X Ray And Photoelectron Spectroscopy Of The Structure Reactivity And Electronic Structure Of Semiconductor Nanocrystals

Author : Alexander Gray
Publisher :
ISBN 13 : 9781267238719
Total Pages : pages
Book Rating : 4.2/5 (387 download)

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Download or read book Depth-resolved Electronic Structure of Spintronic Nanostructures and Complex Materials with Soft and Hard X-ray Photoemission written by Alexander Gray and published by . This book was released on 2011 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: In this dissertation we describe several new directions in the field of x-ray photoelectron spectroscopy, with a particular focus on the enhancement and control of the depth sensitivity and selectivity of the measurement. Enhancement of the depth sensitivity is achieved by going to higher photon energies with hard x-ray excitation and taking advantage of the resulting larger electron inelastic mean-free paths. This novel approach provides a more accurate picture of bulk electronic structure, when compared to the traditional soft x-ray photoelectron spectroscopy (XPS) which, for some systems, may be too strongly influenced by surface effects. We present three case-studies wherein such hard x-ray photoelectron spectroscopy (HAXPES) in the multi-keV regime is used to probe the bulk properties of complex thin-film materials, which would be otherwise impossible to investigate using conventional soft x-ray XPS. Namely, (1) we directly observe the opening of a semiconducting gap in epitaxial Cr[subscript 0.08]Al[subscript 0.20] alloy thin films and confirm this with theory, (2) we study the electronic and structural properties of near-Heusler Fe[subscript x]Si[subscript 1-x] alloy thin films of various composition and degrees of crystallinity, and (3) we observe the Mott metal-to-insulator transition in the ultra-thin epitaxial LaNiO3 films via core-level and valence-band spectroscopies. By performing the experiments at the photon energy of 5.95 keV, the bulk-sensitivity of the measurements, characterized by the inelastic mean-free path of the photoemitted electrons, is enhanced by a factor of 4-7 compared to the conventional soft x-ray photoelectron spectroscopy. The experimental results are compared to calculations performed using various first-principle theoretical approaches, such as the density-functional theory and the one-step theory of photoemission. Furthermore, we present the first results of hard x-ray angle-resolved photoemission measurements (HARPES), at excitation energies of 3.24 and 5.95 keV. Compared to traditional ARPES, carried out in the UPS regime (20-100 eV), our technique enables us to probe on average 10-40 times deeper into the bulk. Single-crystal W(110) is used to demonstrate viability of the technique, with clear dispersive bands seen at 5.95 keV. As a second proof of principle for HARPES, the technologically-relevant material GaAs(001) is used to illustrate the broad applicability of the technique. In order to demonstrate the compositional sensitivity of the new method, we also compare the measurements of pure GaAs to the measurements of the ferromagnetic semiconductor Ga[subscript 0.97]Mn[0.03]As. Clear differences due to Mn-derived states are observed in the bulk band structure measured with the excitation energy of 3.24 keV. All of these experimental HARPES results are compared to free-electron final-state model calculations and more precise fully-relativistic one-step photoemission theory including matrix element effects. As noted, increasing the inelastic mean-free path, or more properly the effective attenuation length [Lambda subscript EAL] of the photoemitted electrons by going to higher photon energies increases the bulk sensitivity of the measurement, and facilitates a wider range of probing depths which can be accessed via variable take-off angle (theta subscript TOA) experiments. However, this approach does not circumvent the fact that no matter how long the photoelectron escape depth is, the photoemission signal originating closer to the surface will prevail over the signal originating from below according to I(z)=I0exp[-z /[Lambda subscript EAL]sin[theta subscript TOA]]. This constraint can be modulated in a useful way, if one were allowed to tailor the intensity profile of the electromagnetic field of the incident x-ray radiation inside the sample. In a second aspect of this dissertation, this type of depth selectivity is achieved by setting-up an x-ray standing wave field in the sample by growing it on a synthetic periodic multilayer mirror substrate, which in first-order Bragg reflection acts as the standing-wave generator. The antinodes of the standing wave function as "epicenters" for photoemission, and can be moved in the direction perpendicular to the sample surface by either scanning the incidence angle [theta subscript inc], or the photon energy through the Bragg condition. Alternatively, provided that one of the underlying layers in the structure is grown in a shape of a wedge with varying thickness, the standing wave can be scanned vertically though the sample simply by moving the sample laterally under the x-ray measurement spot. We present the first study in which the chemical and electronic-structure profiles of a magnetic tunnel junction La[subscript 0.7]Sr[subscript 0.3]MnO3/SrTiO3 (LSMO/STO) have been quantitatively determined by a combination of soft and hard x-ray standing-wave excited photoemission. By comparing experiment to x-ray optical calculations, the detailed chemical profile of the constituent layers and their interfaces is quantitatively derived with Ångstrom precision. Combined with core-hole multiplet theory incorporating Jahn-Teller distortion, these results indicate a change in the Mn bonding state near the LSMO/STO interface. Our results thus further clarify the reduced performance of LSMO/STO magnetic tunnel junction compared to ideal theoretical expectations. Finally, we demonstrate the addition of depth resolution to the usual two-dimensional images in photoelectron emission microscopy (PEEM) as a further aspect of standing-wave photoemission. We show that standing-wave excited photoelectron microscopy can be used to produce element-specific and depth-selective images of patterned samples. In conjunction with x-ray optical theoretical modeling, quantitative information about the depth-dependent chemical composition of the sample can be extracted from the photoemission data. The good agreement between our experimental results and model calculations suggests that future studies with better spatial and spectral resolution will also yield more detailed information about the interfacial regions. This addition of quantitative depth selectivity to the conventional laterally-resolved soft x-ray photoelectron emission microscopy thus should considerably enhance the capabilities of the PEEM as a research, development and metrology tool for science and industry.