The Effect of a Sputtered Al-Doped ZnO Seed Layer on the Morphological, Structural and Optical Properties of Electrochemically Grown ZnO Nanorod Arrays

Abstract

The effect of both a RF sputtered Al-doped ZnO (AZO) thin film seed layer onto a FTO/glass substrate and its growth time onto the morphological, structural and optical properties of the resulting electrochemically grown ZnO nanorod arrays (NRAs) have been studied. ZnO NRs grown onto the different AZO seed layers exhibit smaller mean diameter and length than those grown onto a bare FTO/glass substrate, but ZnO NR density presents an opposite behavior, by using an AZO seed layer ZnO nanorod density can be increased by a factor of six. ZnO nanorods are highly crystalline with a wurtzite hexagonal structure and with a preferential growth perpendicular to the substrate. The c-axis of most of the ZnO NRs grown onto an AZO seed layer is aligned within ±6 • from the substrate surface normal. Both NRAs mean length and density increases light scattering, without greatly affecting the spectra shape. The diffuse reflectance intensity is more sensitive to NR density variations than to length or diameter variations. NR diameter affects directly the shape of these diffuse reflectance spectra: they red-shifts and broadens when NR mean diameter increases. A small influence in the UV edge due to size quantization may be also present. In recent years, ZnO, a wide bandgap semiconductor with a direct bandgap of about 3.37 eV and high exciton binding energy (60 meV) at room temperature, has attracted increasing interest due to its unique ability to form a variety of nanostructures such as nanowires, nanorods, nanobelts, nanocombs, nanospheres, nano-tetrapods.1,2 Among them, the most interesting are nanorods and nanorod arrays (NRAs) vertically arranged with respect to the substrate.1 These ZnO nanostructures present a pseudo-one dimensional (1D) structure, with an enhanced surface-to-volume ratio and confinement effects.3 ZnO nanorods exhibit fewer defects than its thin-film structure, it is, therefore, a promising material for optoelectronic applications. 4 In fact, recently, single crystal ZnO nanowire and nanorod arrays have emerged as promising building blocks for a new generation of devices in different hi-tech domains such as optoelectronics, gas sensing, field emission, piezoelectrics and solar cells. 2,5,6 In particular, ZnO one dimensional nanostructures are good candidates for photovoltaic applications for three straightforward reasons: i) they have a low reflectivity that enhances the light absorption; ii) relatively high surface to volume ratio that enables interfacial charge separation and iii) fast electron transport along the crystalline 1D nanostructures that improves the charge collection efficiency. In fact, ZnO arrays of 1D nanostructures, such as nanowires and nanotubes, have been widely utilized as they provide a direct conduction pathway for the rapid collection of the photogenerated electrons, 7 reducing the non-radiative recombination and carrier scattering loss dramatically, 8 and providing as well a high junction area.9,10 Moreover, electron transport in the crystalline nanorod is expected to be several orde...

Characteristics of Carrier Transport and Crystallographic Orientation Distribution of Transparent Conductive Al-Doped ZnO Polycrystalline Films Deposited by Radio-Frequency, Direct-Current, and Radio-Frequency-Superimposed Direct-Current Magnetron Sputtering

“…The depth profiles of Al concentration in AZO thin films were determined by SIMS measurements. The textures of the films were characterized by measurements of wide-range out-of-plane reciprocal space maps [ 14 , 16 , 17 , 44 ] and pole figures [ 14 , 16 , 17 , 45 , 46 , 47 , 48 ] using the SmartLab XRD system (Rigaku Corp, Tokyo, Japan) equipped with a PILATUS 100 K/R two-dimensional X-ray detector using Cu-K radiation (wavelength λ = 0.15418 nm; weighted average of Cu-Kα 1 λ = 0.154059 nm/Cu-Kα 2 λ = 0.15444 nm with an intensity ratio of 2:1). The width of the X-ray beam on the samples is 10 mm during XRD measurements.…”

Efficiency Improvements in Solution‐Based CuInS2 Solar Cells Incorporating a Cl‐Doped ZnO Nanopillars Array

“…The SP‐ZnO transmittance spectra also shows the typical sharp direct‐like absorption edge of ZnO thin films located at 380 nm . The SP‐ZnO/NP‐ZnO spectrum (dot‐dashed curve) also presents the ZnO absorption edge previously described and a smooth monotonous increment for longer wavelengths caused in the light scattering properties of the sample . The transmittance of the buffer layer (dotted curve) shows an absorption edge located at 490 nm.…”

“…Moreover, both samples with NP (SC2 and SC3) have a greater IPCE in the visible region. This may be due to the light scattering capability of the NP . The IPCE integrated short circuit current density was calculated.…”

Morphological engineering of $$\hbox {SnO}_{2}$$ and $$\hbox {In}_{2}\hbox {O}_{3}$$ films deposited by spray pyrolysis

“…These seed-layers could play the role of growth centres during deposition of metal oxides and thus can affect the structural properties of these films [7]. Later it was shown that this approach can be used in the formation of various metal oxide films by the methods of sol-gel technology, electrochemical deposition, aerosol-assisted CVD, chemical bath deposition and so on [8][9][10][11][12]. It was shown that the pre-coating of glass and silicon substrates by seed-layers affected texturing and crystallinity and was crucial for the formation of metal oxides with controlled properties.…”

“…To demonstrate the possibilities of the proposed approach, we use SnO 2 and In 2 O 3 , which relate to materials promising for a wide range of applications from catalysis and displays to gas sensors [16]. Previously, studies in this area were mainly devoted to ZnO [8,9,12].…”