Stacked Ge nanocrystals with ultrathin SiO₂ separation layers

The aim of this work is the tailored growth of Ge nanocrystals (NCs) in (GeO(x)/SiO(2)) multilayers (ML) for photovoltaic applications. For this purpose the fabrication of regularly stacked Ge NCs separated by ultrathin SiO(2) layers is essential to enable charge carrier transport by direct tunnelling. In this paper we report on the fabrication of (GeO(x)/SiO(2))(50) multilayer stacks via reactive dc magnetron sputtering and Ge NCs formation after subsequent annealing. It is shown that magnetron sputtering allows us to deposit very regular ML stacks with a total thickness of about 300 nm, characterized by ultrathin (down to 1 nm) and very smooth (roughness ～ 0.6 nm) SiO(2) separation layers. A main challenge is to keep these properties for a thermal budget necessary to form Ge NCs. For this reason, the temperature dependence of phase separation. Ge crystallization and ML morphology was investigated by Rutherford backscattering, x-ray scattering, Raman spectroscopy and electron microscopy. The formation of size confined Ge NCs of about 5 nm after annealing of only 550?°C is confirmed. This low thermal budget ensures the suppression of GeO emanation and multilayer stability. Spectroscopic ellipsometry was applied to determine the optical Ge NC bandgap to (1.65 ± 0.5) eV.     

Epitaxial Mn(5)Ge(3) nano-islands on a Ge(001) surface

The growth behavior and atomic structure of Mn germanide, grown on Ge(001), is studied with x-ray diffraction and scanning probe microscopy. The amorphous clusters of as-deposited Mn are crystallized into Mn(5)Ge(3) nano-islands with a size of ～100?nm by solid phase epitaxy. At low coverage, the shape of the nano-islands is plateau-like, while at increased coverage it becomes mound-like. At the flat top of the plateau-like nano-islands, the hexagonal atomic structure is resolved. It is interpreted, with the help of first-principles study, as a Mn-terminated Mn(5)Ge(3)(0001) structure.     

Na+,Ca2+和Al3+对聚丙烯酸钠的自组装行为的影响

采用荧光探针、动态光散射和透射电子显微镜研究了NaCl、CaCl<'2>和AlCl<.3>对水溶液中的聚丙烯酸钠(PAANa)自组装行为的影响.实验结果表明,在含有NaCl的聚丙烯酸钠水溶液中,分子链以线团形式存在;而Ca<'2+>或Al<'3+>与聚丙烯酸钠主链间通过静电相互作用自组装形成疏水胶束.当Ca<'2+>/...     

Growth of Ge islands on Si substrates

We present a study of Ge islands formation on Si(100) substrates using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Samples were prepared by magnetron sputtering of a 5 nm thick Ge layer in a very high vacuum on Si(100) substrate held at different temperatures. The vertical cut (perpendicular to the surface) of the experimental 2D GISAXS pattern has been fitted using a Guinier approximation. The optimum temperature for the islands formation was 650 °C. At this temperature, islands grow in conical shape with very similar dimensions; however, inter-island distances varied significantly.     

Observation of free surface-induced bending upon nanopatterning of ultrathin strained silicon layer

We provide evidence of nanopatterning-induced bending of an ultrathin tensile strained silicon layer directly on oxide. This strained layer is achieved through the epitaxial growth of silicon on a Si(0.84)Ge(0.16) virtual substrate and subsequent transfer onto a SiO(2)-capped silicon substrate by combining hydrophilic wafer bonding and the ion-cut process. Using high resolution transmission electron microscopy, we found that the upper face of the strained silicon nanostructures fabricated from the obtained heterostructure using electron beam lithography and dry reactive ion etching displays a concave shape. This bending results from the free-surface-induced strain relaxation, which implies lattice out-of-plane expansion near the edges and concomitant contraction at the center. For a ～ 110 nm × 400 nm × 20 nm nanostructure, the bending is associated with an angle of 1.5° between the [Formula: see text] vertical atomic planes at the edges of the ～ 110 nm side. No bending is, however, observed at the strained Si/SiO(2) interface. This phenomenon cannot be explained by the classical Stoney's formula or related formulations developed for nanoscale thin films. Here we employed a continuum mechanical approach to describe these observations using three-dimensional numerical calculations of relaxation-induced lattice displacements.     

Semiconductor surface doping by electron beam melting of thin evaporated layers

We have studied the distribution profiles of Cu and Sb introduced into Si substrates, and Sb introduced into Ge substrates by electron beam melting of thin (～30nm) layers of the dopant evaporated onto the substrate. Surface evaporation of the dopant during melting was reduced by a cover layer of the substrate material evaporated on top. We propose a novel model which explains solute surface accumulation phenomena oberved in laser and electron beam surface melting experiments. It is based on a mechanism of solute immobilization at the very surface.     

面向5nm CMOS技术代堆叠纳米线释放工艺研究

针对5 nm CMOS技术代亟待解决的纳米线释放难题, 通过实验探究了HF (6%) ∶H₂O₂ (30%) ∶CH₃COOH (99.8%) =1∶2∶3的混合溶液放置时间对纳米尺度外延堆叠GeSi/Si/GeSi/Si结构释放影响。混合溶液放置时间在48 h内, GeSi层的腐蚀速率会随着溶液的放置时间的增加而变大, 48 h后腐蚀速率趋于稳定。另外, 在厚度相同的情况下, Ge Si层的腐蚀速率会随着Ge含量的增加而变大。本文实现了腐蚀纳米尺度GeSi的同时没有对Si造成损伤, 对于厚度为31.3 nm的GeSi层, 腐蚀的深宽比达到了17∶1, 而且没有出现倒塌现象。该湿法腐蚀工艺对于5 nm及以下技术代堆叠纳米线制造、SON结构、新型MEMS和传感器件制造等具有一定的借鉴和指导意义。     

Electrical characteristics of Ge-based metal-insulator-semiconductor devices with Ge3N4 dielectrics formed by plasma nitridation

We have fabricated pure germanium nitrides (Ge3N4) using high-density plasma nitridation and investigated electrical properties of Au/Ge3N4/Ge capacitors. We achieved equivalent oxide thickness (EOT) of 1.4 nm, and dielectric constant of Ge3N4 was estimated to be 9.7. The gate leakage current density of 4.3 A/cm2 in the accumulation condition at V(fb)-1 V, where V(fb) is the flatband voltage, was one order of magnitude lower than that of conventional poly-Si/SiO2/Si stacks. The interface state density (D(it)) of Ge3N4/Ge interfaces evaluated by a low-temperature conductance method exhibited a minimum value of 9.4 x 10(11) cm(-2)eV(-1) at E - E(v) = 0.27 eV. Furthermore, the insulating property and interface quality of Ge3N4/Ge system was found to be thermally stable up to 650 degrees C. These results indicate that Ge3N4 is a promising candidate for either a gate insulator or an interfacial layer under high-k dielectrics for Ge-MIS devices.     

Effect of the growth rate on the morphology and structural properties of hut-shaped Ge islands in Si(001)

The effect of Ge deposition rate on the morphology and structural properties of self-assembled Ge/Si(001) islands was studied. Ge/Si(001) layers were grown by solid-source molecular-beam epitaxy at 500?°C. We adjusted the Ge coverage, 6?monolayers (ML), and varied the Ge growth rate by a factor of 100, R = 0.02-2?ML?s(-1), to produce films consisting of hut-shaped Ge islands. The samples were characterized by scanning tunnelling microscopy, Raman spectroscopy, and Rutherford backscattering measurements. The mean lateral size of Ge nanoclusters decreases from 14.1?nm at R = 0.02?ML?s(-1) to 9.8?nm at R = 2?ML?s(-1). The normalized width of the size distribution shows non-monotonic behaviour as a function of R and has a minimum value of 19% at R = 2?ML?s(-1). Ge nanoclusters fabricated at the highest deposition rate demonstrate the best structural quality and the highest Ge content (～0.9).     

Nanoscale control of an interfacial metal-insulator transition at room temperature

Experimental and theoretical investigations have demonstrated that a quasi-two-dimensional electron gas (q-2DEG) can form at the interface between two insulators: non-polar SrTiO3 and polar LaTiO3 (ref. 2), LaAlO3 (refs 3-5), KTaO3 (ref. 7) or LaVO3 (ref. 6). Electronically, the situation is analogous to the q-2DEGs formed in semiconductor heterostructures by modulation doping. LaAlO3/SrTiO3 heterostructures have recently been shown to exhibit a hysteretic electric-field-induced metal-insulator quantum phase transition for LaAlO3 thicknesses of 3 unit cells. Here, we report the creation and erasure of nanoscale conducting regions at the interface between two insulating oxides, LaAlO3 and SrTiO3. Using voltages applied by a conducting atomic force microscope (AFM) probe, the buried LaAlO3/SrTiO3 interface is locally and reversibly switched between insulating and conducting states. Persistent field effects are observed using the AFM probe as a gate. Patterning of conducting lines with widths of approximately 3 nm, as well as arrays of conducting islands with densities >10(14) inch(-2), is demonstrated. The patterned structures are stable for >24 h at room temperature.     

Direct solution synthesis of Pd nanowire networks and their application in surface-enhanced Raman scattering

Highly branched Pd 2D nanowire networks are prepared by a template-?and surfactant-free method in an ethylene glycol (EG)-dimethyl sulfoxide (DMSO) mixture under mild conditions. Pd nanowire networks with lengths ranging from hundreds of nanometers to several micrometers and branches with uniform diameters of ～8?nm are grown. The most likely formation mechanism is also proposed. The as-produced Pd nanowire networks exhibit high surface-enhanced Raman scattering activity for 4-mercaptopyridine probe molecules with 5.0 × 10(-8)?M concentration.     

Tip-enhanced Rayleigh scattering and photoluminescence from semiconductor nanoparticles

Tip-enhanced Rayleigh scattering images of Ge quantum dots grown on a Si substrate have been observed at room temperature. Changing the wavelength of the incidence light from 405 to 590 nm, the contrast of the images is reversed. It is found that the scattering intensity depends on the dielectric constants of the materials under the probe. By changing the wavelength of the incident light, we have obtained information about the dielectric constant dispersion of single Ge quantum dots. The spectral peak position of single Ge quantum dots is found to shift to higher energy, compared to that of bulk Ge. Tip-enhanced photoluminescence from an In0.25Ga0.75N film at room temperature has also been reported. The strong local enhancement of the photoluminescence of the localized excitons has been observed in the vicinity of a gold nano-particle attached to the end of the probe. A coupling to plasmons in the gold nano-particle yields strong enhancement of the photoluminescence.     

Fabrication of multilayered Ge nanocrystals by magnetron sputtering and annealing

Multilayered Ge nanocrystals embedded in Si and Ge oxide films have been fabricated on Si?substrate by a (SiO(2)+Ge)/(SiO(2)+GeO(2)) superlattice approach, using an rf magnetron sputtering technique with a Ge+SiO(2) composite target and subsequent thermal annealing in N(2) ambient at 750?°C for 5?min. X-ray diffraction (XRD) measurements indicated the formation of Ge nanocrystals with an average size estimated to be 9.8?nm. Raman scattering spectra showed a peak of the Ge-Ge vibrational mode shifted downwards to 298.8?cm(-1), which was caused by quantum confinement of phonons in the Ge nanocrystals. X-ray photoemission spectroscopy (XPS) analysis demonstrated that the Ge chemical state is mainly Ge(0) in the (SiO(2)+Ge) layer and Ge(4+) in the (SiO(2)+GeO(2)) layer in the superlattice structure. Transmission electron microscopy (TEM) revealed that Ge nanocrystals were confined in (SiO(2)+Ge) layers, and had good crystallinity. This superlattice approach significantly improved both the size uniformity of Ge nanocrystals and their uniformity of spacing on the 'Z' growth direction compared with the conventional Ge-ncs fabrication method using a single and thick SiO(2) matrix film.     

In situ RHEED control of self-organized Ge quantum dots

In situ registration of high-energy electron diffraction patterns was used for constructing the diagram of structural and morphological states of the Ge film on the Si(100) surface. The following regions identified in the diagram: two-dimensional (2D)-growth, ‘hut’- and ‘dome’-clusters, ‘dome’-clusters with misfit dislocations at the interface. Variations in the lattice constants of the Ge film during the MBE growth on the Si(100) surface were determined. An increase in the lattice constant at the (100) surface was attributed to the elastic deformation at the stage of 2D growth and formation of ‘hut’-clusters and to the plastic relaxation for the ‘dome’-clusters. As a result, epitaxial silicon structures with germanium quantum dots of 15 nm base size at the density of 3×10¹¹ cm⁻² were synthesized. The total electron structure of the hole spectrum of Ge quantum dots in Si was established.     

Size-dependent electron transfer from PbSe quantum dots to SnO2 monitored by picosecond Terahertz spectroscopy

We report the direct and unambiguous determination of electron transfer rates and efficiencies from PbSe quantum dots (QDs) to mesoporous SnO2 films. We monitor the time-dependent electron density within the oxide with picosecond time resolution using Terahertz spectroscopy, following optical excitation of the QDs using a femtosecond laser pulse. QD-oxide electron transfer occurs with efficiencies of ～2% in our samples under 800 nm pumping with a marked dependence on QD size, ranging from ～100 ps injection times for the smallest, ～2 nm diameter QDs, to ～1 ns time scale for ～7 nm QDs. The size-dependent electron transfer rates are modeled within the framework of Marcus theory and the implications of the results for device design are discussed.     

Fabrication and characteristics of porous germanium films

Abstract

Porous germanium films with good adhesion to the substrate were produced by annealing GeO₂ ceramic films in H₂ atmosphere. The reduction of GeO₂ started at the top of a film and resulted in a Ge layer with a highly porous surface. TEM and Raman measurements reveal small Ge crystallites at the top layer and a higher degree of crystallinity at the bottom part of the Ge film; visible photoluminescence was detected from the small crystallites. Porous Ge films exhibit high density of holes (10²⁰ cm^?3) and a maximum of Hall mobility at ～225 K. Their p-type conductivity is dominated by the defect scattering mechanism.     

Near-infrared photoluminescence in germanium oxide enclosed germanium nano- and micro-crystals

We have studied the near-infrared photoluminescence properties of free-standing germanium nano-crystals (20?nm on average) and micro-crystals (60?μm on average) at 80-300?K. Two peaks were observed at ～1.0 and ～1.4?eV from both the nano-?and micro-crystals. The integrated PL (I(PL)) intensity of the nano-crystals is about an order of magnitude stronger than that of the micro-crystals and the I(PL) is also enhanced by ageing in air for both crystals. The ～1.0?eV peak position does not change with either the crystal size or temperature. We suggest that the deep traps located at the interfacial region between the surface GeO(2) layer and the bulk crystal Ge is responsible for the near-infrared PL.     

Growth of Ge nanoparticles on SiO2/Si interfaces during annealing of plasma enhanced chemical vapor deposited thin films

Multilayer germanosilicate (Ge:SiO₂) films have been grown by plasma enhanced chemical vapor deposition. Each Ge:SiO₂ layer is separated by a pure SiO₂ layer. The samples were heat treated at 900 °C for 15 and 45 min. Transmission electron microscopy investigations show precipitation of particles in the layers of highest Ge concentration. Furthermore there is evidence of diffusion between the layers. This paper focuses mainly on observed growth of Ge particles close to the interface, caused by Ge diffusion from the Ge:SiO₂ layer closest to the interface through a pure SiO₂ layer and to the interface. The particles grow as spheres in a direction away from the interface. Particles observed after 15 min anneal time are 4 nm in size and are amorphous, while after 45 min anneal time they are 7 nm in size and have a crystalline diamond type Ge structure.     

Chemically isolated graphene nanoribbons reversibly formed in fluorographene using polymer nanowire masks

We demonstrated the fabrication of graphene nanoribbons (GNRs) as narrow as 35 nm created using scanning probe lithography to deposit a polymer mask(1-3) and then fluorinating the sample to isolate the masked graphene from the surrounding wide band gap fluorographene. The polymer protected the GNR from atmospheric adsorbates while the adjacent fluorographene stably p-doped the GNRs which had electron mobilities of ～2700 cm2/(V·s). Chemical isolation of the GNR enabled resetting the device to nearly pristine graphene.     

Effects of Pressure on the Solidification Microstructure of Mg_(65)Cu_(25)Y_(10) Alloy

The Mg65Cu25Y10 melts were quenched at a temperature of 973 K under various pressures in the range of 2-5 GPa and ambient pressure. The microstructure of the solidified specimens has been investigated by X-ray diffraction, transmission electron microscope and electron probe microanalysis. Experimental results show that the pressure has a great influence on the solidification microstructure of the Mg65Cu25Y10. At ambient pressure, the solidification products are Mg2(Cu,Y) and a very small amount of Y2O3 inclusion. As the pressure is above 2 GPa, a new Cu2(Y,Mg) phase appears, while Y2O3 is not observed at the pressure of 3, 4 and 5 GPa. When the pressure increases from 2 GPa to 5 GPa, the grain sizes of Mg2(Cu,Y) and Cu2(Y,Mg) decrease from 125, 96 nm to 80, 7 nm, respectively. The mechanisms for the effects of the pressure on the phase evolution and microstructure during solidification process of Mg65Cu25Y10 alloy have been discussed.