Tokio Takahashi

Self-generated microcracks in an ultra-thin AlN/GaN superlattice interlayer and their influences on the GaN epilayer grown on Si(110) substrates by metal–organic chemical vapor deposition

We investigated the effect of an ultra-thin AlN/GaN superlattice interlayer (SL IL) on the GaN epilayer grown on Si(110) substrates by metal–organic chemical vapor deposition (MOCVD). It is found that microcracks (MCs) are self-generated in the SL IL region, which depend on the thickness of the SL IL. The MCs influence the characteristics of the GaN epilayers grown on the SL IL, such as surface morphologies, strain and structural qualities. Furthermore, different MC configurations depending on the SL IL thickness are observed, which imply the controllability of MC generation. Understanding of the mechanism and the optimization of the SL IL structure make it possible to grow crack-free high-quality GaN films on Si substrates for optical and electronic device applications.

Modulation of Strain States in GaN Films by a Thin AlN/GaN Superlattice Interlayer Grown on Si(110) Substrates

The effect of a thin AlN/GaN superlattice structure (SLs) interlayer on the strain properties in GaN films grown on Si(110) substrates is investigated. By comparing the strain states in GaN films without the interlayer, it is found that the strain in GaN films can be modulated by the SLs interlayer, which can be converted from a tensile state to a compressive one. The realization of the compressive strain state in the GaN film results in the suppression of the crack generation in the thick GaN growth, which is a general problem in the GaN growth on Si substrates. Using this simple technique, we successfully grow a crack-free continuous GaN film exceeding 4 µm thick. Therefore, the SLs interlayer is a promising structure in growing crack-free thick GaN on the Si substrate for optic and electronic device applications.

Nitrogen vacancies as a common element of the green luminescence and nonradiative recombination centers in Mg-implanted GaN layers formed on a GaN substrate

The photoluminescences of ion-implanted (I/I) and epitaxial Mg-doped GaN (GaN:Mg) are compared. The intensities and lifetimes of the near-band-edge and ultraviolet luminescences associated with a MgGa acceptor of I/I GaN:Mg were significantly lower and shorter than those of the epilayers, respectively. Simultaneously, the green luminescence (GL) became dominant. These emissions were quenched far below room temperature. The results indicate the generation of point defects common to GL and nonradiative recombination centers (NRCs) by I/I. Taking the results of positron annihilation measurement into account, N vacancies are the prime candidate to emit GL and create NRCs with Ga vacancies, (VGa) m (VN) n , as well as to inhibit p-type conductivity.

Investigating the bistability characteristics of GaN/AlN resonant tunneling diodes for ultrafast nonvolatile memory

The bistability characteristics of GaN/AlN resonant tunneling diodes (RTDs) grown on a sapphire substrate by metalorganic vapor phase epitaxy (MOVPE) were investigated to better understand their physical origin and explore their use in nonvolatile memories. The bistability current–voltage (I–V) characteristics of GaN/AlN RTDs, which were due to intersubband transitions and electron accumulation in the quantum well, were clearly observed over a wide temperature range between 50 and 300 K. However, the I–V characteristics sometimes degraded at temperatures above 250 K. Complex staircase structures were observed in the voltage region showing a negative differential resistance in the I–V curve, and the forward current increased or decreased rapidly as the forward-bias voltage increased. Repeated measurements of the I–V characteristics over the wide temperature range between 50 and 300 K revealed that the bistability characteristics of GaN/AlN RTDs degraded owing to the leakage of electrons accumulating in the quantum well through a deep level in the AlN barrier associated with crystal defects such as dislocations and impurities. Therefore, reduction in crystal defect and impurity densities in the AlN barrier, and a careful design that considers deep levels are important for realizing realize ultrafast nonvolatile memories based on the bistability characteristics of GaN/AlN RTDs.

Enhanced light extraction in GaN-based light-emitting diodes by evanescent wave coupling effect

Subwavelength-sized truncated cones satisfying the requirement for evanescent wave coupling effect at the emission wavelength were fabricated on the p-side of a GaN-based blue light-emitting diode (LED). The light-extraction efficiency increased by a factor of approximately 2.2 relative to that of a reference LED with a flat light-extraction surface. This light-extraction enhancement ratio is much larger than that realized by using conventional light-extraction techniques. The evanescent wave coupling effect appearing on the surface of the truncated-cone structure is believed to be the mechanism responsible for the large enhancement of light extraction.

Light Source Position Dependence of Evanescent Wave Coupling Effect in Narrow GaAs/AlGaAs Ridge Structure

The light source position dependence of the evanescent wave coupling effect in a sub-wavelength-sized GaAs/AlGaAs ridge structure was investigated using the theoretical simulation and temperature-dependent emission pattern measurements. It was found that the evanescent wave coupling effect could be realized for a light source located at any position of the top-flat quantum well (QWL), although the emissions transformed from evanescent waves began to get emitted in directions that formed an increasingly larger angle with respect to the surface normal of the ridge flat facet when the light source was moved from the center to the edge of the flat QWL.

High quality thin AlN epilayers grown on Si(110) substrates by metal-organic chemical vapor deposition

Thin AlN epilayers are grown on Si(110) by metal-organic chemical vapor deposition, and are intentionally treated not only as conventional seed layers but also as base layers for high quality AlGaN epilayer growth. By controlling the growth conditions, a V-shape pit free AlN surface with atomic steps is achieved. Characterization results of high resolution X-ray diffraction show that the structural qualities of the thin AlN epilayers grown on Si(110) are excellent, and the epilayers are superior to those grown on Si(111) with the same thickness in the literature. In addition, the structural quality of the AlGaN epilayers grown on AlN/Si(110) is confirmed to strongly depend on the underlying AlN epilayer. Furthermore, it is important to control the AlN thickness to prevent the generation of large cracks during the growth process. Our results present potential applications of high quality III-nitrides grown on Si(110) for optical (UV region) and power device applications.

Electrical properties of Ni/n-GaN Schottky diodes on freestanding m-plane GaN substrates

The electrical properties of m-plane Ni/n-GaN Schottky diodes grown via metalorganic chemical vapor deposition were investigated. Under growth at 1,120 °C with a V/III ratio of 1,000 (growth rate of 100 nm/min), the residual Si, O, and C impurity concentrations in the m-plane GaN layer were below the secondary-ion mass spectroscopy detection limit. The surface of the Si-doped n-GaN epitaxial layer on the 5°-off m-plane GaN substrate consisted of steps and terraces. A linear correlation between the carrier concentration and the Si atomic concentration was clearly observed from 1 × 1017 to 5 × 1015 cm−3. The reverse current–voltage curves were fitted using the thermionic field-emission model at the measured carrier concentration and qB. The leakage current of the diodes under a reverse bias was effectively suppressed at a low carrier concentration of 4.6 × 1015 cm−3.

Enhancement of the evanescent wave coupling effect in a sub-wavelength-sized GaAs/AlGaAs ridge structure by low-refractive-index surface layers

We have investigated the three-dimensional emission patterns of GaAs/AlGaAs ridge structures with a sub-wavelength-sized top-flat facet by angle-resolved photoluminescence (PL). We found that the integrated PL intensity, and hence the light-extraction efficiency, can be enhanced by about 34% just by covering the ridge surface with a thin SiO2 layer. A double-coupling effect of evanescent waves that occurs at both the semiconductor-SiO2 and SiO2-air interfaces is suggested to be responsible for the improvement, based on a finite-difference time-domain simulation of the electromagnetic field around the ridge top.

Impact of strain state on the ultrathin AlN/GaN superlattice growth on Si(110) substrates by metalorganic chemical vapor deposition

We investigate the effect of strain state on ultrathin AlN/GaN superlattice (SL) growth by metalorganic chemical vapor deposition. It is found that the strain state (tensile or compressive) plays a role in periodic structure formation. A well-ordered SL growth with excellent periodicity can be realized under a tensile strain. However, a disordered SL structure is formed under a compressive strain. Furthermore, the ordered SL growth can be restored by inverting the strain state. Our findings show the importance of strain engineering in nanoscale structure growth.