Reducing Thermal Carrier Spreading in InP Quantum Dot Lasers

Record low values in this material system of threshold current density, particularly at elevated temperature, are presented for InP quantum dot lasers. Lasers with Ga
_0.58
In
_0.42
P in the dot upper confining layer have the lowest threshold current densities, 138 A·cm
^-2
at 300 K, and 235 A·cm
^-2
at 350 K (77 °C) (2-mm lasers, uncoated facets). Gain-current density data suggests laser performance with an upper confining layer of Ga
_x
In
_1-x
P with x = 0.54, 0.56 or 0.58 would be similar if not for the very low internal optical mode loss, α
_i
of samples with x = 0.56 and 0.58. Gain measurements at fixed inversion level suggest that increasing x content in Ga
_x
In
_1-x
P increases gain at fixed inversion level but samples with x = 0.54 also exhibit reduced recombination current density. The increasing recombination current density at elevated temperature due to thermal carrier spreading is significantly reduced in samples with x = 0.56 and x = 0.58 but measurements at common operating points attribute this largely to the reduced α
_i
for these samples and given the same α
_I
, samples with x = 0.54, 0.56 and 0.58 would all benefit from reduced effects due to thermal carrier spreading compared to x = 0.52.