Takayoshi Takano

Room-temperature deep-ultraviolet lasing at 241.5 nm of AlGaN multiple-quantum-well laser

Room-temperature deep-ultraviolet lasing of AlxGa1−xN multiple-quantum-well lasers with an Al composition x of 0.66 was achieved at 241.5 nm under pulsed optical pumping. The threshold pumping power was approximately 1200 kW/cm2 at room temperature. The shortest lasing wavelength was 231.8 nm at 20 K. The laser structure was grown on a high-quality AlN layer, which was grown on a 4H-SiC substrate by inserting an AlN/GaN multibuffer-layer structure between the substrate and the AlN layer. Temperature dependence of lasing wavelength was also estimated to be 0.01 and 0.03 nm/K in the temperature region from 20 to 150 K and from 160 K to room temperature, respectively. The laser cavity was made of a cleaved facet of AlGaN epitaxial layers and a SiC substrate. For this purpose, it was necessary to polish the wafer to a thickness of less than 100 μm. The optimal wafer thickness for cleaving in our experiments was 60–70 μm.

Dependence of Crystal Quality on Residual Strain in Strain-Controlled Thin AlN Layer Grown by Metalorganic Vapor Phase Epitaxy

Strain-controlled AlN layers were grown on (0001) 6H–SiC with a (GaN/AlN) buffer layer by metalorganic vapor phase epitaxy using an alternating-source-feeding technique (ASF). The successful strain and quality control of the thin AlN layer were experimentally demonstrated down to 0.05 µm by changing the growth conditions of the (GaN/AlN) buffer layer. The quality of the AlN layer was evaluated by not only X-ray diffraction (θ-2θ) but an X-ray rocking curve (ω-scan) from the viewpoint of c-axis tilting. The crystal quality was dependent on the residual strain in the AlN layer. The (GaN/AlN)-buffer layer is effective in improving the quality of the AlN layer.

Improvement of Crystal Quality of AlGaN Multi Quantum Well Structure by Combination of Flow-Rate Modulation Epitaxy and AlN/GaN Multi-Buffer Layer and Resultant Lasing at Deep Ultra-Violet Region

The crystal quality of AlN and AlGaN MQW layers was improved greatly by a combination of flow-rate modulation epitaxy (FME) and the optimized AlN/GaN multi-buffer layer in low-pressure metal organic vapor phase epitaxy (LP-MOVPE). The cross-sectional TEM image indicated that the threading-dislocation density of the AlN template decreased from 109–1010 cm-2 to 107–108 cm-2 by this combination. Resultantly, the lasing wavelength with the same optical pumping power decreased by about 80 nm, and lasing at 241 nm, the shortest reported so far at room temperature, has been achieved.

Possibility of Strain Control in AlN Layer Grown by MOVPE on (0001) 6H‐SiC with GaN/AlN Buffer

A tensile-strained AlN layer along the a-axis was grown on a (0001) 6H-SiC substrate with a (GaN/AlN) buffer layer by metalorganic vapor phase epitaxy using an alternating source feeding (ASF) technique. It was experimentally demonstrated that the strain in the AlN layer was affected by the growth conditions of the buffer layer. On the other hand, the AlN layer grown directly on a substrate without the buffer layer exhibits compressive strain along the a-axis. Strain control in the AlN layer using the ASF buffer is proposed.

Room-Temperature Photoluminescence from BAlGaN-Based Double or Single Heterostructures for UV Laser Diode

A BGaN/AlN double heterostructure or BAlGaN/AlN single was grown on 6H-SiC substrate by metalorganic vapor phase epitaxy (MOVPE). The relation between the band-gap energy and boron composition was experimentally determined for BGaN. Photoluminescence (PL) spectra were observed with 2.7% boron composition at room temperature. The PL spectrum of BAlGaN, which was near the band-edge, was observed.

Improved Optical Quality of BAlGaN/AlN MQW Structure Grown on 6H-SiC Substrate by Controlling Residual Strain Using Multi-Buffer Layer

BAlGaN and (BAlGaN/AlN) multi-quantum-wells (MQWs) structure were grown on 6H-SiC substrate by a low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). Estimated boron compositions of the BAlGaN quantum wells by an Auger electron spectroscopy (AES) analysis were 0% to 13%. Photoluminescence (PL) spectra around 260 nm were observed at room temperature. The full-width at half maximum (FWHM) of PL spectra for BAlGaN/AlN MQW structure(with 2% of boron) was narrowed from 360 meV to 179 meV, as the residual strain in the BAlGaN well layer was decreased from 1.3% to 1.0% by increasing the Al content in the quantum wells.