TSC data-analysis on heavily irradiated silicon detectors

Nuclear Instruments and Methods in Physics Research A 352 (1995)

618-621

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A

ELSEVIER

TSC data-analysis on heavily irradiated silicon detectors Mara Bruzzi Dipartimento di Energetica, Via S.Marta 3, 50139 Firenze, Italy

Received 10 March 1994 Abstract Thermally stimulated current analysis has been performed on silicon detectors irradiated with high fast neutron fluences . The complexity of the TSC spectra required the development of a numerical procedure to deconvolute peak components . The numerical code has been used to directly calculate trap parameters: a comparison between numerical and experimental results is shown. Twentyone peaks related to various lattice defects have been found for the highest fluence of irradiation of 10 14 n/cm 2.

1. Introduction

2. Experimental setup

The thermally stimulated current [1] techniques has been widely used up to now to analyse the presence of defects inside semiconductors devices as pn junctions or Schottky barriers . Recently, it has been furthermore applied in high energy physics to investigate the semiconductor detectors lattice damage after exposures to fast particle beams [2,3]. Previous works [2,3] have shown that the TSC spectrum shape is strongly dependent on the fluence of irradiation, increasing its complexity for high particle doses. In such experimental analysis, the heating ratio variation method [1] and the delayed heating method [4] were applied to obtain from the measured TSC spectra the energy level, the cross section and the concentration related to each defect present inside the detector bulk . These two methods, very effective to determine lattice defect parameters, are however no longer useful if the TSC peak related to the defect is not clearly resolved inside the TSC spectrum as is the case of highly irradiated high resistivity silicon detectors. To analyse the TSC signals of highly irradiated detectors, it is therefore necessary to develop some other methods to evaluate the radiation-induced trap parameters . In this paper the TSC analysis performed on silicon detectors irradiated with high fast neutron fluences (up to 10 14 n/cm Z ) are presented. TSC spectra obtained for such fluences have shown a very complex structure, in which a great number of peaks were grouped in a temperature range between 10 and 200 K. Due to the spectra complexity which makes the two standard elaboration methods not applicable, a numerical code has been developed to obtain from the TSC spectra the energy level, cross section and concentration of the radiation induced lattice defects.

Samples used in this study are p +n junctions detectors produced at the Brookhaven National Laboratory's Silicon Detector Development and Processing Lab (SDDPL) with resistivity between 4 and 6 kf cm . The active area and the thickness of the test devices are respectively 0.25 cm Z and 350 p,m. The samples have been irradiated with fast neutrons with an average energy of 1 MeV obtained from the 7Li(p, n) reaction using 4 MeV protons from a Van de Graaff accelerator at the University of Lowell with neutron fluences ranging from 10 12 to 10 14 n/cm 2. TSC spectroscopy has been carried out using the experimental apparatus designed and performed at the Dipartimento di Energetica di Firenze (DEF), Italy [5].

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3. Numerical treatment Numerical calculations of the thermally stimulated current start from the following equation [1]: ITSC(T) =qS

f W nt (T, x)e,t (T) dx b

W dn t (T,

x)

where nt(T, x) is the fraction of occupied traps, e(T) the emission coefficient, Nt the total trap concentration, q the electronic charge, S the diode surface, W the depletion depth and b the heating rate . The equation describing the trap occupation at tempera-

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M.B ruzzc /Nucl. Instr. and Meth. i n Phys. Res. A 352 (1995) 618-621 6

ture T by majority carriers during the thermal scan when recapturing processes after emission are neglected is : n,(T) = Nt exp ( -

IT

1

b

s,

en(T) dT) ,

where T, is the heating scan initial temperature . Combining Eqs. (1) and (2), TSC current becomes: ITsc(T)=N,qS

2

1

en(T) exp (- b

I

ren(T) dT)

Data-analysis of the experimental TSC spectra of heavily irradiated samples have been developed numerically applying Eq . (3). The program starts by putting trial values for the number of peaks occurring in the spectrum and for the trap parameters of each peak considered . The chosen trial values have usually a physical meaning: trap parameters obtained for the same samples irradiated with lower fluences are used . If m is the total number of peaks, the starting data are Et(i), s(i) and Nt(i) which are respectively the energy level, the cross section and the concentration of the ith peak (1 -< i -< m) in the spectrum . During the thermal scan the temperature T(j) depends on time t(j) by the simple linear law, where j is the index of the scan sampling: T (j) = T, + bt(j) .

(4)

For each peak the thermally stimulated current INUM(c, T(J)) = INUM(c, j) is then calculated at the temperature T(j) by applying the following simple routine: INUM(c,

I) a

=A(i, I) exp( -b X

L

k-2

2

), (5)

where k is the temperature index running from 2 to j, K b is the Boltzmann constant and: 2

aNt (c)s(c)T(j)

(

exp -

b (`)

102t1 hs °

f(i, k)=s(i)T(k)

2

cm2 K2

( exp -

75 Temperature [K]

Fig. 1. TSC spectra obtained by delayed heating method of the A centre peak for a sample irradiated with a neutron fluence of 1.55 X 10 12 n/cm 2 . Deexcitation times are: (O) 0 s; ( X ) 30 s; (+) 90 s, (-----) 150 s, ( . - -) 300 s. marital INUM(J) thermally stimulated currents are then compared using the Y( j) parameter [6]:

I IEXP( I ) -INUM( I) I

Y(j)

INUM(1)

Calculations are repeated by slightly changing trap parameters until the average value of Y( j) over the total heating scan is lower than 5% . If this is not achieved after a fixed number of tries, the program stops asking to change the chosen number of peaks: the problem to fit the experimental current has to be then reconsidered physically by the operator .

s1 ,

E,(i)

K bT(k) j

Before applying the numerical code to a complex spectrum, the sensitivity of the method in calculating the TSC peaks has been tested by comparing the numerical and experimental signals of an isolated peak . The vacancyoxygen complex (A center) with its TSC peak placed at 70 K and well distinguishable in the TSC spectrum of a diode

s

K T(1) ), 6

2

a-16am*

70

4. Experimental results

1

(f(i, k) + f(i, k- 1))(T(k) - T(k- 1)

A(i, j) =

o'

(3)

(7)

(8)

with h the Planck constant and m* the charged carrier effective mass. The ItruM(c, j) values are then summed over i to have the total spectrum IrruM(j) given by the set of TSC peaks considered. Experimental IExp( j) and nu-

= Expenmental - = Namental

4

3 21

o50

60

70

80

e0

100

Temperature [K]

Fig. 2. Comparison between the numerical and experimental TSC signals for the A centre peak observed in a sample irradiated with a neutron fluence of 1 .55 X 10 12 n/cm 2 .

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M. Bruzzi /Nucl. Instr. and Meth . in Phys . Res. A 352 (1995) 618-621

irradiated with 1 .55 X 10 12 n/cm2, has been used for this purpose. It has been analysed by delayed heating method in a temperature range from 50 to 90 K, with 0.2 K/s heating rate and reverse and filling voltage of - 20 V and + 2 V respectively . Deexcitation times are in the range of 10 to 300 s. TSC signals at various deexcitation times are plotted in Fig. l . The energy level and cross section for the level so evaluated are E l = Ec - 0.165 eV (where E, is the conduction band edge) and s = 1 X 10 -14 cm 2, respectively . These estimations are consistent with the data given in Ref. [7] for A centre . The trap concentration N has been calculated by using the Forbes-Sah method [8]:

N=

4

dT, g WSb T , "'ITSC(T)

(10)

resulting in a value of 1.9 X 10 11 cm -3 for the A center shown in Fig. 1 . The experimental and numerical TSC peaks related to the A center are shown in Fig. 2: numerical data calculated from the new numerical code fit very well the experimental ones : the asymmetric shape in temperature of the thermal emission peak seems to be reproducible in detail . Trap concentration obtained by using the energy level and cross section values reported above is 1.65 X 10 11 n/cm 2, in good agreement with the one obtained by Forbes-Sah method, if error propagation is considered [5]. The numerical code described above allows one to analyse very complex TSC spectra. An example is given in Fig. 3, in which the TSC signal of a detector irradiation to 1.0 X 10 14 n/cm 2 is shown. A total of 21 different peaks have been detected, in the temperature range from 8 to 200 K, each of them unresolved with respect to the other. The energy levels and trap concentrations are reported in Table 1. Many of the observed energy levels have never been cited in literature where the levels have been obtained for the most part from low resistivity silicon or lightly irradiated high resistivity silicon. The proposed identities given in the table are indicated only for those levels already measured experimentally in lightly irradiated silicon [7]. 120~

Table 1 Energy levels and concentrations for the TSC peaks observed in the silicon p+ n junctions irradiated with a fluence of 1 .0 X 10 14 n/cm 2 . Indications on the possible related defects are given for peaks already cited in the literature E, [eV]

N[1012 CM-3]

Proposed identity

0 .026 0 .030 0 .034 0 .037 0 .052 0 .068 0 .087 0 .097 0 .105 0 .145 0 .168 0 .175 0 .184 0 .200 0 .250 0 .311 0 .341 0 .370 0 .393 0 .405 0 .432

0 .12 0 .14 0 .19 0 .42 0 .23 0 .45 0 .23 0 .23 0 .15 0 .15 0 .85 0 .45 0 .35 0 .68 2 .20 3 .00 9 .80 3 .00 6 .40 8 .30 15 .30

P B -

vacancy-oxygen (I) -

vacancy-oxygen (II)

V2

H related V2 related Ci-Cs VZ

vacancy-phosphorous -

More details on the radiation-induced lattice disorder dependence on fluence of irradiation have been discussed in Refs . [9,10] . 5. Conclusions TSC analysis has been performed on silicon detectors irradiated with high fluences of fast neutrons . The complexity of the TSC measured spectra required the development of a numerical procedure to deconvolute the peak composing the total signal . The numerical code described in this work which is based on the carrier emission from different traps during the TSC heating scan, allows one to determine the trap parameters of the deconvoluted peaks without using the delayed heating and beta variation methods which are no longer useful if peaks are unresolved. Experimental and numerical TSC signals for vacancyoxygen complex (A centre) peak have been compared : the numerical procedure fits very well the experimental data. Using the numerical code, up to twentyone different levels have been detected in the TSC spectra of the heavily irradiated sample . Acknowledgements

Fig. 3. Experimental TSC signal (+), deconvoluted peaks (-----) ) for a sample irradiated with a and numerical TSC signal ( neutron fluence of 1 .0 X 10 14 n/cm 2.

The author wishes to thank Prof. Emilio Borchi for helpful discussions and Mr . Ugo Biggeri (Dipartimento di Energetica di Firenze, Italy) and Dr. Z. Li (Brookhaven National Laboratories, Upton, New York) for the contribu-

M. Bruzzi/Nucl. Instr. and Meth . in Phys . Res . A 352 (1995) 618-621 tions to the Thermally Stimulated Currents experimental analysis . References [1] M .G. Buehler, Solid State Electronics 15 (1972) 69. [2] A. Baldini, E. Borchi, M . Bruzzi and P . Spillantini, Nucl . Instr. and Meth. A 315 (1992) 182. [3] M . Bruzzi, A . Baldini, E . Borchi and I. Lukianov, Nucl . Instr. and Meth. A 326 (1993) 344. [4] G . Micocci, A. Rizzo and A . Tepore, J. Appl . Phys . 54 (1983) 1924 .

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[5] A . Baldini and M . Bruzzi, Rev . Sci . Instr. 64 (1993) 932 . [6] W .H. Press, B .P . Flannery, S .A . Teukolsky and W.T . Vetterling, Numerical Recipes (Cambridge Univ . Press, 1986). [7] E . Borchi, M . Bruzzi and M.S . Mazzom, Phys . Status Solidi 124 (1991) K17 . [8] L. Forbes and C .T. Sah, Solid State Electronics 14 (1971) 182. [9] U . Biggeri, E . Borchi, M . Bruzzi, Z . Li and S . Lazanu, IEEE Trans . Nucl . Sci . 41 (1994) 964 . [10] H .W . Kraner, Z. Li, S . Lazanu, U . Biggeri, E. Borchi and M . Bruzzi, Proc. Large Scale Applications and Radiation Hardness of Semiconductor Detectors, eds . A . Baldini and E . Focardi, SIF, Bologna, Italy, vol . 46 (1994) pp. 123-130.