Indium arsenide quantum well micro-Hall devices for magnetic detection of biomolecular labels

This dissertation presents work on demonstrating the feasibility of micro-Hall sensors for the detection of specific bimolecular interactions using micrometer and nanometer sized superparamagnetic beads. The first part of this dissertation focuses on fabrication and characterization of micro-Hall sensors from InAs/AlSb quantum well semiconductor heterostructures. Our detailed analysis indicates the significance of having high mobility and high carrier density for the sensor material in order to achieve the optimum magnetic field and magnetic moment sensitivities. Noise characterization of InAs micro-Hall sensors revealed that the main noise sources for InAs micro-Hall sensors are 1/f noise and thermal noise. The measured magnetic moment sensitivities are in the order of 105 muB/Hz1/2 and 10 4 muB/Hz1/2 at 92Hz and 23 kHz respectively, where muB is Bohr magneton. Based on these promising sensitivities, detection of single magnetic bead as small as 100 nm in diameter is predicted. Furthermore, by improving the detection scheme, it has been demonstrated that very high signal to noise ratios can be obtained, above 90 for a single 2.8 mum magnetic bead. The second part of this dissertation focuses on the demonstration of the suitability of InAs micro-Hall sensors for bio-detection. Streptavidin coated magnetic beads having average diameters of 200 nm were self assembled via biotin-streptavidin binding over very specific regions of a pre-functionalized micro-Hall sensor and subsequently magnetically detected. This demonstrates the first explicit magnetic detection of specific bimolecular interactions using magnetic sensor. The signal to noise ratio per bead was 7.1 dB. The second demonstration utilized DNA-DNA interactions to assemble magnetic beads over 3 mum gold pads that were pre-deposited over some of the Hall crosses. The high on off ratio of the detected signals (occupied crosses versus empty crosses) and the use of three DNA strands in this experiment demonstrates the possibility to perform label free detection at the single molecule level. Finally, a demonstration for the feasibly of InAs micro-Hall sensors to effectively work in a wet environment and in real time was achieved by integrating the sensor chip with a microfluidic channel and performing dynamic detection of flowing magnetic beads. Moreover, the detected signal was successfully interpreted based on the magnetic dipole representation of the magnetic fields induced by the magnetized beads.