Crystalline to amorphous phase transition in very low temperature molecular beam epitaxy

Abstract

For the first time, we use in situ time resolved reflectivity (TRR) measurement to determine the thickness for crystalline–amorphous transition. The use of TRR measurements to study the breakdown of the single crystalline growth and the following deposition of amorphous silicon ( α-Si) is based on the higher refraction index of α-Si at the wavelengths used (950 and 670 nm). An increasing α-film thickness causes cycles of constructive and destructive interference and leads to a time-dependent reflectivity. Final amorphous film thicknesses have been confirmed by ex situ spectroscopic ellipsometry. Samples were prepared by the deposition of electron-beam evaporated Si in an MBE equipment. The temperature, as a function of heater power, was carefully calibrated for highly doped substrates, with a thermocouple mounted into a test substrate. Substrates were kept at constant growth temperatures between 100 and 250 °C. From an Arrhenius-type expression, we can deduce an activation energy of 0.36±0.05 eV. The growth rate dependence was investigated between 0.013 and 0.7 nm s −1 at a constant temperature of 200 °C. Rate dependent data can be fitted by a power law. A variation of the deposition rate by a factor of 100 causes a variation in epi thickness by a factor 5. Similar experiments were also performed on low doped substrates and with non-linear temperature ramp. The obtained substrate temperature depends strongly on doping because of free carrier absorption (differences up to ≈50 °C).