Gaku Sugahara

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.

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.

Study on Deformations and Stress Distributions in Air-Bridged Lateral-Epitaxial-Grown GaN Films

Two-dimensional deformations and stress distributions in air-bridged lateral-epitaxial-grown GaN (ABLEG-GaN) films have been studied by atomic force microscopy (AFM), two-dimensional finite element method (FEM) analysis, and micro-Raman spectroscopy. The ABLEG-GaN wings slightly tilt, and the direction of the wing tilt changes when the wings coalesce with each other. After coalescence of the wings, the tilt angle decreases with increasing film thickness. By FEM analysis and Raman spectroscopy, it has been revealed that the deformation of the wings originates from the distributions of thermal stress due to large mismatch of the thermal expansion in the GaN seed layer and in the sapphire substrate. The wing deformation is suppressed with increasing film thickness, since the stress distribution becomes more uniform.