The structural defects and formation of native oxides during thermal treatment on p-type epitaxial-GaAs/Ge have been investigated using spectroscopic measurements and electrical characterization. The performance of epi-GaAs based device depends on the interface quality between epi-GaAs and gate oxide and structural quality of the epi-GaAs layer. P-type epitaxial-GaAs was grown on Ge substrate using MOCVD technique at 675oC. Defective surface native oxides of arsenic and gallium oxides are observed for as-grown epi-GaAs layer. The arsenic oxide significantly reduced after thermal treatment as seen from XPS observations. The structural defects at surface enhanced after thermal treatment which is clearly probed by micro-Raman spectroscopy. Atomic layer deposited (ALD) Al2O3 significantly improved the interface properties after thermal treatment compared with bare epi-GaAs layer. Even though, the interface trap defect density slightly higher for p-type epi-GaAs MOS capacitor compared with bulk p-type GaAs devices, high frequency-dispersion in epi-GaAs based devices observed. This is mainly governs through the formation of p-i-n junction diode in the epi-GaAs layer on Ge substrates.