Effects of prestrain applied to a polyethylene terephthalate substrate before the coating of al-doped zno film on film quality, electrical properties, and pop-in behavior during nanoindentation

Four kinds of polyethylene-terephthalate (PET)/Al-doped zinc oxide (AZO) specimen are prepared to examine the effects of prestrain applied to the PET substrate before the coating of the AZO film on the mechanical, material, and electrical properties, and the start of pop-in in the loading phase of nanoindentation. The electrical contact resistance function is used to measure the variations of electrical current during the nanoindentation process. With the aid of the stress-strain profile, the inflection point of the load-depth profile in the loading phase is identified as the start of pop-in, at or nearby which the electrical current sharply increases due to the significant increase in the indenter-film contact area. The pop-in depth decreases with increasing prestrain. The behavior demonstrated in the pop-in depth due to the change in the prestrain is exactly opposite to those of the quantity and the mean size of submicrometer voids/cracks. Increase in the quantity/size of film voids/cracks generally reduces the specimen's carrier mobility. The carrier mobility is presented to be inversely proportional to the sheet resistance. The electrical current created at the end of the loading process has its value inversely proportional to the number/size of film voids/cracks.