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Next Generation Room Temperature Semiconductor Nuclear Radiation Detectors - WA Node

December 2012

Figure 1
Figure 1: Ramin Rafiei holding a CdMnTe radiation detector (Image courtesy of the Australian Nuclear Science and Technology Organisation (ANSTO)).

Semiconductor gamma-ray and X-ray detectors are being used increasingly in medicine, industry, astronomy and national security. Conventional semiconductor detectors are manufactured from germanium and silicon. Such materials have become less useful in many emerging applications due to their physical limitations such as low detection efficiency or their need to operate at cryogenic temperatures. Next generation nuclear radiation detectors are advanced sensors which utilise innovative technologies developed for the wide band-gap compound semiconductor industry and microelectronics. This research project is aimed at developing room-temperature operating nuclear radiation detectors based on cadmium manganese telluride (CdMnTe). Now in their second generation, major improvements in the performance of these detectors have been demonstrated. This research is paving the way for the realization of advanced CdMnTe sensors for use in nuclear medical imaging and accurate in-field radiological threat detection.

CdMnTe is a promising compound semiconductor. While previously investigated for applications in optical isolators, infrared detectors and tuneable solid state lasers, its application for nuclear radiation detection was first investigated in 1999. Its distinct advantage of excellent compositional homogeneity compared to CdZnTe, which has been the leading room temperature detector candidate for over three decades, enables the growth of large-volume uniform CdMnTe crystals. Inhomogeneity of CdZnTe crystals continues to limit the industrial scale growth of large-volume uniform CdZnTe crystals, where typical yields of usable material remain at 10% or lower, resulting in very high material costs. A collaboration led by Ramin Rafiei between the WA Node of the Australian National Fabrication Facility (ANFF) at The University of Western Australia, The Australian Nuclear Science and Technology Organisation (ANSTO), and Brookhaven National Laboratory (USA), investigates CdMnTe crystal growth, detector fabrication and detector performance.

The CdMnTe crystals were grown by the vertical Bridgman technique. A manganese fraction of 5%, corresponding to a band-gap of 1.59 eV, was chosen for optimum room temperature spectral performance. To grow high resistivity CdMnTe, the crystal was doped with indium. Indium, which is a donor, compensates for the high concentration of Cd vacancies which act as acceptor centres in CdMnTe. Major obstacles towards realising CdMnTe crystals that are advantageous in nuclear radiation detector applications have been high levels of residual impurities in the Mn source material and high concentrations of tellurium (Te) inclusions which are known to act as charge trapping centres. These issues have been overcome in the growth of generation II CdMnTe crystals where the MnTe source material was purified by a zone-refining method with molten Te solvent, and control over the size and distribution of the Te inclusions was also achieved. The fabrication of these devices was carried out at the ANFF WA node utilising the expertise of this node in II-VI semiconductor processing and their experience in II-VI semiconductor device fabrication. A fabricated CdMnTe detector of size 10 × 10 × 2.6 mm3 is shown in Figure 1 above.

A detailed understanding of the fundamental charge transport properties of CdMnTe radiation detectors is essential for detector development. The most useful figure of merit is the mobility-lifetime product which quantifies the charge carrier transport through the detector. A low mobility-lifetime product results in short carrier drift lengths and limits the maximum detector thickness and, hence, its application. Time-resolved transient current measurements and alpha-spectroscopy measurements have been used to measure the charge-collection efficiency of CdMnTe detectors. From the dependence of the charge collection efficiency on the applied bias an average electron mobility-lifetime value has been calculated and shows an improvement from 5 x 10-4 cm2V-1 for a generation I device to 3 x 10-3 cm2V-1 for a generation II device.

Uniform charge-carrier transport is critical to the spectroscopic performance of CdMnTe detectors. Ion beam induced charge (IBIC) measurements, utilising 4He2+ beams from the ANTARES accelerator at ANSTO, have revealed the spatial distribution of charge transport in these devices down to micron scale resolution. Fig. 2 (left panel) is an IBIC image of a generation I detector showing the charge collection efficiency (CCE) across an area of 1450 x 1450 μm2. Such images have quantified how major impurities such as tellurium inclusions present within the detector bulk (and clearly visible in Fig. 2 (left panel) as areas of reduced charge collection), affect the charge collection of these devices. In contrast to the left panel of Fig. 2, the right panel is an IBIC image of a generation II detector showing for the first time uniform large-area charge collection at a value of 100%.

Figure 2a Figure 2b

Figure 2: Spatially resolved charge-collection efficiency maps of generation I (left panel) and generation II (right panel) CdMnTe detectors.
On this colour scale blue signifies low charge collection, while regions of high charge collection are represented in red. In the left panel regions of poor efficiency are spread throughout the bulk and are associated with the high density network of tellurium inclusions present in generation I devices.
In contrast, for the first time large area uniformity with 100% charge collection has been achieved with generation II devices and is presented in the right panel. The edges of the detector are clearly visible in the right-hand panel.

Radiation detector applications require long lifetime of the photo-generated charge carriers. From single charge-transient measurements the drift time of the electrons across the detector has been extracted. From these measurements the room temperature mobility and carrier lifetime of CdMnTe was found to be 718(±55) cm2/Vs and 1.2(±0.1) µs for generation I detectors and 990(±50) cm2/Vs and 3.1(±0.3) µs for generation II detectors, respectively.

Generation II detectors are currently the world's most advanced CdMnTe radiation detectors and their charge transport properties are compared to commercially available CdZnTe (see Table 1). While the current performance of state-of-the-art CdZnTe detectors represents over four decades of research and development, recent advances in CdMnTe detector technology has been the result of a relatively short 12 month intensive international effort.

Table 1: Comparison of electron charge transport properties of generation II CdMnTe detectors with commercially available CdZnTe detectors.

Detector Bandgap (eV) Mobility-Lifetime (cm2/V) Mobility (cm2/Vs) Lifetime (µs)
Cd0.95Mn0.05Te 1.59 3 x10-3 990 (±50) 3.1 (±0.3)
Cd0.90Zn0.10Te 1.57 10-2 1200 5

With continued progress in crystal growth, purity and fabrication processes, the achievement of advanced CdMnTe sensors with spectroscopic performance superior to that of CdZnTe combined with an imaging capability is within reach. It is anticipated that over the next decade investment in next generation nuclear radiation detectors will be mainly driven by their application in nuclear medical imaging. While CdZnTe medical probes have recently become commercially available, their application in more advanced capabilities such as X-ray computed tomography and positron emission tomography is being aggressively pursued. With the increased demand for compound semiconductor nuclear radiation detectors, higher yield advanced CdMnTe sensors may define the economical way forward.

Article by Ramin Rafiei from the School of Electrical, Electronic & Computer Engineering at The University of Western Australia