Nanocavities in Si formed by He ion implantation anneal are of interest for impurity gettering in Si technology, localized lifetime control in power devices, and in layer splitting techniques used in water bonding. We have found that sequential thermal anneal is essential to obtain multiple cavity layers with cascade He implants (40-160 keV, 2×1015-4×1016 cm-2). This behavior is related to the vacancy generation process necessary for cavity formation. Transmission electron microscopy data reveal that, under isothermal anneal, the cavity shape changes from a distinct, aligned hexagonal geometry to a rounded spheroidal shape with increasing anneal time. Photoluminescence (PL) spectra at 77 K reveal a peak at 0.8 eV for all the He-implanted and annealed samples, attributable to band bending around the cavity interfaces. Deep level transient spectroscopy (DLTS) measurements of the cavity region show broad minority carrier (electron in p-type Si) peaks indicative of the presence of defect clusters. Unusual capacitance-temperature (C-T) characteristics with steps and hysterisis are also seen, reflecting metastable behavior arising from change in structural configuration of the cavity defects.