Hydrogen Dilution Conditions Research Articles

Effect of hydrogen dilution on the silicon cluster volume fraction of a hydrogenated amorphous silicon film prepared using plasma-enhanced chemical vapor deposition

We investigated the effect of hydrogen dilution on the Si cluster volume fraction of hydrogenated amorphous films by varying the hydrogen dilution ratio at 0.5 Torr and compared it to that obtained at pure silane discharge at 0.3, 0.4, and 0.5 Torr. The correlation between the plasma emission characteristic, deposition rate, and cluster volume fraction in the hydrogen dilution plasma was described. The cluster volume fractions of films under hydrogen dilution conditions were similar to those of the pure silane but showed a higher deposition rate. The results suggest that under hydrogen dilution conditions, it is possible to maintain a higher deposition rate with a lower cluster incorporation rate.

Structural, optical, and interface properties of sputtered AlN thin films under different hydrogen dilution conditions

In this work, the influence of different hydrogen dilution conditions on the optical, structural and passivation properties of crystalline, hexagonal aluminum nitride is assessed. The layers were deposited using an inline sputter coater in reactive Ar+N2 and Ar+N2+H2 atmosphere mixtures. Elemental composition was determined using energy dispersive spectroscopy. The structural properties were investigated applying Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. The optical characterization was performed through transmittance measurements using a modified envelope method. It could be observed that the incorporation of hydrogen leads to an increase of crystalline texture, grain size and bandgap. The full-width at half-maximum of the A1 transverse optical phonon mode decreases with increasing grain size and optical bandgap induced by the deposition conditions, showing a good correlation between the optical and crystalline properties. The potential of aluminum nitride for surface passivation of silicon is discussed in terms of surface recombination velocity, fixed charge density and defect state density at the c-Si/AlN:H interface.

The Urbach focus and optical properties of amorphous hydrogenated SiC thin films

We report on the optical bandgap engineering of sputtered hydrogenated amorphous silicon carbide (a-SiC:H) thin films under different hydrogen dilution conditions during the deposition process and after post-deposition annealing treatments. The Tauc-gap and Urbach energy are calculated from ultraviolet-visible optical transmittance measurements. Additionally, the effect of the thermal annealing temperature on the hydrogen out-diffusion is assessed through infra-red absorption spectroscopy. A new model for the optical absorption of amorphous semiconductors is presented and employed to determine the bandgap as well as the Urbach energy from a single fit of the absorption coefficient. This model allowed the discrimination of the Urbach tail from the Tauc region without any external bias. Finally, the effect of the hydrogen dilution on the band-edge and the Urbach focus is discussed.

Hydrogen dilution on an undoped silicon oxide layer and Its application to amorphous silicon thin-film solar cells

This paper proposes the use of undoped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) deposited on an n-μc-Si:H layer of amorphous silicon single-junction superstrate-configuration thin-film solar cells produced using 40MHz very high frequency plasma-enhanced chemical vapor deposition. We found that undoped μc-SiOx:H thin film under optimized hydrogen dilution conditions had high crystallinity, high conductivity, a wide optical band gap, and a high refractive index, which are advantageous properties in solar cells. However, deposition at higher hydrogen dilutions degraded the quality and optoelectronic properties of the films, because the morphology of the films changed from microcrystalline to amorphous. These results suggest that the use of an optimized undoped μc-SiOx:H layer improves a-Si:H thin-film solar cell performance through enhancement of the short-circuit current density Jsc. The increased Jsc can be attributed to an improved light-trapping capability in the long wavelength range, between 620 and 680nm, as demonstrated by the external quantum efficiency. This technique also allows optimal conversion efficiency to be achieved. The results demonstrated that hydrogen dilution plays a dominant role in the improvement of film quality and solar cell performance; however, the tradeoff between refractive index and conductivity must be considered.

Influence of hydrogen dilution on a-SiSn:H film growth and solar cell properties

In this contribution we report about improvement of the hydrogenated amorphous silicon tin (a-SiSn:H) thin film solar cells by using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at the high hydrogen (H2) dilution conditions. Our studies revealed that the incorporation of Sn into a-SiSn:H does not depend much on the H2 dilution whereas it strongly depends on the Sn(CH3)4 and SiH4 mixture flow rate and the VHF input power density. On the other hand, the photoconductivity and solar cell performance were strongly improved with H2 dilution due to the better passivation of the Si- and Sn-related dangling bonds as was confirmed by their drastically reduced, broad and featureless ESR signals. Moreover, we believe that the reduction of carbon might be one of the reasons for the improvement of the a-SiSn:H solar cells. Our measurements of the external quantum efficiency (EQE) spectra and the I–V characteristics suggest that the H2 dilution plays an important role in the reduction of the photo-carrier killer sites, which opens a way for the fabrication of high quality a-SiSn:H thin films with enhanced photovoltaic performance.

Improvement of solar cells performance by boron doped amorphous silicon carbide/nanocrystalline silicon hybrid window layers

Boron doped amorphous silicon carbide/nanocrystalline silicon (p-type a-SiC/nc-Si) hybrid window layer has a great potential to improve the solar cells performance for its high optical band gap, carrier mobility and doping efficiency. Thin (<30nm) p-type a-SiC/nc-Si hybrid films have been successfully deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) from silane–methane gas mixtures at a low temperature of 150°C. Raman, FT-IR and HRTEM measurements were employed to investigate the structural characteristics of hybrid structure films. High quality p-type a-SiC/nc-Si hybrid window layer with controllable nano-sized silicon crystals embedded in amorphous silicon carbide matrix was obtained. Hybrid structure films with high values of optical band gap (∼2.2eV) and σd (∼10−3S/cm) were achieved in high hydrogen dilution conditions with optimized doping concentrations. Applying high quality p-type window layers and intrinsic a-Si layers, both high open circuit voltage (>0.96V) and high quantum efficiency values at short wavelengths were achieved for n–i–p type a-Si single junction solar cell. Meanwhile, the efficiency of the amorphous silicon and silicon–germanium (a-Si/a-SiGe) double junction solar cell was significantly improved to 11.8% by incorporating p-type a-SiC/nc-Si hybrid structure window layer.

Open-circuit voltage analysis of p—i—n type amorphous silicon solar cells deposited at low temperature

This paper identifies the contributions of p—a—SiC:H layers and i—a—Si:H layers to the open circuit voltage of p—i—n type a—Si:H solar cells deposited at a low temperature of 125 °C. We find that poor quality p—a—SiC:H films under regular conditions lead to a restriction of open circuit voltage although the band gap of the i-layer varies widely. A significant improvement in open circuit voltage has been obtained by using high quality p—a—SiC:H films optimized at the “low-power regime" under low silane flow rates and high hydrogen dilution conditions.

Structural order of thin film silicon made at 100 °C

AbstractSpectroscopic ellipsometric measurements showed that amorphous silicon thin films made by very high frequency plasma enhanced chemical vapour deposition at 100 °C using an optimum H2 dilution has denseness (A parameter) and structural order (C parameter) that are comparable to the sample made at 200 °C. The materials made by hot wire chemical vapour deposition (HWCVD) at low hydrogen dilution conditions have a less dense structure and higher roughness compared to the plasma deposited samples. The C parameter reaches the lowest value (C=1.67) for the HWCVD sample made at an optimum H2/SiH4 ratio of 20, even though it was made only at 100 °C. The importance of filament to substrate distance in HWCVD process in suppressing the undesirable species reaching at the growing surface has been put forward to achieve materials with very small structural disorder and fabrication of thin film silicon solar cells with high stability. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thin film silicon devices deposited at 100 °C: A study on the structural order of the photoactive layer

The dielectric functions of thin film silicon materials on glass were measured by spectroscopic ellipsometry (SE) and simulated by using the Tauc-Lorentz (TL) dispersion law, which provided information on disorder (C) and density (A). A VHF plasma-deposited sample made at 100°C with an optimum hydrogen dilution shows density (relative packing density of Si–Si bonds as estimated by SE) and structural disorder that are comparable to samples made at 200°C. HWCVD materials made at 100°C at lower hydrogen dilution conditions have a less dense structure and higher roughness compared to the plasma-deposited samples. This can be attributed to the absence of ion bombardment on the growing film. Out of all samples investigated, the HWCVD sample made at a hydrogen to silane flow ratio value of 20 showed a remarkably low structural disorder (C=1.67) even though the deposition temperature was only 100°C. A small bond angle variation of ∼6.4° as determined from its Raman spectrum, the presence of small (1–1.5nm) dispersed crystalline-like islands in the silicon matrix, and sharp rings in the selective area diffraction pattern point towards a special ordered structure. The photoresponse of this material is >105.

Incubation Effects upon Polycrystalline Silicon on Glass Deposited by Hot‐Wire CVD

AbstractA growth mechanism diagnosis for high‐rate, polycrystalline silicon deposition using hot‐wire CVD is explored in this article. The effects of various deposition parameters on the Si film growth are investigated by Raman spectroscopy and transmission electron microscopy (TEM) measurements, with special attention paid to the crystalline and amorphous phases. It is found that the H2/SiH4 ratio and substrate temperature (Ts) are the essential process parameters in determining the crystalline phase and the incubation thickness of the as‐deposited Si film. Under low hydrogen dilution conditions, an amorphous incubation layer was formed and degraded the crystalline fraction of the Si film. The incubation thickness prior to the nucleation decreases with the increase of the hydrogen dilution ratio. Under higher H2/SiH4 ratios (≥50), the amorphous phase deposited on the substrate could be etched selectively by the atomic hydrogen. As a consequence, nucleation of small crystallite occurs directly on the substrate. At low substrate temperatures, the deposition process shows the disadvantages of small grain size and low crystallinity. The poly‐Si film can be deposited from the seed layer under high Ts conditions (&gt;350 °C) where the Si appears singly crystallized at the initial stage and then grows epitaxially.

Deposition of highly efficient microcrystalline silicon solar cells under conditions of low H2 dilution: the role of the transient depletion induced incubation layer

AbstractThis paper addresses the plasma deposition of highly efficient microcrystalline silicon (μc‐Si:H) p‐i‐n solar cells under conditions of high SiH4 utilization and low H2 dilution. It was established that the transient depletion of the initially present SiH4 source gas induces the formation of an amorphous incubation layer that prevents successful crystallite nucleation in the i‐layer and leads to poor solar cell performance. The effect of this transient depletion induced incubation layer on solar cells was made visible through dedicated solar cell deposition series and selected area electron diffraction measurements. Applying a gas flow procedure at plasma ignition it was succeeded to prepare state‐of‐the‐art μc‐Si:H material and solar cells under low hydrogen dilution conditions, highlighted by μc‐Si:H solar cells of up to 9·5% efficiency prepared using an undiluted source gas flow consisting solely of SiH4. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Effect of the Structural Change of Hydrogenated Microcrystalline Silicon Thin Films Prepared by Hot-Wire Chemical Vapor Deposition

Hydrogenated microcrystalline silicon (µc-Si:H) thin films and solar cells were prepared by the hot-wire chemical vapor deposition (HWCVD) with various hydrogen dilution conditions. The surface of the film was observed by scanning electron microscopy (SEM) and the atomic force microscopy (AFM). From the observation, we observed that the films deposited at lower hydrogen dilution ratios [films with lower crystalline volume fractions (XC)] had rough surfaces with large crystalline aggregations with diameters larger than 100 nm. Conversely, the films deposited at higher hydrogen dilution ratios (films with higher XC) had almost smooth surface with smaller aggregations. In other words, the number of boundaries between crystalline aggregations increased with hydrogen dilution ratio. These structural changes agreed with the results of the spin density and ambipolar diffusion length measurements, which showed a tendency to deteriorate with the higher XC. The performance of the fabricated solar cells showed a similar change with hydrogen dilution ratio, i.e., a lower hydrogen dilution ratio resulted in a higher quality device. However, too low a hydrogen dilution ratio resulted in the degradation of the solar cell performance originating from the amorphous incubation layer grown at the p–i interface.

Mechanisms of the growth of nanocrystalline Si:H films deposited by PECVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were deposited using plasma-enhanced chemical vapor deposition from a SiF4/SiH4/H2 gas mixtures. Properties were examined of nc-Si:H films produced by decreasing the deposition temperature (Td) under two different hydrogen dilution ([H2]) conditions. For these films, the X-ray diffraction, the Raman scattering, the Fourier transform infrared absorption and the stress were investigated. Our results show that the decrease in Td has significant effects in the decrease of the average grain size (〈δ〉), the crystalline volume fraction (ρ) values, and an increase in the density of SiH-related bonds (NSiH) values. On the contrary, increases in [H2] decreased the 〈δ〉 and the NSiH, while the ρ were increased. Our experiments also confirmed that the increase in ρ corresponds with the decrease in NSiH. In view of these results, it may be concluded that the use of both low Td and high [H2] conditions might lead to growth of nc-Si:H films with small grains and high crystallinity. In this context, the surface processes (such as diffusion and etching) for the growth of nc-Si:H films were extensively discussed in this current work.

Time resolved photoluminescence of hydrogenated amorphous silicon carbon thin films deposited by HWCVD

Hydrogenated amorphous silicon carbon (a-SiC:H) alloys deposited by Hot Wire Chemical Vapor Deposition (HWCVD) technique have been extensively characterized by room temperature photoluminescence (PL) and time resolved photoluminescence (TRPL). The films were deposited under different sets of conditions of acetylene (C2H2) flow rate and hydrogen dilution. The temporal behavior of photoluminescence decay measured at peak emission energy exhibits a bi-exponential decay processes with lifetime of the order of a few nanoseconds. The variation in the process conditions leads to variations in the compositional and structural properties of the films, which affect the PL decay. The B-parameter which is the slope of the plot of absorption coefficient with photon energy has been determined for these films along with the carbon content. A systematic correlation between the carbon content and the decay time has been seen which is affected by an increase of hydrogen dilution or the C2H2 flow.

Deposition and characterization of μc-Ge 1− xC x thin films grown by hot-wire chemical vapor deposition using organo-germane

Microcrystalline germanium carbon (μc-Ge 1− x C x ) films prepared by hot-wire chemical vapor deposition were characterized by Raman, Fourier transform infra-red and X-ray photoelectron spectroscopy. μc-Ge 1− x C x films ( x = 0.02 to 0.03 by using dimethylgermane and x = 0.07 to 0.08 by using monomethylgermane) were successfully deposited using organo-germane and hydrogen. Raman scattering measurements reveal that the microcrystalline films could be obtained with high hydrogen dilution conditions. X-ray photoelectron spectroscopy measurements suggest the possible bonding of carbon with Ge atoms in the deposited thin films. The conductivity of the films were 10 − 3 ∼10 − 1 S/cm for microcrystalline films and 10 − 8 ∼10 − 5 S/cm for amorphous films. The dissociation efficiency of monomethylgermane is found to be higher than that of dimethylgermane.

Defect Densities, Diffusion Lengths and Device Physics in Nanocrystalline Si:H Solar Cells

ABSTRACTWe report on the properties of nanocrystalline Si:H solar cells. The solar cells were of the p+nn+ type, with the n+ layer deposited first on a stainless steel substrates. The solar cells were prepared under high hydrogen dilution conditions using either ECR plasma deposition, or VHF diode plasma deposition processes. The deposition pressures were kept low, 5 mTorr in the ECR reactor and 50 mTorr in the VHF reactor. All the solar cells reported showed a high Raman ratio of crystalline to amorphous peaks. Properties such as dark current, deep level defects and shallow doping densities, and hole diffusion lengths were measured in these cells. It was found that the base layer was always n type, but that its doping could be changed by adding ppm levels of B during growth. A sufficient B doping even type converted the base layer to p type. It was found that there was a good one-to-one correlation between the shallow doping and deep level defects, suggesting that the same element, probably oxygen, is responsible for generating both shallow dopants and deep levels. The diffusion length of holes was measured in these cells using quantum efficiency vs. voltage techniques, and it was found that the diffusion length data could be explained very well by invoking trap-controlled recombination statistics. The dark I(V) curves could be represented by a standard diode model for highly crystalline materials, but as the degree of crystallinity was reduced, the diode factor increased. Voltage could be improved by reducing the crystallinity of the layer, but doing so resulted in a decrease in quantum efficiency in the infrared regions of the solar spectrum.

Effect of hydrogen and deuterium on the microstructure of amorphous Si solar cell material

We investigate hydrogenated and deuterated amorphous Si (a-Si:H and a-Si:D) solar cell material, prepared under conditions of high hydrogen and deuterium dilution, respectively. Using cross-sectional transmission electron microscopy, we provide direct observation of the microstructure of these materials. We observe that the high hydrogen dilution results in homogeneous a-Si:H material with no evidence of microvoids. In contrast, the replacement of hydrogen with deuterium produces a heterogeneous mixture of amorphous and microcrystalline material. The amorphous phase contains significant numbers of microvoids or low-density deuterium-rich domains. The observed heterogeneity is related to the effect of the substitution of hydrogen with deuterium on the microstructure of the Si film.

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.

Toward stabilized 10% efficiency of large-area (&gt;5000cm2) a-Si/a-SiGe tandem solar cells using high-rate deposition

Abstract This paper reviews recent progress in large-area a-Si/a-SiGe tandem solar cells at Sanyo. Optimized hydrogen dilution conditions for high-rate deposition of hydrogenated amorphous silicon (a-Si:H) films and thinner i-layer structures have been systematically investigated for improving both the stabilized efficiency and the process throughput. As a result, a high photosensitivity of 10 6 for a-Si:H films has been maintained up to the deposition rate of 15 A/s. Furthermore, the world's highest initial conversion efficiency of 11.2% which corresponds to a stabilized efficiency of about 10% has been achieved for a 8252 cm 2 a-Si/a-SiGe tandem solar cell by combining the optimized hydrogen dilution and other successful technologies.

DEPOSITION OF HYDROGENATED NANOCRYSTALLINE SILICON CARBIDE BY ECR-CVD

In this work we report on the deposition of hydrogenated nanocrystalline silicon carbide (nc-SiC:H) films using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) system with silane (SiH4) and methane (CH4) as source gases. It was found that the conditions of strong hydrogen dilution and high microwave power are necessary for the fabrication of nanocrystalline SiC grains, which are found to be embedded in an amorphous matrix. The films have been studied using high resolution transmission electron microscopy, infrared absorption, Raman scattering and x-ray photoelectron spectroscopy. All the results have confirmed the prescence of SiC nanocrystallites. Very strong photoluminescence (PL) could be observed from these films at room temperature, with a peak energy of 2.64 eV. This energy, being higher than the optical bandgap of 3C-SiC, can be possibly due to quantum size effect in these crystals, which are embedded in an amorphous matrix of larger bandgap. Time-resolved PL at the peak emission energy exhibits a bi-exponential decay process with lifetimes that are in the order of picoseconds and nanoseconds, which are at least 2 orders of magnitude faster than that of bound excition transitions in bulk 3C-SiC at low temperature. The strong light emission and short PL lifetimes observed strongly suggest that the radiative recombination is a result of direct optical transitions in the SiC nanocrystallites. It was found that upon ultraviolet irradiation using an Ar + laser (351 nm), the PL intensity of the films was enhanced. After 20 minutes of irradiation, the PL intensity increased by about three times. This result suggests that the UV light may lead to modification of nonradiative recombination centers in the films.