Science Fair Project Encyclopedia
Protocrystalline or ProtocrystallinityDeposition phase diagrams developed by real time spectroscopic ellipsometry have led to the concept of the protocrystalline hydrogenated silicon (Si:H) growth regime, shaded in yellow in the figure below.
The key feature of the protocrystalline material is its relative stability to light induced degradation as observed in the electron-mobility lifetime product and similarly in the solar cell fill factor.
There are five important characteristics of the protocrystalline Si:H regime:
1. Phase EvolutionThe protocrystalline growth regime is one in which predominantly amorphous silicon (a) Si:H is deposited, but the film will ultimately evolve -- given sufficient thickness -- first to mixed-phase amorphous + nanocrystalline silicon (a+nc) Si:H and finally to a single-phase mc-Si:H. Once the a to (a+mc) transition is detected (which is most easily done using a real time probe such as spectroscopic ellipsometry or the more difficult atomic force microscopy and electrical measurements), the accumulating material is no longer considered protocrystalline.
2. Substrate DependenceThe phase of the growing material in the protocrystalline growth regime has an extreme substrate dependence. For example, the Si:H film growing in the protocrystalline regime on a freshly-deposited amorphous film substrate (such as R=0 a-Si:H in the figure above, where R is equal to the hydrogen to silane dilution ratio in plasma enhanced chemical vapor deposition), would lead to single-phase microcrystalline silicon growth on a freshly-deposited mc-Si:H substrate film even with the same deposition conditions. Under protocrystalline growth conditions local epitaxy is favored on a c-Si substrate. On the other hand, crystallite nucleation is suppressed on an amorphous substrate. For this reason, it is difficult if not impossible to grow protocrystalline Si:H i-layers directly on mc-Si:H p and n layers in p-i-n and n-i-p solar cells, respectively, without a conventional amorphous interlayer.
3. Increased Nuclei CoalescenceThe protocrystalline growth regime possesses an enhanced degree of nuclei coalescence which yields smoother surfaces than conventional a-Si:H films. Thus, the dielectric discontinuity between the ambient and bulk film is sharpest under the protocrystalline growth conditions. Such a characteristic can only be detected when using an oxide-covered c-Si substrate (which suppresses Si crystallite nucleation in the protocrystalline regime to a sufficient degree relative to clean c-Si or mc-Si:H in order to observe protocrystallinite state).
In addition to the unique evolutionary growth behavior (1-3) exhibited under the protocrystalline Si:H growth conditions, the protocrystalline material itself exhibits unique optoelectronic properties. These properties are often difficult to measure because one must use the appropriate substrate and thickness to ensure that the film is protocrystalline throughout and has not crossed the a to (a+nc) transition during the growth process. As a result, probes that must employ very thick films, such as small angle x-ray scattering cannot be applied to characterize the protocrystalline Si:H i-layers used in devices since these materials cross the a to (a+nc) after a thickness of ~7000 Å. ---
4. Unique Optical PropertiesProtocrystalline Si:H has unique optical properties. The width of the broad Lorentzian-shaped peak in the imaginary part of the dielectric function e2 is narrowest in protocrystalline Si:H than any other a-Si:H material or alloy we have measured. This suggests that the relaxation time of the excited electron and hole in the bands is longest. However, the Si bond packing density is not the highest, suggesting the presence of voids that apparently are not detrimental to the electronic properties. In addition, the optical gaps of protocrystalline Si:H are larger than conventional materials and increase with increasing H2-dilution ratio R.
5. Unique Electrical PropertiesProtocrystalline Si:H has unique electrical properties. The Urbach tail is narrower for protocrystalline Si:H, and the hole drift mobility is larger. The key feature of the protocrystalline material is its relative stability to light induced degradation as observed in the electron-mobility lifetime product and similarly in the solar cell fill factor. In fact, upon light soaking under AM1.5 at room temperature, these quantities tend to stabilize after 50-100 hrs, in contrast to conventional materials that continue to degrade for up to 1000hrs.
More InformationFor a detailed review article on protocrystalline growth see: R.W. Collins, A.S. Ferlauto, G.M. Ferreira, C. Chen, J. Koh, R.J. Koval, Y. Lee, J.M. Pearce, and C. R. Wronski, “Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry”, Solar Energy Materials and Solar Cells, 78(1-4), pp. 143-180, 2003.
--Jmpearce 23:14, Mar 6, 2005 (UTC)
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