Sanjay K. Ram's Research Page

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

2009-01-24T08:36:00.000-08:00

My Thesis: Influence of Microstructure on the Electronic Transport Behavior of Microcrystalline Silicon Films

View more presentations from Sanjay Ram.

Read the details of the thesis work here.

I wish to thank everyone who has shown an interest in my thesis research, especially students of M. Tech and Ph.D who have written to me. I am often asked for a soft copy of my thesis by email. The soft copy of my thesis can be requested from the library of IIT Kanpur. It is a pdf file of 33 MB, which is why I cannot send it by email. If you are unable to get it from IITK, email me.

Bimodal CSD in Microcrystalline Silicon

2008-06-30T05:34:00.000-07:00

Crystallite Size Distribution in µc-Si:H

Plasma deposition techniques of microcrystalline silicon (µc-Si:H) fabrication inevitably lead to inhomogeneities in the microstructure of the material, as the contents of the constituent phases of mc-Si:H are inﬂuenced by the processing history. These inhomogeneities exist at different length scales and require to be studied with the help of different microstructural characterization tools acting at different length scales. An inhomogeneity in the form of a distribution in the sizes of the crystallites (CSD) is one such well-known feature present in plasma deposited µc-Si:H.

Significance

Electrical transport in µc-Si:H has been conventionally linked to and studied in the context of changing ﬁlm crystallinity. In the fully crystalline single-phase µc-Si:H material, there is no appreciable change in crystallinity with ﬁlm growth. In the absence of an amorphous phase, the CSD and conglomeration of grains in such material may have a signiﬁcant inﬂuence on the electrical transport properties and mechanisms. A detailed knowledge about these microstructural properties would allow a better understanding of the anisotropic nature of electrical transport, which results from the inﬂuence of crystalline orientation and the location of grain boundaries on the transport path. With this knowledge, it would be possible to predict the transport behavior in a particular direction, i.e., planar or perpendicular conﬁguration, which is essential for optimization of device performance.

Issues

Conventionally, Raman spectroscopy proﬁles are deconvoluted assuming a single mean crystallite size and a peak assigned to grain boundary material, and to account for the asymmetric tail, an amorphous phase is included. However, these assumptions could be erroneous for a single phase µc-Si:H material, in which the presence of a CSD is demonstrated by other microstructural characterization studies. The presence of CSD should be accounted for in the analyses of Raman spectra for more physically accurate results and picture of the material structure.

Measured imaginary part of <E₂> spectrum for µc-Si:H sample; and reference spectra of µc-Si, amorphous silicon, and of low-pressure chemical vapor deposited polysilicon with large (pc-Si- l) and ﬁne (pc-Si-f) grains.

What we have done

In our microstructural studies on µc-Si:H, we employed different microstructural characterization tools (spectroscopic ellipsometry, Raman spectroscopy, X-ray diffraction, and atomic force microscopy) to study the presence of CSD.
We acquired quantitative information about the mean crystallite sizes and their volume fractions in highly crystalline µc-Si:H with spectroscopic ellipsometry. Then we determined the actual crystallite size distribution using Raman spectroscopy with the help of a modiﬁcation in the modeling method.

Corroboration of results from different studies

The results of spectroscopic ellipsometry, X-ray diffraction, and atomic force microscopy demonstrate the presence of a distribution in the sizes of crystallites.

The modeling of spectroscopic ellipsometry results using two types of crystallites having two distinct sizes is corroborated with the deconvolution of experimentally observed RS proﬁles using a bimodal size distribution of crystallites having two mean sizes, large and small.

The fractional compositional analyses of the ﬁlms obtained by this methodology are found to be in qualitative agreement with the ﬁndings of spectroscopic ellipsometry.

Important Result

Our study shows that the appearance of a strong and longer low-frequency tail in Raman proﬁles measured from ﬁlm side of single-phase µc-Si:H material, without any distinguishable amorphous hump, can be due to the presence of size distribution in nanocrystallites, instead of a contribution from disordered or amorphous phase.

REFERENCES

Read more about these in these papers:

1. “Evidence of Bimodal Crystallite Size Distribution in µc-Si:H Films”, Sanjay K. Ram, Md. N. Islam, S. Kumar and P. Roca i Cabarrocas, Materials Science and Engineering B (in press, doi: 10.1016/j.mseb.2008.11.048)

2. “Influence of the statistical shift of Fermi level on the conductivity behavior in microcrystalline silicon”, Sanjay K. Ram, P. Roca i Cabarrocas and S. Kumar, Phys. Rev. B. 77, 045212 (2008).

3. “Structural determination of nanocrystalline Si films using ellipsometry and Raman spectroscopy”, Sanjay K. Ram, Md. N. Islam, S. Kumar, and P. Roca i Cabarrocas, Thin Solid Films 516 (2008) 6863.

4. "Influence of crystallite size distribution on the micro-Raman analysis of porous Si", M.N. Islam, S. Kumar, Appl. Phys. Lett. 78 (2001) 715.

5. "Effects of crystallite size distribution on the Raman-scattering profiles of silicon nanostructures", M.N. Islam, A. Pradhan, S. Kumar, J. Appl. Phys. 98 (2005) 024309.

Organic Semiconductors (small molecules)

2007-10-28T22:47:00.000-07:00

Team Members: (Dept. of Physics, Indian Institute of Technology Kanpur, India)

Prof. Satyendra Kumar

Dr. Sanjay K. Ram

Dr. Vivek Shukla

Mr. Girish Gupta

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Organic semiconductors have shown immense potential in terms of their numerous technological applications which were earlier dominated by inorganic semiconductors. These applications started off with electroluminescent devices, but have since diversified to include electronic devices such as transistors as well. The electronic conductivity of these materials lies between that of metals and insulators, spanning a broad range of 10^-9to 10³ (Ωcm)^-1.

Known organic semiconductors can be broadly classified into two groups on the basis of their molecular weight:

Conjugated polycyclic compounds of molecular weight less than 1000, and
Heterocyclic polymers with molecular weight greater than 1000.

Polymers are easily deposited as thin films on large areas making them valuable semiconducting materials. Nevertheless, they suffer from a major drawback in that they are not highly soluble in organic solvents, and they lose their mobility upon functionalization to enhance solubility. This has been a major driving force behind the research on small molecules as semiconductors. With small molecules semiconductors, it is possible to control charge transport in a simpler way by modification of various molecular parameters. For example, the ability of these molecules to pack into well-organized polycrystalline films leads to higher mobility compared to polymeric semiconductors.

The study of organic materials (small molecules) in our research group is focused on:

1. Material synthesis and purification

2. Thin film deposition of organic materials (synthesized & commercially available materials) by thermal evaporation technique on the glass/Si/quartz substrates.

3. Structural characterization: AFM, XRD, Spectroscopic Ellipsometry, FTIR

4. Optical Characterization: normal and transient photoluminescence (PL), photoluminescence excitation (PLE), transmissitance and refelectance.

5. Electrical characterization : Temperature dependent dark and photoconductivity on planar and sandwich configuration

6. Device fabrication:

Organic light emitting diodes (OLED)
Organic thin film transistors (OTFT)
Organic solar cells devices

7. Electroluminescence (EL) studies of OLED devices and transistor characteristic studies of OTFT

Organic Light Emitting Diode (OLED)

The device structure of OLED consists of several layers of organic materials sequentially deposited on glass substrate, each layer having a specific purpose that serves to enhance device quality and performance. The schematic representation of an ideal/standard OLED device is shown below.

We have explored several organic materials (small molecules) in our electroluminescent (EL) devices as emitting as well as electron transporting layer like Alq3, Znq2, Cr-doped Alq3, Inq3. The organic materials are usually susceptible to environmental aging and photo-oxidation, which influence their viability for commercial utility. Our studies demonstrate the effects of oxygen, light and environment on these organic materials to enhance the efficiency and lifetime of OLEDs. A simple device structure for these studies, along with the molecular structures of the materials used are shown below.

Organic Thin Film Transistor (OTFT)

Organic thin film transistors (OTFT) have made impressive progress over the past decade. Organic TFTs provide two principal advantages over TFTs based on inorganic semiconductors; they can be fabricated at lower temperatures, and potentially, at significantly lower cost. Low process temperatures, in particular, may allow organic TFTs to be integrated on inexpensive plastic substrates rather than glass. With field effect mobility and current on/off ratio values comparable to amorphous silicon, it becomes increasing likely that organic electronic devices will find use in broad area electronics applications. OTFTs are of interest for such a number of applications as pixel-access devices in active matrix displays, liquid crystal light valves of organic light emitting diodes, switching devices for logic gate memory arrays in smart cards, and low cost integrated circuits on flexible large area substrates.

We have fabricated OTFTs using Pentacene (C₂₂H₁₄), as the active material. Pentacene (C₂₂H₁₄) is a planar molecule composed of five benzene rings. Pentacene has a strong tendency to form molecular crystals. It forms well-ordered films that can be poly or single crystalline depending upon deposition and substrate conditions using vacuum evaporation even at low substrate temperatures. With Pentacene, ordered films are obtained when deposited by thermal evaporation at substrate temperatures as low as 0 ^οC. While the bulk electrical conductivity of acenes such as Pentacene is very low (~ 10^-15 S/cm), Pentacene has been found to have the highest mobilities for hole transport (p-channel). These devices have field-effect mobility as large as 2.2 m²/V-s, comparable to hydrogenated amorphous silicon TFTs. The upper limits in microscopic mobilities of organic molecular crystals, determined at 300 K by time-of-flight experiments, are falling between 1 and 10 cm²/V-s. The high mobility of pentacene is a result of significant orbital overlap from edge-to-face interactions among the molecules in their crystal lattice. The performance of OTFT depends on orientation of molecules, crystal structure, morphology, grain size and defects. In our study we tried to tune these parameters by changing the deposition parameters like nature of substrate (or surface treatment), substrate temperature, deposition rate, film thickness etc during deposition.

OTFT Fabrication

Once the gate oxide is made over Si there are just two possible structures for the source-drain contacts. One is called bottom gate-bottom contact (BG-BC) or bottom electrode TFT design, where drain and source contact metal is patterned on the gate dielectric prior to the active layer deposition. The other is named as bottom gate-top contact (BG-TC) or top electrode TFT design, where both source and drain pads are deposited on the top of an active layer through a shadow mask. The schematic cross sectional view of these structures is as shown below.

Structural and Optoelectronic Properties of Nanostructured Porous Silicon

2007-10-25T00:00:00.000-07:00

Team Members: (Dept. of Physics, Indian Institute of Technology Kanpur, India)

Prof. Satyendra Kumar

Dr. Md. Nazrul Islam (Present address: QAED-SRG, Space Applications Centre (ISRO), Ahmedabad, India)

Dr. Sanjay K. Ram

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Under special condition partial electrochemical etching (anodization) of the surface of a crystalline silicon wafer leads to the formation of nanoporous holes in its microstructure, resulting in a large surface to volume ratio in the order of 500m²/cm³. The degree of porosification can be controlled by optimizing the anodization parameters. The material thus achieved, Porous silicon (PS), is a network of nanometer sized Si particles surrounded by voids and space, and is remarkable not only in its ease of fabrication, but also in regards to its properties like complementary metal–oxide–semiconductor (CMOS) compatibility and visible photoluminescence at room temperature.

PS is already being used for a large number of applications which include light-emitting devices like photodetectors and solar cells, and sensing devices. It has also shown great promise either in the form of active layer or in combination with other materials (multilayer). PS is being researched for a whole range of applications like microelectromechanical systems (MEMs), anti-reflective coatings, Bragg reflectors, optical waveguides, chemical and biological sensors.

THEME

The microstructure of porous silicon layers plays a crucial role in determining its opto-electronic properties and possible applications. We have fabricated PS layers with a variety of microstructures having thicknesses ranging from about 1 to 200 microns. Our research has been focused on the microstructural characterization of these PS layers to understand the influence of crystallite size effects, surface effects and surrounding media on the Raman and photoluminescence (PL) spectra. The effect of structural inhomogeneities on the electrical properties and light induced metastabilities were also studied.

In particular, our Raman spectroscopy studies of these PS layers have led to the observation of symmetry forbidden Raman modes at room temperature, depending on the thickness and microstructure. We developed a modified approach to the deconvolution of the Raman scattering data by incorporating the effects of crystallite size distribution (CSD) in the data analysis. In order to understand the PL spectra from silicon nanostructures, a phenomenological model was developed to include both size as well as surface effects. We studied the electron transport properties of these well-characterized porous silicon layers in planar geometry as well as across the c-Si/PS/metal junctions over a wide temperature range from 15 to 450 K.

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Fabrication of Porous Silicon Layers

The PS layers were fabricated by electrochemical anodization of p-Si (100) wafers of 6-10 W.cm resistivity in a Teflon cell using HF (48%) and C₂H₅OH (99.9%) (1:1 by volume) as electrolyte and a platinum disc as a counter electrode. The schematic view of the experimental setup is shown below.

For a uniform current distribution over the exposed area, an Ohmic back contact was provided by thermal evaporation of Al, followed by annealing at 450° C for 1 hour, both procedures carried out in high vacuum conditions. The wafers were anodized at a constant current density of about 10 mA.cm^-2 for times varying from 90 to 120 min under white light illumination, resulting in 30–50 micron thick PS layers. Samples were rinsed in deionized water followed by methanol and subsequently soaked in propanol for few minutes to minimize the structural damage during drying.

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Structural Properties of Porous Silicon

The proper understanding of the PS layer microstructure is vital to the study of the optoelectronic properties of this material. Processing history of PS samples would suggest natural incorporation of disorder and inhomogeneities in the PS network. To understand the microstructure requires measurement of the porosity, thickness, crystallite orientations, sizes and their distribution in PS layers at different length scales. In addition, knowledge of strains in PS layers and PS/c-Si interfaces helps to explain the observed optoelectronic properties.

Scanning Electron Microscopy (SEM)

We used SEM to study the morphology and cross-sections of the PS layers. Our studies show the evolution of the PS layers with anodization time and the effect of changes in different anodization parameters. SEM images of PS layers are shown below which depict our observation of crack initiation in PS layers for shorter anodization times and well-developed cracks and fractured surfaces leading to island formation surrounded by channels for longer anodization times.

X-ray Diffraction (XRD)

The powder XRD studies on the PS layers demonstrated that the remnant porous Si skeleton is single crystalline in nature and has the same orientation as that of the substrate Si. Small lattice misorientation in the crystal planes in XRD spectra increases with increasing thickness of PS layers. Further extensive analyses of XRD data helped us to determine the amount and nature of stress and strain in the PS layers, the mean crystallite sizes and the influence of the anodization parameters on them.

Stress Analysis of Porous Silicon / crystalline Silicon interface

Porous silicon (PS) lattice is expanded from its substrate silicon lattice. This lattice expansion of PS generates lattice mismatch induced compressive strains on PS layers at the PS/substrate interface. The strain relaxes gradually away from the interface resulting the strain gradient from PS/substrate interface to PS surface. The value of average strain depends on the PS layer thickness and becomes the maximum at a certain thickness of PS layer. The change in strain with PS layer thickness is due to the cracking and stress relaxation.

Micro Raman Scattering (RS) Spectroscopy

The major outcomes of our RS studies carried out on PS layers were

- Microstructural characterization of PS layers based on crystallite size distributions with incorporation of CSD in the Raman data analysis methodology.

- Observation of symmetry forbidden Raman modes

The RS data of PS layers reveal spatial inhomogeneities over the anodized surface as well as along the thickness of the samples. These features were explained by correlating the surface morphology from SEM and stress information using XRD. RS spectra show clear evidence of nanocrystalline Si but no distinctive features corresponding to amorphous silicon tissues for all the samples under study.

Crystallite Size Distribution (CSD) in Si Nanostructures

Crystallite sizes determined using standard phonon confinement model do not correspond to the sizes obtained by XRD analysis. Further, this model fails to describe the PL spectrum measured on the same spot using quantum confinement models. In order to resolve this problem, a Gaussian distribution in crystallite sizes was explicitly included to calculate the Raman spectra of porous silicon in a model developed by Islam and Kumar (J. Appl. Phys. 98 (2005) 024309). The size distribution (mean size and standard deviation) obtained from fitting the Raman data using our procedure was able to predict the PL accurately in the quantum confinement models. Further, the modified Raman intensity analysis was extended to published reports on directly measured crystallite size distribution and RS data on a variety of Si nanostructures (other than anodized PS also). Our Raman analysis is found to produce good agreement with the mean crystallite sizes obtained from X-ray and high-resolution transmission electron microscopy, especially in the size range of mean crystallite sizes between 2 nm and 5 nm.

The phenomenological model not only is useful to obtain the analytic expression for Raman spectral profile from semiconductor nanostructures having a Gaussian distribution in the crystallite sizes, but also helps us envision in a new light the Raman analysis of such materials where a CSD may be present. The presence of large size dispersion in an ensemble of nanocrystallites was found to give rise to amorphous-like low-frequency tails in the Raman line shapes. Assigning such low-frequency tail in Raman line shapes to a-Si during deconvolution of experimental Raman spectra may lead to a misinterpretation.
(Read more on the concept and calculations of bimodal CSD)

(Our studies on Bimodal CSD in microcrystalline silicon)

Symmetry Forbidden Raman (SFR) Modes in PS

Enhanced microstructural features in thick PS layers led us to the observation of symmetry forbidden Raman scattering modes at room temperature. Information obtained by XRD and SEM on the structural orientation of the PS layer was used to understand the symmetry violations in Raman selection rules. A combination of various mechanisms such as crystallite size effects, lattice mismatch induced micro-misorientations of crystal planes, and multiple reflections and within the porous silicon nanostructures explains our results

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Photo Luminescence (PL) Spectroscopy

In literature various models have been proposed to understand the origin of room temperature PL from nc-Si structures. None of these PL models can explain all the observed experimental results on PL from PS. But a broad consensus has been reached to quantum confinement effect (QCE), which explains most of experimental PL results at least qualitatively. It is generally accepted that the QCE in the nanocrystallites opens up the band gap as well as relaxes the selection rules for radiative transitions, giving rise to above band gap PL in the visible region for crystallite sizes below ~5 nm. However, QCE alone cannot explain the role of various surface treatments and surrounding media.

In our experiments, the PL peak energy was found to vary about ±0.05eV from the mean value with sampling locations on the same samples. However, if we consider the spatial variation of PL peak energy as an error bar, the PL peak energy averaged over the whole PS layer surface remained almost constant for all samples produced over our entire range of anodization times under same anodization conditions. A free fit to our experimental data using simple QC model yields the unreasonable size parameters.

We developed a phenomenological model to analyze the room temperature PL that includes the surface effects and exciton binding energies along with the crystallite size dependent quantum confinement effects. The optical band gap widening is due to QC effects in nanoparticles. On excitation with high-energy photons, photo carriers are generated inside the crystallites and then some relax non-radiatively to the surface states. Subsequently, the relaxed carriers recombine to the ground states radiatively giving PL. We obtained analytical expressions to model the PL line shapes using normal as well as lognormal crystallite size distributions.

Our microstructural studies have revealed that PS layer contains mixed sized crystallites having two different crystallite size distributions. One CSD for smaller crystallites (L<5>L>10 nm). The former CSD only contributes to PL from PS while the latter has no role to play in luminescent properties of PS layers. The quantum confinement and surface states are equally important for efficient visible PL from PS layers. The QCE in nanocrystallites opens up the band gap in nano-particles increases the oscillator strength of radiative transitions while localized surface states take part in radiative de-excitation of photo-excited carriers. The CSD determined from Raman analysis successfully describes the PL line shapes from PS layers using the hybrid (or unified) PL model consisting of QCE and surface states.

This combined mechanism of PL explained most of the observed PL results from PS layers. Further, experimental data on a variety of nanocrystalline silicon (nc-Si) structures with directly measured crystallite size distribution were analyzed satisfactorily. This showed the importance of localized surface states in predicting the PL data from nc-Si. The model is also found useful in understanding the role of surface passivation and surrounding media on the photoluminescence in porous and nanocrystalline Si.

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Electrical Transport in Porous Silicon

For electrical measurements in coplanar configuration, two rectangular Al pads of 1 mm´5 mm sized with a gap of 0.5 mm while for sandwich configuration circular Al pad of 2 mm diameter were thermally evaporated on top of the freshly prepared porous layers in glancing geometry at an angle of 30° between molecular beam and the sample. This precaution prevents shorting of contact between evaporated metal and the silicon skeleton (especially for thick PS layers). In order to make an intimate contact between PS and Al, samples were annealed at ≈200° C for 45 min. All electrical measurements were carried out in a closed-cycle helium cryostat under dark conditions. Care was taken to avoid any light or thermal induced degradation.

Electrical Transport in Coplanar Configuration

We studied the electrical conductivity of electrochemically etched porous silicon over a wide temperature range from 15K to 450K. Applicability of various transport mechanisms has been critically analyzed. Different current transport mechanisms through thick PS layers were found to be predominant in different temperature zones. While the conductivity data above room temperature shows extended state conduction, lowering the temperature leads to Berthelot type conduction (180 - 280 K). Further, Mott’s (140 - 180 K) and Efros-Shklovskii hopping conduction (below 120 K) are found to be operating in lower temperature ranges. A clear cross-over from Mott to Efros-Shklovskii variable range hopping transport is observed at low temperatures.

Electrical Transport in Sandwich Configuration

In our device structure of Al/c‑Si/PS/Al, measured I‑V characteristics may be governed by either c‑Si/PS heterojunction or PS/Al interface, or both. In our studies, we found Al/PS junctions are non-rectifying and quasi-linear whereas Al/PS/c-Si junctions are weakly rectifying. The rectifying behavior is due to PS/c-Si heterojunction. The diode ideality factor (n) is about 8 for bias ≤0.5 V (about 50 for bias ≤5 V) at forward bias and nearly 1 for ≤0.5 V at reverse bias. As the temperature decreases, n at both forward and reverse biases increases. Different current transport mechanisms are found to be operating across the PS/c-Si junctions under forward and reverse biases.

The barrier height measured from I-V data for ≤0.5 V is higher for forward bias than that for reverse bias. I-V results on PS/c-Si junctions are explained by a multi tunneling-recombination model for forward bias. The current transport mechanism in the reverse bias condition is mainly dominated by the carrier generation recombination in the depletion region formed on the PS side. At higher reverse biases, the reverse current transport is governed by the barrier lowering effects. It is found to behave like a Schottky junction with Fermi level pinned to the defect energy levels at the c-Si/PS interface. The conduction band offset is found to be ≈ 0.1 eV. Based on the detailed analysis of I–V data the energy band diagram of the c-Si/PS heterojunction has been presented. Our study provides an easy and useful alternative method of determination of band edge discontinuities in multilayer structures using PS layers.

Persistent Photo Current (PPC) in Porous Silicon

On exposing the samples to infrared filtered white light, PS layers gave an enhanced dark conductivity, known as persistent photo current (PPC), which persisted over long time at 300 K after light illumination was stopped. We studied PPC in details as a function of illumination time, intensity, illumination temperature and sample temperature. We also discovered that PS layers exhibited a decrease in its dark conductivity, similar to Stabler-Wronski effect in a-Si:H, after a prolong illumination. We explained these effects in PS layers by considering inhomogeneities in PS nanostructures.

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Dielectric-Emissive Coatings in High Definition Plasma Display Panels

2007-10-21T23:25:00.000-07:00

Team Members: (Dept. of Physics and Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, India)

Prof. Satyendra Kumar

Dr. Sanjay K. Ram (Sr. Project Scientist)

Dr. Vandana Singh (Sr. Project Scientist)

Mr. Durgesh K. Rai (M. Tech. Student)

Mr. Surajit Sarkar (Ph.D. Student)

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Plasma Display Panels

Alternating current plasma display panel (ac-PDP) technology has ushered in a new era in the manufacture of large, flat, and lightweight displays. The reduced thickness and weight, rich color, with a conveniently wide viewing angle has led the plasma display panels (PDPs) to become an important flat panel display system in the consumer television market.

A PDP works by sandwiching a neon and xenon gas mixture between two sealed glass plates with parallel electrodes deposited on their surfaces. The plates are sealed so that the electrodes form right angles, creating pixels. When a voltage pulse passes between two electrodes, the gas breaks down and produces weakly ionized plasma, which emits UV radiation. The UV radiation activates color phosphors and visible light is emitted from each pixel. A gas discharge in a PDP is a key process for conversion of an electrical representation of an image to visible light information via generation of vacuum ultraviolet (VUV). The schematic diagram of a single pixel of PDP is shown below.

Secondary Electron Emission Coefficient

In ac-PDPs, the metal electrodes are covered with a glass-like dielectric layer upon which a thin sputter-resistant dielectric film is deposited. This protective layer plays a key role in preventing ac-PDPs from sputtering of ions and other plasma particles, like electrons, photons, meta-stable atoms, and emits a large amount of secondary electrons. The secondary electron emission (SEE) coefficient, denoted by γ, provides information on the efficiency of electron emission from the cathode due to ion bombardment. Emission by ions can be one of the major causes of electron emission, but it is never the only cause and often it is not dominant. That is why the coefficient is more often named as effective secondary electron emission coefficient γ_eff.

This SEE yield of the cathode is an important parameter in determining the discharge characteristics in a PDP. A high γ is beneficial in maintaining high plasma density in the PDP cells and is important for lowering of the firing voltage, increase of lifetime, and enhancement of the luminous efficiency. The discharge (firing and sustaining) voltages of PDP are largely dependent on the γ value of the protective layer. These voltages are also important in reducing the product cost.

High Definition Plasma Display Panels (HDPDP) are the future for next generation plasma TVs, which require substantial innovation and improvements in the existing technology platforms to enhance secondary electron emission of protective layers.

Measurement of SEE yield (γ)

Our group’s research is directed towards the measurement of the SEE yield (γ), which is a crucial parameter for characterization of dielectric layers and their applicability in the PDPs. This project is a part of a larger academia-industry collaborative project aiming to develop indigenous HDPDP technology in India.

There is no commercially available set-up to measure the SEE coefficient. We have designed a system, which has been named SEE-tool to measure the effective γ values. In this technique a discharge is produced in a cell (or pixel) (having conditions similar to those existing in real PDPs) in pure noble gases (or their mixtures), and then Paschen curve is obtained that can be matched with model calculations to yield the ion-induced SEE coefficients. The system includes a ultra high vacuum chamber (~10^-9 Torr) inside which the test sample (dielectric-emissive layer coated) is placed during measurements.

The firing voltage in such a setup is uniquely related to the ion-induced γ value, without any fudge factors involved. The breakdown voltage for a particular gas discharge setup depends only on the product pxd, where p is the pressure in the system and d the distance between the electrodes (Paschen law). The γ-coefficient determination is, therefore, based on breakdown voltage measurements as a function of pxd, i.e., the measurement of Paschen curves. Plasma firing voltages vs. the product of pxd are measured using the set up depicted in the figure below.

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Materials for Dielectric-Emissive Coating in HDPDPs

Materials that satisfy the demand for HDPDP must have:

High secondary electron emission coefficient (γ) of ~ 0.5;
Near zero sputtering yield for the lifetime (~60,000 hrs);
High transmission in the visible range with no deterioration over the operating life;
Acceptable degassing behavior.

In addition, the process for making these films must have high deposition rates and large area scaling capabilities suitable for industrial exploitation of the technology. At present we are exploring the following thin films for suitability in emissive layer in PDPs:

MgO based thin films by appropriate changes in composition and doping, such as Si, Ca, Sr etc.
Carbon based thin films such as diamond like carbon and carbon nanotubes.
Combination of CNTs and MgO thin films.

MgO Thin Films

MgO is a material that is highly suitable for use in plasma-related devices due to its properties like high SEE coefficient (that lowers the operating voltage), high insulation, great tolerance for ion bombardment (increasing the lifetime of panel), high transparency for visible light, and relatively intense exoelectron emission realizing ultra-high dark-room contrast.

We are depositing MgO thin films by DC and pulsed-DC reactive magnetron sputtering technique (picture of unit shown below) using metallic magnesium target and combination of Ar and O₂ gases.

We have achieved MgO thin films with preferred growth along the desired crystalline orientation and controlled growth rates. The films show high optical transmittance in visible range. These films are investigated using different microstructural characterization tools like spectroscopic ellipsometry (SE), atomic force microscopy (AFM), scanning electron microscopy (SEM), and (EDAX). The measured γ of these films are seen to differ in MgO films having different preferential orientations. We have measured the γ on MgO single crystals having a particular type of orientation to know the correlation between γ and crystal orientation. A knowledge about the growth of the preferentially oriented MgO thin films is helpful to optimize the deposition conditions that can lead to the desired types of films. Other aspects of the MgO protective layer that are being explored involve a structural modification in the film, so that the altered physical properties result in improved electrical properties.

Diamond-like carbon

New PDP cell structures have been developed that do not need a transparent protective coating. This will enable the development of a complete new class of non-transparent protective layer materials, like diamond. It will also allow the processing of the dielectric layer in PDP at 500°C, thereby enabling the use of soda-lime glass, even for high definition panels.

DLC is an attractive surface protective layer for dielectric materials inside PDPs due to its high γ and good optical transmission (with proper film optimization). We are preparing DLC films by PECVD technique (with DC and pulsed-DC power supplies) using gas mixture of acetylene (C₂H₂) and hydrogen (H₂). We are investigating the effect of several deposition parameters on DLC film microstructure, its optical properties and SEE. These films are characterized by Raman spectroscopy, SE, FTIR, AFM, SEM, and EDAX.

Carbon Nanotubes

Carbon nanotubes (CNTs) are formed from a single sheet of graphite (a hexagonal lattice of carbon) called “graphene” rolled into a seamless cylinder. Multi-layer techniques used in dielectric-emissive layers in PDPs include MgO layers deposited on CNT. Such bilayer MgO-CNT films show very high γ values, and the strong CNT layer acts as a protective shield for the MgO layer.

We have prepared multi walled carbon nanotubes by PECVD (with both DC and Pulsed DC power supplies) technique using C₂H₂ + H₂ gas mixture. The PECVD technique does not require high synthesis-temperature as it uses heat as well high energy electronsin the plasma to dissociate the feed gas and hence it allows CNT growth at significantly lower temperatures. In addition, the electric field in PECVD method enables the growth of more vertically aligned CNTs than possible with other deposition techniques. Thus microstructural modifications in such layers will be investigated for optimization of γ and other properties.

Thesis Related Slideshare Presentations

2007-07-17T04:34:00.000-07:00

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

2007-01-30T00:44:00.000-08:00

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 (T_s = 200^oC), 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 (T_s = 200 ^oC).

T_s 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.

(more on bimodal CSD in µc-Si:H)

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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 (F_cl, 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 F_cl values, and this classification reflects some common microstructural and electrical transport properties shared by each ‘type’,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, F_cl is less than 30% and σ₀ and E_a 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 F_cl varies from 30% to 45% and there is a sharp drop in σ₀ [from ~10³ to 0.1 (Ωcm)^-1] and E_a (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 E_a 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 F_cl 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 (E_a) with the equation: σ₀ = σ₀₀exp(G.E_a), where G and σ₀₀ are called MN parameters. Often G^-1is denoted as E_MN, 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 (E_f) with temperature. Apart from MNR, another interesting and important phenomenon is the anti-MNR, in which a negative value of E_MN 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 E_f 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 E_min, σ_M, and values of σ₀ and E_a where γ_f = 0 and γ_f = γ_c, 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.

The dark electrical conductivity σ_d(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 σ_d. 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 (F_cl) 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 σ_d(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 σ_d(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 F_clvalues 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 σ_ph(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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