The environmental stability of the layered semiconductor black phosphorus (bP) remains a challenge. Passivation of the bP surface with phosphorus oxide, PO x , grown by a reactive ion etch with oxygen plasma is known to improve photoluminescence efficiency of exfoliated bP flakes. We apply phosphorus oxide passivation in the fabrication of bP field effect transistors using a gate stack consisting of a PO x layer grown by reactive ion etching followed by atomic layer deposition of Al 2 O 3 . We observe room temperature top-gate mobilities of 115 cm 2 V −1 s −1 in ambient conditions, which we attribute to the low defect density of the bP/PO x interface.Black phosphorus (bP) is a direct band gap (E g = 0.3 eV) semiconductor with a puckered honeycomb layer structure characterized by van der Waals interlayer bonding 1,2 . The most thermodynamically stable allotrope of phosphorus, bP exhibits ambipolar conduction, anisotropic conductivity, and can be exfoliated down to the atomic monolayer limit 3-6 . Exfoliation of bP in a nitrogen environment followed by encapsulation with hexagonal boron nitride in a vacuum environment has led to the observation of ∼ 45, 000 cm 2 V −1 s −1 hole mobility at cryogenic temperatures 7 . Importantly, bP is subject to degradation by photooxidation with a reaction rate that increases as bP thickness decreases 9 . A number of passivation techniques have been developed with varying degrees of success, including encapsulation with Al 2 O 3 10-13 , hexagonal boron nitride (h-BN) 7,14,15,17 , polymer layers 6,8 and functionalization with nickel nanoparticles 16 . More recently, it has been demonstrated that the formation of a dense phosphorus oxide, PO x , layer by oxygen plasma dry etching followed by Al 2 O 3 deposition results in stable encapsulation of bP without compromising photoluminescence (PL) efficiency 18 . The preservation of PL efficiency indicates that the interface between bP and PO x does not measurably increase non-radiative recombination rates and is thus an effective surface passivation strategy.In this work, we apply the PO x passivation approach of Pei et al. 18 to fabricate top-gated bP field effect transistors (FETs). The use of a native oxide for passivation and gate stack formation in bP FETs is appealing as a direct analogue to the use of silicon oxide in silicon FET technology. Various phases of PO x are known, including a rhombohedral crystal of molecular P 4 O 10 19 , and the most thermodynamically stable form of P 2 O 5 which is itself a layered material composed of a hexagonal network of edge connected (PO) 3− 4 tetrahedra 20 .The PO x layer passivates the bP surface, and acts as a seeding layer for subsequent atomic layer deposition of high-quality gate dielectrics such as Al 2 O 3 . We have fabricated dual-gate bP FETs, a bottom gate formed by a heavily doped, oxidized silicon substrate, and a top gate structure with a PO x /Al 2 O 3 dielectric stack. Room temperature top gate field effect mobilities of up to 115 cm 2 V −1 s −1 are achieved.In our experiments, w...

The layered semiconductor black phosphorus has attracted attention as a 2D atomic crystal that can be prepared in ultra-thin layers for operation as field effect transistors [1][2][3]. Despite the susceptibility of black phosphorus to photo-oxidation [4], improvements to the electronic quality of black phosphorus devices has culminated in the observation of the quantum Hall effect [5]. In this work, we demonstrate the room temperature operation of a dual gated black phosphorus transistor operating as a velocity modulated transistor [6], whereby modification of hole density distribution within a black phosphorus quantum well leads to a two-fold modulation of hole mobility. Simultaneous modulation of Schottky barrier resistance leads to a four-fold modulation of transconductance at a fixed hole density. Our work explicitly demonstrates the critical role of charge density distribution upon charge carrier transport within 2D atomic crystals.Black phosphorus (bP) is an elemental allotrope and a direct bandgap semiconductor with a puckered, honeycomb layer structure [7,8] that can be exfoliated down to atomic few-layer thickness [1-4, 9, 10]. Although bP is the most thermodynamically stable allotrope of phosphorus, photo-oxidation in the presence of water, oxygen and visible light is known to degrade bP with a reaction rate that increases as bP layer thickness decreases [4]. Several materials have been used to encapsulate bP in order to protect it against photo-oxidation, including hexagonal boron-nitride [11][12][13], aluminum oxide [14], parylene [4], and poly-methylmethacrylate [15]. Recent works have also shown that 2D hole transport can be achieved in a single 2D sub-band within an accumulation layer of many-layer bP [11,13,15], effectively combining 2D transport characteristics with the increased chemical stability of many-layer bP. These advances have culminated in the observation of the quantum Hall effect in bP [5]. Nonetheless, further understanding and control of transconductance, carrier mobility and contact resistance in bP field effect transistors (FETs) is desired.We report here an experimental investigation of the transport characteristics of bP FETs with an asymmetric dual gate geometry consisting of top and bottom gate electrodes. The top gate is found to be effective in modulating the back gate FET transfer characteristics, including both field effect mobility and Schottky barrier contact resistance. The mobility modulation effect enables operation of the dual gate bP FET as a velocity modulated transistor (VMT), first proposed by Sakaki [6] [18]. Asymmetric dual-gate bP FETs exhibit a two-fold mobility modulation at room temperature, and the underlying mechanism is modulation of hole density distribution with the naked bP quantum well channel of the bP FET, and a resultant modulation of scattering by charged impurities within the gate oxide, surface roughness, and other spatially dependent scattering mechanisms. Simultaneously, bP FETs exhibit strong Schottky barrier modulation. First conclusive...

Free-standing films of reduced graphene oxide were prepared by evaporative drying of drop-cast graphene oxide followed by thermal reduction. The electrical resistance of reduced graphene oxide films showed a strong temperature dependence, reaching a temperature coefficient of resistance of 44×103 Ω/K at 60 K. The bolometric response under black body illumination was measured from 50 K to 300 K, reaching a voltage responsivity of up to 82 × 103 V/W at 50 K.

Resistivity measurements of a black phosphorus (bP) field‐effect transistor 16 nm thick in parallel magnetic fields up to 45 T are reported as a function of the angle between the in‐plane field and the source–drain (S–D) axis of the device. The crystallographic directions of the bP crystal are determined by Raman spectroscopy, with the zigzag axis found to be within 5° of the S–D axis and the armchair axis in the orthogonal planar direction. A transverse magnetoresistance (TMR) as well as a classically forbidden longitudinal magnetoresistance (LMR) are observed. Both are found to be strongly anisotropic and nonmonotonic with increasing in‐plane field. Surprisingly, the relative magnitude (in %) of the positive LMR is larger than the TMR above ≈32 T. Considering the known anisotropy of bP whose zigzag and armchair effective masses differ by a factor of ≈7, the experiment strongly suggests this LMR to be a consequence of the anisotropic Fermi surface of bP.