SURFACE MICROSTRUCTURE OF CIS THIN-FILMS PRODUCED BY RAPID THERMAL-PROCESSING

The surfaces of polycrystalline CuInSe2 thin films produced by rapid t hermal processing (RTP) have been analyzed by scanning tunnelling micr oscopy and spectroscopy in ambient air. Deviating from standard measur ement techniques the tunnelling microscope is driven by an AC sample v oltage for surface morphology mapping in the constant current mode. Ad ditionally, a Fermi energy mapping of the semiconductor surface is per formed by mapping significant features of the I-V tunnelling character istic. The polarity of the tunnelling current proves to be a reliable measure of the conductivity type of the material (n- or p-type); the o bservation of leakage currents at small bias voltages allows the ident ification of gap states around the Fermi level or metallic phases. Cur rent-voltage curves taken at positions of different conduction type ve rify the spectroscopic information in the maps. Typical areas imaged a re (1.5 mu m)(2). Intra- and inter-granular nonuniformities of the con duction type are observed. Although the bulk material of all samples i nvestigated is p-conductive, abrupt changes of the conductivity type o f the surfaces from p- to n-type are observed as a function of the ove rall copper-to-indium ratio. The dominant current flow direction in sl ightly Cu-rich thin film hulk material is associated with p-type condu ction, whereas In-rich samples exhibit largely n-type conductivity at the surface. Surfaces of copper-rich hulk materials show Fermi level p inning. The spectroscopic results do not depend on material and geomet ry of the tunnelling tip.