Characterization of Single Defects in Ultrascaled MoS₂ Field-Effect Transistors

Abstract

MoS has received a lot of attention lately as a semiconducting channel material for electronic devices, in part due to its large band gap as compared to that of other 2D materials. Yet, the performance and reliability of these devices are still severely limited by defects which act as traps for charge carriers, causing severely reduced mobilities, hysteresis, and long-term drift. Despite their importance, these defects are only poorly understood. One fundamental problem in defect characterization is that due to the large defect concentration only the average response to bias changes can be measured. On the basis of such averaged data, a detailed analysis of their properties and identification of particular defect types are difficult. To overcome this limitation, we here characterize single defects on MoS devices by performing measurements on ultrascaled transistors (∼65 × 50 nm) which contain only a few defects. These single defects are characterized electrically at varying gate biases and temperatures. The measured currents contain random telegraph noise, which is due to the transfer of charge between the channel of the transistors and individual defects, visible only due to the large impact of a single elementary charge on the local electrostatics in these small devices. Using hidden Markov models for statistical analysis, we extract the charge capture and emission times of a number of defects. By comparing the bias-dependence of the measured capture and emission times to the prediction of theoretical models, we provide simple rules to distinguish oxide traps from adsorbates on these back-gated devices. In addition, we give simple expressions to estimate the vertical and energetic positions of the defects. Using the methods presented in this work, it is possible to locate the sources of performance and reliability limitations in 2D devices and to probe defect distributions in oxide materials with 2D channel materials.

Insulators for 2D nanoelectronics: the gap to bridge

“…As a result, in nanoscale 2D FETs capture and emission of a single carrier can strongly perturb the electrostatics inside the channel and thus cause RTN fluctuations in I D and V th . A few recent studies for MoS 2 FETs 94,96 have already established that in general the dynamics of RTN in 2D FETs are very similar to Si technologies 103,104 . For instance, it was demonstrated that charge trapping events causing RTN are the same as those responsible for the hysteresis and BTI 94 , with the unique characteristic of border traps being the exponential V G dependence of the time constants (Fig.…”

“…The most widely observed issues in 2D devices are the hysteresis of the gate transfer characteristics 11,12,45,54,58 and long-term drifts of the threshold voltage [90][91][92] , which are commonly known from Si technologies as BTI, given their strong bias and temperature dependence 85 . Recent analysis of experimental results for MoS 2 58,70,71,93,94 and black phosphorus 23,50 using non-radiative multiphonon (NMP) models 95 (see more details in Box 2) suggests that hysteresis and BTI have the same microscopic origin and result from changes in the charge state of border traps (Fig. 5a).…”

“…In particular, the strong gate bias dependence of the time constants 71 , which is typical for border traps, results in a sizeable hysteresis. Other issues caused by border traps include flicker (1/f) noise 17,52 , which appears as random fluctuations of I D (or, equivalently, V th ) and for nanoscale devices decomposes into discrete steps known as random telegraph noise (RTN) 94,96 , as well as hot-carrier degradation (HCD) during device operation at non-zero drain bias, which is also known from Si technologies 97 and has been already observed for 2D devices 98 .…”

First principle investigation of the influence of sulfur vacancies on thermoelectric properties of single layered MoS2

“…28 This obviously raises the question of the implication of these defects on the physical properties of MoS 2 that is not yet well understood from both the experimental and theoretical point of view. For instance, it has a high impact on the performance of field effect transistors 29 and several attempts have been made to tune the physical properties through defect engineering. [30][31][32] Regarding the thermal properties, non-equilibrium molecular dynamics calculations have shown a remarkable suppression of the thermal conductivity by point defects in MoS 2 nanoribbons.…”

Semi-Automated Extraction of the Distribution of Single Defects for nMOS Transistors

“…Using the proposed methodology the trap bands can be extracted from the measurement data. While the focus of this study is on Si/SiO 2 devices, the methodology is possible on other technologies, given that small devices are available which are also stable enough to record RTN in a reproducible fashion [13][14][15].…”