Ichitaro Waki

Shortest intersubband transition wavelength (1.68 μm) achieved in AlN/GaN multiple quantum wells by metalorganic vapor phase epitaxy

Two-hundred-period high-quality AlN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy are studied using high-resolution x-ray diffraction, transmission electron microscopy, and optical transmission spectroscopy. Excellent interfaces of the MQWs are confirmed by these measurements. The strong intersubband absorption peak at a wavelength of 1.68 μm is achieved for AlN (1.6 nm)/GaN(1.7 nm) MQW. The full width at half-maximum of the absorption peak is estimated to be 27 meV.

Low-temperature activation of Mg-doped GaN using Ni films

The activation of metalorganic chemical vapor deposition-grown Mg-doped GaN by N2 annealing with a thin Ni film has been investigated. p-type conduction in GaN has been obtained at an annealing temperature as low as 200 °C using the proposed technique. A hole concentration of 2×1017 cm−3 has been achieved by the annealing at 400 °C. Secondary ion mass spectroscopy measurements have revealed that hydrogen is effectively removed from the Mg-doped GaN layer. These results suggest that the Ni film significantly enhances hydrogen desorption from the GaN film, which results in the activation of Mg-doped GaN at quite low temperatures.

Mechanism for low temperature activation of Mg-doped GaN with Ni catalysts

The activation mechanism of Mg-doped GaN with Ni catalysts has been investigated by thermal desorption spectroscopy. It has been revealed that Ni deposited on Mg-doped GaN enhances the hydrogen recombination reaction at temperatures below 200 °C with the activation energy of 1.3 eV. The hydrogen desorbed at this temperature can be attributed to a part of the passivating hydrogen in GaN with a weak binding energy. The enhancement of the hydrogen recombination reaction on the GaN surface is essential to decreasing hydrogen concentration efficiently at low temperatures.

The Effect of Surface Cleaning by Wet Treatments and Ultra High Vacuum Annealing for Ohmic Contact Formation of P-Type GaN

The effect of surface cleaning techniques on the properties of the metal/p-GaN contacts has been studied using X-ray photoelectron spectroscopy (XPS) and I–V measurements. It has been clarified that the contact resistance strongly depends on the amount of the surface residues such as oxygen and carbon just before the electrode deposition. We have also found that the annealing in ultra high vacuum (UHV) is effective to reduce the residues. It has turned out that the combination of the hydrochloric acid and sulfuric acid hydrogen peroxide mixtures cleaning followed by the UHV annealing is promising for the improvement in the contact characteristics.

Etching of Ga-face and N-face GaN by Inductively Coupled Plasma

Inductively coupled plasma (ICP) dry etching of metalorganic chemical vapor deposition-grown Ga-face and N-face GaN was performed under various etching conditions. Unlike wet etching, etch rates of the N-face GaN were close to those of the Ga-face GaN for most etching conditions. We also found that BCl3 and SF6 are not suitable for N-face etching, while conventional Cl2/Ar etching provides smoother N-face surfaces even at high etch rates. The dependence of the etch rate on the Cl2 fraction and the plasma condition suggested that adsorption of chlorine onto the GaN surface is the rate-limiting step of the etching at low chlorine coverage conditions, and ion bombardment is the rate-limiting step of the etching at high chlorine coverage conditions.

Low Temperature Metal Organic Vapor Phase Epitaxial Growth of AlN by Pulse Injection Method at 800 °C

High quality AlN layer could be grown at 800 °C by metal organic vapor phase epitaxy (MOVPE) by pulse injection (PI) method in which trimethylaluminium (TMAl) and NH3 are alternately supplied. Crystal quality of AlN layer grown by PI method (PI-AlN) was comparable with AlN layer grown by conventional MOVPE with continuous sources flow sequence (continuous-AlN, C-AlN) at 1240 °C showing similar (0002) symmetric and (1012) skew symmetric full width at half maximum (FWHM) values by high resolution X-ray diffraction (HRXRD). Especially, it is noticeable that best crystal quality was observed when the growth rate was controlled to 1 monolayer/cycle. Moreover, PI method was proved to be effective in improving surface roughness of AlN layer. The root-mean-square (RMS) roughness of PI-AlN layer was 0.9 nm, which is three times smaller than C-AlN layer grown at same temperature with PI-AlN layer.

Generalized Grazing Incidence-Angle X-Ray Diffraction Studies on InAs Quantum Dots on Si(100) Substrates

We have investigated structural properties of InAs quantum dots grown on a Si (100) substrate using Generalized Grazing Incidence-angle X-ray Diffraction (G-GIXD). Strong diffraction spots from the InAs dots were observed in spite of their small volume (37 nm in diameter) and small surface coverage (about 11%). It has been found that the InAs dots are dilated by 1.2% along the perpendicular direction to the surface and compressed by 0.8% in the lateral direction. This finding indicates that the density of the misfit dislocations introduced during the growth is not large enough to relax the lattice mismatch perfectly. We have found that the lateral strain in the InAs dots is smaller than that expected from the macroscopic strain model for flat epitaxial films.

Characterization of crack-free AlN/GaN multiple quantum wells grown by metalorganic vapor phase epitaxy using H2 as a carrier gas

Crack-free 40 pairs of AlN/GaN multiple quantum wells (MQWs) were successfully grown by metalorganic vapor phase epitaxy using H 2 as a carrier gas. From a 2θ-ω scan XRD profile of the 40-pair AlN/GaN MQWs, satellite peaks from -2nd to +1st were observed, which suggests that the MQWs form abrupt interfaces with good crystal quality. The abrupt interfaces were also confirmed by a cross-sectional transmission electron microscopy. Additionally, an excellent electrical property of the AlN(0.7 nm)/GaN(1.7 nm) MQWs with an electron mobility of 1500 cm 2 /Vs and a sheet electron density of 1.4 x 10 13 cm -2 at room temperature was achieved. This electrical property is also an evidence of the high quality AlN/GaN MQWs.

Catalytic effect of Ni for activation of Mg-doped GaN in N2 and N2O

Abstract Catalytic effect of Ni for activation of metalorganic chemical vapor deposition-grown Mg-doped GaN in N2 and N2O has been investigated. P-type GaN has been obtained at an annealing temperature as low as 200°C using the Ni catalytic film in N2O as well as in N2. Moreover, the hole concentrations for the samples obtained by annealing in N2O were in good agreement with those obtained in N2 in the whole annealing temperature range. These results indicate that the Ni film effectively acts as a catalyst to enhance hydrogen desorption at the surface of GaN, which results in the reduction of hydrogen concentration in GaN at low annealing temperatures. It has also been suggested that the catalytic activity of Ni to enhance hydrogen desorption is much stronger than that of N2O. Therefore, an introduction of oxygen in the annealing atmosphere is not necessary for effective removal of hydrogen under the existence of the Ni film.

Low‐Temperature Activation of Mg‐Doped GaN with Pd Thin Films

The activation of metalorganic chemical vapor deposition (MOCVD)-grown Mg-doped GaN by N 2 annealing with thin Pd films has been investigated, p-type GaN with a hole concentration of 7 x 10 16 cm -3 has been obtained at an annealing temperature as low as 200 °C using this technique. Thermal desorption spectroscopy (TDS) measurements have revealed that hydrogen is effectively removed from the Mg-doped GaN layer by the use of the Pd film.