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Positron annihilation signals from VMCO-like samples grown by atomic layer deposition at different temperatures are utilized for the characterization of differences in open volume defects in TiN/TiO 2 /a-Si heterostructures. Doppler and coincidence Doppler mode of positron annihilation spectroscopy combined with a monoenergetic positron beam were used for this study. Differences observed in the Doppler parameters indicate differences in the positron trapping states of the TiO 2 epilayers grown at different temperatures. Furthermore, the coincidence-Doppler results show that these differences cannot be due to intermixing of the TiO 2 and a-Si layers and formation of thin SiO 2 layers at the interface during the growth process. The results indicate that the amount of open volume defects in the TiO 2 layer of the VMCO-structure seems to increase with an increase in the growth temperature. [ABSTRACT FROM AUTHOR]

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium–copper divacancy complex (VSe–VCu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at Ev + 0.98 eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the VSe–VCu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the VSe–VCu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices. [ABSTRACT FROM AUTHOR]

Positron annihilation spectroscopy in the Doppler and coincidence Doppler mode was applied on Ge 1 − x Sn x epitaxial layers, grown by chemical vapor deposition with different total As concentrations (∼ 10 19 – 10 21 cm − 3 ), high active As concentrations (∼ 10 19 cm − 3 ), and similar Sn concentrations (5.9%–6.4%). Positron traps are identified as mono-vacancy complexes. Vacancy-As complexes, V-As i , formed during the growth were studied to deepen the understanding of the electrical passivation of the Ge 1 − x Sn x :As epilayers. Larger mono-vacancy complexes, V-As i (i ≥ 2), are formed as the As doping increases. The total As concentration shows a significant impact on the saturation of the number of As atoms (i = 4) around the vacancies in the sample epilayers. The presence of V-As i complexes decreases the dopant activation in the Ge 1 − x Sn x :As epilayers. Furthermore, the presence of Sn failed to hinder the formation of larger V-As i complexes and thus failed to reduce the donor-deactivation. [ABSTRACT FROM AUTHOR]

We examined the vacancy trapping proficiency of Sn and P atoms in germanium using positron annihilation spectroscopy measurements, sensitive to the open-volume defects. Epitaxial Ge 1 − x Sn x films were grown by chemical vapor deposition with different P concentrations in the ∼ 3.0 × 10 19 – 1.5 × 10 20 cm − 3 range. We corroborate our findings with first principles simulations. Codoping of Ge with a Sn concentration of up to 9% is not an efficient method to suppress the free vacancy concentration and the formation of larger phosphorus–vacancy complexes. Experimental results confirm an increase in the number of P atoms around the monovacancy with P-doping, leading to dopant deactivation in epitaxial germanium–tin layers with similar Sn content. Vice versa, no impact on the improvement of maximum achieved P activation in Ge with increasing Sn-doping has been observed. Theoretical calculations also confirm that P n -V (vacancy) complexes are energetically more stable than the corresponding Sn m P n -V and Sn m -V defect structures with the same number of alien atoms (Sn or P) around the monovacancy. The strong attraction of vacancies to the phosphorus atoms remains the dominant dopant deactivation mechanism in Ge as well as in Ge 1 − x Sn x. [ABSTRACT FROM AUTHOR]

The E centers (dopant-vacancy pairs) play a significant role in dopant deactivation in semiconductors. In order to gain insight into dopant-defect interactions during epitaxial growth of in situ phosphorus doped Ge, positron annihilation spectroscopy, which is sensitive to open-volume defects, was performed on Ge layers grown by chemical vapor deposition with different concentrations of phosphorus (∼ 1 × 10 18 – 1 × 10 20 cm − 3 ). Experimental results supported by first-principles calculations based on the two component density-functional theory gave evidence for the existence of mono-vacancies decorated by several phosphorus atoms as the dominant defect type in the epitaxial Ge. The concentration of vacancies increases with the amount of P-doping. The number of P atoms around the vacancy also increases, depending on the P concentration. The evolution of P n –V clusters in Ge contributes significantly to the dopant deactivation. [ABSTRACT FROM AUTHOR]