Defect centers and optical absorption edge of degenerated semiconductor ZnO thin films grown by a reactive plasma deposition by means of piezoelectric photothermal spectroscopy

Abstract

Undoped and Ga-doped (3 wt %) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under an oxygen flow rate from 0 to 50 SCCM. In this paper, we report on the defect and band edge related signals in the optical absorption spectra for ZnO thin film by using a piezoelectric photothermal (PPT) spectroscopy, which is effective in observing a nonradiative transition process. The PPT peak around 2.5 eV was observed only for the undoped ZnO samples grown under a low oxygen flow rate. This signal is considered to be related to the oxygen vacancies, because it disappears with the increase of the oxygen flow rates. No corresponding peak was found for the Ga-doped samples. This result indicates that Ga doping inhibits the generation of the oxygen vacancies, and it agrees with that from the first-principle electronic band structure calculations. We have also carried out the theoretical calculation for the optical absorption edge of degenerated ZnO as a function of the carrier concentration. Burstein-Moss effect and band-gap-narrowing effect in ZnO should be considered in the case of high carrier concentration. Comparing the experimental results with the theoretical predictions, we found out that the proposed PPT edge energy coincides well with the Fermi level EF.

Structure, Morphology, and Optical Properties of the Compact, Vertically-Aligned ZnO Nanorod Thin Films by the Solution-Growth Technique

“…When the growth time increases from 3 to 6 h, the values o f E g decrease from 3.35 to 3.31 eV which gradually diverges from the intrinsic band gap of ZnO (3.37 eV). It is known that the energy band gap of a ZnO thin film could be affected by the residual strain (Mohamed et al, 2006;Srikant & Clarke, 1997), defects (Burstein, 1954;Dong et al, 2007;Moss, 1954;Sakai et al, 2006), and grain size confinement (Prathap et al, 2008;. For ZnO nanorod thin films fabricated at 75 °C for 3 to 6 h, the average grain sizes enlarge from ~105 to ~200 nm and the film thicknesses increase from ~460 to ~800 nm, which results in the variation of strain from -0.26 to -0.32%.…”

Influence of Oxygen on Zinc Oxide Films Fabricated by Ion-Beam Sputter Deposition

“…The BM effect is the blue-shift phenomenon of the band edge where the Fermi level merges into the conduction band with the increase of the carrier concentration. Yet, the BGN effect is a red-shift phenomenon of the band edge because the electron-electron repulsive interaction and the localization of the electron wave function is weakened by the screening of the potential in the high carrier concentration [7]. Apparently, the band gap shift is affected by the combination of the BM and BGN effects as increasing carrier concentrations.…”

“…Illustrating ZnO-10 to -15 samples from Figures 5 and 7, the absorption edges red-shift from 3.41 to 3.32 eV with respect to the increase sheet concentrations from 3.01 × 10 15 to 1.82 × 10 16 cm −2 with increasing O 2 . The red-shift phenomenon is the electron-electron repulsive interaction from the many electron carriers due to the bandgap-narrowing (BGN) effect [7], that is, the optical band gaps red-shift resulting from the increases of electron carrier concentrations in the films. However, the sheet concentration reduces to 6.15 × 10 15 cm −2 ; the optical band gap still slightly decreases to 3.31 eV due to BM effect at O 2 = 2.0 × 10 −4 Torr.…”

“…Apparently, the band gap shift is affected by the combination of the BM and BGN effects as increasing carrier concentrations. Sakai et al have also investigated that the carrier concentration strongly affects the optical band gap resulting from the oxygen vacancies controlled by the oxygen flow rate during the growth of the polycrystalline ZnO film by a reactive plasma deposition [7]. In this study, the optical band gaps and the ZnO structures were determined by controlling the various oxygen partial pressures during the IBSD at room temperature.…”

Photothermal Investigation of Surface Defects of Pure Semiconducting A2B6 Materials

“…Photothermal methods have become a very useful tool for measurements of the optical and thermal parameters of semiconductors [1][2][3][4]. They have also been used to investigate the surface properties of different materials [5][6][7][8].…”