Edwin J. Koerperick

Advanced near‐infrared monitor for stable real‐time measurement and control of Pichia pastoris bioprocesses

Near‐infrared spectroscopy is considered to be one of the most promising spectroscopic techniques for upstream bioprocess monitoring and control. Traditionally the nature of near‐infrared spectroscopy has demanded multivariate calibration models to relate spectral variance to analyte concentrations. The resulting analytical measurements have proven unreliable for the measurement of metabolic substrates for bioprocess batches performed outside the calibration process. This paper presents results of an innovative near‐infrared spectroscopic monitor designed to follow the concentrations of glycerol and methanol, as well as biomass, in real time and continuously during the production of a monoclonal antibody by a Pichia pastoris high cell density process. A solid state instrumental design overcomes the ruggedness limitations of conventional interferometer‐based spectrometers. Accurate monitoring of glycerol, methanol, and biomass is demonstrated over 274 days postcalibration. In addition, the first example of feedback control to maintain constant methanol concentrations, as low as 1 g/L, is presented. Postcalibration measurements over a 9‐month period illustrate a level of reliability and robustness that promises its adoption for online bioprocess monitoring throughout product development, from early laboratory research and development to pilot and manufacturing scale operation.

Electron and hole spin dynamics in semiconductor quantum dots

We report direct measurement of the spin dynamics of electrons and holes in self-assembled InAs quantum dots (QDs) through polarization-sensitive time-resolved photoluminescence experiments on modulation-doped quantum dot heterostructures. Our measured hole spin decay time is considerably longer than in bulk and quantum well semiconductor systems, indicating that the removal of near degenerate hole states with different spin quantization axes through three-dimensional confinement slows hole spin relaxation in semiconductors. The electron and hole spin decay times we observe (electrons: 120ps; holes: 29ps) are consistent with spin relaxation via phonon-mediated virtual scattering between the lowest two confined levels in the QDs, which have a mixed spin character due to the spin–orbit interaction.

Cascaded Superlattice InAs/GaSb Light-Emitting Diodes for Operation in the Long-Wave Infrared

Superlattice InAs/GaSb light-emitting diodes with peak emission wavelength of 8.6 μm and output power approaching 190 μW at 77 K from a 120 × 120 μm2 mesa are demonstrated. Output power in excess of 600 μ.W was demonstrated from a 520 × 520 μm mesa at 1 A drive current and 50% duty cycle. Devices were grown by molecular beam epitaxy on lightly n-doped GaSb substrates and employed a 16-stage cascaded active region configuration to improve current efficiency and increase optical output. Emitting regions were coupled by semi-metallic tunnel junctions consisting of a p-GaSb layer and a thickness-graded InAs/GaSb superlattice stack.

High-Power MWIR Cascaded InAs–GaSb Superlattice LEDs

Midwave IR LEDs operating at 3.8 mum with output powers approaching 25 mW at 77 K are reported. Devices based on the InAs-GaSb superlattice material system grown by solid source molecular beam epitaxy are demonstrated in a cascaded active region configuration as high-power IR emitters. Optical and electronic characteristics of 16-stage devices with variable mesa size were examined to assess the performance dependence on device size and injection current. The results are suggestive that output power saturation was due to thermal management limitations and carrier leakage out of the active region. Reported output power measurements were taken without the use of an immersion lens or other collection optics, thus representing the upper hemisphere output. Devices were also demonstrated to generate an upper hemisphere power exceeding 1.4 mW at 220 K under quasi-DC excitation conditions.

Active Region Cascading for Improved Performance in InAs–GaSb Superlattice LEDs

Cascading of active regions in InAs-GaSb superlattice light-emitting diodes (LEDs) grown by molecular beam epitaxy is demonstrated as an effective means of increasing optical emission. Devices were fabricated into 120 x 120 mum2 mesas to demonstrate suitability for high resolution projection systems. Devices with 1, 4, 8, and 16 stages were designed for midwave infrared emission at 3.8 mum operating at 77 K, and quasi-continuous-wave output powers in excess of 900 muW from a 16-stage LED have been demonstrated. External quantum efficiency is shown to improve substantially with cascading, approaching 10% for a 16-stage device.

InAs∕GaSb cascaded active region superlattice light emitting diodes for operation at 3.8μm

We report on the growth and characterization of InAs∕GaSb superlattice light emitting diodes (LEDs) operating in the midwave infrared at 3.8μm at 77K. Devices were grown by solid source molecular beam epitaxy on (100) GaSb substrates and were fabricated into 120×120μm2 mesa devices using wet etching. By employing an eight-stage cascaded active region design, output powers in excess of 1.5mW were achieved at 77K with 100mA peak drive current and a 50% duty cycle. Operating characteristics of the devices were examined from room temperature to 77K under quasi-dc excitation conditions.

512

Single element 33×33 μm² InAs/GaSb superlattice light-emitting diodes (SLEDs) operating at 77 K with peak emission at approximately 4.6 μm are demonstrated. A peak radiance of 2.2 W/cm²/sr was measured corresponding to an apparent temperature greater than 1350 K within the 3-5 μm band. A 48 μm pitch, 512 × 512 individually addressable LED array was fabricated from a nominally identical SLED wafer, hybridized with a read-in integrated circuit, and tested. The array exhibited a pixel yield greater than 95%.

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The flip chip bonding process is optimized by varying the bonding pressure, temperature, and time. The 68times68 mid wave infrared (MWIR) LED array was hybridized onto Si-CMOS driver array with same number of pixels. Each pixel has two indium bumps, one for cathode and another for anode. Both LED array and CMOS drivers have 15-mum-square Indium bump contact pads. We used Karl Suss FC150 flip chip machine for bonding of CMOS driver array onto LED array. From the LED current-voltage characteristics, it is concluded that the optimized flip chip bonding process results in uniform contact and very low contact resistance. Both electrical and optical characteristics of LED array after flip chip bonding are presented.

512 Individually Addressable MWIR LED Arrays Based on Type-II InAs/GaSb Superlattices

The authors report measurements of hole spin relaxation in neutral InGaAs quantum dots using polarization-dependent time-resolved photoluminescence experiments. The single-particle hole spin relaxation was isolated from other spin flip processes in the electron-hole system by detecting the initial transfer of population from optically active to dark states. The results indicate that electron-hole exchange interactions play a negligible role in the carrier spin kinetics, and are consistent with a mechanism of hole spin relaxation via phonon-mediated virtual scattering between confined quantum dot states.

Flip Chip Bonding of 68

GaSb-based, 6.1 A lattice-constant, infrared photodetector materials were grown on large diameter, 6-in. GaAs substrates by molecular beam epitaxy. Multiple metamorphic buffer architectures, including bulk GaSb nucleation, AlAsSb superlattices, and graded GaAsSb ternary alloys, were investigated to bridge the 7.8% mismatch gap between the GaAs substrates and the GaSb-based epitaxial layers. Unique surface morphologies and crystal structure properties, as revealed by atomic force microscopy and cross-section transmission electron microscopy, pointed to different relaxation mechanisms for different buffer architectures. GaSb nucleation results in a more island-like surface morphology with a mix of 90° misfit and 60°-type threading dislocations, while the graded ternary buffer results in a cross-hatch surface morphology with effective filtering of the threading dislocations. Low root-mean-square roughness values of 5–20 A were obtained for this type of metamorphic epilayer growth. A generic InAsSb/AlAsSb nBn...