Si Buffer Layer Research Articles

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.

Effect of buffer layer and substrate growth temperature on the microstructural evolution of hexagonal boron nitride thin films

Hexagonal boron nitride (hBN) thin films have excellent optical, electrical, and corrosion resistance properties that can enable a wide range of applications. The current study explores pulsed laser deposition technique for synthesizing hBN thin films. The microstructure, mechanical, and wetting properties of hBN thin films were analyzed as a function of substrate growth temperature (25 °C, 300 °C, 500 °C, and 800 °C) and buffer layer addition (Cu or Ti). The microstructure of hBN thin films was found to be influenced by the deposition temperature. This study revealed that the layered growth (for samples deposited at 25 °C) and layer plus island growth (for samples deposited at ≥300 °C) to be the primary growth modes of the thin films regardless of the buffer layer. The roughness of hBN thin films with the Cu buffer layer, in general, increased with the increase in deposition temperature, whereas no significant differences in roughness were observed for films deposited on Si and Ti buffer layer. Grazing Incidence X-ray diffraction results showed the presence of hexagonal close packed crystal structure for all the deposited thin films. These results were corroborated with Raman spectroscopy. The hBN thin films exhibited the hardness and modulus values of ~10–14 GPa and ~120–180 GPa, respectively. No considerable differences in mechanical properties were observed for hBN thin films deposited at various temperatures except for the films deposited with the Cu buffer layer, which showed lower mechanical properties. The wettability of hBN thin films was analyzed using contact angle measurements. The wettability was found to be directly influenced by the surface roughness of hBN thin films. Characterization of hBN thin films indicates that the deposition temperature and buffer layer addition had an influence on the growth and microstructural evolution of hBN thin films.

High Curie Temperature Achieved in the Ferromagnetic MnxGe1-x/Si Quantum Dots Grown by Ion Beam Co-Sputtering.

Ferromagnetic semiconductors (FMSs) exhibit great potential in spintronic applications. It is believed that a revolution of microelectronic techniques can take off, once the challenges of FMSs in both the room-temperature stability of the ferromagnetic phase and the compatibility with Si-based technology are overcome. In this article, the MnxGe1−x/Si quantum dots (QDs) with the Curie temperature (TC) higher than the room temperature were grown by ion beam co-sputtering (IBCS). With the Mn doping level increasing, the ripening growth of MnGe QDs occurs due to self-assembly via the Stranski–Krastanov (SK) growth mode. The surface-enhanced Raman scattering effect of Mn sites observed in MnGe QDs are used to reveal the distribution behavior of Mn atoms in QDs and the Si buffer layer. The Curie temperature of MnxGe1−x QDs increases, then slightly decreases with increasing the Mn doping level, and reaches its maximum value of 321 K at the doping level of 0.068. After a low-temperature and short-time annealing, the TC value of Mn0.068Ge0.932 QDs increases from 321 K to 383 K. The higher Ge composition and residual strain in the IBCS grown MnxGe1−x QDs are proposed to be responsible for maintaining the ferromagnetic phase above room temperature.

Detection of amino acid with metamaterial perfect absorbers in the terahertz band

At present, most amino acid detection techniques have the shortcomings of laborious equipment operation procedures, long detection periods, and low accuracy, which cannot meet the requirements of high-standard amino acid identification in the food industry. Based on the related literature, some amino acids are readily soluble in water. Therefore, a novel detection and identification method can be designed based on the different refractive indices of aqueous amino acid solutions. We introduce a terahertz metamaterial perfect absorber which component successively are a Si buffer layer, a solid metallic Au reflecting layer, a SiO2 depletion layer, a main structure array layer, and a cover layer from bottom to top. The main structure array layer composed of graphene four-opening square ring structure and a golden hemisphere. The absorber achieves perfect absorption based on the principle of impedance matching. It is innovatively applied for the detection of aqueous amino acid additives solution. The absorber has two modes with absorptivity of 99.86% and 99.99% at 5.9 THz and 8.1 THz, respectively. The sensing sensitivity of both modes can reaches 1.1 THz/RIU and 1.5 THz/RIU, respectively, which has obvious advantages in other reported sensors. Experimental results show that the proposed sensor can accurately identify the four amino acids selected by 100% in this study. The research presented this paper provides an essential reference for further development of metamaterial sensors.

Local excitons in Si/Ge inverted quantum huts (IQHs) embedded Si

We investigate the properties of excitons in the SiGe inverted quantum huts (IQHs) embedded in Si employing high-resolution x-ray photoemission spectroscopy. Ultra-small Si/Ge IQHs (13.3 nm × 6.6 nm) were grown on a Si buffer layer deposited on a Si (001) substrate using molecular beam epitaxy. We study the behavior of the excitons at different depths of the IQH structures by exposing the desired surfaces via controlled sputtering and annealing processes. The Si and Ge core level spectra show interesting properties at different surfaces; additionally, we discover distinct new features at the lower binding energy side of the Ge 3d peak. The emergence of these features is attributed to the final state effects arising from core hole screening by the excitons. The properties of these features in the spectra collected at different locations of the IQHs are found significantly different from each other, indicating the local character of the excitons. These results provide a pathway to study the properties of excitons in such quantum structures. The evidence of the local character of the excitons suggests a type I behavior of the system, which is important for the devices for optoelectronic applications, quantum communications, etc.

Growth of a Ge Layer on a Si/SiO2/Si(100) Structure by the Hot Wire Chemical Vapor Deposition

Ge/Si layers are formed on Si/SiO2/Si(100) substrates and are investigated for different growth temperatures. The Si layer is grown by molecular-beam epitaxy, while the Ge layer is produced by the hot wire chemical vapor deposition. Structural studies are carried out using high-resolution transmission electron microscopy and reflection high-energy electron diffraction. Such structures are promising for the growth of high-quality light-emitting structures on them, which are compatible with silicon radiation-resistant integrated circuits. It is shown that a single-crystal Ge layer can be grown on Si/SiO2/Si(100) via a Si buffer layer by the hot wire chemical vapor deposition, and the difficulties arising during the growth of Ge/Si layers on Si/SiO2/Si(100) are demonstrated.

Inversion Boundary Annihilation in GaAs Monolithically Grown on On‐Axis Silicon (001)

AbstractMonolithic integration of III–V materials and devices on CMOS compatible on‐axis Si (001) substrates enables a route of low‐cost and high‐density Si‐based photonic integrated circuits. Inversion boundaries (IBs) are defects that arise from the interface between III–V materials and Si, which makes it almost impossible to produce high‐quality III–V devices on Si. In this paper, a novel technique to achieve IB‐free GaAs monolithically grown on on‐axis Si (001) substrates by realizing the alternating straight and meandering single atomic steps on Si surface has been demonstrated without the use of double Si atomic steps, which was previously believed to be the key for IB‐free III–V growth on Si. The periodic straight and meandering single atomic steps on Si surface are results of high‐temperature annealing of Si buffer layer. Furthermore, an electronically pumped quantum‐dot laser has been demonstrated on this IB‐free GaAs/Si platform with a maximum operating temperature of 120 °C. These results can be a major step towards monolithic integration of III–V materials and devices with the mature CMOS technology.

Наращивание слоя Ge на структуру Si/SiO-=SUB=-2-=/SUB=-/Si (100) методом "горячей проволоки"

Ge/Si buffer layers grown at different temperatures on Si/SiO2/Si (100) substrates have been fabricated and studied. The Si buffer was grown via molecular beam epitaxy. The Ge layer was produced in a single stage via hot wire chemical vapor deposition process. Structural properties were investigated by high-resolution transmission electron microscopy and reflected high-energy electron diffraction. Such structures can be used in the future as a substrate for growth of high quality light-emitting structures compatible with silicon radiation-resistant integrated circuits. The paper shows the possibility of growth of a single crystal layer of Ge on Si/SiO2/Si (100) through a buffer layer of Si by the hot wire chemical vapor deposition process, and also demonstrates the difficulties that arise in the process of growth of Ge/Si layers on Si/SiO2/Si (100).

Controllable growth of GeSi nanowires on trench patterned Si(001) substrate

Controllable growth of nanowires is a prerequisite for addressability and scalability of nanowire quantum devices. By combining top-down nanofabrication and bottom-up self-assembly, site-controlled GeSi nanowires with two (105) facets can be grown on Si (001) substrate with pre-patterned trenches. Trenches along the [100] or [010] crystallographic direction with 60 nm in width and 6 nm in height are fabricated on Si substrate by electron beam lithography and reactive ion etching. Subsequently, a 60-nm-thick Si buffer layer is grown at 330–400 ℃ on the patterned substrate to improve the surface quality. The facets at the tip of the trenches transform into (11n) after depositing the Si buffer layer. Self-organized GeSi nanowires form inside the trenches by depositing the 6-nm-thick Si<sub>67</sub>Ge<sub>33</sub> film at 450 ℃ followed by 1 h annealing at 510 ℃. The GeSi nanowires are (105)-faceted with an average height of approximately 7 nm. Furthermore, we systematically study the influence of annealing temperature, Ge concentration and pattern period on the formation of site-controllable GeSi nanowire on a patterned Si (001) substrate. The GeSi nanowires can be formed only inside the trenches within a specific annealing temperature ranging from 500 ℃ to 520 ℃. It is also discovered that GeSi nanowires are very sensitive to Ge concentration, as they cannot form at lower Ge concentration due to a large nucleation energy barrier. In contrast, high Ge concentration will lead to the discontinuity of nanowires caused by higher atomic diffusion barrier. The generated GeSi nanowires in the trenches exhibit similar dimensions at different pattern periods, which indicates that the growth process is thermodynamically determined. Overall, we realize the controllable growth of the GeSi nanowires, while the length of nanowires can reach the millimeter even centimeter scales, replying on the patterned trench length. The above results offer a controllable growth method of the Ge nanowires, which could potentially lead to the scalability of the Ge quantum devices on Si substrates.

Selective Growth of Semiconducting Silicide Phase Based on the Growth Parameters

Os silicide thin films were grown on (001) Si substrate using MBE method. The structural characterizations of the grown films were investigated using x-ray diffraction (XRD) measurements, Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). In addition, the surface morphology was depicted by AFM micrographs, and the nature of the grown epilayers was recognized using Transmission electronic microscope (TEM). All the samples had rough surface with various size of hillocks and intermediate valley like regions with a remarkable decrease in surface roughness after annealing. The measurements revealed a significant difference in the formed phases and crystalline quality with the growth parameters. The influence of Si buffer layer with thickness of 20 nm which deposited at relatively low temperature of 650 °C, and high temperature annealing (at 1050 °C) of the grown Os silicide epilayer was investigated. It was confirmed that isolating a metastable phase OsSi2 was established by selecting the required growth parameters.

Effect of High-Temperature Annealing on Epitaxially Grown Ru Silicide Thin Films

Epitaxial growth was carried out to grow Ruthenium (Ru) silicide films. Films were grown on Si (100) substrate utilizing molecular beam epitaxy (MBE) method. Firstly, the low-temperature Si buffer layer was grown at relatively low temperature of 400 °C to accommodate lattice strain and Ru silicides epilayers were grown at 750 °C. Secondly, the effect of high-temperature annealing of 1050 °C on the grown film was depicted. To investigate the surface morphology as well as microstructural characteristics atomic force microscopy (AFM), transmission electron microscopy TEM, and x-ray diffraction (XRD) measurements were employed. Only the peaks of the Ru2Si3 phase were recorded in XRD measurements. X-ray photoelectron spectroscopy (XPS) was used to reveal the chemical and electronic composition of Ru silicide films, and a detectable change in the composition ratio toward the formation of Ru2Si3 was established after annealing. Additionally, Raman spectroscopy was utilized to evaluate the characteristics modes of entity phases.

Formation of poly-crystalline BaSi2 thin films by pulsed laser deposition for solar cell applications

A new growth method for BaSi2 thin films by using pulsed laser deposition (PLD) on transparent SiO2 and CaF2 substrates has been developed. X-ray diffraction and Raman spectroscopy revealed the poly-crystalline property of the deposited films. By introducing a thin Si buffer layer on the SiO2 substrate, the crystalline quality of BaSi2 thin films were improved. BaSi2 thin films exhibited a Ba/Si ratio very close to 0.5, indicating the good stoichiometry control of PLD growth. The absorption coefficient of the poly-BaSi2 thin film reached 105 cm−1 and its band gap was deduced to be 1.32 eV, which are similar to those grown by molecular beam epitaxy or sputtering. A maximum photoresponsivity of 12.5 mA/W was achieved in the BaSi2 thin film, which implies the potential of PLD-deposited BaSi2 for thin film solar cell applications.

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaOx buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm−2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaOx semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Влияние буферного слоя por-Si на оптические свойства эпитаксиальных гетероструктур In-=SUB=-x-=/SUB=-Ga-=SUB=-1-x-=/SUB=-N/Si(111) с наноколончатой морфологией пленки

AbstractIntegrated heterostructures exhibiting a nanocolumnar morphology of the In_ x Ga_1 –_ x N film are grown on a single-crystal silicon substrate ( c -Si(111)) and a substrate with a nanoporous buffer sublayer ( por -Si) by molecular-beam epitaxy with the plasma activation of nitrogen. Using a complex of spectroscopic methods of analysis, it is shown that the growth of In_ x Ga_1 –_ x N nanocolumns on the por -Si buffer layer offer a number of advantages over growth on the c -Si substrate. Raman and ultraviolet spectroscopy data support the inference about the growth of a nanocolumn structure and agree with the previously obtained X-ray diffraction (XRD) data indicative of the strained, unrelaxed state of the In_ x Ga_1 –_ x N layer. The growth of In_ x Ga_1 –_ x N nanocolumns on the por -Si layer positively influences the optical properties of the heterostructures. At the same half-width of the emission line in the photoluminescence spectrum, the emission intensity for the heterostructure sample grown on the por -Si buffer layer is ~25% higher than the emission intensity for the film grown on the c -Si substrate.

Исследование влияния переходного слоя нанопористого кремния на атомное и электронное строение, а также оптические свойства гетероструктур A-=SUP=-III-=/SUP=-N/por-Si, выращенных методом плазменно-активированной молекулярно-пучковой эпитаксии

This paper reports on influence of the nanoporous Si buffer layer on morphological, physical and structural properties of the InxGa1-xN layer with nanocolumnar morphology of the surface, grown by plasma assisted molecular beam epitaxy on the traditional Si(111) substrates. By means of various structural and spectroscopy methods electronic structure, morphology of the surface and optical properties of grown heterostructures was studied. We showed that usage of por-Si ad-layer helps to achieve more isotropic InGaN nanocolumns diameter distribution as well as to increase PL intensity up to 25%.

Epitaxial La0.7Sr0.3MnO3 thin films on silicon with excellent magnetic and electric properties by combining physical and chemical methods

ABSTRACTHalf-metallic ferromagnetic La0.7Sr0.3MnO3 (LSMO) represents an appealing candidate to be integrated on silicon substrates for technological devices such as sensors, data storage media, IR detectors, and so on. Here, we report high-quality epitaxial LSMO thin films obtained by an original combination of chemical solution deposition (CSD) and molecular beam epitaxy (MBE). A detailed study of the thermal, chemical, and physical compatibility between SrTiO3 (STO)/Si buffer layers and LSMO films, grown by MBE and CSD, respectively, enables a perfect integration of both materials. Importantly, we show a precise control of the coercive field of LSMO films by tuning the mosaicity of the STO/Si buffer layer. These results demonstrate the enormous potential of combining physical and chemical processes for the development of low-cost functional oxide-based devices compatible with the complementary metal oxide semiconductor technology.

Structural properties of templated Ge quantum dot arrays: impact of growth and pre-pattern parameters

In this study we analyze the impact of process and growth parameters on the structural properties of germanium (Ge) quantum dot (QD) arrays. The arrays were deposited by molecular-beam epitaxy on pre-patterned silicon (Si) substrates. Periodic arrays of pits with diameters between 120 and 20 nm and pitches ranging from 200 nm down to 40 nm were etched into the substrate prior to growth. The structural perfection of the two-dimensional QD arrays was evaluated based on SEM images. The impact of two processing steps on the directed self-assembly of Ge QD arrays is investigated. First, a thin Si buffer layer grown on a pre-patterned substrate reshapes the pre-pattern pits and determines the nucleation and initial shape of the QDs. Subsequently, the deposition parameters of the Ge define the overall shape and uniformity of the QDs. In particular, the growth temperature and the deposition rate are relevant and need to be optimized according to the design of the pre-pattern. Applying this knowledge, we are able to fabricate regular arrays of pyramid shaped QDs with dot densities up to 7.2 × 1010 cm−2.

720-V/0.35-m<inline-formula> <tex-math notation="LaTeX">$\Omega \cdot$ </tex-math> </inline-formula>cm2Fully Vertical GaN-on-Si Power Diodes by Selective Removal of Si Substrates and Buffer Layers

This letter demonstrates a novel technology to fabricate fully vertical GaN-on-Si power diodes with state-of-the-art performance. Si substrate and buffer layers were selectively removed and the bottom cathode was formed in the backside trenches extending to an n+-GaN layer. A specific differential ON-resistance of 0.35–0.4 $\text{m}\Omega \cdot $ cm2 (normalized to the total device area) and a breakdown voltage of 720 V were demonstrated in this novel fully vertical GaN-on-Si p-n diode, rendering a Baliga’s figure of merit over 1.5 GW/cm2. These results set a new record performance in all vertical GaN power diodes on foreign substrates, and demonstrate the feasibility of making fully vertical GaN-on-Si power diodes and transistors by selective removal of Si substrates and buffer layers.

Growth and characterization of low composition Ge, x in epi-Si1−xGex (x ⩽ 10%) active layer for fabrication of hydrogenated bottom solar cell

Semiconducting epi-Si1−xGex alloys have promising features as solar cell materials and may be equally important for some other semiconductor device applications. Variation of the germanium compositional, x in epi-Si1−xGex, makes it possible to control the bandgap between 1.12 eV and 0.68 eV for application in bottom solar cells. A low proportion of Ge in SiGe alloy can be used for photovoltaic application in a bottom cell to complete the four-terminal tandem structure with wide bandgap materials. In this research, we aimed to use a low proportion of Ge—about 10%—in strained or relaxed c-Si1−xGex/c-Si heterojunctions (HETs), with or without insertion of a Si buffer layer grown by molecular beam epitaxy, to investigate the influence of the relaxed or strained SiGe active layer on the performance of HET solar cells grown using the plasma enhanced chemical vapor deposition system. Thanks to the c-Si buffer layer at the hetero-interface, the efficiency of these SiGe based HET solar cells was improved from 2.3% to 3.5% (fully strained and with buffer layer). The Jsc was improved, from 8 mA cm−2 to 15.46 mA cm−2, which might be supported by strained c-Si buffer layer at the hetero-interface, by improving the crystalline quality.

Influences of ultrathin amorphous buffer layers on GaAs/Si grown by metal–organic chemical vapor deposition

In this work, a technique for the growth of GaAs epilayers on Si, combining an ultrathin amorphous Si buffer layer and a three-step growth method, has been developed to achieve high crystalline quality for monolithic integration. The influences of the combined technique for the crystalline quality of GaAs on Si are researched in this article. The crystalline quality of GaAs epilayer on Si with the combined technique is investigated by scanning electron microscopy, double crystal X-ray diffraction (DCXRD), photoluminescence, and transmission electron microscopy measurements. By means of this technique, a 1.8-µm-thick high-quality GaAs/Si epilayer was grown by metal–organic chemical vapor deposition. The full-width at half-maximum of the DCXRD rocking curve in the (400) reflection obtained from the GaAs/Si epilayers is about 163 arcsec. Compared with only using three-step growth method, the current technique reduces etch pit density from 3 × 106 cm−2 to 1.5 × 105 cm−2. The results demonstrate that the combined technique is an effective approach for reducing dislocation density in GaAs epilayers on Si.