Asymmetric-Barrier-Tunneling Rectifier Arrays for Uncooled MM-Wave Imagers

In this two-year STTR Phase-II project, the primary objective in Year-1 will be to further develop a new type of room-temperature, zero-bias semiconducting rectifier diode for the millimeter-wave (MMW) region between 75 and 300 GHz. This is a frequency region which provides free-space radiation that can penetrate through clothing, luggage and other articles that are opaque to visible or infrared light. This in turn allows the sensing of concealed objects of importance to law-enforcement, counterterrorism, and other security activities. The new rectifier device is based on quantum-mechanical tunneling of electrons through asymmetric double-barrier heterostructures made from the InGaAs/InAlAs material system on InP substrates. Through quantum-transport simulation and experimental demonstration, the Phase-I effort showed that such a device is realizable, with external responsivity potentially greater than the traditional Schottky diode limit. But we also discovered that the new rectifier device has much higher (~26x) zero-bias specific electrical conductance (siemens/square micron) than state-of-the-art zero-bias Schottky diodes, which greatly improves the coupling efficiency to free space (via a planar antenna) at 100 GHz and above. This promises higher sensitivity as measured by the specific noise-equivalent power (NEPÆ). Since the new device has the same inherent broad bandwidth as a Schottky diode, it also promises lower noise equivalent temperature difference (NETD), which is the key metric for passive (thermal) imaging. The proposed Year-1 effort will focus on four activities: (1) continued quantum-transport simulations aimed at achieving good agreement between theory and experiment; (2) continued design and growth of double-barrier InGaAs/InAlAs heterostructures on 2-inch InP substrates by organometallic chemical vapor deposition in concert with the same commercial vendor (Rochester Institute of Technology EPICenter) as in Phase I; (3) microfabrication of micron-scale devices and integration with planar (square spiral) antennas in collaboration with our primary collaborator, Wright State University (WSU), and also with Ohio State University; and (4) dc electrical and MMW characterization (responsivity, NEPÆ and NETD) in the MMW-to-THz laboratory at WSU. The primary objective in Year-2 will be to fabricate, package, and characterize 2D arrays of the new rectifier devices. The baseline array will contain 32x32 elements and be fabricated on 2-inch-diameter InP substrates. The radiation pattern from each element will be combined through an objective lens (or lens sequence) to form the total beam. Since the behavior of the combined beam affects image quality, it must be carefully characterized in the MMW region. This will be done using thermal and coherent sources that exist at WSU. Of key importance is uniformity in the external responsivity, NEPÆ, and NETD across the array. This too will be measured in the WSU laboratory.