Influence of annealing parameters on the ferromagnetic properties of optimally passivated (Ga,Mn)As epilayers

Stanciu, Victor Ioan; Wilhelmsson, Ola; Bexell, Ulf; Adell, Martin; Sadowski, J.; Kanski, J.; Warnicke, Peter; Svedlindh, Peter

doi:10.1103/physrevb.72.125324

Cited by 20 publications

(30 citation statements)

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“…3͑b͔͒ it is apparent that T Curie increases. 19 It remains basically unchanged for further annealing to 180 min, which is consistent with more extensive work presented by Stanciu et al 16 This enhancement of T Curie has been usually traced back to a removal of compensating defects, and thus, to an increase of the hole concentration. 17,20 In fact, channeling Rutherford backscattering 17 and Auger 20 experiments have shown that annealing at low temperature causes a migration of Mn interstitial defects towards the surface of GaMnAs.…”

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confidence: 77%

Correlation effects in the density of states of annealedGa1−xMnxAs

et al. 2007

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We report on an experimental study of low-temperature tunneling in hybrid NbTiN/ GaMnAs structures. The conductance measurements display a ͱ V dependence, consistent with the opening of a correlation gap ͑⌬ C ͒ in the density of states of Ga 1−x Mn x As. Our experiment shows that low-temperature annealing is a direct empirical tool that modifies the correlation gap and thus the electron-electron interaction. Consistent with previous results on boron-doped silicon we find, as a function of voltage, a transition across the phase boundary delimiting the direct and exchange correlation regime. DOI: 10.1103/PhysRevB.75.033308 PACS number͑s͒: 75.50.Pp, 73.40.Gk, 71.30.ϩh The new class of ferromagnetic semiconductors Ga 1−x Mn x As is known 1 to display a metal-insulator transition ͑MIT͒ as function of Mn doping. In conventional doped semiconductors the MIT, which occurs as a function of carrier density, is widely studied and considered to be a prime example of a quantum phase transition. It is understood that the spatial localization of charge carriers, which drives the MIT, reduces the ability of the system to screen charges, leading to a prominent role of the electron-electron interactions. The experimental trace of the Coulomb interactions between the electrons is the depletion of the single-particle density of states ͑DOS͒ N͑E͒ at the Fermi energy. 2-9 For a dirty three-dimensional system it is found that N͑E͒ϳ ͱ E in the metallic regime, 3,4 whereas N͑E͒ϳE 2 in the insulating regime 2 recently observed in different localized systems, 5-7 including magnetically doped materials. 8 Recently, using conductance measurements across the metal-insulator-transition, Lee 9 constructed the phase diagram shown in Fig. 1͑a͒. At low enough temperatures, 10 mK, the energy is controlled by the voltage at which the differential conductance is measured. For low energies, i.e., very close to the Fermi energy where the theory for the MIT is valid, the system is a Coulomb gap insulator below the critical density and a correlated metal above the critical density. For higher energies a mixed state develops around the critical density, in which the density of states on both sides of the transition have a common functional dependence on energies masking the existence of a critical density. The "pure" state at low densities is the regime where exchange correlations describe the Coulomb interactions, whereas above the critical density the direct Coulomb interactions rule. At low energies the DOS is clearly distinct for metallic and insulating samples and the system is in the "pure" state. At high energies the insulating and metallic states are indistinguishable from DOS measurements.The new material system GaMnAs is for low Mn doping an insulator and the resistivity diverges for T → 0, indicating localization effects. In the metallic regime this ͑III, V͒ Mn is characterized by a decreasing resistivity which eventually saturates for T → 0, although these resistivity values remain relatively high ͓ϳ10 −3 ⍀ cm, see Fig. 4͑c͔͒. Thus GaMnAs...

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Correlation effects in the density of states of annealedGa1−xMnxAs

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“…• C for ten hours or longer reduces T C again [6,7,8], and annealing at higher temperatures leads to a more swift lowering of T C [8]. This twofold behavior clearly indicates that besides the Mn i out-diffusion another yet unknown microstructure evolution process takes place.…”

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“…Post-growth annealing of a few hours is an efficient method to remove the interstitial Mn i and thus increase T C [6,7,8,9]. However, extended annealing at temperatures around 250• C for ten hours or longer reduces T C again [6,7,8], and annealing at higher temperatures leads to a more swift lowering of T C [8]. This twofold behavior clearly indicates that besides the Mn i out-diffusion another yet unknown microstructure evolution process takes place.…”

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“…The substitutional Mn (Mn Ga ) act as acceptors, providing spinpolarized holes that mediate the ferromagnetic coupling, while the As Ga and Mn i as donors hamper the ferromagnetism by compensating holes. Post-growth annealing of a few hours is an efficient method to remove the interstitial Mn i and thus increase T C [6,7,8,9]. However, extended annealing at temperatures around 250…”

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Diffusion and clustering of substitutional Mn in (Ga,Mn)As

Raebiger

Ganchenkova

Boehm

2006

Applied Physics Letters

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The Ga vacancy mediated microstructure evolution of (Ga,Mn)As during growth and post-growth annealing is studied using a multi-scale approach. The migration barriers for the Ga vacancies and substitutional Mn together with their interactions are calculated from first principles, and temporal evolution at temperatures ranging from 200 to 350• C is studied using Lattice Kinetic Monte Carlo simulations. We show that at the typical growth and annealing temperatures (i) gallium vacancies provide the diffusion mechanism for substitutional Mn and (ii) in 10-20 h the vacancy mediated diffusion of Mn promotes the formation of substitutional clusters. Clustering reduces the Curie temperature (TC), and therefore the Mn clustering combined with the fast interstitial Mn diffusion explains the experimentally observed twofold annealing behavior of TC.PACS numbers: 75.50.PpUnderstanding of microstructure evolution during growth and post-growth annealing is one of the key issues in materials science. The microstructure and its inhomogeneities largely determine the material properties, including the basic phase transition points for materials ranging from high temperature superconductors to diluted magnetic semiconductors [1,2]. In the ongoing quest for room temperature semiconductor spintronics materials (Ga,Mn)As has been one of the main candidates since the observation of the relatively high Curie temperature T C of 110 K [1]. Typically the (Ga,Mn)As thin films, where Mn substitutionally replaces Ga atoms, are grown by means of low-temperature molecular beam epitaxy, which also leads to the formation of As antisites (As Ga ) [3] and interstitial Mn (Mn i ) [4,5]. The substitutional Mn (Mn Ga ) act as acceptors, providing spinpolarized holes that mediate the ferromagnetic coupling, while the As Ga and Mn i as donors hamper the ferromagnetism by compensating holes. Post-growth annealing of a few hours is an efficient method to remove the interstitial Mn i and thus increase T C [6,7,8,9]. However, extended annealing at temperatures around 250• C for ten hours or longer reduces T C again [6,7,8], and annealing at higher temperatures leads to a more swift lowering of T C [8]. This twofold behavior clearly indicates that besides the Mn i out-diffusion another yet unknown microstructure evolution process takes place. At present, the nature of this process and the mechanism underlying this evolution is not understood.In this Letter we show that the mechanism behind the long-term microstructure evolution is gallium vacancy (V Ga ) mediated Mn Ga diffusion on the Ga sublattice. We also show that this diffusion leads to Mn clustering, which reduces T C [10,11]. Although the formation of Mn Ga clusters has been shown to be energetically * Electronic address: hra@fyslab.hut.fi favorable [11,13,14,15], it requires an abundance of mobile gallium vacancies. The recent discovery of rather high gallium vacancy (V Ga ) concentrations in (Ga,Mn)As up to 10 18 cm −3 [16] gives us good reason to consider this mechanism plausible, but the mobilities...

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“…In these configurations Mn is highly mobile [7] and tends to accumulate on the surface during crystal growth, resulting in non uniform doping profiles and Mn enriched surfaces. Thermal treatments on thin films in gaseous environment, or on As-capped samples [8], are used to remove Mn I thus improving the ferromagnetic properties of the material. These procedures causes Mn I to accumulate on the surface, in the form of a thick, non-magnetic layer.…”

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Surface treatments and magnetic properties of Ga1−xMnxAs thin films

et al. 2007

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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.

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Influence of annealing parameters on the ferromagnetic properties of optimally passivated (Ga,Mn)As epilayers

Cited by 20 publications

References 25 publications

Correlation effects in the density of states of annealedGa1−xMnxAs

Correlation effects in the density of states of annealedGa1−xMnxAs

Diffusion and clustering of substitutional Mn in (Ga,Mn)As

Surface treatments and magnetic properties of Ga1−xMnxAs thin films

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