Study of Mn interstitials in (Ga,Mn)As using HRXRD

L. Horák¹, J. Matějová¹, X. Martí (a)¹, V. Holý¹, V. Novák², Z. Šobáň^2,3, S. Mangold⁴, and F. Jimenez-Villacorta⁵

¹Department of Condensed Matter Physics, Charles University, Prague, Czech Republic

²Institute of Physics ASCR, Prague, Czech Republic

³Department of Microelectronics, The Czech Technical University in Prague, Czech Republic

⁴Karlsruhe Institute of Technology, Karlsruhe, Germany

⁵Department of Chemical Engineering, Northeastern University, Boston, USA

horak@karlov.mff.cuni.cz

The (Ga,Mn)As, belonging to the diluted magnetic semiconductors, is intended for a spintronics application, but the temperature of the magnetic ordering is decreased among others due to the presence of the interstitial Mn. The amount of the interstitials can be reduced by the post-growth annealing[1], which leads to the out-diffusion of the interstitials to the free surface. Although this material has been intensively studied for last decade, the process of the out-diffusion is not yet fully understood [1–2].

Figure 1: The measured diffraction curves (circles) for as-grown, annealed and re-annealed (sequentially 20x etched and annealed) sample with their theoretical fits (solid line).

We present a method for the determination of the concentration depth profiles of Mn interstitial ions in (Ga,Mn)As thin epitaxial layers using high-resolution x-ray diffraction (HRXRD) [3]. The measured diffraction curves for several diffraction maxima hkl were fitted to the theoretical curves based on standard dynamical diffraction theory (figure 1). From the asymmetry of the intensities of the thickness fringes it is possible to characterize an eventual depth inhomogeneity of the interstitial density in the (Ga,Mn)s layer. The depth profiles of the Mn interstitial density obtained for the sample in various annealing states were compared to the numerical drift-diffusion simulations, from this comparison the diffusivity of the interstitials in (Ga,Mn)As host lattice can be estimated.

The epitaxial GaMnAs layers under study were grown on (001)GaAs substrates by molecular beam epitaxy. A nondestructive character of the characterization (in contrast for instance to the transmission electron microscopy) allows to investigate the same sample in various annealing states. We measured an as-grown sample, then after 24 hours of annealing in the air at 160°C and finally after 20 cycles of etching and short annealing (under the same conditions as in the previous case). We have determined the depth profiles of the interstitial density for all samples and these have been compared to the numerical simulations of the interstitial drift-diffusion in the sample.

Figure 2: Depth profiles of the concentration of the Mn interstitials in as-grown sample, annealed and 20x etch-anneled. The concentration profiles determined by HRXRD are represented by gray areas indicating the uncertainty of the profiles. The profiles obtained from the diffusion simulations for the interstitial diffusion constant D_n = 4 × 10−20 m2/s are plotted by solid lines; the concentration profiles simulated for 10 times larger and 10 times smaller values of the Mn diffusion constant are plotted by dotted and dashed lines, respectively. The initial concentration profile for all simulations is given by the concentration of Mn interstitials in as-grown sample.

From these comparisons the diffusivity of the Mn ions in the (Ga,Mn)As lattice has been estimated (figure 2). The results show that the flux of the Mn ions towards the free surface is strongly affected by the internal electric field produced by inhomogeneously distributed holes [4].

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[2] J. Adell, I. Ulfat, L. Ilver, J. Sadowski, K. Karlsson, and J. Kanski, Journal of Physics Condensed Matter 23, 085003 (2011).

[3] L. Horák, Z. Šobáň, and V. Holý, Journal of Physics: Condensed Matter 22, 296009 (2010).

[4] L. Horák, J. Matějová, X. Martí, V. Holý, V. Novák, Z. Šobáň, S. Mangold, and F. Jiménez-Villacorta, Phys. Rev. B – accepted for publishing

This work is a part of the research programme MSM 0021620834 financed by the Ministry of Education of the Czech Republic. The work has been supported by the European Community’s Seventh Framework Programme NAMASTE under grant agreement number 214499. The XANES experiment was carried out at synchrotron ANKA, Germany.