Single-photon avalanche diodes (SPADs) fabricated in conventional CMOS processes typically have limited near infra-red (NIR) sensitivity. This is the consequence of isolating the SPADs in a lowly-doped deep N-type well. In this work, we present a second improved version of the “current-assisted” single-photon avalanche diode, fabricated in a conventional 350 nm CMOS process, having good NIR sensitivity owing to 14 μm thick epilayer for photon absorption. The presented device has a photon absorption area of 30 × 30 µm2, with a much smaller central active area for avalanche multiplication. The photo-electrons generated in the absorption area are guided swiftly towards the central area with a drift field created by the “current-assistance” principle. The central active avalanche area has a cylindrical p-n junction as opposed to the square geometry from the previous iteration. The presented device shows improved performance in all aspects, most notably in photon detection probability. The p-n junction capacitance is estimated to be ~1 fF and on-chip passive quenching with source followers is employed to conserve the small capacitance for bringing monitoring signals off-chip. Device physics simulations are presented along with measured dark count rate (DCR), timing jitter, after-pulsing probability (APP) and photon detection probability (PDP). The presented device has a peak PDP of 22.2% at a wavelength of 600 nm and a timing jitter of 220 ps at a wavelength of 750 nm.
A current assisted avalanche photodetector (CAAPD) is presented with a large detection window of 40 × 40 μm2, having a small 1-fF avalanche diode in its center. To quickly guide the photogenerated electrons to the center for avalanche multiplication, a drift field with associated majority hole current is applied across the neutral detection volume. This first type of CAAPD is fabricated in a conventional 350-nm CMOS process on a high resistive p− epi-layer. The low diode-junction capacitance can be of interest to integrated receivers. The CAAPD is characterized for its basic functionality, including the effects of lateral detection delay and gain.
A current-assisted single-photon avalanche diode (CASPAD) is presented with a large and deep absorption volume combined with a small p-n junction in its middle to perform avalanche trigger detection. The absorption volume has a drift field that serves as a guiding mechanism to the photo-generated minority carriers by directing them toward the avalanche breakdown region of the p-n junction. This drift field is created by a majority current distribution in the thick (highly-resistive) epi-layer that is present because of an applied voltage bias between the p-anode of the avalanching region and the perimeter of the detector. A first CASPAD device fabricated in 350-nm CMOS shows functional operation for NIR (785-nm) photons; absorbed in a volume of 40 × 40 × 14 μm3. The CASPAD is characterized for its photon-detection probability (PDP), timing jitter, dark-count rate (DCR), and after pulsing.
We study the light matter interaction in WS2 nanotube-graphene hybrid devices. Using scanning photocurrent microscopy we find that by engineering graphene electrodes for WS2 nanotubes we can improve the collection of photogenerated carriers. We observe inhomogeneous spatial photocurrent response with an external quantum efficiency of ∼1% at 0 V bias. We show that defects play an important role and can be utilized to enhance and tune photocarrier generation.Heterostructure devices of transition metal dichalcogenides (TMDCs) and graphene have generated considerable research interest recently because of their superior optical and electronic properties.[1, 2] The semiconducting nature of TMDCs combined with the presence of van Hove singularities in their electronic density of states allows for efficient photon absorption and carrier generation under optical excitation.[3] Combining this feature with the high mobility of graphene has led to optoelectronic studies of heterostructure devices comprising graphene and single layer TMDCs. [1,[3][4][5][6][7] These devices have exhibited good quantum efficiency for photocurrent generation in the visible range. However, the fabrication of such heterostructures requires multiple exfoliation and transfer steps. TMDC nanotubes[8] represent another alternative for such applications; nanowires offer an additional advantage because they can enhance the absorption of light through the formation of optical cavities[9, 10] and quasi 1D structures are known to enhance light matter interaction by virtue of an enhanced joint density of states (JDOS).[11] Silicon and carbon nanotubes have been shown to be promising materials for solar-cell applications. [12,13] Similarly, TMDC nanotubes could also allow for large scale integration of on-chip optoelectronic elements. In addition, the curvature of the nanotubes can be used to engineer spin and valley based optoelectronic control in dichalcogenide systems.[14] Here, we investigate the photoresponse of WS 2 nanotubes with field-effect transistor geometry and the enhanced photoresponse properties of hybrid devices of WS 2 nanotubes with graphene electrodes. One of the motivations for using graphene electrodes for the nanotube is to modulate the density of carriers in the electrodes and modify the Schottky barrier;[15] the other motivation is to observe the spatial homogeneity of the photoresponse. We investigate the efficiency of these devices for photoconversion and attempt to understand the role of defects in modifying optoelectronic properties. [16] Prior to studying the hybrid devices, we probe individual WS 2 nanotubes and show that they offer a good optoelectronic platform. [17,18] We used WS 2 multiwalled * deshmukh@tifr.res.in
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