Low thermal conductivity and enhanced thermoelectric performance of nanostructured Al-doped ZnTe

Nanostructured Zn_1-xAl_xTe (0≤x≤0.15) samples have been fabricated using hydrothermal synthesis and evacuating-and-encapsulating sintering. Thermoelectric properties are measured at 300-600 K. The thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The hole concentration increases with increasing Al³⁺ content. Based on the charge neutrality, the hole concentration is expected to decrease upon Al doping. However, the x-ray absorption spectroscopy indicates the electrons transfer from Zn 4s to Te 5d states upon Al doping, which might explain that the increase of hole concentration upon partial Al³⁺ substitution of Zn²⁺. Using Slack's model for estimating the lattice thermal conductivity, the Grüneisen parameter at 300 K is found to increase with Al doping content. The highest power factor (1.52 μW m^-1 K^-2) and highest dimensionless figure of merit (0.075) are attained at 575 K for Zn_0.85Al_0.15Te, representing an improvement of about 508% and 851% with respect to the nondoped ZnTe at the same temperature. These results suggest that aluminum is an effective doping element for improving the thermoelectric properties of ZnTe.