Yuzaburoh Ban

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.

Residual impurities in GaN/Al 2 O 3 grown by metalorganic vapor phase epitaxy

Residual impurities in GaN films on sapphire (A12O3) substrates grown by two-step metalorganic vapor phase epitaxy (MOVPE) have been investigated. We have mainly investigated the incorporation of carbon into the GaN films with GaN buffer layers on A12O3 during MOVPE growth, comparing trimethygallium (TMGa) and triethygallium (TEGa) as the typical gallium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photolu-minescence, and Hall measurements. The carbon, hydrogen, and oxygen concentrations increase with decreasing growth temperature in using TMGa. Especially the carbon concentration increases with decreasing a V/III ratio, for both TMGa and TEGa. There is about two times more carbon in the GaN films grown using TEGa than those using TMGa. The carbon from TMGa mainly enhances the D-A pair emission (∼378 nm), which shows the carbon makes an acceptor level at nitrogen sites in GaN. On the other hand, the carbon from TEGa enhances a deep emission (∼550 nm), which shows the carbon makes not only an acceptor level but deep levels at interstitial sites in GaN. The carbon impurities originate from methyl radicals for TMGa, or ethyl radicals for TEGa. It is supposed that, in the case of TEGa, the carbon impurities are not always located at nitrogen sites, but are also located at interstitial sites because of the C-C bonding in ethyl radicals.

Analysis of GaInP/AlGaInP compressive strained multiple-quantum-well laser

We have analyzed GaInP/AlGaInP compressive strained MQW lasers, with theoretical calculation and experimental results. Our calculations of TE polarized gain, where the valence subband mixing and the heterobarrier leakage current are taken into account, are in good agreement with the experimental results. When a compressive strain of up to 0.5% is induced in the quantum wells, the density of states near the valence band edge is decreased, due to the reduction of heavy-hole and light-hole subband mixing. At the threshold condition, the compressive strain reduces not only the radiative recombination current, but also the hetero-barrier leakage current. Therefore, the threshold current is reduced, and its temperature dependence is found to be small, In the analysis, we also show that when larger compressive strain of more than 0.5% is induced in the 40-/spl Aring/-thick quantum wells, the threshold characteristics are degraded. >

High power and high‐temperature operation of GaInP/AlGaInP strained multiple quantum well lasers

High power and high‐temperature operation of transverse‐mode stabilized 690 nm AlGaInP strained multiple quantum well lasers is described. Three 0.5% biaxially compressive strained GaInP (8 nm) wells were employed in the active region. Low threshold current of 36 mA and very high output power of 60 mW at high temperature up to 100 °C were obtained with 700 μm long lasers, whose facets were coated with antireflection–reflection films. This is, to the best of our knowledge, the first report that an AlGaInP laser has reached a cw output power of 60 mW at a high temperature of 100 °C. Very low degradation rate at 50 °C with 35 mW output power was confirmed.

Room Temperature 339 nm Emission from Al0.13Ga0.87N/Al0.10Ga0.90N Double Heterostructure Light-Emitting Diode on Sapphire Substrate

Room-temperature deep-ultraviolet emission has been observed from Al0.13Ga0.87N/Al0.10Ga0.90N double heterostructure light-emitting diodes (LEDs) on (0001)-oriented sapphire substrate. By introducing undoped barrier layers, which sandwich the active layer, the LED was operated at a peak emission wavelength of 339 nm with a narrow linewidth of 5.6 nm. The dependence of emission intensity on injection current suggests that the nonradiative recombination was suppressed and the diffusion current for the recombination process was dominant at the injection current of over 20 mA.

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.

InP MESFET Grown by MOCVD

MESFET devices with 2 µm gate length have been fabricated on InP epitaxial layers grown by the MOCVD technique on Fe-doped semi-insulating InP substrates. The best electrical properties of InP epitaxial films have been measured and are n=2.7×1015 cm-3 and µ=3500 cm2/Vs at room temperature. The barrier height and the ideality factor of Au-n InP Schottky diode were estimated to be 0.66 eV and 1.18, respectively. Depletion mode n-channel MESFET has shown transconductance of ~90 mS/mm and the saturation drain current of ~20 mA at zero gate bias.

Theoretical analysis of valence subband structures and optical gain of GaInP/AlGaInP compressive strained-quantum wells

The valence subband structures and optical gain of GaInP/AlGaInP strained quantum wells are theoretically analyzed, using the 4*4 Luttinger-Kohn Hamiltonian. The compressive strain reduces the density of states near the valence band edge. As a result, the differential gain is enhanced for low injection carrier density, and the threshold current is reduced due to the reduction of radiative recombination current. For high injection current, the strain reduces the differential gain, although the threshold current is reduced due to the reduction of the hetero-barrier leakage current.<<ETX>>