Growth Of μc-Si Research Articles

Effect of Ion Energy on Microcrystalline Silicon Material and Devices: A Study Using Tailored Voltage Waveforms

The use of tailored voltage waveforms to excite a plasma has been shown to be an effective technique to decouple maximum ion energy from the ion flux on the electrode. We use it here as a way to scan through the maximum ion energy in order to study this quantity's role in the growth of μc-Si:H. We find that at critical energies (30 and 70 eV), a stepwise increase in the a-Si:H/μc-Si:H transition thickness is observed, together with change in the surface morphology. These thresholds correspond to SiH x + - and H 3 + -induced displacement energies, respectively. A model is proposed to account for the impact of these ions on the morphology of μc-Si:H growth and is confirmed by comparison with epitaxial growth on a crystalline wafer.

Substrate Dependence of Microcrystalline Silicon Growth with SiH<sub>4</sub> Diluted by Ar

Microcrystalline Si films were deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) using Ar diluted SiH4gaseous mixture. The effects of the substrate on deposition rate, preferred orientation and roughness of the films were investigated. The results show that, the influence of the substrate surface chemical nature on the deposition rate is significant in the initial stage of the growth. And considering the crystallinity and roughness of the films, the substrate is favored in its preferred orientation with a rougher surface. Based on these results, it is confirmed that the combination of diffusion and etching is indispensable to describe the deposition of μc-Si with SiH4diluted by Ar, and the mechanism of μc-Si growth could be controlled by diffusion of Si and etching of the Ar+on the film surface.

Low-temperature deposition of µc-Si : H thin films by a low-frequency inductively coupled plasma for photovoltaic applications

Low-temperature depositions of Si films from hydrogenated amorphous silicon (a-Si : H) to highly crystallized hydrogenated microcrystalline silicon (µc-Si : H) were realized by the low-frequency inductively coupled plasma (LF-ICP) technique, with low hydrogen dilution (50%) and without any intentional substrate heating. µc-Si : H films with a thin incubation layer (<100 nm), a small grain size (<14 nm), a weak [1 1 1] or [2 2 0] preferred orientation, and a low hydrogen content (4–8%) were obtained (at deposition rates ∼0.4–0.7 nm s−1). The increased crystallinity due to the increasing rf power is accompanied by the improved structural order, lowered impurity contamination and decreased hydrogen content. The compactness of the µc-Si : H film is positively related to crystallinity and independent of the high microstructure factor (>0.8). Low-temperature growth of µc-Si : H is attributed to high atomic H flux and suppression of high-energy ion bombardment due to the high density of low-temperature electrons in the plasma. A µc-Si : H solar cell with a less dense intrinsic layer (on a SnO2 : F glass substrate) exhibits a high Voc (584 mV), showing great potential for photovoltaic applications.

Effects of ion bombardment on microcrystalline silicon growth by inductively coupled plasma assistant magnetron sputtering

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were deposited by inductively coupled plasma assistant magnetron sputtering (ICP-MS) in an Ar-H2 gas mixture. The role of ion bombardment in the growth of μc-Si:H films was studied with increasing negative bias voltages on the substrate holder from 0 to −100 V. Raman scattering, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) were performed to investigate the microstructure changes of deposited Si films. Raman scattering showed that the high energy ion bombardment resulted in crystalline degradation of Si films. The XRD results showed the decrease and even elimination of preferential growth orientation of crystalline Si films with ion bombardment energy increase. The SiH bonding configuration changes and the increase of bonded hydrogen concentration were determined with the analysis of FTIR spectra. Furthermore, the dramatic evolution of cross-sectional morphology of Si thin films was detected by TEM observation.

Determination factor of direction growth in microcrystalline silicon thin film deposition

We have proposed the mechanism of the <110> directional growth of microcrystalline silicon (μc-Si) thin films deposited by PECVD (plasma enhanced chemical vapor deposition) from SiH4 and H2 gas mixture, where dimeric radicals act a key role to form bridge nuclei for the ledge formation on the (110) facet. In order to look further into details of the mechanism, we investigated other important factors that influence the growth of μc-Si in <110> direction in terms of their impact on crystallinity with varying deposition temperature. The enhancement of surface diffusion length of radicals is inferred from the enlargement of the crystalline grain size accompanied with the increase of the deposition temperature. The growth in <110> direction is also promoted as the deposition temperature increases. Therefore, it is suggested that the surface diffusion length of radicals is another key factor that governs the crystalline growth in <110> direction. The growth mechanism of μc-Si thin films in <110> direction is discussed in terms of the relation between the surface diffusion length of monomeric radicals depending on the substrate surface temperature and the average space of bridges depending on the density of dimeric radicals on the growing surface.

New two-step growth of microcrystalline silicon thin films without incubation layer

A new two-step growth method was proposed to fabricate microcrystalline silicon (μc-Si:H) thin films by an electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD). An ultra thin Si film was first deposited and followed by H 2 plasma treatment for few minutes, and then the μc-Si:H film was deposited on it. High-resolution transmission electron microscope (HRTEM) and Raman spectrometer were used to study the microstructures and the crystalline volume fraction of μc-Si:H films. The HRTEM results show that the amorphous silicon thin film with a thickness of 15 nm can be crystallized by H 2 plasma treatment in 2 min, and then it serves as the seed layer for the subsequent growth of μc-Si:H films. By optimizing the deposition parameters, the μc-Si:H film without amorphous incubation layer can be fabricated by this new two-step method and a proper crystalline volume fraction of 50.6% can be obtained.

Modeling of high frequency atmospheric pressure Ar/H 2/SiH 4 glow discharges

In this paper, a one-dimensional self-consistent fluid model is applied to simulate high frequency atmospheric pressure glow discharges. The results show that the plasma density and current density depend strongly on the excitation frequency. When the excitation frequency is below 13.56 MHz, the discharge operates in the α mode, and when the excitation frequency is above 13.56 MHz, the discharge operates in a γ-like mode. The densities of species including SiH 3 +, SiH 3 −, SiH 3, SiH 2, H, Ar +, Ar ⁎ and electron are enhanced with the frequency increasing from 6.78 to 27.12 MHz. Similar discharge mode transition was observed experimentally in radio frequency atmospheric pressure He glow discharges. The effects of excitation frequency on plasma characteristics and densities of precursors for μc-Si:H film are further discussed. This study reveals that an appropriate excitation frequency is important for the growth of μc-Si:H film.

Surface Chemistry of Preferentially (111)- and (220)-Crystal-Oriented Microcrystalline Silicon Films by Radio-Frequency Plasma-Enhanced Chemical Vapor Deposition

The surface chemistry of chlorinated hydrogenated microcrystalline silicon (µc-Si:H:Cl) films with preferred (111) and (220) crystal orientations was investigated by the radio-frequency (rf) plasma-enhanced chemical vapor deposition (PE-CVD) of a dichlorosilane (SiH2Cl2) and H2 mixture. The growing surface of the preferentially (220)-crystal-oriented µc-Si:H:Cl films included many microroughness features, voids, and dangling bonds, and was chemically active to hydrogen and argon plasma exposures. On the other hand, the growing surface with the preferential (111) crystal orientation was chemically stable relatively. These findings suggest that the sticking process of deposition precursors and/or the reconstruction of Si clusters within the subsurface region including microroughness features and dangling bonds determines the growth of the preferential (220) crystal orientation. The determining factor for the preferential crystal orientation is discussed in terms of the growth of µc-Si:H:Cl films.

The relationship between $I_{{\rm H}_{\rm \alpha } } /(I_{{\rm SiH}}^* )^2$ and crystalline volume fraction in microcrystalline silicon growth

AbstractOptical‐emission‐intensity ratio of $I_{{\rm H}_{\rm \alpha } } /(I_{{\rm SiH}}^* )$ during film growth has been used as a simple indicator to predict crystallinity (crystal‐volume fraction: XC) in the resulting microcrystalline silicon (µc‐Si:H) thin films. The relationship between $I_{{\rm H}_{\rm \alpha } } /(I_{{\rm SiH}}^* )$ and XC has been checked under a wide variety of film‐preparation conditions including low‐deposition‐rate (&lt;0.1 nm/s) and high‐deposition‐rate (&gt;5 nm/s) cases. On the basis of theoretical consideration, we have proposed optical‐emission‐intensity ratio of $I_{{\rm H}_{\rm \alpha } } /(I_{{\rm SiH}}^* )^2 $ as a new indicator of XC during film growth of µc‐Si:H.

Deposition of uniform μc-Si : H layers on plasma etched vertical ZnOnanowires

This study introduces the deposition of μc-Si : H layers on vertically well-aligned ZnO nanowires by very high frequency (VHF) plasma enhanced chemical vapor deposition (PECVD). It was found that μc-Si : H deposited on un-etched ZnO nanowires, with a high length-to-diameter aspect ratio, were nail-shaped due to the rapid growth of μc-Si : H along the lateral (0002) plane. By slightly etching the ZnO nanowires prior to μc-Si : H deposition, μc-Si : H films were coated uniformly on the surface of ZnO nanowires. The ZnO nanowires uniformly coated by plasma etching are potentially useful not only for solar cell applications but also for any nanodevice that uses nanowires as a nanoelectrode.

Surface Reactions and Growth Kinetics of Plasma-deposited Amorphous and Microcrystalline Silicon Thin Films

The growth kinetics of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) thin films fabricated by plasma-enhanced chemical vapor deposition (PECVD) is discussed for developing low-cost and high-efficiency solar cells. Surface reactions are reviewed associated with the a-Si growth and the nucleation of μc-Si. With respect to the film growth of μc-Si after the nucleation, roughness evolutions were evaluated using an atomic force microscope, and scaling analyses were carried out on fractal structures of growing surfaces, because scaling exponents give a great insight into the growth kinetics. On the basis of results of scaling analyses in conjunction with Monte Carlo simulations, the growth kinetics of microcrystalline silicon thin films, actually used for the high-efficiency solar cells, is discussed.

Improvement of μc-Si:H n–i–p cell efficiency with an i-layer made by hot-wire CVD by reverse H 2-profiling

The technique of maintaining a proper crystalline ratio in microcrystalline silicon (μc-Si:H) layers along the thickness direction by decreasing the H 2 dilution ratio during deposition (H 2 profiling) was introduced by several laboratories while optimizing either n–i–p or p–i–n μc-Si:H cells made by PECVD. With this technique a great increase in the energy conversion efficiency was obtained. Compared to the PECVD technique, the unique characteristics of HWCVD, such as the catalytic reactions, the absence of ion bombardment, the substrate heating by the filaments and filament aging effects, necessitate a different strategy for device optimization. We report in this paper the result of our method of using a reverse H 2 profiling technique, i.e. increasing the H 2 dilution ratio instead of decreasing it, to improve the performance of μc-Si:H n–i–p cells with an i-layer made by HWCVD. The principle behind this technique is thought to be a compensation effect for the influence of progressing silicidation of the filaments during the growth of μc-Si:H, if the filament current is held constant during growth. The dependence of the material crystallinity on thickness with and without H 2 profiling is discussed and solar cell J– V parameters are presented. Thus far, the best efficiency of μc-Si:H n–i–p cells made on a stainless steel substrate with an Ag/ZnO textured back reflector made in house has been improved to 8.5%, which is the highest known efficiency obtained for n–i–p cells with a hot-wire μc-Si:H i-layer.

Relative importance of hydrogen atom flux and ion bombardment to the growth of μc-Si:H thin films

An investigation of the relative importance of H atoms and ions to the transition from amorphous to microcrystalline silicon growth was performed by applying in situ plasma diagnostics and a 2D simulator of SiH4/H2 discharges. The growth transition was achieved by reducing the % SiH4 in the SiH4/H2 discharges while keeping all the other plasma parameters constant. The distribution of the main species in the discharge space, as well as the flux of H atoms and ions per monolayer and the energy transferred by each to the growing film surface, was estimated from the simulation results. H atoms flux was found to be much higher compared to ions but the total amount of energy transferred from both H atoms and ions was found to be much lower than the activation energy required for crystallization of stable a-Si:H films with low H-content. These results support the theory that in the present conditions the formation of microcrystals proceeds via the initial growth of an unstable a-Si:H with high H content, which reduces significantly the energy barrier for crystallization.

Transport in microcrystalline silicon thin films deposited at low temperature by hot-wire chemical vapor deposition

This work is focused on the determination of the variation of local mobility of charge carriers with thickness (< 1 μm) for undoped microcrystalline silicon layers deposited by the hot-wire chemical vapor deposition technique. We observed that the temperature of the layers T s evolves with the deposition time, once the tungsten filament has been heated from room temperature to a fixed definite value. Thus, experiments have been realized by fixing the gas pressure (41 mTorr), the dilution of silane in hydrogen (50%), by setting the filament temperature (1600 °C) and letting the time run. An average substrate temperature T s,av has been defined, whose value depends on deposition time. As a result, the local mobility deduced from time-resolved microwave conductivity increases almost linearly with T s,av up to 193 °C, i.e. with thickness up to 400 nm corresponding approximately to the amorphous–microcrystalline transition and then increases sublinearly up to T s,av = 221 °C, i.e. a 900-nm-thick layer. These results, compatible with the highest AM1.5 efficiency (> 9%) reported so far for p–i–n μc-Si:H solar cells realized at T s = 185 °C [S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L. Houben, M. Stutzmann, Thin Solid Films 430 (2003) 202], suggest that in the range of T s,av from 190 °C to 220 °C, hydrogen plays a dominant role in the HWCVD growth of μc-Si:H films.

Effects of seeding methods on the fabrication of microcrystalline silicon solar cells using radio frequency plasma enhanced chemical vapor deposition

Single junction p-i-n μc-Si:H solar cells were prepared in a low-cost, large-area single chamber radio frequency plasma enhanced chemical vapor deposition system. The effects of seeding processes on the growth of μc-Si:H i-layers and performance of μc-Si:H solar cells were investigated. Seeding processes, usually featured by highly hydrogen rich plasma, are effective in inducing the growth of μc-Si:H i-layers. It has been demonstrated that p-layer seeding methods are preferable to i-layer seeding. While performance of μc-Si:H solar cells produced by i-layer seeding methods was usually limited by very low fill factors, μc-Si:H solar cells with good initial and stabilized conversion efficiencies were obtained by p-layer seeding.

Crystallographic Study of The Initial Growth Region of μc-Si with Different Preferential Orientations

AbstractGrowth of microcrystalline silicon (μc-Si) thin films with different preferential orientations has been studied employing XRD measurements versus thickness. The focus of this study is on the influence of preferential orientation on the film growth process. The (220) preferential orientation and randomly oriented μc-Si films studied here were prepared by VHF-PECVD at 180 °C. The thickness evolution revealed that the crystallinity was improved in the μc-Si with (220) preferential orientation μc-Si particularly in the first 0.5-μm of growth, while that in the randomly oriented μc-Si was almost completely unchanged. The difference in the crystallinity in the initial growth region arises from a difference in the growth mechanisms. Specifically, the growth of μc-Si with (220) preferential orientation can be elucidated by a hybrid-phase mode consisting of vapor- and solid-phase growth, whereas the growth of randomly oriented μc-Si can be achieved only by vapor-phase growth as occurs conventionally. Based on the growth mechanisms, the microstructures of μc-Si with both orientations and their influence on the photovoltaic performance are discussed.

2-Step Growth Method and Microcrystalline Silicon Thin Film Solar Cells Prepared by Hot Wire Cell Method

Hot Wire Cell (HW-Cell) method has been developed in order to grow microcrystalline silicon (µc-Si:H) thin films. The influence of various deposition parameters on the structural and electrical properties of the films was investigated to improve film quality. It was found that the concentrations of O and C atoms in µc-Si:H films could be reduced from the order of 1021 cm-3 to the order of 1020 cm-3 by decreasing the partial pressure of SiH4 from 100 mTorr to 3 mTorr. Then, a novel 2-step growth method was proposed in order to reduce the incubation layer in the initial growth of µc-Si:H i-layer. By using this method, Jsc largely increased (10.11 → 18.32 mA/cm2), and as a result, the conversion efficiency of 3.9% could be achieved. The influence of the incubation layer on solar cell performances was also investigated by a numerical analysis. To date, a conversion efficiency of 5.3% (Voc: 0.48 V, Jsc: 20.56 mA/cm2, F.F.: 0.54, active area: 0.086 cm2, AM1.5) was obtained for µc-Si:H solar cells with an i-layer thickness of 1.0 µm. Furthermore, high-rate depositions were investigated and a maximum deposition rate of 11.5 nm/s could be achieved. µc-Si:H solar cells fabricated at a high deposition rate of 1.5 nm/s showed a conversion efficiency of 2.8% (Voc: 0.42 V, Jsc: 12.31 mA/cm2, F.F.: 0.54, active area: 0.086 cm2, AM1.5).

Synthesis of Microcrystalline Silicon at Room Temperature Using ICP

Hydrogenated microcrystalline silicon (μc-Si:H) channel layers were synthesized at a temperature below 100°C by inductively coupled plasma (ICP) methods. High ionization efficiency and low ion bombardment from the ICP, enabled the use of high ICP power and, thus, extremely dense plasma or markedly increased electron temperature, promoting the diffusion capability of the reactive radicals that eventually yield ICP μc-Si:H films with large grains of several tens of nanometers, a high deposition rate of 5 A/s, and a smooth roughness of ∼ 1 nm. Accordingly, the epitaxial growth of μc-Si:H films at room temperature is demonstrated.

Microcrystallisation in Si:H: the effect of gas pressure in Ar-diluted SiH4 plasma

Abstract Using noble gas argon as a diluent of SiH 4 in RF glow discharge, undoped μc-Si:H thin films have been developed at a low power density of 30 mW/cm 2 . It has been found that the gas pressure is a critical factor for the growth of μc-Si:H films. Undoped μc-Si:H films having σ D ∼10 −6 S/cm and Δ E 2 , a more crystalline as well as highly conducting ( σ D ∼10 −4 S/cm) μc-Si:H film has been achieved at a deposition rate of 30 A/min, which is much higher than that attained from H 2 -diluted SiH 4 plasma, by conventional approach. The crystallinity of the films has been identified by the sharp Raman peak at ∼520 cm −1 and a large number of micrograins in the TEM micrographs. The metastable state of Ar, denoted as Ar * , plays the crucial role in inducing microcrystallisation by transferring its de-excitation energy at the surface of the growing film. A mechanism has been proposed to explain the dependence of the formation of μc-Si:H film on the working gas pressure in the plasma.

Effect of hydrogen radical on growth of μc-Si in hetero-structured SiC x alloy films

The changes of the crystallinity of μc-Si phase are studied in samples deposited with hydrogen dilution ratio, H 2/SiH 4, from 9.0 to 19.0 by hot-wire CVD (Cat-CVD). In the samples deposited at filament temperature, T f, of 1850 °C, the crystalline fraction and the crystallite size of μc-Si phase increased with increasing the H 2/SiH 4. The carbon content, C/(Si+C), was almost constant. In the XRD patterns, the intensity of Si(1 1 1) peak decreased and that of Si(2 2 0) peak increased with increasing the H 2/SiH 4. In the samples deposited at T f of 2100 °C with H 2/SiH 4 over 11.4, the μc-Si phase was not formed and the C/(Si+C) increased. The growth mechanism of μc-Si in hetero-structured SiC x alloy films is discussed.