Y. SHI et al.: Optical Studies of IR Active Electronic Defects in Silicon 139

phys. stat. sol. (a) 144, 139 (1994)

Subject classification: 61.80 and 71.55; 78.50; S5.11

Department of Physics, Nanjing University ') ( a ) and Institute for Materials Research, Tohoku University, Senhi') (b)

Optical Studies of Infrared Active Electronic Defects in Neutron Irradiated Silicon after Annealing at 450 "C

BY Y. SHI (a), F. M. Wu (a), Y. D. ZHENG (a), M. SUEZAWA (b), M. I M A I ~ ) (b), and K. SUMINO (b)

Using Fourier transform infrared absorption spectroscopy and Hall effect measurements mainly the infrared active electronic defects in fast neutron irradiated float-zone silicon and Czochralski silicon after annealing at 450 "C are investigated. Introducing thermal donors (TD) to alter the Fermi level, the defect associated with the higher-order bands (HOB) is analyzed, which is proposed to have at least three charge states within the band gap. The defect level giving rise to the HOB is located slightly below E , - 0.15 eV, and another level possibly exists near the bottom of the conduction band. Moreover, the measurement results indicate that the characteristic of photoexcitation and decay of the HOB is associated with the slow relaxation process of photoexcited carriers.

1. Introduction

The infrared active electronic defects induced by annealing around 450 "C, namely the well-known thermal donors (TD) in oxygen-rich Czochralski (Cz) silicon [l to 31 and the so-called higher-order bands (HOB) in neutron irradiated silicon [4 to 81, have been studied extensively, because they govern the electronic properties of silicon. Compared to the TD, nevertheless, the understanding of the HOB is far from being complete. The HOB is the main infrared active electronic defect in irradiated silicon with a high fluence of fast neutrons after annealing at 400 to 600 "C, which consists of a series of more than forty sharp peaks in the range of 600 to 1400 cm-'. The defect associated with the HOB comprises a family of at least three intrinsic defect clusters involving vacancies and/or interstitials [5, 71. Although some investigations on the HOB have been made with different experimental techniques such as photoluminescence (PL) [6,9], electron paramagnetic resonance (EPR) [7], and deep-level transient spectroscopy (DLTS) [lo], no direct correlation can be established.

Most of the experimental results were obtained from neutron irradiated float-zone (Fz) silicon, where the Fermi level is pinned around the middle of the band gap due to heavy radiation damage. Obviously, it is very difficult to study the defect state. In order to observe the HOB, a band-edge light illumination leading to the shift of the quasi-Fermi level is required. This optically active defect is attributed to the transformation of the charge states capturing free electrons [4, 51. An interesting process for the photoexcitation and decay of the HOB in an irradiated Fz-silicon was investigated firstly by Corelli et al. [6] with a dual

') Nanjing 210008, People's Republic of China. ') Sendai 980, Japan. ') Permanent address: Komatsu Electronic Metals Co., Ltd., Kanagawa 254, Japan.

140 Y. SHI, F. M. Wu, Y. D. ZHENG, M. SUEZAWA, M. IMAI, and K. SUMINO

beam method. Based upon an examination of the residual absorption and relevant depletion, Corelli et al. suggested the defect related to the HOB to have three charge states, where the intermediate state is a strong trap center and the final state gives rise to the HOB. More recently, such residual absorption of the HOB has been proposed to be associated with a slow relaxation process of photoexcited carriers [8, 111. The annealing at 450 "C of an irradiated Czochralski (Cz) silicon introducing thermal donors (TD) to alter the Fermi level provides an important opportunity for studying the defect state. Furthermore, a comparison with the TD is helpful for understanding the HOB. There are many similar points between the HOB and the TD. The TD comprises a family of more than nine double donors involving oxygen-related aggregates. The donor levels are located around E , - 0.15 eV and E , - 0.07 eV for T D (+/+ +) and TD (O/+), respectively. The absorption bands appear in the ranges of 650 to 1300 cm-' for the single ionized charge state TD' and 350 to 550 cm-' for the neutral charge state TD', which can be well described by the effective mass theory [3].

In this paper, we investigate the infrared active electronic defect HOB in the fast neutron irradiated Fz- and Cz-silicon using Fourier transform infrared absorption spectroscopy and Hall effect measurements at low temperatures. By controlling the annealing time at 450 "C, the Fermi level is altered successfully from the middle of the band gap to the conduction band in irradiated Cz-silicon. The defect associated with the HOB has at least three charge states in the band gap, the defect level giving rise to the HOB is located slightly below E , - 0.15 eV, and another level exists near the bottom of the conduction band. Furthermore, the processes of photoexcitation and decay of the HOB are measured. The observation indicafes that the characteristic of this process is associated with the slow relaxation process of photoexcited carriers.

2. Experimental Details

The samples used in this experiment were Fz- and Cz-silicon crystals irradiated with a high fluence of fast neutrons (8 x 10" cm-2, En > 1.0 MeV). The starting crystals were of n-type with resistivities of 2300 and 5 Cl cm, and oxygen concentrations < 10l6 and 8 x l O I 7 atoms cm-' for irradiated Fz- and Cz-silicon, respectively. The samples were annealed at 450°C for various annealing times from 10min to 100h under vacuum conditions.

Infrared absorption measurements were performed at low temperatures using a JEOL JIR-100 Fourier transform infrared (FTIR) spectrometer. The temperature was controlled by an Air Product liquid helium cryostat in the range from 6 to 300 K. The spectral solutions were from 0.25 to 2 cm-' in accordance with the requirement of the measurements. The light source of the spectrometer was filtered with a germanium wafer at room temperature during all measurements. The samples were cooled in darkness, and were measured without or with an additional illuminating light obtained from a tungsten lamp through a monochromator. The conventional Hall coefficient and four-point resistivity techniques were used to determine the position of the Fermi level in the temperature range 80 to 300 K.

3. Results and Discussion

3.1 Defect state of the HOB

Fig. 1 shows the position of the Fermi level in the samples annealed at 450 "C for various annealing times as measured by Hall effect at 100 and 300 K. Because the resistivities in

Optical Studies of Infrared Active Electronic Defects in Irradiated Si 141

0 1 5 10 50 101

Annealing Time (h)

Fig. 1. Fermi level position vs. annealing time in the irradiated samples annealed at 450 "C. Data for the irradiated Fz-Si at 300 K (0) and the irradiated Cz-Si at 100 K (A) and 300 K (A) are shown

the iradiated Fz-silicon samples at low temperatures are too high to be measured, their data at 100 K are absent. As expected, the Fermi level is pinned near the middle of the band gap in all irradiated Fz-silicon even after annealing for 100 h. Owing mainly to the generation of the TD in irradiated Cz-silicon, on the other hand, the Fermi level approaches gradually the conduction band with the increase of annealing time. The position of the Fermi level at 7 K will be estimated from the measurement of the TD in the following section.

c--- Wave number (cm-1)

Fig. 2. Infrared absorption spectra at 7 K of the irradiated Fz-Si annealed for 40 h. The most prominent absorption bands are labeled

142 Y. S i n , F. M. Wu, Y. D. ZHENG, M. SUEZAWA, M. IMAI, and K. SUMINO

The typical infrared absorption spectra of the HOB in irradiated Fz-silicon annealed for 40 h and measured at 7 K are shown in Fig. 2. The absorption bands of the HOB cannot be detected before illumination (Fig. 2, spectrum A). The Oi at 1136 cm-' means the absorption band which arises from the localized vibrations of interstitial oxygen atoms. More than forty sharp absorption bands are clearly visible under illumination (Fig. 2, spectrum B). The positions of the most prominent absorption bands are labeled here. This behavior is still true as the annealing time rises up to 100 h. Although the intensity ratios of each absorption band vary in the different annealed samples, the positions of the prominent absorption bands do not change. Furthermore, the absorption of the HOB has been measured in the samples annealed in the temperature range of 400 to 600°C, a strong absorption can be obtained as the sample is annealed at 450 to 500 "C.

The infrared absorption spectra of irradiated Cz-silicon are more complicated than those of irradiated Fz-silicon, as seen from Fig. 3, where the spectra are obtained from the irradiated Cz-silicon samples annealed for 20, 40, and 100 h. Spectrum A is measured without additional illumination, and spectrum B is the difference spectrum of the spectrum measured with and without illumination. A serious overlapping of absorption bands is observed in the range of 600 to 1300 cm-', which originates from the electronic transition absorptions of the HOB [S] and TD+ [3], as well as of local vibrational absorptions of interstitial oxygen at 1136 and 1205 cm-', dioxygen-vacancy VOz at 895 cm-', and other VO, complexes at 909, 976, 991, and 1005 cm-' [12]. On the basis of our knowledge, the prominent absorption bands of the HOB and TD+ can be well recognized. In the spectra of the sample annealed for 20 h (Fig. 3a), the absorption bands of the HOB are clearly visible, but those of TD' are almost absent before illumination. Under band edge light illumination, the absorption bands of TD' appear together with an increase of the absorption intensity of the HOB. As the annealing time increases to 40 h, the absorption bands of both HOB and TD+ appear simultaneously, and both intensities increase with illumination (Fig. 3b). Further annealing (100 h) results in the appearance of absorption bands of the TDo besides those of HOB and TD' (Fig. 3c). Here, it is interesting to note that the absorption intensities of the HOB and the TD+ decrease under a strong light illumination, at the same time, those of the TDo and phosphorous donors increase. Since the absorptions of both HOB and TD are due to electronic transitions, the change of the absorption intensity is caused by the charge state transformation of the defect giving rise to the absorption band. It is believed that these phenomena are dependent on the position of the Fermi level, as well as the quasi-Fermi level under illumination [5, 131. The generation of the TD leads to the fact that the Fermi level approaches the conduction band. From the measurement results in Fig. 1 and 3, and the property of the TD as well, the position of the Fermi level at 7 K can be estimated. It is located below, near, and above the TD (+/+ +) ( E , - 0.15 eV) in the samples annealed for 20, 40, and 100 h, respectively. Fig. 4 shows the ratio of the absorption intensities (Zi/Zo) for the HOB and the TD' at different measurement temperatures in irradiated Cz-silicon annealed for 40 h. The similar behavior of the HOB and the TD' provides further evidence that the mechanism of the Fermi level influence on the absorption for the HOB is the same as that for TD'.

Due to heavy radiation damage, the Fermi level is pinned near the middle of the band gap in irradiated Fz-silicon samples even after annealing at 450 "C for 100 h. The fact in Fig. 2 that the absorption bands of the HOB in irradiated Fz-silicon can only be observed with illumination indicates that the defect level giving rise to the HOB is located above the Fermi level. Before illumination the position of the Fermi level is near the middle of

Optical Studies of Infrared Active Electronic Defects in Irradiated Si 143

111 I Cz - S i 1

1200 1000 800 600 - Wave number (cm-' 1 Fig. 3. Infrared absorption spectra at 7 K of the irradiated Cz-Si annealed for a) 20, b) 40, and c) 100 h. The most prominent absorption bands are labeled

the band gap, therefore, the occupation number of the defect state giving rise to the HOB is too low to be observed. Under illumination, the generation of photoexcited carriers leads to the shift of the quasi-Fermi level to the conduction band. As a results, the occupation number of the defect state increases, and then the absorption becomes detectable.

144 Y. SHI, F. M. Wu, Y. D. ZHENG, M. SUEZAWA, M. IMAI, and K. SUMINO

4

8 I 3 \ 27

2

1

\

4

1 I 1 1 I I I 1 I I 0 10 20 30 40 5 0 6 0 70 80 90

Temperature (K) - Fig. 4. Ratio of the absorption intensities for the HOB and the TD' measured with illumination to those without illumination vs. measurement temperature in the irradiated Cz-Si annealed for 40 h

Experimentally, it has been observed that the most effective photon energy of the illuminating light at 7 K is the band edge light, as seen from Fig. 5, which is in agreement with the results of Corelli et al. [6] and Sahu et al. [7]. This dependence of the absorption intensity on the photon energy can be attributed to the characteristic of intrinsic photoexcitation in bulk silicon. Whereas the Fermi level approaches the conduction band in irradiated Cz-silicon samples annealed for 20 and 40 h, the appearance of the absorption bands of the HOB is prior to those of the TD', and both absorption intensities increase simulta- neously under illumination, reflecting that the defect level is slightly below the TD (+ / + +) level E , - 0.15 eV. The features in the irradiated Cz-silicon annealed for 100 h (Fig. 3c) indicate that the position of the Fermi level lies above the defect level. Like the charge state transition of TD' ++ TD', the decrease of absorption intensity of the HOB under a strong illumination suggests that another defect level would exist possibly near the bottom of the conduction band. Meakawa et al. [lo] have investigated the defect levels in a variety

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Photon Energy (eV) -

Fig. 5 . Dependence of the absorption intensity of the HOB on the photon energy as measured at 7 K in the irradiated Fz-Si annealed for 40 h

Optical Studies of Infrared Active Electronic Defects in Irradiated Si 145

of neutron irradiated silicon samples by DLTS measurement. Based on similar features in impurity dependence, annealing, and photoexcitation, the electron traps at E , - 0.22 eV and E , - 0.29 eV were proposed to be related to the HOB. In fact, since the defect associated with the HOB comprises a family of defects, the defect level has a distribution within the band gap. Here, it is reasonable to draw the significant conclusion that the defect associated with the HOB has at least three charge states in the band gap, the defect level giving rise to the HOB is located slightly below E, - 0.15 eV, and another level exists possibly near the bottom of the conduction band.

3.2 Optical active process of the HOB

We have measured the processes of photoexcitation and decay of the HOB in both the irradiated Fz- and Cz-silicon. Fig. 6 presents the time dependence of the photoexcitation (left-hand side) and the decay (right-hand side) of the HOB at 1102 cm-' in irradiated Fz-silicon annealed for 40 h. The measurement temperature is 7 K, and the photon energy of the illuminating light is 1.23 eV. The ratio of the illuminating light intensities for curves 1 to 3 is 1:0.2:0.06; for curve 4 the intensity is the same as that of curve 2, but the illuminating time is only 400 s. It is found experimentally that the relative change rates of the absorption intensities of the prominent bands keep almost constant during photo- excitation and decay. Thus, the band at 1102cm-' with the highest peak is used as a representation here. In the processes of photoexcitation and decay for curves 1 to 3, the variation of the absorption intensity Z can be quite well described by an exponential and a logarithmic function of time, respectively,

Time - Fig. 6. Time dependence of the absorption intensity of the HOB during photoexcitation (left-hand side) and decay (right-hand side) at 7 K in the irradiated Fz-Si annealed for 40 h

10 physica (a) 144/1

146 Y. SHI, F. M. Wu, Y . D. ZHENG, M. SUEZAWA, M. IMAI, and K. SUMINO

where 5 , and zd are the time constants for photoexcitation and decay, respectively, I , is the saturation value that depends on the logarithm of the illuminating light intensity, to and A are parameters related to the defect structure in a sample and the illuminating light intensity [S, 111. The value zd estimated from the present data is about lo5 s, thus, the residual absorption intensity remains at some level practically not changing during our measurement. Furthermore, if the illumination induced absorption intensity is lower than the residual absorption, there is no obvious decay process (curve 4). Further measurements show the identical behavior of photoexcitation and decay in all irradiated Fz-silicon samples annealed for 1 to 100 h. Here, it is important to point out that these characteristics are consistent with those of the persistent residual photoconductivity observed in neutron irradiated silicon at low temperatures, which are caused by the macroscopic potential barrier induced by some defect clusters [14, 151. The optical active process of the HOB presented in Fig. 6 can be well described by the macroscopic barrier model [ll]. One of the main features of the fast neutron irradiation is introducing defect clusters [16 to 181. Investigations on the defect structure in neutron irradiated silicon have been carried out. The defects, such as dislocation loops, voids, rod-like defects, have been observed by electron microscopy in neutron irradiated silicon annealed at temperatures between 400 and 500 “C [19, 201. In the present samples, the cluster-like defects have been detected with the electron-dipole spin resonance method (EDSR). These defects are able to play the role of a “macroscopic potential barrier” and result in the slow relaxation of photoexcited carriers at low temperatures [21, 111.

In Fig. 7, we present the time dependence of photoexcitation and decay of the HOB at 1102 cm- and TD’ at 951 cm- in irradiated Cz-silicon annealed for 40 h. The measure- ment condition is the same as in Fig. 6. Similarly to the optically active process shown in Fig. 6 , the absorption intensities of both HOB and TD’ begin to increase and then approach a saturation value that depends on the light intensity when the illuminating light is turned on. As the light is switched off, the illumination induced intensities decay slowly to the initial value, which takes about 6 x lo3 s. It can be seen clearly that the time dependence of the

0 0 Time -

Fig. 7. Time dependence of the absorption intensities of the HOB and the TD’ during photoexcitation (left-hand side) and decay (right-hand side) in the irradiated Cz-Si annealed for 40 h

Optical Studies of Infrared Active Electronic Defects in Irradiated Si 147

HOB is almost the same as that of the TD' except for a slight time lag of the TD' during photoexcitation. On the basis of the property of TD, it can be confirmed that the characteristic of the photoexcitation and decay of the TD', as well as that of the HOB, is here related to the slow relaxation of photoexcited carriers. In addition, it should be noted that the residual absorption intensity existing in the irradiated Fz-silicon is not observed in the irradiated Cz-silicon. This difference can be interpreted by the different defect structures between fast neutron irradiated Fz- and Cz-silicon [16 to 181. Owing to the interaction with impurities in a sample having high impurity atom concentration, such as oxygen and dopant, the formation efficiency of the irradiation induced defect clusters and the temperature of annealing-out are reduced. Therefore, in an initially high-resistivity silicon sample annealed below 600 "C, the change of electrical parameters could be characterized by the presence of fast neutron irradiation induced defect clusters. In an initially low-resistivity silicon sample, on the other hand, the change is associated with the character of point defects [17]. In order to explain the optically active process of the HOB, Corelli et al. suggested that the defect related to the HOB has three charge states, among which the intermediate state is a very strong electron trap center which is capturing the photoexcited electron at low temperatures [6]. However, it is not able to interpret the present experimental results. Moreover, it has been observed that the difference in the process of photoexcitation and decay of the HOB among various samples is due to different irradiation or annealing conditions. This implies that the behavior of photoexcitation and decay of the HOB is not governed by the defect itself. Additionally, the depletion phenomenon of the residual absorption reported by Corelli et al. [6] is not observed in the present samples. In fact, it could be attributed to an infrared quenching effect [22]. The present results indicate that the characteristic of the optically active process of the HOB is associated with the slow relaxation of photoexcited carriers, which is observed in neutron irradiated silicon at low temperatures.

4. Summary

We have investigated the infrared active electronic defect, higher-order bands, in the fast neutron irradiated Fz- and Cz-silicon. The introduction of the TD provides an important opportunity to alter the position of the Fermi level and to make a comparison of the HOB with the TD. Based upon the infrared absorption and electrical measurements at low temperatures in irradiated Fz- and Cz-silicon annealed at 450 "C with various annealing times, it is proposed strongly that the defect associated with the HOB has at least three charge states in the band gap, the defect level giving rise to the HOB is located slightly below E, - 0.15 eV, and another level possibly exists near the bottom of the conduction band.

The interesting process of photoexcitation and decay of the HOB has been also measured. It is found that the photoexcitation process follows an exponential time dependence and the decay follows a logarithmic one with a decay time constant lo5 s at 7 K in irradiated Fz-silicon. Moreover, the consistency of the characteristics of photoexcitation and decay between HOB and TD+ is observed in irradiated Cz-silicon. These observations indicate that the characteristic of the optically active process is associated with the slow re- laxation of photoexcited carriers, which is caused by the presence of defect clusters and traps.

1 o*

148 Y. SHI et al.: Optical Studies of IR Active Electronic Defects in Silicon

Acknowledgements

We would like to thank Dr. T. Sekiguchi and Dr. 1. Yonenaga for valuable discussions and help. One of us (Y.S.) would like to thank Komatsu Electronic Metals Co., Ltd. for financial support.

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(Received March 14, 1994)