Morphology of quench condensed Pb films near the insulator to metal transition

We present in situ scanning tunneling microscopy topographs of Pb films, fo rmed by vapor deposition onto cold (T < 20 K), inert substrates, near their insulator to metal transition; At the critical mass deposited thickness fo r conduction, d(G) congruent to 5.2 nm, the films consist of approximately two layers of nanoclusters with diameter, 2r approximate to 20 nm and heigh t, 3.5 less than or equal to h less than or equal to 5.5 nm. We discuss how the nanocluster size and formation mechanism dictate the need fur two laye rs to form in order for conduction to commence. [S0031-9007(99)08430-6].