Takayuki Yuasa

Influence of Energetic Oxygen Bombardment on Conductive ZnO Films

Conductive ZnO films were prepared by DC planar magnetron sputtering and DC diode sputtering, and the reason for the high resistivity shown by the films at the substrate positions facing the eroded area of the target was investigated. It was found that bombardment of the film by energetic O atoms or O- ions is responsible for the high film resistivity, since this results in a decrease in both the carrier mobility and the carrier concentration. The arrival of excess O atoms and O- ions with relatively lower energies also induces an increase in the film resistivity as a result of a decrease in carrier concentration. The positional dependence of the film resistivity is explained by considering the above two factors.

Blue Laser Diodes Fabricated on m-Plane GaN Substrates

Blue laser diodes (LDs) were fabricated on m-plane oriented GaN substrates by atmospheric-pressure metalorganic chemical vapor deposition. Typical threshold current for stimulated emission at a wavelength λ of 463 nm was 69 mA. Blueshift of the spontaneous emission peak with increasing injection current was examined in LDs fabricated on m- and c-plane GaN substrates. Blueshifts for the m-plane LD (λ=463 nm) and the c-plane LD (λ=454 nm) with an injection current density just below threshold were about 10 and 26 nm, respectively. These results confirm that the blueshift in quantum-wells fabricated on m-plane oriented substrates is smaller than on c-plane oriented substrates due to the absence of polarization-induced electric fields.

Effect of Slight Misorientation of Sapphire Substrate on Metalorganic Chemical Vapor Deposition Growth of GaN

The effects of slight misorientation from c-plane (0001) sapphire (α-Al2O3) substrates on GaN surface morphologies and electroluminescence (EL) properties were studied. The GaN layers were grown using a two-step growth method under atmospheric pressure by metalorganic chemical vapor deposition (MOCVD). When using around 0.17° misoriented sapphire substrate tilted toward both and directions, a small step-type morphology appeared. On the other hand, using c-plain substrate or substrates with the misorientation angle larger than approximately 0.25° from the c-plane, a scale-shaped morphology appears. We also found that the misorientation of the substrate significantly affects the uniformity of the EL image. On the other hand, the light output powers of the light-emitting diodes (LEDs) fabricated on the substrates with different misorientations were almost the same in spite of the different surface morphologies.

AlGaInN Violet Laser Diodes Grown on GaN Substrates with Low Aspect Ratio

AlGaInN violet laser diodes grown on GaN substrates have been studied. Waveguide simulations have been performed and perpendicular radiation angles have been investigated. A peculiar layer structure has been proposed to reduce the perpendicular radiation angle and compared with a conventional laser. The laser structure has an n-cladding layer consisting of three AlGaN films, the middle film of which has relatively high Al composition. A laser diode with an optimized structure has been fabricated. The threshold current density and slope efficiency have been 2.9 kA/cm2 and 1.4 W/A, respectively. The perpendicular radiation angle has been as small as 16.2°. The AlGaInN violet laser diode with a low aspect ratio of 1.6 and a small threshold current of 29 mA has been realized.

Energy Distribution of Energetic Oxygen Atoms in the Sputtering of ZnO

Energetic neutral oxygen atoms bombarding the substrate in DC diode sputtering and DC planar magnetron sputtering were detected by a time-of-flight apparatus. In analyzing the time-of-flight spectrum, the energy distribution of the energetic oxygen atoms was computed by the fast Fourier transformation (FFT) method. The energy of the oxygen atoms bombarding the substrate was found to be distributed from 300 to eVT electron volts, the maximum energy which negative oxygen ions gain in the cathode fall in DC diode sputtering. The mean energy of the energetic oxygen atoms was estimated to be nearly eVT/2 electron volts. In planar magnetron sputtering, the flux of the energetic oxygen atoms showed a nearly monochromatic energy distribution.

Dislocation reduction in the annealed undercut GaAs on Si

The dislocations in the undercut GaAs on Si (UCGAS) was investigated by the cross‐sectional transmission electron microscope. The UCGAS structure was fabricated by first growing Al0.7Ga0.3As and GaAs on Si substrate and then partially etching the Al0.7Ga0.3As layer in the lateral direction. No dislocation was found in the annealed UCGAS at 800 °C for 10 min, while dislocations were observed in the region where the GaAs layer was connected to the Si substrate. The dislocation reduction mechanism in the UCGAS was also discussed in this letter.

High-Energy Oxygen Atoms in ZnO Film Preparation by Reactive Sputtering of Zn

Using Zn metal disk target, ZnO films were prepared in Ar+O2 mixed gas by both DC planar magnetron sputtering and RF diode sputtering. The dependence of c-axis orientation of ZnO film on O2 partial pressure was examined from the view point of the bombardment of film by energetic oxygen atoms. For DC planar magnetron sputtering, the c-axis orientation of film does not depend on the O2 partial pressure, but for RF diode sputtering, the c-axis orientation becomes high at r=10–20%, where r is the percent ratio of O2 partial pressure to total pressure. It is found that there exists a correlation between the dependence of c-axis orientation on O2 partial pressure and the film bombardment by energetic O atoms.

Energetic Oxygen Atoms in RF Sputtering of ZnO

Energetic particles bombarding the substrate placed in front of the target was observed by time-of-flight method for the film preparation of ZnO by RF planar magnetron sputtering and RF diode sputtering. It was confirmed that energetic oxygen atoms are bombarding the substrate. At the same time, fairly amount of energetic oxygen molecules were also observed. The total energies of the energetic oxygen atoms and molecules were extimated from the analysis of the time-of-flight spectra, and the estimated total energies for RF sputtering were compared with those for DC sputtering. It has been confirmed that the influence of the energetic oxygen atoms and molecules in Rf sputtering on ZnO films is the same order as that of DC sputtering.

GaAs/AlGaAs Single Quantum Well Optical Switch Fabricated on Si Substrate

We report a single-mode waveguide-type optical switch with a double hetero (DH)-structure fabricated on a Si substrate by Metalorganic Chemical Vapor Deposition (MOCVD). This switch utilizes the quantum confined Stark effect (QCSE). The sample consists of an Al0.3Ga0.7As cladding layers and Al0.25Ga0.75As guiding layer with an 8.3-nm-wide GaAs single quantum well (SQW). To measure the light absorption under reverse bias, the photocurrent method was applied out using a cw Ti:sapphire laser. We measured about a 10 nm shift of the absorption edge at -8 V. This switch exhibits a 33.1 dB/mm extinction ratio at 867 nm wavelength under -8 V bias.

A New Reactor for Metalorganic Chemical Vapor Deposition Equipped with an Internal Rotary Flow Selector

A reactor for metalorganic chemical vapor deposition is newly developed. The reactor has an internal flow selector by which two out of four continuously flowing gases are selected. The gas switching is performed just by rotating the flow selector without turning the valves on and off, eliminating the valve lifetime problem. The reduced effective reactor volume enables rapid gas switching at a relatively fast growth rate. GaAs/AlAs quantum wells are grown to check the gas switching ability. The cross-sectional transmission electron microscopy (TEM) image clearly shows 30-, 50- and 100 ?-thick quantum well structures, as expected.