"The Dark Side of Open Access in Google and Google Scholar: The Case of Latin-American Repositories"
Enrique Orduña-Malea et al. have self-archived "The Dark Side of Open Access in Google and Google Scholar: The Case of Latin-American Repositories."
Here's an excerpt:
The main objective of this study is to ascertain the presence and visibility of Latin American repositories in Google and Google Scholar through the application of page count and visibility indicators. For a sample of 137 repositories, the results indicate that the indexing ratio is low in Google, and virtually nonexistent in Google Scholar; they also indicate a complete lack of correspondence between the repository records and the data produced by these two search tools. These results are mainly attributable to limitations arising from the use of description schemas that are incompatible with Google Scholar (repository design) and the reliability of web indicators (search engines). We conclude that neither Google nor Google Scholar accurately represent the actual size of open access content published by Latin American repositories; this may indicate a non-indexed, hidden side to open access, which could be limiting the dissemination and consumption of open access scholarly literature.