Benjamin Schmid (PhD thesis)

Benjamin Schmid (PhD thesis): Surface preparation and Mn states of (Ga,Mn)As investigated by means of soft-and hard x-ray photoemission spectroscopy

Abstract

The present thesis deals with surface treatment, material improvement, and the electronic structure of the diluted magnetic semiconductor (Ga,Mn)As. The two key issues are the preparation of clean surfaces and the observation of potential valence hybridizations in (Ga,Mn)As by means of photoemission spectroscopy. Several cleaning methods are applied individually to (Ga,Mn)As and their effects are compared in detail by various methods. Based on the results of each method, a sophisticated recipe has been elaborated, which provides clean, stoichiometric, and reconstructed surfaces, even if the sample was exposed to air prior to preparation. Moreover, the recipe works equally well for intentionally oxidized surfaces. The individual advantages of ex situ wet-chemical etching and in situ ion-milling and tempering can be combined in an unique way.
In regard to the post-growth annealing in order to optimize the electronic and magnetic properties of (Ga,Mn)As, the effect of surface segregation of interstitial Mn was quantified. It turns out that the Mn concentration at the surface increases by a factor 4:3 after annealing at 190 degrees C for 150 h. The removal of the segregated and oxidized species by wet-chemical etching allows a tentative estimate of the content of interstitial Mn. 19-23% of the overall Mn content in as-grown samples resides on interstitial positions.
The complementary results of core level photoemission spectroscopy and resonant photoemission spectroscopy give hints to the fact that a sizeable valence hybridization of Mn is present in (Ga,Mn)As. This outlines that the simple Mn 3d5-configuration is too naive to reflect the true electronic structure of substitutional Mn in (Ga,Mn)As. Great similarities in the core level spectra are found to MnAs. The bonding is thus dominantly of covalent, not ionic, character.
Transport measurements, in particular for very low temperatures (below 10 K), are in agreement with previous results. This shows that at low temperature, the conduction is mainly governed by variable-range hopping which is in line with the presence of an impurity band formed by substitutional Mn.
In the light of the presented results, it is therefore concluded that a double-exchange interaction is the dominant mechanism leading to ferromagnetic coupling in (Ga,Mn)As. The valence hybridization and the presents of an impurity band, both of which are inherent properties of substitutional Mn, are indications for a double-exchange scenario, being at variance to a RKKY-based explanation. Contributions from a RKKY-like mechanism cannot definitely be excluded, however, they are not dominant.

You find the complete pdf version here.