Effects of Pb Doping on Hole Transport Properties and Thin-Film Transistor Characteristics of SnO Thin Films

Abstract

Effects of Pb doping on the microstructural, optical, and hole transport properties of Sn 1−xf Pb xf O films fabricated by pulsed laser deposition were investigated. It was found that Pb was not solved in bulk ceramic samples, while solved up to x f = 0.035 in the thin films. The Pb doping enlarged the optical bandgap, decreased the hole concentration, and reduced the Hall mobility (μ Hall , from 1.95 to 0.68 cm 2 V −1 s −1 up to x f = 0.035) as x f increased. The field-effect mobility in thin-film transistors also decreased with x f (μ lin , from 0.34 to 0.0024 cm 2 V −1 s −1 ), but the deterioration of μ lin was significantly larger than that of μ Hall . It is speculated that the deterioration of μ Hall would be due to the increase in grain boundary potential barriers. Oxide semiconductors have attracted considerable attention as next-generation channel materials to replace amorphous silicon and polycrystalline silicon for thin-film transistors (TFTs), because of their high electron mobilities >10 cm 2 V −1 s −1 and excellent electrical properties due to the large bandgaps.1−3 However, the reported oxide-based TFTs with excellent performance show n-type characteristics only. The lack of high-performance p-type oxide-based TFTs impedes the realization of transparent low-power-consumption complementary circuits in various TFT applications. 4,5 It is known that the valence band maximum (VBM) of most oxide semiconductors is mainly composed of fully occupied 2p orbitals of oxygen ions with a large electronegativity. 5,6 Hence the dispersion of the VBM band is in general small (i.e., the hole effective mass is large) and the ionization potential is large (i.e., the holes are energetically unstable), causing difficulty in p-type doping for oxide semiconductors.Fortunately, effective p-type doping and relatively high hole mobility is attained in SnO due to the hole contribution through the VBM composed of 5s 2 orbitals of Sn 2+ . 5,7 However, SnO TFTs generally operate in the depletion mode and have a high off-state current owing to the high hole concentration and the low resistivity of the SnO films.7−11 It is reported that the conductivity in SnO is related to tin vacancy and controllable also by impurity doping.5,12−14 However, the previous reports on doping effects are not consistent with each other. Guo et al. reported that doping in SnO with large size dopants such as Sb or Y enhances their hole mobilities and hole concentrations, 13 the latter of which is opposite to usual concept of aliovalent ion doping (i.e., substitutions of the Sn 2+ sites by Sb 3+ /Y 3+ ions are expected to be n-type doping). In contrast, Hosono et al. gave a clear evidence that Sb 3+ doping decreases the hole mobility and the hole concentration in SnO and finally converts the carrier polarity to n-type. 5Liang et al. demonstrated that doping of SnO with Y decreases the field-effect mobility but suppresses the off-state current in SnO TFTs, even though the mechanism for the reduction in the off-state current is not clarified.14 Like SnO, ...

Recent Developments in p‐Type Oxide Semiconductor Materials and Devices

“…Fabrication of SnO thin films has been extensively evaluated using a series of methods, including physical vapor deposition (PVD) routes, such as PLD, EB, RFMS, and DCMS using Sn, SnO, and SnO 2 targets on both rigid and flexible substrates. The Hall mobility and carrier (hole) concentrations, along with the deposition conditions are listed in Table 3 for relatively recent studies.…”

P-type SnO thin films and SnO/ZnO heterostructures for all-oxide electronic and optoelectronic device applications

“…Ogo et al reported an activation energy of 45 meV in epitaxial SnO thin films, which suggests an acceptor energy level90 meV above the VBM [8]. Poly-crystalline thin films show slightly higher activation energy (60 meV) compared to epitaxial SnO thin films [58]. Polycrystalline thin films also show increased barrier potentials at the grain boundaries which cause mobility deterioration in the films [58].…”

“…Poly-crystalline thin films show slightly higher activation energy (60 meV) compared to epitaxial SnO thin films [58]. Polycrystalline thin films also show increased barrier potentials at the grain boundaries which cause mobility deterioration in the films [58]. It has been…”

Electrical, optical, and structural characterization of p‐type N‐doped SnO thin films prepared by thermal oxidation of sputtered SnNx thin films

“…On the other hand, there are works in the literature that report the introduction of impurities into the SnO structure to improve its physical properties. Among the different mentioned impurities that may produce p‐type SnO thin films are Y, Sb, Cu, Pb, and N . Nitrogen has also been used as an impurity to induce a change in the conduction from n‐type to p‐type in oxides such as ZnO, Ta 2 O 5, and SnO 2.…”