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Localized excitonic states in ZnS–ZnSe single quantum wells
Superlattices and Microstructures, Vol. 24, No. 2, 1998 Article No. sm980575
Localized excitonic states in ZnS–ZnSe single quantum wells V. V. T ISHCHENKO , N. V. B ONDAR Institute of Physics, National Academy of Sciences, 252026 Kiev, Ukraine
A. V. K OVALENKO Dnepropetrovsk State University, 320625 Dnepropetrovsk, Ukraine
M. P. H ALSALL Department of Physics, University of Manchester Institute of Science and Technology, Manchester, M60 1QD, U.K. P. L ILLEY Department of Electrical Engineering, The University, Manchester M13 9PL, U.K.
(Received 26 February 1998)
We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well. c 1998 Academic Press
Key words: quantum well, interface disordering, localized exciton, photoluminescence.
1. Introduction ZnSe-based quantum well (QW) structures are considered as promising materials for blue light emitting devices [1]. The quality of these devices depends to a large extent on the quality of the interfaces between the two materials within which QW structures are constructed. One of the most reliable and powerful characterization methods relies on photoluminescence studies of excitonic recombination in the quantum wells. Many previous investigations have demonstrated that both the spectral lineshape and width of the low temperature excitonic luminescence bands are very sensitive to any interface deviation from plane. Studies of both MBEand MOCVD-grown structures have shown that interface disordering leads to inhomogeneous broadening of excitonic photoluminescence (PL) [2–4]. The magnitude of this disorder in the best samples typically corresponds to no more than one monolayer. The present work refers to PL characterization of a single quantum well (SQW) produced by the comparatively simple technique of PAVPE [5]. We have examined both excitonic PL and reflection from (ZnSe)2 (ZnS)11 SQWs and observed localized excitonic states, the spectral extent of which relates to the shift in the 0749–6036/98/080143 + 05
$30.00/0
c 1998 Academic Press
144
Superlattices and Microstructures, Vol. 24, No. 2, 1998
Response (AU)
120
D C 60
B Ehh A E 412
416 Wavelength (nm)
Fig. 1. Photoluminescence (A,B,C,D) and reflectivity (E) spectra from typical sample. A is the measured over the whole surface of the sample, B, C and D are measured with the laser focused to random positions on the sample. The excitation power for B, C and D is 400 times higher than for A. The spectra are normalized and shifted for clarity.
ground energy level of exciton with the QW width. From temperature and intensity dependencies of PL bands measured in random positions over the surface of the samples we found the edge of mobility of localized excitonic states to be 2.991 eV, which is 6 meV below the ground state of a free exciton.
2. Methods and materials The (ZnSe)2 (ZnS)11 SQWs were prepared from the gas phase using high purity ZnSe and ZnS as source materials. A horizontal-type quartz reactor was used with a 250 ◦ C cm−1 temperature gradient in the deposition zone, and negligible elsewhere in the reactor. During the growth, the layers of ZnSe were illuminated by the 441.6 nm line of a cw He–Cd laser in order to improve their quality as described in reference [5]. The SQWs were grown on GaAs(100) substrates and thus are coherently strained since their total thickness is below the critical value (100Å) for the onset of misfit dislocation. As a result, QW width fluctuations do not lead to planar strain deviation. During the measurements the samples were mounted inside an optical cryostat operating in either heliumimmersion or gas-flow mode. The PL was excited by the 325 nm line of a 10 mW cw He–Cd laser and analysed by a computer-controlled double monochromator with photon-counting detection.
3. Results In Fig. 1A we show the photoluminescence spectrum in the wavelength region of the excitonic absorption having taken over the whole of the sample surface. The excitonic nature of the observed PL band is confirmed by reflection spectrum shown in Fig. 1E. This displays the typical excitonic feature in reflection at the same energy as the dominant PL band shown by Fig. 1A. Both PL and reflection spectra are blue shifted by 188 meV relative to the excitonic energy observed in bulk ZnSe. This shift is caused by the combined effects of carrier confinement and compressive strain in the ZnSe well. The contribution of the strain to the observed shift is no more than 5% [6].
Superlattices and Microstructures, Vol. 24, No. 2, 1998
145
3
Em (eV)
2.990
2 2.984
1
40
120
80 (W cm–2)
Iex
Fig. 2. Dependence of the peak energy (E m ) of the photoluminescence on excitation intensity (Iex ). 1, 2 and 3 correspond to curves B, C and D, respectively, on Fig. 1. The lines are drawn through the data points to guide the eye.
2.990
3
Em (eV)
2
2.984 1
10
20
30
Temperature (K) Fig. 3. Dependence of the peak energy (E m ) of the photoluminescence on sample temperature. The curves are labelled as in Fig. 2.
The observed spectral position of the main peak (E m ), shown to be at 415.9 nm (2.980 eV) in Fig. 1A, changes slightly from sample to sample. The shape of this band is asymmetric and displays a series of steps which are not equally spaced in energy and thus cannot be explained by transitions involving phonons. We believe that these steps are due to inhomogeneity of the quantum well interfaces. To try to confirm this conclusion we repeated the PL measurements in random positions on the sample area. It was observed that there was a dependence of E m on the position of the excitation spot on the sample surface (Figs 1B, C and D). Figures 2 and 3 show the dependencies of E m for different regions of the samples surface area on intensity
146
Superlattices and Microstructures, Vol. 24, No. 2, 1998
(Iex ) of excitation light and temperature (T ), respectively. As a rule of thumb, the PL band shifts with Iex with increasing energy and as the intensity becomes larger tends to the position of 2.991 eV. This is accompanied by an increase in the PL band asymmetry as can be clearly observed from the increasing tail at low energy. A similar behaviour is observed when T is changed. However, there are sample regions where E m = 2.991 eV is independent of Iex and T : in these cases the shape of PL band does not depend on Iex .
4. Discussion We explain the observed behaviour of the excitonic emission from our samples by a model involving exciton localization in random potential wells resulting from the interface disordering and resulting well width fluctuations. On the scale of exciton Bohr radius (aB ) the interface disorder leads to a 2D potential profile confining carriers in the plane of the layers. As a result, a tail of localized states for which the occupying carriers obeys the Fermi–Dirac statistic, arises [7]. The utilization of the F–D statistic for excitons, which can be treated as bosons when they are well separated becomes possible due to the exciton’s internal structure: each separate localizing region of quantum well can localize no more than one exciton when the spatial extent of the region is comparable to aB . Using the above model we can interpret the data on Fig. 2 as the effect of increasing/decreasing of chemical potential of the localized excitons system when its density is increasing/decreasing. It is straightforward to see that when the magnitude of Iex is high enough, the chemical potential can reach the mobility edge (E c ) of the localized states. This leads to a complete filling of the localized states and, as a result, to a stabilization of the position of the PL band at E c . It follows that in our case E c is equal to 2.991 eV. We also conclude that the width (35–45 meV) of the observed PL band implies that the magnitude of the interface disordering is of the order of one monolayer. The key question is whether or not E c corresponds to the ground state of the free exciton. To answer this it would be necessary to compare PL and photoluminescence excitation (PLE) measurements. Not having access to PLE data we turn our attention to the existence of a weak PL peak (E hh at 2.997 eV/417.5 nm) in the vicinity of the reflection minimum. This peak can only be distinguished on the integrated spectrum. It is known that the reflection minimum is close (allowing for the uncertainty of the Longitudinal–Transverse (L–T) splitting) to the free exciton level. Taking into account that L–T splitting for ZnSe is only 1.5 meV we can conclude that the E hh peak is due to free exciton recombination. We conclude that the mobility edge is 6 meV below free exciton energy in the case of our 11Å ZnSe quantum wells. There remains a question about the nature of the delocalized states accounting for luminescence between E c and E hh . This emission may be due to excitons in those quantum well areas where RS < aB [8]. Inside these areas excitons behave like free particles and undergo scattering only when they reach the region boundaries. Following this approach the PL shown by Fig. 1D is interpreted as representing mainly the excitonic emission of areas with Rs < aB . Moreover, the observed temperature behaviour of the PL implies that the transition from localized to delocalized excitonic states starts at around T = 15 K. Acknowledgements—The authors would like to acknowledge the financial support of NATO linkage grant HTECH.LG 941302. VVT acknowledges the partial support of INTAS 94-324.
References [1] }R. A. Reynolds, J. Vacuum Sci. Technol. A7, 269 (1989). [2] }P. A. Kopyev, I. N. Uralcev, D. R. Yakovlev, Al. L. Efros, and A. V. Vinokurova, Fiz. Tech. Poluprovodnikov. 22, 424 (1988). [3] }H. Kalt, J. Collett, S. D. Baranovskii, Rosari Saleh, P. Thomas, Le Si Dang, and J. Cibert, Phys. Rev. B45, 4253 (1992).
Superlattices and Microstructures, Vol. 24, No. 2, 1998
[4] [5] [6] [7] [8]
147
}T. Taguchi, Y. Kawakami, and Y. Yamodo, Physica B191, 23 (1993). }A. V. Kovalenko and V. V. Tishchenko, Jap. J. Appl. Phys. 34 (Suppl. 34-1), 209 (1995). }V. V. Tishchenko, Y. Raptis, E. Anastassakis, and N. V. Bondar, Solid State Comm. 96, 793 (1995). }I. A. Kash, M. Zachau, E. E. Mendez, and J. M. Hong, Phys. Rev. Lett. 66, 2274 (1991). }B. M. Askinadze, E. Cohen, Azra Ron, and L. Pfeiffer, Phys. Rev. B47, 10613 (1993).
Localized excitonic states in ZnS–ZnSe single quantum wells V. V. T ISHCHENKO , N. V. B ONDAR Institute of Physics, National Academy of Sciences, 252026 Kiev, Ukraine
A. V. K OVALENKO Dnepropetrovsk State University, 320625 Dnepropetrovsk, Ukraine
M. P. H ALSALL Department of Physics, University of Manchester Institute of Science and Technology, Manchester, M60 1QD, U.K. P. L ILLEY Department of Electrical Engineering, The University, Manchester M13 9PL, U.K.
(Received 26 February 1998)
We present low temperature photoluminescence spectra taken from an 11Å ZnSe quantum well in ZnS barriers. The samples are grown by the technique of photo-assisted vapour phase epitaxy (PAVPE) and the spectra show evidence for interface disorder. The observed dependences of the excitonic luminescence on excitation power and temperature are interpreted by a model involving excitonic localization below an exciton mobility edge. This mobility edge is measured for these samples to be 6 meV below the free exciton energy in the ideal quantum well. c 1998 Academic Press
Key words: quantum well, interface disordering, localized exciton, photoluminescence.
1. Introduction ZnSe-based quantum well (QW) structures are considered as promising materials for blue light emitting devices [1]. The quality of these devices depends to a large extent on the quality of the interfaces between the two materials within which QW structures are constructed. One of the most reliable and powerful characterization methods relies on photoluminescence studies of excitonic recombination in the quantum wells. Many previous investigations have demonstrated that both the spectral lineshape and width of the low temperature excitonic luminescence bands are very sensitive to any interface deviation from plane. Studies of both MBEand MOCVD-grown structures have shown that interface disordering leads to inhomogeneous broadening of excitonic photoluminescence (PL) [2–4]. The magnitude of this disorder in the best samples typically corresponds to no more than one monolayer. The present work refers to PL characterization of a single quantum well (SQW) produced by the comparatively simple technique of PAVPE [5]. We have examined both excitonic PL and reflection from (ZnSe)2 (ZnS)11 SQWs and observed localized excitonic states, the spectral extent of which relates to the shift in the 0749–6036/98/080143 + 05
$30.00/0
c 1998 Academic Press
144
Superlattices and Microstructures, Vol. 24, No. 2, 1998
Response (AU)
120
D C 60
B Ehh A E 412
416 Wavelength (nm)
Fig. 1. Photoluminescence (A,B,C,D) and reflectivity (E) spectra from typical sample. A is the measured over the whole surface of the sample, B, C and D are measured with the laser focused to random positions on the sample. The excitation power for B, C and D is 400 times higher than for A. The spectra are normalized and shifted for clarity.
ground energy level of exciton with the QW width. From temperature and intensity dependencies of PL bands measured in random positions over the surface of the samples we found the edge of mobility of localized excitonic states to be 2.991 eV, which is 6 meV below the ground state of a free exciton.
2. Methods and materials The (ZnSe)2 (ZnS)11 SQWs were prepared from the gas phase using high purity ZnSe and ZnS as source materials. A horizontal-type quartz reactor was used with a 250 ◦ C cm−1 temperature gradient in the deposition zone, and negligible elsewhere in the reactor. During the growth, the layers of ZnSe were illuminated by the 441.6 nm line of a cw He–Cd laser in order to improve their quality as described in reference [5]. The SQWs were grown on GaAs(100) substrates and thus are coherently strained since their total thickness is below the critical value (100Å) for the onset of misfit dislocation. As a result, QW width fluctuations do not lead to planar strain deviation. During the measurements the samples were mounted inside an optical cryostat operating in either heliumimmersion or gas-flow mode. The PL was excited by the 325 nm line of a 10 mW cw He–Cd laser and analysed by a computer-controlled double monochromator with photon-counting detection.
3. Results In Fig. 1A we show the photoluminescence spectrum in the wavelength region of the excitonic absorption having taken over the whole of the sample surface. The excitonic nature of the observed PL band is confirmed by reflection spectrum shown in Fig. 1E. This displays the typical excitonic feature in reflection at the same energy as the dominant PL band shown by Fig. 1A. Both PL and reflection spectra are blue shifted by 188 meV relative to the excitonic energy observed in bulk ZnSe. This shift is caused by the combined effects of carrier confinement and compressive strain in the ZnSe well. The contribution of the strain to the observed shift is no more than 5% [6].
Superlattices and Microstructures, Vol. 24, No. 2, 1998
145
3
Em (eV)
2.990
2 2.984
1
40
120
80 (W cm–2)
Iex
Fig. 2. Dependence of the peak energy (E m ) of the photoluminescence on excitation intensity (Iex ). 1, 2 and 3 correspond to curves B, C and D, respectively, on Fig. 1. The lines are drawn through the data points to guide the eye.
2.990
3
Em (eV)
2
2.984 1
10
20
30
Temperature (K) Fig. 3. Dependence of the peak energy (E m ) of the photoluminescence on sample temperature. The curves are labelled as in Fig. 2.
The observed spectral position of the main peak (E m ), shown to be at 415.9 nm (2.980 eV) in Fig. 1A, changes slightly from sample to sample. The shape of this band is asymmetric and displays a series of steps which are not equally spaced in energy and thus cannot be explained by transitions involving phonons. We believe that these steps are due to inhomogeneity of the quantum well interfaces. To try to confirm this conclusion we repeated the PL measurements in random positions on the sample area. It was observed that there was a dependence of E m on the position of the excitation spot on the sample surface (Figs 1B, C and D). Figures 2 and 3 show the dependencies of E m for different regions of the samples surface area on intensity
146
Superlattices and Microstructures, Vol. 24, No. 2, 1998
(Iex ) of excitation light and temperature (T ), respectively. As a rule of thumb, the PL band shifts with Iex with increasing energy and as the intensity becomes larger tends to the position of 2.991 eV. This is accompanied by an increase in the PL band asymmetry as can be clearly observed from the increasing tail at low energy. A similar behaviour is observed when T is changed. However, there are sample regions where E m = 2.991 eV is independent of Iex and T : in these cases the shape of PL band does not depend on Iex .
4. Discussion We explain the observed behaviour of the excitonic emission from our samples by a model involving exciton localization in random potential wells resulting from the interface disordering and resulting well width fluctuations. On the scale of exciton Bohr radius (aB ) the interface disorder leads to a 2D potential profile confining carriers in the plane of the layers. As a result, a tail of localized states for which the occupying carriers obeys the Fermi–Dirac statistic, arises [7]. The utilization of the F–D statistic for excitons, which can be treated as bosons when they are well separated becomes possible due to the exciton’s internal structure: each separate localizing region of quantum well can localize no more than one exciton when the spatial extent of the region is comparable to aB . Using the above model we can interpret the data on Fig. 2 as the effect of increasing/decreasing of chemical potential of the localized excitons system when its density is increasing/decreasing. It is straightforward to see that when the magnitude of Iex is high enough, the chemical potential can reach the mobility edge (E c ) of the localized states. This leads to a complete filling of the localized states and, as a result, to a stabilization of the position of the PL band at E c . It follows that in our case E c is equal to 2.991 eV. We also conclude that the width (35–45 meV) of the observed PL band implies that the magnitude of the interface disordering is of the order of one monolayer. The key question is whether or not E c corresponds to the ground state of the free exciton. To answer this it would be necessary to compare PL and photoluminescence excitation (PLE) measurements. Not having access to PLE data we turn our attention to the existence of a weak PL peak (E hh at 2.997 eV/417.5 nm) in the vicinity of the reflection minimum. This peak can only be distinguished on the integrated spectrum. It is known that the reflection minimum is close (allowing for the uncertainty of the Longitudinal–Transverse (L–T) splitting) to the free exciton level. Taking into account that L–T splitting for ZnSe is only 1.5 meV we can conclude that the E hh peak is due to free exciton recombination. We conclude that the mobility edge is 6 meV below free exciton energy in the case of our 11Å ZnSe quantum wells. There remains a question about the nature of the delocalized states accounting for luminescence between E c and E hh . This emission may be due to excitons in those quantum well areas where RS < aB [8]. Inside these areas excitons behave like free particles and undergo scattering only when they reach the region boundaries. Following this approach the PL shown by Fig. 1D is interpreted as representing mainly the excitonic emission of areas with Rs < aB . Moreover, the observed temperature behaviour of the PL implies that the transition from localized to delocalized excitonic states starts at around T = 15 K. Acknowledgements—The authors would like to acknowledge the financial support of NATO linkage grant HTECH.LG 941302. VVT acknowledges the partial support of INTAS 94-324.
References [1] }R. A. Reynolds, J. Vacuum Sci. Technol. A7, 269 (1989). [2] }P. A. Kopyev, I. N. Uralcev, D. R. Yakovlev, Al. L. Efros, and A. V. Vinokurova, Fiz. Tech. Poluprovodnikov. 22, 424 (1988). [3] }H. Kalt, J. Collett, S. D. Baranovskii, Rosari Saleh, P. Thomas, Le Si Dang, and J. Cibert, Phys. Rev. B45, 4253 (1992).
Superlattices and Microstructures, Vol. 24, No. 2, 1998
[4] [5] [6] [7] [8]
147
}T. Taguchi, Y. Kawakami, and Y. Yamodo, Physica B191, 23 (1993). }A. V. Kovalenko and V. V. Tishchenko, Jap. J. Appl. Phys. 34 (Suppl. 34-1), 209 (1995). }V. V. Tishchenko, Y. Raptis, E. Anastassakis, and N. V. Bondar, Solid State Comm. 96, 793 (1995). }I. A. Kash, M. Zachau, E. E. Mendez, and J. M. Hong, Phys. Rev. Lett. 66, 2274 (1991). }B. M. Askinadze, E. Cohen, Azra Ron, and L. Pfeiffer, Phys. Rev. B47, 10613 (1993).