  We analyse HST WFC3/$H_{160}$-band observations of a sample of 48
ALMA-detected submillimeter galaxies (SMGs) in the Extended Chandra Deep Field
South field, to study their stellar morphologies and sizes. We detect
79$\pm$17% of the SMGs in the $H_{160}$-band imaging with a median sensitivity
of 27.8 mag, and most (80%) of the non-detections are SMGs with 870$\mu$m
fluxes of $S_{870} < $3 mJy. With a surface brightness limit of $\mu_H \sim $26
mag arcsec$^{-2}$, we find that 82$\pm$9% of the $H_{160}$-band detected SMGs
at $z =$ 1-3 appear to have disturbed morphologies, meaning they are visually
classified as either irregulars or interacting systems, or both. By determining
a S\'ersic fit to the $H_{160}$ surface-brightness profiles we derive a median
S\'ersic index of $n = $1.2$\pm$0.3 and a median half-light radius of $r_e =
$4.4$^{+1.1}_{-0.5}$ kpc for our SMGs at $z = $1-3. We also find significant
displacements between the positions of the $H_{160}$-component and 870$\mu$m
emission in these systems, suggesting that the dusty star-burst regions and
less-obscured stellar distribution are not co-located. We find significant
differences in the sizes and the S\'ersic index between our $z = $2-3 SMGs and
$z \sim $2 quiescent galaxies, suggesting a major transformation of the stellar
light profile is needed in the quenching processes if SMGs are progenitors of
the red-and-dead $z\sim$2 galaxies. Given the short-lived nature of SMGs, we
postulate that the majority of the $z = $2-3 SMGs with $S_{870} \gtrsim $2 mJy
are early/mid-stage major mergers.
