Two-dimensional　(2D)　hexagonal　boron　nitride　(h-BN)　is　a　promising　candidate　as　a　supporting　substrate,　a　gate　dielectric　and　a　protecting　layer　for　2D　electronic　and　photonic　devices.　Transition　metals　were　usually　adopted　as　substrates　for　the　synthesis　of　2D　h-BN,　however,　catalyst-free　growth　of　high-quality　h-BN　on　dielectric　substrates　is　still　very　challenging.　Herein,　we　report　the　catalyst-free　growth　of　2D　h-BN　few-layers　on　sapphire　substrates　by　ion　beam　sputtering　deposition　(IBSD).　We　find　that　the　h-BN　grown　under　conventional　conditions　is　nonstoichiometric　with　an　excess　of　B　element,　resulting　in　a　large　number　of　N　vacancy　defects　and　poor　crystalline　quality.　The　wafer-scale　high　quality　2D　h-BN　layers　were　synthesized　on　sapphire　by　the　combination　of　surface　nitridation　and　N+　sputtering.　Furthermore,　the　2D　h-BN　few-layers　on　sapphire　were　used　to　fabricate　deep　ultraviolet　photodetectors,　which　exhibit　better　performance　in　comparison　with　the　devices　fabricated　by　the　transferred　h-BN.