Three isolation oxide structures have been prepared to study their resistance to the undercut formation during selective epitaxial growth (SEG) processes. The N2O annealed oxide tetraethyoxysilane (TEOS) oxide stacked structure has the best resistance to the undercut formation and exhibits the best electrical characteristics compared to the other two isolation oxide structures prepared in the study, which are a wet oxide and a TEOS oxide. This is ascribed to a smaller interfacial stress between the isolation oxide and the silicon substrate for the stacked structure. The sidewall damage is the predominant factor deteriorating the current-voltage (I-V) characteristics of the N+-P SEG diodes. When treated with a quick HF dip and followed by a low temperature desorption cleaning before the SEG process, the N+-P SEG diode, which has no perceivable undercut, shows satisfactory I-V characteristics.