Effects of free electrons and quantum confinement in ultrathin ZnO films: a comparison between undoped and Al-doped ZnO

Abstract

Band gaps and exciton binding energies of undoped and Al-doped ZnO thin films were determined from optical absorption measurement based on the Elliott's exciton absorption theory. As compared to the undoped films, the doped films exhibit a band gap expansion and a reduction in the exciton binding energies due to the free electron screening effect, which suppresses the excitonic absorption and results in a blue shift of the absorption edge. The undoped and doped films show the same quantum size dependence, i.e. both the exciton binding energies and band gap energies increase with decreasing grain size of the oxides.

A study on the evolution of dielectric function of ZnO thin films with decreasing film thickness

“…4(a) and 4(b), the size dependence of average exciton binding energies and band gap energy could be roughly described by the trend lines of R AV ðmeVÞ ¼ 60 þ 926=DðnmÞ 1:62 and E g ðeVÞ ¼ 3:46 þ3:23=DðnmÞ 1:65 , respectively. These results are consistent with our previous absorption study, 3 which is shown in the insets of Fig. 4.…”

“…The situation is similar to that reported in our previous study. 3 from the SE fittings for the various films are very close to the nominal values of 10, 20, 40, 90, and 185 nm, respectively. Optical absorbance measurement was carried out with an UV-Vis spectrophotometer (Perkin-Elmer 950) on the samples of ZnO thin films deposited on quartz substrate in the wavelength range of 250-800 nm.…”

“…A study on the evolution of dielectric function of ZnO thin films with decreasing film thickness X. D. Li, 1 T. P. Chen, 1,a) P. Liu, 1 Y. Liu, 2 Z. Liu, 3 Dielectric function, band gap, and exciton binding energies of ultrathin ZnO films as a function of film thickness have been obtained with spectroscopic ellipsometry. As the film thickness decreases, both real (e 1 ) and imaginary (e 2 ) parts of the dielectric function decrease significantly, and e 2 shows a blue shift.…”

“…In our previous work, it has been revealed that the quantum confinement effect has a significant impact on the optical properties of ultrathin ZnO and Al-doped ZnO films, in particular, the excitonic absorption near the absorption band edge. 3 On the other hand, it was reported that quantum confinement effect has a significant influence on the dielectric functions of the Si and Ge nanocrystals embedded in a SiO 2 matrix, e.g., the magnitude of the dielectric functions is reduced with decreasing nanocrystal size. 4,5 Therefore, it would be interesting to examine the film-thickness dependence of the complex dielectric function of the ultrathin ZnO films as quantum confinement effect could play an important role in such system.…”

Thickness Dependence of Optical Properties of Amorphous Indium Gallium Zinc Oxide Thin Films: Effects of Free-Electrons and Quantum Confinement

“…It has been shown in our early work that free-electron concentration of a metal oxide thin film could have a significant impact on the optical properties of the oxide film, in particular, the onset absorption near the band edge. 7 When the electron concentration reaches ∼10 18 -10 19 cm −3 , a semiconductor-to-metal transition occurs, and the conduction band of the oxide semiconductor is partially filled with free electrons, resulting in a bandgap expansion as a result of the Burstein-Moss effect. 8 The dependence of carrier concentration on film thickness could further complicate the situation.…”