High Performance Indium-Tin-Oxide Schottky Diodes for Terahertz Band Operation.

Schottky diode, capable of ultrahigh frequency operation, plays a critical role in modern communication systems. To develop cost-effective and widely applicable high-speed diodes, researchers have delved into thin-film semiconductors. However, a performance gap persists between thin-film diodes and conventional bulk semiconductor-based ones. Featuring high mobility and low permittivity, indium-tin-oxide has emerged to bridge this gap. Nevertheless, due to its high carrier concentration, indium-tin-oxide has predominantly been utilized as electrode rather than semiconductor. In this study, a remarkable quantum confinement induced dedoping phenomenon was discovered during the aggressive indium-tin-oxide thickness downscaling. By leveraging such a feature to change indium-tin-oxide from metal-like into semiconductor-like, in conjunction with a novel heterogeneous lateral design facilitated by an innovative digital etch, we demonstrated an indium-tin-oxide Schottky diode with a cutoff frequency reaching terahertz band. By pushing the boundaries of thin-film Schottky diodes, our research offers a potential enabler for future fifth-generation/sixth-generation networks, empowering diverse applications.

Exploring Low Power and Ultrafast Memristor on p-Type van der Waals SnS.

Memristor devices that exhibit high integration density, fast speed, and low power consumption are candidates for neuromorphic devices. Here, we demonstrate a filament-based memristor using p-type SnS as the resistive switching material, exhibiting superlative metrics such as a switching voltage ∼0.2 V, a switching speed faster than 1.5 ns, high endurance switching cycles, and an ultralarge on/off ratio of 108. The device exhibits a power consumption as low as ∼100 fJ per switch. Chip-level simulations of the memristor based on 32 × 32 high-density crossbar arrays with 50 nm feature size reveal on-chip learning accuracy of 87.76% (close to the ideal software accuracy 90%) for CIFAR-10 image classifications. The ultrafast and low energy switching of p-type SnS compared to n-type transition metal dichalcogenides is attributed to the presence of cation vacancies and van der Waals gap that lower the activation barrier for Ag ion migration.

Ge0.95Sn0.05 Gate-All-Around p-Channel Metal-Oxide-Semiconductor Field-Effect Transistors with Sub-3 nm Nanowire Width.

We demonstrate Ge0.95Sn0.05 p-channel gate-all-around field-effect transistors (p-GAAFETs) with sub-3 nm nanowire width (WNW) on a GeSn-on-insulator (GeSnOI) substrate using a top-down fabrication process. Thanks to the excellent gate control by employing an aggressively scaled nanowire structure, Ge0.95Sn0.05 p-GAAFETs exhibit a small subthreshold swing (SS) of 66 mV/decade, a decent on-current/off-current (ION/IOFF) ratio of ∼1.2 × 106, and a high-field effective hole mobility (µeff) of ∼115 cm2/(V s). In addition, we also investigate quantum confinement effects in extremely scaled GeSn nanowires, including threshold voltage (VTH) shift and IOFF reduction with continuous scaling of WNW under 10 nm. The phenomena observed from experimental results are substantiated by the calculation of GeSn bandgap and TCAD simulation of electrical characteristics of devices with sub-10 nm WNW. This study suggests Ge-based nanowire p-FETs with extremely scaled dimension hold promise to deliver good performance to enable further scaling for future technology nodes.

Floating-base germanium-tin heterojunction phototransistor for high-efficiency photodetection in short-wave infrared range.

The floating-base germanium-tin (Ge1-xSnx) heterojunction phototransistor (HPT) is designed and investigated as an efficient optical receiver in the short-wave infrared range. Simulations indicate that as the Sn content increases, the responsivity significantly increases due to a higher absorption coefficient and a larger valence band offset between Ge and Ge1-xSnx. Ge0.935Sn0.065 HPTs that incorporated high-quality Ge0.935Sn0.065 film grown by molecular beam epitaxy were fabricated, demonstrating optical response beyond wavelength of 2003 nm. At a low bias voltage of 1.0 V, optical response enhancement of ~10 times was achieved over the conventional Ge0.935Sn0.065 p-i-n photodiode. High responsivities of ~1.8 A/W at 1550 nm and ~0.043 A/W at 2003 nm were demonstrated with low dark current density of 0.147 A/cm2.

Integration of InGaAs MOSFETs and GaAs/ AlGaAs lasers on Si Substrate for advanced opto-electronic integrated circuits (OEICs).

Lasers monolithically integrated with high speed MOSFETs on the silicon (Si) substrate could be a key to realize low cost, low power, and high speed opto-electronic integrated circuits (OEICs). In this paper, we report the monolithic integration of InGaAs channel transistors with electrically pumped GaAs/AlGaAs lasers on the Si substrate for future advanced OEICs. The laser and transistor layers were grown on the Si substrate by molecular beam epitaxy (MBE) using direct epitaxial growth. InGaAs n-FETs with an ION/IOFF ratio of more than 106 with very low off-state leakage and a low subthreshold swing with a minimum of 82 mV/decade were realized. Electrically pumped GaAs/AlGaAs quantum well (QW) lasers with a lasing wavelength of 795 nm at room temperature were demonstrated. The overall fabrication process has a low thermal budget of no more than 400 °C.

Monolithic integration of InGaAs n-FETs and lasers on Ge substrate.

We report the first monolithic integration of InGaAs channel field-effect transistors with InGaAs/GaAs multiple quantum wells (MQWs) lasers on a common platform, achieving a milestone in the path of enabling low power and high speed opto-electronic integrated circuits (OEICs). The III-V layers used for realizing transistors and lasers were grown epitaxially on the Ge substrate using molecular beam epitaxy (MBE). A Si-CMOS compatible process was developed to realize InGaAs n-FETs with subthreshold swing SS of 93 mV/decade, ION/IOFF ratio of more than 4 orders of magnitude with very low off-state leakage current, and a peak effective mobility of more than 2000 cm2/V·s. In addition, fabrication process uses a low overall processing temperature (≤ 400 °C) to maintain the high quality of the InGaAs/GaAs MQWs for the laser. Room temperature electrically-pumped lasers with a lasing wavelength of 1.03 µm and a linewidth of less than 1.7 nm were realized.

Suppression of dark current in germanium-tin on silicon p-i-n photodiode by a silicon surface passivation technique.

We demonstrate that a complementary metal-oxide-semiconductor (CMOS) compatible silicon (Si) surface passivation technique effectively suppress the dark current originating from the mesa sidewall of the Ge(0.95)Sn(0.05) on Si (Ge(0.95)Sn(0.05)/Si) p-i-n photodiode. Current-voltage (I-V) characteristics show that the sidewall surface passivation technique could reduce the surface leakage current density (Jsurf) of the photodiode by ~100 times. A low dark current density (Jdark) of 0.073 A/cm(2) at a bias voltage of -1 V is achieved, which is among the lowest reported values for Ge(1-x)Sn(x)/Si p-i-n photodiodes. Temperature-dependent I-V measurement is performed for the Si-passivated and non-passivated photodiodes, from which the activation energies of dark current are extracted to be 0.304 eV and 0.142 eV, respectively. In addition, the optical responsivity of the Ge(0.95)Sn(0.05)/Si p-i-n photodiodes to light signals with wavelengths ranging from 1510 nm to 1877 nm is reported.

Theoretical study of thermoelectric properties of few-layer MoS2 and WSe2.

Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) are prototypical layered two-dimensional transition metal dichalcogenide materials, with each layer consisting of three atomic planes. We refer to each layer as a trilayer (TL). We study the thermoelectric properties of 1-4TL MoS2 and WSe2 using a ballistic transport approach based on the electronic band structures and phonon dispersions obtained from first-principles calculations. Our results show that the thickness dependence of the thermoelectric properties is different under n-type and p-type doping conditions. Defining ZT1st peak as the first peak in the thermoelectric figure of merit ZT as doping levels increase from zero at 300 K, we found that ZT1st peak decreases as the number of layers increases for MoS2, with the exception of 2TL in n-type doping, which has a slightly higher value than 1TL. However, for WSe2, 2TL has the largest ZT1st peak in both n-type and p-type doping, with a ZT1st peak value larger than 1 for n-type WSe2. At high temperatures (T > 300 K), ZT1st peak dramatically increases when the temperature increases, especially for n-type doping. The ZT1st peak of n-type 1TL-MoS2 and 2TL-WSe2 can reach 1.6 and 2.1, respectively.

Direct design of ground-state probabilistic logic using many-body interactions for probabilistic computing.

In this work, an innovative design model aimed at enhancing the efficacy of ground-state probabilistic logic with a binary energy landscape (GSPL-BEL) is presented. This model enables the direct conversion of conventional CMOS-based logic circuits into corresponding probabilistic graphical representations based on a given truth table. Compared to the conventional approach of solving the configuration of Ising model-basic probabilistic gates through linear programming, our model directly provides configuration parameters with embedded many-body interactions. For larger-scale probabilistic logic circuits, the GSPL-BEL model can fully utilize the dimensions of many-body interactions, achieving minimal node overhead while ensuring the simplest binary energy landscape and circumventing additional logic synthesis steps. To validate its effectiveness, hardware implementations of probabilistic logic gates were conducted. Probabilistic bits were introduced as Ising cells, and cascaded conventional XNOR gates along with passive resistor networks were precisely designed to realize many-body interactions. HSPICE circuit simulation results demonstrate that the probabilistic logic circuits designed based on this model can successfully operate in free, forward, and reverse modes, exhibiting the simplest binary probability distributions. For a 2-bit × 2-bit integer factorizer involving many-body interactions, compared to the logic synthesis approach, the GSPL-BEL model significantly reduces the number of consumed nodes, the solution space (in the free-run mode), and the number of energy levels from 12, 4096, and 9-8, 256, and 2, respectively. Our findings demonstrate the significant potential of the GSPL-BEL model in optimizing the structure and performance of probabilistic logic circuits, offering a new robust tool for the design and implementation of future probabilistic computing systems.

Graphene-based spin caloritronics.

Thermally induced spin transport in magnetized zigzag graphene nanoribbons (M-ZGNRs) is explored using first-principles calculations. By applying temperature difference between the source and the drain of a M-ZGNR device, spin-up and spin-down currents flowing in opposite directions can be induced. This spin Seebeck effect in M-ZGNRs can be attributed to the asymmetric electron-hole transmission spectra of spin-up and spin-down electrons. Furthermore, these spin currents can be modulated and completely polarized by tuning the back gate voltage. Finally, thermal magnetoresistance of ZGNRs between ground states and magnetized states can reach 10(4)% without an external bias. Our results indicate the possibility of developing graphene-based spin caloritronic devices.

Room-temperature nonlinear Hall effect and wireless radiofrequency rectification in Weyl semimetal TaIrTe4.

The nonlinear Hall effect (NLHE), the phenomenon in which a transverse voltage can be produced without a magnetic field, provides a potential alternative for rectification or frequency doubling1,2. However, the low-temperature detection of the NLHE limits its applications3,4. Here, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. We also observe a temperature-induced sign inversion of the NLHE in TaIrTe4. Our theoretical calculations suggest that the observed sign inversion is a result of a temperature-induced shift in the chemical potential, indicating a direct correlation of the NLHE with the electronic structure at the Fermi surface. Finally, on the basis of the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless radiofrequency (RF) rectification with zero external bias and magnetic field. This work opens a door to realizing room-temperature applications based on the NLHE in Weyl semimetals.

Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions.

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi2Te3 heterojunction which combines monolayer graphene and a Bi2Te3 nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi2Te3 when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi2Te3 bulk band. The increased modulation depth of the graphene/Bi2Te3 heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi2Te3 thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi2Te3 heterojunction and demonstrates its potential for applications.

High oscillator strength interlayer excitons in two-dimensional heterostructures for mid-infrared photodetection.

The development of infrared photodetectors is mainly limited by the choice of available materials and the intricate crystal growth process. Moreover, thermally activated carriers in traditional III-V and II-VI semiconductors enforce low operating temperatures in the infrared photodetectors. Here we demonstrate infrared photodetection enabled by interlayer excitons (ILEs) generated between tungsten and hafnium disulfide, WS2/HfS2. The photodetector operates at room temperature and shows an even higher performance at higher temperatures owing to the large exciton binding energy and phonon-assisted optical transition. The unique band alignment in the WS2/HfS2 heterostructure allows interlayer bandgap tuning from the mid- to long-wave infrared spectrum. We postulate that the sizeable charge delocalization and ILE accumulation at the interface result in a greatly enhanced oscillator strength of the ILEs and a high responsivity of the photodetector. The sensitivity of ILEs to the thickness of two-dimensional materials and the external field provides an excellent platform to realize robust tunable room temperature infrared photodetectors.

Electrically tunable valley polarization in Weyl semimetals with tilted energy dispersion.

Tunneling transport across electrical potential barriers in Weyl semimetals with tilted energy dispersion is investigated. We report that the electrons around different valleys experience opposite direction refractions at the barrier interface when the energy dispersion is tilted along one of the transverse directions. Chirality dependent refractions at the barrier interface polarize the Weyl fermions in angle-space according to their valley index. A real magnetic barrier configuration is used to select allowed transmission angles, which results in electrically controllable and switchable valley polarization. Our findings may pave the way for experimental investigation of valley polarization, as well as valleytronic and electron optic applications in Weyl semimetals.

All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe2/ferromagnet heterostructures.

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals1-3 or topological insulators4,5 provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure6,7; the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation8. The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling6,9 and non-trivial band structures10 with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states9-11. Atomically flat surfaces, which can be produced with high quality12, facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of ~2.96 × 105 A cm-2 suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 ± 0.06 mJ m-2 induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.

A pseudopotential method for investigating the surface roughness effect in ultrathin body transistors.

An atomistic method based on the diffraction pseudopotential model is established, for investigating the surface roughness (SR) effect in ultrathin body double-gate metal-oxide-semiconductor field effect transistors. The scattering of electrons due to atoms and vacancies responsible for roughness results from a three-dimensional effective field, and its planar components provide essentially roughness scattering, while a vertical effective field is the source of scattering in the method developed in which roughness is treated as a semiclassical barrier fluctuation. The present model involves a stronger effect on mobility than the previously developed one and results in an excellent fit, as regards mobility, to the reported experimental data. The extracted SR parameter also matches the observed value.

Inherent orbital spin textures in Rashba effect and their implications in spin-orbitronics.

The Rashba effect gives rise to the key feature of chiral spin texture. Recently it was demonstrated that the orbital angular momentum (OAM) texture forms the underlying basis for Rashba spin texture. Here we solve a model Hamiltonian of a generic p-orbital system in the presence of crystal field, internal spin-orbit coupling (SOC) and inversion symmetry breaking (ISB), and demonstrate, in addition to OAM and spin texture, the existence of orbital projection (OP) of the spin texture in a general Rashba system. The unique form of the OP pattern follows from the same condition for the existence of chirality of the spin texture. From the analytical results, we obtained the spin polarization as a function of parameters such as the SOC strength, crystal field splitting and degree of ISB, and compare them with those from numerical solutions and ab initio calculations. All three methods yield highly consistent results. Our results suggest means of external modulation, and elucidate the multi-orbital nature of the Rashba effect and the underlying OP of the spin texture. The understanding has potential applications in fields such as spin-orbitronics that requires delicate control between orbital occupancy and spin momentum.

Ultrafast and low-energy switching in voltage-controlled elliptical pMTJ.

Switching magnetization in a perpendicular magnetic tunnel junction (pMTJ) via voltage controlled magnetic anisotropy (VCMA) has shown the potential to markedly reduce switching energy. However, the requirement of an external magnetic field poses a critical bottleneck for its practical applications. In this work, we propose an elliptical-shaped pMTJ to eliminate the requirement of providing an external field by an additional circuit. We demonstrate that a 10 nm thick in-plane magnetized bias layer (BL) separated by a metallic spacer of 3 nm from the free layer (FL) can be engineered within the MTJ stack to provide the 50 mT bias magnetic field for switching. By conducting macrospin simulation, we find that a fast switching in 0.38 ns with energy consumption as low as 0.3 fJ at a voltage of 1.6 V can be achieved. Furthermore, we study the phase diagram of switching probability, showing that a pulse duration margin of 0.15 ns is obtained and low-voltage operation (~1 V) is favored. Finally, the MTJ scalability is considered, and it is found that scaling down may not be appealing in terms of both the energy consumption and the switching time for precession based VCMA switching.

Wave Function Parity Loss Used to Mitigate Thermal Broadening in Spin-orbit Coupled Zigzag Graphene Analogues.

Carrier transport through a graphene zigzag nanoribbon (ZNR) is possible to be blocked by a p-n profile implemented along its transport direction. However, we found that in cases of analogous materials with significant intrinsic spin-orbit coupling (SOC), i.e. silicene and germanene, such a profile on ZNR of these materials allows transmission mostly through spin-orbit coupled energy window due to the loss of the parity of wave functions at different energies caused by SOC. Next, a p-i-n scheme on germanene ZNR is proposed to simultaneously permit edge transmission and decimate bulk transmission. The transmission spectrum is shown to mitigate the effect of thermal broadening on germanene and silicene ZNR based spin-separators by improving spin polarization yield by 400% and 785%, respectively, at 300 K. The importance of proper gate voltage and position for such performance is further elucidated. Finally, the modulation the current output of the proposed U-shape p-i-n device while maintaining its spin polarization is discussed.

Single Atomically Sharp Lateral Monolayer p-n Heterojunction Solar Cells with Extraordinarily High Power Conversion Efficiency.

The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p-n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy structures at the interface, limiting the development for high-efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy-free 2D monolayer WSe2 -MoS2 lateral p-n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode-spacing design can lead to environment-independent PV properties. These robust PV properties deriving from the atomically sharp lateral p-n interface can help develop the next-generation photovoltaics.