Electronic Structure of Dangling Bonds in Amorphous Silicon Studied via a Density-Matrix Functional Method

A structural model of hydrogenated amorphous silicon containing an isolated dangling bond is used to investigate the effects of electron interactions on the electronic level splittings, localization of charge and spin, and fluctuations in charge and spin. These properties are calculated with a recently developed density-matrix correlation-energy functional applied to a generalized Anderson Hamiltonian, consisting of tight-binding one-electron terms parametrizing hydrogenated amorphous silicon plus a local interaction term. The energy level splittings approach an asymptotic value for large values of the electron-interaction parameter U, and for physically relevant values of U are in the range 0.3-0.5 eV. The electron spin is highly localized on the central orbital of the dangling bond while the charge is spread over a larger region surrounding the dangling bond site. These results are consistent with known experimental data and previous density-functional calculations. The spin fluctuations are quite different from those obtained with unrestricted Hartree-Fock theory.Comment: 6 pages, 6 figures, 1 tabl

Structural Studies on Semiconducting Hydrogenated Amorphous Silicon Oxide Films

In hydrogenated amorphous silicon oxide (a-SiO:H) films, incorporation of oxygen enhances optical gap due to a large number of St-O-Si bond formation, which lies deep into valence band states. An induction effect of this Si-O on other bonds within the network also takes place. At higher oxygen content micro-void forms and bonded hydrogen accumulates in di and/or polyhydride form. At this stage a phase separation of Si-rich and O-rich region taking place. A peak shift of absorption spectra within 1850 - 2250 cm-1, towards higher wave number is continuous. A gradual increase and broadening of 850 cm-1 absorption band on both sides of peak position indicate higher structural disorder in network formation. It may be considered that the stretching vibration of-OH bonded to Si gives rise to 780 cm-1 absorption band. This Si-OH formation is beneficial which prevents deterioration in photosensitivity due to reduction in bonded hydrogen content. Hydrogen content is found reducing as oxygen content increases from zero to ~15 at.%. A systematic study is carried out to correlate the optoelectronic property with local atomic arrangement

High Resolution Microimaging with Pulsed Electrically-Detected Magnetic Resonance

The investigation of paramagnetic species (such as point defects, dopants, and impurities) in solid-state electronic devices is significant because of their effect on device performance. Conventionally, these species are detected and imaged using the electron spin resonance (ESR) technique. In many instances, ESR is not sensitive enough to deal with miniature devices having small numbers of paramagnetic species and high spatial heterogeneity. This limitation can in principle be overcome by employing a more sensitive method called electrically-detected magnetic resonance, which is based on measuring the effect of paramagnetic species on the electric current of the device while inducing electron spin-flip transitions. However, up until now, measurement of the current of the device could not reveal the spatial heterogeneity of its paramagnetic species. We provide here, for the first time, high resolution microimages of paramagnetic species in operating solar cells obtained through electrically-detected magnetic resonance. The method is based on unique microwave pulse sequences for excitation and detection of the electrical signal under a static magnetic field and powerful pulsed magnetic field gradients that spatially encode the electrical current of the sample. The approach developed here can be widely used in the nondestructive three-dimensional inspection and characterization of paramagnetic species in a variety of electronic devices.Comment: 19 pages, 4 figures +S

A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics

Different hypotheses have been proposed to explain the mechanism for the extremely low friction coefficient of carbon coatings and its undesired dependence on air humidity. A decisive atomistic insight is still lacking because of the difficulties in monitoring what actually happens at the buried sliding interface. Here we perform large-scale ab initio molecular dynamics simulations of both undoped and silicon-doped carbon films sliding in the presence of water. We observe the tribologically-induced surface hydroxylation and subsequent formation of a thin film of water molecules bound to the OH-terminated surface by hydrogen bonds. The comparative analysis of silicon-incorporating and clean surfaces, suggests that this two-step process can be the key phenomenon to provide high slipperiness to the carbon coatings. The water layer is, in fact, expected to shelter the carbon surface from direct solid-on-solid contact and make any counter surface slide extremely easily on it. The present insight into the wettability of carbon-based films can be useful for designing new coatings for biomedical and energy-saving applications with environmental adaptability.Comment: 22 pages, 4 figures, 1 tabl

Developing the knowledge-based human resources that support the implementation of the National Dual Training System (NDTS): evaluation of TVET teacher's competency at MARA Training Institutions

Development in the world of technical and vocational education and training (TVET) on an ongoing basis is a challenge to the profession of the TVET-teachers to maintain their performance. The ability of teachers to identify the competencies required by their profession is very critical to enable them to make improvements in teaching and learning. For a broader perspective the competency needs of the labour market have to be matched by those developed within the vocational learning processes. Consequently, this study has focused on developing and validating the new empirical based TVET-teacher competency profile and evaluating teacher’s competency. This study combines both quantitative and qualitative research methodology that was designed to answer all the research questions. The new empirical based competency profile development and TVET-teacher evaluation was based upon an instructional design model. In addition, a modified Delphi technique has also been adopted throughout the process. Initially, 98 elements of competencies were listed by expert panel and rated by TVET institutions as important. Then, analysis using manual and statistical procedure found that 112 elements of competencies have emerged from seventeen (17) clusters of competencies. Prior to that, using the preliminary TVET-teacher competency profile, the level of TVETteacher competencies was found to be Proficient and the finding of 112 elements of competencies with 17 clusters was finally used to develop the new empirical based competency profile for MARA TVET-teacher. Mean score analysis of teacher competencies found that there were gaps in teacher competencies between MARA institutions (IKM) and other TVET institutions, where MARA-teacher was significantly better than other TVET teacher. ANOVA and t-test analysis showed that there were significant differences between teacher competencies among all TVET institutions in Malaysia. On the other hand, the study showed that teacher’s age, grade and year of experience are not significant predictors for TVET-teacher competency. In the context of mastering the competency, the study also found that three competencies are classified as most difficult or challenging, twelve competencies are classified as should be improved, and eight competencies are classified as needed to be trained. Lastly, to make NDTS implementation a reality for MARA the new empirical based competency profile and the framework for career development and training pathway were established. This Framework would serve as a significant tool to develop the knowledge based human resources needed. This will ensure that TVET-teachers at MARA are trained to be knowledgeable, competent, and professional and become a pedagogical leader on an ongoing basis towards a world class TVET-education system

High growth rate deposition of hydrogenated amorphous silicon-germanium films and devices using ECR-PECVD

Hydrogenated amorphous silicon germanium (a-SiGe:H) films and devices have been extensively studied because of the tunable band gap for matching the solar spectrum and mature the fabrication techniques. a-SiGe:H thin film solar cells have great potential for commercial manufacture because of very low cost and adaptability to large scale manufacturing. Although it has been demonstrated that a-SiGe:H thin films and devices with good quality can be produced successfully, some issues regarding growth chemistry have remained yet unexplored, such as the hydrogen and inert gas dilution, bombardment effect, and chemical annealing, to name a few. The alloying of the SiGe introduces above an order-of-magnitude higher defect density, which degrades the performance of the a-SiGe:H thin film solar cells. This degradation becomes worse when high growth-rate deposition is required. The work presented here uses the Electron-Cyclotron-Resonance Plasma-Enhanced Chemical Vapor Deposition (ECR-PECVD) technique to fabricate a-SiGe:H films and devices with high growth rates. Helium gas, together with small amount of H2, was used as the plasma species. Thickness, optical band gap, conductivity, Urbach energy, mobility-lifetime product, and quantum efficiency were characterized during the process of pursuing good materials. High-quality material was successfully fabricated with the ECR-PECVD technique at high growth rates. The device we made with 1.47 eV band gap has a fill factor of 64.5%. With the graded band gap and graded doping techniques, 70% fill factor was achieved when the band gap was graded from 1.75 to 1.47 eV. We also got 68% fill factor with the band gap graded from 1.75 to 1.38 eV

The Investigation of the Electronic Properties of Si Based Heterojucntions: a First Principle Study of a-Si:H/c-Si and GaP/Si Heterojunctions

abstract: In this dissertation, I investigate the electronic properties of two important silicon(Si)-based heterojunctions 1) hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) which has already been commercialized in Heterojunction with Intrinsic Thin-layer (HIT) cells and 2) gallium phosphide/silicon (GaP/Si) which has been suggested to be a good candidate for replacing a-Si:H/c-Si in HIT cells in order to boost the HIT cell’s efficiency. In the first part, the defect states of amorphous silicon (a-Si) and a-Si:H material are studied using density functional theory (DFT). I first employ simulated annealing using molecular dynamics (MD) to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. I find that H atoms decrease the density of mid-gap states and increase the band gap of a-Si by binding to Si atoms with strained bonds. My results also indicate that Si atoms with strained bonds creates high-localized orbitals in the mobility gap of a-Si, and the binding of H atoms to them can dramatically decrease their degree of localization. In the second part, I explore the effect of the H binding configuration on the electronic properties of a-Si:H/c-Si heterostructure using density functional theory studies of models of the interface between a-Si:H and c-Si. The electronic properties from DFT show that depending on the energy difference between configurations, the electronic properties are sensitive to the H binding configurations. In the last part, I examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen H passivation at the interface in comparison to interface mixing, through DFT calculations. My calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. I find that the addition of H leads to additional bonding at the interface which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Amorphous Silicon Thin Film Transistor Fabrication and Models

One of the primary purposes of this research was to develop techniques to improve the quality of vacuum evaporated amorphous silicon (a-Si), i.e. lower the density of localized states in the mobility gap. The electron beam evaporation of amorphouss silicon and hydrogenation by ion implanting has proved promising. This technique permits independent control of amorphous silicon disorder and the hydrogenation level, thereby separating the process of hydrogenation from that of film deposition. Electrical measurement of field effect conductance changes was used as a probing tool to monitor changes in the properties of a-Si before and after hydrogenation. Field effect data was transcribed by a computer program to determine the density of localized states. Amorphous silicon films were prepared by electron beam evaporation of a high purity silicon onto the surface of a thermally oxidized crystalline silicon substrate. The films were deposited at a fixed rate in a high vacuum. Immediately after deposition, some films were subjected to in situ thermal anneal and some films were not. A Comparison of the results of these two eases revealed the porous nature of evaporated a-Si. Hydrogen incorporation into a- Si films was performed by ion implantation followed by a low temperature thermal activation of the hydrogen. After hydrogenation, a field effect conductance change of four orders of magnitude was observed on the devices which were not in situ thermally annealed. A comparison before and after hydrogenation demonstrates that almost three orders of magnitude reduction (from about 1022 to about 1019/cm3-eV) in the density of localized states near the Fermi level (N F/T) was achieved. Varying the hydrogen implantation dosage between lxlO16 to 1.5xl017/cm2, with all other sample preparation procedures fixed, caused a decrease in NF/T from 8.6xl020 to ixl019/cm3-eV. The effect of in situ thermal annealing prior to hydrogen implantation was also investigated. By performing a 400°C anneal for four hours immediately following film deposition the film porosity was greatly reduced. The film was then implanted with hydrogen to a total dose of lxl017/cm2. A field effect Conductance change of six orders of magnitude was observed which yielded a N|F/T of 4xl017/cm3-eV, approaching that of glow discharge produced films. The second purpose of the research was to develop modeling techniques for the a-Si:H TFT. Despite rapid progress in the TFT performence, [performance] the theoretical basis to determine static- and dynamic-characteristics of TFTs has not yet been determined mainly because the influence of the localized states on TFT operation is very complicated. The theoretical expression of drain current as a function of gate bias and drain voltage was derived. To use the theoretical expressions, the localized state density distribution N(E) must be known, A derived yet practical formula for the N(E) did not exist. A common way is to use the experiment of field effect conductance change to determine the N(E), With the data theoretical expressions the localized state density N(E) could be calculated by using a numerical technique, but it is cumbersome and connot [cannot]be determined uniquely. As a design tool for devices and circuits, a simple theory which can express concisely the TFT characteristics is very important. In this report, several models for N(E) are listed:. Approximate analyses for characteristics pf a-Si:H TFT are derived. In two special cases, i.e. uniform localized state density distribution and exponential localized distribution, some useful approximate expressions was obtained. Compared with the experiment data, the uniform density distribution of localized state model is a good approximate expression for a large density discribution [distribution] of localized states near Fermi level. The exponential model is a good approximate expression for lower density distribution of localized states near Fermi level