I want to clear my doubt. I am helping students in University to make research subjects, which is the part of their study. They'll pay for it. Is it Haram? Is it a type of cheating?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If what you are doing does not exceed helping these students understand the subject matter, or if it is only guiding them to the appropriate references showing them the errors and how to correct them and similar forms of help that the students need, then there is no harm in your getting some financial compensation for that help.
But if you do the research instead of them and let them record it in their names as if they did the work, then this is cheating and it is unlawful. In fact, it has disastrous consequences not only on the student himself but also on the society as a whole. You know that this student might get a degree - he does not deserve - and that entitles him to hold a position he cannot handle properly, hence causing injustice to the community he was supposed to serve according to skills which he never acquired or acquired poorly. Muslim reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever cheats is not one of us" .
Allah knows best.