A systematic study of titanium dioxide (TiO2) nanotubes (NTs) grown by electrochemical anodization in NH4F + glycerol electrolyte has been carried out in a broad range of anodization voltage of 5-350 V and acid concentration of 0.1-0.7 wt%. It is found that NTs can be grown in the voltage range from 10 to 240 V. The maximum NH4F acid concentration at which NTs can be formed decreases with the anodization voltage (V(a)). The maximum NH4F acid concentration is 0.7% for V(a)<60 V, and it decreases to 0.1% at V(a) = 240 V. Glancing angle x-ray diffractometer (GAXRD) measurements show that as-grown amorphous TiO2 transforms to the anatase phase when annealed at 400 degrees C, and further transforms to the rutile phase at annealing temperatures higher than 500 degrees C. The transition temperature from anatase to rutile phase depends on the anodization conditions. The electrical resistivity of the NT increases by eight orders of magnitude when V(a) increases from 10 to 240 V.

Selective area epitaxy has been used to grow pyramidal GaN stripes, followed by InGaN multiple quantum well (MQW) structures, in order to produce long-wavelength green light emission. Stripes oriented along ⟨112¯0⟩ produce smooth {11¯01} sidewall facets. The room-temperature optical properties are investigated by cathodoluminescence spectroscopy using a scanning electron microscope. MQWs grown in unmasked reference regions exhibit emission at 450 nm. The stripe sidewalls emit light with peak wavelength of 500 nm with consistent linewidth and intensity. The stripe ridge emits light with peak intensity at wavelength of ∼550 nm. Based on the spatial extent of the 550 nm emission, the ridge is estimated to be ∼250 nm wide. The large redshift is produced by the enhanced presence of indium species due to lateral vapor diffusion and surface migration in selective area epitaxy.

We have prepared large-area, 0.50×0.55mm2, metal-semiconductor-metal photodetectors based on AlN layers with different density of inversion domains (IDs). AlN layers were grown on (0001) sapphire substrates using gas source molecular beam epitaxy. The introduction of AlN∕GaN short period superlattices after growth of AlN nucleation layer yields significant reduction in the ID density. Photodetectors with ID density of 106cm−2 exhibit a very low dark current of 0.5fA at zero bias, which remains below 50fA up to a bias of ±30V. The peak responsivity of 0.08A∕W was obtained at a wavelength of ∼202nm.

The effect of BCl3 and BCl3/Ar pretreatment on Cl2/Ar and Cl2/Ar/BCl3 dry etching of AlN is investigated using inductively coupled plasma reactive ion etching. The native AlN oxide can be effectively removed by a short exposure to BCl3 or BCl3/Ar plasma. Compared to the chlorine based plasma etching, BCl3/Ar is found to have the highest etch rate for both AlN and its native oxide. Following removal of the native oxide, Cl2/Ar/BCl3 plasma etching with 15% BCl3 fraction results in a high etch rate ˜ 87 nm/min and modest increases in the surface roughness.

The Mg doped AlN/AlxGa1-xN (0.03 ≤ x ≤ 0.05) short period superlattices (SPSLs) were grown by gas source molecular beam epitaxy on (0001) sapphire substrates. The average AlN mole fraction is ∼ 0.7 and the hole concentration is ∼ 7×1017 cm-3. Contacts formed to the SPSLs using Ni/Au bilayer are found to have specific contact resistance ∼ 5×10-5 Ωcm2 near room temperature and to show weak temperature dependence attributed to activation of Mg acceptors in the AlN barriers of SPSLs. These p-SPSLs are attractive for fabrication of transparent low resistive ohmic contacts for deep UV LEDs.