I. Akasaki

Quantum-Confined Stark Effect due to Piezoelectric Fields in GaInN Strained Quantum Wells

We have studied the influence of piezoelectric fields on luminescence properties of GaInN strained quantum wells. Our calculation suggests that an electric field of 1.08 MV/cm is induced by the piezoelectric effect in strained Ga0.87In0.13N grown on GaN. The photoluminescence peak energy of the Ga0.87In0.13N strained quantum wells showed blue shift with increasing excitation intensity. Moreover, the well-width dependence of its luminescence peak energy was well explained when the piezoelectric fields were taken into account. These results clearly showed that the piezoelectric field induced the quantum-confined Stark effect.

Crystal Growth and Conductivity Control of Group III Nitride Semiconductors and Their Application to Short Wavelength Light Emitters.

Recent development of technology and understanding of the growth mechanism in heteroepitaxial growth of nitrides on highly-mismatched substrates have enabled us to grow high-quality GaN, AlGaN, GaInN and their quantum well structures. Conductivity control of both n-type and p-type nitrides has also been achieved. These achievements have led to the commercialization of high-brightness blue, green and white light-emitting diodes and to the realization of short wavelength laser diodes and high-speed transistors based on nitrides. The performance of these devices is still progressing, but still requires advances in many areas of materials science and device fabrication.

Optical properties of strained AlGaN and GaInN on GaN

The composition of alloys in strained ternary alloy layers, Alx Ga1-x N (0<x<0.25) and Ga1-x Inx N (0<x<0.20), on thick GaN was precisely determined using the high-resolution X-ray diffraction profile. The band gap of strained AlGaN is found to increase almost linearly according to the AlN molar fraction, while that of strained GaInN has a large bowing parameter of 3.2 eV.

Deep level defects in n‐type GaN

In n‐type GaN grown by metalorganic chemical vapor deposition two new electronic defects were detected and characterized by deep level transient spectroscopy (DLTS). Schottky‐barrier diodes with Ohmic back contacts and low series resistance were fabricated in GaN layers grown on sapphire. The diodes display well behaved current‐voltage and capacitance‐voltage characteristics and permit unambiguous DLTS evaluation. The new deep levels display thermal activation energies for electron emission of 0.49 and 0.18 eV.

Effect of Si doping on the dislocation structure of GaN grown on the A‐face of sapphire

Transmission electron microscopy, x‐ray diffraction, low‐temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si‐doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three‐dimensional dislocation network and a strain gradient along the c axis of the layer.

Widegap Column‐ III Nitride Semiconductors for UV/Blue Light Emitting Devices

This paper reports on a high-quality AtGaN/GaN double heterostructure (DH) which shows UV emission stimulated by optical pumping at room temperature with low threshold power in both edge and surface modes, and a high-efficiency blue light emitting diode (LED) and UV-LED based on p-n GaN homojunction and A1GaN/GaN DH. The process for the fabrication of LEDs and DH and their characteristics are presented. Demand for a high-power short-wavelength light emitter in the blue, violet, and ultraviolet (UV) regions, such as light emitting diode (LED) and laser diode (LD) has been increasing. Applications of these new devices include new full-color large-scale flat panel display systems, compact and high-density optical storage systems, high-speed printing systems, and small medical and biological apparatuses. Realization of these new devices will require highquality wide-bandgap semiconductor films. Column-III nitrides are promising candidates as materials for fabrication of such short-wavelength light emitters, because the wurtzite polytypes of InN, GaN, and A1N form a continuous alloy system whose direct bandgap ranges from 1.9 eV for InN, 3.4 eV for GaN, and to 6.2 eV for AIN, as shown in Fig. 1. During the two decades before 1985, many pioneering works on these nitrides were conducted on crystal growth and doping, ~-8 on device fabrication and physics, 9 and on physical properties. L~ However, it had been quite difficult to grow high-quality epitaxial nitride films, in particular, with a flat surface free from cracks. This is mainly due to the lack of substrate materials for which the lattice constant and thermal expansion coefficient are close to those of GaN and alloys, n Moreover, it was suggested by many researchers that the column-III nitrides have a strong nonstoichiometry problem. ~2 In 1986, we succeeded in overcoming the problems mentioned above and in growing high-quality GaN with a spec

Shallow donors in GaN—The binding energy and the electron effective mass

Abstract Fourier transform infrared absorption spectroscopy has been performed on a GaN epitaxial film grown by the hydride vapor phase epitaxy on sapphire substrate. We observe a transition at 215 cm −1 with a half width of 2 cm −1 , which we attribute to an electronic transition on the basis of temperature dependent measurements. We assign it to the 1s-2p transition of the residual shallow donors. Using effective mass theory neglecting anisotropies in the effective mass and dielectric constant the binding energy of the shallow donors is calculated to be (35.5 ± 0.5) meV. The electron effective mass is (0.236 ± 0.005) m o . Based on this knowledge, the energy separation between the donor-acceptor and the band-acceptor transitions as seen in photoluminescence at 45 K can be used to evaluate the donor concentration in the epitaxial films.

Hydrogen passivation of Mg acceptors in GaN grown by metalorganic chemical vapor deposition

The effects of the deliberate hydrogenation of GaN were investigated for heteroepitaxial layers grown by metalorganic chemical vapor deposition. The GaN layers were either Mg‐doped, p‐type after thermal activation, or Si‐doped, n type. Elemental depth profiles from secondary ion mass spectroscopy reveal a striking contrast after a deuteration at 600u2009°C: the deuterium concentration in Mg‐doped GaN is ∼1019 cm−3 while there is no detectable deuterium incorporation in the n‐type material. Variable temperature Hall effect measurements provide the most direct evidence to date for Mg–H complex formation with the decrease in the hole concentration upon hydrogenation accompanied by an increase in the hole Hall mobility.

Stimulated Emission by Current Injection from an AlGaN/GaN/GaInN Quantum Well Device

Quantum well structures composed of GaInN well and GaN barrier were fabricated. Room-temperature stimulated emission by pulsed current injection is observed from group III nitride using the very thin active layer, for the first time.

Determination of the Conduction Band Electron Effective Mass in Hexagonal GaN

The electron effective mass in hexagonal GaN films grown by metal organic vapor phase epitaxy on sapphire substrates is determined by cyclotron resonance experiments. Its value is m p* = 0.22±0.005 m o. Taking polaron effects into account the band edge mass is m b* = 0.20±0.005 m o. From the resonance linewidth a mobility of 3500 cm2/Vs at 6 K is obtained.