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AlGaAs core–shell nanowires on Si(1 0 0) substrates
Journal of Luminescence 155 (2014) 27–31
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Shell to core carrier-transfer in MBE-grown GaAs/AlGaAs core–shell nanowires on Si(1 0 0) substrates Maria Herminia Balgos n, Rafael Jaculbia, Michael Defensor, Jessica Pauline Afalla, Jasher John Ibañes, Michelle Bailon-Somintac, Elmer Estacio, Arnel Salvador, Armando Somintac Condensed Matter Physics Laboratory, National Institute of Physics, University of the Philippines, Diliman, Quezon City 1101, Philippines
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2013 Received in revised form 2 June 2014 Accepted 5 June 2014 Available online 16 June 2014
We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowires grown on Si(1 0 0) substrates via molecular beam epitaxy. The nanowires are dominantly zincblende and are tilted with respect to the substrate surface. Photoluminescence (PL) excitation spectrosocopy at 77 K revealed an abrupt increase in the GaAs PL intensity at excitation above the Al0.1Ga0.9As shell bandgap which is attributed to shell to core carrier-transfer. More carriers from the Al0.1Ga0.9As transfer to the GaAs at T490 K, as observed in the time-resolved PL and temperature dependence of the relative PL intensities of GaAs and Al0.1Ga0.9As due to the ionization of the traps within the Al0.1Ga0.9As. Using a coupled rate equation model that takes into account shell to core carrier-transfer, the average recombination time constants of Al0.1Ga0.9As shell τrec,s ¼ 400 ps (580 ps) and GaAs core τrec,c ¼ 600 ps (970 ps) were obtained from the time-resolved PL at 300 K (77 K). Carrier-transfer time constants τCT ¼50 ps (55 ps) at 300 K (77 K) were also obtained. & 2014 Elsevier B.V. All rights reserved.
Keywords: III–V semiconductors Carrier transfer Nanowires Time-resolved luminescence
1. Introduction Semiconductor nanowires (NWs) are quasi-one dimensional structures with diameters in nanometers and length in microns. The wide range of base materials and the possible production of highly crystalline structures from lattice-mismatched materials make NWs a versatile alternative to its bulk counterpart [1-3]. In fact, the integration of III–V semiconductors to Si has been possible through NW growth [4–7]. Due to its interesting properties, NWs cover a wide range of applications such as in energy conversion, sensors, electronics, and optoelectronics. Application of NWs as light emitters such as light emitting diodes [8,9] and lasers [10] has been previously demonstrated. Due to their high surface-to-volume ratio, however, NWs are susceptible to surface states that may degrade their light emission capabilities [11,12]. To reduce the effects of surface states, NWs are coated with high bandgap material (shell) that serves as a passivation barrier from the surface states. Electrical and optical investigations in these coaxial structures, commonly known as core-shell NWs (CSNWs), show enhanced carrier mobility [13,14] and photoluminescence (PL) intensity [15–18], as compared to bare NWs (no shell).
n
Corresponding author. Tel./fax: þ 63 99209749. E-mail address: [email protected] (M.H. Balgos).
http://dx.doi.org/10.1016/j.jlumin.2014.06.008 0022-2313/& 2014 Elsevier B.V. All rights reserved.
The enhancement of the PL intensity in CSNWs is attributed mainly to the reduced interaction of carriers with the surface states which may act as nonradiative recombination sites, electronic traps, and scattering sites [11,12,19]. However, the introduction of the shell can also enhance the PL intensity by acting as another source of carriers for the NW core [20]. The carriers from the shell can transfer to the core and effectively increase the number of recombining carriers thereby producing a more intense PL [21]. Carrier transfer (CT) has already been observed in quantum wells [22–23], laser structures [24], and type II core–shell heterostructures [20] using time resolved PL (TRPL). In this work, we study in detail the shell-to-core CT in GaAs/ Al0.1Ga0.9As CSNWs by utilizing continuous wave (CW) and TRPL for varying temperatures. The relatively low Al mole fraction (x ¼0.1) of the AlxGa1 xAs allows us to observe a distinct PL signal from GaAs and AlGaAs and at the same time, to pump both GaAs and AlGaAs using a Ti:Sapphire CW laser. The effect of the CT on the PL intensity is measured directly using PL excitation (PLE).
2. Methods GaAs/Al0.1Ga0.9As CSNWs coated with n-doped GaAs cap layer were grown on Si (1 0 0) substrates using a Riber 32P molecular beam epitaxy (MBE) machine. The Si substrate was pre-patterned with gold nanodots that serve as catalysts for the vapor–liquid–solid (VLS) growth of NWs [25]. GaAs was deposited at 580 1C for 30 min
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M.H. Balgos et al. / Journal of Luminescence 155 (2014) 27–31
followed by Al0.3Ga0.7As at the same growth temperature and duration. The thin film equivalent thickness is 0.65 μm and 0.84 μm for GaAs and Al0.3Ga0.7As, respectively. Lastly, a 0.1 μm Sidoped GaAs cap ( 1017 cm 3) served as a protective coating for the AlGaAs which readily oxidizes when exposed to air [26]. The ndoping of the GaAs cap decreases the width of the depletion region at the air–AlGaAs interface [13,14] preventing it from penetrating to the AlGaAs. The formation of a depletion shell could narrow the electronic channel producing PL [15] and decreases the carrier lifetime [5]. CW-PL measurements were conducted at temperatures of 10–300 K. The sample was mounted on the coldfinger of a closed-cycle He cryostat equipped with a Lakeshore temperature controller. The excitation source was a 488 nm Ar þ laser and the PL signal was dispersed by a SPEX 500 m monochromator and detected by a Hamamatsu GaAs photomultiplier tube. PLE (at 77 K) and TRPL (at 77 K and 300 K) measurements were conducted at a separate μ-PL setup where a 10 objective was used to focus the laser and collect the PL signal from the sample mounted inside a continuous flow liquid nitrogen cryostat for 77 K measurements. For the PLE, the GaAs PL peak intensity at 1.514 eV was monitored using a Synapse Si CCD detector attached to an iHR550 Horiba Jobin Yvon spectrometer. The excitation energy of a CW Ti: Sapphire laser was tuned from 1.53–1.68 eV (810–740 nm) corresponding to below and above the Al0.1Ga0.9As shell bandgap, respectively. For the TRPL measurements, a frequency doubled Ti:Sapphire femtosecond laser (400 nm) with 100 fs pulses at 80 MHz repetition rate was used as excitation. The fluence was kept at 0.012 J/cm3 to ensure minimal, if any, nonlinear effects [27]. The PL signal was fed to a streak camera system consisting of Digikröm spectrometer and an Optronis streak camera. The measured TRPL resolution is 10 ps. All measurements were performed on the as-grown CSNW sample (i.e., on ensemble of CSNWs) only. TRPL for bare GaAs NWs was not reported because the signal was too weak to be detected by our TRPL setup.
3. Results Fig. 1 shows a representative top view SEM micrograph of the sample. The CSNWs are tilted with respect to the horizontal due to the o1 1 1 4 preferential growth of GaAs NWs [28]. Close inspection of the CSNWs show that they have a tapered tip, as previously observed in Au-assisted GaAs NWs [29]. From various SEM images, the diameter of the CSNWs is estimated to be
Fig. 1. Top view SEM micrograph of the core–shell nanowires (CSNWs) showing high density formation. The CSNWs are predominantly tilted with respect to the substrate surface due to the o 1 1 1 4 preferential growth of the nanowires.
Fig. 2. 300 K PL of GaAs/AlGaAs core-shell nanowires and bare GaAs nanowires. The ( 10) means that the PL spectrum of GaAs is multiplied by 10 for clarity. More than an order of magnitude increase in the GaAs PL peak intensity of the core–shell nanowires compared to bare nanowires is observed. The inset is a schematic of the core–shell–cap structure.
141 714 nm with a density of 1.68 108 cm 2. The GaAs core diameter and the AlGaAs shell thickness is 100 nm and 19 nm, respectively. These values were estimated using the diameter of a bare GaAs NW (1037 25 nm) grown separately using the same parameters as reference. For MBE-grown NWs via the VLS mechanism, the growth elements are decomposed as they are deposited on the substrate. Hence, a film is also formed in areas with no gold catalysts [30]. However, due to the high lattice mismatch between GaAs and Si (4%) [31], this film is defective and emits very weak PL, if any. Fig. 2 shows the 300 K PL spectrum of the CSNWs and bare GaAs NWs. The sharp peaks in the 300 K spectra may be Ar þ laser plasma lines. For the CSNWs, we identified distinct bands at 1.437 eV and 1.559 eV corresponding to the GaAs core and to the Al0.1Ga0.9As shell transitions, respectively. The position of the GaAs band is near the bandgap of bulk zincblende GaAs, suggesting that our sample is predomintantly zincblende (ZB). To verify this structure, we use x-ray diffraction and temperature dependent PL [32]. The low Al mole fraction x¼ 0.1 may be attributed to the different kinetics between 2-D film and NW growth, as observed by Chen et al. [30]. In VLS, the growth depends on the rate at which the adatoms diffuse along the NW sidewalls [30]. Compared to the bare GaAs NWs, there is more than an order of magnitude increase in the GaAs PL intensity for the CSNWs. The enhanced GaAs PL signal in CSNWs may be due to (i) suppressed surface recombination because of the passivation provided by the Al0.1 Ga0.9As shell [11,12,15–19]; and (ii) increased number of recombining electron-hole pairs via the shell to core CT [21]. The shell-to-core CT in our sample is observed via PLE at 77 K. Fig. 3a shows the 77 K PLE spectrum of GaAs where the excitation is varied from below to above the Al0.1Ga0.9As bandgap of 1.65 eV. An abrupt increase in the PLE signal is observed at 1.65 eV which we attribute to the CT. At excitations hʋo1.65 eV (Region I), the carriers that contribute to the GaAs PLE in Region I are those optically excited from the GaAs region only. At excitations hʋ41.65 eV (Region II), carriers optically excited from the Al0.1Ga0.9As can diffuse and transfer to GaAs and can contribute to the PLE signal [33]. Since the PLE spectrum follows absorption spectrum for CW excitation, we expect the PLE spectrum to mimic the √E bulk density of states. The PLE spectrum follows the √h dependence in Region I but deviates drastically in Region II. The deviation can be explained by the added absorption introduced by the Al0.1Ga0.9As. The CT mechanism and band-to-band recombination are schematically shown in Fig. 3b. However, it should be noted that aside from these two, recombination via defects (not included in Fig. 3b) is also possible.
M.H. Balgos et al. / Journal of Luminescence 155 (2014) 27–31
29
Fig. 3. (a) GaAs core PLE spectrum at 77 K. Abrupt increase of PL at excitations above the Al0.1Ga0.9As bandgap indicates carrier transfer. (b) Mechanism for the shell to core carrier transfer.
Fig. 4. Time resolved photoluminescence at 77 K and 300 K of the (a) GaAs core and (b) AlGaAs shell of the core-shell nanowires. Black solid lines are fits to the coupled rate equation (Eqs. (1a) and (1b)). The recombination time constants τrec and carrier transfer time constants τCT obtained from the fits are included in the figure.
The n-doped GaAs cap is too thin to have a significant effect on the PLE signal. We further investigated the CT mechanism using TRPL. Fig. 4a and b shows the TRPL spectra of GaAs and Al0.1Ga0.9As, respectively, at 300 K and 77 K. The spectra were obtained by taking a horizontal segment in the streak image with height equal to the full-width at half maximum of the PL signal. All spectra were fitted using a coupled differential rate equation model to represent the temporal carrier evolution for the Al0.1Ga0.9As shell and GaAs core: d N c ðtÞ N ex;c Nc N s N max N CT ¼ þ ð1aÞ dt τrc τrec;c τCT N max d N s ðtÞ N ex;s Ns N s N max NCT ¼ dt τrs τrec;s τCT Nmax
ð1bÞ
where the subscripts s and c denote shell and core, respectively. N is the number of carriers, Nex is the initial number of excited carriers due to the optical excitation, τr is the rise time constant, τrec is the recombination time constant, τCT is the transfer time constant, Nmax is the maximum number of carriers that can transfer, and NCT is the number of carriers that transferred. Eqs. (1a) and (1b) are modified generation–recombination rate equations for NWs [34]. Generation manifests as the TRPL rise while recombination manifests as the TRPL decay. The first term in the right-hand-side of Eqs. (1a) and (1b) indicates addition of carriers via optical excitation and the subsequent thermalization of the
carriers to the band edges. The second term represents the radiative (R) and nonradiative (NR) recombination with time constant 1/τrec ¼1/τR þ 1/τNR. The last term accounts for of the CT mechanism. The term in brackets is to incorporate the finite number of carriers Nmax from Al0.1Ga0.9As that can transfer. Physically, only carriers near the Al0.1Ga0.9As–GaAs interface can readily transfer; those away from the interface have to initially diffuse before they can undergo CT. Simultaneously, in momentum space, these carriers thermalize to reach the band edges before they recombine. Since thermalization is a faster process (in the order of femtoseconds [35]) than diffusion, then recombination is favored at longer timescales. In our model, when NCT reaches Nmax, the CT process stops (last term in Eqs. (1a) and (1b) goes to zero) leaving recombination as the only carrier exit path in the shell and the optical excitation as the only source of carriers in the core. The black solid lines in Fig. 4a and b are the least square fits of Eqs. (1a) and (1b), respectively. It is important to note that τr is fixed at 10 ps equivalent to the resolution of our TRPL setup. This value is reasonable since the thermalization of the carriers, characterized by τr is in the order of femtoseconds [34] and is beyond the resolution of our TRPL setup. For the GaAs core (Fig. 4a), the TRPL rise has a single rate. Since carriers from Al0.1Ga0.9As transfer to GaAs, we expect a double rise characterized by two different time constants – one from the optical excitation and one from the CT. The single TRPL rise may be explained by the relative contribution to the number of carriers of
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M.H. Balgos et al. / Journal of Luminescence 155 (2014) 27–31
Fig. 5. (a) Integrated PL intensity versus inverse temperature of the AlGaAs shell. Activation energy Ea ¼ 31.4 meV corresponds to the ionization of traps within the AlGaAs. (b) Temperature dependent GaAs–AlGaAs PL intensity ratio showing an increase at 90 K.
the two processes. More carriers are produced via optical excitation, hence, the TRPL rise from the CT is swamped and is not observed. The TRPL of GaAs at both temperatures is characterized by a single TRPL decay rate. We attribute the decay to carrier recombination within the GaAs, The faster decay of 600 ps at 300 K is due to significant NR recombination at 300 K. Since τrec,c which is a combined effect of R and NR recombination [36], that is, 1/τrec ¼1/τR þ1/τNR, we observe a longer time constant of 970 ps at 77 K where NR is minimized and 1/τNR approaches zero. The values obtained at 300 K and 77 K are within the range of reported carrier lifetime values for nearly intrinsic exciton lifetimes of GaAs NWs (0.05–1.1 ns) reported by Perera et al. [37]. For the Al0.1Ga0.9As shell (Fig. 4b), a single exponential TRPL rise, due to the optical excitation, is observed at 77 K and 300 K. The TRPL decay, however, is characterized by a single exponential decay rate at 77 K and a double exponential (fast and slow) decay rate at 300 K. We propose that the appearance of the double decay is a consequence of the CT mechanism. We attribute the fast decay to the combined effect of recombination and carrier transfer while the slow decay is ascribed to the recombination. Numerical fitting using Eq. (1a) revealed τrec,s of 400 ps at 300 K and 580 ps at 77 K. A longer τrec,s at 77 K is again attributed to the minimized NR recombination at low temperatures. The transfer time constant τCT is 50 ps and 55 ps for at 300 K and 77 K, respectively. This is similar to the CT time constants of GaAs/AlGaAs separate confinement heterostructures reported by Morin et al. (19.2 ps at 300 K and 22 ps at 80 K) [38]. Our values of τCT show that the CT time is almost independent of temperature and cannot explain the double decay observed in the 300 K TRPL of Al0.1Ga0.9As. As such, the the maximum number of transferred carriers Nmax relative to the initial number of carriers Nex,s (t¼0) in the shell was investigated. We obtain a value of Nmax/Nex,s(t¼0) of 0.24 at 77 K and 0.69 at 300 K. These values show that a higher fraction of excited carriers transfer at 300K than at 77 K. We believe that defects in the Al0.1Ga0.9As affect the number of carriers that can transfer. Using an Arrhenius plot of the integrated PL intensity vs inverse temperature (Fig. 5a), we calculate an activation energy Ea ¼31 meV for the defects within the AlGaAs at 90 K which can be attributed to an electron trap E9 (30 meV) or a hole trap H4 (32 meV) [39]. At 77 K (o90 K), there is still efficient trapping of carrier by the defects, thus there are not enough transferred carriers to produce a double decay. At 300 K, the defects in the shell are already ionized and are not as efficient at trapping carriers. Thus, more carriers can diffuse until they finally transfer to the GaAs. The temperature dependence of the number of carriers available for transfer to GaAs is investigated using the relative PL intensities of GaAs and Al0.1Ga0.9As at 10K–300 K. Fig. 5b shows
the GaAs–Al0.1Ga0.9As PL intensity ratio (PLIR) as a function of temperature. Owing to the same growth conditions, the defects incorporated in GaAs and AlGaAs have minimal difference. The PL signal for GaAs and Al0.1Ga0.9As therefore should quench at almost the same rate with temperature resulting to a constant PLIR if they are to be treated independently. Constant PLIR is seen at 10–70 K as indicated by the horizontal dash line in Fig. 5b. At 90–150 K, however, there is an apparent increase in the PLIR as indicated by the slanted dash line. At these temperatures, the GaAs PL intensity increases relatively to the Al0.1Ga0.9As. More carriers from the Al0.1Ga0.9As transferred to GaAs at T 490 K. We note that the intersection of the horizontal and slanted line at 90K coincides with the activation temperature of the defects within the Al0.1Ga0.9As. 4. Conclusion The shell-to-core CT mechanism of an ensemble of dominantly zincblende GaAs/AlGaAs CSNWs is investigated through CW-PL, TRPL and PLE spectroscopy for varying temperatures. PLE provides strong evidence of the CT mechanism while CW-PL and TRPL demonstrate that the CT process is dependent of temperature mainly because a high number of carriers participate in the CT process at T 490 K. Coupled rate equations are used to fit the TRPL at 300 K and at 77 K, thereby facilitating the evaluation of the recombination and carrier transfer time constants. The 300 K (77 K) average recombination time constants for GaAs core are τrec,c ¼600 ps (970 ps) while that of the AlGaAs shell are τrec, s ¼400 ps (580 ps). The CT time constant is determined to be τCT ¼50 ps (55 ps) for 300 K (77 K). Acknowledgments This work is supported in part by Grants from PCIEERD-DOST, DOST-GIA, and University of the Philippines OVCRD. The authors would also like to thank Intel for the donation of the streak camera. References [1] B.R. Huang, Y.K. Yang, T.C. Lin, W.L. Yang, Sol. Energy Mater. Sol. Cells 98 (2012) 357. [2] Y. Liang, H. Liang, X. Xiao, S. Hark, J. Mater. Chem. 22 (2012) 1199. [3] S.L. Wu, J.H. Deng, T. Zhang, R.T. Zheng, G.A. Cheng, Diam. Relat. Mater. 26 (2012) 83. [4] T. Mårtensson, C.P.T. Svensson, B.A. Wacaser, M.W. Larsson, W. Seifert, K. Deppert, A. Gustafsson, L.R. Wallenberg, L. Samuelson, Nano Lett. 4 (2004) 1987.
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[5] K. Tomioka, T. Tanaka, S. Hara, K. Hiruma, T. Fukui, IEEE J. Sel. Top. Quantum Electron. 17 (2011) 1112. [6] G.E. Cirlin, V.G. Dubrovskii, Y.B. Samsonenko, A.D. Bouravleuv, K. Durose, Y.Y. Proskuryakov, B. Mendes, L. Bowen, M.A. Kaliteevski, M.A. Kaliteevski, D. Zeze, Phys. Rev. B 82 (2010) 035302. [7] L.C. Chuang, M. Moewe, S. Crankshaw, C. Chang-Hasnain, Appl. Phys. Lett. 92 (2008) 013121. [8] J. Bao, M.A. Zimmler, F. Capasso, X. Wang, Z.F. Ren, Nano Lett. 6 (2006) 1719. [9] R. Konenkamp, R.C. Word, C. Schlegel, Appl. Phys. Lett. 85 (2004) 6004. [10] S. Gradeçak, F. Qian, Y. Li, H.G. Park, C.M. Lieber, Appl. Phys. Lett. 87 (2005) 173111. [11] O. Demichel, M. Heiss, J. Bleuse, H. Mariette, A. Fontcuberta i Morral, Appl. Phys. Lett. 97 (2010) 201907. [12] C.Y. Chen, C.A. Lin, M.J. Chen, G.R. Lin, J.H. He, Nanotechnology 20 (2009) 185605. [13] X. Jiang, Q. Xiong, S. Nam, F. Qian, Y. Li, C.M. Lieber, Nano Lett. 7 (2007) 3214. [14] P. Parkinson, H.J. Joyce, Q. Gao, H.H. Tan, X. Zhang, J. Zou, C. Jagadish, L.M. Herz, M.B. Johnston, Nano Lett. 9 (2009) 3349. [15] J. Noborisaka, J. Motohisa, S. Hara, T. Fukui, Appl. Phys. Lett. 87 (2005) 093109. [16] N. Sköld, L.S. Karlsson, M.W. Larsson, M.E. Pistol, W. Seifert, J. Trägårdh, L. Samuelson, Nano Lett. 5 (2005) 1943. [17] P. Prete, F. Marzo, P. Paiano, N. Lovergine, G. Salviati, L. Lazzarini, T. Sekiguchi, J. Cryst. Growth 310 (2008) 5114. [18] O.D.D. Couto Jr., D. Sercombe, J. Puebla, L. Otubo, I.J. Luxmoore, M. Sich, T.J. Elliott, E.A. Chekhovich, L.R. Wilson, M.S. Skolnick, H.Y. Liu, A. I. Tartakovskii, Nano Lett. 12 (2012) 5269. [19] C. Chen, S. Shehata, C. Fradin, R. LaPierre, C. Couteau, G. Weihs, Nano Lett. 7 (2007) 2584. [20] C.H. Chuang, T. Doane, S. Lo, G. Scholes, C. Burda., Measuring electron and hole transfer in core/shell nanoheterostructures, ACS Nano 5 (7) (2011) 6016. [21] J. Yoo, B. Chon, W. Tang, T. Joo, L.S. Dang, G.C. Yi, Appl. Phys. Lett. 100 (2012) 223103. [22] B. Deveaud, J. Shah, T.C. Damen, W.T. Tsang., Appl. Phys. Lett. 52 (22) (1988) 1886. [23] J.A. Brum, G. Bastard, Phys. Rev. B 33 (2) (1986) 1420.
31
[24] B. Deveaud, F. Clerot, A. Regreny, K. Fujiwara, K. Mitsunaga, J. Ohta., Appl. Phys. Lett. 55 (25) (1989) 2646. [25] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (5) (1964) 89. [26] H.L. Hartnagel, R. Riemenschneider, Properties of Aluminium Gallium Arsenide, in: S. Adachi (Ed.), INSPEC, the Institution of Electrical Engineers, London, United Kingdom, 1993, p. 239. [27] J.P. Callan, A.M.T. Kim, L. Huang, E. Mazur, Chem. Phys. 251 (2000) 167. [28] Seth A. Fortuna, Li Xiuling, Semicond. Sci. Technol. 25 (2) (2010) 024005. [29] M.C. Plante, R.R. LaPierre, Au-assisted growth of GaAs nanowires by gas source molecular beam epitaxy: tapering, sidewall faceting and crystal structure, J. Cryst. Growth 310 (2) (2008) 356. [30] Chen Chen, Shyemaa Shehata, Cécile Fradin, Ray LaPierre, Christophe Couteau, Weihs Gregor, Nano Lett. 7 (no. 9) (2007) 2584. [31] Yu B. Bolkhovityanov, Pchelyakov Oleg Petrovich, Phys. Uspekhi 51 (5) (2008) 437. [32] Michelle F. Bailon-Somintac, Jasher J. Ibañez, Rafael B. Jaculbia, Regine A. Loberternos, Michael J. Defensor, Arnel A. Salvador, Armando S. Somintac, Low temperature photoluminescence and Raman phonon modes of Aucatalyzed MBE-grown GaAs–AlGaAs core–shell nanowires grown on a prepatterned Si (111) substrate, J. Cryst. Growth 314 (1) (2011) 268. [33] B. Ketterer, M. Heiss, M.J. Livrozet, A. Rudolph, E. Reiger, A. Fontcuberta i Morral, Phys. Rev. B 83 (2011) 125307. [34] L. Smith, H. Jackson, J. Yarrison-Rice, C. Jagadish, Semicond. Sci. Technol. 25 (2010) 024010. [35] J.P. Callan, A.M-T. Kim, C.A.D. Roeser, E. Mazur, Semiconductors and Semimetals, in: K.T. Tsen (Ed.), Elsevier B.V., Massachusetts, USA, 2001, pp. 151–203. [36] P.Y. Yu, M. Cardona, Fundamentals of Semiconductors: Physics and Materials Properties, fourth ed., Springer-Verlag, Berlin Heidelberg, New York, 2010. [37] S. Perera, M.A. Fickenscher, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Q. Gao, H.H. Tan, C. Jagadish, X. Zhang, J. Zou, Appl. Phys. Lett. 93 (2008) 053110. [38] S. Morin, B. Deveaud, F. Clerot, K. Fujiwara, K. Mitsunaga, IEEE J. Quantum Electron. 27 (1991) 1669. [39] Sheinkman, T.V. Torchinskayaand M.K., Deep Centers In AlGaAs Heteroepitaxial Diodes.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Shell to core carrier-transfer in MBE-grown GaAs/AlGaAs core–shell nanowires on Si(1 0 0) substrates Maria Herminia Balgos n, Rafael Jaculbia, Michael Defensor, Jessica Pauline Afalla, Jasher John Ibañes, Michelle Bailon-Somintac, Elmer Estacio, Arnel Salvador, Armando Somintac Condensed Matter Physics Laboratory, National Institute of Physics, University of the Philippines, Diliman, Quezon City 1101, Philippines
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2013 Received in revised form 2 June 2014 Accepted 5 June 2014 Available online 16 June 2014
We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowires grown on Si(1 0 0) substrates via molecular beam epitaxy. The nanowires are dominantly zincblende and are tilted with respect to the substrate surface. Photoluminescence (PL) excitation spectrosocopy at 77 K revealed an abrupt increase in the GaAs PL intensity at excitation above the Al0.1Ga0.9As shell bandgap which is attributed to shell to core carrier-transfer. More carriers from the Al0.1Ga0.9As transfer to the GaAs at T490 K, as observed in the time-resolved PL and temperature dependence of the relative PL intensities of GaAs and Al0.1Ga0.9As due to the ionization of the traps within the Al0.1Ga0.9As. Using a coupled rate equation model that takes into account shell to core carrier-transfer, the average recombination time constants of Al0.1Ga0.9As shell τrec,s ¼ 400 ps (580 ps) and GaAs core τrec,c ¼ 600 ps (970 ps) were obtained from the time-resolved PL at 300 K (77 K). Carrier-transfer time constants τCT ¼50 ps (55 ps) at 300 K (77 K) were also obtained. & 2014 Elsevier B.V. All rights reserved.
Keywords: III–V semiconductors Carrier transfer Nanowires Time-resolved luminescence
1. Introduction Semiconductor nanowires (NWs) are quasi-one dimensional structures with diameters in nanometers and length in microns. The wide range of base materials and the possible production of highly crystalline structures from lattice-mismatched materials make NWs a versatile alternative to its bulk counterpart [1-3]. In fact, the integration of III–V semiconductors to Si has been possible through NW growth [4–7]. Due to its interesting properties, NWs cover a wide range of applications such as in energy conversion, sensors, electronics, and optoelectronics. Application of NWs as light emitters such as light emitting diodes [8,9] and lasers [10] has been previously demonstrated. Due to their high surface-to-volume ratio, however, NWs are susceptible to surface states that may degrade their light emission capabilities [11,12]. To reduce the effects of surface states, NWs are coated with high bandgap material (shell) that serves as a passivation barrier from the surface states. Electrical and optical investigations in these coaxial structures, commonly known as core-shell NWs (CSNWs), show enhanced carrier mobility [13,14] and photoluminescence (PL) intensity [15–18], as compared to bare NWs (no shell).
n
Corresponding author. Tel./fax: þ 63 99209749. E-mail address: [email protected] (M.H. Balgos).
http://dx.doi.org/10.1016/j.jlumin.2014.06.008 0022-2313/& 2014 Elsevier B.V. All rights reserved.
The enhancement of the PL intensity in CSNWs is attributed mainly to the reduced interaction of carriers with the surface states which may act as nonradiative recombination sites, electronic traps, and scattering sites [11,12,19]. However, the introduction of the shell can also enhance the PL intensity by acting as another source of carriers for the NW core [20]. The carriers from the shell can transfer to the core and effectively increase the number of recombining carriers thereby producing a more intense PL [21]. Carrier transfer (CT) has already been observed in quantum wells [22–23], laser structures [24], and type II core–shell heterostructures [20] using time resolved PL (TRPL). In this work, we study in detail the shell-to-core CT in GaAs/ Al0.1Ga0.9As CSNWs by utilizing continuous wave (CW) and TRPL for varying temperatures. The relatively low Al mole fraction (x ¼0.1) of the AlxGa1 xAs allows us to observe a distinct PL signal from GaAs and AlGaAs and at the same time, to pump both GaAs and AlGaAs using a Ti:Sapphire CW laser. The effect of the CT on the PL intensity is measured directly using PL excitation (PLE).
2. Methods GaAs/Al0.1Ga0.9As CSNWs coated with n-doped GaAs cap layer were grown on Si (1 0 0) substrates using a Riber 32P molecular beam epitaxy (MBE) machine. The Si substrate was pre-patterned with gold nanodots that serve as catalysts for the vapor–liquid–solid (VLS) growth of NWs [25]. GaAs was deposited at 580 1C for 30 min
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followed by Al0.3Ga0.7As at the same growth temperature and duration. The thin film equivalent thickness is 0.65 μm and 0.84 μm for GaAs and Al0.3Ga0.7As, respectively. Lastly, a 0.1 μm Sidoped GaAs cap ( 1017 cm 3) served as a protective coating for the AlGaAs which readily oxidizes when exposed to air [26]. The ndoping of the GaAs cap decreases the width of the depletion region at the air–AlGaAs interface [13,14] preventing it from penetrating to the AlGaAs. The formation of a depletion shell could narrow the electronic channel producing PL [15] and decreases the carrier lifetime [5]. CW-PL measurements were conducted at temperatures of 10–300 K. The sample was mounted on the coldfinger of a closed-cycle He cryostat equipped with a Lakeshore temperature controller. The excitation source was a 488 nm Ar þ laser and the PL signal was dispersed by a SPEX 500 m monochromator and detected by a Hamamatsu GaAs photomultiplier tube. PLE (at 77 K) and TRPL (at 77 K and 300 K) measurements were conducted at a separate μ-PL setup where a 10 objective was used to focus the laser and collect the PL signal from the sample mounted inside a continuous flow liquid nitrogen cryostat for 77 K measurements. For the PLE, the GaAs PL peak intensity at 1.514 eV was monitored using a Synapse Si CCD detector attached to an iHR550 Horiba Jobin Yvon spectrometer. The excitation energy of a CW Ti: Sapphire laser was tuned from 1.53–1.68 eV (810–740 nm) corresponding to below and above the Al0.1Ga0.9As shell bandgap, respectively. For the TRPL measurements, a frequency doubled Ti:Sapphire femtosecond laser (400 nm) with 100 fs pulses at 80 MHz repetition rate was used as excitation. The fluence was kept at 0.012 J/cm3 to ensure minimal, if any, nonlinear effects [27]. The PL signal was fed to a streak camera system consisting of Digikröm spectrometer and an Optronis streak camera. The measured TRPL resolution is 10 ps. All measurements were performed on the as-grown CSNW sample (i.e., on ensemble of CSNWs) only. TRPL for bare GaAs NWs was not reported because the signal was too weak to be detected by our TRPL setup.
3. Results Fig. 1 shows a representative top view SEM micrograph of the sample. The CSNWs are tilted with respect to the horizontal due to the o1 1 1 4 preferential growth of GaAs NWs [28]. Close inspection of the CSNWs show that they have a tapered tip, as previously observed in Au-assisted GaAs NWs [29]. From various SEM images, the diameter of the CSNWs is estimated to be
Fig. 1. Top view SEM micrograph of the core–shell nanowires (CSNWs) showing high density formation. The CSNWs are predominantly tilted with respect to the substrate surface due to the o 1 1 1 4 preferential growth of the nanowires.
Fig. 2. 300 K PL of GaAs/AlGaAs core-shell nanowires and bare GaAs nanowires. The ( 10) means that the PL spectrum of GaAs is multiplied by 10 for clarity. More than an order of magnitude increase in the GaAs PL peak intensity of the core–shell nanowires compared to bare nanowires is observed. The inset is a schematic of the core–shell–cap structure.
141 714 nm with a density of 1.68 108 cm 2. The GaAs core diameter and the AlGaAs shell thickness is 100 nm and 19 nm, respectively. These values were estimated using the diameter of a bare GaAs NW (1037 25 nm) grown separately using the same parameters as reference. For MBE-grown NWs via the VLS mechanism, the growth elements are decomposed as they are deposited on the substrate. Hence, a film is also formed in areas with no gold catalysts [30]. However, due to the high lattice mismatch between GaAs and Si (4%) [31], this film is defective and emits very weak PL, if any. Fig. 2 shows the 300 K PL spectrum of the CSNWs and bare GaAs NWs. The sharp peaks in the 300 K spectra may be Ar þ laser plasma lines. For the CSNWs, we identified distinct bands at 1.437 eV and 1.559 eV corresponding to the GaAs core and to the Al0.1Ga0.9As shell transitions, respectively. The position of the GaAs band is near the bandgap of bulk zincblende GaAs, suggesting that our sample is predomintantly zincblende (ZB). To verify this structure, we use x-ray diffraction and temperature dependent PL [32]. The low Al mole fraction x¼ 0.1 may be attributed to the different kinetics between 2-D film and NW growth, as observed by Chen et al. [30]. In VLS, the growth depends on the rate at which the adatoms diffuse along the NW sidewalls [30]. Compared to the bare GaAs NWs, there is more than an order of magnitude increase in the GaAs PL intensity for the CSNWs. The enhanced GaAs PL signal in CSNWs may be due to (i) suppressed surface recombination because of the passivation provided by the Al0.1 Ga0.9As shell [11,12,15–19]; and (ii) increased number of recombining electron-hole pairs via the shell to core CT [21]. The shell-to-core CT in our sample is observed via PLE at 77 K. Fig. 3a shows the 77 K PLE spectrum of GaAs where the excitation is varied from below to above the Al0.1Ga0.9As bandgap of 1.65 eV. An abrupt increase in the PLE signal is observed at 1.65 eV which we attribute to the CT. At excitations hʋo1.65 eV (Region I), the carriers that contribute to the GaAs PLE in Region I are those optically excited from the GaAs region only. At excitations hʋ41.65 eV (Region II), carriers optically excited from the Al0.1Ga0.9As can diffuse and transfer to GaAs and can contribute to the PLE signal [33]. Since the PLE spectrum follows absorption spectrum for CW excitation, we expect the PLE spectrum to mimic the √E bulk density of states. The PLE spectrum follows the √h dependence in Region I but deviates drastically in Region II. The deviation can be explained by the added absorption introduced by the Al0.1Ga0.9As. The CT mechanism and band-to-band recombination are schematically shown in Fig. 3b. However, it should be noted that aside from these two, recombination via defects (not included in Fig. 3b) is also possible.
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Fig. 3. (a) GaAs core PLE spectrum at 77 K. Abrupt increase of PL at excitations above the Al0.1Ga0.9As bandgap indicates carrier transfer. (b) Mechanism for the shell to core carrier transfer.
Fig. 4. Time resolved photoluminescence at 77 K and 300 K of the (a) GaAs core and (b) AlGaAs shell of the core-shell nanowires. Black solid lines are fits to the coupled rate equation (Eqs. (1a) and (1b)). The recombination time constants τrec and carrier transfer time constants τCT obtained from the fits are included in the figure.
The n-doped GaAs cap is too thin to have a significant effect on the PLE signal. We further investigated the CT mechanism using TRPL. Fig. 4a and b shows the TRPL spectra of GaAs and Al0.1Ga0.9As, respectively, at 300 K and 77 K. The spectra were obtained by taking a horizontal segment in the streak image with height equal to the full-width at half maximum of the PL signal. All spectra were fitted using a coupled differential rate equation model to represent the temporal carrier evolution for the Al0.1Ga0.9As shell and GaAs core: d N c ðtÞ N ex;c Nc N s N max N CT ¼ þ ð1aÞ dt τrc τrec;c τCT N max d N s ðtÞ N ex;s Ns N s N max NCT ¼ dt τrs τrec;s τCT Nmax
ð1bÞ
where the subscripts s and c denote shell and core, respectively. N is the number of carriers, Nex is the initial number of excited carriers due to the optical excitation, τr is the rise time constant, τrec is the recombination time constant, τCT is the transfer time constant, Nmax is the maximum number of carriers that can transfer, and NCT is the number of carriers that transferred. Eqs. (1a) and (1b) are modified generation–recombination rate equations for NWs [34]. Generation manifests as the TRPL rise while recombination manifests as the TRPL decay. The first term in the right-hand-side of Eqs. (1a) and (1b) indicates addition of carriers via optical excitation and the subsequent thermalization of the
carriers to the band edges. The second term represents the radiative (R) and nonradiative (NR) recombination with time constant 1/τrec ¼1/τR þ 1/τNR. The last term accounts for of the CT mechanism. The term in brackets is to incorporate the finite number of carriers Nmax from Al0.1Ga0.9As that can transfer. Physically, only carriers near the Al0.1Ga0.9As–GaAs interface can readily transfer; those away from the interface have to initially diffuse before they can undergo CT. Simultaneously, in momentum space, these carriers thermalize to reach the band edges before they recombine. Since thermalization is a faster process (in the order of femtoseconds [35]) than diffusion, then recombination is favored at longer timescales. In our model, when NCT reaches Nmax, the CT process stops (last term in Eqs. (1a) and (1b) goes to zero) leaving recombination as the only carrier exit path in the shell and the optical excitation as the only source of carriers in the core. The black solid lines in Fig. 4a and b are the least square fits of Eqs. (1a) and (1b), respectively. It is important to note that τr is fixed at 10 ps equivalent to the resolution of our TRPL setup. This value is reasonable since the thermalization of the carriers, characterized by τr is in the order of femtoseconds [34] and is beyond the resolution of our TRPL setup. For the GaAs core (Fig. 4a), the TRPL rise has a single rate. Since carriers from Al0.1Ga0.9As transfer to GaAs, we expect a double rise characterized by two different time constants – one from the optical excitation and one from the CT. The single TRPL rise may be explained by the relative contribution to the number of carriers of
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Fig. 5. (a) Integrated PL intensity versus inverse temperature of the AlGaAs shell. Activation energy Ea ¼ 31.4 meV corresponds to the ionization of traps within the AlGaAs. (b) Temperature dependent GaAs–AlGaAs PL intensity ratio showing an increase at 90 K.
the two processes. More carriers are produced via optical excitation, hence, the TRPL rise from the CT is swamped and is not observed. The TRPL of GaAs at both temperatures is characterized by a single TRPL decay rate. We attribute the decay to carrier recombination within the GaAs, The faster decay of 600 ps at 300 K is due to significant NR recombination at 300 K. Since τrec,c which is a combined effect of R and NR recombination [36], that is, 1/τrec ¼1/τR þ1/τNR, we observe a longer time constant of 970 ps at 77 K where NR is minimized and 1/τNR approaches zero. The values obtained at 300 K and 77 K are within the range of reported carrier lifetime values for nearly intrinsic exciton lifetimes of GaAs NWs (0.05–1.1 ns) reported by Perera et al. [37]. For the Al0.1Ga0.9As shell (Fig. 4b), a single exponential TRPL rise, due to the optical excitation, is observed at 77 K and 300 K. The TRPL decay, however, is characterized by a single exponential decay rate at 77 K and a double exponential (fast and slow) decay rate at 300 K. We propose that the appearance of the double decay is a consequence of the CT mechanism. We attribute the fast decay to the combined effect of recombination and carrier transfer while the slow decay is ascribed to the recombination. Numerical fitting using Eq. (1a) revealed τrec,s of 400 ps at 300 K and 580 ps at 77 K. A longer τrec,s at 77 K is again attributed to the minimized NR recombination at low temperatures. The transfer time constant τCT is 50 ps and 55 ps for at 300 K and 77 K, respectively. This is similar to the CT time constants of GaAs/AlGaAs separate confinement heterostructures reported by Morin et al. (19.2 ps at 300 K and 22 ps at 80 K) [38]. Our values of τCT show that the CT time is almost independent of temperature and cannot explain the double decay observed in the 300 K TRPL of Al0.1Ga0.9As. As such, the the maximum number of transferred carriers Nmax relative to the initial number of carriers Nex,s (t¼0) in the shell was investigated. We obtain a value of Nmax/Nex,s(t¼0) of 0.24 at 77 K and 0.69 at 300 K. These values show that a higher fraction of excited carriers transfer at 300K than at 77 K. We believe that defects in the Al0.1Ga0.9As affect the number of carriers that can transfer. Using an Arrhenius plot of the integrated PL intensity vs inverse temperature (Fig. 5a), we calculate an activation energy Ea ¼31 meV for the defects within the AlGaAs at 90 K which can be attributed to an electron trap E9 (30 meV) or a hole trap H4 (32 meV) [39]. At 77 K (o90 K), there is still efficient trapping of carrier by the defects, thus there are not enough transferred carriers to produce a double decay. At 300 K, the defects in the shell are already ionized and are not as efficient at trapping carriers. Thus, more carriers can diffuse until they finally transfer to the GaAs. The temperature dependence of the number of carriers available for transfer to GaAs is investigated using the relative PL intensities of GaAs and Al0.1Ga0.9As at 10K–300 K. Fig. 5b shows
the GaAs–Al0.1Ga0.9As PL intensity ratio (PLIR) as a function of temperature. Owing to the same growth conditions, the defects incorporated in GaAs and AlGaAs have minimal difference. The PL signal for GaAs and Al0.1Ga0.9As therefore should quench at almost the same rate with temperature resulting to a constant PLIR if they are to be treated independently. Constant PLIR is seen at 10–70 K as indicated by the horizontal dash line in Fig. 5b. At 90–150 K, however, there is an apparent increase in the PLIR as indicated by the slanted dash line. At these temperatures, the GaAs PL intensity increases relatively to the Al0.1Ga0.9As. More carriers from the Al0.1Ga0.9As transferred to GaAs at T 490 K. We note that the intersection of the horizontal and slanted line at 90K coincides with the activation temperature of the defects within the Al0.1Ga0.9As. 4. Conclusion The shell-to-core CT mechanism of an ensemble of dominantly zincblende GaAs/AlGaAs CSNWs is investigated through CW-PL, TRPL and PLE spectroscopy for varying temperatures. PLE provides strong evidence of the CT mechanism while CW-PL and TRPL demonstrate that the CT process is dependent of temperature mainly because a high number of carriers participate in the CT process at T 490 K. Coupled rate equations are used to fit the TRPL at 300 K and at 77 K, thereby facilitating the evaluation of the recombination and carrier transfer time constants. The 300 K (77 K) average recombination time constants for GaAs core are τrec,c ¼600 ps (970 ps) while that of the AlGaAs shell are τrec, s ¼400 ps (580 ps). The CT time constant is determined to be τCT ¼50 ps (55 ps) for 300 K (77 K). Acknowledgments This work is supported in part by Grants from PCIEERD-DOST, DOST-GIA, and University of the Philippines OVCRD. The authors would also like to thank Intel for the donation of the streak camera. References [1] B.R. Huang, Y.K. Yang, T.C. Lin, W.L. Yang, Sol. Energy Mater. Sol. Cells 98 (2012) 357. [2] Y. Liang, H. Liang, X. Xiao, S. Hark, J. Mater. Chem. 22 (2012) 1199. [3] S.L. Wu, J.H. Deng, T. Zhang, R.T. Zheng, G.A. Cheng, Diam. Relat. Mater. 26 (2012) 83. [4] T. Mårtensson, C.P.T. Svensson, B.A. Wacaser, M.W. Larsson, W. Seifert, K. Deppert, A. Gustafsson, L.R. Wallenberg, L. Samuelson, Nano Lett. 4 (2004) 1987.
M.H. Balgos et al. / Journal of Luminescence 155 (2014) 27–31
[5] K. Tomioka, T. Tanaka, S. Hara, K. Hiruma, T. Fukui, IEEE J. Sel. Top. Quantum Electron. 17 (2011) 1112. [6] G.E. Cirlin, V.G. Dubrovskii, Y.B. Samsonenko, A.D. Bouravleuv, K. Durose, Y.Y. Proskuryakov, B. Mendes, L. Bowen, M.A. Kaliteevski, M.A. Kaliteevski, D. Zeze, Phys. Rev. B 82 (2010) 035302. [7] L.C. Chuang, M. Moewe, S. Crankshaw, C. Chang-Hasnain, Appl. Phys. Lett. 92 (2008) 013121. [8] J. Bao, M.A. Zimmler, F. Capasso, X. Wang, Z.F. Ren, Nano Lett. 6 (2006) 1719. [9] R. Konenkamp, R.C. Word, C. Schlegel, Appl. Phys. Lett. 85 (2004) 6004. [10] S. Gradeçak, F. Qian, Y. Li, H.G. Park, C.M. Lieber, Appl. Phys. Lett. 87 (2005) 173111. [11] O. Demichel, M. Heiss, J. Bleuse, H. Mariette, A. Fontcuberta i Morral, Appl. Phys. Lett. 97 (2010) 201907. [12] C.Y. Chen, C.A. Lin, M.J. Chen, G.R. Lin, J.H. He, Nanotechnology 20 (2009) 185605. [13] X. Jiang, Q. Xiong, S. Nam, F. Qian, Y. Li, C.M. Lieber, Nano Lett. 7 (2007) 3214. [14] P. Parkinson, H.J. Joyce, Q. Gao, H.H. Tan, X. Zhang, J. Zou, C. Jagadish, L.M. Herz, M.B. Johnston, Nano Lett. 9 (2009) 3349. [15] J. Noborisaka, J. Motohisa, S. Hara, T. Fukui, Appl. Phys. Lett. 87 (2005) 093109. [16] N. Sköld, L.S. Karlsson, M.W. Larsson, M.E. Pistol, W. Seifert, J. Trägårdh, L. Samuelson, Nano Lett. 5 (2005) 1943. [17] P. Prete, F. Marzo, P. Paiano, N. Lovergine, G. Salviati, L. Lazzarini, T. Sekiguchi, J. Cryst. Growth 310 (2008) 5114. [18] O.D.D. Couto Jr., D. Sercombe, J. Puebla, L. Otubo, I.J. Luxmoore, M. Sich, T.J. Elliott, E.A. Chekhovich, L.R. Wilson, M.S. Skolnick, H.Y. Liu, A. I. Tartakovskii, Nano Lett. 12 (2012) 5269. [19] C. Chen, S. Shehata, C. Fradin, R. LaPierre, C. Couteau, G. Weihs, Nano Lett. 7 (2007) 2584. [20] C.H. Chuang, T. Doane, S. Lo, G. Scholes, C. Burda., Measuring electron and hole transfer in core/shell nanoheterostructures, ACS Nano 5 (7) (2011) 6016. [21] J. Yoo, B. Chon, W. Tang, T. Joo, L.S. Dang, G.C. Yi, Appl. Phys. Lett. 100 (2012) 223103. [22] B. Deveaud, J. Shah, T.C. Damen, W.T. Tsang., Appl. Phys. Lett. 52 (22) (1988) 1886. [23] J.A. Brum, G. Bastard, Phys. Rev. B 33 (2) (1986) 1420.
31
[24] B. Deveaud, F. Clerot, A. Regreny, K. Fujiwara, K. Mitsunaga, J. Ohta., Appl. Phys. Lett. 55 (25) (1989) 2646. [25] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (5) (1964) 89. [26] H.L. Hartnagel, R. Riemenschneider, Properties of Aluminium Gallium Arsenide, in: S. Adachi (Ed.), INSPEC, the Institution of Electrical Engineers, London, United Kingdom, 1993, p. 239. [27] J.P. Callan, A.M.T. Kim, L. Huang, E. Mazur, Chem. Phys. 251 (2000) 167. [28] Seth A. Fortuna, Li Xiuling, Semicond. Sci. Technol. 25 (2) (2010) 024005. [29] M.C. Plante, R.R. LaPierre, Au-assisted growth of GaAs nanowires by gas source molecular beam epitaxy: tapering, sidewall faceting and crystal structure, J. Cryst. Growth 310 (2) (2008) 356. [30] Chen Chen, Shyemaa Shehata, Cécile Fradin, Ray LaPierre, Christophe Couteau, Weihs Gregor, Nano Lett. 7 (no. 9) (2007) 2584. [31] Yu B. Bolkhovityanov, Pchelyakov Oleg Petrovich, Phys. Uspekhi 51 (5) (2008) 437. [32] Michelle F. Bailon-Somintac, Jasher J. Ibañez, Rafael B. Jaculbia, Regine A. Loberternos, Michael J. Defensor, Arnel A. Salvador, Armando S. Somintac, Low temperature photoluminescence and Raman phonon modes of Aucatalyzed MBE-grown GaAs–AlGaAs core–shell nanowires grown on a prepatterned Si (111) substrate, J. Cryst. Growth 314 (1) (2011) 268. [33] B. Ketterer, M. Heiss, M.J. Livrozet, A. Rudolph, E. Reiger, A. Fontcuberta i Morral, Phys. Rev. B 83 (2011) 125307. [34] L. Smith, H. Jackson, J. Yarrison-Rice, C. Jagadish, Semicond. Sci. Technol. 25 (2010) 024010. [35] J.P. Callan, A.M-T. Kim, C.A.D. Roeser, E. Mazur, Semiconductors and Semimetals, in: K.T. Tsen (Ed.), Elsevier B.V., Massachusetts, USA, 2001, pp. 151–203. [36] P.Y. Yu, M. Cardona, Fundamentals of Semiconductors: Physics and Materials Properties, fourth ed., Springer-Verlag, Berlin Heidelberg, New York, 2010. [37] S. Perera, M.A. Fickenscher, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, H.J. Joyce, Q. Gao, H.H. Tan, C. Jagadish, X. Zhang, J. Zou, Appl. Phys. Lett. 93 (2008) 053110. [38] S. Morin, B. Deveaud, F. Clerot, K. Fujiwara, K. Mitsunaga, IEEE J. Quantum Electron. 27 (1991) 1669. [39] Sheinkman, T.V. Torchinskayaand M.K., Deep Centers In AlGaAs Heteroepitaxial Diodes.