One point not covered by the other answers is Review articles which summarize the state of a particular research field.
As the OP is at an early stage of the process, these are especially useful for gving an overview of the most important work, and are usually written by an authority on the topic. They can be invaluable in preparing a reading list, as the references are the ones that are considered central to the development of the field, and will often be articles that you'll be expected to know.
Even when looking for inter-disciplinary stuff it is usually useful to begin with review articles from all the sub-fields that touch upon the research topic. The Annual Reviews series of journals are a good generic place to start, though you should focus on the leading journals of your field. The number of citations that can be checked on google scholar are also useful in establishing the important research concerns of your field to know before you start working on something brilliant that nobody will care about. For instance, there might be excellent reasons why nobody has done work usng your chosen approach, so getting the most respected/ cited papers will help you identify useful research topics, and also the ones to avoid.