Si Nanotips Research Articles

Observation of enhanced heat transfer between a nanotip and substrate at nanoscale distances via direct temperature probing with Raman spectroscopy

Nanoscale heat transfer between two nanostructured surfaces holds paramount significance in the realms of extreme manufacturing and high-density data storage. However, experimental probing of heat transfer encounters significant challenges, primarily due to limitations in current instrumentation. Here, we report a method based on Raman spectroscopy to directly probe the temperature difference between a Si nanotip and SiC substrate. Results indicate a decrease in substrate temperature, while the temperature of the nanotip remains relatively stable as the nanotip moves away from the substrate from approximately 82.5 to 1320 nm. We trace this enhanced heat transfer to a significant augmentation, by one order of magnitude, in air conduction and thermal radiation energy exchange theoretically, with air conduction being the dominant mode over thermal radiation. This work advances the direct observation of surface temperatures with gaps smaller than 1 μm, utilizing a noncontact and nondestructive Raman technique, which can be extended to studying near-field heat transfer across various Raman-active surfaces.

Investigation of Defect Formation in Monolithic Integrated GaP Islands on Si Nanotip Wafers

Investigation of Defect Formation in Monolithic Integrated GaP Islands on Si Nanotip Wafers

Selective Growth of GaP Crystals on CMOS-Compatible Si Nanotip Wafers by Gas Source Molecular Beam Epitaxy.

Gallium phosphide (GaP) is a III-V semiconductor with remarkable optoelectronic properties, and it has almost the same lattice constant as silicon (Si). However, to date, the monolithic and large-scale integration of GaP devices with silicon remains challenging. In this study, we present a nanoheteroepitaxy approach using gas-source molecular-beam epitaxy for selective growth of GaP islands on Si nanotips, which were fabricated using complementary metal-oxide semiconductor (CMOS) technology on a 200 mm n-type Si(001) wafer. Our results show that GaP islands with sizes on the order of hundreds of nanometers can be successfully grown on CMOS-compatible wafers. These islands exhibit a zinc-blende phase and possess optoelectronic properties similar to those of a high-quality epitaxial GaP layer. This result marks a notable advancement in the seamless integration of GaP-based devices with high scalability into Si nanotechnology and integrated optoelectronics.

Experimental Decoding and Tuning Electronic Friction of Si Nanotip Sliding on Graphene.

Due to the coupled contributions of adhesion and carrier to friction typically found in previous research, decoupling the electron-based dissipation is a long-standing challenge in tribology. In this study, by designing and integrating a graphene/h-BN/graphene/h-BN stacking device into an atomic force microscopy, the carrier density dependent frictional behavior of a single-asperity sliding on graphene is unambiguously revealed by applying an external back-gate voltage, while maintaining the adhesion unaffected. Our experiments reveal that friction on the graphene increases monotonically with the increase of carrier density. By adjusting the back-gate voltage, the carrier density of the top graphene layer can be tuned from -3.9 × 1012 to 3.5 × 1012 cm-2, resulting in a ∼28% increase in friction. The mechanism is uncovered from the consistent dependence of the charge density redistribution and sliding barrier on the carrier density. These findings offer new perspectives on the fundamental understanding and regulation of friction at van der Waals interfaces.

Synergy of cold development and electron-beam exposure for Si nanotip patterning in quantum-electronic devices

We demonstrated an effective method for lithographic patterning of a fanout-shaped Si nanotip array with nanometer-scale feature size/pitch by using a combination of overdose exposure, cold development, and layout design. We optimized process conditions of electron-beam exposure onto poly(methyl methacrylate) resist and methyl isobutyl ketone development temperature (10 to −5 °C) to produce dense Si nanotips with large gradient fanout patterns. A circular array of Si nanotips with a minimum feature size/pitch of 15/30 nm with a high degree of pattern fidelity, uniformity, and reproducibility were demonstrated. Based on the platform of the Si fanout-shaped nanotip array, we have successfully produced an array of self-organized germanium quantum-dots/Si3N4 barriers, which are self-aligned with Si nanotips as coupling barriers or electrodes for implementing advanced quantum-electronic devices.

Fabrication and characterization of silicon nano-tip memristor for low-power neuromorphic application

In this work, a memristor using Si nano-tip bottom electrode has been fabricated and evaluated. Compared with the control device fabricated in a planar structure, the invented Si nano-tip device statistically demonstrates 400 times of area shrinkage and 3 and 8 times of current reductions in the low- and high-resistance states, respectively, owing to the effects of highly confined switching area on the operations of multiple-weak-filament (MWF)-type memristor. The results reveal the validity of structural scaling on the MWF-type memristors as effective as a material approach. Moreover, the fabricated Si memristor has been evaluated as a synaptic device via pattern recognition toward neuromorphic applications in consideration of interconnect resistance. The proposed Si memristor shows substantially reduced inference power consumption by an increase in resistances and improved accuracy by 5% compared with the control device.

One-step fabrication of high-density Si nanotips as SALDI-MS substrate for highly sensitive detection

The high sensitivity of surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) analysis depends on the enhancement of the signal intensity and the reduction of the background. Here, high-density Si nanotips were prepared as SALDI-MS substrate by taking template-free reactive ion etching for sensitive detection. The high-density Si nanotips act as tiny antennas to concentrate the laser energy and generate photo-induced electrons on a small area, which promotes desorption and ionization of molecule, enhancing the signal intensity. Additionally, the high stability of Si nanotips under laser irradiation ensures low background interference. The applicability of the substrate was demonstrated by the detection of various small molecules, e.g., fatty acids, amino acids, environmental pollutants and drugs. In addition, this substrate can be successfully applied for detecting trace small molecules in complex sample, such as 10−9 M verapamil in serum and 10−12 M crystal violet in lake water. Moreover, the banned fungicides left on the fish skin was also detected. The substrate is compatible with imaging, and the endogenous and exogenous substances in fingerprint were visualized by MS imaging. These results demonstrate the easily prepared high-density Si nanotips SALDI-MS substrate is of high detection sensitivity and great value toward large-scale practical applications.

Picosecond-Resolved Raman Response of a Si Nanotip for Probing Temperature and Thermal Stress in the Confined Regime under Laser Heating

Picosecond-Resolved Raman Response of a Si Nanotip for Probing Temperature and Thermal Stress in the Confined Regime under Laser Heating

Omnidirectional and Broadband Antireflection Effect with Tapered Silicon Nanostructures Fabricated with Low-Cost and Large-Area Capable Nanosphere Lithography.

In this report, we present a process for the fabrication and tapering of a silicon (Si) nanopillar (NP) array on a large Si surface area wafer (2-inch diameter) to provide enhanced light harvesting for Si solar cell application. From our N,N-dimethyl-formamide (DMF) solvent-controlled spin-coating method, silica nanosphere (SNS in 310 nm diameter) coating on the Si surface was demonstrated successfully with improved monolayer coverage (>95%) and uniformity. After combining this method with a reactive ion etching (RIE) technique, a high-density Si NP array was produced, and we revealed that controlled tapering of Si NPs could be achieved after introducing a two-step RIE process using (1) CHF3/Ar gases for SNS selective etching over Si and (2) Cl2 gas for Si vertical etching. From our experimental and computational study, we show that an effectively tapered Si NP (i.e., an Si nanotip (NT)) structure could offer a highly effective omnidirectional and broadband antireflection effect for high-efficiency Si solar cell application.

Efficiency enhancement of Si nanostructure hybrid solar cells by optimizing non-radiative energy transfer from Si quantum dots

Non-radiative energy transfer (NRET) from silicon quantum dots (SiQDs) terminated with ligands is an effective way to improve solar cell efficiency. The NRET effect can be enhanced by minimizing the distance between SiQDs and the solar cell surface. SiQDs were inserted between poly (3,4-ethylene dioxythiophene):poly (styrene sulfonate) (PEDOT:PSS) and Si nanostructures of hybrid heterojunction solar cells. To clarify dependence on ligand length, three different ligands of different lengths were investigated. 1-octene, which has the shortest ligand length, showed the greatest enhancement of NRET, resulting in the highest solar cell efficiency. The non-radiative energy transfer (NRET) effect in Si quantum dots (SiQDs) can be enhanced by shortening the ligand length, resulting in enhanced short circuit current, thus significantly improving the energy conversion efficiency of solar cells. • High-efficiency hybrid solar cells were fabricated by inserting SiQDs into hybrid solar cell with Si nanotips and PEDOT:PSS. • Non-radiative energy transfer from SiQDs terminated with ligands effectively enhanced conversion efficiency of solar cells. • SiQDs with shorter ligands had a higher conversion efficiency. • The highest conversion efficiency of 12.4% was given by hybrid solar cells with 1-octene terminated SiQDs.

Downshifting of highly energetic photons and energy transfer by Mn-doped perovskite CsPbCl3 nanocrystals in hybrid organic/silicon nanostructured solar cells

Abstract The downshifting of highly energetic photons in the ultraviolet region has the benefit of extending the spectral response and decreasing the thermalization effect in Si-related solar cells. Herein, we report low-cost hybrid organic/inorganic solar cells with minimized spectral losses with directly-deposited Mn-doped perovskite CsPbCl3 nanocrystals (NCs) on the surface of Si nanotips with PEDOT:PSS acting as the hole transporting and passivating layer. The CsPbCl3 NCs act as highly efficient radiative and non-radiative energy transfer centers. Manganese doping in CsPbCl3 NCs also drastically enhances photoluminescence quantum yields due to the charge transfer of photoinduced excitons from the CsPbCl3 host to the dopant Mn2+ centers. Finally, the Mn doped CsPbCl3 NCs enhance the cell properties and give a high power conversion efficiency of 13.48%.

Quasi-Saturated Arsenic Concentration and Uniform Electron Emission by Regulating Thermal Oxidation of Si Nanotips

The improving of tip-to-tip electron emission uniformity in an array is one of the essential issues for the vacuum microelectronic/nanoelectronic applications. Here, we report the achieving of quasi-saturated ultrahigh arsenic concentration ( $\sim 6.7\times 10^{{21}}$ /cm3) at the Si nanotip apex, in the mechanisms of dopant segregation during the thermal oxidation and the limitation of arsenic solubility in Si. The tips with quasi-saturated high-level arsenic concentration possess a well tip-to-tip uniformity in surface work function. The measured work function for the tips in an array with saturated dopant concentration is in a narrow range of 4.347–4.371 eV (~2.1% fluctuation). This merit not only resulted in high emission current density (~418.36 mA/cm2) from the devices at a relatively lower gate voltage but also brought well spatially uniform electron emission in an array. The work inspired vital insight into the interpretation for mechanisms in dopant density control and the improving of emission uniformity of the Si-tip array. It could be used as an essential approach to acquire high-performance Si-based vacuum electronic devices.

The role of the substrate in Graphene/Silicon photodiodes

The Graphene/Silicon (Gr/Si) junction can function as a Schottky diode with performances strictly related to the quality of the interface. Here, we focus on the substrate geometry and on its effects on Gr/Si junction physics. We fabricate and study the electrical and optical behaviour of two types of devices: one made of a Gr/Si planar junction, the second realized with graphene on an array of Si nanotips. We show that the Gr/Si flat device exhibits a reverse photocurrent higher than the forward current and achieves a photoresponsivity of 2.5 A/W. The high photoresponse is due to the charges photogenerated in Si below a parasitic graphene/SiO2/Si structure, which are injected into the Gr/Si junction region. The other device with graphene on Si-tips displays a reverse current that grows exponentially with the bias. We explain this behaviour by taking into account the tip geometry of the substrate, which magnifies the electric field and shifts the Fermi level of graphene, thus enabling fine-tuning of the Schottky barrier height. The Gr/Si-tip device achieves a higher photoresponsivity, up to 3 A/W, likely due to photocharge internal multiplication.

A tool for automatic recognition of [110] tilt grain boundaries in zincblende-type crystals

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm.

Strain relaxation in epitaxial GaAs/Si (0 0 1) nanostructures

Crystal defects, present in ~100 nm GaAs nanocrystals grown by metal organic vapour phase epitaxy on top of (0 0 1)-oriented Si nanotips (with a tip opening 50–90 nm), have been studied by means of high-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The role of 60° perfect, 30° and 90° Shockley partial misfit dislocations (MDs) in the plastic strain relaxation of GaAs on Si is discussed. Formation conditions of stair-rod dislocations and coherent twin boundaries in the GaAs nanocrystals are explained. Also, although stacking faults are commonly observed, we show here that synthesis of GaAs nanocrystals with a minimum number of these defects is possible. On the other hand, from the number of MDs, we have to conclude that the GaAs nanoparticles are fully relaxed plastically, such that for the present tip sizes no substrate compliance can be observed.

Structural and optical characterization of GaAs nano-crystals selectively grown on Si nano-tips by MOVPE

We present the nanoheteroepitaxial growth of gallium arsenide (GaAs) on nano-patterned silicon (Si) (001) substrates fabricated using a CMOS technology compatible process. The selective growth of GaAs nano-crystals (NCs) was achieved at 570 °C by MOVPE. A detailed structure and defect characterization study of the grown nano-heterostructures was performed using scanning transmission electron microscopy, x-ray diffraction, micro-Raman, and micro-photoluminescence (μ-PL) spectroscopy. The results show single-crystalline, nearly relaxed GaAs NCs on top of slightly, by the SiO2-mask compressively strained Si nano-tips (NTs). Given the limited contact area, GaAs/Si nanostructures benefit from limited intermixing in contrast to planar GaAs films on Si. Even though a few growth defects (e.g. stacking faults, micro/nano-twins, etc) especially located at the GaAs/Si interface region were detected, the nanoheterostructures show intensive light emission, as investigated by μ-PL spectroscopy. Achieving well-ordered high quality GaAs NCs on Si NTs may provide opportunities for superior electronic, photonic, or photovoltaic device performances integrated on the silicon technology platform.

Bi-modal nanoheteroepitaxy of GaAs on Si by metal organic vapor phase epitaxy

Nano-heteroepitaxial growth of GaAs on Si(001) by metal organic vapor phase epitaxy was investigated to study emerging materials phenomena on the nano-scale of III–V/Si interaction. Arrays of Si nano-tips (NTs) embedded in a SiO2 matrix were used as substrates. The NTs had top Si openings of 50–90 nm serving as seeds for the selective growth of GaAs nano-crystals (NCs). The structural and morphological properties were investigated by high resolution scanning electron microscopy, atomic force microscopy, electron backscatter diffraction, x-ray diffraction, and high resolution scanning transmission electron microscopy. The GaAs growth led to epitaxial NCs featuring a bi-modal distribution of size and morphology. NCs of small size exhibited high structural quality and well-defined {111}–{100} faceting. Larger clusters had less regular shapes and contained twins. The present work shows that the growth of high quality GaAs NCs on Si NTs is feasible and can provide an alternate way to the integration of compound semiconductors with Si micro- and opto-electronics technology.

Self-modulated field electron emitter: Gated device of integrated Si tip-on-nano-channel

We report the featured gated field electron emission devices of Si nano-tips with individually integrated Si nano-channels and the interpretation of the related physics. A rational procedure was developed to fabricate the uniform integrated devices. The electrical and thermal conduction tests demonstrated that the Si nano-channel can limit both the current and heat flows. The integrated devices showed the specialties of self-enhancement and self-regulation. The heat resistance results in the heat accumulation at the tip-apex, inducing the thermally enhanced field electron emission. The self-regulated effect of the electrical resistance is benefit for impeding the current overloading and prevents the emitters from a catastrophic breakdown. The nano-channel-integrated Si nano-tip array exhibited emission current density up to 24.9 mA/cm2 at a gate voltage of 94 V, much higher than that of the Si nano-tip array without an integrated nano-channel.

Selective Epitaxy of InP on Si and Rectification in Graphene/InP/Si Hybrid Structure

The epitaxial integration of highly heterogeneous material systems with silicon (Si) is a central topic in (opto-)electronics owing to device applications. InP could open new avenues for the realization of novel devices such as high-mobility transistors in next-generation CMOS or efficient lasers in Si photonics circuitry. However, the InP/Si heteroepitaxy is highly challenging due to the lattice (∼8%), thermal expansion mismatch (∼84%), and the different lattice symmetries. Here, we demonstrate the growth of InP nanocrystals showing high structural quality and excellent optoelectronic properties on Si. Our CMOS-compatible innovative approach exploits the selective epitaxy of InP nanocrystals on Si nanometric seeds obtained by the opening of lattice-arranged Si nanotips embedded in a SiO2 matrix. A graphene/InP/Si-tip heterostructure was realized on obtained materials, revealing rectifying behavior and promising photodetection. This work presents a significant advance toward the monolithic integration of graphene/III-V based hybrid devices onto the mainstream Si technology platform.

Dislocation-free Ge Nano-crystals via Pattern Independent Selective Ge Heteroepitaxy on Si Nano-Tip Wafers.

The integration of dislocation-free Ge nano-islands was realized via selective molecular beam epitaxy on Si nano-tip patterned substrates. The Si-tip wafers feature a rectangular array of nanometer sized Si tips with (001) facet exposed among a SiO2 matrix. These wafers were fabricated by complementary metal-oxide-semiconductor (CMOS) compatible nanotechnology. Calculations based on nucleation theory predict that the selective growth occurs close to thermodynamic equilibrium, where condensation of Ge adatoms on SiO2 is disfavored due to the extremely short re-evaporation time and diffusion length. The growth selectivity is ensured by the desorption-limited growth regime leading to the observed pattern independence, i.e. the absence of loading effect commonly encountered in chemical vapor deposition. The growth condition of high temperature and low deposition rate is responsible for the observed high crystalline quality of the Ge islands which is also associated with negligible Si-Ge intermixing owing to geometric hindrance by the Si nano-tip approach. Single island as well as area-averaged characterization methods demonstrate that Ge islands are dislocation-free and heteroepitaxial strain is fully relaxed. Such well-ordered high quality Ge islands present a step towards the achievement of materials suitable for optical applications.