Tailoring the electrical properties of Ge/GaAs by film deposition rate and preparation of fully compensated Ge films

We investigate the electronic, optical, and structural properties of thin Ge/GaAs(100) films for a variety of growth rates. All of the films have a granular, single-crystal structure, but the electronic properties vary dramatically, with resistivity and carrier concentration changing by more than three orders of magnitude. For high deposition rates, the films are heavily doped and of n-type, with relatively high carrier concentration and low resistivity. The temperature dependence of the resistivity indicates metalliclike transport with degenerate charge carriers. For low deposition rates, the films are p-type, with lower carrier concentrations and higher resistivity whose temperature dependence indicates semiconducting, activation-type transport with an activation energy that can reach up to half of the Ge band gap. Such films are heavily doped and strongly (sometimes fully) compensated Ge. At moderate deposition rates, a metal-insulator transition occurs. The electrical properties of strongly compensated Ge films are analyzed in terms of the two-dimensional percolation of charge carriers through a fluctuating electrostatic potential.