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.

Figure 1

Resistivity $\rho$ (a), free charge carrier concentrations $n$ and $p$ (b), and activation energies for resistivity ${\varepsilon }_1$ (full line) and free charge carrier $\varepsilon$ (dashed line) (c) as a function of Ge film deposition rate $V$.

Figure 2

The characteristic energy $\Delta$ that determines the degree of spreading of the fundamental absorption edge of Ge films versus deposition rate $V$.

Figure 3

3D AFM images of Ge films surfaces. The film deposition rates are 0.025 (a), 0.0337 (b), 0.075 (c), 0.118 (d), and 0.337 (e) nm/s. The insets show the corresponding normalized histograms of grain diameters.

Figure 4

Scanning probe microscope images of Ge film on GaAs grown at 0.0337 nm/s: topography (a) and KPFM at an additional tip DC bias of +5 (b) and −5 (c) V. KPFM scan height was 100 nm.

Figure 5

TEM and HRTEM from the Ge films and Ge/GaAs interface. The cross-section bright field-TEM image is in the [110] direction for films prepared with a low deposition rate (0.025 nm/s) (a) and a high deposition rate (0.337 nm/s) (b). The HRTEM from the Ge/GaAs interface (c) and from the grain boundary region (d) are pointed to by the white arrow in (b). Insets in (c) are digital diffractograms from the substrate and the film showing the epitaxial growth of Ge in the [001] direction on GaAs(001).

Figure 6

Energy diagram of a HDFC semiconductor. $r$ is the size of a typical potential fluctuation, $\gamma$ is the LSFEP amplitude, ${\varepsilon }_p$ is the percolation level (dotted line), and ${\varepsilon }_F$ is the Fermi level.