ALD-grown two-dimensional TiSx metal contacts for MoS2 field-effect transistors

Metal contacts to MoS2 field-effect transistors (FETs) play a determinant role in the device electrical characteristics and need to be chosen carefully. Because of the Schottky barrier (SB) and the Fermi level pinning (FLP) effects that occur at the contact/MoS2 interface, MoS2 FETs often suffer from high contact resistance (Rc). One way to overcome this issue is to replace the conventional 3D bulk metal contacts with 2D counterparts. Herein, we investigate 2D metallic TiSx (x ∼ 1.8) as top contacts for MoS2 FETs. We employ atomic layer deposition (ALD) for the synthesis of both the MoS2 channels as well as the TiSx contacts and assess the electrical performance of the fabricated devices. Various thicknesses of TiSx are grown on MoS2, and the resultant devices are electrically compared to the ones with the conventional Ti metal contacts. Our findings show that the replacement of 5 nm Ti bulk contacts with only ∼1.2 nm of 2D TiSx is beneficial in improving the overall device metrics. With such ultrathin TiSx contacts, the ON-state current (ION) triples and increases to ∼35 μA μm−1. Rc also reduces by a factor of four and reaches ∼5 MΩ μm. Such performance enhancements were observed despite the SB formed at the TiSx/MoS2 interface is believed to be higher than the SB formed at the Ti/MoS2 interface. These device metric improvements could therefore be mainly associated with an increased level of electrostatic doping in MoS2, as a result of using 2D TiSx for contacting the 2D MoS2. Our findings are also well supported by TCAD device simulations.<br/

Dirac-Point Shift by Carrier Injection Barrier in Graphene Field-Effect Transistor Operation at Room Temperature.

A positive shift in the Dirac point in graphene field-effect transistors was observed with Hall-effect measurements coupled with Kelvin-probe measurements at room temperature. This shift can be explained by the asymmetrical behavior of the contact resistance by virtue of the electron injection barrier at the source contact. As an outcome, an intrinsic resistance is given to allow a retrieval of an intrinsic carrier mobility found to be decreased with increasing gate bias, suggesting the dominance of short-range scattering in a single-layer graphene field-effect transistor. These results analytically correlate the field-effect parameters with intrinsic graphene properties

Graphene-based nanolaminates as ultra-high permeation barriers

Permeation barrier films are critical to a wide range of applications. In particular, for organic electronics and photovoltaics not only ultra-low permeation values are required but also optical transparency. A laminate structure thereby allows synergistic effects between different materials. Here, we report on a combination of chemical vapor deposition (CVD) and atomic layer deposition (ALD) to create in scalable fashion few-layer graphene/aluminium oxide-based nanolaminates. The resulting ~10 nm contiguous, flexible graphene-based films are >90% optically transparent and show water vapor transmission rates below 7 × 10−3 g/m2/day measured over areas of 5 × 5 cm2. We deploy these films to provide effective encapsulation for organic light-emitting diodes (OLEDs) with measured half-life times of 880 h in ambient

Parameter Space of Atomic Layer Deposition of Ultrathin Oxides on Graphene.

Atomic layer deposition (ALD) of ultrathin aluminum oxide (AlOx) films was systematically studied on supported chemical vapor deposition (CVD) graphene. We show that by extending the precursor residence time, using either a multiple-pulse sequence or a soaking period, ultrathin continuous AlOx films can be achieved directly on graphene using standard H2O and trimethylaluminum (TMA) precursors even at a high deposition temperature of 200 °C, without the use of surfactants or other additional graphene surface modifications. To obtain conformal nucleation, a precursor residence time of >2s is needed, which is not prohibitively long but sufficient to account for the slow adsorption kinetics of the graphene surface. In contrast, a shorter residence time results in heterogeneous nucleation that is preferential to defect/selective sites on the graphene. These findings demonstrate that careful control of the ALD parameter space is imperative in governing the nucleation behavior of AlOx on CVD graphene. We consider our results to have model system character for rational two-dimensional (2D)/non-2D material process integration, relevant also to the interfacing and device integration of the many other emerging 2D materials.We acknowledge funding from the EPSRC (Grant EP/ K016636/1, GRAPHTED) and ERC (Grant 279342, InsituNANO). J.A.A.-W. acknowledges a Research Fellowship from Churchill College, Cambridge, U.K

Engineering the Photoresponse of InAs Nanowires.

We report on individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either negative or positive photoconductivity (NPC or PPC). The NPC photoresponse time and magnitude is found to be highly tunable by varying the nanowire diameter under controlled growth conditions. Using hysteresis characterization, we decouple the observed photoexcitation-induced hot electron trapping from conventional electric field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Furthermore, we demonstrate surface passivation without chemical etching which both enhances the field-effect mobility of the nanowires by approximately an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an "intrinsic" positive photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications

Engineering the Photoresponse of InAs Nanowires

We report on individual-InAs nanowire optoelectronic devices which can be tailored to exhibit either negative or positive photoconductivity (NPC or PPC). The NPC photoresponse time and magnitude is found to be highly tunable by varying the nanowire diameter under controlled growth conditions. Using hysteresis characterization, we decouple the observed photoexcitation-induced hot electron trapping from conventional electric field-induced trapping to gain a fundamental insight into the interface trap states responsible for NPC. Furthermore, we demonstrate surface passivation without chemical etching which both enhances the field-effect mobility of the nanowires by approximately an order of magnitude and effectively eliminates the hot carrier trapping found to be responsible for NPC, thus restoring an "intrinsic" positive photoresponse. This opens pathways toward engineering semiconductor nanowires for novel optical-memory and photodetector applications.We acknowledge funding from the EPSRC (Grant No. EP/ M009505/1) and the ERC (Grant No. 716471, ACrossWire). S.H. acknowledges funding from the EPSRC (Grant No. EP/ P005152/1). This work was also supported by the Australian Research Council, Australian National Fabrication Facility and Australian Microscopy & Microanalysis Research Facility. J.A.A.-W. acknowledges the support of his Research Fellowships from the Royal Commission for the Exhibition of 1851 and Churchill College, Cambridge. C.K.G acknowledges the support of her scholarship from The Winston Churchill Foundation of the United States

Metal-organic molecular device for non-volatile memory storage

Non-volatile memory devices have been of immense research interest for their use in active memory storage in powered off-state of electronic chips. In literature, various molecules and metal compounds have been investigated in this regard. Molecular memory devices are particularly attractive as they offer the ease of storing multiple memory states in a unique way and also represent ubiquitous choice for miniaturized devices. However, molecules are fragile and thus the device breakdown at nominal voltages during repeated cycles hinders their practical applicability. Here, in this report, a synergetic combination of an organic molecule and an inorganic metal, i.e., a metal-organic complex, namely, palladium hexadecylthiolate is investigated for memory device characteristics. Palladium hexadecylthiolate following partial thermolysis is converted to a molecular nanocomposite of Pd(II), Pd(0), and long chain hydrocarbons, which is shown to exhibit non-volatile memory characteristics with exceptional stability and retention. The devices are all solution-processed and the memory action stems from filament formation across the pre-formed cracks in the nanocomposite film

Ultrafast direct ablative patterning of HOPG by single laser pulses to produce graphene ribbons

Ultrafast, single step and direct patterning of highly oriented pyrolytic graphite (HOPG) is achieved through pulsed laser interference ablation using a near field transmitting phase mask. Periodic arrays of lines are patterned on the HOPG surface over large areas by spatially modulating the laser intensity through the mask. Thus patterned surface serve as a source for multi and few layer graphene ribbons for transferring onto desired substrates using polydimethylsiloxane as transferring agent. The transferred regions are contained with few layer graphene (5–6 layers) ribbons as well as thick graphitic ribbons (30–40 nm), with widths &#8764;1 &#956;m and lengths of several micrometers. Raman, TEM and electrical measurements have confirmed that the transferred ribbons are highly crystalline in nature. Using combinations of shadow and transmitting phase masks, other patterns such as checker boards and diamond-shaped pits are produced

Intricate nature of Pd nanocrystal–hydrogen interaction investigated using thermolysed Pd hexadecylthiolate films

Nanocrystalline Pd films are derived by thermolysing Pd hexadecylthiolate at different temperatures (195–250 &#176;C) to obtain films with different resistances, 3–115 &#8486;. The films essentially consist of Pd nanocrystals (5–40 nm) amidst amorphous carbon as evidenced by electron microscopy and Raman measurements. The response to H2 is seen as a jump in the resistance which varied proportionally with the base resistance, the slope being &#8764;0.2. Interestingly, the change over from H2 to purging N2 (or Ar) atmosphere is accompanied by a kink-like feature in the electrical response, which is linked to depletion of hydrogen from the nanocrystal surface and its backfilling from the core. The presence of O2 in the purging atmosphere affects adversely the formation of the kink