Fabrication Of Si-based Nano-patterned Structures And Its Applications In Light Emitting Device And Solar Cells

As the rapid development of Si planar technology, the integration is significantly promoted and the feature size of device becomes smaller and smaller. As a consequence, low-dimensional Si structures are attracting much attention nowadays. Low-dimensional silicon-based materials exhibit different and promising optical and electrical properties compared with their bulk counterpart, which can be potentially applied in realizing silicon-based light emitting source for opto-electronic integration and high efficiency solar cells. In order to further improve the efficiency of the devices and to cut the cost, many methods have been proposed to make the light management by introducing nanostructures into the devices to reduce the reflection loss and enhance the optical absorption. In the present work, we fabricate nano-patterned substrates by nano-sphere lithography technique and their surface morphology and optical properties are investigated. We study the influence of the nano-patterned substrates with various etching time on the photoluminescence and electroluminescence behaviors of Si QDs/SiO2 multilayers. We also investigate the performance of the amorphous Si/crystalline Si he tero-junction solar cell by using the nano-patterned substrates. The main content and the conclusions are as follows:1. In our work, we prepare the nano-patterned Si substrates with periodical nanocone array on the front surface by using nano-sphere lithography technique, in which, the silicon substrates are etched by RIE with a mask formed by polystyrene (PS) nano-sphere monolayer. The period and the depth of the cone could be controlled by the diameter of the nanosphere and the etching time, respectively. It is found that the nano-patterned substrates keep the periodical structures as revealed by AFM and SEM images. It is also found that the reflection of the nano-patterned Si substrates can be obviously suppressed compared with the flat substrate. More interestingly, the reflection is further reduced by increasing the etching time. For 16min etched nano-patterned substrate, the reflection can be reduced from 40% (flat one) to 5%.2. We prepare a-Si:H/SiO2 multilayers on the above-mentioned nano-patterned Si substrates in PECVD system by alternatively changing the amorphous Si deposition and in-situ plasma oxidation processes. After post-annealing treatment, Si QDs/SiO2 multilayers are achieved. The periodical nano-patterned structure is maintained on the surface of the multilayers due to the conformal deposition as indicated by the AFM observations. The layered structures are well kept and the formation of Si QDs after the annealing can be clearly identified by cross-sectional TEM images. The reflection of multilayer deposited on nano-patterned substrates is highly suppressed than the sample on flat substrate. Consequently, the light extraction efficiency and the absorption of the exciting laser are both improved which resulted in the improvement of the photoluminescence of the nano-patterned devices. The photoluminescence of multilayers on the 16min etched substrate is enhanced by more than one order of magnitude. The electroluminescence of the nano-patterned device is also improved consequently.3. We also apply the nano-patterned Si substrates into amorphous Si/crystalline Si hetero-junction solar cells aimed to improve the device performance. We deposit intrinsic and n-type a-Si on both nano-patterned and flat substrates in PECVD system. The periodical nano-patterned structure formed by conformal deposition can be shown in SEM images. After depositing electrode on the surface, we achieve test device and reference device on the nano-patterned substrate and flat one, respectively. It is found that the electrical properties of the test device are promoted, the short circuit current density is increased from 31.7 mA/cm2 to 37.2 mA/cm2, and the power conversion efficiency is promoted by 110%. It is proved that the introduction of the nano-structures effectively reduces the surface reflection and leads more light penetrate into the device, which results in the more light absorbed by the device to achieve more photo-excited carriers.