Surface Conduction Channel Research Articles

High performance E-mode NiO/β-Ga2O3 HJ-FET with high conduction band offset and thin recessed channel

In this paper, an enhancement-mode (E-mode) NiO/β-Ga2O3 heterojunction field-effect transistor (HJ-FET) with high conduction band offset (ΔEC) and thin recessed channel is proposed and studied by Sentaurus TCAD. Different from the existing HJ-FET with low ΔEC alignment, the High ΔEC HJ-FET can achieve a much lower on-resistance (Ron) due to the strong electron confinement effect. More importantly, the disadvantage in the threshold voltage (Vth) is compensated by reducing the thickness of the recessed channel, maintaining an almost unchanged Ron with the help of the special surface conduction channel. Compared with the corresponding Low ΔEC HJ-FET, at the same Vth (∼ 0.82 V), the Ron is decreased from 135 Ω/mm to 90.7 Ω/mm and the maximum drain current is increased from 14.9 mA/mm to 83.1 mA/mm. By adding a top p-NiO layer for further optimization, a greatly improved power figure of merit (P-FOM) of 2.29 GW/cm2 is achieved among the E-mode HJ-FETs. These results show that the proposed High ΔEC HJ-FET with thin recessed channel is probably a better choice to achieve the high-performance E-mode lateral HJ-FET.

Novel superjunction Fin-based NiO/β-Ga2O3 HJFET with additional surface drift region channels for record-high performance

—In this paper, a novel superjunction fin-based NiO/β-Ga2O3 heterojunction field-effect transistor (SJ Fin-HJFET) is proposed and studied by simulations. Compared with the conventional Ga2O3 SJ FinFET, Ga2O3 SJ Fin-HJFET can generate surface conduction channels instead of depletion zones near the p-n junction interface in the SJ drift region. The additional surface conduction channel significantly improves the specific on-resistance of the SJ Fin-HJFET (from 1.091 mΩ cm2 to 0.397 mΩ cm2, reduced by 63.6 %) and dramatically improves the Baliga figure of merit (BFOM, form 11.75 GW/cm2 to 32.28 GW/cm2) at the same breakdown voltage (3580 V). The effect of different conduction band offset (ΔEC) on the performance of the SJ Fin-HJFET is also investigated. The simulation results show that the on-resistance advantage of the SJ Fin-HJFET applies to a wide range of ΔEC, exhibiting strong applicability and stability. These results indicate that the SJ Fin-HJFET has record-high performance and promising application prospects.

Composition dependence of the specific heat of FeSi

Recently, a high-mobility surface conduction channel and in-gap states were identified in the correlated small-gap semiconductor FeSi using electrical transport measurements and high-resolution tunneling spectroscopy. The mobility of the charge carriers in the surface channel is quantitatively reminiscent of topological insulators, but displays a lack of sensitivity to the presence of ferromagnetic impurities as studied by means of a series of single crystals with slightly different starting compositions. Here, we report measurements of the specific heat of these crystals. At low temperatures, a shallow maximum is observed in the specific heat divided by temperature. This maximum is suppressed under magnetic field, characteristic of a Schottky anomaly associated with magnetic impurities. In comparison, the height of this maximum decreases with increasing initial iron content.

Observation of spin-momentum locked surface states in amorphous Bi2Se3.

Crystalline symmetries have played a central role in the identification and understanding of quantum materials. Here we investigate whether an amorphous analogue of a well known three-dimensional strong topological insulator has topological properties in the solid state. We show that amorphous Bi2Se3 thin films host a number of two-dimensional surface conduction channels. Our angle-resolved photoemission spectroscopy data are consistent with a dispersive two-dimensional surface state that crosses the bulk gap. Spin-resolved photoemission spectroscopy shows this state has an anti-symmetric spin texture, confirming the existence of spin-momentum locked surface states. We discuss these experimental results in light of theoretical photoemission spectra obtained with an amorphous topological insulator tight-binding model, contrasting it with alternative explanations. The discovery of spin-momentum locked surface states in amorphous materials opens a new avenue to characterize amorphous matter, and triggers the search for an overlooked subset of quantum materials outside of current classification schemes.

Electrical detection of the inverse Edelstein effect on the surface of SmB6

We report the measurement of spin current induced charge accumulation, the inverse Edelstein effect (IEE), on the surface of a candidate topological Kondo insulator SmB6 single crystal. Robust surface conduction channel of SmB6 has been shown to exhibit large degree of spin-momentum locking, and spin polarized current through an external ferromagnetic contact induces the spin dependent charge accumulation on the surface of SmB6. The dependences of the IEE signal on the bias current, an external magnetic field direction and temperature are consistent with the anticlockwise spin texture for the surface band in SmB6 in the momentum space, and the direction and magnitude of the effect compared with the normal Edelstein signal are clearly explained by the Onsager reciprocal relation. Furthermore, we estimate spin-to-charge conversion efficiency, the IEE length, as 4.46 nm that is an order of magnitude larger than the efficiency found in other typical Rashba interfaces, implying that the Rashba contribution to the IEE signal could be small. Building upon existing reports on the surface charge and spin conduction nature on this material, our results provide additional evidence that the surface of SmB6 supports spin polarized conduction channel.

Crystal Symmetry, Strain, and Facet-Dependent Nature of Topological Surface States in Mercury Selenide

Spin-filtered surface conduction channels set topological insulator (TI) materials apart as a distinct class and grant them physical properties that are suited to the discovery of new quantum pheno...

Ferromagnetic phase transition in topological crystalline insulator thin films: Interplay of anomalous Hall angle and magnetic anisotropy

In magnetic topological phases of matter, the quantum anomalous Hall (QAH) effect is an emergent phenomenon driven by ferromagnetic doping, magnetic proximity effects and strain engineering. The realization of QAH states with multiple dissipationless edge and surface conduction channels defined by a Chern number $\mathcal{C}\geq1$ was foreseen for the ferromagnetically ordered SnTe class of topological crystalline insulators (TCIs). From magnetotransport measurements on Sn$_{1-x}$Mn$_{x}$Te ($0.00\leq{x}\leq{0.08}$)(111) epitaxial thin films grown by molecular beam epitaxy on BaF$_{2}$ substrates, hole mediated ferromagnetism is observed in samples with $x\geq0.06$ and the highest $T_\mathrm{c}\sim7.5\,\mathrm{K}$ is inferred from an anomalous Hall behavior in Sn$_{0.92}$Mn$_{0.08}$Te. The sizable anomalous Hall angle $\sim$0.3 obtained for Sn$_{0.92}$Mn$_{0.08}$Te is one of the greatest reported for magnetic topological materials. The ferromagnetic ordering with perpendicular magnetic anisotropy, complemented by the inception of anomalous Hall effect in the Sn$_{1-x}$Mn$_{x}$Te layers for a thickness commensurate with the decay length of the top and bottom surface states, points at Sn$_{1-x}$Mn$_{x}$Te as a preferential platform for the realization of QAH states in ferromagnetic TCIs.

P‐6.6: Effects of lightly Doped Drain Structure in n‐channel ELA Bridged‐Grain Poly‐Si TFTs

We lead the lightly doped drain (LDD) structure into the bridged‐grain (BG) poly‐Si thin film transistor (TFT). This combined structure can restrain the gate induced drain leakage current (GIDL) and the electrical characteristics shows not much sacrifice. A high Ion/Ioff current ratio 3.79×108 is achieved. Besides, the device reliability is improved significantly. Because the lateral electrical field decreases by LDD, lower trap state density is generated in the SiO2/poly‐Si interface and the grain boundaries of poly‐Si near the surface conduction channel, resulting in the stable on‐state drive current (Ion) and drain saturation voltage (Vdsat). All test results indicate that the BG poly‐Si TFT with a LDD structure will work well and has a great potential for system‐on‐panel application.

Observation and control of the anomalous Aharonov-Bohm oscillation in enhanced-mode topological insulator nanowire field-effect transistors

Aharonov-Bohm (AB) oscillation is a quantum mechanical phenomenon which reveals the coupling of electromagnetic potentials with the electron wave function, affecting the phase of the wave function. Such a quantum interference effect can be demonstrated through the magnetotransport measurement focusing on low-dimensional electronic states. Here, we report the experimental observation of anomalous AB oscillation in an enhanced-mode topological insulator Bi2Se3 nanowire field-effect transistor (FET) under strong surface disorder, which is different from the reported AB oscillation in topological insulator nanostructures. The surrounding gate of the nanowire FET gives rise to tunability of the chemical potential and introduces strong disorder on the surface states, leading to primary oscillation with an anomalous h/e period. Furthermore, the oscillation exhibits a significant dependence on the gate voltage which has been preliminary explained with the quantization of the surface conduction channel. The experimental demonstration can be very attractive for further exploration of quantum phase interference through electrical approaches, enabling applications in future information and electromagnetic sensing technology.

X-Ray Detector Based on All-Inorganic Lead-Free Cs2AgBiBr6 Perovskite Single Crystal

In this paper, single crystals (SCs) of the Cs2AgBiBr6 double perovskite were grown using the method of crystallization from a supersaturated solution. It was shown that the natural crystal growth surface of the perovskite with perfect crystal quality is along the (111) plane by X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) analyses. The fabricated high-sensitivity X-ray detector based on the Cs2AgBiBr6 SCs with vertical structure achieved detection sensitivity up to $316~\mu $ CGyair−1 cm−2 under bias voltage of 18 V. Furthermore, the frequency noise characteristics of the device under different posttreatment processes were systematically analyzed, which indicated that the combination of thermal annealing and isopropanol rinsing can suppress field-driven ion migration and surface conduction channel, thereby increasing the resistivity of the perovskite material and reducing the noise current of the device. Our results revealed that the Cs2AgBiBr6 SCs-based X-ray detector with high sensitivity and low cost has a great potential for application in manufacturing lead-free and stable radiation detection electronics in the future.

Low-Loss Si-Substrates Enhanced Using Buried PN Junctions for RF Applications

A novel method for increasing the effective resistivity in low-doped silicon substrates is presented. By creating a chain series of p-n depletion junctions beneath the insulator, the parasitic surface conduction channel is interrupted, significantly lowering substrate losses and reducing harmonic distortion in the simulated and measured CPW lines achieving performance close to the widely used trap-rich silicon substrate at RF frequencies.

Composition-dependent conductivity of InxGa1-xAs nanowires

Using the methods of scanning probe microscopy, I-V characteristics of individual InxGa1-xAs nanowires with different In content (x) were measured. A sharp decrease in the conductivity was observed at x=0.7. It is shown that type of contact between nanowire and probe changes from ohmic at x=1 to Schottky at x=0.85. These changes were explained by the formation of surface conductive channel, induced by surface Fermi level pinning in conduction band for x>0.85.

Topological Insulator Based Dual State Photo‐Switch Originating Through Bulk and Surface Conduction Channels

Topological insulators are novel electronic materials, where time reversal symmetry protects the spin‐polarized surface states from backscattering. Although, the bulk offers a trivial semiconducting response to incident light, but the metallic surface states exhibit interesting electrical response towards the incident radiation, such as polarization dependent surface photocurrent and topological phase transitions. Here, we study the temperature dependent near‐infrared photoresponse in bismuth selenide (Bi2Se3) nano‐flakes. A very good photo‐sensitivity to near‐infrared 800 nm wavelength is observed for the temperature range 300–2 K. Voltage responsivity at 2 K (1.481 × 1010 VW−1) is estimated to be four‐order greater than at 300 K (2.095 × 106 VW−1) and is comparatively much higher than the previously reported values of voltage responsivity in other materials. Interestingly, we also find the temperature dependent existence of both positive and negative photoresponse in our device. An anomalous photoresponse reversal is observed at 2 K, where resistance of the nano‐flake increases under light illumination. We propose that this phenomenon is due to the topological surface states, which have dominant transport contribution at very low temperatures. These observations establish the fact that topological insulators can be further engineered to develop a dual state photo‐switch, by manipulating the bulk and surface state conduction contribution.

Multiple surface conduction channels via topological insulator and amorphous insulator thin film multi-stacks

Multi-channel Bi2Se3 thin films were grown by combining molecular beam epitaxy and atomic layer deposition. High-resolution transmission electron microscope images showed that c-axis oriented Bi2Se3 grew on amorphous Al2O3 even after multiple stacking. While the surface morphology degraded for the upper layers, each layer was electrically similar. The electrical transport measurements showed that the weak anti-localization effect was quantitatively enhanced upon increasing the number of Bi2Se3 channels. Our results provide a promising approach to exploit diverse combinations of layered topological insulator films vertically stacked with amorphous insulator films.

Carrier transport characteristics of H-terminated diamond films prepared using molecular hydrogen and atomic hydrogen

The H-terminated diamond films, which exhibit high surface conductivity, have been used in high-frequency and high-power electronic devices. In this paper, the surface conductive channel on specimens from the same diamond film was obtained by hydrogen plasma treatment and by heating under a hydrogen atmosphere, respectively, and the surface carrier transport characteristics of both samples were compared and evaluated. The results show that the carrier mobility and carrier density of the sample treated by hydrogen plasma are 15 cm2·V−1·s−1 and greater than 5 × 1012 cm−2, respectively, and that the carrier mobilities measured at five different areas are similar. Compared to the hydrogen-plasma-treated specimen, the thermally hydrogenated specimen exhibits a lower surface conductivity, a carrier density one order of magnitude lower, and a carrier mobility that varies from 2 to 33 cm2·V−1·s−1. The activated hydrogen atoms restructure the diamond surface, remove the scratches, and passivate the surface states via the etching effect during the hydrogen plasma treatment process, which maintains a higher carrier density and a more stable carrier mobility.

Photoconductivity induced by nanoparticle segregated grain-boundary in spark plasma sintered BiFeO3

Photoconductivity studies on spark plasma sintered BiFeO3 samples with two contrasting morphologies, viz., nanoparticle-segregated grain boundary (BFO-AP) and clean grain boundary (BFO-AA), show that their photo-response is largely influenced by the grain boundary defects. Impedance analyses at 300 K and 573 K clearly demarcate the contributions from grain, grain-boundary, and the nanoparticle-segregated grain-boundary conductivities. I-V characteristics under 1 sun illumination show one order of higher conductivity for BFO-AP, whereas conductivity decreases for BFO-AA sample. Larger photocurrent in BFO-AP is attributed to the extra conduction path provided by oxygen vacancies on the nanoparticle surfaces residing at the grain boundaries. Creation of photo-induced traps under illumination and the absence of surface conduction channels in BFO-AA are surmised to result in a decreased conductivity on illumination.

Probing the interplay between surface and bulk states in the topological Kondo insulatorSmB6through conductance fluctuation spectroscopy

We present results of resistance fluctuation spectroscopy on single crystals of the predicted Kondo topological insulator material SmB$_6$. Our measurements show that at low temperatures, transport in this system takes place only through surface states. The measured noise in this temperature range arises due to Universal Conductance Fluctuations whose statistics was found to be consistent with theoretical predictions for that of two-dimensional systems in the Symplectic symmetry class. At higher temperatures, we find signatures of glassy dynamics and establish that the measured noise is caused by mobility fluctuations in the bulk. We find that, unlike the topological insulators of the dichalcogenide family, the noise in surface and bulk conduction channels in SmB$_6$ are completely uncorrelated. Our measurements establish that at sufficiently low temperatures, the bulk has no discernible contribution to electrical transport in SmB$_6$ making it an ideal platform for probing the physics of topological surface states.

UV Photosensing Characteristics of Nanowire-Based GaN/AlN Superlattices.

We have characterized the photodetection capabilities of single GaN nanowires incorporating 20 periods of AlN/GaN:Ge axial heterostructures enveloped in an AlN shell. Transmission electron microscopy confirms the absence of an additional GaN shell around the heterostructures. In the absence of a surface conduction channel, the incorporation of the heterostructure leads to a decrease of the dark current and an increase of the photosensitivity. A significant dispersion in the magnitude of dark currents for different single nanowires is attributed to the coalescence of nanowires with displaced nanodisks, reducing the effective length of the heterostructure. A larger number of active nanodisks and AlN barriers in the current path results in lower dark current and higher photosensitivity and improves the sensitivity of the nanowire to variations in the illumination intensity (improved linearity). Additionally, we observe a persistence of the photocurrent, which is attributed to a change of the resistance of the overall structure, particularly the GaN stem and cap sections. As a consequence, the time response is rather independent of the dark current.

Enhanced surface transfer doping of diamond by V2O5 with improved thermal stability

Surface transfer doping of hydrogen-terminated diamond has been achieved utilising V2O5 as a surface electron accepting material. Contact between the oxide and diamond surface promotes the transfer of electrons from the diamond into the V2O5 as revealed by the synchrotron-based high resolution photoemission spectroscopy. Electrical characterization by Hall measurement performed before and after V2O5 deposition shows an increase in hole carrier concentration in the diamond from 3.0 × 1012 to 1.8 × 1013 cm−2 at room temperature. High temperature Hall measurements performed up to 300 °C in atmosphere reveal greatly enhanced thermal stability of the hole channel produced using V2O5 in comparison with an air-induced surface conduction channel. Transfer doping of hydrogen-terminated diamond using high electron affinity oxides such as V2O5 is a promising approach for achieving thermally stable, high performance diamond based devices in comparison with air-induced surface transfer doping.

Pentacene phototransistor with gate voltage independent responsivity and sensitivity by small silver nanoparticles decoration

In this study, a Pentacene phototransistor (PT) decorated with silver nanoparticles (Ag-NPs) in bottom gate structure was demonstrated. With the ultra-small Ag-NPs, this device exploits surface plasmon resonance at 445 nm and established a stable surface conduction channel under light illumination. Therefore, the PT exhibit gate voltage independent responsivity and photosensitivity in broad range of gate voltage. The light-induced magnetized in ultra-small Ag-NPs that generate polarized surface electrons were used to explain the gate-voltage independent photocurrent. The maximum photosensitivity and responsivity are 2.1 × 103 and 17.7 mA/W, respectively, under white-light illumination. They are enhanced around 25 times at gate voltage of 20 V than conventional phototransistor. This device may be used in flexible optical interactive display, white-light indoor communication, or optical sensors.