Dust-Sized High-Power-Density Photovoltaic Cells on Si and SOI Substrates for Wafer-Level-Packaged Small Edge Computers.

Advancement in microelectronics technology enables autonomous edge computing platforms in the size of a dust mote (<1 mm), bringing efficient and low-cost artificial intelligence close to the end user and Internet-of-Things (IoT) applications. The key challenge for these compact high-performance edge computers is the integration of a power source that satisfies the high-power-density requirement and does not increase the complexity and cost of the packaging. Here, it is shown that dust-sized III-V photovoltaic (PV) cells grown on Si and silicon-on-insulator (SOI) substrates can be integrated using a wafer-level-packaging process and achieve higher power density than all prior micro-PVs on Si and SOI substrates. The high-throughput heterogeneous integration unlocks the potential of large-scale manufacturing of these integrated systems with low cost for IoT applications. The negative effect of crystallographic defects in the heteroepitaxial materials on PV performance diminishes at high power density. Simultaneous power delivery and data transmission to the dust mote with heteroepitaxially grown PV are also demonstrated using hand-held illumination sources.

Polarization-independent nonuniform grating couplers on silicon-on-insulator.

Grating couplers are proposed for polarization-independent coupling of light between a single-mode fiber and a 220-nm-thick channel waveguide on silicon-on-insulator. The grating couplers have nonuniform grating periods that are composed of the intersection or union of a set of two near-optimal TE- and TM-grating periods. The proposed grating couplers have a coupling efficiency greater than 20% and polarization dependent loss (PDL) lower than 0.5 dB within 3-dB bandwidth in design. For the evaluation of the design concept, a fabricated intersection grating coupler has the PDL of less than 0.8 dB within the wavelength range of 1540 to 1560 nm, and the coupling efficiency is ∼18%.

Efficient waveguide-integrated tunnel junction detectors at 1.6 mum.

Near-infrared detectors based on metal-insulator-metal tunnel junctions integrated with planarized silicon nanowire waveguides are presented, which we believe to be the first of their kind. The junction is coupled to the waveguide via a thin-film metal antenna feeding a plasmonic travelling wave structure that includes the tunnel junction. These devices are inherently broadband; the design presented here operates throughout the 1500-1700 nm region. Careful design of the antenna and travelling wave region substantially eliminates losses due to poor mode matching and RC rolloff, allowing efficient operation. The antennas are made from multilayer stacks of gold and nickel, and the active devices are Ni-NiO-Ni edge junctions. The waveguides are made via shallow trench isolation technology, resulting in a planar oxide surface with the waveguides buried a few nanometres beneath.The antennas are fabricated using directional deposition through a suspended Ge shadow mask, using a single level of electron-beam lithography. The waveguides are patterned with conventional 248-nm optical lithography and reactive-ion etching, then planarized using shallow-trench isolation technology. We also present measurements showing overall quantum efficiencies of 6% (responsivity 0.08 A/W at 1.605 mum), thus demonstrating that the previously very low overall quantum efficiencies reported for antenna-coupled tunnel junction devices are due to poor electromagnetic coupling and poor choices of antenna metal, not to any inherent limitations of the technology.

Ni-NiO-Ni tunnel junctions for terahertz and infrared detection.

We present complete experimental determinations of the tunnel barrier parameters (two barrier heights, junction area, dielectric constant, and extrinsic series resistance) as a function of temperature for submicrometer Ni-NiO-Ni thin-film tunnel junctions, showing that when the temperature-invariant parameters are forced to be consistent, good-quality fits are obtained between I-V curves and the Simmons equation for this very-low-barrier system (measured phi approximately 0.20 eV). A splitting of approximately 10 meV in the barrier heights due to the different processing histories of the upper and lower electrodes is clearly shown, with the upper interface having a lower barrier, consistent with the increased effect of the image potential at a sharper material interface. It is believed that this is the first barrier height measurement with sufficient resolution for this effect to be seen. A fabrication technique that produces high yields and consistent junction behavior is presented as well as the preliminary results of inelastic tunneling spectroscopy at 4 K that show a prominent peak at -59 meV, shifted slightly with respect to the expected transverse optic phonon excitation in bulk NiO but consistent with other surface-sensitive experiments. We discuss the implications of these results for the design of efficient detectors for terahertz and IR radiation.