The first relevant document I managed to discover on this is an undergraduate biology thesis titled The Real "Monster" in Frankenstein. Here is an extract from the abstract:
I argue that that the real “monster” of the story is in fact Victor
Frankenstein who is suffering from paranoid schizophrenia and that the “monster” is really just
a delusions that Victor uses to cope with the idea that he in fact is the killer of the story.
I wasn't sure if an undergraduate level paper was sufficient to answer the question but, fortunately, the author included two supportive references from more mainstream journals.
First is Probing the Psychological Mystery of Frankenstein from Approaches to Teaching Shelley's Frankenstein. This goes into detail as to how a teacher can guide a class into considering that the monster may be a manifestation of Frankenstein's schizophrenia. For example:
I call attention to the many times throughout the
novel that Victor falls asleep or loses consciousness and how he explicitly acknowledges his own culpability when he first sees the corpse of each of the "monster's" victims.
I point out a passage in which the Creature reminds Frankenstein of their physical kinship ("my form is a filthy type of yours, more horrid even from the very resemblance"
I call attention to the similarity of many of their utterances. For example, each in his own way cries to a mocking universe, "if ye really pity me, crush sensation and memory", just as each describes himself repeatedly as a "miserable wretch." Each, motivated by blind revenge, destroys himself as he seeks the annihilation of the other. One is the slave of the
other and at the same time master and executioner.
Second is The Myth of the Monster in Mary's Shelley's Murder Mystery from Journal of South Texas English Studies. This makes a similar argument more from a close reading of the text than a medical examination. For example:
Another interesting comment that throws into doubt the existence of this being is when
Frankenstein says, “Clerval, my friend and dearest companion, had fallen a victim to me and
the monster of my creation”. The monster of his creation can be seen as a metaphor for
his delusions rather than an actual creature.
The ambiguity in Shelley’s novel makes a single, definitive reading impossible. It is not clear
from the text whether a creature exists who stalks his creator, killing all of the people close
to him, or whether Frankenstein is actually a deranged killer with an alter ego who goes on a
killing spree. Either reading is just as plausible.
Of course, both these papers include their own references which may also be of help for further reading.