i am also a recent joinee in Ph.d, my major is into psychometric; i am sure the scope would be there in future for PH.d s in HR, simultaneously you should also have an experience of min 8 -10 years while completing of your studies. if you strongly have a passion and enthusiastic in HR, please go for it also i can advice you choose the best university or college for this unless your research will not have better scope.
all the best
Scope in any field depends upon what knowledge you have in the field and not on what qualification you possess.
Ph.D. will certainly enhance your value but how to encash it, is totally a skilled of individual.
These are my personal views.