In situ observation of surface-limited Ge growth processes by transient optical reflectometry

Optical surface reflectometry was used to monitor the active layer during the epitaxial growth of Ge from pulsed molecular jets of digermane (Ge₂H₆) molecules. The modulation of the digermane delivery facilitates the seperation and independent investigation of the elementary reaction steps, digermane chemisorption and reaction by-product desorption. For (100) surfaces of Si and Ge, both the chemisorption and desorption processes were found to be first-order. The digermane chemisorption was not thermally activated, while the desorption rate was strongly temperature-dependent. For Ge(100) substrates and the temperature ranges 410-480 and 480-570°C, desorption activation energies of 2.2{hermitian conjugative matrix}0.1 and 1.6{hermitian conjugative matrix}0.1 eV were determined, respectively, and the corresponding pre-exponential factors were 4.9×10^{16{hermitian conjugative matrix}0.7} and 3.5×10^{12{hermitian conjugative matrix}0.6} s^-1. The lower activation energy is in agreement with the expected value for desorption of H₂ from the monohydride phase. The difference in kinetic parameters between the two temperature ranges is believed to be caused by the presence of oxygen contamination at lower substrate temperatures.