Tetsuo Narita

Reliability Evaluation of Al2O3 Deposited by Ozone-Based Atomic Layer Deposition on Dry-Etched n-Type GaN

The time-to-breakdown (tBD) of Al2O3 deposited by ozone-based atomic layer deposition (ALD) on dry-etched n-type GaN was evaluated by constant-voltage-stress time-dependent dielectric breakdown (TDDB) measurements. The influence of dry etching was not observed in the TDDB and current–voltage (I–V) measurements at room temperature. The tBD of the ALD-Al2O3 film was estimated to be more than 40,000 years at 3 MV/cm and room temperature. However, the tBD estimated at 250 °C was around 102–103 s.

Interface Properties of Al2O3/n-GaN Structures with Inductively Coupled Plasma Etching of GaN Surfaces

The effects of the Cl2-based inductively coupled plasma (ICP) etching of GaN on the interface properties of Al2O3/GaN structures prepared by atomic layer deposition (ALD) were investigated. We used n-GaN layers grown on freestanding n+-GaN substrates with low dislocation density. The ICP etching caused slight disorder of the chemical bonds at the GaN surface and monolayer-level interface roughness at the Al2O3/GaN interface, resulting in poor capacitance–voltage (C–V) characteristics due to high-density interface states including nitrogen-vacancy (VN) related levels. The postannealing process in N2 at 400 °C drastically improved the C–V characteristics, probably owing to the partial recovery of the VN-related defects and the increased ordering of chemical bonds in the GaN surface region.

P-type doping of GaN by magnesium ion implantation

Magnesium ion implantation has been performed on a GaN substrate, whose surface has a high thermal stability, thus allowing postimplantation annealing without the use of a protective layer. The current–voltage characteristics of p–n diodes fabricated on GaN showed distinct rectification at a turn-on voltage of about 3 V, although the leakage current varied widely among the diodes. Coimplantation with magnesium and hydrogen ions effectively suppressed the leakage currents and device-to-device variations. In addition, an electroluminescence band was observed at wavelengths shorter than 450 nm for these diodes. These results provide strong evidence that implanted magnesium ions create acceptors in GaN.

Band offset of Al1− x Si x O y mixed oxide on GaN evaluated by hard X-ray photoelectron spectroscopy

An Al1− x Si x O y mixed oxide has been deposited on GaN by plasma-enhanced atomic layer deposition. The band diagrams between the mixed oxide and GaN for various Si atom fraction x values are determined by hard X-ray photoelectron spectroscopy for the first time. The band gap of the mixed oxide increases with increasing x. This dependence has a large bowing parameter of 1.5 eV. We have successfully obtained conduction band offset (ΔE C) and valence band offset (ΔE V) as a function of x: ΔE C (eV) = 1.6 + 0.4x + 1.2x 2 and ΔE V (eV) = 1.7 + 0.34x + 0.36x 2. These relationships enable us to design GaN metal–oxide–semiconductor devices using the Al1− x Si x O y mixed oxide.

Hole-Trapping Process at Al 2 O 3 /GaN Interface Formed by Atomic Layer Deposition

The hysteresis of the capacitance–voltage <italic>(C–V)</italic> characteristics of an Al₂O₃/n-GaN metal–insulator–semiconductor structure was evaluated under light (white LED)-irradiation and dark conditions. The hysteresis was not observed under the dark condition but was observed under the light-irradiation condition. The GaN surface was completely depleted in negative bias under the dark condition. The <italic>C–V</italic> characteristics indicated that the hysteresis is caused by hole trapping under the LED irradiation condition and that the holes are generated in the n-GaN surface by LED irradiation and subsequently injected into the Al₂O₃ films. When the holes are generated in the depletion region of GaN for any reason, such as a short generation lifetime, they can be trapped in the Al₂O₃ films.

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

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.

Ion implantation technique for conductivity control of GaN

Ion implantation technique for controlling n- or p-type conduction has been a significant challenge for GaN-based high-power devices to achieve levels approaching their theoretical limits of performance. Previous studies on n-type conduction of GaN through Si ion implantation achieved 86% [1] and approximately 100% [2] of activation rate after annealing at 1250 and 1400 °C, respectively. Such high-temperature annealing results in serious surface degradation of GaN(0001) due to decomposition [1], thereby needing a protective layer. However, it is difficult to make a proper choice of a protective layer which remains unaltered and is removable after annealing above 1200°C. Therefore achieving the high-activation rate at lower temperature is a very important practice. On the other hand, the Mg-ion implantation for p-type conductivity is more challenging due to the higher-temperature annealing required for electrical activation, resulting in a major difficulty protecting the surface. The formation energy of Mg on the Ga site (MgGa) at the position of Fermi level near the valence band is about 1 eV higher than that of SiGa at the Fermi level near the conduction band [4, 5], which may explain the difference of required annealing temperature for different conduction types.