Deposition Of Amorphous Silicon Research Articles

Nanoindentation of Ion Implanted and Deposited Amorphous Silicon

ABSTRACTThe deformation behavior of both ion-implanted and deposited amorphous Si (a-Si) films has been studied using spherical nanoindentation, followed by analysis using Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). Indentation was carried out on both unannealed a-Si films (the so-called unrelaxed state) and in ion implanted films that were annealed to 450°C to fully relax the amorphous film. The dominant mode of deformation in unrelaxed films was via plastic flow of the amorphous phase rather than phase transformation, with measured hardness being typically 75–85% of that of crystalline Si. In contrast, deformation via phase transformation was clearly observed in the relaxed state of ion implanted a-Si, with the load-unload curves displaying characteristic discontinuities and Raman and XTEM indicating the presence of high-pressure crystalline phases Si-III and Si-XII following pressure release. In such cases the measured hardness was within 5% of that of the crystalline phase.

Nanoindentation of Ion Implanted and Deposited Amorphous Silicon

ABSTRACTThe deformation behavior of both ion-implanted and deposited amorphous Si (a-Si) films has been studied using spherical nanoindentation, followed by analysis using Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). Indentation was carried out on both unannealed a-Si films (the so-called unrelaxed state) and in ion implanted films that were annealed to 450°C to fully relax the amorphous film. The dominant mode of deformation in unrelaxed films was via plastic flow of the amorphous phase rather than phase transformation, with measured hardness being typically 75–85% of that of crystalline Si. In contrast, deformation via phase transformation was clearly observed in the relaxed state of ion implanted a-Si, with the load-unload curves displaying characteristic discontinuities and Raman and XTEM indicating the presence of high-pressure crystalline phases Si-III and Si-XII following pressure release. In such cases the measured hardness was within 5% of that of the crystalline phase.

Deposition of high quality amorphous silicon, germanium and silicon-germanium thin films by a hollow cathode reactive sputtering system

High quality hydrogenated amorphous silicon (a-Si:H), germanium (a-Ge:H) and silicon–germanium (a-SiGe:H) thin films have been deposited by means of a d.c. hollow cathode system with magnetic field confinement. High purity single-crystal silicon and germanium nozzles were reactively sputtered in a high-density hollow cathode discharge of argon and hydrogen. This process avoids the use of the toxic and pyrophoric gases, germane and silane. The amorphous silicon thin films had light to dark conductivity ratios >10 6 with light conductivity in the 10 −5 S/cm range. The best a-Si:H films have a Tauc band gap near 1.8 eV with an atomic hydrogen concentration of approximately 14%. The growth rate was in the 2–3 μm/h range. For the a-Ge:H films the FTIR results indicate that these films have hydrogen bonding as a single atom, as did the hydrogenated silicon films. The Tauc bandgap was approximately 1.0 eV for all the germanium films. A slight photoresponse was noted for these films, which were deposited at a rate of from 2 to 6 μm/h. For the a-SiGe:H films, two hollow cathodes of single crystal Si and Ge are reactively sputtered simultaneously. A description of the complete system will be presented. The optical and electronic properties of the initial films are promising. The photoresponse is dependent upon the bandgap, i.e. the germanium content, as expected. A light to dark ratio of 2600 has been achieved for a film with a bandgap of 1.53 eV. The FTIR data indicates that SiH bonds dominate over Ge:H bonds by the absence of peaks at 570 and 1880 cm −1.

Deposition of high quality amorphous silicon, germanium and silicon-germanium thin films by a hollow cathode reactive sputtering system

High quality hydrogenated amorphous silicon (a-Si:H), germanium (a-Ge:H) and silicon–germanium (a-SiGe:H) thin films have been deposited by means of a d.c. hollow cathode system with magnetic field confinement. High purity single-crystal silicon and germanium nozzles were reactively sputtered in a high-density hollow cathode discharge of argon and hydrogen. This process avoids the use of the toxic and pyrophoric gases, germane and silane. The amorphous silicon thin films had light to dark conductivity ratios >106 with light conductivity in the 10−5 S/cm range. The best a-Si:H films have a Tauc band gap near 1.8 eV with an atomic hydrogen concentration of approximately 14%. The growth rate was in the 2–3 μm/h range. For the a-Ge:H films the FTIR results indicate that these films have hydrogen bonding as a single atom, as did the hydrogenated silicon films. The Tauc bandgap was approximately 1.0 eV for all the germanium films. A slight photoresponse was noted for these films, which were deposited at a rate of from 2 to 6 μm/h. For the a-SiGe:H films, two hollow cathodes of single crystal Si and Ge are reactively sputtered simultaneously. A description of the complete system will be presented. The optical and electronic properties of the initial films are promising. The photoresponse is dependent upon the bandgap, i.e. the germanium content, as expected. A light to dark ratio of 2600 has been achieved for a film with a bandgap of 1.53 eV. The FTIR data indicates that SiH bonds dominate over Ge:H bonds by the absence of peaks at 570 and 1880 cm−1.

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H 2 and Si 2H 6, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.

Recent progress of Cat-CVD research in Japan—bridging between the first and second Cat-CVD conferences

We review the recent progress of Cat-CVD research in Japan since the 1st Cat-CVD conference in Kanazawa in 2000. Some groups, including ours, succeeded in realizing large-area deposition of amorphous silicon (a-Si) of approximately 1 m size, and thin film transistors (TFTs) with a mobility over several 10s of cm 2 V −1 s −1 are fabricated using Cat-CVD polycrystalline silicon (poly-Si) films. Extensive studies of in situ cleaning methods revealed that a high rate of chamber cleaning is possible in Cat-CVD systems. Solar cell research is now carried out within the New Energy and Industrial Technology Development Organization (NEDO) project, and the study of Cat-CVD Si 3N 4 films prepared at lower than 100 °C is now a Japan Science and Technology Corporation (JST) project to use them as coatings on organic devices. The feasibility of Cat-CVD for various applications has been widely demonstrated, along with further understanding of the fundamental mechanism of the Cat-CVD process.

Monte Carlo simulations on large-area deposition of amorphous silicon by hot-wire CVD

Scale-up of hot-wire CVD reactors for commercial production of a-Si:H based solar cells requires understanding of the large-area deposition process. Therefore, the process was simulated using the Direct Simulation Monte Carlo-method (G.A. Bird, Clarendon Press, Oxford (1994)), considering reactions at the filaments, in the gas phase and at the substrate, and in particular large-area deposition by modeling the gas shower and the filament grid, which were found to determine the uniformity and quality of the a-Si:H films (Thin Solid Films 395 (2001) 61; Solar Energy Mater. Solar Cells 73 (2002) 321). The distance between the filament grid and the substrate ( d fil–S) and the distance between the filaments ( d fil) were systematically varied, and the simulation results were compared to experimental results obtained in our large-area deposition system (Thin Solid Films 395 (2001) 61; Solar Energy Mater. Solar Cells 73 (2002) 321). The experimentally obtained optimum filament-to-substrate distance was supported by an optima in the simulated Si 2H 4-concentration. For other species, the existence was confirmed but a definite value for optimum d fil–S could not be concluded. The simulations also confirmed the influence of the filament geometry on the uniformity as obtained in the experiments.

Numerical modeling of capacitively coupled hydrogen plasmas: Effects of frequency and pressure

In the field of plasma deposition of amorphous and microcrystalline silicon, the increase of the excitation frequency has often been considered as a way to enhance the deposition rate. Moreover, the increase of pressure has also been shown to enhance the deposition rate and improve the film properties. We attempt to clarify the effects of frequency in the 13.56–40.68 MHz range and to compare them to those of the pressure in the range of 0.5–1.5 Torr. For that purpose we use a numerical modeling of capacitively coupled hydrogen plasma, particularly relevant for the deposition of microcrystalline silicon. We use a one-dimensional time-dependent fluid model for the description of neutrals, positive and negative ions, and electrons, which involves a chemistry model taking into account 32 reactions in the gas phase and on the surface of the electrodes. The results of the model for the symmetrical system show that both pressure and frequency have pronounced influence on the parameters of the discharge: sheath thickness, ratio between power transferred to ions and electrons, and concentration and flux of atomic hydrogen at the electrode surface. We found that increasing the excitation frequency, while keeping constant the power dissipated in the plasma, leads to a more moderate increase of electron density as compared with the case of constant rf-voltage amplitude. The analysis of this phenomenon reveals that, with increase of frequency, the power coupling to the electrons becomes more efficient due to the decrease of the phase shift between voltage and current for both constant power and constant voltage conditions. There is, in addition, a significant drop of the sheath voltage with frequency when the power dissipated in the plasma is kept constant. This leads to the reduction in the drift loss rate for charged species. The increase of pressure mainly reduces the diffusive component of the loss rate for both charged and neutral species and, as a result, electron density enhancement is less pronounced. The increase of pressure leads to a more uniform spatial dissipation of the power coupled to the plasma, whereas the increase in frequency results in a higher amount of power dissipated on the plasma-sheath boundaries due to the decrease of the sheath width.

Optimization of sensitivity and noise in piezoresistive cantilevers

In this article, the sensitivity and the noise of piezoresistive cantilevers were systematically investigated with respect to the piezoresistor geometry, the piezoresistive materials, the doping dose, the annealing temperature, and the operating biased voltage. With the noise optimization results, dimension optimized array cantilevers were designed and fabricated by using single-crystal silicon, low-pressure chemical-vapor deposition (LPCVD) amorphous silicon and microcrystalline silicon as piezoresistive layers. Measurement results have shown that the smallest Hooge factor (α) was 3.2×10−6, the biggest gauge factors was 95, and the minimum detectable deflection (MDD) at 6 V and 200 Hz-measurement bandwidth was 0.3 nm for a single-crystal silicon cantilever. Of the two LPCVD silicon piezoresistive cantilevers, amorphous silicon piezoresistors had relatively lower 1/f noise. The MDD for a LPCVD silicon cantilever at a 200 Hz-measurement bandwidth was 0.4 nm. For all kinds of piezoresistive cantilevers, the 1/f noises were decreased by 35%–50% and the gauge factors were decreased by 60–70% if the doping dose were increased by ten times. The annealing at 1050 °C for 30 min decreased 1/f noise by about 65% compared with the 950 °C for 10 min treatments. The cantilevers with a relatively higher-doping dose gave smaller MDD even though the gauge factors of them were decreased by nearly a factor of 1.8. The higher-biased voltages had no great improvements on the MDD due to the 1/f noise dominance.

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

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

Influence of implantation and annealing on the surface topography of amorphous and polysilicon thin films

In this paper the surface roughness and topography of low-pressure chemical vapor deposition amorphous silicon films grown on (001) silicon substrates after phosphorus/argon/arsenic ion implantation, after annealing at 850 °C in an ambient N2 environment and after further annealing to 1070 °C in O2 environment have been investigated. The influence of the different treatments on the sample surface topography was examined by atomic force microscopy in the noncontact mode. The surface composition of the films was analyzed by x-ray photoelectron spectroscopy for annealed and unannealed phosphorus implanted wafers. It was found that the surface roughness of the films is affected by implantation and annealing and mostly by implantation followed by annealing. Only implantation reduced the surface roughness while only annealing results in an increase of surface roughness. Implantation followed by annealing results in a drastic increase of surface roughness. “Bumps” which appeared following annealing seem to consist of a polysilicon structure. The evolution of surface roughness with annealing temperature is accompanied by a concurrent formation of silicon oxide. Implanted wafers annealed at 850 °C in an ambient N2 grow a thin substochiometric silicon oxide layer which has a higher roughness than the as-deposit amorphous silicon layer. Further annealing to 1070 °C in a O2 environment results in an increase in roughness and formation of a stochiometric SiO2 layer.

Use of real-time ion-mass spectrometry and discharge voltage trending for analysis of a-Si DC plasma CVD processes

DC discharge voltage monitoring is a convenient non-intrusive tool for in-line monitoring of plasma-deposited amorphous silicon deposition processes. We discuss the observed variations of the discharge voltage due to (i) changing hydrogen dilution of silane, (ii) etch back of silicon by a hydrogen plasma, (iii) admixing of germane and methane and correlate the results with in situ ion-mass spectroscopy. We suggest that negative ion formation by electron attachment to silane and/or higher-silane species causes the changes of discharge voltage. Time dependences following discharge initiation are discussed in terms of the relative concentrations of silane versus higher-silane species.

Properties of silicon oversaturated with implanted hydrogen

Structural properties of silicon layers oversaturated with implanted hydrogen (up to 20%) are studied by means of IR Fourier spectroscopy, Raman spectroscopy, and high-resolution electron microscopy (HREM). The data obtained by the different methods are found to agree fairly well with one another. Silicon samples implanted up to fluxes (1–3)×10 17 cm −2 reveal a layered structure. It is found that just after implantation a surface layer forms which manifests the properties of amorphous silicon. During subsequent heat treatments at 600–800 °C, amorphous layers with embedded silicon nanocrystals form, with a simultaneous decrease in the degree of polymerization of SiH bonds and hydrogen transition in a molecular form. Results of electrical studies of the hydrogenated silicon layers allow us to put forward a new production method for Si:H/c-Si heterostructures based on implanted H which is promising for photoelectrical converters. The new technology makes it possible to avoid the stage of deposition of amorphous silicon from ecologically harmful compounds. The density of states both in the amorphous silicon and at the Si:H/c-Si interface is of the order 5×10 17 cm −3 and it is compared to CVD technology.

The influence of the amorphous silicon deposition temperature on the efficiency of the ITO/A-Si:H/C-Si heterojunction (HJ) solar cells and properties of interfaces

Some crucial limiting process steps of ITO/(n)a-Si:H/(p)c-Si solar cell manufacturing were investigated. The influence of amorphous silicon deposition temperature on the efficiency of heterojunction (HJ) solar cells as well as ITO deposition on the properties of ITO/a-Si:H/c-Si interfaces were studied. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and SIMS measurements were employed for the investigations. It was observed that the decrease in a-Si:H deposition temperature from 230 down to 100°C leads to a reduction in solar cell efficiencies to only 1%. It was found that the ITO deposition process itself can significantly modify the ITO/a-Si:H/c-Si interfaces due to the interaction of oxygen, and In and Sn atoms with silicon at the initial stage. It was concluded that this modification has a more significant impact on the quality of the HJ solar cells than on that of the a-Si:H layer.

Crystalline to amorphous phase transition in very low temperature molecular beam epitaxy

For the first time, we use in situ time resolved reflectivity (TRR) measurement to determine the thickness for crystalline–amorphous transition. The use of TRR measurements to study the breakdown of the single crystalline growth and the following deposition of amorphous silicon ( α-Si) is based on the higher refraction index of α-Si at the wavelengths used (950 and 670 nm). An increasing α-film thickness causes cycles of constructive and destructive interference and leads to a time-dependent reflectivity. Final amorphous film thicknesses have been confirmed by ex situ spectroscopic ellipsometry. Samples were prepared by the deposition of electron-beam evaporated Si in an MBE equipment. The temperature, as a function of heater power, was carefully calibrated for highly doped substrates, with a thermocouple mounted into a test substrate. Substrates were kept at constant growth temperatures between 100 and 250 °C. From an Arrhenius-type expression, we can deduce an activation energy of 0.36±0.05 eV. The growth rate dependence was investigated between 0.013 and 0.7 nm s −1 at a constant temperature of 200 °C. Rate dependent data can be fitted by a power law. A variation of the deposition rate by a factor of 100 causes a variation in epi thickness by a factor 5. Similar experiments were also performed on low doped substrates and with non-linear temperature ramp. The obtained substrate temperature depends strongly on doping because of free carrier absorption (differences up to ≈50 °C).

Optimization of process parameters in a large-area hot-wire CVD reactor for the deposition of amorphous silicon (a-Si:H) for solar cell application with highly uniform material quality

Abstract Scale-up of a-Si:H-based thin film applications such as solar cells, entirely or partly prepared by hot-wire chemical vapor deposition (HWCVD), requires research on the deposition process in a large-area HWCVD system. The influence of gas supply and filament geometry on thickness uniformity has already been reported, but their influence on material quality is systematically studied for the first time. The optimization of deposition parameters for obtaining best material quality in our large-area HWCVD system resulted in an optimum filament temperature, T fil ≈1600°C, pressure, p =8 mTorr and silane flow, F(SiH 4 )=100 sccm, keeping the substrate temperature at T S =200°C. A special gas supply (gas shower with tiny holes of uniform size) and a filament grid, consisting of six filaments with an interfilament distance, d fil =4 cm were used. The optimum filament-to-substrate distance was found to be d fil–S =8.4 cm. While studying the influence of different d fil and gas supply configurations on the material quality, the above-mentioned setup and parameters yield best results for both uniformity and material quality. With the setup mentioned, we could achieve device quality a-Si:H films with a thickness uniformity of ±2.5% on a circular area of 20 cm in diameter. The material, grown at a deposition rate of r d ≈4 A/s, was characterized on nine positions of the 30 cm×30 cm substrate area, and revealed reasonable uniformity of the opto-electronic properties, e.g photosensitivity, σ Ph / σ D =(2.46±0.7)×10 5 , microstructure factor, R =0.17±0.05, defect densities, N d(PDS) =(2.06±0.6)×10 17 cm −3 and N d(CPM) =(2.05±0.5)×10 16 cm −3 (film properties are given as mean values and standard deviations). Finally, we fabricated pin solar cells, with the i-layer deposited on small-area p-substrates distributed over an area of 20 cm×20 cm in this large-area deposition system, and achieved high uniformity of the cell parameters with initial efficiencies of η =(6.1±0.2)% on the 20 cm×20 cm area.

Simulations of the gas flux distribution for different gas showers and filament geometries on the large-area deposition of amorphous silicon by hot-wire CVD

For the scaleup of hot-wire CVD for the deposition of a-Si:H based solar cells, simulations regarding the influence of gas supply and filament geometry on the uniformity of a-Si:H films are described to support the design of hot-wire CVD reactors. We simulated the spatial distribution of the gas flux generated by different kinds of gas showers and changes in this spatial distribution with increasing distance from the gas outlet. Regarding the influence of the filaments, we calculated temperature gradients across the substrate caused by the radiative heat and the gas flux, based on the assumption that the gas extends collision-free, from the filament grid. For both simulations, the distances between the filaments and between substrate and filament grid were varied. The simulations of the gas flux from both gas shower and filament grid were combined and compared to experimental results obtained in our large-area deposition system. With an optimized setup, we could achieve good quality a-Si:H films with high thickness uniformity of ±2.5% and p–i–n solar cells with initial efficiencies of η=(6.1±0.2)% on an area of 20×20 cm 2 .

Correlations between properties and applications of the CVD amorphous silicon carbide films

The aim of this paper is to emphasise the correlation between film preparation conditions, film properties and their applications. Low pressure chemical vapour deposition amorphous silicon carbide (a-SiC) and silicon carbonitride (SiCN) films obtained from liquid precursors have different structure and composition depending on deposition conditions. Thus, the films deposited under kinetic working conditions reveal a stable structure and composition. Deposition at moderate temperature leads to stoichiometric SiC, while the films deposited at high temperatures have a composition closer to Si 1− x C x , with x=0.75. These films form a very reactive interface with metallic layers. The films realised under kinetic working regime can be used in Si membrane fabrication process or as coating films for field emission applications. SiC layers field emission properties were investigated; the field emission current density of the a-SiC/Si structures was 2.4 mA/cm 2 at 25 V/μm. An Si membrane technology based on moderate temperatures (770–850 °C) a-SiC etching mask is presented.

Hot-wire produced atomic hydrogen: effects during and after amorphous-silicon deposition

The role of hot-wire atomic hydrogen during a-Si:H deposition is elucidated by investigating hydrogen diffusion into and within hot-wire deposited a-Si:H films. We exposed device-quality a-Si:H layers to atomic hydrogen in order to mimic the atomic hydrogen flux during deposition. Initially, exposure of as-deposited a-Si:H films leads to an enhancement of the hydrogen concentration by a factor of 2. A higher wire temperature or a longer duration of the atomic hydrogen exposure etches the silicon layer. The temperature of the films during these treatments was below the deposition temperature. Deposition of double layers which incorporate either H or D, shows considerable diffusion taking place at the substrate temperature which is usually applied for hot-wire a-Si:H layers in devices. The observation of these effects, the high atomic hydrogen flux and the high diffusion coefficient, indicate the important role of atomic hydrogen during hot-wire deposition.

Hot-wire deposition of photonic-grade amorphous silicon

A new amorphous silicon application related to the patterning of refractive index for the purpose of defining and integrating photonic-device elements is emerging. Photonic device elements include waveguides, splitters, mirrors, optical memories, etc. Hot-wire-deposited amorphous silicon has several attributes that make it an exceedingly attractive matrix for photonic device patterning, including: high hydrogen solubility limits; relatively little sub-gap absorption; low stress; non-peeling films; and fast, economical deposition of thick (≥5 μm) films, as well as optically smooth as-deposited surfaces, even on thick films. The growing catalog of proposed and/or demonstrated amorphous silicon-based optical devices is rapidly expanding.