Si Surface Research Articles

Multiphoton-initiated laser writing of semiconductors using nanosecond mid-infrared pulses

The advent of more powerful mid-infrared nanosecond laser sources allows using them as attractive tools in material laser processing. For semiconductor bulk processing, laser intensities must remain modest to avoid detrimental nonlinear propagation and pre-focal plasma screening effects. Accordingly, nanosecond pulses are usually appropriate to locally initiate energy deposition by multiphoton absorption and subsequent modification, hardly achievable with ultrashort pulses. Employing a 2.8-μm 2.5-ns laser source, we explore the possibility to modify silicon and other semiconductors. With interactions initiated by three-photon absorption, we modify and determine the surface and bulk modification thresholds of Si, GaAs, and InP. For Si, compared to previous studies at telecom wavelengths, we report on a reduction of the energy threshold for volume modification with processing depth, and repeatable rear-surface modifications. Compared to Si, we find for GaAs and InP important fluctuations in the modification responses, which we attribute to an absorption mechanism initially triggered by a greater presence of defects. We also study germanium, as this narrower bandgap semiconductor very interesting for mid-infrared electro-optical applications is transparent at the newly introduced wavelength. Despite the ability to modify the surface, the bulk of Ge remains inaccessible in this two-photon regime, mainly attributed to beam aberrations and nonlinearities. Nonetheless, we demonstrate the ability to process Si chips integrating Ge layers. All these results showcase the potential of mid-infrared wavelengths to offer new degrees of flexibility for 3D laser processing technologies turned to silicon photonics and microelectronics packaging.

Chemoselective Adsorption of Allyl Ethers on Si(001): How the Interaction between Two Functional Groups Controls the Reactivity and Final Products of a Surface Reaction.

Selective adsorption of multifunctional molecules is rarely observed when the different functional groups react via nonactivated reaction channels. Although the latter is also the case for ether cleavage and the adsorption of C=C double bonds on the highly reactive Si(001) surface, we find that allyl ethers, which combine both functional groups, react on Si(001) selectively via the cleavage of the molecules' ether group. In addition, our XPS measurements at 90, 150, and 300 K indicate an increased reactivity of the ether group when compared to monofunctional ethers. STM investigations furthermore reveal different final adsorption configurations after ether cleavage of allyl methyl ether when compared to diethyl ether as the monofunctional reference molecule. The interaction of the two functional groups in one molecule thus leads to new reaction channels with higher reactivity for ether cleavage on Si(001). As a further consequence, the reactivity of the C=C double bond is suppressed up to room temperature, leading to the observed selective adsorption.

Stabilizing the Si/C blend anode by a multifunctional ternary composite binder

Si/C blend anodes hold great promise in commercialized high-energy-density (HED) Li-ion batteries but suffer from volumetric effects and electrode integrity deterioration. This study reports a ternary composite binder consisting of carboxymethyl cellulose (CMC), cationic polyacrylamide (CPAM), and styrene-butadiene rubber (SBR), which can significantly improve the cyclability of a Si@C/graphite blend anode. A high capacity retention of 92.9% after 100 cycles was achieved in a half cell. Moreover, the ternary binder proves suitable for a very high mass loading (4.9 mAh/g) electrode and is effective in a 2 Ah-level full cell, which delivers an excellent capacity retention of 80.6% after 500 cycles. The beneficial effects were ascribed to the multifunctional groups compatible with the surfaces of both Si and carbon, and the designed rigidness and elasticity which restricts and accommodates the volumetric effects of the Si/C blend anode simultaneously.

SiC and Si detectors comparison for high carbon energy spectrometry

An innovative SiC Schottky junction and a traditional p-n Si surface barrier detector have been compared to detect carbon ions with MeVs kinetic energy. To this, a comparison was performed during Rutherford backscattering spectrometry (RBS) using 2–10 MeV carbon ion beams. The energy resolution and detection efficiency for RBS analysis using the two detectors and their detection electronics are presented.The detector parameters dependencies on the surface passivating layers, ion energy and current dependence, ion penetration depth, detection efficiency, energy resolution, and others are discussed.The comparison of RBS analysis with SiC and Si is investigated highlighting the advantages and disadvantages of using SiC with respect to the traditional Si surface barrier detectors. The two detectors employed for proton, helium and carbon RBS spectrometry of different targets have been also compared on the base of the literature data.

Template-assisted growth of Ga-based nanoparticle clusters on Si: effect of post-annealing process on the Ga ion beam exposed 2D arrays fabricated by focused ion beam nanolithography

We demonstrate template-assisted growth of gallium-based nanoparticle clusters on silicon substrate using a focused ion beam (FIB) nanolithography technique. The nanolithography counterpart of the technique steers a focussed 30 kV accelerated gallium ion beam on the surface of Si to create template patterns of two-dimensional dot arrays. Growth of the nanoparticles is governed by two vital steps namely implantation of gallium into the substrate via gallium beam exposure and formation of the stable nanoparticles on the surface of the substrate by subsequent annealing at elevated temperature in ammonia atmosphere. The growth primarily depends on the dose of implanted gallium which is in the order of 107 atoms per spot and it is also critically influenced by the temperature and duration of the post-annealing treatment. By controlling the growth parameters, it is possible to obtain one particle per spot and particle densities as high as 109 particles per square centimetre could be achieved in this case. The demonstrated growth process, utilizing the advantages of FIB nanolithography, is categorized under the guided organization approach as it combines both the classical top-down and bottom-up approaches. Patterned growth of the particles could be utilized as templates or nucleation sites for the growth of an organized array of nanostructures or quantum dot structures.

Cryogenic DRIE processes for high-precision silicon etching in MEMS applications

Cryogenic deep reactive ion etching (Cryo DRIE) of silicon has become an enticing but challenging process utilized in front-end fabrication for the semiconductor industry. This method, compared to the Bosch process, yields vertical etch profiles with smoother sidewalls not subjected to scalloping, which are desired for many microelectromechanical systems (MEMS) applications. Smoother sidewalls enhance electrical contact by ensuring more conformal and uniform sidewall coverage, thereby increasing the effective contact area without altering contact dimensions. The versatility of the Cryo DRIE process allows for customization of the etch profiles by adjusting key process parameters such as table temperature, O2 percentage of the total gas flow rate (O2 + SF6), RF bias power and process pressure. In this work, we undertake a comprehensive study of the effects of Cryo DRIE process parameters on the trench profiles in the structures used to define cantilevers in MEMS devices. Experiments were performed with an Oxford PlasmaPro 100 Estrelas ICP-RIE system using positive photoresist SPR-955 as a mask material. Our findings demonstrate significant influences on the sidewall angle, etch rate and trench shape due to these parameter modifications. Varying the table temperature between −80 °C and −120 °C under a constant process pressure of 10 mTorr changes the etch rate from 3 to 4 μm min−1, while sidewall angle changes by ∼2°, from positive (<90° relative to the Si surface) to negative (>90° relative to the Si surface) tapering. Altering the O2 flow rate with constant SF6 flow results in a notable 10° shift in sidewall tapering. Furthermore, SPR-955 photoresist masks provide selectivity of 46:1 with respect to Si and facilitates the fabrication of MEMS devices with precise dimension control ranging from 1 to 100 μm for etching depths up to 42 μm using Cryo DRIE. Understanding the influence of each parameter is crucial for optimizing MEMS device fabrication.

Long-lifetime aqueous Si-air batteries prepared by growing multi-dimensionally tunable ZIF-8 crystals on Si anodes

Si-air batteries have a high energy density, high theoretical voltage, and long lifetime, but they present a low anode utilization rate in a potassium hydroxide electrolyte. In this work, a ZIF-8 protective layer was prepared and modulated by a secondary growth method and then applied to protect the Si flat and Si nanowire (NW) anodes of a Si-air battery. By adjusting the conversion ratio, particle size, and crystallinity of ZIF-8 on the Si surface, the contact mode of the Si anode with water and OH– was controlled, thus achieving long-term corrosion and passivation resistance. Si NWs@ZIF-8 exhibited the highest average discharge voltage of 1.16 V, and the Si flat@ZIF-8 anode achieved the longest discharge time of 420 h. This work confirms that ZIF-8 acts as an anode protective layer to improve the properties of Si-air batteries and also provides valuable insights into the protection of Si anodes by MOFs.

Advanced-design cross-linked binder enables high-performance silicon-based anodes through in-situ crosslinking based on sodium carboxymethyl cellulose and poly-lysine

Practical employment of silicon (Si) electrodes in lithium-ion batteries (LIBs) is limited due to the severe volume changes suffered during charging-discharging process, causing serious capacity fading. Here, a composite polymer (CP-10) containing sodium carboxymethyl cellulose (CMC-Na) and poly-lysine (PL) is proposed for the binder of Si-based anodes, and a multifunctional strategy of “in-situ crosslinking” is achieved to alleviate the severe capacity degradation effectively. A cross-linked three-dimensional (3D) network is established through the strong hydrogen bonding interaction and reversible electrostatic interactions within CP-10, offering favorable mechanical tolerance for the extreme volume expansion of Si. Moreover, hydrogen bonding interaction along with ion-dipole interaction formed between CP-10 and Si surface enhance the bonding capability of Si-based anodes, promoting the maintenance of anodes' integrity. Consequently, over 800 cycles are achieved for the Si@CP-10 at 0.5C while maintaining a fixed discharge specific capacity of 1000 mAh g−1. Moreover, the Si/C@CP-10 can stably operate over 500 cycles with a capacity retention of 77.12 % at 1C. The prolonged cycling lifetime of Si/C and Si anodes suggests great potential for this strategy in promoting the implementation of high-capacity LIBs.

Yielding high photovoltaic conversion efficiency by Pd-decoration on Si-based interface: A density-functional theory study

There has been showing great interest in boosting the performance of III-V/Si multi-junction solar cell, which is low-cost and offering high photovoltaic conversion efficiency. Adding noble-metal nano-array at the interface between the Si and III-V materials is one of most effective ways. This theoretical work comprehensively studied the atomic structure, electronic properties, and optical response of Pd-decoration on the Si(111) surface by using density-functional theory calculations. The structural stability calculations showed that Pd atoms could exist on Si surface in many forms. The calculated differential charge density with Bader charge showed that the charge transfer between Pd and Si atoms would increase the generation of electron-hole pairs. At the same time, Pd-decoration could reduce the work function and improve the optical conductivity of Si surface. In addition, the modification of Pd atoms would create new energy levels near the Fermi level in the band gap of Si surface. These energy levels enable Si bottom cell to undergo electron inter-band transitions under weak light irradiation, improving the utilization of weak light. This work provided a micro physical explanation for the improvement of III-V/Si solar cell by using noble-metal nano-array.

Observation of Positive Trap Charge by Electron Holography in PMOS Device

Abstract It has been hypothesized that trap charge plays an important role in PMOS (positive-channel metal oxide semiconductor) device reliability. By using electron holography, trap charge in a spacer oxide was observed to cause an inversion of junction in an un-stressed PMOS device prepared using a Ga+ focused ion beam (FIB). Through 400°C annealing for 10 min in vacuum, trap charge in the spacer oxide dispersed and the junction recovered. Technology computer-aided design (TCAD) simulation was used to confirm the positive trap charge effect on the junction. Furthermore, the simulation showed that a low concentration of positive trap charge at the Si/SiO2 interface under the spacer led to a higher tunneling current due to an abrupt junction near the Si surface through a very localized junction inversion.

Effects of chemical action of polishing medium on the material removal of SiC

The chemical action mechanism in the chemical mechanical polishing of SiC substrate remains elusive, which has great limitations on the selection of polishing solution and polishing parameters. In this paper, the cutting process of unreacted SiC (U–SiC) and the SiC reacted with H2O (R-SiC-H2O) and H2O2 (R-SiC-H2O2) was studied with an individual diamond abrasive particle by molecular dynamics simulation. The influence of chemical reaction and cutting speed on the material removal of SiC was explored by focusing on the removal rate, surface quality, machining temperature and machining stress. The simulation results showed that the number of removed atoms increased, and the surface roughness and subsurface damage depth decreased for SiC at the same cutting speed after the chemical action. Although the number of removal atoms increased with the increase of cutting speed, the surface roughness and subsurface damage depth increased at the same time. The effects of the polishing medium on the material removal process for SiC can be attributed to the decrease of surface hardness and Si–C bonding energy after the reaction of the polishing medium and SiC. The results provide a guidance for the selection of chemical mechanical polishing systems and machining parameters.

Platinum-Particle-Assisted Etching of Low-, Moderately-, and Highly-Doped p-Type Silicon: Change of Composite Porous Structure

Metal-assisted etching (metal-assisted chemical etching) is an efficient method to fabricate porous silicon (Si). When using platinum (Pt) particles as metal catalysts in metal-assisted etching, a composite porous structure of straight macropores formed beneath the Pt particles and a mesoporous layer formed on the entire surface of Si can be fabricated. The formation mechanism of the composite structure is still open to discussion. We previously demonstrated that the ratio of mesoporous layer thickness to macropore depth showed a large value (approximately 1.1) in the case of highly-doped p-Si. In this study, we investigated the composite structure formation by using p-Si substrates with different doping densities and etching solutions with different concentrations of hydrogen peroxide (H2O2). There was not significant difference in the structures formed on low- and moderately-doped Si, despite the large difference in doping density. The ratio of mesoporous layer thickness to macropore depth increased within the range approximately from 0.1 to 0.4 with increasing the H2O2 concentration in the case of low- and moderately-doped Si, but it did not change in the case of highly-doped Si. We discussed the observation results based on the spatial distribution of hole consumption and the band structures at Pt/Si and Si/electrolyte interfaces.

Lithium-conducting phosphates as artificial solid-electrolyte interphase on silicon anode for supreme lithium storage

Silicon (Si) anode is an ideal material for next-generation lithium-ion batteries due to its high specific capacity. Unfortunately, the fragmentation and reconstruction of solid electrolyte interface (SEI) film caused by volume expansion during the (de)lithiation is still a problem to be overcome. Constructing a stable SEI film on Si surface through interface engineering is a very effective strategy to improve the electrochemical performance. In this work, we construct lithium-conducting phosphates (Li3PO4, denoted as LPO) as SEI film and SiOx buffer layer on Si surface. The obtained Si@SiOx@LPO anode retains 2015 mAh g−1 after 100 cycles at 0.2 A g−1 with a capacity retention of 73.6%, and 1636 mAh g−1 after 500 cycles at 1 A g−1 with a capacity retention of 81.7%, which is significantly higher than the Si anode under the same conditions, respectively. In addition, in/ex-situ tests demonstrate that Si@SiOx@LPO anode has faster lithium-ion diffusion and structural reversibility. The analysis shows that LPO has higher mechanical strength and provides better ionic conductivity. On the other hand, SiOx buffer layer can not only reduce the volume expansion of Si, but also reduce the energy barrier between Si and LPO interface and increase the electrical conductivity. Therefore, this work provides a novel strategy for boosting electrochemical performance by artificial SEI and surface modification.

Silicon (100) surface passivation-driven tuning of Ag film crystallinity and its impact on the performance of Ag/n-Si mid-infrared Schottky photodetector

The utilization of metal/semiconductor Schottky devices for plasmonic harvesting of hot carriers holds immense potential in the field of sub-bandgap photodetection. In this work, we explore a surface passivation scheme using air plasma exposure to modify the Si (100) surface and subsequently the crystal orientation of the deposited Ag film for photon detection in the mid-infrared (MIR) regime. This tailoring was achieved by varying the plasma exposure duration (0, 150, 300, 450, and 600 s). As a result, we could tune the crystal orientation of Ag from the (200) to the (210) crystal plane, with the Ag (111) orientation present in all devices. Furthermore, the photo-response behavior under MIR exposure at λ = 4.2 µm was studied both experimentally and using COMSOL simulations. It was observed that both photoelectric (PE) and photothermal (PT) effects contributed to the photo-response behavior of all devices. The Ag/Si device exposed to air plasma for 300 s exhibited the maximum PE-driven response (2.73 µA/W), while the device exposed to air plasma for 600 s showed a significant PT-driven response (13.01 µA/W). In addition, this strategy helped reduce the reverse leakage current by up to 99.5%. This study demonstrates that MIR detection at longer wavelengths can be optimized by tailoring the crystal orientation of the metal film.

Strain-induced shape transition of VSi2 clusters on Si(111)

We investigated the growth and shape of vanadium silicide clusters on Si(111) using scanning tunneling microscopy, atomic force microscopy and scanning electron microscopy. The deposition of 8 monolayers of vanadium on Si(111) and subsequent annealing at 1200 K leads to the formation of large elongated VSi2 clusters. This is in marked contrast to the deposition of submonolayer amounts of V where the VSi2 islands have a compact shape. The shape transition can be explained by a competition between edge formation and strain relaxation energy terms. The VSi2 clusters, which are aligned along the three high symmetry directions of the Si(111) surface and surrounded by a denuded zone, form a two-dimensional network.

Modification and strengthening mechanism of high strength-toughness as-cast Al–11Si–3Cu alloy modified with minor Sr and Er content

The strength of Al–11Si–3Cu requires improvement to meet the growing demands of manufacturing and expand its application in the automotive market. A new high strength-toughness as-cast Al–11Si–3Cu alloy with the ultimate tensile strength and elongation up to 294 MPa and 3.5 % was developed by using minor Sr and Er modifiers. The TEM and the first principles calculations showed that Sr and Er atoms adsorbed on the eutectic Si surface and prompted the formation of twins that branched in 70.5° and 39° directions, and the synergistic effects of Sr and Er atoms significantly reduced the size of eutectic Si to ∼500 nm. Moreover, the ∼10 nm Al3Er and ∼200 nm Al8Cu4Er particles formed in the alloy, which have positive effects on enhancing the mechanical properties of the alloy. The dislocation lines were stretched when passing through the Al3Er particles and eventually formed dislocation rings, while the motion of the grain boundary was hindered by the Al8Cu4Er particle. Additionally, the modification mechanism of Sr and Er modifiers on the eutectic Si and the strengthening mechanism of the modified alloy were discussed.

Competitive adsorption mechanism of SiHCl3 with BCl3 under a hydrogen atmosphere: Boron impurities introduction into polysilicon

The primary method for electronic-grade (EG) polysilicon production is the modified Siemens process. Unfortunately, the EG polysilicon produced through this method contains excessive levels of boron impurities (> 0.2 ppb), thereby diminishing the product quality. This issue impedes the large-scale production of EG polysilicon. This study establishes the structure of the Si (100) surface within a hydrogen-rich environment under the BCl3-SiHCl3-H2 system and investigates the adsorption mechanisms of BCl3 and SiHCl3, as well as the competitive adsorption mechanism of BCl3-SiHCl3, on the Si (100) surface. The results indicate that the adsorption energy barriers for BCl3 and SiHCl3 on the Si (100) surface are 7.8383 kJ·mol−1 and 229.6970 kJ·mol−1, respectively, so that the preferential adsorption of BCl3 onto the Si (100) surface over SiHCl3. The B atom and the Si atom are competing for adsorption on the Si (100) surface and creating an ab-B-Si-ab structure, and the optimal adsorption path is as follows: ab-BCl3→ab-BCl2→ab-BCl2-SiHCl3→ab-BCl-SiHCl3→ab-B-SiHCl3→ab-B-SiHCl2→ab-B-SiHCl→ab-B-SiH-ab→ab-B-Si-ab.

Interfacial Si–O coordination for inhibiting the graphite phase enables superior SiC/Nb heterostructure joining by AuNi

The reaction between SiC and Ni-based brazing alloy is extremely intense, where the brittle Ni–Si compound and accompanied dissociate graphite phase as the interfacial reaction product in the joint damage the mechanical properties. In this study, the design of "graphite-controlled" heterogeneous integration component structure on SiC surface is developed to weaken the intense reaction of Ni and SiC. The high-quality surface Si–O coordination was successfully achieved by plasma-enhanced chemical vapor deposition (PECVD) method at relatively low temperature on SiC, which increases the Si bond strength due to that the higher binding energy of Si–O (451 kJ/mol) than that of Si–C (347 kJ/mol). SiO2 films enhance the mechanical properties of the joint through macro-weak side interface interlocking and micro-stress mismatch regulation. The shear strength of SiC/Nb heterostructure joints by AuNi alloy was increased by 44 %, which paves an inspiring way for boosting heterogeneous integration of SiC ceramic components.

Crystal orientation of epitaxial film deposited on silicon surface

Direct growth of oxide film on silicon is usually prevented by extensive diffusion or chemical reaction between silicon (Si) and oxide materials. Thermodynamic stability of binary oxides is comprehensively investigated on Si substrates and shows possibility of chemical reaction of oxide materials on Si surface. However, the thermodynamic stability does not include any crystallographic factors, which is required for epitaxial growth. Adsorption energy evaluated by total energy estimated with the density functional theory predicted the orientation of epitaxial film growth on Si surface. For lower computing cost, the adsorption energy was estimated without any structural optimization (simple total of energy method). Although the adsorption energies were different on simple ToE method, the crystal orientation of epitaxial growth showed the same direction with/without the structural optimization. The results were agreed with previous simulations including structural optimization. Magnesium oxide (MgO), as example of epitaxial film, was experimentally deposited on Si substrates and compared with the results from the adsorption evaluation. X-ray diffraction showed cubic on cubic growth [MgO(100)//Si(100) and MgO(001)//Si(001)] which agreed with the results of the adsorption energy.

Formation of Organic Monolayers on KF-Etched Si Surfaces

Silicon is the most commonly used material in the microelectronics industry, due to its inherent advantages of high natural abundance, low cost, and high purity, coupled with the chemical and electrical stability at the interface with its oxide. For molecular electronics applications, oxide-free Si surfaces are widely used because of the relative ease of removing the oxide (SiOx) by chemical means, yielding a surface which forms strong covalent bonds with a wide range of chemical functional groups; another advantage is that these surfaces remain oxide-free in the absence of oxidising agents. Standard procedures require the use of either HF, NH4F, or a mixture of both as the etching solution; however, these two chemicals are highly corrosive and toxic, posing a significant risk to the experimentalist. Here, we report that for silicon wafers etched by using potassium fluoride, a less toxic chemical, the resulting surface is free of oxides and can be functionalized by self-assembled monolayers of 1,8-nonadiyne. To demonstrate this, Si/SiOx wafers were etched by using either KF or NH4F, followed by hydrosilylation with 1,8-nonadiyne and a click reaction of the terminal alkyne with azidomethylferrocene. The surface coverages and electron transfer kinetics of the ferrocene-terminated KF-etched surfaces are comparable to those formed by acidic fluoride etching procedures. This is the first study comparing the differences between surfaces functionalized by self-assembled monolayers of 1,8-nonadiyne which were etched by KF and NH4F. KF could be used as a replacement chemical for etching silicon wafers when a less corrosive and toxic chemical is required.