A single-photon avalanche diode with high sensitivity in the ultraviolet (UV) wavelength range has been fabricated on Si using pure boron chemical vapor deposition to create both a nanometer-thin anode junction and a robust light entrance window. The device shows high sensitivity at the wavelengths of 330-370 nm when operated in the Geiger mode and good selectivity for UV light without applying a capping filter. The dark count rates can be as low as 5 Hz at room temperature for an active area of 7 mu m(2). An implicit guard ring, using an n-enhancement implantation in the central region of the diode, is applied instead of peripheral diffused p-type guard rings to achieve a high fill factor.