Interfacial characterization and transport conduction mechanisms in Al|HfO$_2$|$p$-Ge structures: energy band diagram

Ge-based metal-oxide semiconductor structures exhibiting thin ALD-grown high-$k$ dielectric HfO$_2$ films were fabricated and characterized chemically, structurally, and electrically. $X$-ray photoelectron (XP) spectroscopy confirms the good stoichiometry of the ALD-grown HfO$_2$ films. Furthermore, through the analysis of the XP spectra, the conduction and valence band offsets of HfO$_2$|$p$-Ge were calculated to be equal to 1.8 $\pm$ 0.2 eV and 2.8 $\pm$ 0.2 eV, respectively. $C(V)$ and $G(V)$ analysis reveals structures with a well-defined MOS behavior with $D_{\mathrm{it}}$ values in the range of 10$^{11}$ eV$^{-1}$cm$^{-2}$ and a dielectric constant of HfO$_2$ films of 20. The dominant carrier transport conduction mechanisms were studied through $J(V)$ analysis, performed at both substrate and gate electron injection. Specifically, in the low voltage region ($V<$ 0.2 V), the prevailing conduction mechanism is Ohmic, with an activation energy of 0.28 eV for both substrate and gate electron injection. In the voltage range 0.4–1.5 V, the dominant conduction mechanism is Frenkel–Poole, through which the trap energy level into HfO$_2$ films $(\varphi_t)$ is calculated to be $\varphi_t$ = 0.36 eV. Schottky conduction mechanism is the prevailing one, for high applied bias voltages ($V>$ 3.0 V) and high temperatures ($>$ 450 K). Applying Schottky’s emission model the energy barrier heights of HfO$_2$|$p$-Ge and Al|HfO$_2$ interfaces were evaluated equal to 1.7 $\pm$ 0.2 eV and 1.3 $\pm$ 0.2 eV, respectively. Combining the XPS and $J(V)$ analysis results, the energy band diagram of Al|HfO$_2$|$p$Ge structures is constructed. The calculated values of conduction and valence band offsets via XPS and $J(V)$ measurements are in very good agreement.