Magnetic Semiconductor Alloys Research Articles

The magnetic state in the binary DyxGe1−x (x ≤ 0.02) alloy semiconductor

Diluted bulk magnetic alloy semiconductors DyxGe1−x (x ≤ 0.02) were prepared by a two-step fabrication procedure and studied experimentally by XRD, EDX and magnetic methods. The polycrystalline materials had the ccp (cubic closed-packed) structure of the host Germanium up to ≈2 at.% of Dysprosium. They exhibited different low-temperature and high-temperature behavior. Low-field dc magnetic susceptibility data showed sharp peaks and irreversibility between zero-field cooled and field-cooled states for x < 0.02. A stable antiferromagnetic phase with temperature of the antiferromagnetic to paramagnetic transition TN = 25 K was observed at x = 2 at.% of Dysprosium. The solubility range of the binary DyxGe1−x alloy system is limited to ∼2 at.% due to the large atomic radius of the solute. The low-temperature magnetic phase is characterized as spin-glass below 2 at.%. The magnetic relaxation and non-linear susceptibility χnl of Dy0.01Ge0.99 were analyzed, and the spin-glass phase was defined with the critical exponents of the phase transition β = 0.52 ± 0.10, γ = 2.85 ± 0.10 and δ = 6.49 ± 0.02. From the Curie-Weiss behavior of the susceptibility at high temperatures, we determined an average effective Dy-Dy exchange constant J1 = −6.90 K and effective magnetic moment per Dy ion µeff = 10.08 µB. The rare-earth Dy atoms behave as isolated in Ge matrix at high-temperatures.

Composition dependence of fundamental properties of $$\hbox {Cd}_{\mathrm{1-x}}\hbox {Co}_\mathrm{x}$$ Cd 1 - x Co x Te magnetic semiconductor alloys

Ab initio calculations based on density functional theory have been performed using the full-potential augmented-plane-wave method so as to investigate the composition dependence of the electronic structure and fundamental properties of hypothetical zinc-blende $$\hbox {Cd}_{\mathrm{1-x}}\hbox {Co}_{\mathrm{x}}\hbox {Te}$$Cd1-xCoxTe magnetic semiconductor alloys at low Co concentrations. To treat the exchange and correlation energies, the generalized gradient approximation (GGA) of Perdew---Burke---Ernzerhof has been used. In addition, the modified Becke---Johnson exchange potential with the GGA approach is used for the band structure providing high accuracy. It is found that the addition of a small amount of Co atoms in the $$\hbox {Cd}_{\mathrm{1-x}}\hbox {Co}_{\mathrm{x}}\hbox {Te}$$Cd1-xCoxTe makes the latter less compressible, ferromagnetic and exhibiting a half metallic character. Besides, the composition dependence of the real and imaginary parts of the dielectric function has been examined and discussed. The information derived from the present study may be useful for spintronics technological applications.

Ab initio predictions of the stability and structural properties of zincblende (III,TM)V magnetic semiconductor alloys

First-principles calculations and statistical methods were combined to study electronic, magnetic, thermodynamic and structural properties of zincblende (III,Mn)V and (III,Cr)V magnetic semiconductor alloys, including both nitride and arsenide alloys. From phase diagrams it was observed that nitride alloys are much less stable than arsenide ones, although the former ones have more localized d-states at the Fermi level. It was observed that all alloys present an anisotropic behavior, with the strongest magnetic interaction in the 〈110〉 direction. The relationship between the structural properties of these alloys and their electronic and magnetic characteristics (i.e., their half-metallicity) was investigated.

Probe of the Band Structure of MBE Grown p-Type InMnAs at Ultrahigh Magnetic Fields

We present a theoretical and experimental study on electronic and magneto-optical properties of p-type paramagnetic InMnAs dilute magnetic semiconductor (DMS) alloys in ultrahigh (&gt; 100 T) external magnetic fields (B). Theoretical calculations are based on an 8-band Pidgeon–Brown model which is generalized to include the wavevector dependence of the electronic states along B as well as s–d and p–d exchange interactions with localized Mn d-electrons. The spin-dependent electronic structure as a function of Mn doping is computed and the dependence of the valence band structure on parameters such as the sp–d exchange interaction strength and effective masses in paramagnetic p- InMnAs alloys are examined. The cyclotron resonance (CR) and magneto-optical properties of InMnAs are calculated using Fermi's golden rule. Two strong CR peaks are observed in p-type InMnAs alloys which correspond to the transitions within either heavy-hole (HH) or light-hole (LH) Landau levels. Furthermore, we also observed strong resonance absorption for electron-active polarization which can occur in p-type semiconductors originating from transitions between the light and heavy hole Landau levels.

Coherent GdN clusters in epitaxial GaN:Gd thin films determined by transmission electron microscopy

The detection of non-uniform magnetic element distribution and phase separation has been highly challenging in dilute magnetic semiconductor alloys. Here, we present a dedicated transmission electron microscopy (TEM) investigation of epitaxial GaN:Gd thin films and unambiguously identify the occurrence of nano-scale coherent GdN clusters. The films were grown by molecular beam epitaxy with Gd concentrations varying between 1016 and 1019 cm−3. The TEM results revealed the presence of nano-scale, coherently strained platelets lying parallel to the (0001) basal planes of the wurtzite GaN matrix. The platelet-shaped clusters consist of GdN as verified by chemically sensitive Z-contrast scanning TEM imaging. The cluster dimension as well as the displacement of the distorted lattice along the [0001] direction are quantitatively determined by high-resolution TEM based on geometric phase analysis. Dynamic strain contrast calculations taking the measured displacement field as an input parameter attained an excellent agreement with the experimental diffraction contrast results, which enables an estimate of average cluster size and distance. The GdN platelet clusters are of two monolayer thickness with a base diameter of about 2 nm for samples with low Gd concentrations (1016 cm−3) and about 4 nm for samples with high Gd concentrations (1019 cm−3). A discussion about the local stress environment establishes that the GdN platelets are incorporated in a metastable wurtzite crystal phase.

Controlling the growth morphology and phase segregation of Mn-doped Ga2Se3on Si(001)

The growth and phase segregation properties of the potential dilute magnetic semiconductor alloy (MnSe)${}_{x}$(Ga${}_{2/3}$Se)${}_{1\ensuremath{-}x}$ are studied as a function of thickness, Mn concentration, postgrowth annealing, and the presence or absence of undoped Ga${}_{2}$Se${}_{3}$ buffer and capping layers. This system is an unusual case in heteroepitaxy where two-phase $\mathrm{MnSe}+{\mathrm{Ga}}_{2}{\mathrm{Se}}_{3}$ has better lattice matching than the (MnSe)${}_{x}$(Ga${}_{2/3}$Se)${}_{1\ensuremath{-}x}$ alloy. Despite this peculiarity, this system shows a modified form of Stranski-Krastonow growth: laminar films are observed up to a certain $x$-dependent critical thickness, above which islands are observed by scanning tunneling microscopy. The island morphology depends on the presence or absence of an undoped Ga${}_{2}$Se${}_{3}$ buffer layer and postgrowth annealing. A kinetically stabilized platelet morphology is observed at the crossover point between laminar and islanded films. Based on Mn and Se $K$-edge extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopy, there are two types of Mn in islanded films: Mn that remains doped in the Ga${}_{2}$Se${}_{3}$ but oxidizes upon exposure to air, and Mn that participates in the islands, which are precipitates of the MnSe phase. Consistent with MnO or MnSe, $L$-edge x-ray absorption on air-exposed films suggests the Mn is in the formal $+$2 oxidation state. No $L$-edge x-ray magnetic circular dichroism signal is observed at 20 K, which may be due to surface effects or to a lack of magnetic order.

Phase decomposition diagram of magnetic alloy semiconductor

We investigate the thermodynamics of phase decomposition in magnetic alloy semiconductors (MASs) using the statistical Flory–Huggins lattice model. Based on this model, we propose a method to determine experimentally the interaction parameter χ and apply it to draw the phase decomposition diagram of (GaMn)As. The interaction energy of (GaMn)As was estimated to be about 5.7 mRy, which is close to the first principle calculation of 4.2 mRy. Using this phase diagram, we fabricated MnAs nanoparticles with a rectangular lattice structure utilizing the spinodal decomposition. We discuss the formation of rectangular and triangular lattice structures at the late stage of spinodal decomposition in MASs using the Ginzburg–Landaulike free energy function.

Anomalous lattice parameter of magnetic semiconductor alloys

The addition of transition metals to III-V semiconductors radically changes their electronic, magnetic, and structural properties. We show by ab initio calculations that in contrast to the conventional semiconductor alloys, the lattice parameter in magnetic semiconductor alloys, including those with diluted concentration, strongly deviates from Vegard’s law. We find a direct correlation between the magnetic moment and the anion-transition metal bond lengths and derive a simple and general formula that determines the lattice parameter of a particular magnetic semiconductor by considering both the composition and magnetic moment. This dependence can explain some experimentally observed anomalies and stimulate other kind of investigations.

Magneto-optical Studies of Spin Phenomena in CdMnTe Doped with Co and Cr

We investigate the magneto-optical and magnetic properties of two quaternary diluted magnetic semiconductor alloys, Cd1−x−y Mn x Cr y Te and Cd1−x−y Mn x Co y Te, with fixed Mn concentration x ∼ 0.37 and, respectively, with concentrations of Cr in the range 0 < y < 0.07 and Co in the range␣0 < y < 0.009. The introduction of Cr and Co leads to very different behaviors, including the occurrence of ferromagnetic order in the case of Cd1−x−y Mn x Cr y Te. We discuss the possible origins leading to the observed behaviors.

Investigation of Mn site configuration in wurtzite and rock-salt Zn1−xMnxO by means of XAS experiments under pressure

In this paper we report on XANES and EXAFS experiments under pressure on the diluted magnetic semiconductor alloy Zn1-x MnxO [1-4]. Zn1-x MnxO powder was prepared from relatively thick films grown by pulsed laser deposition (PLD) on mica monocrystalline substrates. For XAS measurements under pressure at the K-edge of Mn were carried out at the m-XAS LUCIA beam line (SLS), using a membrane diamond anvil cell with partially perforated diamonds anvils. Silicone oil was used as pressure transmitting medium. XAS was measured in the transmission configuration for samples with high Mn content (x=0.25) and in the fluorescence configuration for samples with low Mn content (x=0.05). In the last case, the sample was excited through a partially perforated diamond and the Mn Kα fluorescence was detected in direction perpendicular to the exciting beam, after crossing a Be gasket. EXAFS analysis reveals that the Mn-O bond distance is practically independent on the Mn concentration. On the other side, slight changes are observed in the XANES spectrum when increasing the Mn content, probably related with the increasing proportion of Mn atoms among the 12 second neighbors. In the wurtzite phase, the Mn-O distance is found to be slightly higher and less compressible than the Zn-O distance in pure ZnO. The wurtzite-to-rock-salt transition is observed in both types of samples with very similar features. On the one side, a dramatic change is observed in the XANES spectrum including a drastic decrease of the pre-peak intensity and a complete restructuring of the above edge oscillations. On the other side, the EXAFS analysis shows the expected increase of the Mn-O distance associated to the first neighbor coordination change from tetrahedral to octahedral. A metastable rock-salt phase is retrieved at ambient pressure for samples with 25% Mn Content.

Growth and magnetic properties of self-assembled (In, Mn)As quantum dots

Self-assembled In0.79Mn0.21As quantum dots were successfully grown on GaAs (001) substrates by low-temperature molecular beam epitaxy. Atomic force microscopy and high-resolution transmission electron microscopy confirm the formation of quantum dots. High-resolution lattice image suggests that In0.79Mn0.21As dots are single phase with zinc-blend structure. The dots exhibit typical ferromagnetic state at 5K and demonstrate a Curie temperature of ∼290K which is much higher than those of (In, Mn)As diluted magnetic semiconductor alloys ever reported. The significant increase in Curie temperature can be attributed to the much higher Mn content in the dots, and the possible enhancement of the hybridization strength between the quantum-confined holes in the dots and the itinerant holes in the semiconductor valence band.

Cubic binary compounds MnN and MnAs and diluted magnetic [formula omitted] semiconductor alloys: a first-principle study

In this work we report ab initio calculations of the electronic structures of the binary compounds MnN and MnAs, and of the diluted magnetic semiconductor alloys Ga 1 - x Mn x N ( x = 0.06 ; 0.03) in the zinc-blende phase. We use the theoretical local density approximation and the generalized gradient approximation within the density functional theory. We follow the evolution of the band structures, density of states and magnetic moments as functions of the lattice parameter of the MnN and MnAs compounds. Depending on the lattice parameter, the binary compounds MnAs and MnN are found to be half-metallic, that is, they exhibit metallic conduction by spin-polarized charge carriers; both materials reach a maximum magnetization of 4.0 μ B . We also discuss the electronic structures, local magnetic moments, exchange interactions, and the general features of the Ga 1 - x Mn x N alloys. These ferromagnetic alloys are also candidates to be half-metallic materials: the majority-spin states are metallic and the minority-spin states are insulating. The valence band is ferromagnetically coupled to the Mn atoms, and the total magnetization of the cell is 4.0 μ B , which is independent of the Mn concentration.

Temperature dependence of the band-edge photoluminescence of Zn1−xMnxSe films

We report a systematic investigation of band-edge photoluminescence of the diluted magnetic semiconductor alloy Zn1−xMnxSe for a series of compositions (0 < x < 0.485). The specimens investigated were in the form of thin films fabricated by molecular beam epitaxy. The near band-edge emissions of donor-bound excitons (designated as I2) and of neutral acceptor-bound excitons (I1) were systematically investigated as a function of temperature and of alloy composition. These measurements also allowed us to determine the LO-phonon energy ℏωLO = 31.2 ± 0.2 meV. From the temperature dependence of the bound exciton peak, the Varshni temperature parameter α was found to increase linearly with increasing Mn concentration x, while β decreased with x. From the temperature dependence of the full width at half maximum (FWHM) of the I2 emission line, the broadening factors Γ(T) were determined from the fit to the data. For x > 0.20 the effect of inhomogeneous broadening Γinh on Γ(T) was found to be stronger than that of the longitudinal optical phonon term ΓLO. Furthermore, the activation energies of thermal quenching were obtained for both the I2 and the I1 peaks from the temperature dependence of the bound exciton peaks, and were found to decrease with increasing Mn concentration. Finally, we show that at high temperature the broadening of the FWHM is dominated by the LO-phonon interaction.

Formation of quantized states and spin dynamics in III–V-based ferromagnetic quantum wells

Abstract We have studied quantum size effect and spin dynamics in single quantum wells consisting of III–V magnetic alloy semiconductor with different well width. A blue-shift of the broad negative peak observed in magnetic circular dichroism spectra indicates the formation of quantized states in (Ga,Mn)As well layers. Time-resolved Kerr rotation measurements were employed for ferromagnetic SQW to study the influence of lifting the degeneracy between hole sub-bands on the spin dynamics. Significant enhancement in the magnitude of Mn spin rotation was observed.

Carrier-induced ferromagnetic order in the narrow gap III–V magnetic alloy semiconductor (In,Mn)Sb

Preparation and physical properties of p- and n-InMnSb epitaxial films with Mn contents up to 10% were studied with the aim of seeking phenomena induced by the spin exchange interaction between carrier and Mn spins. For p-type samples with Mn eff=4.5×10 20 and p=1.1×10 20 cm −3 , carrier-induced ferromagnetic order with a Curie temperature of 20 K was observed. The sign of the anomalous Hall coefficient is found to be negative. Tellurium-doped n-type samples (n=8.6×10 18 cm −3) with net Mn contents of 10% are found to be paramagnetic.

Electronic states and cyclotron resonance inn-type InMnAs

We present a theory for electronic and magneto-optical properties of n-type ${\mathrm{In}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ magnetic alloy semiconductors in a high magnetic field $B\ensuremath{\Vert}\mathrm{z\ifmmode \hat{}\else \^{}\fi{}}.$ We use an eight-band Pidgeon-Brown model generalized to include the wave vector ${(k}_{z})$ dependence of the electronic states as well as $s\ensuremath{-}d$ and $p\ensuremath{-}d$ exchange interactions with localized Mn d electrons. Calculated conduction-band Landau levels exhibit effective masses and g factors that are strongly dependent on temperature, magnetic field, Mn concentration $(x),$ and ${k}_{z}.$ Cyclotron resonance (CR) spectra are computed using Fermi's golden rule and compared with ultrahigh-magnetic-field $(&gt;50$ T) CR experiments, which show that the electron CR peak position is sensitive to x. Detailed comparison between theory and experiment allowed us to extract the $s\ensuremath{-}d$ and $p\ensuremath{-}d$ exchange parameters $\ensuremath{\alpha}$ and $\ensuremath{\beta}.$ We find that not only $\ensuremath{\alpha}$ but also $\ensuremath{\beta}$ affects the electron mass because of the strong interband coupling in this narrow-gap semiconductor. In addition, we derive analytical expressions for effective masses and g factors within the eight-band model. Results indicates that $(\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta})$ is the crucial parameter that determines the exchange interaction correction to the cyclotron masses. These findings should be useful for designing novel devices based on ferromagnetic semiconductors.

Photo-carrier-induced magnetism in (In,Mn)As/GaSb magnetic alloy semiconductor heterostructures

We present wavelength dependence of photo-carrier-induced magnetization for (In,Mn)As/GaSb heterostructures, and discuss the problem involved in hole transfer from GaSb to (In,Mn)As across the (In,Mn)As/GaSb heterointerface. The reduction in coercive force by the light illumination has also been shown as a new effect associated with carrier-induced magnetism.

Preparation of quaternary magnetic alloy semiconductor epilayers (Ga, Mn, Fe)As

This paper reports on the successful preparation of quaternary III–V-based magnetic alloy semiconductor layers (Ga, Mn, Fe)As by molecular beam epitaxy. Magnetic and electronic behavior suggest that the competition between ferromagnetic and antiferromagnetic spin exchange interactions takes place between incorporated magnetic atoms.

Local structure and chemical valency of Mn impurities in wide-band-gap III–V magnetic alloy semiconductors Ga1−xMnxN

Local structure and effective chemical valency of Mn impurity atoms incorporated in wide-band-gap (Ga,Mn)N epilayers have been investigated by using x-ray absorption fine structure techniques. The x-ray results provide direct evidence for the substitution of majority Mn atoms for the Ga sites in GaN, with an effective valency close to Mn(II), up to a rather high Mn concentration about 2 at. %. A small fraction of the impurity atoms could also form Mn clusters.

Enhancement of direct (type-I) excitonic transitions in ZnTe/CdSe-based type-II Bragg confining structures

We have used distributed Bragg reflectors (DBRs) designed for electron de Broglie wavelengths to achieve enhanced localization of above-barrier states in the conduction band of a ZnTe/CdSe-based type-II heterostructure. We studied the effect of such DBRs by observing excitonic transitions between above-barrier electron states which are Bragg confined in the barrier layer of the conduction band and heavy holes localized in the quantum well of the valence band. In a type-II system, both carriers are thus confined in the same layer, leading to a direct (type-I-like) excitonic transition. In order to identify the physical region where this transition actually occurs, we have used a diluted magnetic semiconductor (DMS) alloy Zn0.94Mn0.06Te as the confining layer. In such a structure, the giant Zeeman splitting of the band edges in the DMS layer serves to identify the location where the states participating in the transition are confined. To enhance the intensity of such direct transitions, we have sandwiched the central DMS layer between two DBR stacks, each consisting of 10 alternating quarter-wave layers of ZnTe and CdSe tailored to the de Broglie wavelength of the above-barrier state in the conduction band of the central layer. As expected, the transition of interest was observed at an energy slightly higher than the band gap of the DMS layer. We have observed this direct transition in absorption and, more importantly, also in photoluminescence, attesting to the very significant confinement of the above-barrier layer in such DBR-based structures.