## Materials and Methods  
Two silicon micropillar samples were studied. P-type Cz (100) wafers with resistivity of 5-10 \(\Omega\)-cm were used for the experiments. The first was fabricated using RIE and the second using MACE. The procedure for fabricating RIE is as follows. P-type silicon wafer (11-22 \(\upmu\)m) was used to fabricate the vertical silicon micropillars using dry etching. The microdisk arrays were patterned in LOR3A and Shipley S1805 photoresist using photolithography. After the pattern was developed, aluminum was deposited to a thickness of 100 nm and lifted off to leave aluminum microdisk arrays on the starting Si wafer. A highly doped n-type conformal shell for the crystalline p-n junction was formed using the spin-on doping (SOD) method. After spinning an n-type doping source (P509, Filmtronics, Inc.) on a dummy Si wafer, we position the device wafer containing the micropillars so that it faces the dummy wafer coated with doping source, and then anneal the pair of wafers at high temperature (900-950 \({}^{\circ}\)C) for 10 min in a 20% O\({}_{2}\) and 80% N\({}_{2}\) environment. The process of the MACE fabrication is as follows. Au was used as the etch catalyst because it has been known to produce a solid and smooth surface compared with other catalysts [22]. The mesh patterns were generated by soft lithography with a liftoff process. The MACE solution to produce solid micropillar was a mixture of concentrated HF, H\({}_{2}\)O\({}_{2}\), and ethanol. The oxidizing agent generates free holes (h+) when catalyzed by the metal. The holes oxidize Si at the metal-Si interface and HF dissolves oxidized Si [23]. As a result the material directly underneath the metal is preferentially removed and the metal descends into the semiconductor.  
Figure 1 represents the RIE micropillars with a diameter of 2.2 \(\upmu\)m and a spacing of 1.75 \(\upmu\)m. Figure 2 represents the MACE micropillars with diameter of 2.1 \(\upmu\)m and spacing of 2 \(\upmu\)m. All samples were rinsed with deionized water and dipped in an HF solution (45% for 5 min) before any surface treatment. Surface treatment with KOH was performed by rinsing the sample with deionized water and dipping the samples in a KOH solution with concentration of 45% for 30 s. Surface treatment for Hydrofluoric Nitric Acetic (HNA) solution was performed by rinsing the sample with deionized water and dipping the sample in 50% HNA solution for 5 s. The samples were passivated with Al\({}_{2}\)O\({}_{3}\) by depositing 20 nm of the film using the atomic layer deposition approach and annealing the sample at 350 \({}^{\circ}\)C for 10 min. The Savannah ALD from Cambridge NanoTech was used to perform  
the deposition [24]. The system is a thermal deposition system that uses surface adsorption of single mono-layers of reactive precursor gases to form single atomic monolayers. The samples were passivated with SiN by depositing 60 nm of SiN using the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. The composition of silicon nitride is Si\({}_{3}\)N\({}_{4}\). Finally, a set of samples were treated with SiO\({}_{2}\) by depositing 100 nm of SiO\({}_{2}\) using the thermal oxidation technique.