K. R. C. Mok

UV-Sensitive Low Dark-Count PureB Single-Photon Avalanche Diode

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 μm2. 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.

Chemical vapor deposition of PureB layers for solar cell application

A damage-free Si doping method using chemical vapor deposition of pure boron (PureB) layers is investigated for p+ doping of solar cell emitters. After PureB deposition the layer is thermally diffused into the Si and evaluated by determining the dopant profile, sheet resistance and effective carrier lifetime. It was found that dopant profiles and sheet resistance could be controlled by optimizing the thickness of the initial PureB layer and the subsequent annealing conditions. Compared to a boron ion implanted sample of the same sheet resistance, samples fabricated with the PureB method showed higher effective lifetime of about 300 μs as compared to 75 μs. In addition to providing defect-free junctions, PureB deposits selectively on silicon and not on silicon oxide, and may therefore be attractive for novel cell architectures.

Low-pressure chemical vapor deposition of pureb layers on silicon for p + n junction formation

Ultrashallow junctions were fabricated by chemical vapor deposition of pure boron (PureB) at 95 torr as opposed to atmospheric pressure deposition. The low pressure deposition process with hydrogen as carrier gas demonstrated to be a viable process for fabricating ideal diodes with low saturation current and no detrimental effects. The minimum deposition temperature for forming an ideal diode is 500°C. In the 500°C to 700°C temperature regime, the saturation current density decreases with increasing deposition temperature. Diodes fabricated with PureB deposition at 400°C show a superposition of p+n and Al-to-Si Schottky-like behavior. Depositing PureB at 95 torr results in a thicker layer than at atmospheric pressure, implying that hydrogen desorption from silicon surface is limiting in the reaction mechanism of the PureB deposition.

Applications of PureB and PureGaB ultrashallow junction technologies

A review is given of present and potential applications of pure dopant deposition of boron and gallium integrated as the p+-region in p+n ultrashallow junctions. Pure B (PureB) layers have been applied in several large area Si diode applications where nm-shallow junctions are required: high-linearity, high-quality varactor diodes for RF adaptive circuits and photodiode detectors for low-penetration-depth beams such as extreme/ vacuum/deep-ultraviolet (EUV, VUV, DUV) light and low-energy electrons. The integration of these types of detectors in CMOS is discussed along with some points that may make the pure dopant depositions attractive for source/drain fabrication in advanced pMOS transistors. Pure Ga capped with pure B (PureGaB) layers have been demonstrated as the p+-region in p+n Ge-on-Si diodes that are sensitive to infrared wavelengths (> 1 μm) both in avalanche and Geiger mode.

Towards CMOS-compatible photon-counting imagers in the whole 10 nm – 1600 nm spectral range with PureB Si and PureGaB Ge-on-Si technology

The paper gives an overview of the implementation of PureB Si photodiodes in the spectral range 10 nm - 400 nm and PureGaB Ge-on-Si photodiodes in the near infrared (NIR) up to about 1.6 μm. Focus is put on the special properties of the technology in relationship to the integration in CMOS as single-photon avalanche diodes (SPADs).

Reverse biasing and breakdown behavior of PureB diodes

In this paper, the reverse biasing and breakdown properties of the PureB diodes are investigated for different methods of processing the PureB anode window and the metal contacting. In particular, micron-sized devices are examined in order to assess their suitability for use in dense imaging arrays that may require operation as avalanche photodiodes to obtain the necessary photosensitivity [6]. For such small devices implanted guard rings cannot be implemented without paying a penalty in fill-factor. At the same time it is also desirable to position the photosensitive area away from the oxide perimeter where permanent damage can be inflicted by high reverse currents. Therefore, a “virtual” guard, using an n-enhancement implantation in the central region of the diode is applied here.