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Irradiation effects on the low-temperature thermoluminescence (TSL) of quartz crystals
.I. Phys. Ckm. Sdids Vol. 49, No. 5. pp. 511-583, Printed in Great Britain.
0022.3697/t% 13.00 + 0.00 Q 1988 Pergamon Press pie
1988
IRRADIATION EFFECTS ON THE LOO-TEMPERATURE THERMOLUMINESCENCE OF QUARTZ CRYSTALS
(TSL)
A. HALPERIN and S. KATZ Raeah Institute of Physics, Hebrew University, Jerusalem 91904, Israel (Received 30 Jdy
1987; accepted 24 September 1987)
Abstract-The TSL of synthetic quartz below room temperature {RI) was examined for n- and z-growth zone samples. Preirradiation at 320 K by 104, 5 MeV electron pulses reduced very much the TSL peaks in the range 115-230 K and a peak at 70 K. Other TSL peaks below 110 K did not change much, while peaks at 252 and 265 K were enhanced by the electron preirradiation. Room temperature X-preirradiation caused most TSL peaks first to increase, and they started to decrease only after several hours of RT irradiation. New TSL peaks at 136, 161 and 181 K appeared after comparatively short RT irradiations. These peaks and a peak at 172 K could also be excited by cycles of X-irradiation at 20 K followed by annealing at 360 K. On preirradiation at temperatures above 2.50K the peaks in .x-samples increased while those of the z-samples decreased. The restoration by annealing at various temperatures up to above 800 K of the TSL of samples preirradiated at RT is also described. Almost all TSL peaks in the temperature range 115-230 K were observed to emit at 3.26 eV (380 nm) and their emission involved the relesase of electrons into the conduction band and recombination at one luminescence center. We propose that most of these TSL peaks are emitted when electrons are released from defects of the type [X,/Li+]“, in which X, stands for various unspecified defects in the quartz lattice. Our mode1 seems to account well for the observed effects. Keyworc&: Thermoluminescence,
defects, irradiation, quartz.
INTRODUCTION
The phosphorescence and the~olumines~n~ (TSL) of quartz crystals at low temperatures, namely below room temperature (RT), was described in a recent communication from this laboratory [ 11. The report described the low-temperature phosphorescence and TSL of synthetic quartz crystals cut from pure zgrowth and pure x-growth zones. The TSL was shown to differ widely for samples cut from the two growth zones. The previous communication, however, was limited to the TSL excited by irradiation at very low temperatures, mostly in the range of 10-20 K. Further work showed that preirradiation at tem~ratures near or above RT had a strong effect on the TSL below RT; some TSL peaks were enhanced or “sensitized” by the preirradiation and others decreased in intensity, or were “desensitized”. Preliminary results of such measurements carried out in our laboratory have been reported previously [2], and reports from other laboratories on sensitization and desensitization of the low-tem~rature TSL in quartz also appear in literature [3]. The process involved in these effects remained, however, unclear. The measurements in the present work included effects of the dose of the RT preirradiation, and of pre-warming and annealing. The sensitization or desensitization was also examined as a function of the tem~rature of preirradiation covering the range 20-360 K. Several TSL peaks, namely those at 136, 161, 172 and 181 K, barely observable in the virgin crystal, were found to
be enhanced very strongly by pretreatment and became do~nant in the glow curve. In the present work we shall concentrate mainly on these TSL peaks.
EXPERIMENTAL The samples were the same as used in the earlier work [l], and most of the ex~rimental arrangement were described there. This includes the arrangements for X-irradiation with the sample in the cryostat. In this arrangement the samples received a dose of about 300 rad s-l. In some of the experiments in the present work the samples were irradiated by high-energy electrons from a V-7715B Varian Linear Accelerator. The accelerator was operated at 5 MeV, 200 mA in 1.5~s pulses at a rate of 10 pulses s-‘. The samples were pressed in these irradiations to an aluminum plate in order to help to dissipate the heat generated by the electron beam. Still there was some warming, and the actual temperature of the samples during the electron irradiation was estimated to be about 50°C. After the electron irradiation, the samples were cleaned and mounted in an Air Pro&act Displex close cycle double-stage refrigerator. The measurements were carried out as described previously [l]. The rate of heating in the TSL measurements was kept at 10” mini. TSL emission spectra were scanned in about 10 s covering the range 300-700 nm. The heating rate in these measurements was 2.5” min-‘. 577
A. HALPERINand S. KATZ
578
to those in Fig. l(A). The effect on the 76, 83, 92 and 109 K peaks is again small. Other peaks in the glow curve, including the one at 70 K, specific to z-growth zone samples [ 11, disappeared almost completely after the electron irradiation, except for the weak peak at 252 K, which remained almost unchanged.
(b) Eflects of prolonged RT X-irradiation
100
0
300
200
Fig. 1. Glow curves of an x-sample (A) and a z-sample (B) obtained after Smin of X-irradiation at 20K. Curves X,, Z,-nearly virgin; X,, Z,-after exposure of the crystal to lo“, 5 MeV electron pulses at about 320 K.
RESULTS
Effects electrons
(a)
of
RT
irradiation
by
energetic
Figure l(A) illustrates the effect of lo4 electron pulses (5 MeV, 200 mA, 1.5 ps) on the glow curve of an x-growth zone sample (Sample X2). Curve X, in the figure gives the glow curve after annealing the crystal for 15 min at 530°C which eliminated most effects of previous irradiations and brought it back to nearly virgin conditions. For excitation, the sample was X-irradiated for Smin at about 20K when the glow curve was recorded during warming. The curve shows the peaks described previously [l], namely weak peaks at 37 and about 55 K, stronger ones at 76, 83 and 92 K, and a complex peak close to 115 K. At higher temperatures there appear the peaks at 145, 192, 221 and 252K with additional weak peaks appearing as shoulders to the main ones. Curve X, in Fig. l(A) gives the glow curve excited under the same conditions as for Curve X,, but after exposing the sample to lo4 electron pulses. The TSL peaks below 110 K have not changed much, while those in the range 115-230 K disappeared, leaving only comparatively weak peaks at 136, 161 and 181 K. The peak near 252 K and the shoulder at 265 K were enhanced by the electron irradiation. Figure l(B) gives for comparison the glow curves of a z-growth zone sample (Sample 22) before (Curve Z,) and after lo4 electron pulses (Curve Z,). Both curves were obtained under conditions similar
The effect of prolonged (up to 32 h) X-irradiations at RT on the TSL of Sample X2 is shown in Fig. 2(A). For each point in this figure the crystal had accumulated the corresponding total RT irradiation time given on the abscissa and was subjected to a 5-min X-irradiation at about 20 K, when the TSL was recorded during warming. The procedure for the production of the 190 K peak [4] was different. This peak is produced only by a double irradiation [l, 5,6]. In our case the crystal was warmed up to 220 K after the first 5 min irradiation, then retooled to 20 K and subjected to another 5 min X-irradiation, when the 190K peak appeared on warming. Big differences in the behavior of the different TSL peaks can be seen in Fig. 2(A). The 76 K peak is the least affected and the 83 and 92 K peaks [not shown in Fig. 2(A)] behave similarly. The overlapping 109 and 115 K peaks and even the one at 221 K decrease only moderately. More drastic is the decrease in the intensity of the 190 K peak. It came down by more than an order of magnitude by just 1 h of RT irradiation, and continued to decrease after longer RT irradiations. The 136, 161 and 181 K peaks are affected in a completely different way. They first rise sharply with irradiation time, reaching a maximum after a few hours of RT irradiation. Then they decrease but still do not quite reach the low intensities obtained by lo4 electron pulses (Fig. 1, Curve X,). In contrast with other TSL peaks, those at 252 and 265 K rise continuously with time of RT irradiation. The inset in Fig. 2(A) illustrates the glow curve obtained after 7.5 h of RT X-irradiation. The 181 K peak now dominates, with the 161 K peak appearing as a strong shoulder, and the 136 K peak as a very weak shoulder to the main peak. All other glow peaks can hardly be seen on the ordinate scale fitting the very strong 181 K peak. Figure 2(B) is similar to 2(A) but is for Sample 22. In this case the 70K peak shows the most drastic change; it practically disappears after less than 1 h of RT irradiation. The 190 K peak, now weaker compared with that obtained for Sample X2 [Fig. 2(A)], decreases in a way similar to that shown in Fig. 2(A). The 76 K (as well as the 83 and 92 K peaks) again changes very little. Other peaks are seen to change only moderately with RT irradiation. The insert in Fig. 2(B) shows the glow curve obtained after 7 h of RT irradiation [note the difference in ordinate scale compared with the inset in Fig. 2(A)]. The reduction in TSL intensities by
Irradiation effects on the low-temperature
thermoluminescence
(TSL)
579
B Sample
22
Ii 0
10 Total
RT Irradiation
20 Time
30 (hours)
0-
5 Total RT Irradiation
10 Time
15 (hours)
Fig. 2. Effects of prolonged X-irradiations at RT on various TSL peaks of an x-sample (A) and a z-sample (B) excited at 20 K. The insets show the glow curves recorded after about 7 h of RT irradiation in each case.. prolonged RT X-irradiation is again qualitatively the same as that of lo4 pulses of energetic electrons. (c) Eflects of cycles of annealing followed by X-irradiation at 20 K
at 360 K
The behavior of the 136, 161 and 181 K peaks [Fig. 2(A)] looks strange. They first rise with RT irradiation and then decrease for longer irradiation times. This effect has therefore been further examined, when it was observed that the intensities of these peaks can rise without RT irradiation, as shown in Fig. 3. For the curves in this figure the crystal was subjected to subsequent cycles of annealing for 5 min at 360 K followed by 5 min of X-irradiation at about 20 K, when the glow curve was obtained on warming. The effect of such cycles on the glow peaks is shown in the figure starting from the “virgin” crystal (annealed for 30min at 53O”C), namely Cycle 0. The 181 K peak is seen to rise by a factor of 60 after just two cycles, when it reaches its maximum. The 161 K peak becomes observable only after the second cycle. It rises first steeply and then moderately with further cycles. The 136 K peak was not observable in these measurements as it was covered by the stronger peak at 145 K, which also increased, though not much, with the cycling. A TSL peak at 172 K, barely observable in the RT irradiation experiments, was enhanced by about two orders of magnitude by just one cycle when it took over the 181 K in intensity. The inset in Fig. 3 shows the glow curve for the nearly virgin
crystal (Curve A), and that obtained after the second cycle (Curve B). It should be noted that the maxima reached for the 181 K peak (as well as for the I36
‘05-i 10' 0
1
2
3
4 Cycle
5
6
7
8
9
10
Number
Fig. 3. Formation of the 161, 172 and 181 K TSL peaks by cycles of 20 K X-irradiation (5 min) followed by annealing for 5 min at 360 K. The inset shows the glow curve obtained for the nearly virgin crystal (A) and that after the second cycle (B).
580
A. HALPERIN and S. KATZ 1044
I
I
I
‘2;oY
I L 300 ’
400
I
c
B
Sample
I/
Sample
X2 Y is I
206
22
K
I lo’ A+ d ;ob?do
’ Temperature
Temperature
(K)
(K)
Fig. 4. Effect of preirradiation at intermediate temperatures in the range 1 l&360 K (20 min for each point) on the TSL excited at 20 K (5 min) for an x-sample (A) and a z-sample (B).
450
500
550
600
650
Temperature Fig.
700 (KI
750
800
450
500
550
600
650
Temperature
700
750
800
(KI
5. Restoration of the TSL in samples exposed to 104, 5 MeV electron pulses by annealing temperatures up to 820 K, (A) for an x-sample, and (B) tbr a r-sample.
at
581
Irradiation effects on the low-temperature thermoluminescence (TSL) and 161 K peaks) is lower by more than one and a half orders of magnitude compared with those obtained after RT irradiation (compare ordinates at the maxima in Figs 2 and 3). (d) Effects of the preirradiation temperature on the TSL Figure 4(A) illustrates the effect of the preirradiation temperature on the TSL of Sample X2, covering the temperature range 110-360 K. The points at 20 K give the intensities for the nearlyvirgin crystal. Each of the other points was obtained after 20 min of X-irradiation at the temperature given on the abscissa, followed by annealing for 5 min at 360 K which eliminated the formation of the 190 K peak. The crystal was then cooled down and X-irradiated for 5 min at about 20 K when the TSL was recorded on warming. The dashed lines connecting the 20 K points to the solid curves give mainly the enhancement by the low-temperature irradiation plus 360 K annealing cycles (Fig. 3). The 172 K peak is seen in Fig. 4(A) to remain unaffected up to 180K, when it decreases sharply with irradiation at higher temperatures. This explains its absence in glow curves obtained after RT irradiations (Figs 1 and 2). The 161 K peak rises in a way similar to that in Fig. 3 up to 240 K. Irradiation at higher temperatures starts a second stage of increase. The same is true for the 181 K peak, but this time the increase above 240 K is much steeper, and it rises by two orders of magnitude between 240 and 340 K. The 136 K peak was not observable up to 220 K. Above this temperature its behavior was similar to that of the 161 and 181 K peaks. The intensity of the phosphorescence at 20 K is also shown in Fig. 4(A). It remains practically constant up to 240 K and rises at higher temperatures in a way qualitatively similar to that of the 136, 161 and 181 K peaks. Figure 4(B) is similar to 4(A) but is for Sample 22. The enhancement of the TSL by the low-temperature irradiation and 360 K annealing is given by the dashed lines connecting the points at 20 and 110 K. Up to 180 K the TSL peaks at 70, 115, 145 and 206 K remain almost unchanged and they decrease at higher temperatures. The 161 and 181 K peaks are not observable in the z-growth sample up to irradiation temperatures near 200K, when they are seen to rise. In contrast with the behavior of these peaks in the x-growth zone sample [Fig. 4(A)] the 161 and 181 K peaks in the z-growth sample do not rise but decrease in intensity at temperatures above 260 K. (e) Restoration of the TSL by annealing at high temperatures To get more information on the processes involved in the RT irradiations and in the restoration by annealing we carried out pulsed thermal anneals of the heavily irradiated samples. Figure S(A) illustrates
the results obtained with the X2 sample after its exposure to 104, 5 MeV electron pulses. The abscissa in the figure gives the temperature reached in each step of the annealing and the ordinate gives the TSL intensities obtained by 5 min of X-irradiation at about 20 K. The 190K peak (excited by two Xirradiations at 20 K and warming to 230 K between the two irradiations) is seen to start rising at about 480K and its restoration is completed at about 570 K. The 192 and 221 K peaks both appear at 480 K, rise steeply up to 620 K, and continue with a slower rise in the range 700-820 K. The 145 K peak becomes observable below 700 K and rises continuously with temperature up to about 820 K. The 252 K, which was enhanced by the electron irradiation (Fig. l), decreases now in intensity, up to about 700K, when it rises with further increase in temperature. Figure 5(B) is similar to 5(A) but is for the 22 sample. In this case, restoration of the 190K peak takes place in the temperature range 62&700 K. The 145 and 206 K peaks rise up to 670 K, show a minimum near 720 K, and then continue to rise up to 800 K. The 252 K starts to decrease with annealing temperature near 600 K and drops in intensity to below the bottom of the ordinate in Fig. 5(B). The 70K peak appears above 650 K and rises continuously at higher temperatures. Figure 6, again for the X2 sample, shows the restoration of some TSL peaks after subjecting the crystal to 8 h of RT X-irradiation. By pulse annealing, the 190 K peak starts rising steeply near 400 K when the 181 K peak decreases steeply. The 190K peak remains constant between 425 and 105
, Sample
I
,
I
,
I
/
X2
76 KI
t’
500
600 Temperature
’
.
‘_
700 [K)
Fig. 4. As for Fig. 5(A), but after 8 h of RT X-irradiation of the x-sample.
A. HALPERINand S. KATZ
582
500 K, after which it rises again and reaches full restoration at 580K. Other peaks behave in a way similar to that shown in Fig. 5(A). Emission spectra, thermal activation energies and order of kinetics for most of the TSL peaks were given in a previous report [l]. The peaks at 136, 161, 172 and 181 K, however, were not observed before. All the four peaks were found now to emit the 380 nm (3.26 eV) band just as almost [7] all other TSL peaks in the temperature range between 115 K and RT. The kinetics involved in the emission of the 181 K peak was found to be close to second order. That for the 136, 161 and 172 K peaks was difficult to determine because of the interference of other overlapping peaks, mainly the very strong 181 K peak. Thermal activation energies for the 136, 161 and 181 K peaks were 0.36, 0.45 and 0.57eV respectively.
DISCUSSION In previous work [l], we concluded that all the glow peaks emitting at 380 nm involve thermal release of electrons from the various electron-traps and recombination at the same luminescence center. This should be valid for the additional TSL emitting at 380nm, namely the 136, 161, 172 and 181 K peaks. We note, however, that different results have been reported by other investigators. Thus Medlin [8] found the 380 nm band only in one TSL peak at 165 K, while all other peaks up to RT in his samples exhibited a complex emission between 400 and 700 nm. On the other hand, Malik et al. [9] reported that all the TSL peaks between 145 and 270 K emit at 380 nm. Let us discuss now the behavior of the 136, 161, 172 and 181 K peaks. The model for the trapping center related to the 190 K TSL peak was established clearly in an investigation combining TSL and EPR measurements [5] when it has been shown to be related to the [SiO,/Li]’ center. The emission of the 190 K peak then occurs when the electrons are released thermally from this center and recombine with holes at a luminescence center. A double irradiation procedure was found necessary for the production of the 190 K peak [l, 51. The first X-irradiation provided free holes which became trapped at [AIO,/Li]’ centers, thus producing [AlO,/Li]+ centers. The Li+ ions in the latter are only loosely bound to the centers, and above 170 K they can migrate away along the z-axis channels in the quartz lattice. During the second (low temperature) X-irradiation these Li+-ions in the channels can trap electrons, thus forming the [SiOJLi]’ centers responsible for the 190 K peak. However, when the first X-irradiation takes place above 230 K, the Li+-ions released from the [AlO,/Li]+ centers can migrate further away from the aluminum, when part of them are trapped at unspecified defect centers and are not available any more for the production of the
[SiO,/Li]’ centers. Hence there is a drop in intensity of the 190 K peak above 230 K [l, 51. Let us denote these unknown defects at which the Li-ions in the channels are trapped by X;. We propose that the 136, 161, 172 and 181 K TSL peaks are related to various [X,/Li+]’ centers, or centers in which Li+ ions close to Xi defects have trapped electrons during the second low-temperature X-irradiation. Let us examine the proposed model in light of the present results. X-Irradiation at RT [Fig. 2(A)] causes the 181 K peak (and the peaks at 136 and 161 K) to rise steeply with irradiation time when the intensity of the 190 K peak drops sharply. On varying the Xirradiation temperature, the 181 K peak starts to rise above 240 K [Fig. 4(A)] when the formation curve for the 190 K peak passes its maximum [6]. These relations between the 181 and 190 K peaks fit our model well. Additional support is obtained from the restoration of the 190 K on annealing (Fig. 6). The 190K peak shows its first stage of restoration near 425 K, just when the intensity of the 181 K peak drops sharply. The intensity rise of the 190 K peak then continues to full restoration near 550 K when other effects seem to set in. It is interesting to note that when annealing under conditions at which the 181 K peak is absent, e.g. after lo4 pulses of electron irradiation [Fig. 5(A)], the 190 K peak is restored only on reaching 550 K. Another interesting observation is that the full intensity of the 181 K peak [Figs 2(A) and 4(A)] is close to that of the 190 K peak [Fig. 5(A)] which supports our model by which the 181 K peak is created by the same Li-ions which formed the [SiO,/Li]’ centers. The 136, 161 and 181 K peaks are very much alike in their behavior. It is possible that all the three belong to the same type of defect center, possibly with the Li+ at different sites differing in binding energies for the electrons. The latter peaks as well as the 172 K peak have not been clearly observed before. Bernhardt [3] has observed TSL peaks at about 150, 175, 180 and 190 K. It is, however, difficult to compare them with those reported in the present work because Bernhardt’s observations were obtained under uncontrolled conditions, and he did not provide information on the heating rate used in his experiments. It is also not clear whether the samples used by Bernhardt were from x- or z-growth zones, or a mixture of both. The 172 K peak is formed after a cycle of Xirradiation at 20 K followed by warming to 360 K, when it appears after a second irradiation at 20 K (Fig. 3). With respect to this it behaves like the 181 K peak. The 172 K peak, however, decreases in intensity at 180 K [Fig. 4(A)] when the 190 K peak formation curve rises [6]. It seems therefore that the 172 K peak is related to a less stable defect of the type [xi/Li+]’ which under X-irradiation above 180 K loses its Li+ in favor of the defect related to the 190 K peak. The TSL of the z-samples is very much different from that of the x-samples. The 190 K peak is weaker
Irradiation effects on the low-temperature
in the z-samples by about one and a half orders of magnitude, which results from the lower concentration of the aluminum (and lithium) in this growth zone. According to the above model, the same should be true for the peaks in the range 136181 K, which is indeed the case (see Fig. 4). On X-irradiation above 250 K, all the TSL peaks in the z-sample [Fig. 4(B)] decrease in intensity, in striking contrast with their behavior in the x-samples [Fig. 4(A)]. It is possible that in the z-samples there are enough defects of another type which can bind the low concentration of lithium ions in these samples, and so very few of them remain free for the defects related to the TSL peaks which increase on irradiation above 250 K [Fig. 4(A)]. The 70 and 206 K peaks are practically absent in the x-samples. It seems that they are related with defects specific to the z-growth zone. This may also explain why the 190 K peak, observed in the zsamples, though completely identical with the one appearing in the x-samples, is restored [Fig. 5(B)] only at a higher temperature (650 K) compared with that in the x-sample (550 K). In the x-sample, the 252 and 265 K peaks grew continuously with the total RT X-irradiation time [Fig. 2(A)], and came down on annealing in the 500-700 K temperature range. This covers the restoration regions of the 190 K peak (480-570 K) and of the 192 and 221 K peaks [Figs 5(A), 61. Some relation seems to exist between the involved centers. Its nature, however, is still unclear. The restoration of the various TSL peaks takes place at the following annealing temperature ranges: about 400, 48&580, 600-700, and about 800 K. Jani et al. [lo] have observed, above RT, TSL peaks in quartz in the range close to 400 K and at 450-570 K. The latter appeared after irradiation at 273 K (or 273 + 77 K). In our experiments, some of the TSL peaks are restored by annealing just in these temperature ranges. It seems therefore that the transitions producing the TSL emission above RT are also responsible for the restoration by annealing of the TSL below RT. Bernhardt [I 1] classified the TSL of quartz above RT into four temperature regions:
thermoluminescence
(TSL)
583
3OWO0, -500, about 600 and about 800 K. These fit the temperature ranges of restoration by annealing of the TSL below RT observed in the present work. The relation between the TSL peaks in quartz above RT and those below RT was already postulated by Bernhardt [3]. However, the conditions at which the TSL above RT was produced and, of course, the samples used were different in the above referred work, which makes it difficult to draw conclusions on the relation to the TSL in the present work. The phosphorescence was observed to increase with the 190 K peak, which was attributed [l] to the release of the lithium ions from [SiO,/Li+]’ centers, and their return to the original sites at the aluminumhole centers. In the presentwork we have observed a similar effect related to the 181 K peak, namely the phosphorescence at 20 K was found to rise with the increase in intensity of the 181 K peak [Fig. 4(A)]. This gives further support to our proposed model, by which at least some of the TSL peaks observed in the present work are related to defects of the type [X,/Li+]O, which are similar in nature to the [Si04/Li+$ but with the regular Si04 replaced by a defect site in the quartz lattice.
REFERENCES 1. Katz S. and Halperin A., J. Cumin. (in press). 2. Katz S., Halperin A. and Ronen M., Proc. 37th Annual Symp. Frequency Control, p. 181 (1983). 3. Bemhardt H., Crystal Res. Technol. 19, 133 (1984). 4. The exact peak temperature depends on the conditions of excitation. It can change between above 180 and 190 K; see Ref. [S]. 5. Halperin A., Jani M. G. and Halhburton L. E., Phys. Rev. B, 34, 5702 (1986). 6. Halperin A. and Katz S., J. Lumin. 31, 32, 129 (1984). 7. On very strong excitation of the 190 K peak we have observed weak TSL (mainly_ near 260 K) emitting at about 500 nm. 8. Medlin W. L., J. them. Phys. 38, 1132 (1963). 9. Mahk D. M., Kohnke E. E. and Sibley W. A., J. appl. Phys. 52, 3600 (1981). 10. Jani M. G., Halliburton L. E. and Kohnke E. E., J. appl. Phys. 54, 6321 (1983). 11. Bemhardt H., Phys. Status Solidi 78, 61 (1983).
0022.3697/t% 13.00 + 0.00 Q 1988 Pergamon Press pie
1988
IRRADIATION EFFECTS ON THE LOO-TEMPERATURE THERMOLUMINESCENCE OF QUARTZ CRYSTALS
(TSL)
A. HALPERIN and S. KATZ Raeah Institute of Physics, Hebrew University, Jerusalem 91904, Israel (Received 30 Jdy
1987; accepted 24 September 1987)
Abstract-The TSL of synthetic quartz below room temperature {RI) was examined for n- and z-growth zone samples. Preirradiation at 320 K by 104, 5 MeV electron pulses reduced very much the TSL peaks in the range 115-230 K and a peak at 70 K. Other TSL peaks below 110 K did not change much, while peaks at 252 and 265 K were enhanced by the electron preirradiation. Room temperature X-preirradiation caused most TSL peaks first to increase, and they started to decrease only after several hours of RT irradiation. New TSL peaks at 136, 161 and 181 K appeared after comparatively short RT irradiations. These peaks and a peak at 172 K could also be excited by cycles of X-irradiation at 20 K followed by annealing at 360 K. On preirradiation at temperatures above 2.50K the peaks in .x-samples increased while those of the z-samples decreased. The restoration by annealing at various temperatures up to above 800 K of the TSL of samples preirradiated at RT is also described. Almost all TSL peaks in the temperature range 115-230 K were observed to emit at 3.26 eV (380 nm) and their emission involved the relesase of electrons into the conduction band and recombination at one luminescence center. We propose that most of these TSL peaks are emitted when electrons are released from defects of the type [X,/Li+]“, in which X, stands for various unspecified defects in the quartz lattice. Our mode1 seems to account well for the observed effects. Keyworc&: Thermoluminescence,
defects, irradiation, quartz.
INTRODUCTION
The phosphorescence and the~olumines~n~ (TSL) of quartz crystals at low temperatures, namely below room temperature (RT), was described in a recent communication from this laboratory [ 11. The report described the low-temperature phosphorescence and TSL of synthetic quartz crystals cut from pure zgrowth and pure x-growth zones. The TSL was shown to differ widely for samples cut from the two growth zones. The previous communication, however, was limited to the TSL excited by irradiation at very low temperatures, mostly in the range of 10-20 K. Further work showed that preirradiation at tem~ratures near or above RT had a strong effect on the TSL below RT; some TSL peaks were enhanced or “sensitized” by the preirradiation and others decreased in intensity, or were “desensitized”. Preliminary results of such measurements carried out in our laboratory have been reported previously [2], and reports from other laboratories on sensitization and desensitization of the low-tem~rature TSL in quartz also appear in literature [3]. The process involved in these effects remained, however, unclear. The measurements in the present work included effects of the dose of the RT preirradiation, and of pre-warming and annealing. The sensitization or desensitization was also examined as a function of the tem~rature of preirradiation covering the range 20-360 K. Several TSL peaks, namely those at 136, 161, 172 and 181 K, barely observable in the virgin crystal, were found to
be enhanced very strongly by pretreatment and became do~nant in the glow curve. In the present work we shall concentrate mainly on these TSL peaks.
EXPERIMENTAL The samples were the same as used in the earlier work [l], and most of the ex~rimental arrangement were described there. This includes the arrangements for X-irradiation with the sample in the cryostat. In this arrangement the samples received a dose of about 300 rad s-l. In some of the experiments in the present work the samples were irradiated by high-energy electrons from a V-7715B Varian Linear Accelerator. The accelerator was operated at 5 MeV, 200 mA in 1.5~s pulses at a rate of 10 pulses s-‘. The samples were pressed in these irradiations to an aluminum plate in order to help to dissipate the heat generated by the electron beam. Still there was some warming, and the actual temperature of the samples during the electron irradiation was estimated to be about 50°C. After the electron irradiation, the samples were cleaned and mounted in an Air Pro&act Displex close cycle double-stage refrigerator. The measurements were carried out as described previously [l]. The rate of heating in the TSL measurements was kept at 10” mini. TSL emission spectra were scanned in about 10 s covering the range 300-700 nm. The heating rate in these measurements was 2.5” min-‘. 577
A. HALPERINand S. KATZ
578
to those in Fig. l(A). The effect on the 76, 83, 92 and 109 K peaks is again small. Other peaks in the glow curve, including the one at 70 K, specific to z-growth zone samples [ 11, disappeared almost completely after the electron irradiation, except for the weak peak at 252 K, which remained almost unchanged.
(b) Eflects of prolonged RT X-irradiation
100
0
300
200
Fig. 1. Glow curves of an x-sample (A) and a z-sample (B) obtained after Smin of X-irradiation at 20K. Curves X,, Z,-nearly virgin; X,, Z,-after exposure of the crystal to lo“, 5 MeV electron pulses at about 320 K.
RESULTS
Effects electrons
(a)
of
RT
irradiation
by
energetic
Figure l(A) illustrates the effect of lo4 electron pulses (5 MeV, 200 mA, 1.5 ps) on the glow curve of an x-growth zone sample (Sample X2). Curve X, in the figure gives the glow curve after annealing the crystal for 15 min at 530°C which eliminated most effects of previous irradiations and brought it back to nearly virgin conditions. For excitation, the sample was X-irradiated for Smin at about 20K when the glow curve was recorded during warming. The curve shows the peaks described previously [l], namely weak peaks at 37 and about 55 K, stronger ones at 76, 83 and 92 K, and a complex peak close to 115 K. At higher temperatures there appear the peaks at 145, 192, 221 and 252K with additional weak peaks appearing as shoulders to the main ones. Curve X, in Fig. l(A) gives the glow curve excited under the same conditions as for Curve X,, but after exposing the sample to lo4 electron pulses. The TSL peaks below 110 K have not changed much, while those in the range 115-230 K disappeared, leaving only comparatively weak peaks at 136, 161 and 181 K. The peak near 252 K and the shoulder at 265 K were enhanced by the electron irradiation. Figure l(B) gives for comparison the glow curves of a z-growth zone sample (Sample 22) before (Curve Z,) and after lo4 electron pulses (Curve Z,). Both curves were obtained under conditions similar
The effect of prolonged (up to 32 h) X-irradiations at RT on the TSL of Sample X2 is shown in Fig. 2(A). For each point in this figure the crystal had accumulated the corresponding total RT irradiation time given on the abscissa and was subjected to a 5-min X-irradiation at about 20 K, when the TSL was recorded during warming. The procedure for the production of the 190 K peak [4] was different. This peak is produced only by a double irradiation [l, 5,6]. In our case the crystal was warmed up to 220 K after the first 5 min irradiation, then retooled to 20 K and subjected to another 5 min X-irradiation, when the 190K peak appeared on warming. Big differences in the behavior of the different TSL peaks can be seen in Fig. 2(A). The 76 K peak is the least affected and the 83 and 92 K peaks [not shown in Fig. 2(A)] behave similarly. The overlapping 109 and 115 K peaks and even the one at 221 K decrease only moderately. More drastic is the decrease in the intensity of the 190 K peak. It came down by more than an order of magnitude by just 1 h of RT irradiation, and continued to decrease after longer RT irradiations. The 136, 161 and 181 K peaks are affected in a completely different way. They first rise sharply with irradiation time, reaching a maximum after a few hours of RT irradiation. Then they decrease but still do not quite reach the low intensities obtained by lo4 electron pulses (Fig. 1, Curve X,). In contrast with other TSL peaks, those at 252 and 265 K rise continuously with time of RT irradiation. The inset in Fig. 2(A) illustrates the glow curve obtained after 7.5 h of RT X-irradiation. The 181 K peak now dominates, with the 161 K peak appearing as a strong shoulder, and the 136 K peak as a very weak shoulder to the main peak. All other glow peaks can hardly be seen on the ordinate scale fitting the very strong 181 K peak. Figure 2(B) is similar to 2(A) but is for Sample 22. In this case the 70K peak shows the most drastic change; it practically disappears after less than 1 h of RT irradiation. The 190 K peak, now weaker compared with that obtained for Sample X2 [Fig. 2(A)], decreases in a way similar to that shown in Fig. 2(A). The 76 K (as well as the 83 and 92 K peaks) again changes very little. Other peaks are seen to change only moderately with RT irradiation. The insert in Fig. 2(B) shows the glow curve obtained after 7 h of RT irradiation [note the difference in ordinate scale compared with the inset in Fig. 2(A)]. The reduction in TSL intensities by
Irradiation effects on the low-temperature
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Fig. 2. Effects of prolonged X-irradiations at RT on various TSL peaks of an x-sample (A) and a z-sample (B) excited at 20 K. The insets show the glow curves recorded after about 7 h of RT irradiation in each case.. prolonged RT X-irradiation is again qualitatively the same as that of lo4 pulses of energetic electrons. (c) Eflects of cycles of annealing followed by X-irradiation at 20 K
at 360 K
The behavior of the 136, 161 and 181 K peaks [Fig. 2(A)] looks strange. They first rise with RT irradiation and then decrease for longer irradiation times. This effect has therefore been further examined, when it was observed that the intensities of these peaks can rise without RT irradiation, as shown in Fig. 3. For the curves in this figure the crystal was subjected to subsequent cycles of annealing for 5 min at 360 K followed by 5 min of X-irradiation at about 20 K, when the glow curve was obtained on warming. The effect of such cycles on the glow peaks is shown in the figure starting from the “virgin” crystal (annealed for 30min at 53O”C), namely Cycle 0. The 181 K peak is seen to rise by a factor of 60 after just two cycles, when it reaches its maximum. The 161 K peak becomes observable only after the second cycle. It rises first steeply and then moderately with further cycles. The 136 K peak was not observable in these measurements as it was covered by the stronger peak at 145 K, which also increased, though not much, with the cycling. A TSL peak at 172 K, barely observable in the RT irradiation experiments, was enhanced by about two orders of magnitude by just one cycle when it took over the 181 K in intensity. The inset in Fig. 3 shows the glow curve for the nearly virgin
crystal (Curve A), and that obtained after the second cycle (Curve B). It should be noted that the maxima reached for the 181 K peak (as well as for the I36
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Fig. 3. Formation of the 161, 172 and 181 K TSL peaks by cycles of 20 K X-irradiation (5 min) followed by annealing for 5 min at 360 K. The inset shows the glow curve obtained for the nearly virgin crystal (A) and that after the second cycle (B).
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A. HALPERIN and S. KATZ 1044
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Fig. 4. Effect of preirradiation at intermediate temperatures in the range 1 l&360 K (20 min for each point) on the TSL excited at 20 K (5 min) for an x-sample (A) and a z-sample (B).
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5. Restoration of the TSL in samples exposed to 104, 5 MeV electron pulses by annealing temperatures up to 820 K, (A) for an x-sample, and (B) tbr a r-sample.
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Irradiation effects on the low-temperature thermoluminescence (TSL) and 161 K peaks) is lower by more than one and a half orders of magnitude compared with those obtained after RT irradiation (compare ordinates at the maxima in Figs 2 and 3). (d) Effects of the preirradiation temperature on the TSL Figure 4(A) illustrates the effect of the preirradiation temperature on the TSL of Sample X2, covering the temperature range 110-360 K. The points at 20 K give the intensities for the nearlyvirgin crystal. Each of the other points was obtained after 20 min of X-irradiation at the temperature given on the abscissa, followed by annealing for 5 min at 360 K which eliminated the formation of the 190 K peak. The crystal was then cooled down and X-irradiated for 5 min at about 20 K when the TSL was recorded on warming. The dashed lines connecting the 20 K points to the solid curves give mainly the enhancement by the low-temperature irradiation plus 360 K annealing cycles (Fig. 3). The 172 K peak is seen in Fig. 4(A) to remain unaffected up to 180K, when it decreases sharply with irradiation at higher temperatures. This explains its absence in glow curves obtained after RT irradiations (Figs 1 and 2). The 161 K peak rises in a way similar to that in Fig. 3 up to 240 K. Irradiation at higher temperatures starts a second stage of increase. The same is true for the 181 K peak, but this time the increase above 240 K is much steeper, and it rises by two orders of magnitude between 240 and 340 K. The 136 K peak was not observable up to 220 K. Above this temperature its behavior was similar to that of the 161 and 181 K peaks. The intensity of the phosphorescence at 20 K is also shown in Fig. 4(A). It remains practically constant up to 240 K and rises at higher temperatures in a way qualitatively similar to that of the 136, 161 and 181 K peaks. Figure 4(B) is similar to 4(A) but is for Sample 22. The enhancement of the TSL by the low-temperature irradiation and 360 K annealing is given by the dashed lines connecting the points at 20 and 110 K. Up to 180 K the TSL peaks at 70, 115, 145 and 206 K remain almost unchanged and they decrease at higher temperatures. The 161 and 181 K peaks are not observable in the z-growth sample up to irradiation temperatures near 200K, when they are seen to rise. In contrast with the behavior of these peaks in the x-growth zone sample [Fig. 4(A)] the 161 and 181 K peaks in the z-growth sample do not rise but decrease in intensity at temperatures above 260 K. (e) Restoration of the TSL by annealing at high temperatures To get more information on the processes involved in the RT irradiations and in the restoration by annealing we carried out pulsed thermal anneals of the heavily irradiated samples. Figure S(A) illustrates
the results obtained with the X2 sample after its exposure to 104, 5 MeV electron pulses. The abscissa in the figure gives the temperature reached in each step of the annealing and the ordinate gives the TSL intensities obtained by 5 min of X-irradiation at about 20 K. The 190K peak (excited by two Xirradiations at 20 K and warming to 230 K between the two irradiations) is seen to start rising at about 480K and its restoration is completed at about 570 K. The 192 and 221 K peaks both appear at 480 K, rise steeply up to 620 K, and continue with a slower rise in the range 700-820 K. The 145 K peak becomes observable below 700 K and rises continuously with temperature up to about 820 K. The 252 K, which was enhanced by the electron irradiation (Fig. l), decreases now in intensity, up to about 700K, when it rises with further increase in temperature. Figure 5(B) is similar to 5(A) but is for the 22 sample. In this case, restoration of the 190K peak takes place in the temperature range 62&700 K. The 145 and 206 K peaks rise up to 670 K, show a minimum near 720 K, and then continue to rise up to 800 K. The 252 K starts to decrease with annealing temperature near 600 K and drops in intensity to below the bottom of the ordinate in Fig. 5(B). The 70K peak appears above 650 K and rises continuously at higher temperatures. Figure 6, again for the X2 sample, shows the restoration of some TSL peaks after subjecting the crystal to 8 h of RT X-irradiation. By pulse annealing, the 190 K peak starts rising steeply near 400 K when the 181 K peak decreases steeply. The 190K peak remains constant between 425 and 105
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Fig. 4. As for Fig. 5(A), but after 8 h of RT X-irradiation of the x-sample.
A. HALPERINand S. KATZ
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500 K, after which it rises again and reaches full restoration at 580K. Other peaks behave in a way similar to that shown in Fig. 5(A). Emission spectra, thermal activation energies and order of kinetics for most of the TSL peaks were given in a previous report [l]. The peaks at 136, 161, 172 and 181 K, however, were not observed before. All the four peaks were found now to emit the 380 nm (3.26 eV) band just as almost [7] all other TSL peaks in the temperature range between 115 K and RT. The kinetics involved in the emission of the 181 K peak was found to be close to second order. That for the 136, 161 and 172 K peaks was difficult to determine because of the interference of other overlapping peaks, mainly the very strong 181 K peak. Thermal activation energies for the 136, 161 and 181 K peaks were 0.36, 0.45 and 0.57eV respectively.
DISCUSSION In previous work [l], we concluded that all the glow peaks emitting at 380 nm involve thermal release of electrons from the various electron-traps and recombination at the same luminescence center. This should be valid for the additional TSL emitting at 380nm, namely the 136, 161, 172 and 181 K peaks. We note, however, that different results have been reported by other investigators. Thus Medlin [8] found the 380 nm band only in one TSL peak at 165 K, while all other peaks up to RT in his samples exhibited a complex emission between 400 and 700 nm. On the other hand, Malik et al. [9] reported that all the TSL peaks between 145 and 270 K emit at 380 nm. Let us discuss now the behavior of the 136, 161, 172 and 181 K peaks. The model for the trapping center related to the 190 K TSL peak was established clearly in an investigation combining TSL and EPR measurements [5] when it has been shown to be related to the [SiO,/Li]’ center. The emission of the 190 K peak then occurs when the electrons are released thermally from this center and recombine with holes at a luminescence center. A double irradiation procedure was found necessary for the production of the 190 K peak [l, 51. The first X-irradiation provided free holes which became trapped at [AIO,/Li]’ centers, thus producing [AlO,/Li]+ centers. The Li+ ions in the latter are only loosely bound to the centers, and above 170 K they can migrate away along the z-axis channels in the quartz lattice. During the second (low temperature) X-irradiation these Li+-ions in the channels can trap electrons, thus forming the [SiOJLi]’ centers responsible for the 190 K peak. However, when the first X-irradiation takes place above 230 K, the Li+-ions released from the [AlO,/Li]+ centers can migrate further away from the aluminum, when part of them are trapped at unspecified defect centers and are not available any more for the production of the
[SiO,/Li]’ centers. Hence there is a drop in intensity of the 190 K peak above 230 K [l, 51. Let us denote these unknown defects at which the Li-ions in the channels are trapped by X;. We propose that the 136, 161, 172 and 181 K TSL peaks are related to various [X,/Li+]’ centers, or centers in which Li+ ions close to Xi defects have trapped electrons during the second low-temperature X-irradiation. Let us examine the proposed model in light of the present results. X-Irradiation at RT [Fig. 2(A)] causes the 181 K peak (and the peaks at 136 and 161 K) to rise steeply with irradiation time when the intensity of the 190 K peak drops sharply. On varying the Xirradiation temperature, the 181 K peak starts to rise above 240 K [Fig. 4(A)] when the formation curve for the 190 K peak passes its maximum [6]. These relations between the 181 and 190 K peaks fit our model well. Additional support is obtained from the restoration of the 190 K on annealing (Fig. 6). The 190K peak shows its first stage of restoration near 425 K, just when the intensity of the 181 K peak drops sharply. The intensity rise of the 190 K peak then continues to full restoration near 550 K when other effects seem to set in. It is interesting to note that when annealing under conditions at which the 181 K peak is absent, e.g. after lo4 pulses of electron irradiation [Fig. 5(A)], the 190 K peak is restored only on reaching 550 K. Another interesting observation is that the full intensity of the 181 K peak [Figs 2(A) and 4(A)] is close to that of the 190 K peak [Fig. 5(A)] which supports our model by which the 181 K peak is created by the same Li-ions which formed the [SiO,/Li]’ centers. The 136, 161 and 181 K peaks are very much alike in their behavior. It is possible that all the three belong to the same type of defect center, possibly with the Li+ at different sites differing in binding energies for the electrons. The latter peaks as well as the 172 K peak have not been clearly observed before. Bernhardt [3] has observed TSL peaks at about 150, 175, 180 and 190 K. It is, however, difficult to compare them with those reported in the present work because Bernhardt’s observations were obtained under uncontrolled conditions, and he did not provide information on the heating rate used in his experiments. It is also not clear whether the samples used by Bernhardt were from x- or z-growth zones, or a mixture of both. The 172 K peak is formed after a cycle of Xirradiation at 20 K followed by warming to 360 K, when it appears after a second irradiation at 20 K (Fig. 3). With respect to this it behaves like the 181 K peak. The 172 K peak, however, decreases in intensity at 180 K [Fig. 4(A)] when the 190 K peak formation curve rises [6]. It seems therefore that the 172 K peak is related to a less stable defect of the type [xi/Li+]’ which under X-irradiation above 180 K loses its Li+ in favor of the defect related to the 190 K peak. The TSL of the z-samples is very much different from that of the x-samples. The 190 K peak is weaker
Irradiation effects on the low-temperature
in the z-samples by about one and a half orders of magnitude, which results from the lower concentration of the aluminum (and lithium) in this growth zone. According to the above model, the same should be true for the peaks in the range 136181 K, which is indeed the case (see Fig. 4). On X-irradiation above 250 K, all the TSL peaks in the z-sample [Fig. 4(B)] decrease in intensity, in striking contrast with their behavior in the x-samples [Fig. 4(A)]. It is possible that in the z-samples there are enough defects of another type which can bind the low concentration of lithium ions in these samples, and so very few of them remain free for the defects related to the TSL peaks which increase on irradiation above 250 K [Fig. 4(A)]. The 70 and 206 K peaks are practically absent in the x-samples. It seems that they are related with defects specific to the z-growth zone. This may also explain why the 190 K peak, observed in the zsamples, though completely identical with the one appearing in the x-samples, is restored [Fig. 5(B)] only at a higher temperature (650 K) compared with that in the x-sample (550 K). In the x-sample, the 252 and 265 K peaks grew continuously with the total RT X-irradiation time [Fig. 2(A)], and came down on annealing in the 500-700 K temperature range. This covers the restoration regions of the 190 K peak (480-570 K) and of the 192 and 221 K peaks [Figs 5(A), 61. Some relation seems to exist between the involved centers. Its nature, however, is still unclear. The restoration of the various TSL peaks takes place at the following annealing temperature ranges: about 400, 48&580, 600-700, and about 800 K. Jani et al. [lo] have observed, above RT, TSL peaks in quartz in the range close to 400 K and at 450-570 K. The latter appeared after irradiation at 273 K (or 273 + 77 K). In our experiments, some of the TSL peaks are restored by annealing just in these temperature ranges. It seems therefore that the transitions producing the TSL emission above RT are also responsible for the restoration by annealing of the TSL below RT. Bernhardt [I 1] classified the TSL of quartz above RT into four temperature regions:
thermoluminescence
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583
3OWO0, -500, about 600 and about 800 K. These fit the temperature ranges of restoration by annealing of the TSL below RT observed in the present work. The relation between the TSL peaks in quartz above RT and those below RT was already postulated by Bernhardt [3]. However, the conditions at which the TSL above RT was produced and, of course, the samples used were different in the above referred work, which makes it difficult to draw conclusions on the relation to the TSL in the present work. The phosphorescence was observed to increase with the 190 K peak, which was attributed [l] to the release of the lithium ions from [SiO,/Li+]’ centers, and their return to the original sites at the aluminumhole centers. In the presentwork we have observed a similar effect related to the 181 K peak, namely the phosphorescence at 20 K was found to rise with the increase in intensity of the 181 K peak [Fig. 4(A)]. This gives further support to our proposed model, by which at least some of the TSL peaks observed in the present work are related to defects of the type [X,/Li+]O, which are similar in nature to the [Si04/Li+$ but with the regular Si04 replaced by a defect site in the quartz lattice.
REFERENCES 1. Katz S. and Halperin A., J. Cumin. (in press). 2. Katz S., Halperin A. and Ronen M., Proc. 37th Annual Symp. Frequency Control, p. 181 (1983). 3. Bemhardt H., Crystal Res. Technol. 19, 133 (1984). 4. The exact peak temperature depends on the conditions of excitation. It can change between above 180 and 190 K; see Ref. [S]. 5. Halperin A., Jani M. G. and Halhburton L. E., Phys. Rev. B, 34, 5702 (1986). 6. Halperin A. and Katz S., J. Lumin. 31, 32, 129 (1984). 7. On very strong excitation of the 190 K peak we have observed weak TSL (mainly_ near 260 K) emitting at about 500 nm. 8. Medlin W. L., J. them. Phys. 38, 1132 (1963). 9. Mahk D. M., Kohnke E. E. and Sibley W. A., J. appl. Phys. 52, 3600 (1981). 10. Jani M. G., Halliburton L. E. and Kohnke E. E., J. appl. Phys. 54, 6321 (1983). 11. Bemhardt H., Phys. Status Solidi 78, 61 (1983).