The experimental study of the valence band offset (Delta E-v) of a mixed As/Sb type-II staggered gap GaAs0.35Sb0.65/In0.7Ga0.3As heterostructure used as source/channel junction of n-channel tunnel field effect transistor (TFET) grown by molecular beam epitaxy was investigated by x-ray photoelectron spectroscopy (XPS). Cross-sectional transmission electron micrograph shows high crystalline quality at the source/channel heterointerface. XPS results demonstrate a Delta E-v of 0.39 +/- 0.05 eV at the GaAs0.35Sb0.65/In0.7Ga0.3As heterointerface. The conduction band offset was calculated to be similar to 0.49 eV using the band gap values of source and channel materials and the measured valence band offset. An effective tunneling barrier height of 0.21 eV was extracted, suggesting a great promise for designing a metamorphic mixed As/Sb type-II staggered gap TFET device structure for low-power logic applications. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4775606]