Thermally Stimulated Currents Method

Ioana Pintiliea)National Institute of Materials Physics,

Bucharest-Magurele, P.O.Box MG-7, Romania

 b)Institute for Experimental Physics,

Hamburg University, D-22761, Germany

Electrical properties of Point Defects in the space charge region

)()(

)()(;

)()(

)()(

TeTe

TeNTn

TeTe

TeNTn

pn

nTE

donorT

pn

pTE

acceptorT

Tk

ETETTe

b

VCapnpn

,,,

)(exp*)(~)(

)()()()()( 00 TnTeqTnTeqT donorTp

acceptorTnE

Defect´ signature – emission rates

2) Leakage current density

1) Effective doping concentration

- given by the steady state occupancy

of the defect/impurity levels in SCR

Effective generation centers (acceptor like)

P+

N+

Electrons in the conduction band

Holes in the valence band

N

NA

ND (+)

(-)/0

Thermally Stimulated Currents Method

1. cooling the sample

2. filling of the traps:

- during cooling under 0 bias (majority carriers)

- or carrier injection at low T by biasing the diode/ illumination

3. recording the TSCurrent – the current due to charge emission from the traps is recorded as function of temperature during the heating of the sample with constant rate

p+- n – n+ diodes

n+

p+

n

Electrons in the conduction band

Holes in the valence bandI

Current

d

Tk

EENce

vc

nNp

withdt

dpxAqI

dt

dnxAqI

npcpedt

dp

pncnedt

dndt

dn

dt

dp

penpcpncnedt

dn

B

VCTVCpnpn

pnthpnpn

ttt

vhTSC

ceTSC

tptpv

tntnc

tt

ptptntnt

,,,,

,,,,

exp

;

2

2

``)(`)(1

exp(*)(*)(*2

*)(*)(

0

0 dTTeTeTeTnqTxA

TIT

T

pnnfillte

TSC

nt(Tfill), pt(Tfill):

• electrons injection nt(Tfill) = Nt

• holes injectionpt(Tfill) = Nt

• forward injection/illumination under 0Vpt(Tfill) = Ntcp/(cn+cp)

nt(Tfill) = Ntcn/(cn+cp)

j

jt

i

itDeff

eff

bi

TpTnNTN

dTN

TVV

qTx

)()()(

)(

)(2)(

20 40 60 80 100 120 140 160 180 200-5

0

5

10

15

20

25

30

35

40electron traps

15e12

7e9e

8e

5

3e1e

TSC

(pA)

Temperature (K)5 10 15 20 25 30

0.1

1

10

100 12 20 min. at 300 C

3e

5

9e

15e 8e

7e

Ha3

=0.095 eVH

a5=0.166 eV

Ha7

=0.22 eVH

a8=0.3 eV

Ha9

=0.32 eVH

a12=0.37 eV

Ha15

=0.45 eV

Log(

TSC

)

1000/T(K)

• Activation enthalpy - the thermal cleanning procedureFor defects with en(T) >> ep(T) (or viceversa) and for constant SCR width (x(T)=d) the increasing part of the TSC peak is ~ exp (Hn (p)/kT)

Hn (p) can be determined from Arrhenius plots

``)(`)(1

exp(*)(*)(*2

*)(*)(

0

0 dTTeTeTeTnqTxA

TIT

T

pnnfillte

TSC

• Concentration of defects – by integrating the TSC peak

• Capture cross sections – by fitting the TSC peaks

``)(`)(1

exp(*)(*)(*2

*)(*)(

0

0 dTTeTeTeTnqTxA

TIT

T

pnnfillte

TSC

j

jt

i

itDeff

eff

bi

TpTnNTN

dTN

TVV

qTx

)()()(

,)(

)(2)(

x = d

x < dx < d

x < dx < dx =d

Nt >20xNd

Problems - Temperature variation of SCR width (x(T) < d)

60 80 100 120 140 160 180 200 220

100

200

300

400

500

600 RB = 100 V E&H(147K) protons neutrons

TS

C (

pA)

Temperature (K)

80 100 120 140 160 180 2000

100

200

300

400

500proton irradiated RB= 100 V

SCR

wid

th (

m)

Temperature (K)

130 140 150 160 170 180

0

50

100

150

200

250

300

350neutron irradiated

E&H(165)

E&H(147) RB= 200 V RB= 150 V RB= 100 V fit 200 V fit 150 V fit 100 V

TS

C (

pA)

Temperature (K)80 100 120 140 160 180 200

200

400

600

800neutron irradiated RB= 200 V RB= 150 V RB= 100 V

SCR

wid

th (

m)

Temperature (K)

``)(`)(

1exp(*)(*)(*)(~)(

0

i

T

T

ip

in

infill

it dTTeTeTeTnTxTITSC

j

jt

i

itDeff

eff

bi

TpTnNTN

dTN

TVV

qTx

)()()(

,)(

)(2)(

Evaluation of temperature dependent capture cross sections

Diodes with initial doping of Nd = 5·1011 cm-3, Nt ~ 2·1013 cm-3

P+

N+

Electrons in the conduction band

Holes in the valence band

N

NA

ND (+)

(-)/0

Steady state occupancy in the SCREmission from the filled traps

Deep acceptors acting like Generation centers

N+

P+

N

Electrons in the conduction band

Holes in the valence band

I

Current

nT (T) ~ 80%NT

what can be detected is

20% NT emission of electrons

80% NT emission of holes

``)(`)(1

exp(*)(*)(*2

**)(

0

0 dTTeTeTeTnqdA

TIT

T

pnnfillte

TSC

Thermally Stimulated Currents Method

• limitation:

- TSCmin > 0.1 pA

- TSCmax – given by the possibility of depleting the diodes thickness (RBmax=500 V) during T scan

- no direct measurement of the capture cross sections possible

• advantage:

- investigation of diodes with high traps concentration

NT >ND-NA

-    determination of ionisation energy (Thermal cleaning procedure)

-    suitable for the detection of donor activity (Poole-Frenkel effect)

• disadvantage:

- no direct defect identification possible

- combination with other methods (e.g. EPR, FTIR)