Thesis for obtaining the scientific degree of candidate of chemical sciences in the specialty 02.00.21 – Solid State Chemistry (chemical sciences). – Yuriy Fedkovych Chernivtsi National University, Chernivtsi, 2022.
The thesis is devoted to the study of phase transitions in the CdTe–MnTe–ZnTe system from the CdTe side and experimental investigation of the properties of
Cd1-x-yMnxZnyTe single crystals (x = 0.05 – 0.30, y = 0.05–0.15).
The differential thermal analysis (DTA) method was used for investigation of the melting and crystallization kinetic parameters of Cd1-x-yMnxZnyTe alloys (x = 0.10 – 0.30, y = 0.05–0.15). It was found that the changing of the MnTe content from 10 to 30 mol. % decreases a melting temperature from 1367 ± 1 К to 1358 ± 1 К (for Cd0.95-xMnxZn0.05Te, х=0.10 – 0.30), from 1363 ± 1 К to 1360 ± 1 К (for Cd0.90-xMnxZn0.10Te alloys, x=0.10 – 0.30), from 1378 ± 1 К to 1370 ± 1 К for Cd0.85-xMnxZn0.15Te alloys (x=0.10 – 0.20). The ZnTe content increasing led to an increase in the melting temperature of the samples from 1358 ± 1 К to 1375 ± 1 К.
The activation energies of melting and crystallization processes for Cd1-x-yMnxZnyTe alloys under different conditions are established. The dependence of the logarithm of the preexponential factor on the activation energy of melting process for Cd1-x-yMnxZnyTe (х=0.10 – 0.30, y = 0.10 – 0.15) melts has a linear behavior and corresponds to the compensation effect. The values of the activation energy Ea for crystallization process of Cd1-x-yMnxZnyTe melts (x = 0.20 – 0.30, y = 0.05 – 0.15) are 380 – 820 kJ/mol and are commensurate with the activation energies of the crystallization process of pure CdTe and alloys of CdTe–In systems. This means that the dominant role in crystallization processes of these alloys belongs to the structure centered on the role of CdTe bonds. These results are in good agreement with previously published results concerning melting and crystallization processes for CdTe-based melts.
Based on the obtained differential thermal analysis data, Cd1-x-yMnxZnyTe single crystals (x = 0.05 – 0.30, y = 0.05 – 0.15) were grown by the vertical Bridgman method. The structural perfection of the crystals decreases with increasing ZnTe content. This is due to the fact that the process of growing ingots with 15% ZnTe requires higher growth temperatures (Tmelt(ZnTe) = 1568 K). It is also interesting that the presence of twins is not observed in all grown ingots. Only Cd1-x-yMnxZnyTe (x = 0.20, 0.30 and y = 0.05, 0.10) crystals are single-grain with large number of twins. Our date is in agreement with the analyses the morphology and crystallography of twins in Cd1-xMnxTe (x=0.2 and x = 0.35) crystal. Thus, 0.05 and 0.10 mol. % of Zn has no influence at twins formation in process of Cd1-x-yMnxZnyTe (x = 0.20, 0.30 and y = 0.05, 0.10) crystals growth. The typical IR microscope images of all as-grown crystals show that the size of Te inclusions was about 5-20 m in all samples. But the Te inclusions in some ingots were distributed across the entire plane of the crystals uniformly, while in the other ingots Te precipitates show evidently tendency to ordering in quasi parallel lines and stacking faults formation that can be caused by lateral mechanism of the crystal growth. According to XRD-analyses the value of the lattice constant of the Cd1-x-yMnxZnyTe ingots (x = 0.05 – 0.30, y = 0.05–0.15) linearly decreases from 6.43(1) Å to 6.37(5) Å with increasing MnTe and ZnTe concentration. By measuring the transmission spectra, it was determined that the band-gap of the grown semiconductor crystals linearly increases with an increasing amount of MnTe and ZnTe.
According to the results of the high temperature Hall-effect investigations, it was found that the lattice of grown Cd1-x-yMnxZnyTe crystals is unstable and its stabilization is achieved by holding the samples for several hours at 873 K–973 K. The study of the nature of the dominant charge carriers in the investigated ingots at different temperatures showed that there is an increased content of metal vacancies in these crystals.
Keywords: CdTe–MnTe–ZnTe system, differential thermal analysis (DTA), melting temperature, crystallization temperature, solid phase (clusters), phase transitions, Cd1-x-yMnxZnyTe single crystals, XRD-analyses, high-temperature electrical measurements, point defects.
