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Vertical unseeded vapor growth of large CdTe crystals
Journal of Crystal Growth 60 (1982) 343—348 North-Holland Publishing Company
343
VERTICAL UNSEEDED VAPOR GROWTH OF LARGE CdTe CRYSTALS N. YELLIN, D. EGER and A. SHACHNA Soreq Nuclear Research Centre, Yavne, Israel
Received 6 April 1982; manuscript received in final form 25 July 1982
Single crystals of CdTe were grown by the vertical, unseeded vapor growth technique. The faceted, high quality crystals showed higher purity than the 6N source materials. The growth rate was found to be strongly dependent on the excess Te present in the growth charge. A maximum growth rate of about 2 g/day was observed at about 0.00 1—0.02 mol% excess Te at charge preparation. X-ray analysis and metallography of the crystals and mechano-chemical polished slices revealed high quality monocrystallinity. The crystals showed high resistivity (~iø~t2 cm) and did have nuclear radiation detection properties.
1. Introduction Perfect cadmium telluride single crystals of very high purity and high electrical resistance are required for nuclear radiation detectors and optoelectronic devices [1]. Several methods of growing large CdTe crystals from melt and solution have been extensively studied [1,2]. While these techniques provide large single crystals in relatively short growth times, it is difficult to obtain high resistivity material without doping or to obtain material of high crystalline quality. The closed tube vapor transport method appears to be promising for CdTe crystals [3—7]with a high degree of compositional uniformity, low density of imperfections and high resistivity. However, the use of vapor growth is limited because growth rates are generally low and the ultimate crystal size is frequently small. In this study, preliminary results of CdTe crystal growth by the vertical unseeded vapor growth (VUVG) technique are reported. This technique was successfully applied to grow PbTe, Pb1 ~Sn~Te [8,9] and a-GeTe [10]. —
freshly etched 6N Cd, both purchased from Corninco American as DZR materials. No Cd or Te reservoir, which usually controls the partial pressures of Cd and Te2 during growth, is used in the present technique. Instead, the partial pressures are controlled by the initial deviation of the growth charge from stoichiometry. The amounts of Te and Cd by weight determined the deviation from stoichiometry 8Te (0 ~ ôTe ~ 0.1 mol%). The elements were sealed under high vacuum in a cleaned, high purity quartz ampoule (~ 23 mm, 1= 5—10 cm). The ampoule was heated to 900°Cfor 5—7 days and slowly cooled to room temperature. X-ray diffraction analysis showed that the elements reacted completely (sensitivity to elemental Te residues in CdTe is about 0.1 mol%). The material was broken into a few lumps and transferred to the growth ampoule. A detailed description of the experimental VUVG technique has been given previously [8—10]. Application of the VUVG technique to CdTe crystal growth required only minor adjustments. Fig. 1 shows the temperature profile of the furnace and the relative positions of the growth ampoule during the VUVG process. The growth temperature was 950°C. The rate at which the ampoule was raised from the nucleation position was about 0.5 cm/day, up to a maximum temperature difference of 5°Calong the growth ampoule. When about half of the initial charge was transported to -~
2. Experimental Stoichiometric and Te-rich growth charges of 30—60 g were prepared by weighing 6N Te and 0022-0248/82/0000—0000/$02.75
©
1982 North-Holland
—~
344
Quartz rod
5
1 el/in er al.
t~ errical uncee~leJrapor groirth at large ( ~lTe ~rr.itak
1-~1 I
I
Crystal Nucleus
6Te > 0.01 mol~’. It should he noted that the excess Te was unequally distributed in the (dTe charge. The crystal gro~th rate was found to he affected by STe. The results are shown in fig. 2. For ~Te >0.02 mol%, the growth rate (GR) was independent of STe. having a value of OR 0.33 0.15 g/day. Stoichiometric growth charges (STe
I
0) yielded similar values. For charges with 6Te varying in the range 0.001 > 6Te >0.02 mol~. growth rates were higher, up to 2.1 g/day. Experiments with OR 0.33 g/day showed aconstant mass transport rate during the whole growth. However, for 0.001 ~ ~Te ~ 0.02 mol% faster transport was observed in the first few hours of the growth process. then the transport rate de-
Ampule
Charge
c
creased with increasing time of growth. This is shown in fig. 3. The shorter the growth time, the higher the mass transport rate. The growth rates given in fig. 2 for 6Te 0.01 mol~and 6Te 0.001 mol% were measured for a growth time of 7 days. During this time about half of the original charge
b
936~ 950 T(°C) Fig. I. Temperature profile and growth ampoule positions in the furnace during VUVG of CdTe: (a) reverse transporh (h) nucleation. (c) final position.
was transported. The dependence of the crystal growth rate on the Cd and Te 2 partial pressures in closed vapor growth systems has been extensively studied [II — l5J. It was found that the transport rate is considerably lower when P0~1>~P~d or ~ ran’ compared with situations where P( FrOun~
the grown crystal. the ampoule was slowly cooled to room temperature.
3. Results and discussions 3.1. Effects of stoichiometry’ on the crtstal growth rate
The deviation from stoichiometry of the charges varied between 0~8Te~0.l mol%. on weighing.
P(>ajr
or
2
.~‘
I
~
Some minute loss of Te occurred on ampoule loading. It could be observed visually. hut was impossible to prevent. Hence, a charge with a given weight of 6Te had a lower effective ~Te during growth. X-ray diffraction on the annealed CdTe charges before loading growth ampoule sometimes showed elementalthe Te (diffraction angle of 27.6°)if
d
[Ill. Similar results were obtained
“—
—
S
0
—
0
005 ~Te (mol %)
Fig 2. (rvstal growth rate (g. da~)dependence on the de~ia8le lion from stotchiomeirv of the growth charge preparation, nioFi).
N. Ye//in ci al.
/
Vertical unseeded vapor growth of large CdTe crystals
345
expected as well. A stoichiometric charge should 8Te mol
3.0
0
-
•
\
•
exhibit the maximum growth rate, where ~Cd = 2P Tc,. . As for the Cd-rich charges a smaller deviation from stoichiometry will reduce the growth rate compared with that on the Te-rich side, because the Te2 partial pressure in Te-rich CdTe is
0/
0001 ~Ol O.O~
I,. ‘~
2,0
0
-
0
0
I 20
30
Time of growth(doys)
Fig. 3. Crystal growth rate (g/day) dependence on growth time, in days, for growth charges with 0.001 ~ 8Te~0.02mol% at charge preparation.
by Tuller et al. [16] using an open growth system with a gaseous carrier. In the VUVG technique, the Cd and Te2 partial pressures are determined by the initial deviation from stoichiometry, 6Te, of the charge. With ~Te > 0.02 mol% the excess Te present in the charge is assumed to exceed the existence region of solid CdTe at the growth temperature (—~950°C), as expected from the expanded phase diagram [1]. Hence, Te-rich solid CdTe and liquid Te solution saturated with CdTe probably exist as two distinct phases. Any increase in STe will be added to the liquid phase and will not affect the partial pressures of the elements. This explains the constant growth rate observed for charges with 6Te > 0.02 mol%. In these cases, ~Te, >> ~cd’ with the mass transport rate being determined by the Cd atom diffusion in the growth tube [15]. The growth rate is relatively low. When reducing ~Te to Te concentrations in the existence region of Te-rich CdTe at the growth temperature, ~Te ~ 0.02 mol%, an increase in ~(d is expected. Hence, higher mass transport rates are
much lower than that of Cd in Cd-rich CdTe [1]. A higher growth rate is indeed observed for i~Te~0.02mol%, as compared with that for STe >0.02 mol% (fig. 2). However, the maximum transport rate was observed for 6Te>0.00l mol% and not for STe 0. Charges with ôTe = 0 are assumed to represent a Cd-rich system in the crystal growth process. The same arguments explaining the Te-rich charge behavior hold in this case; however, ~Cd > ~Te, and the mass transport rate is limited by the Te2 diffusion. Further studies on growth charges with ~Te<0 at growth preparation are in progress. The results shown in fig. 2 are “shifted” towards higher ~Te values, compared with expected values from the phase diagram because the effective ~Te value during growth is lower than the initial STe, due to Te loss during ampoule loading. The change in growth rate as the growth proceeds (in charges with 0.001 ~~Te~0.02 mol%) may be attributed to the following reasons: (a) The constantly increasing distance between nucleation area and growth interface results in a decreasing temperature difference between growth and evaporation interfaces. (b) The evaporation interface area decreases with growth time, since the initial CdTe lumps form a boule at the bottom of the growth ampoule. The effect of surface reaction on CdTe mass transport rate for low ~Te2 values was mentioned previously [151. (c) The distance between evaporation and growth interfaces increases with growth time. This effect is quite critical; with too long a distance between the two interfaces (d~’ 10 cm), no mass transport was observed. The mass transport rate was found to be independent of ~Te for 0.001 ~~Te~0.02 mol%. This is shown in figs. 2 and 3. In other words, in this ~Te region the partial pressure of Te2 does not affect the crystal growth rate, as was suggested previously [15].
346
N. Ye//in ci al.
/
Vertical unseeded vapor growth of large (‘riTe cri’~,tal,v
3.2. Evaluation of crystals
controlled nucleation occurs. Once a high quality
3.2.1. Crystalline quality In most cases single crystals of about 1 inch in diameter and 15 g in weight were obtained. Occasionally small facets of (100), (Ill) or (110) planes were observed. The crystals were cut with a wire saw, and the cutting damage removed by mechano-chemical polish with Br 5/CH~OH solution, The crystals. as well as polished slices, were optically shiny, smooth and clear, as seen in fig. 4. No voids, microscopic scattering centers or inclusions could be detected by metallography or by IR transmission microscopy. Laue reflection photographs of crystals and polished slices showed monocrystallinity of the entire crystal. Twinning, which is well known in CdTe, was quite limited. Sometimes twin-free crystals were obtained. Berg—Barrett X-ray topography showed very few low angle grain boundaries. The etch pit density (EPD) was determined on
nucleus has formed, a large crystal of the same quality is usually obtained. 3.2.2. Crystal purity Emission spectrography (sensitivity 0.1 1 ppm) was used for the determination of impurities. The results are only semiquantitative. Since growth is stopped after half of the initial charge is transported a disproportionation of the impurity hetween crystal and charge is expected, similar to distillation. Typical results of the spectrographic analysis of source elements, growth charge. crystal and residue are given in table 1. case a. Since the impurity concentrations were close to the sensitivity limit of the analysis. growth charges doped with the common impurities Cu. Pb and Si were studied. The results for these cases are given in table I. case b. Si, Mg, Fe and Al appear in the blank determination because of their presence in the gra-
(Ill) oriented polished slices after etching to reveal dislocations [17]. The EPD was found to be
phite spectrograph electrodes. Therefore, their detection in quantities of < I ppm in the materials is
almost4/cm2. constant in acrystals given crystal, below Some showedusually an EPD as I >< as l0 2 x 103/cm2. This value is among the lowest low reported for CdTe. The high quality of CdTe crystals obtained by the VUVG technique is probably due to the selfnucleation, carried out under controlled conditions. When using a seed, the crystal quality depends critically on the seed quality. Therefore, self-nucleation is superior. In the growth ampoule used [8—10]the quartz rod creates a “colder point” (—~T—~ 2°C) on the ampoule walls, and slow and
questionable. Thetopurity was usually similar that of of the thegrowth sourcecharge elements. except for intentional doping. i.e. no contamination occurred during charge preparation, within the sensitivity of the analysis. The results in table I clearly indicate that purification from Cu. Ph and Si occurs during the vapor transport.
,
Undoped crystals showed room temperature resistivities of > 1O~~2 cm, measured by the Van der Pauw method. Hall effect could not be observed in
~m tittitilti
these samples. Doped crystals showed resistivities
in the range of l0~—l0~’~ cm. IR transmission was recorded between 2—40 p.m with a Beckman lR spectrometer. The samples, doped and undoped. b
Fig. 4. tdl’c single crystal:
3.2.3. Physical properties Physical characterization of the CdTe crystals included electrical and optical measurements. In some cases, lifetime measurements and evaluation for nuclear radiation detection were carried out.
t,i .is-grown crs~ial. (h) mechanochemical polished (Ill) orienied slice.
:~~c~e~t transmission curve up to 30 p.m. Lifetime was measured by the photoconductivity decay method. Short laser (Nd: Yag) pulses of 1.06 p.m (~t = 3 ns) were used either as a direct
N. Yellin ci a!.
/
Vertical unseeded vapor growth of large CdTe crystals
347
Table I Emission spectrography analysis for impurities in CdTe, in ppm (+, traces)
Blank Cd Te (a) Charge Crystal Residue (b) Charge Crystal Residue
Cu
Ag
—
-
-
—I <0.5 <0.5 + —5 —5 >5
+
—
—
—
‘—50
Pb
Mg
Fe
Si
Al
+ <0.5 <0.5
+ ‘—1 <0.5
+ —1
+ + <0.5
<0.5 <0.5 <0.5 “—0.5 1—5
—0.5 <0.5 “—0.5 1—5 —1
—0.5 <0.5
“—10 —10
<0.5 —0.5 <0.5 ~0.5 <0.5
“-100
<0.5
1—S
—
—
—
—
+ + —
+
—
light source or through a KDP frequency multiplier which converted the light wavelength to 0.54 p.m. The absorption coefficient of CdTe for 1.06 p.m wavelength (JR light) is much lower than that for 0.54 p.m (visible light), since in the first case radiation is observed by the two-photon process only, while in the second case by direct band-toband excitation. The decay time of a high quality crystal was T 30 ns, independent on the wavelength used. This shows that ‘r was not affected by the surface conditions. The decay curve was purely exponential with a single decay time, indicating the lack of slow trapping. Based on the value of “-‘
30 ns, for E
500 V/cm, a drift length of
I
mm is expected. Although the T values obtained for the best THM grown CdTe crystals [18] are much higher, 1 ~is,the drift length of our crystal should permit the detection of nuclear radiation. Radiation detection was studied on 2 >< 2 X 2 mm3 samples. Indium contacts were placed on the front face while silverpaint was used for back contacts. The bias was 100 V. The nuclear radiation response was measured with an Elcint multichannel analyzer with 57Co( 127 keV), 37Cs(660 keV) and 60Co(1300 keV) as radiation sources. Only undoped. high purity samples showed radiation response. The pulse heights were proportional to the radiation energies. It was quite difficult to estimate the charge collection efficiency; however, it seems to be lower than that of a similar commercial RMD CdTe detector. No polarization effect was observed, probably due to the high purity of the crystals. —-
“—1
“—5
4. Conclusions (a) Large CdTe single crystals of high quality were obtained by the VUVG technique. (b) Deviation from stoichiometry, 6Te, of the growth charge determines the crystal growth rate for 0~8Te’~0.lmol%. (c) Further purification of the source material is achieved during the vapor transport process. (d) The crystals show promising results for application as nuclear detectors.
Acknowledgments We are indebted to A. Zemel and J. Eisen for nuclear radiation detection measurements, and to H. Shaham and E. Sheinfeld for analytical assistance.
—‘
References [I] K. Zanio, in: Semiconductors and Semimetals, Vol. 13,
CdTe (Academic Press. New York, 1978). [2] Proc. Intern. Conf. on CdTe [Rev. Phys. AppI. 12 (1977)]. [3] W. Akutagawa and A. Zanio, J. Crystal Growth 11(1971) 191. [4] P. Buck and R. Nilsche, J. Crystal Growth 48 (1980) 29. [5] C.B. Norris, J. Electron. Mater. 9 (1980) 499. [6] A.A. Glebbin. A.A. Davydov and N.!. Garba, lzv. Akad. Nauk SSSR, Neorg. Mater. 16 (1980) 28. [7] Z. Gotacki, M. Górska, J. Makowski and A. Szczerbakow, J. Crystal Growth 56 (1981) 213.
348
N. Ye//in ci al. / Vertical unseeded vapor groii’ih o/ large (‘dTe cr1’
[8] N. Tamari and H. Shtrikman. J. (‘rystal Growih 43 (1978) 378. [9] N. Tamari and H. Shtrikman, J. Electron. Mater. 8 (1979) 269. [10] N. Yellin and G. Gafni, J. Crystal Growth 53 (1981) 409. [II] D. de Nobel, Philips Res. Rept. 14 (1959) 361. 431. [12] K. Mochizuki. J. Cry
[IS] K. Igaki and K. Mochizuki. J. Crystal (iro~tli 24, 25 (1974) 162. [16] HF. Tuller, K. t,Jematsu and F1.K. Iloweri. .1. (rystal Growth 42 (1977)15)). [17) M. Inoue. I. Teramoto and S. ‘Fakasanagi .J..Appl. Ph\v 33 (1962) 2578. [181 P. Siffert. J. Phssique 39 1978) (3-40.
343
VERTICAL UNSEEDED VAPOR GROWTH OF LARGE CdTe CRYSTALS N. YELLIN, D. EGER and A. SHACHNA Soreq Nuclear Research Centre, Yavne, Israel
Received 6 April 1982; manuscript received in final form 25 July 1982
Single crystals of CdTe were grown by the vertical, unseeded vapor growth technique. The faceted, high quality crystals showed higher purity than the 6N source materials. The growth rate was found to be strongly dependent on the excess Te present in the growth charge. A maximum growth rate of about 2 g/day was observed at about 0.00 1—0.02 mol% excess Te at charge preparation. X-ray analysis and metallography of the crystals and mechano-chemical polished slices revealed high quality monocrystallinity. The crystals showed high resistivity (~iø~t2 cm) and did have nuclear radiation detection properties.
1. Introduction Perfect cadmium telluride single crystals of very high purity and high electrical resistance are required for nuclear radiation detectors and optoelectronic devices [1]. Several methods of growing large CdTe crystals from melt and solution have been extensively studied [1,2]. While these techniques provide large single crystals in relatively short growth times, it is difficult to obtain high resistivity material without doping or to obtain material of high crystalline quality. The closed tube vapor transport method appears to be promising for CdTe crystals [3—7]with a high degree of compositional uniformity, low density of imperfections and high resistivity. However, the use of vapor growth is limited because growth rates are generally low and the ultimate crystal size is frequently small. In this study, preliminary results of CdTe crystal growth by the vertical unseeded vapor growth (VUVG) technique are reported. This technique was successfully applied to grow PbTe, Pb1 ~Sn~Te [8,9] and a-GeTe [10]. —
freshly etched 6N Cd, both purchased from Corninco American as DZR materials. No Cd or Te reservoir, which usually controls the partial pressures of Cd and Te2 during growth, is used in the present technique. Instead, the partial pressures are controlled by the initial deviation of the growth charge from stoichiometry. The amounts of Te and Cd by weight determined the deviation from stoichiometry 8Te (0 ~ ôTe ~ 0.1 mol%). The elements were sealed under high vacuum in a cleaned, high purity quartz ampoule (~ 23 mm, 1= 5—10 cm). The ampoule was heated to 900°Cfor 5—7 days and slowly cooled to room temperature. X-ray diffraction analysis showed that the elements reacted completely (sensitivity to elemental Te residues in CdTe is about 0.1 mol%). The material was broken into a few lumps and transferred to the growth ampoule. A detailed description of the experimental VUVG technique has been given previously [8—10]. Application of the VUVG technique to CdTe crystal growth required only minor adjustments. Fig. 1 shows the temperature profile of the furnace and the relative positions of the growth ampoule during the VUVG process. The growth temperature was 950°C. The rate at which the ampoule was raised from the nucleation position was about 0.5 cm/day, up to a maximum temperature difference of 5°Calong the growth ampoule. When about half of the initial charge was transported to -~
2. Experimental Stoichiometric and Te-rich growth charges of 30—60 g were prepared by weighing 6N Te and 0022-0248/82/0000—0000/$02.75
©
1982 North-Holland
—~
344
Quartz rod
5
1 el/in er al.
t~ errical uncee~leJrapor groirth at large ( ~lTe ~rr.itak
1-~1 I
I
Crystal Nucleus
6Te > 0.01 mol~’. It should he noted that the excess Te was unequally distributed in the (dTe charge. The crystal gro~th rate was found to he affected by STe. The results are shown in fig. 2. For ~Te >0.02 mol%, the growth rate (GR) was independent of STe. having a value of OR 0.33 0.15 g/day. Stoichiometric growth charges (STe
I
0) yielded similar values. For charges with 6Te varying in the range 0.001 > 6Te >0.02 mol~. growth rates were higher, up to 2.1 g/day. Experiments with OR 0.33 g/day showed aconstant mass transport rate during the whole growth. However, for 0.001 ~ ~Te ~ 0.02 mol% faster transport was observed in the first few hours of the growth process. then the transport rate de-
Ampule
Charge
c
creased with increasing time of growth. This is shown in fig. 3. The shorter the growth time, the higher the mass transport rate. The growth rates given in fig. 2 for 6Te 0.01 mol~and 6Te 0.001 mol% were measured for a growth time of 7 days. During this time about half of the original charge
b
936~ 950 T(°C) Fig. I. Temperature profile and growth ampoule positions in the furnace during VUVG of CdTe: (a) reverse transporh (h) nucleation. (c) final position.
was transported. The dependence of the crystal growth rate on the Cd and Te 2 partial pressures in closed vapor growth systems has been extensively studied [II — l5J. It was found that the transport rate is considerably lower when P0~1>~P~d or ~ ran’ compared with situations where P( FrOun~
the grown crystal. the ampoule was slowly cooled to room temperature.
3. Results and discussions 3.1. Effects of stoichiometry’ on the crtstal growth rate
The deviation from stoichiometry of the charges varied between 0~8Te~0.l mol%. on weighing.
P(>ajr
or
2
.~‘
I
~
Some minute loss of Te occurred on ampoule loading. It could be observed visually. hut was impossible to prevent. Hence, a charge with a given weight of 6Te had a lower effective ~Te during growth. X-ray diffraction on the annealed CdTe charges before loading growth ampoule sometimes showed elementalthe Te (diffraction angle of 27.6°)if
d
[Ill. Similar results were obtained
“—
—
S
0
—
0
005 ~Te (mol %)
Fig 2. (rvstal growth rate (g. da~)dependence on the de~ia8le lion from stotchiomeirv of the growth charge preparation, nioFi).
N. Ye//in ci al.
/
Vertical unseeded vapor growth of large CdTe crystals
345
expected as well. A stoichiometric charge should 8Te mol
3.0
0
-
•
\
•
exhibit the maximum growth rate, where ~Cd = 2P Tc,. . As for the Cd-rich charges a smaller deviation from stoichiometry will reduce the growth rate compared with that on the Te-rich side, because the Te2 partial pressure in Te-rich CdTe is
0/
0001 ~Ol O.O~
I,. ‘~
2,0
0
-
0
0
I 20
30
Time of growth(doys)
Fig. 3. Crystal growth rate (g/day) dependence on growth time, in days, for growth charges with 0.001 ~ 8Te~0.02mol% at charge preparation.
by Tuller et al. [16] using an open growth system with a gaseous carrier. In the VUVG technique, the Cd and Te2 partial pressures are determined by the initial deviation from stoichiometry, 6Te, of the charge. With ~Te > 0.02 mol% the excess Te present in the charge is assumed to exceed the existence region of solid CdTe at the growth temperature (—~950°C), as expected from the expanded phase diagram [1]. Hence, Te-rich solid CdTe and liquid Te solution saturated with CdTe probably exist as two distinct phases. Any increase in STe will be added to the liquid phase and will not affect the partial pressures of the elements. This explains the constant growth rate observed for charges with 6Te > 0.02 mol%. In these cases, ~Te, >> ~cd’ with the mass transport rate being determined by the Cd atom diffusion in the growth tube [15]. The growth rate is relatively low. When reducing ~Te to Te concentrations in the existence region of Te-rich CdTe at the growth temperature, ~Te ~ 0.02 mol%, an increase in ~(d is expected. Hence, higher mass transport rates are
much lower than that of Cd in Cd-rich CdTe [1]. A higher growth rate is indeed observed for i~Te~0.02mol%, as compared with that for STe >0.02 mol% (fig. 2). However, the maximum transport rate was observed for 6Te>0.00l mol% and not for STe 0. Charges with ôTe = 0 are assumed to represent a Cd-rich system in the crystal growth process. The same arguments explaining the Te-rich charge behavior hold in this case; however, ~Cd > ~Te, and the mass transport rate is limited by the Te2 diffusion. Further studies on growth charges with ~Te<0 at growth preparation are in progress. The results shown in fig. 2 are “shifted” towards higher ~Te values, compared with expected values from the phase diagram because the effective ~Te value during growth is lower than the initial STe, due to Te loss during ampoule loading. The change in growth rate as the growth proceeds (in charges with 0.001 ~~Te~0.02 mol%) may be attributed to the following reasons: (a) The constantly increasing distance between nucleation area and growth interface results in a decreasing temperature difference between growth and evaporation interfaces. (b) The evaporation interface area decreases with growth time, since the initial CdTe lumps form a boule at the bottom of the growth ampoule. The effect of surface reaction on CdTe mass transport rate for low ~Te2 values was mentioned previously [151. (c) The distance between evaporation and growth interfaces increases with growth time. This effect is quite critical; with too long a distance between the two interfaces (d~’ 10 cm), no mass transport was observed. The mass transport rate was found to be independent of ~Te for 0.001 ~~Te~0.02 mol%. This is shown in figs. 2 and 3. In other words, in this ~Te region the partial pressure of Te2 does not affect the crystal growth rate, as was suggested previously [15].
346
N. Ye//in ci al.
/
Vertical unseeded vapor growth of large (‘riTe cri’~,tal,v
3.2. Evaluation of crystals
controlled nucleation occurs. Once a high quality
3.2.1. Crystalline quality In most cases single crystals of about 1 inch in diameter and 15 g in weight were obtained. Occasionally small facets of (100), (Ill) or (110) planes were observed. The crystals were cut with a wire saw, and the cutting damage removed by mechano-chemical polish with Br 5/CH~OH solution, The crystals. as well as polished slices, were optically shiny, smooth and clear, as seen in fig. 4. No voids, microscopic scattering centers or inclusions could be detected by metallography or by IR transmission microscopy. Laue reflection photographs of crystals and polished slices showed monocrystallinity of the entire crystal. Twinning, which is well known in CdTe, was quite limited. Sometimes twin-free crystals were obtained. Berg—Barrett X-ray topography showed very few low angle grain boundaries. The etch pit density (EPD) was determined on
nucleus has formed, a large crystal of the same quality is usually obtained. 3.2.2. Crystal purity Emission spectrography (sensitivity 0.1 1 ppm) was used for the determination of impurities. The results are only semiquantitative. Since growth is stopped after half of the initial charge is transported a disproportionation of the impurity hetween crystal and charge is expected, similar to distillation. Typical results of the spectrographic analysis of source elements, growth charge. crystal and residue are given in table 1. case a. Since the impurity concentrations were close to the sensitivity limit of the analysis. growth charges doped with the common impurities Cu. Pb and Si were studied. The results for these cases are given in table I. case b. Si, Mg, Fe and Al appear in the blank determination because of their presence in the gra-
(Ill) oriented polished slices after etching to reveal dislocations [17]. The EPD was found to be
phite spectrograph electrodes. Therefore, their detection in quantities of < I ppm in the materials is
almost4/cm2. constant in acrystals given crystal, below Some showedusually an EPD as I >< as l0 2 x 103/cm2. This value is among the lowest low reported for CdTe. The high quality of CdTe crystals obtained by the VUVG technique is probably due to the selfnucleation, carried out under controlled conditions. When using a seed, the crystal quality depends critically on the seed quality. Therefore, self-nucleation is superior. In the growth ampoule used [8—10]the quartz rod creates a “colder point” (—~T—~ 2°C) on the ampoule walls, and slow and
questionable. Thetopurity was usually similar that of of the thegrowth sourcecharge elements. except for intentional doping. i.e. no contamination occurred during charge preparation, within the sensitivity of the analysis. The results in table I clearly indicate that purification from Cu. Ph and Si occurs during the vapor transport.
,
Undoped crystals showed room temperature resistivities of > 1O~~2 cm, measured by the Van der Pauw method. Hall effect could not be observed in
~m tittitilti
these samples. Doped crystals showed resistivities
in the range of l0~—l0~’~ cm. IR transmission was recorded between 2—40 p.m with a Beckman lR spectrometer. The samples, doped and undoped. b
Fig. 4. tdl’c single crystal:
3.2.3. Physical properties Physical characterization of the CdTe crystals included electrical and optical measurements. In some cases, lifetime measurements and evaluation for nuclear radiation detection were carried out.
t,i .is-grown crs~ial. (h) mechanochemical polished (Ill) orienied slice.
:~~c~e~t transmission curve up to 30 p.m. Lifetime was measured by the photoconductivity decay method. Short laser (Nd: Yag) pulses of 1.06 p.m (~t = 3 ns) were used either as a direct
N. Yellin ci a!.
/
Vertical unseeded vapor growth of large CdTe crystals
347
Table I Emission spectrography analysis for impurities in CdTe, in ppm (+, traces)
Blank Cd Te (a) Charge Crystal Residue (b) Charge Crystal Residue
Cu
Ag
—
-
-
—I <0.5 <0.5 + —5 —5 >5
+
—
—
—
‘—50
Pb
Mg
Fe
Si
Al
+ <0.5 <0.5
+ ‘—1 <0.5
+ —1
+ + <0.5
<0.5 <0.5 <0.5 “—0.5 1—5
—0.5 <0.5 “—0.5 1—5 —1
—0.5 <0.5
“—10 —10
<0.5 —0.5 <0.5 ~0.5 <0.5
“-100
<0.5
1—S
—
—
—
—
+ + —
+
—
light source or through a KDP frequency multiplier which converted the light wavelength to 0.54 p.m. The absorption coefficient of CdTe for 1.06 p.m wavelength (JR light) is much lower than that for 0.54 p.m (visible light), since in the first case radiation is observed by the two-photon process only, while in the second case by direct band-toband excitation. The decay time of a high quality crystal was T 30 ns, independent on the wavelength used. This shows that ‘r was not affected by the surface conditions. The decay curve was purely exponential with a single decay time, indicating the lack of slow trapping. Based on the value of “-‘
30 ns, for E
500 V/cm, a drift length of
I
mm is expected. Although the T values obtained for the best THM grown CdTe crystals [18] are much higher, 1 ~is,the drift length of our crystal should permit the detection of nuclear radiation. Radiation detection was studied on 2 >< 2 X 2 mm3 samples. Indium contacts were placed on the front face while silverpaint was used for back contacts. The bias was 100 V. The nuclear radiation response was measured with an Elcint multichannel analyzer with 57Co( 127 keV), 37Cs(660 keV) and 60Co(1300 keV) as radiation sources. Only undoped. high purity samples showed radiation response. The pulse heights were proportional to the radiation energies. It was quite difficult to estimate the charge collection efficiency; however, it seems to be lower than that of a similar commercial RMD CdTe detector. No polarization effect was observed, probably due to the high purity of the crystals. —-
“—1
“—5
4. Conclusions (a) Large CdTe single crystals of high quality were obtained by the VUVG technique. (b) Deviation from stoichiometry, 6Te, of the growth charge determines the crystal growth rate for 0~8Te’~0.lmol%. (c) Further purification of the source material is achieved during the vapor transport process. (d) The crystals show promising results for application as nuclear detectors.
Acknowledgments We are indebted to A. Zemel and J. Eisen for nuclear radiation detection measurements, and to H. Shaham and E. Sheinfeld for analytical assistance.
—‘
References [I] K. Zanio, in: Semiconductors and Semimetals, Vol. 13,
CdTe (Academic Press. New York, 1978). [2] Proc. Intern. Conf. on CdTe [Rev. Phys. AppI. 12 (1977)]. [3] W. Akutagawa and A. Zanio, J. Crystal Growth 11(1971) 191. [4] P. Buck and R. Nilsche, J. Crystal Growth 48 (1980) 29. [5] C.B. Norris, J. Electron. Mater. 9 (1980) 499. [6] A.A. Glebbin. A.A. Davydov and N.!. Garba, lzv. Akad. Nauk SSSR, Neorg. Mater. 16 (1980) 28. [7] Z. Gotacki, M. Górska, J. Makowski and A. Szczerbakow, J. Crystal Growth 56 (1981) 213.
348
N. Ye//in ci al. / Vertical unseeded vapor groii’ih o/ large (‘dTe cr1’
[8] N. Tamari and H. Shtrikman. J. (‘rystal Growih 43 (1978) 378. [9] N. Tamari and H. Shtrikman, J. Electron. Mater. 8 (1979) 269. [10] N. Yellin and G. Gafni, J. Crystal Growth 53 (1981) 409. [II] D. de Nobel, Philips Res. Rept. 14 (1959) 361. 431. [12] K. Mochizuki. J. Cry
[IS] K. Igaki and K. Mochizuki. J. Crystal (iro~tli 24, 25 (1974) 162. [16] HF. Tuller, K. t,Jematsu and F1.K. Iloweri. .1. (rystal Growth 42 (1977)15)). [17) M. Inoue. I. Teramoto and S. ‘Fakasanagi .J..Appl. Ph\v 33 (1962) 2578. [181 P. Siffert. J. Phssique 39 1978) (3-40.