Shu Goto

Dislocation related issues in the degradation of GaN-based laser diodes

We investigate degraded GaN-based laser diodes (LDs) on epitaxial lateral overgrown GaN layers in terms of dislocations. Almost all of the threading dislocations that appear in the wing regions are a-type dislocations. Their origins are the lateral extension of dislocations from the seed regions that contingently bend upwards to the episurface. Comparing short-lived LDs and long-lived LDs that have almost the same power consumption, we find that the relative levels of dislocation densities in their respective active layers are different. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations are observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. The degradation rate is almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.

GaN-Based High Power Blue-Violet Laser Diodes

The epitaxial lateral overgrowth (ELO) technique is an important technology for improving the characteristics of GaN-based laser diodes (LDs). The photoluminescence intensities from GaN and GaInN multiple quantum well active layers in the ELO-GaN wing region were found to be higher than those in the seed region. This indicates that the density of dislocations in the wing region could be reduced significantly. This is evidenced by dislocation densities of less than 106 cm-2 as determined from transmission emission microscopy and etching-pit-density measurements. The cleaved facets of LDs on ELO-GaN and sapphire were observed by atomic forced microscopy. Although the roughness of GaN cleaved facets on sapphire were high (Ra>10 nm), the roughness in the ELO-GaN wing region was found to be as smooth as that of GaAs cleaved facet (Ra<1 nm). The characteristics of LDs on ELO-GaN were found to be superior to those on sapphire as a result of smoother facets and lower dislocation densities.

Mechanism of Multiatomic Step Formation during Metalorganic Chemical Vapor deposition Growth of GaAs on (001) Vicinal Surface Studied by Atomic Force Microscopy

The detailed behavior and mechanism of multiatomic step formation processes during metalorganic chemical vapor deposition (MOCVD) of GaAs on vicinal surfaces are systematically investigated using an atomic force microscope (AFM). Under the low growth rate condition, the step flow growth mode with regular stripe is obtained, and the final terrace width is almost independent of the substrate misorientation angle. The result suggests that the barrier height difference for surface migrating Ga atoms between the terrace site and the step site is small, and the final terrace width is mainly determined by a migration length. Three-dimensional nucleation and growth mode with irregular steps are also obtained under the high growth rate condition. However, as the AsH3 partial pressure increases, irregular steps are no longer observed. For these results, we discuss the multiatomic step formation mechanism.

High-Power AlGaInN Laser Diodes with High Kink Level and Low Relative Intensity Noise

High-power AlGaInN laser diodes (LDs) with both high kink level and low relative intensity noise (RIN) are successfully fabricated. A kink level of higher than 100 mW is obtained by using a new ridge structure and by narrowing the ridge width down to 1.5 µm. This structure consists of a thin Si on SiO2 multiple buried layers instead of the conventional thick SiO2 buried layer. The low RIN of these devices is obtained by decreasing the threshold current; under optical feedback with high-frequency modulation, the RIN values are as low as -125 dB/Hz. As the RIN of these devices exhibits a tendency to deteriorate with increasing operating current, the lifetime criterion was redefined as the point at which the operating current has increased by 20%. Even under this stricter lifetime criterion, the estimated lifetime of these LDs exceeds 15,000 h under 30 mW cw operation at 60°C.

AlGaInN laser diodes grown on an ELO-GaN substrate vs. on a sapphire substrate

The lifetime of AlGaInN-based violet laser diodes is restricted by the high dislocation densities caused by the large difference between the lattice constants and thermal expansion coefficients of GaN and sapphire. In order to reduce the dislocation density, epitaxial lateral overgrowth (ELO) techniques have been proposed, leading to the higher performance of AlGaInN lasers. In this work, we compared the lasing characteristics of AlGaInN lasers grown on ELO-GaN with those grown on sapphire substrates.

Over 100-mW blue-violet laser diodes for Blu-ray Disc system

400-nm-band GaN-based blue-violet laser diodes (LDs) operating with a high output power of over 100 mW have been successfully fabricated. A new ridge structure, in which the outside of the ridge was covered with a stacked layer of Si on SiO2 and the ridge width was as narrow as 1.4 μm, was applied to realize the stable lateral-mode operation. A layer structure around the active layer was carefully designed so as to ensure a high COD level. The lasers have been operated stably for more than 500 h under 130-mW pulsed operation at 60°C. From ambient temperature dependence of the device lifetime, the empirical activation energy was estimated as 0.32 eV. These results indicate that this LD is suitable for next-generation Blu-ray Disc system.

Effect of Growth Conditions on Electrical Properties of Si-Doped In0.52Al0.48As Grown by Metalorganic Vapor Phase Epitaxy

A systematic study of the electrical properties of Si-doped In0.52Al0.48As grown under various metalorganic vapor phase epitaxy (MOVPE) conditions such as V/III ratio and growth temperature is carried out. It is demonstrated that either high V/III ratios (≥64) or high growth temperatures (≥720°C) are necessary for obtaining good InAlAs electrical properties. For a small V/III ratio (=32) and low growth temperatures (≤700°C), a large discrepancy is found in Hall carrier concentration (nHall), ionized impurity concentration (NC – V), and Si concentration (NSi); NC – V>NSi>nHall; which can be explained by the dual formation of donor and acceptor deep levels. SIMS results suggest that carbon and oxygen impurities are not candidates for these deep levels, and other origins such as intrinsic defects, which are closely related to growth conditions, are applicable.

Schottky Characteristics of InAlAs Grown by Metal-Organic Chemical Vapor Deposition

Schottky characteristics of InAlAs grown by metal-organic chemical vapor deposition (MOCVD) have been evaluated. InAlAs Schottky characteristics are strongly affected by MOCVD growth temperature. The reverse current of InAlAs grown at 700°C is more than one order of magnitude larger than that at 750°C. From deep-level transient spectroscopy (DLTS) measurements, electron traps with activation energies of 0.45, 0.33 and 0.15 eV have been observed in InAlAs grown at 700°C. The results of C-V, Hall and secondary ion mass spectrometry (SIMS) measurements suggest that the trap is acceptor-type and seems to be related not to impurities but to intrinsic defects. The mechanism of the large reverse current in InAlAs grown at 700°C is believed to be due to the conduction through the trap.

Growth Behavior and Mechanism of Alkyl-Desorption-Limited Epitaxial Growth of GaAs on Exactly Oriented and Vicinal Substrates

New atomically controlled epitaxial growth, called alkyl-desorption-limited epitaxial (ADLE) growth, is studied on (001) exactly oriented and vicinal GaAs substrates. In ADLE growth, the growth rate is limited by the desorption rate of alkyl from organometals rather than by saturation of alkyl adsorption. First, the proposed ADLE growth mechanism is quantitatively confirmed by comparing a new theory of growth based on the alkyl-desorption rate equations with the experimental growth data taken on (001) exactly oriented substrates. Next, the behavior of multi-atomic steps on vicinal substrates is studied using an atomic force microscope (AFM). It is found that the multi-atomic-step heights are reduced during ADLE growth. This new phenomenon is explained by the ADLE growth mechanism.

Recent progress in high-power blue-violet lasers

GaN-based 400-nm high-power lasers have been developed as a light source for optical disk recording, laser printer, and displays. The lasers in systems for optical disks with capacities greater than 50 GB are required to operate at high output power exceeding 100 mW in order to write information to a dual-layer disk. To satisfy this performance requirement, we examined the absorption loss in Mg-doped GaN super-lattice layers, and successfully reduced the internal loss by isolating these layers from the active layer.