aa r X i v : . [ c ond - m a t . m e s - h a ll ] A ug Electric-field control of spin accumulation signals in silicon at room temperature
Y. Ando, , Y. Maeda, K. Kasahara, S. Yamada, K. Masaki, Y. Hoshi, K. Sawano, K. Izunome, A. Sakai, M. Miyao, and K. Hamaya , ∗ Department of Electronics, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan Advanced Research Laboratories, Tokyo City University, 8-15-1 Todoroki, Tokyo 158-0082, Japan Principle Technology, Covalent Silicon Corporation, Seiroumachi, Niigata 957-0197, Japan Department of Systems Innovation, Osaka University, Toyonaka 560-8531, Japan and PRESTO, Japan Science and Technology Agency, Sanbancho, Tokyo 102-0075, Japan (Dated: March 13, 2018)We demonstrate spin-accumulation signals controlled by the gate voltage in a metal-oxide-semiconductor field effect transistor structure with a Si channel and a CoFe/ n + -Si contact at roomtemperature. Under the application of a back-gate voltage, we clearly observe the three-terminalHanle-effect signal, i.e., spin-accumulation signal. The magnitude of the spin-accumulation signalscan be reduced with increasing the gate voltage. We consider that the gate controlled spin signalsare attributed to the change in the carrier density in the Si channel beneath the CoFe/ n + -Si con-tact. This study is not only a technological jump for Si-based spintronic applications with gatestructures but also reliable evidence for the spin injection into the semiconducting Si channel atroom temperature. PACS numbers:
The progress of silicon-based spintronics (Si spintron-ics) is splendid in recent years. Many groups have sofar demonstrated electrical spin injection and detectionthrough ferromagnet-insulator-Si heterostructures.[1–6]Recently, spin-related phenomena at room temperaturewere reported in Si-based three- or four-terminal lateraldevices.[4–6] In particular, clear room-temperature spintransport and its manipulation by applying transversemagnetic fields were achieved although the channel usedwas a heavily doped Si.[5]To date, we have explored the spin injection and de-tection in Si-based devices without insulators for sourceand drain contacts in order to reduce the parasiticresistance.[7, 8] Recently, we studied spin accumulationsignals at a ferromagnetic CoFe/ n + -Si interface by mea-suring a Hanle effect in three-terminal lateral devices,[8]and found that there is an electrical detectability ofthe contact for spin accumulation in the Si channel,consistent with the previous work in Fe/GaAs lateraldevices.[11] If the Hanle-effect signals are arising fromthe spin accumulation in the Si channel, we should ob-serve the variation in the Hanle-effect signals by changingcarrier densities in the Si channel.[6, 11]In this letter, we demonstrate spin-accumulation sig-nals controlled by the gate voltage in a Si-metal-oxide-semiconductor field effect transistor (MOSFET) struc-ture with a CoFe/ n + -Si contact at room tempera-ture. Under the application of a back-gate voltage, weclearly observe the three-terminal Hanle-effect signal, i.e.,spin-accumulation signal. The magnitude of the spin-accumulation signals can be reduced with increasing thegate voltage. We consider that the gate controlled spin ∗ E-mail: [email protected] signals can be explained by the change in the carrier den-sity in the Si channel beneath the CoFe/ n + -Si contact.This is reliable evidence for the spin injection into thesemiconducting Si channel at room temperature. Fromthe technological point of view, this study will lead toan acceleration of research and development of Si-basedspintronic applications with gate structures.[9, 10]Ferromagnetic CoFe epitaxial layers with a thickness of ∼
10 nm were grown on (111)-oriented Silicon On Insu-lator (SOI) by low-temperature molecular beam epitaxy(MBE) at ∼ ◦ C,[12] where the thicknesses of the SOIand buried oxide (BOX) layers are about ∼
75 and 200nm, respectively, and the carrier density of the SOI layeris ∼ × cm − (1 ∼ δ -doping technique, an n + -Si layer (Sb :1 × cm − ) was inserted between CoFe and SOI. Herethe Sb δ -doped n + -Si layer on the channel region was re-moved by the Ar + ion milling. An ohmic contact (AuSb)for backside heavily doped Si was formed at less than 300 ◦ C. Conventional processes with electron-beam lithogra-phy, Ar + ion milling, and reactive ion etching were usedto fabricate three-terminal lateral devices with a backsidegate electrode, illustrated in Fig. 1(a). The CoFe/ n + -Sicontact (contact 2) and AuSb ohmic contacts (contact 1and 3) have lateral dimensions of 1 × µ m and 100 × µ m , respectively. The distance between the con-tacts 2 and 1 or 3 is ∼ µ m. The three-terminal Hanlemeasurements were performed by a dc method with thecurrent-voltage configuration shown in Fig. 1(a) at roomtemperature, where a small magnetic field perpendicularto the plane, B Z , was applied after the magnetic momentof the contact 2 aligned parallel to the plane along thelong axis of the contact.First, we confirm the operation as a MOSFET for thefabricated device shown in Fig. 1(a). Figure 1(b) shows B z (a) A u S b B O X S b δ - dop i ng µ m µ m C o F e µ m n - S i ( ) ( ~ c m ) - n - S i ( ) ( ~ c m ) + - µ m V G (b) V SD I V I SD V (V) G (c)
0 20 40 60 80543210 I ( µ A ) S D V = -0.5 V SD V SD (V) -0.2 V-0.1 V I ( µ A ) S D A uSb A uSb V = S D V = S D V = 54 V V =
30 V V = 0 V G G G
FIG. 1: (Color online) (a) Schematic diagram of a Si-MOSFET structure with a CoFe/ n + -Si Schottky-tunnel con-tact fabricated. (b) I SD − V G and (c) I SD − V SD characteristicsat room temperature. The constant V SD and V G values aredenoted in each figure. I SD − V G characteristic measured at room temperature atconstant bias voltages of V SD = -0.1, -0.2, and -0.5 V.With increasing V G , the I SD value gradually increasesfor all V SD . This means that the conduction channel isformed from the vicinity of the interface between SOIand BOX. These results clearly indicate that this devicecan operate as a MOSFET. Because of relatively thickSOI layer, the carrier density of the bulk SOI is still en-hanced by further applying V G . Hence, the I SD valueis not saturated in high V G region. Figure 1(c) displaysrepresentative I SD − V SD characteristics at room temper-ature at various V G . Almost symmetric and nonlinearcharacteristics were observed up to V G = 20 V but smallasymmetric features with respect to the bias polarity canbe seen in V G >
20 V. Since these features are markedlydifferent from those resulting from the thermionic emis-sion examined previously in Ref.[12], tunneling conduc-tion through the CoFe/ n + -Si interface is dominant factorfor the observed I SD − V SD characteristics at room tem-perature. Although we could not make the evident offstate at V G = 0 V for this device, as shown in Fig. 1(b),there is almost no influence on the main claim of thisstudy.Using this device, we measured the three-terminal volt-age, ∆ V , as a function of B Z , i.e., Hanle effect. Figure2(a) shows a ∆ V − B Z curve for V G = 8.0 V at I SD =-1.0 µ A at room temperature, where a quadratic back-ground voltage depending on B Z is subtracted from theraw data. Here in this condition ( I SD <
0) the electronsare injected from the spin-polarized states of CoFe into B Z (Oe)(a) (b) V ( µ V ) ∆ V ( µ V ) ∆ V G = 8.0 V -12-10-8-6-4-202-400 -200 0 200 400 -12-10-8-6-4-202-400 -200 0 200 400 B Z (Oe) V G = 54 V V ∆ V ∆ | | | | FIG. 2: (Color online) Room-temperature spin accumulationsignals measured at (a) V G = 8.0 V and (b) V G = 54 V. Theapplied bias current is a constant value of I SD = -1.0 µ A,which induces spin accumulation in a Si conduction band byspin injection from CoFe into Si. The red curves are fittingresults by the Lorentzian function. the conduction band of Si. When B Z increases from zeroto ±
200 Oe, a clear voltage change ( | ∆ V | ) is observedeven at room temperature. The voltage change is causedby the depolarization of the accumulated spins, that is,a Hanle-type spin precession is detected by the three-terminal voltage measurements.[4, 6, 8, 11, 13, 14] Wecould not obtain such Hanle-effect curves in V G < . I SD = -1.0 µ A for examining the effect of the application of V G ( V G ≥ . V , | ∆ V | , is ∼ µ V inFig. 2(a). Surprisingly, | ∆ V | is decreased to ∼ µ Vwhen the gate voltage is further applied up to V G = 54V in Fig. 2(b). For both V G conditions, a lower limit ofspin lifetime ( τ S ) can be obtained using the Lorentzianfunction, ∆ V ( B Z ) = ∆ V (0)/[1+( ω L τ S ) ], where ω L = gµ B B Z / ~ is the Lamor frequency, g is the electron g -factor ( g = 2), µ B is the Bohr magneton.[4] The fittingresults are denoted by the red solid curves in Fig. 2. The τ S values for V G = 8.0 and 54 V are estimated to be ∼ ∼ τ S value is almost constant despite the change in | ∆ V | .On the basis of the simple spin diffusion model,[15–18] we consider the observed | ∆ V | . For three-terminalmeasurements, the magnitude of the voltage change dueto the Hanle-type spin precession can be expressed asfollows:[14] ∆ VI = P λ N ρ N A , (1)where P is the spin polarization, λ N and ρ N are thespin diffusion length and resistivity of the nonmagnet V (V) G V ( µ V ) ∆ CoFe Si µ µ V G1 (a) CoFe Si(c) µ µ V G2 | | V (V) G
0 20 40 60 (b) R ( Ω ) RR ChannelInterface < I = -1.0 µΑ SD ∆µ ∆µ FIG. 3: (Color online) (a) | ∆ V | as a function of V G for I SD =-1.0 µ A at room temperature. (b) The changes in R Channel and R Interface with increasing V G at room temperature. (c)Schematic diagrams of the change in the spin accumulation(∆ µ ) by the application of V G . used, respectively. A is the contact area. For our fab-ricated device, when we assume D ∼
40 cm s − ( n ∼ cm − ),[19] λ Si ∼ µ m is obtained by using the re-lationship of λ N = √ Dτ S ( τ S ∼ ρ Si =1 ∼ RA ), ∆ V I SD × A , is ob-tained to be ∼ µ m for the data in Fig. 2(a),we can roughly obtain 0.14 < P < P is consistent with that for CoFe alloysexpected.[20] Therefore, our Hanle-effect signals observedhere can roughly be considered within the framework ofthe commonly used diffusion model.[15–18]To discuss the origin of the reduction in | ∆ V | in Fig.2, we explored Hanle-effect signals for various V G in de-tail. Figure 3(a) displays | ∆ V | vs V G at I SD = -1.0 µ Aat room temperature. With increasing V G , nearly lin-ear decrease in | ∆ V | is obtained. Thus, the observedfeature in Fig. 2 is reproduced systematically. Here wealso examine the V G dependence of channel resistance( R Channel ) and interface resistance ( R Interface ) at roomtemperature, where R Channel and R Interface can roughlybe obtained by using local and nonlocal three-terminalmeasurements, respectively. As shown in Fig. 3(b), R Channel decreases with increasing V G while R Interface isalmost constant irrespective of V G .[21] Here we focus onthe change in R Channel by the application of V G . Since the thickness of SOI layer is relatively thick ( ∼
75 nm),all the changed R Channel values after the application of V G are not directly associated with the change in ρ Si ofall the channel regions in this device. However, the appli-cation of V G should affect the partial change in ρ Si of theSi channel at least because R Channel does not saturatewith increasing V G . Thus, the carrier density in the Sichannel beneath the CoFe/ n + -Si contact should be en-hanced by applying V G . From these considerations, themonotonous decrease in | ∆ V | with applying V G can beexplained by the decrease in ρ N in Eq. (1), where ρ N is ρ Si of the Si channel beneath the CoFe/ n + -Si contact.Phenomenological schematic diagrams of the spin ac-cumulation in the Si channel beneath the CoFe/ n + -Sicontact are shown in Fig. 3(c). Under a condition forspin injection into Si, the spin accumulation (∆ µ ) oc-curs in the Si conduction band near the quasi Fermi level(left figure). When we increase V G from V G1 to V G2 ,the carrier density in the Si conduction channel beneaththe CoFe/ n + -Si contact increases, causing the decreasein ρ Si . Namely, even if the same I SD is used for spin in-jection into the Si channel, tunneling probability of spin-polarized electrons and the density of state in Si at theFermi level should be varied by the application of V G ,resulting in the reduction in ∆ µ (right figure). Accord-ingly, the experimental data can also be explained withina framework based on the simple diffusion model [Eq.(1)].We note that the present study including gate-inducedchange in the Hanle-effect signals is exact evidence for thedetection of spin-polarized electrons created not in thelocalized state in the vicinity of the interface but in theSi channel. We convince that the three-terminal Hanle-effect measurements are powerful tool for detection of thespin accumulation in the semiconductor channels.In summary, we have demonstrated electric-field con-trol of spin accumulation in Si using a MOSFET struc-ture with a CoFe/ n + -Si contact. Even at room temper-ature, we observed clear spin accumulation signals un-der an application of the gate voltage. The magnitudeof the spin accumulation signals was reduced by the in-crease in the gate voltage. We consider that the reductionin the spin-accumulation signals is attributed to the in-crease in the carrier density in the Si channel beneaththe CoFe/ n + -Si contact, indicating reliable evidence forthe spin injection into the semiconducting Si channel atroom temperature. This study also includes a technolog-ical jump for Si-based spintronic applications with gatestructures.This work was partly supported by Precursory Re-search for Embryonic Science and Technology (PRESTO)from Japan Science and Technology Agency, andSemiconductor Technology Academic Research Center(STARC). Three of the authors (Y.A. K.K. and S.Y.)acknowledge Japan Society for the Promotion of Science(JSPS) Research Fellowships for Young Scientists. [1] I. Appelbaum, B. Huang, and D. J. Monsma, Nature , 295 (2007); B. Huang, D. J. Monsma, and I. Ap-pelbaum, Phys. Rev. Lett. , 177209 (2007); H.-J. Jangand I. Appelbaum, Phys. Rev. Lett. , 117202 (2009);Y. Lu, J. Li, and I. Appelbaum, Phys. Rev. Lett. ,217202 (2011).[2] B. T. Jonker, G. Kioseoglou, A. T. Hanbicki, C. H. Li,and P. E. Thompson, Nat. Phys. , 542 (2007); O. M.J. van’t Erve, A. T. Hanbicki, M. Holub, C. H. Li, C.Awo-Affouda, P. E. Thompson, and B. T. Jonker, Appl.Phys. Lett. , 212109 (2007).[3] T. Sasaki, T. Oikawa, T. 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