Amir Sammak

Robust UV/VUV/EUV PureB Photodiode Detector Technology With High CMOS Compatibility

This paper gives an assessment of old and new data relevant to the optical and electrical performance of PureB photodiodes for application in the wavelength range 2 nm to 400 nm. The PureB layer, fabricated by depositing pure boron on Si, forms the anode region of devices that function as p+n junction diodes. The results show that the high sensitivity and high stability of the PureB diodes is related to the integrity of the interface with the Si. When measures are taken to retain a complete PureB coverage, thermal processing steps with minute long exposure to temperatures up to 900 °C do not compromise the robustness and a lower-than-ideal but still high responsivity is maintained. Besides the thermal processing considerations, other aspects that impact the integration of PureB in CMOS are reviewed.

VUV/Low-Energy Electron Si Photodiodes With Postmetal 400

Pure boron (PureB) chemical-vapor deposition performed at 400°C is applied as a postmetalization process module to fabricate near-ideal p+n photodiodes with nm-thin PureB-only beam-entrance windows. The photodiodes have near-theoretical sensitivity and high stability for optical characterization performed with either UV light down to a wavelength of 270 nm or low-energy electrons down to 200 eV.

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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 µm2, 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 1µm in avalanche and Geiger modes.

PureB Deposition

A commercial Chemical Vapor Deposition (CVD) system, the ASMI Epsilon 2000 designed for Si and SiGe epitaxy, has, for the first time, been equipped for the growth of GaAs compounds in a manner that does not exclude the use of the system also for Si-based depositions. With the new system, intrinsic, Si-doped and Ge-doped GaAs epitaxial layers with excellent quality have been grown on GaAs substrate wafers by the decomposition of trimethylgallium (TMGa) and AsH3 in the reactor at reduced pressure and at temperatures in the 600-700°C range. A low AsH3 concentration, 0.7 % in H2, is used as one of the precursors, which has the added advantage that the severe safety precautions always associated with MOCVD systems need not be implemented.

A CMOS compatible Ge-on-Si APD operating in proportional and Geiger modes at infrared wavelengths

The Ge APD detectors are fabricated on Si by using a selective chemical-vapor deposition (CVD) epitaxial growth technique. A novel processing procedure was developed for the p+ Ge surface doping by a sequence of pure-Ga and pure-B depositions (PureGaB). Then, PVD Al is used to contact the n-type Si and the anode of p+n Ge diode. Arrays of diodes with different areas, as large as 40×40 μm2, were fabricated. The resulting p+n diodes have exceptionally good IV characteristics with ideality factor of ~1.1 and low saturation currents. The devices can be fabricated with a range of breakdown voltages from a minimum of 9 V to a maximum of 13 V. They can be operated both in proportional and in Geiger mode, and exhibit relatively low dark counts, as low as 10 kHz at 1 V excess reverse bias. The dark current at 1 V reverse bias are as low as 2 pA and 20 pA for a 2×2 μm2 and 2×20 μm2 devices, respectively. Higher IR-induced current than that induced by visible light confirms the sensitivity of Ge photodiodes at room temperature. The 25% peak in Id/Iref at an IR-wavelength of 1100 nm in Geiger mode is measured for excess bias voltages of 3 V and 4 V, where Id refers to the photocurrent of the 2×20 μm2 device at different wavelengths, and Iref is the reference photodiode current. The timing response (Jitter) for the APD when exposed to a pulsed laser at 637 nm and 1 V excess bias is measured as 900 ps at full width of half maximum (FWHM).

Merging Standard CVD Techniques for GaAs and Si Epitaxial Growth

Chemical Vapor Deposition (CVD) of Ga on Si is performed in a commercial Si/SiGe epitaxial reactor at temperatures from 400 – 650 °C and conditions for which the Ga deposits selectivity on Si and the reactivity of the Ga with Si is so low that a thin-film deposition is achieved. Contact windows to c-Si are covered with a thin layer of Ga and contacted by sputtering Al/Si(1%). On the basis of an extensive electrical I–V characterization, it is concluded that the Ga gives a substantial doping of the Si at a temperature of 400°C and the resulting p-n junctions are near-ideal.

A Ge-on-Si single-photon avalanche diode operating in Geiger mode at infrared wavelengths

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 μm2. 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/cm2 at room temperature and below the measurement limit of 2.5 × 10-2 μA/cm2 at 77 K. Spread in dark current levels and optical gain, that reached the range of 106 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.

Chemical Vapor Deposition of Ga dopants for fabricating ultrashallow p-n junctions at 400°C

A CMOS compatible Ge photodetector (Ge-PD) fabricated on Si substrates has been shown to be suitable for near infrared (NIR) sensing; linear and avalanche detection, in both proportional and Geiger modes have been demonstrated, for photon counting at room temperature [1]. This paper focuses on implementations of the technology for the fabrication of imaging arrays of such detectors with high reproducibility and yield. The process involves selective chemical vapor deposition (CVD) of a ~ 1-μm-thick n-type Ge crystal on a Si substrate at 700°C, followed by deposition of a nm-thin Ga and B layer-stack (so-called PureGaB), all in the same deposition cycle. The PureGaB layer fulfills two functions; firstly, the Ga forms an ultrashallow p+n junction on the surface of Ge islands that allows highly sensitive NIR photodiode detection in the Ge itself; secondly, the B-layer forms a barrier that protects the Ge/Ga layers against oxidation when exposed to air and against spiking during metallization. A design for patterning the surrounding oxide is developed to ensure a uniform selective growth of the Ge crystalline islands so that the wafer surface remains flat over the whole array and any Ge nucleation on SiO2 surface is avoided. This design can deliver pixel sizes up to 30×30 μm2 with a Ge fill factor of up to 95 %. An Al metallization is used to contact each of the photodiodes to metal pads located outside the array area. A new process module has been developed for removing the Al metal on the Ge-islands to create an oxide-covered PureGaB-only front-entrance window without damaging the ultrashallow junction; thus the sensitivity to front-side illumination is maximized, especially at short wavelengths. The electrical I-V characteristics of each photodetector pixel are, to our knowledge, the best reported in literature with ideality factors of ~1.05 with Ion/Ioff ratios of 108. The uniformity is good and the yield is close to 100% over the whole array.

CMOS-Compatible PureGaB Ge-on-Si APD Pixel Arrays

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.

A 270×1 Ge-on-Si photodetector array for sensitive infrared imaging

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).