Investigation of the Thermal Properties of Gallium Nitride using the Three Omega Technique

Abstract

Gallium nitride based devices suffer from undesirable heating effects that significantly limit the performance of high electron mobility transistors and laser diodes thereby reducing device lifetime and reliability. An accurate knowledge of the gallium nitride thermal conductivity is crucial to understanding thermal effects.

This work addresses issues related to both the thermal limitations and thermal characterization of gallium nitride alloys and devices. First, theoretical developments concerning the three omega technique applied to a film-on-substrate system are considered when the film-to-substrate thermal conductivity ratio is larger than one. The case of thermal boundary resistance between film and substrate is included. In the case of high thermal conductivity films, it is found that neglecting the presence of thin thermally insulating buffer layer would lead to large uncertainties in extracted film thermal conductivities.

Next, very thick, free standing gallium nitride layers grown by hydride vapor phase epitaxy were precisely measured from 300K to 450K using the three omega method. By comparing the measured values with dislocation density measurements the dependence of the gallium nitride thermal conductivity on dislocation density was obtained and compared with theory. In addition, the thermal conductivity of iron doped semi-insulating gallium nitride was measured to be as high as 230 W•K-1•m-1. In this study, a 2mm thick iron doped gallium nitride substrate was measured to have an average dislocation density of 5.104 cm-2 which represents the present state of the art.

Finally the modeling of the thermal resistance of multifinger AlGaN/GaN HEMTs was examined using the experimentally determined values of the thermal conductivity. Using the three omega method as the starting point, an original accurate closed-form compact expression for the thermal resistance of single and multifinger HEMT device structures was derived. It was found that the thermal performance of a single finger HEMT composed of homoepitaxial GaN on a GaN substrate is superior to that of single finger GaN HEMT heteroepitaxially grown on silicon carbide.