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Bonding of nitrogen in dilute GaInAsN and AlGaAsN studied by Raman spectroscopy
Solid-State Electronics 47 (2003) 461–465 www.elsevier.com/locate/sse
Bonding of nitrogen in dilute GaInAsN and AlGaAsN studied by Raman spectroscopy J. Wagner *, T. Geppert, K. K€ ohler, P. Ganser, M. Maier Fraunhofer-Institut f€ur Angewandte Festk€orperphysik, Tullastrasse 72, D-79108 Freiburg, Germany
Abstract To gain information on the local bonding of the nitrogen, Ga1x Inx As1y Ny with x 6 0:12 and y 6 0:04 and Alx Ga1x As1y Ny with x 6 0:05 and y 6 0:04 have been studied by Raman spectroscopy. When adding In to GaAsN, the nitrogen-induced vibrational mode near 470 cm1 observed in GaAsN was found to split into up to three components, with one of the In–N related modes at higher and the other at lower frequencies than the Ga–N mode. Upon thermal annealing, the relative mode intensities were found to change in favor of the In–N related modes, indicating a redistribution of the III–N bonds. For Alx Ga1x As0:99 N0:01 , in contrast, the almost exclusive formation of complexes with Alto-N bonding was observed already for a low Al content of x ¼ 0:05, as seen from a complete switch in mode intensity from the Ga–N mode at 470 cm1 to a new Al–N related mode near 450 cm1 . This result was confirmed by a corresponding analysis of the quinary compound AlGaInAsN. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dilute group III–arsenide/nitrides; Local bonding; Raman spectroscopy
1. Introduction The group III–AsN alloy system with nitrogen concentrations in the range of a few percent, such as e.g. GaAsN and GaInAsN, is of considerable current interest both from a fundamental point of view as well as for applications in e.g. GaAs-based long-wavelength photodetectors and diode lasers [1–3]. The addition of nitrogen, with highly dissimilar properties compared to the other group V constituent As, leads to material properties that deviate strongly from those expected for a conventional III–V alloy. For a further understanding of these interesting properties of dilute group III–AsN, information on the local bonding of the nitrogen is required. Vibrational mode spectroscopy has been shown to be a powerful tool for the study of the local bonding of impurities and impurity complexes in conventional III–
*
Corresponding author. Tel.: +49-761-5159-352; fax: +49761-5159-677. E-mail address: [email protected] (J. Wagner).
V materials, such as GaAs [4,5]. The incorporation of nitrogen in dilute GaAsN and GaInAsN has been studied both by vibrational mode absorption spectroscopy and Raman spectroscopy [6–11], while there is only one report so far on a Raman spectroscopic study of the bonding of nitrogen in dilute AlGaAsN [12]. The purpose of the present paper is to report on a comparative study of the bonding of nitrogen in dilute GaInAsN, AlGaAsN as well as AlGaInAsN, all materials grown by plasma-assisted molecular-beam epitaxy (MBE) on GaAs substrates.
2. Experiment Nominally undoped bulk-like, typically 100 nm thick layers of GaAs1y Ny , Alx Ga1x As1y Ny , and Alx Ga0:94x In0:06 As0:98 N0:02 with Al-contents x 6 0:10 and N-contents y 6 0:02, were grown by solid-source MBE on (1 0 0) GaAs substrates using an rf nitrogen plasma source. Further, GaAs0:99 N0:01 /GaAs multiple quantum well (MQW) and Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW samples as well as bulk-like layers of Ga0:93 In0:07 As0:98 N0:02 were
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prepared for a comparative analysis of N-incorporation into these materials, including the effect of post-growth rapid thermal annealing. High-resolution X-ray diffraction (HRXRD) was used to determine the strain state of the epitaxial layers, and from this to derive the N-content based on VegardÕs rule, assuming for the GaInAsN samples the same In content as determined for GaInAs reference layers grown with the same Ga-to-In flux ratio. All (AlGaIn)(AsN) layers were found to be pseudomorphically strained to the in-plane lattice parameter of the underlying GaAs. The N-concentrations derived from the HRXRD measurements were cross-checked by secondary ion mass spectroscopy (SIMS), using appropriate calibration standards prepared by implantation of N into GaAs and GaInAs. For SIMS measurement of the Al- and In-contents, AlGaAs and GaInAs calibration standards were used whose Al- or In-concentration was determined by energy dispersive X-ray analysis. Raman spectra were recorded in backscattering geometry from the (1 0 0) growth surface, using the 568.2 nm (2.18 eV photon energy) line of a Kr-ion laser for optical excitation close to resonance with the mostly nitrogen-derived Eþ transition [8,9]. Samples were kept either at room temperature or cooled to 77 K by Heexchange gas in a continuous-flow cryostat. Incident and scattered light were both polarized parallel to the same (1 1 0)-type crystallographic direction, i.e. the xðy 0 ; y 0 Þ–x scattering configuration was employed. These experimental conditions were chosen to maximize the intensity of Raman scattering from nitrogen-related vibrational modes [12].
3. Results and discussion To address the issue of nitrogen bonding in dilute GaAsN and GaInAsN, Fig. 1 shows Raman spectra, covering the range of the nitrogen-induced vibrational modes, recorded from (a) an as-grown GaAs0:99 N0:01 / GaAs MQW serving as a reference, (b) an as-grown Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW, and (c) the same Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW annealed at 950 °C for 15 s. The GaAsN/GaAs MQW sample shows a Ninduced vibrational mode near 470 cm1 , which has been observed previously both by infrared absorption [6] and Raman spectroscopy [7–9] and assigned to the vibration of isolated nitrogen bonded to four Ga neighbors [6]. For GaInAsN, the same experimental techniques gave evidence for the presence of additional modes at slightly lower and higher frequency, which gain in strength upon, or even appear only after postgrowth thermal annealing [9–12]. This is confirmed by the present data displayed in Fig. 1(b)–(d). Upon the addition of In to GaAsN the GaN-like LO2 phonon mode shifts slightly from 470 to 469 cm 1 , accompanied
Fig. 1. Room-temperature Raman spectra of (a) an as-grown GaAs0:99 N0:01 /GaAs MQW, (b) an as-grown Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW, (c) same MQW as (b) but after rapid thermal annealing at 950 °C for 15 s, and (d) the difference spectrum ðcÞ ðbÞ highlighting the effect of thermal annealing.
by the appearance of an additional shoulder at 457 cm1 , which is at the low-frequency side of the LO2 mode (Fig. 1(b)). This low-frequency mode gains intensity upon annealing, accompanied by the appearance of a further mode on the high-frequency side, centered at 488 cm1 (Fig. 1(c)). The changes induced in the Raman spectrum of the GaInAsN/GaAs MQW by the annealing step are highlighted in Fig. 1(d) where the difference spectrum ðcÞ ðbÞ is plotted. The above data indicate that in as-grown GaInAsN a certain portion of the nitrogen can be bonded to In neighbors. Thermal annealing leads to an increase of that fraction, but the GaN-like mode still remains dominant, indicating that a considerable amount of N is still bonded to four Ga neighbors (Ga4 N configuration). The observed change in bonding configuration upon thermal annealing can be explained by an interplay between the larger cohesive energy of the Ga–N bond (2.24 eV) compared to that of the In–N bond (1.93 eV) [13], and reduced local strain energy of e.g. the Ga3 InN configuration relative to that of Ga4 N [14]. It has been argued that at the growth surface, nitrogen incorporation is controlled by chemical bonding favoring the Ga4 N configuration, while under thermal equilibrium conditions in the bulk the Ga3 InN
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configuration becomes more favorable because of the reduced local strain [15]. For dilute AlGaAsN a complete dominance of chemical bonding aspects can be expected because of the even larger cohesive energy of the Al–N bond of 2.88 eV [13], strongly favoring the bonding of the N to Al while, due to the comparable size of Ga and Al, differences in local strain should be negligible. That this expectation is indeed fulfilled can be seen from Fig. 2, where Raman spectra of Alx Ga1x As0:99 N0:01 with x 6 0:05 are displayed, covering the frequency range of the N-related vibrational modes and of the AlAs-like longitudinal optical phonon (LO2 ). For x ¼ 0, i.e. GaAs0:99 N0:01 , only the GaN-like mode (labeled LO3 ) is observed at 470 cm1 , superimposed on a background from second-order phonon scattering by dominantly GaAs-like phonon modes [7–9]. Upon the addition of Al new modes appear at the low-frequency side of the GaN-like LO3 phonon, one at 454 cm1 which is clearly resolved for the x ¼ 0:01 and 0.02 samples, and a second peak at 448 cm1 which dominates for x P 0:04. For the highest Al-contents two additional peaks can be resolved at the high-frequency side of the AlAs-like LO2 phonon, centered at 379 and 397 cm1 . None of these new modes are present in the
Fig. 2. Room-temperature Raman spectra of Alx Ga1x As0:99 N0:01 layers on GaAs with Al-concentrations 0 6 x 6 0:05 as given in the figure. New modes arising from the simultaneous presence of N and Al are marked by vertical arrows.
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Raman spectrum of Alx Ga1x As reference samples with the same Al-contents, which were prepared in the same growth series as the AlGaAsN layers. Thus, these modes are linked to the simultaneous presence of both Al and N, but the exact origin of the two modes lowest in frequency is still unclear. There is an almost complete switch in mode intensity from the GaN-like LO3 mode to the Al–N related mode at 448 cm1 , labeled in the following as LO4 (AlN-like), when increasing the Alcontent from x ¼ 0 to 0.05. This drastic change in mode intensity indicates that for an Al-content of only 5%, almost all of the N atoms are incorporated into AlN-like complexes, whose most simple form could be described as Ga3 AlN, Ga2 Al2 N etc. configurations. This is particularly surprising given the fact that for a random statistical distribution, at an Al-content of x ¼ 0:05, 80% of the N atoms are expected to be present in the Ga4 N configuration and only 17% in Ga3 AlN clusters [16]. Therefore the present findings strongly indicate that the incorporation of nitrogen in dilute AlGaAsN is governed by the large Al–N bond strength [13] rather than by random statistics, resulting in the almost exclusive formation of Al–N complexes for x ¼ 0:05 and y ¼ 0:01. This conclusion is confirmed by the Raman data shown in Fig. 3, where spectra are displayed recorded from bulk-like quinary Alx Ga0:94x In0:06 As0:98 N0:02 layers. For x ¼ 0, i.e. Ga0:94 In0:06 As0:98 N0:02 , the Raman spectrum resembles those plotted in Fig. 1(b) and (c), showing a GaN-like mode (LO3 ) accompanied by two InN-like modes at lower and higher frequencies, respectively. Adding Al leads to a decrease in intensity of all three modes and the AlN-like mode LO4 appears slightly below 450 cm1 . Again, for the highest Al-concentration of 10%, i.e. an Al content five times the Ncontent, the AlN-like LO4 mode is the by far dominant N-related feature in the Raman spectrum. And this bonding situation was found to remain unchanged after rapid thermal annealing at 850 °C for 60 s. Only the modes at lower frequencies around 380 and 400 cm1 show a decrease in intensity upon thermal annealing, which indicates that these modes stem from thermally less stable higher-order Al–N complexes. Fig. 4 shows a comparison of Raman spectra recorded from GaAs0:99 N0:01 at 300 and 77 K, respectively. Upon reduction of sample temperature the GaN-like LO2 mode shifts to higher frequencies by 2 cm1 , which is a significantly smaller shift than that of the GaAs-like LO1 phonon (not shown in Fig. 4) of 3.5 cm1 . This temperature behavior is consistent with that reported by Yu et al. [17], who also observed a smaller temperatureinduced frequency shift of the GaN-like mode than for the GaAs-like LO phonon. Going from 300 to 77 K leads further to an increase in peak intensity of the GaNlike LO2 mode by a factor of 3.6, accompanied by a reduction in line width from 12.3 cm1 at 300 K to 6.4 cm1 at 77 K. Also this reduction in line width by about
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a factor of two is consistent with the data presented in Ref. [17]. The increase in integrated intensity, observed upon cooling down from 300 to 77 K, can be explained by a high-energy shift of the Eþ transition with decreasing temperature, as expected from the band anticrossing model [18,19]. This shift brings the incident photons with an energy of 2.18 eV closer to resonance with the Eþ transition, which in turn provides stronger resonant enhancement for Raman scattering by the III– N like LO phonon modes [8,9]. The overall gain in sensitivity to III–N like modes achieved at low temperatures has been exploited to study the bonding of nitrogen in AlGaAsN in more detail. Fig. 5 shows a sequence of 77 K Raman spectra recorded from the same series of Alx Ga1x As0:99 N0:01 layers (x 6 0:05) as used for recording the room-temperature spectra displayed in Fig. 2. Compared to the room-temperature spectra the successive appearance of two closely spaced AlN-like modes upon increasing the Al-concentration from x ¼ 0 to 0.05 at around 450 cm1 , i.e. at the low-frequency side of the GaN-like mode centered at 472 cm1 , is much better resolved in the lowtemperature spectra. Further, the low-temperature Raman spectra reveal the presence of another AlN-related
Fig. 3. Room-temperature Raman spectra of Alx Ga0:94x In0:06 As0:98 N0:02 layers on GaAs with Al-concentrations 0 6 x 6 0:10 as indicated in the figure.
Fig. 4. Raman spectra of GaAs0:99 N0:01 recorded at 77 K (upper spectrum) and 300 K (lower spectrum).
Fig. 5. Low-temperature (77 K) Raman spectra of Alx Ga1x As0:99 N0:01 layers on GaAs with Al-concentrations 0 6 x 6 0:05 as indicated in the figure (same set of samples as shown in Fig. 2). Vertical dashed lines indicate GaN-and AlN-like modes.
J. Wagner et al. / Solid-State Electronics 47 (2003) 461–465
mode at the high-frequency side of the GaN-like mode (i.e. at 498 cm1 ), which is not resolved in the roomtemperature spectra because of the underlying background from second-order phonon scattering.
4. Conclusions A comprehensive Raman spectroscopic study of nitrogen bonding in dilute GaInAsN, AlGaAsN, and AlGaInAsN has been reported. The main result is that in the as-grown AlGa(In)AsN the bonding of nitrogen is governed by the significant difference in bond cohesive energy for the different III–N bonds, while in GaInAsN bonding is controlled by an interplay between bond cohesive energy and a reduction of local strain energy. An issue, which requires further clarification, is that of the AlN-like and InN-like mode frequencies. Based on simple atomic mass and bond strength arguments, AlN-like modes are expected to appear at higher and InN-like modes at lower frequencies than the GaN-like LO phonon mode. Experimentally, however, the addition of both Al and In has been found to produce modes at frequencies both above and below that of the GaNlike LO phonon. For GaInAsN the appearance of a mode at higher frequency than the GaN-like LO phonon has tentatively been explained by a large reduction in local strain upon formation of In–N bonds, leading to a significant stiffening of those bonds compared to the Ga–N bonds in Ga4 N complexes, and thus an overall increase in mode frequency [10]. However, to shed further light on this issue, theoretical work is required to model the AlN-like in InN-like mode frequencies in dilute AlGaAsN and GaInAsN, including different bonding and complex configurations. Results of such theoretical work can then help in assigning the observed modes to specific bonding configurations.
Acknowledgements We would like to thank T. Fuchs for expert technical assistance with the SIMS measurements as well as N. Herres, H. G€ ullich and L. Kirste for performing the HRXRD analyses. G. Weimann is thanked for continuous support and encouragement.
References [1] Kondow M, Uomi K, Niwa A, Kitatani T, Watahiki S, Yazawa Y. GaInAsN: a novel material for long-wavelength-range laser diodes with excellent high-temperature performance. Jpn J Appl Phys 1996;35(2B):1273–5.
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[2] Kurtz SR, Allermann AA, Jones DE, Gee JM, Banas JJ, Hammons BE. InGaAsN solar cells with 1.0 eV band gap, lattice matched to GaAs. Appl Phys Lett 1999;74(5):729– 31. [3] Heroux JB, Wang X, Wang WI. GaInAsN resonantcavity-enhanced photodetector operating at 1.3 lm. Appl Phys Lett 1999;75(18):2716–8. [4] See, e.g., Newman RC. Local vibrational mode spectroscopy of defects in III/V compounds. In: Semiconductors and semimetals, vol. 38. New York: Academic Press; 1993. [5] For a recent review see, McCluskey MD. Local vibrational modes of impurities in semiconductors. J Appl Phys 2000;87(8):3593–617. [6] Alt HCh, Egorov AYu, Riechert H, Wiedemann B, Meyer JD, Michelmann RW, et al. Infrared absorption study of nitrogen in N-implanted GaAs and epitaxially grown GaAs1x Nx layers. Appl Phys Lett 2000;77(21):3331–3. [7] Prokofyeva T, Sauncy T, Seon M, Holtz M, Qiu Y, Nikishin S, et al. Raman studies of nitrogen incorporation in GaAs1x Nx . Appl Phys Lett 1999;73(10):1409–11. [8] Wagner J, K€ ohler K, Ganser P, Herres N. GaAs interband transitions involving localized and extended states probed by resonant Raman scattering and spectroscopic ellipsometry. Appl Phys Lett 2000;77(22):3592–4. [9] Wagner J, Geppert T, K€ ohler K, Ganser P, Herres N. Ninduced vobrational modes in GaAsN and GaInAsN studied by resonant Raman scattering. J Appl Phys 2001;90(10):5027–31. [10] Kurtz S, Webb J, Gedvilas L, Friedman D, Geisz J, Olson J, et al. Structural changes during annealing of GaInAsN. Appl Phys Lett 2001;78(6):748–50. [11] Kitatani T, Kondow M, Kudo M. Transition of infrared absorption peaks in thermally annealed GaInAsN. Jpn J Appl Phys 2001;44(7B):L750–2. [12] Geppert T, Wagner J, K€ ohler K, Ganser P, Maier M. Preferential formation of Al–N bonds in low N-content AlGaAsN. Appl Phys Lett 2002;80(12):2081–3. [13] Harrison WA. Electronic structure and properties of solids. New York: Dover; 1989. [14] Kim K, Zunger A. Spatial correlations in GaInAsN alloys and their effects on band-gap enhancement and electron localization. Phys Rev Lett 2001;86(12):2609–12. [15] Klar PJ, Gr€ uning H, Koch J, Sch€afer S, Volz K, Stolz W, et al. (Ga,In)(N,As)-fine structure of the band gap due to nearest-neighbor configurations of the isovalent nitrogen. Phys Rev B 2001;64:121203-1–4. [16] See, e.g., Pritchard RE, Newman RC, Wagner J, Fuchs F, € berg S. Bonding of H-CAs pairs in Alx Ga1x As Jones R, O alloys. Phys Rev B 1994;50(15):10628–36. [17] Yu G, Shen ZX, Liu L, Sun WX. Raman scattering study of a GaAsN epitaxial layer. Mat Sci Semicond Process 2001;4:581–4. [18] Shan W, Walukiewicz W, Ager III JW, Haller EE, Geisz JF, Friedman DJ, et al. Band anticrossing in GaInAsN alloys. Phys Rev Lett 1999;82(6):1221–4. [19] Chtourou R, Bousbih F, BenBouzid S, Charfi FF, Harmand JC, Ungaro G, et al. Effect of nitrogen and temperature on the electronic band structure of GaAs1x Nx alloys. Appl Phys Lett 2002;80(12):2075–7.
Bonding of nitrogen in dilute GaInAsN and AlGaAsN studied by Raman spectroscopy J. Wagner *, T. Geppert, K. K€ ohler, P. Ganser, M. Maier Fraunhofer-Institut f€ur Angewandte Festk€orperphysik, Tullastrasse 72, D-79108 Freiburg, Germany
Abstract To gain information on the local bonding of the nitrogen, Ga1x Inx As1y Ny with x 6 0:12 and y 6 0:04 and Alx Ga1x As1y Ny with x 6 0:05 and y 6 0:04 have been studied by Raman spectroscopy. When adding In to GaAsN, the nitrogen-induced vibrational mode near 470 cm1 observed in GaAsN was found to split into up to three components, with one of the In–N related modes at higher and the other at lower frequencies than the Ga–N mode. Upon thermal annealing, the relative mode intensities were found to change in favor of the In–N related modes, indicating a redistribution of the III–N bonds. For Alx Ga1x As0:99 N0:01 , in contrast, the almost exclusive formation of complexes with Alto-N bonding was observed already for a low Al content of x ¼ 0:05, as seen from a complete switch in mode intensity from the Ga–N mode at 470 cm1 to a new Al–N related mode near 450 cm1 . This result was confirmed by a corresponding analysis of the quinary compound AlGaInAsN. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Dilute group III–arsenide/nitrides; Local bonding; Raman spectroscopy
1. Introduction The group III–AsN alloy system with nitrogen concentrations in the range of a few percent, such as e.g. GaAsN and GaInAsN, is of considerable current interest both from a fundamental point of view as well as for applications in e.g. GaAs-based long-wavelength photodetectors and diode lasers [1–3]. The addition of nitrogen, with highly dissimilar properties compared to the other group V constituent As, leads to material properties that deviate strongly from those expected for a conventional III–V alloy. For a further understanding of these interesting properties of dilute group III–AsN, information on the local bonding of the nitrogen is required. Vibrational mode spectroscopy has been shown to be a powerful tool for the study of the local bonding of impurities and impurity complexes in conventional III–
*
Corresponding author. Tel.: +49-761-5159-352; fax: +49761-5159-677. E-mail address: [email protected] (J. Wagner).
V materials, such as GaAs [4,5]. The incorporation of nitrogen in dilute GaAsN and GaInAsN has been studied both by vibrational mode absorption spectroscopy and Raman spectroscopy [6–11], while there is only one report so far on a Raman spectroscopic study of the bonding of nitrogen in dilute AlGaAsN [12]. The purpose of the present paper is to report on a comparative study of the bonding of nitrogen in dilute GaInAsN, AlGaAsN as well as AlGaInAsN, all materials grown by plasma-assisted molecular-beam epitaxy (MBE) on GaAs substrates.
2. Experiment Nominally undoped bulk-like, typically 100 nm thick layers of GaAs1y Ny , Alx Ga1x As1y Ny , and Alx Ga0:94x In0:06 As0:98 N0:02 with Al-contents x 6 0:10 and N-contents y 6 0:02, were grown by solid-source MBE on (1 0 0) GaAs substrates using an rf nitrogen plasma source. Further, GaAs0:99 N0:01 /GaAs multiple quantum well (MQW) and Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW samples as well as bulk-like layers of Ga0:93 In0:07 As0:98 N0:02 were
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prepared for a comparative analysis of N-incorporation into these materials, including the effect of post-growth rapid thermal annealing. High-resolution X-ray diffraction (HRXRD) was used to determine the strain state of the epitaxial layers, and from this to derive the N-content based on VegardÕs rule, assuming for the GaInAsN samples the same In content as determined for GaInAs reference layers grown with the same Ga-to-In flux ratio. All (AlGaIn)(AsN) layers were found to be pseudomorphically strained to the in-plane lattice parameter of the underlying GaAs. The N-concentrations derived from the HRXRD measurements were cross-checked by secondary ion mass spectroscopy (SIMS), using appropriate calibration standards prepared by implantation of N into GaAs and GaInAs. For SIMS measurement of the Al- and In-contents, AlGaAs and GaInAs calibration standards were used whose Al- or In-concentration was determined by energy dispersive X-ray analysis. Raman spectra were recorded in backscattering geometry from the (1 0 0) growth surface, using the 568.2 nm (2.18 eV photon energy) line of a Kr-ion laser for optical excitation close to resonance with the mostly nitrogen-derived Eþ transition [8,9]. Samples were kept either at room temperature or cooled to 77 K by Heexchange gas in a continuous-flow cryostat. Incident and scattered light were both polarized parallel to the same (1 1 0)-type crystallographic direction, i.e. the xðy 0 ; y 0 Þ–x scattering configuration was employed. These experimental conditions were chosen to maximize the intensity of Raman scattering from nitrogen-related vibrational modes [12].
3. Results and discussion To address the issue of nitrogen bonding in dilute GaAsN and GaInAsN, Fig. 1 shows Raman spectra, covering the range of the nitrogen-induced vibrational modes, recorded from (a) an as-grown GaAs0:99 N0:01 / GaAs MQW serving as a reference, (b) an as-grown Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW, and (c) the same Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW annealed at 950 °C for 15 s. The GaAsN/GaAs MQW sample shows a Ninduced vibrational mode near 470 cm1 , which has been observed previously both by infrared absorption [6] and Raman spectroscopy [7–9] and assigned to the vibration of isolated nitrogen bonded to four Ga neighbors [6]. For GaInAsN, the same experimental techniques gave evidence for the presence of additional modes at slightly lower and higher frequency, which gain in strength upon, or even appear only after postgrowth thermal annealing [9–12]. This is confirmed by the present data displayed in Fig. 1(b)–(d). Upon the addition of In to GaAsN the GaN-like LO2 phonon mode shifts slightly from 470 to 469 cm 1 , accompanied
Fig. 1. Room-temperature Raman spectra of (a) an as-grown GaAs0:99 N0:01 /GaAs MQW, (b) an as-grown Ga0:97 In0:03 As0:99 N0:01 /GaAs MQW, (c) same MQW as (b) but after rapid thermal annealing at 950 °C for 15 s, and (d) the difference spectrum ðcÞ ðbÞ highlighting the effect of thermal annealing.
by the appearance of an additional shoulder at 457 cm1 , which is at the low-frequency side of the LO2 mode (Fig. 1(b)). This low-frequency mode gains intensity upon annealing, accompanied by the appearance of a further mode on the high-frequency side, centered at 488 cm1 (Fig. 1(c)). The changes induced in the Raman spectrum of the GaInAsN/GaAs MQW by the annealing step are highlighted in Fig. 1(d) where the difference spectrum ðcÞ ðbÞ is plotted. The above data indicate that in as-grown GaInAsN a certain portion of the nitrogen can be bonded to In neighbors. Thermal annealing leads to an increase of that fraction, but the GaN-like mode still remains dominant, indicating that a considerable amount of N is still bonded to four Ga neighbors (Ga4 N configuration). The observed change in bonding configuration upon thermal annealing can be explained by an interplay between the larger cohesive energy of the Ga–N bond (2.24 eV) compared to that of the In–N bond (1.93 eV) [13], and reduced local strain energy of e.g. the Ga3 InN configuration relative to that of Ga4 N [14]. It has been argued that at the growth surface, nitrogen incorporation is controlled by chemical bonding favoring the Ga4 N configuration, while under thermal equilibrium conditions in the bulk the Ga3 InN
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configuration becomes more favorable because of the reduced local strain [15]. For dilute AlGaAsN a complete dominance of chemical bonding aspects can be expected because of the even larger cohesive energy of the Al–N bond of 2.88 eV [13], strongly favoring the bonding of the N to Al while, due to the comparable size of Ga and Al, differences in local strain should be negligible. That this expectation is indeed fulfilled can be seen from Fig. 2, where Raman spectra of Alx Ga1x As0:99 N0:01 with x 6 0:05 are displayed, covering the frequency range of the N-related vibrational modes and of the AlAs-like longitudinal optical phonon (LO2 ). For x ¼ 0, i.e. GaAs0:99 N0:01 , only the GaN-like mode (labeled LO3 ) is observed at 470 cm1 , superimposed on a background from second-order phonon scattering by dominantly GaAs-like phonon modes [7–9]. Upon the addition of Al new modes appear at the low-frequency side of the GaN-like LO3 phonon, one at 454 cm1 which is clearly resolved for the x ¼ 0:01 and 0.02 samples, and a second peak at 448 cm1 which dominates for x P 0:04. For the highest Al-contents two additional peaks can be resolved at the high-frequency side of the AlAs-like LO2 phonon, centered at 379 and 397 cm1 . None of these new modes are present in the
Fig. 2. Room-temperature Raman spectra of Alx Ga1x As0:99 N0:01 layers on GaAs with Al-concentrations 0 6 x 6 0:05 as given in the figure. New modes arising from the simultaneous presence of N and Al are marked by vertical arrows.
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Raman spectrum of Alx Ga1x As reference samples with the same Al-contents, which were prepared in the same growth series as the AlGaAsN layers. Thus, these modes are linked to the simultaneous presence of both Al and N, but the exact origin of the two modes lowest in frequency is still unclear. There is an almost complete switch in mode intensity from the GaN-like LO3 mode to the Al–N related mode at 448 cm1 , labeled in the following as LO4 (AlN-like), when increasing the Alcontent from x ¼ 0 to 0.05. This drastic change in mode intensity indicates that for an Al-content of only 5%, almost all of the N atoms are incorporated into AlN-like complexes, whose most simple form could be described as Ga3 AlN, Ga2 Al2 N etc. configurations. This is particularly surprising given the fact that for a random statistical distribution, at an Al-content of x ¼ 0:05, 80% of the N atoms are expected to be present in the Ga4 N configuration and only 17% in Ga3 AlN clusters [16]. Therefore the present findings strongly indicate that the incorporation of nitrogen in dilute AlGaAsN is governed by the large Al–N bond strength [13] rather than by random statistics, resulting in the almost exclusive formation of Al–N complexes for x ¼ 0:05 and y ¼ 0:01. This conclusion is confirmed by the Raman data shown in Fig. 3, where spectra are displayed recorded from bulk-like quinary Alx Ga0:94x In0:06 As0:98 N0:02 layers. For x ¼ 0, i.e. Ga0:94 In0:06 As0:98 N0:02 , the Raman spectrum resembles those plotted in Fig. 1(b) and (c), showing a GaN-like mode (LO3 ) accompanied by two InN-like modes at lower and higher frequencies, respectively. Adding Al leads to a decrease in intensity of all three modes and the AlN-like mode LO4 appears slightly below 450 cm1 . Again, for the highest Al-concentration of 10%, i.e. an Al content five times the Ncontent, the AlN-like LO4 mode is the by far dominant N-related feature in the Raman spectrum. And this bonding situation was found to remain unchanged after rapid thermal annealing at 850 °C for 60 s. Only the modes at lower frequencies around 380 and 400 cm1 show a decrease in intensity upon thermal annealing, which indicates that these modes stem from thermally less stable higher-order Al–N complexes. Fig. 4 shows a comparison of Raman spectra recorded from GaAs0:99 N0:01 at 300 and 77 K, respectively. Upon reduction of sample temperature the GaN-like LO2 mode shifts to higher frequencies by 2 cm1 , which is a significantly smaller shift than that of the GaAs-like LO1 phonon (not shown in Fig. 4) of 3.5 cm1 . This temperature behavior is consistent with that reported by Yu et al. [17], who also observed a smaller temperatureinduced frequency shift of the GaN-like mode than for the GaAs-like LO phonon. Going from 300 to 77 K leads further to an increase in peak intensity of the GaNlike LO2 mode by a factor of 3.6, accompanied by a reduction in line width from 12.3 cm1 at 300 K to 6.4 cm1 at 77 K. Also this reduction in line width by about
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a factor of two is consistent with the data presented in Ref. [17]. The increase in integrated intensity, observed upon cooling down from 300 to 77 K, can be explained by a high-energy shift of the Eþ transition with decreasing temperature, as expected from the band anticrossing model [18,19]. This shift brings the incident photons with an energy of 2.18 eV closer to resonance with the Eþ transition, which in turn provides stronger resonant enhancement for Raman scattering by the III– N like LO phonon modes [8,9]. The overall gain in sensitivity to III–N like modes achieved at low temperatures has been exploited to study the bonding of nitrogen in AlGaAsN in more detail. Fig. 5 shows a sequence of 77 K Raman spectra recorded from the same series of Alx Ga1x As0:99 N0:01 layers (x 6 0:05) as used for recording the room-temperature spectra displayed in Fig. 2. Compared to the room-temperature spectra the successive appearance of two closely spaced AlN-like modes upon increasing the Al-concentration from x ¼ 0 to 0.05 at around 450 cm1 , i.e. at the low-frequency side of the GaN-like mode centered at 472 cm1 , is much better resolved in the lowtemperature spectra. Further, the low-temperature Raman spectra reveal the presence of another AlN-related
Fig. 3. Room-temperature Raman spectra of Alx Ga0:94x In0:06 As0:98 N0:02 layers on GaAs with Al-concentrations 0 6 x 6 0:10 as indicated in the figure.
Fig. 4. Raman spectra of GaAs0:99 N0:01 recorded at 77 K (upper spectrum) and 300 K (lower spectrum).
Fig. 5. Low-temperature (77 K) Raman spectra of Alx Ga1x As0:99 N0:01 layers on GaAs with Al-concentrations 0 6 x 6 0:05 as indicated in the figure (same set of samples as shown in Fig. 2). Vertical dashed lines indicate GaN-and AlN-like modes.
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mode at the high-frequency side of the GaN-like mode (i.e. at 498 cm1 ), which is not resolved in the roomtemperature spectra because of the underlying background from second-order phonon scattering.
4. Conclusions A comprehensive Raman spectroscopic study of nitrogen bonding in dilute GaInAsN, AlGaAsN, and AlGaInAsN has been reported. The main result is that in the as-grown AlGa(In)AsN the bonding of nitrogen is governed by the significant difference in bond cohesive energy for the different III–N bonds, while in GaInAsN bonding is controlled by an interplay between bond cohesive energy and a reduction of local strain energy. An issue, which requires further clarification, is that of the AlN-like and InN-like mode frequencies. Based on simple atomic mass and bond strength arguments, AlN-like modes are expected to appear at higher and InN-like modes at lower frequencies than the GaN-like LO phonon mode. Experimentally, however, the addition of both Al and In has been found to produce modes at frequencies both above and below that of the GaNlike LO phonon. For GaInAsN the appearance of a mode at higher frequency than the GaN-like LO phonon has tentatively been explained by a large reduction in local strain upon formation of In–N bonds, leading to a significant stiffening of those bonds compared to the Ga–N bonds in Ga4 N complexes, and thus an overall increase in mode frequency [10]. However, to shed further light on this issue, theoretical work is required to model the AlN-like in InN-like mode frequencies in dilute AlGaAsN and GaInAsN, including different bonding and complex configurations. Results of such theoretical work can then help in assigning the observed modes to specific bonding configurations.
Acknowledgements We would like to thank T. Fuchs for expert technical assistance with the SIMS measurements as well as N. Herres, H. G€ ullich and L. Kirste for performing the HRXRD analyses. G. Weimann is thanked for continuous support and encouragement.
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