Controlling the Structural Properties of Single Step, Dip Coated ZnO

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Controlling the Structural Properties of Single Step, Dip Coated ZnO Seed Layers for Growing Perfectly Aligned Nanowire Arrays Sophie Guillemin, Estelle Appert, Hervé Roussel, Béatrice Doisneau, Romain Parize, Thomas Boudou, Georges Bremond, and Vincent Consonni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06180 • Publication Date (Web): 14 Aug 2015 Downloaded from http://pubs.acs.org on August 30, 2015

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The Journal of Physical Chemistry

Controlling the Structural Properties of Single Step, Dip Coated ZnO Seed Layers for Growing Perfectly Aligned Nanowire Arrays

Sophie Guillemin,†,‡ Estelle Appert,† Hervé Roussel,† Béatrice Doisneau,† Romain Parize,† Thomas Boudou,† Georges Bremond,‡ and Vincent Consonni*,† †Université Grenoble Alpes, CNRS, LMGP, F-38000 Grenoble, France ‡Institut des Nanotechnologies de Lyon, Université de Lyon, UMR 5270 CNRS - INSA Lyon, 7 avenue Jean Capelle 69621 Villeurbanne, France. *Corresponding author: [email protected]

ABSTRACT Mastering the structural ordering of ZnO seed layers by sol-gel process in terms of ultrathin thickness (i.e, < 10 nm), strong c-axis texture, low-mosaicity, low-porosity and lowroughness is a critical challenge for the formation of well-ordered ZnO nanowires in solution. The effects of the solution concentration, of the withdrawal speed, and of the annealing process on the formation mechanisms of ZnO seed layers deposited by single dip process are revealed. The size and density of primary clusters in the sol are found to govern the evolution of the film thickness and nanoparticle average diameter through the solution concentration. The Landau-Levich theory modeling the dragging process accounts for the evolution of the 1 ACS Paragon Plus Environment


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film thickness only before annealing and over a reduced range of withdrawal speeds. The texture mechanisms along the c-axis are driven by particle/particle interactions during annealing and explained in the light of thermodynamic considerations. They are further determined locally by electron backscattered diffraction. Importantly, an alternative annealing process under argon flux is specifically developed for sol-gel process and is shown to form remarkably well-textured, compact ZnO seed layers with a very low mosaicity and porosity as well as a thin thickness as small as 10 nm. These ZnO seed layers lead to the growth of wellordered ZnO nanowires by chemical bath deposition with a remarkable mean tilt angle smaller than 6° as deduced by x-ray pole figures. These findings represent a significant step towards the more efficient integration of ZnO seed layers grown by sol-gel process into ZnO nanowire-based devices.

KEYWORDS ZnO, seed layers, sol-gel, dip coating, nanowires, chemical bath deposition and hydrothermal method.

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I. INTRODUCTION Zinc oxide is as an abundant, nontoxic material with a high exciton binding energy (60 meV), a wide band gap energy (3.3 eV at room temperature) and a high electron mobility.1 Its use has extensively been investigated, in the last decade, in the form of thin films and nanowires (NWs) for nanoscale-engineering devices in the fields of sensing, electronics, and optoelectronics.1-7 In all of these devices, the precise control of the structural uniformity of ZnO NWs in terms of diameter, length, period, vertical alignment and polarity is crucial for improving their overall performances. The development of optimized ZnO thin films as seed layers has thus received increasing interest for the nucleation and formation of ZnO NWs by chemical deposition techniques such as the low-cost, low-temperature, surface scalable chemical bath deposition (CBD) process.3 For both spontaneously and selective area grown ZnO NWs by CBD, their structural uniformity strongly depends on the morphological properties of the ZnO nucleation surface, which is typically ZnO single crystals or, more often, polycrystalline ZnO seed layers consisting of nanoparticles (NPs).8-13 Basically, it has been shown, in the latter case, that spontaneously grown ZnO NWs by CBD are homoepitaxially nucleated on top of ZnO NPs oriented along the polar c-axis.9 Correlatively, their density is directly related to the number of c-plane ZnO NPs when surface nucleation occurs, governing in turn their diameter and length according to the growth regime limited by the mass transport of chemical precursors in solution.9,14 Furthermore, their vertical alignment is also affected by the angle of the c-plane of ZnO NPs with the surface plane (i.e., mosaicity). The control of the structural properties of ZnO seed layers has thus become a critical challenge for the integration of ZnO NWs into complex heterostructures and nanoscale engineering-devices4-7,15-19 The development of ZnO seed layers with controlled morphological, electrical and optical properties has been considered by using a large number of growth techniques



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including physical vapor deposition (PVD), chemical vapor deposition (CVD), and solution deposition.1,3 Among them, PVD and CVD techniques form ZnO seed layers with controlled morphological properties, but typically at the expense of cost and easiness via the use of growth chamber under vacuum. Alternatively, sol-gel processes using dip or spin coating are particularly attractive as low–cost, low-temperature and easily implemented growth techniques. The influence of growth conditions such as the solution concentration, withdrawal speed or number of deposition has widely been reported on the morphological properties of ZnO thin films.20 It is well-known that mixing zinc acetate dihydrate (ZAD, Zn(CH3COO)2.2H2O) and monoethanolamine (MEA, C2H7NO) in absolute ethanol for instance can produce fairly well-textured ZnO thin films. This can further be enhanced by the use of standard annealing process such as backing 10 min at 300°C and then annealing at high temperature around 500°C (i.e., close to the highest temperature achievable on a hot-plate) for 1 h.13,20-58 However, the fundamental mechanisms responsible for the film formation and structural ordering are still not completely understood. The texture along the c-axis as a critical requirement has, for example, been found to increase,27 decrease23-25 or only vary21,22,26,28,29

with

the

solution

concentration.

Particle/substrate

interactions

or

particle/particle interactions have been suggested as playing an important role. Similarly, although widely supported by the experimental results (see for instance Ref. 59 and therein), the Landau-Levich theory has been unable to predict the thickness of ZnO thin films deposited by dip coating.33,60 Currently, the occurrence of controversial results can mainly be explained by the systematic use of multi-dip processes (i.e., multi-layers),20 leading to fairly thick ZnO thin films and, correlatively, to cross-studied parameters. Furthermore, standard annealing processes have been shown to be insufficient owing to the resulting high porosity and roughness of ZnO thin films for instance, limiting in turn their efficient integration as seed layers into ZnO NW-based devices.20-58 The use of high temperature annealing process


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has recently been applied to thick films deposited by dip coating through multi-dip processes to form strongly textured films.13,30,35,44-46,53 By combining multi-dip processes with a annealing temperature higher than 800°C, the formation of thick films consisting of ZnO NPs with an average diameter of almost 200 nm has been reported.30,35,45 In such processes, the film thickness or other parameters such as the solution concentration or the withdrawal speed may only affect slightly the ZnO film texture and its surface morphology.30,35,45 However, despite their high potential, the use of multi-deposit films is strongly detrimental for their integration into ZnO NW-based devices owing to their large thickness and, correlatively, high roughness. The large film thickness can also enhance the optical absorption of these multideposit films when used in nanostructured solar cells, which should be avoided as much as possible. One single layer is also very often required for the management of charge carriers in these devices to limit the number of interfaces and hence the density of nonradiative recombination centers. As a result, a precise list of specifications should carefully be considered for optimizing the morphological properties of ZnO seed layers such as i) their very thin thickness (i.e., typically smaller than 10 nm) combined with much larger NP diameters, ii) their texture along the c-axis and their low mosaicity as well as iii) their low porosity and roughness. This requires a deep investigation of their fundamental, formation and structural ordering mechanisms as well as the development of an alternative annealing process. It is the aim of this paper to deeply investigate the fundamental mechanisms responsible for the formation and structural ordering of ZnO thin films deposited by single dip process. The effects of the solution concentration and of the withdrawal speed on the morphological properties of ZnO thin films are thoroughly shown and discussed in the light of the LandauLevich theory modeling the dragging process. Also, an alternative annealing process is specifically developed in order to fabricate optimized ZnO seed layers allowing the growth of



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perfectly aligned ZnO NWs in accordance with the precise list of specifications required for their efficient integration into nanoscale engineering-devices.

II. EXPERIMENTAL The chemical precursor solution was prepared by dissolving an equal amount of zinc acetate dihydrate (ZnAc2·2H2O) from Merck and monoethanolamine from JT Baker in absolute ethanol. The solution was aged under continuous stirring for 24h, half of this time at 60°C, leading to a clear and homogenous solution. The seed layers were deposited by dip coating (using single dip process) under controlled atmosphere (hygrometry

Controlling the Structural Properties of Single Step, Dip Coated ZnO

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