Two-dimensional Reciprocal Space Research Articles

2D reciprocal space map of etched metalorganic chemical vapor deposited CdTe(001) film surface on miscut GaAs(001)

CdTe thin films have drawn continued interest in research and applications in various fields such as optoelectronics. We report on the study of a two-dimensional (2D) reciprocal space map of Br-methanol etched CdTe(001) epitaxial film grown by metalorganic chemical vapor deposition on miscut GaAs(001) using azimuthal reflection high-energy electron diffraction (ARHEED), which reveals a two-fold symmetric surface with a rectangular unit mesh. This is in contrast to the observed four-fold symmetry with a square unit mesh on an etched miscut single crystal CdTe(001). Two sets of two-fold symmetric diffraction spots with unequal intensities indicate unequal populations of two 90° orientation domains in the etched film. Based on the 2D maps and tilted azimuthal dependent RHEED patterns of the etched film, it unveils that the short length of rectangular unit mesh in the dominant domain aligns with the step edge direction. A simple method to characterize the miscut angle of a heteroepitaxial film grown on a miscut substrate is demonstrated using ARHEED. Both etched single crystal and film surfaces are Te rich with a higher Te to Cd ratio on the elongated hillocks in the etched film disclosed by scanning Auger electron spectroscopy mapping. This methodology can be applied to examine etched epitaxial film's surfaces that might deviate from bulk structure or bulk stoichiometry before interfacing with other materials to form heterojunctions or making ohmic contacts with metals.

Sn distribution in Ge/GeSn heterostructures formed by sputter epitaxy method

Ge/GeSn heterostructure was formed on Ge and Ge virtual substrates using the sputter epitaxy method, and the Sn profiles at the Ge/GeSn heterostructure were investigated using secondary ion mass spectrometry (SIMS). The growth temperature of the Ge/GeSn heterostructure was about 215 and 250 °C, and the thicknesses of the Ge and GeSn layers were about 90 and 70 nm, respectively. The growth rates of Ge and GeSn were relatively high and were about 0.36 and 0.4 nm/sec, respectively. The Sn content of the formed GeSn layers was estimated using an X-ray diffraction (XRD) two-dimensional reciprocal space map and was about 9 %. From the results of the atomic force microscopy and the transmission electron microscopy, the interface of the Ge/GeSn heterostructure was sharp and the surface was flat. At the Ge/GeSn interface in the sample formed at 215 °C, the Sn concentration was decreased at 3.4 nm/dec, and the 1/e decay length was about 15Å, which were smaller than 8 nm/dec and 35 Å in the previous report. The Sn diffusion depended on the growth temperature and the underlayer. The gradient of the Sn content at the 390-nm-thick GeSn layer was estimated using SIMS and was about 0.55 %/μm in the sputter epitaxy method, which was smaller than 6.2 %/μm in the previous reports. From the XRD measurements, the 390-nm-thick GeSn layer had no strain relaxation. It is considered that a high-growth rate and no strain relaxation contribute to limiting the Sn diffusion, and uniform Sn content GeSn layers were obtained.

Investigation of Band Structure in Strained Single Crystalline Si1-X Sn X

1. Background and purpose Silicon tin (SiSn) alloys are attractive candidate for the next-generation group-IV semiconductors. It is well known that Si and Ge change from indirect to direct transition types with the addition of Sn. Among them, SiSn alloys are expected to be applied for the near-infrared optical devices because it is predicted to become a direct band-gap appropriate for the optical communication with sufficient Sn content [1]. Although the Sn composition that changes from indirect to direct band-gap has been reported using several calculations, there are few reports of experimental results. In addition, the optical properties of single crystalline Si1-x Sn x have not been sufficiently clarified owing to the difficulty of crystal growth. In this study, we evaluated the optical properties to clarify the band structure of single crystalline Si1-x Sn x . 2. Experimental method Strained 30 nm-thick Si1-x Sn x films were grown on Si substrate by molecular beam epitaxy (MBE), and epitaxial growth was confirmed by X-ray diffraction (XRD) two-dimensional reciprocal space mapping (2DRSM) [2]. The Sn composition was determined by 2DRSM. The Sn fraction x of Si1-x Sn x samples used in this study were 0.005, 0.009, 0.018, 0.022, and 0.06. The spectroscopic ellipsometry measurements were performed with an incident angle of 70°, using a 70 W xenon lamp with a wavelength range of 200 to 1600 nm for the light source. The functions used in the analysis are the Tauc-Lorentz and Lorentz function. 3. Results and Discussion Figure 1(a) and (b) show real part and imaginary part of the complex dielectric functions for Si1-x Sn x (x=0.005 - 0.06) and pure-Si, respectively. From Fig. 1, it can be confirmed that the spectra shift with increasing Sn composition and the peaks appear around 1.8 eV and 2.8 eV for the samples with high Sn composition (2.2% and 6.0%). The peak shift characteristic with the addition of Sn is similar to the results of germanium tin (GeSn) [3].The complex dielectric functions of the semiconductor include much information about the electronic band structure. In Fig. 1, the sharp peaks that are due to direct band transitions show what is known as critical points (CPs). In the case of Si, E'0 CP (Γ) and E 1 CP (L) energy are around 3.2 eV [4]. Therefore, it can be considered that the peak at 2.8 eV indicates the separation of the E'0 and E 1 CP energies due to the change in the band gap at the Γ point with the addition of Sn. The peak at 1.8 eV is unique to Si1-x Sn x , which is not found in Si. We consider that this peak is due to the split-off valence band caused by the Sn addition. The behavior of the valence band is strongly affected by strain. These results suggest a reduction of the band gap at the Γ point and the formation of an optical transition region due to the Sn addition, and reveal important findings for the application of SiSn for the near-infrared devices. Acknowledgment This work was partly supported by the Japan Society for the Promotion of Science (JSPS) (17J08240, 19K21971, and 21H01366).

Visualization of local strain in 4H-SiC trench metal-oxide-semiconductor field-effect transistor using synchrotron nanobeam X-ray diffraction

We investigated the local strain in a silicon carbide (4H-SiC) (0001) trench metal-oxide semiconductor field-effect transistor (MOSFET) using synchrotron nanobeam X-ray diffraction (nano-XRD) at the SPring-8 BL13XU beamline. Using X-rays incident on the 4H-SiC trench cross section, diffraction measurements were performed on the and planes. Intensity maps of the and diffractions yielded images reflecting the trench structure. The spatial resolution of the 4H-SiC intensity map in the [0001] direction was higher than that for the diffraction because the angle of incidence was close to perpendicular to the sample. From two-dimensional reciprocal space maps of the symmetric and asymmetric diffractions, the tilt and the - and (0004)-plane strain components were individually separated and visualized. A tensile strain of approximately 0.1% was found in the region between the trenches. These results indicate the effectiveness of nano-XRD for strain visualization in trench MOSFETs and similar electronic devices.

Surface and interface structures of epitaxial Sb2Se3 on mica

Sb2Se3 thin film is an emerging photon absorber used in solar cells. We report the study of surface and interface structures of Sb2Se3(120) film grown on mica substrate by a high-rate vapor transport method. The interface epitaxial relationship between Sb2Se3 and mica examined by the cross- sectional TEM images and diffraction patterns along the [001] and [10]2¯ directions of Sb2Se3 reveal a rectangular structure with lengths of 4.03 ± 0.1 Å and 5.29 ± 0.1 Å, consistent with the [120] out-of-plane direction of Sb2Se3 bulk lattice parameters. In contrast, the two-dimensional reciprocal space map (2D map) constructed from azimuthal reflection high-energy electron diffraction (ARHEED) patterns from the surface exhibits a decorated hexagonal structure. This surface structure emerges from six epitaxial orientation domains/rods and each domain has a rectangular unit mesh of 3.94 ± 0.09 Å and 26.95 ± 1.16 Å along the [001] and [10]2¯ directions. The 26.95 Å is consistent with the unit mesh of the outermost layer of the Sb2Se3(120) domains/rods. Our 2D map reveals surface information that are not easily observed by other diffraction techniques.

In-plane growth of germanium nanowires on nanostructured Si(001)/SiO2 substrates

Germanium (Ge) nanowires (NWs) were grown in-plane on nano-structured Si(001)/SiO2 substrates by molecular beam epitaxy using gold (Au) as the solvent. The site-selective NW growth was enabled by a rectangular array of gold droplets on silicon (Si) tips with an Au nuclei density below 0.25 µm−2 on the surrounding silicon oxide (SiO2). The initial growth of Ge NWs starting from Si–Au droplets with SixGe1−x nucleation from ternary alloy is discussed from a thermodynamic point of view. The in-plane NW elongation occurred within 〈110〉 directions on the substrate and NWs were mainly bounded by two 55° inclined 111 facets and a less pronounced planar (001) top facet. Fully relaxed crystal lattices of Ge NWs were observed from two-dimensional reciprocal space maps of x-ray diffraction measurements.

Structural Property Study for GeSn Thin Films.

The structural properties of GeSn thin films with different Sn concentrations and thicknesses grown on Ge (001) by molecular beam epitaxy (MBE) and on Ge-buffered Si (001) wafers by chemical vapor deposition (CVD) were analyzed through high resolution X-ray diffraction and cross-sectional transmission electron microscopy. Two-dimensional reciprocal space maps around the asymmetric (224) reflection were collected by X-ray diffraction for both the whole structures and the GeSn epilayers. The broadenings of the features of the GeSn epilayers with different relaxations in the ω direction, along the ω-2θ direction and parallel to the surface were investigated. The dislocations were identified by transmission electron microscopy. Threading dislocations were found in MBE grown GeSn layers, but not in the CVD grown ones. The point defects and dislocations were two possible reasons for the poor optical properties in the GeSn alloys grown by MBE.

Time-resolved X-ray reciprocal space mapping of a crystal in an external electric field

A reciprocal space mapping technique with the use of triple-crystal time-resolved X-ray diffractometry has been developed and implemented using a laboratory X-ray source for the first time. This technique allows studying fast processes that occur in a sample under external influences that cause reversible deformations of its crystal lattice. It also allows distinguishing these processes in time and distinguishing different types of crystal deformations caused by these actions. The essence of the technique is to measure time dependences of the intensity for each point of the reciprocal space in the vicinity of the diffraction maximum in three-axis diffraction geometry by subjecting the sample to repeated and structurally identical action of a strong electric field, with the subsequent construction of the time evolution of the two-dimensional reciprocal space map. The time resolution is achieved with the use of a high-speed multichannel intensity analyzer synchronized with a high-voltage source. The results of measuring the reciprocal space maps with a laboratory radiation source with a time resolution of up to 10 ms are demonstrated for a piezoelectric crystal of lanthanum gallium silicate subjected to an external electric field with the field strength 3.08 kV mm−1, which is close to the sample breakdown value.

Coherent Electron Trajectory Control in Graphene.

We investigate coherent electron dynamics in graphene, interacting with the electric field waveform of two orthogonally polarized, few-cycle laser pulses. Recently, we demonstrated that linearly polarized driving pulses lead to sub-optical-cycle Landau-Zener quantum path interference by virtue of the combination of intraband motion and interband transition [Higuchi etal., Nature 550, 224 (2017)NATUAS0028-083610.1038/nature23900]. Here we introduce a pulsed control laser beam, orthogonally polarized to the driving pulses, and observe the ensuing electron dynamics. The relative delay between the two pulses is a tuning parameter to control the electron trajectory, now in a complex fashion exploring the full two-dimensional reciprocal space in graphene. Depending on the relative phase, the electron trajectory in the reciprocal space can, e.g., be deformed to suppress the quantum path interference resulting from the driving laser pulse. Intriguingly, this strong-field-based complex matter wave manipulation in a two-dimensional conductor is driven by a high repetition rate laser oscillator, rendering unnecessary complex and expensive amplified laser systems.

Large Area Atomically Flat Surfaces via Exfoliation of Bulk Bi2Se3 Single Crystals

In this work, we present an exfoliation method that produces cm2-area atomically flat surfaces from bulk layered single crystals, with broad applications such as for the formation of lateral heterostructures and for use as substrates for van der Waals epitaxy. Single crystals of Bi2Se3 were grown using the Bridgman method and examined with X-ray reciprocal space maps, Auger spectroscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy. An indium-bonding exfoliation technique was developed that produces multiple ∼100 μm thick atomically flat, macroscopic (>1 cm2) slabs from each Bi2Se3 source crystal. Two-dimensional X-ray diffraction and reciprocal space maps confirm the high crystalline quality of the exfoliated surfaces. Atomic force microscopy reveals that the exfoliated surfaces have an average root-mean-square (RMS) roughness of ∼0.04 nm across 400 μm2 scans and an average terrace width of 70 μm between step edges. First-principles calculations reveal exfoliation energies of Bi2...

Ab initio phonon scattering by dislocations

Heat management in thermoelectric and power devices as well as in random access memories poses a grand challenge and can make the difference between a working and an abandoned device design. Despite the prevalence of dislocations in all these technologies, the modeling of their thermal resistance is based on 50-year-old analytical approximations, whose simplicity was driven by practical limitations rather than physical insight. We introduce an efficient ab initio approach based on Green's functions computed by two-dimensional reciprocal space integration. By combining elasticity theory and density functional theory, we calculate the scattering strength of a ${90}^{\ensuremath{\circ}}$ misfit edge dislocation in Si. Because of the breakdown of the Born approximation, earlier literature models fail, even qualitatively. We find that a dislocation density larger than ${10}^{9}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ is necessary to substantially influence thermal conductivity at room temperature and above. We quantify how much of the reduction of thermal conductivity measured in nanograined samples can be explained by realistic dislocation concentrations.

Analysis of Microscopic Strain and Crystalline Structure in Ge/Ge1−X Sn x Fine Structures by Using Synchrotron X-ray Microdiffraction

The strain engineering is a promising technology not only for low-power CMOS but also for optoelectronic applications of germanium (Ge) devices compatible with the conventional silicon (Si) process technology [1]. In terms of CMOS applications, a uniaxial compressive strained Ge has drawn attention as high-mobility channel material in p-MOSFET which is expected to transcend that of conventional strained Si channel [2]. We are focusing on germanium-tin (Ge1−x Sn x ) as a source/drain (S/D) stressors to apply a uniaxial compressive strain into Ge in analogy with Si1−x Ge x S/D for strained Si channel. However, there are few reports of the local epitaxial growth of Ge1−x Sn x on patterned Ge substrate and the local strain technology of the Ge/Ge1−x Sn x heterostructures. Since the recent device dimensions of Si CMOS devices have reached the sub-100 nm scale in accordance with Moore’s law, the investigation of the microscopic crystalline and the strain structures for various next generation semiconductor materials has grown increasingly important. In this study, we experimentally characterized the local strain structure in Ge/Ge1−x Sn x fine heterostructures by using synchrotron x-ray microdiffraction method [3,4]. We prepared Ge stripe structures with various line widths (25–100 nm) sandwiched with Ge1−x Sn x stressors. Patterned Ge(001) substrate with a SiO­ 2 cap layer was formed by anisotropic wet/dry etching as lines parallel along to the <110> direction. The pitch of a Ge stripe is 500 nm. The Ge1−x Sn x layers were grown with solid-source molecular beam epitaxy (MBE) system and metal-organic chemical vapor deposition (MOCVD) system [5]. The Sn content in the Ge1−x Sn x was 2.9–6.5%. The x-ray microdiffraction measurement was performed at the beamline BL13XU in SPring-8 to analyze the local strain distribution and crystalline structure in Ge/Ge1−x Sn x samples. A synchrotron radiation light with an energy of 8 keV (λ=0.155 nm) was used. The cross-section size of an incident x-ray microbeam was estimated to be 0.16×0.20–0.82×0.26 μm2 in preliminary experiments. Transmission electron microscope observations revealed that Ge1−x Sn x were epitaxially grown on both sides of a Ge line, while the crystalline structure of Ge1−x Sn x on SiO2 on the top of Ge line was polycrystalline. We consider that the stressors of locally grown epitaxial Ge1−x Sn x can induce a local strain into the Ge line. In order to directly characterize the in-plane strain status in strained Ge region, we examined two-dimensional reciprocal space mapping (2DRSM) around the Ge1—1—3 asymmetric Bragg reflection with step scans of the microbeam position across the Ge lines with a step of 50 nm. Three diffraction peaks related to the bulk Ge, the epitaxial Ge1−x Sn x stressors, and the strained Ge are clearly observed in 2DRSM results. The intensity of diffraction peak of the strained Ge periodically varied corresponds to the stripe pitches. The diffraction peak position of the strained Ge in 2DRSM indicates an in-plane compressive strain is induced into a Ge line sandwiched with Ge1−x Sn x stressors. From the 2DRSM results for various Ge stripe widths, an in-plane strain value can be estimated from the lattice spacing with respect to that of bulk Ge. The in-plane compressive strain value in the Ge increases with narrowing the Ge line width and increasing the Sn content in the Ge1−x Sn x stressors, which is consistent with the simulated results of finite element method calculations. For the Ge0.943Sn0.057 stressors, the in-plane compressive strains were estimated to be 0.84%, 0.46% and 0.38% for 30, 60, and 100 nm-wide Ge lines, respectively. In summary, the formation of Ge/Ge1−x Sn x fine heterostructures were achieved by MBE and MOCVD, and experimentally characterized the local strain distribution in the Ge/Ge1−x Sn x by synchrotron radiation microdiffraction method. The microdiffraction enables a quantitative analysis of strain and an investigation of local crystalline structure in sub-100 nm-scale Ge lines sandwiched with Ge1−x Sn x stressors. This work was partly supported by the JSPS through the FIRST Program initiated by the CSTP, a Grant-in-Aid for Scientific Research (S) (Grant No. 26220605) and Core-to-Core Program ICRC-ACP4ULSI from the JSPS in Japan. The synchrotron radiation experiments were performed at SPring-8 with the approval of JASRI (Nos. 2012B1783, 2013A1682, 2013B1779, 2015A1874, and 2015B1813/BL13XU). [1] S. Zaima et al., Sci. Technol. Adv. Mater. 16, 043502 (2015). [2] T. Krishnamohan et al., IEDM Proc., 899 (2008). [3] S. Takeda et al., Jpn. J. Appl. Phys. 45, L1054 (2006). [4] Y. Imai et al., AIP Conf. Proc. 1221, 30 (2010). [5] Y. Inuzuka et al., Thin Solid Films 602, 7 (2016).

Depth-resolved magnetic and structural analysis of relaxing epitaxialSr2CrReO6

Structural relaxation in a ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$ epitaxial film, which exhibits strong spin-orbit coupling, leads to depth-dependent magnetism. We combine two depth-resolved synchrotron x-ray techniques---two-dimensional reciprocal space mapping and x-ray magnetic circular dichroism---to quantitatively determine this effect. An 800-nm-thick film of ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$, grown with tensile epitaxial strain on ${\mathrm{SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}(225\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/{\text{(LaAlO}}_{3}{)}_{0.3}({\text{Sr}}_{2}{\mathrm{AlTaO}}_{6}{)}_{0.7}$, relaxes away from the ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}{\text{/SrCr}}_{0.5}{\mathrm{Nb}}_{0.5}{\mathrm{O}}_{3}$ interface to its bulk lattice parameters, with much of the film being fully relaxed. Grazing incidence x-ray diffraction of the film elucidates the in-plane strain relaxation near the film-substrate interface, while depth-resolved x-ray magnetic circular dichroism at the Re L edge reveals the magnetic contributions of the Re site. The smooth relaxation of the film near the interface correlates with changes in the magnetic anisotropy. This provides a systematic and powerful way to probe the depth-varying structural and magnetic properties of a complex oxide with synchrotron-source x-ray techniques.

Characterization of Ge1-x-ySixSny Ternary Alloys – Comparison of UHV-CVD and Gas Source MBE Growth

Group-IV semiconductor alloys are attractive materials for advanced optoelectronic device applications such as integrated Si photonics, multi-junction solar cells, telecom, and infrared sensing. Over the past decade, ternary alloys of Ge1-x-ySixSny have been developed with attractive materials properties for these applications. These alloys show great promise for strain engineering or as compliant buffer layers since their band gap energy and lattice parameter can be tuned independently [1]. In addition, for sufficient levels of Sn incorporation, the fundamental optical band gap undergoes a crossover from indirect to direct transition [2-4]. The first demonstrated synthesis of Ge1-x-ySixSny was achieved via a UHV-CVD process using deuterated stanane (SnD4) and digermane (Ge2H6) precursors [5]. Low growth temperatures (T < 350 ◦C) are required to stabilize the substitutional Sn in the SiGe lattice because of the large disparity in atomic size. These deposition processes have subsequently been improved and extended to include the use of higher order group-IV hydride precursors such as Ge3H8, Si3H8, Ge4H10, and Si4H10 [6-9]. Alternative growth processes have also been developed to synthesize Ge1-x-ySixSny alloys including gas source molecular beam epitaxy (GS-MBE) [10] and ion implantation of Sn into a SiGe matrix [11].In this work, we report the electrical, optical, structural, and material characteristics of Ge1-x-ySixSny films over a range of alloy compositions. We compare the properties of films deposited by UHV-CVD with those grown by GS-MBE. The elemental compositions of these Ge1-x-ySixSny alloys, determined by X-ray photoelectron spectroscopy (XPS), are in good agreement with values measured using Rutherford backscattering (RBS). The XPS measurements also indicate that these alloys are susceptible to surface oxidation when exposed to laboratory ambient. Crystalline structural characteristics were measured with X-ray diffraction, two-dimensional reciprocal space mapping and X-ray reflectivity. Thin Ge1-x-ySixSny films (~ 300 nm) grown on Ge substrates showed coherent strain, while thicker films (~ 800 nm) showed partial strain relaxation. The thickest films (~ 1000 nm) were completely relaxed. No thickness fringes were observed in X-ray reflectivity due to negligible difference in electron density between the film and substrate.Temperature-dependent Hall-effect measurements show highly conductive layers although care must be exercised to avoid parallel conduction through the substrate for samples grown on Si or Ge wafers. We also report the first measurement of electronic traps in Ge1-x-ySixSny via deep level transient spectroscopy (DLTS) on p-n junction devices. Samples grown by UHV-CVD show two electron traps at low temperature while samples grown by GS-MBE show only one which is more than 10x weaker. Both the UHV-CVD and GS-MBE samples exhibit an activation energy of Ea=0.40 eV for the dark current for 200 K < T < 350 K. In addition, the leakage current under reverse bias conditions increases with increasing Sn content in the films.[1] V. R. D’Costa, Y.-Y. Fang, J. Tolle, J. Kouvetakis, and J. Menendez, Phys. Rev. Lett. 102, 107403 (2009).[2] D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994 (1987).[3] R. A. Soref and C. H. Perry, J. Appl. Phys. 69, 539 (1991)[4] J. D. Gallagher, C. Xu, L. Jiang, J. Kouvetakis, and J. Menendez, Appl. Phys. Lett. 103, 202104 (2013)[5] M. Bauer, J. Taraci, J. Tolle, A. V. G. Chizmeshya, S.Zollner, D. J. Smith, J. Menendez, C. Hu and J. Kouvetakis, Appl. Phys. Lett., 81, 2992 (2002).[6] J. Tolle, A. V. G. Chizmeshya, Y. Y. Fang, J. Kouvetakis, V. R. D’Costa, C. W. Hu, J. Men_endez, and I. S. T. Tsong, Appl. Phys. Lett. 89, 231924 (2006).[7] Y.-Y. Fang, J. Xie, J. Tolle, R. Roucka, V. R. D’Costa, A. V. G. Chizmeshya, J. Menendez, and J. Kouvetakis, J. Am. Chem. Soc. 130, 16095 (2008).[8] J. Xie, J. Tolle, V. R. D’Costa, A. V. G. Chizmeshya, J. Men_endez, and J. Kouvetakis, Appl. Phys. Lett. 95, 181909 (2009).[9] C. Xu, L. Jiang, J. Kouvetakis, and J. Men_endez, Appl. Phys. Lett. 103, 072111 (2013).[10] H. Lin, R. Chen, W. Lu, Y. Huo, T. I. Kamins, and J. S. Harris, Appl. Phys. Lett. 100, 141908 (2012).[11] G. H. Wang, E.-H. Toh, X. Wang, S. Tripathy, T. Osipowicz, T. K. Chan, K.-M. Hoe, S. Balakumar, G.-Q. Lo, G. Samudra, and Y.-C. Yeo, Appl. Phys. Lett. 91, 202105 (2007).

Inelastic light-scattering from spin-density excitations in the regime of the persistent spin helix in a GaAs-AlGaAs quantum well

We have investigated the intrasubband spin-density excitation (SDE) in an asymmetrically doped GaAs-AlGaAs single quantum well with balanced Rashba and Dresselhaus spin-orbit interaction strengths by inelastic light scattering. For this unique symmetry, the combined spin-orbit field is either parallel or antiparallel to the [11¯0] in-plane direction of the quantum well for all wave vectors of the two-dimensional reciprocal space. In backscattering geometry, the SDE is formed by spin-flip intrasubband transitions of the spin-split subband. Via the splitting of the intrasubband SDE, we have directly detected the spin splitting of the conduction band due to the anisotropic spin-orbit field. As expected, the splitting is nonzero if a wave vector is transferred perpendicular to the direction of the spin-orbit field and close to zero for a parallel wave-vector transfer. The extracted values for the spin-orbit strength and for the wavelength of a persistent spin helix compare well with results of previous experiments of direct spatial mapping of the spin helix.

The power ofin situpulsed laser deposition synchrotron characterization for the detection of domain formation during growth of Ba0.5Sr0.5TiO3on MgO

A highly sophisticated pulsed laser deposition (PLD) chamber has recently been installed at the NANO beamline at the synchrotron facility ANKA (Karlsruhe, Germany), which allows for comprehensive studies on the PLD growth process of dielectric, ferroelectric and ferromagnetic thin films in epitaxial oxide heterostructures or even multilayer systems by combining in situ reflective high-energy diffraction with the in situ synchrotron high-resolution X-ray diffraction and surface diffraction methods. The modularity of the in situ PLD chamber offers the opportunity to explore the microstructure of the grown thin films as a function of the substrate temperature, gas pressure, laser fluence and target-substrate separation distance. Ba0.5Sr0.5TiO3 grown on MgO represents the first system that is grown in this in situ PLD chamber and studied by in situ X-ray reflectivity, in situ two-dimensional reciprocal space mapping of symmetric X-ray diffraction and acquisition of time-resolved diffraction profiles during the ablation process. In situ PLD synchrotron investigation has revealed the occurrence of structural distortion as well as domain formation and misfit dislocation which all depend strongly on the film thickness. The microstructure transformation has been accurately detected with a time resolution of 1 s. The acquisition of two-dimensional reciprocal space maps during the PLD growth has the advantage of simultaneously monitoring the changes of the crystalline structure as well as the formation of defects. The stability of the morphology during the PLD growth is demonstrated to be remarkably affected by the film thickness. A critical thickness for the domain formation in Ba0.5Sr0.5TiO3 grown on MgO could be determined from the acquisition of time-resolved diffraction profiles during the PLD growth. A splitting of the diffraction peak into two distinguishable peaks has revealed a morphology change due to modification of the internal strain during growth.

A classroom demonstration of reciprocal space

An array of nanowires and a laser pointer are used for a simple visualization of two-dimensional reciprocal space. The experiment can be performed without any preparation and in any classroom. It aids the teaching of scattering experiments, and illustrates the underlying principles of electron, x-ray, and neutron scattering. A detailed study of the diffraction pattern was performed by mounting the sample with nanowires on a stage designed for x-ray scattering. The setup is well suited for undergraduate students, who get training in sample alignment in a small lab instead of at a large-scale facility. The exact positions of the diffraction spots are calculated and monitored experimentally for a 360° rotation of the sample. By fitting to this set of images, it is possible to determine the lattice vectors of the artificial crystal with an uncertainty of less than 1%.

Mechanical properties of interacting lipopolysaccharide membranes from bacteria mutants studied by specular and off-specular neutron scattering

Specular and off-specular neutron scattering are used to study the influence of molecular chemistry (mutation) on the intermembrane interactions and mechanical properties of the outer membrane of Gram-negative bacteria consisting of lipopolysaccharides (LPSs). For this purpose, solid-supported multilayers of mutant LPS membranes are deposited on silicon wafers and hydrated either at defined humidity or in bulk buffers. The planar sample geometry allows to identify out-of-plane and in-plane scattering vector components. The measured two-dimensional reciprocal space maps are simulated with membrane displacement correlation functions determined by two mechanical parameters (vertical compression modulus and bending rigidity) and an effective cutoff radius for the membrane fluctuation wavelength. Experiments at controlled humidity enable one to examine the influence of the disjoining pressure on the saccharide-mediated intermembrane interactions, while experiments in bulk buffers (i.e., in the absence of an external osmotic stress) reveal the effect of divalent cations on LPS membranes, highlighting the role of divalent cations in the survival mechanism of bacteria in the presence of antimicrobial molecules.

Effects of Mg doping on structural, optical, and electrical properties of CuScO 2(0001) epitaxial films

CuScO 2 thin films with different Mg concentrations were grown on a-plane sapphire substrates by combining the two-step deposition and post-annealing techniques using Cu 2(Sc 1− x Mg x ) 2O y [ X = 0.01, 0.05, 0.10] targets. The effects of the Mg doping in the Sc-site on the structural, optical, and electrical properties of the films were investigated. A Mg-doped CuScO 2[3R](0001) epitaxial film was obtained at a Mg concentration of 1 at%. The slight increase in the a-axis lattice constant and the slight decrease in the c-axis lattice constant of the film were confirmed using two-dimensional X-ray reciprocal space mapping. No significant increase in optical absorption was observed in the film, and the energy gap for direct allowed transition was estimated at 3.7 eV. The film showed an increase in the electrical conductivity and carrier concentration and a decrease in the Hall mobility compared with those of the non-doped epitaxial film. The decrease in the overlap of Cu 3d orbitals due to the increase in the a-axis lattice constant is one cause of the decrease in the Hall mobility of the film. The temperature dependence of the electrical transport properties of the film exhibited semiconducting characteristics, and the activation energy estimated from the temperature dependence of the carrier concentration was 0.55 eV.

Modulation of intermembrane interaction and bending rigidity of biomembrane models via carbohydrates investigated by specular and off-specular neutron scattering

We designed artificial models of biological membranes by deposition of synthetic glycolipid membrane multilayers on planar silicon substrates. In contrast to commonly used phospholipid membranes, this offers the unique possibility to study the influence of membrane-bound saccharide chains (cell glycocalix) on the membrane mechanics. Taking advantage of the planar sample geometry, we carried out specular and off-specular neutron scattering experiments to identify out-of-plane and in-plane scattering vector components. By considering the effects of finite sample sizes, we were able to simulate the measured two-dimensional reciprocal space maps within the framework of smectic liquid-crystal theory. The results obtained both at controlled humidity and in bulk water clearly indicate that a subtle change in the molecular chemistry of the saccharides strongly influences intermembrane interactions and membrane bending rigidities.