Cadmium magnesium telluride (CdMgTe), as a group Ⅱ-Ⅵ CdTe-based compound semiconductor, can be used in room-temperature radiation detection. However, CdMgTe crystals grown by melt methods often exhibit major defects like vacancies, dislocations, twins, and inclusions, severely impacting the crystal quality and detector performance. Some studies show that selenium (Se) can significantly reduce harmful defects like sub-boundary networks and Te inclusions in CdTe-based crystals, enhancing the crystal quality through its favorable segregation coefficient and intense solid-solution hardening effect. Therefore, in this paper, Se was introduced into CdMgTe crystals via vapor phase diffusion as an advanced annealing strategy to enhance the both crystal quality and detector performance. In addition, the effect of annealing time on the crystal defects, microhardness, optical and electrical properties, and detector performance was also investigated.

The annealing source was a high-purity Se (7N), and the slices with the dimensions of 5 mm×5 mm×2 mm were selected from an In-doped CdMgTe ingot grown by a modified vertical Bridgman method under Cd excess condition. Prior to annealing, the slices were mechanically polished with an MgO suspension and then treated with a 2% Br2-MeOH solution to eliminate scratches and any damaged layers. The slices and the source were positioned at opposite ends of a quartz annealing device, with quartz crucibles evacuated to a pressure of 10^–5 Pa. The annealing temperature of both slice and source was selected at 773 K. Effect of annealing time on CdMgTe crystal properties was investigated at different holding time (i.e., 30, 60, 120 h, and 240 h). CdMgTe planar room-temperature radiation detectors were fabricated via evaporating Au electrodes onto the both sides of the annealed crystals.

CdMgTe crystals before and after annealing in Se atmosphere were characterized by near-infrared spectrometry (NIR), X-ray photoelectron spectroscopy (XPS), infrared transmission microscopy (IRTM), field emission scanning electron microscopy (FESEM), infrared (IR) spectrometry, Raman spectrometry, and current-voltage (I–V) measurement. Room-temperature radiation detectors were prepared by evaporation of Au electrodes on both sides of CdMgTe crystals. The carrier mobility was determined by a time-of-flight (TOF) technology. The energy spectrum of the detectors was acquired by an ORTEC measurement system with uncollimated ²⁴¹Am α particles with an energy of 5.48 MeV as an irradiation source.

The band gap of the as-grown crystal is 1.505 eV, and annealing in Se atmosphere does not prevent the volatilization of Cd in CdMgTe crystals or reduce Te inclusions. After annealing, the maximum increase of the band gap is 0.042 eV. Se interacts with CdMgTe matrix via diffusion and chemisorption, forming chemical bonds that effectively result in the partial substitution of Se for Te., The solution strengthening effect of Se is more pronounced than that of Mg as the annealing time increases, causing that the microhardness initially decreases and then increases, which is lower than that of the as-grown crystal. The IR transmittance of the crystal after 120 h annealing is 63.6%, which approaches the theoretical value of 65%, thus satisfying a criterion for high-quality crystals. The peak intensity of A₁(Te) is basically unchanged based on the Raman spectra. However, an increase in the peak intensity of TO(CdTe) and a decrease in that of LO(CdTe) indicate that the lattice integrity in the annealed crystal is compromised.

CdMgTe crystal obtained under optimal annealing conditions is fabricated into an Au/CdMgTe/Au planar structure for the room-temperature radiation detector. TOF measurements show a remarkable increase in electron mobility (μe), up to 344.47 cm²/(V·s). ²⁴¹Am α particle source with an energy of 5.48 MeV is used for detector energy spectrum testing. The optimal CdMgTe detector exhibits the performance with an energy resolution of 17.2% and a carrier mobility lifetime product (μτ)_e of 1.32×10^–4 cm²/V. Annealing in a Se atmosphere can thus offer an effective approach to enhance the quality of CdTe-based crystals.

An advanced annealing strategy was adopted for enhancing the quality of CdMgTe single crystals for room-temperature radiation detectors. After annealing, a decrease in Cd content along with an increase in Mg content resulted in an expanded band gap. The microhardness and the IR transmittance of the crystals firstly decreased and then increased, and the resistivity gradually decreased as the annealing time increased, thus improving the crystal quality. The optimum condition of Se atmosphere annealing was at 773 K for 120 h. The optimal performance of the detector was an electron mobility (μe) of 344.47 cm²/(V·s), an energy resolution of 17.2%, and a (μτ)_e product of 1.32×10^–4 cm²/V for ²⁴¹Am α particles with an energy of 5.48 MeV.

Cadmium magnesium telluride (CdMgTe) crystal is an ideal material for room-temperature radiation detection. However, there are still many defects in as-grown crystals, which degrade the detector performance. At present, a single annealing method is unable to achieve the effect of both eliminating defects and maintaining or increasing resistivity. The multi-step combined annealing method can improve the crystal quality and the detector performance. There are a few studies on the annealing of CdMgTe crystals. In this work, a multi-step combined annealing method was used, i.e., the CdMgTe crystals grown under Te-rich condition were annealed in Cd atmosphere and Te atmosphere. The effect of annealing temperature on the defects, mechanics, photoelectric properties and detector performance of CdMgTe crystals was investigated.

High purity (7N) cadmium (Cd) and tellurium (Te) were selected as raw materials. The selected slices for annealing were derived from In-doped Cd_0.95Mg_0.05Te ingot grown by a modified vertical Bridgman method under Te rich condition, with the size of 5 mm×5 mm×2 mm. The annealing in Cd atmosphere was mainly divided into two steps, i.e., high and low temperature annealing. In the high temperature annealing step, the temperatures of the slices were 873, 923 K and 973 K, respectively. The corresponding temperatures of Cd source were 901, 946 K and 986 K, respectively. The annealing time was 120 h. After high temperature annealing, the temperature of the slices and Cd source reduced to 623 K, and the annealing time was also 120 h. After annealing in Cd atmosphere, it continued annealing in Te atmosphere. The annealing temperature of both slices and Te source was selected to 773 K, and the annealing time was also 120 h.

The CdMgTe crystals before and after annealing were characterized by infrared transmission microscopy, scanning electron microscopy, ultraviolet-visible-near infrared (NIR) spectroscopy, infrared transmittance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Vickers indentation analysis and current–voltage (I–V) measurement. Au/CdMgTe/Au planar detectors were fabricated by an evaporation method, and the energy spectrum was collected in ORTEC test system. The radiation source was ²⁴¹Am α particle source with a non-collimated energy of 5.48 MeV.

The band gap of the as-grown crystal is 1.510 eV, and the maximum band gap is increased by 0.021 eV after annealing. After annealing in Cd atmosphere, Te inclusions in the crystals are greatly reduced. At the annealing temperature of 973 K, Te inclusions can be completely eliminated. The IR transmittance and resistivity are reduced. The density of Te inclusions has little change after annealing in Te atmosphere. However, the IR transmittance and resistivity improve and are better than those of the as-grown crystals. The optimum IR transmittance and resistivity are 63% and 2.41×10¹⁰Ω·cm, respectively. After multi-step combined annealing, there is no extra oxidation on the crystal surface, and the microhardness is increased by 16%. The Raman spectra show that the crystal quality is improved.

Au/CdMgTe/Au planar room temperature radiation detectors are fabricated with the crystals obtained under the optimum annealing conditions. The time-of-flight (TOF) measurements show a significant increase in electron mobility, up to 526.86 cm²/(V·s). The optimum energy resolution and electron mobility lifetime product of the detector are 15.6% and 1.89×10^–4 cm²/V, respectively, using ²⁴¹Am α particles with an energy of 5.48 MeV as a radiation source.

CdMgTe crystals were annealed by a multi-step combined method. After annealing, there was no additional oxidation on the crystal surface, and Te inclusions decreased significantly. The band gap, microhardness, IR transmittance and resistivity of the crystals were all improved, indicating the improvement of crystal quality. The optimum annealing conditions were in Cd atmosphere at high (i.e., 923 K) and low (623 K) temperatures for 120 h, and then in Te atmosphere at a low (i.e., 623 K) temperature for 120 h. The optimum performance of CdMgTe detector was electron mobility (μ_e) of 526.86 cm²/(V·s), the energy resolution of 15.6%, and the mobility lifetime product (μ_τ)_e of 1.89×10^–4 cm²/V.