IMPLEMENTATION OF STRONG LIGHT-MATTER INTERACTION FOR FABRICATION AND LIGHT MANAGEMENT OF THIN CRYSTAL SILICON SOLAR CELLS

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2021-7-29
Zolfaghari Borra, Mona
The integration of photonic components with electrical elements on the same silicon chip may lead to the development of new technologies. One limitation is the space available on the wafer surface, which is restricted. Currently, conventional fabrication techniques produce devices only on the top thin layer of the wafer surface. As a result, new architectural designs are required. Producing functional components deep inside Si without creating damage to the surfaces is a potential technique for overcoming the space constraint in electronic-photonic integration since the bulk of the wafer can be used with this method. In different transparent materials, such as glasses and polymers, laser-written devices have been shown. High-intensity laser pulses may cause a nonlinear breakdown when focused and can alter the morphology of the material's interaction area. This method is capable of fabricating a broad range of devices, including interconnects, optical waveguides, and quantum photonic devices. However, similar techniques have not been successful in Si. We developed a similar enabling technique inside Si by using nonlinear phenomena to create very controlled modifications deep inside Si.Recently-demonstrated direct laser modification of high-quality three-dimensional (3D) subsurface inside of crystalline silicon (c-Si) wafers opens the doors to a wide range of novel applications in multidisciplinary research areas. A specially developed selective wet chemical etching step is usually required to follow subsurface processing of c-Si by laser in order to reveal the desired 3D structures. Proper development of such a selective etchant that leaves remaining unprocessed Si surfaces with smooth features is critical for subsequent applications at micro- and nanoscales. Achieving practically useful etch rate, etch selectivity, and final surface morphology of subsurface laser processed Si is essential and yet highly complicated due to intricate interdependence of these aspects through the etchant composition. Moreover, the well-accepted definitions of etch rate and selectivity in semiconductor micro-processing are univocally valid for surface etch in 2D in literature. As laser processing of crystalline semiconductors is an emerging technique, there appears to be a need for redefinition of both etch rate and selectivity since the etch takes place subsurface in 3D where etch thickness/time is not an applicable measure. Here, we report on development of a novel chromium-free (Cr-free) chemical etching recipe based on copper nitrate which yields substantially smooth surfaces at a high etch rate and selectivity of the both surface and subsurface laser processed Si without inducing significant etching of unmodified Si. The results show that etch rate and surface morphology are interrelated and highly affected by the composition of the adopted etching solution. After an extensive compositional study performed at room temperature, we achieved to develop such an etchant with a composition of HF:HNO3:CH3COOH:H2O – 56:65:72:207 (vol%), and 1 g of Cu(NO3)2.3H2O in 100 ml solution. This champion etchant exhibits more than 1600 selectivity for laser modified Si with respect to unmodified Si. Etch pit size distribution of surface defect analysis shows pit sizes to reside in the range of 1 – 10 µm indicating relatively smooth and low defective surface. In order to demonstrate the application potential of the etchant, c-Si solar cells based on thin absorber slices produced with a minimum loss of Si material. Additionally, we etched the laser processed parts of a c-Si to reveal high aspect ratio micro holes all the way through a wafer of 250 µm thickness, which has the potential to be utilized in the fabrication and development of microfluidic devices and photonic devices on Si, and also fabricated micro pillars with various sizes/depth to reach the minimum reflection which is crucial for solar cells. This work is the first to propose and use new definitions of etch rate and selectivity in etching of 3D structures.

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Citation Formats
M. Zolfaghari Borra, “IMPLEMENTATION OF STRONG LIGHT-MATTER INTERACTION FOR FABRICATION AND LIGHT MANAGEMENT OF THIN CRYSTAL SILICON SOLAR CELLS,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.