Epitaxial growth of Si(Ge) materials on Si and GaAs by low temperature PECVD: towards tandem devices

Abstract : This thesis focuses on epitaxial growth of Si and SiGe at low temperature (200°C) by Plasma Enhanced Chemical Vapor Deposition (PECVD), and its application in thin film crystalline solar cells. Our goal is to gain insight into this unusual growth process, as well as to investigate the potential of such low temperaturedeposited material for single and multi-junction solar cells. First, we have proposed a one pump-down plasma process to clean out-of-the-box c-Si wafer surface and grow epitaxial layers of up to 8µm thick, without ultra-high vacuum, in a standard RF-PECVD reactor. By exploring the experimental parameters space, the link between layer quality and important physical variables, such as silane dilution, ion energy, or deposition pressure, has been confirmed. Both material and electrical properties were analyzed, and we found that epitaxial quality improves with film thickness. Furthermore, we could bring evidence of SiGe and Ge epitaxial growth under similar conditions. Then, with the whole process steps <200°C, we have achieved PIN heterojunction solar cells on highly doped substrates with 1-4µm epitaxial absorber, reaching 8.8% efficiency (without light trapping) and 80.5% FF. Replacing Si absorber by epitaxial Si0:73Ge0:27 resulted in 11% boost in Jsc. The use of an engineered wafer/epitaxial layer interface and stress enables easy lift-off: e.g. we successfully bonded 1.5µm thick 10cm^2 epi-Si to glass. Additionally, we have considered the impact of photonic nanostructures on device properties. Together, the control of growth, transfer and advanced light trapping are paving the way toward highly efficient, ultrathin (<10µm) and low cost c-Si cells. Finally, in contrast with general trend of growing III-V semiconductors on Si, we have studied the hetero-epitaxial growth of Si on III-V. Good crystal quality was achieved by direct Si deposition on GaAs, thanks to reduced thermal load and suppressed polarity issues in this approach. Using MOCVD, we could build GaAs cells with 20% efficiency and III-V tunnel junctions reaching 55A/cm^2. Tunneling improvement upon H-plasma exposure was shown. Those results, combined with III-V layer lift-off, validate milestones toward high efficiency tandem AlGaAs(MOVD)/SiGe(PECVD) metamorphic solar cells.
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Romain Cariou. Epitaxial growth of Si(Ge) materials on Si and GaAs by low temperature PECVD: towards tandem devices. Physics [physics]. Ecole Polytechnique, 2014. English. ⟨tel-01113794⟩

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