Quantum size effect in core-shell structured silicon-germanium nanowires

Abstract

First-principles density functional theory has been employed to study the composition dependent quantum size effect in a series of silicon-germanium core-shell structured nanowires with the diameters ranging from 0.5 to 3.2 nm. Analysis of the calculated band gap energies in Si-core/Ge-shell and Ge-core/Si-shell structured nanowires shows a nonlinear composition dependence for nanowires with fixed diameter ͑fixed total number N = N Core + N Shell of Si and Ge atoms in the unit cell͒. In contrast, for nanowires with fixed core size and varying shell thickness, our calculation results reveal a striking linear blueshift of the direct band gap with composition. The obtained linear composition effect implies an inverse square relation between the energy of the fundamental band gap and the size of nanowire, in agreement with experimental observations. Our results provide useful guidelines for experimental gap engineering in the core-shell structured nanowire heterostructures.There has been fast-growing interest in nanometer-scale semiconductor materials such as quantum dots ͑0-D͒, quantum wires ͑1-D͒, and quantum wells ͑2-D͒. 1-3 These nanostructures constitute an intriguing class of semiconductor materials due to unique size dependent electronic and optical properties that are intrinsically associated with their low dimensionality and quantum confinement effect. 3,4 It has been established that the optical properties of nanocrystals strongly depend on the ratio of the nanocrystal radius, R, to the Bohr radius of the bulk exciton, R B . 5 Three different regions, R ӷ R B , R ϳ R B , and R Ӷ R B , corresponding to weak, medium, and strong confinement regimes, respectively, have been proposed. The confinement of electrons and holes in nanoscale crystallites leads to an increase in the electronic band gap with decreasing crystallite size and, consequently, a blueshift of the absorption and emission spectra. A qualitative quantum mechanical approach based on the effective mass approximation and particle-in-a-box models 1-5 predicts that the dependence of the band gap on the radius R of crystallites is of the form E g ϳ 1/R 2 . More detailed analysis of the quantum confinement effect in the framework of firstprinciples and semiempirical methods shows that size dependence of band gaps can be described by an inverse power law E g ϳ 1/R n with a smaller parameter n being between 1 and 2. 6-8 The ability to control the physical and chemical properties of low dimensional nanomaterials by variation of their size is of crucial importance for nanotechnology applications. 2,9 With the progress in synthesis and fabrication of low dimensional nanomaterials, semiconductor nanowires have attracted much attention because of their exceptional electronic, optical, and transport properties. 10 In these quasi-onedimensional materials, the motions of electrons and holes are confined in the radial plane, while the charge carriers are free to move along the nanowire's axis. Semiconductor nanowires can be used as building blocks in the engineeri...

SiGe nanowires: Structural stability, quantum confinement, and electronic properties

“…In recent years, various methods of synthesis have been developed, [17][18][19][20] which guarantee a precise control of composition, morphology, and electronic properties. Several theoretical [21][22][23][24][25] and experimental 5,12,26,27 studies have shown that the electronic, transport, and optical properties of SiGe NWs, like other nanostructures, are strictly related to their low dimensionality and to the quantum confinement effect ͑QCE͒. [28][29][30] Moreover, they offer, unlike the corresponding pure single material NWs, the possibility of modulating band structure, band gap, and optical spectra by changing not only the size of the material but even the composition and the geometry of the system.…”

Ge–Si and Si–Ge core–shell nanocrystals: Theoretical study

“…They are promising materials for building blocks of field effect transistors [24][25][26]. Several experimental [27,28] and theoretical [29][30][31] investigations have been performed to study of the quantum confinement effects in the Si-Ge core shell nanowires. For instance the Peng et al [22] have studied the band structure properties of the Si-Ge core-shell nanowires in different situations of applied uniaxial strain.…”

Crystallography and cathodoluminescence of ultra‐long GaN nanowires

“…One dimensional (1D) semiconductor nanostructures are currently the subject of material study due to their promising applications in the field of nano-optoelectronics, sensors and environmental cleaning, as well as their scientific interest for quantum size effect [1][2][3][4][5]. Compared with other materials, group III-nitride nanostructures have been intensively studied because of their wide band-gap ranging from 0.7 eV for InN to 6.2 eV for AlN, and the intriguing properties behind them [6][7][8].…”