  In this paper, we have successfully identified the triangular-shaped defect
structures with stacking fault tetrahedra. These structure often appeared in
hard-sphere (HS) crystals grown on a square pattern under gravity. We have, so
far, performed Monte Carlo simulations of the HS crystals under gravity. Single
stacking faults as observed previously in the HS crystals grown on a flat wall
were not seen in the case of square template. Instead, defect structures with
triangular appearance in $xz$- and $yz$- projections were appreciable. We have
identified them by looking layer by layer. Those structures are surrounded by
stacking faults along face-centered cubic (fcc) {111}. Also, we see isolated
vacancies and vacancy-interstitial pairs, and we have found octahedral
structures surrounded by stacking faults along fcc {111}.
