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Room-temperature formation of magnesium–oxygen complex impurities in silicon
Physica B 302–303 (2001) 197–200
Room-temperature formation of magnesium–oxygen complex impurities in silicon L.T. Ho* Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan
Abstract Magnesium which is thermally diffused into silicon is well-known to behave like an interstitial donor. Recent study indicates that magnesium in silicon containing proper amount of oxygen can pair with oxygen to form magnesium– oxygen complex impurity, which is also an interstitial donor in silicon. Our study on this subject further shows that such magnesium–oxygen complex impurities can even be formed by interstitial magnesium and dispersed oxygen in silicon at quite low temperatures such as room-temperature. Experimentally observed results clearly demonstrating this newly found phenomenon are given. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 71.55.Cn; 78.30.Am Keywords: Complex impurity; Magnesium; Oxygen; Silicon
1. Introduction It is well-known that the group-II element magnesium behaves like an interstitial impurity when diffused into silicon [1,2]. This behavior is very interesting since all the group-III and -V chemical impurities occupy substitutional lattice sites in silicon. Even beryllium, another group-II element, has also been shown to be a substitutional impurity in silicon [3]. Such an unusual behavior of magnesium makes it a divalent donor impurity rather than an acceptor impurity in silicon [1]. Recently, we have found that, for silicon containing proper amount of oxygen, the interstitial magnesium diffused into the crystal can pair with dispersed oxygen to form magnesium–oxygen complex impurities and such complex impurities *Tel.: +886-2-2789-6708; fax: +886-2-2783-4187. E-mail address: [email protected] (L.T. Ho).
are also interstitial donors [4]. During the course of studying the behavior of this novel complex impurity, we have further found an unexpected phenomenon, which indicates that this complex impurity pair can be formed in silicon even at temperatures as low as ordinary room-temperature. Considering that silicon is the material to be widely used in many important semiconductor industries today, it should be quite interesting to be aware of the possibility that the basic characteristic of this material might change on its own at room-temperature. The purpose of the present paper is to report our experimentally observed results on this matter.
2. Experimental procedure Magnesium was introduced into silicon by hightemperature thermal diffusion. The detailed pro-
0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 2 8 - 8
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L.T. Ho / Physica B 302–303 (2001) 197–200
cedure, which involves mainly to evaporate magnesium onto the sample surfaces and then to use a sandwich technique to diffuse magnesium into the sample, has been described elsewhere [2]. The magnesium-doped silicon samples reported here were prepared basically following the same procedure. A floating-zone grown silicon crystal containing a proper amount of oxygen was used in order to form magnesiun–oxygen complex impurities. The oxygen concentration of the crystal has not been determined but believed to be of the order of 1016 cm 3 from the nine-micron vibrational band of oxygen observed in the absorption spectrum measured at low temperature. Highpurity silicon crystals without oxygen were also used as reference material for the purpose of comparison. Bomem DA3 Fourier-transform infrared spectrometer (Bomem Inc., Quebec, Canada) equipped with a KBr beamsplitter and liquid-nitrogencooled MCT detector was used for obtaining the absorption spectrum of the sample. An Oxford continuous flow cryostat (Oxford Instruments, Oxon, UK) equipped with CsI optical windows
was used for the low temperature measurements. The sample was mounted on the copper tailpiece of the cryostat using a strainfree mounting technique [5] to avoid the broadening of the spectral lines due to the stress produced in the sample at low temperatures. All optical measurements were carried out at a temperature of about 10 K.
3. Experimental results Fig. 1 Shows the absorption spectrum of a magnesium-doped silicon sample measured with liquid helium as coolant. The left side of Fig. 1 shows several strong absorption lines within the spectral range of 95 and 107 meV. They have been reported previously and identified as the excitation lines due to transitions from the ground state to various excited states of isolated interstitial neutral magnesium donor impurities [1], thus are denoted as Si(Mg) in Fig. 1. The positions of the absorption lines shown in Fig. 1 for Si(Mg) are 95.81 (2p0), 101.12 (2p ), 101.96 (3p0), 104.39 (3p ), and 105.32 (4p ) meV, respectively.
Fig. 1. Low-temperature absorption spectrum of silicon containing magnesium and oxygen. Measurement was made right after magnesium had been diffused into silicon. Liquid helium was used as coolant.
L.T. Ho / Physica B 302–303 (2001) 197–200
A small but clear absorption peak near 140.8 meV (or 1136 cm 1) appears at the right side of Fig. 1. It is attributed to the vibrational mode of interstitial oxygen in silicon [6] and is denoted as Si(O). This is also an unambiguous proof that the sample being investigated does contain oxygen. Besides the absorption lines mentioned above, three additional absorption peaks which are very weak and barely visible can be observed to appear within the spectral range of 112–124 meV in Fig. 1 as well. These additional peaks appear only for samples prepared by diffusing magnesium into oxygen-containing silicon. For samples prepared following the same diffusion procedure but starting with high-purity, oxygen-free silicon, they do not appear and only the excitation spectrum of Si(Mg) can be observed. Presumably, they are due to magnesium–oxygen complexes in silicon. For this reason, they are denoted as Si(Mg–O) in Fig. 1. The spectrum shown in Fig. 1 was typically obtained by measuring the silicon sample within a few days right after diffusing magnesium into the crystal. After the measurement, the sample was stored in a sample box and kept at room temperature. After some time, however, when the sample was re-measured, it is interesting to find that the spectrum was different clearly indicating that some changes had occurred, even though the sample was only left alone by keeping at ordinary room temperature. A typical example to show the change of the spectrum is given in Fig. 2. Fig. 2(A) shows the three Si(Mg–O) absorption peaks obtained from the measurement of a sample right after the introduction of magnesium into the silicon crystal. After keeping the sample at room temperature for two years, the sample was re-measured with the result given in Fig. 2(B). It is evident that the three peaks shown in Fig. 2(A) become obviously stronger in Fig. 2(B). Furthermore, besides the much stronger absorption for 2p0, 2p and 3p , two additional peaks, i.e., 3p0 and 4p also appear to be clearly observable in Fig. 2(B). The same sample was measured after another three years. The spectrum is shown in Fig. 2(C). Compared with those shown in Fig. 2(B), the absorption lines observed in Fig. 2(C) are again slightly stronger.
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Fig. 2. Low-temperature absorption spectrum of magnesium– oxygen complex impurities in silicon. Measurements were made for a sample (A) right after diffusing magnesium into silicon, (B) two years later, and (C) five years later, respectively.
The positions of the absorption lines shown in Fig. 2 for Si(Mg–O) are 112.99 (2p0), 118.27 (2p ), 119.14 (3p0), 121.54 (3p ), and 122.46 (4p ) meV, respectively. Compared with the absorption lines observed for Si(Mg) in Fig. 1, the energy spacings between any two corresponding lines for Si(Mg–O) and Si(Mg) are identical within the experimental error. The relative intensities of the absorption lines for Si(Mg–O) are also strikingly similar compared to those for Si(Mg). The labeling of the absorption lines for Si(Mg–O) is based on this comparison. Because of the similarity of the two spectra, it is clear that, like magnesium donor, magnesium–oxygen complex impurity is a donor in silicon as well.
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The experimental result shown in Fig. 2(A) clearly indicates that, during the high-temperature diffusion process, the thermal energy apparently seems too high to allow the formation of magnesium–oxygen complex pairs easily. Thus, when the sample was measured right after introducing magnesium into silicon, only very small amount of magnesium–oxygen pairs could be formed to show very weak absorption for Si(Mg–O) while most magnesium impurities still remained isolated to give very strong absorption for Si(Mg), as clearly shown in Fig. 1. Later, when the sample was re-measured after storing at room temperature for a quite long time, the result observed in Figs. 2(B) and (C) showing stronger absorption for Si(Mg–O) obviously demonstrates that the interstitial magnesium could gradually pair with dispersed oxygen to form magnesium– oxygen complex impurities. It is interesting to note that such a pair formation can continue for very long time, as we can see the difference between Figs. 2(A) and (B) as well as the difference between Figs. 2(B) and (C), where the time intervals between the measurements are as long as two and three years, respectively. It is more interesting to find that the pair formation can take place at temperatures as low as ordinary room temperature, which also means that it is possible for the characteristic of the silicon crystal to change gradually at room temperature. This unexpected behavior certainly needs us to pay special attention since silicon is well-known to be the most important material used today for making computer chips and many electronic devices. In conclusion, from measuring the absorption spectrum of silicon containing magnesium and
oxygen, we have found that the interstitial magnesium can pair with dispersed oxygen to form magnesium–oxygen complex donor impurities in silicon. By re-measuring the same sample, we have also found that the intensity of the absorption lines for Si(Mg–O) keeps on getting stronger for several years, which indicates that the number of magnesium–oxygen pairs formed, i.e., the concentration of magnesium–oxygen complex donor impurities in the sample, continues to increase, but very slowly, for years. The formation of such magnesium–oxygen pairs has also been found to be able to take place at temperatures as low as ordinary room temperature.
Acknowledgements This work was partially supported by the National Science Council of the Republic of China under contract numbers NSC 88-2112-M-001-002 and NSC 89-2112-M-OO1-034.
References [1] R.K. Franks, J.B. Robertson, Solid State Commun. 5 (1967) 479. [2] L.T. Ho, A.K. Ramdas, Phys. Rev. B 5 (1972) 462. [3] R.K. Crouch, J.B. Robertson, T.E. Gilmer Jr., Phys. Rev. B 5 (1972) 3111. [4] L.T. Ho, Phys. Status Solidi B 210 (1998) 313. [5] C. Jagannath, Z.W. Grabowski, A.K. Ramdas, Phys. Rev. B 23 (1981) 2080. [6] B. Pajot, in: F. Shimura (Ed.), Oxygen in Silicon, Academic Press, San Diego, 1994, p. 200.
Room-temperature formation of magnesium–oxygen complex impurities in silicon L.T. Ho* Institute of Physics, Academia Sinica, Nankang, Taipei, Taiwan
Abstract Magnesium which is thermally diffused into silicon is well-known to behave like an interstitial donor. Recent study indicates that magnesium in silicon containing proper amount of oxygen can pair with oxygen to form magnesium– oxygen complex impurity, which is also an interstitial donor in silicon. Our study on this subject further shows that such magnesium–oxygen complex impurities can even be formed by interstitial magnesium and dispersed oxygen in silicon at quite low temperatures such as room-temperature. Experimentally observed results clearly demonstrating this newly found phenomenon are given. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Tt; 71.55.Cn; 78.30.Am Keywords: Complex impurity; Magnesium; Oxygen; Silicon
1. Introduction It is well-known that the group-II element magnesium behaves like an interstitial impurity when diffused into silicon [1,2]. This behavior is very interesting since all the group-III and -V chemical impurities occupy substitutional lattice sites in silicon. Even beryllium, another group-II element, has also been shown to be a substitutional impurity in silicon [3]. Such an unusual behavior of magnesium makes it a divalent donor impurity rather than an acceptor impurity in silicon [1]. Recently, we have found that, for silicon containing proper amount of oxygen, the interstitial magnesium diffused into the crystal can pair with dispersed oxygen to form magnesium–oxygen complex impurities and such complex impurities *Tel.: +886-2-2789-6708; fax: +886-2-2783-4187. E-mail address: [email protected] (L.T. Ho).
are also interstitial donors [4]. During the course of studying the behavior of this novel complex impurity, we have further found an unexpected phenomenon, which indicates that this complex impurity pair can be formed in silicon even at temperatures as low as ordinary room-temperature. Considering that silicon is the material to be widely used in many important semiconductor industries today, it should be quite interesting to be aware of the possibility that the basic characteristic of this material might change on its own at room-temperature. The purpose of the present paper is to report our experimentally observed results on this matter.
2. Experimental procedure Magnesium was introduced into silicon by hightemperature thermal diffusion. The detailed pro-
0921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 4 2 8 - 8
198
L.T. Ho / Physica B 302–303 (2001) 197–200
cedure, which involves mainly to evaporate magnesium onto the sample surfaces and then to use a sandwich technique to diffuse magnesium into the sample, has been described elsewhere [2]. The magnesium-doped silicon samples reported here were prepared basically following the same procedure. A floating-zone grown silicon crystal containing a proper amount of oxygen was used in order to form magnesiun–oxygen complex impurities. The oxygen concentration of the crystal has not been determined but believed to be of the order of 1016 cm 3 from the nine-micron vibrational band of oxygen observed in the absorption spectrum measured at low temperature. Highpurity silicon crystals without oxygen were also used as reference material for the purpose of comparison. Bomem DA3 Fourier-transform infrared spectrometer (Bomem Inc., Quebec, Canada) equipped with a KBr beamsplitter and liquid-nitrogencooled MCT detector was used for obtaining the absorption spectrum of the sample. An Oxford continuous flow cryostat (Oxford Instruments, Oxon, UK) equipped with CsI optical windows
was used for the low temperature measurements. The sample was mounted on the copper tailpiece of the cryostat using a strainfree mounting technique [5] to avoid the broadening of the spectral lines due to the stress produced in the sample at low temperatures. All optical measurements were carried out at a temperature of about 10 K.
3. Experimental results Fig. 1 Shows the absorption spectrum of a magnesium-doped silicon sample measured with liquid helium as coolant. The left side of Fig. 1 shows several strong absorption lines within the spectral range of 95 and 107 meV. They have been reported previously and identified as the excitation lines due to transitions from the ground state to various excited states of isolated interstitial neutral magnesium donor impurities [1], thus are denoted as Si(Mg) in Fig. 1. The positions of the absorption lines shown in Fig. 1 for Si(Mg) are 95.81 (2p0), 101.12 (2p ), 101.96 (3p0), 104.39 (3p ), and 105.32 (4p ) meV, respectively.
Fig. 1. Low-temperature absorption spectrum of silicon containing magnesium and oxygen. Measurement was made right after magnesium had been diffused into silicon. Liquid helium was used as coolant.
L.T. Ho / Physica B 302–303 (2001) 197–200
A small but clear absorption peak near 140.8 meV (or 1136 cm 1) appears at the right side of Fig. 1. It is attributed to the vibrational mode of interstitial oxygen in silicon [6] and is denoted as Si(O). This is also an unambiguous proof that the sample being investigated does contain oxygen. Besides the absorption lines mentioned above, three additional absorption peaks which are very weak and barely visible can be observed to appear within the spectral range of 112–124 meV in Fig. 1 as well. These additional peaks appear only for samples prepared by diffusing magnesium into oxygen-containing silicon. For samples prepared following the same diffusion procedure but starting with high-purity, oxygen-free silicon, they do not appear and only the excitation spectrum of Si(Mg) can be observed. Presumably, they are due to magnesium–oxygen complexes in silicon. For this reason, they are denoted as Si(Mg–O) in Fig. 1. The spectrum shown in Fig. 1 was typically obtained by measuring the silicon sample within a few days right after diffusing magnesium into the crystal. After the measurement, the sample was stored in a sample box and kept at room temperature. After some time, however, when the sample was re-measured, it is interesting to find that the spectrum was different clearly indicating that some changes had occurred, even though the sample was only left alone by keeping at ordinary room temperature. A typical example to show the change of the spectrum is given in Fig. 2. Fig. 2(A) shows the three Si(Mg–O) absorption peaks obtained from the measurement of a sample right after the introduction of magnesium into the silicon crystal. After keeping the sample at room temperature for two years, the sample was re-measured with the result given in Fig. 2(B). It is evident that the three peaks shown in Fig. 2(A) become obviously stronger in Fig. 2(B). Furthermore, besides the much stronger absorption for 2p0, 2p and 3p , two additional peaks, i.e., 3p0 and 4p also appear to be clearly observable in Fig. 2(B). The same sample was measured after another three years. The spectrum is shown in Fig. 2(C). Compared with those shown in Fig. 2(B), the absorption lines observed in Fig. 2(C) are again slightly stronger.
199
Fig. 2. Low-temperature absorption spectrum of magnesium– oxygen complex impurities in silicon. Measurements were made for a sample (A) right after diffusing magnesium into silicon, (B) two years later, and (C) five years later, respectively.
The positions of the absorption lines shown in Fig. 2 for Si(Mg–O) are 112.99 (2p0), 118.27 (2p ), 119.14 (3p0), 121.54 (3p ), and 122.46 (4p ) meV, respectively. Compared with the absorption lines observed for Si(Mg) in Fig. 1, the energy spacings between any two corresponding lines for Si(Mg–O) and Si(Mg) are identical within the experimental error. The relative intensities of the absorption lines for Si(Mg–O) are also strikingly similar compared to those for Si(Mg). The labeling of the absorption lines for Si(Mg–O) is based on this comparison. Because of the similarity of the two spectra, it is clear that, like magnesium donor, magnesium–oxygen complex impurity is a donor in silicon as well.
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L.T. Ho / Physica B 302–303 (2001) 197–200
The experimental result shown in Fig. 2(A) clearly indicates that, during the high-temperature diffusion process, the thermal energy apparently seems too high to allow the formation of magnesium–oxygen complex pairs easily. Thus, when the sample was measured right after introducing magnesium into silicon, only very small amount of magnesium–oxygen pairs could be formed to show very weak absorption for Si(Mg–O) while most magnesium impurities still remained isolated to give very strong absorption for Si(Mg), as clearly shown in Fig. 1. Later, when the sample was re-measured after storing at room temperature for a quite long time, the result observed in Figs. 2(B) and (C) showing stronger absorption for Si(Mg–O) obviously demonstrates that the interstitial magnesium could gradually pair with dispersed oxygen to form magnesium– oxygen complex impurities. It is interesting to note that such a pair formation can continue for very long time, as we can see the difference between Figs. 2(A) and (B) as well as the difference between Figs. 2(B) and (C), where the time intervals between the measurements are as long as two and three years, respectively. It is more interesting to find that the pair formation can take place at temperatures as low as ordinary room temperature, which also means that it is possible for the characteristic of the silicon crystal to change gradually at room temperature. This unexpected behavior certainly needs us to pay special attention since silicon is well-known to be the most important material used today for making computer chips and many electronic devices. In conclusion, from measuring the absorption spectrum of silicon containing magnesium and
oxygen, we have found that the interstitial magnesium can pair with dispersed oxygen to form magnesium–oxygen complex donor impurities in silicon. By re-measuring the same sample, we have also found that the intensity of the absorption lines for Si(Mg–O) keeps on getting stronger for several years, which indicates that the number of magnesium–oxygen pairs formed, i.e., the concentration of magnesium–oxygen complex donor impurities in the sample, continues to increase, but very slowly, for years. The formation of such magnesium–oxygen pairs has also been found to be able to take place at temperatures as low as ordinary room temperature.
Acknowledgements This work was partially supported by the National Science Council of the Republic of China under contract numbers NSC 88-2112-M-001-002 and NSC 89-2112-M-OO1-034.
References [1] R.K. Franks, J.B. Robertson, Solid State Commun. 5 (1967) 479. [2] L.T. Ho, A.K. Ramdas, Phys. Rev. B 5 (1972) 462. [3] R.K. Crouch, J.B. Robertson, T.E. Gilmer Jr., Phys. Rev. B 5 (1972) 3111. [4] L.T. Ho, Phys. Status Solidi B 210 (1998) 313. [5] C. Jagannath, Z.W. Grabowski, A.K. Ramdas, Phys. Rev. B 23 (1981) 2080. [6] B. Pajot, in: F. Shimura (Ed.), Oxygen in Silicon, Academic Press, San Diego, 1994, p. 200.