The manipulation of the spin degree of freedom is highly sought after in the field of spintronics. This study looks at the emergence of Rashba physics in group IV materials, such as p-i-n diodes that contain Ge quantum wells and Si0.15Ge0.85barriers. By using optical spin orientation, it was found that the circular polarization degree of the direct emission can be increased by increasing the power of the optical pump, while the device remains unbiased. This is attributed to the optical-induced changes in the built-in Rashba field due to the asymmetric doping of the diode structure. These findings can provide a new way to fine-tune the material properties for spin quantum electronic and optical applications.
We have measured a helicity-dependent photocurrent at zero external magnetic field in a device based on a semiconductor quantum well embedded in a p-i-n junction. The device is excited under vertical incidence with circularly polarized light. The spin filtering effect is evidenced in the range 77–300 K at B = 0 owing to a CoFeB/MgO spin filter with out-of-plane magnetization in remanence. The helicity-dependent photocurrent is explored as a function of the temperature and bias. These characteristics are compared with those of a spin photocurrent device with in-plane magnetized CoFeB/MgO spin filter, excited under oblique incidence with circularly polarized light. In contrast to the in-plane spin filter device, the circularly polarized light asymmetry of the photocurrent in the out-of-plane device depends weakly on the external bias. This may be attributed to the large spin relaxation anisotropy in III-V zinc-blende quantum wells in the presence of a vertical electric field.
Henri Jaffrès, Jean-Marie George, Shiheng Liang, Huaiwen Yang, Bingshan Tao, Stefan McMurtry, Sébastien Petit Watelot, Stephane Mangin, Pierre Renucci, Xavier Marie, Yuan Lu
KEYWORDS: Molybdenum, Semiconductors, Spintronics, Magnetic semiconductors, Interfaces, Resistance, Semiconductor materials, Nanoelectronics, Optoelectronics, Electron transport
Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nano-electronic, opto-electronic and spintronic applications. However, the demonstration of an electron spin transport through a semiconducting MoS2 channel remains challenging. Here we show the evidence of the electrical spin injection and detection in the conduction band of a multilayer MoS2 semiconducting channel using a two-terminal spin-valve configuration geometry. A magnetoresistance around 1% has been observed through a 450 nm long, 6 monolayer thick MoS2 channel with a Co/MgO tunnelling spin injector and detector [1]. It is found that keeping a good balance between the interface resistance and channel resistance is mandatory for the observation of the two-terminal magnetoresistance. Moreover, the electron spin-relaxation is found to be greatly suppressed in the multilayer MoS2 channel with an in-plane spin polarization. The long spin diffusion length (approximately 235 nm) could open a new avenue for spintronic applications using multilayer transition metal dichalcogenides. In this talk, we will present the main issues of the spin-injection problem at ferromagnet/Tunnel barrier/MoS2 interfaces as well as the spin-propagation in multilayered channel like played by MoS2 involving Schottky depletion layers giving thus an extension to earlier modeling of spin-injection in ferromagnet/ssource/ferromagnet systems [2-3].
[1] Shiheng Liang et al., Nat. Comm. 8, 14947 (2017)
[2] A. Fert, H. Jaffrès, Phys Rev B64, 184420, 2001
[3] A. Fert, J.-M. George, H. Jaffrès, R. Mattana, IEEE, 54, 921-932 (2007)
The hexagonal close-packed (hcp) Co(0001)/MoS2 and face-centered cubic (fcc) Ni(111)/WSe2 interface atomic, magnetic and electronic structures are investigated using first-principles methods based on the density functional theory. We show that the MoS2 and WSe2 single layers are covalently bond to the Co(0001) and Ni(111) metal surfaces. We describe the consequences of this bonding on the spin magnetic moments and on the electron states at the vicinity of these interfaces, where MoS2 and WSe2 become metallic due to hybridization between Co (or Ni) and S (or Se) atomic orbitals. A finite spin-polarization at the Fermi level is calculated in the MoS2 and WSe2 layers at these two interfaces. We also give and estimation of the Schottky barrier height that may appear at the border between the metallic and semiconducting phases of MoS2 (or WSe2) near the edge of a Co/MoS2 or Ni/WSe2 metallic contact.
In this paper, we demonstrate a very efficient electrical spin injection into an ensemble of InAs/InGaAs quantum dots at zero magnetic field. The circular polarization of the electroluminescence coming from the dots, which are embedded into a GaAs-based Spin Light Emitting diode reaches a value as large as 20% at low temperature. In this device, no external magnetic field is required in order to inject or read spin polarization thanks to the use of an ultrathin CoFeB electrode (1.1 nm), as well as p-doped quantum dots (with one hole per dot in average) as an optical probe. The electroluminescence circular polarization of the dots follows the hysteresis loop of the magnetic layer and decreases as a function of bias for large voltages. In a reverse way, we have also investigated the possibility to use such a device as a photodetector presenting a photon helicity-dependent photocurrent. We observe a weak asymmetry of photocurrent under right and left polarized light that follows the hysteresis cycle of the magnetic layer, and the effect decreases for increasing temperatures and can be controlled by the bias.
The spectacular progress in controlling the electronic properties of graphene has triggered research in alternative atomically thin two-dimensional crystals. Monolayers (ML) of transition-metal dichalcogenides such as MoS2 have emerged as very promising nanostructures for optical and spintronics applications. Inversion symmetry breaking together with the large spin-orbit interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (σ+ or σ−) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two nonequivalent K valleys (K+ or K−, respectively).
We have investigated the spin and valley properties for both neutral and charged excitons in transition metal dichalcogenide monolayer MoS2, MoSe2 and WSe2 with cw and time-resolved polarized photoluminescence spectroscopy [1,2]. The key role played by exciton exchange interaction will be presented [3]. We also demonstrate that the optical alignment of excitons (“exciton valley coherence”) can be achieved following one or two photon excitation [1,4].
Finally recent results on magneto-photoluminescence spectroscopy on MoSe2 and WSe2 in Faraday configuration up to 9 T will be presented; the results will be discussed in the framework of a k.p theory [5].
[1] G. Wang et al, PRL 114, 97403 (2015)
[2] G. Wang et al, Nature Com. 6, 10110 (2015)
[3] J. P. Echeverry, ArXiv 1601.07351 (2016)
[4] G. Wang et al, PRL 115, 117401 (2015)
[5] G. Wang et al, 2D Mat. 2, 34002 (2015)
We report on optical orientation experiments in GaAs epilayers with excitation energies in the 3 eV region,
leading the photo-generation of spin-polarized electrons in the satellite L valley. From both continuous-wave and
time resolved measurements we show that a significant fraction of the electron spin memory can be conserved
when the electron is scattered from the L to the Γ valley following an energy relaxation of several hundreds
of meV. A typical L-valley electron spin relaxation time of 200 fs is deduced, in agreement with theoretical
calculations.
Lattice-matched GaP-based nanostructures grown on silicon substrates is a highly rewarded route for coherent
integration of photonics and high-efficiency photovoltaic devices onto silicon substrates. We report on the structural and
optical properties of selected MBE-grown nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a
first stumbling block, the GaP/Si interface growth has been optimised thanks to a complementary set of thorough
structural analyses. Photoluminescence and time-resolved photoluminescence studies of self-assembled (In,Ga)As
quantum dots grown on GaP substrate demonstrate a proximity of two different types of optical transitions interpreted as
a competition between conduction band states in X and Γ valleys. Structural properties and optical studies of
GaAsP(N)/GaP(N) quantum wells coherently grown on GaP substrates and GaP/Si pseudo substrates are reported. Our
results are found to be suitable for light emission applications in the datacom segment. Then, possible routes are drawn
for larger wavelengths applications, in order to address the chip-to-chip and within-a-chip optical interconnects and the
optical telecom segments. Finally, results on GaAsPN/GaP heterostructures and diodes, suitable for PV applications are
reported.
The photocurrent obtained under polarized optical excitation and the polarized electroluminescence recorded under
forward electric bias have been measured in the same hybrid Semiconductor/Ferromagnetic metal structures (Spin-Light
Emitting Diode). Systematic investigations have been performed on devices with different ferromagnetic spin injectors,
consisting e.g. of MgO tunnel barriers with a CoFeB ferromagnetic layer. Though a very efficient electrical spin
injection is demonstrated, very weak polarization of the photocurrent is evidenced: the photocurrent polarization
measured under continuous resonant circularly polarized excitation of the quantum well excitons is below 3%. This
demonstrates that the investigated devices do not act as efficient spin filters for the electrons flowing from the
semiconductor part to the ferromagnetic part of these structures though these systems are very efficient spin aligners for
electrical spin injection. We interpret the weak measured photocurrent polarization of the as a consequence of the
Zeeman splitting of the quantum well excitons which yields different absorption coefficients for the polarized excitation
laser with different helicities. This leads to different intensities of photocurrent collected for the two different circularly
polarized excitations. This interpretation is confirmed by an experiment exhibiting the same results for photocurrent
measured on a device with a non ferromagnetic electrical contact.
Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence
experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged
exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.
The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently
controlled with resonant laser pulses. Long coherence times allow the observation of Rabi-flopping of a single
dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole
moment and the excitation laser power. We report on the robust preparation of a quantum state using an optical
technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser
pulses whose frequency is swept through the resonance.
Thanks to optimized growth techniques, a high density of uniformly sized InAs quantum dots (QD) can be grown on
InP(113)B substrates. Low threshold currents obtained at 1.54 μm for broad area lasers are promising for the future. This
paper is a review of the recent progress toward the understanding of electronic properties, carrier dynamics and device
modelling in this system, taking into account materials and nanostructures properties. A first complete analysis of the
carrier dynamics is done by combining time-resolved photoluminescence experiments and a dynamic three-level model,
for the QD ground state (GS), the QD excited state (ES) and the wetting layer/barrier (WL). Auger coefficients for the
intradot assisted relaxation are determined. GS saturation is also introduced. The observed double laser emission for a
particular cavity length is explained by adding photon populations in the cavity with ES and GS resonant energies. Direct
carrier injection from the WL to the GS related to the weak carrier confinement in the QD is evidenced. In a final step,
this model is extended to QD GS and ES inhomogeneous broadening by adding multipopulation rate equations
(MPREM). The model is now able to reproduce the spectral behavior in InAs-InP QD lasers. The almost continuous
transition from the GS to the ES as a function of cavity length is then attributed to the large QD GS inhomogeneous
broadening comparable to the GS-ES lasing energy difference. Gain compression and Auger effects on the GS transition
are also be discussed.
An electron spin confined to a semiconductor quantum dot is not subject to the classical spin relaxation mechanisms
known for free carriers but it strongly interacts with the nuclear spin system via the hyperfine interaction.
We show in time resolved photoluminescence spectroscopy experiments on ensembles of self assembled InAs
quantum dots in GaAs that this interaction leads to strong electron spin dephasing. By analysing the polarization
state of photons absorbed or emitted by individual dots we show how optical pumping of electron spins
leads in turn to a strong nuclear polarisation that can be measured via a drastic change in the Zeeman splitting
in magneto-photoluminescence experiments.
We have investigated the electron and hole spin dynamics in p-doped semiconductor InAs/GaAs quantum dots by time resolved photoluminescence. We observe a decay of the average electron spin polarisation down to 1/3 of its initial value with a characteristic time of TΔ ≈ 500ps. We attribute this decay to the hyperfine interaction of the electron spin with randomly orientated nuclear spins. Magnetic field dependent studies reveal that this efficient spin relaxation mechanism can be suppressed by a field in the order of 100mT. In pump-probe like experiments we demonstrate that the resident hole spin, "written" with a first pulse, remains stable long enough to be "read" 15ns later with a second pulse.
The ground and excited state luminescent transitions in self-organized InAs/InP(001) quantum islands (QIs) grown in two different matrices (In0.52Al0.48As and InP), have been studied by cw photoluminescence (PL) and time resolved photoluminescence (TRPL). PL excitation (PLE) shows that the multi-component PL spectrum measured for the InAs/InAlAs QIs is associated to ground and related excited state transitions of QIs having monolayer-height fluctuation whereas for InAs/InP QIs the multi-component PL spectrum is only due to one ground state and their related excited states. This attribution is confirmed by the recombination life times measured by TRPL which are in the 1.2-1.4 ns range for the ground state transitions and in the 90-600 ps range for the excited state transitions.
We report on the dynamics of resonantly excited polariton states in semiconductor quantum microcavities. We show that the spin orientation of cavity polaritons can be coherently manipulated. The measurements of the optical dephasing time and the decay time of the radiant states as a function of the cavity detuning and the lattice temperature give further insights into the relaxation mechanisms of cavity polaritons. The influence of the polariton lifetime on the optical dephasing time is demonstrated. For zero detuning, a quenching of the polariton-acoustic phonon scattering is evidenced at low temperature.
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