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We performed electron-spin-resonance (ESR) investigations of defects related to dangling bonds (DBs) around the insulator/semiconductor interface in Al2O3/AlGaN/GaN MIS-HEMT structures on Si. The Al2O3 layer was fabricated by atomic layer deposition, and we studied the effects of the Al2O3 layer thickness and the post-deposition annealing (PDA) temperature on the number of DBs (Ndb). The Ndb of the AlGaN/GaN HEMT structure was reduced by deposition of the 10-nm-thick Al2O3 layer. The Ndb value at the 5-nm-thick Al2O3/AlGaN interface decreased to a minimum value with PDA at 500 °C. We show that defects related to DBs around ALD-Al2O3/AlGaN interfaces can be reduced by optimizing the ALD-Al2O3 thickness and PDA temperature.

A GaN/InGaN double quantum well (DQW) gate structure was designed on a conventional GaN-on-Si based AlGaN/GaN heterostructure. The electrical characteristics of the devices exhibited a normally-off or E-mode (enhancement-mode) operation with a threshold voltage ( V t h ) of +0.53 V. We achieved a drain current density ( I d ) of 171 mA/mm and a specific on-resistance ( R o n , s p ) of 2.45 m Ω -cm 2 . The leakage current densities of the devices, under the off-state condition, were of the order of 10−7 mA/mm. This DQW gate also prevents electron leakage through the gate, which improves the current gain ( h F E ) significantly. The fabricated normally-off i -DQW HEMTs showed excellent 3-terminal off-state breakdown characteristics ( > 1000 V).

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.

Frequency dispersion of capacitance-voltage (C-V) characteristics of a GaN metal-oxide-semiconductor (MOS) capacitor was systematically investigated. A high frequency C-V curve without including capacitance associated with interface traps and negligibly small C-V hysteresis gave us an accurate relationship between the surface Fermi level and the gate voltage in the C-V curve. We found that obvious frequency dispersion was observed even at a gate bias condition of 1V, which has very long electron capturing time compared with time corresponding to inverse of the measurement frequencies. Furthermore, calculation based on an equivalent circuit of the MOS capacitor including surface potential fluctuation (s) clarified that the frequency dispersion is owing to the large s. This indicates that consideration of the impact of s is quite important for accurate understanding of the C-V characteristics for wide-bandgap semiconductor. Display Omitted Accurate relationship between surface Fermi level and gate voltage was determined.The relationship enables to derive time constant for AC response of interface traps.The AC response was observed even at a gate bias with the very long time constant.We clarified the AC response is owing to large surface potential fluctuation.The large fluctuation could derive significant mislead of the interface quality.

A normally-Off metal-oxide-semiconductor field-effect transistor (MOS-FET) is proposed by using AlInN/AlGaN heterostructure grown on silicon. The AlGaN channel MOSFET with Al2O3 layer as gate insulator exhibits a drain current of 90 mA/mm at the gate bias of +8 V, an ON/OFF drain-current ratio of ${1.4} \times {10}^{8}$ , a peak field-effect mobility of 97 cm2/ $\text {V} \cdot \text {s}$ , and threshold voltage of +2.4 V, respectively. With significant reduction in leakage currents, the MOS-FET with a gate-to-drain spacing of 20 $\mu \text{m}$ delivers a high breakdown of 993 V. The present results of Al2O3/AlGaN normally-Off MOSFET signifies promising prospects for future high power and high voltage applications.

We investigated the effect of well layer thicknesses on the external quantum efficiency (EQE) and energy conversion efficiency (ECE) for InGaN/GaN multiple quantum well (MQW) solar cells grown on sapphire substrates by metalorganic chemical vapor deposition. The results indicated that EQE and ECE have maximum values at a specific well thickness. When the well thickness is sufficiently thin, EQE and ECE increase with an increase in the well thickness owing to an increase in light absorption. Then, once the well thickness surpasses a critical thickness, EQE and ECE begin to decrease owing to the influence of nonradiative recombination processes, which was indicated by the static and dynamic photoluminescence analyses. The critical well thickness probably depends not only on the MQW design but also on growth conditions. Further, we confirmed that the increased total thickness of the stacked well layers leads to increased light absorption and thereby contributes to the improvement of solar cell performance. A high short circuit current density of 1.34 mA/cm2 and a high ECE of 1.31% were achieved for a InGaN/GaN MQW solar cell with a 3.2-nm-thick InGaN well with total well thickness of 115 nm.

GaN-based light emitting diodes (LEDs) have been grown by metalorganic chemical vapor deposition on Si(111) substrate with and without 3C-SiC intermediate layer (IL). Structural property has been characterized by means of atomic force microscope, X-ray diffraction, and transmission electron microscope measurements. It has been revealed that a significant improvement in crystalline quality of GaN and superlattice epitaxial layers can be achieved by using 3C-SiC as IL. Regarding of electrical and optical characteristics, it is clearly observed that the LEDs with its IL have a smaller leakage current and higher light output power comparing with the LEDs without IL. The better performance of LEDs using 3C-SiC IL can be contributed to both of the improvements in epitaxial layers quality and light extraction efficiency. As a consequence, in terms of optical property, a double enhancement of the light output power and external quantum efficiency has been realized.

In this article, we have demonstrated the fabrication of first GaN-on-Si based fully-vertical metal-oxide-semiconductor field-effect transistor (V-MOSFET) by introducing a $p$ -GaN current aperture (CA) in the device structure to control the vertical current conduction. The $p$ -GaN CA was constructed by using plasma-based dry etching and epitaxial regrowth technique. The electrical measurements on the device exhibited its capability in both vertical and lateral modes of operation. We achieved a drain current density $(I_{D})$ of 0.25 A/mm with an ON-resistance $(R_{ON})$ of 44 $\Omega$ -mm and a threshold voltage $(V_{th})$ of − 20 V for the vertical mode of operation. The comparison of the electrical results indicates a relatively higher ON-resistance of the device in the vertical configuration. In spite of relatively lower drain current and higher $R_{ON}$ , this device structure would be a potential and cost-effective prototype for the realization of fully-vertical GaN MOSFETs on Si for the next generation power device applications.

In order to consider applying them to cladding layers in visible GaN laser diodes (LDs), we conducted the growth and characterization of quaternary AlGaInN films with alloy compositions around lattice-matched to GaN grown on sapphire. Unlike ternary AlInN films, it was indicated that a relatively flat epitaxial film is grown even with an in-plane compressive strain.

Novel Al 2 O 3 gate metal-oxide-semiconductor heterostructure field-effect transistors (MIS-HFETs) were fabricated using an AlGaInN/AlGaN channel two-dimensional-electron gas (2DEG) heterostructure. The fabricated MIS-HFETs exhibited good pinch-off characteristics. Furthermore, an extremely high off-state breakdown voltage of approximately 2.5 kV was obtained for a device with a gate-to-drain length of $20\ \mu \mathrm{m}$ , which corresponds to a breakdown field of $122\ \mathrm{V}/\mu \mathrm{m}$ . In addition to the characterization of MIS-HFETs, C-V characteristics were evaluated for an Al 2 O 3 /AlGaInN/AlGaN diode structure to understand its MIS interface state.

This Letter reports the polarization induced hole conduction in composition-graded AlInN epitaxial layers grown by metalorganic chemical vapor deposition. First, the composition-graded AlInN layer with an InN mole fraction from 0.12 to 0.20 was formed on c-plane GaN on sapphire, and they were confirmed to show the p-type hole conduction with a less temperature dependence, which is a feature of polarization-induced carriers. Then, blue light-emitting diodes (LEDs) with the composition-graded AlInN layers inserted in the p-type side were fabricated and their vertical current injection was investigated. The electroluminescence (EL) spectra confirmed that the fabricated LEDs exhibited a single-peak blue-light emission with the help of the impurity Mg doping. The LED simulation indicated that the impurity and polarization co-doping effectively compensated and overcame the residual oxygen donors in the AlInN layer and promoted the carrier recombination at the light-emitting layers. Finally, LEDs with the Mg-doped and composition-graded AlInN insertion layer exhibited good current–voltage characteristics with a low forward voltage drop of approximately 3 V in addition to the good EL spectra.

In this study, the initial AlN layer and the vertical-direction breakdown voltage (VDBV) of AlGaN/GaN high-electron-mobility transistors (HEMTs) were characterized. Prior to the formation of the interface between the AlN layer and the Si substrate, only trimethylaluminum (TMA) was introduced without ammonia to control the crystal quality of initial AlN layer (TMA preflow). HEMT structures were simultaneously grown on identical AlN layers on Si substrates (AlN/Si templates) grown using different TMA preflow temperatures. The density of screw- or mixed-type dislocations in the initial AlN layer decreased as the TMA preflow temperature increased. Further, the VDBV of the HEMT structure increased as the TMA preflow temperature increased. It is supposed that the screw- or mixed-type dislocations are the possible source of the vertical leakage current in the HEMT structures. The improvement in the crystal quality of the initial AlN layer affects the increase in the VDBV of the AlGaN/GaN HEMTs on Si substrates.

A comprehensive analysis of the degradation behavior of (In)AlGaN-based deep-ultraviolet light-emitting diode (DUV LED) stressed at a constant dc current of 60 mA has been presented. This work is based on combined X-ray diffraction (XRD), atomic force microscopy (AFM), electroluminescence (EL), cathodoluminescence (CL), current-voltage (I–V) and capacitance-voltage (C–V) characteristics. We used XRD and AFM data to investigate the structural properties of the epilayer. The EL spectrum shows two clear emission peaks at around 4.71 and 3.77 eV and a plateau region within 4.24 to 3.82 eV. To know the origin of the plateau region and the peak at 3.77 eV, we performed CL measurement. As a consequence of constant current stress, we obtained the following results: (1) a gradual degradation in EL power due to increase of Shockley–Read–Hall (SRH) recombination and the generation of new point defects in the active region; (2) a significant improvement in the leakage current, that is attributed to the enhance in trap assisted tunneling; and (3) a strong increment in the device capacitance in both reverse and forward bias regions, that is ascribed to the improvement in the net charge concentration and their redistribution in the active region of the LED.

In this study, Si-doped conductive AlInN films with a thickness of 300 nm were grown nearly lattice-matched to c-plane GaN-on-sapphire templates by metalorganic chemical vapor deposition. A high net donor concentration of approximately 1 × 1019 cm−3 was observed for a highly Si-doped AlInN film. To evaluate its vertical-direction electrical resistivity without being affected by polarization-induced carriers, the transfer length measurement (TLM) model was applied to two kinds of test element groups. By analyzing the TLM results, the vertical-direction resistivity of the 300-nm-thick n-type AlInN film was estimated to be 5.8 × 10−4 Ω cm2.

The influence of quantum-well (QW) number on electroluminescence properties was investigated and compared with that of AlN electron blocking layer (EBL) for deep ultraviolet light-emitting diodes (DUV-LEDs). By increasing the QW number, the band emission around 265 nm increased and the parasitic peak around 304 nm was suppressed. From the theoretical calculation, the electron current overflowing to the p-type layer was decreased as the QW number increased under the same injection. Correspondingly, the light output power also increased. The increment of output power from 5 QWs to 10 QWs was less than that from 10 QWs to 40 QWs, which was very different from what has been reported for blue and near-UV LEDs. The parasitic peak was still observed even when the QW number increased to 40. However, it can be suppressed efficiently by 1 nm AlN EBL for LEDs with 5 QWs. The simulation showed that the insertion of a thin EBL increased the barrier height for electron overflow and the electron current in p-type layers decreased significantly. The results contributed to the understanding of behavior of electron overflow in DUV-LEDs.

The influence of quantum-well (QW) width on electroluminescence properties of AlGaN deep ultraviolet light-emitting diodes (DUV LEDs) was studied at different temperatures. The maximum external quantum efficiency (EQE) ratios of LED with 3.5 nm QW to that with 2 nm increased from 6.8 at room temperature (RT) to 8.2 at 5 K. However, the ratios for LED with 3.5 nm QW to that with 5 nm QW decreased from 4.8 at RT to 1.6 at 5 K. The different changes of EQE ratios were attributed to the decrease of non-radiative recombination and the increase of volume of the active region. From theoretical analysis, the LED with 2-nm wells had a shallowest barrier for electron overflow due to the quantum-confined effect, whereas the LED with 5-nm wells showed the least overlap of electron and hole due to the large internal field. Therefore, the LED with 3.5 nm QW had the highest maximum EQE at the same temperature. As temperature decreased, the current for maximum EQE decreased for all the LEDs, which was believed to be due to the increase of electron which overflowed out of QWs and the decrease of hole concentration. The results were helpful for understanding the combination of polarization effect and electron overflow in DUV LEDs.

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

The surface passivation for InGaN/GaN multilayer solar cells was investigated, and it was confirmed that the device with an atomic-layer-deposited (ALD) Al2O3 passivation film showed high internal and external quantum efficiencies of 99 and 84%, respectively, along with a high energy conversion efficiency of 1.31% under a 1-sun air-mass 1.5 global illumination. The current−voltage characteristics indicated that the ALD Al2O3 film improved the surface electrical stability. The carrier lifetime measurements revealed that the ALD Al2O3 film reduced the surface carrier recombination rate and thereby contributed to the improvement of the solar cell performance in a short wavelength region.

Recently, LEDs based on heteroepitaxial GaN on Si substrates (GaN/Si) are attracting a great attention and are industrially developed by many companies in order to compete with LEDs based on GaN/sappire and GaN/SiC which are commercially marketed advancing GaN/Si LEDs, though GaN/Si LEDs have been realized early in 2002. The recent attention on GaN/Si is due the availability of low cost and large wafer size availability (up to 300 mm diameter) of Si substrates. In this article, we review the development of the GaN epitaxial growth on Si and summarize the development made in our laboratory including the device structures achieving the GaN/Si LEDs with higher emission efficiency. We describe on GaN/Si using (i) high-temperature (HT) AlN/AlGaN intermediate layers, (ii) HT intermediate layers (ILs) and multilayers (MLs), and (iii) strained-layer superlattices (SLS) interlayers and their LED performances, respectively. We believe that GaN/Si LEDs with low prices will become important LEDs for general lighting in the near future.

GaN-based light emitting diodes (LEDs) grown on 4-in. Si (111) substrate by metal–organic chemical vapor deposition have been systematically characterized. The significantly smooth surface of the sample has been confirmed by an atomic force microscope. In X-ray diffraction measurement, two kinds of fringe peaks were observed, which are believed to originate from the strained-layer superlattice (SLS) and multiple quantum wells. Moreover, from cross-sectional scanning electron microscope images of the sample, it is found that the interfaces of SLS are smooth. The 100-pair AlN/GaN SLS can be employed to modulate the strain between the GaN layer and substrate, resulting in the improvement of GaN crystalline quality. The full width at half maximum of ω -scan of the GaN (0002) diffraction is around 630ʺ. In addition, the device properties have been investigated in detail, and the maximum light output power can reach 2.02 mW with a high saturation injection current of 320 mA.

This paper presents a novel GaN switching power device-based DC-AC converter using Heterodyne Technique. Compared with the traditional DC/AC converters, the proposed converter has a simpler circuit topology with less number of high speed switching devices, thus making more efficient and cost-effective converter with reduced size and weight. By the aid of Heterodyne Technique, DC power is converted to the AC power by two half-bridge high frequency inverters operating at different frequency, which is directly transformed to the commercial frequency AC power through a small high frequency transformer. This means only one power conversion is necessary to obtain the commercial frequency alternating power from DC source. The basic theoretical working principle is explained and the circuit operations are confirmed by PSPICE circuit simulation. The prototype has been made to verify the validity of the proposed circuit topology.

We report the fabrication of fully-vertical GaN-based metal-oxide-semiconductor field-effect transistors (V-MOSFETs) on Si. A alt;iagt;palt;/iagt;-GaN current aperture was introduced in the vertical device epi-structure using plasma-based dry etching and epitaxial regrowth technique to control the vertical current conduction. The fabricated V-MOSFET exhibited drain current density of 2.5 kA/cmalt;supagt;2alt;/supagt; with ON-resistance (alt;iagt;Ralt;subagt;ONalt;/subagt;alt;/iagt;) of 4.3 mΩ-cmalt;supagt;2alt;/supagt;. The transfer characteristics of the device showed a peak trans-conductance (alt;iagt;Galt;subagt;m,maxalt;/subagt;alt;/iagt;) of 248 S/cmalt;supagt;2alt;/supagt; with a threshold voltage (alt;iagt;Valt;subagt;thalt;/subagt;alt;/iagt;) of -18.7 V during OFF-to-ON state sweeping. However, a blocking voltage of 36.5 V (drain current density 0.3 kA/cmalt;supagt;2alt;/supagt;) was observed under an OFF-state condition of the device which needs further improvement for the high-power device applications.

In this study, Al2O3-gate-insulated metal-insulator-semiconductor (MIS) heterostructure field-effect transistors (HFETs) were fabricated using an Al0.61Ga0.37In0.02N/Al0.18Ga0.82N two-dimensional-electron-gas heterostructure, and their electrical properties were characterized. It was confirmed that the thermally stable quaternary AlGaInN barrier layer contributed to a good ohmic contact resistance of 10.5 Ω mm. This value seemed to be considerably small as an AlGaN-channel heterostructure. The fabricated MIS-HFETs showed good pinch-off characteristics and exhibited a maximum drain current (IDSmax) of approximately 180 mA/mm at the gate bias of +2 V. A high off-state breakdown voltage of 2.5 kV was obtained for the device with a gate-to-drain length of 20 μm.

We used conductive atomic force microscopy (C-AFM) and aqueous potassium hydroxide (KOH) solution etching to investigate the origin of vertical leakage current through an AlN nucleation layer in AlGaN/GaN high-electron mobility transistors (HEMTs) grown on a Si substrate. The micro area vertical leakage current in the AlN nucleation layer was observed in the presence of large hexagonally shaped etch pits (Each etch pit was about 1 μm in diameter) using C-AFM measurements after KOH etching. It is considered that large hexagonally shaped etch pits originate from screw-type dislocations from the etch pit shape. Vertical leakage current in the AlN nucleation layer increased by two orders of magnitude with increasing density of large hexagonally shaped etch pits in the AlN nucleation layer. Consequently, with increased vertical leakage current in the AlN nucleation layer, the vertical leakage current in the HEMT structure also increased. These results indicate that screw-type threading dislocations in AlN nucleation layers creates the vertical leakage current path in HEMT structures on Si substrates.

The electrical properties of metal-oxide-semiconductor (MOS) capacitors with Al2O3/GaN interfaces formed by atomic layer deposition at various deposition temperatures (T d ) were systematically investigated through comparison with the interface properties of a Si MOS capacitor. Although interface trap densities (D it ) for the GaN MOS capacitors are almost the same as for the Si MOS capacitor, surface potential fluctuation (σ s ) for the GaN MOS capacitors formed at various T d are much larger than for the Si MOS capacitor. Comparison between a theoretical calculation related to σ s and the measured results clarified that the energy dependences of σ s in the bandgap of GaN cannot be explained by electron trapping to the interface traps with the single acceptor nature (0/−). We found that the result is experimental evidence of the existence of multiple charge states at the Al2O3/GaN interfaces. The existence of multiple charge states suggests that it may be difficult to obtain good carrier transport in future GaN devices with a MOS structure.

A high current gain cutoff frequency (fT ) of 90 GHz and a peak maximum oscillation frequency (f MAX) as high as 150 GHz are reported for a rectangular-shaped gate AlGaN/GaN high-electron mobility transistor (HEMT) on a high resistive silicon (HR-Si) substrate. The combined high fT /f MAX values for 100 nm unpassivated gate device demonstrate the high-quality heterostructure on silicon substrate. The reported high-performance RF device characteristics are comparable and even superior to the existing passivated AlGaN/GaN HEMTs of similar gate length. In addition, good DC characteristics have been recorded with the drain current density and an extrinsic transconductance of 0.6 A/mm and 157 mS/mm, respectively.

This paper reports on the improved crystalline quality of In 0.11 Ga 0.89 N/In 0.02 Ga 0.98 N multi-quantum well (MQW) light-emitting diode (LED) structure with the insertion of n-AlN/n-GaN multilayer (ML) and n-Al 0.06 Ga 0.94 N/n-GaN strained-layer superlattices (SLSs) cladding under-layer. The LED sample structure was epitaxially grown on Si (1 1 1) substrate by metal organic chemical vapor deposition (MOCVD) system with laminar-flow horizontal growth reactor. From X-ray rocking curves (XRCs) omega-scan ( ω -scan), the n-Al 0.06 Ga 0.94 N/n-GaN SLS under-layer facilitate in improving the crystalline quality of the LED sample structures by reducing the threading dislocation density (TDD). Reciprocal space mapping (RSM) confirms the strain state of the GaN epitaxial layers. Triple axis (TA) X-ray diffraction (XRD) omega-2Theta ( ω -2 θ ) scan reveals a higher-order fringes indicate the ML and MQW layers with sharp layer interfaces. Photoluminescence (PL) measurement of the sample with n-Al 0.06 Ga 0.94 N/n-GaN SLS under-layer shows band-to-band photon energy of 2.97 eV with narrower full-width at half-maximum (FWHM) as compared to the sample with n-Al 0.03 Ga 0.97 N under-layer. Surface morphology examination by atomic force microscopy (AFM) demonstrate smoother sample surface for the sample with n-Al 0.06 Ga 0.94 N/n-GaN SLS as compared to conventional sample without the SLS layer.

The vertical leakage current of AlGaN/GaN high-electron-mobility transistors on Si substrates was studied. The effects of the Al content in the AlGaN buffer layer and pit density on the vertical leakage current were not as significant as the effect of the initial AlN layer's crystal quality.

SUMMARY Current collapse of AlGaN/GaN heterostructure fieldeffect transistors (HFETs) formed on qualified epitaxial layers on Si substrates was successfully suppressed using graded field-plate (FP) structures. To improve the reproducibility of the FP structure manufacturing process, a simple process for linearly graded SiO2 profile formation was developed. An HFET with a graded FP structure exhibited a significant decrease in an on-resistance increase ratio of 1.16 even after application of a drain bias of 600 V.

An extensive study on the use of Si as a substrate for the growth of AlGaN/GaN layers for High-Electron-Mobility Transistor (HEMT) were studied and reported in this article. We have used thick buffers to grow high resistive i-GaN by MOCVD which offers a high breakdown voltage. While the leakage through buffer and substrate can be controlled by thick buffer, the leakage through gate is controlled using a thin 2-nm in-situ grown i-GaN cap layer. We have evidenced a high figure of merit (BV2/RON) of 2.6 x 108 V2Ω-1cm-2 for AlGaN/GaN HEMTs grown on 4-inch Si substrate. The challenges before the MOCVD growth of GaN on Si is also discussed in detail.

The dependence of electron overflow on injection current, operating temperature, and structure of p-type layers was investigated in 264 nm AlGaN light-emitting diodes (LEDs). Both increasing current and decreasing temperature resulted in the increase of electron overflow due to the insufficient barrier height and the increase of electrical field in p-type layers, respectively. The use of heterostructure as p-type layer was more favorable to suppress the overflow than single layer owing to the higher barrier for electron overflow and the lower barrier for hole injection induced by the polarization field. Both simulation and experiment showed that the insert of thin i-AlN interlayer between active region and p-type layers can suppress the electron overflow effectively due to the further increase of barrier height. The optical properties of such LEDs were improved significantly and the maximum output power was increased by two orders of magnitude.

Two kinds of substrates, sapphire and AlN/sapphire template (AlN template), are used for the growth of InGaN/GaN multi-quantum-well solar cell structures by metalorganic chemical vapor deposition, and their material and device properties are investigated. The results show that the samples grown on AlN template have a better crystal quality with a larger in-plane compressive strain than those on sapphire, and solar cells fabricated on sapphire mostly exhibit better performance than those on AlN template. An analysis of the photoluminescence measurements indicates that a critical InGaN well thickness related to the generation of nonradiative recombination centers, which affects the internal and external quantum efficiencies, is thinner in samples grown on AlN template than in samples on sapphire. The critical thickness is speculated to be related to the large in-plane compressive strain in the samples on AlN template. By contrast, in comparison between samples with a sufficiently thin InGaN well thickness of 1.0 nm, the sample on AlN template exhibits better solar cell performance than on sapphire. This implies that the improved crystal quality contributes to the improvement of internal quantum efficiency as long as the well layer is thinner than the critical thickness.

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.

In this paper, we demonstrate a novel method to integrate photonic crystals (PhCs) on GaN-based blue light-emitting diodes (LEDs) using a silicon substrate as a mold for forming the PhCs. This method starts with fabricating a 2D grooved Si substrate as that mold. Subsequently, GaN-based epitaxial layers are grown on the Si mold-substrate, which effectively reduces the dislocation density in GaN by enhanced lateral epitaxial growth. After the epitaxial layers are bonded onto a highly reflective substrate, the Si mold-substrate is removed. This substrate-transfer technique replicates PhC from the mold-substrate on the LED surface free from processing damages. The resultant LEDs with PhC have outperformed the LEDs without PhC in the optical output power by 80%, taking advantage of the enhanced light extraction by PhC.

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.

We present results of some novel AlGaN/GaN heterojunction field-effect transistors (HFETs) specifically developed for RF front-end and power applications. To reduce the parasitic resistance, two unique techniques: selective Si doping into contact area and a superlattice (SL) cap structure, are developed. With the selective Si doping method, a transistor with an on-state resistance as low as 1.86 Ω·mm and a Tx/Rx switch IC with very low insertion loss (0.26 dB) and very high power handling capability (P1dB over 40 dBm) were obtained. With the SL cap HFETs, an ultra low source resistance of 0.4 Ω·mm was achieved and excellent DC and RF performances were demonstrated. The typical characteristics of these HFETs are: maximum transconductance of over 400 mS/mm, maximum drain current of 1.2 A/mm, cut-off frequency of 60 GHz, maximum oscillation frequency of 140 GHz, and a very low noise figure of 0.7 dB with 15 dB gain at 12 GHz. For power applications, in order to significantly reduce fabrication cost, we fabricated the AlGaN/GaN HFET on a conductive Si substrate with a source-via grounding (SVG) structure. The device has a very low on-state sheet resistance of 1.9 mΩ·cm2, a high off-state breakdown voltage of 350 V, and a current handling capability of 150 A. In addition, a sub-nano second switching response with t r of 98 ps and t f of 96 ps with a current density as high as 2.0 kA/cm2 is demonstrated for the first time.

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.

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.

We report the improved characteristics of GaInN multiple-quantum-well (MQW) light-emitting diodes (LEDs) on Si substrate by an insertion of distributed Bragg reflector (DBR). The DBR-based GaInN MQW LED structures were grown on 2-in-diameter n-Si (1 1 1) substrates using a metalorganic chemical vapor deposition method. The number of Al 0.3 Ga 0.7 N/AlN pairs in DBR was changed from 1 to 5. No cracks were observed over the entire area of the DBR-based LED structure with less than 3-pair DBR. The light output power of the 3-pair DBR-based LED is approximately two times larger than that of the non-DBR-based LED.

The concentration of F center in high-surface-area anatase titania nanoparticles prepared by MOCVD are studied with electron paramagnetic resonance measurements at different time and different temperatures. The results show that the kinetics of F center is dominated by surface process rather than bulk process. With increasing temperature, two temperature dependent regions for the concentration of F center can be clearly identified. The starting temperature and activation energy of the surface process are determined to be 357.4 K and 0.085 eV, respectively. The reaction of F center with oxygen is of zero order and the concentration of F center can be well described by C ( T , t ) = 1.7 × 10 18 exp ( − 986.87 T ) ( 1 − t 386.1 ) ( g − 1 ) .

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.

For the purpose of the practical use of GaN-based electronic devices, we demonstrated the growth of 100-mm-diameter AlGaN/GaN heterostructures on c-face sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). The obtained films have specular surface, good crystal quality and good uniformity of the Al composition across the entire epitaxial wafer. These epitaxial wafers showed a relatively high electron mobility, 1320 ± 80 cm2/Vs with the sheet carrier concentration of 8.4 ± 0.5 × 1012/cm3 at room temperature, across the entire epitaxial wafer. High-electron-mobility transistors (HEMTs) fabricated on the 100-mm-diameter epitaxial wafer also exhibited good dc characteristics. Typical values of the peak extrinsic transconductance and the maximum drain current density were 200 mS/mm and 534 mA/mm, respectively, for the device with the gate length of 2.0 μm and the gate width of 15.0 μm. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recessed gate AlGaN/GaN high electron mobility transistors (HEMTs) grown on sapphire substrate have been fabricated. In order to improve FET performances, we optimized the layer structure and the electrode arrangement of the HEMT, and hence 0.15 μm gate-length AlGaN/GaN-HEMTs with recessed gate were successfully fabricated and the obtained transconductance was as high as 450 mS/mm. In this paper we describe the improvement of HEMT layer structures on sapphire substrate, the optimisation of an offset arrangement of gate electrodes, and the results of DC/RF measurements of our HEMTs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An excellent on/off current ratio of 1010 and a nearly ideal subthreshold slope of 65 mV/dec was confirmed in a p-InGaN/AlGaN/GaN high-electron-mobility transistor. Favorable I-V characteristics were achieved with the p-InGaN cap layer under the gate electrode. A dry etching technique with a low-damage p-InGaN cap layer resulted in a significantly low leakage current of 10-11 A/mm.

We demonstrate high-performance ultraviolet photodetectors (UV-PDs) based on lattice-matched (LM) InAlN/AlGaN heterostructure field-effect transistors (HFETs) gated by transparent ITO films. Low dark currents of 6.8 × 10−8 and 6.1 × 10−7 A/mm and high photocurrent gains over four and three orders of magnitude were obtained for the LM In0.12Al0.88N/Al0.21Ga0.79N and In0.10Al0.90N/Al0.34Ga0.66N HFETs, respectively. The negative threshold voltage shifts under illumination indicate that most of the photo-generated carriers are transported in the two-dimensional gas (2DEG) region around the InAlN/AlGaN interface. High peak responsivities of 2.2 × 104 and 5.4 × 104 A/W and large UV-to-visible rejection ratios greater than 104 and 103 were achieved for the LM In0.12Al0.88N/Al0.21Ga0.79N and In0.10Al0.90N/Al0.34Ga0.66N HFET-type UV-PDs, respectively. These improved performances with respect to other AlGaN UV-PDs around the same wavelength detection range may possibly be attributed to the greater contribution of t...