Selective growth of N-polar InN by exploiting the similar to 100 degrees gap between the upper limits of the growth temperatures of In- and N- polarities has been introduced. An InN epilayer grown at a temperature in this gap on a sapphire substrate covered with an ultrathin AlN layer has demonstrated N-polarity. The dislocation density of such-grown InN layer has been significantly reduced (one order of magnitude lower than the conventional InN epilayers). The results have demonstrated great potential for improving the crystalline quality of hetero-epitaxial InN films. This concept can be easily adopted for other substrates. (C) 2014 AIP Publishing LLC.

High-quality thick GaN layers were directly grown by hydride vapor phase epitaxy on sapphire substrates preliminary processed by a laser beam focused within the substrate. The critical thickness for crack generation was improved drastically from 10-20 mu m for regular substrates to 220-250 mu m by using the laser processed sapphire substrates. Layers of 200 mu m thickness grown on the laser processed sapphire substrates exhibited a crack-free surface over the entire 2-in. area and a threading dislocation density as low as 1 x 10(7) cm(2), which is one order of magnitude lower than that achievable with a regular sapphire substrate. (C) 2013 The Japan Society of Applied Physics

Recently ozone is one of natural hazards which comes from cars, industry using ozone for sterilization of organic and inorganic materials and for water purification. So, ozone sensing becomes very important, and convenient and accurate ozone sensor is required. A new high sensitivity ozone sensing system using an deep ultra-violet light emitting diode (DUV-LED) operated at the wavelength of 280 nm has been successfully constructed. The fabrication of diode operated at 280 nm is much easier than that of DUV-LED operated at Hg lamp wavelength of 254 nm. The system is compact and possible to sense the ozone concentration less than 0.1 ppm with an accuracy of 0.5% easily with low power DUV-LED of around 200 micro Watts operated at 280 nm without any data processing circuit.

A drastic increase in the p-type carrier concentration and decrease in the activation energy of an acceptor are achieved by using a method of pulse supply of the source gases for AlGaN and GaN. This method offers a new way of producing a low-resistance p-AlGaN and p-GaN epitaxial layer.

The importance of the atomically well-controlled surface of sapphire substrate with slight misorientation and ideally minimized surface roughness for III–nitride epitaxy is discussed in detail. An atomically controlled surface of sapphire substrate with slight misorientation angle is modeled and an almost ideal level of atomic surface roughness of sapphire substrate is found to be obtained by a chemical mechanical polishing (CMP) with colloidal silica. Cathodoluminescence (CL) imaging indicated the complete absence of subsurface damage induced by mechanical polishing. GaN and AlN thin films are grown on misoriented sapphire substrate with an atomically controlled surface by the CMP to investigate the misorientation angle of both sapphire and grown GaN and AlN thin films. An interface model is proposed to explain the difference in misorientation angle between sapphire and III–nitride thin films, providing strong evidence of the necessity of atomically controlled surface of sapphire substrate for III–nitride epitaxy.

Disinfection of bacterial viruses MS2, Q beta, and phi X174 in water was successfully carried out using deep ultraviolet light/emitting diodes (DUV/LEDs) operated at 280 nm and 255 nm. It was shown that the 280/nm DUV/LEDs are much more suitable than the 255/nm DUV/LEDs for the disinfection of water because, even though the disinfection efficiency of the former DUV/LEDs is only half that of the latter ones, the production of high/power 280/nm DUV/LEDs is much easier than that of the 255/nm ones because the external quantum efficiency of the 280/nm DUV/LEDs is more than ten times that of the 255/nm DUV/LEDs. We propose the use of vertical emitting DUV/LEDs for the design of an efficient DUV/LED disinfection chamber. This disinfection technique using DUV/LEDs will open new avenues for water purification. DOI: 10.1061/(ASCE)EE.1943/7870.0000442. (C) 2011 American Society of Civil Engineers.

A method of alternative co-doping in metal organic chemical deposition was used to realize high hole carrier concentrations of about 6 × 1018/cm3 in AlxGa1−xN (x = 0.4) and 2 × 1019/cm3 for GaN at room temperature. This technique opens up a new avenue for fabricating electronic p-channel devices, such as p-channel high electron mobility transistor, and vertical current flow type devices, such as deep ultra violet light emitting diodes.

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A channel layer substitution of a wider bandgap AlGaN for a conventional GaN in high electron mobility transistors (HEMTs) is an effective method of enhancing the breakdown voltage. Wider bandgap AlGaN, however, should also increase the ohmic contact resistance. Si ion implantation doping technique was utilized to achieve sufficiently low resistive source/drain contacts. The fabricated AlGaN channel HEMTs with the field plate structure demonstrated good pinch-off operation with sufficiently high drain current density of 0.5 A/mm without noticeable current collapse. The obtained maximum breakdown voltages was 1700 V in the AlGaN channel HEMT with the gate-drain distance of 10 μm. These remarkable results indicate that AlGaN channel HEMTs could become future strong candidates for not only high-frequency devices such as low noise amplifiers but also high-power devices such as switching applications.

Large-area (∼1 cm2) laser lift-off (LLO) wafer separation of Al0.45Ga0.55N layers from AlN/sapphire templates has been demonstrated by using 200-period AlN/Al0.22Ga0.78N short-period superlattice (SPSL) sacrificial layers instead of conventional GaN photoabsorbing layers. The SPSL functions as the photoabsorbing and mechanically weakened layer in the LLO process. This SPSL-assisted LLO technique promises future progress of vertical-type deep ultraviolet light emitting diodes and freestanding AlN–AlGaN bulk substrates.

A channel layer substitution of a wider bandgap AlGaN for conventional GaN in high electron mobility transistors (HEMTs) is one possible method of enhancing the breakdown voltage for higher power operation. Wider bandgap AlGaN, however, should also increase the ohmic contact resistance. We utilized a Si ion implantation doping technique to achieve sufficiently low resistive source/drain contacts, and realized the first HEMT operation with an AlGaN channel layer. This result is very promising for the further higher power operation of high-frequency HEMTs. (c) 2008 The Japan Society of Applied Physics.

The channel layer substitution of a wider bandgap AlGaN for a conventional GaN in high electron mobility transistors (HEMTs) is an effective method of enhancing the breakdown voltage. We demonstrated a remarkable breakdown voltage enhancement in these AlGaN channel HEMTs. The obtained maximum breakdown voltages were 463 and 1650 V in the Al0.53Ga0.47N/Al0.38Ga0.62N HEMT with the gate-drain distances of 3 and 10 mu m, respectively. This result is very promising for the further higher power operation of high-frequency HEMTs. (c) 2008 American Institute of Physics.

To investigate the effect of Si doping on AlxGa1-xN/AlyGa1-yN multiple quantum wells (MQWs), photoluminescence (PL) measurements of Al0.15Ga0.85N/Al0.20Ga0.80N MQWs with an undoped Al0.20Ga0.80N barrier layer (undoped MQWs) and with a Si-doped layer (Si-doped MQWs) are compared in weak (1.8 ×10-4 to 5.2 ×10-3 W/cm2) and strong (360 to 1.28 ×104 W/cm2) optical excitation ranges at 10 K. Based on the results of PL measurement, the carrier injection into the undoped MQWs induced by light illumination in the strong optical excitation range and that into the Si-doped Al0.15Ga0.85N/Al0.20Ga0.80N MQWs in the weak optical excitation range show an increase in the PL intensity and PL peak energy, and a narrowing of the full width at half maximum (FWHM) of the PL peak. Although the internal polarization fields of the undoped and Si-doped MQWs are screened in the strong optical excitation range, the PL intensity of the undoped MQWs is higher than that of the Si-doped MQWs. The difference in the PL intensity is attributed to the formation of a nonradiative center due to Si doping.

Improvement of Al-polar AIN layer quality was accomplished by three-stage flow-modulation metalorganic chemical vapor deposition (FM-MOCVD). In this method, the unit of the FM-MOCVD sequence was composed of three stages; Stage I for simultaneous source supply, Stage II for trimethylaluminum supply, and Stage III for ammonia supply, which were cyclically repeated. The AIN quality revealed by X-ray diffraction strongly depended on the time of Stage I, A growth model was proposed considering the surface coverage of the islands nucleated during Stage I. Exciton fine structures were eventually observed by low-temperature cathodoluminescence reflecting the tremendously improved crystalline quality. (c) 2008 The Japan Society of Applied Physics.

2007 International Conference on Solid State Devices and Materials,First Operation of AlGaN Channel High Electron Mobility Transistors with Sufficiently Low Resistive Source/Drain Contact formed by Si Ion Implantation

The thermal stabilities of Al- and N-polarity AlN layers grown on (0001) sapphire substrates were investigated at temperatures ranging from 1100 to 1400°C in various gas flows (He, H2 and H2+NH3). Decomposition of AlN occurred in flowing H2, while it did not occur in He or H2+NH3 flow. The decomposition rate in H2 increased with an increase in the temperature over 1200°C, and the decomposition rate of the Al-polarity AlN layer was found to be lower than that of the N-polarity AlN layer at each temperature. The decomposition reaction of AlN and the relationship between the polarities of the AlN layers and their different decomposition rates are discussed.

We report barrier-height and well-width dependence of room-temperature photoluminescence from AlGaN-based quantum well structures for deep-UV light emitting device applications. A quantum well (QW) sample consisting of 1.5nm-Al0.15Ga0.85N wells/7nm-Al0.40Ga0.60N barriers showed the largest emission intensity in our samples. Quantum confined Stark effect on the emission energy and intensity in wide-well conditions, carrier penetration into the barrier regions in narrow well conditions and heterointerface defect formation were comprehensively considered in the discussion. If the heterointerface defect formation is ideally suppressed by optimal growth conditions, QW structures with narrow wells and high barriers will be a better design for highly efficient deep-UV light emitting devices.