[1] Fraunhofer Inst Phys Measurement Tech IPM, Georges Kohler Allee 301, D-79110 Freiburg, Germany
[2] Leibniz Inst Kristallzuchtung IKZ, Max Born Str 2, D-12489 Berlin, Germany
[3] Univ Freiburg, IMTEK, Georges Kohler Allee 102, D-79110 Freiburg, Germany
[4] Univ Freiburg, Inst Phys, Freiburg, Germany