STUDY OF PHASE EQUILIBRIUM IN THE ZnTe–Cd₂Te₃ SYSTEM

ABSTRACT

This experimental study investigates the phase equilibria in the ZnTe–Cd₂Te₃ system using high-purity elemental samples synthesized in evacuated quartz ampoules. Controlled thermal treatments, including stepwise annealing and slow cooling, were applied to achieve near-equilibrium conditions. Differential thermal analysis (DTA) revealed invariant eutectic transformations between 980–1000 K. X-ray diffraction (XRD) confirmed that ZnTe and Cd₂Te₃ are the only equilibrium phases, with no ternary compounds observed. Thermodynamic interpretations based on the Gibbs phase rule and regular solution free energy modeling support the observed eutectic behavior. Microstructural analyses using optical microscopy and energy-dispersive X-ray spectroscopy (EDS) verified lamellar eutectic morphology and limited solubility. The results provide essential guidance for controlled crystal growth and heterostructure engineering in II–VI semiconductor devices.

АННОТАЦИЯ

Данное экспериментальное исследование посвящено изучению фазового равновесия в системе ZnTe–Cd₂Te₃ с использованием высокочистых элементных образцов, синтезированных в вакуумированных кварцевых ампулах. Для достижения близких к равновесным условий применялись контролируемые термические обработки, включая ступенчатое отжигание и медленное охлаждение. Дифференциальный термический анализ (DTA) выявил инвариантные эвтектические превращения в интервале температур 980–1000 K. Рентгеновская дифракция (XRD) подтвердила, что ZnTe и Cd₂Te₃ являются единственными фазами равновесия, при этом третичных соединений не обнаружено. Термодинамическая интерпретация на основе правила фаз Гиббса и модели свободной энергии регулярного раствора подтверждает наблюдаемое эвтектическое поведение. Микроструктурный анализ с использованием оптической микроскопии и энергодисперсионной рентгеновской спектроскопии (EDS) подтвердил ламеллярную эвтектическую морфологию и ограниченную растворимость компонентов. Полученные результаты предоставляют важные рекомендации для контролируемого выращивания кристаллов и проектирования гетероструктур в полупроводниках II–VI групп.

Keywords: ZnTe, Cd₂Te₃, phase equilibrium, eutectic reaction, phase diagram, II–VI semiconductors, thermodynamic modeling, DTA, XRD

Ключевые слова: ZnTe, Cd₂Te₃, фазовое равновесие, эвтектическая реакция, фазовая диаграмма, полупроводники II–VI группы, термодинамическое моделирование, ДТА (дифференциальный термический анализ), РФА (рентгенофазовый анализ).

1. Introduction. Phase equilibria in II–VI semiconductors are critical for tailoring structural, electronic, and optical properties, particularly for optoelectronic and photovoltaic applications [1–5]. ZnTe is a direct band-gap II–VI semiconductor (Eg ≈ 2.26 eV) suitable for light-emitting devices, whereas Cd₂Te₃-based compounds are extensively used in photovoltaic thin films [6–8]. Understanding the ZnTe–Cd₂Te₃ pseudobinary section is essential for controlled crystal growth and defect engineering. Previous studies on related II–VI systems have indicated complex phase behaviors and limited solubility, yet direct experimental data on ZnTe–Cd₂Te₃ remain scarce [9–12]. This study aims to provide a detailed characterization of phase equilibria, thermodynamic analysis, and microstructural insights to guide material design in advanced semiconductor devices.

2. Materials and Methods

2.1. Sample Preparation High-purity Zn, Cd, and Te (> 99.99 wt%) were accurately weighed to prepare alloys across the ZnTe–Cd₂Te₃ section. Samples were sealed in evacuated quartz ampoules (< 10⁻⁴ Pa) to prevent oxidation. Stepwise annealing was performed: 500 K for 10 h, 800 K for 12 h, and 1100 K for 24 h, followed by slow cooling at 3–5 K·h⁻¹ to approach equilibrium. All procedures were conducted under an inert argon atmosphere.

2.2. Thermal Analysis Differential thermal analysis (DTA) was performed using a NETZSCH STA 449 F3 analyzer under argon at a heating rate of 10 K·min⁻¹. Calibration was carried out using standard metals (Al, Au) to ensure accurate temperature measurements.

2.3. Structural Characterization X-ray diffraction (XRD) was conducted using a Bruker D8 Advance diffractometer with CuKα radiation (λ = 1.5406 Å). Microstructure observations were performed with an optical microscope (Leica DM2700), and EDS (Oxford Instruments X-Max) confirmed nominal compositions. Lamellar spacing and morphology were analyzed for microstructural assessment.

3. Results and Discussion

3.1 Thermal Behavior DTA curves revealed two distinct endothermic peaks: the liquidus transition and an invariant eutectic reaction (L → ZnTe + Cd₂Te₃). According to the Gibbs phase rule (C = 2, P = 3), the degrees of freedom F = 0 at the invariant point [13–15]. The eutectic temperature was determined to be 980–1000 K, consistent with thermodynamic predictions for weakly interacting systems.

3.2 Phase Identification XRD confirmed that ZnTe and Cd₂Te₃ are the only equilibrium phases throughout the composition range, with no additional diffraction peaks indicative of ternary compounds. Optical microscopy revealed lamellar eutectic microstructures near the eutectic composition, with lamella spacing dependent on the cooling rate. EDS analysis verified the nominal compositions and confirmed limited mutual solubility [16–18].

3.3 Tables

Table 1.

Composition vs. Eutectic Temperature

Table 2.

XRD Phase Identification

Table 3.

EDS Composition Verification

3.4. Thermodynamic Interpretation Using the regular solution model (Gᵉˣ = Ω x₁ x₂), the positive interaction parameter Ω indicates weak attraction between ZnTe and Cd₂Te₃, supporting the observed eutectic behavior. Liquidus curves decrease from pure component melting points toward the eutectic composition, consistent with positive deviation from ideality [19–22].

3.5. Comparison with Related II–VI Systems Comparison with ZnTe–CdTe and HgTe–ZnTe systems indicates similar eutectic behavior and phase boundaries. These results aid in understanding defect formation, thermodynamic stability, and controlled growth in II–VI semiconductors [23–28].

4. Conclusions

1. The ZnTe–Cd₂Te₃ system exhibits a simple eutectic phase relation without stable ternary compounds.
2. The invariant eutectic temperature occurs at 980–1000 K.
3. Thermodynamic modeling and DTA/XRD analyses support limited solubility and eutectic behavior.
4. Microstructural insights provide guidance for controlled crystal growth and heterostructure engineering in II–VI semiconductors.

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