It is well known that vertical GaN devices could surpass current lateral GaN switch technology due to higher critical electric fields and higher breakdown voltages from its different geometry, and lower impurity concentration from the superior quality of homoepitaxial films. However, the inconsistency of GaN substrate properties, both within wafer and vendor-to-vendor, makes reliable device fabrication difficult. Here we implement long-range spectroscopic studies of GaN substrates and epitaxial wafers using Raman, photoluminescence, and optical profilometry to assess incoming material and correlate to electrical performance of vertical diodes. We have classified incoming wafers into two general types, and determined that inhomogeneities in the wafers can negatively affect the reverse leakage current of PiN diodes.