Vladimir I. Litvinov

GaN : based terahertz sources

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.

Resonant Tunneling in III-Nitrides

Wide-bandgap semiconductors can sustain high temperatures and high power operation in various important applications such as transistors, light-emitting diodes, and lasers. Although in embryonic stage, one can expect such a resilience in GaN resonant tunneling diodes (RTDs) and superlattices as well with distinct applications. Because of the negative differential conduction, the double barrier resonant tunneling structures could be the basis for new high-power coherent microwave sources operating in W-band and terahertz. In this paper, recent progress in wide-bandgap semiconductor RTDs is discussed.

Conductive coating with infrared pass band

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.

Spin-orbit coupling in AlGaN/AlN/GaN heterostructures with a polarization induced two-dimensional electron gas

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 + γκF 3) 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.

Electronically reconfigurable aperture (ERA): A new approach for beam-steering technology

In order to satisfy the demanding SWaP requirements of modern microwave/MMW instruments, designers are looking for novel integrated solutions. In particular, phased array packaging, especially at Ka and higher frequencies, is extremely challenging as half-wavelength spacing is prohibitively small. The cost of implementing high density phased array packaging is another impediment to wider use. This communication presents a new electronically beam-steering technology that is compatible with highly integrated antenna design and dramatically simplifies packaging. It is based on the coherent scattering of the evanescent field associated with waves propagating through a dielectric waveguide. The antenna scattering elements are controlled electronically and constitute a dynamically reconfigurable hologram. The switching time from one hologram pattern (one beam position) to another is on the order of tens of nanoseconds. Because of the hologram nature of this approach, the beam-forming capabilities of the electronically reconfigurable aperture (ERA) approach are comparable to those of phased arrays: 1D and 2D beam-forming and beam-steering, multiple simultaneous individually controlled beams, steerable nulls and variable beam width(s).

Electronically steerable semiconductor-based MMW antennas

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.

Digital beam former based on an electronically steerable antenna for HWIL target simulation for US Army AMRDEC

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 SUTs field of view. ECB is compatible with currently used retransmitter and waveform simulator. We present the results of a simulation of the devices 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.

Electrical domains and submillimeter signal generation in AlGaN/GaN superlattices

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.

Electrical domains and sub-millimeter signal generation in AlGaN/GaN superlattices

This paper discusses the feasibility of a terahertz-signal source made of an AlGaN/GaN superlattice. The negative differential conductivity, electrical domain formation, current oscillations, and power efficiency of a perspective source are described. The superlattice geometry and conduction band profile, which are distorted by polarization fields, are related to the oscillation frequency and power efficiency of the device. The optimal Al content, superlattice period, and the parameters of the external circuit that favor submillimeter wave generation are determined.

Millimeter-wave reflection from electron-hole plasma in short P-I-N structures

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.