Spin mosfets using ferromagnetic schottky barrier contacts for the source and drain

Abstract

Spin transistors, which utilize two ferromagnetic layers as a spin injector and as a spin detector, are very attractive as a basis for spin-electronic integrated circuits, owing to their additional spin degree of freedom in controlling output currents. In particular, recently proposed spin MOSFETs that are spin transistors analogous to MOSFETs are promising, since its prospected large current-drive capability, large magnetocurrent ratio, and compatibility with present MOS technology can lead to novel nonvolatile memory and reconfigurable logic architectures [1-3]. The spin MOSFETs can be classified by the structure of the source/drain and the source/drain material [1]. The simplest and most feasible way to realize a spin MOSFET is to replace the source/drain material of an Schottky barrier (SB) MOSFET with a ferromagnetic metal that forms a ferromagnetic Schottky junction with the Si channel (Fig.l). In this paper, we present the theoretical analysis and experimental demonstration of spin MOSFETs using a ferromagnetic metal for the SB source/drain. Fig.1 shows the structure of our model device used in the calculations. Assuming the ballistic transport of spin-polarized carriers in the nanometer-scale channel, the device performance was theoretically analyzed. Solid and dashed curves in Fig. 2 show calculated drain currents IDP and IDAP as a function of drain bias VDs in the parallel and antiparallel magnetization configurations, respectively, where gate bias VGS varied from 0.0 to 1.0 V in steps of 0.2 V. When the relative magnetization configuration of the source/drain is parallel, the spin MOSFET shows a large output current comparable to that of high performance MOSFETs, indicating the high transcondactance of the spin MOSFET. The output current for small VDS conditions (less than the pinch-off voltage) can be reduced by changing the relative magnetization of the source/drain from the parallel to antiparallel configuration. Figs. 3 and 4 show magnetocurrent ratio Nc [=(IDP-IDAP)/IDAP] as a function of VDs and VGS, respectively. Nc falls with increasing VDs, but it is enhanced with increasing VGS. These phenomena can be rationalized by spin dependent transport similar to the tunneling magnetoresistance effect and the gate-induced enhancement of spin injection efficiency [1]. The SB height SB of the source/drain is important to obtain large output currents and also large Nc. IDP dramatically increases with decreasing S5B in comparison with IDAP, and thus Nc increases with decreasing B, as shown in Fig. 5. This means that IDP and Nc can simultaneously increase without any trade-off, which is important for spin-electronic integrated circuit applications. In order to demonstrate the transistor action of a spin MOSFET using a ferromagnetic metal for the SB source/drain, a prototypic spin MOSFET was fabricated by using ferromagnetic silicide Fe,-1Si,. A bottom-gate structure with a SOI substrate was employed for simplicity, where the buried oxide and the Si substrate were used as a gate dielectric and a gate electrode, respectively (Fig.6). The fabrication procedures are as follows: A p-type (001) SOI wafer with resistivity 10-20 Q-cm was used as a substrate. The SOI thickness was 50nm. The channel region was defined by using a sio2 hard mask, and then a 70-nm-thick Fe layer was deposited on the SOI surface by electron beam evaporation. Successively, Fe,-,Si, was formed by rapid thermal annealing (RTA) at 700C for 4 min in an N2 ambient. Finally, the mesa isolation of the device was done to define the active region. The physical gate length and width of the fabricated spin MOSFET were 1.6 gm and 10 gm, respectively (Fig.7). The fabricated Fe1 xSi,/Si junction showed clear Schottky diode characteristics, and it was found from preliminary experiments that the Schottky barrier height for p-type Si was estimated at less than 0.2 eV. Fig. 8 shows the output characteristics of the fabricated spin MOSFET, where gate bias VGS varied from 0 to 25 V in steps of 5 V. The output characteristics exhibited the inversion-channel mode operation of p-type FET. The accumulation-channel mode operation was also observed by the application of negative VGS and VDS. These results indicate that the fabricated spin MOSFET can operate as a SB MOSFET. The detailed characteristics including the magnetic transport properties of the fabricated spin MOSET will be presented at the conference. [1] S. Sugahara, to be published in LEE Proc. Circuits, Devices & Systems. [2] S. Sugahara and M. Tanaka, to be published in J. Appl. Phys. [3] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84 (2004) 2307.