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Defects and their influence on the luminescence of rare-earth ions implanted in single crystal Si
Physica B 308–310 (2001) 333–336
Defects and their influence on the luminescence of rare-earth ions implanted in single crystal Si N.A. Sobolev* Ioffe Physicotechnical Institute, Polytechnicheskaya 26, 194021 St. Petersburg, Russia
Abstract With respect to the use of (1 1 1)-oriented silicon substrates, the modification of structural defects as well as electrically and optically active center spectra in solid-phase epitaxial recrystallized rare-earth doped Si layers has been investigated. TEM and X-ray diffraction revealed the presence of microtwins and dislocations in very high densities exceeding 1010 cm2 in (1 1 1) Si : (Er,O). The formation of microtwins and dislocations in very high densities in (1 1 1) samples enabled us to observe some new effects and improve the characteristics of rare-earth doped Si-based lightemitting diodes (LEDs), in particular, to prepare avalanching and tunneling diodes characterized by uniform breakdown over the whole area of p–n junction and working at room temperature. Temperature enhancement of the rare-earth-related electroluminescence (EL) intensity in (1 1 1) LEDs is observed under avalanche and tunnel breakdown of p–n junctions. The enhancement is related to the formation of process-induced deep level centers. New centers emitting under reverse bias and characterized by the highest effective cross section for the excitation of Er3+ ions were first observed. The LEDs with EL of Ho3+ ions at room temperature have firstly been prepared. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Defects; Ion implantation; Rare-earth doped Si; Light-emitting structures
1. Introduction Light-emitting structures based on Er-doped monocrystalline Si have attracted considerable interest for optoelectronic applications. To produce such structures, Polman with co-authors [1] suggested to use the solid phase epitaxial (SPE) technique. In the case of conventional dopant ions, the implantation is carried out in (1 0 0)-oriented substrates at room temperature. SPE recrystallization of layers amorphized by the Er-ion implantation under these conditions is followed by the formation of end-of-range defects and hairpin dislocations, which may be responsible for a relatively low intensity of Er-related luminescence. To prevent the structural defects formation, Franzo et al. [2] suggested implanting Er ions at 77 K. It allowed them to prepare light-emitting diodes (LEDs) which demonstrate a high intensity of Er-related electroluminescence (EL) under *Tel.: +7-812-24-73885; fax: +7-812-24-71017. E-mail address: [email protected]ffe.rssi.ru (N.A. Sobolev).
tunneling breakdown of the p–n junction at room temperature. To prevent the hairpin dislocation formation, we suggested to implant Er ions into (1 1 1)oriented substrates at room temperature some years ago [3]. The purpose of this paper is to review our recent results demonstrating the possibilities of the modification of the SPE technique for the engineering of structural defects and electrically and optically active centers as well as improvements of luminescence properties of rare-earth doped Si-based LEDs.
2. Experimental Er was introduced into 5 O cm (1 1 1)- and (1 0 0)oriented n-Cz–Si wafers with a double implant of 2.0 and 1.6 MeV ions at room temperature. The samples were implanted to a dose of 1 1014, 3 1014 or 9 1014 cm2, and the doses for each pair of implants were identical. To enhance the luminescence response,
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 1 - 3
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N.A. Sobolev / Physica B 308–310 (2001) 333–336
oxygen ions were coimplanted at 0.28 and 0.22 MeV energies, and implant doses were one order of magnitude larger than the erbium doses used. The oxygen implant energies were chosen such that their projected ranges matched those of the Er implants. To fabricate Si : (Ho,O) LEDs, the co-implantation of Ho ions into (1 1 1) n-Cz–Si at 2.0 and 1.6 MeV energies and 1 1014 cm2 dose as well as of oxygen ions at 0.29 and 0.23 MeV energies and 1 1015 cm2 dose was carried out at room temperature. The implantation produced a continuous amorphous layer extending from the surface to a depth of B0.7 mm. Annealings were made in a chlorine-containing atmosphere at 6201C for 1 h to induce the SPE recrystallization of the amorphous layer and at 9001C for 0.5 h to produce optically active Er- and Ho-related centers and reduce the density of regrowth defects. The p++–n+ junction was produced by 40 keV boron implantation at a dose of 5 1015 cm2 into the front side of the wafer. The back side was implanted with phosphorus at 80 keV and a 1 1015 cm2 dose. Titanium and gold sputtering, photolithography, as well as chemical etching, were performed to prepare LEDs with a mesa-like edge contour. SIMS, RBS, TEM, X-ray diffraction, transientstimulated capacitance, capacitance-voltage, photoluminescence (PL) and EL techniques were used to study the properties of Si : (Er,O) structures. Structural, electrical and optical properties in Ho and O ion co-implanted Si were also studied to determine some details of defect formation and luminescence processes.
3. Results TEM and X-ray diffraction techniques were used to study the microstructure of (1 1 1) Si : (Er,O) [4]. TEM showed that the SPE-regrown layer consists of at least two sublayers. The 0.2 mm-thick sublayer lies at the bottom of the regrown layer and contains small looplike defects of high density (so-called end-of-range (EOR) defects). The 0.6 mm-thick sublayer spreads from the band of the EOR defects to the wafer surface. Its defect structure is more complex and consists of microtwins and dislocations. TEM and X-ray diffraction revealed that numerous microtwins formed on the (1 1 1) planes parallel and inclined to the surface are observed over the entire upper sublayer. The twins were found to be platelets with lateral dimensions of 15–30 nm and a thickness of about 2–9 nm, and their densities throughout the regrown layer were nonuniform. Dislocations form a dense spatial network propagating through the upper sublayer. The dislocation lines are rather highly curved than being linear. It does not allow us to identify them as hairpin (V-shaped) dislocations, which are observed in (1 0 0) Si layers recrystallized by SPE following amorphization using Si and Er ions. The
curvature of the dislocation lines can be due to the interaction between the dislocations and microtwins. The dislocation densities observed in the regrown layers were very high, with densities exceeding 1010 cm2. Within the implant fluence range studied, between 1 1014 and 9 1014 Er/cm2, the twin and dislocation densities were observed to increase with fluence, while the twin dimensions were found to decrease. The changes in the spectrum of structural defects in (1 1 1) samples, as compared with the (1 0 0) sample spectrum, allowed us to observe some new effects and improve the characteristics of LEDs. On the basis of (1 1 1) structures, we have prepared avalanching and tunneling diodes (ADs and TDs). The first (1 1 1) ADs and TDs have been investigated in Refs. [3,5], respectively. The EL spectra of the reversebiased samples contain a sharp luminescence line at a wavelength of lB1:538 mm accounted by intra-4f shell transitions of Er3+ ions and a relatively weak radiation, practically independent of the wavelength. This radiation was observed in both avalanche and tunnel breakdown due to intraband transitions of hot carriers in Si. Fig. 1 shows the temperature dependence of the EL intensity at lB1:538 mm (curve 2) in AD. An enhancement of the Er-related EL intensity takes place. On the contrary, a weak temperature quenching of the EL intensity takes place in (1 0 0) Si : (Er,O) diodes [2,6] (Fig. 1, curve 1). Capacitance spectroscopy measurements showed that the unusual temperature dependence of the EL intensity under fixed reverse current is related to the formation of deep level centers (hole traps) in the lower part of the forbidden gap [3]. The effective concentration of the traps are several times higher than the ionized center concentration in the space charge layer before the diode was switched into the breakdown regime. The non-exponential character of the measured capacitance decay may be related to the existence of several levels with different ionization energies or the zone of traps in the lower half. We suggest that the process-induced dislocations are responsible for the
Fig. 1. Temperature dependence of rare-earth-related EL intensity in ADs and TDs at reverse bias in: 1F(1 0 0) Si : Er : O AD [6], 2F(1 1 1) Si : Er : O AD [3], 3F(1 1 1) Si : Er : O TD [7], and 4F(1 1 1) Si : Ho : O AD [10].
N.A. Sobolev / Physica B 308–310 (2001) 333–336
levels. The drop of Er3+-related EL intensity, as the temperature decreases from 190 to 150 K (Fig. 1, curve 2), is associated with the smaller width of the space charge layer and the respective decrease in the concentration of Er-related centers which may be excited. The tunneling (1 1 1) LEDs have been investigated in Refs. [5,7]. Fig. 1 (curve 3) shows the temperature dependence of the EL intensity at lB1:538 mm [7]. A temperature enhancement of the Er-related EL intensity also takes place. The EL intensity as a function of the reverse current density passing through the diode can be described by solving the rate equation accounting for the excitation and deexcitation processes of Er3+ ions: dN =dt ¼ sjðN N Þ=q N =t;
ð1Þ
where N and N are the total and excited amount of Er ions, s is the effective cross section for excitation, t is the lifetime of the excited state 4I13/2, j is the current density passing through the p–n structure, and q is the electron charge. The EL intensity is proportional to N =trad ; where trad is the radiative lifetime of Er3+ ions. At steady state, solving Eq. (1) gives the dependence ELðjÞ=ELmax ¼ ðstj=qÞ=ðstj=q þ 1Þ;
ð2Þ
where ELmax is the maximum EL intensity. From a fit to the data measured in Ref. [8], we have obtained st ¼ 8:7 1020 cm2 s at 300 K. This value is one order of magnitude higher than that obtained by the authors in Ref. [9] for tunneling (1 0 0) diodes. Er-related EL intensity saturation achieved under the avalanche regime at current density is one order of magnitude lower than that under the tunnel regime. In order to separate the contribution of the excitation and deexcitation processes in the increase of st values, it is possible to make independent measurements of the cross section s and lifetime t: The EL transient during the diode turn-on is described by solving Eq. (1) as ELðtÞ ELmax ¼ ½ðstj=qÞ=ðstj=q þ 1Þ f1 exp½ðsj=q þ 1=tÞt g:
ð3Þ
The EL signal approaches the steady state with a characteristic time ton given by 1=ton ¼ sj=q þ 1=t:
ð4Þ
According to Eq. (4), the dependence of (1=ton ) on the reverse current is linear. Its slope is proportional to the excitation cross section of Er3+ ions, s; and its intersection with the ordinate axis is equal to the reciprocal of the lifetime of the Er3+ excited state (1=t). Fig. 2 (curve 2) shows 1=ton as a function of reverse current for AD [8]. A fitting of the experimental time dependencies of EL intensity rise at different current densities with Eq. (4) has given s ¼ 2:3 1016 cm2 and t ¼ 380 ms at 300 K [8]. An increase of s and t by B3.8 times as compared with the
335
Fig. 2. Reciprocal time constant of EL intensity rise (ton ) vs. the difference in the operating ( j) and threshold ( jth ) reverse current densities for centers studied in Si : Er : O ADs and TDs; 1F(1 0 0) TD [9], 2F(1 1 1) AD [8], 3-1 and 3-2F(1 1 1) TD [5].
corresponding data measured in tunneling diodes (Fig. 2, curve 1, stun ¼ 6 1017 cm2 and ttun ¼ 100 ms [8]) takes place. The higher excitation efficiency under the avalanche regime than that under the tunnel breakdown may be due to a change of the energetic spectrum of hot carriers at the changes of the breakdown mechanisms. The study of the kinetics of EL intensity when the reverse current is turned on in (1 1 1) TDs shows that the same kind of Er-related light-emitting centers is introduced in (1 0 0) TDs as well as in (1 1 1) ADs and TDs (Fig. 2, curves 1, 2 and 3-1). In (1 1 1) tunneling LEDs, we have also observed the formation of new kind of centers emitting under reverse bias (Fig. 2, curve 3-2). These centers are characterized by the highest sB7 1016 cm2. Such diodes exhibit the shortest characteristic rise time for Er-related EL in Si : (Er,O) LEDs. The main physical relationships established in defect formation and luminescence in (1 1 1) Si : (Er,O) samples enabled us to prepare the first LEDs with EL of Ho3+ ions at room temperature [10]. The EL related to the internal 4f-shell transitions of Ho3+ ions from the first excited state (5I7) to the ground state (5I8) is observed at 300 K in the range from 1.9 to 2.2 mm [10]. The maximum intensity of the Ho-related EL in Si : (Ho,O) takes place at B1.96 mm. Fig. 1 (curve 4) shows the temperature dependence of the EL intensity at lB1:96 mm [10]. A temperature enhancement of the Ho-related EL intensity is also observed. The comparison of the experimental data concerned with the structural defects and optical properties of (1 1 1) Si : (Er,O) and Si : (Ho,O) LEDs invites us to suppose that electrically and optically active centers belong to the complexes formed near the dislocations
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and containing rare-earth ions and intrinsic point defects.
4. Summary The use of the (1 1 1)-oriented substrates enabled us to modify spectra of structural defects, electrically and optically active centers. The effect of temperature enhancement of the Er- and Ho-related EL intensity is associated with the formation of deep level centers introduced in the lower part of the forbidden gap. The new Er-related light-emitting centers have been observed. They are responsible for the shortest-everrecorded characteristic rise time for Er-related EL in LEDs under breakdown conditions.
Acknowledgements This work was supported in part by the Russian Foundation for Basic Research (Grant No. 99-02-17750) and INTAS (Grant No. 99-01872).
References [1] A. Polman, J.S. Custer, E. Snoeks, G.N. van den Hoven, Appl. Phys. Lett. 62 (1993) 507. [2] G. Franzo, F. Priolo, S. Coffa, A. Polman, A. Carnera, Appl. Phys. Lett. 64 (1994) 2235. [3] A.M. Emel‘yanov, N.A. Sobolev, A.N. Yakimenko, Appl. Phys. Lett. 72 (1998) 1223. [4] R.N. Kyutt, N.A. Sobolev, Yu.A. Nikolaev, V.I. Vdovin, NIMS B173 (2001) 319. [5] A.M. Emel‘yanov, N.A. Sobolev, M.A. Trishenkov, P.E. Khakuashev, Semiconductors 34 (2000) 927. [6] N.A. Sobolev, A.M. Emel‘yanov, K.F. Shtel‘makh, Appl. Phys. Lett. 71 (1997) 1930. [7] N.A. Sobolev, A.M. Emel‘yanov, Yu.A. Nikolaev, Semiconductors 34 (2000) 1027. [8] N.A. Sobolev, Yu.A. Nikolaev, A.M. Emel‘yanov, K.F. Shtel‘makh, P.E. Khakuashev, M.A. Trishenkov, J. Lumin. 80 (1999) 315. [9] G. Franzo, S. Coffa, F. Priolo, J. Appl. Phys. 81 (1997) 2784. [10] N.A. Sobolev, A.M. Emel‘yanov, R.N. Kyutt, Yu.A. Nikolaev, Solid State Phenom. 69–70 (1999) 371.
Defects and their influence on the luminescence of rare-earth ions implanted in single crystal Si N.A. Sobolev* Ioffe Physicotechnical Institute, Polytechnicheskaya 26, 194021 St. Petersburg, Russia
Abstract With respect to the use of (1 1 1)-oriented silicon substrates, the modification of structural defects as well as electrically and optically active center spectra in solid-phase epitaxial recrystallized rare-earth doped Si layers has been investigated. TEM and X-ray diffraction revealed the presence of microtwins and dislocations in very high densities exceeding 1010 cm2 in (1 1 1) Si : (Er,O). The formation of microtwins and dislocations in very high densities in (1 1 1) samples enabled us to observe some new effects and improve the characteristics of rare-earth doped Si-based lightemitting diodes (LEDs), in particular, to prepare avalanching and tunneling diodes characterized by uniform breakdown over the whole area of p–n junction and working at room temperature. Temperature enhancement of the rare-earth-related electroluminescence (EL) intensity in (1 1 1) LEDs is observed under avalanche and tunnel breakdown of p–n junctions. The enhancement is related to the formation of process-induced deep level centers. New centers emitting under reverse bias and characterized by the highest effective cross section for the excitation of Er3+ ions were first observed. The LEDs with EL of Ho3+ ions at room temperature have firstly been prepared. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Defects; Ion implantation; Rare-earth doped Si; Light-emitting structures
1. Introduction Light-emitting structures based on Er-doped monocrystalline Si have attracted considerable interest for optoelectronic applications. To produce such structures, Polman with co-authors [1] suggested to use the solid phase epitaxial (SPE) technique. In the case of conventional dopant ions, the implantation is carried out in (1 0 0)-oriented substrates at room temperature. SPE recrystallization of layers amorphized by the Er-ion implantation under these conditions is followed by the formation of end-of-range defects and hairpin dislocations, which may be responsible for a relatively low intensity of Er-related luminescence. To prevent the structural defects formation, Franzo et al. [2] suggested implanting Er ions at 77 K. It allowed them to prepare light-emitting diodes (LEDs) which demonstrate a high intensity of Er-related electroluminescence (EL) under *Tel.: +7-812-24-73885; fax: +7-812-24-71017. E-mail address: [email protected]ffe.rssi.ru (N.A. Sobolev).
tunneling breakdown of the p–n junction at room temperature. To prevent the hairpin dislocation formation, we suggested to implant Er ions into (1 1 1)oriented substrates at room temperature some years ago [3]. The purpose of this paper is to review our recent results demonstrating the possibilities of the modification of the SPE technique for the engineering of structural defects and electrically and optically active centers as well as improvements of luminescence properties of rare-earth doped Si-based LEDs.
2. Experimental Er was introduced into 5 O cm (1 1 1)- and (1 0 0)oriented n-Cz–Si wafers with a double implant of 2.0 and 1.6 MeV ions at room temperature. The samples were implanted to a dose of 1 1014, 3 1014 or 9 1014 cm2, and the doses for each pair of implants were identical. To enhance the luminescence response,
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 6 9 1 - 3
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N.A. Sobolev / Physica B 308–310 (2001) 333–336
oxygen ions were coimplanted at 0.28 and 0.22 MeV energies, and implant doses were one order of magnitude larger than the erbium doses used. The oxygen implant energies were chosen such that their projected ranges matched those of the Er implants. To fabricate Si : (Ho,O) LEDs, the co-implantation of Ho ions into (1 1 1) n-Cz–Si at 2.0 and 1.6 MeV energies and 1 1014 cm2 dose as well as of oxygen ions at 0.29 and 0.23 MeV energies and 1 1015 cm2 dose was carried out at room temperature. The implantation produced a continuous amorphous layer extending from the surface to a depth of B0.7 mm. Annealings were made in a chlorine-containing atmosphere at 6201C for 1 h to induce the SPE recrystallization of the amorphous layer and at 9001C for 0.5 h to produce optically active Er- and Ho-related centers and reduce the density of regrowth defects. The p++–n+ junction was produced by 40 keV boron implantation at a dose of 5 1015 cm2 into the front side of the wafer. The back side was implanted with phosphorus at 80 keV and a 1 1015 cm2 dose. Titanium and gold sputtering, photolithography, as well as chemical etching, were performed to prepare LEDs with a mesa-like edge contour. SIMS, RBS, TEM, X-ray diffraction, transientstimulated capacitance, capacitance-voltage, photoluminescence (PL) and EL techniques were used to study the properties of Si : (Er,O) structures. Structural, electrical and optical properties in Ho and O ion co-implanted Si were also studied to determine some details of defect formation and luminescence processes.
3. Results TEM and X-ray diffraction techniques were used to study the microstructure of (1 1 1) Si : (Er,O) [4]. TEM showed that the SPE-regrown layer consists of at least two sublayers. The 0.2 mm-thick sublayer lies at the bottom of the regrown layer and contains small looplike defects of high density (so-called end-of-range (EOR) defects). The 0.6 mm-thick sublayer spreads from the band of the EOR defects to the wafer surface. Its defect structure is more complex and consists of microtwins and dislocations. TEM and X-ray diffraction revealed that numerous microtwins formed on the (1 1 1) planes parallel and inclined to the surface are observed over the entire upper sublayer. The twins were found to be platelets with lateral dimensions of 15–30 nm and a thickness of about 2–9 nm, and their densities throughout the regrown layer were nonuniform. Dislocations form a dense spatial network propagating through the upper sublayer. The dislocation lines are rather highly curved than being linear. It does not allow us to identify them as hairpin (V-shaped) dislocations, which are observed in (1 0 0) Si layers recrystallized by SPE following amorphization using Si and Er ions. The
curvature of the dislocation lines can be due to the interaction between the dislocations and microtwins. The dislocation densities observed in the regrown layers were very high, with densities exceeding 1010 cm2. Within the implant fluence range studied, between 1 1014 and 9 1014 Er/cm2, the twin and dislocation densities were observed to increase with fluence, while the twin dimensions were found to decrease. The changes in the spectrum of structural defects in (1 1 1) samples, as compared with the (1 0 0) sample spectrum, allowed us to observe some new effects and improve the characteristics of LEDs. On the basis of (1 1 1) structures, we have prepared avalanching and tunneling diodes (ADs and TDs). The first (1 1 1) ADs and TDs have been investigated in Refs. [3,5], respectively. The EL spectra of the reversebiased samples contain a sharp luminescence line at a wavelength of lB1:538 mm accounted by intra-4f shell transitions of Er3+ ions and a relatively weak radiation, practically independent of the wavelength. This radiation was observed in both avalanche and tunnel breakdown due to intraband transitions of hot carriers in Si. Fig. 1 shows the temperature dependence of the EL intensity at lB1:538 mm (curve 2) in AD. An enhancement of the Er-related EL intensity takes place. On the contrary, a weak temperature quenching of the EL intensity takes place in (1 0 0) Si : (Er,O) diodes [2,6] (Fig. 1, curve 1). Capacitance spectroscopy measurements showed that the unusual temperature dependence of the EL intensity under fixed reverse current is related to the formation of deep level centers (hole traps) in the lower part of the forbidden gap [3]. The effective concentration of the traps are several times higher than the ionized center concentration in the space charge layer before the diode was switched into the breakdown regime. The non-exponential character of the measured capacitance decay may be related to the existence of several levels with different ionization energies or the zone of traps in the lower half. We suggest that the process-induced dislocations are responsible for the
Fig. 1. Temperature dependence of rare-earth-related EL intensity in ADs and TDs at reverse bias in: 1F(1 0 0) Si : Er : O AD [6], 2F(1 1 1) Si : Er : O AD [3], 3F(1 1 1) Si : Er : O TD [7], and 4F(1 1 1) Si : Ho : O AD [10].
N.A. Sobolev / Physica B 308–310 (2001) 333–336
levels. The drop of Er3+-related EL intensity, as the temperature decreases from 190 to 150 K (Fig. 1, curve 2), is associated with the smaller width of the space charge layer and the respective decrease in the concentration of Er-related centers which may be excited. The tunneling (1 1 1) LEDs have been investigated in Refs. [5,7]. Fig. 1 (curve 3) shows the temperature dependence of the EL intensity at lB1:538 mm [7]. A temperature enhancement of the Er-related EL intensity also takes place. The EL intensity as a function of the reverse current density passing through the diode can be described by solving the rate equation accounting for the excitation and deexcitation processes of Er3+ ions: dN =dt ¼ sjðN N Þ=q N =t;
ð1Þ
where N and N are the total and excited amount of Er ions, s is the effective cross section for excitation, t is the lifetime of the excited state 4I13/2, j is the current density passing through the p–n structure, and q is the electron charge. The EL intensity is proportional to N =trad ; where trad is the radiative lifetime of Er3+ ions. At steady state, solving Eq. (1) gives the dependence ELðjÞ=ELmax ¼ ðstj=qÞ=ðstj=q þ 1Þ;
ð2Þ
where ELmax is the maximum EL intensity. From a fit to the data measured in Ref. [8], we have obtained st ¼ 8:7 1020 cm2 s at 300 K. This value is one order of magnitude higher than that obtained by the authors in Ref. [9] for tunneling (1 0 0) diodes. Er-related EL intensity saturation achieved under the avalanche regime at current density is one order of magnitude lower than that under the tunnel regime. In order to separate the contribution of the excitation and deexcitation processes in the increase of st values, it is possible to make independent measurements of the cross section s and lifetime t: The EL transient during the diode turn-on is described by solving Eq. (1) as ELðtÞ ELmax ¼ ½ðstj=qÞ=ðstj=q þ 1Þ f1 exp½ðsj=q þ 1=tÞt g:
ð3Þ
The EL signal approaches the steady state with a characteristic time ton given by 1=ton ¼ sj=q þ 1=t:
ð4Þ
According to Eq. (4), the dependence of (1=ton ) on the reverse current is linear. Its slope is proportional to the excitation cross section of Er3+ ions, s; and its intersection with the ordinate axis is equal to the reciprocal of the lifetime of the Er3+ excited state (1=t). Fig. 2 (curve 2) shows 1=ton as a function of reverse current for AD [8]. A fitting of the experimental time dependencies of EL intensity rise at different current densities with Eq. (4) has given s ¼ 2:3 1016 cm2 and t ¼ 380 ms at 300 K [8]. An increase of s and t by B3.8 times as compared with the
335
Fig. 2. Reciprocal time constant of EL intensity rise (ton ) vs. the difference in the operating ( j) and threshold ( jth ) reverse current densities for centers studied in Si : Er : O ADs and TDs; 1F(1 0 0) TD [9], 2F(1 1 1) AD [8], 3-1 and 3-2F(1 1 1) TD [5].
corresponding data measured in tunneling diodes (Fig. 2, curve 1, stun ¼ 6 1017 cm2 and ttun ¼ 100 ms [8]) takes place. The higher excitation efficiency under the avalanche regime than that under the tunnel breakdown may be due to a change of the energetic spectrum of hot carriers at the changes of the breakdown mechanisms. The study of the kinetics of EL intensity when the reverse current is turned on in (1 1 1) TDs shows that the same kind of Er-related light-emitting centers is introduced in (1 0 0) TDs as well as in (1 1 1) ADs and TDs (Fig. 2, curves 1, 2 and 3-1). In (1 1 1) tunneling LEDs, we have also observed the formation of new kind of centers emitting under reverse bias (Fig. 2, curve 3-2). These centers are characterized by the highest sB7 1016 cm2. Such diodes exhibit the shortest characteristic rise time for Er-related EL in Si : (Er,O) LEDs. The main physical relationships established in defect formation and luminescence in (1 1 1) Si : (Er,O) samples enabled us to prepare the first LEDs with EL of Ho3+ ions at room temperature [10]. The EL related to the internal 4f-shell transitions of Ho3+ ions from the first excited state (5I7) to the ground state (5I8) is observed at 300 K in the range from 1.9 to 2.2 mm [10]. The maximum intensity of the Ho-related EL in Si : (Ho,O) takes place at B1.96 mm. Fig. 1 (curve 4) shows the temperature dependence of the EL intensity at lB1:96 mm [10]. A temperature enhancement of the Ho-related EL intensity is also observed. The comparison of the experimental data concerned with the structural defects and optical properties of (1 1 1) Si : (Er,O) and Si : (Ho,O) LEDs invites us to suppose that electrically and optically active centers belong to the complexes formed near the dislocations
336
N.A. Sobolev / Physica B 308–310 (2001) 333–336
and containing rare-earth ions and intrinsic point defects.
4. Summary The use of the (1 1 1)-oriented substrates enabled us to modify spectra of structural defects, electrically and optically active centers. The effect of temperature enhancement of the Er- and Ho-related EL intensity is associated with the formation of deep level centers introduced in the lower part of the forbidden gap. The new Er-related light-emitting centers have been observed. They are responsible for the shortest-everrecorded characteristic rise time for Er-related EL in LEDs under breakdown conditions.
Acknowledgements This work was supported in part by the Russian Foundation for Basic Research (Grant No. 99-02-17750) and INTAS (Grant No. 99-01872).
References [1] A. Polman, J.S. Custer, E. Snoeks, G.N. van den Hoven, Appl. Phys. Lett. 62 (1993) 507. [2] G. Franzo, F. Priolo, S. Coffa, A. Polman, A. Carnera, Appl. Phys. Lett. 64 (1994) 2235. [3] A.M. Emel‘yanov, N.A. Sobolev, A.N. Yakimenko, Appl. Phys. Lett. 72 (1998) 1223. [4] R.N. Kyutt, N.A. Sobolev, Yu.A. Nikolaev, V.I. Vdovin, NIMS B173 (2001) 319. [5] A.M. Emel‘yanov, N.A. Sobolev, M.A. Trishenkov, P.E. Khakuashev, Semiconductors 34 (2000) 927. [6] N.A. Sobolev, A.M. Emel‘yanov, K.F. Shtel‘makh, Appl. Phys. Lett. 71 (1997) 1930. [7] N.A. Sobolev, A.M. Emel‘yanov, Yu.A. Nikolaev, Semiconductors 34 (2000) 1027. [8] N.A. Sobolev, Yu.A. Nikolaev, A.M. Emel‘yanov, K.F. Shtel‘makh, P.E. Khakuashev, M.A. Trishenkov, J. Lumin. 80 (1999) 315. [9] G. Franzo, S. Coffa, F. Priolo, J. Appl. Phys. 81 (1997) 2784. [10] N.A. Sobolev, A.M. Emel‘yanov, R.N. Kyutt, Yu.A. Nikolaev, Solid State Phenom. 69–70 (1999) 371.