Unhydrogenated Amorphous Silicon Research Articles

Optical Characterization of H-Free a-Si Layers Grown by rf-Magnetron Sputtering by Inverse Synthesis Using Matlab: Tauc–Lorentz–Urbach Parameterization

Several, nearly-1-µm-thick, pure, unhydrogenated amorphous-silicon (a-Si) thin layers were grown at high rates by non-equilibrium rf-magnetron Ar-plasma sputtering (RFMS) onto room-temperature low-cost glass substrates. A new approach is employed for the optical characterization of the thin-layer samples, which is based on some new formulae for the normal-incidence transmission of such a samples and on the adoption of the inverse-synthesis method, by using a devised Matlab GUI environment. The so-far existing limiting value of the thickness-non-uniformity parameter, Δd, when optically characterizing wedge-shaped layers, has been suppressed with the introduction of the appropriate corrections in the expression of transmittance. The optical responses of the H-free RFMS-a-Si thin films investigated, were successfully parameterized using a single, Kramers–Krönig (KK)-consistent, Tauc–Lorentz oscillator model, with the inclusion in the model of the Urbach tail (TLUC), in the present case of non-hydrogenated a-Si films. We have also employed the Wemple–DiDomenico (WDD) single-oscillator model to calculate the two WDD dispersion parameters, dispersion energy, Ed, and oscillator energy, Eso. The amorphous-to-crystalline mass-density ratio in the expression for Ed suggested by Wemple and DiDomenico is the key factor in understanding the refractive index behavior of the a-Si layers under study. The value of the porosity for the specific rf-magnetron sputtering deposition conditions employed in this work, with an Ar-pressure of ~4.4 Pa, is found to be approximately 21%. Additionally, it must be concluded that the adopted TLUC parameterization is highly accurate for the evaluation of the UV/visible/NIR transmittance measurements, on the H-free a-Si investigated. Finally, the performed experiments are needed to have more confidence of quick and accurate optical-characterizations techniques, in order to find new applications of a-Si layers in optics and optoelectronics.

Spectroscopic ellipsometry study of non-hydrogenated fully amorphous silicon films deposited by room-temperature radio-frequency magnetron sputtering on glass: Influence of the argon pressure

The complex dielectric functions of unhydrogenated amorphous silicon (a-Si) thin films, grown by the radio-frequency magnetron-sputtering (RFMS) technique, at room temperature, upon glass substrates, have been accurately determined by using ex-situ variable-angle spectroscopic ellipsometry, in the photon energy range from 0.73 to 4.96 eV (i.e. 1700 to 250 nm). Mass density of sputter-deposited amorphous silicon films was changed by a simple argon-working-gas pressure control: higher Ar gas pressure results in reduced film mass density, or increased porosity. The optical response of the hydrogen-less amorphous silicon films under study can be successfully parameterized employing a single, Kramers-Kronig (K-K)-consistent Cody-Lorentz (CL) oscillator dispersion model; the Urbach absorption tail included in the CL model, is certainly needed in the present case of hydrogen-free sputtered a-Si films. It has been unambiguously demonstrated that the obtained values of CL-model parameters are reasonably correlated with the particular argon-gas sputtering pressure utilized for each deposition. The behavior of these best-fit CL-model parameters with varying argon pressure from 0.13Pa up to 4.4Pa, can be clearly understood in terms of the effect of the existing micro-voids, within the as-deposited a-Si film material.

Structural, optical and electrical properties of Ti doped amorphous silicon prepared by co-sputtering

Titanium-doped unhydrogenated amorphous silicon (a-Si: Ti) films were prepared by rf co-sputtering. Structural, optical and electrical properties of a-Si: Ti as a function of Ti content were investigated by RBS(Rutherford Backscattering Spectrometry), XPS(X-ray Photoelectron Spectroscopy), Raman, ESR (Electron Spin Resonance), Spectroscopic Ellipsometry, thermoelectric tests and temperature dependent I–V tests. Introducing Ti impurity in amorphous Si results in n-type material, room temperature conductivity increases more than three orders of magnitude and activation energy decreases obviously. When Ti content is lower than 2 at.%, the degree of short-range order and defect state density of amorphous Si network improve while the optical bandgap changes little. However, with Ti content higher than 2 at.%, the degree of short-range order decreases and the optical gap of a-Si shrinks. The role of Ti atoms in amorphous Si network is discussed. ESR results confirm that introducing low content of Ti can compensate the dangling bonds of amorphous Si, while with Ti content higher than 2%, most Ti atoms compensate the unpaired electrons in the conduction band tail.

Microstructure Characterization of Amorphous Silicon Films by Effusion Measurements of Implanted Helium

ABSTRACTAn important property of thin film silicon and related materials is the microstructure which may involve the presence of interconnected and isolated voids. We report on effusion measurements of implanted helium (He) to detect such voids. Several series of hydrogenated and unhydrogenated amorphous silicon films prepared by the methods of plasma deposition, hot wire deposition and vacuum evaporation were investigated. The results show common features like a He effusion peak at low temperatures attributed to He out-diffusion through a compact material or through interconnected voids, and a He effusion peak at high temperatures attributed to He trapped in isolated voids. While undoped plasma-grown device-grade hydrogenated amorphous silicon (a-Si:H) films show a rather low concentration of such isolated voids, its concentration can be rather high in doped a-Si:H, in unhydrogenated evaporated material and others.

Effect of stress on the aluminum-induced crystallization of hydrogenated amorphous silicon films

The effect of stress, resulting from the presence of hydrogen, on the aluminum-induced crystallization of hydrogenated amorphous silicon films was studied. Layered thin films of hydrogenated and unhydrogenated amorphous silicon and aluminum, deposited by sputtering, were used to study this effect. The stress of the deposited films was determined by measuring the radius of curvature of c-Si substrates before and after deposition of the films. It was observed that unhydrogenated amorphous silicon films exhibit a high compressive stress compared with hydrogenated ones. The amount of stress is shown to decrease with increasing hydrogen content. It was also observed that aluminum always provides tensile stress. After the initial stress measurements, all the samples were annealed for 30 min at temperatures between 200 °C and 400 °C. X-ray diffraction was used to determine the crystallinity of the silicon films. The results of the study show that the temperature at which crystallization of amorphous silicon is initiated is lower for films with a lower initial stress.

Hydrogen-induced modification of the medium-range structural order in amorphous silicon films

We use fluctuation electron microscopy to determine changes in the medium-range structural order of un-hydrogenated amorphous silicon thin films after they are exposed to atomic hydrogen at a substrate temperature of 230 °C. The films are deposited by magnetron sputtering at either 230 or 350 °C substrate temperature to obtain starting states with small or large initial medium-range order, respectively. The in-diffusion of atomic hydrogen causes the medium-range order to decrease for the small initial order but to increase for the large initial order. We suggest that this behavior can be understood in terms of classical nucleation theory: The ordered regions of small diameter are energetically unstable and can lower their energy by evolving towards a continuous random network, whereas the ordered regions of large diameter are energetically stable and can lower their energy by coarsening towards the nanocrystalline state.

Evidence from atomistic simulations of fluctuation electron microscopyfor preferred local orientations in amorphous silicon

Simulations from a family of atomistic structural models for unhydrogenated amorphous silicon suggest that fluctuation electron microscopy experiments have observed orientational order of paracrystalline grains in amorphous silicon. This order may consist of correlations in the orientation of nearby paracrystalline grains or anisotropy in the grain shape. This observation makes a natural connection to the known growth modes of microcrystalline silicon and may be useful for other materials systems.

ICP ETCHING OF RF SPUTTERED AND PECVD SILICON CARBIDE FILMS

In this paper, we report the Inductive Coupled Plasma (ICP) etching of RF sputtered unhydrogenated amorphous silicon carbide film (a-Si0.5C0.5) , and plasma enhanced chemical vapour deposited (PECVD) hydrogenated amorphous silicon carbide films (a-Si0.3C0.7:H and a-Si0.7C0.3:H) , asprepared and annealed, using CF4/O2 chemistry. The etch rate of amorphous SiC is observed to be closely related to the hydrogen content. The mechanism of etching is studied by varying the RIE power, ICP power and pressure. It has been suggested that the removal of surface polymers (CFx) is a key factor in the etching process. In order to study the difference in etch rate between sputtered and PECVD SiC and the effects of changing Si/C ratio, IR spectra are used to reveal the bonds density in the samples.

Nano-oxidation of an amorphous silicon surface with an atomic force microscope

A surface anodization technique was applied to an amorphous silicon (a-Si) surface by means of an atomic force microscope (AFM). The oxide line height increased with increasing applied voltage. The height of these lines on the un-hydrogenated amorphous silicon (a-Si:H) film was greater than that on crystalline silicon (c-Si) due to the large defect density in the former. Although the height of the oxide lines did not depend on film thickness, their width increased with decreasing film thickness. The photoluminescence (PL) intensity of the nano-scale lattice pattern drawn on a-Si:H was measured by micro-scale PL equipment. The PL intensity from the area having narrow oxide lines was smaller than that from the non-oxidized area. This indicates that anodization affects not only the area observed by the AFM, but its effects also spread over a sub-micron region.

Comparative EPR study of hydrogenated and unhydrogenated amorphous silicon carbide thin films

In this work, electron paramagnetic resonance (EPR) is used to study paramagnetic centers in stoichiometric hydrogenated a-SiC:H synthesized by plasma enhanced chemical vapor deposition (PECVD) and unhydrogenated a-SiC grown by pulsed laser deposition. While the spin density of the a-SiC films was determined to be of the order of 10 20 spins/cm 3, it was found to be more than one order of magnitude lower (10 19 spins/cm 3) for the as-deposited a-SiC:H films and decreases to less than 10 17 spins/cm 3 upon annealing at 650 °C. For both types of films, the dominant paramagnetic centers in the films are carbon related defects, as deduced from the g-value of the EPR signal. The PLD a-SiC films have a symmetric exchange narrowed Lorentzian EPR line that indicates some clustering of the spins. For a-SiC:H films, the EPR shows some asymmetry that is probably due to the presence of silicon related defects in the layers. No evidence of spin clustering was found for a-SiC:H and the EPR linewidth is most likely determined by dipolar interaction and some unresolved hydrogen hyperfine structure for the as-deposited and annealed a-SiC:H films, respectively. The temperature dependence of the EPR linewidth for both types of films and the power saturation behavior of the EPR signal are also presented and discussed.

Investigation of hydrogen induced phase transition from a-Si:H to μc-Si:H using real time infrared spectroscopy

In this paper, we present the use of real time process monitors to study hydrogen induced phase transition from hydrogenated amorphous silicon (a-Si:H) to hydrogenated microcrystalline silicon (μc-Si:H) films. We determine the H bonding configurations and H concentration in the film using a real time, high sensitivity infrared reflectance spectroscopy. Spectroscopic ellipsometry is used to determine whether the film is amorphous or crystalline, and to measure the film thickness. The Si–H bonding and film microstructure are monitored in three sets of experiments: during deposition of μc-Si:H using DC reactive magnetron sputtering; exposure of unhydrogenated amorphous silicon (a-Si) to atomic hydrogen; exposure of a-Si to hydrogen plasma. In the first experiment, a transition layer of a-Si:H is deposited before μc-Si:H growth starts. Results show no evidence of enhanced etching during μc-Si:H growth compared to a-Si:H deposition under same processing conditions. In the second experiment, hydrogen coverage of the a-Si:H surface drops at higher temperatures due to thermal desorption. Lastly, when exposed to hydrogen plasma a-Si film is first hydrogenated, and then it transforms to microcrystalline phase with a concomitant decrease in H content. Based on these results, we conclude that the role of H is to assist in the nucleation of μc-Si phase and provide enough surface coverage to facilitate the diffusion of surface species. We also suggest that introduction of hydrogen treatment (using atomic hydrogen/hydrogen plasma) during the initial growth of μc-Si:H on glass or oxide substrates will be able to eliminate the a-Si:H transition layer. Role of the real time process monitors will be crucial in developing such processes.

Noise characteristics of radio frequency sputtered amorphous silicon carbide films

Noise measurements of radio frequency sputtered hydrogenated (a-SiC:H) and unhydrogenated (a-SiC) amorphous silicon carbide films have been carried out. Two Lorenztian components were found in the noise spectra of the a-SiC:H and a-SiC films. Discrete traps, created by the sputtering process, were suggested to be responsible for the Lorenztian spectra observed. As only a relatively small amount of hydrogenation was achieved in our a-SiC:H films, passivation of the discrete traps was therefore not significant. This accounts for the Lorenztian spectra of the a-SiC:H films. We propose that furnace annealing has reduced the discrete traps substantially so that only the 1/f noise was observed in the annealed samples.

Observation of a conducting filament within unhydrogenated amorphous silicon

Random telegraph switching (RTS) noise in unhydrogenated amorphous silicon was observed and analysed. Switching is observed as a step-like fluctuation in the material resistance and is shown to result from a filament-like microstructure where intermittent carrier transport is dictated by a bistable defect. One state allows conduction through the filament while the other state inhibits transport. Analysis of the RTS signal over a range of temperature and current provides the opportunity to examine the transport properties of the microstructure. The results show that this filament has a smaller temperature coefficient of resistance than the bulk material.

Grain growth in laser dehydrogenated and crystallized polycrystalline silicon for thin film transistors

Selective dehydrogenation and crystallization are realized by a three-step incremental increase in laser energy density. X-ray diffraction and transmission electron microscopy show that the polycrystalline grains formed with this three-step process are similar to those after a conventional one-step laser crystallization of unhydrogenated amorphous silicon. The grain size increases with increasing laser energy density up to a peak value of a few micrometers. The grain size decreases with further increases in laser energy density. The transistor field effect mobility is correlated to the material properties, increasing gradually with laser energy density until reaching its maximum value. Thereafter, the transistors suffer from leakage through the gate insulators. A dual dielectric gate insulator has been developed for these bottom-gate thin film transistors. Our structure simplifies fabrication of both high quality amorphous and polycrystalline thin film transistors on the same glass substrate. We discuss the application of this process for producing hybrid amorphous and polycrystalline silicon thin film transistors from hydrogenated amorphous silicon on glass substrates.

Considerations for Large Area Fabrication Of Integrated a-Si and Poly-Si TFTs

AbstractThe combination of a-Si low leakage pixel TFTs with poly-Si TFTs in peripheral circuits provides an excellent method for reducing the number of external connections to large-area imaging arrays and displays. To integrate the fabrication of the peripheral poly-Si TFTs with the a-Si pixel TFTs, we have developed a threestep laser process which enables selective crystallization of PECVD a-Si:H. X-ray diffraction and transmission electron microscopy show that the polycrystalline grains formed with this three-step process are similar to those crystallized by a conventional one step laser crystallization of unhydrogenated amorphous silicon. The grain size increases with increasing laser energy density up to a peak value of a few microns. The grain size decreases with further increases in laser energy density. The transistor field effect mobility is correlated with the grain size, increasing gradually with laser energy density until reaching its maximum value. Thereafter, the transistors suffer from leakage through the gate insulators. A dual dielectric gate insulator has been developed for these bottom-gate thin film transistors to provide the correct threshold voltages for both a-Si and poly-Si TFTs.

Considerations for Large Area Fabrication of Integrated a-Si and Poly-Si TFTs

ABSTRACTThe combination of a-Si low leakage pixel TFTs with poly-Si TFTs in peripheral circuits provides an excellent method for reducing the number of external connections to large-area imaging arrays and displays. To integrate the fabrication of the peripheral poly-Si TFTs with the a-Si pixel TFTs, we have developed a three-step laser process which enables selective crystallization of PECVD a-Si:H. X-ray diffraction and transmission electron microscopy show that the polycrystalline grains formed with this three-step process are similar to those crystallized by a conventional one step laser crystallization of unhydrogenated amorphous silicon. The grain size increases with increasing laser energy density up to a peak value of a few Microns. The grain size decreases with further increases in laser energy density. The transistor field effect mobility is correlated with the grain size, increasing gradually with laser energy density until reaching its maximum value. Thereafter, the transistors suffer from leakage through the gate insulators. A dual dielectric gate insulator has been developed for these bottom-gate thin film transistors to provide the correct threshold voltages for both a-Si and poly-Si TFTs.

Ellipsometry and x-ray photoelectron spectroscopy study of SnO2 reduction at the interface with sputtered a-Si:H

Very thin layers of hydrogenated and unhydrogenated amorphous silicon have been deposited on tin oxide substrates under different temperatures and H2 partial pressures by dc magnetron sputtering. The deposition processes are monitored by real time in situ ellipsometry. We observe the reduction reaction of the SnO2 exposed to a H- or Si-containing deposition flux in the early stages of film growth by real time ellipsometry. The chemical states of Sn at the a-Si:H (or a-Si)/SnO2 interface have been studied by x-ray photoelectron spectroscopy (XPS). XPS confirms that a Si flux alone can reduce SnO2 to elemental Sn, and that this reaction is temperature dependent. However, the contribution of Si is secondary to that of H atoms in the reduction reaction.

Dependence of Thermally Induced Metastability on Hydrogen Content in Amorphous Silicon

The thermally induced metastability was observed in unhydrogenated doped amorphous silicon prepred by Low Pressure Chemical Vapour Deposition (LPCVD). The equilibration temperature (T*) was found to be 380°± 20°C. The preliminary study of the influence of hydrogen content on thermally induced metastability in amorphous silicon is reported. In the hydrogenated doped films the equilibration temperature changes from 380° ± 20°C to 170° ± 10°C, as H content increases to 12 at.%.

Influence of hydrogen on the structure and surface morphology of pulsed ArF excimer laser crystallized amorphous silicon thin films

In this study, we compare the microstructure and surface morphology of hydrogenated and unhydrogenated amorphous silicon thin films deposited onto amorphous substrates (glass, SiO 2) after pulsed ArF excimer laser crystallization with energy densities ranging typically between 60 and 600 mJ/cm 2. Characterization of the films is performed by optical reflectivity, profilometry (Talystep), elastic recoil detection analysis, grazing X-ray diffraction and transmission electron microscopy. The amorphous-to-polycrystalline transition of amorphous silicon with and without hydrogen is achieved at a threshold energy density of around 80–100 mJ/cm 2. Above this value both the microstructure and surface morphology of the crystallized layers appear to be strongly related to the hydrogen content in the as-deposited films.

Annealing of Irreversible Defects in Hydrogenated and Unhydrogenated Amorphous Silicon Thin Films

ABSTRACTWe have prepared unhydrogenated and hydrogenated ([H] = 14 at.%) amorphous silicon thin films using magnetron sputtering with substrate temperature, TS = 40°C. After deposition, the films were annealed at temperatures between 150 and 200°C and conductivity was measured as a function of anneal time. We find that for that for both materials, the conductivity changes non-exponentially with annealing time. The characteristic time constant for annealing at 175°C is approximately the same in unhydrogenated films (τ≈100min) as found in films containing 14 atomic % hydrogen (τ≈200min).