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GaAs quantum well structures containing Sb atoms
Applied Surface Science 253 (2006) 152–157 www.elsevier.com/locate/apsusc
Contactless electroreflectance spectroscopy of Ga(In)NAs/GaAs quantum well structures containing Sb atoms R. Kudrawiec a,b,*, M. Gladysiewicz a, M. Motyka a, J. Misiewicz a, H.B. Yuen b, S.R. Bank b, M.A. Wistey b, H.P. Bae b, James S. Harris Jr.b a
Institute of Physics, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland b Solid State and Photonics Laboratory, Department of Electrical Engineering, 126X CISX, Via Ortega, Stanford University, Stanford, CA 94305-4075, USA Available online 17 July 2006
Abstract Contactless electroreflectance (CER) spectroscopy has been applied to investigate the optical transitions in Ga(In)NAs/GaAs quantum well (QW) structures containing Sb atoms. The identification of the optical transitions has been carried out in accordance with theoretical calculations which have been performed within the framework of the effective mass approximation. Using this method, the bandgap discontinuity for GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/GaAs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs QW structures has been determined. It has been found that the conduction-band offset is 50 and 80% for GaN0.027As0.863Sb0.11/GaAs and Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QWs, respectively. It corresponds to 264 and 296 meV depth QW for electrons and heavy-holes in GaN0.027As0.863Sb0.11/GaAs QW; and 520 and 146 meV depth QW for electrons and heavy-holes in Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QW. In the case of the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs step-like QW structure it has been shown that the depth of electron and heavy-hole Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW is 144 and 127 meV, respectively. # 2006 Elsevier B.V. All rights reserved. Keywords: Dilute nitrides; Quantum wells; Electroreflectance; Band offset
1. Introduction GaAs-based structures with few percent of nitrogen (N) are being intensively investigated as promising active materials for the 1.3–1.6 mm wavelength emission due to several advantages in comparison to InP-based structures, including improved temperature-dependent characteristics and the feasibility of monolithic integration with AlAs/GaAs-based Bragg reflectors [1,2]. Among these materials, GaInNAs is an attractive alloy which has been used in the active regions of lasers with emission at 1.3 mm. Recent progress on the use of GaInNAs for these applications has succeeded in producing lasers operating up to 1.4 mm wavelength [3–5]. However, it remains difficult to obtain good performance for these lasers and to shift the emission to longer wavelengths, because with increasing nitrogen concentration, especially above 2.5%, the optical
* Corresponding author. Tel.: +48 71 320 42 80; fax: +48 71 328 36 96. E-mail address: [email protected] (R. Kudrawiec). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.05.111
quality of the material deteriorates due to a restriction in the optimal growth parameters [1,2]. The degradation results in a higher threshold current density for lasers. In the past few years, Sb-containing Ga(In)NAs/GaAs QW structures has been found to be a potentially superior material to GaInNAs for long wavelength laser applications [6–12]. It has been shown that when using Sb as a surfactant, the quality of highly strained GaInNAs/GaAs QWs improves significantly [8,12]. In addition, the incorporation of Sb atoms into Ga(In)NAs helps to achieve emission at longer wavelengths. However, there are few reports on the optical properties of GaInNAs/GaAs structures containing Sb atoms [6–23], and a lot of properties important for laser structures, especially the band gap discontinuity, still pose important questions. Modulation spectroscopy, such as photoreflectance (PR) and contactless electroreflectance (CER), is a powerful tool to investigate optical properties of semiconductor systems [24–26]. PR and CER spectroscopies are particularly useful because they are performed in a contactless mode that is nondestructive for samples. In addition, these techniques are very
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sensitive at room temperature, an important aspect of material characterization since devices normally operate at room temperature. The derivative nature of these experimental methods enables observation of a large number of sharp spectral features including those related to excited state transitions in low-dimensional structures, in contrast to common emission-type experiments such as photoluminescence (PL), which usually probes only the ground state. The analysis of QW energy transitions together with theoretical calculations makes it possible to determine material parameters such as the band gap discontinuity. Such procedures have been often applied in studies for different semiconductor structures [27–30]. In our previous papers we have applied PR spectroscopy supported by theoretical calculations in order to determine the energy level structure in GaNAsSb/GaAs [16,31], GaInNAsSb/GaAs [32] and GaInNAsSb/GaNAs/GaAs QWs [33]. In this paper, similar QW systems have been studied in CER spectroscopy, which in addition to PR information is free of below band gap oscillation features typical of GaAsbased structures grown on n-type GaAs substrate [34,35].
sensitivity the post-growth annealing do not influence the strain and content of investigated samples. The CER measurements were performed in the so called ‘bright configuration’ where the sample is illuminated by white light instead the monochromatic light used in the standard ‘dark configuration’. A tungsten halogen lamp (150 W) was used as the probe light source and a single grating 0.55 m monochromator with a thermo-electrically cooled InGaAs pin photodiode was used to analyze the reflected light. Other details of the ‘bright configuration’ setup can be found elsewhere [36]. Samples were mounted in a capacitor with the top electrode made from a copper mesh and the bottom electrode made from a copper block. The top electrode is semitransparent and was kept at a distance of 0.1 mm from the sample surface, while the sample itself was fixed on the bottom copper electrode. A maximum peak-to-peak alternating voltage of 1.8 kV was applied to this capacitor. Phase sensitive detection of the CER signal was made using a lock-in amplifier.
2. Experimental
Figs. 1–3 show CER spectra measured at room temperature for as-grown (upper panel) and annealed (bottom panel) GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/ GaAs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs QW structures, respectively. All these spectra are dominated by GaAs bulk-like bandgap signal at 1.42 eV. In addition, a PR feature at 1.37 eV is resolved in Figs. 1–3. This feature confirms the presence of an intermediate GaAs:N layer due to the nitrogen rf plasma ignition and stabilization prior to
The QW structures used to this study were grown by solidsource molecular beam epitaxy (MBE) in a Varian Mod GenII system on semi-insulating (1 0 0) GaAs substrates. Four structures are presented in this paper. The first structure is a GaNAsSb/GaAs SQW; it is composed of a 250 nm thick GaAs buffer layer, 50 nm thick GaAs:N layer with nitrogen concentration of 0.1%, 10 nm thick GaN0.027As0.863Sb0.11 QW and 50 nm thick GaAs cap layer. The second structure is composed of a 250 nm thick GaAs buffer layer, 8 nm thick GaInNAsSb QW and 50 nm thick GaAs cap layer. The GaInNAsSb layer has composition of 38% In, 2.6% N, 2% Sb. This sample has been reported in Ref. [32]. The third structure is a GaInNAsSb/GaNAs/GaAs step-like QW structure reported in Ref. [33]. This sample is composed of a 250 nm thick GaAs buffer layer, 50 nm thick GaAs:N layer with nitrogen concentration of 0.1%, GaNAs step-like barriers, GaInNAsSb QW and 50 nm thick GaAs cap layer. The GaInNAsSb/GaNAs QW has a nominal composition of 39% In, 1.7% N, 2% Sb in the GaInNAsSb layer and 2.7% N in the GaNAs barriers. The nominal thickness of the GaNAs step-like barriers and GaInNAsSb QW is 20 and 7.5 nm, respectively. The fourth structure is a 20 nm thick GaN0.027As0.973/GaAs QW. This sample is a reference structure useful in the interpretation of PR spectra for the step-like QW structures. The composition of QW layers and the width of the QWs were determined by secondary ion mass spectroscopy and the high resolution X-ray diffraction measurements as being close to nominal values. Note that the GaAs:N layer is not an intentional part of these structures, but a result of the growth method. Details of the growth conditions can be found in Refs. [11,16,19,32,33]. In order to improve the optical quality a sample of each QW structure was also annealed ex situ at 760 8C for 60 s. It has been observed that within the XRD
3. Results and discussion
Fig. 1. Room temperature CER spectra of the as-grown (upper panel) and annealed (bottom panel) GaN0.027As0.863Sb0.11/GaAs SQWs.
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Fig. 2. Room temperature CER spectra of the as-grown (upper panel) and annealed (bottom panel) Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs.
the QW. Below the GaAs:N-related transition CER resonances associated with the optical transitions in the QW region are clearly observed. All these features blue-shift due to annealing of the sample. The QW transitions and the phenomenon of their blue-shift are discussed in appropriate sub-sections of this article. The identification of CER resonances was possible on the basis of calculations performed in the framework of the effective mass approximation [40]. In the calculations, the influence of strain on the band structure was taken into account, but excitonic effects were neglected. The biaxial strain was calculated based on the Pikus–Bir Hamiltonian [41]. All the parameters necessary for the calculations have been obtained by linear interpolation between the parameters of a relevant binary semiconductor taken after Ref. [42]. According to the band anti-crossing model (BAC) model [1,2,43], the influence of nitrogen localized states on the valence-band structure is neglected. Hence, it can be assumed that the effective mass of light- and heavy-hole does not change after adding nitrogen atoms, i.e. the hole effective mass is the same as for Ga(In)AsSb. The electron effective mass has been assumed to be 0.09m0 for GaNAsSb after Ref. [17] and 0.12m0 for GaInNAsSb [33]. This assumption indicates an increase in the electron effective mass in comparison to the N free sample. Note that such an electron effective mass is in accordance with the BAC model prediction [1,2]. However, it has been found that the BAC model with usual parameters does not satisfactorily describe the Ga(In)NAsSb bandgap. Therefore the bandgap energy of the Ga(In)NAsSb layer has been adjusted to the experimental value of the QW ground state transition. The conduction-band offset QC is defined as QC = DEC/(DEC + DEV), where DEC and DEV are the conduction- and valence-band discontinuities at the heterojunction. Note that in our calculations, we assume QC before taking into account the strain effects, treating QC as a fitting parameter. The notation nmH(L) denotes the transition between nth heavy-hole (light-hole) valence subband and mth conduction sub-band. 3.1. GaNAsSb/GaAs
Fig. 3. Room temperature CER spectra of the as-grown (upper panel) and annealed (bottom panel) Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs (curves (i) and (iii)) and GaN0.027As0.973/GaAs (curves (ii) and (iv)) QW structures.
Three CER resonances have been easily resolved for the asgrown and annealed GaN0.027As0.863Sb0.11/GaAs SQW structures below the GaAs:N feature, i.e. below 1.35 eV. Such an observation suggests that this structure is Type-I with deep confined potential for both electrons and holes. The three resonances have been attributed to 11H, 22H, and 33H transitions. CER features associated with the partially forbidden transitions (e.g. 31H transition) are not excluded in our spectra; however they can be neglected at the first approach due to their lower intensity. Also CER features associated with transitions between light-hole and electron levels are neglected at the first approach. From comparison of experimental energies of QW transitions (11H, 22H, and 33H) with these from calculations, QC has been determined as being close to 50%. The energy level diagram calculated for this GaN0.027As0.863Sb0.11/GaAs SQW with QC = 50% is shown in Fig. 4. For this situation we have found three, six, and three confined states for electrons,
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Fig. 4. Band diagram of the GaN0.027As0.863Sb0.11/GaAs SQW obtained for QC = 50%.
heavy-holes, and light-holes, respectively. From the obtained energy level diagram, optical transitions between light-hole and electron levels are expected at 0.938 eV (11 L), 1.096 eV (22 L), and 1.325 eV (33 L). Weak CER features related to these transitions can be found at the indicated energies (upper panel in Fig. 1). Note that for the annealed sample they are observed at higher energies by 50 meV, similarly as the 11H, 22H, 33H transitions. The obtained QC = 50% agrees with our previous investigations for GaNAsSb/GaAs QWs using PR spectroscopy [16]. It is worth noting that a GaAs0.9Sb0.1/GaAs QW is close to Type-II [9,44] while a GaN0.02As0.98 QW is Type-I with QC 80% [45]. Our results for the intermediate situation, i.e. GaN0.027As0.863Sb0.11/GaAs QW, matches expectations and shows that GaNAsSb/GaAs QW is a system where the QC is very sensitive to the content of the GaNAsSb layer. 3.2. GaInNAsSb/GaAs In the case of Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQW (see in Fig. 2), three and four CER resonances have been identified for the as-grown and annealed samples, respectively. The resonances have been attributed to 11H, 22H, 33H, and 44H transitions and have a complex character due to the contribution of partially forbidden transitions and the multigap character of GaInNAsSb compound originating in from the different nitrogen nearest-neighbors environments in GaInNAsSb [1,2]. The energy level structure determined for this SQW is shown in Fig. 5. There is good agreement between experiment and calculation 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. For the fourth transition, a different behaviour has been observed. Calculations show that the fourth transition is only possible for a narrow range of QC values. For QC smaller than 70%, only three confined states for electrons have been obtained and for QC greater than 85%, only three confined states for heavy-holes have been found. Therefore we have
Fig. 5. Band diagram of the Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQW obtained for QC = 80%.
concluded that the QC for our SQW has to be close to 80 5%, because this value has four confined states for both electrons and heavy-holes. Disagreement between the experimental and calculated energies for the 44H transition exists due to the nonsquare 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 theory that the energy difference between energy levels is smaller for a non-square QW than for a perfectly square QW and is especially important in higher order levels. Therefore, the energy of the 44H transition observed experimentally is smaller than that calculated, assuming a square QW. 3.3. GaInNAsSb/GaNAs/GaAs For the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs step-like QW structures, CER features below 1.35 eV, are associated with the optical transitions within the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW and optical transitions between hole and electron states confined above the step-like GaNAs barriers (see in Fig. 3). The above barrier transitions have been observed in PR spectroscopy for GaInNAs/GaInNAs/GaAs step-like barrier QW structures [46]. In order to identify the section of the CER spectrum associated with the above step-like barrier transitions, reference samples have been measured and their CER spectra have been analyzed. It has been observed that CER spectra obtained for the
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reference structures are composed of two main resonances associated with absorption between the light-hole (LH) (heavyhole (HH)) sub-band and the electron sub-band. The LH transition is at lower energy than HH transition due to the tensile strain in this layer [23,30]. CER signals between these features and the GaAs:N feature are associated with the optical transitions between excited states, which are confined in this broad QW. It has been found that this QW possesses six, six, and three confined states for electrons, heavy-holes, and light-holes, respectively. In the case of step-like QW structures, similar CER signals are expected in this spectral region. It has been found that the CER signal associated with the above step-like barrier transitions starts from lower energies but is very similar to this observed for the reference structures. The difference in the energy is associated with the presence of GaInNAsSb layer within the GaNAs/GaAs QW. Finally, it has been concluded that the CER spectra below 1.0 eV is associated with the optical transitions between heavy-hole and electron levels confined within the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW. In this spectral region (0.75–1.0 eV), three CER resonances can be resolved. Also three resonances at the corresponding energies (within experimental error) have been resolved in PR spectra measured for these samples [33]. According to Ref. [33], the three CER resonances have been attributed to 11H, 31H and 22H transitions. Fig. 6 shows the energy level diagram for the asgrown step-like QW structure obtained from matching theoretical calculations with experimental data obtained from PR measurements [33]. It has been found that the depth of the electron and heavy-hole QWs are 144 and 127 meV, respectively. For such a situation, the Ga0.61I-
n0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW has two and three confined states for electrons and heavy-holes, respectively. The Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW for lightholes is Type-II due to the compressive strain in GaInNAsSb layer (e = 2.61%) [33]. 3.4. Blue-shift of quantum well transitions For all structures shown in this paper, post-growth annealing leads to a blue-shift of the QW transitions. The annealing induced blue-shift of the bandgap energy has been observed many times for GaNAs, GaInNAs, GaNAsSb layers, and QWs including these layers [47–54]. The origin of the blue-shift is complex and changes from sample to sample. It has been proposed that one of the reasons for the blue-shift is a reduction of the tail of density of states by a reduction of the number of structural defects, i.e. the number of energy levels associated with these defects [52]. For GaInNAs(Sb) compound also the phenomenon associated with the annealing induced change in the nitrogen nearest-neighbour environment leads to a blue-shift of the bandgap energy [50,51,54]. For QW structures, the phenomenon of atomic inter-diffusion across QW interfaces is another possible origin of the blue-shift in QW transitions [53]. All mentioned phenomena could be responsible for the blue-shift of QW transitions of the samples measured in this study. However, a quantitative analysis of the contribution related to an individual phenomenon is not the aim of this paper and will be discussed elsewhere. We have decided to show CER spectra for annealed samples in this article because in some cases CER features related to QW transitions are better resolved/separated for the annealed samples. Note that CER spectroscopy enables the possibility to investigate the blue-shift of optical transitions associated with excited states besides the blue-shift of the fundamental transition which is usually investigated by photoluminescence spectroscopy. 4. Conclusions
Fig. 6. Band diagram of the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs step-like QW structure obtained for following parameters: QC = 0.8 for GaN0.027As0.973/GaAs and QC = 0.85 for Ga0.61In0.39As0.963N0.017Sb0.02/GaAs interfaces.
It has been shown that CER spectroscopy is a powerful tool to investigate the optical transitions in Ga(In)NAs/GaAs QW structures containing Sb atoms such as GaN0.027As0.863Sb0.11/ GaAs and Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs step-like QW structures. CER features related to the optical transitions between the ground and excited states confined in the QW have been observed. Also CER features associated with the optical transitions between hole and electron levels confined above the step-like barriers have been resolved for the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs QW structure. In addition, CER resonances related to optical transitions in the GaAs barrier and the intermediate GaAs:N layer have been clearly observed. The last finding confirms the presence of the intermediate GaAs:N layer in investigated samples. From comparison of experimental of QW transitions with calculated energies, the bandgap discontinuity for GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/
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GaAs and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs QW structures have been estimated. Acknowledgments 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). Also we acknowledge the support from the Foundation for Polish Science through a Subsidy 8/2005. 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] I.A. Buyanova, W.M. Chen, Physics and Applications of Dilute Nitrides, Taylor & Francis Books, Inc., New York, 2004. [2] M. Henini, Dilute Nitride Semiconductors, Elsevier Ltd., Oxford, 2005. [3] H. Riechert, A. Ramakrishnan, G. Steinle, Semicond. Sci. Technol. 17 (2002) 892. [4] D. Gollub, S. Moses, M. Fischer, M. Kamp, A. Forchel, Electron. Lett. 40 (2004) 427. [5] G. Jaschke, R. Averbeck, L. Geelhaar, H. Riechert, J. Cryst. Growth 278 (2005) 224. [6] G. Ungaro, G. Le Roux, R. Teissier, J.C. Harmand, Electron. Lett. 35 (1999) 1246–1248. [7] X. Yang, M.J. Jurkovic, J.B. Heroux, W.I. Wand, Appl. Phys. Lett. 75 (1999) 178. [8] H. Shimizu, K. Kumada, S. Uchiyama, A. Kasukawa, Electron. Lett. 36 (2000) 1701. [9] J.C. Harmand, A. Caliman, E.V.K. Rao, L. Largeau, J. Ramos, R. Teisser, L. Travers, G. Ungaro, B. Theys, I.F.L. Dias, Semicond. Sci. Technol. 17 (2002) 778. [10] M.A. Wistey, S.R. Bank, H.B. Yuen, L.L. Goddard, J.S. Harris Jr., Electron. Lett. 39 (2003) 1822. [11] 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. [12] V. Gambin, W. Ha, M.A. Wistey, H.B. Yuen, S.R. Bank, S. Kim, J.S. Harris Jr., IEEE J. Sel. Top. Quantum Electron. 8 (2002) 795. [13] L.H. Li, V. Sallet, G. Patriarche, L. Largeau, S. Bouchoule, L. Travers, J.C. Harmand, Appl. Phys. Lett. 83 (2003) 1298. [14] X. Yang, J.B. Heroux, L.F. Mei, W.I. Wang, Appl. Phys. Lett. 78 (2001) 4068. [15] T.S. Kim, J.Y. Park, T.V. Cuong, H.J. Lee, E.-K. Suh, C.-H. Hong, J. Cryst. Growth 270 (2004) 340. [16] R. Kudrawiec, K. Ryczko, J. Misiewicz, H.B. Yuen, S.R. Bank, M.A. Wistey, H.P. Bae, S. James, Harris Jr., Appl. Phys. Lett. 86 (2005) 141908. [17] R.T. Senger, K.K. Bajaj, E.D. Jones, N.A. Modine, K.E. Waldrip, F. Jalali, J.F. Klem, G.M. Peake, X. Wei, S.W. Tozer, Appl. Phys. Lett. 83 (2003) 5425. [18] G.M. Peake, K.E. Waldrip, T.W. Hargett, N.A. Modine, D.K. Serkland, J. Cryst. Growth 261 (2004) 398. [19] H.B. Yuen, S.R. Bank, M.A. Wistey, A. Moto, J.S. Harris Jr., J. Appl. Phys. 96 (2004) 6375. [20] V. Lordi, H.B. Yuen, S.R. Bank, J.S. Harris Jr., Appl. Phys. Lett. 85 (2004) 902. [21] F. Bousbih, S. Ben Bouzid, R. Chtourou, F.F. Charfi, J.C. Harmand, G. Ungaro, Mater. Sci. Eng. C 21 (2002) 251.
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Contactless electroreflectance spectroscopy of Ga(In)NAs/GaAs quantum well structures containing Sb atoms R. Kudrawiec a,b,*, M. Gladysiewicz a, M. Motyka a, J. Misiewicz a, H.B. Yuen b, S.R. Bank b, M.A. Wistey b, H.P. Bae b, James S. Harris Jr.b a
Institute of Physics, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland b Solid State and Photonics Laboratory, Department of Electrical Engineering, 126X CISX, Via Ortega, Stanford University, Stanford, CA 94305-4075, USA Available online 17 July 2006
Abstract Contactless electroreflectance (CER) spectroscopy has been applied to investigate the optical transitions in Ga(In)NAs/GaAs quantum well (QW) structures containing Sb atoms. The identification of the optical transitions has been carried out in accordance with theoretical calculations which have been performed within the framework of the effective mass approximation. Using this method, the bandgap discontinuity for GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/GaAs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs QW structures has been determined. It has been found that the conduction-band offset is 50 and 80% for GaN0.027As0.863Sb0.11/GaAs and Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QWs, respectively. It corresponds to 264 and 296 meV depth QW for electrons and heavy-holes in GaN0.027As0.863Sb0.11/GaAs QW; and 520 and 146 meV depth QW for electrons and heavy-holes in Ga0.62In0.38As0.954N0.026Sb0.02/GaAs QW. In the case of the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs step-like QW structure it has been shown that the depth of electron and heavy-hole Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW is 144 and 127 meV, respectively. # 2006 Elsevier B.V. All rights reserved. Keywords: Dilute nitrides; Quantum wells; Electroreflectance; Band offset
1. Introduction GaAs-based structures with few percent of nitrogen (N) are being intensively investigated as promising active materials for the 1.3–1.6 mm wavelength emission due to several advantages in comparison to InP-based structures, including improved temperature-dependent characteristics and the feasibility of monolithic integration with AlAs/GaAs-based Bragg reflectors [1,2]. Among these materials, GaInNAs is an attractive alloy which has been used in the active regions of lasers with emission at 1.3 mm. Recent progress on the use of GaInNAs for these applications has succeeded in producing lasers operating up to 1.4 mm wavelength [3–5]. However, it remains difficult to obtain good performance for these lasers and to shift the emission to longer wavelengths, because with increasing nitrogen concentration, especially above 2.5%, the optical
* Corresponding author. Tel.: +48 71 320 42 80; fax: +48 71 328 36 96. E-mail address: [email protected] (R. Kudrawiec). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.05.111
quality of the material deteriorates due to a restriction in the optimal growth parameters [1,2]. The degradation results in a higher threshold current density for lasers. In the past few years, Sb-containing Ga(In)NAs/GaAs QW structures has been found to be a potentially superior material to GaInNAs for long wavelength laser applications [6–12]. It has been shown that when using Sb as a surfactant, the quality of highly strained GaInNAs/GaAs QWs improves significantly [8,12]. In addition, the incorporation of Sb atoms into Ga(In)NAs helps to achieve emission at longer wavelengths. However, there are few reports on the optical properties of GaInNAs/GaAs structures containing Sb atoms [6–23], and a lot of properties important for laser structures, especially the band gap discontinuity, still pose important questions. Modulation spectroscopy, such as photoreflectance (PR) and contactless electroreflectance (CER), is a powerful tool to investigate optical properties of semiconductor systems [24–26]. PR and CER spectroscopies are particularly useful because they are performed in a contactless mode that is nondestructive for samples. In addition, these techniques are very
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sensitive at room temperature, an important aspect of material characterization since devices normally operate at room temperature. The derivative nature of these experimental methods enables observation of a large number of sharp spectral features including those related to excited state transitions in low-dimensional structures, in contrast to common emission-type experiments such as photoluminescence (PL), which usually probes only the ground state. The analysis of QW energy transitions together with theoretical calculations makes it possible to determine material parameters such as the band gap discontinuity. Such procedures have been often applied in studies for different semiconductor structures [27–30]. In our previous papers we have applied PR spectroscopy supported by theoretical calculations in order to determine the energy level structure in GaNAsSb/GaAs [16,31], GaInNAsSb/GaAs [32] and GaInNAsSb/GaNAs/GaAs QWs [33]. In this paper, similar QW systems have been studied in CER spectroscopy, which in addition to PR information is free of below band gap oscillation features typical of GaAsbased structures grown on n-type GaAs substrate [34,35].
sensitivity the post-growth annealing do not influence the strain and content of investigated samples. The CER measurements were performed in the so called ‘bright configuration’ where the sample is illuminated by white light instead the monochromatic light used in the standard ‘dark configuration’. A tungsten halogen lamp (150 W) was used as the probe light source and a single grating 0.55 m monochromator with a thermo-electrically cooled InGaAs pin photodiode was used to analyze the reflected light. Other details of the ‘bright configuration’ setup can be found elsewhere [36]. Samples were mounted in a capacitor with the top electrode made from a copper mesh and the bottom electrode made from a copper block. The top electrode is semitransparent and was kept at a distance of 0.1 mm from the sample surface, while the sample itself was fixed on the bottom copper electrode. A maximum peak-to-peak alternating voltage of 1.8 kV was applied to this capacitor. Phase sensitive detection of the CER signal was made using a lock-in amplifier.
2. Experimental
Figs. 1–3 show CER spectra measured at room temperature for as-grown (upper panel) and annealed (bottom panel) GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/ GaAs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs QW structures, respectively. All these spectra are dominated by GaAs bulk-like bandgap signal at 1.42 eV. In addition, a PR feature at 1.37 eV is resolved in Figs. 1–3. This feature confirms the presence of an intermediate GaAs:N layer due to the nitrogen rf plasma ignition and stabilization prior to
The QW structures used to this study were grown by solidsource molecular beam epitaxy (MBE) in a Varian Mod GenII system on semi-insulating (1 0 0) GaAs substrates. Four structures are presented in this paper. The first structure is a GaNAsSb/GaAs SQW; it is composed of a 250 nm thick GaAs buffer layer, 50 nm thick GaAs:N layer with nitrogen concentration of 0.1%, 10 nm thick GaN0.027As0.863Sb0.11 QW and 50 nm thick GaAs cap layer. The second structure is composed of a 250 nm thick GaAs buffer layer, 8 nm thick GaInNAsSb QW and 50 nm thick GaAs cap layer. The GaInNAsSb layer has composition of 38% In, 2.6% N, 2% Sb. This sample has been reported in Ref. [32]. The third structure is a GaInNAsSb/GaNAs/GaAs step-like QW structure reported in Ref. [33]. This sample is composed of a 250 nm thick GaAs buffer layer, 50 nm thick GaAs:N layer with nitrogen concentration of 0.1%, GaNAs step-like barriers, GaInNAsSb QW and 50 nm thick GaAs cap layer. The GaInNAsSb/GaNAs QW has a nominal composition of 39% In, 1.7% N, 2% Sb in the GaInNAsSb layer and 2.7% N in the GaNAs barriers. The nominal thickness of the GaNAs step-like barriers and GaInNAsSb QW is 20 and 7.5 nm, respectively. The fourth structure is a 20 nm thick GaN0.027As0.973/GaAs QW. This sample is a reference structure useful in the interpretation of PR spectra for the step-like QW structures. The composition of QW layers and the width of the QWs were determined by secondary ion mass spectroscopy and the high resolution X-ray diffraction measurements as being close to nominal values. Note that the GaAs:N layer is not an intentional part of these structures, but a result of the growth method. Details of the growth conditions can be found in Refs. [11,16,19,32,33]. In order to improve the optical quality a sample of each QW structure was also annealed ex situ at 760 8C for 60 s. It has been observed that within the XRD
3. Results and discussion
Fig. 1. Room temperature CER spectra of the as-grown (upper panel) and annealed (bottom panel) GaN0.027As0.863Sb0.11/GaAs SQWs.
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Fig. 2. Room temperature CER spectra of the as-grown (upper panel) and annealed (bottom panel) Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs.
the QW. Below the GaAs:N-related transition CER resonances associated with the optical transitions in the QW region are clearly observed. All these features blue-shift due to annealing of the sample. The QW transitions and the phenomenon of their blue-shift are discussed in appropriate sub-sections of this article. The identification of CER resonances was possible on the basis of calculations performed in the framework of the effective mass approximation [40]. In the calculations, the influence of strain on the band structure was taken into account, but excitonic effects were neglected. The biaxial strain was calculated based on the Pikus–Bir Hamiltonian [41]. All the parameters necessary for the calculations have been obtained by linear interpolation between the parameters of a relevant binary semiconductor taken after Ref. [42]. According to the band anti-crossing model (BAC) model [1,2,43], the influence of nitrogen localized states on the valence-band structure is neglected. Hence, it can be assumed that the effective mass of light- and heavy-hole does not change after adding nitrogen atoms, i.e. the hole effective mass is the same as for Ga(In)AsSb. The electron effective mass has been assumed to be 0.09m0 for GaNAsSb after Ref. [17] and 0.12m0 for GaInNAsSb [33]. This assumption indicates an increase in the electron effective mass in comparison to the N free sample. Note that such an electron effective mass is in accordance with the BAC model prediction [1,2]. However, it has been found that the BAC model with usual parameters does not satisfactorily describe the Ga(In)NAsSb bandgap. Therefore the bandgap energy of the Ga(In)NAsSb layer has been adjusted to the experimental value of the QW ground state transition. The conduction-band offset QC is defined as QC = DEC/(DEC + DEV), where DEC and DEV are the conduction- and valence-band discontinuities at the heterojunction. Note that in our calculations, we assume QC before taking into account the strain effects, treating QC as a fitting parameter. The notation nmH(L) denotes the transition between nth heavy-hole (light-hole) valence subband and mth conduction sub-band. 3.1. GaNAsSb/GaAs
Fig. 3. Room temperature CER spectra of the as-grown (upper panel) and annealed (bottom panel) Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs (curves (i) and (iii)) and GaN0.027As0.973/GaAs (curves (ii) and (iv)) QW structures.
Three CER resonances have been easily resolved for the asgrown and annealed GaN0.027As0.863Sb0.11/GaAs SQW structures below the GaAs:N feature, i.e. below 1.35 eV. Such an observation suggests that this structure is Type-I with deep confined potential for both electrons and holes. The three resonances have been attributed to 11H, 22H, and 33H transitions. CER features associated with the partially forbidden transitions (e.g. 31H transition) are not excluded in our spectra; however they can be neglected at the first approach due to their lower intensity. Also CER features associated with transitions between light-hole and electron levels are neglected at the first approach. From comparison of experimental energies of QW transitions (11H, 22H, and 33H) with these from calculations, QC has been determined as being close to 50%. The energy level diagram calculated for this GaN0.027As0.863Sb0.11/GaAs SQW with QC = 50% is shown in Fig. 4. For this situation we have found three, six, and three confined states for electrons,
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Fig. 4. Band diagram of the GaN0.027As0.863Sb0.11/GaAs SQW obtained for QC = 50%.
heavy-holes, and light-holes, respectively. From the obtained energy level diagram, optical transitions between light-hole and electron levels are expected at 0.938 eV (11 L), 1.096 eV (22 L), and 1.325 eV (33 L). Weak CER features related to these transitions can be found at the indicated energies (upper panel in Fig. 1). Note that for the annealed sample they are observed at higher energies by 50 meV, similarly as the 11H, 22H, 33H transitions. The obtained QC = 50% agrees with our previous investigations for GaNAsSb/GaAs QWs using PR spectroscopy [16]. It is worth noting that a GaAs0.9Sb0.1/GaAs QW is close to Type-II [9,44] while a GaN0.02As0.98 QW is Type-I with QC 80% [45]. Our results for the intermediate situation, i.e. GaN0.027As0.863Sb0.11/GaAs QW, matches expectations and shows that GaNAsSb/GaAs QW is a system where the QC is very sensitive to the content of the GaNAsSb layer. 3.2. GaInNAsSb/GaAs In the case of Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQW (see in Fig. 2), three and four CER resonances have been identified for the as-grown and annealed samples, respectively. The resonances have been attributed to 11H, 22H, 33H, and 44H transitions and have a complex character due to the contribution of partially forbidden transitions and the multigap character of GaInNAsSb compound originating in from the different nitrogen nearest-neighbors environments in GaInNAsSb [1,2]. The energy level structure determined for this SQW is shown in Fig. 5. There is good agreement between experiment and calculation 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. For the fourth transition, a different behaviour has been observed. Calculations show that the fourth transition is only possible for a narrow range of QC values. For QC smaller than 70%, only three confined states for electrons have been obtained and for QC greater than 85%, only three confined states for heavy-holes have been found. Therefore we have
Fig. 5. Band diagram of the Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQW obtained for QC = 80%.
concluded that the QC for our SQW has to be close to 80 5%, because this value has four confined states for both electrons and heavy-holes. Disagreement between the experimental and calculated energies for the 44H transition exists due to the nonsquare 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 theory that the energy difference between energy levels is smaller for a non-square QW than for a perfectly square QW and is especially important in higher order levels. Therefore, the energy of the 44H transition observed experimentally is smaller than that calculated, assuming a square QW. 3.3. GaInNAsSb/GaNAs/GaAs For the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs step-like QW structures, CER features below 1.35 eV, are associated with the optical transitions within the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW and optical transitions between hole and electron states confined above the step-like GaNAs barriers (see in Fig. 3). The above barrier transitions have been observed in PR spectroscopy for GaInNAs/GaInNAs/GaAs step-like barrier QW structures [46]. In order to identify the section of the CER spectrum associated with the above step-like barrier transitions, reference samples have been measured and their CER spectra have been analyzed. It has been observed that CER spectra obtained for the
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reference structures are composed of two main resonances associated with absorption between the light-hole (LH) (heavyhole (HH)) sub-band and the electron sub-band. The LH transition is at lower energy than HH transition due to the tensile strain in this layer [23,30]. CER signals between these features and the GaAs:N feature are associated with the optical transitions between excited states, which are confined in this broad QW. It has been found that this QW possesses six, six, and three confined states for electrons, heavy-holes, and light-holes, respectively. In the case of step-like QW structures, similar CER signals are expected in this spectral region. It has been found that the CER signal associated with the above step-like barrier transitions starts from lower energies but is very similar to this observed for the reference structures. The difference in the energy is associated with the presence of GaInNAsSb layer within the GaNAs/GaAs QW. Finally, it has been concluded that the CER spectra below 1.0 eV is associated with the optical transitions between heavy-hole and electron levels confined within the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW. In this spectral region (0.75–1.0 eV), three CER resonances can be resolved. Also three resonances at the corresponding energies (within experimental error) have been resolved in PR spectra measured for these samples [33]. According to Ref. [33], the three CER resonances have been attributed to 11H, 31H and 22H transitions. Fig. 6 shows the energy level diagram for the asgrown step-like QW structure obtained from matching theoretical calculations with experimental data obtained from PR measurements [33]. It has been found that the depth of the electron and heavy-hole QWs are 144 and 127 meV, respectively. For such a situation, the Ga0.61I-
n0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW has two and three confined states for electrons and heavy-holes, respectively. The Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973 QW for lightholes is Type-II due to the compressive strain in GaInNAsSb layer (e = 2.61%) [33]. 3.4. Blue-shift of quantum well transitions For all structures shown in this paper, post-growth annealing leads to a blue-shift of the QW transitions. The annealing induced blue-shift of the bandgap energy has been observed many times for GaNAs, GaInNAs, GaNAsSb layers, and QWs including these layers [47–54]. The origin of the blue-shift is complex and changes from sample to sample. It has been proposed that one of the reasons for the blue-shift is a reduction of the tail of density of states by a reduction of the number of structural defects, i.e. the number of energy levels associated with these defects [52]. For GaInNAs(Sb) compound also the phenomenon associated with the annealing induced change in the nitrogen nearest-neighbour environment leads to a blue-shift of the bandgap energy [50,51,54]. For QW structures, the phenomenon of atomic inter-diffusion across QW interfaces is another possible origin of the blue-shift in QW transitions [53]. All mentioned phenomena could be responsible for the blue-shift of QW transitions of the samples measured in this study. However, a quantitative analysis of the contribution related to an individual phenomenon is not the aim of this paper and will be discussed elsewhere. We have decided to show CER spectra for annealed samples in this article because in some cases CER features related to QW transitions are better resolved/separated for the annealed samples. Note that CER spectroscopy enables the possibility to investigate the blue-shift of optical transitions associated with excited states besides the blue-shift of the fundamental transition which is usually investigated by photoluminescence spectroscopy. 4. Conclusions
Fig. 6. Band diagram of the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/ GaAs step-like QW structure obtained for following parameters: QC = 0.8 for GaN0.027As0.973/GaAs and QC = 0.85 for Ga0.61In0.39As0.963N0.017Sb0.02/GaAs interfaces.
It has been shown that CER spectroscopy is a powerful tool to investigate the optical transitions in Ga(In)NAs/GaAs QW structures containing Sb atoms such as GaN0.027As0.863Sb0.11/ GaAs and Ga0.62In0.38As0.954N0.026Sb0.02/GaAs SQWs, and Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs step-like QW structures. CER features related to the optical transitions between the ground and excited states confined in the QW have been observed. Also CER features associated with the optical transitions between hole and electron levels confined above the step-like barriers have been resolved for the Ga0.61In0.39As0.963N0.017Sb0.02/GaN0.027As0.973/GaAs QW structure. In addition, CER resonances related to optical transitions in the GaAs barrier and the intermediate GaAs:N layer have been clearly observed. The last finding confirms the presence of the intermediate GaAs:N layer in investigated samples. From comparison of experimental of QW transitions with calculated energies, the bandgap discontinuity for GaN0.027As0.863Sb0.11/GaAs, Ga0.62In0.38As0.954N0.026Sb0.02/
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