Topological wetting states of microdroplets on closed-loop structured surfaces: Breakdown of the Gibbs equation at the microscale.

Microdroplets are a class of soft matter that has been extensively employed for chemical, biochemical, and industrial applications. However, fabricating microdroplets with largely controllable contact-area shape and apparent contact angle, a key prerequisite for their applications, is still a challenge. Here, by engineering a type of surface with homocentric closed-loop microwalls/microchannels, we can achieve facile size, shape, and contact-angle tunability of microdroplets on the textured surfaces by design. More importantly, this class of surface topologies (with universal genus value = 1) allows us to reveal that the conventional Gibbs equation (widely used for assessing the edge effect on the apparent contact angle of macrodroplets) seems no longer applicable for water microdroplets or nanodroplets (evidenced by independent molecular dynamics simulations). Notably, for the flat surface with the intrinsic contact angle ~0°, we find that the critical contact angle on the microtextured counterparts (at edge angle 90°) can be as large as >130°, rather than 90° according to the Gibbs equation. Experiments show that the breakdown of the Gibbs equation occurs for microdroplets of different types of liquids including alcohol and hydrocarbon oils. Overall, the microtextured surface design and topological wetting states not only offer opportunities for diverse applications of microdroplets such as controllable chemical reactions and low-cost circuit fabrications but also provide testbeds for advancing the fundamental surface science of wetting beyond the Gibbs equation.

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.

Diverse field-effect characteristics and negative differential transconductance in a graphene/WS2/Au phototransistor with a Ge back gate.

We propose an infrared-sensitive negative differential transconductance (NDT) phototransistor based on a graphene/WS2/Au double junction with a SiO2/Ge gate. By changing the drain bias, diverse field-effect characteristics can be achieved. Typical p-type and n-type behavior is obtained under negative and positive drain bias, respectively. And NDT behavior is observed in the transfer curves under positive drain bias. It is believed to originate from competition between the top and bottom channel currents in stepped layers of WS2 at different gate voltages. Moreover, this phototransistor shows a gate-modulated rectification ratio of 0.03 to 88.3. In optoelectronic experiments, the phototransistor exhibits a responsivity of 2.76 A/W under visible light at 532 nm. By contrast, an interesting negative responsivity of -29.5 µA/W is obtained and the NDT vanishes under illumination by infrared light at 1550 nm. A complementary inverter based on two proposed devices of the same structure is constructed. The maximum voltage gain of the complementary inverter reaches 0.79 at a supply voltage of 1.5 V. These results demonstrate a new method of realizing next-generation two- and three-dimensional electronic and optoelectronic multifunctional devices.

Self-Powered Bidirectional Photoresponse in High-Detectivity WSe2 Phototransistor with Asymmetrical van der Waals Stacking for Retinal Neurons Emulation.

An artificial retina system shows a promising potential to achieve fast response, low power consumption, and high integration density for vision sensing systems. Optoelectronic sensors, which can emulate the neurobiological functionalities of retinal neurons, are crucial in the artificial retina systems. Here, we propose a WSe2 phototransistor with asymmetrical van der Waals (vdWs) stacking that can be used as an optoelectronic sensor in artificial retina systems. Through the utilization of the gate-tunable self-powered bidirectional photoresponse of this phototransistor, the neurobiological functionalities of both bipolar cells and cone cells, as well as the hierarchical connectivity between these two types of retinal neurons, are successfully mimicked by a single device. This self-powered bidirectional photoresponse is attributed to the asymmetrical vdWs stacking structure, which enables the transition from an n-p to p+-p homojunction in the WSe2 channel under different polarities of gate bias. Moreover, the detectivity and ON/OFF ratio of this phototransistor reach as high as 1.8 × 1013 Jones and 5.3 × 104, respectively, and a rise/fall time <80 µs is achieved, as well, which reveals good photodetection performance. The proof of this device provides a pathway for the future development of neuromorphic vision devices and systems.

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.

Infrared broadband enhancement of responsivity in Ge photodetectors decorated with Au nanoparticles.

A broadband, high-performance infrared Ge photodetector decorated with Au nanoparticles (NPs) is proposed. Photoelectronic characterization demonstrated that the responsivity of devices decorated with Au NPs is as high as 3.95 A/W at a wavelength of 1550 nm. Compared with a Ge photodetector without Au NPs, the responsivity of a device decorated with Au NPs is significantly increased, i.e., by more than 10 times in the entire range of infrared communication wavelengths, including the O, E, S, C, L, and U bands. The increase is ascribed to type-II energy-band alignment between Ge covered with Au NPs and bare Ge, instead of the localized surface-plasmon-resonance effect. The type-II energy-band alignment enhances the spatial electron-hole separation and restrains the electron-hole recombination, thus a larger photocurrent is observed. These results reflect the potential of this approach for achieving broadband, high-performance Ge photodetectors operating in the near-infrared communication band.

Photoelectrical Properties Investigated on Individual Si Nanowires and Their Size Dependence.

Periodically ordered arrays of vertically aligned Si nanowires (Si NWs) are successfully fabricated with controllable diameters and lengths. Their photoconductive properties are investigated by photoconductive atomic force microscopy (PCAFM) on individual nanowires. The results show that the photocurrent of Si NWs increases significantly with the laser intensity, indicating that Si NWs have good photoconductance and photoresponse capability. This photoenhanced conductance can be attributed to the photoinduced Schottky barrier change, confirmed by I-V curve analyses. On the other hand, electrostatic force microscopy (EFM) results indicate that a large number of photogenerated charges are trapped in Si NWs under laser irradiation, leading to the lowering of barrier height. Moreover, the size dependence of photoconductive properties is studied on Si NWs with different diameters and lengths. It is found that the increasing magnitude of photocurrent with laser intensity is greatly relevant to the nanowires' diameter and length. Si NWs with smaller diameters and shorter lengths display better photoconductive properties, which agrees well with the size-dependent barrier height variation induced by photogenerated charges. With optimized diameter and length, great photoelectrical properties are achieved on Si NWs. Overall, in this study the photoelectrical properties of individual Si NWs are systematically investigated by PCAFM and EFM, providing important information for the optimization of nanostructures for practical applications.

Nanoscale growth of a Sn-guided SiGeSn alloy on Si (111) substrates by molecular beam epitaxy.

Here, SiGeSn nanostructures were grown via molecular beam epitaxy on a Si (111) substrate with the assistance of Sn droplets. Owing to the thermal effect and the compressive strain induced by a lattice mismatch, Si and Sn atoms were successfully incorporated into the Ge matrix during the Sn-guided Ge deposition process. A low growth temperature of 350 °C produced a variety of SiGeSn nanostructures of different sizes, attributed to the variation of the initial Sn droplet size. Using energy-dispersive X-ray spectroscopy, the Sn, Si and Ge contents of a defect-free SiGeSn nanoisland were approximately determined to be 0.05, 0.09 and 0.86, respectively. Furthermore, as the growth temperature increased past 600 °C, the growth direction of the nanostructure was changed thermally from out-of-plane to in-plane. Meanwhile, the stacked SiGeSn nanowires grown along the 〈112〉 direction remained defect-free, though some threading dislocations were observed in the smooth SiGeSn nanowires along the 〈110〉 direction. These results offer a novel method to grow Si-based SiGeSn nanostructures while possessing important implications for fabricating further 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.

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.

GexSi1-x virtual-layer enhanced ferromagnetism in self-assembled Mn0.06Ge0.94 quantum dots grown on Si wafers by molecular beam epitaxy.

Self-assembled Mn0.06Ge0.94 quantum dots (QDs) on a Si substrate or GexSi1-x virtual substrate (VS) were grown by molecular beam epitaxy. The GexSi1-x VS of different thicknesses and Ge compositions x were utilized to modulate the ferromagnetic properties of the above QDs. The MnGe QDs on GexSi1-x VS show a significantly enhanced ferromagnetism with a Curie temperature above 220 K. On the basis of the microstructural and magnetization results, the ferromagnetic properties of the QDs on GexSi1-x VS are believed to come from the intrinsic MnGe ferromagnetic phase rather than any intermetallic ferromagnetic compounds of Mn and Ge. At the same time, we found that by increasing the Ge composition x of GexSi1-x VS, the ferromagnetism of QDs grown on VS will markedly increase due to the improvements of hole concentration and Ge composition inside the QDs. These results are fundamentally important in the understanding and especially in the realization of high Curie temperature MnGe diluted magnetic semiconductors.

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.

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.

The photoelectric response of the graphene/GeSi QDs hybrid structure.

In this work, the photoelectric response properties of the graphene/GeSi QDs hybrid structure were demonstrated by measuring the I-V curve, and the incident photon-to-current conversion efficiency (IPCE). The maximal on-off ratio of the current value reaches 1500 at 10 K, due to the competition between the carrier freeze-out effect and the recombination center effect. The IPCE of the hybrid structure under different incident light indicated that the photoelectric response of hybrid structure is most sensitive to the ultraviolet light (325 nm), which is attributed to the enhanced ultraviolet absorption of graphene surface plasmon in the hybrid structure. Hence, our results represent that the graphene/GeSi QDs hybrid structure has potential application as a novel ultraviolet photoelectric device.

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.

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.

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.