Growth Of Microcrystalline Silicon Films Research Articles

Influence of excitation frequency and electrode separation on the growth of microcrystalline silicon films and their application in single junction microcrystalline solar cell

In this paper we present the role of plasma excitation frequency and electsrode separation on the growth of microcrystalline silicon thin films at two different hydrogen dilutions of silane and different power densities. We optimized the process conditions to develop device quality microcrystalline material. Optoelectronic and structural properties of the developed material have been correlated with the solar cell properties. Growth rate ~7 A/s has been achieved using plasma excitation frequency of 27.12 MHz at 15 mm electrode separation. We have noticed the positive effects after reducing the electrode separation in higher frequency (27.12 MHz). Optimized microcrystalline film of activation energy 0.55 eV and grain size of 14.61 nm has been developed and is applied to fabricate single junction microcrystalline solar cell. Solar cell with initial cell efficiency of 7.75% with short circuit current density of 24.98 mA/cm2 and open circuit voltage of 0.47 V and fill factor of 0.66 has been achieved.

Understanding the amorphous-to-microcrystalline silicon transition in SiF4/H2/Ar gas mixtures.

We report on the growth of microcrystalline silicon films from the dissociation of SiF4/H2/Ar gas mixtures. For this growth chemistry, the formation of HF molecules provides a clear signature of the amorphous to microcrystalline growth transition. Depositing films from silicon tetrafluoride requires the removal of F produced by SiF4 dissociation, and this removal is promoted by the addition of H2 which strongly reacts with F to form HF molecules. At low H2 flow rates, the films grow amorphous as all the available hydrogen is consumed to form HF. Above a critical flow rate, corresponding to the full removal of F, microcrystalline films are produced as there is an excess of atomic hydrogen in the plasma. A simple yet accurate phenomenological model is proposed to explain the SiF4/H2 plasma chemistry in accordance with experimental data. This model provides some rules of thumb to achieve high deposition rates for microcrystalline silicon, namely, that increased RF power must be balanced by an increased H2 flow rate.

The study of amorphous incubation layers during the growth of microcrystalline silicon films under different deposition conditions

The structural un-uniformity of μc-Si:H films prepared using a very high frequency plasma-enhanced chemical vapour deposition method has been investigated by Raman spectroscopy, spectroscopic ellipsometer and atomic force microscopy. It was found that the formation of amorphous incubation layer was caused by the back diffusion of SiH4 and the amorphous induction of glass surface during the initial ignition process, and growth of the incubation layer can be suppressed and uniform μc-Si:H phase is generated by the application of delayed initial SiH4 density and silane profiling methods.

An optical emission spectroscopy study on the high rate growth of microcrystalline silicon films

The growth process of microcrystalline silicon thin films deposited at high growth rate was monitored online by optical emission spectroscopy. The properties of the material were studied by Raman and FTIR spectroscopy. The results indicated that the I［SiH*］/I［Hβ*］ ratio decreased during the process, particularly at low total gas flows, which was consistent with the Raman results. The I［Hβ*］/I［Hα*］ratio detected after the plasma glowed for 5 minutes showed that the electronic temperature first decreased then increased with the increasing Ftotal. The FTIR spectra showed that the microstructure defect fraction R first decreased then increased with increasing Ftotal. This means that the electronic temperature in hydrogen plasma plays an important role in determining the properties of the microcrystalline silicon thin films.

Effects of silyl concentration, hydrogen concentration, ion flux, and silyl surface diffusion length on microcrystalline silicon film growth

Two sets of μc-Si : H films as a function of pressure were fabricated by very-high-frequency plasma en- hanced chemical vapor deposition (VHF-PECVD). Deposition rate, Raman crystallinity, and photo/dark conductivity were investigated under both low and high power conditions. A plasma fluid model and a surface hydride-dependent precursor diffusion model were constructed to understand the evolution of microcrystalline silicon under low and high power conditions. Silyl, hydrogen, ion flux, silyl surface diffusion length are believed to have much influence on film growth rate, crystallinity and photo electronic properties. But the interesting point is that under a certain condition one or more of these parameters dominate μc-Si : H growth, while other parameters have weak influence. Short-life radicals are found to be the possible major factor on the deterioration of photo sensitivity of μc-Si : H films.

Advances in the deposition of microcrystalline silicon at high rate by distributed electron cyclotron resonance

We report on the growth of microcrystalline silicon films at high rates (14 Å/s) in a MDECR plasma reactor. Our studies show that the growth process can be described by the same mechanisms as in RF deposition. However, the higher deposition rate achieved in MDECR leads to a higher substrate temperature requirement (225–250 °C) in order to allow hydrogen to diffuse within the film and induce its crystallization. Moreover we also found that microcrystalline silicon growth requires high microwave power (depletion mode) to produce enough atomic hydrogen and low sheath potential to limit ion bombardment. Combining these conditions we could achieve deposition rates up to 28 Å/s (limited by the silane flow rate) in films with a crystalline fraction of the order of 65% which is well adapted for solar cell applications.

Improved transport properties of microcrystalline silicon films grown by HWCVD with a variable hydrogen dilution process

The structure and the transport properties of microcrystalline silicon films prepared by hot-wire/catalytic chemical vapor deposition (HWCVD/Cat-CVD), using different dilution ratios of silane in hydrogen, were investigated. Spectroscopic ellipsometry analysis revealed an increase in the thickness of amorphous incubation layer formed before nucleation and a reduction of the void volume fraction when hydrogen dilution decreases. Thus, a specific microcrystalline silicon film growth process was proposed, based on a variable dilution of silane in hydrogen. For films prepared in such conditions, the formation of the incubation layer was inhibited, which led to a drastic improvement in carrier transport along the growth direction as proved by the diffusion-induced time-resolved microwave conductivity data.

Silane versus silicon tetrafluoride in the growth of microcrystalline silicon films by standard radio frequency glow discharge

Properties of microcrystalline silicon (μc-Si) films produced by standard radio frequency glow discharge from SiH 4 or SiF 4 precursors were studied. Spectroscopic ellipsometry (SE), Raman spectroscopy and time resolved microwave conductivity (TRMC) were used for their characterisation. Although modelling of SE spectra shows that μc-Si films with crystalline fractions up to 100% can be deposited from both types of precursors, we have found that SiF 4 allows obtaining films with improved electronic properties. A detailed analysis of the SE data reveals that such improvement relates to the larger grain size for films deposited from SiF 4 in comparison with the ones deposited from SiH 4. Interestingly, the deposition rate, the fraction of large grains, and the mobility of carriers for the films grown from SiF 4 display close dependencies on the product of pressure and inter-electrode distance. These results are discussed with respect to the contribution of the plasma-synthesised nanocrystals to deposition.

High rate growth of device-grade microcrystalline silicon films at 8nm/s

Abstract We have developed a novel technique for large-area high rate growth of microcrystalline silicon films by plasma-enhanced chemical vapor deposition, designing a novel cathode with interconnected multi-holes, which leads to produce uniformly flat-distributed stable high-density plasma spots near cathode surface. The spatial distribution of plasma at holes on cathode surface was analyzed using optical emission spectroscopy for SiH 4 /H 2 plasma with various pressures with a view to optimizing deposition conditions. Improvement of properties of high-rate-grown films was discussed with regard to silane depletion as well as the temperature of film-growing surface. Microcrystalline silicon films with a low defect density of 5×10 15 cm −3 obtained at a high rate approaching 8 nm/s demonstrate the effectiveness of the novel cathode.

Hydrogenated Microcrystalline Silicon Film Growth by Inductively Coupled Plasma–Chemical Vapor Deposition on ZrO2 Gate Dielectric for Thin Film Transistors

Hydrogenated microcrystalline silicon (µc-Si:H) film was fabricated by inductively coupled plasma–chemical vapor deposition (ICP–CVD) at 150 °C on ZrO2 gate dielectric which was made by atomic layer deposition (ALD) method. µc-Si:H film with very thin incubation layer less than 10 nm was deposited on ZrO2 film. Polycrystalline ZrO2 played a role of crystal seed layer to enhance the crystalline fraction of µc-Si:H film. Surface roughness of ZrO2 film increased as deposition temperature of ZrO2 was increased. Rougher surface of ZrO2 film provided nucleation site of µc-Si:H film at early growth stage to reduce the thickness of the incubation layer. The crystalline fraction of µc-Si:H film depended on crystallinity of ZrO2. When µc-Si:H film was deposited on ZrO2 of 100 and 50 nm thickness at 250 °C, (200) and (111) peak intensity was increased respectively.

The application of very high frequency inductively coupled plasma to high-rate growth of microcrystalline silicon films

High-rate growth of microcrystalline silicon films (μc-Si:H) from inductively coupled plasma (ICP) of H2 diluted SiH4 generated with a very high frequency (VHF: 60MHz) power source has been studied from the viewpoint of efficient gas dissociation. From the VHF power and gas pressure dependences of the film growth rate and optical emission intensities, we have found that the Si and SiH emission intensities and the intensity ratio of Hα to SiH are good indicators for the film growth rate and crystallinity, respectively. The generation rate of film precursors is reflected by the Si and SiH emission intensities while the flux ratio of atomic hydrogen to film precursors, which plays an important role on the structural relaxation for the crystalline network formation, is characterized by the intensity ratio of Hα to SiH. An increase in SiH emission while keeping the intensity ratio of Hα to SiH at a certain level enables us to enhance the film growth rate without significant deterioration in the crystallinity. In this study, a growth rate as high as 10nm/s was obtained for highly crystallized films.

High-rate growth of microcrystalline silicon films using a high-density SiH 4/H 2 glow-discharge plasma

We developed a novel technique for high-rate growth of microcrystalline silicon films by plasma-enhanced chemical vapor deposition, designing a novel cathode with interconnected multi-holes, which leads to generate uniformly flat-distributed stable high-density-plasma spots near cathode-surface. Improvement of quality of high-rate grown films was discussed, and microcrystalline silicon films with a low defect density of 1.2×10 16 cm −3 were obtained at a high rate of 7.7 nm/s, demonstrating the efficient gas dissociation and the effectiveness of the novel cathode. The spatial distribution of plasma at cathode-surface holes was analyzed using optical emission spectroscopy for further optimization of plasma conditions.

Effect of deposition conditions and dielectric plasma treatments on the electrical properties of microcrystalline silicon TFTs

We study the growth of microcrystalline silicon films on silicon nitride as a function of the deposition conditions and the dielectric plasma treatment. For thin film transistors processed in the bottom gate configuration, we obtain stable transistors with mobilities of 0.7 cm 2 V −1 s −1, indicating that we have indeed achieved a microcrystalline channel even in the bottom gate approach. Moreover, we found an opposite correlation between the linear mobility and the crystallization kinetics of the material deduced from in-situ ellipsometry measurements; i.e. the faster the crystallization kinetics, the lower the mobility. This result is discussed in terms of smaller grain size for films with fast crystallization, and is supported by Raman spectroscopy measurements. These results provide a guideline for further improving the mobility of the transistors.

Ion bombardment effects on microcrystalline silicon growth mechanisms and on the film properties

The role of ions on the growth of microcrystalline silicon films produced by the standard hydrogen dilution of silane in a radio frequency glow discharge is studied through the analysis of the structural properties of thick and thin films. Spectroscopic ellipsometry is shown to be a powerful technique to probe their in-depth structure. It allows to evidence a complex morphology consisting of an interface layer, a bulk layer, and a subsurface layer. The ion energy has been tuned by codepositing series of samples on the grounded electrode and on the powered electrode, as functions of pressure and power. On the one hand, reducing the ion energy through the increase of the total pressure and depositing on the grounded electrode, favors the formation of large grains and results in improved bulk transport properties, but leaves an amorphous interface layer with the substrate. On the other hand, we achieve fully crystallized films on glass substrates under conditions of high energy ion bombardment. We suggest that ion bombardment, and particularly the implantation of hydrogen ions, favors the formation of a porous layer where the nucleation of crystallites takes place. These results are further supported by in situ spectroscopic ellipsometry measurements of the film morphology as a function of the ion energy.

Fundamental processes of microcrystalline silicon film growth: a molecular dynamics study

The fundamental processes of microcrystalline silicon film growth in plasma enhanced chemical vapor deposition (PECVD) with SiH 4 gas diluted by H 2 have been studied using molecular dynamics (MD) simulations with a realistic many-body semiclassical potential developed to describe interatomic interactions in the Si:H system. In this study the fundamental processes have been investigated from the following three viewpoints: (i) Si crystallization processes from SiH 3 radicals; (ii) hydrogen desorption processes; and (iii) stability of the crystallized Si atoms. As a result of MD simulations related to these three points, it was found that when a substrate temperature is ca. 500 K, a SiH 3 radical migrates to a crystalline Si site and is stabilized by bonding two surface Si atoms after releasing one H atom after ca. 200–300 ps, and that surface H desorption occurs as H 2 molecules, SiH 3 radicals and SiH 4 molecules due to atomic H exposure. In addition it was observed that the stabilized Si atom in a crystalline Si site mentioned above cannot be etched by incident atomic H but unrelaxed Si atoms in surface layers can be etched.

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.

Real time analysis of amorphous and microcrystalline silicon film growth by multichannel ellipsometry

Real time spectroscopic ellipsometry (SE) has been applied to obtain insights into the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) thin films by plasma-enhanced chemical vapor deposition as a function of the H2-dilution gas flow ratio R=[H2]/[SiH4], the accumulated film thickness db, and the substrate material. For depositions with 15≤R≤80 on clean amorphous semiconductor surfaces, for example, initial film growth occurs in a predominantly amorphous phase, as deduced from analyses of the real time SE data. However, after an accumulated thickness ranging from 3000 Å for R=15 to 30 Å for R=80, a roughening transition is observed in the SE analysis results as the Si film begins to develop a predominantly microcrystalline structure. We have identified this roughening transition as an amorphous-to-microcrystalline phase boundary in the deposition parameter space of db and R. The thickness at which this boundary occurs decreases continuously with increasing R, and the position of the boundary is strongly substrate dependent. Based on these real time SE studies and detailed device analyses, we have found that the highest performance p–i–n solar cells are obtained in i-layer deposition processes maintained at the highest possible R value versus thickness without crossing the deposition phase boundary into the microcrystalline regime.

Early stages of microcrystalline silicon film growth on amorphous substrate with SiH 4 gas heating

The effect of SiH 4 gas heating has been investigated for the growth of microcrystalline silicon (μc-Si) thin film by plasma-enhanced chemical vapor deposition (PECVD) and low pressure CVD to improve the crystallinity and inhomogenities at the early stage of growth on amorphous substrate such as SiO 2. By using the cathode heating technique, T c=550°C, the crystallinity was enhanced from the early stage of growth in the PECVD of SiH 4 highly diluted in H 2 at substrate temperature, T s=200°C.

Initial stages of microcrystalline silicon film growth

μc-Si:H films with thicknesses increasing from 5 to 350 nm, which are deposited by the new closed-chamber cyclic chemical vapor deposition method, have been studied. The structure of the samples versus film thickness has been investigated by scanning tunneling microscopy (STM), scanning electron microscopy (SEM), Raman scattering, and ultra-violet (UV)-reflection spectroscopy. The carrier transport is characterised by dark and photoconductivities, as well as by the ambipolar diffusion length obtained from the steady-state photocarrier grating method (SSPG). Optical absorption is measured by transmission spectroscopy and by the constant photocurrent method (CPM). A gradual change of all properties has been found with increasing film thickness. While the transition from amorphous to highly crystalline structure is completed within some 50–80 nm, the lateral carrier transport properties saturate at about 200 nm. The transition layer thickness can be reduced by soft deposition conditions during the nucleation phase. A growth model of initially cone-shaped crystallites merging into a compact crystalline layer at higher thicknesses is proposed. Transport could then proceed on percolation paths along the crystallites.