Annealing behaviors of vacancy in varied neutron irradiated Czochralski silicon

   CHEN Gui-feng(陈贵锋), LI Yang-xian(李养贤), LIU Li-li(刘丽丽),

NIU Ping-juan(牛萍娟), NIU Sheng-li(牛胜利), CHEN Dong-feng(陈东风)

1. School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China;

2. School of Information and Communication, Tianjin Polytechnic University, Tianjin 300160, China;

3. China Institute of Atomic Energy, Beijing 102413, China

Received 10 April 2006; accepted 25 April 2006

Abstract: The difference of annealing behaviors of vacancy-oxygen complex (VO) in varied dose neutron irradiated Czochralski silicon: (S1 5×10¹⁷ n/cm³ and S2 1.07×10¹⁹ n/cm³) were studied. The results show that the VO is one of the main defects formed in neutron irradiated Czochralski silicon (CZ-Si). In this defect, oxygen atom shares a vacancy, it is bonded to two silicon neighbors. Annealed at 200 ℃, divacancies are trapped by interstitial oxygen(O_i) to form V₂O (840 cm^-1). With the decrease of the 829 cm^-1 (VO) three infrared absorption bands at 825 cm^-1 (V₂O₂), 834 cm^-1 (V₂O₃) and 840 cm^-1 (V₂O) will rise after annealed at temperature range of 200-500 ℃. After annealed at 450-500 ℃ the main absorption bands in S1 sample are 834 cm^-1, 825 cm^-1 and 889 cm^-1 (VO₂), in S2 is 825 cm^-1. Annealing of A-center in varied neutron irradiated CZ-Si is suggested to consist of two processes. The first is due to trapping of VO by O_i in low dose neutron irradiated CZ-Si (S1) and the second is due to capture the wandering vacancy by VO, etc, in high dose neutron irradiated CZ-Si (S2), the VO₂ plays an important role in the annealing of A-center. With the increase of the irradiation dose, the annealing behavior of A-center is changed.

Key words: Czochralski silicon; neutron irradiation; A-center, FTIR

1 Introduction

Despite more than three decades of work on vacancies and divacancies in crystalline silicon, some important aspects of these point defects are not completely known and understood. During irradiation of silicon with high dose neutron, lattice atoms are displaced[1]. In the dense displacement regions, mainly divacancies are formed and mainly clusters are created. A migration of these defects take place after the primary generation of silicon interstitial (I) and vacancies (V) in the crystal lattice. The damage produced by high-energy neutron results in degradation of the parameters of silicon, and it arises mainly from the introduction of defects with deep levels in the energy band gap.

Oxygen is a very common and important impurity in silicon materials. In as-grown crystals oxygen is mainly present in the form of an electrically inactive interstitial defect (O_i), where it binds with two neighboring Si atoms. Oxygen does not normally occupy a substitutional site. However, in irradiated crystals the mobile vacancy can be trapped at the O_i atom to form the vacancy-oxygen complex[2,3]. In this configuration oxygen bridges a pair of Si neighbors of the vacancy and is called an off-center substitutional oxygen, or A-center. Upon further irradiation, the trapping of vacancies by the A-center results in the formation of a divacancy-oxygen V₂O complex[3]. Annihilation of both centers, VO and V₂O, occurs in the temperature range 300-400 ℃ and is accompanied by the appearance of a number of new more complicated vacancy-oxygen complexes. Among them the VO₂ complex, formed via the capture of a mobile A-center by a O_i atom, is the dominant one. In this complex two-oxygen atoms share a vacancy, each was bonded between two Si neighbors. Fourier transform infrared spectrometer(FTIR) is a most powerful and sensitive characterization technique to analyze the changes of the irradiation defects at different annealing temperatures. Infrared (IR) absorption spectra of vacancy-oxygen, which was related to complexes in Si, have been intensively studied and a number of vibrational bands have been reported.

In the present paper the annealing mechanism of A-center and the formation of VO₂ were investigated by FTIR in varied fast neutron irradiated CZ-Si.

2 Experimental

In this experiment, N-type /111S CZ-Si wafers were used whose resistivity was in the range of 40-60 Ω?cm. The concentrations of  oxygen and carbon are about 1.03×10¹⁸ atoms/cm³ and <1.0×10¹⁶ atoms/cm³, respectively. Original interstitial oxygen and substitutional carbon concentrations measured by WQF-410 FTIR at room temperature. Monitored the concentration of the O_i and C_s by measuring the intensity of the absorption peaks at 1 107 cm^-1 and 605 cm^-1, respectively. The sample was irradiated with 500 MeV fast neutron at China Institute of Atomic Energy, irradiation flux was 6.6×10¹² n?cm^-2?s^-1, irradiation temperature was 45 ℃, and the irradiation dose was

5×10¹⁷ n/cm³ (S1)and 1.17×10¹⁹ n/cm³ (S2). The samples were cut into circular shape, 2 cm in radius and 0.55 mm in thickness. All the samples were cleaned and polished with wright etching (V(HNO₃₎ :V (HF)= 6:1) after neutron irradiation. The samples were furnace annealed in a pure argon atmosphere in the temperature range of 200-600 ℃ for 1 h. After thermal annealing, all the samples were treated with HF acid to remove the oxide formed on the surface of these samples and washed with de-ionized water repeatedly. In this study we use a WQF-410-type Fourier transform IR spectrometer to analyze the IR absorption. The measurements were performed at room temperature and 10 K, with a spectral resolution of 1.0 cm^-1.

3 Results and discussion

It is well-known that the irradiation temperature[4] is the main factor which determines the type of irradiation defects. When the irradiation temperature is low (<170 ℃) the main vacancy-type defects are V, V₂ and V₄-type defects, while irradiated at high temperature (>170 ℃) the main multivacancy-type defect is V₅-type defect. In this experiment the irradiation temperature is 45 ℃, so the main defects are V, V₂ and V₄-type. The vacancy-oxygen complex is one of the dominant defects in CZ-Si after RT irradiation with fast neutron[5,6]. Its maximum concentration is achievable at high doses of irradiation but it is usually limited by trapping of diffusing Si self-interstitial (I) by VO, due to the occurrence of the reaction as

VO+I→O_i                                   (1)

Fig.1 shows the fragments of FTIR absorption spectrums for the low dose fast neutron irradiated CZ-Si (S1) annealed at the temperature rang of 200-500 ℃. The strong and well-known line positioned originating from VO at 829 cm^-1 is observed. We have performed a careful analysis of a shape of the 829 cm^-1. It has been found that there is a clearly pronounced shoulder at about 840 cm^-1 on the high-energy side of the band after annealed at 200 ℃. The existence of such a shoulder (840 cm^-1), with a relative intensity of about 50%, is expected due to the V₂O. The V₂O complex was identified via a detailed EPR study of the A14 spectrum in heavily electron-irradiated silicon. Since V₂O contains a Si-O-Si bonding structure like VO, it has been suggested[7] that an oxygen vibrational band of V₂O is very similar to that of VO. At this annealing temperature, V can wander in the silicon and be easily trapped by VO to form V₂O.

VO+V→V₂O                               (2)

Fig. 1 FTIR spectrua for S1 sample annealed at temperature range of 200-500 ℃

Disappearance of the A-center at 300 ℃ is followed by the appearance of a series of bands at 825, 834 and 889 cm^-1 and the increasing intensity of 840 cm^-1. It has been suggested that those IR absorption lines should be related to A-center or divacancy-oxygen complex formed via trapping of mobile V, V₂ and O_i atom. The V₂ behavior during isochronal anneals by measuring the intensity of the band at 2 755 cm^-1[8,9] is shown in Fig.2. It is found that the V₂ begins to disappear at 150 ℃and disappeare completely at 300 ℃. These are followed by the increasing in the strength of the band at 840 cm^-1, so it can be concluded that the growth of 840 cm^-1 band is associated with the disappearance of is V₂, the wandering V₂ trapped by O_i:

V₂+O_i→V₂O                               (3)

Fig.2 Absorption spectra at 10 K for divacancy annealed at temperature range of 100-300 ℃

The IR absorption band at 825 cm^-1 is related to the V₂O₂ complex, it starts to appear and abound in neutron-irradiated silicon after annealed at the temperature of 300 and 400 ℃, respectively. The VO trapped by other VO at low annealing temperature (300 ℃) and the wandering V trapped by VO₂ or O—V—O at higher annealing temperature (400 and 450 ℃) is suggested to form V₂O₂, respectively.

When annealing temperature reaches 500 or 550 ℃, there exist only two absorption bands at the wave number of 825 (V₂O₂) and 834 cm^-1 (V₃O₂), and a groups of absorption bands appear at the wavenumbers of 902(VO₃), 928, 958 and 980 cm^-1(VO₄). At this temperature range the interstitial oxygen has a high diffusion speed, which is the so-called abnormal diffusion. Since in this experiment trapping of O_i by V or VO subsequently occurs due to the mobility of O_i to form V₂O₂ and V₃O₂, and they will trap more O_i to form a series of V_nO_m defects. So it can be suggest that V₂O₂ and V₃O₂ play a key role in the oxygen precipitation process at the next high temperature annealing.

Fig.3 shows the FTIR absorption spectrums for the high dose fast neutron irradiated CZ-Si (S2) annealed at the temperature rang of 200-500 ℃. After annealed at the temperature of 400-450 ℃, the IR absorption bands at 834 (V₃O₂) and 889 cm^-1(VO₂) disappears and the main absorption bands are 825 cm^-1 (V₂O₂).

CO+VO→V₂O₂                              (4)

VO+O_i→O-V-O                             (5)

O-V-O+V→V₂O₂                            (6)

With increasing irradiation dose, the vacancy will abound in the silicon. It is known that vacancies are easy to combine with O_i to form VO, so the intensity of the O_i will decrease, and when the irradiation dose increases to

Fig.3 FTIR spetra for S2 sample annealed at temperature range of 200-500 ℃

6×10¹⁸ n/cm³, two-part of the O_i will transform into the configuration of VO. Annealing of VO in high dose neutron irradiated sample (S2) is suggested to consist of two processes. The first is migration of other wandering A-center to form V₂O₂, and the second is that A-center is trapped at the neighborhood O_i and then trapped V to form the V₂O₂.

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(Edited by LONG Huai-zhong)

Foundation item: Project(50472034) supported by the National Natural Science Foundation of China; Project(E2005000048) supported by the Natural Science Foundation of Hebei Province, China; Project(20050080006) supported by the Specialized Research Fund for the Doctoral Program of Higher Education, China.

  Corresponding author: LI Yang-xian, Tel: +86-22-26582214; E-mail: admat@jsmail.hebut.edu.cn