Initial nucleation stage in photo-CVD of GeH4 on SiO2 substrate monitored by real-time spectroscopic ellipsometry and photo reflectance: Accurate determination of incubation time

The initial stage of photoexcited chemical vapor deposition (CVD) of Ge layers on SiO2 substrate was studied through a combined analysis of real-time spectroscopic ellipsometry and photoreflectance (PR). Just after GeH4 gas was admitted into the growth chamber, there was a certain period during which both the ellipsometric (Psi, Delta) angles and PR intensity remained unchanged from their initial values. This phenomenon prior to steady-state growth is known as incubation, and it is due to the very slow reactive sticking of GeHx species on the dangling-bond free SiO2 surfaces. The incubation time was accurately determined from the time derivatives of (Psi, Delta) and the PR intensity. In most cases, the incubation times independently derived from (Psi, Delta) and the PR intensity coincided, but for other cases, those derived from the PR intensity were slightly longer than those derived from (Psi, Delta). The incubation time decreased as thermally activated decomposition of GeHx species became prevalent above 300 degrees C. An activation energy of 18 kJ/mol was obtained by assuming that the rate of creating nucleation seeds on SiO2 is proportional to the inverse of the incubation time. Such a low activation energy, much lower than that of thermal CVD, is due to partial decomposition of GeH4 by photoexcitation to create reactive GeHx fragments. At the end of incubation, the Ge layer wetting the SiO2 surface changed into self-assembled Ge dots when more Ge atoms were deposited. The dot density was maximum at the beginning of dot formation and gradually decreased as the dot size became larger. The activation energy of continuous Ge growth on Ge-covered SiO2 surface was derived from the time evolution of D to be 13 kJ/mol. The minimum PR intensity for three-dimensional (3D) dot formation was lower than that of two-dimensional (2D) layer growth, reflecting greater surface roughness. Contrasting behaviors between 2D and 3D growth were also observed in terms of the recovery level of the PR intensity after prolonged growth; continued 2D growth recovered up to 90% of the initial level, while 3D dot formation recovered only 50 - 60%. Published by the AVS.