Resonance Plasma-enhanced Chemical Vapor Deposition Research Articles

The Effects of Deposition and Processing Parameters on the Electronic Structure and Photoluminescence from Nitride-Passivated Silicon Nanoclusters

Silicon nanoclusters (Si-ncs) embedded in silicon nitride films have been studied to determine the effect that deposition and processing parameters have on their growth, luminescent properties, and electronic structure. Luminescence was observed from Si-ncs formed in silicon-rich silicon nitride films with a broad range of compositions and grown using three different chemical vapour deposition (CVD) based systems: plasma-enhanced CVD (PECVD), inductively coupled plasma CVD (ICP CVD), and electron cyclotron resonance PECVD (ECR PECVD). Photoluminescence experiments revealed broad, tunable emissions with peaks ranging from the near-infrared across the full visible spectrum. The emission energy was highly dependent on the film composition and changed only slightly with annealing, which primarily affected the emission intensity. X-ray absorption spectra at the Si K- and L3,2-edges exhibited composition dependent phase separation and structural re-ordering of the Si-ncs and silicon nitride host matrix under different post-deposition annealing conditions.

Structural and optical properties of Si nanocrystals embedded in SiO 2/SiN x multilayers

Si nanocrystals (Si-ncs) embedded in SiO 2/SiN x multilayer structures can be of interest for optoelectronic devices such as solar cells. However, controlling the size and density of the Si-ncs is strongly requested for an efficient use in a functioning solar cell device. In this work, SiO 2 and Si-rich Si 3N 4 (SRN) layers have been deposited alternatively on a silicon wafer in an electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) using N 2O, NH 3 and SiH 4 gases. The Si-ncs can be obtained in the SRN layers after thermal annealing, thanks to the phase separation between the Si atoms in excess and the matrix. The SiO 2 layers allow the Si clusters’ spatial confinement in the SRN film and are also used as a barrier of diffusion. The amount of Si atoms in excess can be controlled by the gas flow ratio. The structural characteristics of Si-ncs density have been investigated by conventional TEM and HR-TEM for different silicon nitride layer thicknesses. Their optical properties were studied by photoluminescence (PL) spectroscopy. Results clearly show a room temperature PL in the visible range, which is of interest for the solar cells applications.

Light Emission from Rare-Earth Doped Silicon Nanostructures

Rare earth (Tb or Ce)-doped silicon oxides were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). Silicon nanocrystals (Si-ncs) were formed in the silicon-rich films during certain annealing processes. Photoluminescence (PL) properties of the films were found to be highly dependent on the deposition parameters and annealing conditions. We propose that the presence of a novel sensitizer in the Tb-doped oxygen-rich films is responsible for the indirect excitation of the Tb emission, while in the Tb-doped silicon-rich films the Tb emission is excited by the Si-ncs through an exciton-mediated energy transfer. In the Ce-doped oxygen-rich films, an abrupt increase of the Ce emission intensity was observed after annealing at 1200∘C. This effect is tentatively attributed to the formation of Ce silicate. In the Ce-doped silicon-rich films, the Ce emission was absent at annealing temperatures lower than 1100∘C due to the strong absorption of Si-ncs. Optimal film compositions and annealing conditions for maximizing the PL intensities of the rare earths in the films have been determined. The light emissions from these films were very bright and can be easily observed even under room lighting conditions.

Deposition of Ge-doped silica thin films for an integrated optic application using a matrix distributed electron cyclotron resonance PECVD reactor

Optical quality Ge-doped SiO2 thin films, suitable for an integrated optic version of a gain equalizer for erbium-doped fibre amplifiers (EDFAs), have been deposited using a matrix distributed electron cyclotron resonance plasma-enhanced chemical vapour deposition (MDECR-PECVD) system. Using spectroscopic ellipsometry and infrared transmission spectroscopy, the optical constants and hydroxyl content of the films were calculated. Losses due to the hydroxyl overtone at 1.37μm are found to be approximately 0.251dB/cm. An RBS analysis determined the germanium content of the films to be in the vicinity of 4 at.%. A comparison of the atomic percentage of germanium in the films and their corresponding refractive indices with values obtained using other deposition methods is also discussed.

Electrical conduction of silicon oxide containing silicon quantum dots

Current–voltage measurements have been made at room temperature on aSi-rich silicon oxide film deposited via electron-cyclotron resonance plasmaenhanced chemical vapour deposition (ECR-PECVD) and annealed at750–1000 °C. The thickness of the oxide between Si quantum dots embedded in the film increaseswith increasing annealing temperature. This leads to a decreasing current densityas the annealing temperature is increased. Assuming the Fowler–Nordheimtunnelling mechanism in large electric fields, we obtain an effective barrier heightϕeff of∼0.7 ± 0.1 eV for an electron tunnelling through an oxide layer between Si quantum dots. TheFrenkel–Poole effect can also be used to adequately explain the electrical conduction of thefilm under the influence of large electric fields. We suggest that at room temperature Siquantum dots can be regarded as traps that capture and emit electrons by means oftunnelling.

Large-grained poly-silicon thin films by aluminium-induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si (μc-Si:H) thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) on SiO 2-coated Si wafers has been studied. The starting structure was substrate/μc-Si:H/Al. Annealing this structure at a temperature of 520 °C resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. Grain sizes exceeding ∼60 μm have been achieved in films displaying a preferential (1 0 0) orientation. The length of time the samples are kept under ambient conditions before annealing plays a key role in controlling grain size and orientation. It is likely that this time delay influences the formation of the interface between the Si and Al layers and, hence, the crystallisation process. These poly-Si layers exhibit an average surface roughness ( R a) generally in the range ∼7–12 nm.

Poly-silicon thin films by aluminium induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR–PECVD) on glass and SiO 2 coated Si wafers has been studied. The starting structure was substrate/μc-Si/Al. Annealing this structure in the temperature range 370–520 °C, immediately following deposition of the Al layer, resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. The top poly-Si layer exhibited grain sizes generally in the range ∼2–6 μm, although larger grains were also sparsely present. The films did not exhibit any appreciable degree of preferred orientation. The surface roughness was relatively high with a R a value of ∼20 nm.

Light emission from Si nanoclusters formed at low temperatures

Photoluminescence (PL) from amorphous Si nanoclusters (Si-ncls) formed by thin-film deposition via electron-cyclotron resonance plasma-enhanced chemical vapor deposition followed by annealing at temperatures ⩽875°C has been investigated. We find that Si-ncls grow very slowly after their initial nucleation at low temperatures. An increase in the size of Si-ncls, which can be controlled by the annealing temperature, induces a redshift in the Si-ncl PL peak. While the emitted optical power is more than 100 times smaller than that of Si nanocrystals formed in an identically deposited film, it is increased by a factor of up to approximately four times following hydrogen passivation. The incorporation of hydrogen causes a redshift in the PL peak position, suggesting a partial hydrogenation induced bond distortion of the Si-ncls. This redshift decreases with increasing hydrogen ambient annealing temperature.

Low-temperature electron cyclotron resonance plasma-enhanced chemical-vapor deposition silicon dioxide as gate insulator for polycrystalline silicon thin-film transistors

Silicon dioxide films have been deposited at temperatures below 270°C in an electron cyclotron resonance (ECR) plasma reactor from O2, SiH4, and He gas mixture. Pinhole density analysis as a function of substrate temperature for different microwave powers was carried out. Films deposited at higher microwave power and at room temperature show defect densities (<7pinhole∕mm2), ensuring low-temperature process integration on large area. From Fourier transform infrared analysis and thermal desorption spectrometry we also evaluated very low hydrogen content if compared to conventional rf-plasma-enhanced chemical-vapor-deposited (PECVD) SiO2 deposited at 350°C. Electrical properties have been measured in metal-oxide-semiconductor (MOS) capacitors, depositing SiO2 at RT as gate dielectric; breakdown electric fields >10MV∕cm and charge trapping at fields >6MV∕cm have been evaluated. From the study of interface quality in MOS capacitors, we found that even for low annealing temperature (200°C), it is possible to considerably reduce the interface state density down to 5×1011cm−2eV−1. To fully validate the ECR-PECVD silicon dioxide we fabricated polycrystalline silicon thin-film transistors using RT-deposited SiO2 as gate insulator. Different postdeposition thermal treatments have been studied and good device characteristics were obtained even for annealing temperature as low as 200°C.

H-sensitive radiative recombination path in Si nanoclusters embedded in SiO2

The room-temperature photoluminescence (PL) from silicon nanocrystals embedded in a SiO2 matrix fabricated by electron cyclotron resonance plasma-enhanced chemical vapor deposition and subsequent annealing in Ar and (Ar+5%H2) was studied. In addition to strong increases of the integrated PL intensity (factors of ∼4 to 10), the selective enhancement of contributions to the PL spectra at long wavelengths was observed for (Ar+5%H2) annealings. The selective H passivation of Si dangling bonds in disordered Si nanoclusters where radiative recombination proceeds through disorder-induced shallow states is proposed as a possible explanation for the observed effects.

Low-Temperature Deposition of Hydrogenated Amorphous Silicon in an Electron Cyclotron Resonance Reactor for Flexible Displays

Electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) is investigated as a technique for depositing hydrogenated amorphous silicon (a-Si : H) at a temperature of 80/spl deg/C, which is compatible with the use of transparent, plastic substrates. The ECR-PECVD reactor is described and the principles underlying its operation explained. In particular, the factors controlling the deposition of a-Si : H by this technique are investigated, and it is shown that control of gas phase reactions between silane and hydrogen species is essential. High-quality a-Si : H is deposited in a narrow processing window with a photosensitivity greater than 10/sup 6/. Thin-film transistors (TFTs) fabricated at 125/spl deg/C incorporating low-temperature a-Si : H as the channel layer have a switching ratio of almost 10/sup 5/. With further optimization of the other material layers, such TFTs could be used for the active matrix transistors in flexible liquid crystal displays on plastic substrates.

Gate oxide induced switch-on undershoot current observed in thin-film transistors

The transient drain current of the single-grain silicon thin-film transistor with gate oxide deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition has been measured by applying a square signal on the gate and a constant low voltage between source and drain. Switch-on undershoot current has been observed, which can be attributed to the motion of space charge in gate oxide. Assuming there are some mobile ions in the gate oxide, we find the drift kinetics of the ions is quite similar to the mobile protons in SiO2, as reported in the literature.

Compositional study of silicon oxynitride thin films deposited using electron cyclotron resonance plasma-enhanced chemical vapor deposition technique

We have used backscattering spectrometry and N15(H1,α,γ)C12 nuclear reaction analysis techniques to study in detail the variation in the composition of silicon oxynitride films with deposition parameters. The films were deposited using 2.45GHz electron cyclotron resonance plasma-enhanced chemical vapor deposition (PECVD) technique from mixtures of precursors argon, nitrous oxide, and silane at deposition temperature 90°C. The deposition pressure and nitrous oxide-to-silane gas flow rates ratio have been found to have a pronounced influence on the composition of the films. When the deposition pressure was varied for a given nitrous oxide-to-silane gas flow ratio, the amount of silicon and nitrogen increased with the deposition pressure, while the amount of oxygen decreased. For a given deposition pressure, the amount of incorporated nitrogen and hydrogen decreased while that of oxygen increased with increasing nitrous oxide-to-silane gas flow rates ratio. For nitrous oxide-to-silane gas flow ratio of 5, we obtained films which contained neither chemically bonded nor nonbonded nitrogen atoms as revealed by the results of infrared spectroscopy, backscattering spectrometry, and nuclear reaction analysis. Our results demonstrate the nitrogen-free nearly stoichiometric silicon dioxide films can be prepared from a mixture of precursors argon, nitrous oxide, and silane at low substrate temperature using high-density PECVD technique. This avoids the use of a hazardous and an often forbidden pair of silane and oxygen gases in a plasma reactor.

A method for the analysis of multiphase bonding structures in amorphous SiOxNy films

A tetrahedral model is presented to explain the bonding properties of nonstoichiometric amorphous silicon oxynitride (a-SiOxNy) alloys, grown under highly nonequilibrium conditions, whose structures obey neither the random bonding model nor the random mixture model. Based on our approach, a numerical procedure is proposed to obtain the relative atomic percentages of each component structural phase from the deconvolution of the high-resolution x-ray photoelectron spectroscopy (XPS) spectra in the Si 2p3∕2 region. The tetrahedral model is then used to study the bonding properties of a-SiOxNy films grown by electron-cyclotron resonance plasma-enhanced chemical-vapor deposition, having relatively low values of the O/Si atomic ratio (⩽0.37) incorporated in their networks. The experimental results show that five tetrahedral phases (tetrahedrons Si–Si4, Si–Si2ON, Si–N4, Si–Si3O, and Si–O4) are present in a-SiOxNy films with low N/Si atomic ratios (⩽0.93), while only three phases (Si–SiON2, Si–N4, and Si–O2N2) are present in samples with higher N/Si atomic ratios (⩾1.12). The Si3N4 phase is the most important bonding unit and it is the only phase present in all our samples. These results are corroborated by survey scans and by comparison with the high-resolution XPS spectra in the N 1s region. They support the validity of the model proposed for a-SiOxNy alloys and the XPS analysis, correlated with growth conditions, presented in this work.

Polarization-dependent Raman spectra of thin crystalline silicon films

Polarization-dependent Raman spectra are presented to analyze the structural properties of epitaxial, quasiepitaxial, and microcrystalline silicon films deposited on (100) single-crystal silicon substrates by electron cyclotron resonance plasma-enhanced chemical-vapor deposition. We rotate the sample with respect to the polarization direction of the incident laser light and observe the changes in Raman scattering. The measurements are very effective in differentiating between films with a preferred orientation and those that are randomly oriented or amorphous. An interesting anisotropy has been observed for quasiepitaxial films for which the spectra resemble those of single-crystal wafers under normal measurement conditions. However, at sample orientations where the scattering intensity is minimized, additional structure on the low-energy side of the main silicon TO-phonon peak becomes visible. This is tentatively attributed to scattering from crystalline and amorphouslike grain-boundary regions. Polarization-dependent Raman spectra are able to provide useful information on the structure of these films which would normally be masked by the main silicon optical-phonon peak.

Analysis of stress and composition of silicon nitride thin films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition for microfabrication processes

We investigated the influence of process parameters in electron cyclotron resonance plasma-enhanced chemical-vapor deposition (ECR-PECVD) of silicon nitride on the intrinsic stress of thin SiNx films and on their composition, to obtain SiNx films suitable for micromechanical applications. The silane-to-nitrogen gas flow ratio R, along with the addition of helium to the gas mixture, was found to be a critical parameter for the tuning of intrinsic stress in ECR-PECVD SiNx films, from compressive to tensile stress within a large window of R from 0.3 to 0.7, with a maximum related to the largest Si–N bond density in the film.

New designs for graded refractive index antireflection coatings

The incessant progress in thin layers technology, especially the graded index inhomogeneous dielectrics, allows the realization of antireflection coatings (ARCs). Graded refractive index silicon oxynitrides are deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition (ECR-PECVD) controlled in situ by monochromatic ellipsometry. While avoiding the complexity of the classical multilayer ARCs, graded coatings permit to obtain the same performances, or furthermore to improve solar cells efficiency. A theoretical model, validated by confrontation to ellipsometric spectra, and reflectance measurements were used to optimize different suggested profiles. Calculation predicts an enhancement of photogenerated current exceeding 45% and a weighted reflectance (between 300 and 1100 nm) around 5.6%. On texturized surfaces, these ARCs should enhance short-circuit current by 52.79%.

XPS and NEXAFS characterisation of plasma deposited vertically aligned N-doped MWCNT

This paper is dedicated to carbon nanotubes grown by electron cyclotron resonance plasma-enhanced chemical vapour deposition. It has been shown that carbon nanotubes (CNTs) can be grown at temperature as low as 550 °C by this technique. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy were used to analyse nitrogen-doped CNTs. Independently of the synthesis temperature, about 4 at.% nitrogen are incorporated in the CNTs. A good correlation was found between XPS and NEXAFS results. Four components (397.3, 398.5, 400.8 and 402.5 eV) compose N 1s XPS spectra related to different chemical environments of nitrogen. This interpretation can be used as a simple check method of the presence of CNTs on as-deposited PECVD samples.

Enhancement of the green, visible Tb3+ luminescence from Tb-doped silicon-rich silicon oxide by C co-doping

The effect of C co-doping on the Tb3+ luminescence from Tb-doped silicon-rich silicon oxide (SRSO) films is investigated. Tb-doped SRSO films co-doped with C (SRSO:C) were deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition. The Tb3+ photoluminescence intensity is enhanced by the presence of nanocluster Si (nc-Si), and C co-doping further increases the Tb3+ photoluminescence intensity by more than an order of magnitude. The maximum enhancement is observed at the C content of ∼5 at. %, at which the Tb3+ luminescence is bright enough to be observed by the naked eye under ambient conditions. The 543 nm Tb3+ lifetimes were in the range of 0.5–1.2 ms, comparable to those from Tb-doped silica. Based on the results, we conclude that nanometer-sized nc-Si can excite Tb3+ ions via an Auger-type energy transfer, and that C co-doping greatly increases the efficiency of such exciton-mediated excitation of Tb3+.

Enhanced formation of luminescent nanocrystal Si embedded in Si/SiO2 superlattice by excimer laser irradiation

The effect of excimer laser annealing on the formation of luminescent nanocrystal Si (nc-Si) embedded in Si/SiO2 superlattice is investigated. An amorphous Si/SiO2 superlattice consisting of 20 periods of 2 nm thin Si layers and 5 nm thin SiO2 layers was deposited on Si using electron cyclotron resonance plasma-enhanced chemical vapor deposition. Excimer laser annealing alone did not result in any nc-Si luminescence even at an energy density sufficient to melt the Si layers. However, if the nc-Si is preformed by a thermal anneal, subsequent excimer laser annealing will result in a threefold increase of the nc-Si luminescence intensity. The temperature dependence of the nc-Si luminescence spectrum, lifetime, and intensity indicates that excimer laser annealing activates luminescent nc-Si by removing defects and amorphous regions in thermally crystallized Si layers without significant changes in the size or shape of nc-Si.