Dilute Magnetic Semiconductor Quantum Wells Research Articles

Interface inversion asymmetry in Cd1–xMnxTe quantum wells

AbstractSpin‐flip Raman spectroscopy (SFRS) reveals an intrinsic in‐plane anisotropy of the conduction band spin splitting in dilute magnetic semiconductor quantum wells based on cadmium telluride, containing a twodimensional electron gas, and intended for studies of extrinsic anisotropies induced through nanomagnets.We consider various possible sources of this intrinsic anisotropy (interface, bulk or structural inversion asymmetries) and we ascribe the present results to an interface‐related inversion asymmetry of the quantum wells. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sub‐ns electrical control of spin polarization in a semiconductor by microscale current loops

AbstractWe demonstrate a method to electrically control the spin polarization in a diluted magnetic semiconductor (DMS) quantum well (QW) on a micrometer length scale. A variable magnetic field generated from a microscale current loop atop a single CdMnTe/CdMgTe QW magnetizes the Mn2+ ion spins in the DMS QW and, via the sp–d exchange interaction, polarizes the spins of photon‐generated carriers. In the absence of an external field, the spin polarization can be switched on a time scale clearly below 1 ns.

Multi-spin-subband structure of dilute magnetic semiconductor quantum wells: Feedback mechanism

Using a fully self-consistent envelope function approach, we focus on wide conduction band NMS (non-magnetic semiconductor)/DMS (dilute magnetic semiconductor)/NMS quantum wells, under weak external parallel magnetic field, where many spin-subbands are usually present. We concentrate on small values of the magnetic field because we want to investigate the influence of the feedback mechanism due to the difference of the concentrations of spin-up and spin-down carriers which could induce spontaneous spin-polarization i.e. in the absence of a magnetic field. We study the spin-subband structure, the spin-subband populations and the spin-polarization as functions of the sheet carrier concentration, N s , for different values of the magnitude of the exchange interaction, | J | , between the itinerant carriers and the magnetic impurities. Our calculations for 0.01 T show that at 20 K the values of | J | necessary to make this feedback mechanism sufficiently strong are too high compared to the | J | values of common Mn-doped systems in the conduction band. However, the feedback mechanism will be sufficiently strong at low enough temperatures below 20 K for realistic values of | J | . Moreover, we explain how increasing the sheet carrier concentration the heterostructure is transformed from an almost square quantum well to a system of two coupled heterojunctions with an intermediate soft barrier.

Micro-photoluminescence study of electron-spin injection in self-assembled semiconductor quantum dots

We present a micro-photoluminescence (m-PL) study of electron-spin injection under magnetic fields in self-assembled semiconductor quantum dots (QDs) of CdSe. A characteristic band line-up of the CdSe QDs coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of ZnCdMnSe through a ZnSe barrier layer enabled us to inject the electron spins from the DMS-QW into QDs. An experimental evidence of the electron-spin injection was provided by observations of circularly polarized m-PL spectra from individual QDs in this coupled QD structure. We find anti-correlation of PL intensity in between the DMS-QW (spin injector) and the individual QDs (spin receiver).

Coherent Raman Spectroscopy of Cd1-xMnxTe Quantum Wells

The first coherent Raman electron spin resonance (ESR) results obtained in dilute magnetic semiconductor quantum wells are presented and the sensitivity and selectivity of the coherent Raman-detected ESR (CRESR) technique for probing the local environment of magnetic ions in a semiconductor host is demonstrated. The lineshapes and linewidths that are observed are considered in the context of previous work on bulk Cd1-x,MnxTe alloys and on CdTe/MnTe heterostructures.

Photoluminescence of “dark” excitons in CdMnTe quantum well, embedded in a microcavity

A diluted magnetic semiconductor (DMS) quantum well (QW) microcavity operating in the limit of the strong coupling regime is studied by magnetoptical experiments. The interest of DMS QW relies on the possibility to vary the excitonic resonance over a wide range of energies by applying an external magnetic field, typically about 30 meV for 5 T in our sample. In particular, the anticrossing between the QW exciton and the cavity mode can be tuned by the external field. We observe the anticrossing and formation of exciton polaritons in magneto-reflectivity experiments. In contrast, magneto-luminescence exhibits purely excitonic character. Under resonant excitation conditions an additional emission line is observed at the energy of the dark exciton. The creation of dark excitons is made possible due to heavy hole–light hole mixing in the QW. The emission at this energy could be due to a combined spin flip of an electron and a bright exciton recombination.

Magneto-Optical Properties of Non-Magnetic Semiconductor Quantum dot and Magnetic Quantum well Coupled Structures

The magneto-optical properties of quantum-dot (QD) and quantum-well (QW) coupled structures are investigated using magneto-photoluminescence (PL). The coupled system consists of a self-assembled non-magnetic CdSe quantum-dot layer and a 7-nm ZnCdMnSe dilutedmagnetic-semiconductor (DMS) quantum well (QW) layer. We have observed two well-separated PL peaks, one corresponding to emission from the CdSe QD layer and the other to the emission for ZnCdMnSe QW layer. The temperature dependence of the PL intensity clearly demonstrates the characteristics of zero- and two-dimensional structures for the CdSe QDs and the ZnCdMnSe QW, respectively. The significant PL energy shift and intensity variation observed for the DMS QW in a magnetic field can be understood based on the giant Zeeman splitting of the band edges of the DMSs. Furthermore, we have observed a type-II transition between the conduction band of the DMS QW and the valence band of non-DMS QD, which enables us to determine the band alignment of the coupled systems.

Magnetic-field-dependent type-II transition induced by resonant electron tunneling from a diluted magnetic semiconductor quantum well into self-assembled quantum dots

Magnetic-field-dependent type-II transition has been observed between electrons in self-assembled quantum dots (QDs) of CdSe and heavy holes (hhs) in a diluted magnetic semiconductor quantum well (DMS-QW) of Zn1−x−yCdxMnySe, originating from an electron tunneling from the DMS-QW into the QDs. The electron tunneling is resonantly assisted by LO-phonon scattering in the QDs. As a result, the type-II emission energy depends on a magnetic field, since the electron energy in the DMS-QW is a function of field strength. The magnetic-field dependence of intensity of the type-II emission suggests a level-crossing field of the hh states between the QD and DMS-QW.

Spin injection in self-assembled quantum dots coupled with a diluted magnetic quantum well

Spin injection is studied in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of Zn 1− x − y Cd x Mn y Se, by means of time-resolved circularly polarized photoluminescence (PL). Excitonic PL from the CdSe QDs shows σ −-circular polarization in magnetic fields, mainly due to negative g-values of individual dots, when the energy difference of excitons between the QDs and DMS-QW is large as 300 meV. However, when such energy difference is comparable with LO-phonon energy in the QD, we observe an additional PL peak with the long lifetime as 3.5 ns and σ +-polarization in magnetic fields. It can be attributed to a type-II transition between the down-spin electron injected from the DMS-QW into the QDs, via LO-phonon-assisted resonant tunneling, and the down-spin heavy hole in the DMS-QW. In addition, the electron spin-injection is also evidenced by σ +-polarized PL with the fast rise-time of 20 ps in the QDs.

Nanoscale magnet for semiconductor spintronics

A hybrid nanostructure of diluted magnetic semiconductor quantum well (DMS-QW) has been fabricated by using a ferromagnetic Co∕Pt-multilayered film with perpendicular magnetization. The nanoscale disk of DMS-QW with a diameter of 80nm was embedded in the magnetic film generating magnetic fields perpendicular to the DMS-QW plane. As a result, exciton photoluminescence with circular-polarization properties arises in a zero external field, due to the giant Zeeman effects of excitons. It indicates that the perpendicular magnetic fields applied from the Co∕Pt film align the exciton spins in the DMS nanoscale disk, acting as a nanoscale magnet for semiconductor spintronics.

Efficient spin injection in self‐assembled CdSe quantum dots coupled with a diluted magnetic semiconductor quantum well

AbstractElectron‐spin injection has been studied from a diluted magnetic semiconductor quantum well (DMS‐QW) of Zn1–x–yCdxMnySe into self‐assembled quantum dots (QDs) of CdSe through a tunneling barrier of ZnSe. The spin injection is experimentally evidenced by time‐resolved circularly polarized photoluminescence (PL) in the QDs with the circular polarization degree up to 40%. The fast injection time of 20 ps can be attributed to the tunneling probability of the electron from the DMS‐QW side. In addition to this, circularly polarized PL with the lifetime of 3.5 ns is simultaneously observed, indicating type‐II transition between the electron in the QDs and the spin‐polarized heavy hole in the DMS‐QW. Those PL energies directly indicate that the electron tunneling is assisted by a LO phonon in the CdSe QD. The spin injection via LO‐phonon‐assisted resonant electron tunneling provides an efficient spin‐injection process into the self‐assembled QD exhibiting strong quantum confinement effects. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Suppression of electron‐spin relaxation induced by magnetic fields in a Cd 1–x Mn x Te quantum well

Carrier-spin relaxation under a magnetic field has been studied in a diluted magnetic semiconductor quantum well (DMS-QW) of Cd0.95Mn0.05Te, by means of pump-probe absorption spectroscopy. With resonant excitation using a circularly polarized pump-pulse, dynamical spin-flip processes of an electron and heavy-hole (hh) are clearly observed in time dependences of the circularly polarized differential absorbance. In 0 T, time constants of 17 ps and 0.4 ps for the electron- and hh-spin relaxations in the Cd0.95Mn0.05Te QW are shorter than those of 46 ps and 1.0 ps in a non-magnetic CdTe QW, due to s-d exchange interactions between the carriers and Mn spins. The spin-flip time of the electron due to the exchange mechanism is determined as 27 ps. In an intense magnetic field of 5 T, the electron-spin relaxation due to the exchange mechanism is found to be strongly suppressed in the DMS-QW by magnetic-field-induced pinning of the Mn spins, exhibiting a longer time constant of 29 ps than that of 17 ps in 0 T. The role of LO-phonon is also discussed in the electron-spin relaxation under magnetic fields. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magneto‐optical properties in hybrid nanostructures of II–VI diluted magnetic semiconductors with perpendicular ferromagnetic films

AbstractHybrid nanostructures of a diluted magnetic semiconductor quantum well (DMS‐QW) of Zn1–x –y Cdx Mny Se have been fabricated with a ferromagnetic Co/Pt‐multilayered film, by electron beam lithography. The disk structure of the DMS‐QW with the diameter of 100 nm was embedded in the magnetic thin film with the thickness of 50 nm. Giant Zeeman effects of excitons in the DMS‐QW in the hybrid structure are studied by micro‐photoluminescence (PL) spectroscopy. The Co/Pt film shows perpendicular magnetization without external field, and therefore it can generate magnetic fields nearly perpendicular to the DMS‐QW plane in the hybrid structure. As the result, the circularly polarized excitonic PL arises in zero external field, indicating that the perpendicular magnetic field applied from the Co/Pt film aligns the exciton spins in the DMS‐disk in the present hybrid structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient spin injection into self-assembled quantum dots via LO-phonon-assisted resonant electron tunneling

Spin injection from a diluted magnetic semiconductor quantum well (DMS-QW) into self-assembled quantum dots (QDs) of CdSe is demonstrated via LO-phonon-assisted resonant electron tunneling. The experimental evidence for the spin injection is clearly shown by time-resolved circularly polarized exciton photoluminescence (PL) with the polarization degree up to 40% in QDs. In addition, a type II transition with the lifetime of 3.5ns between electrons in the QDs and heavy holes in the DMS-QW is observed. These PL energies directly indicate that the electron tunneling is resonantly assisted by LO-phonon scattering, which realizes an efficient spin-injection process into QDs.

Giant Zeeman effect of excitons in a hybrid nanostructure of diluted magnetic semiconductor quantum well sandwiched in Co wires

Giant Zeeman effects of excitons in a hybrid nanostructure of diluted magnetic semiconductor (DMS) quantum well (QW) with ferromagnetic Co have been demonstrated, where the DMS-QW wire with the width of 200 nm is sandwiched in the Co wires and magnetic fields nearly perpendicular to the well plane can efficiently be applied from the Co wires to all of the DMS QW. The spectral shape of excitonic photoluminescence and the field dependence show clearly that the microscopic perpendicular fields generated from the Co wires induce the giant Zeeman shifts of excitons in the DMS QW.

MAGNETO-PHOTOLUMINESCENCE STUDY AT A FRACTIONAL QUANTUM HALL REGIME OF CHARGED EXCITONS IN A DILUTE MAGNETIC SEMICONDUCTOR

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1-x Mn x Te / Cd 1-y Mg y Te single quantum wells. In low magnetic fields (the electrons filling factor ν>1), we observed several photoluminescence lines associated with spin-singlet and spin-triplet charged excitons. In high magnetic fields (ν<1), we observed only one of the spin-triplet charged excitons that are called as a bright type due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells, where the photoluminescence intensity increased at a fractional value of [Formula: see text]. We consider that the screening effect plays a crucial role for this behavior in high magnetic fields.

Spin sensitive dynamics of a charged exciton in magnetic fields

We have performed magneto-photoluminescence measurements on modulation doped n-type Cd 1− x Mn x Te/Cd 1− y Mg y Te single quantum wells. We observed several transitions arising from spin-singlet and spin-triplet charged excitons. The transient photoluminescence data showed characteristic decay time depending on the spin configuration of the charged excitons. The spin-triplet charged excitons of a bright type are known to realize due to characteristic spin states of electrons and holes in dilute magnetic semiconductor quantum wells. We have also found that the decay-time of charged excitons depends much on photon energies used for excitation.

Properties of conduction-band dilute-magnetic-semiconductor quantum wells in an in-plane magnetic field: A density of states profile that is not steplike

Leibniz Institute for Neurobiology, Special Lab for Non-Invasive Brain Imaging, Brenneckestrasse 6, D-39118 Magdeburg, Germanyand Physics Department, Solid State Section, University of Athens, Panepistimiopolis, Zografos, GR-15784 Athens, Greece~Received 6 September 2003; published 12 March 2004!We examine how an in-plane magnetic fieldB modifies the density of states ~DOS! in narrow-to-wide,conduction-band dilute-magnetic semiconductor quantum wells. We demonstrate that the DOS diverges sig-nificantly from the ideal steplike two-dimensional electron gas form and this causes severe changes to thephysical properties, e.g., to the spin-subband populations, the internal and free energy, the Shannon entropy,and the in-plane magnetizationM. We predict a considerable fluctuation of M in cases of vigorous competitionbetween spatial and magnetic confinement.DOI: 10.1103/PhysRevB.69.113302 PACS number~s!: 73.20.2r, 85.75.2d, 75.75.1a, 65.40.Gr

Optical studies of Mn 2+ spin resonance in (Cd,Mn)Te quantum wells

We report optical investigations of Mn 2+ spin resonance in (Cd,Mn)Te dilute magnetic semiconductor quantum wells. Raman scattering spin–flip measurements and optically detected magnetic resonance were performed on these samples. Both methods show features suggestive of Mn ion correlations—multiple Raman spin flips are observed in both Faraday and Voigt configurations. An unexpected multiplet of hyperfine split lines is also observed in the magnetic resonance spectra.

Non-equilibrium dynamical behavior of exciton/biexciton in a diluted magnetic semiconductor asymmetric quantum well

Optical properties of a (Cd,Mg)Te/CdTe/(Cd,Mn)Te quantum well is investigated systematically using photoluminescence (PL) and time-resolved photoluminescence (TRPL). The PL intensity ratio of biexciton to exciton showed a linear dependence. The biexciton TRPL showed a longer lifetime as compared with that of the exciton. These results suggested an important role of dark excitons and their spin scattering by Mn localized spins in this system. The rate equations based on our model taking account of the dark exciton level ( J=±2) were used to analyze the time decay of the exciton and biexciton TRPL in various temperatures and excitation densities. Our analysis leads to a conclusion that the new channel from a dark exciton has an essential role when biexcitons are created in dilute magnetic semiconductor quantum wells containing Mn in the barrier.