Deposition Of Silicon Thin Films Research Articles

The effects of RF plasma excitation frequency and doping gas on the deposition of polymorphous silicon thin films

Polymorphous silicon (pm-Si:H) is a nanostructured material that has attracted much attention due to its unique structure and electronic properties that make it an excellent alternative to a-Si:H for photovoltaic applications. In this study we present the effects of the plasma excitation frequency (13.56, 27.12 and 40.68 MHz) and of dopants on the formation of clusters in the plasma, and correlate them to the optical and electronic properties of the films. While previous studies have focused on films incorporating crystallites and clusters with sizes of 1–3 nm, the analysis of the roughness by in situ spectroscopic ellipsometry suggests that more ordered films result from the incorporation of agglomerates with sizes up to 10 nm. Moreover, the increase of the excitation frequency does not enhance the deposition rate, it only shifts the maximum value of deposition rate to the lower deposition pressure.

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.

Processes in silicon deposition by hot-wire chemical vapor deposition

Hot-wire chemical vapor deposition is a rapidly developing CVD technique for the deposition of silicon thin films and silicon alloys and may become a competitor of the plasma-enhanced (PE) CVD method due to significant advantages such as high deposition rate, efficient source gas utilization, lack of ion bombardment, and low equipment cost. Little is known, however, about the mechanisms for catalytic decomposition of the source gases, gas phase reactions at commonly used pressures, and the growth reactions. In this article, the differences in the reactions at various filament materials are discussed and it is shown that the subsequent reactions in the gas phase and reactions contributing to film growth can be substantially different from those in PE-CVD, due to the lack of energetic electrons and ions. Further work is necessary to identify the role of each precursor for the deposition of amorphous and microcrystalline films.

Plasma enhanced chemical vapor deposition of silicon thin films for large area electronics

In the past 2 years major advances have been made in the understanding of silane–hydrogen plasmas. In particular, the control of the formation of clusters and even crystallites at room temperature has lead researchers to change their approach and to look for plasma conditions where clusters and crystallites contribute to the growth. In addition, hydrogen has become a key element for the growth of amorphous and microcrystalline silicon films as it can easily diffuse through the growing layers and induce their crystallization below the surface.

Investigation of the rf and dc hollow cathode plasma-jet sputtering systems for the deposition of silicon thin films

Both rf and dc hollow cathode plasma-jet sputtering systems have been investigated for thin film semiconductor deposition. These systems were studied as a modification of the well-known rf hollow cathode plasma jet system. The aim of this modification was to provide low temperature deposition of semiconductor silicon and silicon-based alloys as thin films with these plasma jet systems. As a first step, the deposition of an already well explored, hydrogenated amorphous silicon material, a-Si:H, was chosen for experimentation. Plasma erosion of single crystal silicon nozzles in an Ar and H 2 working gas mixture was utilized for this purpose. A comparison of both dc and rf hollow cathode plasma jets has been made and correlated to the a-Si:H thin film properties. As a preliminary result, large differences between the properties of a-Si:H thin films deposited using dc and rf plasmas have been found. Monohydride Si:H composition was found for a-Si:H films fabricated using the dc plasma jet system under certain experimental conditions. However, predominantly di-hydride and multi-hydride structures and strong oxidization were found for the a-Si:H films deposited using rf plasma excitation. The sputtering efficiencies of both the rf and dc jet sources for silicon films have been found to be similar.

Power density in RF PECVD: a factor for deposition of amorphous silicon thin films and successive solid phase crystallization

Solid phase crystallization of amorphous silicon thin films deposited by radio frequency plasma enhanced chemical vapour deposition (RF PECVD) has been studied. The effects of power density variation on the properties of amorphous and microcrystalline films (after crystallization) have been discussed. A fast growth rate of the amorphous layer (12.5 Ås-1) and large grain (~350Å) microcrystalline film has been reported. A high value of electrical dark conductivity (~10-6S cm-1) is also obtained for the microcrystalline (intrinsic) film. It has been observed that deposition rate and bonded hydrogen content in the amorphous films play important roles in the crystallization process and properties of the crystallized films. Electron resonance spectra indicate an increase in defect density of amorphous films with increase in power density and better crystallization is obtained for films with higher density of defects. Scanning electron microscopy reveals that the nature of film surfaces and distribution of grain in the crystallized films are strongly affected by thepower density regime in the deposition process.

Detection of hydrogen atoms in SiH4–H2 radio-frequency plasmas using two-photon laser-induced fluorescence

Two-photon laser-induced fluorescence spectroscopy is used to detect ground-state atomic hydrogen in highly diluted SiH4–H2 radio-frequency discharges currently used for microcrystalline silicon thin film deposition. Along with the fluorescence coming from the hydrogen atoms created in the discharge, additional atoms arising from photodissociation of silicon containing species at 205 nm are observed. This parasitic effect is identified as resulting mainly from the disilane molecules formed in the plasma. Hydrogen atoms created in the plasma and originating from the photodissociation are discriminated by a spectral analysis of the laser-induced fluorescence line profile and relative densities are deduced. Some results on the hydrogen atom and disilane concentrations as functions of the silane dilution, radio-frequency power, gas pressure, and gas temperature are given to illustrate the potentiality of such a diagnostic in SiH4–H2 discharges containing up to 6% of silane.

Mechanism and activation energy barrier for H abstraction by H(D) from a-Si:H surfaces

Hydrogen atoms are abstracted from the surface of hydrogenated amorphous silicon (a-Si:H) films by impinging H(D) atoms through an Eley–Rideal mechanism that is characterized by a zero activation energy barrier. This has been revealed by systematic analysis of the interactions of H(D) atoms with a-Si:H films during exposure to an H 2(D 2) plasma using synergistically molecular-dynamics simulations and attenuated total reflection Fourier transform infrared spectroscopy combined with spectroscopic ellipsometry. Understanding such interactions is of utmost importance in optimizing the plasma deposition of silicon thin films.

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

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

Combined effect of electrode gap and radio frequency on power deposition and film growth kinetics in SiH4/H2 discharges

The combined effect of the variation of the interelectrode gap (1.3–2.5 cm) and radio frequency (13.56–50 MHz) on the properties of highly diluted silane in hydrogen discharges used for the deposition of microcrystalline silicon thin films is presented. The investigation included electrical and optical discharge measurements as well as the in situ determination of the film growth rate. In the lower frequencies regime, the increase of the interelectrode gap for the same applied voltage results in higher current flows and higher total power dissipation. On the other hand, at 50 MHz the variation of the interelectrode space has only a slight effect on the total power dissipation, due to the low excitation voltage. However, at all frequencies, the increase of the interelectrode space results in a drop of the power dissipation per discharge volume. This is related to the less effective energy transfer to the electrons that is due to the enhancement of the bulk relative to the sheath ohmic heating. The variation of the relative importance of the electron heating modes is reflected in the discharge radical production efficiency and the film growth rate.

Ab initio analysis of silyl precursor physisorption and hydrogen abstraction during low temperature silicon deposition

The energetics of silyl (SiH 3) precursor surface adsorption and hydrogen abstraction on a monohydride terminated silicon surface are described. The abstraction of surface hydrogen by H radicals is more exothermic, and proceeds with a smaller kinetic barrier than H abstraction by silyl. Surface adsorption and abstraction were analyzed using both multi-parent configuration interaction (CI) and several density functional approaches using the Si 4H 10 cluster representing a monohydride terminated silicon (1 1 1) surfaces, and results from the two techniques are critically compared and evaluated. Hydrogen abstraction by H is found to proceed through a kinetic barrier that is between 0 kcal/mol predicted by DFT and 7.2 kcal/mol determined from CI, consistent with experimental values of ∼2 kcal/mol. The barrier height for H abstraction by silyl (without zero point and H tunneling corrections) is determined to be between 4.1 kcal/mol calculated using DFT, and 14.2 kcal/mol determined from the multi-parent CI. These calculations indicate that during typical low temperature silicon deposition processes, H abstraction by impinging hydrogen atoms dominates H abstraction by SiH 3 and plays an important role in creation of surface dangling bonds. None of the Si–H/silyl potential energy surfaces obtained from CI and DFT methods show evidence for stable physisorbed three-center Si–H–(SiH 3) p bond, which is commonly presumed in several models of silicon thin film deposition. We discuss these results in relation to experimental analysis of surface diffusion kinetics in film deposition, and suggest alternate growth models, including H-mediated Si–Si bond breaking and/or direct silyl insertion, to describe activated low temperature silicon-based film deposition.

Surface transport kinetics in low-temperature silicon deposition determined from topography evolution

In this article, surface transport kinetics during low-temperature silicon thin film deposition are characterized using time dependent surface topography and dynamic scaling models. Analysis of surface morphology indicates that diffusion of adsorbed species dominates surface transport, with a characteristic diffusion length that increases with surface temperature. A diffusion activation barrier of $\ensuremath{\sim}0.2 \mathrm{eV}$ is obtained, consistent with hydrogen-mediated adspecies diffusion on the growth silicon surface. Samples are compared over a range of deposition temperatures (25 to $350\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and film thickness (20 to $5000 \AA{})$ deposited using silane with helium or argon dilution, on glass and silicon substrates. Self-similar surface structure is found to depend on detailed film growth conditions, but is independent of film thickness after nuclei coalescence. For films deposited using helium dilution, static and dynamic scaling parameters are consistent with self-similar fractal geometry scaling, and the lateral correlation length increases from 45 to 150 nm as temperature increases from 25 to $150\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ These results are discussed in relation to current silicon deposition models and with topography evolution observed during low temperature growth of other amorphous material systems.

Effect of frequency in the deposition of microcrystalline silicon from silane discharges

The influence of frequency in the range from 13.56 to 50 MHz, on the properties of 2% silane in hydrogen 0.5 Torr discharges used for the deposition of microcrystalline silicon thin films, has been investigated. The experiments were carried out under constant power conditions as determined through Fourier transform voltage and current measurements. The increase of frequency leads to a decrease of the rf field, an extension of the bulk, and a marked increase of the electron density and the amount of power consumed by electrons. These changes induce a decrease of the rate of high-energy electron–molecule collision processes (>10.5 eV) at higher frequencies and an enhancement of lower energy processes. Thus, there is a significant increase in the hydrogen flux toward surfaces, which can explain the beneficial effect of frequency to the crystallinity of μc-Si:H thin films. At the same time, SiH4 electron impact dissociation is enhanced mainly due to the increase of electron density. On the contrary, ionization is not favored by the increase of frequency and the calculated ion flux toward the film surface indicates that the role of ions in a possible enhancement of the surface mobility of the film precursors is minor. The observed increase of the deposition rate is further discussed in terms of the nature of the film precursors and the spatial distribution of their production.

The influence of different catalyzers in hot-wire CVD for the deposition of polycrystalline silicon thin films

Polycrystalline silicon thin films grown by hot-wire chemical vapour deposition, using tungsten and tantalum as filament material, show different material properties. The different filament materials can cause this difference, due to different surface reactions and catalytic properties. X-Ray photoelectron spectroscopy on used-filaments shows a much lower silicon content in the near-surface region of the tantalum filaments, as compared to the tungsten ones. Furthermore, it is shown that the silicon content on the tungsten filament increases linearly with time, while the silicon content on the tantalum filament saturates quickly. A comparison of the silicon contents on the different filaments shows the possible presence of a liquid phase on the tungsten filament's surface during deposition. This liquid phase can cause the short lifetime of tungsten filaments compared to tantalum ones.

High-rate deposition of polycrystalline silicon thin films by hot wire cell method using disilane

A new process, hot wire cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low-temperature and high deposition rate. In the hot wire cell method, reactant gases are decomposed by a heated tungsten filament. Polycrystalline silicon films can be deposited at deposition rates of 1.2 nm/s for mono-silane (SiH 4) and 2.8 nm/s for disilane (Si 2H 6). By using disilane as a reactant gas, it is possible to achieve a high deposition rate without any change in the quality of the films.

High-density microwave plasma for high-rate and low-temperature deposition of silicon thin film

A novel high-density microwave plasma utilizing a spokewise antenna was produced and applied for high-rate and low-temperature deposition of hydrogenated microcrystalline silicon (μc-Si : H) film. The plasma maintains a uniform state, i.e., high electron density, n e>10 11 cm −3 and low temperature, T e of 2–2.5 eV in pure Ar plasma over 20 cm in a diameter. High deposition rate was achieved of 47 Å/s in the growth of highly crystallized and photoconductive μc-Si : H film at the axial distance, Z=6 cm from the quartz glass plate from SiH 4 and Ar without the use of H 2 dilution method at low substrate temperature, T s of 250°C. The effects of the total pressure, H 2 dilution ratio, flow rate of SiH 4, Fr(SiH 4) and the axial distance, Z from the quartz glass plate on the μc-Si : H film deposition are discussed along with the plasma diagnostics.

Low-temperature polysilicon deposition by ionized magnetron sputtering

Ionized magnetron sputtering was successfully applied to polycrystalline silicon thin-film deposition on glass substrate at temperatures lower than 250 °C maintaining a deposition rate of about 133 Å/min. Hydrogen mixing was effective up to Ar:H2=10:6 by mass flow rate. Prior to deposition, H2 inductively coupled plasma was used for precleaning the substrate with −40 V bias. During Si deposition, the substrate biasing scheme was in two steps; +20 V for an initial stage and +20 to −40 V bipolar pulse bias for the rest of the deposition time. The crystallinity was evaluated by both x-ray diffraction analysis and Raman spectroscopy; the average crystalline fraction was calculated as 70%. Grain size was measured in plan-view scanning-electron micrographs after selective etching of the amorphous phase by chemical solution. In 800-nm-thick samples, grains are 500–700 Å in diameter. Optical emission spectroscopy was used as real-time diagnostics, and ionization of sputtered silicon atoms distinctly increased as the hydrogen partial pressure increased. The successful deposition of polycrystalline silicon was explained as being due to enhanced ionization of sputtered and reflected neutrals and resultant energy control by bipolar substrate bias.

Plasma enhanced chemical vapor deposition of amorphous, polymorphous and microcrystalline silicon films

The plasma processes and growth reactions involved in the deposition of amorphous, polymorphous and microcrystalline silicon thin films are reviewed. The reference being a-Si:H deposition through surface reactions of SiH 3 radicals, we study the growth of microcrystalline silicon films produced by the layer-by-layer and standard hydrogen dilution techniques. We show that subsurface reactions play a key role, particularly during the incubation phase where hydrogen is responsible for the formation of a porous layer in which nucleation takes place. The evolution of the film properties is related to the long range effects of hydrogen. Coming back to a-Si:H deposition, we further consider the deposition at low substrate temperature (<200°C) and pressure (<5 Pa) where the role of ions is dominant and at deposition rates where powder formation takes place. We propose that rather than a drawback, nanoparticle formation in silane plasmas might be considered as a potential for obtaining new silicon films. We address in particular the deposition of polymorphous silicon consisting of an a-Si:H matrix with silicon nanocrystallites produced in the gas phase. Despite their heterogeneity polymorphous silicon films have improved transport properties and stability with respect to a-Si:H.

Study of hot wire chemical vapor deposition technique for silicon thin film

Micro and multi-crystalline silicon thin films have been prepared in the growing rate of more than 10 Å/S by hot wire (HW) CVD. The crystallite size of the films ranges from 0.1 to 1 μm. The effects of processing parameters on the property of the films have been systematically studied. The energy gap ranges from 1.5 to 1.12 eV when substrate temperature changes from 250°C to 450°C. The conductivity of the doped films is as high as 10 (Ω cm) −1. An anomalous behavior of hot palladium wire in an atmosphere of silane at low pressure is reported and preliminarily explained. The reason why tungsten wire is easy to break during silicon thin film deposition by hot wire CVD is discussed.