In order to improve the crystal quality of GaN-based light
emitting devices, photoluminescence (PL) characterization of
below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum
well (QW) structures has been done by utilizing a below-gap excitation
(BGE) light in addition to an above-gap excitation light. The
decrease of the band-edge PL intensity due to the addition of the
BGE of 1.17 eV indicates the presence of an energy-matched below-gap
state in the two-wavelength excited PL. In continuation to
our previous efficiency improvement by applying modulation-doping
to GaAs/AlGaAs QW's, we focused on several undoped and Si-doped GaN/AlGaN
QW's. Experimental results showed that Si modulation-doping reduces
the density of below-gap states in the QW region, hence it is
promising for increasing internal quantum efficiency of GaN-based QW's.