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The frequency dispersion in admittance measurements in AlGaN/GaN high-electron-mobility transistors, which is typically interpreted to result from an interface trap density Dit, is also known to be caused by effects other than Dit. To study the origin of two peaks of conductance component “ Gp/ ω” as a function of frequency in lateral gated diodes using AlGaN/GaN structures on Si wafers grown by metal–organic chemical vapor deposition, we measured capacitance C and conductance Gp/ ω–voltage V as a function of angular frequency ω ( C– ω– V and Gp/ ω– ω– V, respectively) of long-gate field-effect transistors with a varied gate length. We also simulated the C– ω– V and Gp/ ω– ω– V curves using an equivalent circuit that consisted of actually measured component parameters without Dit. We confirmed that the Gp/ ω– ω curves show two peaks caused by the two-dimensional electron gas channel resistance and the gate current leakage and quantitatively determined the two ωpeak positions and intensities. We also discussed the effect of acceptor concentration in the GaN channel-layer on lowering of ωpeak at weak inversion.

A crack-free indium gallium nitride (InGaN) based light emitting diode (LED) grown on silicon (Si) substrate was successfully demonstrated by introducing aluminium nitride/gallium nitride (AlN/GaN) superlattice structure (SLS) in the growth of the LED. The luminescence and the crystalline properties of the LED were discussed. From photoluminescence (PL) surface mapping measurement, the emission wavelength of the LED (453 nm) was almost uniform across the LED epi-wafer area. Temperaturedependent PL revealed that the dominant emission peak of the LED was 2.77 eV at all temperatures. The emission peak was related to the quantum wells of the LED. Some additional peaks were also observed, in particular at lower temperatures. These peaks were associated to alloy fluctuations in the In0.11Ga0.89N/ In0.02Ga0.98N multiquantum wells (MQWs) of the LED. Furthermore, the dependence of PL intensity and PL decay time on temperature revealed the evidence related to indium and/or interface fluctuations of the quantum wells. From X-ray diffraction (XRD) ω-scan measurements, fringes of the AlN/GaN SLS were clear, indicating the SLS were grown with good interface abruptness. However, the fringes for the MQWs were less uniform, indicating another evidence of the alloy fluctuations in the MQWs. XRD-reciprocal surface mapping (RSM) measurement showed that all epitaxial layers of the LED were grown coherently, and the LED was fully under strain.

In this study, we fabricated and characterized heterojunction field-effect transistors (HFETs) based on an Al0.36Ga0.64N-channel heterostructure with a dual AlN/AlGaInN barrier layer. The device fabrication was accomplished by adopting a regrown n++-GaN layer for ohmic contacts. The fabricated HFETs with a gate length of 2 μm and a gate-to-drain distance of 6 μm exhibited an on-state drain current density as high as approximately 270 mA mm−1 and an off-state breakdown voltage of approximately 1 kV, which corresponds to an off-state critical electric field of 166 V μm−1. This breakdown field, as a comparison in devices without field-plate electrodes, reaches approximately four-fold higher than that for conventional GaN-channel HFETs and was considered quite reasonable as an Al0.36Ga0.64N-channel transistor. It was also confirmed that the devices adopting the dual AlN/AlGaInN barrier layer showed approximately one order of magnitude smaller gate leakage currents than those for devices without the top AlN barrier layer.

Transfer-free graphene synthesis was performed using the catalyst metal agglomeration technique. X-ray diffraction and electron backscattering diffraction measurements indicated that the quality of the Ni catalyst film was enhanced with the increase of the crystallite size of Ni (111) by heating treatments during and following Ni deposition. Moreover, Raman scattering measurements revealed that the graphene film synthesized using heating treatments for the Ni film had better structural qualities. We measured the maximum field-effect mobilities to be 1540 cm2/(V⸱s) for electrons and 1600 cm2/(V⸱s) for holes, which was approximately twice as large as those fabricated without the heating treatments.

GaInN-based photovoltaic (PV) devices are highly promising for application to optical wireless power transmission (OWPT) systems as well as solar cells. This paper reports the research results of Ga0.9In0.1N multiple-quantum-well (MQW) PV cells on sapphire, focusing primarily on the growth temperature managements in metalorganic chemical vapor deposition (MOCVD) processes. As a result of the MOCVD study, the epilayer qualities in the PV cell structures improved significantly through the adoption of an optimized growth temperature for the GaInN MQWs and the two-step growth for the top p-GaN layers. Furthermore, the improved epilayer qualities resulted in the decrease in carrier recombination currents and series resistance for the forward diode characteristics without a light illumination. Subsequently, a sample with the improved qualities exhibited a higher open-circuit voltage and a higher fill factor in the PV characteristics. Eventually, the highest power conversion efficiency (PCE) in this study was measured to be 1.6% at a 1-sun solar spectrum and 42.7% at a monochromatic light illumination with 389 nm in wavelength and 5 mW cm−2 in optical power density. The dependency of the PV performance on the optical power densities at a monochromatic light illumination predicted that a higher PCE value may be achievable at a higher optical-power-density illumination. This is a very promising prediction when considering the practical application to OWPT systems.

We have demonstrated the use of scanning internal photoemission microscopy (SIPM) to characterize crystal defects in an AlGaN/GaN heterostructure grown on Si substrates. SIPM enabled the visualization of unusually grown regions owing to cracking of the Si substrates. In these regions, photocurrent was large, which was consistent with leaky current–voltage characteristics. We also found smaller photoyield regions, which may originate from the Al-rich AlGaN regions on hillocks. We confirmed the usefulness of SIPM for investigating the inhomogeneity of crystal quality and electrical characteristics from macroscopic viewpoints.

Al x In y Ga ( 1 − x − y ) N / GaN heterostructures were grown on 4-in. p-type Si wafers to investigate the effect of In composition in the quaternary nitride layer on the electrical performance of Al 2 O 3 / AlInGaN / GaN-based normally-ON metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). From the comparative study of the electrical measurements, it was observed that the transport properties of the devices were relatively poor in the presence of higher In composition in the quaternary-N layer. The deterioration of the electrical characteristics of MIS-HEMTs originated from the formation of deep pits on the AlInGaN epilayer surface caused by the segregation of In atoms during epitaxial growth. However, the formation of such pits was reduced for the quaternary epilayer with lower In content and exhibited better transport performance. A maximum current density ( I d , m a x) of 780 mA/mm with a specific ON-resistance of 0.71 m Ω cm 2 was observed for the device fabricated on the wafer with an In composition of 9% in the AlInGaN epilayer. We have achieved a high breakdown voltage of 793 V with a device with the gate-to-drain distance ( L g d) of 20 μ m under the off-state condition. Al x In y Ga ( 1 − x − y ) N / GaN heterostructures were grown on 4-in. p-type Si wafers to investigate the effect of In composition in the quaternary nitride layer on the electrical performance of Al 2 O 3 / AlInGaN / GaN-based normally-ON metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). From the comparative study of the electrical measurements, it was observed that the transport properties of the devices were relatively poor in the presence of higher In composition in the quaternary-N layer. The deterioration of the electrical characteristics of MIS-HEMTs originated from the formation of deep pits on the AlInGaN epilayer surface caused by the segregation of In atoms during epitaxial growth. However, the formation of such pits was reduced for the quaternary epilayer with lower In content and exhibited better transport performance. A maximum current density ( I d , m a x) of 780 mA/mm with a specific ON-resistance of 0.71 m Ω cm 2 was observed fo...

In order to improve the thermal stability in AlGaN-channel two-dimensional electron gas (2DEG) heterostructures, we newly designed strain-controlled quaternary AlGaInN barrier layers, and then we grew them by metalorganic chemical vapor deposition. A 2DEG density as high as 2.5 × 1013 cm−2 and a good surface morphology were obtained for an (Al0.64Ga0.36)0.976In0.024N/Al0.19Ga0.81N heterostructure, in which in-plane tensile strain in the barrier layer was estimated to be 0.51%. It was also confirmed that the (Al0.64Ga0.36)0.976In0.024N/Al0.19Ga0.81N heterostructure showed an excellent thermal stability in its 2DEG properties even at high temperatures up to 900 °C. As a result of their thermal stability improvement, we were also able to confirm that the contact resistance was improved by applying high-temperature annealing.

The temperature dependence of specific contact resistivity of recessed ohmic contacts was investigated to examine the carrier conduction mechanism in Ti/Al/W ohmic contacts on InAlN/AlN/GaN on Si wafer. The lowest specific contact resistivity of 2.36 × 10−5 Ωcm2 was obtained from a sample with a half-etched InAlN layer. The barrier heights and donor concentrations of recessed contacts with various recess depths were calculated using a thermionic field emission model that showed good fitting with the experimental data. It was found that the rate of tunneling in carrier conduction decreased with increasing recess depth.

AlInN epitaxial films with film thicknesses up to approximately 300 nm were grown nearly lattice-matched to a c-plane GaN-on-sapphire template by metalorganic chemical vapor deposition. The AlInN films showed relative good crystal qualities and flat surfaces, despite the existence of surface pits connected to dislocations in the underlying GaN film. The refractive index derived in this study agreed well with a previously reported result obtained over the whole visible wavelength region. The extinction coefficient spectrum exhibited a clear absorption edge, and the bandgap energy for AlInN nearly lattice-matched to GaN was determined to be approximately 4.0 eV.

We attempted to improve the mobility of InAlN/AlGaN two-dimensional electron gas (2DEG) heterostructures by achieving an atomically smooth heterointerface in metalorganic chemical vapor deposition processes. In the result, it was confirmed that the high-growth-rate AlGaN layer was very effective to improve the surface morphology. The atomically smooth surface morphology with a root-mean-square roughness of 0.26 nm was achieved for an Al0.15Ga0.85N layer under the growth rate of approximately 6 µm/h. Furthermore, nearly lattice-matched In0.17Al0.83N/Al0.15Ga0.85N 2DEG heterostructures with the atomically smooth heterointerface exhibited a 2DEG mobility of 242 cm2 V−1 s−1 with a 2DEG density of 2.6 × 1013/cm2, which was approximately 1.5 times larger than the mobility in a sample grown under original conditions.

The electrical interface properties of GaN metal–oxide–semiconductor (MOS) capacitors with an Al2O3 gate insulator formed by atomic layer deposition method using three kinds of oxidants were investigated by the capacitance–voltage technique, Terman method, and conductance method. We found that O3 and the alternate supply of H2O and O3 (AS-HO) are effective for reducing the interface trap density (D it) at the energy range of 0.15 to 0.30 eV taking from the conduction band minimum. On the other hand, we found that surface potential fluctuation (σs) induced by interface charges for the AS-HO oxidant is much larger than that for a Si MOS capacitor with a SiO2 layer formed by chemical vapor deposition despite the small D it values for the AS-HO oxidant compared with the Si MOS capacitor. This means that the total charged center density including the fixed charge density, charged slow trap density, and charged interface trap density for the GaN MOS capacitor is higher than that for the Si MOS capacitor. Therefore, σs has to be reduced to improve the performances and reliability of GaN devices with the Al2O3/GaN interfaces.

The AlGaN/GaN heterostructures (HSs) with reduced dislocation density were grown on 4-in. a-plane sapphire substrate. The two-dimensional electron gas (2DEG) was observed for AlGaN/GaN HSs on a-plane sapphire. A complete pinch off AlGaN/GaN high electron mobility transistors (HEMTs) were demonstrated with a maximum drain current density (IDSmax) of 551 mA/mm and transconductance maximum (gmmax) of 134 mS/mm. Compared to c-plane grown HEMTs, two orders of magnitude low gate leakage current was observed on a-plane grown HEMTs with enhanced barrier height. The low gate leakage is due to enhanced Schottky barrier height and reduced dislocation density of GaN grown on a-plane sapphire.

AlGaN films and deep ultraviolet light-emitting diode (UV-LED) structures with AlGaN multi-quantum wells (MQWs) were grown directly on 2-in.-diameter epitaxial AlN/sapphire template (AlN template) by metalorganic chemical vapor deposition (MOCVD). It was confirmed that crack-free and good-crystal-quality AlGaN films and UV-LED structures can be grown on AlN template without the use of any buffer layer technique such as superlattice structures or pulsed atomic layer deposition. This seemed to be mainly due to the in-plane compressive stress caused by using AlN template as underlying substrates. Deep UV-LED devices were successfully fabricated using MOCVD-grown wafers. Electroluminescence spectra for those LEDs exhibited sharp near-band emissions ranging from 265 to 277 nm, depending on their Al compositions in MQWs. It was also confirmed that the insertion of a thin AlN layer between the active region and the p-type cladding layer can drastically suppress a parasitic sub-band emission around 320 nm.

Influences of gate-recess etching with BCl3 plasma in drain current (ID) collapse were performed on different cap layers (i-GaN, n-GaN, and p-GaN) grown AlGaN/GaN high-electron-mobility transistors (HEMTs). Due to the decrease of dynamic-source-resistance by gate-recess, the increase of maximum drain current density and maximum extrinsic transconductance were observed in all cap layers grown AlGaN/GaN HEMTs. After gate-recess etching, about 14 and 17% of decrease in ID collapse were observed on n-GaN and p-GaN cap layers HEMTs, respectively when compared to non-recessed HEMTs. However, increase (~47%) of ID collapse was observed in i-GaN cap layer HEMTs. The decrease of ID collapse in doped GaN cap layer HEMTs is possibly due to the compensation of dopant related traps with plasma induced traps. The increase of ID collapse in i-GaN cap layer HEMTs may be due to the incorporation of damage related traps by gate-recess etching. The decrease and increase of trapping effects were qualitatively confirmed by white-light illuminated IDS–VDS characteristics. An increase of gate leakage current in all recessed gate AlGaN/GaN HEMTs are due to the BCl3 plasma induced damage.

Transverse-optical (TO) and longitudinal-optical (LO) phonons in Si-doped and undoped n-GaN films have been studied by infrared reflection spectroscopy. The former film has been grown by metalorganic chemical vapor deposition (MOCVD) to a thickness of 2.3 µm, while the latter by hydride vapor phase epitaxy (HVPE) to a thickness of 35 µm. Both films are deposited on the c-surface of sapphire substrates with the surface of the c-plane of the wurtzite structure. Oblique-incidence infrared reflection spectroscopy has permitted us to observe the A1 mode, as well as the E1 mode, of polar optical phonons. The reflection spectra observed for s- and p-polarizations can be described well in terms of Gervais and Piriou's four-parameter semi-quantum oscillator model taking account of the LO-phonon–plasmon coupling. It is found that in the MOCVD-grown film, the damping energies of the phonons are markedly graded along the growth direction of the film. It is suggested that the lattice strain is quite large in the interface zone but is reduced rapidly upon going beyond about 1 µm from the interface. Accordingly, the HVPE film has a good lattice quality throughout the film.

Different-Al-content AlxGa1-xN/GaN (0.26≤x≤0.52) high-electron-mobility transistor (HEMT) structures were grown on large-area 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A low sheet resistance of approximately 380 Ω/sq. and an in-wafer total variation of 7.6% with a standard deviation of 1.6% were obtained for the sample with the highest Al content of 0.52. The two-dimensional electron gas (2DEG) density and room-temperature Hall mobility of the Al0.52Ga0.48N/GaN HEMT structures were 1.76×1013/cm2 and 971 cm2/Vs with in-wafer total variations of 7.8% and 8.1%, respectively. HEMTs were successfully fabricated and they exhibited good dc characteristics. The dependence of maximum source–drain current density on Al content was consistent with the Al content dependence of sheet carrier concentration. The highest source–drain current density of 1033 mA/mm with a maximum extrinsic transconductance of 228 mS/mm was obtained for 2-µm-gate-length Al0.52Ga0.48N/GaN HEMTs.

We have systematically studied the effect of various interlayers on epiwafer bowing in AlGaN/GaN high-electron-mobility transistor (HEMT) structures on sapphire substrates by using an interlayer insertion method. The interlayers including aluminum increase the epiwafer bowing independently of growth temperature. However, both InGaN-based interlayers and low-temperature-deposited GaN interlayers suppress the epiwafer bowing. When multilayers are used as interlayers, the effect on epiwafer bowing is enhanced compared with that in the case of using single interlayers. The minimum bowing values were lower than 10 µm, which were accomplished by using multilayers. The crystalline qualities of AlGaN/GaN HEMTs with interlayers were satisfactory and the performance of these HEMTs was better than that of conventional AlGaN/GaN HEMTs without interlayers. Epiwafers with reduced bowing exhibited a reduced strain in GaN layers compared with that of GaN layers with larger bowing, which indicates that strain control in the epitaxial layers is important for reducing the epiwafer bowing.

The effects of rapid thermal annealing (RTA) on the optical properties of GaNAs∕GaAs triple quantum wells grown by chemical beam epitaxy are studied in detail by photoluminescence (PL) spectroscopy at 77K. Special emphasis is put on the RTA-induced PL peak blueshift. It is found that the blueshift is neither due to nitrogen diffusion from well layer to barrier layer nor due to homogenization of nitrogen composition fluctuations. The blueshift is due to the coupling between the radiative recombination of PL emission and the nonradiative recombination of nonradiative centers. A quantitative model is proposed in which the blueshift is proportional to the relative change of the concentration of nonradiative centers. This model quantitatively explains not only our present results but also previous observations.

InGaN multiple-quantum-well light-emitting diodes (LEDs) were grown on an AlN/sapphire template by metalorganic chemical vapor deposition. The crystalline quality was investigated by x-ray diffraction and electron-beam-induced current. The thermal stability of the LED was demonstrated by measurements of current-voltage, light output power-current, and electroluminescence (EL) spectra at different temperatures. The output power at 200 mA decreased by 7.3% for the LED on the template upon increasing temperature from 25 to 95°C, while that for the LED on sapphire decreased by 23.9%. The peak external quantum efficiency decreased from 0.23% to 0.22% and from 0.15% to 0.10% for the LEDs on the template and on sapphire, respectively. The EL spectrum peak at 20 mA shifted to lower energy by 17.2 meV for the LED on the template upon increasing temperature, while that for the LED on sapphire shifted by 32.7 meV. The LED on the template exhibited a higher output power and a better thermal stability with respect to the conventional LED on sapphire using a low-temperature GaN buffer layer, which is due to the low threading dislocation density in the active layer and the high thermal conductivity of AlN layer.

Highly resistive layers are formed by the implantation of Zn ion along the c axis of GaN and AlGaN/GaN epitaxial layers. Heavy ions such as Zn have been desirable for the formation of highly resistive layers, because ions effectively transferred their energy to the crystal atoms rather than the electrons in GaN. A sheet resistance Rs as high as 3.8×1011 Ω/sq was obtained on GaN layers after the ion implantation. Rs increased up to 2.2×1013 Ω/sq after the annealing at 500 °C for 300 s in an N2 atmosphere. The thermal activation energy Er for this sample was 0.67 eV. It was found that the experimental data in current–voltage characteristics were fitted to the equation included the Poole–Frenkel current and resistive (ohmic) current. The difference of Rs between the as-implanted and 500 °C annealed samples was due to the Poole–Frenkel current. The Poole–Frenkel current overcame the resistive one, and dominated the current mechanism in the case of the samples annealed at 200 °C or less. On the other hand, for...

Lightly doped n-GaN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. The grown n-GaN epilayers were characterized using atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence and Hall effect measurements. Enhanced PL intensity and low dark spot density (DSD) were observed on the aligned step structure n-GaN. The samples with these aligned step structure showed high electron mobilities with good structural and optical properties, while the samples with the anisotropic step structure showed broadened XRD FWHM values, low mobilities, and poor structural and optical properties. The low Hall mobility of n-GaN is due to the scattering of charged threading dislocations. A clear correlation was observed between Hall mobility and DSD by AFM. The AFM surface observation is also a better method for the evaluation of the electron mobility of lightly doped MOCVD grown n-GaN.

Anatase titania nanoparticles are prepared at 700°C by metalorganic chemical vapor deposition (MOCVD) and the nanoparticles are collected by thermophoretic collection method. The effect of collection distance on the lattice structure of the nanoparticles is investigated, followed by discussion. The results show that the collection distance exerts a significant influence on the lattice structure of the nanoparticles. With decreasing collection distance, the lattice constant a increases while c decreases simultaneously, resulting in more significant lattice distortion. X-ray photoelectron spectroscopy and electron paramagnetic resonance results show that both oxygen and titanium vacancies exist in the nanoparticles. Vacancies in the nanoparticles are responsible for the lattice distortion.

Anatase titania nanoparticles with a particle size of about 10 nm are prepared by metalorganic chemical vapor deposition (MOCVD) and the anatase-rutile tranformation of the nanoparticles is investigated and discussed. The results show that anatase-rutile transformation takes place in a wide range of temperature. When annealing time is 1 h, the anatase-rutile transformation temperature of the nanoparticles is as high as 900 to 1200°C, which is comparable to that of micrometer-sized titania. Based upon X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) results, a possible transformation mechanism is proposed. The high transformation temperature of the nanoparticles is primarily attributed to the retardation of the initial particle growth of the anatase phase due to the lattice strains in the nanoparticles before transformation proceeds.

Atmospheric pressure-metal organic chemical vapour deposition (AP-MOCVD) was used to grow InAs and InGaAs quantum dots (QDs) on GaAs/Si substrate. The influence of growth temperature on the QD density and distribution was investigated. Scanning probe microscopy (AFM and DFM) was used to investigate the growth behaviour of the QD structure. The growth properties of InAs/InGaAs QDs on the highly stressed GaAs/Si substrate under S–K growth mode at different deposition temperatures have been analysed. It was identified that the growth temperature plays major role in determining the growth and distribution of both InAs and InGaAs QDs due to the temperature-dependent dislocation propagation from the GaAs/Si substrate. The growth of InGaAs QDs was found “In” composition dependent. A high QD density of ∼5×10 10 –6×10 10 cm −2 was obtained on GaAs/Si at a growth temperature of 500 °C.

The normal-incidence reflectance measurement was employed to obtain the free exciton transition energy (EFX) of AlGaN alloys in AlxGa1-xN/GaN/sapphire heterostructure grown by metalorganic chemical vapor deposition. It was found that the thickness variation of the AlGaN layer may cause a noticeable change in the line shape of reflectance spectrum and impede the identification of the desired excitonic position. By using a reflection model of two absorbing layers with a transparent substrate, the experimental reflectance spectra were theoretically simulated and utilized to explain the reflection mechanism in AlxGa1-xN/GaN heterostructures. On the basis of the above analysis, the feasibility of the reflectance measurement for such heterostructures is confirmed. At room temperature, the EFXs obtained from the fitting showed an excellent agreement with the corresponding peak energies in the photoluminescence spectra. Furthermore, at the optical energy position about 100 meV above the EFX, the spectral feature of exciton-LO phonon interaction was observed in the reflectance spectrum record for low Al composition (x?0.16). Using the Al mole fraction derived from x-ray diffraction measurement, the bowing parameter of the epitaxial AlGaN layer was determined. In the range of 0?x

We report the growth of self-formed InGaAs quantum dot (QD) and the fabrication of self-formed In0.2Ga0.8As QD-like laser on Si substrate by metal organic chemical vapor deposition (MOCVD). The dots are grown by Stransky–Krastanov (S–K) growth mode. The laser shows room temperature continuous wave (CW) light output with a threshold current density of 1.32 kA/cm2 and a lasing wavelength of 854 nm.

We report on the fabrication and characterization of high-performance AlGaN solar-blind p–i–n photodiodes grown on AlN/sapphire templates by metal organic chemical vapor deposition. The realized devices demonstrate a sharp spectral cutoff with a drop of four orders of magnitude from 260 to 310 nm under the 1 µW/cm2 illumination. Dark current density as low as 2.6×10-11 A/cm2 at -5 V bias has been measured. The zero bias external quantum efficiency peaks at 258 nm with a value of 30%. The upper-bound detectivity limited by Johnson noise is calculated to be 7.1×1014 cmHz1/2/W at 300 K.

We report characteristics and degradation of an InGaN/AlGaN double-heterostructure light-emitting diode (LED) grown by metalorganic chemical vapor deposition on a sapphire substrate. The InGaN/ AlGaN LED exhibited an optical output power of 0.17 mW, an external quantum efficiency of 0.2%, a peak emission wavelength at 440 nm with a full width at half-maximum of 63 nm, and a stable operation up to 3000 h under 30 mA dc operation at 30 °C. However, the InGaN/AlGaN LED showed electrical and optical degradations under high injected current density and high ambient temperature. Electroluminescence, electron-beam-induced current and cathodoluminescence observations showed that the degraded InGaN/AlGaN LED exhibited formation and propagation of dark regions, which act as nonradiative recombination centers. The values of the degradation rate were determined to be 1.1×10-3, 1.9×10-3, and 3.9×10-3 h-1 under the injected current density of 100 A/cm2, and 1.6×10-2, 3.6×10-2, and 8×10-2h-1 under 200 A/cm2 at ambient temperatures of 30, 50, and 80 °C, respectively. The activation energy of degradation was also determined to be 0.23-0.25 eV. The degradation of electrical and optical characteristics was caused by the growth of dark regions. It was also observed that GaN-based LEDs on sapphire substrates have longer lifetime than the ZnSe-based LED, but shorter than the AlGaAs and InGaAsP LEDs.

An AlGaAs/GaAs multi-quantum well vertical-cavity surface-emitting laser diode (VCSELD) has been grown on a Si substrate using metalorganic chemical vapor deposition. The VCSELD structure grown on a Si substrate consists of ten quantum well active layers and a 23-pairs of AlAs/Al0.1Ga0.9As distributed Bragg reflector (DBR). The VCSELD on a Si substrate exhibited a threshold current of 82 mA and a threshold current density of 4.2 kA/cm2 under continuous-wave (cw) condition at 150 K. Cross-sectional scanning electron microscopy observation showed quasi-periodic zigzag roughness and nonuniformity in the DBR structure. Auger-electron spectroscopy also showed compositional transitions at the heterointerfaces of DBR. A low reflectivity of the DBR on Si substrate is caused by the degraded heterointerfaces, which prevent 300 K cw operation for the VCSELD grown on Si.

Single crystalline epitaxial layers of Al0.22Ga0.78As have been grown on Si substrates. Silicon diffusion in the heteroepitaxial layers has been explained by a pipe diffusion model and a gas phase reaction model. The dislocation bending at the AlGaAs Si interfaces has been explained in terms of electrical potential difference, thermal stress and point defect complexes. The Schottky diode characteristics of AlGaAs Si have been studied and the ideality factor (n) and Schottky barrier height (φb) estimated in the present study were 1.2 and 0.89 eV, respectively. Capture cross-sections, activation energies and concentrations of the deep levels have been calculated by deep level transient spectroscopy. Two deep levels with thermal emission energies of 0.43 and 0.64 eV have been observed in the 900°C thermal cycle annealed samples. Influence of silicon complex defects on the nature of the deep levels has been addressed.

We have reported that the crystallinity of GaAs on Si (GaAs/Si) grown by chemical beam epitaxy (CBE) depends on the growth temperatures of the top GaAs and nucleation layers. The GaAs layer on Si, grown at 500°C for the top GaAs layer and 400°C for the nucleation layer, exhibited a high crystal quality even at a lower growth temperature of 500°C. By performing in-situ thermal cycle annealing for thisGaAs/Si, the values of etch pit density and the full width at half maximum of X-ray double-crystal measurement were2.3 × 107cm−2and 147 arcsec, respectively. These samples were superior to those grown by the conventional techniques such as metal organic chemical vapor deposition and molecular beam epitaxy.

Using droplet-epitaxy, self-formed GaAs islands were successfully grown onGaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD). The shapes and sizes of the islands were observed by atomic force microscope (AFM). In addition, a GaAs-based light emitting diode (LED) on Si with self-formed GaAs islands active region was fabricated for the first time. This LED was operated up to 27μW at 190 mA under room-temperature direct current (dc) conditions. Compared with a GaAs-based LED on Si with a GaAs quantum well active region, the reliability of this LED was much superior because of low dislocation density due to reduction of the active region by use of a GaAs island structure.

GaNAs/GaAs triple quantum wells (QWs) grown by chemical beam epitaxy are annealed by two methods, ex situ rapid thermal annealing (RTA) and in situ annealing in a growth chamber followed by ex situ RTA (NRTA). The effects of annealing method on the structural, morphological, and optical properties of the QWs are studied comparatively. The results show that although there is surface desorption for both RTA and NRTA, the holes on the sample surface after desorption are clearly different. RTA is better than NRTA from the viewpoints of both surface morphology and optical properties.

We report novel GaN fully vertical p–n diode on Si grown by metalorganic chemical vapor deposition. The thick strained layer superlattice is effective in controlling a doping level of 1016 cm−3 in an n−-GaN drift layer. The GaN p–n diode exhibits a differential on-resistance R on of 7.4 mΩ cm2, a turn-on voltage of 3.4 V, and a breakdown voltage V B of 288 V. The corresponding Baliga’s figure of merit (FOM) is 11.2 MW/cm2. A good FOM value for the GaN-on-Si vertical p–n diode is realized for a drift layer thickness of 1.5 µm without using substrate removal technology.

Dislocations in a GaN layer grown on 4-in. Si(111) with AlGaN/AlN strained layer superlattices using a horizontal metal–organic chemical vapor deposition system were characterized by transmission electron microscopy and scanning transmission electron microscopy. Pure screw dislocations were not found in the observed area but mixed and edge dislocations were found. The dislocation density in the GaN layer decreased from the bottom (∼2 × 1010 cm−2) to the top (∼6 × 109 cm−2). Some dislocations were inclined from the c-axis, and half-loop dislocations were observed in the GaN layer. Plan-view weak-beam dark-field analysis indicated that the dislocation inclination was caused by climb and glide motions.

The behavior of dislocations in a GaN layer grown on a 4-inch Si(111) substrate with an AlGaN/AlN strained layer superlattice using horizontal metal-organic chemical vapor deposition was observed by transmission electron microscopy. Cross-sectional observation indicated that a drastic decrease in the dislocation density occurred in the GaN layer. The reaction of a dislocation (b=1/3[-211-3]) and anothor dislocation (b =1/3[-2113]) to form one dislocation (b =2/3[-2110]) in the GaN layer was clarified by plan-view observation using weak-beam dark-field and large-angle convergent-beam diffraction methods.

We found that inhomogeneous epitaxial growth in a superlattice near a Si substrate creates an area where local leakage current occurs at the interface between an AlGaN surface and a Schottky electrode. Here, electroluminescence (EL) through a transparent gate of an AlGaN/GaN high-electron-mobility transistor enables us to identify the area in the entire gate periphery. Further, the superlattice near the Si substrate supports clear observation of inhomogeneous growth under the EL spots. The energy-dispersive X-ray spectroscopy profile indicates that a Ga-rich layer was grown in the early stage of inhomogeneous area creation.

InGaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs) on Si (111) substrate were grown by metalorganic chemical vapor deposition. Crack-free films on 2-inch wafer were obtained using AlN/GaN multilayers with a thin AlN/AlGaN buffer layer. High-resolution X-ray diffraction (HRXRD) reveals the LED on Si is of high crystalline quality. Dislocation density of MQW active layer was investigated by cathodoluminescence (CL) at room temperature. The operating voltages of 3.7 V and 4.2 V and the output powers of 34.8 µW and 34.5 µW at 20 mA were obtained for the lateral and the vertical conduction, respectively. The forward series resistances are 33 Ω and 42 Ω for the lateral and the vertical conduction, respectively. The EL peaks at 453 nm with a full width at half maximum (FWHM) of 22 nm at 20 mA current. These characteristics are comparable to those of LED on sapphire. Especially, the LED on Si shows a high saturation operating current due to the good thermal conductivity of Si substrate.

GaN/AlGaN distributed Bragg reflectors (DBRs) have been grown on sapphire by metal-organic chemical vapor deposition. A GaN/AlGaN superlattice (SL) was introduced prior to the growth of the DBR to suppress crack generation. By introducing the SL, GaN layers in the DBR were highly compressed and the in-plane lattice constants were close to those of AlGaN layers in the DBR. For the 30 pairs of GaN/Al0.41Ga0.59N DBRs, the reflectivity was improved from 93% to 98% by the introduction of the 100 periods of GaN/AlGaN SL, and the generation of cracks was effectively suppressed.

The thermal stabilities of metal-organic chemical vapor deposition-grown lattice-matched InAlN/GaN/Si heterostructures have been reported by using slower and faster growth rates for the InAlN barrier layer in particular. The temperature-dependent surface and two-dimensional electron gas (2-DEG) properties of these heterostructures were investigated by means of atomic force microscopy, photoluminescence excitation spectroscopy, and electrical characterization. Even at the annealing temperature of 850 °C, the InAlN layer grown with a slower growth rate exhibited a smooth surface morphology that resulted in excellent 2-DEG properties for the InAlN/GaN heterostructure. As a result, maximum values for the drain current density (IDS,max) and transconductance (gm,max) of 1.5 A/mm and 346 mS/mm, respectively, were achieved for the high-electron-mobility transistor (HEMT) fabricated on this heterostructure. The InAlN layer grown with a faster growth rate, however, exhibited degradation of the surface morphology at...

Schottky diode characteristics of a boron trichloride (BCl3) plasma-etched n-GaN layer by reactive ion etching (RIE) were investigated using current-voltage measurements. BCl3 plasma etching degraded the Schottky diode characteristics due to plasma-induced damage and a discharge-gas-related residue near the surface of the GaN layer. To remove the residue and plasma damage from the GaN layer, various surface treatments were carried out on the BCl3 plasma-etched GaN layer. Improved Schottky characteristics were observed in nitrogen plasma-treated n-GaN Schottky diodes. X-ray photoelectron spectroscopy confirmed the removal of the discharge-gas-related residue.

GaN-based metal–semiconductor–metal photodetectors (MSM-PDs) with a recessed-electrode structure have been fabricated and characterized. The photodiodes exhibited low leakage currents with a typical value of 51 pA at 10 V. Spectral response revealed a responsivity up to 0.14 A/W at 350 nm under 10 V bias, corresponding to an internal quantum efficiency of about 90%, which is a 55.6% improvement over that of conventional MSM-PDs with planar electrodes. The improvement was attributed to the enhanced and uniform electric field due to the recessed electrodes in the photoactive region, leading to a more efficient carrier collection.

We report the characteristics of InGaN multiple-quantum-well (MQW) green light-emitting diodes (LEDs) on Si (111) substrates. The MQW LEDs were grown on Si by metalorganic chemical vapor deposition using Al0.27Ga0.73N/AlN intermediate layers. The LED on Si showed an operating voltage of 7 V, a series resistance of 100 Ω, an optical output power of 20 μW, and a peak emission wavelength of 505 nm with a full width at half maximum of 33 nm at 20 mA drive current. The optical output power was half as compared to that of green LED on sapphire. The LED also exhibited a stable operation over 500 h under automatic current control (20 mA) condition at 27 °C.

The crystallinity of GaAs grown on Si with various intermediate layers by metalorganic chemical vapor deposition and characteristics of Schottky diodes fabricated on the grown GaAs/Si have been studied. The GaAs/Si with an Al0.55Ga0.45P intermediate layer has both good crystallinity, x‐ray full width at half maximum of 188 arc sec, and a smooth surface. The Schottky diode fabricated on the GaAs/Si with an Al0.55Ga0.45P intermediate layer also shows good forward and reverse current‐voltage characteristics with an ideality factor of 1.06, as good as for an n‐type GaAs substrate. However, a relatively large leakage current is observed under reverse and small forward bias. This leakage current is caused by generation and recombination through the centers related to dislocations in the GaAs/Si.

Two-dimensional electron mobilities in AlGaN-channel nitride heterostructures were numerically and experimentally analyzed. The calculation and experimental results indicated that mobility for InAlN/AlGaN heterostructures grown by metalorganic chemical vapor deposition was limited by interface roughness scattering as well as alloy disorder scattering. The mobility limits for AlGaN-channel heterostructures with a two-dimensional electron gas (2DEG) density of 3 × 1013/cm2 were estimated to be, for instance, 249 cm2 V−1 s−1 for an Al0.3Ga0.7N channel and 192 cm2 V−1 s−1 for an Al0.5Ga0.5N channel. The calculated values showed no major disagreement with the results reported on AlGaN-channel heterostructures.

The trapping and emission of carriers in the gate-to-drain region of an AlGaN/GaN high-electron-mobility transistor (HEMT) have been investigated using a bias-controllable field plate (CFP). Once an instantaneous positive CFP voltage is applied after bias stress in a transient drain current measurement, carrier trapping occurs, which can subsequently be observed as a drain current discontinuity. Numerical analysis of carrier trapping using the Shockley?Read?Hall process also provides a trapped carrier density of 5.1 ? 1012 cm?2 and an energy level of 0.6 eV.