Deposition Of Microcrystalline Silicon Research Articles

Effect of the RF Power of PECVD on the Crystalline Fractions of Microcrystalline Silicon (μc-Si:H) Films and Their Structural, Optical, and Electronic Properties

In this work, we report on the deposition of microcrystalline silicon (µc-Si:H) films produced from silane (SiH4), hydrogen (H2), and argon (Ar) mixtures using the plasma-enhanced chemical vapor deposition (PECVD) technique at 200 °C. Particularly, we studied the effect of RF power on the crystalline fraction (XC) of the deposited films, and we have correlated the XC with their optical, electrical, and structural characteristics. Different types of characterization were performed in the µc-Si:H film series. We used several techniques, such as Raman scattering spectroscopy, Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM), among others. Our results show that RF power had a strong effect on the XC of the films, and there is an optimal value for producing films with the largest XC.

Highly Microcrystalline Phosphorous‐Doped Si:H Very Thin Films Deposited by RF‐PECVD

Fine‐tuning the crystallinity and doping of the hydrogenated microcrystalline silicon thin films are the key points in obtaining high‐quality devices for above‐integrated circuit (above‐IC) image sensors. This study focuses on the structural and electrical properties of radio‐frequency plasma enhanced chemical vapor deposition (RF‐PECVD) deposited microcrystalline silicon (μc‐Si:H) contact layers. Herein, phosphorus‐doped μc‐Si:H is deposited by capacitively coupled plasma RF‐PECVD (13,56 MHz) from a SiH4 + H2 + PH3gas mixture with varying phosphine concentrations. The influence of phosphine concentration on dopant concentration, active dopant concentration, and crystallinity is studied by secondary ion mass spectrometry, Hall effect measurement, and Raman spectroscopy, respectively. A high doping level is attained, reaching up to 1.7 × 1020 cm−3. Furthermore, “low‐power” and “high‐power” deposition conditions are studied which lead to incubation layers of different nature.

Investigation of deposition conditions on the structural properties of µc-Si:H

This article is concerned with Raman study of µc-Si:H obtained by PECVD under different deposition conditions. The analysis of crystal fracture and bonded hydrogen content in the layer was carried out. It is shown that as the amorphous phase increases, the content of Si-H bonds prevails. Optimal conditions for deposition of microcrystalline silicon with a low content of hydrogen bonds were defined.

RF bias to suppress post-oxidation of μc-Si:H films deposited by inductively-coupled plasma using a planar RF resonant antenna

One challenge for microcrystalline silicon (μc-Si:H) deposition is to achieve high deposition rates while maintaining high quality films. In this work, an inductively-coupled plasma (ICP) is used to deposit μc-Si:H on glass substrates by means of a novel planar resonant antenna at 13.56 MHz. No particle formation occurs in the low pressure (5 Pa) plasma, but the films suffer post-oxidation. By embedding a 5 MHz RF-biased substrate, films deposited simultaneously with and without RF bias are compared. It is shown that large area, low pressure (5 Pa), particle-free ICP deposition at 1 nm/s of μc-Si:H films can be obtained without post-oxidation by means of a planar resonant antenna, provided that RF substrate bias is included for independent control of the ion energy.

Disilane as a growth rate catalyst of plasma deposited microcrystalline silicon thin films

The effect of small disilane addition on the gas phase properties of silane-hydrogen plasmas and the microcrystalline silicon thin films growth is presented. The investigation was conducted in the high pressure regime and for constant power dissipation in the discharge with the support of plasma diagnostics, thin film studies and calculations of discharge microscopic parameters and gas dissociation rates. The experimental data and the calculations show a strong effect of disilane on the electrical properties of the discharge in the pressure window from 2 to 3 Torr that is followed by significant raise of the electron number density and the drop of the sheaths electric field intensity. Deposition rate measurements show an important four to six times increase even for disilane mole fractions as low as 0.3 %. The deposition rate enhancement was followed by a drop of the material crystalline volume fraction but films with crystallinity above 40 % were deposited with different combinations of total gas pressure, disilane and silane molar ratios. The enhancement was partly explained by the increase of the electron impact dissociation rate of silane which rises by 40% even for 0.1% disilane mole fraction. The calculations of the gas usage, the dissociation and the deposition efficiencies show that the beneficial effect on the growth rate is not just the result of the increase of Si-containing molecules density but significant changes on the species participating to the deposition and the mechanism of the film growth are caused by the disilane addition. The enhanced participation of the highly sticking to the surface radical such as disilylene, which is the main product of disilane dissociation, was considered as the most probable reason for the significant raise of the deposition efficiency. The catalytic effect of such type of radical on the surface reactivity of species with lower sticking probability is further discussed, while it is also used to explain the restricted and sensitive process window where the disilane effect appears.

ECWR plasma enhanced chemical vapour deposition of microcrystalline silicon thin films

An ECWR plasma source was used in order to deposit microcrystalline silicon thin films. The effect of input power and silane content on the deposition rate and the materials properties was investigated. Deposition rates, up to 25Å/sec, and high crystallinity degree were achieved using high silane content which is in contrast to the conventional CCP method. The amorphous to crystalline transition zone was determined revealing that in ECWR discharges microcrystalline silicon growth is favoured in a wide range of experimental conditions.

Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition

The contribution of ions to the growth of microcrystalline silicon thin films has been investigated in the well-known high-pressure-depletion (HPD) regime by coupling thin-film analysis with plasma studies. The ion flux, measured by means of a capacitive probe, has been studied in two regimes, i.e., the amorphous-to-microcrystalline transition regime and a low-to-high power regime; the latter regime had been investigated to evaluate the impact of the plasma power on the ion flux in collisional plasmas. The ion flux was found not to change considerably under the conditions where the deposited material undergoes a transition from the amorphous to the microcrystalline silicon phase; for solar-grade material, an ion-to-Si deposition flux of ∼0.30 has been determined. As an upper-estimation of the ion energy, a mean ion energy of ∼19 eV has been measured under low-pressure conditions (<1 mbar) by means of a retarding field energy analyzer. Combining this upper-estimate with an ion per deposited Si atom ratio of ∼0.30, it is concluded that less than 6 eV is available per deposited Si atom. The addition of a small amount of SiH4 to an H2 plasma resulted in an increase of the ion flux by about 30% for higher power values, whereas the electron density, deduced from optical emission spectroscopy analysis, decreased. The electron temperature, also deduced from optical emission spectroscopy analysis, reveals a slight decrease with power. Although the dominant ion in the HPD regime is SiH3+, i.e., a change from H3+ in pure hydrogen HPD conditions, the measured larger ion loss can be explained by assuming steeper electron density profiles. These results, therefore, confirm the results reported so far: the ion-to-Si deposition flux is relatively large but has neither influence on the microcrystalline silicon film properties nor on the phase transition. Possible explanations are the reported high atomic hydrogen to deposition flux ratio, mitigating the detrimental effects of an excessive ion flux.

The influence of the electrical asymmetry effect on deposition uniformity of thin silicon film

The deposition of amorphous and microcrystalline silicon is an important step in the production of thin silicon film solar panels. Deposition rate, layer uniformity and material quality are key attributes for achieving high efficiency in such panels. Due to the multilayer structure of tandem solar cells (more than 6 thin silicon layers), it is becoming increasingly important to improve the uniformity of deposition without sacrificing deposition rate and material quality. This paper reports the results of an investigation into the influence of the electrical asymmetry effect (EAE) on the uniformity of deposited layers. 13.56MHz+27.12MHz excitation frequencies were used for thin silicon film deposition in a Gen5 reactor (1100×1400mm). To change the plasma properties, the DC self bias voltage on the RF electrode was varied by adjustment of the phase angle between the two frequencies applied. It was found that the layers deposited by EAE method have better uniformity than layers deposited in single frequency 27.12MHz discharge. The EAE provides additional opportunities for improvement of uniformity, deposition rate and material quality.

Analysis on steady plasma process of high-rate microcrystalline silicon by optical emission spectroscopy

The ratio of I(Hα*)/I(SiH*), obtained from the real time optical emission spectroscopy (OES) measurement in the high-rate microcrystalline silicon deposition process, as a function of time is used to analyze the cause of increasing crystallinity along the growth direction. Hydrogen dilution gradient method which means silane concentration gradient and hydrogen flow gradient method is adopted to improve vertical structure uniformity of the material. High-quality microcrystalline material around 53%-62% of Xc can be prepared through silane concentration gradient compared with 55%-75% of Xc prepared in the traditional method. In the silane depleted cases, by increasing the hydrogen flow the longitudinal uniformity of the material can be effectively improved. The vertical crystallinity around 53%-60% can be obtained. This is mainly due to the increase of the hydrogen flow that makes the collision probability increased, as a result, electron temperature of plasma reduced. Thus, the decomposition of hydrogen decreases and the reaction of hydrogen annihilation is suppressed. At the same time, the influence of back diffusion of SiH4 is suppressed. The gradually increasing trend of the ratio of I(Hα*)/I(SiH*) is controlled during the deposition of microcrystalline silicon film.

Critical flux ratio of hydrogen radical to film precursor in microcrystalline silicon deposition for solar cells

The properties of thin-film silicon grown by plasma enhanced chemical vapor deposition were investigated with respect to the flux ratio of hydrogen radical to film precursor. The absolute density and translational temperature of H radicals in SiH4/H2 capacitively coupled plasma were measured using vacuum ultraviolet laser absorption spectroscopy. The flux of effective H radicals to the surface reactions was estimated from the results. The flux of effective film precursor for deposition was estimated from the deposition rate, so that the critical flux ratio of H radicals to film precursor to obtain suitable film properties was identified to be approximately 65–70.

In-situ Raman spectroscopy used to study and control the initial growth phase of microcrystalline absorber layers for thin-film silicon solar cells

The continuous deposition of microcrystalline silicon has been monitored with in-situ Raman spectroscopy. The process and measurement settings were chosen such that one spectrum was taken during approximately 9nm of layer growth. This allows observing the crystallinity in the initial growth phase of microcrystalline silicon absorber layers. The influence of different p-doped seed layers has been studied. Under constant deposition conditions, an initial decrease in crystallinity was observed over the first tens of nanometers. By profiling the process gas flows during the initial phase it was possible to reduce the amount of amorphous material that was detected during the initial phase of deposition.

EFFECTS OF INTERELECTRODE SPACING ON THE PROPERTIES OF MICROCRYSTALLINE SILICON ABSORBER AND SOLAR CELLS

ABSTRACTWe study the effect of the spacing between electrodes in very high frequency plasma enhanced chemical vapor deposition on the properties of microcrystalline silicon films and their related n-i-psolar cells. We vary the spacing from 0.2 to 1.0 cm to deposit microcrystalline silicon at 67.8 MHz while maintaining other growth parameters. The spacing between the electrodes significantly changes the plasma conditions, which govern film precursor chemistry as well as introduce etching and ion bombardment to the film; thereby, influencing nucleation and growth of the microcrystalline Si films. The resulting films were characterized by UV-Vis spectrometry, atomic force microscopy, X-ray diffraction, and transmission electron microscopy. We found that deposition rate decreases, while surface roughness and short circuit current density increase with smaller spacing.

<I>N</I>-Type Crystalline Silicon Films Free of Amorphous Silicon Deposited on Glass by HCl Addition Using Hot Wire Chemical Vapour Deposition

Since n-type crystalline silicon films have the electric property much better than those of hydrogenated amorphous and microcrystalline silicon films, they can enhance the performance of advanced electronic devices such as solar cells and thin film transistors (TFTs). Since the formation of amorphous silicon is unavoidable in the low temperature deposition of microcrystalline silicon on a glass substrate at temperatures less than 550 degrees C in the plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition (HWCVD), crystalline silicon films have not been deposited directly on a glass substrate but fabricated by the post treatment of amorphous silicon films. In this work, by adding the HCl gas, amorphous silicon-free n-type crystalline silicon films could be deposited directly on a glass substrate by HWCVD. The resistivity of the n-type crystalline silicon film for the flow rate ratio of [HCl]/[SiH4] = 7.5 and [PH3]/[SiH4] = 0.042 was 5.31 x 10(-4) ohms cm, which is comparable to the resistivity 1.23 x 10(-3) ohms cm of films prepared by thermal annealing of amorphous silicon films. The absence of amorphous silicon in the film could be confirmed by high resolution transmission electron microscopy.

At the limit of total silane gas utilization for preparation of high-quality microcrystalline silicon solar cells at high-rate plasma deposition

It was aimed to find a regime for high-rate deposition of microcrystalline silicon with a silane gas utilization rate close to 100%. It is found that state-of-the art solar cells can be prepared at such conditions. The interdependencies of the relevant deposition parameters were identified in an experimental study in a multidimensional parameter space in which for each condition the μc-Si crystalline volume fraction was optimized to find the “optimum phase mixture.” It is concluded that choice of the deposition pressure has a critical influence on the silane gas utilization rate and deposition rate.

The effect of disturbed PECVD electrode surfaces on the homogeneity of microcrystalline silicon films

In this work we present a novel electrode design for the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films that enables optical access to the growing layer under normal incidence. The optical access is realized by piercing the electrode with a conical feed through of 10mm diameter at the electrode side facing the plasma. The influence of the feed through on deposition homogeneity is studied in different pressure regimes from 8mbar to 24mbar on intrinsic layers optimized for state of the art thin-film silicon solar cells. The homogeneity of the layers was determined by spatially resolved thickness measurements and evaluation of the crystalline volume fraction by Raman spectroscopy. With the aim to minimize the influence of the disturbance of the electrode surface the effect of different insets in the feed through on the homogeneity is studied. To find a maximum in optical transmission of the insets at optimal film homogeneity different designs of metallic grids were tested. We show that using the modified electrode it is possible to deposit microcrystalline silicon layers which are comparable in homogeneity to those fabricated with an unchanged standard electrode. This was achieved by covering only 19% of the area of the feed through by a metallic inset.

Hydrogen-dominated plasma, due to silane depletion, for microcrystalline silicon deposition

Plasma conditions for microcrystalline silicon deposition generally require a high flux of atomic hydrogen, relative to SiHα=0→3 radicals, on the growing film. The necessary dominant partial pressure of hydrogen in the plasma is conventionally obtained by hydrogen dilution of silane in the inlet flow. However, a hydrogen-dominated plasma environment can also be obtained due to plasma depletion of the silane in the gas mixture, even up to the limit of pure silane inlet flow, provided that the silane depletion is strong enough. At first sight, it may seem surprising that the composition of a strongly depleted pure silane plasma consists principally of molecular hydrogen, without significant contribution from the partial pressure of silane radicals. The aim here is to bring some physical insight by means of a zero-dimensional, analytical plasma chemistry model. The model is appropriate for uniform large-area showerhead reactors, as shown by comparison with a three-dimensional numerical simulations. The SiHα densities remain very low because of their rapid diffusion and surface reactivity, contributing to film growth which is the desired scenario for efficient silane utilization. Significant SiHα densities due to poor design of reactor and gas flow, on the other hand, would result in powder formation wasting silane. Conversely, hydrogen atoms are not deposited, but recombine on the film surface and reappear as molecular hydrogen in the plasma. Therefore, in the limit of extremely high silane depletion fraction (>99.9%), the silane density falls below the low SiHα densities, but only the H radical can eventually reach significant concentrations in the hydrogen-dominated plasma.

Impact of secondary gas-phase reactions on microcrystalline silicon solar cells deposited at high rate

The role of secondary gas-phase reactions during plasma-enhanced chemical vapor deposition of microcrystalline silicon is a controversial subject. In this paper, we show that the enhancement of such reactions is associated with the improvement of material properties of absorber layers deposited at high constant rate. We detect powder, a product of secondary gas-phase reactions, via infrared laser absorption spectroscopy, laser light scattering, and optical emission spectroscopy. As the powder formation is increased, we measure a systematic improvement of device performance. This demonstrates that secondary gas-phase reactions are not detrimental to the material quality of microcrystalline silicon deposited at high rate.

Reduction of amorphous incubation layer by HCl addition during deposition of microcrystalline silicon by hot-wire chemical vapor deposition

The amorphous incubation layer, which is formed in the initial growth stage of hydrogenated microcrystalline silicon (μc-Si:H) thin film deposited at low temperature, is harmful to the electric properties of film. In this study, the effect of the addition of HCl gas on the reduction of such an amorphous incubation layer was investigated during the silicon deposition on a glass substrate at 220 °C by hot-wire chemical vapor deposition process using the Raman spectroscopy, the X-ray diffraction and the field-emission scanning electron microscopy. In the initial stage of deposition where the silicon film deposited without HCl addition consisted almost entirely of the amorphous incubation layer; highly crystalline silicon films could be deposited with HCl addition. As the flow rate of HCl increased, the crystallinity of silicon films increased but the film growth rate decreased. The surface morphology of films prepared with HCl addition became smoother with smaller grain size than that prepared without HCl.

A new method used to control the structure of high rate microcrystalline silicon thin films

AbstractWe report a systematic study of plasma heating effect on microcrystalline silicon (μc‐Si:H) deposition. Normally, substrate surface temperature increases with time during a high rate deposition of μc‐Si:H thin film, especially under a high power and high pressure condition. We deposited μc‐Si:H films using a very high frequency discharge under the high pressure and high power condition at a fixed heater temperature or a profiled heater temperature. Raman spectra with different wavelength excitations showed that a proper heater temperature profiling during μc‐Si:H deposition is an effective method to modify the structure of μc‐Si:H films, which can control the structure evolution to form a uniform crystallinity along the growth direction and reduce the amorphous incubation layer thickness (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microcrystalline silicon films and solar cells deposited at high rate by Matrix Distributed Electron Cyclotron Resonance (MDECR) plasma

AbstractThe latest results obtained in a new promising Matrix Distributed Electron Cyclotron Resonance (MDECR) reactor for the deposition of microcrystalline silicon (μc‐Si:H) films and cells are reported in this article. The reactor is presented with a focus on the importance and the difficulties of temperature measurements. The unique properties of this high density plasma allowed us to demonstrate high deposition rates of 28 Å/s. The chemical composition of the MDECR material has been compared to a standard material and in particular, the effect of the oxygen level will be discussed. The installation of a load‐lock allowed a reduction of the oxygen con‐tamination by a factor of 10. Lower microwave powers helped to improve the electrical properties of the material. So far, the cell performance unfortunately seems decoupled from these successive optimizations of the film quality, and stayed limited (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)