Misaichi Takeuchi

Remarkable breakdown voltage enhancement in AlGaN channel high electron mobility transistors

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 1650V in the Al0.53Ga0.47N∕Al0.38Ga0.62N HEMT with the gate-drain distances of 3 and 10μm, respectively. This result is very promising for the further higher power operation of high-frequency HEMTs.

First Operation of AlGaN Channel High Electron Mobility Transistors

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.

Vertical AlGaN deep ultraviolet light emitting diode emitting at 322nm fabricated by the laser lift-off technique

A vertical AlGaN deep ultraviolet (DUV) light emitting diode (LED) emitting at 322nm was fabfricated by the laser lift-off technique. The emission area extended to the entire electrode uniformly, and the current crowding was suppressed effectively in the devices. As a result, the differential conductance of the vertical LED was improved by a factor of 5 and the operation voltage was reduced to half, compared to that of the lateral LED. The self-heating effect was effectively suppressed even at high-current-density operation. The vertical structure in the high resistive AlGaN LED has potential application in high-power AlGaN DUV devices.

Improvement of Al-Polar AlN Layer Quality by Three-Stage Flow-Modulation Metalorganic Chemical Vapor Deposition

Improvement of Al-polar AlN 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 AlN 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.

Remarkable Breakdown Voltage Enhancement in AlGaN Channel HEMTs

We demonstrated a remarkable breakdown voltage enhancement in a new high-electron-mobility transistor (HEMT) with a wider bandgap AlGaN channel layer. A Si ion implantation doping technique was utilized to achieve sufficiently low resistive source/drain contacts. The obtained maximum breakdown voltage was 1650 V with a gate-drain distance of 10 mum. This result is very promising for the further higher-power operation of high-frequency HEMTs.

Selective growth of N-polar InN through an in situ AlN mask on a sapphire substrate

Selective growth of N-polar InN by exploiting the ∼100° 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.

Excitation-Density Dependence of Photoluminescence from Si-Doped AlGaN/AlGaN Multiple Quantum Wells at Low Temperature

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.

Photonic crystal and III-N quantum dot laser

Deep UV light emitting devices are important for future medical, environmental and IT applications. Generation of DUV light using quantum dots GaN-related materials and nonlinear photonic crystals are demonstrated, and both techniques are compared.