Dense, Regular GaAs Nanowire Arrays by Catalyst-Free Vapor Phase

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Dense, regular GaAs nanowire arrays by catalystfree vapour phase epitaxy for light harvesting Jiehong Jin, Toma Stoica, Stefan Trellenkamp, Yang Chen, Nicklas Anttu, Vadim Migunov, Rudy M. S. Kawabata, Pio John S. Buenconsejo, Yeng Ming Lam, Fabian Haas, Hilde Helen Hardtdegen, Detlev Grützmacher, and Beata Kardynal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05581 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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ACS Applied Materials & Interfaces

Dense, regular GaAs nanowire arrays by catalyst-free vapour phase epitaxy for light harvesting Jiehong Jin,1 Toma Stoica,1,2 Stefan Trellenkamp,3 Yang Chen,4 Nicklas Anttu,4 Vadim Migunov,5 Rudy M. S. Kawabata,1 Pio J. S. Buenconsejo,6 Yeng M. Lam,6 Fabian Haas,1 Hilde Hardtdegen,1 Detlev Grützmacher,1 Beata E. Kardynał1* 1

Peter Grünberg Institute 9 (PGI 9) and JARA-Fundamentals of Future Information Technologies, Forschungszentrum Juelich, 52425 Juelich, Germany

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National Institute of Materials Physics, P.O. Box MG-7, Magurele, Bucharest 077125, Romania 3

Peter Grünberg Institute 8 (PGI 8) and JARA-Fundamentals of Future Information Technologies, Forschungszentrum Juelich, 52425 Juelich, Germany

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Division of Solid State Physics and NanoLund, Lund University, Box 118, 22100 Lund, Sweden

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Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons and Peter Grünberg Institute , Forschungszentrum Juelich, 52425 Jülich, Germany 6

Materials Science and Engineering, Nanyang Technological University, Singapore 1

ACS Paragon Plus Environment


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E-mail: [email protected] E-mail: [email protected]

Keywords: GaAs nanowires; metalorganic vapour phase epitaxy; selective area growth; Raman spectroscopy; optical spectroscopy

Abstract: Density dependent growth and optical properties of periodic arrays of GaAs nanowires (NWs) by fast selective area growth MOVPE are investigated. As the period of the arrays is decreased from 500 nm down to 100 nm, a volume growth enhancement by a factor of up to four compared with the growth of a planar layer is observed. This increase is explained as resulting from increased collection of precursors on the side walls of the nanowires due to the gas flow redistribution in the space between the NWs. Normal spectral reflectance of the arrays is strongly reduced compared with a flat substrate surface in all fabricated arrays. Electromagnetic modeling reveals that this reduction is caused by antireflective action of the nanowire arrays and nanowire-diameter dependent light absorption. Irrespective of the periodicity and diameter, Raman scattering and grazing angle X-ray diffraction show signal from zinc blende and wurtzite phases, the latter originating from stacking faults as observed by high resolution transmission electron microscopy. Raman spectra contain intense surface phonons peaks, whose intensity depends strongly on the nanowire diameters as a result of potential structural changes and as well as variations of optical field distribution in the nanowires.

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INTRODUCTION Semiconducting nanowires (NWs) have attracted a lot of attention due to their potential applications in electronics and optoelectronics.1-8 These applications can benefit from the flexibility in a design of nanowire heterostructures and semiconducting devices. For example, it has been found that some crystalline structures (polytypes), which are unstable in bulk materials, become present in NWs. This is also the case for GaAs, whose stable phase in the bulk is the cubic zinc blende (ZB), but wurtzite (WZ) phase and other higher polytypes have also been observed in NWs.9 The change of the crystalline symmetry alters the electronic band structure, and a modulation of the polytypes in NWs can produce, for example, quantum dots or superlattices for many applications.10-15 Heterostructures, both in the radial and axial directions, have been developed for optoelectronic devices.16-18 For many applications, such as solar cells or high power light emitting diodes it is desirable to grow the nanowires in regular arrays.19-21 Such arrays provide periodic modulation of the refractive index and thus allow optical engineering to be incorporated in the material design.22 Almost all of the incoming light in a wide range of wavelengths can be absorbed in arrays that cover only 11% of the surface area.21 It is expected that with a choice of the dimensions of the nanowires in the array one achieves selective, wavelength dependent light absorption.20 Large and controlled surface-to-volume ratio in nanowire arrays is also attractive for sensors and for fabrication of hybrid materials such as inorganic-organic semiconductor junctions.4, 22 Regular arrays of nanowires can be grown for example using selective area growth (SAG): growth in the openings in a mask, typically SiOx or SiNx layers deposited on the growth substrate. Lithographically defined, regular, hexagonal arrays of holes in the mask offer identical growth environment for every NW, so that periodically arranged NWs with regular dimensions can be 3



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produced by this method. NWs grown using SAG show narrow size distribution compared with random NWs.23-24 Since different applications may require widely different nanowire densities and diameters, the growth and optical properties of regular, hexagonal arrays of GaAs NWs with wide range of periods are studied here. The arrays are grown using a catalyst-free SAG MOVPE method with a mask composed of holes of different diameters in a SiOx layer with periodicities ranging from 105 nm to 450 nm. Special emphasis is placed on the understanding of the growth mechanism of the densest nanowire arrays as the growth of arrays with periodicity as small as 100 nm has to our knowledge not been reported up to now. Such dense arrays can be very attractive for sensors, hybrid photovoltaics and other devices in which large surface-to-volume ratios are needed as well as in application that require large densities of nanowires (for example light emitting diodes or pixelated photodetectors). Structural characteristics of the NW arrays are determined based on Raman spectroscopy, X-ray Diffraction (XRD), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM).

Spectral reflectance measurements supported by

electromagnetic simulations are used to study antireflective properties as well as absorption resonances in the nanowire arrays. RESULTS AND DISCUSSION Growth of Periodic Arrays. Figure 1 shows a representative SEM image of a nanowire array. In general, a uniform filling of holes with nanowires was obtained, as can be seen in this figure, with no parasitic growth on the mask between them. Coalescence of the NWs in dense arrays with large holes resulted in a growth of layers. Slight asymmetry of the nanowires reflects the shape of the holes in the mask. Typically, from in-plane SEM images of the nanowires in the apertures in the mask, the NWs appear 10 nm larger in diameter than the aperture. However, 4


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close examination of a SEM image of the cross section through the NW in the aperture in the mask (inset of Figure 1b) shows that the diameter of the nanowire follows an increasing diameter of the aperture and continues to grow above the mask with the diameter of the largest of the aperture of the top. There is no evidence of the radial growth contributing to the diameter in a significant way.

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SiOx

GaAs Substrate 50nm

d p l

Figure 1. SEM top (a) and side view (b) image of NW array with 150 nm period, with indicated definitions of length l, diameter d and period p. The inset in (b) shows SEM image of the cross section through the NW near the base showing the relationship between the size of the aperture in the mask and of the NW. For this study, a set of arrays was grown on the same substrate in the same growth run in order to ensure the same growth conditions. For each studied periodicity, arrays with different NW 5



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diameters were grown by varying the sizes of the apertures in the mask. SEM images have been used to evaluate the length (l) and the diameter (d) of nanowires for all periodicities (p) of the studied arrays. The diameters and lengths of NWs of the investigated arrays are listed in Table 1 (in section ‘Preparation and Growth of GaAs NWs’) and shown in Figure 2a. It can be seen from Figure 2a that the length of the nanowires scales approximately inversely proportionally with the diameter, as expected for a diffusion of the adatoms (radicals) on the surface of the side facets, toward the NW top surface.23, 25 Due to small periods, the nanowire length for the same diameter scales additionally with the array periodicity. This results from the proximity effect in which the collection of atoms from the surrounding space is affected by the distance between neighboring NWs. The effect of periodicity on the growth of NWs in a hexagonal array can be evaluated by √

calculating the ratio of the volume of a NW in an array of period p, to the volume of the √

layer that would grow on an unpatterned GaAs in the area of one unit cell of the array, . The thickness l0 = 40nm of the layer, that grows in the absence of the mask, is determined by measuring the growth in a 1x1mm2 large opening in the mask on the same wafer. This ratio, (ld2)/(l0p2), will be referred to as a growth efficiency and it is plotted in Figure 2b as a function of the diameter, d, for all analyzed periodicities. In the case of large array periodicities (350 nm and 450 nm), it is smaller than 100% and only approaches 100% when d approaches p, in which case the growth tends to the conditions of a planar growth. In contrast, for small periodicities (p ≤ 220 nm), a growth efficiency of up to 400% is observed meaning that the dense NW arrays grow more efficiently than a planar layer.

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Figure 2. NW sizes for different periods p: (a) the NW length, l, and (b) growth enhancement, defined as (ld2)/(l0p2), as a function of the NW diameter d, l0 = 40nm; (c) growth enhancement, defined as [(l-l0)d2]/(l0p2) vs the relative surface area of NW side facets 4ld/p2; (d) Scaling factor, c, (see eq.1) as a function of diameter d; points (d=p, c=0) are extrapolated upon noticing that for d=p (growth of the layer) there is no contribution to the growth from the side facets. Note that all these arrays were grown on a single substrate in a single growth run, and thus under equivalent growth conditions, for a growth time of 1 min. Efficiencies above 100% can be correlated with the emergence of side facets which increase the total surface area for collection of adatoms. The growth contribution from the side facets, referred to the volume of the material that would grow on unpatterned substrate, [(l-l0)d2]/(l0p2), 7



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can be compared with the relative surface area of the NWs side facets which is defined as the ratio of GaAs NW side facet surface area 2√3) and the corresponding array unit cell area √

: [(l-l0)d2]/(l0p2) = c·4ld/p2

(1).

(a) z=c l l

(b)

Figure 3. A schematic diagram of the precursor distribution for (a) different periodicities p and for (b) different diameters d (and the corresponding lengths, l). Precursors from a dark shaded region are collected directly on the top (111) surface, while precursors from a light shaded region are contributing to the axial growth by diffusion along the side walls from up to the depth cl. Precursors in the white region desorb from the mask and don’t participate in the NW growth. A scaling factor c is included in the equation to reflect the sub-linear dependence of the additional volume on the additional surface area, seen in Figure 2c. The sub-linear dependence c 870 nm), where the NWs do not absorb light (see SI Figure S2) and the array functions as an anti-reflection layer. Both the p = 150 nm and d = 97 nm and the p = 450 nm and d = 149 nm array show a dip in the reflectance, at λ ≈ 560 nm and λ ≈ 660 nm, respectively. For the p = 450 nm and d = 149 nm array, the dip in the reflectance at λ ≈ 660 nm is accompanied by a peak in the absorbance (see Figure 5). This absorption peak thus originates from absorption at the waveguide resonance at HE11 hybrid mode in the constituent, individual nanowires.27 Such an absorption peak leads to less light remaining for reflection, and causes consecutively a complementary reflectance dip. Additional modeling showed that this peak in the absorbance red-shifts slightly toward λ ≈ 700 nm and becomes stronger and more pronounced with increasing l (not shown). In contrast, for the p = 150 nm and d = 97 nm array, the dip in the reflectance at λ ≈ 560 nm is not accompanied by a peak in the absorbance (see Figure 5). This reflectance dip is thus assigned to a destructive interference of light reflected from the top and bottom of the nanowire array, as in a conventional planar anti-reflection coating. Note, that the dense array can be thought of in 11



terms of an effective medium approximation. The HE11 resonance in these NWs might occur near 480nm (SI Figure S2) as a dip in reflectance at 480 nm is accompanied by a peak in absorbance (SI Figure S2). The simulated absorption of the nanowire arrays is strongly enhanced compared with the 40 nm layer that grows on an unpatterned substrate at the same growth conditions, also shown in Figure 5. Even when perfect antireflective coating is assumed to be present on the GaAs layer, the absorption in the NWs is stronger. For example, an absorption of 65% at a wavelength of 550 nm is achieved for an array with p =150 nm, d =97 nm and l = 240 nm compared with the upper limit of 24% for a 40 nm thick GaAs layer that grows on the unpatterned substrate. 1.0

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bare GaAs no reflection GaAs array, p=450nm array, p=150nm

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Figure 5. Simulated absorption in the NWs of two arrays. Red line: p = 450nm, d = 149nm, l = 240nm. Blue line: p = 150nm, d = 97nm, l = 223nm. Black (green) line shows absorption in the 40nm thick layer of GaAs without (with perfect) antireflective coating. While the absorption enhancement for the sparse arrays has been a subject of previous reports, our results on dense nanowires are equally encouraging when considering the achieved absorption per efficiency of material usage. The growth efficiency compared with the layer is enhanced by over 300% (reduced to 40%) for the 150nm (450nm) period array. Therefore, the

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dense nanowire arrays are very attractive in terms of absorption efficiency related to the efficiency of precursor use (growth efficiency). Structural analysis. Raman spectra were obtained in order to assess the effect of highly varying growth rates on the structure of the nanowires in different arrays. Since the measurements were performed on as grown samples, the contribution to the Raman signal from the substrate has to be also considered. In order to eliminate the possible modification of the NWs due to heating with the laser light, it was confirmed that the shape of the spectrum did not change when the excitation intensity was increased from the value used for the experiments reported here. Figure 6a shows a set of spectra from NWs grown in arrays of 150 nm period but of different NW diameters. For comparison, the spectrum of the bulk GaAs n-doped substrate is included. Lorentzian fitting of the spectra was used to find the sets of peak positions. An example of the fitting is shown in Supporting Information (Figure S3). The peak found at 267 cm-1 matches the reported value for the TO peak of zinc blende (ZB) GaAs as well as for A1(TO)-E1(TO) of the GaAs wurtzite (WZ) structure. The LO peak is found at slightly lower energy (~287cm-1) than the literature value of 291cm-1 for the bulk undoped ZB GaAs.28 This red shift of the LO peak is frequently reported for GaAs NWs with high density of twins and WZ-ZB short segments and can be explained by the presence of crystalline disorder in NWs and tensile strain induced in this heterostructure.14 Note, that a small red shift of the LO peak is also observed in the spectrum of the n-doped GaAs (111) substrate in Figure 6a. The spectra of arrays with thinner NWs in Figure 6a are dominated by the peak at around 284 cm-1, which, as will be discussed later, is a surface phonon SO mode.

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Figure 6. (a) A set of Raman spectra on NW arrays with a period of 150 nm and different NW diameters (indicated as labels near the spectra). The spectrum of the n-doped GaAs substrate is included for comparison. The vertical dotted lines show the literature energy values of the main Raman modes, (labels above the figure). (b) An effect of the surrounding medium (air or PMMA) in which the NWs are embedded, for the array with p = 350 nm and NW diameter of 107 nm is limited to the SO mode, whose shift is shown in the figure. For clarity, the literature energy values of the main Raman modes are marked with vertical dashed lines and labels in both figures. A peak present close to 256 cm-1 is usually assigned to the E2H mode in WZ (polymorph H2) phase of NWs, while the peak at ~ 263 cm-1 is most likely the E1 mode of ZB in NWs. A peak at 263 cm-1 was also observed in µ-Raman measurements on individual GaAs NWs with high 14


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density of twins in a ZB structure, in WZ segments due to fluctuations in the TO and E2H peak position of 1-3 cm-1

29-30

and have been explained as resulting from the strain and quantum

confinement induced by structural defects and ZB-WZ heterostructures.30-31 The presence of the WZ phase in the NWs has been observed with glancing angle XRD measurements as shown in Figure 7. Broadening of the peaks is such that the signal can be interpreted as WZ GaAs or mixture of WZ and ZB. The peaks at 27° and 54° are slightly shifted compared with the ones measured using 2theta/theta method on the (111) GaAs substrate and the tabulated spectra, all of which are shown in Figure 7. The in-plane compressive (tensile) strain of the WZ (ZB) phase can explain the shift compared to the tabulated values.32 The presence of WZ phase is clearly confirmed by peaks at 61° and 91°, which are not expected for ZB phase. The transmission electron micrograph (TEM) nanowires from two arrays (SI Figure S4) shows ZB structure with high density of stacking faults. Since the twin boundaries can be considered as single units of WZ phase (and periodic twin boundaries as 2H, 4H polytypes contribute to WZ-like features in Raman spectra and XRD. GaAs zinc blende ICSD-43424 GaAs wurtzite ICSD-67773

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Figure 7. XRD of the 220 nm period NW array (black line) and GaAs (111) substrate (red). Note, that for clarity some XRD peaks of the substrate are scaled.

Blue and red lines in the

panels above show standard tabulated positions of XRD peaks for wurtzite and zinc blende GaAs. Dashed blue and red lines are added to show experimental peaks assignment. In order to confirm the assignment of the SO mode, the Raman signal was further acquired after changing the refractive index of the medium at the NWs’ surface by embedding them in poly(methyl methacrylate) (PMMA). The NW array with p = 350nm was chosen to ensure full polymer penetration. A clear shift of the SO peak (1.7 cm-1) was observed, while the other Raman peak positions remained unchanged, as illustrated in Figure 6. The same results were obtained for other arrays (SI Figure S5). The unchanged position of the bulk modes means that no strain on the NWs was induced by the PMMA. The red shift of the SO mode observed in our experiments is in agreement with the reported value of 1.8 cm-1 measured by Spirkoska et al.32 and with the predictions from the simplified formula for the shift ∆ωSO of the surface phonon frequency when chancing the surrounding medium from air to a medium with dielectric constant

εm:

∆ω SO =

2 2 ) (ε m − 1) / (1 + 1 / ε ∞ ) (ω LO − ωTO (ε ∞ + ε m f (qr )) 2ω SO

(2)

where ε∞ is the high frequency dielectric constant of GaAs, and ωSO is the mean value of the two surface phonon frequencies. The factor f(qr) is about 0.932 for NWs of diameter larger than 100 nm as in the analyzed array. The PMMA has a dielectric constant of 2.8. A shift of the SO mode from air to PMMA of about 2 cm-1 is estimated using the above formula, in good agreement with the measured value of 1.7 cm-1.

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The Raman results for all measured arrays are summarized in Figure 8. The diameter dependence of the peak energies of the identified Raman modes is shown in Figure 8a for different arrays. One can see that the TO peak shows almost constant energy of about 266.7 cm-1 for all diameters and periodicities. In contrast, the positions of remaining peaks vary in the range of a few cm-1 with both diameters and periodicities. For example, the two peaks E2H and E1 vary in the ranges of 253-257 cm-1 and 261-264 cm-1, respectively. Both peak positions are monotonically red-shifting with increasing NW diameter for arrays with small periodicities. The trend is opposite for the arrays with larger periodicities (350 and 450 nm). The SO peak energy increases with the NW diameter, approaching the LO peak energy, as expected from theory.32 The LO peak for the nanowires is at about 287 cm-1, red-shifted compared with the bulk crystal value. However, since the LO peak is very close to the SO peak, it is not always well resolved. For the largest NW diameters, the superposition of SO and LO modes results in a single asymmetric peak. The observed increase of the SO energy with the decrease of the wire diameter (Figure 6a) agrees well with the theoretical predictions. The Raman scattering on the surface mode is activated by the breaking of the translation symmetry on the NW surface.34-35 This is produced by the surface roughness. It is well known that the surface roughness is increased in GaAs NWs in the presence of structural defects such as twin boundaries, which are present in our NWs (SI Figure S4). Figure 8b shows the dependence of the intensity ratio of SO and E1 modes. There is a steady increase of the SO mode intensity when reducing the NW diameter from 200 nm to 100 nm. The increase of the surface to volume ratio could be one of the reasons for the relative increase of the SO intensity but it cannot explain the change fully since a decrease of the nanowire diameter by 50% results in more than an order of magnitude increase of the SO 17



intensity. An additional factor contributing to this trend is a distribution of the electric field of the probe laser light in the nanowires. The HE11 resonance observed in reflectance spectra has maximum field amplitude at the surface of the NWs.11, 36 Electromagnetic simulations of 97 nm diameter NWs identified a HE11 resonance at wavelength of 480 nm. The tail of this peak at 532 nm is expected to manifest itself as a very strong SO signal from the 97nm diameter NWs. (a)

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d (nm) Figure 8. The diameter dependence of (a) different mode frequencies and (b) the ratio of SO and E1 peak areas. CONCLUSIONS Selective area growth of regular arrays of GaAs NWs on GaAs (111) substrate with a SiOx mask using MOVPE is strongly affected by the redistribution of precursors between nanowires. It leads to strong enhancement of the nanowire growth rate in dense arrays and for the thinner nanowires, with the highest measured volume growth enhancement, compared with a growth of a layer, of 400%. In contrast, the volume growth rate is lower than that of the layer in sparse arrays, the reduction being strongest for thinner nanowires. All NW arrays showed reduced spectral reflectance at normal incidence which according to full electromagnetic field simulations is caused by a combination of antireflective action of the nanowires arrays and leaky 18


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waveguide resonances (HE11 mode). The latter is accompanied by strong light absorption in the nanowires irrespective of the array periodicity. The single-nanowire waveguide mode HE11 resonance, which maximizes light absorption near the surface as shown in simulations and observed in Raman scattering, can be used for applications such as sensors, or organic-inorganic hybrid devices. Raman spectra of all studied arrays contain a number of peaks typical for the bulk-like ZB structure and WZ polytype, as well as surface phonons. Glancing angle XRD confirmed the presence of the WZ phase structure in the NW array. Since no WZ phase was observed in high resolution TEM images of selected, random NWs, the WZ signal in Raman and XRD spectra is likely to originate from the high density of stacking faults in the NWs. All Raman peaks, except from the TO peak, showed wavenumber shift dependent on NW diameter and array period, indicating the dependence of the disorder in the material on array periodicity and NW diameter. This structural disorder is also clear from strong intensity of SO Raman mode. Quantitative analysis of Raman signal intensities was not possible due to a strong dependence of the distribution of the electric field of laser light in the nanowires on the geometry of the arrays. EXPERIMENTAL SECTION Preparation and Growth of GaAs NWs. Before patterning, the n-GaAs (111) B substrate was covered with a 20 nm thick SiOx layer by spin coating of the diluted hydrogen silsesquioxane followed by baking at 275˚C for 20 min. This mask layer was subsequently patterned into hexagonally arranged arrays of holes by electron beam lithography followed by reactive ion etching (RIE) with CHF3 gas. Each array covered an area of 100x100 µm2. These hole arrays differed from each other with the period and/or the diameter of the hole in the mask. Arrays of 105nm, 150 nm, 220 nm, 350 nm and 450 nm in period were chosen for the analysis. The smallest hole diameter used in the study was 60 nm and the largest was limited by the array 19



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period. The investigated NW arrays were grown in the same growth run on the same patterned wafer. The selective area growth of the GaAs nanowires was carried out in an Aixtron AIX-200 low-pressure MOVPE system. Prior to the growth, the patterned wafer was deoxidized by immersion in H2SO4 for 5 min, followed by a rinse in deionized water and then transferred to a N2 ambient.37 N2 at a working pressure of 20 mbar and total flow rate of 3100 ml/min was used as a carrier gas for the growth. The partial pressures of the precursors, trimethylgallium (TMGa) and arsine (AsH3), were set at 0.254 Pa and 4.323 Pa, respectively. The growth temperature was set at 750oC, which combined with the low V/III partial pressure ratio of 19, promotes a fast growth in the axial [111] direction and suppresses radial growth (deposition time was 1 min).24, 38 The investigated sets of arrays are listed in Table 1 which contains the corresponding mean values of NW diameters and lengths (so, an array in a set Pxxx is defined by the period xxx and the NW mean size is given by the pair: d and l). The statistic fluctuations of the sizes for each array are included in Figure 2 as error bars. The statistic fluctuations ∆d and ∆l in Table 1 are average values for all arrays with the same period. A very likely origin of the NW size fluctuations is the variation of the diameters of the holes in the mask caused by the patterning process in which the proximity effects in e-beam exposure lead to small processing window and thus electron beam stability, development of the resist play an important role in opening of holes in oxide. We can see in Table 1 that the statistic fluctuations are reduced for larger periods for which the proximity effects are not important. Note, that at these conditions the MOVPE growth of GaAs is in the diffusion controlled regime, thus growth rate is limited by mass transport from the gas phase to the surface via the boundary layer.

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Table 1 – The dimensions of the investigated set of arrays: for each period p, there are arrays with different NW diameters d1;d2…dn and corresponding lengths l1;l2…ln both listed with the statistic fluctuations Δd and Δl averaged for the arrays of the given period. Array set P105

p (nm) 105

P150

150

P220

223

P350

350

P450

450

d1;d2…dn

l1;l2…ln

(nm) 66;77;89 72;85;97; 105;123;137 130;154;190 107;121;136; 199;278 128;130;149; 182;226;318

(nm) 415;274;83 498;334;223; 203;117;100 192;125;80 225;179;131; 92;51 279;237;176; 137;89;67

Δd (%) 4.5

Δl (%) 10.0

3.0

7.1

2.0

8.1

2.8

7.7

1.6

5.3

Reflectance Measurement. The reflectance has been measured by illuminating the array with white light of a tungsten lamp through a lens with NA=0.9. The light was focused to a spot of 5 – 6 µm in diameter. The reflected light was collected with the same lens and measured using a spectrometer in the wavelength range of 530 nm to 870 nm. Reflectance and Absorption Simulations. The simulations use tabulated values39 for the refractive index of GaAs and consider light incident at normal angle (along z-direction) on an array of NWs with circular cross-section. The reflectance R(λ) of light back to the incident air side and the transmittance T(λ) into the GaAs substrate at a given wavelength λ was calculated. The absorbance of the NWs, that is, the fraction of incident light absorbed in the NWs, is then given by A(λ) = 1 - R(λ) - T(λ). To represent the unpolarized light in the experiments, the simulated results are averaged over x and y polarized incident light. Raman Measurement. The unpolarized µ-Raman measurements have been performed at room temperature using a defocused Ar-ion laser emitting at 532nm, The laser beam has a spot diameter on the order of 6-10 µm in order to measure an average signal from many NWs and to 21



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avoid excessive heating of the sample. The laser beam is parallel to the axial direction of NWs in the array on the GaAs substrate. The back-scattered signal was then collected from using a triple grating spectrometer. XRD Measurement. The measurement was performed for an array covering an area of 1 mm x 1 mm with 220 nm period and 105 nm NW diameter and grown under the same conditions as the ones studied with SEM and Raman spectroscopy. When the nanowire array was exposed to an X-ray beam of 1 mm diameter at a glancing angle of 1°, a set of broad peaks originating from the NW layer was present.

ASSOCIATED CONTENT Supporting Information The Supporting Information contains information about optical reflectance of the other NW arrays, the absorption simulations of 150nm period array, an example of the deconvolution of Raman curves, the TEM image and another example for the SO peak shift for different surrounding medium is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS B.K. and Y.M.L. acknowledge the support from BMBF and NTU under the 1° N Programme. We thank Doris Meertens from Ernst Ruska-Centre for TEM specimen preparation. REFERENCES 22


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1.

Dasgupta, N. P.; Sun, J.; Liu, C.; Brittman, S.; Andrews, S. C.; Lim, J.; Gao, H.; Yan, R.;

Yang, P., 25th Anniversary Article: Semiconductor Nanowires--Synthesis, Characterization, and Applications. Advanced materials 2014, 26, 2137-2184. 2.

Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M., Nanowire Electronic and Optoelectronic

Devices. Materials Today 2006, 9, 18-27. 3.

Lysov, A.; Vinaji, S.; Offer, M.; Gutsche, C.; Regolin, I.; Mertin, W.; Geller, M.; Prost,

W.; Bacher, G.; Tegude, F.-J., Spatially Resolved Photoelectric Performance of Axial GaAs Nanowire pn-Diodes. Nano Research 2011, 4, 987-995. 4.

Ren, S.; Zhao, N.; Crawford, S. C.; Tambe, M.; Bulovic, V.; Gradecak, S.,

Heterojunction Photovoltaics Using GaAs Nanowires and Conjugated Polymers. Nano letters 2011, 11, 408-413. 5.

Saxena, D.; Mokkapati, S.; Parkinson, P.; Jiang, N.; Gao, Q.; Tan, H. H.; Jagadish, C.,

Optically Pumped Room-Temperature GaAs Nanowire Lasers. Nature Photonics 2013, 7, 963968. 6.

Soini, M.; Zacharias, M.; Uccelli, E., Thermal Conductivity of GaAs NWs Studied by

Micro-Raman Spectroscopy Combined with Laser Heating. Applied Physics Letters 2010, 97, 263107. 7.

Thunich, S.; Prechtel, L.; Spirkoska, D.; Abstreiter, G.; Fontcuberta i Morral, A.;

Holleitner, A. W., Photocurrent and Photoconductance Properties of a GaAs Nanowire. Applied Physics Letters 2009, 95, 083111.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8.

Page 24 of 28

Tomioka, K.; Motohisa, J.; Hara, S.; Hiruma, K.; Fukui, T., GaAs/AlGaAs Core

Multishell Nanowire-Based Light-Emitting Diodes on Si. Nano letters 2010, 10, 1639-1644. 9.

Harmand, J. C.; Patriarche, G.; Péré-Laperne, N.; Mérat-Combes, M. N.; Travers, L.;

Glas, F., Analysis of Vapor-Liquid-Solid Mechanism in Au-Assisted GaAs Nanowire Growth. Applied Physics Letters 2005, 87, 203101. 10. Akiyama, T.; Sano, K.; Nakamura, K.; Ito, T., An Empirical Potential Approach to Wurtzite-Zinc-Blende Polytypism in Group III-V Semiconductor Nanowires. Japanese Journal of Applied Physics 2006, 45, L275-L278. 11. Anttu, N.; Lehmann, S.; Storm, K.; Dick, K. A.; Samuelson, L.; Wu, P. M.; Pistol, M. E., Crystal Phase-Dependent Nanophotonic Resonances in InAs Nanowire Arrays. Nano letters 2014, 14, 5650-5655. 12. Belabbes, A.; Panse, C.; Furthmüller, J.; Bechstedt, F., Electronic Bands of III-V Semiconductor Polytypes and Their Alignment. Physical Review B 2012, 86, 075208. 13. Lehmann, S.; Wallentin, J.; Jacobsson, D.; Deppert, K.; Dick, K. A., A General Approach for Sharp Crystal Phase Switching in InAs, GaAs, InP, and GaP Nanowires Using Only Group V Flow. Nano letters 2013, 13, 4099-4105. 14. Spirkoska, D.; Arbiol, J.; Gustafsson, A.; Conesa-Boj, S.; Glas, F.; Zardo, I.; Heigoldt, M.; Gass, M. H.; Bleloch, A. L.; Estrade, S.; Kaniber, M.; Rossler, J.; Peiro, F.; Morante, J. R.; Abstreiter, G.; Samuelson, L.; Fontcuberta i Morral, A., Structural and Optical Properties of High Quality Zinc-Blende/Wurtzite GaAs Nanowire Heterostructures. Physical Review B 2009, 80, 245325. 24


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15. Spirkoska, D.; Efros, A. L.; Lambrecht, W. R. L.; Cheiwchanchamnangij, T.; Fontcuberta i Morral, A.; Abstreiter, G., Valence Band Structure of Polytypic Zinc-Bblende/Wurtzite GaAs Nanowires Probed by Polarization-Dependent Photoluminescence. Physical Review B 2012, 85, 045309. 16. Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M., Growth of Nanowire Superlattice Structures for Nanoscale Photonics and Electronics. Nature 2002, 415, 617-620. 17. Hyun, J. K.; Zhang, S.; Lauhon, L. J., Nanowire Heterostructures. Annual Review of Materials Research 2013, 43, 451-479. 18. Nakai, E.; Yoshimura, M.; Tomioka, K.; Fukui, T., GaAs/InGaP Core–Multishell Nanowire-Array-Based Solar Cells. Japanese Journal of Applied Physics 2013, 52, 055002. 19. Fan, H. J.; Werner, P.; Zacharias, M., Semiconductor Nanowires: From Self-Organization to Patterned Growth. Small 2006, 2, 700-717. 20. Kupec, J.; Witzigmann, B., Dispersion, Wave Propagation and Efficiency Analysis of Nanowire Solar Cells. Optics express 2009, 17, 10399-10410. 21. Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Aberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgström, M. T., InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit. Science 2013, 339, 1057-1060.

25



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Page 26 of 28

22. Hu, S.; Chi, C.-Y.; Fountaine, K. T.; Yao, M.; Atwater, H. A.; Dapkus, P. D.; Lewis, N. S.; Zhou, C., Optical, Electrical, and Solar Energy-Conversion Properties of Gallium Arsenide Nanowire-Array Photoanodes. Energy & Environmental Science 2013, 6, 1879-1890. 23. Motohisa, J.; Noborisaka, J.; Takeda, J.; Inari, M.; Fukui, T., Catalyst-Free SelectiveArea MOVPE of Semiconductor Nanowires on (111)B Oriented Substrates. Journal of Crystal Growth 2004, 272, 180-185. 24. Ikejiri, K.; Noborisaka, J.; Hara, S.; Motohisa, J.; Fukui, T., Mechanism of Catalyst-Free Growth of GaAs Nanowires by Selective Area MOVPE. Journal of Crystal Growth 2007, 298, 616-619. 25. Sladek, K.; Klinger, V.; Wensorra, J.; Akabori, M.; Hardtdegen, H.; Grützmacher, D., MOVPE of n-Doped GaAs and Modulation Doped GaAs/AlGaAs Nanowires. Journal of Crystal Growth 2010, 312, 635-640. 26. Anttu, N.; Xu, H. Q., Scattering Matrix Method for Optical Excitation of Surface Plasmons in Metal Films with Periodic Arrays of Subwavelength Holes. Physical Review B 2011, 83, 165431. 27. Anttu, N.; Xu, H. Q., Efficient Light Management in Vertical Nanowire Arrays for Photovoltaics. Optics express 2013, 21 A558-A575. 28. Mooradian, A.; Wright, G. B., First Order Raman Effect in III-V Compounds. Solid State Communications 1966, 4, 431-434. 29. Smirnov, M. B.; Koshkin, A. O.; Karpov, S. V.; Novikov, B. V.; Smirnov, A. N.; Shtrohm, I. V.; Cirlin, G. E.; Bouravleuv, A. D.; Samsonenko, Y. B., Computer Simulation of 26


Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the Structure and Raman Spectra of GaAs Polytypes. Physics of the Solid State 2013, 55, 12201230. 30. Zardo, I.; Conesa-Boj, S.; Peiro, F.; Morante, J. R.; Arbiol, J.; Uccelli, E.; Abstreiter, G.; Fontcuberta i Morral, A., Raman Spectroscopy of Wurtzite and Zinc-Blende GaAs Nanowires: Polarization Dependence, Selection Rules, and Strain Effects. Physical Review B 2009, 80, 245324. 31. Yeh, C.-Y.; Lu, Z. W.; Froyen, S.; Zunger, A., Zinc-Blende–Wurtzite Polytypism in Semiconductors. Physical Review B 1992, 46, 10086-10097. 32. Spirkoska, D.; Abstreiter, G.; Fontcuberta, I. M. A., Size and Environment Dependence of Surface Phonon Modes of Gallium Arsenide Nanowires as Measured by Raman Spectroscopy. Nanotechnology 2008, 19, 435704. 33. Kriegner, D.; Panse, C.; Mandl, B.; Dick, K. A.; Keplinger, M.; Persson, J. M.; Caroff, P.; Ercolani, D.; Sorba, L.; Bechstedt, F.; Stangl, J.; Bauer, G., Unit Cell Structure of Crystal Polytypes in InAs and InSb Nanowires. Nano letters 2011, 11, 1483-1489. 34. Gupta, R.; Xiong, Q.; Mahan, G. D.; C., E. P., Surface Optical Phonons in Gallium Phosphide Nanowires. Nano letters 2003, 3, 1745-1750. 35. Zardo, I.; Abstreiter, G.; Fontcuberta i Morral, A., Raman Spectroscopy on Semiconductor Nanowires. In Nanowires, Prete, P., Ed. InTech: 2010. Available from: http://www.intechopen.com/books/nanowires/raman-spectroscopy-on-semiconductor-nanowires

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36. Svensson, J.; Anttu, N.; Vainorius, N.; Borg, B. M.; Wernersson, L. E., DiameterDependent Photocurrent in InAsSb Nanowire Infrared Photodetectors. Nano letters 2013, 13, 1380-1385. 37. Hardtdegen, H.; Hollfelder, M.; Meyer, R.; Carius, R.; Munder, H.; Frohnhoff, S.; Szynka, D.; Luth, H., Movpe Growth of Gaas Using a N2 Carrier. Journal of Crystal Growth 1992, 124, 420-426. 38. Haas, F.; Sladek, K.; Winden, A.; von der Ahe, M.; Weirich, T. E.; Rieger, T.; Luth, H.; Grutzmacher, D.; Schapers, T.; Hardtdegen, H., Nanoimprint and Selective-Area MOVPE for Growth of GaAs/InAs Core/Shell Nanowires. Nanotechnology 2013, 24, 085603. 39. Palik, E. D., Gallium Arsenide. In Handbook of Optical Constants of Solids; Palik, E. D., Eds.; Academic Press: Florida, 1985; Vol. 1; pp 429-443.

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Dense, Regular GaAs Nanowire Arrays by Catalyst-Free Vapor Phase

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