J.Vaitkus. RD50 Workshop, Liverool, 23-25 May, 2011 Deep level system Gaussian approximation according the extrinsic photoconductivity in irradiated Si

J.Vaitkus. RD50 Workshop, Liverool, 23-25 May, 2011

Deep level system Gaussian approximation according the extrinsic

photoconductivity in irradiated Si diodes.

J. VAITKUS, V. KAZUKAUSKAS, N. VAINORIUSDepartment of New Materials Research and

Measurement Technology Institute of Applied Research,

Vilnius University


Why photoconductivity?• If the deep centers are important, the optical

excitation excites ALL centers (except of those, that due to a strong electron-phonon interaction create the metastable state without a possible optical transition in the extrinsic range, e.g., EL2* center in GaAs).

• Electrical methods feel the centers that are active – (e.g., if the center is surrounded by the repulsive barrier, it is

almost not seen (except by carrier scattering, or its capture cross-section is

very small) )

• Therefore, if a correct model of semiconductor is awaited, then optical absorption (low sensitivity), or photoconductivity (high sensitivity) methods are applicable.


J.L. Hastings et al. Vacancy aggregates in silicon. Phys. Rev. B, 56, p. 10215 (1997).

The valence tail is primarily due to structural disorder, the conduction tail is much more sensitive to temperature and originates in thermal disorder. J. Dong, D. A. Drabold. Atomistic Structure of Band-Tail States in a-Si. Phys. Rev. Lett. 80,. 1928 (1998).

The actual deep level energy spectrum models:The kick off by neutron Si atom creates a track, consisting vacancies and Si disorder, some vacancies can be generated in a bulk (if the Si is irradiated by protons, then vacancies in the bulk concentration is bigger) (Huhtinen)

Three type of defects have to be created:1) the vacancies, multi-vacancies, vacancy-interstitials complex2) Vacancy-impurity complexes,3) Clusters, consisting different complexes inside

Initial distribution of vacancies produced by 1 MeV neutron in Si, a – a result of kick of Si atom, and b – if a fluence was 1014 cm-2, according M.Huhtinen. NIMA 491 (2002) 194–215

0.05 m

G.Davies, RD50


Earlier results (related to this talk)

• Investigation of deep centers in the irradiated Si– Our results presented in RD50

Workshops

• This presentation, due to high number of centers, was found too complicated or not enough evident.

• Therefore a possible distribution of deep level energies was analyzed and the approximation by Gaussian model is presented in this talk.

Con

cent

rati

on, r

el. u

.


Basics about the deep levels

• If the deep level property -potential then Lukovsky model can be applied for the data analyze.

I ~ m x EM 0,5(h - EM )1,5/(h)3)

• This model (at low temperatures) does not valid:

– for the hydrogen type defect &

– for the inter-deep level state transitions

• Low temperature requires the attention on the filling of the traps and to avoid the influence of electron-phonon coupling

0,5 1,0 1,5

0,0

0,4

0,8

1,2

1

Ph

oto

n c

ross-s

ectio

n, a

.u.

Photon energy, eV

41

2

3


In this series of analyze we modified the measurement equipment:

Now the equal number of light quantum was kept at each wavelength of the excitation. In earlier measurements the linear

dependence of photoresponse on excitation was proposed (and checked for some cases, but not for all samples).

Prism Monochromator for spectral measurements

Grating monochromator for the additional excitation

Helium cryostat

Controlable light source

Measurement equipment


Investigation of Traps and Recombination centers

Traps

Recombination centers (RC)

E

EC

EV

The traps recognition in homogeneous sample –most common - by the Thermally Stimulated Current methods – illumination at low T, - Wait until free carrier recombine (if some of them do not recombine and the current does not disappear, it is a signal that the free e and localized h are separated by a barrier),- linear increase of T OR- Multiple heating and cooling by ΔT

All levels can be recognized by photo-ionization spectra (the recombination-capture and two-step excitation makes the problem to measure the concentration, but not the activation energy !!!


Two step excitation of carriers(the effect observed in the CMS CEC proton irradiated samples)

Measurement at higher T reveled the additional photoconductivity band due to the thermal excitation of electrons excited TO the deep center (1.086O eV) and then by thermal excitation To the conductivity band. This trap ΔET ~ 0,257 eV (from TSC analysis) 0,6 0,8 1,0 1,2 1,4

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

I(A

)h(eV)

2011-05-12 experiment, 150K, E

0=1.086eV,

N=2.79E-7, =0.006

U=50VT=150K

50 100 150 200 250 30010-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

I (A

)

T (K)

0 100 200 300 4001.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18

E ,

eV

T, K

Si


If electron-phonon coupling is important:

• At higher T measurement the electron-phonon coupling has to be included.

• If electron-phonon coupling is important, the configuration coordinate presentation has to be used. -15 -10 -5 0 5 10 15 20

0

400

800

1200

1600

2000

En

erg

y, a

.u.

Q, config. coordinate

ET

ET EOpt

EOpt

EV

EM

EC

-15 -10 -5 0 5 10 15 20

0

400

800

1200

1600

2000

En

erg

y, a

.u.


-15 -10 -5 0 5 10 15 20

0

400

800

1200

1600

2000

En

erg

y, a

.u.


ET

ET EOpt

EOpt

EV

EM

EC

ΔEOPT - ΔETH = Frank-Condon shift, in Si ~ 50-70 meV


Wodean samples 1e13 cm-2

0,6 0,8 1,0 1,2 1,41E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

I(A

)

h(eV)

ITotal

(center) IPH

(IDC

=7.65E-14 A)

approximation E

0=0.845eV, N=2.9E-11, =0.037

E0=1.03eV, N=3.2E-11, =0.012

0

20

40

U=-50VT=18K

W337-B111E13

180oC 15 val.

0,01 0,02 0,0310-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0,01

10-13

10-12

10-11

I (A

)

1/T (1/K)

0,430

0,272

0,280

W337-B11

1E13, 180oC 15 h.

I (A

)

1/T (1/K)

U=-50V0,563

0,435

0,272

0,280

0,6 0,8 1,0 1,2 1,41E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7 I

Total (edge)

IPH

( Itams

=7.65E-14 A)

approximation E

0=0.813eV,

N=1.31E-11, =0.029 E

0=0.833eV,

N=3.2E-11, =0.012

I(A

)

h(eV)

0

10

20

30

40

U=-50VT=18K

W337-B111E13

180oC 15 val.


Annealing


1e13 cm-2 with a peculiarity

0.4 0.6 0.8 1.0 1.2 1.410-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

0

20

40

I (A

)

h (eV)

ITotal

IPH

approximation

E0=0.475eV, N=2.2E-12, =0.005

E0=0.859eV, N=1.12E-10, =0.042

0.4 0.6 0.8 1.0 1.2 1.410-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

W337-E81E13

250oC 15 val.

I (A

)

h (eV)

to 1.4eV, 6s, 18K from 1.4eV, 6s, 18K to 1.4eV, 20s from 1.4eV, 20s to 1.4eV, 40s,18K to 1.4eV, 6s,18K to 1.4eV, 6s, 80K from 1.4eV, 6s, 80K to 1.4eV, 6s, 40K from 1.4eV, 6s, 40K to 1.4eV, 6s,100K from 1.4eV, 6s, 100K to 1.4eV, 6s,125K from 1.4eV, 6s, 125K to 1.4eV, 6s,140K from 1.4eV, 6s,140K to 1.4eV, 6s,160K from 1.4eV, 6s,160K to 1.4eV, 6s,140K from 1.4eV, 6s,140K to 1.4eV, 6s,170K from 1.4eV, 6s,170K to 1.4eV, 6s,150K from 1.4eV, 6s,150K

0,005 0,01010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

W337-E81E13

250oC 15 val.

I (A

)

1/T (1/K)

0.257eV

0.256eV

0.359eV

0.185eV

0.6 0.8 1.0 1.2 1.41E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

I(A

)

h(eV)

T=140K E0=1.088eV, N5.23E-7=, =0.01 T=125K E0=1.092eV, N=2.65E-7, =0.009 T=100K E0=1.092eV, N=7.1E-8, =0.008


Annealing


1e14 cm-2

0,4 0,6 0,8 1,0 1,2 1,41E-16

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

I(A

)

h(eV)

spektras atemus I

tams

suma E

0=0.51eV, N=9.8E-13, =0.009

E0=0.696eV, N=8.2E-12, =0.039

E0=0.824eV, N=7.3E-10, =0.035

E0=1.091eV, N=2E-8, =0.005

U=-50VT=90K

0

20

40

60

80

W337-G131E14

250oC 15 val.

0,4 0,6 0,8 1,0 1,2 1,41E-16

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

W337-G131E14

250oC 15 val.

U=-50VT=18K

I(A

)

h(eV)

ITotal

IPH

approximation

E0=0.507eV, N=8.2E-13, =0.005

E0=0.652eV, N=2.7E-12, =0.005

E0=0.84eV, N=8.7E-10, =0.032

0

10

20

30

40

50

60

70

80

0,01 0,02 0,03 0,0410-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

W337-G131E14

250oC 15 val.

I (A

)

1/T (1/K)

0.08eV

0.307eV

0.32eV

0,05 eV

0,4 0,6 0,8 1,0 1,2 1,41E-16

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

I(A

)

h(eV)

spektras atemus I

tams

suma E

0=0.508eV, N=3.4E-13, =0.044

E0=0.693eV, N=6E-13, =0.035

E0=0.802eV, N=6.2E-13, =0.027

E0=0.910eV, N=5.7E-13, =0.011

T=18K

0

10

20

30

40

W337-Q51E14

120oC 15val.


Annealing


1e15 cm-2

0,00 0,01 0,02 0,03 0,04 0,05 0,0610-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

0,01

10-12

10-11

10-10

W337-H2

1E15 250oC 15 h.

I (A

)

1/T (1/K)

77 meV

130 meV

287 meV

328 meV

313 meV

U=-50V

W337-H2 1E15 250oC 15 h.

I (A

)

1/T (1/K)

77 meV7 meV

130 meV287 meV

328 meV

313 meV

U=-50V

0,006 0,008 0,010 0,012 0,014

10-13

10-12

10-11

10-10

10-9

10-8

W337-H31E15250C 15val.

I (A

)

1/T (1/K)

J Linear Fit of Data7_J Linear Fit of Data7_J Linear Fit of Data7_J Linear Fit of Data7_J ###

0.181eV

0.317eV

0.348eV

0.408eV

0.367eV

0,4 0,6 0,8 1,0 1,2 1,41E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

I(A

)

h(eV)

ITotal

IPH

(IDC

=9.450E-14A)

approximation E

0=0.561eV, N=8.6E-13,=0.049

E0=0.721eV, N=6.49E-11,=0.038

E0=0.758eV, N=5.7E-10,=0.02

E0=0.841eV, N=1.4E-9,=0.027

E0=0.993eV, N=1.62E-9,=0.02

T=100K

0

10

20

30

40W337-H31E15250C 15val.

0,4 0,6 0,8 1,0 1,2 1,41E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7W337-H21E15

250oC 15val.

I(A

)

h(eV)

ITotal

IPH

(IDC

=6.480E-13 A)

approximation E

0=0.656eV, N=5.15E-12, 0.051

E0=0.817eV, N=7.66E-10, 0.026

E0=0.94eV, N=3.1E-9, 0.033

T=18K

0

10

20

30

40


Annealing


1e16 cm-2

0,4 0,6 0,8 1,0 1,2 1,41E-17

1E-16

1E-15

1E-14

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

h(eV)

experiment (total current) photocurrent (I

DC=1.580E-13 A)

approximation E

0=0.523eV, N=7.4E-13, =0.033

E0=0.7eV, N=1.4E-11, =0.044

E0=0.816eV, N=9.5E-10, =0.027

E0=1.009eV, N=8.1E-9, =0.036

W337-Q61E16

250oC 15 val.

T=18K

0

10

20

30

40

0,01 0,02

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

I (A

)

1/T (1/K)

0.037eV

0.317eV

0.161eV

0.113eV

0.326eV

W337-Q61E16

250oC 15 val


Intriguing low activation energy values

8 meV increased excitation

9 meV 100 V bias

15 meV 50 V bias

Dependence on the bias and on the excitation (persistent photoconductivity) showc the existence of conductivity band bottom modulation (microinhomogeneities)


The similar photo-excitation spectra was

measured at T=300 K (excitation by the fs

light pulse) – the deep traps are partly filled.

(Taken from E.Gaubas talk)The shallowestE1 =h 0.3 eV

E2

h 0.41eV

E3

h 0.51eV


Conclusions:

• The deep levels in the irradiated Si are distributed in 3-4 bands, and these bands are as “fingeprints”. Therefore the modeling of the device properties should take it into account (especially in analyze of the generation lifetime)

• The micro-inhomogeneities of conductivity are responsible for the small values of the TSC activation energies (and it confirms earlier reported data obtained from Hall and magnetoresistance effects investigation)

• It would be interesting to analyze all data according the WODEAN program and to fix which contribution goes from “the bulk” and from “the clusters”


Thank you for your attention!

Documents

J.Vaitkus. RD50 Workshop, Liverool, 23-25 May, 2011 Deep level system Gaussian approximation according the extrinsic photoconductivity in irradiated Si