Positron Beam Application to Materials Science and Intense Accelerator …positron.physik.uni-halle.de › talks › KEK2002.pdf · 2006-06-15 · Positron Beam Application to Materials

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


• 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


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


• 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


• 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


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


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


• 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


The Nature of the EL2-Defect in GaAs

Krause et al., Phys. Rev. Lett. 65 (1990) 3329


• 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


Gebauer et al., Phys. Rev. Lett. 78, 3334-3337 (1997)

electron-irradiated GaAs


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


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)


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


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


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


C. Hugenschmidt, 2002

Design of the Positron Source at FRM-II

Scanning Positron Microscope in Munich


• 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






• Semiconductor test structure

Nature 412 (2001)764Phys. Rev. Lett. 87 () 067402

• defects near a crack in fatigued Cu

τ (ps)


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


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


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]

Documents

Positron Beam Application to Materials Science and Intense Accelerator …positron.physik.uni-halle.de › talks › KEK2002.pdf · 2006-06-15 · Positron Beam Application to Materials