Bias Dependence of the Electrical Spin Injection into GaAs from $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}$ Injectors with Different MgO Growth Processes

We investigate the influence of the MgO growth process on the bias dependence of the electrical spin injection from a $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}$ spin injector into a GaAs-based light-emitting diode (spin LED). With this aim, textured MgO tunnel barriers are fabricated either by sputtering or molecular-beam-epitaxy (MBE) methods. For the given growth parameters used for the two techniques, we observe that the circular polarization of the electroluminescence emitted by spin LEDs is rather stable as a function of the injected current or applied bias for the samples with sputtered tunnel barriers, whereas the corresponding circular polarization decreases abruptly for tunnel barriers grown by MBE. We attribute these different behaviors to the different kinetic energies of the injected carriers linked to differing amplitudes of the parasitic hole current flowing from GaAs to Co-Fe-B in both cases.

Figure 1

(a) Schematics of the spin-LED structure with a single $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{GaAs}$ quantum well. (b) TEM cross section of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}/\mathrm{GaAs}$ interfaces for (left panel) a sputtering sample and (right panel) a MBE sample. (c) Polarized electroluminescence spectra ($I^{+}$ and $I^{-}$) for the sputtering sample ($T_{\mathrm{an}}=300°\mathrm{C}$) at $T=25\mathrm{K}$ for $B=0.8\mathrm{T}$. (d) EL circular polarization as a function of the applied longitudinal magnetic field for the same sample used in (c).

Figure 2

(a) EL circular polarization as a function of the current for sputtering and MBE samples annealed at $300°\mathrm{C}$. (Inset) EL circular polarization as a function of the current for the sputtering sample for different annealing temperature $T_{\mathrm{an}}$. (b) EL circular polarization as a function of the applied bias. (Inset) $I\text{−}{V}_{\mathrm{bias}}$ curves for both samples. $T=25\mathrm{K}$, $B=0.8\mathrm{T}$.

Figure 3

Spin LEDs annealed at $300°\mathrm{C}$. Central wavelength of the EL emission as a function of the injected current for a sputtering sample (the black spheres) and a MBE sample (the red spheres). The spectral resolution is 1 nm. (Top-left inset) EL spectra as a function of current for the sputtering sample. (Top-right inset) EL spectra as a function of current for the MBE sample.

Figure 4

Band structure calculations using a one-dimensional Poisson-Schrödinger solver [39] at 25 K. (a) Conduction band as a function of $z$ (the growth axis) under applied bias. (b) The same physical quantity with an enlargement of the first 250 Å. (c) Valence band as a function of $z$ under applied bias with an enlargement of the first 250 Å. The open circles represent the hole accumulation.

Figure 5

(a) EL intensity as a function of the current for a sputtering sample (the black spheres) and a MBE sample (the red spheres) in the low current regime ($\le 1\mathrm{mA}$). (b) Scheme of spin injection with parasitic current; see the text. (c) EL intensity (log scale) as a function of the current for the sputtering sample (the black spheres) and the MBE sample (the red spheres). (d) Figure of merit: EL circular polarization versus EL intensity for the sputtering sample (the black spheres) and the MBE sample (the red spheres). (e) $1/C^2$ curves as a function of $V$ extracted from the high-frequency (1 MHz) $CV$ curves measured at room temperature for $\mathrm{Au}/\mathrm{MgO}(2.5\mathrm{nm})/n\text{−}\mathrm{GaAs}(001)$ samples annealed at $300°\mathrm{C}$ (the contact diameter is $300\mu \mathrm{m}$), where the MgO layers are deposited either by sputtering (the black line) or by MBE (the red line). The vertical arrows indicate the intercept voltage $V_I$ for the sputtering (black) and MBE (red) samples. (Inset) Band scheme at the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}/\mathrm{MgO}/\mathrm{GaAs}$ interface; see the text. The spin LEDs are annealed at $300°\mathrm{C}$. $T=25\mathrm{K}$.