Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Deposition of phosphorus doped a-Si:H and μc-Si:H using a novel linear RF source

A roll-to-roll PECVD system for thin film silicon solar cells on steel foil has been developed by ECN in collaboration with Roth and Rau AG. It combines MW–PECVD for fast deposition of intrinsic Si and novel linear RF sources, which apply very mild deposition conditions, for the growth of doped Si layers. The RF and MW sources can be easily scaled up to deposition widths of up to 150 cm. Here, we report on n-type doping, achieved by RF–PECVD from a H₂/SiH₄/PH₃ mixture in the reaction chamber. The best n-type a-Si:H layers showed E_act = 0.27 eV and σ_d = 2.7 × 10^?3 S/cm. Also thin layers down to 20 nm were of device quality and were deposited at a rate of 0.4 Å/s. Furthermore, n-type μc-Si:H layers with thicknesses of 150 nm, with E_act = 0.034 eV and σ_d = 2 S/cm were grown. Good quality n-type μc-Si:H layers can be made for layer thicknesses down to 50 nm at a rate of 0.15 Å/s. To conclude, the novel RF source is well-suited for the growth of n-doped a-Si:H and μc-Si:H layers for roll-to-roll solar cell production.     

Photocarrier diffusion lengths of high-growth-rate microcrystalline silicon

We report on photocarrier transport of high-growth-rate microcrystalline Si (μc-Si) in conjunction with the lateral size, σ_L, of crystallites’ conglomerate (grain) determined from the atomic force microscope (AFM) topographic images on the basis of fractal concepts. μc-Si films were prepared using very-high-frequency plasma-enhanced chemical vapor deposition at a high deposition rate of 6.8 ± 0.5 nm/s. μc-Si thicknesses, d, were varied from 0.53 μm to 5.6 μm. With an increase in d, σ_L increased from 70 nm to 590 nm. At the same time, the ambipolar diffusion lengths, L_amb, of photocarriers, observed using the steady-state photocarrier grating (SSPG) technique, increased from 50 nm to 420 nm. Log–log plots of L_amb versus d and σ_L versus d were both expressed as a power law with an exponent of 0.9, yielding a simple linear relation between L_amb and σ_L. Moreover, their ratio, L_amb/σ_L, was below unity, implying the intra-grain carrier diffusion. From these results, the role of the grain (column) boundaries for photocarrier diffusion in μc-Si is discussed.     

Fabrication and optimization of single-junction nc-Si:H n–i–p solar cells using Si:H phase diagram concepts developed by real time spectroscopic ellipsometry

In this study, we have developed and applied deposition phase diagrams in the plane of the bulk layer thickness d_b and the H₂-dilution ratio R = [H₂]/[Si₂H₆] for Si:H materials deposited by 70 MHz VHF PECVD from [H₂] + [Si₂H₆] mixed gases on c-Si/(native-oxide)/n-layer substrates. To establish the phase diagrams, series of Si:H depositions having different R values over the range of 60–150 were measured in real time using a rotating-compensator multichannel ellipsometer. Using phase diagram concepts for guidance, we have fabricated high efficiency single-junction nc-Si:H n–i–p solar cells with ～3 Å/s intrinsic layers using the VHF PECVD process. We have found that the nc-Si:H solar cells with the best performance are obtained by incorporating i-layers deposited in the single-phase nanocrystalline silicon regime near the transition boundary to mixed-phase (a + nc)-Si:H. Applying insights from real time spectroscopic ellipsometry moreover, we have investigated in detail the effects of the phase of the underlying n-layer on the phase evolution of the overdeposited Si:H i-layer and on the overall device performance. With the strategy developed here, a stabilized efficiency of η = 9.46% (V_oc = 0.516 V, J_sc = 24.65 mA/cm², FF = 0.744) has been achieved for nc-Si:H solar cells (0.25 cm² in active area) fabricated with an i-layer deposition rate of ～2.2 Å/s.     

Characteristics of p-type nanocrystalline silicon thin films developed for window layer of solar cells

Different rf-power and chamber pressures have been used to deposit boron doped hydrogenated silicon films by the PECVD method. The optoelectronic and structural properties of the silicon films have been investigated. With the increase of power and pressure the crystallinity of the films increases while the absorption decreases. As a very thin p-layer is needed in p–i–n thin film solar cells the variation of properties with film thickness has been studied. The fraction of crystallinity and thus dark conductivity vary also with the thickness of the film. Conductivity as high as 2.46 S cm⁻¹ has been achieved for 400 Å thin film while for 3000 Å thick film it is 21 S cm⁻¹. Characterization of these films by XRD, Raman Spectroscopy, TEM and SEM indicate that the grain size, crystalline volume fraction as well as the surface morphology of p-layers depend on the deposition conditions as well as on the thickness of the film. Optical band gap varies from 2.19 eV to 2.63 eV. The thin p-type crystalline silicon film with high conductivity and wide band gap prepared under high power and pressure is suitable for application as window layer for Silicon thin film solar cells.     

Nanocrystalline silicon TFT process using silane diluted in argon–hydrogen mixtures

We have investigated the effect of Ar dilution on the deposition process of intrinsic nc-Si:H (hydrogenated nanocrystalline silicon) thin films used as active layers of top-gate TFTs, in order to improve the TFTs performances. The nc-Si:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature (165 °C) and the related TFTs were fabricated with a maximum process temperature of 200 °C. During the nc-Si:H films deposition, the SiH₄ fraction and the total flow of the diluting gases Ar + H₂ mixture was kept constant, H₂ being substituted by Ar. We have pointed out the active role played by the metastable states of excited Ar atoms in both the dissociation of SiH₄ and H₂ by quenching reactions in the plasma. The role of the atomic hydrogen during the film deposition seems to be promoted by the addition of argon into the discharge, leading to an increase of the deposition rate by a factor of about three and an enhancement of the crystalline quality of the nc-Si:H films. This effect is maximized when the Ar fraction in the Ar + H₂ gases mixture reaches 50%. The evolution with Ar addition of the carriers mobility of the related TFTs is closely connected to the evolution of the crystalline fraction of the intrinsic nc-Si:H film. Mobilities values as high as 8 cm² V^?1 s^?1 are obtained at the Ar fraction of 50%. For higher Ar fractions, the fall of the mobility comes with a degradation of the I_D–V_G transfer characteristics of the processed TFTs due to a degradation of the nc-Si:H films quality. OES measurements show that the evolution of the H_α intensity is closely connected to the evolution of the deposition rate, intrinsic films crystalline fraction and TFTs mobility, providing an interesting tool to monitor the TFTs performances.     

Hydrogenated amorphous and nanocrystalline silicon solar cells deposited by HWCVD and RF-PECVD on plastic substrates at 150 °C

In this work we present a study of the structural, optoelectronic and transport properties of a series of Si films deposited in a parameter region (namely hydrogen dilution) corresponding to a transition from amorphous-to-nanocrystalline silicon by hot-wire (HW) and radio-frequency plasma enhanced chemical vapor deposition (RF) on plastic substrates at 150 °C. To achieve a higher deposition rate of Si films by RF we used a relatively high power density (350 mW/cm²) and deposition pressure (1.5 Torr). For certain hydrogen dilution values, these deposition conditions can lead to the formation of Si crystals in the silane plasma and to a growth of polymorphous silicon film. This material has improved carrier transport properties (ambipolar diffusion length = 220 nm) and very high photosensitivity (>5 × 10⁶). The best HW amorphous silicon films exhibited lower photosensitivity (7 × 10⁴) and an ambipolar diffusion length of only 100 nm. For solar cell fabrication, we optimized the RF deposition conditions to produce very thin amorphous and nanocrystalline phosphorous and boron doped silicon layers. Our best n–i–p solar cell, with a polymorphous Si intrinsic layer deposited on plastic, has an efficiency of 5.5%, FF = 52.5%, V_OC = 920 mV, J_SC = 11.6 mA/cm². For solar cells with a nanocrystalline Si active layer deposited on glass the following results were achieved: efficiency = 3.4%, FF = 43.5%, V_OC = 460 mV, J_SC = 17.2 mA/cm²; and on plastic substrate: efficiency = 2.2%, FF = 32.7%, V_OC = 397 mV, J_SC = 17.2 mA/cm².     

Effect of illumination on hydrogenated amorphous silicon thin films doped with chalcogens

Hydrogenated amorphous silicon thin films doped with chalcogens (Se or S) were prepared by the decomposition of silane (SiH₄) and H₂Se/H₂S gas mixtures in an RF plasma glow discharge on 7059 corning glass at a substrate temperature 230 °C. The illumination measurements were performed on these samples as a function of doping concentration, temperature and optical density. The activation energy varied with doping concentration and is higher in Se-doped than S-doped a-Si:H thin films due to a low defect density. From intensity versus photoconductivity data, it is observed that the addition of Se and S changes the recombination mechanism from monomolecular at low doping concentration films to bimolecular at higher doping levels. The photosensitivity (σ_ph/σ_d) of a-Si, Se:H thin films decreases as the gas ratio H₂Se/SiH₄ increased from 10^?4 to 10^?1, while the photosensitivity of a-Si, S:H thin films increases as the gas ratio H₂S/SiH₄ increased from 6.8 × 10^?7 to 1.0×10^?4.     

High diffusion length silicon germanium alloy thin films deposited by pulsed rf PECVD method

Hydrogenated amorphous silicon germanium (a-Si_1?xGe_x:H) alloy thin films were deposited in a rf (13.56 MHz) powered plasma CVD system from a mixture of silane, germane and hydrogen. The rf power was pulsed at a frequency of 1356 Hz with the duty cycle (Q) ranging from 50% to 100%. The Ge content in the films, found in the range 40–45%, was deduced from Raman spectra. The band gaps E_g of the films were in between 1.44 and 1.48 eV. We find a good correlation between the ambipolar diffusion length (L_d) and the microstructure factor ($R^1$) for different duty cycles. At Q = 75% the $R^1$ value is lowest (0.14) which correlates well with the maximum value of L_d (100 nm), the highest L_d value reported for a-Si_1?xGe_x:H films with E_g = 1.44 eV. It was also found to be very stable under light soaking. Powder incorporation within the films was also studied. Quality of the films is determined by two opposing effects viz. size of the powders incorporated into the films from the plasma and the bombardment of energetic ions and neutrals. Best transport properties were found at an optimum duty cycle of 75%.     

Characterization of a-B5C:H prepared by PECVD of orthocarborane: Results of preliminary FTIR and nuclear reaction analysis studies

The electronic properties of a-Si:H vary with hydrogen passivation of dangling bond defects. It appears this effect is also operative in semiconducting amorphous hydrogenated boron carbide (a-B₅C:H). Therefore, the ability to quantify the amount of hydrogen will be key to development of the materials science of a-B₅C:H. The results of an initial investigation probing the ability to quickly correlate hydrogen concentration in a-B₅C:H films with infrared spectroscopy are reported. a-B₅C:H thin films were growth on Si (1 1 1) substrates by plasma-enhanced chemical vapor deposition (PECVD) using sublimed orthocarborane and argon as the precursor gas. Nuclear reaction analysis (NRA) was performed to quantify the atomic concentration of H in the a-B₅C:H films. While the observed vibronic structure does not show stretches due to terminal C–H or bridging B–H–B, analysis of the terminal B–H stretch at ～2570 cm^?1 gives a proportionality constant of A = 2 × 10²² cm^?2. We conclude that the methods previously developed for correlating H concentration to infrared data in a-Si:H are similarly viable for a-B₅C:H films.     

Effect of tellurium deposition rate on the properties of Cu–In–Te based thin films and solar cells

To investigate the effects of tellurium (Te) deposition rate on the properties of Cu–In–Te based thin films (Cu/In=0.30–0.31), the films were grown on both bare and Mo-coated soda-lime glass substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy and X-ray diffraction. The crystalline quality of the films was improved with increase in the deposition rate of Te, and exhibited a single CuIn₃Te₅ phase with a highly preferred (1 1 2) orientation. Te-deficient film (Te/(Cu+In)=1.07) grown with a low Te deposition rate showed a narrow bandgap of 0.99 eV at room temperature. The solar cell performance was affected by the deposition rate of Te. The best solar cell fabricated using CuIn₃Te₅ thin films grown with the highest deposition rate of Te (2.6 nm/s) yielded a total area (0.50 cm²) efficiency of 4.4% (V_oc=309 mV, J_sc=28.0 mA/cm², and FF=0.509) without light soaking.     

Electron beam poling in amorphous Ge-doped H:SiO2 films

Amorphous Ge-doped H:SiO₂ films on silica, deposited by matrix-distributed electron cyclotron resonance – plasma enhanced chemical vapor deposition, were irradiated with an electron beam while varying the dose. Using the Maker fringe method, second-harmonic generation was measured in the irradiated regions of the films. With a current of 5 nA, and an acceleration voltage of 25 kV for 25 s, a Ge-doped H:SiO₂ film (3.8 at.% Ge) showed a maximum second-order nonlinearity of d₃₃ = 0.0005 pm/V. In contrast, a H:SiO₂ film with a smaller Ge content (1.0 at.% Ge), showed a large SHG: d₃₃ = 0.06 pm/V when irradiated for 15 s. The second-harmonic generation in the films is caused by a frozen-in electric field induced by charge implantation from the electron beam. The strength of the electric field is determined by two conditions: the trapping centers (numbers, depth) and the remaining conductivity under large electric field.     

Influence of hydrogen on the thermomechanical properties of a-CNx:H and a-CNx films deposited by glow discharge and ion beam assisted deposition

The coefficient of thermal expansion (CTE), Young’s modulus, Poisson’s ratio, stress and hardness of a-CN_x and a-CN_x:H were investigated as a function of nitrogen concentration. Hydrogenated films were prepared by glow discharge, GD, and unhydrogenated films were prepared by ion beam assisted deposition, IBAD. Using nanohardness measurements and the thermally induced bending technique, it was possible to extract separately, Young’s modulus and Poisson’s ratio. A strong influence of hydrogen, in a-CN_x:H films, was observed on the CTE, which reaches about ∼9 × 10⁻⁶ C⁻¹, close to that of graphite (∼8 × 10⁻⁶ C⁻¹) for nitrogen concentration as low as 5 at.%. On the other hand, the CTE of unhydrogenated films increases with nitrogen concentration at a much lower rate, reaching 5.5 × 10⁻⁶ C⁻¹ for 33 at.% nitrogen.     

Effect of hot-wire passivation on the properties of hydrogenated microcrystalline silicon films to reduce post-deposition oxidation

Hydrogenated microcrystalline silicon (μc-Si:H) films have a large number of grain boundaries that oxidize after deposition, leading to deterioration of device performance. In this study, post-treatment of μc-Si:H thin films was carried out with methane-related radicals generated by a hot wire. The effect of the hot-wire passivation on the properties of the μc-Si:H thin films was investigated using Fourier-transform infrared (FT-IR) transmission spectroscopy. Through post-treatment, hydrogen on the silicon-crystallite surface was substituted with hydrocarbon. Further, an increase in filament temperature (T_ft) was found to enhance passivation. For films treated at T_ft above 1700 °C, post-oxidation and nitridation hardly occurred, whereas films treated at T_ft below 1400 °C were oxidized and nitrided even after post-treatment.     

Enhanced spectral response by silicon nitride index matching layer in amorphous silicon thin-film solar cells

We employed the low temperature hydrogenated amorphous silicon nitride (a-SiN_x:H) prepared by plasma-enhanced chemical vapor deposition as a refractive index (n) matching layers in a silicon-based thin-film solar cell between glass (n = 1.5) and the transparent conducting oxide (n = 2). By varying the stoichiometry, refractive index and thickness of the a-SiN_x:H layers, we enhanced the spectral response and efficiency of the hydrogenated amorphous silicon thin-film solar cells. The refractive index of a-SiN_x:H was reduced from 2.32 to 1.78. Optimizing the a-SiN_x:H thickness to 80 nm increased the J_SC from 8.3 to 9.8 mA/cm² and the corresponding cell efficiency increased from 4.5 to 5.3%, as compared to the cell without the a-SiN_x:H index-matching layer on planar substrate. The a-SiN_x:H layers with graded refractive indices were effective for enhancing the cell performance.     

Nanocalorimetric investigation of light-induced metastable defects in hydrogenated amorphous silicon

The electrical properties of hydrogenated amorphous silicon, a-Si:H, are degraded by light-induced metastable defects after exposure to visible light for extended periods. Using nanocalorimetry, we have directly measured the heat released when these defects are annealed. Although these low level measurements were close to the instrument noise limit, and were affected by extraneous signals from adsorbed gas, a total heat release of only a few tens of nJs could be resolved. For a heating rate of 12 000 K s⁻¹, a single broad peak of heat release, centered at 180 °C, was observed. The integrated heat release indicates that ∼8 × 10¹⁶ defects cm⁻³ h⁻¹ were generated. Polycrystalline Si samples, in which no defects are created by light-soaking, showed no heat release.     

Optical and structural proprieties of nc-Si:H prepared by argon diluted silane PECVD

Using argon as a diluent of Silane, hydrogenated amorphous and nanorocrystalline silicon films Si:H were prepared by radio-frequency (13.56 MHz) plasma enhanced chemical vapor deposition (rf-PECVD). The deposition rate and crystallinity varying with the deposition pressure and rf power, were systematically studied. Structural analysis (Raman scattering spectroscopy and X-ray diffraction), combined with optical measurements spectroscopy were used to characterize the films. The argon dilution of silane for all samples studied was 95% by volume, and the substrate temperature was 200 °C. The deposition pressure was varied from 400 mTorr to 1400 mTorr and varying rf power from 50 to 250 W. The structural evolution studies, shows that beyond 200 W of rf power, an amorphous-nanocrystalline transition was observed, with an increase in crystalline fraction by increasing rf power and working pressure. The films were grown at high deposition rates. The deposition rates of the films near the amorphous-nanocrystalline phase transition region were found in the range 6–10 Å/s. A correlation between structural and optical properties has been found and discussed.     

Effect of deposition pressure on microstructure and properties of hydrogenated carbon nitride films prepared by DC-RF-PECVD

Hydrogenated carbon nitride (a-CN:H films) were deposited on n-type (1 0 0) silicon substrates making use of dual direct current radio frequency plasma enhanced chemical vapor deposition (DC-RF-PECVD), at working pressure of 2–20 Pa, using a mixed gas of CH₄ and N₂ as the source gas. The growth rate, composition, bonding structure of the deposited films were characterized by means of XPS and FTIR, and the mechanical properties of the deposited films were investigated by nano-indentation test. It was found that the parameters for the DC-RF-PECVD process had significant effects on the growth rate, structure and properties of the deposited films. The growth rate of the deposited films increased at first with increasing deposition pressure, then saturated with further increase of the deposition pressure. The N/C ratio inside the deposited films increased with increasing working pressure except that it was as much as 0.50 at a working pressure of 5.0 Pa. The nano-hardness of the films decreased with increasing deposition pressure. CN radicals were remarkably formed in the deposited films at higher pressures, and their contents are related to the nitrogen concentrations in the deposited films.     

Surface morphology of spin-coated As–S–Se chalcogenide thin films

Surface morphology and roughness of amorphous spin-coated As–S–Se chalcogenide thin films were determined using atomic force microscopy. Prepared films were coated from butylamine solutions with thicknesses d ∼ 100 nm and then annealed in a vacuum furnace at 45 °C and 90 °C for 1 h for their stabilization. The root mean square surface roughness analysis of surfaces of as-deposited spin-coated As–S–Se films indicated a very smooth film surface (with R_q values 0.42–0.45 ± 0.2 nm depending on composition). The nanoscale images of as-deposited films confirmed that surface of the films is created by domains with dimensions 20–40 nm, which corresponds to diameters of clusters found in solutions. The domain character of film surfaces gradually disappeared with increasing annealing temperature while the solvent was removed from the films. Middle-infrared transmission spectra recorded a decrease of intensities of vibration bands connected to N–H (at 3367 and 3292 cm⁻¹) and C–H (at 2965, 2935 and 2880 cm⁻¹) stretching vibrations. Temperature regions of solvent evaporation T = 60–90 °C and glass transformation temperatures T_g = 135–150 °C of spin-coated As–S–Se thin films were determined using a modulated differential scanning calorimetry.     

Growth of pure ZnO thin films prepared by chemical spray pyrolysis on silicon

Structural, morphological, optical and electrical properties of ZnO thin films prepared by chemical spray pyrolysis from zinc acetate (Zn(CH₃COO)₂ 2H₂O) aqueous solutions, on polished Si(1 0 0), and fused silica substrates for optical characterization, have been studied in terms of deposition time and substrate temperature. The growth of the films present three regimes depending on the substrate temperature, with increasing, constant and decreasing growth rates at lower, middle, and higher-temperature ranges, respectively. Growth rate higher than 15 nm min⁻¹ can be achieved at T_s=543 K. ZnO film morphological and electrical properties have been related to these growth regimes. The films have been characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy.