Influence of Microstructure on the Electronic Transport Behavior of Microcrystalline Silicon Films

Team Members: (Dept. of Physics, Indian Institute of Technology Kanpur, India)
Prof. Satyendra Kumar
Dr. Sanjay K. Ram
Our Collaborators:
Dr. Pere Roca i Cabarrocas (CNRS Research Director, LPICM, UMR-7647 CNRS, Ecole Polytechnique, France)


Theme:

Plasma deposited hydrogenated microcrystalline silicon (µc-Si:H) offers the possibilities of high carrier mobilities and stability against light and current induced degradation along with an ease of large area processing capabilities making it attractive for use in solar cells and thin film transistors. However, µc-Si:H material is heterogeneous in nature consisting of crystalline and amorphous phases with presence of density deficient regions. It does not have a unique microstructure due to the processing history. Moreover, optoelectronic properties being intimately linked with the detailed nature of film microstructure makes it difficult to compare the results obtained from various laboratories. Though µc-Si:H has been studied for well over two decades, the presence of significant disorder in terms of size and shape variations in crystallites (grains) and the nature of amorphous or disordered phase (boundaries) complicates a comprehensive description of the optoelectronic properties in this heterogeneous material. In particular, little is known about the recombination mechanisms and the nature of the density of gap states.
Our group's research has been focussed on the issues of correlation between the microstructural characteristics and optoelectronic properties of plasma deposited single-phase microcrystalline silicon. I briefly describe below the specific issues regarding the various aspects of structural and transport properties of µc-Si:H, and the outcome of our research.




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Film Deposition:
The hydrogenated microcrystalline silicon films were fabricated at Laboratoire de Physique des Interfaces et des Couches Minces at Ecole Polytechnique in France. The µc-Si:H films were prepared by a parallel plate PECVD reactor operating at 13.56 MHz RF using a mixture of SiF4, H2 and Ar. Stainless steel chamber was evacuated down to a base pressure of ~1x10-6 Torr before flushing it with Ar for a sufficient time. Cleaned Corning 1737 or c-Si substrates were used for film deposition at the desired substrate temperature. A brief hydrogen plasma exposure was given before deposition. Deposition conditions were optimized to obtain high crystallinity in the samples as characterized by in-situ ellipsometry.
To create a large microstructural variety of film, the µc-Si:H samples were prepared under some fixed deposition parameters while other parameters such as gas flow ratio and substrate temperature were varied. As the microstructure of µc-Si:H is known to vary as a function of deposition time, films of different thicknesses at the same deposition conditions were prepared for this study. The microcrystalline silicon films used for this study can be clubbed into three sets of samples in the following way:
Thickness series: Samples prepared using a particular flow ratio (denoted by R = SiF4 / H2), a constant substrate temperature (Ts = 200oC), analyzed at different stages of growth, and hence, having different thicknesses. Three different flow ratios (R values) were used to produce this set of ‘thickness series’.
R series: This set consists of films produced with different flow ratios, for a particular thickness range, with substrate temperature remaining constant (Ts = 200 oC).
Ts series: The samples of this set were deposited at different substrate temperatures, with constant gas flow ratio (R = 1/5).

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Structural characterization of single-phase undoped µc-Si:H
Although microcrystalline silicon has been studied for many decades, the controversies regarding the various aspects of the material are numerous. The main reason for this is that plasma deposited µc-Si:H is not a unique material in terms of its microstructure and contents of its constituent phases: crystallites, amorphous tissues and density deficit. Moreover, the crystallites themselves may have a distribution in sizes, shapes and orientation. The quantitative analysis of the µc-Si:H microstructure is difficult and often ambiguous due to presence of microstructural features and disorder at different length scales. Therefore, it requires a variety of characterization tools at different length scales for a complete picture of the material. A consistent microstructural characterization of the material with reliable tools is essential before investigating and understanding the electronic transport in the material.
Raman scattering (RS) is a non-destructive optical characterization tool that is extensively used to determine the crystalline nature and the crystallite size in nano- or micro- crystalline silicon. An issue that has been an important part of our research is that while the inherent distribution in the sizes of crystallites (CSD) in plasma deposited µc-Si:H is well-recognized, it is not taken into account while deconvoluting the Raman spectra of the material.

In literature, Gaussian deconvolution of the RS profile of µc-Si:H is typically performed using three components/peaks: a broad peak at lower wave number ~ 480 cm-1 characterizing the TO mode in a-Si:H, a second peak at ~ 520 cm-1 corresponding to TO mode in crystalline silicon (c-Si) and a third peak, basically the intermediate component at ~510 cm-1 (in the range of 505-515 cm-1) arising due to the bond dilation at the grain boundaries and is attributed to disorder phase/ grain boundary region. Thus, RS data is conventionally analyzed to obtain the mean crystallite size only and the existing deconvolution methodology does not take the CSD into account. The inclusion of a non-existent amorphous phase in the deconvolution of RS profiles of single-phase µc-Si:H, and ignoring the size distribution of crystallites may lead to errors in the estimation of the total crystallinity (false lowering) and an over-estimation of the mean crystallite size.
The results of spectroscopic ellipsometric (SE) studies carried out on our wide microstructural variety of µc-Si:H films demonstrated the presence of two distinct mean sizes of crystallites in the films after a certain growth stage, depending on the H2 dilution level and other deposition factors. This further substantiates the rationale of taking into account a crystallite size distribution while analyzing microstructural data. Our microstructural studies of the material shows that the percentage volume fractions of the large and small crystallite grains vary with film thickness, and deposition parameters. In our single-phase material, having no distinguishable amorphous phase, the presence of two mean sizes of crystallites demonstrated by RS, SE and X-ray diffraction assumes significance not only from a structural point of view, but also in context of the optoelectronic properties.

Therefore, in our microstructural studies of plasma deposited highly crystallized single-phase undoped µc-Si:H, a deconvolution model incorporating the distribution in the sizes of crystallites has been applied to explain the experimentally observed RS profiles. The deconvolution of RS profiles distinguishes two distinct large and small-sized crystallites in the material. Our study demonstrates that in the absence of an amorphous phase, the asymmetric low frequency tail in RS profiles of single-phase µc-Si:H can indicate the presence of a distribution of nanocrystallites. The fractional compositional analysis of the films obtained by this methodology are found to be in qualitative agreement with the findings of other microstructural studies and the overall microstructural picture that emerges from the results of spectroscopic ellipsometry, atomic force microscopy and X-ray diffraction is found to be self-consistent.



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Dark electrical transport in single-phase undoped µc-Si:H
The heterogeneous nature of µc-Si:H makes the study and characterization of its electrical properties complicated. In mixed-phase µc-Si:H material consisting of predominantly small crystallites, the electrical transport is influenced and dominated by the amorphous phase, while in highly crystallized µc-Si:H material with large crystallites, electrical properties similar to c-Si are observed. Therefore, it is not surprising that the electronic transport in µc-Si:H films has been variously claimed to be analogous to that observed in a-Si:H films and poly-silicon (poly-Si) films. An additional complexity regarding the transport routes in single-phase µc-Si:H is that an interconnected percolation network formed by the columnar boundary tissues acts as a conducting path, while in absence of such a well-connected network, the disorder tissues act as barriers to the charge carriers that pass between the conducting grains/ elements. In the conductivity studies in µc-Si:H, different transport mechanisms and routes have been implicated to explain the various data gathered on some limited or partial aspects of electrical transport in this material. A comprehensive picture of electrical transport and its dependence on the widely variegated film microstructure in µc-Si:H still eludes us.

Earlier works have reported on the correlation between electronic transport properties and the microstructural attributes of µc-Si:H films such as crystallinity, crystallite size, crystalline orientation and conglomerate crystallite sizes. These correlations have been explained using models that invoke potential barriers at grain boundaries (GB) and percolation. Many studies have been devoted to understanding how variation in film thickness and deposition parameters (e.g., H2 dilution, substrate temperature, etc.) can influence transport properties and can be used to tune them.

In single-phase µc-Si:H materials, where the crystallinity is constant, and the ‘amorphous phase’ is absent, the transport in the material is not easily understood in terms of rising crystallinity or amorphous content. The parameters such as film thickness, feed gas flow ratios, substrate temperature, etc., basically influence the film microstructure, and consequently may alter the transport routes or mechanisms or properties. Therefore, a single deposition parameter in isolation may not have a unique causal relationship with any specific film microstructure, as the same microstructure can be achieved by the adjustment of one or more deposition parameters. In such a situation, the question arises, what then can be such a microstructural parameter, that is well-correlated to change in deposition conditions and the electrical transport as well. It follows then, that there must exist one or more unifying microstructural characteristics resulting from the alteration in deposition parameters, independent of film thickness, which influence the electrical properties.

Our microstructural studies using Raman spectroscopy have shown that our plasma deposited single-phase µc-Si:H material has a bimodal distribution of crystallite sizes. The large and small crystallite grains (LG and SG) show significant variation in their percentage volume fractions with film growth, depending on the deposition conditions. But generally, a rising trend of fractional composition of LG (Fcl, determined from spectroscopic ellipsometric studies) and evolution of conglomerates is observed with film growth.

Our results show that the variety of µc-Si:H films can be empirically segregated into some categories on the basis of the Fcl values, and this classification reflects some common microstructural and electrical transport properties shared by eachtype’,which we found useful from a practical standpoint for the systematic study of the microstructurally varied µc-Si:H materials. Understanding the film microstructure in terms of three broad categories with some definite microstructural features and unique transport properties helps in a better elucidation of the underlying transport routes and mechanisms in each case.

To summarize this classification, the type-A films have small grains, low amount of conglomeration (without column formation), and high density of inter grain boundary regions containing disordered phase. In this type, Fcl is less than 30% and σ0 and Ea are constant [~103 (Ωcm)-1and ~ 0.55 eV respectively]. The type-B films contain a fixed ratio of mixed grains in the bulk. There is a marked morphological variation in these films due to the commencement of conglomeration of grains resulting in column formation, and a moderate amount of disordered phase is present in the columnar boundaries. Here Fcl varies from 30% to 45% and there is a sharp drop in σ0 [from ~103 to 0.1 (Ωcm)-1] and Ea (from ~ 0.55 to 0.2 eV). The type-C µc-Si:H material is fully crystallized, crystallite conglomerates are densely packed with significant fraction of large crystallites (>50%) and preferential orientation is seen. Here σ0 shows a rising trend [from 0.05 to 1 (Ωcm)-1] and the fall in Ea is slowed down (from 0.2 to 0.10 eV).

The significance of this classification is that where film thickness (films of same thickness can have very different microstructures), total crystalline volume and deposition parameters fail to correlate to transport properties in any systematic way, the classification based on Fcl used in our studies provides consistent correlation that can be explained using known models.




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MNR and Anti-MNR in single-phase
undoped µc-Si:H
Meyer-Neldel Rule (MNR) is a well-known phenomenon seen in many thermally activated processes, including electronic conduction in amorphous or disordered semiconductors, where it correlates exponentially the conductivity prefactor (
σ0) and the conductivity activation energy (Ea) with the equation: σ0 = σ00exp(G.Ea), where G and σ00 are called MN parameters. Often G-1is denoted as EMN, the Meyer-Neldel characteristic energy. Various theories have been put forward for explaining the observed MNR in amorphous silicon, the most popular among these being the model invoking a statistical shift of Fermi-level (Ef) with temperature. Apart from MNR, another interesting and important phenomenon is the anti-MNR, in which a negative value of EMN is seen. Anti MNR, has been reported in heavily doped µc-Si:H and heterogeneous Si thin film transistors. This phenomenon has been explained by the Ef moving deep into the band tail.

In our studies of electrical conductivity behavior of highly crystallized undoped µc-Si:H films having different microstructures, the dark conductivity was seen to follow MNR in some films and anti MNR in others, depending on the details of microstructural attributes and corresponding changes in the effective density of states distributions. A band tail transport and statistical shift of Fermi level were used to explain the origin of MNR as well as anti-MNR in our samples.

We further analyzed a sizeable amount of reported experimental transport data of µc-Si:H materials to establish evidence of MNR and anti-MNR and demonstrate the consistency and physical plausibility of statistical shift model in explaining these phenomena. We have derived well-substantiated and generalized values of Emin, σM, and values of σ0 and Ea where γf = 0 and γf = γc, which hold true for a wide microstructural range of µc-Si:H system, and can further add to our understanding of the electrical transport in this heterogeneous material.


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Low temperature dark electrical transport in single-phase undoped µc-Si:H

The dark electrical conductivity
σd(T) of µc-Si:H is intricately linked to the film microstructure. Different transport models are used to explain conduction at low temperatures, depending on the observed temperature dependence (T -1/2 or T -1/4) of σd. Our studies demonstrate that at low temperatures (300–15 K), dark conductivity in highly crystallized undoped µc-Si:H films follows different temperature dependences for different microstructures. The fractional composition of large crystallite grains (Fcl) in our material emerges as a dominant parameter in the study of correlation between film microstructure and its transport properties.



A T -1/2 dependence of low temperature σd(T) is observed in highly crystallized µc-Si:H films (type-C) having tightly packed large columnar grains and high percentage fraction of large crystallite grains, suggesting a presence of tunneling of carriers between neighboring conducting crystals, similar to the metal-insulator composite systems.


The T -1/4 dependence of low temperature σd(T) is observed in types-A and B µc-Si:H films having a low percentage volume fraction of large crystallite grains and low degree of conglomeration, which is well addressed by Godet’s variable range hopping (VRH) model that proposes hopping in exponential band tail states.

The application of these two different transport models leads to the deduction of physically rational parameters (for each model) and material properties of µc-Si:H material, including DOS values with an exponential distribution near Fermi level, which are in good agreement with film microstructure.

A significant observation is that the
Fcl values which were initially found to be helpful in correlating, segregating and understanding the different film microstructures, were equally useful to correlate the observed dark transport behavior (at both high and low temperature ranges) to the three types of film microstructures, albeit empirically.


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Phototransport in single-phase undoped µc-Si:H: Experimental & Numerical Modeling Study of SSPC

In spite of the attractive optoelectronic properties of
µc-Si:H, the heterogeneous nature of its microstructure hinders a comprehensive interpretation of those properties. The recombination mechanisms and the nature of density of gap states (DOS) of µc-Si:H are still inadequately understood, as a single unique effective DOS profile might not be suitable for the whole microstructural range of µc-Si:H system and explain all the intricacies involved in the transport mechanisms. The DOS approximations presently used for µc-Si:H material, for academic purposes and in industry (e.g., in various semiconductor device simulations), are based on the known DOS of crystalline and amorphous silicon. Therefore, the accurate DOS profiles of µc-Si:H would not only add to our knowledge of the physics of this material, but also are essential for further improvement in µc-Si:H based device technology.

In our study of phototransport properties of plasma deposited highly crystalline undoped µc-Si:H films using steady state photoconductivity (SSPC), different phototransport behaviors were observed in films belonging to different types of the material. In type-A µc-Si:H films, light intensity exponent (γ ) lies between 0.5 and 1, and temperature dependent photoconductivity σph(T) shows thermal quenching (TQ) effect. In type-B µc-Si:H material, γ lies between 0.5 and 1 without a TQ effect, and in type-C µc-Si:H material, γ lies below 1, reaching as low as a surprising 0.15, with a TQ effect.


We employed numerical modeling of SSPC using Shockley-Read statistics in steady state conditions to determine the recombination processes to corroborate and further elucidate the experimental results. Our study indicates that the different phototransport behaviors are linked to different features of the proposed density of states maps of the material which are different for µc-Si:H films having different types of microstructure. We have used a band tail transport to construct an experimentally and theoretically consistent picture of the transport in our µc-Si:H material and to explain the observed phototransport behaviors. The different DOS profiles proposed by us have significant implications for semiconductor technology, as they offer a more accurate substitute to approximated DOS distributions, and can be applied to a wide range of microstructural types of µc-Si:H materials.

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