The material presented in this thesis concerns the growth and characterizationof III-V semiconductor heterostructures. Studies of the interactions betweenbound states in coupled quantum wells and between well and barrier bound statesin AlAs/GaAs heterostructures are presented. We also demonstrate the broad arrayof novel tunnel structures realizable in the InAs/GaSb/AlSb material system.Because of the unique broken-gap band alignment of InAs/GaSb these structuresinvolve transport between the conduction- and valence-bands of adjacent layers.These devices possess a wide range of electrical properties and are fundamentallydifferent from conventional AlAs/GaAs tunnel devices. We report on thefabrication of a novel tunnel transistor with the largest reported room temperaturecurrent gains. We also present time-resolved studies of the growth frontsof InAs/GainSb strained layer superlattices and investigations of surface anionexchange reactions.
Chapter 2 covers tunneling studies of conventional AlAs/GaAs RTD's. Theresults of two studies are presented: (i) A test of coherent vs. sequential tunnelingin triple barrier heterostructures, (ii) An optical measurement of the effect ofbarrier X-point states on Γ-point well states. In the first it was found if twoquantum wells are separated by a sufficiently thin barrier, then the eigenstatesof the system extend coherently across both wells and the central barriers. Forthicker barriers between the wells, the electrons become localized in the individualwells 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 statesinteract strongly. The barrier X-point states modify the energies of the well statesand increase the escape rate for carriers in the quantum well.
The results of several experimental studies of a novel class of tunnel devicesrealized in the InAs/GaSb/AlSb material system are presented in Chapter 3.These interband tunnel structures involve transport between conduction- andvalence-band states in adjacent material layers. These devices are compared andcontrasted with the conventional AlAs/GaAs structures discussed in Chapter 2and experimental results are presented for both resonant and nonresonant devices.These results are compared with theoretical simulations and necessary extensionsto 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 arediscussed.
Chapters 5 and 6 are studies of the growth of structures involving layers withdifferent anions. Chapter 5 covers the growth of InAs/GainSb superlattices forfar infrared detectors and time resolved, in-situ studies of their growth fronts.It was found that the bandgap of superlattices with identical layer thicknessesand compositions varied by as much as 40 meV depending on how their internalinterfaces are formed. The absorption lengths in superlattices with identicalbandgaps but whose interfaces were formed in different ways varied by as muchas a factor of two. First the superlattice is discussed including an explanationof the device and the complications involved in its growth. The experimentaltechnique of reflection high energy electron diffraction (RHEED) is reviewed,and the results of RHEED studies of the growth of these complicated structuresare presented. The development of a time resolved, in-situ characterization ofthe internal interfaces of these superlattices is described. Chapter 6 describesthe result of a detailed study of some of the phenomena described in chapter 5.X-ray photoelectron spectroscopy (XPS) studies of anion exchange reactions onthe growth fronts of these superlattices are reported. Concurrent RHEED studiesof the same physical systems studied with XPS are presented. Using the RHEEDand XPS results, a real-time, indirect measurement of surface exchange reactionswas developed.
