Inhibited recombination of charged magnetoexcitons

Abstract

Time-resolved photoluminescence measurements show that the decay time for charged excitons in a GaAs twodimensional electron gas increases by an order of magnitude at high magnetic fields. Unlike neutral excitons, the charged exciton center-of-mass is spatially confined in a "magnetically-adjustable quantum dot" by the cyclotron orbit and the quantum well. The inhibited recombination is explained by a reduced phase coherence volume of the magnetically-confined charged excitons.PACS numbers: 71.35. Ji, 8.47.+p, 73.20.D Charged excitons or "trions" were first identified in optical absorption experiments on electron-doped CdTe quantum well (QW) structures through their polarization properties in a magnetic field. 1 The negatively charged exciton (X − ) transition in a narrow QW was manifest in the spectra as a peak lying several meV below the uncharged exciton (X 0 ) peak. Although both X 0 and X − transitions may had been observed previously in PL spectra of GaAs QWs, the high electron density precluded their identification. 2 In hindsight, it is not surprising that the recently identified X − is often the most common exciton found in a system with excess electrons, similar to the X 0 in a system without excess electrons. An X 0 in the presence of excess electrons becomes polarized by a nearby electron and binds the electron by a dipolar attraction. The properties of X − transitions in GaAs QWs have been explored in several recent experimental 3-7 and theoretical [8][9][10][11][12] studies.An interesting facet of the charged exciton that has yet to be explored is the confinement produced by the cyclotron motion in a magnetic field. The X − complex (two electrons plus one hole) is singly-charged and a magnetic field confines the X − center-of-mass motion to a cyclotron orbit, unlike the neutral exciton (X 0 ) which is free to move in a magnetic field. This will be referred to as the magnetically-confined charged exciton (MCX). A magnetic field applied perpendicular to a twodimensional (2D) QW effectively confines the exciton to a quantum dot (QD) whose size is adjustable with magnetic field. The 3D MCX volume, defined roughly by the QW width and the area of the cyclotron orbit in the plane of QW, is inversely proportional to the perpendicular magnetic field, V M CX ∝ 1/B ⊥ . At high magnetic fields this volume is typically smaller than QDs currently available via patterned nanostructures.The purpose of the present study is to examine the radiative recombination of excitons confined in these MCX QDs. Exciton recombination times were determined by measuring the photoluminescence (PL) decay times in low-density GaAs/AlGaAs electron gases in magnetic fields to 18 T, at temperatures 0.5-7 K. At low temperatures, the X − decay time was found to increase by an order of magnitude for increasing perpendicular magnetic field. In contrast, the recombination is rapid for both the X − in fields applied in the plane of the QW and for the uncharged X 0 . In the latter two cases the exciton is not confined to a QD. The linear d...

Negatively charged trion in ZnSe single quantum wells with very low electron densities

“…A similar decrease of the X s Ϫ line intensity accompanied by an increase of the X hh line intensity was also found in Ref. 28 for a GaAs QW with a large total band offset. However, this is not further commented upon in this paper.…”

Charged Magnetoexcitons in Two Dimensions: Isolated X? and Many-Electron Effects

“…For the isolated MX, it is achieved at a finite momentum Kl B ' 1:19; an inverse square-root van Hove singularity in the X 10 density of states is associated with this extremum in 2D. There is another extremum [41] in the X 10 dispersion at K ¼ 0 producing a finite jump in the X 10 Fig. 1 for states in the ðn e n h Þ ¼ ð01Þ LLs.…”

“…Such absorption processes were observed [45] in the low-density 2DEG in magnetic fields and called a ''Combined Exciton-Cyclotron Resonance" (ExCR). Due to the selection rule (10), the transitions to the bound triplet state e À 0 þ photon ! X À t10 and transitions to the excited bound states (2e)-h are prohibited.…”

“…Experimentally, X À has been identified in magneto-optical spectra of a low-density quasi-two-dimensional electron gas (2DEG) in quantum wells (QWs) [5]. The observation in magnetic fields B of spin-singlet and spin-triplet X À and X þ charged excitons in QWs has been subsequently reported by many groups [6][7][8][9][10][11][12][13]. The bulk of the experimental work to date has been concentrated on interband magneto-optics.…”

Formation and Recombination Dynamics of Charged Excitons in a GaAs Quantum Well

“…These excess carriers can be provided by either remote doping of the barrier layers [11][12][13] or through optical excitation of a neighbouring QW [14]. Even though there have been several studies on the recombination process of localised and free trions [14][15][16][17][18][19], very little has been reported about X À [15,20] and X þ [21] formation dynamics.…”