A. Desalvo

Optoelectronic properties, structure and composition of a-SiC:H films grown in undiluted and H2 diluted silane-methane plasma

a-SiC:H films with energy gap in the range 2.00–2.65 eV have been grown by plasma enhanced chemical vapor deposition in undiluted and H2 diluted SiH4+CH4 gas mixtures, by making use of optimized deposition conditions. A complete picture of structural, compositional, optoelectronic, and defective properties for high quality films has been drawn for the first time. We show that the addition of H2 to the gas mixture leads to a different chemical composition of the deposited films; in particular, carbon incorporation is enhanced and a carbon fraction in the solid matrix up to C/(C+Si)≈0.45 can be obtained. These films have a higher mass density, a reduced microvoid and carbon cluster concentration, a better structural connectivity, and improved optoelectronic properties. For samples with optical gap below 2.4 eV, the reduced defect concentration of H2 diluted films results in an increase of the photoconductivity gain and the steady-state (ημτ)ss values up to two orders of magnitude.

Optical, structural and electrical properties of device-quality hydrogenated amorphous silicon-nitrogen films deposited by plasma-enhanced chemical vapour deposition

Abstract High-electronic quality hydrogenated amorphous silicon-nitrogen (a-Si1-xNx: H) films with an energy gap in the range 1.9-2.7eV have been deposited by plasma-enhanced chemical vapour deposition in silane-ammonia gas mixtures at two different gas residence times and in hydrogen-diluted silane-ammonia gas mixtures. Compositional, structural, electrical and optical properties have been investigated. For the first time the effects of hydrogen dilution of SiH4 + NH3 gas mixtures on the a-Si1-xNx: H network is reported. We have observed that hydrogen dilution decreases hydrogen incorporation and increases nitrogen incorporation, promoting a higher connectivity of the a-Si1-xNx :H network. All films show good electronic properties, comparable with or superior to those of amorphous silicon-carbon films, which are improved in films deposited from hydrogen-diluted gas mixtures.

Wide band-gap silicon-carbon alloys deposited by very high frequency plasma enhanced chemical vapor deposition

The use of very high frequency (VHF) plasma enhanced chemical vapor deposition in a capacitive discharge is investigated to fabricate hydrogenated amorphous silicon carbon alloys, using silane and methane as silicon and carbon precursors, respectively, and hydrogen dilution of the gas mixture. The properties of samples differ significantly from that is normally observed for rf deposition. A wide band-gap material is obtained, with a carbon ratio ranging from 0.2 to 0.63. An energy gap up to 3.4eV is measured, indicating a large sp3 content. The most interesting properties are observed using 90% hydrogen dilution and 350°C as substrate temperature. In this case, a SiC bond concentration up to 6×1022cm−3 was measured for stoichiometric samples, associated to a highly crosslinked structure and no detectable SiCH3 bending signal. The role of hydrogen in determining the optical properties of the film is established and is shown to affect mainly the valence electron concentration. Based on the free energy mod...

Homojunction and heterojunction silicon solar cells deposited by low temperature-high frequency plasma enhanced chemical vapour deposition

Abstract Plasma enhanced chemical vapour deposition (PECVD) is widely used to deposit materials on a variety of substrates at low temperature. However, examples of epitaxial growth on silicon with this technique are scarce. In this paper, we present homojunction silicon solar cells, epitaxially grown by PECVD, and μc-Si/a-Si:H/c-Si heterojunctions deposited with the same technique, manufactured by a completely low temperature process. All cells incorporate an intrinsic buffer layer, whose deposition conditions were varied. It is shown that the best Voc is obtained when the intrinsic layer is deposited under two extreme conditions, i.e. zero or very high (99.4%) hydrogen dilution of the gas mixture, resulting in a totally amorphous or epitaxial i-layer, respectively. Intermediate conditions result in Voc degradation. Efficiencies as high as 13.7% were obtained in planar devices that include an amorphous i-layer, and 13.1% in homojunction devices.

Very high frequency hydrogen plasma treatment of growing surfaces: a study of the p-type amorphous to microcrystalline silicon transition

Abstract The deposition of microcrystalline silicon (μc-Si) in a 100 MHz plasma, in condition close to equilibrium between etching and deposition, is studied. Chemical transport in a pure H 2 plasma is shown to occur in presence of a lower density, a-Si:H precursor layer, and is used to deposit p-type silicon thin (17.5–40 nm) films with microcrystalline fraction >70% for a 17.5 nm thick film, and up to 90% for thicker films, with dark conductivity up to 0.1 S/cm and much better optically measured homogeneity with respect to 100 MHz plasma deposited samples under high dilution (0.5% silane-to-hydrogen flow ratio). Transmission electron microscopy on the 17.5 nm sample shows that crystalline grains extend to the interface. Within the 2 nm detection limit, no continuous interface amorphous layer is detected.

Open circuit voltage in homojunction and heterojunction silicon solar cells grown by VHF-PECVD

We present homojunction and μc-Si/a-Si:H/c-Si heterojunction silicon solar cells fabricated by PECVD. The H 2 dilution used during the i-layer growth strongly affects the device efficiency. While intermediate H 2 dilution of the gas mixture results in V oc degradation, the best V oc is obtained under zero or very high (=99.4%) H 2 dilution, resulting in totally amorphous or epitaxial i-layer respectively. A maximum value of 638 mV, with 13.7% efficiency, is observed in the case of an amorphous i-layer, indicating an improvement of interface quality. If the i-layer is deposited using a 99.4% H 2 dilution, a 608 mV V oc is observed and for homojunction solar cells a 13.1% efficiency is obtained.

Ultrathin μc-Si films deposited by PECVD

Abstract The crystalline fraction of microcrystalline silicon films 18–200 nm thick, deposited by VHF plasma and by chemical transport deposition (CTD) was characterized by Raman and optical measurements. On a p-type CTD sample, thinner than 20 nm, a crystalline fraction as large as 78%, to our knowledge the largest obtained by VHF plasma on p-type films in this thickness range, was measured. Transmission electron microscopy shows crystallites extending to the interface with the substrate. Electrical conductivities in the range 10 −2 –10 0 S/cm, and 10 −1 –10 1 S/cm after annealing at 250°C, were measured. Weak dependence of crystalline fraction and electrical properties on thickness was observed.

Plasma-enhanced chemical vapour deposition of microcrystalline silicon : on the dynamics of the amorphous-microcrystalline interface by optical methods

Abstract Very-high-frequency plasma-enhanced chemical vapour deposition was used to produce p-type microcrystalline samples. Spectroscopic ellipsometry measurements and transmission electron microscopy observations on the deposited samples are compared and discussed. Continuous deposition is observed to result in a growth which is initially amorphous and then evolves to microcrystalline. At this stage, the grains are observed to propagate towards the interface with the substrate. In order to obtain very thin layers, a deposition + hydrogen etching + deposition sequence was also used. This technique produces an increase in the microcrystalline fraction by a factor of more than ten with respect to continuous deposition: a crystalline fraction as large as 48% at the film–substrate interface for a 20 nm film is detected. Electrical measurements are correlated with the sample structure. The dark conductivity confirms the microcrystalline nature of samples, but is also shown to depend on the distribution of the microcrystalline phase.

Laser induced crystallization of hydrogenated amorphous silicon-carbon alloys

Laser induced crystallization of hydrogenated amorphous silicon carbon alloy (a-Si1−xCx:H) films has been investigated by means of synchrotron x-ray diffraction. The a-Si1−xCx:H films were deposited on (100) silicon wafers by very high frequency plasma enhanced chemical vapor deposition at 100MHz in hydrogen diluted silane-methane gas mixtures. The substrate was kept at 250°C or 350°C and the stoichiometry was changed from x=0.20 to 0.63. The structural characterization of the as-grown films has been carried out by Rutherford backscattering (hydrogen concentration) and infrared spectroscopy (film ordering). The films were irradiated by a KrF excimer laser (248nm) with varying energy density and number of pulses. After irradiation, the formation of SiC crystallites has been revealed by synchrotron x-ray diffraction. Besides SiC nanocrystals, the formation of crystalline Si and graphite is observed for under- (x 0.50) samples, respectively. The essential role played by hydro...

Photoinduced absorption in B-doped hydrogenated amorphous silicon alloys applied to all-optical modulators

All-optical modulators have been fabricated, based on the infrared photoinduced absorption produced within an optical waveguide upon visible light illumination. The modulation data are analyzed by means of simulation software based on a numerical mode solver. It is found that the modulation depth increases for pump illumination energy closer to the energy gap of the guiding material, while illumination at varying intensity shows a sublinear dependence of the photoinduced absorption. The results are discussed in terms of occupation statistics of gap states. It is shown that the major contribution to the photoinduced signal derives from the modulation of the occupation of tail states under illumination. Modeling of the phenomenon allows one to predict the behavior of a given device and opens the way to practical applications.