Hiroyuki Kano

Raman scattering from LO phonon‐plasmon coupled modes in gallium nitride

Raman spectra of n‐type gallium nitride with different carrier concentrations have been measured. The LO phonon band shifted towards the high‐frequency side and broadened with an increase in carrier concentration. Results showed that the LO phonon was coupled to the overdamped plasmon in gallium nitride. The carrier concentrations and damping constants were determined by line‐shape fitting of the coupled modes and compared to values obtained from Hall measurements. The carrier concentrations obtained from the two methods agree well. As a result, the dominant scattering mechanisms in gallium nitride are deformation‐potential and electro‐optic mechanisms.

Thermal stress in GaN epitaxial layers grown on sapphire substrates

Thermal stress in GaN epitaxial layers with different thicknesses grown on sapphire substrates by metalorganic vapor phase epitaxy using an AlN buffer layer was investigated. Biaxial compressive stress in the GaN layer, due to the difference in the thermal expansion coefficients between GaN and sapphire, was obtained by measuring the curvature of wafer bending, and the observed stress agreed with the calculated stress. In Raman measurements, the E2 phonon peak of GaN was found to shift and broaden with the stress as a consequence of the change of the elastic constants with strain. The frequency shift Δω (in cm−1) was obtained for the first time, given by the relation: Δω=6.2 σ, where the biaxial stress σ is expressed in GPa.

Time‐resolved measurement of tunneling and energy relaxation of hot electrons in GaAs/AlGaAs double quantum well structures

The tunneling and cooling times of photoexcited hot electrons in AlGaAs/GaAs double (one narrow and the other wide) quantum well structures have been measured using photoluminescence excitation correlation spectroscopy. The tunneling time was of the order of 200 ps for a 60 A barrier. The tunneling is the indirect process assisted by the emission of optical phonons. The relaxation time of electrons as a function of the kinetic energy shows a threshold for cooling via the emission of optical phonons.

Production of the midgap electron trap (EL2) in molecular‐beam‐epitaxial GaAs by rapid thermal processing

Rapid thermal processing (RTP) using halogen lamps for molecular‐beam‐epitaxial (MBE) n‐GaAs layers was investigated by deep‐level transient spectroscopy. RTP was performed at 800u2009°C for 6 s with proximity capping method. It was found that the Ec −0.82 eV electron trap (EL2) was produced by RTP. The depth profile of EL2 was flat. The spatial variations of EL2 produced by RTP were observed across the MBE layers. The EL2 concentration increased by about two orders of magnitude toward the edge from the center of the samples (∼18×16 mm2). It was thought that the spatial distribution of EL2 corresponded to that of thermal stress induced by RTP.

Effects of rapid thermal processing on electron traps in molecular‐beam‐epitaxial GaAs

Variations of deep levels in Si‐doped molecular‐beam‐epitaxial (MBE) n‐GaAs layers by rapid thermal processing (RTP) using halogen lamps were investigated by deep level transient spectroscopy. RTP was performed at 700, 800, and 900u2009°C with the face‐to‐face configuration. Native deep levels M1 (Ec−0.18 eV), M3 (Ec−0.33 eV), and M4 (Ec−0.51 eV) in MBE n‐GaAs are annealed out by RTP at 900u2009°C. The metastable electron trap N1 (Ec−0.5∼0.7 eV) and the midgap electron trap EL2 (Ec−0.82 eV) are produced by RTP at 700, 800, and 900u2009°C. Two electron traps N2 (Ec−0.36 eV) and N3 (Ec−0.49 eV) are produced by RTP at 900u2009°C. The peculiar spatial distribution of N1 and EL2 are observed across the RTP layers. In particular, the EL2 distribution is found to be a W‐shaped pattern. It is supposed that this peculiar shape of the spatial variation is consistent with that of the thermal stress induced by RTP. In addition the spatial variations of EL2 are suppressed by use of the guard ring composed of GaAs pieces, since it pre...

Interface-Roughness Scattering in GaAs/AlxGa1-xAs Superlattices

The electron mobility limited by the interface-roughness scattering in a GaAs/AlxGa1-xAs superlattice is studied as a function of the period (the well width LW and the barrier thickness LB) and the temperature. If the superlattice period is short and the electronic states resemble that of the 3D state, the mobility is limited by the interface-roughness scattering at low temperatures and insensitive to the temperature, in good agreement with experimental results. Using a finite potential-barrier height, it is shown that the variation of the mobility as a function of the quantum well width LW is not so steep as that of the single-quantum-well structure assuming an infinite potential-barrier height.

Antiphase domains in GaAs grown on a (001)‐oriented Si substrate by molecular‐beam epitaxy

Antiphase domains (APDs) in the GaAs layer grown by molecular‐beam epitaxy on a nominally (001)‐oriented Si substrate were easily observed by molten potassium hydroxide etching or photoelectrochemical etching. The APD boundaries are almost parallel to {100} or {110} planes. The density of APDs decreases with the GaAs layer thickness in the 0.5–1.0‐μm region from the GaAs/Si interface. The appearance of APDs depends on the preheating conditions of the substrate. Preheating at 950u2009°C for 30 min or at 1000u2009°C for 5 min was sufficient for the suppression of APDs. This may be due to the change of the Si surface structure and the following complete annihilation of APDs in the GaAs layer near the heterointerface.

Molecular beam epitaxial growth of a GaAs layer free from antiphase domains on an exactly (100)-oriented Si substrate preheated at 1000°C

A preheat process of the Si substrate in ultra high vacuum has been found to control the antiphase disorder in the GaAs crystalline layer, which occurs during the molecular beam epitaxial growth of GaAs on Si. The growth layers with different preheat temperatures were characterized by Nomarski microscopy, anisotropic etching, Rutherford backscattering, transmission electron microscopy, photoluminescence, Hall and C−V measurements. A GaAs layer free from antiphase domains with excellent surface morphology was obtained on an exactly (100)-oriented Si substrate preheated over 1000°C for 30 min. This is probably due to the change of atomic steps in the Si surface from single to double caused by the preheating process.

Real space transfer of two dimensional electrons in double quantum well structures

Abstract Hot electrons and real space transfer effect in double quantum well structures are studied. It is shown that the electrons in the two wells have the same temperature, but the manner of the temperature rise depends on the structure. Assuming the parallel conduction in the two channels, the Hall mobility and the electron density in each well are estimated. The electron density in the narrow well is increased by the field, and the mobility in the wide well is decreased by the field. The mechnism and the traversal time of the tunneling transition are discussed in terms of the intersubband matrix elements.

Photo-luminescence studies of hot electrons and real space transfer effect in a double quantum well superlattice

Abstract A GaAsue5f8AlGaAs superlattice of which unit cell has two GaAs quantum wells shows a negative differential resistance under electric fields applied parallel to the superlattice plane at E ≳ 1 kV/cm (300 K) or at E ≳ 0.5 kV/cm (77 K). The photo-luminescence at 77 K shows a dominant peak IA followed by a small peak IB. Under the electric field the high energy tail of the dominant peak is enhanced and the intensity of the second peak IB is increased, which confirms the occurence of the real space transfer of hot electrons from the wide quantum well to the narrow one.