Nakao Akutsu

AN ANALYSIS OF TEMPERATURE DEPENDENT PHOTOLUMINESCENCE LINE SHAPES IN INGAN

Photoluminescence (PL) line shapes in InGaN multiple quantum well structures have been studied experimentally and theoretically between 10 and 300 K. The higher temperature PL spectra can be fitted quantitatively with a thermalized carrier distribution and a broadened joint-density-of-states. The low temperature PL line shapes suggest that carriers are not thermalized, as a result of localization by band-gap fluctuations. We deduce a localization energy of ∼7 meV as compared with an activation energy of ∼63 meV from thermal quenching of the PL intensity. We thus conclude that this activation energy and the band-gap fluctuation most likely have different origins.

Quantum chemical mechanism in parasitic reaction of AlGaN alloys formation

The mechanism of parasitic reactions among trimethylaluminum (TMA), trimethylgallium (TMG), and NH3 in atmospheric pressure (AP) MOVPE for growth of AlGaN is theoretically studied using the quantum chemical method. The calculations show that metal–nitrogen chain growth reaction easily proceeds through the successive reactions of ‘complex formation with NH3’ and ‘CH4 elimination by the bimolecular mechanism’. Additionally, a parasitic reaction in APMOVPE using other raw material is also investigated. The calculated result shows that small change of raw material raises activation energy of parasitic reaction, and, thus, the parasitic reaction is suppressed. This result suggests a way to improve APMOVPE by a suitable choice of substituent.

High breakdown voltage AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistor with TiO2/SiN gate insulator

We report the fabrication of an AlGaN/GaN high-electron-mobility transistor (HEMT) with a high breakdown voltage by employing a metal–insulator–semiconductor (MIS) gate structure using TiO2/SiN insulators. We employed the TiO2/SiN gate insulator for the first time in a multilayered insulator structure MIS-HEMT. The gate leakage current was significantly reduced by employing the MIS structure, and the breakdown voltage characteristics of the fabricated MIS-HEMTs were 1.1 kV with an on-resistance of 15 mΩ cm2 for a gate-drain length of Lgd = 28 µm. The current collapse in the TiO2/SiN MIS-HEMT has been improved by employing a thin SiN film under the TiO2 insulator. AlGaN/GaN MIS-HEMTs are promising not only for high-speed applications but also for high-power switching applications.

p-Type InGaN Cap Layer for Normally Off Operation in AlGaN/GaN Heterojunction Field Effect Transistors

The normally off operation of AlGaN/GaN heterojunction field effect transistor (HFET) devices with a p-type InGaN cap layer under a gate electrode was demonstrated. The threshold gate voltage VGth was 0.5 V, and the maximum transconductance gm was about 120 mS/mm. The maximum drain current IDmax was more than 210 mA/mm. RF characteristics were also measured, and it was found that fT is 2.1 GHz and fmax is 6.4 GHz when the gate length is 2 µm.

INFLUENCE OF SI DOPING ON THE INFRARED REFLECTANCE CHARACTERISTICS OF GAN GROWN ON SAPPHIRE

Si-doped GaN films grown on sapphire are investigated by infrared reflectance. A damping behavior of the interference fringes is observed, and interpreted to be due to the presence of an interface layer between the film and the substrate. A theoretical calculation using a two-layer model to take into account the interface layer resulted in this damping in agreement with the experiment. The damping behavior and an improvement of interface properties by Si incorporation are demonstrated.

An analysis of temperature dependent piezoelectric Franz–Keldysh effect in AlGaN

Strong Franz–Keldysh oscillations near the band gap of AlGaN are observed in the contactless electroreflectance (CER) studies of a GaN/InGaN/AlGaN multilayer structure. The line shape analysis of the CER spectra at different temperatures provides an accurate determination of the AlGaN band gap energies and the built-in electric fields. Using the existing data of the thermal expansion coefficients of GaN and sapphire, and the piezoelectric constants of AlGaN, the temperature dependence of the electric field is estimated and is in good agreement with the experimental results between 15 and 300 K. We attribute such electric field to the piezoelectric strain effect.

Gate-Length Dependence of DC Characteristics in Submicron-Gate AlGaN/GaN High Electron Mobility Transistors

We investigated the gate-length dependence of room-temperature DC operation in submicron-gate AlGaN/GaN high-electron-mobility transistors (HEMTs). The threshold voltage was reduced by the short-channel effect, which is only dependent on the aspect ratio of the gate length to the barrier-layer thickness. On the other hand, the transconductance was restricted by the source resistance, and was dependent on not only the gate length LG but also the source?gate length LSG. The transconductance was increased by reducing LSG rather than LG in the submicron-gate HEMTs.

Material properties of GaN grown by MOCVD

n-Type GaN thin films grown by metal-organic chemical vapour deposition (MOCVD) were studied using photoluminescence (PL), photoreflectance (PR) and Raman scattering. In the PL spectra, the peak position of the band-edge transition shifts to the red side monotonically with increasing doping concentrations. It may be explained theoretically in terms of many-body effects, namely the renormalization of the bandgap. In the meantime, the line width of the PL peak is increased monotonically with the doping concentration. The luminescence line broadening can be modelled in terms of potential fluctuations caused by the random distribution of doping impurities. In the PR spectra the magnitude decreases monotonically with increasing doping concentration. This result can be explained by a simple model based on an assumption that low-density surface states exist in the surface of GaN. In the Raman scattering spectra, both A 1 (LO) mode and E 2 mode were observed in the back scattering configuration. The A 1 (LO) mode shifted towards the high-frequency side and broadened with an increase in carrier concentration. This phenomenon can be interpreted by the LO phonon being coupled to the overdamped plasmon in GaN.

Epitaxial Growth and Properties of Mg-Doped Gan Film Produced by Atmospheric Mocvd System With Three Layered Lammar Flow Gas Injection

Mg-doped GaN films with a variety of Mg concentrations were grown on sapphire (0001) by horizontal atmospheric metalorganic chemical vapor deposition (MOCVD) system with three layered laminar flow gas injection in an attempt to study the Mg doping effects on film quality. The increase of Mg concentration induced an increase of x-ray rocking curve full width at half maximum (FWHM) and degradation of surface morphology. Secondary ion mass spectroscopy (SIMS) analysis shows increase of Si and O, associated with Mg-doping concentration. Si and O concentrations of Mg-doped film are up to 5×10 16 cm −3 and 5×10 17 cm −3 at Mg concentration of 4.5×10 19 cm −3 , respectively. Strong 380nm emission and weak 430nm emission were observed by photoluminescence (PL) measurement at room temperature for as-grown Mg-doped GaN films which shows p-type conductivity after thermal annealing. While, in highliy Mg-doped GaN films which do not show the p-type conduction after thermal annealing, 430nm and/or 450nm emission were dominating. The highest room temperature free hole concentration achieved was p=2.5× 10 18 cm −3 with mobility μ p =l.9cm 2 /V s.

PHOTOVOLTAIC SPECTROSCOPIC STUDY OF GAN EPILAYERS AND INGAN QUANTUM WELL STRUCTURES

Room-temperature photovoltaic spectroscopy was applied to study undoped GaN, n -type GaN, and InGaN quantum well structures. Clear exciton absorption was observed in the photovoltaic spectra of the undoped GaN, and polarization measurements were made to identify the exciton absorption. For the n -type GaN sample, instead of the exciton absorption we observed only bulk absorption edge, which may be due to the free carrier screening effect. For the InGaN quantum well structures, the photovoltaic spectra showed relatively complicated line shape due to the overlap of the signals from different layers. By changing the reference phase of the lock-in amplifier, we were able to suppress some of the signals and thus identify the origin of the corresponding signal.