Characterization of Low Temperature Solid Phase Crystallization of Amorphous Silicon




Thin film solar cells and transistors fabricated from polycrystalline Si hold promise for reducing cost due to reduced processing temperature and therefore possibility for using low cost glass and plastic substrates.  Polycrystalline silicon derived from crystallization of amorphous silicon (a-Si) can be reduced from 600°C to 150°C with the help of a metal, such as Al.  Research on aluminum induced crystallization (AIC) has been ongoing at the Arkansas Advanced Photovoltaic Lab of University of Arkansas for several years.  X-ray diffraction has confirmed crystallization of the a-Si film by showing the Si [111] peak at a 2Θ angle of 28.5°.  Scanning electron microscopy (SEM) with Energy Dispersive X-ray (EDX) mapping has shown changes in surface morphology and a complex mix of Si and Al near the surface before and after annealing and at different stages of crystallization.  To date however, the mechanism of AIC is not clearly understood yet and several competing mechanisms have been proposed.  Some reports indicate that Al diffuses through a-Si inducing crystallization, while other work suggests Si diffusion through Al takes place before crystallization. Most likely both mechanisms are at work but one dominates over the other at different temperature regimes. 




Accomplishments and Progress:


The goal of this Materials Analysis Center (MAUC) User Project was to use Auger depth profiles to measure composition as a function of depth over a much finer scale than previously done with EDX.   Early efforts within this Project showed that Auger depth profiling, while providing a good measure of the composition, it was difficult to reproduce the data from different areas of the same sample.  The complexity of these systems was revealed by interrupting the depth profiles and mapping the lateral distribution of Si, Al, and O and showed that the distribution was more complex than anticipated.  A second set of samples, prepared to bracket the AIC anneal time for a given annealing temperature, was recently analyzed by Auger microanalysis to further explore the nature of the Al/Si intermixing.  All of the samples for this group were prepared by depositing a 300 nm Al layer onto the Si wafer, followed by a 300 nm layer of a-Si.  The samples were annealed at 525°C for times ranging from 0 min to 120 min.   Auger elemental maps of sample cross-sections 0 min of annealing clearly showed the individual layers, with a-Si on top.  The figure below shows a series of secondary electron images for 5 min, 10 min, 15 min, and 30 min annealed samples.  These images clearly show changes occurring on the surface after 10 min of annealing, as evidenced by the bright patches that are emerging.  The area of the bright patches increases for the 15 min sample and dominate on the 30 min sample.  Two points were selected for each of these samples and atomic concentration as a function of depth was determined by Ar-ion depth profiling.  The profiles on the left were for points labeled 1, and correspond to bright patches for the 10, 15, and 30 min samples.  Conversely, points labeled 2 are shown on the right-hand side and were selected within darker areas.  It is clear that the bright patches are becoming Al-rich near the surface as the AIC is proceeding.  Likewise, the darker areas remained relatively unchanged in Al/Si depth distribution until 30 min of annealing has occurred.  These data are being used at the University of Arkansas to verify model calculation for describing the mechanism al aluminum induced crystallization of amorphous Si.

MAUC collaborative research by H. M. Meyer (ORNL) with K. Sharif and Prof. H.A. Naseem (University of Arkansas).


    Click on images to enlarge picture [pdf]

Other Examples from the Scanning Auger Nanoprobe




 Oak Ridge National Laboratory