Thick-film chalcogenide semiconductors such as mercury iodide (HgI2) and thallium bromide (TlBr) have emerged as promising materials for direct X-ray detection in medical imaging applications. These materials exhibit high atomic numbers, strong X-ray absorption coefficients, and favorable charge transport properties, making them suitable for converting X-ray photons directly into electrical signals without the need for intermediate scintillation processes. Unlike silicon or germanium detectors, chalcogenides offer superior stopping power for high-energy X-rays, enabling efficient detection at lower thicknesses while maintaining high spatial resolution.

A critical performance metric for direct X-ray detectors is the mobility-lifetime (μτ) product of charge carriers, which determines the charge collection efficiency. In HgI2, the electron μτ product typically ranges from 1×10⁻⁴ to 5×10⁻⁴ cm²/V, while holes exhibit values around 1×10⁻⁵ cm²/V. TlBr demonstrates higher μτ products, with electrons reaching 1×10⁻³ cm²/V and holes around 2×10⁻⁴ cm²/V. These values are influenced by material purity, crystalline defects, and fabrication conditions. Optimizing the μτ product involves minimizing trap states through controlled growth processes, such as physical vapor transport (PVT) or zone refining, to reduce charge carrier recombination. Post-growth treatments, including thermal annealing and halogenation, further enhance carrier transport by passivating defects.

Pixelation is a key challenge in developing practical medical imaging detectors. Thick-film chalcogenides require fine-pitch electrode patterning to achieve high spatial resolution, but material brittleness and interfacial reactions with metal contacts complicate fabrication. HgI2 detectors often employ palladium or carbon-based contacts to mitigate chemical degradation, while TlBr devices use gold or platinum electrodes to minimize interdiffusion. Pixel sizes below 100 μm are achievable, but charge sharing between adjacent pixels due to lateral diffusion limits resolution. Monte Carlo simulations indicate that for a 500 μm thick TlBr detector, charge sharing becomes significant at pixel pitches below 150 μm, necessitating corrective algorithms in readout electronics.

Environmental stability is another concern. TlBr is hygroscopic and degrades in humid conditions, requiring hermetic encapsulation. HgI2 is more stable but suffers from polarization effects under prolonged bias, leading to signal fading. Operating detectors at moderate electric fields (0.1–1 V/μm) balances between efficient charge collection and minimizing field-driven degradation.

The energy resolution of chalcogenide detectors is another critical parameter. TlBr exhibits better energy resolution than HgI2 due to its higher μτ products, with reported values of 5–10% FWHM at 60 keV for TlBr compared to 10–15% for HgI2. This makes TlBr more suitable for spectral imaging applications where energy discrimination is required. However, HgI2’s lower dark current and better long-term stability give it an advantage in continuous imaging modalities.

Table: Comparison of key parameters for HgI2 and TlBr X-ray detectors

Parameter HgI2 TlBr
μτ product (e⁻) 1–5×10⁻⁴ cm²/V 1×10⁻³ cm²/V
μτ product (h⁺) 1×10⁻⁵ cm²/V 2×10⁻⁴ cm²/V
Density (g/cm³) 6.4 7.5
Bandgap (eV) 2.1 2.7
Energy Resolution 10–15% FWHM 5–10% FWHM

Future advancements in thick-film chalcogenide detectors will focus on improving material purity, developing robust pixelation techniques, and integrating advanced readout architectures. Hybrid approaches combining TlBr’s high performance with HgI2’s stability may offer a viable path toward commercial medical imaging systems. The ongoing development of low-noise application-specific integrated circuits (ASICs) tailored for chalcogenide detectors will further enhance their viability in clinical environments.

In summary, thick-film chalcogenides present a compelling alternative to conventional direct detection materials for medical X-ray imaging. Their high stopping power, tunable charge transport properties, and potential for high-resolution pixelation make them well-suited for next-generation diagnostic systems. Continued research into defect engineering and device integration will be essential to overcome current limitations and unlock their full potential.