SiH4 Flow Research Articles

Plasma diagnostic approach for high rate nanocrystalline Si synthesis in RF/UHF hybrid plasmas using a PECVD process

Hydrogenated nanocrystalline silicon (nc-Si : H) films intended for efficient nc-Si : H solar cells are usually made at the transition to the nanocrystalline regime using the plasma-enhanced chemical vapor deposition (PECVD) process. This change occurs within a sensitive process window and is affected by various deposition parameters. This paper reports a study of nc-Si : H films' fabrication by utilizing systematic plasma diagnostics. This work presents a novel approach for plasma processing using radio frequency (RF), ultra high frequency (UHF) and RF/UHF hybrid plasmas. Using careful analysis, efforts are made to investigate the radicals and plasma formation by changing the operating source power and silane (SiH4) concentration. The aim of this work is also to investigate the PECVD process and conditions favorable for the synthesis of nc-Si : H film. For the present study, we systematically use the optical emission spectroscopy (OES), normal, and RF-compensated Langmuir probe (LP) and vacuum ultraviolet absorption spectroscopy diagnostics. Measurements reveal that the OES diagnostic is consistent with the LP measurements. Investigation reveals that UHF power in addition to RF enables higher dissociation of H or SiH radicals and the production of higher plasma density. The combined effect of both RF and UHF sources is used as the hybrid plasma source. Measurements also reveal that inbetween SiH4 flow rates ∼20–30 sccm, there is significant change in the plasma characteristics that denotes the nc-Si : H−a-Si : H transition region. An atomic hydrogen density (nH) in the range ≈(8 − 10) × 1011 cm−3 and plasma density n0 ≈ (2 − 3) × 1011 cm−3 with a silane to hydrogen ratio of 1–2% with high crystallinity has been obtained. Along with the discussion on the effect of frequency on plasma chemistry, this explains the RF power coupling and the role of electrons and ions in plasmas with increasing frequency.

The microstructure and optical properties of SiNx deposited by linear microwave chemical vapor deposition

SiNx films are synthesized on Si substrates in a home-made linear microwave plasma enhanced chemical vapor deposition system at different microwave powers, duty cycles, substrate temperatures, and ratios of silane (SiH4) flow to ammonia (NH3) gas flow. The effects of technological parameters on morphology of film surface, stoichiometric proportion, refractive index and deposition rate of SiNx film are characterized by scanning electron microscopy (SEM) and elliptical polarization instrument, and the relationships among stoichiometric proportion, refractive index and deposition rate are investigated. The results from SEM analysis indicate that the surfaces are smooth and the elements are homogeneously distributed in the films obtained under different deposition parameters. The ratio of SiH4 flow to NH3 gas flow and the duty cycle are the most critical factors determining the refractive index which can be changed from 1.92 to 2.33. The thickness measurements show that the deposition rate of SiNx film is affected by microwave power, duty cycle, substrate temperature and flow ratio. The maximum deposition rate achieved in the paper is 135 nm·min-1.

Self-assembled growth of inclined GaN nanorods on (10−10) m-plane sapphire using metal–organic chemical vapor deposition

We report the self-assembled growth of inclined and highly ordered GaN nanorods on (10−10) m-plane sapphire by metal–organic chemical vapor deposition, without metal catalyst. To determine the growth mechanism we performed a systematic study of the effect of the SiH4 flow, V/III ratio, growth temperature and growth time on growth behavior, demonstrating that optimized parameters were required for the growth of nanorods with high aspect ratios. High resolution X-ray diffraction showed that the nanorods were inclined at an angle of 58.4° with respect to the substrate normal and followed a well-defined epitaxial relationship with respect to the on-axis plane of the nanorods, the (11–22) semipolar plane, and the (10−10) m-plane sapphire. Finally cathodoluminescence showed that the near band edge emission of the Si-doped nanorod was asymmetric and broad owing to the band filling effect resulting from high carrier concentration, compared to the undoped GaN.

Anomalous junctions characterized by Raman spectroscopy in SixGe1−x nanowires with axially degraded components

The characterization of junctions in nanowires by high-resolution transmission electron microscopy with spherical aberration correction is tricky and tedious. Many disadvantages also exist, including rigorous sample preparation and structural damage inflicted by high-energy electrons. In this work, we present a simple, low-cost, and non-destructive Raman spectroscopy method of characterizing anomalous junctions in nanowires with axially degraded components. The Raman spectra of SixGe1−x nanowires with axially degraded components are studied in detail using a confocal micro-Raman spectrometer. Three Raman peaks (νSi–Si = 490 cm−1, νSi–Ge = 400 cm−1, and νGe–Ge = 284 cm−1) up-shift with increased Si content. This up-shift originates in the bond compression induced by a confined effect on the radial direction of nanowire. The anomalous junctions in SixGe1−x nanowires with axially degraded components are then observed by Raman spectroscopy and verified by transmission electron microscopy energy-dispersive X-ray spectroscopy. The anomalous junctions of SixGe1−x nanowires with axially degraded components are due to the vortex flow of inlet SiH4 and GeH4 gas in their synthesis. The anomalous junctions can be used as raw materials for fabricating devices with special functions.

Silicon nanoparticles synthesized using double tube reactor and inductive coupled plasma

Silicon (Si) nanoparticles were synthesized using a double tube reactor and inductive coupled plasma (ICP), and their microstructures with various process conditions were investigated in terms of plasma density and residence time of reactive gas in the plasma zone. Homogeneous amorphous or single crystalline Si nanoparticles were synthesized depending on applied power to ICP coil. The SiH4 flow rate mainly determined the particle size. Average particle sizes of the synthesized Si nanoparticles were 14–34 nm, and the maximum production rate was as high as 1000 mg h−1. The purity of synthesized Si nanoparticles was higher than 99.99%. When Si nanoparticles were applied as an active material in the anode of a Li-ion battery (LIB), the high first reversible capacities of 2540 (initial coulombic efficiency (ICE) = 61.4%) and 1358 mAh g−1 (ICE = 62.5%) for crystalline and amorphous Si nanoparticles were attained, respectively. However, capacity fadings were detected in both nanoparticles, and capacity retentions at 100th cycle were only 17.1% and 34.8% for crystalline and amorphous Si nanoparticles.

Spectroscopic studies on nanocrystalline silicon thin films prepared from H2-diluted SiH4-plasma in inductively coupled low pressure RF PECVD

A comprehensive analysis on the evolution of the microstructure as well as optical constants and dielectric functions of intrinsic hydrogenated nano-crystalline silicon thin films prepared by highly H2 diluted SiH4 plasma in a planar inductively coupled RF plasma chemical vapor deposition (ICP-CVD) reactor has been performed by spectroscopic ellipsometry. Films are assumed to have a three-layer structure, with a thin incubation layer at substrate/bulk interface, the bulk layer and a thin growth zone and surface roughness layer. Individual composition and the thickness of each layer have been estimated from the simulation of the ellipsometry data using Bruggeman effective medium approximation (BEMA). The ellipsometry results are correlated with atomic force microscopy and micro-Raman data of these films. The effect of the flow rate of SiH4 and the key role of hydrogen dilution on growth dynamics, optical constants and dielectric functions of highly crystalline nanosilicon films is discussed elaborately. The bulk crystalline volume fraction of the deposited films varies considerably (~67–84%) with the change in flow rate of SiH4. With increasing SiH4 flow rate the overall bulk crystallinity reduces; however the ultra-nanocrystalline component (Xunc) enhances substantially that helps reducing the porosity and surface roughness.

4H-SiC ホモエピタキシャル成長における表面モフォロジーのオフ角依存性

We investigated homoepitaxial growth on 4H-SiC substrates with a vicinal off-angle lower than 1°. The small off angle difference of a tenth part of a degree has an influence on surface morphology of epitaxial layers. This tendency also depends on the face polarity and a C-face can be obtained that has a specular surface with a lower vicinal off angle than a Si-face. Low C/Si ratio conditions are also important to control the surface morphology of these substrates. Especially, controlling C/Si ratio with SiH4 flow rate is effective to suppress the generation of step bunching on Si-face epitaxial layers. These results indicate the control of surface free energy is also important to controlling the surface morphology of homoepitaxial growth on these substrates.

High-Performance Low-Cost Back-Channel-Etch Amorphous Gallium–Indium–Zinc Oxide Thin-Film Transistors by Curing and Passivation of the Damaged Back Channel

High-performance, low-cost amorphous gallium-indium-zinc oxide (a-GIZO) thin-film-transistor (TFT) technology is required for the next generation of active-matrix organic light-emitting diodes. A back-channel-etch structure is the most appropriate device structure for high-performance, low-cost a-GIZO TFT technology. However, channel damage due to source/drain etching and passivation-layer deposition has been a critical issue. To solve this problem, the present work focuses on overall back-channel processes, such as back-channel N2O plasma treatment, SiOx passivation deposition, and final thermal annealing. This work has revealed the dependence of a-GIZO TFT characteristics on the N2O plasma radio-frequency (RF) power and frequency, the SiH4 flow rate in the SiOx deposition process, and the final annealing temperature. On the basis of these results, a high-performance a-GIZO TFT with a field-effect mobility of 35.7 cm(2) V(-1) s(-1), a subthreshold swing of 185 mV dec(-1), a switching ratio exceeding 10(7), and a satisfactory reliability was successfully fabricated. The technology developed in this work can be realized using the existing facilities of active-matrix liquid-crystal display industries.

Optical and Structural Properties of Ammonia-Free Amorphous Silicon Nitride Thin Films for Photovoltaic Applications

ABSTRACT Hydrogenated amorphous silicon nitride (a-SiNx:H) thin films have been deposited through the green chemistry route using silane (SiH4) and nitrogen (N2) as process gases with SiH4 flow being variable and N2 flow being constant without the use of pollutant and corrosive ammonia (NH3) by the plasma-enhanced chemical vapor deposition technique at 13.56 MHz. Fourier transform infrared spectroscopy analysis shows various possible vibrational modes of Si-H, Si-N, and N-H bonds present in the film. Raman spectroscopy is performed on these samples to calculate volume fractions corresponding to amorphous phases present in the a-SiNx:H films. The refractive index (η) values are calculated using Swanepoel's method, which are in the range of 2.89 to 3.17. The thickness of the deposited films has been evaluated using transmission spectra. Absorption coefficient and band gap (E g) values are obtained from optical absorption studies. An increase in the E g and a decrease in the η value have been observed for the samples grown with decreasing SiH4 flow.

Substrate with Si Nanoparticles Prepared by Low Pressure Chemical Vapor Deposition for Application in Si Solar Cells

Nanostructure strategies are frequently used to enhance the light absorption in solar cells. For improving the efficiency of absorption in solar cells, an industrial-feasible processing technique, i.e. low-pressure chemical vapor deposition was used to form a substrate with large-area silicon nanoparticles (Si-NPs). It was shown that the density and size of Si-NPs can be modulated by controlling the flow of pure SiH4, the deposition temperature and the deposition time. The substrate with large-area Si-NPs can be applied in photovoltaic devices since they can increase the effective absorption path of the incident sunlight.

Optimization of a buffer layer for cubic silicon carbide growth on silicon substrates

A procedure for the optimization of a 3C–SiC buffer layer for the deposition of 3C–SiC/(001) Si is described. After a standard carbonization at 1125°C, SiH4 and C3H8 were added to the gas phase while the temperature was raised from 1125°C to the growth temperature of 1380°C with a controlled temperature ramp to grow a thin SiC layer. The quality and the crystallinity of the buffer layer and the presence of voids at the SiC/Si interface are related to the gas flow and to the heating ramp rate. In order to improve the buffer quality the SiH4 and C3H8 flows were changed during the heating ramp. On the optimized buffer no voids were detected and a high-quality 1.5μm 3C–SiC was grown to demonstrate the effectiveness of the described buffer.

Low-Interface-Trap-Density and High-Breakdown-Electric-Field SiN Films on GaN Formed by Plasma Pretreatment Using Microwave-Excited Plasma-Enhanced Chemical Vapor Deposition

We investigated the SiN/gallium nitride (GaN) interface properties formed by the microwave-excited plasma-enhanced chemical vapor deposition (PECVD) with SiH4/N2/H2 gases. The interface and insulating properties of SiN films on GaN formed by the microwave-excited PECVD strongly depend on SiH4 flow rate. Although the interface trap density is lower than $10^{11}$ cm $^{\mathrm {-2}}$ eV $^{\mathrm {-1}}$ at the relatively high SiH4 flow rate of 1 sccm, the breakdown electric field is very low ( $\sim 1$ MV/cm). Using the SiH4 plasma pretreatment before the stoichiometric SiN deposition, both low interface trap density and high breakdown voltage greater than 2 MV/cm are obtained. In this case, the clearly ordered Ga bonding near the SiN/GaN interface is estimated by an electron energy loss spectroscopy. The formation of SiN film on GaN using the microwave-excited PECVD is a very useful technique for the high-quality interface properties.

Raman and FTIR Studies on PECVD Grown Ammonia-Free Amorphous Silicon Nitride Thin Films for Solar Cell Applications

Ammonia- (NH3-) free, hydrogenated amorphous silicon nitride (a-SiNx:H) thin films have been deposited using silane (SiH4) and nitrogen (N2) as source gases by plasma-enhanced chemical vapour deposition (PECVD). During the experiment, SiH4 flow rate has been kept constant at 5 sccm, whereas N2 flow rate has been varied from 2000 to 1600 sccm. The effect of nitrogen flow on SiNx:H films has been verified using Raman analysis studies. Fourier transform Infrared spectroscopy analysis has been carried out to identify all the possible modes of vibrations such as Si–N, Si–H, and N–H present in the films, and the effect of nitrogen flow on these parameters is correlated. The refractive index of the above-mentioned films has been calculated using UV-VIS spectroscopy measurements by Swanepoel’s method.

Growth mode control of InGaN on GaN by using SiH4 post-treatment

We report on a way to control the growth mode of an In0.43Ga0.57N layer on GaN by using SiH4 post-treatment. In0.43Ga0.57N islands were formed on the GaN layers by a using Stranski-Krastanov growth mode without SiH4 post-treatment. When the SiH4 flow rate was increased from 0 to 10 sccm, the InGaN island density decreased from 3.3 × 109 to 1.9 × 109 then to zero, resulting in two-dimensional growth. However, a further increase in the SiH4 flow rate to 15 sccm again led to the formation of InGaN islands. Temperature- and excitation-power-dependent photoluminescence spectra showed that the InGaN islands provided strong localized recombination centers for electrical charge carriers and that the InGaN layers were composed of InGaN islands with different numbers of monolayers. This abnormal growth behavior with SiH4 post-treatment was explained by the combined effects of surfactant-mediated growth modification and the formation of Si x N y with the injection of SiH4.

Deposition of CrSiN/AlTiSiN nano-multilayer coatings by multi-arc ion plating using gas source silicon

Quinary CrAlTiSiN coatings have been prepared on cemented carbide and Si substrates using Cr and AlTi (67:33at.%) alloy targets in the mixture flow of N2 and SiH4 gas by multi-arc ion plating method. X-ray diffraction and transmission electron microscopy revealed that the multi-element coatings had a nc-CrSiN/nc-AlTiSiN multilayered structure with a thickness period of 10nm. The microhardness and wear of the coatings were found to be strongly influenced by SiH4 flow rate. The hardness increased monotonically with the increase of SiH4 flow rate and reached 37GPa at 23sccm. The coatings exhibit friction coefficients varying between 0.58 and 0.76 against Si3N4 ball.

Study on the Correlation between Film Composition and Piezoresistive Properties of PECVD Si<sub>x</sub>C<sub>y</sub> Thin Films

The effects of carbon content on the piezoresistive properties of non-stoichiometric silicon carbide (SixCy) films deposited on thermally oxidized (100) Si substrates by Plasma Enhanced Chemical Vapor Deposition (PECVD) from silane (SiH4) and methane (CH4) gas mixtures have been investigated. Four different film compositions have been obtained by varying SiH4 flow ratios from 1.0 to 4.0 sccm, while the other parameters were kept constant. In order to evaluate the piezoresistive properties of the SixCy films, we have developed test structures consisting of SixCy thin-film resistors defined by reactive ion etching (RIE) with Ti/Au pads formed by lift-off process. The gauge factor (GF) and temperature coefficient of resistance (TCR) of each SixCy film were measured.

Comparative studies of polar (0001) and semipolar [formula omitted] n-type GaN with different Si doping concentrations

We investigated the electrical and crystalline properties of polar (0001) GaN and semipolar (112¯2) n-type GaN with different Si doping concentrations grown on c-plane and m-plane sapphire substrates, respectively. Regardless of the polar and semipolar GaN, the electron concentrations of polar (0001) and semipolar (112¯2) GaN increased proportionally with increasing silane (SiH4) flow rates. However, the mobility of polar (0001) n-type GaN was decreased from 391 to 242 cm2/V s, whereas that of semipolar (112¯2) n-type GaN unexpectedly increased from 110 to 180 cm2/V s on increasing the SiH4 flow rate. The full width at half maximum of X-ray rocking curves of semipolar (112¯2) n-GaN decreased on increasing the SiH4 flow rate, whereas that of polar (0001) n-GaN slightly increased with increasing SiH4 flow rate. In addition, the crystal domain size of semipolar (112¯2) n-GaN increased with increasing SiH4 flow rate, which was contrary to the trend seen in polar (0001) n-GaN. Based on these results, we suggest that Si doping of semipolar (112¯2) n-GaN would improve the crystalline qualities of semipolar (112¯2) GaN, resulting in an increase in the electron mobility and concentration.

Annealing temperature dependence of photoluminescent characteristics of silicon nanocrystals embedded in silicon-rich silicon nitride films grown by PECVD

Recently, light emission from silicon nanostructures has gained great interest due to its promising potential of realizing silicon-based optoelectronic applications. In this study, luminescent silicon nanocrystals (Si–NCs) were in situ synthesized in silicon-rich silicon nitride (SRSN) films grown by plasma-enhanced chemical vapor deposition (PECVD). SRSN films with various excess silicon contents were deposited by adjusting SiH4 flow rate to 100 and 200sccm and keeping NH3 one at 40sccm, and followed by furnace annealing (FA) treatments at 600, 850 and 1100°C for 1h. The effects of excess silicon content and post-annealing temperature on optical properties of Si–NCs were investigated by photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). The origins of two groups of PL peaks found in this study can be attributed to defect-related interface states and quantum confinement effects (QCE). Defect-related interface states lead to the photon energy levels almost kept constant at about 3.4eV, while QCE results in visible and tunable PL emission in the spectral range of yellow and blue light which depends on excess silicon content and post-annealing temperature. In addition, PL intensity was also demonstrated to be highly correlative to the excess silicon content and post-annealing temperature due to its corresponding effects on size, density, crystallinity, and surface passivation of Si–NCs. Considering the trade-off between surface passivation and structural properties of Si–NCs, an optimal post-annealing temperature of 600°C was suggested to maximize the PL intensity of the SRSN films.

Surface loss probability of H radicals on silicon thin films in SiH4/H2 plasma

The surface loss probability of H radicals was investigated in SiH4/H2 plasma using vacuum ultraviolet resonance absorption spectroscopy. The surface loss probability was calculated from the decay curve of the H radical density in the plasma afterglow and increased with the SiH4 flow rate. Silicon thin films deposited on the chamber wall were analyzed to investigate the relation between the surface loss probability and the surface condition. The surface reaction of H radicals is influenced by deposition precursors, such as SiH3 radicals. The density of H radicals significantly decreased with heating of the chamber wall up to 473 K. The surface loss probability of H radicals was estimated to be ca. 1 at 473 K. Quantitative measurements of the surface loss probability of H radicals in SiH4/H2 plasma are expected to be particularly important for understanding the surface reactions that occur during the deposition of silicon thin films.

4H-SiC Homoepitaxial Growth on Substrate with Vicinal Off-Angle Lower than 1°

We investigated homoepitaxial growth on 4H-SiC Si-face substrate with a vicinal off-angle lower than 1°. The control of the wafer off-angle to a precision of 0.1° is important to controlling the surface morphology of homoepitaxial growth on this substrate. Keeping the C/Si ratio low by controlling the SiH4 flow rate was effective in striking balance between surface morphology and polytype homogeneity. Homoepitaxial layers were successfully grown on whole, 2-inch Si-face vicinal off-angled substrate without large step bunching or polytype inclusions. The quality of the epitaxial layer was confirmed by I-V characteristics of Schottky barrier diodes fabricated on the epitaxial wafer.