Mechanical stress effects on the electrical characteristics of III-nitride devices.

摘要

Hydrostatic pressure and uniaxial stress have long been used to study the properties of semiconductors. The changes in the properties of semiconductors with stress have contributed vitally to the understanding of semiconductors at atmospheric conditions. The strong piezoelectricity of III-nitrides has important implications for device characteristics. Furthermore, it can be exploited in designing stress sensors, particularly devices that can operate at high temperature and in harsh environments. This thesis presents a study of mechanical stress effects on the electrical characteristics of several III-nitride devices, and it explores their potential as stress sensors.; The electrical conductivity of a p-GaN epitaxial layer is measured as a function of uniaxial stress and as a function of hydrostatic pressure. The observed piezoconductivity is consistent with theoretical calculations based on stress induced changes in the band structure near the top of the valence band.; Uniaxial stress and hydrostatic pressure are applied to Ni Schottky contacts on Ga-polarity n-GaN, N-polarity n-GaN, and Ga-polarity n-AlGaN. The changes in barrier heights with stress are attributed to a combination of band structure and piezoelectric effects.; Hydrostatic pressure is applied to AlGaN/GaN heterojunction field effect transistors (HFETs). The drain current is found to increase with pressure owing to a negative shift of threshold voltage. The pressure effect is attributed to an increase of the polarization charges at the AlGaN/GaN interface as well as a shift of the gate contact Schottky barrier height.; The effect of hydrostatic pressure on the current-voltage characteristics of GaN/AlGaN/GaN heterostructure devices is examined. Theoretical modeling suggests a decrease of the current with pressure due to the piezoelectric effect. The concept is verified experimentally by examining a GaN/Al 0.2Ga0.8N/GaN device under hydrostatic pressure. The current is found to decrease approximately linearly with pressure in agreement with the model results.; The thesis demonstrates the importance of piezoelectricity in III-nitride device characteristics, and their considerable potential as stress sensors. Of all the devices tested under hydrostatic pressure, Ga-polarity n-AlGaN Schottky diodes have the largest gauge factor at room temperature, while AlGaN/GaN HFETs appear to have the advantage of good thermal stability and suitability for high temperature operation.