Short Notes K15

phys. stat. sol. (a) 113, K15 (1989) Subject classification: 61.80; S5.11

Department of Physics, Nanjing University' ) (a) and Hebei Semiconductor Research Institute* ) (b) Influence of Previous Defects on the Formation of Irradiation Defects in NTD Si

BY Y. SHI (a) , F.M. WU (a ) , D.X. SHEN (a) , M.K. DENG (a) , K . J . CHENG (a) , and C.H. WANG (b)

Introduction Neutron-transmutation doped (NTD) silicon has been used for the manufacture of high-power devices extensively, which are sometimes used under irradiation conditions I 1 to 31. Since this material has been subjected to the processes of neutron irradiation damage and high temperature annealing, it would show different characteristics to ordinary crystals during further irradiation 14 to 71. In this note a special NTD silicon sample irradiated by electrons and neutrons is investigated, the properties of interaction of previous defects with irradiation induced defects are discussed.

Experiments The samples used in this experiment were neutron-transmutation doped (NTD) float-zoned silicon (p = 40 Qcm) which were prepared by special processes. The samples were fabricated upon p-n diodes by the deep diffusion method. In the n-region the oxygen content was about 1017 and the minority carrier lifetime about 5 ps. Irradiations were carried out, respectively, by electrons with energy Ee = 2 and 1 2 MeV, and reactor neutrons with an average energy of En = 2.5 MeV near room temperature. Isochronal annealings for the irradiated samples from 50 to 400 O C were performed. After each treatment deep level transient spectroscopy (DLTS) was used to determine the nature and concentration of irradiation induced defects.

Results and discussion The DLTS spectrum obtained from unirradiated samples is shown in Fig. 1, four electron traps (El through E4) with high concentration were observed, which survived after above 1000 O C annealing. Their electrical parameters are given in Table 1. However, any hole trap signal was not detected.

Fig. 2 shows the DLTS spectra for the samples after irradiation by electrons, neutrons, and isochronal annealing at 400 O C . About six main electron traps were observed in all irradiated samples, all EXI peaks had similar level (Ec - (0.18 +_

0.02) eV), Ex2 (Ec - (0.23 +_ 0.02) eV), Ex3 (Ec - (0.28 i: 0.02) eV), Ex4 (Ec - (0.35 +_ 0.02) eV), Ex5 (Ec - (0.40 ?r 0.02) eV), and Ex6 (Ec - (0.50 k 0.02) eV)

l ) Nanjing, People's Republic of China. ' ) Hebei, People's Republic of China.

K16

level

Et

El

E2

E3

E4

physica status solidi (a) 113

activation energy capture cross-section concentration

(eV) ( em2 (Ntl Nd)

Ec - 0.18 1~10-l 0.005 to 0.01

Ec - 0.28 lX10-l4 0.008 to 0.012

Ec - 0.35 1x10-16 0.015 to 0.025

Ec - 0.50 8 ~ 1 0 - l ~ 0.015 to 0.025

T a b l e 1 Tha electrical parameters of previous defects in unirradiated samples

also, moreover, Exl, Ex3, ExQ, and Ex6 levels were alike to El, E2, E3, and E4 levels of the unirradiated sample, respectively. After irradiation, El and E3 peaks increased evidently, but the E2 and E4 peaks did not alter basically. The increase of El and E3 peaks could be attributed to the introduction of a lot of vacancies produced by irradiation, it might be suggested that the El, E3 peaks are due to vacancy-related defects. After isochronal annealing at 400 'C, some levels were annealed, and several new levels appeared, but Ex3, Ex4, and Ex6 peaks did not change evidently. According to the level parameters and annealing behaviors, Exl peaks could correspond mainly to the oxygen-vacancy ( A center), the Ex2 peaks to divacancy ( V i ) and impurity-related defects, the Ex5 peaks to divacany (V,) and phosphorus-vacancy complex ( E center) /8 / . The identification of the defects associated with El through E4 peaks in unirradiated samples was difficult. In fact, each peak consisted of many types of defects 121.

E3 I I

80 150 200 2 73

uIo - Fig. 1. DLTS spectrum obtained from unirradiated samples (T = 14 ms)

Short Notes K17

En 5 , 80 150 200 273

Fig. 2. DLTS spectra of the irradiated samples: a) 2.0 MeV electron irradiated (@e

lx1014 e/cm2; b) 12 MeV electron irradiated (4, = 2 ~ 1 0 ~ ~ elcm ); c) 2.5 MeV reactor neutron irradiated ($n = 6 ~ 1 0 ' ~

after iso- nlcm ; - before annealing; - - - chronal anneahng at 400 O C (T = 14 ms)

2

2

From Fig. 1 and 2 we can see that the DLTS spectra are different to those of ordinary crystals, and the introduction rates of the main electron traps in NTD silicon are lower than others 12, 7 to 13/. The processes of defect formation in semiconductor crystals are known to be governed by the irradiation condition and by the previous history of the crystal. Here the experimental results could be explained by the characteristic of NTD silicon. After neutron transmutation the silicon crystal was subjected to high-temperature heat treatment and other processes of primary irradiation damage annealing. It resulted in the presence of some additional electric defects (e.g. El) and other neutral defects in the bulk of the crystal. These defects would create elastic stress fields surrounding them, these fields cause the migration of primary

defects (e. g. vacancies and interstitials) generated by further irradiation towards these defects 15, 61, where they may be annihilated or form complexes, the formation was resisted for some defects including A and E centers. There was nearly no signal detected for irradiation induced electron traps in 2 MeV electron irradiated samples, because the primary irradiated point defects were absorbed almost by previous defects. Therefore, the distribution and annealing behavior of irradiation induced defects were altered, the rates of introduction were lowered in NDT Si: However, the work of Brotherton and Bradley 171 did not show similar results, because any previous electric defects were not observed in their NTD sample. The difference could

2 physica (a)

physica status solidi (a) 113 K18

indicate that previous electric defects play a main role in the influence on the formation of defects during irradiation.

Conclusions The NTD silicon used in this experiment showed that there were four

high concentration previous electron traps before irradiation. These previous electric defects created elastic stress fields around them, and influenced the process of radiation defect formation during irradiation and subsequent annealing. As a result, the NTD silicon reduced effectively the rates of introduction of irradiation induced electron traps as well as lowered the irradiation damage as compared with ordinary crystals. This material is interesting for investigating the interaction of previous defects with irradiation defects, and may be useful for the manufacture of antiradiation devices.

The authors would like to thank the colleagues of the Physics Department of the Nanjing University and of the Hebei Semiconductor Research Institute who helped us in preparing and irradiating the samples.

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Moscow 1981 (in Russian). / 2 / J . M . MEESE, in: Defects in Semiconductors, Ed. J. NARAYAN, Elsevier/North

Holland, New York 1980 (p. 225). /3/ C .H. WANG and S.M. SU, Chin. Semicond. Inform. 2, 1 (1986). / 4 / Y. SHI, F.M. WU, D . X . SHEN, M.K. CHENG, K . J . CHENG, and

C.H. WANG, J. Nanjing Univ. (Natural Sci.) 1989, to be published. / 5 / P.F. LUGAKOV and V.V. LUKYANITSA, Fiz. Tekh. Poluprov. l7, 1601 (1983). / 6 / A.V. VASILIEV, S.A. SMAGULOVA, and S.S. SHAIMEEV, Fiz. Tekh. Poluprov.

/ 7 / S.D. BROTHERTON and P. BRADLEY, J. appl. Phys. 5 3 , 5720 (1982). /8/ X .C . YAO and G . G . QIN, in: Materials and Process Characterization

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(Received November 28, 1988)