Mahesh Pandikunta

The anodization voltage influence on the properties of TiO2 nanotubes grown by electrochemical oxidation

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.

Green light emission from InGaN multiple quantum wells grown on GaN pyramidal stripes using selective area epitaxy

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.

High quality AlN for deep UV photodetectors

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.

GaN stripes on vertical {111} fin facets of (110)-oriented Si substrates

Selective sidewall epitaxy of AlN/GaN is reported on vertical fins of silicon using metallorganic vapor phase epitaxy. Silicon (110) wafers are structured to form fins with {111} sidewall facets. AlN buffer layers are grown with uniform thickness on vertical {111} surfaces, followed by GaN which grows selectively on the AlN to form the sidewall fin structures. Raman measurements of the GaN show very narrow line widths, consistent with excellent material quality. Spatial dependence from microcathodoluminescence mapping of the GaN band gap emission shows compressive strain in the GaN relaxes closer to the fin corners.

Reflection high energy electron diffraction investigation and comparison of the initial stage during molecular beam epitaxy of AlN on Si(111) and Si(110) substrates

The onset of AlN nucleation on Si(111) and Si(110) surfaces during gas source molecular beam epitaxy with ammonia was carried out by reflection high energy electron diffraction. Exposing the clean Si surfaces to NH3 flux at 600 °C yields the formation of crystalline Si3N4 on both (111) and (110) surfaces. An 8×8 Si3N4 structure was observed for the Si(111) surface. On the Si(110) surface a 2.87 A periodic structure was observed for electron beam directed along [001] azimuth and 2.46 A periodic structure for the [1¯12] azimuth. Together, these periodic structures confirm the formation of Si3N4 (0001) plane on both Si(111) and Si(110) surfaces.

Influence of phonons on the temperature dependence of the band gap of AlN and AlxGa1−xN alloys with high AlN mole fraction

The temperature dependence of the optical band gap of AlN and AlxGa1−xN alloys, with x ∼ 0.6, has been studied using optical absorption spectroscopy. The band gap shrinkage is interpreted based on electron-phonon interactions using a two-phonon oscillator model. The two-oscillator model includes average acoustic and optic phonon energies and thermal occupation factor described by the Bose function. The temperature dependence of the transition width, from transparent to opaque, is also described based on electron-phonon interactions.

N-polar AlN thin layers grown on Si(111) by plasma-assisted MBE

The influence of RF power on the growth of N-polar AlN thin layers on Si(111) by plasma-assisted molecular beam epitaxy was systematically studied. The surface reconstruction of AlN was 1 × 1 for samples grown at RF power of 350–450 W, while samples grown at lower power of 150–300 W had 3 × 6 reconstruction. All samples demonstrate smooth surface with root mean square roughness less than 1 nm. The 45 nm thick AlN sample grown at RF power of 150 W has the best structural quality. The screw and edge dislocation densities estimated for this sample were 1.4 × 109 and 5.9 × 109 cm−2, respectively.

Characterization of Si3N4/Si(111) thin films by reflectance difference spectroscopy

Si3N4 has become an important material with great technological and scientific interests. The lattice symmetry and the crystallinity quality of Si3N4 thin films are fundamental parameters that must be determined for different applications. In order to evaluate the properties of Si3N4 films, we used reflectance difference spectroscopy/reflectance anisotropy spectroscopy (RDS/RAS) to measure the optical anisotropy of Si3N4 thin films (1–2 nm) grown by nitridation of two different Si(111) substrates, one with a 4.2° miscut off towards the direction and another one with a nonintentional miscut. We demonstrate that, by modifying the measurement optical setup, we could increase the RD sensitivity and clearly display the optical response corresponding to the hexagonal symmetry of the Si3N4 thin layer. Our results are in good agreement with reflection high energy electron diffraction (RHEED) measurements for both misoriented and oriented substrates.

Green light emission from selectively grown InGaN multiple quantum well stripes oriented along [11-20] direction

InGaN multiple quantum wells (MQWs) with green light emission have been grown on GaN stripes oriented along the [11-20] direction by selective metal-organic vapor phase epitaxy (MOVPE). Several different window widths were designed in the SiO2 mask. Completed pyramidal InGaN stripes with flat and smooth {1-101} sidewall were produced on 2-μm windows while trapezoidal stripes with both {1-101} sidewall and (0001) top surface were obtained on the 5-μm windows. The former has uniform CL emissions at 500 nm on the {1-101} sidewall and at 550 nm on the narrow ridge. The latter exhibits similar CL emissions at 500 nm on the sidewall and at 570 nm on the top surface. These wavelength shifts relative to the CL spectrum peak (450 nm) from the reference region are attributed to thickness enhancement and indium enrichment in selective MOVPE. The short-wavelength shoulder near 500 nm in the spectrum from the ridge of the completed pyramidal strip is attributed to overlapping excitation of the sidewall by the SEM incident beam.