Hydrothermal deposition of zinc oxide nanostructures on silicon substrates under different conditions

Zinc oxide (ZnO) is a wide bandgap semiconductor promising for ultraviolet (UV) and blue range optoelectronics. Its major advantages is a possibility of formation of ZnO nanostructures by simple low-temperature methods of electrochemical and chemical deposition also in a form of nanostructures without lithography step. Unique devices based on ZnO nanostructures can be used for random laser. The random laser requires disordered scattering medium with specific properties containing semiconductor particles of high crystalline quality. Such medium can be created by hydrothermal deposition technique. That method is widely used for ZnO deposition, but key dependencies of ZnO nanocrystal properties on deposition process parameters are determined only for specific cases and cannot be used for development of random laser medium formation technology. The aim of this work is to study the correlation between ZnO hydrothermal deposition process parameters and ZnO nanostructures morphology and optical properties.

SEM images of ZnO nanostructures deposited on the Si substrate at different temperatures and constant deposition time of 120 min are presented in the Fig. 1. Size and density of ZnO nanocrystals depend on the solution temperature. Increasing the temperature at constant deposition time reduces the diameter and increases the lengthof crystals, due to increased rate of their formation. Time dependence of the thickness of ZnO layer and the average diameter of ZnO nanocrystals deposited at different temperature presented in Fig. 2. At 75 °Сaverage diameter reaches 800 nm. At 85 °С and 95 °С it isrespectively600 and500 nm. Increasing of density of ZnO nanocrystals associated with the activation of a larger number of nucleation centers with the increase of the thermal energy of the system.

Figure 1. SEM images of ZnO nanostructures deposited on the surface of Si substrate for 120 min at: 75 °C (a), 85 °C (b) and 95 °C (c).

ZnO nanocrystals formed at constant temperature (Fig. 3) retained the same diameter and density with varying formation time, but the length of individual crystals, and hence the thickness of ZnO layer is different. By increasing the deposition time from 90 to 210 min, the film thickness increased from 1.6 to 2.8 μm. For larger deposition time, agglomerates of ZnO nanocrystals appeared on the substrate surface. The thickness of agglomerates reaches 15-20 μm. The presence of agglomerates explains disproportional change in ZnO layer thickness over time. This tendency is also valid for lower deposition temperatures (Fig. 2).

Figure 2. Temperature and time dependencies of average diameter of ZnO nanocrystals (left) and thickness of deposited ZnO layer (right).

Figure 3. SEM images of cross-section Si substrate with ZnO nanostructures deposited at 95 °C for different deposition time: 90 min (a), 210 min (b).

Photoluminescence spectra of ZnO nanostructures fabricated at different temperatures during the same time are presented in Fig. 4. Distinct UV photoluminescence peak at 380 nm (i.e. 3.35 eV – ZnO bandgap energy) corresponding to the band-to-band recombination process is clearly seen. Broad peak of the lower intensity in the visible range at 580 nm associated with recombination via oxygen related defects of ZnO crystal lattice is also registred. The higher photoluminescence intensity of the samples obtained at higher temperatures can be explained by the larger quantity of deposited material on the Si surface. For longer deposition time photoluminescence intensity also increases for the same reason. For different deposition temperatures UV and visible photoluminescence band changes. The higherrelativeintensityofvisibleluminescence forthe samplefabricatedat95 °C suggestsa higher oxygen defects concentration in ZnO nanostructures.

Figure 4. Photoluminescence spectra of ZnO nanostructures deposited on Si substrate at different temperatures.

Temperature and time dependencies of structural and optical properties of ZnO nanostructures deposited on Si substrate by the hydrothermal method has allowed to determine optimal condition for random laser medium formation. High temperature and long deposition time from the experimentally studied ranges are preferable because of the increased density of ZnO crystals and their length. Thus one can get the longer optical path for the light passing through the structure and thereby the higher efficiency of ZnO based random lasers.