Kentaro Nagamatsu

Low-Leakage-Current Enhancement-Mode AlGaN/GaN Heterostructure Field-Effect Transistor Using p-Type Gate Contact

An enhancement-mode AlGaN/GaN heterostructure field-effect transistor (HFET) using a p-type GaN gate was fabricated. Its leakage current density was 18.2 µA/mm at a gate–source bias of 0 V and a drain–source bias of 20 V, indicating a good pinched-off operation.

Growth and conductivity control of high quality AlGaN and its application to high-performance ultraviolet laser diodes

A microstructure in AlGaN with a whole compositional range on an AlN/sapphire template is systematically investigated. At the interface between AlGaN and AlN, misfit dislocation density increases with Ga composition in AlGaN. However, most misfit dislocations bend owing to the compressive stress and form loops, by which threading dislocation density is markedly reduced if the thickness exceeds several microns. The effective acceptor energy of Mg in Al0.5Ga0.5N is found to depend on Mg concentration with a negative one-third power. A SiO2/AlN dielectric multilayer mirror is very effective for controlling the reflectivity of the Fabry-Perot resonator mirrors at the cleaved edge of the ultraviolet (UV) laser diodes (LDs). A UV LD with an emission wavelength of 358 nm shows a threshold current density as low as 3.9 KA/cm2 at room temperature.

The interface analysis of GaN grown on 0° off 6H-SiC with an ultra-thin buffer layer

Previously, we reported a growth method by metalorganic vapor phase epitaxy using a single two-dimensional growth step, resulting in 1.2-µm crack-free GaN directly grown on 6H-SiC substrate. The introduction of Al-treatment prior to the standard GaN growth step resulted in improved surface wetting of gallium on the SiC substrate. Transmission electron microscope and energy dispersive spectrometer analysis of the epitaxial interface to the SiC determined that an ultra-thin AlGaN interlayer had formed measuring around 2–3 nm. We expect our growth technique can be applied to the fabrication of GaN/SiC high frequency and high power devices.

Comparison of the simulation and experiments of the nitride-based UV light emitting diodes

In attempt to prepare a high performance AlxGa1-xN based UV-B LED, a computer simulation has been performed on a typical UV-LED structure to find out the effect of threading dislocations on non-radiative recombination process. UVB LED structures were formed on using GaN and AlN based layers for comparison. Cracks were generated in the device structure formed on GaN underlayer. No cracks were observed on the device structure formed on AlN under layer. Much better structure was formed when the base AlN was grown by high temperature MOVPE.

Correlation between dislocations and leakage current of p-n diodes on a free-standing GaN substrate

Dislocations that cause a reverse leakage current in vertical p-n diodes on a GaN free-standing substrate were investigated. Under a high reverse bias, dot-like leakage spots were observed using an emission microscope. Subsequent cathodoluminescence (CL) observations revealed that the leakage spots coincided with part of the CL dark spots, indicating that some types of dislocation cause reverse leakage. When etch pits were formed on the dislocations by KOH etching, three sizes of etch pits were obtained (large, medium, and small). Among these etch pits, only the medium pits coincided with leakage spots. Additionally, transmission electron microscopy observations revealed that pure screw dislocations are present under the leakage spots. The results revealed that 1c pure screw dislocations are related to the reverse leakage in vertical p-n diodes.

Preflow trimethylaluminum treatment effect on GaN growth on SiC with an ultrathin interlayer

Previously, we reported a growth method for GaN on SiC by metalorganic vapor phase epitaxy. By using a preflow trimethylaluminum treatment, the poor wetting problem of gallium on the SiC surface was alleviated, resulting in a 1.2-µm-thick crack-free GaN grown on an on-axis 6H-SiC(0001) substrate via an ultrathin AlGaN interlayer. In this study, the impact of the preflow trimethylaluminum treatment time is investigated to understand why a crack-free epilayer was realized. To demonstrate the electrical performance of devices formed by our technique, GaN/SiC vertical Schottky barrier diodes were fabricated and compared with GaN/AlN/SiC and GaN/GaN vertical Schottky barrier diodes. Compared with diodes including a high-resistance AlN interlayer, the series resistance of GaN/SiC Schottky barrier diodes incorporating the ultrathin interlayer with 5 s of TMAl treatment showed a marked reduction from 4.0 × 107 to 2.0 × 10−1 Ωcm2. The ultrathin interlayer growth technique is expected to be applied in future GaN/SiC hybrid high-power and high-frequency devices.

Activation of Mg-Doped p-Type Al0.17Ga0.83N in Oxygen Ambient

Annealing in oxygen ambient was found to be effective for realizing a high hole concentration in p-type Al0.17Ga0.83N:Mg. The maximum hole concentration obtained was 1.3×1016 cm-3 at room temperature. Hydrogen dissociation from Mg-doped Al0.17Ga0.83N is found to be enhanced by annealing in a flow of O2 compared with annealing in a flow of N2.

Nitride-Based UV Lasers

The ultraviolet (UV) laser diode (LD) is attracting much attention for various novel applications such as in medical engineering, sterilization and high density optical storage. Group III nitrides are one of the most promising candidates to realize UV LD. The external quantum efficiency of group-Ill nitride-based light-emitting diodes (LEDs) with emission wavelength shorter than 360 nm is still far inferior to that of nitride-based visible-short-wavelength LEDs. Simulation results show that there is no theoretical barrier which hiders the realization of short wavelength UV LDs (Chow, 2005). At the moment, however, the shortest emission wavelength of nitride-based LD is limited to 343nm on SiC (Edmond, 2004) and 350.9 nm on a sapphire substrate (Iida, 2004). In order to overcome the barrier for emission wavelength and to realize much shorter wavelength UV LDs, in addition to the control of conductivity for both n-type and p-type layers, the growth of low-dislocation-density high-Al-content AlGaN with low internal loss is essential.

Evaluation of excess In during metal organic vapor-phase epitaxy growth of InGaN by monitoring via in situ laser scattering

Using an in situ laser absorption and scattering method, the surface roughness and incorporation of In in InGaN layers grown by metal organic vapor-phase epitaxy (MOVPE) were monitored. We observed that the laser light with energy higher than the GaN bandgap was fully absorbed in a GaN layer with a smooth film surface. On the other hand, we observed that the scattering laser light from the surface when the roughness of the InGaN surface increased owing to the formation of In droplets. Laser light with energy lower than the GaN bandgap was weakly absorbed by the GaN layer and was scattered at the back surface of the wafer. Furthermore, laser light intensity decreased during InGaN growth because of In incorporation. The threshold of trimethyl-In (TMIn) for the formation of In droplets as a function of growth temperature was determined using our in situ system. Moreover, we observed that the In droplets were removed by thermal or H2 treatment. The results indicate that multiwavelength laser absorption and scattering enable the optimization of the growth conditions for In-rich InGaN.

Increased pressure digital metalorganic vapor phase epitaxy system with high-speed switching valves for growing high-In-content GaInN

Increased-pressure metalorganic vapor phase epitaxy (MOVPE) system with high speed switching valves is found to be effective for growing In-rich GaInN at a high temperature. High-speed switching valves enable the atomic layer epitaxy of high quality AlGaN at a low temperature, by which we can grow thin AlGaN capping layer without the thermal decomposition of underlying In-rich GaInN. This new growth technology sheds light on the digital alloy growth for the development of high-efficiency nitride-based visible long -wavelength light emitters.