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Carbon centres in β rhombohedral boron
285
Journal of the Less-Common
Metals, 82 (1981) 285 - 289
CARBON
IN fl RHOMBOHEDRAL
CENTRES
BORON*
A. J. NADOLNY Institute of Physics, Polish Academy of Sciences,
Warsaw (Poland)
Summary Carbon is the main impurity in boron, but no “carbon centre” has been identified until now. However, high densities of two kinds of trapping centres for electrons have been found in this material. Therefore we attempted to answer the question whether there is a relation between the carbon atoms and these trapping centres. Photoinduced electron spin resonance excited under special conditions that were chosen so as to obtain the occupation by electrons of nearly all trapping centres existing in the sample was investigated in samples with different carbon contents. The preliminary results indicate that one type of trapping centre may be related to carbon atoms.
1. Introduction Carbon is the main impurity in boron, its amount usually ranging between 50 and 5000 ppm. Attempts have been made for several years to determine the relations between the carbon content and the properties of boron crystals (see, for example, ref. 1). A correlation between the carbon content in boron and electron spin resonance (ESR) has been found [2, 31, but no “carbon centre” has been identified. Gercke and Siems [3] found two ESR lines, called D and E, in carbondoped polycrystalline p rhombohedral boron, their spin density ratio being dependent on carbon content and temperature. Both lines are photoactive and seem to correspond to the photoinduced ESR that we [4] observed in monocrystalline p rhombohedral boron; this was attributed to two trapping centres. The low temperature line E [3] may correspond to traps of type A [4] and the high temperature line D to traps of type B. *Paper presented at the 7th International Symposium Compounds, Uppsala, Sweden, June 9 - 12, 1981. 0022-5088/81/0000-0000/$02.50
on Boron, Borides and Related
6 Elsevier Sequoia/Printed
in The Netherlands
286
The densities of these trapping centres, which were estimated in ref. 4, were of the order of 1Ol9 cmw3 for type A and lOi7 crnm3 for type B. They seemed to be independent of impurity content. Such a conclusion is not well founded, however, for the following reasons. (1) No chemical analysis (mainly for carbon) was carried out on the material under investigation (except for one sample). (2) The impurities were distributed inhomogeneously within the boron rods, and this caused the impurity concentration in a particular sample to differ appreciably from the results of the chemical analysis. (3) There were non-equivalent conditions for the photogeneration of carriers in samples with different thicknesses; this is related to the considerable photoabsorption caused by the exciting light (/i = 930 nm). (We discuss this photoabsorption later. In the previous paper dealing with photoabsorption in boron [5], the photoabsorption was excited only by light with A 6 800 nm.) Gercke and &ems [3] stated that the two centres associated with the lines D and E are not directly related to the carbon content. The experimental evidence for this conclusion also, however, is somewhat doubtful (see ref. 4).
As the problem of carbon centres in p rhombohedral boron still remains open, an attempt was made to elucidate it. For this purpose, investigations of the correlation between the carbon content and the maximum density of photoinduced spins were made. Special attention was paid to 11) an exact determination of the amount of carbon in the samples and (2) the saturation of all trapping centres with electrons.
2. Experimental details
Monocrystalline p rhombohedral boron made by Wacker-Chemie, Munich, and the Research Centre for Crystals, Warsaw, was used. The material from Wacker-Chemie contained not only carbon but also small amounts of other impurities (less than about 1 ppm Mg, Al, Si, Ca, Cu and Cr) [6,7]. The impurities in the crystals from the Research Centre for Crystals are listed in Table 1. TABLE
1
The impurities (with the exception of hydrogen and carbon) in the boron rods produced by the Research Centre for Crystals (determined by a mass spectroscopy analysis) Rod
Elements present I _ 20
B17 826
u~.ppm
N. Si, Cl, Ca, Fe, Mg, Cr, Ni, W N, 0, Cl, K, Ca
20
200 at.ppm
0, K, Na -_
The carbon content was determined by a combustion method using a Leco WR 12 carbon determinator with copper as the combustion accelerator. Small pieces of boron (of mass less than 100 mg) cut from the boron rods in the vicinity of the samples to be investigated were used for this analysis in order to avoid the influence of an inhomogeneous distribution of carbon. Samples used for the photoinduced ESR experiments were in the form of platelets with a thickness of about 0.1 mm and a volume of the order of 1 mm’.
The excitation conditions were chosen so that almost all the traps were occupied by electrons, the following parameters being properly adjusted (see ref. 4) : (1) the wavelength and intensity of illumination; (2) the sample geometry; (3) the temperature; (4) the time of illumination. The samples were placed in a double-walled quartz tube and were cooled with a nitrogen stream to keep their temperature between 150 and 180 K. Both sides of the sample were simultaneously illuminated by lightemitting GaAs diodes (General Electric, type SSL55C) with i.,,, = 940 nm and Ai,,,,, = 50 nm (the photon flux falling onto the sample was about 2 x 10” cm-- ’ s- ‘). (The effects observed using a much stronger excitation with filtered light‘ from a 150 W halogen lamp (830 nm < j. < 1400 nm) were quite similar.) It has been proved that the absorption coefficient for a wavelength of 940 nm (approximately 20 cm-’ without excitation) increases more than fivefold during the excitation. Nevertheless, the penetration of light in the samples was satisfactory because they were thin and illuminated from both sides. The time of excitation was about 20 h. After the excitation the sample was moved and placed in a TE,,,, resonator of an X band ESR spectrometer (Bruker B-ER 4185) inside a double-walled quartz tube cooled by a stream of nitrogen; the sample temperature was kept in the vicinity of 150 K. Special care was taken to avoid an increase in the sample temperature during its replacement. The low temperature ensured that the decay of the number of photoindu~ed spins in the sample before the registration of the ESR signal was almost negligible. The differentiated absorption line was measured under constant experimental conditions for all samples and the number of spins was calculated from a comparison with the integrated intensity of the absorption for Bruker carbon standards.
3. Results and discussion A photoinduced ESR line with g = 2.003 and a linewidth ranging between 30 and 36 G (at T = 150 K) was observed for all samples. An example is shown in Fig. 1. The density Ns of spins generated in three samples using the excitation conditions described is plotted against the
288
density NC of carbon atoms in these samples in Fig. 2. It should be noted that the N, scale is accurate to within a factor of about 2.
A
30G ,
NC
Fig. 1. The ESR signal after optical excitation, density is approximately 3 x 1OL9cmm3.
i1020cm~3)-
recorded at 7’ 5 150 K. The corresponding
spin
Fig. 2. The density of photoinduced spins plotted against the density of carbon atoms for three samples. The samples were cut from the boron rods B18 (Wacker-Chemie), B26 (Research Centre for Crystals) and Bl7 (Research Centre for Crystals) (in a sequence of experimental points from left to right).
From the above preliminary results we can see that the density of traps increases with an increase in carbon content. It is possible that, for a small density of carbon atoms (less than about 10” cme3), one trap corresponds to each carbon atom. The accuracy of the spin density determination, as well as that of the carbon determination, is too small to allow such a precise conclusion, however. For a higher density of carbon atoms (greater than about 10” cm-3) the spin density seems to increase with carbon content at a rate slower than linear. The reason may be physical. However, we cannot exclude that the real density of traps is higher than the measured spin density but that the trap occupation needs stronger excitation than that used in the experiment. As was pointed out in ref. 4, ESR lines can be due to either trapped electrons or excess holes in localized valence states, both these carrier populations being equal. The saturation density of photoinduced spins, however, corresponds well to electron traps of one type; these were called type A in ref. 4. From investigations of the photoinduced ESR and the photoconductivity [4,8], the density of traps of type B is found to be much lower than that of type A and thus traps of type B cannot play any significant role here.
4. Conclusions The results obtained
indicate
that there are electron-trapping
centres
289
which can be related t,o carbon atoms in p rhombohedral boron. As was shown in ref. 4, the corresponding trapping levels lie about 0.4 eV below the band of excited states (the conduction band). The energy needed for trap emptying, however, is spread over 0.1 eV. These traps control the slow relaxation processes observed, for example, in photoconductivity, photoabsorption [5] and the field effect [9], particularly at low temperatures (‘I’ < 300 K) and therefore they are of great importance to the properties of boron crystals. Further investigations on a larger number of samples (particularly with a carbon content below 1000 ppm) carried out under improved experimental conditions would therefore be very useful.
Acknowledgments The author wishes to express his gratitude to Mr. A. Sienkiewicz for his help in the ESR measurements and to Mr. K. Molenda, Institute of Precision Mechanics, Warsaw, for the carbon analysis. Special thanks are due to Mr. J. Jun, Research Centre for Crystals, Warsaw, for supplying the boron samples.
References 1 2 3 4 5 6 7 8 9
W. Klein and D. Geist, 2. Phys., 201 (1967) 411. J. J. Koulmann, N. Perol and P. Taglang, C. R. Acad. Sci., S&r. B, 267 (1968) 15. U. Gercke and C. D. Siems, J. Less-Common Met., 67 (1979) 245. A. J. Nadolny, Phys. Status Solidi B, 65 (1974) 801. H. Werheit, Phys. Status Solidi, 39 (1970) 109. W. Borchert, W. Dietz and H. KGlker, 2. Angew. Phys., 29 (1970) 277. H. Werheit, A. Hausen and H: Binnenbruck, Phys. Status Solidi B, 51 (1972) 115. A. J. Nadolny, A. Zarqba, M. Igalson and N. T. Dung, J. Less-Common Met., 47 (1976) 119. A. Szadkowski, Phys. Status Solidi A, 53 (1979) 95.
Journal of the Less-Common
Metals, 82 (1981) 285 - 289
CARBON
IN fl RHOMBOHEDRAL
CENTRES
BORON*
A. J. NADOLNY Institute of Physics, Polish Academy of Sciences,
Warsaw (Poland)
Summary Carbon is the main impurity in boron, but no “carbon centre” has been identified until now. However, high densities of two kinds of trapping centres for electrons have been found in this material. Therefore we attempted to answer the question whether there is a relation between the carbon atoms and these trapping centres. Photoinduced electron spin resonance excited under special conditions that were chosen so as to obtain the occupation by electrons of nearly all trapping centres existing in the sample was investigated in samples with different carbon contents. The preliminary results indicate that one type of trapping centre may be related to carbon atoms.
1. Introduction Carbon is the main impurity in boron, its amount usually ranging between 50 and 5000 ppm. Attempts have been made for several years to determine the relations between the carbon content and the properties of boron crystals (see, for example, ref. 1). A correlation between the carbon content in boron and electron spin resonance (ESR) has been found [2, 31, but no “carbon centre” has been identified. Gercke and Siems [3] found two ESR lines, called D and E, in carbondoped polycrystalline p rhombohedral boron, their spin density ratio being dependent on carbon content and temperature. Both lines are photoactive and seem to correspond to the photoinduced ESR that we [4] observed in monocrystalline p rhombohedral boron; this was attributed to two trapping centres. The low temperature line E [3] may correspond to traps of type A [4] and the high temperature line D to traps of type B. *Paper presented at the 7th International Symposium Compounds, Uppsala, Sweden, June 9 - 12, 1981. 0022-5088/81/0000-0000/$02.50
on Boron, Borides and Related
6 Elsevier Sequoia/Printed
in The Netherlands
286
The densities of these trapping centres, which were estimated in ref. 4, were of the order of 1Ol9 cmw3 for type A and lOi7 crnm3 for type B. They seemed to be independent of impurity content. Such a conclusion is not well founded, however, for the following reasons. (1) No chemical analysis (mainly for carbon) was carried out on the material under investigation (except for one sample). (2) The impurities were distributed inhomogeneously within the boron rods, and this caused the impurity concentration in a particular sample to differ appreciably from the results of the chemical analysis. (3) There were non-equivalent conditions for the photogeneration of carriers in samples with different thicknesses; this is related to the considerable photoabsorption caused by the exciting light (/i = 930 nm). (We discuss this photoabsorption later. In the previous paper dealing with photoabsorption in boron [5], the photoabsorption was excited only by light with A 6 800 nm.) Gercke and &ems [3] stated that the two centres associated with the lines D and E are not directly related to the carbon content. The experimental evidence for this conclusion also, however, is somewhat doubtful (see ref. 4).
As the problem of carbon centres in p rhombohedral boron still remains open, an attempt was made to elucidate it. For this purpose, investigations of the correlation between the carbon content and the maximum density of photoinduced spins were made. Special attention was paid to 11) an exact determination of the amount of carbon in the samples and (2) the saturation of all trapping centres with electrons.
2. Experimental details
Monocrystalline p rhombohedral boron made by Wacker-Chemie, Munich, and the Research Centre for Crystals, Warsaw, was used. The material from Wacker-Chemie contained not only carbon but also small amounts of other impurities (less than about 1 ppm Mg, Al, Si, Ca, Cu and Cr) [6,7]. The impurities in the crystals from the Research Centre for Crystals are listed in Table 1. TABLE
1
The impurities (with the exception of hydrogen and carbon) in the boron rods produced by the Research Centre for Crystals (determined by a mass spectroscopy analysis) Rod
Elements present I _ 20
B17 826
u~.ppm
N. Si, Cl, Ca, Fe, Mg, Cr, Ni, W N, 0, Cl, K, Ca
20
200 at.ppm
0, K, Na -_
The carbon content was determined by a combustion method using a Leco WR 12 carbon determinator with copper as the combustion accelerator. Small pieces of boron (of mass less than 100 mg) cut from the boron rods in the vicinity of the samples to be investigated were used for this analysis in order to avoid the influence of an inhomogeneous distribution of carbon. Samples used for the photoinduced ESR experiments were in the form of platelets with a thickness of about 0.1 mm and a volume of the order of 1 mm’.
The excitation conditions were chosen so that almost all the traps were occupied by electrons, the following parameters being properly adjusted (see ref. 4) : (1) the wavelength and intensity of illumination; (2) the sample geometry; (3) the temperature; (4) the time of illumination. The samples were placed in a double-walled quartz tube and were cooled with a nitrogen stream to keep their temperature between 150 and 180 K. Both sides of the sample were simultaneously illuminated by lightemitting GaAs diodes (General Electric, type SSL55C) with i.,,, = 940 nm and Ai,,,,, = 50 nm (the photon flux falling onto the sample was about 2 x 10” cm-- ’ s- ‘). (The effects observed using a much stronger excitation with filtered light‘ from a 150 W halogen lamp (830 nm < j. < 1400 nm) were quite similar.) It has been proved that the absorption coefficient for a wavelength of 940 nm (approximately 20 cm-’ without excitation) increases more than fivefold during the excitation. Nevertheless, the penetration of light in the samples was satisfactory because they were thin and illuminated from both sides. The time of excitation was about 20 h. After the excitation the sample was moved and placed in a TE,,,, resonator of an X band ESR spectrometer (Bruker B-ER 4185) inside a double-walled quartz tube cooled by a stream of nitrogen; the sample temperature was kept in the vicinity of 150 K. Special care was taken to avoid an increase in the sample temperature during its replacement. The low temperature ensured that the decay of the number of photoindu~ed spins in the sample before the registration of the ESR signal was almost negligible. The differentiated absorption line was measured under constant experimental conditions for all samples and the number of spins was calculated from a comparison with the integrated intensity of the absorption for Bruker carbon standards.
3. Results and discussion A photoinduced ESR line with g = 2.003 and a linewidth ranging between 30 and 36 G (at T = 150 K) was observed for all samples. An example is shown in Fig. 1. The density Ns of spins generated in three samples using the excitation conditions described is plotted against the
288
density NC of carbon atoms in these samples in Fig. 2. It should be noted that the N, scale is accurate to within a factor of about 2.
A
30G ,
NC
Fig. 1. The ESR signal after optical excitation, density is approximately 3 x 1OL9cmm3.
i1020cm~3)-
recorded at 7’ 5 150 K. The corresponding
spin
Fig. 2. The density of photoinduced spins plotted against the density of carbon atoms for three samples. The samples were cut from the boron rods B18 (Wacker-Chemie), B26 (Research Centre for Crystals) and Bl7 (Research Centre for Crystals) (in a sequence of experimental points from left to right).
From the above preliminary results we can see that the density of traps increases with an increase in carbon content. It is possible that, for a small density of carbon atoms (less than about 10” cme3), one trap corresponds to each carbon atom. The accuracy of the spin density determination, as well as that of the carbon determination, is too small to allow such a precise conclusion, however. For a higher density of carbon atoms (greater than about 10” cm-3) the spin density seems to increase with carbon content at a rate slower than linear. The reason may be physical. However, we cannot exclude that the real density of traps is higher than the measured spin density but that the trap occupation needs stronger excitation than that used in the experiment. As was pointed out in ref. 4, ESR lines can be due to either trapped electrons or excess holes in localized valence states, both these carrier populations being equal. The saturation density of photoinduced spins, however, corresponds well to electron traps of one type; these were called type A in ref. 4. From investigations of the photoinduced ESR and the photoconductivity [4,8], the density of traps of type B is found to be much lower than that of type A and thus traps of type B cannot play any significant role here.
4. Conclusions The results obtained
indicate
that there are electron-trapping
centres
289
which can be related t,o carbon atoms in p rhombohedral boron. As was shown in ref. 4, the corresponding trapping levels lie about 0.4 eV below the band of excited states (the conduction band). The energy needed for trap emptying, however, is spread over 0.1 eV. These traps control the slow relaxation processes observed, for example, in photoconductivity, photoabsorption [5] and the field effect [9], particularly at low temperatures (‘I’ < 300 K) and therefore they are of great importance to the properties of boron crystals. Further investigations on a larger number of samples (particularly with a carbon content below 1000 ppm) carried out under improved experimental conditions would therefore be very useful.
Acknowledgments The author wishes to express his gratitude to Mr. A. Sienkiewicz for his help in the ESR measurements and to Mr. K. Molenda, Institute of Precision Mechanics, Warsaw, for the carbon analysis. Special thanks are due to Mr. J. Jun, Research Centre for Crystals, Warsaw, for supplying the boron samples.
References 1 2 3 4 5 6 7 8 9
W. Klein and D. Geist, 2. Phys., 201 (1967) 411. J. J. Koulmann, N. Perol and P. Taglang, C. R. Acad. Sci., S&r. B, 267 (1968) 15. U. Gercke and C. D. Siems, J. Less-Common Met., 67 (1979) 245. A. J. Nadolny, Phys. Status Solidi B, 65 (1974) 801. H. Werheit, Phys. Status Solidi, 39 (1970) 109. W. Borchert, W. Dietz and H. KGlker, 2. Angew. Phys., 29 (1970) 277. H. Werheit, A. Hausen and H: Binnenbruck, Phys. Status Solidi B, 51 (1972) 115. A. J. Nadolny, A. Zarqba, M. Igalson and N. T. Dung, J. Less-Common Met., 47 (1976) 119. A. Szadkowski, Phys. Status Solidi A, 53 (1979) 95.