Hexagonal-Ge Nanostructures with Direct-Bandgap Emissions in a Si-Based Light-Emitting Metasurface.

Si-based emitters have been of great interest as an ideal light source for monolithic optical-electronic integrated circuits (MOEICs) on Si substrates. However, the general Si-based material is a diamond structure of cubic lattice with an indirect band gap, which cannot emit light efficiently. Here, hexagonal-Ge (H-Ge) nanostructures within a light-emitting metasurface consisting of a cubic-SiGe nanodisk array are reported. The H-Ge nanostructure is naturally formed within the cubic-Ge epitaxially grown on Si (001) substrates due to the strain-induced nanoscale crystal structure transformation assisted by far-from-equilibrium growth conditions. The direct-bandgap features of H-Ge nanostructures are observed and discussed, including a rather strong and linearly power-dependent photoluminescence (PL) peak around 1562 nm at room temperature and temperature-insensitive PL spectrum near room temperature. Given the direct-bandgap nature, the heterostructure of H-Ge/C-Ge, and the compatibility with the sophisticated Si technology, the H-Ge nanostructure has great potential for innovative light sources and other functional devices, particularly in Si-based MOEICs.

Unique Enhancement of the Whispering Gallery Mode in Hexagonal Microdisk Resonator Array with Embedded Ge Quantum Dots on Si.

The coupling between the quantum dots (QDs) and silicon-based microdisk resonator facilitates enhancing the light-matter interaction for the novel silicon-based light source. However, the typical circular microdisks embedded with Ge QDs still have several issues, such as wide spectral bandwidth, difficult mode selection, and low waveguide coupling efficiency. Here, by a promising structural modification based on the mature nanosphere lithography (NSL), we fabricate a large area hexagonal microdisk array embedded with Ge QDs in order to enhance the near-infrared light emissions by a desired whispering gallery modes (WGMs). By comparing circular microdisks with comparable sizes, we found the unique photoluminescence enhancement effect of hexagonal microdisks for certain modes. We have confirmed the WGMs which are supported by the microdisks and the well-correlated polarized modes for each resonant peak observed in experiments through the Finite Difference Time Domain (FDTD) simulation. Furthermore, the unique enhancement of the TE5,1 mode in the hexagonal microdisk is comparatively analyzed through the simulation of optical field distribution in the cavity. The larger enhanced region of the optical field contains more effectively coupled QDs, which significantly enhances the PL intensity of Ge QDs. Our findings offer a promising strategy toward a distinctive optical cavity that enables promising mode manipulation and enhancement effects for large-scale, cost-effective photonic devices.

Competitive Growth of Ge Quantum Dots on a Si Micropillar with Pits for a Precisely Site-Controlled QDs/Microdisk System.

Semiconductor quantum dots (QDs)/microdisks promise a unique system for comprehensive studies on cavity quantum electrodynamics and great potential for on-chip integrated light sources. Here, we report on a strategy for precisely site-controlled Ge QDs in SiGe microdisks via self-assembly growth of QDs on a micropillar with deterministic pits and subsequent etching. The competitive growth of QDs in pits and at the periphery of the micropillar is disclosed. By adjusting the growth temperature and Ge deposition, as well as the pit profiles, QDs can exclusively grow in pits that are exactly located at the field antinodes of the corresponding cavity mode of the microdisk. The inherent mechanism of the mandatory addressability of QDs is revealed in terms of growth kinetics based on the non-uniform surface chemical potential around the top of the micropillar with pits. Our results demonstrate a promising approach to scalable and deterministic QDs/microdisks with strong light-matter interaction desired for fundamental research and technological applications.

Direct observation of hot-electron-enhanced thermoelectric effects in silicon nanodevices.

The study of thermoelectric behaviors in miniatured transistors is of fundamental importance for developing bottom-level thermal management. Recent experimental progress in nanothermetry has enabled studies of the microscopic temperature profiles of nanostructured metals, semiconductors, two-dimensional material, and molecular junctions. However, observations of thermoelectric (such as nonequilibrium Peltier and Thomson) effect in prevailing silicon (Si)-a critical step for on-chip refrigeration using Si itself-have not been addressed so far. Here, we carry out nanothermometric imaging of both electron temperature (Te) and lattice temperature (TL) of a Si nanoconstriction device and find obvious thermoelectric effect in the vicinity of the electron hotspots: When the electrical current passes through the nanoconstriction channel generating electron hotspots (with Te~1500 K being much higher than TL~320 K), prominent thermoelectric effect is directly visualized attributable to the extremely large electron temperature gradient (~1 K/nm). The quantitative measurement shows a distinctive third-power dependence of the observed thermoelectric on the electrical current, which is consistent with the theoretically predicted nonequilibrium thermoelectric effects. Our work suggests that the nonequilibrium hot carriers may be potentially utilized for enhancing the thermoelectric performance and therefore sheds new light on the nanoscale thermal management of post-Moore nanoelectronics.

Extensive Broadband Near-Infrared Emissions from GexSi1-x Alloys on Micro-Hole Patterned Si(001) Substrates.

Broadband near-infrared (NIR) luminescent materials have been continuously pursued as promising candidates for optoelectronic devices crucial for wide applications in night vision, environment monitoring, biological imaging, etc. Here, graded GexSi1-x (x = 0.1-0.3) alloys are grown on micro-hole patterned Si(001) substrates. Barn-like islands and branch-like nanostructures appear at regions in-between micro-holes and the sidewalls of micro-holes, respectively. The former is driven by the efficient strain relation. The latter is induced by the dislocations originating from defects at sidewalls after etching. An extensive broadband photoluminescence (PL) spectrum is observed in the NIR wavelength range of 1200-2200 nm. Moreover, the integrated intensity of the PL can be enhanced by over six times in comparison with that from the reference sample on a flat substrate. Such an extensively broad and strong PL spectrum is attributed to the coupling between the emissions of GeSi alloys and the guided resonant modes in ordered micro-holes and the strain-enhanced decomposition of alloys during growth on the micro-hole patterned substrate. These results demonstrate that the graded GexSi1-x alloys on micro-hole pattered Si substrates may have great potential for the development of innovative broadband NIR optoelectronic devices, particularly to realize entire systems on a Si chip.

Artificial Graphene on Si Substrates: Fabrication and Transport Characteristics.

Artificial graphene (AG) based on a honeycomb lattice of semiconductor quantum dots (QDs) has been of great interest for exploration and applications of massless Dirac Fermions in semiconductors thanks to the tunable interplay between the carrier interactions and the honeycomb topology. Here, an innovative strategy to realize AG on Si substrates is developed by fabricating a honeycomb lattice of Au nanodisks on a Si/GeSi quantum well. The lateral potential modulation induced by the nanoscale Au/Si Schottky junction results in the formation of quantum dots arranged in a honeycomb lattice to form AG. Nonlinear current-voltage curves of the AG reveal conductance phase transitions with switch on/off voltages, a large electric hysteresis loop, and a strong sharp current peak accompanied by a group of differential-conductance peaks and negative differential conductance around the switch-on voltage, which can be modulated by temperature and light. These features are interpreted by a model based on the Coulomb blockade effect, the collective resonant tunneling, and the coupling of holes in the AG. Our results not only demonstrate an approach to the formation but also will greatly stimulate the characterizations and the applications of innovative semiconductor-based AG.

Promising modulation of self-assembled Ge-rich QDs by ultra-heavy phosphorus doping.

Self-assembled Ge-rich quantum dots (QDs) can not only act as a prototype model for the fundamental studies of heteroepitaxial growth but also have great potential in optoelectronic devices for telecommunication and monolithic optical-electronic integrated circuits. Here, we report the unique features of Ge-rich QDs ultra-heavily doped with phosphorus (P) and embedded in a thin SiGe alloy film on Si (001) substrates. The ultra-heavy P doping considerably reduces the size of Ge-rich QDs and improves their uniformity. The inherent mechanism is associated with the reductions of both surface energy and the diffusion length of adatoms during QD growth promoted by the P dopants. Raman spectra indicate that the Ge content and strain in QDs are essentially not modified by the P doping. Particularly, the power- and temperature-dependent photoluminescence (PL) spectra demonstrate a type-I band alignment of Ge-rich QDs/SiGe alloy film due to the ultra-heavy P doping, which gives rise to additional low energy levels of electrons in QDs. Moreover, the PL of Ge-rich QDs is remarkably enhanced by ultra-heavy P doping at temperatures over 80 K. Over 3 times enhancement is obtained at 245 K. These results indicate that the overall quantum efficiency of Ge-rich QDs is substantially improved by the ultra-heavy P doping, which facilitates the applications of Ge-rich QDs in Si-based innovative optoelectronic devices.

Manipulations of light by ordered micro-holes in silicon substrates.

Ordered micro-holes with controllable period, diameter and depth are fabricated in Si (001) substrates via a feasible approach based on nanosphere lithography. They dramatically reduce the reflectance in a broad wavelength range of 400-1000 nm, which can be deliberately modulated by tailoring their geometrical parameters. The simulated reflectance via finite-difference time-domain (FDTD) method agrees well with the experimental data. The FDTD simulations also demonstrate substantially enhanced light absorption of a Si thin film with ordered micro-holes. Particularly, the light-filled distributions around micro-holes disclose fundamental features of two types of modes, channel modes and guided modes, involving the wavelength-dependence, the origin, the dominant location region and the interference pattern of the light field around micro-holes. Our results not only provide insights into the antireflection and the substantially enhanced absorption of light by ordered micro-holes, but also open a door to optimizing micro-hole arrays with desired light field distributions for innovative device applications.

Fabrication of High-Quality and Strain-Relaxed GeSn Microdisks by Integrating Selective Epitaxial Growth and Selective Wet Etching Methods.

GeSn is a promising material for the fabrication of on-chip photonic and nanoelectronic devices. Processing techniques dedicated to GeSn have thus been developed, including epitaxy, annealing, ion implantation, and etching. In this work, suspended, strain-relaxed, and high-quality GeSn microdisks are realized by a new approach without any etching to GeSn alloy. The GeSn alloy was grown on pre-patterned Ge (001) substrate by molecular beam epitaxy at low temperatures. The transmission electron microscopy and scanning electron microscopy were carried out to determine the microstructures of the GeSn samples. The microdisks with different diameters of Ge pedestals were fabricated by controlling the selective wet etching time, and micro-Raman results show that the microdisks with different dimensions of the remaining Ge pedestals have different extents of strain relaxation. The compressive strain of microdisks is almost completely relaxed under suitable conditions. The semiconductor processing technology presented in this work can be an alternative method to fabricate innovative GeSn and other materials based micro/nano-structures for a range of Si-compatible photonics, 3D-MOSFETs, and microelectromechanical device applications.

Association between SFRP promoter hypermethylation and different types of cancer: A systematic review and meta-analysis.

Abnormal methylation of secreted frizzled-related proteins (SFRPs) has been observed in various human cancer types. The loss of SFRP gene expression induces the activation of the Wnt pathway and is a vital mechanism for tumorigenesis and development. The aim of the present systematic review was to assess the association between SFRP methylation and cancer risk. A meta-analysis was systematically conducted to assess the clinicopathological significance of SFRP methylation in cancer risk. The Cochrane Library, PubMed and Web of Science databases were comprehensively searched, and 83 publications with a total of 21,612 samples were selected for the meta-analysis. The pooled odds ratios (ORs) and corresponding 95% confidence intervals (CIs) were calculated to evaluate the degree of associations between SFRP promoter methylation and cancer risk. Subgroup analysis, meta regression and sensitivity analysis were used to identify the potential sources of heterogeneity. SFRP1, SFRP2, SFRP4 and SFRP5 hypermethylation was significantly associated with cancer risk, with ORs of 8.48 (95% CI, 6.26-11.49), 8.21 (95% CI, 6.20-10.88), 11.41 (95% CI, 6.42-20.30) and 6.34 (95% CI, 3.86-10.42), respectively. SFRP2 methylation was significantly associated with differentiation in colorectal cancer (OR, 2.16; 95% CI, 1.02-4.56). The results of the present study demonstrated that SFRP methylation may contribute to carcinogenesis, especially in certain cancer types, including hepatocellular carcinoma and colorectal cancer.

Optical properties demonstrating strong coupling of compactly arranged Ge quantum dots.

Coupled quantum dots (QDs) have been extensively investigated for their unique collective properties and potential applications in optoelectronic devices. Herein, dense Ge QDs with a compact arrangement in-plane are readily obtained. Systematic studies on power-dependent photoluminescence (PL) from the QD ensemble demonstrate a PL peak with a superior intensity, a constant peak energy and width as a function of the excitation power. Moreover, the temperature-dependent PL spectra exhibit a pronounced red-shift and a rapid PL quenching with increasing temperature. Such PL properties are attributed to the formation of miniband and the delocalization of holes in the QD ensemble due to the strong coupling between closely adjacent QDs.

An array of SiGe nanodisks with Ge quantum dots on bulk Si substrates demonstrating a unique light-matter interaction associated with dual coupling.

Si-Based microdisks with Ge quantum dots (QDs) have been of great interest due to their potential as the desired light source for monolithic optical-electronic integrated circuits (MOEICs), as well as in the studies of cavity quantum electrodynamics (CQED). Here, we report on unique SiGe nanodisk arrays with embedded Ge QDs directly realized on bulk Si substrates. Their superior optoelectronic properties are demonstrated by remarkably enhanced photoluminescence due to the coupling between QD emissions and cavity modes of the nanodisk, even though the size of the nanodisk is much smaller than the wavelengths of cavity modes. Moreover, spectral shifts of cavity modes and an intensity modulation related to the interference of in-phase emissions from QDs in the nanodisk array are observed due to alternative coupling between nanodisks. A hybridized mode, originating from the spectral overlap between the anapole mode of individual nanodisks and the guided mode of periodic nanodisks, results in strong luminescence even at room temperature. Our results shed new light on the fundamental physics of CQED in nanodisk arrays with embedded QDs. Given their superior optoelectronic properties, the feasibility of carrier injection and thermal dissipation through the Si pedestal, the presented SiGe nanodisks with embedded Ge QDs will have great potential for application in innovative optoelectronic devices, particularly as the light source for MOEICs.

Toward precise site-controlling of self-assembled Ge quantum dots on Si microdisks.

A feasible route is developed toward precise site-controlling of quantum dots (QDs) at the microdisk periphery, where most microdisk cavity modes are located. The preferential growth of self-assembled Ge QDs at the periphery of Si microdisks is discovered. Moreover, both the height and linear density of Ge QDs can be controlled by tuning the amount of deposited Ge and the microdisk size. The inherent mechanisms of these unique features are discussed, taking into account both the growth kinetics and thermodynamics. By growing Ge on the innovative Si microdisks with small protrusions at the disk periphery, the positioning of Ge QDs at the periphery can be exactly predetermined. Such a precise site-controlling of Ge QDs at the periphery enables the location of the QD right at the field antinodes of the cavity mode of the Si microdisk, thereby achieving spatial matching between QD and cavity mode. These results open a promising door to realize the semiconductor QD-microdisk systems with both spectral and spatial matching between QDs and microdisk cavity modes, which will be the promising candidates for exploring the fundamental features of cavity quantum electrodynamics and the innovative optoelectronic devices based on strong light-matter interaction.

Promising features of low-temperature grown Ge nanostructures on Si(001) substrates.

High-quality Ge nanostructures are obtained by molecular beam epitaxy of Ge on Si(001) substrates at 200 °C and ex situ annealing at 400 °C. Their structural properties are comprehensively characterized by atomic force microscopy, transmission electron microscopy and Raman spectroscopy. It is disclosed that they are almost defect free except for some defects at the Ge/Si interface and in the subsequent Si capping layer. The misfit strain in the nanostructure is substantially relaxed. Dramatically strong photoluminescence (PL) from the Ge nanostructures is observed. Detailed analyses on the power- and temperature-dependent PL spectra, together with a self-consistent calculation, indicate the confinement and the high quantum efficiency of excitons within the Ge nanostructures. Our results demonstrate that the Ge nanostructures obtained via the present feasible route may have great potential in optoelectronic devices for monolithic optical-electronic integration circuits.

Electrostatic effect of Au nanoparticles on near-infrared photoluminescence from Si/SiGe due to nanoscale metal/semiconductor contact.

Photoluminescence (PL) from Si and SiGe is comprehensively modified by Au NPs under excitation without surface plasmon resonance. Moreover, the PL sensitively depends on the size of the Au NPs, the excitation power and the thickness of the Si layer between the Au NPs and SiGe. A model is proposed in terms of the electrostatic effects of Au NPs naturally charged by electron transfer through the nanoscale metal/semiconductor Schottky junction without an external bias or external injection of carriers. The model accounts well for all the unique PL features. It also reveals that Au NPs can substantially modify the energy band structures, distribution and transition of carriers in the nanoscale region below the Au NPs. Our results demonstrate that Au NPs on semiconductors can efficiently modulate light-matter interaction.

Large-Area Au-Nanoparticle-Functionalized Si Nanorod Arrays for Spatially Uniform Surface-Enhanced Raman Spectroscopy.

In this study, large-area hexagonal-packed Si nanorod (SiNR) arrays in conjunction with Au nanoparticles (AuNPs) were fabricated for surface-enhanced Raman spectroscopy (SERS). We have achieved ultrasensitive molecular detection with high reproducibility and spatial uniformity. A finite-difference time-domain simulation suggests that a wide range of three-dimensional electric fields are generated along the surfaces of the SiNR array. With the tuning of the gap and diameter of the SiNRs, the produced long decay length (>130 nm) of the enhanced electric field makes the SERS substrate a zero-gap system for ultrasensitive detection of large biomolecules. In the detection of R6G molecules, our SERS system achieved an enhancement factor of >107 with a relative standard deviation as small as 3.9-7.2% over 30 points across the substrate. More significantly, the SERS substrate yielded ultrasensitive Raman signals on long amyloid-ß fibrils at the single-fibril level, which provides promising potentials for ultrasensitive detection of amyloid aggregates that are related to Alzheimer's disease. Our study demonstrates that the SiNRs functionalized with AuNPs may serve as excellent SERS substrates in chemical and biomedical detection.

Fabrication and ferromagnetism of Si-SiGe/MnGe core-shell nanopillars.

Si-Si0.5Ge0.5/Mn0.08Ge0.92 core-shell nanopillar samples were fabricated on ordered Si nanopillar patterned substrates by molecular beam epitaxy at low temperatures. The magnetic properties of the samples are found to depend heavily on the growth temperature of the MnGe layer. The sample grown at a moderate temperature of 300 °C has the highest Curie temperature of 240 K as well as the strongest ferromagnetic signals. On the basis of the microstructural results, the ferromagnetic properties of the samples are believed to come from the intrinsic Mn-doped amorphous or crystalline Ge ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. After being annealed at a temperature of 500 °C, all the samples exhibit the same Curie temperature of 220 K, which is in sharp contrast to the different Curie temperature for the as-grown samples, and the ferromagnetism for the annealed samples comes from Mn5GeSi2 compounds which are formed during the annealing.

Evolution and Engineering of Precisely Controlled Ge Nanostructures on Scalable Array of Ordered Si Nano-pillars.

The scalable array of ordered nano-pillars with precisely controllable quantum nanostructures (QNs) are ideal candidates for the exploration of the fundamental features of cavity quantum electrodynamics. It also has a great potential in the applications of innovative nano-optoelectronic devices for the future quantum communication and integrated photon circuits. Here, we present a synthesis of such hybrid system in combination of the nanosphere lithography and the self-assembly during heteroepitaxy. The precise positioning and controllable evolution of self-assembled Ge QNs, including quantum dot necklace(QDN), QD molecule(QDM) and quantum ring(QR), on Si nano-pillars are readily achieved. Considering the strain relaxation and the non-uniform Ge growth due to the thickness-dependent and anisotropic surface diffusion of adatoms on the pillars, the comprehensive scenario of the Ge growth on Si pillars is discovered. It clarifies the inherent mechanism underlying the controllable growth of the QNs on the pillar. Moreover, it inspires a deliberate two-step growth procedure to engineer the controllable QNs on the pillar. Our results pave a promising avenue to the achievement of desired nano-pillar-QNs system that facilitates the strong light-matter interaction due to both spectra and spatial coupling between the QNs and the cavity modes of a single pillar and the periodic pillars.

Towards promising modification of GeSi nanostructures via self-assembly on miscut Si(001) substrates.

Self-assembled GeSi nanostructures on miscut Si(001) substrates are studied systematically with regard to the miscut angle and azimuth, the amount of Ge and the growth temperature. The comprehensive dependence of the spatial arrangement, which can exhibit one- and two-dimensional (1D and 2D) ordering, as well as the shape and density, of GeSi nanostructures on the miscut angle is observed. The orientation and side-walls of the 1D ordered in-plane GeSi nanowires on miscut Si(001) substrates are intimately associated with the miscut azimuth towards the 〈110〉 or 〈010〉 directions. Furthermore, the unique evolution of the GeSi nanostructures with the amount of Ge and the growth temperature on miscut Si (001) substrates towards the 〈010〉 direction is discovered. Such promising features of self-assembled GeSi nanostructures on miscut Si (001) substrates are explained in terms of the thermodynamics and growth kinetics, which are both affected significantly by the substrate vicinality. These results demonstrate that the miscut substrates offer a promising degree of freedom for the feasible modification of self-assembled nanostructures.

Plasmon-gating photoluminescence in graphene/GeSi quantum dots hybrid structures.

The ability to control light-matter interaction is central to several potential applications in lasing, sensing, and communication. Graphene plasmons provide a way of strongly enhancing the interaction and realizing ultrathin optoelectronic devices. Here, we find that photoluminescence (PL) intensities of the graphene/GeSi quantum dots hybrid structures are saturated and quenched under positive and negative voltages at the excitation of 325 nm, respectively. A mechanism called plasmon-gating effect is proposed to reveal the PL dependence of the hybrid structures on the external electric field. On the contrary, the PL intensities at the excitation of 405 and 795 nm of the hybrid structures are quenched due to the charge transfer by tuning the Fermi level of graphene or the blocking of the excitons recombination by excitons separation effect. The results also provide an evidence for the charge transfer mechanism. The plasmon gating effect on the PL provides a new way to control the optical properties of graphene/QD hybrid structures.