Optical spin injection and tunneling in asymmetric coupled II–VI quantum wells

Abstract

We give an overview of recent results concerning exciton and carrier tunneling and relaxation in II-VI asymmetric double quantum wells structures with semimagnetic (diluted magnetic) wells or barriers. We undertake the issue of the spin relaxation and injection in the tunneling process and we discuss the dependence of the tunneling efficiency and spin injection on the energy detuning between the initial and final state of the process. The fast tunneling of excitons as whole entities was observed with efficiency strongly dependent on the barrier width, exciton detuning and the relation between detuning and optical phonon energy. In the energy transfer processes an important role of the charged excitons formation is observed. As a result of high sensitivity of energy levels in semimagnetic semiconductor to magnetic field, the thermodynamical redistribution of neutral and charged excitons dependence in magnetic field is observed and must be taken into account in the analysis.1 Introduction Asymmetric double quantum well (ADQW) structure forms an excellent laboratory for studying tunneling and related spin effects. It consists of two quantum wells (QWs) of different depth and/or width coupled through a barrier having width comparable to QW widths. In this system, usually the two lowest excitonic states are separated spatially but the overlap between the wave functions enables tunneling. ADQW structure based on the II-VI compounds is particularly useful in such investigations. It results from the possibility of technological realization of diverse designed structures due to a great number of available materials and their combinations. Simultaneously exciton binding energies in wide-gap II-VI semiconductors, few times larger compared to III-V materials, enable investigations of the influence of excitonic effects on the tunneling process. In addition, due to stronger coupling between excitons and longitudinal optical (LO) phonons in II-VI compounds than in III-V ones, phonon assisted tunneling plays a major role in the energy relaxation process and can be studied.The detuning between initial and final state of the tunneling process proves to be the parameter that controls the tunneling mechanism and, consequently, its efficiency. With the development of crystal growth techniques it became possible to accurately control the structure parameters and, as a consequence, the energy levels and wave function penetration through the barrier. However, to perform a detailed study of tunneling, it is preferable to tune continuously the energy levels and thus change tunneling conditions. A straightforward way of tuning the levels is using a diluted magnetic semiconductor (DMS) as a QW material. Due to s,p-d exchange interaction between manganese ions and band carriers, a giant Zeeman effect leads to 100 times larger band splitting than in a nonmagnetic material [33]. Therefore, a very efficient tuning of energy levels is possible with little modification of single particle or exci-

Tunnel coupling in asymmetric semimagnetic double quantum wells

“…That results in a huge negative hh g and positive e g effective g-factors of the hh and electron states, respectively. The ground exciton transition in the MW is σ + -polarized at B > 0 and corresponds to the optical transition between the upmost state with a 3 2 z J = -/ moment projection in the valence band 8 Γ and the lowest state with 1 2 z S = -/ spin projection in the conductivity band 6 , Γ according to usual notation [14]. Very quick exciton relaxation from the excited 1 J = -state to the ground 1 J = + state with a picosecond time scale is induced by a spin-flip due to the strong s,p d -exchange [2] and gives rise to observation of only σ + -polarized exciton transition in the PL at B ≥ 1 T, both in 3D and 2D cases.…”

“…This allows to vary the interwell coupling in the double quantum wells (QWs) [4] and quantum dots (QDs) [5] structures after growing, when a barrier width B L is fixed. ADQWs are highly attractive objects, in first order as an excellent system for studying of the tunnelling phenomena in solids, e.g., a direct measuring of the tunnelling times in the case of weakly tunnel coupled double wells (see comprehensive review in [6]). In the opposite case a pronounced influence of strong tunnel coupling on the exciton transition energies was found that in ADQWs controlled by electric or magnetic fields [7,8].…”

Exciton states and tunneling in semimagnetic asymmetric double quantum wells

“…Carriers tunneling was intensively studied in coupled quantum wells (QWs) 8–15 and in pairs of correlated quantum dots 16–19. A special interest arose to a spin dependent interwell tunneling 20. It was found that excitonic effects are essential in tunneling process 3.…”

Spin-Conserving Tunneling of Excitons in Diluted Magnetic Semiconductor Double Quantum Wells

“…4) DMS DQWs of II-VI compounds further enable us an investigation of the influence of the excitonic effect in the spin injection due to the large exciton binding energy. Considerable number of works have been made up to now on the spin injection: [5][6][7][8][9] Individual spin injection of electrons and heavy holes (hhs) were reported due to the large difference of their tunneling probabilities. 6) The carrier spin injections were accelerated by emission of an longitudinal optical (LO) phonon, when the detuning energy of the electron/hh levels exceeds one LO-phonon energy.…”