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

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.