Yijie Huo

Increased photoluminescence of strain-reduced, high-Sn composition Ge1−xSnx alloys grown by molecular beam epitaxy

We synthesized up to Ge0.914Sn0.086 alloys on (100) GaAs/InyGa1−yAs buffer layers using molecular beam epitaxy. The buffer layers enable engineered control of strain in the Ge1−xSnx layers to reduce strain-related defects and precipitation. Samples grown under similar conditions show a monotonic increase in the integrated photoluminescence (PL) intensity as the Sn composition is increased, indicating changes in the bandstructure favorable for optoelectronics. We account for bandgap changes from strain and composition to determine a direct bandgap bowing parameter of b = 2.1 ± 0.1. According to our models, these are the first Ge1−xSnx samples that are both direct-bandgap and exhibit PL.

Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown by molecular beam epitaxy

Highly tensile-strained layers of Ge were grown via molecular beam epitaxy using step-graded InxGa1−xAs buffer layers on (100) GaAs. These layers have biaxial tensile-strain of up to 2.33%, have surface roughness of <1.1 nm, and are of high quality as seen with transmission electron microscopy. Low-temperature photoluminescence (PL) suggests the existence of direct-bandgap Ge when the strain is greater than 1.7%, and we see a greater than 100× increase in the PL intensity of the direct transition with 2.33% tensile-strain over the unstrained case. These results show promise for the use of tensile-strained Ge in optoelectronics monolithically integrated on Si.

Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30.

Hydrogen production via electrochemical water splitting is a promising approach for storing solar energy. For this technology to be economically competitive, it is critical to develop water splitting systems with high solar-to-hydrogen (STH) efficiencies. Here we report a photovoltaic-electrolysis system with the highest STH efficiency for any water splitting technology to date, to the best of our knowledge. Our system consists of two polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolysers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolysers to optimize the system efficiency. The system achieves a 48-h average STH efficiency of 30%. These results demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage.

Investigation of the direct band gaps in Ge1−xSnx alloys with strain control by photoreflectance spectroscopy

Unstrained and compressive-strained Ge1−xSnx alloys were grown on InGaAs buffer layers by molecular beam epitaxy. Photoreflectance at room temperature determines the direct bandgap energies of Ge1−xSnx alloys from the maxima of the light- and heavy-hole bands to the bottom of Γ valley. The lowest transition energies from photoreflectance are consistent with the energies derived from photoluminescence. The calculated bowing parameter is 2.42 ± 0.04 eV for the direct band gap of Ge1−xSnx alloys. The dilational and shear deformation potentials of the direct band gap are −11.04 ± 1.41 eV and −4.07 ± 0.91 eV, respectively. These basic material parameters are important in designing optoelectronic devices based on Ge1−xSnx alloys.

High-efficiency nanostructured window GaAs solar cells.

Nanostructures have been widely used in solar cells due to their extraordinary optical properties. In most nanostructured cells, high short circuit current has been obtained due to enhanced light absorption. However, most of them suffer from lowered open circuit voltage and fill factor. One of the main challenges is formation of good junction and electrical contact. In particular, nanostructures in GaAs only have shown unsatisfactory performances (below 5% in energy conversion efficiency) which cannot match their ideal material properties and the record photovoltaic performances in industry. Here we demonstrate a completely new design for nanostructured solar cells that combines nanostructured window layer, metal mesa bar contact with small area, high quality planar junction. In this way, we not only keep the advanced optical properties of nanostructures such as broadband and wide angle antireflection, but also minimize its negative impact on electrical properties. High light absorption, efficient carrier collection, leakage elimination, and good lateral conductance can be simultaneously obtained. A nanostructured window cell using GaAs junction and AlGaAs nanocone window demonstrates 17% energy conversion efficiency and 0.982 V high open circuit voltage.

Raman study of strained Ge1−xSnx alloys

The Ge-Ge longitudinal optical Raman peak has been measured in strained Ge1−xSnx alloy layers grown on top of relaxed InyGa1−yAs buffer layers on GaAs substrates by molecular beam epitaxy. The experimental result shows that the peak frequency shift increases linearly from the value for bulk Ge with the Sn fraction x and the strain ɛ, Δω = ω − ωGe = ax + bɛ. In these experiments alloy and strain contributions are decoupled and measured separately, and a and b are determined to be a = − 82 ± 4 cm−1 and b = − 563 ± 34 cm−1, over the entire composition and strain range investigated.

Demonstration of a Ge/GeSn/Ge Quantum-Well Microdisk Resonator on Silicon: Enabling High-Quality Ge(Sn) Materials for Micro- and Nanophotonics

We theoretically study and experimentally demonstrate a pseudomorphic Ge/Ge0.92Sn0.08/Ge quantum-well microdisk resonator on Ge/Si (001) as a route toward a compact GeSn-based laser on silicon. The structure theoretically exhibits many electronic and optical advantages in laser design, and microdisk resonators using these structures can be precisely fabricated away from highly defective regions in the Ge buffer using a novel etch-stop process. Photoluminescence measurements on 2.7 μm diameter microdisks reveal sharp whispering-gallery-mode resonances (Q > 340) with strong luminescence.

Enhanced second-harmonic generation in AlGaAs/Al x O y tightly confining waveguides and resonant cavities

We demonstrate second-harmonic generation (SHG) from sub-micrometer-sized AlGaAs/AlxOy artificially birefringent waveguides. The normalized conversion efficiency is the highest ever reported. We further enhanced the SHG using a waveguide-embedded cavity formed by dichroic mirrors. Resonant enhancements as high as approximately 10x were observed. Such devices could be potentially used as highly efficient, ultracompact frequency converters in integrated photonic circuits.

Ultra-Compact and Low-Loss Polarization Rotator Based on Asymmetric Hybrid Plasmonic Waveguide

We propose a novel hybrid plasmonic polarization rotator based on mode interference. Operating at the telecommunication wavelength of 1.55 μm, the rotation length is very short (3.2 μm), while the polarization conversion efficiency is as high as 99.5%. The total device insertion loss is only 1.38 dB, much smaller than the common level of plasmonic devices. It also has potential to realize integrated waveplates for various polarization states.

Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si

In this paper, we present observations of quantum confinement and quantum-confined Stark effect electroabsorption in Ge quantum wells with SiGe barriers grown on Si substrates. Though Ge is an indirect gap semiconductor, the resulting effects are at least as clear and strong as seen in typical III-V quantum well structures at similar wavelengths. We also designed and fabricated a coplanar high-speed modulator, and demonstrated modulation at 10 GHz and a 3.125-GHz eye diagram for 30-¿m-sized modulators.