M. J. Lukitsch

Optical and electrical properties of Al1−xInxN films grown by plasma source molecular-beam epitaxy

Epitaxial Al1−xInxN thin films with 0⩽x⩽1 have been grown by plasma source molecular beam epitaxy on sapphire (0001) substrates at a low temperature of 375 °C. Both reflection high-energy electron diffraction and x-ray diffraction measurements confirm the c-plane growth with the following epitaxial relations: nitride [0001] ∥ sapphire [0001] and nitride 〈0110〉 ∥ sapphire 〈2110〉. However, the degree of crystalline mosaicity and the compositional fluctuation increase with increasing x. The observed direct energy band gap, determined using optical transmission and reflection measurements show relatively less bowing compared to some earlier studies. Electrical resistivity and Hall effect measurements show n-type electrical conductivity in these alloys with carrier concentrations n⩾1019 cm−3 for In-rich alloys and n⩽1010 cm−3 for Al-rich alloys.

Ultraviolet and visible resonance-enhanced Raman scattering in epitaxial Al1−xInxN thin films

We report the observation of ultraviolet and visible near-resonance enhanced Raman scattering in epitaxial wurtzite Al1−xInxN (0001) (0⩽x<0.7) thin films. The films (thickness∼150 nm) were grown by plasma source molecular beam epitaxy on sapphire (0001) substrates. A substantial spectral enhancement is seen for Al-rich samples using 244 nm (5.01 eV) radiation due to the closeness of their band gap energy to the excitation energy. On the other hand, samples with x∼0.6 (energy band gap ∼2.5 eV) show significant enhancement with 514.5 nm (2.41 eV) excitation. The A1(LO) and E2 zone center phonons have been observed for all the samples. The A1(LO) phonon frequency shows the expected decrease with increasing x. The E2 mode shows a two-mode behavior supporting the recent theoretical predictions. Due to increased resonance enhancement, strong second- and third-order spectra are seen in some films.

Development of Wide Bandgap Semiconductor Photonic Device Structures by Excimer Laser Micromachining

Excimer laser ablation rates of Si (111) and AlN films grown on Si (111) and r-plane sapphire substrates were determined. Linear dependence of ablation rate of Si (111) substrate, sapphire and AlN thin films were observed. Excimer laser micromachining of the AlN thin films on silicon (111) and SiC substrates were micromachined to fabricate a waveguide structure and a pixilated structure. This technique resulted in clean precise machining of AlN with high aspect ratios and straight walls.

Processing and Characterization of Al 2 Ti/Al 3 Ti Two-Phase Alloys

Several buttons of each of three binary Ti-Al alloys containing nominally 70, 71 and 72 at.% Al were prepared from elemental Ti and Al by plasma arc-melting. One button of each composition was then either solution heat-treated, subjected to a two-step heat treatment, or hot forged. Each composition and material condition was then characterized using SEM/EDS and microhardness testing. The results show that while all three compositions have a predominantly two-phase, coarse microstructure in the as-cast condition, it is possible to produce an essentially single phase material through an appropriate solution heat treatment for the two lower Al content alloys. A fine two-phase microstructure can be achieved through an additional, lower temperature heat treatment step. The microhardness results show that the solution heat treatment reduced the hardness for all three compositions compared to the as-cast condition while hot forging as-cast samples increased hardness. The 70% Al alloy has the highest hardness for all four material conditions studied while the 71% Al alloy has the lowest hardness in three of the four material conditions.

Fracture Toughness of Stoichiometric, Non-Stoichiometric and Ternary-Alloyed Al 2 Ti

Polycrystalline stoichiometric Al{sub 2}Ti was produced via casting or powder metallurgy and further processed yielding material in six conditions. The fracture toughness of the six material conditions was determined from the critical load to initiate cracks with a Vickers indenter. The results show a strong dependence on material condition. Powder processed Al{sub 2}Ti exhibits the highest fracture toughness value among the material conditions studied. Polycrystalline non-stoichiometric Al{sub 2{+-}y}Ti{sub 1{+-}y} and ternary-alloyed Al{sub 2}Ti + X were prepared in as-cast and cast and annealed conditions. Each material condition exhibited a multiphase microstructure. The composition of the phases present in each alloy was analyzed with SEM/EDS. Fracture toughness values of cast and annealed stoichiometric and non-stoichiometric binary alloys are 20--30% greater than as-cast stoichiometric Al{sub 2}Ti. For the ternary alloys, the fracture toughness values show a strong dependence on the ternary element used and heat treatment condition. The fracture toughness values of three hot forged ternary alloys were also determined.