Design and growth characteristics of broadly tunable hetero-cascading quantum cascade lasers are discussed. We show the influence of electric field variation, and interface roughness on the gain spectra. Interface roughness is shown to play an important role for these broadband lasers. External cavity configuration has been used to extract narrow-band emission, thus rendering wide tuning range of the device. We achieve a single-chip tuning range of 400 cm(-1) (from 1030 cm(-1) to 1430 cm(-1), namely 7.0-9.7 mu m in wavelength) by using tri-stack active regions.