We have investigated an influence of positive polarization charges generated at an interface between GaN barrier/p-AlGaN EB (Electron Blocking) layer in a blue-LED. Simulation results suggested that such polarization charges caused an electron overflow from QWs. The simulation results also indicated that sufficient acceptor doping at the interface could neutralize the positive polarization charges and suppress the electron overflow. We then demonstrated the electron overflow caused by the positive polarization charges and its suppression with sufficient Mg doping at the interface by monitoring emissions from an additional second QW inserted between the p-EB layer and the p-GaN layer. Finally we conclude that the contribution of the electron overflow is not significant for the efficiency droop in blue-LEDs.

The impact of device processing and plasma treatments at different plasma conditions on the electronic transport properties of GaN/AlGaN/GaN heterostructures was investigated as well as annealing in nitrogen atmosphere at 425°C. The electrical properties are characterized by Hall-effect measurements while electron spectroscopy and X-ray measurements are used to investigate changes in the surface chemical composition and in the layer structure, respectively. It is demonstrated that these layer structures are quite sensitive even to non-plasma based processing. Furthermore, treatments in SF6 and N2 based plasmas strongly affect the 2DEG properties of the heterostructure due to altering of the surface barrier accompanied by thinning of the layer structure. Depending on the layer structure and the plasma conditions used the electronic properties may be recovered by annealing.

We have fabricated light emitting diodes (LEDs) in which two active regions separated with a Mg-doped GaN intermediate layer were placed in a single pn junction toward periodic gain structures (PGS) for blue vertical-cavity surface emitting lasers (VCSELs). By current density dependence on a emission intensity ratio from two different active regions, we obtained a very stable emission intensity ratio over 1 kA/cm2. This result is also confirmed with the simulation result. Furthermore, we found that the difference of emission wavelength affect the carrier injection and the emission intensity ratio. On the basis of this result, the optimized well-balanced Mg concentration in the intermediate layer for the two identical active regions were estimated approximately 5 x 1018 cm-3.

Stress control using AlN/GaN superlattices (SLs) for epitaxy of GaN on 200 mm Si (111) substrates is reported. Crack-free 2 μm GaN layers were grown over structures containing 50 to 100 pairs of 3-5 nm AlN/10-30 nm GaN SLs. Compressive and tensile stress can be precisely adjusted by changing the thickness of the AlN and GaN layers in the SLs. For a constant period thickness, the effects of growth conditions, such as growth rate of GaN, V/III ratio during AlN growth, and growth temperature, on wafer stress were investigated.

Non-polar a-plane InGaN films were grown on a r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The growth temperature and Indium flux were varied to optimize the desired composition of In0.23Ga0.77N on the (11-20) a-plane GaN epilayer grown on a (1-102) r-plane sapphire substrate. The structural, morphological and optical properties of the optimized composition have been studied. It was found that highly a-axis oriented InGaN epilayers with no phase separation can be grown at 540 °C with In/Ga flux ratio of 0.72. The composition of indium incorporation in single phase InGaN films was found to be 23% as estimated by high resolution X-ray diffraction. The room temperature band gap energy of single phase InGaN layers was determined by photoluminescence measurement and found to be around 2.56 eV.

GaN epilayers were grown on m-plane (10-10) sapphire substrates using plasma assisted molecular beam epitaxy. Impact of nitridation on structural and optical properties of GaN film was investigated. The film grown on a nitridated surface resulted in a nonpolar (10-10) orientation while without nitridation caused a semipolar (11-22) orientation. The high resolution X-ray diffraction studies confirmed the orientation of the GaN films. X-ray rocking curve showed better crystallinity of semipolar as compared to nonpolar GaN. Atomic force microscopy showed smoother films in case of nonpolar GaN which might be in account of the nitridation treatment. Room temperature photoluminescence study showed nonpolar GaN to have higher value of compressive strain as compared to semipolar GaN film, which was further confirmed by room temperature Raman spectroscopy. Despite the fact that it is difficult to obtain high-quality nonpolar material due to the planar anisotropic nature of the growth mode, we hereby report the development of non-polar GaN of usable quality, on an m-plane sapphire, involving controlled steps of nitridation.

Gallium nitride (n-type) films of thickness 300nm were grown on c-plane sapphire substrates using plasma assisted molecular beam epitaxy (PA-MBE). High resolution X-ray diffraction and photoluminescence measurements were used to confirm the crystalline and optical qualities of the grown films. Metal-semiconductor Schottky diodes were fabricated using Pt as the Schottky metal and Al as the Ohmic metal contact. Metal-insulator-semiconductor Schottky diodes were also fabricated using HfO2 (10nm) as the insulator material. Diode parameters like barrier height and ideality factor were extracted from I-V measurements. Introduction of HfO2 as the insulator layer leads to better rectifying behavior (forward to reverse current ratio improves from 5.1 to 8.9) with a reduction in reverse leakage current (by 7.4 times), increase in barrier height (from 0.62eV to 0.74eV) and a reduction in ideality factor (from 6 to 4.1) of the Schottky diode.

Growth of GaN on Si(111) and Ge coated Si(111) using pulsed electron beam deposition (PED) process is reported. GaN was deposited on Si(111) and Ge/Si(111) at 600°C in an N2 environment without any surface pre-treatment such as pre-nitridation. X-ray diffraction confirmed that c-plane oriented GaN was grown. Photoluminescence showed near-band-edge emission, the intensity of which was improved with hydrogen passivation. Electrical characterization showed n-type conductivity with room temperature electron mobilities in the range of 300 cm2/V-sec.

Metal organic chemical vapor deposition, as well as material and basic device properties of nitride-based high electron mobility transistor structures on (111) silicon substrates varying in diameter from 4 to 8 inch were studied using in-situ and ex-situ characterization techniques. All substrates used for the growth of the nitride structures in this study were of SEMI standard thicknesses. The total thickness of the nitride structures was in the range of 1.5 – 5 µm. It is reported that nitride structures can be grown on 4, 6 and 8 inch diameter substrates with very similar post-growth wafer shape, material and device characteristics. It is also shown that their crystal quality, 2DEG transport properties and isolation blocking voltages can be improved by increasing nitride structure thickness while maintaining post-growth wafer bow and warp less than 50 µm. The maximum thickness of nitride structures that can be successfully grown on 8 inch diameter SEMI standard substrates seems to be limited to about 4.5 µm due to plastic deformation of Si. Blocking voltages of more than 700 V were achieved using 4.5 µm thick nitride-based high electron mobility transistor structures grown on 8 inch Si substrate.

We fabricated single-crystalline microspheres of wide-gap semiconductors with anisotropic crystal structures, such as ZnO and ZnSe, by laser ablation in superfluid helium and investigated their lasing properties. Whispering gallery mode lasing at their band edges in ultraviolet region was clearly observed under the optical excitation, reflecting their high sphericity and crystal quality.

We have investigated two approaches for an alternative hole injection with a tunnel junction targeting deep UV-LEDs. One was an AlGaN-based tunnel junction. We fabricated the AlGaN-based tunnel junctions with various AlN mole fractions (0～0.2) grown on conventional blue-LEDs by MOVPE. A 7.5 nm heavily Mg-doped GaN/15 nm heavily Si-doped Al0.2Ga0.8N tunnel junction showed a large voltage drop, 5.31 V at 20 mA, under reverse bias. The other was a GaInN-based tunnel junction. We prepared Ga0.6In0.4N tunnel junctions with various thicknesses and Si doping levels grown on the blue LEDs by MOVPE. A 2 nm heavily Mg-doped Ga0.6In0.4N/3 nm heavily Si-doped GaN tunnel junction showed only 0.12 V drop at 20mA under reverse bias. Since an absorption of the thin GaInN tunnel junction was estimated to be less than 10 %, such a tunnel junction with small bandgap and thin layer thickness is a practical approach to obtain a low resistive and low absorptive hole injection in the deep UV-LEDs.

In this contribution we report on the optical properties of cubic AlN/GaN asymmetric multi quantum wells (MQW) structures on 3C-SiC/Si (001) substrates grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) and spatially resolved cathodoluminescence (CL) at room temperature and at low temperature are used to characterize the optical properties of the cubic AlN/GaN MQW structures. An increasing CL emission intensity with increasing film thickness due to the improved crystal quality was observed. This correlation can be directly connected to the reduction of the linewidth of x-ray rocking curves with increasing film thickness of the c-GaN films. Defects like stacking faults (SFs) on the {111} planes, which also can be considered as hexagonal inclusions in the cubic crystal matrix, lead to a decrease of the CL emission intensity. With low temperature CL line scans also monolayer fluctuations of the QWs have been detected and the observed transition energies agree well with solutions calculated using a one-dimensional (1D) Schrödinger-Poisson simulator.

We investigated MOVPE growth conditions for AlInN layers with high growth rates and obtained 0.5µm/h with smooth surfaces. We found that short gas mixing time, relatively high growth temperature, and very low In/Al supply ratio were key growth parameters in order to obtain the AlInN layers with high growth rate and smooth surface simultaneously. AlInN/GaN DBRs grown under such growth conditions showed smooth surfaces and a reflectivity of over 99%.

In this paper, we propose a wrap around field termination technique for a vertical Schottky barrier diode fabricated on a free standing GaN substrate. Unlike conventional field plate designs, in the wrap around structure the field plate surrounds the active device area. This allows for better control of the electric field distribution, and reduces field-crowding. 2D finite element simulations using ATLAS show a uniform field distribution across the device. Calculations show that the Ron increases relative to the conventional field plate. A break down voltage of 1300V was predicted for a 5um thick epilayer.

In this study, we investigated GaN channel layer quality to suppress drain-lag, which is an important parameter for switching performance. In this experiment, we confirmed that drain-lag performance has dependence on the tilt of the GaN channel layer. GaN channel layer with the tilt angle of 243 arcsec showed faster drain-lag recovery than the tilt angle of 209 arcsec. The results of the drain-lag test and isolation leakage current measurement indicated that the tilt angle and hopping distance contributed to drain-lag recovery. We proposed the mechanism of trap effect during the drain-lag test.

We developed a new GaN on SiC growth method by metalorganic vapour phase epitaxy (MOVPE) using of a single 2-dimension-growth step. Prior to epitaxy, to inhibit pre-reaction of Si-face SiC substrate with TMGa and NH3, TMAl was flowed without NH3. 1.5 μm of undoped crack-free GaN was grown on 6H-SiC (Si-face). Without buffer layer, the vertical resistance of GaN/SiC structure was found to be around 82.1Ω as determined by I-V characteristic. Further reduction in vertical resistance is expected by growth of n-GaN (1.5μm)/SiC structure (300μm). We also expect a SiC-based GaN heterostructure vertical FET will achieve high power and high switching speed performance.

We have studied photoluminescence (PL) from undoped GaN films grown by HVPE technique on sapphire. Several defect-related PL bands are observed in the low-temperature PL spectrum. The concentrations of the defects responsible for these PL bands are determined from the dependence of PL intensity on excitation intensity. The RL band with a maximum at 1.8 eV is often the dominant PL band in HVPE GaN. It is caused by an unknown defect with the concentration of up to ∼1017 cm-3. The concentrations of defects responsible for other defect-related PL bands rarely exceed 1015 cm-3.