We investigated effects of base pressure (Pbase) of the deposition chamber on electrical properties and defect states of amorphous In-Ga-Zn-O (a-IGZO) thin films deposited by pulsed laser deposition. The impurity hydrogen concentration was increased by an order of magnitude when Pbase was deteriorated from <10-5 to 10-3Pa. The optimum oxygen partial pressures (PO2) were 2-4 Pa for an optimized deposition condition with the good Pbase; on the other hand, off-optimized and/or poor Pbase require much higher PO2. This result provides an experimental evidence for a charge compensation model by excess oxygen for H-containing a-IGZO. Thermal desorption spectra indicated that the impurity hydrogens originate mainly from water molecules in the residual gas and exist as -OH chemical bonding states in the a-IGZO films. Hard X-ray photoemission spectroscopy revealed that these -OH states form deep defects above the valence band maximum.
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