## Overview of THM and Bridgman growth methods
Thallium-mediated Bridgman (THM) and conventional Bridgman (including horizontal and vertical Bridgman variants) are common methods to grow
Cadmium Zinc Telluride (CZT) crystals. Both produce bulk CZT but differ in thermodynamic driving forces, melt/recrystallization dynamics, impurity segregation behavior, and thermal gradients — all of which shape the type, density, and spatial distribution of point defects, extended defects (dislocations, twins), precipitates, and compositional nonuniformities that affect detector performance.
## Thermodynamics and process differences that set defect landscapes
- Solvent/mediator presence:
- THM uses a thallium-containing solvent (Tl-rich melt or flux) that lowers effective melting temperature and modifies solute segregation and solubility behavior; Bridgman typically solidifies from a stoichiometric melt without such a flux.
- Solidification rate and thermal gradients:
- Bridgman processes commonly use higher thermal gradients and faster pull rates than THM. THM favors slower growth with smaller thermal gradients due to flux-mediated lower undercooling.
- Constitutional supercooling and interface stability:
- Faster Bridgman solidification is more prone to constitutional supercooling and cellular/striated growth, which fosters compositional inhomogeneity and arrays of growth-related defects. THM’s fluxing action and slower solidification produce more stable planar solidification fronts, reducing segregation-induced instabilities.
- Impurity incorporation and segregation coefficients:
- The solvent in THM alters partitioning behavior of impurities and native defects (e.g., Cd vacancies, Te antisites), often enabling lower net background impurity concentrations or more favorable compensation compared with conventional Bridgman.
These process distinctions lead to characteristic differences in defect types and spatial distributions.
## Point defects and stoichiometry-related defects
- Cadmium vacancies (VCd) and Te-related antisites:
- Bridgman-grown CZT tends to have higher concentrations of native vacancies and antisites in regions affected by non-equilibrium solidification or steep thermal gradients. These defects act as acceptors/trapping centers that reduce electron μτ and increase leakage.
- THM growth, via flux-mediated equilibrium and slower kinetics, generally yields lower concentrations of such point defects or more uniform compensation, improving μτ homogeneity across the boule.
- Unintentional dopants and contaminants:
- Bridgman melts can trap and redistribute metallic impurities non-uniformly, producing localized regions of higher defect-related trapping. THM’s solvent can scavenge or redistribute impurities differently, often reducing some contaminants but potentially introducing Tl-related species that must be controlled.
## Extended defects: dislocations, twins, and sub-grain boundaries
- Dislocation density and networks:
- Rapid solidification and thermal stress in Bridgman-grown crystals commonly produce higher dislocation densities, dislocation tangles, and slip-related networks, often with spatial correlation to seed orientation and thermal gradients.
- THM’s slow growth and lower thermal stresses generally yield lower dislocation densities and fewer extended defect clusters; when present, dislocations are often more isolated.
- Twins and microtwins:
- Both methods can produce twinning, but Bridgman’s non-uniform interface motion can enhance twin formation and stacking-fault-related features in localized bands; THM tends to suppress widespread twinning due to more uniform growth fronts.
- Sub-grain boundaries:
- Bridgman boules may show sub-grain boundaries from recrystallization or stress relaxation; these boundaries localize trapping centers and can cause abrupt μτ variations. THM boules typically have larger single-crystal domains with fewer sub-grain boundaries.
## Precipitates and secondary phases
- Telluride or cadmium-rich precipitates:
- Rapid solidification and solute rejection in Bridgman can cause supersaturation and formation of secondary precipitates (e.g., Te inclusions), particularly if thermal gradients or cooling profiles promote solute segregation.
- THM growth with solvent can reduce the driving force for precipitate formation and yield fewer or smaller inclusions, though solvent chemistry must be well controlled to avoid introducing new phases.
- Metallic inclusions and oxide pockets:
- Both methods can trap contaminants forming inclusions; Bridgman higher solidification rates and convection patterns can aggregate impurities into localized regions more readily than THM.
## Compositional homogeneity and Zn segregation
- Zn compositional uniformity:
- Bridgman growth often shows stronger axial and radial Zn segregation because of finite segregation coefficients and less mixing, producing banding or grading of bandgap across the boule; Zn variation causes spatial variation in detector response (energy calibration shifts, mobility-lifetime changes).
- THM’s solvent and slower kinetics promote improved Zn homogenization and reduced banding, resulting in more uniform bandgap and charge-transport properties across slices.
- Striations and banding:
- Bridgman crystals frequently exhibit growth striations (compositional bands) tied to interface instabilities and flow; THM tends to exhibit reduced striation amplitude.
## Spatial patterns of defects across the boule
- Bridgman:
- Defects often correlate with axial position (near seed or tail), radial inhomogeneities, and visible striations. Edge regions near crucible walls can have distinct defect densities due to wetting, friction, and thermal boundary conditions.
- Localized “hot spots” of high leakage or poor μτ are common and can map to dislocation tangles or inclusion clusters.
- THM:
- Boules generally show larger low-defect regions and more gradual spatial variation; defect concentrations and μτ tend to be more uniform, reducing the incidence of unusable regions.
- End effects:
- Both methods produce end regions (seed and tail) with atypical defect populations; THM end effects are often less severe due to gentler solidification.
## Impact on detector-relevant electronic properties
- Mobility-lifetime (μτ) product:
- Lower and more spatially uniform μτ typically results from THM growth, improving charge collection efficiency and spectral uniformity. Bridgman-grown material commonly shows larger μτ variability and lower average μτ in defect-rich zones.
- Leakage current and resistivity:
- Bridgman crystals with higher point-defect and impurity concentrations can exhibit higher dark currents and lower resistivity uniformity. THM tends to produce higher resistivity material with lower surface- and bulk-leakage propensity when processed correctly.
- Trapping centers and afterglow:
- Higher trap densities and a broader trap-spectrum in Bridgman material cause more pronounced tailing in spectra (low-energy tails) and longer detrapping times, worsening energy resolution and pulse-shape distortions. THM material typically exhibits reduced trap-induced tailing.
## Sensitivity to post-growth processing and annealing
- Bridgman material:
- More sensitive to post-growth anneals, chemical etching, and contact processing because existing high defect densities can interact with processing-induced stress and surface damage to produce variable outcomes.
- THM material:
- Generally more robust to processing, but solvent residues or incorporated Tl must be managed. THM crystals respond well to careful annealing and passivation with fewer emergent defect-related artifacts.
## Manufacturability, yield, and crystal size trade-offs
- Boule size and yield:
- Bridgman techniques can produce large boules economically but with variable usable-area yield due to defect bands. THM can yield higher-quality wafers with larger contiguous regions suitable for detector fabrication but historically at lower throughput and potentially higher per-wafer cost.
- Scalability:
- Improvements in both processes have narrowed differences, but THM remains preferred for high-performance spectroscopic crystals where uniformity and low defect density are critical; Bridgman variants are used where cost and throughput dominate and downstream selection/filtering is acceptable.
## Modeling and characterization correlations
- Mapping techniques:
- Defect distributions from growth are characterized using IR microscopy (to image Te inclusions), X-ray topography (dislocation mapping), micro-PL or cathodoluminescence (compositional and defect contrasts), and μτ mapping via collimated-beam spectroscopy or transient-current techniques.
- Correlation with growth method:
- Empirical studies consistently show better inclusion control, lower dislocation densities, and improved μτ uniformity in well-executed THM boules versus standard Bridgman, particularly when growth parameters are optimized.
## Practical detector-design implications
- For high-resolution spectroscopy:
- Prefer THM-grown material where low trap density, uniform μτ, and low inclusion/dislocation densities directly translate to improved FWHM and lower low-energy tailing.
- For large-area or cost-constrained arrays:
- Bridgman-grown material may be acceptable with careful selection and mapping to use only high-quality regions, combined with device-level mitigations (segmentation, DOI correction, coplanar grids) that reduce sensitivity to localized defects.
- Process combination:
- Some manufacturers combine improved Bridgman variants (vertical gradient freeze, modified thermal profiles) with post-growth anneals and stringent selection to approach THM-like qualities at lower cost.
## Summary
THM and Bridgman growth routes produce distinct defect landscapes in CZT: THM generally yields lower point- and extended-defect densities, fewer inclusions, and superior compositional uniformity, producing more uniform μτ and lower leakage — outcomes favorable for high-performance planar detectors. Bridgman growth more commonly exhibits higher dislocation densities, compositional banding, and localized precipitates/inclusions that degrade charge transport and create spatial nonuniformities; however, modern Bridgman variants and careful post-growth processing can mitigate many issues. Selection of growth method therefore directly impacts detector yield, uniformity, and ultimate spectroscopic performance, and must be weighed against throughput and cost constraints.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-does-crystal-growth-method-thm-or-bridgman-influence-defect-distribution-in-a-czt-planar-detector.html
