Investigation of aluminum gallium nitride films and nickel/aluminum gallium nitride Schottky diodes using depth-dependent cathodoluminescence spectroscopy and secondary ion mass spectrometry.

摘要

AlGaN/GaN heterostructures and high Al mole fraction AlGaN films are used in a wide variety of applications, such as high power/high frequency transistors, UV photodetectors, solar-blind detectors, light-emitting diodes, and laser diodes. However, there are several important issues that need to be addressed in AlGaN/GaN heterostructures, such as the impact of surface roughness, impurities, and defect states on electronic properties such as mobility and two-dimensional electron gas (2DEG) sheet charge density and the role of surface processing on the Schottky barrier height. Also, Si doping of AlGaN with high Al mole fraction has been shown to be difficult and may be restricted by non-intentional impurities and their associated deep levels (such as O), as well as an increasing dopant donor energy with higher Al mole fraction. For the AlGaN/GaN heterostructures, correlations have been made between deep level defects and the 2DEG sheet charge density, interface broadening, surface roughness, and Ga-N ratios. Depth-dependent cathodoluminescence spectroscopy (CLS) and secondary ion mass spectrometry (SIMS) reveal the nature of deep level defects and their effect on Si doping of high Al mole fraction (25%–100%) AlGaN. SIMS results provide correlations between AlGaN deep level emissions from CLS and elemental impurities distributed through the epitaxial bulk films. The highest Al mole fraction (x_Al) samples exhibit deep level optical emissions that correlate with O and C impurities measured by SIMS. The CLS energy onset of near band edge peak emissions track the b = 1 theoretical band gap for 0 ≤ x_Al ≤ 0.98 while their peak emissions deviate monotonically. The absence of free carriers for x_Al > 0.80 is consistent with Si donor compensation due to deep levels associated with oxygen. Cross-sectional CLS measurements of the AlGaN/sapphire interface reveal luminescence signatures which correlate with oxygen diffusing from the sapphire into the AlGaN. Internal photoemission spectroscopy (IPE) reveals changes in the Schottky barrier height of Ni on AlGaN/GaN heterojunction field effect transistor structures (HFETs) with pre-metallization processing conditions and post-metallization ultra-high vacuum (UHV) annealing. These variations in the IPE Schottky barrier height are correlated with AlGaN near band edge emissions from low energy electron-excited nanoluminescence spectroscopy (LEEN) and Ni/AlGaN interface impurities by SIMS. It is shown that changes in the Schottky barrier height and the appearance of dual barriers are dominated by changes in the local Al mole fraction. (Abstract shortened by UMI.)