Two-stage photobioreactor process for the metabolic insertion of nanostructured germanium into the silica microstructure of the diatom Pinnularia sp.

There is significant interest in imbedding nanoscale germanium (Ge) into dielectric silica for optoelectronic applications. In this study, a bioreactor process was developed to metabolically insert nanostructured Ge into a patterned silica matrix of the diatom Pinnularia sp. at levels ranging from 0.24 to 0.97 wt.% Ge. In Stage I, the diatom cell culture was grown up to silicon starvation. In Stage II, soluble silicon and germanium were co-fed to the silicon-starved culture to promote one cell division during Ge uptake. In Stage II, soluble Si and Ge were transported into the silicon-starved diatom cell by a surge uptake process, and Ge uptake preceded its incorporation into the frustule. STEM–EDS line scans of the frustule in the newly-divided cells revealed that the Ge was uniformly incorporated into the biosilica. The overall shape of the new frustule was intact, but Si–Ge oxides filled the frustule areolae and altered their nanoscale pore size and geometry. Ge-rich pockets imbedded within the silica frustule and Ge-rich nanoparticles littering the frustule surface were also found. These results suggest that a two-stage diatom cultivation process can biologically fabricate and self-assemble new types of Ge–Si nanocomposite hierarchical materials that possess intricate submicron features.     

Electronic structural details of donor-vacancy complexes in Si-doped Ge and Ge-doped Si

We present a detailed picture of the electronic structure of donor-vacancy complexes in Ge-doped silicon and Si-doped germanium clusters to mimic Si-rich and Ge-rich SiGe alloys, respectively. Jahn-Teller effects and electrical levels were investigated in both Si-rich and Ge-rich end compositions. It is shown that while Ge atoms act as efficient traps for mobile E-centers in Si-rich alloys, Si atoms in Ge-rich material do not have this ability. A detailed linear combination of atomic orbitals model is proposed for VP-Ge and VP-Si complexes in Si_1−xGe_x and Ge_1−xSi_x, where the orbital filling order is driven by the larger electron affinity of Si atoms when compared to Ge.     

Evolution of composition distribution of Si-capped Ge islands on Si(001)

The evolution of the composition distribution and microstructures of Ge islands on Si(001) during the Si overgrowth was investigated by atomic force microscopy combined with selective wet etching procedures. With increasing Si coverage to 5.4 nm, the uncapped Ge islands were found to change their shapes dramatically from domes to truncated pyramids, nanorings and eventually to the fully buried islands. Different atomic composition profiles in SiGe islands were observed at different Si coverages. Especially, the nanorings were found to have a Ge-rich core with a Si-rich periphery. Based on the experimental results, the Ge redistribution in islands during Si capping is not only correlated with the intermixing between Si capping layer and Ge islands, but also a strain-driven process.     

Microphotoluminescence and perfect ordering of SiGe islands on pit-patterned Si(001) substrates

We show that both the morphology and the optoelectronic properties of SiGe islands growing in the pits of periodically pre-patterned Si(001) substrates are determined by the amount of Ge deposited per unit cell of the pattern. Pit-periods (p) ranging from 300 to 900 nm were investigated, and Ge growth was performed by molecular beam epitaxy (MBE) at temperatures of 690 and 760?°C. The ordered SiGe islands show photoluminescence (PL) emission, which becomes almost completely quenched, once a critical island volume is exceeded. By atomic force and transmission electron microscope images we identify the transition from pyramid-shaped to dome-shaped islands with increasing p. Eventually, the nucleation of dislocations in the islands leads to PL quenching. Below a critical Ge coverage a narrowing and a blue shift of the PL emission is observed, as compared to islands grown on a planar reference area of the same sample.     

Ge-rich islands grown on patterned Si substrates by low-energy plasma-enhanced chemical vapour deposition

Si(1-x)Ge(x) islands grown on Si patterned substrates have received considerable attention during the last decade for potential applications in microelectronics and optoelectronics. In this work we propose a new methodology to grow Ge-rich islands using a chemical vapour deposition technique. Electron-beam lithography is used to pre-pattern Si substrates, creating material traps. Epitaxial deposition of thin Ge films by low-energy plasma-enhanced chemical vapour deposition then leads to the formation of Ge-rich Si(1-x)Ge(x) islands (x > 0.8) with a homogeneous size distribution, precisely positioned with respect to the substrate pattern. The island morphology was characterized by atomic force microscopy, and the Ge content and strain in the islands was studied by μRaman spectroscopy. This characterization indicates a uniform distribution of islands with high Ge content and low strain: this suggests that the relatively high growth rate (0.1 nm s(-1)) and low temperature (650?°C) used is able to limit Si intermixing, while maintaining a long enough adatom diffusion length to prevent nucleation of islands outside pits. This offers the novel possibility of using these Ge-rich islands to induce strain in a Si cap.     

Serial and parallel Si, Ge, and SiGe direct-write with scanning probes and conducting stamps

Precise materials integration in nanostructures is fundamental for future electronic and photonic devices. We demonstrate Si, Ge, and SiGe nanostructure direct-write with deterministic size, geometry, and placement control. The biased probe of an atomic force microscope (AFM) reacts diphenylsilane or diphenylgermane to direct-write carbon-free Si, Ge, and SiGe nano and heterostructures. Parallel direct-write is available on large areas by substituting the AFM probe with conducting microstructured stamps. This facile strategy can be easily expanded to a broad variety of semiconductor materials through precursor selection.     

Composition and atomic ordering of Ge/Si(001) wetting layers

A combination of X-ray diffraction with anomalous X-ray scattering at the Ge K edge and specular reflectivity measurements is used to reveal both composition and atomic ordering in Ge:Si wetting layers. By comparing the intensity distribution close to the (400) and (200) surface reflections we show that the Ge wetting layer is composed of a SiGe alloy which exhibits atomic ordering. Due to the Si interdiffusion the wetting layer thickness is larger than the nominal 3 ML Ge deposition. The chemical depth distribution is obtained from X-ray reflectivity measurements and confirms the enhanced Ge interdiffusion. These phenomena evidence the crucial interplay between surface kinetics and intermixing in SiGe thin films and nanostructures on Si(001) substrates.     

SiGe-based FETs: buffer issues and device results

SiGe quantum well structures gain increasing interest in the Si technology. The preparation of a Si channel or a Ge-rich or even a pure Ge channel with a respective two-dimensional carrier gas opens the attractive possibility to fabricate high performance n- or p-type field effect transistors. For both device types, a virtual substrate surface is required which is created by a strain relieved buffer layer grown on a Si standard wafer. The paper reviews various approaches of SiGe buffers including special attempts to reduce the thickness and to improve the quality. N- and p-type modulation-doped field-effect transistors are presented which show comparably good device characteristics and cut-off frequencies in the range of 100–120 GHz.     

UV Raman spectroscopy of group IV nanocrystals embedded in a SiO<Subscript>2</Subscript> matrix

Nanostructures of both Ge nanocrystals formed by thermal oxidation of SiGe layers, and SiGe nanocrystals formed by crystallization of amorphous SiGe nanoparticles deposited by LPCVD have been analyzed by Raman spectroscopy. The nanostructures are formed on a silicon substrate. Raman spectra have been acquired with visible (514.5 nm) and UV (325 nm) excitation lines. When the amount of material is very small, as it has happens in these nanostructures, the visible line is not able to excite the characteristic peaks of the Ge or SiGe in the Raman spectrum; instead the Si second order spectrum of the substrate appears and it can be misinterpreted by attributing it to the Ge–Ge band associated with the nanocrystals. In this work, the use of UV excitation has been demonstrated to enhance the sensitivity respect to the conventional visible excitation, allowing the characteristic peaks of the Ge or SiGe nanocrystals to appear in the spectrum. We attributed this effect to the resonance effects.     

Growth of high Ge content SiGe on (110) oriented Si wafers

Growth of high (above 40%) Ge content SiGe by applying silane and dichlorosilane as Si precursors on (110) Si is investigated. In the case of silane based processes Ge concentration is ~ 20% higher, whereas for dichlorosilane based processes it is ~ 30% lower on (110) Si compared to (100) Si. The morphology of the grown layers is found to be dependent on Ge concentration, layer thickness and process temperature. Use of optimized deposition parameters and adequate thickness results in high quality strained SiGe layers. Integration of high Ge content SiGe layers in multiple gate filed-effect transistor structures shows the expected differences in Ge content on the different Si planes forming Si fin. These differences can be avoided by adjusting the fin orientation on the Si wafer resulting in equal planes on the fin's top and sidewalls. When the investigated SiGe layers are incorporated in the buried channel field effect transistor structures on (110) Si wafers a significant thickening at the active windows edge is observed. It is speculated that this effect is connected with elastic SiGe relaxation caused by a non optimized process temperature.     

Stranski–Krastanov growth of (Si)Ge/Si(001): transmission electron microscopy compared with segregation theory

ABSTRACT

We performed transmission electron microscopy of SiGe/Si(001) and Ge/Si(001) samples that undergo the Stranski–Krastanov transition from flat layer to island growth. With the help of quantitative X-ray maps of those layers, we have determined the total amount of deposited germanium at which islanding commences. The maximum amount of Ge buried in a flat layer amounts to 2.3 monolayers. We show by modelling that it is the strain due to the total amount of Ge atoms deposited that drives the islanding process. At 600°C [400°C], 1.62 [1.74] monolayers of Ge are expected from simulations to segregate towards the surface, the strain of which is sufficient to trigger plastic relaxation by islanding, in agreement with our electron microscope observations.

This is part of a thematic issue on Nanoscale Materials Characterisation and Modeling by Advances Microscopy Methods - EUROMAT.     

Band structure analysis in SiGe nanowires

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.     

Si基Si_(1-x-y)Ge_xC_y合金生长中C的影响

报道了Si基Si1 x yGexCy 合金生长中C对Ge组分和生长速率的抑制作用 ,提出一个Si、Ge、C原子的排列构型 ,从理论上给出了C对Ge组分的抑制度和Ge/C原子比的关系 ,并指出在富Ge情况下C对Ge的抑制作用会趋向于饱和。     

Si/SiGe hetero-junction solar cell with optimization design and theoretical analysis

Performances of solar cells, such as short circuit current density, open-circuit voltage, fill factor, and efficiency of solar cells on the multi-crystalline (mc)-SiGe on the Si with different Ge contents, are compared and investigated in this paper. The average Ge concentration was varied from 0% to ~ 20%. Appropriate addition of Ge in crystal Si is a very effective method to enhance the short circuit current density without degrading the open-circuit voltage owing to the modulation of the SiGe band-gap. The band-gap of the SiGe can be extracted by electron-hole plasma (EHP) model. With an optimization of Ge content and clean process condition, the overall efficiency of a Si/SiGe hetero-junction solar cell with Ge content of 8% is found to be ~ 16% and ~ 4% improvement achieved, as compared to the control multi-crystalline (mc)-Si solar cell. The theoretical simulations and analyses can help design the high efficiency Si/SiGe hetero-junction solar cell.     

Geordnete Nanoinseln für schnellere Transistoren

The spontaneous formation of self‐assembled nanostructures in strained‐layer epitaxy offers the possibility to fabricate perfectly passivated nanostructures on planar substrate surfaces in a very simple way. However, as plentiful investigations are on these nano‐islands, as limited is the control over their exact position. Since fabrication is based on a self‐assembling process the nanostructures nucleate more or less randomly on the surface. Hence, one of the biggest challenges in this research field is the controlled ordering of self‐assembled nanostructures on a planar substrate surface. Taking the SiGe material system it is shown that pre‐patterning in combination with self‐assembly leads to long‐range ordered lines of self‐assembled nanoislands. The ordering opens the way to a new concept for faster Si‐based field effect transistors, which exploits the specific electronic properties of self‐assembled Ge/Si islands. The transistor is called 'DotFET'.     

SiGe/Si异质结光电器件

SiGe/Si异质结光电器件及其光电集成(OEIC)是硅基光电研究的一个非常引人注目的领域.综述了SiGe/Si异质结材料的基本性质,SiGe/Si异质结光电器件的结构、性能、应用及其光电集成.重点介绍了SiGe/Si光电探测器及其与其他相关器件的集成.     

Ge/SiGe superlattices for thermoelectric energy conversion devices

Ge-rich multiple quantum well heterostructures have been investigated as engineered material for efficient thermoelectric generators monolithically integrated on silicon substrates. Thick Ge/SiGe multilayers on Si substrates designed for lateral thermoelectric devices have been grown and characterized in which electrical and thermal conduction occur parallel to the heterostructure interfaces. In this study, an overview of the investigated structures is presented together with results from X-ray scattering and transmission electron microscopy experiments. These analyses confirm the high quality of the material and the uniformity of the structure over the whole deposited thickness. Important parameters in terms of the optimization of the material quality which could affect thermoelectric properties, such as the interfaces roughness and the threading dislocation density, have also been evaluated. Preliminary electrical and Seebeck coefficient measurements indicate the viability of this material for the realization of thermoelectric devices.     

Ge oxidation in the remaining cores of Si(1-x)Ge(x) nanowires after prolonged oxidation

For this investigation of the Ge behavior of condensed Si(1-y)Ge(y) (y > x) cores during the oxidation of Si(1-x)Ge(x) nanowires, Si(1-x)Ge(x) nanowires were grown in a tube furnace by the vapor-liquid-solid method and thermally oxidized. The test results were characterized using several techniques of transmission electron microscopy. The two types of Ge condensation are related to the diameter and Ge content of the nanowires. The consumption of Si atoms in prolonged oxidation caused the condensed SiGe cores to become Ge-only cores; and the continuous oxidation resulted in the oxidation of the Ge cores. The oxidation of Ge atoms was confirmed by scanning transmission electron microscopy.     

Growth of one-dimensional Si/SiGe heterostructures by thermal CVD

The first results on a simple new process for the direct fabrication of one-dimensional superlattices using common CVD chambers are presented. The experiments were carried out in a 200?mm industrial Centura reactor (Applied Materials). Low dimensionality and superlattices allow a significant increase in the figure of merit of thermoelectrics by controlling the transport of phonons and electrons. The monocrystalline nanowires produced according to this process are both one-dimensional and present heterostructures, with very thin layers (40?nm) of Si and SiGe. Concentrations up to 30?at.% Ge were obtained in the SiGe parts. Complementary techniques including transmission electronic microscopy (TEM), selected area electron diffraction (SAED), energy dispersive x-ray spectroscopy (EDS), scanning transmission electron microscopy (STEM) in bright field and high angle annular dark field (HAADF STEM), and energy-filtered transmission electron microscopy (EF-TEM) were used to characterize the nanoheterostructures.     

Band-offset driven efficiency of the doping of SiGe core-shell nanowires

Impurity doping of semiconducting nanowires has been predicted to become increasingly inefficient as the wire diameter is reduced, because impurity states get deeper due to quantum and dielectric confinement. We show that efficient n- and p-type doping can be achieved in SiGe core-shell nanowires as thin as 2 nm, taking advantage of the 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.