Crossover from Spin Accumulation into Interface States to Spin Injection in the Germanium Conduction Band

Electrical spin injection into semiconductors paves the way for exploring new phenomena in the area of spin physics and new generations of spintronic devices. However the exact role of interface states in the spin injection mechanism from a magnetic tunnel junction into a semiconductor is still under debate. In this Letter, we demonstrate a clear transition from spin accumulation into interface states to spin injection in the conduction band of $n$-Ge. We observe spin signal amplification at low temperature due to spin accumulation into interface states followed by a clear transition towards spin injection in the conduction band from 200 K up to room temperature. In this regime, the spin signal is reduced to a value compatible with the spin diffusion model. More interestingly, the observation in this regime of inverse spin Hall effect in germanium generated by spin pumping and the modulation of the spin signal by a gate voltage clearly demonstrate spin accumulation in the germanium conduction band.

Figure 1

(a) Schematic drawing of the multiterminal device used for electrical spin injection, detection, and manipulation in germanium. BOX denotes the buried oxide layer which is made of $\mathrm{Si}{\mathrm{O}}_2$. The inset shows the geometry for spin pumping measurements. (b) Cross section TEM image of the full stack $\mathrm{Ta}/\mathrm{CoFeB}/\mathrm{MgO}/\mathrm{Ge}$. MgO is poorly crystallized and appears amorphous in the TEM images. (c) Temperature dependence of the magnetic tunnel junction resistance-area $RA$ product measured at a bias current of $50\mu \mathrm{A}$. Inset: $I(V)$ curves recorded at different temperatures between the tunnel junction and an Ohmic contact.

Figure 2

(a) and (b) Hanle and inverted Hanle curves recorded at 10 K, and 250 and 300 K, respectively. Solid lines are Lorentz fits. $\Delta H_{\perp }$ and $\Delta H_{\parallel }$ are the full width at half maximum (FWHM) of the Hanle and inverted Hanle curves. (c) Spin resistance-area product as a function of temperature and applied voltage. This three-dimensional plot is the result of more than $5\times {10}^5$ measurements on the same device. The inset shows the temperature dependence of $R_{S}A$ at four different bias voltages. (d) Sketch of the energy band diagram of $\mathrm{CoFeB}/\mathrm{MgO}/\mathrm{Ge}$ and its modification upon bias voltage and increasing temperature. Following Refs. 21, 36, the interface density of states (DOS) has a characteristic $U$-shape with a minimum at the Ge midgap. At low $T$ and for positive bias voltage, the detailed resistor model is given at the $\mathrm{CoFeB}/\mathrm{MgO}/\mathrm{Ge}$ interface showing spin accumulations into interface states and in the Ge conduction band.

Figure 3

(a) FMR line and $V_{\mathrm{ISHE}}$ measured at room temperature. The applied field is along $x$ (${\theta }_H=90°$). $H_{\mathrm{res}}$ and $\Delta H_{pp}$ are the resonance field and FMR linewidth respectively. (b) Temperature dependences of the asymmetric voltage $V_{\mathrm{Asym}}$ and $V_{\mathrm{ISHE}}$.

Figure 4

(a) Effect of a backgate voltage (0 and $-50\mathrm{V}$) on Hanle and inverted Hanle spin signals at 300 K. Open symbols are for $V_G=0\mathrm{V}$ and solid lines for $V_G=-50\mathrm{V}$. (b) Temperature dependence of $\Delta V/V=[V_S(-50\mathrm{V})-V_S(0\mathrm{V})]/V_S(0\mathrm{V})$ in percent. The dashed curve is a guide for the eye. (c) Spin resistance-area product converted into spin diffusion length $l_{sf}$ using the diffusive model of Ref. 12 developed for spin injection in the germanium channel. Only $T=200$, 250, and 300 K values are reported when spin polarized electrons are injected in the Ge conduction band. (d) Results of the triple fit over $r=V_{\perp }/V_S$, $\Delta H_{\perp }$, and $\Delta H_{\parallel }$ yielding the mean depth at which spin polarized electrons are injected and their spin lifetime. The depths and spin lifetimes are almost identical at 200, 250, and 300 K.