Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σ_D, photoconductivity, σ_ph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O₂ gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.     

Evaluation of charge trapping properties of microcrystalline germanium thin films by Kelvin force microscopy

We have prepared highly-crystallized germanium (Ge) films on quartz and evaluated their local charge trapping and electrical conduction properties from topographic and surface potential images simultaneously taken by a conductive atomic force microscopy (AFM) during and after current application to Ge films. By applying a bias of 10 V at which the current of ~ 8 mA flows between the co-planer electrodes on Ge films, the surface potential image which was uniform before bias application shows in-plane inhomogeneity within ~ 1.0 mV commensurate with the surface morphology. Such potential images remained inhomogeneous at zero bias for more than two hours after bias application. The inhomogeneous potential images can be interpreted in terms of the difference in electron concentration in highly-crystallized Ge films presumably caused by electron charging in the grain boundaries, indicating direct detection of electrically separated grain structures and resultant percolation current pass.     

Peculiarities of photoluminescence excited by 157 nm wavelength F2 excimer laser in fused and unfused silicon dioxide

Photoluminescence (PL) spectra and kinetics of high purity amorphous silicon dioxide with ultra low hydroxyl content is studied under the excitation by F₂ excimer laser (157 nm wavelength) pulses. Materials synthesized in the SPCVD plasma chemical process are studied before and after fusion. Two bands are found in the PL spectra: one centered at 2.6–2.9 eV (a blue band) and the other at 4.4 eV (a UV band). Luminescence intensity of unfused material is found to increase significantly with exposure time starting from a very small level, whereas in fused counterpart it does not depend on irradiation time. Both bands show complicated decay kinetics, to which add exponential and hyperbolic functions. The UV band of the unfused material is characterized by decay with exponential time constant τ  4.5 ns and hyperbolic function t⁻ⁿ, where n = 1.5 ± 0.4. For the blue band the hyperbolic decay kinetics with n  1.5 extends to several milliseconds, gradually transforming to the exponential one with τ = 11 ± 0.5 ms. In fused glass relative contribution of the fast component to the UV band is small whereas for the blue one it is great, that allows one to more accurately determine the hyperbolic law factor n = 1.1 ± 0.1 typical for tunneling recombination. Simultaneous intracenter and recombination luminescence, the later occurring with the participation of laser radiation induced defects, add particular features to the decay kinetics. Spectra of the above luminescence processes are different. A less sharp position of bands is associated with the recombination luminescence. The origin of the observed PL features we attribute to the presence of oxygen deficient centers in glass network in the form of twofold coordinated silicon. Such centers being affected by network irregularities can be responsible for the recombination PL component. A great variety of network irregularities is responsible for centers’ structural inequivalence, which causes a non-uniform broadening of PL spectral and kinetic parameters.     

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si-H_x stretching (2000-2100 cm⁻¹), Si-O-Si (1000-1200 cm⁻¹), and O_xSi-H_y modes (2130-2250 cm⁻¹). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O-H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.     

A narrow process window for the preparation of polytypes of microcrystalline silicon carbide thin films by hot-wire CVD method

Effects of deposition conditions on the structure of microcrystalline silicon carbide (μc-SiC) films prepared by hot-wire chemical vapor deposition (hot-wire CVD) method have been investigated. It is found from X-ray diffraction patterns of the film that a diffraction peak from crystallites from hexagonal polytypes of SiC is observed in addition to those of 3 C-SiC crystallites. This result is obtained in the film under a narrow deposition conditions of SiH₃CH₃ gas pressure of 8 Pa, the H₂ gas pressure of 80–300 Pa and the total gas pressure of 40–300 Pa under fixed substrate and filament temperatures employed in this study. Furthermore, the grain size of hexagonal crystallites (about 20 nm) on c-Si substrates becomes larger than that of 3 C-SiC crystallites (about 10 nm) for the films deposited under the total gas pressure of 36–88 Pa. The fact that microcrystalline hexagonal SiC can be deposited under limited deposition conditions could be interpreted in the context of a result for c-SiC polytypes prepared by thermal CVD method.     

Mapping of mechanical stress in silicon thin films on silicon cantilevers by Raman microspectroscopy

We have used plasma enhanced chemical vapor deposition (PECVD) to deposit silicon thin films (～0.2–1 μm) with different crystallinity fractions on Nanosensors PtIr5 coated atomic force microscopy (AFM) cantilevers (450 × 50 × 2 μm³). Microscopic measurements of Raman scattering were used to map both internal stress and extrinsic stress induced in the films by bending the cantilevers using a nanomanipulator (Kleindiek Nanotechnik MM3A). Thanks to the excellent elasticity of the cantilevers, the films could be bent to curvature radii down to 300 μm. We observed the stress induced shift of the TO–LO phonon Raman band of more than 3 cm^?1 for fully microcrystalline film corresponding to the stress ～0.8 GPa. The shift of the similar film with amorphous structure was ～2.5 cm^?1.     

Characterization of single-crystal sapphire substrates by X-ray methods and atomic force microscopy

The possibility of characterizing a number of practically important parameters of sapphire substrates by X-ray methods is substantiated. These parameters include wafer bending, traces of an incompletely removed damaged layer that formed as a result of mechanical treatment (scratches and marks), surface roughness, damaged layer thickness, and the specific features of the substrate real structure. The features of the real structure of single-crystal sapphire substrates were investigated by nondestructive methods of double-crystal X-ray diffraction and plane-wave X-ray topography. The surface relief of the substrates was investigated by atomic force microscopy and X-ray scattering. The use of supplementing analytical methods yields the most complete information about the structural inhomogeneities and state of crystal surface, which is extremely important for optimizing the technology of substrate preparation for epitaxy.     

Microstructures of high-growth-rate (up to 8.3 nm/s) microcrystalline silicon photovoltaic layers and their influence on the photovoltaic performance of thin-film solar cells

Microstructures of microcrystalline silicon (μc-Si) deposited at a high-growth-rate have been investigated in order to apply to the photovoltaic i-layer. μc-Si films were prepared by very-high-frequency (100 MHz) plasma-enhanced chemical vapor deposition at 180 °C. High growth rates of 3.3–8.3 nm/s have been achieved utilizing high deposition pressures up to 24 Torr and large input powers. Applying μc-Si to n–i–p junction solar cells, as the optimum result in this experimental series, a conversion efficiency of 6.30% (J_SC: 22.1 mA/cm², V_OC: 0.470 V, and FF: 60.7%) has been achieved employing the i-layer deposited at 8.1 nm/s. Raman scattering and X-ray diffraction measurements revealed the crystalline volume fraction of around 50% with the (2 2 0) crystallographic preferential orientation, respectively. The cross-sectional transmission electron microscope image shows densely columnar structure grown directly on the underlying n-layer. These structural features are basically in good agreement those of low-growth-rate μc-Si used for a high efficiency solar cell as previously reported, implying advantages of the use of high pressures with regard to providing the photovoltaic i-layers. Finally, the implication is discussed from the photovoltaic performance as a function of the crystalline volume fraction of i-layer, and current problems in improving the photovoltaic performance are extracted.     

Crystallinity of the mixed phase silicon thin films by Raman spectroscopy

Raman spectra of the mixed phase silicon films were studied for a sample with transition from amorphous to fully microcrystalline structure using four excitation wavelengths (325, 514.5, 632.8 and 785 nm). Factor analysis showed the presence of two and only two spectrally independent components in the spectra within the range from 250 to 750 cm^?1 for all four excitation wavelengths. The 785 nm excitation was found optimal for crystallinity evaluation and by comparison with surface crystallinity obtained by atomic force microscopy, we have estimated the ratio of integrated Raman cross-sections of microcrystalline and amorphous silicon at this wavelength as y = 0.88 ± 0.05.     

Selective crystallization of amorphous silicon thin film by a CW green laser

A simple and effective method for selective CW laser crystallization of a-Si (CLC) without pre-patterning of a-Si has been reported. By using a metallic shadow mask instead of a photolithographic process, we can reduce the process steps and time compared with a conventional CLC process. It shows very high performance – mobility of 173 cm²/s, I_off of ∼10⁻¹³ A @ V_d = −5 V, I_on/I_off of >10⁸ – as a p-channel poly-Si TFT even without any pre-/post-treatment to improve TFT characteristics such as plasma hydrogenation.     

Nanostructural and optical features of amorphous AlxSi0.45−xO0.55 (0 ⩽ x ⩽ 0.05) thin films prepared by RF-magnetron sputter techniques

The nanostructural, chemical, and optical features of Al_xSi_0.45−xO_0.55 (0 ⩽ x ⩽ 0.05) thin films were investigated in terms of Al concentration and post-deposition annealing conditions; the films were prepared by co-sputtering a Si main target and Al-chips, and the annealing was carried out at temperatures of 400–1100 °C. The a-Si_0.45O_0.55 films prepared without Al-chips and annealed at 800 °C contain ∼3.5 nm-sized Si nanocrystallites. The photoluminescence (PL) intensity as well as the volume fraction of Si nanocrystallites increased with increasing the concentration of Al to a certain level. In particular, the intensity of the PL spectra of the Al_0.025Si_0.425O_0.550 films which were annealed at 800 °C increased significantly at wavelengths of ∼580 nm. It is highly likely that the observed increase in the PL intensity is caused by the raise in the total volume of the ∼3.5 nm-sized nanocrystallites in the films. The addition of Al as well as the post-deposition annealing allow adjustment and control of the nanostructural and light-emission features of the a-SiO_x films.     

Investigation into the correlation factor of substrate and multilayer film surfaces by atomic force microscopy

A method for studying the correlations between substrate and film-coating profiles by atomic force microscopy, which makes it possible to calculate the correlation factor (a function of spatial frequency), has been developed. The spatial-frequency range in which the correlation factor can be reliably calculated is established. The method proposed is used to calculate the dependence of the correlation factor on spatial frequency for multilayer interference mirror elements.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

Study of the surface heterogeneity of topographic and electric nature by atomic force microscopy by the example of triglycine sulfate

Triglycine sulfate crystals with an ideal (010) cleavage plane are used as model objects to reveal problems in interpreting atomic force microscopy (AFM) images of surfaces with nonuniform charge distribution. Specific microrelief features of two types are found: lenslike formations with different contrast and rounded protrusions/valleys of different size but fixed height. An analysis of their evolution with a change in temperature and under an electric field and mechanical impacts has made it possible to separate relief elements from the crystal domain structure. The interpretation proposed is confirmed by the multimode AFM data. The specific features of the images of dynamic domains and aged domains (which cannot undergo polarization reversal) are studied. The domain-wall width found in the AFM measurements depends on the technique used and the specificity of probe-surface interaction; it varies from 9 to 2000 nm. The most reliable data on the domain-wall width in triglycine sulfate crystals are provided by piezoelectric force microscopy, according to which the wall width does not exceed 30 nm.     

Synthesis of polycrystalline silicon thin films with ‘icosahedral’ symmetry by ceramics hot wire chemical vapor deposition

We report on synthesis and materials physics of polycrystalline silicon thin films deposited on glass with rarely observed ‘five-fold’ symmetry or ‘icosahedral’ symmetry. We invented these ‘novel form’ of polycrystalline silicon thin films by ceramics hot wire chemical vapor deposition (hot-wire CVD). A new physical effect in hot-wire CVD technology has been proposed that controls the nucleation and growth of silicon thin films on glass substrate.     

Mechanical properties of thin silicon films deposited on glass and plastic substrates studied by depth sensing indentation technique

This work concerns the characterisation of mechanical properties of thin amorphous and microcrystalline silicon films deposited on glass and polyethylene terephthalate substrates. The film/substrate indentation response has been investigated from the near surface up to film/substrate interface using depth sensing indentation technique. The universal hardness, Vickers hardness, elastic modulus, fracture toughness and creep resistance of the studied films have been determined. Particular attention has been paid to the effects of the flexible viscoelastic-plastic substrate on the indentation response of the film/substrate system.     

Rotary target method to prepare thin films of CdS/SiO2 by pulsed laser deposition

Thin films of CdS-doped SiO₂ glass were prepared by using the conventional pulsed laser deposition (PLD) technique. The laser target consisted of a specially constructed rotary wheel which provided easy control of the exposure-area ratio to expose alternately the two materials to the laser beam. The physical target assembly avoided the potential complications inherent in chemically mixed targets such as in the sol–gel method. Time-of-flight (TOF) spectra confirmed the existence of the SiO₂ and CdS components in the thin-film samples so produced. X-ray diffraction (XRD) and atomic force microscopy(AFM) results showed the different sizes and structures of the as-deposited and annealed films. The wurtzite phase of CdS was found in the 600^oC-annealed sample, while the as-deposited film showed a cubic–hexagonal mixed structure. In the corresponding PL (photoluminescence) spectra, a red shift of the CdS band edge emission was found, which may be a result of the interaction between the CdS nanocrystallite and SiO₂ at their interface.     

Properties of ZnO films grown on (0 0 0 1) sapphire substrate using H2O and N2O as O precursors by atmospheric pressure MOCVD

In this paper, we compare the properties of ZnO thin films (0 0 0 1) sapphire substrate using diethylzinc (DEZn) as the Zn precursor and deionized water (H₂O) and nitrous oxide (N₂O) as the O precursors, respectively in the main ZnO layer growth by atmospheric pressure metal–organic chemical vapor deposition (AP-MOCVD) technique. Surface morphology studied by atomic force microscopy (AFM) showed that the N₂O-grown ZnO film had a hexagonal columnar structure with about 8 μm grain diameter and the relatively rougher surface compared to that of H₂O-grown ZnO film. The full-widths at half-maximum (FWHMs) of the (0 0 0 2) and () ω-rocking curves of the N₂O-grown ZnO film by double-crystal X-ray diffractometry (DCXRD) measurement were 260 and 350 arcsec, respectively, indicating the smaller mosaicity and lower dislocation density of the film compared to H₂O-grown ZnO film. Compared to H₂O-grown ZnO film, the free exciton A (FX_A) and its three phonon replicas could be clearly observed, the donor-bound exciton A⁰X (I₁₀):3.353 eV dominated the 10 K photoluminescence (PL) spectrum of N₂O-grown ZnO film and the hydrogen-related donor-bound exciton D⁰X (I₄):3.363 eV was disappeared. The electron mobility (80 cm²/V s) of N₂O-grown ZnO film has been significantly improved by room temperature Hall measurement compared to that of H₂O-grown ZnO film.