Effect Of Hydrogen Dilution Research Articles

Effect of hydrogen dilution on carrier transport in hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloys

The effect of the hydrogen dilution ratio on characteristics of hydrogenated boron-doped nanocrystalline silicon-silicon carbide alloy (p-nc-Si–SiC:H) films is investigated. Hydrogen coverage near the growing surface causes nanocrystallization by retarding the reactions of the precursors. It was found that p-nc-Si–SiC:H alloys have two different kinds of carrier transport mechanisms: one is the thermally activated hopping conduction between neighboring crystallites near room temperature and the other is the band tail hopping conduction below 150K. However, the film at the onset of the nanocrystalline growth exhibits a different behavior due to a large band tail disorder.

Effect of hydrogen dilution on structure and optical properties of polycrystalline silicon films

Polycrystalline silicon films were prepared from SiCl4 diluted with hydrogen by plasma-enhanced chemical vapor deposition at a low temperature of 250℃. The effect of hydrogen dilution on their structure and optical properties were investigated. It was found that the effect of hydrogen on the growth behavior of the films deposited using SiCl4+H2 are completely different from that of using SiH4/H2. The crystalline fraction increases with decreasing the hydrogen dilution ratio R and reaches a maximum value of 85% at a low hydrogen dilution. However, further decreasing hydrogen dilution ratio leads the crystallinity to deteriorate and a phase transition between microcrystalline silicon and amorphous silicon is observed. The optical band gap gradually decreases from ～1.5eV to ～1.2eV and then increases up to 1.8eV with the decrease of the hydrogen dilution ratio. The deposition rate initially increases and then decreases with decreasing hydrogen dilution ratio, and no film is formed by using pure SiCl4. According to these results, we conclude that under the condition of optimum hydrogen dilution ratio, the enhanced crystalline fraction and the increase in grain size are attributed to the Cl radicals which play important roles in the low-temperature growth of crystalline silicon films.

Effect of Hydrogen Dilution on Structure and Electronic Properties of Ge:H and GeYSi1-Y Films Deposited by Low Frequency Plasma

AbstractWe report on a systematical study of growth rate, surface morphology, hydrogen and oxygen incorporation, optical and electrical properties in Ge:H and GeYSi1-Y:H, Y> 0.85 films, deposited in a capacitive reactor by low frequency PE CVD. Silane and germane were used as feed gases diluted by hydrogen. Hydrogen dilution characterized by R= QH2/[QSiH4+QGeH4], where QH2, QSiH4, and QGeH4 are gas flows of hydrogen, silane and germane, respectively. The flow was varied in the range of R=20 to 80. Composition of the films was characterized by SIMS profiling. We did not observed a significant change of the deposition rate Vd in GeYSi1-Y:H films in all the range of R, while for Ge:H films Vd was significantly reduced after R=50. AFM characterization of the surface morphology demonstrated that at R=50 average height <H>(R) reached maximum in both Ge:H and GeYSi1-Y:H films, while average diameter <D>(R) had a minimum in GeYSi1-Y:H films and maximum in Ge:H films. Both Ge:H and GeYSi1-Y:H films demonstrated change of E04 in the studied range of R, and a minimum clearly appeared in E at R=50-60 suggesting significant reduction in weak bonds of these films. The activation energy of conductivity Ea slightly increases with R in Ge:H films and shows no definitive trend in GeYSi1-Y:H: films. Both FTIR and SIMS data show a general trend of reducing hydrogen and oxygen content with R. These two types of films showed different behavior and correlations between surface morphology and optical and electrical properties.

Amorphous silicon nitride deposited by hot-wire chemical vapor deposition

High-quality amorphous silicon nitrides were deposited by hot-wire chemical vapor deposition using SiH4, NH3, and H2 gases. These films show a high deposition rate of 5Å∕s, a low processing temperature of 300°C, an excellent conformal coverage, a low etching rate of 7Å∕min, an index of refraction of 2.1, an optical band gap of 4.0eV, and a high breakdown field of 3MV∕cm. The effects of hydrogen dilution, substrate temperature, chamber pressure, and filament temperature on silicon nitride film property were studied to optimize the process. We found that adding H2 to the processing significantly enhances the silicon nitride films’ properties. The N content in the film increased significantly based on the infrared measurement. Hydrogen dilution is believed to play a key role for the conformal silicon nitride film. Hydrogen dilution also improves the process in that the gas ratio of NH3∕SiH4 has been greatly reduced with the assistance of the H2 gas. With substrate temperatures varying from 23°to400°C, this study showed a best film at near 300°C. However, a good-quality silicon nitride can be grown even if starting with a substrate at room temperature. Furthermore, we found that increasing chamber pressure and a high filament temperature result in higher deposition rate and better quality in the films. The optimized films were grown with hydrogen dilution at about 300°C substrate temperature, 83mTorr pressure, and 2100°C filament temperature.

A 3D model for PEM fuel cells operated on reformate

A three-dimensional mathematical model for PEM fuel cells operated on reformate is developed based on our previous established fuel cell model [Int. J. Transport Phenomena 3 (2001) 177], by incorporating the adsorption and oxidation kinetics of CO on platinum surface proposed by Springer et al. [Proceedings of the Electrochemical Society, Montreal, Canada, 1997; J. Electrochem. Soc. 148 (2001) A11]. This model is capable of studying the effect of CO poisoning as well as the hydrogen dilution effect by inert gases. The adsorption and oxidation kinetics of CO on a platinum surface are incorporated in the source terms of the species equations; thus, the basic form of the mathematical equations are the same as those used for PEM fuel cells operated on pure hydrogen. With this model, we can obtain detailed information on the CO poisoning and variation of CO and hydrogen concentrations inside the anode. The results from this 3D model reveal many new phenomena that cannot be obtained from previous 1D or 2D models. Results of the effects of various operating and design parameters, such as anode flow rate, gas diffuser porosity, gas diffuser thickness, and the width of the collector plate shoulder, are also presented. The modeling results demonstrate the value of this model as a design and optimization tool for the anode of PEM fuel cells operating on reformate.

A 3D model for PEM fuel cells operated on reformate

A three-dimensional mathematical model for PEM fuel cells operated on reformate is developed based on our previous established fuel cell model [Int. J. Transport Phenomena 3 (2001) 177], by incorporating the adsorption and oxidation kinetics of CO on platinum surface proposed by Springer et al. [Proceedings of the Electrochemical Society, Montreal, Canada, 1997; J. Electrochem. Soc. 148 (2001) A11]. This model is capable of studying the effect of CO poisoning as well as the hydrogen dilution effect by inert gases. The adsorption and oxidation kinetics of CO on a platinum surface are incorporated in the source terms of the species equations; thus, the basic form of the mathematical equations are the same as those used for PEM fuel cells operated on pure hydrogen. With this model, we can obtain detailed information on the CO poisoning and variation of CO and hydrogen concentrations inside the anode. The results from this 3D model reveal many new phenomena that cannot be obtained from previous 1D or 2D models. Results of the effects of various operating and design parameters, such as anode flow rate, gas diffuser porosity, gas diffuser thickness, and the width of the collector plate shoulder, are also presented. The modeling results demonstrate the value of this model as a design and optimization tool for the anode of PEM fuel cells operating on reformate.

Highly conductive n+ hydrogenated microcrystalline silicon and its application in thin film transistors

n + μc- Si:H films were deposited using 13.56 MHz plasma enhanced chemical vapor deposition from a gas mixture of silane, phosphine, and hydrogen. To study the effect of deposition conditions and thickness dependence on film properties, and to optimize deposition regimes, we varied the rf power and doping ratio for two different hydrogen dilutions. The film properties were investigated using Raman, x-ray diffraction, secondary-ion-mass spectroscopy, and optical transmittance measurements, as well as dark conductivity. The growth mechanism for a formation of n+ μc-Si:H is explained in terms of a hydrogen dilution effect using the combination of surface diffusion and selective etching models. The optimal conductivity [25 (Ωcm)−1] 50 nm n+ μc-Si:H film was obtained with 99.6% hydrogen dilution of silane. Thin film transistors with this n+ μc-Si:H ohmic contact layer demonstrate a device mobility of 0.9 cm2/Vs, a threshold voltage of 3 V, an ON/OFF current ratio of above 107, a subthreshold slope of 0.5 V/dec, and a leakage current of the order of 10−13 A.

Characteristics of the electron temperature in VHF-excited SiH 4/H 2 plasma produced using a ladder-shaped electrode

Discharge frequency was selected at 80 MHz to generate VHF-excited SiH 4/H 2 plasma using a ladder-shaped electrode. The plasma parameters were measured using a heated Langmuir probe. The effect of hydrogen dilution on SiH 4/H 2 VHF plasma characteristics was investigated. The electron temperature took the minimum value at SiH 4 flow rate/(SiH 4 flow rate + H 2 flow rate)=4–9%, which is corresponding to the H 2 dilution rate near the deposition condition of hydrogenated microcrystalline silicon films. The rate of H 2 dilution where electron temperature takes the minimum value depended on RF power.

The effect of hydrogen dilution on the field emission from hydrogenated amorphous carbon films

The microstructures and field emission characteristics of hydrogenated amorphous carbon films prepared by using different hydrogen dilution ratio were investigated. It was found that a very low threshold electric field emission could be achieved for samples with moderated hydrogen dilution ratio. However, the field emission characteristics became worse for samples with high hydrogen dilution ratio. The change of field emission can be attributed to the change of electronic structures due to the hydrogen dilution in addition to the increase of the hydrogen surface termination.

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

The effect of the dilution of silane and nitrogen with hydrogen on the optical properties of hydrogenated amorphous silicon-nitrogen films prepared by plasma deposition has been investigated as functions of the gas-volume ratio γ (= ([SiH4] + [N2])/([SiH4] + [N2] + [H2]) and the substrate temperature. The prepared films are characterized by the values of the deposition rate, the optical gap, the Urbach energy, the defect density, the integrated infrared absorption intensity and the refractive index, and by correlations between these parameters and the type of hydrogen- and nitrogen-bonding configurations estimated from infrared absorption spectra. The hydrogen dilution effect is discussed in terms of the above and compared with that in hydrogenated amorphous silicon reported in a previous paper by the present authors. It is pointed out that nitrogen atoms incorporated into the silicon network cause more disorder than incorporated hydrogen atoms, from the γ dependence of the Urbach energy and the integrated infrared intensities associated with the hydrogen and nitrogen bondings.

Characteristics of VHF-excited SiH 4/H 2 plasmas using a ladder-shaped electrode

A VHF-excited SiH 4/H 2 plasma was produced using a ladder-shaped electrode. By measuring the plasma parameters with a heated Langmuir probe, the effect of hydrogen dilution on SiH 4/H 2 VHF plasma characteristics was investigated. The ion density of the SiH 4/H 2 plasma was higher than that of the H 2 plasma. The electron temperature tended to decrease for high H 2 dilution rate and became minimum at H 2 flow rate/(SiH 4 flow rate+H 2 flow rate)=91%, corresponding to the H 2 dilution rate near the deposition condition of hydrogenated microcrystalline silicon films. Negative ions exist in the SiH 4/H 2 plasma. The negative ion density increased with increases in the radio frequency power. At the H 2 dilution rate where the negative ions exist, the electron temperature was high approximately 1.5 eV compared with that at other dilution rates.

Effect of hydrogen dilution on electronic properties of a-SiHx films deposited by low-frequency plasma

The effect of hydrogen dilution during plasma deposition on hydrogen incorporation and the optical and electrical properties of a-SiHx films were studied. The films were grown in capacitive low-frequency (f = 10 and 110 kHz) discharge in SiH4 diluted with H2, varying the ratio RH of the gases H2/SiH4 from RH = 0 to 40. The optical absorption coefficient and optical bandgap were changed with RH. Si–H bonding, studied by infrared spectroscopy, depended on RH. Hydrogen concentration in the films estimated from infrared spectra was in the range 20–30%. We observed the significant effect of RH on the temperature dependence of conductivity σ(T) and on the subgap absorption spectra measured by the constant photocurrent method. The reduction of subgap absorption up to 1.5 order of magnitude was observed with increasing RH.

Effect of hydrogen dilution on the open-circuit voltage of hydrogenated amorphous silicon solar cells

The open-circuit voltage (Voc) of hydrogenated amorphous silicon solar cells has been studied experimentally and by computer simulation. We find that band gap and defect states affect Voc significantly. Hydrogen (H2) dilution widens the band gap and reduces density of defect states; both contribute to larger Voc and better fill factor in H2 diluted solar cells. Valence band tail narrowing could affect Voc, but is not included in this study due to lack of available data.

Effect of hydrogen dilution on the growth of nanocrystalline silicon films at high temperature by using plasma-enhanced chemical vapor deposition

Two hundred-nm-thick nanocrystalline silicon films were deposited at 620 °C under different H 2 flow rates, ([H 2]), by plasma-enhanced chemical vapor deposition using SiH 4/H 2 mixtures. When [H 2] increased, the grain size decreased. On the other hand, no crystallization was found at [H 2]=5 sccm. The photoluminescence spectra changed from a single peak of approximately 2.0–2.1 eV at [H 2]=10 sccm to two separated lines of approximately 1.7 and 2.1 eV at [H 2] between 15 and 20 sccm. However, the 2.1-eV band decreased with increasing [H 2], and at [H 2]=25 sccm, we observed a single peak at approximately 1.8 eV. The hydrogen content decreased and two infrared absorption bands approximately at 850 and 1000 cm −1 also decreased with [H 2].

High quality amorphous silicon materials and cells grown with hydrogen dilution

Hydrogen dilution of the active gas during deposition has been found to be a very effective way to improve the quality of amorphous silicon-based materials and solar cells. With increasing hydrogen dilution, the material is characterized by an improved order, and at a certain threshold dilution, the amorphous to microcrystalline transition takes place. The best material is grown just below the threshold and is heterogeneous consisting of tiny crystallites embedded in an amorphous matrix of improved order. In this paper, we discuss the effects of hydrogen dilution on the material and cell properties of amorphous silicon-based alloys and provide an explanation for their improved stability against light-induced degradation. We also discuss some special properties of the on-the-edge materials that are not seen in the conventional amorphous or microcrystalline alloys.

High Rate Deposition of Stable Hydrogenated Amorphous Silicon in Transition from Amorphous to Microcrystalline Silicon

AbstractHigh rate deposition of high quality and stable hydrogenated amorphous silicon (a-Si:H) films were performed near the threshold of amorphous to microcrystalline phase transition using a very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The effect of hydrogen dilution on optic-electronic and structural properties of these films was investigated by Fourier-transform infrared (FTIR) spectroscopy, Raman scattering and constant photocurrent method (CPM). Experiment showed that although the phase transition was much influenced by hydrogen dilution, it also strongly depended on substrate temperature, working pressure and plasma power. With optimized condition high quality and high stable a-Si:H films, which exhibit σph/σd of 4.4×106 and deposition rate of 28.8Å/s, have been obtained.

Optimization of 75°C amorphous silicon nitride for TFTs on plastics

AbstractAmorphous silicon nitride (a-SiNx) is widely used as the gate dielectric and passivation layer in a-Si:H based electronics. For devices on plastic substrates deposited at low temperature, the a-SiNx quality seems to determine the device performance. This paper investigates the effects of hydrogen dilution, helium dilution, ammonia-silane gas flow ratio, and RF power on the properties of PECVD silicon nitrides deposited in large-area parallel-plate reactors at substrate temperatures of 75°C. The chemical composition and bonding of the SiNx:H films was studied using FTIR spectroscopy. The physical properties were investigated, and the density, growth rate, and compressive stress of the films were determined. The electrical properties such as leakage current, breakdown, stability, trap density, and dielectric constant of the films were characterized by I-V and C-V measurements of metal-insulator semiconductor (MIS) structures. Analysis of Variance (ANOVA) was performed on the results, and the deposition conditions for the optimal film properties were determined. The optimum film had SiNx:H stoichiometry of x=1.56 with hydrogen concentrations of 17 at.%, and exhibited compressive stress of -220 MPa. The film displayed good stability under electrical stress with ohmic leakage of Rleak ∼1016 Ωcm. Strong relationships between the film properties and deposition conditions were observed, and are discussed within the paper. A-Si:H bottom gate TFTs were fabricated using the optimized nitrides for gate dielectrics and passivation layers, and the influence of a-SiNx on TFT performance is discussed.

Thermodynamic model of nucleation and growth of plasma deposited microcrystalline silicon

Hydrogen dilution is used to promote the nucleation and growth of microcrystalline Si (μc–Si) by plasma-enhanced chemical-vapor deposition. The free energy of μc–Si and hydrogenated amorphous silicon (a–Si:H) is analyzed as a function of the Si:H composition in order to understand 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 subsurface nucleation of μc–Si.

Effect of hydrogen on adsorbed precursor diffusion kinetics during hydrogenated amorphous silicon deposition

Fractal analysis of the surface topography is used to study the effects of hydrogen dilution on the surface transport kinetics during the plasma deposition of hydrogenated amorphous silicon. Images obtained from atomic force microscopy are examined using dimensional fractal analysis, and surface diffusion lengths of growth precursors are estimated from the measured correlation lengths. The addition of small amounts of hydrogen (H2/SiH4 ratios <10/1) during deposition leads to a decrease in the diffusion length, but larger hydrogen dilutions result in increased diffusion length. Moreover, the measured surface diffusion activation barrier is reduced from 0.20 eV for deposition from pure SiH4 to 0.13 eV with high hydrogen dilution. Results are consistent with recent models for precursor surface transport during low-temperature deposition, and give insight into critical processes for low-temperature silicon crystallization.

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.