Niedertemperatur-Wachstum von ZnO-Nanosäulen für optoelektronische Anwendungen (English shop)

Preview

Table of Contents, Datei (49 KB)
Extract, Datei (120 KB)

ZnO nanocolumns as a basis for miniaturised devices play an important role in the field of forward-looking nanotechnologies. In recent years, vapour-phase epitaxial growth methods in particular have become established in the area of nanocolumn growth. However, these are associated with high growth temperatures and considerable equipment requirements. As an alternative to these methods, wet-chemical and electrochemical fabrication approaches were developed within the scope of this work. These approaches are of particular interest because of their low growth temperatures of at most 90 °C and the low equipment requirements, which make these methods especially cost-effective. To date, the literature lacks systematic investigations of the wet-chemical growth process, which are indispensable for controlled fabrication and for the further establishment of the method, particularly with regard to device applications. The aim of the present work was therefore to close this gap and to gain a deeper understanding of wet-chemical growth.

ZnO nanocolumns were synthesised in a specially designed setup in an aqueous solution of zinc nitrate (Zn(NO3)2) and HMT (C6H12N4). Initial experiments on Si showed that surface activation is necessary for homogeneous growth of ZnO nanocolumn ensembles. A layer of ZnO nanocrystals served as a seed layer. This made it possible to reproducibly produce homogeneous and dense ensembles across the entire wafer. On the basis of transmission electron microscopy (TEM) investigations at the substrate/nanocolumn interface, it was possible to develop a clear model for the growth mechanisms.

As a further option, metal films were successfully employed for surface activation. Silver layers in particular led to well-aligned ZnO nanocolumn ensembles. Growth on glass substrates pre-structured with silver exhibited a high selectivity for a suitable choice of growth parameters. This was characterised by dense growth on the silver surface and no deposition on the glass surface. It was thus possible to fabricate structured nanocolumn ensembles without subsequent elaborate photolithographic structuring steps.

For a fundamental understanding of the wet-chemical growth method, it was important to determine the influence of the growth parameters on the morphology and on the crystalline and optical properties of the samples. Reactant concentrations between 0.01 and 0.1 mol/l, growth times of up to 8 h, the number of growth cycles, growth temperatures between 60 and 90 °C and different HMT/zinc nitrate ratios were used and their influence on growth was systematically investigated. On the basis of the results obtained, ZnO nanocolumns with diameters between 20 and 300 nm and lengths from 0.4 to 6 µm could be fabricated in a controlled manner under the stated conditions. Higher concentrations led to larger diameters, whereas the length was determined primarily by the growth duration.

Despite the low growth temperatures, the nanocolumns possess good crystalline and optical quality. TEM and X-ray diffraction (XRD) investigations show that the ZnO nanocolumns are single-crystalline and dislocation-free. Photoluminescence (PL) spectra at room temperature show the UV emission at 3.3 eV typical of ZnO (linewidth ~ 120 meV) and, in addition, a broad band in the visible spectral range with a maximum at 2.1 eV (orange luminescence), which is attributed to point defects in the crystal. The occurrence of phonon replicas in low-temperature spectra is evidence of the good optical quality of the samples. The broad UV emission results from a high donor concentration, which is also present in the nominally undoped crystal. This is likewise confirmed by electrical measurements. Investigations on individual nanocolumns show low resistivities of 4–5 Ωcm, which also indicate a high charge carrier concentration.

The low growth temperatures and the use of only slightly reactive chemicals enabled growth on a wide variety of substrate materials. Thus, in addition to the growth of ZnO nanocolumn ensembles on (100)Si, successful deposition on Al2O3, SiC, ITO (In-doped Sn2O) coated glass, silicone and PEN foil (polymer foil) could also be demonstrated. No significant differences in the growth morphology and in the optical properties of the samples were observed. It was also remarkable that the growth of ZnO nanocolumns on three-dimensional surfaces was possible. Thus, in addition to glass fibres, textured silicon surfaces were also provided with dense ZnO nanocolumns. Such coatings are very difficult or even impossible to realise with conventional vapour-phase epitaxial growth methods and open up new application possibilities for ZnO nanostructures in the field of functional coatings.

Ideally, one would like to deposit nanostructures directly between metal contacts. This makes it possible to completely avoid one of the main problems of nanotechnology, the subsequent contacting of individual nanostructures. For this purpose, metal conductor tracks were fabricated on Si/SiO2 in finger structures. ZnO nanocolumns were then grown between adjacent conductor tracks by means of electrodeposition. The nanocolumns produced possessed structural and optical properties similarly good to those of the wet-chemically deposited structures. In the samples fabricated by electrodeposition, a clear influence of UV illumination and ambient atmosphere on the I–V characteristics was observed. This behaviour forms the basis for possible applications in sensor technology.

After the wet-chemical growth of ZnO nanocolumns had been successfully demonstrated, the fabricated structures were investigated with regard to their applicability in dye-sensitised solar cells (DSSC). DSSCs based on ZnO nanocolumns on ITO-coated glass, the dye Ru 535 bis-TBA and the solid hole conductor copper thiocyanate (CuSCN) were developed. It was found that Zn2+–Ru complexes form during the dye coating. Owing to the acidic environment caused by the deprotonation of Ru 535 bis-TBA, ZnO dissociates into Zn2+ ions, which can form the aforementioned complexes with the deprotonated dye. This mechanism limits electron injection into the ZnO nanocolumns. Furthermore, it was shown that coating with CuSCN is not readily crack-free and reproducible. The ZnO/dye/CuSCN solar cells fabricated thus achieved an open-circuit voltage of 0.41 V, a responsivity of 1.4·10-2 A/W, a fill factor of 47 % and an efficiency of 0.3 %. However, by optimising the dyes with regard to adaptation to ZnO and by further improving the CuSCN coating, an increase in efficiency can be expected.