S. Vignoli

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

Very low densities of localized states at the Fermi level in hydrogenated polymorphous silicon from capacitance and space-charge-limited current measurements

The density of states at the Fermi level N(E-F) has been measured on hydrogenated polymorphous (pm-Si:H) silicon samples using both capacitance measurements on Schottky barriers and space-charge-limited current measurements on n(+)/i/n(+) structures. From both techniques, N(E-F) values of 7-8 x 10(14) cm(-3) eV(-1) have been obtained, which is significantly lower than reported in the literature for hydrogenated amorphous silicon (a-Si:H). Such values demonstrate that pm-Si:H is a very low defect density material which should be able to replace a-Si:H in the field of applications like photovoltaics

Some electronic and metastability properties of a new nanostructured material: hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapour deposition in a plasma regime close to that of the formation of powder, a new type of material, called polymorphous silicon (pm-Si), is obtained. We present here the optoelectronic and stability properties of pm-Si films deposited from a mixture of silane diluted with hydrogen at total gas pressures in the range 800-1600 mTorr. A comparison with the properties of standard hydrogenated amorphous silicon (a-Si:H) is made. While some properties of both materials are similar, many others differ in a striking manner. Characterizations of as-deposited pm-Si films show that the best samples exhibit enhanced transport properties, such as the fact that the quantum efficiency-mobility-lifetime product eta mu tau is increased by a factor of 200-700 compared with that measured on a-Si:H under the same conditions. This correlates with a lower density of deep states. The kinetics of creation of defects, performed under 670 mW cm(-2) white light illumination and at a high temperature (100 degrees C) in order to attain a final steady state, have been studied, pm-Si samples exhibit faster kinetics of creation as well as of annealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit eta mu tau products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhanced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embedded in an amorphous matrix.

Structural, optical and electronic properties of hydrogenated polymorphous silicon films deposited at 150°C

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to the formation of powder, a new type of material, called hydrogenated polymorphous silicon (pm-Si:H) is obtained. This material has increased transport properties with respect to device-grade hydrogenated amorphous silicon (a-Si:H). To understand the origin of such improved transport properties, we made electrical measurements from which we deduced that the density of states at the Fermi level N(E-F) and the carrier capture cross-section, sigma(c), in pm-Si:H films are at least 10 times lower and 5 times lower, respectively, than in a-Si:H films. The crystallite sizes deduced from Raman spectra confirm high-resolution transmission electron microscopy measurements. The infrared stretching modes of pm-Si:H films have a band at similar to 2035 cm(-1) which is attributed to hydrogen platelets. The smaller density of states at the Fermi level N(E-F) is explained in terms of improved amorphous matrix as confirmed by optical measurements. We suggest that the low capture cross-section, sigma(c), observed in these films results from a preferential carrier recombination path at grain boundary dangling bonds as predicted by theoretical calculations

Structural properties depicted by optical measurements in hydrogenated polymorphous silicon

Hydrogenated polymorphous silicon (pm-Si:H) is a new material obtained by plasma-enhanced chemical vapour deposition by running the plasma close to powder formation. Preliminary studies have revealed the presence of silicon nanocrystallites embedded in an amorphous matrix but only in a limited range of deposition conditions. In this work we have investigated the structural properties of such films by means of spectroscopic optical measurements. The analysis of transmission spectra in the transparent region has shown that pm-Si:H films have indeed a more ordered structure than state-of-the-art hydrogenated amorphous silicon (a-Si:H) films. This has been observed in the whole range of deposition conditions leading to pm-Si:H films. On a final point the implication of structural properties on the excellent optoelectronic properties previously reported in pm-Si:H films is discussed.

Hydrogen related bonding structure in hydrogenated polymorphous and microcrystalline silicon

We investigate the hydrogen related bonding structure in hydrogenated polymorphous silicon films (pm-Si:H) and hydrogenated microcrystalline silicon. Infra-red spectra reveal some new features for both kinds of films, namely new modes appearing in the stretching band. We propose that this peculiar hydrogen bonding occurs at the surface of crystallites in a platelet-like configuration. Increasing the size of the crystallites increases the size of the platelets so that the compactness of the resulting films decreases as shown by mass density measurements. We show that this peculiar hydrogen bonding is responsible for the low temperature (LT) effusion peak. These results point towards a metastable nature of the crystallites contributing to the growth of pm-Si:H films.

Kinetics of defect creation in hydrogenated amorphous silicon by light pulses

Abstract The creation of metastable defects in hydrogenated amorphous silicon subjected to laser pulses with laser pulse energy density in the range U = (2.5–33) 10−3 J cm−2 has been investigated. After a rise time nearly independent of U, saturation of the defect density N sat is observed, and N sat varies linearly with U in the range of energies considered. A model including the light-induced annealing of defects fits the present experimental data.

Properties of Si:H thin films deposited by rf-PECVD of silane–argon mixtures with variation of the plasma condition

Abstract Structural evolution of hydrogenated silicon (Si:H) films deposited from silane–argon mixtures varying the rf power density in the conventional plasma enhanced chemical vapour deposition (PECVD) process were studied by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Fourier transform infrared (FTIR) absorption and Raman spectroscopy. It has been observed that the evolution is analogous to that seen with hydrogen dilution and the transition from amorphous (a-Si:H) to microcrystalline (μc-Si:H) phase is faciliated by high argon dilution. The results have been explained on the basis of bombardment of the growth surface by excited Ar atoms ( Ar ∗ ) from the plasma.

Determination of the midgap density of states and capture cross-sections in polymorphous silicon by space-charge-limited conductivity and relaxation

A new type of material consisting of an amorphous silicon matrix, in which silicon nanoparticules are embedded, has recently been obtained. This material, named polymorphous silicon (pm-Si), exhibits enhanced transport and stability properties with respect to hydrogenated amorphous silicon (a-Si:H). In order to progress in the understanding of such improved properties, we combine space-charge-limited current and space-charge relaxation measurements which allow us to show that the density of states at the Fermi level and their capture cross sections in pm-Si are at least ten times and five times lower respectively than in a-Si:H. This is in good agreement with photoconductivity results.

Thermoelectric power in undoped hydrogenated polymorphous silicon

Thermoelectric power and conductivity measurements have been made as a function of temperature on a new nanostructured material, hydrogenated polymorphous silicon (pm-Si:H). The thermoelectric power is negative, so electrons are the dominant carriers. The activation energy of the thermopower is less than that of the dark conductivity. However, the short-circuit Seebeck current activation energy agrees with the conductivity-activation energy. These results are consistent with a model involving long-ranged fluctuations at the mobility edges. The magnitude of the fluctuations is larger than that measured in highly-doped hydrogenated amorphous silicon (a-Si:H) in relation to the peculiar structure of pm-Si:H