Overview of Areas of Interest
Photoactive semiconductor devices such as lasers, light-emitting diodes, detectors and solar cells form a large, diverse and key area of solid state electronics and high technology industries in general. All of my work is based on such devices with a specific area of interest in the characteristics, mechanisms and optimisation of the formation of Schottky barrier semiconductor junctions for particular device applications. The two primary applications of my research are high efficiency solar cells and precision optical position sensitive detectors.
• Solar cells
Solar cells have been and are the focal point of a world-wide research effort because of their appeal as a clean and renewable energy and also because of their usefulness as a reliable power source in remote regions and for space applications. Solar energy cannot compete on a cost basis with fossil fuels at this stage because of their inherent fabrication and materials costs. There are essentially two ways to make them a more competitive option - to make them more efficient or to make them less costly. The projects I have worked on have focused on making high efficiency solar cells.
• Position Sensitive Detectors
Position sensitive detectors, or PSDs, are used for a variety of optical applications such as robotic vision, machine tool alignment, medical instrumentation, remote optical alignment and other applications requiring precise position measurements. PSDs are configured as continuous junctions and so are different to photodiode and other device arrays such as those formed using charged coupled devices (CCDs), in that they can provide continuous information with no internal discontinuities
In essence an electronic device configuration can be set up where a lateral photovoltage is produced such that its magnitude gives a very precise measure of the location of an impinging light beam. These devices behave in many ways similarly to solar cells and so are a natural progression of the group's extensive work in junction optimisation for solar cells.
The simplest model of a PSD is that of a crystal-based device with a highly conducting top layer on a lower conductivity substrate, with appropriately placed contacts made to the lower layer. PSDs can detect position in either a one-dimensional or a two-dimensional sense. When light is shone onto any p-n junction, electron-hole pairs are generated on both sides of the junction and carriers move down the potential gradient, with minority electrons moving from the p-side to the n-side and minority holes moving from the n-side to the p-side. This sets up a forward bias and so a transverse photovoltage is set up - this is the basic mechanism of a solar cell output voltage, when the whole receiver surface is illuminated with light. If now, a spot of light is shone on a junction that has one layer much more conductive than the other, a lateral photovoltage is also set up and we can observe a voltage parallel to the plane of the junction. As before, electrons and holes are generated on both sides of the junction, and the minority carriers are swept across the junction. The photogenerated carriers are swept down the built-in potential to the highly conducting layer, and spread rapidly in this plane, forming an equipotential layer. These carriers from the equipotential layer then re-inject into the less conductive layer. In this low conductivity layer, slow moving excess carriers are still located in the region of the light spot. It is the tendency of these bunched carriers to recombine with the re-injected carriers that constitutes the lateral photovoltage. The magnitude of this photovoltage is a measure of the location of the light beam. Furthermore, the linear behaviour of the incremental change in the voltage with distance along the receiver surface indicates the sensitivity, or the usefulness of the sensor. At this point, the best crystal based devices have a sensitivity of 3.1 mV/mm and a nonlinearity of <5% for measurement increments down to 10µm.
Significance of amorphous silicon thin film devices: Thin film amorphous silicon (a-Si:H) is the subject of an international research effort to utilise it in advanced, large area photoactive devices. Much of the research has focused on solar cells but we believe that the best use of this material is in the area of PSDs. These detectors are the future PSDs as they can be formed into large area devices, and so provide continuous optical information. This is an important advantage over small discrete optical devices, such as photodiodes, which by their nature have regions of zero response between adjacent devices.
1. 2001 J. Henry and J. Livingstone, Thin Film amorphous silicon position sensitive detectors, Advanced Materials, Vol 13, No12-13 p1023-1026
2. 2001 J. Henry and J. Livingstone, A comparative study of position sensitive detectors based on Schottky barrier crystalline and amorphous silicon structures. Journal of Materials Science: Materials in Electronics, Vol 12, p387 - 393, 2001
pdf version3. 2001 J. Henry and J. Livingstone, Electron-beam fabricated titanium and ITO position sensitive detectors, International Journal of Electronics, Vol 88, No 10, p1057-1065
4. 2001 J. Henry and J. Livingstone, Sputtered thin film a-Si position sensitive detectors, Journal of Physics D: Applied Physics , Vol 34, p1-4
5. 2002 J. Henry and J.
Livingstone , A comparison of layered metal-semiconductor optical position sensitive detectors
, IEEE Sensors Journal, Vol 2, No 4, p372-376.
6. 2002 J. Henry and J.
Livingstone, Wavelength response of thin film optical position sensitive detectors, Journal of Optics A: Pure and Applied Optics, 4, p527-534
7. 2003 J. Henry and J. Livingstone, A comparison of Schottky barrier position sensitive detectors as a function
of light wavelength, IEEE Sensors Journal Vol3, No 4, p519-524
pdf version
8. 2003 J. Henry and J. Livingstone, Optical wavelength response of Ta/p-Si position sensitive detectors , International Journal of Electronics 5 Vol 90, No 10, p613-625
9. 2004 J. Henry and J. Livingstone, Optimising the device response of Schottky Barrier Position Sensitive Detectors, Journal of Physics D: Applied Physics Vol 37 p3180-3184
pdf version
10. 2006 J. Henry and J. Livingstone, Aging Effects of Schottky Barrier Position Sensitive Detectors , Sensors Journal, IEEE Volume: 6 Issue: 6, 2006 Page(s): 1557-1563
2007 J. Henry, Teaching introductory foundation engineering units to a common first year
Oral presentation: International Conference on Engineering Education & Research, December 2-7, 2007 Melbourne Australia
AINSE (Australian Institute for Nuclear Science and Engineering) Councillor
Dr. J Henry is the AINSE Councillor for UWA.
Interested in accessing ANSTO facilities? How about taking a look at ANSTO facilities via the WInter School? Click here for more information!