You can study for a doctorate after graduate school. Postgraduate is a kind of academic qualification in higher education. After graduation, you can continue to study for a master's degree, and a master's degree can continue to study for a doctor.

Master graduates get master's degrees, doctoral graduates get doctor's degrees, and stay in the laboratory after postdoctoral graduation, so there is no degree. Postgraduates include master's education and doctoral education. After a master's degree, he is a master, and after a doctor's degree, he is a doctor.