  A key component for optical on-chip communication is an efficient light
source. However, to enable low energy per bit communication and local
integration with Si CMOS, devices need to be further scaled down. In this work,
we fabricate micro- and nanolasers of different shapes in InP by direct wafer
bonding on Si. Metal-clad cavities have been proposed as means to scale
dimensions beyond the diffraction limit of light by exploiting hybrid
photonic-plasmonic modes. Here, we explore the size scalability of
whispering-gallery mode light sources by cladding the sidewalls of the device
with Au. The metal clad cavities demonstrate room temperature lasing upon
optical excitation for Au-clad devices with InP diameters down to 300 nm, while
the purely photonic counterparts show lasing only down to 500 nm. Numerical
thermal simulations support the experimental findings and confirm an improved
heat-sinking capability of the Au-clad devices, suggesting a reduction in
device temperature of 473 K for the metal-clad InP nanodisk laser, compared to
the one without Au. This would provide substantial performance benefits even in
the absence of a hybrid photonic-plasmonic mode. These results give us insight
into the benefits of metal-clad designs to downscale integrated lasers on Si.
