Surface Band-Gap Narrowing in Quantized Electron Accumulation Layers

Abstract

An energy gap between the valence and the conduction band is the defining property of a semiconductor, and the gap size plays a crucial role in the design of semiconductor devices. We show that the presence of a two-dimensional electron gas near to the surface of a semiconductor can significantly alter the size of its band gap through many-body effects caused by its high electron density, resulting in a surface band gap that is much smaller than that in the bulk. Apart from reconciling a number of disparate previous experimental findings, the results suggest an entirely new route to spatially inhomogeneous band-gap engineering.

Conductivity in transparent oxide semiconductors

“…This causes the conduction band states to become quantized into two-dimensional subbands (the electron accumulation can be seen as a two-dimensional electron gas (2DEG)). This two-dimensional electronic structure at the surface of CdO has been observed directly by angle-resolved photoemission spectroscopy [150,151], as shown in Fig. 9.…”

“…In addition to this Moss-Burstein shift, which widens the optical energy gap, it has been found that many-body interactions lead to a shrinkage of the fundamental band gap with increasing free-carrier concentration [183]. This has been employed in the analysis of optical absorption in many TCOs [184][185][186][187][188][189], and has recently been shown to have a pronounced depth-dependent effect on the band gap size within their surface electron accumulation layers [150], as mentioned above. However, the origins of such band gap narrowing have also recently been questioned [190], with hybridization between states from the dopant impurity atoms and the host conduction band states rather than many-body interactions between the free carriers attributed as the cause of the bandgap shrinkage.…”

“…Such measurements [150] have recently revealed an unexpectedly large importance of many-body interactions within the electron accumulation layer, which leads to a depth-dependent shrinkage of the semiconductor band gap. Consequently, the band gap at the interface of TCOs with other materials may also be modified from its bulk value, and this should be investigated for its potential influence on the properties of these electrical contacts.…”

H-tailored surface conductivity in narrow band gap In(AsN)

“…From the analysis of the Hall resistance using the two channels model, we find that the H-induced increase of the electron density is negligible and that the surface accumulation layer has an electron sheet density of $10 16 m À2 , significantly smaller than for In(AsN). Furthermore, whereas in InAs the charge is located on the surface thus leading to a bending of the bands and the formation of two-dimensional (2D) accumulation layer, 16 in In(AsN), the spatially uniform density of donors across the thin ($100 nm) subsurface region of the epilayer and large electron sheet density lead to a 3D electron gas near the surface (Figure 3(c)). …”

Two-dimensional electron gas generated by La-doping at SrTiO3(001) surface: A first-principles study

“…In addition, the LDA+U scheme can approximately describe the effect of electron correlation, which may significantly affect the electronic properties of 2DEG. 15,25 An U − J of 10 eV was used for the La f orbitals to push them far from the Fermi level, avoiding the spurious hybridization between them and the Ti d orbitals. The SrTiO 3 (001) surfaces were modelled into a supercell which consists of a SrTiO 3 slab and a vacuum region of about 12 Å.…”