Power-law photoconductivity time decay in nanocrystalline TiO₂thin films

Abstract

The sub-band-gap excited photoconductivity (PC) time decay and the film structure of rf-sputter deposited nanocrystalline TiO2 thin films have been studied. Atomic force microscopy and x-ray diffraction measurements were used to assess roughness, crystalline structure and mean grain size of the films. Samples fabricated under different deposition conditions exhibit different microstructures and absolute PC, but similar persistent PC behaviour after switching off the light source. The very slow PC decay can be well represented by a function that is nearly constant for short times and decreases as a power law for times longer than about 100 s. This function is shown to be consistent with a rate equation characterized by a relaxation time that increases linearly with time. This behaviour, in turn, agrees with predictions of a previously reported model that assumes electron–hole recombination limited by carrier-density-dependent potential barriers associated with inhomogeneities. These results may have important implications on attempts to determine distributions of trap energies from PC decay curves in TiO2.

Changes in the electrical transport of ZnO under visible light

“…These changes due to the structural quality affect the photoconductivity. Similar dependence of the photoconductivity with the microstructure was obtained in TiO2 films [11]. …”

Electrical, photoelectrical and morphological properties of ZnO nanofiber networks grown on SiO2 and on Si nanowires

“…The first is very fast (characteristic time much lower than a second), and the second one is very slow and non-exponential, which tends to a conductance value a 30% higher than the conductance in initial darkness. This behavior is usually observed in ZnO and other semiconductor oxides films/sheets [18][19][20][21] , and is explained by the band-band recombination (which occurs in very short times, in the order of 10 -9 seconds) limited by the extremely slow release of charges from a deep state in the bandgap, due to defects (see Figure 4a).…”

“…The broad peak is related to recombination of electrons trapped in deep states within the bandgap, due to defects, such as oxygen vacancies or Zn intersticials. The great intensity of this peak indicates that the nanofibers have a large number of defects, which could be related to their large surface area 18 . Photoconductivity effects are observed in the ZnO NFN when the sample is illuminated with UV, as it can be seen in Figure 4c.…”

“…From Mott's VRH expressions 18 we estimated the density of localized states at the Fermi energy (g F ) and the mean hopping distance (R) from the fit values of T 0A and T 0B (See Table 2). The g F value obtained for mechanism B is reasonable, but for mechanism A, it is too large.…”

The Hall effect measurement in the high resistance In₂O₃-ZnO polycrystalline films

“…The temperature dependence of  in amorphous In 2 O 3 -ZnO is explained by the electronphonon scattering, the electron-electron interaction and weak localization [3,4,5]. On the other hand, the conductance enhancements due to lights have been reported in various metal oxide semiconductors such as titanium oxides [6,7,8], zinc oxides [9,10,11] and indium oxides [12]. P. Görm at el.…”