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Optical emission of InAs nanowires
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 23 (2012) 375704 (6pp)
doi:10.1088/0957-4484/23/37/375704
Optical emission of InAs nanowires M M¨oller1 , M M de Lima Jr1,2 , A Cantarero1 , T Chiaramonte3,4 , M A Cotta3 and F Iikawa3 1
Instituto de Ciencia de los Materiales, Universidad de Valencia, E-46071 Valencia, Spain Fundaci´o General de la Universitat de Val`encia, ES-46010 Valencia, Spain 3 Instituto de F´ısica Gleb Wataghin, UNICAMP, CEP-13083-859, Campinas-SP, Brazil 4 Departamento de Ciˆencias Naturais, Universidade Federal de S˜ao Jo˜ao del Rei, 36301-160 S˜ao Jo˜ao del Rei, MG, Brazil 2
E-mail:
[email protected]
Received 14 May 2012, in final form 18 July 2012 Published 24 August 2012 Online at stacks.iop.org/Nano/23/375704 Abstract Wurtzite InAs nanowire samples grown by chemical beam epitaxy have been analyzed by photoluminescence spectroscopy. The nanowires exhibit two main optical emission bands at low temperatures. They are attributed to the recombination of carriers in quantum well structures, formed by zincblende–wurtzite alternating layers, and to the donor–acceptor pair. The blue-shift observed in the former emission band when the excitation power is increased is in good agreement with the type-II band alignment between the wurtzite and zincblende sections predicted by previous theoretical works. When increasing the temperature and the excitation power successively, an additional band attributed to the band-to-band recombination from wurtzite InAs appears. We estimated a lower bound for the wurtzite band gap energy of approximately 0.46 eV at low temperature. (Some figures may appear in colour only in the online journal)
Semiconductor nanowires are attracting considerable attention due to their potential application in advanced nanophotonic devices [1, 2]. In addition, it has been shown that III–V nanowires, such as InP, GaAs and InAs, can exhibit wurtzite crystal structure in contrast to the most stable zincblende phase of their bulk materials [3–6]. The change in crystal structure alters the optical and electronic properties of the material and results in different fundamental physical parameters such as the band gap energy, exciton binding energy and phonon energies. As a consequence of the novelty brought by the nanowire growth, even for GaAs, which is among the most well-known materials, much controversy regarding the band gap energy of the wurtzite structure still exists. While some authors have reported that the band gap of the GaAs wurtzite structure is lower than that of the zincblende one [7, 8], others have claimed that the wurtzite band gap is higher [9, 10]. In the case of InAs much less information is available. Theoretical studies predict a wurtzite band gap 40–66 meV higher than the one of the zincblende phase [11–13]. Tr¨ag˚ardh et al have predicted a wurtzite band gap of 0.54 eV by extrapolating fitted photocurrent measurements on InAs1−x Px nanowires [14] and Bao et al observed a value of 0.52 eV in two-dimensional-like wurtzite 0957-4484/12/375704+06$33.00
structures [15]. Very recently Sun et al reported the first photoluminescence (PL) characterization of pure wurtzite InAs nanowires in which the observed blue-shifted band gap has been assigned to quantization effects rather than to the difference in crystal structures due to the small diameters of the nanowires [16]. In this paper, we investigate the low-temperature optical emission of wurtzite InAs nanowires using PL spectroscopy. A detailed study of the excitation power and temperature dependence of two InAs nanowire samples, grown at different temperatures, is presented. The InAs nanowires have been grown by the vapor liquid solid method in a chemical beam epitaxy system using gold nanoparticles as catalysts on an GaAs(100) substrate. Growth precursors are trimethyl indium [(CH3 )3 In] diluted with hydrogen (H2 ) as a carrier gas, and thermally decomposed arsine (AsH3 ). The growth temperatures of the InAs nanowires are 420 ◦ C for sample A and 450 ◦ C for sample B. The results of performed transmission electron microscopy (TEM) measurements show that the nanowires exhibit predominantly a pure wurtzite phase and in some nanowires we find few stacking faults (few monolayers of 1
c 2012 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 23 (2012) 375704
M M¨oller et al
20 nm
10 nm
Figure 1. (a) TEM image of an InAs nanowire from sample A close to the nanowire tip with pure wurtzite (WZ) structure (alternating layer stacking). (b) TEM image of an InAs nanowire from sample B. The structure is predominantly wurtzite with few thin segments of zincblende (ZB) phases.
zincblende phase) along the wire (cf figure 1), as confirmed by high-resolution TEM and electron diffraction analysis (not shown). The nanowires are grown along the [0001] direction. Additionally, we carried out scanning electron microscopy measurements and both analyses reveal that the nanowires of the two samples present a needle-like shape with an apex diameter over 30 nm and the base diameter is larger for sample A than for sample B. The average diameters of the nanowires are 103 ± 26 nm and 67 ± 16 nm and their average lengths are 1.5 ± 0.6 µm and 1.8 ± 0.5 µm for sample A and B, respectively. PL measurements of the as-grown nanowire ensembles (on the GaAs(100) substrate) of sample A and B were carried out using a pulsed Ti-sapphire laser (pulse width of ∼3 ps and repetition of ∼80 MHz) as the excitation source in order to avoid nanowire heating. The laser was syntonized at 760 nm and focused on a spot of approximately 1 mm2 . The luminescence was detected using a 0.5 m monochromator with 300 g mm−1 , blazed at 2 µm and a liquid-N2 -cooled InSb photodiode. The sample was cooled with a cold-finger He cryostat with CaF2 optical windows, where the sample temperature was varied from 5 to 100 K. The whole setup was mounted in a nitrogen-purged box in order to minimize the water absorption between 0.445 and 0.485 eV. In figures 2(a) and (b), PL spectra from sample A and sample B measured for different excitation powers at 5 K are presented, respectively. Two optical emission bands are observed in both samples. The low-energy emission band (LEB, dashed line) does not shift with increasing excitation power while we detect a pronounced blue-shift for the high-energy emission band (HEB, dotted line). In both samples, LEB dominates for low excitation powers and HEB dominates for high powers, suggesting a saturation effect of the LEB at high excitation power. Additionally, for high excitation power a shoulder at the high-energy side (denoted BtB) is identified which has an emission energy of 0.488 eV, far above the zincblende band gap energy of 0.415 eV [17]. The PL spectra at different positions on both samples were fitted with various Gaussian functions. The data between 0.445 and 0.485 eV and between 0.369 and 0.375 eV were
not included in the fit in order to avoid contributions from the water absorption band and a sharp substrate emission band, respectively. The obtained PL peak energies are depicted in figure 3(a) for the three emission bands. As mentioned before, for high excitation power the HEB dominates and the LEB saturates (see figures 3(b) and (c)). The latter suggests that impurity-like recombination processes are involved, as previously reported for donor–acceptor pair and band-to-acceptor transitions in bulk InAs [18, 19]. Therefore, we attributed the LEB to the donor–acceptor pair overlapped with the band-to-acceptor recombinations, which are usually not well resolved. Although the precise identification of the shallow impurity centers is beyond the scope of this paper, we point out that carbon is often observed as an impurity in samples grown by chemical beam epitaxy [20]. The HEB peak position blue-shifts approximately 15–30 meV as the average excitation power raises from 10 to 500 mW. This large energy shift is a behavior usually observed in quantum wells with type-II band alignment, which is attributed to the band bending induced by the carrier accumulation at the interfaces and the band filling effect. Similar blue-shifts have been observed in InP [21, 22] and GaAs [23] nanowires containing wurtzite and zincblende phases, where type-II interfaces between these structures were present. In our InAs nanowires, in fact, two phases along the wire axis are observed in TEM images forming a quantum well potential profile as depicted in the schematic diagram in figure 2(c). It has been predicted theoretically that InAs zincblende/wurtzite quantum well structures have type-II band alignments [11, 24, 25] which have a crucial effect on the optical properties of the nanowires [22]. Photoexcited electrons and holes get spatially separated in the zincblende and wurtzite section, respectively. At low excitation powers, these electrons recombine with the holes, leading to an emission below both the zincblende and wurtzite band gap energies. However, if the carriers are confined within the quantum wells in the zincblende structure, the minimum possible recombination energy corresponds to a transition from the confined ground state in the zincblende conduction band to the top of the wurtzite valence band (transition ¬ in 2
Nanotechnology 23 (2012) 375704
M M¨oller et al
Figure 2. The PL spectra for different excitation powers measured at 5 K are shown from (a) sample A and (b) sample B. Two optical emission bands (LEB, dashed line and HEB, dotted line) can be clearly observed in both samples. For high excitation power a shoulder at the high energy side (BtB) can be identified. (c) Schematic diagram of the valence band (VB) and the conduction band (CB) of the wurtzite InAs nanowires with zincblende sections. The numbering denotes possible transitions in the InAs nanowires; ¬ quantum well related recombination, band-to-band transition in wurtzite phase and ® donor–acceptor recombinations.
Figure 3. (a) PL peak energies of the low-energy (LEB) and high-energy (HEB) emission band and the band-to-band emission (BtB) as a function of the excitation power for sample A (squares) and sample B (circles). Integrated PL intensities of (b) the HEB and (c) the LEB for both samples. For increasing excitation powers, a saturation of the PL emission can be observed. Symbols with distinct fillings denote measurements at different positions on the same sample.
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Nanotechnology 23 (2012) 375704
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Figure 4. (a) PL spectra from sample B measured at different temperatures. (b) PL spectra from sample B measured at 100 K for different excitation powers. The hatched area represents the data excluded in the fits due to the water absorption in this spectral range. (c) PL peak energies of the HEB of sample A (squares) and sample B (circles) as a function of temperature. (d) PL peak energies at 100 K as a function of excitation power.
figure 2(c)). When the excitation power is increased, band bending and state filling of electrons in the zincblende section and holes in the wurtzite interface result in a blue-shift and broadening of the HEB (see figure 2(a)). Thus, we attribute the HEB to the quantum well related emission. Another effect that may be present in nanowires is the radial confinement. In our case most of the nanowires have a diameter much larger than the effective Bohr radius, such that the radial quantum confinement is negligible. To clarify this point, we calculated the energy gap between the quantized ground states of the conduction and valence band for an infinite long cylinder according to an overly simplified effective mass model [26, 27], where the band gap is given by E = Eg + (2h¯ 2 ξ 2 /d2 )(1/me + 1/mh ), where Eg is the bulk band gap, ξ the zero-point of the cylindrical Bessel function (2.4048), d the diameter of the nanowires and me = 0.042 and mh = 0.084 [13] the effective electron and hole masses, respectively. The calculations reveal a band gap difference between nanowires with diameters of 70 and 50 nm of approximately 5 meV, which is within the error of our measurements. The volume fraction of the nanowire extension that have diameters smaller than 50 nm located at the nanowire tip corresponds to less than 5% and can be neglected. Furthermore, photoexcited carriers at the tip will diffuse to wurtzite band edges with lower potential in thicker nanowire sections, where they will recombine.
In figure 4(a), we present the temperature-dependent PL spectra measured for a fixed intermediate excitation power (50 mW) for sample B. Under these conditions the LEB is still dominant at low temperature. Similar results were obtained for sample A. When the temperature is increased, the LEB decreases rapidly and the HEB becomes dominant, in agreement with the assignation of the peaks to the donor–acceptor pair/band-to-acceptor and the excitonic quantum well related recombinations. Similar effects have also been observed for bulk InAs [19] and GaAs nanowires [28]. In figure 4(b) we display the PL spectra for different excitation powers measured at 100 K for sample B. The corresponding PL peak energies for the HEB as a function of temperature for a fixed intermediate excitation power (50 mW) and as a function of different excitation powers measured at 100 K are depicted in figures 4(c) and (d) for sample A (squares) and sample B (circles), respectively. A clear blue-shift with increasing temperature is observed. This behavior, which is opposite to the usual reduction of the band gap of semiconductors with increasing temperature [16, 29], may be attributed to the thermal excitation of the confined electrons to higher energy states in the zincblende phase. In addition, raising the excitation power at 100 K (see figures 4(b) and (d)) further blue-shifts the emission band. We associate this behavior with the band filling of the zincblende quantum well states due to the increased excitation power. This leads to a blue-shift of the emission towards the band gap 4
Nanotechnology 23 (2012) 375704
M M¨oller et al
energy of the wurtzite structure (transition in figure 2(c)). The HEB seems to saturate at high excitation power for both samples, suggesting that the saturation energy is the band-to-band transition energy of the wurtzite InAs phase. For high excitation power the emission energy is determined to be 0.447 ± 0.010 eV, which is a lower bound for the fundamental band gap of wurtzite InAs at 100 K. Note that due to the water absorption in the spectral range between 0.445 and 0.485 eV the data has been excluded in the fits, leading to larger uncertainties in the determination of the emission energies (cf error bars in figure 4(d)). If we assume that the temperature dependence of the band gap of InAs is the same for the wurtzite (WZ) and zincblende [17] (ZB) phases, the lower bound for the wurtzite band gap at 5 K can be estimated as ZB ZB WZ WZ − k T/2 + E − E = E Eg,5 B g,5 K g,100 K g,100 K K
band gap measurements of InAs nanowire ensembles. Due to the low optical efficiency of infrared photodetectors in this spectral range the signal to noise may be too low in order to measure individual nanowires to address this point. In summary, we have investigated the optical emission from wurtzite InAs nanowires grown by chemical beam epitaxy. The blue-shift of the HEB observed when the excitation power is increased is an indication of the type-II band alignment between wurtzite and zincblende interfaces, in agreement with the theoretical prediction. Additional temperature-dependent PL studies allowed us to estimate a lower bound for the wurtzite band gap to be 0.458 ± 0.010 eV at low temperature. This value is in good agreement with previously reported experimental and theoretical works.
Acknowledgments
= 0.443 eV + 0.015 eV = 0.458 eV
We thank the Spanish Ministry of Science and Innovation (grant Nos TEC2009-12075 and MAT2009-10350), CNPq, CAPES, FAPESP for financial support and P Motisuke, R L Carvalho and M J S P Brasil for experimental support. We also thank the Electron Microscopy Laboratory (LME) at the Brazilian Synchrotron Light Laboratory for support during microscopy experiments.
where the term kB T/2 accounts for the thermal excitation and kB represents the Boltzmann constant. This value is 43 meV higher than that of the zincblende structure and is in good agreement with the available theoretical works which predict a 40–66 meV higher value for the wurtzite structure [11–13]. It is close to the value suggested by Koblm¨uller et al [30], who recently observed a PL peak shoulder at 0.50 eV that they attributed to the band transition in the wurtzite segments of predominant zincblende InAs nanowires, and to the experimental values of 0.52 eV [15] and 0.54 eV [14] mentioned before. In addition, the lower bound value is only slightly lower than the observed BtB emission, shown in figures 2(a) and (b), that may be associated with the band-to-band transition with an average emission energy of 0.488 ± 0.010 eV for high excitation power at T = 5 K (cf figure 3(a)). Although the BtB emission energy has been estimated with an elevated error due to the low intensity of the shoulder and the water absorption in the spectra, it corroborates the assignment of the lower threshold for the wurtzite InAs band gap at 5 K. Finally, our results are higher than the PL emission energies of 0.426 eV assigned to the band gap energy with small confinement effect by Sun et al [16]. These authors have also observed an emission at 0.43–0.45 eV that has been attributed to surface-related effects. Although the pinning of the Fermi level at the surface, which leads to an electron accumulation layer close to the surface, should be present in our nanowires, this is not likely to be the origin for the high emission energy detected for the HEB since the large blue-shift observed for increasing excitation power and increasing temperature cannot be explained by that. Furthermore, no significant difference between the results from nanowires of sample A and B can be observed, suggesting that their different diameters and, thus, the surface effect is not responsible for the blue-shift of the HEB. These features are rather more consistent with the type-II band alignment of wurtzite and zincblende phases. Furthermore, broad values of the band gap of wurtzite InAs obtained by several groups indicate that distinct effects, such as excess of carriers, lateral confinement, polytypism, surface effects and the substrate may affect the
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