AVS 53rd International Symposium
    Thin Film Friday Sessions
       Session TF+EM-FrM

Paper TF+EM-FrM9
In Situ Defect Spectroscopy: Probing Dangling Bonds during a-Si:H Film Growth by Subgap Absorption

Friday, November 17, 2006, 10:40 am, Room 2022

Session: In-Situ/Ex-Situ & Real-Time Monitoring and Characterization
Presenter: M.C.M. Van De Sanden, Eindhoven University of Technology, The Netherlands
Authors: I.M.P. Aarts, Eindhoven University of Technology, The Netherlands
A.C.R. Pipino, Eindhoven University of Technology, The Netherlands
M.C.M. Van De Sanden, Eindhoven University of Technology, The Netherlands
W.M.M. Kessels, Eindhoven University of Technology, The Netherlands
Correspondent: Click to Email
Detecting ultralow defect concentrations in a-Si:H thin films is of great importance for improving the efficiency of a-Si:H-based solar cells. Likewise, from a more fundamental point of view, it is essential to obtain understanding of the role of surface defect states during the growth process. Yet, a paucity of experimental techniques capable of detecting these surface and or bulk defect states is readily available. Using the technique of evanescent-wave cavity ringdown spectroscopy, we have realized a unique and absolute absorption technique capable of detecting defect states such as dangling bonds with an unprecedented sensitivity that can be applied in situ and during film growth as we will demonstrate for hotwire chemical vapor deposited a-Si:H. We deposited a thin a-Si:H film (from 0 up to 800 nm thickness), onto a total-internal reflection surface of a ultralow-loss monolithic folded optical resonator. Subgap absorption spectra between wavelengths of 1170 and 1245 nm are obtained and show the typical broad absorption feature due to dangling bonds defect states present in the bulk and at the interfaces. The minimal detectable absorption of the technique is 3 10@super -8@ optical loss, which is equivalent to 3 10@super 8@ dangling bonds/cm@super 2@. Furthermore, from the real time experiments the defect distribution in the film could be established and showed that the highest defect concentration was present at the interface of the a-Si:H with the substrate while the surface defect density was approximately ten times smaller. Moreover, changes in surface dangling bond concentration (formation- and decay-curves) could be monitored in real-time under various growth conditions.