IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 6, JUNE 2010 1637 The Effect of Doping Concentration of Si on the Nature of Barrier of Co MnSi/MgO/n-Si Junctions M. A. I. Nahid, M. Oogane, H. Naganuma, and Y. Ando Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan In this work, we have presented the electrical characteristic of Co MnSi/MgO/n-Si junctions as a function of the doping concentration of Si. Films were fabricated by dc sputtering and post annealed at 400 C for 1 h without breaking the vacuum. The Co MnSi/MgO/n-Si junctions exhibited diode like characteristics at low doping concentration 10 /cc. This can be attributed due to oxide charges or in- terface traps close to the silicon interface, which causes the bend bending and forms the large extended depletion region. The junction characteristic was found to change with the increase of doping concentration and became symmetric at doping concentration of 10 /cc. Therefore, with the same thickness of MgO barrier, the junction characteristic was changed from Schottky to symmetric tunneling with the doping density of Si. The origin of the change of junction characteristic might be due to the change of the depletion width with the doping density. Index Terms—Doping concentration, interface traps, oxide charges, Schottky, spin injection. I. INTRODUCTION F OR THE implementation of future semiconductor based spintronic devices, it is an essential requirement to obtain efficient spin injection into semiconductor [1]–[4]. One of the ways for achieving efficient spin injection is by using high spin polarized electrode [5] into the spin conserving semiconductor. Recently, Si containing full-Heusler alloys, such as Co MnSi attract considerable attention as a spin injector electrode since they have high spin polarization, high Curie temperature and large saturation magnetization [6]–[8]. Large TMR ratio is ob- tained using Co MnSi as a tunneling electrode [9]. These results demonstrate the suitability of the Co MnSi materials as a spin injector. On the other hand, Si is suitable semiconductor for spin conserving because of small spin-orbit coupling, low lattice in- version symmetry and mature technology [10], [11]. The main obstacle in achieving efficient spin injection using these mate- rials is the interfacial diffusion or intermixing or chemical re- action when Co MnSi directly deposited on n-Si [12]. This can be circumvented by introducing the insertion of insulating layer [13], [14]. The presence of the insulating layer not only inhibits material inter-diffusion but also helps to overcome the conduc- tivity mismatch problem. The thickness of the insulating layer needs to be optimized considering the resistance-area product and chemical reaction or diffusion. Recently, it was reported that 2-nm-thick MgO is necessary for stopping the diffusion of Co MnSi/MgO/n-Si junction and in this case, the MgO effec- tively acts as a tunnel barrier [15]. However, it has been seen in the previous reports that with the similar MgO thickness, the metal-insulator-semiconductor junction can be Schottky type [16], [17]. By the insertion of another material like Gd, the junc- tion characteristics become symmetric [18]. It is known that the junction characteristic is found sensitive to the doping type Manuscript received October 30, 2009; revised February 03, 2010; accepted February 07, 2010. Current version published May 19, 2010. Corresponding author: M. A. I. Nahid (e-mail: mainahid@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2043223 and concentration and therefore, it is requisite to know the elec- trical properties of Co MnSi/MgO/n-Si junction depending on the doping concentration prior to spin injection. In this paper, the effect of doping concentration on the nature of barrier of Co MnSi/MgO/n-Si junctions has been discussed. II. EXPERIMENTAL SETUP By means of magnetron sputtering, the MgO layer was deposited at room temperature on antimony doped n-Si sub- strates. The doping concentration of the Si substrates was varied from 10 /cc to 10 /cc. The substrates were dipped into low concentration HF solution in order to remove native oxide. Then, the substrates were cleaned by commercial solution, Semico-clean in ultrasonic bath and finally by distilled water. Prior to deposition, the substrates were heated at 500 C for 20 m and cooled down in the chamber. The MgO thickness was varied from 1 nm to 4 nm. The 30-nm-thick Co MnSi films were then deposited by dc sputtering in another chamber without breaking the vacuum at an argon pressure of 0.1 Pa. The samples were in situ annealed at 400 C for 1 hour and cooled down for 1 hour. Finally 5-nm-thick capping layer was deposited. The magnetic measurements were carried out by vibrating sample magnetometer. III. RESULTS Fig. 1 shows the saturation magnetization of Co MnSi alloy thin films on different doped Si substrates as a function of the various MgO thicknesses. It is observed that the of Co MnSi films grown on low doped (10 /cc) and high doped (10 /cc) Si substrates are nearly same with the variation of MgO thickness. The is found to increase with MgO thick- ness and becomes constant above 2 nm. The maximum value of was about 900 emu/cc comparable to previous reports [12]. This suggests that the diffusion or chemical reaction may occur until 2-nm thick-MgO. Therefore, the MgO thickness was kept fixed at 2 nm and the electrical characteristics were studied varying the doping concentration. Patterning for the electrical characterization was done by e-beam lithography followed by 0018-9464/$26.00 © 2010 IEEE