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Photoluminescence and electroluminescence of GaS : Tm single crystals
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
Solid State Communications, Vol. 55, No. 4, pp. 385-386, 1985. Printed in Great Britain.
PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE OF GaS: Tm SINGLE CRYSTALS B.G. Tagiev, G.M. Niftiev and F.Sh. Aidaev Institute of Physics, Academy of Sciences of the Azerbaidzhan SSR, Prospekt Narimanova 33, 370143 Baku, USSR
(Received 6 February 1985 by F. Bassani) Photoluminescence and electroluminescence spectra of the single crystal GaS : Tm have been studied. Five series of the intra-central transition have been detected. It is shown that transfer of energy to Tm ion takes place through broad bands which are related to the matrix itself. It may be suggested that electro luminescence is due to carrier injection. OWING TO THE DEVELOPMENT of quantum electronics and to the prospects of producing an inverse population by means of impact excitation of impurity centres, special interest is attracted by luminescence investigations of rare earths (RE) in AmBVI-type wideband semiconductors under electric-field excitation. Photoluminescence and electroluminescence spectra of Nd 3÷ and Yb a+ in AraB vI were studied earlier [1,2]. The present report deals with an investigation of photoluminescence spectra of thalium-activated GaS single crystals. p-GaS single crystals were obtained by the Bridgeman method. The concentrations of the impurity introduced while synthetizing varied in the range from 0.01 to 1 at %. Contacts were produced by fusing-in indium on the opposite mirror surfaces perpendicular to the single crystal axis. The specimen thicknesses and the contact areas varied in the ranges of 10 to 250gm and (1 to 4) x 10 -2 cm 2, respectively. The spectra were taken with the use of an SDL-1type spectrometer. Photoluminescence was excited with an LPM-11-1aser (Xb=0.4416#m), whereas electroluminescence was excited both by an a.c. and a d.c. field. Photoelectronic multipliers FEU-39, and FEU-62 and a PbS photoresistor served as radiation receivers. In the 0.68 to 1.54#m wavelength region the photoluminescence (PL) and electroluminescence (EL) spectra of GaS: 0.1 at.% Tm single crystals exhibit a large number of narrow-band lines relating to Tm a+ ions, namely aF:-aH6 (0.68 to 0.70gm), aFa-aH~ (0.72 to 0.73/,tm), 3114-31-16(0.78 to 0.82um) (Fig. 2a), aHs-aH~ (1.17 to 1.29pm) and aH4-aF4 (1.42 to 1.54#m) (Fig. 2). As seen from Fig. la, the pattern of the EL spectrum copies that of the PL spectrum. In the d.c. field the radiation started at a current density j_ = 3.8 x 10-aA cm -~ and a field intensity E_ = 8.3 x 10a V c m -~ , whereas in the a.c. field it started at j ~ = 6 . 3 x l 0 - a A c m -: and E~ = 1.42 x 104 V cm -~ . The spectra were identified according to the known spectra ofintracentral transitions of Tm a+ ions [ 3 - 5 ] .
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2 Fig. 1. (a) 1,2-Photolurninescence spectrum at 77 and 300K respectively. 3-Electroluminescence spectrum at 77 K. (b) Electroluminescence intensity dependence on current, flowing through the sample, (1) for alternating current, (2) for direct current. (c) EL intensity dependence on the applied voltage. 1, 2, 3,4, 5 are respectively with 380, 370, 360, 350 and 330V. 1', 2', 3', 4', 5' at high frequency region with respect to the said curves in log-log scale. Figure lb depicts the dependences of EL intensities on the intensities of alternating (1) and direct (2) current. With the alternating current the dependence is linear (n = 1), whereas with the direct current it is superlinear (n = 1.3). In the EL-producing field range
386
GaS : Tm SINGLE CRYSTALS
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Fig. 2. Photoluminescence spectrum at 1 - 3 0 0 K and 2 - 7 7 K. under study a squaredaw section (U2), a cubic section (U 3) and a sharp-growth section (U n, n = 3 to 6.5) are revealed in the current-voltage characteristic. The square-law section precedes the linear section. The existence of the above sections suggests that the current passage mechanism is due to a double injection [6]. In the frequency dependence of EL intensity a maximum (about 750Hz) has been found (Fig. lc). The high-frequency decays represent hyperbolas inasmuch as they rectify well on a log-log scale (Fig. lc). This is an indication of the capacitive nature of the decays [7]. Radiation lifetimes have been determined for the 3F2-3H6, 3/73-3/-/6 and 3//4-3//6 transitions; they amount to 18, 25 and 100/asec. With the 3//4-3//6 transition an intensification section (20/asec) is observed which points to the fact that the excitation of the 3//4 state is associated with relaxation processes from the higher-lying levels. The non-existence of intensification sections with the 3F2-3H6 and 3173-31-16 transitions is due to a simultaneous energy transfer to both levels via a wide band (0.52 to 0.72/~m). A large number of experiments on AraBY1: REcrystals were performed by us to reveal the Ln 3÷ ion excitation mechanisms at PL and EL. PL and EL spectra of non-activated GaS single crystals display wide bands with maxima at 0.483, 0.585 and 1.20/~m. The first of the three bands is narrower than the other two bands. The second band covers the 0.52 to 0.72#m region, whereas the third band embraces the 0.96 to 1.5/am
Vol. 55;No. 4
region. In GaS : Ho and GaS : Nd single crystals the highfrequency part of the wide band (0.585 pm) is partially absorbed by the intracentral transitions Sla-sS2 (Ho 3÷) and 419/z-4Gs/2 + 4G7/2 (Nd3+). The PL spectra of GaS:Yb show an intensive luminescence of the intracentral transition of Yb 3+ in the 0.97 to 1.05/am region [2]. In GaSe:Yb single crystals the Yb 3+ ions fail to be excited because of the non-existence of wide-band radiation in this region, even though the band gap is sufficiently wide. In REactivated GaTe single crystals, neither intracentral transitions nor wide radiation bands have been found (Ee = 1.7 eV at 300 K). The 0.483 and 0.585/am bands have been studied in rather considerable detail [8, 9] ; they are associated with an indirect edge and an electron transition from the conduction band to the acceptor levels. The energy position of the 1.20#m band corresponds to a transition between donor-acceptor pairs [10]. It is believed that the 3/72, 3F3 and 3//4 states are excited via 0.585/1m bands, whereas 3Hs is excited via the 1.20/am band. Based on the foregoing it is concluded that the transfer of energy to the Tm 3+ ions in GaS single crystals is effected via wide radiation bands.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
G.B. AbduUaev, S.A. Abushov, I.M. Briskina, V.F. Zolin, V.M. Markushev, G.M. Neftiev & B.G. Tagiev, Quantum Electronics, II, 605 (1984). B.G. Tagiev, G.M. Niftiev & S.A. Abushov. Phys. Status. Solidi. (b), 118, K13-16 (1983). P.J. Cresswell, D.J. Roblins & A.J. Tomson, J. Luminescence, 17, 311 (1978). H~. Jenssen, A. Linz, K.P. Leavitt, C.A. Morrisson & D.E. Worfmann, J. Phys. Rev. B., 11, 92 (1975). B.M. Antipenko, A.A. Mak, O.B. Raba, K.B. Seiranian & T.V. Uvarova, Quantum Electronics, 10,889 (1983). R. Baron & J.W. Mayer, Semiconductors and Semimetals, Injection Phenomena., 6,360. V.L. Rabotnik, N.D. Abrosimova, V.A. Burobin & V.V. Demiyanov, Electroluminescence in Solid State and its Application., Kiev, 125 (1972). G.A. Akhundov, I.G. Aksiyanov & G.M. Kasumov, F.T.P. 6, 912 (1969). E. Aulich, J.I. Brebner & E. Mooser, Phys. Status. Solidi. 31,129 (1969). R.M.A. Lieth & E.F. Van der Maesen, Phys. Stat. Sol. (a), 10, 73 (1972).
Solid State Communications, Vol. 55, No. 4, pp. 385-386, 1985. Printed in Great Britain.
PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE OF GaS: Tm SINGLE CRYSTALS B.G. Tagiev, G.M. Niftiev and F.Sh. Aidaev Institute of Physics, Academy of Sciences of the Azerbaidzhan SSR, Prospekt Narimanova 33, 370143 Baku, USSR
(Received 6 February 1985 by F. Bassani) Photoluminescence and electroluminescence spectra of the single crystal GaS : Tm have been studied. Five series of the intra-central transition have been detected. It is shown that transfer of energy to Tm ion takes place through broad bands which are related to the matrix itself. It may be suggested that electro luminescence is due to carrier injection. OWING TO THE DEVELOPMENT of quantum electronics and to the prospects of producing an inverse population by means of impact excitation of impurity centres, special interest is attracted by luminescence investigations of rare earths (RE) in AmBVI-type wideband semiconductors under electric-field excitation. Photoluminescence and electroluminescence spectra of Nd 3÷ and Yb a+ in AraB vI were studied earlier [1,2]. The present report deals with an investigation of photoluminescence spectra of thalium-activated GaS single crystals. p-GaS single crystals were obtained by the Bridgeman method. The concentrations of the impurity introduced while synthetizing varied in the range from 0.01 to 1 at %. Contacts were produced by fusing-in indium on the opposite mirror surfaces perpendicular to the single crystal axis. The specimen thicknesses and the contact areas varied in the ranges of 10 to 250gm and (1 to 4) x 10 -2 cm 2, respectively. The spectra were taken with the use of an SDL-1type spectrometer. Photoluminescence was excited with an LPM-11-1aser (Xb=0.4416#m), whereas electroluminescence was excited both by an a.c. and a d.c. field. Photoelectronic multipliers FEU-39, and FEU-62 and a PbS photoresistor served as radiation receivers. In the 0.68 to 1.54#m wavelength region the photoluminescence (PL) and electroluminescence (EL) spectra of GaS: 0.1 at.% Tm single crystals exhibit a large number of narrow-band lines relating to Tm a+ ions, namely aF:-aH6 (0.68 to 0.70gm), aFa-aH~ (0.72 to 0.73/,tm), 3114-31-16(0.78 to 0.82um) (Fig. 2a), aHs-aH~ (1.17 to 1.29pm) and aH4-aF4 (1.42 to 1.54#m) (Fig. 2). As seen from Fig. la, the pattern of the EL spectrum copies that of the PL spectrum. In the d.c. field the radiation started at a current density j_ = 3.8 x 10-aA cm -~ and a field intensity E_ = 8.3 x 10a V c m -~ , whereas in the a.c. field it started at j ~ = 6 . 3 x l 0 - a A c m -: and E~ = 1.42 x 104 V cm -~ . The spectra were identified according to the known spectra ofintracentral transitions of Tm a+ ions [ 3 - 5 ] .
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2 Fig. 1. (a) 1,2-Photolurninescence spectrum at 77 and 300K respectively. 3-Electroluminescence spectrum at 77 K. (b) Electroluminescence intensity dependence on current, flowing through the sample, (1) for alternating current, (2) for direct current. (c) EL intensity dependence on the applied voltage. 1, 2, 3,4, 5 are respectively with 380, 370, 360, 350 and 330V. 1', 2', 3', 4', 5' at high frequency region with respect to the said curves in log-log scale. Figure lb depicts the dependences of EL intensities on the intensities of alternating (1) and direct (2) current. With the alternating current the dependence is linear (n = 1), whereas with the direct current it is superlinear (n = 1.3). In the EL-producing field range
386
GaS : Tm SINGLE CRYSTALS
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55
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Fig. 2. Photoluminescence spectrum at 1 - 3 0 0 K and 2 - 7 7 K. under study a squaredaw section (U2), a cubic section (U 3) and a sharp-growth section (U n, n = 3 to 6.5) are revealed in the current-voltage characteristic. The square-law section precedes the linear section. The existence of the above sections suggests that the current passage mechanism is due to a double injection [6]. In the frequency dependence of EL intensity a maximum (about 750Hz) has been found (Fig. lc). The high-frequency decays represent hyperbolas inasmuch as they rectify well on a log-log scale (Fig. lc). This is an indication of the capacitive nature of the decays [7]. Radiation lifetimes have been determined for the 3F2-3H6, 3/73-3/-/6 and 3//4-3//6 transitions; they amount to 18, 25 and 100/asec. With the 3//4-3//6 transition an intensification section (20/asec) is observed which points to the fact that the excitation of the 3//4 state is associated with relaxation processes from the higher-lying levels. The non-existence of intensification sections with the 3F2-3H6 and 3173-31-16 transitions is due to a simultaneous energy transfer to both levels via a wide band (0.52 to 0.72/~m). A large number of experiments on AraBY1: REcrystals were performed by us to reveal the Ln 3÷ ion excitation mechanisms at PL and EL. PL and EL spectra of non-activated GaS single crystals display wide bands with maxima at 0.483, 0.585 and 1.20/~m. The first of the three bands is narrower than the other two bands. The second band covers the 0.52 to 0.72#m region, whereas the third band embraces the 0.96 to 1.5/am
Vol. 55;No. 4
region. In GaS : Ho and GaS : Nd single crystals the highfrequency part of the wide band (0.585 pm) is partially absorbed by the intracentral transitions Sla-sS2 (Ho 3÷) and 419/z-4Gs/2 + 4G7/2 (Nd3+). The PL spectra of GaS:Yb show an intensive luminescence of the intracentral transition of Yb 3+ in the 0.97 to 1.05/am region [2]. In GaSe:Yb single crystals the Yb 3+ ions fail to be excited because of the non-existence of wide-band radiation in this region, even though the band gap is sufficiently wide. In REactivated GaTe single crystals, neither intracentral transitions nor wide radiation bands have been found (Ee = 1.7 eV at 300 K). The 0.483 and 0.585/am bands have been studied in rather considerable detail [8, 9] ; they are associated with an indirect edge and an electron transition from the conduction band to the acceptor levels. The energy position of the 1.20#m band corresponds to a transition between donor-acceptor pairs [10]. It is believed that the 3/72, 3F3 and 3//4 states are excited via 0.585/1m bands, whereas 3Hs is excited via the 1.20/am band. Based on the foregoing it is concluded that the transfer of energy to the Tm 3+ ions in GaS single crystals is effected via wide radiation bands.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
G.B. AbduUaev, S.A. Abushov, I.M. Briskina, V.F. Zolin, V.M. Markushev, G.M. Neftiev & B.G. Tagiev, Quantum Electronics, II, 605 (1984). B.G. Tagiev, G.M. Niftiev & S.A. Abushov. Phys. Status. Solidi. (b), 118, K13-16 (1983). P.J. Cresswell, D.J. Roblins & A.J. Tomson, J. Luminescence, 17, 311 (1978). H~. Jenssen, A. Linz, K.P. Leavitt, C.A. Morrisson & D.E. Worfmann, J. Phys. Rev. B., 11, 92 (1975). B.M. Antipenko, A.A. Mak, O.B. Raba, K.B. Seiranian & T.V. Uvarova, Quantum Electronics, 10,889 (1983). R. Baron & J.W. Mayer, Semiconductors and Semimetals, Injection Phenomena., 6,360. V.L. Rabotnik, N.D. Abrosimova, V.A. Burobin & V.V. Demiyanov, Electroluminescence in Solid State and its Application., Kiev, 125 (1972). G.A. Akhundov, I.G. Aksiyanov & G.M. Kasumov, F.T.P. 6, 912 (1969). E. Aulich, J.I. Brebner & E. Mooser, Phys. Status. Solidi. 31,129 (1969). R.M.A. Lieth & E.F. Van der Maesen, Phys. Stat. Sol. (a), 10, 73 (1972).