Thermally Induced Defects on WSe₂

Abstract

The 2D nature of transition metal dichalcogenides (TMDs) makes their electronic and optical performance highly susceptible to the presence of defects. At elevated temperatures, which can be reached during growth or in operation, additional defects can be introduced and lead to further material degradation. Therefore, by studying the impact of temperature on 2D-TMDs, the formation of defects and their respective degradation pathways can be established. The electronic and geometric structure and density of thermally induced defects on 2D tungsten diselenide (WSe 2 ) layers were examined using scanning tunneling microscopy/spectroscopy (STM/STS). WSe 2 layers were grown on highly ordered pyrolytic graphite (HOPG), via molecular beam epitaxy (MBE) and annealed at 600 °C, which caused a 7-fold increase in overall defect density. A layer-dependent trend emerged whereby the defect density on the first layer was greater than the second, suggesting that the TMD−graphite and TMD−TMD van der Waals interactions influence the formation energy of thermally growth defects. The defect inventory included single-point vacancies and a collection of larger defects with complex geometric and electronic signatures. These defects were classified by matching their unique electronic structures with their respective topographical presentation via spatially resolved STS maps. Defect states at the conduction and valence band edges introduced n-or p-type character and generally lowered the local band gap around each defect site. A unique defect structure displayed an increased band gap, likely as a consequence of local delamination of the TMD due to subsurface Se− cluster formation. Density functional theory (DFT) was used to examine select defects and supported the interpretation of the STM/STS work with density of states (DOS) and local-integrated DOS calculations. The assessment of the geometric and electronic signatures and details of the local doping profile around all defect sites deepened our understanding of the thermal stability of WSe 2 .

Impact of Al2O3 Passivation on the Photovoltaic Performance of Vertical WSe2 Schottky Junction Solar Cells

“…Interestingly, the simulated curve for 2 × 10 11 cm –2 surface doping shows a good match with our experimental results (black symbols), suggesting that unpassivated devices may have some fixed positive charge at the surface that causes electrons in WSe 2 to accumulate near the surface to maintain charge neutrality at the interface. These positive fixed charges are likely due to selenium vacancies , induced on the surface during our 350 °C annealing step during fabrication. The simulated curves also show that increases in positive surface charge lead to large improvements in photocurrent collection away from the top electrode.…”

Recent Progress on the Scanning Tunneling Microscopy and Spectroscopy Study of Semiconductor Heterojunctions

“…Blades et al studied the electronic band structure and defect density of 2D WSe 2 before and after annealing by using STM/STS. [ 98 ] They found that the bandgap distribution was significantly broadened because of a seven‐fold increase in defect density and the introduction of n ‐type and p ‐type defect after 600 °C annealing, as shown in Figure ,b. Four different types of defects were identified (Figure 6c,e).…”

van der Waals epitaxy of transition metal dichalcogenides via molecular beam epitaxy: looking back and moving forward

“…From the scanning tunnelling spectroscopic (STS) measurements, it is observed that an asymmetry exists between the valence band maximum and conduction band maximum, with respect to the Fermi level, revealing slightly n-type doping of the grown WSe 2 films. In another report, Blades et al 86 have comprehensively studied the electronic and geometric structure of defects in WSe 2 thin films grown on highly oriented pyrolytic graphite (HOPG) with STS and STM, and the effect of temperature on the nature of these defects. In 2018, Wang et al 87 reported the transport properties of highly crystalline epitaxial TiSe 2 thin films on a sapphire substrate grown by MBE.…”