John Spann

Integrated tunable CMOS laser

An integrated tunable CMOS laser for silicon photonics, operating at the C-band, and fabricated in a commercial CMOS foundry is presented. The III-V gain medium section is embedded in the silicon chip, and is hermetically sealed. The gain section is metal bonded to the silicon substrate creating low thermal resistance into the substrate and avoiding lattice mismatch problems. Optical characterization shows high performance in terms of side mode suppression ratio, relative intensity noise, and linewidth that is narrow enough for coherent communications.

Experimental results from performance improvement and radiation hardening of inverted metamorphic multi-junction solar cells

This paper discusses results from continued development of inverted metamorphic multi-junction (IMM) solar cells with air mass zero (AM0) conversion efficiencies greater than 34%. An experimental best four-junction IMM (IMM4J) design is presented. In an effort to improve IMM performance in space radiation environments, 1-MeV electron irradiation studies are conducted on the individual IMM4J subcells. This data is used to engineer an IMM4J structure with beginning of life (BOL) AM0 conversion efficiency of approximately 34% and an end of life (EOL) remaining factor greater than 82%, where EOL is defined as performance after exposure to 1-MeV electron irradiation at 1E15 e/cm2 fluence. Next generation IMM designs are explored and an avenue toward AM0 conversion efficiencies of greater than 35% is presented.

Evolution of a 2.05 eV AlGaInP top sub-cell for 5 and 6J-IMM applications

This paper discusses the evolution of our 2.05 eV AlGaInP top sub-cell. Although we were not successful in introducing a more optimum 1.94 eV AlGaInP alloy into the top sub-cell of our 3 or 4J-IMM (inverted metamorphic multi-junction) structure because of insufficient current density capacity, we did successfully develop a 2.05 eV AlGaInP top sub-cell structure for 5 or 6J-IMM application. To achieve the desired current capacity of 11.3mA/cm2, we fabricated 2.05 eV hetero-junctions composed of a 1.91 eV GaInP emitter and a 2.05 eV AlGaInP base. Such a hetero-junction top sub-cell incorporated in a 3J-IMM iso-type structure exhibited sufficient Jsc but reduced Voc=1.466 V, being only 43 mV greater than the 1.91 eV GaInP homo-junction sub-cell. Minor modification to the emitter of our AlGaInP homo-junction permitted the fabrication of a 2.05 eV sub-cell in a 5J-IMM iso-type structure with sufficient current density and improved Voc=1.537 V. 5 and 6J-IMM cells incorporating these top sub-cells demonstrated maximum Vocs= 4.839 and 5.195 V, respectively.

Lattice-Matched Solar Cells With 40% Average Efficiency in Pilot Production and a Roadmap to 50%

A commercial lattice-matched InGaP/InGaAs/Ge solar cell has reached an average efficiency of 40% at 500 kW/m2. The design changes that lead to this result are discussed. These data are complemented with a presentation of the latest new solar cell development results from the laboratory. Inverted metamorphic multijunction (IMM) solar cells have been prototyped with 42.4% efficiency at 325 suns for concentrator applications and 33.6% efficiency at 1 sun AM0 for space applications. Six subcell devices are now under development. These results are used, along with other experimental data and other industrial constraints, as input to a computer model to predict what practical efficiency might be achievable with this device approach. The computer model suggests that 45% and 50% efficiencies are technologically feasible with a three-junction and five-junction device, respectively, at an irradiance of 500 kW/m2 and 25 °C using known materials, device architectures, and manufacturing methods.

Investigation of quantum dot enhanced triple junction solar cells

InAs quantum dots have been incorporated into the middle junction of an InGaP/(In)GaAs/Ge triple junction solar cell (TJSC) on four inch wafers, in aims of band gap engineering a high efficiency solar cell to even higher limits. Results of QD growth on 4” diameter Ge templates gave densities near 1×1011 cm−3 and QD height between 2–5 nm. Arrays of 10 layers of InAs QDs have been grown between the base and emitter in the middle cell of a full triple junction solar cell. Control triple junction cells that received growth interrupts without QD growth showed similar results (within 5 mV open circuit voltage) to standard triple junction cells without an interrupt. Integrated current of the (In)GaAs junction with 10 layers of strain balanced InAs QD layers shows a gain of 0.37 mA/cm2 beyond the band edge. One sun AM0 current-voltage measurements of QD TJSC show an efficiency of 26.9% with a Voc of 2.57 V.

Radiation effects on quantum dot enhanced solar cells

Radiation tolerance of quantum dot (QD) enhanced solar cells has been measured and modeled. GaAs solar cells enhanced with 10, 20, 40, 60, and 100X layers of strain compensated QDs are compared to baseline devices without QDs. Radiation resistance of the QD layers is higher than the bulk material. Increasing the number of QD layers does not lead to a systematic decrease in QD response throughout the course of radiation exposure. Additionally, InGaP/(In)GaAs/Ge triple junction solar cells with and without 10 layers of strain compensated QDs in the (In)GaAs triple junction solar cells are analyzed. Triple junction solar cells with QDs have a better resistance to Voc degradation but these samples have a degradation in Isc that leads to lower radiation resistance for power output.

High power, narrow linewidth, low noise, integrated CMOS tunable laser for long haul coherent applications

An integrated-CMOS-tunable-laser with 15-dBm output power is presented. Fabrication is realized in commercial CMOS foundry. Laser shows high power, low RIN, and ultra-narrow linewidth. Performance over fiber is comparable with best-in-class, market-leading ITLA, proving suitability for long haul coherent applications.

100Gb/s CWDM transmitter and receiver chips on a monolithic Si-photonics platform

We present fully integrated 100Gb/s CWDM transmitter and receiver chips, including uncooled lasers, 25Gb/s modulators and Ge PDs, wavelength multiplexer and de-multiplexer, all fabricated on a monolithic Si-photonics platform with low-cost wafer-scale manufacturing.

Composite-CMOS Integrated Photonics for High Bandwidth WDM Optical Interconnects

Bandwidth requirements continue to drive the need for low-power, high speed interconnects. Harnessing the mature CMOS technology for high volume manufacturing, Silicon Photonics is a top candidate for providing a viable solution for high bandwidth, low cost, low power, and high packing density, optical interconnects. The major drawback of silicon, however, is that it is an indirect bandgap material, and thus cannot produce coherent light. Consequently, different integration schemes of III/V materials on silicon are being explored. An integrated CMOS tunable laser is demonstrated as part of a composite-CMOS integration platform that enables high bandwidth optical interconnects. The integration platform embeds III-V into silicon chips using a metal bonding technique that provides low thermal resistance and avoids lattice mismatch problems. The performance of the laser including side mode suppression ratio, relative intensity noise, and linewidth is summarized.