In this paper, we demonstrate why cubic Rashba spin splitting is observed within inverted doped strained germanium (sGe) hetrostructures. Magnetotransport measurements showed beating within the SdH oscillation, with fast Fourier analysis revealing cubic Rashba spin splitting to be present. A cubic Rashba coefficient of β=7.97×10−29 eVm3β=7.97×10−29 eVm3 and a spin-splitting energy of Δ=1.17 meVΔ=1.17 meV were determined. The source of the cubic Rashba spin splitting was identified from a combination of ultra low energy secondary ion mass spectrometry analysis and subsequent band structure modelling using Nextnano3. Ultra-low eneIn this paper we demonstrate an origin for cubic Rashba spin splitting observed within inverted doped strained germanium (sGe) hetrostructures. Magnetotransport measurements showed beating within the SdH data, with ensuing Fast Fourier analysis revealing cubic Rashba spin splitting to be present. A spin orbit interaction value of and spin splitting energy were determined. The source of the cubic Rashba spin splitting was identified from a combination of ultra low energy secondary ion mass spectrometry analysis and subsequent band structure modelling using Nextnao3. Ultra low energy secondary ion mass spectrometry revealed an unintentional, highly B doped near surface region to be present. By incorporating this information into the Nextnano3 modelling, two single subband triangular QWs were predicted, one at the upper and the other at the lower interface of the sGe QW. Moreover, these triangular wells are expected to be asymmetric due to the difference in B doping levels and spacer layer thicknesses, and it is this asymmetry which induces the cubic Rashba spin splitting observed.