Hisao Saito

Efficient and high-power AlGaN-based ultraviolet light-emitting diode grown on bulk GaN

By introducing thick bulk GaN as a substrate, we improved the performance of an AlGaN-based ultraviolet (UV) light-emitting diode (LED). The output power exceeds 3 mW at the injection current of 100 mA under a bare-chip geometry. Internal quantum efficiency is estimated as more than 80%, and the peak wavelength is 352 nm. The maximum power exceeds 10 mW at a large current injection of 400 mA, with an operation voltage of less than 6 V. These results indicate that an efficient UV LED is intrinsically possible by the combination of appropriate device design and the nitride substrate. By introducing packaging technology to enhance extraction efficiency, we will have a compact and efficient UV light source in the wide wavelength range of 200–360 nm, similar to conventional longer-wavelength LEDs.

(AlAs)0.5(GaAs)0.5 fractional‐layer superlattices grown on (001) vicinal surfaces by metalorganic chemical vapor deposition

(AlAs)0.5(GaAs)0.5 fractional‐layer superlattices with a new periodicity perpendicular to the growth direction was successfully grown by metalorganic chemical vapor deposition on (001) GaAs substrates slightly misoriented toward [110]. The atomic structures were analyzed by x‐ray superlattice satellite diffraction. Superlattice periods were exactly the same as the mean distance of each atomic step on the (001) vicinal surface. The results indicate that lateral growth from nucleation at the step edge is the dominant process compared with the two‐dimensional nucleation on atomically flat terraces.

(AlAs)1/2(GaAs)1/2 fractional‐layer superlattices grown on (001) vicinal GaAs substrates by metal–organic chemical vapor deposition

(AlAs)1/2(GaAs)1/2 fractional‐layer superlattices with a new periodicity perpendicular to the growth direction are successfully grown, by metal–organic chemical vapor deposition on (001) GaAs substrates, slightly misoriented towards [110]. The periodic structure in the lateral [110] direction is analyzed by x‐ray superlattice satellite diffraction and high‐resolution transmission electron microscopy(TEM). Superlattice width along [110] direction are exactly the same as the mean distance of each monolayer step on the (001) vicinal surface. The satellite peak intensity increases with increasing AsH3 partial pressure, and is stronger for substrates misoriented towards [110] than those misoriented towards [110]. The result can be explained using the lateral growth model, taking into account the dangling bond at the step edge. The superlattice image is clearly observed by TEM, which shows that the superlattice periods are almost uniform everywhere.

Excitonic luminescence and absorption in dilute GaAs1−xNx alloy (x<0.3%)

Dilute GaAs1−xNx alloys (x<0.3%) were grown by metalorganic vapor phase epitaxy to investigate their photoluminescence and photoluminescence excitation characteristics. Photoluminescence excitation spectra show clear excitonic absorption peaks at low temperatures and their peak energy drastically decreases with increasing nitrogen concentration due to the band-gap bowing in the GaAsN system. This result indicates that the band-gap bowing starts at a nitrogen concentration as low as 1018 cm−3, and its bowing parameter is −22 eV. According to this band-gap bowing, the GaAsN alloys show two photoluminescence lines whose peak energy decreases with increasing nitrogen concentration. Their dependence on the nitrogen concentration suggests that these lines correspond to excitonic and carbon-related transitions in the GaAsN alloy.

Milliwatt operation of AlGaN-based single-quantum-well light emitting diode in the ultraviolet region

By introducing a single-quantum-well active layer and a high-Al-content carrier blocking layer, the output power of an AlGaN-based ultraviolet light-emitting diode has been improved by one order of magnitude. Optical output of 1 mW was achieved at the emission peak wavelength of 341–343 nm.

InAs)1(GaAs)1 Layered Crystal Grown by MOCVD

(InAs)1(GaAs)1 layered crystals (LC) were successfully grown on (100) InP substrate for the first time. The growth method was metallorganic chemical vapor deposition (MOCVD). Monolayer growth was achieved by low growth rate and rapid change of gas composition. Layered crystals were confirmed by X-ray diffraction peaks at the (100) and (300) positions. The electrical and optical properties of (InAs)1(GaAs)1 LC were also studied.

Submilliwatt operation of AlGaN-based ultraviolet light-emitting diode using short-period alloy superlattice

Over 0.1 mW ultraviolet output was achieved by an AlGaN-based light-emitting diode. To realize a highly conductive and ultraviolet-transparent layer, a short-period alloy superlattice was introduced. The device was fabricated on SiC substrate. Low electric resistivity due to the short-period alloy superlattice and the high thermal conductivity of the SiC substrate enabled large current injection of up to 1.7 kA/cm2. The emission was monochromatic band-edge emission about 350 nm in wavelength without significant D–A and/or deep emissions.

AlGaN-based ultraviolet light-emitting diodes grown on bulk AlN substrates

We have confirmed the potential of a bulk AlN substrate for high current operation of nitride ultraviolet-light-emitting diodes (UV-LEDs). For the high flux UV extraction from nitride UV-LEDs, transparency and high thermal conductivity of the substrates are important issues. The bulk AlN is one of the best candidates, because it satisfies requirements above, and has the same crytallographic symmetry with those of AlGa(In)N families, which is beneficial to the high-quality crystal growth of the nitride device structures. We formed AlGaN-based UV-LEDs on a bulk AlN substrate and compared its performance with that of a reference device grown on an AlN-template grown on a sapphire substrate. The output power linearly increases with a saturation injection current of 300 mA, which is two times higher than that of the reference device. The emission spectrum under high current injection is much more stable than that of conventional substrate.

Ideal Crystal Growth from Kink Sites on a GaAs Vicinal Surface by Metalorganic Chemical Vapor Deposition

(AlAs)1/2(GaAs)1/2 fractional-layer superlattices are grown on a (001) vicinal substrate by metalorganic chemical vapor deposition. The substrate is misoriented to [10] by 1.92° and [110] by 0.10{degree}. A large surface area of the superlattice is observed by bright field transmission electron microscopy (TEM). The superlattice periods are uniform and no undulation is observed, which is the same quality as that of normal superlattices. The results suggest the ideal growth from a kink site occurs during MOCVD growth, and the distances between kink sites are equal.

Terrace width ordering mechanism during epitaxial growth on a slightly tilted substrate

Abstract The change of parallel terrace widths during epitaxial growth is examined. The terraces are formed on a substrate slightly tilted to [ 1 10] direction on (001) GaAs surface, and are several tens of nanometers wide. In an analysis of the kinetics of step growth, terrace width ordering is shown by introducing an anisotropy of the Ga diffusion constant. This anisotropy is explained by a microscopic surface structure near the steps. The adsorbed impurities are shown to weaken the ordering mechanism and also to cause instability in a short range fluctuation of the terrace width.