Yoshinobu Narita

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.

Vertical GaN p-n Junction Diodes With High Breakdown Voltages Over 4 kV

Vertical structured GaN power devices have recently been attracting a great interest because of their potential on extremely high-power conversion efficiency. This letter describes increased breakdown voltages in the vertical GaN p-n diodes fabricated on the free-standing GaN substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, the record breakdown voltages (VB) of 4.7 kV combined with low specific differential ON-resistance (RON) of 1.7 mΩcm2 were achieved. With reducing the Si-doping concentration of the top n-GaN drift layer adjacent to the p-n junction using well-controlled metal-organic vapor phase epitaxy systems, the peak electric field at the p-n junction could be suppressed under high negatively biased conditions. The second drift layer with a moderate doping concentration contributed to the low RON. A Baligas figure of merit (VB2/RON) was 13 GW/cm2. These are the best values ever reported among those achieved by GaN p-n junction diodes on the free-standing GaN substrates.

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.

Wafer-level nondestructive inspection of substrate off-angle and net donor concentration of the n−-drift layer in vertical GaN-on-GaN Schottky diodes

In the mass production of GaN-on-GaN vertical power devices, a nondestructive simple inspection of the net donor concentration (N D − N A) of the n−-drift layer in the range of 1015 cm−3 is required. In this study, we demonstrate the wafer-level nondestructive inspection of GaN Schottky barrier diode epi-structures grown by metal organic vapor phase epitaxy (MOVPE) on free-standing GaN substrates. We found that the normalized yellow (YL) photoluminescence peak intensity of the near band edge (NBE), I YL/I NBE, is strongly related to the acceptor concentration N A of the n−-drift layer. This means that the N D − N A of the n−-drift layer can be inspected by photoluminescence measurement at a high speed, when Si concentration is not varying across the GaN wafers. Noncontact capacitance–voltage and secondary ion mass spectrometry measurements were used to investigate the cause of N D − N A variation across the GaN wafers. The discrepancy between C and N A indicates that compensation could be due to another electron trap.

Nondestructive measurement of homoepitaxially grown GaN film thickness with Fourier transform infrared spectroscopy

In vertical devices containing GaN homoepitaxial layers on GaN substrates, the layer thickness is a key parameter that needs to be clarified before starting the device process. We applied Fourier transform infrared spectroscopy (FT-IR) to a homoepitaxially grown GaN film that consisted of an n−-GaN layer. The estimated film thickness from the FT-IR spectrum agreed well with the results of cross-sectional scanning electron microscope cathodoluminescence images. This is the first report of nondestructive film thickness measurements for homoepitaxially grown GaN and indicates the applicability of FT-IR to the nondestructive inspection of vertical GaN power devices.