Photodetectors convert light into electrical signals and are at the heart of any optical link. In silicon photonics, traditionally germanium (Ge) [1,2] or III-V compound semiconductors [3,4] are used for photodetection and both technologies have reached a high level of maturity. Nevertheless, the direct monolithic integration of III-V photodetectors on Si wafers remains a challenge because of the large lattice constant mismatch and different thermal coefficients. Ge can be directly grown on crystalline Si, but pushing the bandwidth of Ge photodetectors to higher and higher frequencies [5,6,7] becomes increasingly difficult because of the material's poor electrical quality. One of the most promising routes to a new era of chip performance is the monolithic 3D integration of electronic and photonic components on the same chip, where a promising material for the photonic layers is SiN [8]. On these amorphous SiN layers, crystalline Ge (the prerequisite for realizing high

Heterostructures based on atomically thin semiconductors are considered a promising emerging technology for the realization of ultrathin and ultralight photovoltaic solar cells on flexible substrates. Much progress has been made in recent years on a technological level, but a clear picture of the physical processes that govern the photovoltaic response remains elusive. Here, we present a device model that is able to fully reproduce the current-voltage characteristics of type-II van der Waals heterojunctions under optical illumination, including some peculiar behaviors such as exceedingly high ideality factors or bias-dependent photocurrents. While we find the spatial charge transfer across the junction to be very efficient, we also find a considerable accumulation of photogenerated carriers in the active device region due to poor electrical transport properties, giving rise to significant carrier recombination losses. Our results are important to optimize future device architectures and increase power conversion efficiencies of atomically thin solar cells.

Layered transition metal dichalcogenide semiconductors, such as MoS and WSe , exhibit a range of fascinating properties and are being currently explored for a variety of electronic and optoelectronic devices. These properties include a low thermal conductivity and a large Seebeck coefficient, which make them promising for thermoelectric applications. Moreover, transition metal dichalcogenides undergo an indirect-to-direct bandgap transition when thinned down in thickness, leading to strong excitonic photo- and electroluminescence in monolayers. Here, it is demonstrated that a MoS monolayer sheet, freely suspended in vacuum over a distance of 150 nm, emits visible light as a result of Joule heating. Due to the poor transfer of heat to the contact electrodes, as well as the suppressed heat dissipation through the underlying substrate, the electron temperature can reach ≈1500-1600 K. The resulting narrow-band light emission from thermally populated exciton states is spatially located to an only ≈50 nm wide region in the center of the device and goes along with a negative differential electrical conductance of the channel.

We study electric field modulation of the thermovoltage in single-layer MoS2. The Seebeck coefficient generally increases for a diminishing free carrier concentration, and in the case of single-layer MoS2 reaches considerable large values of about S = −5160 μV/K at a resistivity of 490 Ω m. Further, we observe time dependent degradation of the conductivity in single layer MoS2, resulting in variations of the Seebeck coefficient. The degradation is attributable to adsorbates from ambient air, acting as p-dopants and additional Coulomb potentials, resulting in carrier scattering increase, and thus decrease of the electron mobility. The corresponding power factors remain at moderate levels, due to the low conductivity of single layer MoS2. However, as single-layer MoS2 has a short intrinsic phonon mean free path, resulting in low thermal conductivity, MoS2 holds great promise as high-performance 2D thermoelectric material.

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