Layered transition metal dichalcogenides have shown tremendous potential for photodetection due to their non-zero direct bandgaps, high light absorption coefficients and carrier mobilities, and ability to form atomically sharp and defect-free heterointerfaces. A critical and fundamental bottleneck in the realization of high performance detectors is their trap-dependent photoresponse that trades off responsivity with speed. This work demonstrates a facile method of attenuating this trade-off by nearly 2x through integration of a lateral, in-plane, electrostatically tunable p-n homojunction with a conventional WSe2 phototransistor. The tunable p-n junction allows modulation of the photocarrier population and width of the conducting channel independently from the phototransistor. Increased illumination current with the lateral p-n junction helps achieve responsivity enhancement upto 2.4x at nearly the same switching speed (14–16 µs) over a wide range of laser power (300 pW–33 nW). The added benefit of reduced dark current enhances specific detectivity (D*) by nearly 25x to yield a maximum measured flicker noise-limited D* of 1.1×1012 Jones. High responsivity of 170 A/W at 300 pW laser power along with the ability to detect sub-1 pW laser switching are demonstrated.

Two-dimensional MoS2 gas sensors have conventionally relied on a change in field-effect-transistor (FET) channel resistance or in the Schottky contact/pn homojunction barrier. We demonstrate an enhancement in sensitivity (6×) and dynamic response along with a reduction in detection limit (8×) and power (104×) in a gate-tunable type-II WSe2(p)/MoS2(n) heterodiode gas sensor over an MoS2 FET on the same flake. Measurements for varying NO2 concentration, gate bias, and MoS2 flake thickness, reinforced with first-principles calculations, indicate dual-mode operation due to (i) a series resistance-based exponential change in the high-bias thermionic current (high sensitivity), and (ii) a heterointerface carrier concentration-based linear change in near-zero-bias interlayer recombination current (low power) resulting in sub-100 μW/cm2 power consumption. Fast and gate-bias tunable recovery enables an all-electrical, room-temperature dynamic operation. Coupled with the sensing of trinitrotoluene (TNT) molecules down to 80 ppb, this study highlights the potential of the WSe2/MoS2 pn heterojunction as a simple, low-overhead, and versatile chemical-sensing platform.

In this work, we demonstrate a deep mesa etch design for efficient edge field termination in β-Ga2O3 Schottky barrier diodes (SBDs). The proposed design enabled parallel plate fields higher than 4.1 MV/cm with negligible change to the device ON characteristics. The effect of BCl3/Cl2-based dry etch on (100) and (010) etched vertical sidewalls is also analyzed. A remarkable anisotropy in depletion was observed for etch along (100) and (010) sidewalls. This work provides insight into the impact of etching on n-type Ga2O3 and shows a promising method to realize efficient field termination for high breakdown field strength SBDs.

In-situ etching using Ga flux in an ultra-high vacuum environment like MBE is introduced as a method to make high aspect ratio 3 dimensional structures in 𝛽-Ga2O3. Etching of 𝛽-Ga2O3 due to excess Ga adatoms on the epilayer surface had been viewed as non-ideal for epitaxial growth especially since it results in plateauing and lowering of growth rate. In this study, we use this well-known reaction from epitaxial growth to intentionally etch 𝛽-Ga2O3. We demonstrate etch rate ranging from 2.9 nm/min to 30 nm/min with the highest reported etch rate being only limited by the highest Ga flux used. Patterned in-situ etching is also demonstrated and used to study the effect of fin orientation on the sidewall profiles and dopant (Si) segregation on the etched surface. Using in-situ Ga etching, we also demonstrate 150 nm wide fins and 200 nm wide nano pillars with high aspect ratio. This new etching method could enable future development of highly scaled vertical and lateral 3D devices in 𝛽-Ga2O3.The ultra-wide band gap semiconductor, 𝛽-Ga2O3 has attracted a lot of interest owing to its large breakdown field strength of 8 MV/cm 1-3 . When compared to existing state of the art technologies like Si, SiC and GaN, the high breakdown field strength theoretically predicts better performance for 𝛽-Ga2O3 based devices especially for applications like high voltage switching and high frequency power amplification 3 . In addition, the availability of bulk substrates grown from melt based techniques 4-7 and the wide range of controllable doping (10 15 cm -3 to 10 20 cm -3 ) 8,9 has led to rapid development of 𝛽-Ga2O3 devices in both vertical and lateral topology with excellent performance [10][11][12][13] .Due to the lack of p-type doping, most vertical devices in 𝛽-Ga2O3 require confined and scaled regions that control the flow of current between the source and drain, like vertical fins and pillars 10,14 . Using vertical fin structures also improves the electric field distribution in vertical Schottky barrier diodes by reducing the electric field seen at the Schottky metal semiconductor interface 15,16 . In addition, moving to a fin geometry would also result in increased power density and possibly enhancement-mode operation in lateral devices. Fabrication of these 3-dimensional structures require controlled, damage free etching that ideally provides vertical sidewalls (90 o

No abstract