SiGe Nanoclusters Research Articles

Cluster-Assisted Mesoplasma Chemical Vapor Deposition for Fast Epitaxial Growth of SiGe/Si Heterostructures: A Molecular Dynamics Simulation Study.

Co-condensation of mixed SiGe nanoclusters and impingement of SiGe nanoclusters on a Si substrate were applied using molecular dynamics (MD) simulation in this study to mimic the fast epitaxial growth of SiGe/Si heterostructures under mesoplasma chemical vapor deposition (CVD) conditions. The condensation dynamics and properties of the SiGe nanoclusters during the simulations were investigated first, and then the impingement of transient SiGe nanoclusters on both Si smooth and trench substrate surfaces under varying conditions was studied theoretically. The results show that the mixed nanoclusters as precursors demonstrate potential for enhancing epitaxial SiGe film growth at a high growth rate, owing to their loosely bound atomic structures and high mobility on the substrate surface. By varying cluster sizes and substrate temperatures, this study also reveals that smaller clusters and higher substrate temperatures contribute to faster structural ordering and smoother surface morphologies. Furthermore, the formed layers display a consistent SiGe composition, closely aligning with nominal values, and the cluster-assisted deposition method achieves the epitaxial bridging of heterostructures during cluster impingement, highlighting its additional distinctive characteristics. The implications of this work make it clear that the mechanism of fast alloyed epitaxial film growth by cluster-assisted mesoplasma CVD is critical for extending it as a versatile platform for synthesizing various epitaxial films.

Local charge trapping in Ge nanoclustersdetected by Kelvin probe force microscopy

The understanding of local charge trapping on the nanoscale is crucial for the design of novel electronic devices and photodetectors based on SiGe nanoclusters (NCs). Here, the local spatial distribution of the surface potential of the Ge NCs was detected using Kelvin probe force microscopy (KPFM). Different surface potentials between Ge NCs and the wetting layer (WL) surface were detected at room temperature. Changes of the local contact potential differences (CPD) were studied after injection of electrons or holes into single Ge NCs on top of the Si layer using a conductive atomic force microscopy tip. The CPD image contrast was increased after electron injection by applying a forward bias to the n-tip/i-Ge NC/p-Si junction. Injecting holes into a single Ge NC was also accompanied by filling of two-dimensional states in the surrounding region, which is governed by leakage currents through WL or surface states and Coulomb charging effects. A long retention time of holes trapped by the Ge NC was found.

Transport and Photoelectric Effects in Structures with Ge and SiGe Nanoclusters Grown on Oxidized Si (001)

Crystalline germanium nanoclusters (NCs) are grown by a molecular-beam epitaxy technique on chemically oxidized Si (100) surface at 700oC. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters core becomes tetragonal SiGe alloy. Possible mechanisms for nanoclusters growth are discussed. Selective photoexcitation of Ge or SiGe nanoclusters or space-charge layer of underlying Si allows to observe two non-equilibrium steady-states with higher and lower conductivity values as compared to the equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after excitation of electron-hole pairs in Si (001) substrate. This effect may be attributed to spatial carrier separation by macroscopic fields in the depletion layer of the near-surface Si. Decreasing of surface conductivity, driven by optical recharging of NCs and Si/SiO2 interface states, is observed in the spectral range from 0.6 to 1.0 eV. Conductivity drop is discussed in the terms of hole accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.

(Invited) Fast and Slow Light-Emitting Silicon-Germanium Nanostructures

Epitaxially-grown three-dimensional Si/SiGe nanostructures (NSs) produce photoluminescence (PL) and electroluminescence in the desired spectral range of 1.3-1.6 μm. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast (PL lifetime <20 ns) and hence more efficient SiGe light-emitting devices. The results presented here demonstrate that in such Si/SiGe 3D NSs with a nominal Ge concentration approaching ~35% the PL peaked near 0.78 eV strongly depends on the Si/SiGe heterointerface abruptness. In other Si/SiGe NS/quantum-well samples with a Ge concentration approaching ~40%, we find two PL bands peaked at ~0.8 eV and ~0.9 eV at low temperatures. The PL peaked at 0.8 eV rises and decays slowly, and it quickly saturates as the excitation intensity increases. In contrast, the PL peaked at 0.9 eV shows a much shorter lifetime and exhibits a linear dependence versus excitation intensity. The slow/delayed PL at 0.8 eV is attributed to carrier recombination at the SiGe NS/Si transition layer while the faster and more efficient PL at 0.9 eV is associated with SiGe quantum wells. More complicated and similarly fast (~10-7 s) decays are observed at very high excitation intensities due to electron-hole droplet formation. The physics of carrier recombination in these Si/SiGe NSs is discussed.

Fast Luminescence in Silicon-Germanium Nanostructures

Epitaxially-grown three-dimensional Si/SiGe nanostructures produce photoluminescence and electroluminescence in the desired spectral range of 1.3-1.6 μm. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast and hence more efficient SiGe light-emitting devices. The physics of carrier recombination in these Si/SiGe nanostructures is discussed. The present results provide another possible route toward CMOS compatible light emitters.

Fast Light-Emitting Silicon-Germanium Nanostructures

Epitaxially-grown three-dimensional Si/SiGe nanostructures produce photoluminescence and electroluminescence in the desired spectral range of 1.3-1.6 μm. We show that by controlling and modifying such Ge-rich SiGe nanoclusters during growth it is possible to fabricate very fast and hence more efficient SiGe light-emitting devices. Models for the physics of carrier recombination in these Si/SiGe nanostructures are presented, and a new route toward CMOS compatible light emitters is proposed.

Visible photoluminescence of nanometer-sized SiGe/Si heterostructure fabricated by ion implantation

An innovative fabrication technique for the nanometer-sized SiGe/Si heterostructure was developed in this study. Ge was induced in Si substrate by two-step ion implantation. The spherical SiGe nanoclusters are self-assembled in the Si substrate by subsequent rapid thermal annealing at 1,100 °C. The diameter of the spherical SiGe nanoclusters is 5–7 nm. Visible photoluminescence from this nanometer-sized SiGe/Si heterostructure at room temperature was investigated. We found three peak energies of visible luminescence spectra at 1.97, 2.13, and 2.16 eV, respectively. The luminescence intensity depends on the number of the nanoclusters and will be decreased because of the micro-defects around the heterostructure, which is discussed in detail.

Evolution of SiGe nanoclusters and micro defects in the Si 1− xGe x layer fabricated by two-step ion implantation and subsequent thermal annealing

The Si 1− x Ge x thin layer is fabricated by two-step Ge ion implantation into (0 0 1) silicon. The embedded SiGe nanoclusters are produced in the Si 1− x Ge x layer upon further annealing. The number and size of the nanoclusters changed due to the Ge diffusion during annealing. Micro defects around the nanoclusters are illustrated. It is revealed that the change of Si–Si phonon mode is causing by the nanoclusters and micro defects.

Calculating the energy spectrum and electronic structure of two holes in a pair of strained Ge/Si coupled quantum dots

We theoretically investigate the energy spectrum and electronic structure of two vertically stacked Ge/Si quantum dots loaded with a pair of interacting holes, and study the dependence of basic physical characteristics on dot size and interdot separation. In particular, we focus on the splitting of the lowest spin singlet and triplet states and spatial correlations between holes. Here the term ``spin'' is rather an index for two Kramers degenerate states. The two holes are treated by six-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations combined with the configuration-interaction model taking into account the realistic situation when both Si matrix and Ge nanoclusters are inhomogeneously strained. It is shown that asymmetry of strain distribution and spin-orbit coupling of the valence band introduce characteristic features in both single- and many-particle hole states, which are not captured by the usual one-band heavy-hole approximation. We find a level anticrossing between the lowest singlet and triplet states with a zero anticrossing energy gap as a function of interdot spacing. It is demonstrated that the level anticrossing between many-particle states is accompanied by a level crossing between single-particle bonding and antibonding molecular orbitals. We argue that both phenomena of level crossing and anticrossing originate from the asymmetry-induced localization of single-particle hole wave functions on different dots. Above a certain dot separation, the Mott-type delocalized-to-localized transition for the many-particle hole states is observed. We show that the transition is caused by the combined action of correlations and strain asymmetry.

Raman Characterization of SiGe Nanostructures Formed by Rapid Thermal Annealing

This work presents the micro-Raman (μR) analysis in SiGe nanoclusters formed by rapid thermal annealing (RTA). Heterostructure of a-Si:H/Ge/a-Si:H was grown on p-Si (001) substrate by Electron Cyclotron Resonance ECR plasma deposition and e-beam technique. Different parts of the sample were heated at 1000°C during 40, 50, 60 and 70 seconds. The samples as-deposited and RTA processed were characterized by Raman measurements in order to evaluate the Ge concentration and strain for different annealing times. The same parameters were extracted from high-resolution x-ray diffraction (HRXRD) performed in grazing incidence x-ray reflection mode (GIXRR). The μR spectra were sensitive to film strain changes due to the Ge diffusion during RTA process. The association of both characterization techniques provides information about some features of the SiGe nanoclusters at different annealing times. The results show the potential micro-Raman applicability for SiGe nanoclusters embedded in amorphous structure characterization. [DOI: 10.1380/ejssnt.2009.301]

Structural characterization of SiGe nanoclusters formed by rapid thermal annealing

This work presents the structural characterization of nanoclusters formed from a-Si:H/Ge heterostructures processed by rapid thermal annealing (RTA) at 1000 °C for annealing times varying between 30 s and 70 s. The a-Si:H layers were grown on electron cyclotron resonance (ECR) using SiH 4 and Ar precursor gases. The Ge layer was grown in an e-beam evaporation system. The structural characterizations were performed by high-resolution X-ray diffractometer (HRXRD) on grazing incidence X-ray reflection mode (GIXRR) and micro-Raman measurements. The average grain size, Ge concentration ( x Ge) and strain were estimated from Lorentzian GIXRR peak fit. The average grain size varied from 3 nm to 7.5 nm and decreased with annealing time. The x Ge increase with annealing time and varied from 8% to 19%, approximately. The strain calculated for (1 1 1), (2 2 0) and (3 1 1) peaks at 40 s, 50 s, 60 s and 70 s annealing time suggest the geometrical changes in nanoclusters according to process time.

Strain effects and microstructural evolution in Ge–Si system materials prepared by ion implantation and by ultrahigh vacuum chemical vapor deposition

Appropriately utilizing some microstructures may be very helpful to acquire desirable Ge–Si system materials. In this work, the Ge–Si system materials have been prepared either by ion implantation or ultrahigh vacuum chemical vapor deposition (UHVCVD). The interesting microstructures including half-loop dislocations, SiGe nanoclusters, and dislocation dipoles have been found in these two kinds of Ge–Si system materials. It is demonstrated that the evident surface strain state and adequate surface layer quality have been realized by employing these microstructures. Compared with the dipole dislocations in the Ge–Si systems deposited by UHVCVD on the compliant silicon on insulator, the half-loop dislocations and the SiGe nanoclusters induced by Ge ion implantation and subsequent annealing can relax the SiGe layer more effectively and lead to relatively large strain in the surface silicon. It may provide some new approaches to the control of misfit strains for fabricating desirable Ge–Si system materials.