Doping of SiGe core-shell nanowires (original) (raw)

Band-Offset Driven Efficiency of the Doping of SiGe Core−Shell Nanowires

Nano Letters, 2011

Impurity doping of semiconducting nanowires is expected to become increasingly inefficient as the wire diameter shrinks down, because impurity states get deeper due to quantum and dielectric confinement. Here we show that efficient n-and p-type doping can be achieved in strongly confined SiGe core-shell nanowires, taking advantage of the type-II band offset at the Si/Ge interface. A one-dimensional electron (hole) gas is created at the band-edge and the carrier density is uniquely controlled by the impurity concentration with no need of thermal activation. Additionally, SiGe core shell nanowires provide naturally the separation between the different types of carriers, electron and holes, and are ideally suited for photovoltaic applications.

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

Physical Review B, 2009

We report first-principles calculations of ͓110͔ SiGe NWs; we discuss the effect of geometry and composition on their thermodynamic stability, on their electronic properties, and on the nature of the quantum confinement effect. The analysis of formation enthalpy reveals that Ge core / Si shell NWs represent the most stable structure at any diameter, as a confirmation of the results of many experimental works. The study of the dependence of the energy band gap on the composition and geometry shows how abrupt NWs ͑wires with a clear flat interface between Si and Ge͒ present strongly reduced quantum confinement effect and offer a very easy way to predict and to engine energy band gap, which can have a strong relevance from a technological point of view. A careful analysis of the influence of composition on the wave-function localization and quantum confinement effect is also presented, in particular, for core-shell NWs.

From bare Ge nanowire to Ge/Si core/shell nanowires: A first-principles study

Physical Review B, 2009

Germanium/Germanium-Silicon core/shell nanowires are expected to play an important role in future electronic devices. We use first-principles plane-wave calculations within density-functional theory in the generalized gradient approximation to investigate the structural and electronic properties of bare and H-passivated Ge nanowires and core/shell Ge/Ge-Si, Ge/Si, and Si/Ge nanowires. The diameters of the nanowires considered are in the range of 0.6-2.9 nm and oriented along ͓110͔ and ͓111͔ directions. The diameter, the surface passivation, and the substitutional effects on the binding energy, band structure, and effective mass are extensively investigated considering the relative contribution of quantum confinement and surface effects.

Band structure analysis in SiGe nanowires

Materials Science and Engineering: B, 2012

One of the main challenges for Silicon-Germanium nanowires (SiGe NWs) electronics is the possibility to modulate and engine their electronic properties in an easy way, in order to obtain a material with the desired electronic features. Diameter and composition constitute two crucial ways for the modification of the band gap and of the band structure of SiGe NWs. Within the framework of density functional theory we present results of ab initio calculations regarding the band structure dependence of SiGe NWs on diameter and composition. We point out the main differences with respect to the case of pure Si and Ge wires and we discuss the particular features of SiGe NWs that are useful for future technological applications.

Reduced quantum confinement effect and electron-hole separation in SiGe nanowires

Physical Review B, 2009

Using first-principles methods, we investigate the structural and electronic properties of SiGe nanowiresbased heterostructures, whose lattice contains the same number of Si and Ge atoms but arranged in a different manner. Our results demonstrate that the wires with a clear interface between Si and Ge regions not only form the most stable structures but show a strongly reduced quantum confinement effect. Moreover, we, with the inclusion of many-body effects, prove that these nanowires-under optical excitation-display a clear electronhole separation property which can have relevant technological applications.

Electronic properties of strained Si/Ge core-shell nanowires

Applied Physics Letters, 2010

We investigated the electronic properties of strained Si/Ge core-shell nanowires along the [110] direction using first principles calculations based on density-functional theory. The diameter of the studied core-shell wire is up to 5 nm. We found the band gap of the core-shell wire is smaller than that of both pure Si and Ge wires with the same diameter. This reduced band gap is ascribed to the intrinsic strain between Ge and Si layers, which partially counters the quantum confinement effect. The external strain is further applied to the nanowires for tuning the band structure and band gap. By applying sufficient tensile strain, we found the band gap of Sicore/Ge-shell nanowire with diameter larger than ~3 nm experiences a transition from direct to indirect gap.

Ab initio optoelectronic properties of SiGe nanowires: Role of many-body effects

Physical Review B, 2010

The self-energy and electron-hole interaction corrections to the one-particle approximation for SiGe nanowires have been calculated for different geometries and diameters. We show that, at fixed nanowire diameter and orientation, the self-energy corrections for the SiGe nanowires can be obtained as a weighted average, on the relative composition of one type of atom with respect to the total numbers of atoms in the unit cell, of the corrections for the pure ͑Si and Ge͒ nanowires, thus circumventing cumbersome computations and allowing a direct and practical determination of the electronic band gap. Moreover we show that particular geometrical configurations are at the origin of an enhancement of the optical oscillator strength that should be important for optoelectronic applications.

Understanding doping at the nanoscale: the case of codoped Si and Ge nanowires

Journal of Physics D: Applied Physics, 2014

Results of first-principles DFT calculations of the structural and electronic properties of B-P codoped Si and Ge NWs are presented and discussed. We find that, according to experiments, for both Si and Ge NWs, impurities tend to get closer together and to occupy edge positions, as a result of minor structural relaxation and hence lower formation energy. The study of the electronic structure shows that the simultaneous addition of B and P only slightly modifies the energy band gap value with respect to the pure wire, and is strongly dependent on the particular codoping configuration considered.

Tailoring Strain and Morphology of Core-Shell SiGe Nanowires by Low-Temperature Ge Condensation

Nano letters, 2017

Selective oxidation of the silicon element of silicon germanium (SiGe) alloys during thermal oxidation is a very important and technologically relevant mechanism used to fabricate a variety of microelectronic devices. We develop here a simple integrative approach involving vapor-liquid-solid (VLS) growth followed by selective oxidation steps to the construction of core-shell nanowires and higher-level ordered systems with scalable configurations. We examine the selective oxidation/condensation process under nonequilibrium conditions that gives rise to spontaneous formation of core-shell structures by germanium condensation. We contrast this strategy that uses reaction-diffusion-segregation mechanisms to produce coherently strained structures with highly configurable geometry and abrupt interfaces with growth-based processes which lead to low strained systems with nonuniform composition, three-dimensional morphology, and broad core-shell interface. We specially focus on SiGe core-she...

Hole mobility in Ge/Si core/shell nanowires: What could be the optimum?

Applied Physics Letters, 2014

Recent experimental works have shown that Ge/Si core/shell nanowires (NWs) are very attractive for nanoelectronics and for low-temperature quantum devices, thanks to the confinement of holes in the Ge core. Reported hole mobilities of the order of 200 cm 2 /V/s are promising for high-performance field-effect transistors. However, we demonstrate that mobilities more than ten times higher, up to 8000 cm 2 /V/s, could be reached in Ge/Si NWs. Atomistic calculations reveal the considerable influence of the strains induced by the Si shell on the hole transport, whatever the NW orientation. The enhancement of electron-phonon interactions by confinement, which usually degrades the mobility in NWs, is therefore outbalanced by the effect of strains.