Papers

AVS2001 Session EL+MI-WeM: Spintronics III: Ferromagnetic Semiconductors

Wednesday, October 31, 2001 8:20 AM in Room 111

Wednesday Morning

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8:20 AM		EL+MI-WeM-1 Tailoring Spin Ordering in Magnetic Semiconductors T. Dietl (Polish Academy of Sciences, Poland) Recent advances¹ in the field of carrier-controlled ferromagnetism in tetrahedrally coordinated diluted magnetic semiconductors and their nanostructures will be reviewed with a focus on the phenomena important for prospective spintronic devices. Experimental results for III-V materials, where the Mn atoms introduce both spins and holes, will be compared to the case of II-VI compounds, in which the Curie temperatures T_C above 1 K have been observed for the uniformly and modulation-doped p-type structures but not in the case of n-type films. The experiments demonstrating the tunability of T_C by electrostatic gates as well as by light will be presented. The tailoring of domain structures and magnetic anisotropy by strain engineering and confinement will be discussed emphasizing the role of the spin-orbit coupling in the valence band. The question of designing modulated magnetic structures in low dimensional semiconductor systems will be addressed. Recent progress in search for semiconductors with T_C above room temperature and hopes associated with materials containing magnetic ions other than Mn will be presented. ¹ T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 (2000); H. Ohno et al., Nature 408, 944 (2000); P. Kossaki et al., Physica E 6, 709 (2000); D. Ferrand et al., Phys. Rev. B 63, 085201 (2001); T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev. B 63, 195205 (2001).
9:00 AM		EL+MI-WeM-3 Dilute Magnetic Semiconductors Based Upon GaP M.E. Overberg, C.R. Abernathy, S.J. Pearton, N. Theodoropoulou, A.F. Hebard (University of Florida); S.N.G. Chu (Agere Systems); R.G. Wilson (Consultant) Dilute magnetic semiconductors (DMS), where a semiconductor host material is heavily doped with magnetic ions, could potentially be used in a variety of interesting applications and devices where the spin degree of freedom of the electron is exploited, such as quantum-based computation, electro-optic switches and modulators, to name a few. Recent theoretical calculations based upon a 5% concentration of Mn have predicted a Curie temperature for (Ga,Mn)P of roughly 100 K.¹ The challenge is to incorporate such a large amount of magnetic ions while still maintaining the integrity of the host semiconductor. In this paper, we will report on the growth of (Ga,Mn)P:C thin films by gas source molecular beam epitaxy (GSMBE) utilizing phosphine as the group V source, and co-doped with C via a CBr₄ source for enhanced p-type doping. Results of the epitaxially grown films will be compared to (Ga,Mn)P films produced via direct implantation of Mn into GaP:C, particularly in regard to the formation of alternate phases and how this correlates with the observed magnetic behavior. X-ray diffraction (XRD) of the epitaxial films indicates the presence of the orthorhombic MnP phase in layers grown at a temperature of 600°C, and in addition the hexagonal Mn_5.64P₃ phase in films grown at a temperature of 400°C. At lower temperatures, only the Mn_5.64P₃ phase exists. The use of superlattices appears to help suppress the second phase formation, particularly in the low temperature regime. Analysis of the GaMnP:C by SQUID magnetometry suggests the presence of a ferromagnetic phase with a T_C above 50 K. This behavior is most likely due to the presence of ferromagnetic MnP. In an effort to increase the T_C above that which is possible when using Mn, the properties of Ni implanted GaP:C will also be presented. ¹ T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science, 287, p. 1019 (2000).
9:20 AM		EL+MI-WeM-4 Epitaxial Growth of a Group IV Ferromagnetic Semiconductor: Mn_xGe_1-x A.T. Hanbicki, Y.D. Park, A. Wilson, G. Spanos, B.T. Jonker (Naval Research Laboratory) Ferromagnetic semiconductors promise to provide spin-dependent functionality to the well-established technology of semiconductor device heterostructures. While much effort has focused on the III-Mn-V materials such as GaMnAs, where hole density is believed to play a critical role, the origins of ferromagnetic (FM) order remain elusive. This is due in part to complications arising from use of compound semiconductor hosts. Mean field calculations predict that FM order should be stabilized in many other semiconductor materials, if certain Mn concentrations and hole densities can be realized.¹ We have chosen one of the simplest semiconductor hosts, Ge, in which to investigate and better understand such effects. We report here the epitaxial growth of the first Group IV ferromagnetic semiconductor, Mn_xGe_1-x, and describe the structural, magnetic, and magneto-transport properties. Single crystal films were grown for x < 0.1 on GaAs(001) and Ge(001) substrates by MBE at low substrate temperatures from elemental sources. RHEED and x-ray diffraction confirm crystallinity and orientation. Samples exhibit hysteretic M vs H loops with significant remanence, and coercivities of several hundred Gauss. Curie temperatures range from 30 to 120 K with increasing Mn concentration, as determined from SQUID magnetometry. Films exhibit a non-metallic temperature dependent resistivity, as well as a pronounced extraordinary Hall effect. They are strongly p-type with hole densities of 10¹⁹ - 10²⁰ cm^-3. Gated structures confirm that the hole density can be varied. Results will be presented on efforts to toggle ferromagnetism by application of a gate voltage at temperatures significantly higher than recently reported for InM nAs.² This work was supported by the DARPA SpinS program and ONR. ¹ T. Dietl, et al., Science 287, 1019 (2000). ² H. Ohno et al, Nature 408, 944 (2000).
9:40 AM		EL+MI-WeM-5 Metalorganic Chemical Vapor Deposition of ZnO-based Diluted Magnetic Semiconductors A.C. Tuan (University of Washington); D. McCready, S. Thevuthasan, J.W. Rogers, Jr., S.A. Chambers (Pacific Northwest National Laboratory) One of the most attractive means of adding the electron-spin degree of freedom to electronic and photonic devices is by spin injection and transport in semiconducting structures involving ferromagnetic metals or ferromagnetic dilute magnetic semiconductors (DMSs), as spin injectors. A great deal of work has been done in this area and thus far, DMSs have proven to be more efficient spin sources than ferromagnetic metals. This is because the conductivities of DMSs are better matched to those of the channel material. However, because of the very low ferromagnetic ordering temperature of current DMSs, efficient spin injection is only observed at cryogenic temperatures. A recent calculation predicts that heavily nitrogen-doped Mn_XZn_1-XO will exhibit a Curie temperature of ~320K,¹ and serves as partial motivation for this work. We have grown Mn_XZn_1-XO films by metalorganic chemical vapor deposition (MOCVD), using a direct liquid injection system and β-diketonate metal sources. Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) show that compositions ranging from pure ZnO to ~Mn_0.30Zn_0.70O are achievable on Al₂O₃(0001) substrates, without carbon contamination. X-ray diffraction (XRD) was used to confirm that there was no phase segregation. XRD also indicated that all films grown below 500 °C had a preferred (0001) orientation, while epitaxy of ZnO was possible at 575 °C on both Al₂O₃ (0001) and ZnO(0001). Preliminary attempts at nitrogen incorporation show that NH₃ is a promising nitrogen source. ¹T. Dietl, H. Ohno, F. Matsukura et al., "Zener model description of ferromagnetism in zinc-blende magnetic semiconductors," Science 287, 1019-22 (2000).
10:00 AM		EL+MI-WeM-6 Heterostructures of Mn/GaN and MnGa/GaN Grown by ECR-Plasma Assisted MBE Y. Cui, L. Li (University of Wisconsin) Ferromagnetic semiconductor GaMnN is very promising for developing spintronic devices.¹ To investigate the mechanism of Mn incorporation in GaN, we have grown digital heterostructures by alternately depositing GaN (10 to 50 monolayers) and monolayer of either Mn or MnGa using ECR plasma assisted molecular beam epitaxy. The heterostructures are grown on 6H-SiC(0001) substrate with plasma power of 30 W at 550 °C. Adsorption and desorption of the Mn and MnGa on the non-growing surface, surface reactions and reconstruction are monitored by reflection high-energy electron diffraction (RHEED) and in situ scanning tunneling microscopy (STM). All the surfaces immediately following MBE are composed of spiral hillocks, with the edges of the spirals form bilayer steps. On the terraces, a gallium rich (1x1) structure is observed with a lattice spacing of 3.2 Å. At temperatures between 500 and 550 °C, deposition of approximately 1 ML of Mn on this surface results in a domain superstructure with a periodicity of ~ 32 Å. Within the domains the surface atoms are in (√3x√3) geometry. By closely monitoring the surface reconstruction present during MBE, heterostructures with high crystalline quality are grown, as confirmed by high-resolution x-ray diffraction. These results and their implications for Mn doping of GaN will be presented at the meeting. This research is supported by NSF DMR-0094105. ¹T. Dietl et al., Science 287, 1019 (2000).
10:20 AM		EL+MI-WeM-7 Epitaxial Growth of GaMnN G.T. Thaler, M.E. Overberg, C.R. Abernathy, S.J. Pearton, N. Theodoropoulou, A.F. Hebard (University of Florida) Dilute magnetic semiconductors (DMS) offer the use of the spin degree of freedom of the electron in addition to its charge in device applications. Recent theoretical calculations have predicted a Curie temperature for GaMnN of roughly 400 K.¹ In this talk we will discuss the feasibility of growing GaMnN via gas-source molecular beam epitaxy. Mn levels up to 47% as determined by Auger electron spectroscopy (AES) have been obtained in GaMnN. X-ray diffraction (XRD) shows no evidence of second phase formation in films with Mn concentrations less than 9%. Addition of Mn to the GaN changes the conductivity from highly conductive n-type to highly resistive, suggesting that at least some of the Mn behaves as a deep acceptor. Nominally semi-insulating GaMnN with a Mn concentration of ~5% shows paramagnetic behavior with a saturation moment per Mn of 3.9 Bohr magnetons, suggesting that much of the Mn is substitutional. Increasing the growth temperature increases the electron concentration due to enhanced incorporation of nitrogen vacancies. The increased carrier concentration produces what appears to be a ferromagnetic material, but with a low Curie temperature of ~100K. Similar studies on p-GaMnN will be discussed in this talk as will the effect of adding Al to the GaMnN. ¹ T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science, 287, p. 1019 (2000).
10:40 AM		EL+MI-WeM-8 The Investigation of MnScN Grown by Molecular Beam Epitaxy H.A.H. Al-Brithen, H. Yang, A.R. Smith (Ohio University) Nitride semiconductors are well known as important materials due to their unique electronic and optical properties. If it is possible to incorporate magnetic species (i.e. Mn or Fe) into nitrides, these may also be used as magnetic semiconductors for spintronics. Scandium nitride, shown to be a semiconductor, has been studied lately for its potential electronic applications.¹ Since both MnN and ScN are known to exist with octohedral bonding,² it seems likely that Mn may be soluble in ScN. Thus, we have undertaken a study of the growth of MnScN by molecular beam epitaxy. Mn_xSc_1-xN growth is initiated on a buffer of ScN grown on MgO(001). Growth has been performed for samples at substrate temperature between 350°C and 650°C with Mn/Sc intended flux ratios of 10 and 20%. For all samples, XRD reveals a clear alloy peak which is distinguishable from the ScN and MgO peaks. From the comparison of the alloy peak position with that of ScN and the expected peak position of MnN (based on separate studies of this binary compound³), the values of x are estimated and found to be in good agreement with the Mn/Sc flux ratios. RHEED patterns shows smooth growth on the ScN buffer at the initial stage. The evolution of the RHEED pattern depends on the substrate temperature and the Mn flux. For Mn/Sc ratio of 10%, the RHEED pattern show only a single phase with smooth growth; but for Mn/Sc ratio of 20%, some weak ring structures are observed after extended growth time, indicating a possible limit to the solubility of Mn in ScN. Studies of the magnetic properties of the MnScN alloy are underway. Work is supported by NSF. ¹ H. A. Al-Brithen and A. R. Smith, Appl. Phys. Lett., 77, 2485 (2000); A.R. Smith et al., to be published. ² Suzuki et al., J. Alloys and Compounds 306, 66 (2000) ³ H. Yang et al., Appl. Phys. Lett. 78, 3860 (2001).
11:00 AM		EL+MI-WeM-9 MBE Growth and Properties of Co-doped TiO₂ Anatase S.A. Chambers, S. Thevuthasan, D. McCready (Pacific Northwest National Laboratory); R.F.C. Farrow, R.F. Marks, L. Folks (IBM Almaden Research Center); N. Ruzycki, D.L. Ederer, U. Diebold (Tulane University) The realization of fully functional spintronics requires semiconductors that are magnetic at ambient temperature. In a recent publication¹, Co-doped anatase TiO₂ (Co_xTi_1-xO₂) epitaxial films grown on SrTiO₃(001) and LaAlO₃(001) by laser ablation were shown to exhibit weak ferromagnetism at room temperature for x up to 0.08. We have grown Co_xTi_1-xO₂ by oxygen-plasma-assisted molecular beam epitaxy on SrTiO₃(001) and SrAlLaO₄(001) substrates, for which the lattice mismatches are -3.1% and -0.8%, respectively. Preliminary results have confirmed that this material can be ferromagnetic at room temperature. Kerr effect measurements on some samples show larger remanence (>40%) than that seen in fig. 3 of ref. 1. In all cases, reflection high-energy electron diffraction measured during growth reveals that the film surface becomes progressively more disordered with increasing thickness, with secondary phases and/or complete disordering occurring after several tens of nm. X-ray diffraction reveals the presence of rutile in some cases. X-ray photoemission and x-ray absorption spectroscopy carried out at the LBNL Advanced Light Source reveal that Co is extremely mobile in the anatase lattice, and in most cases has a tendency to concentrate in the near-surface region. In addition, these spectroscopies reveal that Co in the ferromagnetic films is nearly 100% Co(II), whereas a mix of Co(II) and Co(III) is found in nonmagnetic films. Hall effect and TEM measurements are being carried out at the time of abstract preparation, and will be discussed at the conference. These preliminary results reveal that the single largest obstacle to reproducibility is the extremely facile diffusion of Co in the anatase lattice. ¹Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P, Ahmet, T. Chikyow, S.-Y. Koshihara, and H. Koinuma, Science, 291, 854 (2001).
11:20 AM		EL+MI-WeM-10 Epitaxial Growth of an n-Type Ferromagnetic Semiconductor: CdCr₂Se₄^* Y.D. Park, A.T. Hanbicki, J.E. Mattson, B.T. Jonker (Naval Research Laboratory) Ferromagnetic semiconductors (FMSs) provide unprecedented opportunity to tune and optimize spin-dependent behavior in semiconductor device heterostructures. Most efforts have focused on III-Mn-V materials such as GaMnAs, which are p-type only.¹ Since efficient spin injection and very long spin scattering lengths have been confirmed for electrons rather than holes in semiconductors such as GaAs,^2-4 one would like to realize FMS materials which are both n-type and can be epitaxially grown on a readily available device-quality substrate. We report here the epitaxial growth of FMS CdCr₂Se₄(001) films on both GaAs and GaP(001) substrates, and describe the structural, magnetic and electronic properties. The samples were grown by molecular beam epitaxy from elemental K-cell sources, and exhibit a 1x1 RHEED pattern during growth. The film structure, orientation and composition were determined by post-growth x-ray diffraction and fluorescence measurements. SQUID magnetometry data confirm ferromagnetic order with a Curie temperature of 130 K, as in the bulk material, and hysteretic behavior with significant remanence. The easy axis is in-plane with a coercive field of ~125 Oe. Temperature dependent transport data show that the films are semiconducting in character, and lightly n-type as grown. We further describe efforts at controlled doping and electrical spin injection from CdCr₂Se₄ contacts into GaAs-based LED heterostructures. ^* This work was supported by the DARPA SpinS program and ONR. ¹ Ohno, Science 281, 951 (1998). ² Fiederling et al, Nature 402, 787 (1999). ³ Jonker et al, PRB 62, 8180 (2000); Park el al, APL 77, 3989 (2000). ⁴ Oestreich et al, APL 74, 1251 (1999).
11:40 AM		EL+MI-WeM-11 Characterization of High Dose Mn, Fe and Ni Implantation into p-GaN S.J. Pearton, N. Theodoropoulou, A.F. Hebard (University of Florida); S.N.G. Chu (Bell Laboratories, Lucent Technologies); M.E. Overberg, C.R. Abernathy (University of Florida); R.G. Wilson (Consultant); J.M. Zavada (U. S. Army European Research Office, UK) High concentrations (3-5at.%) of Mn, Fe and Ni were incorporated into p-GaN by direct implantation at elevated substrate temperature (350°C). Subsequent annealing at 700°C produced apparent ferromagnetic behavior below ~175 K for the 3at.% Fe sample and ~100 K for the 5at.% Fe sample. Selected area diffraction patterns did not reveal the presence of any other phases in the Fe-implanted region. For Mn-implantation, ferromagnetic contributions to the magnetization were observed below 250K in 3at.% samples. In this material, platelets consistent with the formation of GaMnN were observed by TEM. The direct implantation process appears promising for examining the properties of magnetic semiconductors with application to magnetotransport and magneto-optical devices.