First-principles prediction of the magnetism of 3d transition-metal-doped Rocksalt MgO

Using first-principles band structure methods, we have systematically studied the electronic structures, magnetic stabilities, and half-metal properties of 3d transition-metal (TM) doped Rocksalt MgO compounds TMMg3O4 (TM = V, Cr, Mn, Fe, Co, and Ni). The calculations reveal that only CrMg3O4 has a ferromagnetic stability among the six compounds, which is explained by double-exchange mechanism. The magnetic stability is affected by the doping concentration of TM if the top valance band is composed of partially occupied t(2g) states. In addition, CrMg3O4 is a half-metallic ferromagnet. The origins of half-metallic and ferromagnetic properties are explored. The Curie temperature (T-c) of CrMg3O4 is 182 K. And it is hard for CrMg3O4 to deform due to the large bulk modulus and shear modulus, so it is a promising spintronic material. Our calculations provide the first available information on the magnetic properties of 3d TM-doped MgO.

Intrinsically limited critical temperatures of highly doped Ga1-xMnxAs thin films

Ga1-xMnxAs films with exceptionally high saturation magnetizations of approximate to 100 emu/cm(3) corresponding to effective Mn concentrations of x(eff)approximate to 0.10 still have a Curie temperature T-C smaller than 195 K contradicting mean-field predictions. The analysis of the critical exponent beta of the remnant magnetization-beta = 0.407(5)-in the framework of the models for disordered/amorphous ferromagnets suggests that this limit on T-C is intrinsic and due to the short range of the ferromagnetic interactions resulting from the small mean-free path of the holes. This result questions the perspective of room-temperature ferromagnetism in highly doped GaMnAs.

Band-tail shape and transport near the metal-insulator transition in Si-doped

In the present work, an infrared light-emitting diode is used to photodope molecular-beam-epitaxy-grown Si: Al0.3Ga0.7As, a well-known persistent photoconductor, to vary the effective electron concentration of samples in situ. Using this technique, we examine the transport properties of two samples containing different nominal doping concentrations of Si [1 x 10(19) cm(-3) for sample 1 (S1) and 9 x 10(17) cm(-3) for sample 2 (S2)] and vary the effective electron density between 10(14) and 10(18) cm(-3). The metal-insulator transition for S1 is found to occur at a critical carrier concentration of 5.7 x 10(16) cm(-3) at 350 mK. The mobilities in both samples are found to be limited by ionized impurity scattering in the temperature range probed, and are adequately described by the Brooks-Herring screening theory for higher carrier densities. The shape of the band tail of the density of states in Al0.3Ga0.7As is found electrically through transport measurements. It is determined to have a power-law dependence, with an exponent of -1.25 for S1 and -1.38 for S2.

Origins of magnetism in transition metal doped Cul

Cupric iodide is a p-type semiconductor and has a large band gap. Doping of Mn, Co, and Ni are found to make gamma-CuI ferromagnetic ground state, while Cr-doped and Fe-doped CuI systems are stabilized in antiferromagnetic configurations. The origins of the magnetic ordering are demonstrated successfully by the phenomenological band coupling model based on d-d level repulsions between the dopant ions. Furthermore, using a molecular-orbital bonding model, the electronic structures of the doped CuI are well understood. According to Heisenberg model, high-T-C may be expected for CuI:Mn and CuI:Ni if there are no native defects or other impurities.

The formation and electronic structures of 3d transition-metal atoms doped in silicon nanowires

Using first-principles methods, we systematically study the mechanism of defect formation and electronic structures for 3d transition-metal impurities (V, Cr, Mn, Fe, and Co) doped in silicon nanowires. We find that the formation energies of 3d transition-metal impurities with electrons or holes at the defect levels always increase as the diameters of silicon nanowires decrease, which suggests that self-purification, i.e., the difficulty of doping in silicon nanowires, should be an intrinsic effect. The calculated results show that the defect formation energies of Mn and Fe impurities are lower than those of V, Cr, and Co impurities in silicon nanowires. It indicates that Mn and Fe can easily occupy substitutional site in the interior of silicon nanowires. Moreover, they have larger localized moments, which means that they are good candidates for Si-based dilute magnetic semiconductor nanowires. The doping of Mn and Fe atom in silicon nanowires introduces a pair of energy levels with t(2) symmetry. One of which is dominated by 3d electrons of Mn or Fe, and the other by neighboring dangling bonds of Si vacancies. In addition, a set of nonbonding states localized on the transition-metal atom with e symmetry is also introduced. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000445]

Enhanced mid-infrared transmission in heavily doped n-type semiconductor film based on surface plasmons

Transmission of electromagnetic wave in a heavily doped n-type GaAs film is studied theoretically. From the calculations, an extraordinary transmission of p-polarized waves through the film with subwavelength grooves on both surfaces at mid-infrared frequencies is found. This extraordinary transmission is attributed to the coupling of the surface-plasmon polariton modes and waveguide modes. By selecting a set of groove parameters, the transmission is optimized to a maximum. Furthermore, the transmission can be tuned by dopant concentrations. As the dopant concentration increases, the peak position shifts to higher frequency but the peak value decreases.

Interface as the origin of ferromagnetism in cobalt doped ZnO film grown on silicon substrate

We have investigated the magnetic properties of Co-doped zinc oxide (ZnO) film deposited on silicon substrate by magnetron sputtering. Co ions have a valence of 2+ and substitute for Zn sites in the lattice. By using a chemical etching method, an extrinsic ferromagnetism was demonstrated. The observed ferromagnetism is neither associated with magnetic precipitates nor with contamination, but originates from the silicon/silicon oxide interface. This interface ferromagnetism is characterized by being temperature independent and by having a parallel magnetic anisotropy. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2989128]

Enhanced electroluminescence intensity of InGaN/GaN multi-quantum-wells based on Mg-doped GaN annealed in O-2

InGaN/GaN multi-quantum-well blue (461 +/- 4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 C in O-2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg-H complex by forming H2O, annealing in O-2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated Mg-Ga-O-N complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen. (C) 2008 American Institute of Physics.

Enhancement of field emission properties in La-doped ZnO films prepared by magnetron sputtering

Field emissions (FE) from La-doped zinc oxide (ZnO) films are both experimentally and theoretically investigated. Owing to the La-doped effect, the FE characteristic of ZnO films is remarkably enhanced compared with an undoped sample, and a startling low turn-on electric field of about 0.4 V/mu m (about 2.5 V/mu m for the undoped ZnO films) is obtained at an emission current density of 1 mu A/cm(2) and the stable current density reaches 1 mA/cm(2) at an applied field of about 2.1 V/mu m. A self-consistent theoretical analysis shows that the novel FE enhancement of the La-doped sample may be originated from its smaller work function. Due to the effect of doping with La, the Fermi energy level lifts, electrons which tunnelling from surface barrier are consumedly enhancing, and then leads to a huge change of field emission current. Interestingly, it suggests a new effective method to improve the FE properties of film materials.

Doped polycrystalline 3C-SiC films deposited by LPCVD for radio-frequency MEMS applications

Polycrystalline 3C-SiC films are deposited on SiO2 coated Si substrates by low pressure chemical vapour deposition (LPCVD) with C3H8 and SiH4 as precursors. Controlled nitrogen doping is performed by adding NH3 during SiC growth to obtain the low resistivity 3C-SiC films. X-ray diffraction (XRD) patterns indicate that the deposited films are highly textured (111) orientation. The surface morphology and roughness are determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface features are spherulitic texture with average grain size of 100 nm, and the rms roughness is 20nm (AFM 5 x 5 mu m images). Polycrystalline 3C-SiC films with highly orientational texture and good surface morphology deposited on SiO2 coated Si substrates could be used to fabricate rf microelectromechanical systems (MEMS) devices such as SiC based filters.

Organic light emitting diodes using magnesium doped organic acceptor as electron injection layer and silver as cathode

Organic light emitting diodes employing magnesium doped electron acceptor 3, 4, 9, 10 perylenetetracarboxylic dianhydride (Mg: PTCDA) as electron injection layer and silver as cathode were demonstrated. As compared to Mg: Ag cathode, the combination of the Mg: PTCDA layer and silver provided enhanced electron injection into tris (8-quinolinolato) aluminium. The device with 1: 2 Mg: PTCDA and Ag showed an increase of about 12% in the maximum current efficiency, mainly due to the improved hole-electron balance, and an increase of about 28% in the maximum power efficiency, as compared to the control device using Mg: Ag cathode. The properties of Mg: PTCDA composites were studied as well.

New observations on the pressure dependence of luminescence from Eu2+-doped MF2 (M = Ca, Sr, Ba) fluorides

The luminescence from Eu2+ ions in MF2 (M = Ca, Sr, Ba) fluorides has been investigated under the pressure range of 0-8 GPa. The emission band originating from the 4f(6)5d(1) -> 4f(7) transition of Eu2+ ions in CaF2 and SrF2 shows the red-shift as increasing pressure with pressure coefficients of -17 meV/GPa for CaF2 and -18 meV/GPa for SrF2. At atmospheric pressure, the emission spectrum of BaF2:Eu2+ comprises two peaks at 2.20 and 2.75 eV from the impurity trapped exciton (ITE) and the self-trapped exciton (STE), respectively. As the pressure is increased, both emission peaks shift to higher energies, and the shifting rate is slowed by the phase transition from the cubic to orthorhombic phase at 4 GPa. Due to the phase transition at 4-5 GPa pressure, the ITE emission disappears gradually, and the STE emission is gradually replaced by the 4f(6)5d(1) -> 4f(7) transition of Eu2+. Above 5 GPa, the pressure behavior of the 4f(6)5d(1) -> 4f(7) transition of EU2+ in BaF2: EU2+ is the same as the normal emission of Eu2+ in CaF2 and SrF2 phosphors.

Ultrafast carrier dynamics in undoped and p-doped InAs/GaAs quantum dots characterized by pump-probe reflection measurements

We investigate the dependence of the differential reflection on the structure parameters of quantum dot (QD) heterostructures in pump-probe reflection measurements by both numerical simulations based on the finite-difference time-domain technique and theoretical calculations based on the theory of dielectric films. It is revealed that the value and sign of the differential reflection strongly depend on the thickness of the cap layer and the QD layer. In addition, a comparison between the carrier dynamics in undoped and p-doped InAs/GaAs QDs is carried out by pump-probe reflection measurements. The carrier capture time from the GaAs barrier into the InAs wetting layer and that from the InAs wetting layer into the InAs QDs are extracted by appropriately fitting differential reflection spectra. Moreover, the dependence of the carrier dynamics on the injected carrier density is identified. A detailed analysis of the carrier dynamics in the undoped and p-doped QDs based on the differential reflection spectra is presented, and its difference with that derived from the time-resolved photoluminescence is discussed. (C) 2008 American Institute of Physics.

Magnetic coupling properties of mn-doped ZnO nanowires: First-principles calculations

Based on the density functional theory, we study the magnetic coupling properties of Mn-doped ZnO nanowires. For the nanowires with passivated surfaces, the antiferromagnetic state is found and the Mn atoms have a clustering tendency. When the distance between two Mn atoms is large, the system energetically favors the paramagnetic or spin-glass state. For the nanowires with unpassivated surfaces, the ferromagnetic (FM) coupling states appear between the two nearest Mn atoms, and the zinc vacancies can further stabilize the FM states between them. The electrons with enough concentration possibly mediate the FM coupling due to the negative exchange splitting of conduction band minimum induced by the s-d coupling, which could be useful in nanomaterial design for spintronics. (C) 2008 American Institute of Physics.