Ternary Alloys Based on IV-VI and IV-VI2 Semiconductors

Vasyl Tomashyk

• Éditeur: CRC Press
• Année: 2022
• ISBN:
  9781000597790
• Édition: 1
• Sujet: Biological Sciences
• Sous-sujet: Science Research & Methodology

IV-VI and IV-VI2 semiconductors are among the most interesting materials in semiconductor physics. The electrical properties of these semiconductors can also be tuned by adding impurity atoms. These semiconductors either have already found use or are promising materials for infrared sensors and sources, thermoelectric elements, solar cells, memory elements, etc. The basic characteristics of these compounds, namely, narrow bandgap, high permittivity, relatively high radiation resistance, high mobility of charge carriers, and high bond ionicity, are unique among semiconductor substances. Because of their wide application in various devices, the search for new semiconductor materials and the improvement of existing materials is an important field of study. Doping with impurities is a common method of modifying and diversifying the properties of physical and chemical semiconductors. This book covers all known information about phase relations in ternary systems based on IV-VI and IV-VI2 semiconductors, providing the first systematic account of phase equilibria in ternary systems and making research originally published in Russia accessible to the wider scientific community. This book will be of interest to undergraduate and graduate students studying materials science, solid state chemistry, and engineering. It will also be relevant for researchers at industrial and national laboratories, in addition to phase diagram researchers, inorganic chemists, and solid-state physicists.

FEATURES

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• Provides up-to-date experimental and theoretical information

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• Allows readers to synthesize semiconducting materials with predetermined properties

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• Delivers a critical evaluation of many industrially important systems presented in the form of two-dimensional sections for the condensed phases

1Systems Based on Silicon Sulfides

DOI: 10.1201/​9781003123507-1

1.1 Silicon–Lithium–Sulfur

SiS₂–Li₂S: The phase diagram of this system is given in Figure 1.1 (Ahn and Huggins 1990, 1991). The system has two eutectics which crystallize at 610°C ± 10°C and 680°C ± 10°C and one peritectic at 710°C ± 10°C. Two compounds, Li₂SiS₃ and Li₄SiS₄, are formed in the SiS₂–Li₂S system. Li₂SiS₃ melts congruently at 745°C ± 10°C and depending on cooling conditions from the melt, three different phases of this compound (equilibrium, metastable crystalline and glass) were formed. The equilibrium and metastable crystalline phases crystallize in the orthorhombic structure with the lattice parameters a = 1166.4, b = 673.5, c = 592.6 pm, and a calculated density of 1.97 g⋅cm⁻³ and a = 1143.6, b = 660.5, c = 648.7 pm, respectively (Ahn and Huggins 1989, 1990, 1991). Metastable crystalline Li₂SiS₃ has a stretched one-dimensional chain structure compared to the equilibrium phase. When the melt cooled down to room temperature continuously in a 5–6 h period, a metastable phase was formed. It transforms into the equilibrium phase after 6 months at room temperature.

FIGURE 1.1 Phase diagram of the SiS₂–Li₂S system. (From Ahn, B.T., and Huggins, R.A., Mater. Res. Bull., 25(3), 381, 1990.)

When the Li₂SiS₃ melt was cooled down to 780°C just above the melting point and was quenched in water, it became a glassy phase. The glass transition temperature of this phase is about 320°C, the crystallization temperature is about 450°C, and the heat of crystallization is 12.6 kJ⋅mol⁻¹ (Ahn and Huggins 1990, 1991). The recrystallization of the glass phase formed the metastable crystalline phase.

Li₄SiS₄ melts incongruently at 710°C ± 20°C and apparently has two polymorphic modifications. First of them crystallizes in the orthorhombic structure with the lattice parameters a = 1373.5, b = 776.9, c = 614.7 pm, and a calculated density of 1.864 g⋅cm⁻³ (Ahn and Huggins 1989, 1990, 1991). The second modification crystallizes in the monoclinic structure with the lattice parameters a = 689.34 ± 0.03, b = 776.75 ± 0.03, c = 612.41 ± 0.02 pm, and β = 91.225 ± 0.005° (Murayama et al. 2002a).

In order to prepare various compositions in the SiS₂–Li₂S system, mixtures of SiS₂ and Li₂S were melted in pyrolitic graphite-coated ampoules (Ahn and Huggins 1990, 1991). All the materials were handled in an He-filled glove box to avoid oxidation. Li₂SiS₃ and Li₄SiS₄ were also prepared by the interaction of Li₂Si and Li₁₅Si₄, respectively, with sulfur (Weiss and Rocktäschel 1960).

The isothermal section of the Si–Li–S ternary system at room temperature is shown in Figure 1.2 (Ahn and Huggins 1989). To prepare the sample for the investigation of this system, a powder mixture of SiS₂, Si, and Li₂S was reacted at 1000°C for 1 h with the next annealing at 720°C for half a day and cooling down to room temperature for a day.

FIGURE 1.2 Isothermal section of the Si–Li–S ternary system at room temperature. (From Ahn, B.T., and Huggins, R.A., Mater. Res. Bull., 24(8), 889, 1989.)

The glasses with the composition (Li₂S)_x(SiS₂)_1−x (x ≤ 0.6) have been prepared by twin roller quenching (Pradel and Ribes 1986). The glass transition temperature for such glasses is within the range of 331°C–341°C. Starting materials were obtained from a mixture of sulfide powders (SiS₂ and Li₂S), placed in vitreous carbon crucible inside a silica tube and sealed under vacuum (10⁻²–10⁻³ Pa). The powders were melted for 2 h at 1050°C before being quenched at room temperature. The materials obtained were then crushed into small pieces and remelted using a twin roller apparatus. The glasses were obtained as small flakes, orange to brown in color. All these materials are very hygroscopic and the experiments were carried out in an Ar-filled glove box.

1.2 Silicon–Sodium–Sulfur

Four ternary compounds, Na₂SiS₂, Na₄SiS₄, Na₄Si₄S₁₀, and Na₆Si₂S₆, are formed in the Si–Na–S system (Cade et al. 1972; Ribes et al. 1973; Feltz and Pfaff 1983). Na₄Si₄S₁₀ crystallizes in the orthorhombic structure with the lattice parameters a = 1268.1 ± 0.8, b = 1272.0 ± 0.4, and c = 1036.4 ± 0.5 pm and the calculated and experimental densities of 2.12 and 2.09 g⋅cm⁻³, respectively (Cade et al. 1970; Ribes et al. 1973). To obtain the single crystals of this compound, a mixture of Na₂S and SiS₂ (molar ratio 1:2) was heated in quartz ampoule under vacuum at 800°C for 48 h with next slowly cooling to room temperature.

Na₆Si₂S₆ was obtained by a reaction of stoichiometric amounts of the elements in closed silica ampoules under vacuum at 650°C (Feltz and Pfaff 1983). After a 5–h reaction time, the reaction products, which were not yet uniform, were finely ground under inert conditions and subjected again to the described temperature treatment. The repetition gave a uniform reaction product which completely dissolved in MeOH. After concentration of the solution, Na₆Si₂S₆ segregated out as finely crystalline colorless precipitates. This compound decomposes due to hydrolysis.

To prepare the glasses in the SiS₂–Na₂S system, 2–3 g of sulfides were mixed in the correct proportions and put in a vitreous carbon crucible (Ribes et al. 1980). This crucible was placed inside the silica tube and sealed under a vacuum. After melting at a temperature of 700°C–1000°C, the orange–brown glasses were obtained by cooling down to room temperature or by immersing the silica tube in cold water. The obtained glasses contained up to 60 mol% Na₂S.

1.3 Silicon–Rubidium–Sulfur

The Rb₄Si₂S₆ ternary compound, which crystallizes in the monoclinic structure with the lattice parameters a = 1323 ± 2, b = 686.4 ± 0.2, c = 953 ± 1 pm, and β = 125.15 ± 0.05°, is formed in the Si–Rb–S system (Kolb et al. 2004). It was obtained by reacting a mixture of Rb₂S (4.4 mM), Si (4.4 mM), and S powder (13 mM) in alumina crucibles which were sealed into evacuated silica ampoules at 700°C, followed by cooling at a constant rate of 3°C⋅h⁻¹. The reaction product consisted of colorless prismatic crystals which are sensitive to air and humidity.

1.4 Silicon–Cesium–Sulfur

The Cs₄Si₂S₆ ternary compound, which crystallizes in the monoclinic structure with the lattice parameters a = 986.9 ± 0.3, b = 713.8 ± 0.3, c = 1132.3 ± 0.6 pm, and β = 100.12 ± 0.04°, is formed in the Si–Cs–S system (Feldmann et al. 1998). The colorless prisms of this compound were obtained as a result of a solid-state reaction of equimolar amounts of Cs₂S and SiS₂ in a sealed quartz tube at 400°C for 4 days.

1.5 Silicon–Copper–Sulfur

SiS₂–Cu₂S: The phase diagram of this system was constructed by Venkatraman et al. (1995) and Olekseyuk et al. (2005). The more reliable phase diagram in the 0–60 mol% SiS₂ concentration range is presented in Figure 1.3 (Olekseyuk et al. 2005). The eutectic near Cu₂S side contains ≈ 8 mol% SiS₂ and crystallizes at 1062°C. According to the data of Venkatraman et al. (1995), two eutectics in this system contain ≈ 40 and ≈ 90 mol% Cu₂S and crystallize at 567°C ± 10°C and 792°C ± 5°C, respectively.

FIGURE 1.3 Phase diagram of th...