Browsing by Subject "Semiconductors"

Now showing 1 - 6 of 6
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    Device modeling of field-effect transistors with nanocrystalline channels.
    (2011-06) Steinke, Isaiah Peter
    Due to certain limitations of silicon for particular device applications, there has been increasing interest in the use of compound semiconductors. However, the growth of compound semiconductors in single crystal form is not always feasible on a large scale or even wanted for particular applications, such as solar cells or thin film transistors. The material for these applications is usually polycrystalline, and the presence of grain boundaries limits the performance of these devices. In our work, we present two models that take into account the effect of grain boundaries in nanocrystalline field-effect transistors. In our "macroscopic" model, we modify the field-effect mobility to include terms dependent upon the local carrier density and the longitudinal field along the channel. These terms are motivated by the expected carrier density and field dependences of transport across grain boundaries. In general, we find that the addition of each mobility term separately changes the carrier profile along the channel in opposite ways, and the inclusion of these terms increases the magnitude of the current. Furthermore, the addition of the longitudinal field dependent mobility term is only significant for large values of drain bias, i.e. near saturation. The limitation of the macroscopic model is that it inherently averages over the grains present in the channel. In order to further study the role of grains in the channel, we developed our "mesoscopic" model that incorporates ideas from percolation theory. Here, individual grains are represented as sites in our percolation problem, while the bonds represent the energy barriers between neighboring grains. The relative occupation of sites and bonds is connected to the carrier statistics of the device, whereby the carriers can be either free carriers in the grain or trapped carriers at the grain boundary. The relative occupations are controlled by the applied gate bias. Through the combination of a site-bond percolation problem and the carrier statistics, we describe the behavior of the transistor near threshold and illustrate a method to determine the threshold voltage.
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    Electronic Properties of Oxide and Semiconductor Heterostructures
    (2019-08) Sammon, Michael
    The modern world's dependency on electronics provides a constant need to discover new materials and devices. A promising technique to fabricate a new device is to create a heterostructure; a device consisting of two bulk crystals joined at an interface. These materials often support a low dimensional electron gas confined to the interface, which exhibits properties different than both the parent materials. These materials have led to the creation of MOSfets, the discovery of the quantum Hall effect, and in recent years the discovery of Majorana edge modes in nanowires. In this thesis, we study several different heterostructures. We begin with one of the most famous heterostructures, AlGaAs/GaAs. Modern AlGaAs/GaAs heterostructures support a high mobility two-dimensional electron gas (2DEG) in a quantum well. The 2DEG is provided by two remote donor $\delta$-layers placed on both sides of the well. Each $\delta$-layer is located in the midplane of a narrow GaAs well, flanked by narrow AlAs layers which capture excess electrons from donors. We show that each excess electron is localized in a compact dipole atom with the nearest donor. The excess electrons screen both the remote donors and background impurities, and are responsible for the observed high mobility. Still, we find that the mobility is substantially lower than theoretical estimates, which may be due to significant disorder in the donor layers, most likely roughness of the interfaces or spreading of the donors out of the midplane of the layer. Thus one should take care to make sure that the donor layers are as ideal as possible. We next move on to oxide heterostructures involving SrTiO$_3$ (STO). More specifically, we study the electron gas in accumulation layers of these heterostructures characterized by a density profile $n(x)$, where $x$ is the distance from the STO surface. SrTiO$_3$ at liquid helium temperatures has the highest dielectric constant which strongly enhances the role of nonlinear dielectric effects. It was recently shown that the nonlinear dielectric response results in an electron density profile $n(x)$ that slowly decays as $1/x^{12/7}$. We show that such a long tail of $n(x)$ causes the magnetization and the specific heat of the accumulation layer to diverge at large $x$. We explore the truncation of the tail by the finite sample width $W$, the transition from the nonlinear to linear dielectric response with dielectric constant $\kappa$, and the use of a back gate with a negative voltage $-\abs{V}$. We find that as a result both the magnetization and specific heat are anomalously large and obey nontrivial power law dependences on $W$, $\kappa$, or $\abs{V}$. In the linear dielectric regime under a strong magnetic field, the large dielectric constant of STO makes it easy to reach a quasi-one-dimensional state known as the extreme quantum limit (EQL) in which all electrons occupy the lowest Landau level. We present a theory of the EQL phase in STO accumulation layers. We find a phase diagram of the electron gas in the plane of the magnetic field strength and the electron surface concentration for different orientations of the magnetic field. In addition to the quasi-classical metallic phase (M), there is a metallic EQL phase, as well as an insulating Wigner crystal state (WC). Remarkably, the insulating Wigner crystal phase depends on the orientation of the magnetic field. We show that these effects can be measured through quantum capacitance measurements of the STO accumulation layer. The third material we study is semiconducting quantum wires. Though it is not a heterostructure, it supports a low dimensional electron gas which is often tuned with an external gate, making it similar to many of the devices we have studied. We have theoretically investigated the influence of interface roughness scattering on the low temperature mobility of electrons in quantum wires when electrons fill one or many subbands. We find the Drude conductance of the wire as a function of the linear concentration $\eta$ has a sharp peak. The height of this peak grows as a large power of the wire radius $R$, so that at large $R$ the conductance $G_{max}$ exceeds $e^2/h$ and a window of concentrations with delocalized states (which we call the metallic window) opens around the peak. Thus, we predict an insulator-metal-insulator transition with increasing concentration for large enough $R$. Furthermore, we show that the metallic domain can be sub-divided into three smaller domains: 1) single-subband ballistic conductor, 2) many-subband ballistic conductor 3) diffusive metal, and use our results to estimate the conductance in these domains. Finally we estimate the critical value of $R_c(\mathcal{L})$ at which the metallic window opens for a given length $\mathcal{L}$. We conclude the thesis with a discussion of a newer class of materials known as transition metal dichalcogenides (TMDs). We study a capacitor made of three monolayers TMD separated by hexagonal boron nitride (hBN). We assume that the structure is symmetric with respect to the central layer plane. The symmetry includes the contacts: if the central layer is contacted by the negative electrode, both external layers are contacted by the positive one. As a result a strong enough voltage $V$ induces electron-hole dipoles (indirect excitons) pointing towards one of the external layers. Antiparallel dipoles attract each other at large distances. Thus, the dipoles alternate in the central plane forming a 2D antiferroelectric with negative binding energy per dipole. The charging of a three-layer device is a first order transition, and we show that if $V_1$ is the critical voltage required to create a single electron-hole pair and charge this capacitor by $e$, the macroscopic charge $Q_c = eSn_c$ ($S$ is the device area) enters the three-layer capacitor at a smaller critical voltage $V_{c} < V_{1}$. In other words, the differential capacitance $C(V)$ is infinite at $V = V_{c}$. We also show that in a contact-less three-layer device, where the chemically different central layer has lower conduction and valence bands, optical excitation creates indirect excitons which attract each other, and therefore form antiferroelectric exciton droplets. Thus, the indirect exciton luminescence is red shifted compared to a two-layer device.
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    Green Chemistry Approach for The Synthesis of Transition Metal Sulfides based on Cu2ZnSnS4 (CZTS) and Etching of Their Impurities
    (2017-07) Pinto, Alexandre
    Green Chemistry comprises a set of good practices leading to more sustainable and environmentally friendly chemical processes. In the first chapter, this dissertation introduces the Green Chemistry Principles and shows how these Principles can be applied towards the synthesis of transition metal chalcogenides. The next chapter presents results focused on the synthesis of the multinary sulfide Cu2ZnSnS4 (CZTS) using microwave as heating source. The control over the crystalline phase of CZTS was studied as a function of variations in synthetic conditions, such as source of sulfur excess, initial oxidation states of Cu and Sn sources, temperature, and time. A model explaining two different behaviors according to the sulfur excess source is proposed. The third chapter uses the concepts learned from the previous one to develop the synthesis of solid solutions between Cu2ZnSnS4 and Cu2CoSnS4, generating compounds with the formula Cu2(Zn1-xCox)SnS4. Thin films were prepared from aqueous dispersions of these Cu2(Zn1-xCox)SnS4 compounds, and the stability of the films upon annealing in sulfur atmosphere was analyzed. The fourth chapter describes the development of a milder etching solution based on a mixture of ethylenediamine and 2-mercaptoethanol to eliminate undesired copper sulfide (Cu2-XS) or copper selenide (Cu2-XS) phases from CZTS thin films. The development of this etching solution represents a viable alternative to the widely used etching methods based on potassium cyanide (KCN) use. The fifth chapter extends the application of the etching solution to etch other common undesired phases such as ZnS, SnS2, and CuxZnySnz, and a possible mechanism is proposed for the etching process based in a Lewis acid-base reaction between ethylenediamine and 2-mercaptoethanol
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    Light-matter interactions in optical nanostructures based on organic semiconductors
    (2012-12) Lodden, Grant H.
    The confinement of a semiconductor material to an optical microcavity leads to an inherent coupling between light and matter. Depending on the lifetime of the excited state of the semiconductor (the exciton) and the cavity photon, two distinct regimes of interaction are possible. The system is said to be weakly coupled if either the exciton or the cavity photon decay before the two species interact. Weak exciton-photon coupling results in a modification of the exciton lifetime, the spectral shape, and the angular dispersion of emission from the microcavity. Conversely, when the lifetimes of the exciton and cavity photon are long enough so that an interaction occurs prior to either state decaying, the regime of strong exciton-photon coupling is realized. The timescale for coupling is the Rabi period, which depends on exciton and cavity parameters including the exciton oscillator strength and transition linewidths. The eigenstates of the strongly coupled system are known as microcavity polaritons. Microcavity polaritons have unique properties arising from their mixed exciton-photon character, permitting the realization of novel optoelectronic devices. Organic semiconductors are attractive for application in strongly coupled systems due to their large exciton binding energy (~1 eV), which permits a robust coupled state that is stable at room temperature and under electrical excitation. In addition, organic semiconductors exhibit large exciton oscillator strengths (~1015 cm-2) resulting in a strong interaction between the cavity photon and the exciton. We aim to better the understanding of polaritons in organic semiconductor microcavities to push the field towards novel optoelectronic devices.
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    Oral history interview with Carl Rench
    (Charles Babbage Institute, 1984-04-18) Rench, Carl F.
    Rench, an NCR employee since l946, surveys the company's growth from a manufacturer of cash registers to one of the largest suppliers of business computers. He begins with NCR's l946 experiments with vacuum tube arithmetic devices, work during the Korean war on the A-1-A bombing navigational system, and the acquisition in 1952 of the Computer Research Corporation. Rench points to Joseph Desch's role in moving NCR into electronics. Rench highlights the major products of the l950s: the Post-Tronic machine for reading magnetic strips on ledger cards and doing financial transactions, and the Magnetic Ink Character Recognition (MICR) device. He mentions a l959 joint venture with General Electric to produce one of the first all-transistorized business computers. He explains how, in the 1960s, NCR returned to its earlier specialty in peripheral devices, and contrasts this approach with IBM's concentration on the sale of systems. Rench focuses on the company in the early 1970s as a major producer of metal oxide semicon- ductor chips and as a multinational corporation. He discusses at length NCR president William Anderson's decentralization of the company, the resistance among Dayton employees, and the advantages of this policy to the company's livelihood.
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    Spin-dependent transport phenomena in ferromagnet/semiconductor heterostructures
    (2014-06) Geppert, Chad Christopher
    This dissertation examines several aspects of spin-dependent transport phenomena in epitaxially grown ferromagnet/n-GaAs heterostructures. Further maturation of the field of semiconductor-based spintronics is hindered by difficulties in evaluating device performance across materials systems. Using Fe/n-GaAs and Co2MnSi/n-GaAs heterostructures as a test case, the main goal of this work is to demonstrate how such difficulties may be overcome by (1) specifying a more quantitative framework for evaluating transport parameters and (2) the introduction of a new spin-to-charge conversion phenomenon which may be parameterized by bulk semiconductor parameters. In the introductory chapter, this work is placed in the broader context of developing improved methods for the generation, modulation, and detection of spins. The lateral spin-valve geometry is presented as a concrete example of the typical measurement procedures employed. Chapter 2 presents the charge-based transport properties of these samples and establishes the notation and calculation techniques to be employed in subsequent chapters. In particular, we examine in detail the calculation of the electrochemical potential for a given carrier concentration. Chapter 3 provides a full derivation of the equations governing spin-dependent transport in the large polarization regime. This is applied to the case of extracting spin lifetimes and diffusion rates, demonstrating how quantitative agreement with theoretical predictions may be obtained upon properly accounting for both device geometry and material parameters. Further examination of the boundary conditions applicable to the heterojunctions of these samples demonstrates to what extent device performance may be parameterized across materials systems. Chapter 4 presents experimental observations of a new spin-to-charge conversion phenomenon using a non-magnetic probe. In the presence of a large non-equilibrium spin accumulation, the combination of a non-constant density of states and energy-dependent conductivity generates an electromotive force (EMF). It is shown that this signal dephases in the presence of applied and hyperfine fields, scales quadratically with the polarization, and is comparable in magnitude to the spin-splitting. Since this spin-generated EMF depends only on experimentally accessible parameters of the bulk material, its magnitude is used to quantify the injected spin polarization in absolute terms, independent of any assumptions regarding the spin-resistance of the interface. Chapter 5 examines spin-dependent contributions to signals measured in the Hall geometry. In particular, a large scattering asymmetry develops in the presence of hyperfine interactions with dynamically polarized nuclei. A pulsed measurement technique is introduced which allows the polarization of the electron spin system and nuclear spin system to be manipulated independently. Based on these results, a possible mechanism is presented based on inhomogeneities in the nuclear polarization. This motivates a phenomenological model which is compared against experimental data using the modeling techniques of the previous chapters.