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Characterization of cadmium telluride crystals grown by different techniques from the vapour phase
,. . . . . . . .
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 146 (1995) 125-129
Characterization of cadmium telluride crystals grown by different techniques from the vapour phase M . L a a s c h a,., R . S c h w a r z a, W . J o e r g e r a, C. E i c h e a, M . F i e d e r l e a, K . W . B e n z a, K. Grasza b a Kristallographisches Institut, Universitiit, Freiburg, Hebelstrasse 25, D-79104 Freiburg, Germany b Institute of Physics, Polish Academy of Sciences, AI. Lotnikow 32/46, PL-02-668 Warsaw, Poland
Abstract Semi-insulting CdTe bulk crystals were grown from the vapour phase in both closed and semi-open arrangements. The results of the growth experiments are discussed in terms of various electrical and optical characterization methods. Van der Pauw measurements and time dependent charge measurements (TDCM) were used to determine the resistivity. Deep level defects were investigated by means of photoinduced current transient spectroscopy (PICTS). For one of the most important fields of application, detector spectra of the vapour phase material are measured and discussed.
1. Introduction The applicability of CdTe as a material for room-temperature X-ray and y-ray detectors has generated interest in high resistivity bulk material and its connection with crystalline perfection and low residual impurity content. Vapour phase growth of CdTe has been shown to offer the opportunity of obtaining high quality crystals by avoiding imperfections due to the high temperatures necessary in melt growth [1,2]. To succeed in the growth of high resistivity material, it is necessary to either prevent intrinsic electrical active defects occurring or compensate for free carriers. Semi-open systems generally allow for the possibility of influencing the vapour composition by removing an excess component or
* Corresponding author.
volatile impurities and thus facilitate the prevention of defect formation whereas in cases of closed ampoules, compensation is most frequently achieved by chlorine doping. In recent studies, other halogens such as bromine and iodine have been shown to be possible dopants [2]. Experiments both on seeded growth in closed ampoules and on unseeded growth in semi-open arrangements were performed. Our work focused on the study of the electrical and optical properties of the CdTe crystals obtained by the different growth techniques.
2. Growth techniques 2.1. Vapour growth in closed ampoules
Chlorine-doped CdTe crystals were grown in closed ampoules using the sublimation travelling
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00474-9
M. Laasch et al. /Journal of Crystal Growth 146 (1995) 125-129
126
heater method (STHM). The evacuated growth ampoule containing both an undoped seed and d o p e d (1019-10 20 c m -3, obtained by Bridgman method) feed material, separated by an 8 mm
[~J
vapour zone, is moved vertically through a temperature maximum (825°C) with a pulling rate of 1-2 mm per day; the sublimation and deposition temperatures are 815 and 770°C, respectively.
[~33 2. l~=E 4-09
2 2~;. 0 -
2 . 4 7 E ~ 20.0
-
2 . GfTE÷~ 15 • 0 -
1 • 81E +09 1. ~'~1~
10.0
1. ~.~D~ 1.41E-HD9
5.0
1.28E.HD9 1 • l:$E.e.Og
(a)
,,
12.0
.£W
, , , ,
, , , ,
, , , ,
, , , i
I
,
tW
[S~(:=n]
]
[n~]] 3.. 96E[+09
10.0
.IL. 8 7 E + 0 9 8.0
1.78E+09 6.0
1.69E
4.0
+09
.11.. 6 0 E + 0 9 1.51E+09
2.0
1. 421[ ÷OS --0.0
4
--2.0
-4.o
1.34E
-I-09
.I.. ~
+09
1.16E+09
I
1.07E+09
-6.0q
9 • 80E+08 -8.01 --10.0
(b)
8.9.1LE+08
],,,, --10.0
g • 02E~*08
--51-0
-o1.0
5[0
rx
[l~rt ]
Fig. 1. Resistivity mappings obtained by TDCM. (a) CdTe : CI, axial section, y = growth direction; (b) Nominally undoped CdTe, radial section, x - y plane perpendicular to the growth direction.
M. Laasch et al. /Journal of Crystal Growth 146 (1995) 125-129
This technique is described in principal in Ref. [2]. Additionally, experiments using a self-seeding procedure were performed.
2.2. Vapour growth in semi-open arrangements The problem of diffusion and heat transfer leading to growth instabilities and crystal imperfections when growing crystals from the vapour has been discussed in great detail (cf. Refs. [3,6]). Using semi-open growth arrangements (cf. Refs. [4,5]), the effect of forming a third phase and impurity incorporation can be avoided by removing both excess species as well as impurities from the growth region applying, for example, a heat sink or an effusion hole. A hydrogen atmosphere (130 Torr at room temperature) additionally facilitates the growth without the risk of constitutional supersaturation [17]. A detailed description of the technique used for unseeded CdTe crystal growth is given in Ref. [1]. Here, the formation of a feed cone and the creation of a high quality seed on a quartz crystal holder attached to the ampoule is governed by a suitable temperature regime. Wall-free crystal growth takes place in a low temperature gradient using a pulling velocity of 4 mm per day and a sublimation temperature of 960°C. The cooling rate applied to the crystal investigated in this work was 300°C/h.
3. Material characterization and discussion
For physical characterization, the crystals were sliced both parallel and perpendicular to the growth axis. The samples were lapped and polished mechanically down to a surface roughness of 1 tzm. Gold (PICTS) and silver were used as contact materials.
3.1. Resistivity determination The resistivities measured by means of the Van der Pauw method at 303 K were 3 x 109 l~.cm (Cl-doped material, closed system) and 2 x 10 9 [-~'cm (nominally undoped material, semi-open system).
127
Time dependent charge measurements (TDCM) allow for contact free resistivity determination within the range from 1 x ]07 to 2 x 10 ~° 11. cm. Axial and radial crystal sections were used for measuring the two-dimensional resistivity distribution (see Fig. 1) within the vapour phase crystals with a spatial resolution of about 0.5 x 0.5 mm 2. From the two-dimensional image, maximum deviations of about 50% in the radial resistivity distribution can be seen in both crystals. Decreases in resistivity in the outer parts of the samples are probably due to wall effects in the CdTe : CI material, whereas in the undoped material, defects may occur in these regions due to instabilities caused by the vapour stream towards the temperature sink. In the axial section of the chlorine-doped crystal (Fig. la), no significant segregation behaviour is observed.
3.2. Photoinduced current transient spectroscopy (PICTS) The investigation of deep levels in high-resistivity CdTe has been performed by recording the photoinduced current transients [7] stepwise within a temperature range 80-300 K. The activation energy, E, as well as the capture cross-section, o-, but not the concentration of the deep levels can be determined mathematically from the temperature dependence of the thermal relaxation times. Arrhenius plots of samples grown by the different methods from the vapour are shown in Fig. 2. Generally, the defect number is clearly higher in undoped CdTe. In this respect, due to the related purification effect, using Bridgman grown source material, as is done in CdTe:C1 growth, seems to be advantageous. On the other hand, tensions related to the fast cooling rate and the different volume expansion coefficients of CdTe and quartz seem to be an important defect origin. Two traps (T6, T8), identical in both undoped and doped material, have been detected. The height distribution of the peak maxima in the PICTS spectrum (which is related to the defect concentration) is quite uniform for most defects. T6 traps in undoped CdTe as well as T4, T5 and T7 traps show significantly higher intensities.
M. Laasch et al. /Journal of Crystal Growth 146 (1995) 125-129
128
10
, "
' I ' ['
'
9-_
I ' I ' I ' I ' I'
I ' I ' li''
T9
7-
c '--'
6" 5-
~
4.
vIT4:TI:' 0.210"08 I' I eveV T6:0.28
eV
T9:0.55
eV
T8
3'
T1
2-
'0
I") -i
,i
2
,I
3
,i
4
5
i ,I ,i ,~, 6 7 8 9
i ,i ,i ,i ,i, i 10 11 12 13 14 15 16
IO00/T [K-']
9TIO
T8
8v
7-
c
6-
T7 v
5-
7--I-.
4-
c
3"
1- £o) 0
2
3
4
5
6
7
~ 8 9
10 11
12 13 14 15 16
1000/T [K-'] Fig. 2. Arrhenius plot obtained by means of PICTS in C d T e : C1 grown in closed ampoules (a) and in nominally undoped C d T e grown in a semi-open arrangement (b).
The traps T1 and T4 are related to compensation defects and in good agreement with published results (e.g. Refs. [8,9], see Table 1). The complex [V2dCl~e] -, the so-called A-center [10], may be responsible. Even in nominally undoped material, low amounts of halogens incorporated as impurities could probably cause the PICTS signal (T5 [11]). This observation underlines the importance of using high purity source material for vapour transport. In both undoped and doped material, the traps T2 and T6 probably arise from residual impurities such as Au, Ag, Cu, whereas T7 traps could be caused by both impurities and structural defects [9,12]. In particular, Au is assumed to be a possible origin of T6 traps due to diffusion of the contact material (Au) into the crystal matrix [13]. T7 traps have not been detected in CdTe:C1, a possible explanation is the dopant incorporation (e.g. formation of the [V2d-Cl~e] - complex). The origins of T8 traps detected both in undoped and doped material are structural defects such as Cd~ + [9]. The T9 and T10 traps are hitherto unidentified. However, a deep donor level (due to an intrinsic defect) seems to be particularly important for compensation and is probably responsible for the high resistivity of the material [14]. An overview of the detected deep levels is given in Table 1.
Table 1 Activation energies and capture cross-sections of the deep levels found in vapour grown undoped and chlorine-doped C d T e Trap
STHM (Cl-doped)
Semi-open
Literature data
Energy
(nominally undoped)
Energy
Energy
(eV)
tr (cm 2)
(eV)
~r (cm 2)
Assignation Ref.
(eV) T1 T2 T3 T4 T5 T6 T7 T8 T9 T10
0.08
10-16
0.21
10 I1
0.28
10 - 12
0.56 0.55
10- 9 10-15
* = hitherto unidentified.
0.11 0.14
10-17 10 -12
0.21 0.28 0.34 0.56
10 . 9 10 - 12 10-13 10- 8
0.51
10 -14
0.06 0.1-0.2 0.1-0.2 0.28 0.3-0.35 0.56
[81 [16] e.g. [9] [12] e.g. [9,11] [12]
[V~d- 2Cl{e ] Cu * [Vdd2- CITe ]+ -
_
_
*
-
-
*
Au, Ag Au, Ag, Al Cd 2 +
M. Laasch et al. / Journal of Crystal Growth 146 (1995) 125-129
3. 3. Detector properties In addition to high resistivities, the quality enhancement of detector devices arises from increasing the mobility-lifetime (~zz) product of the free carriers. Due to structural defects, which are more difficult to suppress than impurities, maximum /zr products are approximately 10 -3 cm 2 V -1 [15]. /zz products for electrons and holes measured in chlorine-doped CdTe are/ze% = 5.66 × 10 -5 cm 2 V -I a n d / z h r h = 1 × 10 -5 cm 2 V -1, respectively. In undoped material, lower values have been measured: p.eZe=9.5 × 10 -6 cm 2 V -~ and /zhr h = 1 x 10 -6 cm 2 V -1. These results correlate with the high number of defects detected. Accordingly, the detector efficiency (a-radiation) of the CdTe : CI crystal is 90%, whereas only 12% efficiency has been measured for nominally undoped material.
4. Conclusion High resistivity (up to 109 D . cm) cadmium telluride crystals have been grown from the vapour both in closed and semi-open arrangements. By means of TDCM, the high homogeneity of the resistivity distribution has been shown. In contrast to chlorine-doped material, the detector efficiency of the undoped material grown within a semi-open arrangement is less than satisfactory, this correlates with the high number of defects which were found by PICTS. The main reason is probably the fast cooling rate applied to this crystal.
Acknowledgements The authors are indebted to Mrs. I. Koch and Mr. M. Kranz-Probst for skilled experimental
129
help. The work has been supported financially by the Federal Ministry of Research and Technology (BMFT) under project management by D L R and D A R A (contract no. 50QV88083).
References [1] K. Grasza, U. Zuzga-Grasza, A. Jedrzejczak, R.R. Galazka, J. Majewski, A. Szadkowski and E. Grodzicka, J. Crystal Growth 123 (1992) 519. [2] M. Laasch, R. Schwarz, P. Rudolph and K.W. Benz, J. Crystal Growth 141 (1994) 81. [3] D.A. Louchev, J. Crystal Growth 140 (1994) 219. [4] E.V. Markov and A.A. Davydov, Neorg. Mater. 11 (1975) 1755. [5] R. Lauck, G. Miiller-Vogt and W. Wendl, J. Crystal Growth 74 (1986) 520. [6] K. Grasza, Mass and Heat Transfer in Crystal Growth, in: Elementary Crystal Growth, Ed. K. Sangwall (SAAN, Lublin, 1994). [7] Ch. Hurtes, M. Boulou, A. Mitonneau and D. Bois, Appl. Phys. Lett. 32 (1978) 821. [8] R. Struck and A. Cornet, J. Phys. Chem. Solids 37 (1976) 989. [9] M. Samini, B. Biglari, M. Hage-Ali, J.M. Koebel and P. Siffert, Nucl. Instrum. Methods A 283 (1989) 243. [10] D.M. Hofmann, P. Omling, H.G. Grimmeiss, B.K. Meyer, K.W. Benz and D. Sinerius, Phys. Rev. 13 45 (1992) 6247. [11] G.M. Khattak and C.G. Scott, J. Phys. Condens. Matter 3 (1991) 8619. [12] P. Siffert, Cadmium Telluride Detectors and Applications, in: Mater. Res. Soc. Proc., Vol. 16, Nuclear Radiation Detector Materials, Eds. E.E. Hailer and H.W. Kraner (1983) p. 87. [13] H. Zimmermann, R. Boyn, P. Rudolph, J. Bollmann, A. Klimakow and B. Krause, Mater. Sci. Eng. B 16 (1993) 139. [14] B.K. Meyer, D.M. Hofmann, W. Stadler, M. Salk, C. Eiche and K.W. Benz, Mater. Res. Soc. Symp. Proc., Vol. 302 (Pittsburgh, 1993) p. 189. [15] M. Hage-Ali and P. Siffert, Nucl. Instrum. Methods A 322 (1992) 313. [16] B. Biglari, M. Samini, M. Hage-Ali, J.M. Koebel and P. Siffert, Nucl. Instrum Methods A 283 (1989) 249. [17] K. Grasza, J. Crystal Growth 146 (1995) 65.
CRYSTAL G R O W T H
ELSEVIER
Journal of Crystal Growth 146 (1995) 125-129
Characterization of cadmium telluride crystals grown by different techniques from the vapour phase M . L a a s c h a,., R . S c h w a r z a, W . J o e r g e r a, C. E i c h e a, M . F i e d e r l e a, K . W . B e n z a, K. Grasza b a Kristallographisches Institut, Universitiit, Freiburg, Hebelstrasse 25, D-79104 Freiburg, Germany b Institute of Physics, Polish Academy of Sciences, AI. Lotnikow 32/46, PL-02-668 Warsaw, Poland
Abstract Semi-insulting CdTe bulk crystals were grown from the vapour phase in both closed and semi-open arrangements. The results of the growth experiments are discussed in terms of various electrical and optical characterization methods. Van der Pauw measurements and time dependent charge measurements (TDCM) were used to determine the resistivity. Deep level defects were investigated by means of photoinduced current transient spectroscopy (PICTS). For one of the most important fields of application, detector spectra of the vapour phase material are measured and discussed.
1. Introduction The applicability of CdTe as a material for room-temperature X-ray and y-ray detectors has generated interest in high resistivity bulk material and its connection with crystalline perfection and low residual impurity content. Vapour phase growth of CdTe has been shown to offer the opportunity of obtaining high quality crystals by avoiding imperfections due to the high temperatures necessary in melt growth [1,2]. To succeed in the growth of high resistivity material, it is necessary to either prevent intrinsic electrical active defects occurring or compensate for free carriers. Semi-open systems generally allow for the possibility of influencing the vapour composition by removing an excess component or
* Corresponding author.
volatile impurities and thus facilitate the prevention of defect formation whereas in cases of closed ampoules, compensation is most frequently achieved by chlorine doping. In recent studies, other halogens such as bromine and iodine have been shown to be possible dopants [2]. Experiments both on seeded growth in closed ampoules and on unseeded growth in semi-open arrangements were performed. Our work focused on the study of the electrical and optical properties of the CdTe crystals obtained by the different growth techniques.
2. Growth techniques 2.1. Vapour growth in closed ampoules
Chlorine-doped CdTe crystals were grown in closed ampoules using the sublimation travelling
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00474-9
M. Laasch et al. /Journal of Crystal Growth 146 (1995) 125-129
126
heater method (STHM). The evacuated growth ampoule containing both an undoped seed and d o p e d (1019-10 20 c m -3, obtained by Bridgman method) feed material, separated by an 8 mm
[~J
vapour zone, is moved vertically through a temperature maximum (825°C) with a pulling rate of 1-2 mm per day; the sublimation and deposition temperatures are 815 and 770°C, respectively.
[~33 2. l~=E 4-09
2 2~;. 0 -
2 . 4 7 E ~ 20.0
-
2 . GfTE÷~ 15 • 0 -
1 • 81E +09 1. ~'~1~
10.0
1. ~.~D~ 1.41E-HD9
5.0
1.28E.HD9 1 • l:$E.e.Og
(a)
,,
12.0
.£W
, , , ,
, , , ,
, , , ,
, , , i
I
,
tW
[S~(:=n]
]
[n~]] 3.. 96E[+09
10.0
.IL. 8 7 E + 0 9 8.0
1.78E+09 6.0
1.69E
4.0
+09
.11.. 6 0 E + 0 9 1.51E+09
2.0
1. 421[ ÷OS --0.0
4
--2.0
-4.o
1.34E
-I-09
.I.. ~
+09
1.16E+09
I
1.07E+09
-6.0q
9 • 80E+08 -8.01 --10.0
(b)
8.9.1LE+08
],,,, --10.0
g • 02E~*08
--51-0
-o1.0
5[0
rx
[l~rt ]
Fig. 1. Resistivity mappings obtained by TDCM. (a) CdTe : CI, axial section, y = growth direction; (b) Nominally undoped CdTe, radial section, x - y plane perpendicular to the growth direction.
M. Laasch et al. /Journal of Crystal Growth 146 (1995) 125-129
This technique is described in principal in Ref. [2]. Additionally, experiments using a self-seeding procedure were performed.
2.2. Vapour growth in semi-open arrangements The problem of diffusion and heat transfer leading to growth instabilities and crystal imperfections when growing crystals from the vapour has been discussed in great detail (cf. Refs. [3,6]). Using semi-open growth arrangements (cf. Refs. [4,5]), the effect of forming a third phase and impurity incorporation can be avoided by removing both excess species as well as impurities from the growth region applying, for example, a heat sink or an effusion hole. A hydrogen atmosphere (130 Torr at room temperature) additionally facilitates the growth without the risk of constitutional supersaturation [17]. A detailed description of the technique used for unseeded CdTe crystal growth is given in Ref. [1]. Here, the formation of a feed cone and the creation of a high quality seed on a quartz crystal holder attached to the ampoule is governed by a suitable temperature regime. Wall-free crystal growth takes place in a low temperature gradient using a pulling velocity of 4 mm per day and a sublimation temperature of 960°C. The cooling rate applied to the crystal investigated in this work was 300°C/h.
3. Material characterization and discussion
For physical characterization, the crystals were sliced both parallel and perpendicular to the growth axis. The samples were lapped and polished mechanically down to a surface roughness of 1 tzm. Gold (PICTS) and silver were used as contact materials.
3.1. Resistivity determination The resistivities measured by means of the Van der Pauw method at 303 K were 3 x 109 l~.cm (Cl-doped material, closed system) and 2 x 10 9 [-~'cm (nominally undoped material, semi-open system).
127
Time dependent charge measurements (TDCM) allow for contact free resistivity determination within the range from 1 x ]07 to 2 x 10 ~° 11. cm. Axial and radial crystal sections were used for measuring the two-dimensional resistivity distribution (see Fig. 1) within the vapour phase crystals with a spatial resolution of about 0.5 x 0.5 mm 2. From the two-dimensional image, maximum deviations of about 50% in the radial resistivity distribution can be seen in both crystals. Decreases in resistivity in the outer parts of the samples are probably due to wall effects in the CdTe : CI material, whereas in the undoped material, defects may occur in these regions due to instabilities caused by the vapour stream towards the temperature sink. In the axial section of the chlorine-doped crystal (Fig. la), no significant segregation behaviour is observed.
3.2. Photoinduced current transient spectroscopy (PICTS) The investigation of deep levels in high-resistivity CdTe has been performed by recording the photoinduced current transients [7] stepwise within a temperature range 80-300 K. The activation energy, E, as well as the capture cross-section, o-, but not the concentration of the deep levels can be determined mathematically from the temperature dependence of the thermal relaxation times. Arrhenius plots of samples grown by the different methods from the vapour are shown in Fig. 2. Generally, the defect number is clearly higher in undoped CdTe. In this respect, due to the related purification effect, using Bridgman grown source material, as is done in CdTe:C1 growth, seems to be advantageous. On the other hand, tensions related to the fast cooling rate and the different volume expansion coefficients of CdTe and quartz seem to be an important defect origin. Two traps (T6, T8), identical in both undoped and doped material, have been detected. The height distribution of the peak maxima in the PICTS spectrum (which is related to the defect concentration) is quite uniform for most defects. T6 traps in undoped CdTe as well as T4, T5 and T7 traps show significantly higher intensities.
M. Laasch et al. /Journal of Crystal Growth 146 (1995) 125-129
128
10
, "
' I ' ['
'
9-_
I ' I ' I ' I ' I'
I ' I ' li''
T9
7-
c '--'
6" 5-
~
4.
vIT4:TI:' 0.210"08 I' I eveV T6:0.28
eV
T9:0.55
eV
T8
3'
T1
2-
'0
I") -i
,i
2
,I
3
,i
4
5
i ,I ,i ,~, 6 7 8 9
i ,i ,i ,i ,i, i 10 11 12 13 14 15 16
IO00/T [K-']
9TIO
T8
8v
7-
c
6-
T7 v
5-
7--I-.
4-
c
3"
1- £o) 0
2
3
4
5
6
7
~ 8 9
10 11
12 13 14 15 16
1000/T [K-'] Fig. 2. Arrhenius plot obtained by means of PICTS in C d T e : C1 grown in closed ampoules (a) and in nominally undoped C d T e grown in a semi-open arrangement (b).
The traps T1 and T4 are related to compensation defects and in good agreement with published results (e.g. Refs. [8,9], see Table 1). The complex [V2dCl~e] -, the so-called A-center [10], may be responsible. Even in nominally undoped material, low amounts of halogens incorporated as impurities could probably cause the PICTS signal (T5 [11]). This observation underlines the importance of using high purity source material for vapour transport. In both undoped and doped material, the traps T2 and T6 probably arise from residual impurities such as Au, Ag, Cu, whereas T7 traps could be caused by both impurities and structural defects [9,12]. In particular, Au is assumed to be a possible origin of T6 traps due to diffusion of the contact material (Au) into the crystal matrix [13]. T7 traps have not been detected in CdTe:C1, a possible explanation is the dopant incorporation (e.g. formation of the [V2d-Cl~e] - complex). The origins of T8 traps detected both in undoped and doped material are structural defects such as Cd~ + [9]. The T9 and T10 traps are hitherto unidentified. However, a deep donor level (due to an intrinsic defect) seems to be particularly important for compensation and is probably responsible for the high resistivity of the material [14]. An overview of the detected deep levels is given in Table 1.
Table 1 Activation energies and capture cross-sections of the deep levels found in vapour grown undoped and chlorine-doped C d T e Trap
STHM (Cl-doped)
Semi-open
Literature data
Energy
(nominally undoped)
Energy
Energy
(eV)
tr (cm 2)
(eV)
~r (cm 2)
Assignation Ref.
(eV) T1 T2 T3 T4 T5 T6 T7 T8 T9 T10
0.08
10-16
0.21
10 I1
0.28
10 - 12
0.56 0.55
10- 9 10-15
* = hitherto unidentified.
0.11 0.14
10-17 10 -12
0.21 0.28 0.34 0.56
10 . 9 10 - 12 10-13 10- 8
0.51
10 -14
0.06 0.1-0.2 0.1-0.2 0.28 0.3-0.35 0.56
[81 [16] e.g. [9] [12] e.g. [9,11] [12]
[V~d- 2Cl{e ] Cu * [Vdd2- CITe ]+ -
_
_
*
-
-
*
Au, Ag Au, Ag, Al Cd 2 +
M. Laasch et al. / Journal of Crystal Growth 146 (1995) 125-129
3. 3. Detector properties In addition to high resistivities, the quality enhancement of detector devices arises from increasing the mobility-lifetime (~zz) product of the free carriers. Due to structural defects, which are more difficult to suppress than impurities, maximum /zr products are approximately 10 -3 cm 2 V -1 [15]. /zz products for electrons and holes measured in chlorine-doped CdTe are/ze% = 5.66 × 10 -5 cm 2 V -I a n d / z h r h = 1 × 10 -5 cm 2 V -1, respectively. In undoped material, lower values have been measured: p.eZe=9.5 × 10 -6 cm 2 V -~ and /zhr h = 1 x 10 -6 cm 2 V -1. These results correlate with the high number of defects detected. Accordingly, the detector efficiency (a-radiation) of the CdTe : CI crystal is 90%, whereas only 12% efficiency has been measured for nominally undoped material.
4. Conclusion High resistivity (up to 109 D . cm) cadmium telluride crystals have been grown from the vapour both in closed and semi-open arrangements. By means of TDCM, the high homogeneity of the resistivity distribution has been shown. In contrast to chlorine-doped material, the detector efficiency of the undoped material grown within a semi-open arrangement is less than satisfactory, this correlates with the high number of defects which were found by PICTS. The main reason is probably the fast cooling rate applied to this crystal.
Acknowledgements The authors are indebted to Mrs. I. Koch and Mr. M. Kranz-Probst for skilled experimental
129
help. The work has been supported financially by the Federal Ministry of Research and Technology (BMFT) under project management by D L R and D A R A (contract no. 50QV88083).
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