Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V10: Focus Session: Spin Injection into Semiconductors |
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Sponsoring Units: DMP GMAG Chair: Chris Palmstrom, University of Minnesota Room: LACC 153B |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V10.00001: Spin injection from CoFe/MgO tunnel injectors into GaAs Roger Wang, Xin Jiang, Robert Shelby, Seth Bank, James Harris, Stuart Parkin The primary goal of research in spin injection has been to create a population of highly spin-polarized carriers inside a semiconductor at room temperature for potential manipulation in a spin-based device. Using quantum well electroluminescence detection, the CoFe/MgO tunnel spin injector has demonstrated greater than 50 {\%} polarization of electrons inside GaAs at 100 K as well as polarizations exceeding 30 {\%} at 290 K. In addition, the structures are thermally stable, showing no decrease in injected polarization even after exposure to temperatures as high as 400 $^{o}$C. Both spin relaxation rate and recombination lifetime play a role in determining the measured polarization. The temperature and bias dependence of the polarization between 1.4 K - 290 K will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V10.00002: Spin Injection and Relaxation in Ferromagnet-Semiconductor Heterostructures X. Lou, C. Adelmann, J. Strand, C. J. Palmstr{\O}m, P. A. Crowell We present a detailed description of spin injection and detection in Fe/Al$_x$Ga$_{1-x}$As/GaAs heterostructures for temperatures from 2 to 295~K. Experimental measurements of the full bias and temperature dependence of the steady-state spin polarization in the semiconductor indicate three distinct temperature regimes for spin transport and relaxation. At temperatures below 70~K, spin-polarized electrons injected into quantum well (QW) structures form excitons, and the spin polarization in the QW depends strongly on the electrical bias conditions as well as the temperature. At intermediate temperatures, the spin polarization is determined primarily by the spin relaxation rate for free electrons in the QW. This process is slow relative to the spin relaxation rate for excitons at lower temperatures and is responsible for a broad maximum in the spin polarization between 100 and 200~K. The spin injection efficiency of the Fe/Al$_x$Ga$_{1-x}$As Schottky barrier decreases at higher temperatures, although a steady- state spin polarization of at least 6\% is observed at 295~K. The difference in spin relaxation effects between QW and bulk systems is also investigated over the full bias and temperature range. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V10.00003: Measurement of Oblique Hanle Effect in {100} Fe Spin LEDs R. Mallory, S. Delikanli, M. Yasar, A. Petrou, G. Kioseoglou, C.H. Li, A.T. Hanbicki, O. van't Erve, B.T. Jonker In previous oblique Hanle work (1) on CoFe spin LEDs a prefactor equal to T$_{S}$/$\tau $ remained undetermined. Here $\tau $ and T$_{S}$ are the electron lifetime and spin lifetime, respectively. We have removed this uncertainty by carrying out a combined Hanle and magneto-EL study in the Faraday geometry of surface emitting [100] Fe spin LEDs. These experiments yield the values for $\tau $ and T$_{S }$ from which we can extract the spin relaxation time $\tau _{S}$. These parameters were determined in the 5-120 K temperature range. A parallel experiment on [110] Fe spin LEDs gave a null result even though these devices show comparable polarization in the Faraday geometry. This result is attributed to the short spin lifetime of spin oriented perpendicular to the [110] axis (2). This work was supported by the DARPA SpinS program, ONR, and NSF. 1. Motsnyi et. al. Appl. Phys. Lett. \underline {81}, 265, (2002). 2. D\"{o}hrmann et. al., Phys. Rev. Lett. \underline {93}, 147405-1, (2004). [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:27PM |
V10.00004: Electrical Spin Injection into InAs/GaAs Quantum Dots at Room Temperature Invited Speaker: Efficient electrical injection of spin-polarized electrons from a magnetic contact into a semiconductor is an essential requirement for utilizing the spin degree of freedom in semiconductor spintronic devices. The spin- polarized light emitting diode (spin-LED) is used as the platform for demonstrating spin injection since it provides a quantitative and model independent measure of the electron spin polarization. Using this method, we have observed an electron spin polarization 32\% in GaAs (001) QWs due to electrical spin injection across an Fe/AlGaAs reverse- biased Schottky contact [1]. Recent attention has focused on the unpaired spin of an electron in a charged quantum dot (QD). The lack of available energy states due to the discrete energy level spectrum inhibits both elastic spin flip and inelastic phonon scattering mechanisms, resulting in long spin relaxation times [2]. This is an important requirement for developing spin-based electronics and certain implementations of quantum information technology. We report here room temperature electron spin polarization in InAs/GaAs self assembled QDs by electrical injection of spin-polarized electrons from an Fe Schottky contact. The quantum dots, formed by Stranski-Krastanov strain driven self- assembly during MBE growth, are embedded in the intrinsic region of an AlGaAs/GaAs QW LED structure with an epitaxial Fe film as the surface contact for injection of spin-polarized electrons. The circular polarization of the QD electroluminescence tracks the out-of-plane magnetization of the Fe, and shows that a 5\% electron spin polarization is achieved in the InAs QDs. The QD spin polarization persists to room temperature. Nonmagnetic reference samples show a background polarization on the order of 1\% which has little dependence on magnetic field. TEM images indicate that the Fe/AlGaAs interface is rougher than expected possibly due to QD incorporation, which is likely to limit spin injection, and may be solved by refining growth procedures. These results demonstrate that practical regimes of spin-based operation are clearly attainable in solid state semiconductor devices. \newline \newline [1] A.T. Hanbicki et al., APL 80, 1240 (2002), 82, 4092 (2003). [2] M. Paillard et al. PRL 86, 1634(2001). [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V10.00005: Spin Injection From The Heusler Alloy Co2MnGe INTO Al0.1Ga0.9As/GaAs Heterostructures Xuying Dong, X. Lou, C. Adelmann, J. Strand, P.A. Crowell, C.J. Palmstrom, J.P. Barnes, A.K. Petford-Long The Heusler alloy Co$_{2}$MnGe has been predicted to be half- metallic [1]. The ability to grow Co$_{2}$MnGe epitaxially on GaAs, the predicted half-metallicity and the high Curie temperature ($\sim $900K), make it an ideal candidate for a spin injecting contact. Co$_{2}$MnGe epitaxial films were grown by molecular beam epitaxy (MBE) on Al$_{x}$Ga$_{1x}$As (001) surfaces. \textit{In-situ} and \textit {ex-situ} structural characterization, such as RHEED, XRD and TEM, demonstrate the epitaxial single crystallinity of the films. In order to measure the spin injection, tunneling Schottky barrier contact spin-LED structures were fabricated from MBE-grown Al/Co$_{2}$MnGe(70{\AA})/$n$-Al$_{0.1}$Ga$_{0.9}$As/ GaAs(100{\AA})/$p$-Al$_{0.1}$Ga$_{0.9}$As heterostructures. The epitaxial heterostructures were processed into LED devices and the devices were operated with the Schottky contact under reverse bias and the $p-i-n$ LED under forward bias. Electroluminescence was collected along the sample normal. The circular polarization of the observed electroluminescence was 14 {\%} indicating a spin injection of 14{\%} at 2K. The injected spin polarization at 2 K was calculated to be 27 {\%} based on a calibration of the spin detector using Hanle effect measurements. The authors thank NSF-MRSEC, DARPA, and ONR for financial support. [1] S. Fujii, S. Sugimura, S. Ishida, and S. Asano., J. Phys.:Condens. Matter 2, 8583 (1990). [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V10.00006: Gate Controlled Current Oscillation observed in a Fe/In(Ga)As Field-Effect Spin Transistor Structure Kanji Yoh, Marhoun Ferhat, Alexandru Riposan, Joanna Millunchick We report anomalous transport behavior in strained InGaAs heterojunction spin transistor structure embedded with ferromagnetic electrodes (Fe) as spin injector/detector. The meta-stable InGaAs channel layer contains 81{\%} of indium, which enables increased spin-orbit interaction as well as ideal non-alloyed Ohmic contact with Fe spin injector/detector[1][2]. We have observed clear current oscillations when the electrodes were magnetized along the channel current at room temperature. The drain current oscillation dependence on gate voltage agreed with the estimation of spin precession and spin relaxation in the channel based on Monte Carlo simulation. We believe that this is the first observation of spin transistor operation. Our structure design made it possible to tune the Rashba and Dresselhaus effect [3] enabling long lived spin polarization in the 2DEG channel. References [1] H.Ohno, Kanji Yoh et al, Jpn.J.Appl.Phys. Express Lett. Vol.42 pp.L1-L3 (2003) [2] Kanji Yoh, Hiroshi Ohno, et al, ``Spin polarization in photo- and electro-luminescence of InAs and metal/InAs hybrid structures'', Semi.Sci.Technol. 19, 1-4 (2004) [3] Schlieman et al, Phys.Rev.Lett.90, 146801 (2003) [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V10.00007: Vertical spin transport in MnAs/GaMnAs heterostructures S. H. Chun, J. P. Yu, Y. S. Kim, H. K. Choi, Y. D. Park, Z. G. Khim, J. C. Woo Electrical spin injection and detection across metal-semiconductor interface is an important step for practical semiconductor spintronic devices. However, the problem of huge conductance mismatch between metals and semiconductors prohibited the progress. Intensive studies were carried out to find ways to overcome this problem. The use of tunneling or Schottky barrier is one of them and turns out to be quite successful. For example, AlAs tunneling barrier at MnAs/GaMnAs interface or GaMnAs/GaMnAs interface allows efficient spin-polarized tunneling [1][2]. Here, we discuss the effect of barrier strength on the tunneling efficiency in MnAs/GaMnAs heterostructures grown by low-temperature molecular beam epitaxy. Work supported by the Korean Science and Engineering Foundation through the Center for Strongly Correlated Materials Research at Seoul National University and by the KIST Vision 21 program. [1] S. H. Chun \textit{et al}., Phys. Rev. B \textbf{66}, 100408(R) (2002) [2] M. Tanaka and Y. Higo, Phys. Rev. Lett. \textbf{87}, 026602 (2001) [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V10.00008: Interface Effects on Spin Injection in Fe/AlGaAs spin-LEDs T. Zega, R. Stroud, A. Hanbicki, G. Kioseoglou, O. van't Erve, C. Li, B. Jonker, M. Yasar, R. Mallory, A. Petrou We have measured spin-injection efficiency (SIE) at room temperature for Fe/AlGaAs spin light-emitting diodes ranging from 1 to 32{\%}. Here we report results from transmission electron microscope (TEM) studies of the interface between Fe and AlGaAs. Measurements of the interface thickness show an inverse correlation with SIE, i.e., samples with thicker interfaces have lower SIEs. The correlation predicts that as the interface thickness decreases to zero the SIE approaches 37{\%}, comparable to the 40{\%} spin polarization commonly accepted for bulk Fe. The results imply that greater SIE can only be achieved by specifically altering the interface structure to achieve spin filtering. In the other limit, complete suppression of the injected spin polarization occurs if the interface thickness exceeds 2 nm. The possibility of determining atomic-scale interface structure by comparing simulated- to experimental-TEM images will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V10.00009: Effect of interfacial reactions on spin injection from Fe contacts into (Al,Ga)As C. Adelmann, X. Lou, X.Y. Dong, J. Strand, B.D. Schultz, P.A. Crowell, C.J. Palmstrom, S. Park, M.R. Fitzsimmons The effect of interfacial Fe/Al$_{0.1}$Ga$_{0.9}$As reactions on spin injection from Fe into Al$_{0.1}$Ga$_{0.9}$As has been studied by annealing of the heterostructures. For Fe layers grown at $0^\circ$C, annealing at 250$^\circ$C leads to an increase in spin injection by up to a factor of 3. Interfacial modifications are evidenced by an increase in the Fe/Al$_{0.1}$Ga$_{0.9}$As Schottky barrier height but no Fe/Al$_{0.1}$Ga$_{0.9}$As reactions were evident from X-ray diffraction at annealing temperatures $<$350$^\circ$C. The influence of Fe growth temperature on spin injection has also been studied and found to be different from annealing effects. \emph{In situ} X-ray photoelectron spectroscopy showed that Fe growth at 175$^\circ$C leads to the formation of a $\sim 20$ \AA{} thick interfacial reacted layer, which appears to be correlated with spin injection with the opposite sign than for low temperature grown Fe layers or after annealing. These findings are in qualitative agreement with changes in the interfacial magnetic properties of Fe$_{0.5}$Co$_{0.5}$/GaAs determined by polarized neutron reflectometry. Supported by DARPA, ONR, and NSF-MRSEC. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V10.00010: Spin injection and detection in silicon Igor Zutic, Steven Erwin, Jaroslav Fabian Silicon has long spin-relaxation and spin-decoherence times, properties which make it very attractive for spintronics and quantum-information applications [1]. Unfortunately, the underlying origin of these properties---indirect band gap and weak spin-orbit coupling---preclude conventional optical methods of spin injection and detection in semiconductors. We propose two schemes to overcome these difficulties and realize robust spin injection and detection in silicon: one using magnetic semiconductors, and one using direct-gap conventional semiconductors, both in a heterojunction geometry [2]. To analyze the operation of these schemes, we develop an analytical model for spin-polarized transport across a heterojunction. Under realistic operating assumptions, we find that the symmetry properties of charge current with respect to the reversal of equilibrium and nonequilibrium spin polarization can be used to detect spin injection in silicon. [1] I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] I. Zutic, J. Fabian, and S. C. Erwin, preprint cond- mat/0412580. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V10.00011: Spin Tunneling through an Indirect-Gap semiconductor Barrier Subodha Mishra, Sunita Thulasi, Sashi Satpathy We study the spin dependent tunneling of electrons through an indirect conduction minimum of a zinc-blende semiconductor and show that both the transmission coefficient as well as the spin polarization can be substantially large at the same time, unlike the case for tunneling through a direct minimum.$^1$ The spin polarization is calculated using a simple barrier tunneling model. The parameter describing the linear $k$ spin splitting for the indirect minimum is computed using density- functional method. The basic difference is the linear $k$ spin splitting for the indirect minimum, as opposed to the Dresselhaus $k^3$ splitting for the direct minimum at the $\gamma$ point. \newline \newline $^1$V. I. Perel, S. A. Tarasenko, and I. N. Yassievich. Phys. Rev. B {\bf 67}, 201304 (2003). [Preview Abstract] |
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V10.00012: Circular polarization of electroluminescence from InAs-based Spin-LED's C.J. Meining, B.D. McCombe, P. Grabs, I. Chado, G. Schmidt, L.W. Molenkamp We have investigated circular polarized electroluminescence (EL) from InAs-based spin-LED's utilizing CdMnSe to spin-polarize electrons, which are injected into an InAs quantum well. For comparison, non- magnetic structures with CdSe replacing the CdMnSe were also investigated to elucidate the inherent band structure and strong spin-orbit interaction effects in this system. Both magnetic and non-magnetic structures show qualitatively similar dependences of the circular polarization as a function of magnetic field. This behavior will be discussed in terms of a rate equation model that includes the band-structure of electrons and holes in a magnetic field, a finite ratio of recombination time and spin-lifetime, as well as the polarization efficiency of the CdMnSe spin-aligner as a function of injection current, which is determined by in-situ circular polarized photoluminescence measurements. The similarity of the magnetic and non-magnetic aligner results is partially due to the low polarization of the CdMnSe resulting from a low effective Mn concentration.\\ Work supported by DARPA ONR\# N00014-00-1-0951 [Preview Abstract] |
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