Enhancement of the Spin Accumulation at the Interface between a Spin-Polarized Tunnel Junction and a Semiconductor

We report on spin injection experiments at a $\mathrm{Co}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{GaAs}$ interface with electrical detection. The application of a transverse magnetic field induces a large voltage drop $\Delta V$ at the interface as high as 1.2 mV for a current density of $0.34\mathrm{nA}·\mu {\mathrm{m}}^{-2}$. This represents a dramatic increase of the spin accumulation signal, well above the theoretical predictions for spin injection through a ferromagnet/semiconductor interface. Such an enhancement is consistent with a sequential tunneling process via localized states located in the vicinity of the ${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{GaAs}$ interface. For spin-polarized carriers these states act as an accumulation layer where the spin lifetime is large. A model taking into account the spin lifetime and the escape tunneling time for carriers traveling back into the ferromagnetic contact reproduces accurately the experimental results.

Figure 1

(a) Diagram of the structure described in the text. The region within the dashed line represents the depletion layer. (b) Current vs bias curve of a typical $15\times 196\mu {\mathrm{m}}^2$ $\mathrm{Co}/{\mathrm{Al}}_2{\mathrm{O}}_3$ interface. (c) $RA$ product vs bias measured at the same contact.

Figure 2

Voltage drop $\Delta V$ vs transverse magnetic field measured at 10 K on a typical contact (size $15\times 196\mu {\mathrm{m}}^2$, bias of $V=-430\mathrm{mV}$ and current $I=-1\mu \mathrm{A}$). We have substracted a small positive quadratic background to the bare signal corresponding to the intrinsic tunneling anisotropic magnetoresistance of the ${\mathrm{Al}}_2{\mathrm{O}}_3$ barrier. The red line corresponds to a typical fit obtained with Eq. (3) for ${\tau }_{sf}^{*}=230\mathrm{ps}$ and ${\tau }_n=15\mathrm{ns}$. Left inset: bias dependence of $\Delta V$ for 2 typical $\mathrm{Co}/{\mathrm{Al}}_2{\mathrm{O}}_3$ electrodes (width 6 and $15\mu \mathrm{m}$ and similar $RA$). Right inset: sketch of the model described in the discussion.

Figure 3

Bias dependence of (a) the MR for 2 typical $\mathrm{Co}/{\mathrm{Al}}_2{\mathrm{O}}_3$ interfaces with different sizes $6\times 196\mu {\mathrm{m}}^2$ (squares, black) and $15\times 196\mu {\mathrm{m}}^2$ (triangles, red) and (b) the spin lifetime ${\tau }_{sf}^{*}$ and tunneling escape time ${\tau }_n$ for the $6\times 196\mu {\mathrm{m}}^2$ junction. The dashed red line is an extrapolation of the behavior of ${\tau }_n$. Figure 3a insets: band bendings A, B, and C corresponding to different applied bias and described in text.

Figure 4

Temperature dependence of the MR ($\Delta V/V$) acquired at $V=-0.3\mathrm{V}$ on a $30\times 196\mu {\mathrm{m}}^2$ contact. The $RA$ product is 1.5 times larger than the 2 junctions studied in Fig. 3a. Inset: ${\tau }_{sf}^{*}$ and ${\tau }_n$ as a function of temperature and as determined by fitting the $\Delta V/V$ vs $H$ curves by Eq. (3).