Phase evolution in nanocrystalline silicon films: Hydrogen dilution and the cone kinetics model

Abstract

Our cone kinetics model of heterogeneous thin film growth explains the evolution of the crystalline inclusions that form during chemical vapor deposition (CVD) growth of silicon films. Various morphologies, including isolated crystallites and conical nanocrystalline formations, form during plasma-enhanced (PE) and other CVD techniques when isotropic growth is coupled with the point nucleation of a second phase with a higher growth rate. Cone angles are determined by the relative growth rates alone. By generalizing the physics of cone formation, we create a qualitative phase diagram that predicts film morphology from two factors during deposition: the relative amorphous and crystalline growth rates and the rate of crystallite nucleation. It is well known that both of these factors are influenced by the H-dilution of the film precursor gases. Analysis of the statistics of cone geometries and densities in published experimental data on PECVD-grown nanocrystalline silicon shows that the cone angle increases monotonically with H-dilution. In the context of the cone kinetics model, this implies that increased H-dilution results in an increase of the ratio of the nanocrystalline silicon growth rate (ν_nc) to the amorphous silicon growth rate (ν_a). At a specific H-dilution, ν_nc/ν_a exceeds unity and nanocrystalline cone formation results.

Keywords:

• a-Si:H
• CVD
• microcrystalline Si:H
• nanocrystals
• photovoltaics
• Si

Acknowledgements

We gratefully acknowledge support from the US DOE under Contract DE-AC36-08GO28308, and from a United Solar Ovonic subcontract from their DOE Solar America Initiative Contract DE-FC36-07GO17053. We also acknowledge helpful discussions with Ina Martin and Chun-Sheng Jiang (NREL), Baojie Yan (United Solar), and Antonin Fejfar (Czech Academy of Sciences). The pioneering works by the late Walter Spear on both doping and filament switching in amorphous silicon have been extremely valuable and a great influence on our own research over the years.

Notes

Note

1. Note in proof: The validity of Equation (1) has recently been confirmed by measurements of amorphous and coalesced-nanocrystalline silicon deposition rates at identical growth conditions, for which a cone angle was also measured. [see J.A. Stoke, L.R. Dahal, J. Li, N.J. Podraza, X. Cao, X. Deng, and R.W. Collins, Proc. 33rd IEEE Photovoltaics Specialists Conference, San Diego CA USA, May 11–16, 2008 (IEEE, Piscataway, NJ, 2008) Art. No. 413.]