Size effect on electronic transport in nC–Si/SiO x core/shell quantum dots Debajyoti Das *, Arup Samanta Nano-Science Group, Energy Research Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India 1. Introduction Persistent efforts toward progressive miniaturization of devices have resulted in the reduction in dimension, from bulk material to quantum well, then to quantum wire, and eventually to quantum dot (QD) structures. Being the ultimate limit in carrier confine- ment, semiconductor QDs are the center of research attention due to their unique electronic and optical properties. Silicon as a semiconductor is the material of perpetual choice in micro- and nano-electronic device fabrication. However, as an indirect band- gap semiconductor, bulk silicon is an inefficient optical emitter and absorber and is not suitable for photonic devices. As the purely electronic optical transition at the band edge is forbidden due to momentum conservation requirements, the involvement of phonons is essentially required for an optical recombination process to occur in bulk silicon. With the advent of structural miniaturization, however, a breakthrough in Si-research has occurred. Silicon-rich silicon oxide, in particular SiO x :H network containing silicon nanocrystallites (Si-NCs) or quantum dots has been the subject of considerable interest for optoelectronic applications compatible with the consolidated silicon technology. In earlier studies silicon nanocrystals were reported within semi- insulating polycrystalline silicon doped with oxygen atoms, when grown at 650 8C and annealed at 1000 8C [1] and the possible application of this material in metal-insulator-semiconductor (MIS) capacitor structures was explored [2]. A promising approach based on the quantum confinement in low-dimensional systems has originated from the first reports of room-temperature photoluminescence from porous silicon [3,4] and subsequently by Si nanocrystals [5], Si nanopillers [6] etc., which triggered a strong effort of research in this field. Optical gain observed in Si nanocrystals embedded in SiO 2 [7] and electroluminescence demonstrated using ac-field effect injection in nano-silicon [8] have given further impulses toward potential use of silicon nanoparticles in optoelectronic devices. Quantum confinement is known to alter the electrical conductivity in Si-NCs [9,10]. Possible applications in numerous other fields e.g., in photovoltaics [11], photodetectors [12], data storage [13] and optical waveguide [14] have also been demonstrated. Various solid, liquid and gas phase synthesis methods have been proposed to produce Si nanocrystals. Following solid phase annealing of silicon suboxide materials, silicon nanocrystals embedded in oxide matrices have been prepared [15,16]. Si nanoparticles wrapped in silicon oxide layer obtained from solution process showed stable PL properties [17,18]. By introduc- ing a small amount of oxygen on the surface cell, the possibility of fabricating stable nC–Si core has been indicated [19]. In gas phase synthesis methods, silicon nanocrystals are produced from laser or plasma decomposition of SiH 4 [20–23] or other silicon precursors like SiBr 4 [24]. However, the formation of Si-QDs, reported till the recent past, generally proceeded through multi-step routes, the most fundamental one being the high temperature processing as either pre-deposition or post-deposition annealing at 1000 8C and/or etching with hydrofluoric acid (HF). Recently, the instant growth of silicon quantum dots has been reported at low substrate temperatures, using H 2 [25] or He [26] as an alternative to H 2 as the Materials Research Bulletin 47 (2012) 3625–3629 A R T I C L E I N F O Article history: Received 27 February 2012 Received in revised form 28 May 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: A. Nanostructures A. Semiconductor A. Thin film B. Plasma deposition D. Electrical properties A B S T R A C T Electronic transport in silicon quantum dots (Si-QDs) in core/shell configuration was studied. The nC–Si cores encapsulated by protective SiO x shells embedded in a-Si matrix were obtained from one-step and spontaneous plasma processing, at low substrate temperature (300 8C) compatible for device fabrication. The size, density and distribution of nC–Si QDs were controlled by optimizing the plasma parameters. Very high electrical conductivity, s  4  10 2 S cm 1 , was achieved at a total number density of Si-QDs, N  4.8  10 11 cm 2 , corresponding to the lowering in its average core size, d  3.7 nm, to the order of the bulk Si exciton Bohr radius and the associated quantum confinement effects. The electrical conductivity was demonstrated to exhibit quantum size (3 < d (nm) < 10) effect in zero dimensional quantum dots. The underlying electronic transport was explained using hetero- quantum-dot model, the nC–SiO x :H QDs possess hetero-junction like band structure in the interface regions, due to their different band gaps. ß 2012 Elsevier Ltd. All rights reserved. * Corresponding author. E-mail address: erdd@iacs.res.in (D. Das). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.051