Ingrain and grain boundary scattering effects on electron mobility of transparent conducting polycrystalline Ga-doped ZnO films

Abstract

Transparent conducting polycrystalline Ga-doped ZnO (GZO) films with different thicknesses were deposited on glass substrates at a substrate temperature of 200 °C by ion-plating deposition with direct current arc-discharge. The dependences of crystal structure, electrical, and optical properties of the GZO films on thickness have been systematically studied. Optical response due to free electrons of the GZO films was characterized in the photon energy range from 0.73 to 3.8 eV by spectroscopic ellipsometry (SE). The free electron response was expressed by the simple Drude model combined with the Tauc–Lorentz model. From the SE analysis and the results of Hall measurements, electron effective mass, m∗, and optical mobility, μopt, of the GZO films were determined, based on the assumptions that the films are homogeneous and optically isotropic. By comparing the μopt and Hall mobility, μHall, an indication on the effect of ingrain and grain boundary scattering limiting the electron mobility has been obtained. Moreover, the variation in scattering mechanism causing thickness dependence of μHall was correlated with the development of polycrystalline grain structure.

Electron scattering mechanisms in fluorine-doped SnO2 thin films

“…The later refers to ionized impurity scattering and to a lower extent to phonons. 13,25,26 Single crystalline samples generally exhibit higher mobility than their polycrystalline variant, especially for low charge carrier densities, indicating that grain boundary scattering can be important. 1 However, the GB scattering dominance is still a question of debate.…”

“…42,43 In order to at least partially disentangle the effects of ingrain and GB scattering on electron transport in TCOs, a frequent approach consists in measuring the electron mobility in two different ways. 2,25,32,44,45 The Hall measurements describe the mobility of electrons (l Hall ), which are moving across many grains and grain boundaries in the conduction path. l Hall is then limited by both scattering processes within The dotted-dashed line shows fit using the Drude model associated with a x À3 behavior at low frequencies.…”

“…25,26 Besides, a process of twin boundary scattering is shown to play a role. Indeed, as discussed in this paper, the presence of a high density of lamellar twins in FTO can contribute to lower the observed electro-optical properties of FTO as compared with other TCOs.…”

Modeling the transport properties of epitaxially grown thermoelectric oxide thin films using spectroscopic ellipsometry

“…14,19 Optical methods such as spectroscopic ellipsometry (SE) is used to determine effective mass and scattering effects in conducting thin films, for example degenerate transparent conducting oxides. 20,21 SE, being a non-destructive and non-invasive technique, can be effectively used in extracting the thickness, composition, and phase of the films. By employing suitable theoretical dielectric functions to fit the ellipsometry data, contactless determination of electrical properties of the films is possible.…”

“…25 For electron concentration above 10 20 cm À3 , the Hall mobility is dominated by scattering due to ionized impurities. 26 Since the mean free path (l) of electrons is much smaller (see , Table II) than the crystallite size ($50 nm) in the films, in-grain (ionized impurity) scattering dominates C s and hence the optical mobility. Also, the Debye temperature 27 (h D ) of STO is 513 K and scattering by ionized impurities is the predominant mechanism at temperatures below h D .…”