Persistent High Polarization of Excited Spin Ensembles During Light Emission in Semiconductor Quantum-Dot-Well Hybrid Nanosystems

Kazuki Takeishi, Satoshi Hiura, Junichi Takayama, Kodai Itabashi, Masayuki Urabe, Akihiro Washida, Takayuki Kiba, and Akihiro Murayama

Phys. Rev. Applied 10, 034015 – Published 10 September 2018

Abstract

We demonstrate persistent high degrees of spin polarization (SPD) up to 70% during light emission in $({In}_{1 - x} {Ga}_{x}) As$ quantum-dot–well (QD–QW) hybrid nanosystems, where QD excited states are laterally tunnel coupled through the adjacent two-dimensional QW potential depending on the QW thickness. Spin-polarized electrons are photo-excited by using circularly polarized light pulses. The decay time of spin relaxation, obtained by that of the SPD, is 70 times larger than the photoluminescence decay time. The temporally constant SPD is sustained by a selective transfer of minority spins among QDs after flipping from the majority spins, which is promoted by a moderate state filling of lower-energy spin sublevels in surrounding QDs. The spin transfer times are deduced as functions of the QW thickness and excited-spin density. These results can provide a precise control of lateral interdot spin-transfer dynamics and resultant suppression of spin relaxation in QD ensembles.

Received 5 March 2018
Revised 2 July 2018

DOI:https://doi.org/10.1103/PhysRevApplied.10.034015

Physics Subject Headings (PhySH)

Optoelectronics Spin dynamics Spin polarization Spintronics

Devices III-V semiconductors Quantum dots Quantum wells

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Kazuki Takeishi¹, Satoshi Hiura¹, Junichi Takayama¹, Kodai Itabashi¹, Masayuki Urabe¹, Akihiro Washida¹, Takayuki Kiba², and Akihiro Murayama^1,*

¹Graduate School of Information Science and Technology, Hokkaido University, Kita 14, Nishi 9, Kita-ku, Sapporo 060-0814, Japan
²Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan

^*murayama@ist.hokudai.ac.jp

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Vol. 10, Iss. 3 — September 2018

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Images

Figure 1
(a) AFM image of the ${In}_{0.5} {Ga}_{0.5} As$ QDs. (b) 2D FFT image from the AFM image in (a). (c) Cross-sectional HAADF-STEM lattice image of the ${In}_{0.5} {Ga}_{0.5} As$ QD in the QD-QW hybrid nanostructure, where the degree of brightness reflects the In concentration. (d) Cross-sectional TEM image of the QD-QW hybrid nanostructure with a 6-nm-thick $Ga As$ barrier and 20-nm-thick QW, where the dashed white lines are introduced to help the visualization. (e) Schematic drawing of the sample structure. (f) Schematic band alignment along the thickness direction and the resultant energy levels of an electron and a hole in the coupled QD-QW structure with a QW thickness of 20 nm. 3D spatial maps of the existence probabilities of electron wavefunction at the first excited state (ES) between two neighboring QDs, (g) without a QW, and with (h) 6-nm-, (i) 10-nm-, and (j) 20-nm-thick QWs.
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Figure 2
Circularly polarized PL spectra and corresponding CPD as a function of the photon energy at 6 K from the QD-QW hybrid nanosystems with (a) 6-nm-, (b) 10-nm-, and (c) 20-nm-thick QWs at an excitation power of 2.5 mW, where the spectral peak energy of the QD ground state (GS) is indicated by the black arrows. Circularly polarized transient PL and corresponding CPD as a function of the time in the samples with (d) 6-nm-, (e) 10-nm-, and (f) 20-nm-thick QWs, measured at detection energy windows corresponding to the ESs in the QD ensembles, outlined by the black dashed lines in (a)–(c).
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Figure 3
Time constants (a) τ_CPD, (b) $τ_{CPD} / τ_{PL}^{σ +}$ , and (c) $τ_{PL}^{σ \pm}$ of the circularly polarized QD ES PL, where τ_CPD is the CPD decay time and $τ_{PL}^{σ \pm}$ are the PL decay times of the σ^± polarizations, as a function of the QW thickness t.
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Figure 4
(a) Spin-split rate equation model including electron-spin tunneling through coupling of electron wavefunctions in ESs and subsequent energy relaxation into a lower-energy ground state in a single QD, where the ESs detected by the time-resolved PL is expressed as first ES, while the ground state is expressed as GS (left-hand side). A schematic illustration of this model is also shown at the right-hand side. Simulation results for the transient CPD of the first ES for various (b) $τ_{tr}^{eff}$ with a fixed parameter $N_{b} / D_{QD 2}^{σ^{\pm}} = 3$ , and (c) $N_{b} / D_{QD 2}^{σ^{\pm}}$ with $τ_{tr}^{eff} = 50 ps$ .
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Figure 5
Circularly polarized time-resolved PL and corresponding CPD, best fitted by rate equations (solid lines) for the samples with a 20-nm-thick QW at an excitation power of (a) 2.5, (b) 7.5, and (c) 10 mW. The results at an excitation power of 3 mW for the samples with QW thicknesses t of (d) 6 , (e) 10 , and (f) 20 nm are shown.
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Figure 6
Effective spin transfer time $τ_{tr}^{eff}$ as a function of the excitation power for various QW thicknesses t = 6 nm (green squares), 10 nm (blue triangles), and 20 nm (red circles).
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