The interface formed between the ferromagnetic semiconductor GaMnAs and the organic semiconductor N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'- diamine (NPB) was investigated using current transport measurement and ultraviolet photoemission spectroscopy (UPS). The hole injection barrier at a GaMnAs and NPB interface was measured as 0.77eV by modelling the measured current density-voltage (J-V) characteristics in a GaMnAs/NPB/Al structured device. The vacuum level shift at a GaMnAs/NPB interface was deduced to be 0.54 eV, indicating that a dipole layer exists at the interface. A UPS study gave a vacuum level shift of 0.53 eV and a band offset between the GaMnAs valence band and the highest occupied molecular orbital of NPB of 0.79 eV, in good agreement with the results of J-V measurements. We attribute the vacuum level shift to charge transfer across the interface.
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