Phosphorus oxide gate dielectric for black phosphorus field effect transistors (original) (raw)

Stabilizing ultra-thin black phosphorus with in-situ-grown 1 nm-Al2O3 barrier

Applied Physics Letters, 2017

Exfoliated black phosphorus is a 2D semiconductor with promising properties for electronics, spintronics, and optoelectronics. Nevertheless, its rapid degradation in air renders its integration and use in devices particularly challenging-even more so for smaller thicknesses for which the degradation rate is tremendously enhanced. In order to effectively protect the thinnest flakes, we present here an approach based on an in-situ dielectric capping to avoid all contact with air. Optical microscopy, Raman spectroscopy, and atomic force microscopy studies confirm that 1 nm of Al 2 O 3 efficiently passivates exfoliated black phosphorus (below 5 layers) on Si/SiO 2 substrates. Such an ultrathin and transparent passivation layer can act as a tunnel barrier allowing for black phosphorus devices processing without passivation layer removal. V

Effects of plasma-treatment on the electrical and optoelectronic properties of layered black phosphorus

Applied Materials Today, 2018

Exfoliated few-layer black-phosphorus (BP) has been explored for a variety of electrical and optoelectronic applications. Plasma-assisted thinning of BP has emerged as an exciting pathway to achieve BP crystals of desired thickness. However, to fully realise the true potential of plasma-assisted thinning of BP and other emerging 2D materials, it is critical to understand the effects of different plasma environments on the electrical and optoelectronic properties of the resultant material. Here, we investigate the influence of Ar and O 2 plasma on the electrical and optoelectronic properties of plasma-treated BP flakes. It is revealed that by manipulating the environment under which BP is exposed to the plasma, it is possible to engineer defects that lead to new photoluminescence (PL) emission peaks without compromising the switching ratios or carrier mobilities of BP-based field effect transistors (FETs). Overall, our study finds the use of O 2 plasma as a more suitable approach to retain and enrich the intrinsic (opto)electronic properties of BP. Additionally, our study, for the first time, experimentally reveals the ability of BP to respond to UV excitation.

Creating a Stable Oxide at the Surface of Black Phosphorus

ACS applied materials & interfaces, 2015

The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atmosphere is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atmosphere exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by <0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielectric deposition.

Ultrathin and Flat Layer Black Phosphorus Fabricated by Reactive Oxygen and Water Rinse

Ultrathin black phosphorus (BP) is one of the promising two-dimensional (2D) materials for future optoelectronic devices. Its chemical instability in ambient conditions and lack of a bottom-up approach for its synthesis necessitate efficient etching methods that generate BP films of designed thickness with stable and high-quality surfaces. Herein, reporting a photochemical etching method, we demonstrate a controlled layer-by-layer thinning of thick BP films down to a few layers or a single layer and confirm their Raman and photoluminescence characteristics. Ozone molecules generated by O 2 photolysis oxidize BP, forming P 2 O 5-like oxides. When the resulting phosphorus oxides are removed by water, the surface of BP with preset thickness is highly flat and self-protective by surface oxygen functional groups. This method provides a fabrication strategy of BP and possibly other 2D semiconductors with band gaps tuned by their thickness.

Reliable Passivation of Black Phosphorus by Thin Hybrid Coating

Nanotechnology, 2017

Black phosphorus (BP) possesses several extraordinary physical properties, which include in-plane anisotropy, thickness dependent direct bandgap and high carrier mobility. These physical properties make BP highly desirable from the point of view of fundamental science and modern optoelectronics applications. The excitement about this material has always been accompanied by unreserved skepticism due to its extraordinary degradation under ambient conditions. Here we show ambient degradation of exfoliated BP can be effectively suppressed using thin layer of hybrid MOCVD coating of BN followed by ALD coating of Al2O3. We have extensively studied the time dependent surface, optical and electrical properties of BP encapsulated by BN and/or Al2O3 using nanoscale infrared imaging and I-V characterizations. Our results show hybrid thin layer (~5 nm) BN/Al2O3 coated BP exfoliated on SiO2 substrate is protected from degradation in ambient for over 6 months, much longer than those coated only b...

Isolation and characterization of few-layer black phosphorus

2014

Isolation and characterization of mechanically exfoliated black phosphorus flakes with a thickness down to two single-layers is presented. A modification of the mechanical exfoliation method, which provides higher yield of atomically thin flakes than conventional mechanical exfoliation, has been developed. We present general guidelines to determine the number of layers using optical microscopy, Raman spectroscopy and transmission electron microscopy (TEM) in a fast and reliable way. Moreover, we demonstrate that the exfoliated flakes are highly crystalline and that they are stable even in free-standing form through Raman spectroscopy and TEM measurements. A strong thickness dependence of the band structure is found by density functional theory (DFT) calculations. The exciton binding energy, within an effective mass approximation, is also calcu- lated for different number of layers. Our computational results for the optical gap are consistent with preliminary photoluminescence results on thin flakes. Finally, we study the environmental stability of black phosphorus flakes finding that the flakes are very hydrophilic and that long term exposure to air moisture etches black phosphorus away. Nonetheless, we demonstrate that the aging of the flakes is slow enough to allow fabrication of field-effect transistors with strong ambipolar behavior. DFT calculations also give us insight into the water-induced changes of the structural and electronic properties of black phosphorus.

Accessing the transport properties of pristine few-layer black phosphorus by van der Waals passivation in inert atmosphere

Ultrathin black phosphorus, or phosphorene, is the second known elementary two-dimensional material that can be exfoliated from a bulk van der Waals crystal. Unlike graphene it is a semiconductor with a sizeable band gap and its excellent electronic properties make it attractive for applications in transistor, logic, and optoelectronic devices. However, it is also the first widely investigated two dimensional electronic material to undergo degradation upon exposure to ambient air. Therefore a passivation method is required to study the intrinsic material properties, understand how oxidation affects the physical transport properties and to enable future application of phosphorene. Here we demonstrate that atomically thin graphene and hexagonal boron nitride crystals can be used for passivation of ultrathin black phosphorus. We report that few-layer pristine black phosphorus channels passivated in an inert gas environment, without any prior exposure to air, exhibit greatly improved n-type charge transport resulting in symmetric electron and hole trans-conductance characteristics. We attribute these results to the formation of oxygen acceptor states in air-exposed samples which drastically perturb the band structure in comparison to the pristine passivated black phosphorus.

Long-Term Stability and Reliability of Black Phosphorus Field-Effect Transistors

ACS Nano, 2016

Black phosphorus has been recently suggested as a very promising material for use in 2D field-effect transistors. However, due to its poor stability under ambient conditions, this material has not yet received as much attention as for instance MoS 2. We show that the recently demonstrated Al 2 O 3 encapsulation leads to highly stable devices. In particular, we report our long-term study on highly stable black phosphorus field-effect transistors, which show stable device characteristics for at least eight months. This high stability allows us to perform a detailed analysis of their reliability with respect to hysteresis as well as the arguably most important reliability issue in silicon technologies, the bias-temperature instability. We find that the hysteresis in these transistors depends strongly on the sweep rate and temperature. Moreover, the hysteresis dynamics in our devices are reproducible over a long time, which underlines their high reliability. Also, by using detailed physical models for oxide traps developed for Si technologies, we are able to capture the channel electrostatics of the black phosphorus FETs and determine the position of the defect energy band. Finally, we demonstrate that both hysteresis and bias-temperature instabilities are due to thermally activated charge trapping/ detrapping by oxide traps and can be reduced if the device is covered by Teflon-AF.

Transport properties of pristine few-layer black phosphorus by van der Waals passivation in an inert atmosphere

Nature communications, 2015

Ultrathin black phosphorus is a two-dimensional semiconductor with a sizeable band gap. Its excellent electronic properties make it attractive for applications in transistor, logic and optoelectronic devices. However, it is also the first widely investigated two-dimensional material to undergo degradation upon exposure to ambient air. Therefore a passivation method is required to study the intrinsic material properties, understand how oxidation affects the physical properties and enable applications of phosphorene. Here we demonstrate that atomically thin graphene and hexagonal boron nitride can be used for passivation of ultrathin black phosphorus. We report that few-layer pristine black phosphorus channels passivated in an inert gas environment, without any prior exposure to air, exhibit greatly improved n-type charge transport resulting in symmetric electron and hole transconductance characteristics.