Influence of disorder in compensation-doped germanium on the critical indices of the metal-insulator transition

We present a critical review of the present state of the critical exponent puzzle of the metal-insulator transition of doped semiconductors with empha sis on the role of meso- and macroscopic inhomogeneity caused by the disord er of intended or unintended acceptors and donors in crystals. By using bot h isotopic engineering and neutron transmutation doping (NTD) of germanium we found for low compensations (at K = 1.4 and 12%) that the critical expon ents of the localization length and the dielectric constant are nearly nu = 1/2 and zeta = 1, which double for medium compensations (at K = 38 and 54% ) to nu = 1 and zeta = 2, respectively. (C) 1999 American Institute of Phys ics. [S1063-7834(99)01905-X].