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Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods
Journal of Crystal Growth 240 (2002) 340–346
Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods A.M. Samoylova,*, M.K. Sharova, S.A. Buchneva, A.M. Khoviva, E.A. Dolgopolovab a
Department of Chemistry, Voronezh State University, Universitetskaya Sq. 1, 394693 Voronezh, Russia b Department of Physics, Voronezh State University, Universitetskaya Sq. 1, 394693 Voronezh, Russia Received 26 September 2001; accepted 28 December 2001 Communicated by T. Hibiya
Abstract The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO2/Si heterostructures, which were previously formed by the modified ‘‘hot wall’’ technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO2/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.J; 73.61.E Keywords: A1. Crystal structure; A1. Doping; A1. Impurities; A1. Point defects; A3. Hot wall epitaxy; B2. Semiconducting lead compounds
1. Introduction Because of its interesting physical properties, lead telluride has been studied intensively for more than 40 years. Its small band gap and high carrier *Corresponding author. Department of Physics, Voronezh State University, Universitetskaya Sq. 1, 394693 Voronezh, Russia. Tel./fax: +7-73-2-789445. E-mail address: [email protected] (A.M. Samoylov).
mobilities identify it as a basic material for infrared (IR) optoelectronic devices [1]. To make IR sensors, a low carrier concentration material is necessary. Preparation of the AIVBVI compounds with low carrier densities is difficult because of the stoichiometric deviation. The IR sensitivities of these materials are similar to that of Cd1xHgxTe, but processing procedures are much less demanding [2]. It is well known that the way to decrease the carrier concentration in AIVBVI materials is
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 0 9 1 2 - 0
A.M. Samoylov et al. / Journal of Crystal Growth 240 (2002) 340–346
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 lead telluride bulk and thin films for doped with Ga and In [3–6]. However, many aspects concerning the influence of Ga and In impurity atoms on the PbTe crystal structure and energy spectrum are still unknown. Some fundamental aspects of the formation, saturation and influence of quasi-local impurity levels in PbTe heavily doped with In and Ga have not been fully understood yet. Therefore, the main purposes of this study are to discuss the crystal structure, electronic properties and IR photoconductivity of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by means of two different techniques.
2. Experimental procedure The modified ‘‘hot wall’’ technique (HWE) has been employed for preparation of mirror-smooth surface PbTe thin films both doped with Ga and undoped (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 [7,8]. Our basic design of the HWE apparatus is similar to the system by Kinoshita et al. [9] with the essential improvements using an additional source of pure group-IV and group-VI elements. The partial pressure of residual gases of about 5 107 Pa can be realized in a graphite reaction chamber during the evaporation process. An exposure of the substrate to Te2 molecules during 20–30 min directly before the condensation of the binary semiconductor has been used for the removal of a SiO2 natural layer from the Si substrate surface. On the other hand, lead telluride layers were grown on Si substrates with the help of an intermediate buffer layer consisting of 100715, 200720 and 300730 nm thick previously formed SiO2. As we cannot pose the problem of highquality PbTe film growth in this paper and can only determine the evolution of the lattice parameter and electrical properties with contamination of Ga impurity atoms in PbTe(Ga) films fabricated by two different techniques, we decided to use
341
SiO2 buffer layers on Si substrates. In other words, the simple and reliable procedure of thermal oxidation of Si wafers in dry O2 does not make the solution to the main problem of this study very difficult. Different experimental methods were employed for the investigation of the lead telluride thin films with and without doping Ga on Si substrates. The chemical composition of as-grown and Ga-doped PbTe/Si and PbTe/SiO2/Si layers was analyzed by the electron probe microanalysis (EPMA) method on JEOL-JCA-840. X-ray diffraction (XRD) patterns were obtained with Cu-radiation (kðCuKa1 Þ ¼ 0:154051 nm) on a DRON-4-07 diffractometer. During XRD experiments, single crystal Si(1 0 0) and Si(1 1 1) substrates were used as internal standards. The (4 0 0), (4 4 4) and (2 0 0), (4 0 0), (6 0 0) X-ray reflection profiles of Si substrates of various orientations and PbTe films, respectively, were obtained with special care by the DRON-4-07 diffractometer with 0.011 step-bystep movement. The lattice parameter of PbTe films values have been calculated precisely by extrapolation to y ¼ 901 using the approximation function [10]: f ðyÞ ¼ 0:5 ðcos2 y=y þ cos2 y=sinyÞ;
ð1Þ
where y is the diffraction angle. The crystal microstructure and thickness of all the prepared samples have been studied by scanning electron microscopy (SEM) on CAMSCAN-4. The electrical parameters of these thin films have been determined by the standard Van der Pauw technique and by investigation of the C–V curves (‘‘Hg probe’’ method) too. An IR spectrometer was employed for studying the IR photoconductivity of PbTe/Si and PbTe/SiO2/Si heterostructures at the temperature range of 4–300 K. The source of IR-radiation is characterized as the Gaussian energy distribution within the wave length range of 2–20 mm.
3. Results and discussion In this work Ga-doped lead telluride films have been prepared by two different ways. One of them is principally based on the vapour-phase doping
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procedure of PbTe/Si and PbTe/SiO2/Si heterostructures, which were previously formed by modified HWE technique. For the doping procedure, lead telluride samples with p-type of conductivity and charge carrier densities of about 1016–1017 cm3 at 77 K have been chosen. Firstly, the doping experiments have been carried out by two zones annealing in vapour phase for Ga(L)–V equilibrium. However, the investigations of these specimens by X-ray analysis, high-power optical microscopy and EPMA show that the film surface has been contaminated by Ga precipitates after annealing. The 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, practically in all cases, it resulted in complete elimination of lead telluride layers from Si substrates. Therefore, in this work we have employed the doping technique, which is based on the results of the thermodynamic analysis of vapour-phase composition in the Ga–Te system [11]. It has been established that mole fraction of Ga2Te molecules in vapour phase is much higher than that of other molecules. The gallium atomic fraction xGa in the vapour phase rises as a result of the decrease in temperature and may approach the value of about 0.6670.02 at 800 K. Thus, the doping process of the PbTe layers has been performed by annealing them under different pressures of Ga2Te molecules in the vapour phase. These processes were carried out by vapour phase diffusion method under vacuum in quartz ampoules which had been placed in a resistively heat furnace, which has two isothermal zones. Ga1xTex heterogeneous alloy 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 the first doping technique consists of two stages. Using Ga2Te molecules as the dopant of Ga in PbTe films is appropriate for two reasons. First, the high-power optical microscopy and EPMA investigations of these specimens show that the
film surface has not been contaminated by any precipitate after annealing. The results of X-ray analysis have exhibited only (2 0 0), (4 0 0), (6 0 0), and (8 0 0) peaks of lead telluride. In other words, PbTe(Ga)/Si films were homogeneous after annealing. Second, this technique allows to control the values of Te partial pressure for L1–GaTe(S)–V equilibrium by setting the temperature of Ga1xTex heterogeneous alloy. The values of Te partial pressure for L1–GaTe(S)–V equilibrium were approximately equal to those of Te partial pressure for PbTe(S)QV equilibrium (conditions of lead telluride congruent sublimation). Thus, the concentration of Te atoms in PbTe(Ga)/Si films has not been changed practically after annealing. The results of the quantitative chemical analysis by EPMA and atomic absorption analysis confirmed this fact. 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 contamination and unit cell parameter have risen monotonically with the increase of Ga partial pressure in 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. The solid-state chemical reactions, which would probably take place during the annealing and diffusion processes, can be expressed within the frameworks of the quasichemical method through the following schemes [12]: Ga2 TeðgÞ þ 0$2Ga Pb þ TeTe þ VTe ;
ð2Þ
Ga2 TeðgÞ þ V Pb $2GaPb þ TeTe þ VTe ;
ð3Þ
0 V Te $VTe þ e
ð4Þ
and according to the theory of spontaneous dissociation of impurity centers [4]: 0 2Ga Pb $GaPb þ GaPb ;
ð5Þ
GaPb $Ga Pb þ h ;
ð6Þ
0 Ga0Pb $Ga Pb þ e ;
ð7Þ
5500 6200 5100 5300 6350 — 2.34 1014 0.66 1014 5.62 1014 2.65 1014 0.78 1014 B1013 0.6467270.0006 0.6470070.0006 0.6464670.0006 0.6466870.0006 0.6468070.0006 0.6471070.0006 240 600 180 360 120 480 823 823 803 803 873 873 2.81 101 2.81 101 5.32 102 5.32 102 1.356 1.356 1.66 1016 1.66 1016 1.23 1016 1.23 1016 6.94 1015 6.94 1015 0.6461270.0006 0.6461270.0006 0.6461070.0006 0.6461070.0006 0.6460870.0006 0.6460870.0006
0.00170.0005 0.00370.0008 0.00170.0005 0.001570.0008 0.00270.0008 0.00470.001
Mobility (77 K) m (cm2/V s) Charge carrier densities (77 K) p (cm3) Concentration of Lattice parameter Ga impurity atoms aPbTe (nm) xGa (mole fraction) Duration of annealing s (min) Temperature of PbTe film TPbTe (K) Ga partial pressure PGa (Pa) Charge carrier densities (77 K) p (cm3) Lattice parameter aPbTe (nm)
Parameters of PbTe/SiO2/Si films doped with Ga Experimental conditions of doping procedure Parameters of as-grown PbTe/SiO2/Si films
Table 1 The values of physical parameters of as-grown PbTe/SiO2/Si films and PbTe(Ga)/SiO2/Si films doped with Ga by annealing in saturated vapour phase for GaTe(S)– L1–V heterogeneous equilibrium
A.M. Samoylov et al. / Journal of Crystal Growth 240 (2002) 340–346
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Ga2 TeðgÞ þ V i $GaPb þ TeTe þ Gai ;
ð8Þ
ddd Ga þ 3e0 : i $Gai
ð9Þ
Eqs. (7) and (9) show that Ga impurity atoms exhibit essentially the donor properties in PbTe crystal structure under the doping technique conditions. As it can be seen from Eq. (4), the VTe ionization process provides the increase in electron densities in the PbTe films too. It should be pointed out that the second goal of this paper is connected with the possibility of gaining freer and wider control over the lead telluride thin-film structure and properties during the formation process. Thus, the second method of preparing PbTe(Ga)/Si and PbTe(Ga)/SiO2/Si heterostructures is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer-condensation process directly. In this technique the Pb1xGax (0:15pxGa p0:95) liquid alloys have been employed as the sources of gallium and lead vapours coincidentally. The quantitative chemical composition of PbTe(Ga) films by the wave dispersion (WD) spectrometers has shown the presence of the Ga atoms in all the deposited films. As it is shown in Table 2, the Ga mole fraction in these Pb1yGayTe/SiO2/Si layers were about 0.00270.0002–0.01570.001 or 0:004oyGa o0:03: It is necessary to note that the values of Ga atoms concentration in Pb1yGayTe/SiO2/Si layers may be strictly controlled by setting the composition and the temperature of Pb1xGax liquid alloys during the condensation process. This doping technique consists of one stage only. As it can be seen in Table 2, it is unable to represent as the monotonic function the dependence the lattice parameter aPbTe on the concentration of Ga impurity atoms yGa in Pb1yGayTe/ SiO2/Si films. The first section of this curve exhibits the decrease in aPbTe values within the concentration interval 0oyGa o0:0037: At yGa ¼ 0:003770:0002 the lattice parameter minimum is observed. The second section of this dependence shows the increase in the unit cell parameter aPbTe within the concentration interval 0:0037oyGa o0:012: The results of resistivity and Hall coefficient measurements, which are
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Table 2 The quantitative chemical composition and physical parameters of Pb1yGayTe films, which have been doped with Ga during the condensation process directly Impurity concentration yGa 0.00 0.001 0.0022 0.0031 0.0037 0.0065 0.008a a
Lattice parameter aPbTe (nm) 0.64606 0.64593 0.64581 0.64563 0.64548 0.64580 0.64585a
Charge carrier densities p, n (cm3)
Resistivity r (O cm)
Mobility m (cm2/V s)
77 K
77 K
298 K
77 K
298 K
1.11 0.75 0.21 0.36 11.02 16.44 0.087a
0.067 0.060 0.018 0.034 0.223 0.396 0.006a
8700 2496 757 865 7054 9502 11000a
1329 877 456 490 3112 3089 2100a
298 K 15
0.65 10 0.95 1016 4.00 1016 2.51 1016 0.10 1015 0.40 1014 6.50 1015a
16
0.70 10 0.75 1017 0.80 1018 0.47 1018 0.90 1016 0.55 1016 4.80 1017a
This sample has n-type of conductivity.
Fig. 1. The typical resistance log10 R temperature dependence of doped with Ga PbTe/SiO2/Si films (1, 2—under IR radiation; 3—without IR radiation ): 1—the power of IR radiation is equal to 0.016 W; 2—the power of IR radiation is equal to 0.008 W.
presented in Fig. 1, show that evolution of charge carrier densities with Ga atoms content has nonmonotonic character too. It is beneficial to consider the impurity concentration dependence of Pb1yGayTe films electrical properties together with dependence of the lattice parameter. The hole densities slowly mounted at impurity concentration yGa o0:0031; for example, from p ¼ 0:95 1016 to 4:0 1016 cm3 at 77 K with yGa rising from 0.001 to 0.0022 in Pb1yGayTe films (Table 2). However, the following rise in Ga atoms contamination 0:0031oyGa o0:0065 is accompanied by the hole densities decrease from p ¼ 2:5 1016 to almost intrinsic values
E4.0 1013 cm3 at 77 K. It is necessary to emphasise that once the Ga impurity concentration yGa ¼ 0:008 is achieved, the inversion of the type of conductivity has been established in Pb1yGayTe/SiO2/Si films. Under these circumstances, all the fabricated films exhibit n-type conductivity with electron densities n ¼ 3:5 1015 24:0 1016 cm3 at 77 K (Table 2). Summarizing all the experimental data allows us to assume that non-monotonic character of aPbTe ¼ f ðyGa Þ curve as well as charge carrier densities evolution is connected with 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 [13]. Thus the substitution of Pb by Ga impurity in its regular positions in PbTe crystal structure for concentration interval 0oyGa o0:0037 can give the lattice parameter decrease (Table 2). 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 concentration interval 0oyGa o0:0031 in Pb1yGayTe films. In this case neutral or acceptor behaviour of Ga atoms is primarily responsible for slight increase of hole densities. It seems reasonable to say that the following rise in impurity concentration in Pb1yGayTe/SiO2/Si films yGa > 0:0037 has been associated with the change of forming mechanism of real crystal structure. Simultaneous presence of Te, Pb, and Ga vapours allows to reveal the self-organization tendency of crystal structure of PbTe(Ga) films.
A.M. Samoylov et al. / Journal of Crystal Growth 240 (2002) 340–346
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4. Conclusions
Fig. 2. The charge carrier densities temperature dependence of Pb1yGayTe films (1–3, 5, 6—p-type of conductivity; 4—n-type of conductivity): 1FyGa ¼ 0:0065; 2FyGa ¼ 0:0037; 3—undoped PbTe; 4FyGa ¼ 0:008; 5FyGa ¼ 0:001; and 6FyGa ¼ 0:0022:
The position of Pb, Ga, and Te atoms in PbTe(Ga) real crystal structure would be determined by the aspiration for the minimum of Gibbs’ free energy DG: The increase in lattice parameter is the evidence for build-up of the distortion in PbTe crystal structure. The appearance of this distortion can be caused probably by the Ga3+ ions occupied in the tetrahedral voids in PbTe structure. As mentioned above, the impurity atoms according to Eq. (9) exhibit only the donor properties in this case. As it can be seen in Fig. 2, these PbTe films doped with Ga, which were prepared with the help of both methods, had a good infrared sensitivity at the temperature range of 4–200 K with ‘‘signal-tonoise’’ ratio of about 40–100. Fig. 2 shows that the PbTe films doped with Ga are able to indicate the changes in IR radiation power. The comparison of experimental data presented in Tables 1 and 2 makes it possible to come to the conclusion that the complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by the different mechanisms of substitu tion (GaPb ) or implantation ðGaddd Þ of Ga atoms i in the PbTe crystal structure.
The comparison of the results of chemical composition, crystal structure, electronic properties and IR photoconductivity investigations of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by two different techniques will be important in the understanding the behaviour of III A metals impurities in AIVBVI narrow band gap semiconductors. One of these techniques is principally based on vapour phase doping procedure of PbTe/Si and PbTe/SiO2/Si heterostructures which were previously formed by the modified HWE technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO2/Si heterostructures preparation is based on the fabrication of lead telluride films, which have been doped with Ga atoms in the layers condensation process directly. 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(Ga)/Si 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 ðGaddd Þ of i impurity atoms in the crystal structure of lead telluride.
Acknowledgements The authors are thankful to B.A. Akimov and V.P. Zlomanov for very helpful discussions. We are also grateful to Dr. Taketoshi Hibiya for assistance in publication.
References [1] V. Munoz, A. Lasbley, S. Klotz, R. Triboulet, J. Crystal Growth 196 (1999) 71. [2] H. Zogg, S. Blunier, A. Fach, C. Maissen, P. Muller, S. Teodoropol, V. Meyer, G. Kostorz, A. Dommann, T. Richmond, Phys. Rev. B. 50 (1994) 10801. [3] B.A. Akimov, A.V. Dmitriev, D.R. Khokhlov, L.I. Ryabova, Phys. Stat. Sol. A 137 (1993) 9. [4] I.A. Drabkin, B.Ya. Moyjes, Phys. Techn. Semicond. 15 (1981) 625.
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[5] D.E. Onopko, A.I. Ryskin, Phys. Techn. Semicond. 34 (2000) 270. [6] D.E. Doljenko, V.N. Demin, I.I. Ivanchik, D.R. Khohlov, Phys. Techn. Semicond. 34 (2000) 1194. [7] Ya.A. Ugai, A.M. Samoylov, M.K. Sharov, A.V. Tadeev, Thin Solid Films 336 (1998) 196. [8] Ya.A. Ugai, A.M. Samoylov, O.B. Yatsenko, Yu.V. Synorov, Inorg. Mater. 36 (2000) 550. [9] H. Kinoshita, T. Sakashita, H. Fujiyasu, J. Appl. Phys. 52 (1981) 2869.
[10] S.S. Gorelik, L.N. Rastorguev, Yu.A. Skakov, X-ray Analysis and Electron Microscopy, Nauka, Moscow, 1994, p. 328. [11] V.P. Zlomanov, A.V. Novoselova, PTx Phase Diagrams of Metal–Chalcogen Systems, Nauka, Moscow, 1987, p. 207. . [12] F.A. Kroger, The Chemistry of Imperfect Crystals, NorthHolland, Amsterdam, 1964, p. 650. [13] G.B. Bokiy, Crystallochemistry, Nauka, Moscow, 1971, p. 400.
Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods A.M. Samoylova,*, M.K. Sharova, S.A. Buchneva, A.M. Khoviva, E.A. Dolgopolovab a
Department of Chemistry, Voronezh State University, Universitetskaya Sq. 1, 394693 Voronezh, Russia b Department of Physics, Voronezh State University, Universitetskaya Sq. 1, 394693 Voronezh, Russia Received 26 September 2001; accepted 28 December 2001 Communicated by T. Hibiya
Abstract The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO2/Si heterostructures, which were previously formed by the modified ‘‘hot wall’’ technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO2/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.J; 73.61.E Keywords: A1. Crystal structure; A1. Doping; A1. Impurities; A1. Point defects; A3. Hot wall epitaxy; B2. Semiconducting lead compounds
1. Introduction Because of its interesting physical properties, lead telluride has been studied intensively for more than 40 years. Its small band gap and high carrier *Corresponding author. Department of Physics, Voronezh State University, Universitetskaya Sq. 1, 394693 Voronezh, Russia. Tel./fax: +7-73-2-789445. E-mail address: [email protected] (A.M. Samoylov).
mobilities identify it as a basic material for infrared (IR) optoelectronic devices [1]. To make IR sensors, a low carrier concentration material is necessary. Preparation of the AIVBVI compounds with low carrier densities is difficult because of the stoichiometric deviation. The IR sensitivities of these materials are similar to that of Cd1xHgxTe, but processing procedures are much less demanding [2]. It is well known that the way to decrease the carrier concentration in AIVBVI materials is
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 0 9 1 2 - 0
A.M. Samoylov et al. / Journal of Crystal Growth 240 (2002) 340–346
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 lead telluride bulk and thin films for doped with Ga and In [3–6]. However, many aspects concerning the influence of Ga and In impurity atoms on the PbTe crystal structure and energy spectrum are still unknown. Some fundamental aspects of the formation, saturation and influence of quasi-local impurity levels in PbTe heavily doped with In and Ga have not been fully understood yet. Therefore, the main purposes of this study are to discuss the crystal structure, electronic properties and IR photoconductivity of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by means of two different techniques.
2. Experimental procedure The modified ‘‘hot wall’’ technique (HWE) has been employed for preparation of mirror-smooth surface PbTe thin films both doped with Ga and undoped (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 [7,8]. Our basic design of the HWE apparatus is similar to the system by Kinoshita et al. [9] with the essential improvements using an additional source of pure group-IV and group-VI elements. The partial pressure of residual gases of about 5 107 Pa can be realized in a graphite reaction chamber during the evaporation process. An exposure of the substrate to Te2 molecules during 20–30 min directly before the condensation of the binary semiconductor has been used for the removal of a SiO2 natural layer from the Si substrate surface. On the other hand, lead telluride layers were grown on Si substrates with the help of an intermediate buffer layer consisting of 100715, 200720 and 300730 nm thick previously formed SiO2. As we cannot pose the problem of highquality PbTe film growth in this paper and can only determine the evolution of the lattice parameter and electrical properties with contamination of Ga impurity atoms in PbTe(Ga) films fabricated by two different techniques, we decided to use
341
SiO2 buffer layers on Si substrates. In other words, the simple and reliable procedure of thermal oxidation of Si wafers in dry O2 does not make the solution to the main problem of this study very difficult. Different experimental methods were employed for the investigation of the lead telluride thin films with and without doping Ga on Si substrates. The chemical composition of as-grown and Ga-doped PbTe/Si and PbTe/SiO2/Si layers was analyzed by the electron probe microanalysis (EPMA) method on JEOL-JCA-840. X-ray diffraction (XRD) patterns were obtained with Cu-radiation (kðCuKa1 Þ ¼ 0:154051 nm) on a DRON-4-07 diffractometer. During XRD experiments, single crystal Si(1 0 0) and Si(1 1 1) substrates were used as internal standards. The (4 0 0), (4 4 4) and (2 0 0), (4 0 0), (6 0 0) X-ray reflection profiles of Si substrates of various orientations and PbTe films, respectively, were obtained with special care by the DRON-4-07 diffractometer with 0.011 step-bystep movement. The lattice parameter of PbTe films values have been calculated precisely by extrapolation to y ¼ 901 using the approximation function [10]: f ðyÞ ¼ 0:5 ðcos2 y=y þ cos2 y=sinyÞ;
ð1Þ
where y is the diffraction angle. The crystal microstructure and thickness of all the prepared samples have been studied by scanning electron microscopy (SEM) on CAMSCAN-4. The electrical parameters of these thin films have been determined by the standard Van der Pauw technique and by investigation of the C–V curves (‘‘Hg probe’’ method) too. An IR spectrometer was employed for studying the IR photoconductivity of PbTe/Si and PbTe/SiO2/Si heterostructures at the temperature range of 4–300 K. The source of IR-radiation is characterized as the Gaussian energy distribution within the wave length range of 2–20 mm.
3. Results and discussion In this work Ga-doped lead telluride films have been prepared by two different ways. One of them is principally based on the vapour-phase doping
342
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procedure of PbTe/Si and PbTe/SiO2/Si heterostructures, which were previously formed by modified HWE technique. For the doping procedure, lead telluride samples with p-type of conductivity and charge carrier densities of about 1016–1017 cm3 at 77 K have been chosen. Firstly, the doping experiments have been carried out by two zones annealing in vapour phase for Ga(L)–V equilibrium. However, the investigations of these specimens by X-ray analysis, high-power optical microscopy and EPMA show that the film surface has been contaminated by Ga precipitates after annealing. The 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, practically in all cases, it resulted in complete elimination of lead telluride layers from Si substrates. Therefore, in this work we have employed the doping technique, which is based on the results of the thermodynamic analysis of vapour-phase composition in the Ga–Te system [11]. It has been established that mole fraction of Ga2Te molecules in vapour phase is much higher than that of other molecules. The gallium atomic fraction xGa in the vapour phase rises as a result of the decrease in temperature and may approach the value of about 0.6670.02 at 800 K. Thus, the doping process of the PbTe layers has been performed by annealing them under different pressures of Ga2Te molecules in the vapour phase. These processes were carried out by vapour phase diffusion method under vacuum in quartz ampoules which had been placed in a resistively heat furnace, which has two isothermal zones. Ga1xTex heterogeneous alloy 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 the first doping technique consists of two stages. Using Ga2Te molecules as the dopant of Ga in PbTe films is appropriate for two reasons. First, the high-power optical microscopy and EPMA investigations of these specimens show that the
film surface has not been contaminated by any precipitate after annealing. The results of X-ray analysis have exhibited only (2 0 0), (4 0 0), (6 0 0), and (8 0 0) peaks of lead telluride. In other words, PbTe(Ga)/Si films were homogeneous after annealing. Second, this technique allows to control the values of Te partial pressure for L1–GaTe(S)–V equilibrium by setting the temperature of Ga1xTex heterogeneous alloy. The values of Te partial pressure for L1–GaTe(S)–V equilibrium were approximately equal to those of Te partial pressure for PbTe(S)QV equilibrium (conditions of lead telluride congruent sublimation). Thus, the concentration of Te atoms in PbTe(Ga)/Si films has not been changed practically after annealing. The results of the quantitative chemical analysis by EPMA and atomic absorption analysis confirmed this fact. 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 contamination and unit cell parameter have risen monotonically with the increase of Ga partial pressure in 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. The solid-state chemical reactions, which would probably take place during the annealing and diffusion processes, can be expressed within the frameworks of the quasichemical method through the following schemes [12]: Ga2 TeðgÞ þ 0$2Ga Pb þ TeTe þ VTe ;
ð2Þ
Ga2 TeðgÞ þ V Pb $2GaPb þ TeTe þ VTe ;
ð3Þ
0 V Te $VTe þ e
ð4Þ
and according to the theory of spontaneous dissociation of impurity centers [4]: 0 2Ga Pb $GaPb þ GaPb ;
ð5Þ
GaPb $Ga Pb þ h ;
ð6Þ
0 Ga0Pb $Ga Pb þ e ;
ð7Þ
5500 6200 5100 5300 6350 — 2.34 1014 0.66 1014 5.62 1014 2.65 1014 0.78 1014 B1013 0.6467270.0006 0.6470070.0006 0.6464670.0006 0.6466870.0006 0.6468070.0006 0.6471070.0006 240 600 180 360 120 480 823 823 803 803 873 873 2.81 101 2.81 101 5.32 102 5.32 102 1.356 1.356 1.66 1016 1.66 1016 1.23 1016 1.23 1016 6.94 1015 6.94 1015 0.6461270.0006 0.6461270.0006 0.6461070.0006 0.6461070.0006 0.6460870.0006 0.6460870.0006
0.00170.0005 0.00370.0008 0.00170.0005 0.001570.0008 0.00270.0008 0.00470.001
Mobility (77 K) m (cm2/V s) Charge carrier densities (77 K) p (cm3) Concentration of Lattice parameter Ga impurity atoms aPbTe (nm) xGa (mole fraction) Duration of annealing s (min) Temperature of PbTe film TPbTe (K) Ga partial pressure PGa (Pa) Charge carrier densities (77 K) p (cm3) Lattice parameter aPbTe (nm)
Parameters of PbTe/SiO2/Si films doped with Ga Experimental conditions of doping procedure Parameters of as-grown PbTe/SiO2/Si films
Table 1 The values of physical parameters of as-grown PbTe/SiO2/Si films and PbTe(Ga)/SiO2/Si films doped with Ga by annealing in saturated vapour phase for GaTe(S)– L1–V heterogeneous equilibrium
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Ga2 TeðgÞ þ V i $GaPb þ TeTe þ Gai ;
ð8Þ
ddd Ga þ 3e0 : i $Gai
ð9Þ
Eqs. (7) and (9) show that Ga impurity atoms exhibit essentially the donor properties in PbTe crystal structure under the doping technique conditions. As it can be seen from Eq. (4), the VTe ionization process provides the increase in electron densities in the PbTe films too. It should be pointed out that the second goal of this paper is connected with the possibility of gaining freer and wider control over the lead telluride thin-film structure and properties during the formation process. Thus, the second method of preparing PbTe(Ga)/Si and PbTe(Ga)/SiO2/Si heterostructures is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer-condensation process directly. In this technique the Pb1xGax (0:15pxGa p0:95) liquid alloys have been employed as the sources of gallium and lead vapours coincidentally. The quantitative chemical composition of PbTe(Ga) films by the wave dispersion (WD) spectrometers has shown the presence of the Ga atoms in all the deposited films. As it is shown in Table 2, the Ga mole fraction in these Pb1yGayTe/SiO2/Si layers were about 0.00270.0002–0.01570.001 or 0:004oyGa o0:03: It is necessary to note that the values of Ga atoms concentration in Pb1yGayTe/SiO2/Si layers may be strictly controlled by setting the composition and the temperature of Pb1xGax liquid alloys during the condensation process. This doping technique consists of one stage only. As it can be seen in Table 2, it is unable to represent as the monotonic function the dependence the lattice parameter aPbTe on the concentration of Ga impurity atoms yGa in Pb1yGayTe/ SiO2/Si films. The first section of this curve exhibits the decrease in aPbTe values within the concentration interval 0oyGa o0:0037: At yGa ¼ 0:003770:0002 the lattice parameter minimum is observed. The second section of this dependence shows the increase in the unit cell parameter aPbTe within the concentration interval 0:0037oyGa o0:012: The results of resistivity and Hall coefficient measurements, which are
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Table 2 The quantitative chemical composition and physical parameters of Pb1yGayTe films, which have been doped with Ga during the condensation process directly Impurity concentration yGa 0.00 0.001 0.0022 0.0031 0.0037 0.0065 0.008a a
Lattice parameter aPbTe (nm) 0.64606 0.64593 0.64581 0.64563 0.64548 0.64580 0.64585a
Charge carrier densities p, n (cm3)
Resistivity r (O cm)
Mobility m (cm2/V s)
77 K
77 K
298 K
77 K
298 K
1.11 0.75 0.21 0.36 11.02 16.44 0.087a
0.067 0.060 0.018 0.034 0.223 0.396 0.006a
8700 2496 757 865 7054 9502 11000a
1329 877 456 490 3112 3089 2100a
298 K 15
0.65 10 0.95 1016 4.00 1016 2.51 1016 0.10 1015 0.40 1014 6.50 1015a
16
0.70 10 0.75 1017 0.80 1018 0.47 1018 0.90 1016 0.55 1016 4.80 1017a
This sample has n-type of conductivity.
Fig. 1. The typical resistance log10 R temperature dependence of doped with Ga PbTe/SiO2/Si films (1, 2—under IR radiation; 3—without IR radiation ): 1—the power of IR radiation is equal to 0.016 W; 2—the power of IR radiation is equal to 0.008 W.
presented in Fig. 1, show that evolution of charge carrier densities with Ga atoms content has nonmonotonic character too. It is beneficial to consider the impurity concentration dependence of Pb1yGayTe films electrical properties together with dependence of the lattice parameter. The hole densities slowly mounted at impurity concentration yGa o0:0031; for example, from p ¼ 0:95 1016 to 4:0 1016 cm3 at 77 K with yGa rising from 0.001 to 0.0022 in Pb1yGayTe films (Table 2). However, the following rise in Ga atoms contamination 0:0031oyGa o0:0065 is accompanied by the hole densities decrease from p ¼ 2:5 1016 to almost intrinsic values
E4.0 1013 cm3 at 77 K. It is necessary to emphasise that once the Ga impurity concentration yGa ¼ 0:008 is achieved, the inversion of the type of conductivity has been established in Pb1yGayTe/SiO2/Si films. Under these circumstances, all the fabricated films exhibit n-type conductivity with electron densities n ¼ 3:5 1015 24:0 1016 cm3 at 77 K (Table 2). Summarizing all the experimental data allows us to assume that non-monotonic character of aPbTe ¼ f ðyGa Þ curve as well as charge carrier densities evolution is connected with 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 [13]. Thus the substitution of Pb by Ga impurity in its regular positions in PbTe crystal structure for concentration interval 0oyGa o0:0037 can give the lattice parameter decrease (Table 2). 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 concentration interval 0oyGa o0:0031 in Pb1yGayTe films. In this case neutral or acceptor behaviour of Ga atoms is primarily responsible for slight increase of hole densities. It seems reasonable to say that the following rise in impurity concentration in Pb1yGayTe/SiO2/Si films yGa > 0:0037 has been associated with the change of forming mechanism of real crystal structure. Simultaneous presence of Te, Pb, and Ga vapours allows to reveal the self-organization tendency of crystal structure of PbTe(Ga) films.
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4. Conclusions
Fig. 2. The charge carrier densities temperature dependence of Pb1yGayTe films (1–3, 5, 6—p-type of conductivity; 4—n-type of conductivity): 1FyGa ¼ 0:0065; 2FyGa ¼ 0:0037; 3—undoped PbTe; 4FyGa ¼ 0:008; 5FyGa ¼ 0:001; and 6FyGa ¼ 0:0022:
The position of Pb, Ga, and Te atoms in PbTe(Ga) real crystal structure would be determined by the aspiration for the minimum of Gibbs’ free energy DG: The increase in lattice parameter is the evidence for build-up of the distortion in PbTe crystal structure. The appearance of this distortion can be caused probably by the Ga3+ ions occupied in the tetrahedral voids in PbTe structure. As mentioned above, the impurity atoms according to Eq. (9) exhibit only the donor properties in this case. As it can be seen in Fig. 2, these PbTe films doped with Ga, which were prepared with the help of both methods, had a good infrared sensitivity at the temperature range of 4–200 K with ‘‘signal-tonoise’’ ratio of about 40–100. Fig. 2 shows that the PbTe films doped with Ga are able to indicate the changes in IR radiation power. The comparison of experimental data presented in Tables 1 and 2 makes it possible to come to the conclusion that the complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by the different mechanisms of substitu tion (GaPb ) or implantation ðGaddd Þ of Ga atoms i in the PbTe crystal structure.
The comparison of the results of chemical composition, crystal structure, electronic properties and IR photoconductivity investigations of PbTe/Si and PbTe/SiO2/Si heterostructures doped with Ga atoms by two different techniques will be important in the understanding the behaviour of III A metals impurities in AIVBVI narrow band gap semiconductors. One of these techniques is principally based on vapour phase doping procedure of PbTe/Si and PbTe/SiO2/Si heterostructures which were previously formed by the modified HWE technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO2/Si heterostructures preparation is based on the fabrication of lead telluride films, which have been doped with Ga atoms in the layers condensation process directly. 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(Ga)/Si 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 ðGaddd Þ of i impurity atoms in the crystal structure of lead telluride.
Acknowledgements The authors are thankful to B.A. Akimov and V.P. Zlomanov for very helpful discussions. We are also grateful to Dr. Taketoshi Hibiya for assistance in publication.
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