In this study, the hump in the capacitance-voltage (C-V) curves, variation of leakage current, interfacial layer increase, and electron trapping in non-surface treated hafnium oxide (HfO2) samples were observed and investigated. From the results of the investigation, it was found that both rapid thermal oxidation and NH3 surface treatments improved the C-V curves. In addition, it was observed that samples treated with ammonia exhibited a lower leakage current when compared with the others. From the results of the dielectric leakage current study, a severe electron trapping effect was exhibited under higher electric field stress. Finally, the conduction mechanism in the HfO2 thin film was dominated by Frenkel-Poole emission in a high electric field.