The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT property superior to the other two samples, with a resistivity change as large as 9 × 104 times and a transition window as narrow as 3.9 K, and it has the highest resistivity with the lowest free carrier density in the insulating phase. THz transmission measurements on VO2 films grown on r-plane sapphire substrates revealed intensity modulation depth as large as 98% over a broadband THz region, suggesting that VO2 films are ideal material candidates for THz modulation applications.

Integration of diamond and AlGaN/GaN highelectron mobility transistors (HEMTs) terminated with an in situ grown SiN x interface layer via metal organic chemical vapor deposition is investigated. The effect of diamond growth on the structure and interface properties of the HEMT is studied using high-resolution X-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, and scanning transmission electron microscopy (STEM). No structural or physical damage is observed to the HEMT device layers as a result of the hot filament chemical vapor deposition diamond fabrication process. The TEM cross section confirms the smooth and abrupt interface of in situ SiN x /AlGaN/GaN before and after the diamond growth, with no detectable carbon diffusion into the GaN buffer layer. However, selective degradation of the in situ SiN x dielectric adhesion layer was observed at the SiN x /diamond interface. Using time domain thermoreflectance (TDTR), the effective isotropic thermal conductivity of the diamond was determined to be 176 −35 +40 W/m•K. The effective thermal boundary resistance of the diamond/ GaN interface (including the SiN x and additional layers) was 52.8 −3.2 +5.1 m 2 •K/GW.

Spectroscopic ellipsometry studies are reported for vanadium dioxide grown on c, m, and r-plane sapphire substrates. The crystallographic orientation of the VO 2 depends strongly on the substrate, producing diverse strains in the layers which affect the interband transition energies and the phase transition temperatures. For the m and r-plane substrates, the VO 2 appears to transform abruptly from the monoclinic phase to the rutile R structure as temperature is increased. In contrast, VO 2 deposited on c-plane sapphire exhibits a sluggish transformation. These structural differences correlate with distinct variations of the optical transitions observed in the ellipsometry results. For the m-plane sample, the energy gap collapses over a narrow temperature range. For the c-plane case, a broad temperature range is obtained between the onset and completion of the transformation. Raman studies of the vibrational structure show that internal stresses due to expansion and contraction across the phase transitions impacts the observed phonon energies.2

Full-wafer stress mapping is accomplished using visible and ultraviolet (UV) micro-Raman spectroscopy of a 730-nm thick GaN layer integrated with diamond grown by chemical vapor deposition. The UV measurements taken from both sides of the wafer reveal a higher tensile stress of 0.86 6 0.07 GPa at the free GaN surface compared to 0.23 6 0.06 GPa from the GaN/diamond interface, each with good cross-wafer uniformity. Factors influencing the overall stress and stress gradient are understood based on relaxation from dislocations in the GaN which vary in density along the growth direction. Simulations incorporating a model for stress relaxation in the GaN elastic modulus adequately describe the observed dependence. Published by AIP Publishing.

We consider the problem of estimating the 3D orientation of a user, using the downlink mmWave signals received from multiple base stations. We show that the received signals from several base stations, having known positions, can be used to estimate the unknown orientation of the user. We formulate the estimation problem as a maximum likelihood estimation problem in the the manifold of rotation matrices. In order to provide an initial estimate to solve the non-linear non-convex optimization problem, we resort to a least squares estimation problem that exploits the underlying geometry. Our numerical results show that the problem of orientation estimation can be solved when the signals from at least two base stations are received. We also provide the orientation lower error bound, showing a narrow gap between the performance of the proposed estimators and the bound.