Ambient aging effects on the effective energy gap of ZnO thin films

Abstract

Using photoconductance spectroscopy, we have studied the influence of different types of thermal annealing on epitaxial ZnO thin films where band bending effects play a major role. Once the film is exposed to ambient air conditions after a simple thermal annealing in oxygen at 600 C, the effective energy gap is stable with a value of ≃ 3:15 eV, while after a corresponding annealing in vacuum and subsequent air exposure, it starts at ≃ 3:24 eV, and then it evolves along the days until it reaches the bulk energy gap value of ZnO. By means of valence band x-ray photoemission spectroscopy (XPS), we have confirmed that these phenomena are related via the Franz-Keldysh effect to a downward band bending in the former case and a time dependent upward band bending in the latter one that slowly tends to a flat band condition, tracking the behavior observed in the effective energy gap. Core level XPS measurements suggest that for each type of thermal annealing, a different adsorption kinetics of water and hydrogen take place.

The role of defects in the persistent photoconductivity of BaSnO3 thin films

“…, where E G is the effective energy gap [26], see figure 2. This implies that the obtained energy gaps are direct ones [25]. In the case of the BSO film on MgO we get E G = 3.9 eV in agreement with the band to band transition value found for thick BSO films and single crystals [14,44,45].…”

“…The photoconductivity, PC, is defined as PC = (σ − σ dark )/σ dark . If a proportionality between PC and the optical absorption coefficient is assumed it is possible to extract the energy gaps involved in both films [25]. For both spectra, the best fit above the energy onset is achieved using a linear relation between PC 2 and the energy:…”

“…Moreover, these defects can also modify the effective energy gap of oxide semiconductors in different ways [9,21,24]. In this regard, PC spectroscopy [25,26] is an adequate tool to observe energy gap transitions. In particular, the use of this technique in BaSnO 3 can help to clarify the dispersion of energy gap values found in this material [27].…”

Modification of the photoconducting properties of ZnO thin films via low-temperature annealing and air exposure

“…2b. Taking into account that τ is related to the activation energy for adsorption via 40 Finally, it is worth to make a general comparison between this work and previous studies in ZnO 9,20,21 where high-temperature annealing (above ∼ 500 • C) were performed. One of the main differences resides in the fact that at such a high temperatures oxygen diffusion occurs and it can modify the number of oxygen vacancies in the proximities of the surface.…”

“…On the other hand, it would be of interest to confirm the presence of a band bending after the thermal annealing and to determine its value and sign, that is, if there is an upward or a downward band bending after the TA 21 . In order to do that, we have measured the valence band XPS spectrum of the film a day after TA and air exposure, see Fig.…”