Yaohua Tan

First principles study and empirical parametrization of twisted bilayer MoS2 based on band-unfolding

We explore the band structure and ballistic electron transport in twisted bilayer MoS2 using the density functional theory. The sphagetti like bands are unfolded to generate band structures in the primitive unit cell of the original 2H MoS2 bilayer and projected onto the original bands of an individual layer. The corresponding twist angle dependent bandedges are extracted from the unfolded band structures. Based on a comparison within the same primitive unit cell, an efficient two band effective mass model for indirect ΓV and ΛC valleys is created and parametrized by fitting to the unfolded band structures. With the two band effective mass model, we calculate transport properties—specifically, the ballistic transmission in arbitrarily twisted bilayer MoS2.

Modeling tunnel field effect transistors—From interface chemistry to non-idealities to circuit level performance

We present a quasi-analytical model for Tunnel Field Effect Transistors (TFETs) that includes the microscopic physics and chemistry of interfaces and non-idealities. The ballistic band-to-band tunneling current is calculated by modifying the well known Simmons equation for oxide tunneling, where we integrate the Wentzel-Kramers-Brillouin (WKB) tunneling current over the transverse modes. We extend the Simmons equation to finite temperature and non-rectangular barriers using a two-band model for the channel material and an analytical channel potential profile obtained from Poissons equation. The two-band model is parametrized first principles by calibrating with hybrid Density Functional Theory calculations, and extended to random alloys with a band unfolding technique. Our quasi-analytical model shows quantitative agreement with ballistic quantum transport calculations. On top of the ballistic tunnel current we incorporate higher order processes arising at junctions coupling the bands, specifically interface trap-assisted tunneling and Auger generation processes. Our results suggest that both processes significantly impact the off-state characteristics of the TFETs - Auger in particular being present even for perfect interfaces. We show that our microscopic model can be used to quantify the TFET performance on the atomistic interface quality. Finally, we use our simulations to quantify circuit level metrics such as energy consumption.

Toward deterministic construction of low noise avalanche photodetector materials

Over the past 40+ years, III-V materials have been intensively studied for avalanche photodetectors, driven by applications including optical communications, imaging, quantum information processing, and autonomous vehicle navigation. Unfortunately, impact ionization is a stochastic process that introduces noise, thereby limiting sensitivity and achievable bandwidths, leading to intense effort to mitigate this noise through the identification of different materials and device structures. Exploration of these materials has seen limited success as it has proceeded in a largely ad hoc fashion due to little consensus regarding which fundamental properties are important. Here, we report an exciting step toward deterministic design of low-noise avalanche photodetector materials by alternating the composition at the monolayer scale; this represents a dramatic departure from previous approaches, which have concentrated on either unconventional compounds/alloys or nanoscale band-engineering. In particular, we demonstrate how to substantially improve upon the noise characteristics of the current state-of-the art telecom avalanche multipliers, In0.52Al0.48As grown on InP substrates, by growing the structure as a strain-balanced digital alloy of InAs and AlAs layers, each only a few atomic layers thick. The effective k-factor, which has historically been considered a fundamental material property, was reduced by 6–7× from k = 0.2 for bulk In0.52Al0.48As to k = 0.05 by using the digital alloy technique. We also demonstrate that these “digital alloys” can significantly extend the photodetector cutoff wavelength well beyond those of their random alloy counterparts.Over the past 40+ years, III-V materials have been intensively studied for avalanche photodetectors, driven by applications including optical communications, imaging, quantum information processing, and autonomous vehicle navigation. Unfortunately, impact ionization is a stochastic process that introduces noise, thereby limiting sensitivity and achievable bandwidths, leading to intense effort to mitigate this noise through the identification of different materials and device structures. Exploration of these materials has seen limited success as it has proceeded in a largely ad hoc fashion due to little consensus regarding which fundamental properties are important. Here, we report an exciting step toward deterministic design of low-noise avalanche photodetector materials by alternating the composition at the monolayer scale; this represents a dramatic departure from previous approaches, which have concentrated on either unconventional compounds/alloys or nanoscale band-engineering. In particular, we demon...

Graphene Klein tunnel transistors for high speed analog RF applications

We propose Graphene Klein tunnel transistors (GKTFET) as a way to enforce current saturation while maintaining large mobility for high speed radio frequency (RF) applications. The GKTFET consists of a sequence of angled graphene p-n junctions (GPNJs). Klein tunneling creates a collimation of electrons across each GPNJ, so that the lack of substantial overlap between transmission lobes across successive junctions creates a gate-tunable transport gap without significantly compromising the on-current. Electron scattering at the device edge tends to bleed parasitic states into the gap, but the resulting pseudogap is still sufficient to create a saturated output (ID–VD) characteristic and a high output resistance. The modulated density of states generates a higher transconductance (gm) and unity current gain cut-off frequency (fT) than GFETs. More significantly the high output resistance makes the unity power gain cut-off frequency (fmax) of GKTFETs considerably larger than GFETs, making analog GKTFET potentially useful for RF electronics. Our estimation shows the fT/fmax of a GKTFET with 1 μm channel reaches 33 GHz/17 GHz, and scale up to 350 GHz/53 GHz for 100 nm channel (assuming a single, scalable trapezoidal gate). The fmax of a GKTFET is 10 times higher than a GFET with the same channel length.

Spintronic signatures of Klein tunneling in topological insulators

Klein tunneling, the perfect transmission of normally incident Dirac electrons across a potential barrier, has been widely studied in graphene and explored to design switches, albeit indirectly. We show that Klein tunneling maybe easier to detect for spin-momentum locked electrons crossing a PN junction along a three dimensional topological insulator surface. In these topological insulator PN junctions (TIPNJs), the spin texture and momentum distribution of transmitted electrons can be measured electrically using a ferromagnetic probe for varying gate voltages and angles of current injection. Based on transport models across a TIPNJ, we show that the asymmetry in the potentiometric signal between PP and PN junctions and its overall angular dependence serve as a direct signature of Klein tunneling.

First principles study of twisted bilayer MoS 2 through band unfolding

Two dimensional(2D) materials such as graphene and transition metal dichalcogenides(TMDs) are exciting candidates for electronic and optoelectronic device applications. In particular, there is growing interest in stacked 2D materials that often arise naturally, and also provide added possibilities for desired functionalities with varying thickness and composition. It is essential to understand the electronic properties of stacked 2D materials such as twisted multilayer TMDs and TMD het-erostructures are sensitive to inter-layer interactions [3,4]. The translational symmetry of a twisted multilayer TMD is compromised due to the twist angle. Consequently a supercell much larger than the primitive unit cell needs to be considered, creating a spaghetti-like band structure from band folding. The challenge for theoretical studies of twisted multilayer systems is to extract inter-layer interactions from the folded band structures. In this work, band structures of twisted bilayer TMDs are studied using first principles calculations. In order to extract the band-edge splittings relavent to inter-layer interactions, we apply a band unfolding technique to the twisted bilayer TMDs. Multi-valley effective mass models are then created to model the bandedges at the Γ point as well as indirect conduction bands along K directions.