Highly Efficient Room-Temperature Electron-Photon Spin Conversion Using a Semiconductor Hybrid Nanosystem with Gradual Quantum Dimensionality Reduction

Improved electron-photon spin conversion efficiency is a key component of technological platforms for optospintronics integration in information processing; this concept is based on optical devices transmitting and receiving spin information superimposed on light. Semiconductor quantum dots (QDs) are the most promising materials for optospintronic devices; however, in addition to their weak room-temperature luminescence, their electron-photon spin conversion efficiencies are lower than 50%. Here, we present semiconductor QDs embedded in quantum wells (QWs) containing quasi-QDs. The proposed semiconductor hybrid nanosystem with gradual quantum dimensionality reduction demonstrates luminescence one order of magnitude stronger than that of conventional QDs and an electron-photon spin conversion efficiency of almost 80% at room temperature. Optical characterization reveals that efficient carrier capture, suppressed depolarized-spin reinjection, and quasi-three-dimensional quantum confinements in the QWs facilitate the highly efficient electron-photon spin conversion. This study constitutes a significant advance towards the realization of QD-based spin-functional optical devices for electron-spin-based quantum information platforms.

Figure 1

Structural characterization of the DIW containing quasi-QD. (a),(b) HAADF STEM images and EDX elemental maps of (c),(d) $\mathrm{In}$ and (e),(f) $\mathrm{Al}$ for DIW-30 nm and DIW-15 nm, respectively. (g) Schematic illustration of the conduction band profile in the semiconductor hybrid nanosystem with gradual quantum dimensionality reduction. The highly efficient electron-photon spin conversion achieved in the proposed nanosystem is attributed to blocked reinjection of the depolarized spins from the adjacent QW (blue dashed arrow) due to efficient radiative recombination, along with suppressed relaxation of the reinjected electron spins (black solid arrow) due to the quasi-three-dimensional quantum confinements at the quasi-QD state. (h) Schematic illustration of the DIW structure indicating wide $\mathrm{In}$-atom distribution around QDs. An intermediate $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ layer corresponding to a quasi-QD with quasi-three-dimensional quantum confinements is formed within the $\mathrm{Ga}\mathrm{As}$ QW.

Figure 2

Contour maps of the temperature-dependent PL intensity as a function of photon energy for (a) the reference QD, (b) DIW-30 nm, and (c) DIW-15 nm. The energy regions surrounded by the white dashed lines indicate the ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ QD ES focused on in this study and the $\mathrm{Ga}\mathrm{As}$ barrier or QW states, which are shifted with changing temperature in accordance with Varshni’s law with the appropriate parameter for ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ and $\mathrm{Ga}\mathrm{As}$ [41]. (d) PL peak energies with the predicted temperature-dependent energy shift by Varshni’s law (dashed lines), (e) integrated PL intensities of QD ESs with Arrhenius fits (solid lines), and (f) PL intensity ratios of $\mathrm{Ga}\mathrm{As}$ to QD ES as functions of temperature. For the reference QD, the PL intensity ratio at temperatures above 200 K is not plotted because the PL intensity is weak under those conditions.

Figure 3

Contour maps of temperature-dependent $P_e$ as a function of photon energy for (a) the reference QD, (b) DIW-30 nm, and (c) DIW-15 nm. The energy regions surrounded by the white dashed lines indicate the ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ QD ES focused on in this study, which is shifted with changing temperature in accordance with Varshni’s law with the appropriate parameter for ${\mathrm{In}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{As}$ [41]. The areas surrounded by the yellow dashed lines show the temperature and energy ranges where the PL intensities larger than 1200 are obtained in the PL intensity maps of Figs. 2–2. (d) Averaged $P_e$ values of QD ESs as functions of temperature. (e) Circularly polarized time-integrated PL spectra and corresponding $P_e$ values of DIW-30 nm (left) and DIW-15 nm (right), measured at 293 K. For comparison, the corresponding $P_e$ of the reference QD measured at 230 K is indicated by the gray symbols.

Figure 4

(a) Circularly polarized PL time profiles and corresponding $P_e$ values of QD ESs for DIW-30 nm (left) and DIW-15 nm (right), measured at 293 K. The sum of ${\sigma }^{\pm }$-polarized PL intensity as a function of time with the single-exponential decay fittings (black solid lines) is also shown. For comparison, the corresponding $P_e$ of the reference QD measured at 230 K is indicated by the gray symbols. The gray and green solid lines show the single-exponential decay fittings for the time-dependent CPD. (b) Radiative lifetimes (decay time constant of the sum of ${\sigma }^{\pm }$-polarized PL intensity) and (c) electron-spin relaxation times as functions of temperature. The inset in (b) shows the suppressed reinjection of electron spins from the QW due to the efficient radiative recombination there. The gray, cyan, and magenta solid lines in (c) show the $T^{-1.8}$, $T^{-1.2}$, and $T^{-0.8}$ dependencies, respectively. (d) Circularly polarized PL time profiles and corresponding $P_e$ values of the $\mathrm{Ga}\mathrm{As}$ QW, measured at 293 K. The green solid lines show the single-exponential decay fittings for the time-dependent CPD. (e) Electron-photon spin conversion efficiencies as functions of temperature.