Shallow defect states in hydrogenated amorphous silicon oxide

The theoretical cluster-Bethe-lattice method is used in this study to investigate the shallow defect states in hydrogenated amorphous silicon oxide. The electronic density of states (DOS) for the SiO₂ Bethe lattice of various Si–O–Si angles, non-bridging oxygen Si–O^√, peroxyl radical Si–O–O^√, threefold coordinated O₃ and Si–H bonds are calculated. The variation of the Si–O–Si bond angle causes the bandgap fluctuation and induces tail states near the conduction band minimum. The Si–O^√ and Si–O–O^√ bonds introduce shallow defect states in the energy gap near the top of the valence band. The Si–H bond induces a defect state, in the energy gap near the conduction band minimum, in a-SiO_x with high oxygen concentration, but not low oxygen concentration. The O₃ bond itself does not induce defect state in the energy gap. The O₃⁺D⁻ complex, formed by the O₃ and threefold coordinated silicon, induces shallow state in the energy gap near the conduction band minimum. This defect state can explain the energy shift of photoluminescence of a-SiO_x:H under annealing.     

An empirical density of states and joint density of states analysis of hydrogenated amorphous silicon: a review

A number of empirical models for the valence band and conduction band hydrogenated amorphous silicon density of states functions are presented. Then, a relationship between these density of states functions and the imaginary part of the dielectric function is developed. The joint density of states function, which plays a key role in determining the spectral dependence of the imaginary part of the dielectric function, and is directly related to the valence band and conduction band density of states functions, is defined in the process. Joint density of states results, corresponding to a specific empirical density of states model, are then presented. Finally, the modeling results for the imaginary part of the dielectric function are contrasted with those of experiment and satisfactory agreement is found.     

A quantitative characterization of the optical absorption spectrum associated with hydrogenated amorphous silicon

We propose a quantitative means of characterizing the optical absorption spectrum associated with an amorphous semiconductor. In particular, for a representative hydrogenated amorphous silicon optical absorption experimental data set, through a series of least-squares linear fits of an exponential function to this experimental data set, taken over a number of optical absorption ranges, we determine how the breadth of the optical absorption tail varies along the optical absorption spectrum of hydrogenated amorphous silicon. We find that the quantitative variations in the breadth of the optical absorption tail that are found provide for a clear delineation between the different regions of the optical absorption spectrum of hydrogenated amorphous silicon. We complete this analysis by theoretically determining the form of the optical absorption spectrum using a recently developed empirical model for the density of states functions corresponding to hydrogenated amorphous silicon, this analysis providing a theoretical basis for the interpretation of our results.

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Metastable defects in hydrogenated amorphous silicon

The electronic structure of hydrogenated amorphous silicon (a-Si:H) is in a state of metastable equilibrium and can change upon application of external stimuli. We study the effect of thermal quenching and light soaking in lithium-doped a-Si:H, on its conductivity and thermopower. We present evidence showing that the metastable state obtained after fast quenching is different than that obtained after light exposure. Experiments on chalcogenides show that they are not affected by thermal quenching although they change upon light soaking. This is in contrast with lithium doped a-Si:H in which both effects are observed. Our experiments suggest that hydrogen present in a-Si:H plays an important role by controlling heterogeneities and potential fluctuations in a-Si:H. Light soaking appears to enhance these potential flucutations, whereas fast cooling seems to have little effect on them.     

Stress induced crystallization of hydrogenated amorphous silicon

Hydrogenated amorphous silicon films were grown on to thermally oxidized silicon wafers by Radio Frequency magnetron sputtering, and SiN_x and Al₂O₃ capping layers were used to control the residual thermal stress. After annealing, a comparison of the silicon films with and without capping layers indicates that tensile stress induced by the capping layer enhances the crystallinity of the annealed amorphous silicon film. The stress is due to the mismatch between the coefficients of thermal expansion for the capping layer and amorphous silicon film. These results highlight the potential of thermal stress as a means to alter the crystallization in thin film architectures and suggest that even larger effects can be obtained with suitable choices of capping layer chemistry.     

Raman study of ion-implanted hydrogenated amorphous silicon

The effect of silicon and hydrogen ion implantations on the structural properties of hydrogenated amorphous silicon films was studied by means of Raman spectroscopy, with the aim of revealing the influence of hydrogen atoms inserted into the silicon matrix on its short-range order. To separate the implantation-induced increase in the structural disorder from the effect of the implanted hydrogen, the implantation doses of silicon and hydrogen ions were selected to create closely similar numbers of host-atom displacements. The results obtained suggest that the presence of hydrogen in amorphous silicon reduces the structural disorder related to variations in the silicon bond length, but affect the bond-angle deviations to a lesser extent.     

Broadband wavelength conversion in hydrogenated amorphous silicon waveguide with silicon nitride layer

Broadband wavelength conversion based on degenerate four-wave mixing is theoretically investigated in a hydrogenated amorphous silicon (a-Si:H) waveguide with silicon nitride inter-cladding layer (a-Si:HN). We have found that enhancement of the non-linear effect of a-Si:H waveguide nitride intermediate layer facilitates broadband wavelength conversion. Conversion bandwidth of 490 nm and conversion efficiency of 11.4 dB were achieved in a numerical simulation of a 4 mm-long a-Si:HN waveguide under 1.55 μm continuous wave pumping. This broadband continuous-wave wavelength converter has potential applications in photonic networks, a type of readily manufactured low-cost highly integrated optical circuits.     

Photoconductivity studies of n-type hydrogenated amorphous silicon and microcrystalline silicon

Photoconductivity techniques serve as useful tools for the characterization of amorphous and microcrystalline silicon. From the link between the majority carrier mobility–lifetime product from steady-state photoconductivity and the position of the Fermi level, useful insight can be gained when comparing sample properties. The temperature dependence of the minority carrier mobility–lifetime product implies that the band-tail region of the density-of-states (DOS) is steeper in microcrystalline silicon than in amorphous silicon. Transient and modulated photoconductivity determine the DOS in the upper half of the band gap, for which we find an exponential variation. We indicate that the Fermi level or quasi-Fermi level impose limitations on the DOS extraction from the measured data. In samples in which the Fermi level is shifted towards the conduction band, the DOS calculation then yields values that are too low.     

Temperature dependence of hydrogenated amorphous silicon thin-film transistors

The temperature dependence characteristics of hydrogenated amorphous silicon thin-film transistors were investigated. The results indicate that as the temperature was increased, the threshold voltage and the field-effect mobility were first increased, and then decreased, which may be controlled by different mechanisms at low and high temperatures. In addition, if the temperature was higher than 420 K, the Fermi level was promoted to the degenerate-like states, the current channel always existed due to the temperature effect, and the threshold voltage became negative.     

Optical properties of doped hydrogenated amorphous silicon films

The effect of doping on the reflectance and on the imaginary part of the dielectric function in the UV-visible range was investigated for a series of hydrogenated amorphous silicon films. The samples, doped with phosphorus or boron, were prepared by a reactive evaporation technique. Doping, with either p-or n-type dopants, strongly affects the spectra causing both a red shift and a decrease in peak height. Analysis of the results in terms of the oscillator strength and the excitation energy indicates that there is a decrease in the short-range order which, according to the literature, is attributed to a degree of structural disorder induced by the dopants.     

Potential fluctuations and metastabilities in hydrogenated amorphous silicon

The inhomogeneities present in hydrogenated amorphous silicon (a-Si : H) give rise to potential fluctuations. They influence the electronic properties of a-Si : H in a profound manner. Nevertheless, very few theoretical and experimental studies acknowledge their presence, because of the inherent difficulties in dealing with them. We find that the width of the potential fluctuations obtained from transport measurements is much smaller than that obtained by optical methods. This may be because the former (transport) depends on long-range potential fluctuations, whereas the latter is sensitive to short-range ones. External perturbations, e.g. light soaking and thermal quenching, are found to have different effects on the long-range potential fluctuations. Light soaking (the Staebler–Wronski effect) seems to increase them, whereas thermal quenching leaves them unchanged.     

Investigation on schottky diodes on amorphous hydrogenated silicon

Schottky barrier diodes with near-ideal characteristics have been fabricated on amorphous hydrogenated silicon prepared by decomposition of a mixture of 10% silane and 90% hydrogen. The interface properties are found to be stable up to heat treatment of 300°C. From a detailed investigation of dark and photovoltaic properties it is concluded that the density of states in the mobility gap is sufficiently small so that there is no significant carrier recombination in the space charge region.     

Electronic properties of bottom gate silicon nitride/hydrogenated amorphous silicon structures

The electronic properties of silicon nitride/hydrogenated amorphous silicon (SiN/a-Si:H) interfaces are studied with complementary techniques: quasistatic capacitance measurements achieved on c-Si/SiN/a-Si:H/Al MIS structures, and dark conductivity, steady-state photoconductivity and modulated photocurrent (MPC) experiments performed on glass/a-Si:H and glass/SiN/a-Si:H samples fitted with two coplanar Al electrodes, using the same SiN and aSi:H layers as in the MIS structures. Results of bias annealing experiments on the MIS structures are explained in the framework of the defect-pool model taking account of a fixed positive charge in the insulator, which should yield a slight electron accumulation in the a-Si:H close to the SiN/a-Si:H interface under zero bias equilibrium conditions. This electron accumulation is clearly put into evidence from the experiments carried out on the coplanar samples, where we observe that the conductivities in the dark and under illumination are much higher in presence of the bottom SiN layer. The SiN layer also induces a significant decrease of the density of states above the Fermi level determined from MPC, which also confirms the changes in the defect density stated by the capacitance measurements and in agreement with the defect-pool model predictions.     

Photovoltaic characterization of amorphous hydrogenated and chlorinated silicon films

Hydrogenated and chlorinated silicon films were used to deposit Schottky barrier solar cells. Photovoltaic characterization, together with the results of electronic transport measurements, led to the conclusion that the presence of chlorine is detrimental to the properties of this kind of device.     

Room temperature photoluminescence spectrum modeling of hydrogenated amorphous silicon carbide thin films by a joint density of tail states approach and its application to plasma deposited hydrogenated amorphous silicon carbide thin films

Room temperature photoluminescence (PL) spectrum of hydrogenated amorphous silicon carbide (a-SiC_x:H) thin films was modeled by a joint density of tail states approach. In the frame of these analyses, the density of tail states was defined in terms of empirical Gaussian functions for conduction and valance bands. The PL spectrum was represented in terms of an integral of joint density of states functions and Fermi distribution function. The analyses were performed for various values of energy band gap, Fermi energy and disorder parameter, which is a parameter that represents the width of the energy band tails. Finally, the model was applied to the measured room temperature PL spectra of a-SiC_x:H thin films deposited by plasma enhanced chemical vapor deposition system, with various carbon contents, which were determined by X-ray photoelectron spectroscopy measurements. The energy band gap and disorder parameters of the conduction and valance band tails were determined and compared with the optical energies and Urbach energies, obtained by UV-Visible transmittance measurements. As a result of the analyses, it was observed that the proposed model sufficiently represents the room temperature PL spectra of a-SiC_x:H thin films.     

Crystallization control of the amorphous buffer layer in hydrogenated nanocrystalline silicon

Hydrogenated nanocrystalline silicon film have been deposited by RF sputtering in a pure hydrogen plasma at a substrate temperature of 150°C. The unintentionally grown amorphous buffer layer was almost completely crystallized at an annealing temperature as low as 150°C. By combining infrared absorption measurements and electron microscopy observations, the critical role of dihydride species, SiH₂, in the crystallization process have been clearly evidenced and discussed.     

Stress in hydrogenated amorphous silicon determined by X-ray diffraction

The residual strain in an a-Si:H layer has been directly determined with X-ray diffraction techniques from variations in the diffraction angle of the first amorphous peak using CuK radiation. The layer was deposited by HW-CVD on glass substrates at a growth temperature of 300 °C, and is known from previous studies to be highly disordered. It was found to have an average compressive stress of 750 MPa, using the c-Si lattice parameter as a reference, and typical values of the elastic constants for a-Si:H, increasing strongly towards the surface.     

Pyrolytic transformation from polydihydrosilane to hydrogenated amorphous silicon film

The fabrication of thin film silicon devices based on solution processes rather than on conventional vacuum processes is of substantial interest since cost reductions may result. Using a solution process, we coated substrates with polydihydrosilane solution and studied the pyrolytic transformation of the material into hydrogenated amorphous silicon (a-Si:H). From thermal gravimetry and differential thermal analysis data a significant reduction in weight of the material and a construction of SiSi bonds are concluded for the pyrolysis temperature T_p = 270 to 360 °C. The appearance of amorphous silicon phonon bands in Raman spectra for films prepared at T_p ≥ 330 °C suggests the construction of a three-dimensional amorphous silicon network. Films prepared at T_p ≥ 360 °C exhibit a hydrogen content near 10 at.% and an optical gap near 1.6 eV similar to device-grade vacuum processed a-Si:H. However, the infrared microstructure factor, the spin density, and the photosensitivity require significant improvements.     

Conductivity of amorphous hydrogenated silicon in the planar and sandwich configurations

The temperature dependence of the conductivity of amorphous hydrogenated silicon in planar and sandwich configurations prepared under identical conditions is measured. The planar conductivity is measured on heat-dried samples; the sandwich conductivity is obtained from (i) the ohmic series resistance of forward-biased Schottky diodes and (ii) the ohmic conductivity of n⁺/n/n⁺ structures. We find that the conductivities for the two configurations compare favourably, thus ruling out any appreciable effect of space charge layers on the conductivity of the planar samples.