Changes in optical properties of MnAs thin films on GaAs(001) induced by α- to β-phase transition

MnAs layers with 45 nm thickness were grown epitaxially on GaAs(001) substrates. Ellipsometry measurements were made in the spectral range 0.045 eV to 6 eV as a function of temperature (between -10 degrees C and 50 degrees C) at 70 degrees of incidence. In this way the transition from the hexagonal alpha-phase to the orthorhombic beta-phase could be monitored. Non-zero off-diagonal elements of the Jones matrix for an azimuth of 38 degrees off the [1 (1) over bar0] axis of the substrate indicate that the optical functions of MnAs are anisotropic in both phases. The optical conductivity exhibits low-energy interband transitions around 0.3 eV, more clearly seen in the alpha-phase than in the beta-phase. Extrapolation of the optical conductivity to zero frequency confirms that the alpha-phase is about two times more conducting than the beta-phase. A broad structure is observed in the visible range around 3 eV. The a-phase is characterised by an anisotropy induced energy difference of this structure with a maximum at 2.8 eV for the extraordinary index and at 3.15 eV for the ordinary index. This difference vanishes in the beta-phase in which anisotropy mainly induces changes in amplitude of the 3 eV structure. The assignment of the structures will be discussed. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.