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. 22           April 25, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. RING STUDIES INDICATE BEAM CHANGES IN 2-BUNCH MODE
    3. NEW USER ADVISORIES ON VACUUM POLICY AND VACUUM PUMPS
    4. X-RAY PROCESSING OF SEMICONDUCTOR MATERIALS
    5. WOULD YOU LIKE TO RECEIVE THE ALS REPORT?

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

Beam availability last week was 94.4% overall and 95.2% during user shifts. Short outages during accelerator physics shifts were caused by the booster extraction kicker and a storage ring quadrupole power supply replacement. An unscheduled beam dump occurred on Sunday because of a power dip.

**NOTE SCHEDULE CHANGE**
May 2, 08:00-23:15 and May 3, 08:00-16:00 -- 1.5-GeV, 400-mA, 320-bunch operations for users. This time, previously scheduled for accelerator physics, has been changed to user time because many of the accelerator physics staff will be attending a particle accelerator conference.

**EVENING SHUTDOWN TIME**
ALS Users are reminded that evening operations cease at 23:15 (11:15 p.m.) as scheduled, and requests for a later turn-off time cannot be granted by the operations staff on shift.

Operations summary for April 25 - May 14
1.5-GeV, 400-mA, 320-bunch operations for users:
    April 26, 16:00-23:15
    April 27-30, 08:00-23:15
    May 2, 08:00-23:15
    May 3, 08:00-16:00
    May 10, 16:00-23:15
1.5-GeV, 2-bunch operations for users (see #2 below concerning 2-bunch
conditions):
    May 3, 16:00-23:15
    May 4-7, 08:00-23:15
    May 11-14, 08:00-23:15
Maintenance: 
    May 1 & 8, 08:00-16:00, with startup 16:00-23:15
Accelerator Physics:
    April 25 and May 9, 08:00-23:15
    April 26 and May 10, 08:00-16:00

2. RING STUDIES INDICATE BEAM CHANGES IN 2-BUNCH MODE
(contact: ajackson@lbl.gov)

Accelerator physics studies conducted April 18 indicate that the beam lifetime in 2-bunch operation will be shorter (~30-90 minutes at 2 x 20 mA) than has been usual for this mode. Also, due to betatron tune changes and possible beam loss when some undulators are closed to their minimum gaps, there may be gap restrictions on Sector 7 and possibly Sector 8 undulators during 2-bunch operation. The long-term solution to undulator-generated tune shifts will be to compensate with quadrupole tuning. The ALS will attempt to implement this concept before the next 2-bunch run, scheduled for August.

3. NEW USER ADVISORIES ON VACUUM POLICY AND VACUUM PUMPS
(contact: alsuser@lbl.gov)

The ALS has recently issued two User Advisories to explain ALS policy in two important areas: vacuum policy for user endstation and beamline equipment, and the requirements for the use of mechanical vacuum pumps. Copies of the advisories are available in the User Services Area near Beamline 9 and from ALS Administration [Tel: (510) 486-4257; Fax: (510) 486-4960, Email: alsuser@lbl.gov].

"Vacuum Policy for User Endstations for Protection of Beamline Components and Storage Ring Vacuum (ALS Advisory 9/Rev. 0)" gives an overview of the ALS vacuum policy for endstations and provides guidelines to follow to ensure compliance with ALS requirements. Areas covered include operation of non-UHV endstations; vacuum pumps and gauges; venting and purging vacuum chambers; viewports; interlocks; gas cells; and rules and recommendations for avoiding contamination of beamline components.

The second advisory, "ALS Policy for Exhausting of Mechanical Vacuum Pumps (ALS Advisory 10/Rev. 0)," details the ALS requirements for operating and exhausting mechanical pumps in order to minimize the amount of potentially hazardous gases, pump fluids, smoke, and oil released in the experiment floor area.

4. X-RAY PROCESSING OF SEMICONDUCTOR MATERIALS
(contact: kmy@csa.lbl.gov)

A group working at Beamline 10.3 has conducted experiments in the use of x rays for processing semiconductors, aiming to improve several aspects of semiconductor quality. Semiconductors are the basic materials for diodes, transistors, and most other elements of integrated circuits. Production of these circuit elements requires that a substrate such as silicon be implanted with dopants -- atoms of another element that contribute free charge carriers (either extra electrons or positive "holes" which allow conduction of electricity) to the semiconductor. Ideally, a dopant should be distributed through the substrate as single atoms, substituting for atoms of the substrate in a near-perfect lattice structure.

The ion bombardment used to implant the dopant atoms, however, typically leaves the lattice at the surface of the substrate heavily damaged. Also, implanted atoms often precipitate into clumps within the substrate, bonding with each other instead of with substrate atoms, and thus not contributing free charge carriers. The first problem is correctable by annealing, most commonly thermal annealing in which the sample is heated to very high temperatures (but not to its melting point). This heating gives atoms in the damaged layers enough energy to reorganize into roughly crystalline structures based on those in the underlying, undamaged layers, a process called solid-phase epitaxy (SPE). The problem with thermal annealing is that it affects the entire sample, with no selectivity by location or chemical species, so it provides little control over the precipitation of dopants.

Researchers from the Center for Advanced Materials at Lawrence Berkeley Laboratory, who wished to improve both the quality of SPE in damaged silicon layers and the uniform dispersion of dopant atoms, sought solutions using x rays. In one experiment, they implanted silicon wafers with gallium dopant ions, producing surface damage. They irradiated some of the samples with x rays for several hours, and then thermally annealed all the samples at 570 degrees Celsius to produce SPE. They found that the irradiated wafers reorganized into single-crystal forms with far fewer defects than the non-irradiated control samples. They also found that after x-ray irradiation but before the thermal annealing step, Ga atoms in the irradiated samples had higher coordination numbers (a measure of their bonding with neighboring Si atoms and thus of their ability to contribute free charge carriers) than those in the control samples. There is not yet a microscopic model of this x-ray annealing, but the group suggests that absorption of x-ray photons leads to large local excitations in the crystal lattice, allowing more perfect near-crystalline reorganization than thermal annealing alone.

In a second experiment, the researchers used samples of indium phosphide (InP) doped with carbon atoms. Most of the carbon was electrically inactive (not contributing to changes in conductivity) because it had precipitated into large graphitic clusters. They irradiated the samples with x rays, using an energy above the carbon K-edge to encourage selective excitation of the graphitic clusters, with comparatively little excitation of the InP or of single carbon atoms. No subsequent thermal annealing was used. The results of preliminary studies, using Raman spectroscopy to analyze the irradiated samples, showed a significant decrease in the concentration of carbon precipitates, with an indication of an increased free charge carrier concentration. These results suggest that x-ray annealing may be used to increase the effectiveness of semiconductor doping with previously unusable dopants. This method may also prove useful in writing patterns in semiconductor wafers, since x rays can be focused to produce sharply localized variations in conductivity within a sample.

5. WOULD YOU LIKE TO RECEIVE THE ALS REPORT?
(contact: alsnews@lbl.gov)

The ALS Report is the twice-yearly newsletter of the ALS, written for a broad readership and published in printed form. It spans a wider range of events and issues at the ALS, some of them in more depth, than ALSNews -- and it includes pictures!

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