Deep Level Transient
Spectroscopy
Eduard Monakhov, UiO
Introduction
The effect of deep levels
Electronic levels of main vacancy
related traps in Si:
Ec
Ec-0.18 eV
VO
V2
-/0
=/-/0
0/+
Ec-0.23 eV
Ec-0.44 eV
Ev+0.20 eV
Ev
Leakage current as a function of the bias in silicon detector with
different types of defects (SILVACO Virtual Wafer Fab).
V2 is important!
Introduction
The effect of deep levels: Light Induced
Degradation (B-doped, mc-Si)
Electronic levels of Fei and B-Fei:
Ec
B-Fe Fe
After fabrication (in dark):
-/0
Fei + B  B-Fei
0/+
Ev+0.4 eV
Under illumination:
EV+0.1 eV
0/+
B-Fei  Fei + B
Ev
Fei is important!
DLTS technique
b) V=VR, t~0
a) V=0, t<0
c) V=VR, t>0
WR0
WR
W0
e-
p+
p+
Ef
p+
n
n
n
DLTS technique
q 0 ( N D  nT )
CA
2(Vbi  V )
if nT<<ND:
nT
q 0N D (1 
)
nT
ND
CA
 C0 1 
2(Vbi  V )
ND
 1 nT 
  C0  C
C  C0 1 
 2 ND 
 1 nT 0

C  CR 1 
exp( et )
 2 ND

C0
1 nT
C  C0
2 ND
e~exp[-(Ec-Et)/kT]
CR
CR0
0
D.K.Schroder "Semiconductor material and device characterization"
t
DLTS technique
C
T
0
t1
t2
t
C(t2)-C(t1)
DLTS technique
U0
filling
pulse
Height and width of
filling pulse can be
varied
UR
t
C0
CR
CR0
t
DLTS technique
Typical DLTS spectrum for n-type Si with radiation-induced defects.
VO, 0.18 eV
V2(=/-)
0.23 eV
V2(-/0)
0.44 eV
DLTS technique
C
T
em
Tm
em’
0
t1
t2
Tm’
t3
t
DLTS signal
DLTS technique
Spectra for different measurement times (time windows)
0.45
0.40
DLTS signal
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: (e1,T1); (e2,T2); (e3,T3); …
Expected: e~exp[-(Ec-Et)/kT], more precisely: e=NCnvth,nexp[-(Ec-Et)/kT]
e=nAT2exp[-(Ec-Et)/kT]
ln(e/T2)=ln(nA)-(Ec-Et)/kT
V2(0/-):
8h at 200oC
3.5h at 250oC
7.5h at 250oC
Plot ln(e/T2) vs 1/T
X(0/-):
8h at 200oC
3.5h at 250oC
7.5h at 250oC
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)=(t2)-(t1)
Mostly used (’lock-in’ weighting function):
Electron capture cross-section
filling
pulse
U0
Height and width of
filling pulse can be
varied
UR
a) V=VR, t=0
b) V=0, 0<t<tfilling
WR
W0
e-
p+
p+
n
n
electron capture rate: rn=nnvth,n
Electron capture cross-section
DLTS signal
DLTS signal: S=Smax[1-exp(-rntfiling)]
log(time of filling)
Electron capture cross-section
a)
fast capture kinetics
b)
slow capture kinetics
9.6x10-15 cm2
3.1x10-15 cm2
1.3x10-16 cm2
E.V. Monakhov, B.S. Avset, A. Hallen, B.G. Svensson, Phys. Rev. B 65, 233207 (2002)
Laplace DLTS
The technique is based on solving the following integral equation:
y(t )   F ( , t ) s( )d  A  
If one chooses F(,t)=exp(-t)
C
the solution s() in case of DLTS is a
sum of delta functions
t
s

Laplace DLTS
Laplace-DLTS setup
SemiLab bath type
cryostat with a silicon
diode temperature sensor
BioRad
Boonton bridge
capacitance meter
LakeShore
temperature
controller
(temperature
stability ~0.02 K)
Analog
output
National
Instruments
analog-digital
converter
(PCI card, 12-bit
resolution, 5 MHz
sampling rate)
Digitized transient
(normally ~106 points)
averaging to
103 points
* S. W. Provencher, Computer Physics Communications, 27 (1982) 213
Transient analysis
(program CONTIN*)
DLTS technique
Typical DLTS spectrum for n-type Si with radiation-induced defects.
VO, 0.18 eV
V2(=/-)
0.23 eV
V2(-/0)
0.44 eV
Interaction of V2 with defects and impurities
V2
X
X
V2
15min at 220oC
15min at 300oC
1h at
300oC
15at 325oC
E.V. Monakhov, B.S. Avset, A. Hallen, B.G. Svensson, Phys. Rev. B 65, 233207 (2002)
Tentative
identification of X:
X=V2O
V2+OV2O
Laplace DLTS
Typical Laplace-DLTS spectrum for the overlapping V2 and X peaks.
V2(-/=)
0.45
0.40
DLTS signal
0.35
0.30
0.25
0.20
X(-/=)
0.15
0.10
0.05
0.00
-0.05
100
150
200
250
T, K
6
8
10
20
40
Laplace DLTS
Transformation of V2 to X at 250oC.
A: [O]~(2-3)x1017 cm-3
DLTS signal (pF)
C: [O]~(1-2)x1016 cm-3
Transformation rate is
proportional to [O].
0.1
sample A
V2(=/-)
X(=/-)
0
1000
2000
3000
sample C
V2(=/-)
Proves that
X=V2O
V2+OV2O
X(=/-)
4000
Annealing time (min)
5000
6000
Minority carrier DLTS (MCTS)
U0
filling
pulse
Height and width of
filling pulse can be
varied
UR
t
if U0>0?
Injection of holes to n-type region will occur
Minority carrier DLTS (MCTS)
b) V>0, t~0
a) V=0, t<0
c) V=VR, t>0
WR
W0
e
p+
Ef
h
p+
n
p+
h
n
C0
CR0
CR
0
t
h
n
Minority carrier DLTS (MCTS)
VO
V2(-)
V2(=)
CiOi
Boron – Iron interaction
FeB
Fei
SUMMARY
DLTS is a powerful technique for studies of 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 << doping concentration
D.K.Schroder "Semiconductor material and device characterization"