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Growth and electrical properties of PbTe bulk crystals grown by the Bridgman method under controlled tellurium or lead vapor pressure
, ........
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 165 (1996) 402-407
Growth and electrical properties of PbTe bulk crystals grown by the Bridgman method under controlled tellurium or lead vapor pressure N u g r a h a a, K e n Suto a, O s a m u Itoh c, Jun-ichi N i s h i z a w a b,c, * , Y a s u t o s h i Y o k o t a a a Department of Materials Science, Faculty of Engineering, Tohoku Unit,ersiO', Aramakiaza Aoba, Sendai 980, Japan b Tohoku University, Katahira 2-1-1 Aoba, Sendai 980, Japan c Semiconductor Research Institute, Kawauchi Aoba, Sendai 980, Japan Received 19 September 1995; accepted 23 December 1995
Abstract PbTe bulk crystals have been grown by the Bridgman method under controlled tellurium or lead vapor pressure. Both p-type crystals, and n-type crystals which have been difficult to grow by the conventional Bridgman method can be grown by this method. For PTe = 10.6 and 3.03 Torr, the grown crystals are p-type. For PTe = 1.34 and 0.86 Torr, large portions of grown crystals are n-type. For PTe = 0.26 Torr and lead vapor pressures of PPb = 4.92 × 10 - 7 and 8.4 × 10 -5 Torr, grown crystals are n-type. Average pit sizes of p-type crystals are larger than those of n-type crystals, but etch pit densities of p-type crystals are lower than those of n-type.
1. Introduction PbTe is a well known I V - V I group semiconductor used as a basic material for infrared optoelectronic devices. The excess of one constituent, lead or tellurium, in a PbTe crystal, i.e., deviation from stoichiometry greatly influences the electrical properties. It was reported that the excess of lead in PbTe causes n-type conductivity and the excess of tellurium causes p-type conductivity [1]. The ability to control the excess of a constituent in the crystal lattice, which means the ability to control electrical properties of crystals through controlling densities of
* Corresponding author. Fax: +81 22 2237289.
nonstoichiometric defects, is a very important point in device fabrication. The growth of PbTe by the Bridgman method from various melt compositions have been done by many authors. Grown crystals usually have p-type conductivity, and growth of n-type single crystals have been very difficult. Sato et al. [1] and Kanai et al. [2] reported that single crystals grown from nearly stoichiometric melts were always p-type. Although the growths were performed from melts with excess Pb, the n-type conductivity was found only at the top portion of a grown crystal [3] [4]. Another author reported that n-type crystals cannot readily be grown from melt [5]. Lawson [6] reported that there is a possibility to grow n-type crystals by using a treatment which removes surface oxide layers from start-
0022-0248/96515.00 Copyright © 1996 Elsevier Science B.V. All fights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 2 2 0 - 5
Nugraha et al. / Journal qf Crystal Growth 165 (1996) 402-407
ing materials, but the result of measurement was not shown. The deviation from stoichiometry can be controlled by a heat-treatment process. Sato et al. [1] reported that, at a heat-treatment temperature of 600°C, the samples heat-treated under tellurium vapor pressures lower than 3 X 10 -5 Torr were n-type and those heat-treated under tellurium vapor pressures higher than 3 X 10 -5 Torr were p-type, but this method needs two step works, the growth process and the heat-treatment process. Nishizawa et al. [7-10] reported that stoichiometry controlled nearly perfect crystals of III-V compound semiconductors can be grown by using a temperature difference method under controlled vapor pressure (TDM-CVP). In this method, the vapor pressure of one element (usually a very volatile element) is applied on the growth solution. They applied this method to liquid phase epitaxy (LPE) [7-10] and melt growth [11]. An optimum pressure was found for GaAs as well as for GaP at which the grown crystals show minimum densities of point defects, dislocations, and carrier concentrations. The optimum arsenic pressure for GaAs obtained by this method was the same as the stoichiometric pressure obtained by heat-treatment experiments [8,12]. The essential point of this method is that the applied vapor pressure fixes the chemical potential of a volatile element in the gas phase, and the chemical potentials of the element in the liquid and solid phase are in equilibrium with the chemical potentials of the element in the gas phase, i.e., ~AL = ~
= ~A,"
(l)
In other words, the saturation solubility depends on the applied vapor pressure and influences the stoichiometry of the segregating crystals [10]. The TDM-CVP method was also applied to the growth of ZnSe that is one of II-VI compound semiconductors [13-15]. The growth of ZnSe by using conventional methods always revealed n-type conductivity. The TDM-CVP method enabled the growth of p-type ZnSe crystals and the fabrication of pn junctions which emitted blue light. However, the application of the TDM-CVP method in the growth of lead telluride has not yet been reported. We have firstly applied the TDM-CVP method in liquid phase epitaxial growth of lead
403
telluride [16]. Epitaxial layers grown from a lead solution have been always n-type, but the free electron concentration of the grown crystals can be controlled by applied tellurium pressure and becomes minimum at an optimum tellurium vapor pressure. In this paper, we report the growth of bulk single crystals of lead telluride by the Bridgman method from a nearly stoichiometric melt under controlled tellurium or lead vapor pressure. The main purpose of this experiment is to control the conductivity type of lead telluride single crystals.
2. Experimental procedure Lead telluride source crystals for Bridgman growth are synthesized from 0.1 mole nearly stoichiometric melts which have been mixed from 99.9999% purity lead and tellurium. Lead has been etched by C H 3 C O O H + H 2 0 2 (4: 1) to remove the surface oxide layer. Quartz ampoules (10 mm in diameter) are rinsed by organic and acid solutions, etched by HF, and baked at about 10 -6 Torr and 1000°C before use. The ampoule is sealed off with source elements at approximately 10 -6 Tort. The ampoule is then placed in a furnace at about 1000°C for 24 h to allow reaction of the elements and homogeneously mixing of the compound. After that, the melt is rapidly cooled by immersing the ampoule into room temperature water in order to obtain a homogeneous polycrystal ingot. The polycrystals are then transferred into an ampoule for Bridgman growth (13 mm in diameter) which is shown in Fig. 1. Pellets of lead or tellurium (about 0.3 g) are placed in the vapor pressure control region of the ampoule to control the vapor pressure of one constituent of the melt during the growth. A quartz rod is inserted between the lead telluride melt region and the vapor pressure control region to narrow the free space which connects the two regions. As a result, it is thought that Eq. (2) holds. The ampoule is then sealed off at about 10 -6 Torr and placed in a furnace which has the temperature profile shown in Fig. 1. The temperature profile is divided into two regions, a growth region and a pressure control region. The temperature gradient of the growth region at the
404
Nugraha et al. / Journal of C~stal Growth 165 (1996) 402-407
~ or P~URE
CONTROL
RF~ION
i -
/ / ~ 13ram PbTe
GROWTH REGION
lOOmm
~Meit
2 sink
T~qm-mture Fig. 1. Illustration of ampoule and temperature profile for Bridgman growth under controlled vapor pressure.
melting temperature of lead telluride, 924°C, is 3 5 40°C/cm. The thermocouple reading for the pressure control region has been controlled to + 0.1°C and that for the growth region to + I°C. After holding the growth ampoule at the initial position for about 24 h to make a homogenous melt and to permit diffusion of insoluble impurities to top or bottom of the melt, the ampoule is then lowered with a speed of 1.4 m m / d a y . When the growth has been finished, the crystal is slowly cooled to room temperature for 24 h, because rapid cooling can create strains in the crystal which make it weak and fragile. The temperature of the pressure control region used as a parameter for this experiment is 445,495, 515, 555 or 625°C for tellurium (TTe), and 445 or 555°C for lead (Tpb). The grown crystals ingot is cut in a direction normal to the growth direction to wafers with a
thickness of 1.2 mm. The samples for Hall and resistivity measurements and etch pit observation are taken from each wafer to obtain a cartier concentration profile along the crystal length. The Hall and resistivity measurements have been carried out by a conventional dc Hall measurement with indium as a contact material. Measurements have been performed at 300 K on 4 × 4 × 0.1 mm samples cut out from each wafer, polished and etched by a Norr etch solution. [17]. For etch pit observation, freshly cleaved surfaces from each of the wafers are etched by a solution NaOH (5 g ) + 12 (0.2 g ) + H 2 0 (10 ml) [18] at 95°C for about 5 min. Observation has been made by a scanning electron microscope.
3. R e s u l t s a n d d i s c u s s i o n
Crystals of 13 mm in diameter and about 30 mm in length have been grown from a nearly stoichiometry melt. All grown crystals have metallic appearance and cleave quite readily parallel to the (100) plane. The top region of a crystal ingot with a thickness of about 0.5 mm are tough and cannot be cleaved and are obviously composed of lead and lead telluride for n-type crystals and tellurium and lead telluride for p-type crystals. Fig. 2 shows a carrier concentration profile at 300 K along the growth axis for crystals grown under various temperatures of the vapor pressure control region. The vapor pressure applied to the melt during growth can be calculated from the temperature of the vapor pressure control region, Tve or Pb, by using the following equation which holds when the tube that connects the melt and the vapor pressure region is narrow enough [10]: eTe orPb = POeorPb( TG/TTeor Pb)1/2,
(2)
where Pve or Pb is the vapor pressure of Te or Pb applied to the melt, Tve or Pb is the temperature of the Te or Pb vapor pressure control region, POe or Pb the vapor pressure of Te or Pb at temperature Tve or Pb and TG is the growth temperature (nearly equal to the melting temperature of lead telluride, 924°C). In the present temperature range, the main molecular species in the tellurium gas phase are Te 2, so that Pve means the pressure of Te 2. For growth under tellurium vapor pressure, the
Nugraha et al. / Journal of Co'stal Growth 165 (1996) 402-407
highest tellurium vapor pressure applied to the melt (PTe) in this experiment is 10.6 Tort (TTe = 625°C). This growth condition always produces a p-type crystal and hole concentrations are of the order of l018 cm -3, but they show fluctuation along the crystal length as indicated by ( A ) in Fig. 2. For TTe = 555°C (PTe = 3.03 Ton'), a grown crystal is also p-type in all portions and hole concentrations are of the order of 1018 c m - 3 , but more homogeneously distributed along the crystal length and lower than those for TT, = 625°C, as indicated by ( Q ) in Fig. 2. For TT~ = 515°C (PTe = 1.34 Ton') and TT~ = 495°C (PT~ = 0.86 Ton'), the grown crystals have both p- and n-type conductivities. The p-type crystals regions are grown at the bottom portion of the crystals (the first-to-freeze end regions) as indicated by ( ~ ) (TTe = 515°C) and (El) (TTe = 495°C), but large portions of the crystals are n-type as indicated by ( 0 ) (TT~ = 515°C) and ( I ) (TT~ = 495°C) in Fig. 2. The hole concentrations of these p-type crystals regions are lower than those for TT, = 625°C and TTe = 555°C. The electron concent.rations of the n-type crystal regions are of the order of l017 c m -3 and nearly homogeneous along the crystal length. For TT~ =445°C (PTe =0.26 Ton-), the grown crystal is n-type in all portions. The carrier concen1019
~i..
T,,-55~*C
.•
t~
O 13
\ \
OTT.~IS°C • TT.=495*C • TT.=445°C
10r
"
I
10
,
I
20
,
I
I
30
h
Pmltiel (am) Fig. 2. Carder concentration distributions along PbTe crystal lengths for various temperatures of the tellurium vapor pressure control region (TTe).
1 0 "1
405
~
,
~, ".~ 10"2
,
-
~
° TT.~625C
o X.r,=SS~C •~..
~
,~ 0 • T~.~51~C • TT,~9~C
~, 80(
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'
Position (ram) Fig. 3. Mobility and resistivity distributions along crystal lengths for various temperatures of the tellurium vapor pressure control region (TTe).
trations are of the order of 10 L7 cm -3 and more homogeneously distributed, except at the bottom portion of the crystal. This result has shown that the conductivity of grown crystals can be controlled by the tellurium vapor pressure. For PTe > 3.03 Torr, the melt allows the segregation of tellurium excess crystals. At the highest pressure Pre = 10.6 Torr, the top portion of the crystal (about 4 mm) separates into two phases, that is, lead telluride crystals surrounded by tellurium crystals. However, for 1.34 > Pxe >_ 0.86 Torr, the melt slightly allows segregation of lead excess crystals. The p-type crystal region appears only at the bottom portion of the crystals. There is a possibility that the tellurium vapor pressure when the bottom portion of the melt solidifies first is a little higher than when the other portion solidifies. For PT~ < 0.26 Torr, the melt allows segregation of lead excess crystals at all portions of the melt. Fig. 3 shows the mobility and the resistivity distribution at 300 K of grown crystals for various applied tellurium pressures. The mobility of n-type crystals grown at T.r~ = 515 and 495°C is about 1200 cm2/V • s when n = 1 × 1017 cm -3, which is higher
Nugraha et al. / Journal of Crystal Growth 165 (1996) 402-407
406
10 -1
I
I
I
Q 2.10 -2 1200
s s e s s ~
800
• T~=450°C • Tr,=550°C
3.10
i
~'-,,,~
lOl:
rJ
I
/ I 10
I ,
I
20
,
I
30
Position(ram)
Fig. 4. Carrier concentrauon, mobility, and resistivitydistributions along crystal lengths for various temperatures of the lead vapor pressure control region (Tpb).
than the mobilities observed for n-type heat-treated crystals [1]. In that heat treatment experiment, the mobility reaches its maximum value, 1000 c m 2 / V • s, at a carrier concentration of about 4 X 1017 cm -3 but decreases as the carrier concentration decreases due to the formation of compensating point defects. So it can be said that the densities of the point defects of n-type crystals grown at TTe = 515 and 495°C are less than those of n-type heat-treated crystals. The electrical properties at 300 K of n-type crystals grown under controlled lead vapor pressures are shown in Fig. 4. The electron concentrations are of the order of 1017 c m - 3 and homogeneously distributed along the crystal length. The electron concentrations for Tpb = 555°C are lower than those for Tpb = 445°C, and the mobilities are higher. The mobilities are as high as those for n-type crystals grown at TTe = 515 and 495°C. Although we have done experiments for only two different Tpb, it can be said that n-type lead telluride crystals can be grown from
nearly stoichiometric melts under controlled lead vapor pressure, as well as growth under controlled tellurium vapor pressures PTe < 0.86 Torr. Comparing to n-type crystals grown under tellurium vapor pressure, the carrier concentrations and the mobilities of crystals grown under lead vapor pressure are more homogeneously distributed, especially for a crystal grown at Tpb = 555°C which has the highest mobility among the grown n-type crystals. Lead vapor pressures at Tpb = 445 and Tpb = 555°C are PPb = 4.92 × 10 - 7 Torr and PPb = 8.4 × 10- S Torr, respectively. Typical etch pits have been observed on (100) surfaces of PbTe for p-type and n-type crystals. There are two shapes of pits, pyramidal and flat, with various sizes. Average pit sizes for p-type crystals (T.re = 625°C and TTe = 555°C) are larger than those for n-type crystals (TTe = 515°C, TTe = 495°C, and TTe = 445°C). However, etch pit densities for p-type crystals are lower than those for n-type crystals. The etch pit density counted for all sizes and all shapes of etch pits as a function of applied tellurium vapor pressure is shown in Fig. 5. The etch pit density tends to become minimum at the tellurium vapor pressure region where a conductivity-type transition occurs. However, there has been no experimental evidence for the relation between dislocations and etch pits for this etch pit solution. In
|08
l0 0 i
. . . . .
I
'
'
I'
l 'o o ........ ....... Tellurium Pressure (tort)
'I
101
Fig. 5. Etch pit densities of PbTe crystals grown by the Bridgman method under controlled tellurium vapor pressure as a function of applied tellurium pressure.
Nugraha et al. / Journal of Crystal Growth 165 (1996) 402-407
particular, no report is present for n-type crystals. Therefore, a more detailed analysis for etch pits will be necessary. From the measurement of carrier concentration and mobility, as well as etch pit density, it is evident that the tellurium or lead vapor influences the stoichiometry of the grown crystals, which is the same phenomenon as that observed for III-V compounds [8]. It is thought that the optimum pressure for the growth of a nearly stoichiometric crystal lies in the range 3.03 > PTe > 1.34 Torr (555 > TTe > 515°C) where a conductivity-type transition occurs.
4. Conclusion The growth of PbTe from nearly stoichiometric melts by the Bridgman method under controlled tellurium or lead vapor pressure has been performed. The conductivity of the grown crystals can be controlled by the tellurium vapor pressure. For PTe > 3.03 Tort the growing crystals are p-type, for 3.03 > PTe > 0.86 Torr the growing crystals are n-type in large portions of the crystals, and for PTe < 0.26 Torr the grown crystal is n-type in all portions. The crystals grown under controlled lead vapor pressure are always n-type in all portions. Etch pits of p-type crystals are larger in average size but lower in density than those of n-type crystals. The etch pit density tends to become minimum at a vapor pressure region where a conductivity-type transition occurs. The tellurium vapor pressure at which the conductivity-type transition occurs lies in the range 3.03 >
407
Pve > 1.34 Torr (555 > TTe > 515°C), and the etch pit density tends to become minimum in this range. It is thought that the optimum pressure for the growth of nearly stoichiometyic crystals lies in the range 3.03 > PTe > 1.34 Torr (555 > TTe > 515°C).
References [1] Y. Sato, M. Fujimoto and A. Kobayashi, Jpn. J. Appl. Phys. 2 (1963) 688. [2] Y. Kanai and K. Shohno, Jpn. J. Appl. Phys. 2 (1963) 3. [3] R. Breschi, A, Camanzi and V. Fano, J. Crystal Growth 58 (1982) 399. [4] M.P. Gomez, D.A. Stevenson and R.A. Huggins, J. Phys. Chem. Solids 32 (1971) 335. [5] J.F. Miller, J.W. Moody and R.C. Himes, Trans. Met. Soc. AIME 239 (1967) 342. [6] W.D. Lawson, J. Appl. Phys. 23 (1952) 495. [7] J. Nishizawa, S. Shinozaki and K. Ishida, J. Appl. Phys. 44 (1973) 168. [8] J. Nishizawa and Y. Okuno, IEEE Trans. ED-22 (1975) 716. [9] J. Nishizawa and Y. Okuno, in: Semiconductor Optoelectronics, Ed. M.A. Herman (PWN-Polish, Warsawa, 1980). [10] J. Nishizawa, Y. Okuno and K. Suto, in: JARECT 19 Semiconductor Technology, Ed. J. Nishizawa (OHM and North-Holland, Tokyo, 1986). [1 l] K. Tomizawa, K. Sassa, Y. Shimanuki and J. Nishizawa, J. Electrochem. Soc. 131 (1984) 2394. [12] J. Nishizawa, H. Otsuka, S. Yamakoshi and K. lshida, Jpn. J. Appl. Phys. 13 (1974) 46. [13] J. Nishizawa, K. Itoh, Y. Okuno and F. Sakurai, J. Appl. Phys. 57 (1985) 2210. [14] J. Nishizawa and R. Suzuki, J. Appl. Phys. 59 (1986) 2256. [15] F. Sakurai, H, Fujishiro, K. Suto and J. Nishizawa, J. Crystal Growth 112 (1991) 153. [16] Nugraha, O. ltoh, K. Suto and J. Nishizawa, to be published. [17] M.K. Norr, J. Electrochem. Soc. 109 (1962) 433. [18] M.K. Norr, J. Appl. Phys. 31 (1960) 615.
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 165 (1996) 402-407
Growth and electrical properties of PbTe bulk crystals grown by the Bridgman method under controlled tellurium or lead vapor pressure N u g r a h a a, K e n Suto a, O s a m u Itoh c, Jun-ichi N i s h i z a w a b,c, * , Y a s u t o s h i Y o k o t a a a Department of Materials Science, Faculty of Engineering, Tohoku Unit,ersiO', Aramakiaza Aoba, Sendai 980, Japan b Tohoku University, Katahira 2-1-1 Aoba, Sendai 980, Japan c Semiconductor Research Institute, Kawauchi Aoba, Sendai 980, Japan Received 19 September 1995; accepted 23 December 1995
Abstract PbTe bulk crystals have been grown by the Bridgman method under controlled tellurium or lead vapor pressure. Both p-type crystals, and n-type crystals which have been difficult to grow by the conventional Bridgman method can be grown by this method. For PTe = 10.6 and 3.03 Torr, the grown crystals are p-type. For PTe = 1.34 and 0.86 Torr, large portions of grown crystals are n-type. For PTe = 0.26 Torr and lead vapor pressures of PPb = 4.92 × 10 - 7 and 8.4 × 10 -5 Torr, grown crystals are n-type. Average pit sizes of p-type crystals are larger than those of n-type crystals, but etch pit densities of p-type crystals are lower than those of n-type.
1. Introduction PbTe is a well known I V - V I group semiconductor used as a basic material for infrared optoelectronic devices. The excess of one constituent, lead or tellurium, in a PbTe crystal, i.e., deviation from stoichiometry greatly influences the electrical properties. It was reported that the excess of lead in PbTe causes n-type conductivity and the excess of tellurium causes p-type conductivity [1]. The ability to control the excess of a constituent in the crystal lattice, which means the ability to control electrical properties of crystals through controlling densities of
* Corresponding author. Fax: +81 22 2237289.
nonstoichiometric defects, is a very important point in device fabrication. The growth of PbTe by the Bridgman method from various melt compositions have been done by many authors. Grown crystals usually have p-type conductivity, and growth of n-type single crystals have been very difficult. Sato et al. [1] and Kanai et al. [2] reported that single crystals grown from nearly stoichiometric melts were always p-type. Although the growths were performed from melts with excess Pb, the n-type conductivity was found only at the top portion of a grown crystal [3] [4]. Another author reported that n-type crystals cannot readily be grown from melt [5]. Lawson [6] reported that there is a possibility to grow n-type crystals by using a treatment which removes surface oxide layers from start-
0022-0248/96515.00 Copyright © 1996 Elsevier Science B.V. All fights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 6 ) 0 0 2 2 0 - 5
Nugraha et al. / Journal qf Crystal Growth 165 (1996) 402-407
ing materials, but the result of measurement was not shown. The deviation from stoichiometry can be controlled by a heat-treatment process. Sato et al. [1] reported that, at a heat-treatment temperature of 600°C, the samples heat-treated under tellurium vapor pressures lower than 3 X 10 -5 Torr were n-type and those heat-treated under tellurium vapor pressures higher than 3 X 10 -5 Torr were p-type, but this method needs two step works, the growth process and the heat-treatment process. Nishizawa et al. [7-10] reported that stoichiometry controlled nearly perfect crystals of III-V compound semiconductors can be grown by using a temperature difference method under controlled vapor pressure (TDM-CVP). In this method, the vapor pressure of one element (usually a very volatile element) is applied on the growth solution. They applied this method to liquid phase epitaxy (LPE) [7-10] and melt growth [11]. An optimum pressure was found for GaAs as well as for GaP at which the grown crystals show minimum densities of point defects, dislocations, and carrier concentrations. The optimum arsenic pressure for GaAs obtained by this method was the same as the stoichiometric pressure obtained by heat-treatment experiments [8,12]. The essential point of this method is that the applied vapor pressure fixes the chemical potential of a volatile element in the gas phase, and the chemical potentials of the element in the liquid and solid phase are in equilibrium with the chemical potentials of the element in the gas phase, i.e., ~AL = ~
= ~A,"
(l)
In other words, the saturation solubility depends on the applied vapor pressure and influences the stoichiometry of the segregating crystals [10]. The TDM-CVP method was also applied to the growth of ZnSe that is one of II-VI compound semiconductors [13-15]. The growth of ZnSe by using conventional methods always revealed n-type conductivity. The TDM-CVP method enabled the growth of p-type ZnSe crystals and the fabrication of pn junctions which emitted blue light. However, the application of the TDM-CVP method in the growth of lead telluride has not yet been reported. We have firstly applied the TDM-CVP method in liquid phase epitaxial growth of lead
403
telluride [16]. Epitaxial layers grown from a lead solution have been always n-type, but the free electron concentration of the grown crystals can be controlled by applied tellurium pressure and becomes minimum at an optimum tellurium vapor pressure. In this paper, we report the growth of bulk single crystals of lead telluride by the Bridgman method from a nearly stoichiometric melt under controlled tellurium or lead vapor pressure. The main purpose of this experiment is to control the conductivity type of lead telluride single crystals.
2. Experimental procedure Lead telluride source crystals for Bridgman growth are synthesized from 0.1 mole nearly stoichiometric melts which have been mixed from 99.9999% purity lead and tellurium. Lead has been etched by C H 3 C O O H + H 2 0 2 (4: 1) to remove the surface oxide layer. Quartz ampoules (10 mm in diameter) are rinsed by organic and acid solutions, etched by HF, and baked at about 10 -6 Torr and 1000°C before use. The ampoule is sealed off with source elements at approximately 10 -6 Tort. The ampoule is then placed in a furnace at about 1000°C for 24 h to allow reaction of the elements and homogeneously mixing of the compound. After that, the melt is rapidly cooled by immersing the ampoule into room temperature water in order to obtain a homogeneous polycrystal ingot. The polycrystals are then transferred into an ampoule for Bridgman growth (13 mm in diameter) which is shown in Fig. 1. Pellets of lead or tellurium (about 0.3 g) are placed in the vapor pressure control region of the ampoule to control the vapor pressure of one constituent of the melt during the growth. A quartz rod is inserted between the lead telluride melt region and the vapor pressure control region to narrow the free space which connects the two regions. As a result, it is thought that Eq. (2) holds. The ampoule is then sealed off at about 10 -6 Torr and placed in a furnace which has the temperature profile shown in Fig. 1. The temperature profile is divided into two regions, a growth region and a pressure control region. The temperature gradient of the growth region at the
404
Nugraha et al. / Journal of C~stal Growth 165 (1996) 402-407
~ or P~URE
CONTROL
RF~ION
i -
/ / ~ 13ram PbTe
GROWTH REGION
lOOmm
~Meit
2 sink
T~qm-mture Fig. 1. Illustration of ampoule and temperature profile for Bridgman growth under controlled vapor pressure.
melting temperature of lead telluride, 924°C, is 3 5 40°C/cm. The thermocouple reading for the pressure control region has been controlled to + 0.1°C and that for the growth region to + I°C. After holding the growth ampoule at the initial position for about 24 h to make a homogenous melt and to permit diffusion of insoluble impurities to top or bottom of the melt, the ampoule is then lowered with a speed of 1.4 m m / d a y . When the growth has been finished, the crystal is slowly cooled to room temperature for 24 h, because rapid cooling can create strains in the crystal which make it weak and fragile. The temperature of the pressure control region used as a parameter for this experiment is 445,495, 515, 555 or 625°C for tellurium (TTe), and 445 or 555°C for lead (Tpb). The grown crystals ingot is cut in a direction normal to the growth direction to wafers with a
thickness of 1.2 mm. The samples for Hall and resistivity measurements and etch pit observation are taken from each wafer to obtain a cartier concentration profile along the crystal length. The Hall and resistivity measurements have been carried out by a conventional dc Hall measurement with indium as a contact material. Measurements have been performed at 300 K on 4 × 4 × 0.1 mm samples cut out from each wafer, polished and etched by a Norr etch solution. [17]. For etch pit observation, freshly cleaved surfaces from each of the wafers are etched by a solution NaOH (5 g ) + 12 (0.2 g ) + H 2 0 (10 ml) [18] at 95°C for about 5 min. Observation has been made by a scanning electron microscope.
3. R e s u l t s a n d d i s c u s s i o n
Crystals of 13 mm in diameter and about 30 mm in length have been grown from a nearly stoichiometry melt. All grown crystals have metallic appearance and cleave quite readily parallel to the (100) plane. The top region of a crystal ingot with a thickness of about 0.5 mm are tough and cannot be cleaved and are obviously composed of lead and lead telluride for n-type crystals and tellurium and lead telluride for p-type crystals. Fig. 2 shows a carrier concentration profile at 300 K along the growth axis for crystals grown under various temperatures of the vapor pressure control region. The vapor pressure applied to the melt during growth can be calculated from the temperature of the vapor pressure control region, Tve or Pb, by using the following equation which holds when the tube that connects the melt and the vapor pressure region is narrow enough [10]: eTe orPb = POeorPb( TG/TTeor Pb)1/2,
(2)
where Pve or Pb is the vapor pressure of Te or Pb applied to the melt, Tve or Pb is the temperature of the Te or Pb vapor pressure control region, POe or Pb the vapor pressure of Te or Pb at temperature Tve or Pb and TG is the growth temperature (nearly equal to the melting temperature of lead telluride, 924°C). In the present temperature range, the main molecular species in the tellurium gas phase are Te 2, so that Pve means the pressure of Te 2. For growth under tellurium vapor pressure, the
Nugraha et al. / Journal of Co'stal Growth 165 (1996) 402-407
highest tellurium vapor pressure applied to the melt (PTe) in this experiment is 10.6 Tort (TTe = 625°C). This growth condition always produces a p-type crystal and hole concentrations are of the order of l018 cm -3, but they show fluctuation along the crystal length as indicated by ( A ) in Fig. 2. For TTe = 555°C (PTe = 3.03 Ton'), a grown crystal is also p-type in all portions and hole concentrations are of the order of 1018 c m - 3 , but more homogeneously distributed along the crystal length and lower than those for TT, = 625°C, as indicated by ( Q ) in Fig. 2. For TT~ = 515°C (PTe = 1.34 Ton') and TT~ = 495°C (PT~ = 0.86 Ton'), the grown crystals have both p- and n-type conductivities. The p-type crystals regions are grown at the bottom portion of the crystals (the first-to-freeze end regions) as indicated by ( ~ ) (TTe = 515°C) and (El) (TTe = 495°C), but large portions of the crystals are n-type as indicated by ( 0 ) (TT~ = 515°C) and ( I ) (TT~ = 495°C) in Fig. 2. The hole concentrations of these p-type crystals regions are lower than those for TT, = 625°C and TTe = 555°C. The electron concent.rations of the n-type crystal regions are of the order of l017 c m -3 and nearly homogeneous along the crystal length. For TT~ =445°C (PTe =0.26 Ton-), the grown crystal is n-type in all portions. The carrier concen1019
~i..
T,,-55~*C
.•
t~
O 13
\ \
OTT.~IS°C • TT.=495*C • TT.=445°C
10r
"
I
10
,
I
20
,
I
I
30
h
Pmltiel (am) Fig. 2. Carder concentration distributions along PbTe crystal lengths for various temperatures of the tellurium vapor pressure control region (TTe).
1 0 "1
405
~
,
~, ".~ 10"2
,
-
~
° TT.~625C
o X.r,=SS~C •~..
~
,~ 0 • T~.~51~C • TT,~9~C
~, 80(
I 1'0
20
3'0
'
Position (ram) Fig. 3. Mobility and resistivity distributions along crystal lengths for various temperatures of the tellurium vapor pressure control region (TTe).
trations are of the order of 10 L7 cm -3 and more homogeneously distributed, except at the bottom portion of the crystal. This result has shown that the conductivity of grown crystals can be controlled by the tellurium vapor pressure. For PTe > 3.03 Torr, the melt allows the segregation of tellurium excess crystals. At the highest pressure Pre = 10.6 Torr, the top portion of the crystal (about 4 mm) separates into two phases, that is, lead telluride crystals surrounded by tellurium crystals. However, for 1.34 > Pxe >_ 0.86 Torr, the melt slightly allows segregation of lead excess crystals. The p-type crystal region appears only at the bottom portion of the crystals. There is a possibility that the tellurium vapor pressure when the bottom portion of the melt solidifies first is a little higher than when the other portion solidifies. For PT~ < 0.26 Torr, the melt allows segregation of lead excess crystals at all portions of the melt. Fig. 3 shows the mobility and the resistivity distribution at 300 K of grown crystals for various applied tellurium pressures. The mobility of n-type crystals grown at T.r~ = 515 and 495°C is about 1200 cm2/V • s when n = 1 × 1017 cm -3, which is higher
Nugraha et al. / Journal of Crystal Growth 165 (1996) 402-407
406
10 -1
I
I
I
Q 2.10 -2 1200
s s e s s ~
800
• T~=450°C • Tr,=550°C
3.10
i
~'-,,,~
lOl:
rJ
I
/ I 10
I ,
I
20
,
I
30
Position(ram)
Fig. 4. Carrier concentrauon, mobility, and resistivitydistributions along crystal lengths for various temperatures of the lead vapor pressure control region (Tpb).
than the mobilities observed for n-type heat-treated crystals [1]. In that heat treatment experiment, the mobility reaches its maximum value, 1000 c m 2 / V • s, at a carrier concentration of about 4 X 1017 cm -3 but decreases as the carrier concentration decreases due to the formation of compensating point defects. So it can be said that the densities of the point defects of n-type crystals grown at TTe = 515 and 495°C are less than those of n-type heat-treated crystals. The electrical properties at 300 K of n-type crystals grown under controlled lead vapor pressures are shown in Fig. 4. The electron concentrations are of the order of 1017 c m - 3 and homogeneously distributed along the crystal length. The electron concentrations for Tpb = 555°C are lower than those for Tpb = 445°C, and the mobilities are higher. The mobilities are as high as those for n-type crystals grown at TTe = 515 and 495°C. Although we have done experiments for only two different Tpb, it can be said that n-type lead telluride crystals can be grown from
nearly stoichiometric melts under controlled lead vapor pressure, as well as growth under controlled tellurium vapor pressures PTe < 0.86 Torr. Comparing to n-type crystals grown under tellurium vapor pressure, the carrier concentrations and the mobilities of crystals grown under lead vapor pressure are more homogeneously distributed, especially for a crystal grown at Tpb = 555°C which has the highest mobility among the grown n-type crystals. Lead vapor pressures at Tpb = 445 and Tpb = 555°C are PPb = 4.92 × 10 - 7 Torr and PPb = 8.4 × 10- S Torr, respectively. Typical etch pits have been observed on (100) surfaces of PbTe for p-type and n-type crystals. There are two shapes of pits, pyramidal and flat, with various sizes. Average pit sizes for p-type crystals (T.re = 625°C and TTe = 555°C) are larger than those for n-type crystals (TTe = 515°C, TTe = 495°C, and TTe = 445°C). However, etch pit densities for p-type crystals are lower than those for n-type crystals. The etch pit density counted for all sizes and all shapes of etch pits as a function of applied tellurium vapor pressure is shown in Fig. 5. The etch pit density tends to become minimum at the tellurium vapor pressure region where a conductivity-type transition occurs. However, there has been no experimental evidence for the relation between dislocations and etch pits for this etch pit solution. In
|08
l0 0 i
. . . . .
I
'
'
I'
l 'o o ........ ....... Tellurium Pressure (tort)
'I
101
Fig. 5. Etch pit densities of PbTe crystals grown by the Bridgman method under controlled tellurium vapor pressure as a function of applied tellurium pressure.
Nugraha et al. / Journal of Crystal Growth 165 (1996) 402-407
particular, no report is present for n-type crystals. Therefore, a more detailed analysis for etch pits will be necessary. From the measurement of carrier concentration and mobility, as well as etch pit density, it is evident that the tellurium or lead vapor influences the stoichiometry of the grown crystals, which is the same phenomenon as that observed for III-V compounds [8]. It is thought that the optimum pressure for the growth of a nearly stoichiometric crystal lies in the range 3.03 > PTe > 1.34 Torr (555 > TTe > 515°C) where a conductivity-type transition occurs.
4. Conclusion The growth of PbTe from nearly stoichiometric melts by the Bridgman method under controlled tellurium or lead vapor pressure has been performed. The conductivity of the grown crystals can be controlled by the tellurium vapor pressure. For PTe > 3.03 Tort the growing crystals are p-type, for 3.03 > PTe > 0.86 Torr the growing crystals are n-type in large portions of the crystals, and for PTe < 0.26 Torr the grown crystal is n-type in all portions. The crystals grown under controlled lead vapor pressure are always n-type in all portions. Etch pits of p-type crystals are larger in average size but lower in density than those of n-type crystals. The etch pit density tends to become minimum at a vapor pressure region where a conductivity-type transition occurs. The tellurium vapor pressure at which the conductivity-type transition occurs lies in the range 3.03 >
407
Pve > 1.34 Torr (555 > TTe > 515°C), and the etch pit density tends to become minimum in this range. It is thought that the optimum pressure for the growth of nearly stoichiometyic crystals lies in the range 3.03 > PTe > 1.34 Torr (555 > TTe > 515°C).
References [1] Y. Sato, M. Fujimoto and A. Kobayashi, Jpn. J. Appl. Phys. 2 (1963) 688. [2] Y. Kanai and K. Shohno, Jpn. J. Appl. Phys. 2 (1963) 3. [3] R. Breschi, A, Camanzi and V. Fano, J. Crystal Growth 58 (1982) 399. [4] M.P. Gomez, D.A. Stevenson and R.A. Huggins, J. Phys. Chem. Solids 32 (1971) 335. [5] J.F. Miller, J.W. Moody and R.C. Himes, Trans. Met. Soc. AIME 239 (1967) 342. [6] W.D. Lawson, J. Appl. Phys. 23 (1952) 495. [7] J. Nishizawa, S. Shinozaki and K. Ishida, J. Appl. Phys. 44 (1973) 168. [8] J. Nishizawa and Y. Okuno, IEEE Trans. ED-22 (1975) 716. [9] J. Nishizawa and Y. Okuno, in: Semiconductor Optoelectronics, Ed. M.A. Herman (PWN-Polish, Warsawa, 1980). [10] J. Nishizawa, Y. Okuno and K. Suto, in: JARECT 19 Semiconductor Technology, Ed. J. Nishizawa (OHM and North-Holland, Tokyo, 1986). [1 l] K. Tomizawa, K. Sassa, Y. Shimanuki and J. Nishizawa, J. Electrochem. Soc. 131 (1984) 2394. [12] J. Nishizawa, H. Otsuka, S. Yamakoshi and K. lshida, Jpn. J. Appl. Phys. 13 (1974) 46. [13] J. Nishizawa, K. Itoh, Y. Okuno and F. Sakurai, J. Appl. Phys. 57 (1985) 2210. [14] J. Nishizawa and R. Suzuki, J. Appl. Phys. 59 (1986) 2256. [15] F. Sakurai, H, Fujishiro, K. Suto and J. Nishizawa, J. Crystal Growth 112 (1991) 153. [16] Nugraha, O. ltoh, K. Suto and J. Nishizawa, to be published. [17] M.K. Norr, J. Electrochem. Soc. 109 (1962) 433. [18] M.K. Norr, J. Appl. Phys. 31 (1960) 615.