Vertical double heterostructures based on GaN were prepared and investigated for their current voltage characteristics
and compared to theory. In our quest to observe negative differential resistance (NDR) phenomenon based on quantum
mechanical tunneling to, we fabricated resonant tunneling diode (RTD) like structures grown on low defect density and
high quality templates prepared by metal organic chemical vapor deposition using in situ SiN nanonetwork induced
epitaxial lateral overgrowth. The measured threading dislocation density of the template was in the range of 107 cm-2.
Inductively coupled plasma reactive ion etching (ICP-RIE) where in enhanced chemical etching mode was used for
reducing the detrimental surface defects on the mesa walls which otherwise contribute to current. Double barrier
structures with varying barrier and quantum well thicknesses as well as doping profiles were tested for their I-V
characteristics. The rectifying phenomenon occurred as a result of depletion region in GaN above the top Al(Ga)N layer
and asymmetric barrier shape of GaN RTD-like structure due to polarization. With the aid of calculated band structure
and resultant doping profile optimization, we now observe what appears to be resonant increase in current, the source of
which is not yet clear, at quantum states of the well.
Spin-orbit coupling is investigated by magnetoconductivity measurements in wurtzite AlxGa1-xN/AlN/GaN
heterostructures with a polarization induced two-dimensional electron gas with different Al concentrations ranging from
x = 0.1 to 0.35. By employing the persistent photoconductivity effect and by gating we are able to vary the carrier
density of the samples in a controllable manner from 0.8
×1012 cm-2 to 10.6 ×1012 cm-2. The samples are characterized
using magnetoresistance measurements. To characterize the spin-orbit interaction we measured quantum corrections to
conductance at low magnetic fields. All the samples we studied exhibit a weak antilocalization feature at liquid He
temperatures. The zero-field electron spin-splitting energies extracted from the weak antilocalization measurements are
found to scale with the Fermi wavevector kF as 2( ακF + γκF3) with effective linear and cubic spin-orbit parameters of
-α= 5.01×10−13 eV • m and γ= 1.6 ×10−31 eV •m3, respectively. The linear spin-orbit coupling arises from both the bulk
inversion asymmetry of the crystal and the structural inversion asymmetry of the heterostructure whereas the cubic spinorbit
coupling parameter is purely due to the bulk inversion asymmetry of the wurtzite crystal. We also extracted phase
coherence times from the amplitude of the weak antilocalization feature. The measured phase coherence times ranged
from 3-40 ps and were in agreement with the theory of decoherence based on electron-electron interactions.
The paper discusses the feasibility of a terahertz-signal source made of AlGaN/GaN superlattice. Negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. We relate the superlattice geometry and conduction band profile, distorted by polarization fields, to the oscillation frequency and power efficiency of the device. We also determine the optimal Al content, superlattice period, and the parameters of external circuit that favor sub-millimeter wave generation.
A W-band target glint and background scene generator is developed for compact range hardware-in-the-loop (HWIL) seeker testing and characterization. The device comprises an Electronically Controlled Beamformer (ECB) capable of real time generation of wide variety of wavefronts in the near field of the system under test (SUT). The fine-pixelized ECB aperture with individual control of each pixel allows (in particular) formation of radar returns in a compact range by focusing and steering the (focused) Millimeter Wave (MMW) beam on the SUT aperture. Unlike compact range systems using limited number of radiators and focal plane optics, fine-pixelized ECB allows full glint simulation over SUT's field of view. ECB is compatible with currently used retransmitter and waveform simulator.
We present the results of a simulation of the device's operation and compare them with the experiment. Major attention in both the simulations and the measurements was paid to the field distribution in the near-field region of the device.
This work has been conducted under US Army Phase II Small Business Innovation Research (SBIR) effort, under the technical management of Mr. James A. Buford Jr., US Army Aviation & Missile Research, Development & Engineering Center (AMRDEC), Redstone Arsenal, Alabama.
Due to their unique physical properties GaN-based heterostructures show a great promise for spintronics applications. This stimulates the search for GaN-based ferromagnetic semiconductors which can be used for injection of spin polarized carriers in device structures. In this study, magnetic properties of GaN layers implanted with Gd+ ions to various doses were investigated. Magnetization curves of samples with Gd content nGd = 2x1017 and 2x1018 cm-3 show clear hysteresis, while the samples with nGd = 2x1016 and 2x1019 cm-3 exhibit no ferromagnetism. Most likely, the lowest Gd concentration produced magnetization below the detection limit, whereas the absence of ferromagnetism in the sample with the highest Gd content may be resulted from heavy implantation-induced damage. Curie temperatures for samples with Gd contents of 2x1017 and 2x1018 cm-3 were estimated to be larger than 300 K. Saturation magnetizations of 1550 μB and 1350 μB per Gd-atom were found at 5 K and 300 K, respectively, for the sample with nGd=2x1018 cm-3.
Semiconductor electronically beam-steering antennas based on a plasma hologram have been demonstrated as a possible cost- effective alternative to phase arrays. The paper addresses the design issues critical for antenna operation.
A semiconductor electron-hole plasma effectively reflects microwave radiation that is used in electromagnetic shields and semiconductor millimeter wave antennas. Silicon-based P- I-N structures allow us to engage in engineering the conductive properties of plasma crated under current injection. We calculate the I-V characteristics of short P- I-N structures and model their reflective properties in the millimeter wave band.
We discuss possible new sub-millimeter sources based on group-III Nitrides superlattices. It is shown that traveling dipole domains in biased GaN/InGaN and GaN/AlGaN short- period superlattices can generate electromagnetic power in the terahertz region.
Transparent conductors find applications in electrodes for flat panel displays, electromagnetic shielding, optical filters, antennas embedded in windshields, transparent electrodes for light-emitting diodes and lasers. Shields transparent in the atmospheric spectral windows of (3- 5)μm or (10-12)μm are in demand for electromagnetic protection of infrared displays in thermo-imaging devices. We propose semiconductor/dielectric or metal/dielectric multilayers that can serve as shield against RF and microwave radiation, while being transparent in the mid-infrared spectral region.