Room Temperature Field Effect Research Articles

Electrode-Induced Self-Healed Monolayer MoS2 for High Performance Transistors and Phototransistors.

Contact engineering for monolayered transition metal dichalcogenides (TMDCs) is considered to be of fundamental challenge for realizing high-performance TMDCs-based (opto) electronic devices. Here, an innovative concept is established for a device configuration with metallic copper monosulfide (CuS) electrodes that induces sulfur vacancy healing in the monolayer molybdenum disulfide (MoS2 ) channel. Excess sulfur adatoms from the metallic CuS electrodes are donated to heal sulfur vacancy defects in MoS2 that surprisingly improve the overall performance of its devices. The electrode-induced self-healing mechanism is demonstrated and analyzed systematically using various spectroscopic analyses, density functional theory (DFT) calculations, and electrical measurements. Without any passivation layers, the self-healed MoS2 (photo)transistor with the CuS contact electrodes show outstanding room temperature field effect mobility of 97.6 cm2 (Vs)-1 , On/Off ratio > 108 ,low subthreshold swing of 120mV per decade, high photoresponsivity of 1 × 104 A W-1 , and detectivity of 1013 jones, which are the best among back-gated transistors that employ 1L MoS2 . Using ultrathin and flexible 2D CuS and MoS2 , mechanically flexible photosensor is also demonstrated, which shows excellent durability under mechanical strain. These findings demonstrate a promising strategy in TMDCs or other 2D material for the development of high performance and functional devices including self-healable sulfide electrodes.

In-plane InSb nanowires grown by selective area molecular beam epitaxy on semi-insulating substrate

In-plane InSb nanostructures are grown on a semi-insulating GaAs substrate using an AlGaSb buffer layer covered with a patterned SiO2 mask and selective area molecular beam epitaxy. The shape of these nanostructures is defined by the aperture in the silicon dioxide layer used as a selective mask thanks to the use of an atomic hydrogen flux during the growth. Transmission electron microscopy reveals that the mismatch accommodation between InSb and GaAs is obtained in two steps via the formation of an array of misfit dislocations both at the AlGaSb buffer layer/GaAs and at the InSb nanostructures/AlGaSb interfaces. Several micron long in-plane nanowires (NWs) can be achieved as well as more complex nanostructures such as branched NWs. The electrical properties of the material are investigated by the characterization of an InSb NW MOSFET down to 77 K. The resulting room temperature field effect mobility values are comparable with those reported on back-gated MOSFETs based on InSb NWs obtained by vapor liquid solid growth or electrodeposition. This growth method paves the way to the fabrication of complex InSb-based nanostructures.

(Invited) Black Phosphorus Field Effect Transistors: Passivation By Oxidation, and the Role of Anisotropy in Magnetotransport

Black phosphorus (bP) is an elemental allotrope with an anisotropic layered crystal structure consisting of a puckered honeycomb lattice that can be mechanically exfoliated down to atomic layer thickness. bP is the most thermodynamically stable allotrope of phosphorus but is nonetheless subject to photo-­oxidation in the presence of water, oxygen and visible light with a reaction rate that increases as bP layer thickness decreases [1]. In the first part of my talk, I will describe our experiments showing how oxidation can be used for passivation of black phosphorus field effect transistors. In the second part of my talk, I will describe magnetotransport measurements of black phosphorus transistors and their relation to the anisotropy of the bP crystal lattice. Motivated by the experimental observation that reactive ion etching produces a phosphorus oxide / black phosphorus interface that does not inhibit photo-luminescence yield [2], we have fabricated bP field effect transistors with an encapsulating phosphorus oxide layer that acts as a passivation layer. X-ray photoemission spectroscopy (XPS) reveals the oxide layer to have a P205 stoichiometry. Air stable, top gated bP field effect transistors with room temperature field effect mobilities exceeding 100 cm2/Vs are demonstrated using a gate oxide formed by reactive ion etching and subsequent atomic layer deposition of aluminum oxide. We have also investigated the magnetotransport properties of black phosphorus at modest fields. Weak localization was observed in a black phosphorus field-effect transistor 65 nm thick [3]. The weak localization behavior was found to be in excellent agreement with the Hikami-Larkin-Nagaoka [4] model for fields up to 1 T, from which characteristic scattering lengths could be inferred. The temperature dependence of the phase coherence length L ϕ was investigated, and above 1 K, it was found to decrease weaker than the L ϕ ~ T −1/2 dependence characteristic of electron-electron scattering in the presence of elastic scattering in two dimensions. Rather, the observed power law was found to be close to that observed previously in quasi-one-dimensional systems such as metallic nanowires [5] and carbon nanotubes [6]. The potential role of strong in-plane anisotropy resulting from a puckered honyecomb structure on localization will be discussed. [1] A. Favron et al., Nat. Mater. 14, 826 (2015). [2] J. Pei et al., Nat. Comm. 7, 10450 (2016). [3] N. Hemsworth et al., Phys. Rev. B 94, 245404 (2016). [4] S. Hikami et al., Prog. Theor. Phys. 63, 707 (1980). [5] D. Natelson et al., Phys. Rev. Lett. 86, 1821 (2001). [6] J. Appenzeller et al., Phys. Rev. B 64, 121404(R) (2001).

Room-temperature field effect transistors with metallic ultrathin TiN-based channel prepared by atomic layer delta doping and deposition

Metallic channel transistors have been proposed as the candidate for sub-10 nm technology node. However, the conductivity modulation in metallic channels can only be observed at low temperatures usually below 100 K. In this study, room-temperature field effect and modulation of the channel resistance was achieved in the metallic channel transistors, in which the oxygen-doped TiN ultrathin-body channels were prepared by the atomic layer delta doping and deposition (AL3D) with precise control of the channel thickness and electron concentration. The decrease of channel thickness leads to the reduction in electron concentration and the blue shift of absorption spectrum, which can be explained by the onset of quantum confinement effect. The increase of oxygen incorporation results in the increase of interband gap energy, also giving rise to the decrease in electron concentration and the blue shift of absorption spectrum. Because of the significant decrease in electron concentration, the screening effect was greatly suppressed in the metallic channel. Therefore, the channel modulation by the gate electric field was achieved at room temperature due to the quantum confinement and suppressed screening effect with the thickness down to 4.8 nm and the oxygen content up to 35% in the oxygen-doped TiN ultrathin-body channel.

Screening limited switching performance of multilayer 2D semiconductor FETs: the case for SnS.

Gate tunable p-type multilayer tin mono-sulfide (SnS) field-effect transistor (FET) devices with SnS thickness between 50 and 100 nm were fabricated and studied to understand their performance. The devices showed anisotropic inplane conductance and room temperature field effect mobilities ∼5-10 cm2 V-1 s-1. However, the devices showed an ON-OFF ratio ∼10 at room temperature due to appreciable OFF state conductance. The weak gate tuning behavior and finite OFF state conductance in the depletion regime of SnS devices are explained by the finite carrier screening length effect which causes the existence of a conductive surface layer from defect induced holes in SnS. Through etching and n-type surface doping by Cs2CO3 to reduce/compensate the not-gatable holes near the SnS flake's top surface, the devices exhibited an order of magnitude improvement in the ON-OFF ratio, and a hole Hall mobility of ∼100 cm2 V-1 s-1 at room temperature is observed. This work suggests that in order to obtain effective switching and low OFF state power consumption, two-dimensional (2D) semiconductor based depletion mode FETs should limit their thickness to within the Debye screening length of the carriers in the semiconductor.

Diameter dependent transport properties of gallium nitride nanowire field effect transistors

The authors report transport property measurements of individual GaN nanowire field effect transistors and the correlation of the electron mobilities with the existence of grain boundaries in these nanowires. Room temperature field effect electron mobilities as high as 319cm2V−1s−1 were obtained for the 200nm diameter nanowires. Mobilities calculated from these reliable nanowire field effect transistors indicated that the surface scattering plays a dominant role in smaller diameter nanowires, whereas for intermediate diameter devices transport is dominated by grain boundary scattering. Reduction of the mobility with decreasing diameter of nanowires can be explained using “continuous surface” model.

High sensitivity polyacrylic acid films for ammonia detection with field effect devices

Thin polyacrylic acid (PAA) films, with defined geometry, have been produced by spray deposition from dilute polymer solutions on metal plated wafers. The layer morphology was characterized by standard methods: optical, atomic force and scanning electron microscopy; stylus and optical profiler. Their sensing properties toward ammonia as target gas and the related cross sensitivities towards the most relevant interferents have been determined by the Kelvin Probe (KP) technique. The influence of the deposition parameters, layer thickness, operating temperature and aging were also investigated. The study was made in order to investigate the potential of PAA as sensing material for room temperature field effect gas sensors.

Interface state density and channel mobility for 4H-SiC MOSFETs with nitrogen passivation

Interface state density and channel mobility have been characterized for 4H-SiC MOSFETs fabricated with dry thermal oxides and subsequently passivated with nitric oxide. The interface trap density at 0.1 eV below the conduction band edge decreases from approximately 8×10 12 to 1×10 12 eV −1 cm −2 following anneals in nitric oxide (NO) at 1175 °C for 2 h. The room temperature field effect channel mobility increases by an order of magnitude to approximately 35 cm 2/V s following the passivation anneal. The field effect channel mobility of passivated MOSFETs shows almost no change with increasing temperature, while the mobility for unpassivated devices increases with increasing temperature and is thermally activated (∼ T 1.9) due to decreased Coulomb scattering by electrons trapped at the acceptor-like interface states near the conduction band. Over the temperature range 300–473 K, threshold voltage changes of about −0.8 and −3.7 V, respectively, are observed for devices processed with and without NO passivation.

Excimer laser crystallization and doping of source and drain regions in high quality amorphous silicon thin film transistors

A method has been proposed to fabricate a bottom gate hydrogenated amorphous silicon thin film transistor with highly conductive polycrystalline silicon source and drain regions. Dopant atoms were supplied during selective crystallization of the amorphous silicon layer, via a spin-on-dopant film coated on top of it. A suitably patterned thin layer of silicon nitride served to restrict laser-induced crystallization and doping of the amorphous silicon to the required source and drain regions. The sheet resistance of the resulting doped polycrystalline areas of the film was found to be 1.8 kΩ/⧠. The fabricated transistors had a room temperature field effect mobility of up to 1.35 cm2 V−1 s−1 and featured on/off current ratios of more than 106.

Field effect on silicon

Field effect experiments were performed on silicon to evaluate the characteristics of etched and thermally oxidized surfaces. The majority of the experiments were performed at 100°C in order to eliminate the effects of the low density of minority carriers at room temperature. An observed lack of a unique minimum of conductance for thermally oxidized silicon surfaces is reported and discussed. The effect of a high transverse electric field on conductance of thermally oxidized surfaces is discussed. This high electric field effect is the opposite of what one would expect from surface state shielding. A model to account for these effects is presented. The equilibrium surface potentials and fast state energy distributions of etched surfaces are presented. Room temperature field effect on silicon is discussed.