Properties of Surface States on GaN and Related Compounds and Their Passivation by Dielectric Films
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Properties of Surface States on GaN and Related Compounds and Their Passivation by Dielectric Films Hideki HASEGAWA and Tamotsu HASHIZUME Research Center for Integrated Quantum Electronics (RCIQE) and Graduate School of Electronics and Information Engineering, Hokkaido University N-13, W-8, Kita-ku, Sapporo, 060-8628 Japan ABSTRACT This paper reviews the authors' recent efforts to clarify the properties of electronic states near surfaces of GaN and AlGaN by using various in-situ and ex-situ characterization techniques, including UHV contactless C-V, photoluminescence surface state spectroscopy (PLS3), cathode luminescence in-depth spectroscopy (CLIS),and gateless FET techniques that have been developed by the authors’ group. As a result, a model including a U-shaped surface state continuum, having a particular charge neutrality level, combined with frequent appearance of near-surface N-vacancy related deep donor states having a discrete level at Ec - 0.37eV is proposed as a unified model that can explain large gate leakage currents and current collapse in AlGaN/GaN HFETs. Hydrogen plasma treatment and SiO2 deposition increase Nvacancy related deep donors. Reasonably good surface passivation can be achieved by ECR-plasma SiNx films and by ECR-plasma oxidized Al2O3films both combined with ECR N2 plasma treatment. INTRODUCTION GaN and related compounds such as AlGaN and InGaN have recently established their position as the key materials for applications to high-power microwave/millimeter-wave electronic devices and blue/UV optoelectronic devices. Great progress has been made on metal-organic vapor phase epitaxy (MOVPE) growth of these materials on sapphire and SiC substrates. Molecular beam epitaxy (MBE)-growth method has also demonstrated unique features such as achievements of high electron mobility values [1], a lower growth temperature reducing thermally induced strain, availability of larger In fractions in InGaN and reduced hydrogen passivation of dopants such as Mg, making it also a seemingly viable growth technique. However, AlGaN/GaN heterostructure field effect transistor (HFET) devices are known to exhibit various anomalous behavior including anomalously large gate leakage currents [2], I-V dispersion, drain current collapse [3,4], gate- and drain-lag [4], etc which are closely related to details of processing applied to the surface during device fabrication. Thus, understanding and control of surface are essentially important to produce viable devices with acceptable performance and reliability. The purpose of the present paper is to review the results of our recent efforts to clarify and control the properties of near-surface electronic states of GaN and AlGaN using various in-situ and ex-situ characterization techniques. The topics discussed here include characterization techniques, properties of free surfaces, current transport in Schottky barriers and processes for surface passivation. EXPERIMENTAL Sample structures and their preparation GaN and AlGaN epitaxial wafers grown on sapphire substrate
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