Limiting factor of defect-engineered spin-filtering effect at room temperature

Y. Puttisong, I. A. Buyanova, and W. M. Chen

Phys. Rev. B 89, 195412 – Published 12 May 2014

Abstract

We identify hyperfine-induced electron and nuclear spin cross-relaxation as the dominant physical mechanism for the longitudinal electron spin relaxation time $(T_{1})$ of the spin-filtering $G a_{i}^{2 +}$ defects in GaNAs alloys. This conclusion is based on our experimental findings that $T_{1}$ is insensitive to temperature over 4–300 K, and its exact value is directly correlated with the hyperfine coupling strength of the defects that varies between different configurations of the $G a_{i}^{2 +}$ defects present in the alloys. These results thus provide a guideline for further improvements of the spin-filtering efficiency by optimizing growth and processing conditions to preferably incorporate the $G a_{i}^{2 +}$ defects with a weak hyperfine interaction and by searching for new spin-filtering defects with zero nuclear spin.

Received 3 February 2014
Revised 25 April 2014

DOI:https://doi.org/10.1103/PhysRevB.89.195412

Authors & Affiliations

Y. Puttisong, I. A. Buyanova, and W. M. Chen

Department of Physics, Chemistry and Biology, Linköping University, S-581 83 Linköping, Sweden

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Issue

Vol. 89, Iss. 19 — 15 May 2014

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Images

Figure 1
(a) Atomic structure of a $G a_{i}^{}$ defect in GaNAs, for example, a $G a_{i}^{}$ residing on the T $_{d}$ site surrounded by the group-III sublattice. The exact atomic structures of the spin-filtering $G a_{i}^{}$ defects studied in this work are still unknown in terms of their exact locations and their neighboring atoms. Therefore, the position of the N atom is for illustrative purposes and does not indicate its actual location. (b) Schematic illustration of the defect-engineered spin-filtering effect via SDR under $σ^{-}$ excitation. The band-to-band optical transition should exhibit strong optical polarization, reflecting spin polarization of CB electrons, and stronger intensity as compared with the case (c) under $σ^{X}$ excitation without the spin-filtering effect.
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Figure 2
(a) A schematic picture of the Hanle measurements performed in this work. Hanle curves obtained at RT by monitoring $P_{P L}$ (b) and SDR ratio $I^{σ^{-}} / I^{σ^{X}}$ (c). The solid lines are the fitting curves assuming two (b) or one (c) Lorentzian line(s), with the former being a sum of the Hanle curves for CB electrons (denoted by the thin broad line) and the defect electrons (the thin narrow line). (d) $T_{S C}^{eff}$ as a function of optical excitation power, which approaches $T_{1}^{eff}$ at the lowest excitation power.
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Figure 3
(a), (b) Hanle curves (open circles) obtained from Sample V at 300 K and 4 K by monitoring $I^{σ^{-}} / I^{σ^{X}}$ . The solid lines are fitting curves assuming a Lorentzian lineshape. (c) $1 / T_{1}^{eff}$ as a function of measurement temperature. The error bars are deduced from the uncertainty in the fitting procedure. All the measurements were done under very weak optical excitation.
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Figure 4
(a), (b) Hanle curves (open circles) obtained at RT from Sample I and II. The solid lines are the simulation curves obtained by a best fit of the coupled rate equations to the experimental data, with the specified fitting parameters of $T_{1}^{x}$ . $T_{1}^{eff}$ is estimated from the effective Hanle width. (c), (d) ODMR spectra of the $G a_{i}^{2 +}$ defects obtained at 4 K from Samples I and II. The solid lines are the simulation curves obtained by a best fit of the spin Hamiltonian Eq. (1) to the experimental data. The involved configurations of the $G a_{i}^{2 +}$ defects are given in (c) and (d), together with the degrees of their contributions and the resulting $〈 A_{eff} 〉$ . The microwave frequencies used in ODMR are 33.92 GHz in (c) and 9.27 GHz in (d). (e) ${〈 A_{eff} 〉}^{2}$ as a function of $1 / T_{1}^{eff}$ , with each symbol representing a specific sample. The solid line is a linear fitting following the Fermi-golden rule. The dotted lines mark the associated parameters of each $G a_{i}^{2 +}$ configuration, obtained from the rate equation and spin Hamiltonian analysis of the experimental data.
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Figure 5
Spin polarization degree of CB electrons $(P_{e})$ as a function of the HFI parameter A, calculated by the coupled rate equation analysis described in Eq. (2). The simulations are performed with 5% of initial spin polarization of CB electrons ( $P_{e}^{i}$ ), generated by optical orientation before the defect-enabled spin filtering takes effect, and two values of the defect concentration $(N_{C}),$ as examples.
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