Material characterization and process development for indium-arsenide / indium-gallium-antimonide channel, high electron mobility transistors for low power, high speed applications

Efforts to push the performance of transistors for millimeter-wave and microwave applications have borne fruit through device size scaling and the use of novel material systems. III-V semiconductors and their alloys hold a distinct advantage over silicon because they have much higher electron mobility which is a prerequisite for high frequency operation. InGaAs/InP pseudomorphic heterojunction bipolar transistors (HBTs) have demonstrated fT of 765 GHz at room temperature and InP based high electron mobility transistors (HEMTs) have demonstrated fMax of 1.2 THz. The 6.1 A lattice family of InAs, GaSb, AlSb covers a wide variety of band gaps and is an attractive future material system for high speed device development. Extremely high electron mobilities ~ 30,000 cm^2 V^-1s^-1 have been achieved in modulation doped InAs-AlSb structures. The work described in this thesis involves material characterization and process development for HEMT fabrication on this material system.

Growth and application of planar III-V semiconductor nanowires grown with MOCVD

The semiconductor nanowire has been widely studied over the past decade and identified as a promising nanotechnology building block with application in photonics and electronics. The flexible bottom-up approach to nanowire growth allows for straightforward fabrication of complex 1D nanostructures with interesting optical, electrical, and mechanical properties. III-V nanowires in particular are useful because of their direct bandgap, high carrier mobility, and ability to form heterojunctions and have been used to make devices such as light-emitting diodes, lasers, and field-effect transistors. However, crystal defects are widely reported for III-V nanowires when grown in the common out-of-plane <111>B direction. Furthermore, commercialization of nanowires has been limited by the difficulty of assembling nanowires with predetermined position and alignment on a wafer-scale. In this thesis, planar III-V nanowires are introduced as a low-defect and integratable nanotechnology building block grown with metalorganic chemical vapor deposition. Planar GaAs nanowires grown with gold seed particles self-align along the <110> direction on the (001) GaAs substrate. Transmission electron microscopy reveals that planar GaAs nanowires are nearly free of crystal defects and grow laterally and epitaxially on the substrate surface. The nanowire morphology is shown to be primarily controlled through growth temperature and an ideal growth window of 470 +\- 10 °C is identified for planar GaAs nanowires. Extension of the planar growth mode to other materials is demonstrated through growth of planar InAs nanowires. Using a sacrificial layer, the transfer of planar GaAs nanowires onto silicon substrates with control over the alignment and position is presented. A metal-semiconductor field-effect transistor fabricated with a planar GaAs nanowire shows bulk-like low-field electron transport characteristics with high mobility. The aligned planar geometry and excellent material quality of planar III-V nanowires may lead to highly integrated III-V nanophotonics and nanoelectronics.

Atomic resolution studies of oxide superlattices and ultrathin films

The ability to grow ultrathin films layer-by-layer with well-defined epitaxial relationships has allowed research groups worldwide to grow a range of artificial films and superlattices, first for semiconductors, and now with oxides. In the oxides thin film research community, there have been concerted efforts recently to develop a number of epitaxial oxide systems grown on single crystal oxide substrates that display a wide variety of novel interfacial functionality, such as enhanced ferromagnetic ordering, increased charge carrier density, increased optical absorption, etc, at interfaces. The magnitude of these novel properties is dependent upon the structure of thin films, especially interface sharpness, intermixing, defects, and strain, layering sequence in the case of superlattices and the density of interfaces relative to the film thicknesses. To understand the relationship between the interfacial thin film oxide atomic structure and its properties, atomic scale characterization is required. Transmission electron microscopy (TEM) offers the ability to study interfaces of films at high resolution. Scanning transmission electron microscopy (STEM) allows for real space imaging of materials with directly interpretable atomic number contrast. Electron energy loss spectroscopy (EELS), together with STEM, can probe the local chemical composition as well as local electronic states of transition metals and oxygen. Both techniques have been significantly improved by aberration correctors, which reduce the probe size to 1 Å, or less. Aberration correctors have thus made it possible to resolve individual atomic columns, and possibly probe the electronic structure at atomic scales. Separately, using electron probe forming lenses, structural information such as the crystal structure, strain, lattice mismatches, and superlattice ordering can be measured by nanoarea electron diffraction (NED). The combination of STEM, EELS, and NED techniques allows us to gain a fundamental understanding of the properties of oxide superlattices and ultrathin films and their relationship with the corresponding atomic and electronic structure. In this dissertation, I use the aforementioned electron microscopy techniques to investigate several oxide superlattice and ultrathin film systems. The major findings are summarized below. These results were obtained with stringent specimen preparation methods that I developed for high resolution studies, which are described in Chapter 2. The essential materials background and description of electron microscopy techniques are given in Chapter 1 and 2. In a LaMnO3-SrMnO3 superlattice, we demonstrate the interface of LaMnO3-SrMnO3 is sharper than the SrMnO3-LaMnO3 interface. Extra spectral weights in EELS are confined to the sharp interface, whereas at the rougher interface, the extra states are either not present or are not confined to the interface. Both the structural and electronic asymmetries correspond to asymmetric magnetic ordering at low temperature. In a short period LaMnO3-SrTiO3 superlattice for optical applications, we discovered a modified band structure in SrTiO3 ultrathin films relative to thick films and a SrTiO3 substrate, due to charge leakage from LaMnO3 in SrTiO3. This was measured by chemical shifts of the Ti L and O K edges using atomic scale EELS. The interfacial sharpness of LaAlO3 films grown on SrTiO3 was investigated by the STEM/EELS technique together with electron diffraction. This interface, when prepared under specific conditions, is conductive with high carrier mobility. Several suggestions for the conductive interface have been proposed, including a polar catastrophe model, where a large built-in electric field in LaAlO3 films results in electron charge transfer into the SrTiO3 substrate. Other suggested possibilities include oxygen vacancies at the interface and/or oxygen vacancies in the substrate. The abruptness of the interface as well as extent of intermixing has not been thoroughly investigated at high resolution, even though this can strongly influence the electrical transport properties. We found clear evidence for cation intermixing through the LaAlO3-SrTiO3 interface with high spatial resolution EELS and STEM, which contributes to the conduction at the interface. We also found structural defects, such as misfit dislocations, which leads to increased intermixing over coherent interfaces.

Low temperature magneto-photoluminescence characterization of high purity gallium arsenide and indium phosphide

Low-temperature magneto-photoluminescence is a very powerful technique to characterize high purity GaAs and InP grown by various epitaxial techniques. These III-V compound semiconductor materials are used in a wide variety of electronic, optoelectronic and microwave devices. The large binding energy differences of acceptors in GaAs and InP make possible the identification of those impurities by low-temperature photoluminescence without the use of any magnetic field. However, the sensitivity and resolution provided by this technique rema1ns inadequate to resolve the minute binding energy differences of donors in GaAs and InP. To achieve higher sensitivity and resolution needed for the identification of donors, a magneto-photoluminescence system 1s installed along with a tunable dye laser, which provides resonant excitation. Donors 1n high purity GaAs are identified from the magnetic splittings of "two-electron" satellites of donor bound exciton transitions 1n a high magnetic field and at liquid helium temperature. This technique 1s successfully used to identify donors 1n n-type GaAs as well as 1n p-type GaAs in which donors cannot be identified by any other technique. The technique is also employed to identify donors in high purity InP. The amphoteric incorporation of Si and Ge impurities as donors and acceptors in (100), (311)A and (3ll)B GaAs grown by molecular beam epitaxy is studied spectroscopically. The hydrogen passivation of C acceptors in high purity GaAs grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) 1s investigated using photoluminescence. Si acceptors ~n MBE GaAs are also found to be passivated by hydrogenation. The instabilities in the passivation of acceptor impurities are observed for the exposure of those samples to light. Very high purity MOCVD InP samples with extremely high mobility are characterized by both electrical and optical techniques. It is determined that C is not typically incorporated as a residual acceptor ~n high purity MOCVD InP. Finally, GaAs on Si, single quantum well, and multiple quantum well heterostructures, which are fabricated from III-V semiconductors, are also measured by low-temperature photoluminescence.

Electrical and optical studies on carbon nanotube arrays

Single-walled carbon nanotubes (SWNTs) have been studied as a prominent class of high performance electronic materials for next generation electronics. Their geometry dependent electronic structure, ballistic transport and low power dissipation due to quasi one dimensional transport, and their capability of carrying high current densities are some of the main reasons for the optimistic expectations on SWNTs. However, device applications of individual SWNTs have been hindered by uncontrolled variations in characteristics and lack of scalable methods to integrate SWNTs into electronic devices. One relatively new direction in SWNT electronics, which avoids these issues, is using arrays of SWNTs, where the ensemble average may provide uniformity from device to device, and this new breed of electronic material can be integrated into electronic devices in a scalable fashion. This dissertation describes (1) methods for characterization of SWNT arrays, (2) how the electrical transport in these two-dimensional arrays depend on length scales and spatial anisotropy, (3) the interaction of aligned SWNTs with the underlying substrate, and (4) methods for scalable integration of SWNT arrays into electronic devices. The electrical characterization of SWNT arrays have been realized by polymer electrolyte-gated SWNT thin film transistors (TFTs). Polymer electrolyte-gating addresses many technical difficulties inherent to electrical characterization by gating through oxide-dielectrics. Having shown polymer electrolyte-gating can be successfully applied on SWNT arrays, we have studied the length scaling dependence of electrical transport in SWNT arrays. Ultrathin films formed by sub-monolayer surface coverage of SWNT arrays are very interesting systems in terms of the physics of two-dimensional electronic transport. We have observed that they behave qualitatively different than the classical conducting films, which obey the Ohm’s law. The resistance of an ultrathin film of SWNT arrays is indeed non-linear with the length of the film, across which the transport occurs. More interestingly, a transition between conducting and insulating states is observed at a critical surface coverage, which is called percolation limit. The surface coverage of conducting SWNTs can be manipulated by turning on and off the semiconductors in the SWNT array, leading to the operation principle of SWNT TFTs. The percolation limit depends also on the length and the spatial orientation of SWNTs. We have also observed that the percolation limit increases abruptly for aligned arrays of SWNTs, which are grown on single crystal quartz substrates. In this dissertation, we also compare our experimental results with a two-dimensional stick network model, which gives a good qualitative picture of the electrical transport in SWNT arrays in terms of surface coverage, length scaling, and spatial orientation, and briefly discuss the validity of this model. However, the electronic properties of SWNT arrays are not only determined by geometrical arguments. The contact resistances at the nanotube-nanotube and nanotube-electrode (bulk metal) interfaces, and interactions with the local chemical groups and the underlying substrates are among other issues related to the electronic transport in SWNT arrays. Different aspects of these factors have been studied in detail by many groups. In fact, I have also included a brief discussion about electron injection onto semiconducting SWNTs by polymer dopants. On the other hand, we have compared the substrate-SWNT interactions for isotropic (in two dimensions) arrays of SWNTs grown on Si/SiO2 substrates and horizontally (on substrate) aligned arrays of SWNTs grown on single crystal quartz substrates. The anisotropic interactions associated with the quartz lattice between quartz and SWNTs that allow near perfect horizontal alignment on substrate along a particular crystallographic direction is examined by Raman spectroscopy, and shown to lead to uniaxial compressive strain in as-grown SWNTs on single crystal quartz. This is the first experimental demonstration of the hard-to-achieve uniaxial compression of SWNTs. Temperature dependence of Raman G-band spectra along the length of individual nanotubes reveals that the compressive strain is non-uniform and can be larger than 1% locally at room temperature. Effects of device fabrication steps on the non-uniform strain are also examined and implications on electrical performance are discussed. Based on our findings, there are discussions about device performances and designs included in this dissertation. The channel length dependences of device mobilities and on/off ratios are included for SWNT TFTs. Time response of polymer-electrolyte gated SWNT TFTs has been measured to be ~300 Hz, and a proof-of-concept logic inverter has been fabricated by using polymer electrolyte gated SWNT TFTs for macroelectronic applications. Finally, I dedicated a chapter on scalable device designs based on aligned arrays of SWNTs, including a design for SWNT memory devices.

Beyond hope: Rhetorics of mobility, possibility, and literacy

When writing teachers enter the classroom, they often bring with them a deep faith in the power of literacy to rectify social inequalities and improve their students’ social and economic standing. It is this faith—this hope for change—that draws some writing teachers to locations of social and economic hardship. I am interested in how teachers and theorists construct their own narratives of social mobility, possibility, and literacy. My dissertation analyzes the production and expression of beliefs about literacy in the narratives of a diverse group of writing teachers and theorists, from those beginning their careers to those who are published and widely read. The central questions guiding this study are: How do teachers’ and theorists’ narratives of becoming literate intersect with literacy theories? and How do such literacy narratives intersect with beliefs in the power of literacy to improve individuals’ lives socially, economically, and personally? I contend that the professional literature needs to address more fully how teachers’ and theorists’ personal histories with literacy shape what they see as possible (and desirable) for students, especially those from marginalized communities. A central focus of the dissertation is on how teachers and theorists attempt to resolve a paradox they are likely to encounter in narratives about literacy. On one hand, they are immersed in a popular culture that cherishes narrative links between literacy and economic advancement (and, further, between such advancement and a “good life”). On the other hand, in professional discourse and in teacher preparation courses, they are likely to encounter narratives that complicate an assumed causal relationship between literacy and economic progress. Understanding, through literacy narratives, how teachers and theorists chart a practical path through or around this paradox can be beneficial to literacy education in three ways. First, it can offer direction in professional development and teacher education, addressing how teachers negotiate the boundaries between personal experience, theory, and pedagogy. Second, it can help teachers create spaces wherein students can explore the impact of paradoxical views about the role of literacy on their own lives. Finally, it can offer direction in public policy discourse, extending awareness of what we want—and need—from English language arts education in the twenty-first century. To explore these issues, I draw on case studies and ethnographic observation as well as narrative inquiry into teachers’ and theorists’ published literacy narratives. I situate my findings within three interrelated frames: 1) the narratives of new teachers, 2) the published works of literacy educators and theorists, and 3) my own literacy narrative. My first chapter, “Beyond Hope,” explores the tenuous connections between hope and critique in literacy studies and provides a methodological overview of the study. I argue that scholarship must move beyond a singular focus on either hope or critique in order to identify the transformative potential of literacy in particular circumstances. Analyzing literacy narratives provides a way of locating a critically informed sense of possibility. My second chapter, “Making Teachers, Making Literacy,” explores the intersection between teachers’ lives and the theories they study, based on qualitative analysis of a preservice course for secondary education English teachers. I examine how these preservice English teachers understood literacy, how their narratives of becoming literate and teaching English connected—and did not connect—with theoretical and pedagogical positions, and how these stories might inform their future work as practitioners. Centering primarily on preservice teachers who resisted Nancie Atwell’s pedagogy of possibility because they found it too good to be true, this research concentrates on moments of disjuncture, as expressed in class discussion and in one-on-one interviews, when literacy theories failed to align with aspiring teachers’ understandings of their own experiences and also with what they imagined as possible in disadvantaged educational settings. In my third and fourth chapters, I analyze the narratives of celebrated teachers and theorists who put forth an agenda that emphasizes possibilities through literacy, examining how they negotiate the relationship between their own literacy stories and literacy theories. Specifically, I investigate the narratives of three proponents of critical literacy: Mike Rose, Paulo Freire, and Myles Horton, all highly respected literacy teachers whose working-class backgrounds influenced their commitment to teaching in disenfranchised communities. In chapter 3, “Reading Lives on the Boundary,” I demonstrate how Mike Rose’s 1989 autobiographical text, Lives on the Boundary, juxtaposes rhetorics of mobility with critiques of such possibility. Through an analysis of work published in professional journals, I offer a reception history of Rose’s narrative, focusing specifically on how teachers have negotiated the tension between hope and critique. I follow this analysis with three case studies, drawn from a larger sampling, that inquire into the personal connections that writing teachers make with Lives on the Boundary. The teachers in this study, who provided written responses and participated in audio-recorded follow-up interviews, were asked to compare Rose’s story to their own stories, considering how their personal literacy histories influenced their teaching. My findings illustrate how a group of teachers and theorists have projected their own assessments of what literacy and higher education can and cannot accomplish onto this influential text. In my fourth chapter, “Horton and Freire’s Road as Literacy Narrative,” I concentrate on Myles Horton and Paulo Freire’s 1990 collaborative spoken book, We Make the Road by Walking. Central to my analysis are the educators’ stories about their formative years, including their own primary and secondary education experiences. I argue that We Make the Road by Walking demonstrates how theories of literacy cannot be divorced from personal histories. I begin by examining the spoken book as a literacy narrative that fuses personal and theoretical knowledge, focusing specifically on its authors’ ideas on theory. Drawing on Bakhtin’s notion of the chronotope—the intersection of time and space within narrative—I then explore the literacy narratives emerging from the production process of the book, in a video production about Horton and Freire’s meeting, and ultimately in the two men’s reflections on their childhood years (Dialogic). Interspersed with these accounts is archival material on the book’s editorial production that illustrates the value of increased dialogue between personal history and theories of literacy. My fifth chapter is both a reflective analysis and a qualitative study of my work at a men’s medium-high security prison in Illinois, where I conducted research and served as the instructor of an upper-level writing course, “Writing for a Change,” in the spring of 2009. Entitled “Doing Time with Literacy Narratives,” this chapter explores the complex ways in which literacy and incarceration are configured in students’ narratives as well as my own. With and against students’ stories, I juxtapose my own experiences with literacy, particularly in relation to being the son of an imprisoned father. In exploring the intersections between such stories, I demonstrate how literacy narratives can function as a heuristic for exploring beliefs about literacy between teachers and students both inside and outside of the prison-industrial complex. My conclusion pulls together the various themes that emerged in the three frames, from the making of new teachers to the published literacy narratives of teachers and theorists to my own literacy narrative. Writing teachers encounter considerable pressure to align their curricula with one or another theory of literacy, which has the effect of negating the authority of knowledge about literacy gleaned from experience as readers and writers. My dissertation contends that there is much to be gained by finding ways of articulating theories of literacy that encompass teachers’ knowledge of reading and writing as expressed in personal narratives of literacy. While powerful cultural rhetorics of upward social mobility often neutralize the critical potential of teachers’ own narratives of literacy—potential that has been documented by scholars in writing studies and allied disciplines—this is not always the case. The chapters in this dissertation offer evidence that hopeful and critical positions on the transformational possibilities of literacy are not mutually exclusive.

In situ transmission electron microscopy ion irradiations of model iron-chromium alloys

Iron-chromium alloys are used as a model to study the microstructural evolution of defects in irradiated structural steel components of a nuclear reactor. We examine the effects of temperature and chromium concentration on the defect evolution and segregation behavior in the early stages of damage. In situ irradiations are conducted in a transmission electron microscope (TEM) at 300°C and 450°C with 150keV iron ions in single crystal Fe14Cr and Fe19Cr bicrystal to doses of 2E15 ions/cm^2. The microstructures resulting from annealing and irradiation of the alloy are characterized by analysis of TEM micrographs and diffraction patterns and compared with those of irradiated pure iron. We found the irradiation temperature to have little effect on the microstructural development. We also found that the presence of chromium in the sample leads to defect populations with small average loop size and no extended or nested loop structures, in contrast to the populations of large extended loops seen in irradiated pure iron. A very weak dependence was found on the specific chromium content of the alloy. Chromium was shown to suppress defect growth by inhibiting defect mobility in the alloy. While defects in pure iron are highly mobile and able to grow, those in the FeCr alloys remained small and relatively motionless due to the pinning effect of the chromium.

A study at high energy-resolution and at low primary energies of the secondary emission from (100) tungsten

This work was supported by the Joint Services Electronics Program (U.S. Army, U.S. Navy, and U.S. Air Force) under Contract No. DA 28 043 AMC 00073(E).