Lisa Sugiura

Room Temperature Pulsed Operation of Nitride Based Multi-Quantum-Well Laser Diodes with Cleaved Facets on Conventional C-Face Sapphire Substrates

We demonstrate room temperature pulsed operation of nitride based multi-quantum-well (MQW) laser diodes with cleaved mirror facets grown on a conventional C-face sapphire substrate. Cleavage was performed along the direction of the sapphire substrate, and the resultant facet was analyzed using an atomic force microscope (AFM) and theoretical calculation. A single peak emisson, at a wavelength of 417.5 nm, with a full width at half-maximum of 0.15 nm, was obtained. The threshold current density of the laser was 50 kA/cm2 and a voltage for the threshold current was 20 V.

Dislocation motion in GaN light-emitting devices and its effect on device lifetime

In this study, dislocation motions in GaN-based materials and devices were quantitatively estimated in order to determine why GaN-based light-emitting diodes have remarkable reliability and longevity in spite of extremely high dislocation density. The dislocation velocity of GaN-based materials was calculated by estimating the activation energy of dislocation, and comparing it with that of GaAs, which are typically used for light-emitting devices. It was estimated that the dislocation mobility of GaN-related materials was lower than that of GaAs by a factor of approximately 10−10–10−16, at room temperature. Furthermore, dislocation velocity under current injection became about 10−20 times lower than that of GaAs, under the assumption that the dislocations in GaN-related materials do not act as nonradiative recombination centers. The possibility of degradation under high current densities and high temperature, as would be found in GaN-based laser diodes, is also discussed.

Effects of thermal treatment of low-temperature GaN buffer layers on the quality of subsequent GaN layers

The surface morphology and crystalline quality of GaN layers grown by metalorganic chemical vapor deposition on sapphire (0001) substrates were investigated for various thermal treatment conditions of low-temperature (LT)-grown GaN buffer layers. The surface morphology and crystalline quality of subsequently grown high-temperature (HT) GaN layers strongly depended on thermal effects during the temperature ramping process after LT growth of the buffer layers. We have found that the defect density and structure are affected by this temperature ramping process, and that the generation of growth pits is closely related to defects in the HT-GaN layers. High-quality HT-GaN layers with specular surface morphology were obtained with optimum growth and ramping conditions for the LT-GaN buffer layers. Furthermore, the role of thermal treatment during the temperature ramping process was identified, and mechanisms of nucleus formation, HT-growth initiation on the LT-GaN buffer layers, and defect formation are propose...

p-type conduction in as-grown Mg-doped GaN grown by metalorganic chemical vapor deposition

We have clarified the effect of H2 and NH3 on the passivation of Mg acceptor in p-type GaN films grown by metalorganic chemical vapor deposition. It has been found that the small amount of H2 carrier gas strongly influences the electrical property of the Mg-doped GaN films. Low-resistivity p-type GaN has been obtained by H2-free growth without any post-treatments. Its acceptor concentration is as high as that obtained by conventional H2-rich growth with subsequent thermal annealing. It has also been clarified that hydrogen produced by NH3 dissociation does not prevent Mg from electrically activating in H2-free growth.

Comparison of degradation caused by dislocation motion in compound semiconductor light-emitting devices

Dislocation glide velocities in GaAlAs/GaAs, InGaAsP/InP, and GaN-based light-emitting devices are estimated. These results are consistent with device degradation rates related to dislocation motion. It is clarified that the long lifetime of GaN-based devices with high dislocation density is principally due to extremely small dislocation mobility, partly due to small shear stress for dislocation motion, and due little to the radiation enhancement effect.

Characterization of InGaN multiquantum well structures for blue semiconductor laser diodes

In0.15Ga0.85N/GaN and In0.15Ga0.85N/In0.05Ga0.95N multi quantum well (MQW) structures grown on (0001) sapphire substrates were investigated by high-resolution x-ray diffraction and cross-sectional transmission electron microscopy. The results show that ultrathin MQWs with fairly good crystallinity and precisely controlled clear parallel interfaces were grown. Laser diode structures with MQWs were also studied, and the results suggest that these superlattice structures retain their high quality even after being subjected to high temperatures during the subsequent growth of p-type GaN as the optical guiding layer, p-type GaAlN as the cladding layer, and p-type GaN as the contact layer.

Reactive Ion Beam Etching and Overgrowth Process in the Fabrication of InGaN Inner Stripe Laser Diodes

Dry etching and overgrowth process techniques have been developed for the fabrication of InGaN inner stripe laser diodes. The dry etch damage at the etched surface and the overgrown interface was estimated using X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS). As a result, the etch damage at the overgrown interface was shown to be repaired during the overgrowth. Groove stripes formed on the GaN layer by reactive ion beam etching (RIBE) were overgrown by GaN using metalorganic chemical vapor deposition (MOCVD). The morphology of the overgrown surface was observed by scanning electron microscopy (SEM). The grooves of widths less than 10 µm were buried completely at 1100°C and smooth, flat surfaces were obtained. We have fabricated the InGaN inner stripe (IS) lasers and reported on the first pulsed lasing operation of InGaN inner stripe lasers, fabricated by dry etching and overgrowth techniques.

Characteristics of Mg-Doped GaN and AlGaN Grown by H2-Ambient and N2-Ambient Metalorganic Chemical Vapor Deposition

The characteristics of Mg-doped GaN and AlGaN grown by H2-ambient and N2-ambient metalorganic chemical vapor deposition are compared and discussed. While as-grown Mg-doped GaN and AlGaN grown under H2-ambient show high resistivity, p-type GaN and AlGaN with high acceptor concentration were obtained by N2-ambient growth without any post-treatment. The hydrogenation process of the Mg acceptor and the dissociation process of hydrogen atoms from Mg–H complexes might occur simultaneously in the case of N2-ambient growth. The films grown under N2-ambient were clearly shown to have superior characteristics in uniformity and reproducibility to those grown under H2-ambient, since the characteristics of the films grown under N2-ambient are steady over the wide range of growth temperature. Our results indicate that the N2-ambient growth is suitable for the mass production of GaN-based light-emitting devices in the respects that the device fabrication process can be simplified and that the uniformity and the reproducibility of the layer properties are greatly improved.

Band-gap separation in InGaN epilayers grown by metalorganic chemical vapor deposition

Two energetically-separated resonance structures were found in photoreflectance (PR) spectra of three-dimensional InxGa1−xN epilayers (0.05⩽x⩽0.2) grown by metalorganic chemical vapor deposition. Energy difference between the two structures was nearly constant about 50 meV for all x examined. The broadening parameter of each structure was nearly independent of x (∼50 meV), indicating that each separated region has rather homogeneous net potential fluctuation. The photoluminescence (PL) peak energy agreed with the resonance energy of the lower-energy PR structure, showing nearly zero Stokes-like shift at room temperature. Observation of a single PL peak indicated that photoexcited carriers were effectively corrected into the separated regions having lower PR resonance energy.

Orientation dependence of HgCdTe epitaxial layers grown by MOCVD on Si substrates

Orientation dependence of HgCdTe epilayers grown by MOCVD on Si substrates was studied. Substrate orientation is considered to be one of the most sensitive factors to enable hetero-epitaxial growth on silicon substrates, especially in the case of a low temperature growth process. The present work was carried out with characterized features of a low temperature process for HgCdTe growth on Si and using a thin CdTe buffer layer. The (100), (100) misoriented toward [110], (311), (211), (111), and (331) oriented Si substrates were used in the present work. The best results were obtained on (211)Si substrates with an x-ray full width at half maximum of 153 arc sec for a 5 (im thickness HgCdTe layer and 69 arc sec for a 10 um thickness layer. It was found that the effective lattice mismatch of CdTe/Si heterosystem was reduced to 0.6% (for the 611 lattice spacing of CdTe and 333 spacing of Si) in the case of (133)CdTe/(211)Si.