Tetsuro Ishiguro

Low dislocation density InAlN/AlN/GaN heterostructures grown on GaN substrates and the effects on gate leakage characteristics

This paper reports on the electrical characterization of Ni/Au Schottky diodes fabricated on InAlN high-electron-mobility transistor (HEMT) structures grown on low dislocation density free-standing GaN substrates. InAlN HEMT structures were grown on sapphire and GaN substrates by metal-organic vapor phase epitaxy, and the effects of threading dislocation density on the leakage characteristics of Ni/Au Schottky diodes were investigated. Threading dislocation densities were determined to be 1.8 × 104 cm−2 and 1.2 × 109 cm−2 by the cathodoluminescence measurement for the HEMT structures grown on GaN and sapphire substrates, respectively. Leakage characteristics of Ni/Au Schottky diodes were compared between the two samples, and a reduction of the leakage current of about three to four orders of magnitude was observed in the forward bias region. For the high reverse bias region, however, no significant improvement was confirmed. We believe that the leakage current in the low bias region is governed by a dislo...

New Model of Fe Diffusion in Highly Resistive Fe-Doped Buffer Layer for GaN High-Electron-Mobility Transistor

We demonstrated the suppression of Fe incorporation into undoped GaN layers grown on an Fe-doped GaN layer by metal organic chemical vapor deposition (MOCVD). Our systematic study of the depth profile of Fe doping by secondary-ion mass spectrometry (SIMS) revealed that Fe segregation on the growth surface is responsible for Fe incorporation into the upper undoped GaN layer. Moreover, we confirmed from the Fe doping profiles of strain-varied undoped GaN layers grown on an Fe-doped AlxGa1-xN layer that the compressive strain on the growth surface could effectively suppress Fe segregation. In this study, we propose a new model showing that the suppression is due to an increase in the extent of thermal desorption of Fe from the growth surface by compressive strain.

Suppression of gate leakage current in in-situ grown AlN/InAlN/AlN/GaN heterostructures based on the control of internal polarization fields

This paper investigates the gate leakage characteristics of in-situ AlN capped InAlN/AlN/GaN heterostructures grown by metal-organic vapor phase epitaxy. It was revealed that the leakage characteristics of AlN capped InAlN/AlN/GaN heterostructures are strongly dependent on the growth temperature of the AlN cap. For an AlN capped structure with an AlN growth temperature of 740 °C, the leakage current even increased although there exists a large bandgap material on InAlN/AlN/GaN heterostructures. On the other hand, a large reduction of the gate leakage current by 4–5 orders of magnitudes was achieved with a very low AlN growth temperature of 430 °C. X-ray diffraction analysis of the AlN cap grown at 740 °C indicated that the AlN layer is tensile-strained. In contrast to this result, the amorphous structure was confirmed for the AlN cap grown at 430 °C by transmission electron microscopy. Furthermore, theoretical analysis based on one-dimensional band simulation was carried out, and the large increase in two...

Advantages of the AlGaN spacer in InAlN high-electron-mobility transistors grown using metalorganic vapor phase epitaxy

We demonstrate the advantages of an AlGaN spacer layer in an InAlN high-electron-mobility transistor (HEMT). We investigated the effects of the growth parameters of the spacer layer on electron mobility in InAlN HEMTs grown by metalorganic vapor phase epitaxy, focusing on the surface roughness of the spacer layer and sharpness of the interface with the GaN channel layer. The electron mobility degraded, as evidenced by the formation of a graded AlGaN layer at the top of the GaN channel layer and the surface roughness of the AlN spacer layer. We believe that the short migration length of aluminum atoms is responsible for the observed degradation. An AlGaN spacer layer was employed to suppress the formation of the graded AlGaN layer and improve surface morphology. A high electron mobility of 1550 cm2 V−1 s−1 and a low sheet resistance of 211 Ω/sq were achieved for an InAlN HEMT with an AlGaN spacer layer.

Origin of unintentional gallium incorporation into AlN layers grown by metalorganic vapor phase epitaxy

This paper presents the origin of gallium (Ga) incorporation into AlN layers grown by metalorganic vapor phase epitaxy (MOVPE). We systematically investigated Ga incorporation from GaN deposits on the inner walls of a growth reactor and an underlying GaN layer. The Ga incorporation is not affected by the GaN deposits on the inner walls of the reactor, but is strongly affected by the underlying GaN layer. We found that the AlN layer incorporates Ga atoms which are desorbed from the underlying GaN layer, and the Ga incorporation decreases with the growth temperature. We conclude that the dominant source of unintentional Ga incorporation into an AlN layer is the underlying GaN layer. Furthermore, we successfully achieved an aluminum composition of over 0.95, almost without unintentional Ga incorporation, by growing the AlN layers at a low temperature.