Conductivity and noise in thin films of nonhydrogenated amorphous silicon in the hopping regime

Excess noise measurements have been carried out in nonhydrogenated amorphous silicon material at room temperature. The temperature dependence of the conductivity has shown that, at room temperature, noise measurements deal with transport mechanisms dominated by hopping to nearest neighbors. At lower temperature there is variable range hopping from which the number of states near the Fermi level was determined. The ohmic behavior of structures such as molybdenum-amorphous silicon-molybdenum, and the V² dependence of the noise spectral density indicate that $\text{1}\text{f}$ noise is characteristic for this material. A simple theory of phonon assisted hopping $\text{1}\text{f}$ noise is presented. From this the fluctuations. Some preliminary data are shown for hydrogenated amorphous silicon. The noise shows a Lorentzian, indicative of generation-recombination noise.     

Conductivity and structure of Er-doped amorphous hydrogenated silicon films

A columnar structure of a-Si:H(Er) film serving as a working layer in electroluminescent structures has been demonstrated. The diameter of columns is in the range of 60–100 Å. In a structure of this kind, the conductivity depends on the direction of current. In the planar configuration, room-temperature transport occurs through hopping via localized states near the conduction band edge, within the band tail. In the sandwich configuration, the conduction occurs along the column boundaries, where the conductivity is higher, via hopping conduction at the Fermi level.     

PECVD nanocrystalline silicon films versus porous silicon: Structural and optical properties

A comparative experimental study is conducted of nanocrystalline-and porous-silicon films from the viewpoint of their structural, luminescent, and optical properties, the films being fabricated by PECVD and electrochemical etching, respectively. The x-ray-diffraction data obtained imply that the PECVD films consist of nanocrystals that are uniformly (004) oriented and measure on average 4.8 nm across. It is shown by an AFM investigation that the films have a very rough surface occupied by sharply pointed nanoscale asperities. The films can exhibit room-temperature photoluminescence that is strongest at a photon energy of 1.55 eV. Optical properties of the nanocrystalline-and porous-silicon films are examined in the visible range and the IR. The nanocrystalline-silicon films are found to have similar physical and chemical properties to the porous-silicon films.     

Properties of thin LPCVD silicon oxynitride films

Thin, uniform silicon oxynitride films with films thicknesses of ≤ 10 nm were successfully deposited by low pressure chemical vapor deposition (LPCVD). The reactant gases were SiH₂Cl₂, N₂O, and NH₃. The compositional uniformity of these films as a function of depth was good. The structure of these oxynitride films was found to be dominated by the mixed matrix of Si, N, and 0, rather than a physical mixture of SiO₂ and Si₃N₄ clusters. N-H bonding was observed and the total amount of hydrogen in the as-deposited film was on the order of 5 x l0²⁰/cm³. No H-OH or Si-OH bonds were detected. Excellent dielectric breakdown distributions were found for oxynitride films with equivalent oxide film thicknesses as low as 7.5 nm. The conduction of Si-N-0 films depended on film composition. A small capacitor-voltage (C-V) window (< 0.1 V) was observed for the Si-N-O/Si structures. The midgap surface state density was on the order of 5 x 10¹⁰/cm² /eV. Either trapping of holes or the generation of positive states were found after high field stressing of the oxynitride films.     

Microstructure evolution of hydrogenated silicon thin films

This paper describes the microstructure evolution of hydrogenated silicon films containing various amounts of hydrogen. Microcrystalline silicon films were produced when the hydrogen content of the films was adjusted by using the diluted hydrogen and hydrogen atom treatment methods. Polycrystalline silicon films having grain sizes in the micrometre range were deposited at low temperatures (250°C) by electron cyclotron resonance chemical vapour deposition with the hydrogen dilution method. The micro crystalline and polycrystalline films were characterized by NMR, FTIR, Raman, X-ray and optical spectroscopy and electrical measurements. The results suggest the possibility of even larger grain silicon films suitable for high-performance solar cells which avoid the fundamental difficulties of amorphous Si:H solar cells.     

Enhancing optical properties of nanocrystalline silicon films with air exposure

We report the effect of air exposure and deposition temperatures, T_d, on the optical property of nanocrystalline silicon (nc-Si). The nc-Si thin films were investigated by photoluminescence (PL), optical absorption, X-ray diffraction (XRD), Fourier-transform infrared (FTIR) absorption and Raman scattering. Experimental results show the structural change from an amorphous to a nanocrystalline phase at T_d=80 °C. In addition, it suggests that T_d low condition leads to the increase in the density of SiH-related bonds and a decrease in the average grain size, δ. The oxygen absorption peak increases with the air-exposure time. The PL exhibited two peaks at around 1.75–1.78 and 2.1–2.3 eV. The PL increases and blue shifts consistently with the decrease of δ and increase of oxygen content. The first peak may be related to nanocrystallites in nc-Si films and the origin of another one may be due to defect-related oxygen. Thus, by the plasma-enhanced chemical vapor deposition (PECVD) technique at low T_d, we can produce the nc-Si films with different grain sizes, causing the corresponding luminescent properties. The new method processes advantages of low deposition temperature and effective oxidation of nc-Si on inexpensive substrates, thus making it more suitable for developing low-cost array or flexible nc-Si optoelectronic devices.     

Lithium intercalation into amorphous silicon thin films

Electrochemical lithium intercalation into a-Si:H thin films grown on stainless steel substrates at temperatures of 100 and 250°C was studied. The intercalation capacity of films grown at 250°C is ～1750 and ～500 mA h/g in the first and hundredth cycles. The lithium diffusivity in films is ～ 10^?12 cm²/s.     

Optoelectronic study in porous silicon thin films

A photoconductivity (PC) study in as deposited porous silicon (PS) thin films is presented in this work. PS thin films were produced by the electrochemical anodizing method at different anodizing times. The films surfaces were characterized by SEM and porosity was determined by gravimetric methods. Photoluminescence and PC measurements were taken at room temperature. The maximum of the photoluminescence spectra are located around 650 nm, whereas those of PC are placed around 400 nm. The maximum of the photoluminescence signal shifts toward short wavelengths as the quantum dimension of the material skeleton diminishes, while any spectral displacement of the photocurrent signal as the porosity of the material increases is not observed. The spectral position of the PC signal does not change because it is strongly affected by the large quantity of defects present in the sample surface which diminishes the mean free path of the carriers to reach the electrodes. In all the samples photocurrent is small around 10^?1 μA and the intensity of the signal goes down as the porosity increases. Two mechanisms exist that compete with one another, the carrier generation and recombination through light emission centers which diminish the photocurrent.     

BF3-doped amorphous silicon thin films

BF₃ has been used as a p-type dopant in thin films of hydrogenated amorphous silicon (a-Si:H). The films were deposited in a capacitively coupled radio frequency (13.56 MHz) glow discharge system in which silane (SiH₄) was the principal gaseous component. The boron content in the films was measured by secondary ion mass spectroscopy (SIMS) using a calibrated boron-implanted silicon standard. The optical and transport properties of these films were measured and compared with films of similar boron film content deposited using B₂H₆ BF₃-doped films showed less sub-band gap absorption (as determined by photothermal deflection spectroscopy), no band gap narrowing, and higher conductivity prefactors σ_o for low doping levels compared to B₂H₆-doped films. The possible utility of lightly BF₃-doped a-Si:H as the i-layer in a p/i/n photovoltaic device is suggested.     

ESR studies of nanocrystalline silicon films obtained by pulsed laser ablation of silicon targets

Nanocrystalline silicon films formed using laser ablation of silicon targets were studied using electron spin resonance. The measurements were performed in the X band with modulation of the magnetic field at a frequency of ～100 kHz at temperatures of 300 and 77 K. Two types of spectra were observed. The first type of spectra is related to the high concentration of dangling silicon bonds in Si nanocrystals and SiO_x sheaths of nanocrystals and are inherent in nanocrystalline silicon (nc-Si) films that do not exhibit photoluminescence in the visible region of the spectrum. The second type of spectra is related to the presence of E′ centers, nonbridging oxygen hole centers (NBOHC), and peroxide radicals and is characteristic of films with photoluminescence in the visible region of the spectrum, which indicates that high-barrier SiO₂ layers exist in these films. An increase in the photoluminescence intensity and a decrease in the signal of electron spin resonance were observed in porous nc-Si films exposed to atmospheric air for a long time.     

衬底温度对纳米晶SnO_2薄膜结晶特性的影响

阐述了金属氧化物SnO2纳米薄膜研究的发展情况及其应用前景,介绍了采用磁控溅射技术,使用混合气体Ar和O2,在衬底温度为150～400℃的耐热玻璃基片上制备了纳米晶SnO2∶Sb透明导电薄膜。通过测定x射线衍射谱,表明薄膜择优取向为[110]和[211]方向,SnO2∶Sb薄膜的结晶特性随衬底温度变化。     

Effect of irradiation on the properties of nanocrystalline silicon carbide films

The effect of irradiation with electrons on the optical and electrical properties of the nanocrystalline rhombohedral 21R-SiC and 27R-SiC films is studied. The cycles of irradiation of nanocrystalline SiC films on sapphire substrates with electrons with the energy 10 MeV are conducted in the range of fluences from 5 × 10¹⁴ to 9 × 10¹⁹ cm^?2. It is established that, at the irradiation doses above 10¹⁹ cm^?2, the optical absorption of the films at the photon energies E > E _g becomes less efficient than the optical absorption of unirradiated films. It is established that the Urbach energy as a function of the irradiation dose exhibits a minimum at the dose ～10¹⁷ cm^?2 for the 21R-SiC films and ～5 × 10¹⁷ cm^?2 for the 27R-SiC films, suggesting that radiation induces some ordering in the films. As the dose is increased from 5 × 10¹⁷ up to 9 × 10¹⁹ cm^?2, an increase in the Urbach energy and a decrease in the optical band gap are observed. The effect is attributed to an increase in the concentration of radiation defects in the films.     

Titanium dioxide nanocrystalline bactericidal thin films grown by sol-gel technique

Titanium dioxide thin films were grown by sol-gel technique in order to obtain films to be used in photocatalytic reactions. We present results on the structural and optical characterization measured for series of films in which number of layers and sintering temperature were changed. Using X-ray diffraction we have characterized the crystalline phase in the films depending on the growth conditions and by using the spectrophotometer we characterized the resulting bandgap energy. The results on the photocatalytic activity were obtained by immersing these films in a methylene blue solution with the presence of ultraviolet light, which is a standard used to characterize the efficiency of the reaction. High bactericidal activity with these films in wastewater samples was observed subtracting the action of used ultraviolet light, through gas evolution measurements using photoacoustic (PA) technique.     

Polycrystalline silicon thin films on glass by aluminum-inducedcrystallization

This work focuses on the development and characterization of device quality thin-film crystalline silicon layers directly onto low-temperature glass. The material requirements and crystallographic quality necessary for high-performance device fabrication are studied and discussed. The processing technique investigated is aluminum-induced crystallization (AIC) of sputtered amorphous silicon on Al-coated glass substrates. Electron and ion beam microscopy are employed to study the crystallization process and the structure of the continuous polycrystalline silicon layer. The formation of this layer is accompanied by the juxtaposed layers of Al and Si films exchanging places during annealing. The grain sizes of the poly-Si material are many times larger than the film's thickness. Raman and thin-film X-ray diffraction measurements verify the good crystalline quality of the Si layers. The electrical properties are investigated by temperature dependent Hall effect measurements. They show that the electrical transport is governed by the properties within the crystallites rather than the grain boundaries. The specific advantages of AIC are: (1) its simplicity and industrial relevance, particularly for the processes of sputter deposition and thermal evaporation, (2) it requires only low-temperature processing at 500°C, (3) its short processing times, and (4) its ability to produce polycrystalline material with good crystallographic and electrical properties. These advantages make the poly-Si material formed by AIC highly interesting and suitable for subsequent device fabrication such as for poly-Si thin-film solar cells     

Metal-to-metal antifuses with very thin silicon dioxide films

Antifuse samples with very thin insulating oxide were fabricated using a technique of two-step PECVD oxide deposition. Dielectric strength as high as 13 MV/cm was obtained for our samples. Defect density and uniformity have been improved in this way. The on-state resistance of the programmed antifuses shows a stronger dependence on the oxide thickness when it was programmed at the lower current than when it was programmed at the higher current     

Conductivity of structures based on doped nanocrystalline SnO2 films with gold contacts

The conductivity of nanocrystalline Pt-, Pd-, and Ni-doped tin dioxide films on insulating SiO₂ substrates is investigated in the temperature range 77–400 K. Doping allows variation of the resistance from 10⁴ to 10⁷Ω. It is established that, in contrast with a Au/single-crystal SnO₂ contact, the gold contacts for the nanocrystalline material are ohmic in the entire temperature range and their contribution to the conductance of all the structures investigated does not exceed 5%. Fiz. Tekh. Poluprovodn. 33, 205–207 (February 1999)     

Paramagnetic structural defects and conductivity in hydrogenated nanocrystalline carbon-doped silicon films

The behavior of paramagnetic defects and dark conductivity in heterogeneous samples of hydrogenated nanocrystalline carbon-containing silicon (nc-SiC:H) prepared by the photo-CVD method is studied. It is shown that, with increasing carbon content in nc-SiC:H, a phase transition from a nanocrystalline to an amorphous structure occurs, producing a reduction in paramagnetic defect density and a significant decrease in the dark conductivity.     

Optical and electrical properties of thin wafers fabricated from nanocrystalline silicon powder

The infrared transmittance spectra and dark conductivity of wafers fabricated from nanocrystalline silicon powder (nc-Si) have been studied. The initial nc-Si powder is first synthesized by laser-induced silane dissociation, with the temperature of the surrounding buffer gas varied from 20 to 250°C, and then compacted at pressures from 10⁸ to 10⁹ Pa. It is found that compaction of the nc-Si powder results in formation of Si-H, Si-CH_x, and O_y-Si-H_x structures (x, y=1–3). The formed structures break down under annealing, with the Si-H and Si-CH_x complexes disintegrating at the lowest annealing temperature (t = 160°C). The dark conductivityof the nc-Si wafers is shown to increase along with the buffer gas temperature at which the starting powder was prepared. Two temperature regions are found in which the dark conductivity behaves in radically different ways. At wafer temperatures T ≥ 270 K, conductivity is mediated by free carriers, whereas, at lower temperatures, electron transport is governed by hopping conduction over localized states in the band gap.     

Dielectric characterization of ferroelectric thin films deposited on silicon

This paper describes some experimental results concerning the dielectric characterizations of PZT, PT and BT ferroelectric thin films deposited on silicon. Firstly, the ferroelectric properties of these films are checked, notably the high dielectric constant values. Secondly, assuming a simple electrical model, we derive from impedance measurements the conductivity of the platinum deposited as a bottom electrode.     

Diffusion of chromium in thin hydrogenated amorphous silicon films

The diffusion of a chromium bottom contact has been studied through thin 10-nm amorphous silicon film. The concentration of the diffused impurity has been analyzed by an X-ray photon spectroscopy technique and the diffusion coefficient was estimated. Diffusion annealing was carried out in vacuum (10^?6 mTorr), the temperature was kept at 400°C, and the annealing time was varied from 0 to 300 min. The authors propose that diffusion of chromium in thin hydrogenated amorphous film is limited by silicide formation at the metal-silicon interface.