Properties Of Polycrystalline Silicon Films Research Articles

Effects of laser parameters on optoelectronic properties of polycrystalline silicon films prepared by two-step annealing process

A two-step method combining furnace and laser annealing is proposed for improving the electrical and optical properties of amorphous silicon thin films on glass substrates. It is shown that the optical transmittance of the as-deposited silicon film increases from 27 to 31% following furnace annealing at 600 °C. However, the electrical resistance is too high to be measured using a four-point probe. The sheet resistance can be reduced to 270 kΩ/□ by annealing the as-deposited film using an ultra-violet (UV) laser. However, the resistance is still too high for TFT applications. The as-deposited silicon film is first furnace annealed at 600 °C for 12 h and then annealed using a UV laser with a laser power of 41 mW and a scanning speed of 60 mm/s. The optical transmittance and sheet resistance of the annealed film are found to be 29% and 1.17 kΩ/□, respectively. The X-ray diffraction (XRD) results suggest that the improved optical and electrical properties are the result of an amorphous-to-crystalline transformation of the silicon microstructure.

Micro-structural defects in polycrystalline silicon thin-film solar cells on glass by solid-phase crystallisation and laser-induced liquid-phase crystallisation

The microstructural properties of polycrystalline silicon films obtained by either solid-phase crystallisation (SPC) or laser-induced liquid-phase crystallisation (LPC) were investigated by transmission electron microscopy (TEM). In SPC films, the most common intra-grain defects are dislocations with the density as high as 1E10cm−2 determined from cross-sectional weak-beam dark-field images. The highest dislocation density in LPC film is at least two orders of magnitude lower than the SPC film, 1E8cm−2 and typically it is below 1E6cm−2. The most common defect type in LPC films is twin boundaries and other junctions of different coincidence site lattice (CSL) boundaries. Such differences in the material structural properties result in far superior electrical performance of solar cells made of LPC films, such as mobility up to 400cm2V−1s−1, similar to c-Si wafers, and the higher open-circuit voltage up to 585mV.

Элементы твердотельной электроники на основе КНИ-структур и нитевидных кристаллов Si для криогенных температур

The paper presents the study results of electrical properties of polycrystalline silicon films in silicon-on-insulator structures and Si whiskers in the temperature range of 4,2—70 K obtained by impedance measurements in the frequency range from 10 Hz to 250 kHz and the possibility of their use in solid-state electronics, functioning at cryogenic temperatures. Characteristics of samples obtained with impedance measurements allow to predict certain specifications of reactive elements of solid state electronics based on polycrystalline and single crystalline silicon, operable at low temperatures. Using the established dependencies, separate elements in the form of solid-state electronics capacitive and inductive elements as well as a combined system in an oscillatory circuit, operable at cryogenic temperatures, have been suggested. The features of developed system depend on the structure of samples and their doping level, which allows to change the required parameters of the elements of solid state electronics in a wide range.

Influence of annealing temperature on the properties of polycrystalline silicon films formed by rapid thermal annealing of a-Si:H films

In this work, rapid thermal annealing (RTA) was employed to crystallize the amorphous silicon films deposited by hot-wire chemical vapor deposition. The influence of annealing temperature on structural and electrical properties was studied by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy and temperature-dependent conductivity measurement. The results show that the amorphous silicon films can be successfully crystallized by RTA in a very short time. The crystallinity and electrical properties of the poly-Si films was greatly improved as the RTA temperature increasing. When the temperature higher than 900 °C, the poly-Si films obtained the crystalline fraction above 95 %, and the hydrogen atoms almost disappeared in the poly-Si films. At the temperature of 1,100 °C, polycrystalline silicon films with conductivity of 16.4 S cm−1 is obtained, which is seven orders in magnitude higher than that of the film annealed at 700 °C.

Effect of Rapid Thermal Annealing Condition on the Structure and Conductivity Properties of Polycrystalline Silicon Films on Glass

The effect of rapid thermal annealing (RTA) temperature (700~1200 °C) and time (1~8 min) on the structure and conductivity properties of polycrystalline silicon (Si) films on glass, grown by RTA crystallization of magnetron sputtering (MS) deposited amorphous Si (a-Si) films, was investigated by means of X-ray diffraction (XRD) and UV reflectance. It was observed the critical temperature for crystallizing a-Si films was ~750 °C and ~700 °C based on XRD and reflectance measurements, respectively. As soon as RTA temperature reached and exceeded the critical value, the structural property of polycrystalline Si films increased with RTA temperature or time. The above results were related to the thermal and photon effects induced by RTA. Moreover, it was revealed that the resistivity of polycrystalline Si films decreased with RTA temperature, however, even the resistivity of the polycrystalline Si films annealed at 1200 °C was 2 orders of magnitude higher than that of Si target, attributed to the carrier scattering by grain boundaries or defects. The polycrystalline Si films on glass fabricated by MS deposition combined with RTA crystallization may endow them with great application potentials in Si thin-film solar cells.

Electrical properties of polycrystalline silicon films formed from amorphous silicon films by flash lamp annealing

The electrical properties of polycrystalline silicon (poly-Si) films formed from amorphous silicon (a-Si) films by flash lamp annealing (FLA) are investigated by Hall effect measurement. The impurity-doping concentration dependences of resistivity, carrier density, and Hall mobility of such flash-lamp-crystallized (FLC) poly-Si films show the effect of carrier trapping at grain boundaries (GBs). Potential barrier height formed at GBs, estimated from the temperature dependences of Hall mobility and electrical conductivity, decreases with an increase in doping concentration, due to the complete filling of trapping states at GBs. The density of trapping states at GBs is estimated to be on the order of 10 12 cm −2 from such barrier heights, which is almost equivalent to those of poly-Si films prepared by other techniques such as solid-phase crystallization or laser annealing of a-Si films.

Characterization of local electrical properties of polycrystalline silicon thin films and hydrogen termination effect by conductive atomic force microscopy

We observed local electrical properties of polycrystalline silicon films by conductive atomic force microscopy. Moreover, we investigated the effects of hydrogen termination on the polycrystalline silicon films. Before hydrogen termination, conductive regions in grain disappeared with the repeated scanning of the cantilever, while conductive regions in grain boundary almost unchanged. It is considered that hopping conduction is a major electrical conduction mechanism at grain boundaries. After 5 min hydrogen termination, locally nonterminated regions were observed near grain boundaries. This suggests that hydrogen termination of the polycrystalline silicon does not randomly progress, and there are regions that cannot be easily inactivated near grain boundaries.

Roles of SiH 4 and SiF 4 in growth and structural changes of poly-Si films

The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH 4/SiF 4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH 4 and SiF 4 radicals show that the SiH 4 radicals are important in the nucleation and growth of grains. However, SiF 4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF 4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films.

Roles of SiH 4 and SiF 4 in growth and structural changes of poly-Si films

The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH 4/SiF 4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH 4 and SiF 4 radicals show that the SiH 4 radicals are important in the nucleation and growth of grains. However, SiF 4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF 4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films.

Poly-Si films prepared by rapid thermal CVD on boron and phosphorus silicate glass coated ceramic substrates

This study investigates the crystallographic properties of polycrystalline silicon films formed on doped (boron or phosphorus) or undoped silicate glass coated ceramic substrates using the chemical vapor deposition method at high temperatures (> 1000 °C). First, FT-IR analysis of the silicate oxide layers is presented in order to monitor the presence of the Si–O, B–O and/or P–O bonds in the layers prior to deposition. Then, the average grain size, crystalline orientation and boundary defects of these poly-Si films on silicate coated mullite and alumina were evaluated as a function of deposition parameters and boron/phosphorus content. The results show a significant increase of the grain size and narrower size distribution after CVD on BSG or PSG intermediate layers, whatever is the substrate. Large grains up to 7 μm are observed for 9 μm thick films. The enhancement in grain size is attributed to the nucleation change by defects or impurities rather due to the flowability of the silicate glass during the silicon deposition at high temperature. The open-circuit voltage measurements of these fine-grained poly-silicon films before hydrogenation shows a dependence on the grain size through the choice of the substrate. The open-circuit voltage increases substantially from 250 mV to 410 mV after 1 h plasma hydrogenation, despite partial etching of the emitter by the hydrogen atoms.

Effect of hydrogen dilution on structure and optical properties of polycrystalline silicon films

Polycrystalline silicon films were prepared from SiCl4 diluted with hydrogen by plasma-enhanced chemical vapor deposition at a low temperature of 250℃. The effect of hydrogen dilution on their structure and optical properties were investigated. It was found that the effect of hydrogen on the growth behavior of the films deposited using SiCl4+H2 are completely different from that of using SiH4/H2. The crystalline fraction increases with decreasing the hydrogen dilution ratio R and reaches a maximum value of 85% at a low hydrogen dilution. However, further decreasing hydrogen dilution ratio leads the crystallinity to deteriorate and a phase transition between microcrystalline silicon and amorphous silicon is observed. The optical band gap gradually decreases from ～1.5eV to ～1.2eV and then increases up to 1.8eV with the decrease of the hydrogen dilution ratio. The deposition rate initially increases and then decreases with decreasing hydrogen dilution ratio, and no film is formed by using pure SiCl4. According to these results, we conclude that under the condition of optimum hydrogen dilution ratio, the enhanced crystalline fraction and the increase in grain size are attributed to the Cl radicals which play important roles in the low-temperature growth of crystalline silicon films.

Properties of polycrystalline silicon films obtained by rapid thermal processing for micromechanical sensors

The mechanical and structural properties of as-deposited polycrystalline Si films, grown by rapid thermal processing low-pressure chemical vapor deposition at a high deposition rate of 0.4 μm/min on oxidized Si wafer substrates, were investigated using Raman spectroscopy and transmission electron microscopy (TEM) measurements. The effect of high phosphorus doping using a spin-on technique has also been studied for applications in electrically conducting microstructures. The thermal treatment of the samples, with rapid thermal annealing at 1000 and 1050 °C, as well as with furnace annealing at 1000 °C, results in a reduction of the intra-grain defects in the films. Furthermore, spin-on-coated phosphorus was easily incorporated in the films resulting in an effective electrical activation. All films have shown an overall marginal compressive stress of about 20 MPa; however, the SiO 2, in pad form, beneath the SOI structures, introduces a small (about 10 MPa) tensile stress. Finally, cantilever and doubly supported beams, of useful length with a deflection, out of the horizontal plane, of less than 0.1 μm, were fabricated using rapid thermal processing compatible with the standard silicon microelectronics technologies.

Structural properties of polycrystalline silicon films formed by pulsed rapid thermal processing

A novel pulsed rapid thermal processing (PRTP) method has been used for realizing the solid-phase crystallization of amorphous silicon films prepared by PECVD. The microstructure and surface morphology of the crystallized films are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results indicate that this PRTP is a suitable post-crystallization technique for fabricating large-area polycrystalline silicon films with good structural qualities such as large grain size, small lattice microstain and smooth surface morphology on low-cost substrate.

Structural and electrical properties of polycrystalline silicon films deposited by hot-wire CVD

The polycrystalline silicon (poly-Si) films are deposited on low-temperature glass substrate by hot-wire CVD. The SiH 4 gas flow rate {F(SiH 4 )} is a critical parameter both for deposition rate and for crystalline properties. The poly-Si films deposited at low F(SiH 4 ) with moderate deposition rate (about 3 μm/h) have superior crystalline properties and crystalline volume fraction that exceeds 90%. The films also have natural texture structure on the surface that is strongly recommended in thin-film solar cells in order to obtain high current density by increasing incident light trapping. By increasing F(SiH 4 ) to 10 sccm, the high deposition rate (20 μm/h) is obtained, but crystalline properties deteriorated in these samples. However, these films have high potentials for thick (20-30 μm) solar cell applications due to the high deposition rate and enhanced grain growth; average grain sizes are larger than that of low-F(SiH 4 ) samples although small nano-sized grains still exist, by which crystalline volume fraction was <70%. The secondary ion mass spectroscopy and Fourier transform infrared spectroscopy analysis showed that the films have considerable amount of O and C within the films and this is originated from impurity penetration during storing the samples in atmosphere.

Grain boundary defects in RTCVD polycrystalline silicon for solar cells

The electrical and optical properties of polycrystalline silicon films are dominated by grain-boundary defects. It is therefore of particular interest to obtain columnar grain growth during the deposition process. In this work, we studied the properties of RTCVD deposited polycrystalline silicon films suitable for solar cell production. The effect of substrate temperature on texture and paramagnetically active defects of polycrystalline Si-films deposited on mono-Si substrate were studied in the temperature range 870–1240°C. Preferential grain orientation is found for all polycrystalline films in the whole range of depositing temperature, with the 〈1 1 0〉 orientation as the main texture component. The volume fraction of this texture component reaches a maximum for 1030–1100°C, while weak 〈3 1 1〉 and 〈1 1 1〉 components clearly display slight enhancement for 960°C. For deposition temperatures above 1100–1240°C the density of the main 〈1 1 0〉-texture component diminishes in favour of isotropic grain orientations. The concentration of paramagnetically active dangling bonds matches with the volume fraction of 〈1 1 0〉 oriented grains. It is shown that the spreading in growth direction causes the formation of highly defective grain boundary regions that are the main source of dangling bonds.

Physical properties of polycrystalline silicon films related to LPCVD conditions

The properties of polycrystalline silicon films have been investigated in relation to low pressure chemically vapour deposited (LPCVD) conditions, such as gas chemical content, pressure and temperature in the range of 500–615°C. A careful investigation of the 250–400-nm reflection spectra indicates a definite dependence on the temperature and growth rate for LPCVD poly-Si films. The disorder in as-deposited samples produces changes to the main peaks of optical transition in c-Si described by the complex band theory. The percentage of the disorder has been associated with changes in Γ′23–Γ15 at 365-nm and in X4–X1 at 280-nm optical transitions obtained by a deconvolution approach to UV spectra. The most disordered LPCVD poly-Si samples have been obtained at 550°C for a deposition pressure of 400 mtorr. Optical properties are discussed in relation to CVD conditions and are related to XRD data.

Free carrier optical absorption used to analyze the electrical properties of polycrystalline silicon films formed by plasma enhanced chemical vapor deposition

The electrical properties of (400) oriented polycrystalline silicon films fabricated at 300°C by 100-MHz plasma enhanced chemical vapor deposition from gaseous mixture of SiF4/H2/SiH4 are reported. A double layered structure of phosphorus-doped poly-Si/H/F film (200 nm)/undoped poly-Si/H/F film was adopted to research the changes in electrical properties in the doped layer induced by the undoped layer thickness. The carrier mobility in the crystalline grain of the doped layer, analyzed by free carrier optical absorption, increased from 10 to 35 cm2/Vs as the undoped film thickness increased from 0 to 1000 nm. The carrier density in the crystalline grain was 2.5×1020 cm−3 for each sample. The grain properties in the doped layer improved as the undoped film thickness increased.

Structural properties of polycrystalline silicon films having varied textures fabricated with intentional control of surface reactions using SiF 4/H 2/SiH 4 mixing gas

Polycrystalline silicon (poly-Si) films were fabricated from SiF 4 and H 2 gas mixtures by very high frequency (100 MHz) plasma enhanced chemical vapor deposition. The relationship between the deposition condition and film orientation was studied over a wide range of growth temperatures and SiF 4/H 2 gas flows. We found that (4 0 0) oriented films were obtained at the larger SiF 4/H 2 ratios and at temperatures higher than 150°C while (2 2 0) oriented films were obtained at other SiF 4/H 2 flow ratios. The optimized SiF 4/H 2 ratio for (4 0 0) oriented growth shifts to smaller ratios as the growth temperature decreases. (2 2 0) preferential growth is observed at high temperatures independent of the SiF 4/H 2 flow ratio. A phenomenological explanation is proposed for these preferential growths. We conclude that (2 2 0) growth requires larger thermal energy compared to those of (4 0 0) and randomly oriented growths. Also, the larger high H 2 mixing inhibits the (4 0 0) growth.

Electronic transport properties of polycrystalline silicon films deposited on ceramic substrates

In this paper we first present our results on growth of silicon onto silicon oxide and ceramic (alumina, mullite) substrates in three chemical vapor deposition (CVD) reactors using chlorosilanes as precursor gases. The effect of operational parameters (substrate temperature and gas concentration) on growth rate and grain size were analyzed using scanning electron microscopy (SEM) technique. We show that deposition rates up to 5μm/min can be reached. Furthermore, preferential (up to 80%) (220) oriented structure with grain size ranging from 0.5 to 10μm can be generated for a 10μm thick Si film. In the second part we report on electronic properties of the deposited poly-Si films through the layer resistivity, open-circuit voltage of photovoltaic devices, minority carrier diffusion length and spectral response data. We show that the grain size and the post-hydrogenation treatment affect mostly the open-circuit voltage and weakly the spectral response.

Effects of inductively coupled plasma oxidation on the properties of polycrystalline silicon films and thin film transistors

We investigated the effects of inductively coupled plasma (ICP) oxidation on the properties of polycrystalline silicon (poly-Si) films and thin film transistors (TFTs). The ICP oxidation in oxygen plasma passivated the dangling bonds in the poly-Si films, not by oxygen incorporation but by hydrogen incorporation; but the incorporated hydrogen diffused out during the TFT fabrication, so that the effect of the dangling bond passivation was not obtained in the TFT. The ICP oxidation did not remove the intragranular defects such as microtwins and stacking faults, but it reduced the interface trap density and also improved the performance of the poly-Si TFT. The field effect mobility of TFT with an ICP oxide and low-pressure chemical vapor deposited (LPCVD) oxide double layer was 30.6 cm2/V s, while that of TFT with only a LPCVD oxide was 17.2 cm2/V s.