Satoshi Kurai

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.

Pressure-Controlled Solution Growth of Bulk GaN Crystals under High Pressure

We have developed the pressure-controlled solution growth (PC-SG) method and indicated the validity of this method for the growth of GaN single crystals. We have investigated the effect of the supersaturation of nitrogen atoms on the size of a GaN single crystal and its morphology. GaN single crystals with a surface area of about 120 mm2 and/or with good morphology were obtained at a rate of increase of nitrogen pressure less than 49 MPa/h. Structural and photoluminescence properties of GaN single crystals were examined and determined that these crystals with good morphology had good crystallinity. Furthermore, we have introduced a new high-pressure furnace and developed the multi-crucible system. The GaN single crystal with a surface area of 300 mm2 was obtained by natural nucleation. The GaN crystal with 47 mm diameter, whose normal axis was the (0001) plane, could be grown on a sapphire substrate.

Photopumped Stimulated Emission from Homoepitaxial GaN Grown on Bulk GaN Prepared by Sublimation Method

We have observed stimulated emission at room temperature from a photopumped homoepitaxial GaN for the first time. A homoepitaxial layer was grown by atmospheric metalorganic chemical vapor deposition (MOCVD) on small hexagonal bulk GaN prepared by the sublimation method. The lasing threshold of the pumping power density is 0.86 MW/cm2 and the stimulated emission is polarized with its electric vector perpendicular to the c-axis.

Superior Illuminant Characteristics of Color Rendering and Luminous Efficacy in Multilayered Phosphor Conversion White Light Sources Excited by Near-Ultraviolet Light-Emitting Diodes

We have demonstrated that a phosphor conversion (PC) white light-emitting diode (LED) light source composed of multi-layered (ML) red, green, and blue (RGB) phosphors with a near-ultraviolet (n-UV) LED as an excitation source shows excellent color rendering, luminous efficacy, and luminous flux. The ML-phosphor structure consists of vertically stacked layers and could significantly reduce cascade excitation loss compared with a mixed RGB PC white LED. A high luminous efficacy of 82 lm/W with a high color-rendering index (CRI) of over 90 was achieved using a ML-RGB PC white LED. The ML-RGB structure also enables a white LED with an ultrahigh CRI of 99 and a high luminous efficacy comparable to that of a high CRI fluorescent lamp. We have also fabricated a high-power ML-PC white LED excited by an n-UV LED. In one package, a high luminous flux of 61 lm at a high CRI of 94 was obtained.

Growth and characterization of thick GaN by sublimation method and homoepitaxial growth by metalorganic chemical vapor deposition

Thick GaN, 10-30 µm, was grown on sapphire substrate by a sublimation method for the first time. GaN was homoepitaxially grown on this thick layer by metalorganic chemical vapor deposition to obtain a high-quality layer. Stimulated emission from a photopumped homoepitaxial GaN grown on thick GaN prepared by the sublimation method at room temperature was demonstrated. The threshold of the stimulated emission was estimated to be 1.04 MW/cm2.

GaN Bulk Substrates for GaN Based LEDs and LDs

In the present work, we discuss the importance of bulk GaN substrates for GaN based LEDs and LDs. As crystal growth methods for growing bulk GaN, we study the hydride vapor phase epitaxial (HVPE) method and the pressure-controlled solution growth (PC-SG) method. The recent results obtained by these two methods are presented. The maximum size of the grown single crystals was 3 × 4 cm2 for the HVPE method and 12 mm diameter for the PC-SG method. Both methods can provide high quality GaN substrates, with a low dislocation density of the level of less than 106 cm–2.

Nucleation Control in the Growth of Bulk GaN by Sublimation Method

Nucleation of GaN crystallites by sublimation method on sapphire (0001), scratched sapphire (0001), SiO2, Si (111), and metalorganic chemical vapor deposition(MOCVD)-GaN/sapphire (0001) was investigated. The density of the nuclei on SiO2 and Si (111) was estimated to be 6 × 103/cm2. On the other hand, a continuous film rather than discrete crystallites was obtained on both the MOCVD-GaN and the scratched sapphire. Growth nucleation control was performed by partly covering the MOCVD-GaN or scratched sapphire (0001) with SiO2. As a result, hexagonal columns about 200 µm in diameter and 200 µm high were selectively and uniformly grown at the window sites. The diameter of each hexagonal column was much larger than the width of each window. This method can be used for device processing utilizing the crystallites.

Characterization of bulk GaN grown by sublimation technique

We have characterized a bulk GaN grown by sublimation technique by means of the high resolution X-ray diffraction measurements, the secondary ion mass spectrometry measurements, the Raman spectroscopy measurements and the resistivity measurements. The results indicated that a bulk GaN was of relatively high purity with a very high crystal perfection.

Surface Pretreatment of Bulk GaN for Homoepitaxial Growth by Metalorganic Chemical Vapor Deposition

Homoepitaxial growth of GaN on bulk GaN prepared by the sublimation method was performed by metalorganic chemical vapor deposition (MOCVD). Two kinds of bulk substrates were used for homoepitaxial growth: (i) a polished bulk GaN C-face, selectively grown on MOCVD-GaN on a sapphire (0001) substrate which was partly covered by SiO2 (?quasi-bulk? GaN) and (ii) a bulk GaN spontaneously nucleated on the source powder used in the sublimation method (?free-standing? bulk GaN). A smooth surface and strong CL band-edge emission from the homoepitaxial layer were obtained when proper surface pretreatment of the substrate, such as H3PO4 etching, RIE and photoassisted wet chemical etching (PAW), was adapted before loading into the MOCVD reactor. The CL intensity of the band-edge emission was stronger and the CL image more uniform for homoepitaxial GaN compared to that of heteroepitaxial GaN on sapphire. The dislocation density of the homoepitaxial layer was also decreased after proper surface treatment of the substrate.

Inhomogeneous distribution of defect-related emission in Si-doped AlGaN epitaxial layers with different Al content and Si concentration

The spatial distribution of luminescence in Si-doped AlGaN epitaxial layers that differ in Al content and Si concentration has been studied by cathodoluminescence (CL) mapping in combination with scanning electron microscopy. The density of surface hillocks increased with decreasing Al content and with increasing Si concentration. The mechanisms giving rise to those hillocks are likely different. The hillocks induced surface roughening, and the compositional fluctuation and local donor-acceptor-pair (DAP) emission at hillock edges in AlGaN epitaxial layers were enhanced irrespective of the origin of the hillocks. The intensity of local DAP emission was related to Si concentration, as well as to hillock density. CL observation revealed that DAP emission areas were present inside the samples and were likely related to dislocations concentrated at hillock edges. Possible candidates for acceptors in the observed DAP emission that are closely related in terms of both Si concentration and hillock edges with lar...