Adam Charnas

Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe5

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However, the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows a significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculations. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with a reduced thickness. A low-temperature Hall experiment sheds light on more intrinsic anisotropic electrical transport, with a hole mobility of 3000 and 1500 cm2/V·s along the a-axis and c-axis, respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with the highest electron mobility up to 44 000 cm2/V·s.

Controlled Growth of a Large-Size 2D Selenium Nanosheet and Its Electronic and Optoelectronic Applications

Selenium has attracted intensive attention as a promising material candidate for future optoelectronic applications. However, selenium has a strong tendency to grow into nanowire forms due to its anisotropic atomic structure, which has largely hindered the exploration of its potential applications. In this work, using a physical vapor deposition method, we have demonstrated the synthesis of large-size, high-quality 2D selenium nanosheets, the minimum thickness of which could be as thin as 5 nm. The Se nanosheet exhibits a strong in-plane anisotropic property, which is determined by angle-resolved Raman spectroscopy. Back-gating field-effect transistors based on a Se nanosheet exhibit p-type transport behaviors with on-state current density around 20 mA/mm at Vds = 3 V. Four-terminal field-effect devices are also fabricated to evaluate the intrinsic hole mobility of the selenium nanosheet, and the value is determined to be 0.26 cm2 V-1 s-1 at 300 K. The selenium nanosheet phototransistors show an excellent photoresponsivity of up to 263 A/W, with a rise time of 0.1 s and fall time of 0.12 s. These results suggest that crystal selenium as a 2D form of a 1D van der Waals solid opens up the possibility to explore device applications.

Few-layer black phosporous PMOSFETs with BN/AI 2 O 3 bilayer gate dielectric: Achieving I on =850μA/μm, g m =340μS/μm, and R c =0.58kΩ·μm

In this paper, high-performance few-layer black phosphorus (BP) PMOSFETs have been demonstrated by using MOCVD BN and ALD Al<inf>2</inf>O<inf>3</inf> as the top-gate dielectric as well as the passivation layer. Highest Ion of 850μA/μm (V<inf>ds</inf> = −1.8 V) and g<inf>m</inf> of 340μS/μm (Vds = −0.8 V) have been achieved with the 200nm chancel length (L<inf>ch</inf>) devices. Record low contact resistance (R<inf>c</inf>) of 0.58kΩ·μm has been obtained on BP transistors by contact engineering. The gate leakage of the BN/Al<inf>2</inf>O<inf>3</inf> bilayer gate dielectric is less than 10⁻¹²A/μm² (V<inf>g</inf> = −1V) with an EOT of 3nm. SS and hysteresis voltage as low as 70mV/dec and 0.1V have been achieved, indicating a high quality interface between BP and BN.

Ultrafast Laser‐Shock‐Induced Confined Metaphase Transformation for Direct Writing of Black Phosphorus Thin Films

Few-layer black phosphorus (BP) has emerged as one of the most promising candidates for post-silicon electronic materials due to its outstanding electrical and optical properties. However, lack of large-scale BP thin films is still a major roadblock to further applications. The most widely used methods for obtaining BP thin films are mechanical exfoliation and liquid exfoliation. Herein, a method of directly synthesizing continuous BP thin films with the capability of patterning arbitrary shapes by employing ultrafast laser writing with confinement is reported. The physical mechanism of confined laser metaphase transformation is understood by molecular dynamics simulation. Ultrafast laser ablation of BP layer under confinement can induce transient nonequilibrium high-temperature and high-pressure conditions for a few picoseconds. Under optimized laser intensity, this process induces a metaphase transformation to form a crystalline BP thin film on the substrate. Raman spectroscopy, atomic force microscopy, and transmission electron microscopy techniques are utilized to characterize the morphology of the resulting BP thin films. Field-effect transistors are fabricated on the BP films to study their electrical properties. This unique approach offers a general methodology to mass produce large-scale patterned BP films with a one-step manufacturing process that has the potential to be applied to other 2D materials.

How Important Is the Metal–Semiconductor Contact for Schottky Barrier Transistors: A Case Study on Few-Layer Black Phosphorus?

Black phosphorus (BP) is a recently rediscovered layered two-dimensional (2D) semiconductor with a direct band gap (0.35–2 eV), high hole mobility (300–5000 cm2/Vs), and transport anisotropy. In this paper, we systematically investigated the effects of metal–semiconductor interface/contacts on the performance of BP Schottky barrier transistors. First, a “clean” metal–BP contact is formed with boron nitride (BN) passivation. It is found that the contact resistance of the clean metal–BP contact is seven times less than the previously reported values. As a result, high-performance top-gate BP transistors show a record high ON-state drain current (Ion) of 940 μA/μm. Second, BN tunneling barriers are formed at the source/drain contacts to help understand the abnormally high OFF-state drain current (Ioff) in devices with clean metal–BP contacts. This high Ioff is attributed to the electron tunneling current from the drain to the channel. Finally, the Ion/Ioff of BP field-effect transistors can be significantly improved by using an asymmetric contact structure. By inserting a thin BN tunneling barrier at the drain side, Ioff is reduced by a factor of ∼120 with a cost of 20% reduction in Ion. This case study of contacts on BP reveals the importance of understanding the metal–semiconductor contacts for 2D Schottky barrier transistors in general.