An apparatus is usually a physical device or machine. In engineering or science papers, there is usually a section called the "literature review" that summarizes the previous research on the topic. In this context, "literature" means writings in general, and not novels, plays, and so on. So, I would call it the "relevant literature" or "the texts/source materials that were used/examined/analyzed in this research," unless you need to be more specific.

An apparatus in a scientific paper is more specifically some type of set up used to conduct the actual experiment. Such as a titration apparatus in a chemistry experiment. It is most likely something a student must set up to conduct the experiment. Materials is more general and includes the apparatus and all other materials used for conducting the experiment.