Fixed oxide charges in atomic layer deposited (ALD) Al2O3 on hydrogen-terminated Si were observed by ultra-high vacuum (UHV) non-contact AFM with a conductive tip. Comparison of bias-dependent images with a spherical tip model showed the importance of the tip image in the substrate for a quantitative understanding of the image contrast. The Contact Potential Difference and differential capacitance were probed by means of bias modulation and lock-in detection at the 1st and 2nd harmonic, respectively. A difference in topography-capacitance cross-correlation was found between Al2O3 deposited on hydrogen-terminated Si and thermal SiO2, which can be attributed to substrate-inhibited versus non-inhibited deposition.