  GeSn lasers enable monolithic integration of lasers on the Si platform using
all-group-IV direct-bandgap materials. Although optically pumped GeSn lasers
have made significant progress, the study of the electrically injected lasers
has just begun only recently. In this work, we present explorative
investigations of electrically injected GeSn heterostructure lasers with
various layer thicknesses and material compositions. The cap layer total
thickness was varied between 240 and 100 nm. At 10 K, a 240-nm-SiGeSn capped
device had a threshold current density Jth = 0.6 kA/cm2 compared to Jth = 1.4
kA/cm2 of a device with 100-nm-SiGeSn cap due to an improved modal overlap with
the GeSn gain region. Both devices had a maximum operating temperature Tmax =
100 K. Device with cap layers of Si0.03Ge0.89Sn0.08 and Ge0.95Sn0.05,
respectively, were also compared. Due to less effective carrier (electron)
confinement, the device with a 240-nm-GeSn cap had a higher threshold Jth = 2.4
kA/cm2 and lower maximum operating temperature Tmax = 90 K, compared to those
of the 240-nm-SiGeSn capped device with Jth = 0.6 kA/cm2 and Tmax = 100 K. In
the study of the active region material, the device with Ge0.85Sn0.15 active
region had a 2.3 times higher Jth and 10 K lower Tmax, compared to the device
with Ge0.89Sn0.11 in its active region. This is likely due to higher defect
density in Ge0.85Sn0.15 rather than an intrinsic issue. The longest lasing
wavelength was measured as 2682 nm at 90 K. The investigations provide guidance
to the future structure design of GeSn laser diodes to further improve the
performance.
