Yes,after a pH.D one can join ISRO and become a research scientist. If you visit the official website of ISRO you will find a "live register" button. PG and pH.D holders with the relevant specialisations such as aeronautics, agriculture, atmospheric science, engineering branches such as civil,electrical,remote sensing,computational fluid mechanics,space science, electrical,electro-optics etc,as well as some other specialisations such as geology, oceanography, metallurgy can apply for ISRO to work as a scientist
HOW TO APPLY.
if lyou scroll towards the end of the official government website of ISRO,various options such as site map,time line,FAQ.....etc will be displayed,in the same line,there's the option called "career",click on that and then another window opens,scroll to the bottom of the page and click on the live register button.
Please feel free to drop any further query.
As a doctor you can work in ISRO but not as research scientist.
You can work as AMO (Authorised Medical Officers.
You can also work as a Panel doctor on contract basis.
I hope this helps.