On the stability of Cu x Te polytypes: phase transitions, vibrational and electronic properties.

Based on the experimental structures reported for the Cu x Te (1 ⩽ x ⩽ 2) system, a theoretical study on stability and phase transitions has been performed. Three theoretical structures derived from rickardite (Cu1.5Te) were considered to represent different Cu/Te ratios (1, 1.5 and 2). The structural, electronic, and vibrational properties were calculated by density functional theory and compared to the experimental data available to date. This analysis showed that the proposed CuTe and Cu1.5Te structures are energetically and dynamically stable (unlike Cu2Te), and that their vibrational modes may play an important role in the reported Raman spectra for Cu x Te films. As well, it was found that being vulcanite the most stable phase for x = 1, the addition of Cu atoms to this structure induces a gradual flattening of the Cu planes, producing significant changes in the electronic band structure. A thorough review of the experimental reports on the electrical properties of the system was carried out. The experimental data showed that, in agreement with the calculations, the electrical conductivity is higher for phases with x ⩽ 1.5, decreasing as x gets closer to 2. Hence, the low-copper concentration phases are the best choice for solar cell applications due to their electrical properties and stability.

Vibrational and electrical properties of Cu2-xTe films: experimental data and first principle calculations.

Vibrational and electrical properties of sputtered films of the copper telluride system are presented. Despite of its technological importance in photovoltaics, the fundamental properties of copper tellurides are poorly understood. Films were deposited at 200 °C by rf sputtering from targets containing mixtures of copper and tellurium powders at nominal concentrations of Cu1.25Te, Cu1.5Te, Cu1.75Te and Cu2Te. Remarkably for the copper telluride system, it was possible to obtain single-phase vulcanite (CuTe) from the Cu1.25Te target. Two-phase mixtures of rickardite (Cu7Te5) and weissite (Cu2-xTe) were achieved for other cases. Raman spectra were obtained using two laser lines: 633 and 488 nm. Density functional theory was employed to calculate the phonon dispersion curves and density of states for vulcanite. The Raman bands were in good correspondence with the calculated frequencies. In general, the Raman spectra consisted of high-intensity totally symmetric modes superimposed on monotonically decaying signals. These were explained in terms of three contributing phenomena: convolution of vibrational normal modes, phonon-coupled charge density fluctuations and time-varying local-field contributions to the electric susceptibility. Studies on the conductivity, mobility and carrier concentration were carried out by the Van der Pauw method. Micro/nano scale surface potential studies were performed through Kelvin probe force microscopy mapping.