Monocrystalline Thin Films Research Articles

A new approach for AFM cantilever elaboration with 3C-SiC

The recent achievement of a continuous silicon monocrystalline thin film on 3C-SiC epilayers deposited on silicon substrates has opened the field for new microstructures. In this work, this original hetero-structure is the basis for the elaboration of an entire cantilever for atomic force microscopy. The hetero-epitaxially grown silicon layer is used to define the tip of the cantilever fabricated from the 3C-SiC epilayer deposited on silicon. The complete cantilever is elaborated by plasma etching using a nickel mask. The use of a full dry etching process is very promising as it is independent of the crystalline orientation of the silicon epilayer contrary to process based on wet etching solutions. Moreover, based on such hetero-structure, new MEMS devices can be considered.

Elaboration of Monocrystalline Si Thin Film on 3C-SiC(100)/Si Epilayers by Low Pressure Chemical Vapor Deposition

The growth of continuous silicon monocrystalline thin films on 3C-SiC epilayers deposited on silicon substrates is presented in this study. Such heterostructures can be beneficial for the fabrication of Micro Electro Mechanical Systems or electronic applications. The elaboration of these heterostructures was carried out using Low Pressure Chemical Vapor Deposition. X-ray Diffraction, Fourier Transformed Infra-Red spectroscopy and Scanning Electron Microscopy have been used to investigate the structural properties of Si epilayers and their dependence on growth conditions. Monocrystalline Si (110) films are obtained on 3CSiC(100)/Si (100) substrates, only when using growth temperatures close to 850°C. The strong influence of the underlying 3C-SiC film on the final structural properties of Si epilayer is evidenced.

Loss analysis of back-contact back-junction thin-film monocrystalline silicon solar cells

We investigate power losses in back-contact back-junction monocrystalline thin-film silicon solar cells fabricated using the porous silicon layer transfer process. Our loss analysis combines two-dimensional finite element modeling and resistance network simulations. The input parameters of the finite element modeling are determined experimentally by measuring saturation current densities and sheet resistances on test samples prepared identically to the solar cells. Characteristic solar cell parameters such as short circuit current, open circuit voltage, fill factor, and efficiency of measured and network simulated current voltage characteristics investigated in this study match within an uncertainty of 5%. Free energy loss analysis serves as comparison of all losses in units of power per area at the maximum power point. The largest loss is bulk recombination due to a carrier lifetime of 2 μs in the epitaxial Si layer. Further significant losses result from recombination at the base contacts characterized by a diode saturation current density of 50 000 fA cm−2 as well as resistive losses due to lateral majority carrier current flows within the solar cell base and contact resistance losses.

A Charge Based Compact Modeling Technique for Monocrystalline TFTs on Glass

A thin-film monocrystalline CMOS display technology has been realized by implementing a conventional NMOS inversion device and a PMOS accumulation device. In this paper, a charge based modeling technique is introduced which provides the dc current-voltage characteristics for both inversion and accumulation devices. Since conduction occurs at the surface and in the bulk of accumulation devices, traditional charge sheet models are invalid. Starting directly from the Pao-Sah equation by applying the 1-D Gauss' Law, C-∞ expressions able to capture the effect of both surface and bulk conduction suitable for circuit simulations are derived. The model is validated against the two-dimensional device simulation.

A Charge-Based Compact Model for Thin-Film Monocrystalline Silicon on Glass PMOSFETs Operated in Accumulation

A thin-film monocrystalline CMOS display technology has been realized by implementing a conventional NMOS inversion device and a PMOS accumulation device or PACC. In this paper, a charge-based model is introduced which provides the dc current-voltage characteristics of PACC devices. Derived directly from the Pao-Sah equation by applying the 1D Gauss' law, the model provides a C-∞ expression for drain current valid from cutoff through accumulation. The model correctly predicts the influence of fixed charge at the silicon-glass interface on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</i> characteristics and shows excellent agreement for both transfer and output characteristics with results from 2D device simulation. The core model coupled with previously developed equations describing channel length modulation, subthreshold slope degradation, drain induced barrier lowering, and mobility degradation form a complete PACC model which is compared to measured results.

H-induced subcritical crack propagation and interaction phenomena in (001) Si using He-cracks templates

H and He ion implantations allow the formation of nanocracks within controlled subsurface depths in semiconducting materials. Upon annealing, crack propagation and coalescence provides a way of cutting monocrystalline thin films. Here, the mechanisms of coalescence by crack-tip interactions are depicted in (001) Si wafers. Starting from overpressurized He-cracks, subcritical propagation was activated by diffusional H. Nanocrack interaction can occur by elastic forces, causing tip folding, or by plastic deformation forming extended defects. These observations are discussed and modeled using elasticity and fracture mechanics. The model suggests that kinetic effects in the cutting process depend on the crack interplanar separations.

Diamond detectors for time-of-flight measurements in laser-generated plasmas

CVD mono-crystalline diamond thin films deposited on a doped diamond and aluminum backing were employed as detectors of the radiation emitted from the laser-generated plasma. Laser-matter interactions were obtained by the use of an Nd:Yag repetitive laser at INFN-LNS in Catania operating at 1010 W/cm2 pulse intensity, and a high-power iodine PALS laser in Prague operating at 1015 W/cm2 pulse intensity. Plasmas were obtained by ablating Al, Ta, Au, and CF2 bulk targets. Plasma characterization was carried out using diamond detectors and ion collectors placed at different distances and angles in relation to the position of the ablated target. Photons, electrons, and ions hitting the sensible volume of the detector generate electronhole pairs (loosing 13 eV for a pair), resulting in an arising of the voltage signal at the device electrodes, which is proportional to the deposited energy. Diamond detectors can measure UV, X-rays, electrons, and ions. The time-of-flight (TOF) technique was exploited to separate photon, electron, and particle contributions. Because of the high gap of the diamond band structure (5.48 eV), the detectors are blind to visible and IR light from the plasma, which results in very low background current. The TOF diamond spectra were compared with traditional ion collector spectra using the experimental data obtained from the experiments conducted at the Catania and Prague laboratories. The results indicate that the ion energy resolution of the employed diamond detectors is high and that the fast electrons can be detected from a deconvolution procedure applied to the fast photo-peak. Information about the mean energy of soft X-rays could be obtained by analyzing the fast contribute of the spectra acquired by using different absorber films.

Modeling the Effect of P-N Junction Depth on the Output of Planer and Rectangular Textured Solar Cells

Problem statement: High cost of the solar cells is one of the important limitations in extensively using of the photovoltaic panels. Thin monocrystalline silicon solar cell could be reduce the cost but lost the absorption efficiency. Surface texturing help to enhance absorption. Using of advance texturing by diffraction grating was suggested for high absorption. It is necessary to investigate the scattering effect of diffraction grating with other solar cell parameter for optimization. In first step we concentrate on p-n junction position impact by modeling. Approach: The effect of position of p-n junction on the output current for both micro rectangular texturing and planer surface in solar cell has been investigated by ray tracing. Modeling of nine pairs solar cells with the same texture and planer surfaces but with different p-n junction position are done by using Atlas software. The output short current is a criterion for determining of efficiency performance. By comparing of the short current for each pair we was find the impacts of texturing and p-n junction depth on the monocrystalline thin film. Results: Light scattering due to diffraction grating inside the silicon with rectangular depth of 5 µm and a range of 5-40 µm p-n junction depths are investigated. The difference of short current in textured to bare silicon showed the enhancement from 4-8 µA when the p-n junction depths vary from 5-45 µm. Conclusions: Comparison of short current output confirms the correlation between p-n junction depth and texturing. Advanced texturing improve the solar cell efficiency but the effectiveness change with the p-n junction depth and need a simultaneous optimization for getting the high efficiency solar cell.

Demonstration of enhanced absorption in thin film Si solar cells with textured photonic crystal back reflector

Herein the authors report the experimental application of a powerful light trapping scheme, the textured photonic crystal (TPC) backside reflector, to thin film Si solar cells. TPC combines a one-dimensional photonic crystal as a distributed Bragg reflector with a diffraction grating. Light absorption is strongly enhanced by high reflectivity and large angle diffraction, as designed with scattering matrix analysis. 5 μm thick monocrystalline thin film Si solar cells integrated with TPC were fabricated through an active layer transfer technique. Measured short circuit current density Jsc was increased by 19%, compared to a theoretical prediction of 28%.

Comparative study on stress in AlGaN/GaN HEMT structures grown on 6H-SiC, Si and on composite substrates of the 6H-SiC/poly-SiC and Si/poly-SiC

The stresses in GaN-based HEMT structures grown on both single crystal 6H SiC(0001) and Si(111) have been compared to these in the HEMT structures grown on new composite substrates engendered as a thin monocrystalline film attached to polycrystalline 3C-SiC substrate. By using HRXRD technique and wafer curvature method we show that stress of monocrystalline layer in composite substrates of the type mono-Si/poly-SiC is lower than 100 MPa and residual stress of epitaxial GaN buffer grown on the composite substrate does not exceed 0.31 GPa, but in the cases of single crystal SiC or Si substrates the GaN buffer stress is compressive in the range of -0.5 ÷ -0.75 GPa. The total stress of the HEMT structure calculated from strains is consistent with the averaged stress of the multilayers stack measured by wafer curvature method. The averaged stress of HEMT structure grown on single crystals is higher than those in structures grown on composites substrates.

Lateral homogeneity of porous silicon for large area transfer solar cells

Transfer solar cells are monocrystalline silicon thin film cells grown epitaxially on annealed double layer porous silicon then transferred from the host wafer onto a foreign substrate. Porous silicon forms by electrochemical etching of the silicon wafer in hydrofluoric acid. An upper low porosity layer forms at low etch current density and a buried high porosity layer forms by increasing the etch current density. After heat treatment, the upper layer serves as a seed for the epitaxy growth, where a buried high porosity layer transforms to the separation layer. The separation layer is mechanically weak and allows the transfer of the monocrystalline epitaxy layer to a foreign substrate by gluing it and applying a mechanical force. After the transfer, the processing temperature is limited to T = 220 °C due to the change of the optical properties of the epoxy glue. Moreover, the yield Y of the transfer process is about Y = 33% due to the lateral inhomogeneity of the etched porous silicon. Therefore, increasing the yield of the transfer process requires the increase of the lateral homogeneity of porous silicon formation. The present contribution introduces a new etching setup which improves the lateral homogeneity of porous silicon films by about 10% and hence enables the fabrication of a stand alone 47.4 m thin large area circular epitaxy membranes with diameter D ≈ 13 cm. The new etching setup has increased the transfer process yield Y from Y = 33% to Y = 65%. Solar cells with an area A = 2 × 2 cm 2 are fabricated by epitaxy growth of both p-type absorber and n-type emitter prior to the transfer process to investigate the stability of the membranes. First experiment results in stand alone 47.4 µm thin solar cell with a maximum efficiency η = 12.3%. With the new etching process, it is possible to fabricate stand alone thin film solar cells with large area. Avoiding the epoxy glue enables a higher temperature limit T = 420 °C for the back side processing and an easier module connection.

Study of the structural evolutions of crystalline tungsten oxide films prepared using hot-filament CVD

Structural evolutions of tungsten oxide(WO3) samples on different substrates are studied using Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction and x-ray photoelectron spectroscopy. The WO3 samples are prepared using hot-filament CVD techniques. The focus of the study is on the evolutions of nano structures at different stages following deposition time. The experimental measurements reveal evolutions of the surface structures from uniform film to fractal-like structures, and eventually to nano particles, and crystalline structures from mono (0 1 0) crystalline thin film to polycrystalline thick film developments. The effect of high temperature on the nanostructured WO3 is also investigated. Well-aligned nanoscale WO3 rod arrays are obtained at a substrate temperature of up to 1400 °C. Further increasing the substrate temperature yields microscale crystalline WO3 particles.

Experimental studies of O 2–SnO 2 surface interaction using powder, thick films and monocrystalline thin films

Surface properties of solids and the interactions between molecules and solid surfaces are important for many technical applications. They also involve a range of physical and chemical phenomena of fundamental scientific interest. The importance of oxygen chemistry at SnO 2 surfaces follows from the fact that SnO 2 is used as an active material in gas sensor applications. The operation principle of these sensors is usually based on measurable conductance response of the material, which is understood in terms of reactions of gas molecules with different oxygen species adsorbed onto the surface. The role of the lattice oxygen, but in particular, the bridging oxygen atoms on SnO 2 surfaces, is also active. Detailed understanding of the reaction mechanisms of various oxygen species at SnO 2 surfaces is important, as it offers a way to improve the sensitivity and selectivity of the sensors. Oxygen adsorption–desorption kinetics at the SnO 2 surface is studied experimentally using O 2-temperature-programmed desorption (TPD) method together with conductance measurements in the case of SnO 2 powder and polycrystalline thick films made from the powder. In addition, CO-TPD is studied and the transient behaviour of various oxygen species is considered. Molecular beam epitaxy (MBE) was also used to fabricate polycrystalline and monocrystalline thin films with the SnO 2(101) face on single crystal sapphire substrate. Simultaneous surface potential and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO 2(101) surface after various surface treatments.

Characterization of free-standing thin crystalline films on porous silicon for solar cells

Thin film monocrystalline silicon solar cells based on porous silicon (PS) layer transfer processes could be cost-effective because of their lower consumption of material use and the potential for high efficiency. The process is based on our ability to separate thin porous films (p +) of desired thickness and area from Si substrate. Crystallinity of these porous films is studied with XRD for their suitability for Si epitaxy. AFM is employed to study the Si substrate surface roughness after one or more porous film separations. The roughness stabilizes after four film separations. Annealing of the porous films at temperatures above 1050 °C results in a closed surface, suitable for Si epitaxy. XRD analysis of epitaxially deposited Si layer on annealed PS film revealed its high mono-crystalline quality. A FMS solar cell of 12% efficiency has been realized in an 18 μm thin epitaxial layer.

Monocrystalline thin films of ZnSe and ZnO grown by atomic layer epitaxy

We report on the growth of monocrystalline thin films of ZnSe and ZnO by atomic layer epitaxy by simple reaction between elemental precursors. Structural and optical properties of these films are discussed with reference to the investigations performed with atomic force microscopy, scanning electron microscopy, cathodoluminescence and photoluminescence.

Non-stoichiometry in SnTe thin films and temperature instabilities of thermoelectric properties

The temperature dependences of the Seebeck coefficient and the electrical conductivity for (0 0 1)KCl/SnTe monocrystalline thin films with thicknesses d=20–600 nm were obtained in the range of 80–300 K. In contrast with SnTe bulk crystals, these dependences exhibited anomalies (plateaus, steps, bends). It is suggested that as temperature increases, the system passes through a number of intermediate states of quantum origin and/or corresponding to different distributions of non-stoichiometric defects (cation vacancies) over the crystal lattice. The observed effect is attributed to a high concentration of non-stoichiometric defects, which leads to the defect interaction stimulating their ordering, to loosening of the crystal lattice and to an increase in the diffusion rate. It is suggested that the observed temperature instabilities of properties are typical for other phases with high concentrations of non-stoichiometric defects.

Flexible monocrystalline Si films for thin film devices from transfer processes

AbstractTransfer of monocrystalline silicon films to arbitrary foreign substrates is a promising way for the fabrication of high quality silicon films on foreign substrates, demonstrated by solar cell efficiencies on glass as high as 16.6 % in the past. Transfer technologies also enable the use of flexible substrates. This paper investigates the mechanical stability of the separation layer for two different morphologies. First measurements on the minimum bending radius of unsupported silicon films are presented that allow us to estimate minimum curvatures for flexible monocrystalline devices. Finally, we report the first flexible monocrystalline thin film silicon solar cell of 4 cm2 with an independently confirmed efficiency of 14.6 %.

Effects of surface scattering on the thermal-noise properties and AC conductance of whiskers

A theory is developed for the equilibrium Johnson–Nyquist noise and the related AC conductance in thread-like metallic monocrystals (whiskers) with circular cross-section whose diameter is comparable to, or much smaller than, the (bulk) electron mean-free path, but larger than the screening length. The method of the kinetic theory of fluctuations previously applied to one-dimensional conductors and thin metallic films is used here to calculate the basic characteristics of the noise, in both the time and frequency domains. It is shown that in case of entirely diffusive electron surface scattering the size effect results in a non-exponential time decay of the autocorrelation function of the fluctuations, which, in its turn, originates a deviation of the noise spectral density from its standard Lorentzian shape. The above phenomena are shown to be accompanied by a ‘blue’ shift of the cut-off frequency in the noise spectrum as well as by the redistribution of noise intensity over the spectrum towards higher frequencies. The results obtained indicate that both these effects are strongly dependent on the geometrical size of the sample and are much more pronounced in whiskers than in monocrystalline thin films. A simple intuitive picture to elucidate the origin of these effects is presented. The illustrative numerical examples lead us to infer that size-dependent variations of the noise properties or of the AC conductance predicted herein should be observable experimentally.

Why Can Smart Cut® Change the Future of Microelectronics?

Deposition techniques like chemical vapor deposition (CVD) offer to the semiconductor industry the initial flexibility to deposit thin films of key materials on many kinds of substrates. The homoepitaxy or hetroepitaxy techniques using CVD or molecular beam epitaxy (MBE) add the flexibility to get a pure monocrystalline thin film but with a major limitation: the starting substrate has to be monocrystalline. The missing technology has always been the one which allows the growth of a thin monocrystalline film on any kind of substrate. Hydrogen induced splitting (known today as Smart Cut®), discovered at the LETI laboratory in 1991, provides a unique opportunity to get crystalline layers on any kind of substrate. Therefore, a new tool is offered to the semiconductor industry, for new material developments and new structures. This technique is in use in production today on a first application: silicon-on-insulator (SOI) wafers which consist of a monocrystalline film of silicon on a thin amorphous silicon dioxide layer, on top of a silicon wafer. We will discuss the SOI application of the Smart Cut® technology and present other recently demonstrated breakthroughs in new material development, including SiC, compound semiconductor or 3D structures.

WHY CAN SMART CUT® CHANGE THE FUTURE OF MICROELECTRONICS?

Deposition techniques like chemical vapor deposition (CVD) offer to the semiconductor industry the initial flexibility to deposit thin films of key materials on many kinds of substrates. The homoepitaxy or hetroepitaxy techniques using CVD or molecular beam epitaxy (MBE) add the flexibility to get a pure monocrystalline thin film but with a major limitation: the starting substrate has to be monocrystalline. The missing technology has always been the one which allows the growth of a thin monocrystalline film on any kind of substrate. Hydrogen induced splitting (known today as Smart Cut®), discovered at the LETI laboratory in 1991, provides a unique opportunity to get crystalline layers on any kind of substrate. Therefore, a new tool is offered to the semiconductor industry, for new material developments and new structures. This technique is in use in production today on a first application: silicon-on-insulator (SOI) wafers which consist of a monocrystalline film of silicon on a thin amorphous silicon dioxide layer, on top of a silicon wafer. We will discuss the SOI application of the Smart Cut® technology and present other recently demonstrated breakthroughs in new material development, including SiC, compound semiconductor or 3D structures.