III-Nitride based superlattice sub-millimeter wave source.

Abstract

In recent years, interest in the use of terahertz (THz) radiation in civilian and military applications, ranging from biomedical imaging to space-communications, has increased greatly. However, several roadblocks stand in the way of integration of the THz radiation into practical applications. One of the biggest challenges is the development of a compact coherent high-power THz source. In order to progress into the THz frequency range and higher power regions, researchers are looking for new materials such as GaN and meta-materials including superlattices and carbon nanotubes. In this thesis, I explore a high-power high-frequency source of radiation based on a new regime of operation of short-period III-Nitride superlattice devices. I begin by extending a simple analytical miniband transport model to include energy-dependent effective mass and momentum relaxation time, as well as polarization and strain related effects found in Stark III-Nitride material systems. Using this analytical framework, I investigate various modes of superlattice transport that can be utilized for generation of high-frequency signals. The most significant results presented in my thesis arise from the investigation of a new mode of operation: high-harmonic generation in low-mobility superlattices under below-critical bias. This regime is very important since it allows high-frequency generation in the III-Nitride material system, in which both Bloch oscillations and electrical domains are extremely difficult to achieve. Analysis of superlattice transport in a biharmonic field reveals the presence of high-harmonic instabilities with respect to current oscillations, related to negative absorption (gain). These instabilities along with a feedback circuit allow for fabrication of a THz source. In addition, I find that even simple frequency conversion, resulting from high-harmonic superlattice response, can produce high output power levels in III-Nitrides. For example, second harmonic response can generate output powers on the order of 10--100 kW/cm<super>2</super> for the second harmonic at 1.6 THz with high intrinsic generation efficiency of nearly 50%. My experimental development of the two-terminal vertical III-Nitride source devices shows that bulk GaN Gunn-type devices, under the biasing conditions necessary for the onset of oscillations, dissipate powers that are prohibitive for device operation. On the other hand, III-Nitride superlattice sources may dissipate significantly smaller powers, since the energy scale of minibands is nearly an order of magnitude smaller than the relevant scale in bulk GaN devices. Therefore, I suggest that high-harmonic generation in III-Nitride superlattice devices is an excellent avenue for the development of a practical compact high-power high-frequency source.