Mototaka Taneya

Local vibrational modes as a probe of activation process in p-type GaN

Raman spectra for a series of Mg-doped GaN films grown by metal organic chemical vapor deposition and annealed in N2 ambiance at different temperatures have been investigated. Some local vibrational modes related to hydrogen were observed, showing drastic changes with the annealing temperature. The spectra show clearly that H impurities incorporated in as-grown films, which passivate Mg acceptors, are released from the Mg–N bonding at above ∼600 °C, and diffuse in the film to form new chemical bondings. We have also observed a local mode related to activated Mg acceptors. This mode is conveniently used as a probe of the activation process of Mg acceptors.

Effect of Slight Misorientation of Sapphire Substrate on Metalorganic Chemical Vapor Deposition Growth of GaN

The effects of slight misorientation from c-plane (0001) sapphire (α-Al2O3) substrates on GaN surface morphologies and electroluminescence (EL) properties were studied. The GaN layers were grown using a two-step growth method under atmospheric pressure by metalorganic chemical vapor deposition (MOCVD). When using around 0.17° misoriented sapphire substrate tilted toward both and directions, a small step-type morphology appeared. On the other hand, using c-plain substrate or substrates with the misorientation angle larger than approximately 0.25° from the c-plane, a scale-shaped morphology appears. We also found that the misorientation of the substrate significantly affects the uniformity of the EL image. On the other hand, the light output powers of the light-emitting diodes (LEDs) fabricated on the substrates with different misorientations were almost the same in spite of the different surface morphologies.

Novel electron‐beam lithography for in situ patterning of GaAs using an oxidized surface thin layer as a resist

The first demonstration of in situ electron‐beam (EB) lithography is reported, where a photo‐oxidized surface thin layer of GaAs is used for a resist. The in situ EB lithography sequence consists of five processes, i.e., preparation of a clean GaAs surface, photo‐oxidation for a resist film formation, direct patterning of the oxide resist by EB‐induced Cl2 etching, Cl2 gas etching of GaAs surface for pattern transfer, and thermal treatment in an arsenic ambient for resist removal and surface cleaning. The GaAs wafer is never exposed to air throughout all of the above processes to avoid an unintentional surface contamination. The minimum electron dose required for patterning of the GaAs oxide resist is about 5×1016 cm−2. An overgrown layer on the patterned GaAs surface shows a good surface morphology, which strongly indicates that this technology makes it possible to repeat crystal growth and surface patterning.

0° phase mode operation in phased‐array laser diode with symmetrically branching waveguide

An AlGaAs phased‐array laser with a novel index waveguide structure like the letter ‘‘Y’’ is described. In all arrays with such a structure, controllable selection of only 0° phase mode and complete suppression of 180° phase shift mode is achieved up to ∼65 mW in 2%–96% coated devices. The far‐field patterns, near‐field patterns, and spectra of the array are in good agreement with the theoretical results. The cw threshold currents are ∼100 mA, and the external differential quantum efficiencies are 57% per facet.

Characterization of subsurface damage in GaAs processed by Ga+ focused ion‐beam‐assisted Cl2 etching using photoluminescence

Subsurface damage in GaAs processed by a Ga+ focused ion‐beam‐assisted Cl2 etching is studied by photoluminescence (PL) measurement. The PL intensity of the processed sample decreased to (1)/(30) – (1)/(40) compared to that of the unprocessed sample. The recovery of PL intensity by a step removal of the damaged layer is observed as a function of the removed layer thickness. The removal of a 0.7‐μm‐thick surface layer enables the PL intensity to be recovered perfectly, which leads to the postulate that the damaged layer thickness is 0.7 μm at least, which is much larger than the ion range (about 0.01 μm). The recovery of PL intensity is analyzed on a one‐dimensional model in the direction normal to the sample surface. Computer simulations of PL intensity are carried out. The calculated result fully explains the experimental PL intensity recovery as a function of the removed layer thickness, which gives the profile of subsurface damage in the sample.

Electron-Beam-Induced Cl2 Etching of GaAs

Electron-beam (EB)-induced Cl2 etching of GaAs is performed for the first time. Etching occurs only in the area exposed to both the Cl2 molecules and the EB. The etching rate is equal to that of a Cl2 gas phase etching. The morphologies of the etched surfaces are slightly rough, but the photoluminescence intensity of the processed sample does not change as compared with that of the unprocessed sample. The etching characteristics predict that surface adsorbates act as a mask for a gas phase etching and that the EB plays an important part in patterning of the adsorbate mask.

Single-lobed far-field pattern operation in a phased array with an integrated phase shifter

A phased array with an integrated phase shifter which can convert a 180° phase mode into an in‐phase mode was demonstrated. An Al2O3 facet coating with a different thickness on each array element was performed. It should act as the phase shifter. A stable single‐lobed far‐field pattern was obtained from the array with the phase shifter.

Reduction of induced damage in GaAs processed by Ga+ focused‐ion‐beam‐assisted Cl2 etching

Damage in GaAs induced by Ga+ focused‐ion‐beam‐assisted Cl2 etching is studied by photoluminescence (PL) intensity measurements as functions of ion energy, ion dose, and substrate temperature. By decreasing the ion energy from 10 to 1 keV, the damage depth decrease to 1/5, where damage depth is taken as the thickness at which the PL intensity recovers by wet etching. The damage depth is shallower when the etching yield is larger with the same ion energy. By increasing the ion dose, the normalized PL intensity decreases, but damage depth is nearly constant. Over 1015 ion dose, the normalized PL intensity shows a constant value. By increasing the sample temperature, the damage depth becomes shallower. At 150  °C with ion energy of 1 keV, the damage depth is less than 0.5 μm, which is the detection limit of the PL measurement in GaAs substrate.

Electronic properties in p-type GaN studied by Raman scattering

Raman spectra from p-type GaN have been systematically studied in the hole density range of 5×1016–1×1018 cm−3. Contrary to the case of n-type samples, spectral profiles of the LO-phonon-plasmon coupled mode in p-type show no remarkable change with the hole density. Thus, precise evaluation of electrical transport parameters such as carrier density and mobility from the coupled mode profile is difficult. However, a continuum band has been observed in the low-frequency range of the spectra, becoming intense with the increase of the hole density. This band has been attributed to the inter-valence-band transition of holes, and the intensity can be used as a good measure of the hole density.

Photo‐oxidation of GaAs for in situ patterned‐mask formation prior to chlorine gas etching

This is the demonstration of ‘‘in‐situ masking’’ concept. In situ patterning of GaAs is carried out by using a photo‐oxidized surface layer as a mask for subsequent Cl2 etching. Clean GaAs surface provided by molecular beam epitaxy is exposed to pure oxygen and is simultaneously irradiated with an Ar+ ion laser of the 514.5 nm line for photo‐oxidation of the surface. Subsequent Cl2 gas etching of the photo‐oxidized sample reveals that the GaAs oxide fills the role of an etching mask. The resistance of the oxide mask against Cl2 etching varies depending on the laser fluence with which the oxide of GaAs is formed.