Silicon Carbide Detectors

The Space Research Centre is collaborating with the Microelectronic Technology Group, University of Newcastle to produce a Silicon Carbide imaging spectroscopy detector. There is considerable basic experimental evidence that Silicon Carbide (SiC) exceeds the radiation tolerance limitations and cooling constraints for Silicon.

Silicon carbide has a bandgap, (3.65ev), sufficiently large to allow for the spectroscopic detection of X-rays at room temperature. (dark current in semiconductor devices is a function of both temperature and bandgap).

SiC detectors have been reported good spectral resolution (315eV @ 10keV) at room temperature, and their devices continued to offer good energy resolution even at operational temperatures of 100°C (797eV FWHM).

Detectors fabricated with SiC should be intrinsically radiation hard by virtue of their wide bandgap energy, although careful optimisation of process conditions during fabrication will be required to achieve this in a functional device.

One obstruction to the wider use of SiC has been the difficulty in producing high quality wafers. This now seems to have been largely solved with a new technique for growing essentially defect free wafers using the "repeated a-face" growth process. The availability of defect-free SiC wafers will undoubtedly lead to devices of much higher quality than are currently possible.

For many spectroscopic applications, high quantum efficiency for photons with incident energies between 1and10keV is essential. It is in this range where the elements Na(Z=11, Ek=1.04keV) to Zn(Z=30, Ek=8.64keV) have their K-shell emission lines. The need for high efficiency, low energy spectroscopy was the driving force behind the development of a new ultra-thin Schottky contact - the “Semi-Transparent SiC Schottky Diode (STSSD)”. The recent paper, Lees et al 2007 (doi:10.1016/j.nima.2007.05.172) describes the X-ray characterisation of these new SiC structures.

Radiation-hard X-ray imaging spectrometers capable of operating at high temperatures and in extreme radiation environments would be an ideal match for the mission to the outer planets and their moons such as  Jupiter – Europa  and  Titan - Enceladus which were possible missions of the Cosmic Vision CV2015 Assessment Studies

Semi-Transparent SiC Schottky Diode (STSSD)

The semi-transparent SiC Schottky diode has an “ultra-thin” (18nm Ni/Ti) Schottky contact, a gold annular overlayer and a gold corner-contact pad. We have show that the new architecture exhibits the same essential characteristics as a more conventional ‘thick-contact’ Schottky diode (>100nm). Such diodes will have a higher efficiency for low energy (<5keV) X-rays than that of conventional structures combined with minimal self-fluorescence from the electrode materials.

Figure1Leesetal_000.jpg

Fig.1. Cross-section of 4H-SiC diode with semi-transparent Schottky contact (not to scale). Epitaxial thickness 20 microns and resistive n+ layer thickness of 370 microns.

Publications

  1. J.E. Lees, A.M. Barnett, D.J. Bassford, R. C. Stevens, A. B. Horsfall, SiC X-ray Detectors for Harsh Environments, J. Inst. 6 (2011) C01032
  2. Rupert Stevens, Konstantin Vassilevski, John E. Lees, Nicholas G. Wright, Alton B. Horsfall, Effect of Proton Irradiation Induced Defects on 4H-SiC Schottky Diode X-ray Detectors, Materials Science Forum 679 - 680 (2011): 547-550.
  3. J.E. Lees, Spectroscopic model for SiC X-ray detectors, Nucl. Inst. Meth. A, 613 (2010) 98-105
  4. J.E. Lees, D. Bassford,E.J. Bunce, M.R. Sims, A.B. Horsfall, Silicon Carbide X-ray Detectors for Planetary Exploration, Nucl. Inst. Meth. A, 604 (2009) 174-176
  5. J.E. Lees, D. Bassford, G.W. Fraser, A.B. Horsfall, K.V. Vassilevski, N.G. Wright and A. Owens, Semi-transparent SiC Schottky Diodes for X-ray spectroscopy, Nucl. Inst. Meth. A, 578 (2007) 226-234
Space Research Centre, Department of Physics,
University of Leicester, University Road, Leicester, LE1 7RH, UK.
Contact: Dr John Lees, +44 (0)116 252 5519, lee@le.ac.uk

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