Polycrystalline SiC Films Research Articles

Physical properties of ECR-CVD polycrystalline SiC films for micro-electro-mechanical systems

Polycrystalline 3C–SiC films deposited by ECR-CVD on Si(1 0 0) wafers and on thermally oxidized silicon wafers were comparatively analyzed. Transmission electron microscopy and X-ray diffractometry were used to evaluate the substrate influence on the structural properties. Substrate effects are present: films grown on oxidized substrates are polycrystalline and films on polished Si(1 0 0) exhibit etheroepitaxial structures embedded in a more disordered matrix. Defect characterization was attained by electron-spin-resonance spectroscopy at variable temperature, with special emphasis on the spin relaxation processes. It shows that a same defect type (probably related to grain boundaries in disordered polycrystalline phase) is present in both cases.

Role of Ge on film quality of SiC grown on Si

We find that the incorporation of Ge into SiC during the metalorganic chemical vapor deposition growth process improves the crystalline quality of SiC films grown on Si (111) substrates at 1000 °C. Secondary ion mass spectroscopy results indicate that Ge does not act as a surfactant, but rather it incorporates throughout the entire film. Transmission electron microscopy results show that high quality, single crystalline SiC films of up to 80 nm thickness are repeatably obtained for GeH4 flow rates ranging from 20 to 30 sccm. Higher GeH4 flow rates induce twinning and result in a reduced growth rate and increased surface roughness. Lower GeH4 flow rates result in polycrystalline and/or amorphous-like SiC films similar to what is normally obtained at such a low growth temperature. We discuss the role of Ge during growth and how its presence in the reactor during Si substrate carbonization results in higher quality SiC films than normally achieved for growth temperatures of 1000 °C.

Texturing of polycrystalline SiC films on graphitic carbon in laminated composites grown by forced-flow chemical vapor infiltration

The growth and orientation of SiC on pyrocarbon using forced-flow thermal gradient chemical vapor infiltration to deposit alternate layers of these materials has been examined by transmission electron microscopy. The SiC layers were determined to be predominantly of the cubic (β) polytype, and the close-packed {111} SiC planes were found to be mostly oriented parallel and next to the highly dense {0001} C basal planes of the pyrocarbon layers at the interface. A model involving close-packed plane sequencing is proposed for the C/SiC interface.

Compositional and structural studies of DC magnetron sputtered SiC films on Si(111)

Polycrystalline SiC films were deposited on Si(111) by reactive DC magnetron sputtering using a four inch elemental silicon target. Composition of the deposited films was studied by AES (auger electron spectroscopy) and RBS (Rutherford backscattering spectroscopy) which showed that stoichiometric SiC could be obtained. Both AES depth profile and RBS indicated the existence of a transition layer. XRD (X-ray diffraction) analysis revealed the formation of polycrystalline SiC films at a substrate temperature as low as 1123 K, and the absence of other peaks in XRD patterns except the SiC(111) peak implied that the films were (111) oriented. Furthermore, the films were found to consist of columnar nanometer sized crystallites by cross-section TEM (transmission electron microscopy) study.

Growth of polycrystalline SiC films on SiO2 and Si3N4 by APCVD

The deposition of undoped and phosphorus-doped polycrystalline silicon carbide (poly-SiC) films on SiO2 and Si3N4 substrates by atmospheric pressure chemical vapor deposition (APCVD) was studied. Silane and propane were used as source gases, hydrogen was used as the carrier gas, and phosphine was used as the doping gas. The deposition temperature was fixed at 1050°C and deposition times of 30 s and 30 min were used. Stylus profilometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and selected area electron diffraction (SAD) were used to compare the surface morphology and microstructure of poly-SiC films deposited on the two amorphous substrate materials. In general, doped and undoped poly-SiC films deposited on Si3N4 exhibit a smoother surface morphology and smaller grain size than doped and undoped poly-SiC deposited on SiO2. Based on the thermodynamic theory, the large grain size and rough surface texture for poly-SiC films deposited on SiO2 may be due to a higher energy barrier and larger critical nucleus size than for poly-SiC films deposited on Si3N4. These differences have been exploited to develop a lift-off patterning technique for poly-SiC films.

β-SiC Thin film growth using microwave plasma activated CH 4-SiH 4 sources

The activation of a source gas by plasma is used to decrease the growth temperature of a highly oriented epitaxial SiC film on a Si(100) substrate. A mixed source gas of 1% CH 4–0.5% SiH 4–H 2 is activated by a microwave plasma at 100 Torr with the power between 400 and 900 W. No pre-deposition processes such as carbonization is applied. A substrate is immersed in the plasma, whose temperature is controlled between 795 and 1000°C by varying the microwave power. Polycrystalline SiC films, whose grains appear equiaxed and randomly oriented, forms under substrate temperatures below 900°C. However, the grains begin to change their shape into elongated ones above 935°C, whose longitudinal axes are aligned to the 〈011〉 direction of the Si substrate. The growth rate of the SiC film decreases with increasing temperature. The stoichiometry, C/Si concentration ratio, is maintained as 1.1 throughout the temperature range.

Surface micromachining of polycrystalline SiC films using microfabricated molds of SiO/sub 2/ and polysilicon

As an alternative to conventional SiC reactive ion etching (RIE), polycrystalline (poly-SiC) films were patterned into micron-sized structures using sacrificial SiO/sub 2/ and polycrystalline silicon (polysilicon) molds in conjunction with mechanical polishing. The molds were made from thermally grown SiO/sub 2/ and LPCVD polysilicon films and were fabricated using conventional patterning techniques. The poly-SiC micromolding process combines film deposition, polishing, and selective wet chemical etching of the molds to achieve the desired pattern. The process is simple and does not suffer from the difficulties associated with RIE of SiC. Micrometer sized lines, spaces, and complex device structures have been patterned using this technique. The micromolding technique has been used in a SiC surface micromachining process to fabricate fully released lateral resonant structures.

Characterization of polycrystalline silicon carbide films grown by atmospheric pressure chemical vapor deposition on polycrystalline silicon

X-ray diffraction, transmission electron microscopy, and Rutherford backscattering spectroscopy were used to characterize the microstructure of polycrystalline SiC films grown on as-deposited and annealed polysilicon substrates. For both substrate types, the texture of the SiC films resembles the polysilicon at the onset of SiC growth. During the high temperature deposition process, the as-deposited polysilicon recrystallizes without influencing the crystallinity of the overlying SiC. An investigation of the SiC/polysilicon interface reveals that a heteroepitaxial relationship exists between polysilicon and SiC grains. From this study, a method to control the orientation of highly textured polycrystalline SiC films has been developed.

Temperature dependence of conductivity and Hall carrier concentration of polycrystalline SiC deposited on fused silica by laser ablation

The conductivity and Hall coefficient of polycrystalline SiC films deposited on quartz are studied in the temperature ranges of 13–1275 K and 17–800 K, respectively. Since conductivities were monotonically increasing with the increasing temperature, these films can be used as thermistors over the entire temperature range. While Hall mobility shows a weak temperature dependence, the electron concentration, as computed from n=1/qRH, increases exponentially with temperature from 1019 to 4×1020 cm−3. Three well-separated activation energies, computed from the measured conductivity and Hall concentration, are in the ranges of 0.4–3.8 meV, 7–20 meV, and 55.5±5 meV. Due to high electron concentration measured at temperatures as low as 17 K, impurity/defect related conduction or thermally activated hopping may be dominant at low temperatures.

Improvement of the Young’s modulus of SiC film by low-pressure chemical vapor deposition with B2H6 gas

SiC is one of the potential materials for use as an x-ray mask substrate. It is preferable for an x-ray mask substrate to have a large elastic modulus, so as to suppress any distortion of the extremely minute and precise patterns. The improvement of the Young’s modulus of polycrystalline SiC film using low-pressure chemical vapor deposition with the introduction of B 2H6 in the source gas was investigated. The Young’s modulus increased with the addition of B2H6, and a maximum value of 600 GPa, which was 25% higher than in the case without B2H6, was reached at a source gas ratio B/Si=0.02. Two models which would possibly explain this phenomenon are discussed, and the theory which takes into account the interaction between carrier and ion core is found to provide a more plausible explanation of the results.

Characteristics of amorphous silicon precipitated by means of argon excimer laser irradiation on SiC films

Amorphous silicon precipitated on SiC films by means of argon excimer laser irradiation were investigated. Raman spectra show that the observed bands of the amorphous silicon depended on crystalline quality of SiC film. In case of a preferentially oriented polycrystalline SiC film, precipitated amorphous silicon is similar to the amorphous silicon produced by a radio frequency (rf) discharge method based on similarities of Raman scattering data.

Growth and characterisation of SiC films deposited on a-SiO2/Si (100) substrates

The growth of polycrystalline SiC films has been carried out by low pressure chemical vapour deposition in a horizontal quartz reaction chamber using tetramethylsilane and H2 as the precursor gas mixture. Silicon (100) wafers were used as substrates. A thin Si O2 amorphous layer of ~6 nm was formed before SiC deposition to reduce the strain induced by the 8% difference in thermal expansion coefficients between SiC and Si. Samples were. analysed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared reflectivity. The structure of films grown at temperatures between 950 and 1150°C varies from amorphous to polycrystalline SiC. Preferential [111] orientation and columnar growth of polycrystalline films develops with increasing temperature.MST/3317

Effect of reactant depletion on the microstructure and preferred orientation of polycrystalline SiC films by chemical vapor deposition

Polycrystalline SiC films were deposited on graphite substrates from the gaseous mixture of CH 3SiCl 3 and H 2 at temperatures ranging from 1273 to 1673 K by low-pressure chemical vapor deposition. Variations of deposition rate, surface morphology and preferred orientation of the films with temperature and depletion of reactants (distance from inlet in the isothermal zone of the reactor) have been studied. The deposition had two growth kinetics evidently. The first is controlled by surface reaction for 1273–1473 K and the second by mass transport for 1523–1673 K. For each growth region, depletion of reactants is correlated to the decrease of the deposition rate and for the surface reaction region to the decrease of the activation energy. The change of the surface morphology of the films with depletion is shown by scanning electron microscopy. From X-ray diffraction experiments, it is concluded that the growth direction of the SiC films is always [111]for the surface reaction control but changes from [220]to [111]with depletion of the reactants for the mass transport control.

Optical absorption and electronic transport in ion-implantation-doped polycrystalline SiC films

Fine-grained (d≈0.1 μm), polycrytalline SiC films were prepared on top of insulating and optically transparent sapphire substrates by means of a thermal crystallization technique. Optical absorption measurements indicate that the individual SiC grains consist of relatively defect-free β-SiC surrounded by high-defect density grain-boundary material. Nominally undoped material exhibits a low de conductivity (δ≈10−8 Ω−1 cm−1) in the dark and an efficient photoconductivity apon illumination with short-wavelength UV light. The temperature dependence of the de transport exhibits a quasi-Arrhenius-type behaviour with average activation energies of the order to 0.6 eV. A characteristic feature of this kind of transport is a continuous increase in activation energy with increasing film temperature. Upon doping with N, P and Al ions, the average activation energy is decreased and room temperature conductivities of the order of 0.1 Ω−1 cm−1 are reached. Doping with B ions, on the other hand, only leads to high-resistivity material. It is shown that the electronic transport in doped SiC-On-Sapphire (SiCOS) films can be successfully modelled in terms of a grain-boundary-dominated conduction process. In this process thermal activation across potential barriers at the grain-boundary surfaces competes with funneling through these same barriers.

Optical absorption and electronic transport in ion-implantation-doped polycrystalline SiC films

Optical absorption and electronic transport in ion-implantation-doped polycrystalline SiC films

X-ray, transmission electron microscopy and atomic force microscopy characterization of SiC thin films on Si(111)

Thin polycrystalline SiC films grown on Si(111) at 830 degrees C using solid state evaporation were characterized by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The thin-film crystallites have a preferred growth orientation azimuthal and nearly perpendicular to the substrate surface. The orientation distribution function was characterized to be Gaussian with full width at half maximum of 2.5 degrees . The size of crystallites normal to the surface (the length of columns) is of the order of the thin film thickness, whilst the size parallel to the surface is in the range 20-70 nm.

Diagnostic Techniques For Polycrystalline Thin Film Growth

Abstract

Electronic transport in thermally crystallized SiC films on sapphire

Fine-grained ( d ≈ 0.1 μm), polycrystalline (pc) SiC films were prepared on top of insulating and optically transparent sapphire substrates by a thermal crystallization technique (SiCOS films). Unlike high-temperature deposited pc-SiC films, SiCOS films exhibit a very low d.c. conductivity in the dark (σ ≈ 10 −8 Ω −1 cm −1) and an efficient photoconductivity on illumination with short-wavelength UV light. Relatively high n- or p-type conductivities (σ ≈ 1 Ω −1 cm −1) were obtained after implantation of N, P and Al ions. It is argued that the electronic transport in the thermally crystallized films is limited by a grain-boundary-dominated conduction process in which thermal activation across potential barriers competes with tunnelling through these same barriers.

Ion implantation doping of polycrystalline SiC thin films prepared by PECVD

Ion implantation doping experiments have been performetion fine-grained, polycrystalline SiC films deposited onto insulating and optically transparent sapphire substrates. The efficiency of implantation doping has been found to be critically dependent on the actual implantation and annealing sequence. Using optim and implantation and annealing sequences efficient n-type doping has been obtained after implantation of N and P impurities. Implantation of Al under the same conditions leads to p-type doping. All films exhibit UV photoconductivity. Plasma hydrogenation has been found to reduce the density of grain boundary defects in the polycrystalline SiC films.

Influence of Al and P doping on optical and electrical properties of ion beam synthesized SiC

Ion implantation of 3 × 10 17 C/cm 2 at 25 keV into Si has been applied to synthesize thin films of SiC. The starting material, 130 nm thick layers of Si on sapphire substrates, was held at 900°C during ion implantation. After annealing at 1300°C for 15 min the resulting SiC films consist of crystallites of ∼ 10–30 nm diameter. Dopant species, P and Al for n- and p-type doping, have been introduced by ion implantation either before and after synthesis of SiC. Optical and electrical measurements have been performed in order to characterize the films. As concluded from nearly identical absorption characteristics, the crystallization behaviour of both types of specimens is fairly the same, indicating that the presence of dopants does not influence the reordering of the crystal matrix. Also the electrical properties of the doped specimens do not depend on the sequence of preparation steps. The experimental results will be discussed in some detail by considering the structural properties of the polycrystalline SiC films.