8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Intense blue-violet photoluminescence from nanocrystalline silicon in silicon-oxide matrix prepared by mechanical milling
Samata Sarkar
School of Materials Science and Engineering, Bengal engineering and Science University, Shibpur, Howrah: 711103, India
Email: samata_online@rediffmail.com
Abstract
Silicon is the building block of most of the modern solid state electronic devices like ICs, transistors, photovoltaic solar cells, sensing devices etc. The promotion of silicon from being the basic electronic material for microelectronic devices to a potential light emitter has emerged as a consequence of the possibility to reduce its dimensionality by different techniques [1,2]. Quantum confinement of photo excited carriers that yields a band gap widening and an increased radiative transition rate has been cited as the most probable reason for visible light emission of Si nanostructures with dimensions less than 7 nm [1-4]. The present study involved synthesis of Si nanocrystals (Si-nc) by mechanical milling of crystalline (-c) Si followed by chemical oxidation to reduce the dimension of the nanocrystals to the desired level. Colloidal solutions of oxidized Si-ncs were formed in isopropanol and intense room temperature photoluminescence (PL) detectable with unaided eye was observed. 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 nm) was obtained after 75 h of milling. However as-milled samples did not show any visible PL but intense visible PL was observed from colloidal suspension of the chemically treated and annealed nc-Si prepared by mechanical milling. The PL band peaks were obtained at 3.14, 3.11, 2.93 and 2.79 eV under UV excitation. Phase contrast AFM and HRTEM 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 (a-) Si and Si oxide shell. The thickness of the oxide layer and the diameter of the core crystalline region were measured by AFM line profile analyses. It is proposed that invasive oxidation takes place at the interface of the nc-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 sub-oxides 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.
References
[1] L. T. Canham, Appl. Phys. Lett. 57, 1045 (1990).
[2] V. Lehmann and U. Gosele, Appl. Phys. Lett. 58, 856 (1991).
[3] B. Delley and E.F. Steigmeier, Phys. Rev. B 47, 1397 (1992).
[4] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan and C. Delerue, Phys. Rev. Lett. 82, 197 (1999)
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