Spectroscopic and microscopic studies of self-assembled nc-Si/a-SiC thin films grown by low pressure high density spontaneous plasma processing

Abstract

In view of suitable applications in the window layer of nc-Si p-i-n solar cells in superstrate configuration, the growth of nc-Si/a-SiC composite films was studied, considering the trade-off relation between individual characteristics of its a-SiC component to provide a wide optical-gap and electrically conducting nc-Si component to simultaneously retain enough crystalline linkages to facilitate proper crystallization to the i-nc-Si absorber-layer during its subsequent growth. Self-assembled nc-Si/a-SiC thin films were spontaneously grown by low-pressure planar inductively coupled plasma CVD, operating in electromagnetic mode, providing high atomic-H density. Spectroscopic simulations of ellipsometry and Raman data, and systematic chemical and structural analysis by XPS, TEM, SEM and AFM were performed. Corresponding to optimized inclusion of C essentially incorporated as Si-C bonds in the network, the optical-gap of the a-SiC component widened, void fraction including the incubation layer thickness reduced. While the bulk crystallinity decreased only marginally, Si-ncs diminished in size with narrower distribution and increased number density. With enhanced C-incorporation, formation of C-C bonds in abundance deteriorates the Si continuous bonding network and persuades growth of an amorphous dominated silicon-carbon heterostructure containing high-density tiny Si-ncs. Stimulated nanocrystallization identified in the Si-network, induced by a limited amount of carbon incorporation, makes the material most suitable for applications in nc-Si solar cells. The novelty of the present work is to enable spontaneous growth of self-assembled superior quality nc-Si/a-SiC thin films and simultaneous spectroscopic simulation-based optimization of properties for utilization in devices.

Superior optical response of size-controlled silicon nano-crystals in a-Si:H/nc-Si:H superlattice films for multi-junction solar cells

“…Fig. 24 It has been identied by the TEM studies mentioned in the next section that the size distribution became narrower when the grain size reduced within a thinner t nc , which might be supported by similar earlier reports as well. It is notable that the average grain sizes (d), as estimated from Raman spectroscopy, are comparable with the thickness (t nc ) of the nc-Si:H active layer sandwiched between two adjacent a-Si:H barrier layers.…”

“…For 6 nm # t nc # 8 nm, the average grain size varied in the range 5.7 nm # d # 7.5 nm where quantum connement has feeble effects on the limit of Bohr's radius, while size distribution effects may play a signicant role. 24 It has been identied by the TEM studies mentioned in the next section that the size distribution became narrower when the grain size reduced within a thinner t nc , which might be supported by similar earlier reports as well. 10 Thus the deviation of the FWHM data-point for t nc ¼ 8 nm from the regular nature of its variation in Fig.…”

Self-assembled nc-Si-QD/a-SiC thin films from planar ICP-CVD plasma without H2-dilution: a combination of wide optical gap, high conductivity and preferred 〈220〉 crystallographic orientation, uniquely appropriate for nc-Si solar cells

“…[17][18][19][20][21][22] The inclusion of C in the amorphous network leads to a lowering in the degree of crystallinity and this tradeoff relation becomes a perpetual hindrance for device application. 11,23 Inductively coupled plasma CVD with very high density of plasma species, low plasma sheath potentials and excellent uniformity of the plasma parameters in the radial and axial directions can provide a high density of atomic hydrogen over the growing lm surface, which could be instrumental in strategic promotion of crystallization in the silicon network even with the C-inclusion. 6,9,13,[24][25][26][27] In this context, we report on the formation of self-assembled nanocrystalline silicon quantum dots in amorphous silicon carbide matrix (nc-Si-QD/a-SiC) with a high deposition rate by inductively coupled plasma CVD from (SiH 4 + CH 4 )-plasma without any hydrogen dilution, 28 and retaining the crystallinity at a high magnitude even aer inclusion of a signicant amount of carbon into the network.…”

Quantum size effects on the optical properties of nc-Si QDs embedded in an a-SiOx matrix synthesized by spontaneous plasma processing

“…The volume fraction of crystallinity (X C ) has been estimated from the Raman studies for samples prepared at various RF powers, using typical spectroscopic analysis for nanocrystalline silicon films demonstrated in our earlier reports. 27,29 Fig. 1 displays a nature of variation of the crystalline volume fraction similar to the nature of change in number density of the nc-Si QDs, while the former changes in a linear scale and the latter in a logarithmic scale.…”

“…25,26 For thin film systems where Si-ncs are embedded in an amorphous SiC dielectric matrix, the chemical composition of which changes from sample to sample leading to consistent changes in the optical property of the amorphous component, the BEMA model accompanied by the TL model for the amorphous part has been successfully applied for each layer. 27 The present work deals with a comprehensive study on nc-Si QDs embedded in an a-SiO x matrix, by optical simulation of the ellipsometry data in the photon-energy range B1.5-6.0 eV. The optical modeling of the complex system consisting of nc-Si QDs and the a-SiO x matrix has been done with the combination of Bruggmann effective medium (BEMA) approximation [28][29][30] and the Tauc-Lorentz oscillator (TLO) model 31 to find out the changes in the compositional characteristics of the ensemble and various optical constants of the nc-Si QDs, in particular, which have been found to be strongly dependent on its size.…”