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Suppression of the green photoluminescence band in ZnO embedded into porous opal by spray pyrolysis
ARTICLE IN PRESS
Journal of Luminescence 109 (2004) 25–29
Suppression of the green photoluminescence band in ZnO embedded into porous opal by spray pyrolysis S.M. Abrarova,*, Sh.U. Yuldasheva, S.B. Leeb, T.W. Kanga a
Dongguk University, Quantum-functional Semiconductor Research Center, Seoul, South Korea b Nanotechnology Institute, University of Texas at Dallas, Richardson, TX 75083, USA
Received 8 July 2003; received in revised form 7 November 2003; accepted 15 December 2003
Abstract The photoluminescence (PL) and transmittance characteristics of the zinc oxide embedded into voids of FCC submicron packed silicon dioxide spheres by using technologically simple and inexpensive spray pyrolysis are reported. The uniform formation of ZnO nanocrystalline particles inside of the porous opal takes place after deposition in aqueous solution with zinc nitrite hexahydride precursor followed by thermal annealing. The decrease of green PL is observed due to the inhibition of spontaneous emission through oxygen vacancies in ZnO. The strong red shift of the transmittance characteristics signifies the essential filling of voids in the opal matrix. r 2004 Elsevier B.V. All rights reserved. PACS: 78.55.–m; 42.70.Qs; 82.30.Lp Keywords: ZnO; Photoluminescence; Photonic band-gap; Opal
1. Introduction Among wide-band-gap semiconductors ZnO is one of the promising material for the fabrication of UV and visible light emitting devices, which recently attracted a particular attention due to remarkable optical properties [1–3]. It has been shown already that the low power threshold (2 mW) polariton laser operating at room temperature can be realized on the basis of ZnO [3]. The applications of photonic crystals, proposed by Yablonovitch [4], can improve and even reveal *Corresponding author. Tel./fax: +82-2-2260-3205. E-mail addresses: [email protected], absanj@yahoo. co.uk (S.M. Abrarov).
the new optical properties of materials, useful for the novel device fabrications. The interesting application of photonic crystals is related with ability to suppress the radiative emission, which has been observed in photoluminescence (PL) spectrum. For instance, the strong modification in PL of CdS embedded into porous opal as a result of inhibition of the spontaneous emission was reported [5]. Typically the PL spectra of ZnO nanocrystals exhibit dominant deep level (DL) emission commonly observed at around 2.3 eV is attributed to radiative recombination through the oxygen vacancy defects [6–8] and interstitial Zn ions [8]. This paper presents the experimental evidence of suppression in the green PL region of ZnO
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2003.12.050
ARTICLE IN PRESS 26
S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
nanocrystalline particles embedded into porous opal and the transmittance characteristics of the samples. In order to suppress DL recombinations contributing for a very wide green to yellow spectral region, the ZnO was deposited using the spray pyrolysis in aqueous solution [6,9]. The suppression of the green PL band arises due to inhibition of spontaneous emission, originated from DL recombinations since the emitted light matches a forbidden photonic band of the matrix.
2. Experimental details The deposition of ZnO in the voids of submicron packed SiO2 spheres using spray pyrolysis method includes two stages. Since the hydroscopic porous opal effectively absorbs the deionised water solution, the uniform ZnO crystallization can be obtained inside of the matrix. The powder and film deposition technique and detailed chemical synthesis of ZnO nanocrystals from with zinc nitrite hexahydrate, Zn(NO3)2 6H2O, precursor are described elsewhere [6,9]. Initially samples A and B were kept during 24 and 48 h, respectively, in 0.1 M aqueous solution with zinc nitrite precursor at room temperature. Afterwards the samples were thermally annealed in ambient atmosphere during 1 h at 500 C. As a reference sample the ZnO powder was deposited from the solution under similar conditions. In order to examine the influence of the interface on PL spectrum due to possible changes in the surface morphology and boundary defects, the ZnO film was deposited on the flat crystalline SiO2 substrate (a-quartz). The PL measurements were obtained under 325 nm line of 10 mW He–Cd laser excitation using SPEX spectrometer equipped with 0.75 m monochromator and broadband-sensitive photomultiplier tube. Transmittance characteristics of the samples were measured using xenon lamp as a white light source. The structural study of the samples was carried out with help of XL-30 PHILIPS scanning electron microscope (SEM).
3. Results and discussion After deposition of ZnO, the samples A and B were cleaved and their cross-sections were analyzed. SEM cross-section images showed that ZnO has been formed quite uniformly inside of the porous opal. Fig. 1a shows the SEM image of FCC sub-micron packed silicon dioxide (SiO2) of the bare opal where pores between spheres of 210– 220 nm are clearly seen. Fig. 1b shows the SEM image after ZnO formation on the surface of SiO2 spheres. During the growing process, the shape transformation of spheres in FCC structure results to honeycomb type cells in the plane (1 1 1) and square type cells in the plane (1 0 0). Because of the dislocations in the FCC structure, the seams were formed across (1 1 1) plane as shown in Fig. 1b.
Fig. 1. (a) SEM image of bare porous opal in (1 1 1) plane. (b) SEM image in (1 1 1) plane of the opal embedded with ZnO. Insert shows the SEM image in (1 0 0) plane of the opal embedded with ZnO.
ARTICLE IN PRESS S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
Fig. 2 shows the PL spectra measured at room temperature for the ZnO powder and samples A and B deposited by using the spray pyrolysis during 24 and 48 h, respectively. The emission line with the highest energy in the PL spectra at 3.26 eV of ZnO powder and the samples A and B is attributed to the recombination through free exciton [10]. The free exciton peak manifests stronger in sample A compared with sample B. The next emission line in ZnO powder near 3.17 eV is attributed to the nitrogen impurity [11]. The strong emission line observed at 3.1 eV is attributed to the recombination of the donor–acceptor pairs [12]. Very broad PL of ZnO powder in the region from 2.5 eV towards lower energies, is referred to as the green and yellows bands centered approximately at 510 and 640 nm and attributed to the recombination on impurities such as oxygen vacancies and interstitial Zn ions [8]. The special treatment controlling the growth conditions of ZnO crystals can be used to obtain the reduced green ZnO PL. For instance, the high quality heteroepitaxial film grown in sapphire that manifests weak PL originated from DLs was reported [13]. However, the relevant technologies are very expensive and the optic device fabrication may not be compatible with such treatment methods. Furthermore, if the sample contains disoriented micro- or nano-crystals, it is very difficult to reduce considerably the green PL [7, 14]. In order to suppress the recombination
PL Intensity (a. u.)
ZnO Powder Sample A Sample B
2.25
2.50
2.75
3.00
3.25
Energy (eV)
Fig. 2. PL of the pure ZnO powder and samples A, B.
27
through oxygen vacancies, the opal matrix is chosen with photonic band gap that overlaps the green PL spectrum of ZnO. As shown in Fig. 2, the broad spectral range below 2.5 eV corresponding to green PL band is significantly suppressed in samples A and B. The samples A and B exhibit the strong broadening from blue to UV region that probably is related to following factors. Because of the inhibition of spontaneous emission, the oscillating strength in the green PL region decreases. As a result the radiative emission releases chiefly through allowed transitions leading to the redistribution and broadening of the PL shape as depicted by dashed and dotted lines in the Fig. 2. Secondly, the broadening can be also due to the increase of the surface defects [15]. Comparing the PL of samples A and B one can see that the contribution from the surface defects in the sample B is lower since it was kept longer in the precursor solution forming a bigger size of crystals. Therefore the spectrum for the sample B is narrower and closer to that for the ZnO powder. It is worth remarking that without influence of the photonic band gap, the increase of surface defects could exhibit stronger the green PL. Fig. 3 shows the transmittance characteristics of bare opal and samples A and B with thickness of 1 mm. The minimum corresponding to the middle of the photonic band gap is 505 nm shown by the arrow. The transmittance depends on the refractive index contrast [5,16] and the strong red shift indicates that the significant fraction of ZnO is embedded. To verify that the suppression of green PL is not related to changes in the surface morphology and boundary defects, the pure ZnO film was deposited on the flat SiO2 crystalline substrate from the diluted deionised water solution with zinc nitrite precursor followed by thermal annealing. Insert in the Fig. 3 shows the transmittance of the ZnO film, which is consistent with PL spectrum of pure ZnO powder. Comparing the transmittance of ZnO film and the PL of reference sample one can see that absorption lines at around 2.1 and 2.3 eV are consistent with the emission lines. The wide and relatively deep absorption in the green region shows the high density of DLs,
ARTICLE IN PRESS S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
28
100 Transmittance (%)
Transmittance (%)
100
80
60
Pure ZnO
90 80 70 60 50 40 1.75
2.00
2.25
40
2.50 2.75 3.00 Energy (eV)
3.25
3.50
3.75
20
0 1.75
2.00
2.25
2.50
2.75
3.00
3.25
Energy (eV) Fig. 3. Transmittance of the bare opal (solid line) and samples A, B (dashed and dotted lines). Insert shows the transmittance of ZnO film deposited on crystalline SiO2 substrate.
which indicates that the film formed by polycrystalline particles and the absence of the green PL is not related to decrease of defects like oxygen vacancies and interstitial Zn ion locations. The filling extent can be estimated using the red shift at FWHM of transmittance for the samples with respect to that for the bare opal. The corresponding red shifts for samples A and B are 53 and 61 nm, respectively. Taking into account that bare opal has 26% of voids and 74% of silicon dioxide and using [5] pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi l ¼ 2 0:816D eSiO2 0:74 þ eZnO f pffiffiffiffiffiffi þ eair ð0:26 f Þ ; where l is the optic wavelength, D is the diameter of spheres, eSiO2 ; eZnO ; eair are dielectric constants of SiO2, ZnO and air, the estimated filling factor f of ZnO are found to be 6% and 7% for samples A and B, respectively. This technique also suggests that the enhancement of the radiative emission through DLs for
green to yellow region of the PL spectrum can be observed if the light from blue to UV range matches with forbidden photonic band. This can be achieved by depositing the ZnO particles into the porous opal with diameter of the spheres 140– 160 nm.
4. Conclusion Technologically simple and inexpensive spray pyrolysis method is used to fill voids between silicon dioxide spheres. Two samples were prepared in aqueous solution with zinc nitrite precursor followed by thermal annealing. The green PL is suppressed due to inhibition of spontaneous emission since the photonic band gap of opal overlaps the DL recombination spectrum of ZnO. The samples revealed significant red shift of transmittance whereby the estimated filling factors were determined.
ARTICLE IN PRESS S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
Acknowledgements This work is supported by Korea Science and Engineering Foundation through Quantum-functional Semiconductor Research Center, and research program and fund of Dongguk University, 2003. References [1] Y. Nakanishi, A. Miyake, H. Kominami, T. Aoki, Y. Hatanaka, G. Shimaoka, Appl. Surf. Sci. 142 (1999) 233. [2] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (19) (2002) 3648. [3] M. Zamfirescu, A. Kavokin, B. Gill, G. Malpuech, M. Kaliteevski, Phys. Rev. B 65 (R) (2002) 161201. [4] E. Yablanovitch, Phys. Rev. Lett. 58 (2) (1987) 2059. [5] A. Blanco, C. Lopez, R. Mayoral, H. Migues, F. Meseguer, A. Mifsud, J. Herrero, Appl. Phys. Lett. 73 (13) (1998) 1781. [6] S.A. Studenikin, N. Golero, M. Cocivera, J. Appl. Phys. 84 (4) (1998) 2287.
29
[7] A. van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 87–89 (2000) 454. [8] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, J. Appl. Phys. 79 (10) (1996) 7983. [9] S.A. Studenikin, N. Golero, M. Cocivera, J. Appl. Phys. 83 (4) (1998) 2104. [10] G. Xiong, J. Wilkinson, J. Lyles, K.B. Ucer, R.T. Williams, Radiat. Eff. Defects Solids 158 (2003) 83. [11] A.B.M. Almamun Ashrafi, I. Suemune, H. Kumano, S. Tanaka, Jpn. J. Appl. Phys. 41 (2002) L1281. [12] K. Tamura, T. Makino, A. Tsukazaki, M. Sumiya, S. Fuke, T. Furumochi, M. Lippmaa, C.H. Chia, Y. Segawa, H. Koinuma, M. Kawasaki, Solid State Commun. 127 (2003) 265. [13] S. Bethke, H. Pan, B.W. Wessels, Appl. Phys. Lett. 52 (2) (1988) 138. [14] Sh. Mahamuni, K. Borgohain, B.S. Bendre, V.J. Leppert, S.H. Risbud, J. Appl. Phys. 85 (5) (1999) 2861. [15] A. Wander, N.M. Harrison, Surf. Sci. Lett. 529 (2003) L281. [16] S.G. Romanov, A.V. Fokin, Phys. Solid State 45 (3) (2003) 444.
Journal of Luminescence 109 (2004) 25–29
Suppression of the green photoluminescence band in ZnO embedded into porous opal by spray pyrolysis S.M. Abrarova,*, Sh.U. Yuldasheva, S.B. Leeb, T.W. Kanga a
Dongguk University, Quantum-functional Semiconductor Research Center, Seoul, South Korea b Nanotechnology Institute, University of Texas at Dallas, Richardson, TX 75083, USA
Received 8 July 2003; received in revised form 7 November 2003; accepted 15 December 2003
Abstract The photoluminescence (PL) and transmittance characteristics of the zinc oxide embedded into voids of FCC submicron packed silicon dioxide spheres by using technologically simple and inexpensive spray pyrolysis are reported. The uniform formation of ZnO nanocrystalline particles inside of the porous opal takes place after deposition in aqueous solution with zinc nitrite hexahydride precursor followed by thermal annealing. The decrease of green PL is observed due to the inhibition of spontaneous emission through oxygen vacancies in ZnO. The strong red shift of the transmittance characteristics signifies the essential filling of voids in the opal matrix. r 2004 Elsevier B.V. All rights reserved. PACS: 78.55.–m; 42.70.Qs; 82.30.Lp Keywords: ZnO; Photoluminescence; Photonic band-gap; Opal
1. Introduction Among wide-band-gap semiconductors ZnO is one of the promising material for the fabrication of UV and visible light emitting devices, which recently attracted a particular attention due to remarkable optical properties [1–3]. It has been shown already that the low power threshold (2 mW) polariton laser operating at room temperature can be realized on the basis of ZnO [3]. The applications of photonic crystals, proposed by Yablonovitch [4], can improve and even reveal *Corresponding author. Tel./fax: +82-2-2260-3205. E-mail addresses: [email protected], absanj@yahoo. co.uk (S.M. Abrarov).
the new optical properties of materials, useful for the novel device fabrications. The interesting application of photonic crystals is related with ability to suppress the radiative emission, which has been observed in photoluminescence (PL) spectrum. For instance, the strong modification in PL of CdS embedded into porous opal as a result of inhibition of the spontaneous emission was reported [5]. Typically the PL spectra of ZnO nanocrystals exhibit dominant deep level (DL) emission commonly observed at around 2.3 eV is attributed to radiative recombination through the oxygen vacancy defects [6–8] and interstitial Zn ions [8]. This paper presents the experimental evidence of suppression in the green PL region of ZnO
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2003.12.050
ARTICLE IN PRESS 26
S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
nanocrystalline particles embedded into porous opal and the transmittance characteristics of the samples. In order to suppress DL recombinations contributing for a very wide green to yellow spectral region, the ZnO was deposited using the spray pyrolysis in aqueous solution [6,9]. The suppression of the green PL band arises due to inhibition of spontaneous emission, originated from DL recombinations since the emitted light matches a forbidden photonic band of the matrix.
2. Experimental details The deposition of ZnO in the voids of submicron packed SiO2 spheres using spray pyrolysis method includes two stages. Since the hydroscopic porous opal effectively absorbs the deionised water solution, the uniform ZnO crystallization can be obtained inside of the matrix. The powder and film deposition technique and detailed chemical synthesis of ZnO nanocrystals from with zinc nitrite hexahydrate, Zn(NO3)2 6H2O, precursor are described elsewhere [6,9]. Initially samples A and B were kept during 24 and 48 h, respectively, in 0.1 M aqueous solution with zinc nitrite precursor at room temperature. Afterwards the samples were thermally annealed in ambient atmosphere during 1 h at 500 C. As a reference sample the ZnO powder was deposited from the solution under similar conditions. In order to examine the influence of the interface on PL spectrum due to possible changes in the surface morphology and boundary defects, the ZnO film was deposited on the flat crystalline SiO2 substrate (a-quartz). The PL measurements were obtained under 325 nm line of 10 mW He–Cd laser excitation using SPEX spectrometer equipped with 0.75 m monochromator and broadband-sensitive photomultiplier tube. Transmittance characteristics of the samples were measured using xenon lamp as a white light source. The structural study of the samples was carried out with help of XL-30 PHILIPS scanning electron microscope (SEM).
3. Results and discussion After deposition of ZnO, the samples A and B were cleaved and their cross-sections were analyzed. SEM cross-section images showed that ZnO has been formed quite uniformly inside of the porous opal. Fig. 1a shows the SEM image of FCC sub-micron packed silicon dioxide (SiO2) of the bare opal where pores between spheres of 210– 220 nm are clearly seen. Fig. 1b shows the SEM image after ZnO formation on the surface of SiO2 spheres. During the growing process, the shape transformation of spheres in FCC structure results to honeycomb type cells in the plane (1 1 1) and square type cells in the plane (1 0 0). Because of the dislocations in the FCC structure, the seams were formed across (1 1 1) plane as shown in Fig. 1b.
Fig. 1. (a) SEM image of bare porous opal in (1 1 1) plane. (b) SEM image in (1 1 1) plane of the opal embedded with ZnO. Insert shows the SEM image in (1 0 0) plane of the opal embedded with ZnO.
ARTICLE IN PRESS S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
Fig. 2 shows the PL spectra measured at room temperature for the ZnO powder and samples A and B deposited by using the spray pyrolysis during 24 and 48 h, respectively. The emission line with the highest energy in the PL spectra at 3.26 eV of ZnO powder and the samples A and B is attributed to the recombination through free exciton [10]. The free exciton peak manifests stronger in sample A compared with sample B. The next emission line in ZnO powder near 3.17 eV is attributed to the nitrogen impurity [11]. The strong emission line observed at 3.1 eV is attributed to the recombination of the donor–acceptor pairs [12]. Very broad PL of ZnO powder in the region from 2.5 eV towards lower energies, is referred to as the green and yellows bands centered approximately at 510 and 640 nm and attributed to the recombination on impurities such as oxygen vacancies and interstitial Zn ions [8]. The special treatment controlling the growth conditions of ZnO crystals can be used to obtain the reduced green ZnO PL. For instance, the high quality heteroepitaxial film grown in sapphire that manifests weak PL originated from DLs was reported [13]. However, the relevant technologies are very expensive and the optic device fabrication may not be compatible with such treatment methods. Furthermore, if the sample contains disoriented micro- or nano-crystals, it is very difficult to reduce considerably the green PL [7, 14]. In order to suppress the recombination
PL Intensity (a. u.)
ZnO Powder Sample A Sample B
2.25
2.50
2.75
3.00
3.25
Energy (eV)
Fig. 2. PL of the pure ZnO powder and samples A, B.
27
through oxygen vacancies, the opal matrix is chosen with photonic band gap that overlaps the green PL spectrum of ZnO. As shown in Fig. 2, the broad spectral range below 2.5 eV corresponding to green PL band is significantly suppressed in samples A and B. The samples A and B exhibit the strong broadening from blue to UV region that probably is related to following factors. Because of the inhibition of spontaneous emission, the oscillating strength in the green PL region decreases. As a result the radiative emission releases chiefly through allowed transitions leading to the redistribution and broadening of the PL shape as depicted by dashed and dotted lines in the Fig. 2. Secondly, the broadening can be also due to the increase of the surface defects [15]. Comparing the PL of samples A and B one can see that the contribution from the surface defects in the sample B is lower since it was kept longer in the precursor solution forming a bigger size of crystals. Therefore the spectrum for the sample B is narrower and closer to that for the ZnO powder. It is worth remarking that without influence of the photonic band gap, the increase of surface defects could exhibit stronger the green PL. Fig. 3 shows the transmittance characteristics of bare opal and samples A and B with thickness of 1 mm. The minimum corresponding to the middle of the photonic band gap is 505 nm shown by the arrow. The transmittance depends on the refractive index contrast [5,16] and the strong red shift indicates that the significant fraction of ZnO is embedded. To verify that the suppression of green PL is not related to changes in the surface morphology and boundary defects, the pure ZnO film was deposited on the flat SiO2 crystalline substrate from the diluted deionised water solution with zinc nitrite precursor followed by thermal annealing. Insert in the Fig. 3 shows the transmittance of the ZnO film, which is consistent with PL spectrum of pure ZnO powder. Comparing the transmittance of ZnO film and the PL of reference sample one can see that absorption lines at around 2.1 and 2.3 eV are consistent with the emission lines. The wide and relatively deep absorption in the green region shows the high density of DLs,
ARTICLE IN PRESS S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
28
100 Transmittance (%)
Transmittance (%)
100
80
60
Pure ZnO
90 80 70 60 50 40 1.75
2.00
2.25
40
2.50 2.75 3.00 Energy (eV)
3.25
3.50
3.75
20
0 1.75
2.00
2.25
2.50
2.75
3.00
3.25
Energy (eV) Fig. 3. Transmittance of the bare opal (solid line) and samples A, B (dashed and dotted lines). Insert shows the transmittance of ZnO film deposited on crystalline SiO2 substrate.
which indicates that the film formed by polycrystalline particles and the absence of the green PL is not related to decrease of defects like oxygen vacancies and interstitial Zn ion locations. The filling extent can be estimated using the red shift at FWHM of transmittance for the samples with respect to that for the bare opal. The corresponding red shifts for samples A and B are 53 and 61 nm, respectively. Taking into account that bare opal has 26% of voids and 74% of silicon dioxide and using [5] pffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffi l ¼ 2 0:816D eSiO2 0:74 þ eZnO f pffiffiffiffiffiffi þ eair ð0:26 f Þ ; where l is the optic wavelength, D is the diameter of spheres, eSiO2 ; eZnO ; eair are dielectric constants of SiO2, ZnO and air, the estimated filling factor f of ZnO are found to be 6% and 7% for samples A and B, respectively. This technique also suggests that the enhancement of the radiative emission through DLs for
green to yellow region of the PL spectrum can be observed if the light from blue to UV range matches with forbidden photonic band. This can be achieved by depositing the ZnO particles into the porous opal with diameter of the spheres 140– 160 nm.
4. Conclusion Technologically simple and inexpensive spray pyrolysis method is used to fill voids between silicon dioxide spheres. Two samples were prepared in aqueous solution with zinc nitrite precursor followed by thermal annealing. The green PL is suppressed due to inhibition of spontaneous emission since the photonic band gap of opal overlaps the DL recombination spectrum of ZnO. The samples revealed significant red shift of transmittance whereby the estimated filling factors were determined.
ARTICLE IN PRESS S.M. Abrarov et al. / Journal of Luminescence 109 (2004) 25–29
Acknowledgements This work is supported by Korea Science and Engineering Foundation through Quantum-functional Semiconductor Research Center, and research program and fund of Dongguk University, 2003. References [1] Y. Nakanishi, A. Miyake, H. Kominami, T. Aoki, Y. Hatanaka, G. Shimaoka, Appl. Surf. Sci. 142 (1999) 233. [2] C.J. Lee, T.J. Lee, S.C. Lyu, Y. Zhang, H. Ruh, H.J. Lee, Appl. Phys. Lett. 81 (19) (2002) 3648. [3] M. Zamfirescu, A. Kavokin, B. Gill, G. Malpuech, M. Kaliteevski, Phys. Rev. B 65 (R) (2002) 161201. [4] E. Yablanovitch, Phys. Rev. Lett. 58 (2) (1987) 2059. [5] A. Blanco, C. Lopez, R. Mayoral, H. Migues, F. Meseguer, A. Mifsud, J. Herrero, Appl. Phys. Lett. 73 (13) (1998) 1781. [6] S.A. Studenikin, N. Golero, M. Cocivera, J. Appl. Phys. 84 (4) (1998) 2287.
29
[7] A. van Dijken, E.A. Meulenkamp, D. Vanmaekelbergh, A. Meijerink, J. Lumin. 87–89 (2000) 454. [8] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, J. Appl. Phys. 79 (10) (1996) 7983. [9] S.A. Studenikin, N. Golero, M. Cocivera, J. Appl. Phys. 83 (4) (1998) 2104. [10] G. Xiong, J. Wilkinson, J. Lyles, K.B. Ucer, R.T. Williams, Radiat. Eff. Defects Solids 158 (2003) 83. [11] A.B.M. Almamun Ashrafi, I. Suemune, H. Kumano, S. Tanaka, Jpn. J. Appl. Phys. 41 (2002) L1281. [12] K. Tamura, T. Makino, A. Tsukazaki, M. Sumiya, S. Fuke, T. Furumochi, M. Lippmaa, C.H. Chia, Y. Segawa, H. Koinuma, M. Kawasaki, Solid State Commun. 127 (2003) 265. [13] S. Bethke, H. Pan, B.W. Wessels, Appl. Phys. Lett. 52 (2) (1988) 138. [14] Sh. Mahamuni, K. Borgohain, B.S. Bendre, V.J. Leppert, S.H. Risbud, J. Appl. Phys. 85 (5) (1999) 2861. [15] A. Wander, N.M. Harrison, Surf. Sci. Lett. 529 (2003) L281. [16] S.G. Romanov, A.V. Fokin, Phys. Solid State 45 (3) (2003) 444.