Highly-stable black phosphorus field-effect transistors with low density of oxide traps

Abstract

Black phosphorus is considered a very promising semiconductor for two-dimensional field-effect transistors. Initially, the main disadvantage of this material was thought to be its poor air stability. However, recent studies have shown that this problem can be solved by suitable encapsulation. As such, long-term studies of the outstanding properties of black phosphorus devices have become possible. In particular, here we examine highly-stable black phosphorus field-effect transistors and demonstrate that they can exhibit reproducible characteristics for at least 17 months. Furthermore, we notice some improvement in the performance of black phosphorus devices after this long time, i.e., positive aging. Although our black phosphorus devices are stable at room temperature, we show that their performance is affected by thermally activated charge trapping by oxide traps into the adjacent SiO 2 substrate layer. Aiming to analyze the dynamics of these defects in detail, we perform an accurate mapping of oxide traps with different time constants using the 'extended incremental hysteresis sweep method'. Our results show that at room temperature the extracted oxide trap densities are (i) few orders of magnitude lower than for MoS 2 /SiO 2 transistors and (ii) close to those reported for more mature Si/SiO 2 devices (~10 17 cm). Taking into account the novelty of black phosphorus and recent issues with its stability, these values must be considered unexpectedly low.

Recent Progress on Stability and Passivation of Black Phosphorus

“…Some devices have also shown what is known as “positive aging,” where there is slight improvement in electrical performance over time. “Positive aging” might arise from slow annealing of traps and defects or desorption of moisture that may have been trapped shortly before or during the process of encapsulation …”

Engineering Field Effect Transistors with 2D Semiconducting Channels: Status and Prospects

“…In order to solve this problem, several attempts of encapsulation of phosphorene FETs with an AlO x layer, polymers and 2D h‐BN have been reported. An efficient solution has been found only using conformal encapsulation schemes (see Figure ) . Recently, at least 17 months of phosphorene FETs stability has been achieved using conformal Al 2 O 3 and Al 2 O 3 /Teflon‐AF encapsulation, with the latter scheme also leading to an improved device performance and reliability .…”

“…An efficient solution has been found only using conformal encapsulation schemes (see Figure ) . Recently, at least 17 months of phosphorene FETs stability has been achieved using conformal Al 2 O 3 and Al 2 O 3 /Teflon‐AF encapsulation, with the latter scheme also leading to an improved device performance and reliability . However, it is shown that the performance of phosphorene could be significantly degraded by Al 2 O 3 capping, resulting in a 5 times lower ON‐state current and 3 times lower ON/OFF ratios, likely due to the moisture introduced during the ALD encapsulation process .…”

“…b) Encapsulation with 25 nm Al 2 O 3 leads to at least 17 months stability of BP FETs even if the devices are intensively stressed and alternatively stored in the ambient c). Reproduced under the terms and conditions of the Creative Commons CC BY 4.0 International License . Copyright 2017, The Authors, published by Springer Nature.…”

Enhancing the ambient stability of few-layer black phosphorus by surface modification

“…For example, although Al 2 O 3 encapsulated BP-based FETs can be stable at room temperature for 17 months. 25 However, such protection is too challenging to consider potential damage and contamination during the deposition. [25][26][27][28][29][30][31] Wang et al suggested that doping with tellurium (Te) can enhance the ambient stability and transport performance of few-layer BP devices.…”