Session B12: Focus Session: Spin Injection

Highly Efficient Room Temperature Spin Injection Using Spin Filtering in MgO

Efficient electrical spin injection into GaAs/AlGaAs quantum well structures was demonstrated using CoFe/MgO tunnel spin injectors at room temperature. The spin polarization of the injected electron current was inferred from the circular polarization of electroluminescence from the quantum well. Polarization values as high as 57{\%} at 100 K and 47{\%} at 290 K were obtained in a perpendicular magnetic field of 5 Tesla. The interface between the tunnel spin injector and the GaAs interface remained stable even after thermal annealing at 400 $^{o}$C. The temperature dependence of the electron-hole recombination time and the electron spin relaxation time in the quantum well was measured using time-resolved optical techniques. By taking into account of these properties of the quantum well, the intrinsic spin injection efficiency can be deduced. We conclude that the efficiency of spin injection from a CoFe/MgO spin injector is nearly independent of temperature and, moreover, is highly efficient with an efficiency of $\sim $ 70{\%} for the temperature range studied (10 K to room temperature). Tunnel spin injectors are thus highly promising components of future semiconductor spintronic devices. \newline \newline Collaborators: Roger Wang$^{1, 3}$, Gian Salis$^{2}$, Robert Shelby$^{1}$, Roger Macfarlane$^{1}$, Seth Bank$^{3}$, Glenn Solomon$^{3}$, James Harris$^{3}$, Stuart S. P. Parkin$^{1}$ \newline $^{1 }$\textit{IBM Almaden Research Center, San Jose, CA 95120} \newline $^{2}$\textit{ IBM Zurich Research Laboratory, S\"{a}umerstrasse 4, 8803 R\"{u}schlikon, Switzerland } \newline $^{3}$\textit{ Solid States and Photonics Laboratory, Stanford University, Stanford, CA 94305}   

Electrical spin injection from Fe into Al$_{x}$Ga$_{1-x}$As quantum well spin-LEDs

We have studied Fe spin LEDs in which electron-hole recombination takes place either in GaAs or in Al$_{x}$Ga$_{1-x}$As quantum wells (QW). The dependence of the electroluminescence circular polarization P on temperature T in these two types of devices at fixed magnetic field is compared. The polarization in the Al$_{x}$Ga$_{1-x}$As QW LEDs decreases much more slowly with temperature compared with the GaAs QW LEDs; the polarization of the former persists up to room temperature. The improved high temperature performance of the Al$_{x}$Ga$_{1-x}$As spin LEDs is tentatively attributed to the localization of the recombining electron-hole pairs by potential fluctuations in the QW. These sites have zero-dimensional character suppressing the Dyakonov-Perel spin scattering mechanism.   

Spin injection from Fe into GaAs quantum wells populated by electrons or holes: A comparison

We have studied the circular polarization of band-edge electroluminescence (EL) from three types of AlGaAs(n)/GaAs(i)/AlGaAs(p) light emitting diodes (LEDs) in which the electrons are injected from ferromagnetic Fe contacts. In the first (second) group the GaAs quantum well is populated by electrons (holes) due to excess n-type (p-type) doping in the n-AlGaAs (p-AlGaAs) barrier. In the third device type the GaAs quantum wells are empty and these LEDs are used as reference samples. We have compared the magneto-optical characteristics (dependence of the EL circular polarization P as function of magnetic field, current, and photon energy) of these three groups. Significant differences have been identified which must be taken into account in order to determine accurately the spin injection efficiency of these devices.   

Generation and detection of spin current in GaAs with MgO tunnel barriers Replace

The MgO tunnel barrier has been proven as one of best candidates for the spintronic memory and switching devices. When one injects and detects spin polarized carriers efficiently into (and out of) semiconductors, the use of tunnel barrier (TB) is expected to avoid the conductivity mismatch and provide a high feasibility for the fabrication of a spin transistor. To reach this goal evaluation of the TB on a semiconductor is an important issue. In this work, we report the combination of spin dependent photocurrent generation and electrical detection as an efficient technique for understanding the role of the MgO TB grown on GaAs. We used (100)GaAs/MgO/Fe structures prepared in an MBE chamber. Our results show that spin filtering effects are largely influenced by the quality of MgO TBs. The estimated photocurrent polarization reaches up to approximately 80{\%} at RT in a certain forward bias region which is associated with transport processes. The efficient room temperature spin filtering for GaAs/MgO/Fe structures observed here has not been reported yet for either Fe/GaAs or Fe/Al$_{2}$O$_{3}$/GaAs structure. The possible origin will be discussed in detail.   

Electrical spin injection from Fe$_{1-x}$Ga$_{x}$ (001) films into AlGaAs/GaAs(001) LEDs

Electron spin polarizations of 40-70{\%} have been obtained in GaAs due to electrical injection from Fe or FeCo contacts using surface-emitting spin-LEDs. In such LEDs, since Fe has its magnetization easy axis in the substrate plane, a large magnetic field ($>$2.2 tesla) along the surface normal is required to saturate the magnetization out-of-plane. We have grown epitaxial films of Fe$_{1-x}$Ga$_{x}$ (0 $<$ x $<$ 0.75), a material noted for its high magnetostriction, on AlGaAs/GaAs (001) heterostructures, and summarize the structure, magnetization, spin polarization, and results for electrical spin injection into AlGaAs/GaAs. The out-of-plane saturation field and magnetization decrease rapidly with Ga content, but the point contact spin polarization remains near that of Fe for x $\le $ 0.5. Electrical spin injection from an Fe$_{0.5}$Ga$_{0.5}$ contact produces an electron spin polarization of 30{\%} in the GaAs at 20 K, similar to that obtained from Fe contacts, but with out-of-plane saturation fields as low as 0.4 T. Post-annealing at low temperature increases the electron spin polarization up to 40{\%} and it will be discussed at the meeting.   

Electron distribution among the $\Gamma $, L, and X GaAs conduction band valleys in an Fe/GaAs(n) Schottky barrier.

The electroluminescence (EL) spectra from Fe/GaAs(n)/InGaAs/GaAs(p) spin-LEDs have a complicated composition. In addition to the $e_1 \ell_1 $light and $e_1 h_1 $ heavy hole excitonic features the band-edge EL contains the following phonon replicas: $e_1 h_1 -TA$, $e_1 \ell_1 -LO$, $e_1 h_1 -LO$, $e_1 h_1 -LO-TA$, and $e_1 h_1 -LO-LA$. The replicas are interpreted as due to recombination processes that involve electrons occupying the L and X valleys of the GaAs(n) conduction band. The high momentum electrons are promoted to the higher energy L and X valleys by the strong electric field at the Fe/semiconductor interface [1]. \newline [1] S. Saikin et al, J. Phys: Condens.Matter \textbf{18}, 1535, (2006)   

Electrical spin injection into the ground and excited states of uniform InAs quantum dots

Spin-polarized electrons from Fe contacts were injected into zero-dimensional InAs quantum dots (QDs) where they recombine with unpolarized holes. Using standard MBE growth techniques, the QD density was reduced resulting in uniform dot-size distribution. In these new samples the broad electroluminescence (EL) observed previously in the high QD density LEDs is replaced by distinct features associated with the atomic-like s-, p-, d-, and f-shells of the QDs. As the diode bias voltage was increased, higher energy shells became populated. The circular polarization of these features was studied as function of applied magnetic field, bias, and temperature. Significant differences were observed in the behavior of the various EL features. The polarization dependence on magnetic field confirms spin injection from Fe. Furthermore, higher energy shells exhibit correspondingly higher polarization values consistent with optical pumping studies.   

Measurement of the spin detection efficiency of the s, p,d, and f shells in InAs QDs using optical pumping

We have carried out an optical pumping study of n-i-p AlGaAs(n)/GaAs(i)/AlGaAs(p) quantum wells (QW) structures that incorporate a single layer of InAs quantum dots (QDs) in the GaAs QW. The electron-hole pairs were photo-excited in the GaAs QWs and were subsequently captured by the InAs QDs. The resulting photoluminescence (PL) spectra contain features associated with recombination processes that involve electrons occupying the s- , p-, d-, and f-shells. The circular polarization of these features varies from 10 {\%} to 30 {\%} and increases monotonically from the s- to the f-shell. From these circular polarization measurements the spin detection efficiency for each shell was determined as function of temperature and longitudinal magnetic field applied along a direction perpendicular to the QD plane (z-axis). The optical pumping results, in combination with magneto-EL studies of Fe spin LEDs that have the same parameters and were grown under identical conditions were used to determine the injected electrons spin polarization in these devices.   

Model for the Bias Dependence of the Sign of Spin Injection in Ferromagnetic Metal/Semiconductor Schottky Tunnel Contacts

We examine theoretically the spin-polarized electron injection across a ferromagnetic metal/semiconductor Schottky tunnel barrier. The example structures we model consist of thin, heavily doped regions near an Fe/GaAs interface in which the band bending associated with the Schottky barrier is accommodated, and lightly doped bulk regions.$^{1,2}$ Under reverse bias electrons tunnel from the Fe through the space charge layer into the lightly doped region; under forward bias electrons tunnel from the GaAs into the Fe. Because Fe is ferromagnetic, the transmission is spin-dependent. The shape of the tunnel barrier depends strongly on bias. By considering the asymptotic forms of the wave functions, we show that the bias dependence of the spin-dependent transmission coefficients generally can induce a change in the sign of the spin-polarized current. Model calculations reveal that spin-polarized transport is sensitive to the tunnel barrier properties. The results are in good agreement with recent experimental data. 1) A.T. Hanbicki et al., Appl. Phys. Lett. 80, 1240 (2002). 2) S.A. Crooker et al., Science 309, 2191 (2005).   

Hot electron injection, vertical transport, and electrical spin detection in Silicon

In our devices, spin-dependent hot electron transport through metallic ferromagnetic thin films is used to polarize a charge current injected into the conduction band of Si, and then to analyze the remaining polarization after vertical drift. Our measurements of a clear spin-valve signature indicate substantial electron spin polarization after transport through several microns of Si.   

All Epitaxial Heterostructure for Spin Injection from a Half Metal into Silicon

Using reactive molecular-beam epitaxy, epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ / SrTiO$_{3}$ / Si heterostructures have been grown. The SrTiO$_{3}$ layer, just a few unit cells thick, serves simultaneously as a tunnel barrier and as a means to reduce reaction between the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and the underlying Si. The growth of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ at MBE-compatible pressures requires ozone, which readily oxidizes bare Si and would destroy the chances for epitaxial growth. In contrast, epitaxial SrTiO$_{3}$ can be grown on (001) Si using molecular oxygen via a complex, but established process. Once the SrTiO$_{3}$ film is complete, ozone is turned on for the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ growth. The thin SrTiO$_{3}$ layer acts as a diffusion barrier for oxygen limiting the formation of SiO$_{2}$ at the SrTiO$_{3}$/Si interface. X-ray diffraction measurements show that the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer has good crystalline quality with rocking curve full width at half maximum values of the 200 peak of less than 0.5\r{ }. Furthermore, electrical transport measurements indicate that the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer is ferromagnetic and metallic below $\sim $370 K with a resistivity $<$100 $\mu \Omega $--cm at 4.2 K. Possible devices for tunneling spins into Si and for detecting spin carrier density inside a Si channel will be discussed.   

Non-equilibrium Fe-Si thin films as potential spin injection materials

Fe-Si thin films are potential spintronics materials for its tunable structural, magnetic, and electric properties [1]. Our goal is to inject spin polarized electrons from iron-silicides into Si through a Schottky barrier, which is formed by choosing the proper doping level for Si, and a suitable Iron-silicide composition. We prepared Fe$_{1-x}$Si$_{x}$ (x = 0.25 - 0.5) films by electron beam co-evaporation from Fe and Si sources onto Si substrates under ultra-high vacuum conditions. Growth at 300 \r{ }C leads to a homogeneous Fe$_{1-x}$Si$_{x}$ magnetic alloy with both Tc and room temperature magnetization monotonically decrease with an increasing x. X-ray diffraction patterns show that a thin seed layer of FeSi at interface reduces the lattice mismatch between Si substrate and the bcc Fe$_{1-x}$Si$_{x}$ film, so that epitaxial growth can be realized. The seed layer also plays the role of reducing interdiffusion. A clear interface at Iron-silicide and silicon is obtained, resulting in a good Schottky barrier with height around 0.7 eV. By adjusting the Fe$_{1-x}$Si$_{x}$ composition, the resistivity of iron-silicide can be tailored. Ferromagnetic Fe$_{55}$Si$_{45}$ shows resistivity of 10$^{-3}$ ohm-cm, and magnetization 100 emu/cc at 300 K. By increasing the iron-silicide resistivity, one of the main obstacles for spin-injection from metal to semiconductor -- the resistivity mismatch -- can be overcome. Experiments based on spin-valve-type magneto-resistance for spin injection detection will be discussed. [1]. Ionescu et al. Physical Review B 71, 94401 (2005).   

Spin Injection and Spin Dynamics at CuPc/GaAs (100) Interface

Spin injection from GaAs (100) to organic semiconductor copper phthalocyanine (CuPc) has been investigated experimentally with spin-resolved two-photon photoemission (SR-2PPE) spectroscopy. The spin-polarized electrons are originally generated in GaAs through optical pumping with femtosecond time resolution and injected into CuPc film. We observed an enhancement in spin polarization at the interface after initial CuPc deposition. This demonstrates that interface spin scattering is insignificant, which is similar to our previous results of spin injection at CuPc/Co interface. The spin polarization dropped when the CuPc film became thick, an effect attributed to bulk attenuation in CuPc. The lifetime of the unoccupied orbits in CuPc was also studied with red-blue excitation of photon energy of 1.6 eV and 3.2 eV, respectively. There was a strong asymmetry in the time-resolved spectra, and an unexpected long lifetime for the low intermediate state was observed. A simple explanation of this phenomenon will be discussed.