Hydrogenated Silicon Thin Films Research Articles

Observation of electrically detected electron nuclear double resonance in amorphous hydrogenated silicon films

We report on the electrical detection of electron nuclear double resonance (EDENDOR) through spin-dependent tunneling transport in an amorphous hydrogenated silicon thin film. EDENDOR offers a many orders of magnitude improvement over classical ENDOR and is exclusively sensitive to paramagnetic defects involved in electronic transport. We observe hyperfine interactions with 1H nuclei very close to silicon dangling bond defects. These observations substantially extend recent EDENDOR observations involving silicon vacancy defects and 14N hyperfine interactions with fairly distant nitrogen atoms in 4H-SiC bipolar junction transistors. We have improved the detection scheme utilized in the earlier study by combining magnetic field modulation with RF amplitude modulation; this combination significantly improves the operation of the automatic power leveling scheme and the overall sensitivity.

Analysis of surface adsorption kinetics of SiH4 and Si2H6 for deposition of a hydrogenated silicon thin film using intermediate pressure SiH4 plasmas

For the future mass production of silicon-based devices, the deposition process of hydrogenated silicon (Si:H) thin films must not only satisfy several important material properties, it must also be highly productive. Therefore, a deep understanding of the reactivities of silicon precursors such as SiH4 and Si2H6 on substrate surfaces is required to obtain a rapid growth rate for the Si:H thin film. Thus, we performed first-principles density functional theory calculations to investigate the reactivities of the silicon precursors in detail. Our results show that the exposure of the Si(001) surface to Si2H6 is the most efficient method for depositing Si:H thin films among the different cases tested, including SiH4 on Si(001), Si2H6 on Si(001), SiH4 on Si(111), and Si2H6 on Si(111). The dissociative reactions of Si2H6 on the Si(001) surface were energetically the most favorable. The energy difference between the initial adsorbate (Si2H6) and the most stable adsorbate (Si2H2) on the Si(001) surface was 1.45 eV, which was the highest value among the four cases tested. In addition, it was numerically demonstrated that although Si2H6 is not added to a source mixture, Si2H6 can be observed to a non–negligible degree during the deposition process using an intermediate pressure SiH4/He plasma.

Formation of micro structured doped and undoped hydrogenated silicon thin films

The microcrystalline hydrogenated-silicon (μc-Si: H) (also called polymorphous silicon) consisting a two-phase mixture of amorphous and structured silicon is being used for electronic or optoelectronic based thin-film devices. The μc-Si: H thin films are deposited using radio frequency (13.56 MHz) Plasma Enhanced Chemical Vapour Deposition (RF-PECVD) by varying doping gases (diborane (B2H6) and phosphine (PH3)) flow and hydrogen-silane dilution ratio (R = H2/SiH4) to optimize the crystalline fraction and electrical conductivity. Micro-Raman spectroscopy is used to investigate these effect on the transition fraction regime from amorphous into micro-structured silicon. Qualitative and quantitative properties have been studied by deconvolution of the micro Raman spectra which allows to determine the crystalline fraction in the film and also some investigation regarding the correlation between electrical and structural properties are presented for different annealing temperature (from 300 to 550 °C) and various film thickness ranges (10–100 nm). In this work, we present the characterization of thin films (both doped and undoped) deposited at the temperature of 250 °C on quartz substrate after annealed at 550 °C in N2-ambient, as a result crystallinity percentage up to 90% for p-type, 96% for n-type and 80% for undoped films are achieved. A detailed characterization of the microcrystalline silicon (μc-Si: H) has been demonstrated in this paper: structural properties through Raman spectroscopy, electrical properties through Four-point probe station and optical properties using Ellipsometer.

Optical, structural evolution and surface morphology studies of hydrogenated silicon films synthesized by rf-magnetron sputtering: Effects of pressure and radio frequency power at low temperature

In the present work, three series of hydrogenated silicon thin films were prepared by radiofrequency magnetron sputtering method at low temperature, various deposition pressures (2, 4, 6 Pa) and different rf powers (200, 300, 400, 500 W) while all the other plasma parameters were fixed. The deposited films were characterized using atomic force microscopy (AFM), Fourier transform infrared (FTIR), UV–vis-NIR spectroscopy, Raman spectroscopy, X-ray diffraction (XRD) and DC electrical conductivity. The results show that rf power and pressure play an important role on optical band gap, the content of hydrogen, crystalline volume fraction, grain size and the surface roughness. The samples prepared at 6 Pa show the phase transition from amorphous, polymorphous to microcrystalline with increasing rf power. This transition is accompanied by an increase of the crystalline volume fraction and grain size, where hydrogen rich amorphous matrix enhances the crystallization. The variation of the optical band gap with the rf power was found to be dependent on the structure and hydrogen content. We conclude that increasing rf power favors the growth of microcrystalline films where the substrate was reheated by a high energy ions bombardment. The activation energy decreases linearly with increasing crystalline volume fraction.

Comparison of FTPS Performed on Thin Films and Solar Cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been used to investigate on the absorption coefficient versus photon energy (α(hν)) spectroscopy of thin film semiconductors. We have developed an FTPS experiment to study the α(hν) spectrum of hydrogenated amorphous silicon (a-Si:H) thin films, to subsequently deduce information on the density of states in the sub bandgap region. In this work we studied whether the same α(hν) spectroscopy could be found both for coplanar a-Si:H films deposited on glass and for NIP diodes incorporating the same material as the intrinsic layer. The back and front contacts of the diodes are made of transparent conductive oxides, either ITO or ZnO. The results show that the Urbach tails of both coplanar and NIP diodes are very similar but that α at low photon energy is slightly lower for the diodes than for the coplanar samples. In addition, we noticed that the diode with ZnO contacts presents a rapid increase of α below 0.95 eV. Transmission-reflection measurements performed on the diodes show that the diode with ZnO contacts exhibits a weaker transmission in the same wavelength range where we observed an increase of α, though the reflection with both types of contacts remains the same, indicating a parasitic absorption of the back ZnO layer. These results show that FTPS measurements performed on diodes must be taken very cautiously and that one has to take into account the optical properties (e.g., absorption) of the contacts to properly deduce reliable α(hν) spectroscopy.

Improved thermoelectric properties of nanocrystalline hydrogenated silicon thin films by post-deposition thermal annealing

The influence of post-deposition thermal annealing on the thermoelectric properties of n- and p-type nanocrystalline hydrogenated silicon thin films, deposited by plasma enhanced chemical vapour deposition, was studied in this work. The Power Factor of p-type films was improved from 7×10−5 to 4×10−4W/(m.K2) as the annealing temperature, under vacuum, increased up to 400°C while for n-type films it has a minor influence. Optimized Seebeck coefficient values of 460μV/K and −320μV/K were achieved for p- and n-type films, respectively, with crystalline size in the range of 10nm, leading to remarkable low thermal conductivity values (<10W.m−1.K−1) at room temperature.

Evolution of nanostructure in hydrogenated amorphous silicon thin films with substrate temperature studied by Raman mapping, Raman scattering and spectroscopic ellipsometry

Raman mapping and Raman scattering studies have been performed on hydrogenated silicon thin films deposited at different substrate temperature by rf-PECVD technique. In Raman mapping a clear difference in contrast, corresponding to different phases, was observed for films deposited at substrate temperature (Ts) ≥ 130 °C. The Raman spectra recorded at bright and dark spots in Raman image correspond to nanocrystalline and amorphous phase respectively. The fractional area of bright spots in Raman mapping was found to increase with increasing Ts. Also, the nanocrystalline fraction and size of nanocrystallites is found to increase with increasing Ts. These studies suggest that nucleation sites for the growth of nc-Si:H are formed at substrate temperature as low as 130 °C. With further increase in Ts more and more nucleation sites are formed which subsequently enhance crystallinity in these films. Films deposited at Ts ≥ 150 °C have larger band gap, higher roughness and higher photosensitivity and are of excellent device quality.

Electrode-selective deposition/etching processes using an SiF4/H2/Ar plasma chemistry excited by sawtooth tailored voltage waveforms

We report on the electrode-selective deposition and etching of hydrogenated silicon thin films using a plasma enhanced chemical vapour deposition process excited by sawtooth-shaped tailored voltage waveforms (TVWs). The slope asymmetry of such waveforms leads to a different rate of sheath expansion and contraction at each electrode, and therefore different electron power absorption near each electrode. This effect was employed with an SiF4/H2/Ar plasma chemistry, as the surface processes that result from this gas mixture depend strongly on the local balance between multiple precursors. For a specific gas flow ratio, a deposition rate of 0.82 Å s−1 on one electrode and an etching rate of 1.2 Å s−1 on the other were achieved. Moreover, this deposition/etching balance is controlled by the H2 flow rate, which limits the deposition rate at low flows. When the H2 injection is sufficiently high, the processes are then limited by the dissociation of SiF4, and the relative rate of the surface processes on the two electrodes are reversed, i.e. a higher net deposition rate is observed on the electrode where the fast sheath contraction occurs due to the electronegative character of the plasma.

Effects of ion bombardment on the structural and optical properties in hydrogenated silicon thin films

Structural and optical properties of two series of hydrogenated silicon films prepared by reactive radiofrequency magnetron sputtering technique at low substrate temperature, and with applied rf-powers of 150W (series A) and 300W (series B) were investigated. The Fourier transform infrared spectroscopic analysis showed changes in the nature of SiH bonding absorption spectra for both wagging and stretching vibration modes between the two series films. By varying the electrode gap spacing, D, (D=5, 6 and 7cm) and keeping all the parameters of the plasma constant, a gradual change in the hydrogen-bonding configurations was also observed. The effects of this change, on the structural and the optical properties of the material, were studied by means of Raman spectroscopy and spectroscopic ellipsometry (1.5–5eV) measurements, complemented with standard optical transmission. The results of this investigation clearly show that the films of the series A present a completely amorphous structure whatever are the D value, with a constant band gap ET of 1.68eV, typical of hydrogenated amorphous silicon (a-Si:H) films; the decrease in D leads only to an increase in the film compactness. However, the films of the series B are well crystallized, and exhibit a mixture of small and large Si crystallite sizes. Both the volume fractions of these crystallites, Fc, increase with decreasing in D. The optical gap shows decreasing trend, with constant hydrogen content, as Fc increases. The values of ET lie between those of a-Si:H and c-Si materials, and consequently they have been attributed to the increase in Fc values. It is suggested that ion bombardment plays a crucial role in increasing the compactness of the films and one or more specific properties.

Varied longitudinal microstructure of μc-Si:H films by tuning substrate temperature

Hydrogenated silicon thin films were deposited using plasma enhanced chemical vapour deposition method. The effects of substrate temperature on the microstructure and electrical properties of the films were investigated. Results show that the as received crystalline silicon thin film is a typical two-phase structure consisting of an amorphous phase and a crystalline phase. At a lower substrate temperature, the thickness of the initial amorphous incubation layer is dozens of nanometers, and few crystalline grains inset in the film after phase transition. As the substrate temperature increases, the initial incubation stage shortens to several nanometers, and the phase transition to crystalline starts earlier. After the phase transition, abundant crystallites appear, and the amorphous phase exists in the film just as the grain boundary. However, at overhigh substrate temperature, the average crystallinity and grain size of the film decrease. Deposition rate abidingly decreases with the increase in substrate temperature. The films with more ordered structure exhibit superior electrical properties.

Influence of Hydrogen Dilution on Two-phase Structure and Electrical Properties of Hydrogenated Silicon Thin Films

Influence of Hydrogen Dilution on Two-phase Structure and Electrical Properties of Hydrogenated Silicon Thin Films

Low Temperature Sub-micron Gap Thin-film Silicon Resonators on Glass Substrate

Flexural and in-plane vibrating thin-film resonators fabricated on glass substrates using low temperature surface micromachining and hydrogenated silicon structural layers with sub-micron gaps are demonstrated. Using hydrogenated silicon thin-films with distinct types of mechanical stress as the resonator structural layer allows controlling the quality factor and resonance frequencies. Sub-micron actuation gaps are a key feature to optimize the electromechanical coupling between the actuation/ capacitive sensing schemes and the resonating mechanical structure. Modeling the resonant system as a RLC circuit with parasitic parallel capacitances allows extracting the motional parameters. Strategies for reducing the influence of the parasitic parallel capacitance are shown.

Influence of cw CO2-laser radiation on the amorphous-to-microcrystalline phase transition in a-Si:H films: a Raman spectroscopic study

A detailed Raman spectroscopic study of the amorphous-to-microcrystalline phase transition in hydrogenated silicon thin films on glass substrates is presented. Crystallization is induced by exposing the film surface to a continuous wave CO2-laser working at 10.6 μm, at constant irradiation time/variable power density and at constant power density/variable irradiation time. The induced crystallization is followed quantitatively by analyzing the Raman spectra of the exposed area. The crystallite size distribution and the film stress are then estimated using a parametric fitting procedure. The pertinent microcrystal geometry of the samples after CO2-laser treatment has been directly correlated to Raman spectroscopic data. Variations of several spectral features, such as bandwidths and band frequencies have been interpreted in terms of three different mechanisms including the local heating due to the CO2-laser heating, the reduction of crystallite size and the tensile stress of μc-Si:H films. The results are discussed in the context of recent experimental and theoretical works concerning the bond polarizability.

Effect of Deposition Pressure on the Properties of Silicon Thin Films

Hydrogenated silicon thin film was prepared by plasma enhanced chemical vapor deposition (PECVD). The effects of the deposition pressure on the growth rate, the photoelectronic and microstructure properties of the thin films were investigated via transmission, photo/dark conductivity, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) measurements. The results indicate that the increase of the deposition pressure increases the bandgap and the growth rate, while makes the photosensitivity get worse, decreasing from more than ~103to ~102. And at the same time, the crystalline volume fraction (Xc) in the film decreases from 70% to 61%, when the deposition pressure increases from 100 Pa to 500 Pa. The order degree of the microstructure was deteriorated with pressure increasing.

Study of hydrogenated silicon thin film deposited by using dual-frequency inductively-coupled plasma-enhanced chemical-vapor deposition

Microcrystalline silicon thin films were deposited using an inductively-coupled plasma source with an internal linear-type antenna in the dual frequency mode (2 MHz/13.56 MHz), and the characteristics of the thin film and the plasma were investigated as functions of the relative power ratio. The deposition was performed in the SiH4 depletion condition at a deposition rate of about 10 A/s to improve the microstructural properties of the film. In the dual-frequency mode, the crystalline volume fraction could be increased by increasing the low-frequency power, which is added to the fixed 13.56 MHz rf power without changing the microstructure factor (R*), which is related to defects in the crystal structure. The differences appear to be related to the lower-energy ion bombardment of the substrate in the dual-frequency mode. In addition, by increasing the low-frequency power from 0 to 1.5 kW while keeping 3 kW at 13.56 MHz, we were able to change the uniformity of the deposition from 15.5% to less than 10% an improvement.

Effect of the hydrogen concentration on the growth mechanism of sputtered hydrogenated silicon thin films

The method of elastic recoil detection (ERD) has been utilized to analyze the actual hydrogen concentration in hydrogenated silicon thin films and has been demonstrated to provide more stable and accurate analysis for quantification of the hydrogen concentration than does Fourier Transform Infrared Spectroscopy. In order to realize the quality of thin films, spectroscopic ellipsometry and the applied effective medium approximation (EMA) theory of the Maxwell-Garnett and Bruggeman models are applied to deducing the effective refractive index of the films. The relative void fractions corresponding to amorphous silicon films with lowest hydrogen flow could be obtained based on the EMA theory. Therefore, this study found an important linear relationship between the hydrogen concentration and structural relaxation as a consequence of the void fraction induced by the hydrogen flow.

The effect of original crystalline phase on solid phase crystallization of hydrogenated silicon thin films

Hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) films fabricated on glass substrate are crystallized through solid phase crystallization process. In this study, the crystalline phase in μc-Si:H films could decrease process temperature, but prevent the film from crystallized completely. In counterpart, the a-Si:H films need higher annealing temperature to achieve crystallization, but could reach a better crystallinity. The fact that μc-Si:H films do not crystallize completely is attributed to the initial microcrystals in the μc-Si:H films, which avoid the relaxation of the film structure and inhibit the crystallization process. In addition, the effusion of hydrogen could promote the relaxation of a-Si but not induce direct crystallization.

Effect of silane flow rate on structural, electrical and optical properties of silicon thin films grown by VHF PECVD technique

Hydrogenated silicon thin films deposited by VHF PECVD process for various silane flow rates have been investigated. The silane flow rate was varied from 5 sccm to 30 sccm, maintaining all other parameters constant. The electrical, structural and optical properties of these films were systematically studied as a function of silane flow rate. These films were characterized by Raman spectroscopy, Scanning Electron Microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and UV–visible (UV–Vis) spectroscopy. Different crystalline volume fraction (22%–60%) and band gap (∼1.58 eV–∼1.96 eV) were achieved for silicon thin films by varying the silane concentration. A transition from amorphous to nanocrystalline silicon has been confirmed by Raman and FTIR analysis. The film grown at this transition region shows the high conductivity in the order of 10−4 Ω−1 cm−1.

Effects of H2 and Ar flow rates on the deposition of hydrogenated silicon thin films by an inductive coupled plasma-chemical vapor deposition system

Amorphous hydrogenated silicon films were deposited on quartz substrates in an inductive coupled plasma-chemical vapor deposition system with four internal low inductance antennas units. Different Ar and hydrogen flow rates were tested for their influences on the structures of deposited films. For monitoring purposes, Langmuir probe and optical emission spectrometer were installed to detect the variation of electrical field in plasma during deposition. Data from Langmuir probe and optical emission spectrometer were analyzed subsequently. After deposition, the films were examined by X-ray diffraction and Raman spectrometer for their microstructures. Results indicate that under the supply of pure Ar flow, the deposition rate can be expedited to 3.5nm/s and amorphous films were formed on quartz substrates. With the supply of mixed hydrogen and argon (Ar 15sccm+H2 50sccm+SiH4 50sccm), the deposition rate can reach 4.5nm/s. Although it is well known that a high supply of H2 helps the formation of micro-crystalline silicon, these deposited hydrogenated Si films, confirmed by X-ray diffraction patterns and Raman spectroscopy, all maintained their amorphousness under various range of Ar and H2 flow rates.

Interphase chemistry of Si electrodes used as anodes in Li-ion batteries

The effect of the Si electrode morphology (amorphous hydrogenated silicon thin films – a-Si:H as a model electrode and Si nanowires – SiNWs electrode) on the interphase chemistry was thoroughly investigated by the surface science techniques: X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS analysis shows a strong attenuation and positive shift of the Si 2p peaks after a complete charge/discharge performed in PC- and EC:DMC-based electrolytes for both electrodes (a-Si:H and SiNW), confirming a formation of a passive film (called solid electrolyte interphase – SEI layer). As evidenced from the XPS analysis performed on the model electrode, the thicker SEI layer was formed after cycling in PC-based electrolyte as compared to EC:DMC electrolyte. XPS and ToF-SIMS investigations reveal the presence of organic carbonate species on the outer surface and inorganic salt decomposition species in the inner part of the SEI layer. Significant modification of the surface morphology for the both electrodes and a full surface coverage by the SEI layer was confirmed by the scanning electron microscopy (SEM) analysis.