A recent article in Scientific Reports proposed the use of neutron Bragg-dip imaging and the time-of-flight method for pulsed-laser-induced charge carriers to evaluate thallium bromide (TlBr) crystal quality and carrier transport characteristics. Neutron Bragg-dip imaging effectively revealed grain boundaries and crystal imperfections, while the time-of-flight method provided spatial variation in carrier mobility.
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Background
Gamma-ray detectors are typically of two types: scintillators and semiconductors. Scintillator-based detectors allow for comparatively straightforward manufacturing of large detectors but offer limited energy resolution. In contrast, semiconductor-based detectors provide higher energy resolution. However, aside from high-purity germanium (HPGe) detectors, no fabrication process is well-established for producing large semiconductor detectors.
HPGe detectors are considered the standard in gamma-ray detection, but they require cooling, which is a significant drawback. Consequently, researchers are investigating alternatives such as CdTe and CdZnTe detectors, which can operate at room temperature. TlBr is another promising candidate for semiconductor detectors due to its wide bandgap energy, enabling room-temperature operation.
The high atomic numbers of Tl (81) and Br (35), along with TlBr's high density, are expected to contribute to high detection efficiency. However, achieving high-quality, large-volume detectors necessitates reliable crystal evaluation methods. This study, therefore, examined the relationship between the crystal orientation distribution of TlBr and its carrier transport properties.
Methods
Two disk-shaped TlBr crystal wafers were prepared from commercially sourced TlBr powder. The tilted-horizontal traveling zone technique was used for crystal growth, after which the crystal was cut perpendicular to the solidification direction into disks. Two samples, upstream (2.88 mm thick) and downstream (2.81 mm thick), were cut from different locations within the ingot.
Neutron Bragg-dip measurements were performed on the polished disk wafers using a two-dimensional time-resolving neutron detector comprising a boron neutron converter (boron µ-NID). The pattern-matching method was employed to estimate the crystal orientation based on the dip patterns in the transmission spectrum. The results were validated using electron backscatter diffraction-scanning electron microscopy (EBSD-SEM).
Carrier mobility in the TlBr crystals was measured using gold electrodes and a system that included a pulsed laser (405 nm), laser beam transport optics, detector, signal amplification units, and data acquisition equipment. The induced signal current in the electrodes was amplified with a trans-impedance amplifier and digitized by an analog-to-digital converter.
Data were collected and analyzed on a personal computer, with the current pulse signals averaged over 1000 pulses to reduce noise. Carrier velocity was determined from the detector thickness and the width of the induced current pulse. Carrier mobility was subsequently calculated by recording carrier velocity across different applied voltages.
Results and Discussion
The neutron transmission images revealed imperfections and distortions within the TlBr crystal, including grain boundaries present in both upstream and downstream samples. This indicates variations in crystal growth across different positions.
Both EBSD maps and neutron Bragg-dip patterns showed similar trends. While the EBSD provided surface-level information, neutron Bragg-dip imaging revealed details along the neutron beam path through the specimen. Despite minor differences between the two imaging techniques, neutron Bragg-dip imaging effectively determined the crystal orientations.
Electron mobility maps were generated using the time-of-flight method for pulsed-laser-induced carriers in the TlBr samples. The upstream specimen displayed nearly uniform electron mobility, whereas the downstream specimen showed reduced mobility in certain upper areas. These regions, likely containing dislocations and defects, exhibited lower mobility due to unaligned crystal orientations that likely acted as carrier trap sites.
The experimental data, however, did not show a direct correlation between carrier mobility and crystal alignment. Despite minor grain boundaries and some misaligned crystal regions, the upstream specimen exhibited high carrier mobility.
In contrast, the downstream specimen had areas of lower carrier mobility, even with comparable crystal quality to the upstream sample. This lower mobility in the downstream specimen was largely attributed to impurities introduced by segregation during crystal growth.
Conclusion
The researchers conducted a detailed analysis of the carrier transport properties and crystal orientation distribution in TlBr semiconductor detectors, using neutron Bragg-dip imaging and time-of-flight methods for pulsed-laser-induced carriers.
Neutron Bragg-dip imaging results aligned closely with EBSD findings. Unlike EBSD, which provides only surface-level information, neutron Bragg-dip transmission captured details from deeper regions within the TlBr specimens. The orientation maps generated through neutron Bragg-dip imaging highlighted imperfections in crystal growth, such as grain boundaries.
Within the range of carrier mobility studied, the impact of crystal integrity on carrier mobility was less pronounced than that of impurities. Nevertheless, enhancing crystal integrity can significantly improve carrier mobility. Thus, refining the crystal growth process is essential. Further, optimizing carrier collection efficiency, a critical factor in detector performance, requires advancements in both carrier mobility and lifetime.
Journal Reference
Watanabe, K., et al. (2024). Comparison between carrier transport property and crystal quality of TlBr semiconductors. Scientific Reports. DOI: 10.1038/s41598-024-76005-9, https://www.nature.com/articles/s41598-024-76005-9
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