On photoluminescence and photoreflectance of 1-eV GaInNAs-on-GaAs epilayers

Journal of Luminescence 141 (2013) 67–70

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On photoluminescence and photoreflectance of 1-eV GaInNAs-onGaAs epilayers E.-M. Pavelescu a,b,n, R. Kudrawiec c, M. Dumitrescu d a

National Institute for Research and Development in Microtechnologies, Erou Iancu Nicolae 126A, 077190 Bucharest, Romania Faculty of Exact Sciences and Engineering, Hyperion University, Calea Că lă ras- ilor 169, 030615, Bucharest, Romania c Institute of Physics, University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland d Optoelectronics Research Centre, Tampere University of Technology, P.O. Box 692, 33101 Tampere, Finland b

art ic l e i nf o

a b s t r a c t

Article history: Received 12 July 2012 Received in revised form 8 February 2013 Accepted 28 February 2013 Available online 15 March 2013

The effects of growth temperature (410 and 470 1C) and subsequent rapid thermal annealing (RTA) on luminescence performance of lattice-matched 1-eV GaInNAs-on-GaAs epilayers, grown by molecular beam epitaxy under constant fluxes, has been studied experimentally by 9-K photoluminescence (PL) and 300-K photoreflectance (PR) spectroscopy. The near band edge PL has been found to noticeably decrease and red-shift as the growth temperature (Tgr) is reduced from 470 to 410 1C. This red-shift is the consequence of a reduction in the alloy band gap with decreasing Tgr, as confirmed by PR. An N-related broad PL band, whose intensity remarkably enhances with increasing Tgr, has been detected at longer wavelengths in the 9-K PL's. Post-growth RTA (1 min at 800 1C) significantly enhanced the near band edge PL but it did so at the expense of a PL blue-shift: the more the PL enhancement and blue-shift the lower the Tgr. This annealing-induced blue-shift is due to In–N bonds formation, whose magnitude enhanced as the Tgr is decreased. The broad PL band also enhanced upon the RTA, but the ratio between its intensity and the intensity of the near band edge PL reduced, especially for the sample grown at 410 1C. 7-MeV electron irradiation to the dose of around 1015 cm−2 applied prior to annealing considerably reduced the broad PL band upon annealing. & 2013 Elsevier B.V. All rights reserved.

Keywords: Dilute nitrides Photoreflectance Photoluminescence Thermal treatment

1. Introduction Besides a growing interest of researchers toward the fabrication of GaInNAs/GaAs quantum-well (QW) heterostructures for 1.3-μm GaAs-based laser diodes [1], thick GaInNAs films lattice matched to GaAs substrates are also very attractive for a number of applications, e.g., heterojunction bipolar transistors [2] (HBTs) or photovoltaic cell production [3]. In HBTs, a small bandgap is desirable in the base region for reduced power dissipation. In HBTs with the base made of GaInAs, pseudomorphically grown on a GaAs substrate, the turn on voltage is reduced by about 100 meV, compared to GaAs HBTs. However, for a base thickness of 500 nm, the In composition is restricted to be less than 0.1% to avoid the formation of strain-induced misfit dislocations. GaInNAs could be used as a base layer, thus enabling thick base layers with reduced strain and smaller bandgaps. As to the solar cells, an addition of a 1-eV bandgap GaInNAs cell to a multijunction InGaP–GaAs structure can improve the internal quantum efficiency to record values beyond 50% [3]. This is because the GaInNAs section collects the

long-wavelength part of the solar spectrum, which passes unabsorbed through the higher bandgap InGaP and GaAs cells. Unfortunately, alloying even a small amount of N with Ga(In)As is challenging due to a low efficiency of N incorporation combined with a large alloy miscibility gap in the phase diagram [4]. These problems, responsible for creation of N-related non-radiative centers and structural inhomogeneities, could become more accentuated for low indium concentrations (≤10%) when the nitrogen mole fraction could readily exceed the theoretical solubility limit (2%) to obtain a 1-eV bandgap, while satisfying the lattice matching conditions. One way for alleviating these problems is to grow GaInNAs by molecular beam epitaxy (MBE) under accentuated non-equilibrium conditions at relatively low temperatures, 50–100 1C lower than normally used for growing generic N-free compounds. Lowering the growth temperature, which promotes non-radiative point defects, could have a dramatic influence on the optical performance of GaInNAs alloys.

2. Methods n

Corresponding author at: National Institute for Research and Development in Microtechnologies, Erou Iancu Nicolae 126A, 077190 Bucharest, Romania. Tel.: þ 40 212690775. E-mail address: [email protected] (E.-M. Pavelescu). 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.02.041

The structures investigated consist of 300 nm Ga0.942In0.058NAs layers grown at 410 and 470 1C on n-type GaAs substrates by solid source MBE under constant molecular fluxes. The samples are

E.-M. Pavelescu et al. / Journal of Luminescence 141 (2013) 67–70

nominally undoped and capped with 10 nm GaAs, grown at the same temperatures as the N-containing layer beneath. PL was recorded at 9 K using a closed-cycle He cryostat and the 488-nm line of an Ar-ion laser for excitation. PR experiments were performed with a tungsten halogen lamp used as a probe light source. For photo-modulation, the 635 nm line of a semiconductor laser was employed as a pump beam. Thermal annealing was carried out ex-situ at 800 1C for 60 s in dry N2 atmosphere using a GaAs proximity cap.

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The PL spectra of the as-grown samples taken at 9 K are shown in Fig. 1. The PL of as-grown S410 is clearly red-shifted (with 55 nm) and less intense as compared to the PL of the as-grown S470. The red-shift is most likely the consequence of the increase in nitrogen content with decreasing growth temperature (Tgr), as previously observed by x-ray diffraction measurements [5]. The deterioration of PL indicates an increase with decreasing Tgr in amount of non-radiative defects, which “incorporate” during growth likely due to reduced migration length of adatoms on the growing surface. At longer wavelengths, one can also observe in Fig. 1 a very broad PL band, whose intensity noticeably increased as Tgr was raised. Low-temperature long-wavelength broad emission bands in PL from as-grown dilute nitride material were previously seen only in high-In GaInNAs/GaAs (N ¼1.7%) [6] and GaNAs/GaAs (N ¼ ¼2%) [7] quantum wells prepared by MBE. The origin of this broad band is unknown at the moment, but we believe it involves a bandgap defect, whose formation is growth temperature dependent. The fact that such broad band has not been observed in the 9-K PL, shown in Fig. 2, from two 100-nm thick InGaAs-on-GaAs epilayers grown at 410 1C and 450 1C under the same fluxes as the studied samples except for the nitrogen, leads to the conclusion that the nitrogen is involved in the appearance of such a broad PL band. It is known that the poor optical properties of as-grown dilute nitride can, in general, be improved by thermal annealing. After an RTA treatment at 800 1C for 1 min, the PL of both samples considerably enhanced, indicating a significant improvement of the optical quality of the samples, and noticeably blue-shifted. This annealinginduced PL enhancement and blue-shift were much more pronounced for S410 as compared to S470. As a consequence, the near

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Wavelength (nm) Fig. 1. 9-K Photoluminescence spectra taken from S410 (continuous line) and S470 (dotted line) before (as-grown) and after annealing (RTA@800 1C). Also shown is the spectra (thick dashed line) taken from S470 after 7-MeV electron irradiation to the dose of 1  1015 cm−2 and, subsequently, rapid thermally annealed at 800 1C for 1 min (RTA@800 1C).

Fig. 2. 9-K Photoluminescence spectra recorded from two 100-nm InGaAs-on-GaAs epilayers grown at 410 and 450 1C under the same elemental fluxes as S410 or S470 except for the nitrogen.

band edge PL of S410 became almost three times higher and remained only 10 nm red-shifted than that of S470 upon annealing. The broad PL band also enhanced, especially for the sample grown at 470 1C, and marginally blue-shifted upon annealing, but, however, the ratio between its intensity and the intensity of the PL reduced, especially for the sample grown at 410 1C. Fig. 1 also depicts the 9-K PL recorded from the S470 after 7-MeV irradiation stage to the dose of 1015 cm−1 and, subsequently, rapid thermally annealed at 800 1C for 1 min. One can observed that the electron irradiation promoted a substantial decrease of the PL broad band as compared to the annealed non-irradiated S470. Knowing that electron irradiation is a straightforward way to generate point defects or to alter the existing ones, the remarkable influence of the electron irradiation on the long-wavelength broad PL band supports the suggestion that a (complex) defect is involved in its mechanism of formation. One can also notice that the RTA lead to observation of two overlapping yet distinguished peaks composing the near band edge PL of annealed S410. In contrast, no such peaks can be distinguished in the near band edge PL of annealed S410. The energy separation of these two peaks amounts to 14.3 meV. This energy is very close to 14.8 meV representing the calculated energy of separation of the strain-induced splitting between the light and heavy holes of S410, taking into account the values of 5.8% and 3.2% for indium and nitrogen concentrations, respectively, for S410 [5]. Therefore, we attributed the left-hand side and right-hand side peaks of annealed S410 to electrons–heavy holes and electrons–light holes radiative transitions, respectively, taking into account that the epilayer is under slight tensile strain. The observation of these two distinctive transitions upon annealing only in the near band edge PL of S410 could be related to a more efficient reduction compared to S470 of the as-grown N-related conduction band potential fluctuations [8], which impede resolving radiative transitions relatively close in energy, as it happens in the PL from the as-grown samples. The very close similitude between the calculated value of the strain-induced light–heavy holes splitting with the indium and nitrogen concentrations derived from as-grown S410 and that observed experimentally from annealed S410 suggests that the epilayer strain and, hence, its macroscopic composition remained unchanged after the thermal treatment. This implies that the annealing induced blue-shift observed in our samples is due to a mechanism which takes place within the epilayers themselves without affecting their macroscopic composition. Fig. 3 illustrates room-temperature difference PR spectra recorded from the present S410 and S470 samples, before and after annealing, after removing the long-period oscillatory behavior of the signal, throughout the energy range studied, appeared

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Fig. 3. (a,c) 300-K Photoreflectance (PR) spectra after subtracking an oscillating background from S410 and S470, respectively; (b,d) Kramers–Kronig modulus of PR signals from S410 and S470, respectively.

due to interference effects caused by the presence of GaAs substrate beneath the studied layer [8]. In the difference spectra with removed long-period oscillations the enhanced critical point features are clearly resolved, as can be seen in Fig. 3(a) and (c). The critical points or resonance energies were extracted from the difference spectra shown in Fig. 3(a) and (c) by a standard PR line shape fitting procedure assuming Lorenzian oscillators, followed by a Kramers–Kronig analysis which produces a well-resolved PR module, as shown in Fig. 3(b) and (d). It can be seen that bandgaps around 1 eV could be obtained at both temperatures. Also, the near band edge PR features redshifted as Tgr decreased indicating a shrink in bandgap energy as previously suggested by the PL measurements. A set of at least three discrete transitions at well defined energies is resolved with the maximum in oscillator strength hoping to higher energies upon annealing, especially for the sample grown at the lower temperature. For the as-grown S410 and S470 sample, the main PR features can be interpreted as being due to band-to-band transitions related to the N–Ga4 configuration (“4Ga” for short). At higher energies, a long tail can be seen for S410 whereas for S470 a PR peak can be observed centered at about 25 meV higher energy than the corresponding main peak. This second PR feature can be assigned to the presence of a second nitrogen configuration, such as “3Ga1In”, coexisting in this sample with the dominant “4Ga” configuration. The high-energy tail for S410 and the well-defined peak of S470 indicate that In–N bonds are formed during the growth within the 410–470 1C temperature range besides the Ga–N majority ones [8] and this formation process enhances as the growth temperature increases, as theoretically predicted [9]. This could also account for the observed increased of the alloy bandgap at elevated growth temperatures. Upon rapid thermal annealing, the signal arising from the “4Ga” configuration significantly decreased for both samples and at higher energies the growth temperature dependent changes in the PR moduli can clearly be seen. Thus for S470 the “3Ga1In” configuration is significantly enhanced, and a new peak, less intense than the previous one, assigned to the “2Ga2In” configuration appears at around 25 meV higher energy apart from the enhanced PR peak assigned to the “3Ga1In” configuration. For S410 the dominant “4Ga” peak became negligible whereas two new well-defined PR peaks, separated one from another by about

22 meV in energy and having different intensities, appeared on the high-energy tail of the dominant peak. The less intense peak is the closest to the former “4Ga” dominating peak of the as-grown S410 and can be assigned to the “3Ga1In” configuration, whereas the more intense peak can be assigned to the “2Ga2In” configuration. This clearly indicates that upon annealing the dominant clusters for S410 and S470 are N–In2Ga2 and N–In1Ga3, respectively, and, hence, that more In–N bonds were promoted in S410 compared to S470 upon annealing. This explains the larger annealing-induced BS observed for S410 as compared to S470 [9] and emphasizes the important role of growth temperature in determining the dominant nearest neighborhood of N atoms upon annealing. An enhancement in annealing-induced formation of In–N bonds in GaInNAs grown at lower temperatures was also seen previously by Raman spectroscopy. However, compared to Raman spectroscopy, which evidentiated so far only the annealing-induced formation of In–N bonds belonging to N–In1Ga3 coordination [5], the photoreflectance spectroscopy technique and subsequent Kramers–Kroning analysis prove capable to resolve at the same time several possible N–IniGai−4 (0≤i≤4) configurations of substitutional N in GaInNAs and, hence, to determine to which dominant configuration the formed In–N bonds belong upon annealing.

4. Conclusions The influence of growth temperature (410 and 470 1C) and subsequent thermal annealing on the luminescence properties of lattice-matched 1-eV GaInNAs epilayers grown on GaAs substrates by molecular-beam epitaxy under constant fluxes has been investigated. The study has been done by means of 9-K photoluminescence (PL) and 300-K photoreflectance (PR). The near band edge PL has been found to noticeably shift toward longer wavelengths and decrease in intensity as the growth temperature is decreased. Besides near band edge PL, a broad PL emission band, whose intensity noticeably enhanced with rising growth temperature, has been found at longer wavelengths in the 9-K PL. This broad PL band was assigned to a radiative recombination process involving an N-related defect, whose formation enhances as growth temperature is increased. Post-growth rapid thermal annealing (1 min at 800 1C) remarkably enhanced and blue-shifted the near band

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edge PL, the more the PL enhancement and blue-shift the lower the growth temperature. The intensity of the broad PL band also enhanced upon annealing but the ratio between its intensity and the intensity of the PL reduced. 7-MeV electron irradiation to the dose of around 1015 cm−2 applied prior to annealing considerably reduced the PL broad band upon annealing. The annealing-induced PL blue-shift in both samples was found to be due to In–N bonds formation, whose magnitude enhances as the growth temperature is decreased. The formed In–N bonds upon annealing belong mainly to the N–Ga3In configuration for the sample grown at 470 1C and to the N–Ga2In2 configuration for the sample grown at the lower temperature of 410 1C. Acknowledgment The research presented in this paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), financed from the European Social Fund and by the

Romanian Government under the Contract number POSDRU/89/ 1.5/S/63700. N. Bălţăţeanu from Hyperion University is thanked for helping with electron irradiation. References [1] M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, Y. Yazawa, Jpn. J. Appl. Phys. 35 (1996) 1273. [2] R.E. Welser, R.S. Setzko, K.S. Stevens, E.M. Rehder, C.R. Lutz, D.S. Hill, P. J. Zampardi, J. Phys.: Condens. Matter 16 (2004) S3373. [3] K. Tanabe, Energies 2 (2009) 504. [4] V.A. Odnoblyudov, A.Yu. Egorov, A.R. Kovsh, A.E. Zhukov, N.A. Maleev, E. S. Semenova, V.M. Ustinov, Semicond. Sci. Technol. 16 (2002) 831. [5] E.-M. Pavelescu, J. Wagner, H.-P. Komsa, T.T. Rantala, M. Dumitrescu, M. Pessa, J. Appl. Phys. 98 (2005) 083524. [6] M. Albrecht, V. Grillo, T. Remmele, H.P. Strunk, A.Yu. Egorov, Gh. Dumitras, H. Riechert, A. Kaschner, R. Heitz, A. Hoffmann, Appl. Phys. Lett. 81 (2002) 2719. [7] Q.X. Zhao, S.M. Wang, Y.Q. Wei, M. Sadechi, A. Larsson, M. Villander, Appl. Phys. Lett. (2005)121910-1–121910-3. [8] R. Kudrawiec, E.M. Pavelescu, J. Wagner, G. Sek, J. Misiewicz, M. Dumitrescu, J. Konttinen, A. Gheorghiu, M. Pessa, J. Appl. Phys. 96 (2006) 2576. [9] K. Kim, A. Zunger, Phys. Rev. Lett. 86 (2001) 2609.