Apart from XRD analysis, I want to know the other techniques used for identifying the oxide phases (even when a particular oxide is having very less volume fraction) present in a thermally grown oxide.
If the layer contains different oxide phases you should consider Analytical TEM (composition from EDS, EELS) and crystallographic information from Selected Area Electron Diffraction.
If you want to study the structural properties of samples, you need X-Ray, FE-SEM and Afm measurements, while optical measurements need UV and electrical Hall effect.
Raman mapping would be the best technique to identify and quantify the phases of oxide on the surface. This technique can detect the low content of solid phase/or form impuritues down to ca. 10 to 100ppm , and even lower subject to the compound and the instrument.