The chemical composition, crystalline structure and optical properties of un-doped ZnO (UZO) and indium (6%)-doped ZnO (IZO) thin films grown on Si substrate were studied using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and photoluminescence (PL) techniques. The results are complementary and confirm each other. The surface is cleaned using checked ultra-high vacuum (UHV) treatment (argon ion sputtering followed by successive heating). For IZO, the XPS analysis displays that the indium incorporates in the ZnO matrix to form the In-O-Zn-type chemical bonds. The PL of UZO reveals structural defects, including oxygen interstitial (O[Formula: see text], oxygen vacancies (V[Formula: see text], zinc vacancies (V[Formula: see text] and interstitial zinc (Zn[Formula: see text], and they decrease with the In doping and UHV treatment. For IZO, the PL measurements show the great interest of UHV treatment to stimulate the incorporation of indium into the ZnO matrix. There is an increase in the UV emission intensity and improvement of its physical structure. The In (6%) doping of ZnO is convenient to compensate the zinc vacancies (V[Formula: see text], eliminate Zni and VO, and ensure the structural homogeneity of IZO film. All the detected peaks of the XRD patterns are matched to the wurtzite crystalline structure for both UZO and IZO thin films grown mainly along the (002) orientation plane.