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Nonstoichiometry and solubility of impurity in In-doped PbTe films on Si substrates
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 6 (2003) 327–333
Nonstoichiometry and solubility of impurity in In-doped PbTe films on Si substrates$ Alexander M. Samoylova,*, Sergey A. Buchneva, Nikolay N. Dement’eva, Yury V. Synorova, Vladimir P. Zlomanovb a
Department of Chemistry, Voronezh State University, Universitetskaya Sq. 1, Voronezh 394006, Russia b Department of Chemistry, Moscow State University, Leninskie Gory, 1/3. GSP 119, Moscow, Russia
Abstract The chemical quantitative composition, phase constitution, and crystal structure of doped with In lead telluride films on Si (1 0 0) or SiO2/Si (1 0 0) substrates have been studied in this work. By EPMA and atomic absorption measurements, it has been found that the concentration of In atoms yIn varied from 0.0011 to 0.045 in these deposited Pb1yInyTe films. The results of EPMA, SEM, and X-ray diffraction (XRD) measurements show that formation of In solid solutions in lead telluride matrix revealed not only in PbTe–InTe cross-section, but in PbTe–In2Te3 pseudobinary system also. The results of XRD show that the lattice parameter aPbTe of PbTe/InS/Si and PbTe/InS/SiO2/Si heterostructures is described by nonmonotone function and does not obey the Vegard’s law within concentration interval 0.0011pyIn p0.045. r 2003 Published by Elsevier Ltd. PACS: 68.55.J; 73.61.E Keywords: Nonstoichiometry; Heterostructures; Lead telluride; Indium; Doping; Solid solutions
1. Introduction The small band gap and high carrier mobilities of AIVBVI semiconductors identify them as the perspective materials for infrared (IR) optoelectronic devices. Under the influence of the presence of III A group metals, the electrical parameters of PbTe and its solid solutions can change significantly [1]. As it was demonstrated by numerous investigations, the effect of Fermi level pinning has been established for doped with Ga and In lead telluride single crystals and thin films [2–4]. However, until now, the many problems about the influence of III A group metals’ impurity atoms upon $
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.: +7-0732-208445; fax: +70732-208445. 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.2003.07.013
the crystal structure and energy spectrum of PbTe are still unknown. Some fundamental aspects of the formation and saturation of quasi-local impurity levels in Indoped PbTe have not been resolved yet. As a practical matter, the growth of AIVBVI thin films on Si substrates would allow to fabricate the monolithic structures for IR sensor arrays with the charge storage and signal multiplexing performed on the same chip [5–6]. Therefore, the main purposes of this study are to discuss the experimental results, which have been received during the examination of the chemical quantitative composition and the real crystal structure of PbTe/InS films deposited on Si (1 0 0) and SiO2/Si (1 0 0) substrates, and to evaluate the solubility of In atoms in lead telluride thin films at different temperatures.
2. Experimental procedure The modified ‘‘hot wall’’ technique (HWE) has been employed to prepare the mirror-smooth surface PbTe
ARTICLE IN PRESS 328
A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 327–333
thin films doped with In (thickness was about 0.5–5.0 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 HWE apparatus is similar to the system by Kinoshita et al. [9] with some improvements: additional sources of pure group-IV, group-III and group-VI elements were installed in the reaction chambers. Under optimal experimental conditions, the previously synthesized Pb1xInx (0.05pxIn p0.50) liquid alloys have been employed as the sources of indium and lead vapours coincidentally. To form ternary Pb1yInyTe layers, an additional source of the tellurium vapours has been activated. The values of lead and tellurium partial pressures were kept at the same level as at fabrication of undoped PbTe/Si and PbTe/SiO2/Si films (which were characterized by p-type of conductivity with charge carrier densities 5 1016– 2 1018 cm3 at 298 K) described elsewhere [8]. For preparation of PbTe/InS films high-purity Pb (99.999%), In (99.999%), and Te (99.99%) were used. The partial pressure of residual gases of about 5 107 Pa can be realized in graphite reaction chamber during the evaporation process. The direct exposure of the Si substrate to Te2 molecules during 20–30 min before the condensation of the binary semiconductor has been used to remove of a SiO2 natural layer from substrate surface. On the other hand, lead telluride layers were grown on Si substrates with the help of an intermediate buffer layers, which consist of 300730 nm thick previously formed SiO2. The presence of dielectric SiO2 buffer layers is needed for further correct Hall coefficient and resistivity measurements in order to isolate PbTe/InS films and Si wafers. As it can be seen from X-ray diffraction (XRD) patterns, SiO2 buffer layers were amorphous with trace amount of orthorombic phase (space group Cmcm) crystallines. The precise estimation of the quantitative chemical composition of deposited PbTe/InS films on Si and SiO2/Si substrates has been carried out sequentially with the special carefulness. Firstly, in all deposited films, the presence of the In impurity has been established with the help of energy dispersion (ED) spectrometers on CAMSCAN-4. Secondly, in scanning and local mode, we have analysed the quantitative chemical composition of PbTe/InS layers with the help of wavelength dispersion (WD) spectrometers on JEOL-JCA-840 using Ka1 and La1 emissions of the elements. Thirdly, some of PbTe/InS/Si and PbTe/InS/SiO2/Si samples have been studied by atomic absorption method. During EPMA measurements 99.999%—pure Pb metal, 99.99% Te, 99.999%—pure In metal and single crystal PbTe and InAs have being served as reference standards. XRD patterns were obtained with filtered CoKa- and CuKa-radiation on a computer-interfaced DRON-4-07 diffractometer. During the XRD phase analysis, the high-resolution experimental technique has been
employed. The values of lattice parameter of PbTe films have been precisely calculated by extrapolation to a diffraction angle y ¼ 90 . To eliminate systematic errors, we tried different approximation functions. The best results have been obtained with the Nelson–Riley function [10]: f ðyÞ ¼ 0:5ðcos2 y=y þ cos2 y=sin2 yÞ;
ð1Þ
where y is the diffraction angle. Structural identifications have been performed with the help of JCPDS database [11]. The thickness and the crystal microstructure of the etched samples have been studied by a scanning electron microscopy (SEM) on CAMSCAN-4.
3. Results and discussion The experimental data, which were obtained earlier during the examination of the evaporation process of Pb1xInx (0.05pxIn p0.70) liquid alloys [12], have been used to prepare PbTe layers doped with In by modified HWE technique. Under optimal experimental conditions, the Pb1xInx (0.05pxIn p0.50) liquid alloys have been employed as the sources of indium and lead vapours coincidentally. In previous works, the appropriate Si substrate temperature, in which relatively the PbTe/Si films with high crystallinity perfection could be deposited by modified HWE method, has been determined [7,8]. As an example, the substrate temperature should be regulated at the range between 48373 and 62373 K in order to produce (1 0 0) PbTe films with mosaic single crystal structure more than 40 mm in diameter on Si wafers. As it can be seen from the results analysed by EPMAWDS and atomic absorption results presented in Table 1, the indium concentration in these fabricated Pb1yInyTe/SiO2/Si and Pb1yInyTe/Si layers varied from yIn ¼ 0:002270.0002 to yIn ¼ 0:045070.0002 or mole fraction of In atoms varied from 0.0011 to 0.0225. The experimental results (Table 1) show that for all deposited Pb1yInyTe layers, the In concentration rises with increase in vapour source temperature Tsource at a fixed melt composition and Tsub or with increase in In concentration in initial Pb1xInx liquid alloys at the fixed temperatures Tsource and Tsub : It is evident that In concentration in grown films may be strictly controlled by adjusting the initial composition and the Tsource of Pb1xInx liquid alloys. The precise estimation of the quantitative chemical composition shows that the all deposited Pb1yInyTe/SiO2/Si and Pb1yInyTe/Si films have been characterized by little excess of Te atoms with regard to stoichiometric ratio (mole fraction of Te varied within the range from 0.500470.0002 to 0.503570.0002).
Number of Composition of sample metals vapour source
Substrate
Temperature T (K)
Source of Substrate Te vapours
Concentration of In atoms in Pb1yInyTe films, yIn
Results of XRD and microstructural observations
Homogeneous, single cryst. Homogeneous, polycryst. Homogeneous, single cryst. Homogeneous, single cryst. Homogeneous, polycryst. Homogeneous, single cryst. Heterogeneous, mixture of PbTe+InTe Heterogeneous, mixture of PbTe+InTe+In2Te3 Heterogeneous, mixture of PbTe+InTe+In2Te3 Heterogeneous, mixture of PbTe+InTe+In2Te3 Heterogeneous, mixture of PbTe+InTe+In2Te3
58-t 59-t 63-t 60-t 61-t 64-t 49-t
Pb0.90In0.10 Pb0.90In0.10 Pb0.90In0.10 Pb0.75In0.25 Pb0.75In0.25 Pb0.75In0.25 Pb0.60In0.40
Si (1 0 0) SiO2/Si (1 0 0) Si (1 0 0) Si (1 0 0) SiO2/Si (1 0 0) Si (1 0 0) SiO2/Si (1 0 0)
110373 110373 110373 110373 110373 110373 110373
56873 56873 56873 56873 56873 56873 56873
58373 58373 58373 58373 58373 58373 58373
0.6459070.00005 0.6458870.00005 0.6458870.00005 0.6455770.00005 0.6456870.00005 0.6455970.00005 0.6459670.00005
0.0034 0.0032 0.0035 0.0068 0.0076 0.0069 0.0159
N 54-t
Pb0.60In0.40
SiO2/Si (1 0 0)
110373
56873
58373
0.6461570.00005
0.0032
N 55-t
Pb0.60In0.40
SiO2/Si (1 0 0)
113373
56873
58373
0.6461670.00005
0.0032
N 103-t
Pb0.60In0.40
SiO2/Si (1 0 0)
110373
56873
62373
0.6461370.00005
0.0022
N 105-t
Pb0.60In0.40
SiO2/Si (1 0 0)
113373
56873
62373
0.6461170.00005
0.0032
N N N N N N N
ARTICLE IN PRESS
Pb1xInx vapour source
Values of lattice parameter aPbTe/InS ; (nm)
A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 327–333
Table 1 The experimental conditions of the deposition, the quantitative chemical composition, and XRD and microstructural observations of Pb1yInyTe films, which have been doped with In during the condensation process directly
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The XRD patterns of the deposited Pb1yInyTe/Si and Pb1yInyTe/SiO2/Si films are presented in Fig. 1. The results of microstructural observations and XRD measurements of these films are summarized in Table 1. It has been found that all Pb1yInyTe/Si films have mosaic single crystal structure with (1 0 0) orientation (Fig. 1a), if In concentration does not exceed the value yIn ¼ 0:013–0.014 for layers prepared at Tsub: ¼ 583 K or the value yIn ¼ 0:017–0.018 for layers prepared at Tsub ¼ 623 K. On the contrary, the Pb1yInyTe films, which were deposited on Si substrates with SiO2 intermediate buffer layers under the same experimental conditions, reveal clearly defined PbTe (2 1 1), (2 2 2),
and (3 1 1) reflections in addition to (h 0 0) peaks (Fig. 1b). It is necessary to note that the intensity of these (h k l) lines is not significant. For example, the I200 to I211 ratio is always more than 400. Thus, it is clear that the presence of SiO2 buffer layers on Si substrates brings the polycrystalline structure into the Pb1yInyTe films at yIn o0:013–0.014 (Tsub ¼ 583 K) or at yIn o0:017–0.018 (Tsub ¼ 623 K). This fact is in agreement with the results reported earlier for undoped PbTe films on Si and SiO2/Si substrates [7]. It is important to keep in mind that concentration of Te atoms is the key condition of the homogeneity of Pb1yInyTe layers for yIn o0:013–0.014 (Tsub ¼ 583 K) or yIn o0:017–0.018
Fig. 1. XRD patterns of Pb1yInyTe films on Si and SiO2/Si substrates prepared at Tsub ¼ 58373 and 62373 K by the modified HWE technique: (a) yIn o0:013; Si substrate; (b) yIn o0:013; SiO2/Si substrate; (c) yIn > 0:017; SiO2/Si substrate (mixture of PbTe/InS+InTe); (d) yIn > 0:017; SiO2/Si substrate (mixture of PbTe/InS+InTe+In2Te3).
ARTICLE IN PRESS A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 327–333
(Tsub ¼ 623 K). Investigated PbTe/InS samples with Te concentration over 0.5000 were heterogeneous (Table 1). X-ray reflections of this group of Pb1yInyTe/Si and Pb1yInyTe/SiO2/Si films exhibit the presence of slight excess of tellurium phase. As it can be seen in Table 1, at higher indium contents yIn > 0:013–0.014 (Tsub ¼ 583 K) or yIn > 0:017–0.018 (Tsub ¼ 623 K), the crystal structures of Pb1yInyTe films are defined as polycrystalline and heterogeneous without regard to the nature of the substrate. These heterogeneous Pb1yInyTe/Si and Pb1yInyTe/SiO2/Si
331
samples can be divided into two groups (Table 1, Fig. 1c and d). The first group of these two-phase samples is characterized by the presence of the little amount of tetragonal indium monotelluride InTe (space group I m4 cm) (Fig. 1c). The second group of the specimens is formed from the three-phase heterogeneous Pb1yInyTe films. X-ray spectra of these samples consist of PbTe, tetragonal InTe, and cubic In2Te3 (space group F 4% 3m) reflections (Fig. 1d). The results of the precise determination of the lattice parameter aPbTe/InS values are listed in Table 1. It is
Fig. 2. (a) The binary phase diagrams and Gibbs’ compositional triangle of Pb–In–Te ternary system. (b) The isothermal cross section through the phase microdiagram of Pb–In–Te ternary system at T ¼ 58373 K: 1—homogeneous Pb1yInyTe films; 2—heterogeneous Pb1yInyTe films (PbTe/InS+Te); 3—heterogeneous Pb1yInyTe films (PbTe/InS+InTe); 4—heterogeneous Pb1yInyTe/SiO2/Si films (PbTe/InS+InTe+In2Te3).
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necessary to note that during XRD investigations we have studied over 100 Pb1yInyTe/Si and Pb1yInyTe/ SiO2/Si films prepared at substrate temperatures Tsub ¼ 583 and 62373 K by modified HWE technique. These experimental results allow us to observe the variation of the unit cell parameter aPbTe/InS in Pb1yInyTe films on Si and SiO2/Si substrates. It has been found that the lattice constants of Pb1yInyTe/Si and Pb1yInyTe/SiO2/ Si layers are the same within the accuracy of XRD experiments, when these layers have been prepared under the identical growth conditions (Table 1). The Pb–Te–In phase diagram (Fig. 2a) provides a useful framework for understanding of solid-state interaction in this system. The In solubility limit in PbTe matrix has been drawn based on the microstructural observations, XRD and EPMA results of PbTe/InS/Si and PbTe/InS/SiO2/Si layers fabricated at Tsub ¼ 583 K (Fig. 2b). From the representation on a Gibbs’ compositional triangle, it is clear that the analysis of the PbTe–InTe pseudobinary cross-section only [13] does not exhaust the process of indium’s solubility in PbTe matrix in the whole. As it can be seen in Fig. 2b, for the explanation the formation of In solid solutions in ternary Pb–In–Te system, it is also necessary to consider the PbTe–In2Te3 pseudobinary cross-section. In view of the fact that all Pb1yInyTe films were deposited at approximately constant values of Te2 partial pressure (Table 1), it is possible to say that Fig. 2b represents the isothermal and isobaric pTe2 sections of Pb–In–Te phase microdiagram. A closer look to Fig. 2 allows us to assume that at fixed temperature borders of homogeneity region and phase constitution of PbTe/InS films depend upon the two main factors, which are the ratio between In and Pb concentration and content of Te atoms. During the formation of the PbTe/InS layers from the vapour phase, the heterogeneous chemical reactions can be expressed through the following schemes: ðsÞ ðsÞ PbðgÞ þ InðgÞ þ TeðgÞ 2 -PbTe þ InTe :
ð2Þ
Within the frameworks of the quasi-chemical method [14], this reaction may be written as PbðgÞ þInðgÞ þTe2ðgÞ -Pb Pb þInPb þTeTe þTeTe :
3+
d IndPb "In Pb þ h
ð5Þ
As it has been established earlier [1,2,4], the influence of indium impurity atoms on the energy spectrum of PbTe shows the dependence upon its concentration and, in the whole, may be classified as ambiguous. Summarizing of the experimental data, which have been received in this work, allows us to assume that amphoteric (donor and acceptor) character of indium impurity levels is connected with its different values of oxidation number (+2) or (+3) as well as its different position in PbTe crystal matrix also.
4. Conclusions The boundary of indium’s limited solubility region in lead telluride matrix has been drawn based on the microstructural observations, XRD and EPMA results. From the representation on a Gibbs’ compositional triangle, it is clear that the analysis of the PbTe–InTe pseudobinary cross-section only does not exhaust the process of indium’s solubility in PbTe matrix in the whole. The formation of In solid solutions has revealed in PbTe–In2Te3 pseudobinary system also. It is necessary to assume that at fixed temperature borders of homogeneity region and phase constitution of PbTe/InS films depend upon the two main factors, which are the ratio between In and Pb concentration and content of Te atoms. In the case where Te2 partial pressure goes beyond the stoichiometric ratio, the formation of In3+ ions with oxidation number (+3) is possible. Summarizing of the experimental data, which have been collected in this work, allows us to assume that amphoteric (donor and acceptor) character of indium impurity levels is connected with its different values of oxidation number (+2) or (+3) as well as its different position in PbTe crystal matrix also.
ð3Þ
It is necessary to note that Eqs. (2) and (3) satisfy the fabrication process of PbTe/InS films with stoichiometric ratio of metal components and Te atoms only and correspond the formation of In solid solutions along PbTe–InTe pseudobinary section. If tellurium partial pressure goes beyond the stoichiometric ratio the following reaction occurs: ðsÞ ðsÞ 2PbðgÞ þ2InðgÞ þ5=2TeðgÞ 2 -2PbTe þ In2 Te3 :
These In3+ ions have surplus positive charge relative to PbPb atoms in regular positions and may affect the charge carrier density in PbTe:
ð4Þ
ions may exist together with lead Thus, In vacancies VPb within the region of nonstoichiometry outside PbTe–InTe pseudobinary section (Fig. 2b).
References [1] Akimov BA, Dmitriev AV, Khokhlov DR, Ryabova LI. Carrier transport and non-equilibrium phenomena in doped PbTe and related materials. Phys Stat Sol A 1993;137(8):9–55. [2] Drabkin IA, Moyjes BY. The spontaneous dissociations of the neutral states into positive and negative states. Phys Tech Semicond 1981;15(4):625–44. [3] Vasilyeva LF, Klimov AE, Petikov NI, Shumskiy VN. Correction of PbSnTe/InS epitaxial films properties by low temperature diffusion annealing. Inorg Mater 2001;37(2):193–8.
ARTICLE IN PRESS A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 327–333 [4] Ravich YI, Nemov SA. Hopping on hard-localized impurity states of indium in PbTe and its solid solutions. Phys Tech Semicond 2002;36(1):3–23. [5] Zogg H, Blunier S, Fach A, Maissen C, Muller P, Teodoropol S, Meyer V, Kostorz G, Dommann A, Richmond T. Thermalmismatch-strain relaxation in epitaxial CaF2, BaF2/CaF2, and PbSe/BaF2/CaF2 layers on Si(1 1 1) after many temperature cycles. Phys Rev B 1994;50(15):10801–10. [6] 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. [7] 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. [8] 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.
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[9] Kinoshita H, Sakashita T, Fujiyasu H. PbTe multiplelayer films prepared by a hot wall technique. J Appl Phys 1981;52(4):2869–71. [10] Gorelik SS, Rastorguev LN, Skakov YA. X-ray analysis and electron microscopy. Moscow: Nauka; 1994. [11] JCPDS—International Centre for Diffraction Data. r 1987–1995. JCPDS-ICDD. Newtown Square, PA 19073. USA. [12] Samoylov AM, Sharov MK, Buchnev SA, Khoviv AM, Synorov YV. Pb and In co-deposition in the vacuum growth of PbTe/InS films on Si substrates. Inorg Mater 2002;38(7):655–60. [13] Rogacheva EI, Gorne GV, Panasenko NM. Phase interaction and nature of solid solutions in PbTe–InTe system. Inorg Mater 1979;15(8):1366–9. . [14] Kroger FA. The chemistry of imperfect crystals. Amsterdam: North-Holland Publishing Company; 1964.
Materials Science in Semiconductor Processing 6 (2003) 327–333
Nonstoichiometry and solubility of impurity in In-doped PbTe films on Si substrates$ Alexander M. Samoylova,*, Sergey A. Buchneva, Nikolay N. Dement’eva, Yury V. Synorova, Vladimir P. Zlomanovb a
Department of Chemistry, Voronezh State University, Universitetskaya Sq. 1, Voronezh 394006, Russia b Department of Chemistry, Moscow State University, Leninskie Gory, 1/3. GSP 119, Moscow, Russia
Abstract The chemical quantitative composition, phase constitution, and crystal structure of doped with In lead telluride films on Si (1 0 0) or SiO2/Si (1 0 0) substrates have been studied in this work. By EPMA and atomic absorption measurements, it has been found that the concentration of In atoms yIn varied from 0.0011 to 0.045 in these deposited Pb1yInyTe films. The results of EPMA, SEM, and X-ray diffraction (XRD) measurements show that formation of In solid solutions in lead telluride matrix revealed not only in PbTe–InTe cross-section, but in PbTe–In2Te3 pseudobinary system also. The results of XRD show that the lattice parameter aPbTe of PbTe/InS/Si and PbTe/InS/SiO2/Si heterostructures is described by nonmonotone function and does not obey the Vegard’s law within concentration interval 0.0011pyIn p0.045. r 2003 Published by Elsevier Ltd. PACS: 68.55.J; 73.61.E Keywords: Nonstoichiometry; Heterostructures; Lead telluride; Indium; Doping; Solid solutions
1. Introduction The small band gap and high carrier mobilities of AIVBVI semiconductors identify them as the perspective materials for infrared (IR) optoelectronic devices. Under the influence of the presence of III A group metals, the electrical parameters of PbTe and its solid solutions can change significantly [1]. As it was demonstrated by numerous investigations, the effect of Fermi level pinning has been established for doped with Ga and In lead telluride single crystals and thin films [2–4]. However, until now, the many problems about the influence of III A group metals’ impurity atoms upon $
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.: +7-0732-208445; fax: +70732-208445. 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.2003.07.013
the crystal structure and energy spectrum of PbTe are still unknown. Some fundamental aspects of the formation and saturation of quasi-local impurity levels in Indoped PbTe have not been resolved yet. As a practical matter, the growth of AIVBVI thin films on Si substrates would allow to fabricate the monolithic structures for IR sensor arrays with the charge storage and signal multiplexing performed on the same chip [5–6]. Therefore, the main purposes of this study are to discuss the experimental results, which have been received during the examination of the chemical quantitative composition and the real crystal structure of PbTe/InS films deposited on Si (1 0 0) and SiO2/Si (1 0 0) substrates, and to evaluate the solubility of In atoms in lead telluride thin films at different temperatures.
2. Experimental procedure The modified ‘‘hot wall’’ technique (HWE) has been employed to prepare the mirror-smooth surface PbTe
ARTICLE IN PRESS 328
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thin films doped with In (thickness was about 0.5–5.0 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 HWE apparatus is similar to the system by Kinoshita et al. [9] with some improvements: additional sources of pure group-IV, group-III and group-VI elements were installed in the reaction chambers. Under optimal experimental conditions, the previously synthesized Pb1xInx (0.05pxIn p0.50) liquid alloys have been employed as the sources of indium and lead vapours coincidentally. To form ternary Pb1yInyTe layers, an additional source of the tellurium vapours has been activated. The values of lead and tellurium partial pressures were kept at the same level as at fabrication of undoped PbTe/Si and PbTe/SiO2/Si films (which were characterized by p-type of conductivity with charge carrier densities 5 1016– 2 1018 cm3 at 298 K) described elsewhere [8]. For preparation of PbTe/InS films high-purity Pb (99.999%), In (99.999%), and Te (99.99%) were used. The partial pressure of residual gases of about 5 107 Pa can be realized in graphite reaction chamber during the evaporation process. The direct exposure of the Si substrate to Te2 molecules during 20–30 min before the condensation of the binary semiconductor has been used to remove of a SiO2 natural layer from substrate surface. On the other hand, lead telluride layers were grown on Si substrates with the help of an intermediate buffer layers, which consist of 300730 nm thick previously formed SiO2. The presence of dielectric SiO2 buffer layers is needed for further correct Hall coefficient and resistivity measurements in order to isolate PbTe/InS films and Si wafers. As it can be seen from X-ray diffraction (XRD) patterns, SiO2 buffer layers were amorphous with trace amount of orthorombic phase (space group Cmcm) crystallines. The precise estimation of the quantitative chemical composition of deposited PbTe/InS films on Si and SiO2/Si substrates has been carried out sequentially with the special carefulness. Firstly, in all deposited films, the presence of the In impurity has been established with the help of energy dispersion (ED) spectrometers on CAMSCAN-4. Secondly, in scanning and local mode, we have analysed the quantitative chemical composition of PbTe/InS layers with the help of wavelength dispersion (WD) spectrometers on JEOL-JCA-840 using Ka1 and La1 emissions of the elements. Thirdly, some of PbTe/InS/Si and PbTe/InS/SiO2/Si samples have been studied by atomic absorption method. During EPMA measurements 99.999%—pure Pb metal, 99.99% Te, 99.999%—pure In metal and single crystal PbTe and InAs have being served as reference standards. XRD patterns were obtained with filtered CoKa- and CuKa-radiation on a computer-interfaced DRON-4-07 diffractometer. During the XRD phase analysis, the high-resolution experimental technique has been
employed. The values of lattice parameter of PbTe films have been precisely calculated by extrapolation to a diffraction angle y ¼ 90 . To eliminate systematic errors, we tried different approximation functions. The best results have been obtained with the Nelson–Riley function [10]: f ðyÞ ¼ 0:5ðcos2 y=y þ cos2 y=sin2 yÞ;
ð1Þ
where y is the diffraction angle. Structural identifications have been performed with the help of JCPDS database [11]. The thickness and the crystal microstructure of the etched samples have been studied by a scanning electron microscopy (SEM) on CAMSCAN-4.
3. Results and discussion The experimental data, which were obtained earlier during the examination of the evaporation process of Pb1xInx (0.05pxIn p0.70) liquid alloys [12], have been used to prepare PbTe layers doped with In by modified HWE technique. Under optimal experimental conditions, the Pb1xInx (0.05pxIn p0.50) liquid alloys have been employed as the sources of indium and lead vapours coincidentally. In previous works, the appropriate Si substrate temperature, in which relatively the PbTe/Si films with high crystallinity perfection could be deposited by modified HWE method, has been determined [7,8]. As an example, the substrate temperature should be regulated at the range between 48373 and 62373 K in order to produce (1 0 0) PbTe films with mosaic single crystal structure more than 40 mm in diameter on Si wafers. As it can be seen from the results analysed by EPMAWDS and atomic absorption results presented in Table 1, the indium concentration in these fabricated Pb1yInyTe/SiO2/Si and Pb1yInyTe/Si layers varied from yIn ¼ 0:002270.0002 to yIn ¼ 0:045070.0002 or mole fraction of In atoms varied from 0.0011 to 0.0225. The experimental results (Table 1) show that for all deposited Pb1yInyTe layers, the In concentration rises with increase in vapour source temperature Tsource at a fixed melt composition and Tsub or with increase in In concentration in initial Pb1xInx liquid alloys at the fixed temperatures Tsource and Tsub : It is evident that In concentration in grown films may be strictly controlled by adjusting the initial composition and the Tsource of Pb1xInx liquid alloys. The precise estimation of the quantitative chemical composition shows that the all deposited Pb1yInyTe/SiO2/Si and Pb1yInyTe/Si films have been characterized by little excess of Te atoms with regard to stoichiometric ratio (mole fraction of Te varied within the range from 0.500470.0002 to 0.503570.0002).
Number of Composition of sample metals vapour source
Substrate
Temperature T (K)
Source of Substrate Te vapours
Concentration of In atoms in Pb1yInyTe films, yIn
Results of XRD and microstructural observations
Homogeneous, single cryst. Homogeneous, polycryst. Homogeneous, single cryst. Homogeneous, single cryst. Homogeneous, polycryst. Homogeneous, single cryst. Heterogeneous, mixture of PbTe+InTe Heterogeneous, mixture of PbTe+InTe+In2Te3 Heterogeneous, mixture of PbTe+InTe+In2Te3 Heterogeneous, mixture of PbTe+InTe+In2Te3 Heterogeneous, mixture of PbTe+InTe+In2Te3
58-t 59-t 63-t 60-t 61-t 64-t 49-t
Pb0.90In0.10 Pb0.90In0.10 Pb0.90In0.10 Pb0.75In0.25 Pb0.75In0.25 Pb0.75In0.25 Pb0.60In0.40
Si (1 0 0) SiO2/Si (1 0 0) Si (1 0 0) Si (1 0 0) SiO2/Si (1 0 0) Si (1 0 0) SiO2/Si (1 0 0)
110373 110373 110373 110373 110373 110373 110373
56873 56873 56873 56873 56873 56873 56873
58373 58373 58373 58373 58373 58373 58373
0.6459070.00005 0.6458870.00005 0.6458870.00005 0.6455770.00005 0.6456870.00005 0.6455970.00005 0.6459670.00005
0.0034 0.0032 0.0035 0.0068 0.0076 0.0069 0.0159
N 54-t
Pb0.60In0.40
SiO2/Si (1 0 0)
110373
56873
58373
0.6461570.00005
0.0032
N 55-t
Pb0.60In0.40
SiO2/Si (1 0 0)
113373
56873
58373
0.6461670.00005
0.0032
N 103-t
Pb0.60In0.40
SiO2/Si (1 0 0)
110373
56873
62373
0.6461370.00005
0.0022
N 105-t
Pb0.60In0.40
SiO2/Si (1 0 0)
113373
56873
62373
0.6461170.00005
0.0032
N N N N N N N
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Pb1xInx vapour source
Values of lattice parameter aPbTe/InS ; (nm)
A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 327–333
Table 1 The experimental conditions of the deposition, the quantitative chemical composition, and XRD and microstructural observations of Pb1yInyTe films, which have been doped with In during the condensation process directly
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The XRD patterns of the deposited Pb1yInyTe/Si and Pb1yInyTe/SiO2/Si films are presented in Fig. 1. The results of microstructural observations and XRD measurements of these films are summarized in Table 1. It has been found that all Pb1yInyTe/Si films have mosaic single crystal structure with (1 0 0) orientation (Fig. 1a), if In concentration does not exceed the value yIn ¼ 0:013–0.014 for layers prepared at Tsub: ¼ 583 K or the value yIn ¼ 0:017–0.018 for layers prepared at Tsub ¼ 623 K. On the contrary, the Pb1yInyTe films, which were deposited on Si substrates with SiO2 intermediate buffer layers under the same experimental conditions, reveal clearly defined PbTe (2 1 1), (2 2 2),
and (3 1 1) reflections in addition to (h 0 0) peaks (Fig. 1b). It is necessary to note that the intensity of these (h k l) lines is not significant. For example, the I200 to I211 ratio is always more than 400. Thus, it is clear that the presence of SiO2 buffer layers on Si substrates brings the polycrystalline structure into the Pb1yInyTe films at yIn o0:013–0.014 (Tsub ¼ 583 K) or at yIn o0:017–0.018 (Tsub ¼ 623 K). This fact is in agreement with the results reported earlier for undoped PbTe films on Si and SiO2/Si substrates [7]. It is important to keep in mind that concentration of Te atoms is the key condition of the homogeneity of Pb1yInyTe layers for yIn o0:013–0.014 (Tsub ¼ 583 K) or yIn o0:017–0.018
Fig. 1. XRD patterns of Pb1yInyTe films on Si and SiO2/Si substrates prepared at Tsub ¼ 58373 and 62373 K by the modified HWE technique: (a) yIn o0:013; Si substrate; (b) yIn o0:013; SiO2/Si substrate; (c) yIn > 0:017; SiO2/Si substrate (mixture of PbTe/InS+InTe); (d) yIn > 0:017; SiO2/Si substrate (mixture of PbTe/InS+InTe+In2Te3).
ARTICLE IN PRESS A.M. Samoylov et al. / Materials Science in Semiconductor Processing 6 (2003) 327–333
(Tsub ¼ 623 K). Investigated PbTe/InS samples with Te concentration over 0.5000 were heterogeneous (Table 1). X-ray reflections of this group of Pb1yInyTe/Si and Pb1yInyTe/SiO2/Si films exhibit the presence of slight excess of tellurium phase. As it can be seen in Table 1, at higher indium contents yIn > 0:013–0.014 (Tsub ¼ 583 K) or yIn > 0:017–0.018 (Tsub ¼ 623 K), the crystal structures of Pb1yInyTe films are defined as polycrystalline and heterogeneous without regard to the nature of the substrate. These heterogeneous Pb1yInyTe/Si and Pb1yInyTe/SiO2/Si
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samples can be divided into two groups (Table 1, Fig. 1c and d). The first group of these two-phase samples is characterized by the presence of the little amount of tetragonal indium monotelluride InTe (space group I m4 cm) (Fig. 1c). The second group of the specimens is formed from the three-phase heterogeneous Pb1yInyTe films. X-ray spectra of these samples consist of PbTe, tetragonal InTe, and cubic In2Te3 (space group F 4% 3m) reflections (Fig. 1d). The results of the precise determination of the lattice parameter aPbTe/InS values are listed in Table 1. It is
Fig. 2. (a) The binary phase diagrams and Gibbs’ compositional triangle of Pb–In–Te ternary system. (b) The isothermal cross section through the phase microdiagram of Pb–In–Te ternary system at T ¼ 58373 K: 1—homogeneous Pb1yInyTe films; 2—heterogeneous Pb1yInyTe films (PbTe/InS+Te); 3—heterogeneous Pb1yInyTe films (PbTe/InS+InTe); 4—heterogeneous Pb1yInyTe/SiO2/Si films (PbTe/InS+InTe+In2Te3).
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necessary to note that during XRD investigations we have studied over 100 Pb1yInyTe/Si and Pb1yInyTe/ SiO2/Si films prepared at substrate temperatures Tsub ¼ 583 and 62373 K by modified HWE technique. These experimental results allow us to observe the variation of the unit cell parameter aPbTe/InS in Pb1yInyTe films on Si and SiO2/Si substrates. It has been found that the lattice constants of Pb1yInyTe/Si and Pb1yInyTe/SiO2/ Si layers are the same within the accuracy of XRD experiments, when these layers have been prepared under the identical growth conditions (Table 1). The Pb–Te–In phase diagram (Fig. 2a) provides a useful framework for understanding of solid-state interaction in this system. The In solubility limit in PbTe matrix has been drawn based on the microstructural observations, XRD and EPMA results of PbTe/InS/Si and PbTe/InS/SiO2/Si layers fabricated at Tsub ¼ 583 K (Fig. 2b). From the representation on a Gibbs’ compositional triangle, it is clear that the analysis of the PbTe–InTe pseudobinary cross-section only [13] does not exhaust the process of indium’s solubility in PbTe matrix in the whole. As it can be seen in Fig. 2b, for the explanation the formation of In solid solutions in ternary Pb–In–Te system, it is also necessary to consider the PbTe–In2Te3 pseudobinary cross-section. In view of the fact that all Pb1yInyTe films were deposited at approximately constant values of Te2 partial pressure (Table 1), it is possible to say that Fig. 2b represents the isothermal and isobaric pTe2 sections of Pb–In–Te phase microdiagram. A closer look to Fig. 2 allows us to assume that at fixed temperature borders of homogeneity region and phase constitution of PbTe/InS films depend upon the two main factors, which are the ratio between In and Pb concentration and content of Te atoms. During the formation of the PbTe/InS layers from the vapour phase, the heterogeneous chemical reactions can be expressed through the following schemes: ðsÞ ðsÞ PbðgÞ þ InðgÞ þ TeðgÞ 2 -PbTe þ InTe :
ð2Þ
Within the frameworks of the quasi-chemical method [14], this reaction may be written as PbðgÞ þInðgÞ þTe2ðgÞ -Pb Pb þInPb þTeTe þTeTe :
3+
d IndPb "In Pb þ h
ð5Þ
As it has been established earlier [1,2,4], the influence of indium impurity atoms on the energy spectrum of PbTe shows the dependence upon its concentration and, in the whole, may be classified as ambiguous. Summarizing of the experimental data, which have been received in this work, allows us to assume that amphoteric (donor and acceptor) character of indium impurity levels is connected with its different values of oxidation number (+2) or (+3) as well as its different position in PbTe crystal matrix also.
4. Conclusions The boundary of indium’s limited solubility region in lead telluride matrix has been drawn based on the microstructural observations, XRD and EPMA results. From the representation on a Gibbs’ compositional triangle, it is clear that the analysis of the PbTe–InTe pseudobinary cross-section only does not exhaust the process of indium’s solubility in PbTe matrix in the whole. The formation of In solid solutions has revealed in PbTe–In2Te3 pseudobinary system also. It is necessary to assume that at fixed temperature borders of homogeneity region and phase constitution of PbTe/InS films depend upon the two main factors, which are the ratio between In and Pb concentration and content of Te atoms. In the case where Te2 partial pressure goes beyond the stoichiometric ratio, the formation of In3+ ions with oxidation number (+3) is possible. Summarizing of the experimental data, which have been collected in this work, allows us to assume that amphoteric (donor and acceptor) character of indium impurity levels is connected with its different values of oxidation number (+2) or (+3) as well as its different position in PbTe crystal matrix also.
ð3Þ
It is necessary to note that Eqs. (2) and (3) satisfy the fabrication process of PbTe/InS films with stoichiometric ratio of metal components and Te atoms only and correspond the formation of In solid solutions along PbTe–InTe pseudobinary section. If tellurium partial pressure goes beyond the stoichiometric ratio the following reaction occurs: ðsÞ ðsÞ 2PbðgÞ þ2InðgÞ þ5=2TeðgÞ 2 -2PbTe þ In2 Te3 :
These In3+ ions have surplus positive charge relative to PbPb atoms in regular positions and may affect the charge carrier density in PbTe:
ð4Þ
ions may exist together with lead Thus, In vacancies VPb within the region of nonstoichiometry outside PbTe–InTe pseudobinary section (Fig. 2b).
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