JAERI-Conf 95-021

PB52
Time-of-Flight Measurements of Positron-Annih ilation
Induced Auger Electrons
Toshiyuki Ohdaira, Ryoichi Suzuki, Tomohisa Mikado, Hideaki Ohgaki,Mitsukuni Chiwaki, and Tetsuo YamazakiElectrotechnical Labomtory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan

ABSTRACT

II. EXPERIMENTAL SET UP

We have developed an apparatus for positronannihilation induced Auger electron spectroscopy(PAES) with a time-of-flight (TOF) technique. We
report the details of the experimental set up and an
example of the TOF-PAES analysis for a MoS 2 (0001)
surface.

A slow positron beam (rv108 e+ js) was generated with the electron linac [2]. A 70 MeV electron
beam from the linac was incident onto a Ta converter
where the bremsstrahlung 'Y rays generate high-energy(keV-MeV) positrons through pair productions. The
positrons were then moderated with a set of thin W
foils to form a slow (rv6 e V) positron beam. The
slow positron beam was guided to the sample chamber in an axial magnetic field (rv0.01 T) with a series of solenoid coils. The positron beam generated
with the linac was pulsed since the electron beam fromthe linac was pulsed at a pulse width of 1 /);S and arepetition rate of 100 pulsejs. However, this pulsed
beam cannot be used for the TOF-PAES measurements, because the pulse width is too wide and therepetition rate is too low. Thus, the "-'/LS positronpulse was stretched to rv10 ms with a linear storage
section [2]. The quasi-continuous beam was used in
the TOF-PAES measurements.
Figure 1 shows a schematic of the TOF-PAES apparatus. The apparatus consists of a pulsing system,
an ExB plates deflector, a magnetic field electron paralleliser, and a time-of-flight energy analyzer [3]. Anaxial magnetic field of 4x 10- 3 T was applied to thewhole apparatus for the beam guiding and for theparallelisation of the emitted electrons. The sample
chamber was pumped by a cryo-pump to a base pressure of 2x1o- 9 Torr.

I. INTRODUCTION
Auger electron spectroscopy (AES) is widely usedfor elemental analysis of solid surfaces. In the conventional AES measurements, the core-hole excitations
are formed by the bombardment of electrons or photons with energies larger than the core-electron binding energies. The observation depth is determinedby the inelastic mean free path of Auger electrons,
and is about 1G-30 A. The positron-annihilatio n induced Auger electron spectroscopy which was developed by Weiss et al. [1], uses the annihilations ofpositrons with core electrons for creating core holes.
PAES has the following advantages over the conventional AES. Firstly, the core-holes are created mainlyby the positrons trapped in the image-potential welloutside the surface, so that the PAES signal originates preferentially from the surface top-layer. For
this reason, PAES can easily identify the elements.
at the surface top-layer. Secondly, the use the primary positron beams with energies less than thecore-electron binding energies eliminates the large
secondary-electron background around the Auger signals. Thirdly, the high signal-to-background ratio reduces radiation damages.
At the Electrotechnical Laboratory (ETL), a highintensity slow positron beam is generated with an electron linac. Using this beam, we have developed anapparatus for PAES with a time-of-flight technique,which enables high energy-resolution and high countrate measurements. In this paper, we show the experimental set up and the performance of the TOFPAES apparatus, and an example of the analysis fora MoS2(0001) surface.

----*8 (0.004T)

68cm

FIG. 1. Schematics of the TOF-PAES apparatus.

-395-

JAERI-Conf 95-021

The quasi-continuous positron beam was pulsedwith a chopper and a buncher. The chopper consistsof three mesh grids. A pulsed voltage was applied tothe middle grid to make a positron pulse with a pulse
width of 30 ns. The buncher consists of two cylindrical tubes, ·as shown in fig.1. A 20 MHz sine wave
potential was applied to the inner tube. The positronpulse was modulated at the gaps between the innerand outer tubes, and was compressed at the sample.
The compressed positron beam was deflected with aExB plates, and then incident onto the sample. The
pulse width at the sample was monitored with a BaF2
scintillator and photomultiplier (PMT) detector, andwas 5 ns for the present set up.
An incident energy of 75 eV was used in the presentmeasurements. At this energy, the implantation depth
is shorter than the diffusion length of the thermalized
positrons. Therefore, a large fraction of the positrons
diffuse back to the surface, and are trapped into theimage potential outside the surface. Some fraction(less than a few%) of the surface-state positrons annihilate with core electrons of the surface top-layer
atoms [4], resulting in the Auger electron emission.
ANd-Fe-B permanent magnet (0.2 T) was mounted
behind the sample to reduce the angular spread of theemitted electrons and to improve the energy resolution. The electrons emitted over 211" steradians fromthe sample in the high (0.2 T) magnetic field are parallelised in the weaker (4 x w- 3 T) uniform field [5].
The parallelized electrons were separated from the incident positron beam with the ExB plates, and weredetected with a micro-channel-plate (MCP) detector
which was placed 68 em away from the sample. Theenergy distributions of the electrons were determinedby measuring the time interval between the trigger
signal from the pulsing system and the detector signal with a time-to-amplitude converter.

spectrum, the strong S-LVV Auger peak is seen, but
Auger peaks from the Mo atoms that exist 1.6 Abelow
the top layer are absent. In contrast, the electroninduced AES showed Mo-NMM Auger peaks at rv180
and 220 eV [7]. This is because PAES probes preferentially the surface top-layer whereas the electroninduced AES probes a few atomic layers beneath thesurface.
In the PAES spectrum, small Auger peaks from C(KLL) and 0 (KLL) contaminants are seen also. Itshould be noted that before the measurement the sample was annealed up to rv800 °C for surface cleaning,
and that the electron-induced AES spectrum taken
for the same surface showed no Auger peak from thecontaminants. The result demonstrates that PAES isextremely sensitive to impurities adsorbed on the surface. The high sensitivity to impurities implies thatthe surface-state positrons are efficiently trapped atimpurities and defects on the surface [7].

SecondaryElectrons

400.

SLW• •Qic:c:

PositronPulse

ns.s::(.):I.l!lc: 200..I'''..

OKLL CKLLje/::l0~(.)

~;II

0050

100

Time of Flight (nsec)

III. RESULTS

400 r-To demonstrate the ability of the apparatus, wechose a layered material, MoS 2 as the target. It isknown that a basal plane (0001) surface of MoS 2 iscomposed of S as the topmost layer and Mo as thesecond one [6]. The layered structure is suitable forthe tests of the surface sensitivity of PAES.
Figure 2 (a) shows a time-of-flight spectrum of
the positron-annihilation induced Auger electrons for
MoS 2 (0001). The incident positron pulse measuredwith the BaF2 scintillation detector is also plotted.
Figure 2 (b) shows the corresponding energy spectrum. The count rate for the S-LVV Auger peak at"' 150 eV was rv2 countsjs, and the duration of themeasurement was "'1000 s. A large component at energies below 100 eV is due to the secondary electrons
excited by the primary positron beam. In the PAES

(a)

Qi

Secondary_,Electrons

(I:))

..

•tsLW

c:
c:
ns.s::

I•(.).l!lc: 200::l0·ll
I. i~!

(.)

C KLL~.......

0 1LL~-0I100200300 400 500
Energy (eV)600700

FIG. 2. (a)Time-of-fiight spectrum of positron annihilation induced Auger electrons for MoS2(000l). (b) Corresponding energy spectrum.

-396-JAERI-Conf 95-021

It is clear from fig. 2 (a) that the energy resolutionof the present apparatus is determined mainly by thepulse width of the incident positrons. For the positronpulse of 5 ns width, the energy resolutions at 150 eV
and 500 eV are rv20 eV and 100 eV, respectively. Theresolution of the present apparatus is better than thatof the previously reported apparatus [5,8], but is notgood compared with the conventional AES apparatus with the CMA. The use of a shorter pulse would
improve the energy resolution. For the present apparatus, however, it was not possible to obtain thepositron pulse with width less than 5 nsec. This is because the magnetically guided positron beam was notmonoenergetic but had an energy spread of rv2 eV.
In order to overcome this problem and improve theenergy resolution, we are constructing a new TOFPAES apparatus with a longer flight distance and amore efficient pulsing system.
In summary, we developed the apparatus for PAES
with the TOF technique. We demonstrated the TOFPAES analysis for the MoS 2 (0001) surface, and foundthat PAES probes only the surface top-layer, and that
PAES is very sensitive to surface impurities. Further
improvements are needed to increase the energy resolution.

[1] A. Weiss, R. Mayer, M. Jibaly, C. Lei, D. Mehl, andK. G. Lynn, Phys. Rev. Lett. 61, 2245 (1988).
[2] T. Akahane, T. Chiba, N. Shiotani, S. Tanigawa,T. Mikado, R. Suzuki, M. Chiwaki, T. Yamazaki, andT. Tomimasu, Appl. Phys. A51, 146 (1990).
[3] R. Suzuki, T. Ohdaira, T. Mikado, H. Ohgaki, M. Chiwaki, and T. Yamazaki, submitted to Appl. Surf. Sci.
[4] Kjeld 0. Jensen and A. Weiss, Phys. Rev. B41, 3928
(1990).
[5] Chun Lei, David Mehl, A.R. Koymen, Fred Gotwald,
M. Jibaly, and Alex Weiss, Rev. Sci. Instrum 60, 3656
(1989).
[6] B. J. Mrstik, R. Kaplan, T. Reinecke, M. Van Hove,and S. Y. Tong, Phys. Rev. B15, 897 (1977).
[7] T. Ohdaira, R. Suzuki, T. Mikado, H. Ohgaki, M. Chiwaki, T. Yamazaki, and M. Hasegawa, submitted toAppl. Surf. Sci.
[8] E. Soininen, A. Schwab, and K. G. Lynn, Phys. Rev.
B43, 10051 (1991).

-397-