Tomoya Sugahara

DIRECT EVIDENCE THAT DISLOCATIONS ARE NON-RADIATIVE RECOMBINATION CENTERS IN GAN

Plan-view transmission electron microscopy (TEM) and cathodoluminescence (CL) images were taken for the same sample at exactly the same location in n-type GaN grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). There was a clear one to one correspondence between the dark spots observed in CL images and the dislocations in TEM foils, indicating that the dislocations are non-radiative recombination centers. The hole diffusion length in n-type GaN was estimated to be neighboring 50 nm by comparing the diameters of the dark spots in thick samples used for CL and samples that were thinned for TEM observation. The efficiency of light emission is high as long as the minority carrier diffusion length is shorter than the dislocation spacing.

Photoluminescence investigation of InGaN/GaN single quantum well and multiple quantum wells

The photoluminescence investigation at a low temperature was carried out in In0.13Ga0.87N/GaN single quantum well (SQW) and multiple quantum wells with 10 (10QW) or 5 periods. With decreasing number of wells, the emission peak shows a redshift. In the case of a low excitation power, the emission intensity is enhanced by an increase in the number of wells while it decreases in the case of a high excitation power. With increasing excitation power, the emission peak of the SQW exhibits a blueshift and its linewidth decreases, but the emission peak of the 10QW remains unchanged and its linewidth increases. Based on the theory of the quantum confined Stark effect, the behavior of the SQW and the 10QW can be well explained. This result should be highly emphasized in designing InGaN/GaN based optical devices.

Role of Dislocation in InGaN Phase Separation

The role of dislocation for luminescence in InGaN grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD) method was investigated by cathodoluminescence (CL) and atomic force microscopy (AFM). The CL emission area and dark spots between InGaN and GaN layers in InGaN/GaN single quantum well (SQW) and multiple quantum well (MQW) structures showed completely one to one correspondence. These results indicate that dislocations in InGaN work as non-radiative recombination centers. Furthermore it was confirmed that the phase separation in InGaN is caused by spiral growth due to mixed dislocations, and such a growth mechanism is discussed.

Large Schottky barriers for Ni/p-GaN contacts

Large Schottky barriers were measured for Ni contacts formed on low Mg-doped p-GaN. In order to improve leaky Schottky characteristics, low-Mg doping was examined. This provided atomically flat surfaces and a low dislocation density of 5.5×108 cm−2. The Schottky barrier height (qφB) as high as 2.4±0.2 eV and n values of 1.84±0.06 were obtained from current–voltage measurements. These results are in good agreement with the prediction that the sum of qφB of n and p types adds up to the band gap. In the capacitance–voltage measurements, a transient response of capacitance was observed. This indicates that the evaluation of deep levels close to the valence band is possible, which could result in improvement of p-GaN growth.

Photoluminescence studies of InGaN/GaN multi-quantum wells

We report measurements of photoluminescence, photoluminescence excitation spectroscopy and photoluminescence time decay on three MOVPE-grown InGaN/GaN multiple quantum well structures with 13% In in the wells and well widths Lz = 1.25, 2.5 and 5.0 nm. The PL spectra are dominated by single emission peaks, together with phonon sidebands spaced by a GaN LO phonon energy (92 meV). The peak energies are red-shifted with respect to energies calculated for exciton recombination in square quantum wells and the wide well sample also shows a significant Stokes shift between emission and absorption. Recombination lifetimes measured at 6 K are energy dependent, increasing as the photon energy is scanned downwards through the emission line. They also depend strongly on well width. On the low energy side of the 5 nm well emission line we measure lifetimes as long as 100 ns. Raising the temperature from 6 to 300 K results in a strong reduction of emission intensity for all samples and reduction of the lifetimes, though by a much smaller factor. The peak positions shift slightly to lower energy but by far less than the shift in the band edge. We consider three different theoretical models in an attempt to interpret this data, an exponential tail state model, a model of localization due to In/Ga segregation within the wells and the quantum confined Stark effect model. The QCSE model appears able to explain most of the data reasonably well, though there is evidence to suggest that, in addition, some degree of localization occurs.

Optical investigation of InGaN/GaN multiple quantum wells

Optical investigation was performed on InGaN/GaN multiple quantum well (MQW) structures with different well thicknesses. At low temperature, the excitation power dependence of the photoluminescence (PL) emission energy of a MQW with 5 nm well thickness was found to be different from that of a MQW with 2.5 nm well thickness. Their temperature dependence of the optical behaviors including the PL line shapes and the internal quantum efficiencies also showed distinct features. The optical behaviors of the quantum well with a thickness above 2.5 nm can be explained by a model based on the formation of self-organized small In-rich regions, rather than by the piezoelectric field-induced quantum-confined Stark effect.

V-shaped defects in InGaN/GaN multiquantum wells

InGaN/GaN multiquantum well (MQW) structures have been grown on (0001) sapphire substrate by metalorganic chemical vapor deposition. From cross-sectional transmission electron microscopy (TEM), a number of V-shaped defects has been observed on the surface which are associated with mixed or pure-edge screw dislocations, as well as with inversion domains. Atomic force microscopy (AFM) reveals that these are hexagonal pits with a width of about 70 nm. The mechanism of formation of these defects has been discussed in terms of stress induced by lattice mismatch and reduced In incorporation on the {1011} planes in comparison to the (0001) surface. A decrease in In concentration and also in well thickness during growth has been found. No optical emission has been observed from these defects by cathodoluminescence (CL) studies.

COMPOSITIONAL INHOMOGENEITY OF INGAN GROWN ON SAPPHIRE AND BULK GAN SUBSTRATES BY METALORGANIC CHEMICAL VAPOR DEPOSITION

The compositional inhomogeneity of the InGaN layers in GaN/InGaN/GaN double-hetero (DH) and InGaN/GaN single-hetero (SH) structures grown by metalorganic chemical vapor deposition (MOCVD) on sapphire (0001) and bulk GaN was investigated by means of cathodoluminescence (CL) and energy dispersive X-ray (EDX) spectroscopy. Dotlike CL image of the band edge emission from InGaN was observed. The bright spots were found to have higher indium content compared to that on the outside of the spots. The compositional inhomogeneity increased and the density of the spot decreased with increasing film thickness. Hexagonal hillocks, which had higher indium content and emitted stronger CL, were observed on the surface of the SH structure. Compositional inhomogeneity of homoepitaxial InGaN on bulk GaN substrate was much less compared to that of InGaN on sapphire revealing that dislocation plays a key role in producing an inhomogeneity. A possible mechanism that explains these phenomena is proposed.

Current transport mechanism of p-GaN Schottky contacts

Transient measurements of I–V and depletion layer capacitance were conducted to clarify the leaky current flow mechanism in Ni Schottky contacts formed on Mg-doped p-GaN. We found that carrier capture and emission from acceptor-like deep level defects cause depletion layer width (Wdep) to vary significantly. Upon ionization of the defects by white light, which results in small Wdep, current can go through the Schottky barrier and a leaky I–V curve is observed. Upon filling by current injection, Wdep becomes larger and the large original Schottky barrier height is seen. The time constant of carrier emission is as long as 8.3×103 min.

Investigation on the P-Type Activation Mechanism in Mg-doped GaN Films Grown by Metalorganic Chemical Vapor Deposition

An investigation on the p-type activation in Mg-doped GaN epilayers has been carried out in relation to the defect structure. The samples were grown by the metalorganic chemical vapor deposition method. Sapphire with (0001) orientation (C-face) was used as the substrate. After growth, the samples were heat-treated under flowing N2, at temperatures ranging from 600 to 850°C. The p-type activation arises from the dissociation of electrically inactive Mg–H complexes and the neutralization of the dissociated H+ during the annealing process. The annealing temperature dependence of hole concentration and hole mobility was studied. The p-type activation process resulted in a different maximum hole concentration and an optimum annealing temperature. Subsequent microstructural characterization of our samples revealed that the dislocations play a key role in p-type conductivity and may explain the difference observed in the electrical properties. Indeed, the analyses of transmission electron microscopy (TEM) images and X-ray diffraction (XRD) data show that Mg-doped GaN exhibits a different X-ray rocking curve full width at half maximum (FWHM) and dislocation density. Furthermore, it was found that the higher the dislocation density, the higher the hole concentration. Therefore, we suggest that dislocations could act as a migration path or a neutralizing source for dissociated hydrogen impurities.