H. Stuchlíková

Basic features of transport in microcrystalline silicon

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed.

Model of transport in microcrystalline silicon

Abstract Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.

Optical and Electrical Properties of Undoped Microcrystalline Silicon Deposited by the VHF-GD with Different Dilutions of Silane in Hydrogen

Note: IMT-NE Number: 237 Reference PV-LAB-CONF-1997-001 Record created on 2009-02-10, modified on 2017-05-10

The structure and growth mechanism of Si nanoneedles prepared by plasma-enhanced chemical vapor deposition

Silicon nanowires and nanoneedles show promise for many device applications in nanoelectronics and nanophotonics, but the remaining challenge is to grow them at low temperatures on low-cost materials. Here we present plasma-enhanced chemical vapor deposition of crystalline/amorphous Si nanoneedles on glass at temperatures as low as 250 °C. High resolution electron microscopy and micro-Raman spectroscopy have been used to study the crystal structure and the growth mechanism of individual Si nanoneedles. The H(2) dilution of the SiH(4) plasma working gas has caused the formation of extremely sharp nanoneedle tips that in some cases do not contain a catalytic particle at the end.

Amorphous/microcrystalline silicon superlattices—the chance to control isotropy and other transport properties

Preparation of amorphous silicon/microcrystalline silicon superlattices allowed us a systematic study of transition from isotropic amorphous silicon to microcrystalline silicon with anisotropic (columnar) microstructure. The fact that just a few nm of amorphous interlayers are sufficient to interrupt columnar growth of crystallites is reflected in a clearly demonstrated isotropy of transport properties of all superlattice samples. Values of dark conductivity and diffusion length as well as grain size vary with changing crystallinity and so we can tailor the properties of the resulting material by adjusting thicknesses of amorphous and microcrystalline layers repeated to achieve a total desired thickness. Properly selected design of superlattice can lead to transport properties more suitable for solar cells than with pure microcrystalline silicon.

Microcrystalline silicon -- Relation of transport properties and microstructure

Understanding of transport in hydrogenated microcrystalline silicon ({micro}c-Si:H) is difficult due to its complicated microstructure (grains, grain boundaries, amorphous tissue). {micro}c-Si:H layers often exhibit preferential orientation leading to transport anisotropy. Furthermore, specific {micro}c-Si:H growth features lead to the thickness dependence of the structure and properties. {micro}c-Si:H incubation layer was studied by AFM with conductive cantilever measuring simultaneously morphology and local conductivity maps with submicron resolution. Clear identification of Si crystallites (with size of few tens of nanometers) is demonstrated. The crystalline fraction at the surface may be easily evaluated. For the charge collection in solar cells they need to study transport perpendicular to the substrate. Measurement of frequency spectra of A.C. conductivity is introduced as a new tool which can exclude the influence of contact barriers in sandwich geometry and can be used for finding the true conductivity perpendicular to the substrate. Using this technique transport anisotropy in some {micro}c-Si:H samples was clearly demonstrated. Finally, it is shown how the transport properties change with growing {micro}c-Si:H thickness and how these changes correlate with the structure observed by AFM.

Charge transport in microcrystalline Si – the specific features

Abstract The microcrystalline hydrogenated silicon has one clear advantage – stability against light-induced changes – but rather complicated growth and microstructure. As a result there are many problems and specific features of transport in microcrystalline silicon. These features – like inhomogeneity, anisotropy, influence of substrate and thickness dependence are discussed in detail.

New method of drift mobility evaluation in μc-Si:H, basic idea and comparison with time-of-flight

We illustrate problems of measurement of the drift mobility by time-of- flight method on microcrystalline silicon. A new method, based on charge extraction in linearly increasing voltage, is introduced. Simple theory as well as numerical modelling of this new method are presented together with the first experimental results on microcrystalline silicon.

Importance of the transport isotropy in μc-Si:H thin films for solar cells deposited at low substrate temperatures

The influence of the substrate temperature during μc-Si:H deposition on the material structure and optoelectronic properties was explored in the range from 150 to 350 °C. The low temperature material is especially interesting with regard to the suppressed oxygen-related donor formation and high efficiency of the resulting solar cells. Surprisingly, in this material, conductivity is the same parallel and perpendicular to the substrate (as measured by dc and ac techniques). The same is true for the ambipolar diffusion length which was measured by steady-state photocarrier grating (SSPG) (L∥) and by surface photovoltage (SPV) (L⊥) methods. Finally, the relevance of the SPV method extended to the measurement in complete thin film solar cell structures is demonstrated.