Epitaxially Oriented Sn:In2O3 Nanowires Grown by the VaporâLiquid

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Epitaxially Oriented Sn: InO Nanowires Grown by the Vapor Liquid Solid Mechanism on m-, r- , a-Al O as Scaffolds for Nanostructured Solar Cells 2

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Andreas Charalampous, Matthew Zervos, Joseph Kioseoglou, Katerina Tsagaraki, Maria Androulidaki, George Konstantinidis, Eugenia Tanasa, and Eugeniu Vasile ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00519 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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ACS Applied Energy Materials

Epitaxially Oriented Sn: In2O3 Nanowires Grown by the Vapor Liquid Solid Mechanism on m-, r-, a-Al2O3 as Scaffolds for Nanostructured Solar Cells Andreas Charalampous §, Matthew Zervos *,⁋, Joseph Kioseoglouǂ, Katerina Tsagaraki $, Maria Androulidaki $ , George Konstantinidis $ , Eugenia Tanasă ǁ, Eugeniu Vasileǁ §, ⁋

Nanostructured Materials and Devices Laboratory, School Of Engineering, University of

Cyprus, PO Box 20537, Nicosia, 1678, Cyprus. ǂ

Department of Physics, Aristotle University of Thessaloniki, Nanostructured Materials

Microscopy Group, Thessaloniki, 54124, Greece. $

Foundation Of Research and Technology Hellas, Institute of Electronic Structure and LASER,

Microelectronics Group, PO Box 527, Heraklion, Crete, GR 711 10, Greece. ǁ

Department of Science and Engineering of Oxide Materials and Nanomaterials, Politehnica

University of Bucharest, 313 Splaiul Independentei, Bucharest, 060042, Romania.

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ABSTRACT We have grown highly directional, epitaxial Sn: In2O3 nanowires via the vapor-liquid-solid mechanism on m-, r- and a-Al2O3, between 800°C and 900°C at 1 mbar. The Sn: In2O3 nanowires have the cubic bixbyite crystal structure and are tapered with lengths up to 80 µm, but they are inclined at  ≈ 60° along one direction on m-Al2O3 while those on r-Al2O3 are inclined at  ≈ 45° and oriented along two mutually orthogonal directions. In contrast, vertical Sn: In2O3 nanowires were obtained on a-Al2O3. We obtain an excellent uniformity and reproducible growth of Sn: In2O3 nanowires up to 15 mm x 15 mm on m- and r-Al2O3 which is important for the fabrication of nanowire solar cells. . All of the Sn: In2O3 nanowires had a resistivity of 10-4 cm and carrier densities of the order of 1021 cm-3 in which case the charge distribution has a maximum at the surface of the Sn: In2O3 nanowires, due to the occupancy of sub-bands residing well below the Fermi level, as shown via the self-consistent solution of the Poisson-Schrödinger equations in the effective mass approximation. Besides we show that the Sn: In2O3 nanowires are capable of light emission and exhibited room temperature photoluminescence at 3.1 eV due to band-to-band radiative transitions but also at 2.25 eV due to donor-like states residing energetically in the upperhalf of the energy band gap. We discuss the advantages of using ordered networks of Sn: In2O3 nanowires in solar cell devices and issues pertaining to their fabrication.

Keywords: Epitaxial, Ordered, Nanowires, Indium Tin Oxide, Solar Cells


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1. INTRODUCTION Indium tin oxide (ITO) is a transparent conducting oxide (TCO) with high transparency and conductivity that is used extensively in solar cells and displays. The epitaxial growth, structural, electrical and optical properties of ITO epitaxial layers on c-, m-, r- and a- Al2O3 have been investigated extensively in the past

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and remains an active topic of interest 6. In addition to

epitaxial layers, ITO i.e. Sn: In2O3 nanowires (NWs) have also been investigated for the fabrication of etc.

nanowire 7-14

solar

cells,

light

emitting

diodes,

sensors,

photo

catalysis

In the past we have grown Sn: In2O3 NWs at 800°C and 10-1 mbar via the vapor liquid

solid (VLS) mechanism on Si, fused SiO2 as well as on C fibers, using metallic Sn and In

15.

However, they were not ordered in any way, which prohibits the realization of nanoscale devices such as nanowire solar cells (NWSCs). Only a few have obtained epitaxial, ordered Sn: In2O3 NWs. More specifically vertical Sn: In2O3 NWs with a cubic crystal structure have been obtained by Nguyen et al.

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on a-Al2O3 which

enabled their integration into a vertical field effect transistor. Furthermore Wan et al. 17, 18 obtained epitaxial, ordered Sn: In2O3 NWs, with square sections and the cubic crystal structure on a Sn: In2O3 buffer layer that was deposited on Y: ZrO2 i.e. yttrium stabilized zirconia (YSZ) at 900°C. The electrical properties of individual Sn: In2O3 NWs were measured by Wan et al. 17, 18 in a two and four point probe terminal configuration, and the average resistivity was found to be of the order of 10-4  cm. However one of the drawbacks of YSZ is that it is not transparent like m-, rand a-Al2O3. Consequently their Sn: In2O3 NWs were dispersed on glass forming a 500 nm thick layer and the optical transmission was measured to be 80% all the way from 300 nm to 800 nm.



In addition to these efforts Gao et al. 19 obtained highly ordered, vertical Sn: In2O3 NWs, via the VLS mechanism on a-Al2O3 at 840° C by using In2O3, SnO2 and C. These also had the cubic crystal structure, diameters of 180 nm, lengths of a few µm, and were highly transparent with a transmission greater than 80% above 600 nm, but also exhibited blue emission under ultra violet (UV) excitation suggesting that they are attractive, not just for their conductivity and transparency, but also for light emission. More recently, Shen et al 20 obtained Sn: In2O3 NWs via the VLS mechanism on YSZ at 800°C using a mixture of In2O3, SnO2 and C (0.6:0.4:1) under a small flow of Ar at 1 mbar. The Sn: In2O3 NWs had cubic and orthorhombic crystal structures, square sections with sides of 140 nm ± 18 nm, lengths up to 1 µm and were oriented at  = 90°,  = 45° and  = 35°. Besides, Chen et al. 21, 22

has grown vertical, un-doped In2O3 NWs with a cubic crystal structure but hexagonal cross

sections via the VLS mechanism on a-Al2O3 at 620°C using C and In2O3 under a flow of Ar and O2. These In2O3 NWs exhibited photoluminescence at 378 nm ( 3.28 eV) which was attributed to radiative transitions from oxygen donor levels to the valence band. Finally, it is important to mention that Hsin et al. 23 obtained lateral self-aligned In2O3 NWs on Si (111), while Shen et al. 24

has grown laterally aligned Sn: In2O3 NWs on YSZ.

All other TCO NWs that have been obtained in the past e.g. Sb: SnO2, Al: ZnO and In: ZnO NWs 25, 26, 27

were not ordered along any particular direction. Only Sn: In2O3 NW ordered networks have

been obtained on a-Al2O3 and YSZ via the reduction of In2O3, SnO2 or SnO by C, which is not best for achieving a high uniformity over areas greater than 10 mm2. To the best of our knowledge no one has investigated the growth and properties of Sn: In2O3 NWs on m- and r-Al2O3 although these have oxygen terminated surfaces that are suitable for the growth of cubic, tetragonal and orthorhombic crystal structures while being optically transparent. 4 ACS Paragon Plus Environment

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Consequently here we have carried out a detailed investigation into the structural, electrical and optical properties of Sn: In2O3 NWs grown on m-, r- and a-Al2O3 by the VLS mechanism. We obtain highly directional, epitaxial Sn: In2O3 NWs with excellent uniformity up to 15 mm x 15 mm on m- and r-Al2O3, in a reproducible fashion but to a less extent on a-Al2O3. We describe their structural, electrical, and optical properties in conjunction with theoretical calculations and discuss the advantages of these Sn: In2O3 NWs for the fabrication of NWSCs 28.

2. EXPERIMENTAL 2.1. Epitaxial growth of Sn: In2O3 NWs on m-, r- and a-Al2O3. The Sn: In2O3 NWs were grown using a 1˝ hot wall, low pressure chemical vapour deposition (LPCVD) reactor capable of reaching 1100°C, which was fed via a micro flow leak valve positioned on the upstream side just after the gas manifold which consists of four mass flow controllers. A chemically resistant, rotary pump that can reach 10-4 mbar was connected downstream. For the growth of the Sn: In2O3 NWs, 5 mg of Sn and 95 mg of In (Aldrich, 100 Mesh, 99.9%) were weighed with an accuracy of ± 1 mg. Square samples of 10 mm x 10 mm, m-, r- and a- Al2O3 were cleaned sequentially in trichloroethylene, methanol, acetone, isopropanol, rinsed with de-ionised water, dried with nitrogen and then coated with  1 nm Au. The elemental Sn and In as well as the clean m-, r- and a-Al2O3 substrates were loaded in the same quartz boat that was positioned at the centre of the 1˝ LPCVD reactor, pumped down to 10-4 mbar and purged with 1000 sccm of Ar for 10 min at 1 mbar. Subsequently, the temperature was ramped up to 800°C at 30°C/min using the same flow of Ar. Upon reaching 800°C a flow of 10 sccm O2 was added to the flow of Ar in order to grow the Sn: In2O3 NWs over 10 min at 1 mbar, followed by cool down without O2. We have also grown



the Sn: In2O3 NWs on m-, r- and a-Al2O3 at 700°C, 750°C, 800°C, 900°C and 1000°C, keeping all else equal, in order to find out if this has an effect on the ordering.

2.2. Structural, Electrical and Optical Characterization of the Sn: In2O3 NWs. The morphology and crystal structure of the Sn: In2O3 NWs was determined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). High resolution transmission electron microscopy (HRTEM) was carried out using a TECNAI F30 G2 S-TWIN operated at 300 kV and equipped for energy dispersive x-ray (EDX) analysis. In addition, the electrical properties and carrier density were measured by the Hall effect in the Van der Pauw geometry, at B = 0.2 Tesla using a Keithley 2182 Nano Voltmeter and Keithley 2635A constant current source, after the formation of ≈ 1 mm diameter Ag paint contacts over the Sn: In2O3 NWs at the four corners of the 10 mm x 10 mm Al2O3

29.

The room temperature photoluminescence (PL) spectra were

measured with a He-Cd laser operating at 325 nm and 20 mW.

3. RESULTS AND DISCUSSION In the past, we have shown that the reaction of Sn and In with O2 at 800°C and 10-1 mbar results into the growth of Sn: In2O3 NWs on 1 nm Au/Si (001) with average diameters of  50 nm and lengths up to  100 m 15. These Sn: In2O3 NWs had square sections and the cubic crystal structure, but they were not oriented in any particular direction due to the fact that they do not obey any epitaxial relation with the underlying Si which is also not transparent. This is a constraint in exploiting the full potential of Sn: In2O3 NWs as a TCO for the realization of devices such as NWSCs. Highly directional, epitaxial Sn: In2O3 NWs have only been obtained on a-Al2O3 and YSZ, although m- and r-Al2O3 have oxygen terminated surfaces that are suitable for the growth of cubic, tetragonal and orthorhombic crystal structures and are also optically transparent. 6 ACS Paragon Plus Environment

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Consequently we have grown Sn: In2O3 NWs on m-, r- and a-Al2O3 and investigated their structural, electrical and optical properties.

3.1. Structural Properties of Sn: In2O3 NWs on m-, r- or a-Al2O3 We readily obtained highly ordered Sn: In2O3 NWs on m-, r- and a-Al2O3 at 800°C and 1 mbar as shown by the SEM images in Figure 1, 2 and 3 respectively. No Sn: In2O3 NWs were grown without supplying O2 during the growth step suggesting that the residual O2 after purging and the temperature ramp was negligibly small. In other words the growth of the Sn: In2O3 NWs occurred only during the growth step at 800°C, upon supplying a small flow of O2 at 1 mbar, which also prevented the oxidation of the metallic Sn and In that melt at 232°C and 157°C respectively, during the temperature ramp. As a result we obtained epitaxial ordered Sn: In2O3 NWs on m-, r-Al2O3 in a highly reproducible fashion and with excellent uniformity over areas up to 15 x 15 mm2 but not to the same extent on a-Al2O3. We did not obtain ordered networks of Sn: In2O3 NWs on m-, r- or a-Al2O3 at 800°C by reducing the growth pressure down to 10-1 mbar due to the fact that this results into a larger amount or flux of Sn and In arriving at the Au particles which seems to be incompatible with epitaxial growth. Furthermore temperatures below 800°C or above 900°C resulted into the growth of Sn: In2O3 NWs that were not oriented in any particular direction. A similar dependence on growth temperature and pressure has been observed by Chern et al.

5

Consequently the optimum temperature range for the epitaxial growth of highly directional Sn: In2O3 NWs on m-, r- or a-Al2O3 falls between 800°C and 900°C at 1 mbar. We find that the majority of Sn: In2O3 NWs on m-Al2O3 are aligned along one direction as shown in Figure 1(a) and (b) but one may also observe the existence of a minority of Sn: In2O3 NWs in two other directions which form a herringbone like pattern. The Sn: In2O3 NWs are tapered as 7 ACS Paragon Plus Environment


shown in Figure 1(c) and we do not observe any Au particles on their ends. Upon closer inspection we find that the majority of the Sn: In2O3 NWs appear to be vertical upon viewing from one side, as shown in Figure 1(d), but in fact they are inclined at  = 60° with respect to the surface of mAl2O3 when viewed from the side after an in-plane rotation of  = 90° as shown in Figure 1(e). A higher magnification image of the side views of the Sn: In2O3 NWs are shown in Figures 1(f) and (g) from which it is clear that the minority Sn: In2O3 NWs are also inclined at  = 60°. Most of the Sn: In2O3 NWs have square sections, as shown in Figure 1(h), with lengths up to 80 µm, but we have also observed the existence of a few with hexagonal like sections and flat tops as shown in Figure 1(i). In addition one may observe a periodic variation in the diameter of the Sn: In2O3 NWs, as shown in Figure 1(j), which gives a saw tooth like profile along their length. In the past Nguyen et al. 30 has tried to grow Sn: In2O3 NWs on m-Al2O3 via the reduction of In2O3 and SnO2 by C but obtained SnO2 NWs despite the fact that they used more In2O3 than SnO2. In essence they obtained oriented SnO2 NWs with diameters of 86 nm ± 12 nm and average lengths of 2.5 µm on m- and a-Al2O3 substrates at 840°C. The In: SnO2 NWs on m-Al2O3 were oriented along two mutual perpendicular directions similar to the un-doped SnO2 NWs obtained by Mathur et al. 31 and Leonardy et al. 32 To the best of our knowledge no one else has previously obtained epitaxial ordered Sn: In2O3 NWs on m-Al2O3. However, it is worthwhile noting that Chern et al. 5 has grown Sn: In2O3 epitaxial layers by pulsed laser deposition on m-Al2O3 which consisted of elongated grains oriented along one direction, consistent with the alignment of the Sn: In2O3 NWs on m-Al2O3 shown in Figure 1. In contrast, we find that the Sn: In2O3 NWs on r-Al2O3 are oriented along two mutually perpendicular directions, as shown in Figure 2 (a) and (b). The Sn: In2O3 NWs are inclined at  =


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45 ° with respect to the surface of r-Al2O3 as shown from the side views in Figure 2(c) and (d) and have lengths up to 80 µm. Higher magnification images of the side views are shown in Figures 2(e) and (f). Most of the Sn: In2O3 NWs have square sections as shown in Figure 2(g) but again we observed a few that have hexagonal sections with flat tops as shown in Figure 2(h). The Sn: In2O3 NWs are tapered, as shown in Figure 2(i), but one may also observe the formation of pyramidal heads on their ends as opposed to spherical Au particles as shown in Figure 2(g). In the past Sn: In2O3 epitaxial layers with a cubic crystal structure have been deposited on r-Al2O3 4, 5 but to the best of our knowledge no one has tried to grow Sn: In2O3 NWs on r-Al2O3. Nevertheless Chern et al.

5

observed that Sn: In2O3 epitaxial layers on r-Al2O3 consisted of elongated grains

aligned along two mutually perpendicular directions consistent with the ordering of the Sn: In2O3 NWs on r-Al2O3 shown in Figure 2. Finally, we obtained vertical Sn: In2O3 NWs on a-Al2O3 as shown in Figure 3 (a), (b). Side view images of these are shown in Figure 3(c) and (d) from which we find that they have lengths up to 50 µm and sides up to 300 nm as shown in Figure 3(e). The vertical Sn: In2O3 NWs on a-Al2O3 are similar to those obtained by Gao et al.

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on a-Al2O3 at 840° C by using In2O3, SnO2 and C.

However, we also observed on occasions the formation of larger nanowires as shown in Figure 3 (f), which are inclined with respect to the surface of a-Al2O3 as shown in Figure 3(g). Consequently the growth was not so reproducible in the sense that we did not always obtain vertical Sn: In2O3 NWs all over the a-Al2O3. In all of the above cases the majority of the Sn: In2O3 NWs had square sections but a few appeared to be hexagonal with flat tops. One would be inclined then to suggest that the Sn: In2O3 NWs with hexagonal and square sections have different crystal structures. Before considering the structural properties of the Sn: In2O3 NWs in further detail it is useful to note that In2O3 is polymorphic and 9 ACS Paragon Plus Environment


has the cubic bixbyite crystal structure i.e. body centre cubic bcc-In2O3 with a unit cell consisting of 32 In and 48 O atoms, that has a lattice constant of a = 10.11Å under ambient conditions. The rhombohedral or corundum structure of In2O3 was first obtained by Shannon et al. 33 at 1250 °C and 6.5 GPa and is a metastable form that belongs to the hexagonal family and consists of hexagonal closed pack oxygen ions. As such the rhombohedral cell can also be described in terms of a hexagonal cell with a= 5.487 Å, b = 5.487 Å, c = 15.51Å. Corundum-type rh-In2O3 has been used to make sensors 34 but also for energy storage 35 and photo catalysis 36. It has been shown that bcc- In2O3 may be transformed into rh-In2O3 upon compression 37 but Wang et al. 38 obtained bccIn2O3 as well as rh-In2O3 layers on c-Al2O3 by metal organic chemical vapor deposition between 400°C and 600°C. Similarly, Nishinaka et al. 6 obtained epitaxial layers of rh-In2O3 on m-, r- and a- Al2O3 by mist chemical vapor deposition at 475°C and 1 bar using SnCl4.5H2O and In (OCCH3CHOCCH3)3, suggesting that rh-In2O3 may be obtained under moderate growth conditions via a suitable choice of the underlying substrate. Consequently the formation of Sn: In2O3 NWs with bcc- In2O3 and rh-In2O3 crystal structures that have square and hexagonal sections would not be surprising. However it is important to emphasize that Chen et al. 21 obtained In2O3 NWs on a-Al2O3 at 620°C with a cubic crystal structure but hexagonal sections consisting of a horizontal triangular top-surface surrounded by tilted side-surfaces which is possible by consideration of the crystallographic symmetry of the cubic crystal of In2O3. Therefore the Sn: In2O3 NWs with square and hexagonal like sections shown in Figures 1, 2 and 3 may both correspond to bcc- In2O3. All of the Sn: In2O3 NWs on m-, r- and a-Al2O3 exhibited one or two major peaks in the XRD as shown in Figures 4, 5 and 6 respectively due to the fact that they grow in an epitaxial fashion along specific crystallographic directions. In contrast the Sn: In2O3 NWs we obtained previously on Si 10 ACS Paragon Plus Environment

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displayed a multitude of peaks as they do not obey any epitaxial relation with the underlying Si. Note that the XRD traces of the m-, r- or a-Al2O3 without any Sn: In2O3 NWs are also shown for comparison in Figures 4, 5 and 6 respectively. The Sn: In2O3 NWs on m-Al2O3 exhibit one major peak at  = 21° and a weaker but nevertheless clearly resolved peak at  = 43.5° as shown in Figure 4. These correspond to the (422) and (211) crystallographic planes of bcc-In2O3 but can’t be indexed to rh-In2O3 in agreement with the findings of Nishinaka et al. 6 who obtained bcc Sn: In2O3 layers on m-Al2O3. Similarly the Sn: In2O3 NWs obtained on r-Al2O3 exhibit one major peak at  = 51° and a weaker one at  = 30.5° as shown in Figure 5 corresponding to (440) and (222) crystallographic planes of bcc-In2O3 similar to the XRD obtained by Vogt et al. 3 from a Sn: In2O3 epitaxial layer that was grown on r-Al2O3. The same peaks as well as the strong side peak belonging to Al2O3 (024) were also observed by Chern et al. 5 who obtained (110) oriented Sn: In2O3 layers on r-Al2O3 by pulsed laser deposition. It is also consistent with the fact that the Sn: In2O3 epitaxial layers obtained by Nishinaka et al. 6 directly on r-Al2O3 also had a cubic crystal structure. A rocking curve around the (440) peak is shown as an inset in Figure 5 giving a full width half maximum of 1.6° due to the fact that not all of the Sn: In2O3 NWs are perfectly parallel. We did not observe any peaks belonging to the tetragonal rutile crystal structure of SnO2 in Figures 4 and 5. This is due to the fact that we used a mass ratio of In: Sn ≈ 20 which results into the formation Sn: In2O3 NWs where Sn is incorporated into the In2O3 as a substitutional donor impurity. 15 Finally the Sn: In2O3 NWs obtained on a-Al2O3 exhibit two major peaks at  = 30.5° and  = 63.5° corresponding to the (222) and (444) crystallographic planes of bcc-In2O3 as shown in Figure 6 11 ACS Paragon Plus Environment


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These can’t be indexed to rh-In2O3 or o-In2O3 and are identical to those observed by Chen et al. 21 who obtained un-doped In2O3 NWs with hexagonal sections on a-Al2O3. The formation of bcc Sn: In2O3 NWs on a-Al2O3 is also consistent with the fact that Nishinaka et al. 6 and Chern et al.

4

obtained bcc Sn: In2O3 epitaxial layers directly on a-Al2O3. The minor peak at  = 27.4° can’t be indexed to bcc-, rh- or o- In2O3 but surprisingly appears to belong to the tetragonal rutile crystal structure of SnO2. However this peak was not always observed consistent with the fact that the growth on a-Al2O3 was not highly reproducible compared to that on m- and r-Al2O3. From the above we conclude that the highly ordered Sn: In2O3 NWs obtained on m-, r- and aAl2O3 have a cubic crystal structure similar to all epitaxial, ordered Sn: In2O3 NWs obtained previously on a-Al2O3 and Y: ZrO2. For completeness a high magnification TEM image of an individual Sn: In2O3 NW after being removed from the m-Al2O3 is shown in Figure 7(a) while a HRTEM image of the same Sn: In2O3 NW is shown in Figure 7(b) from which we find that the lattice spacing between the (211) crystallographic planes is 4.13 Å. A typical EDX spectrum is shown in Figure 7(c) from which we observed peaks corresponding to In, Sn and O. Similarly a TEM image of an individual Sn: In2O3 NW that was grown on r-Al2O3 is shown in Figure 8(a). HRTEM images are shown in Figures 8(b), (c) from which we observe the [440] crystallographic planes of In2O3 consistent with the XRD in Figure 5 where the lattice spacing between the (440) crystallographic planes is 1.79 Å. Finally a typical TEM image of a Sn: In2O3 NW on a-Al2O3 is shown in Figure 9(a). The Sn: In2O3 NWs grow along the [111] direction as shown by the HRTEM in Figure 9(b) where the lattice spacing between the (222) crystallographic planes is 2.9 Å. An EDX spectrum is also shown for completeness in Figure 9(c) confirming the existence of In, O as well as Sn.


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3.2. Epitaxial Relation and Growth Mechanism of Sn: In2O3 NWs on m-, r- , a-Al2O3 At first sight the epitaxial growth of bcc Sn: In2O3 on m-, r- a-Al2O3 appears to be impossible due to the large lattice constant of cubic Sn: In2O3 i.e. 10.067Å but it is possible to maintain epitaxial growth by matching integral multiples of lattice planes of In2O3 with the underlying lattice of m-, r- or a-Al2O3 shown as insets in Figures 7, 8 and 9. In order to investigate the epitaxial relationships between the bcc-In2O3 (space group: Ia3 (206)) and the Al2O3 (space group: R3c (167)) all the possible atomistic models were constructed and shown in Figure 10. From these we conclude that the majority of Sn: In2O3 NWs grow on m-Al2O3 by a stacking of [211] crystallographic planes at an angle of  ≈ 60°. We also deduce that the in-plane epitaxial relationships are Sn: In2O3 [001] || [1010] Al2O3 and Sn: In2O3 [110] || [0001] Al2O3 with lattice mismatch of 5.24% and 10.1% respectively. Consequently the Sn: In2O3 NWs are preferentially oriented along the direction of smaller lattice mismatch. The Sn: In2O3 NWs on r-Al2O3 grow by a stacking of (110) planes at an 1

angle of  ≈ 45°. In this case the in-plane epitaxial relationships are Sn: In2O3 [001] ||2[4261] Al2O3 and Sn: In2O3 [110] ||

2

3[2201]

Al2O3 with lattice mismatches of 4.09% and 5.49 %

respectively which are comparable. Consequently the Sn: In2O3 NWs are oriented along two mutually perpendicular directions. The epitaxial relationships for the Sn: In2O3 NWs on m- and r-Al2O3 are very similar to those observed by Chern et al. 5 who deposited Sn: In2O3 epitaxial layers on m- and r-Al2O3. In contrast the Sn: In2O3 NWs on [1120] Al2O3 grow vertically along the [111] direction so they obey the epitaxial relation Sn: In2O3 (111)||(1120) Al2O3. 13 ACS Paragon Plus Environment


All of the Sn: In2O3 NWs grow by the VLS mechanism whereby Sn and In enter the Au catalyst particles and form liquid Au: Sn: In particles at elevated temperatures. Upon saturation solid Sn: In2O3 NWs forms beneath the liquid Au particles via the reaction with O2 at the triple phase junction as shown in Figure 11 leading to one dimensional growth. We did not obtain Sn: In2O3 NWs on m-, r- and a-Al2O3 with no Au. Instead we observed the deposition of a textured layer of Sn: In2O3 as shown in Figure 3(h) which grows by a vapor solid (VS) mechanism. This textured layer that grows by the VS mechanism directly on the m-, r- and a-Al2O3 between the Sn: In2O3 NWs is not an impediment for the realization of NWSCs as discussed in more detail later. Considering the above it is reasonable to suggest that the Sn: In2O3 NWs grow via the VLS mechanism but a sideways deposition must also occur via the VS mechanism which explains why we do not observe clearly the Au particles on the ends of the Sn: In2O3 NWs.

3.3. Electrical and Optical Properties of Sn: In2O3 NWs on m-, r- or a-Al2O3 The carrier density of the Sn: In2O3 NWs obtained at 800°C and 1 mbar on m-, r- and a-Al2O3 with In: Sn = 20 was measured by the Hall effect in the Van der Pauw geometry by forming  1 mm diameter Ag contacts at the corners of the 10 mm x 10 mm sample . The contacts showed excellent, linear and ohmic behavior due to the metallic-like conductivity of the Sn: In2O3 NWs so no annealing was carried out. In this case the current is expected to flow through the Sn: In2O3 layer that exists between the free-standing Sn: In2O3 NWs and the underlying Al2O3. However we expect that the current will also flow through the free-standing ordered network of Sn: In2O3 NWs as many of these are in contact with each other and the Sn: In2O3 NWs, which have lengths of many tens of m, are essentially buried in the Ag paint contact at the corners. For comparison we removed the Sn: In2O3 NWs from their parent substrate by dry-transfer onto 10 mm x 10 mm fused


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SiO2. This resulted into an interconnected, planar, network of Sn: In2O3 NWs which also exhibited metallic like conductivity and we obtained carrier densities between 1020-1021 cm-3 in very good agreement with Gao et al. 19 who also measured the carrier density of Sn: In2O3 NWs in the Van der Pauw geometry. We obtained a maximum carrier density of ~ 1021 cm-3 but did not observe a significant variation in the carrier density of the Sn: In2O3 NWs on the different m-, r- or a- orientations of Al2O3. The Sn: In2O3 NWs constitute in essence a highly degenerate n-type semiconductor with a large carrier density between 1020 and 1021 cm-3 that is related to the substitutional Sn donor impurities. The metallic like conductivity is of the order of 106 S/m similar to that obtained by Wan et al. 17, 18 who measured the resistivity of individual Sn: In2O3 NWs in a two and four point probe terminal configuration and found that the average resistivity was of the order of 10-4  cm. The Fermi level as such resides in the conduction band as discussed in more detail below. Consequently a reduction in temperature will not result into a significant change in carrier density similar to the case of metals. This has been observed in temperature dependent measurements of the carrier density and conductivity of both Sn: In2O3 NWs and ITO layers.39-42 As such the native defects in the Sn: In2O3 NWs are not expected to contribute strongly towards the overall conductivity. In order to gain a better understanding of the charge distribution in the Sn: In2O3 NWs we have calculated the conduction band (CB) potential profile along the radial direction but also the energetic position of the one dimensional sub-bands and one dimensional electron gas (1DEG) charge distribution via the self-consistent solution of the Poisson - Schrödinger (SCPS) equations in the effective mass approximation. In such a calculation one begins with the solution of Schrödinger’s equation taking a trial potential, subject to specific boundary conditions, which gives the sub-band energies and wave functions



that are normalized in order to obtain the one dimensional electron gas (1DEG) using the one dimensional density of states (1D DOS) in conjunction with Fermi Dirac statistics. Finally Poisson’s equation is solved to get a correction potential that is added onto the initial trial potential, and the process is repeated until convergence is reached, after which charge neutrality is evaluated for completeness. The SCPS calculations were carried out by taking into account the effective mass and dielectric constants of In2O3 i.e. me* = 0.35 mo and r = 9.3 43, 44. Note that the energy band gap and effective masses of cubic and rhombohedral In2O3 are very close to each other 45. In addition, we have taken into account the work function  = 4.5 eV and electron affinity  = 3.3 eV of In2O3 .46 Finally the Fermi level was taken to reside ≈ 0.4 eV above the CB edge in accordance with the theoretical and experimental investigation of King et al. 47 on bulk In2O3. The band profile and charge distribution of a 40 nm diameter Sn: In2O3 NW taking a uniform distributon of donors i.e. ND = 1 x 1019 cm-3 throughout is shown in Figure 12. The potential is inverted and one may observe a downward band bending towards the surface of the Sn: In2O3 . We find a total of eight sub-bands residing below the Fermi level; the lowest one at -0.26 eV and the shallowest at -0.015 eV or 15 meV below the Fermi level. The 1DEG charge distribution resulting from the occupancy of the lowest energy sub-bands is confined near the vicinity of the surface as shown in Figure 12. In contrast the shallower energy sub-bands are occupied by electrons with wave functions that have a maximum near the core which gives rise to a peak of the 1DEG charge distribution near r = 0. The number of sub-bands occupied by electrons in equilibrium is strongly dependent on the diameter of the Sn: In2O3 NW and is reduced to just two only when the radius is 2 nm. When the radius is greater than 100 nm we find that most of the charge resides near the surface similar to the charge distribution in bulk Sn: In2O3. The large carrier density and small


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resistivity which was found to be ≈ 10-4  cm gives a mobility of 10 cm2/Vs at 300K due to ionized impurity scattering. However before describing in detail the advantages of using these highly conductive and ordered networks of Sn: In2O3 NWs in NWSCs it is worthwhile pointing out that the Sn: In2O3 NWs exhibited room temperature PL as shown in Figure 13. It has been shown by Boer et al. 45 that In2O3 has energy band gaps of 3.2 ± 0.3, 3.1 ± 0.3, and 2.9 ± 0.3 eV corresponding to the Ia3, R3c, and Pbcn In2O3 polymorphs, respectively in excellent agreement with experiment. We suggest then that the PL emission at 400 nm ( 3.1 eV) is attributed to band to band radiative transitions while the PL at 550 nm ( 2.25 eV) is attributed to (a) radiative transitions via donorlike states that are energetically located in the upper half of the energy band-gap and which are related to crystal imperfections and/or (b) radiative recombination of photo excited core holes and conduction band electrons in the metal Au particles.48 The PL at 3.1 eV and 2.25 eV were observed at different points of the ordered network of Sn: In2O3 NWs but not that at 800 nm (1.55 eV). It should also be noted that we have not observed PL at 1.55 eV in the past from randomly oriented networks of Sn: In2O3 NWs grown using an identical thickness of Au catalyst 15. Similarly other like Gao et al. 19 have not observed near infra-red PL from Sn: In2O3 NWs. It is highly unlikely that the PL at 1.55 eV is related to Au particles as these will emit around 2.0 eV.48 In addition the near infra-red PL at 800 nm is not related to the laser excitation or surface plasmon resonance (SPR) effects related to the ordered network of Sn: In2O3 NWs for otherwise we would have observed it consistently at different positions.49 Instead it is more likely that the near IR emission at 800 nm (1.55 eV) is related to some kind of defect. Indium vacancies (VIn) give rise to acceptor like levels residing in the lower half of the energy bandgap as shown by Wang et al.50 and Reunchan et al.51 More specifically In vacancies in n-type In2O3 act as triple acceptors with low formation energy and in fact may reduce the n-type carrier density. In contrast oxygen vacancies 17 ACS Paragon Plus Environment


VO give rise to deep donor, mid gap states starting around 1.5 eV above the valence band edge.52 From the above it appears reasonable to suggest that the near infra-red emission at 800 nm (1.55 eV) is related to radiative transitions occurring between the conduction band and VO mid-gap states or alternatively between the VO mid gap states and the valence band. Despite this it is evident that the Sn: In2O3 NWs are capable of light emission and are promising for the realization of light emitting devices in addition to acting as TCOs in NWSCs.

3.4. Advantages of ordered Sn: In2O3 NW networks in NWSCs All of the epitaxial ordered Sn: In2O3 NWs that were grown on m-, r- and a-Al2O3 are attractive for the fabrication of high performance NWSCs due to their large surface area. We estimate that the ordered network of Sn: In2O3 TCO NWs grown on a 15 mm x 15 mm i.e. 225 mm2 r-Al2O3 has an effective area of 195 x 195 mm2 which is ~ 170 times larger than the area of a 15 x 15 mm2 film of ITO on glass. Consequently the junction area formed by the deposition of a barrier, absorber and p-type layer over the ordered network of Sn: In2O3 TCO NWs is expected to be considerably larger compared to a planar device which in turn is expected to lead to a significant increase in the short circuit current. In the past Sn: In2O3 NWs have been used mainly for the fabrication of dyesensitized solar cells but they were not grown in an epitaxial fashion and as such were not ordered 53, 54. To

the best of our knowledge no one has previously used ordered networks of Sn: In2O3 NWs

in NWSCs despite the fact that periodic photonic nanostructures have been shown to outperform their random counterparts in trapping light in solar cells 55. Although other metal oxide (MO) NWs such as ZnO NWs have been used for the fabrication of all-solid state NWSCs but they are not as highly conductive as the Sn: In2O3 NWs described here which in turn limits the short circuit current and performance of NWSCs. Short ZnO NWs with lengths of 300 nm have been grown by Jean et


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al. 56 on top of a 50 nm ZnO layer that was deposited on an ITO film on soda lime glass (SLG). The ZnO NWs were subsequently coated with PbS quantum dots (QDs) that filled the space in between leading to a planar structure that was completed by the deposition of MoO3 and a 100 nm thick metal contact of Au. 56 A similar structure employing graphene on quartz as opposed to ITO on SLG was fabricated by Park et al. 57. More recently ZnO NWs were grown on a compact layer of TiO2 on ITO/SLG after which a perovskite, hole transport layer (HTL) and metal contact of Au were deposited also resulting into a planar structure 58. In all of the above cases the ZnO NWs did not act as the TCO per se but were grown on a film of ITO on SLG in order to increase the surface area of the junction and promote light trapping. It is desirable then to use highly conductive, ordered networks of Sn: In2O3 NWs such as those described here for the fabrication of all-solid state NWSCs. We propose the architecture shown in Figure 14 which could consist of (i) TiO2 as a barrier (ii) CH3NH3PbI3 as an absorber and (iii) CuxZn1-xS as the p-type layer but one could envisage other materials too. The device must include separate metal contacts to the n-type Sn: In2O3 NWs and p-type layer. In order to fabricate the device shown in Figure 14 it would be necessary to deposit a layer of photoresist (PR) all over the ordered network of n-type Sn: In2O3 NWs and then use a photomask (PM) to expose with ultra violet light the PR over the right part of the network as shown in Figure 14(a) followed by development as depicted in Figure 14(b). Alternatively one could apply an acrylic polymer adhesive tape over the left part of the n-type Sn: In2O3 NW network instead of using PR which would allow the deposition of the barrier and absorber shells as shown in Figure 14(c). Subsequently a p-type layer would be deposited to fill in the voids between the Sn: In2O3 NWs and create a planar device that would allow also the deposition of a metal contact on top of the p-type layer as shown in Figure 14(d) and (e). Finally the PR would be lifted off in order to deposit a metal contact such as Au over the n-type Sn: In2O3.



We have found that it is possible to apply tape and remove Sn: In2O3 NWs from certain areas of the ordered network leaving a conductive layer on the Al2O3 that is connected with the free standing ordered network of Sn: In2O3 NWs. In other words it is possible to make a planar contact to the side of the three dimensional, ordered network of Sn: In2O3 NWs. In general the overall device structure has to be carefully prepared to avoid direct contact between the n-type Sn: In2O3 NWs and the metal contact deposited on top of the p-type layer through voids. This would inadvertently lead to shorting and the device would not be able to sustain a voltage upon exposure to light.


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 CONCLUSIONS

We have grown highly ordered Sn: In2O3 NWs by the VLS mechanism on m-, r- and a-Al2O3 between 800°C and 900°C using metallic Sn and In with In: Sn ≈ 20 under a flow of Ar and O2 at 1 mbar. All of the Sn: In2O3 NWs have a cubic crystal structure, they are tapered with lengths up to 80 m and are inclined at =60° on m-Al2O3. The Sn: In2O3 NWs on r-Al2O3 are inclined at = 45° along two mutually perpendicular directions while those on a-Al2O3 are vertical. The Sn: In2O3 NWs have a high carrier density of the order of 1021 cm-3 and resistivity of 10-4  cm but low mobility of 10 cm2/Vs at 300K due to the large density of donor impurities and ionized impurity scattering. We show that the charge distribution is confined in the vicinity of the surface due to the occupation of sub-bands that fall well below the Fermi level via the self-consistent solution of the Poisson-Schrödinger equations in the effective mass approximation. In addition the Sn: In2O3 NWs exhibited room temperature photoluminescence at 3.1 eV due to band-to-band radiative transitions and 2.25 eV due to donor like states residing energetically in the upper half of the energy band gap. These epitaxial ordered Sn: In2O3 NWs can be grown with excellent uniformity up to 15 mm x 15 mm on m- and r- Al2O3 which is important for the realization of high performance NWSCs.



 AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

ORCID Matthew Zervos:

0000-0002-6321-233X

Joseph Kioseoglou: 0000-0002-6933-2674 Maria Androulidaki:0000-0002-6772-8851 Eugeniu Vasile:

0000-0002-5868-1932

Author Contributions All authors have approved the final version of the manuscript. Notes The authors declare no competing interests. There is no funding to be reported.


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(47) King, P.D.C.; Veal, T.D.; Fuchs, F.; Wang, C.Y.; Payne, D.J.; Bourlange, A.; Zhang, H.; Bell, G.R.; Cimalla, V.; Ambacher, O. Egdell, R.G.; Bechstedt, F.; McConville, C.F. Band Gap, Electronic Structure, and Surface Electron Accumulation of Cubic and Rhombohedral In2O3. Phys. Rev. B 2009, 79, 205211. (48) Shahbazyan, T.V. Theory of Plasmon-Enhanced Metal Photoluminescence. Nano Lett., 2012, 13, 194-198. (49) Dapuzzo, F.; Esposito, M.; Cuscuna,̀ M.; Cannavale, A.; Gambino, S.; Lio, G.E.; De Luca, A.; Gigli, G.;Lupi S. Mid-Infrared Plasmonic Excitation in Indium Tin Oxide Microhole Arrays. ACS Photonics 2018, 5, 2431−2436. (50) Wang, D.; Du, J.; Li, X.; Xu, Y.; Li, Y. Photoluminescence properties of indium tin oxide films deposited on glass, Chinese Journal of Lasers 2011, 1, 38. (51) Reunchan, P.; Zhou, P.R.X.; Limpijumnong, S.; Janotti, A.; Van de Walle, C.G. Vacancy defects in indium oxide: An ab-initio study. Curr. Appl. Phys. 2011, 11 296-300. (52) Ghuman, K.K.; Hoch, L.B.; Szymanski, P.; Loh, J.Y.Y.; Kherani, N.P.; El-Sayed, M.A.;, Ozin, G.A.; Singh, C.V. Photoexcited Surface Frustrated Lewis Pairs for Heterogeneous Photocatalytic CO2 Reduction, J. Am. Chem. Soc. 2016, 138, 1206−1214. (53) Joanni, E.; Savu, R.; Goes, M.S.; Bueno, P.R.; Freitas, J.N.; Nogueira, A.F.; Longo, E.; Varela, J.A.; Dye-Sensitized Solar Cell Architecture Based on Indium-Tin Oxide Nanowires Coated with Titanium Dioxide. Scripta Mater. 2007, 57, 277-280. (54) Zervos, M.; Vasile, E.; Vasile, E.; Karageorgou, E.; Othonos, A.; Current Transport Properties of CuS/Sn: In2O3 versus CuS/SnO2 Nanowires and Negative Differential Resistance in Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C, 2016, 120, 11-20.



(55) Battaglia, C.; Hsu, C.M.; Soderstrom, K.; Escarre, J.; Haug, F.J.; Charriere, M.; Boccard, M.; Despeisse, M.; Alexander, D.T.L.; Cantoni, M.; Cui, Y.; Ballif, C. Light Trapping in Solar Cells: Can Periodic Beat Random? ACS Nano 2012, 6, 2790-2797. (56) Jean, J.; Chang, S.; Brown, P.R.; Cheng, J.J.; Rekemeyer, P.H.; Bawendi, M.G.; Gradecak, S.; Bulovic, V. ZnO Nanowire Arrays for Enhanced Photocurrent, in PbS Quantum Dot Solar Cells. Adv. Materials 2013, 25, 2790-2796. (57) Park, H.; Chang, S.; Jean, J.; Cheng, J.J.; Araujo, P.T.; Wang, M.; Bawendi, M.G.; Dresselhaus, M.S.; Bulovic, V.; Kong, J.; Gradecak, S. Graphere Cathode Based ZnO Nanowire Hybrid Solar Cells. Nano Lett. 2013, 13, 233-239. (58) Fan, J.; Jia, B.; Gu, M. Perovskite-Based Low-Cost and High-Efficiency Hybrid Halide Solar Cells. Photonics Res. 2014, 2, 111-120.


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 TOC GRAPHIC



Figure 1. (a), (b) and (c) SEM images of the Sn: In2O3 NWs obtained at 900°C on m-Al2O3 with In: Sn = 20 (d) side view of the Sn: In2O3 NWs which appear to be vertical (e) side view of the same Sn: In2O3 NWs showing that they actually grow at =60° after a rotation by =90° (f), (g) high magnification images of side views (h), (i) SEM images showing the square and hexagonal sections of Sn: In2O3 NWs after removing them from m-Al2O3 (j) SEM image of the Sn: In2O3 NWs showing that their surface is not smooth.


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Figure 2. (a), (b) SEM images of the Sn: In2O3 NWs obtained at 900°C on r-Al2O3 with In: Sn = 20 (c) side view of the Sn: In2O3 NWs showing that they grow at an angle of =45° (d) side view after rotation by =90° (e), (f) high magnification images of side views (g), (h) SEM images showing the square and hexagonal sections of the Sn: In2O3 NWs after removing them from the rAl2O3 (i) SEM image showing tapering and steps on the surface of the Sn: In2O3 NWs.



Figure 3. (a), (b) SEM images of the vertical Sn: In2O3 NWs obtained at 900°C on a-Al2O3 with In: Sn = 20 (c) side view showing that the Sn: In2O3 NWs grow at  ≈ 90° (d) side view after rotation of =90° (e) square section of the vertical Sn: In2O3 NWs after removing them from a-Al2O3 (f) longer, tilted Sn: In2O3 NWs (g) side view of longer tilted Sn: In2O3 NWs (h) representative image of a textured layer of Sn: In2O3 grown on a-Al2O3 with no Au. Similar morphology was obtained on m- and r-Al2O3.


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Figure 4. XRD of Sn: In2O3 NWs obtained on m-Al2O3; also shown the trace from plain m-Al2O3.



Figure 5. XRD of Sn: In2O3 NWs obtained on r-Al2O3; inset shows rocking curve around the (440) peak of the cubic bixbyite Sn: In2O3.


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Figure 6. XRD of Sn: In2O3 NWs obtained on a-Al2O3; inset shows the trace from a-Al2O3.



Figure 7 (a) TEM and (b) HRTEM images of a Sn: In2O3 NW after being removed from m-Al2O3 (c) EDX spectrum showing peaks belonging to In and Sn (d) two dimensional arrangement of Al and O on the oxygen terminated surface of m-Al2O3.


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Figure 8 (a) TEM image of Sn: In2O3 NW after removing it from r-Al2O3 (b) HRTEM showing the (440) crystallographic planes of Sn: In2O3 (c) HRTEM image showing that the lattice planes are not vertical to the sides of the Sn: In2O3 NW; note that the sides are not straight consistent with the SEM images (d) two dimensional arrangement of Al and O on the oxygen terminated surface of r-Al2O3.



Figure 9 (a) TEM image of a Sn: In2O3 NW after removing it from a-Al2O3 (b) HRTEM image of Sn: In2O3 NW showing the (222) crystallographic planes that grow on top of a-Al2O3 (c) EDX spectrum showing that the Sn is incorporated into the In2O3 NWs (d) two dimensional arrangement of Al and O on the oxygen terminated surface of a-Al2O3.


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Figure 10 The atomistic models of In2O3 and Al2O3 constructed by VESTA (a) m-Al2O3 (b) rAl2O3 and (c) a-Al2O3 (d) (110) In2O3 (e) (100) In2O3 (f) (111) In2O3. The small, red spheres represent O, while big mauve and blue spheres represent In and Al atoms respectively; the black line outline denotes the unit cell.



Figure 11. Schematic representation of the growth of Sn: In2O3 NWs via the VLS mechanism.


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Figure 12. SCPS CB edge potential profile relative to the Fermi level i.e. EC - EF (eV) and 1DEG charge distribution (x1019 cm-3) versus distance along the radial direction r for a 40 nm diameter Sn: In2O3 NW taking a doping level of ND = 1 x 1019 cm-3 at 300 K. The Fermi level resides above the CB edge at the surface i.e. EC - EF = - 0.4 eV



Figure 13. Room temperature PL of Sn: In2O3 NWs grown on m-Al2O3.


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Figure 14. Schematic outline of process for the fabrication of a NWSC using an order network of Sn: In2O3 NWs (a) deposition of photoresist (PR) and exposure through photomask (PM) (b) development of PR (c) sequential deposition of barrier and absorber shells (d) planarization via the deposition of a p-type layer (e) formation of a contact to the p-type layer (f) lift-off PR (g) formation of contact to the underlying n-type Sn: In2O3. The final device is depicted at the center where light enters from the back through the Al2O3 and is absorbed. The top contact to the p-type layer may also serve as a mirror to reflect back light not absorbed during its first passage through the device.



Figure 1. (a), (b) and (c) SEM images of the Sn: In2O3 NWs obtained at 900°C on m-Al2O3 with In: Sn = 20 (d) side view of the Sn: In2O3 NWs which appear to be vertical (e) side view of the same Sn: In2O3 NWs showing that they actually grow at an angle of φ=60° after a rotation by ω=90° (f), (g) high magnification images of side views (h), (i) SEM images showing the square and hexagonal sections of Sn: In2O3 NWs after removing them from m-Al2O3 (j) SEM image of the Sn: In2O3 NWs showing that their surface is not smooth. 265x200mm (150 x 150 DPI)

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Figure 2. (a), (b) SEM images of the Sn: In2O3 NWs obtained at 900°C on r-Al2O3 with In: Sn = 20 (c) side view of the Sn: In2O3 NWs showing that they grow at an angle of φ=45° (d) side view after rotation by ω=90° (e), (f) high magnification images of side views (g), (h) SEM images showing the square and hexagonal sections of the Sn: In2O3 NWs after removing them from the r-Al2O3 (i) SEM image showing tapering and steps on the surface of the Sn: In2O3 NWs. 266x201mm (150 x 150 DPI)



Figure 3. (a), (b) SEM images of the vertical Sn: In2O3 NWs obtained at 900°C on a-Al2O3 with In: Sn = 20 (c) side view showing that the Sn: In2O3 NWs grow at φ ≈ 90° (d) side view after rotation of ω=90° (e) square section of the vertical Sn: In2O3 NWs after removing them from a-Al2O3 (f) longer, tilted Sn: In2O3 NWs (g) side view of longer, tilted Sn: In2O3 (h) representative image of a textured layer of Sn: In2O3 grown on a-Al2O3 with no Au. Similar morphology was obtained on m- and r-Al2O3. 265x199mm (150 x 150 DPI)


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Figure 4. XRD of Sn: In2O3 NWs obtained on m-Al2O3 ; also shown the trace from plain m-Al2O3. 254x190mm (150 x 150 DPI)



Figure 5. XRD of Sn: In2O3 NWs obtained on r-Al2O3 ; inset shows rocking curve around the (440) peak of the cubic bixbyite Sn: In2O3. 254x191mm (150 x 150 DPI)


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Figure 6. XRD of Sn: In2O3 NWs obtained on a-Al2O3; inset shows the trace from a-Al2O3. 254x191mm (150 x 150 DPI)



Figure 7 (a) TEM and (b) HRTEM images of a Sn: In2O3 NW after being removed from m-Al2O3 (c) EDX spectra showing peaks belonging to In and Sn (d) two dimensional arrangement of Al and O on the oxygen terminated surface of m-Al2O3. 254x190mm (150 x 150 DPI)


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Figure 8 (a) TEM image of Sn: In2O3 NW after removing it from r-Al2O3 (b) HRTEM showing the (440) crystallographic planes of Sn: In2O3 (c) HRTEM image showing that the lattice planes are not vertical to the sides of the Sn: In2O3 NW ; note that the sides are not straight consistent with the SEM images (d) two dimensional arrangement of Al and O on the oxygen terminate surface of r-Al2O3. 164x125mm (220 x 220 DPI)



Figure 9 (a) TEM image of a Sn: In2O3 NW after removing it from a-Al2O3 (b) HRTEM image Sn: In2O3 NW showing the (222) crystallographic planes that grow on top of a-Al2O3 (c) EDX spectra showing that the Sn is incorporated into the In2O3 NWs (d) two dimensional arrangement of Al and O on the oxygen terminated surface of a-Al2O3. 164x123mm (220 x 220 DPI)


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Figure 10 The atomistic models of In2O3 and Al2O3 constructed by VESTA (a) m-Al2O3 (b) r-Al2O3 and (c) a-Al2O3 (d) (110) In2O3 (e) (100) In2O3 (f) (111) In2O3. The small, red spheres represent O, while big mauve and blue spheres represent In and Al atoms respectively; the black line outline denotes the unit cell. 165x120mm (220 x 220 DPI)



Figure 11. Schematic representation of the growth of Sn:In2O3 NWs via the VLS mechanism. 127x95mm (220 x 220 DPI)


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SCPS CB edge potential profile relative to the Fermi level i.e. EC - E F(eV) and 1DEG charge distribution (x10 19cm-3) versus distance along the radial direction r for a 40 nm diameter Sn: In O NW taking a doping 2 3

level of N D = 1 x 10 19cm-3 at 300 K. The Fermi level resides above the CB edge at the surface i.e. EC - E F = - 0.4 eV 254x190mm (150 x 150 DPI)



Figure 13. Room temperature PL of Sn: In2O3 NWs grown on m-Al2O3. 279x197mm (150 x 150 DPI)


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Figure 14. Schematic outline of process for the fabrication of a NWSC using an order network of Sn: In2O3 NWs (a) deposition of photoresist (PR) and exposure through photomask (PM) (b) development of PR (c) sequential deposition of barrier and absorber shells (d) planarization via the deposition of a p-type layer (e) formation of a contact to the p-type layer (f) lift-off PR (g) formation of contact to the underlying n-type Sn: In2O3. The final device is depicted at the center where light enters from the back through the Al2O3 and is absorbed. The top contact to the p-type layer may also serve as a mirror to reflect back light not absorbed during its first passage through the device. 254x190mm (150 x 150 DPI)



TOC Graphic 165x124mm (220 x 220 DPI)


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Epitaxially Oriented Sn:In2O3 Nanowires Grown by the VaporâLiquid

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