Well-ordered Si Research Articles

Crater-free monolayer graphene above the 2D Si film on SiC(0001) formed via SiSn cointercalation and Sn deintercalation

When Si atoms are intercalated under the graphene-like zero layer (ZL) generated on the SiC(0001) substrate, the resulting quasi-free-standing monolayer graphene (QFMLG) has lots of crater-like defects due to a strong Si-C reaction. Here, a method for creating crater-free QFMLG above a two-dimensional Si film is investigated using scanning tunneling microscopy and photoemission spectroscopy. The method involves forming a SiSn interfacial film as a precursor via cointercalation performed by sequential Sn and Si deposition on the ZL at room temperature and annealing at 650–700 °C. The Sn atoms prevent reactive Si atoms from directly contacting the ZL, which would otherwise cause SiC defects. The sample is then annealed at 750 °C to selectively remove the Sn atoms from the SiSn interfacial film, leaving a well-ordered Si film. The SiSn film composed of a bottom Si layer and top Sn atoms has a short-range-ordered “2 × 2” or a long-range-ordered 2×7 superstructure depending on the annealing temperature and the Sn coverage. Both the superstructures are semiconducting and induce less n-doped QFMLG than the pure Si film.

Silicon Nanowire Array Weaved by Carbon Chains for Stretchable Lithium-Ion Battery Anode.

To manufacture flexible batteries, it can be a challenge for silicon base anode materials to maintain structural integrity and electrical connectivity under bending and torsion conditions. In this work, 1D silicon nanowire array structures combined with flexible carbon chains consisting of short carbon nanofibers (CNFs) and long carbon nanotubes (CNTs) are proposed. The CNFs and CNTs serve as chain joints and separate chain units, respectively, weaving the well-ordered Si nanowire array into a robust and integrated configuration. The prepared flexible and stretchable silicon array anode exhibits excellent electrochemical performance during dynamic operation. A high initial specific capacity of 2856mAhg-1 is achieved. After 1000 cycles, a capacity retention of 60% (1602mAhg-1) is maintained. Additionally, the capacity attenuation is less than 1% after 100 bending cycles. This excellent cycling stability is obtained with a high Si loading of 6.92mgcm-2. This novel approach offers great promise for the development of high-loading flexible energy-storage devices.

GaAsP/Si tandem solar cells: In situ study on GaP/Si:As virtual substrate preparation

Realization of high efficiency III-V-on-Si solar cells requires preparation of an oxygen free Si substrate surface, which allows for subsequent antiphase domain free III-V epitaxy. For GaAsP/Si tandem cells, the impact of As on established Si processing in metal organic vapor phase epitaxy (MOVPE) is essential. Here, we study the interaction of As with vicinal Si(100) surfaces, the formation of atomically well-ordered As-modified Si(100) surfaces, and its impact on subsequently grown GaP epilayers. We apply optical in situ spectroscopy during MOVPE and show that process routes strongly affect the formation of double-layer steps on Si(100). We demonstrate that exposure to As, together with HF pre-treatment, enables processing of suitable Si(100) 2° and 6° offcut surfaces at temperatures below 820°C. Subsequently grown GaP epilayers exhibit smooth surfaces free of antiphase disorder and distinct Laue oscillations in X-ray diffraction. Such low temperature processing, which is controllable in situ, is promising for further metamorphic GaAsP integration.

High efficiency n-Si/p-Cu2O core-shell nanowires photodiode prepared by atomic layer deposition of Cu2O on well-ordered Si nanowires array

A highly efficient n-Si/p-Cu2O core-shell (C-S) nanowire (NW) photodiode was fabricated using Cu2O grown by atomic layer deposition (ALD) on a well-ordered Si NW array. Ordered Si nanowires arrays were fabricated by nano-sphere lithography to pattern metal catalysts for the metal-assisted etching of silicon, resulting in a Si NW arrays with a good arrangement, smooth surface and small diameter distribution. The ALD-Cu2O thin films were grown using a new non-fluorinated Cu precursor, bis(1-dimethylamino-2-methyl-2-butoxy)copper (C14H32N2O2Cu), and water vapor (H2O) at 140°C. Transmission electron microscopy equipped with an energy dispersive spectrometer confirmed that p-Cu2O thin films had been coated over arrayed Si NWs with a diameter of 150 nm (aspect ratio of ∼7.6). The C-S NW photodiode exhibited more sensitive photodetection performance under ultraviolet illumination as well as an enhanced photocurrent density in the forward biasing region than the planar structure diode. The superior performance of C-S NWs photodiode was explained by the lower reflectance of light and the effective carrier separation and collection originating from the C-S NWs structure.

Amorphous oxygen-rich molybdenum oxysulfide Decorated p-type silicon microwire Arrays for efficient photoelectrochemical water reduction

We report the fabrication of p-type silicon (Si) photocathodes consisting of well-ordered Si microwire (Si-MW) arrays coupled with non-precious and earth-abundant amorphous oxygen-rich molybdenum oxysulfide (MoOxSy) as both a hydrogen evolution catalyst and a passivation layer. The MoOxSy is conformally grown on the Si-MW surface through photo-assisted cyclic voltammetric (CV) deposition. By adjusting the cycle numbers of the CV deposition, Si-MW array electrodes with different MoOxSy catalyst loadings (Si-MWs@MoOxSy) have been obtained and comprehensively characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman and Fourier-transform infrared spectroscopies. The photoelectrochemical performance of the Si-MWs@MoOxSy cathodes toward water reduction is investigated and compared with that of platinum nanoparticle decorated Si-MW array electrodes (Si-MWs@PtNPs). An optimized Si-MWs@MoOxSy photocathode is found to exhibit activity comparable to that of the Si-MWs@PtNPs one, with a much better stability in acidic medium. In neutral electrolyte, Si-MWs@MoOxSy outperforms Si-MWs@PtNPs in terms of both activity and stability. Given the low materials cost, easy and well-established electrode fabrication procedure, and high demonstrated photoelectrochemical performance, the Si-MWs@MoOxSy arrays reported here hold substantial promise for use as low-cost and efficient photocathodes for water reduction.

Selective Epitaxial Si:P Film for nMOSFET Application: High Phosphorous Concentration and High Tensile Strain

Semiconductor industry is in the era of transition from 2D transistors to 3D transistors (for example, FinFETs). The scaling–down of transistor source/ drain (S/D) contact area causes more challenges in reducing the S/D parasitic resistance which becomes comparable to (or even higher than) the channel resistance itself. A highly phosphorous doped Si epitaxial film on S/D is crucial to reduce the parasitic resistance in nMOSFET transistors. Besides that, 2D/3D nMOSFET transistors favor the tensile strain induced in the channel to enhance the electron channel mobility. Here we present a selective Si:P epitaxial film growth which provides both high phosphorous concentrations (>1E+21 at/cc) and high tensile strain (comparable to ~1 at.% of Csub in Si:CP).This selective Si:P epitaxial process was performed using dichlorosilane (DCS), phosphine (PH3), and hydrochloride (HCl) gases in Applied Materials Centura RP Epi system. Grown highly concentrated, highly tensile-strained Si:P (called HS Si:P) films were analyzed by techniques of high-resolution XRD (HR-XRD), four-point Rs probe, SEM, TEM, and SIMS, etc.Table 1 compares two types of Si:P epitaxial films: conventional Si:P Epi versus HS Si:P Epi. In Fig. 1(a), the total [P] by SIMS in HS Si:P epitaxial film is 1.75E+21 at/cc (~ 3 at.% in silicon), about one magnitude order higher than in conventional Si:P film, much higher than the solid solubility (~3E+20 at/cc) of phosphorous in silicon at 700°C [1]. The 0.6 mΩ-cm resistivity in HS Si:P epitaxial film indicates that only ~1.3E+20 at/cc phosphorous atoms are electrically active. In Fig. 1(b), the HR-XRD profile from HS Si:P film shows a strong tensile strain equivalent to ~0.8 at.% Csub from a Si:CP film. We assume that majority of phosphorous atoms are covalently bonded with adjacent Si atoms in a stable Si-P compound phase –pseudocubic Si3P4 which is energetically favored relative to other Si3P4 phases [2]. With the Vegard’s law (of linear relationship between lattice parameter and alloy concentration) applied between Si and pseudocubic Si3P4 (which has a smaller lattice constant than Si), the tensile strain induced in HS Si:P film matches well the HR-XRD data, as previously reported by Z. Ye et. al.[3].Fig. 2 plots out selective HS Si:P epitaxial process sensitivity to growth temperature (675-775°C) regarding resistivity, strain, and total [P]. HS Si:P epitaxial films are stable without obvious phosphorous out-diffusion in this temperature range, which is confirmed by SIMS measurements. Both total [P] level and tensile strain decrease with the increasing temperature. Meanwhile, resistivity drops from 0.7 mΩ-cm @675°C to 0.5 mΩ-cm @725°C as more phosphorous atoms are electrically activated at higher temperature.Fig. 3 characterizes the HS Si:P film epitaxial growth on the (110) orientated substrate. The HR-XRD profile in Fig. 3(a) indicates a well-ordered HS Si:P epitaxial film grown on the (110) substrate. The TEM image in Fig. 3(b) shows a HS Si:P epitaxial film grown on (110) substrate without defects at interface, which is very significant for epitaxial growth around non-planar structures. Furthermore, Fig. 4(a) and (b) present two high-quality HS Si:P films with few defects, epitaxially grown on a planar structure and a Si fin structure, respectively.HS Si:P epitaxial films have been studied after the millisecond annealing treatment @900-1300°C in Fig. 5. After annealing, resistivity drops from 0.65 mΩ-cm @1050°Cto ~ 0.3 mΩ-cm @1150°C and above. Tensile strain in film is stable up to 1200°C. At 1250°C and above, HS Si:P epitaxial film obviously starts to partially lose the strain likely because some phosphorous atoms are released from the structure of pseudocubic Si3P4 at such high temperatures.Overall, this selective HS Si:P epitaxial process demonstrates its great potential in 2D/3D nMOSFET application.

High efficiency n-ZnO/p-Si core–shell nanowire photodiode based on well-ordered Si nanowire array with smooth surface

A highly efficient n-ZnO/p-Si core–shell nanowire (NW) photodiode was fabricated using ZnO grown by atomic layer deposition (ALD) on a well-ordered Si NW array. Si NW arrays were prepared by metal-assisted chemical etching, for which a metal mesh with a well-organized nanohole array was made using anodic aluminum oxide. This resulted in a good arrangement, smooth surface, and small diameter distribution of the Si NW array. Consequently, ZnO layers with various thicknesses from 15 to 30nm were deposited by the ALD method. Because of the smooth surface of the well-ordered Si NWs yielding low surface roughness scattering, the resulting photodiode showed significantly improved device characteristics.

Preparation of clean and well-ordered hydrogen-terminated Si(1 1 0)-(1 × 1) surfaces and the measurements of vibrational modes

We propose electrochemical and wet chemical processes to prepare the clean and well-ordered hydrogen-terminated Si(110)-(1×1) surfaces, with our evaluation by low-energy electron-diffraction (LEED), high-resolution electron-energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). Clear (1×1) LEED patterns indicated the two-dimensional space group of p2mg. The intrinsic SiH bending and stretching modes and surface phonon modes, as well as hydrocarbon impurities were observed by HREELS. Among the procedures tested, the electrochemical etching in a NH4F solution yielded the best ordered surface. The wet chemical etching produced the clean samples suitable for HREELS measurements.

Film-thickness-dependent conduction in ordered Si quantum dot arrays

In recent years, silicon nanostructures have been investigated extensively for their potential use in photonic and photovoltaic applications. So far, for silicon quantum dots embedded in SiO2, control over inter-dot distance and size has only been observed in multiple bilayer stacks of silicon-rich oxides and silicon dioxide. In this work, for the first time the fabrication of spatially well-ordered Si quantum dots (QDs) in SiO2 is demonstrated, without using the multilayer approach. This ordered formation, confirmed with TEM micrographs, depends on the thickness of the initially deposited sub-stoichiometric silicon oxide film. Grazing incidence x-ray diffraction confirms the crystallinity of the 5 nm QDs while photoluminescence shows augmented bandgap values. Low-temperature current–voltage measurements demonstrate film thickness and order-dependent conduction mechanisms, showing the transition from temperature-dependent conduction in randomly placed dots to temperature-independent tunnelling for geometrically ordered nanocrystals. Contrary to expectations from dielectric materials, significant conduction and photocarrier generation have been observed in our Si QDs embedded in SiO2 demonstrating the possibility of forming initial film-thickness-controlled conductive films. This conduction via the silicon quantum dots in thick single layers is a promising result for integration into photovoltaic devices.

Structural and electronic properties of double Pb chains on the Si(553)-Au surface

We report on the structural and the electronic properties of Pb atomic chains fabricated on the prepatterned well-ordered Si(553)-Au surface. Using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission (ARPES) supplemented by first-principles density functional theory (DFT) calculations, we show that submonolayer coverage of Pb (0.35 ML) leads to a regular array of several nanometers long double Pb chains. The electronic band structure and the simulated STM topography images obtained from the DFT calculations agree well with the ARPES and STM/STS data.

Dehydration of the natural zeolite goosecreekite CaAl2Si6O16{middle dot}5H2O upon stepwise heating: A single-crystal and powder X-ray study

The zeolite mineral goosecreekite CaAl 2 Si 6 O 16 ·5H 2 O has been structurally investigated between 25 and 600 °C to monitor structural modifications upon partial dehydration. Temperature-dependent in situ powder and single-crystal X-ray techniques were combined with TG/DTA experiments. Goosecreekite has a porous framework structure with well-ordered Si,Al distribution. In its natural form extraframework Ca is seven-coordinated. Up to ca. 100 °C, 1 H 2 O molecule is expelled reducing the Ca coordination from 7 to 6 accompanied by very minor changes of the tetrahedral framework. Depending on humidity and equilibration time a second structural change begins between 150 and 250 °C due to loss of additional 2.8 H 2 O pfu. The space group P 2 1 is maintained but the c axis doubles and the structure is highly twinned. This new phase may be interpreted with a disordered model having two different framework topologies: one with the original T-O-T linkage and a second one with broken T-O-T links. Starting at ca. 300 °C, another 1.2 H 2 O are lost and this new orthorhombic, strongly contracted phase of P 2 1 2 1 2 1 symmetry has five-coordinated Ca. The X-ray powder signal of the orthorhombic phase persists up to 600 °C, but the structure becomes increasingly X-ray amorphous until complete destruction at 675 °C. A single crystal ( P 2 1 phase with doubled c axis) partly dehydrated at 200 °C recovers the structure of natural goosecreekite under ambient conditions. Similar rehydration occurs after one month under ambient humidity for a powder sample of the orthorhombic P 2 1 2 1 2 1 phase produced by heating of goosecreekite to 325 °C.

Amorphous Carbon Coated Silicon Nanotips Fabricated by MPCVD Using Anodic Aluminum Oxide as the Template

We have used nanoporous anodic aluminum oxide (AAO) as a template to fabricate amorphous carbon coated silicon nanotips by microwave plasma chemical vapor deposition (MPCVD). During the preparation of the well-ordered AAO pore channel array, an underlying TiN layer was anodically oxidized as well in the late stage of the AAO anodization, forming titanium oxide nanomasks for Si nanotip fabrication. The titanium oxide nanomasks were then used to transfer the arrangement pattern of the AAO pore channel array to the Si substrate by plasma etch in the MPCVD system and, therefore, a well-ordered Si nanotip array was produced. An layer thick was in situ deposited on the Si nanotips during the MPCVD process. The layer was rich in nanocrystalline graphitic carbons according to Raman and Auger electron spectroscopies. The nanocrystalline graphitic carbons in the coating and the sharp tip shape made the Si nanotip a good field emitter and a field enhancement factor of was obtained.

Stability and fragmentation of organic silicon functionalizations studied by laser desorption mass spectrometry

A method is introduced to investigate organic functionalizations on silicon by laser-induced thermal desorption (LITD), where well-ordered Si(1 1 1)-(1 × 1):H(D) surfaces are used to determine the desorption temperature as a function of laser fluence. To demonstrate the potential of this technique silicon surfaces with ultrathin oxide layers were functionalized with organic end groups. The species desorbed with focused XeCl laser pulses were monitored at an oblique angle and their time-of-flight (TOF) distributions were measured with a quadrupole mass analyzer after electron impact ionization. By assuming a negligible contribution of the oxide and organic layers to the heating effect, the TOF temperatures measured for Si(1 1 1)-(1 × 1):H(D) could be used to determine the mass of the desorbed species. Detailed results are presented for dimethylsilyl (DMS), bromomethyldimethylsilyl (BMDMS), and chloromethyldimethylsilyl (CMDMS) terminated surfaces which were prepared by silanization with suitable chloro and disilazane compounds. While for the DMS termination dimethylsilanol (76 u) is desorbed as a single species, clearly identifying the terminating group, in the case of BMDMS and CMDMS further fragmentation of the end group occurs at the surface.

Growth of SrTiO 3 films on Si(0 0 1)–Sr(2 × 1) surfaces

The growth and characterization of a SrTiO 3 film by molecular beam deposition process have been studied. After Sr deposition on the chemically formed SiO 2/Si surface, a stable and well-ordered Si(0 0 1)–Sr(2 × 1) surface was formed. The SrTiO 3 films were grown on the Si(0 0 1)–Sr(2 × 1) surface at 80 °C in a molecular oxygen partial pressure of approximately 3 × 10 −7 Torr and subsequently in situ post-annealed at various high temperatures without oxygen supply in ultra-high vacuum (<2.5 × 10 −9 Torr). The combined reflection high-energy electron diffraction, X-ray diffraction and atomic force microscopy analyses suggest that the SrTiO 3 film grown at 80 °C is amorphous nature. The lowest post-annealing temperature from amorphous-to-crystal transformation of the SrTiO 3 film is approximately 485 °C. The quality of the crystalline SrTiO 3 films is further improved by post-annealing at temperatures between 600 and 700 °C.

Monolayer Nitridation of Si(001) Surfaces

Well-ordered nitrogen-covered Si(001) surfaces were formed for the first time by thermal nitridation using an N2/H2 gas mixture. The scanning tunneling microscope images showed clear Si(001)-2×2 reconstruction over the entire surface. The ordered structure of the nitrided surfaces can be explained by a model in which the surface nitridation by N2 and the termination of the remaining Si dangling bonds by hydrogen atoms occur simultaneously to minimize the surface free energy during the nitridation reaction.

Structural Transformations During Sb Adsorption on Si(111)–In(4×1) Reconstruction

Understanding of initial stages of compound semiconductor growth on Si substrates is important for achieving atomically flat heterointerfaces, which are necessary for the fabrication of many advanced devices. Present report details the formation of InSb(2×2) reconstruction on the Si(111) substrate by Sb adsorption on Si(111)–In(4×1) surface phase and illustrates the structural transformations caused by the reordering of excess In. Higher Sb adsorption destroys the InSb phase with the coalescence of most of the In into 3D islands. This coalescence is accompanied by the formation of well-ordered Si(111)–Sb(2×1) phase between 260 and 350°C. Possible atomistic mechanisms responsible for the structural transformations are detailed.

Raman scattering from very thin Si layers of Si/SiO2 superlattices: Experimental evidence of structural modification in the 0.8–3.5 nm thickness region

Raman study of very thin (⩽3.5 nm) Si layers constituting Si/SiO2 superlattices and grown by molecular beam epitaxy is described. The Raman spectra show systematic dependence on thickness of the Si layers, which highlights the variety of disordered microstructures in the Si/SiO2 superlattices. A clear change in the vibrational properties is found to occur in the 0.8–3.5 nm thickness region. In particular, the Raman spectra are typical for amorphous silicon for the thicker layers, and the characteristic phonon band disappears for the thinner layers, presumably representing another form of Si coordination with a small Raman scattering cross section. In addition, absorption of the material changes essentially with the Si-layer thickness. Photoluminescence is detected from the Si/SiO2 superlattices, the superlattices with 1.2 and 1.8 nm Si layers being the most efficient emitters among our samples, and the photoluminescence is blueshifted with the decrease of the Si-layer thickness. The Raman spectra show no sign of nanocrystalline structure at any thickness of the as-deposited Si layers so that the observed photoluminescence cannot be connected with Si nanocrystallities. Annealing strongly changes the Raman and photoluminescence spectra, a well-ordered Si phase appears in the superlattices, but its increase does not correlate with the photoluminescence, which further disregard it as an emitter. Nevertheless, the emitting phase is not identified in the Raman spectra.

Synergetic effects in annealing and low energy ion bombardment of Si(100) surfaces

Synergetic effects have been observed for low energy Ar+ ion bombardment combined with low temperature annealing for producing clean and well-ordered Si(100) surfaces. Low energy, mass-selected Ar+ bombardment of Si(100) surfaces in the range of 50–300 eV was performed over the temperature range 25–500 °C. The surfaces were characterized by in situ Auger electron spectroscopy and reflection high energy electron diffraction and ex situ atomic force microscopy. It is found that a clean and well-ordered Si surface can be obtained at a lower temperature when ion bombardment and annealing are applied simultaneously than if they are applied separately or in sequence. Nearly ideal surfaces can be obtained for the conditions of 100–200 eV Ar+ bombardment at 400–500 °C. This synergy between ion bombardment and annealing is discussed in terms of a subplantation model that includes Ar-defect complexes. Additionally, formation of SiC by 100 eV Ar+ ion bombardment of the carbon contaminated silicon surface was observed.

Evolution of implanted carbon in silicon upon pulsed excimer laser annealing: epitaxial Si1−yCy alloy formation and SiC precipitation

Formation of epitaxial Si1−yCy alloy layers on monocrystalline silicon surfaces with y≈1 at% is reported. The preparation method was carbon ion implantation, followed by KrF excimer laser annealing in air. Results of Rutherford backscattering (RBS) measurements, secondary ion mass spectrometry (SIMS), Raman spectroscopy, infrared (IR) absorption analysis, and transmission electron microscopy (TEM) are compared. The evolution of the implanted carbon was affected by the implantation conditions, i.e., implantation energies, ion current, and implanted ion distribution (single-energy versus flat profile implantation), as well as by the laser annealing parameters. Up to approximately 1 at% carbon content, the dominant process was nonequilibrium trapping of carbon atoms in substitutional lattice sites upon fast resolidification, allowing perfect epitaxial recovery of the crystal lattice by laser annealing. Above this concentration, silicon carbide precipitates were formed, embedded into an otherwise well-ordered substitutional Si–C alloy matrix. Upon multipulse processing, the complex carbon redistribution processes were influenced by trapping of carbon in SiC precipitates, dissolution of SiC in the melt and segregation effects in the near-surface region.

Surface electronic structure of Si(001)2 x 2-In studied by angle-resolved photoelectron spectroscopy.

Angle-resolved photoelectron spectroscopy (ARPES) using synchrotron radiation was employed to study the electronic structure of a well-ordered single-domain Si(001)2\ifmmode\times\else\texttimes\fi{}2-In surface. The existence of five surface state bands, denoted as ${S}_{1}$, ${S}_{2}$, ${S}_{2}^{\ensuremath{'}}$, ${S}_{3}$, and ${S}_{3}^{\ensuremath{'}}$ is revealed within the bulk band gap between 0.6 and 2.2 eV in binding energy (${E}_{B}$). The dispersions of these surface states are determined for most of the symmetry axes in the 2\ifmmode\times\else\texttimes\fi{}2 surface Brillouin zone (SBZ), which turn out to be essentially identical to those observed for Si(001) 2\ifmmode\times\else\texttimes\fi{}2-Al recently [Surf. Sci. Lett. 321, L177 (1994)]. Symmetries of the surface states with respect to two mirror axes of the SBZ are determined by the polarization dependence of ARPES intensities. A comparison to a theoretical calculation makes it possible to determine that the smallest ${E}_{B}$ state ${S}_{1}$ is due to the In dimer bond and ${S}_{2}$, ${S}_{2}^{\ensuremath{'}}$, ${S}_{3}$, and ${S}_{3}^{\ensuremath{'}}$ are due to bonds between In dimers and topmost Si atoms, and that the In dimers are parallel to the substrate Si dimers. Besides these five surface state bands, two other spectral features are observed within the bulk band gap, which can be related to similar features observed for a clean Si(001) surface.