Si Layer Research Articles

Significant conductivity enhancement in Al-rich n-AlGaN by modulation doping

Enhancing the conductivity in Al-rich n-AlGaN is a key issue for realizing AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) with low operating voltage and high wall-plug efficiency, especially in a planar geometry of flip–chip configuration. An approach of modulation doping is herein proposed, where an alternating-layer structure consisting of Si-doped and unintentionally doped AlGaN is assembled to achieve the spatial separation of electron activation and transport. As massive electrons diffuse from the AlGaN:Si layer into the neighboring i-AlGaN ones and then drift, the ionized-donor scattering is effectively weakened, leading to a significant enhancement of mobility as well as conductivity. An impressive electrical property of n-Al0.6Ga0.4N with a lateral conductivity of 201.7 S/cm is realized as a consequence, being 2.1 times of that in the continuously doped one. Furthermore, the operating voltage of 280 nm UV-LEDs is correspondingly reduced by 0.1–0.2 V at 100 mA by adopting modulation-doped n-AlGaN in the n-cladding layer.

Modeling of Photovoltaic Characteristics of a TiO<sub>2</sub>/Porous-Si/Si-Based Heterojunction Solar Cell

This work is focused on the study of photosensitive structures based on porous Si and film TiO2, which are promising for solar energy. For numerical simulation of the transportation and accumulation of charge carriers in the considered heterostructure, the drift-diffusion approximation of the semiclassical approach was proposed. A device scheme of a solar cell model based on TiO2/porous-Si/Si heterostructures is proposed. Production of photoconverters of solar cells based on the TiO2/porous-Si/Si heterostructure can be carried out according to the standard method supplemented by additional technological operations. Ohmic contacts are formed in the upper and lower parts of the structure above the TiO2 and Si layers. The strip system of contacts is a contact grid, with hatching, the surface coefficient should not exceed 5%. The thickness of the applied layer of photoresist should be 1 μm. Using the PC1D program, the light characteristics of the fabricated structure were calculated (open circuit voltage VOC, short circuit current ISC, fill factor FF and efficiency η), and current-voltage characteristics were plotted. The influence of the thickness and doping level Nd and Na of porous Si and TiO2 layers on the productivity of a heterojunction solar cell TiO2/porous-Si/Si was studied in order to obtain a device with a good conversion efficiency. It was found that the energy conversion efficiency of a TiO2/porous-Si/Si solar cell can reach 22.5 %. Based on the optimized simulation conditions, it was found that the maximum solar cell efficiency is achieved at thicknesses of 100 and 200 nm and donor concentration of Nd=1∙1017 cm-3 and acceptor concentration of Na=1∙1018 cm-3 for TiO2 and porous Si buffer layer, accordingly.

COMPUTER SIMULATION OF LI AND BE WETTING LAYERS ON THE SI (100) SURFACE

Within the framework of the of density functionality theory and the pseudopotential method, the atomic and electronic structure of the two-dimensional Li-Si and Be-Si systems forming on the Si (100) surface is calculated, with a metal thickness of one to three monolayers (ML). At the first ML, the formation of ordered silicide wetting layer of Li (with atoms embedded inside the top layer Si) and Be (with atoms embedded between the top two Si layers) was detected. At 2 ML, the systems are modified: Li atoms occupy positions between the top two Si layers, and Be atoms rise at positions above the top Si layer. After that, with a coating thickness of 3 ML, in the case of Li, a continuous disordered wetting layer is formed, and in the case of Be, a wetting layer in the form of disordered chains along the length. At 1 ML, an energy gap appears in the electronic structure of the studied systems in the density of electronic states near the Fermi level, the width of which is 1.02 eV and 0.36 eV, respectively, for Li-Si and Be-Si systems. Then the gap disappears, first for the lithium system (at 2 ML), and then, for the beryllium system (at 3 ML).

Deep-subwavelength multilayered meta-coatings for visible-infrared compatible camouflage

Abstract Camouflage is a common technique in nature, enabling organisms to protect themselves from predators. The development of novel camouflage technologies, not only in fundamental science, but also in the fields of military and civilian applications, is of great significance. In this study, we propose a new type of deep-subwavelength four-layered meta-coating consisting of Si, Bi, Si, and Cr from top to bottom with total thickness of only ∼355 nm for visible-infrared compatible camouflage. The visible color and the infrared emission properties of the meta-coating can be independently adjusted. Colorful meta-coating for visible camouflage can be obtained by changing the thickness of top Si layer, while the selective high emissivity in non-atmospheric window for infrared camouflage remains. Due to the deep-subwavelength properties, the meta-coating shows high angle tolerance in both visible and infrared regions. The compatible camouflage capability of our proposed meta-coating in the visible-infrared region is validated under different environments. The deep-subwavelength, angular insensitivity, visible-infrared compatibility and large-area fabrication feasibility promise the meta-coating an effective solution for camouflage in various applications such as military weapons and anti-counterfeiting.

Design and performance analysis of tri-layered strained Si/Si1–x Ge x /Si heterostructure DG feedback FET

This work presents the design and performance analysis of a tri-layered strained Si/Si1−x Ge x /Si heterostructure double gate feedback field-effect transistor (DG FBFET). The proposed DG FBFET is designed by introducing biaxial strain in the device by sandwiching a Si1−x Ge x layer between two thin Si layers to provide high ON current as well as ultra-steep switching characteristics. The device offers a significantly high ON current (3.4 x 10−3 A/μm), high I ON /I OFF ratio (∼1010), a large memory window of 1.06 V, and an extremely low subthreshold swing (∼0.3 μ V/decade), which can be very useful for memory and neuromorphic applications. Furthermore, the ON/OFF switching of the device has been accomplished at a lower threshold voltage (0.287 V), allowing it to be utilized in low-power electronics. Synopsys TCAD tool has been used to create the device structure and analyze the electrical performances of the device.

High detectivity Ge photodetector at 940 nm achieved by growing strained-Ge with a top Si stressor.

We have developed a self-powered near-infrared photodetector (PD) with high detectivity using a tensile strained Ge layer capped with a thick Si layer. The Si layer acts as a stressor and maintains the strain of Ge with minimal dislocations by creating a rough surface. By using Raman spectroscopy, we confirmed that the Ge layer has a 1.83% in-plane tensile strain. The Ge PD exhibits a high responsivity of 0.45 A/W at -1 V bias voltage for 940 nm wavelength. The PD's dark current density is as low as ∼1.50 × 10-6 A/cm2 at -1 V. The high responsivity and low dark current result in a detectivity as high as 6.55 × 1011 cmHz1/2/W. This Ge PD has great potential for applications in light detection and ranging (LiDAR), Internet of Things (IoTs), and Optical Sensing Networks.

Low power silicon evaporation source—Construction, performances, and applications

The construction of a low power silicon evaporation source for thin film deposition applications is proposed in this article. A few differently shaped tungsten filaments were mounted inside a quartz glass crucible, which assured an effective heating of silicon wafer pieces. The sublimation process was monitored at filament powers in the range of 8–22 W, which corresponds to temperatures far below the melting point of Si. The operation of the evaporation source requires only the use of a low voltage power supply. All considered models of evaporation sources are characterized by an easy construction. The measurements carried out with the use of a quartz crystal microbalance sensor enabled to determine the deposition rate at different filament powers for all constructed Si sources and confirm the long-term stability of the silicon flux. The experimental data exhibit the Polany–Wigner dependence of the deposition rate as a function of inverse of power at higher filament powers. Auger electron spectroscopy was used to monitor the deposition of Si on Ag(100) under constant silicon flux. The Auger signal recorded from Si and Ag reflects the growth of subsequent silicon layers. This enabled the determination of the Frank–van der Merwe growth mode of Si on Ag(100) at early stages of growth, the formation time of one Si monolayer, and, thus, the deposition rate. The presented designs of the Si source exhibit long time stable evaporation fully controlled by the applied filament power, which is crucial in the precise adsorption of ultrathin Si layers.

Nanocrystalline Silicon Layers for the Application in Silicon Heterojunction Solar Cells

After application in thin-film silicon tandem solar cells and in lab-scale silicon heterojunction (SHJ) devices, doped nanocrystalline silicon (nc) layers now arrived on the industrial stage. Despite their challenging deposition, the benefits they hold with respect to even higher device performance compared to their amorphous counterparts are significant and justify additional effort. In this contribution we report on developments towards industrially applicable processes for n- and p-doped silicon layers, nc-Si(n) and nc-Si(p), and their implementation in SHJ cells. Our investigation focuses on the impact of deposition temperature (Tdep) and the need for a thin oxide layer to promote fast nucleation of thin, sufficiently crystalline, doped nc-Si films in a single deposition chamber powered at 13.56 MHz. We identified main challenges for thin film and contact engineering and reached efficiencies of 23.0% with n- and 23.1% with p-type nc-Si approaching cell performances of our process of record based on amorphous Si (a-Si) layers.

Highly oriented crystalline si on epitaxial Gd2O3/Si(111) substrate using low-cost Radio Frequency sputtering for Silicon on Insulator application

Silicon-on-Insulator (SOI) technology has been the center of attraction with the advancement in Radio Frequency (RF) technology and the advent of Internet-of-Things due to its low-power operation with reduced parasitic and short-channel effects. However, the state-of-the-art smart-cut method used for the SOI wafer is costly. Alternatively, the notion of using the Si channel layer on epitaxial rare-earth oxide has been proposed. In literature, the existing techniques used to achieve the same, such as Molecular beam Epitaxy or reduced-pressure chemical vapor deposition technique, are either costly or non-high-volume manufacturable. In this context, we propose a method to fabricate the highly oriented crystalline Si(111) channel layer (Top-Si) on an epitaxial Gd2O3/Si(111) virtual substrate (SOXI) utilizing the solid phase epitaxy using the high-volume manufacturing friendly (HVM), low-cost RF magnetron sputtering technique. First, we have shown the multicrystalline Si and epitaxial Gd2O3 layer grown on Si(111) substrate, as confirmed using high-resolution X-ray diffraction (HRXRD) and Transmission Electron Microscopy (TEM). Second, we investigated the impact of rapid thermal annealing at 850 °C on the heterostructure using HRXRD and TEM. It is observed that the high-temperature annealing transforms the Top-Si layer into the highly oriented crystalline Si(111) layer by fusing smaller grains towards larger grains. Hence, we demonstrate a low-cost, HVM-friendly technique for the fabrication of SOXI wafers.

Characteristics of high-order silane based Si and SiGe epitaxial growth under 600 ℃

Low temperature Si and SiGe epitaxy have been crucial in the semiconductor industry, prompting interest in high-order silanes as Si precursors. In this study, we investigated the growth characteristics of Si and SiGe epitaxial layers using Si2H6, Si3H8, and Si4H10 precursors by ultra-high vacuum chemical vapor deposition at temperatures between 500℃ and 600 ℃. We examined the effects of each precursor on the growth behaviors through varying temperature and flow rate conditions. For these three precursors, activation energies for Si epitaxy were measured to be below surface H desorption energy, and an increase in the order led to a greater adatom density at equivalent flow rates. In SiGe epitaxy, we observed Ge composition variations with temperature at each Ge-to-Si flow ratio depending on the Si precursors. Si2H6 exhibited almost consistent Ge compositions across temperatures, and the activation energy for SiGe epitaxy was decreased to 1.73 eV when the flow ratio was increased to 1. In comparison, Si3H8 and Si4H10 exhibited negligible changes in Ge compositions for the flow ratios below 0.5, with activation energies of 1.88 and 1.62 eV, respectively. Based on our experimental results, we proposed potential reaction pathways for high-order silane precursors.

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.

Laser-enabled dual-state transformation on PANI:PSS/Si for wavelength-selective detection

Position-sensitive detectors are widely used in industry due to accurate sensing of changes in light position. The introduction of additional wavelength selectors can cause deviations in light position and affect the performance of the devices. In this work, an organic–inorganic heterojunction of polyaniline-poly (p-styrenesulfonic acid) (PANI:PSS)/p-Si is proposed to achieve selective detection at optical wavelengths using the lateral photovoltaic effect. The maximum reduction of sensitivity in the detection of 405 nm laser can reach up to 127.59 mV/mm, with a decrease in about 99%. When the device is exposed to blue-violet light, the absorption layer changes from a Si layer to a PANI:PSS layer, which weakens the photon utilization efficiency and charges carrier transport. The self-generated longitudinal voltage further enhances the absorption of the PANI:PSS layer for blue-violet light. PANI absorbs photon energy to generate electron–hole pairs and transitions from the reduced state to the oxidized state in the depletion region at the contact interface. The oxide layer hinders carrier separation and increases carrier recombination. Due to the inherent properties of the structure, the wavelength selection function is realized by the detector itself, enriching its original single function. The sensitivity in other light bands has been improved, including more than three times at 520 nm compared to p-Si. These results have provided the theoretical foundation for highly selective and tunable optoelectronic devices while helping to overcome the challenges of high manufacturing costs and customized application scenarios. They provide a viable solution for the intelligent development of optoelectronic devices.

Interfacial structure changes between amorphous silicon anode/liquid electrolyte using a highly dense and flat model electrode

AbstractChanges in the interfacial structures of the amorphous silicon (a-Si) anode/organic electrolyte interfaces in lithium-ion batteries were investigated using highly dense a-Si films fabricated by cathodic arc plasma deposition as a model electrode. Raman spectroscopy, transmission electron microscopy (TEM), and X-ray reflectivity revealed that the Si films were grown in an amorphous state with an atomically flat surface. The a-Si films exhibited lithium alloying and de-alloying reactions, with a large irreversible capacity during the first cycle. The irreversible capacity was derived from the formation of a solid electrolyte interphase (SEI) along with an incompletely de-alloyed Li-Si phase, as confirmed by ex situ TEM and X-ray photoelectron spectroscopy observations. The discharge and charge capacities of the Si films gradually decreased in the subsequent cycles, despite the fact that no further SEI formation or cracking of the Si layer occurred. Scanning electron microscopy observations combined with energy-dispersive X-ray spectroscopy revealed the miniaturization of the a-Si film in the surface region to generate domains measuring less than a few hundred nanometers. These results suggest that delamination and miniaturization of the a-Si nanodomains from the electrode surface are partly responsible for degradation of the a-Si anode.

Formation of ultra-thin nickel silicide on SiO2 using Si/Ni/Si structures for oxidation control

We have demonstrated the formation of ultrathin Ni silicide on SiO2 by annealing Si/Ni/Si structures and have systematically evaluated the impacts of the Si layer thickness on oxidation, surface roughening and the silicidation reaction. X-ray photoelectron spectroscopy analyses revealed that suppression of Ni oxidation due to the top Si layer makes it possible to form an ultrathin Ni silicide layer with a thickness of around 2 nm. Then, it turned out that the composition ratio of Ni and Si depends on not only the annealing temperature but also the initial thickness ratio of the top and bottom Si layers. Furthermore, this work clarified that the ultra-thin top Si layer has a large impact on the surface morphology during Ni silicide formation with diffusion and preferential oxidation.

Simulation study of Perovskite/Si Monolithic Multijunction Solar Cell

The utilization of perovskite and crystalline-silicon (c-Si) light absorbers in multijunction solar cells presents an exciting opportunity to surpass the efficiency limit of the industry's leading single-junction c-Si solar cells. In this work, we used the Solar Cell Capacitance Simulator (SCAPS-1D) to simulate a monolithic tandem junction solar cell. The solar cell consists of two types of materials: low bandgap and high bandgap. These are layers of perovskites and Si, divided by a window layer of zinc oxide (ZnO), a buffer layer of cadmium sulfide (CdS), a recombination layer of Spiro-MeOTAD/silicon, and a heavily doped back surface field layer formed from n++Si to prevent recombination at the back surface. The impact of di fferent series and shunt resistances, as well as perovskite layer thickness and bandgap, on solar cells' photovoltaic performance has been investigated.

Effect of Thickness and Thermal Treatment on the Electrical Performance of 2D MoS2 Monolayer and Multilayer Field-Effect Transistors

We deposited high-quality molybdenum disulfide (MoS2) monolayer and multilayer crystals on SiO2/Si substrates, by means of a chemical vapor deposition (CVD) process at atmospheric pressure. Notably, NaCl salt was used as component of the precursors to assist the growth of MoS2 crystals, which were intended for use as the active channel layer in the fabrication of field-effect transistors (FETs). The resulting MoS2 crystals from this CVD process were analyzed by optical, scanning electron, and atomic force microscopies, and by Raman and photoluminescence spectroscopies. The optical images and the micrographs obtained by SEM revealed the formation of dispersed MoS2 crystals with a triangular shape all over the SiO2 surface. The thickness of the MoS2 crystals, analyzed by atomic force microscopy, showed minimum values of around 0.7 nm, confirming the formation of monolayers. Additionally, multilayers with larger thickness were also identified. The Raman and photoluminescence spectra of the MoS2 crystals corroborated the formation of single and multiple layers. The fabrication of the FET back-SiO2 -gate configuration was made by depositing patterned source and drain Ti contacts on the dispersed MoS2 crystals to achieve the Ti/MoS2/SiO2/Si layer stacks. MoS2-based FETs with one and three layers were assembled and their electrical response analyzed by I–V output and transfer curves showing the typical characteristics of an n-type semiconductor channel operating in depletion mode. The electrical performance parameters of the devices, such as mobility and threshold voltage, were also determined from this analysis. Finally, to enhance their electrical response, the MoS2-based devices were thermally annealed at 200 °C for 30 min in Ar atmosphere. The increase in the mobility of the device was 176% compared to the device before the treatment.

Review of: "Surface processes of the Si layer, including burning, are carried out by plasma and ion beam technology"

Review of: "Surface processes of the Si layer, including burning, are carried out by plasma and ion beam technology"

Tunable magneto-transport properties in ultra-high Bi-doped Si prepared by liquid phase epitaxy

During the past decades, excavating the novel functional materials towards CMOS-compatible spin-transport devices has been treated as one of the most urgent tasks to revolutionize the current Si-based nonmagnetic spintronics. Among various strategies, triggering the exotic physics in Si via utilizing heavy-element hyperdoping is the most straightforward way to achieve the abovementioned expectation. In this work, by using liquid phase epitaxy growth, we have achieved the highest Bi doping level in Si up to 2.0 × 1021 cm−3. The resulting Bi-hyperdoped Si layers are confirmed to be single-crystalline and epitaxially regrown without extended defects or Bi agglomerates via various microstructural analysis. The Bi-hyperdoped Si layers exhibit a metallic-like behavior with the carrier densities up to 1.55 × 1015 cm−2 and a prominent electrical activation yield around 50 %. Negative magnetoresistance due to magnetic-field suppressed weak localization at both sides of the insulator-to-metal transition is observed. Tunable magneto-transport properties are realized in ultra-high Bi doped Si epilayers by controlling the donor concentration. This provides new possibilities for designing Si devices that integrate spin-charge conversion and spin transport. This work promises to enable heavy-element hyperdoped semiconductor epilayers as novel spin-based electronics material platform via a chip-technology compatible approach.

Effect of temperature on the emission properties of quasi 2D glass-like carbon placed on silicon and silicon dioxide substrates

In this work, in silico methods are used to study the temperature effect of the emission properties, including the electron work function, of quasi 2D glass-like carbon (GLC) films deposited on few/multi-layer Si/SiO2 substrates of two types. The equilibrium configuration of the structures under study at different temperatures is established by molecular dynamics (MD) method using reactive force field (ReaxFF) due to the polyatomic nature of supercells. The electronic structure is determined using the self-consistent charge density-functional based tight-binding (SCC-DFTB) method. The temperature range under consideration is 50–1000 K. The effect of one substrate layer and several substrate layers on the electronic structure and emission properties of GLC is considered. In the case of one layer of Si and SiO2, in fact, the functionalization of the GLC film occurs. This leads to a decrease in the work function by ∼ 6 % and ∼ 21 %, respectively, at a temperature of 300 K. As the temperature increased to 1000 K, the work function of the GLC with one SiO2 layer further decreased by ∼ 7 %. An increase in the substrate thickness to five Si layers and two SiO2 layers leads to a decrease in the work function by 7.3 % and 20.5 %, respectively. An increase in temperature leads to a change in the work function of the GLC with five Si substrate layers and two SiO2 layers by 0.2 % and 3.8 %, respectively.

Enhancement of hardness and corrosion resistance of Al-Si-N multilayer color coating via SiN/AlSiN/AlN compositional gradient interlayer

The color of AlN/Si/Al coating can be controlled by the thickness of the AlN layer according to the interference effect. However, the loading capacity is affected by the large hardness difference between the Si and Al layers. Corrosion resistance is relatively weak due to penetration defects in the AlN surface layer. In this work, therefore, a SiN/AlSiN/AlN interlayer is sputtered in between the Si/Al layer in the AlN/Si/Al coating as the transition layer. The chemical state, structure, morphology, color, hardness, and corrosion resistance of the as-deposited Al-Si-N coating are carefully characterized using x-ray photoelectron spectrometry, grazing incident x-ray diffraction, atomic force microscopy, scanning electron microscope, colorimeter, nanoindentation, and electrochemical corrosion meter, respectively. To evaluate the long-term corrosion resistance, the uncoated, AlN/Si/Al-coated, and AlN/Si/SiN/AlSiN/AlN/Al-coated AZ31B Mg alloys are immersed in salt solution for different durations, followed by characterization of morphology and composition. The results show that the SiN/AlSiN/AlN interlayer is of a gradient structure in both composition and hardness. The AlN crystals grow continuously from the Al bonding layer into the AlSiN layer, resulting in internal longitudinal grain boundaries. The coating surface becomes smoother with a roughness (Rq) of 12.6 nm. The color of the coating is controlled by the AlN surface layer thickness. The coating hardness increases from 6.5 to 20.6 GPa. The corrosion current density of the coating decreases from 2.02 × 10−6 to 1.99 × 10−8 A/cm2. The coating could withstand corrosion in salt solution for at least 192h. The gradient structure of the interlayer effectively alleviates the hardness difference between the Si layer and the Al layer and inhibits the penetration of the corrosive medium from the surface. The mechanism for the enhanced corrosion resistance is explained through a model.