Isao Kidoguchi

Air-bridged lateral epitaxial overgrowth of GaN thin films

A promising technique of selective lateral epitaxy, namely air-bridged lateral epitaxial overgrowth, is demonstrated in order to reduce the wing tilt as well as the threading dislocation density in GaN thin films. A seed GaN layer was etched to make ridge-stripe along 〈1100〉GaN direction and a GaN material was regrown from the exposed (0001) top facet of the ridged GaN seed structures, whose sidewalls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor phase epitaxy. The density of dislocations in the wing region was reduced to be <107 cm−2, which was at least two orders of magnitude lower than that of underlying GaN. The magnitude of the wing tilt was determined to be 0.08° by x-ray diffraction (XRD) measurements, which was smaller than other lateral epitaxial overgrown GaN thin films. The full width at half maximum of XRD for the wing region was 138 arc sec, indicating high uniformity of c-axis orientation.

High-quality GaN films obtained by air-bridged lateral epitaxial growth

High-quality GaN films with low dislocation density and low wing tilt of c-axis orientation have been successfully obtained by a promising technique of selected area growth, namely air-bridged lateral epitaxial growth (ABLEG). A GaN film was grown from the exposed (0001) top facet of the ridged GaN seed structures, whose side walls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor-phase epitaxy. The ridge-stripe structures of the GaN seed were constructed in the GaN direction. At the optimum growth temperature of 950°C, only the {1120} and {0001} facets were obtained. Continuing the growth led to fabricating the air-bridged structure, where the coalescence of the wing region occurred. From the transmission electron microscopy study, it was found that most of the vertical dislocations along the c-axis were confined to the seed region, while the horizontal dislocations were newly generated in the vicinity of coalescence boundary. The densities of the vertical dislocations were about 9 x 10 8 cm -2 in the seed region, while below 1 x 10 6 cm -2 in other regions. The densities of the horizontal dislocations were about 1 x 10 6 cm -2 in the wing region and 4 x 10 7 cm -2 in the vicinity of the coalescence boundary, respectively. The X-ray diffraction (XRD) measurements revealed that the tilt angle of c-axis relative to underlying seed GaN was about 297 arcsec (0.083°), and the full-width at half-maximum of the XRD curve for the wing region was 138 arcsec, indicating that the wing region has high uniformity of c-axis orientation. Both of the wing and the coalescence boundary region exhibited atomically smooth surfaces with stepped terraces, whose root mean square roughness was found to be 0.089 nm by atomic force microscopy measurements.

Improvement of Crystalline Quality in GaN Films by Air-Bridged Lateral Epitaxial Growth

Air-bridged lateral epitaxial growth (ABLEG), a new technique of lateral growth of GaN films, has been developed using low-pressure metalorganic vapor phase epitaxy. A previously grown 1-µm-thick GaN film is grooved along the GaN direction, and the bottoms of the trenches and the sidewalls are covered with a silicon nitride mask. A free-standing GaN material is regrown from the exposed (0001) surface of the ridged GaN seed structure. Cross-sectional transmission electron microscopy analysis reveals that the dislocations originating from the underlying seed GaN extend straight in the direction and dislocations do not propagate into the wing region. The wing region also exhibits a smooth surface and the root mean square roughness is found to be 0.088 nm by atomic force microscopy measurement of the (0001) face of the wing region.

Extremely high quantum efficiency (86%) operation of AlGaInP visible laser diode with lateral leaky waveguide structure

AlGaInP visible laser diode with lateral leaky waveguide structure has been demonstrated for the first time. The laser has differential quantum efficiency as high as 43% from one facet without coating in addition to stable fundamental transverse‐mode oscillation. The high differential quantum efficiency is due to the propagation loss as low as 4.2 cm−1 of this laser, which is the lowest value for AlGaInP visible diode to the best of our knowledge. This laser can be successfully fabricated by selective growth of AlGaInP quarternary alloy.

Low-noise 650-nm-band AlGaInP visible laser diodes with a highly doped saturable absorbing layer

Stable self-sustained pulsating and low-noise 650-nm-band AlGaInP visible laser diodes were demonstrated by adopting a novel structure, which has a highly doped saturable absorbing (SA) layer. Short carrier lifetime, which is indispensable for self-pulsation, was realized by applying high doping concentration to the p-type SA layer. 500-/spl mu/m-long devices with 51%/51% coated facets were fabricated, resulting in the threshold current of 75 mA at 20/spl deg/C. The temporal output power was measured at the average output power of 5 mW and the stable self-pulsation was observed up to an ambient temperature of 60/spl deg/C. Therefore, the relative intensity noise (RIN) was kept about -140 dB/Hz in temperature ranging from 20/spl deg/C to 60/spl deg/C. Since the refractive index difference could be kept large and the optical mode could be confined effectively, the astigmatism in this work was below 3 /spl mu/m at 5 mW against dc injection current. The lasers operated over 1350 h under the average output power of 5 mW at 60/spl deg/C.

Complex Flow and Gas-Phase Reactions in a Horizontal Reactor for GaN Metalorganic Vapor Phase Epitaxy

Three-dimensional fluid simulations are performed in a horizontal reactor for GaN epitaxy. Attention is paid to the effect of gas flow velocity at the inlet and gas pressure. It is found that the gas flow rate rather than the velocity or the pressure is a key parameter which controls the spatial distribution of streamlines, temperature and gas-phase species. As the gas flow rate increases, the size of return-flow or flow-separation appearing near the gas entrance of the expansion region with a tapering angle increases. This causes velocity peaking near the reactor symmetry plane and complicated transport of gas-phase species along the streamlines of the return-flow. If an optimum gas flow rate which gives minimum return-flow and uniform macroscopic spatial distribution for flow pattern and gas-phase species can be determined, then it is desirable to change the gas flow velocity and the gas pressure on the condition that the gas flow rate is maintained.

Stable operation of self‐sustained pulsation in 650‐nm‐band AlGaInP visible lasers with highly doped saturable absorbing layer

Self‐sustained pulsating 650‐nm‐band AlGaInP visible lasers were fabricated by adopting a new structure, which has a highly doped saturable absorbing (HDSA) layer. Since the doping level and the thickness of the HDSA layer could be controlled accurately by using metalorganic vapor phase epitaxy growth technique, a stable operation of self‐sustained pulsation up to 60 °C was obtained. The lasers operated over 1350 h under the average output power of 5 mW at 60 °C. The relative intensity noise values were below −136 dB/Hz in the temperature ranging from 20–60 °C and remained unchanged after life test.

Nonradiative recombination processes of carriers in InGaN/GaN probed by the microscopic transient lens spectroscopy

Temporally and spatially resolved observations of the nonradiative recombination (NR) processes of carriers in low dislocated GaN and InGaN/GaN were successfully obtained by using microscopic transient lens spectroscopy. The heat generations and conductivities of NR processes were detected by the signal intensities and the time profiles. We found that the thermal conductivities were not so different at the seed region (threading dislocation density (TDD)=1–2×109 cm−2) and the wing region (TDD=1–2×106 cm−2) of air-bridged lateral epitaxially grown GaN and InGaN/GaN, but the amount of heat generated at the wing regions was much smaller than that at the seed regions.

Systematic studies on optical gain spectra in GaInN/GaN-MQWs

Abstract The origin of optical gain in Ga 1− x In x N/GaN multiple quantum wells with x =0.05, 0.10 and 0.16 was investigated systematically by using a variable excitation-stripe method under various excitation conditions, such as power density and temperature. It was found that the alloy inhomogeneity of In content increases the width of the gain spectrum and decreases its value. It is supposed that the stimulated emission observed at the high-energy end of spontaneous emission spectrum originates from the optical gain due to electron–hole plasma around the mobility edge for all the samples.

Suppression of Zn diffusion due to hydrogen passivation in p ‐AlGaInP

The behaviors of Zn acceptors in p‐AlGaInP layer during thermal annealing in AsH3+H2 mixture gas have been investigated. The electrical activities of Zn acceptors were greatly affected by the hydrogen atoms originating from AsH3 during thermal annealing. In addition, from observation of the atomic disordering, hydrogen passivation of Zn acceptors was found to suppress the atomic diffusion. These phenomena can be consistently explained by a simple hydrogen passivation model, which is the termination of dangling bonds by hydrogen atoms.