Influence of crystalline quality of low-temperature GaN cap layer on current collapse in AlGaN/GaN heterostructure field-effect transistors

The correlation between the crystalline quality of low-temperature GaN (LT-GaN) films and current collapse in AlGaN/GaN heterostructure field-effect transistors with a LT-GaN cap layer was investigated. LT-GaN cap layers were grown at various temperatures ranging from 450 to 700 degrees C. No current collapse was observed for heterostructure field-effect transistors with LT-GaN cap layers grown at 500 and 550 degrees C. The crystalline quality of the 1-mu m-thick LT-GaN films grown at 450-700 degrees C was then studied through x-ray diffraction and selected area electron diffraction measurements. The crystalline quality, considered by measuring the full width at half maximum through x-ray diffraction, deteriorated as the growth temperature decreased. The crystal structure of LT-GaN films grown at 500 and 550 degrees C was confirmed as polycrystalline. Thus, the results demonstrated that in AlGaN/GaN heterostructure field-effect transistors, a polycrystalline structure provides the most effective LT-GaN cap layer for suppressing current collapse. (C) 2008 American Institute of Physics.