Study of the structure of GaMnAs thin layers

L. Horák, V. Holý

The Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic

Lukas.horak@napismi.cz

In present time there is a big focus on magnetic semiconductors, in which gallium arsenide doped with manganese belongs. It will have a wide application if it is succeeded to raise its Curie temperature to the room temperature. To reach this aim it is necessary to know its inner structure because magnetic properties are connected with positions of manganese atoms in a lattice. According to the theoretical predictions Curie temperature could reach a room temperature for the concentration of manganese ten percents [1].

It results from various studies (e.g. [1]) that a magnetic behavior is caused by added manganese atoms at gallium sites. These atoms act as acceptors and an interaction between their spins is mediated by charge carriers - holes. Donors present in material compensate holes and reduce T_C. Two major problems arise when GaMnAs layer grows. The first problem is a creation of EL2 defect; it means antisites arsenide atoms, which are donors [2]. Their concentration is correlated to the concentration of Mn_Ga [1]. The second problem is positioning of Mn atoms, they can occupy not only Ga sites but they can be present as double donor interstitials in As tetrahedrons. Their concentration and influence can be reduced by annealing as described e.g. in [3].

The goal of our work is to obtain a concentration of Mn atoms in substitional and interstitial positions with x-ray diffraction measurement and a concentration of other defects as well. These values should be compared with magnetic measurements to optimize growing layers of desired properties.

We measure rocking curves on different diffractions and we compare them with dynamical calculations of the chosen model. This model comes from GaAs lattice disturbed by added Mn atoms and As antisite defects. It leads to change of lattice constant [4-5] and to local shift of atoms near lattice disturbance changing a structure factor. It is suitable to measure more reflections considering that there are many parameters to fit. Atomic scattering factors of gallium and arsenide differ not too much, so it is useful to measure quazi-forbidden reflections, because change of structure factor due to the defects is the biggest.

Our measured samples are tens nanometers thin layers of GaMnAs containing approximately 5% Mn deposited on low-temperature GaAs buffer on GaAs substrate. Direction [001] is perpendicular to surface. Measurement are done with laboratory difractometer for monocrystals, asymmetric diffractions are measured in coplanar grazing-exit geometry.

I compared measured data with their simulations and there is shown difference of kinematical and dynamical approach in my presentation too. GaMnAs layer is thin enough to use kinematical formulas as described in [6], but there are so many parameters such as shifts of atoms, which make conclusions from kinematical calculations very problematical.

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