- Email: [email protected]
Infrared photoluminescence from Er-doped a-GaAsN alloys
Journal of Non-Crystalline Solids 266±269 (2000) 854±858
www.elsevier.com/locate/jnoncrysol
Infrared photoluminescence from Er-doped a-GaAsN alloys A.R. Zanatta Instituto de Fõsica de S~ ao Carlos, Universidade de S~ ao Paulo, P.O. Box 369, 13560-250 S~ ao Carlos, S.P., Brazil
Abstract Amorphous gallium±arsenic±nitrogen (a-GaAsN) ®lms were deposited by co-sputtering from a crystalline GaAs wafer partially covered with metallic Er pieces. The ®lms were deposited at room temperature under dierent partial pressures of Ar and N2 . After deposition, measurements of optical transmission in the visible±ultraviolet (VIS±UV) energy range, photoluminescence (PL) in the infrared (IR) region, and Raman scattering spectroscopy were made. Compositional analysis was also performed indicating an Er content of 0.5 at.% and a N concentration that scales with the N2 partial pressure during deposition. According to the experimental results, larger N content samples have larger optical band-gaps and more intense Er-related PL signals at 1540 nm. This dependence is analyzed in terms of the compositional, electronic and structural properties of each ®lm. Ó 2000 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Most of the current eorts on the study of Erdoped semiconductors stem from their possible application as light sources operating at 1540 nm due to the Er3 4 I13=2 ! 4 I15=2 optical transition [1]. At this wavelength, which coincides with the minimum transmission loss of silica-based optical ®bers, the realization of hybrid opto-electronic devices can provide improvements in systems of communications [1]. However, while the Er-related emission wavelength is almost independent of temperature, its luminescence intensity decreases as the temperature is increased. This thermal quenching eect observed in various semiconductor hosts [2] limits the light emission eciency at room temperature and, as a consequence, prevents
E-mail address: [email protected] (A.R. Zanatta).
the employment of the existing Er-based devices in some applications [1]. The suggestion [3] that the thermal quenching of intra-4f-shell emissions from Er3 ions in semiconductor hosts decreases in wide band-gap (WBG) materials has motivated several recent studies of these compounds [4±7]. Additionally, WBG materials have advantages in the design of opto-electronic devices because of chemical stability, carrier generation, and physical stability over a wide temperature range. Moreover, in the speci®c case of Er-doped semiconductors, it is assumed that the bonding (or atomic arrangement) found in WBG compounds may be better for forming the required environment for ecient light emission [7]. In other words, besides the energetic aspects involved in the mechanisms of excitation and de-excitation of carriers, the Er3 atomic environment should be a factor. In this respect, the present contribution deals with a series of Er-doped a-GaAsN alloys with diering atomic compositions and optical band-gaps. In view of
0022-3093/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 5 5 - 8
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
855
the present experimental results, and compared with other hosts [2], Er-doped amorphous GaAsN ®lms can have interesting applications in the ®eld of photonics. 2. Experimental The a-GaAs(N) ®lms were deposited at room temperature (RT) by radio frequency (RF) cosputtering a crystalline (c-)GaAs wafer covered at random with small metallic Er pieces. All deposition runs employed polished c-Si and soda±lime glass substrates and took place under a total pressure of 0.7 Pa with dierent mixtures of Ar and N2 . Films deposited with pure Ar or N2 atmospheres were also prepared for comparison purposes. The atomic composition of the aGaAs(N) series was investigated by means of either energy dispersive X-ray spectroscopy (EDS) or X-ray photoelectron spectroscopy (XPS). The E04 optical band-gap (energy at which the absorption coecient is 104 cm-1 ) was obtained from optical transmission measurements in the visible± ultraviolet (VIS±UV) energy range. Photoluminescence (PL) spectroscopy was carried out at RT and 77 K with the samples being excited with the 514.5 nm line of an Ar -laser ( 6 100 mW). The emitting radiation was dispersed with a 0.275 m Czerny±Turner type monochromator, detected with a liquid-nitrogen cooled p±i±n Ge detector and standard in-phase techniques. RT Raman scattering measurements employed the same laser line ( 6 200 mW), a double 0.85 m monochromator, a cooled photomultiplier tube and photon counting. Despite the bene®cial eects of thermal anneals on the light emission at 1540 nm in Erdoped semiconductors, the data in this report are for as-deposited samples. 3. Results Fig. 1 displays the transmission spectra in the 300±1500 nm wavelength range of the series of Er-doped a-GaAs(N) ®lms deposited on soda±lime glass substrates. The spectrum of a non-intentionally Er-doped a-GaAs ®lm is also represented
Fig. 1. Optical transmission in the 300±1500 nm wavelength range of a-GaAs(N) samples. The spectra have been vertically shifted for clarity. The following apply: 1 (non-intentionally doped a-GaAs), 2 (Er-doped a-GaAs), 3 (Er-doped a-GaAs deposited under a N2 partial pressure PN2 of 7 10ÿ3 Pa), 4 (Er-doped a-GaAs with PN2 1:4 10ÿ2 Pa), 5 (Er-doped aGaAs with PN 2 7 10ÿ2 Pa), 6 (Er-doped a-GaAs with PN2 0:14 Pa), and 7 (Er-doped a-GaAs with PN2 0:7 Pa). For all Er-doped ®lms the Er content remained at 0.5 at.%.
for comparison. As can be seen from Fig. 1 the insertion of both Er and N atoms induces changes in the optical transmission of the samples. Either EDS or XPS analysis indicates a N concentration that scales with the N2 partial pressure used during deposition. Regardless of the N concentration of each sample, the Er content remained around 0.5 at.% in all Er-doped samples. Concerning the atomic structure of these asdeposited a-GaAsN samples, all of them had a Raman feature at 250 cmÿ1 characteristic of distorted GaAs-based matrices [8]. Fig. 2 illustrates the emission at 1540 nm due to the 4 I13=2 ! 4 I15=2 Er3 transition in several dierent Er-containing hosts. According to Fig. 2, at 77
856
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
Fig. 2. Photoluminescence PL spectra of dierent Er-doped ahosts. The emission at 1540 nm is due to the 4 I13=2 ! 4 I15=2 Er3 transition. The spectra were achieved at 77 K under excitation with 514.5 nm photons [10]. Notice the multiplying factors of each spectrum that has been vertically shifted for clarity.
and a-GeN alloys [12]. Indeed, considering the dierence in Ga±N and As±N bond energies, it is not surprising that the N atoms readily replace the As atoms with the formation of Ga±N bonds [13]. The present Er-doped a-GaAs samples have a greater intensity at 1540 nm, with no background contribution due to micro-defect formation, contrary to previous reports on Er-implanted c-GaAs [14]. Another interesting aspect of light emission in the IR region, observed in the most nitrogenated a-GaAsN ®lm, was the existence of light emission at about 1200 nm (not shown in Fig. 2). This emission was observed [15] and is attributed, in the present case, to defects either due to the amorphous state of the matrix or due to the N atoms. Similarly to other Er-doped a-semiconductors, RT light emission at 1540 nm is a factor of 5 (or even more) smaller in our a-GaAsN samples and we attribute this decrease to losses by non-radiative carrier de-excitation in the a-matrix. A summary of the above experimental results can be found in Fig. 3 that contains the E04 optical band-gap and Er-related PL intensity at 1540 nm as a function of the N2 partial pressure during the deposition of the a-GaAs(N) samples. As can be
K, the most nitrogenated ([N] 35 at.%) Er-doped a-GaAsN sample has a signal intensity comparable to that of other Er-doped a-Si:H [9] and a-SiN [5,6] hosts. Er-related emission in an N-free aGaAs matrix was observed only at low temperatures being, at least, one order of magnitude less intense than those of the a-GaAsN hosts. 4. Discussion Most of the optical features veri®ed in Fig. 1 are related to the atomic composition imposed by the N2 partial pressure adopted during deposition. Since N atoms are inserted in the a-GaAs matrix at the expense of As [8,11], large amounts of N are assumed to decrease the energy of the top of the valence band inducing an optical band-gap increase in a way similar to that observed in a-SiN
Fig. 3. E04 optical band-gap and Er-related PL intensity at 1540 nm as a function of the nitrogen partial pressure during deposition of the a-GaAs(N) samples. Besides the optical band-gap increase, notice the Er-related PL improvement achieved for samples deposited at higher partial pressures of N2 . All Erdoped a-GaAs(N) samples present an Er concentration of 0.5 at.%. Undoped stands for a non-intentionally Er-doped N-free a-GaAs ®lm (sample 1). The lines are guides to the eye.
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
seen from Fig. 3, while the insertion of 0.5 at.% of Er in the a-GaAs matrix causes a reduction of E04 , increasing amounts of N induce changes on both the optical band-gap and Er-related light emission. The initial decrease veri®ed in the E04 optical gap of the Er-doped N-free a-GaAs sample is ascribed, partially, to AsEr precipitates that are known to be semi-metallic [16]. Increasing amounts of N atoms in the a-GaAs matrix prevent the formation of such precipitates and induce the replacement of the electronic states at the top of the valence band which, at larger N contents, are assumed to be at the origin of the observed E04 optical band-gap widening [12]. Still related to Fig. 3, the Er-related PL is consistent with the fact that Er3 ions are ecient recombination centers that eectively compete with other (non-)radiative processes taking place in the a-host [17]. Furthermore, in an a-host we should expect the existence of Er3 ions in association with other atomic species and/or defects (or with groups of them) in forming stable complexes rather than isolated Er3 sites. Consequently, the mechanism of excitation and de-excitation can be described by the following. As a result of the electron-hole pair capture, and recombination, ground state Er3 4fshell electrons are excited to the 4 I13=2 (or even higher) energy levels from which they can radiatively recombine to the 4 I15=2 level giving rise to light emission at 1540 nm. Along with the localization of carriers, this host-mediated recombination mechanism also depends on the ionization energy of the excitons bound to the Er-related centers [18]. In this respect, compounds with a large optical band-gap will have larger exciton energies (and less thermalization of carriers to the conduction band edge), leading to an improvement in the Er-related emission [18]. As a consequence of the compositional (and electronic) modi®cations induced by increasing amounts of N atoms in our Erdoped a-GaAs samples, the existence of Er3 complexes can act either localizing carriers near the Er3 ions or increasing the associated exciton energy. In this sense, further improvements in the Errelated emission of the present ®lms are expected to occur due to the insertion of even larger amounts of nitrogen and/or as a function of thermal anneals and will be performed in the near future.
857
5. Conclusions In conclusion, this work presents experimental results of Er-doped amorphous gallium±arsenic± nitrogen (a-GaAsN) alloys prepared by rf cosputtering. In view of our results, we conclude that insertion of N in the Er-doped a-GaAs matrix induces changes in the optical properties. Optical band-gap widening and a substantial increase in the Er-related emission eciency are among the most important ones. The improvement observed in the Er-related emission is attributed to higher exciton energies that are characteristic of WBG materials. As a consequence of the compositional (and electronic) modi®cations induced by increasing amounts of N atoms in the Er-doped a-GaAs ®lms, the presence of Er3 complexes can act either to localize carriers near the Er3 ions or increasing the associated exciton energy. In view of the present experimental results, a-GaAsN (or even aGaN [19]) alloys may be semiconducting hosts with new and interesting properties. Moreover, if doped with Er, they can have useful applications in the ®eld of photonics.
Acknowledgements The author is indebted to Professor F. Alvarez and Dr P. Hammer (UNICAMP) for the XPS measurements. Professor L.A.O. Nunes is also acknowledged for the use of the facilities of the Laboratory of Optical Spectroscopy (USP). This work was supported by the Brazilian Agencies FAPESP and CNPq.
References [1] A. Polman, J. Appl. Phys. 82 (1997) 1. [2] P.N. Favennec, H.L. Harridon, M. Salvi, D. Moutonnet, Y.L. Cuillo, Electron. Lett. 25 (1989) 718. [3] A.J. Neuhalfen, B.W. Wessels, Appl. Phys. Lett. 60 (1992) 2657. [4] S.J. Pearton, C.R. Abernathy, J.D. Mackenzie, U. H ommerich, X. Wu, R.G. Wilson, R.N. Schwartz, J.M. Zavada, F. Ren, Appl. Phys. Lett. 71 (1997) 1807. [5] A.R. Zanatta, L.A.O. Nunes, Appl. Phys. Lett. 72 (1998) 3127.
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A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
[6] A.R. Zanatta, M.J.V. Bell, L.A.O. Nunes, Phys. Rev. B 59 (1999) 10091. [7] See, for example, J.M. Zavada, D. Zhang, Solid-State Electron. 38 (1995) 1285. [8] J. Bandet, K. Aguir, D. Lollman, A. Fennouh, H. Carchano, Jpn. J. Appl. Phys. 36 (1997) 11. [9] A.R. Zanatta, L.A.O. Nunes, L.R. Tessler, Appl. Phys. Lett. 70 (1997) 511. [10] As pointed out in Refs. [5,6], Er-related light emission in aSiN alloys can be considerably enhanced under optical pumping with photons of 488.0 nm which resonantly populates the 4 F7=2 energy levels of the Er3 ions. This wavelength was employed in the a-GaAsN samples producing no signi®cant modi®cations. [11] I. Aksenov, Y. Nakada, H. Okumura, J. Appl. Phys. 84 (1998) 3159.
[12] See, for instance, I. Chambouleyron, A.R. Zanatta, J. Appl. Phys. 84 (1998) 1. [13] R.C. West (Ed.), CRC Handbook of Chemistry and Physics, CRC, Boca Raton, FL, 1987, p. E102. [14] See, for example, R.L. Maltez, Z.L. Weber, J. Washburn, M. Behar, P.B. Klein, P. Specht, E.R. Weber, Appl. Phys. Lett. 73 (1998) 2170. [15] S. Kim, S.J. Rhee, D.A. Turnbull, E.E. Reuter, X. Li, J.J. Coleman, S.G. Bishop, Appl. Phys. Lett. 71 (1997) 231. [16] A.R. Peaker, F. Coppinger, H. Efeoglu, J.H.E. Freeman, D.K. Maude, J.C. Portal, P. Rutter, K.E. Singer, A. Schols, A.C. Wright, Mater. Sci. Forum 258±263 (1997) 1551. [17] X.Z. Wang, B.W. Wessels, Appl. Phys. Lett. 64 (1994) 1537. [18] X.Z. Wang, B.W. Wessels, Appl. Phys. Lett. 67 (1995) 518. [19] P. Stumm, D.A. Drabold, Phys. Rev. Lett. 79 (1997) 677.
www.elsevier.com/locate/jnoncrysol
Infrared photoluminescence from Er-doped a-GaAsN alloys A.R. Zanatta Instituto de Fõsica de S~ ao Carlos, Universidade de S~ ao Paulo, P.O. Box 369, 13560-250 S~ ao Carlos, S.P., Brazil
Abstract Amorphous gallium±arsenic±nitrogen (a-GaAsN) ®lms were deposited by co-sputtering from a crystalline GaAs wafer partially covered with metallic Er pieces. The ®lms were deposited at room temperature under dierent partial pressures of Ar and N2 . After deposition, measurements of optical transmission in the visible±ultraviolet (VIS±UV) energy range, photoluminescence (PL) in the infrared (IR) region, and Raman scattering spectroscopy were made. Compositional analysis was also performed indicating an Er content of 0.5 at.% and a N concentration that scales with the N2 partial pressure during deposition. According to the experimental results, larger N content samples have larger optical band-gaps and more intense Er-related PL signals at 1540 nm. This dependence is analyzed in terms of the compositional, electronic and structural properties of each ®lm. Ó 2000 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Most of the current eorts on the study of Erdoped semiconductors stem from their possible application as light sources operating at 1540 nm due to the Er3 4 I13=2 ! 4 I15=2 optical transition [1]. At this wavelength, which coincides with the minimum transmission loss of silica-based optical ®bers, the realization of hybrid opto-electronic devices can provide improvements in systems of communications [1]. However, while the Er-related emission wavelength is almost independent of temperature, its luminescence intensity decreases as the temperature is increased. This thermal quenching eect observed in various semiconductor hosts [2] limits the light emission eciency at room temperature and, as a consequence, prevents
E-mail address: [email protected] (A.R. Zanatta).
the employment of the existing Er-based devices in some applications [1]. The suggestion [3] that the thermal quenching of intra-4f-shell emissions from Er3 ions in semiconductor hosts decreases in wide band-gap (WBG) materials has motivated several recent studies of these compounds [4±7]. Additionally, WBG materials have advantages in the design of opto-electronic devices because of chemical stability, carrier generation, and physical stability over a wide temperature range. Moreover, in the speci®c case of Er-doped semiconductors, it is assumed that the bonding (or atomic arrangement) found in WBG compounds may be better for forming the required environment for ecient light emission [7]. In other words, besides the energetic aspects involved in the mechanisms of excitation and de-excitation of carriers, the Er3 atomic environment should be a factor. In this respect, the present contribution deals with a series of Er-doped a-GaAsN alloys with diering atomic compositions and optical band-gaps. In view of
0022-3093/00/$ - see front matter Ó 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 8 5 5 - 8
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
855
the present experimental results, and compared with other hosts [2], Er-doped amorphous GaAsN ®lms can have interesting applications in the ®eld of photonics. 2. Experimental The a-GaAs(N) ®lms were deposited at room temperature (RT) by radio frequency (RF) cosputtering a crystalline (c-)GaAs wafer covered at random with small metallic Er pieces. All deposition runs employed polished c-Si and soda±lime glass substrates and took place under a total pressure of 0.7 Pa with dierent mixtures of Ar and N2 . Films deposited with pure Ar or N2 atmospheres were also prepared for comparison purposes. The atomic composition of the aGaAs(N) series was investigated by means of either energy dispersive X-ray spectroscopy (EDS) or X-ray photoelectron spectroscopy (XPS). The E04 optical band-gap (energy at which the absorption coecient is 104 cm-1 ) was obtained from optical transmission measurements in the visible± ultraviolet (VIS±UV) energy range. Photoluminescence (PL) spectroscopy was carried out at RT and 77 K with the samples being excited with the 514.5 nm line of an Ar -laser ( 6 100 mW). The emitting radiation was dispersed with a 0.275 m Czerny±Turner type monochromator, detected with a liquid-nitrogen cooled p±i±n Ge detector and standard in-phase techniques. RT Raman scattering measurements employed the same laser line ( 6 200 mW), a double 0.85 m monochromator, a cooled photomultiplier tube and photon counting. Despite the bene®cial eects of thermal anneals on the light emission at 1540 nm in Erdoped semiconductors, the data in this report are for as-deposited samples. 3. Results Fig. 1 displays the transmission spectra in the 300±1500 nm wavelength range of the series of Er-doped a-GaAs(N) ®lms deposited on soda±lime glass substrates. The spectrum of a non-intentionally Er-doped a-GaAs ®lm is also represented
Fig. 1. Optical transmission in the 300±1500 nm wavelength range of a-GaAs(N) samples. The spectra have been vertically shifted for clarity. The following apply: 1 (non-intentionally doped a-GaAs), 2 (Er-doped a-GaAs), 3 (Er-doped a-GaAs deposited under a N2 partial pressure PN2 of 7 10ÿ3 Pa), 4 (Er-doped a-GaAs with PN2 1:4 10ÿ2 Pa), 5 (Er-doped aGaAs with PN 2 7 10ÿ2 Pa), 6 (Er-doped a-GaAs with PN2 0:14 Pa), and 7 (Er-doped a-GaAs with PN2 0:7 Pa). For all Er-doped ®lms the Er content remained at 0.5 at.%.
for comparison. As can be seen from Fig. 1 the insertion of both Er and N atoms induces changes in the optical transmission of the samples. Either EDS or XPS analysis indicates a N concentration that scales with the N2 partial pressure used during deposition. Regardless of the N concentration of each sample, the Er content remained around 0.5 at.% in all Er-doped samples. Concerning the atomic structure of these asdeposited a-GaAsN samples, all of them had a Raman feature at 250 cmÿ1 characteristic of distorted GaAs-based matrices [8]. Fig. 2 illustrates the emission at 1540 nm due to the 4 I13=2 ! 4 I15=2 Er3 transition in several dierent Er-containing hosts. According to Fig. 2, at 77
856
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
Fig. 2. Photoluminescence PL spectra of dierent Er-doped ahosts. The emission at 1540 nm is due to the 4 I13=2 ! 4 I15=2 Er3 transition. The spectra were achieved at 77 K under excitation with 514.5 nm photons [10]. Notice the multiplying factors of each spectrum that has been vertically shifted for clarity.
and a-GeN alloys [12]. Indeed, considering the dierence in Ga±N and As±N bond energies, it is not surprising that the N atoms readily replace the As atoms with the formation of Ga±N bonds [13]. The present Er-doped a-GaAs samples have a greater intensity at 1540 nm, with no background contribution due to micro-defect formation, contrary to previous reports on Er-implanted c-GaAs [14]. Another interesting aspect of light emission in the IR region, observed in the most nitrogenated a-GaAsN ®lm, was the existence of light emission at about 1200 nm (not shown in Fig. 2). This emission was observed [15] and is attributed, in the present case, to defects either due to the amorphous state of the matrix or due to the N atoms. Similarly to other Er-doped a-semiconductors, RT light emission at 1540 nm is a factor of 5 (or even more) smaller in our a-GaAsN samples and we attribute this decrease to losses by non-radiative carrier de-excitation in the a-matrix. A summary of the above experimental results can be found in Fig. 3 that contains the E04 optical band-gap and Er-related PL intensity at 1540 nm as a function of the N2 partial pressure during the deposition of the a-GaAs(N) samples. As can be
K, the most nitrogenated ([N] 35 at.%) Er-doped a-GaAsN sample has a signal intensity comparable to that of other Er-doped a-Si:H [9] and a-SiN [5,6] hosts. Er-related emission in an N-free aGaAs matrix was observed only at low temperatures being, at least, one order of magnitude less intense than those of the a-GaAsN hosts. 4. Discussion Most of the optical features veri®ed in Fig. 1 are related to the atomic composition imposed by the N2 partial pressure adopted during deposition. Since N atoms are inserted in the a-GaAs matrix at the expense of As [8,11], large amounts of N are assumed to decrease the energy of the top of the valence band inducing an optical band-gap increase in a way similar to that observed in a-SiN
Fig. 3. E04 optical band-gap and Er-related PL intensity at 1540 nm as a function of the nitrogen partial pressure during deposition of the a-GaAs(N) samples. Besides the optical band-gap increase, notice the Er-related PL improvement achieved for samples deposited at higher partial pressures of N2 . All Erdoped a-GaAs(N) samples present an Er concentration of 0.5 at.%. Undoped stands for a non-intentionally Er-doped N-free a-GaAs ®lm (sample 1). The lines are guides to the eye.
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
seen from Fig. 3, while the insertion of 0.5 at.% of Er in the a-GaAs matrix causes a reduction of E04 , increasing amounts of N induce changes on both the optical band-gap and Er-related light emission. The initial decrease veri®ed in the E04 optical gap of the Er-doped N-free a-GaAs sample is ascribed, partially, to AsEr precipitates that are known to be semi-metallic [16]. Increasing amounts of N atoms in the a-GaAs matrix prevent the formation of such precipitates and induce the replacement of the electronic states at the top of the valence band which, at larger N contents, are assumed to be at the origin of the observed E04 optical band-gap widening [12]. Still related to Fig. 3, the Er-related PL is consistent with the fact that Er3 ions are ecient recombination centers that eectively compete with other (non-)radiative processes taking place in the a-host [17]. Furthermore, in an a-host we should expect the existence of Er3 ions in association with other atomic species and/or defects (or with groups of them) in forming stable complexes rather than isolated Er3 sites. Consequently, the mechanism of excitation and de-excitation can be described by the following. As a result of the electron-hole pair capture, and recombination, ground state Er3 4fshell electrons are excited to the 4 I13=2 (or even higher) energy levels from which they can radiatively recombine to the 4 I15=2 level giving rise to light emission at 1540 nm. Along with the localization of carriers, this host-mediated recombination mechanism also depends on the ionization energy of the excitons bound to the Er-related centers [18]. In this respect, compounds with a large optical band-gap will have larger exciton energies (and less thermalization of carriers to the conduction band edge), leading to an improvement in the Er-related emission [18]. As a consequence of the compositional (and electronic) modi®cations induced by increasing amounts of N atoms in our Erdoped a-GaAs samples, the existence of Er3 complexes can act either localizing carriers near the Er3 ions or increasing the associated exciton energy. In this sense, further improvements in the Errelated emission of the present ®lms are expected to occur due to the insertion of even larger amounts of nitrogen and/or as a function of thermal anneals and will be performed in the near future.
857
5. Conclusions In conclusion, this work presents experimental results of Er-doped amorphous gallium±arsenic± nitrogen (a-GaAsN) alloys prepared by rf cosputtering. In view of our results, we conclude that insertion of N in the Er-doped a-GaAs matrix induces changes in the optical properties. Optical band-gap widening and a substantial increase in the Er-related emission eciency are among the most important ones. The improvement observed in the Er-related emission is attributed to higher exciton energies that are characteristic of WBG materials. As a consequence of the compositional (and electronic) modi®cations induced by increasing amounts of N atoms in the Er-doped a-GaAs ®lms, the presence of Er3 complexes can act either to localize carriers near the Er3 ions or increasing the associated exciton energy. In view of the present experimental results, a-GaAsN (or even aGaN [19]) alloys may be semiconducting hosts with new and interesting properties. Moreover, if doped with Er, they can have useful applications in the ®eld of photonics.
Acknowledgements The author is indebted to Professor F. Alvarez and Dr P. Hammer (UNICAMP) for the XPS measurements. Professor L.A.O. Nunes is also acknowledged for the use of the facilities of the Laboratory of Optical Spectroscopy (USP). This work was supported by the Brazilian Agencies FAPESP and CNPq.
References [1] A. Polman, J. Appl. Phys. 82 (1997) 1. [2] P.N. Favennec, H.L. Harridon, M. Salvi, D. Moutonnet, Y.L. Cuillo, Electron. Lett. 25 (1989) 718. [3] A.J. Neuhalfen, B.W. Wessels, Appl. Phys. Lett. 60 (1992) 2657. [4] S.J. Pearton, C.R. Abernathy, J.D. Mackenzie, U. H ommerich, X. Wu, R.G. Wilson, R.N. Schwartz, J.M. Zavada, F. Ren, Appl. Phys. Lett. 71 (1997) 1807. [5] A.R. Zanatta, L.A.O. Nunes, Appl. Phys. Lett. 72 (1998) 3127.
858
A.R. Zanatta / Journal of Non-Crystalline Solids 266±269 (2000) 854±858
[6] A.R. Zanatta, M.J.V. Bell, L.A.O. Nunes, Phys. Rev. B 59 (1999) 10091. [7] See, for example, J.M. Zavada, D. Zhang, Solid-State Electron. 38 (1995) 1285. [8] J. Bandet, K. Aguir, D. Lollman, A. Fennouh, H. Carchano, Jpn. J. Appl. Phys. 36 (1997) 11. [9] A.R. Zanatta, L.A.O. Nunes, L.R. Tessler, Appl. Phys. Lett. 70 (1997) 511. [10] As pointed out in Refs. [5,6], Er-related light emission in aSiN alloys can be considerably enhanced under optical pumping with photons of 488.0 nm which resonantly populates the 4 F7=2 energy levels of the Er3 ions. This wavelength was employed in the a-GaAsN samples producing no signi®cant modi®cations. [11] I. Aksenov, Y. Nakada, H. Okumura, J. Appl. Phys. 84 (1998) 3159.
[12] See, for instance, I. Chambouleyron, A.R. Zanatta, J. Appl. Phys. 84 (1998) 1. [13] R.C. West (Ed.), CRC Handbook of Chemistry and Physics, CRC, Boca Raton, FL, 1987, p. E102. [14] See, for example, R.L. Maltez, Z.L. Weber, J. Washburn, M. Behar, P.B. Klein, P. Specht, E.R. Weber, Appl. Phys. Lett. 73 (1998) 2170. [15] S. Kim, S.J. Rhee, D.A. Turnbull, E.E. Reuter, X. Li, J.J. Coleman, S.G. Bishop, Appl. Phys. Lett. 71 (1997) 231. [16] A.R. Peaker, F. Coppinger, H. Efeoglu, J.H.E. Freeman, D.K. Maude, J.C. Portal, P. Rutter, K.E. Singer, A. Schols, A.C. Wright, Mater. Sci. Forum 258±263 (1997) 1551. [17] X.Z. Wang, B.W. Wessels, Appl. Phys. Lett. 64 (1994) 1537. [18] X.Z. Wang, B.W. Wessels, Appl. Phys. Lett. 67 (1995) 518. [19] P. Stumm, D.A. Drabold, Phys. Rev. Lett. 79 (1997) 677.