Activation energy and spectroscopy of the growth of germanium films by ultraviolet laser-assisted chemical vapor deposition

Abstract

Emission and absorption spectroscopic studies of a reactor used to grow germanium thin films by photodissociating GeH4 at 248 nm with an excimer laser are described here. For every Ge or GeH (A→X) transition examined, the dependence of the emission line intensity on pump laser fluence was found to be quadratic, indicating that Ge and GeH have a common precursor which is itself produced by the simultaneous absorption of two 5-eV photons. This conclusion is supported by the known photochemistry of SiH4 and CH4 in the ultraviolet and the laser wavelength and intensity thresholds for the growth of Ge films. The short (∼2 μs) lifetime of GeH2 in the reactor suggests that the immediate precursor to Ge and GeH is the germyl radical, GeH3. The activation energy for Ge film growth in the temperature range 300≤T≤500 K has been measured to be 2.0±0.5 kcal/mole which suggests that the limiting mechanisms for the film growth rate are a gas-phase reaction and surface desorption of residual gases rather than surface diffusion. Radiation trapping on several Ge emission lines and absorption measurements at 422.6 nm indicate that the steady-state Ge number density in the reactor is ∼1012 cm−3 which is sufficient to account for the observed film growth rates. Spatial profiles of the atomic Ge and GeH emission from the reactor have been measured with a resolution of ∼100 μm.