Efficient electron spin detection with positively charged quantum dots

Abstract

We report the application of time-and polarization-resolved photoluminescence up-conversion spectroscopy to the study of spin capture and energy relaxation in positively and negatively charged, as well as neutral, InAs self-assembled quantum dots. When compared to the neutral dots, we find that carrier capture and relaxation to the ground state is much faster in the highly charged dots, suggesting that electron-hole scattering dominates this process. The long spin lifetime, short capture time, and high radiative efficiency of the positively charged dots, indicates that these structures are superior to both quantum well and neutral quantum dot light-emitting diode (LED) spin detectors for spintronics applications.The efficient detection of spin-polarized carriers is a crucial issue for the design of semiconductor-based spintronic devices. 1 A light-emitting diode (LED) configuration employing a quantum well as an optical marker has proven to be an effective means of detecting spin polarized carriers. 2,3 In a spin LED, the degree of circular polarization of the quantum well luminescence provides a direct measure of the spin polarization of the carriers arriving at the spatial location of the quantum well.The application of semiconductor quantum dots (QDs) in such a spin detection scheme is expected to provide a substantial improvement in spin sensitivity over the use of a quantum well. 4,5,6,7 Recent studies of electron spin dynamics in neutral QDs 4,5,8 have revealed that the discrete energy levels in quantum dots arising from three-dimensional quantum confinement blocks the dominant spin relaxation channels present in higher dimensional bulk and quantum well systems, resulting in considerably longer spin relaxation times. Combined with the high optical luminescence efficiency observed in QDs, 9,10,11 these long spin relaxation times should lead to larger spin-dependent luminescence signatures in spin detection applications incorporating QDs as an optical marker. The first spin LED using neutral QDs was recently demonstrated. 6 Few experiments have examined electron spin dynamics in charged quantum dots. 12,13 Through a comparison of spin capture and relaxation dynamics in neutral, positively (+QDs) and negatively (-QDs) charged QDs, we demonstrate that +QDs act as a highly efficient detector for spin-polarized electrons. Our room temperature time-resolved measurements reveal that, following capture of spin-polarized electrons, the initial degree of circular polarization of the QD ground state luminescence is more than four times larger in +QDs compared to neutral QDs. The larger spin signature in +QDs originates from an increased rate of capture of spin polarized electrons through interaction with the built-in hole population, 14,15 as indicated by the early time dynamics of the QD photoluminescence. Rapid electron capture into +QDs reduces spin relaxation in the GaAs barriers prior to capture, resulting in a six-fold enhancement in the time-integrated spin detection efficiency with the incorporation of positiv...

Optical spin polarization and exchange interaction in doubly charged InAs self-assembled quantum dots

“…This assumption is based on a simple observation that a charged QD attracts particles of the opposite charge and repels like-charged ones and also on the experimental results of Ref. 26. An evident consequence is that the opposite-charge carrier lives for some time in the QD before the coming of the like-charge one; during this time interval, the QD contains not a tetron, but a trion.…”

Entanglement transfer from electron spins to photons in spin light-emitting diodes containing quantum dots

“…The rate for the injection and the subsequent relaxation of electrons into the conduction band ground state in the dot α is denoted by W eα . It has been demonstrated that this entire process is spin conserving and occurs much faster than the optical recombination 5,6 , which is described by the rates W pα . Typically, W pα ∼ 1 (ns)…”

“…Spin light-emitting diodes (spin-LEDs), 1,2,3,4,5,6,7,8 in which electron recombination is accompanied by the emission of a photon with well-defined circular polarization, provide an efficient interface between electron spins and photons. The operation of such devices at the single-photon level would allow one to convert the quantum state of an electron encoded in its spin state into that of a photon with a wide range of possible applications.…”

Electron Scattering Through a Quantum Dot

“…The spin-dependent transport properties are of particular interest for its possible applications, e.g., the QD spin valves (Konig & Martinek, 2003), the quantum logic gates using coupled QDs, as well as the spin-dependent transport in single-electron devices (Seneor et al, 2007), etc.. In such systems, the electron-electron exchange potential and the electron spin states have been utilized and manipulated (Burkard et al, 2000;Gundogdu et al, 2004;Sarma et al, 2001;Wolf et al, 2001). A thorough quantitative understanding of spin-dependent transport properties due to electron-electron interaction is therefore important for a successful construction of these devices.…”