Wide-band-gap　layered　semiconductor　hexagonal　boron　nitride　(h-BN)　is　attracting　intense　interest　due　to　its　unique　optoelectronic　properties　and　versatile　applications　in　deep　ultraviolet　optoelectronic　and　two-dimensional　electronic　devices.　However,　it　is　still　a　great　challenge　to　directly　grow　high-quality　hBN　on　dielectric　substrates,　and　an　extremely　high　substrate　temperature　or　annealing　is　usually　required.　In　this　work,　high-quality　few-layer　h-BN　is　directly　grown　on　sapphire　substrates　via　ion　beam　sputtering　deposition　at　a　relatively　low　temperature　of　700　degrees　C　by　introducing　NH3　into　the　growth　chamber.　Such　low　growth　temperature　is　attributed　to　the　presence　of　abundant　active　N　species,　originating　from　the　decomposition　of　NH3　under　ion　beam　irradiation.　To　further　tailor　the　properties　of　h-BN,　carbon　was　introduced　into　the　h-BN　layer　by　simultaneously　introducing　CH4　and　NH3　during　the　growth　process,　indicating　the　wide　applicability　of　this　approach.　Moreover,　a　deep　ultraviolet　(DUV)　photodetector　is　also　fabricated　from　a　C-doped　h-BN　layer　and　exhibits　superior　performance　compared　with　an　intrinsic　h-BN　device.