Material-Selective Doping of 2D TMDC through Al_xO_y Encapsulation

Abstract

For the integration of two-dimensional (2D) transition metal dichalcogenides (TMDC) with high-performance electronic systems, one of the greatest challenges is the realization of doping and comprehension of its mechanisms. Low-temperature atomic layer deposition of aluminum oxide is found to n-dope MoS2 and ReS2 but not WS2. Based on electrical, optical, and chemical analyses, we propose and validate a hypothesis to explain the doping mechanism. Doping is ascribed to donor states in the band gap of Al x O y , which donate electrons or not, based on the alignment of the electronic bands of the 2D TMDC. Through systematic experimental characterization, incorporation of impurities (e.g., carbon) is identified as the likely cause of such states. By modulating the carbon concentration in the capping oxide, doping can be controlled. Through systematic and comprehensive experimental analysis, this study correlates, for the first time, 2D TMDC doping to the carbon incorporation on dielectric encapsulation layers. We highlight the possibility to engineer dopant layers to control the material selectivity and doping concentration in 2D TMDC.

Benchmarking monolayer MoS2 and WS2 field-effect transistors

“…Median V t lin of 2.9 V with a standard deviation of σ V t = 0.8 V is obtained for MoS 2 , and median V t lin of 6.4 V with a σ V t = 0.8 V is obtained for WS 2 . Threshold voltage was found to be more positive for WS 2 FETs compared to MoS 2 FETs, which can be attributed to higher intrinsic n-type doping of MoS 2 either due to the specific nature of the impurity present in the MOCVD grown MoS 2 film or due to surface charge transfer induced 31 .…”

Dual‐Gated MoS2 Memtransistor Crossbar Array

“…However, a drain bias of 30 V could potentially produce a sufficiently high electric field to inject electrons into the mid‐gap states of the Al 2 O 3 top dielectric that are just above the MoS 2 conduction band edge. [ 22 ] To further elucidate the switching mechanism, we note that the dual‐gated devices show distinct charge transport regimes ( Figure ) in contrast to the gradual changes in I D observed during switching of single‐gated MoS 2 memtransistors. [ 8b ] The distinct segments of the I – V curves exhibit well‐defined power‐law behavior (i.e., I ∝ V m ) with clear transition points (i.e., kinks) reminiscent of complex oxide memristors.…”

“…Clockwise switching results from resistive switching occurring at forward‐biased Schottky contacts in the dual‐gated memtransistor. Growth of high‐κ metal‐oxide dielectrics is known to increase electron doping of MoS 2 (and other transition metal dichalcogenides) [ 22,25 ] resulting in doping‐induced lowering of the Schottky barrier height. [ 26 ] Thus, devices start in LRS and gradually switch to HRS through reversible changes near the drain contact for V D > 0 V and source contact for V D < 0 V. Indeed, SCLC and TFL currents during resistive switching in symmetric Pt/TiO 2 /Pt memristors are correlated with filamentary switching near the anode electrode (i.e., the drain electrode in memtransistor).…”

“…Indeed, ALD‐grown amorphous Al x O y has been shown to have mid‐gap states slightly above the MoS 2 conduction band and is accessible for trapping at relatively low biases. [ 22 ] Thermally assisted charge trapping in the oxide has also been shown to induce nonvolatile memory effects. [ 27 ] On the other hand, recent computational work on MoS 2 memtransistors also predicts that the diffusion barrier of sulfur vacancies in MoS 2 can be as low as 0.68 eV, resulting in a significant hopping rate (>10 2 s −1 ) that increases rapidly with increasing temperature.…”

Trap Density Assessment on Multilayer WS2 using Power-Dependent Indirect Photoluminescence

“…To further investigate the impact of midgap traps on the indirect-to-direct peak ratio, aluminum oxide (Al x O y ) is deposited by atomic layer deposition (ALD) directly on the WS 2 flakes. It has been demonstrated that ALD Al x O y introduces trap states inside the bandgap of WS 2 [30], making this encapsulating layer an interesting case study for in-gap defectivity assessments. Photoluminescence spectra are acquired after ALD, presented in Fig 4, comparing the same flake before and after oxide deposition.…”