Alan P. Morrison

Development of a microfluidic device for fluorescence activated cell sorting

This paper describes the development towards a miniaturized analytical system that can perform the major key functions of a flow cytometer. The development aims at diagnostic applications for cell counting and sorting with the ultimate goal of a low-cost portable instrument for point of care diagnosis. The present systems configuration consists of a disposable microfluidic device, that enables injection, single file cell flow through a miniaturized laser induced fluorescence detection system as well as sorting of identified samples. The microfluidic devices were fabricated by means of rapid prototyping technologies based on thick film photo-polymers. This paper reports various approaches on cell sorting and demonstrates sorting of single cells by means of an off-chip valve switching technique. The miniaturized fluorescence detection system employs active and passive micro-optical components, including semiconductor laser and ultra bright LED sources, highly sensitive avalanche photodiodes as well as micro-prism, holographic diffraction gratings and fibre optics for transmission and collection of light. Furthermore we demonstrate the feasibility of integrating solid-state components as part of an on-chip detection system.

Solid-state nanopore technologies for nanopore-based DNA analysis

Nanopore-based DNA analysis is a new single-molecule technique that involves monitoring the flow of ions through a narrow pore, and detecting changes in this flow as DNA molecules also pass through the pore. It has the potential to carry out a range of laboratory and medical DNA analyses, orders of magnitude faster than current methods. Initial experiments used a protein channel for its pre-defined, precise structure, but since then several approaches for the fabrication of solid-state pores have been developed. These aim to match the capabilities of biochannels, while also providing increased durability, control over pore geometry and compatibility with semiconductor and microfluidics fabrication techniques. This review summarizes each solid-state nanopore fabrication technique reported to date, and compares their advantages and disadvantages. Methods and applications for nanopore surface modification are also presented, followed by a discussion of approaches used to measure pore size, geometry and surface properties. The review concludes with an outlook on the future of solid-state nanopores.

Toward integrated single-photon-counting microarrays

Silicon, shallow junction, Geiger-mode avalanche photo- diodes (APDs) can be manufactured with complementary metal-oxide semiconductor (CMOS) compatible processing steps and provide single- photon-counting sensitivity. As we move toward providing integrated de- tection of increasingly nanoscopic-sized emissions, small-area detectors and arrays that can be easily integrated into marketable systems will be required. Geiger-mode diodes with diameters of 10, 15, and 20 mm are manufactured and the dark counts measured at 10 V above breakdown are 9, 95, and 990, respectively, at room temperature. The simulated and measured optical crosstalk is found to be significantly reduced for detec- tor pixel pitches beyond 300 mm. The activation energy of the dark count with temperature is found to be 0.58 eV, representing an order of mag- nitude drop in dark count for every 27°C decrease in temperature. The responsivity of the detectors, without antireflection coatings, is found to peak between 550 and 650 nm with a photon detection probability of 43% at 10 V above the breakdown voltage. The low dark counts of the detectors and high photon detection probability highlight the potential these detectors have for fluorescence decay experiments and also in future integrated photonic detection systems.

Polystyrene Particles Reveal Pore Substructure As They Translocate

In this article, we report resistive-pulse sensing experiments with cylindrical track-etched PET pores, which reveal that the diameters of these pores fluctuate along their length. The resistive pulses generated by polymer spheres passing through these pores have a repeatable pattern of large variations corresponding to these diameter changes. We show that this pattern of variations enables the unambiguous resolution of multiple particles simultaneously in the pore, that it can detect transient sticking of particles within the pore, and that it can confirm whether any individual particle completely translocates the pore. We demonstrate that nonionic surfactant has a significant impact on particle velocity, with the velocity decreasing by an order of magnitude for a similar increase in surfactant concentration. We also show that these pores can differentiate by particle size and charge, and we explore the influence of electrophoresis, electroosmosis, and pore size on particle motion. These results have practical importance for increasing the speed of resistive-pulse sensing, optimizing the detection of specific analytes, and identifying particle shapes.

A novel silicon Geiger-mode avalanche photodiode

Dark count nonlinearity in CMOS compatible, single photon counting, Geiger-mode avalanche photodiodes (GM-APD) has been investigated. A novel structure was designed, fabricated, and characterized to allow dark count optimization. Dark count levels for the proposed structure are shown to scale linearly with area.

Characterization of Geiger Mode Avalanche Photodiodes for Fluorescence Decay Measurements

Geiger mode avalanche photodiodes (APD) can be biased above the breakdown voltage to allow detection of single photons. Because of the increase in quantum efficiency, magnetic field immunity, robustness, longer operating lifetime and reduction in costs, solid-state detectors capable of operating at non-cryogenic temperatures and providing single photon detection capabilities provide attractive alternatives to the photomultiplier tube (PMT). Shallow junction Geiger mode APD detectors provide the ability to manufacture photon detectors and detector arrays with CMOS compatible processing steps and allows the use of novel Silicon-on-Insulator(SoI) technology to provide future integrated sensing solutions. Previous work on Geiger mode APD detectors has focused on increasing the active area of the detector to make it more PMT like, easing the integration of discrete reaction, detection and signal processing into laboratory experimental systems. This discrete model for single photon detection works well for laboratory sized test and measurement equipment, however the move towards microfluidics and systems on a chip requires integrated sensing solutions. As we move towards providing integrated functionality of increasingly nanoscopic sized emissions, small area detectors and detector arrays that can be easily integrated into marketable systems, with sensitive small area single photon counting detectors will be needed. This paper will demonstrate the 2-dimensional and 3-dimensional simulation of optical coupling that occurs in Geiger mode APDs. Fabricated Geiger mode APD detectors optimized for fluorescence decay measurements were characterized and preliminary results show excellent results for their integration into fluorescence decay measurement systems.

Theoretical performance of multi-junction solar cells combining III-V and Si materials.

A route to improving the overall efficiency of multi-junction solar cells employing conventional III-V and Si photovoltaic junctions is presented here. A simulation model was developed to consider the performance of several multi-junction solar cell structures in various multi-terminal configurations. For series connected, 2-terminal triple-junction solar cells, incorporating an AlGaAs top junction, a GaAs middle junction and either a Si or InGaAs bottom junction, it was found that the configuration with a Si bottom junction yielded a marginally higher one sun efficiency of 41.5% versus 41.3% for an InGaAs bottom junction. A significant efficiency gain of 1.8% over the two-terminal device can be achieved by providing an additional terminal to the Si bottom junction in a 3-junction mechanically stacked configuration. It is shown that the optimum performance can be achieved by employing a four-junction series-connected mechanically stacked device incorporating a Si subcell between top AlGaAs/GaAs and bottom In0.53Ga0.47As cells.

Comparing leakage currents and dark count rates in Geiger-mode avalanche photodiodes

~Received 9 January 2002; accepted for publication 9 April 2002!This letter presents an experimental study of dark count rates and leakage current in Geiger-modeavalanche photodiodes ~GM APD!. Experimental results from circular diodes over a range of areas~20–500 mm diam!, exhibit leakage current levels orders of magnitude higher than anticipated fromdark count rates. Measurements of the area and peripheral components of the leakage currentindicate that the majority of the current in reverse bias does not enter the high-field region of thediode, and therefore, does not contribute to the dark count rate. Extraction of the area leakagecurrent term from large-area devices~500 mm! corresponds well with the measured dark count rateson smaller devices ~20 mm!. Finally, the work indicates how dark count measurements represent10

InAlAs solar cell on a GaAs substrate employing a graded InxGa1−xAs–InP metamorphic buffer layer

Single junction In0.52Al0.48As solar cells have been grown on a (100) GaAs substrate by employing a 1 μm thick compositionally graded InxGa1−xAs/InP metamorphic buffer layer to accommodate the 3.9% mismatch. Cells processed from the 0.8 μm thick InAlAs layers had photovoltaic conversion efficiency of 5% with an open circuit voltage of 0.72 V, short-circuit current density of 9.3 mA/cm2, and a fill factor of 74.5% under standard air mass 1.5 illumination. The threading dislocation density was estimated to be 3 × 108 cm−2.

Characterization of germanium/silicon p-n junction fabricated by low temperature direct wafer bonding and layer exfoliation

The current transport across a p-Ge/n-Si diode structure obtained by direct wafer bonding and layer exfoliation is analysed. A low temperature anneal at 400 °C for 30 min was used to improve the forward characteristics of the diode with the on/off ratio at −1 V being >8000. Post anneal, the transport mechanism has a strong tunnelling component. This fabrication technique using a low thermal budget (T ≤ 400 °C) is an attractive option for heterogeneous integration.