Narendra,
This can be a difficult structure to model as the mode solver works on the whole structure and you are correct most of your dominant modes are going to be found in the Si layer. A way of working around this would be to use the FEM mode solver as it allows you to set a “Mode Index Estimate”. As your layer of interest is 2.25, you would expect the fundamental mode to be slightly below this. You can set that to 2.25 and request a large number of modes (30ish) and sort through them to find the one you are interested in.
Based on a quick test it looks like your fundamental mode is at 2.2405825. Please note that if you are injecting this you must have you number of modes and settings selected such that the mode you want injected is the last one in the list.
Scott