Ultra-shallow junctions formed by quasi-epitaxial growth of boron and phosphorous-doped silicon films at 175 °C by rf-PECVD

In this paper, we use in-situ and ex-situ spectroscopic ellipsometry as an optical tool to optimize the process conditions that lead to epitaxial growth of undoped and doped silicon films in a standard radio-frequency Plasma Enhanced Chemical Vapor Deposition (rf-PECVD) reactor at temperatures below 200 °C. The influence of the plasma conditions (such as pressure, inter-electrode distance, hydrogen dilution and dopant precursor gas concentration) on the nature of the films is studied. This optimization allows us to achieve epitaxial growth of phosphorous-doped and boron-doped silicon films at temperatures as low as 175 °C and 140 °C, respectively. The epitaxial films possess thicknesses on the order of a few tens of nanometers, and sheet resistance values below 150 Ω/□. Annealing in air at 275 °C helps to further improve the conductivity of boron-doped layers. Four-point probe sheet resistance measurements and secondary ion mass spectrometry profiles are used to assess the dopant profile in the epitaxial layers. The junctions thus obtained are also characterized through their application in a solar cell, resulting in a fill factor over 76% and an efficiency exceeding 14%.