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: h.wolf@mx.uni-saarland.de

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