Yaser Abdulraheem

Optical bandgap of ultra-thin amorphous silicon films deposited on crystalline silicon by PECVD

An optical study based on spectroscopic ellipsometry, performed on ultrathin hydrogenated amorphous silicon (a-Si:H) layers, is presented in this work. Ultrathin layers of intrinsic amorphous silicon have been deposited on n-type mono-crystalline silicon (c-Si) wafers by plasma enhanced chemical vapor deposition (PECVD). The layer thicknesses along with their optical properties –including their refractive index and optical loss- were characterized by spectroscopic ellipsometry (SE) in a wavelength range from 250 nm to 850 nm. The data was fitted to a Tauc-Lorentz optical model and the fitting parameters were extracted and used to compute the refractive index, extinction coefficient and optical bandgap. Furthermore, the a-Si:H film grown on silicon was etched at a controlled rate using a TMAH solution prepared at room temperature. The optical properties along with the Tauc-Lorentz fitting parameters were extracted from the model as the film thickness was reduced. The etch rate for ultrathin a-Si:H layers i...

Heterojunction Interdigitated Back-Contact Solar Cells Fabricated on Wafer Bonded to Glass

Future wafer-based silicon solar cells will be fabricated on thin (<;140 μm) wafers. However, technologies to handle thin wafers during cell processing are not yet available for industry. In this paper, a flow to handle thin wafers during rear side cell processing is developed and demonstrated on 4-in 200 μm-thick wafers. The flow involves bonding the wafers to glass after front-side processing followed by a low-temperature p-n heterojunction formation on the rear side. 2.5 × 2.5 cm2 amorphous/crystalline silicon heterojunction interdigitated back-contact solar cells are fabricated by use of lithography while bonded to glass, and they show an efficiency of up to 17.7%. Shunts, infrared light absorption, and rear side interface passivation are identified as the main efficiency losses. Dedicated experiments suggest that the passivation losses are related to the degradation of the adhesive during wafer cleaning. Hence, methods to improve the compatibility of the adhesive with the cleaning process are discussed.

Aggregation Behavior of Titanium Dioxide Nanoparticles in Aqueous Environments

We report on the stability of titanium dioxide nanoparticles suspended in various aqueous solutions with and without the presence of a model organic contaminant—salicylic acid. The stability of the suspended nanoparticles was revealed by measuring the isoelectric point by means of zetapotential scans and measuring the growth kinetics of the hydrodynamic particle size by means of dynamic light scattering (DLS) under different pH conditions. Aggregated nanoparticle sizes were also examined using atomic force microscopy. It was found that salicylic acid had a dramatic impact on the aggregation behavior of TiO2 nanoparticles, suggesting that salicylic acid adsorbs to TiO2 nanoparticles surfaces.

Morphology, electrical, and optical properties of heavily doped ZnTe:Cu thin films

We report on a study of the physical properties of ZnTe:Cu films with Cu content up to ∼12 at. % prepared using rf magnetron sputtering. The composition and lateral homogeneities are studied using X-ray photoelectron spectroscopy (XPS). Atomic force microscopy measurements on films deposited at different substrate temperatures (up to 325 °C) yielded activation energy of 12 kJ/mole for the grains growth. The results of XPS and electrical and optical measurements provide evidence for the formation of the ternary zinc copper telluride alloy in films containing Cu concentration above ∼4 at. %. The XPS results suggest that copper is incorporated in the alloy with oxidation state Cu1+ so that the alloy formula can be written Zn1−yCuy Te with y = 2−x, where x is a parameter measuring the stoichiometry in the Cu site. The formation of this alloy causes appreciable shift in the binding energies of the XPS peaks besides an IR shift in the energy band gap. Detailed analysis of the optical absorption data revealed th...

The effects of iron as a dopant on the dielectric properties of ferroelectric potassium tantalate niobate (KTaxNb1−xO3)

Presented here is a preliminary study on the effects of iron as a dopant in potassium tantalate niobate (KTaxNb1−xO3) (KTN). Crystals of high purity were grown using a top-seeded isothermal method with starting materials that were purified ensuring that the impurity content was controlled down to less than 10ppm. The study included introducing iron to KTN single crystals of high purity both during crystal growth and through thermal diffusion after growth. Dielectric measurements versus temperature were used to reveal the origins of the ferroelectric phase transition in the KTN:Fe system. Large shifts in the Curie temperature have been shown to be a direct consequence of the presence of iron impurities in KTN. The conventional theories of ferroelectricity do not explain this effect; however, a model is proposed to explain the observed behavior. The model is based on the concept of “polar microregions” proposed previously by Bidault and Maglione [J. Phys. I France 7, 543 (1997)] and others to explain ferroe...

Module-level cell processing of silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells with efficiencies above 22%: Towards all-dry processing

Module-level processing of silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells while bonded to glass, in the so-called i2-module concept, is discussed. In this approach, a key challenge is the interdigitated patterning of a-Si:H without compromising on the rear-surface passivation. Process adaptations involving more resistant bonding agents and a milder wet etchant enabled bonded cells with the best efficiency of 21.7% and Voc of 734 mV, which are the highest reported for bonded cells processed partially at module level, and which undoubtedly proves the potential of this i2-module concept to reach high Voc and efficiency. Yet another challenge is to make the process flow cost-effective and industrially-relevant, i.e. a litho-free, all-dry process flow, analogous to thin-film PV module fabrication. As a first step, dry etching of a-Si:H was developed to replace wet etching, and was successfully incorporated in a SHJ-IBC process flow to fabricate freestanding cells with the best efficiency of 22.9% and above 20% on thick (190 μm) and thin (56 μm) EVA-bonded silicon.

The Effect of Annealing on the Structural and Optical Properties of Titanium Dioxide Films Deposited by Electron Beam Assisted PVD

Titanium dioxide thin films were deposited on crystalline silicon substrates by electron beam physical vapor deposition. The deposition was performed under vacuum ranging from 10−5 to 10−6 Torr without process gases, resulting in homogeneous layers with a thickness of around 100 nm. Samples were then annealed at high temperatures ranging from C to C for 4 hours under nitrogen, and their structural and optical properties along with their chemical structure were characterized before and after annealing. The chemical and structural characterization revealed a substoichiometric film with oxygen vacancies, voids, and an interface oxide layer. It was found from X-ray diffraction that the deposited films were amorphous and crystallization to anatase phase occurred for annealed samples and was more pronounced for annealing temperatures above C. The refractive index obtained through spectroscopic ellipsometry ranged between 2.09 and 2.37 in the wavelength range, 900 nm to 400 nm for the as-deposited sample, and jumped to the range between 2.23 and 2.65 for samples annealed at C. The minimum surface reflectance changed from around 0.6% for the as-deposited samples to 2.5% for the samples annealed at C.

Simplified Cleaning for a-Si:H Passivation of Wafers Bonded to Glass

Silicon photovoltaic (PV) roadmaps indicate the reduction of wafer thicknesses and the need for innovation in wafering method and cell processing. Within this framework, Imec proposes the i2-module device [1], i.e. an heterojunction interdigitated back-contact (HJ i-BC) solar module [2] processed on 40-μm thick epitaxial wafers bonded to carriers by means of silicone. In the i2-module concept, the Rear Side (RS) of the solar cell is passivated while the wafer is bonded to the module glass and the influence of the silicone on the passivation process is reduced by an O2 plasma realized in an Reactive Ion Etching (RIE) chamber [3]. In this contribution, the effect of different post-bonding cleaning sequences on the passivation of wafers/silicone/glass stacks treated with an O2 plasma is investigated and a simplified post-bonding cleaning sequence leading to state-of-the-art passivation is proposed.