Post on 23-Feb-2016
description
Origin of unusual Ag diffusion profiles in CdTe
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J. Lehnert, M. Deicher, K. Johnston, Th. Wichert, H. WolfExperimentalphysik, Universität des Saarlandes, 66123 Saarbrücken,Germany
The ISOLDE CollaborationCERN, Geneva, Switzerland
ISOLDE Workshop and Users meeting 2013
Support by BMBF, contract 05KK7TS2
Outline
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Diffusion
Uphill diffusion of Ag in CdTe
Influence of metal layers on Ag diffusion
Influence of oxidized crystal surfaces
Summary
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Radiotracer technique
aCdTeØ = 6 mmd = 800 µm
Implantation
CdTethermal
treatment
550 … 900 K60 keV
30 nm
111Ag+
Resolution > 1 µm T1/2 > 1 h
Mechanicalsectioning
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4
Diffusion in solids
Monotonously decreasing profiles
( ) X
XJ X D
x
( )X
J Xt x
24( , )
2
xDtNC x t e
Dt
for
0 200 400 600 800
1011
1012
1013
1014
conc
entra
tion
(cm
-3
)
depth (µm)
111AgCdTe
)()0,( xNtxC
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Diffusion of 111Ag in CdTe
5(H. Wolf et al., Phys. Rev. Lett. 94 (2005) 125901)
Monotonously decreasing profile
Uphill diffusion profile
0 200 400 600 8001011
1012
1013
1014
0 200 4001011
1012
1013
1014
Tdiff = 800 Ktdiff = 60 minCd pressure
depth (µm)
depth (µm)
con
cent
ratio
n (c
m
-3
) Tdiff = 570 Ktdiff = 30 minArgon
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ModelDeviation from stoichiometry
Initial state of the crystal Te rich
Highly mobile Cd interstitialsHighly mobile Ag dopant (interstitial)
Source of Cd interstitials during diffusion
DC < 0
Cd Cd
i Cd[ΔC]=[Cd ]-[V ]
Ag dopants images the profile of the intrinsic defects
0 200 400 600 8001011
1012
1013
1014
depth (µm)
DC < 0
DC < 0DC > 0 DC > 0
DC > 0 DC > 0
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Alternative sources for Cdi defects?
Metal layers
Oxidized crystal surfaces
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Diffusion without Cu layer at 500 KDiffusion with Cu layer at 500 K
DC < 0
111Ag in CdTe
Metal layers can initialize uphill diffusion
(H. Wolf et al., Appl. Phys. Lett. 100 (2012) 171915 )
Influence of metal layers (Cu)
without metal layerwith metal layer
30 nm Cu layer
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metal layer CdTe
Cdi source
CdTemetal layer metal-Tealloy
550 K
Model
Ag dopant images the profile of deviation of stoichiometry again?
ΔC < 0
CdTe
metal layer
ΔC > 0
CdTe
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Can Ag image a previously prepared deviation of stoichiometry?
ΔC < 0
CdTe
Metal layer
2nd Annealing 550 K 60 min
Concentrationmeasurement
Creating the deviation of stoichiometry before implantation of Ag
1st Annealing 550 K 30 min
ΔC > 0
CdTe
ΔC < 0
Implantation111Ag
CdTe
ΔC < 0ΔC > 0
ΔC < 0
EtchingHCl
CdTe
ΔC > 0
Ar atmosphere
Ar atmosphere
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Cd source is created by Te-metal alloy
Concentration of the dopant images the profile of deviation from stoichiometry again
Can we create a Cd source at the surface without evaporated metals?
Experimental result
ΔC < 0
CdTe
metal layer
ΔC > 0
CdTe
550 K 60 minArgon
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Influence of the condition of the crystal surface
As delivered samples show uphill diffusion
Br-methanol, KOH etching suppresses uphill diffusion
Surface condition has significant effect on Ag diffusion
as delivered etched (Br-methanol, KOH)
550 K 30 minArgon
570 K 30 minArgon
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CdTe CdTeTe oxide
RT+air
Cdi source
Natural oxidation at room temperature and air 1)
TEM measurements: TeO2 at the surface 2)
No evidence of Cd at the surface
1) T. B. Wu, J. S. Chen, C. D. Chiang, Y. M. Pang, and S. J. Yang, J. Appl. Phys. 71, 5212 (1992)
2) F. A. Ponce, R. Sinclair, and R. H. Bube, Appl. Phys. Lett. 39, 951 (1981)
Mechanism
Cd source is created by natural oxidation process
Can we create a Cd source by oxidizing etching?
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Experimental idea
Oxidation creates a Cd source
ΔC < 0
etched CdTe
Etching HCl/H2O2 CdTe
ΔC < 0
CdTe
ΔC < 0
Implantation
Annealing 550 K 30 min
ΔC<0ΔC>0
Concentration measurement
550 K 30 minArgon
550 K 30 minArgonas delivered
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- Uphill diffusion is initialized by a Cd source- Ag dopant images the profile of the deviation from stoichiometry- Cd source can be provided by:
Summary
CdTeTe oxide
Cdi source
Cd Cd
DC <0DC >0 DC >0
external Cd pressure
Cdi source
CdTemetal layer metal-Tealloy
evaporated metal layer natural and forced oxidation
Outlook Verifying of the results by diffusion experiments AND observation of
the state of the surface (XPS or similar method)
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Thanks to
Uni Freiburg:M. FiederleA. Fauler
Uni Bonn:R. Vianden
CERN:ISOLDE Collaboration
BMBF for financial support
Uni Prague:R. GrillE. Belas