SCANNING-TUNNELING-MICROSCOPY STUDIES OF GE SI FILMS ON SI(111) - FROM LAYER-BY-LAYER TO QUANTUM DOTS/

We have followed by scanning tunneling microscopy (STM) the growth of thin Ge films obtained by reactive deposition epitaxy on the Si(111) s urface kept at 500 degrees C. For Ge thickness smaller than 0.45 monol ayers (ML), STM images show large 7X7 flat regions without protrusions while at higher coverages flat, triangular 5X5 islands start nucleati ng, We have followed the evolution of this wetting layer up to its com pletion and investigated its surface composition at 3 ML by current im aging tunneling spectroscopy measurements. At larger coverages thick G e islands (quantum dots) start to nucleate according to the Stranski-K rastanov mechanism. We analyze the evolution of the lattice strain bot h on the wetting layer and on the islands up to 15 ML coverage. A clea r expansion of the lattice parameter as a function of the coverage is evidenced both on the islands' top and on the wetting layer. The lumin escence yield measured at 10 K on samples covered by 40 Angstrom- of G e and capped with 10 Angstrom of Si evidences a structure that could b e assigned to Ge quantum dots. (C) 1998 American Vacuum Society.