Ultrathin (10-30 Å) SiO2 layers with large interface-state densities were used as the dielectric between aluminum and degenerate silicon. The presence of interface states resulted in current-vs-voltage curves characteristic of metal-insulator-metal (MIM) tunnel structures. MIM tunneling theory was used to estimate the Si-SiO2 (φSi-SiO2) and the Al-SiO2 (φAl-SiO2) barrier heights. We found that the Si-SiO2 barrier height increased from 0.42 eV at 10 Å to 0.65 eV for 25.5 Å of SiO2 on degenerate p-type Si, and from 0.64 eV at 14 Å to 1.27 eV for 29.3 Å of SiO2 on degenerate n-type Si. The Al-SiO2 barrier height could not be consistently determined but was about 0.61±0.16 eV. A smooth transition from Schottky barrier to MOS tunnel structure was observed. The thickness dependence of φSi-SiO2 is most likely due to the recently observed 15-20-Å nonstoichiometric SiO transition region at the Si-SiO2 interface.