A. Le Donne

About the D1 and d2 dislocation luminescence and its correlation with oxygen segregation

It is well known that the dislocation luminescence (DL) in silicon consists of four main bands, conventionally labelled D1 to D4, where E 1 = 0.807 eV, E 2 = 0.874 eV, E 3 = 0.95 eV and E 4 = 0.99 eV, of which the D1 is considered of interest for optoelectronic devices working in the third window of optical communications. Although DL has been the subject of a number of investigations in the last twenty years, still some questions remain open, concerning both the origin of the dislocation luminescence and its intrinsic or extrinsic nature. We report in this paper the results of a combination of complementary dislocation generation processes (deformation and oxide segregation) and characterisation procedures (photoluminescence and surface photovoltage), which give a strong evidence that the Dl band is correlated in a very complex manner with the presence of optically active silicon self-interstitials and oxygen clusters.

Dislocation luminescence in nitrogen-doped Czochralski and float zone silicon

This paper reports the results of a study of the effect of nitrogen on the optical properties of dislocations in nitrogen-doped Czochralski and nitrogen-doped float zone silicon samples where the nitrogen doping was carried out by adding Si3N4 in the molten silicon charge or by nitrogen gas dissolution. Dislocations were introduced by plastic deformation at 650?C. In nitrogen-doped plastically deformed samples, emissions in the range of the D1?D4 bands of dislocations are present with a significant shifting from the energies and intensities of the corresponding bands in nitrogen-free samples. It has been shown that the main effect of nitrogen could be the enhancement of the oxygen precipitation. The results confirm the suggestion of some of the present authors that luminescence at 0.830 eV is associated with some intrinsic properties of oxygen precipitates.

Crystallinity and microstructure in Si films grown by plasma-enhanced chemical vapor deposition: A simple atomic-scale model validated by experiments

A joint theoretical and experimental analysis of the crystalline fraction in nanocrystalline films grown by low-energy plasma enhanced chemical vapor deposition is presented. The effect of key growth parameters such as temperature, silane flux, and hydrogen dilution ratio is analyzed and modeled at the atomic scale, introducing an environment-dependent crystallization probability. A very good agreement between experiments and theory is found, despite the use of a single fitting parameter.

Key Success Factors and Future Perspective of Silicon-Based Solar Cells

Today, after more than 70 years of continued progress on silicon technology, about 85% of cumulative installed photovolatic (PV) modules are based on crystalline silicon (c-Si). PV devices based on silicon are the most common solar cells currently being produced, and it is mainly due to silicon technology that the PV has grown by 40% per year over the last decade. An additional step in the silicon solar cell development is ongoing, and it is related to a further efficiency improvement through defect control, device optimization, surface modification, and nanotechnology approaches. This paper attempts to briefly review the most important advances and current technologies used to produce crystalline silicon solar devices and in the meantime the most challenging and promising strategies acting to increase the efficiency to cost/ratio of silicon solar cells. Eventually, the impact and the potentiality of using a nanotechnology approach in a silicon-based solar cell are also described.

Optical spectroscopy study of type 1 natural and synthetic sapphires

Low iron, pale blue natural and synthetic sapphire samples were studied by low temperature absorption and luminescence spectroscopy. For comparison, a bicolour pink and blue corundum from Vietnam was also considered. From radioluminescence and photoluminescence excitation spectra of both Cr3+ and Ti3+, experimental evidence was obtained for attributing an absorption band at 17 500 cm−1, currently interpreted as intervalence charge transfer, to overlapping crystal field transitions of Cr3+ and Ti3+. An important role was also proposed for Cr2+; indeed, it is possible to propose that the colour of pale blue sapphires is mainly determined by chromium in its two valence states while Ti3+ and Fe3+ have a minor role.

Effects of CdS buffer layers on photoluminescence properties of Cu2ZnSnS4 solar cells

Cu2ZnSnS4 (CZTS) absorber layers grown by sputtering were investigated by photoluminescence before and after the chemical bath deposition of CdS in order to evaluate the possible passivation of point defects by Cd atoms at the absorber/buffer layer interface. According to the literature, a broad emission around 1.21 eV was observed at low temperature under above bandgap excitation of the as-grown CZTS samples. Broad bands at 1.075 eV and 0.85 eV were detected for the first time under below bandgap excitation of the as-grown CZTS samples at low temperature, which were explained in terms of radiative transitions involving point defect-related levels determined in the literature by first-principles calculations. The emissions observed in the as-grown samples were monitored by both above and below bandgap excitations also in standard CZTS solar cells produced on the same layers. The obtained results suggest that, as in the case of Cu(In, Ga)Se2, Cd atoms passivate point defects at the absorber/buffer layer interface also in CZTS.

Structural Homogeneity of nc-Si Films Grown by Low-Energy PECVD

Nanocrystalline silicon is considered one of the most promising materials for thin-film solar cells. For such an application, one of the critical issues yet unsolved is to obtain a good structural uniformity along the film growth direction to yield high fill factors and open-circuit voltages. In this article, Raman spectroscopy was used to obtain crystallinity in-depth profiles of samples grown at a high growth rate by low-energy plasma-enhanced chemical vapor deposition (PECVD) using different SiH 4 and H 2 fluxes, all yielding the same dilution factor. The results showed that the total flow rate strongly affects the structural uniformity of nanocrystalline silicon films.

Optical properties of shuffle dislocations in silicon

The radiative recombination processes in dislocated float zone silicon samples deformed under gigapascal stresses were studied by photoluminescence (PL) spectroscopy. The observed shuffle dislocations present a reconstructed core and their generation is accompanied by the introduction of point defects and point defect clusters, whose signature is evident in the PL spectra. A broad band around 1eV is the only PL feature which could be directly related to shuffle dislocations and it is explained conjecturing strain field induced gap changes, as confirmed by molecular dynamics simulations.

Fabricating Cu(In,Ga)Se2 solar cells on flexible substrates by a new roll-to-roll deposition system suitable for industrial applications

In this work, a new hybrid sputtering–evaporation system providing a scalable process for deposition of Cu(In,Ga)Se2 (CIGS) layers is presented. The growth apparatus has been designed and realized to fit a size suitable for direct industrial transfer. In this process the metal precursors are first of all sputtered on rotating transfer devices, then evaporated on the substrate by local heating in a Se atmosphere. The desired thickness and composition of the CIGS film are obtained by repeated sputtering–evaporation cycles. The cylindrical geometry of the deposition chamber has been designed to accommodate different types of flexible substrates with a maximum size of 20 × 120 cm2 in a roll-to-roll configuration. Several techniques, including secondary ion mass spectrometry, Raman and photoluminescence spectroscopies, x-ray diffraction, scanning electron microscopy, external quantum efficiency, and I–V under 1 Sun illumination, have been used to test both the as-grown CIGS layers and the solar cell devices based on them. A significant performance and good control of Ga grading and Na content were obtained for solar cells grown at 450 °C on polyimide substrates with high deposition rates. In spite of the fact that the present efficiency record for CIGS solar cells on polyimide substrates is 20.4%, the 10.1% obtained using the hybrid method presented in this work is significant because the growth apparatus meets the requirements for direct industrial transfer. In fact, this process is being transferred in a 1 MW production line, where standard CIGS layers are deposited at low temperature on flexible substrates in a single-step process with a 1 mm sec−1 substrate velocity.

Diffusion length and junction spectroscopy analysis of low-temperature annealing of electron irradiation-induced deep levels in 4H-SiC

The effects of low-temperature annealing in 8.2MeV electron-irradiated 4H-SiC Schottky diodes were investigated. Deep-level transient spectroscopy and minority-carrier diffusion length (Ld) measurements were carried out on not-irradiated samples and on irradiated samples before and after thermal treatments up to T=450°C. We found that several deep levels in the upper half band gap (S1 with enthalpy ET=0.27eV, S2 with ET=0.35eV, S4 with ET=0.71eV, and S5 with ET=0.96eV) anneal out or modify at temperature values lower or equal to T=450°C, whereby their progressive annealing out is accompanied by a net increase of Ld, up to 50% of the value in the as-irradiated sample. We drew some conclusions regarding the microscopic nature of the defects related to the deep levels, according to their annealing behavior.