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

Nanostructured Zn1−xAlxTe (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 Gruneisen 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 Zn0.85Al0.15Te, 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.