Yuzaburo Ban

Comparison of Forward and Reverse Reactions in Hydrogen Generation between GaN, InGaN, Nanocrystalline TiO2 and Pt Electrodes during Water Electrolysis

The electron transport between n-, p-GaN, and p-In0.12Ga0.88N electrodes and electrolyte solution is studied by cyclic voltammetry under dark conditions, and compared with that between Pt and nanocrystalline TiO2/Ti electrodes. Both forward and reverse reactions of 2H+ + 2e-\rightleftharpoonsH2 occur for Pt and TiO2/Ti electrodes. In contrast, in n-, p-GaN and p-In0.12Ga0.88N electrodes, only the forward reaction of 2H+ + 2e-→H2 occurs and no reverse reaction is observed. These results are consistent with the finding that the conduction band-edge potentials of n-, p-GaN, and p-In0.12Ga0.88N are higher than the reduction potential of H+(aq) in electrolyte, determined by flat-band potential measurement. These differences in potential work as an energy barrier for the reverse electron transfer from H2 molecules to electrodes. These results also support the experimental results of spontaneous H2 generation caused by the band-gap excitation of p-GaN and p-In0.12Ga0.88N electrodes without applying bias voltage.

Room-Temperature Absorption Edge of InGaN/GaN Quantum Wells Characterized by Photoacoustic Measurement

The absorption edges of five periods of InxGa1-xN (3 nm)/GaN (15 nm) (x=0.07–0.23) quantum wells (QWs) are characterized by photoacoustic (PA) measurement at room temperature. The absorption edge is determined by differentiating the PA signal curve to obtain the inflection point on the assumption that the signal curve consists of Urbach tail in the low-energy region and Elliotts equation in the high-energy region. The constant absorption edge of GaN is observed at 3.4 eV and an absorption edge redshift with increasing In composition is observed for InGaN QWs. As a result, the Stokes shift increases with In composition and the highest shift of 435 meV is observed at x=0.23. From the energy calculation of optical transition in the InGaN/GaN QWs under an internal polarization field, the transition between the ground states confined in the well with a triangular potential causes a low-energy shift in the photoluminescence peak from the bulk band-gap energy, and the excited bound states whose wave functions are confined by the step-linear potential extending over the GaN barrier lead to the high-energy shift in the absorption edge.

High growth rate of AlGaN for buffer structures for GaN on Si to increase throughput

Throughput requirement of the epitaxial process of GaN on Si is described. The impact of the growth rate of AlGaN for the buffer layer of GaN on Si is highlighted. In the attempt of growing GaN on Si, we have tested a production scale high flow speed MOVPE reactor (TAIYO NIPPON SANSO UR25k) for 6 inch X 7 wafers. Al0.58Ga0.42N was grown with the growth rate of 1.85μm/hr at 30 kPa. AlN was grown with the growth rate of 1.4μm/hr at 13kPa. AlN/GaN SLS (5nm/20nm) was also grown at the growth rate of 1.4μm/hr. An excellent uniformity of aluminum concentration of less than 0.5% was also obtained for Al0.58Ga0.42N. The challenge which we are facing to further increase of the throughput is summarized.

Characterization of Conduction Band Offset in Nanocrystalline TiO2/p-InGaN for Application to Water Photolysis Using Visible Light

We fabricate a rutile-rich TiO2 nanocrystalline layer by the sol–gel method on an epitaxial p-InGaN layer and characterize their conduction band offsets by photocurrent spectroscopy. The conduction band edge of p-In0.12Ga0.88N is higher in energy than that of the TiO2 nanocrystalline layer. The excitation of the TiO2/p-In0.12Ga0.88N electrode by 405 nm light causes a cathodic photocurrent and an increase in scattering light intensity owing to the hydrogen bubble formation on the electrode surface. These results indicate electron injection from the TiO2/p-InGaN electrode to the electrolyte and the possibility of hydrogen generation using visible light by TiO2/p-InGaN electrode.

Criteria for versatile GaN MOVPE tool: high growth rate GaN by atmospheric pressure growth

Growth rate has a direct impact on the productivity of nitride LED production. Atmospheric pressure growth of GaN with a growth rate as high as 10 μm/h and also Al0.1Ga0.9N growth of 1 μm/h by using 4 inch by 11 production scale MOVPE are described. XRD of (002) and (102) direction was 200 arcsec and 250 arcsec, respectively. Impact of the growth rate on productivity is discussed.

p-GaN cathode degradation correlated with hydrogen passivation during water photolysis under UV light

We investigated the mechanism of p-GaN cathode degradation in Na2SO4 solution under UV light irradiation. During water photolysis, photocurrent and hydrogen production decreased with time at a zero bias. Samples degraded by UV light irradiation showed an increase in photoluminescence (PL) intensity and a decrease in hole concentration compared with those before UV light irradiation. Annealing the degraded samples at 850 °C in N2 ambient resulted in a decrease in PL intensity and an increase in hole concentration. These changes suggest that hydrogen passivation occurs as the mechanism of p-GaN cathode degradation during water photolysis.