Study of nanocomposites and nanowire devices for THz circuit applications

Interest in terahertz (1011-10 13Hz) spectral region is driven by the possibility of exploiting unique interaction between electromagnetic fields and materials in this spectral regime. Potentials of THz have been examined using quasi-optical table top systems. There is significant interest to minimize the bench-top quasi-optical systems to integrated circuit level in order to realize similar functions and benefits as in the digital and RF integrated circuit areas. Integration of both passive and active devices at the wafer level is necessary to meet this challenge. Conventional integration approaches (e.g., microstrip transmission lines) do not directly lend themselves to the design and fabrication of THz circuits and new design approaches are required. This work proposes and demonstrates novel approaches to achieve both active and passive element integration at the wafer level that are compatible with large-area and low-temperature processes, and paves the path to realize highly functional, compact, low-cost THz systems.;THz waveguide and interconnects are one of the fundamental building blocks of THz passives. This research investigates the use of thin dielectric ribbons made from polymer-ceramic nanocomposite for the fabrication of planar, low-loss, and large area compatible THz waveguides. Simulations show the ribbon waveguides provide low loss THz wave propagation when a combination of high dielectric constant (high-k) core and low dielectric constant cladding are used. This combination provides stronger field confinement and reduces losses at waveguide bends. Two different fabrication approaches are investigated: photopatterning of tailorable nanocomposite thin films, and laser cutting of dry nanocomposite thin films. Measurements of different waveguide samples validate the simulated results and prove that low cost, wafer-level planar THz integrated circuits can be realized with proposed waveguides.;THz active devices are the core elements required to build THz circuits. Diode is a key component that is needed to form a basic THz active circuit. Semiconducting n-type GaAs nanowires are utilized in the fabrication of THz Schottky diodes. Nanowire based devices can be used to achieve high cut-off frequency devices, but individual nanowire has high impedance that is not suitable for wide-band impedance matching. To overcome this challenge, multiple nanowires placed in parallel are integrated together to achieve desired impedance while maintaining high cut-off frequency. A novel low-cost process using photolithography is applied to fabricate sub-micron devices. Fabrication of nanowire based devices is compatible with integration on a host of large area substrates at low processing temperature. These diodes are first utilized in the design of THz detectors, calculated and measured results show strong nonlinear rectification behavior and high sensitivity over a wide frequency band (0.1 - 1 THz).;In parallel, an alternative method of fabricating THz detector was also investigated. Active devices are embedded within the dielectric layers forming the waveguides. This avoids the use of flip-chip or wire bonds to connect the devices and thus minimizes the parasitics. GaAs Schottky Barrier Diodes (SBDs) are directly integrated with broadband log-periodic antennas to design a highly sensitive broad-band THz detector. Calculated and measured sensitivity of the detector closely matches the performance of existing commercial THz detectors fabricated using elaborate micromachining techniques. A THz image sensor is fabricated and demonstrated in this work to prove the feasibility of this concept. This fabrication approach is large-area, low-cost, and low-temperature process compatible and can also be implemented in heterogeneous integration of THz devices on a host of substrates.