Tatsuya Fujishima

GaN-on-Si Vertical Schottky and p-n Diodes

This letter demonstrates GaN vertical Schottky and p-n diodes on Si substrates for the first time. With a total GaN drift layer of only 1.5-μm thick, a breakdown voltage (BV) of 205 V was achieved for GaN-on-Si Schottky diodes, and a soft BV higher than 300 V was achieved for GaN-on-Si p-n diodes with a peak electric field of 2.9 MV/cm in GaN. A trap-assisted space-charge-limited conduction mechanism determined the reverse leakage and breakdown mechanism for GaN-on-Si vertical p-n diodes. The ON-resistance was 6 and 10 mQ · cm2 for the vertical Schottky and p-n diode, respectively. These results show the promising performance of GaN-on-Si vertical devices for future power applications.

Electrothermal Simulation and Thermal Performance Study of GaN Vertical and Lateral Power Transistors

In this paper, we present self-consistent electrothermal simulations of single-finger and multifinger GaN vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) and lateral AlGaN/GaN high-electron-mobility transistors (HEMTs) and compare their thermal performance. The models are first validated by comparison with experimental dc characteristics, and then used to study the maximum achievable power density of the device without the peak temperature exceeding a safe operation limit of 150°C (P150°C). It is found that the vertical MOSFETs have the potential to achieve a higher P150°C than the lateral HEMTs, especially for higher breakdown voltages and higher scaling level designs.

Radio-frequency plasma-excited molecular beam epitaxy growth of GaN on graphene/Si(100) substrates

Strong c-axis-oriented hexagonal (0001) GaN was grown on graphene/Si(100) substrates by radio-frequency plasma-excited molecular beam epitaxy. The hexagonal symmetry of graphene transferred onto the Si(100) surface enabled the growth of a highly c-axis-oriented GaN film. The GaN showed a full width at half maximum of 11.3 arcmin for a (0002) rocking curve measured by X-ray diffraction. Strong luminescence at 3.4 eV was also observed by cathodoluminescence with a luminescence at 3.2 eV, which originated from a cubic-phase inclusion. A microstructural study using transmission electron microscopy also confirmed the growth of hexagonal (0001) GaN on a graphene/Si(100) substrate.

Formation of low resistance ohmic contacts in GaN-based high electron mobility transistors with BCl3 surface plasma treatment

A BCl3 surface plasma treatment technique to reduce the resistance and to increase the uniformity of ohmic contacts in AlGaN/GaN high electron mobility transistors with a GaN cap layer has been established. This BCl3 plasma treatment was performed by an inductively coupled plasma reactive ion etching system under conditions that prevented any recess etching. The average contact resistances without plasma treatment, with SiCl4, and with BCl3 plasma treatment were 0.34, 0.41, and 0.17 Ω mm, respectively. Also, the standard deviation of the ohmic contact resistance with BCl3 plasma treatment was decreased. This decrease in the standard deviation of contact resistance can be explained by analyzing the surface condition of GaN with x-ray photoelectron spectroscopy and positron annihilation spectroscopy. We found that the proposed BCl3 plasma treatment technique can not only remove surface oxide but also introduce surface donor states that contribute to lower the ohmic contact resistance.

Optically active vacancies in GaN grown on Si substrates probed using a monoenergetic positron beam

Native defects in GaN layers grown on Si substrates by metal organic chemical vapor deposition have been studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation for GaN layers showed that optically active vacancy-type defects were formed in the layers. Charge transition of the defects due to electron capture was found to occur when the layers were irradiated by photons with energy above 2.71 eV. The concentration of such defects increased after 600–800 °C annealing, but the defects have not been annealed out even at 1000 °C. They were identified as Ga-vacancy-type defects, such as complexes between Ga vacancies and carbon impurities, and the relationship between their charge transition and optical properties were discussed.

Thickness Dependence of Structural and Electrical Properties of Thin InN Grown by Radio-Frequency Plasma-Assisted Molecular Beam Epitaxy

We have succeeded in achieving high-quality thin InN films (<500 nm) by low-plasma-power growth. In this study, we investigated the thickness dependence of the structural and electrical properties of relatively thin InN films grown by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). InN films were grown by RF-MBE on GaN/sapphire templates at 80 W, which is lower than that for conventional InN growth (200 W). The films had thicknesses of 5, 20, 100, 200, and 460 nm. The mobility and carrier concentration of InN degraded with decreasing InN film thickness, although even 200-nm-thick InN demonstrated relatively good characteristics (µ= 1540 cm2 V-1 s-1 and n= 2.1 ×1018 cm-3). On the other hand, the FWHM values of the (002) and (302) X-ray rocking curves (XRCs) were minimum in 5-nm-thick InN and increased with film thickness up to approximately 100–200 nm. This seems to be due to the formation of small grains with better crystal quality at the initial stage. It is also suggested that these grain structures with poor coalescence and roughness caused the deterioration of InN electronic properties in a very thin region.

Annealing behaviors of vacancy-type defects near interfaces between metal contacts and GaN probed using a monoenergetic positron beam

Vacancy-type defects near interfaces between metal contacts and GaN grown on Si substrates by metal organic chemical vapor deposition have been studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation for Ti-deposited GaN showed that optically active vacancy-type defects were introduced below the Ti/GaN interface after annealing at 800 °C. Charge transition of those defects due to electron capture was observed and was found to correlate with a yellow band in the photoluminescence spectrum. The major defect species was identified as vacancy clusters such as three to five Ga-vacancies coupled with multiple nitrogen-vacancies. The annealing behaviors of vacancy-type defects in Ti-, Ni-, and Pt-deposited GaN were also examined.

Vacancy clusters introduced by CF4-based plasma treatment in GaN probed with a monoenergetic positron beam

Vacancy-type defects introduced by CF4-plasma treatment in GaN grown on Si substrates have been studied using a monoenergetic positron beam. By positron annihilation spectroscopy, it was found that vacancies were introduced below the surface (≤30 nm), and this region expanded to a depth of 50 nm after 400 °C annealing. The major species of such defects were identified as vacancy clusters coupled with fluorine. The charge transition between these defects and optically active native defects was studied. The defects introduced by plasma treatment were found to introduce deep levels into the bandgap of GaN and act as nonradiative recombination centers.

Reduction in threshold voltages in GaN-based metal oxide semiconductor field effect transistors

The dc characteristics, such as on-resistances (Ron) and threshold voltages (Vth), of gallium nitride-based metal oxide semiconductor field effect transistors with vertical trench gates have been theoretically derived. The optimized acceptor density and the thickness of p-type layers for n channels (channel length) were estimated to be 3×1017cm−3 and 0.5μm, respectively, in order to realize Ron in the sub-mΩcm2 range. On the other hand, this resulted in a high Vth of 18V due to the wide bandgap. To achieve low Ron and moderate Vth less than 10V simultaneously, the insertion of an additional p−-type or n-type layer with finite thickness between the gate insulator and the p-type layer was suggested.

Native and process induced defects in GaN films grown on Si substrates probed using a monoenergetic positron beam

Positron annihilation is a non-destructive tool for investigating vacancy-type defects in materials. Detectable defects are monovacancies to vacancy clusters, and there is no restriction onsample temperature or conductivity. Using this technique, we studied native and plasma-treatment induced defects in GaN layers grown on Si substrates deposited by metal organic chemical vapor deposition. Measurements of Doppler broadening spectra of the annihilation radiation for 1-μm-thick GaN layers showed that optically active vacancy-type defects were formed during their growth. These defects were identified as complexes of vacancies and carbon impurities. For plasma treated samples, we found the introduction of vacancy-type defects in the subsurface region (≤2.5 nm). These results show that positron annihilation spectroscopy is a useful tool for identifying vacancy-type defects in GaN-based devices.