Analysis of Controlled Mixed-Phase „Amorphous+Microcrystalline… Silicon Thin films by Real Time Spectroscopic Ellipsometry ͒ N
Total Page：16
File Type：pdf, Size：1020Kb
Analysis of controlled mixed-phase „amorphous+microcrystalline… silicon thin films by real time spectroscopic ellipsometry ͒ N. J. Podrazaa Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Jing Li Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 C. R. Wronski Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 E. C. Dickey Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 R. W. Collins Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo, Ohio 43606 ͑Received 1 June 2009; accepted 3 August 2009; published 14 September 2009͒ Engineered thin films consisting of periodic arrays of silicon microcrystallites in a hydrogenated amorphous silicon host matrix have been prepared by plasma-enhanced chemical vapor deposition where the hydrogen dilution of silane is modulated in multiple cycles. These types of films have been guided by a phase evolution diagram, depicting the deposition conditions and film thickness at which the material exhibits amorphous, microcrystalline, or mixed-phase ͑amorphous ͒ +microcrystalline characteristics, developed for intrinsic Si:H prepared with varying H2 dilution on unhydrogenated a-Si:H. Real time spectroscopic ellipsometry ͑RTSE͒ has been used in situ to noninvasively determine the phase evolution of the resulting hydrogenated mixed-phase ͑amorphous+microcrystalline͒ silicon thin films and corroborated with dark-field transmission electron microscopy. Such tailored microstructures are of growing interest as components of thin film photovoltaic devices, and RTSE is shown to be a key technique for structure verification. © 2009 American Vacuum Society. ͓DOI: 10.1116/1.3212893͔ I. INTRODUCTION Improvements in thin film Si:H solar cell efficiency have been achieved using these optimization principles facilitated ͑ ͒ Thin-film hydrogenated silicon Si:H based n-i-p and by phase evolution diagrams, depicting the deposition condi- p-i-n photovoltaic devices may incorporate amorphous sili- tions and film thickness at which the material exhibits amor- ͑ ͒ ͑ ͒ con a-Si:H , microcrystalline silicon c-Si:H , and even phous, microcrystalline, or mixed-phase ͑amorphous mixed-phase ͑amorphous+microcrystalline͒ silicon ͓͑a +microcrystalline͒ characteristics, as determined in real time +c͒-Si:H͔ intrinsic layer ͑i-layer͒ components. Optimum spectroscopic ellipsometry ͑RTSE͒ studies.2,5–7 Further im- single-step a-Si:H i-layers, as assessed by the stabilized ef- provements in the stability of a-Si:H based thin film solar ficiency of the resulting photovoltaic devices, are produced ͑ ͒ ͑ ͒ cells have been reported by incorporating a+ c -Si:H by plasma-enhanced chemical vapor deposition PECVD 8–10 from SiH at the maximum H dilution possible while avoid- i-layers into the device structure; however, optimization 4 2 ͑ ͒ ing the nucleation of crystallites from the amorphous phase and reproducible control of a+ c -Si:H deposition are a 1–3 challenge due to the rapid evolution of the microcrystallite throughout the i-layer thickness. The high H2 dilution en- sures a well-ordered amorphous network that rapidly reaches content, the extreme substrate dependence, and thus the dif- its fully light-soaked state in less than 100 h at air mass 1.5. ficulty of controlling the volume fraction of crystallites at a In contrast, the corresponding optimum single-step c-Si:H constant average value throughout the thickness. Although i-layers are produced by PECVD at the minimum H2 dilution continuous hydrogen dilution profiling has been widely while maintaining continuous crystal growth on a microcrys- adopted in order to optimize grain boundary passivation in 4,5 11 talline surface. In this case, the low H2 dilution ensures c-Si:H i-layer materials throughout the thickness, the that grain boundaries are well passivated with an amorphous more challenging problem of modulating hydrogen dilution phase. to generate ͑a+c͒-Si:H with a controlled fraction of em- bedded microcrytallites has yet to be widely applied. The ͒ ͑ ͒ a Author to whom correspondence should be addressed; electronic mail present work demonstrates that a+ c -Si:H i-layer films [email protected] can be fabricated with periodic arrays of microcrystallites in 1255 J. Vac. Sci. Technol. A 27„6…, Nov/Dec 2009 0734-2101/2009/27„6…/1255/5/$25.00 ©2009 American Vacuum Society 1255 1256 Podraza et al.: Analysis of controlled mixed-phase „amorphous+microcrystalline… silicon thin films 1256 an amorphous matrix, whereby the average crystallite size as well as the in-plane and out-of-plane crystallite spacings can be separately modified solely by deposition conditions estab- lished and verified by RTSE. II. EXPERIMENTAL DETAILS The ͑a+c͒-Si:H i-layer films studied here were depos- ited on single-phase a-Si:H layers, which were prepared, in turn, on native oxide coated crystalline silicon ͑c-Si͒ sub- strates using a single-chamber rf ͑13.56 MHz͒ PECVD sys- tem. The deposition parameters included a modulated hydro- ͓ ͔/͓ ͔ gen dilution ratio, R= H2 SiH4 , with all other parameters ͑ ͒ fixed as follows: a low substrate temperature Ts =200 °C , the minimum rf power possible for a stable plasma ͑ / 2͒ 0.08 W cm , a low partial pressure of the SiH4 source gas ͑ϳ0.06 Torr͒, and a low total pressure ͑Ͻ1.0 Torr͒. The initial a-Si:H layer was prepared under the same conditions but with fixed R=0. This layer serves to erase the memory of the c-Si wafer substrate and also mimics the underlying p-or FIG. 1. Surface roughness layer thickness ͑top͒ and microcrystallite volume n-layer for i-layer deposition in p-i-n or n-i-p solar cell con- fraction in the near-surface of the bulk layer ͑bottom͒ plotted vs bulk layer ͓ ͔/͓ ͔ figurations, respectively. An initial series of Si:H films was thickness for a high H2-dilution R= H2 SiH4 =40 film deposited on a crystalline silicon ͑c-Si͒ wafer substrate coated with undiluted R=0 amor- prepared on these structures at hydrogen dilution ratios of phous silicon ͑a-Si:H͒. The evolution of the microcrystallite fraction as 0ՅRՅ170 for phase evolution diagram development, while predicted by a cone growth geometrical model is also shown ͑solid line͒;the multilayer mixed-phase ͑a+c͒-Si:H films were prepared inset shows parametrizations of the imaginary parts of the dielectric function ͑ ͒ ͑ by modulating R during the deposition. The Si:H deposition spectra, 2, for the fully amorphous dotted line and microcrystalline solid line͒ R=40 hydrogenated silicon ͑Si:H͒ material. process was investigated in situ and in real time using a rotating-compensator multichannel ellipsometer.12 Films prepared at low H2 dilution, for example, R=0, parameters that include the volume fractions of Si:H crystal- remain in the amorphous growth regime throughout the lites ͑f ͒ and voids ͑f ͒ in the outer layer, the outer-layer thickness. For such films, the time evolution of the bulk and c void growth rate r, and the surface roughness layer thickness ds. surface roughness layer thicknesses, as well as the bulk layer After this information is obtained, additional structural pa- ͑ ͒ dielectric functions 1 , 2 , were extracted from the RTSE rameters such as the areal density of microcrystallites in the data using a global ͚ minimization procedure, where is ͑ ͒ ͑͒ 13 amorphous matrix Nd and the half-angle of the inverted the unweighted error function. Films prepared with high H2 cones can be estimated using a geometric model.15 dilution tend to develop in the amorphous ͑protocrystalline͒ phase for an initial period of growth but subsequently nucle- ate isolated microcrystallites from within the amorphous ma- III. RESULTS trix at a well-defined thickness. These nuclei grow in the Figure 1 shows the evolution of the surface roughness form of inverted cones, and eventually coalesce to single- thickness and microcrystallite volume fraction in the outer phase c-Si:H. Consequently, the final films exhibit an layer ͑top 3 Å͒ for an R=40 Si:H film prepared on a native amorphous phase near the substrate interface and a microc- oxide/c-Si substrate coated with R=0 a-Si:H. Microcrystal- rystalline phase at the top of the film, each with distinct lites first nucleate from the amorphous phase at a bulk layer dielectric functions. An intervening structurally graded ͑a thickness of db =160 Å, which is denoted as the amorphous +c͒-Si:H phase exists, wherein the microcrystallite frac- to mixed-phase ͑amorphous+microcrystalline͓͒a→͑a ͔͒ tion increases with bulk layer thickness from zero ͑initial + c transition. At db =500 Å, the microcrystallites coa- nucleation͒ to near unity ͑full coalescence͒. lesce to form single-phase c-Si:H, which is referred to as RTSE data collected during the growth of such structur- the mixed-phase to single-phase microcrystalline ͓͑a+c͒ ally graded ͑a+c͒-Si:H are conveniently interpreted using →c͔ transition. The microcrystallite nucleation density is ͑ ͒ 14,15 ϳ ϫ 10 −2 virtual interface analysis VIA . This analysis utilizes a estimated at Nd 6.0 10 cm , which corresponds to an four medium optical model consisting of ͑i͒ the ambient, ͑ii͒ initial in-plane microcrystallite spacing of ϳ400 Å, and the a surface roughness layer, ͑iii͒ an outer layer that contains cone half-angle is estimated at ϳ35°. the most recently deposited material, and a virtual interface The inset in Fig. 1 shows the spectra in the imaginary part ͑ ͒ to iv the pseudosubstrate which contains the past history of of the dielectric function, 2, for the near-substrate amor- the deposition. The surface roughness layer is modeled using phous and near-surface microcrystalline R=40 Si:H materi- the Bruggeman effective medium theory as a 0.5/0.5 volume als.