Real time observation of tellurium inclusion drift and annealing in CdZnTe crystals under infrared laser irradiation

CdZnTe crystals are largely employed as substrates for IR devices, X-ray detectors, and electro-optic modulators. One of the main problems affecting device quality is the presence of tellurium inclusions. These form during crystal cooling due to the retrograde shape of the liquidus curve or are incorporated during growth. In fact, the maximum melting point is actually on the Te side of the phase diagram. Moreover, high resistivity is obtained by doping with In or Cl and by growing crystals from a Te deviated charge. Different approaches have been suggested for the elimination of tellurium inclusions by means of post growth treatments. One of them relies on the observation that when CdZnTe crystals are annealed in a temperature gradient, Te inclusions drift towards the higher temperature region, due to the fact that CdZnTe solubility in tellurium increases with temperature. However, it was reported by many authors that after thermal treatment high resistivity can't be reproducibly recovered. It was also shown that Te inclusions can experience thermomigration under the influence of a CO2 micron laser irradiation at 10.6 micron [1]. The effect of the laser irradiation was evaluated comparing samples before and after irradiation. Recently, our group has developed a new system for the 3D reconstruction of the position of tellurium inclusions in CdZnTe crystals that is based on IR microscopy [2]. In this work, we have studied the tellurium inclusion thermomigration induced by a pulsed Nd:YAG laser irradiation (1.064 micron). The inclusion movement was live-monitored in situ using our new IR microscopy system. First of all, with respect to the use of the CO2 laser, it should be noted that a much larger thermomigration speed was in general observed, without the need to heat the sample. Secondly, thanks to the in situ observation, it was possible to conclude that inclusions migrate at different speed according to their dimension. Moreover, inclusions separates into smaller parts and finally solve in the CdZnTe matrix. In conclusion, it was shown that laser irradiation at 1.064 micron is a powerful method to eliminate tellurium inclusions by CdZnTe crystals without heating the sample and that thanks to in situ monitoring it is possible to study the thermomigration dynamics and, thus, to tailor the process at best