Positron Positron BeamBeam ApplicationApplication to Materials Science and to Materials Science and IntenseIntense Accelerator Accelerator oror ReactorReactor basedbased
Positron Positron BeamBeam FacilitiesFacilities in Germanyin Germany
R. Krause-Rehberg
• Introduction: Positrons detect lattice defects• Examples:
- new getter centers in Si after high-energy self-implantation (Rp/2 effect)
- study of defects in GaAs• Large Positron Facility Projects in Germany• Conclusions
Martin-Luther-Universität Halle
Martin-Luther-Universität
Halle-Wittenberg
Martin-Luther-Universität Halle-Wittenberg, Germany
The positron lifetime spectroscopy
Martin-Luther-Universität Halle
• positron wave-function can be localized in the attractive potential of a defect
• annihilation parameters change in the localized state
• e.g. positron lifetime increases in a vacancy
• lifetime is measured as time difference between 1.27 and 0.51 MeV quanta
• defect identification and quantification possible
0 1 2 3 4 5
103
104
105
106
τb = 218 ps
τ3 = 520 ps
τ2 = 320 ps
As–grown Cz Si Plastically deformed Si
Cou
nts
Time [ns]
Positron lifetime spectroscopy
Martin-Luther-Universität Halle
positron lifetime spectra consist of exponential decay components
positron trapping in open-volume defects leads to long-lived components
longer lifetime due to lower electron density
analysis by non-linear fitting: lifetimes τi and intensities Ii
N t I ti
i ii
k
( ) exp= −FHGIKJ=
+
∑ τ τ1
1
κ µτ τd d
b d
= = −FHG
IKJC I
I2
1
1 1
trapping rate defect concentration
trapping coefficient
positron lifetimespectrum:
(divacancies)
(vacancyclusters)(bulk)
Monoenergetic positron beam by moderation
Martin-Luther-Universität Halle
• positron annihilation was very successful in defect identification in last decades• semiconductor technology: thin layers (epitaxy, ion implantation)• broad energy distribution due to β+ decay• some surfaces: negative workfunction ⇒ moderation (but rather inefficient)
Energy distribution after β+ decayEffect of moderation
Conventional positron beam technique
Martin-Luther-Universität Halle
• positron beam can be formed using mono-energetic positrons• often: magnetically guided for simplicity
• defect studies by Doppler-broadening spectroscopy• characterization of defects only by line-shape parameters or positron diffusion length• for positron lifetime spectroscopy: beam can be bunched
Martin-Luther-Universität Halle
Defects in high-energy self-implanted Si The Rp/2 effect
• after high-energy (3.5 MeV) self-implantation of Si (5 ×1015 cm-2) and RTA annealing (900°C, 30s): two new gettering zones appear at Rp and Rp/2 (Rp = projected range of Si+)
• visible by SIMS profiling after intentional Cu contamination
0 1 2 3 41015
1016
1017
Cu c
once
ntra
tion
(cm
-3)
Depth (µm)
RpRp/2
SIMS
• at Rp: gettering by interstitial-type dislocation loops (formed by excess interstitials during RTA)
• no defects visible by TEM at Rp/2• What type are these defects?
TEM image by P. Werner, MPI Halle
Interstitial type [3,4]
Vacancy type [1,2]
[1] R. A. Brown, et al., J. Appl. Phys. 84 (1998) 2459[2] J. Xu, et al., Appl. Phys. Lett. 74 (1999) 997[3] R. Kögler, et al., Appl. Phys. Lett. 75 (1999) 1279[4] A. Peeva, et al., NIM B 161 (2000) 1090
Enhanced depth resolution by using the Munich Scanning Positron Microscope
Martin-Luther-Universität Halle
scan direction
positronmicrobeamE = 8 keV
lateral resolution1 ... 2 mµ
α = 0.6°
posi
tron
lifet
ime
(ps)
scan width0 1 mm
defect depth10 mµ
τbulk
τdefect
• sample is wedge-shaped polished (0.5…2°)
• layer of polishing defects must be thin compared to e+
implantation depth
• best: chemo-mechanical polishing
0 1 2 3 4 5 6260
280
300
320
340
360
380
0,4
0,6
0,80 1 2 3 4 5 6
divacancy-typedefect
microvoids
defect-relatedlifetime
fraction of trapped positrons
Rp/2 R
p
Silicon self-implantation - 3.5 MeV, 5×1015 cm-2
- annealed 30s 900°C- Cu contaminated
surface bulk silicon
aver
age
lifet
ime
(ps)
depth (µm)
350
400
450
τ 2 (ps
)
η
First defect depth profile using Positron Microscopy
Martin-Luther-Universität Halle
• 45 lifetime spectra: scan along wedge
• separation of 11 µm between two measurements corresponds to depth difference of 155 nm (α = 0.81°)
• beam energy of 8 keV ⇒ mean penetration depth is about 400 nm; represents optimum depth resolution
• no further improvement possible due to positron diffusion: L+(Si @ 300K) ≈ 230 nm
• both regions well visible:
• vacancy clusters with increasing density down to 2 µm (Rp/2 region)
• in Rp region: lifetime τ2 = 330 ps; corresponds to open volume of a divacancy; must be stabilized or being part of interstitial-type dislocation loops
5 µm0
SIMS profile of Cu
stabil metastabil(Dabrowski 1988, Chadi 1988)
• one of the most frequently studied crystal lattice defects at all
• responsible for semi-insulating properties of GaAs: large technological importance
• is deep donor, compensates shallow acceptors, e.g. C- impurities
• defect shows metastable state after illumination at low temperatures
• IR-absorption of defect disappears during illumination at T < 100 K
• ground state recovers during annealing at about 110 K
• many structural models proposed
• Dabrowski, Scheffler and Chadi, Chang (1988): simple AsGa-antisite defect responsible
• must show a metastable structural change
Martin-Luther-Universität Halle
The Nature of the EL2-Defect in GaAs
Krause et al., Phys. Rev. Lett. 65 (1990) 3329
Martin-Luther-Universität Halle
• in metastable state at low temperature: Ga vacancy
• should disappear during annealing at about 110 K
• confirmed by positron lifetime measurements
• kinetics of recovery of ground state is identical for IR- und positron experiment: EA = (0.37 ± 0.02) eV
• evidence of the vacancy in metastable state confirms the proposed structural model
8 9 101000/ [K ]Ta
−1
104
103
102Rec
over
y tim
e [s
]t 1/
2
Positron annihilationIR absorption
The Nature of the EL2-Defect in GaAs
1 2 3 4 5 6 7 8 90.0
0.1
0.2
1 2 3 4 5 6 7 8 9lattice spacing in [110] direction
Hei
ght [
nm]
-2.0 V +1.4 V
occupied empty states
• Scanning tunneling microscopy at GaAs (110)-cleavages planes (by Ph. Ebert, Jülich)
• Defect complex identified as VGa-SiGa
1018 1019
1017
1018
1019
Si concentration (cm-3)
Positrons - cvac STM - [SiGa-VGa]
Def
ect c
once
ntra
tion
(cm
-3)
• Quantification → Agreement
Mono-vacancies in GaAs:Si are VGa- SiGa-complexes
Identification von VIdentification von VGaGa--SiSiGaGa--Complexes in Complexes in GaAs:SiGaAs:Si
Martin-Luther-Universität Halle
Gebauer et al., Phys. Rev. Lett. 78, 3334-3337 (1997)
electron-irradiated GaAs
Martin-Luther-Universität Halle
A. Polity et al., Phys. Rev. B 55 (1997) 10467
• electron irradiation generates vacancies in both sublattices
• very complex annealing behavior• main annealing stage at 300 K• similar annealing stage found for doped
GaAs
100 200 300 400 500 600
230
235
240
245
250
255
Measurement temperature 90 K
τb
Undoped GaAs, n = 1×1015 cm−3
electron irradiated, 2 MeV, 4 K
Irradiation dose 1×1015 cm−2
1×1016 cm−2
1×1017 cm−2
1×1018 cm−2
1×1019 cm−2
Ave
rage
pos
itron
life
time
[ps]
Annealing temperature [K]
Defects in Defects in epitaxiallyepitaxially grown LTgrown LT--GaAsGaAs--LayerLayer
Martin-Luther-Universität Halle
200 300 400 500 600
1.00
1.01
1.02
1.03
1.04
1.05 TGrowth
200°C 210°C 240°C 250°C 270°C 310°C350°C
Nor
mal
ized
S p
aram
eter
Annealing temperature (°C)
Onset of precipitation• MBE-Growth of GaAs can be performed at 200°C
• thickness of layer ≈ 1 µm
• has unique properties, e.g. very short recombination time
• layer extremely rich of defects
• up to 1% As-excess is compensated by AsGa und VGa
• Ga vacancies are seen by positrons
• during annealing: As-preci-pitation starts at 400°C
• positrons detect then small vacancy clusters
• clusters are probably bound to precipitates Gebauer et al., Appl. Phys. Lett. 71, 638 (1997)
Martin-Luther-Universität Halle
Large Positron Facility Projects in Germany
• FRM-II (Positron source at Research Reactor-II in Garching near Munich)
- continous positron beam for different experiments, mainly for:
• Scanning Positron Microscope at “Universität der Bundeswehr”, Munich
- system already working using 22-Na source
- positron lifetime measurement; lateral resolution about 2 µm
• EPOS – European Positron Source for applied Research (project at Research Center Rossendorf, near Dresden)
- positron source for materials research at superconducting 40 MeV-FEL in Rossendorf
- primary time structure suitable for positron lifetime spectroscopy
Martin-Luther-Universität Halle
Research reactorFRM-II, Munich
experimental hall
reactor vessel moderator tank• positron generation by a nuclear reaction: 113Cd(n,γ)114Cd
• three γ rays → pair production
• continuous positron beam
• ≈1010 slow e+/s expected
Martin-Luther-Universität Halle
Principle of the Positron Source at FRM-II
Cd
Pt
Pt
n
γ
slow e fast e
therm
++
n - capture
γ - emission
γ −> e+e-
γ −> e+e- e+ - moderatione+ - emission
C. Hugenschmidt, 2002
Martin-Luther-Universität Halle
C. Hugenschmidt, 2002
Design of the Positron Source at FRM-II
Scanning Positron Microscope in Munich
Martin-Luther-Universität Halle
• Semiconductor devices nm-sized ⇒ Positron microprobes required
• images show directly distribution of positron traps, i.e. nanoscopic lattice defects
• However: positron diffusion length is fundamental limit for lateral resolution
• no sense to improve resolution much below 500 nm
• first instrument was realized at Univ. Bonn (20 µm; Doppler spectroscopy)
• first realization of scanning positron microscope for lifetime spectroscopy: in Munich
Scheme of the Munich Microscope
• moderated positrons are electrostatically focused, choppered and bunched
• second moderator stage allows focus down to about 2 µm
• positron penetration energy adjustable for depth information
• instrument shall be adopted to the FRM-II positron source when available
Martin-Luther-Universität Halle
Scanning Positron Microscope in Munich
Martin-Luther-Universität Halle
Scanning Positron Microscope in Munich
Martin-Luther-Universität Halle
• Semiconductor test structure
Nature 412 (2001)764Phys. Rev. Lett. 87 () 067402
• defects near a crack in fatigued Cu
τ (ps)
Martin-Luther-Universität Halle
EPOS - Positron source at the Free-Electron Laser at ELBE
• electron beam at ELBE FEL is bunched
(length: ≈ 1 ps, repetition time: ≈ 100 ns, cw-mode, up to 108 e-/bunch)
• beam energy: 40 MeV power: 40 kW
• FEL-system in Rossendorf under construction (ELBE)
• primary electron beam already available
psns ≈
• direct positron lifetime measurement using time structure of e- beam possible
• about 2…5 x 108 slow e+/s; multidetector system for high counting rate
• combination with Doppler-coincidence spectroscopy (DOCOS) and Age-momentum correlation (AMOC)
possiblee source+
about 50m
Martin-Luther-Universität Halle
Positron Laboratory
ca. 7 m
ca. 8 m
Remoderation Stage
ca. 4 m
Beam dump andpositron extraction
First Soa Lense
Second Soa Lense
ps Buncher
First buncher Chopper
Moderator 5 keV+
Beamline 0 keV
Sample -1 ... 30 keV-
(schematic drawing)
Positron Lab at ELBE
Martin-Luther-Universität Halle
Conclusions
• vacancy-type defects can be detected in solids by means of positron annihilation
• method very sensitive for early stage of vacancy agglomeration• tools for thin layers (mono-energetic positron beams)• scanning positron microbeams available• intense positron sources under construction in Germany too
This presentation can be found as pdf-files on our Website:http://www.ep3.uni-halle.de/positrons
contact: [email protected]