Exotic diffusion phenomena in II-VI semiconductors
description
Transcript of Exotic diffusion phenomena in II-VI semiconductors
Exotic diffusion phenomenain II-VI semiconductors
H. Wolf, F. Wagner, J. Kronenberg,M. Deicher, and Th. Wichert
Technische Physik, Universität des Saarlandes, D-66123 Saarbrücken, Germany
ISOLDE Workshop 14.02.2007
e-mail: [email protected]
Outline
Quantitative description
defect reactionsdrift-diffusion of Ag in CdTesimulation of Ag profiles
Exotic diffusion profiles of 111Ag in CdTe
Experimental results
New exotic diffusion profiles
short-lived isotopes at ISOLDEused for diffusion experiments
„simple“ diffusion process
Au in Si
monotonously decreasing
profile
solution for finite source:
depth
con
cen
trat
ion
2x
4DtC e
0 100 200 300 400
1011
1012
1013
1014
depth (µm)
con
cen
trat
ion
(cm
-3)
CdTe:111Ag
Tdiff = 570 K tdiff = 30 min
experimental procedure
2 gm
1...30 µm
0 100 200 300 400
1011
1012
1013
1014
depth (µm)
con
cen
trat
ion
(cm
-3)
CdTe:111Ag
Tdiff = 570 K tdiff = 30 min
CdTeØ = 6 mmd = 800 µm
111Ag
111Ag Implantation
CdTethermal
treatment
550 … 900 K
Tdiff = 825 K
tdiff = 60 min
„exotic“ diffusion profiles in CdTe
Shapes of diffusion profiles strongly depend on external vapor pressure
H. Wolf, F. Wagner, Th. Wichert, and ISOLDE Collaboration, Phys. Rev. Lett. 94, 125901, 2005
0 200 400 600 8001011
1012
1013
0 100 200 300 4001011
1012
1013
0 200 400 600 800
1012
1013
1014
0 200 400 600 800
1012
1013
1014
Ag
- c
on
ce
ntr
ati
on
(c
m-3)
Cd-pressure vacuum
Cd-pressure
depth (µm)
pre-treated underCd pressure (800 K, 24 h)
Te-pressure
depth (µm)
111Ag
30 nm CuCdTe (550 µm)
codiffusion experiments
0 100 200 300 400 500
1011
1012
1013
1014
depth (µm)
con
cen
trat
ion
(cm
-3)
111Ag
Tdiff = 550 K tdiff = 30 min
0 200 400 600 8001011
1012
1013
depth (µm)co
nce
ntr
atio
n (
cm-3
)
111Ag
111Ag
30 nm Au
Tdiff = 550 K tdiff = 120 min
CdTe (750 µm)
Cu (Au) layer pushes Ag to the reverse of the crystal
more „exotic“ diffusion profiles
„exotic“ diffusion profiles are not restricted to Ag diffusion in CdTe
„normal“ diffusion profile observed in CdTe:In
0 100 200 300 400
1011
1012
1013
1014
depth (µm)
con
cen
trat
ion
(cm
-3)
CdTe:In
Tdiff = 800 K tdiff = 60 min
111Ag
0 200 400 600 800
1011
1012
1013
depth (µm)
con
cen
trat
ion
(cm
-3)
CdTe
Tdiff = 800 K tdiff = 60 min
67CuCd pressure
0 200 400 600
1010
1011
1012
1013
depth (µm)
con
cen
trat
ion
(cm
-3)
Tdiff = 900 K tdiff = 49 h
ZnTe
111AgZn pressure vacuum
Quantitative description
Defects of consideration
Defect interactions
Quantities of conservation
Drift of charged defects
Defects of consideration
Cd sublattice
intrinsic
Te sublattice: „perfect“
extrinsic
donors
acceptors
Defect reactions
kick-out dissociative annihilation
Defect concentrations
Formation energies:
F(Agi), F(AgCd)F(Cdi), F(VCd)
Energy levels:
ED (donors)EA (acceptors)
(Chemical) potentials:Ag,Cd, F
Charge states:
additional:
(electrons)
(holes)
Quantities of conservation
No changes upon defect reactions:
Total Ag concentration
Deviation from stoichiometry
Charge density
Drift of charged defects
Poisson equation:
Drift of charged defects:
µF: electric potential
: electric field
Quantitative description of Ag profiles
to be solved simultaneously for CAg, C, and /e
Flux of defect Y:
diffusion drift
Particle conservation:
Drift-Diffusion equation:
Model parameters
Free parameters:
FAg = FAgi – FAgCd
Cinit
D(Agi), D(AgCd), D(Cdi), D(VCd)
[1] R.Grill et al., Nuclear Instruments and Methods in Physics A 487 (2002) 40[2] M.A. Berding, Phys. Rev. B 60 (1999) 8943
known from literature [1,2]:
FCdi, FVCd
, and energy levels of Cdi, VCd, Agi, AgCd
FAg
boundary conditions:
surface is in equilibrium with external Cd pressureconservation of total Ag content
Profile simulation
Excellent simulation of profiles with a common set of parameters
F(Agi)-F(AgCd) < 1.5 eV
Cinit = -3.3∙1016 cm-3
D(Agi) = 510-7 cm2/s
D(AgCd) = 0
D(Cdi) = 110-6 cm2/s
D(VCd) = 510-9 cm2/s
Tdiff = 825 K tdiff = 60 min
0 200 400 600 8001011
1012
1013
0 100 200 300 4001011
1012
1013
0 200 400 600 800
1012
1013
1014
0 200 400 600 800
1012
1013
1014
Ag
- c
on
cen
trat
ion
(cm
-3)
Cd-pressure vacuum
Cd-pressure
depth (µm)
pre-treated underCd pressure (800 K, 24 h)
Te-pressure
depth (µm)
At diffusion temperature:
High mobility of Agi, and Cdi
Low mobility of AgCd, and VCd
Preferred incorporation of Ag as Agi
Requirements for „exotic“ diffusion
Propagation of Ag is strongly supportet by
Drift of charged defects in internal electric field(generated by inhomogeneous distribution of defects)
Strong variation of C during diffusion
(Te-excess Cd-excess)external source of intrinsic defects: PCd
codiffusion experiments
0 100 200 300 400 500
1011
1012
1013
1014
depth (µm)
con
cen
trat
ion
(cm
-3)
111Ag
111Ag
30 nm Cu
Tdiff = 550 K tdiff = 30 min
CdTe (550 µm)
0 200 400 600 8001011
1012
1013
depth (µm)co
nce
ntr
atio
n (
cm-3
)
111Ag
111Ag
30 nm Au
Tdiff = 550 K tdiff = 120 min
CdTe (750 µm)
External source of intrinsic defects?
codiffusion experiments
Cu-layer CdTe
Cd-layer acting asstrong source for
intrinsic defects (Cdi)
CdTeCu-layer
Cu-Tealloy
550 K
(1 ML Cd at the interface might be sufficient!)
New „exotic“ diffusion profiles
Investigated materials:
CdTe, ZnTe
111Ag, 67Cu,
New „exotic“ diffusion profiles
111Ag, 67Cu,
24Na, 43K, 56Mn59Fe (59Mn), 65Ni
Investigated materials:
CdTe, ZnTe, CdZnTe, CdS
Alkali dopants in CdTe
CdTe:43K
0 100 200 300 400
depth (µm)
co
nc
en
tra
tio
n
(a.u
.)
Tdiff = 750 K tdiff = 9 h
vacuum
43K: slow diffusion no „exotic“ diffusion profile24Na: fast diffusion „exotic“ diffusion profile
0 200 400 600 800
depth (µm)c
on
ce
ntr
ati
on
(a
.u.)
Tdiff = 750 K tdiff = 10 h
CdTe:24Na
vacuum
0 100 200 300 400
depth (µm)co
nce
ntr
atio
n (
a.u
.)
CdS:24Na
Tdiff = 850 K tdiff = 60 min
CdS:43K
0 50 100 150 200
depth (µm)
co
nc
en
tra
tio
n
(a.u
.)
Tdiff = 850 K tdiff = 30 min
vacuumvacuum
Alkali dopants in CdS
43K: slow diffusion no „exotic“ diffusion profile24Na: fast diffusion „exotic“ diffusion profile
Diffusion of Na (K) in CdS much faster than in CdTe!
0 50 100 150 200 250
depth (µm)
co
nc
en
tra
tio
n
(a.u
.)
CdZnTe:65Ni
Tdiff = 800 K tdiff = 60 min
Cd pressure
Magnetic dopants in Cd(Zn)Te
No diffusion profile detected for
CdTe:59Fe, CdZnTe:56Mn:( )
diffusion temperature too low ?diffusion time too short ?
investigation at a smaller depth scalesputter chamber under construction!
depletion of 65Ni at the surface
curious dip at 60 µmsample inhomogeneity?
Summary
Quantitative understanding of exotic diffusion profiles observed for Ag and Cu in CdTe
Short-lived isotopes at ISOLDE:
Exotic diffusion phenomena are not restricted to Ag and Cu in CdTe
During diffusion:
High mobility of Agi, and Cdi
Preferred incorporation of Ag as Agi
Strong variation of CDrift of charged interstitial defects in internal electric field
External source of intrinsic defects:
Vapor pressure (PCd)interface to metal layer
Defect concentrations
C id F Cd0 kTI 0C C e
CdCd F V
0 kTV 0C C e
g iA
i
F Ag
0 kTAg 0C C e
Cd
Cd
Ag F Ag
0 0 kTAg VC C e
Neutral defects: (C0 = 1.48·1022 cm-3)
D Fi iE0 D kT kT
D D 0D
gC C e e
g
Fi A iE0 A kT kT
A A 0A
gC C e e
g
Charged defects: (i: intrinsic Fermi energy)
donors Agi, Cdi
acceptors AgCd, VCd
Chemical potentials:Ag dopant concentrationCd concentration of intrinsic defectsF carrier concentration
Formation energies:
F(Agi), F(AgCd)F(Cdi), F(VCd)
Energy levels:
ED (donors)EA (acceptors)
(gD, gA: degeneracy)
Incorporation sites of Ag
Height of profile requires F < 1.5 eV
Profile depends only weakly on FAg for FAg < 1.5 eV
Ag is present as Agi+
to a large extent
0 200 400 600 8001011
1012
1013
Ag-Cd
co
nc
en
tra
tio
n (
cm
-3)
depth (m)
Ag+i
Agtot
Drift and diffusion
0 200 400 600 800
-4
-2
0
2
4
total
drift
diffusion
(d/d
t) [
Ag
] (
arb
. un
its)
depth (m)
0 200 400 600 800
1011
1012
1013
-0,3
-0,2
-0,1
0,0
0,1
[Ag
] (
cm
-3)
depth (m)
(F -
i)
(e
V)
Agi+ behaves like a free carrier
i ii i i
i i
i
2 2 2Ag Ag0 0
i A
2
i Ag2 g Ag Ag0 2 2 0 2Ag
AAg
gd
D AC C1 d d d
D Ag C C C2 C dx dx C dx
d
dC
dg C
xt
strong compensation of drift and diffusion
Ag profile reflectsFermi energy F
111Ag diffusion in CdTe
un-etched crystal!
depletion layerof about 100 m
Tdiff = 550 K tdiff = 30 min
Systematc investigations
pre-treatment: etching in bromine-methanol solution
Tdiff = 800 Ktdiff = 60 minCd pressure
symmetric, centredconcentration profiledepletion layers
„common“ concentration profile
Tdiff = 570 Ktdiff = 30 minVacuum
Time dependent measurement of diffusion
t = 15 min t = 60 min t = 240 min
t = 420 min t = 600 min t = 900 min
Tdiff = 700 K
Tdiff = 800 K tdiff = 60 min
Ag Konzentration
NAg = 1016 cm-2 NAg = 2∙1017 cm-2NAg = 2.5∙1011 cm-2
Experiment
Äußere Bedingungen
Tdiff = 800 K tdiff = 60 min111Ag Flächenkonzentration: N = 2∙1017 cm-2
Cd Druck
Starker Einfluss von äußerem Cd / Te Druck
auch bei hoher Ag Konzentration
Zusammenhang mit intrinsischen Defekten (Stöchiometrieabweichung)
Vakuum Te Druck
Codiffusion of Ag and Cu in CdTe
111Ag
Custrongly enhanced
diffusion of Ag by
codiffusion of Cu
67Cu
Agno significant
influence of Ag on
the diffusion of Cu
Tdiff = 550 K, tdiff = 30 min
0 200 400 600 8000,0
2,0x1013
4,0x1013
6,0x1013
8,0x1013
1,0x1014
0 200 400 600 8001011
1012
1013
1014
Kon
zent
ratio
n (c
m-3
)
Tiefe (m)
Kon
zent
ratio
n (c
m-3
)
Tiefe (m)
0 200 400 600 8000,0
2,0x1013
4,0x1013
6,0x1013
8,0x1013
1,0x1014
0 200 400 600 800
1011
1012
1013
K
onze
ntra
tion
(cm
-3)
Tiefe (m)
Kon
zent
ratio
n (c
m-3
)
Tiefe (m)
30 min 120 min
Codiffusion of Ag and Au in CdTe
111Ag
Au
550 K (Vak.)
Ag diffusion in ZnTe
Tdiff = 900 K
tdiff = 2 h tdiff = 4 h tdiff = 49 h
similar effects like in CdTe
Agi diffusion slower than in CdTe
chemical self diffusion significantly slower than in CdTe
B.O. Wartlick et al., Philosophical Magazine B75 (1997) 639 I. Lyobomirsky et al., J. Appl. Phys 81 (1997) 6684 M.A. Kovalets et al., Fizika i Khimiya Obrabotki Materialov 21 (1987) 125 I. Lyobomirsky et al., Journal of Electronic Materials 26 (1997) 97
Diffusion coefficients from literature
irregular!
Ag in CdTe
effects from sample boundary
1) E. Belas, R. Grill, A.L.Toth, P. Moravec, P. Horodysky, J. Franc, P. Höschel, H. Wolf, T. WichertII-VI Workshop, 04.-08.10 2004, Chikago, to be published in IEEE Transactions on Nuclear Science
EBIC on CdTe:In (2∙1016 cm-3) 1)
pn-transition after annealingunder Cd pressure (TA = 773 K)
CdTe:In
1)
2) annealing
3)
diffusion of Cdfrom all sides
tA = 16 h
tA = 57 h
p-type (C < 0)
n-type (C > 0)
Uni Münster:
H. MehrerN.A. StolwijkH. BrachtA. Rodriguez Schachtrup
Uni Bonn:
R. ViandenD. Eversheim
CERN:
ISOLDE Collaboration
BMBF, DFG
Uni Prague:
R. GrillE. Belas