T. Mates

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.

Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy

Abstract We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures (∼80 ° C ) . Coincidence of the maxima in surface roughness and ambipolar diffusion length ( ≳100 nm ) with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries.

Internal structure of mixed phase hydrogenated silicon thin films made at 39°C

A combined cross-sectional transmission electron microscope (XTEM) and atomic force microscope (AFM) study of a hydrogen to silane dilution series of thin silicon films deposited by very high frequency plasma enhanced chemical vapor deposition at a substrate temperature that is low enough (39°C) to neglect the role of the surface diffusion in the growth process is reported. XTEM images of a mixed amorphous/microcrystalline layer reveal a structure of isolated conically shaped crystalline conglomerates (surface diameter ∼570±75nm) embedded in an amorphous phase of columns with diameters of ∼51±3nm. Detailed closeups of these crystallites, combined with AFM images of the hydrogen dilution dependent evolution of the surface, reveal similarities between the nucleation of amorphous and crystalline columnar structures at this low substrate temperature.

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.

Rapid crystallization of amorphous silicon at room temperature

Abstract A way in which thin films of hydrogenated amorphous silicon (a-Si: H) can be instantaneously crystallized at room temperature is reported. The metal-induced solid-phase crystallization (MISPC) method with nickel surface coverage is used. In comparison with previous reports on the MISPC of a-Si: H, the crystallization temperature is reduced by more than 350°C. This is achieved by introducing two novel technological steps: firstly, we use hydrogen-rich a-Si: H films (hydrogen content between 20 and 45at.% H) and, secondly, we apply a high transverse electric field. Polycrystalline silicon islands as large as 3 mm across appear instantaneously after having reached a threshold electric field of about 105Vcm−1. We report macroscopic visualization of the crystallization process as well as microscopic investigation (micro-Raman measurements and scanning electron microphotography) of the crystallized films. We have found that appropriate patterning of the nickel electrode helps to increase homogeneity of the resulting polycrystalline 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.

Influence of combined AFM/current measurement on local electronic properties of silicon thin films

Abstract Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by plasma enhanced chemical vapour deposition (PECVD) and transferred without breaking vacuum into an ultra high vacuum ( 10 −10 mbar ) atomic force microscope (AFM). The AFM is used to characterize surface morphology and electronic properties with high lateral resolution. This combined AFM/current measurement leads to a modification of the local electronic properties. This modification is detected as a significant decrease in local current compared to that in newly scanned regions. Kelvin probe microscopy shows that the contact potential difference is reduced by 0.25 eV in the area with decreased conductivity. This area with decreased conductivity remains on the surface for at least several hours. When the cantilever is held at a particular point on the surface, the current exhibits a long-term decay ( ≈10 min ). It is suggested that may be due to a local change of a gap state distribution.

Patterning of hydrogenated microcrystalline silicon growth by magnetic field

A way of influencing growth of silicon films by magnetic field is demonstrated. Permanent magnet(s) placed under the substrate influenced the discharge in a mixture of silane and hydrogen and led to formation of microcrystalline regions in otherwise amorphous film. The pattern of microcrystalline regions varied with the orientation of the magnetic field. Microscopic study by atomic force microscopy and by micro-Raman spectroscopy revealed that the microcrystalline regions resulted from a higher density of crystalline grain nuclei, increased at the locations where the magnetron effect could be expected. This phenomenon could be used to study the transition between amorphous and microcrystalline growth. Moreover, we suggest it as a kind of “magnetic lithography” for the preparation of predefined microcrystalline patterns in otherwise amorphous silicon films.

Microscopic Aspects Of Charge Transport In Hydrogenated Microcrystalline Silicon

Charge transport in hydrogenated microcrystalline silicon (µc-Si:H) is determined by structure on several size scales: i) local atomic arrangement (