Growth Of Silicon Films Research Articles

A Phase Diagram for Morphology and Properties of Low Temperature Deposited Polycrystalline Silicon Grown by Hot-wire Chemical Vapor Deposition

ABSTRACTThe fabrication of low temperature polycrystalline silicon with lifetimes close to single crystalline silicon, but with internal surface passivation similar to that observed in deposited microcrystalline silicon, is a promising direction for thin film polycrystalline silicon photovoltaics. To achieve this, large grains with passivated grain boundaries and intragranular defects are required. We investigate the low-temperature (250-550°C) epitaxial growth of thin silicon films by hot-wire chemical vapor deposition (HWCVD) on Si(100) substrates and large-grained polycrystalline silicon template layers formed by selective nucleation and solid phase epitaxy (SNSPE). Using reflection high energy electron diffraction (RHEED) and transmission electron microscopy (TEM), we have observed epitaxial, twinned epitaxial, mixed epitaxial/polycrystalline and polycrystalline phases in the 50 nm–15 μm thickness regime. HWCVD growth on Si(100) was performed using a mixture of diluted silane (4% in He) and hydrogen at a H2/SiH4 ratio of 50:1 at substrate temperatures from 300–475°C. We will discuss the relationship between the microstructure and photoconductive decay lifetimes of these undoped layers on Si(100) and SNSPE templates as well as their suitability for use in thin-film photovoltaic applications.

Control of grain size during low-temperature growth of polycrystalline silicon films

Polycrystalline silicon thin films were prepared at high-speed by plasma-enhanced chemical vapor deposition technique at low temperatures using SiCl4 and H2 as source gases. It was found that the grain growth is strongly affected by the relative concentration of different active radicals in the gas-phase space. On the other hand, the relative concentration depends on the deposition conditions. With the decrease of the rf power and the H2/ SiCl4 flow ratio, and the increase in the reaction pressure, the grain size increases. By changing the deposition conditions, variations of the relative concentration were analyzed. It is suggested that the “gas-phase crystalline" is of crucial importance to the grain growth.

Surface Processes during Growth of Hydrogenated Amorphous Silicon

ABSTRACTHydrogenated amorphous silicon films for photovoltaics and thin film transistors are deposited from silane containing discharges. The radicals generated in the plasma such as SiH3 and H impinge on the surface and lead to silicon film growth through a complex network of elementary surface processes that include adsorption, abstraction, insertion and diffusion of various radicals. Mechanism and kinetics of these reactions determine the film composition and quality. Developing deposition strategies for improving the film quality requires a fundamental understanding of the radical-surface interaction mechanisms. We have been using in situ multiple total internal reflection Fourier transform infrared spectroscopy and in situ spectroscopic ellipsometry in conjunction with atomistic simulations to determine the elementary surface reaction and diffusion mechanisms. Synergistic use of experiments and atomistic simulations elucidate elementary processes occurring on the surface. Herein, we review our current understanding of the reaction mechanisms that lead to a-Si:H film growth with special emphasis on the reactions of the SiH3 radical.

Low-temperature silicon homoepitaxial growth by pulsed magnetron sputtering

Undoped silicon (Si) films have been deposited on Si (100) substrates by pulsed magnetron sputtering in pure Ar atmosphere at substrate temperatures TS between 300 and 450°C. Rutherford backscattering channeling (RBS-C) experiments reveal high structural order indicative of epitaxial growth at TS of 375–400°C. The disorder depth profiles derived from RBS-C exhibit defective initial film growth which is markedly reduced in the film volume. Homoepitaxial silicon film growth is observed in this optimal temperature range up to a thickness of 1.6μm and with growth rates of about 20nm/min. At higher and lower substrate temperatures, the disorder in the films is considerably enhanced.

Temperature dependence of the surface reactivity of SiH 3 radicals and the surface silicon hydride composition during amorphous silicon growth

For hydrogenated amorphous silicon (a-Si:H) film growth governed by SiH 3 plasma radicals, the surface reaction probability β of SiH 3 and the silicon hydride (–SiH x ) composition of the a-Si:H surface have been investigated by time-resolved cavity ringdown and attenuated total reflection infrared spectroscopy, respectively. The surface hydride composition is found to change with substrate temperature from –SiH 3-rich at low temperatures to SiH-rich at higher temperatures. The surface reaction probability β, ranging from 0.20 to over 0.40 and with a mean value of β=0.30±0.03, does not show any indication of temperature dependence and is therefore not affected by the change in surface hydride composition. It is discussed that these observations can be explained by a-Si:H film growth that is governed by H abstraction from the surface by SiH 3 in an Eley-Rideal mechanism followed by the adsorption of SiH 3 at the dangling bond created.

Low temperature, high growth rate epitaxial silicon and silicon germanium alloy films

Epitaxial growth of silicon and silicon germanium alloy films on Si(1 0 0) substrates in an ASM Epsilon ® single wafer reactor system using trisilane and germane has been investigated. The results obtained reveal that it is possible to achieve epitaxial silicon and silicon germanium growth rates at 630 °C and below that are substantially higher than those possible using silane, while maintaining high quality crystalline structure. The films grown were characterized using Rutherford backscattering spectroscopy (RBS), high resolution X-Ray diffraction (HR-XRD), atomic force microscopy (AFM) and high resolution cross-sectional transmission electron microscopy (X-TEM).

Effect of hydrogen dilution on the growth of nanocrystalline silicon films at high temperature by using plasma-enhanced chemical vapor deposition

Two hundred-nm-thick nanocrystalline silicon films were deposited at 620 °C under different H 2 flow rates, ([H 2]), by plasma-enhanced chemical vapor deposition using SiH 4/H 2 mixtures. When [H 2] increased, the grain size decreased. On the other hand, no crystallization was found at [H 2]=5 sccm. The photoluminescence spectra changed from a single peak of approximately 2.0–2.1 eV at [H 2]=10 sccm to two separated lines of approximately 1.7 and 2.1 eV at [H 2] between 15 and 20 sccm. However, the 2.1-eV band decreased with increasing [H 2], and at [H 2]=25 sccm, we observed a single peak at approximately 1.8 eV. The hydrogen content decreased and two infrared absorption bands approximately at 850 and 1000 cm −1 also decreased with [H 2].

Effects of silicon tetrachloride concentration on nanocrystalline silicon films growth

The effects of the silicon tetrachloride (SiCl 4) concentration on the growth of the nanocrystalline silicon film using SiCl 4/hydrogen (H 2) reactants are investigated here. An almost twice enhancement of the deposition rate of the silicon film, while still maintaining the film with a high crystal fraction at a level of 96%, is achieved by increasing the SiCl 4 concentration from 17 to 26% at a substrate temperature of 270 °C. The similar trends of the crystal fraction and deposition rate with the SiCl 4 concentration are also observed from those films deposited at 180 °C. The morphologies of the nanocrystalline silicon films with a thickness of 460 nm which deposited at 270 °C from various SiCl 4 concentrations are similar whereas they are drastically different as the film thickness increases to 2.6 μm. The roles of the SiCl 4 concentration, which relates to Cl radical and H radical concentrations in gas phase, in the growth behaviors of the nanocrystalline silicon films are discussed.

Epitaxial silicon films deposited at high rates by gas-jet electron beam plasma CVD

We discuss a new method for low temperature epitaxial growth of silicon films by gas-jet electron beam plasma chemical vapor deposition in SiH 4–Ar mixture. The growth rate up to 0.8 μm/min becomes possible at the substrate temperature below 650 °C, without using ultra-high vacuum chamber. Epitaxial Si films were deposited with the thickness up to 2–10 μm. The structure of the films has been studied after deposition by transmission electron microscopy, transmission high energy electron diffraction and reflection high energy electron diffraction. All films were of epitaxial types, but they were characterized by a high density of threading dislocations (≈10 7 cm −2).

High-rate deposition of highly crystallized silicon films from inductively coupled plasma

We have studied microcrystalline silicon (μc-Si) film growth from inductively coupled RF plasma (ICP) of monosilane (SiH 4) and hydrogen gas mixtures and demonstrated the feasibility of ICP for a high rate growth of highly crystallized Si films at a temperature as low as 250 °C. Using 15% SiH 4 diluted with H 2, a deposition rate of ∼1.0 nm s −1 was achieved for crystalline films, for which Raman scattering spectra show the TO phonon peak intensity ratio for the crystalline/disordered phase is >5 for film thickness >1 μm. By measuring optical emission from the SiH 4 ICP, we suggest that the relative flux of hydrogen radicals to the growing film surface with respect to the flux of film precursors during monatomic layer growth is a key parameter in obtaining highly crystallized films at a higher growth rate. Strong influence of the crystallinity on the optical and electrical properties has also been confirmed for films prepared with different SiH 4 concentrations or different film thickness.

Hot-wire chemical vapor deposition for epitaxial silicon growth on large-grained polycrystalline silicon templates

We investigate low-temperature epitaxial growth of thin silicon films by HWCVD on Si [1 0 0] substrates and polycrystalline template layers formed by selective nucleation and solid phase epitaxy (SNSPE). We have grown 300-nm thick epitaxial layers at 300 °C on silicon [1 0 0] substrates using a high H 2:SiH 4 ratio of 70:1. Transmission electron microscopy confirms that the films are epitaxial with a periodic array of stacking faults and are highly twinned after approximately 240 nm of growth. Evidence is also presented for epitaxial growth on polycrystalline SNSPE templates under the same growth 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.

Temperature dependence of the surface roughness evolution during hydrogenated amorphous silicon film growth

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.

The properties of fluoride/glass and fluoride/silicon

Various fluoride films on a glass substrate were prepared and characterized to provide a seed layer for silicon (Si) film growth. The X-ray diffraction analysis on CaF 2/glass illustrated (2 2 0) preferential orientation and showed lattice mismatch less than 5% with Si. We achieved a fluoride film with breakdown electric field higher than 1.27 MV/cm, leakage current density less than 10 −8 A/cm 2, and relative dielectric constant less than 5.6. This paper demonstrates microcrystalline silicon (μc-Si) film growth at a low substrate temperature of 300 °C by using a CaF 2/glass substrate. The μc-Si films exhibited crystallization in (1 1 1) and (2 2 0) planes, grain size of 700 Å, crystalline volume fraction over 65%, dark- and photo-conductivity ratio of 124, activation energy of 0.49 eV, and dark conductivity less than 4×10 −7 S/cm.

Plasma studies under polymorphous silicon deposition conditions

The relationship between the characteristics of silane–hydrogen plasmas and the growth of polymorphous silicon films has been studied by means of three simultaneous diagnostics: spectral analysis of the harmonics of the RF current, laser light scattering from silicon powders in the plasma, and in situ ellipsometry. The deposition rate of the films has been measured as a function of the plasma pressure in the range of 50 up to 400 Pa. In this way we could identify the polymorphous silicon deposition regime. Using a high hydrogen dilution allows to avoid a sharp transition and thus define a wide range of pressure (from 160 up to 270 Pa) in which pm-Si:H films are obtained. The spectral analysis of the RF current has been identified as the most sensitive technique to characterize the plasma allowing to determine the optimal conditions of cluster and nanoparticle formation while avoiding powders.

Hot-Wire Chemical Vapor Deposition for Epitaxial Silicon Growth On Large-Grained Polycrystalline Silicon Templates

AbstractWe investigate low-temperature epitaxial growth of thin silicon films on Si [100] substrates and polycrystalline template layers formed by selective nucleation and solid phase epitaxy (SNSPE). We have grown 300 nm thick epitaxial layers at 300°C on silicon [100] substrates using a high H2:SiH4 ratio of 70:1. Transmission electron microscopy confirms that the films are epitaxial with a periodic array of stacking faults and are highly twinned after approximately 240 nm of growth. Evidence is also presented for epitaxial growth on polycrystalline SNSPE templates under the same growth conditions.

Comparison of two surface preparations used in the homoepitaxial growth of silicon films by plasma enhanced chemical vapor deposition

The effect of surface preparation on the growth of epitaxial Si films by plasma enhanced chemical vapor deposition was investigated. The surface preparations considered were an ex situ ozone scrub and an in situ Ar/H2-plasma clean. Both methods were found to be effective at removing carbon contamination from the substrate surface which is critical for epitaxial growth. The thin-film quality was determined by Rutherford backscatter spectrometry, high-resolution x-ray diffraction, and transmission electron microscopy. To gain insight into mechanisms controlling the in situ cleaning process, hydrogen was replaced by deuterium in the plasma clean prior to film growth. The film/substrate interface was then analyzed by secondary ion mass spectrometry. Surprisingly, the plasma clean had little influence on the interfacial hydrogen concentration established by the previous hydrofluoric acid dip. It was found that hydrogen remains bound to C and O contaminants at the interface caused by the initial growth surface, and that neither an ex situ process containing an ozone scrub nor an in situ process containing a hydrogen-plasma clean could completely remove them.

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.

Highly 〈100〉-oriented growth of polycrystalline silicon films on glass by pulsed magnetron sputtering

Nominally undoped polycrystalline silicon (poly-Si) thin films were deposited on glass at 450 °C at high deposition rate (>100 nm/min) by pulsed dc magnetron sputtering. The pulse frequency was found to have a significant influence on the preferred grain orientation. The x-ray diffraction pattern exhibits a strong enhancement of the (400) reflex with increasing pulse frequency. The quantitative evaluation reveals that over 90% of the grains are 〈100〉 oriented. The observed change in preferred grain orientation in poly-Si films at low temperatures is associated with concurrent ion bombardment of the growing film.

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.