Nanophase nc-Si/a-SiC films that contain Si quantum dots (QDs) embedded in an amorphous SiC matrix were deposited on single-crystal silicon substrates using inductively coupled plasma-assisted chemical vapor deposition from the reactive silane and methane precursor gases diluted with hydrogen at a substrate temperature of 200 °C. The effect of the hydrogen dilution ratio X (X is defined as the flow rate ratio of hydrogen-to-silane plus methane gases), ranging from 0 to 10.0, on the morphological, structural, and compositional properties of the deposited films, is extensively and systematically studied by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman spectroscopy, Fourier-transform infrared absorption spectroscopy, and X-ray photoelectron spectroscopy. Effective nanophase segregation at a low hydrogen dilution ratio of 4.0 leads to the formation of highly uniform Si QDs embedded in the amorphous SiC matrix. It is also shown that with the increase of X, the crystallinity degree and the crystallite size increase while the carbon content and the growth rate decrease. The obtained experimental results are explained in terms of the effect of hydrogen dilution on the nucleation and growth processes of the Si QDs in the high-density plasmas. These results are highly relevant to the development of next-generation photovoltaic solar cells, light-emitting diodes, thin-film transistors, and other applications.

Si quantum dots embedded in an amorphous SiC matrix : nanophase control by non-equilibrium plasma hydrogenation

As the most fundamental material for microelectronics, silicon (Si) has bourgeoned in the past more than half a century. However, given the indirect bandgap of Si, the use of Si in optoelectronics is relatively limited due to its mediocre optical absorption and rather poor optical emission. During many years of efforts for extending the use of Si in optoelectronics Si nanocrystals (NCs) that are one type of the most important Si nanostructures have attracted significant attention owing to their remarkable electronic and optical properties. Si NCs are actually crystalline Si nanoparticles, which may be called Si quantum dots if their size is smaller than ∼10 nm. With the manipulation of the size, surface and doping of Si NCs great tunability for the light emission from Si NCs with the quantum yield of more than 60% has been realized. Based on the efficient light emission from Si NCs high-performance Si-NC light-emitting devices have been demonstrated. In the meantime, the efficient light emission from Si NCs has also been utilized for synaptic simulations in neuromorphic computing and down-shifting in photovoltaics. Broadband optical absorption ranging from the ultraviolet to mid-infrared has been recently obtained for Si NCs mainly by taking advantage of doping. This has enabled the use of Si NCs in novel solar cells, photodetectors and optoelectronic synaptic devices. The continuous improvement of the electronic and optical properties of Si NCs has made Si NCs unfading Si materials for optoelectronics, contributing to the development of Si-based optoelectronic integration.

Silicon nanocrystals: unfading silicon materials for optoelectronics

Hydrogenated amorphous silicon (a-Si:H) has been effectively utilized as photoactive and doped layers for quite a while in thin-film solar applications but its energy conversion efficiency is limited due to thinner absorbing layer and light degradation issue. To overcome such confinements, it is expected to adjust better comprehension of device structure, material properties, and qualities since a little enhancement in the photocurrent significantly impacts on the conversion efficiency. Herein, some numerical simulations were performed to characterize and optimize different configuration of amorphous silicon-based thin-film solar cells. For the optical simulation, two-dimensional finite-difference time-domain (FDTD) technique was used to analyze the superstrate (p-i-n) planar amorphous silicon solar cells. Besides, the front transparent contact layer was also inquired by using SnO2:F and ZnO:Al materials to improve the photon absorption in the photoactive layer. The cell was studied for open-circuit voltage, external quantum efficiency, and short-circuit current density, which are building blocks for solar cell conversion efficiency. The optical simulations permit investigating optical losses at the individual layers. The enhancement in both short-circuit current density and open-circuit voltage prompts accomplishing more prominent power conversion efficiency. A maximum short-circuit current density of 15.32 mA/cm2 and an energy conversion efficiency of 11.3% were obtained for the optically optimized cell which is the best in class amorphous solar cell.

Efficient amorphous silicon solar cells: characterization, optimization, and optical loss analysis

Progressive development towards all-silicon solar cells insists on the top cell of the tandem structure having a wide band gap p-layer in the superstrate configuration. With the advent of nc-Si solar cells having improved stability, the efficient growth of an i-nc-Si layer of the top cell prefers a nc-p-layer as its substrate. Accordingly, a conducting crystalline silicon alloy material with a wide band gap is a basic requirement at the p-layer. The present investigation deals with the development of a nc-Si/a-SiC:H hetero-structure wherein Si–C bonds in an amorphous matrix widen the optical band gap and the embedded high density tiny Si ultranano-crystallites of mostly 〈220〉 crystallographic orientation provide high electrical conductivity as well as an enhanced optical band gap due to a quantum size effect. A typical nc-Si/a-SiC:H network is obtained that contains 47% crystallinity with a prevailing ultranano-crystalline component, [(XC)unc/(XC)nc] > 1, average crystallite size ∼3.6 nm, number density ∼1.2 × 1013 cm−2 and Si–C bond density ∼1.2 × 1022 cm−3. The hetero-structure contributes a wide optical band gap, Eg ∼ 2.07 eV, high conductivity, σD ∼ 3 × 10−10 S cm−1, σPh ∼6 × 10−8 S cm−1, Ea ∼ 0.73 eV and a very low microstructure factor R ∼ 0.075. Imminent steps for doping will indeed lead to prospective window layers in nc-Si solar cells.

Conducting wide band gap nc-Si/a-SiC:H films for window layers in nc-Si solar cells

The nonstoichiometric ITO/n-SiC/i-SiC/p-Si/Al light-emitting diodes (LEDs) with dense Si quantum dots (Si-QDs) embedded in the Si-rich SixC1-x -based i-SiC layer are demonstrated. The Si-rich SixC1-x films with buried Si-QDs are grown by the plasma-enhanced chemical vapor deposition with varying substrate temperatures. After the annealing process, the average Si-QD size in the Si-rich Si0.52C0.48 film is 2.7 ± 0.4 nm with a corresponding volume density of 1.43 × 1018 cm-3. By increasing the deposition temperatures from 300°C to 650°C, the turn-on voltage and turn-on current of the ITO/n-SiC/i-SiC/p-Si/Al LEDs are found to decrease from 13 to 4.2 V and from 0.63 to 0.34 mA, respectively. In addition, these Si-rich SixC1-x LEDs provide the maximal electroluminescent (EL) power intensity increasing from 1.1 to 4.5 μW/cm2. The yellow (at 570 nm) EL emission power of the ITO/n-SiC/i-SiC/p-Si/Al LEDs reveals a saturated phenomenon due to the Auger effect. The dissipated energy by the lattice thermal vibration contributes to a decayed EL emission power at higher biased currents. The corresponding power-current slope is observed to enhance from 0.45 to 0.61 μW/A with the substrate temperature increasing to 650°C.

Si-Rich $\hbox{Si}_{\rm x}\hbox{C}_{1 - {\rm x}}$ Light-Emitting Diodes With Buried Si Quantum Dots

Si quantum dots (QDs) were formed by thermal annealing the hydrogenated amorphous silicon carbide films (a-SiCx:H) with different C/Si ratio x, which were controlled by using a different gas ratio R of methane to silane during the deposition process. By adjusting x and post annealing temperature, the QD size can be changed from 1.4 to 4.2 nm accordingly, which was verified by the Raman spectra and transmission electron microscopy images. Size-dependent electroluminescence (EL) was observed, and the EL intensity was higher for the sample containing small-sized Si QDs due to the quantum confinement effect (QCE). The EL peak energy as a function of the Si QDs size was in good agreement with a modified effective mass approximation (EMA) model. The calculated finite barrier potential of the Si QDs embedded in SiC matrix is 0.4 and 0.8 eV for conduction and valence band, respectively. Moreover, the current-voltage properties and the linear relationship between the integrated EL intensity and injection current i...

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Hydrogenated amorphous silicon-carbide thin films with high photo-sensitivity prepared by layer-by-layer hydrogen annealing technique

Structural and electroluminescent properties of Si quantum dots/SiC multilayers

Si quantum dots (Si QDs) films were prepared by annealing amorphous SiC single layer and amorphous Si/SiC multilayers fabricated in plasma enhanced chemical vapor deposition system. The microstructures were characterized by Raman spectroscopy as well as Fourier transforms infrared spectroscopy for both samples. It was found that Si QDs with average size of 2.7 nm were formed after annealing and the electroluminescence (EL) band centered at 650 nm can be observed at room temperature. The EL intensity from the Si/SiC multilayers was obviously improved by one order of magnitude and the corresponding EL band width was reduced compared with that from SiC single layer film. The improved electroluminescence behavior can be attributed to the formation of the denser Si QDs, good size distribution and the strong confinement effect of carriers in multilayerd structures.

Comparative study of electroluminescence from annealed amorphous SiC single layer and amorphous Si/SiC multilayers

A solar battery with a band gap gradual changing silicon quantum dot multilayer film comprises a p type silicon substrate, wherein a thickness gradual changing multilayer amorphous silicon/silicon carbide film structure, a thickness gradual changing amorphous silicon/silicon carbide grease silicon quantum dot/silicon carbide multilayer film structure and a p-i-n battery structure are arranged on the p type silicon substrate, the p-i-n battery structure is formed by the p type silicon substrate, a silicon carbide intrinsic layer (namely an i layer) and the outermost n type nanocrystalline silicon film, and an electrode is guided out on the surface of the p type silicon substrate to form a battery. The thickness of amorphous silicon sub layers in the period of each hydrogenation amorphous silicon/silicon carbide growing towards the surface of the p type silicon substrate is thinned gradually. The amorphous silicon sub layer growing on the p type silicon substrate or close to the p type silicon substrate is the thickest, and the amorphous silicon sub layer growing towards the surface of the p type silicon substrate is the thinnest.

Shuxin Li

Papers

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Hydrogenated amorphous silicon-carbide thin films with high photo-sensitivity prepared by layer-by-layer hydrogen annealing technique

Structural and electroluminescent properties of Si quantum dots/SiC multilayers

Comparative study of electroluminescence from annealed amorphous SiC single layer and amorphous Si/SiC multilayers

Solar battery with band gap gradual changing silicon quantum dot multilayer film and production method thereof