Spin-Dependent Transport in Fe/GaAs(100)/Fe Vertical Spin-Valves

Abstract

The integration of magnetic materials with semiconductors will lead to the development of the next spintronics devices such as spin field effect transistor (SFET), which is capable of both data storage and processing. While the fabrication and transport studies of lateral SFET have attracted greatly attentions, there are only few studies of vertical devices, which may offer the opportunity for the future three-dimensional integration. Here, we provide evidence of two-terminal electrical spin injection and detection in Fe/GaAs/Fe vertical spin-valves (SVs) with the GaAs layer of 50 nanometers thick and top and bottom Fe electrodes deposited by molecular beam epitaxy. The spin-valve effect, which corresponds to the individual switching of the top and bottom Fe layers, is bias dependent and observed up to 20 K. We propose that the strongly bias- and temperature-dependent MR is associated with spin transport at the interfacial Fe/GaAs Schottky contacts and in the GaAs membranes, where balance between the barrier profiles as well as the dwell time to spin lifetime ratio are crucial factors for determining the device operations. The demonstration of the fabrication and spin injection in the vertical SV with a semiconductor interlayer is expected to open a new avenue in exploring the SFET.

Figure 1. Two-terminal Fe/GaAs(100)/Fe vertical spin-valves and spatially-resolved magneto-optical Kerr effect measurements of Fe electrodes on etched GaAs membrane.

(a) Schematic representation of the spin-valve (SV) devices with two Fe electrodes sandwiching a chemically etched GaAs(100) membrane; (b) Magnetic hysteresis loops of 150 ML and 10 ML thick Fe electrodes characterized by magneto-optical Kerr effect (MOKE) at room temperature. (c) Optical microscope image of a 50 nm thick GaAs membrane showing the position where the MOKE measurements were taken at and the crystallographic directions of the GaAs. The external magnetic fields, H, were applied in-plane along the [001] direction of the membrane for both MOKE and magnetoresistance (MR) measurements. Inset shows light illumination from an optical microscope transmitting through the thin membrane, prior to in-situ Fe depositions.

Figure 2. Electrical characterization of two-terminal Fe/GaAs/Fe SV device.

(a) Four-point I–V characteristics of the SV device at 5 and 300 K, respectively. Inset shows a back-to-back diode circuit to account for the observed I–V responses. D1 refers to a forward-biased Fe/GaAs Schottky contact, whereas D2 a reverse-biased contact. The parallel resistors R_D represent conduction paths due to potential diode imperfections at the Fe/GaAs interfaces. (b) Logarithmic plot of the I–V curve at 300 K. The linear dependence of the current on both forward and reverse bias is numerically fitted.

Figure 3. Magnetoresistance measurement of Fe/GaAs/Fe device at 5 K.

The MR measurements were performed using a forward bias of 0.32 V with in-plane magnetic field along the [001] direction of the GaAs membrane. The arrows in the figure indicate the relative alignments of the magnetizations of the 150 ML and 10 ML Fe electrodes. (b) Temperature dependent MR for the device measured using the same bias as in (a).

Figure 4. Bias dependence of MR signal.

(a) Bias dependence of the device MR at 5 K for both forward and reverse bias; (b,c) Schematics of the device band diagrams under zero and low bias conditions. φ_b1 and φ_b1 are the respective Schottky barrier heights for the two Fe/GaAs contacts, whereas qV₁ and qV₂ refer to the band offsets of the contacts under a low bias. Operation of the two-terminal device depends on the subtle balance between the energy profiles of the back-to-back Fe/GaAs Schottky contacts. For an observable MR, spin-polarized electrons have to be injected into the GaAs membrane by tunneling via a reverse biased Schottky contact, and be detected at a forward biased contact by spin-filtering (i.e. electron tunneling). In case of relatively high biases in which strong band bending occurs, the injected spin-polarized electrons may surmount the detector barrier height, not contributing any spin-dependent signal in the device.