Felix Law

Integration of β-FeSi2 with poly-Si on glass for thin-film photovoltaic applications

Aluminum-alloyed polycrystalline p-type β-phase iron disilicide p-β-FeSi2(Al) films with different thicknesses are successfully integrated with n-type polycrystalline silicon films on glass for thin-film solar cell applications. A sharp and high-quality interface is formed between 49 nm thick β-FeSi2(Al) and poly-Si, through the formation of a thin layer (∼7 nm) of Al-doped p+ epitaxial Si. The quality of the interface between poly-Si and p-β-FeSi2(Al) is found to degrade with increasing p-β-FeSi2(Al) thickness. An ∼5 nm thick amorphous layer is observed at the interface for the 145 nm thick p-β-FeSi2(Al) layer. The structural and photovoltaic characteristics of the p-type β-FeSi2/p+ Si/n− Si/n+ Si solar cell samples are investigated in detail. For a sample annealed at 650 °C, a one-Sun open-circuit voltage of 320 mV and pseudo fill factor of 67% are obtained, using an ∼49 nm p-type β-FeSi2 film on n-type poly-Si. The efficiency of the investigated solar cell structure decreases with increasing annealing temperature and thickness of the β-FeSi2 film.

Identification of geometrically necessary dislocations in solid phase crystallized poly-Si

In this work, the presence of geometrically necessary dislocations (GNDs) in polycrystalline silicon (poly-Si) thin films was detected, suggesting that plastic deformation occurs during the solid phase crystallization (SPC) process of amorphous silicon (a-Si:H). Electron backscatter diffraction was used to characterize dislocations in SPC poly-Si thin films. The elevated temperatures during SPC allow the GNDs to rearrange into arrays, forming low angle grain boundaries. We found that GNDs start forming in poly-Si grains with sizes >∼3u2009μm, suggesting that larger grains are more defective. GNDs are extra defects in addition to the existing statistically stored dislocations that form during grain growth and hence more care needs to be taken to minimize the formation of GNDs.

Study of large-grained n-type polycrystalline silicon thin films made by the solid phase crystallization method

N-type polycrystalline silicon (poly-Si) films with large grains exceeding 30 μm in width are successfully prepared by the solid phase crystallization (SPC) technique on glass through a control of the PH₃ gas flow rate. The grains in the poly-Si films are investigated by electron backscatter diffraction (EBSD) and are found to be randomly oriented, whereby the average grain size ranges from 4.3 to 18 μm. The grain size in the poly-Si film increases with increasing PH₃ gas flow rate. The impact of the PH₃ flow rate on the crystal quality and the electronic properties of the poly-Si thin-film solar cells are systematically investigated using UV reflectance and Hall effect measurements. A Hall mobility of about 71.5 cm²/Vs for n⁺ doped poly-Si films with a carrier concentration of 2.3×10¹⁹ cm^-3 is obtained.

Medium range order engineering in amorphous silicon thin films for solid phase crystallization

In recent years, it has been recognized that medium range ordering (MRO) in amorphous silicon (a-Si:H) plays a role in controlling its solid phase crystallization (SPC) behavior. Information on the MRO can be obtained from the width of the first X-ray diffraction (XRD) peak of a-Si:H centered around 2θu2009=u200927.5°. The broader the full width half maximum (FWHM) of the first XRD peak, the less ordered the a-Si:H material in the medium range length scale (up to 5u2009nm). In this work, it was found that the FWHM of the first XRD peak changes with the pressure used during the deposition of a-Si:H. A threshold SPC behavior was observed as a function of the a-Si:H deposition pressure and a good correlation between the SPC behavior and the a-Si:H XRD peak width was found. Results in this study indicate that higher MRO in a-Si:H led to faster SPC rates and smaller grain sizes, suggesting the presence of relatively active and high density of nucleation sites. High angle annular dark field scanning transmission electron m...

Static Large-Area Hydrogenation of Polycrystalline Silicon Thin-Film Solar Cells on Glass Using a Linear Microwave Plasma Source

Hydrogenation of polycrystalline silicon thin-film solar cells on glass is performed to improve the open-circuit voltage <i>V</i>_oc of the devices. The hydrogenation process is performed using linear microwave plasma sources that are capable of generating a uniform hydrogen-argon plasma over a large area. The substrate is fixed (i.e., does not move) during the hydrogenation process. The optical emission intensities from the hydrogen-argon plasma are recorded at two wavelengths using an optical emission spectroscopy system and are used to study the impact of several process parameters. The impact of these process parameters on the devices <i>V</i>_oc is also studied. We demonstrate that this plasma reactor is able to hydrogenate samples with a size of up to 400 cm². The uniformity of the hydrogenation process is evaluated by measuring the 1-sun <i>V</i>_oc with a suns-<i>V</i>_oc tester, giving voltage variations of less than ±3%. The highest average <i>V</i>_oc achieved in this study is 465 mV on a 10 cm × 10 cm textured sample and 428 mV on a 20 cm × 20 cm textured sample. In addition, secondary ion mass spectroscopy is used to measure the hydrogen concentration in the poly-Si films, giving an average concentration of about 6 × 10¹⁹ cm^-3.

Synthesis and characterization of large-grain solid-phase crystallized polycrystalline silicon thin films

n-type polycrystalline silicon (poly-Si) films with very large grains, exceeding 30u2009μm in width, and with high Hall mobility of about 71.5u2009cm2/V s are successfully prepared by the solid-phase crystallization technique on glass through the control of the PH3 (2% in H2)/SiH4 gas flow ratio. The effect of this gas flow ratio on the electronic and structural quality of the n-type poly-Si thin film is systematically investigated using Hall effect measurements, Raman microscopy, and electron backscatter diffraction (EBSD), respectively. The poly-Si grains are found to be randomly oriented, whereby the average area weighted grain size is found to increase from 4.3 to 18u2009μm with increase of the PH3 (2% in H2)/SiH4 gas flow ratio. The stress in the poly-Si thin films is found to increase above 900u2009MPa when the PH3 (2% in H2)/SiH4 gas flow ratio is increased from 0.025 to 0.45. Finally, high-resolution transmission electron microscopy, high angle annular dark field-scanning tunneling microscopy, and EBSD are used t...