xMnxAs Thin Films Research Articles

Intrinsic magneto-optical spectra of GaMnAs

In the spectrum of reflection magnetic circular dichroism (MCD) of Ga1−xMnxAs, the E0 peak energy, which is assigned to the band gap of GaMnAs, is higher than the band gap energy of GaAs. In the past, this blue shift was attributed to the Moss-Burstein effect, in which the Fermi level moves toward lower energy in the valence band as the Mn concentration x increases; however, this picture is inconsistent with the impurity-band conduction picture, which has been verified in recent studies. Here, by measuring reflection MCD spectra of Ga1−xMnxAs thin films (x = 1%, 2%, and 8%) with various thicknesses, we derive the off-diagonal element of the dielectric tensor of GaMnAs and obtain the intrinsic MCD spectra of GaMnAs that are free from optical interference. We find that optical interference is significantly strong even in the extremely thin (10–100 nm) GaMnAs films and that the E0 peak of the intrinsic MCD spectra is located close to the band gap energy of GaAs even in highly Mn-doped GaMnAs. This is consistent with the recent understanding of the band structure of GaMnAs; the Fermi level exists in the impurity band in the band gap regardless of x.

Storage moduli, loss moduli and damping factor of GaAs and Ga1−xMnxAs thin films using DMA 2980

The spin injector part of spintronic FET and diodes suffers from fatigue due to rising heat on the depletion layer. In this study the stiffness of Ga1−xMnxAs spin injector in terms of storage modulus with respect to a varying temperature, 45°C≤T≤70°C was determined. It was observed that the storage modulus for MDLs (Manganese Doping Levels) of 0%, 1% and 10% decreased with increase in temperature while that with MDLs of 20% and 50% increase with increase in temperature. MDLs of 20% and 50% appear not to allow for damping but MDLs ≤20% allow damping at temperature range of 45°C≤T≤70°C. The magnitude of storage moduli of GaAs is smaller than that for ferromagnetic Ga1−xMnxAs systems. The loss moduli for GaAs were found to reduce with increase in temperature. Its magnitude of reducing gradient is smaller than Ga1−xMnxAs systems. The two temperature extremes show a general reduction in loss moduli for different MDLs at the study temperature range. From damping factor analysis, damping factors for ferromagnetic Ga1−xMnxAs was found to increase with decrease in MDLs contrary to GaAs which recorded the largest damping factor at 45°C≤T≤70°C. Hence, MDL of 20% shows little damping followed by 50% while MDL of 0% has the most damping in an increasing trend with temperature.

Structural, optical and morphological properties of Ga1−xMnxAs thin films deposited by magnetron sputtering for spintronic device applications

In this work, GaMnAs alloy materials were deposited on 7059 Corning glass and GaAs (100) substrates via RF magnetron sputtering technique. A concentration of Mn about 0.28 was obtained by Energy Dispersive X-ray spectroscopy. The substrate temperature was changed from 440 to 520°C and layer thicknesses between 172 and 514nm were obtained. Characterization by atomic force microscopy and X-ray diffraction were performed to determinate surface morphology and crystal structure, respectively. From transmittance spectral measurements we were able to determine optical constants: band gap energy (Eg), absorption coefficient (α), and refraction index (n). A correlation between morphological properties and substrate type was also studied. Diluted magnetic semiconductors like GaMnAs are considered among promising materials for the development of new spin-electronic devices.

Surface treatments and magnetic properties of Ga 1− xMn xAs thin films

As a diluted magnetic semiconductor (DMS), (Ga,Mn)As is a possible candidate for the realization of spintronics devices, due to its intrinsic compatibility with GaAs based electronics. The low Curie temperature still limits its use for practical devices. Despite the huge knowledge on GaAs surface, the (Ga,Mn)As surface is still not well understood and difficult to handle. Standard surface cleaning techniques have many drawbacks, mainly because thermal treatments changes crystal structure. We will compare the magnetic and spectroscopic properties of differently processed (Ga,Mn)As surfaces with X-ray photoemission (XPS), X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD). Samples as-grown, chemically etched, Ar + sputtered and annealed in oxygen will be compared.

Magnetotransport and magnetocrystalline anisotropy inGa1−xMnxAs epilayers

We present an analysis of the magnetic anisotropy in epitaxialGa1−xMnxAs thin films through electrical transport measurements on multiterminal microdevices. Thefilm magnetization is manipulated in 3D space by a three-axis vector magnet. Anomalousswitching patterns are observed in both longitudinal and transverse resistance data. Intransverse geometry in particular we observe strong interplay between the anomalous Halleffect and the giant planar Hall effect. This allows direct electrical characterization ofmagnetic transitions in the 3D space. These transitions reflect a competition between cubicmagnetic anisotropy and an effective out-of-plane uniaxial anisotropy, with a reversalmechanism that is distinct from the in-plane magnetization. The uniaxial anisotropy field isdirectly calculated with high precision and compared with theoretical predictions.

Magneto-optical and magnetotransport properties of heavily Mn-doped GaMnAs

The authors have studied the magneto-optical and magnetotrasnport properties of Ga1−xMnxAs thin films with high Mn concentrations (x=12.2%–21.3%) grown by molecular-beam epitaxy. These heavily Mn-doped GaMnAs films were formed by decreasing the growth temperature to as low as 150–190°C and by reducing the film thickness to 10nm in order to prevent precipitation of hexagonal MnAs clusters. Magnetic circular dichroism and anomalous Hall effect measurements indicate that these GaMnAs films have the nature of intrinsic ferromagnetic semiconductors with high ferromagnetic transition temperature up to 170K.

Mesoscopic conductance effects in InMnAs structures

Quantum corrections to the electrical conduction of magnetic semiconductors are comparatively unexplored. The authors report measurements of time-dependent universal conductance fluctuations (TDUCFs) and magnetic field dependent universal conductance fluctuations (MFUCFs) in micron-scale structures fabricated from two different In1−xMnxAs thin films. TDUCF and MFUCF increasing in magnitude with decreasing temperature are observed. At 4K and below, TDUCFs are suppressed at finite magnetic fields independent of field orientation.

Optical properties of molecular-beam-epitaxy-grown InGaMnAs thin films

The authors have determined the dielectric functions of a series of molecular-beam-epitaxy-grown (In0.5Ga0.5)1−xMnxAs thin films deposited on InP substrates. Two variable angle spectroscopic ellipsometers, covering both the IR and the UV range (0.2–30μm), were used to obtain optical spectra for each of the samples. Using a standard inversion technique, the experimental data were modeled to obtain the dielectric function for each of the quaternary samples. By using a parametric semiconductor model, they deduced the critical point parameters corresponding to the electronic transitions in the Brillouin zone. Their analysis indicates that in this particular quaternary system, while the critical point associated with the fundamental gap, E0, blueshifts as a function of Mn concentration, the E1 critical point shows a redshift with respect to the Mn concentration.

Magnetic anisotropy of Ga1−xMnxAs thin films on GaAs (311)A probed by ferromagnetic resonance

We have studied the magnetic anisotropy of Ga1−xMnxAs thin films grown by low-temperature molecular beam epitaxy on GaAs (311)A substrates by means of ferromagnetic resonance spectroscopy. The angular dependence of the ferromagnetic resonance fields observed can be explained by two main contributions to the magnetic anisotropy: a cubic magnetic anisotropy field oriented along the crystallographic ⟨001⟩ axes caused by the symmetry of the GaAs host lattice, and an effective uniaxial magnetic anisotropy field along [311] presumably caused by the homoepitaxial growth of the layer. Additional smaller magnetic anisotropy contributions are discussed. Consequently, the dominating magnetic anisotropy of Ga1−xMnxAs on GaAs (311)A substrate appears to have the same origin as on GaAs (100) substrate.

Enhanced annealing effect in an oxygen atmosphere on Ga1−xMnxAs

We report on in situ resistivity measurements on Ga1−xMnxAs during post-growth annealing in different atmospheres. A drop in the resistivity is observed when the Ga1−xMnxAs is exposed to oxygen, which indicates that the passivation of Mn interstitials (MnI) at the free surface occurs through oxidation. The presence of oxygen can therefore be an important annealing condition for the optimization of Ga1−xMnxAs thin films, all the more since the oxidation appears to be limited to the sample surface. Annealing in an oxygen-free atmosphere leads to an increase in the resistivity indicating a second annealing mechanism besides the outdiffusion of MnI. According to our magnetization and Hall effect data, this mechanism reduces the amount of magnetically and electrically active Mn atoms.

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.

Dielectric functions of molecular-beam-epitaxy-grown Ga[sub 1−x]Mn[sub x]As thin films

We have measured the dielectric functions of a series of molecular-beam-epitaxy-grown Ga1−xMnxAs thin films directly deposited on GaAs (100) substrates. Initially, x-ray diffraction experiments were employed to determine the alloy compositions of these samples. A rotating analyzer spectroscopic ellipsometer was used subsequently to measure the complex reflection ratio for each of the films in the energy range between 0.9 and 6.5eV. By modeling the ellipsometric data in the transparent region, we were able to determine the film thickness precisely. Extending the analysis into the absorption region, we were able to identify the dielectric functions for each of the Ga1−xMnxAs samples in the energy region of our measurement. All of the dielectric functions displayed the critical point structures related to the higher-order electronic transitions. To determine the characteristics associated with the higher-order electronic transitions, we fit the dielectric functions with a model that incorporates the energy band structure near critical points as well as discrete and continuum exciton states associated with each critical point. This enabled us to determine that both E1 and E1+Δ1 critical points blueshift slightly in the Ga1−xMnxAs alloy system as Mn is incorporated into the lattice.

Magnetoresistance and Hall effect in the ferromagnetic semiconductor Ga1−xMnxAs

The resistivity, temperature, and magnetic field dependence of the anomalous Hall effect in a series of metallic Ga1−xMnxAs thin films with x=0.015 to 0.08 is presented. A quadratic dependence of the anomalous Hall resistance on the resistivity is observed, with a magnitude which is in agreement with Berry phase theories of the anomalous Hall effect in dilute magnetic semiconductors.

Spin wave resonance in Ga1−xMnxAs

We report ferromagnetic resonance experiments on Ga1−xMnxAs thin films. For the dc magnetic field perpendicular to the sample plane, we observe up to eight distinct resonances, which we attribute to spin wave modes. To account for the spacing of the resonances, we infer a linear gradient in the magnetic properties, which is ascribed to a linear variation of the uniaxial magnetic anisotropy with film thickness. Values of D=(1±0.4)×10−9 Oe cm2 for the spin stiffness and JMnMn≈1 meV for the exchange integral between Mn spins are obtained.

Ferromagnetic Resonance in Ga1−xMnxAs

We report on ferromagnetic resonance measurements of Ga1−x Mn x As thin films with Mn contents 0.022 ≤ x ≤ 0.051. For x ≥ 0.036 and the external magnetic field normal to the thin film, we observe several resonances, which we identify as spin wave resonances. The non-quadratic mode spacing can be consistently explained by a linear gradient in the magnetic properties of the films. From the measurements, the exchange constant A can be deduced for different Mn contents x.