Transcript The steady and transient photoconductivity, and related

The steady and transient
photoconductivity, and related
phenomena in the neutron irradiated Si.
J.Vaitkus, E.Gaubas, A.Kadys, V.Kalendra,
V.Kazukauskas, A.Mekys, J.Storasta, E.Zasinas
Vilnius University, Institute of Materials Science and Applied Research
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Outline:
• A few general words about the photoconductivity
& related transport phenomena
• The results:
– general data
– the peculiarities
• Attempts to conclude
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Definition:
If the sample is uniform, then the chosen effects:
photoconductivity, light induced transient gratings,
Hall and photo-Hall effects, magnetoresistance effects
allow to characterize the behavior of free carriers
and the local levels in the sample and their parameters .
The inhomogegenities differently influence these
effects, therefore the complex application gives a
possibility to recognize what happens in the
sample after a certain treatment.
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Photoconductivity
Photoconductivity spectra allows to identify the deep level and
characterize a role of electron-phonon interaction in its
environment.
E
2000
11
E
4
Energy, a.u.
Photon cross-section, a.u.
C
1600
1,2
0,8
1200
M

EOpt
 ET
800
E
V
0,4
3
400

EOpt
2
0
 ET
0,0
0,5
1,0
Photon energy, eV
1,5
-15
-10
-5
0
5
10
15
20
Q, config. coordinate
The ionizing energy of deep centre in thermal and optical experiments are different
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Spectra in irradiated Si
-6
10
-5
10
-6
10
-7
10
Si 8556-01-36
1E13n
-8
10
-7
-8
10
U= -50V
delay 6s
Normalised
-9
18K
18K
50K
50K
110K
110K
175K
175K
-9
10
-10
10
-11
10
0.4
0.6
0.8
Delay 6s
18K
10
1.0
1.2
 (eV)
1.4
1.6
1.8
I, A
I (A)
10
Si 8556-01-51
1E15n
-10
10
~0.82 eV
-11
10
-12
starting from low energies
return
minus persistant current
minus persistant current
10
-13
10
~0.51 eV
0.6
0.8
1.0
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
1.2
heV
1.4
1.6
1.8
Transitions
light on
0,05mm
EC
0,2mm
0,1mm
light off
1E-10
ET
ER
abs I (A)
1E-11
0V set
1E-12
2007.05.16
T=~15,73K
U= -1V
1,0eV
1mm
-1V
set
1E-13
EV
1E-14
0
1000
2000
t (s)
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
3000
4000
Intrinsic photoconductivity
A=b+ck.
1
1 
L p 1  kLp
j p
-1
1000
s
100
k cm
Where jp is the photocurrent, jp is the photocurrent
independent on the light aborption coefficient, s – surface
recombination velocity, p free carrier lifetime. This
expression that can be transformed into the relation
s p
jp
According to the classical theory of photoconductivity
spectral dependence (T.Smith. Semiconductors. 1959)
Si absorption at 77 K
10
Jellison, Jr., G. E. and F. A. Modine,
Appl. Phys. Lett- 41, 2 (1982) 180-182.
1E-7
1
1,2
abs I (A)
1,8
2,0
2000
1E-9
1E-10
1,6
h eV
Si 8556-01-51
1E15n
A slit width
0.2 mm
0.2 mm
0.07 mm
0.07 mm
Delay 6s
-50V
18K
1E-11
1500
X 25
A, a.u.
1E-8
1,4
0.15
0.15
0.1
0.1
0.05
0.05
1E-12
1000
500
0
0.6
0.8
1.0
1.2
eV
1.4
1.6
1.8
400
600
-1
800
1000
k, cm
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Recombination rate
3 1016 cm-2
Si
and summary
1. Recombination at the
surface mush faster;
1E-7
Si 8556-01-65
3E16n
1E-8
2. Induced PC from deep
levels ~0.5 and 0.62-4 eV
U=50V 2007.02.27
U=50V 2007.02.27
U=50V 2007.02.27
U=50V 2007.02.27
1E-9
-5
10
-6
10
1E-10
580.9±0.1 meV
-7
10
A
B
1E-11
0,6
0,8
1,0
6.78143
-2926.6455
1,2
1,4
0.00153
0.35145
1,6
I, A
abs I (A)
Delay 6s
T=18K
1,8
-8
10
-9
10
(eV)
E=0,82 eV @1,2 eV at 18 K
-10
photoconductivity peak
10
12
10
13
10
14
15
10
10
2
Fluence, n/cm
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
16
10
Steady state lifetime dependence on fluence
C
Linear Fit of Data1_C
1/ rel.u.
1E-4
1E-5
0.5
0.54+-0.03
1E-6
12
10
13
10
14
10
15
10
16
10
Fluence,
The lifetime extracted from the peak photocurrent dependence on the irradiation by
neutrons depends as a square root from the fluence.
It can be proposed that it means that the lifetime depends on the distance between the
defects in the plane. (50 V bias).
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Extrinsic photoconductivity
photoresponse @1200 meV @ 18 K
inverse peak value of photoconductivity - effect of saturation of
the level 0.82 eV
photoresponse @1200 meV @ 18 K
inverse peak value of photo-conductivity
inverse lifetime by MW
-5
10
-6
I, A; other data, rel.u.
10
inverse photoresponse value (steady
state lifetime) (~to RC1 concentration)
-7
10
inverse lifetime by MW (~to RC2
concentration)
-8
10
-9
10
At high centre concentration
the two step PC can appear,
then signal becomes dependent
on the lifetime
-10
10
10
12
13
10
10
14
Fluence, n/cm
15
2
10
16
10
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Transient photoconductivity (TP)
Depends on free carrier concentration, mobility, internal electric field distribution
• Light pulse excitation
• TP can be measured by:
–
–
–
–
DC circuit (contact problems)
Microwave technique
Free carrier absorption (~ non-sensitive to mobility)
Transient gratings: measure free carrier
concentration profile amplitude, i.e. allows to measure
concentration decay law and diffusitivity, separately.
• TP gives information about recombination
cannels and traps
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Transient gratings
1
-2
14
sample
I-1
-2
14
I
I0
L
I+1
-2
14
-2
14
-2
14
CH 2210, ( phi_p [cm ] 1.06 x 10 ), R > 15 ns
CE 2419, ( phi_p [cm ] 1.06 x 10 ), R > 15 ns
Diffraction efficiency. (a. u.)
Il
1
CH 2219, ( phi_p [cm ] 1.84 x 10 ), R > 15 ns
CE 2437, ( phi_p [cm ] 1.84 x 10 ), R > 15 ns
CH 2237, ( phi_p [cm ] 4.25 x 10 ), R = 9 ns
0.1
-2
14
-2
14
-2
14
CH 2253, ( phi_p [cm ] 6.36 x 10 ), R = 7 ns
-2
14
CH 2259, ( phi_p [cm ] 9.80 x 10 ), R = 6 ns
CE 2458, ( phi_p [cm ] 4.25 x 10 ), R = 11 ns
0.1
CE 2459, ( phi_p [cm ] 6.36 x 10 ), R = 9 ns
200
Two features:
400
600
800
1000
1200
200
400
Delay time (ps)
600
800
1000
1200
Delay time (ps)
1. The decrease of generated by the light pulse carrier concentration with the fluence.
2. The decay of the signal may consist two versions:
1. A simple decay with one time constant;
2. The decay can be separated into the two components (fast a few ns and a “slow”
– tens of ns or more)
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Transient gratings
4.5
The recombination channel
which causes the capture time
less than 20 ps.
4.0
~0.7
3.5
1 / sqrt 
3.0
2.5
2.0
We plan direct measurement
this trapping with fs pulses
CE
CH
linear fit
linear fit
Confidence limits
1.5
1.0
0.5
0.0
14
2.0x10
14
4.0x10
14
6.0x10
Fluence, cm
-2
14
8.0x10
15
1.0x10
A peculiarity of this capture:
These levels (we propose –
the disordered region of
cluster) it becomes filled and
the remaining e-h pairs
participate in the recombination
and ambipolar diffusion.
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Transport phenomena: Hall, photo-Hall,
magnetoresistance effects
• Comparison Hall and magnetoresistance effects – a role
of inhomogeneity
• Temperature dependence of mobility – depends on
scattering mechanism
1
H t 

1
0
 vS t N t 
SN   S0 N 0  SN 

UH 0
Y t   1 
U H

1  1
1 
wBE 
UH 0
 
1 
 
v  0  t   vU H 0   U H



1

Y
n   Y 
~ n


 0 H As v 2Z 0 
  

n
N s   n t
Ns

J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06



1
Hall mobility and magnetoresistance vs
temperature
1200
400
1200
2
600
2
H (cm /Vs)
800
1400
m (cm /Vs)
1000
61
62
63
64
54
56
58
60
1600
Sample No.
61
62
63
64
54
56
58
60
200
0
1000
800
-200
100
120
140
160
180
200
T (K)
220
240
260
280
300
255
260
265
270
275
T (K)
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
280
285
290
295
Hall mobility, annealing, excitation effects
2
H (cm /Vs)
JV61AK
0
500
400
initial
0
4 min 80 C
0
+26 min 80 C
0
+24h 80 C
-200
-2 -1
J (cm s )
23
2,83*10
23
1,22*10
22
7,04*10
22
3,03*10
22
1,40*10
21
6,02*10
21
3,72*10
21
1,71*10
20
7,38*10
-400
-800
300
2
-1000
200
0
10
20
30
t (s)
JV61AK
-2 -1
100
J (cm s )
23
2,83*10
23
1,22*10
22
7,04*10
22
3,03*10
22
1,40*10
21
6,02*10
21
3,72*10
21
1,71*10
20
7,38*10
10
-1
cm
-1
)
-1
0
200
250
300
350
 (
H (cm /Vs)
-600
-2
10
T (K)
-3
10
2,54 s
2 4 6 8 10 12 14 16
t (s)
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
40
50
Conductivity vs T
10
-3
61
62
63
64
54
56
58
60
-4
1x10
-5
-1
,  cm
-1
1x10
10
-6
10
-7
10
-8
10
-9
1,4
1,5
(1000/T)
1,6
1/4
(K)
1,7
-1/4
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
1,8
Photoconductivity decay
Fotolaidumo (prifitintu) eksponentiniu relaksaciniu laiku
priklausomybe nuo dozes esant skirtingos fotozadinimo galioms
-2 -1
J (cm s )
23
2,83*10
23
1,22*10
22
7,04*10
22
3,03*10
22
1,40*10
21
6,02*10
21
3,72*10
21
1,71*10
20
7,38*10
-6
 (s)
10
-7
10
12
10
13
10
14
15
10
10
16
10
-2
 (cm )
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Compare the dependences on fluence the data
measured by photoconductivity and by
microwaves technique
-3
10
-4
10
8
10
-5
10
-7
10
7
10
-8
10
E=0,9 eV @1,2 eV at 18 K
-9
10
10
-11
10
-12
10
12
10
13
10
14
10
15
10
Fluence, n/cm
16
10
23
10
12
10
17
10
-2
2.4 10 cm
20
-2
7.4 10 cm
6
photoconductivity peak
1/photoconductivity
Linear Fit of Data1_C
photoconductivity/
inverse lifetime (MW)
-10
1/, 1/s
I, A; otther - rel.u.
-6
10
13
10
14
10
15
10
Fluence, cm
16
10
-2
2
The lifetime measured by microwave technique linearly depends on the fluence while from the
photoconductivity – as a square root of fluence!!!
It shows that the recombination process is more complicated but the MW measurements were
performed at room temperature, and PC – at 18 K (no retraping).
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Conclusions:
1. Spectral dependences,
different transient phenomena reveals the details of the roles
of radiation induced changes in semiconductor.
2. It seems that clusters can be seen by a specific trapping
and by the appearance of percolation.
3. A lot of work ahead …
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Thanks for Your attention !!!
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
•
•
•
•
•
•
•
The peculiarities of dark conductivity, photoconductivity and other transport phenomena were investigated in the
neutron irradiated Si. The photoconductivity mechanism, observed deep levels and the effects related to the nanoand micro- defects are discussed.
At low temperature (18K)
The measurement pf photoconductivity spectral dependence in the high absorption region shows the decrease of
photoconductivity related with an increase of recombination at the surface, but the comparison with a standard
surface velocity model show that the main recombination is going not at the surface, but in the layer near to
surface. Probably there is different doping of the bulk and the layer near to surface.
The photoconductivity at the absorption edge linearly depends on the intensity of light that shows that in the
investigated range of intensities on type of recombination canters plays the main role. The steady state lifetime
depends as a square root on the fluence. The microwave date showed its linear dependence, therefore it is
necessary to predict the more complicated process of recombination. It could be proposed that this peculiarity is
related with the very fast recombination process observed in the measurements with the picosecond resolution.
The extrinsic photoconductivity shows two different regions: on – clearly expressed impurity photoconductivity
band from the level at 0,8-0,9 eV that at low fluence linearly depends on the fluence and saturate at high fluence.
The second band (level ~0,5-0,6 eV) is caused by the induced photoconductivity by capture of carriers generated
from the deeper level or the intrinsic photogeneration. These levels can be related with clusters because they
demonstrate the accumulation effect that cause the partial compensation of the deeper level (the impurity
photoconductivity becomes linear to the excitation).
Near to the room temperature and down to the liquid nitrogen temperature.
The measurement of Hall effect mobility by Hall effect and by magnetoresistance shows more or less the same
data in the lowest fluence sample. For higher fluences the difference becomes high that can be explained only by
model of inhomogeneous sample and percolation type of conductivity. The data of photo-Hall effect support this
model. The heating cycle up to 80 C shows the significant change of the percolation and the important role of the
overlap of the space charge regions in the bulk of sample. The clusters with the effective charge of a few electron
charge was evaluated.
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
Si bandgap f(T)_
Si
1.18
1.16
1.14
E , eV
1.12
1.10
1.08
1.06
1.04
1.02
1.00
0
100
200
300
400
T, K
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
light on 0,1mm
0,05mm
7.00E-013
0,2mm
1E-11
-50V
set
light on
0,05mm
0,1mm
0,2mm
6.00E-013
5.00E-013
light off
-1V set
light off
1E-12
abs I (A)
abs I (A)
4.00E-013
3.00E-013
2007.05.16
T=~15,75 K
U= -1V
0,6eV
1mm
2.00E-013
1.00E-013
0V set
2007.05.16
T=~15,7076K
U= -50V
0,6eV
1mm
1E-13
0.00E+000
-1.00E-013
-1000
0
1000
2000
3000
4000
5000
6000
7000
0
2000
0V set
4000
t, s
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
6000
t (s)
8000
10000
Dark current vs T
-3
10
-3
10
-4
10
-4
-5
abs I (A)
Linear Fit of Temp001_G
10
-6
10
-6
-7
Si 8556-01-51
15
-2
10 cm n
-8
-9
10
0.605 eV
0.581 eV
-7
10
-8
I, A
I, A
10
-10
-5
10
10
10
10
G
Linear Fit of temp001_G
10
10
-9
10
-10
10
-11
10
Si 8556-01-65
-2
16
3 10 cm n
-11
-12
10
-13
10
-14
10
10
-12
10
-13
10
0,003 0,004 0,005 0,006 0,007 0,008 0,009
1/T, K
0.004
0.006
0.008
0.010
1/T, K
J.Vaitkus et al., WOEDAN Workshop, Vilnius, 2007.06.06
-1
0.012
0.014