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Comparative study of point defects induced in PbTe thin films doped with Ga by different techniques
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 6 (2003) 481–485
Comparative study of point defects induced in PbTe thin films doped with Ga by different techniques$ Alexander M. Samoylova,*, Sergey A. Buchneva, Alexander M. Khoviva, Emma A. Dolgopolovaa, Vladimir P. Zlomanovb a
Department of Chemistry, Voronezh State University, Universitetskaya Sq., 1. 394006, Voronezh, Russia b Department of Chemistry, Moscow State University, Leninskie Gory, 1/3., GSP 119, Moscow, Russia
Abstract The comparison of the experimental results, which have been received in studies of chemical composition, crystal structure, and electronic properties of Ga-doped films on Si (1 0 0) and SiO2/Si (1 0 0) substrates, is presented in the present work. The Ga-doped PbTe films have been fabricated by two different ways. In contradistinction to the films, which have been doped by two zones annealing, the films synthesized by one-stage method using PbTe/GaS layers demonstrate the non-monotone dependence of lattice parameter and charge carrier densities with the Ga impurity concentration. Complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution GaPb or implantation Gay i of impurity atoms in the crystal structure of lead telluride. r 2003 Published by Elsevier Ltd. PACS: 68.55.J; 73.61.E Keywords: Crystal structure; Doping; Impurities; Gallium; Point defects; Hot wall epitaxy
1. Introduction The small band gap and high carrier mobilities of AIVBVI semiconductors identify them as the perspective materials for infrared (IR) optoelectronic devices [1]. The IR sensitivities of these materials are similar to that of Cd1xHgxTe, but processing procedures are much less demanding [2]. To make IR sensors, low carrier concentration material is necessary. It is well known that the way to decrease the carrier concentration in AIVBVI materials is doping with III A Periodic system group metals [3]. As it was demonstrated by numerous investigations, the effect of Fermi level pinning has been
established in the doped with Ga lead telluride bulk crystals and thin films [3,4]. However, many aspects are still unknown concerning the influence of Ga impurity atoms on the crystal structure and energy spectrum of lead telluride. Therefore, the main purposes of this study are to compare the experimental results, which have been received during the examination of the crystal structure and electronic properties of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by means of two different techniques and to find the associations between these properties and the nature of the point defects.
$
This work was supported by Research Scientific Program of the Ministry of Education of Russian Federation. Grant Number E02-5.0-289. *Corresponding author. Tel.: +7-73-2-789445; fax: +7-73-2789445. E-mail addresses: [email protected] (A.M. Samoylov), [email protected] (V.P. Zlomanov). 1369-8001/$ - see front matter r 2003 Published by Elsevier Ltd. doi:10.1016/j.mssp.2003.07.014
2. Experimental procedure In this work the gallium-doped PbTe films on Si (1 0 0) and SiO2/Si (1 0 0) substrates have been fabricated by two different ways. One of them is based on vapour phase doping procedure of PbTe/Si and PbTe/SiO2/Si
ARTICLE IN PRESS 482
A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 481–485
heterostructures, which were previously formed by modified ‘‘hot wall’’ (HWE) technique. Our basic design of HWE apparatus is similar to the system by Kinoshita et al. [5] with the additional sources of pure group-IV and group-VI elements improvements. With the help of HWE technique we can prepare the mirror-smooth surface PbTe thin films (thickness was about 0.5–7 mm), which were deposited directly on (1 0 0) Si high-ohmic substrates both with and without any buffer layer [6,7]. The partial pressure of residual gases of about 5 107 Pa can be realized in graphite reaction chamber during the evaporation process. An exposure of a substrate to Te2 molecules during 20–30 min directly before the condensation of the binary semiconductor has been used to remove the SiO2 natural layer from Si substrate surface. On the other hand, lead telluride layers were grown on Si substrates with the help of an intermediate buffer layers consisting of 200720 and 300730 nm thick previously formed SiO2. For further doping procedure PbTe samples with ptype of conductivity and charge carrier densities of about 1016–1017 cm3 at 77 K have been chosen. In pursuing these aims the two isothermal zones annealing them under different pressure of Ga2Te molecules in the vapour phase has performed. Heterogeneous alloy Ga0.60Te0.40 was used as a source of Ga2Te molecules during the diffusion process. In order to suppress sublimation process of the PbTe layers the values of saturated vapour pressures for L1–GaTe(S)–V equilibrium must be higher than those of integral pressure for PbTe(S)–V equilibrium. Thus, it can be seen that first doping technique consists of two stages. The second method of preparation of PbTe/GaS/Si and PbTe/GaS/SiO2/Si heterostructures offers the direct one-stage synthesis by HWE technique, in which the doping and the layer condensation processes proceed simultaneously. In this way the Pb1xGax (0.15pxGap0.95) liquid alloys have been employed as the sources of gallium and lead vapours coincidentally. High-purity Pb (99.999%), Ga (99.999%), and Te (99.99%) were used to prepare PbTe/GaS films by this way. The chemical composition of as-grown and Ga-doped PbTe/Si and PbTe/SiO2/Si layers was analyzed by electron probe microanalysis (EPMA). X-ray diffraction (XRD) patterns were obtained with CuKa-radiation on a computer-interfaced DRON-4-07 diffractometer. During XRD experiments single crystal Si (1 0 0) and Si (1 1 1) wafers were used as internal standards. The X-ray (4 0 0), (4 4 4) reflection profiles of Si substrates with various orientation, and (2 0 0), (4 0 0), (6 0 0) reflection profiles of PbTe films, respectively, were obtained with 0.01 step-by-step movement and sample rotation. The lattice parameter of PbTe films values have been calculated precisely by extrapolation to y=90 using the approximation of Nelson–Riley function [8].
The real microstructure and thickness of all the prepared samples were studied by a scanning electron microscopy (SEM) on CAMSCAN-4. The electrical properties of these thin films were measured by Van der Pauw method and by investigation of the C–V curves (‘‘Hg probe’’ method) too. As it is known [9], that oxidation process of AIVBVI semiconductors, which intentionally or unintentionally occurs on the surface, affects the electronic properties’ measurements of these materials greatly. In order to reduce this undesirable influence the silver contact pads have been prepared by thermal deposition in vacuum immediately after the plasma etching of PbTe/GaS/Si surface in Ar atmosphere within the same technological cycle.
3. Results and discussion As it has been mentioned above, Ga-doped lead telluride films were prepared by two different ways in the present study. Using Ga2Te molecules as the dopant of Ga in PbTe films is appropriate due to the two reasons. At first the doping experiments were carried out by two zones annealing in atmosphere of saturated vapour phase over gallium melts at different temperatures. However, the investigations of these specimens by X-ray analysis, high-power optical microscopy and EPMA show that the film surface was contaminated by Ga precipitates after annealing. Rather small thickness of PbTe layers on Si substrate is commensurable with the diameters of Ga precipitates. For this reason, it is impossible to carry out the chemical polishing of PbTe/ Si and PbTe/SiO2/Si surfaces, because in all cases practically it resulted in complete elimination of lead telluride condensate from Si substrate. Therefore, we employed the doping technique, which was based on the results of the thermodynamic analysis of vapour phase composition in Ga–Te system [10]. It has been established, that in vapour phase mole fraction of Ga2Te molecules is much higher than that of other molecules. The gallium mole fraction xGa in the vapour phase rises as a result of the decrease of the temperature and may approach the value of about 0.6670.02 at 800 K. Furthermore, the high-power optical microscopy, SEM, and EPMA investigations of these specimens show that after annealing the film surface has not been contaminated by any precipitates. The results of X-ray analysis have exhibited (2 0 0), (4 0 0), and (6 0 0) reflections of PbTe only. By another words, after annealing PbTe/GaS films were homogeneous. Second, this technique allows us to control the values of tellurium partial pressure for L1–GaTe(S)–V equilibrium by adjusting the temperature of Ga0.60Te0.40 heterogeneous alloy. The values of Te partial pressure for L1– GaTe(S)–V equilibrium were approximately equal to those for PbTe(S)QV equilibrium (conditions of lead
ARTICLE IN PRESS
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Eq. (4) shows that doped impurity atoms in interstitials Gai exhibit essentially the donor properties in PbTe crystal structure. As it can be seen from Eq. (5), the ionization of the Te vacancies VTe provides the increase in electron densities also. The electrical activity of GaPb atoms, which are substituted in the cation sublattice, seems to be unable to be established for certain. According to the theory of spontaneous dissociation of impurity centers [4] these GaPb atoms can be involved in reaction of disproportionation: 0 2Ga Pb #GaPb þ GaPb ;
ð7Þ
GaPb #Ga Pb þ h ;
ð8Þ
0 Ga0Pb #Ga Pb þ e :
ð9Þ
5500 6200 5100 5300 6350 — 2.34 1014 0.66 1014 5.62 1014 2.65 1014 0.78 1014 B1013 0.6467270.00006 0.6470070.00006 0.6464670.00006 0.6466870.00006 0.6468070.00006 0.6471070.00006 0.00170.0005 0.00370.0008 0.00170.0005 0.001570.0008 0.00270.0008 0.00470.001
Mobilitya (77 K) m (cm2/V s) Lattice parameter Charge carrier densities (77 K) p aPbTe (nm) (cm3) Concentration of Ga impurity atoms xGa (mole fraction)
The presented values of charge carrier mobilities m were calculated only for Ga-doped PbTe films on Si substrates. The values of this parameter for Ga-doped PbTe films on SiO2/ Si substrates are significantly lower.
0 V Pb #VPb þ h :
483
a
This doping method leads to decrease in hole densities of initial PbTe films due to some reasons. Firstly, according to Eq. (2) Ga atoms may occupy the positions of Pb vacancies VPb , which usually are ionized and act as the acceptors in PbTe
240 600 180 360 120 480
ð5Þ
823 823 803 803 873 873
0 V Te #VTe þ e :
2.81 101 2.81 101 5.32 102 5.32 102 1.356 1.356
ð4Þ
1.66 1016 1.66 1016 1.23 1016 1.23 1016 6.94 1016 6.94 1016
? 0 Ga i #Gai þ 3e ;
0.6461270.00006 0.6461270.00006 0.6461070.00006 0.6461070.00006 0.6460870.00006 0.6460870.00006
Some of these generated point defects may be distinctly ionized:
Duration of annealing t (min)
ð3Þ
Temperature of PbTe film TPbTe (K)
Ga2 TeðgÞ þ V i #GaPb þ TeTe þ Gai :
Ga partial pressure PGa (Pa)
ð2Þ
Lattice parameter Charge carrier densities (77 K)p aPbTe (nm) (cm3)
Ga2 TeðgÞ þ V Pb #2GaPb þ TeTe ;
Parameters of doped with Ga PbTe/Si and PbTe/SiO2/Si films
ð1Þ
Experimental conditions of doping procedure
Ga2 TeðgÞ þ 0#2Ga Pb þ TeTe þ VTe ;
Parameters of as-grownPbTe/Si and PbTe/SiO2/Si films
telluride congruent sublimation). Thus, after annealing the results of the quantitative chemical analysis confirmed that Te concentration in PbTe/GaS films has not been changed practically. The evolution of the lattice parameter, Ga impurity concentration and electrical properties with the certain experimental conditions and treatment duration are presented in Table 1. As it can be seen from Table 1, the values of Ga impurity content and unit cell parameter have risen monotonically with the increase in Ga partial pressure in saturated vapour phase and treatment duration. It is necessary to emphasize that hole densities have been decreased to the almost intrinsic values of about 1013 cm3 at 77 K under the same condition. Taking into account that the initial PbTe films correspond to the homogeneous region with excess of tellurium atoms before Ga doping procedure, the solidstate chemical reactions and diffusion processes can be expressed through the following schemes within the frameworks of the quasi-chemical method [11]:
Table 1 The values of physical parameters of as-grown PbTe films and PbTe/GaS films doped with Ga by annealing in saturated vapour phase for GaTe(S)–L1–V hetrogeneous equlibrium
A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 481–485
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Since the hole density decrease monotonically and the lattice parameter increase with increasing of Ga concentration, as presented in Table 1, it is reasonable to assume that the implantation of Ga impurities in lead vacancies and interstitials during vapour phase doping would predominate according to Eqs. (2)–(4). As it is demonstrated in Fig. 1, the values of Ga mole fraction in Pb1yGayTe films, which had been prepared by second one stage method, were about 0.00270.00020.01570.001 or 0.004oyGao0.03. It is necessary to note that the Ga concentration in Pb1yGayTe layers may be strictly controlled by setting the initial composition and the growth temperature of Pb1xGax liquid alloys. As it can be seen in Fig. 1, it is unable to represent the dependence the lattice parameter aPbTe/GaS on the concentration yGa as the monotonic function. The first section of this curve exhibits the decrease in aPbTe/GaS values within the concentration range 0oyGao0.0037. At yGa=0.003770.0002 the lattice parameter minimum is observed. The second section of this dependence shows the increase in aPbTe/GaS within the concentration range 0.0037oyGao0.012. As shown in Fig. 2, the charge carrier densities of the films exhibit non-monotone variation with Ga contamination also. The hole densities slowly mounted at impurity concentration yGao0.0031, for example, from p=0.95 1016 to 4.0 1016 cm3 at 77 K with yGa rising from 0.001 to 0.0022 in Pb1yGayTe films (Fig. 2). However, the increase of Ga atoms content at the range 0.0031oyGao0.0065 is accompanied by the decrease of hole densities from p=2.5 1016 to almost intrinsic values approximately 4.0 1013 cm3 at 77 K. It is necessary to emphasize that once the Ga impurity concentration yGa=0.008 is achieved, the inversion of the type of conductivity established in Pb1yGayTe films. The films fabricated under these circumstances exhibit n-type of conductivity with electron densities within the range from 3.5 1015 to 4.0 1016 cm3 at 77 K (Fig. 2).
Fig. 1. The evolution of lattice parameter aPbTe/GaS values with gallium concentration yGa of Pb1yGayTe films, which have been fabricated by one-stage direct synthesis method.
Fig. 2. The charge carrier densities temperature dependence of Pb1yGayTe films, (1–3, 5, 6—p-type of conductivity; 4—n-type of conductivity): 1—yGa=0.0065; 2—yGa=0.0037; 3—undoped PbTe ; 4—yGa=0.008; 5—yGa=0.001; 6—yGa=0.0022.
Summarizing all the experimental results allows us to assume that non-monotone character of the variation of the lattice parameter and the charge carrier densities is caused by the different position of Ga impurity atoms in PbTe crystal structure. As it is known, Ga atoms and ions are smaller in diameter than Pb ones [12]. Thus, the substitution of Pb by Ga impurity in its regular positions in PbTe crystal structure brings the decrease of the lattice parameter at 0oyGao0.0037 (Fig. 1). A closer look at these data allows us to make the proposal that saturation of Ga quasi-local impurity levels resulting in Fermi energy EF pinning is accomplished for the concentration range 0oyGao0.0031 in Pb1yGayTe films. In this case neutral or acceptor behaviour of GaPb atoms (Eq. (7)) is primarily responsible for slight increase of the hole densities. It seems reasonable to say that the following rise in impurity concentration in Pb1yGayTe films yGa>0.0037 has been associated with the change of forming mechanism of real crystal structure. The increase in lattice parameter (Fig. 1) is the evidence for built-up of the distortion in PbTe crystal structure. The appearance of this distortion can be caused probably by the Ga? atoms occupied in the tetrahedral voids in i PbTe structure. As it is mentioned above, the interstitial Ga impurity atoms according to Eq. (4) exhibit only the donor properties in this case. Mathematical treatment of the experimental curves ln s ¼ f ðTÞ for Ga-doped PbTe layers allows us to determine the values of thermal activation energy of conductivity Ea. It has been found that for all PbTe/GaS films, which have been doped by annealing in vapour phase over L1–GaTe(S) alloys, two values of Ea1=0.1170.005 and Ea2=0.1870.005 eV exist at different temperatures. Fabricated by one-stage
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method Pb1yGayTe layers have exhibited single value of Ea=0.10570.005 eV, if yGao0.004270.0002. On the contrary, if yGa>0.004870.0002 two sections with Ea1=0.105 eV and Ea2=0.16 eV can be clearly defined on the curves ln s ¼ f ðTÞ: This fact may also be considered as the evidence of the existence of two types of Ga impurity levels. The comparison of all experimental data presented in this paper makes it possible to come to the conclusion that the complicate amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by the different mechanisms of substitution GaPb or implantation Ga? i of impurity atoms in the PbTe crystal structure.
4. Conclusions The comparison of the experimental results, which have been received by investigation of chemical composition, crystal structure and electronic properties of PbTe/GaS films doped on the one hand, by vapour phase annealing, and on the other hand, by one stage direct synthesis, will be important in the understanding of the amphoteric behaviour of III A metals impurities in AIVBVI narrow band gap semiconductors. The results of this study clearly demonstrate the different character in the lattice parameter and the charge carrier densities evolutions with the Ga impurity concentration for PbTe/ GaS films prepared by these techniques. It is possible that the ambiguous amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution (GaPb ) or implantation (Ga? i ) of impurity atoms in the crystal structure of lead telluride.
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References [1] Munoz V, Lasbley A, Klotz S, Triboulet R. Synthesis and growth of PbTe crystals at low temperature and their characterization. J Cryst Growth 1999;196:71–6. [2] Zogg H, Fach A, Maissen C, Masec J, Blunier S. Photovoltaic lead-chalcogenide on silicon infrared sensor arrays. Opt Eng 1994;33(5):1440–9. [3] Akimov BA, Dmitriev AV, Khokhlov DR, Ryabova LI. Carrier transport and non-equilibrium phenomena in doped PbTe and related materials. Phys Status Sol A 1993;137(8):9–55. [4] Drabkin IA, Moyjes BY. The spontaneous dissociations of the neutral states into positive and negative states. Phys Technol Semicond 1981;15(4):625–44. [5] Kinoshita H, Sakashita T, Fujiyasu H. PbTe Multiplelayer films prepared by a hot wall technique. J Appl Phys 1981;52(4):2869–71. [6] Ugai YA, Samoylov AM, Sharov MK, Tadeev AV. Crystal microstructure of PbTe/Si and PbTe/SiO2/Si thin films. Thin Solid Films 1998;336:196–200. [7] Ugai YA, Samoylov AM, Synorov YV, Yatsenko OB. Electrical properties of PbTe thin films formed on Si substrates. Inorg Mater 2000;36(5):550–5. [8] Gorelik SS, Rastorguev LN, Skakov YA. X-ray analysis and electron microscopy. Moscow: Nauka; 1994. [9] Yashina LV, Kobeleva SP, Shatalova TB, Zlomanov VP, Shtanov VI. XPS study of fresh and oxidized GeTe and (Ge, Sn)Te surface. Solid State Ionics 2001;141–142: 513–22. [10] Zlomanov VP, Novoselova AV. P–T–x phase diagrams of metal–chalcogen systems. Moscow: Nauka; 1987. . [11] Kroger FA. The chemistry of imperfect crystals. Amsterdam: North-Holland Publishing Company; 1964. [12] Emsley J. The elements. Oxford: Clarendon Press; 1991.
Materials Science in Semiconductor Processing 6 (2003) 481–485
Comparative study of point defects induced in PbTe thin films doped with Ga by different techniques$ Alexander M. Samoylova,*, Sergey A. Buchneva, Alexander M. Khoviva, Emma A. Dolgopolovaa, Vladimir P. Zlomanovb a
Department of Chemistry, Voronezh State University, Universitetskaya Sq., 1. 394006, Voronezh, Russia b Department of Chemistry, Moscow State University, Leninskie Gory, 1/3., GSP 119, Moscow, Russia
Abstract The comparison of the experimental results, which have been received in studies of chemical composition, crystal structure, and electronic properties of Ga-doped films on Si (1 0 0) and SiO2/Si (1 0 0) substrates, is presented in the present work. The Ga-doped PbTe films have been fabricated by two different ways. In contradistinction to the films, which have been doped by two zones annealing, the films synthesized by one-stage method using PbTe/GaS layers demonstrate the non-monotone dependence of lattice parameter and charge carrier densities with the Ga impurity concentration. Complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution GaPb or implantation Gay i of impurity atoms in the crystal structure of lead telluride. r 2003 Published by Elsevier Ltd. PACS: 68.55.J; 73.61.E Keywords: Crystal structure; Doping; Impurities; Gallium; Point defects; Hot wall epitaxy
1. Introduction The small band gap and high carrier mobilities of AIVBVI semiconductors identify them as the perspective materials for infrared (IR) optoelectronic devices [1]. The IR sensitivities of these materials are similar to that of Cd1xHgxTe, but processing procedures are much less demanding [2]. To make IR sensors, low carrier concentration material is necessary. It is well known that the way to decrease the carrier concentration in AIVBVI materials is doping with III A Periodic system group metals [3]. As it was demonstrated by numerous investigations, the effect of Fermi level pinning has been
established in the doped with Ga lead telluride bulk crystals and thin films [3,4]. However, many aspects are still unknown concerning the influence of Ga impurity atoms on the crystal structure and energy spectrum of lead telluride. Therefore, the main purposes of this study are to compare the experimental results, which have been received during the examination of the crystal structure and electronic properties of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by means of two different techniques and to find the associations between these properties and the nature of the point defects.
$
This work was supported by Research Scientific Program of the Ministry of Education of Russian Federation. Grant Number E02-5.0-289. *Corresponding author. Tel.: +7-73-2-789445; fax: +7-73-2789445. E-mail addresses: [email protected] (A.M. Samoylov), [email protected] (V.P. Zlomanov). 1369-8001/$ - see front matter r 2003 Published by Elsevier Ltd. doi:10.1016/j.mssp.2003.07.014
2. Experimental procedure In this work the gallium-doped PbTe films on Si (1 0 0) and SiO2/Si (1 0 0) substrates have been fabricated by two different ways. One of them is based on vapour phase doping procedure of PbTe/Si and PbTe/SiO2/Si
ARTICLE IN PRESS 482
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heterostructures, which were previously formed by modified ‘‘hot wall’’ (HWE) technique. Our basic design of HWE apparatus is similar to the system by Kinoshita et al. [5] with the additional sources of pure group-IV and group-VI elements improvements. With the help of HWE technique we can prepare the mirror-smooth surface PbTe thin films (thickness was about 0.5–7 mm), which were deposited directly on (1 0 0) Si high-ohmic substrates both with and without any buffer layer [6,7]. The partial pressure of residual gases of about 5 107 Pa can be realized in graphite reaction chamber during the evaporation process. An exposure of a substrate to Te2 molecules during 20–30 min directly before the condensation of the binary semiconductor has been used to remove the SiO2 natural layer from Si substrate surface. On the other hand, lead telluride layers were grown on Si substrates with the help of an intermediate buffer layers consisting of 200720 and 300730 nm thick previously formed SiO2. For further doping procedure PbTe samples with ptype of conductivity and charge carrier densities of about 1016–1017 cm3 at 77 K have been chosen. In pursuing these aims the two isothermal zones annealing them under different pressure of Ga2Te molecules in the vapour phase has performed. Heterogeneous alloy Ga0.60Te0.40 was used as a source of Ga2Te molecules during the diffusion process. In order to suppress sublimation process of the PbTe layers the values of saturated vapour pressures for L1–GaTe(S)–V equilibrium must be higher than those of integral pressure for PbTe(S)–V equilibrium. Thus, it can be seen that first doping technique consists of two stages. The second method of preparation of PbTe/GaS/Si and PbTe/GaS/SiO2/Si heterostructures offers the direct one-stage synthesis by HWE technique, in which the doping and the layer condensation processes proceed simultaneously. In this way the Pb1xGax (0.15pxGap0.95) liquid alloys have been employed as the sources of gallium and lead vapours coincidentally. High-purity Pb (99.999%), Ga (99.999%), and Te (99.99%) were used to prepare PbTe/GaS films by this way. The chemical composition of as-grown and Ga-doped PbTe/Si and PbTe/SiO2/Si layers was analyzed by electron probe microanalysis (EPMA). X-ray diffraction (XRD) patterns were obtained with CuKa-radiation on a computer-interfaced DRON-4-07 diffractometer. During XRD experiments single crystal Si (1 0 0) and Si (1 1 1) wafers were used as internal standards. The X-ray (4 0 0), (4 4 4) reflection profiles of Si substrates with various orientation, and (2 0 0), (4 0 0), (6 0 0) reflection profiles of PbTe films, respectively, were obtained with 0.01 step-by-step movement and sample rotation. The lattice parameter of PbTe films values have been calculated precisely by extrapolation to y=90 using the approximation of Nelson–Riley function [8].
The real microstructure and thickness of all the prepared samples were studied by a scanning electron microscopy (SEM) on CAMSCAN-4. The electrical properties of these thin films were measured by Van der Pauw method and by investigation of the C–V curves (‘‘Hg probe’’ method) too. As it is known [9], that oxidation process of AIVBVI semiconductors, which intentionally or unintentionally occurs on the surface, affects the electronic properties’ measurements of these materials greatly. In order to reduce this undesirable influence the silver contact pads have been prepared by thermal deposition in vacuum immediately after the plasma etching of PbTe/GaS/Si surface in Ar atmosphere within the same technological cycle.
3. Results and discussion As it has been mentioned above, Ga-doped lead telluride films were prepared by two different ways in the present study. Using Ga2Te molecules as the dopant of Ga in PbTe films is appropriate due to the two reasons. At first the doping experiments were carried out by two zones annealing in atmosphere of saturated vapour phase over gallium melts at different temperatures. However, the investigations of these specimens by X-ray analysis, high-power optical microscopy and EPMA show that the film surface was contaminated by Ga precipitates after annealing. Rather small thickness of PbTe layers on Si substrate is commensurable with the diameters of Ga precipitates. For this reason, it is impossible to carry out the chemical polishing of PbTe/ Si and PbTe/SiO2/Si surfaces, because in all cases practically it resulted in complete elimination of lead telluride condensate from Si substrate. Therefore, we employed the doping technique, which was based on the results of the thermodynamic analysis of vapour phase composition in Ga–Te system [10]. It has been established, that in vapour phase mole fraction of Ga2Te molecules is much higher than that of other molecules. The gallium mole fraction xGa in the vapour phase rises as a result of the decrease of the temperature and may approach the value of about 0.6670.02 at 800 K. Furthermore, the high-power optical microscopy, SEM, and EPMA investigations of these specimens show that after annealing the film surface has not been contaminated by any precipitates. The results of X-ray analysis have exhibited (2 0 0), (4 0 0), and (6 0 0) reflections of PbTe only. By another words, after annealing PbTe/GaS films were homogeneous. Second, this technique allows us to control the values of tellurium partial pressure for L1–GaTe(S)–V equilibrium by adjusting the temperature of Ga0.60Te0.40 heterogeneous alloy. The values of Te partial pressure for L1– GaTe(S)–V equilibrium were approximately equal to those for PbTe(S)QV equilibrium (conditions of lead
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Eq. (4) shows that doped impurity atoms in interstitials Gai exhibit essentially the donor properties in PbTe crystal structure. As it can be seen from Eq. (5), the ionization of the Te vacancies VTe provides the increase in electron densities also. The electrical activity of GaPb atoms, which are substituted in the cation sublattice, seems to be unable to be established for certain. According to the theory of spontaneous dissociation of impurity centers [4] these GaPb atoms can be involved in reaction of disproportionation: 0 2Ga Pb #GaPb þ GaPb ;
ð7Þ
GaPb #Ga Pb þ h ;
ð8Þ
0 Ga0Pb #Ga Pb þ e :
ð9Þ
5500 6200 5100 5300 6350 — 2.34 1014 0.66 1014 5.62 1014 2.65 1014 0.78 1014 B1013 0.6467270.00006 0.6470070.00006 0.6464670.00006 0.6466870.00006 0.6468070.00006 0.6471070.00006 0.00170.0005 0.00370.0008 0.00170.0005 0.001570.0008 0.00270.0008 0.00470.001
Mobilitya (77 K) m (cm2/V s) Lattice parameter Charge carrier densities (77 K) p aPbTe (nm) (cm3) Concentration of Ga impurity atoms xGa (mole fraction)
The presented values of charge carrier mobilities m were calculated only for Ga-doped PbTe films on Si substrates. The values of this parameter for Ga-doped PbTe films on SiO2/ Si substrates are significantly lower.
0 V Pb #VPb þ h :
483
a
This doping method leads to decrease in hole densities of initial PbTe films due to some reasons. Firstly, according to Eq. (2) Ga atoms may occupy the positions of Pb vacancies VPb , which usually are ionized and act as the acceptors in PbTe
240 600 180 360 120 480
ð5Þ
823 823 803 803 873 873
0 V Te #VTe þ e :
2.81 101 2.81 101 5.32 102 5.32 102 1.356 1.356
ð4Þ
1.66 1016 1.66 1016 1.23 1016 1.23 1016 6.94 1016 6.94 1016
? 0 Ga i #Gai þ 3e ;
0.6461270.00006 0.6461270.00006 0.6461070.00006 0.6461070.00006 0.6460870.00006 0.6460870.00006
Some of these generated point defects may be distinctly ionized:
Duration of annealing t (min)
ð3Þ
Temperature of PbTe film TPbTe (K)
Ga2 TeðgÞ þ V i #GaPb þ TeTe þ Gai :
Ga partial pressure PGa (Pa)
ð2Þ
Lattice parameter Charge carrier densities (77 K)p aPbTe (nm) (cm3)
Ga2 TeðgÞ þ V Pb #2GaPb þ TeTe ;
Parameters of doped with Ga PbTe/Si and PbTe/SiO2/Si films
ð1Þ
Experimental conditions of doping procedure
Ga2 TeðgÞ þ 0#2Ga Pb þ TeTe þ VTe ;
Parameters of as-grownPbTe/Si and PbTe/SiO2/Si films
telluride congruent sublimation). Thus, after annealing the results of the quantitative chemical analysis confirmed that Te concentration in PbTe/GaS films has not been changed practically. The evolution of the lattice parameter, Ga impurity concentration and electrical properties with the certain experimental conditions and treatment duration are presented in Table 1. As it can be seen from Table 1, the values of Ga impurity content and unit cell parameter have risen monotonically with the increase in Ga partial pressure in saturated vapour phase and treatment duration. It is necessary to emphasize that hole densities have been decreased to the almost intrinsic values of about 1013 cm3 at 77 K under the same condition. Taking into account that the initial PbTe films correspond to the homogeneous region with excess of tellurium atoms before Ga doping procedure, the solidstate chemical reactions and diffusion processes can be expressed through the following schemes within the frameworks of the quasi-chemical method [11]:
Table 1 The values of physical parameters of as-grown PbTe films and PbTe/GaS films doped with Ga by annealing in saturated vapour phase for GaTe(S)–L1–V hetrogeneous equlibrium
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Since the hole density decrease monotonically and the lattice parameter increase with increasing of Ga concentration, as presented in Table 1, it is reasonable to assume that the implantation of Ga impurities in lead vacancies and interstitials during vapour phase doping would predominate according to Eqs. (2)–(4). As it is demonstrated in Fig. 1, the values of Ga mole fraction in Pb1yGayTe films, which had been prepared by second one stage method, were about 0.00270.00020.01570.001 or 0.004oyGao0.03. It is necessary to note that the Ga concentration in Pb1yGayTe layers may be strictly controlled by setting the initial composition and the growth temperature of Pb1xGax liquid alloys. As it can be seen in Fig. 1, it is unable to represent the dependence the lattice parameter aPbTe/GaS on the concentration yGa as the monotonic function. The first section of this curve exhibits the decrease in aPbTe/GaS values within the concentration range 0oyGao0.0037. At yGa=0.003770.0002 the lattice parameter minimum is observed. The second section of this dependence shows the increase in aPbTe/GaS within the concentration range 0.0037oyGao0.012. As shown in Fig. 2, the charge carrier densities of the films exhibit non-monotone variation with Ga contamination also. The hole densities slowly mounted at impurity concentration yGao0.0031, for example, from p=0.95 1016 to 4.0 1016 cm3 at 77 K with yGa rising from 0.001 to 0.0022 in Pb1yGayTe films (Fig. 2). However, the increase of Ga atoms content at the range 0.0031oyGao0.0065 is accompanied by the decrease of hole densities from p=2.5 1016 to almost intrinsic values approximately 4.0 1013 cm3 at 77 K. It is necessary to emphasize that once the Ga impurity concentration yGa=0.008 is achieved, the inversion of the type of conductivity established in Pb1yGayTe films. The films fabricated under these circumstances exhibit n-type of conductivity with electron densities within the range from 3.5 1015 to 4.0 1016 cm3 at 77 K (Fig. 2).
Fig. 1. The evolution of lattice parameter aPbTe/GaS values with gallium concentration yGa of Pb1yGayTe films, which have been fabricated by one-stage direct synthesis method.
Fig. 2. The charge carrier densities temperature dependence of Pb1yGayTe films, (1–3, 5, 6—p-type of conductivity; 4—n-type of conductivity): 1—yGa=0.0065; 2—yGa=0.0037; 3—undoped PbTe ; 4—yGa=0.008; 5—yGa=0.001; 6—yGa=0.0022.
Summarizing all the experimental results allows us to assume that non-monotone character of the variation of the lattice parameter and the charge carrier densities is caused by the different position of Ga impurity atoms in PbTe crystal structure. As it is known, Ga atoms and ions are smaller in diameter than Pb ones [12]. Thus, the substitution of Pb by Ga impurity in its regular positions in PbTe crystal structure brings the decrease of the lattice parameter at 0oyGao0.0037 (Fig. 1). A closer look at these data allows us to make the proposal that saturation of Ga quasi-local impurity levels resulting in Fermi energy EF pinning is accomplished for the concentration range 0oyGao0.0031 in Pb1yGayTe films. In this case neutral or acceptor behaviour of GaPb atoms (Eq. (7)) is primarily responsible for slight increase of the hole densities. It seems reasonable to say that the following rise in impurity concentration in Pb1yGayTe films yGa>0.0037 has been associated with the change of forming mechanism of real crystal structure. The increase in lattice parameter (Fig. 1) is the evidence for built-up of the distortion in PbTe crystal structure. The appearance of this distortion can be caused probably by the Ga? atoms occupied in the tetrahedral voids in i PbTe structure. As it is mentioned above, the interstitial Ga impurity atoms according to Eq. (4) exhibit only the donor properties in this case. Mathematical treatment of the experimental curves ln s ¼ f ðTÞ for Ga-doped PbTe layers allows us to determine the values of thermal activation energy of conductivity Ea. It has been found that for all PbTe/GaS films, which have been doped by annealing in vapour phase over L1–GaTe(S) alloys, two values of Ea1=0.1170.005 and Ea2=0.1870.005 eV exist at different temperatures. Fabricated by one-stage
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method Pb1yGayTe layers have exhibited single value of Ea=0.10570.005 eV, if yGao0.004270.0002. On the contrary, if yGa>0.004870.0002 two sections with Ea1=0.105 eV and Ea2=0.16 eV can be clearly defined on the curves ln s ¼ f ðTÞ: This fact may also be considered as the evidence of the existence of two types of Ga impurity levels. The comparison of all experimental data presented in this paper makes it possible to come to the conclusion that the complicate amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by the different mechanisms of substitution GaPb or implantation Ga? i of impurity atoms in the PbTe crystal structure.
4. Conclusions The comparison of the experimental results, which have been received by investigation of chemical composition, crystal structure and electronic properties of PbTe/GaS films doped on the one hand, by vapour phase annealing, and on the other hand, by one stage direct synthesis, will be important in the understanding of the amphoteric behaviour of III A metals impurities in AIVBVI narrow band gap semiconductors. The results of this study clearly demonstrate the different character in the lattice parameter and the charge carrier densities evolutions with the Ga impurity concentration for PbTe/ GaS films prepared by these techniques. It is possible that the ambiguous amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution (GaPb ) or implantation (Ga? i ) of impurity atoms in the crystal structure of lead telluride.
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