In-situ probing and modeling atomic layer deposition processes on Ge surface

Germanium (Ge) is a promising CMOS compatible channel material with a low effective-mass of holes. One of the major challenges in developing Ge-FETs is integrating a high-quality gate-stack on Ge. A direct high-k dielectric deposition like HfO$_{2}$ on Ge has resulted in poor electrical characteristics of the semiconductor-dielectric interface.$^{1}$ GeO$_{2}$/Ge interface has been found low in interface-trap density, but its quality rapidly degraded when scaling down to ultrathin GeO$_{2}$.$^{2}$ Takagi \textit{et al }showed that such interface quality can be preserved using an ultrathin Al$_{2}$O$_{3}$ layer on GeO$_{2}$/Ge, but the detailed mechanism has not been addressed and remained elusive.$^{3}$ In this work, we studied this problem by combining (a) \textit{in-situ} spectroscopic ellipsometry for real-time monitoring of atomic-layer-deposition (ALD) processes on Ge, (b) \textit{ex-situ} X-ray photoelectron spectroscopy (XPS) to probe the interface chemistry, and (c) reactive force field (ReaxFF) simulations to directly model the growth kinetics and interface formation. A strong surface-chemistry dependence (hydrogenated Ge vs oxidized Ge) has been found in the Al$_{2}$O$_{3}$-ALD nucleation (Trimethylaluminum$+$H$_{2}$O), which is well reproduced by ReaxFF simulation. Furthermore, both experiments and simulations revealed that the Al$_{2}$O$_{3}$ capping on GeO$_{2}$/Ge interface prevents oxygen diffusion into Ge, and therefore stabilizes the interface. [1] Appl. Phys. Lett. 87, 032107 (2005). [2] J. Appl. Phys. 106, 104117 (2009). [3] 2012 Symp. VLSI Technol. 2011--2012 (2012).