Wafer-Scale Synthesis and Optical Characterization of InP Nanowire Arrays for Solar Cells

Direct on-Chip Optical Communication between Nano Optoelectronic Devices

On-chip optical communication between individual nano optoelectronic components is important to reduce the footprint and improve energy efficiency of photonic neuromorphic solutions. Although nanoscale photon emitters and receivers have been reported separately, communication between them remains largely unexplored. We demonstrate direct on-chip directional broadcasting of light between individual InP nanowire photodiodes on silicon. The performance of multiple wire-to-wire communication circuits is mapped, demonstrating robust performance with up to 5 bit resolution as needed in biological networks and a minimum component driving power for continuous operation of 0.5 μW which is below that of conventional hardware. The results agree well with theoretical modeling that allows us to understand network performance limits and identify where significant improvements could be achieved. We estimate that an energy per operation of ∼1 fJ and signal fan-out from one emitter to hundreds of other nodes is possible. We find that the nanowire circuit performance parameters can satisfy the quantitative requirements to run the tasks of neural nodes in a bioderived neural network for autonomous navigation.

Composition, Optical Resonances, and Doping of InP/InGaP Nanowires for Tandem Solar Cells: a Micro-Raman Analysis

We present a micro-Raman study of InP/InGaP tandem junction photovoltaic nanowires. These nanowires render possible InGaP compositions that cannot be made in thin films due to strain. The micro-Raman spectra acquired along the nanowires reveal the existence of compositional changes in the InGaP alloy associated with the doping sequence. The heavily Zn-doped InxGa1-xP (x is the In molar fraction) side of the tunnel diode is Ga rich, x = 0.25, with respect to the n-type and intrinsic segments of the top cell, which are close to the nominal composition of the NWs (x = 0.35). The p-type end segment is still Ga-rich. Electromagnetic resonances are observed in the tunnel diode. The Raman signal arising from the InGaP side of the tunnel diode is significantly enhanced. This enhancement permits the observation of a Raman mode that can be associated with an LO phonon plasmon coupled mode (LOPCM). This mode has not been previously reported in the literature of InGaP, and it permits the Raman characterization of the tunnel diode. The analysis of this mode and its relation to the LO phonon modes of the alloy, InP-like and GaP-like, allows to establish an apparent one-mode behavior for the phonon plasmon coupling. It indicates that hole plasma couples to the GaP-like LO mode. The LOPCMs are modeled using the Lindhard Mermin formalism for the dielectric function.

Vertically Processed GaInP/InP Tandem-Junction Nanowire Solar Cells

We present vertically processed photovoltaic devices based on GaInP/InP tandem-junction III-V nanowires (NWs), contacting approximately 3 million NWs in parallel for each device. The GaInP and InP subcells as well as the connecting Esaki tunnel diode are all realized within the same NW. By processing GaInP/InP tandem-junction NW solar cells with varying compositions of the top junction GaInP material, we investigate the impact of the GaInP composition on the device performance. External quantum efficiency (EQE) measurements on devices with varying GaInP composition provide insights into the performance of the respective subcells, revealing that the GaInP subcell is current-limiting for all devices. I-V measurements under AM1.5G illumination confirm voltage addition of the subcells, resulting in an open-circuit voltage of up to 1.91 V. However, the short-circuit current density is low, ranging between 0.24 and 3.44 mA/cm2, which leads to a resulting solar conversion efficiency of up to 3.60%. Our work shows a path forward toward high-efficiency NW photovoltaics and identifies critical issues that need improvement.