Yasuo Ohba

Growth of High-Quality AlN and AlN/GaN/AlN Heterostructure on Sapphire Substrate

Metalorganic chemical vapor deposition (MOCVD) growth of AlN on sapphire substrates was investigated, with the aim of device quality AlN/GaN/AlN double heterostructures. Growth temperature as high as 1300° C was required to obtain AlN epitaxial layers with sharp X-ray diffraction peaks. By growing AlN at the high growth temperature, residual stress at the heterointerface was effectively reduced. The AlN epitaxial layers with smooth surfaces were grown using a low V/III ratio together with the high growth temperature. AlN/GaN/AlN double heterostructures with appropriate layer thicknesses for DH lasers and flat heterointerfaces were grown on sapphire substrates.

H-atom incorporation in Mg-doped GaN grown by metalorganic chemical vapor deposition

H-atom incorporation was studied for Mg-doped GaN grown by metalorganic chemical vapor deposition. H-atom incorporation was found to increase linearly with Mg concentration, suggesting the formation of simple complex between Mg and H atoms in GaN. Decrease of H-atom concentration was observed after thermal treatment in Ar, supporting the hypothesis that H-atom extraction plays an important role in obtaining low-resistivity p-type conduction.

Growth of High-Quality AlN, GaN and AlGaN with Atomically Smooth Surfaces on Sapphire Substrates

Metalorganic chemical vapor deposition (MOCVD) growth of AlN, GaN and AlGaN on sapphire substrates was investigated, with the aim of realizing high-quality heterostructures with atomically smooth interfaces. AlN with surfaces constructed of single-atomic-layer steps was grown by the two-step growth technique, in which the thin first layer was grown at a low temperature (1250° C) and then the second layer was grown at a high temperature (1350° C). GaN with surfaces consisting of simple atomic-layer steps were successively grown on the AlN layer by optimizing the growth conditions. Marked degradation of surface flatness was not observed for Al0.2Ga0.8N grown on GaN.

Estimation of wave‐number spectrum parameters from fixed probe‐pair data

Spectrum parameter estimation concerning a stationary and homogeneous random field is examined in terms of quantities obtainable from a pair of fixed probes. The estimators of the mean wave number and the wave‐number spectral width are derived by relating wave‐number spectral moments to spatial derivatives of the complex covariance of the field, the effect of finite probe size being taken into account. The estimate biases associated with the probe separation can be greatly reduced by employing an algorithm based on the polar‐form representation of the complex covariance, while the variances of the estimates have a tendency to be enhanced with the decrease of the probe separation. An optimum range of the probe separation is theoretically determined.

Mechanism for Reducing Dislocations at the Initial Stage of GaN Growth on Sapphire Substrates Using High-Temperature-Grown Single-Crystal AlN Buffer Layers

The initial stage of GaN metalorganic chemical vapor deposition on high-temperature grown AlN (HT-AlN) was studied. A clear three-dimensional island growth was observed. At the first growth phase, the islands rapidly grew and coalesced forming a non-uniform defect distribution. For the second growth phase, deposition balanced with evaporation and a large-scale island/space structure was formed through the selective enlargement of the low defect parts. At the final growth phase, the spaces were filled by lateral growth and the surface was covered by alternately arranged defective and nearly defect-free domains, resulting in an average defect density of 2×108 cm-2. The selective processes at the second growth phase were found to play an important role in reducing the defects. The elevation of growth temperature utilizing high-stability HT-AlN is a promising technique for further reduction of the defects.

Fabrication and Characterization of AlGaN/GaN Double-Heterolaser Structures on Sapphire Substrates Using Single Crystalline AlN Buffer Layers

High-Al-composition AlGaN/GaN double-heterolaser structures were fabricated on sapphire substrates using single crystalline AlN buffer layers. Structures having Al0.25Ga0.75N cladding layers with a 2-µm total thickness were grown without cracks. Device performance was mainly investigated for the structure with Al0.2Ga0.8N cladding layers and a multiquantum-well-structure active layer. Optical emission was intense for devices with a multiquantum-well-structured active layer. However, no sign of stimulated emission was observed in this study.

Two-Dimensional Growth of AlN and GaN on Lattice-Relaxed Al0.4Ga0.6N Buffer Layers Prepared with High-Temperature-Grown AlN Buffer on Sapphire Substrates and Fabrication of Multiple-Quantum-Well Structures

Lattice mismatch effects on AlN and GaN growth were studied, aiming at the realization of multiple-quantum-well (MQW) structures. Lattice-relaxed AlN, GaN and Al0.4Ga0.6N were prepared as buffer layers. Microcrystal islands were observed for AlN and GaN respectively grown on the GaN and AlN buffer layers, due to lattice mismatch. However, two-dimensional growth was observed for both layers on the Al0.4Ga0.6N buffer layer. This growth-mode change was ascribable to the fact that a lateral lattice constant for the Al0.4Ga0.6N surface, with residual in-plane compression, is almost the center between those of AlN and GaN. For the MQW structures grown on Al0.4Ga0.6N, it was thought that the AlN and GaN grew two-dimensionally and coherently without significant dislocation generation.

Modulation-Doped In0.5Al0.5P/GaAs Field-Effect Transistors

In0.5Al0.5P/GaAs modulation-doped field-effect transistors (MODFETs) have been fabricated to study the applicability of In0.5(Ga1-xAlx)0.5P/GaAs heterostructures to electronic devices. A DC transconductance of 250 mS/mm and a noise figure of 1.7 dB with associated gain of 10 dB at 12 GHz were obtained for a MODFET with a 0.25 µm-long and 200 µm-wide gate. Improvements in the performance are expected by Al-composition optimization for the InGaAlP. These results show the promising potential of In0.5(Ga1-xAlx)0.5P/GaAs MODFETs as high-frequency and high-speed devices.

Room-Temperature CW Operation of GaN-Based Blue-Violet Laser Diodes Fabricated on Sapphire Substrate Using High-Temperature-Grown Single-Crystal AlN Buffer Layer

Room-temperature continuous-wave (cw) laser operation has been demonstrated for the first time for the test devices fabricated on sapphire substrates by metalorganic chemical vapor deposition using a high-temperature-grown single-crystal AlN (HT-AlN) buffer. The device structure was the ridge-stripe type with a 1.5-µm-wide and 750-µm-long cavity without facet coating. The threshold current, operating voltage, and wavelength at 20°C were 102 mA, 5.37 V, and 403 nm, respectively. It was expected that the device performance would be significantly improved by refining the layer structure, adding to adoption of the facet coating. These findings support the promising potential of the HT-AlN buffer technique for production of advanced short wavelength light emitting devices on sapphire substrates.

A New Photoacoustic Method to Characterize Wider-Gap Epitaxial Layers on Narrower-Gap Substrates

A new photoacoustic method is demonstrated to determine the principal band gap energy together with higherlying band edges of thin epitaxial layers of Ga1-xAlxAs. The output signal intensity is proportional to an optical absorption coefficient at least to the extent of 104 cm-1; correspondingly, a thickness less than 1 µm of epitaxial layers is detected without any processing of as-grown samples.