Osamu Ishiguro

GaN-Based Trench Gate Metal Oxide Semiconductor Field-Effect Transistor Fabricated with Novel Wet Etching

A novel method for fabricating trench structures on GaN was developed. A smooth non-polar (1100) plane was obtained by wet etching using tetramethylammonium hydroxide (TMAH) as the etchant. A U-shape trench with the (1100) plane side walls was formed with dry etching and the TMAH wet etching. A U-shape trench gate metal oxide semiconductor field-effect transistor (MOSFET) was also fabricated using the novel etching technology. This device has the excellent normally-off operation of drain current–gate voltage characteristics with the threshold voltage of 10 V. The drain breakdown voltage of 180 V was obtained. The results indicate that the trench gate structure can be applied to GaN-based transistors.

A Vertical Insulated Gate AlGaN/GaN Heterojunction Field-Effect Transistor

We fabricated a vertical insulated gate AlGaN/GaN heterojunction field-effect transistor (HFET), using a free-standing GaN substrate. This HFET has apertures through which the electron current vertically flows. These apertures were formed by dry etching the p-GaN layer below the gate electrodes and regrowing n--GaN layer without mask. The HFET exhibited a specific on-resistance of as low as 2.6 mΩcm2 with a threshold voltage of -16 V. This HFET would be a prototype of a GaN-based high-power switching device.

Reduction of Mg segregation in a metalorganic vapor phase epitaxial grown GaN layer by a low-temperature AlN interlayer

The redistribution behavior of Mg in a sequentially regrown GaN epilayer on a p-type doped GaN template was studied. All samples in this study were regrown by metalorganic vapor phase epitaxy on the sapphire substrates. A high density and a slow tail of Mg concentration were observed in a nominally undoped layer due to the surface segregation. We found that the insertion of a low-temperature (LT) AlN interlayer was effective to suppress the Mg redistribution in the GaN regrown layer. Analyzing the temperature dependence of the surface segregation, the activation energy of the Mg segregation was estimated to be 0.63eV in GaN and 2.47eV in a LT-AlN layer, respectively.

Characterization of plasma etching damage on p-type GaN using Schottky diodes

The plasma etching damage in p-type GaN has been characterized. From current-voltage and capacitance-voltage characteristics of Schottky diodes, it was revealed that inductively coupled plasma (ICP) etching causes an increase in series resistance of the Schottky diodes and compensation of acceptors in p-type GaN. We investigated deep levels near the valence band of p-type GaN using current deep level transient spectroscopy (DLTS), and no deep level originating from the ICP etching damage was observed. On the other hand, by capacitance DLTS measurements for n-type GaN, we observed an increase in concentration of a donor-type defect with an activation energy of 0.25eV after the ICP etching. The origin of this defect would be due to nitrogen vacancies. We also observed this defect by photocapacitance measurements for ICP-etched p-type GaN. For both n- and p-type GaN, we found that the low bias power ICP etching is effective to reduce the concentration of this defect introduced by the high bias power ICP etching.

Excess Carrier Lifetime Measurement for Plasma-Etched GaN by the Microwave Photoconductivity Decay Method

We measured the excess carrier lifetimes in GaN by the microwave photoconductivity decay (µ-PCD) method. We characterized undoped GaN, Si-doped n-GaN, and Mg-doped p-GaN before and after etching using inductively coupled plasma (ICP). For all the samples, decay curves had both fast and slow components, and carrier lifetimes for the p-GaN were significantly shorter than those for the other samples. For the undoped GaN, ICP etching enhanced the slow component compared with the as-grown sample. This would be due to generation of hole traps by ICP etching.

Different Bias-Voltage Dependences of Photocurrent in Pt/InGaN/GaN and Pt/GaN Schottky Photodetectors on Sapphire

Two structures under back illumination showed opposite bias polarity dependence in the photocurrent of Schottky barrier contacts, where a combination of Pt/Au metal films was formed on unintentionally doped n-type layers. The contact with a 2-µm-thick GaN layer exhibited a higher photocurrent for reverse biasing as expected, whereas the same contact with an additional 20-nm-thick InGaN layer on GaN exhibited a much higher current for forward biasing. This current was maintained down to a small reverse bias voltage, which indicates that the thin InGaN layer with an In content of 15% behaves like a p-type semiconductor. The result can be understood by the internal electric field in the InGaN layer as well as the fact that 400 nm light illuminated from the back side is absorbed in the thin layer just under the contact metal.

Selective Detection of Blue and Ultraviolet Light by An InGaN/GaN Schottky Barrier Photodiode

Light in wavelength ranges of 400 and 350 nm was selectively detected by changing the bias voltage of the diode. Piezoelectric field in a 20-nm-thick InGaN layer on an n-GaN/sapphire structure in combination with a back-incidence configuration was successfully utilized. The principle is based on a fact that a strained InGaN layer has an inward electric field, whereas the n-type underlayer has an outward field. Using a circular Schottky electrode of 1 mm diameter surrounded by an ohmic electrode, a responsivity of 0.06 A/W was obtained for both wavelength ranges by choosing a bias of either 0 or -5 V.

Characterization of Traps in GaN pn Junctions Grown by MOCVD on GaN Substrate Using Deep-Level Transient Spectroscopy

Minority- and majority-carrier traps were studied in GaN pn junctions grown homoepitaxially by MOCVD on n+ GaN substrates. Two majority-carrier traps (MA1,MA2) and three minority-carrier traps (MI1, MI2, MI3) were detected by deep-level transient spectroscopy. MA1 and MA2 are electron traps commonly observed in n GaN on n+ GaN and sapphire substrates. No dislocation-related traps were observed in n GaN on n+ GaN. Among five traps in GaN pn on GaN, MI3 is the main trap with the concentration of 2.5x1015 cm-3.