High Mobility Devices Research Articles

Charge-transfer contacts for the measurement of correlated states in high-mobility WSe2.

Two-dimensional semiconductors, such as transition metal dichalcogenides, have demonstrated tremendous promise for the development of highly tunable quantum devices. Realizing this potential requires low-resistance electrical contacts that perform well at low temperatures and low densities where quantum properties are relevant. Here we present a new device architecture for two-dimensional semiconductors that utilizes a charge-transfer layer to achieve large hole doping in the contact region, and implement this technique to measure the magnetotransport properties of high-purity monolayer WSe2. We measure a record-high hole mobility of 80,000 cm2 V-1 s-1 and access channel carrier densities as low as 1.6 × 1011 cm-2, an order of magnitude lower than previously achievable. Our ability to realize transparent contact to high-mobility devices at low density enables transport measurements of correlation-driven quantum phases including the observation of a low-temperature metal-insulator transition in a density and temperature regime where Wigner crystal formation is expected and the observation of the fractional quantum Hall effect under large magnetic fields. The charge-transfer contact scheme enables the discovery and manipulation of new quantum phenomena in two-dimensional semiconductors and their heterostructures.

Knot Architecture for Biocompatible and Semiconducting 2D Electronic Fiber Transistors.

Wearable devices have generally been rigid due to their reliance on silicon-based technologies, while future wearables will utilize flexible components for example transistors within microprocessors to manage data. Two-dimensional (2D) semiconducting flakes have yet to be investigated in fiber transistors but can offer a route toward high-mobility, biocompatible, and flexible fiber-based devices. Here, the electrochemical exfoliation of semiconducting 2D flakes of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) is shown to achieve homogeneous coatings onto the surface of polyester fibers. The high aspect ratio (>100) of the flake yields aligned and conformal flake-to-flake junctions on polyester fibers enabling transistors with mobilities μ ≈1 cm2 V-1 s-1 and a current on/off ratio, Ion/Ioff ≈102-104. Furthermore, the cytotoxic effects of the MoS2 and WSe2 flakes with human keratinocyte cells are investigated and found to be biocompatible. As an additional step, a unique transistor 'knot' architecture is created by leveraging the fiber diameter to establish the length of the transistor channel, facilitating a route to scale down transistor channel dimensions (≈100 µm) and utilize it to make a MoS2 fiber transistor with a human hair that achieves mobilities as high as μ ≈15 cm2 V-1 s-1.

Review on remote phonon scattering in transistors with metal-oxide-semiconductor structures adopting high-k gate dielectrics

One main obstacle to obtaining high carrier mobility in transistors with metal-oxide-semiconductor (MOS) structures is carrier scattering, which has been systematically investigated. In the past few decades, much attention was preferentially paid to the scatterings arising from the region near the semiconductor/oxide interface because they can affect the carrier transport in the semiconductor channel more directly and effectively, e.g., polaronic effect, Coulomb scattering, surface-roughness scattering, and intrinsic phonon scattering resulted from the thermal vibration of the semiconductor channel. However, scattering originated from hybrid interface plasmon/optical-phonon excitations, so-called remote phonon scattering, has been neglected to some extent, but is especially severe for gate oxides with high dielectric constants due to the easy vibrations of their atoms. On the other hand, plasmons generated from the oscillations of majority carriers in the gate electrode can couple with the remote phonons to suppress the remote phonon scattering, which is called the gate screening effect. However, when the frequency of the gate-electrode plasmon is close/equal to that of the gate-dielectric phonon, the resonance between the gate electrode and the gate dielectric greatly enhances the remote phonon scattering to severely degrade the carrier mobility (so-called gate antiscreening effect). This work intends to give a comprehensive review on the origins, effects, suppression methods, and recent advances of the remote phonon scattering, with a view to achieving high-mobility MOS devices (including those based on two-dimensional semiconductors) with high-k gate dielectrics for future high-speed electronic applications.

Machine Learning for Physical Layer in 5G and beyond Wireless Networks: A Survey

Fifth-generation (5G) technology will play a vital role in future wireless networks. The breakthrough 5G technology will unleash a massive Internet of Everything (IoE), where billions of connected devices, people, and processes will be simultaneously served. The services provided by 5G include several use cases enabled by the enhanced mobile broadband, massive machine-type communications, and ultra-reliable low-latency communication. Fifth-generation networks potentially merge multiple networks on a single platform, providing a landscape for seamless connectivity, particularly for high-mobility devices. With their enhanced speed, 5G networks are prone to various research challenges. In this context, we provide a comprehensive survey on 5G technologies that emphasize machine learning-based solutions to cope with existing and future challenges. First, we discuss 5G network architecture and outline the key performance indicators compared to the previous and upcoming network generations. Second, we discuss next-generation wireless networks and their characteristics, applications, and use cases for fast connectivity to billions of devices. Then, we confer physical layer services, functions, and issues that decrease the signal quality. We also present studies on 5G network technologies, 5G propelling trends, and architectures that help to achieve the goals of 5G. Moreover, we discuss signaling techniques for 5G massive multiple-input and multiple-output and beam-forming techniques to enhance data rates with efficient spectrum sharing. Further, we review security and privacy concerns in 5G and standard bodies’ actionable recommendations for policy makers. Finally, we also discuss emerging challenges and future directions.

High field-effect performance and intrinsic scattering in the two-dimensional MoS2 semiconductors

The mobility of MoS2 nanosheets plays an important role in the application of nanoelectronics. The two-dimensional (2D) MoS2 semiconductors have been investigated while the reported mobility varies enormously. We adopt a standard four-terminal electrical measurement and eliminate any contact contributions. We fabricated several tens of field-effect transistors (FETs) with MoS2 nanosheets exfoliated from the same MoS2 bulk. The MoS2 FETs can be grouped into either high- or low-mobility devices according to their temperature behaviors of mobilities. With an RT mobility higher than ~ 25 or ~ 30 cm2V-1s−1, the MoS2 FETs exhibit an increasing mobility with decreasing temperature, implying phonon scattering in these high-mobility (HM) devices. The HM devices are accompanied with metallic states and their conductivities close to ~ 1 e2/h. A low specific contact resistivity of ~ 450 Ω·μm is observed in these HM devices. In contrast, the MoS2 FETs with an RT mobility lower than ~ 30 or ~ 35 cm2V-1s−1, categorized as low-mobility (LM) devices, reveal a decreasing mobility with decreasing temperature. These LM devices show an insulating state, carriers suffering from impurity scattering, and a high contact resistance. Their temperature dependent resistances are highly concordant with the 2D Mott’s variable range hopping. These observations corroborate a large variation of intrinsic disorders in MoS2 nanosheets.

Isotopic disorder in integer and fractional quantum Hall effects

The energy of the activation gaps in the quantum Hall effect has been thought to be reduced by the broadening of the Landau levels due to disorder. The isotopic mass can affect the electron-phonon interactions and the 0 K renormalization of the conduction and valence bands, leading to appreciable energy offsets between lattice sites with different nuclear masses. The isotopic disorder originating from the natural occurrence of nuclear masses could be the dominant broadening mechanism remaining, leading to the experimentally observed quantum Hall effect gaps in high-mobility devices. The Landau level broadening due to the isotopic disorder has been calculated microscopically without fitting parameters.

Simulation of THz Oscillations in Semiconductor Devices Based on Balance Equations

Instabilities of electron plasma waves in high-mobility semiconductor devices have recently attracted a lot of attention as a possible candidate for closing the THz gap. Conventional moments-based transport models usually neglect time derivatives in the constitutive equations for vectorial quantities, resulting in parabolic systems of partial differential equations (PDE). To describe plasma waves however, such time derivatives need to be included, resulting in hyperbolic rather than parabolic systems of PDEs; thus the fundamental nature of these equation systems is changed completely. Additional nonlinear terms render the existing numerical stabilization methods for semiconductor simulation practically useless. On the other hand there are plenty of numerical methods for hyperbolic systems of PDEs in the form of conservation laws. Standard numerical schemes for conservation laws, however, are often either incapable of correctly handling the large source terms present in semiconductor devices due to built-in electric fields, or rely heavily on variable transformations which are specific to the equation system at hand (e.g. the shallow water equations), and can not be generalized easily to different equations. In this paper we develop a novel well-balanced numerical scheme for hyperbolic systems of PDEs with source terms and apply it to a simple yet non-linear electron transport model.

Perovskite Core-Shell Nanowire Transistors: Interfacial Transfer Doping and Surface Passivation.

While halide perovskite electronics are rapidly developing, they are greatly limited by the inferior charge transport and poor stability. In this work, effective surface charge transfer doping of vapor-liquid-solid (VLS)-grown single-crystalline cesium lead bromide perovskite (CsPbBr3) nanowires (NWs) via molybdenum trioxide (MoO3) surface functionalization is achieved. Once fabricated into NW devices, due to the efficient interfacial charge transfer and reduced impurity scattering, a 15× increase in the field-effect hole mobility (μh) from 1.5 to 23.3 cm2/(V s) is accomplished after depositing the 10 nm thick MoO3 shell. This enhanced mobility is already better than any mobility value reported for perovskite field-effect transistors (FETs) to date. The photodetection performance of these CsPbBr3/MoO3 core-shell NWs is also investigated to yield a superior responsivity (R) up to 2.36 × 103 A/W and an external quantum efficiency (EQE) of over 5.48 × 105% toward the 532 nm regime. Importantly, the MoO3 shell can provide excellent surface passivation to the CsPbBr3 NW core that minimizes the diffusion of detrimental water and oxygen molecules, improving the air stability of CsPbBr3/MoO3 core-shell NW devices. All these findings evidently demonstrate the surface doping as an enabling technology to realize high-mobility and air-stable low-dimensional halide perovskite devices.

Commercial GaN-Based Power Electronic Systems: A Review

Wide bandgap semiconductor technology is gaining widespread acceptance in the area of high-power and high-temperature power electronics. Gallium nitride (GaN) not only has a wide bandgap of 3.4 eV and all the associated superior electronic properties but also enables the development of high-mobility power devices which is critical in increasing the power density of a power electronics system. Since a commercial GaN power transistor has a lateral structure as opposed to the traditional vertical device structure, commercially available devices are rated below 1000 V breakdown voltage with a maximum value of 900 V and typical value around 650 V. The primary focus of this review will be to introduce readers to the commercially available power electronic systems developed by various manufacturers which employ GaN-based power devices and highlight their remarkable performance which surpasses existing technology. This review also includes a brief introduction on GaN technology followed by current market study showing the roadmap of integration of GaN-based power electronics in the power industry.

Understanding the Role of Temperature and Drain Current Stress in InSnZnO TFTs with Various Active Layer Thicknesses.

Thin-film transistor (TFT) devices composed of metal oxide semiconductors have attracted tremendous research attention globally in recent years. Owing to their ability to offer mobility, metal oxide semiconductor materials can enable high-performance TFTs for next-generation integrated display devices. Nevertheless, further breakthroughs of metal oxide TFTs are mainly obstructed by their long-term variability, the reason for which is not yet fully understood. Herein, TFTs based on InSnZnO (ITZO) with various thicknesses (TITZO) were prepared and their long-term stabilities under test temperatures and drain current stress were investigated. The results indicate that ITZO TFTs exhibit outstanding electrical properties regardless of the TITZO, including a high saturated mobility of over 35 cm2V−1s−1 and sharp subthreshold swing. Note that the transfer and output characteristic curves of the device with a thick TITZO of 100 nm express an abnormal current surge when high gate and drain voltages are exerted, which is attributed to the floating body effect, caused when the imposed electric field induces impact ionization near the drain side. More interestingly, these drain current stress results further suggest that the abnormal shift behavior of the electrical properties of the ITZO TFTs with a TITZO of greater than 75 nm is observed to deteriorate gradually with increasing temperature and drain current bias. This study addresses that such a degradation effect should be restrained for the operation of high-mobility devices.

Resistively detected NMR in a triple-gate quantum point contact: Magnetic field dependence

We report a resistively detected nuclear magnetic resonance (RDNMR) measurement in a GaAs-based quantum point contact in the quantum Hall breakdown regime. We focus on the detection of the $^{75}\mathrm{As}$ RDNMR signal at different perpendicular magnetic fields in the presence of electrons occupying the lowest Landau level. We confirm successful RDNMR detection down to $B=1.25\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ ($B=3.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$) using a high-mobility (low-mobility) device. All the RDNMR signals exhibit a threefold spectra attributed to electric quadrupole interaction. We find that the separation between the central transition and its satellite, ${\mathrm{\ensuremath{\Delta}}}_{f}\ensuremath{\approx}20\phantom{\rule{0.16em}{0ex}}\mathrm{kHz}$, remains unaffected by magnetic-field variation. Interestingly, the central NMR linewidths vary with the field strength. Above $B=3\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, the linewidth tends to saturate around 13 kHz. Below $B=3\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, the linewidth decreases in proportion with field and approaches a lower limit of 1 kHz corresponding to the GaAs nuclear dipole interaction as the field decreases to $B=1.25\phantom{\rule{0.16em}{0ex}}\mathrm{T}$.

Quantum Hall Effect Measurement of Spin-Orbit Coupling Strengths in Ultraclean Bilayer Graphene/WSe2 Heterostructures.

We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe2 heterostructure devices. Contact mode atomic force microscopy (AFM) cleaning yields ultraclean interfaces and high-mobility devices. In a perpendicular magnetic field, we measure the quantum Hall effect to determine the Landau level structure in the presence of out-of-plane Ising and in-plane Rashba SOC. A distinct Landau level crossing pattern emerges when tuning the charge density and displacement field independently with dual gates, originating from a layer-selective SOC proximity effect. Analyzing the Landau level crossings and measured inter-Landau level energy gaps yields the proximity-induced SOC energy scale. The Ising SOC is ∼2.2 meV, 100 times higher than the intrinsic SOC in graphene, whereas its sign is consistent with theories predicting a dependence of SOC on interlayer twist angle. The Rashba SOC is ∼15 meV. Finally, we infer the magnetic field dependence of the inter-Landau level Coulomb interactions. These ultraclean bilayer graphene/WSe2 heterostructures provide a high mobility system with the potential to realize novel topological electronic states and manipulate spins in nanostructures.

Electron and hole transport in disordered monolayerMoS2: Atomic vacancy induced short-range and Coulomb disorder scattering

Atomic disorder is a common limiting factor for the low-temperature mobility in monolayer transition-metal dichalcogenides (TMDs; $M{X}_{2}$). Here, we study the effect of often occurring atomic vacancies on carrier scattering and transport in $p$- and $n$-type monolayer ${\mathrm{MoS}}_{2}$. Due to charge trapping in vacancy-induced in-gap states, both neutral and charged vacancies resembling, respectively, short-range and combined short-range and long-range Coulomb scatterers must be considered. Using the $T$-matrix formalism, we demonstrate a strong renormalization of the Born description of short-range scattering, manifested in a pronounced reduction and a characteristic energy dependence of the scattering rate. As a consequence, carrier scattering in TMDs with charged vacancies is dominated by the long-range Coulomb disorder scattering, giving rise to a strong screening-induced temperature and density dependence of the low-temperature carrier mobility. For TMDs with neutral vacancies, the absence of intrinsic Coulomb disorder results in significantly higher mobilities as well as an unusual density dependence of the mobility which decreases with the carrier density. Our work illuminates the transport-limiting effects of atomic-vacancy scattering relevant for high-mobility TMD devices.

(Invited) Electrical Activity of Extended Defects in III-V Semiconductors

The electrical activity of a defect in a semiconductor material is usually expressed in terms of the Shockley-Read-Hall (SRH) theory which provides an expression for the generation or recombination lifetime. Along the same lines, one can calculate the leakage current generated by a single defect in the depletion region of a junction, as shown in Fig. 1 [1]. It is clear that for III-V based ternary alloys, like the InxGa1-xAs system, the generation current depends proportionally on the intrinsic carrier concentration ni (see equation in Fig. 1). However, equally important is also the variation of the defect energy level ET as a function of the In content x: for ET pinned to the intrinsic Fermi level Ei(x), the leakage current will follow ni. On the other hand, when ET is pinned to either the valence or conduction band, the leakage current will decrease exponentially with the difference |ET-Ei| [2]. It is thus important to derive the variation of ET with alloy composition x for a certain type of defect and to establish what could be consideredas a kind of Vegard’s law for specific deep levels. In addition, when integrating III-V compounds on a silicon CMOS platform, extended defects will be inevitably introduced [3]; for a sufficiently high density, they will govern the reverse current [3,4]. It is thus of vital importance to study the electrical parameters of extended defects in III-V-on-silicon materials and their evolution with composition x. The aim of the this work is to review what is known about the electrical activity of extended defects in InxGa1-xAs-based hetero-epitaxial layers. Results will be described about a combined p-n diode current-voltage (I-V) and lifetime analysis on a set of nearly strain-free layers where x=0.53 with varying densities of extended defects [5]. Based on this, it is demonstrated that for an extended defect density exceeding ~2×107 cm-2 the SRH lifetime becomes dominated by a near mid-gap trap level, associated with them. A detailed Deep-Level Transient Spectroscopy (DLTS) analysis provides further information about the electrical activity of these extended defects and confirms the presence of a dominant electron trap. The corresponding activation energy is represented in Fig. 2 versus x and compared with available literature data. In addition, it is shown that this deep level indeed belongs to an extended defect, based on the electron capture behavior, varying approximately logarithmically with the capture time [4]. Moreover, it can be shown that the electron traps belong to a one-dimensional band-of-states, associated with a ‘perfect’ extended defect. According to Fig. 2, this level appears to follow approximately Ei(x), so that it will dominate the leakage current of a p-n junction over a wide composition range, at least up to 0.53, when present in a sufficient density. Moreover, the defects will become more efficient leakage centers for increasing x. Finally, the implications of the band-like nature of the energy states of extended defects on the carrier generation/recombination will be discussed. [1] J. Vanhellemont and E. Simoen, J. Electrochem. Soc, 154, H572 (2007). [2] E. Simoen, J. Lauwaert and H. Vrielinck, Semiconductors and Semimetals, Eds. L. Romano, V. Privitera and C. Jagadish, 91, pp. 205-250, Elsevier 2015. [3] E. Simoen, “Impact of defects on the performance of high-mobility semiconductor devices”, In: High Mobility Materials for CMOS Applications, Ed. N. Collaert, Elsevier, Ch. 5, July 2018. [4] C. Claeys et al., ECS J. Solid State Sci. and Technol., 5, P3149 (2016). [5] P.-C. Hsu et al., J. Appl. Phys., 124, 165707 (2018). Figure 1

Simultaneous voltage and current density imaging of flowing electrons in two dimensions.

A variety of physical phenomena associated with nanoscale electron transport often results in non-trivial spatial voltage and current patterns, particularly in nonlocal transport regimes. While numerous techniques have been devised to image electron flows, the need remains for a nanoscale probe capable of simultaneously imaging current and voltage distributions with high sensitivity and minimal invasiveness, in a magnetic field, across a broad range of temperatures and beneath an insulating surface. Here we present a technique for spatially mapping electron flows based on a nanotube single-electron transistor, which achieves high sensitivity for both voltage and current imaging. In a series of experiments using high-mobility graphene devices, we demonstrate the ability of our technique to visualize local aspects of intrinsically nonlocal transport, as in ballistic flows, which are not easily resolvable via existing methods. This technique should aid in understanding the physics of two-dimensional electronic devices and enable new classes of experiments that image electron flow through buried nanostructures in the quantum and interaction-dominated regimes.

Fabry\u2013P\xe9rot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts

We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry–Pérot (FP) resonances due to np’n(pn’p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p–n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices.

Group-Based Sidelink Communication for Seamless Vehicular Handover

With the evolution of communication technique, international mobile telecommunication (IMT) indicates that 5G services will be broadly deployed and be popular in 2020. It is imperative to tackle the challenges of high-mobility devices and dynamic communication demands in the real world. Vehicle-to-everything (V2X) is one of the most important parts; hence, approaches to achieve higher communication quality and better spectrum utilization in V2X have become important issues. Presently, it is a common practice to group the users in vehicle-to-vehicle (V2V) communication based on device-to-device (D2D) techniques. In this paper, we consider the handover of groups of moving vehicles. To address the mobility problem, we assign three types of user equipments (UEs), i.e., the main leader, a sub-leader, and followers, in a group with different functions for different roles. The simulation result shows that our proposed group-based mobility management mechanism can decrease the handover failure ratio as well as increase V2V service continuity.

High-mobility indirect excitons in wide single quantum well

Indirect excitons (IXs) are bound pairs of electrons and holes confined in spatially separated layers. We present wide single quantum well (WSQW) heterostructures with high IX mobility, spectrally narrow IX emission, voltage-controllable IX energy, and long and voltage-controllable IX lifetime. This set of properties shows that WSQW heterostructures provide an advanced platform both for studying basic properties of IXs in low-disorder environments and for the development of high-mobility excitonic devices.

High-Mobility Helical Tellurium Field-Effect Transistors Enabled by Transfer-Free, Low-Temperature Direct Growth.

The transfer-free direct growth of high-performance materials and devices can enable transformative new technologies. Here, room-temperature field-effect hole mobilities as high as 707 cm2 V-1 s-1 are reported, achieved using transfer-free, low-temperature (≤120 °C) direct growth of helical tellurium (Te) nanostructure devices on SiO2 /Si. The Te nanostructures exhibit significantly higher device performance than other low-temperature grown semiconductors, and it is demonstrated that through careful control of the growth process, high-performance Te can be grown on other technologically relevant substrates including flexible plastics like polyethylene terephthalate and graphene in addition to amorphous oxides like SiO2 /Si and HfO2 . The morphology of the Te films can be tailored by the growth temperature, and different carrier scattering mechanisms are identified for films with different morphologies. The transfer-free direct growth of high-mobility Te devices can enable major technological breakthroughs, as the low-temperature growth and fabrication is compatible with the severe thermal budget constraints of emerging applications. For example, vertical integration of novel devices atop a silicon complementary metal oxide semiconductor platform (thermal budget <450 °C) has been theoretically shown to provide a 10× systems level performance improvement, while flexible and wearable electronics (thermal budget <200 °C) can revolutionize defense and medical applications.

Molecular Arrangement and Charge Transfer in C60/Graphene Heterostructures.

Charge transfer at the interface between dissimilar materials is at the heart of electronics and photovoltaics. Here we study the molecular orientation, electronic structure, and local charge transfer at the interface region of C60 deposited on graphene, with and without supporting substrates such as hexagonal boron nitride. We employ ab initio density functional theory with van der Waals interactions and experimentally characterize interface devices using high-resolution transmission electron microscopy and electronic transport. Charge transfer between C60 and the graphene is found to be sensitive to the nature of the underlying supporting substrate and to the crystallinity and local orientation of the C60. Even at room temperature, C60 molecules interfaced to graphene are orientationally locked into position. High electron and hole mobilities are preserved in graphene with crystalline C60 overlayers, which has ramifications for organic high-mobility field-effect devices.