From my experience index files wether it be .html .htm .php or what have you, are requried by default and the server will look for these files in a certain order from a list of about 26 options.
Now i have seen empty index.php files in dir before and i am not sure if that is required to do this next step or not.
But you can specify in your .htaccess file the file name you want to use as your top tier file instead of an index.
some thing like this in your .htaccess
Which the server will now look for that file name and will longer look for any other name as the index.
This was my source as i have not done this before so if you want to refer to this here ya go..