ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol. 52           May 15, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. ACCELERATOR STATUS HOTLINE AVAILABLE MAY 20
    3. GALLIUM NITRIDE GIVES US THE BLUES
    4. HIGH-RESOLUTION COMPUTED MICROTOMOGRAPHY WORKSHOP

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

The ALS is in the last week of a scheduled shutdown for major installations and maintenance (more on major shutdown activities appears in ALSNews Vol. 48, March 20, 1996). This week is devoted to startup and conditioning, and user time is scheduled to resume on May 22.

Operations Summary for May 21 - June 3

May 21, 00:00-24:00           Accelerator Physics
May 22, 00:00-08:00           User Scrubbing & Special Operations
May 22, 08:00- May 27, 07:15  1.9-GeV/260-mA/320-bunch user operations
May 27, 07:30- May 28, 07:15  Holiday
May 28, 07:30-24:00           Maintenance & Startup
May 29, 00:00-24:00           Accelerator Physics
May 30, 00:00-08:00           User Scrubbing & Special Operations
May 30, 08:00- June 3, 07:15  1.9-GeV/260-mA/320-bunch user operations

2. ACCELERATOR STATUS HOTLINE AVAILABLE MAY 20
(contact: rmmiller@lbl.gov)

Starting May 20, 1996, ALS users will be able to call the Accelerator Status Hotline at 486-6766 (ext. 6766 from Lab phones) to get a recorded message giving up-to-date information on the operational status of the accelerator. This service will be maintained 24 hours a day by members of the Accelerator Operations group, and will give current operating conditions (including status updates in case of an accelerator outage). The recorded message will be updated immediately to reflect any changes in the operational status of the accelerator.

3. GALLIUM NITRIDE GIVES US THE BLUES
(contact: cwetzel@mh1.lbl.gov)

Ah, those tiny-wavelength, pure and stable blues, we've been missing them. The development of high-efficiency color light-emitting diode (LED) displays has been limited by the lack of a material with the right electronic structure to emit a true blue. Moreover, compact disc technology has been longing for light with a short enough wavelength to read the closely packed signals on higher density discs. The semiconductor gallium nitride (GaN), which can emit the blue light heretofore missing from the spectrum, promises to bring us these technologies. Work at the Center for X-Ray Optics fluorescence microprobe beamline is helping to guide its development.

With the help of the microprobe, researchers from Berkeley Lab's Materials Science Division are refining a method of preparing GaN crystals. Making GaN a working semiconductor requires introducing atoms of other elements, such as silicon or magnesium, in a process called doping. These added elements, called dopants, replace gallium or nitrogen atoms in the crystal's structure and increase the conductance of the material. To create a semiconductor with uniform electronic properties, the dopant atoms must be homogeneously distributed throughout the crystal.

The electronic properties of a crystal are also affected by the presence of unwanted contaminants such as iron and nickel. These can significantly reduce the quality of the semiconductor by reducing conductance through it or by causing short circuits. Their effect on the function of the semiconductor depends on their concentration and how they are distributed within the material.

Thus, testing the success of a semiconductor preparation method involves determining both the concentrations and the distributions of impurities. The scientists need a tool that combines chemical analysis with spatial resolution. The fluorescence microprobe on Beamline 10.3.1 provides an elegant solution. It offers elemental specificity and a resolution of 2 microns.

Inside the microprobe, photons from the ALS are absorbed by atoms in a 2-micron-square area of the sample. The absorbing atoms then fluoresce, giving off photons with energies that are characteristic of the elements present. From the intensity of photons emitted at any one element's characteristic energy, the concentration of that element in the spot can be determined. Plotting the concentrations for thousands of spots side by side gives a map of the element's concentration in the crystal.

Using the microprobe to test the success of their crystal preparation methods, the researchers found that nickel and iron contaminants in GaN were agglomerated along grain boundaries (boundaries between individual crystals in a sample). Such groupings of contaminant atoms can cause short circuiting within the crystal. The clear view of what's going on inside their crystals will help the researchers to optimize their methods for making usable GaN.

This work was conducted by C. Wetzel (principal investigator) and W. Walukiewicz (Berkeley Lab), A. Thompson (Center for X-Ray Optics), and E. Haller (University of California Berkeley and Berkeley Lab).
Funding: Office of Basic Energy Sciences of the U.S. Department of Energy.

4. HIGH-RESOLUTION COMPUTED MICROTOMOGRAPHY WORKSHOP

A workshop on High Resolution X-Ray Computed Microtomography will be held at Berkeley Lab on August 12-13, 1996. The purposes of the workshop are to assess the status of this field of research, to discuss microtomography applications and radiation requirements required by users of the technique, and to define technical challenges, instrumental requirements, and strategies toward achieving high resolution (~1 micron) tomography.

Topics of discussion will include biomedical, biological, materials, and earth sciences as well as new fields of applications; computed tomography theory; radiation characteristics; x-ray optics; detector requirements; reconstruction algorithms; and 3-D rendering and visualization.

For more information please contact Erika Schlueter (erika@csa1.lbl.gov) or Fred Schlachter (fred_schlachter@lbl.gov).