1 G .Chrobok and M .H ofm ann, Phys.Lett.A 57 (1976)257. 2 K .K oike and K .H ayakaw a,J.Appl.Phys.57 (1985)4244. 3 E.K isker,W .G adatand K .Schroder,Solid State Com m un.44 (1982)591. 4 J.U nguris,D .T.Pierce,A .G alejsand R.J.Celotta,Phys.Rev.Lett.49 (1982)72. 5 O .Paul,M .Taborelliand M .Landolt, Surf.Sci.211/212 (1989)724. 6 J.K irschnerand K .K oike,Surf.Sci.273 (1992)147. 7 A .Bringer,M .Cam pagna,R.Feder,W .G udat,E.K iskerand E.K uhlm ann,Phys.Rev.Lett.42 (1979)1705. 8 Z.J.D ing and R.Shim izu, Scanning 18 (1995)92. 9 N .F.M ott,Proc.Roy.Soc.London Ser.A 124 (1929)425. 10 D .R.Penn,Phys.Rev.B 35 (1987)482. 11 J.H ong and D .L.M ills,Phys.Rev.B 62 (2000)5589. 12 N .R.W hetten,M ethods in Experim entalPhysics(V ol.IV ,A cadem ic Press,N Y ,1962)p.69. 13 I.M .Bronstein and B.S.Fraim an,Vtorichnaya Elektronnaya Em issiya (N auka,M oskva,1969)p.340.%