What are band-edge exciton and indirect exciton ? what's the difference?

2017-12-03 10:47:09

I know that exciton is an electron-hole pair excited from the external photon.

but recently I saw the terms "band-edge exciton" "indirect exciton".

I am not very understanding them and I hope somebody could explain the property of the them, when we will get such exciton and the differences between them. It would be better if there are some references included.

Thanks very much