Detecting the Crystallization of Forsterite from Amorphous Silicate thin Films - IR Spectroscopy vs. DIC and SE Images

The analysis of infrared signals from circumstellar disks is the major source of information on their mineralogical compositions. The abundance of minerals and their spectroscopic properties (mainly related to the degree of crystallinity) controls the opacity and the thermal structure of the disks. In the frame of a study on the crystallization kinetics of forsterite (Mg₂SiO₄), one of the major constituent of circumstellar dust, we have performed annealing experiments on amorphous thin films deposited by pulsed laser deposition (PLD). The substrates of the samples are rectangular pieces of polished (111) silicon wafers, 20 by 10 mm in size and 300 μm thick. The highly stoichiometric, 100-150 nm thick films cover 50% of the front surfaces of the wafers. The samples were thermally treated in air at different temperatures between 750 and 850 °C for durations between 0.5 to 680h. After annealing, the surface of the thin films was examined using Difference Interference Contrast Microscopy (DIC) and Secondary Electron (SE) images obtained with a Scanning electron microscope (SEM). Infrared spectra were measured in transmission and reflection mode in the mid-infrared (2.5 - 27 μm) and far-infrared (14-200 μm) ranges. After relatively short annealing times at the lower temperatures (e.g. 6 h at 800 °C), the MIR spectra display only a broad band with a distinct shoulder to lower wave numbers at about 1100 to 850 cm^-1, which can be interpreted as resulting from amorphous or nano crystalline silicates. In contrast, the DIC and SE images show micron sized crystals as well as a few larger (a few μm in diameter) pyramids and spherulites. Distinct forsterite bands can be resolved in the IR spectra only when nearly the whole sample surface is covered with forsterite crystals that exceed a critical size. At this stage, the intensity of some forsterite bands seems to be related to the relative abundance of spherulites vs pyramidic crystals. Hence, the appearance of sharp forsterite bands in the IR spectra does not mark the beginning of crystallization, as assumed by previous workers, but may rather represent a certain state of Ostwald ripening. The physical background of these observations and their possible impact on infrared measurements of circumstellar disks will be discussed. This project is part of the DFG "Forschergruppe 759": "The Formation of Planets: The Critical First Growth Phase".