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Discovery of a New X-ray Beam Splitting Phenomenon While Passing Through a Semiconductor Crystal (Press Release)

Release Date
31 Jan, 2013
  • BL29XU (RIKEN Coherent X-ray Optics)
-New light shed on a breakthrough toward next-generation semiconductor technology-


Main points of the research
• X-ray incidence on a crystal with islands of lattice deformations causes the X-ray beam splitting
• The splitting of X-ray beams through Berry-phase translation generates two beams running parallel, separated by a distance of 400μm.
• This phenomenon could provide a high sensitivity measurement of microscopic lattice deformation—the key to next-generation semiconductor technology.

Researchers at RIKEN (director: Ryoji Noyori) have made an unprecedented observation: an X-ray beam, produced at SPring-8,*1 irradiated on semiconductor crystal splits into two directions and propagates repeating lateral translations. The finding is expected to have a useful implication for developing next-generation semiconductor technology, a new X-ray trajectory control method, and basic technology for novel optical elements. The finding was the result of a collaborative effort of researchers at RIKEN SPring-8 Center, including Dr. Tetsuya Ishikawa (center director), Dr. Yoshiki Kohmura (leader, SR Imaging Instrumentation Unit), Dr. Kei Sawada (research scientist, XFEL Research and Development Division), and Prof. Susumu Fukatsu (Graduate school of Arts and Science, University of Tokyo).

X-ray trajectory control requires optical elements with outstanding accuracy, making it much more difficult as compared with that for visible light. Berry-phase translation of an X-ray,*3 a potentially promising phenomenon for X-ray trajectory control, was theoretically predicted in 2006. The phenomenon is produced by lattice deformation present in crystal materials, and has the effect of causing a large bend in X-ray trajectory. Dr. Kohmura (unit leader) and others succeeded in demonstrating this phenomenon in 2010. Following this, the collaborative research group tackled the challenge of developing an accurate X-ray trajectory controlling method through the application of lattice deformation.

In this experiment, a heteroepitaxial crystal*4 was prepared by depositing germanium on a silicon crystal for producing quantum dots*5 on its surface, and an X-ray beam was irradiated on them. It was discovered, through this experiment, that the X-ray, when incident at a fixed angle, not only exhibits Berry-phase translation, but also splitting of its path into two. The two beams, after splitting, run nearly parallel to each other, with an intervening distance equal to or larger than 400μm. The lateral translation was found to occur as the beam somewhat surfs on many quantum dots successively.

In addition to the fact that the research unveiled a new optical phenomenon—the X-ray’s surfing on quantum dots and resultant splitting of the X-ray’s path—the knowledge obtained is expected to exert advantageous implications over the development of next-generation technologies. For example, superimposing the once-split two beams can provide the working principle for a novel X-ray interferometer. The application of lattice deformation is expected to become one of the key next-generation semiconductor technologies, and X-ray interferometry may grow into an essential tool for high sensitivity measurement in this area, promising a technological breakthrough.

This study was conducted under the auspices of Grants-in-Aid for Scientific Research (B) ("Observation of abnormal shift of X-ray wave packet by crystal under the Bragg reflection condition and its application to X-ray waveguide tubes"). The research results were published in Physical Review Letters (a U.S. scientific journal) as a highlighted article (issued on the 1st of February).

"Controlling the Propagation of X-Ray Waves inside a Heteroepitaxial Crystal Containing Quantum Dots using Berry's Phase"
Yoshiki Kohmura, Kei Sawada, Susumu Fukatsu and Tetsuya Ishikawa.
Physical Review Letters 110 5, 057402 (2013), published 28 January 2013


Fig. 1. Germanium (Ge) quantum dots distributed on a Silicon (Si) substrate
Fig. 1. Germanium (Ge) quantum dots distributed on a Silicon (Si) substrate

Depositting germanium on a silicon crystal automatically promotes the formation of island structures, called quantum dots, on its surface. The formations of quantum dots accompany upwardly growing lattice deformation right below each of them (green arrows).

Fig. 2. Schematic diagram of the experimental setup
Fig. 2. Schematic diagram of the experimental setup

The X-ray that has transmitted through the heteroepitaxial crystal was captured and imaged by an X-ray image detector.

Fig. 3. Intensity profiles of an X-ray beam passing through a crystal
Fig. 3. Intensity profiles of an X-ray beam passing through a crystal

The figure shows seven X-ray intensity profiles, each corresponding to different incident angles: the lime green line represents Bragg-angle incidence (denoted as “0”) and others represent deviations from it. The profiles with larger deviation from the Bragg angle, such as the black and purple lines, tend to show a merged single peak at the center. As the incident angle approaches the Bragg angle, the peak separates into two (upper and lower), and at - 1 arcsecond deviation (green line), two well-defined peaks are captured by the detector. Considering the effect of oblique X-ray incidence onto the crystal, the separation of the two peaks corresponds to 400μm or more along the crystal surface (1 arcsecond = 1/3600 degree).

Fig. 4. Lateral translation of an X-ray beam into two directions
Fig. 4. Lateral translation of an X-ray beam into two directions

The quantum dots, or lattice deformations in this case, on a silicon crystal surface produce an array of isolated convexes. A parallel X-ray beam incident on the crystal surface is deflected to opposite directions depending on the local risings and fallings around the lattice deformations: lower than the Bragg angle (blue arrow: Δθ<0) and higher than the Bragg angle (red arrow: Δθ>0).

Fig. 5. Total lateral translation is much larger than the inter-quantum dot distance.
Fig. 5. Total lateral translation is much larger than the inter-quantum dot distance.

The green and orange dots represent silicon and germanium atoms, respectively. The darker the color, the larger the displacement from normal regular arrangement, i.e. larger lattice deformation. On the silicon crystal surface, an upward convex is produced at each quantum dot location. Because the lattice deformation runs deep into the silicon crystal, up/down deflection takes place multiple times, resulting in large lateral displacement of the X-ray beam. After penetrating deeper than a certain distance, where the lattice deformation becomes negligibly small, the displaced beam runs parallel to the incident X-ray beam.

*1 SPring-8

A RIKEN facility located in Harima Science Garden City (Hyogo prefecture) is capable of producing the world's highest intensity synchronous radiation. The management and promotion of utilization of this facility are undertaken by JASRI. The name “SPring-8” comes from “Super Photon ring-8GeV.” An electron flying at nearly the speed of light, if deflected from its original trajectory through the effect exerted by a magnet, emits an electromagnetic wave in a direction tangential to its trajectory, which is called radiation light (or synchrotron radiation). At present, there are three “3rd Generation” large scale synchronous radiation facilities in the world: SPring-8 (Japan), APS (USA) and ESRF (France). The acceleration energy available at SPring-8 (8 billion electron volts) enables the provision of an extremely wide spectrum of radiation light: from far infrared to visible, vacuum ultraviolet, and soft X-ray up to hard X-ray. SPring-8 provides a theater for collaborative works involving researchers inside and outside Japan, and the research conducted at this facility cover such diverse areas as material science, geoscience, life science, environmental science, and various applications in industrial sectors.

*2 Crystal lattice deformation
Crystal represents a material made up of atoms or molecules arranged at regular intervals throughout. Force upon a crystal, or the addition of impurity into it, can cause deformation in the regular array of atoms or molecules, which is called lattice deformation. Attempts have been made to utilize controlled lattice deformation for the betterment of electronic properties. Lattice deformation in semiconductor crystals is playing an essential role in the ultra high-speed driving of semiconductor devices, such as those implemented in smartphones.

*3 Berry-phase translation of X-ray
A phenomenon in which an X-ray beam incident on a lattice-distorted single crystal at a near-Bragg angle suffers a large bend.

*4 Heteroepitaxial crystal
A crystal growth on a substrate crystal, in which a material dissimilar to that in the substrate grows and maintains substantially the same lattice arrangement. Heteroepitaxial crystal growth enables controlled strain distribution on a minute spatial scale basis (≤ 100nm) on the substrate crystal.

*5 Quantum dot
A quantum dot is an island-like portion inside a matter (typically a semiconductor) whose electrons are confined in their movements in all three spatial dimensions. Sometimes called an artificial atom because of is spectral similarity to isolated atoms, it is also called a quantum box.

For more information, please contact:
  Dr. Yoshiki Kohmura (RIKEN SPring-8 Center)

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