A. Usui

On the lattice parameters of GaN

The lattice parameters of low-defect density, undoped bulk GaN fabricated by hydride vapor phase epitaxy (HVPE) on (0001) sapphire and subsequent substrate removal, are precisely determined using high-resolution x-ray diffraction. The obtained values, c=5.18523A and a=3.18926A, are compared with the lattice parameters of freestanding HVPE GaN from different sources and found to be representative for state-of-the-art undoped HVPE bulk GaN material. A comparison with bulk GaN fabricated by the high-pressure technique and homoepitaxial GaN is made, and significant differences in the lattice parameters are found. The observed differences are discussed and a possible explanation is suggested.

Metastable behavior of the UV luminescence in Mg-doped GaN layers grown on quasibulk GaN templates

Metastability of near band gap UV emissions in Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on thick GaN templates grown by halide vapor phase epitaxy has been studied by cathodoluminescence (CL). The CL spectrum changes its initial shape within a few minutes under electron irradiation. The acceptor bound exciton line vanishes while the emissions related to the stacking faults (SFs) of different geometry rise significantly. The increase of the defect-related luminescence is likely caused by recombination enhanced SF formation under electron irradiation. The CL spectrum transformation is permanent at low temperatures; however, the metastable process is reversible if samples are heated to room temperature.

Resonant light delay in GaN with ballistic and diffusive propagation.

We report on a strong delay in light propagation through bulk GaN, detected by time-of-flight spectroscopy. The delay increases resonantly as the photon energy approaches the energy of a neutral-donor bound exciton (BX), resulting in a velocity of light as low as 2100 km/s. In the close vicinity of the BX resonance, the transmitted light contains both ballistic and diffusive components. This phenomenon is quantitatively explained in terms of optical dispersion in a medium where resonant light scattering by the BX resonance takes place in addition to the polariton propagation.

Effect of annealing on metastable shallow acceptors in Mg-doped GaN layers grown on GaN substrates

Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on GaN substrates produced by the halide vapor phase technique demonstrate metastability of the near-band-gap photoluminescence (PL). ...

Radiation-induced defects in GaN

Radiation-induced defects in n-type GaN irradiated by 2 MeV electrons at room temperature were studied by electron paramagnetic resonance (EPR). Four EPR spectra, labelled D1-D4, were observed in i ...

Optical properties of metastable shallow acceptors in Mg-doped GaN layers grown by metal-organic vapor phase epitaxy

GaN layers doped by Mg show a metastable behavior of the near‐band‐gap luminescence caused by electron irradiation or UV excitation. At low temperatures <30 K the changes in luminescence are permanent. Heating to room temperature recovers the initial low temperature spectrum shape completely. Two acceptors are involved in the recombination process as confirmed by transient PL. In as‐grown samples a possible candidate for the metastable acceptor is CN, while after annealing a second more stable acceptor related to Mg became active.

Time-resolved Spectroscopy of Excitons Bound at Shallow Neutral Donors in HVPE GaN

Time-resolved photoluminescence (TRPL) spectroscopy has been performed in the temperature range 2 K to 300 K on thick (300 μm – 1 mm) nominally undoped bulk HVPE grown GaN layers. The PL spectra are dominated by the free excitons (FEs) and the two neutral donor bound excitons (DBEs) at about 3.4712 eV and 3.4721 eV at 2 K, assigned to the residual O and Si donors, respectively. The zero-phonon decay curves for both FEs and DBEs are nonexponential, with a fast initial decay and a slower tail at longer times. The initial decay time of the FE is about 130 ps at 2 K in such samples, the corresponding initial decay time for the DBEs for O and Si is close to 300 ps. The different lines corresponding to the so called two-electron transitions for the DBE show a different behaviour, with a longer decay time than the corresponding principal DBE states. The LO phonon replicas of the DBEs also have a decay time in excess of 1 ns in lightly doped samples.