Graphene is an attractive material for broadband photodetection but suffers from weak light absorption. Coating graphene with quantum dots can significantly enhance light absorption and create extraordinarily high photo gain. This high gain is often explained by the classical gain theory which is unfortunately an implicit function and may even be questionable. In this work, we managed to derive explicit gain equations for hybrid graphene-quantum-dot photodetectors. Due to the work function mismatch, lead sulfide (PbS) quantum dots coated on graphene will form a surface depletion region near the interface of quantum dots and graphene. Light illumination narrows down the surface depletion region, creating a photovoltage that gates the graphene. As a result, high photo gain in graphene is observed. The explicit gain equations are derived from the theoretical gate transfer characteristics of graphene and the correlation of the photovoltage with the light illumination intensity. The derived explicit gain equations fit well with the experimental data, from which physical parameters are extracted.Graphene is a zero-bandgap semimetal with extraordinarily high carrier mobility, 1, 2, 3, 4, 5 as a result of which graphene is an attractive material for broadband photodetection. Photodetectors based on graphene operating in the mid-infrared spectrum have been demonstrated in recent years. 6,7,8,9 However, due to its nature of being atomically thin, graphene suffers from weak light absorption, resulting in poor photoresponsivity. 10,11,12,13 Coating graphene with semiconducting quantum dots (QDs) can strongly enhance the light absorption and introduce an interesting high photo gain at an order of 10 8 , 14, 15, 16 several orders of magnitude larger than photodetectors based on pure semiconducting QDs (often have a photo gain of 10 2 -10 3 ). 17,18,19 The classical carrier-recycling gain mechanism is often used to explain the origin of high gain, 14,15,16 that is, the high gain originates from the photoexcited carriers circulating the circuits many times before recombination due to the long response time and short transit time. 20 However, this classical gain theory is an implicit function and may even be questionable. 21 It is implicit in that it is a function of carrier lifetime and transit time and cannot quantitively fit the light-intensitydependent photo gains. More importantly, the classical gain theory was derived on two questionable assumptions. 21,22 Firstly, the classical theory assumes no metal-semiconductor boundary confinement, which leads to the questionable conclusion that high gain can be obtained as long as the minority recombination lifetime is much longer than the transit time. After the metal-semiconductor boundary confinement is

Ultrashallow doping is required for both classical field-effect transistors in integrated circuits and revolutionary quantum devices in quantum computing. In this review, we give a brief overview on recent research advances in three technologies to form ultrashallow doping, namely molecular monolayer doping, molecular beam epitaxy, and low energy ion implantation. A research perspective will be provided at the end of this review.

Developing a scalable method to fabricate atomic wires is an important step for building solid-state semiconductor quantum computers. In this work, we developed a selective doping strategy by patterning the selfassembled monolayer to a few nanometers using standard nanofabrication processes, which significantly improves the lateral doping resolution of monolayer doping from microscale to nanoscale. Using this method, we further explore the possibility to fabricate phosphorus wires in silicon by patterning self-assembled diethyl vinylphosphonate monolayers into lines with a width ranging from 500 to 10 nm. The phosphorus dopants are driven into silicon by rapid thermal annealing, forming dopant wires. Four-probe and Hall effect measurements are employed to characterize the dopant wires. The results show that the conductance is linear with the width for the wires, showing the success of the monolayer patterning process to nanoscale. To fabricate atomic wires made of one or a few lines of phosphorus atoms, we need to significantly shorten the thermal diffusion length and increase the dopant incorporation rate at the same time. Pulsed laser annealing may be a promising solution. The present work provides a promising pathway for mass fabrication of atomic wires in silicon that may find important applications in quantum computing.

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Fabrication of atomic dopant wires at large scale is challenging. We explored the feasibility to fabricate atomic dopant wires by nano-patterning self-assembled dopant carrying molecular monolayers via a resist-free lithographic approach. The resist-free lithography is to use electron beam exposure to decompose hydrocarbon contaminants in vacuum chamber into amorphous carbon that serves as an etching mask for nanopatterning the phosphorus-bearing monolayers. Dopant wires were fabricated in silicon by patterning diethyl vinylphosphonate monolayers into lines with a width ranging from 1 μm down to 8 nm. The dopants were subsequently driven into silicon to form dopant wires by rapid thermal annealing. Electrical measurements show a linear correlation between wire width and conductance, indicating the success of the monolayer patterning process at nanoscale. The dopant wires can be potentially scaled down to atomic scale if the dopant thermal diffusion can be mitigated.