Transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopy have been used to examine the defect structure in the upper-half of the bandgap of CdTe solar cells, with an emphasis on understanding the effect of copper. TPC spectra reveal two defects in the CdTe devices at optical energies of 1.2eV and 0.9eV with respect to the valence band. The origin of the 1.2eV defect could not be associated with a particular element, although copper and zinc were ruled out as sources. TPI spectra were used to observe that the density of the 1.2eV defect was dramatically reduced by thermally annealing the devices, suggesting that the defect itself is annealed during the treatment. The set of CdTe samples examined used a rapid thermal processing treatment to control the amount of copper that diffused into the CdTe layer from the Cu:ZnTe interfacial layer at the back of the device. Comparison of devices with varying amounts of copper in the CdTe layer revealed that the 0.9eV defect seen in TPC was associated with the presence of copper in the absorber layer. TPI spectra confirmed the association of the 0.9eV with copper and showed that the magnitude of the 0.9eV defect signal increased as more copper was diffused into the CdTe layer. A proportional link between the density of the 0.9eV defect observed in TPI spectra and the amount of copper in the absorber layer observed via ToF-SIMS further established that copper is responsible for the existence of the defect. Numerical modeling of the CdTe devices was used to confirm that the spatial distribution of copper observed in ToF-SIMS was consistent with the relative variation of defect magnitudes observed in TPI. The fact that the copper-associated 0.9eV defect lies close to mid-gap suggests that it will act as an efficient recombination center in CdTe. Therefore, it is suggested that this work has detected the deep defect that is responsible for the decreased minority carrier lifetime that has been previously associated with the amount of copper in the CdTe layer

This dissertation consists of six chapters: In the first chapter, electrical and material properties and charge transport in organic semiconductors and graphene based field-effect transistors (FETs) are introduced. In the second chapter, device architectures of indenofluorene-phenanthrene copolymer based thin-film transistors (TFTs) are discussed. The combination of recessed source/drain and surface treatments on electrical contact and low-voltage-operated TFTs with solution-processed high-k dielectric are investigated. In the third chapter, device physics and charge transport of diketopyrrolopyrrole-naphthalene copolymer based TFTs are discussed. Top-gate TFTs with the polymer dielectric exhibit mobilities of ~1 cm2/V-s and charge transport measurements in steady-state and under non-quasi-static conditions reveal device physics in dual-gate configuration. In the fourth chapter, device characteristics and charge transport in ambipolar diketopyrrolopyrrole-benzothiadiazole copolymer based TFTs are focused. The ambipolar polymer TFTs possess balanced electron and hole mobilities which are both > 0.5 cm2/V-s. The trap density of states is calculated using two analytical methods developed by Lang et al. and Kalb and Batlogg. In the fifth chapter, charge transport of diketopyrrolopyrrole-thiophene copolymer based TFTs employing 4-point-probe configuration is studied. Such polymer TFTs possess the mobilities of up to 3 cm2/V-s. The activation energy as a function of carrier concentration represents multiple trapping and thermally release model or Monroe-type model of charge transport. In the sixth chapter, transformation of electrical characteristics of graphene FETs with an interacting capping layer of fluoropolymers and pi-conjugated organic semiconductors is investigated. The electrical properties of graphene by wafer-scale chemical vapor deposition can be favorably tuned by fluorocarbon capping methods. / text

This work focuses on device design and process integration of high-performance Ge-based devices for CMOS applications and beyond. Here we addressed several key challenges towards Ge-based devices, such as, poor passivation, underperformance of nMOSFETs, and incompatibility of fragile Ge wafers for mass production. We simultaneously addressed the issues of bulk Ge and passivation for pMOSFETs, by fabricating Si-capped epitaxial Ge:C(C<0.5%) devices. Carbon improves the crystalline quality of the channel, while Si capping prevents GeOx formation, creates a quantum well for holes and thus improves mobility. Temperature-dependent characterization of these devices suggests that Si cap thickness needs to be optimized to ensure highest mobility. We developed a simple approach to grow GeO₂ by rapid thermal oxidation, which provides improved passivation, especially for nMOSFETs. The MOSCAPs with GeO₂ passivation show ~10× lower Dit (~8×10¹¹ cm⁻²eV⁻¹) than that of the HF-last devices. The Ge (111) nMOSFETs with GeO₂ passivation show ~2× enhancement in mobility (~715 cm²V⁻¹s⁻¹ at peak) and ~1.6× enhancement in drive current over control Si (100) devices. For improved n⁺/p junctions, we proposed a simple technique of rapid thermal diffusion from "spin-on-dopants" to avoid implantation damage during junction formation. These junctions show a high ION/IOFF ratio (~10⁵⁻⁶) and an ideality factor of ~1.03, indicating a low defect density, whereas, ion-implanted junctions show higher Ioff (by ~1-2 orders) and a larger ideality factor (~1.45). Diffusion-doped and GeO₂-passivated Ge(100) nMOSFETs show a high ION/IOFF ratio (~10⁴⁻⁵) , a low SS (111 mV/decade), and a high [mu]eff (679 cm²V⁻¹s⁻¹ at peak). Moreover, diffusion-doped Ge (111) nMOSFETs show even higher [mu]eff (970 cm²V⁻¹s⁻¹ at peak) that surpasses the universal Si mobility at low Eeff. For Beyond CMOS devices, we investigated Mn-doped Ge:C-on-Si (100), a novel Si-compatible ferromagnetic semiconductor. The investigation suggests that the magnetic properties of these films depend strongly on crystalline structure and Mn concentration. On a different approach, we developed LaOx/SiOx barrier for Spin-diodes that reduces contact resistance by ~10⁴, compared to Al₂O₃ controls and hence is more conducive for spin injection. These ferromagnetic materials and devices can potentially be useful for novel spintronic devices. / text

abstract: Semiconductor manufacturing economics necessitate the development of innovative device measurement techniques for quick assessment of products. Several novel electrical measurement techniques will be proposed for screening silicon device parameters. The studied parameters range from oxide reliability, and carrier lifetime in MOS capacitors to the power MOSFET reverse recovery. It will be shown that positive charge trapping is a dominant process when thick oxides are stressed through the ramped voltage test (RVT). Exploiting the physics behind positive charge generation/trapping at high electric fields, a fast I-V measurement technique is proposed that can be used to effectively distinguish the ultra-thick oxides' intrinsic quality at low electric fields. Next, two novel techniques will be presented for studying the carrier lifetime in MOS Capacitor devices. It will be shown that the deep-level transient spectroscopy (DLTS) can be applied to MOS test structures as a swift mean for screening the generation lifetime. Recombination lifetime will also be addressed by introducing the optically-excited MOS technique as a promising tool. The last part of this work is devoted to the reverse recovery behavior of the body diode of power MOSFETs. The correct interpretation of the LDMOS reverse recovery is challenging and requires special attention. A simple approach will be presented to extract meaningful lifetime values from the reverse recovery of LDMOS body-diodes exploiting their gate voltage and the magnitude of the reverse bias. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2015

No description available.

The physics of electron devices is investigated within the framework of Semiclassical Monte Carlo and Path-Sum Monte Carlo analysis. Analyses of shortchannel III-V trigate nanowire and planar graphene FETs using a Semiclassical Monte Carlo algorithm are provided. In the case of the nanowire FETs, the bandstructure and scattering effects of a survey of materials on the drain current and carrier concentration are investigated in comparison with Si FETs of the same geometry. It is shown that for short channels, the drain current is predominantly determined by associated change in carrier velocity, as opposed to changes in the carrier concentration within the channel. For the graphene FETs, we demonstrate the effects of Zener tunneling and remote charged impurities on the device performance. It is shown that, commensurate with experimental evidence, the devices have great difficulty turning off as a result of the Zener tunneling, and have a conductivity minimum which is affected by remote impurities inducing charge puddling. Each material modeled is matched with experimental data by calibrating the scattering rates with velocity-field curves. Material and geometry specific parameters, models, and methods are described, while discussion of the basic semiclassical Monte Carlo method is left to the extensive volume of publications on the subject. Finally, a novel quantum Path-Sum Monte Carlo algorithm is described and applied to a test case of two layered 6 atom rings (to mimic graphene), to demonstrate the effectiveness of the algorithm in reproducing phase transitions in collective phenomena critical to possible beyond-CMOS devices. First, the method and its implementation are detailed showing its advantages over conventional Path Integral Monte Carlo and other Quantum Monte Carlo approaches. An exact solution of the system within the framework of the algorithm is provided. A Fixed Node derivative of the Path Sum Monte Carlo method is described as a work-around of the infamous Fermion sign problem. Finally, the Fixed Node Path-Sum Monte Carlo algorithm is implemented to a set of points showing the accuracy of the method and the ability to give upper and lower bounds to the phase transition points. / text

This dissertation includes work done on graphene and related materials, examining their electronic and topological properties using first-principles methods. Ab-initio computational methods, like density functional theory (DFT), have become increasingly popular in condensed matter and material science. Motivated by the search for novel materials that would help us devise fast, low-power, post-CMOS transistors, we explore the properties of some of these promising materials. We begin by studying graphene and its interaction with dielectric oxides. Graphene has recently inspired a flurry of research activity due to its interesting electronic and mechanical properties. For the device community, graphene's high charge carrier mobility and continuous gap tunability can have immense use in novel transistors. In Chapter 3 we examine the properties of graphene placed on two oxides, namely quartz and alumina. We find that oxygen-terminated quartz is a useful oxide for the purpose of graphene based FETs. Inspired by a recent surge of interest in topological insulators, we then explore the topological properties of two-dimensional materials. We conduct a theoretical study to examine the relationship between crystal space group symmetry and the electric polarization of a two-dimensional crystal. We show that the presence of symmetry restricts the polarization values to a small number of distinct groups. There groups in turn are topologically inequivalent, making polarization a topological index. We also conduct density functional theory calculations to obtain actual polarization values of materials belonging to C3 symmetry and show that our results are consistent with our theoretical analysis. Finally we prove that any transformation from one class of polarization to another is a topological phase transition. In Chapter 5 we use density functional theory to examine the electronic properties of graphene intercalation compounds. Bilayer pseudospin field effect transistor (BiSFET) has been proposed as an interesting low-power, efficient post-CMOS switch. In order to implement this device we need bilayer graphene with reduced interlayer interaction. One way of achieving that is by inserting foreign molecules between the layers, a process which is called intercalation. In this chapter we examine the electronic properties of bilayer graphene intercalated with iodine monochloride and iodine monobromide molecules. We find that intercalation of graphene indeed makes it promising for the implementation of BiSFET, by reducing interlayer interaction. As an interesting side problem, we also use hybrid, more extensive approaches in DFT, to examine the electronic and optical properties of dilute nitrides. Dilute nitrides are highly promising and interesting materials for the purposes of optoelectronic applications. Together, we hope this work helps in elucidating the electronic properties of promising material systems as well as act as a guide for experimentalists. / text

abstract: Scaling of the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) towards shorter channel lengths, has lead to an increasing importance of quantum effects on the device performance. Until now, a semi-classical model based on Monte Carlo method for instance, has been sufficient to address these issues in silicon, and arrive at a reasonably good fit to experimental mobility data. But as the semiconductor world moves towards 10nm technology, many of the basic assumptions in this method, namely the very fundamental Fermi’s golden rule come into question. The derivation of the Fermi’s golden rule assumes that the scattering is infrequent (therefore the long time limit) and the collision duration time is zero. This thesis overcomes some of the limitations of the above approach by successfully developing a quantum mechanical simulator that can model the low-field inversion layer mobility in silicon MOS capacitors and other inversion layers as well. It solves for the scattering induced collisional broadening of the states by accounting for the various scattering mechanisms present in silicon through the non-equilibrium based near-equilibrium Green’s Functions approach, which shall be referred to as near-equilibrium Green’s Function (nEGF) in this work. It adopts a two-loop approach, where the outer loop solves for the self-consistency between the potential and the subband sheet charge density by solving the Poisson and the Schrödinger equations self-consistently. The inner loop solves for the nEGF (renormalization of the spectrum and the broadening of the states), self-consistently using the self-consistent Born approximation, which is then used to compute the mobility using the Green-Kubo Formalism. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

abstract: Advanced and mature computer simulation methods exist in fluid dynamics, elec- tromagnetics, semiconductors, chemical transport, and even chemical and material electronic structure. However, few general or accurate methods have been developed for quantum photonic devices. Here, a novel approach utilizing phase-space quantum mechanics is developed to model photon transport in ring resonators, a form of en- tangled pair source. The key features the model needs to illustrate are the emergence of non-classicality and entanglement between photons due to nonlinear effects in the ring. The quantum trajectory method is subsequently demonstrated on a sequence of elementary models and multiple aspects of the ring resonator itself. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

No description available.