Polycrystalline Silicon Films Research Articles

Structural properties of a-Si films and their effect on aluminum induced crystallization

In this paper, we report the influence of the structural properties of amorphous silicon (a-Si) on its subsequent crystallization behavior via the aluminum induced crystallization (AIC) method. Two distinct a-Si deposition techniques, electron beam evaporation and plasma enhanced chemical vapor deposition (PECVD), are compared for their effect on the overall AIC kinetics as well as the properties of the final poly-crystalline (poly-Si) silicon film. Raman and FTIR spectroscopy results indicate that the PECVD grown a-Si films has higher intermediate-range order, which is enhanced for increased hydrogen dilution during deposition. With increasing intermediate-range order of the a-Si, the rate of AIC is diminished, leading larger poly-Si grain size.

The formation of poly-Si films on flat glass substrates by flash lamp annealing

We have succeeded the formation of polycrystalline silicon (poly-Si) films by flash lamp annealing (FLA) of 4-μm-thick intrinsic amorphous silicon (a-Si(i)) films deposited directly on flat glass substrates by tuning catalytic chemical vapor deposition conditions. The use of a-Si(i) films deposited without intentional substrate heating leads to the suppression of Si film peeling during FLA. The a-Si(i) films deposited at room temperature have low film density, low film stress, and high defect density, compared to a-Si(i) films deposited at higher temperatures. The prevention of Si film peeling may be due to the low film stress and/or the suppression of the emergence of lateral explosive crystallization by using a-Si(i) films with low film density.

Cross-Plane Phonon Conduction in Polycrystalline Silicon Films

Silicon films of submicrometer thickness play a central role in many advanced technologies for computation and energy conversion. Numerous thermal conductivity data for silicon films are available in the literature, but they are mainly for the lateral, or in-plane, direction for both polycrystalline and single crystalline films. Here, we use time-domain thermoreflectance (TDTR), transmission electron microscopy, and semiclassical phonon transport theory to investigate thermal conduction normal to polycrystalline silicon (polysilicon) films of thickness 79, 176, and 630 nm on a diamond substrate. The data agree with theoretical predictions accounting for the coupled effects of phonon scattering on film boundaries and defects related to grain boundaries. Using the data and the phonon transport model, we extract the normal, or cross-plane thermal conductivity of the polysilicon (11.3 ± 3.5, 14.2 ± 3.5, and 25.6 ± 5.8 W m−1 K−1 for the 79, 176, and 630 nm films, respectively), as well as the thermal boundary resistance between polysilicon and diamond (6.5–8 m2 K GW−1) at room temperature. The nonuniformity in the extracted thermal conductivities is due to spatially varying distributions of imperfections in the direction normal to the film associated with nucleation and coalescence of grains and their subsequent columnar growth.

Impact of repeated uniaxial mechanical strain on p-type flexible polycrystalline thin film transistors

This letter investigates the effect of repeated bending of flexible p-channel low-temperature polycrystalline–silicon thin-film transistors employing an ultra-low-temperature process (<673 K). Experimental results reveal that interface state density (Nit) and grain boundary trap density (Ntrap) after 10 000 width-axis tensile strain bending iterations are more pronounced than after equivalent width-axis compressive strain bending. Extracted interface and grain boundary traps both increase, which elevate trap assisted leakage. Furthermore, the bending distorts the Si–Si bonds in the polycrystalline silicon (Poly-Si) film, which causes more significant negative bias temperature instability (NBTI) degradation because strain-induced weak Si–Si bonds can react with dissociated H during NBTI stress.

(Invited) High-Performance Top-Gate and Double-Gate Low-Temperature Polycrystalline-Silicon Thin-Film Transistors Fabricated Using Continuous-Wave-Laser Lateral Crystallization on a Glass Substrate

A high-quality polycrystalline silicon (poly-Si) film is a technology for improving the performance of low-temperature (LT) poly-Si thin-film transistors (TFTs). We have already developed continuous-wave-laser lateral crystallization (CLC) using a diode-pumped solid-state laser to improve the crystalline quality of the thin poly-Si film. In this paper, we report on two approaches for improving the performance of LT poly-Si TFTs, which are based on a thin CLC poly-Si film and are fabricated at 550 ºC on a glass substrate. The first approach is a combination of a CLC poly-Si film and sputtered HfO2 as the gate dielectric. The other approach is a combination of a CLC poly-Si film and a double-gate structure for realizing four-terminal LT poly-Si TFTs. The former exhibited an enhancement in the on-current that was four times greater than a device with a silicon dioxide gate stack. In the latter, high controllability of the threshold voltage was confirmed.

Etching characteristics of hydrogenated amorphous silicon and poly crystalline silicon by hydrogen hyperthermal neutral beam

A hydrogen hyperthermal neutral beam (HNB) generated by an inclined slot-excited antenna electron cyclotron resonance plasma source has been used to etch hydrogenated amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) films. In this work, we present selective etching of a-Si:H with respect to poly-Si by hydrogen plasma and hydrogen HNB under various substrate temperatures, gas pressures, and bias voltages of the neutralizer. We have observed that the etch rate of a-Si:H is considerably higher than that of poly-Si. The etch rate is largely dependent upon the substrate temperature. In this experiment, the optimal substrate temperature for improving the etch rate is approximately at 150°C. The root mean square surface roughness of the etched material reaches a maximum at 150°C and decreases rapidly. The etch rate of poly-Si is not sensitive to changes in the experimental condition, such as the substrate temperatures and gas pressures. However, as the hydrogen HNB energy is increased, the etch rate of poly-Si also increases gradually. The hydrogen HNB energy contributes in improving the etch rate of a-Si:H and poly-Si films.

Effect of Secondary Annealing on the Electrical Properties of Polysilicon Thin Films

In this work, we study the electrical characteristics of polysilicon (resistivity, free carrier concentration and Hall mobility of carriers), depending on the annealing temperature of implantation for films of polycrystalline silicon having undergone a long heat treatment after implantation, and also the secondary annealing temperature (900-1100 ∘C) of longer periods. The results have shown that the longer periods of secondary annealing have permitted the improvement of electrical characteristics of our films (decrease of the resistivity, increase of the free carriers concentration and the carriers Hall mobility).

Effect of hydrogen on low temperature epitaxial growth of polycrystalline silicon by hot wire chemical vapor deposition

Polycrystalline silicon (poly-Si) films were prepared by hot-wire chemical vapor deposition (HWCVD) at a low substrate temperature of 525 °C. The influence of hydrogen on the epitaxial growth of ploy-Si films was investigated. Raman spectra show that the poly-Si films are fully crystallized at 525 °C with a different hydrogen dilution ratio (50%–91.7%). X-ray diffraction, grazing incidence X-ray diffraction and SEM images show that the poly-Si thin films present (100) preferred orientation on (100) c-Si substrate in the high hydrogen dilution condition. The P-type poly-Si film prepared with a hydrogen dilution ratio of 91.7% shows a hall mobility of 8.78 cm2/(V·s) with a carrier concentration of 1.3 × 1020 cm−3, which indicates that the epitaxial poly-Si film prepared by HWCVD has the possibility to be used in photovoltaic and TFT devices.

Geometric Design of Microbolometers Made From CMOS Polycrystalline Silicon

In this paper, microbolometers with different geometric designs were fabricated using polycrystalline silicon (polysilicon) films from TSMC 0.35 μm (D35 type) and 0.18 μm (T18 type). D35 microbolometers with high-resistance films showed negative temperature coefficient of resistance (TCR) values, whereas T18 microbolometers with low-resistance films showed positive TCR values. Results indicate that conventional designs with large areas of infrared absorbers can be optimized using uniform suspended arms throughout the microbolometer in the two types of devices. Finite element modeling indicates that the heat flux of microbolometers with uniform beams is evenly distributed and thus results in improved TCR values.

Formation and characterization of silver nanoparticles embedded in optical transparent materials for plasmonic sensor surfaces

• We analyzed particle forming behavior of thin Ag seed films before/after annealing. • We examined passivation layers produced via different deposition methods. • Ag particles embedded in Al 2 O 3 films were tested as LSPR-sensor-surface. • LSPR shifted 6.15 nm for a refractive index change of 0.07 using glucose solutions. Plasmonic nanostructures promise sensing capabilities with the potential for ultrasensitive and robust assays in life sciences and biomedicine. Silver island films represent an interesting and straightforward alternative for the implementation of substrate-attached plasmonic nanostructures. The temperature-induced particle-forming behavior of thin silver seed films deposited on glass substrates and on polycrystalline silicon films is represented. The measured extinction spectra reflect the different size distributions and shapes. The covering of the particles with different optical transparent film materials like ZnO, Al 2 O 3 , SiN x , and SiO x leads to a further shift in the resonance maximum due to their refractive index. The SiO x system shows an additional long term change in the extinction spectrum in contrast to ZnO, Al 2 O 3 , and SiN x . Thin silver films covered with Al 2 O 3 were used in order to proof the system as a sensor element for analyte detection (glucose solution).

Formation of polycrystalline-silicon films with hemispherical grains for capacitor structures with increased capacitance

The effect of formation conditions on the morphology of silicon films with hemispherical grains (HSG-Si) obtained by the method of low-pressure chemical vapor deposition (LPCVD) is investigated by atomic-force microscopy. The formation conditions for HSG-Si films with a large surface area are found. The obtained HSG-Si films make it possible to fabricate capacitor structures, the electric capacitance of which is twice as large in comparison to that of capacitors with “smooth” electrodes from polycrystalline silicon.

Role of Al2O3 intermediate layer for improving the quality of polycrystalline-silicon film in inverted aluminum-induced layer exchange

A thin Al2O3 intermediate layer prepared by atomic layer deposition was introduced into inverted aluminum-induced layer exchange (inverted-ALILE) to form high-quality polycrystalline silicon (poly-Si) thin layer. It was demonstrated that the continuity and quality of poly-Si were obviously improved by the Al2O3 layer. The fraction of (100)-oriented crystals reached 93%, and the average grain size of 28μm with uniform surface morphology and low defect density were achieved at the optimal Al2O3 thickness of 4nm. It was also found that an a-AlOx layer always existed at the poly-Si/Al interface after inverted-ALILE process, which is independent on the original surface states. The results suggested that the thin poly-Si layer would be a promising epitaxial template for Si based thin film solar cells.

Synthesis and characterization of large-grain solid-phase crystallized polycrystalline silicon thin films

n-type polycrystalline silicon (poly-Si) films with very large grains, exceeding 30 μm in width, and with high Hall mobility of about 71.5 cm2/V s are successfully prepared by the solid-phase crystallization technique on glass through the control of the PH3 (2% in H2)/SiH4 gas flow ratio. The effect of this gas flow ratio on the electronic and structural quality of the n-type poly-Si thin film is systematically investigated using Hall effect measurements, Raman microscopy, and electron backscatter diffraction (EBSD), respectively. The poly-Si grains are found to be randomly oriented, whereby the average area weighted grain size is found to increase from 4.3 to 18 μm with increase of the PH3 (2% in H2)/SiH4 gas flow ratio. The stress in the poly-Si thin films is found to increase above 900 MPa when the PH3 (2% in H2)/SiH4 gas flow ratio is increased from 0.025 to 0.45. Finally, high-resolution transmission electron microscopy, high angle annular dark field-scanning tunneling microscopy, and EBSD are used to identify the defects and dislocations caused by the stress in the fabricated poly-Si films.

Multiscale modelling framework for the fracture of thin brittle polycrystalline films: application to polysilicon

Micro-electro-mechanical systems (MEMS) made of polycrystalline silicon are widely used in several engineering fields. The fracture properties of polycrystalline silicon directly affect their reliability. The effect of the orientation of grains on the fracture behaviour of polycrystalline silicon is investigated out of the several factors. This is achieved, firstly, by identifying the statistical variation of the fracture strength and critical strain energy release rate, at the nanoscopic scale, over a thin freestanding polycrystalline silicon film having mesoscopic scale dimensions. The fracture stress and strain at the mesoscopic level are found to be closely matching with uniaxial tension experimental results. Secondly, the polycrystalline silicon film is considered at the continuum MEMS scale, and its fracture behaviour is studied by incorporating the nanoscopic scale effect of grain orientation. The entire modelling and simulation of the thin film is achieved by combining the discontinuous Galerkin method and extrinsic cohesive law describing the fracture process.

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.

Growth and electrical properties of in situ phosphorus-doped polycrystalline silicon films using Si3H8 and PH3

In situ phosphorus-doped polycrystalline silicon (polysilicon) films grown on silicon oxide layers using trisilane (Si3H8) and phosphine (PH3) as precursors are investigated as a function of the Si3H8/PH3 gas flow ratio and the growth temperature. At a high flow rate for Si3H8 in the temperature range of 600–700 °C, the deposition process is controlled by the rate of desorption of hydrogen molecules on the surface, which has an activation energy of 1.13 eV. For a low Si3H8 flow rate at growth temperatures >650 °C, however, the deposition is limited by the diffusion of Si3H8 gas to the surface. The presence of phosphorus decreases the crystallization temperature of the polysilicon layers during growth. In addition, the ratio of phosphorus incorporated into the polysilicon decreases with increasing growth temperature because of the increase in the growth rate. The resistivity of the phosphorus-doped polysilicon films decreases with increasing deposition temperature at the same phosphorus concentration, indicating that the use of a high growth temperature results in an enhancement in the activation of phosphorus in the polysilicon films during growth.

High-performance poly-Si thin film transistors with highly biaxially oriented poly-Si thin films using double line beam continuous-wave laser lateral crystallization

Highly biaxially oriented poly-Si thin films were formed by double-line beam continuous-wave laser lateral crystallization (DLB-CLC). The crystallinities of the DLB-CLC poly-Si thin films were (110), (111), and (211) for the laser scan, transverse, and surface directions, respectively, and an energetically stable Σ3 grain boundary was observed to be dominant. All silicon grains were elongated in the laser scan direction and one-dimensionally very large silicon grains with lengths of more than 100 µm were fabricated. Using these biaxially oriented polycrystalline silicon (poly-Si) films, low-temperature poly-Si TFTs (LTPS-TFTs) were fabricated at low temperatures (≦550 °C) by a metal gate self-aligned process. As a result, a TFT with a high electron field effect mobility of μFE = 450 cm2 V−1 s−1 in a linear region was realized.

An Improved Matlab-Simulink Model of PV Module considering Ambient Conditions

A photovoltaic (PV) model is proposed on Matlab/Simulink environment considering the real atmospheric conditions and this PV model is tested with different PV panels technologies (monocrystalline silicon, polycrystalline silicon, and thin film). The meteorological data of Istanbul—the location of the study—such as irradiance, cell temperature, and wind speed are taken into account in the proposed model for each technology. Eventually, the power outputs of the PV module under real atmospheric conditions are measured for resistive loading and these powers are compared with the results of proposed PV model. As a result of the comparison, it is shown that the proposed model is more compatible for monocrystal silicon and thin-film modules; however, it does not show a good correlation with polycrystalline silicon PV module.

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.

Improving the quality factor of polycrystalline Si thin-film micromechanical resonators by metal-induced lateral crystallization using biomineralized Ni nanoparticles

We employed metal-induced lateral crystallization (MILC), using Ni nanoparticles synthesized within cage-shaped protein molecules, to crystallize an amorphous Si film into a polycrystalline film. This process allowed us to choose the crystallization sites of the polycrystalline silicon (poly-Si) film and enlarged the average grain size. We fabricated cantilever resonators from the poly-Si film to characterize them and extract quality factors. The reference resonator, fabricated without MILC, had a quality factor of 12 100, while the resonator crystallized along the axial direction had a quality factor of 26 200. Two-fold increase was achieved by tuning the crystal structure of the poly-Si film.