Ground State of a Two-Dimensional Coupled Electron-Hole Gas inInAs/GaSbNarrow Gap Heterostructures

Abstract

The contributions of wave function hybridization and spontaneous exciton formation to the ground state of closely spaced electron and hole gases in InAs͞GaSb heterostructures were investigated using cyclotron resonance (CR) spectroscopy. Strongly hybridized samples exhibit two electronlike CR absorptions at all perpendicular magnetic fields. The high frequency mode neither disappears at high temperatures ͑ϳ100 K͒ nor is affected by changes in electron or hole density, but is eliminated by a high parallel magnetic field ͑ϳ7 T͒. These effects can be understood as signatures of electron-hole wave function hybridization, and cannot be explained in terms of an excitonic gas.[S0031-9007(99)08733-5] PACS numbers: 73.20.Dx, 73.40.Kp, 78.66.Fd The formation of a thermodynamically stable excitonic ground state in a narrow gap semiconductor is a possible precursor to Bose-Einstein condensation in such systems. In order to achieve this, the band gap energy E g of the semiconductor must be smaller than the exciton binding energy E b , making it energetically favorable to spontaneously form excitons. To achieve E g , E b , InAs͞GaSb heterostructures have been proposed [1-3] because the gap between the InAs conduction band and the GaSb valence band can be tuned from ϳ 2150 meV to positive values [4]. Thus, the spontaneous formation of an excitonic gas might be realized using closely spaced InAs and GaSb quantum wells, containing two-dimensional electron and hole states, respectively. To achieve the large E b needed for experimental observation of such excitons, the distance between the gases should be minimized, maximizing the Coulombic attraction between the electrons and holes. Whether such an excitonic gas forms the ground state of the system depends on other competing mechanisms. In particular, as the interwell separation decreases, conduction and valence band wave function overlap results in hybridized states characterized by a new band gap energy D [4]. Additional considerations such as high and unequal extrinsic populations of electrons and holes may also inhibit exciton formation. Recent far-infrared (FIR) cyclotron resonance (CR) studies [5,6] of adjacent electron-hole gases in InAs͞Al x Ga 12x Sb heterostructures revealed two absorption peaks near the electron CR energy at x 0.1 and 0.2. These were not attributable to two occupied electron subbands as in early CR studies of InAs͞GaSb multiheterojunctions [7]. Instead, the lower energy peak was assigned to electron CR, and the higher energy peak to a cyclotron-shifted internal transition of a ground excitonic state. However, subsequent reinterpretation of these experimental results by other investigators [8,9] has suggested hybridized electron and hole Landau levels as a possible description. FIR CR spectroscopy is ideally suited to elucidate the nature of this ground state because it is a sensitive probe of the internal modes (e.g., 1s to 2p) of excitons [10] as well as wave function hybridization [11].In the present study, FIR CR spectroscopy is used to investigat...

Spin states in InAs/AlSb/GaSb semiconductor quantum wells

“…These studies show that due to the overlap of InAs conduction band and GaSb valence band, the energy dispersion may exhibits an anticrossing behavior at a finite in-plane wave vector k 0 3,4,5,6,7,8,9,10 . The tunneling between InAs and GaSb layers opens a mini hybridization gap, which was observed experimentally 11,12,13,14 . In addition, the electrons in GaSb can move across InAs/GaSb interface into InAs layer, forming a two-dimensional electron gas in InAs side and a two-dimensional hole gas in GaSb side, which is promising for observing the Bose-Einstein condensation of excitons 14,15,16 .…”

“…The tunneling between InAs and GaSb layers opens a mini hybridization gap, which was observed experimentally 11,12,13,14 . In addition, the electrons in GaSb can move across InAs/GaSb interface into InAs layer, forming a two-dimensional electron gas in InAs side and a two-dimensional hole gas in GaSb side, which is promising for observing the Bose-Einstein condensation of excitons 14,15,16 . In practical applications, there have been many proposals for electronic and optical devices utilizing the unique characteristics of InAs/AlSb/GaSb system such as resonant tunneling structures 17,18 , infrared detectors 19 , and interband cascade laser diodes 20 .…”

“…In the past decades, the properties of InAs/GaSb broken-gap supperlattices and quantum wells (QWs) were investigated both theocratically and experimentally 3,4,5,6,7,8,9,10,11,12,13,14 . These studies show that due to the overlap of InAs conduction band and GaSb valence band, the energy dispersion may exhibits an anticrossing behavior at a finite in-plane wave vector k 0 3,4,5,6,7,8,9,10 .…”

Partial hybridisation of electron-hole states in an InAs/GaSb double quantum well heterostructure

“…Furthermore, previous studies of the hybridised state through cyclotron resonance in a strongly hybridised system 25 and studies of the 2DTI state observed in a strongly hybridised system 6 show that the hybridised nature of this state is largely independent of temperature, demonstrated here by the fact that a model using a temperature-independent effective mass describes the data well.…”

Closely spaced and separately contacted two-dimensional electron and hole gases by in situ focused-ion implantation

“…However, many proposed experiments require separate contacts to the two layers which, to our knowledge, have not yet been realised in this system. Furthermore, it seems to be difficult to adjust the densities of the two gases by external gates and to obtain large mean free paths of the holes and electrons [3].…”