresponding to the absorption maxima. To follow the change with temperature in the luminescence and the photoconductivity, the dependence of both on temperature was recorded continuously using various wavelengths for excitation.
The results are shown in Figs. 10 and 11 respectively for the luminescence and photoconductivity. In Fig. 10, the curves (a), (b), (c), (d), (e) and (f) were obtained on excitation with 218, 223, 235.5, 2.50, 404 and 415 rryl. respectively. It appears that on excitation with wavelengths between 220-240 rnp there is a strong drop in the luminescence on warming, while on using 250 and 404 rnp the luminescence remains nearly constant up to above 500”K, where temperature quenching seems to start. The drop in curve (f), which was taken just at the peak of the 415 rnp band, should be attributed to broadening of the sharp band. This broadening causes an increase in the luminescence on excitation near 415 rnp as can be seen in curve (e) of Fig. 10. Wavelengths between 250 and 400 rnp also gave curves similar to (d) and (e). This can be seen from Fig. 12 which shows the luminescence excitation spectra (360-420 rnp) at the temperatures 80, 300 and 550°K [curves (a), (b) and (c) respectively]. It is obvious that except at the vicinity of the 415 v band the luminescence is nearly independent of temperature up to 550°K. The curves in Figs. 10 and 11 were obtained with T3 (type I), but intermediate type diamonds also gave similar results. Returning now to Fig. 11, we see that the photoconductivity behaves in a different way. Here curves (a), (b), (c), (d) and (e) were recorded on excitation with the same wavelengths as the corresponding curves in Fig. 10. We get an increase in the photoconductivity with temperature on excitation with wavelengths up to 240 rnp, and independence of temperature up to 400°K for longer wavelengths. This behavior will be further discussed below. (e) Thermduminescezce
excitation near the edge
Thermoluminescence excitation spectra were obtained with the same slit widths as used in the luminescence measurements in order to secure the high resolution needed to reveal the fine structure. The details of the thermoluminescence of diamonds will be described elsewhere, and only one curve will be given here to help in establishing a model which will account for the experimental results described above. Fig. 13 describes the excitation spectrum for the thermoluminescence peak at 150°K of the diamond T5. The points in the curve were obtained in separate excitations for
EXCITATION
SPECTRA
AND
TEMPERATURE
DEPENDENCE
OF LUMINESCENCE
-
e t f
50
-
25
-
.E 3
\ 2: -
FIG. 6. Luminescence spectrum of T5. {a) 80, (b) 300 and (c) 430°K.
25
325
350
375
FIG. 7. Excitation spectra as in Fig. 1 for the range 230-420 mp.
400
4 .
353
NAHUM
354
J. and A. HALPERIN
90
01
1
300
I
325
I
I
350
I 375
I
I
I
400
v FIG. 8. Absorption(A)
and luminescenceexcitation (L) of Tl.
each wavelength, each time with the same light intensity and the same excitation time (10 min). The main maximum appears now at about 226 q. Of special interest, however, are the minima, or inversed maxima, which appear again just at the wavelengths of the main maxima in the luminescence excitation spectra (at 230,236 and 240 q). DISCUSSION The fine structure revealed in the luminescence response curves near the absorption edge was found to fit the structure in absorption. While this structure is present in practically all type I diamonds, it can be observed directly in this type of diamond only in very thin specimens (N 100 rnp thick) because of the strong and broad nitrogen absorption bands in the same region. Intermediate diamonds also possess the fine structure near the edge (by definition) and, because of the absence of the nitrogen bands, direct absorption measurements are possible. Even so, measurements cannot be carried out below 225 q, where all diamonds start to absorb strongly. It is of advantage in
luminescence measurements that they are not subjected to all these limitations. Fine band structure near the edge has been observed in other crystals.(lv-1s) In CdS, for example, some of the observed lines have been foundus) to be associated with excitons. Other lines, however, were found to be too weak and their intensity varied from one specimen to another. Accordingly, they were assumed to be due to excitons in the field of an imperfection (impurity excitons). It seems that the fine structure near the absorption edge of diamond is also due to impurity excitons. This fits the hypothesis made by CIQMPION et aZ.(20) according to which exciton states in the field of defects are responsible for the luminescence in diamond. It is of interest that one of the arguments for this hypothesis was the behavior of the scintillation response of diamonds, which was found to fall in magnitude when the magnitude of the conduction pulses increased. This is in accord with our results showing opposite behavior of the luminescence and the photoconductivity.
EXCITATION
SPECTRA
AND TEMPERATURE
D~PRND~NCE
OF LUMINESCENCE
355
A schematic energy level mode1 might help in &rifying the meaning of the results. We shall confine ourselves to only one system of luminescence bands, e.g. the 415 q system. The mirror image between the emission and absorption bands, and the lack of a Frank-Condon shift between the 415 m,~ line in emission and in absorption, suggest that electronic transitions within a localized center not strongly coupled to the lattice vibrations are involved. In semiconducting diamonds, the conductivity is known to be due to free holes in the valence band [see Ref. f13)f. Mobilities of electrons and positive holes in diamond are, however, nearly equal to L each other,@s) and it is not possible, therefore, to decide from our experiments what the sign of the carriers of the photocurrent is. Just for the sake of drawing, we have chosen a model (Fig. 14) with electrons as free carriers. We assume several defect levels to exist above the valence band which in our drawing are represented by D. Excitation near the absorption edge raises the electrons to the excited state D’. The latter should be located below but near enough to \ the conduction band to account for the ionization which becomes notable at about 150°K (see Figs, 10, 11). At 8O”.K, thermal ionization can be neglected and an electron excited to D’ can return to the ground state (transition D’ -+ 0) or fall into B’--the excited state of the system 3 trepresented \. p again by only one line in the diagram)-and emit blue luminescence by the transition 23’ --f B, This 23c should involve a previous transition 23 -+ D to empty the center B. FIG. 9. Excitation spectra near the edge for TS. Curve Excitation with wavelengths fitting the fine P’ photoconductivity at 80°K; P, the same at 300°K; and structure bands near the edge (tr~sitions D -+I))) L, luminescence at 80°K. should, at low temperatures, contribute very little to the photocurrent because of the very fast reWhatever the exact nature of the centers and the laxation by the emission of luminescence. On associated impurities be, our results seem to supabsorption of light which does not fit exactly any port the “exciton hypothesis”. of the transitions D + D',it is more likely to get Photoconducti~~ exciton spectra showing inan indirect transition to the conduction band by verted peaks which fit absorption maxima were phonon interaction. This explains the inverted observed also in other crystals, e.g. in organic peaks in the excitation of photoconductivity at casts and in CusCYa~). In CusO, the effect low temperatures. At higher tem~ratures, ionizawas temperature independent in contrast with the tion of excited electrons enters, and the chance for strong temperature dependence in our case. This an electron at D'to luminescence by the transition indicates that thermal dissociation is of importance B’ + B is then reduced. This fits the increase in in this case. photoeondu~ti~~ and decrease in luminescence r-
I
J. NAHUM
and A. HALPERLN
FIG. IO. Temperature dependence of the hminescence (of T3) on excitation with variow wavelengths: (a) 218, (b)223, (c)235*5, (d) 250, (e) 404 and (f) 415 m+
(a
L /. ,.,,,_,.._. p< ..I..
/
,._........... I....... (e)
.I’ _...-..._._.-.;-i-__;
1
400
FXG. 11. As Fig. 10 but for the photoconductivity. Excitation with the s~o~~~~~ as in curves (x)-(e)
.
.
FIG. 12. The luminescence excitation in the 415 mjb absorption system. (a) 80, (h) 300 and (c) 550% (with T3).
EXCITATION
SPECTRA
AND TEMPERATURE
DEPENDENCE
OF LUMINESCENCE
357
h
0
FIG. 13. The thermoluminescenceexcitation spectrumfor the glowpeak at 150°K of T5.
on warming the crystal. Under these conditions, we may expect the photoconductivity to increase with the absorption, and the inverted peaks at low temperatures turn therefore to real maxima at high temperatures (Fig. 9, curve P). The inverted peaks in the thermoluminescence excitation spectra are explained similarly. Excitation at low temperatures within the fine structure near the edge is not likely to cause ionization of the electron as expIained above. Less electrons are, therefore, available in the conduction band to be trapped in the trapping levels T (Fig. 14), and hence the inverted peaks in the thermoluminescence (Fig. 13). If this is the case we should obtain, for thermoluminescence appearing above room temperature, inverted peaks in excitation spectra measured after excitation at 80°K and direct maxima in those measured after excitation
at room temperature. This, as well as other experiments with thermoluminescence of diamond, is now under investigation in our laboratory. In our model, each center has both D and B levels. The excitation spectrum in the 415 w should, therefore, be related in intensity to that near the edge. Experimentally, however, type I diamonds gave relatively much stronger excitation in the 415 nq~ system compared to the intermediate diamonds (Fig. 7). Nevertheless, the two systems are closely related. RAAL@) found that the integrated density of the 230 and 236 rnp bands was proportional to that of the 415 q system. The apparent independence of the two systems in the excitation of the luminescence should be attributed to the difference in the strength of the broad nitrogen bands in the different specimens. In type I, incident light of wavelengths near the edge is
J. NAHUM
358
and A. HALPERIN MATTHEWScontained
Conduction
A
____------____ A D’
>,
bond
--_t
1
-I
wavelengths below 250 rq.~ One has to attribute it, therefore, to differences in conditions of illumination; namely, to the much higher excitation intensities used by DYER and MATTHEWScompared to those obtained with the highly monochromatic light in our experiments.
B
REFERENCES
0’
D
Volence
bond
FIG. 14. Schematic energy model.
absorbed mainly by the strong nitrogen bands, which do not contribute to the luminescence. One therefore, expect the luminescence should, efficiency of a type I diamond to decrease with an increase in its intensity of nitrogen absorption which explains RAM&S observation(5) stated in the Introduction. Another point which warrants a short discussion is the following: DYER and h&rTHEws(ll) used filtered 366 q mercury light for excitation and found that the luminescence intensity increased by 34 per cent for all type I and 170 per cent for all type IIa diamonds on cooling from room temperature to 90°K. This disagrees with our results in which practically no dependence on temperature was found between 80 and 500°K on excitation with wavelengths within the 415 r@ absorption system (see Figs. 10 and 12). The origin of this discrepancy is not clear. It seems hard to believe that the filtered light used by DYEX and
1. ROBERWIN R., Fox J. J. and MARTIN A. E., Trans. Roy. Sot. 263, 463 (1934). 2. SUT~ G. B. B. M., BLACICWELLD. E. and SIMEXALW. G., Nature, Lond. 174.901 (1954). 3. CLARK C. D., DITCHBURN R. W. and D&R H.. B., Proc. Rov. Sot. A234. 363 (1956). 4. CUSTERS J.-F. H., Physi& ‘s &av.‘18, 489 (1952). 5. RAMANC. V., Proc. Indian Acad. Sci. A19, 199 (1944). 6. BAI K. S., Proc. Indian Acud. Sci. Al9,253 (1944). 7. KAISJJRW. and BONDW. L., Phys. Rew. 115, 857 (1959). 8. RAMANATHANK. G., Proc. Indian Acad. Sci. A24, 137 (1946). RML F. A., Proc. Phys. Sot. Lond. 74, 647 (1959). 10”: HALPERINA. and AREIELLH., Phys. Rew. 113, 1216 (1959). I. G., Proc. Roy. Sot. Il. DYER H. B. and MATTHEWS AZ43, 320 (1958). MALE J. C., Proc. Phys. Sot. Lond. 77, 869 (1961). ::: HALPERIN A! and NAHUM J., J. Phys. Chem. Solids 18, 297 (1961). 14. PANTD. D., Proc. Indian Acad. Sci. Al9,325 (1944). 15. BRANERA. A. and HALPERIN A., Phys. Rev. 108, 932 (1957). 16. MANI A., Proc. Indian Acud. Sci., A19, 231 (1944). 17. GROSSE. F.. NuowoCim. CRUDDY.)3. 672 (1956). 18. TI-IOMA~ D. k., J. Phys. C?zem~-SoE&15,8‘6 (19kO). 19. THOMASD. G. and HOPFIELDJ. J., Phys. Rev. 116, 573 (1959). 20. CHAMPION F. C., DEANP. J. and LI~~TOWLZRS E., Proceedings of the I&rnational Cot&~ence on Color Centers and Crvstal Luminescence. Torino (1960). p. 234. 21. KOMANDELIR J., paper presented at the International Conference on Photoconductivitv, Cornell Uniw&y (1961). To he published. -. 22. APWL J. H. and PORTISA. M., J. Phys. Chem. Solids 15, 34 (1960). 23. Moss T. S., Optical Properties of Semiconductors, p. 107 Butterworth, London (1959).
Pergamon
EXCITATION
Press 1962. Vol. 23, pp. 345-358.
Printed in Great Britain.
SPECTRA AND
DEPENDENCE
TEMPERATURE
OF LUMINESCENCE
PHOTOCONDUCTIVITY
AND
OF DIAMOND
J. NAHUM* and A. HALPEIUNt Department
of Physics, The Hebrew University of Jerusalem,
Jerusalem,
Israel
(Receiwed 8 September 1961)
Abstract-Luminescence excitation spectra of type I and intermediate diamonds were measured at 80°K and at room temperature, and were found to show a fine band structure near the absorption The photoconductivity excitation spectrum at 80°K showed inverted peaks edge (210-240 q). (minima) at the wavelengths of the maxima in the luminescence excitation. At room temperature,
the inverted peaks turned into direct peaks (maxima) in the excitation spectra of the photoconductivity. Inverted peaks were also observed in the thermoluminescence excitation spectra at low temperatures. On excitation with wavelengths of 220-240 rnp the photoconductivity was found to rise, and the luminescence to decrease on warming the crystals. On excitation with longer wavelengths, both were found to be temperature independent up to considerably above room temperature. The results are discussed and a model to account for the results is proposed.
IN’I’RODUCTION
THE luminescence of diamond has been investigated by many authors. ROBERTSONet al.(l) were the first to examine extensively the physical properties of diamond. They have classified diamonds into two types differing from each other with regard to practically all their structure sensitive properties. One of the characteristic properties of type I diamonds was the absorption in the U.V. up to about 3000 A, while type II transmitted down to about 2250 A. More recently this classification was found insufficient.@4) Type II diamonds were then further divided(s) into: type IIa showing only a weak continuous absorption tail from the edge at about 2250 A towards longer wavelengths, type IIb distinguished by its semiconducting properties,@) and intermediate diamonds(s) showing only a comparatively weak band structure in the region 2250-2400 A. *Detached from the Israel Atomic Energy Commission. t Present address : The Mellon Institute, Pittsburgh 13, Pa. 345
The relation between this classification and the luminescence still remained obscure. ROBERTSON et al.(l) found that type II diamonds did not luminesce, while type I luminesced strongly, mainly irr the blue. RAM&~) and BAI@) claimed that although pure type II diamonds luminesced very weakly, the luminescence intensity of type I diamonds generally decreased with increasing absorption in the range 2200-3000 A. BAI@) suggested that the luminescence depended on the strength of the band structure at 2250-2400 A. This system of bands is difficult to measure in most diamonds (type I) because of the much stronger absorption in the region 220&3000& This absorption was found(7) to be due to nitrogen impurities present in type I diamonds, and seems to contribute slightly, if at all, to the background luminescence but not to the main luminescence appearing with a band structure in the blue and green regions. In intermediate diamonds, the nitrogen absorption is absent and the band structure near the edge can be measured in transmission, and using very thin specimens this absorption could also be detected in type I diamonds.(W)
346
J. NAHUM
and
If the luminescence depends on the bands at 2250-240011, these bands should appear in the excitation spectrum for the luminescence. That the excitation spectrum indeed reveals a band structure in regions of high absorption, and even within the lattice absorption edge, has been shown pre~ously(lO~ for ZnS. This was done in the present work for diamonds and the band structure revealed was found to fit well the one found in direct measurements. These measurements, besides proving the relation between the band structure and the luminescence systems in diamond, are of advantage compared with the direct absorption measurements also because of the better resolution obtainable in the luminescence measurements. A detailed investigation on the luminescence of diamond was carried out by DYER and ~A~HEWS(~~). These authors, however, used in most of their measurements the 3660 A mercury lines for excitation, and their excitation spectra measurements were confined only to wavelengths higher than 3000 A. Very recently MAKE reported on the luminescence excitation spectra of diamond near the absorption edge. The rne~ur~en~ were, however, confined to room temperature, and so the fine structure in the bands was not fully revealed. The present work, which was carried out independent of ,Ws contains measurements at different temperatures down to liquid air temperature. In addition to the better resolution of the fine structure these measurements also provided valuable information on the temperature dependence of the effects. Type IIb (semiconducting dismonds) did not show any fine structure in the region 2250-2400 A at 80°K. Some properties of type IIb diamonds were reported already in an earlier publication,(ls) and this type was therefore omitted in the present paper. The present paper deals mainly with the luminescence. It was, however, interesting to compare the excitation spectra of the luminescence with those of the photoconductivity and the thermoluminescence. PANT states that “there is a clear inverse correlation between photoconductivPANT based his statement ity and luminescence”. on comparison of the intensities of luminescence
A.
HALPERIN
and phot~ondu~ivi~ in different specimens. It will be shown in the present work that the above statement has a much broader aspect. A detailed description of the photoconductivity and of the thermoluminescence of diamond seems to deserve a separate description and will not be given in the present paper. EXPERIMENTAL Experimental arrangements were similar to those described previously. (13,15)In short, monochromatic light was obtained using a Beckman DU monochrom~or with a Hanovia type 507C, 8OOW Xenon compact arc-lamp. Crystals were mounted in the holder of a metal vacuum cryostat.(15) An E.M.I. 62.5613 photomultiplier having a quartz envelope served as a detector. A nitrobenzene cell O-5 cm thick was put between the crystal and the detector to absorb the U.V. exciting light. This set-up enabled us to obtain luminescence excitation spectra with spectral slit widths of less than 2A throughout the region of 2100-42OOA.. This corresponds to less than 0.006 eV at 2100 A and less than 0.0015 eV at 4200 A. Amplification, recording devices and temperature me~urements were as reported pre~ously.(l5~ Some of the luminescence excitation spectra were taken point by point, keeping the exciting light intensity constant by changing the monochromator slit widths, which were calibrated for this purpose. Later it became evident that details of the fine structure in the excitation spectra could be obtained with higher accuracy by continuous records keeping the monochromator slits constant for comparatively narrow spectral regions. Monochromator slits were changed in such measurements on passing from one spectral region to another in order to secure a proper luminescence intensity in each region (see Fig. 2). The wavelengths of the maxima in the sharp peaks in the excitation spectrum were not affected by the comparatively slow change with wavelength of the exciting light intensity. *Cant should he taken in such measurements to assure that the light source shows a smooth variation of intensity with wavelength. Using an old Xe lamp the excitation spectrum in the U.V. near 2OOOA exhibited a line structure, probably due to W vapors in the light source. These were eliminated on replacing the light source by a new one.
EXCITATION
SPECTRA
AND TEMPERATURE
In the phatoconductivi~ measurements, contacts to the crystal were made with heat resisting silver paint. Currents were measured with a Keithley model 410 micro-microammeter, and recorded on a 20 mV Speedomax recorder. A Cary model 31 vibrating reed electrometer has been used in some of the measurements. The diamonds used were unpolished, and some of them had rough surfaces. All of them were colorless. The distance between the silver paint electrodes varied between 2 and 6 mm according to the size of the specimen. Each diamond was cleaned by immersion in several solvents and in nitric acid. Finally it was washed in distilled water and dried before mounting in the vacuum cryostat.
DEPENDENCE
OF LUMINESCENCE
347
RESULTS (a) LunGzescence excitation spectra near the edge (200-245 rnp) Luminescence excitation spectra at 80”K, as obtained by the point by point method, are given in Fig. 1. Measurements were taken at intervals of only 5 A. The experimental points are not shown in the plotted curves to avoid the indistinctness the many points would necessarily cause. Curves (a) and (c) were obtained with type I diamonds (specimens 1‘3 and Tl respectively), and curve (b) gives the excitation spectrum for an intermediate type diamond (T5). The latter had a comparatively strong band structure in absorption near the edge (22524Omp). Its luminescence can be seen
FIG. 1. Luminescenceexcitationspectranear the absorptionedge at 80°K. Curves (a) and (c) for type I diamonds(T3 and Tl respectively), curve (b) for an intermediate diamond (T.5).
348
J.
I
1
0 210
NAHUM
215
and
A.
I 220
HALPERIN
I 225
I 230
I
235
240
FIG. 2. Luminescence excitation spectrum of T5 taken in each interval with a constant monochromator slit width. Slits were 0.28, 0.17 and 0.1 mm respectively for the three sections in the Figure from left to right.
[Fig. l(b)] to be comparatively strong, and on excitation with wavelengths of 225-240 rnp it is even stronger than that of the type I diamonds [curves (a) and (c)l. Other intermediate diamonds, which showed a weaker absorption system in this region, luminesced much weaker. Type IIa dismonds seem to provide the limiting case of diamonds with experimentally undetectably weak absorption systems near the edge, which explains the very weak luminescence of diamonds of this type.(lz 5,~) Of interest is that one of our examined type I diamonds (4al) luminesced very weakly. It seems that in spite of its strong absorption up to about 300 n-q its system of bands near the edge was very weak, and hence its weak luminescence. The luminescence excitation spectrum as obtained by a continuous record with fixed monochromator slits is given (for T5) in Fig. 2. Slits were kept at 0*1,0*17 and O-28 mm for the regions 234-242, 227-234 and 209-227 respectively. Curves (a), (c) and (e) were taken with the crystal at 80”K, and (b), (d) and (f) at room temperature.
The bands become broadened, shift slightly towards longer wavelengths and their intensity is reduced on warming. At 420°K (not shown in Fig. 2), the whole structure almost disappears. It is convenient to separate the bands into groups at 240,236,230 rnp and the groups of band with shorter wavelengths. The 230 and 236 rnp bands seem to keep the same relative intensities in different specimens (see Fig. 1). This conclusion has already been reached by MALE. On the other hand, the relative intensities of the 240 rnp bands change from one crystal to another, and similarly the intensities of the bands near the edge seem not to keep any relation to that of the other bands. Moreover, the relative intensities of the bands near the absorption edge themselves also change from crystal to crystal, and even in the same crystal they depend on the way of illumination of the diamond. This is shown in Fig. 3. Here curve (a) shows the luminescence excitation spectrum of T3 (type I) at 215-227 q on excitation in a given position, and curve (b) gives the same
EXCITATION
SPECTRA
AND
TEMPERATURE
after turning the diamond so that the opposite face was exposed to the exciting beam. The main maximum appears to shift from about 223 to about 225 mp. It seems to be caused by inhomogeneities within the crystal. Such inhomogeneities may be expected to affect the excitation spectrum near the
I I I t---I I I
I I
I I I
?
\
DEPENDENCE
OF LUMINESCENCE
349
the measuring beam at the rough diamond surfaces made it necessary to use widely open slits in the spectrophotometer (Beckman DK). Better resolved absorption spectra were taken by BAI(~) and RAMANATHAN(*). Table 1 gives the wavelengths of the lines in absorption and those obtained from the luminescence excitation spectra in the present work. The wavelengths in absorption fit well those in the luminescence measurements. It should be noted that the wavelengths in our measurements were taken from the calibrated recorder charts and their accuracy is estimated to be within about 1 A. The temperature dependence of the 236 rnp bands in the luminescence excitation spectrum is given in Fig. 5, where curves (a), (b), (c) and (d) were taken at 80, 200, 295 and 375°K respectively. The energy shift in the peaks was found to be nearly -4 x 10-s eV/deg. The same shift was found in other bands in the system which were isolated enough from the neighboring bands to allow accurate measurements. Thus the shift in the band at 24Omt~ was found to amount to -3.6 x 10-s eV/deg. These values fit well the data on the shifts in the absorption.@? s) (b) The emission spectrum
220
;
5
mP FIG. 3. Effect of changing the illuminated side of the diamond (T3) on its luminescence excitation near the absorption edge.
edge, where the depth of penetration of the exciting light changes rapidly with wavelength. That diamonds may differ in their absorption near the edge is shown in Fig. 4, in which curves A and A’ give the absorption at 80°K of diamonds T4 and H3 respectively. Although both the diamonds are of the intermediate type, their absorption near the edge is not the same. T4 has an absorption peak at about 225 rnp but cuts off below 224 rnp, while H3 does not show the band at 225 rnt~ but cuts off very near to 225 ml_c. To compare the absorption of intermediate diamonds to the luminescence excitation spectrum, the latter is given again in Figure 4, curve L. It is obvious that the peaks in L fit those in A and A’. The fine structure, however, does not appear in the absorption curves, for the high scattering of
The emission spectrum of diamond has been investigated recently in detail by DYER and MATTHEWS, and earlier by MANI( Both found that all luminescing diamonds emitted the blue system which consists of a very sharp band at 415 rnp and a set of broader bands up to nearly 500 q. Some diamonds, mainly of the intermediate type, showed also thegreen system, starting with a sharp band at 503 rnt~ and spreading towards longer wavelengths. In addition, more sharp bands were observed occasionally in some diamonds. The emission spectrum of T5 is shown in Fig. 6. Curves (a), (b) and (c) give the spectra at 80, 295 and 430°K respectively. The 415.2 rnp band has at 80”K, a half-width of only 5 A (less than 0.004 eV). Two other equally sharp bands appear at 490.8 and 496 mp. It is of interest that the width of the sharp lines in type I diamonds was generally larger, probably because of self absorption by the stronger absorption bands at the same wavelengths. It is well known [see, for example, Ref. (ll)] that the blue luminescing diamonds have an absorption band which coincides in wavelength
350
NAHUM
J.
T&e
and
A.
HALPERIN
1. Comparism of wavelengths of reported absorption maxima to those in the luminescence excitation near the edge as obtained in the present work Absorption BAI(S)
Luminescence
(A)
Wavelength
(A)
s.
2388 2359.9 2356.5
2314 2309 2306 2300 2296
V.S. V.S.
2360 2355.9
s.
2309 s. 2306 2301-2295
s.
excitation at 80°K
Present work
~MANATHAN(*)
Wavelength 2405 2399 2396 2395
at 93°K
::::
s.
2294
Wavelength 2405.5 2400 2396 2393 2392 2387 2374 2362 2357 2351 2350 2335 2327 2321 2316 2310 2306 2300 2296 2294 2284 2280
(A)
s.
V.S. V.S.
V.S.
s.
Energy (eV) 5*153 5.165 5.173 5.180 5.182 5.193 5.221 5.248 5.258 5.270 5.272 5.308 5.327 5 a340 5.352 5.366 5.375 5.389 5.398 5.403 5 -427 5.436
2271.4 2267 2260 2252.8
5.468
2254 22.50
V.S. V.S.
5.499 5 *SO9
2242 2231 2219 2209 2203 2198 2165 2110
S. S.
5.528 5.556 5 *586 5.611 5.626 5.639 5.727 5.877
2246 5
8. -
strong
with the 415 q emission band and a set of absorption bands towards shorter wavelengths exhibiting a mirror image of the blue emission system with symmetry (in frequencies) about the 415 mp band. A similar absorption system is related to the green emission system with symmetry about the 503 mp band, which again appears both in emission and absorption. In earlier work, the blue luminescence was usually excited by light
V.S.
-
8.
9.
very strong
within the 415 q absorption system (mostly 3660 d mercury light), and the green emission was found to respond to light absorbed in the 503 mt~ absorption system. It was, therefore, of interest to examine the excitation near the absorption edge separately for the blue and green emission. This was done in the present work, but no differences in the excitation spectra were observed; both the blue and green emission gave just the same
EXCITATION
SPECTRA
AND TEMPERATURE
DEPENDENCE
OF LUMINESCENCE
351
I-
/
),_
I
220
i
I
230
240
i
FIG. 4. Comparison
between the absorption near the edge of intermediate diamonds (curves A and A’ for diamonds T4 and H3, respectively), and the luminescence excitation spectrum (curve L).
structure in their excitation spectra near the edge (Figs. 1 and 2). (c) Luminescence excitation near the edge compared with that in the 415 nyl system Luminescence excitation spectra at 80°K covering the region 230-420 nq.~are shown in Fig. 7. The curves were recorded continuously and replotted with the proper correction for the changes in the intensity* of the exciting light with wavelength. The diamonds were the same as those used in Fig. 1. Again curves (a) and (c) belong to T3 and Tl respectively {both type I) and curve (b) was obtained with T5 (intermediate). T5, which showed the strongest luminescence on excitation near the edge, gives the weakest excitation bands in the 415 nq system. This fits its weak, barely detectable, 415 nq~ system in absorp* No correction was made for the change in photon energy. Thus the number of photons per second used at 230 rnp was nearly half of that used at 420 mp.
tion. The type I diamonds, on the other hand, showed a distinct 415 ~J.Labsorption system. This is shown for Tl in curve A of Fig. 8, while curve L in the same figure gives the luminescence excitation spectrum of the same crystal for comparison. The two curves fit well in the region of the 415 nq~ absorption system, but deviate from each other at shorter wavelengths where the tail of the strong nitrogen absorption starts.
(d) Photoconductivity and luminescence excitation spectra Figure 9 shows the excitation spectrum near the edge for diamond T5. Curve P’ was obtained at 80”K, and curve P at room temperature. P is given on a reduced ordinate scale (by a factor of 2). The luminescence excitation spectrum at 80°K is given for comparison (curve L). There are a few interesting featuresin these curves : (a) on excitation with X < 240 w, the photocurrent rises with rising temperature, in contrast with the decrease
352
J.
1 234
NAHUM
and A. HALPERIN
I 236
238 w
FIG. 5. The effect of temperature on the excitation bands at 236 rnp. (a) 80, (b) 200, (c) 295 and (d) 375°K. in luminescence as described above ; (b) the main maximum in photoconductivity appears at 218 rnp (not shown in the Figure) at 80”K, and moves to 223 rnp at room temperature (curve P); (c) at wavelengths at which the luminescence (curve L) reaches maxima (corresponding to maxima in absorption), the photoconductivity at 80°K (curve P’) shows distinct inverse peaks (minima) ; (d) at room temperature the conductivity peaks appear again as maxima. It follows that warming the crystal from 80 to 300°K changes the peaks in the photoconductivity from inversed peaks to maxima corresponding to the absorption maxima. To follow the change with temperature in the luminescence and the photoconductivity, the dependence of both on temperature was recorded continuously using various wavelengths for excitation.
The results are shown in Figs. 10 and 11 respectively for the luminescence and photoconductivity. In Fig. 10, the curves (a), (b), (c), (d), (e) and (f) were obtained on excitation with 218, 223, 235.5, 2.50, 404 and 415 rryl. respectively. It appears that on excitation with wavelengths between 220-240 rnp there is a strong drop in the luminescence on warming, while on using 250 and 404 rnp the luminescence remains nearly constant up to above 500”K, where temperature quenching seems to start. The drop in curve (f), which was taken just at the peak of the 415 rnp band, should be attributed to broadening of the sharp band. This broadening causes an increase in the luminescence on excitation near 415 rnp as can be seen in curve (e) of Fig. 10. Wavelengths between 250 and 400 rnp also gave curves similar to (d) and (e). This can be seen from Fig. 12 which shows the luminescence excitation spectra (360-420 rnp) at the temperatures 80, 300 and 550°K [curves (a), (b) and (c) respectively]. It is obvious that except at the vicinity of the 415 v band the luminescence is nearly independent of temperature up to 550°K. The curves in Figs. 10 and 11 were obtained with T3 (type I), but intermediate type diamonds also gave similar results. Returning now to Fig. 11, we see that the photoconductivity behaves in a different way. Here curves (a), (b), (c), (d) and (e) were recorded on excitation with the same wavelengths as the corresponding curves in Fig. 10. We get an increase in the photoconductivity with temperature on excitation with wavelengths up to 240 rnp, and independence of temperature up to 400°K for longer wavelengths. This behavior will be further discussed below. (e) Thermduminescezce
excitation near the edge
Thermoluminescence excitation spectra were obtained with the same slit widths as used in the luminescence measurements in order to secure the high resolution needed to reveal the fine structure. The details of the thermoluminescence of diamonds will be described elsewhere, and only one curve will be given here to help in establishing a model which will account for the experimental results described above. Fig. 13 describes the excitation spectrum for the thermoluminescence peak at 150°K of the diamond T5. The points in the curve were obtained in separate excitations for
EXCITATION
SPECTRA
AND
TEMPERATURE
DEPENDENCE
OF LUMINESCENCE
-
e t f
50
-
25
-
.E 3
\ 2: -
FIG. 6. Luminescence spectrum of T5. {a) 80, (b) 300 and (c) 430°K.
25
325
350
375
FIG. 7. Excitation spectra as in Fig. 1 for the range 230-420 mp.
400
4 .
353
NAHUM
354
J. and A. HALPERIN
90
01
1
300
I
325
I
I
350
I 375
I
I
I
400
v FIG. 8. Absorption(A)
and luminescenceexcitation (L) of Tl.
each wavelength, each time with the same light intensity and the same excitation time (10 min). The main maximum appears now at about 226 q. Of special interest, however, are the minima, or inversed maxima, which appear again just at the wavelengths of the main maxima in the luminescence excitation spectra (at 230,236 and 240 q). DISCUSSION The fine structure revealed in the luminescence response curves near the absorption edge was found to fit the structure in absorption. While this structure is present in practically all type I diamonds, it can be observed directly in this type of diamond only in very thin specimens (N 100 rnp thick) because of the strong and broad nitrogen absorption bands in the same region. Intermediate diamonds also possess the fine structure near the edge (by definition) and, because of the absence of the nitrogen bands, direct absorption measurements are possible. Even so, measurements cannot be carried out below 225 q, where all diamonds start to absorb strongly. It is of advantage in
luminescence measurements that they are not subjected to all these limitations. Fine band structure near the edge has been observed in other crystals.(lv-1s) In CdS, for example, some of the observed lines have been foundus) to be associated with excitons. Other lines, however, were found to be too weak and their intensity varied from one specimen to another. Accordingly, they were assumed to be due to excitons in the field of an imperfection (impurity excitons). It seems that the fine structure near the absorption edge of diamond is also due to impurity excitons. This fits the hypothesis made by CIQMPION et aZ.(20) according to which exciton states in the field of defects are responsible for the luminescence in diamond. It is of interest that one of the arguments for this hypothesis was the behavior of the scintillation response of diamonds, which was found to fall in magnitude when the magnitude of the conduction pulses increased. This is in accord with our results showing opposite behavior of the luminescence and the photoconductivity.
EXCITATION
SPECTRA
AND TEMPERATURE
D~PRND~NCE
OF LUMINESCENCE
355
A schematic energy level mode1 might help in &rifying the meaning of the results. We shall confine ourselves to only one system of luminescence bands, e.g. the 415 q system. The mirror image between the emission and absorption bands, and the lack of a Frank-Condon shift between the 415 m,~ line in emission and in absorption, suggest that electronic transitions within a localized center not strongly coupled to the lattice vibrations are involved. In semiconducting diamonds, the conductivity is known to be due to free holes in the valence band [see Ref. f13)f. Mobilities of electrons and positive holes in diamond are, however, nearly equal to L each other,@s) and it is not possible, therefore, to decide from our experiments what the sign of the carriers of the photocurrent is. Just for the sake of drawing, we have chosen a model (Fig. 14) with electrons as free carriers. We assume several defect levels to exist above the valence band which in our drawing are represented by D. Excitation near the absorption edge raises the electrons to the excited state D’. The latter should be located below but near enough to \ the conduction band to account for the ionization which becomes notable at about 150°K (see Figs, 10, 11). At 8O”.K, thermal ionization can be neglected and an electron excited to D’ can return to the ground state (transition D’ -+ 0) or fall into B’--the excited state of the system 3 trepresented \. p again by only one line in the diagram)-and emit blue luminescence by the transition 23’ --f B, This 23c should involve a previous transition 23 -+ D to empty the center B. FIG. 9. Excitation spectra near the edge for TS. Curve Excitation with wavelengths fitting the fine P’ photoconductivity at 80°K; P, the same at 300°K; and structure bands near the edge (tr~sitions D -+I))) L, luminescence at 80°K. should, at low temperatures, contribute very little to the photocurrent because of the very fast reWhatever the exact nature of the centers and the laxation by the emission of luminescence. On associated impurities be, our results seem to supabsorption of light which does not fit exactly any port the “exciton hypothesis”. of the transitions D + D',it is more likely to get Photoconducti~~ exciton spectra showing inan indirect transition to the conduction band by verted peaks which fit absorption maxima were phonon interaction. This explains the inverted observed also in other crystals, e.g. in organic peaks in the excitation of photoconductivity at casts and in CusCYa~). In CusO, the effect low temperatures. At higher tem~ratures, ionizawas temperature independent in contrast with the tion of excited electrons enters, and the chance for strong temperature dependence in our case. This an electron at D'to luminescence by the transition indicates that thermal dissociation is of importance B’ + B is then reduced. This fits the increase in in this case. photoeondu~ti~~ and decrease in luminescence r-
I
J. NAHUM
and A. HALPERLN
FIG. IO. Temperature dependence of the hminescence (of T3) on excitation with variow wavelengths: (a) 218, (b)223, (c)235*5, (d) 250, (e) 404 and (f) 415 m+
(a
L /. ,.,,,_,.._. p< ..I..
/
,._........... I....... (e)
.I’ _...-..._._.-.;-i-__;
1
400
FXG. 11. As Fig. 10 but for the photoconductivity. Excitation with the s~o~~~~~ as in curves (x)-(e)
.
.
FIG. 12. The luminescence excitation in the 415 mjb absorption system. (a) 80, (h) 300 and (c) 550% (with T3).
EXCITATION
SPECTRA
AND TEMPERATURE
DEPENDENCE
OF LUMINESCENCE
357
h
0
FIG. 13. The thermoluminescenceexcitation spectrumfor the glowpeak at 150°K of T5.
on warming the crystal. Under these conditions, we may expect the photoconductivity to increase with the absorption, and the inverted peaks at low temperatures turn therefore to real maxima at high temperatures (Fig. 9, curve P). The inverted peaks in the thermoluminescence excitation spectra are explained similarly. Excitation at low temperatures within the fine structure near the edge is not likely to cause ionization of the electron as expIained above. Less electrons are, therefore, available in the conduction band to be trapped in the trapping levels T (Fig. 14), and hence the inverted peaks in the thermoluminescence (Fig. 13). If this is the case we should obtain, for thermoluminescence appearing above room temperature, inverted peaks in excitation spectra measured after excitation at 80°K and direct maxima in those measured after excitation
at room temperature. This, as well as other experiments with thermoluminescence of diamond, is now under investigation in our laboratory. In our model, each center has both D and B levels. The excitation spectrum in the 415 w should, therefore, be related in intensity to that near the edge. Experimentally, however, type I diamonds gave relatively much stronger excitation in the 415 nq~ system compared to the intermediate diamonds (Fig. 7). Nevertheless, the two systems are closely related. RAAL@) found that the integrated density of the 230 and 236 rnp bands was proportional to that of the 415 q system. The apparent independence of the two systems in the excitation of the luminescence should be attributed to the difference in the strength of the broad nitrogen bands in the different specimens. In type I, incident light of wavelengths near the edge is
J. NAHUM
358
and A. HALPERIN MATTHEWScontained
Conduction
A
____------____ A D’
>,
bond
--_t
1
-I
wavelengths below 250 rq.~ One has to attribute it, therefore, to differences in conditions of illumination; namely, to the much higher excitation intensities used by DYER and MATTHEWScompared to those obtained with the highly monochromatic light in our experiments.
B
REFERENCES
0’
D
Volence
bond
FIG. 14. Schematic energy model.
absorbed mainly by the strong nitrogen bands, which do not contribute to the luminescence. One therefore, expect the luminescence should, efficiency of a type I diamond to decrease with an increase in its intensity of nitrogen absorption which explains RAM&S observation(5) stated in the Introduction. Another point which warrants a short discussion is the following: DYER and h&rTHEws(ll) used filtered 366 q mercury light for excitation and found that the luminescence intensity increased by 34 per cent for all type I and 170 per cent for all type IIa diamonds on cooling from room temperature to 90°K. This disagrees with our results in which practically no dependence on temperature was found between 80 and 500°K on excitation with wavelengths within the 415 r@ absorption system (see Figs. 10 and 12). The origin of this discrepancy is not clear. It seems hard to believe that the filtered light used by DYEX and
1. ROBERWIN R., Fox J. J. and MARTIN A. E., Trans. Roy. Sot. 263, 463 (1934). 2. SUT~ G. B. B. M., BLACICWELLD. E. and SIMEXALW. G., Nature, Lond. 174.901 (1954). 3. CLARK C. D., DITCHBURN R. W. and D&R H.. B., Proc. Rov. Sot. A234. 363 (1956). 4. CUSTERS J.-F. H., Physi& ‘s &av.‘18, 489 (1952). 5. RAMANC. V., Proc. Indian Acad. Sci. A19, 199 (1944). 6. BAI K. S., Proc. Indian Acud. Sci. Al9,253 (1944). 7. KAISJJRW. and BONDW. L., Phys. Rew. 115, 857 (1959). 8. RAMANATHANK. G., Proc. Indian Acad. Sci. A24, 137 (1946). RML F. A., Proc. Phys. Sot. Lond. 74, 647 (1959). 10”: HALPERINA. and AREIELLH., Phys. Rew. 113, 1216 (1959). I. G., Proc. Roy. Sot. Il. DYER H. B. and MATTHEWS AZ43, 320 (1958). MALE J. C., Proc. Phys. Sot. Lond. 77, 869 (1961). ::: HALPERIN A! and NAHUM J., J. Phys. Chem. Solids 18, 297 (1961). 14. PANTD. D., Proc. Indian Acad. Sci. Al9,325 (1944). 15. BRANERA. A. and HALPERIN A., Phys. Rev. 108, 932 (1957). 16. MANI A., Proc. Indian Acud. Sci., A19, 231 (1944). 17. GROSSE. F.. NuowoCim. CRUDDY.)3. 672 (1956). 18. TI-IOMA~ D. k., J. Phys. C?zem~-SoE&15,8‘6 (19kO). 19. THOMASD. G. and HOPFIELDJ. J., Phys. Rev. 116, 573 (1959). 20. CHAMPION F. C., DEANP. J. and LI~~TOWLZRS E., Proceedings of the I&rnational Cot&~ence on Color Centers and Crvstal Luminescence. Torino (1960). p. 234. 21. KOMANDELIR J., paper presented at the International Conference on Photoconductivitv, Cornell Uniw&y (1961). To he published. -. 22. APWL J. H. and PORTISA. M., J. Phys. Chem. Solids 15, 34 (1960). 23. Moss T. S., Optical Properties of Semiconductors, p. 107 Butterworth, London (1959).