Electron gas in semiconductor multiple quantum wires: Spatially indirect optical transitions

Abstract

In optical experiments with laterally patterned GaAs quantum wells we observe confinement of free carriers to motion in one dimension. Optical recombination of band-gap transitions shows that free electrons and holes are confined in spatially indirect type-II multiple quantum wires. Resonant-inelasticlight-scattering spectra show intersubband excitations of the one-dimensional electron gas. From the optical measurements we obtain the Fermi energy as well as subband spacings and determine the linear free-electron density. PACS numbers: 78.55.Cr, 73.20.Dx, 78.30.Fs Advances in epitaxial growth and in nanometer-scale lithography have made it possible to fabricate semiconductor structures which exhibit one-and zero-dimensional properties. Semiconductor quantum wires and boxes have been found to exhibit new structure 1 " 4 or energy shifts 5 ' 6 in their photoluminescence (PL) spectra which have been ascribed to excitonic confinement effects. Transport properties of modulation-doped quasi-one-dimensional structures such as electron waveguiding, 7 quantization of resistance, 8 and quenching of the Hall effect 9 have also been observed. Reed et al. 10 observed resonant tunneling into quantum boxes. Less work has been done on the optical properties of one-and zero-dimensional modulation-doped structures. Hansen et al. 11 and, more recently, Heitman et al} 2 have observed intersubband resonances in a quasi-one-dimensional electron gas by means of infrared absorption.This Letter reports the observation of optical properties of a semiconductor one-dimensional (ID) electron gas that are markedly different from closely related two-dimensional (2D) systems. Band-gap optical absorption and emission in GaAs-(AlGa)As multiplequantum-wire structures reveal properties of conductionand valence-band states that are not obtainable from transport or infrared absorption measurements. Resonant inelastic light scattering is used to measure the energy spacings between the electron-gas states confined to one dimension. In these experiments we have determined that the lower states of electrons in conduction subbands and of holes in valence subbands show ID confinement in adjacent, spatially separated quantum wires. We describe this behavior as a type-II multiplequantum-wire heterostructure. 13 ' 14 Multiple quantum wires are fabricated from modulation-doped GaAs/AlGaAs single-quantum-well heterostructures grown by molecular-beam epitaxy on a GaAs substrate. The procedure is schematically shown in Fig. 1. The as-grown 2D electron-gas structure consists of a 250-A-thick GaAs quantum well, separated from the surface by a 1000-A-thick /z-type delta-doped Alo.3Gao.7As layer and a 40-A-thick GaAs cap layer.

Single-particle excitations and many-particle interactions in quantum wires and dots

“…There are also very interesting Raman experiments on quantum wires and dots. [6][7][8][9][10][11][12][13] Strenz et al reported on SPE in quantum wires and observed SPE in dots. 11 Onedimensional electron systems which are nearly in the 1D quantum limit have been investigated by Schmeller et al 9 They found that the relative energy renormalizations of the 1D intersubband SDE and CDE with respect to the simultaneously observed corresponding SPE are quite similar to those found for 2D systems.…”

Influence of the interface corrugation on the subband dispersions and the optical properties of (113)-oriented GaAs/AlAs superlattices

“…This reduction results in a concentration of the density of states at the band edge and in reduced scattering in the remaining directions of free motion, which is both of interest for optical and electrical devices such as laser diodes and fast transistors. [1][2][3][4] It can be achieved by structural sizes in the nanometer range, resulting in an energy difference of the confined states larger than both the thermal energy and the excitonic binding energy.…”

Electrical transport and far-infrared transmission in a quantum wire array

“…A number of reports have been devoted to the probing of quasi-2DEG and quasi-1DEG energy spectrum using different techniques such as Fourier transform infrared spectroscopy ͑FTIR͒, [36][37][38][39][40][41][42][43][44][45][46][47][48] Raman scattering spectroscopy, [49][50][51][52][53][54][55][56][57] far-infrared ͑FIR͒ laser spectroscopy [58][59][60] and photoluminescence. [61][62][63][64][65][66][67][68][69][70] In all these techniques a collective excitation of the 1DEG is measured, the Raman scattering also giving information about single particle excitations. There is a large diversity of theoretical treatments for the optical response of an electron gas in reduced dimensionality, ranging from classical hydrodynamic to quantum manybody approaches.…”