Sodium beta-alumina thin films as gate dielectrics for AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors

Sodium beta-alumina (SBA) is deposited on AlGaN/GaN by using co-deposition process with using sodium and Al₂O₃ as the precursors. X-ray diffraction (XRD) spectrum reveals that the deposited thin film is amorphous. The binding energy and composition of the deposited thin film, obtained from the X-ray photoelectron spectroscopy (XPS) measurement are consistent with those of SBA. The dielectric constant of the SBA thin film is about 50. Each of the capacitance-voltage characteristics obtained at five different frequencies shows a high-quality interface between SBA and AlGaN. The interface trap density of metal-insulator-semiconductor high-electron-mobility transistor (MISHEMT) is measured to be (3.5～9.5)×10¹⁰ cm^-2·eV^-1 by the conductance method. The fixed charge density of SBA dielectric is on the order of 2.7×10¹² cm^-2. Compared with the AlGaN/GaN metal-semiconductor heterostructure high-electron-mobility transistor (MESHEMT), the AlGaN/GaN MISHEMT usually has a threshold voltage that shifts negatively. However, the threshold voltage of the AlGaN/GaN MISHEMT with using SBA as gate dielectric shifts positively from -5.5 V to -3.5 V. From XPS results, the surface valence-band maximum (VBM-E_F) of AlGaN is found to decrease from 2.56 eV to 2.25 eV after the SBA thin film deposition. The possible reasons why the threshold voltage of AlGaN/GaN MISHEMT with the SBA gate dielectric shifts positively are the influence of SBA on surface valence-band maximum (VBM-E_F), the reduction of interface traps and the effects of sodium ions, and/or the fixed charges in SBA on the two-dimensional electron gas (2DEG).