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Structural and magnetic properties of Mn-doped GaAs(110) surface

We have investigated STM images of the (110) cross-sectional surface of Mn-doped GaAs using first principles total-energy pseudopotential calculations. We focus on configurations with Mn interstitial in the uppermost surface layers. In particular, we have found that Mn impurities, surrounded by Ga or As atoms, introduce in both cases strong local distortions in the GaAs(110) surface, with bond length variations up to 8 % on surface and non-negligible relaxations effects propagating up to the third sub-surface layer. In both cases interstitial Mn induces a spin-polarization on its nearest neighbors, giving rise to a ferromagnetic Mn-As and to antiferromagnetic Mn-Ga configuration.


INTRODUCTION
Diluted magnetic semiconductors (DMS's) have been considered of tremendous scientific and technological importance. [1][2][3] This is essentially due to the combination of ferromagnetism with semiconducting properties in the same host material which enable the use of the spin degree of freedom to process, to transfer as well as to store information, giving rise to the emerging field of Spintronic. [4] Among DMS's materials, ferromagnetic (Ga,Mn)As has attracted considerable attention.
Substitution of Mn for Ga in GaAs introduces a local spin 5 2 magnetic moment, and acts as an acceptor, providing itinerant holes which mediate the ferromagnetic order. [5] An important step toward near future device applications was achieved some years ago, when it was recognized that annealing at temperatures close to the growth temperature can result in an important improvement of the Curie temperature (the highest T C for the past few years was 110 K). The observed changes have been attributed to out diffusion of Mn interstitials towards the surface. [6] Therefore, it is of great importance for practical applications to clearly understand the role of Mn-dopant in determining the magnetic and electronic properties.
It is clear that the properties of such systems strongly depend on the type and concentration of defects. [6] In this perspective, experimental and theoretical studies of the atomicscale structure of (Ga,Mn)As are highly motivated. This paper is organized as follows: in the next section we describe the computational method; in Sect. 3 we present our results for the structural and magnetic properties; in Sect. 4 we discuss the XSTM images; finally, in Sect. 5 we draw our conclusions.

COMPUTATIONAL DETAILS
Our calculations have been performed within Density Functional Theory (DFT) framework in the Local Density Approximation for the exchange-correlation functional [8,9], using state-of-the-art first-principles pseudopotential self-consistent calculations, as implemented in the ESPRESSO/PWscf code [10]. Ultrasoft (US) pseudopotential (PP) [11] has been used for Mn atom, while norm-conserving PPs have been used for where V is the voltage applied between the sample and the tip. In this model the tip near the surface does not influence the electronic states.

Structural properties
The GaAs(110) relaxed structure is well known from experimental as well as theoretical point of view. In the relaxed surface, the electronic charge is transferred from Ga to As atoms with the occupied state density being localized around surface As atoms and the unoccupied density around the Ga atoms. Throughout this work, we have considered only tetrahedral interstitial position, as the total energy corresponding to the hexagonal interstitial one is higher by more than 0.5 eV. [6,17,18] In the bulk zinc-blende crystal structure, there are two inequivalent tetrahedral interstitial position which differ in the local environment. We call them Int Ga(As) , to denote that Mn is surrounded by four Ga(As) atoms. The tetrahedral interstitial positions in the ideal geometry is equidistant from its four nearest-neighbor (NN) atoms with a distance equal to the ideal host bond length d 1 . There are six next-nearest-neighbor (NNN) atoms at the distance d 2 = 2 √ 3 d 1 , which are As(Ga) atoms for Int Ga(As) , respectively.
At surface, the tetrahedral interstitial position has three NNs and four NNNs instead of four and six respectively as in the bulk case. To start with, we consider the clean and After relaxations, the two configurations, Int Ga and Int As , are almost degenerate, differing by ∼ 130 meV/Mn atom (Int Ga is favoured).
In Fig.1 we show a ball and stick side (a) and top (b) view of the relaxed Int Ga and Int As configurations. Only the three topmost layers and the atoms closest to Mn are shown.
Grey spheres are cations (Ga atoms), white spheres are anions (As atoms); Mn is explicitly indicated. It is easy to see that the presence of Mn strongly reduces the surface buckling. In Fig.1b, atomic moments are also indicated for atoms close to Mn and the numbers in parenthesis specify the atomic layer from the surface. To characterize the relaxed configurations, in Table I  Mn-Ga distance is strongly reduced with respect to the initial one. From Fig. 1a, we see that small relaxations effects are still present in the third layer, in both configurations.
In conclusion, the largest local distortions with respect to the clean surface occur in the Int Ga configuration resulting in a remarkable repulsion of the NNs and NNNs whereas, in Int As , the lattice relaxations around the Mn impurity involve mainly the NNN Ga atom on surface.

Magnetic properties
In the following, we analyze the magnetic properties for the two configurations. In Fig.1b (top views), we report the spin-polarizations for Mn and for the NN and NNN atoms. These features change a little bit according to the specific positive bias applied, but do not disappear.

Isolated Mn Interstitial (Int As )
In Fig.3, we show the simulated XSTM images for Mn in Int As configuration. At negative bias Mn appears as an additional bright spot close to its neighbouring surface As atoms. giving rise to a ferromagnetic Mn-As and to an antiferromagnetic Mn-Ga configuration.
Comparison of simulated XSTM images with experimental ones preliminary available seem to indicate an Int As configuration in the experimental samples.