Two-dimensional electron gas at the metastable twisted interfaces of CdTe/PbTe (111) single heterojunctions

Abstract

The lattice-structure-mismatched CdTe/PbTe heterostructures are emerging materials with unique properties and promising applications in midinfrared optoelectronics and spintronics. High-density two-dimensional electron gas at the metastable twisted interfaces of CdTe/PbTe (111) is investigated experimentally and theoretically. CdTe thin films grown on PbTe(111) using molecular-beam epitaxy are found to have high electron mobilities and carrier concentrations from Hall-effect measurements: $2.02\ifmmode\times\else\texttimes\fi{}{10}^{4}$ cm${}^{2}$/V s and $4.5\ifmmode\times\else\texttimes\fi{}{10}^{18}$ cm${}^{\ensuremath{-}3}$ at 2 K, and $6.70\ifmmode\times\else\texttimes\fi{}{10}^{3}$ cm${}^{2}$/V s and $6.0\ifmmode\times\else\texttimes\fi{}{10}^{19}$ cm${}^{\ensuremath{-}3}$ at 77 K. Our density-functional theory modeling reveals that the epitaxially grown CdTe/PbTe (111) heterostructures form metastable twisted interfaces that exhibit unusual structural and electronic properties: (1) The PbTe epilayer on CdTe(111) substrates is structurally highly distorted from the rocksalt structure within the first \ensuremath{\sim}4.5 nm from the interface, which is caused by the stereochemical activity of the Pb ${s}^{2}$ lone pair. (2) The CdTe eplilayer on the PbTe(111) forms spontaneously a high density two-dimensional electron gas (2DEG) over 10${}^{13}$ cm${}^{\ensuremath{-}2}$ near the interface, without the need for doping, which explains the experimentally observed high carrier density and mobility. It is a much simpler heterostructure yet able to offer high electron mobility comparable to and one or two order magnitude higher sheet carrier density than the best achieved values for those of Si and II-VI based quantum well structures relying on modulation doping.