Growth and characterization of novel, III-V semiconductor heterostructures

The material presented in this thesis concerns the growth and characterization of III-V semiconductor heterostructures. Studies of the interactions between bound states in coupled quantum wells and between well and barrier bound states in AlAs/GaAs heterostructures are presented. We also demonstrate the broad array of novel tunnel structures realizable in the InAs/GaSb/AlSb material system. Because of the unique broken-gap band alignment of InAs/GaSb these structures involve transport between the conduction- and valence-bands of adjacent layers. These devices possess a wide range of electrical properties and are fundamentally different from conventional AlAs/GaAs tunnel devices. We report on the fabrication of a novel tunnel transistor with the largest reported room temperature current gains. We also present time-resolved studies of the growth fronts of InAs/GainSb strained layer superlattices and investigations of surface anion exchange reactions.

Chapter 2 covers tunneling studies of conventional AlAs/GaAs RTD's. The results of two studies are presented: (i) A test of coherent vs. sequential tunneling in triple barrier heterostructures, (ii) An optical measurement of the effect of barrier X-point states on Γ-point well states. In the first it was found if two quantum wells are separated by a sufficiently thin barrier, then the eigenstates of the system extend coherently across both wells and the central barriers. For thicker barriers between the wells, the electrons become localized in the individual wells and transport is best described by the electrons hopping between the wells. In the second, it was found that Γ-point well states and X-point barrier states interact strongly. The barrier X-point states modify the energies of the well states and increase the escape rate for carriers in the quantum well.

The results of several experimental studies of a novel class of tunnel devices realized in the InAs/GaSb/AlSb material system are presented in Chapter 3. These interband tunnel structures involve transport between conduction- and valence-band states in adjacent material layers. These devices are compared and contrasted with the conventional AlAs/GaAs structures discussed in Chapter 2 and experimental results are presented for both resonant and nonresonant devices. These results are compared with theoretical simulations and necessary extensions to the theoretical models are discussed.

In chapter 4 experimental results from a novel tunnel transistor are reported. The measured current gains in this transistor exceed 100 at room temperature. This is the highest reported gain at room temperature for any tunnel transistor. The device is analyzed and the current conduction and gain mechanisms are discussed.

Chapters 5 and 6 are studies of the growth of structures involving layers with different anions. Chapter 5 covers the growth of InAs/GainSb superlattices for far infrared detectors and time resolved, in-situ studies of their growth fronts. It was found that the bandgap of superlattices with identical layer thicknesses and compositions varied by as much as 40 meV depending on how their internal interfaces are formed. The absorption lengths in superlattices with identical bandgaps but whose interfaces were formed in different ways varied by as much as a factor of two. First the superlattice is discussed including an explanation of the device and the complications involved in its growth. The experimental technique of reflection high energy electron diffraction (RHEED) is reviewed, and the results of RHEED studies of the growth of these complicated structures are presented. The development of a time resolved, in-situ characterization of the internal interfaces of these superlattices is described. Chapter 6 describes the result of a detailed study of some of the phenomena described in chapter 5. X-ray photoelectron spectroscopy (XPS) studies of anion exchange reactions on the growth fronts of these superlattices are reported. Concurrent RHEED studies of the same physical systems studied with XPS are presented. Using the RHEED and XPS results, a real-time, indirect measurement of surface exchange reactions was developed.

Design, Synthesis, and Study of Novel Platforms for Iron-N2 Chemistry and Photoinduced, Copper-mediated C-N Bond Formation

Several new ligand platforms designed to support iron dinitrogen chemistry have been developed. First, we report Fe complexes of a tris(phosphino)alkyl (CP^iPr₃) ligand featuring an axial carbon donor intended to conceptually model the interstitial carbide atom of the nitrogenase iron-molybdenum cofactor (FeMoco). It is established that in this scaffold, the iron center binds dinitrogen trans to the C_alkyl anchor in three structurally characterized oxidation states. Fe-C_alkyl lengthening is observed upon reduction, reflective of significant ionic character in the Fe-C_alkyl interaction. The anionic (CP^iPr₃)FeN₂^- species can be functionalized by a silyl electrophile to generate (CP^iPr₃)Fe-N₂SiR₃. This species also functions as a modest catalyst for the reduction of N₂ to NH₃. Next, we introduce a new binucleating ligand scaffold that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N₂-Fe(μ-SAr)Fe-N₂) across at least three oxidation states (Fe^IIFe^II, Fe^IIFe^I, and Fe^IFe^I). Despite the sulfur-rich coordination environment of iron in FeMoco, synthetic examples of transition metal model complexes that bind N₂ and also feature sulfur donor ligands remain scarce; these complexes thus represent an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. The (N₂-Fe(μ-SAr)Fe-N₂) system undergoes reduction of the bound N₂ to produce NH₃ (~50% yield) and can efficiently catalyze the disproportionation of N₂H₄ to NH₃ and N₂. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Next, inspired by the importance of secondary-sphere interactions in many metalloenzymes, we present complexes of iron in two new ligand scaffolds ([SiP^NMe₃] and [SiP^iPr₂P^NMe]) that incorporate hydrogen-bond acceptors (tertiary amines) which engage in interactions with nitrogenous substrates bound to the iron center (NH₃ and N₂H₄). Cation binding is also facilitated in anionic Fe(0)-N₂ complexes. While Fe-N₂ complexes of a related ligand ([SiP^iPr₃]) lacking hydrogen-bond acceptors produce a substantial amount of ammonia when treated with acid and reductant, the presence of the pendant amines instead facilitates the formation of metal hydride species.

Additionally, we present the development and mechanistic study of copper-mediated and copper-catalyzed photoinduced C-N bond forming reactions. Irradiation of a copper-amido complex, ((m-tol)₃P)₂Cu(carbazolide), in the presence of aryl halides furnishes N-phenylcarbazole under mild conditions. The mechanism likely proceeds via single-electron transfer from an excited state of the copper complex to the aryl halide, generating an aryl radical. An array of experimental data are consistent with a radical intermediate, including a cyclization/stereochemical investigation and a reactivity study, providing the first substantial experimental support for the viability of a radical pathway for Ullmann C-N bond formation. The copper complex can also be used as a precatalyst for Ullmann C-N couplings. We also disclose further study of catalytic C_alkyl-N couplings using a CuI precatalyst, and discuss the likely role of [Cu(carbazolide)₂]^- and [Cu(carbazolide)₃]^- species as intermediates in these reactions.

Finally, we report a series of four-coordinate, pseudotetrahedral P₃Fe^II-X complexes supported by tris(phosphine)borate ([PhBP₃Fe^R]^-) and phosphiniminato X-type ligands (-N=PR'₃) that in combination tune the spin-crossover behavior of the system. Low-coordinate transition metal complexes such as these that undergo reversible spin-crossover remain rare, and the spin equilibria of these systems have been studied in detail by a suite of spectroscopic techniques.