Bilyk T. Structural, electrical and optical properties of porous silicon for solar cells

The thesis is devoted to the complex study of porous layers obtained by chemical etching in different solutions and its application in solar cell technology as antireflective coating. Porous silicon has a wide range of properties that depend on its microstructure and etching method. Particular attention of scientists is attracted to photoluminescence properties of porous layers. However, wider study of porous silicon properties has shown possibility for many other applications of porous silicon. There are solar energy conversion, biotechnology and sensors among them. Despite more than 20-year study of porous silicon, nature of its properties and the relationship between the properties of porous structure and etching conditions still need to be investigated. Till now investigation of porous silicon is carried out fragmentary. Most attention is paid to the study of layers obtained by electrochemical etching and their photoluminescence in particular. Thus, many questions related to the properties of porous silicon layers, especially obtained by chemical etching remain open. Special attention must be paid to investigation of possibility of replacing anodic etching by chemical etching for fabrication fees reduction. From this point of view, wide analysis of porous layers obtained by chemical etching and their comparison with layers obtained by electrochemical etching is needed. It was shown that the surface morphology and thickness of layers of porous silicon depend on the etching parameters (etching time and etchant composition) and the substrate parameters (crystallographic orientation, conductivity, surface pretreatment). Control of porous layers properties can be reached by changing these parameters. According to the results of scanning microscopy and electrical properties of porous layers it was found that chemically etched porous layers have porosity of at least 50% and are composed of individual microcrystallines, covered with smaller nanoscale formations and amorphous matrix of complex chemical composition. Material structure and nature of conductivity undergo significant changes which results in possibility of creating porous layers similar in terms of properties with amorphous silicon and dielectric films. According to temperature dependence of resistance the conductivity of porous layers is similar to the conductivity of amorphous silicon. The hopping conductivity is observed at the temperature below 300 K. Conductivity change obeys the Mott's law within this temperature range. At higher temperatures conductivity becomes thermally activated with the activation energies of about 0.4 - 0.6 ?V (p-type Si) and 0.13 - 0.15 ?V (n-type Si). The resistivity of porous layers at indoor temperature is at least 35 - 45 ??cm for p-type substrates and 15 - 30 ??cm for n-type substrates. The study shows that the growth rate of porous layer for conventional etchants HF:HNO3:H2O is lower and pores are larger, which leads to more reflection from the surface (3 - 8 %) compared to layers obtained by using the etchant HF:NaNO2:H2O (0.2 - 2 %). Porous layers formed on silicon-on-sapphire structures have the rate of absorption twice as much in comparison with crystalline silicon. The band-gap of porous layers was evaluated based on absorption spectra. It was shown that with increasing of etching time the band-gap increases from 1.14 ?V (crystalline silicon-on-sapphire) to 1.26 ?V that approximates the porous silicon absorption spectrum to the solar spectrum. Photoresponse spectra of Me/por-Si structures shows a marked increase in sensitivity in short-wavelength range, even for the smallest thickness of the porous layer. In the work it is also shown that depending on the etching conditions porous silicon can have several types of photoluminescence spectra with different intensity: heterogeneous, with a maximum at a wavelength of 650 - 670 nm (etchant HF:NaNO2:H2O) or 675 nm (etchant HF:HNO3:H2O) and uniform, with greater intensity and with a maximum at the 690 - 700 nm (observed for the layers obtained by using the etchant HF:NaNO2:H2O only). Porous layers formed by using the etchant HF:NaNO2:H2O, can be successfully applied in photovoltaic technology as antireflective coating. Technology of the obtaining of porous layer by chemical etching can be adapted for industrial mass production. It can reduce its costs by replacing energy-intensive process of deposition of silicon nitride. Studies of solar cells with porous layer revealed that the optimal etching time is 7 - 10 s. It is shown that as a result of a porous layer formation main parameters of photovoltaic cells increased by 15 - 20 % on average, especially noticeable is the increase of sensitivity within the UV spectral range (50 - 60 %). The use of porous silicon leads to increase of solar cells efficiency from 8% to 10%. Solar cells characteristics show sufficient stability both in endurance at the air and under the influence of thermal cycles and UV radiation.