Electron tunneling spectroscopy has been used to study the phonon modes of the GaAs electrode and the AlGaAs barrier of single barrier GaAs/AlGaAs/GaAs heterostructures. The barriers were spiked doped with Si or Be to determine whether defects or impurities in the barrier have an effect on the measured line shapes. The phonon line shapes and intensities have been observed to change after shining light on the devices to photoionize defects in the barrier. The results demonstrate that the charge state of defects in a heterostructure barrier can affect the interaction between a tunneling electron and the phonon modes of a tunnel barrier.