Effects of short-range order on phase equilibria and opto-electronic properties of ternary alloy ZnxCd1-xTe

We employ first principles methods based on density functional theory and beyond to study ZnxCd1-xTe, 0 <= x <= 1, alloys in the zinc blende (B3) crystal structure. Cluster Expansion and Monte Carlo formalisms were deployed to provide a phase diagram determining consolute temperature of 387 K at 40% Zn concentration. Opto-electronic properties are computed with the hybrid HSE06 functional for disordered ZnxCd1-xTe alloys, which were simulated using special quasi-random structures. Bowing effects in the bandgap and effective carrier masses were observed in alloying which can be attributed to local geometrical distortions as portrayed by bond length distributions. Downward bowing in the electronic bandgap will be beneficial in photovoltaic applications through increased net photocurrent. Absorption coefficients show robust absorption in ZnxCd1-xTe as indicated by substantial optical absorption throughout all Zn concentrations, aided by low reflectivity that ensures a high absorption. Presence of short-range order in Zn0.25Cd0.75Te was observed through clustering and anti-clustering among different cationic species of Zn and Cd. It has a value of 0.11 at 1000 K compared to -0.79 at 400 K. The bandgap of Zn0.25Cd0.75Te at 1000 K was found to be slightly higher, 0.05 eV, than at 400 K, consistent with the value of short-range order. Thus, short-range order and possibly composition can be used in the design and synthesis of the appropriate solar cell, thereby improving the performance in this system.

Automated summary

In this study, first principles methods based on density functional theory and Monte Carlo simulations were used to investigate the properties and phase diagram of ZnxCd1-xTe alloys. The alloying effects on the opto-electronic properties were studied, and the presence of short-range order was observed. It was found that the bandgap and effective carrier masses exhibit bowing effects, which can be attributed to local geometrical distortions. The study also revealed the importance of short-range order and composition in the design and synthesis of solar cells.