Si/Ge nanostructures

Abstract

A review is given on the formation mechanisms and the properties of Si/Ge nanostructures that have been synthesized by self-assembling and self-ordering during heteroepitaxy of silicon-germanium alloys on single-crystal silicon substrates. The properties of electronic subbands in smooth strained Si/SiGe quantum well structures are presented as a basis for characterizing coherent Si/Ge nanostructures with free motion of carriers in a reduced number of dimensions. The low-dimensional band structure of valence band states confined in strained Si/Ge and Si/SiGe nanostructures is analysed by optical and electrical spectroscopy.The nanostructures presented were fabricated by self-assembly induced by elastic strain relaxation without applying any patterning technique. Misfit lattice strain of SiGe material deposited on Si substrates can relax by bunching of atomic surface steps with SiGe agglomeration at the step edges or by nucleation of Ge-rich islands in the Stranski-Krastanow growth mode. The size, density and composition of such Si/Ge nanostructures representing quantum wires and dots, respectively, can be tuned in a wide range by the growth parameters. Local strain fields extending into the Si host influence the nucleation and the lateral arrangement of nanostructures in subsequent layers and can be applied for self-ordering of nanostructures in the vertical as well as the lateral direction.Interband and intra-valence-band photocurrent, absorption and photoluminescence spectroscopy as well as C-V and admittance measurements reveal a consistent view of the band structure in Si/Ge quantum dot structures. This is in good agreement with model calculations based on band offsets, deformation potentials and effective electron masses known from earlier studies of Si/SiGe quantum well structures. The effective valence band offsets of hole states within Si/Ge nanostructures reach about 400 meV. Typical quantization energies of about 40 meV due to lateral confinement and Coulomb charging energies up to about 15 meV were observed for holes confined in 20 nm sized Si/Ge dots. Future applications of Si/Ge nanostructures such as photodetectors with improved performance or novel functionality are discussed.

Predicting and Understanding Order of Heteroepitaxial Quantum Dots

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] SAQDs are the result of a transition from 2D growth to 3D growth in strained epitaxial films such as Si x Ge 1Àx =Si and In x Ga 1Àx As/GaAs: This process is known as Stranski-Krastanow growth or VolmerWebber growth. 3,[15][16][17] In applications, order of SAQDs is a key factor. There are two types of order, spatial and size.…”

Mechanical Strain in Capped and Uncapped Self-Assembled Ge/Si Quantum Dots

“…The most used methods involve the continuous elasticity [7,8] or atomistic models [9,10]. Several theoretical and experimental works on germanium (Ge)/silicon (Si) QDs can be found in the literature [11,12,13,14], although none of them describes in detail the features of the strain and tension in uncapped QDs.…”

Anomalous x-ray scattering study of the growth of inverted quantum hut structures in a Si-Ge superlattice emitting strong photoluminescence

“…Due to ∼4% lattice mismatch of Si and Ge, such a quantum structure carries an inherent strain that can be controlled to tailor the band gap to the optical communication window. Ge quantum structures embedded in Si-Ge superlattice systems have already exhibited remarkable photoluminescence properties [3][4][5][6][7][8]. In recent years, developments in electron microscopy and synchrotron x-ray scattering techniques have enabled us to understand the growth mechanism by which these Ge quantum structures form in each Si-Ge interface over the Ge wet layer with the apex pointing towards the top surface and get ordered vertically in a Si-Ge superlattice due to propagation of strain.…”