Satoshi Nakazawa

Chemical and potential bending characteristics of SiNx/AlGaN interfaces prepared by in situ metal-organic chemical vapor deposition

We investigate the chemical and potential-bending characteristics of in situ SiNx/AlGaN interfaces prepared by metal-organic chemical vapor deposition. X-ray photoelectron spectroscopy showed that the in situ SiNx layer had typical chemical binding energies corresponding to the Si–N bonds. The in situ SiNx deposition brought no chemical degradation on the AlGaN surface at the SiNx/AlGaN interface, whereas the ex situ deposition of SiNx by a plasma process induced chemical disorder on the AlGaN surface including a composition change and the formation of interfacial oxides. A significant reduction in the surface band bending was observed on the AlGaN surface after the in situ SiNx passivation, probably due to a decrease in the surface state density.

Present and future prospects of gan-based power electronics

GaN/AlGaN device technologies are reviewed aiming at the applications to power switching systems. SL (Super Lattice) capping and QA (Quaternary Alloy) over-layer techniques have been developed to reduce the on-resistance of GaN/ AlGaN HFET. Further, we achieved GaN on Silicon epitaxial growth technology with almost the same mobility keeping the same 2DEG density, which will make the cost comparable to conventional Si one. The experimentally obtained RonA of the FET is 1.9 m¿cm2, which is 14 times lower than that of Si ones. Additionally, a novel approach to realize enhancement mode operation of GaN/AlGaN FET is proposed using minority carrier injection by the gate.

Comprehensive study on initial thermal oxidation of GaN(0001) surface and subsequent oxide growth in dry oxygen ambient

Initial oxidation of gallium nitride (GaN) (0001) epilayers and subsequent growth of thermal oxides in dry oxygen ambient were investigated by means of x-ray photoelectron spectroscopy, spectroscopic ellipsometry, atomic force microscopy, and x-ray diffraction measurements. It was found that initial oxide formation tends to saturate at temperatures below 800u2009°C, whereas the selective growth of small oxide grains proceeds at dislocations in the epilayers, followed by noticeable grain growth, leading to a rough surface morphology at higher oxidation temperatures. This indicates that oxide growth and its morphology are crucially dependent on the defect density in the GaN epilayers. Structural characterizations also reveal that polycrystalline α- and β-phase Ga2O3 grains in an epitaxial relation with the GaN substrate are formed from the initial stage of the oxide growth. We propose a comprehensive model for GaN oxidation mediated by nitrogen removal and mass transport and discuss the model on the basis of ex...

Recessed-gate AlGaN/GaN HFETs with lattice-matched InAlGaN quaternary alloy capping layers

In this paper, we present recessed AlGaN/GaN heterojunction field-effect transistors (HFETs) with lattice-matched InAlGaN capping layers, which reduce both ohmic contact resistance and series resistance between the AlGaN and the capping layer. The lattice-matched alloy epitaxial layer with both In and Al high compositions are successfully grown by metal-organic chemical vapor deposition. The grown lattice-matched In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N capping layer has close total polarization and bandgap to those of the underlying Al/sub 0.26/Ga/sub 0.74/N layer. The balanced polarization eliminates the depletion of electrons at the In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N/Al/sub 0.26/Ga/sub 0.74/N interface, which can reduce the series resistance across it. It is also noted that the fabricated HFET exhibits very low ohmic contact resistance of 1.0/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ or less. Detailed analysis of the source resistance reveals that the series resistance at the In/sub 0.09/Al/sub 0.32/Ga/sub 0.59/N/Al/sub 0.26/Ga/sub 0.74/N interface is one fifth as low as the resistance at the conventional GaN/Al/sub 0.26/Ga/sub 0.74/N interface.

AlGaN/GaN devices for future power switching systems

GaN/AlGaN device technologies are presented aiming at the applications to power switching systems. In order to reduce on-resistance (Ron), we developed SL (super lattice) capping and QA (quaternary alloy) over-layer techniques for GaN/AlGaN HFET. Further, we achieved almost the same mobility keeping the same 2DEG density for GaN/AlGaN hetero structure grown on Si (111) substrates, which will make the cost comparable to conventional Si one. The experimentally obtained RonA of the FET is 1.9 mOmegacm2, which is 14 times lower than that of Si ones. Additionally, a novel approach to realize enhancement-mode operation of GaN/AlGaN FET is examined over R-plane sapphire, where non-polar AlGaN/GaN heterostructure, free from polarization charge, can be grown

Effect of nitrogen incorporation into Al-based gate insulators in AlON/AlGaN/GaN metal-oxide-semiconductor structures

The superior physical and electrical properties of aluminum oxynitride (AlON) gate dielectrics on AlGaN/GaN substrates in terms of thermal stability, reliability, and interface quality were demonstrated by direct AlON deposition and subsequent annealing. Nitrogen incorporation into alumina was proven to be beneficial both for suppressing intermixing at the insulator/AlGaN interface and reducing the number of electrical defects in Al2O3 films. Consequently, we achieved high-quality AlON/AlGaN/GaN metal–oxide–semiconductor capacitors with improved stability against charge injection and a reduced interface state density as low as 1.2 × 1011 cm−2 eV−1. The impact of nitrogen incorporation into the insulator will be discussed on the basis of experimental findings.

High fmax with High Breakdown Voltage in AlGaN/GaN MIS-HFETs using In-Situ SiN as Gate Insulators

AlGaN/GaN heterojunction transistors (HFETs) with metal-insulator-semiconductor (MlS)-type gate structure is promising for high frequency and high power applications owing to the superior material properties together with the reduced gate leakage current. In this paper, we present a novel AlGaN/GaN MIS-HFET using so-called in-situ SiN as a gate insulator. The in-situ SiN with a crystalline structure is formed subsequently after the epitaxial growth in the same reactor without any exposure in the air. The formation of the in-situ SiN greatly enhanced the sheet carrier concentration, which helps the reduction of the parasitic resistances. The fabricated MIS- HFET exhibits very high maximum oscillation frequency (fmax) of 203 GHz for the device with the gate length of 100 nm. The device exhibits the off-state breakdown voltages of 190 V at highest maintaining the high fmax over 190 GHz, and is thus promising for high frequency and high power applications including future millimeter wave communication systems.

200 W Output Power at S-Band in AlGaN/GaN Heterojunction Field Effect Transistors with Field Plates on Si Substrates

Use of Si substrates for the fabrication of microwave AlGaN/GaN heterojunction field effect transistors (HFETs) has been strongly desired for the low cost fabrication. The performance so far has never been satisfactory in view of the output power and the gain as compared with those on SiC substrates. In this paper, AlGaN/GaN HFETs on Si with high output power of 203 W and high linear gain of 16.9 dB at 2.5 GHz are demonstrated. The HFETs have field plates to reduce the feedback capacitance leading to higher gain, of which a new design of the field plates enables high power as well. The structural design is based on the equivalent circuit model using the device parameters extracted from the small signal RF performances. Here, it is found that shortening the field plate length down to 0.6 µm results in the high output power owing to the stable output impedance for various drain voltage. Note that the conditions of the epitaxial growth are optimized to achieve high current density of 850 mA/mm with both the high mobility and high sheet carrier concentration. The device processing is established so as to achieve the high power operation free from the current collapse. The device can be operated at the drain voltage as high as 50 V, which enables the 200 W output power. The presented AlGaN/GaN HFETs are very promising for various microwave applications including cellular base stations, which would lower the system cost taking advantage of cost-effective Si substrates.

High-Power and High-Gain S-band AlGaN/GaN HFETs with Source Field Plates on Si Substrate

1. Introduction AlGaN/GaN heterojunction field-effect transistors (HFETs) have been widely investigated for high-frequency and high-power applications such as base stations of cellular phones, taking advantages of the superior material properties. Higher gain is also required in such applications, which reduces the number of amplifiers leading to smaller system in size. Introduction of field plate structures in the HFETs increases the gain by reducing gate-drain feedback capacitances (C gd) [1]. However, there has been a limitation of increasing the maximum output power keeping the high gain in the reported devices. In this paper, we present 203W output power with high gain of 16.9dB at 2.5GHz in AlGaN/GaN HFETs on Si substrates with source field plates. The detailed simulation using the device parameters at various biasing conditions reveals that shortening the field plate length achieves high output power together with the high gain, which well agrees with the experimental esults. r

Design and control of interface reaction between Al-based dielectrics and AlGaN layer in AlGaN/GaN metal-oxide-semiconductor structures

Important clues for achieving well-behaved AlGaN/GaN metal-oxide-semiconductor (MOS) devices with Al-based gate dielectrics were systematically investigated on the basis of electrical and physical characterizations. We found that low-temperature deposition of alumina insulators on AlGaN surfaces is crucial to improve the interface quality, thermal stability, and variability of MOS devices by suppressing Ga diffusion into the gate oxides. Moreover, aluminum oxynitride grown in a reactive nitric atmosphere was proven to expand the optimal process window that would improve the interface quality and to enhance immunity against charge injection into the gate dielectrics. The results constitute common guidelines for achieving high-performance and reliable AlGaN/GaN MOS devices.