H2 Dilution Research Articles

Correlation of Material Properties and Open-Circuit Voltage of Amorphous Silicon Based Solar Cells

AbstractCorrelation of hydrogenated amorphous silicon (a-Si:H) alloy material properties and solar cell characteristics have been studied experimentally and by computer simulation. Simulation results show that all three solar cell parameters, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF), decrease with increased defect density. For a given intrinsic layer thickness, a larger band gap (Eg) results in a higher Voc but a lower Jsc. However, FF does not depend on band gap. This allows us to distinguish the effect of change in band gap from that in defect density on the variation in Voc. For solar cells with good interface characteristics, a linear relation FF = βVoc + γ is obtained by light soaking experiments and simulation with different defect densities. The slope β is in the range from 2 to 3 V-1 depending on cell properties and light soaking condition, and the intersect γ depends mainly on the band gap. Comparing cells made with high H2 dilution to no H2 dilution, we find that a 58 mV enhancement in Voc with H2 dilution is due to both widening of band gap and reduced defect density. Simulation results also show that a narrower valence band tail leads to a higher Voc. We did not include this effect in the analysis due to lack of available data for correlation between H2 dilution and band tail narrowing.

Helium Versus Hydrogen Dilution of Silane in the Deposition of Polymorphous Silicon Films: Effects on the Structure and the Transport Properties

AbstractWe compare the deposition rate, hydrogen incorporation and optoelectronic properties of hydrogenated polymorphous silicon films produced either by the decomposition of silane-hydrogen or of silane-helium mixtures. Our results clearly show that He dilution allows to drastically reduce the RF power needed to achieve the same deposition rate as in the case of H2 dilution. Infrared spectroscopy and hydrogen effusion experiments show clear differences in the hydrogen bonding and content in both series of films. Interestingly, both He and hydrogen dilution result in films with improved transport properties, in particular the hole diffusion length, with respect to standard amorphous silicon. These results indicate that He dilution is a good alternative to H2 dilution to prepare intrinsic layers for solar cells.

Effects of Excitation Frequency and H2 Dilution on Cluster Generation in Silane High-Frequency Discharges

AbstractReduction of cluster amount in silane discharges is the key to decreasing microstructure parameter Rα of a-Si:H films deposited with the discharges. The cluster amount is found to be reduced more than one order of magnitude using 60 MHz discharges instead of 28 MHz ones or using H2 dilution of an H2/SiH4 ratio of 5. The cluster-suppressed plasma CVD using 60 MHz discharges realizes deposition of a-Si:H films of Rα~ 0 at a fairly high rate of 0.55 nm/s. Moreover, a downstream cluster collection method of high sensitivity has been developed for detecting a small amount of clusters formed under deposition conditions of Rα < 0.01.

Microcrystalline Silicon Films and Solar Cells Prepared by Photochemical Vapor Deposition on Textured SnO2 with High Haze Factors

Microcrystalline silicon (µc-Si) films and solar cells were prepared by mercury-sensitized photochemical vapor deposition (photo-CVD). The changes in the structural properties of a series of µc-Si films grown under various H2 dilution and deposition pressure conditions were discussed. The results indicated that the properties of µc-Si films depend strongly on the atomic hydrogen. The microstructures of µc-Si films on textured SnO2 with different haze factors (from 13% to 65%) were observed with a scanning electron microscope and a transmission electron microscope. The observations revealed that the µc-Si layers grew from the initial stage of deposition with columnar grains and that they were conformal to the surface of textured SnO2. The grain boundary density of µc-Si film on SnO2 with higher haze factors was lower than that on SnO2 with lower haze factors. The effect of textured SnO2 with different haze ratios on p–i–n µc-Si cell characteristics was discussed and it was found that the higher haze factors showed a higher degree of light trapping in our µc-Si cells. We deposited µc-Si cells with an intrinsic µc-Si layer thickness of 430 nm on a textured SnO2 sample with a haze factor of 42% and achieved a conversion efficiency of 6.55%.

Analytical model for the optical functions of amorphous semiconductors from the near-infrared to ultraviolet: Applications in thin film photovoltaics

We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (ε1,ε2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (ε1,ε2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (ε1,ε2) can be simulated assuming a single Lorentz oscillator. The expression developed here provides an improved description of ε2 (or α) in the below-band gap and near-band gap regions compared with previous approaches. Although the expression is more complicated analytically, it has numerous applications in the analysis and simulation of thin film a-Si:H based p-i-n and n-i-p multilayer photovoltaic devices. First, we describe an approach whereby, from a single accessible measure of the optical band gap, the optical functions can be generated over the full solar spectrum for a sample set consisting of the highest quality intrinsic a-Si:H based alloys prepared by plasma-enhanced chemical vapor deposition using the principle of maximal H2 dilution. Second, we describe quantitatively how such an approach can be modified for sample sets consisting of lower quality alloy materials. Finally, we demonstrate how the generated optical functions can be used in simulations of the absorption, reflection, and quantum efficiency spectra of a-Si:H based single-junction and multijunction solar cells.

Hydrogenated amorphous silicon carbide deposition using electron cyclotron resonance chemical vapor deposition under high microwave power and strong hydrogen dilution

We have investigated the growth of a-Si1−xCx:H using the electron cyclotron resonance chemical vapor deposition (ECR-CVD) technique, under the conditions of high microwave power and strong hydrogen (H2) dilution. The microwave power used is 900 W and a gas mixture of CH4 and SiH4 diluted in H2 is varied to give carbon (C) fractions x ranging from 0 to 1. We aim to understand the effects of these deposition conditions on the characteristics of ECR-CVD grown a-Si1−xCx:H films at different x. Their microstructure and optical properties are investigated using infrared absorption, Raman scattering, UV-visible spectrophotometry, and photothermal deflection spectroscopy. Information on the atomic fraction x is obtained with Rutherford backscattering spectrometry. The B parameter in the Tauc relation is found to decrease and the Urbach energy Eu increase with x, which are indicative of a higher degree of disorder with C incorporation. At intermediate x, the presence of Si–C bonds can be clearly seen from the IR absorption and Raman scattering results. The T peak around 1200 cm−1 is observed in the Raman spectra of the C-rich samples, with a redshift noted at increasing x. This suggests an increased presence of sp3 C–C bonds in these films, which is attributed to the high microwave power and strong H2 dilution that enhance C sp3 bonding and indirectly limit the number of C sp2 sites. This accounts for the large E04 gaps of more than 3.2 eV observed in such films, which are nearly saturated at large x, instead of exhibiting a maximum at an intermediate x as are commonly reported. Blue photoluminescence (PL) is observed, and the PL peak energies (EPL) are correlated to the E04 gap. The full width at half maximum of the PL are also correlated to the Urbach energy Eu. These results support that the PL broadening is attributed to the disorder broadening arising from the broad band tails.

Fast deposition of microcrystalline silicon films with preferred (220) crystallographic texture using the high-density microwave plasma

Abstract A novel high-density and low-temperature microwave discharge utilizing a spoke antenna has been applied for the fast deposition of microcrystalline silicon (μc-Si:H) films with preferred (2 2 0) orientation. Systematic deposition studies were performed from pure and H2-diluted SiH4 systems with microwave power, total pressure, H2 dilution ratio and substrate temperature as variables, combined with plasma diagnostics using optical emission spectroscopy and Langmuir probe techniques. The effects of deposition parameters on the film crystallinity, crystal orientation and defect density are demonstrated.

Extended phase diagrams for guiding plasma-enhanced chemical vapor deposition of silicon thin films for photovoltaics applications

Real time spectroscopic ellipsometry has been applied to develop extended phase diagrams that can guide the deposition of hydrogenated silicon (Si:H) thin films for highest performance solar cells. Previous such studies have shown that optimization of amorphous Si:H intrinsic layers by rf plasma-enhanced chemical vapor deposition (PECVD) is achieved using the maximum possible H2 dilution of SiH4 while avoiding a transition to the mixed-phase (amorphous+microcrystalline) growth regime. In this study, we propose that optimization of amorphous Si:H in higher rate rf PECVD processes further requires the largest possible thickness onset for a surface roughening transition detected in the amorphous film growth regime.

Hydrogen desorption properties of multiwall carbon nanotubes with closed and open structures

The multiwall carbon nanotubes (MWNTs) synthesized by microwave plasma enhanced chemical vapor deposition in 0.1% CH4 (H2 dilution) reaction gas were shaped in curly structures with the blocked nanoholes and closed caps. On the contrary, the carbon nanotubes with different structures could be obtained in oxygen loaded reaction gas, showing aligned structures with connected holes and open caps. The hydrogen desorption properties of the carbon nanotubes (CNTs) with these closed and open structures were compared by thermal desorption technique. The MWNTs with closed structure desorbed hydrogen at two different temperature ranges such as 290–330 and ∼420 K, where the evolved hydrogen amount were ∼0.64 and ∼0.03 wt %, respectively. In case of aligned and open MWNTs, hydrogen as high as ∼1.97 wt % was released at ambient temperature. The hydrogen desorption of MWNTs with open and closed structure at ambient temperature showed the hydrogen desorption activation energies of −16.5 kJ/mol H2 and −18.5 kJ/mol H2, respectively. The high temperature hydrogen desorption observed only in CNTs with closed and highly defective structure was high desorption activation energy of −124.4 kJ/mol.

Microcrystalline Silicon for Solar Cells at High Deposition Rates by Hot Wire Cvd

AbstractWe have explored which deposition parameters in Hot Wire CVD have the largest impact on the quality of microcrystalline silicon (μc-Si) made at deposition rates (Rd) < 10 Å/s for use in thin film solar cells. Among all parameters, the filament temperature (Tfil) appears to be crucial for making device quality films. Using two filaments and a filament-substrate spacing of 3.2 cm, μc-Si films, using seed layers, can be deposited at high Tfil (∼2000°C) with a crystalline volume fraction < 70-80 % at Rd's < 30 Å/s. Although the photoresponse of these layers is high (< 100), they appear not to be suitable for incorporation into solar cells, due to their porous nature. n-i-p cells fabricated on stainless steel with these i-layers suffer from large resistive effects or barriers, most likely due to the oxidation of interconnected pores in the silicon layer. The porosity is evident from FTIR measurements showing a large oxygen concentration at ∼1050 cm-1, and is correlated with the 2100 cm-1 signature of most of the Si-H stretching bonds. Using a Tfil of 1750°C, however, the films are more compact, as seen from the absence of the 2100 cm-1 SiH mode and the disappearance of the FTIR Si-O signal, while the high crystalline volume fraction (< 70-80 %) is maintained. Using this Tfil and a substrate temperature of 400°C, we obtain an efficiency of 4.9 % for cells with a Ag/ZnO back reflector, with an i-layer thickness of only ∼0.7 μm. High values for the quantum efficiency extend to very long wavelengths, with values of 33 % at 800 nm and 15 % at 900 nm, which are unequalled by a-SiGe:H alloys. Further, by varying the substrate temperature to enable deposition near the microcrystalline to amorphous transition (‘edge’) and incorporating variations in H2 dilution during deposition of the bulk, efficiencies of 6.0 % have been obtained. The Rd's of these i-layers are 8-10 Å/s, and are the highest to date obtained with HWCVD for microcrystalline layers used in cells with efficiencies of ∼6 %.

Particle Production in High Density Silane Plasma

ABSTRACTTo understand the mechanisms of nanoparticle formation and its potential applications, we have investigated silicon particles formed in various gasses in an inductive coupled plasma (ICP) system and have measured their structural properties by electron microscopy. Particle generation in pure SiH4 and SiH4/H2 are reported. ICP silane plasmas are shown to be an interesting and versatile source of nanoparticles. Three regimes are mapped out: a regime of no observable particle growth at the lowest pressures, a regime of polydisperse and agglomerated particles at the highest pressures, and a regime yielding highly monodisperse particles at intermediate pressures. Conditions that generate these nonagglomerated, extremely monodisperse silicon particles are emphasized. For H2 dilutions less than 92%, the growth rate is almost independent of H2 partial pressure. Particle growth decreases steadily when the H2 dilution is increased further. TEM images, however, indicate that the addition of hydrogen decreases the particle density. At higher dilution ratios, polycrystalline particles are obtained. Under all other conditions the particles are amorphous. Reasons for this behavior are explored.

The Effect of Hydrogen Incorporation on the Performance of Nanocrystalline Silicon Thin Film Transistors Fabricated by Microwave ECR Plasma CVD

ABSTRACTNanocrystalline silicon (nc-Si:H) TFT's with the active layers deposited by microwave ECR-CVD were fabricated and characterized using hydrogen dilution as a variable during the Si film deposition. The bottom-gated TFT's were fabricated on SiO2, glass and polyimide substrates at a temperature of 200–400°C with the introduction of 4–12 mTorr H2. The correlation between the H2 dilution and the TFT characteristics was studied and the experimental results were correlated with some theoretical considerations. The TFT on a glass substrate deposited at 400°C with 10 mTorr H2 showed a field effect mobility of 13.3 cm2/V-s and an ON/OFF current ratio of 4.9×106 with the OFF-state leakage current of 3.7×10−11 A. We found that, for the same Si film deposition conditions, the TFT's fabricated on polyimide foil have comparable characteristics to the TFT's on glass substrates.

The Use of Seed Layers in Hot Wire Chemical Vapor Deposition of Microcrystalline Silicon Films

AbstractThe effect of thin seed layers on the subsequent growth of thick, high growth rate bulk μc-Si is investigated by XRD, SEM and cross sectional TEM. All layers were deposited by hotwire chemical vapor deposition (HWCVD). When the seed layer as observed by XRD is highly crystalline, made by using high H2 dilution (H2:SiH4 100:1), the amorphous incubation layer typical of μc-Si growth is virtually eliminated. Furthermore, with this seed layer, bulk layer deposition conditions that would otherwise produce highly amorphous material enable a composite film with significant crystallinity. When the seed layer is predominantly amorphous, made using a much lower H2 dilution (10:1), there is evidence that very small crystallites, undetected by XRD, still facilitate immediate nucleation and enable the formation of larger grains in the subsequent bulk layer. In concurrence with other HWCVD results, lowering the filament temperature results in significant improvements in film compactness, photoresponse and grain size, while maintaining significant crystallinity. Such films have been incorporated into high deposition rate solar cells.

Hot-wire CVD-grown microcrystalline silicon films with and without initial growing layer modification by transformer-coupled plasma

Microcrystalline Si (μc-Si) films have been deposited using five W wire filaments, 0.5 mm in diameter, for hot-wire chemical vapor deposition (HWCVD). We compared the HWCVD-grown films with and without modification of the initial growing layer by using a transformer-coupled plasma system. The W-wire filament temperature was maintained below 1600°C to avoid metal contamination by thermal evaporation of the filament. Deposition conditions were varied, including the H2 dilution ratio, with and without a seed-layer plasma treatment. From Raman analysis, we observed that the film crystallinity was strongly influenced by the H2 dilution ratio and weakly depended on the distance between the wire and the substrate. We were able to achieve a crystalline volume fraction of approximately 79% with a H2/SiH4 ratio of 80, a wire temperature of 1514°C, a substrate distance of 4 cm, and a chamber pressure of 38 mtorr. We investigated the influence of a plasma treatment during the initial stage of Si film formation for property variations in the HWCVD-grown μc-Si film. This article also deals with the influence of the H2 dilution ratio in crystallinity modification.

Effect of diluted ammonia flow rate on undoped GaN epitaxial films grown by MOCVD

We have found that the electrical and optical properties of GaN epilayers strongly depend on the NH3 flow rates with Hydrogen dilutions. The growth rate decreases with increasing NH3 flow rate. The electron mobility increased from 90 to 104cm2/Vs as the NH3 flow rate is increased from 1.5 to 2.0SLM without H2 dilutions. With NH3 being diluted, the electron mobility can be further increased to 124cm2/Vs. The electron carrier concentration also decreases from 2×1018 to 3.8×1017cm−3. The flow patterns can be observed on the surface but becomes more flat and broader after the dilution of NH3. After dilution, the photoluminescence spectra show no evidence of the yellow band emission related to those without NH3 dilution. From detailed PL investigations at temperatures from 12 to 300K, we find that shallow bound excitons and donor–acceptor pair recombination have stronger emission than yellow band at low temperatures.

Microcrystalline silicon by plasma enhanced chemical vapor deposition from silicon tetrafluoride

The amorphous and microcrystalline silicon growth from SiF4–H2–He plasmas has been studied by exploring various parameters, such as He addition, rf power, H2 dilution, and frequency. The dilution of SiF4–H2 plasmas with He gas has been found to promote the crystalline phase and entirely microcrystalline silicon films have been obtained at low temperature (120 °C) under optimized experimental conditions. All the obtained results are discussed on the basis of etching/growth competition mechanism, in which the etching process by F atoms is selective for the amorphous phase. As such, all the experimental conditions enhancing the etching channel let the microcrystalline growth prevail. The net deposition rate, monitored by the laser reflectance interferometry, results from the difference between growth rate, rG, and etching rate, rE. Optical emission spectroscopy is a powerful in situ diagnostic tool to monitor the crystallinity degree in the material, since the [F*]⋅[He*]/[SiF2*]⋅[Hα] ratio, obtained from the peak intensities and called the Γ ratio, has been found to be proportional to the rE/rG ratio.

Annealing Kinetics of Amorphous Silicon Alloy Solar Cells Made at Various Deposition Rates

ABSTRACTA systematic study has been made of the annealing kinetics of amorphous silicon (a-Si) alloy solar cells. The cells were deposited at various rates using H2 dilution with radio frequency (RF) and modified very high frequency (MVHF) glow discharge. In order to minimize the effect of annealing during light soaking, the solar cells were degraded under 30 suns at room temperature to quickly reach their saturated states. The samples were then annealed at an elevated temperature. The J-V characteristics were recorded as a function of annealing time. The correlation of solar cell performance and defect density in the intrinsic layer was obtained by computer simulation. Finally, the annealing activation energy distribution (Ea) was deduced by fitting the experimental data to a theoretical model. The results show that the RF low rate solar cell with high H2 dilution has the lowest Ea and the narrowest distribution, while the RF cell with no H2 dilution has the highest Ea and the broadest distribution. The MVHF cell made at 8Å/s withhigh H2 dilution shows a lower Ea and a narrower distribution than the RF cell made at 3 Å/s, despite the higher rate. We conclude that different annealing kinetics plays an important role in determining the stabilized performance of a-Si alloy solar cells.

The Effect of H2 Dilution On Thin Film SiN Deposited by Hot Wire Cvd Using SiH4 and NH3 Gas Mixtures

ABSTRACTThe structure of thin film SiN, deposited by the hot wire chemical vapor deposition (HWCVD) technique using SiH4 and NH3 gas mixtures, has been examined as a function of the amount of H2 dilution of the gas mixture. For NH3/SiH4 gas ratios > 0.5/1, all films are a-SiN:H. While H2 dilution does not change the basic film structure, in that the films are amorphous with all dilutions, H2 dilution does increase the efficiency of NH3 dissociation in the gas phase, and causes a further reduction in the already small amount of N-H bonding in a-SiN:H films deposited by HWCVD. For NH3/SiH4 gas ratios typically <0.5/1 and with high H2 dilution, the first deposition of µc-SiN is demonstrated. X-ray diffraction (XRD) measurements demonstrate that the structure of these films consists of silicon crystallites embedded in an a-SiN:H matrix. An upper limit for N incorporation with the preservation of microcrystallinity was found, beyond which the films again became amorphous. The existence of this limit is explained in terms of structural disorder in the a-SiN:H tissue brought about by N incorporation.

Quantitative Modelling of Nucleation Kinetics in Experiments for Poly-Si Growth on Sio2 by Hot-Wire Chemical Vapor Deposition

ABSTRACTWe apply a rate-equation pair binding model of nucleation kinetics [1] to the nucleation of Si islands grown by hot-wire chemical vapor deposition on SiO2 substrates. Previously, we had demonstrated an increase in grain size of polycrystalline Si films with H2 dilution from 40 nm using 100 mTorr of 1% SiH4 in He to 85 nm with the addition of 20 mTorr H2. [2] This increase in grain size is attributed to atomic H etching of Si monomers rather than stable Si clusters during the early stages of nucleation, decreasing the nucleation density. Atomic force microscopy (AFM) measurements show that the nucleation density increases sublinearly with time at low coverage, implying a fast nucleation rate until a critical density is reached, after which grain growth begins. The nucleation density decreases with increasing H2 dilution (H2:SiH4), which is an effect of the etching mechanism, and with increasing temperature, due to enhanced Si monomer diffusivity on SiO2. From temperature-dependent measurements, we estimate the activation energy for surface diffusion of Si monomers on SiO2 to be 0.47 ± 0.09 eV. Simulations of the temperature-dependent supercritical cluster density lead to an estimated activation energy of 0.42 eV ± 0.01 eV and a surface diffusion coefficient prefactor of 0.1 ± 0.03 cm2/s. H2-dilution-dependent simulations of the supercritical cluster density show an approximately linear relationship between the H2 dilution and the etch rate of clusters.

Growth Rate and Crystallinity of Nanocrystalline Silicon Film Grown by Electron Beam Excited Plasma Chemical Vapor Deposition

Using electron beam excited plasma chemical vapor deposition, nanocrystalline Si films can be grown without H2 dilution. This paper describes the effects of growth parameters on the growth rate and the crystallinity of Si films.