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Vibronic spectrum of the U2 isoelectronic center in Si: In
Journal of Luminescence 26 (1982) 227—232 North-Holland Publishing Company
227
VIBRONIC SPECTRUM OF THE U2 ISOELECTRONIC CENTER IN Si:In R.E. STAHLBUSH * and R.A. FORMAN National Bureau of Standards, Washington, D.C. 20234, USA Original manuscript received 3 August 1981 Revised manuscript received 5 November 1981
The photoluminescence spectrum of Si : In measured at 2 and 4 K using samples from several suppliers has been found to be preparation sensitive. In particular, intensity variations allow us to distinguish a sharp no-phonon line at 1.118 eV, variously referred to as U2 or P. and its associated vibronic spectrum from the In(NP) lines and their phonon replicas. Whereas the intensity of the latter did not show preparation sensitivity, the former has been observed to change by three orders of magnitude. The U2 vibronics form a broad-structured spectrum containing density-of-states features. The appearance of phonons other than those conserving crystal momentum demonstrates the exciton is bound to a low symmetry site. In addition, the spectrum includes a peak at 1.109 eV, called R, and a shoulder at 1.107 eV which involves too small an energy loss to be density-of-states related, and these features are most likely modes of the U2 impurity complex. This complex has been tentatively identified as an isoelectronic center composed of an indium—phosphorus nearest-neigbbor substitutional pair.
1. Introduction While a number of reports have been made on the luminescent properties of silicon doped with indium [1—4],no identifications have yet been possible for the numerous lines which appear to be sample dependent. The relative paucity of material suppliers has made the problem more difficult, as it is generally acknowledged that unintentional impurities may play a significant role in forming luminescing centers. In our own studies we have been able to draw upon samples from four different U.S. crystal growth efforts and have examined the line previously described by other authors as either P [4] or U2 [2] and its associated spectral features. Hereafter we will use the designation U2. The luminescence spectrum of indium-doped silicon contains features seen in all samples. The lines at 1.141 eV and 1.144 eV, identified as originating from excitons bound to isolated substitutional indium centers [1], have been *
NBS-NRC Postdoctoral Research Associate.
0022-2313/82/0000—0000/$02.75 © 1982 North-Holland
228
R. E. Stahlbush and R.A. Forman / Vibronic spectrum of LI
2 in Si.~In
seen both in absorption and luminescence [5]. In addition, the luminescence spectrum contains phonon replicas which for this simple center are the momentum-conserving phonons spanning the crystal momentum between the I’ valence band and the i~conduction band minima, and which are found 18.5 and 58.0 meV below the no-phonon lines. The luminescence spectrum is relatively weak, and hence with the limited signal-to-noise available, previous attempts to analyze the remainder of the spectrum in terms of no-phonon lines and their phonon replicas have not been definitive. Attempts at double doping with both boron and indium [6] as well as studies of the effects of electron irradiation and subsequent annealing [7] have been reported; this latter paper also reviews earlier related work. In this paper we report a broad, structured vibromc spectrum associated with the U2 line and comment on earlier interpretations in light of our data.
2. Experimental measurements and results Both float-zone and Czochralski samples were used in this study. Each sample was polished with Syton and/or etched with CP-4. Luminescent excitation was performed using a 200-W super high pressure mercury arc which with appropriate solution, dielectric and glass filters provided wide-band excitation from 280 to 700 nm. The light2 was focused in area withwith an quartz optics forming an image of the arc about 10 mm intensity of a few tenths of a watt. The luminescence from the illuminated surface was collected by a mirror which focused the light through a glass IR-transparent filter and onto the entrance slits of a 3/4 m grating spectrometer. An S- 1 photomultiplier cooled with dry ice was used with photon counting equipment, and the discriminator output was digitally recorded in a signal averager. Samples were immersed in liquid helium at either 4.2 or 2 K in a quartz window cryostat. Variations in the spectral intensity allow us to distinguish features associated with the U 2 line. The variations occur not only from sample to sample but often between successive surface preparations for any given sample. Also, during photoluminescence measurements the relative intensity of lines can be changed by moving the illuminated area of the sample. While these observations are consistent with clustering of the center responsible for the U2 line, our interest in this communication is not to prove clustering but to use spectral intensity variations to demonstrate that there is a broad vibronic spectrum associated with the U2 line. In figs. la—d, we display spectra illustrating the variability. Many of the features noted in earlier work are apparent. Spectrum 1 a consists primarily of lines previously identified as arising from the indium bound exciton In(NP) (no phonon) plus its vibrational satellites, whereas spectrum Id is dominated
ENERGY (eVJ 1.04
1.06
229
1.10
1.06
1.12
1.14
(a)
bi(NP}
)b) 1.5
1.0
010
(c)
a
0I0
a
xio
U
3
Go
010
1.10
1.16
3.14 WAVELENGTH
1.12
1.10
1.06
(pm)
Fig. I. Photoluminescence of Si: In measured at 4 K. In each case the excitation power is the same and the In(NP) line has about the same amplitude. The peak height ratios between U2 (or P) line at 1.118 eV and the In(NP) line at 1.141 eV are the following: (a) 0.07, (b) 1.5, (c) 13 and (d) 66.
230
R.E. Stahibush and R.A. Forpnan
/
Vibronic spectrum of U
2 in Si; In
by the line U2 and vibronic features which clearly scale with it. In this sequence of spectra for which the excitation power is constant, the intensity of the In(NP) line is roughly constant, while the intensity ratio between U2 and In(NP) varies three orders of magnitude, from 0.07 to 70. Interpreting the features below U2 as vibronics is also supported by reported luminescencelifetime measurements which show similar long decay times for the line-like features of the vibronics and the U2 line, and by the essential equality of the thermal activation behavior of these lines [8]. Note the coincidence of the ENERGY (meV) 10
60
50
40
30
20
10
(a)
0 U2
U3
~iIIIX1~$$IJ:~
TOIL) 0)r4 ~ $T0)W)
(bJ
‘~IHI~II
500
400
~
200
100
0
WAVENUMBER (cm-’) Fig. 2. Photoluminescence of the U2 (or P) line and its associated vibronics in Si: In measured at 4 K. Zero energy has been shifted to coincide with U2 and the energy below this line is shown. Two local-mode features, U3 and a shoulder at 1.107, are indicated. No correction for detector response has been made. (b) Two.phonon Rainan spectrum of silicon [10] with the energy scale divided by two. Except for the peak a 260 cm~which is due to single phonon scattering by the optic mode, 0(r), the curve shows the density-of-states features of the phonon spectrum. The line assignments are those given in ref. [I I].
R. E. Stahlbush and R.A. Forman / Vibronic spectrum of U
2 in Si: In
231
indium TO(z~)line with the significantly broader vibronic feature associated with the U2 line. As has been done recently for the case of an unknown center prominent in aluminum-doped silicon [9], this spectrum can be analyzed as a no-phonon line and its associated vibronic spectrum involving primarily density-of-states features. In figs. 2a and b, we have displayed, respectively, the U2 spectrum and the phonon density-of-states spectrum obtained by dividing the energy of the two-phonon Raman spectrum of silicon by two [10]. The appearance of a density-of-states-like spectrum is a clear indication of the breakdown of momentum selection rules. There are normally three ways that such a breakdown occurs, from mosaic strain, from random electric field effects, or from a center of low symmetry. Since the case of mosaic strain or electric field effects would still allow the presence of an undistorted spectrum broadened somewhat by phonon dispersion, the U2 spectrum indicates a low symmetry center. Thus we have spectral verification of an earlier suggestion [7] that U2 is a no-phonon line of an exciton bound to a low symmetry site. In addition to the density-of-states features, the line U3 and a shoulder near 1.107 eV are seen. These lines scale linearly with the density-of-states spectra but appear to be too low in energy to be density-of-states related. Similar features were not seen in the aluminum-doped materials. Based upon earlier calculations of localized impurity modes [12], it appears that these transitions are the modes of a heavy impurity complex and thus further support a characterization of this system as indium related. The broad feature at 1.084 eV is not obvious in the Raman spectrum but is at an energy where impurity activation of acoustic modes (probably LA) [9,13] could produce a peak. A similarly shaped peak was seen at a different energy in the case of the “unknown” center of ref. [9]. We further note that the line U3 is significantly broader than U2 and is not the same line reported in double-doped Si : (In, B) [6]. Again the coincidence of energy position has complicated the spectrum’s interpretation.
3.
Interpretation
The similarity of the centers found in both the indium-doped material and the earlier so-called isoelectromc center seen in Si: Al by the Stuttgart group [3] leads us to return to an interpretation for both of these centers which the Stuttgart group discounted. In all of our materials trace quantities of phosphorus are to be expected and, in fact, are known to exist in some of our samples. We tentatively attribute the U2 line in Si: In as well as the isoelectronic line in Si: Al to nearest neighbor phosphorus—acceptor complexes. Our interpretation is in accord with the earlier data which suggested a correlation between the isoelectromc center in the aluminum-doped material
232
R.E. Stahlbush and R.A. Forman / Vibronic spectrum of U
2 in Si: In
and carbon content. Carbon and phosphorus are often found to be spatially correlated [14]. It has also been determined by infrared measurements that phosphorus will tend to pair at high concentration with boron acceptors forming nearest neighbor complexes [15]. As pointed out in the early work on GaP, one can then consider isoelectromc centers in silicon consisting of a deep acceptor (In or Al) and a shallow donor P [16]. It is chemically reasonable that these centers could be incorporated at growth and not be produced simply by random association. Heat treatments and prior thermal history could then be expected to produce significant variations in the luminescence spectrum. By observing spectral intensity variations, we have identified a vibronic spectrum associated with the U2 no-phonon line. Furthermore, the density-ofstates character of this spectrum demonstrates the low symmetry of the luminescene center which, we argue, is an isoelectronic center consisting of an indium—phosphorus nearest-neighbor substitutional pair.
Acknowledgement We would like to thank Walter Scott of Honeywell for supplying some of the material used in this study. We have recently learned of similar studies by M.L.W. Thewalt, U.O. Ziemeis and R.R. Parsons [17].
References [I] P.J. Dean, J.R. Haynes and W.F. Flood, Phys. Rev. 161 (1967) 711. [2] MA. Vouk and E.C. Lightowlers, J. Lumin. 15 (1977) 357. [3] R. Sauer, W. Schmid and J. Weber, Solid State Commun. 27 (1978) 705. [4] S.A. Lyon, DL. Smith,and T.C. McGill, Phys. Rev. B17 (1978) 2620. [5] K.R. Elliott, G.C. Osbourn, D.L. Smith and T.C. McGill, Phys. Rev. B17 (1978) 1808. [6] U.0. Ziemelis, R.R. Parsons and M. Voos, Solid State Commun. 32 (1979) 445. [7] D.H. Brown and SR. Smith, J. Lumin. 21(1980)329. [8] G.S. Mitchard, S.A. Lyon, K.R. Elliot and T.C. McGill, Solid State Commun. 29(1979) 425. [9] J. Weber, W. Schmid and R. Sauer, Phys. Rev. B2 I (1980) 2401. [10] R.A. Forman and M.I. Bell, Raman Spectrum of 300 tI cm Silicon, unpublished. [II] P.A. Temple and C.E. Hathaway, Phys. Rev. B7 (1973) 3685. [12] PG. Dawber and R.J. Elliott, Proc. Phys. Soc. 81(1963) 453. [13] M. Asche and 0.G. Sarbei, Phys. Stat. Sol. (b) 103 (1981) II. [14] K. Graff, J. Hilgarth and H. Neubrand, Semiconductor Silicon 1977, H.R. Huff and E. Sirtl, eds., Proc. Vol. 77-2, Electrochem. Soc., p. 575. [IS] M. Jouanne, J.F. Morhange and M. Balkanski, Phys. Stat. Sol. (b) 92 (1979) 225. [16] T.N. Morgan, B. Welber and R.N. Barghara, Phys. Rev. 166 (1968) 751. [17] M.L.W. Thewalt, U.0. Ziemelis and R.R. Parsons, Solid State Commun. 39 (1981) 27.
227
VIBRONIC SPECTRUM OF THE U2 ISOELECTRONIC CENTER IN Si:In R.E. STAHLBUSH * and R.A. FORMAN National Bureau of Standards, Washington, D.C. 20234, USA Original manuscript received 3 August 1981 Revised manuscript received 5 November 1981
The photoluminescence spectrum of Si : In measured at 2 and 4 K using samples from several suppliers has been found to be preparation sensitive. In particular, intensity variations allow us to distinguish a sharp no-phonon line at 1.118 eV, variously referred to as U2 or P. and its associated vibronic spectrum from the In(NP) lines and their phonon replicas. Whereas the intensity of the latter did not show preparation sensitivity, the former has been observed to change by three orders of magnitude. The U2 vibronics form a broad-structured spectrum containing density-of-states features. The appearance of phonons other than those conserving crystal momentum demonstrates the exciton is bound to a low symmetry site. In addition, the spectrum includes a peak at 1.109 eV, called R, and a shoulder at 1.107 eV which involves too small an energy loss to be density-of-states related, and these features are most likely modes of the U2 impurity complex. This complex has been tentatively identified as an isoelectronic center composed of an indium—phosphorus nearest-neigbbor substitutional pair.
1. Introduction While a number of reports have been made on the luminescent properties of silicon doped with indium [1—4],no identifications have yet been possible for the numerous lines which appear to be sample dependent. The relative paucity of material suppliers has made the problem more difficult, as it is generally acknowledged that unintentional impurities may play a significant role in forming luminescing centers. In our own studies we have been able to draw upon samples from four different U.S. crystal growth efforts and have examined the line previously described by other authors as either P [4] or U2 [2] and its associated spectral features. Hereafter we will use the designation U2. The luminescence spectrum of indium-doped silicon contains features seen in all samples. The lines at 1.141 eV and 1.144 eV, identified as originating from excitons bound to isolated substitutional indium centers [1], have been *
NBS-NRC Postdoctoral Research Associate.
0022-2313/82/0000—0000/$02.75 © 1982 North-Holland
228
R. E. Stahlbush and R.A. Forman / Vibronic spectrum of LI
2 in Si.~In
seen both in absorption and luminescence [5]. In addition, the luminescence spectrum contains phonon replicas which for this simple center are the momentum-conserving phonons spanning the crystal momentum between the I’ valence band and the i~conduction band minima, and which are found 18.5 and 58.0 meV below the no-phonon lines. The luminescence spectrum is relatively weak, and hence with the limited signal-to-noise available, previous attempts to analyze the remainder of the spectrum in terms of no-phonon lines and their phonon replicas have not been definitive. Attempts at double doping with both boron and indium [6] as well as studies of the effects of electron irradiation and subsequent annealing [7] have been reported; this latter paper also reviews earlier related work. In this paper we report a broad, structured vibromc spectrum associated with the U2 line and comment on earlier interpretations in light of our data.
2. Experimental measurements and results Both float-zone and Czochralski samples were used in this study. Each sample was polished with Syton and/or etched with CP-4. Luminescent excitation was performed using a 200-W super high pressure mercury arc which with appropriate solution, dielectric and glass filters provided wide-band excitation from 280 to 700 nm. The light2 was focused in area withwith an quartz optics forming an image of the arc about 10 mm intensity of a few tenths of a watt. The luminescence from the illuminated surface was collected by a mirror which focused the light through a glass IR-transparent filter and onto the entrance slits of a 3/4 m grating spectrometer. An S- 1 photomultiplier cooled with dry ice was used with photon counting equipment, and the discriminator output was digitally recorded in a signal averager. Samples were immersed in liquid helium at either 4.2 or 2 K in a quartz window cryostat. Variations in the spectral intensity allow us to distinguish features associated with the U 2 line. The variations occur not only from sample to sample but often between successive surface preparations for any given sample. Also, during photoluminescence measurements the relative intensity of lines can be changed by moving the illuminated area of the sample. While these observations are consistent with clustering of the center responsible for the U2 line, our interest in this communication is not to prove clustering but to use spectral intensity variations to demonstrate that there is a broad vibronic spectrum associated with the U2 line. In figs. la—d, we display spectra illustrating the variability. Many of the features noted in earlier work are apparent. Spectrum 1 a consists primarily of lines previously identified as arising from the indium bound exciton In(NP) (no phonon) plus its vibrational satellites, whereas spectrum Id is dominated
ENERGY (eVJ 1.04
1.06
229
1.10
1.06
1.12
1.14
(a)
bi(NP}
)b) 1.5
1.0
010
(c)
a
0I0
a
xio
U
3
Go
010
1.10
1.16
3.14 WAVELENGTH
1.12
1.10
1.06
(pm)
Fig. I. Photoluminescence of Si: In measured at 4 K. In each case the excitation power is the same and the In(NP) line has about the same amplitude. The peak height ratios between U2 (or P) line at 1.118 eV and the In(NP) line at 1.141 eV are the following: (a) 0.07, (b) 1.5, (c) 13 and (d) 66.
230
R.E. Stahibush and R.A. Forpnan
/
Vibronic spectrum of U
2 in Si; In
by the line U2 and vibronic features which clearly scale with it. In this sequence of spectra for which the excitation power is constant, the intensity of the In(NP) line is roughly constant, while the intensity ratio between U2 and In(NP) varies three orders of magnitude, from 0.07 to 70. Interpreting the features below U2 as vibronics is also supported by reported luminescencelifetime measurements which show similar long decay times for the line-like features of the vibronics and the U2 line, and by the essential equality of the thermal activation behavior of these lines [8]. Note the coincidence of the ENERGY (meV) 10
60
50
40
30
20
10
(a)
0 U2
U3
~iIIIX1~$$IJ:~
TOIL) 0)r4 ~ $T0)W)
(bJ
‘~IHI~II
500
400
~
200
100
0
WAVENUMBER (cm-’) Fig. 2. Photoluminescence of the U2 (or P) line and its associated vibronics in Si: In measured at 4 K. Zero energy has been shifted to coincide with U2 and the energy below this line is shown. Two local-mode features, U3 and a shoulder at 1.107, are indicated. No correction for detector response has been made. (b) Two.phonon Rainan spectrum of silicon [10] with the energy scale divided by two. Except for the peak a 260 cm~which is due to single phonon scattering by the optic mode, 0(r), the curve shows the density-of-states features of the phonon spectrum. The line assignments are those given in ref. [I I].
R. E. Stahlbush and R.A. Forman / Vibronic spectrum of U
2 in Si: In
231
indium TO(z~)line with the significantly broader vibronic feature associated with the U2 line. As has been done recently for the case of an unknown center prominent in aluminum-doped silicon [9], this spectrum can be analyzed as a no-phonon line and its associated vibronic spectrum involving primarily density-of-states features. In figs. 2a and b, we have displayed, respectively, the U2 spectrum and the phonon density-of-states spectrum obtained by dividing the energy of the two-phonon Raman spectrum of silicon by two [10]. The appearance of a density-of-states-like spectrum is a clear indication of the breakdown of momentum selection rules. There are normally three ways that such a breakdown occurs, from mosaic strain, from random electric field effects, or from a center of low symmetry. Since the case of mosaic strain or electric field effects would still allow the presence of an undistorted spectrum broadened somewhat by phonon dispersion, the U2 spectrum indicates a low symmetry center. Thus we have spectral verification of an earlier suggestion [7] that U2 is a no-phonon line of an exciton bound to a low symmetry site. In addition to the density-of-states features, the line U3 and a shoulder near 1.107 eV are seen. These lines scale linearly with the density-of-states spectra but appear to be too low in energy to be density-of-states related. Similar features were not seen in the aluminum-doped materials. Based upon earlier calculations of localized impurity modes [12], it appears that these transitions are the modes of a heavy impurity complex and thus further support a characterization of this system as indium related. The broad feature at 1.084 eV is not obvious in the Raman spectrum but is at an energy where impurity activation of acoustic modes (probably LA) [9,13] could produce a peak. A similarly shaped peak was seen at a different energy in the case of the “unknown” center of ref. [9]. We further note that the line U3 is significantly broader than U2 and is not the same line reported in double-doped Si : (In, B) [6]. Again the coincidence of energy position has complicated the spectrum’s interpretation.
3.
Interpretation
The similarity of the centers found in both the indium-doped material and the earlier so-called isoelectromc center seen in Si: Al by the Stuttgart group [3] leads us to return to an interpretation for both of these centers which the Stuttgart group discounted. In all of our materials trace quantities of phosphorus are to be expected and, in fact, are known to exist in some of our samples. We tentatively attribute the U2 line in Si: In as well as the isoelectronic line in Si: Al to nearest neighbor phosphorus—acceptor complexes. Our interpretation is in accord with the earlier data which suggested a correlation between the isoelectromc center in the aluminum-doped material
232
R.E. Stahlbush and R.A. Forman / Vibronic spectrum of U
2 in Si: In
and carbon content. Carbon and phosphorus are often found to be spatially correlated [14]. It has also been determined by infrared measurements that phosphorus will tend to pair at high concentration with boron acceptors forming nearest neighbor complexes [15]. As pointed out in the early work on GaP, one can then consider isoelectromc centers in silicon consisting of a deep acceptor (In or Al) and a shallow donor P [16]. It is chemically reasonable that these centers could be incorporated at growth and not be produced simply by random association. Heat treatments and prior thermal history could then be expected to produce significant variations in the luminescence spectrum. By observing spectral intensity variations, we have identified a vibronic spectrum associated with the U2 no-phonon line. Furthermore, the density-ofstates character of this spectrum demonstrates the low symmetry of the luminescene center which, we argue, is an isoelectronic center consisting of an indium—phosphorus nearest-neighbor substitutional pair.
Acknowledgement We would like to thank Walter Scott of Honeywell for supplying some of the material used in this study. We have recently learned of similar studies by M.L.W. Thewalt, U.O. Ziemeis and R.R. Parsons [17].
References [I] P.J. Dean, J.R. Haynes and W.F. Flood, Phys. Rev. 161 (1967) 711. [2] MA. Vouk and E.C. Lightowlers, J. Lumin. 15 (1977) 357. [3] R. Sauer, W. Schmid and J. Weber, Solid State Commun. 27 (1978) 705. [4] S.A. Lyon, DL. Smith,and T.C. McGill, Phys. Rev. B17 (1978) 2620. [5] K.R. Elliott, G.C. Osbourn, D.L. Smith and T.C. McGill, Phys. Rev. B17 (1978) 1808. [6] U.0. Ziemelis, R.R. Parsons and M. Voos, Solid State Commun. 32 (1979) 445. [7] D.H. Brown and SR. Smith, J. Lumin. 21(1980)329. [8] G.S. Mitchard, S.A. Lyon, K.R. Elliot and T.C. McGill, Solid State Commun. 29(1979) 425. [9] J. Weber, W. Schmid and R. Sauer, Phys. Rev. B2 I (1980) 2401. [10] R.A. Forman and M.I. Bell, Raman Spectrum of 300 tI cm Silicon, unpublished. [II] P.A. Temple and C.E. Hathaway, Phys. Rev. B7 (1973) 3685. [12] PG. Dawber and R.J. Elliott, Proc. Phys. Soc. 81(1963) 453. [13] M. Asche and 0.G. Sarbei, Phys. Stat. Sol. (b) 103 (1981) II. [14] K. Graff, J. Hilgarth and H. Neubrand, Semiconductor Silicon 1977, H.R. Huff and E. Sirtl, eds., Proc. Vol. 77-2, Electrochem. Soc., p. 575. [IS] M. Jouanne, J.F. Morhange and M. Balkanski, Phys. Stat. Sol. (b) 92 (1979) 225. [16] T.N. Morgan, B. Welber and R.N. Barghara, Phys. Rev. 166 (1968) 751. [17] M.L.W. Thewalt, U.0. Ziemelis and R.R. Parsons, Solid State Commun. 39 (1981) 27.