Si-H Bond Density Research Articles

Optimized amorphous silicon nitride layers for the front side passivation of c-Si PERC solar cells

Plasma-enhanced chemical vapour deposition (PECVD) SiNx is the typical choice as anti-reflection coating (ARC) for Silicon based solar cells. However, there still exists a room for improvement in passivation quality of SiNx while maintaining good optics for the front side of a solar cell. In this paper, we studied in detail the optical and electrical properties of SiNx layers by varying the chamber pressure and substrate temperature in an industrially used inline PECVD tool. Both the optical as well as electrical properties of SiNx layers were found to be significantly influenced by the chamber pressure and substrate temperature. A trade-off between excellent optics and low surface recombination is observed with an increase in chamber pressure, whereas higher substrate temperature generally led to better passivation quality. The Si-H bond density, which is expected to directly influence the quality of surface passivation, increased at high pressure and at low substrate temperature. Based on our investigations, a good compromise between optics and surface passivation is struck to prepare optimized SiNx layers and apply them as passivation layers for the front side of passivated emitter and rear cell (PERC) solar cells. The best solar cells show high short-circuit current density (jSC) of 39.9 mA/cm2 corresponding to the SiNx layers with low parasitic absorption, good antireflection property, and excellent passivation of the surface and bulk silicon. The current-voltage (I-V) results are found to be in agreement with internal quantum efficiency (IQE) measurements of the solar cells.

Impact of auxiliary capacitively coupled plasma on the properties of ICP-CVD deposited a-SiNx:H thin films

In this study, we report on the properties of amorphous hydrogenated silicon nitride (a-SiNx:H) thin films synthesized by inductively coupled plasma enhanced chemical vapour deposition (ICP-CVD), combined with an auxiliary capacitively coupled plasma excitation. Physical properties, such as the deposition rate, refractive index and film stress, as well as the chemical composition, microstructure and etch resistance of the samples were investigated. Depending on the capacitively coupled plasma power PRF, two regions can be distinguished which are dominated either by an increased generation of plasma radicals or by an enhanced ion bombardment. At PRF=35W, a maximum in the deposition rate is observed as well as an abrupt change in the chemical composition, resulting in an extremely high (>2.5GPa) compressive stress, in comparison to the layers deposited with PRF=0. In addition, the wet etch rate in hydrofluoric acid (HF) decreases substantially. Above, up to PRF=150W, ion bombardment of the growing film surface is more pronounced due to an increased self-bias of the substrate decreasing the effective deposition rate. Fourier-transform infrared spectroscopy measurements indicate that this behaviour is in close relation to the NH bond density in the samples. Additionally, the continuous increase of the refractive index and SiH bond density, accompanied by the redshift of the SiN infrared absorption peak show that with higher PRF the Si content of the films increases.

Amorphous-Silicon-Based Thin-Film Solar Cells Exhibiting Low Light-Induced Degradation

We have applied a triode electrode configuration in the plasma-enhanced chemical vapor deposition (PECVD) process to grow intrinsic hydrogenated amorphous silicon (a-Si:H) light absorbers for the fabrication of p–i–n junction solar cells. Although the deposition rate is lower (0.1–0.3 Å/s) than that of the conventional diode PECVD process, the light-soaking stability of the solar cell is markedly improved and less sensitive to the cell thickness due to the reduced Si–H2 bond density in the a-Si:H i-layer. The a-Si:H single-junction solar cells exhibit low light-induced degradation of conversion efficiency (Δη/ηini∼10%) in comparison with that of high-efficiency solar cells reported to date. By applying the improved a-Si:H layers as top-cell absorbers in a-Si:H/hydrogenated microcrystalline silicon (µc-Si:H) tandem device, the light-induced degradation can be reduced even further (Δη/ηini\\lesssim5%). As a result, we obtain confirmed stabilized efficiencies of 9.6 and 11.3% for a-Si:H single-junction and a-Si:H/µc-Si:H tandem solar cells, respectively.

Si–H Bond Breaking Induced Retention Degradation During Packaging Process of 256 Mbit DRAMs With Negative Wordline Bias

Data retention degradation of a 256-Mbit DRAM during the packaging process is investigated in this paper. Electrical measurement and device simulation show that a trap-assisted leakage degrades the retention time even in packaging process at about 250/spl deg/C. Retention time of the degraded chip is strongly dependent on the negative wordline voltage and operation temperature, but less sensitive to the substrate bias. Trap-assisted gate induced drain leakage is proposed as the mechanism of retention loss in the degraded chip. The degraded chips usually can be repaired by another thermal baking process. We propose Si-H bond breaking and the subsequent trap generation at the gate and drain overlap region as the root cause of retention degradation according to the fact that the Si-H bond density of backend passivation oxide and nitride layers correlate well with the retention performance of DRAM chips with negative wordline bias. Moreover, the packaged chip shows variable retention behavior during a thermal baking of 250/spl deg/C. Theoretical calculation indicates that the trap generation or movement to the high electrical field region beneath the gate can increase the trap-assisted gate induced drain leakage by about an order of magnitude.

Improved Stability of Hydrogenated Amorphous Silicon Solar Cells Fabricated by Triode-Plasma CVD

AbstractWe have employed a triode-type plasma CVD system to fabricate highly stabilized hydrogenated amorphous silicon (a-Si:H) solar cells. The p-i-n type solar cells were fabricated on a textured SnO2/glass substrate (ASAHI VU type). By applying a triode system, the Si-H2 bond density in the film decreased to about one third (from 1.7 at.% for conventional parallel-plate-electrode to 0.6 at.% for a triode configuration), and correspondingly the degradation ratio decreased from 13 % to 10 %. We have achieved the degradation ratio of 5 % by optimizing the player deposition conditions. In case of a triode system, there were minor effects of higher hydrogen dilution in the stabilized efficiency. We have experimented the effects of the substrate temperature for a higher stabilized efficiency. Further improvement in solar efficiency has been made by applying antireflection layers to air/glass and TCO/p interfaces. As a result, we have achieved the stabilized efficiency of 9.22 % (Jsc = 15.9 mA/cm2, Voc = 0.863 V, FF = 0.672) with a degradation ratio of 7.8 %. We have also employed the triode-deposited a-Si:H solar cell to a tandem type solar cell with a structure of a-Si:H/hydrogenated microcrystalline silicon (μc-Si:H). We have achieved the stabilized efficiency of 10.9 % (Jsc = 12.0 mA/cm2, Voc = 1.31 V, FF = 0.691) with a degradation ratio of 7.3 %.

Data Retention Improvement of Metal-Oxide-Nitride-Oxide-Semiconductor Memories Using Silicon-Tetrachloride-Based Silicon Nitride with Ultralow Si-H Bond Density

We have improved the data retention of metal-oxide-nitride-oxide-semiconductor (MONOS) memories by reducing the Si-H bond density in silicon nitride. This marked improvement was achieved by depositing silicon nitride with NH3 and SiCl4 (silicon tetrachloride, STC), instead of with NH3 and the conventionally used SiCl2H2 (dichlorosilane, DCS). The Si-H bond density of the STC-based nitride was less than 1% that of the DCS-based nitride. We clarified that the improvement was due to the suppression of the leakage of trapped electrons through shallow traps which are related to Si-H bonds with an activation energy of 0.1–0.2 eV.

Correlation between Deposition Parameters and Structures of the SiNx Films Deposited at Extremely High Rates by Atmospheric Pressure Plasma CVD

Using the atmospheric pressure plasma chemical vapor deposition (CVD) technique, further investigations were performed to improve the density of silicon nitride (SiNx) films deposited at extremely high deposition rate. The films were prepared on Si(001) wafers at atmospheric pressure in a very high frequency (150MHz) plasma of gas mixtures containing He, H2, SiH4 and NH3. Si-N, Si-H and N-H bond densities of the SiNx films were studied as functions of NH3/SiH4 ratio and the H2 concentration by Fourier transformation infrared (FTIR) absorption spectroscopy. Etching rate with buffered HF (BHF) solution, refractive index and relative permittivity of the SiNx films were also studied as a function of Si-N bond density of those films. It is found that by decreasing NH3/SiH4 ratio or increasing H2 concentration, Si-N and Si-H bond densities increase, while N-H bond density decreases. It is also noticed that N-H bond density is larger than Si-H bond density within the present deposition conditions, indicating that the SiNx films are N-rich. These facts suggest that elimination of excessive hydrogen atoms bonded with N atoms at the film-growing surface is an important factor to form SiNx film with a dense Si-N network. The highest Si-N bond density and then the lowest hydrogen content are obtained for the film deposited with the H2 concentration of 20% and the input power of 1000W.

Temperature Effects on the Structure of Polycrystalline Silicon Films by Glow-Discharge Decomposition Using SiH4/SiF4

Polycrystalline silicon (poly-Si) films were prepared on glass substrates by the plasma- enhanced chemical vapor deposition method using SiH4/SiF4 mixtures as a function of deposition temperature, Td, from 150 to 400°C, and the structural properties were investigated. In addition, the effects of addition of H2 to the SiH4/SiF4 gas were also examined. The crystallinity and grain size of Si films with added were found to have maximum values at around Td = 250–300°C. However, poly-Si films without H2 addition contain numerous microvoids, and exhibit easy O contamination, and their crystallinity monotonically increased with Td. The change in the SiH and SiH2 bond density with H2 addition was consistently interpreted in terms of this model. As a result, H2 addition at low Td was suggested to suppress O contamination and improve the crystalline quality. By contrast, H2 addition at high Td is likely to deteriorate crystalline quality. The results were discussed in terms of fluorine and hydrogen chemistry.

Atomic Structure Impacts on Dielectric and Mechanical Properties of Hydrogen-Silsesquioxane During Thermal Processing

AbstractAtomic structure impacts on the dielectric and mechanical properties of hydrogen-silsesquioxane (HSQ) were investigated to determine the sensitivity of HSQ to Si-H bond degradation during thermal processing. It was demonstrated that the dielectric properties of HSQ films are not sensitive to Si-H bond degradation up to 50 % if polar silanol or water species are not induced during thermal processing at a temperature of ≤ 450 °C. However, as the Si-H bond density is reduced, the mechanical properties are changed due to the increasingly formed Si-O-Si networks. This provides an opportunity for tuning mechanical properties per application requirements by process optimization.

Characterization of Spin -on Glasses by Microindentation.

ABSTRACTSpin-on glasses are candidates in the microelectronics industry as low dielectric constant insulating layers. Spin-on glasses are very brittle materials. This paper discusses measurement problems as relevant to the characterization of a brittle material by the indentation technique. As for all polymeric materials curing temperature is the most important preparation parameter. There is a correlation between hardness, Young's modulus, the onset of cracking with curing temperature. This dependence on curing temperature is also expressed by the change in Si-H bond density as shown by FTIR data. Life expectancy or aging characteristics were also investigated for these features. As an example results on silsesquioxane spin -on glasses are presented.

96/06259 Solar photocatalytic processes for the purification of water: State of development and barriers to commercialization

In order to improve the conversion efficiency of a-Si solar cells, a superlattice structure p-layer and the use of B(CH3)3 as a dopant gas have been investigated for the first time. The collection efficiency spectrum of a glass/TCO/p-superlattice structure (a-SiC/a-Si)/in/Metal cell shows a remarkable increase in the short wavelength region. This result suggests that the superlattice structure p-layer is an active photovoltaic layer.It is also found that the photoconductivity of p-type a-Si:H films increases by using B(CH3)3 as a dopant gas. A conversion efficiency of 10.0 % (module efficiency 9.02 %) is obtained for 10 cm × 10 cm integrated single junction a-Si solar cell submodule using B(CH3)3.As for reliability it is found that the light induced degradation of a-Si solar cells can be reduced by simultaneous reduction of the impurity concentrations in a-Si with the super chamber (separated UHV reaction chamber) and the SiH2 bond density.

Characteristics of a‐Si solar cells prepared by the super chamber at a high substrate temperature

AbstractA new approach to high‐performance a‐Si solar cells was studied. a‐Si films prepared at a high substrate temperature (> 250°C) have a higher absorption coefficient and a low SiH2 bond density. the effect of deposition temperature on the open‐circuit voltage (Voc) has been investigated systematically for glass/SnO2 Ipin/metal and glass/metal/nip/indium tin oxide (ITO) structure a‐Si solar cells. The Voc is found to depend strongly on the thermal history of the p/i interface. A short‐circuit current of 19.5 mA/cm−−2 was achieved for an a‐Si solar cell using an a‐Si i‐layer with a thickness of 4000 Å, which was prepared at a substrate temperature of 270°C.

Contributions of Silicon-Hydride Radicals to Hydrogenated Amorphous Silicon Film Formation in Windowless Photochemical Vapor Deposition System

Hydrogenated amorphous silicon (a-Si:H) films are prepared using a windowless photochemical vapor deposition system. Vacuum ultraviolet light of 121.6 nm from hydrogen-helium mixture plasma is thought to decompose silane molecules into mainly SiH and SiH2 radicals. Contributions of these radicals to film growth result in growth rate dependence upon substrate temperature (T s). The apparent sticking probability of both radicals is determined to be 0.84 at T s=100° C and 0.68 at T s=250° C, from the variation of step coverage as a function of T s. Adjusting the substrate position close to the light source brings about an increase of incident SiH and SiH2 radicals through primary photolysis by 121.6 nm light, as well as decrease of gas-phase reactions, which allows us to control the Si–H2 bond density in the film. The effect of the Si–H2 bond in the film is also shown.

Direct Photochemical Vapor Deposition of Hydrogenated Amorphous Silicon -Effects of Excitation Wavelengths and Source Gases-

Hydrogenated amorphous silicon films have been prepared by a direct photochemical vapor deposition method using different excitation wavelengths and source gases. The films deposited with the combination of Si2H6+147 nm and Si2H6+185 nm showed good properties indicated by low value of Si-H2 bond density, Urbach energy and defect-state density compared with the films deposited with SiH4+147 nm. Results of simulation demonstrated that these good properties are caused by the smaller contribution of SiH2 and larger contribution of monoradicals to the deposition, and that interconnection of surface sites created from SiH2 is difficult compared with that from monoradicals. Detailed analysis of the simulation revealed that the reactions which control radical density are not those of initial photolysis but instead are secondary reactions, such as those involving atomic hydrogen.

Irradiation-Temperature Dependence of the Light-Induced Effect in a-Si Solar Cells

The irradiation-temperature dependence of light-induced degradation has been studied at temperatures between 300 K and 25 K. Although Si-H2 bonds have a strong influence on degradation at an irradiation temperature of 300 K, their influence is reduced at low irradiation temperatures. Si-H2 bonds are thought to have a stabilization effect on newly created defects. Also, the hydrogen atom itself was found to have an influence on light-induced degradation in a-Si solar cells with low Si-H2 bond density or in the region of low irradiation temperatures below about 60 K.

High-Quality Amorphous Silicon Carbide Prepared by a New Fabrication Method for a Window P-Layer of Solar Cells

ABSTRACTA total area conversion efficiency of 11.1% has been achieved for a 1Ocm×1Ocm integrated-type single-junction a-Si solar cell submodule using a high-quality wide-bandgap p-layer doped with B(CH3)3 and other advanced techniques. This is the highest conversion efficiency ever reported for an a-Si solar cell with an area of 100cm2. As for a multi-junction solar cell, 12.1% was obtained for a 1cm2 cell with a high-quality wide-bandgap a-Si i-layer. The layer was fabricated by a hydrogen dilution method at a low substrate temperature for a front active layer of an a-Si/a-Si/a-SiGe stacked solar cell.For further improvement in conversion efficiency, a wider-bandgap a-SiC was developed using a novel plasma CVD method, called the CPM (Controlled Plasma Magnetron) method. From XPS and IR measurements, the resultant films were found to have high Si-C bond density and low Si-H bond density, p-type a-SiC was fabricated using the post-doping technique, and dark conductivity more than 10-5(Q. cm)-1 was obtained (Eopt3 ≥ 2eV; Eopt2 2.2eV), whereas that of conventional p-type a-SiC is less than 10-6(Ω·cm)-1. These properties are very promising for application to the p-layers of advanced a-Si solar cells.

Amorphous Silicon-Carbon Alloy Prepared by the CMP (Controlled Plasma Magnetron) Method

Hydrogenated amorphous silicon-carbon (a-SiC) films were fabricated using a novel plasma CVD method, called the CPM (Controlled Plasma Magnetron) method. The films obtained had a large Si-C bond density and a low Si-H bond density. The absorption coefficient at the photon energy level of 2.5 to 3.5 eV for these films was one order of magnitude lower than that for conventional a-SiC films. Furthermore, the dark conductivity of boron-doped films increased to more than 10-4 (Ω·cm)-1, which is the highest value achieved for a-SiC with a carbon content of 55%.

Comments on the Determination of Defect Density in a-Si:H Alloys by Integrating the Sub-Bandgap Optical Absorption Coefficient

ABSTRACTThe validity of determining the defect density in a-Si:H and its alloys by integrating the sub-bandgap optical absorption coefficient is examined. It is shown that a formula derived for estimating Si-H bond density has been incorrectly applied to defect related absorption. In addition, the complex defect absorption processes have been oversimplified, by assumption, to the case of a single type of transition. Further, in practice, the defect absorption spectrum can not be unambiguously obtained at energies near and above the low end of the optical absorption edge. It is concluded that die integration method for quantitative defect density determination should be viewed at best as an empirical practice with considerable arbitrariness.

Fabrication of Device-Quality Wide-Gap a-Si:H Films at Very Low Substrate Temperatures

ABSTRACTWide-gap a-Si:H films with device quality (Tauc’s optical gap > 1.9eV, σph under AMI.5, 100mW/cm2 illumination ≥ 10−5, Ω−1cm−1, a σph/σ a≥106) have been fabricated. These films are deposited at low substrtate temperatures (TS≤80°C ) either by diluting SiH4 with H2 or optimizing the plasma parameters in a capacitively–coupled RF plasma–CVD reactor. Reduction in the SiH2 bond density and the ESR spin density are also observed. In this study, good film quality is always accompanied by a small deposition rate. Furthermore, σph is nearly the same if the deposition rate and Ts is the same, regardless of other deposition parameters. This suggests that the surface reactions or structural relaxations at the film-growing surface can produce high–quality a–Si:H films even at low TsS, if the deposition rate is low. Results in thermal annealing, light exposure, and solar cell performance confirm that these films have device quality and wide bandgap.

The Influence of the Si-H2 Bond on the Light-Induced Effect in a-Si Films and a-Si Solar Cells

The influence of the Si-H2 bond on light-induced degradation and the thermal recovery of a-Si films and a-Si solar cells were studied. The influence of the Si-H2 bond on light-induced degradation depends on the impurity content in a-Si films, and light-induced degradation can be reduced by decreasing the Si-H2 bond density in a-Si films with impurity content of 1018 cm-3. The activation energy of the thermal recovery process was about 1.0 eV, and it did not depend on the Si-H2 bond density. However, an irreversible phenomenon was observed in film properties and solar cell characteristics with high Si-H2 bond density. It is thought that the structural flexibility of the Si-H2 bond is related to this irreversible phenomenon.