Session H14: Focus Session: Spins in Semiconductors - Electrical Spin Injection

Electrical Spin Injection and Detection in Silicon: Effect of Interface States

We have observed (using the local Hanle method) electrical spin injection into n and p type Si through a Fe/MgO/Si tunnel, with an effective spin lifetime of $\sim $130ps and an extremely large spin RA product as high as $\sim $0.1 M$\Omega $*$\mu $m$^{2}$ at low bias and temperature. Both the spin-RA and the differential resistance decrease exponentially with bias at temperatures below 150K. The effective spin lifetime weakly depends on temperature, decreasing by $\sim $30{\%} from 10K to 300K. We observe the inverse Hanle effect when an external magnetic field is applied parallel to the magnetization, possibly indicating the presence of stray fields near the Si surface. These observations roughly agree with other local Hanle spin injection studies in Silicon and GaAs, but differ strongly from the results expected for injection into bulk Silicon. The two stage tunneling model through localized states (LS) developed by Tran \textit{et al }(\textbf{PRL} 102; p. 036601) can explain the large magnitude of the observed spin RA, and we have developed an extended LS model which can explain the voltage dependence, which will be discussed in another talk.   

Non-local spin transport and accumulation measurements in Si:AlGaAs with tunable carrier density

The spin lifetime in GaAs varies strongly with carrier density near the insulator to metal transition (IMT), possibly peaking at the transition [1]. However, determining the optimal spin lifetime in this material is challenging because many replica samples need to be fabricated and measured. This difficulty can be circumvented by employing Si:Al$_{0.3}$Ga$_{0.7}$As, a persistent photoconductor, as the spin transport medium. This material has been characterized and has an effective carrier density which can be tuned \textit{in situ} via photo-excitation from 10$^{14}$ to 10$^{18 }$cm$^{-3}$ and a critical carrier density for the IMT of 9.0 x 10$^{16 }$cm$^{-3}$ at 5K [2]. Heterostructures have been grown by MBE, consisting of 2 $\mu $m Si:AlGaAs, a thin epitaxial Fe layer, and an AlGaAs graded junction to create Schottky tunnel barrier contacts. Non-local spin devices have been fabricated and measured. Based on non-local 4 terminal (NL 4T) and local and NL 3T Hanle effect measurements, the initial electrical spin transport and accumulation measurements in this material are reported. The spin lifetimes range from 600 ps to 2.8 ns for multiple carrier densities, ranging from 3.5 x 10$^{16}$ to 2.4 x 10$^{17 }$cm$^{-3}$. [1] J. M. Kikkawa et al., Phys. Rev. Lett. 80, 4313 (1998). [2] J. Misuraca et al., Phys. Rev. B. 82, 125202 (2010).   

Electrical spin injection and detection in Fe/MgO/Si: influence of interface states

We report electrical spin injection and detection in Fe/MgO/Si tunnel diodes using a 3-terminal (3T) geometry. Analysis of our Hanle curves yields an effective spin life-time of $\sim $0.1 ns and a spin-RA product $\sim $1 M$\Omega *\mu $m$^{2}$, both of which are in rough agreement with previous 3T studies. However, according to our analysis the spin-RA is $\sim $ 6 orders of magnitude larger than expectations for bulk Si, and the 0.1 ns effective spin life-time is much smaller than reported value in Si by ESR or non-local methods. Here we provide a detailed analysis of electrical injection and detection in the 3T geometry. We present an alternative expression for the 3T spin signal than is usually used, and we propose that spin is accumulating in localized states (LS) at the MgO/Si interface rather than just in bulk Si. Incorporating a theory of spin accumulation in LS developed by M. Tran \textit{et al} (\textbf{PRL} 102, 036601), we propose an energy distribution for the density of localized states, and introduce a model that agrees well with our anomalously large spin-RA and can explain the strong bias dependence of both spin and charge transport.   

Spin accumulation in Fe/MgO/Ge heterostructures

We have investigated the injection of spins into n-type Ge(001) from Fe through an MgO tunnel barrier using 3-terminal Hanle measurements. While significant progress has been made in Si, spin research in Ge is still at a nascent stage, due in part to the fact that significant Fermi level pinning at the Ge interface makes it difficult to efficiently inject carriers. We observe here precessional dephasing of the spin accumulation in an applied magnetic field (the Hanle effect) in Fe/MgO/Ge structures for both forward and reverse bias. We determine spin lifetimes and corresponding spin diffusion lengths for injection into Ge substrates of varying carrier concentration and see a trend of increasing spin lifetime with decreasing doping density. At room temperature, spin lifetimes range from $\tau_{s}$ = 50 ps to 123 ps as the carrier concentration is reduced from n=8x10$^{17}$cm$^{-3}$ to 2x10$^{16}$cm$^{-3}$. We will discuss the spin-RA product as a function of carrier concentration and the role of interface states. The observed room temperature injection of spins shows that despite persistent Fermi level pinning, spin accumulation is possible in the surface of Ge. This work was supported by core programs at NRL.   

Spin Transfer from a Ferromagnet into a Semiconductor through an Oxide barrier

We present results on the magnetoresistance of the system Ni/Al203/n-doped Si/Al2O3/Ni in fabricated nanostructures. The results at temperature of 14K reveal a 75{\%} magnetoresistance that decreases in value up to approximately 30K where the effect disappears. We observe minimum resistance in the antiparallel configurations of the source and drain of Ni. As a possibility, it seems to indicate the existence of a magnetic state at the Si/oxide interface. The average spin diffusion length obtained is of 650 nm approximately. Results are compared to the window of resistances that seems to exist between the tunnel barrier resistance and two threshold resistances but the spin transfer seems to work in the range and outside the two thresholds.   

Comparing nonlocal and three terminal Hanle experiments in Silicon

We have recently shown electrical spin injection in the technologically important material Si up to 500K and demonstrated a dependence of the spin lifetime with carrier concentration. In previous work on GaAs, we have seen excellent agreement between the spin lifetime derived from Hanle data measured directly at the injector contact using a three terminal measurement and the simultaneously measured nonlocal signal outside of the charge path. Unfortunately, simultaneous measurement of three terminal and nonlocal measurements is impractical on the Silicon devices due to the high resistance of the 6x100 $\mu $m$^{2}$Fe/AlO$_{x}$/2e18 n-type Si injector contact. Instead we used a separate 150x100 $\mu $m$^{2}$Fe/AlO$_{x}$/Si contact on the same substrate to do three terminal measurements and nonlocal measurements independently. Lorentzian fits to the data shows a spin lifetime of 280 ps measured directly underneath the spin injecting contact. This data is in excellent agreement with the spin lifetime vs. carrier concentration for NiFe/SiO$_{2}$/Si contacts, however it is a factor of 3 lower than the spin lifetime of $\sim $ 1ns measured at the nonlocal contact. We discuss this observed difference and other properties such as temperature dependence and bias dependence of the three terminal vs. nonlocal experiments.   

Electrical injection and detection of spin accumulation in Si at 500 K with magnetic metal/SiO$_{2}$ contacts

Electrical spin injection into Si (001) from a ferromagnetic metal through an Al$_{2}$O$_{3}$ tunnel barrier has been demonstrated.\footnote{B. T. Jonker et al., Nature Phys. 3, 542 (2007); S. P. Dash et al., Nature 462, 491 (2009).} However, the utilization of SiO$_{2}$ as the tunnel barrier can have significant impact on the development of Si based spintronics. Here we demonstrate the electrical injection, detection and precession of spin accumulation in Si, via injection from ferromagnetic contacts such as Ni$_{0.8}$Fe$_{0.2}$ and Co$_{0.9}$Fe$_{0.1}$ through a SiO$_{2}$ tunnel barrier.\footnote{C. H. Li et al., Appl. Phys. Lett. 95, 172102 (2009).} The injection of spin-polarized carriers produce a net spin polarization and an imbalance in the spin-dependent electrochemical potential under the contact, which is detected as a voltage at the same contact. The decrease of this voltage with increasing out-of-plane magnetic field due to spin dephasing, i.e., Hanle precession of the electron spin, is observed up to 500 K. We observe Hanle precession of electron spin accumulation in Si for a wide range of bias, and demonstrate that the spin lifetime (extracted from the Lorentzian fit to the Hanle data) varies with Si carrier density. Details of the bias and temperature dependence of the spin lifetime and spin diffusion length will also be presented at the meeting. These results confirm spin accumulation in the Si transport channel up to 500 K rather than trapping in localized interface states, and demonstrate the practical aspect of spin-based semiconductor device technology. Supported by ONR.   

Spin-filter effect of quantum dot with spin-orbit interaction in magnetic field

We theoretically investigate a spin-polarized current generation in a semiconductor quantum dot (QD) with spin-orbit interaction in a magnetic field. In the absence of magnetic field, a spin-polarized current is generated only when the QD is connected to more than two leads.\footnote{M.\ Eto and T.\ Yokoyama, J.\ Phys.\ Soc.\ Jpn.\ {\bf 79}, 123711 (2010).} In the presence of magnetic field, on the other hand, we show that the two-terminal QD works as a spin filter due to the spin-orbit interaction even if the Zeeman effect is negligibly small. First, we focus on the vicinity of current peaks of Coulomb oscillation due to the resonant tunneling, considering the two energy levels around the Fermi level in the leads, and obtain an analytical form of spin-dependent current. The spin-polarization of the current is largely enhanced when the spacing between the two levels is smaller than the level broadening due to the tunnel coupling to the leads. Second, we perform a numerical simulation using a realistic model for the confining potential of the QD. We find more than 40\% spin-filtering efficiency around some current peaks.   

Tailoring Chirp in Spin-Lasers

The interplay of spin injection in lasers and their nonlinear response leads to novel spintronic devices [1]. Such spin-lasers can enable desirable properties including threshold reduction, bandwidth enhancement, and low chirp [1-3]. These lasers can also be viewed as spin-amplifiers, since high circular polarization in the output can be achieved even with nearly spin-unpolarized injection [2,3]. In the present work, we study chirp in spin-lasers and suggest new modulation schemes to improve their performance. Supported by NSF-ECCS, U.S. ONR, AFOSR-DCT, and NSF-NEB 2020. \\[4pt] [1] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007); J. Rudolph et al., Appl. Phys. Lett. 87, 241117 (2005). \\[0pt] [2] J. Lee, W. Falls, R. Oszwadowski, and I. \v{Z}uti\'c, Appl. Phys. Lett. 97, 041116 (2010).\\[0pt] [3] C. G{\o}thgen, R. Oszwadowski, A. Petrou, and I. \v{Z}uti\'c, Appl. Phys. Lett. 93, 042513 (2008).\\[0pt] [4] G. Boeris, J. Lee, K. V\'yborn\'y, and I. \v{Z}uti\'c, preprint (2011).