Enhanced p-type conductivity and band gap narrowing in heavily Al doped NiO thin films deposited by RF magnetron sputtering

Abstract

Stoichiometric NiO, a Mott-Hubbard insulator at room temperature, shows p-type electrical conduction due to the introduction of Ni(2+) vacancies (V(Ni)('')) and self-doping of Ni(3+) ions in the presence of excess oxygen. The electrical conductivity of this important material is low and not sufficient for active device fabrication. Al doped NiO thin films were synthesized by radio frequency (RF) magnetron sputtering on glass substrates at a substrate temperature of 250 °C in an oxygen + argon atmosphere in order to enhance the p-type electrical conductivity. X-ray diffraction studies confirmed the correct phase formation and also oriented growth of NiO thin films. Al doping was confirmed by x-ray photoelectron spectroscopic studies. The structural, electrical and optical properties of the films were investigated as a function of Al doping (0-4 wt%) in the target. The room temperature electrical conductivity increased from 0.01-0.32 S cm (-1) for 0-4% Al doping. With increasing Al doping, above the Mott critical carrier density, energy band gap shrinkage was observed. This was explained by the shift of the band edges due to the existence of exchange and correlation energies amongst the electron-electron and hole-hole systems and also by the interaction between the impurity quasi-particle system.

Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions

“…The total bandgap energy shift is due to (a) the exchange energy for holes and electrons in the valance bad and conduction band, respectively, and (b) due to the interaction between impurity ions and charge carriers. 63 It is obvious that the observed bandgap narrowing from DRS absorption and NBE emission from PL support each other. Present results could be crucial for future applications of ZnO based optoelectronic devices and to get further insight into the origin of narrowing bandgap effect in lighter elements doped ZnO nanoparticles.…”

“…71 In our Li doped nanoparticles, we attribute the bandgap narrowing to the combined effects of disorder and the formation of band tails as described in other p-type systems. 41,63,72 Thus, we suggest that the narrowing of the bandgap occurs due to a structural disorder (defects), carrier-donor and carrier-acceptor impurity interactions, electron-phonon and hole-phonon interactions. At high Li doping concentrations, the impurity band merges with the valence band edge, and it becomes band tail states.…”

“…61 The electronelectron, hole-hole, and carrier-impurity interactions could lead to the lowering of the conduction band and an upward shift of the valence band, which causes the band-gap narrowing. 28,[61][62][63][64] The band-gap narrowing due to Mott critical density is another way for the explanation of decreasing the band gap in semiconductors. 65 For the p-type conduction, …”

Advanced Characterization Techniques

“…Generally, argon is used as the sputtering gas. Depending on the source, two different sputtering techniques exist: (1) DC sputtering and (2) Radio frequency (RF) sputtering [9,10]. Though the methods are different, the underlying philosophy is almost same.…”

“…On the other hand, at lower wavelength with energy greater than the band gap, light will be absorbed by the sample, hence I < I 0 . Therefore, using the following equation, absorbance (A) of the sample can easily be obtained, A ¼ log 10 I 0 I Thus using this procedure, it is possible to determine the A in the whole range of the wavelength. If we have the knowledge of the depth of the material, then we may calculate the absorption coefficient (α) of the material.…”