Showing papers by "Tsutomu Uesugi published in 2018"

Sources of carrier compensation in metalorganic vapor phase epitaxy-grown homoepitaxial n-type GaN layers with various doping concentrations

Abstract: The source of carrier compensation in metalorganic vapor phase epitaxy (MOVPE)-grown n-type GaN was quantitatively investigated by Hall-effect measurement, deep-level transient spectroscopy, and secondary ion mass spectrometry. These analysis techniques revealed that there were at least three different compensation sources. The carrier compensation for samples with donor concentrations below 5 × 1016 cm−3 can be explained by residual carbon and electron trap E3 (E C − 0.6 eV). For samples with higher donor concentrations, we found a proportional relationship between donor concentration and compensating acceptor concentration, which resulted from a third source of compensation. This is possibly due to the self-compensation effect.

Parallel-Plane Breakdown Fields of 2.8-3.5 MV/cm in GaN-on-GaN p-n Junction Diodes with Double-Side-Depleted Shallow Bevel Termination

Abstract: We report homoepitaxial GaN p-n junction diodes with novel beveled-mesa structures. The n-layers and p-layers, the doping concentrations of which are comparable, were prepared. We found that electric field crowding does not occur in the structure using TCAD simulation. The fabricated devices showed the breakdown voltages of 180–480 V, small leakage currents, and excellent avalanche capabilities. The breakdown voltages increased at elevated temperature. At the breakdown, nearly uniform luminescence in the entire p-n junctions was observed in all the devices. These results are strong evidences that the uniform avalanche breakdowns occurred in the devices. We carefully characterized the depletion layer width at the breakdown, and the parallel-plane breakdown electric fields of 2.8-3.5 MV/cm were obtained, which are among the best of the reported non-punch-through GaN vertical devices.

Franz-Keldysh effect in GaN p-n junction diode under high reverse bias voltage

Abstract: Photocurrent induced by sub-bandgap light absorption due to the Franz-Keldysh effect was observed in GaN p-n junction diodes under a high reverse bias voltage. The photocurrent increased with the reverse bias voltage and the increase was found to be more significant as the wavelength approached the absorption edge of GaN. The photocurrent was calculated with consideration of light absorption induced by the Franz-Keldysh effect in the depletion layer. The calculated curves showed excellent agreement with the experimental curves. The photocurrent also increased with an increase in temperature and this could be quantitatively explained by the red-shift of the GaN absorption edge with the increase in temperature.Photocurrent induced by sub-bandgap light absorption due to the Franz-Keldysh effect was observed in GaN p-n junction diodes under a high reverse bias voltage. The photocurrent increased with the reverse bias voltage and the increase was found to be more significant as the wavelength approached the absorption edge of GaN. The photocurrent was calculated with consideration of light absorption induced by the Franz-Keldysh effect in the depletion layer. The calculated curves showed excellent agreement with the experimental curves. The photocurrent also increased with an increase in temperature and this could be quantitatively explained by the red-shift of the GaN absorption edge with the increase in temperature.

Cathodoluminescence Study on Thermal Recovery Process of Mg-Ion Implanted N-Polar GaN

Abstract: Thermal recovery of N-polar GaN(0001¯) samples implanted with magnesium and hydrogen ions was investigated by cathodoluminescence (CL) spectroscopy. The high thermal stability of N-polar GaN allowed annealing process over 1200 °C without protective overlayer. The CL emissions from acceptor-bound excitons and donor-bound excitons were observed in the near-band-edge (NBE) after annealing over 1000 °C of the ion implanted samples, which indicates the formation of Mg acceptors. The emission intensities both in the NBE and in the green luminescence (GL) band increased with the annealing temperature, resulting from reducing the non-radiative recombination centers. On the other hand, the enhancement of the GL band (nitrogen vacancy complexes) by prolonging the annealing duration above 1200 °C was not significant as compared to the band-edge emission, probably resulting from the enhancement of activating the implanted Mg ions. These results give clear directions to improve the quality of Mg-ion implanted GaN.