Here, we explore experimentally and theoretically the possibility to prolong the upper hole state nonradiative lifetime of Si/SiGe quantum cascade (QC) structures by using a spatially indirect diagonal transition between two SiGe quantum well ground states. With the recent observation of well resolved midinfrared electroluminescence from heavy hole intersubband transitions in Si/SiGe valence-band QC structures, a Si-based QC laser seems no longer to be out of reach. A long carrier lifetime and maybe population inversion, however, appear to be impossible for structure designs with a vertical intersubband transition studied so far. This is due to the nonresonant behavior of deformation potential scattering dominant in unipolar SiGe. We report on calculations of the band structure using a six-band k.p model and of hole deformation potential scattering that predict significantly increased nonradiative lifetimes for large barrier thickness, reaching about 20 ps for 35 A Si barrier layer width. Electroluminesence measurements of a series of QC structures with varied barrier width reveal comparable efficiencies and the deduced lifetimes confirm our model calculations. (C) 2003 American Institute of Physics.