Effect Of Hydrogen Dilution Research Articles

Gas Phase Chemistry Study During Deposition of a-Si:H and μc-Si:H Films by HWCVD using Quadrupole Mass Spectrometry

AbstractAmorphous and microcrystalline silicon films were deposited by HWCVD under different deposition conditions and the gas phase chemistry was studied by in situ Quadrupole Mass Spectrometry. Attempt is made to correlate the properties of the films with the gas phase chemistry during deposition. Interestingly, unlike in PECVD, partial pressure of H2 is higher than any other species during deposition of a-Si:H as well as μc-Si:H. Effect of hydrogen dilution on film properties and on concentration of various chemical species in the gas phase is studied. For low hydrogen dilution [H2]/ [SiH4] from 0 to 1 (where [SiH4] is 10 sccm), all films deposited are amorphous with photoconductivity gain of ∼ 106. During deposition of these amorphous films SiH2 was dominant in gas phase next to [H2]. Interestingly [Si]/[SiH2] ratio increases from 0.4 to 0.5 as dilution increased from 0 to 1, and further to more than 1 for higher hydrogen dilution leading to [Si] dominance. At hydrogen dilution ratio 20, consequently films deposited were microcrystalline.

Study of the optical properties and the density-of-states distribution of hydrogenated amorphous silicon-nitrogen alloy

Abstract The constant-photocurrent method (CPM) and photothermal deflection spectroscopy (PDS) have been used as complementary techniques to measure the density-of-states distribution and the different optical parameters in hydrogenated amorphous silicon-nitrogen alloy (a-SiN:H). Constants such as the optical gap E g the Urbach edge E u or valence-band edge E 0V were obtained directly from the CPM or photothermal deflection spectra. The height of the midgap-defect density of States, its wideness or the conduction-band edge have been deduced by applying a deconvolution procedure to the measured absorption spectra. The density of surface states was also calculated. The results were based on the different sensitivities of the CPM and PDS to transitions involving unoccupied defects and surface states. We have studied the influence of nitrogen incorporation in a-SiN: H samples and the effect of hydrogen dilution.

Study of the optical properties and the density-of-states distribution of hydrogenated amorphous silicon-nitrogen alloy

Abstract The constant-photocurrent method (CPM) and photothermal deflection spectroscopy (PDS) have been used as complementary techniques to measure the density-of-states distribution and the different optical parameters in hydrogenated amorphous silicon-nitrogen alloy (a-SiN:H). Constants such as the optical gap E g the Urbach edge E u or valence-band edge E 0V were obtained directly from the CPM or photothermal deflection spectra. The height of the midgap-defect density of States, its wideness or the conduction-band edge have been deduced by applying a deconvolution procedure to the measured absorption spectra. The density of surface states was also calculated. The results were based on the different sensitivities of the CPM and PDS to transitions involving unoccupied defects and surface states. We have studied the influence of nitrogen incorporation in a-SiN: H samples and the effect of hydrogen dilution.

Gap states of hydrogenated amorphous silicon near and above the threshold of microcrystallinity with subtle boron compensation

The effects of hydrogen dilution, subtle boron compensation, and light-soaking on the gap states of hydrogenated amorphous silicon films (a-Si:H) near and above the threshold of microcrystallinity have been investigated in detail by the constant photocurrent method and the improved phase-shift analysis of modulated photocurrent technique. It is shown that high hydrogen dilution near the threshold of microcrystallinity leads to a more ordered network structure and to the redistribution of gap states; it gives rise to a small peak at about 0.55 eV and a shoulder at about 1.2 eV below the conduction band edge, which are associated with the formation of microcrystallites embedded in the amorphous silicon host matrix. A concurrent subtle boron compensation is demonstrated to prevent excessive formation of microcrystallinity, and to help promote the growth of the ordered regions and reduce the density of gap defect states, particularly those associated with microcrystallites. Hydrogen-diluted and appropriately boron-compensated a-Si:H films deposited near the threshold of microcrystallinity show the lowest density of the defects in both the annealed and light-soaked states, and hence, the highest performance and stability.

Hot-Wire Chemical Vapor Deposition of Silicon from Silane: Effect of Process Conditions

We have examined the deposition of uniform polycrystalline silicon films over large surface areas for application in photovoltaics and flat panel displays. Depositions were conducted using a commercially available hot-wire chemical vapor deposition chamber. We investigated the effect of the silane flow rate (4−60 sccm), filament temperature (1550−1850 °C), total pressure (25−1000 mTorr), substrate temperature (400−600 °C), and hydrogen dilution on the exit gas-phase composition, film growth rate, and film crystalline fraction. Experiments show that the growth rate increases with silane flow rate and is independent of the substrate temperature. The growth rate variation with pressure and filament temperature is observed to change with the axial position of the substrate. A transition from amorphous (a-Si) to polycrystalline (poly-Si) silicon films is observed with increasing total pressure, filament temperature, and substrate temperature and with decreasing silane flow rate. The effect of hydrogen dilution...

Optical characterization of wide band gap amorphous semiconductors (a-Si:C:H): Effect of hydrogen dilution

The effect of hydrogen dilution on the optical properties of a wide band gap amorphous semiconductor (a-Si:C:H) was investigated. The samples were prepared by glow discharge decomposition of tetramethylsilane and were characterized primarily by optical techniques: spectroscopic ellipsometry, Raman scattering, infrared absorption, spectrophotometry, and UV photoluminescence. The deposition rate decreased with hydrogen dilution, while the silicon to carbon ratio remained constant with the addition of hydrogen. The optical band gap of this material increased as the hydrogen flow rate increased. Infrared absorption studies show that the concentration of hydrogen which is bonded to carbon decreases systematically upon hydrogen dilution. Hydrogen dilution appears to reduce the size and concentration of sp2 bonded carbon clusters, possibly caused by the etching of sp2 clusters by atomic hydrogen. The result was also supported by the shift of the Raman G peak position to a lower wave number region. Room temperature photoluminescence in the visible spectrum was observed with UV excitation.

Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy

ABSTRACTa-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.

Towards Stabilized 10% Efficiency of Large-Area (> 5000cm2) a-Si/a-SiGe Tandem Solar Cells using High-Rate Deposition

ABSTRACTThis paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells in Sanyo. Much effort has been devoted to increasing both the stabilized efficiency and the process throughput. A key issue in increasing the stabilized efficiency is thinner i-layer structure with an improved optical confinement effect. High-rate deposition of the i-layers has been investigated using rf (13.56MHz) plasma-CVD method while keeping the substrate temperature below 200 °C. A high photosensitivity of 106 of a-Si:H films maintain up to the deposition rate (Rd) of 15 Å/s by optimizing hydrogen dilution and other deposition conditions. It is of great importance to utilize the effect of hydrogen dilution which can reduce the incorporation of excess hydrogen in the films. The world's highest conversion efficiency of 11.2% has been achieved for a large-area (8252cm2) a-Si/a-SiGe tandem by combining the optimized hydrogen dilution and other solar cell related technologies.

High Quality Amorphous Silicon Germanium Alloy Solar Cells Made by Hot-wire CVD at 10 Å/s

ABSTRACTHigh quality amorphous silicon germanium (a-SiGe:H) alloys have been obtained using the hot wire chemical vapor deposition (HWCVD) from a gas mixture of SiH4, GeH4, and H2 at a deposition rate of ∼10 Å/s. Solar cells in a SS/n-i-p/ITO configuration are evaluated in which the n- and i-layers are deposited by HWCVD at NREL and the microcrystalline p-layer by conventional RF glow discharge in a separate reactor by United Solar. Effects of hydrogen dilution and step-wise bandgap profile have been studied and optimized. The best cell has an average optical bandgap of 1.6 eV and incorporates multi-bandgap steps where the narrow-most bandgap is near the p-i interface. J-V characteristics are measured under AM 1.5 illumination with a λ>530 nm filter. The best initial power output obtained exceeds 4 mW/cm2, which is usually used as an indicator for a good quality middle-gap cell. Double-junction cells are made on textured Ag/ZnO back reflectors. The bottom cell uses the optimized a-SiGe:H alloy cell by HWCVD, and the top cell uses an optimized a-Si:H cell near the amorphous-to-microcrystalline transition by PECVD at ∼1 Å/s. The best double-junction cell made to date exhibits an initial AM 1.5 active-area efficiency of 11.7%, and a stable efficiency after 1000 hours of one sun light soaking of 9.6%.

Thermodynamic Model of the Role of Hydrogen Dilution in Plasma Deposition of Microcrystalline Silicon

ABSTRACTHydrogen dilution is used to promote the nucleation and growth of microcrystalline Si (μc-Si) by plasma enhanced chemical vapour deposition (PECVD). The free energy of μc-Si and hydrogenated amorphous silicon (a-Si:H) is analysed as a function of Si:H composition in order to derive the effect of hydrogen dilution. It is shown that increasing the hydrogen content of the a-SiHx precursor phase increases the relative stability of μc-Si slightly, but strongly increases the driving force for nucleation. The higher stability of μc-Si is the fundamental origin of the higher etch rates of a-Si:H, while surface mobility models do not account for sub-surface nucleation of μc-Si.

Computational Fluid Dynamics Modeling of Proton Exchange Membrane Fuel Cells

A transient, multidimensional model has been developed to simulate proton exchange membrane fuel cells. The model accounts simultaneously for electrochemical kinetics, current distribution, hydrodynamics, and multicomponent transport. A single set of conservation equations valid for flow channels, gas‐diffusion electrodes, catalyst layers, and the membrane region are developed and numerically solved using a finite‐volume‐based computational fluid dynamics technique. The numerical model is validated against published experimental data with good agreement. Subsequently, the model is applied to explore hydrogen dilution effects in the anode feed. The predicted polarization curves under hydrogen dilution conditions are in qualitative agreement with recent experiments reported in the literature. The detailed two‐dimensional electrochemical and flow/transport simulations further reveal that in the presence of hydrogen dilution in the fuel stream, hydrogen is depleted at the reaction surface, resulting in substantial anode mass transport polarization and hence a lower current density that is limited by hydrogen transport from the fuel stream to the reaction site. Finally, a transient simulation of the cell current density response to a step change in cell voltage is reported. © 2000 The Electrochemical Society. All rights reserved.

Effect of hydrogen dilution on the structure of SiOF films prepared by remote plasma enhanced chemical vapor deposition from SiF4-based plasmas

Structural and electrical properties of fluorinated silicon dioxide (SiOF) films prepared by remote plasma enhanced chemical vapor deposition from the SiF4–O2–H2–He gas mixture have been studied using ellipsometry, Fourier transform infrared spectroscopy, transmission electron microscopy, and current–voltage measurements. It has been found that the level of hydrogen dilution strongly affects the microstructure of deposited SiOF films. The films prepared at the H2 flow rate below about 0.8 sccm have a biphase structure consisting of an amorphous matrix with the incorporation of 5–30 nm sized particles. The main origin of these particles seems to be gas phase oxidation of SiFx species (with x=1, 2, 3) in plasma and downstream regions. Resulting films are characterized by extremely low density, reduced structural homogeneity, and poor electrical properties. Increase in the H2 flow rate above 0.8 sccm completely suppresses the incorporation of particles into the growing film probably due to effective hindering gas phase oxidation process and results in dense homogeneous amorphous SiOF films with good electrical properties.

Some indications of different film forming radicals in a-Si:H deposition by the glow discharge and thermocatalytic CVD processes

In this paper we present some experimental results which support the hypothesis that different film forming radicals (precursors) are responsible for the deposition of hydrogenated amorphous silicon (a-Si:H) films by the plasma enhanced chemical vapor deposition (PECVD) and thermocatalytic chemical vapor deposition (TCCVD). Indications for the above conjecture come from investigations and comparison of the growth of a-Si:H films prepared by the two methods. Basic differences in the silane dissociation process and subsequent radical collisions in the gas phase obviously produce different precursors. These produce deviations during the coalescence phase of the initial film growth and a very different effect of hydrogen dilution on the film preparation. As a result dense high quality a-Si:H is formed in both methods at quite different optimum substrate temperatures (∼250°C and ∼400°C) and with large differences in hydrogen content (∼10 and ∼2 at.%, respectively).

Electro- and photo-luminescence spectra from a-Si:H and a-SiGe p–i–n solar cells

Electroluminescence (EL) and photoluminescence (PL) spectroscopies were used to study the effects of hydrogen dilution on a-Si:H and of varying Ge content on a-SiGe p–i–n cells. We found that (a) in the case of hydrogen dilution, the EL peak energy (EL peak) increased with H-dilution that correlated with an increase of open circuit voltage ( V oc), but the corresponding PL peak energy (PL peak) did not change; and (b) different from observations that EL peak<PL peak, the EL peak can be the same or larger than the PL peak. The results were explained by the model of dispersive-transport-controlled recombination for EL. We suggest that the V oc is not only related to the optical gap and the tail states but also to the transport processes.

Influence of hydrogen dilution on low-temperature polycrystalline silicon formation using RF excitation SiH 4/H 2 plasma

The effect of hydrogen dilution was investigated on polycrystalline silicon formation using radio frequency excitation SiH 4/ H 2 plasma. The hydrogen dilution reduces the growth rate of the a-Si : H films. The dark conductivity of the a-Si : H films increases with increasing H 2 dilution. The dark conductivity of the poly-Si films formed by recrystallization annealing of the a-Si : H film decrease with H 2 dilution. The grain size, with X-ray diffraction spectroscopy and scanning electron microscopy images of the poly-Si films, is in reverse ratio to the H 2 dilution.

Amorphous and microcrystalline silicon films grown at low temperatures by radio-frequency and hot-wire chemical vapor deposition

The effect of hydrogen dilution on the optical, transport, and structural properties of amorphous and microcrystalline silicon thin films deposited by hot-wire (HW) chemical vapor deposition and radio-frequency (rf) plasma-enhanced chemical vapor deposition using substrate temperatures (Tsub) of 100 and 25 °C is reported. Microcrystalline silicon (μc-Si:H) is obtained using HW with a large crystalline fraction and a crystallite size of ∼30 nm for hydrogen dilutions above 85% independently of Tsub. The deposition of μc-Si:H by rf, with a crystallite size of ∼8 nm, requires increasing the hydrogen dilution and shows decreasing crystalline fraction as Tsub is decreased. The photoconductivity, defect density, and structure factor of the amorphous silicon films (a-Si:H) are strongly improved by the use of hydrogen dilution in the Tsub range studied. a-Si:H films with a photoconductivity-to-dark conductivity ratio above 105, a deep defect density below 1017 cm−3, an Urbach energy below 60 meV and a structure factor below 0.1 were obtained for rf films down to 25 °C (at growth rates ∼0.1–0.4 Å/s) and for HW films down to 100 °C (at growth rates ∼10 Å/s), using the appropriate hydrogen dilution. In the low Tsub range studied, the growth mechanism, film properties, and the amorphous to microcrystalline silicon transition depend on the flux of atomic hydrogen available. The properties of the films are compared to those of samples produced at 175 and 250 °C in the same reactors.

Hydrogen dilution effect on the properties of coplanar amorphous silicon thin-film transistors fabricated by inductively-coupled plasma CVD

The electrical and optical properties of the hydrogenated amorphous silicon (a-Si:H) films deposited by inductively-coupled plasma (ICP) chemical vapor deposition (CVD) with a variation of H/sub 2/ flow rate have been studied. The photosensitivity of a-Si:H is /spl sim/10/sup 7/ when the H/sub 2//SiH/sub 4/ ratio is between 3 and 8. With increasing H/sub 2//SiH/sub 4/, the SiH/sub 2/ mode infrared absorption has a minimum at a H/sub 2//SiH/sub 4/ ratio of 8. Coplanar a-Si:H thin-film transistors (TFT's) were fabricated using a triple layer of thin a-Si:H, silicon-nitride, and a-Si:H deposited by ICP-CVD using ion doping and low resistivity Ni silicide. After patterning the thin a-Si:H/silicon-nitride layers on the channel region, the gate and source/drain regions were ion-doped and then heated at 230/spl deg/C to form Ni silicide layers. The low resistive Ni silicide formed on the a-Si:H reduces the offset length between gate and source/drain, leads to a coplanar a-Si:H TFT. The TFT exhibited a field effect mobility of 0.6 cm/sup 2//Vs and a threshold voltage of 2.3 V at the H/sub 2//SiH/sub 4/ ratio of 8. The effect of H/sub 2/ dilution in SiH/sub 4/ on the coplanar a-Si:H TFT performance has been investigated. We found that the performance of the TFT is the best when the SiH/sub 2/ mode density in a-Si:H is the minimum. The coplanar TFT is very suitable for large-area, high density TFT displays because of its low parasitic capacitance between gate and source/drain contacts.

Effect of hydrogen dilution on the optical properties of hydrogenated amorphous silicon prepared by plasma deposition

Abstract A series of experimental studies has been made on the effect of the dilution of silane with hydrogen on optical properties of hydrogenated amorphous silicon films (a-Si:H) prepared by plasma deposition as functions of the gas-volume ratio γ (=[SiH4]/([SiH4] + [H2])) and the substrate temperature T s. The effect of hydrogen dilution has been discussed in terms of the obtained values of the deposition rate R d, the optical gap E g, the Urbach energy E u, the defect density N d, the hydrogen content C H and the refractive index n 0 and their correlations between them and the hydrogen-bonding configuration estimated from infrared absorption spectra. Its effect strongly depends on T s and decreases N d and E u from γ = 100% to about 20%. The effect for reducing them is discussed in terms of the surface-limited optimal growth model containing the hydrogen dilution effect. On decreasing γ from about 10%, both N d and E u increase in spite of the decrease in R d. This result is due to the onset of the formation of microcrystalline phase and is discussed in terms of the ‘selective etching’ model. An a-Si: H film with E u less than 50 meV is prepared at our optimal growth conditions: γ ≈ 20% and T s, ≈ 200°C.

Effect of hydrogen dilution on the optical properties of hydrogenated amorphous silicon prepared by plasma deposition

Effect of hydrogen dilution on the optical properties of hydrogenated amorphous silicon prepared by plasma deposition

In Situ Diagnostics of Methane/Hydrogen Plasma Interactions with Si(100)

ABSTRACTMethane/hydrogen plasmas have been reported to be sources both for a-C:H film deposition and for compound semiconductor etching. In this work, an in situ diagnostic study of methane/hydrogen plasma interactions with a silicon surface is carried out, focusing on the effect of hydrogen dilution. A remote electron cyclotron resonance (ECR) plasma using a H2/Ar mixture excites methane gas near a Si(l 00) substrate. In situ multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy is used to probe the surface species at different hydrogen dilution ratios. We find that at low methane pressure without hydrogen dilution, a-C:H films are deposited. With H2 dilution, the results suggests that some sputter/etching of the silicon surface occurs. Hence, methyl groups are identified as potential etchants for silicon materials. The data suggest that there is a competition between etching and deposition chemistry which depends strongly upon the methane pressure and hydrogen ratio in the plasma.