Band alignment at interfaces of two-dimensional materials: internal photoemission analysis

Abstract

The article overviews experimental results obtained by applying Internal PhotoEmission (IPE) spectroscopy methods to characterize electron states in single-or few-monolayer twodimensional (2D) materals and at their interfaces. Several conducting (graphene) and semiconducting (transitional metal dichalcogenides MoS2, WS2, MoSe2, and WSe2) films have been analyzed by IPE, which reveals significant sensitivity of interface band offsets and barriers to the details of the material and interface fabrication indicating violation of the Schottky-Mott rule. This variability is associated with charges and dipoles formed at the interfaces with van der Waals bonding as opposed to the chemically bonded interfaces of three-dimensional semiconductors and metals. Chemical modification of the underlying SiO2 surface is shown to be a significant factor, affecting interface barriers due to violation of the interface electroneutrality.

Improving stability in two-dimensional transistors with amorphous gate oxides by Fermi-level tuning

“… 40 ). This evidence suggests that there might be a loss of charge neutrality at the ill-defined interfaces between van der Waals–bonded 2D layers and amorphous oxides, causing deviations from the Schottky–Mott rule 25 . These deviations result in offsets to the band alignments that can be determined, for example, with internal photoemission measurements 41 or scanning probe techniques 42 .…”

“…In the case of metals and semi-metals, the work function, the energetic distance of E F to the vacuum level, determines the energetic location of charge carriers. In this regard, we use the Schottky–Mott rule to determine the band alignments shown as a zero-order approximation, thereby neglecting interface-specific reactions and charge imbalances that would lead to additional shifts on the order of a few hundred millielectronvolts 25 . Knowledge of the energetic position of the oxide’s defect bands and alignment to E F is the core of our design approach.…”

“…It is worth noting that several studies have reported high densities of fixed charges at the interfaces of 2D materials with amorphous oxides, for example, MoS 2 /SiO 2 (refs. 25 , 38 ), WS 2 /SiO 2 (ref. 39 ) or graphene/SiO 2 (ref.…”

“…At the same time, the intentional placement of charges at the interface could be used to shift the band edges away from the oxide defect bands using, for example, surface charge transfer doping 43 . However, fixed charges at the interfaces would also degrade the mobility in the semiconducting 2D channel 25 . This could be avoided by using more complex gate stacks with electric dipoles at the interfaces between different oxides.…”

Challenges for Nanoscale CMOS Logic Based on Two-Dimensional Materials

“…In order to improve the electrical stability of 2D FETs, the probability for charge trapping can be considerably reduced if the defect bands in the gate insulator are energetically far away from the conduction and valence band edges of the 2D semiconductor [ 77 ]. For example, n-type WS /HfO FETs are expected to be electrically unstable due to frequent charge trapping events in the electron trapping bands, whereas the stability is likely considerably improved for p-type WS /HfO FETs due to the increased energy barrier for charge trapping, see Figure 4 d. By selecting a suitable combination of 2D semiconductor and gate insulator or by tuning this respective alignment with fixed charges at the interface [ 89 ] or electric dipoles within the gate stack [ 87 ], the number of electrically active charge traps can be minimized. An overview over the band alignment of various 2D semiconductors to the three commonly used amorphous oxides is shown in Figure 4 e.…”