F. Zignani

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.

The influence of hydrogen dilution on the optoelectronic and structural properties of hydrogenated amorphous silicon carbide films

Abstract Amorphous silicon carbide films were deposited by the plasma-enhanced chemical vapour deposition technique in SiH4-CH4-H2 gas mixtures and the effect of hydrogen dilution on the optoelectronic properties investigated using photo-thermal deflection spectroscopy, photoconductivity and dark electrical conductivity, photoluminescence and Fourier transform infrared spectroscopy. Large H2 dilution leads to materials of improved quality whose E g is about 2.0eV. The materials were also incorporated into a solar cell device structure to confirm our conclusions.

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.

Ion implantation and hydrogen passivation in amorphous silicon films

Abstract Films of hydrogenated amorphous silicon deposited by rf glow discharge have been implanted with P, B and Si ions in order to study the effect of doping by ion implantation and the effects of implantation-induced damage. The films were either subsequently annealed by a conventional low-temperature thermal treatment (260 ° C) or further implanted with 640-eV H ions by a Kaufmann ion source in order to neutralize the negative effect of the damage induced by the implantation process on the electrical and optical characteristics of the films. The results show that only a partial recovery occurs upon annealing whereas post-implant hydrogenation efficiently neutralizes the residual damage and improves the photoconductivity of the films.

Boron doping and hydrogenation by ion implantation of a-Si:H

Abstract We have studied the effects of ion implanting boron into glow-discharge-deposited a-Si:H. Electrical activity more than two orders of magnitude higher than previously reported is measured in our samples. Implantations of Si ions are used to study the effect of post annealing on the radiation damage. Hydrogen introduced by low energy implantation and diffusion is found to completely recover electrical and optical characteristics in Si-implanted specimens even at the highest concentrations (10 21 /cm 3 ), where annealing for 1 h at 260 °C was insufficient. Introduction of H in B-implanted samples was found to de-activate the boron, which can be re-activated by low temperature annealing.

Doping of amorphous silicon by potassium ion implantation

Abstract Highly efficient n-type doping of hydrogenated amorphous silicon films has been achieved by ion implantation of potassium. A high dark conductivity, after implantation and annealing at the optimal temperature, could be obtained over the entire range of concentrations investigated (1018−4 × 1020cm−3). Post-hydrogenation with a Kaufmann ion source produced an increase in conductivity by a factor of up to four. The effect is opposite to that found in phosphorus-implanted samples, in which a decrease in dark conductivity after post-hydrogenation was observed. The highest conductivity obtained without microcry-stallization of the film was 1·6 × 10−l Ω−1 cm−1. The conductivities of potassium-doped samples were higher than those of phosphorus-implanted samples by two orders of magnitude at high concentrations and four orders of magnitude at Iow concentrations. Thermal equilibration experiments were carried out on both annealed and post-hydrogenated samples which confirmed that the eauilibration behaviou...

Photoluminescence and photothermal deflection spectroscopy in potassium doped a-Si:H

Abstract Highly efficient n-type doping of a-Si:H films can be achieved by potassium implantation. Conductivities in the dark are two to four orders of magnitude larger than in the case of phosphorus doping. Moreover, an increase of up to a factor of four is detected when more hydrogen is introduced in the potassium doped samples. For high dose implantations (≥10 20 atoms / cm 3 )1.3 eV spectral photoluminescence, carried out at 77 K, shows peaks of about a factor of three larger in potassium implanted films than in the phosphorus implanted ones at the same dose. If P and K implanted samples with the same electrical conductivity are compared, the same amount of disorder is detected by photothermal deflection spectroscopy measurements, but the photoluminescence of the K implanted samples is more than one order of magnitude higher in the K doped samples than in the P doped ones. The effect is tentatively interpreted in terms of dependence of photoluminescence quenching on total impurity concentration.