Page - 64 - in Photovoltaic Materials and Electronic Devices

large areas. Doped (n-type and p-type) and undoped (intrinsic) hydrogenated silicon (Si:H) thin films are used in single, tandem, and multijunction solar cell applications inbothn-i-psubstrateandp-i-nsuperstrateconfigurations [5–10]. TheseSi:Hfilms prepared by plasma enhanced chemical vapor deposition (PECVD) may exhibit several structural transitions during growth in the PV device configuration. The structuralevolutionofSi:Hcanbecontrolledbydilutionofareactivesiliconcarrying gases likesilane (SiH4)withhydrogen(H2)duringdeposition. Themicrostructural evolution of Si:H has been studied for layers deposited on different bulk and thin film substrates with varying degrees of surface roughness, including native and thermal oxide coated crystalline silicon, glass, polyethylene naphthalate (PEN) polymer, as well as underlying structurally distinct Si:H layers prepared under different deposition conditions. A primary technique for studying this growth evolution is the use of near infrared (IR) to ultraviolet (UV) in situ, real time spectroscopic ellipsometry (RTSE) applied during Si:H thin film growth [2,4,7,11,12]. RTSE involves collection of ellipsometric spectra as a function of time during a process such as thin film deposition, typically using a multichannel instrument whichcollectsallphotonenergies inparallelwithserial readoutofpixelsarranged in a one dimensional (1-D) detector. The data acquisition time is typically chosen such that highest signal-to-noise is obtained via averaging of multiple optical cycles over a period of time in which typically only 0.1 to 10 Å of material accumulates. Koh et al. [7] reported the growth evolution of intrinsic Si:H on native oxide covered crystalline silicon, amorphous Si:H (a-Si:H) films prepared without additional hydrogen dilution, and on newly deposited ~200 Å p-type microcrystalline or nanocrystalline Si:H (nc-Si:H). These results demonstrate that the nature of the underlying material influences nucleation of crystallites, suppressing nucleation with underlying a-Si:H and promoting nucleation with underlying nc-Si:H. The growthevolutionofp-layersonspecularzincoxide(ZnO)coatedglassandtheability to promote a high nucleation density of nc-Si:H were studied by Rovira et al. [11]. In another study of the growth evolution of p-type Si:H on ZnO coated glass and ZnO over-coating tin oxide (SnO2), Koval et al. [12] reported that valid material propertiesanddeviceperformancecorrelationscanbebetter realizedforanygiven material when the properties are obtained from deposition on similar substrates with similar thicknesses as those used in the respective device. The generation of so-called “deposition phase diagrams” or “growth evolution diagrams” for vhf and rf PECVD of Si:H films determined that vhf PECVD shows significant differences in structural evolution with processing conditions, namely the plasma excitation frequency [13]. Growth evolution diagrams have been developed by Stoke et al. for intrinsic a-Si:H, amorphous silicon germanium alloys, and nc-Si:H for top, middle, and bottom cell i-layers used in triple junction devices [14]. Dahal et al. reported growth evolution diagrams for intrinsic and p-type Si:H deposited on 64