Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

Effect of cobalt substitution on microstructure and magnetic properties in ZnO nanoparticles

Ferromagentic semiconductors have been actively pursued because of their potential as spin polarized carrier sources and easy integration into semiconductor technology. One such material, ZnO has been shown to be a potential Diluted Magnetic Semiconductor (DMS). The appearance of ferromagnetism, however, is found to be sensitive to the processing conditions. We report synthesis of ZnO nanoparticles of size ∼20 nm by a simple co-precipitation technique using metal nitrates and NaOH as precipitant. The particles are self-organised and reveal single crystalline behaviour in electron diffraction pattern. Incorporation of Co in ZnO matrix leads not only to the reduction in crystallite size but also to the modification of the structure. At 5% Co, the particles are highly textured. The particles also aggregate and the aggregated mass have nearly rectangular shape as seen through TEM. Increasing Co to 10%, results into further reduction of particle size and the particles self organize in a line, which looks like nanofibers. This alignment of particles increases by increasing the Co content further. This type of growth of nanofibers above Co ≥ 10% is well correlated with the anisotropic peak broadening observed in the XRD spectra. In addition, Co substitute Zn site up to 20% without showing any extra phase in XRD spectra as compared to 7 to 10% in case of bulk. Transport and magnetic studies indicate that conductivity increases with increasing Co content, but carrier mediated ferromagnetism is absent down to 10 K.      

Photo-induced magnetic polarons in semiconductors

Exciton magnetic polarons observed in dilute magnetic semiconductors were investigated by steady-state and pico-second time-resolved photoluminescence measurements and have shown characteristic behavior of exciton localization processes in bulk-Cd_1-x Mn _x Te and also in the quantum structures composed of the dilute magnetic semiconductors. For the quantum structures spin-dependent coherent polarizations associated with excitons and biexcitons were studied by degenerate four-wave mixing experiment. Also investigated for different chalcogenide spinel ferromagnetic semiconductors was photo-induced enhancement of exchange interaction between magnetic ions by direct magnetic flux detection in the vicinity of the Curie temperatures.     

Unconventional ferromagnetism and transport properties of (In,Mn)Sb dilute magnetic semiconductor

Narrow-gap higher mobility semiconducting alloys In_1-xMn_xSb were synthesized in polycrystalline form and their magnetic and transport properties have been investigated. Ferromagnetic response in In_0.98Mn_0.02Sb was detected by the observation of clear hysteresis loops up to room temperature in direct magnetization measurements. An unconventional (reentrant) magnetization versus temperature behavior has been found. We explained the observed peculiarities within the frameworks of recent models which suggest that a strong temperature dependence of the carrier density is a crucial parameter determining carrier-mediated ferromagnetism of (III,Mn)V semiconductors. The correlation between magnetic states and transport properties of the sample has been discussed. The contact spectroscopy method is used to investigate a band structure of (InMn)Sb near the Fermi level. Measurements of the degree of charge current spin polarization have been carried out using the point contact Andreev reflection (AR) spectroscopy. The AR data are analyzed by introducing a quasiparticle spectrum broadening, which is likely to be related to magnetic scattering in the contact. The AR spectroscopy data argued that at low temperature the sample is decomposed on metallic ferromagnetic clusters with relatively high spin polarization of charge carriers (up to 65% at 4.2 K) within a cluster.     

Monte Carlo simulation of spin relaxation in core-shell nanowires of dilute magnetic semiconductors

Spin transport behaviour in stand-alone and core-shell nanowire (NW) structure composed of dilute magnetic semiconductor (DMS) has been analysed using Semi-classical Monte Carlo approach. Inspired by recent attempts on exploring various factors instrumental in determining the spin dynamics, we have employed four DMS materials, namely, CdMnS, CdMnSe, ZnMnSe and CdMnTe for our study. Dominant mechanisms for spin relaxation, D'yakonov-Perel and Elliot-Yafet, have been actively employed in our heuristic model to simulate the spin transport. The dependence of spin relaxation length (SRL) on the diameter of the core has been observed and explained. The first order calculations used to develop the model shows the superiority of the core-shell structure over stand-alone nanowire (NW) structure in terms of spin transport.     

Optical and magnetic properties of InFeP layers prepared by Fe~+ implantation

InFeP layers are prepared by ion implantation of InP with 100-keV Fe＋ ions to a dose of 5 ×10^16 cm-2 and investigated by optical, magnetic, and ion beam analysis measurements. Photoluminescence measurements show a deep-level peak at 1.035 eV due to Fe in InP and two exciton-related luminescences at 1.426 eV and 1.376 eV in the implanted samples annealed at 400℃. Conversion electron Mossbauer spectroscopy reveals a doublet corresponding to Fe3＋ ions in the indium sites. Atomic force microscopy and magnetic force microscopy show that magnetic clusters are formed in the annealing process. The magnetization-field hysteresis loops show ferromagnetic properties persisting up to room temperature with a coercive field of 100 0e （10e = 79.5775 A-m-1）, saturation magnetization of 4.35 × 10-5 emu, and remnant magnetization of 4.4× 10 6 emu.     

Multi-spin-subband structure of dilute magnetic semiconductor quantum wells: Feedback mechanism

Using a fully self-consistent envelope function approach, we focus on wide conduction band NMS (non-magnetic semiconductor)/DMS (dilute magnetic semiconductor)/NMS quantum wells, under weak external parallel magnetic field, where many spin-subbands are usually present. We concentrate on small values of the magnetic field because we want to investigate the influence of the feedback mechanism due to the difference of the concentrations of spin-up and spin-down carriers which could induce spontaneous spin-polarization i.e. in the absence of a magnetic field. We study the spin-subband structure, the spin-subband populations and the spin-polarization as functions of the sheet carrier concentration, _Ns$N_s$, for different values of the magnitude of the exchange interaction, |J|$\left|J\right|$, between the itinerant carriers and the magnetic impurities. Our calculations for 0.01 T show that at 20 K the values of |J|$\left|J\right|$ necessary to make this feedback mechanism sufficiently strong are too high compared to the |J|$\left|J\right|$ values of common Mn-doped systems in the conduction band. However, the feedback mechanism will be sufficiently strong at low enough temperatures below 20 K for realistic values of |J|$\left|J\right|$. Moreover, we explain how increasing the sheet carrier concentration the heterostructure is transformed from an almost square quantum well to a system of two coupled heterojunctions with an intermediate soft barrier.     

Stabilization of organic thin film transistors by ion implantation

We report on the effects of low energy ion implantation (N and Ne) in the reduction and control of the degradation of pentacene organic thin film transistors (OTFTs) due to the exposure to atmosphere (i.e. oxygen and water). We have observed that a controlled damage depth distribution preserves the functionality of the devices, even if ion implantation induces significant molecular structure modifications, in particular a combination of dehydrogenation and carbonification effects. No relevant changes in the pentacene thin film thickness have been observed. The two major transport parameters that characterize OTFT performance are the carrier mobility and the threshold voltage. We have monitored the effectiveness of this process in stabilizing the device by monitoring the carrier mobility and the threshold voltage over a long time (over 2000 h). Finally, we have assessed by depth resolved X-ray Photoemission Spectroscopy analyses that, by selectively implanting with ions that can react with the hydrocarbon matrix (e.g. N⁺), it is possible to locally modify the charge distribution within the organic layer.     

Room-temperature ferromagnetism of Cu ion-implanted Ga-doped ZnO

1 MeV Cu²⁺ ions have been implanted into un-doped ZnO and Ga-doped ZnO films with a dose of 1 × 10¹⁷ ions/cm² at room-temperature. Cu ion-implanted Ga-doped ZnO had ferromagnetism at room-temperature and the saturation magnetization of this sample was estimated to be 0.12 μ_B per Cu, while the Cu ion-implanted un-doped ZnO did not show ferromagnetic behavior. Near-edge X-ray fine structure (NEXAFS) spectroscopy revealed that a partial amount of implanted Cu ions existed as Cu²⁺ (d⁹) state in Ga-doped ZnO film. On the other hand, almost Cu atoms existed as Cu¹⁺ (d¹⁰) state in un-doped ZnO film. However, the subsequent annealing at temperature above 800 °C on this ferromagnetic sample induced the annihilation of ferromagnetism due to the formation of non-ferromagnetic Cu₂O phase.     

Influence of background carriers on magnetic properties of Mn-doped dilute magnetic Si

Using hybrid exchange density functional calculations we show that the type of background carriers has profound effects on magnetic interactions in Mn doped dilute magnetic Si. The p- and n-type Si were simulated by introducing an extra hole and an extra electron, respectively in the 64 atoms Si supercell. In case of p-type Si compensated by a homogeneous background potential and 1.6% Mn, the ground state is ferromagnetic, whereas other conditions remaining the same, the ground state becomes antiferromagnetic for the n-type Si. The exchange energies in Mn-doped extrinsic Si are higher by about 1 eV/Mn atom compared to the Mn doped intrinsic Si. Calculated electronic structures reveal that in p-type Si:Mn the hole localises over Mn and the short range magnetic coupling increases. Our calculations indicate that localisation of magnetic polarons at the Mn site is likely, which in turn enhances long range magnetic interaction between Mn ions and responsible for FM stabilisation. On the other hand, in the n-type host electron–electron repulsion increases within Mn–Si impurity band and the short range coupling decreases, which destroys the long range spin polarisation. These calculations explain the observed ferromagnetism in the p-type Si:Mn at higher temperatures than in the n-type Si:Mn and the magnetic moments of the systems compare well with experiments.     

Combinatorial fabrication and magnetic properties of homoepitaxial Co and Li co-doped NiO thin-film nanostructures

Magnetic properties of nanostructured epitaxial thin layers of a series of Co and Li co-doped NiO on MgO(1 0 0) substrate with NiO buffer layer have been investigated. Thin films were synthesized by combinatorial laser molecular beam epitaxy (CLMBE) in the continuous binary composition spread approach. Large and linear variation of x was achieved in the growth of Co_xLi_0.2Ni_0.8−xO_, onto 9 mm of single substrate. Homoepitaxial growth with smooth surface morphology was confirmed by grazing incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM). Linear decrease in the band gap and optical transparency was observed with increasing cobalt concentration. The magneto-optical Kerr effect revealed a strong photon energy dependency with negative Kerr rotation for all the Co-concentrations in the film, suggesting intra-valence charge transfer (IVCT) between low spin state Co²⁺ with host Ni²⁺. Ferromagnetic (FM)-like ordering was observed at low temperatures, while antiferromagnetism predominates at room temperature in the Co and Li co-doped nickel oxide epitaxial films.     

Structural and magnetic properties of Fe doped NiO

Mott-Hubbard anti-ferromagnetic insulator, NiO shows p-type semiconducting behaviour due to vacancy at Ni²⁺ site in its bunsenite structure. We report the modification of structural and magnetic order in NiO on Fe doping. NiO samples at different Fe concentrations in the range 0 to 5 at.% have been prepared by chemical co-precipitation and post thermal decomposition method. Both structural and magnetic characterization reveal that with increasing Fe doping concentration, NiO evolves as a magnetically inhomogeneous state out of the parent homogeneous antiferromagnetic state. In addition, structural inhomogeneity was also observed with Fe precipitating to γ-Fe₂O₃ phase, the signature of which could be clearly seen for Fe content beyond 2 at.%. At lower Fe content however, some amount of Fe occupies lattice and interstitial sites in the NiO matrix and drive the latter to acquire ferromagnetic ordering, which was evident from a clear hysteresis loop at 300 K.      

The magnetic mechanism of Zn<Subscript>0.93</Subscript>Co<Subscript>0.07</Subscript>O thin films

Zn_0.93Co_0.07O thin films infiltrated with nitrogen and aluminum were prepared by means of magneton sputtering. The structural and magnetic properties of the films were studied systematically. The materials were single phase (wurtzite structure) with surfaces showing signs of homogeneous growth. The films were ferromagnetic at room temperature, and magnetic domains could be clearly observed on the surfaces. In the case of Al infiltration, saturated magnetization increased with Al concentration increasing; whereas in the case of N infiltration, saturated magnetization decreased with the increase in N concentration. The results show that ferromagnetic interactions in Co-doped ZnO diluted magnetic semiconductor may be transferred by electrons. Supported by the National Natural Science Foundation of China (Grant No. 10674059) and the Major Project of National Basic Research Program of China (Grant No. 2005CB623605)     

Effect of hydrogen plasma implantation on the micro-structure and magnetic properties of hcp-Co_(80)~(57)Fe_4Ir_(16) thin films

We present detailed investigations of structural and static/dynamic magnetic properties of hydrogenated hcpCo_(80)~(57)Fe_4Ir_(16) soft magnetic thin films. Two different kinds of defects, i.e., destructive and non-destructive, were demonstrated by controlling the negative bias voltage of the hydrogenation process. Our results show that the structure and magnetic properties of our sample can be tuned by the density of the induced defects. These results provide better understanding of the hydrogenation effect and thus can be used in the future for materials processing to meet the requirements of different devices.     

A possible origin of room temperature ferromagnetism in Indium-Tin oxide thin film: Surface spin polarization and ferromagnetism

Room temperature ferromagnetism in both transition-metals doped and undoped semiconductor thin films and nanostructures challenges our understanding of the magnetism in solids. In this report, we performed the magnetic measurement and Andreev reflection spectroscopy study on undoped Indium-Tin oxide (ITO) thin films and bulk samples. The magnetic measurement results of thin films show that the total magnetization/cm² is thickness independent. Prominent ferromagnetism signal was also discovered in bulk samples. Spin polarized electron transports were probed on ITO thin film/superconductor interface and bulk samples surface/superconductor interface. Based on the magnetic measurement results and spin polarization measurement data, we propose that the ferromagnetism in this material originates from the surface spin polarization and this surface polarization may also explain the room temperature ferromagnetism discovered in other undoped oxide semiconductor thin films and nanostructures.     

Investigations of cobalt and carbon codoping in gallium nitride for spintronic applications

Extensive theoretical investigations have been carried out to study the ferromagnetic properties of transition metal doped wurtzite GaN using the Tight Binding Linear Muffin-tin Orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in cobalt doped GaN when one gallium is replaced by cobalt in a 3×3×2 supercell of GaN, which gives rise to a cobalt concentration of 2.77%. The system is half-metallic with a magnetic moment of 4.0 μ_B. When Co is bonded with one carbon, there is a drastic decrease in magnetic moment and the system becomes metallic. When Co dimer is introduced via nitrogen which corresponds to the Co concentration of 5.5% the magnetic moment is 3.99 μ_B and the system is half-metallic. Same trend is observed when Co is bonded via nitrogen with unequal distance. When cobalt dimer is formed via carbon, the moment becomes 2.95 μ_B and it shows metallic character. For dimer via carbon with unequal distance, the moment is 3.0 μ_B and the system becomes semiconductor. For higher percentage of cobalt dopant the system shows metallic character. C and Co doped GaN samples have been synthesized experimentally and characterized with X-ray diffraction, transmission electron microscopy, micro-Raman and superconducting quantum interface device measurements. The observed results are correlated with the theoretical studies.     

Experimental and theoretical assessment of Fe-doped indium-oxide-based dilute magnetic semiconductors

Specimens of iron-doped indium oxide (In_1-xFe_x)₂O₃ with x?=?0.015, 0.03, 0.045 and 0.06, amalgamated through a traditional solid-state reaction method followed by H₂/air sintering, were characterised using an X-ray diffractometer (XRD), a vibrating sample magnetometer (VSM), and a scanning electron microscope (SEM) to investigate their structural, magnetic and morphological properties respectively. According to XRD plots, all the specimens exhibit cubic bixbyite structures along with ancillary phases. Magnetic assessment showed that In₂O₃ has a negative susceptibility, exhibiting diamagnetic behaviour at room temperature. The doping of Fe ions induces ferromagnetic (FM) ordering, which is enhanced with increasing doping content. The strength of the magnetisation increases when the specimens are exposed to H₂ but is reduced on further air sintering. A bound magnetic polaron (BMP) model is successfully fitted to the observed FM data involving localised carriers and magnetic cations. A multivariate assessment viz. a hierarchical cluster analysis (HCA) was used to corroborate and strengthen the experimental determined magnetic properties. A homogeneous particle distribution was observed in all SEM micrographs and is validated through MATLAB-based simulation by applying a watershed segmentation algorithm. Surface plots also confirm the change in magnetic properties with increase in doping concentration.     

Controlled gold doping of silicon by using ion implantation

A new technology is suggested to dope silicon devices with gold for carrier life time adjustment with high accuracy and reproducibility. Ion implantation is used to dope the sample with a well defined total amount of gold. This gold dose is redistributed by high temperature anneal. A computer simulation of gold diffusion and experimental radio tracer results show that highly accurate gold doping can be obtained in bulk material when temperatures around 1000 C are applied during the redistribution anneal. Damage free surfaces must be used to reduce the gold content gettered in surfaces.     

Structural stability at high pressure,electronic, and magnetic properties of BaFZnAs:A new candidate of host material of diluted magnetic semiconductors

The layered semiconductor BaFZnAs with the tetragonal ZrCuSiAs-type structure has been successfully synthesized.Both the in-situ high-pressure synchrotron x-ray diffraction and the high-pressure Raman scattering measurements demonstrate that the structure of BaFZnAs is stable under pressure up to 17.5 GPa at room temperature. The resistivity and the magnetic susceptibility data show that BaFZnAs is a non-magnetic semiconductor. BaFZnAs is recommended as a candidate of the host material of diluted magnetic semiconductor.     

Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce–Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin–orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.