P/n and n/p junctions with depths of 200 nm to several micrometers have been created in flat silicon substrates as well as on 3D microstructures by means of a variety of methods, including solid source dotation (SSD), low-pressure chemical vapor deposition (LPCVD), atmospheric pressure chemical vapor deposition, and plasma-enhanced chemical vapor deposition. Radial junctions in Si micropillars are inspected by optical and scanning electron microscopies, using a CrO3-based staining solution, which enables visualization of the junction depth. When applying identical-doping parameters to flat substrates, ball grooving, followed by staining and optical microscopy, yields similar junction depth values as high-resolution scanning electron microscopy imaging on stained cross-sections and secondary ion mass spectrometry depth profilometry. For the investigated 3D microstructures, doping based on SSD and LPCVD give uniform and conformal junctions. Junctions made with SSD-boron doping and CVD-phosphorus doping could be accurately predicted with a model based on Fick's diffusion law. 3D-microstructured silicon pillar arrays show an increased efficiency for sunlight capturing. The functionality of micropillar arrays with radial junctions is evidenced by improved short-circuit current densities and photovoltaic efficiencies compared with flat surfaces, for both n- and p-type wafers (average pillar arrays efficiencies of 9.4% and 11%, respectively, compared with 8.3% and 6.4% for the flat samples).
- Deep reactive-ion etching (DRIE)
- P/n junctions
- Silicon microwires
- Solar cells