Umicore magnifies substrate dimensions
Triple-junction solar cells for terrestrial concentrator systems are attracting an unprecedented level of interest. Trials in Spain are evaluating the performance of these assemblies for power generation to electrical grids, while cell manufacturers are seeking options to lower the technology s cost per kilowatt-hour.
Germanium substrates form the basis of all triple-junction cells, which feature individual germanium, GaAs and GaInP subcells connected in series. The standard diameter for these substrates is 100 mm, but a switch to 150 mm equivalents would enable cell manufacturers to cut the processing cost per die, as this can more than double the number of devices extracted from each wafer.
At Umicore Electro-Optic Materials, which is based in Olen, Belgium, we have started to respond to rising market demand through the development and sampling of 150 mm germanium substrates that are tailored to the needs of triple-junction solar cell manufacturers. These substrates, which we plan to launch in early 2009, are thin, have a low resistivity and feature epiready surfaces for the growth of high-quality epitaxial films by MOCVD.
Low resistivity is essential to limit the solar cell s series resistance in the high-current regime that occurs in concentrator photovoltaics, and thin substrates are needed to improve heat dissipation. Thinner substrates have the additional advantage that less material is needed, but this must be balanced against the substrate s strength and its adherence to well defined geometrical specifications.
We grow our dislocation-free monocrystalline germanium ingots by the Czochralski technique. A rotating seed crystal is pulled from a bath of molten, purified germanium, leading to the growth of material with a diameter of up to 300 mm.
Adding a gallium dopant into the melt provides the p-type resistivity required for making terrestrial cells. Good control of this process leads to 150 mm diameter ingots with a resistivity of 10 ± 4 mΩ cm over the entire ingot. Variations of the within-wafer resistivity, expressed as a radial dispersion, are less than 10%.
The 150 mm substrates that we produce from our monocrystalline germanium are formed by first rounding and aligning the ingot. After wire sawing, the substrates undergo edge and surface grinding, chemical etching, polishing and epicleaning to form a platform that s 200 µm thick.
The resulting substrates are slightly thicker than our 100 mm germanium, which is 140–180 µm thick. However, the penalty of using thicker material is offset by improvements in manufacturing efficiencies when processing larger ingots.
We characterize the thickness and geometry with a tool that maps the capacitance over the entire surface and converts this into a thickness profile. 150 mm substrates, with a center thickness of 200 µm, show comparable total thickness variation, bow and warp to 100 mm substrates with a thickness of 175 µm (figure 1). The additional thickness strengthens the substrate and gives it a wafer fracture force of 60 N.
Our 150 mm substrates have a surface that corresponds to a (100) plane off-orientated by 6° to the nearest (111) plane. This geometry, which has been confirmed by X-ray diffraction, aids the growth of epilayers that are free from anti-phase domains.
These substrates are polished on one side to create a surface with a very low defect density, according to laser light scattering measurements. The roughness on the epiready side is very similar to that of our 100 mm germanium, which ensures its suitability for high-quality MOCVD epitaxy.
We are now focusing on improving our 150 mm manufacturing process throughput, which will cut production costs. We are also looking at surface contamination levels, in terms of defects and metals, and the impact that they could have on cell performance. We believe that we are also well placed for any demand for even larger diameters, such as 200 or 300 mm germanium substrates.
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