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 µm 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.Index Terms-Avalanche breakdown, boron, chemical vapor deposition (CVD), Geiger-mode avalanche photodiode, singlephoton avalanche diode (SPAD), ultrashallow junctions, ultraviolet (UV).

A CMOS compatible Ge avalanche photodiode is fabricated on Si by using a selective chemical-vapor deposition (CVD) epitaxial growth technique. At a temperature of 700˚C, single crystal islands of As-doped Ge are grown in windows to the Si with sizes up to hundreds of μm 2 , to a layer thickness of approximately 1 µm, This is followed by a pure Ga and then a pure B (PureGaB) deposition for the creation of an ultrashallow p + -region that can be metallized by Al. The resulting p + n diodes have exceptionally good I-V characteristics with ideality factors of ~ 1.1 and reliably low saturation currents. The measurements show high sensitivity of the photodiodes to IR wavelengths above 1 µm in avalanche and Geiger modes.

Pure gallium and pure boron (PureGaB) Ge-on-Si photodiodes were fabricated in a CMOS compatible process and operated in linear and avalanche mode. Three different pixel geometries with very different area-to-perimeter ratios were investigated in linear arrays of 300 pixels with each a size of 26 × 26 µm 2. The processing of anode contacts at the anode perimeters leaving oxide covered PureGaB-only light-entrance windows, created perimeter defects that increased the vertical Ge volume but did not deteriorate the diode ideality. The dark current at 1 V reverse bias was below 35 µA/cm 2 at room temperature and below the measurement limit of 2.5 × 10 −2 µA/cm 2 at 77 K. Spread in dark current levels and optical gain, that reached the range of 10 6 at 77 K, was lowest for the devices with largest perimeter. All device types were reliably operational in a wide temperature range from 77 K to room temperature. The spectral sensitivity of the detectors extended from visible to the telecom band with responsivities of 0.15 and 0.135 A/W at 850 and 940 nm, respectively. Index Terms-Avalanche photodiode (APD), Ge-on-Si, nearinfrared photodiode, pure gallium and pure boron (PureGaB). I. INTRODUCTION A VALANCHE photodiodes (APDs) and single-photon avalanche diodes (SPADs), or Geiger-mode APDs, are widely used in optical telecommunications, imaging, and medical diagnostics, where high sensitivity to light in the visible or near-infrared (NIR) ranges is needed [1]. Silicon photomultipliers (SiPMs) and imaging arrays of APDs and SPADs have seen a fast growth in the last decade, and are used today in a variety of applications, including time-offlight imaging, positron emission tomography, fluorescence microscopy, and Raman spectroscopy, to name a few [2], [3]. Nevertheless, photomultiplier tubes, the predecessors Manuscript

Abstract-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 µm 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.Index Terms-Avalanche breakdown, boron, chemical vapor deposition (CVD), Geiger-mode avalanche photodiode, singlephoton avalanche diode (SPAD), ultrashallow junctions, ultraviolet (UV).

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