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GaAs single quantum well tailored at 1.5 μm
Solid State Communications 137 (2006) 138–141 www.elsevier.com/locate/ssc
Photoreflectance spectroscopy of a Ga0.62In0.38N0.026As0.954Sb0.02/GaAs single quantum well tailored at 1.5 mm R. Kudrawiec a,*, M. Gladysiewicz a, J. Misiewicz a, H.B. Yuen b, S.R. Bank b, M.A. Wistey b, H.P. Bae b, James S. Harris b a b
Institute of Physics, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland Solid State and Photonics Laboratory, Department of Electrical Engineering, Stanford University, 126X CISX, Via Ortega, Stanford, CA 94305-4075, USA Received 28 October 2005; accepted 2 November 2005 by S. Das Sarma Available online 18 November 2005
Abstract Optical properties of a Ga0.62In0.38As0.954N0.026Sb0.02/GaAs single quantum well (SQW) tailored at w1.5 mm have been investigated by photoreflectance (PR) spectroscopy. The identification of the optical transitions was carried out in accordance with theoretical calculations, which were performed within the framework of the usual envelope function approximation. Using this method, four confined states for both electrons and heavy holes have been found and the optical transitions between them have been determined. The obtained result corresponds to a conduction band offset ratio close to 80%. In addition, the effect of ex situ annealing has been investigated. Lineshape analysis of the PR transitions shows that one of the phenomena responsible for the blueshift of QW transitions is the change in the nitrogen nearest-neighbour environment from Ga-rich to In-rich environments. q 2006 Elsevier Ltd. All rights reserved. PACS: 78.67.De Keywords: A. Semiconductors; A. Quantum wells; D. Photoreflectance
Long wavelength (1.3 and 1.55 mm) laser diodes have attracted much attention in recent years due to their application in optical fiber communication. However, the conventional InP-based system exhibits a relatively low characteristic temperature due to poor electron confinement. In 1996, Kondow et al. proposed the GaInNAs/GaAs quantum well (QW) as a novel GaAs-based material system for this application [1]. One of the main advantages of this system is the monolithic integration of highly-reflective distributed Bragg reflectors which enable the fabrication of low-cost vertical cavity lasers as well as resonant cavity detectors and modulators. Recent progress on the use of GaInNAs for these applications has succeeded in producing devices operating up to 1.4 mm wavelength [2–4]. However, it remains difficult to obtain good performance for these lasers and to shift the emission to longer wavelengths, because with increasing
* Corresponding author. Tel.: C48 71 320 42 80; fax: C48 71 328 36 96. E-mail address: [email protected] (R. Kudrawiec).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.11.006
nitrogen concentration, especially above 2.5%, the optical quality of the material deteriorates due to a restriction in the optimal growth parameters. The degradation results in a higher threshold current density for lasers. In the past few years, GaInNAsSb has been found to be a potentially superior material to GaInNAs for long wavelength laser applications [5–9]. It has been shown that when using Sb as a surfactant, the quality of highly strained GaInNAs/GaAs QWs improves significantly [6,8]. However, to date no photoreflectance (PR) investigations of GaInNAsSb/GaAs QWs have been reported. Moreover, only a few papers report optical properties of GaInNAsSb/GaAs QWs [10–12] and none discuss the energy level structure for such QWs, i.e. the number of confined states for electrons and holes as well as the energy differences between them. In this paper PR spectroscopy has been applied to investigate the number of confined states for a Ga0.62In0.38N0.026As0.954Sb0.02/GaAs single QW (SQW) and to determine the conduction band offset ratio (QC) for this structure. In addition, the effect of post-growth annealing is studied. The lineshape of PR transitions is discussed in the context of the annealing induced change in the nitrogen nearest-neighbor
R. Kudrawiec et al. / Solid State Communications 137 (2006) 138–141
environments [13] and associated with this phenomenon a blue shift of the QW transitions [14–17]. The GaInNAsSb/GaAs SQW was grown by solid-source molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate. The sample was composed of 250 nm thick GaAs buffer layer, 8 nm thick GaInNAsSb QW, and 50 nm thick GaAs cap layer. The GaInNAsSb layer had the composition of 38% In, w2.6% N, 2% Sb. Details of the growth conditions and the content calibration can be found in Ref.[7,8]. A piece of the sample was annealed at 760 8C for 60 s. For PR measurements, details of the setup can be found in Ref.[18,19]. Fig. 1 shows room temperature PR spectra of the as-grown (curve (i)) and annealed (curve (ii)) GaInNAsSb/GaAS SQWs, respectively. The notation nmH denotes the transition between n-th heavy-hole valence subband and m-th conduction subband. The resonance at the lowest energy we connect with the 11H transition which is a fundamental one in such QWs. Besides the 11H transition, PR spectra show resonances related to higher QW transitions such as 22H, 33H and 44H. The identification of the resonances was carried out in accordance with theoretical calculations, which were performed within the framework of the usual envelope function approximation. Following the band anticrossing (BAC) model [20,21], the influence of nitrogen localized states on the valence band structure is neglected. Therefore, we assumed that the effective mass of heavy hole does not change after adding nitrogen atoms, i.e. the hole effective mass is the same as for GaInAsSb (mhhZ0.37m0) [22]. The electron effective mass has been assumed to be 0.12m0. Such electron effective mass is in accordance with the BAC model prediction [21] and literature data for Sb free GaInNAs alloys [23]. In order to find the bandgap energy of the GaInNAsSb layer we avoid the BAC model, because the knowledge about BAC parameters for GaInNAsSb is rather poor. We adjusted the bandgap energy of
Fig. 1. Room temperature PR spectra of as-grown (i) and annealed (ii) Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs.
139
Fig. 2. Energy level structure of the Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQW.
the GaInNAsSb layer to the experimental value of the ground state transition. The influence of strain on the band structure is taken into account as in Ref.[23], but excitonic effects are neglected. The compressive strain present in our sample has been determined on the basis of high resolution X-ray diffraction measurements to be 3Z2.54%. The energy level structure obtained from calculations for the as-grown GaInNAsSb/GaAs SQW is shown in Fig. 2. In addition, the calculated energies of the QW transitions are marked by arrows in Fig. 1 (arrows at top axis). It is seen that the agreement between experiment and theory is very good for the 11H, 22H, and 33H transitions but poor for the 44H transition. It is worth noting that the first three QW transitions are very weakly sensitive to a change in the band offset because the electron and heavy hole levels involved with these transitions are deeply confined in the QW. In the case of the fourth transition, a different behaviour has been observed. Regardless, calculations show that the fourth transition is only possible for a narrow range of QC value. For QC smaller than w70%, only three confined states for electrons have been obtained and for QC greater than w85%, only three confined states for heavy holes have been found. We have concluded that the QC for our SQW has to be close to 80G5% because this value has four confined states for both electrons and heavy holes. The reason for the disagreement between the experimental and calculated energies for the 44H transition (see in Fig. 1) is the non-square shape of the QW profile. The real shape of the potential for electrons and holes could be smoother at the interfaces due a non-abrupt profile of the In, Sb, or N concentrations at QW interfaces. Such imperfections could lead to some disagreements between measured values and calculations, especially for deep QWs where there are many confined states for electrons and holes. It is known from calculations that the energy difference between energy levels is smaller for a non-square QW than for a perfectly square QW. This is especially important for higher order levels. Therefore, the energy of 44H transition observed experimentally is smaller than what is calculated assuming a square QW. Note that if we take into account non-square profile of the SQW we are able to match experimental data with theoretical calculations for smaller electron effective mass. In general, we expect that the electron effective mass for GaInNAsSb
140
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compound is smaller than the electron effective mass predicted by BAC model for GaNAs compound because GaInNAsSb has significantly smaller band gap energy than GaNAs. Transitions related to light-holes are not considered in this paper because they probably are not clearly resolved in PR spectra. On the basis of the calculations it has found that for QCZ80% only one confined state exists for light holes and 11L transition interfere with 22H transition. Thus the second resonance, observed at w0.94 eV for the as-grown SQW, could be attributed to both 11L and 22H transitions. As it is seen in Fig. 1, the QW transitions blueshift with anneal. In addition, the shape of the PR resonances changes significantly, which is rather unusual for the annealed SQW. Such shape of the PR signal suggests that the QW transitions are composed of more than one PR resonance. In order to explore this possibility, we have measured PR spectra at low temperatures. Fig. 3 shows the PR spectra for the as-grown (curve (i)) and annealed (curve (ii)) SQWs. In this figure, the visible PR features related to 11H, 22H and 33H transitions are very broad and complex despite the decrease in temperature. Our fits with one Gaussian- or one Lorenzian-like PR resonance [18] fail, confirming that these transitions must be composed of more than one resonance. A very good fit has been obtained for two and more PR resonances. However, quantitative analysis of such a fit with more than two PR resonances can be controversial, especially since we have to consider two possible origins for the complex character of PR transitions. First, we must consider the multiple bandgap energies of GaInNAsSb compounds due to short range ordering effects [13]. It is expected, such as for GaInNAs/GaAs QWs [14–16] and GaInNAs layers [17], that particular nitrogen nearestneighbour environments could lead to individual PR resonances. Hence PR features related to QW transitions are
Fig. 3. PR spectra of as-grown (i) and annealed (ii) Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs measured at 10 K.
composed of more than one resonance. The second origin of the complex character in PR transitions is associated with forbidden transitions. Well known nominally forbidden transitions, such as 21H, can be observed in a real system due to imperfections in the QW shape and/or mixing of wave functions. In our SQW, the heavy hole levels are separated by energies which are comparable with the broadening of PR resonances (h2Kh1Z29 meV, h3Kh2Z46 meV, and h4K h3Z54 meV). It leads to a small energy separation between such transitions like 11H and 21H or 22H and 32H, etc. This means forbidden transitions are not resolved in the PR spectrum because they are convoluted with the allowed ones. The energy of such PR features of allowed and forbidden transitions is related mostly to the energy of the allowed transition due to larger oscillator strength for this transition. The presence of forbidden transitions changes mostly the shape of the PR features. Such behaviour could take place in our spectra, but we believe the main phenomenon responsible for the PR line shape change is associated with the annealing induced change in the nitrogen nearest-neighbour environment. For as-grown samples, N atoms are present in Ga-rich environments, as they are randomly distributed due to nonequlibrum growth conditions. After annealing, the N environment changes from Ga-rich to In-rich because [13] it is more favourable in the terms of local strain and the crystal energy. This leads to an effective blueshift of the GaInNAsSb bandgap energy and hence a blueshift of QW transitions. In the case of a QW structure, the phenomenon of atomic interdiffusion across QW interfaces is another possible origin of the blueshift in QW transitions. It has been shown for GaInNAs/GaAs QWs [15] the contribution from this effect to the total blueshift of QW transitions is smaller than the contribution from changes in nitrogen nearest-neighbour environment. Similar behaviour probably takes place for GaInNAsSb/GaAs QWs. However, a quantitative analysis of the two contributions is difficult for SQW discussed in this paper. The lineshape of the QW transitions shows that the 11H, 22H, 33H, and 44H features are composed of a few PR resonances. The intensity of these resonances changes after annealing; such changes lead to an effective blueshift of the QW transitions (see Figs. 1 and 3). Our observation confirms that one of the origins of the blueshift is a redistribution in the intensities of individual resonances related to different nitrogen environments. However, this does not eliminate the fact that the total blueshift of the QW transitions has no contribution from the change in the QW shape due to an atom interdiffusion across QW interfaces. But we suppose that in our case the interdiffusion process could be neglected, because HRXRD spectra are identical for the as-grown and annealed samples. In addition, it is well known that the interdiffusion process is a defect related phenomenon, which is enhanced by defects like vacancies. It has been shown that that a significant concentration of vacancies exists only when hydrogen and nitrogen are both present during growth [24]. Growing GaInNAs(Sb) by solid-source MBE, a hydrogen-free growth method, minimizes the formation of the vacancy complexes
R. Kudrawiec et al. / Solid State Communications 137 (2006) 138–141
[24]. Therefore, we expect that for our samples the process of atom interdiffusion could be neglected. In conclusion, the observation of four optical transitions (11H, 22H, 33H, and 44H) in GaInNAsSb/GaAs SQWs by PR spectroscopy shows that the QC for this QW is close to 80%, very promising for laser applications. The lineshape analysis of the PR transitions suggests that the bandgap energy of GaInNAsSb alloy depends on the nitrogen nearest-neighbor environments and the change in the environment from Ga-rich to In-rich environment is one of the origins of the annealing induced blueshift of QW transitions. Acknowledgements This work was supported in the United States of America under DARPA and ARO contracts MDA972-00-1-024, DAAD17-02-C-0101 and DAAD199-02-1-0184, ONR contract N00014-01-1-00100, as well as the Stanford Network Research Center (SNRC). R. Kudrawiec acknowledges the financial support from the Foundation for Polish Science. H. Yuen would like to thank the Stanford Graduate Fellowships for funding assistance. References [1] M. Kondow, K. Uomi, A. Niwa, T. Kikatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (2B) (1996) 1273. [2] J.S. Harris Jr, Semicond. Sci. Technol. 17 (2002) 880. [3] H. Riechert, A. Ramakrishnan, G. Steinle, Semicond. Sci. Technol. 17 (2002) 892. [4] G. Jaschke, R. Averbeck, L. Geelhaar, H. Riechert, 278 (2005) 224. [5] X. Yang, M.J. Jurkovic, J.B. Heroux, W.I. Wand, Appl. Phys. Lett. 75 (1999) 178. [6] H. Shimizu, K. Kumada, S. Uchiyama, A. Kasukawa, Electron. Lett 36 (2000) 1701.
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[7] S.R. Bank, M.A. Wistey, H.B. Yuen, L.L. Goddard, W. Ha, J.S. Harris Jr, Electron. Lett 39 (2003) 1445. [8] S.R. Bank, M.A. Wistey, L.L. Goddard, H.B. Yuen, V. Lordi, J.S. Harris Jr, IEEE J. Quantum Electron. 40 (2004) 656. [9] L.H. Li, V. Sallet, G. Patriarche, L. Largeau, S. Bouchoule, L. Travers, J.C. Harmand, Appl. Phys. Lett. 83 (2003) 1298. [10] X. Yang, J.B. Heroux, L.F. Mei, W.I. Wang, Appl. Phys. Lett. 78 (2001) 4068. [11] T.S. Kim, J.Y. Park, T.V. Cuong, H.J. Lee, E.-K. Suh, C.-H. Hong, J. Cryst. Growth 270 (2004) 340. [12] V. Lordi, H.B. Yuen, S.R. Bank, J.S. Harris Jr, Appl. Phys. Lett. 85 (2004) 902. [13] V. Lordi, S. Friedrich, T. Funk, T. Takizawa, K. Uno, J.S. Harris, Phys. Rev. Lett. 90 (2003) 145505. [14] P.J. Klar, H. Gru¨ning, J. Koch, S. Scha¨fer, K. Volz, W. Stolz, W. Heimbrodt, A.M. Kamal Saadi, A. Lindsay, E.P. O’Reilly, Phys. Rev. B 64 (2001) 121203(R). [15] R. Kudrawiec, G. Sek, J. Misiewicz, D. Gollub, A. Forchel, Appl. Phys. Lett. 83 (2003) 2772. [16] R. Kudrawiec, E.-M. Pavelescu, J. Andrzejewski, J. Misiewicz, A. Gheorghiu, T. Jouhti, M. Pessa, J. Appl. Phys. 96 (2004) 2909. [17] R. Kudrawiec, E.-M. Pavelescu, J. Wagner, G. Se˛k, J. Misiewicz, M. Dumitrescu, J. Konttinen, A. Gheorghiu, M. Pessa, J. Appl. Phys. 96 (2004) 2576. [18] F.H. Pollak, in: T.S. Moss (Ed.), Modulation Spectroscopy of Semiconductors and Semiconductor Microstructures Handbook on Semiconductors vol. 2, Elsevier Science, Amsterdam, 1994, pp. 527–635. [19] J. Misiewicz, P. Sitarek, G. Sek, R. Kudrawiec, Mater. Sci. 21 (2003) 263. [20] W. Shan, W. Walukiewicz, J.W. Ager III, E.E. Haller, J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Krutz, Phys. Rev. Lett. 82 (1999) 1221. [21] W. Shan, W. Walukiewicz, K.M. Yu, J.W. Ager III, W.E. Haller, J.F. Geisz, D.J. Friedman, J.M. Olson, S.R. Kurtz, H.P. Xin, C.W. Tu, Phys. Status Solidi B 223 (2001) 75. [22] I. Vurgaftman, J.R. Mayer, L.-R. Ram-Mohan, J. Appl. Phys. 89 (2001) 5815 (and references there in). [23] J. Misiewicz, R. Kudrawiec, K. Ryczko, G. Se˛k, A. Forchel, J.C. Harmand, M. Hammar, J. Phys.: Condens. Matter 16 (2004) 3071 (and there in). [24] A.J. Ptak, S. Kurtz, M.H. Weber, K.G. Lynn, J. Vac. Sci. Technol., B 22 (2004) 1584.
Photoreflectance spectroscopy of a Ga0.62In0.38N0.026As0.954Sb0.02/GaAs single quantum well tailored at 1.5 mm R. Kudrawiec a,*, M. Gladysiewicz a, J. Misiewicz a, H.B. Yuen b, S.R. Bank b, M.A. Wistey b, H.P. Bae b, James S. Harris b a b
Institute of Physics, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland Solid State and Photonics Laboratory, Department of Electrical Engineering, Stanford University, 126X CISX, Via Ortega, Stanford, CA 94305-4075, USA Received 28 October 2005; accepted 2 November 2005 by S. Das Sarma Available online 18 November 2005
Abstract Optical properties of a Ga0.62In0.38As0.954N0.026Sb0.02/GaAs single quantum well (SQW) tailored at w1.5 mm have been investigated by photoreflectance (PR) spectroscopy. The identification of the optical transitions was carried out in accordance with theoretical calculations, which were performed within the framework of the usual envelope function approximation. Using this method, four confined states for both electrons and heavy holes have been found and the optical transitions between them have been determined. The obtained result corresponds to a conduction band offset ratio close to 80%. In addition, the effect of ex situ annealing has been investigated. Lineshape analysis of the PR transitions shows that one of the phenomena responsible for the blueshift of QW transitions is the change in the nitrogen nearest-neighbour environment from Ga-rich to In-rich environments. q 2006 Elsevier Ltd. All rights reserved. PACS: 78.67.De Keywords: A. Semiconductors; A. Quantum wells; D. Photoreflectance
Long wavelength (1.3 and 1.55 mm) laser diodes have attracted much attention in recent years due to their application in optical fiber communication. However, the conventional InP-based system exhibits a relatively low characteristic temperature due to poor electron confinement. In 1996, Kondow et al. proposed the GaInNAs/GaAs quantum well (QW) as a novel GaAs-based material system for this application [1]. One of the main advantages of this system is the monolithic integration of highly-reflective distributed Bragg reflectors which enable the fabrication of low-cost vertical cavity lasers as well as resonant cavity detectors and modulators. Recent progress on the use of GaInNAs for these applications has succeeded in producing devices operating up to 1.4 mm wavelength [2–4]. However, it remains difficult to obtain good performance for these lasers and to shift the emission to longer wavelengths, because with increasing
* Corresponding author. Tel.: C48 71 320 42 80; fax: C48 71 328 36 96. E-mail address: [email protected] (R. Kudrawiec).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.11.006
nitrogen concentration, especially above 2.5%, the optical quality of the material deteriorates due to a restriction in the optimal growth parameters. The degradation results in a higher threshold current density for lasers. In the past few years, GaInNAsSb has been found to be a potentially superior material to GaInNAs for long wavelength laser applications [5–9]. It has been shown that when using Sb as a surfactant, the quality of highly strained GaInNAs/GaAs QWs improves significantly [6,8]. However, to date no photoreflectance (PR) investigations of GaInNAsSb/GaAs QWs have been reported. Moreover, only a few papers report optical properties of GaInNAsSb/GaAs QWs [10–12] and none discuss the energy level structure for such QWs, i.e. the number of confined states for electrons and holes as well as the energy differences between them. In this paper PR spectroscopy has been applied to investigate the number of confined states for a Ga0.62In0.38N0.026As0.954Sb0.02/GaAs single QW (SQW) and to determine the conduction band offset ratio (QC) for this structure. In addition, the effect of post-growth annealing is studied. The lineshape of PR transitions is discussed in the context of the annealing induced change in the nitrogen nearest-neighbor
R. Kudrawiec et al. / Solid State Communications 137 (2006) 138–141
environments [13] and associated with this phenomenon a blue shift of the QW transitions [14–17]. The GaInNAsSb/GaAs SQW was grown by solid-source molecular beam epitaxy (MBE) on a semi-insulating GaAs substrate. The sample was composed of 250 nm thick GaAs buffer layer, 8 nm thick GaInNAsSb QW, and 50 nm thick GaAs cap layer. The GaInNAsSb layer had the composition of 38% In, w2.6% N, 2% Sb. Details of the growth conditions and the content calibration can be found in Ref.[7,8]. A piece of the sample was annealed at 760 8C for 60 s. For PR measurements, details of the setup can be found in Ref.[18,19]. Fig. 1 shows room temperature PR spectra of the as-grown (curve (i)) and annealed (curve (ii)) GaInNAsSb/GaAS SQWs, respectively. The notation nmH denotes the transition between n-th heavy-hole valence subband and m-th conduction subband. The resonance at the lowest energy we connect with the 11H transition which is a fundamental one in such QWs. Besides the 11H transition, PR spectra show resonances related to higher QW transitions such as 22H, 33H and 44H. The identification of the resonances was carried out in accordance with theoretical calculations, which were performed within the framework of the usual envelope function approximation. Following the band anticrossing (BAC) model [20,21], the influence of nitrogen localized states on the valence band structure is neglected. Therefore, we assumed that the effective mass of heavy hole does not change after adding nitrogen atoms, i.e. the hole effective mass is the same as for GaInAsSb (mhhZ0.37m0) [22]. The electron effective mass has been assumed to be 0.12m0. Such electron effective mass is in accordance with the BAC model prediction [21] and literature data for Sb free GaInNAs alloys [23]. In order to find the bandgap energy of the GaInNAsSb layer we avoid the BAC model, because the knowledge about BAC parameters for GaInNAsSb is rather poor. We adjusted the bandgap energy of
Fig. 1. Room temperature PR spectra of as-grown (i) and annealed (ii) Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs.
139
Fig. 2. Energy level structure of the Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQW.
the GaInNAsSb layer to the experimental value of the ground state transition. The influence of strain on the band structure is taken into account as in Ref.[23], but excitonic effects are neglected. The compressive strain present in our sample has been determined on the basis of high resolution X-ray diffraction measurements to be 3Z2.54%. The energy level structure obtained from calculations for the as-grown GaInNAsSb/GaAs SQW is shown in Fig. 2. In addition, the calculated energies of the QW transitions are marked by arrows in Fig. 1 (arrows at top axis). It is seen that the agreement between experiment and theory is very good for the 11H, 22H, and 33H transitions but poor for the 44H transition. It is worth noting that the first three QW transitions are very weakly sensitive to a change in the band offset because the electron and heavy hole levels involved with these transitions are deeply confined in the QW. In the case of the fourth transition, a different behaviour has been observed. Regardless, calculations show that the fourth transition is only possible for a narrow range of QC value. For QC smaller than w70%, only three confined states for electrons have been obtained and for QC greater than w85%, only three confined states for heavy holes have been found. We have concluded that the QC for our SQW has to be close to 80G5% because this value has four confined states for both electrons and heavy holes. The reason for the disagreement between the experimental and calculated energies for the 44H transition (see in Fig. 1) is the non-square shape of the QW profile. The real shape of the potential for electrons and holes could be smoother at the interfaces due a non-abrupt profile of the In, Sb, or N concentrations at QW interfaces. Such imperfections could lead to some disagreements between measured values and calculations, especially for deep QWs where there are many confined states for electrons and holes. It is known from calculations that the energy difference between energy levels is smaller for a non-square QW than for a perfectly square QW. This is especially important for higher order levels. Therefore, the energy of 44H transition observed experimentally is smaller than what is calculated assuming a square QW. Note that if we take into account non-square profile of the SQW we are able to match experimental data with theoretical calculations for smaller electron effective mass. In general, we expect that the electron effective mass for GaInNAsSb
140
R. Kudrawiec et al. / Solid State Communications 137 (2006) 138–141
compound is smaller than the electron effective mass predicted by BAC model for GaNAs compound because GaInNAsSb has significantly smaller band gap energy than GaNAs. Transitions related to light-holes are not considered in this paper because they probably are not clearly resolved in PR spectra. On the basis of the calculations it has found that for QCZ80% only one confined state exists for light holes and 11L transition interfere with 22H transition. Thus the second resonance, observed at w0.94 eV for the as-grown SQW, could be attributed to both 11L and 22H transitions. As it is seen in Fig. 1, the QW transitions blueshift with anneal. In addition, the shape of the PR resonances changes significantly, which is rather unusual for the annealed SQW. Such shape of the PR signal suggests that the QW transitions are composed of more than one PR resonance. In order to explore this possibility, we have measured PR spectra at low temperatures. Fig. 3 shows the PR spectra for the as-grown (curve (i)) and annealed (curve (ii)) SQWs. In this figure, the visible PR features related to 11H, 22H and 33H transitions are very broad and complex despite the decrease in temperature. Our fits with one Gaussian- or one Lorenzian-like PR resonance [18] fail, confirming that these transitions must be composed of more than one resonance. A very good fit has been obtained for two and more PR resonances. However, quantitative analysis of such a fit with more than two PR resonances can be controversial, especially since we have to consider two possible origins for the complex character of PR transitions. First, we must consider the multiple bandgap energies of GaInNAsSb compounds due to short range ordering effects [13]. It is expected, such as for GaInNAs/GaAs QWs [14–16] and GaInNAs layers [17], that particular nitrogen nearestneighbour environments could lead to individual PR resonances. Hence PR features related to QW transitions are
Fig. 3. PR spectra of as-grown (i) and annealed (ii) Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs measured at 10 K.
composed of more than one resonance. The second origin of the complex character in PR transitions is associated with forbidden transitions. Well known nominally forbidden transitions, such as 21H, can be observed in a real system due to imperfections in the QW shape and/or mixing of wave functions. In our SQW, the heavy hole levels are separated by energies which are comparable with the broadening of PR resonances (h2Kh1Z29 meV, h3Kh2Z46 meV, and h4K h3Z54 meV). It leads to a small energy separation between such transitions like 11H and 21H or 22H and 32H, etc. This means forbidden transitions are not resolved in the PR spectrum because they are convoluted with the allowed ones. The energy of such PR features of allowed and forbidden transitions is related mostly to the energy of the allowed transition due to larger oscillator strength for this transition. The presence of forbidden transitions changes mostly the shape of the PR features. Such behaviour could take place in our spectra, but we believe the main phenomenon responsible for the PR line shape change is associated with the annealing induced change in the nitrogen nearest-neighbour environment. For as-grown samples, N atoms are present in Ga-rich environments, as they are randomly distributed due to nonequlibrum growth conditions. After annealing, the N environment changes from Ga-rich to In-rich because [13] it is more favourable in the terms of local strain and the crystal energy. This leads to an effective blueshift of the GaInNAsSb bandgap energy and hence a blueshift of QW transitions. In the case of a QW structure, the phenomenon of atomic interdiffusion across QW interfaces is another possible origin of the blueshift in QW transitions. It has been shown for GaInNAs/GaAs QWs [15] the contribution from this effect to the total blueshift of QW transitions is smaller than the contribution from changes in nitrogen nearest-neighbour environment. Similar behaviour probably takes place for GaInNAsSb/GaAs QWs. However, a quantitative analysis of the two contributions is difficult for SQW discussed in this paper. The lineshape of the QW transitions shows that the 11H, 22H, 33H, and 44H features are composed of a few PR resonances. The intensity of these resonances changes after annealing; such changes lead to an effective blueshift of the QW transitions (see Figs. 1 and 3). Our observation confirms that one of the origins of the blueshift is a redistribution in the intensities of individual resonances related to different nitrogen environments. However, this does not eliminate the fact that the total blueshift of the QW transitions has no contribution from the change in the QW shape due to an atom interdiffusion across QW interfaces. But we suppose that in our case the interdiffusion process could be neglected, because HRXRD spectra are identical for the as-grown and annealed samples. In addition, it is well known that the interdiffusion process is a defect related phenomenon, which is enhanced by defects like vacancies. It has been shown that that a significant concentration of vacancies exists only when hydrogen and nitrogen are both present during growth [24]. Growing GaInNAs(Sb) by solid-source MBE, a hydrogen-free growth method, minimizes the formation of the vacancy complexes
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[24]. Therefore, we expect that for our samples the process of atom interdiffusion could be neglected. In conclusion, the observation of four optical transitions (11H, 22H, 33H, and 44H) in GaInNAsSb/GaAs SQWs by PR spectroscopy shows that the QC for this QW is close to 80%, very promising for laser applications. The lineshape analysis of the PR transitions suggests that the bandgap energy of GaInNAsSb alloy depends on the nitrogen nearest-neighbor environments and the change in the environment from Ga-rich to In-rich environment is one of the origins of the annealing induced blueshift of QW transitions. Acknowledgements This work was supported in the United States of America under DARPA and ARO contracts MDA972-00-1-024, DAAD17-02-C-0101 and DAAD199-02-1-0184, ONR contract N00014-01-1-00100, as well as the Stanford Network Research Center (SNRC). R. Kudrawiec acknowledges the financial support from the Foundation for Polish Science. H. Yuen would like to thank the Stanford Graduate Fellowships for funding assistance. References [1] M. Kondow, K. Uomi, A. Niwa, T. Kikatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (2B) (1996) 1273. [2] J.S. Harris Jr, Semicond. Sci. Technol. 17 (2002) 880. [3] H. Riechert, A. Ramakrishnan, G. Steinle, Semicond. Sci. Technol. 17 (2002) 892. [4] G. Jaschke, R. Averbeck, L. Geelhaar, H. Riechert, 278 (2005) 224. [5] X. Yang, M.J. Jurkovic, J.B. Heroux, W.I. Wand, Appl. Phys. Lett. 75 (1999) 178. [6] H. Shimizu, K. Kumada, S. Uchiyama, A. Kasukawa, Electron. Lett 36 (2000) 1701.
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