Enhancement of thermopower in GaN by ion irradiation and possible mechanisms

Abstract

A detailed analysis of defect assisted transport properties of GaN after irradiation is carried out. Unintentionally doped GaN samples were irradiated by a 100 MeV oxygen ion beam and a 200 MeV silver ion beam at various fluences between 1 × 1010 ions/cm2 and 5 × 1011 ions/cm2. Thermopower and Hall effect measurements were done on pristine and irradiated GaN samples at variable temperatures. An increase in the thermopower with temperature as well as on irradiation (with respect to pristine GaN) was observed. Electrical conductivity and Hall mobility decreased on irradiation, suggesting scattering of charge carriers due to the formation of defects. A qualitative study of activation energy due to thermopower and conductivity is done by the scattering mechanism using different theoretical models to determine the transport path of the carriers. The observed increase in thermopower of GaN after irradiation suggests a probable hopping mechanism due to the presence of energy levels in the bandgap.

Thermoelectric properties of GaN with carrier concentration modulation: an experimental and theoretical investigation

“…Both p-and n-type materials were successfully achieved depending upon the amount or type of the additional impurity introduced in the pure GaN material [24]. Our group previously reported enhancements in the Seebeck coefficient and power factor of n-GaN thin films and other materials by optimized ion beam irradiations [25,26]. Yamamoto et al prepared bulk GaN material with micron size of particles, which showed a negative S value of −50 μV/K at room temperature that further increased with temperature [27].…”

Wide range temperature-dependent (80–630 K) study of Hall effect and the Seebeck coefficient of β -Ga2O3 single crystals

“…In these materials, it is expected that the thermal conductivity might be decreased by introducing the disorder with the short and longwavelength phonon scatterings for both low and high-temperature regimes. Similarly, in GaN controlled defect introduction by swift heavy ion irradiation have resulted in increased thermopower and power factor [15]. An optimized ion energy and fluence can improve the thermoelectric properties of β-Ga2O3.…”

Tuning the Electrical and Thermoelectric Properties of N Ion Implanted SrTiO3 Thin Films and Their Conduction Mechanisms

“…The concept of large E a over E s lies in such a way that in the resistivity measurements, the current flows through the film in a closed loop because the charge carriers need to overcome the highest potential barrier during the conduction. However, in thermopower, it is an open circuit measurements wherein the charge carriers have to cross only the local fluctuations 10,38 . The difference ΔE quantifies the magnitude of potential fluctuations.…”

“…To conclude, N ion implantation creates defects and vacancies, which affect the transport properties of the carriers. It modifies the transport mechanism by creating trap centers and defects which act as scattering centers 10 . The defects induced leads to the reduction of thermal conductivity and have less effect on S, which in turn improves the ZT value.…”

“…This technique provides independent control of dose and penetration depth. However, ion implantation has been scarcely explored in thermoelectric materials 9,10 . It is reported that ion beam technique can contribute in the modification of lattice thermal conductivity effectively by radiation induced defects engineering or nanostructuring 11,12 .…”