Yes, one can definitely join PSU's after M.Tech, but one must keep in mind some factors before opting for PSU's after M.Tech:

1. One should be in age limit provided by the GATE and PSU's organisation, exceeding that won't give you any opportunity
2. If you wish join PSU, then you are given chance to do that just after your under-graduation through scoring well in GATE
3. There are very less opportunities in the Computer Science-Information technology field if considering for PSU's jobs.
4. After M.Tech from IIT's there are various new opportunities gates opened for you, with higher pay scale and work related to your field.
5. To join PSU after M.Tech you will have to appear for GATE and prepare for that

Thank you