IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT039

CHARACTERIZATION OF DIAMOND WITH BURIED BORON-DOPEDLAYER DEVELOPED FOR Q-SWITCHING AN X-RAY OPTICAL CAVITY∗
R. A. Margraf† 1 , J. Krzywinski, T. Sato, J. P. MacArthur, D. Zhu, M. L. Ng, A. Halavanau,
R. Robles1 , A. Robert2 , Z. Huang1 , G. Marcus, SLAC, Menlo Park, USA
M. D. Ynsa, Center for Micro Analysis of Materials, Autónoma, Madrid, Spain
F. Ke, Stanford University, Stanford, USA
S.-K. Mo, Y. Zhong1 , Lawrence Berkeley National Laboratory, Berkeley, USAP. Pradhan, Argonne National Laboratory, Lemont, USA
1also at Stanford University, Stanford, USA, 2 also at MAX IV, Lund, Sweden
Abstract
X-ray Free-Electron Laser Oscillators (XFELOs) and Xray Regenerative Amplifier FELs (XRAFELs) are currently
in development to improve longitudinal coherence and spectral brightness of XFELs [1]. These schemes lase an electron
beam in an undulator within an optical cavity to produce
X-rays. X-rays circulate in the cavity and interact with fresh
electron bunches to seed the FEL process over multiple
passes, producing progressively brighter and more spectrally pure X-rays. Typically, the optical cavities used arecomposed of Bragg-reflecting mirrors to provide high reflectivity and spectral filtering. This high reflectivity necessitates special techniques to out-couple X-rays from thecavity to deliver them to users. One method involves “Qswitching” the cavity by actively modifying the reflectivityof one Bragg-reflecting crystal. To control the crystal lattice
constant and thus reflectivity, we use an infrared (IR) laser
to heat a buried boron layer in a diamond crystal. Here, webuild on earlier work [2] and present the current status of ourQ-switching diamond, including implantation with 9 MeV
boron ions, annealing and characterization.

Micro Analysis of Materials (CMAM) at the Autonomous
University of Madrid. 200 x 200 µm areas were implantedwith fluences of 5 × 1015 , 1 × 1016 ,1.5 × 1016 2 × 1016 and2.5 × 1016 ions/cm2 by scanning a focused beam of boron
ions in a spiral rastering pattern.
Following implantation, five high-temperature in-vacuum
annealings at 900, 950,1150, 1300, and 1450 °C were performed to heal implantation damage. 900 and 950 °C annealings were performed in a UHV chamber heated by a filament
under 2.7 × 10−8 mbar or better vacuum. The filament washeated to the target temperature, then annealed for 1 hr. The
1150, 1300, and 1450 °C annealings were performed in aRed Devil G vacuum furnace manufactured by R. D. Webb
Company Inc under 1 × 10−4 mbar or better vacuum, similar
to [4]. The furnace was ramped up at 2 °C/min, annealed for
3 hr, then ramped down at 3 °C/min to 700 °C and cooled.
During this process, we did see some graphitization, as
shown in Fig. 1, but not on our regions of interest.

INTRODUCTION
In Q-switching an optical cavity, as described in [2], anIR laser heats a region of a Bragg-reflecting mirror, actively
modifying its reflectivity, and enabling out-coupling of Xrays. A crystal used for this purpose must have a uniform
rocking curve (less than the rocking curve width of 8 µradfor 9.831 keV X-rays reflecting off diamond 400 planes) overa region the size of the X-ray beam ∼40 µm. The crystal
must also absorb IR light strongly. Diamond has thermal
dissipation properties which make it ideal for use in in acavity-based XFEL. A buried boron layer can be implanted
to increase the IR absorption of diamond for Q-switching.

IMPLANTATION AND ANNEALING
A high-temperature-high pressure (HPHT) diamond, described in [2] was implanted as described in [3] with 9
MeV boron ions using the microbeam line at the Center for∗ This work was supported by the Department of Energy, Laboratory Di-rected Research and Development program at SLAC National Accelerator
Laboratory, under contract DE-AC02-76SF00515.
† rmargraf@stanford.edu

MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers

Figure 1: 1x images of HPHT diamond. A) Pre-implantation,B) Post-implantation, Post 1450 °C annealing.

CHARACTERIZATION
We performed several measurements to characterize healing in the boron-implanted regions.

IR Transmittance
To show implantation increases IR absorption, we measured IR transmittance. A Thorlabs CPS780S laser diodebeam was expanded, collimated and transmitted through oursample. A Thorlabs FL780-10 780 nm filter selected thesignal wavelength before a Mako G-319C POE camera.
The post 1450 °C annealing case is shown in Fig. 2. The
IR transmittance of un-implanted diamond and the regionof lowest doping is similar, 66 - 69 %, and the more highly
doped regions transmit less IR light, 15 - 26 %. To accountfor variation in transmittance across the diamond, we also
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13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1

Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1

IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT039

Figure 2: IR transmittance, post 1450 °C annealing. A) IR transmittance image. B) IR transmittance by region. C)
Transmission of regions versus a region of un-doped diamond directly outside it. Error bars are standard error of the mean.
compared transmittance in each boron-implanted region to
the un-implanted diamond directly surrounding it. We foundthe lowest doping reduced transmission of 6 %, versus 45 51 % for the highest doping. This suggests the least doped
region absorbs only slightly more IR than pure diamond,
while the highly doped regions absorb much more IR.

Micro-Raman Spectroscopy
To assess healing of boron-implanted regions, we performed micro-Raman spectroscopy. A Renishaw inVia Raman Microscope with a 514 nm laser and a Leica 20x/.40 N
Plan Epi objective was used on a ∼1 - 4 µm area spot.
Pure diamond has a sharp peak in the micro-Raman specta
at 1332 cm−1 , as shown in Fig. 3. After implantation, the1332 cm−1 peak decreased for all implanted regions. After
1450 °C annealing, the 1332 cm−1 peak for the lowest borondoped region recovered to 83 % of the height of the diamondreference peak. However, the more highly boron-doped
regions have a much smaller 1332 cm−1 peak, less than 3 %of the diamond reference peak. This suggests annealing at1450 °C may somewhat heal the crystal lattice of a region
doped with boron at a 5 × 1015 ions/cm2 fluence, but higher
fluences do not heal. Annealing also changes the microRaman background, consistent with earlier studies [5].

White-Light Topography
We performed a topography measurement on the implanted regions using a Zygo NewView, a 3d white lightinterferometer. For measuring the entire surface, stitchingmeasurement was performed using a 2.75x objective.
As shown in Fig. 4, pre-annealing, the boron-implanted
regions had a surface bulge of 50-160 nm. Post-annealing,
the lowest boron-doped region decreased to a 10 nm bulge
in some regions, while increasing to 100 nm on one edge.
Additionally, annealing increased the surface bulge in thehigher boron-doped regions. This suggests annealing promoted rearrangement of the diamond crystal lattice in theboron-implanted regions, and the diamond was not able tofully heal, especially in the highly doped regions.

at the Advanced Photon Source at Argonne National Laboratory by reflecting 10 keV X-rays off diamond 400 latticeplanes. The post 1450 °C annealing RCI was performed atbeamline 10-2 of the Stanford Synchrotron Radiation Light
Source at SLAC National Accelerator Laboratory by reflecting 9.831 keV X-rays off diamond 400 lattice planes [6]. Images were collected as the crystal was rotated, and a Gaussian
fit was performed to find rocking curve center and FWHM.
In Fig. 5, annealing decreased overall deviation in therocking curve center. It also decreased deviation of therocking curve center in the implanted regions. However,even in the lowest boron-doped region there are still 10 µraddeviations in the rocking curve center, which are detrimental
to Q-switching. We note that for pre-annealing dataset, thecrystal was rocked left/right, versus up/down in the post
1450 °C dataset, explaining differences in RCI structure.

CONCLUSION
Characterization of the boron-implanted and 1450 °C
annealed diamond showed moderate healing of the5 × 1015 ions/cm2 fluence region, and little healing at higher
fluencies. While the increase in IR transmittance, recoveryof the 1332 cm−1 micro-Raman peak, and reduction in surface bulge in white-light topography are all signs of healing
in the lowest doped region, the 10 µrad deviations in RCI
still make the diamond challenging for Q-switching use.

Rocking Curve Imaging

In future work, we must increase uniformity of the RCI
within the boron-doped regions while keeping IR absorption high. The structure in the RCI within the boron-doped
regions is likely due to implanting ions in a rastering pattern.
For a more uniform RCI, future studies should use a large ion
beam with a mask, rather than rastering. To avoid reductionin IR absorption with annealing, one might forgo annealing
and reflect off the opposite surface of the crystal. The backis less damaged by the boron implantation, as boron ionspassed through the front as they were implanted in the crystal. One could machine a diamond drumhead from the backsurface of the crystal such that the boron-implanted regions
are 5 µm below the reflecting surface.

To assess X-ray reflectivity in the Boron-doped regions,
we performed RCI. The pre-annealing RCI was performed

Future diamond boron-implantions should produce a uniform RCI, enabling Q-switching of a cavity-based XFEL.

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A06: Free Electron Lasers

IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT039

Figure 3: Micro-Raman Spectroscopy. A) Pre-annealing, B-C) post 1450 °C annealing.

Figure 4: White light topography. Left plots show surface height across full area of crystal, center and right plots show line
scans where a quartic background term has been removed. A) Pre-annealing, B) post 1450 °C annealing.

Figure 5: RCI Imaging, including RCI center and FWHM for spots 2 and 1. A) Pre-annealing, B) post 1450 °C annealing.
MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers

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Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1

Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1

IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT039

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MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers