- Email: [email protected]
Spectral-kinetic study of self-trapping and multiplication of electronic excitations in Al2O3 crystals
Journal of Electron Spectroscopy and Related Phenomena 101–103 (1999) 587–591
Spectral-kinetic study of self-trapping and multiplication of electronic excitations in Al 2 O 3 crystals a, a a,b a a c A. Lushchik *, E. Feldbach , M. Kirm , P. Liblik , Ch. Lushchik , I. Martinson , F. Savikhin a , G. Zimmerer b a
b
Institute of Physics, University of Tartu, Riia 142, EE 2400 Tartu, Estonia II. Institute of Experimental Physics, Hamburg University, Luruper Chaussee 149, D 22761 Hamburg, Germany c Department of Physics, Lund University, Professorsgatan 1, S 22100 Lund, Sweden
Abstract The luminescence spectra and the excitation spectra of intrinsic (7.6 and 3.77 eV) and extrinsic emissions for a-Al 2 O 3 , Al 2 O 3 :Sc and Al 2 O 3 :Ga crystals have been measured by means of synchrotron radiation of 4–40 eV at 8 and 80 K. The luminescence of self-trapped excitons (7.6 eV) can be excited at the direct photocreation of excitons or at the recombination of electrons with unrelaxed holes. Fast intraband luminescence (t ,2 ns) has been detected in a spectral region from 2.5 to 6.5 eV. The absorption of one photon of 25–37 eV leads to the formation of two or three electron-hole pairs causing a sharp increase of the efficiency of extrinsic emission of Sc 31 (5.6 eV), Ga 31 (4.6 eV) and F 1 centers (3.85 eV). 1999 Elsevier Science B.V. All rights reserved. Keywords: Exciton; Self-trapping; Luminescence; Synchrotron radiation; Al 2 O 3
1. Introduction In a number of dielectric materials VUV radiation is needed for the creation of intrinsic electronic excitations (IEEs). In some wide-gap crystals even the luminescence appearing after vibrational relaxation of IEEs lies in the VUV spectral region. This luminescence is the luminescence of free excitons in rare gas solids, in several alkali halide crystals and some metal oxides (BeO, MgO, CaO). The luminescence of self-trapped excitons (STEs) in rare gas solids and in a number of wide-gap oxides is located in the VUV region as well. By means of lowtemperature VUV spectroscopy with time resolution *Corresponding author. Tel.: 1372-7-428-946; fax: 1372-7-383033. E-mail address: [email protected] (A. Lushchik)
we made an attempt to study the IEEs in pure and doped a-Al 2 O 3 single crystals and ceramics that are widely used in various applications. The processes of IEE self-trapping and multiplication, studied earlier for rare gas solids and other ionic crystals [1–3], were of special interest.
2. Experimental The a-Al 2 O 3 single crystals grown by the Czochralski technique were the main objects of the present study. In our a-Al 2 O 3 samples, in the region of extrinsic absorption there are no selective absorption bands (the value of the absorption constant is less than 1–2 cm 21 at 295 K) [4]. The Al 2 O 3 :Sc (about 150 or 600 ppm of Sc 31 impurity ions [5]) and
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00337-5
588
A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591
Al 2 O 3 :Ga crystals and ceramics were also investigated. The spectra of steady cathodoluminescence were recorded through a double vacuum monochromator [6] in the region 1.8–9.0 eV using crystal excitation by 6 keV electrons at 8–300 K. The fast emissions excited by single powerful electron pulses (t 53 ns, E5300 keV, the current density 1–150 A cm 22 ) were detected through a monochromator by a system consisting of a photomultiplier, a fast oscilloscope and a telecamera [7]. The photoluminescence experiments were carried out at beamline 52 in the MAX I Laboratory in Lund and at the SUPERLUMI station of HASYLAB at DESY, Hamburg. The experimental set-ups are described in Refs. [2,5]. The excitation spectra were normalized to equal quantum intensities of synchrotron radiation (SR) falling onto the crystal. The excitation spectra were measured for time-integrated (steady) luminescence, as well as for the emission detected within a time window (length Dt) correlated with the excitation pulses of SR (delayed by dt). The delay dt and the length Dt were varied between 0.6 to 20 and 1.5 to 50 ns, respectively. The excitation spectra were measured at SR incidence angle of 17.58 (DESY) or 208 (MAX I). In case of electron excitation an electron beam was oriented at an angle 458 to a sample surface.
(Dt55 ns, dt51.5 ns), that does not have selective excitation bands from 4.5 to 8.8 eV, i.e. out of the region of IEE formation. This fast emission can be effectively excited at the photocreation of excitons (E'c) or due to the recombination of electron-hole (e-h) pairs. The main experimental results are presented in Ref. [5]. Below, we will present new additional data on the intrinsic and impurity luminescence of Al 2 O 3 . Fig. 1 shows steady luminescence spectra for a number of Al 2 O 3 single crystals in case of excitation by 6 keV electrons (current density 1mA mm 22 ). The broad luminescence band with the maximum at E I 5 7.6 eV and half-width d 50.8 eV dominates in the emission spectrum for an Al 2 O 3 crystal of high purity and perfection. The heating of the sample from 8 to 295 K causes the decrease of the 7.6 eV emission intensity by two orders of magnitude. It has been recently shown [5] that the 7.6 eV emission intensity depends superlinearly (approximately quadratically) on the density of excitation by nanosecond electron pulses at 8, 80 or 295 K. In Al 2 O 3 :Sc (600 ppm), the broad band E I 55.6 eV dominates in the cathodoluminescence spectrum at 80 and 295 K, while the 7.6 eV STE luminescence is weakened. The appearance of 5.6 eV emission is connected with the Sc 31 impurity centers. The efficiency of Sc 31
3. Results The reflection spectra have been measured for the electric field E of incident SR (6–40 eV) parallel or perpendicular to the optic axis c of a uniaxial Al 2 O 3 single crystal [8]. At 10 K, the long-wavelength edge of the absorption at 8.85–8.95 eV is caused by the Urbach tail of exciton absorption for Eic [8]. The efficiency of 7.6 eV STE emission (the so-called STE A emission) is extremely high just in the region of 8.85–9.1 eV [5,9,10]. The emission with the maximum at 3.8 eV was tentatively ascribed to another configuration of STEs in Al 2 O 3 [10]. However, the emission of F 1 centers lies also in the region of 3.8 eV. Therefore, it is difficult to separate the emission of F 1 centers and the supposed luminescence of STEs at 3.8 eV. For the first time we succeeded in detecting the fast component of 3.8 eV emission
Fig. 1. Spectra of steady luminescence of Al 2 O 3 , Al 2 O 3 :Ga and Al 2 O 3 :Sc (600 ppm) crystals on the excitation by 6 keV electrons at 8 K (solid lines), 80 K (dashed lines) and 300 K (dotted line). Spectrum of the decay time t for Al 2 O 3 emission under irradiation by single nanosecond pulses of 300 keV electrons at 80 K.
A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591
center emission h 5.6 is extremely high at the direct excitation of Sc 31 by 8.5 eV photons. The 5.6 eV emission does not undergo the thermal quenching up to 295 K. The presence of Ga 31 centers in Al 2 O 3 crystals leads to the appearance of the additional emission in the region 4–5 eV. The STE A emission is dominant in the emission spectrum under the excitation of highly pure Al 2 O 3 crystals by single nanosecond pulses of 300 keV electrons with the penetration depth into a crystal about 300 mm. A single electron pulse (t 53 ns) forms about 10 18 e-h pairs in 1 cm 3 of a crystal and mainly provides the appearance of recombination STE emission of 7.6 eV. The values of decay time t for Al 2 O 3 emission registered at 80 K are depicted also in Fig. 1. We obtained the value t 522 ns in the spectral region from 6.5 to 7.6 eV (the limit of our registration system) where the VUV emission of STEs is dominating. The continuous weak emission with t ,2 ns was detected in the region of 4.2–6.0 eV. Similar to other wide-gap dielectrics [7] this temperature-independent emission can be tentatively interpreted as the intraband luminescence of Al 2 O 3 due to the radiative transitions of hot electrons and holes inside of a conduction and a valence band. Fig. 2 presents the excitation spectra of 3.8 and 7.6 eV fast emissions measured at the exciting photon energies of 8 to 10 eV for two different crystals of Al 2 O 3 at 8 K. The arrows indicate the maxima of the exciton doublet in the reflection spectra measured for Eic (R i ) and E'c (R ' ) [8]. The efficiency of fast 7.6 eV emission h 7.6 is extremely high in the region of a long-wavelength component of the exciton doublet (8.9–9.1 eV) while the value of h 7.6 is lower in the region of a shortwavelength doublet component (9.1 to 9.3 eV). In the region of band-to-band transitions (hn .Eg 59.4 eV) the value of h 7.6 is by tens of times lower than at the direct optical formation of excitons. A highdensity excitation (e.g. an intensive line of the krypton discharge, hn 510 eV) is needed in order to obtain the intensive recombination luminescence of 7.6 eV. According to Fig. 2 the fast 3.8 eV luminescence of STEs can be effectively excited in the region of a short-wavelength component of the exciton doublet and at the band-to-band transitions. A typical decay curve of STE A emission for Al 2 O 3 excited by 8.9860.02 eV photons at 8 K is
589
Fig. 2. Time-resolved excitation spectra of STE emissions for two Al 2 O 3 crystals at 8 K. Al 2 O 3 no. 1 — fast emissions (Dt58 ns, dt51.4 ns) of 7.6 eV (11) and 3.8 eV (ss); Al 2 O 3 no. 2 — time-integrated (– –) and fast 7.6 eV emissions (Dt58 ns, dt51.4 ns — curve dd and Dt520 ns, dt59.8 ns — solid curve). The inset shows the decay curve of 7.6 eV emission in Al 2 O 3 (no. 2) excited by 8.98 eV photons at 8 K.
depicted in the inset of Fig. 2. The intensity of the 7.6 eV emission decreases by three orders of magnitude within 50 ns. The excitation spectra for A emission detected within two time windows (Dt58 ns, dt51.4 ns and Dt520 ns, dt59.8 ns) or in time-integrated regime differ from each other very slightly. Fig. 3 shows the excitation spectra of 5.6 eV steady emission measured for Al 2 O 3 :Sc (600 ppm) at 31 8 and 295 K. The emission of Sc centers can be efficiently excited by 7.8 to 8.8 eV photons that directly generate near-impurity electronic excitations. The photocreation of excitons or e-h pairs leads also to the appearance of 5.6 eV emission. The value of h 5.6 is high in the region of 9.4–10.5 eV and 25–37 eV where the values of absorption constant do not exceed k53310 4 cm 21 [8]. However, in the region of 12–24 eV the value of k reaches 10 5 –10 6 cm 21 , the penetration depth of exciting photons into a crystal is r#0.1 mm and the near-surface losses for impurity emissions are high and strongly depend on the surface quality. The value of k for 10.5 and 30 eV photons are practically equal [8], while the value of h sharply increases (at least doubles) from 10 to 30 eV for the 5.6 eV emission in Al 2 O 3 :Sc and 4.6
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A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591
Fig. 3. Excitation spectra of steady emission of Sc 31 centers (5.6 eV, curves 1 and 2) and Ga 31 centers (4.6 eV, curve 3) for Al 2 O 3 :Sc (600 ppm) and Al 2 O 3 :Ga crystals at 8 and 300 K (curve 2). A fragment of the absorption spectrum for Al 2 O 3 crystal (E'c) [8].
eV emission in Al 2 O 3 :Ga. So, we interpret this sharp increase of the emission efficiency as the process of multiplication of electronic excitations (MEE) in Al 2 O 3 . An analysis of the above-mentioned excitation spectra (and these in Ref. [5]) for various emissions allows us to conclude that the absorption of one photon of 26–29 eV leads to the creation of two e-h pairs, while one photon of 30 to 37 eV creates up to three e-h pairs.
in the conduction band is able to form the second e-h pair, while the Auger recombination of an electron from 2p 6 O 22 with a 2s 2 O 22 hole provides the creation of the third e-h pair. The experimentally revealed quadratic dependence of STE A emission intensity on the excitation density in a-Al 2 O 3 [5] is of special interest. Such behavior of 7.6 eV emission can be explained by a small value of an effective cross-section s eh for radiative e-h recombination that causes the appearance of this luminescence. The value of s eh is significantly smaller than that for the recombination of free electrons with self-trapped holes (VK centers) in alkali iodide crystals. It is necessary to mention that the intensity of STE emission in KI linearly depends on the density of electron current [5]. Considering the results of the investigation of edge emissions in semiconductors, we suggest that the small value of s eh in Al 2 O 3 testifies to the recombination of free electrons and free (unrelaxed) holes. Similar to a Y 2 O 3 crystal (see Ref. [12]), an electron and a hole in Al 2 O 3 do not separately undergo self-trapping while the sum of their deformation potentials is sufficient for the formation of a self-shrunk STE (SSTEs). Our hypothesis is based on the Sumi diagram for the possible interaction of IEEs with the field of acoustic phonons [13], that allows the existence of such SSTEs. Further investigation of the process of exciton self-trapping in uniaxial a-Al 2 O 3 is needed.
4. Discussion The simplified energy band diagram of an aAl 2 O 3 crystal, based on theoretical calculations [11] and experimentally determined values of Eg and the width for 2p 6 (DE57–9 eV) and 2s 2 oxygen valence bands (DE53–4 eV), was suggested in Ref. [5] for the interpretation of the MEE process. The energy of a hot conduction electron, formed after the absorption of one photon of 26–29 eV, is sufficient to create the second e-h pair while a hot hole in the lower part of 2p 6 O 22 valence band gains a smaller part of the photon energy and is not able to form a secondary electronic excitation. The value of the threshold energy of this MEE process Et 525 eV significantly exceeds 2Eg . If the energy of the absorbed photon equals 30–37 eV, the ionization process starts from the 2s 2 shell of an oxygen ion. A hot photoelectron
Acknowledgements This work has been partly supported by the Estonian Science Foundation (Grant 1931), the Crafoord Foundation (Sweden) and the STINT Foundation (Sweden).
References [1] G. Zimmerer, J. Low Temp. Phys. 111 (1998) 629. [2] A. Lushchik, E. Feldbach, R. Kink, Ch. Lushchik, M. Kirm, I. Martinson, Phys. Rev. B 53 (1996) 5379. [3] M. Kirm, E. Feldbach, R. Kink, A. Lushchik, A. Maaroos, I. Martinson, J. Electron Spectrosc. Relat. Phenom. 79 (1996) 91.
A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591 [4] V.N. Abramov, B.G. Ivanov, A.I. Kuznetsov, I.A. Meriloo, M.I. Musatov, Phys. Status Solidi A 48 (1978) 287. [5] M. Kirm, G. Zimmerer, E. Feldbach, A. Lushchik, Ch. Lushchik, F. Savikhin, Phys. Rev. B, in press. [6] I. Kuusmann, P. Liblik, R. Mugur, V. Tiit, E. Feldbach, R. Shatskina, J. Edula, Trudy Inst. Fiz. Akad. Nauk Estonian SSR 51 (1981) 57. [7] K.U. Ibragimov, F.A. Savikhin, Sov. Phys. Solid State (USA) 35 (1993) 744. [8] T. Tomiki, Y. Ganaha, T. Shikenbaru, T. Futemma, M. Yuri, Y. Aiura, S. Sato, H. Fukutani, H. Kato, T. Miyahara, A. Yonesu, J. Tamashiro, J. Phys. Soc. Jpn. 62 (1993) 573.
591
¨ [9] A. Kuznetsov, B. Namozov, V. Murk, Proc. Acad. Sci. Estonian SSR, Phys., Math. 36 (1987) 193. [10] P. Kulis, Z. Rachko, M. Springis, I. Tale, J. Jansons, Radiat. Eff. Defects Solids 119–121 (1991) 963. [11] W.Y. Ching, Y.-N. Xu, J. Am. Ceram. Soc. 77 (1994) 404. [12] A. Ch.B. Lushchik, in: Decay of Electronic Excitations With Defect Formation in Solids (in Russian), Nauka, Moscow, 1989. [13] A. Sumi, J. Phys. Soc. Jpn. 43 (1977) 1286.
Spectral-kinetic study of self-trapping and multiplication of electronic excitations in Al 2 O 3 crystals a, a a,b a a c A. Lushchik *, E. Feldbach , M. Kirm , P. Liblik , Ch. Lushchik , I. Martinson , F. Savikhin a , G. Zimmerer b a
b
Institute of Physics, University of Tartu, Riia 142, EE 2400 Tartu, Estonia II. Institute of Experimental Physics, Hamburg University, Luruper Chaussee 149, D 22761 Hamburg, Germany c Department of Physics, Lund University, Professorsgatan 1, S 22100 Lund, Sweden
Abstract The luminescence spectra and the excitation spectra of intrinsic (7.6 and 3.77 eV) and extrinsic emissions for a-Al 2 O 3 , Al 2 O 3 :Sc and Al 2 O 3 :Ga crystals have been measured by means of synchrotron radiation of 4–40 eV at 8 and 80 K. The luminescence of self-trapped excitons (7.6 eV) can be excited at the direct photocreation of excitons or at the recombination of electrons with unrelaxed holes. Fast intraband luminescence (t ,2 ns) has been detected in a spectral region from 2.5 to 6.5 eV. The absorption of one photon of 25–37 eV leads to the formation of two or three electron-hole pairs causing a sharp increase of the efficiency of extrinsic emission of Sc 31 (5.6 eV), Ga 31 (4.6 eV) and F 1 centers (3.85 eV). 1999 Elsevier Science B.V. All rights reserved. Keywords: Exciton; Self-trapping; Luminescence; Synchrotron radiation; Al 2 O 3
1. Introduction In a number of dielectric materials VUV radiation is needed for the creation of intrinsic electronic excitations (IEEs). In some wide-gap crystals even the luminescence appearing after vibrational relaxation of IEEs lies in the VUV spectral region. This luminescence is the luminescence of free excitons in rare gas solids, in several alkali halide crystals and some metal oxides (BeO, MgO, CaO). The luminescence of self-trapped excitons (STEs) in rare gas solids and in a number of wide-gap oxides is located in the VUV region as well. By means of lowtemperature VUV spectroscopy with time resolution *Corresponding author. Tel.: 1372-7-428-946; fax: 1372-7-383033. E-mail address: [email protected] (A. Lushchik)
we made an attempt to study the IEEs in pure and doped a-Al 2 O 3 single crystals and ceramics that are widely used in various applications. The processes of IEE self-trapping and multiplication, studied earlier for rare gas solids and other ionic crystals [1–3], were of special interest.
2. Experimental The a-Al 2 O 3 single crystals grown by the Czochralski technique were the main objects of the present study. In our a-Al 2 O 3 samples, in the region of extrinsic absorption there are no selective absorption bands (the value of the absorption constant is less than 1–2 cm 21 at 295 K) [4]. The Al 2 O 3 :Sc (about 150 or 600 ppm of Sc 31 impurity ions [5]) and
0368-2048 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 98 )00337-5
588
A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591
Al 2 O 3 :Ga crystals and ceramics were also investigated. The spectra of steady cathodoluminescence were recorded through a double vacuum monochromator [6] in the region 1.8–9.0 eV using crystal excitation by 6 keV electrons at 8–300 K. The fast emissions excited by single powerful electron pulses (t 53 ns, E5300 keV, the current density 1–150 A cm 22 ) were detected through a monochromator by a system consisting of a photomultiplier, a fast oscilloscope and a telecamera [7]. The photoluminescence experiments were carried out at beamline 52 in the MAX I Laboratory in Lund and at the SUPERLUMI station of HASYLAB at DESY, Hamburg. The experimental set-ups are described in Refs. [2,5]. The excitation spectra were normalized to equal quantum intensities of synchrotron radiation (SR) falling onto the crystal. The excitation spectra were measured for time-integrated (steady) luminescence, as well as for the emission detected within a time window (length Dt) correlated with the excitation pulses of SR (delayed by dt). The delay dt and the length Dt were varied between 0.6 to 20 and 1.5 to 50 ns, respectively. The excitation spectra were measured at SR incidence angle of 17.58 (DESY) or 208 (MAX I). In case of electron excitation an electron beam was oriented at an angle 458 to a sample surface.
(Dt55 ns, dt51.5 ns), that does not have selective excitation bands from 4.5 to 8.8 eV, i.e. out of the region of IEE formation. This fast emission can be effectively excited at the photocreation of excitons (E'c) or due to the recombination of electron-hole (e-h) pairs. The main experimental results are presented in Ref. [5]. Below, we will present new additional data on the intrinsic and impurity luminescence of Al 2 O 3 . Fig. 1 shows steady luminescence spectra for a number of Al 2 O 3 single crystals in case of excitation by 6 keV electrons (current density 1mA mm 22 ). The broad luminescence band with the maximum at E I 5 7.6 eV and half-width d 50.8 eV dominates in the emission spectrum for an Al 2 O 3 crystal of high purity and perfection. The heating of the sample from 8 to 295 K causes the decrease of the 7.6 eV emission intensity by two orders of magnitude. It has been recently shown [5] that the 7.6 eV emission intensity depends superlinearly (approximately quadratically) on the density of excitation by nanosecond electron pulses at 8, 80 or 295 K. In Al 2 O 3 :Sc (600 ppm), the broad band E I 55.6 eV dominates in the cathodoluminescence spectrum at 80 and 295 K, while the 7.6 eV STE luminescence is weakened. The appearance of 5.6 eV emission is connected with the Sc 31 impurity centers. The efficiency of Sc 31
3. Results The reflection spectra have been measured for the electric field E of incident SR (6–40 eV) parallel or perpendicular to the optic axis c of a uniaxial Al 2 O 3 single crystal [8]. At 10 K, the long-wavelength edge of the absorption at 8.85–8.95 eV is caused by the Urbach tail of exciton absorption for Eic [8]. The efficiency of 7.6 eV STE emission (the so-called STE A emission) is extremely high just in the region of 8.85–9.1 eV [5,9,10]. The emission with the maximum at 3.8 eV was tentatively ascribed to another configuration of STEs in Al 2 O 3 [10]. However, the emission of F 1 centers lies also in the region of 3.8 eV. Therefore, it is difficult to separate the emission of F 1 centers and the supposed luminescence of STEs at 3.8 eV. For the first time we succeeded in detecting the fast component of 3.8 eV emission
Fig. 1. Spectra of steady luminescence of Al 2 O 3 , Al 2 O 3 :Ga and Al 2 O 3 :Sc (600 ppm) crystals on the excitation by 6 keV electrons at 8 K (solid lines), 80 K (dashed lines) and 300 K (dotted line). Spectrum of the decay time t for Al 2 O 3 emission under irradiation by single nanosecond pulses of 300 keV electrons at 80 K.
A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591
center emission h 5.6 is extremely high at the direct excitation of Sc 31 by 8.5 eV photons. The 5.6 eV emission does not undergo the thermal quenching up to 295 K. The presence of Ga 31 centers in Al 2 O 3 crystals leads to the appearance of the additional emission in the region 4–5 eV. The STE A emission is dominant in the emission spectrum under the excitation of highly pure Al 2 O 3 crystals by single nanosecond pulses of 300 keV electrons with the penetration depth into a crystal about 300 mm. A single electron pulse (t 53 ns) forms about 10 18 e-h pairs in 1 cm 3 of a crystal and mainly provides the appearance of recombination STE emission of 7.6 eV. The values of decay time t for Al 2 O 3 emission registered at 80 K are depicted also in Fig. 1. We obtained the value t 522 ns in the spectral region from 6.5 to 7.6 eV (the limit of our registration system) where the VUV emission of STEs is dominating. The continuous weak emission with t ,2 ns was detected in the region of 4.2–6.0 eV. Similar to other wide-gap dielectrics [7] this temperature-independent emission can be tentatively interpreted as the intraband luminescence of Al 2 O 3 due to the radiative transitions of hot electrons and holes inside of a conduction and a valence band. Fig. 2 presents the excitation spectra of 3.8 and 7.6 eV fast emissions measured at the exciting photon energies of 8 to 10 eV for two different crystals of Al 2 O 3 at 8 K. The arrows indicate the maxima of the exciton doublet in the reflection spectra measured for Eic (R i ) and E'c (R ' ) [8]. The efficiency of fast 7.6 eV emission h 7.6 is extremely high in the region of a long-wavelength component of the exciton doublet (8.9–9.1 eV) while the value of h 7.6 is lower in the region of a shortwavelength doublet component (9.1 to 9.3 eV). In the region of band-to-band transitions (hn .Eg 59.4 eV) the value of h 7.6 is by tens of times lower than at the direct optical formation of excitons. A highdensity excitation (e.g. an intensive line of the krypton discharge, hn 510 eV) is needed in order to obtain the intensive recombination luminescence of 7.6 eV. According to Fig. 2 the fast 3.8 eV luminescence of STEs can be effectively excited in the region of a short-wavelength component of the exciton doublet and at the band-to-band transitions. A typical decay curve of STE A emission for Al 2 O 3 excited by 8.9860.02 eV photons at 8 K is
589
Fig. 2. Time-resolved excitation spectra of STE emissions for two Al 2 O 3 crystals at 8 K. Al 2 O 3 no. 1 — fast emissions (Dt58 ns, dt51.4 ns) of 7.6 eV (11) and 3.8 eV (ss); Al 2 O 3 no. 2 — time-integrated (– –) and fast 7.6 eV emissions (Dt58 ns, dt51.4 ns — curve dd and Dt520 ns, dt59.8 ns — solid curve). The inset shows the decay curve of 7.6 eV emission in Al 2 O 3 (no. 2) excited by 8.98 eV photons at 8 K.
depicted in the inset of Fig. 2. The intensity of the 7.6 eV emission decreases by three orders of magnitude within 50 ns. The excitation spectra for A emission detected within two time windows (Dt58 ns, dt51.4 ns and Dt520 ns, dt59.8 ns) or in time-integrated regime differ from each other very slightly. Fig. 3 shows the excitation spectra of 5.6 eV steady emission measured for Al 2 O 3 :Sc (600 ppm) at 31 8 and 295 K. The emission of Sc centers can be efficiently excited by 7.8 to 8.8 eV photons that directly generate near-impurity electronic excitations. The photocreation of excitons or e-h pairs leads also to the appearance of 5.6 eV emission. The value of h 5.6 is high in the region of 9.4–10.5 eV and 25–37 eV where the values of absorption constant do not exceed k53310 4 cm 21 [8]. However, in the region of 12–24 eV the value of k reaches 10 5 –10 6 cm 21 , the penetration depth of exciting photons into a crystal is r#0.1 mm and the near-surface losses for impurity emissions are high and strongly depend on the surface quality. The value of k for 10.5 and 30 eV photons are practically equal [8], while the value of h sharply increases (at least doubles) from 10 to 30 eV for the 5.6 eV emission in Al 2 O 3 :Sc and 4.6
590
A. Lushchik et al. / Journal of Electron Spectroscopy and Related Phenomena 101 – 103 (1999) 587 – 591
Fig. 3. Excitation spectra of steady emission of Sc 31 centers (5.6 eV, curves 1 and 2) and Ga 31 centers (4.6 eV, curve 3) for Al 2 O 3 :Sc (600 ppm) and Al 2 O 3 :Ga crystals at 8 and 300 K (curve 2). A fragment of the absorption spectrum for Al 2 O 3 crystal (E'c) [8].
eV emission in Al 2 O 3 :Ga. So, we interpret this sharp increase of the emission efficiency as the process of multiplication of electronic excitations (MEE) in Al 2 O 3 . An analysis of the above-mentioned excitation spectra (and these in Ref. [5]) for various emissions allows us to conclude that the absorption of one photon of 26–29 eV leads to the creation of two e-h pairs, while one photon of 30 to 37 eV creates up to three e-h pairs.
in the conduction band is able to form the second e-h pair, while the Auger recombination of an electron from 2p 6 O 22 with a 2s 2 O 22 hole provides the creation of the third e-h pair. The experimentally revealed quadratic dependence of STE A emission intensity on the excitation density in a-Al 2 O 3 [5] is of special interest. Such behavior of 7.6 eV emission can be explained by a small value of an effective cross-section s eh for radiative e-h recombination that causes the appearance of this luminescence. The value of s eh is significantly smaller than that for the recombination of free electrons with self-trapped holes (VK centers) in alkali iodide crystals. It is necessary to mention that the intensity of STE emission in KI linearly depends on the density of electron current [5]. Considering the results of the investigation of edge emissions in semiconductors, we suggest that the small value of s eh in Al 2 O 3 testifies to the recombination of free electrons and free (unrelaxed) holes. Similar to a Y 2 O 3 crystal (see Ref. [12]), an electron and a hole in Al 2 O 3 do not separately undergo self-trapping while the sum of their deformation potentials is sufficient for the formation of a self-shrunk STE (SSTEs). Our hypothesis is based on the Sumi diagram for the possible interaction of IEEs with the field of acoustic phonons [13], that allows the existence of such SSTEs. Further investigation of the process of exciton self-trapping in uniaxial a-Al 2 O 3 is needed.
4. Discussion The simplified energy band diagram of an aAl 2 O 3 crystal, based on theoretical calculations [11] and experimentally determined values of Eg and the width for 2p 6 (DE57–9 eV) and 2s 2 oxygen valence bands (DE53–4 eV), was suggested in Ref. [5] for the interpretation of the MEE process. The energy of a hot conduction electron, formed after the absorption of one photon of 26–29 eV, is sufficient to create the second e-h pair while a hot hole in the lower part of 2p 6 O 22 valence band gains a smaller part of the photon energy and is not able to form a secondary electronic excitation. The value of the threshold energy of this MEE process Et 525 eV significantly exceeds 2Eg . If the energy of the absorbed photon equals 30–37 eV, the ionization process starts from the 2s 2 shell of an oxygen ion. A hot photoelectron
Acknowledgements This work has been partly supported by the Estonian Science Foundation (Grant 1931), the Crafoord Foundation (Sweden) and the STINT Foundation (Sweden).
References [1] G. Zimmerer, J. Low Temp. Phys. 111 (1998) 629. [2] A. Lushchik, E. Feldbach, R. Kink, Ch. Lushchik, M. Kirm, I. Martinson, Phys. Rev. B 53 (1996) 5379. [3] M. Kirm, E. Feldbach, R. Kink, A. Lushchik, A. Maaroos, I. Martinson, J. Electron Spectrosc. Relat. Phenom. 79 (1996) 91.
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