Si columnar structures were fabricated using Si MBE on Si substrates with column sizes in the order of ∼ 100 Å. The objective is to explore a viable approach to fabricate quantum wire structures. The growth of the structures, which was due to the growth instability, was an excellent example of a self-limiting process. The dependence of column morphology on the critical parameters, e.g., Si molecular beam incident angle, substrate temperature, substrate rotation, speed, etc., were demonstrated. Comparison between the experimental and the computer simulation results demonstrated the importance of the latent heat related atom migration as compared to the normal surface diffusion at low substrate temperatures and several A/s beam fluxes. A substrate temperature window (≈125°C) was observed which allowed the fabrication of crystalline micro-columns on Si (100) substrates. RHEED studies indicated that the crystalline micro- columns were heavily twined. The twinning phenomenon was also observed in the computer simulation results and interpreted as a result of the reduction in twin formation energy due to the extremely small dimension of the columns. Thermal stability of the columnar structures is discussed. Finally, photoluminescence studies and some potential applications are also discussed.