Synthesis & Characterisation of Silicon nano Particles

Abstract

Silicon is the backbone of semiconductor industry. It is the building block of most of the modern solid slate electronic devices like ICs, transistors, photovoltaic solar cells, Sensing devices etc. The promotion of silicon from being {he key substrate material for icroelectronic devices to a potential light emitter has emerged as a consequence of the possibility to reduce its dimensionality by different techniques. Quantum confinement of photoexcited carriers that yields a band gap widening and an increased radiative transition rate is the most probable reason for visible light emission of Si nanostructures with dimensions less than 7 nm. The present study involved synthesis of Si nanocrystals (Si-nc) in two different routes: (i) by electrochemical etching in HF based solutions 10 produce porous silicon (PS) structures and (ii) by mechanical milling of Si powder followed by chemical oxidation to produce colloidal solution of oxidized Si-ncs Structural and luminescence properties of nanostructured PS produced by electrochemical etching were investigated. Visible photoluminescence (PL) was observed from oxidised PS and the PL band peak was found to be dependent on the various formation and post-formation parameters. AFM and STM measurements revealed that the pores ore highly non-uniform. Mechanical stability of nanostructured PS was investigated by nanoindentation and it appeared that these structures ruptured under an external load < 0.5 mN A macroporous Si sample was prepared with DMF treatment ofhigh resistivity Si wafer by anodic etching. The macroporous Si exhibited uniform pore growth and could potentially be used as templates. The second part of the study involved preparation of nanocrystalline Si by mechanical milling followed by oxidation to reduce the dimension of the nanocrystals to the desired level. Ball milled silicon nanocrystals at different milling hours (25, 50. 75 and 100 h) in toluene medium were studied. Chemical and thermal treatment to promote oxidation of these nanocrystals was carried out for further size reduction. XRD spectrum of milled Si nanocrystals at different stages of milling time revealed that the least size (~40 run) was obtained after 75 h of milling. However as-milled samples did not show any visible PL but intense visible PL detectable by unaided- eye, was observed from colloidal suspension of the chemically treated and annealed silicon nanocrystals prepared by mechanical milling. The PL bands peak were obtained at 3.14, 3.11, 2.93 and 2.79 e V under UV excitation. Phase contrast AFM and TEM investigations of the light emitting Si nanocrystals revealed the existence of Si crystallites with dimensions < 5 nm embedded in an oxide matrix. Some of the particles formed a core-shell structure with a c-Si core surrounded by an amorphous Si and Si oxide shell. It is proposed that invasive oxidation takes place at the interface of the nanocrystalline Si and amorphous silicon oxide leading to the formation of large number of oxide and defect related states. The origin of violet blue PL is discussed in relation to the oxide related surface states, non-stoichiometric suboxides and defect related states. Initial investigation in this route has been successful and encouraging since visible blue-violet PL with PL band peaks at around 400 nm has been observed from the colloidal suspension Si nanocrystals. This preparation route is novel and can be a very good substitute for expensive techniques like plasma enhanced chemical vapour deposition for the preparation of Si quantum dots. These Si-ncs can also have potential application in a standard solar cell where they may be used to transform the un-utilized UV light to visible light which a solar cell can use for generation of electricity.