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Exciton luminescence in TlCl and TlBr: Electro-modulated excitation spectroscopy
Journal of Luminescence 12/13 (1976) 291 —296 © North-Holland Publishing Company
EXCITON LUMINESCENCE IN TiC! AND T1Br: ELECTRO-MODULATED EXCITATION SPECTROSCOPY R. SHIMIZU and T. KODA Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Effect of an electric field is investigated on the exciton luminescence of TICI and TIBr by use of the modulation method. The characteristic features of the clectro-modulated luminescence-excitation spectra are interpreted in terms of the field-induced dissociation effect of the direct-excitons and the perturbation effect of the indirect-forbidden excitons. The radiative decay process of excitons is discussed in relation to the band structure of thallous halides.
1. Introduction Study of the exciton luminescence in thallous halides offers several interesting problems with regard to the exciton decay process. When excited by light, the photogenerated electron—hole pairs undergo relaxation process via two different channels~ the one being formation of the direct-excitons and the other through the indirectexcitons. These hot excitons decay down to the respective exciton ground states with successive emission of plionons and then make radiative transitions [1]. They are also captured by localized centres, giving rise to the broad luminescence bands in the low energy region. The luminescence excitation (LE) spectra are informative of these exciton decay processes [1,2]. The purpose of this paper is to investigate the effect of an electric field on the LE spectra of TIC! and TlBr. The modulation technique was employed to reveal the exciton-related fine structures in the spectra. Furthermore, it was expected that an electric field might influence the selection rule for the suggested X~,,,— indirect transition, which is parity-forbidden without a pertorbing field.
2. Experimental 3 in size, were prepared from zoneParallelepiped samples, about 3 X 2 )< 1 mm refined single crystals of TIC! and T1Br. The surfaces were oriented by the X-ray diffraction method and etched by hot waterjust prior to measurements. The unmodulated luminescence and LE spectra were measured in a conventional way. In the electro-modulation ineasurernents, the sample was placed between two parallel291
292
R. Shimizu, T. Koda / Exciton luminescence in TICI and T1Br
plate metal electrodes. Thin mica sheets were used to block the photo-current between the sample and the electrodes. An ac electric field F, up to about 10 kV/cm with frequency 1 kHz, was applied to the electrodes. The modulated luminescence signal was observed on the surface perpendicular to the electrodes for the polarized components of E II F and EL F, E being the electric vector of the exciting light. We have also measured the electro-reflectance spectra with the same experimental set-up, as reported elsewhere [31. Several precautions were necessary in these electromodulation measurements, since the high photo-conductivity at low temperatures short-circuits the applied voltage near the surface. Experimentally, the electromodulated LE spectra are less affected by the field-screening effect due to the photocarriers than the electro-reflectance spectra, presumably because the modulation of luminescence takes place well inside the surface region. Although the electro-reflectance signal in a zone-refined TIC! becomes undetectable below about 10 K [31, the electro-modulated LE signal is still observable in the same sample even below 4 K. AJI electro-modulated LE measurements were made at frequency 2f, since no !inear response is expected to exist in the bulk crystal for the centrosymmetric CsCI structure. Linear signals, which were actually observed in the direct-exciton region, were considered to arise from a non-uniform field distribution near the surface, and were disregarded in the following discussion.
3. Experimental results and discussion Typical results of unmodulated and electro-modulated spectra are assembled in fig. I and fig. 2 for T1C1 and T1Br, respectively.
3.1. Unmodulated spectra The luminescence spectra, shown in the top of figs. 1 and 2, are composed of three dominant groups of emissions. The highest energy one, A, has been attributed to the radiative decay of the direct-excitons [1,4], while another group of emission lines, B, has been ascribed to the recombination of the indirect-excitons [11. Broad luminescence bands, C (not shown in figs. 1 and 2) are observed on the lower energy side of these exciton emissions, at about 2.67 eV in T1CI (the blue band) and at about 2.45 and 2.54 eV in T1Br. The LE spectra were measured for the B and C emissions. The most noticeable feature of the spectra is a steep rise of the luminescence yield at energy just above the B emission lines. Above this threshold, the spectra exhibit an oscillatory structure, 2hwLo in TlBr, where ~IWLO is the energy of with the spacing of hwLo in TIC! and the longitudinal optical phonon at the F-point of the respective halides. The mechanism of the double LO phonon emission in TIBr has been interpreted in terms of the decay process of the hot indirect-excitons [2]. This phonon structure is then followed by the second threshold associated with the direct-edge. Such features quite
R. Shimizu, T Koda / Exciton luminescence in TIC1 and TIBr
390 I
WAVELENGTH (nm) 370 360 I
380 I
293
350 I
~:JL 7~’TT~ f
‘.
I~ii I
:
e~ectro-modutatedLE F= 2.7 kV/cm rII(loOl 2f(f=lkHz)
~
3.2
3.3 3.4 PHOTON ENERGY (eV)
3.5
3.6
Fig. 1. The luminescence (top), and unnsodulated (middle) and electro-modulated (bottom) excitation spectra of TICL at 1.8 K. Arrows in the last spectrum indicate the positions of the direct-exciton structures from the thin-film-absorption measurements by Kurita and Kobayashi (ref. [7]).
resemble to those of GaP [5j, a well-known indirect-gap semiconductor, and are considered to be inherent in the indirect-gap materials. The indirect-gap energies are estimated to be about 3.20 eV in T1C1 and about 2.63 eV in T1Br, which are consistent with a recent absorption measurements by Nakahara et a!. [6]. 3.2. Electro -modulated LE spectra The electro-modulated LE spectra were measured forE I [100], 1110] and [1111, but since the results are qualitatively the same for the three geometries, only the [100] spectra are shown in figs. I and 2. The signal is proportional to the square of the applied voltage in the low field region, but tends to saturate above ~5 kV/cm. In order to discuss the field effect on the luminescence excitation spectra, we have to consider three separate steps in the excitonic process~the first is the optical creation of the hot excitons by the exciting light, the second is the relaxation process
294
R. Shimizu, T. Koda / Exciton luminescence in TIC1 and T1Br
470 I
460 I
WAVELENGTH (nm) 450 440 430 I I I
420 I
410
P~IB Luminescence
16
~
PHOTON ENERGY (eV)
A
\~
Fig. 2. The luminescence (top), and unmodulated (middle) and electro-modulated (bottom) excitation spectra of T1Br at 1.8 K.
of these hot excitons, and finally the radiative recombination of the excitons, either freely moving in crystal or bound at localized centres. The observed modulation signal should be accounted for as the superposition of the field-perturbation effects of these three processes. In the present case, however, the last effect needs not be considered since the luminescence takes place at the deep-lying localized centres which are almost insensitive to the electric field. Hence, the dominant contributions come from a possible change in the transition probability of the exciton formation and the field-induced dissociation effect of the free excitons during migration in crystal before captured by the luminescence centres. When excitation is made in the direct-exciton region, the former effect is not important since the transition is ab initio dipole-allowed for the X~ X~cedge. Also the absorption coefficient is so large in this spectral region that most of the excitons are created near the surface where the field is almost screened by the photo-carriers, as described before. Hence, the effective modulation mechanism is the field-induced dissociation effect while the photo-generated excitons are diffusing inside from the
R. Shimizu, T. Koda / Exciton luminescence in T1CI and T1Br
295
surface region. We define the critical field Ec by Ec = G/ae, with the exciton binding energy G and the exciton radius a. Using the parameters derived by Kurita and Kobayashi [7], Ec is estimated to be about 15 kV/cm for TIC! and 12 kV/cm for TIBr, respectively, so that the dissociation effect is by no means negligible for the direct-excitons at the field strength presently used. Once dissociated, the free electron and hole follow a decay process which will be considerably different from that of the excitons. For the blue luminescence centres in T1C1, we consider that the luminescence efficiency is much smaller for the free carriers than for the excitons. We expect then that the luminescence intensity is appreciably quenched by electric field, in accord with the negative modulated signals of TlCl observed in the directexciton region. To support this interpretation, the modulated signal saturates in the high field region, F> ~5 ky/cm, indicating the field-screening effect by the dissociated carriers. In T1Br, where the luminescence spectrum is composed of two broad bands, the radiative decay process for the free carriers is supposed to be more complicated than T1C1. On the other hand, for excitation in the indirect-exciton region, the dissociation effect is less important because of the smaller radius of the indirect-exciton than that of the direct-exciton (about one half in T1Br from the estimated binding energies of both excitons [6], and probably of the same order in T1C1, too). Instead, we expect that the parity-forbidden indirect-excitons are significantly perturbed by the electric field giving rise to an increased transition probability for the exciton formation. We believe this explains the positive modulation signals in the indirectexciton region for both halides. Furthermore, it can be shown, from a group-theoretical consideration, that the observed polarization-dependence in this region is reasonably accounted for by the second-order Stark effect of the relevant band edges, details of which will be described elsewhere. As a final remark, it is worth noting that the fine structures observed in the directexciton region of T1CI can be identified with the exciton-related structures, indicated by arrows in fig. 1, observed in the thin-film-absorption measurements [7]. These structures show up as the negative peaks in the electro-modulated LE spectra for the reason mentioned above, but their energies and the spectral profiles reflect the details of the absorption spectrum quite well. In TIBr, the corresponding structure is not well resolved yet, but is likely to be detectable by further investigation.
References [11 121 131 141 [51
R. Shimizu,T. Kodaand T.Murahashi,J. Phys. Soc.Japan 33 (1972) 866;36 (1974) 161. R. Shimizu and T. Koda, J. Phys. Soc. Japan 37 (1974) 1468. R. Shimizu and T. Koda, J. Phys. Soc. Japan 38 (1975) 1550. L. Grabner, Phys. Rev. B4 (1971) 1335. P.J. Dean, Phys. Rev. 168 (1968) 889. 161 J. Nakahara, K. Kobayashi and A. Fujii, J. Phys. Soc. Japan 37 (1974) 1312. [71 S. Kurita and K. Kobayashi, J. Phys. Soc. Japan 30 (1971) 1645.
296
R. Shimizu, T. Koda
/ Exciton luminescence in T1C1 and T1Br
Discussion P.J. Dean: The exact form of these excitation spectra depends rather critically in general on various factors. There are bulk effects such as the influence of changes in the surface to bulk recombination rate ratio and perhaps the ambipolar diffusion length with the condition of the sample, particularly of its surface. There is also the question of the nature of the detected luminescence. For example, in GaP there are systematic differences between excitation spectra for luminescence of distant donor-acceptor pairs and of excitons at shallow isoelectronic traps. Do you find evidence for these effects in these thallium compounds? R. Shimizu: Surely the relative shapes of the luminescence excitation spectra were found to be somewhat dependent on the sample conditions in thallous halides, too, But the sample dependence of the spectra is not so significant in these halides, as compared to your results on GaP. With proper treatment of samples to remove the damaged surface layer and the effect of residual strain, we could obtain well reproducible results which are not so influenced by the factors you pointed out. Heavier electron and hole masses and larger ionicity of these halides seem to be favorable to observe less sample-dependent luminescence excitation spectra, as compared to the case of GaP. The emission we used as monitor of the exciton decay process is a broad luminescence band presumably due to some impurity, such as isoelectronic halogen, or native defect. No emission line has been observed as yet in these halides which is ascribable to the donor—acceptor pairs. R.Z. Bachrach: Your comparison with GaP is not fully clear since GaP is an allowed indirect edge. One would expect the excitation curve to be much weaker than what you found. T. Koda: The absorption coefficient of thallous halides near the forbidden indirect edge is 1, which is certainly considerably smaller than that of GaP near the alof the order of edge. 1 cmHowever, we should note that, in the luminescence excitation spectra, we lowed indirect are observing the total emitted photons which are excited by the incident photons absorbed within the whole volume of a sample. Taking account of the actual sample thickness of about 3 mm and the internal reflection effect from the back surface of the crystal as well, we consider that it is the photon energy dependence of the luminescence efficiency of the excitons rather than the absorption coefficient that determines the relative shapes of the excitation spectra near the indirect threshold. This explains, we believe, the similarity between the excitation spectra of thallous halides and that of Gap, in spite of the appreciable difference in their absorption coefficients near the respective indirect edge.
EXCITON LUMINESCENCE IN TiC! AND T1Br: ELECTRO-MODULATED EXCITATION SPECTROSCOPY R. SHIMIZU and T. KODA Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Effect of an electric field is investigated on the exciton luminescence of TICI and TIBr by use of the modulation method. The characteristic features of the clectro-modulated luminescence-excitation spectra are interpreted in terms of the field-induced dissociation effect of the direct-excitons and the perturbation effect of the indirect-forbidden excitons. The radiative decay process of excitons is discussed in relation to the band structure of thallous halides.
1. Introduction Study of the exciton luminescence in thallous halides offers several interesting problems with regard to the exciton decay process. When excited by light, the photogenerated electron—hole pairs undergo relaxation process via two different channels~ the one being formation of the direct-excitons and the other through the indirectexcitons. These hot excitons decay down to the respective exciton ground states with successive emission of plionons and then make radiative transitions [1]. They are also captured by localized centres, giving rise to the broad luminescence bands in the low energy region. The luminescence excitation (LE) spectra are informative of these exciton decay processes [1,2]. The purpose of this paper is to investigate the effect of an electric field on the LE spectra of TIC! and TlBr. The modulation technique was employed to reveal the exciton-related fine structures in the spectra. Furthermore, it was expected that an electric field might influence the selection rule for the suggested X~,,,— indirect transition, which is parity-forbidden without a pertorbing field.
2. Experimental 3 in size, were prepared from zoneParallelepiped samples, about 3 X 2 )< 1 mm refined single crystals of TIC! and T1Br. The surfaces were oriented by the X-ray diffraction method and etched by hot waterjust prior to measurements. The unmodulated luminescence and LE spectra were measured in a conventional way. In the electro-modulation ineasurernents, the sample was placed between two parallel291
292
R. Shimizu, T. Koda / Exciton luminescence in TICI and T1Br
plate metal electrodes. Thin mica sheets were used to block the photo-current between the sample and the electrodes. An ac electric field F, up to about 10 kV/cm with frequency 1 kHz, was applied to the electrodes. The modulated luminescence signal was observed on the surface perpendicular to the electrodes for the polarized components of E II F and EL F, E being the electric vector of the exciting light. We have also measured the electro-reflectance spectra with the same experimental set-up, as reported elsewhere [31. Several precautions were necessary in these electromodulation measurements, since the high photo-conductivity at low temperatures short-circuits the applied voltage near the surface. Experimentally, the electromodulated LE spectra are less affected by the field-screening effect due to the photocarriers than the electro-reflectance spectra, presumably because the modulation of luminescence takes place well inside the surface region. Although the electro-reflectance signal in a zone-refined TIC! becomes undetectable below about 10 K [31, the electro-modulated LE signal is still observable in the same sample even below 4 K. AJI electro-modulated LE measurements were made at frequency 2f, since no !inear response is expected to exist in the bulk crystal for the centrosymmetric CsCI structure. Linear signals, which were actually observed in the direct-exciton region, were considered to arise from a non-uniform field distribution near the surface, and were disregarded in the following discussion.
3. Experimental results and discussion Typical results of unmodulated and electro-modulated spectra are assembled in fig. I and fig. 2 for T1C1 and T1Br, respectively.
3.1. Unmodulated spectra The luminescence spectra, shown in the top of figs. 1 and 2, are composed of three dominant groups of emissions. The highest energy one, A, has been attributed to the radiative decay of the direct-excitons [1,4], while another group of emission lines, B, has been ascribed to the recombination of the indirect-excitons [11. Broad luminescence bands, C (not shown in figs. 1 and 2) are observed on the lower energy side of these exciton emissions, at about 2.67 eV in T1CI (the blue band) and at about 2.45 and 2.54 eV in T1Br. The LE spectra were measured for the B and C emissions. The most noticeable feature of the spectra is a steep rise of the luminescence yield at energy just above the B emission lines. Above this threshold, the spectra exhibit an oscillatory structure, 2hwLo in TlBr, where ~IWLO is the energy of with the spacing of hwLo in TIC! and the longitudinal optical phonon at the F-point of the respective halides. The mechanism of the double LO phonon emission in TIBr has been interpreted in terms of the decay process of the hot indirect-excitons [2]. This phonon structure is then followed by the second threshold associated with the direct-edge. Such features quite
R. Shimizu, T Koda / Exciton luminescence in TIC1 and TIBr
390 I
WAVELENGTH (nm) 370 360 I
380 I
293
350 I
~:JL 7~’TT~ f
‘.
I~ii I
:
e~ectro-modutatedLE F= 2.7 kV/cm rII(loOl 2f(f=lkHz)
~
3.2
3.3 3.4 PHOTON ENERGY (eV)
3.5
3.6
Fig. 1. The luminescence (top), and unnsodulated (middle) and electro-modulated (bottom) excitation spectra of TICL at 1.8 K. Arrows in the last spectrum indicate the positions of the direct-exciton structures from the thin-film-absorption measurements by Kurita and Kobayashi (ref. [7]).
resemble to those of GaP [5j, a well-known indirect-gap semiconductor, and are considered to be inherent in the indirect-gap materials. The indirect-gap energies are estimated to be about 3.20 eV in T1C1 and about 2.63 eV in T1Br, which are consistent with a recent absorption measurements by Nakahara et a!. [6]. 3.2. Electro -modulated LE spectra The electro-modulated LE spectra were measured forE I [100], 1110] and [1111, but since the results are qualitatively the same for the three geometries, only the [100] spectra are shown in figs. I and 2. The signal is proportional to the square of the applied voltage in the low field region, but tends to saturate above ~5 kV/cm. In order to discuss the field effect on the luminescence excitation spectra, we have to consider three separate steps in the excitonic process~the first is the optical creation of the hot excitons by the exciting light, the second is the relaxation process
294
R. Shimizu, T. Koda / Exciton luminescence in TIC1 and T1Br
470 I
460 I
WAVELENGTH (nm) 450 440 430 I I I
420 I
410
P~IB Luminescence
16
~
PHOTON ENERGY (eV)
A
\~
Fig. 2. The luminescence (top), and unmodulated (middle) and electro-modulated (bottom) excitation spectra of T1Br at 1.8 K.
of these hot excitons, and finally the radiative recombination of the excitons, either freely moving in crystal or bound at localized centres. The observed modulation signal should be accounted for as the superposition of the field-perturbation effects of these three processes. In the present case, however, the last effect needs not be considered since the luminescence takes place at the deep-lying localized centres which are almost insensitive to the electric field. Hence, the dominant contributions come from a possible change in the transition probability of the exciton formation and the field-induced dissociation effect of the free excitons during migration in crystal before captured by the luminescence centres. When excitation is made in the direct-exciton region, the former effect is not important since the transition is ab initio dipole-allowed for the X~ X~cedge. Also the absorption coefficient is so large in this spectral region that most of the excitons are created near the surface where the field is almost screened by the photo-carriers, as described before. Hence, the effective modulation mechanism is the field-induced dissociation effect while the photo-generated excitons are diffusing inside from the
R. Shimizu, T. Koda / Exciton luminescence in T1CI and T1Br
295
surface region. We define the critical field Ec by Ec = G/ae, with the exciton binding energy G and the exciton radius a. Using the parameters derived by Kurita and Kobayashi [7], Ec is estimated to be about 15 kV/cm for TIC! and 12 kV/cm for TIBr, respectively, so that the dissociation effect is by no means negligible for the direct-excitons at the field strength presently used. Once dissociated, the free electron and hole follow a decay process which will be considerably different from that of the excitons. For the blue luminescence centres in T1C1, we consider that the luminescence efficiency is much smaller for the free carriers than for the excitons. We expect then that the luminescence intensity is appreciably quenched by electric field, in accord with the negative modulated signals of TlCl observed in the directexciton region. To support this interpretation, the modulated signal saturates in the high field region, F> ~5 ky/cm, indicating the field-screening effect by the dissociated carriers. In T1Br, where the luminescence spectrum is composed of two broad bands, the radiative decay process for the free carriers is supposed to be more complicated than T1C1. On the other hand, for excitation in the indirect-exciton region, the dissociation effect is less important because of the smaller radius of the indirect-exciton than that of the direct-exciton (about one half in T1Br from the estimated binding energies of both excitons [6], and probably of the same order in T1C1, too). Instead, we expect that the parity-forbidden indirect-excitons are significantly perturbed by the electric field giving rise to an increased transition probability for the exciton formation. We believe this explains the positive modulation signals in the indirectexciton region for both halides. Furthermore, it can be shown, from a group-theoretical consideration, that the observed polarization-dependence in this region is reasonably accounted for by the second-order Stark effect of the relevant band edges, details of which will be described elsewhere. As a final remark, it is worth noting that the fine structures observed in the directexciton region of T1CI can be identified with the exciton-related structures, indicated by arrows in fig. 1, observed in the thin-film-absorption measurements [7]. These structures show up as the negative peaks in the electro-modulated LE spectra for the reason mentioned above, but their energies and the spectral profiles reflect the details of the absorption spectrum quite well. In TIBr, the corresponding structure is not well resolved yet, but is likely to be detectable by further investigation.
References [11 121 131 141 [51
R. Shimizu,T. Kodaand T.Murahashi,J. Phys. Soc.Japan 33 (1972) 866;36 (1974) 161. R. Shimizu and T. Koda, J. Phys. Soc. Japan 37 (1974) 1468. R. Shimizu and T. Koda, J. Phys. Soc. Japan 38 (1975) 1550. L. Grabner, Phys. Rev. B4 (1971) 1335. P.J. Dean, Phys. Rev. 168 (1968) 889. 161 J. Nakahara, K. Kobayashi and A. Fujii, J. Phys. Soc. Japan 37 (1974) 1312. [71 S. Kurita and K. Kobayashi, J. Phys. Soc. Japan 30 (1971) 1645.
296
R. Shimizu, T. Koda
/ Exciton luminescence in T1C1 and T1Br
Discussion P.J. Dean: The exact form of these excitation spectra depends rather critically in general on various factors. There are bulk effects such as the influence of changes in the surface to bulk recombination rate ratio and perhaps the ambipolar diffusion length with the condition of the sample, particularly of its surface. There is also the question of the nature of the detected luminescence. For example, in GaP there are systematic differences between excitation spectra for luminescence of distant donor-acceptor pairs and of excitons at shallow isoelectronic traps. Do you find evidence for these effects in these thallium compounds? R. Shimizu: Surely the relative shapes of the luminescence excitation spectra were found to be somewhat dependent on the sample conditions in thallous halides, too, But the sample dependence of the spectra is not so significant in these halides, as compared to your results on GaP. With proper treatment of samples to remove the damaged surface layer and the effect of residual strain, we could obtain well reproducible results which are not so influenced by the factors you pointed out. Heavier electron and hole masses and larger ionicity of these halides seem to be favorable to observe less sample-dependent luminescence excitation spectra, as compared to the case of GaP. The emission we used as monitor of the exciton decay process is a broad luminescence band presumably due to some impurity, such as isoelectronic halogen, or native defect. No emission line has been observed as yet in these halides which is ascribable to the donor—acceptor pairs. R.Z. Bachrach: Your comparison with GaP is not fully clear since GaP is an allowed indirect edge. One would expect the excitation curve to be much weaker than what you found. T. Koda: The absorption coefficient of thallous halides near the forbidden indirect edge is 1, which is certainly considerably smaller than that of GaP near the alof the order of edge. 1 cmHowever, we should note that, in the luminescence excitation spectra, we lowed indirect are observing the total emitted photons which are excited by the incident photons absorbed within the whole volume of a sample. Taking account of the actual sample thickness of about 3 mm and the internal reflection effect from the back surface of the crystal as well, we consider that it is the photon energy dependence of the luminescence efficiency of the excitons rather than the absorption coefficient that determines the relative shapes of the excitation spectra near the indirect threshold. This explains, we believe, the similarity between the excitation spectra of thallous halides and that of Gap, in spite of the appreciable difference in their absorption coefficients near the respective indirect edge.