Single Crystal Si Research Articles

The role of SiO2 buffer layer in the molecular beam epitaxy growth of CsPbBr3 perovskite on Si(111)

Here we present the growth of molecular beam epitaxy (MBE) CsPbBr3 perovskite films in the orthorhombic crystal structure, with unique structural and morphological properties. CsPbBr3 MBE perovskite films, with thickness ranging from a few nm to 300 nm, were grown in ultra-high vacuum on a Si(111)7 × 7 reconstructed surface, and after the formation of about 2 nm of SiO2, obtained exposing the clean reconstructed Si surface to molecular oxygen that serves to decouple the film from substrate. X-ray diffraction, and electron microscopies, such as scanning electron microscopy and high-angle annular dark-field scanning transmission electron microscopy measurements showed remarkable structural, as well as morphological features, indicating extremely high crystallinity over a large area and across the bulk of the perovskite film. Through the X-ray diffraction patterns we found very narrow (002) and (110) reflections of CsPbBr3 in pure orthorhombic phase, exhibiting a full width at half maximum of only 0.035°, value similar to a bulk Si single crystals, and a surface morphology composed of flat areas up to micrometres in lateral size. Our results shed new light both on preparation of high crystal quality perovskite films, and on the intrinsic properties of this striking fully-inorganic materials, which are exploitable for potential applications in electronic/optoelectronic devices and next generation photovoltaic solar cells.

Effect of the crystallographic orientation of the surface of single-crystal Si wafers on the endotaxial growth of NiSi2 nanoplates

A multi-technique analysis was used to investigate how the orientation of single-crystal Si wafer surfaces affects the size, shape and orientation of NiSi2 nanocrystals grown within the wafers through the thermal diffusion of Ni atoms from a nickel-doped thin film deposited on the surface. Nickel-doped thin films were prepared on silicon wafers with three distinct crystallographic orientations, [001], [110] and [111]. Three sets of samples were then annealed at 500, 600 and 700°C for 2 h. Regardless of crystallographic orientation or annealing temperature, NiSi2 nanoplates with a nearly hexagonal shape grew close to the external surface of the wafers, aligning their larger surfaces parallel to one of the planes of the Si{111} crystallographic form. The crystallographic orientation and annealing temperature in the 500–700°C range did not significantly affect the final values of the average diameter and thickness of the nanoplates. However, significant differences were noted in the number of nanoplates formed in Si wafers with different crystallographic orientations. The results indicate that these observed differences are correlated with the number of pre-existing defects in the wafers that influence the heterogeneous nucleation process. In addition, the average size and size dispersion were determined for pores at the surface of the Si wafers formed due to the etching process used for native oxide removal.

Atomistic understanding of ductile-to-brittle transition in single crystal Si and GaAs under nanoscratch

Ensuring ductile removal in a grinding process is crucial for achieving the desired finish on a hard and brittle single crystal. This study provides new insights into the material removal processes in Si and GaAs single crystals, exploring their deformation behaviour using Berkovich and Conical tips to replicate contact from a fixed abrasive grit. Experimental observations are compared with Molecular Dynamic (MD) simulations to uncover the atomistic deformation mechanisms during the ductile-to-brittle transition (DBT). Notable plastic deformation and minimal cracking were consistently observed in Si, irrespective of the tips used. MD simulations supported this observation, revealing pronounced chip formation indicative of ductile material removal. The resistance to cracking in Si was attributed to amorphization induced by localized high contact stresses. In contrast, GaAs showed a propensity for cracking, with MD simulations revealing dislocation and slip band formation, and cracks emerging in the areas of substantial plastic deformation. These findings address phenomena not previously discernible in experimental studies due to the challenge of real-time observation. Moreover, the tip geometry was shown to significantly influence stress distribution, impacting deformation and crack formation in GaAs. This study also reveals limitations in predicting the critical depth for DBT in both Si and GaAs throught the amended Bifano, Dow, and Scattergood (aBDS) models and MD simulation, offering nuanced insights into these challenges that have not been extensively explored. It was found that the experimental results exceeded predictions by an order of magnitude. These discrepancies underscore the aBDS model's disregard for essential material properties and tip geometry, while the disparities between MD simulation and experiment are primarily attributed to the inherent limitations in the simulated length scales and challenges in detecting initial subsurface cracks.

Impact of Binder Thin-Films on Surface Chemistry of Silicon Anodes in Lithium-Ion Batteries

The effect of polymer binder on the nature and location of solid-electrolyte interphase (SEI) formation on Si active material was investigated. Thin layers of polymeric binder (polyvinylidene fluoride and sodium carboxymethyl cellulose) were spin-coated onto polished single crystal Si wafers. The samples were cycled against a Li counter/reference electrode under various electrochemical conditions in a coin cell configuration. The electrolyte was 1 M of LiPF6 in 1:1:1 wt% EC:DEC:DMC. After cycling, the samples were extracted from the coin cell under inert conditions and X-ray photoelectron spectroscopy (XPS) was performed. Electron microscopy and atomic force microscopy indicate that the binder films are smooth and continuous across the wafer surface, and no impact on the electrochemical behavior was observed. However, notable differences were detected using XPS, which revealed both difference in SEI composition and location relative to the binder-electrolyte and binder-substrate interfaces. These observations and insights will be useful in cell designs, binder selection, and reliability estimates because the location of SEI formation may influence cyclic performance of electrodes.

Evaluation of Thermal Transport Characteristics of Three-Dimensional Self-Ordered Multilayered Silicon-Germanium Nanodots

1. Background and purpose Silicon-germanium (SiGe) alloys have much lower thermal conductivity than Si and Ge single crystals and are expected to be candidates for next-generation thermoelectric device materials of internet of things society. It is important to achieve both high carrier mobility and low thermal conductivity on the basis of the dimensionless figure of merit [1]. There are several studies on the thermoelectric properties of SiGe with low-dimensional structures such as superlattice and nanodots [2,3] to dramatically decrease thermal conductivity. However, the relationship between the nanoscale thermal properties and the phonon scattering mechanism of self-ordered multilayer SiGe nanodots with high uniformity grown by Stranski-Krastanov [4] mode is unclear. In this study, we demonstrated the thermal transport characteristics of the self-ordered multilayer SiGe nanodots. 2. Experiments Si/SiGe superlattice and nanodots samples were fabricated by reduced pressure chemical vapor deposition. We set to 20 cycles of Si0.65Ge0.35 / Si superlattice, the staggered Si0.65Ge0.35 nanodots (alternately stacked) and the dot-on-dot Si0.65Ge0.35 nanodots (vertically stacked) by changing growth temperature and precursors for Si spacer growth [4], as shown in Fig. 1. Thermal properties were evaluated by the Time Domain Thermoreflectance (TDTR) method. The strain states and Ge fraction were evaluated by Raman spectroscopy. The focal length and wavenumber resolution of the Raman spectrometer were 2,000 mm and 0.1 cm-1, respectively. The excitation light source was a green laser (wavelength: 532 nm). We obtained the Raman spectra of Si-Si, Si-Ge, and Ge-Ge vibrational modes derived from the SiGe regions. 3. Results and discussion Figure 2 shows the thermal conductivity for the SiGe region of the superlattice and nanodots (dot-on-dot and staggered structures) obtained by TDTR method. We found that the SiGe nanodots with the dot-on-dot structure have the highest thermal conductivity, while the SiGe nanodots with the staggered structure and SiGe superlattice have almost the same thermal conductivity. We consider that the heat flow easily penetrates from the top (surface) to the bottom (Si substrate interface) in the dot-on-dot structures. On the other hand, it is possible that the staggered SiGe nanodots can play a sufficient role as a phonon scatterer compared to the dot-on-dot structures since the superlattice structure and the staggered SiGe nanodots have almost the same thermal properties. In conclusion, we demonstrated the possibility of controlling phonon scattering by changing the arrangement of the self-ordered multilayer SiGe nanodots. Acknowledgement This work was supported by JSPS KAKENHI Grant Number 21K14201, Japan.

Effect of bias voltage on the structure and properties of CoCrFeNi high entropy alloy thin films prepared by magnetron sputtering

In this study, CoCrFeNi high entropy alloy thin films were prepared on 304 stainless steel and single crystal Si substrates using the direct-current magnetron sputtering process under different bias voltages. The effects of these bias voltages on the film micrographs, phase structure, surface roughness, corrosion resistance, and mechanical characteristics of thin films were studied by a scanning electron microscope, transmission electron microscope, X-ray diffractometer, atomic force microscope, electrochemical workstation and nanoindentation tester, respectively. The results indicated that all films exhibited equiaxed nano-scale grains and developed a columnar structure. Fe, Co, Cr, and Ni were uniformly dispersed in all films and all films displayed face centered cubic solid solution (FCC). At a bias voltage of 0 V, the CoCrFeNi HEAs film formed a FCC single phase. With increasing bias voltage, the crystallinity decreased and then increased, reaching the worst crystalline state at bias voltage of −200 V due to the fact that the structure of the film was composed of FCC nanocrystalline and amorphous biphasic structure. Meanwhile, the surfaces of the film prepared at bias voltage of −200 V exhibited the densest structures and the least roughness, with grain sizes reaching a minimum value of approximately 10 nm. In addition, columnar grains of the film prepared at a bias voltage of −200 V displayed the densest state and had the least size, thus this film displayed the best corrosion resistance of a minimum value 1.51 × 10−7 A/cm2, a maximum hardness of approximately 11.18 ± 0.2 GPa and a maximum elastic modulus of approximately 193 ± 2.8 GPa. The passivation films of CoCrFeNi HEAs films were mainly composed of Cr2O3.

Enhanced thermoelectric properties of nanostructured Si by excess P-doping

In this study, nanostructures were introduced into bulk Si, and the P content in Si was increased. Arc melting, ball milling, and spark plasma sintering were used to prepare excess P-doped Si samples (Si1−xPx, x = 0.01, 0.03, 0.06, 0.09, and 0.12). In Si0.94P0.06, thermal conductivity decreased to 18 W m−1 K−1 at room temperature, which is about 10% of that of bulk Si. Transmission electron microscopy observation of Si0.94P0.06 revealed a linearly localized alignment, in addition to the nanostructures observed in the previous studies of P-doped Si samples. Linear alignments were composed of Si-P nanoprecipitates and defects linearly localized around dislocations. These sub-microstructures can reduce thermal conductivity; as a result, ZT at 973 K increased to 0.43, which is approximately three times higher than that of single-crystal Si.

Application of laboratory micro X-ray fluorescence devices for X-ray topography

It is demonstrated that high-resolution energy-dispersive X-ray fluorescence mapping devices based on a micro-focused beam are not restricted to high-speed analyses of element distributions or to the detection of different grains, twins and subgrains in crystalline materials but can also be used for the detection of dislocations in high-quality single crystals. Si single crystals with low dislocation densities were selected as model materials to visualize the position of dislocations by the spatially resolved measurement of Bragg-peak intensity fluctuations. These originate from the most distorted planes caused by the stress fields of dislocations. The results obtained by this approach are compared with laboratory-based Lang X-ray topographs. The presented methodology yields comparable results and it is of particular interest in the field of crystal growth, where fast chemical and microstructural characterization feedback loops are indispensable for short and efficient development times. The beam divergence was reduced via an aperture management system to facilitate the visualization of dislocations for virtually as-grown, non-polished and non-planar samples with a very pronounced surface profile.

Effects of crystallinity of silicon channels formed by two metal-induced lateral crystallization methods on the cell current distribution in NAND-type 3D flash memory

Two metal-induced lateral crystallization (MILC) methods are proposed as candidate techniques to enhance cell current in future ultra-high-density NAND-type 3D flash memory devices. The channel crystallinity differs depending on the MILC method. In a single MILC, the channel is composed of single-crystal Si, whereas in a regional MILC, the channel comprises multiple crystal grains that are larger than those of the conventional polycrystalline Si. Using transmission electron microscopy, the inhibiting factor of MILC was modeled to reveal that the two MILC approaches result in different cell current distributions that are related to their degree of crystallinity. A comparison of these two cell current distributions in a 3D flash memory with over 900 word-line stacks showed that the single MILC delivers a higher median cell current with outliers on the lower side. In contrast, the regional MILC delivers a lower median cell current without outliers on the lower side.

Development of low-stress SiC coating on single crystal Si using low elastic modulus SiOC amorphous buffer layer

In this study, we successfully prepared low-stress SiC coatings on single-crystal Si substrates using the organometallic chemical vapor deposition (MOCVD) technique. For this purpose, the SiOC buffer layer (with low hardness and low elastic modulus) was initially deposited on the substrate and then the SiC coating was directly deposited on the buffer layer. A series of both, the buffer layer and the SiC coating, were separately deposited at various process conditions to pinpoint their optimized combination. Results revealed that the residual compressive stress value for the optimized SiC composite coating (containing low elastic modulus SiOC buffer layer) was 87.9% lower than that of the single SiC layer. The findings of this study are of great significance for the production and integration of the SiOC buffer layer and the SiC coating which not only improves the production efficiency but also reduces the production costs low-cost power devices.

Dendritic Ag@c-Si structure for influenza A virus detection by SERS and machine learning

The combination of Surface-enhanced Raman Scattering (SERS) method and machine learning algorithms has been successfully demonstrated for the detection and differentiation of influenza A virus in buffer medium. Dendritic Ag nanostructures on single-crystal Si wafer have been proposed as SERS-active substrates, which are fabricated using a low-cost and reproducible AgNO3 reduction method. Depending on the time of Ag deposition, structures of different morphologies were obtained. These structures were studied by spectroscopic ellipsometry (SE). To analyze the spectra obtained by SE, we used the Bruggeman effective medium approximation to determine the volume fraction and height of the metal, the Drude model to describe free electrons, the Tauc-Lorentz model to describe the influence of interband transitions, as well as Lorentz model and Gauss model to describe localized surface plasmon resonance (LSPR). SERS was achieved through LSPR excitation and the presence of “hot spots” in the regions around the tips and in the spaces between close-packed dendritic Ag nanostructures. The sample with the most developed surface is the most promising for SERS, since the spectral position of the LSPR is at 658 nm, which is close to the excitation wavelength of the He–Ne laser. The SERS spectra of influenza A virus in buffer medium are difficult to distinguish, so a machine learning algorithms (principal component analysis and support vector machine) were used to directly classify them. For the sample with the most developed morphology, the detection accuracy of influenza A virus was 76.6 ± 4.2 % at a total protein concentration in the test analyte of 300 μg/ml (according to the Lowry method).

Self-powered high-performance UV–vis–IR hybrid photovoltaic photodetector based on Epilobium angustifolium L. (Onagraceae)

Water-soluble Epilobium angustifolium (EA) extract was successfully coated on n-type Si single crystal by spin coating method to obtain EA/n-Si heterojunction device. UV–vis absorption and SEM analysis of the EA film were performed and the optical band gap was calculated as 2.71 eV. Electro-optical measurements of the fabricated device were performed and was found to give very good photoresponse in the UV–vis-IR regions. According to the experimental results, the EA/n-Si heterojunction showed not only high-performance photodetector properties but also self-driven properties under visible light as well as under UV and IR lights. At 150 mW/cm2 visible light, the device demonstrated excellent photovoltaic properties with Voc and Isc values of 0.40 V and 4.65 A, respectively. Under IR light, the responsivity, specific detectivity and NPDR reached 0.65 A/W and 1.41 × 1012 Jones, 4.23 × 108 W−1 respectively. Finally, a high-performance organic/inorganic photodetector in the broadband range has been successfully fabricated.

Characterization of electromigration-induced short-range stress development in Al(0.25 at. % Cu) conductor line

Scanning x-ray microbeam topography and fluorescence experiments were conducted in situ to study the electromigration behavior of a 0.5 μm thick, 10 μm wide, and 200 μm long Al(0.25 at. % Cu) conductor line with 1.5 μm-thick SiO2 passivation on a single crystal Si substrate. The strain sensitivity of x-ray topography measurement allowed detailed examination of the electromigration-induced stress distribution and evolution in the conductor line in response to the depletion of Cu solute early in the electromigration process. Upon electromigration at 0.4 MA/cm2 and 303 °C, a short-range stress gradient was quickly induced by Al migration in the Cu-depleted cathode region to counteract further Al flow. The stress gradient was fully developed during the 5.3 h incubation time, extending over the critical Blech length of about 66 μm from the cathode end. Plastic deformation then occurred at the downstream end of the Cu-depleted region. The preferential electromigration of Cu did not cause detectable stress change outside the Cu-depleted region, except for the significant stress development from the Al2Cu precipitation at the anode end which appeared to initiate the fracture in the passivation. Preliminary finite difference modeling was undertaken to simulate the experimental observations, from which important parameters dictating electromigration in Al(Cu) line were extracted: an apparent effective valence of −5.6 and −1.9 for Cu and Al in Al(Cu), respectively, and a critical Cu concentration of 0.16 at. % above which Al grain boundary diffusion is effectively blocked.

Controlled synthesis and characterization of porous silicon nanoparticles for dynamic nuclear polarization.

Si nanoparticles (NPs) have been actively developed as a hyperpolarized magnetic resonance imaging (MRI) contrast agent with an imaging window close to one hour. However, the progress in the development of NPs has been hampered by the incomplete understanding of their structural properties that correspond to efficient hyperpolarization buildup and long polarization decays. In this work we study dynamic nuclear polarization (DNP) of single crystal porous Si (PSi) NPs with defined doping densities ranging from nominally undoped to highly doped with boron or phosphorus. To develop such PSi NPs we perform low-load metal-assisted catalytic etching for electronic grade Si powder followed by thermal oxidation to form the dangling bonds in the Si/SiO2 interface, the Pb centers. Pb centers are the endogenous source of the unpaired electron spins necessary for DNP. The controlled fabrication and oxidation procedures allow us to thoroughly investigate the impact of the magnetic field, temperature and doping on the DNP process. We argue that the buildup and decay rate constants are independent of size of Si crystals between approximately 10 and 60 nm. Instead, the rates are limited by the polarization transfer across the nuclear spin diffusion barrier determined by the large hyperfine shift of the central 29Si nuclei of the Pb centers. The size-independent rates are then weakly affected by the doping degree for low and moderately doped Si although slight doping is required to achieve the highest polarization. Thus, we find the room temperature relaxation of low boron doped PSi NPs reaching 75 ± 3 minutes and nuclear polarization levels exceeding ∼6% when polarized at 6.7 T and 1.4 K. Our study thus establishes solid grounds for further development of Si NPs as hyperpolarized contrast agents.

Influence of Accumulation of Impurity Atoms Ni and Fe on the Electrophysical Properties of Si Single Crystals

Influence of Accumulation of Impurity Atoms Ni and Fe on the Electrophysical Properties of Si Single Crystals

Effects of yttrium doping on high-temperature oxidation, friction, and wear properties of CrAlN films

This study investigates the deposition of CrAlYN nanomultilayer films with different yttrium (Y) contents on M2 tool steel and single-crystal Si wafer using unbalanced magnetron-sputtering ion-plating technology. Transmission and scanning electron microscopic and scanning morphologies, x-ray diffraction pattern, energy dispersive spectra, nanoindentation, high-temperature oxidation, and high-temperature tribological analyses show that CrAlYN nanomultilayer films have a face-centered cubic (fcc) crystal structure with a modulation period of CrN/YN+AlN/CrN/AlN. CrAlYN films exhibit optimal mechanical performance when Y content is 1.13at%. However, a substantial drop occurs when Y content increases to 2.67at%. As Y content increases, the high-temperature oxidation resistance, friction, and wear of CrAlYN films first increase and then decrease. Notably, films with the Y content of 1.13at% have optimal resistance to high-temperature oxidation, friction, and wear.

Origin of ultralow thermal conductivity in amorphous Si thin films investigated using nanoindentation, 3ω method, and phonon transport analysis

The origin of the ultralow thermal conductivity in amorphous Si thin films was investigated by comparing their phonon transport properties with those of single-crystal Si. The group velocity and thermal conductivity were measured at 300 K using nanoindentation and the 3ω method, respectively. The phonon mean free path (MFP) and phonon frequency were determined using the measured properties and models. The scattering in the disordered structure of amorphous Si thin films caused a significant decrease in the phonon MFP with an increase in the phonon frequency, leading to ultralow thermal conductivity. However, the group velocity was unaffected by the disordered structure.

Ultrasonic-assisted soldering of Si and Cu joint with SnAgTi solder in air

The reliable connection of single crystal Si and Cu at low temperature is widely utilized and plays a crucial role in the field of electronic packaging. In this study, Sn–Ag–Ti active filler is ultrasonically coated onto the Si surface to facilitate the subsequent ultrasonic soldering process to realize the bonding of Cu to pre-metallized Si. Furthermore, a detailed analysis was conducted on the influence of ultrasonic time on both the bonding mechanism and mechanical properties of the joint. The results revealed that Ti–Sn compound within the solder decomposed with ultrasonic action duration time of 20 s, resulting that Ti elements enriched at the Si/solder interface and reacted with SiO2 to form a mixture comprising TiO2 and SiTi phase. It contributed to higher strength (43.9 MPa) at the Si/solder interface. Additionally, it has been found that microstructure and property at the Cu/solder interface were directly affected by ultrasonic time. Specifically, when subjected to 3 s of ultrasonic action, poor load-bearing capacity exhibited by thin infiltration layers caused fracture at the Cu/solder interface; When the ultrasonic action time is 6 s, the joint strength reaches its maximum value of 41.3 MPa, leading to fracture occurring within the Si material. However, when the ultrasonic holding time is extended to 20 s, the joint strength decreases to 20.3 MPa. This phenomenon arises from impaired plasticity of Cu6Sn5 near the Cu–Sn layer, which is attributed to the presence of attached dendritic Cu6Sn5 phases and shed bulk Cu6Sn5 that are prone to crack initiation. This research presents a promising approach for ceramic/metal heterogeneous joining.

Generational assessment of EBSD detectors for cross-correlation-based analysis: From scintillators to direct detection

Introduced over ten years ago, cross-correlation-based electron backscatter diffraction has enabled high precision measurements of crystallographic rotations and elastic strain gradients at high spatial resolution. Since that time, there have been remarkable improvements in electron detector technology, including the advent of ultra-high speed detectors and the commercialization of direct detectors. In this study, we assess the efficacy of multiple generations of electron detectors for cross-correlation-based analysis using a single crystal Si sample as a reference. We show that, while improvements in precision are modest, there have been significant gains in the rate at which high-quality diffraction patterns can be collected. This has important implications in the size of datasets that can be collected and reduces the impact of drift and sample contamination.

Study of the dependence of the degree of disordering of the surface layers of Si(111) and Ge(111) single crystals upon bombardment with low-energy ions

Study of the dependence of the degree of disordering of the surface layers of Si(111) and Ge(111) single crystals upon bombardment with low-energy ions