Tony Ivanov

Vertical 2D/3D Semiconductor Heterostructures Based on Epitaxial Molybdenum Disulfide and Gallium Nitride

When designing semiconductor heterostructures, it is expected that epitaxial alignment will facilitate low-defect interfaces and efficient vertical transport. Here, we report lattice-matched epitaxial growth of molybdenum disulfide (MoS2) directly on gallium nitride (GaN), resulting in high-quality, unstrained, single-layer MoS2 with strict registry to the GaN lattice. These results present a promising path toward the implementation of high-performance electronic devices based on 2D/3D vertical heterostructures, where each of the 3D and 2D semiconductors is both a template for subsequent epitaxial growth and an active component of the device. The MoS2 monolayer triangles average 1 μm along each side, with monolayer blankets (merged triangles) exhibiting properties similar to that of single-crystal MoS2 sheets. Photoluminescence, Raman, atomic force microscopy, and X-ray photoelectron spectroscopy analyses identified monolayer MoS2 with a prominent 20-fold enhancement of photoluminescence in the center regions of larger triangles. The MoS2/GaN structures are shown to electrically conduct in the out-of-plane direction, confirming the potential of directly synthesized 2D/3D semiconductor heterostructures for vertical current flow. Finally, we estimate a MoS2/GaN contact resistivity to be less than 4 Ω·cm(2) and current spreading in the MoS2 monolayer of approximately 1 μm in diameter.

Development of a PZT MEMS Switch Architecture for Low-Power Digital Applications

A lead zirconate titanate (PZT) microelectromechanical systems (MEMS) digital switch was designed as a low-power low-frequency control system intended to create energy-efficient microcontrollers, control higher voltage systems, and provide integrated control over other MEMS platforms. In addition, the technology is inherently insensitive to high energy radiation and has been shown to operate over a wide range of temperatures. Initial devices were fabricated with three design variables of interest-actuator length, width, and contact metallurgy (Au/Pt, Au/Ru, and Au/Au). To assess the impact of each variable on device performance, device wafers were measured using a SUSS semiautomated probe station and associated control hardware and software. Devices were evaluated based on contact resistance, actuation voltage, minimum actuation voltage using a voltage bias, propagation delay, dynamic power, and static power consumption. The measurements were then analyzed to determine the optimal switch geometry and contact material combination for digital applications. With the data collected, a software model was developed for accurate simulation of higher complexity circuits composed of these switches.

Analysis and measurement of hard-horn feeds for the excitation of quasi-optical amplifiers

Measurement and analysis of hard horn feeds for excitation of quasi-optical amplifiers has been performed. A computer program based on the mode matching technique has been developed in order to determine the aperture field distribution for pyramidal hard horns. This program can be used to optimize a hard horns field distribution and bandwidth. Simulation and measurement results for a 31 GHz hard horn feed are presented.

A CPW-fed microstrip patch quasi-optical amplifier array

A CPW fed microstrip patch quasi-optical power combining amplifier array is introduced. The amplifier array uses CPW fed patches for both input and output antennas. This structure is not only compatible with MMIC implementations but can also provide a greater bandwidth for the patch antennas. A 4/spl times/4 amplifier array was designed and fabricated at X-band. Theoretical and experimental results are presented.

A 26-MESFET spatial power-combining oscillator

Several spatial power-combining oscillators based on an extended resonance technique was designed and fabricated. Experimental results for these combiners are provided and their performance is compared. An effective isotropic radiated power (EIRP) of 19.9 W at 9.923 GHz was obtained from a 26-MESFET spatial power combiner. The total radiated power for this combiner was 160 mW. The measured and predicted radiation patterns in E- and H-planes are also compared.

Structural and electrical analysis of epitaxial 2D/3D vertical heterojunctions of monolayer MoS2 on GaN

Integration of two-dimensional (2D) and conventional (3D) semiconductors can lead to the formation of vertical heterojunctions with valuable electronic and optoelectronic properties. Regardless of the growth stacking mechanism implemented so far, the quality of the formed heterojunctions is susceptible to defects and contaminations mainly due to the complication involved in the transfer process. We utilize an approach that aims to eliminate the transfer process and achieve epitaxial vertical heterojunctions with low defect interfaces necessary for efficient vertical transport. Monolayers of MoS2 of approximately 2 μm domains are grown epitaxially by powder vaporization on GaN substrates forming a vertical 2D/3D heterojunction. Cross-sectional transmission electron microscopy (XTEM) is employed to analyze the in-plane lattice constants and van der Waals (vdW) gap between the 2D and 3D semiconductor crystals. The extracted in-plane lattice mismatch between monolayer MoS2 and GaN is only 1.2% which correspon...

Low-power 3-bit piezoelectric MEMS analog to digital converter

This paper reports on a novel MEMS analog to digital converter (ADC) designed within a low-power, piezoelectric MEMS based digital logic process. The 3-bit ADC achieves full operation without the use of additional solid-state circuitry, allowing it to operate with minimal, 3.83 pW, static power dissipation. In addition, two piezoelectric property exploiting operational modes are presented. These increase the total effective number of bits and the system range without increasing the device area.

A compact, low loss piezoelectric RF MEMS relay with sub 100-ns switching times

This work reports on the effort to develop a low voltage, high performance RF MEMS relay within an existing MEMS process. The 54 × 43 µm2 relay achieved an insertion loss of under 0.2 dB from DC to 23.75 GHz at 5-V and an isolation from DC to 18-GHz of better than 30-dB. Additionally, the smal stiff structure achieved switching times of sub 100-ns with a 7-V actuation voltage and temperature measurements indicate stable operation from 0 °C to 125 °C. The relay was fabricated within an existing digital MEMS relay process, enabling integrated digital control for future RF systems.

Piezoelectric micro-scale mechanical computing system

Near-zero leakage current, nearly ideal on to off-state current ratios, sharp voltage to current relationship slopes and potential for intrinsic immunity to radiation have made MEMS-based mechanical logic an attractive area of research. This work builds upon an ongoing research effort, which aims to develop a lead zirconate titanate (PZT)-based, mechanical logic architecture, in order to demonstrate a fully capable digital MEMS system. The work presented here demonstrates an integration of combinational and sequential mechanical logic elements to provide a 1-bit counter without the aid of semiconductor technologies. The system demonstrates both proper functionality and data retention over multiple cycles.

Nano-electromechanical storage element for a low power complimentary logic architecture using PZT relays

A mechanical dynamic latching element has been developed within a novel, low power piezoelectric MEMS digital architecture. The latch, which has demonstrated reliable and repeatable operation over multiple cycles, consumes only 2.2 pW of leakage power and 0.18 nJ of dynamic energy while utilizing no additional transistor components.