P. Roca i Cabarrocas

Substrate selectivity in the formation of microcrystalline silicon: Mechanisms and technological consequences

We report the results of an in situ spectroscopic ellipsometry study concerning the substrate dependence of the evolution of microcrystalline silicon films deposited by alternating amorphous silicon deposition and hydrogen plasma treatment. The evolution of the composition of the films during growth, up to thicknesses of ∼100 nm, indicates that besides etching, the diffusion of atomic hydrogen efficiently promotes the growth (and/or nucleation) of buried crystallites. Moreover, the evolution of the films strongly depends on the nature of the substrate. This substrate selectivity is discussed in terms of initial growth processes. The effect of the hydrogen plasma well below the film surface, which produces the thickness‐dependent film composition, along with the substrate selectivity, may be of prime importance in technological applications of microcrystalline silicon.

Plasma enhanced chemical vapor deposition of amorphous, polymorphous and microcrystalline silicon films

The plasma processes and growth reactions involved in the deposition of amorphous, polymorphous and microcrystalline silicon thin films are reviewed. The reference being a-Si:H deposition through surface reactions of SiH 3 radicals, we study the growth of microcrystalline silicon films produced by the layer-by-layer and standard hydrogen dilution techniques. We show that subsurface reactions play a key role, particularly during the incubation phase where hydrogen is responsible for the formation of a porous layer in which nucleation takes place. The evolution of the film properties is related to the long range effects of hydrogen. Coming back to a-Si:H deposition, we further consider the deposition at low substrate temperature (<200°C) and pressure (<5 Pa) where the role of ions is dominant and at deposition rates where powder formation takes place. We propose that rather than a drawback, nanoparticle formation in silane plasmas might be considered as a potential for obtaining new silicon films. We address in particular the deposition of polymorphous silicon consisting of an a-Si:H matrix with silicon nanocrystallites produced in the gas phase. Despite their heterogeneity polymorphous silicon films have improved transport properties and stability with respect to a-Si:H.

Ion bombardment effects on microcrystalline silicon growth mechanisms and on the film properties

The role of ions on the growth of microcrystalline silicon films produced by the standard hydrogen dilution of silane in a radio frequency glow discharge is studied through the analysis of the structural properties of thick and thin films. Spectroscopic ellipsometry is shown to be a powerful technique to probe their in-depth structure. It allows to evidence a complex morphology consisting of an interface layer, a bulk layer, and a subsurface layer. The ion energy has been tuned by codepositing series of samples on the grounded electrode and on the powered electrode, as functions of pressure and power. On the one hand, reducing the ion energy through the increase of the total pressure and depositing on the grounded electrode, favors the formation of large grains and results in improved bulk transport properties, but leaves an amorphous interface layer with the substrate. On the other hand, we achieve fully crystallized films on glass substrates under conditions of high energy ion bombardment. We suggest th...

Growth and optoelectronic properties of polymorphous silicon thin films

Polymorphous silicon is a nanostructured thin film consisting of a small fraction of nanocrystalline silicon particles and/or clusters embedded in a relaxed amorphous matrix. This heterogeneous material is produced under plasma conditions close to powder formation. The hydrogen bonding in this material was probed by infrared absorption and hydrogen evolution measurements. Surprisingly, the heterogeneous nature of the microstructure has no deleterious effect on the electronic and transport properties of these films; rather the transport and the hole transport in particular are better than those of standard amorphous silicon. These properties suggest polymorphous silicon is an excellent alternative to amorphous silicon. The performance and stability of the material and solar cells are discussed in terms of the hydrogen bonding structures as well as the preparation conditions used to produce the material. The obtaining of highly stable wide gap single junction polymorphous silicon solar cells suggests that polymorphous films are well adapted for the top cell in tandem or triple junction devices.

Stable microcrystalline silicon thin-film transistors produced by the layer-by-layer technique

Microcrystalline siliconthin films prepared by the layer-by-layer technique in a standard radio-frequency glow discharge reactor were used as the active layer of top-gate thin-film transistors(TFTs). Crystalline fractions above 90% were achieved for silicon films as thin as 40 nm and resulted in TFTs with smaller threshold voltages than amorphous siliconTFTs, but similar field effect mobilities of around 0.6 cm2/V s. The most striking property of these microcrystalline silicontransistors was their high electrical stability when submitted to bias-stress tests. We suggest that the excellent stability of these TFTs, prepared in a conventional plasma reactor, is due to the stability of the μc-Si:H films. These TFTs can be used in applications that require high stability for which a-Si:HTFTs cannot be used, such as multiplexed row and column drivers in flat-panel display applications, and active matrix addressing of polymer light-emitting diodes.

Midgap density of states in hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n(+)-i-n(+) structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3-4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material

Contribution of ions to the growth of amorphous, polymorphous, and microcrystalline silicon thin films

The growth of amorphous, microcrystalline, and polymorphous silicon has been investigated by studying the species contributing to the growth and resulting film structure. The surface reaction probability of the radicals and the contribution of ions to the growth have been determined. In a-Si:H deposition by hot wire chemical vapor deposition, the surface reaction probability (β=0.29) of the depositing radical is compatible with SiH3, whereas the surface reaction probability in microcrystalline silicon growth is higher (0.36⩽β⩽0.54). On the contrary, the deposition of amorphous silicon by plasma enhanced chemical vapor deposition indicates the contribution of more reactive radicals than SiH3. The deposition of polymorphous and microcrystalline silicon by plasma is dominated by ions, which can contribute up to 70% of the deposited film. This is attributed to efficient ionization of silane in charge exchange reactions with hydrogen ions. The surface reaction probability in the case of polymorphous silicon deposition (β≈0.30) is intermediate between that of a-Si:H deposition (β≈0.40) and that of microcrystalline silicon deposition (β≈0.20). Etching of amorphous silicon by means of a hydrogen plasma shows that ions may hinder the process.The growth of amorphous, microcrystalline, and polymorphous silicon has been investigated by studying the species contributing to the growth and resulting film structure. The surface reaction probability of the radicals and the contribution of ions to the growth have been determined. In a-Si:H deposition by hot wire chemical vapor deposition, the surface reaction probability (β=0.29) of the depositing radical is compatible with SiH3, whereas the surface reaction probability in microcrystalline silicon growth is higher (0.36⩽β⩽0.54). On the contrary, the deposition of amorphous silicon by plasma enhanced chemical vapor deposition indicates the contribution of more reactive radicals than SiH3. The deposition of polymorphous and microcrystalline silicon by plasma is dominated by ions, which can contribute up to 70% of the deposited film. This is attributed to efficient ionization of silane in charge exchange reactions with hydrogen ions. The surface reaction probability in the case of polymorphous silicon de...

In situ investigation of the growth of rf glow‐discharge deposited amorphous germanium and silicon films

An in situ investigation of the growth of rf glow‐discharge amorphous germanium (a‐Ge:H) and silicon (a‐Si:H) films using fast real‐time spectroscopic phase‐modulated elipsometry is presented. The influence of the conditions of preparation is studied in both cases. The same behavior is obtained in a‐Ge:H and a‐Si:H films deposited in similar conditions. In particular, the initial stage of the growth can be described by a nucleation process in both cases, whatever the conditions of preparation. The incomplete coalescence of the nuclei leads to the formation of a surface roughness on a ∼40‐A scale which is observed during film growth. In comparing real‐time ellipsometry measurements performed at different wavelengths, a correlation between the internuclei distance and the thickness of the surface roughness is observed. An enhancement of the surface mobility of the reactive species due to an increase of substrate temperature and/or ion‐bombardment energy results in an increase in the density of the nucleatio...

In situ investigation of the optoelectronic properties of transparent conducting oxide/amorphous silicon interfaces

Transparent conducting oxide (TCO)/hydrogenated amorphous silicon (a‐Si:H) interfaces are investigated combining kinetic ellipsometry and Kelvin probe measurements. It is shown that the correlation between both in situ techniques allows a detailed description of the optoelectronic behavior of these interfaces. The Schottky barrier at the TCO/a/Si:H interfaces, as revealed by Kelvin probe measurements, is correlated with the chemical reduction of the TCO surface during the early stage of a:Si:H growth, as evidenced by kinetic ellipsometry. In particular, indium tin oxide (ITO) and SnO2 are found to be reduced by the silane plasma at 250 °C. On the countrary, ZnO is found highly resistant upon plasma reduction. The influence of the substrate temperature during a‐Si:H deposition is analyzed. Finally, the technical consequences of this study are outlined.

Determination of the conduction band offset between hydrogenated amorphous silicon and crystalline silicon from surface inversion layer conductance measurements

The presence of an electron rich inversion layer in p-type crystalline silicon (c-Si) at the interface with n-type hydrogenated amorphous silicon (a-Si:H) is experimentally demonstrated from the coplanar conductance of (n) a-Si:H∕(p) c-Si structures, which is shown to be very sensitive to the band mismatch between the two semiconductors. The temperature dependence of the corresponding sheet electron density allows to determine with a very good precision the conduction band offset ΔEC between a-Si:H and c-Si:ΔEC=0.15±0.04eV.