Akihiko Ishibashi

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.

Temperature dependence of localized exciton transitions in AlGaN ternary alloy epitaxial layers

The optical properties of Ga-rich AlxGa1−xN (x=0.019, 0.038, 0.057, 0.077, and 0.092) ternary alloy epitaxial layers have been studied by means of temperature-dependent photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence intensity of excitons in five epitaxial layers indicated a thermal quenching process with two activation energies. The two quenching activation energies were attributed to the delocalization of excitons and thermal dissociation of excitons. Anomalous temperature dependence of the PL peak energy was also observed in the epitaxial layers, which enabled the evaluation of the localization energy of the excitons. The localization energy increased as the 1.7th power of the PL linewidth, which reflected a broadening of the density of localized exciton states. In addition, the luminescence decay of the localized excitons for the five epitaxial layers became longer with decreasing emission energy. These observations suggest that the decay of excitons is caused not only by ra...

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.

Localization-induced inhomogeneous screening of internal electric fields in AlGaN-based quantum wells

The influence of both localization and internal electric field on the microscopic photoluminescence (PL) properties of AlGaN-based quantum wells (QWs) has been studied by means of scanning near-field optical microscopy (SNOM). SNOM-PL images of three QWs with different well-layer thicknesses were measured under an illumination-collection mode. A correlation was observed between the PL intensity and the PL peak wavelength: a shorter-PL wavelength indicates a stronger intensity for a wider QW. The correlation is caused by an inhomogeneous screening of the internal electric field.

Biexciton luminescence from AlxGa1−xN epitaxial layers

Excitonic optical properties of Ga-rich AlxGa1−xN ternary alloy epitaxial layers (x=0.019, 0.038, 0.057, 0.077, and 0.092) have been studied by means of photoluminescence (PL) and time-resolved PL spectroscopy. The luminescence line due to radiative recombination of biexcitons was clearly observed for all of the five ternary alloy epitaxial layers with different Al compositions. The energy separation between exciton luminescence and biexciton luminescence increased with increasing Al composition, which indicated the increase in the binding energy of biexcitons in ternary alloys.

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.

Evolution of excitonic states and localized exciton luminescence in Pb1-xCdxI2 solid solutions

Emission spectra and decay characteristics of exciton luminescence in Pb 1- x Cd x I 2 solid solutions have been investigated for a wide range of the composition x . Continuous shift of the absorption edge toward higher energy with increasing x is ascribed to reduction in the band-width of the exciton associated with 6s-6p transition of Pb ions. The emission band appearing near the absorption edge of the solid solutions with x below 0.5 is attributed to the luminescence of excitons localized at various potential wells induced by the composition fluctuation. An essential change in the excited state character from the excitonic state to the excitation of Pb ion clusters is found to take place at the composition x around 0.5.

Metalorganic Vapor Phase Epitaxy Growth of a High-Quality GaN/InGaN Single Quantum Well Structure Using a Misoriented SiC Substrate

A high-quality GaN/InGaN single quantum well (SQW) structure has been successfully grown using a misoriented 6H-SiC substrate, the face of which is tilted from (0001) toward [1120] by 3.5°, by low pressure metalorganic vapor phase epitaxy (MOVPE). A sharp emission, whose full-width at half maximum (FWHM) was 24.3 meV, from the GaN/InGaN SQW structure was observed at 385nm in the 77K photoluminescence spectrum. From the transmission electron microscopy (TEM) analysis, the dislocations in the GaN film grown on the misoriented substrate were bent from the c-direction, and the threading dislocations to the InGaN film on the GaN film were decreased. For the InGaN film grown on the misoriented substrate, only the sharp band edge emission, whose FWHM was 92.3 meV, was observed at 385 nm in the PL spectrum at 77K. The dislocation density, which was estimated from TEM photographs, in the InGaN film on the GaN film grown on the misoriented substrate was about 5 x 10 8 cm -2 , which was nearly half of that grown on the (0001) substrate.