xMnxAs Layers Research Articles

Phase Separation in (Ga,Mn)As Layers Obtained by Ion Implantation and Subsequent Laser Annealing

In this paper, we present the results of studies of the spectral, temperature, and field dependences of the transversal Kerr effect in Ga1 – xMnxAs (x = 0.0066–0.033) layers produced by ion implantation and subsequent pulsed laser annealing. The complicated nonmonotonous nature of the temperature dependences of the transversal Kerr effect and its dependence on the measurement range indicate a magnetic inhomogeneity of the layers. The reasons for the inhomogeneity can be the Gaussian distribution of Mn over the thickness of the layers and the electron phase separation in them. The appearance of new features in the spectra of the transversal Kerr effect is explained by the presence in the doped semiconductor matrix of nanoregions with a higher carrier concentration and a higher Curie temperature and a shift of the Fermi level into the valence band leading to an increase in the energy of optical transitions.

Magneto-optical spectroscopy of diluted magnetic semiconductors GaMnAs prepared by ion implantation and further impulse laser annealing

Ga1−xMnxAs layers prepared by ion implantation and subsequent pulsed laser annealing with the planned Mn concentrations of x = 0.01–0.08 have been studied using the magneto-optical transversal Kerr effect (TKE) and spectral ellipsometry. The spectral dependences of the diagonal and nondiagonal components of the permittivity tensor (PT), as well as the spectrum of magnetic circular dichroism (MCD) have been calculated for the layers. The obtained spectra of the diagonal PT components show that the layers under study maintain the zinc-blende crystal structure of the parent GaAs semiconductor. All studied samples reveal a strong TKE response at low temperatures with a dependence of an effective Curie temperature (at which TKE appears) on the Mn concentration. A number of extrema in the low-temperature TKE and MCD spectra are close to the energies of transitions in the Γ and L critical points of the parent semiconductor band structure that confirms the intrinsic ferromagnetism of the Ga1−xMnxAs layers. The MCD spectra shape and its change with Mn concentration increasing are discussed on a base of the valence-band model of ferromagnetism in DMS.

Transversal Kerr effect of In1−xMnxAs layers prepared by ion implantation followed by pulsed laser annealing

In1−xMnxAs (x = 6.9%) layers prepared by ion implantation and subsequent pulsed laser annealing have been studied using the magnetooptical transversal Kerr effect (TKE) and spectral ellipsometry. Ellipsometry data reveal the good crystal quality of the layers. The samples show ferromagnetic behaviour below 77 K. Near the absorption edge of the parent InAs semiconductor, large TKE values are observed. In the energy regions of the transitions in the Γ and L critical points of the InAs Brillouin zone, there are several clearly defined structures in the low-temperature TKE spectra. We have calculated the spectral dependences of the diagonal and nondiagonal components of the permittivity tensor (PT), as well as the spectrum of magnetic circular dichroism (MCD) for our samples. A number of extrema in the obtained MCD and PT spectra are close to the energies of transitions in the critical points of the parent semiconductor band structure, which confirms the intrinsic ferromagnetism of the Mn-doped InAs layers.

What has been measured by reflection magnetic circular dichroism in Ga1–xMnxAs/GaAs structures?

An explicit expression of reflection magnetic circular dichroism (RMCD) has been derived, taking into account the interference effect that arises from multiple internal reflections in an air/Ga1–xMnxAs/GaAs dielectric layered system. It unambiguously shows that the RMCD signal is composed by three terms. In addition to the conventional term, which is sufficient in the absence of interference, an oscillatory term is required. Both of them are related to the imaginary part ε″xy of the off-diagonal element of the dielectric tensor. One also becomes aware that in this case RMCD is not actually determined only by the imaginary part ε″xy of the off-diagonal element of the dielectric tensor, as has been widely accepted. In fact, the real part ε′xy of the off-diagonal element will substantially mix into the measured RMCD results by another oscillatory cos θ form. It can even reverse the sign of RMCD, when the Ga1–xMnxAs layer becomes thicker. The main aspects of these predictions were used to reasonably explain the RMCD results measured in three different types of samples. Our work will bring about a reconsideration of how to correctly explain RMCD results.

Measurement of magnetization of Ga1−xMnxAs by ferromagnetic resonance

In this paper, we extend ferromagnetic resonance (FMR) studies of thin layers of the ferromagnetic semiconductor Ga1−xMnxAs to the analysis of the integrated intensity of the resonance in order to obtain information on the total spin in the sample directly involved in ferromagnetically-ordered magnetization. A theoretical model is proposed for the dependences of the FMR integrated intensity and linewidth on the orientation of the applied magnetic field as the field direction is varied from in-plane to normal-to-the-plane of the Ga1−xMnxAs layer. The strain-induced magnetic anisotropy of Ga1−xMnxAs presents a significant challenge to conventional FMR linewidth and integrated intensity models. The new model predicts that the integrated FMR intensity is proportional to the saturation magnetization MS of the sample, with the constant of proportionality varying as a function of the polar and azimuthal angles of the applied magnetic field. The angular and temperature behaviors of the integrated intensity and linewidth of the FMR predicted by the proposed model are in good qualitative agreement with measurements.

Imaging of interstitial atoms in Ga1−xMnxAs layers by means of X-ray diffuse scattering

The local atomic structure of a Ga1−xMnxAs (x= 0.07) layer during the annealing process was studied by means of X-ray diffuse scattering. The difference between the pair-distribution functions before and after annealing indicated the fraction of atoms that changed concentration and identified them to be exclusively interstitial atoms at the centres of gallium and/or arsenic tetrahedra in the GaMnAs unit cell.

Properties of Ga1−xMnxAs with high x (>0.1)

We have investigated the magnetic and the crystalline properties of a set of Ga1−xMnxAs layers with high nominal Mn compositions (x=0.101–0.198). Magnetization measurements and combined channeling Rutherford backscattering (c-RBS) and particle induced x-ray emission (c-PIXE) measurements have been performed to determine the effective Mn composition xeff and the fraction of Mn atoms at various lattice sites. Here, xeff determined from magnetization measurements, which increases with increasing x, is consistent with the results determined from c-RBS-PIXE measurements.

Magnetization reversal of Ga1−xMnxAs layers separated by a nonmagnetic spacer

We have used polarized neutron reflectometry to individually examine the magnetization reversals of ferromagnetic Ga1−xMnxAs layers separated by a nonmagnetic GaAs spacer layer of varying thickness. For each of the samples studied, the top Ga1−xMnxAs layer is adjacent to a Be-doped Al0.25Ga0.75As capping layer on one side and the GaAs spacer on the other, while the bottom Ga1−xMnxAs layer is surrounded by GaAs on either side. For samples with spacer thicknesses of 12 and 6nm, antiparallel alignment of the two Ga1−xMnxAs layer magnetizations was observed at multiple fields, implying that hole doping from the capping layer strongly affects the coercivity of the top Ga1−xMnxAs layer but has a weaker effect on the coercivity of the bottom Ga1−xMnxAs layer. However, for a spacer thickness of 3nm, both top and bottom Ga1−xMnxAs layers appear to be equally influenced by the capping layer, as virtually identical coercivities were observed. This behavior is evidence of coupling between the Ga1−xMnxAs layers across the 3nm GaAs spacer.

Structure and magnetism of MnAs nanocrystals embedded in GaAs as a function of post-growth annealing temperature

Self-organized Ga(Mn)As nanoclusters, embedded in GaAs, were formed during post-growth thermal annealing of Ga1−xMnxAs layers. Structural and magnetic properties of such composites were systematically studied as a function of the annealing temperature. Small (∼3 nm) Mn-rich zinc-blende Mn(Ga)As clusters, coherent with the GaAs matrix, were formed at the annealing temperature of 500 °C. An increase of the annealing temperature of up to 600 °C led to the creation of 10–20 nm large NiAs-type hexagonal MnAs nanocrystals. Magnetization measurements showed that the MnAs nanoprecipitates were superparamagnetic, with a distribution of blocking temperatures that depended on the MnAs cluster size. Some intermediate paramagnetic clusters (structurally disordered clusters) were also observed.

Properties of Ga1−xMnxAs with high Mn composition (x>0.1)

A series of Ga1−xMnxAs layers with high Mn compositions x (=0.075–0.200) has been grown and investigated. Magnetization, magnetotransport, and magneto-optical properties reveal that the layers have single ferromagnetic phase as in the case of typical (Ga,Mn)As. The authors also describe the variation of magnetic anisotropy with x and the effect of low temperature annealing on Curie temperature.

Location of Mn sites in ferromagnetic Ga1−xMnxAs studied by means of X-ray diffuse scattering holography

A three-dimensional image of the local neighbourhood of Mn atoms in a Ga1−xMnxAs (x= 0.02) layer has been obtained by using X-ray diffuse scattering holography. The first and second nearest neighbours of the Mn atoms correspond to the local structure around Ga atoms in the zinc-blende GaAs structure. Accordingly, the Mn atoms are situated in substitutional positions.

Optical studies of carrier and phonon dynamics in Ga1−xMnxAs

We present a time-resolved optical study of the dynamics of carriers and phonons in Ga1−xMnxAs layers for a series of Mn and hole concentrations. While band filling is the dominant effect in transient optical absorption in low-temperature-grown GaAs, induced absorption by trapped electrons become important with increasing Mn concentration in Ga1−xMnxAs, as inferred from the sign of the absorption change. We find that carrier trapping and trapped carrier recombination becomes faster as Mn concentration increases in GaMnAs. We also report direct observation on lattice vibrations in Ga1−xMnxAs layers via the reflective electro-optic sampling technique. The data show increasingly fast dephasing of longitudinal-optical phonon oscillations for samples with increasing Mn and hole concentration, which can be understood in term of phonon scattering by the holes.

Analysis of magnetic field dependent mobility in ferromagnetic Ga 1− xMn xAs layers

The magneto-transport properties of ferromagnetic Ga 1− x Mn x As epilayers with Mn mole fractions in the range of x≈2.2–4.4% were investigated through Hall effect measurements. The magnetic field-dependent Hall mobility for a metallic sample with x≈2.2% in the temperature range of T=0–300 K was analyzed by magnetic field-dependent mobility model including an activation energy of Mn acceptor level. This model provides outstanding fits to the measured data up to T=300 K. It was found that the acceptor levels with activation energies of 112 meV at B=0 Oe decreased to 99 meV at B=5 kOe in the ferromagnetic region. The decrease in acceptor activation energy was due to the spin splitting of the Mn acceptor level in the ferromagnetic region, and was responsible for increase in carrier concentration.

Perpendicular magnetization reversal, magnetic anisotropy, multistep spin switching, and domain nucleation and expansion in Ga1−xMnxAs films

We present a comprehensive study of the reversal process of perpendicular magnetization in thin layers of the ferromagnetic semiconductor Ga1−xMnxAs. For this investigation we have purposely chosen Ga1−xMnxAs with a low Mn concentration (x≈0.02), since in such specimens contributions of cubic and uniaxial anisotropy parameters are comparable, allowing us to identify the role of both types of anisotropy in the magnetic reversal process. As a first step we have systematically mapped out the angular dependence of ferromagnetic resonance in thin Ga1−xMnxAs layers, which is a highly effective tool for obtaining the magnetic anisotropy parameters of the material. The process of perpendicular magnetization reversal was then studied by magnetotransport (i.e., Hall effect and planar Hall-effect measurements). These measurements enable us to observe coherent spin rotation and noncoherent spin switching between the (100) and (010) planes. A model is proposed to explain the observed multistep spin switching. The agreement of the model with experiment indicates that it can be reliably used for determining magnetic anisotropy parameters from magnetotransport data. An interesting characteristic of perpendicular magnetization reversal in Ga1−xMnxAs with low x is the appearance of double hysteresis loops in the magnetization data. This double-loop behavior can be understood by generalizing the proposed model to include the processes of domain nucleation and expansion.

Magnetic anisotropy and magnetization reversal in Ga1−xMnxAs layers studied by polarized neutron reflectometry

The magnetic anisotropy and the in-plane magnetization reversal mechanism of Ga0.945Mn0.055As thin films, grown on GaAs(001), were investigated. Samples A and B, with sample A grown 45 °C lower than sample B, were analyzed by magnetization hysteresis measurements and polarized neutron reflectivity (PNR) magnetization reversal experiments. Magnetization hysteresis loops and temperature-dependent magnetization curves accounted for an in-plane uniaxial magnetic anisotropy, with a significant out-of-plane magnetization, for sample A, while an in-plane biaxial magnetic anisotropy with in-plane ⟨100⟩ as easy axes was evidenced for sample B. PNR magnetization reversal experiments showed the occurrence of a spin-flip maximum upon magnetization reversal only for sample B. A mechanism of magnetization reversal proceeding by 180° domain-wall nucleation and propagation is proposed for sample A, while an incoherent rotation mechanism by 90° domains is proposed for sample B. The reversal mechanism is shown to be correlated to the anisotropy of the samples which depends on the T∕Tc ratio.

Observation of Combined Ferromagnetic/Paramagnetic Phase in Ga1?xMnxAs by Magnetic Circular Dichroism

Magnetic properties of a series of Ga1−xMnxAs layers with different Mn and hole concentrations has been studied by measuring magneto-absorption and magnetic circular dichroism (MCD). We first focus on comparing the MCD spectra of samples with unambiguous ferromagnetic or paramagnetic magnetization. We then investigate MCD in a sample with parameters between these two extremes, and interpret the observed behavior in terms of the coexistence of ferromagnetic domains and paramagnetic regions that may result, for example, from inhomogeneities in the sample caused, e.g., by compositional or doping fluctuations.

Coherent Optical Phonon Oscillations in GaMnAs

Coherent phonon oscillations are investigated in a series of Ga1−xMnxAs layers with different Mn and hole concentrations. The longitudinal optical (LO)-phonon-hole-plasmon coupled mode oscillations are observed in layers with large Mn concentrations. The energies of these coupled modes depend on the hole concentrations, which may prove useful for determining this parameter in ferromagnetic alloys. Finally, electron-plasmon coupling with LO phonon, observed under optical excitation, is compared in GaMnAs and in LT-GaAs.

Effect of film thickness on the incorporation of Mn interstitials in Ga1−xMnxAs

We have investigated the effect of film thickness on the distribution of Mn atoms at various lattice sites in Ga1−xMnxAs thin films. We find that the growth surface acts as a sink facilitating the outdiffusion of Mn interstitials (MnI), and thus reducing its concentration in the film. The outdiffused MnI accumulate on the surface in a surface oxide layer and do not participate in the ferromagnetism of the film. For thin films less than 15 nm thick, no MnI can be detected. Because of the absence of compensating MnI defects, higher TC can be achieved for such extremely thin Ga1−xMnxAs layers. These results agree with our previously suggested Fermi-level-governed upper limit of the TC of III–Mn–V ferromagnetic semiconductors.

Mn diffusion in Ga1−xMnxAs∕GaAs superlattices

Ga 1 − x Mn x As ∕ GaAs superlattices with Mn concentrations of 1% and 5% in the Ga1−xMnxAs layers and a GaAs spacer thickness of 4 and 60 GaAs monolayers have been studied by cross-sectional scanning tunneling microscopy. By achieving atomic resolution of the superlattices, we observe individual Mn atoms in the Ga1−xMnxAs layers and in the GaAs spacer. We find that about 20% of the total amount of Mn diffuses from the GaMnAs layers into the GaAs spacer layers. Our results can be related to previous measurements of the magnetic properties of short period Ga1−xMnxAs∕GaAs superlattices.

Properties of arsenic antisite defects in Ga1−xMnxAs

We report the results of optical absorption measurements on Ga1−xMnxAs layers grown by low-temperature molecular beam epitaxy. In the paramagnetic layers grown at very low temperatures (below 250 °C) the experiments reveal an absorption band at 1.2 eV arising from the presence of neutral arsenic antisites, AsGa. From the magnitude of the absorption we determine the concentration of AsGa to be between 4×1019 and 8×1019 cm−3 in these paramagnetic samples. These values are typical for GaAs specimens grown below 250 °C. Extrapolating the AsGa concentration from low-temperature-grown GaAs to Ga1−xMnxAs, we determine the concentration of this defect in ferromagnetic Ga1−xMnxAs layers grown at temperatures above 250 °C as 1×1019 down to 1×1018 cm−3. We conclude that the compensating role of arsenic antisites in Ga1−xMnxAs becomes gradually less important with increasing growth temperature.