Nanostructure, electrical and optical properties of p-type hydrogenated nanocrystalline silicon films

In this paper, p-type hydrogenated nanocrystalline (nc-Si:H) films were prepared on corning 7059 glass by plasma-enhanced chemical vapor deposition (PECVD) system. The films were deposited with radio frequency (RF) (13.56 MHz) power and direct current (DC) biases stimulation conditions. Borane (B₂H₆) was a doping agent, and the flow ratio η of B₂H₆ component to silane (SiH₄) was varied in the experimental. Films’ surface morphology was investigated with atomic force microscopy (AFM); Raman spectroscopy, X-ray diffraction (XRD) was performed to study the crystalline volume fraction X_c and crystalline size d in films. The electrical and optical properties were gained by Keithly 617 programmable electrometer and ultraviolet-visible (UV-vis) transmission spectra, respectively. It was found that: there are on the film surface many faulty grains, which formed spike-like clusters; increasing the flow ratio η, crystalline volume fraction X_c decreased from 40.4% to 32.0% and crystalline size d decreased from 4.7 to 2.7 nm; the optical band gap E_g^opt increased from 2.16 to 2.4 eV. The electrical properties of p-type nc-Si:H films are affected by annealing treatment and the reaction pressure.     

Role of argon in hot wire chemical vapor deposition of hydrogenated nanocrystalline silicon thin films

Structural, optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited from silane (SiH₄) and argon (Ar) gas mixture without hydrogen by hot wire chemical vapor deposition (HW-CVD) method were investigated. Film properties are carefully and systematically studied as a function of argon dilution of silane (R_Ar). We observed that the deposition rate is much higher (4-23 Å/s) compared to conventional plasma enhanced chemical vapor deposited nc-Si:H films using Ar dilution of silane (0.5-0.83 Å/s). Characterization of these films with Raman spectroscopy revealed that Ar dilution of silane in HW-CVD endorses the growth of crystallinity and structural order in the nc-Si:H films. The Fourier transform infrared spectroscopic analysis showed that with increasing Ar dilution, the hydrogen bonding in the films shifts from di-hydrogen (Si-H₂) and (Si-H₂)_n complexes to mono-hydrogen (Si-H) bounded species. The hydrogen content in the films increases with increasing Ar dilution and was found to be < 4 at.% over the entire range of Ar dilutions of silane studied. However, the band gap shows decreasing trend with increase in Ar dilution of silane and it has been attributed to the decrease in the percentage of the amorphous phase in the film. The microstructure parameter was found to be > 0.4 for the films deposited at low Ar dilution of silane and ~ 0.1 or even less for the films deposited at higher Ar dilution, suggesting that there is an enhancement of structural order and homogeneity in the film. From the present study it has been concluded that the Ar dilution of silane is a key process parameter to induce the crystallinity and to improve the structural ordering in the nc-Si:H films deposited by the HW-CVD method.     

Effect of hydrogen plasma treatment on the surface morphology, microstructure and electronic transport properties of nc-Si:H

Hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited by reactive radio-frequency sputtering with 33% hydrogen dilution in argon at 200 °C, were treated with low-power hydrogen plasma at room temperature at various power densities (0.1-0.5 W/cm²) and durations (10 s-10 min). Plasma treatment reduced the surface root mean square roughness and increased the average grain size. This was attributed to the mass transport of Si atoms on the surface by surface and grain boundary diffusion. Plasma treatment under low power density (0.1 W/cm²) for short duration (10 s) caused a significant enhancement of crystalline volume fraction and electrical conductivity, compared to as-deposited film. While higher power (0.5 W/cm²) hydrogen plasma treatment for longer durations (up to 10 min) caused moderate improvement in crystalline fraction and electrical properties; however, the magnitude of improvement is not significant compared to low-power (0.1 W/cm²)/short-duration (10 s) plasma exposure. The results indicate that low-power hydrogen plasma treatment at room temperature can be an effective tool to improve the structural and electrical properties of nc-Si:H.     

Optimization of n nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H₂ dilutions. High H₂ dilution resulted in n⁺ nanocrystalline silicon films (n⁺ nc-Si:H) with the lower resistivity (ρ ∼0.7 Ω cm) compared to that of doped amorphous silicon films (∼900 Ω cm) grown at low H₂ dilution. The change of the lateral ρ of n⁺ nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The ρ values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n⁺ nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (∼92 Ω cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiN_x:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH₃/SiH₄ gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiN_x:H film. Having lowest leakage currents, two a-SiN_x:H films with NH₃/SiH₄ ratios of ∼19 and ∼28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH₃/SiH₄∼19 ratio showed higher device performance than the TFT containing a-SiN_x:H with the NH₃/SiH₄∼28 ratio. This was correlated with the N−H/Si−H bond concentration ratio optimized for the TFT application.     

Effect of composition on mechanical behaviour of diamond-like carbon coatings modified with titanium

In this study, diamond-like carbon (DLC) films modified with titanium were deposited by plasma decomposition of metallorganic precursor, titanium isopropoxide in CH₄/H₂/Ar gas atmosphere. The obtained films were composed of amorphous titanium oxide and nanocrystalline titanium carbide, embedded in an amorphous hydrogenated (a-C:H) matrix. The TiC/TiO₂ ratio in the DLC matrix was found to be dependent on the deposition parameters. The dependence of the films chemical composition on gas mixture and substrate temperature was investigated by X-ray photoelectron spectroscopy, whereas the crystallinity of TiC nanoparticles and their dimension were evaluated by X-ray diffraction. The size of TiC crystallites varied from 10 to 35 nm, depending on the process parameters. The intrinsic hardness of 10-13 GPa, elastic modulus of 170-200 GPa and hardness-to-modulus ratio of obtained coatings were measured by the nanoindentation technique. Obtained results demonstrated a correlation of mechanical properties with the chemical composition and the ratio of amorphous/crystalline phases in the films. In particular, the formation of nanocrystalline TiC with atomic concentration not exceeding 10% and with grain size between 10 nm and 15 nm resulted in significantly enhanced mechanical properties of composite material in comparison with ordinary DLC films.     

Nanocrystalline silicon films with high conductivity and the application for PIN solar cells

Intrinsic nanocrystalline silicon films (nc-Si:H) were prepared by plasma enhanced chemical vapor deposition (PECVD) method. Films’ microstructures and characteristics were studied with Raman spectroscopy and Atom Force Microscope (AFM). The electronic conductivity of nc-Si:H films was found to be 4.9×10⁰Ω⁻¹ cm⁻¹, which was one order of magnitude higher thanthe reported 10⁻³-10⁻¹ Ω⁻¹ cm⁻¹. And PIN solar cells with nc-Si:H film as intrinsic thin-layer (ITO/n⁺-nc-Si:H/i-nc-Si:H/p-c-Si/Ag) were researched. The cell's performances were measured, the open-circuit voltage V_oc was 534.7 mV, short-circuit current I_sc was 49.24 mA (3 cm²) and fill factor FF was 0.4228.     

Nanocrystalline silicon prepared at high growth rate using helium dilution

Growth and optimization of the nanocrystalline silicon (nc-Si: H) films have been studied by varying the electrical power applied to the helium diluted silane plasma in RF glow discharge. Wide optical gap and conducting intrinsic nanocrystalline silicon network of controlled crystalline volume fraction and oriented crystallographic lattice planes have been obtained at a reasonably high growth rate from helium diluted silane plasma, without using hydrogen. Improving crystallinity in the network comprising ∼ 10 nm Si-nanocrystallites and contributing optical gap widening, conductivity ascending and that obtained during simultaneous escalation of the deposition rate, promises significant technological impact.     

Preparation and photoluminescence of nanocrystalline Si-rich silicon oxide films by PECVD

Nanocrystalline Si-rich silicon oxide films were deposited using plasma enhanced chemical vapor deposition technique with the mixture of silane (SiH₄), nitrous oxide (N₂O) and hydrogen (H₂) as gas source on quartz glass substrate at the substrate temperature of 300 °C. The effect of the ratio N₂O/SiH₄ on the oxidation, microstructures and photoluminescence (PL) of the as-deposited Si-rich silicon oxide films was investigated with FTIR, XRD and HRTEM. The results reveal that with the increasing ratio of N₂O/SiH₄, more amounts of oxygen are incorporated in the as-deposited films and more nanosized silicon particles are embedded in the films, forming nanocrystalline Si-rich silicon oxide films. The quantum confinement effect or the cooperation of quantum confinement and luminescence center results in the nanocrystalline Si-rich silicon oxide films of higher PL intensity.     

Nanocrystalline silicon fabrication by conventional plasma enhanced chemical vapor deposition for bottom gate thin film transistor

Nanocrystalline silicon was successfully fabricated using conventional plasma enhanced chemical vapor deposition (PECVD) for bottom gate thin film transistor. This was accomplished by promoting nucleation rate in the initial stage of silicon growth by H₂ or SF₆ plasma treatment of the surface of silicon nitride (SiN_x) films. Microstructure of hydrogenated nanocrystalline silicon (nc-Si:H) films confirmed the crystallization of silicon, and nanocrystalline silicon thin film transistor exhibited excellent stability.     

Highly conductive p-type nanocrystalline silicon films deposited by RF-PECVD using silane and trimethylboron mixtures at high pressure

In this paper we present a study of boron-doped nc-Si:H films prepared by PECVD at high deposition pressure (≥4 mbar), high plasma power and low substrate temperature (≤200 °C) using trimethylboron (TMB) as a dopant gas. The influence of deposition parameters on electrical, structural and optical properties is investigated. We determine the deposition conditions that lead to the formation of p-type nanocrystalline silicon thin films with very high crystallinity, high value of dark conductivity (>7 (Ω cm)⁻¹) and high optical band gap (≥1.7 eV). Modeling of ellipsometry spectra reveals that the film growth mechanism should proceed through a sub-surface layer mechanism that leads to silicon crystallization.The obtained films are very good candidates for application in amorphous and nanocrystalline silicon solar cells as a p-type window layer.     

Capacitance analyses of hydrogenated nanocrystalline silicon based thin film transistor

The capacitance-voltage (C-V) measurements within 10⁶-10^− 2 Hz frequency range were performed on the hydrogenated nanocrystalline silicon (nc-Si:H) bottom-gate thin film transistor (TFT) and metal-insulator-amorphous silicon (MIAS) structure, mechanically isolated from the same TFT. It was found that the conducting thin layer in nc-Si:H film expands the effective capacitor area beyond the electrode in the TFT structure, which complicates its C-V curves. Considering the TFT capacitance-frequency (C-F) curves, the equivalent circuit of the TFT structure was proposed and mechanism for this area expansion was discussed. On the other hand, the MIAS C-F curves were fitted by the equivalent circuit models to deduce its electrical properties. nc-Si:H neutral bulk effect was revealed by the dependence of the MIAS capacitance on frequency within 10⁶-10³ Hz at both accumulation and depletion regimes. The inversion in MIAS was detected at 10²-10^− 2 Hz for relatively low negative gate bias without any external activation source. The presence of the ac hopping conductivity in the nc-Si:H film was inferred from the fitting. In addition, the density of the interface traps and its energy distribution were determined.     

Highly reflective nc-Si:H/a-CNx:H multilayer films prepared by r.f. PECVD technique

Multilayer thin films consisting of a-CN_x:H/nc-Si:H layers prepared by radio-frequency plasma enhanced chemical vapour (r.f. PECVD) deposition technique were studied. High optical reflectivity at a specific wavelength is one of major concern for its application. By using this technique, a-CN_x:H/nc-Si:H multilayered thin films (3-11 periods) were deposited on substrates of p-type (111) crystal silicon and quartz. These films were characterized using ultra-violet-visible-near infrared (UV-Vis-NIR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, field effect scanning electron microscopy (FESEM) and AUGER electron spectroscopy (AES). The multilayered films show high reflectivity and wide stop band width at a wavelength of approximately 650 ± 60 nm. The FTIR spectrum of this multilayered structure showed the formation of Si-H and Si-H₂ bonds in the nc-Si:H layer and CC and N-H bonds in a-CN_x:H layer. SEM image and AES reveal distinct formation of a-CN_x:H and nc-Si:H layers in the cross section image with a decrease in interlayer cross contamination with increasing number of periods.     

Structural changes in tungsten wire and their effect on the properties of hydrogenated nanocrystalline cubic silicon carbide thin films

We investigated the structural changes in tungsten wire heated to 1800 °C in SiH₄/CH₄/H₂/N₂ atmosphere and the effect of the aging tungsten wire on the properties of N-doped hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) thin films. The aged tungsten wire had two parts: hot parts of the middle of the wire and relatively cold parts connected to clamps. Tungsten carbide (W₂C) layer formed in the wire of the hot parts, while crystalline silicon and cubic silicon carbide (c-Si/3C-SiC) layer deposited on the wire of the cold parts. N-doped nc-3C-SiC:H thin films were deposited for 5 min (thickness: ~ 30 nm) after the tungsten wire was heated under the same condition as during the film deposition for given times (exposure time). No changes in the structural, electrical and optical properties of the nc-3C-SiC:H thin films were observed for the exposure time up to 450 min.     

Optical and structural characterization of inhomogeneities in a-Si:H TO μc-Si transition

The a-Si:H films with different thickness and microstructure have been deposited with rf-PECVD using a plasma of silane diluted with hydrogen. The structure and optical analysis were carried out by X-ray diffraction, UV-VIS and Raman spectroscopy. Spectral refractive indices, optical energy band gaps, extinction coefficients, phases ratio and grain size were determined as a function of the hydrogen dilution (R = H₂/SiH₄). Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to micro-crystalline hydrogenated silicon (μc-Si:H). XRD analysis indicated that films with R = 0 and R = 20 were amorphous and homogeneous, while films with R = 40 and higher were micro-crystalline consisting medium range ordered silicon hydride (Si₄H) and μc-Si phases with different size of crystallites, which was confirmed also by Raman spectroscopy.     

Size modulation of nanocrystalline silicon embedded in amorphous silicon oxide by Cat-CVD

Different issues related to controlling size of nanocrystalline silicon (nc-Si) embedded in hydrogenated amorphous silicon oxide (a-SiO_x:H) deposited by catalytic chemical vapor deposition (Cat-CVD) have been reported. Films were deposited using tantalum (Ta) and tungsten (W) filaments and it is observed that films deposited using tantalum filament resulted in good control on the properties. The parameters which can affect the size of nc-Si domains have been studied which include hydrogen flow rate, catalyst and substrate temperatures. The deposited samples are characterized by X-ray diffraction, HRTEM and micro-Raman spectroscopy, for determining the size of the deposited nc-Si. The crystallite formation starts for Ta-catalyst around the temperature of 1700 °C.     

Optical properties of nanocrystalline SnS2 thin films

Thin films of nanocrystalline SnS₂ on glass substrates were prepared from solution by dip coating and then sulfurized in H₂S (H₂S:Ar = 1:10) atmosphere. The films had an average thickness of 60 nm and were characterized by X-ray diffraction studies, scanning electron microscopy, EDAX, transmission electron microscopy, UV-vis spectroscopy, and Raman spectroscopy. The influence of annealing temperature (150-300 °C) on the crystallinity and particle size was studied. The effect of CTAB as a capping agent has been tested. X-ray diffraction analysis revealed the polycrystalline nature of the films with a preferential orientation along the c-axis. Optical transmission spectra indicated a marked blue shift of the absorption edge due to quantum confinement and optical band gap was found to vary from 3.5 to 3.0 eV with annealing temperature. Raman studies indicated a prominent broad peak at ∼314 cm⁻¹, which confirmed the presence of nanocrystalline SnS₂ phase.     

Wide optical bandgap p-type μc-Si:Ox:H prepared by Cat-CVD and comparisons to p-type μc-Si:H

Oxygen-impurity boron-doped hydrogenated microcrystalline silicon (p-μc-Si:O_x:H) films have been deposited using catalytic chemical vapor deposition (Cat-CVD). Pure silane (SiH₄), hydrogen (H₂), oxygen (O₂), and diluted diborane (B₂H₆) gases were used. The tungsten catalyst temperature (T_fil) was varied from 1900 to 2100 °C and films were deposited on glass substrates at temperatures of 100 to 300 °C. Different catalyst-to-substrate distances were employed and single- or double-coiled filaments were used. In addition to p-μc-Si:O_x:H deposition, we have also deposited conventional p-type microcrystalline silicon (p-μc-Si:H) in order to compare their electrical and optical properties to p-μc-Si:O_x:H.     

High-speed μc-Si films deposition and large-grain poly-Si films deposition by surface wave discharge

Microcrystalline silicon (μc-Si) and polycrystalline silicon (poly-Si) films are deposited by surface wave (SW) discharge at 2.45 GHz in H₂/SiH₄ gas. This high density SW plasma at relatively low pressures (4-60 Pa) enables strong dissociation of feedstock gas. The films deposited on substrate are investigated by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The SW discharge in 10% SiH₄ at total pressure of ∼ 30 Pa gives μc-Si films on a substrate at 250 °C, at a fairly high deposition rate of 4-20 nm/s, with a crystalline volume fraction of 0.5-0.8 and a grain size of 10-40 nm. Furthermore, poly-Si film with crystalline volume fraction of > 99% is deposited at higher substrate temperature (400 °C) in 2% SiH₄ discharge at lower pressure (4 Pa). X-ray diffraction and SEM results revealed that the grain size of poly-Si films is as large as 600 nm, which is almost 6 times larger than previously reported values.     

Study on variable capacitance diode of (p)nc-Si:H/(n)c-Si heterojunction

Hydrogenated nanocrystalline silicon (nc-Si:H) layers of boron-doped increasing step by step was deposited on n-type crystalline silicon substrate using Plasma Enhanced Chemical Vapor Deposition (PECVD) system. After evaporating Ohm contact electrode on the side of substrate and on the side of nc-Si:H film, a structure of electrode/(p)nc-Si:H/(n)c-Si/electrode was obtained. It is confirmed by electrical measurement such as I-V curve, C-V curve and DLTS that this is a variable capacitance diode.     

Structural and optical properties of four-hexagonal polytype nanocrystalline silicon carbide films deposited by plasma enhanced chemical vapor deposition technique

Four-hexagonal polytype films of nanocrystalline silicon carbide (4H-nc-SiC) were deposited by plasma enhanced chemical vapor deposition method with more than 3×10⁴ W m⁻² threshold of power density, high hydrogen dilution ratio, and bias pretreatment. The source gases were silane, methane and hydrogen. Our work showed that under conditions similar to those used for the growth of μc-SiC—except a higher power densities over a threshold, a bigger bias pretreatment on substrates, and a moderate bias deposition—nc-SiC films could indeed be achieved. The Raman spectra and transmission electron microscopy diffraction patterns demonstrated that the as-grown films from the H₂-CH₄-SiH₄ plasma consist of amorphous network and phase-pure crystalline silicon carbide which has the 4H polytype structure. The microcolumnar 4H-SiC nanocrystallites of a mean size of approximately 1.6×10⁻⁸ m in diameter are encapsulated by amorphous SiC networks. The photoluminescence spectra of 4H-SiC at room temperature, peaking at 8.10×10⁻⁷ m using a wavelength of 5.145×10⁻⁷ m of argon ion laser, were obtained at room temperature; the luminescence mechanism is thought to be related to transitions in the energy band gap which could be ascribed to the surface states and defects in the structure of 4H-SiC nanocrystalline in these films due to its small size. The as-grown films showed an optical transmittance of 89% at 6.58×10⁻⁷ m. This higher transmittance is believed to be from the small size and amorphous matrix.