| Nanometer-scale materials and devices are under intense scrutiny at present and often depend on buried interfaces between two media. We have studied theoretically a novel method, x-ray standing-wave (SW) photoelectron spectroscopy, which is used to non-destructively investigate buried nanometerscale interfaces. The SW is generated via Bragg reflection from a multilayer mirror, on which the multilayer sample is grown. One layer of the sample is grown as a wedge, permitting the scanning of the SW through the layers above it. Prior experiments have focused on soft x-rays at 1 keV or less. However, advances in instrumentation have recently overcome difficulties with intensity and resolution involving hard x-ray SW experiments up to 10 keV or more. In order to support future experimental studies on these samples, we have investigated theoretically the systematics of SW experiments as the photon source is increased from 1 keV to 10 keV. We have studied a prototypical tunnel magnetoresistance (TMR) sample, whose layers from the top layer down to the substrate are: a 1 A pseudo-carbon contaminant layer of Be (chosen simply to give a slightly different core binding energy from the C in the multilayer mirror), 20 A of MgO, 20 A of a 70:30 alloy of CoFe, and finally a 78:22 IrMn alloy wedge that spans from 100 A to 400 A. The mirror substrate consists of 30 repetitions of B4C (20 A thick)/W (20 A thick). The photon energies explored are 1, 2, 4, 6, 8, and 10 keV. The photoelectron intensities in each layer investigated are Be1s, Mg2s, O1s, Co2p 3/2, Fe2p3/2, Ir4f 7/2, Mn2p3/2, B1s, C1s, and W4f 7/2. Via x-ray optical theory, we examine the reflectivity giving rise to the SW. Constructive and destructive interference associated with internal reflection at the top and bottom of the mirror and the wedge cause Kiessig fringes, which are more important for higher energies. For lower energies, the highest absolute peak intensities stem from the oxide layer; for the high energies (8 and 10 keV), the highest intensities arise from the wedge layer due to its higher thickness. The modulations of the 40 A-period SW roughly double in going from 1 keV to 10 keV. These results will aid in the design, understanding, and analysis of future standing-wave photoelectron spectroscopy experiments. |
