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Thermally Stimulated Luminescence (TSL) and Conductivity (TSC) of synthetic crystalline quartz
Radiation Measuwnettts,
23, Nos
361-365, 1954
Copyright 0 1994Elsevier Science Ltd Printed in Great Britain. All rights resewed 1350-4487/94 s7.00 + .oo
Pergamon
13504487(93)E0032-H
THERMALLY STIMULATED LUMINESCENCE (TSL) AND CONDUCTIVITY (TSC) OF SYNTHETIC CRYSTALLINE QUARTZ
Dipartimento
M. CASTIGLIONI,M. MARTINI,G. SPINOU)and A. VEDDA di Fisica, Universitzi di Milano, Via Celoria 16, I-20133 Milano, Italy
Abstract-A
comparative and simultaneous study of TSL and TSC above room temperature (20-400°C) has been performed on “as-grown” and “hydrogen-swept” synthetic quartz crystals. Following Xirradiations, TSL spectra (heating rate = l”C/s) feature a number of peaks: at 75°C an intense structure is observed (the well-known “IOO”c” peak of quartz); the analysis of this peak obtained by numerical methods has shown that it follows monomolecular kinetics, giving a value of 0.83 eV for the trap depth. Additional peaks are observed at 110°C and 160°C. followed by weaker and less resolved emissions above 200°C. TSC peaks at 8O”C, 120°C and 16WC, particularly evident in as-grown samples when measured with the electric field applied along the x-axis, can be associated to the corresponding TSL peaks. However, spectra performed with the electric field applied along the z-axis evidence different features. In as-grown samples a strong and broad peak at approximately 132°C is observed, while hydrogen-swept samples are characterized by two peaks at 180°C and 275°C. Such an anisotropic character, and the fact that no TSL structures are observed in the same temperature range, support the hypothesis of an ionic nature for the latter peaks. TSC “pre-dose” measurements of the 75°C peak show that no current enhancement is observed upon irradiation and heating treatment: this result is in accordance with previous radioluminescence and thermally stimulated exoelectron emission experiments and supports the proposed model of the dynamics of this effect.
1. INTRODUCTION
strong background current prevented the extension of the study of the TSC spectrum to higher temperatures (Halperin et al., 1970). In this work, we present a comparative study of TSL and TSC of quartz above room temperature, with the aim of providing useful complementary information on the traps-recombination centers dynamics. In order to reach a better understanding of the phenomena under study, we have used high purity synthetic single crystals; furthermore, we have performed the TSC measurements in different crystal directions, to take into account the influence of crystal anisotropy on the drift of the carriers.
THE TSL glow curve of crystalline quartz above room temperature features a number of structures: of these, the most studied is undoubtedly the so-called “100°C” peak, because of its ubiquity in almost all
types of crystals and its importance in archaeological dating. The intensity of this structure, and the occurrence of additional peaks at higher temperatures, seem to depend strongly upon different factors, such as crystal origin, impurities, sample conditions (namely powder or single crystal) and annealing treatments. This marked sensitivity of the glow curve depending on several parameters, and the lack of a satisfactory control of the material, have prevented the develop-
2. EXPERIMENTAL CONDITIONS
ment of a structural description of the point defects involved in the TSL of crystalline SiO:. Several previous studies have dealt with the thermoluminescent properties of quartz. either below or above room temperature (McKeever. 1984). At low temperatures, TSC measurements have been discussed in detail in relation to TSL. leading to a complex interpretation of the TSC spectrum in which electronic as well as ionic traps are involved (Katz and Halpcrin, 1987); above room temperature. a TSL-TSC study has been performed for the 100°C
peak (Bohm et al., 1980), but the occurrence
of a
The synthetic quartz considered in this investigation was purchased from Sawyer Research Products Inc., Eastlake, Ohio. “Premium Quality” (PQ)
quartz, cut from the z-growth zone, was used, both “as-grown” and “hydrogen-swept” by Sawyer. The impurity content of this material appears to be quite low: for instance, the aluminium concentration is approximately 1-3 ppm (data supplied by Sawyer). Samples were single crystals cut in the form of discs, 10 mm diameter and 1 mm thickness, with the major surfaces orthogonal to the z- or to the x-axis of the 361
M. CASTIGLIONI
362
bars (z-cut and x-cut, respectively). For the TSC measurements, the electrical contacts were made by applying a conductive paste (JMI 4613) on the surfaces: in most cases, the upper electrode was 2 mm in diameter at the centre of the sample, in order to keep sufficient area for the simultaneous detection of the TSL signal, while the other electrode covered the whole opposite surface; in some measurements, when a particularly weak current signal had to be detected, the diameter of the upper electrode was I mm. Irradiations were performed at room temperature with a Machlett OEG 50 X-ray tube operated at 32 kV, 20 mA or with a %r-9ou source delivering 2 Gy/min. Measurements were performed from RT to 400°C by means of a home-made oven and a computer-interfaced TSC-TSL apparatus by AeDi, Milano, using a heating rate of 1“C/s: the TSL signal was detected by photon counting by an EMI 9635 QB photomultiplier tube; an optical filter (BG12, CWL = 405 run) was used to eliminate blackbody radiation. The TSC was simultaneously detected by a Keithley 617 electrometer, the electric field bias being 90 V/cm.
3.1. Unirradiated samples As expected in the case of synthetic specimens, the TSL glow curves of unirradiated samples did not show any structure up to 400°C; on the other hand, the TSC measurements show a monotonically increasing signal, presented in Fig. 1. A few remarks
1
1
4
40 3
50
-
E 0 p
= 3 f B
2
0
0
c d
:i I
20 -
ifiizl: -2 200
JO0
TEYPERATURE
0
100
200
TEMPERATURE
30% Y
20 E
10 0 0
200
100
TEMPERATURE
300
40:
(‘C)
FIG. 2. Thermally stimulated luminescence (left ordinate scale) and conductivity (right ordinate scale) of a z-cut PQ as-grown quartz X-irradiated at a dose of several thousand Gray. The figure also shows for comparison the thermally stimulated conductivity of an x-cut PQ as-grown sample irradiated at the same X-ray dose. Thin line, TSL; dashed line, TSC; heavy line, TSC of the x-cut sample.
can be made about background current:
the principal
features of the
(9 the magnitude
3. RESULTS
50
et al.
400 (‘C)
300
i
,! 4
of the current decreases with repeated heating cycles; (ii) the magnitude is sample dependent: in fact, PQ as-grown samples have a much higher conductivity than PQ swept samples (see curves (a) and (c) in the inset of Fig. 1); and (iii) the magnitude is also anisotropic, being much higher when the electric field is oriented along the z-axis with respect to the x-axis orientation (see the comparison between curves (a) and (b) for PQ as-grown and (c), (d) for PQ swept). The above-described characteristics are consistent with an ionic interpretation of this signal, due to alkali ions (M+ ) freed from (Al-M+ )’ sites and moving preferentially along the open channels existing in the z-axis crystal direction (Lazzari et al., 1988): the decrease with repeated heating cycles should be ascribed to a polarization mechanism, operated by the ions being collected at the negative electrode; the much higher intensity in PQ as-grown samples reflects their greater alkali content with respect to the swept ones, while the difference between z-cut and x-cut specimens is consistent with the anisotropy of this type of transport.
400
(‘C)
FIG. 1. Thermally stimulated conductivity from RT to 400°C of unirradlaled z-cut PQ as-grown quartz: the curves labelled I, 2,3 and 4 are subsequencheating cycles. The inset shows a comparison from ZOO’Cto 4OO’Cof the first heating cycle betwe& diRerent types of samples: (a) z-cut PG as-grown; (b) x-cut PQ as-grown; (c) z+zut PQ swept; (d) x-cut PQ swept.
3.2. Irradiated samples Upon X-irradiation, several peaks appear in the TSL glow curve. Starting from as-grown samples, an irradiation of several thousand Gray X-rays, illustrated in Fig. 2, induces main structures at 75 and 110°C. a weaker emission at approximately 16O”C, and other unresolved structures above 200°C. An
TSL AND TSC OF SYNTHETIC analysis of the first peak with a curve fitting program has shown that it follows first-order kinetics, giving the value 0.83 f 0.01 eV for the trap depth: this result (McKeever et ol., 1985), together with the observation of a pre-dose effect, identifies the latter peak with the well-known “100°C” peak of quartz, here detected at a lower temperature due to the slow heating rate (1 “C/s). No anisotropy of the TSL signal has been observed, and for this reason, in Fig. 2, only the TSL glow curve of the z-cut sample is reported; in contrast, striking differences result in the TSC curves between the z- and the x-orientations. The x-cut sample shows a peak at 80°C and weak shoulders at approximately 120°C and 160°C; above 200°C the ionic background markedly increases. Due to the strong similarity with the temperatures of the TSL glow peaks, we are confident that the TSC structures are due
CRYSTALLINE
QUARTZ
363
sity of the first structure is reduced. At higher temperatures, the glow curve features broad peaks at approximately 200°C and 320°C. Also in this case, no anisotropy of the TSL signal has been observed. The reduced background current of the swept samples allows the detection of radiation-induced peaks in the TSC up to 300°C; a high degree of anisotropy of the signals is again observed, leading to a complex description of the TSC spectrum. The measurement performed with the x-cut sample shows a weak shoulder in the 80°C region; at higher temperatures, a complex structure is observed, apparently composed of two peaks at approximately 160°C and at 200°C. Above 200°C the background current is superimposed to a further weak structure in the 280°C region. The TSC of the z-cut sample is similar in the 5&1OO”C range, showing only a weak unresolved shoulder; at higher temperatures, however, two intense peaks appear, at 180°C and at 275”C, respectively. The monotonically decreasing signal at low temperatures, already observed in the PQ asgrown sample, is again detected in swept quartz, both x- and z-cut, although at a lower intensity. 3.3. Pre -dose measurements
ObseNed
0
100
200
TEMPERATURE
300
400
(‘C)
FIG. 3. Thermally stimulated luminescence (left ordinate scale) and conductivity (right ordinate scale) of z-cut FQ swept quartz X-irradiated at a dose of several thousand Gray. The figure also shows for comparison the thermally stimulated conductivity of an x-cut PQ swept sample irradiated at the same X-ray dose. Thin line, TSL; dashed line, TSC; heavy line, TSC of the x-cut sample.
l-he peak observed at 75°C exhibits the so-called “pre-dose” effect, its sensitivity being increased by irradiations and subsequent thermal treatments. This effect has been studied in detail (Yang and McKeever, 1990 and references therein), also because of its application in archaeological and environmental dosimetry. In order to study this effect, we operated in the following way: a sample of x-cut PQ as-grown quartz, prepared with the upper electrode of 7 mm diameter, was irradiated with a beta dose of 100 Gy, and then the TSL and TSC measurements were performed, heating the sample up to 400°C. The resulting CUNeS are reported in Fig. 4(a), where an enlargement up to 200°C is shown for better clarity; then, the sample was irradiated for the second time with the same beta dose and the TSL/TSC measurement repeated. The result of this second measurement is reported in Fig. 4(b): an increase in intensity of approximately a factor of four is observed by comparing the two TSL measurements (“pre-dose” effect). In contrast, the two TSC measurements have similar intensities, proving that electronic traps have no active role in this TSL sensitization mechanism. Although with some difficulty due to the weakness of the TSC signal, the same behaviour has been observed at lower doses, up to 10 Gy, and in swept samples. 4. DISCUSSION
AND CONCLUSIONS
In this work, we have presented a detailed comparative study of the TSL and TSC properties of pure synthetic commercial quartz-both as-grown and hydrogen-swept-above room temperature. The fol-
M. CASTIGLIONI
364
0 TEMPERATURE (‘C) FIG. 4(a). Thermally stimulated luminescence and conductivity of an x-cut PQ as-grown quartz sample irradiated with a beta dose of 100 Gy. Continuous line, TSL; dashed line, TX.
60120 50 15 z
40
n : 30
10
Y
z
20
5 10 0
0
50
100
150
200
TEMPERATURE (‘C) FIG. 4(b). Thermally stimulated luminescence and conductivity of an x-cut FQ as-grown quartz sample irradiated with a beta dose of 100 Gy after a previous irradiation at the same dose and subsequent heating at 4OO’C (“pre-dose” sensitization). Continuous line, TSL; dashed line, TSC.
lowing discussion will focus on the most significant aspects of the results here obtained. i.e. (i) the nature of the TSC peaks, (ii) the influence of hydrogen sweeping on the TSLiTSC properties and (iii) the pre-dose effect. 4.1. Nature of the TX
peaks
A simultaneous TSL/TSC signal is to be expected in the simple case in which the detrapping of electrons from shallow levels. followed by recombination at thermoluminescent centres. involves the conduction band; in our quartz samples, we did in fact observe at least three TSC structures which can be associated with TSL peaks, namely at 80-C. 12O’C and approximately 160°C. particularly evident in PQ as-grown x-cut quartz. Also, the TSC signal observed in PQ swept quartz below 2OO’C could be associated with
et al.
TSL, although with a higher degree of uncertainty due to the complex shape of the signal. We observed, however, in the z-cut samples, additional structures, whose properties appear strikingly different: specifically, these are the 132°C peak in z-cut as-grown quartz, and the 180°C and 275°C peaks in z-cut swept quartz: these structures do not seem to be directly related to TSL peaks; furthermore, they are strongly anisotropic, being preferentially detected in the measurements performed with the electric field along the z-axis. The electronic transport in SiO, appears to be isotropic (Hughes, 1975) and thus we can suggest the hypothesis that these features have an ionic character, possibly due to alkalis and, particularly in the case of swept quartz, to hydrogen. In quartz, the variations in the concentrations of radiation-induced extrinsic and intrinsic point defects upon annealing treatments seem to reflect strongly the trapping-detrapping dynamics of alkalis and hydrogen (Halliburton et al., 1981; Jani et al., 1983; Subramanian et al., 1984). Future investigations should be concerned with the nature of these possibly ionic TSC structures and on their relation to the dynamics of other defects. An ionic mechanism could be attributed also to the decreasing current observed at temperatures lower than 50°C: it has in fact already been shown that in this temperature range, alkali ions freed by irradiation from Al’+ give rise- to a measurable transient “radiation-induced” conductivity, which decays at RT, similarly to phosphorescence (Martini er al., 1986).
4.2. Injuence of hydrogen sweeping It appears evident from our results that the hydrogen sweeping treatment strongly affects the intensity of the TSL peak at 75°C and of the associated TSC peak at 80% (compare Figs 2 and 3). In fact, for the same dose of several thousand Gray, the intensity of the TSL of the swept sample is reduced by more than one order of magnitude with respect to that of the as-grown sample; correspondingly, the TSC peak is also markedly reduced. This result is in contrast to others already obtained in the study of the TSL properties of swept quartz, which showed an increase in the TSL intensity upon sweeping treatments (Martini er al.. 1987; Yang and McKeever, 1988). However, care should be taken in comparing results obtained on samples prepared under different experimental conditions, such as temperature and atmosphere during the sweeping treatments, that could play a critical role. The observation that, as a consequence of sweeping, there is a decrease not only of the TSL signal, but also of the TSC one, could indicate a reduction in the trap concentration. On the other hand, one should recognize that this experimental result can also be interpreted by making the hypothesis of an increase, due to the sweeping treatment, of competing recombination centres: these would increase the probability of recombination and therefore
TSL AND TSC OF SYNTHETIC
CRYSTALLINE
QUARTZ
365
reduce the concentration of electrons in the conduction hand during the glow curve, thus reducing both TSL and TSC. These could be non-radiative centres; alternatively, their emission band could be situated outside the spectral region of sensitivity of our appar-
We thank also E. Rosetta for performing some of the measurements.
atus. In this respect, a study of the distribution of the TSL emission in a wide spectral range, both of the unswept and swept crystals, could provide significant information.
Bohm M., Peschke W. and Scharmann A. (1980) Thermolu-
4.3. Pre-dose
effect
Although the study of the “pre-dose” effect has been mostly carried out on powdered samples, a similar behaviour has been observed in the bulk material (Martini et al., 1987). A phenomenological model concerning the dynamics of this phenomenon, supported by TSL, radioluminescence (RL) and thermally stimulated exoelectron emission (TSEE) experiments, has been proposed (Zimmerman, 1971). It involves an increase in the concentration of the radiative recombination centres. The consistency of the proposed model has been checked by our simultaneous TSL and TSC measurements: in fact, if, contrary to Zimmerman’s model, the sensitivity enhancement were caused by an increase in the concentration of the electron traps, an enhancement in the TSC at 80°C associated with the 75°C TSL peak, should be observed. The comparison between Fig. 4(a) and (b) shows that no TSC enhancement occurs after a pre-dose treatment. A few comments can be made: (i) the irradiation-heating cycles do not alter the concentration of electrons in the conduction band; (ii) the TSL sensitivity enhancement is due to an increase in available luminescent centres, as proposed in Zimmerman’s model; (iii) the TSL sensitivity enhancement appears to involve a new emission wavelength (McKeever et al., 1985); recent measurements provide evidence that it is related only to the 75°C glow peak (Martini et al., unpublished results): this would mean that a spatial correlation exists between traps and luminescent centres. Point (i) indicates, however, that no tunnelling occurs in the recombination process.
REFERENCES minescence and thermally stimulated conductivity in quartz induced by thermal treatment after Xirradiation. Rud. Efl 53, 67-72. Halliburton L. E., Koumvakalis N., Markes M. E. and Martin J. J. (1981) Radiation effects in crystalline SiO,: the role of aluminum. J. Appl. Phys. 52, 35653574.
Halperin A., Braner A. A. and Shapira J. (1970) Thermoluminescence and thermally stimulated currents in quartz. J. Luminescence 1, 385-397. Hughes R. C. (1975) Electronic and ionic charge carriers in irradiated single crystals and fused quartz. Rud. Eff. 26, 225-235. Jani M. G., Bossoli R. B. and Halliburton L. E. (1983) Further characterization of the E; center in crystalline SiO,. Phvs. Rev. B 27. 2285-2293. Katz S. *and ’ Halperin A.’ (1987) Thermally stimulated luminescence and its related thermally stimulated currents in quartz. Phys. Rev. B 36, 66466650. Lazzari S., Martini M., Paleari A., Spinolo G. and Vedda A. (1988) DC and AC ionic conductivity in quartz: a new high temperature mechanism and a general assessment. Nucl. Instrwn. Meth. Phys. Res. B32, 299-302.
McKeever S. W. S. (1984) Thermoluminescence of quartz and silica. Radiur. Prof. Dosim. 8, 81-98. McKeever S. W. S. (1985) Thermoluminescence of Solids. Cambridge University Press, Cambridge (U.K.). McKeever S. W. S.. Chen C. Y. and Halliburton L. E. (1985) Point defects and the pre-dose effect in natural quartz. Nucl. Tracks 10, 489-495. Martini M., Spinolo G. and Vedda A. (1986) Radiation induced conductivity of as-grown and electrodiffused quartz. J. Appl. Phjs. 60, I?‘051708. Martini M., Suinolo G. and Vedda A. (1987) Defects dynamics in as grown and electrodiffuxd quartz: an interpretation of the pre-dose effect. J. Appl. Phys. 61, 2486-2488.
Subramanian B., Halliburton L. E. and Martin J. J. (1984) Radiation effects in crystalline SiO,: infrared absorption from OH--related defects. J. Phys. Chem. Solirls 45, 575-579.
Yang X. H. and McKeever S. W. S. (1988) Characterization of the pre-dose effect using ESR and TL. Nucl. Tracks Radiat. Meas. 14, 75-79.
Yang X. H. and McKeever S. W. S. (1990) The pre-dose effect in crystalline quartz. J. Phys. D: Appt. Phys. 23, 237-244.
Acknowledgemenrs-Thanks are due to a referee for a few suggestions in his review of the manuscript regarding the nature of the TSC peaks.
Zimmerman J. (1971) The radiation induced increase of the 100°C thermoluminescence sensitivity of fired quartz J. Phys. C: Solid St. Phys. 4, 3265-3276.
23, Nos
361-365, 1954
Copyright 0 1994Elsevier Science Ltd Printed in Great Britain. All rights resewed 1350-4487/94 s7.00 + .oo
Pergamon
13504487(93)E0032-H
THERMALLY STIMULATED LUMINESCENCE (TSL) AND CONDUCTIVITY (TSC) OF SYNTHETIC CRYSTALLINE QUARTZ
Dipartimento
M. CASTIGLIONI,M. MARTINI,G. SPINOU)and A. VEDDA di Fisica, Universitzi di Milano, Via Celoria 16, I-20133 Milano, Italy
Abstract-A
comparative and simultaneous study of TSL and TSC above room temperature (20-400°C) has been performed on “as-grown” and “hydrogen-swept” synthetic quartz crystals. Following Xirradiations, TSL spectra (heating rate = l”C/s) feature a number of peaks: at 75°C an intense structure is observed (the well-known “IOO”c” peak of quartz); the analysis of this peak obtained by numerical methods has shown that it follows monomolecular kinetics, giving a value of 0.83 eV for the trap depth. Additional peaks are observed at 110°C and 160°C. followed by weaker and less resolved emissions above 200°C. TSC peaks at 8O”C, 120°C and 16WC, particularly evident in as-grown samples when measured with the electric field applied along the x-axis, can be associated to the corresponding TSL peaks. However, spectra performed with the electric field applied along the z-axis evidence different features. In as-grown samples a strong and broad peak at approximately 132°C is observed, while hydrogen-swept samples are characterized by two peaks at 180°C and 275°C. Such an anisotropic character, and the fact that no TSL structures are observed in the same temperature range, support the hypothesis of an ionic nature for the latter peaks. TSC “pre-dose” measurements of the 75°C peak show that no current enhancement is observed upon irradiation and heating treatment: this result is in accordance with previous radioluminescence and thermally stimulated exoelectron emission experiments and supports the proposed model of the dynamics of this effect.
1. INTRODUCTION
strong background current prevented the extension of the study of the TSC spectrum to higher temperatures (Halperin et al., 1970). In this work, we present a comparative study of TSL and TSC of quartz above room temperature, with the aim of providing useful complementary information on the traps-recombination centers dynamics. In order to reach a better understanding of the phenomena under study, we have used high purity synthetic single crystals; furthermore, we have performed the TSC measurements in different crystal directions, to take into account the influence of crystal anisotropy on the drift of the carriers.
THE TSL glow curve of crystalline quartz above room temperature features a number of structures: of these, the most studied is undoubtedly the so-called “100°C” peak, because of its ubiquity in almost all
types of crystals and its importance in archaeological dating. The intensity of this structure, and the occurrence of additional peaks at higher temperatures, seem to depend strongly upon different factors, such as crystal origin, impurities, sample conditions (namely powder or single crystal) and annealing treatments. This marked sensitivity of the glow curve depending on several parameters, and the lack of a satisfactory control of the material, have prevented the develop-
2. EXPERIMENTAL CONDITIONS
ment of a structural description of the point defects involved in the TSL of crystalline SiO:. Several previous studies have dealt with the thermoluminescent properties of quartz. either below or above room temperature (McKeever. 1984). At low temperatures, TSC measurements have been discussed in detail in relation to TSL. leading to a complex interpretation of the TSC spectrum in which electronic as well as ionic traps are involved (Katz and Halpcrin, 1987); above room temperature. a TSL-TSC study has been performed for the 100°C
peak (Bohm et al., 1980), but the occurrence
of a
The synthetic quartz considered in this investigation was purchased from Sawyer Research Products Inc., Eastlake, Ohio. “Premium Quality” (PQ)
quartz, cut from the z-growth zone, was used, both “as-grown” and “hydrogen-swept” by Sawyer. The impurity content of this material appears to be quite low: for instance, the aluminium concentration is approximately 1-3 ppm (data supplied by Sawyer). Samples were single crystals cut in the form of discs, 10 mm diameter and 1 mm thickness, with the major surfaces orthogonal to the z- or to the x-axis of the 361
M. CASTIGLIONI
362
bars (z-cut and x-cut, respectively). For the TSC measurements, the electrical contacts were made by applying a conductive paste (JMI 4613) on the surfaces: in most cases, the upper electrode was 2 mm in diameter at the centre of the sample, in order to keep sufficient area for the simultaneous detection of the TSL signal, while the other electrode covered the whole opposite surface; in some measurements, when a particularly weak current signal had to be detected, the diameter of the upper electrode was I mm. Irradiations were performed at room temperature with a Machlett OEG 50 X-ray tube operated at 32 kV, 20 mA or with a %r-9ou source delivering 2 Gy/min. Measurements were performed from RT to 400°C by means of a home-made oven and a computer-interfaced TSC-TSL apparatus by AeDi, Milano, using a heating rate of 1“C/s: the TSL signal was detected by photon counting by an EMI 9635 QB photomultiplier tube; an optical filter (BG12, CWL = 405 run) was used to eliminate blackbody radiation. The TSC was simultaneously detected by a Keithley 617 electrometer, the electric field bias being 90 V/cm.
3.1. Unirradiated samples As expected in the case of synthetic specimens, the TSL glow curves of unirradiated samples did not show any structure up to 400°C; on the other hand, the TSC measurements show a monotonically increasing signal, presented in Fig. 1. A few remarks
1
1
4
40 3
50
-
E 0 p
= 3 f B
2
0
0
c d
:i I
20 -
ifiizl: -2 200
JO0
TEYPERATURE
0
100
200
TEMPERATURE
30% Y
20 E
10 0 0
200
100
TEMPERATURE
300
40:
(‘C)
FIG. 2. Thermally stimulated luminescence (left ordinate scale) and conductivity (right ordinate scale) of a z-cut PQ as-grown quartz X-irradiated at a dose of several thousand Gray. The figure also shows for comparison the thermally stimulated conductivity of an x-cut PQ as-grown sample irradiated at the same X-ray dose. Thin line, TSL; dashed line, TSC; heavy line, TSC of the x-cut sample.
can be made about background current:
the principal
features of the
(9 the magnitude
3. RESULTS
50
et al.
400 (‘C)
300
i
,! 4
of the current decreases with repeated heating cycles; (ii) the magnitude is sample dependent: in fact, PQ as-grown samples have a much higher conductivity than PQ swept samples (see curves (a) and (c) in the inset of Fig. 1); and (iii) the magnitude is also anisotropic, being much higher when the electric field is oriented along the z-axis with respect to the x-axis orientation (see the comparison between curves (a) and (b) for PQ as-grown and (c), (d) for PQ swept). The above-described characteristics are consistent with an ionic interpretation of this signal, due to alkali ions (M+ ) freed from (Al-M+ )’ sites and moving preferentially along the open channels existing in the z-axis crystal direction (Lazzari et al., 1988): the decrease with repeated heating cycles should be ascribed to a polarization mechanism, operated by the ions being collected at the negative electrode; the much higher intensity in PQ as-grown samples reflects their greater alkali content with respect to the swept ones, while the difference between z-cut and x-cut specimens is consistent with the anisotropy of this type of transport.
400
(‘C)
FIG. 1. Thermally stimulated conductivity from RT to 400°C of unirradlaled z-cut PQ as-grown quartz: the curves labelled I, 2,3 and 4 are subsequencheating cycles. The inset shows a comparison from ZOO’Cto 4OO’Cof the first heating cycle betwe& diRerent types of samples: (a) z-cut PG as-grown; (b) x-cut PQ as-grown; (c) z+zut PQ swept; (d) x-cut PQ swept.
3.2. Irradiated samples Upon X-irradiation, several peaks appear in the TSL glow curve. Starting from as-grown samples, an irradiation of several thousand Gray X-rays, illustrated in Fig. 2, induces main structures at 75 and 110°C. a weaker emission at approximately 16O”C, and other unresolved structures above 200°C. An
TSL AND TSC OF SYNTHETIC analysis of the first peak with a curve fitting program has shown that it follows first-order kinetics, giving the value 0.83 f 0.01 eV for the trap depth: this result (McKeever et ol., 1985), together with the observation of a pre-dose effect, identifies the latter peak with the well-known “100°C” peak of quartz, here detected at a lower temperature due to the slow heating rate (1 “C/s). No anisotropy of the TSL signal has been observed, and for this reason, in Fig. 2, only the TSL glow curve of the z-cut sample is reported; in contrast, striking differences result in the TSC curves between the z- and the x-orientations. The x-cut sample shows a peak at 80°C and weak shoulders at approximately 120°C and 160°C; above 200°C the ionic background markedly increases. Due to the strong similarity with the temperatures of the TSL glow peaks, we are confident that the TSC structures are due
CRYSTALLINE
QUARTZ
363
sity of the first structure is reduced. At higher temperatures, the glow curve features broad peaks at approximately 200°C and 320°C. Also in this case, no anisotropy of the TSL signal has been observed. The reduced background current of the swept samples allows the detection of radiation-induced peaks in the TSC up to 300°C; a high degree of anisotropy of the signals is again observed, leading to a complex description of the TSC spectrum. The measurement performed with the x-cut sample shows a weak shoulder in the 80°C region; at higher temperatures, a complex structure is observed, apparently composed of two peaks at approximately 160°C and at 200°C. Above 200°C the background current is superimposed to a further weak structure in the 280°C region. The TSC of the z-cut sample is similar in the 5&1OO”C range, showing only a weak unresolved shoulder; at higher temperatures, however, two intense peaks appear, at 180°C and at 275”C, respectively. The monotonically decreasing signal at low temperatures, already observed in the PQ asgrown sample, is again detected in swept quartz, both x- and z-cut, although at a lower intensity. 3.3. Pre -dose measurements
ObseNed
0
100
200
TEMPERATURE
300
400
(‘C)
FIG. 3. Thermally stimulated luminescence (left ordinate scale) and conductivity (right ordinate scale) of z-cut FQ swept quartz X-irradiated at a dose of several thousand Gray. The figure also shows for comparison the thermally stimulated conductivity of an x-cut PQ swept sample irradiated at the same X-ray dose. Thin line, TSL; dashed line, TSC; heavy line, TSC of the x-cut sample.
l-he peak observed at 75°C exhibits the so-called “pre-dose” effect, its sensitivity being increased by irradiations and subsequent thermal treatments. This effect has been studied in detail (Yang and McKeever, 1990 and references therein), also because of its application in archaeological and environmental dosimetry. In order to study this effect, we operated in the following way: a sample of x-cut PQ as-grown quartz, prepared with the upper electrode of 7 mm diameter, was irradiated with a beta dose of 100 Gy, and then the TSL and TSC measurements were performed, heating the sample up to 400°C. The resulting CUNeS are reported in Fig. 4(a), where an enlargement up to 200°C is shown for better clarity; then, the sample was irradiated for the second time with the same beta dose and the TSL/TSC measurement repeated. The result of this second measurement is reported in Fig. 4(b): an increase in intensity of approximately a factor of four is observed by comparing the two TSL measurements (“pre-dose” effect). In contrast, the two TSC measurements have similar intensities, proving that electronic traps have no active role in this TSL sensitization mechanism. Although with some difficulty due to the weakness of the TSC signal, the same behaviour has been observed at lower doses, up to 10 Gy, and in swept samples. 4. DISCUSSION
AND CONCLUSIONS
In this work, we have presented a detailed comparative study of the TSL and TSC properties of pure synthetic commercial quartz-both as-grown and hydrogen-swept-above room temperature. The fol-
M. CASTIGLIONI
364
0 TEMPERATURE (‘C) FIG. 4(a). Thermally stimulated luminescence and conductivity of an x-cut PQ as-grown quartz sample irradiated with a beta dose of 100 Gy. Continuous line, TSL; dashed line, TX.
60120 50 15 z
40
n : 30
10
Y
z
20
5 10 0
0
50
100
150
200
TEMPERATURE (‘C) FIG. 4(b). Thermally stimulated luminescence and conductivity of an x-cut FQ as-grown quartz sample irradiated with a beta dose of 100 Gy after a previous irradiation at the same dose and subsequent heating at 4OO’C (“pre-dose” sensitization). Continuous line, TSL; dashed line, TSC.
lowing discussion will focus on the most significant aspects of the results here obtained. i.e. (i) the nature of the TSC peaks, (ii) the influence of hydrogen sweeping on the TSLiTSC properties and (iii) the pre-dose effect. 4.1. Nature of the TX
peaks
A simultaneous TSL/TSC signal is to be expected in the simple case in which the detrapping of electrons from shallow levels. followed by recombination at thermoluminescent centres. involves the conduction band; in our quartz samples, we did in fact observe at least three TSC structures which can be associated with TSL peaks, namely at 80-C. 12O’C and approximately 160°C. particularly evident in PQ as-grown x-cut quartz. Also, the TSC signal observed in PQ swept quartz below 2OO’C could be associated with
et al.
TSL, although with a higher degree of uncertainty due to the complex shape of the signal. We observed, however, in the z-cut samples, additional structures, whose properties appear strikingly different: specifically, these are the 132°C peak in z-cut as-grown quartz, and the 180°C and 275°C peaks in z-cut swept quartz: these structures do not seem to be directly related to TSL peaks; furthermore, they are strongly anisotropic, being preferentially detected in the measurements performed with the electric field along the z-axis. The electronic transport in SiO, appears to be isotropic (Hughes, 1975) and thus we can suggest the hypothesis that these features have an ionic character, possibly due to alkalis and, particularly in the case of swept quartz, to hydrogen. In quartz, the variations in the concentrations of radiation-induced extrinsic and intrinsic point defects upon annealing treatments seem to reflect strongly the trapping-detrapping dynamics of alkalis and hydrogen (Halliburton et al., 1981; Jani et al., 1983; Subramanian et al., 1984). Future investigations should be concerned with the nature of these possibly ionic TSC structures and on their relation to the dynamics of other defects. An ionic mechanism could be attributed also to the decreasing current observed at temperatures lower than 50°C: it has in fact already been shown that in this temperature range, alkali ions freed by irradiation from Al’+ give rise- to a measurable transient “radiation-induced” conductivity, which decays at RT, similarly to phosphorescence (Martini er al., 1986).
4.2. Injuence of hydrogen sweeping It appears evident from our results that the hydrogen sweeping treatment strongly affects the intensity of the TSL peak at 75°C and of the associated TSC peak at 80% (compare Figs 2 and 3). In fact, for the same dose of several thousand Gray, the intensity of the TSL of the swept sample is reduced by more than one order of magnitude with respect to that of the as-grown sample; correspondingly, the TSC peak is also markedly reduced. This result is in contrast to others already obtained in the study of the TSL properties of swept quartz, which showed an increase in the TSL intensity upon sweeping treatments (Martini er al.. 1987; Yang and McKeever, 1988). However, care should be taken in comparing results obtained on samples prepared under different experimental conditions, such as temperature and atmosphere during the sweeping treatments, that could play a critical role. The observation that, as a consequence of sweeping, there is a decrease not only of the TSL signal, but also of the TSC one, could indicate a reduction in the trap concentration. On the other hand, one should recognize that this experimental result can also be interpreted by making the hypothesis of an increase, due to the sweeping treatment, of competing recombination centres: these would increase the probability of recombination and therefore
TSL AND TSC OF SYNTHETIC
CRYSTALLINE
QUARTZ
365
reduce the concentration of electrons in the conduction hand during the glow curve, thus reducing both TSL and TSC. These could be non-radiative centres; alternatively, their emission band could be situated outside the spectral region of sensitivity of our appar-
We thank also E. Rosetta for performing some of the measurements.
atus. In this respect, a study of the distribution of the TSL emission in a wide spectral range, both of the unswept and swept crystals, could provide significant information.
Bohm M., Peschke W. and Scharmann A. (1980) Thermolu-
4.3. Pre-dose
effect
Although the study of the “pre-dose” effect has been mostly carried out on powdered samples, a similar behaviour has been observed in the bulk material (Martini et al., 1987). A phenomenological model concerning the dynamics of this phenomenon, supported by TSL, radioluminescence (RL) and thermally stimulated exoelectron emission (TSEE) experiments, has been proposed (Zimmerman, 1971). It involves an increase in the concentration of the radiative recombination centres. The consistency of the proposed model has been checked by our simultaneous TSL and TSC measurements: in fact, if, contrary to Zimmerman’s model, the sensitivity enhancement were caused by an increase in the concentration of the electron traps, an enhancement in the TSC at 80°C associated with the 75°C TSL peak, should be observed. The comparison between Fig. 4(a) and (b) shows that no TSC enhancement occurs after a pre-dose treatment. A few comments can be made: (i) the irradiation-heating cycles do not alter the concentration of electrons in the conduction band; (ii) the TSL sensitivity enhancement is due to an increase in available luminescent centres, as proposed in Zimmerman’s model; (iii) the TSL sensitivity enhancement appears to involve a new emission wavelength (McKeever et al., 1985); recent measurements provide evidence that it is related only to the 75°C glow peak (Martini et al., unpublished results): this would mean that a spatial correlation exists between traps and luminescent centres. Point (i) indicates, however, that no tunnelling occurs in the recombination process.
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Halperin A., Braner A. A. and Shapira J. (1970) Thermoluminescence and thermally stimulated currents in quartz. J. Luminescence 1, 385-397. Hughes R. C. (1975) Electronic and ionic charge carriers in irradiated single crystals and fused quartz. Rud. Eff. 26, 225-235. Jani M. G., Bossoli R. B. and Halliburton L. E. (1983) Further characterization of the E; center in crystalline SiO,. Phvs. Rev. B 27. 2285-2293. Katz S. *and ’ Halperin A.’ (1987) Thermally stimulated luminescence and its related thermally stimulated currents in quartz. Phys. Rev. B 36, 66466650. Lazzari S., Martini M., Paleari A., Spinolo G. and Vedda A. (1988) DC and AC ionic conductivity in quartz: a new high temperature mechanism and a general assessment. Nucl. Instrwn. Meth. Phys. Res. B32, 299-302.
McKeever S. W. S. (1984) Thermoluminescence of quartz and silica. Radiur. Prof. Dosim. 8, 81-98. McKeever S. W. S. (1985) Thermoluminescence of Solids. Cambridge University Press, Cambridge (U.K.). McKeever S. W. S.. Chen C. Y. and Halliburton L. E. (1985) Point defects and the pre-dose effect in natural quartz. Nucl. Tracks 10, 489-495. Martini M., Spinolo G. and Vedda A. (1986) Radiation induced conductivity of as-grown and electrodiffused quartz. J. Appl. Phjs. 60, I?‘051708. Martini M., Suinolo G. and Vedda A. (1987) Defects dynamics in as grown and electrodiffuxd quartz: an interpretation of the pre-dose effect. J. Appl. Phys. 61, 2486-2488.
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Acknowledgemenrs-Thanks are due to a referee for a few suggestions in his review of the manuscript regarding the nature of the TSC peaks.
Zimmerman J. (1971) The radiation induced increase of the 100°C thermoluminescence sensitivity of fired quartz J. Phys. C: Solid St. Phys. 4, 3265-3276.