- Email: [email protected]
Low supersaturation nucleation and “contactless” growth of photorefractive ZnTe crystals
,. . . . . . . . C R Y S T A L GROWTH
Journal of Crystal Growth 174 (1997) 719 725
ELSEVIER
Low supersaturation nucleation and "contactless" growth of photorefractive ZnTe crystals K. Grasza a'*'l, S.B. Trivedi a, Zengchen
Y u a,
S.W. Kutcher a, W. Palosz b, G.A. Brost c
"Brimrose Corporation of America, 5020 Campbell Blvd., Baltimore, Maryland 21236-4968, USA bSpace Science Laboratory, (USRA) NASA/Marshall Space Flight Center, Huntsville, Alabama 35812, USA CRome Laboratory, 25 Electronic Parkway, Rome, New York 13441, USA
Abstract
New photorefractive ZnTe crystals have been successfully grown by methods previously applied only for the growth of CdTe single crystals [K. Grasza, J. Crystal Growth 146 (1995) 65]. The main feature of this method lies in the novel procedure of self-seeding in conditions of permanently low supersaturation, growth with no contact between the crystal and ampoule wall, self-purification and improvement of stoichiometry of the source material during growth. Additionally, a new procedure of lowering the optimal growth temperature was applied and an optimal growth rate with stable growth conditions was investigated. The ZnTe crystals had resistivity exceeding 108 ft. cm. Photorefractive characterization was performed using the two-beam mixing technique, and the photorefractive gain in the range 0.63-1.5 ~tm was measured. PACS: 81.05.Dz; 81.10.Bk; 61.72.Vv; 42.70.Nq Keywords: Crystal growth from the vapor; Low supersaturation nucleation; ZnTe; Photorefractive materials: Doping by transition metals
1. I n t r o d u c t i o n
ZnTe, a direct band gap II-VI semiconductor, has been identified as a very important material for photonic applications such as laser generation,
*Corresponding author: Fax: + 1 410 668 4835; e-mail: [email protected]. On leave from: IFPAS, Warsaw, Poland.
modulation, nuclear radiation detection and devices which make use of novel optical nonlinearities [2~4]. However, production of crystals of this material in reasonably large sizes with critical control over crystallographic, chemical, electrical and optical properties has not been successfully achieved. The developments in various epitaxial growth techniques are still not sufficient to produce device grade ZnTe. Also, certain non-linear optical applications require longer interaction lengths which
0022-0248/97/$17.00 Copyright ,(C~ 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 0 2 6 2
720
K. Grasza et al. / Journal o f Crystal Growth 174 (1997) 719-725
epitaxial layers may not provide. Hence, for these types of applications, bulk crystals are required. Behavior of the transition metal impurities in II-VI compounds is very complex [5]. The transition metal impurity vanadium has been shown to behave as a deep donor in CdTe and lends it the property of photorefractivity [6, 7]. There are few reports in the literature regarding the behavior of vanadium impurities in ZnTe, however, vanadium doped ZnTe is a superior photorefractive material for optical processing in the wavelength range of 0.6-1.3 ~tm [8, 9]. Manganese is a deep acceptor in CdTe [10]. Like vanadium, the behavior of manganese-vanadium co-doped ZnTe has not been extensively studied nor reported in the literature. We used vanadium and manganese co-doping in ZnTe during this work. Co-doping was found to improve the photorefractive properties of the ZnTe crystals [11]. This paper deals with the growth of large size photorefractive ZnTe crystals from the vapor phase using a relatively new 'contactless' technique rl].
2. Experimental procedure Both intrinsic and extrinsic defects affect the optoelectronic properties of ZnTe. In order to improve the producibility of photorefractive zinc telluride, extensive purification of the constituent elements zinc and tellurium was carried out. Synthesis of the material was performed at gradually increasing temperatures up to 1090°C. The initial concentration of vanadium and manganese were 5 x 1019 and 1 x 1019cm-S, respectively. In order to improve the stoichiometry, the unreacted elements were removed by heating the powdered ZnTe material at low temperature (600°C) under dynamic vacuum. Crystal growth of vanadium doped ZnTe (ZnTe: V) and vanadium-manganese co-doped ZnTe ( Z n T e : V : M n ) was carried out using low supersaturation nucleation and "eontactless" growth [1, 12] by vertical physical vapor transport (PVT) in closed ampoules. Approximately 45 g of source material was vacuum sealed in fused silica ampoules that were 20 cm long with an inner diameter of 25 mm - Fig. la. An inert atmosphere of approximately 50 mbar of high purity hydrogen
was created in the ampoule prior to sealing. The ampoule consisted of three chambers separated by two plugs. The diameter of these plugs was slightly smaller than the inner diameter of the ampoule. The plug located between the second and third zones was sealed to the ampoule wall such that only a narrow slit was left unsealed, making possible the slow transport of gases between zones II and III. The plug located between the first and second zones was fixed to the ampoule wall, but the slit between the plug and the ampoule wall was mostly unsealed. Prior to growth, the source material was placed in zone II. The ampoule was placed in a furnace with a sharp parabola-like temperature profile such that zone I1 was located at the maximum temperature (1050°C), zone I was located at a temperature approximately 50°C lower, and zone III at a temperature 4000C lower. Such a configuration allowed the transport of gases from zone II to both zones I and III. However, the difference in cross section of the slits between the plugs and ampoule wall, as well as the difference in temperature between the zones, caused a majority of the source ZnTe material to be transported to zone I, and deposition of the excessive component in zone III. After the source material was fully transported from zone I1, the ampoule was shifted such that the material was transported from zone I back into zone II. Since the temperature of the plug between zones I and II was only a few degrees lower than the surface of the source material, spontaneous nucleation did not occur on the front of the plug. Both axial and radial temperature gradients across the source material caused a change in the shape of its surface such that a cone with a monocrystalline peak aimed at the plug was formed - Fig. lb. A further increase in the temperature gradient between the source material and the plug separating zones 1 and II (also called the crystal holder) led to adhesion of the monocrystalline part of the source to the crystal holder - Fig. 2a, separation from the source material, and growth of the ZnTe crystal by PVT - Fig. lc. For further study, we oriented the photorefractive crystals using X-ray diffraction. The crystals were then lapped and mechanically polished. The samples had an orientation of
721
K. Grasza et al. / Journal o f C~stal Growth 174 (1997) 719-725
~'~
volatile impurities excessive c o m p o n e n t
o
n a r r o w slit b e t w e e n plug a n d a m p o u l e
~s
ponent
wall purities
Li
~
source material
.
w i d e slit b e t w e e n plug and ampoule wall
~
stoichiometric ZnTematerial
pure
~= o N
~
(a)
votatile impurities , excess ve component
J J
non-volatile impurities
crystal source
(c) Fig. 1. Crystal growth ampoule. The stage of purification, low supersaturation nucleation and crystal growth: (a) source material is transported to zone I; (b) monocrystalline top of the conus touches the crystal holder; (c) the crystal grows on the crystal holder with no contact to the ampoule wall.
( 1 1 0 ) x (1 i 0 ) x ( 1 0 0 ) . The (110) optical face was used for two beam mixing measurements. For good photorefractivity, very high resistivity samples were required. The resistivity of our samples was measured using a simple circuit in which the sample was connected in series with known and thermally stable resistors. The nature of the spectral absorption of the sample in the vicinity of the band-edge provided a preliminary indication of the
photorefractive properties of the sample. Detailed optical absorption spectroscopy was carried out in the 0.6(~2.0 ~tm wavelength range for each sample. This measurement range provided valuable information about the deep centers, native defects, shallow levels and the scattering properties of the ZnTe samples. The photorefractive properties were characterized by two beam coupling [9]. Two S-polarized unexpanded Gaussian beams with
722
K. Grasza et al. / Journal o f Crystal Growth 174 (1997) 719-725
•
(a)
~ii
(b)
Fig. 2. (a) An ampoule with ZnTe : V material removed from the furnace at the moment of low supersaturation nucleation. (b) A ZnTe:V crystal grown by 'contactless' technique.
a beam ratio of 20 " l in the < 1 1 0 > plane and forming a grating along < 0 0 1 > direction was used.
3. Results During the present work we grew three 40g ZnTe boules. Two of the crystals were doped with vanadium (Fig. 2b), the third was doped with both vanadium and manganese. One of these crystals was one single grain comprising a few twin planes, and the other crystals produced photorefractive twin-free samples up to 10 x 10 x 5 mm a volume with orientation ( l 1 0 ) x ( 1 1 0 ) x ( 0 0 1 ) . A section of the vanadium-manganese co-doped crystal is shown in Fig. 3a and Fig. 3b. The first grown part of the crystal, grown close to the crystal holder, is characterized by a strong tendency to twinning. Additionally, adhesion of the crystal to the crystal holder resulted in cracking of this part of the crystal. The last grown part of the crystal,
located far from the crystal holder, showed less twinning behavior. Traces of cracking were not found in this part of the crystal. The investigation of growth rate as a function of furnace profile, ampoule geometry, material stoichiometry and hydrogen pressure was investigated. The maximum furnace temperature was in the range of 815-1050°C. We were looking for optimal conditions for the highest growth rate at the lowest possible temperature. The highest transport rate (a few mm/day at 815°C and a temperature gradient of 10°C/cm) was obtained in an ampoule where carefully prepared near stoichiometric material was sealed under high vacuum (10 -6 Torr) without hydrogen. However, at such a low temperature the crystal was not single. Additionally, under these growth conditions, most of the material was transported to zone II and the size of the crystal was relatively small. Using the ampoule shown in Fig. 1, material containing an excess of tellurium, a growth temperature of 1050°C and a growth rate in the range of
K. Grasza et al. / Journal Of C~stal Growth 174 (1997) 719- 725
(a)
723
(b)
Fig. 3. Cross section of the ZnTe : M n : V crystal (a) close to the crystal holder, and (b) far from the crystal holder.
5-10 ram/day, good reproducibility of the growth conditions was achieved. Growth of "contactless" crystals at this high temperature (1050°C) decreases the risk of constitutional supersaturation instabilities [13]. However, the fast transport rate must be limited by an excess component in the vapor phase such as tellurium or an inert gas. The size and shape of the Laue spots were indicative of reasonably good quality crystals. All asgrown vanadium doped crystals were p-type and had a very high resistivity in the range of 5 x 108-1 x 101° f~-cm. A vanadium-manganese co-doped crystal had a resistivity of 105-106 ~ ' cm. Features in the optical absorption spectra between 0.8 and 1.6 ~tm appear to be due to the presence of vanadium. Measurements of photorefractive gain by two beam mixing were carried out at 0.633, 0.7 to 0.85 gm (using semiconductor laser diodes), and 1.3 and 1.5 gm (important communication wavelengths). All the measurements revealed a strong photorefractive nonlinear effect. In vanadium doped crystals the photorefractive gain was 0.6 cm-1 at a grating spacing of 1.0 gm and a wavelength of 0.80 gm. In vanadium manganese co-doped samples, the photorefractive gain was 1.52 c m - 1 at a wavelength of 0.63 gm and 1.3 c m - 1 at a wavelength of 0.80 gm. The values obtained from both crystals are the highest ever reported [8, 9]. The results of the photorefractive gain indicate that the native defect density in these samples
2.5 ZnTe:Mn:Te- 1 1 2.0 t
~ = 0.75 /zm
1.5
I
E
,
o
1.0
0.5
0.0
0
r
I
I
I
[
1
2
3
4
5
GRATING PERIOD (/l, rn) Fig. 4. The two beam coupling gain coefficient versus the grating period at 0.75 gm for a ZnTe : Mn : V crystal.
was reduced, and the density of deep level traps increased. The two beam coupling gain coefficient versus the grating period at a wavelength of 0.75 p.m for a Z n T e : M n : V crystal is shown in Fig. 4. The solid curve in this figure is fit using the single carrier, single trap model. This fit gives an effective
724
K. Grasza et al. / Journal o f Crystal Growth 174 (1997) 719-725
concentration of N E = 1.3 x electro-optic coefficient of reff
1016 cm -3 =
and an
2.3 pm/V.
4. Discussion The growth of ZnTe crystals by PVT from near stoichiometric source material is possible over a relatively wide range of temperatures due to its high saturated vapor pressure at temperatures far below the melting point [14]. However, it is extremely difficult to control the transport rates in nearly stoichiometric materials. If the 'in situ' purified material is removed from the ampoule and mechanically crushed and powdered, its surface oxidizes relatively fast in the presence of air, and the material is apt to lose its stoichiometry during sealing of the growth ampoule. If compacted, the material stoichiometry is generally poor and hard to improve. The proposed method of improving stoichiometry of the source material during crystal growth satisfactorily stabilizes the growth conditions by lowering the partial pressure of the excessive component to levels controlled by the temperature of the cold zone, pulling rate of the ampoule and crystal growth rate. The procedure of pre-subliming the material from zone II to zone I considerably improves the uniformity of distribution of the transition metal dopants. It is transported from the non-compacted source through the slit in amounts adequate to impose transport conditions. The excess of non-volatile transition metal elements stays in zone II and does not disturb further growth of the crystal. Also, the eventual non-volatile impurities of the source material stay in zone II and cannot be transported to zone I by convection. The method of low supersaturation nucleation ensures formation of good quality seeds. The investigation of temperatures suitable for nucleation and 'contactless' growth also leads to optimal conditions for stability of the growth interface. According to our constitutional supersaturation criterion [13, 15], the stable growth interface results from a low ratio of the gradient of vapor above the interface and the gradient of temperature along the crystal close to the interface. Thus, more stable
growth is always expected for low growth rates. An increase in the growth rate with a stable interface is possible only at higher temperatures 1-14, 15] or steeper furnace temperature gradients. However, an increase in temperature as well as an increase in the furnace temperature gradient is limited by temperature-dependent thermal properties of the ZnTe material.
5. Conclusions In this work it was shown (for the first time) that it is possible to grow large ZnTe crystals using a modified 'contactless' method in which nucleation occurs at low supersaturation. Nucleation at low supersaturation provides a nucleus with very good crystallographic qualities and the resulting ZnTe crystals are high quality. Crystals 24 mm in diameter and up to 40 g in weight were grown in specially designed ampoules. These ampoules were constructed in such a way that purification and improvement of stoichiometry of the source material during growth was obtained. Vanadium and manganese doping at a level sufficient to produce photorefractive material was achieved. The photorefractive measurements conducted during this research were carried out at 0.633 ~tm, 700-850 jam, and the important optical communication wavelengths of 1.3 and 1.5 ~tm. In vanadium doped ZnTe, we observed a photorefractive gain of 0.6 cm"1 at a wavelength of 0.80 jam and a grating spacing of 1 jam. This is the highest gain (at this wavelength and grating spacing) ever reported in ZnTe:V. However, the photorefractive gain observed in vanadium-manganese codoped ZnTe during this work was much higher than the gain in ZnTe:V. The photorefractive gain in ZnTe : V : Mn at 0.80 ~tm and 1 lam grating spacing was approximately 0.85 cm- ~.
Acknowledgements Brimrose Corporation gratefully acknowledges the support provided for this work by Rome Laboratories.
1<2 Grasza et al. / Journal of Crystal Growth 174 (1997) 719 725
References [1] K. Grasza, J. Crystal Growth 146 (1995) 65. [2] H.E. Ruda, Ed., Widegap II-VI compounds for Optoelectronic Applications (Chapman & Hall, London, 1992). [3] C.F. Dewy, Jr., Rev. Phys. Appl. 15 (1977) 405. [4] M.R. Squillante, J. Zhang, C. Zhou, P. Bennett and L. Moy, MRS Symp. Proc. 302 t1993) 319. [5] D. Heiman, M. Dahl, X. Wang, P.A. Wolf, P. Becla, A. Petrov and A. Mycielski, Mater. Res. Soc. Syrup. Proc. 161 (1990) 479. [6] R.B. Bylsma, P.M. Bridenbaugh, D.M. Olson and A.M. Glass, Appl. Phys. Lett. 51 (1987) 889. [7] A. Partovi, J. Millerd, E.M. Garmire, M. Ziari, W.H. Steiter, S.B. Trivedi and M.B. Klein, Appl. Phys. Lett. 57 (1990) 846. [8] M. Ziari, W.H. Steier, P.N. Ranon, M.B. Klein and S. Trivedi, Paper presented at the Annual Meeting of The Optical Society of America, San Jose, CA (1991).
725
[9] M. Ziari, H.W. Steiter, P.N. Ranon, S. Trivedi and M.B. Klein, Appl. Phys. Lett. 60 (1992) 1052. [10] E. Rzepka, Y. Marfaing, M. Cuniot and R. Triboulet, Mater. Sci. Eng. B16 (1993) 262. [11] G. Brost, K. Magde, M. Ziari, W.H. Steier, R.N. Schwartz, M.B. Klein, S.B. Trivedi and G.V. Jagannathan, Digest of Photorefractive Materials Effects and Devices, Estes Park, CO (1995) p. 32. [12] K. Grasza, U. Zuzga-Grasza, A. Jedrzcjczak, R.R. Galazka, J. Majewski, A. Szadkowski and E. Grodzicka, J. Crystal Growth 123 (1992) 510. [13] K. Grasza, in: Elementary Crystal Growth, Ed. K. Sangwal (SAAN, Lublin, 1994). [14] O. Knacke, O. Kubaschewski and K. Hesselmann, Thermochemical Properties of Inorganic Substances. (Springer, Berlin, 1991). [15] K. Grasza and A. Jedrzejczak, J. Crystal Growth 162 (19961 173.
Journal of Crystal Growth 174 (1997) 719 725
ELSEVIER
Low supersaturation nucleation and "contactless" growth of photorefractive ZnTe crystals K. Grasza a'*'l, S.B. Trivedi a, Zengchen
Y u a,
S.W. Kutcher a, W. Palosz b, G.A. Brost c
"Brimrose Corporation of America, 5020 Campbell Blvd., Baltimore, Maryland 21236-4968, USA bSpace Science Laboratory, (USRA) NASA/Marshall Space Flight Center, Huntsville, Alabama 35812, USA CRome Laboratory, 25 Electronic Parkway, Rome, New York 13441, USA
Abstract
New photorefractive ZnTe crystals have been successfully grown by methods previously applied only for the growth of CdTe single crystals [K. Grasza, J. Crystal Growth 146 (1995) 65]. The main feature of this method lies in the novel procedure of self-seeding in conditions of permanently low supersaturation, growth with no contact between the crystal and ampoule wall, self-purification and improvement of stoichiometry of the source material during growth. Additionally, a new procedure of lowering the optimal growth temperature was applied and an optimal growth rate with stable growth conditions was investigated. The ZnTe crystals had resistivity exceeding 108 ft. cm. Photorefractive characterization was performed using the two-beam mixing technique, and the photorefractive gain in the range 0.63-1.5 ~tm was measured. PACS: 81.05.Dz; 81.10.Bk; 61.72.Vv; 42.70.Nq Keywords: Crystal growth from the vapor; Low supersaturation nucleation; ZnTe; Photorefractive materials: Doping by transition metals
1. I n t r o d u c t i o n
ZnTe, a direct band gap II-VI semiconductor, has been identified as a very important material for photonic applications such as laser generation,
*Corresponding author: Fax: + 1 410 668 4835; e-mail: [email protected]. On leave from: IFPAS, Warsaw, Poland.
modulation, nuclear radiation detection and devices which make use of novel optical nonlinearities [2~4]. However, production of crystals of this material in reasonably large sizes with critical control over crystallographic, chemical, electrical and optical properties has not been successfully achieved. The developments in various epitaxial growth techniques are still not sufficient to produce device grade ZnTe. Also, certain non-linear optical applications require longer interaction lengths which
0022-0248/97/$17.00 Copyright ,(C~ 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 0 2 6 2
720
K. Grasza et al. / Journal o f Crystal Growth 174 (1997) 719-725
epitaxial layers may not provide. Hence, for these types of applications, bulk crystals are required. Behavior of the transition metal impurities in II-VI compounds is very complex [5]. The transition metal impurity vanadium has been shown to behave as a deep donor in CdTe and lends it the property of photorefractivity [6, 7]. There are few reports in the literature regarding the behavior of vanadium impurities in ZnTe, however, vanadium doped ZnTe is a superior photorefractive material for optical processing in the wavelength range of 0.6-1.3 ~tm [8, 9]. Manganese is a deep acceptor in CdTe [10]. Like vanadium, the behavior of manganese-vanadium co-doped ZnTe has not been extensively studied nor reported in the literature. We used vanadium and manganese co-doping in ZnTe during this work. Co-doping was found to improve the photorefractive properties of the ZnTe crystals [11]. This paper deals with the growth of large size photorefractive ZnTe crystals from the vapor phase using a relatively new 'contactless' technique rl].
2. Experimental procedure Both intrinsic and extrinsic defects affect the optoelectronic properties of ZnTe. In order to improve the producibility of photorefractive zinc telluride, extensive purification of the constituent elements zinc and tellurium was carried out. Synthesis of the material was performed at gradually increasing temperatures up to 1090°C. The initial concentration of vanadium and manganese were 5 x 1019 and 1 x 1019cm-S, respectively. In order to improve the stoichiometry, the unreacted elements were removed by heating the powdered ZnTe material at low temperature (600°C) under dynamic vacuum. Crystal growth of vanadium doped ZnTe (ZnTe: V) and vanadium-manganese co-doped ZnTe ( Z n T e : V : M n ) was carried out using low supersaturation nucleation and "eontactless" growth [1, 12] by vertical physical vapor transport (PVT) in closed ampoules. Approximately 45 g of source material was vacuum sealed in fused silica ampoules that were 20 cm long with an inner diameter of 25 mm - Fig. la. An inert atmosphere of approximately 50 mbar of high purity hydrogen
was created in the ampoule prior to sealing. The ampoule consisted of three chambers separated by two plugs. The diameter of these plugs was slightly smaller than the inner diameter of the ampoule. The plug located between the second and third zones was sealed to the ampoule wall such that only a narrow slit was left unsealed, making possible the slow transport of gases between zones II and III. The plug located between the first and second zones was fixed to the ampoule wall, but the slit between the plug and the ampoule wall was mostly unsealed. Prior to growth, the source material was placed in zone II. The ampoule was placed in a furnace with a sharp parabola-like temperature profile such that zone I1 was located at the maximum temperature (1050°C), zone I was located at a temperature approximately 50°C lower, and zone III at a temperature 4000C lower. Such a configuration allowed the transport of gases from zone II to both zones I and III. However, the difference in cross section of the slits between the plugs and ampoule wall, as well as the difference in temperature between the zones, caused a majority of the source ZnTe material to be transported to zone I, and deposition of the excessive component in zone III. After the source material was fully transported from zone I1, the ampoule was shifted such that the material was transported from zone I back into zone II. Since the temperature of the plug between zones I and II was only a few degrees lower than the surface of the source material, spontaneous nucleation did not occur on the front of the plug. Both axial and radial temperature gradients across the source material caused a change in the shape of its surface such that a cone with a monocrystalline peak aimed at the plug was formed - Fig. lb. A further increase in the temperature gradient between the source material and the plug separating zones 1 and II (also called the crystal holder) led to adhesion of the monocrystalline part of the source to the crystal holder - Fig. 2a, separation from the source material, and growth of the ZnTe crystal by PVT - Fig. lc. For further study, we oriented the photorefractive crystals using X-ray diffraction. The crystals were then lapped and mechanically polished. The samples had an orientation of
721
K. Grasza et al. / Journal o f C~stal Growth 174 (1997) 719-725
~'~
volatile impurities excessive c o m p o n e n t
o
n a r r o w slit b e t w e e n plug a n d a m p o u l e
~s
ponent
wall purities
Li
~
source material
.
w i d e slit b e t w e e n plug and ampoule wall
~
stoichiometric ZnTematerial
pure
~= o N
~
(a)
votatile impurities , excess ve component
J J
non-volatile impurities
crystal source
(c) Fig. 1. Crystal growth ampoule. The stage of purification, low supersaturation nucleation and crystal growth: (a) source material is transported to zone I; (b) monocrystalline top of the conus touches the crystal holder; (c) the crystal grows on the crystal holder with no contact to the ampoule wall.
( 1 1 0 ) x (1 i 0 ) x ( 1 0 0 ) . The (110) optical face was used for two beam mixing measurements. For good photorefractivity, very high resistivity samples were required. The resistivity of our samples was measured using a simple circuit in which the sample was connected in series with known and thermally stable resistors. The nature of the spectral absorption of the sample in the vicinity of the band-edge provided a preliminary indication of the
photorefractive properties of the sample. Detailed optical absorption spectroscopy was carried out in the 0.6(~2.0 ~tm wavelength range for each sample. This measurement range provided valuable information about the deep centers, native defects, shallow levels and the scattering properties of the ZnTe samples. The photorefractive properties were characterized by two beam coupling [9]. Two S-polarized unexpanded Gaussian beams with
722
K. Grasza et al. / Journal o f Crystal Growth 174 (1997) 719-725
•
(a)
~ii
(b)
Fig. 2. (a) An ampoule with ZnTe : V material removed from the furnace at the moment of low supersaturation nucleation. (b) A ZnTe:V crystal grown by 'contactless' technique.
a beam ratio of 20 " l in the < 1 1 0 > plane and forming a grating along < 0 0 1 > direction was used.
3. Results During the present work we grew three 40g ZnTe boules. Two of the crystals were doped with vanadium (Fig. 2b), the third was doped with both vanadium and manganese. One of these crystals was one single grain comprising a few twin planes, and the other crystals produced photorefractive twin-free samples up to 10 x 10 x 5 mm a volume with orientation ( l 1 0 ) x ( 1 1 0 ) x ( 0 0 1 ) . A section of the vanadium-manganese co-doped crystal is shown in Fig. 3a and Fig. 3b. The first grown part of the crystal, grown close to the crystal holder, is characterized by a strong tendency to twinning. Additionally, adhesion of the crystal to the crystal holder resulted in cracking of this part of the crystal. The last grown part of the crystal,
located far from the crystal holder, showed less twinning behavior. Traces of cracking were not found in this part of the crystal. The investigation of growth rate as a function of furnace profile, ampoule geometry, material stoichiometry and hydrogen pressure was investigated. The maximum furnace temperature was in the range of 815-1050°C. We were looking for optimal conditions for the highest growth rate at the lowest possible temperature. The highest transport rate (a few mm/day at 815°C and a temperature gradient of 10°C/cm) was obtained in an ampoule where carefully prepared near stoichiometric material was sealed under high vacuum (10 -6 Torr) without hydrogen. However, at such a low temperature the crystal was not single. Additionally, under these growth conditions, most of the material was transported to zone II and the size of the crystal was relatively small. Using the ampoule shown in Fig. 1, material containing an excess of tellurium, a growth temperature of 1050°C and a growth rate in the range of
K. Grasza et al. / Journal Of C~stal Growth 174 (1997) 719- 725
(a)
723
(b)
Fig. 3. Cross section of the ZnTe : M n : V crystal (a) close to the crystal holder, and (b) far from the crystal holder.
5-10 ram/day, good reproducibility of the growth conditions was achieved. Growth of "contactless" crystals at this high temperature (1050°C) decreases the risk of constitutional supersaturation instabilities [13]. However, the fast transport rate must be limited by an excess component in the vapor phase such as tellurium or an inert gas. The size and shape of the Laue spots were indicative of reasonably good quality crystals. All asgrown vanadium doped crystals were p-type and had a very high resistivity in the range of 5 x 108-1 x 101° f~-cm. A vanadium-manganese co-doped crystal had a resistivity of 105-106 ~ ' cm. Features in the optical absorption spectra between 0.8 and 1.6 ~tm appear to be due to the presence of vanadium. Measurements of photorefractive gain by two beam mixing were carried out at 0.633, 0.7 to 0.85 gm (using semiconductor laser diodes), and 1.3 and 1.5 gm (important communication wavelengths). All the measurements revealed a strong photorefractive nonlinear effect. In vanadium doped crystals the photorefractive gain was 0.6 cm-1 at a grating spacing of 1.0 gm and a wavelength of 0.80 gm. In vanadium manganese co-doped samples, the photorefractive gain was 1.52 c m - 1 at a wavelength of 0.63 gm and 1.3 c m - 1 at a wavelength of 0.80 gm. The values obtained from both crystals are the highest ever reported [8, 9]. The results of the photorefractive gain indicate that the native defect density in these samples
2.5 ZnTe:Mn:Te- 1 1 2.0 t
~ = 0.75 /zm
1.5
I
E
,
o
1.0
0.5
0.0
0
r
I
I
I
[
1
2
3
4
5
GRATING PERIOD (/l, rn) Fig. 4. The two beam coupling gain coefficient versus the grating period at 0.75 gm for a ZnTe : Mn : V crystal.
was reduced, and the density of deep level traps increased. The two beam coupling gain coefficient versus the grating period at a wavelength of 0.75 p.m for a Z n T e : M n : V crystal is shown in Fig. 4. The solid curve in this figure is fit using the single carrier, single trap model. This fit gives an effective
724
K. Grasza et al. / Journal o f Crystal Growth 174 (1997) 719-725
concentration of N E = 1.3 x electro-optic coefficient of reff
1016 cm -3 =
and an
2.3 pm/V.
4. Discussion The growth of ZnTe crystals by PVT from near stoichiometric source material is possible over a relatively wide range of temperatures due to its high saturated vapor pressure at temperatures far below the melting point [14]. However, it is extremely difficult to control the transport rates in nearly stoichiometric materials. If the 'in situ' purified material is removed from the ampoule and mechanically crushed and powdered, its surface oxidizes relatively fast in the presence of air, and the material is apt to lose its stoichiometry during sealing of the growth ampoule. If compacted, the material stoichiometry is generally poor and hard to improve. The proposed method of improving stoichiometry of the source material during crystal growth satisfactorily stabilizes the growth conditions by lowering the partial pressure of the excessive component to levels controlled by the temperature of the cold zone, pulling rate of the ampoule and crystal growth rate. The procedure of pre-subliming the material from zone II to zone I considerably improves the uniformity of distribution of the transition metal dopants. It is transported from the non-compacted source through the slit in amounts adequate to impose transport conditions. The excess of non-volatile transition metal elements stays in zone II and does not disturb further growth of the crystal. Also, the eventual non-volatile impurities of the source material stay in zone II and cannot be transported to zone I by convection. The method of low supersaturation nucleation ensures formation of good quality seeds. The investigation of temperatures suitable for nucleation and 'contactless' growth also leads to optimal conditions for stability of the growth interface. According to our constitutional supersaturation criterion [13, 15], the stable growth interface results from a low ratio of the gradient of vapor above the interface and the gradient of temperature along the crystal close to the interface. Thus, more stable
growth is always expected for low growth rates. An increase in the growth rate with a stable interface is possible only at higher temperatures 1-14, 15] or steeper furnace temperature gradients. However, an increase in temperature as well as an increase in the furnace temperature gradient is limited by temperature-dependent thermal properties of the ZnTe material.
5. Conclusions In this work it was shown (for the first time) that it is possible to grow large ZnTe crystals using a modified 'contactless' method in which nucleation occurs at low supersaturation. Nucleation at low supersaturation provides a nucleus with very good crystallographic qualities and the resulting ZnTe crystals are high quality. Crystals 24 mm in diameter and up to 40 g in weight were grown in specially designed ampoules. These ampoules were constructed in such a way that purification and improvement of stoichiometry of the source material during growth was obtained. Vanadium and manganese doping at a level sufficient to produce photorefractive material was achieved. The photorefractive measurements conducted during this research were carried out at 0.633 ~tm, 700-850 jam, and the important optical communication wavelengths of 1.3 and 1.5 ~tm. In vanadium doped ZnTe, we observed a photorefractive gain of 0.6 cm"1 at a wavelength of 0.80 jam and a grating spacing of 1 jam. This is the highest gain (at this wavelength and grating spacing) ever reported in ZnTe:V. However, the photorefractive gain observed in vanadium-manganese codoped ZnTe during this work was much higher than the gain in ZnTe:V. The photorefractive gain in ZnTe : V : Mn at 0.80 ~tm and 1 lam grating spacing was approximately 0.85 cm- ~.
Acknowledgements Brimrose Corporation gratefully acknowledges the support provided for this work by Rome Laboratories.
1<2 Grasza et al. / Journal of Crystal Growth 174 (1997) 719 725
References [1] K. Grasza, J. Crystal Growth 146 (1995) 65. [2] H.E. Ruda, Ed., Widegap II-VI compounds for Optoelectronic Applications (Chapman & Hall, London, 1992). [3] C.F. Dewy, Jr., Rev. Phys. Appl. 15 (1977) 405. [4] M.R. Squillante, J. Zhang, C. Zhou, P. Bennett and L. Moy, MRS Symp. Proc. 302 t1993) 319. [5] D. Heiman, M. Dahl, X. Wang, P.A. Wolf, P. Becla, A. Petrov and A. Mycielski, Mater. Res. Soc. Syrup. Proc. 161 (1990) 479. [6] R.B. Bylsma, P.M. Bridenbaugh, D.M. Olson and A.M. Glass, Appl. Phys. Lett. 51 (1987) 889. [7] A. Partovi, J. Millerd, E.M. Garmire, M. Ziari, W.H. Steiter, S.B. Trivedi and M.B. Klein, Appl. Phys. Lett. 57 (1990) 846. [8] M. Ziari, W.H. Steier, P.N. Ranon, M.B. Klein and S. Trivedi, Paper presented at the Annual Meeting of The Optical Society of America, San Jose, CA (1991).
725
[9] M. Ziari, H.W. Steiter, P.N. Ranon, S. Trivedi and M.B. Klein, Appl. Phys. Lett. 60 (1992) 1052. [10] E. Rzepka, Y. Marfaing, M. Cuniot and R. Triboulet, Mater. Sci. Eng. B16 (1993) 262. [11] G. Brost, K. Magde, M. Ziari, W.H. Steier, R.N. Schwartz, M.B. Klein, S.B. Trivedi and G.V. Jagannathan, Digest of Photorefractive Materials Effects and Devices, Estes Park, CO (1995) p. 32. [12] K. Grasza, U. Zuzga-Grasza, A. Jedrzcjczak, R.R. Galazka, J. Majewski, A. Szadkowski and E. Grodzicka, J. Crystal Growth 123 (1992) 510. [13] K. Grasza, in: Elementary Crystal Growth, Ed. K. Sangwal (SAAN, Lublin, 1994). [14] O. Knacke, O. Kubaschewski and K. Hesselmann, Thermochemical Properties of Inorganic Substances. (Springer, Berlin, 1991). [15] K. Grasza and A. Jedrzejczak, J. Crystal Growth 162 (19961 173.