Yoshiki Naoi

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.

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.

MOCVD growth of InAsN for infrared applications

The ternary alloy InAsN has been grown by MOCVD for the first time. The growth was performed at 70 Torr using TMI, AsH3 and NH3 as source gases and GaAs(100) as substrate. The growth temperature and AsH3 to NH3 gas phase ratio, NH3AsH3, were changed from 550 to 750°C and 100 to 1000, respectively. InAs was also grown at 100 Torr and 500°C. All the samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) at room temperature. The increase in growth temperature and gas phase ratio of NH3 made the nitrogen solid composition larger. All the samples have a direct band gap which decreases with increasing nitrogen content. The smallest band gap energy of 0.12 eV at room temperature was obtained for InAs0.939N0.061.

Copper gate AlGaN/GaN HEMT with low gate leakage current

Copper (Cu) gate AlGaN/GaN high electron mobility transistors (HEMTs) with low gate leakage current were demonstrated. For comparison, nickel/gold (Ni/Au) gate devices were also fabricated with the same process conditions except the gate metals. Comparable extrinsic transconductance was obtained for the two kinds of devices. At gate voltage of -15 V, typical gate leakage currents are found to be as low as 3.5/spl times/10/sup -8/ A for a Cu-gate device with gate length of 2 /spl mu/m and width of 50 /spl mu/m, which is much lower than that of Ni/Au-gate device. No adhesion problem occurred during these experiments. Gate resistance of Cu-gate is found to be about 60% as that of NiAu. The Schottky barrier height of Cu on n-GaN is 0.18 eV higher than that of Ni/Au obtained from Schottky diode experiments. No Cu diffusion was found at the Cu and AlGaN interface by secondary ion mass spectrometry determination. These results indicate that copper is a promising candidate as gate metallization for high-performance power AlGaN/GaN HEMT.

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.

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.

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.

High-Performance 348 nm AlGaN/GaN-Based Ultraviolet-Light-Emitting Diode with a SiN Buffer Layer.

A 348 nm ultraviolet-light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW) with a high optical power is reported. In this structure, a thin SiN buffer is introduced before the growth of a conventional low-temperature GaN buffer layer. Such a buffer layer can dramatically reduce the density of threading dislocation as we have previously reported. Since the optical performance of UV-LED is generally known to be sensitive to the density of threading dislocations, unlike the InGaN/GaN- based blue LED, our UV-LED has a higher optical power than that of a similar structure but without a SiN buffer layer. Since our new buffer technology is much easier than the so-called epitaxial lateral overgrowth (ELO) or pendeo-epitaxy method, it is highly recommended for use in the fabrication of GaN-based optical devices, particularly AlGaN/GaN-based UV-LED.

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.