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One of the main issues for high-temperature operation of GaN- based devices is the development of reliable metal contacts. We have investigated the thermal stability and device performance of AlGaN/GaN high-electron mobility transistors (HEMTs) with both conventional (Ti/Al/Ni/Au) and Ir-based (Ti/Al/Ir/Au) Ohmic contacts. Both dc and rf measurements showed better performance with Ir-based contacts, from room temperature up to 500{degree sign}C. We also fabricated AlGaN/GaN HEMTs with Ti/Al/TiB2/Ti/Au source/drain Ohmic contacts and a variety of gate metal schemes (Pt/Au, Ni/Au, Pt/TiB2/Au or Ni/TiB2/Au) and subjected them to long-term annealing at 350{degree sign}C. By comparison with companion devices with conventional Ti/Al/Pt/Au Ohmic contacts and Pt/Au gate contacts, the HEMTs with boride- based Ohmic metal and either Pt/Au, Ni/Au or Ni/TiB2/Au gate metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350{degree sign}C.

In the paper we present band-gap alignment diagrams for type-II/type-I heterostructures incorporating the strong piezoelectric and spontaneous polarization fields in ZnCdMgO and AlGaN-based materials and interfaces. The paper describes some of the recent progress made in the development of high quality ZnCdO layers and CdZnO/ZnO heterostructures grown epitaxially by RF-plasma molecular beam epitaxy (MBE). We summarize optical and electrical properties of high quality CdxZn1-xO alloys with Cd mole fraction from 0.02 to 0.78 and discuss phase separation phenomenon, which may be present in ternary alloys. A single-crystal wurtzite structure of CdZnO alloys has been confirmed. We achieved a strong optical emission at RT in the 380 nm to 574 nm spectral range from CdxZn1-xO with various compositions. Dependence of the fundamental optical band gap on the composition of CdxZn1- xO alloys, band gap bowing, and the possible effect of composition micro-fluctuations in ternary CdxZn1-xO alloys on the optical bandgap is also discussed. Time resolved photoluminescence shows multi-exponential decay with 21 psec. and 49{plus minus}3 psec. lifetimes, suggesting that composition micro- fluctuations may be present in Cd0.16Zn0.84O film. We also report on crystallographic and optical properties of CdZnO/ZnO multiple quantum wells.

We have carried out comprehensive optical studies to evaluate structural and bandgap properties of Zn1-xCdxO alloys with x {less than or equal to} 0.17 grown by molecular beam epitaxy. High crystalline quality of the alloys was concluded from cathodoluminescence measurements. Based on absorption and reflectance measurements, the compositional dependence of the bandgap energy of ZnCdO, estimated without taking into account excitonic effects, was found to follow the trend Eg(x)=3.28- 2.23x+0.45x2. Degradation in the alloy quality due to possible phase separation was found to cause deviations from this trend, evident from a more rapid red shift of the absorption edge. Effects of Cd incorporation on the variation of the bandgap energies with temperature are also discussed.