Direct Observation of Group-V Dopant Substitutional Defects in CdTe Single Crystals.

Point defect chemistry strongly affects the fundamental properties of materials and has a decisive impact on device performance. The Group-V dopant is prominent acceptor species with high hole concentration in CdTe; however, its local atomic structure is still not clear owing to difficulties in definitive measurements and discrepancies between experimental observations and theoretical models. Herein, we report on direct observation of the local structure for the As dopant in CdTe single crystals by the X-ray fluorescence holography (XFH) technique, which is a powerful tool to visualize three-dimensional atomic configurations around a specific element. The XFH result shows the As substituting on both Cd (AsCd) and Te (AsTe) sites. Although AsTe has been well known as a shallow acceptor, AsCd has not attracted much attention and been discussed so far. Our results provide new insights into point defects by expanding the experimental XFH study in combination with theoretical first-principles studies in II-VI semiconductors.

General Model for Defect Dynamics in Ionizing-Irradiated SiO2 -Si Structures.

Irradiation damage is a key issue for the reliability of semiconductor devices under extreme environments. For decades, the ionizing-irradiation-induced damage in transistors with silica-silicon (SiO2 -Si) structures at room temperature has been modeled by a uniform generation of E'γ centers in the bulk silica region through the capture of irradiation-induced holes, and an irreversible conversion from E'γ to Pb centers at the SiO2 /Si interface through reactions with hydrogen molecules (H2 ). However, the traditional model fails to explain experimentally-observed dose dependence of the defect concentrations, especially at low dose rate. Here, it is proposed that the generation of E'γ centers is decelerated because the holes migrate dispersively in disordered silica and the diffusion coefficient decays as the irradiation goes on. It is also proposed that the conversion between E'γ and Pb centers is reversible because the huge activation energy of the reverse reaction can be reduced by a "phonon-kick" effect of the vibrational energy of H2 and Pb centers transferred from nearby nonradiative recombination centers. Experimental studies are carried out to demonstrate that the derived analytic model based on these two new concepts can consistently explain the fundamental but puzzling dose dependence of the defect concentrations for an extremely wide dose rate range.

Doping Limits of Phosphorus, Arsenic, and Antimony in CdTe.

Low p-type doping is a limiting factor to increase CdTe thin-film solar-cell efficiency toward the theoretical Shockley-Queisser limit of 33%. Previous calculations predict relatively high ionization energies for group-V acceptors (P, As, and Sb), and they are plagued by self-compensation, forming AX centers, severely limiting hole concentration. However, recent experiments on CdTe single crystals indicate a much more favorable scenario, where P, As, and Sb behave as shallow acceptors. Using hybrid functional calculations, we solve this puzzle by showing that the ionization energies significantly decrease with the supercell size. When including the effects of spin-orbit coupling and extrapolating the results to the dilute limit, we find these impurities behave as hydrogenic-like shallow acceptors, and AX centers are unstable and do not limit p-type doping. We address the differences between our results and previous theoretical predictions and show that our ionization energies predict hole concentrations that agree with recent temperature-dependent Hall measurements.