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According to Moores law, the number of transistor per square inch on integrated circuits had doubled every year since the integrated circuit was invented and the rapid miniaturization of the device is approaching in such a limit that the heat generated from the transistor cannot be dissipated fast enough, and unwanted quantum-mechanical effect prevent the circuits from function properly(1). To overcome this problem, research interest on spintronics materials is developing fast in recent years(2).
Spintronics is also called spin transport electronics or spin based electronics. For spintronics, it is not the electron charge but the electron spin that carries information, and this offers opportunities for a new generation of devices combining standard microelectronics with spin-dependent effects that arise from the interaction between spin of the carrier and the magnetic properties of the material.(3). The manipulation of spins in spintronics consists of three processes, mainly; spin generation, transport, and detection(4). By understanding and merging electronics, photonics, and magnetic , a new revolution to new spin-based multifunctional devices such as spin-FET (field effect transistor), spin-LED (light-emitting diode), spin RTD (resonant tunneling device), optical switches operating at terahertz frequency, modulators, encoders, decoders, and quantum bits for quantum computation and communication can be achieved(3).
Diluted magnetic semiconductors (DMSs) are materials that exhibit ferromagnetic and semiconducting properties (3-4). DMSs are usually common semiconductor materials containing a few percentages of transition metal (TM) ions substituted onto the cation sites. DMSs are studied extensively because of potential application in spintronics. The initial idea of DMS was to doped magnetic elements such as Mn into a semiconductor host to make magnectic semiconductor(5). Mn-doped GaAs is a successful DMS which has been studied in the context of spintronics application. (2, 6) However, the curie temperature of Mn-doped GaAs is 173K(7) . This limits application at room temperature. For practical application, a DMS exhibiting ferromagnetism at room temperature (>300K) is required.
The theory dealing with ferromagnetism driven by the exchange interaction between carriers and localized magnetic ions was first proposed by Zener(8).By using mean-filed Zener model, Dietl et al. theoretically predicted that Tc of ZnO could be increased above room temperature for p type DMSs and ferromagnetism was stable in DMSs based on wide-band gap semiconductor(9). Co and Mn are among the transition metal that has been studied widely among the researchers. From previous study, it is noted that much larger magnetic moments have been measured for Co at room temperature (10-11).
Properties of ZnO
Since Dietl et al. predicted that TM-ZnO could exhibit ferromagnetism above room temperature upon doping with transition elements such as Mn, Co, Cu etc, there is a revolution in this field(9). Initial theory of origin of ferromagnetism is due to strong sp-d hybridization, which involves the valence and conduction band in host material, owing to small distance from its nearest neighbor and small spin dephasing spin-orbit interaction. Figure 1 shows the calculated ordering temperature of several DMSs materials by Dietl et al.(2000).
Figure 1: Computed values of the Curie temperature for various p-type semiconductors containing 5% of Mn and 3.5x1020 holes per cm3 (9).
ZnO is wurtzite structure which is formed by tetrahedral (s-p3) bonding and the TM elements have valence electrons corresponding to the 4s orbital, and have partially filled 3d shells .From Figure 2, wurtzite structure of ZnO composed of hexagonal unit cell with two lattice parameters, a and c, in the ratio of 1.633 and belongs to the space group of .The structure consists of two interpenetrating hexagonal-close-packed (hcp) sublattices, each consists of one type of atom displaced with respect to each other along the threefold c-axis by amount of u=0.375(in an ideal wurtzite structure) in fractional coordinates. Each sublattice includes four atoms per unit cell and every atom of one kind (group II atom) is surrounded by four atoms of the other kind (groud VI) or vice versa, which are coordinated at the edges of a tetrahedron.(12)
Figure 2: Schematic representative of a wurtzitic ZnO structure having lattice constants a in the basal plane and c in the basal direction; u parameter is expressed as bond length or the nearest-neighbor distance b divided by c (0.375 in ideal crystal, Î± and Î² (109.47 in ideal crystal) are bond angles(12).
Besides that, ZnO has large exciton binding energy (60meV), small excitonic Bohr raidus(rB ~ 1.8nm) which makes excitions stable even at room temperature and sharp transitions facilitating very low threshold semiconductor laser. ZnO also has high energy radiation stability and amenability to wet chemical etching which makes ZnO preferable over other wide band gap materials(13). Table 1 shows the various properties of ZnO at 300k.
Table 1: Properties of ZnO(14)
Lattice parameters at 300 K
Â Â 0.324Â 95 nm
0.520Â 69 nm
Â Â 1.602 (1.633 for ideal structure )
Â Â 0.345
Â Â 5.606 g cmÂ -Â 3
Stable phase at 300 K
Â Â Wurtzite
Â Â 1975 Â°C
Â Â 0.6, 1-1.2
Linear expansion coefficient (/Â°C)
Â Â a0:6.5 Ã- 10Â -Â 6
c0:3.0 Ã- 10 Â -Â 6
Static dielectric constant
Â Â 8.656
Â Â 2.008, 2.029
Â Â 3.4eV, direct
Intrinsic carrier concentration
<106 cmÂ -Â 3
Exciton binding energy
Â Â 60 meV
Electron effective mass
Â Â 0.24
Electron Hall mobility at
300 K for low n-type conductivity
Â Â 200 cm2 (V s)Â -Â 1
Hole effective mass
Â Â 0.59
Hole Hall mobility at 300 K
for low p-type conductivity
Â Â 5-50 cm2 (V s)Â -Â 1
High-quality epitaxial films have exhibited electron moblilities of 300Â cm2Â V âˆ’1Â sâˆ’1 at room temperature. Ideally, 3d transition metal ions such as Co2+ will substitute for the cations of the host semiconductor, i.e. Zn sites in ZnO; the particular TM element, for example, Mn, Co, Cu etc, contributes its 4s electrons to the s-p3 bonding, and can therefore substitutionally replace the Zn in the tetrahedral bonding to form a TM2+ charge state. The 3d orbital of the Mn2+ ion is exactly half-filled with 5 electrons among the 10 available states, with an energy gap between up spin occupied states and empty down-spin states. For other TM, such as Co and Cu one of the bands is usually partially filled (up or down) as shown in Figure 3. From the Figure 3, Co has electronics states of 3d7 and Sato et al. predicted that the ferromagnetic state of Co2+ in Co-doped ZnO could be stabilized by s-d hybridization , pointing out that high curie temperature ferromagnetic materials could be realized in n-type ZnO as well(15).
Figure 3: Electronics configurations of the 3d-states and 4s-states of transition metal elements (from V to Cu)(16)
For ZnO, the solubility of TM elements especially Mn and Co, can reach up to 35% into ZnO. Despite that Dietl et al. predicted that only p-type leading to ferromangnetism(9), experimental observation of ferromagnetism for insulating ZnO and n-type ZnO haven been reported by recent paper(17).
Highly sensitive characterization techniques are critical for understanding the local structure, the magnetic behaviour of TM-doped ZnO, and to accurately detect chemical information for doping elements especially at very low concentration. In this report, I will outline a number of characterization techniques that has been done for the past decade.
X-Ray diffraction (XRD)
XRD is frequently applied to characterize film structure and crystalline quality of TM-doped ZnO. Besides that, by comparing the position of diffraction peaks between doped and undoped ZnO films can help to predict the state and site of doping elements(11, 18). The peaks in an XRD diffraction pattern are directly related to inter-planer spacing, d; the schematic diagram of X-ray diffraction of a periodic lattice is shown in Figure 4a. For a given set of lattice plane with an inter-planer spacing d, the condition for diffraction (peak) to occur can be written as Bragg's law, where, nÎ»=2dsinÎ¸
Figure 4:(a) Bragg's Law reflection. The diffracted X-rays exhibit constructive interference when the distance between paths ABC and A'B'C' differs by an integer number of wavelengths (Î»). (b)XRD pattern of Co:ZnO film at 90nm thick on R-cut sapphire. Extended count between 70o and 80o 2Î¸ reveals the 1 1 0 reflection of cobalt metal at 76.7o.
Diffraction peaks that are caused by secondary phase in a TM-doped ZnO matrix can be observed from the XRD pattern. This is important in a way that the origin of ferromagnetism can be determined. However, Coey pointed out that if the films are prepared in reducing condition, the nanoparticles of metallic Fe and Co are difficult to detect by conventional X-ray technique(19). In Dorneles et al. paper, they showed that no trace of secondary phase is detected in linear scale or logarithmic scale. However, when long count is made with multidetector, a cobalt refection stands out at 76.7o (Figure 4b).They estimate that Cobalt exist in cluster some 4-8nm in size by using Scherrer's formula(20).
High Resolution Transmission Electron Microscopy (HRTEM)
HRTEM is an imaging mode of TEM that allow imaging of crystallographic structure of a sample at an atomic scale. It is used to study nanoscale properties of crystalline material such as semiconductor and metals. HRTEM images are formed from a number of diffracted beams, know as phase-contrast imaging, and are necessary to construct an image of crystal lattice. By using HRTEM, we can analysing crystalline defects and interfaces at the atomic scales observing and verifying devices, multilayer, nanocrystals and nanostructure.
The present of small nanoclusters in the film can be observed in HRTEM images (Figure 5a). HRTEM are performed to investigate different phases that might have form in nanosize range and to determine the state of Co atoms that cannot be detected by XRD. By using HRTEM, Sudakar et al., found a Co cluster with diameter 5nm which is not detected when using XRD(21).
Figure:5(a) HRTEM image of a selected region showing the existence of an impurity phase(22). (c) A HRTEM image of Co:ZnO thin film.The white arrow is pointing at the edge dislocations. Inset is the corresponding selected area electron diffraction (SAED) pattern of the ZnO:Co thin film(23).
Besides that, HRTEM is used to observe RTFM that is induced by strain. By using HRTEM(Figure 5b), Zhang et al. proposed that Oxygen vacancies and Zn interstitial induced by edge dislocation may also contribute to ferromagnetic properties in Co:ZnO(23).
Electron Energy Loss Spectroscopy (EELS)
EELS is an analytical technique that measures the change in kinetic energy of electrons. EELS measures the energy distribution of electron that have interacted with a specimen and lost energy due to inelastic scattering. When carried out in conjunction with TEM, EELS is capable of giving structural and chemical information about a solid, with spatial resolution down to atomic level(24). Sharma et al. used HRTEM to detect cluster and defects in the Mn-doped ZnO film and EELS confirmed that the valence state of Mn is +2. (25). A typical graph for EELS is shown in Figure 6:
Figure 6: Background-reduced EELS from the bulk of the film, inset: STEM-Z contrast image.
From the Figure 6, L2 and L3 peaks is observed in addition to the peaks from Zinc and Oxygen. Calculations on the ratio of integrated intensities of L2 and L3 peaks yielded an oxidation state of +2 for Cobalt. STEM-Z shows that there is no indication of any extra phase. The results from EELS and Z contrast indicate that the cobalt has occupied the substitutional sites, replacing Zinc(26).
X-ray Photoelectron Spectroscopy (XPS)
XPS is one of the most powerful techniques that measure elemental composition, empirical formula, chemical state and electronics state of TM in ZnO thin film. XPS spectra are obtained by irradiating a material with a beam of X-rays while measuring the kinetic energy and number of electron that escape from the surface(27).
By looking at XPS graph of Co:ZnO thin film(Figure 7),it shows that the Co ion in the ZnO thin film is in the +2 formal oxidation state. The 2p3/2 and 2p1/2 peaks were fitted using Gaussian method and this resulting the Co:ZnO 2p3/2 and 2p1/2 core levels for Co-O bonding were found to be at is 780.07eV and 796.07eV (23). This is different than Co 2p3/2 core energy Co metal cluster which is at 778.3eV.
Figure 7: XPS studies of Co 2p3/2 and 2p1/2 peaks for the ZnO:Co thin film
X-ray Absorption Spectroscopy (XAS)
When the x-rays hit a sample, the oscillating electric field of the electromagnetic radiation interacts with the electrons bound in an atom. The radiation will be absorbed and excite the electrons or scattered by electrons. This is show in Figure 8
Figure 8: A narrow parallel monochromatic x-ray beam of intensity I0 passing through a sample of thickness x will get a reduced intensity I according to the expression:
From Figure 8, we can deduce that
ln (I0 /I) = Î¼ x (1)
At certain energies where the absorption increases drastically and gives rise to an absorption edge. Each such edge occurs when the energy of the incident photons is just sufficient to cause excitation of a core electron of the absorbing atom to a continuum state, i.e. to produce a photoelectron. Thus, the energies of the absorbed radiation at these edges correspond to the binding energies of electrons in the K, L, M, etc, shells of the absorbing elements. The absorption edges are labelled in the order of increasing energy, K, LI, LII, LIII, MI. When the photoelectron leaves the absorbing atom, its wave is backscattered by the neighbouring atoms.
There are 4 regions found on a spectra: 1) pre-edge, 2) x-ray absorption near edge structure (XANES), 3)near edge x-ray absorption fine structure(NEXAFS) and 4) extended x-ray absorption fine structure. An example of XAS graph is shown in Figure:
Figure 9: A typical XAS graph showing pre-edge, XANES, NEXAFS and EXAFS
X-ray Magnetic Circular Dichroism (XMCD)
XMCD is a difference spectrum of XAS taken in to magnetic field. By analysing the difference in the XMCD spectrum, information such as magnetic properties of atom, its spin and orbital magnetic moment can be obtained. The magnetic properties for the 3d transition metals are mainly determined by d valence electron(28). For the case of Co doped ZnO, the absorption spectrum are usually measured at the L-edge, which associated with 2p to 3d transition(29).
The basic concepts of XMCD spectroscopy are illustrated is Figure 10. The properties of 3d electrons are best probed in a x-ray absorption experiment by excitation of 2p core electrons to unfilled 3d states(28) as illustrated in Figure 10. The sum of the intensities (L3 and L2) is directly proportional to the number N of empty d states (holes). The d valence shell can hold up to 10 electrons which are filled into band states up to the Fermi level and the number of filled states is therefore 10 -N.
Figure 10:(a) Electronic transitions in conventional L-edge x-ray absorption ,(b) and (c) X-ray magnetic circular x-ray dichroism illustrated in a one-electron model. The transitions occur from the spin-orbit split 2p core shell to empty conduction band states above the Fermi level. In conventional x-ray absorption the total transition intensity of the two peaks is proportional to the number of d holes. By use of circularly polarized x-rays the spin moment (b), and orbital moment (c) can be determined from linear combinations of the dichroic difference intensities A and B (28).
The L3 and L2 resonance intensities and their differences for parallel and anti-parallel orientation of photon spin and magnetization directions are quantitatively related by sum rules to the number of d holes and the size of the spin and orbital magnetic moments(28). Angle dependent measurements in external magnetic fields give the anisotropies of the spin density and orbital moment.
Figure 11: XMCD spectra under different magnetic fields at 20K. Closed circle shows the ferromagnetic component(30).
From the Figure 11, the paramagnetic components is the part which linearly increases with H while ferromagnetic component is the part where H=0T. The ferromagnetic component obtained by subtracting the appropriate paramagnetic component from the XMCD spectrum at H=2.0. The line shape of the ferromagnetic component is nearly identical to that of the paramagnetic spectrum. Therefore, Co ions have similar electronic structures in the paramagnetic and the ferromagnetic components. XMCD spectra also show a multiplet structure, unlike those of Co metal, indicating that, though only a part of the doped transition-metal ions are ferromagnetic, the magnetism in the present sample is not due to metallic Co clusters but due to Co ions with localized 3d electrons(30).
Room Temperature Photoluminescence Spectroscopy (PL)
PL is a non destructive method of probing the electrical properties of materials. By using PL, information about electronic band structure, recombination mechanism and defects within the sample can be obtained. In PL, electron-hole pairs (exciton) are generated by application of laser beam on the surface of the sample. As a result of excess energy caused by photo-excitation, electron will jump to permissible excited states .When the electron move back to ground state, excess energy is released through emission of light with energy equal to the energy difference between equilibrium and excited states. By measuring the wavelength of the emitted photon from the observed recombination, information such as electronics band structure, crystalline quality, impurity levels and defect densities within the materials system can be known (31).
Figure 12: PL spectra of Cu:ZnO thin film at 300k(32)
From Figure 12, the emission peak at 3.29eV (UV) originates from the NBE transition in band gap of ZnO due to the recombination of free excitons through an exciton- exciton collision process(32). The redshift by 11meV in the graph is due to an increase in Cu concentration. Besides that ,near band gap edge(NBE) narrowing that is observed in the graph is due to sp-d exchange interaction between the d electrons of transition metal and the band electrons of ZnO; the strength of this interaction strongly depends on the number of d electrons(33). The s-d and p-d exchange gives rise to negative and positive corrections to the conduction and valance band edges, respectively, leading to the NBE narrowing(32) .Zhu et al. proposed that the green emission 2.60eV is observed in Cu doped (>1%) ZnO samples is due to surface defects, Cu impurities, and the oxygen vacancies(34).
Raman Scattering Spectroscopy
Raman spectroscopy is a powerful light scattering technique used to diagnose the internal structure of molecules and crystals. Raman scattering measures the interaction of light via inelastic scattering from an incident laser beam off of a material. Inelastic scattering is defined as transfer of energy into lattice vibration or phonons. The energy of lattice vibrations is quantized and a function of the local bonding and atoms involved in the structure. Information regarding crystalline quality and lattice dynamics of the material can be gain by measuring the energy transferred to or from phonons to photons, which is known as Stokes or Anti-stokes shift in the inelastic scattered light source(31).
Figure 13: Room temperature Raman Spectra of Co:ZnO thin films(35)
From Figure 13, the and are associated with the motion of oxygen atom and Zn sublattice respectively. The peak position of ZnO was shifted towards the lower frequency (up to 10%) and there was an increase in FWHM for up to 10% of Co substituted ZnO.The Due to Co substitutions, broad band centred at 549cm-1 and an additional mode at 470 cm-1 in ceramic targets is observed(35)
Superconducting Quantum Interference Device (SQUID)
SQUID are use for magnetization study in the temperature range 5-350K. By using SQUID, DMSs samples are drawn through a coil of superconducting wire in the presence of magnetic field. The moving magnetic field from the sample induces a current in the wire, which through signal processing can be analysed and converted to a signal proportional to be the magnetization of the sample (36). Due to the noise floor of SQUID can be as low as 8 emu, it is the ideal magnetization study for DMSs. The superconducting magnet and coil be cooled to cryogenic temperatures, as the critical temperature for the superconducting wire coil is 20k.
Figure 14: Magnetization curves of Zn0.92Co0.04Eu0.04O and Zn0.96Co0.04O thin film measured by A SQUID magnetometer at room temperature(29).
From the Figure 14, the magnetization of Zn0.92Co0.04Eu0.04O and Zn0.96Co0.04O as a function of the external magnetic field applied parallel to the film surface. Zn0.92Co0.04Eu0.04O and Zn0.96Co0.04O showed room temperature ferromagnetism with a coercive field of 60 Oe as shown in the hysteresis loop.Pat et al. suggest that Zn0.96Co0.04O shows higher magnetic moment than Zn0.92Co0.04Eu0.04O is due to Co clustering(29).
Vibrating Sample Magnetometer (VSM)
VSM operates on Faraday's Law of induction, which means changing magnetic field will produce an electric field. VSM is used when magnetization study is carried out at high temperature (300-800K). In VSM, the sample is usually mounted on a sample tail and placed between the coils of can electromagnet. The magnet will supply a constant magnetic field. If the sample is magnetic, the constant magnetic field will magnetize the sample by aligning the magnetic domains with the field. The stronger the constant field, the larger the magnetization will be. The magnetic dipole moment of sample will create a magnetic field around the samples. By moving the sample up and down, this will induces a current in the pick-up coils, which is send, amplified , and converted to a know magnetic signal . VSM is not suitable to perform at low temperature because it needs an additional cryostat. Figure 15 reveals the magnetic hysteresis loops (M vs H) at 300k of Zn0.96Co0.04O films(18).
Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
ICP-AES is an analytical technique used for detection of trace metal. ICP-AES produce excited atoms and ions that emit electromagnetic radiation at wavelength characterictic of a particular elements(37). The intensity of the emission is indicative of the concentration of the elements. ICP-AES can used to determine the Tc of DMSs materials(18). Inset of Figure 15 shows the
Hall Effect measurement - van der Pauw geometry
Hall Effect is the production of voltage difference across electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. In ferromagnetic materials, the hall resistivity includes an additional contribution, known as anomalous Hall effect (AHE)(38). AHE is depends directly on the magnetization of the materials. The origin of AHE is still under debate. The AHE can be either extrinsic (disorder-related) effect dur to spin-dependent scattering of charge carriers, or intrinsic effect which can be described in terms of the Berry phase effect in the crystal momentum space(39).
Hall Effect is carried out by using indium metal for contact electrodes and current supplied by dc-voltage source. There is a question here, due to ZnO has wide band gap, it's not easy to measure the Hall effect. Most researchers do not include hall measurement in their jouna while others are using Indium Tin Oxide (ITO). By measuring the Hall effect, carrier concentration can be known.
Rutherford Backscattering Spectroscopy
Rutherford backscattering spectrometry (RBS) is the measurement of energies of these backscattered particles. These energies depend on the identity of the atom from which the alpha particle scatters, the angle of scatter, and the depth into the sample to which the particle travels before scattering. Thus, RBS can be used for elemental analysis, stoichiometry changes, detecting surface/bilk contamination and interdiffusion of solid films(40). From Figure 15, the graph shows that ZnO film is not homogenous and interdiffusion happen at 1150x1015at/cm2. Besides that, we can know the depth of penetration of Eu ions by using ion implantation.
Figure 15: Typical RBS graph for Eu doped ZnO on Al2O3 substrate
Origin of Ferromagnetism in TM:ZnO
Computational work based on ab initio calculation can provide some prediction and explanations of electronics structure and nature of ferromagnetic in TM-doped ZnO. In Photongkam et al. paper, they used ab initio calculations showed that Eu dopants are more preferable substitution with Zn site(29). Total energy calculation in their paper show that in supercell, Zn0.875Co0.0625Eu0.0625O, the ferromagnetic interaction between Co and Eu is stronger than the ferromagnetic where spin alignment of Eu and Co ions is antiparellel(29).
Density functional theory (DFT) is a quantum mechanical theory used in physics and chemistry ,based on pseudopotentials with localised atomic-orbital basis sets, in which the total energy of a many-electron system is described as a function of the electron density(41). The limitation of DFT is that the exact functional for exchange and correlation are not known except for free electron gas. Local density approximation (LDA) exchange correlation function is the most widely used approximation. The local spin-density approximation (LSDA) is a relation of the constraint of an equal occupation of spin-up and spin-down states.
However, both LDA and LDSA calculations could not predict the insulating behaviour of many TM oxides, but produce metallic ground state. LDA + U which add in the effective on-site Coulomb interaction LDA and LDSA calculations greatly improve the interaction of TM oxides. By taking into account the gradient of the density at the same coordinate we will have generalized gradient approximation (GGA). GGA has very good results for molecular geometries and ground-state energy.
By using mean field approach, Dietl et al. predicted that 5% Mn doped p-type ZnO would show room temperature ferromagnetism(9).Following Dietl work, Sato et al. use Korringa-Kohn-Rostoker (KKR) Green's function method based on local density approximation to calculate the properties of ZnO doped with TMs such as V, Cr, Fe, Co and Ni(42).
Recent paper by Sato et al, they stated that these approximations are not always sufficient for reproducing the electronic structure of a given material. In order to improve on this approximation a mean-ï¬eld treatment of the Coulomb repulsion of electrons situated on the same atom has been suggested. Sato et al believe that coherent potential approximation(CPA) is the most efficient method to determine the substitutional of TM impurities at cation sites of host semiconductor(43).
Besides that, first principle full potential linearized augmented plan-wave (FP-LAPW) method within LDA and LDA+U schemes is used by Zhang et al. for the investigation of electronic structure and magnetic properties of Co-doped zinc-blende ZnO(44) .FP-LAPW is one among most accurate schemes for band structure calculations, which allow inclusion of local orbits in basis, improving upon linearization and making possible a consistent treatment of semi-core and valence in one energy window(44).
6.0 Proposed Spintronics Devices
There are few proposed spintronics devices. The goal of spintronics is to combine standard microelectronics with spin-dependant effects that arise from the interaction between spin of the carrier and magnetic properties of the material (3). Traditional approaches are based on alignment of spin (either "up" or "down") relative to an applied magnetic field or magnetization orientation of ferromagnetic film. By using electrical current, the degree of alignment is predictable. Adding spin degree of freedom to conventional semiconductor charge based electronics will lead to a more capability and performance devices. The advantages are nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices(3). There are 3 proposed devices which are spin light emitting diode (SLED), spin transistor and spin field effect transistor (SFET)
6.1 Spin light emitting (SLED)
In Figure(45) , spin polarized electrons are injected from a ferromagnetic layer (pale blue) into a semiconductor structure (orange) recombine with holes in the active region (yellow) to produce circularly polarized light (brown, where the arrow indicated the direction of polarization), and it could be useful for encrypted communication.
Figure: Schematic diagram of SLED (p-n junction) (45)
6.2 Spin Transistor
As shown in Figure(45), spin transistor is the magnetic tunnel transistor, in which the injected electrons are filtered depending on their spin as they tunnel through a thin insulating layer (red), as happens in a magnetic tunnel junction, before passing through a Schottky barrier. By changing the spin alignment of the "emitter" and "base" ferromagnetic layers, the output current in the "collector" semiconductor can therefore be controlled
Figure: Schematic diagram of spin transistor (n-p-n)(45)
6.3 Spin Field Effect Transistor (SFET)
Datta and Das proposed spin field effect transistor (SFET) in 1990(46), which is shown is Figure. The spin polarized current is injected from source side of the device. The gate voltage is used to control the precession of spins via the Rashaba-spin orbit interaction from the ferromagnetic source to ferromagnetic drain (47). The transport through the device is affected by the end of channel spin alignment of the current relative to that of the ferromagnetic collector at the drain end of the device because degree of spin precession is dependent on the voltage. This control of the drain current through application of a gate voltage is analogous to what is seen in a charge based field effect transistor. The advantages of SFET are the low operational currents and higher speeds than traditional FETs. This has a great impact on the overall research and development of spin-based devices. Implementation of these structures has been slow due to difficulties in the fabrication and lack of suitable materials. In Ohno and Yoh paper, their study on Datta and Das device suggest that Datta and Das device can successfully operate in nonballistic regime of spin transport in a 2DEG system(47).
Figure: A schematic diagram of a spin field effect transistor (Datta-Das transistor). In this device, the gate voltage is used to control the precession of spin from a ferromagnetic source to ferromagnetic drain(46)
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