Nonthermal plasmas have emerged as a viable synthesis technique for nanocrystal materials. Inherently solvent and ligand-free, nonthermal plasmas offer the ability to synthesize high purity nanocrystals of materials that require high synthesis temperatures. The nonequilibrium environment in nonthermal plasmas has a number of attractive attributes: energetic surface reactions selectively heat the nanoparticles to temperatures that can strongly exceed the gas temperature; charging of nanoparticles through plasma electrons reduces or eliminates nanoparticle agglomeration; and the large difference between the chemical potentials of the gaseous growth species and the species bound to the nanoparticle surfaces facilitates nanocrystal doping. This paper reviews the state of the art in nonthermal plasma synthesis of nanocrystals. It discusses the fundamentals of nanocrystal formation in plasmas, reviews practical implementations of plasma reactors, surveys the materials that have been produced with nonthermal pla...

Nonthermal Plasma Synthesis of Nanocrystals: Fundamental Principles, Materials, and Applications

In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The ...

Crystal size and temperature measurements in nanostructured silicon using Raman spectroscopy

The deposition of hydrogenated microcrystalline silicon (µc-Si:H) at a relatively high working pressure is performed using a conventional radio-frequency plasma-enhanced chemical vapor deposition method. Correlation of the deposition rate and crystallinity with deposition parameters, such as working pressure, flow rate, dilution ratio and input RF power, are studied. It was found that the deposition rate exhibits a maximum at around 4 Torr and that the crystallinity of films decreases monotonically with increasing pressure. The combination of SiH4 depletion and high working pressure in the plasma is necessary to improve the crystallinity of films deposited at a high rate. Consequently, a high deposition rate of 9.3 A/s is achieved with high crystallinity and low defect density.

http://iopscience.iop.org/article/10.1143/JJAP.37.L1116/pdf

High rate deposition of microcrystalline silicon using conventional plasma-enhanced chemical vapor deposition

Hydrogenated amorphous silicon (a-Si:H) has been used for decades—doped and as intrinsic absorber layers—in thin-film silicon solar cells. Whereas their effiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to a better understanding of the interfaces, allowing for their specific engineering. In this review, we briefly resume the state-of-the-art of a-Si:H solar cell technology from growth and characterization of single layers to full solar cells and multijunction devices. Focusing on the absorber layer quality first, we highlight thereafter aspects of interface problematics and discuss the growth and role of doped microcrystalline silicon-oxide layers and approaches of 3D-solar-cell designs in more detail. Although the findings summarized in this review were obtained from thin-film solar cells, we show that a-Si:H is a very versatile material with properties that are of high interest for application in other devices such as heterojunction solar cells, detectors, or optoelectronic devices.

Review: Progress in solar cells from hydrogenated amorphous silicon

Thin film silicon is an attractive and versatile material for photovoltaics whose manufacturing reached a high level of maturity. Owing to its moderate efficiency compared to crystalline technologies, it should target either power plants with low installation cost or applications with added value like building-integration. Since the technology relies on very thin films of a weakly absorbing material, light-management is, and always has been, a key aspect of the technology. In this review, we briefly describe the class of materials that is summed up under the name “thin film silicon”, point out requirements on device design, and discuss functionalities that enhance the absorption in the silicon films, addressing both their theoretical understanding as well as experimental realization.

Light management in thin film silicon solar cells

We present a study on the development and the evaluation of a fully automated radio‐frequency glow discharge system devoted to the deposition of amorphous thin film semiconductors and insulators. The following aspects were carefully addressed in the design of the reactor: (1) cross contamination by dopants and unstable gases, (2) capability of a fully automated operation, (3) precise control of the discharge parameters, particularly the substrate temperature, and (4) high chemical purity. The new reactor, named ARCAM, is a multiplasma‐monochamber system consisting of three separated plasma chambers located inside the same isothermal vacuum vessel. Thus, the system benefits from the advantages of multichamber systems but keeps the simplicity and low cost of monochamber systems. The evaluation of the reactor performances showed that the oven‐like structure combined with a differential dynamic pumping provides a high chemical purity in the deposition chamber. Moreover, the studies of the effects associated with the plasma recycling of material from the walls and of the thermal decomposition of diborane showed that the multiplasma‐monochamber design is efficient for the production of abrupt interfaces in hydrogenated amorphous silicon (a‐Si:H) based devices. Also, special attention was paid to the optimization of plasma conditions for the deposition of low density of states a‐Si:H. Hence, we also present the results concerning the effects of the geometry, the substrate temperature, the radio frequency power and the silane pressure on the properties of the a‐Si:H films. In particular, we found that a low density of states a‐Si:H can be deposited at a wide range of substrate temperatures (100 °C≤Ts≤300 °C).

/pdf/a-fully-automated-hot-wall-multiplasma-monochamber-reactor-p36v7yi0pb.pdf

A fully automated hot-wall multiplasma-monochamber reactor for thin film deposition

Polycrystalline silicon thin films have been deposited at fast growth rates (50 A/s) by hotwire chemical vapour deposition (HW-CVD) from SiH4/H2 gas mixtures at low substrate temperature (400–500°C). The surface morphology of these films consists of 0.5 – 2.0μm dendritic grains as seen by electron microscopy. The films have a columnar morphology with grains starting from the substrate either on glass or c-Si. Even the 150 nm thick initial layer is polycrystalline. The preferential crystalline orientation of the poly-Si film is apparently not governed by the radiative source but strongly depends on the type and orientation of the substrate. A strong hydrogen dilution (>90%) of silane is essential to obtain poly-Si films with optimal crystalline structure.

Fast Deposition of Polycrystalline Silicon Films by Hot-Wire CVD

The uses of diborane, trimethylboron and trimethylgallium gases have been systematically compared in order to obtain p-type hydrogenated amorphous silicon grown in silane rf glow discharges. The doping properties and the contamination effects due to the thermal CVD have been investigated by an in situ Kelvin probe. This study, which has been completed by electrical (dark conductivity, activation energy and fill factor of standard p-i-n devices), and optical (a combination of transmission and photothermal deflection spectroscopy) measurements, indicates that trimethylboron doped layers have semiconducting properties similar to those of diborane doped layers. When trimethylboron is used, the contamination is shown to be reduced by at least a factor 50. In contrast, trimethylgallium, despite its acceptable doping efficiency, produces a contamination intermediate between the diborane and the trimethylboron ones. The effects of C incorporation in the doped layers have also been studied, in particular by optical absorption measurements in the band-edge region.

Hydrogenated amorphous silicon p-doping with diborane, trimethylboron and trimethylgallium

A hot‐cathode silane dc discharge with electrostatic confinement devoted to the deposition of hydrogenated amorphous silicon is analyzed by using a Langmuir probe The electron energy distribution of the plasma shows the existence of two pseudo‐Maxwellian populations with densities depending on the electrostatic confinement The electron density of electrostatically confined plasma is higher than in the case of an unconfined one In the electrostatically confined plasma, the hydrogenated amorphous silicon deposition rate is high (033 nm/s at 6 μbar)

Distribution of electron energy in an electrostatically confined silane plasma

Hydrogen content, transport properties and light degradation of a-Si: H films containing artificially generated interfaces

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