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Exciton luminescence of spherical GaAs nanocrystals studied by persistent hole-burning spectroscopy
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Physica E 17 (2003) 79 – 81 www.elsevier.com/locate/physe
Exciton luminescence of spherical GaAs nanocrystals studied by persistent hole-burning spectroscopy K. Taniguchi∗ , Y. Morishige, Y. Kanemitsu Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan
Abstract We have studied free-exciton photoluminescence (PL) properties of spherical GaAs nanocrystals embedded in SiO2 matrices fabricated by ion-beam synthesis. Under resonant excitation at energies within the free-exciton PL band, LO-phonon structures appear in the PL hole-burning spectra. By deuterium implantation to GaAs nanocrystals, both the impurity-related PL band and the LO-phonon-assisted luminescence disappear. It is concluded that the exciton–LO-phonon coupling in GaAs nanocrystals is enhanced by local electric 4elds caused by impurities ionization. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 78.67.Hc; 78.55.Cr Keywords: Spherical nanocrystals; Persistent luminescence hole-burning; Exciton–phonon coupling
1. Introduction In semiconductor nanocrystals, electrons and holes generated by optical absorption are spatially con4ned to a small volume. Exciton luminescence properties in nanocrystals are di
Corresponding author. E-mail address: [email protected] (K. Taniguchi).
provides important information for the nature of exciton–phonon coupling. In this work, we have studied exciton luminescence properties of spherical GaAs nanocrystals by means of persistent hole-burning spectroscopy. 2. Experiment Spherical GaAs nanocrystals were fabricated by sequential ion implantation followed by thermal annealing. 3 × 1016 cm−2 Ga+ and 2 × 1016 cm−2 As+ ions were implanted at 75 keV into 100-nm SiO2 4lm on Si (1 0 0) substrate. After thermal annealing at 900◦ C for 60 min, spherical nanocrystals appear (∼6 nm average diameter) in the SiO2 matrix [2]. In addition, deuterium ions were implanted (6 × 1015 cm−2 ) at 600 eV into the GaAs nanocrystals. Two different types of samples, the GaAs sample and the D-implanted GaAs sample were prepared. In PL
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 2 ) 0 0 7 7 8 - 6
K. Taniguchi et al. / Physica E 17 (2003) 79 – 81
measurements, the samples were excited by 325-nm laser at 10 K. In persistent luminescence hole-burning (PLHB) measurements, the samples were irradiated by burning laser (1:6 W cm−2 ) for 30 min at energies within the free-exciton band. The PLHB spectrum is de4ned as (Ia − Ib )=Ib , where Ib and Ia are PL intensities (488-nm laser excitations) measured before and after the burning laser irradiation.
GaAs LO
ZPL
ZPL
Ex.(eV) 1.771 1.746 1.722 1.698
Ex. (eV) 1.771 1.746 1.722 1.698
3. Results and discussion The PL spectrum in the GaAs sample consists of two broad Gaussian bands. The low-energy structure (∼1:6 eV) is attributed to bound-exciton emission, and the high-energy structure (∼1:8 eV) is free-exciton emission [2]. On the other hand, the D-implanted GaAs sample shows one Gaussian PL band: the bound-exciton emission disappears by deuterium implantation. From comparison of the PL spectra between the GaAs and the D-implanted GaAs samples, it is considered that impurities states decrease by hydrogen passivation. This is because hydrogenation of semiconductor is e
D-implanted GaAs
(Ia-Ib) / Ib
80
1.6 (a)
1.65 1.7 1.75 1.8 Photon Energy (eV)
1.6 (b)
1.65 1.7 1.75 1.8 Photon Energy (eV)
Fig. 1. Persistent luminescence hole-burning spectra of: (a) the GaAs nanocrystals sample and (b) the D-implanted GaAs nanocrystals sample.
irradiation, photo-generated carriers neutralize ionized impurities, and then local electric 4elds are reduced in nanocrystals. As a result, the exciton–phonon coupling strength becomes weak in nanocrystals during laser irradiation and the peak shifts higher energy after the burning laser irradiation.
4. Conclusion We have studied exciton luminescence from spherical GaAs nanocrystals fabricated by ion-beam synthesis. The PLHB spectroscopy suggests that the enhancement of the exciton–phonon coupling is mainly caused by local electric 4elds due to impurities ionization.
Acknowledgements The authors would like to thank Prof. H. A. Atwater and Dr. K. S. Min for discussions and sample preparation. This work was supported by The Foundation of Nara Institute of Science and Technology, The Mazda Foundation, and a Grant-in-Aid for Scienti4c Research (KAKENHI 14340093) from Japan Society for the Promotion of Science.
K. Taniguchi et al. / Physica E 17 (2003) 79 – 81
References [1] R. Heiz, I. Mukhametzhanov, O. Stier, A. Madhukar, D. Bimberg, Phys. Rev. Lett. 83 (1999) 4654. [2] Y. Kanemitsu, H. Tanaka, Y. Fukunishi, T. Kushida, K.S. Min, H.A. Atwater, Phys. Rev. B 62 (2000) 5100.
81
[3] J. Weber, S.J. Pearton, W.C. Dautremont-Smith, Appl. Phys. Lett. 49 (1986) 1181. [4] Y. Kanemitsu, K. Taniguchi, unpublished.
Physica E 17 (2003) 79 – 81 www.elsevier.com/locate/physe
Exciton luminescence of spherical GaAs nanocrystals studied by persistent hole-burning spectroscopy K. Taniguchi∗ , Y. Morishige, Y. Kanemitsu Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan
Abstract We have studied free-exciton photoluminescence (PL) properties of spherical GaAs nanocrystals embedded in SiO2 matrices fabricated by ion-beam synthesis. Under resonant excitation at energies within the free-exciton PL band, LO-phonon structures appear in the PL hole-burning spectra. By deuterium implantation to GaAs nanocrystals, both the impurity-related PL band and the LO-phonon-assisted luminescence disappear. It is concluded that the exciton–LO-phonon coupling in GaAs nanocrystals is enhanced by local electric 4elds caused by impurities ionization. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 78.67.Hc; 78.55.Cr Keywords: Spherical nanocrystals; Persistent luminescence hole-burning; Exciton–phonon coupling
1. Introduction In semiconductor nanocrystals, electrons and holes generated by optical absorption are spatially con4ned to a small volume. Exciton luminescence properties in nanocrystals are di
Corresponding author. E-mail address: [email protected] (K. Taniguchi).
provides important information for the nature of exciton–phonon coupling. In this work, we have studied exciton luminescence properties of spherical GaAs nanocrystals by means of persistent hole-burning spectroscopy. 2. Experiment Spherical GaAs nanocrystals were fabricated by sequential ion implantation followed by thermal annealing. 3 × 1016 cm−2 Ga+ and 2 × 1016 cm−2 As+ ions were implanted at 75 keV into 100-nm SiO2 4lm on Si (1 0 0) substrate. After thermal annealing at 900◦ C for 60 min, spherical nanocrystals appear (∼6 nm average diameter) in the SiO2 matrix [2]. In addition, deuterium ions were implanted (6 × 1015 cm−2 ) at 600 eV into the GaAs nanocrystals. Two different types of samples, the GaAs sample and the D-implanted GaAs sample were prepared. In PL
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 2 ) 0 0 7 7 8 - 6
K. Taniguchi et al. / Physica E 17 (2003) 79 – 81
measurements, the samples were excited by 325-nm laser at 10 K. In persistent luminescence hole-burning (PLHB) measurements, the samples were irradiated by burning laser (1:6 W cm−2 ) for 30 min at energies within the free-exciton band. The PLHB spectrum is de4ned as (Ia − Ib )=Ib , where Ib and Ia are PL intensities (488-nm laser excitations) measured before and after the burning laser irradiation.
GaAs LO
ZPL
ZPL
Ex.(eV) 1.771 1.746 1.722 1.698
Ex. (eV) 1.771 1.746 1.722 1.698
3. Results and discussion The PL spectrum in the GaAs sample consists of two broad Gaussian bands. The low-energy structure (∼1:6 eV) is attributed to bound-exciton emission, and the high-energy structure (∼1:8 eV) is free-exciton emission [2]. On the other hand, the D-implanted GaAs sample shows one Gaussian PL band: the bound-exciton emission disappears by deuterium implantation. From comparison of the PL spectra between the GaAs and the D-implanted GaAs samples, it is considered that impurities states decrease by hydrogen passivation. This is because hydrogenation of semiconductor is e
D-implanted GaAs
(Ia-Ib) / Ib
80
1.6 (a)
1.65 1.7 1.75 1.8 Photon Energy (eV)
1.6 (b)
1.65 1.7 1.75 1.8 Photon Energy (eV)
Fig. 1. Persistent luminescence hole-burning spectra of: (a) the GaAs nanocrystals sample and (b) the D-implanted GaAs nanocrystals sample.
irradiation, photo-generated carriers neutralize ionized impurities, and then local electric 4elds are reduced in nanocrystals. As a result, the exciton–phonon coupling strength becomes weak in nanocrystals during laser irradiation and the peak shifts higher energy after the burning laser irradiation.
4. Conclusion We have studied exciton luminescence from spherical GaAs nanocrystals fabricated by ion-beam synthesis. The PLHB spectroscopy suggests that the enhancement of the exciton–phonon coupling is mainly caused by local electric 4elds due to impurities ionization.
Acknowledgements The authors would like to thank Prof. H. A. Atwater and Dr. K. S. Min for discussions and sample preparation. This work was supported by The Foundation of Nara Institute of Science and Technology, The Mazda Foundation, and a Grant-in-Aid for Scienti4c Research (KAKENHI 14340093) from Japan Society for the Promotion of Science.
K. Taniguchi et al. / Physica E 17 (2003) 79 – 81
References [1] R. Heiz, I. Mukhametzhanov, O. Stier, A. Madhukar, D. Bimberg, Phys. Rev. Lett. 83 (1999) 4654. [2] Y. Kanemitsu, H. Tanaka, Y. Fukunishi, T. Kushida, K.S. Min, H.A. Atwater, Phys. Rev. B 62 (2000) 5100.
81
[3] J. Weber, S.J. Pearton, W.C. Dautremont-Smith, Appl. Phys. Lett. 49 (1986) 1181. [4] Y. Kanemitsu, K. Taniguchi, unpublished.