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The optical properties of amorphous-nano-crystalline thin films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) were studied in correlation with the size distribution of individual crystal sizes and the crystalline to amorphous fraction. A possible application as active part in solar cells was tested by integrating the actual Si laye...
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... examined structure as active part in solar cells was tested by integration in a typical p-i-n solar cell structure. The most important property, besides high absorption, reflects itself in the possibility of changing the spectral distribution of the opto-electrical response (Quantum efficiency, QE) with changing the crystals size in thin film. In Fig. 6 are compared a-Si, a-nc-Si:H and μc-Si solar cells. As can be seen, the QE of the solar cell with nano-crystals has a "blue shift" comparing to amorphous and microcrystalline cells which enable the use of this kind of material in multigap-multilayer solar cells where each the of individual cells covers a part of the solar spectrum, and ...
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... The microcrystalline/nanocrystalline silicon ( c-Si:H/nc-Si:H) or a mixed phase material has gained importance in the last decade due to its light stability, good charge transport characteristics and tunable band gap [2][3][4][5]. A lot of work has been done on effect of hydrogen dilution and optical characterization of individual thin films of a-Si:H and c-Si:H/nc-Si:H [6][7][8][9][10][11][12][13]. The absorption coefficient of amorphous silicon dominates in the higher energy part of visible spectrum and for crystalline Silicon it is more in lower energy range of the visible spectrum [14]. ...
... For increased size of nano crystals 'a' here, the expected increase in band gap due to QSE is not seen and the band gap remains near to 1.6 -1.8 eV. Similar results were shown by [11][12]. ...
Silicon multilayer thin films with stack of amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) of varied hydrogen dilution were prepared by hot wire chemical vapor deposition (HWCVD). The optical properties of the thin films were measured by UV-visible spectroscopy. The smooth transmittance spectra measured in the range of 500-1100nm show films with uniform deposition. The optical constants such as refractive index, absorption coefficient, tauc band gap, extinction coefficient together with thickness were determined using envelope method. It is found that the optical band gap decreases from 1.78eV to 1.6eV as hydrogen dilution increases.
Different processes are used to improve the deposition of a nanocrystalline silicon layer to enhance optical absorption in different devices. The difficulty encountered lies in the reproducibility and cost of the process. In the present study, we have investigated the effect of a crystalline seed layer on the structural properties of a subsequent hydrogenated silicon (Si:H) layer (top layer). Structural properties of deposited films were investigated by absorption and Raman spectroscopy, grazing incidence X-ray diffraction measurements and spectroscopic ellipsometry. They show that using the seed layer approach promotes the structural evolution of the subsequent layer by improving its size and fraction of crystallites. UV–visible reflectance spectra simulations using Bruggeman effective medium approximation were used in order to investigate the composition evolution of the studied samples. They predict that the optical model of the elaborated layers consists of (i) the crystalline substrate (ii) the incubation layer (a mixture of amorphous silicon and voids) (iii) the bulk crystalline layer (a mixture of amorphous silicon, crystalline silicon and voids) and finally (iv) the surface layer. The incubation layer observed at the beginning of growth of the subsequent layer is absent in the optical model in the presence of the seed layer or it may be very thin in thickness, so that it has not been presented in the optical model. The optical model shows also an increase of void fraction in the top layer in the presence of the seed layer; this has been linked to the increase of crystallite size. It is noted also that the top layer is rougher in the presence of the seed layer and this has been attributed to the presence of larger silicon crystallites on its surface. Such deposition process can be used to enhance crystalline phase in absorber layer for optoelectronic devices.
In this study, an intrinsic hydrogenated amorphous silicon (a-Si:H) thin film was deposited using the plasma-enhanced chemical vapor deposition technique. The samples were annealed at 700 °C and 900 °C for 1 h. Optical measurements indicated the formation of nanocrystallites embedded in an amorphous a-Si:H matrix and the density increased with the annealing temperature. The electrical properties of the Schottky diodes Au/a-Si:H were investigated using admittance spectroscopy over a temperature range of 300–500 K. The conductivity at room temperature in the dark increased by several orders of magnitude after the transition from an amorphous to nanocrystalline structure. The activation energy was deduced based on the variations in the conductivity with temperature. The responses of the amorphous and nanocrystalline phases were identified by complex impedance analysis. The amorphous phase was dominant at low frequencies and high temperatures, whereas the response of the nanocrystalline phase was observed at high frequencies and low temperatures. Semicircular arcs were observed in the impedance plot at different temperatures and an electrical equivalent circuit was proposed. Complex impedance analysis demonstrated that the relaxation phenomenon was a thermally activated process. Moreover, we deduced the temperature at which the available density of the trapped charge states vanished based on the temperature dependence of the average normalized change.
Quantification of the short-range order in amorphous silicon has been formulized using Raman scattering by taking into account established frameworks for studying the spectral line-shape and size dependent Raman peak shift. A theoretical line-shape function has been proposed for representing the observed Raman scattering spectrum from amorphous-Si-based on modified phonon confinement model framework. While analyzing modified phonon confinement model, the term “confinement size” used in the context of nanocrystalline Si was found analogous to the short-range order distance in a-Si thus enabling one to quantify the same using Raman scattering. Additionally, an empirical formula has been proposed using bond polarizability model for estimating the short-range order making one capable to quantify the distance of short-range order by looking at the Raman peak position alone. Both the proposals have been validated using three different data sets reported by three different research groups from a-Si samples prepared by three different methods making the analysis universal.
The purpose of this paper is to compare the influence of boron and phosphor doping on the optical, structural and electrical properties of hydrogenated amorphous silicon (a-Si:H) thin films elaborated at low radiofrequency power and low hydrogen dilution. Structural properties of deposited films were investigated by Scanning Electron Microscopy (SEM), Grazing Incidence X-Ray Diffraction (GI-XRD) and Raman spectroscopy. The SEM analysis shows that the best homogeneity is observed in the layer doped with low diborane flow rate. Reflectance, XRD and Raman measurements show that the doping of thin films induces the formation of nanocrystallites (nc-Si) embedded in the amorphous matrix with higher density in the phosphorus-doped layers compared to those of boron-doped one. The temperature dependent electrical properties of Au/a-Si:H Schottky diodes were investigated using current-voltage characteristics (I-V), admittance spectroscopy technique and capacitance-voltage characteristics (C(V)) in the temperature range of 100-400K. From the I-V analyses based on thermionic emission (TE) theory we notice the heterogeneity of the barrier height of the Schottky diodes. From the electrical conductivity measurements, we plotted the evolution of the logarithm of the conductivity .T as a function of 1000/T. Activation energies and of the samples in low and high temperature ranges respectively are estimated. The decrease of with increasing boron flow rate is attributed to an increase in the density of the traps in the band gap of silicon. On the other hand, we found that increases when the flow rate of boron increases and decreases when the flow rate of phosphorus increases. We attributed these behaviors to the increase in the crystallinity of the n-type layers and to the presence of a deep defect in the p-type layers. C-2(V) plots show the presence of two linear regions. We found that the response of the nanocrystalline phase becomes more pronounced at high doping flow rate. Optical, structural and electrical measurements confirmed that the type and the density of doping are important parameters that influence the electrical properties of nc-Si:H Schottky diodes.
