Determination of the local concentrations of Mn interstitials and antisite defects in GaMnAs

We present a method for the determination of the local concentrations of interstitial and substitutional Mn atoms and As antisite defects in GaMnAs. The method relies on the sensitivity of the structure factors of weak reflections to the concentrations and locations of these minority constituents. High spatial resolution is obtained by combining structure factor measurement and x-ray analysis in a transmission electron microscope. We demonstrate the prevalence of interstitials with As nearest neighbors in as-grown layers.     

Mn interstitial diffusion in (ga,mn)as

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.     

Magnetic linear dichroism in the angular dependence of core-level photoemission from (Ga,Mn)As using hard x rays

We report a study of the electronic properties of the ferromagnetic semiconductor (Ga,Mn)As using magnetic linear dichroism in the angular dependence of Mn 2p photoemission under hard x-ray excitation. Bulk plasmon loss satellites demonstrate that the probed Mn ions are incorporated deep within the GaAs lattice, while the observed large dichroism indicates that the spectra originate from ferromagnetic substitutional Mn. Simulations of the spectra using an Anderson impurity model show that the ferromagnetic Mn 3d electrons of substitutional Mn in (Ga,Mn)As are intermediate between localized and delocalized.     

(Ga,Mn)As 体系中Mn自补偿效应的第一性原理研究

基于密度泛函理论的赝势平面波方法计算了处于填隙位置磁性原子(MnI)对(Ga,Mn) As体系电子结构和磁性的影响. 计算结果表明MnI在GaAs中是施主；代替Ga位的MnGa 与MnI的自旋按反铁磁序排列；静电相互作用使MnGa，MnI倾向于形成MnGa_MnI对. MnI的存在一方面补偿了(Ga,Mn)As中的空穴，降低了空穴浓度；同时还使邻近的MnG a失活. MnI的存在对获得高居里温度的(Ga,Mn)As是极为不利的. 关键词： (Ga Mn)As 稀磁半导体 密度泛函理论     

Observation of a new gettering mechanism in (Hg,Cd)Te

A new gettering mechanism is proposed for substitutional impurities which diffuse by an interstitial process. In this mechanism, an externally imposed gradient of self interstitials generates a gradient in impurity interstitials leading to the segregation of fast diffusing impurities to low interstitials, high vacancy regions. Data are presented to support this model in (Hg, Cd)Te alloys, as well as explicitly rule out gettering by dislocations, by segregation to precipitates, or by enhanced solubility arising from the interaction of the impurity with a varying Fermi level.     

Dilute moment n-type ferromagnetic semiconductor Li(Zn,Mn)As

We propose to replace Ga in (Ga,Mn)As with Li and Zn as a route to high Curie temperature, carrier mediated ferromagnetism in a dilute moment n-type semiconductor. Superior material characteristics, rendering Li(Zn,Mn)As a realistic candidate for such a system, include high solubility of the isovalent substitutional Mn impurity and carrier concentration controlled independently of Mn doping by adjusting Li-(Zn,Mn) stoichiometry. Our predictions are anchored by ab initio calculations and comparisons with the familiar and directly related (Ga,Mn)As, by the physical picture we provide for the exchange interaction between Mn local moments and electrons in the conduction band, and by analysis of prospects for the controlled growth of Li(Zn,Mn)As materials.     

Current-driven magnetization reversal in a ferromagnetic semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As tunnel junction

Current-driven magnetization reversal in a ferromagnetic semiconductor based (Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K. Magnetoresistance measurements combined with current pulse application on a rectangular 1.5 x 0.3 microm2 device revealed that magnetization switching occurs at low critical current densities of 1.1-2.2 x 10(5) A/cm2 despite the presence of spin-orbit interaction in the p-type semiconductor system. Possible mechanisms responsible for the effect are discussed.     

Dopant-assisted concentration enhancement of substitutional Mn in Si and Ge

The influence of p- and n-type electronic dopants on Mn incorporation in bulk Si and Ge is studied using first-principles calculations within density functional theory. In Si, it is found that the site preference of a single Mn atom is reversed from interstitial to substitutional in the presence of a neighboring n-type dopant. In Ge, a Mn atom is more readily incorporated into the lattice when an n-type dopant is present in its immediate neighborhood, forming a stable Mn-dopant pair with both impurities at substitutional sites. A detailed analysis of the magnetic exchange interactions between such pairs reveals a new type of magnetic anisotropy in both systems.     

Investigation of Mn incorporation in fifteen-period InGaAs/GaAs quantum well system

A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 °C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.     

Exploration the p-type doping mechanism of GaAs nanowires from first-principles study

Using first-principle calculations, we present a systematic investigation upon the influence of p-type doping on the structural and electronic properties of H-passivated GaAs nanowires with wurtzite structure. The GaAs nanowire models of different doping types, different doping elements, different doping positions and different doping concentrations are established. The calculated formation energies show that Zn element becomes more competitive or even slightly favored in realizing p-type doping compared to Be element. For an individual Zn incorporation model, Zn atom tends to substitute the subsurface Ga atom. As increasing Zn doping concentration, the p-type doping process becomes more and more difficult. Besides, both interstitial and substitutional doping lead to the distortion of atomic structure near impurity atoms and cause the ionicity of GaAs nanowires enhanced. The p-type doped GaAs nanowires models are all direct band gap semiconductors. After substitutional doping, the total density of state curves shift toward higher energy sides and the Fermi level entering valence bands. Our calculations provide a significant reference for the preparation of p-type doping GaAs nanowire, which has a promising potential application in the field of photocathodes.     

Magnetization-switched metal-insulator transition in a (Ga,Mn)as tunnel device

We observe the occurrence of an Efros-Shklovskii gap in (Ga,Mn)As based tunnel junctions. The occurrence of the gap is controlled by the extent of the hole wave function on the Mn acceptor atoms. Using k.p-type calculations we show that this extent depends crucially on the direction of the magnetization in the (Ga,Mn)As (which has two almost equivalent easy axes). This implies one can reversibly tune the system into the insulating or metallic state by changing the magnetization.     

Prediction of enhanced ferromagnetism in (Ga,Mn)As by intrinsic defect manipulation

In the diluted magnetic semiconductor (Ga,Mn)As the excess of As incorporated as As antisites (As_Ga) is responsible for the hole compensation. The As_Ga defect can be transformed into a As interstitial–Ga vacancy pair (As_i–V_Ga) upon illumination. In this paper we study the effects of such a transition on the ferromagnetism of (Ga,Mn)As using density functional theory within the local spin density approximation. We find that the ferromagnetic order in (Ga,Mn)As is strongly enhanced if As_Ga are transformed into As_i–V_Ga pairs, since the hole compensation is reduced. This suggests a valuable way to tune the carrier concentration and hence the T_c in (Ga,Mn)As, without changing the Mn concentration nor the microscopic configuration of the Mn ions.     

Influences of As flux on the lattice constants, magnetic and transport properties of (Ga, Mn)As epilayers

A series of (Ga, Mn)As epilayers have been prepared on semi-insulating GaAs (001) substrates at 230  ^°C by molecular-beam epitaxy under fixed temperatures of Ga and Mn cells and varied temperatures of the As cell. By systematically studying the lattice constants, magnetic and magneto-transport properties in a self-consistent manner, we find that the concentration of As antisites monotonically increases with increasing As flux, while the concentration of interstitial Mn defects decreases with it. Such a trend sensitively affects the properties of (Ga, Mn)As epilayers.     

Uniaxial strain-dependent magnetic and electronic properties of (Ga,Mn)As nanowires

Variations in magnetic and electronic properties as a function of uniaxial strain in wurtzite(Ga,Mn)As nanowires(NWs) grown along the [0001] direction were investigated based on density functional theory(DFT). We found that(Ga,Mn)As NWs are half-metal, and the ferromagnetic state is their stable ground state. The magnetism of the NWs is significantly affected by the strain and by the substituent position of Mn impurities. By examining charge densities near the Fermi level, we found that strain can regulate the conductive region of the NWs. More interestingly, the size of spin-down band gap of the NWs is tunable by adjusting uniaxial stress, and the NWs can be converted from indirect to direct band gap under tension.     

Ga sublattice defects in (Ga,Mn)As: thermodynamical and kinetic trends

We have used positron annihilation spectroscopy and infrared absorption measurements to study the Ga sublattice defects in epitaxial Ga(1-x)MnxAs with Mn content varying from 0% to 5%. We show that the Ga vacancy concentration decreases and As antisite concentration increases with increasing Mn content. This is in agreement with thermodynamical considerations for the electronic part of the formation energy of the Ga sublattice point defects. However, the absolute defect concentrations imply that they are determined rather by the growth kinetics than by the thermodynamical equilibrium. The As antisite concentrations in the samples are large enough to be important for compensation and magnetic properites. In addition, the Ga vacancies are likely to be involved in the diffusion and clustering of Mn at low annealing temperatures.     

Magnetic susceptibility of CdSb doped with Mn,Fe, Ni and La

This paper is devoted to the study of the temperature dependence of the magnetic susceptibility of CdSb doped with Mn, Fe, Ni and La. Analyzing the measured curves on the assumption that the impurities are substitutional and using the Hall constant measurements, we were able to determine the magnetic moments of Fe and Ni impurities and to obtain some information concerning the bond of these atoms with their nearest neighbours. The impurities Mn and La are dissolved in CdSb only partly as substitutional impurities, the major part most probably forming with antimony a new phase which is strongly paramagnetic and fully overgrows the diamagnetism of the parent semiconductor compound.     

High temperature ferromagnetism in GaAs-based heterostructures with Mn delta doping

We show that suitably designed magnetic semiconductor heterostructures consisting of Mn delta (delta)-doped GaAs and p-type AlGaAs layers, in which the locally high concentration of magnetic moments of Mn atoms are controllably overlapped with the two-dimensional hole gas wave function, realized remarkably high ferromagnetic transition temperatures (T(C)). A significant reduction of compensative Mn interstitials by varying the growth sequence of the structures followed by low-temperature annealing led to high T(C) up to 250 K. The heterostructure with high T(C) exhibited peculiar anomalous Hall effect behavior, whose sign depends on temperature.     

Ab initio study of the diffusion mechanisms of gallium in a silicon matrix

We present the results of a study into the diffusion mechanisms of Ga defects in crystalline Si using ab initio techniques. Five stable neutral configurations for single and multi-atom defects are identified by density-functional theory (DFT) calculations within the local density approximation and using a localized basis set as implemented in the SIESTA package. Formation energy (E _F ) calculations on these stable structures show the most likely neutral single-atom defect to be the Ga substitutional, with an E _F of 0.7 eV in good agreement with previous work. Charge state studies show the Ga tetrahedral interstitial defect to be in a + 1 state for most doping conditions. They also indicate the possibility for a gallium substitutional-tetrahedral interstitial complex to act as a deactivating center for the Ga dopants except in n-doped regime, where the complex adopts a − 1 charge state. Migration pathway calculations using SIESTA coupled with the activation relaxation technique (ART nouveau) allow us to determine possible migration paths from the stable configurations found, under various charge states. In general, diffusion barriers decrease as the charge state becomes more negative, suggesting that the presence of Si self-interstitials can enhance diffusion through the kicking out of substitutional Si and by adding negative charge carriers to the system. An overall picture of a possible Ga diffusion and complex formation mechanism is presented based on these results.     

MFM and Raman studies in PEMBE-grown (Ga,Mn)N thin films showing room-temperature ferromagnetism

We present the room-temperature ferromagnetism in the (Ga,Mn)N films grown on n-type GaN templates by plasma-enhanced molecular beam epitaxy for semiconductor spintronic device applications. Despite of the possible interface effects between the (Ga,Mn)N layers and n-type GaN templates, the (Ga,Mn)N films were found to exhibit the ferromagnetic ordering above room temperature. The magnetic force microscopy identified the magnetic domains with the different magnetic orientations at room temperature, indicating the existence of the ferromagnetic long-range ordering. In Raman spectra, an additional peak at 578 cm⁻¹ was observed, which is attributed to the local vibration of substitutional Mn in the (Ga,Mn)N lattice. Therefore, it is believed that the ferromagnetic ordering in (Ga,Mn)N is due to the carrier-mediated Ruderman-Kittle-Kasuya-Yosida interaction.     

Density-functional theory study of half-metallic heterostructures: interstitial Mn in Si

Using density-functional theory within the generalized gradient approximation, we show that Si-based heterostructures with 1/4 layer delta doping of interstitial Mn (Mn(int)) are half-metallic. For Mn(int) concentrations of 1/2 or 1 layer, the states induced in the band gap of delta-doped heterostructures still display high spin polarization, about 85% and 60%, respectively. The proposed heterostructures are more stable than previously assumed delta layers of substitutional Mn. Contrary to widespread belief, the present study demonstrates that interstitial Mn can be utilized to tune the magnetic properties of Si, and thus provides a new clue for Si-based spintronics materials.