Ryuji Matsuo

Quantum-Confined Stark Effect in an AlGaN/GaN/AlGaN Single Quantum Well Structure

Excitonic absorption was observed in a transmittance spectrum of AlGaN/GaN/AlGaN single quantum well structure with a well width of 5 nm at room temperature. The total internal electric field strength in the well was about 0.73 MV/cm, which was estimated from the absorption peak position based on a simple calculation, neglecting excitons. The observation is clearly due to the quantum-confined Stark effect. While excitonic absorption was clearly observed even in such a high internal field, no light emission was detected under a He-Cd laser excitation at temperatures ranging from room temperature to T = 10 K. Light emission accompanied by a blue shift of the emission peak and an increase of emission intensity was observed under higher excitation power density. The obvious conclusion in the present case is that the presence of a high internal electric field in the well is a disadvantage for light emission.

Structural and vibrational properties of GaN

Structural and vibrational properties of device quality pure GaN substrate grown using a lateral epitaxial overgrowth (LEO) technique were studied using x-ray diffraction, Brillouin, Raman, and infrared spectroscopy. Lattice constants were found to be a=3.1896±0.0002 A and c=5.1855±0.0002 A. Comparing the results with those on GaN epilayer directly grown on sapphire substrate, it is shown that the GaN substrate is indeed of high quality, i.e., the lattice is relaxed. However the GaN substrate has a small enough but finite residual strain arising from the pileup of the lateral growth front on SiO2 masks in the course of LEO. It was also found that the elastic stiffness constants C13 and C44, are more sensitive to the residual strain than the optical phonon frequencies. The high frequency and static dielectric constants were found to be 5.14 and 9.04. The Born and Callen effective charges were found to be 2.56 and 0.50.

Epitaxial Growth of InAs on Single-Crystalline Mn–Zn Ferrite Substrates

We have grown InAs on single-crystalline Mn–Zn ferrite substrates by molecular beam epitaxy (MBE). In spite of a large lattice mismatch (about 30%), we succeeded in the growth of InAs epitaxial films. Reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) measurements have revealed that (111)InAs grows on (110)Mn–Zn ferrite substrates with the in-plane alignment of [112]InAs//[001]Mn–Zn ferrite.

Optical Properties of an InGaN Active Layer in Ultraviolet Light Emitting Diode

Optical properties of a 5 nm thick InGaN active layer with In content less than a few percent in an ultraviolet light emitting diode, have been studied by employing reflectance, transmittance, and photoluminescence spectroscopy. The co-existence of the localized electronic states and the usual confined ones in the active layer has been demonstrated. A possible origin of the co-existence of those states is attributed to the non-random alloy potential fluctuation in the InGaN active layer.