Deep Level Transient Spectroscopy ... - IRTG Heidelberg

Deep Level Transient
Spectroscopy: Characterization
of Radiation Damage
Eduard Monakhov, UiO

Introduction
Electronic levels of main vacancy
related traps in Si:
E c
E c -0.18 eV
E v
VO V 2
-/0
=/-
-/0
0/+
The effect of deep levels
E c -0.23 eV
E c -0.44 eV
E v +0.20 eV
V 2 is important!
Leakage current as a function of the bias in silicon detector with
different types of defects (SILVACO Virtual Wafer Fab).

Introduction
The effect of C and O on radiation hardness of detectors
standart FZ-Si: [O]~10 15 -10 16 cm -3
oxygenated FZ-Si: [O]~10 17 -10 18 cm -3

Introduction
The neutron puzzle
No radiation hardening
for neutron radiation!
.

p +
DLTS technique
a) V=0, t0
p +
W R
e -
n
E f

C
C
=
A
DLTS technique
qε
0ε
( N D − n
2(
V −V
)
bi
T
)
⎛ 1
⎜ −
⎝
qε
0εN
if nT

C
DLTS technique
0
t 1
t 2
t
C(t 2 )-C(t 1 )
T

U 0
U R
C 0
C R
C R0
DLTS technique
filling
pulse
Height and width of
filling pulse can be
varied
t
t

DLTS technique
Typical DLTS spectrum for n-type Si with radiation-induced defects.
VO, 0.18 eV
V 2 (=/-)
0.23 eV
V 2 (-/0)
0.44 eV

C
DLTS technique
e m
e m ’
0 t1 t2 t 3
t
DLTS signal
T
T m
T m ’

DLTS technique
DLTS signal
Spectra for different measurement times (time windows)
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
100 150 200 250
T, K
In the analysis we determine temperatures of the peaks

DLTS technique
From experiment: (e 1 ,T 1 ); (e 2 ,T 2 ); (e 3 ,T 3 ); …
Expected: e~exp[-(E c -E t )/kT], more precisely: e=N Cσ n v th,n exp[-(Ec-Et)/kT]
e=σ n AT 2 exp[-(Ec-Et)/kT]
ln(e/T 2 )=ln(σ n A)-(Ec-Et)/kT
Plot ln(e/T 2 ) vs 1/T
8h at 200oC 3.5h at 250oC 7.5h at 250o X(0/-):
C
V 2 (0/-):
8h at 200 o C
3.5h at 250 o C
7.5h at 250 o C

DLTS technique
Alternative ways to convert transients to a spectrum
S(T)=∫ C(T,t)w(t)dt
In previous example (box-car):
w(t)=δ(t2)-δ(t1)
Mostly used (’lock-in’ weighting function):

Electron capture cross-section
a) V=V R , t=0
p +
W R
n
p +
b) V=0, 0

Electron capture cross-section
DLTS signal
DLTS signal: S=S max [1-exp(-r n t filing )]
log(time of filling)

Electron capture cross-section
fast capture kinetics slow capture kinetics
a) b)
9.6x10 -15 cm 2
3.1x10 -15 cm 2
E.V. Monakhov, B.S. Avset, A. Hallen, B.G. Svensson, Phys. Rev. B 65, 233207 (2002)
1.3x10 -16 cm 2

Laplace DLTS
The technique is based on solving the following integral equation:
y ( t)
= ∫ F(
λ , t)
s(
λ)
dλ
+ A + ε
If one chooses F(λ,t)=exp(-λt)
the solution s(λ) in case of DLTS is
a sum of delta functions
C
s
t
λ

Laplace DLTS
BioRad
Boonton bridge
capacitance meter
Analog
output
National
Instruments
analog-digital
converter
(PCI card, 12-bit
resolution, 5 MHz
sampling rate)
Laplace-DLTS setup
SemiLab bath type cryostat
with a silicon diode
temperature sensor
Digitized transient
(normally ~10 6 points)
* S. W. Provencher, Computer Physics Communications, 27 (1982) 213
averaging to
10 3 points
LakeShore
temperature
controller
(temperature
stability ~0.02 K)
Transient analysis
(program CONTIN*)

DLTS technique
Typical DLTS spectrum for n-type Si with radiation-induced defects.
VO, 0.18 eV
V 2 (=/-)
0.23 eV
V 2 (-/0)
0.44 eV

Interaction of V 2 with defects and impurities
X
V 2
15min at 220 o C
15min at 300 o C
1h at 300 o C
15at 325 o C
E.V. Monakhov, B.S. Avset, A. Hallen, B.G. Svensson, Phys. Rev. B 65, 233207 (2002)
V 2
X
Tentative
identification of X:
X=V 2 O
V 2 +OV 2 O

DLTS signal
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
Laplace DLTS
Typical Laplace-DLTS spectrum for the overlapping V 2 and X peaks.
100 150 200 250
T, K
V 2 (-/=)
X(-/=)
6 8 10 20 40

Laplace DLTS
DLTS signal (pF)
0.1
Transformation of V 2 to X at 250 o C.
sample A sample C
V 2 (=/-) V 2 (=/-)
X(=/-) X(=/-)
0 1000 2000 3000 4000 5000 6000
Annealing time (min)
A: [O]~(2-3)x10 17 cm -3
C: [O]~(1-2)x10 16 cm -3
Transformation rate
is proportional to [O].
Proves that
X=V 2 O
V 2 +OV 2 O

U 0
U R
Minority carrier DLTS (MCTS)
filling
pulse
if U 0 >0?
Injection of holes to n-type region will occur
Height and width of
filling pulse can be
varied
t

p +
Minority carrier DLTS (MCTS)
a) V=0, t0, t~0
+ p n
0
t
c) V=V R , t>0
p +
h
W R
h
h
e
n
E f

Minority carrier DLTS (MCTS)
VO
V 2 (=)
V 2 (-)
C i O i

SUMMARY
DLTS is a powerful technique for studies of radiation induced defects
Capabilities:
- position of defect levels in the band gap
- concentration of specific defects
- capture cross-sections
- depth distribution
Limitations:
- problems with accurate measurements on minority carriers
- concentration of the defects should be