A. X. Gray

Probing bulk electronic structure with hard X-ray angle-resolved photoemission

Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.

Bulk electronic structure of the dilute magnetic semiconductor Ga1−xMnxAs through hard X-ray angle-resolved photoemission

A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga(0.97)Mn(0.03)As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga(0.97)Mn(0.03)As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga(1-x)Mn(x)As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material.

Optical detection and characterization of graphene by broadband spectrophotometry

The spectra of optical constants, index of refraction (n), and extinction coefficient (k) of graphene and graphite are obtained in the wavelength range of 190–1000 nm (6.53–1.24 eV) using broadband optical spectrophotometry in conjunction with the Forouhi–Bloomer dispersion relations for n and k. Measurement is made possible by the use of a multilayer substrate consisting of bulk Si and a 3000 A SiO2 film. The effect of multiple internal reflections between the Si/SiO2 and SiO2/graphene interfaces amplifies the attenuating effect of the graphene layer, thereby improving the sensitivity of the reflectance measurement by a factor of 27 in the deep ultraviolet region of the spectrum. Maximum sensitivity is observed in the deep ultraviolet region of the spectrum, where a strong peak in the spectrum of the extinction coefficient of graphene is identified. The proposed method enables fast nondestructive angstrom-level thickness measurements of graphene and graphite. In this work, layers ranging in thickness bet...

Observation of boron diffusion in an annealed Ta/CoFeB/MgO magnetic tunnel junction with standing-wave hard x-ray photoemission

The CoFeB/MgO system shows promise as a magnetic tunnel junction with perpendicular magnetization and low critical current densities for spin-torque driven magnetization switching. The distribution of B after annealing is believed to be critical to performance. We have studied the distribution of B in a Ta/Co0.2Fe0.6B0.2/MgO sample annealed at 300 °C for 1 h with standing-wave hard x-ray photoemission spectroscopy (SW-HXPS). Comparing experimental rocking curve data to x-ray optical calculations indicates diffusion of 19.5% of the B uniformly into the MgO and of 23.5% into a thin TaB interface layer. SW-HXPS is effective for probing depth distributions in such spintronic structures.

Temperature-driven nucleation of ferromagnetic domains in FeRh thin films

The evolution of ferromagnetic (FM) domains across the temperature-driven antiferromagnetic (AF) to FM phase transition in uncapped and capped epitaxial FeRh thin films was studied by x-ray magnetic circular dichroism and photoemission electron microscopy. The coexistence of the AF and FM phases was evidenced across the broad transition and the different stages of nucleation, growth, and coalescence were directly imaged. The FM phase nucleates into single domain islands and the width of the transition of an individual nucleus is sharper than that of the transition in a macroscopic average. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4730957]

Making use of x-ray optical effects in photoelectron-, Auger electron-, and x-ray emission spectroscopies: Total reflection, standing-wave excitation, and resonant effects

We present a general theoretical methodology and related open-access computer program for carrying out the calculation of photoelectron, Auger electron, and x-ray emission intensities in the presence of several x-ray optical effects, including total reflection at grazing incidence, excitation with standing-waves produced by reflection from synthetic multilayers and at core-level resonance conditions, and the use of variable polarization to produce magnetic circular dichroism. Calculations illustrating all of these effects are presented, including in some cases comparisons to experimental results. Sample types include both semi-infinite flat surfaces and arbitrary multilayer configurations, with interdiffusion/roughness at their interfaces. These x-ray optical effects can significantly alter observed photoelectron, Auger, and x-ray intensities, and in fact lead to several generally useful techniques for enhancing surface and buried-layer sensitivity, including layer-resolved densities of states and depth profiles of element-specific magnetization. The computer program used in this study should thus be useful for a broad range of studies in which x-ray optical effects are involved or are to be exploited in next-generation surface and interface studies of nanoscale systems.

Energetic, spatial, and momentum character of the electronic structure at a buried interface : The two-dimensional electron gas between two metal oxides

The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO3 (STO) and GdTiO3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6 unit cells (u.c.) STO/3 u.c. of GTO](20) using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry.

Band offsets in complex-oxide thin films and heterostructures of SrTiO3/LaNiO3 and SrTiO3/GdTiO3 by soft and hard X-ray photoelectron spectroscopy

The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν ≤ 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν > 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations.

Standing-wave excited soft x-ray photoemission microscopy: Application to Co microdot magnetic arrays

Standing-wave excited soft x-ray photoemission microscopy: Application to Co microdot magnetic arrays Alexander X. Gray, 1,2,a Florian Kronast, 3 Christian Papp, 2,4 See-Hun Yang, 5 Stefan Cramm, 6 Ingo P. Krug, 6 Farhad Salmassi, 7 Eric M. Gullikson, 7 Dawn L. Hilken, 7 Erik H. Anderson, 7 Peter Fischer, 2 Hermann A. Durr, 3 Claus M. Schneider, 6 and Charles. S. Fadley 1,2 Department of Physics, University of California, Davis, California 95616, USA Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Helmholtz-Zentrum Berlin, Albert-Einstein-Strase 15, 12489 Berlin, Germany Lehrstuhl fur Physikalische Chemie II, Universitat Erlangen-Nurnberg, Egerlandstrase 3, D-91054 Erlangen, Germany IBM Almaden Research Center, San Jose, California 95120, USA Institute of Solid State Research IFF-9, Research Center Julich, Julich D-52425, Germany Center for X-Ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA We demonstrate the addition of depth resolution to the usual two-dimensional images in photoelectron emission microscopy (PEEM), with application to a square array of circular magnetic Co microdots. The method is based on excitation with soft x-ray standing-waves generated by Bragg reflection from a multilayer mirror substrate. Standing wave is moved vertically through sample simply by varying the photon energy around the Bragg condition. Depth-resolved PEEM images were obtained for all of the observed elements. Photoemission intensities as functions of photon energy were compared to x-ray optical calculations in order to quantitatively derive the depth-resolved film structure of the sample. Soft x-ray photoelectron emission microscopy (PEEM) is an established powerful technique, enabling element- specific studies of surfaces and nanostructures through images obtained via core-level excitations of photoelectrons and secondary electrons. PEEM has been used extensively in recent years to study element-specific physical, chemical, structural, and magnetic properties of surfaces, resulting in a plethora of literature revealing interesting surface and nano- structure phenomena. 1–6 Most of these studies involve image formation from low-energy secondary electrons, although a growing number of microscopes can image with an energy- resolved photoelectron or Auger intensity that is thus also element-specific. PEEM images to date are inherently two- dimensional in space, with resolutions in the lateral x and y directions that are now typical 10–20 nm but which promise in the future to be in the few nanometer regime. 7 Very recently, a method has been suggested, by which soft x-ray standing-wave excitation extends the dimensionality of the PEEM, giving this technique depth resolution. 8 This depth resolution is achieved by setting-up an x-ray standing wave (SW) field in the sample by growing it on a synthetic periodic multilayer mirror substrate which in first- order Bragg reflection acts as the SW-generator. 9 The SW can then be moved vertically through the sample in several following ways: by varying the photon energy or the incidence angle through the Bragg condition, or in a previous study of this type, also by growing one layer of the sample as a wedge and looking along the wedge. 8 As the antinodes of the electromagnetic field shift vertically through the sample, they highlight various parts of the sample, introducing depth- selectivity in the photoemission process. Exploiting this depth-selectivity in an element-specific way requires an energy-selection of photoelectrons contributing to the PEEM image so as to image with a given core-electron intensity. In a previous exploratory study, this SW-PEEM technique has been used to determine the depth-resolved properties of a Ag- wedge/Co/Au multilayer nanostructure. 8 It was shown that with a suitable wedge-profile sample, the vertical resolution in the PEEM images approaches - ±3–4 A thus about 1/10 of the SW period, which is in turn equal to the period of the multilayer of 3–4 nm. In this study, we demonstrate the next step in the SWPEEM approach; imaging a lateral array of microstructures with depth resolution, as a prelude to future applications with true nanometer resolution in three dimensions. In contrast with the previous study 8 the approach used here, namely, moving the SW through the structure by means or varying photon energy, is simpler and more versatile because it does not require one to grow the layer of interest in a shape of a wedge. Due to this improvement, it is possible to measure patterned structures which are more technologically relevant, such as arrays of microdots or pillars described in this paper. One of the major advantages of the SW-PEEM technique is the enhanced interface-sensitivity of the measurement due to the possibility of translating the antinodes of the SW vertically through the sample and therefore choosing the “epicenter” of the photoemission. This effect cannot be achieved with other methods, such as, for example, using harder x-rays in order to increase the probing depth. We have investigated a nanostructured system consisting of square arrays of circular magnetic cobalt microdots, n o m i n al ly 4 n m in th ic k ne s s an d 1 µ m in di a m e ter , g r o w n o n a m u l t i l a y e r s u b s t r a t e o f c o n f i g u r a t i o n [23.6 A -Si/ 15.8 A -Mo] X 40, as depicted in cross section in

Identifying the electronic character and role of the Mn states in the valence band of (Ga,Mn)As.

We report high-resolution hard x-ray photoemission spectroscopy results on (Ga,Mn)As films as a function of Mn doping. Supported by theoretical calculations we identify, for both low (1%) and high (13%) Mn doping values, the electronic character of the states near the top of the valence band. Magnetization and temperature-dependent core-level photoemission spectra reveal how the delocalized character of the Mn states enables the bulk ferromagnetic properties of (Ga,Mn)As.