Yasushi Nanishi

Unusual properties of the fundamental band gap of InN

The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.

RF-Molecular Beam Epitaxy Growth and Properties of InN and Related Alloys

The fundamental band gap of InN has been thought to be about 1.9 eV for a long time. Recent developments of metalorganic vapor phase epitaxy (MOVPE) and RF-molecular beam epitaxy (RF-MBE) growth technologies have made it possible to obtain high-quality InN films. A lot of experimental results have been presented very recently, suggesting that the true band-gap energy of InN should be less than 1.0 eV. In this paper, we review the results of the detailed study of RF-MBE growth conditions for obtaining high-quality InN films. The full widths at half maximum (FWHMs) of ω-mode X-ray diffraction (XRD), ω–2θ mode XRD and E2 (high-frequency)-phonon-mode peaks in the Raman scattering spectrum of the grown layer were 236.7 arcsec, 28.9 arcsec and 3.7 cm-1, respectively. The carrier concentration and room temperature electron mobility were 4.9×1018 cm-3 and 1130 cm2/Vs, respectively. Photoluminescence and optical absorption measurements of these high-quality InN films have clearly demonstrated that the fundamental band gap of InN is about 0.8 eV. Studies on the growth and characterization of InGaN alloys over the entire alloy composition further supported that the fundamental band gap of InN is about 0.8 eV.

Direct observation of dislocation effects on threshold voltage of a GaAs field‐effect transistor

The threshold voltage of a GaAs metal‐semiconductor field‐effect transistor (MESFET) fabricated on a liquid encapsulated Czochralski grown, semi‐insulating substrate was found to be influenced by growth‐induced dislocations. Field‐effect transistors located at less than 20–30 μm from a dislocation exhibited a threshold voltage lower than that of FET’s located far from a dislocation. The maximum difference in threshold voltage of FET’s located at less than and more than this critical distance was obtained to be about 300 mV. The first definitive correlation between dislocations and FET threshold voltage is reported.

Marked enhancement of 320-360 nm ultraviolet emission in quaternary InxAlyGa1-x-yN with In-segregation effect

We demonstrated room-temperature (RT) intense ultraviolet (UV) emission in the wavelength range of 315–370 nm from quaternary InxAlyGa1−x−yN alloys grown by metalorganic vapor-phase epitaxy. We found that the UV emission is considerably enhanced by the In-segregation effect upon introducing 2%–5% of In into AlGaN. The In incorporation in quaternary InxAlyGa1−x−yN is markedly enhanced with the increase of Al content when using a relatively high growth temperature (830–850 °C), resulting in efficient RT UV emission. Maximally efficient emission was obtained at around 330–360 nm from the fabricated quaternary InxAlyGa1−x−yN (x=2.0%–4.8%,y=12%–34%). The intensity of the 330 nm emission from quaternary In0.034Al0.12Ga0.85N was as strong as that of the 430 nm emission from In0.22Ga0.78N at RT. We clearly observed In segregation of submicron size from cathode luminescence images of quaternary InAlGaN films.

Growth temperature dependence of indium nitride crystalline quality grown by RF-MBE

We have investigated the growth temperature dependence of InN crystalline quality grown by RF-MBE. It was confirmed that the crystalline quality improved by increasing the growth temperature within the dissociation limit of InN. We obtained FWHM as narrow as 3.7 cm -1 for E 2 (high frequency) phonon mode peak of Raman spectroscopy and 24 meV for the band-edge luminescence at 77 K. These values prove excellent quality of our samples. In this study, we obtained optical band-gap energy of around 0.8 eV at room temperature from the high-quality InN samples.

Growth of High-Electron-Mobility InN by RF Molecular Beam Epitaxy.

We succeeded in growing InN films two-dimensionally by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE), using a low-temperature-grown InN buffer layer. From the results of reflection high-energy electron diffraction (RHEED) observation and X-ray diffraction (XRD) measurement, it was found that a single crystal of InN films with a wurtzite structure was obtained. Moreover, from the results of Hall effect measurement, it was found that the InN films had quite high electron mobilities. The best electron mobility at room temperature obtained in this study was 760 cm2/Vs and the corresponding carrier density was 3.0×1019 cm-3. To our knowledge, this electron mobility is the highest value ever reported for single crystal InN films.

Universality of electron accumulation at wurtzite c- and a-plane and zinc-blende InN surfaces

Electron accumulation is found to occur at the surface of wurtzite (112¯0), (0001), and (0001¯) and zinc-blende (001) InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low Г-point conduction band minimum lying significantly below the charge neutrality level.

Raman-scattering study of the InGaN alloy over the whole composition range

We present Raman-scattering measurements on InxGa1−xN over the entire composition range of the alloy. The frequencies of the A1(LO) and E2 modes are reported and show a good agreement with the one-mode behavior dispersion predicted by the modified random-element isodisplacement model. The A1(LO) mode displays a high intensity relative to the E2 mode due to resonant enhancement. For above band-gap excitation, the A1(LO) peak displays frequency shifts as a function of the excitation energy due to selective excitation of regions with different In contents, and strong multiphonon scattering up to 3LO is observed in outgoing resonance conditions.

Inhomogeneous GaAs FET Threshold Voltages Related to Dislocation Distribution

Clear correlation between dislocation distribution and FET threshold voltage distribution in undoped LEC GaAs was observed directly for the first time by automatic computer-controlled measurement of drain-source current (Ids) and threshold voltage (Vth). FETs fabricated in the high EPD area, which covers the center and the periphery of (100) wafers, showed high Ids and low Vth, whereas FETs fabricated in the low EPD area showed low Ids and high Vth.

GaN-Based Trench Gate Metal Oxide Semiconductor Field Effect Transistors with Over 100 cm2/(V s) Channel Mobility

Enhancement-mode metal oxide semiconductor field effect transistors (MOSFETs) with trench gate structures have been developed. These MOSFETs show excellent DC characteristics with on-voltage of 5.1 V, i.e., enhancement-mode operation and extremely high channel mobilities of 133 cm2/(V s). This structure enables us to realize vertical switching devices with high breakdown voltage and highly integrated low on-resistance with the usage of excellent physical parameters of GaN. This excellent performance of these devices breaks though the realization of GaN-based power switching transistors.