Multimodal characterization of Te inclusions in Cd1-xZnxTe and Cd1-xZnxTe1-ySey for gamma and X-ray detectors

Abstract

While CdZnTe (CZT) and CdZnTeSe (CZTS) semiconductors have emerged as compounds for room-temperature gamma and X-ray detection materials, they continue to be constrained by the formation of Te-inclusion defects generated during the growth and post-growth phases of the material, which adversely affect the detector performance. We demonstrate the utility of multimodal microscopic imaging and analysis for the characterization of the optical and electronic properties of Te inclusions in CZT and CZTS crystals at both micron and nanometer length scales. Having first identified regions with micron-scale Te inclusions using confocal Raman microscopy techniques, optically coupled infrared scattering near-field optical microscopic mapping was performed to map the distribution of these inclusions with nanometer spatial resolution and correlate the presence of Te inclusions in the matrix with other properties. Kelvin probe force microscopy was then utilized to characterize the variations of the work function associated with the presence of Te inclusions. Here, we observe an increase of ~ 240 mV in the work function associated with Te inclusions compared to the bulk CZT/CZTS crystals. Additionally, we observe that individual bulk grains in CZT can exhibit slight potential variations. Our findings develop a portrait of the charge trapping mechanisms in CZT and CZTS that act to degrade detector performance, while the demonstration of these combined microscopy techniques provides a new analytical tool that can be utilized for further optimization of the detector performance for these semiconducting compounds.

Fig. 1

Multimodal imaging of a single Te inclusion within a bulk Cd_0.9Zn_0.1Te_0.985Se_0.015 crystal. (a) Optical microscopy image with correlated (b) Raman spectral mapping can be utilized to image large inclusions with a diffraction limited resolution of several hundred nm. (c) Nanometer-scale resolution scanning probe imaging techniques can then be employed using conventional AFM, which maps the topography of the Te inclusion (a). (d) Optical coupled IR s-SNOM imaging can be applied to observe nanoscale variations of the inclusion’s infrared optical response, while electrical (e) KPFM imaging can be used to relate information on the relative variations of the surface potential associated with the presence of a Te inclusion. The scale bar is 5 μm for all images.

Fig. 2

micro-Raman mapping and spectroscopy of Te inclusion embedded within bulk CZT crystal. (a) Optical microscopy image with correlated (b) line-trace of the Raman response over an individual Te inclusion showing the intensity variation and spectral shifts of the Raman response. The location of the line trace is marked by green dashed line in panel (a). A comparison of the Raman spectra measured at (c) a Te inclusion (position marked by the red circle in (a) and (d)) and the bulk CZT (position marked by black circle in (a)). Integrating the Raman response over a spectral range spanning from 110 to 150 cm⁻¹, the intensity variation of the Raman response increases by a factor of ~ 6 × at the Te inclusion with respect to the bulk CZT.

Fig. 3

IR s-SNOM mappings of Te inclusions embedded within bulk CZT crystals. (a, b) Topography and associated s-SNOM scattering amplitude images recorded over a large hexagonal Te inclusion embedded in a bulk CZT crystal. Panels (c, d) and (e–g) demonstrate imaging of the spatial correlation of the presence of Te inclusions with grain boundaries (blue-dashed line). (c, d) shows topography and s-SNOM amplitude of a Te inclusion located at a sub-grain boundary. Panels (e–g) show topography, s-SNOM amplitude, and s-SNOM phase of a series of Te inclusions located along a sub-grain boundary (blue dashed line) in a bulk CZT crystal. All IR s-SNOM data were recorded using lock-in filtering at the 3^rd harmonic of the AFM dither frequency ().

Fig. 4

KPFM mapping image of a hexagonal Te inclusion within a bulk CZTS crystal. (a, b) Topography and KPFM potential images, respectively. (c) Line trace showing the spatial variation of the CPD as the location denoted by the dashed white line in panels (a, b). The dashed blue line displays an exponential fit of band bending in the bulk crystal in proximity to the inclusion. (d) Illustration of the band bending induced carrier funneling to Te-inclusion trap sites.

Fig. 5

(a) Topography and corresponding (b) KPFM imaging of surface potential changes of Te inclusion defects located along a sub-grain boundary located in a bulk CZT crystal. Zoom in (c) topography and (d) KPFM scans of the area marked by the white box in (a, b), illustrating the surface potential change, which occurs at the sub-grain boundary (blue dashed line). (e) Line-trace of the variation of the surface height and measured KPFM contact potential difference along the white line shown in (c, d). Moving from the grain on the left to the grain on the right, the surface topography varies by only a few nanometers while the surface potential increases by ~ 85 mV.