Electrical detection of light helicity using a quantum-dot-based hybrid device at zero magnetic field

Photon helicity-dependent photocurrent is measured at zero magnefic field thanks to a device based on an ensemble of (In,Ga)As/GaAs quantum dots that are embedded into a GaAs-based $p\text{−}i\text{−}n$ diode. Our main goal is to take advantage of the long electron spin-relaxation time expected in these nano-objects. In these experiments, no external magnetic field is required thanks to the use of an ultrathin magnetic CoFeB/MgO electrode, presenting perpendicular magnetic anisotropy. We observe a clear asymmetry of the photocurrent measured under respective right and left polarized light that follows the hysteresis of the magnetic layer. The amplitude of this asymmetry at zero magnetic field decreases with increasing temperatures and can be controlled with the bias. Polarization-resolved photoluminescence is detected in parallel while the device is operated as a photodetector. This demonstrates the multifunctional capabilities of the device and gives valuable insights into the spin relaxation of the electrons in the quantum dots.

Figure 1

Schematics of measurement setup. Spin light-emitting-diode device that can be operated either in LED mode when supplied by a voltage generator or in the photocurrent mode when excited by a circularly polarized laser. Polarization-resolved photoluminescence is detected in parallel while the device is operated in the photocurrent mode.

Figure 2

(a) AFM image of the (In,Ga)As QD layer without capping with GaAs layer. The (In,Ga)As QDs have a density of $1.6\times {10}^{10}\mathrm{c}{\mathrm{m}}^{-2}$. The QDs have an average diameter of about 30 nm and height of about 9 nm. (b) STEM large-scale annular dark-field (ADF) image showing three planes of QDs and homogeneous FM/SC interface. (c) STEM bright-field (BF) image showing the stack structure of spin detector. (d) EELS map showing the chemical component variation in three QD planes.

Figure 3

EL circular polarization as a function of external magnetic field. $T=15\mathrm{K}$. $I=80\mu \mathrm{A}$. Inset. Left axis: electroluminescence spectra of the ensemble of quantum dots. ${\sigma }^{+}$ and ${\sigma }^{–}$ are the right and left components (respectively). $B=0\mathrm{T}$. $T=15\mathrm{K}$. Right axis: Corresponding circular polarization $P_{\mathrm{c}}$.

Figure 4

(a) $I-V$ characteristics of the device at different temperatures. (b) Photocurrent (black squares, left axis) and photoluminescence signals (open red circles, right axis) as a function of bias. ${\lambda }_{\mathrm{laser}}=850\mathrm{nm}$. The gray zone indicates the bias window for which the photocurrent asymmetry $A$ displayed in (e) is maximal. (c) Circular polarization of the photoluminescence as a function of the bias. ${\lambda }_{\mathrm{laser}}=850\mathrm{nm}$. $T=10\mathrm{K}$. $B=0\mathrm{T}$. Inset: σ+ (black line) and σ− (red line) components of the luminescence for $V=+1\mathrm{V}$ and −0.7 V. (d) Photocurrent helicity asymmetry $A$ as a function of the external magnetic field. ${\lambda }_{\mathrm{laser}}=910\mathrm{nm}, V=+0.4\mathrm{V}, T=30\mathrm{K}$. (e) Photocurrent helicity asymmetry $A$ as a function of applied bias at zero external magnetic field. ${\lambda }_{\mathrm{laser}}=850\mathrm{nm}$. $T=30\mathrm{K}$. (f) Photocurrent helicity asymmetry $A$ (black squares, left axis) as a function of temperature at fixed bias. ${\lambda }_{\mathrm{laser}}=850\mathrm{nm}, V=+0.4\mathrm{V}, B=0\mathrm{T}$. PL circular polarization (red triangles, right axis). ${\lambda }_{\mathrm{laser}}=850\mathrm{nm}, V=0\mathrm{V}, B=0\mathrm{T}$. EL circular polarization (open blue squares, right axis) as a function of temperature. $V=+2.0\mathrm{V}, I=54\mu \mathrm{A}, B=0\mathrm{T}$.

Figure 5

Energy band diagram simulations using a one-dimensional Poisson-Schrödinger solver for the forward and reverse bias conditions at 10 K.