This kind of mixed methods is sometimes referred to an "embedded" design because the qualitative portion of the project takes place within the context of the larger intervention study.

Vicky Plano-Clark has written about this design, so you could look her articles, as well as the section on embedded designs in the textbook on mixed methods by Creswell and Plano-Clark.

Thanks a lot @David for the suggestion. This is helpful, my first time to work on quali within clinical trial.