Fan-Yong Ran

n -type conversion of SnS by isovalent ion substitution: Geometrical doping as a new doping route

Tin monosulfide (SnS) is a naturally p-type semiconductor with a layered crystal structure, but no reliable n-type SnS has been obtained by conventional aliovalent ion substitution. In this work, carrier polarity conversion to n-type was achieved by isovalent ion substitution for polycrystalline SnS thin films on glass substrates. Substituting Pb2+ for Sn2+ converted the majority carrier from hole to electron, and the free electron density ranged from 1012 to 1015 cm−3 with the largest electron mobility of 7.0 cm2/(Vs). The n-type conduction was confirmed further by the position of the Fermi level (EF) based on photoemission spectroscopy and electrical characteristics of pn heterojunctions. Density functional theory calculations reveal that the Pb substitution invokes a geometrical size effect that enlarges the interlayer distance and subsequently reduces the formation energies of Sn and Pb interstitials, which results in the electron doping.

Route to n-type doping in SnS

SnS is intrinsically a p-type semiconductor, and much effort has been made to attain n-type conduction. In this letter, we performed density functional theory calculations to seek an effective doping route for n-type SnS. It was found that aliovalent doping of SnS by Sb or Bi is not effective due to their high formation enthalpies; while the isovalent Pb-substitution of the Sn sites largely reduces formation enthalpies of Sn and Pb interstitials, which explain the recently demonstrated n-type conduction in the Sn1−xPbxS films fabricated under low H2S pressures.

Growth of high-quality SnS epitaxial films by H2S flow pulsed laser deposition

SnS epitaxial films were grown on MgO (100) substrates by pulsed laser deposition using a H2S gas as an S source. High growth temperature and high H2S gas flow rate caused re-evaporation and etching of the deposited films; therefore, the optimum condition was limited to a narrow region around 400 °C. The measured bandgap 1.08 eV is consistent with the previously reported theoretical calculation. The films with a S/Sn ratio of ∼1.0 showed the largest mobility of ∼37 cm2/(Vs). The hole transport was dominated by domain boundary potential barriers ∼0.05 eV in height.

Narrow Bandgap in β-BaZn2As2 and Its Chemical Origins

β-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that β-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of β-BaZn2As2 (previously reported value ~0.2 eV) is 1 order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of β-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the temperature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemical bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the nonbonding Ba 5d(x(2)-y(2)) orbitals form an unexpectedly deep conduction band minimum (CBM) in β-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between β-BaZn2As2 and LaZnAsO.

Detection of dead layers and defects in polycrystalline Cu2O thin-film transistors by x-ray reflectivity and photoresponse spectroscopy analyses

Polycrystalline Cu2O films were fabricated on amorphous SiO2 glass by pulsed laser deposition at room temperature and postdeposition thermal annealing in N2 + O2 mixing gases. The authors made a phase map in annealing temperature (Tann) vs RO2 = [O2]/([O2] + [N2]) ratio and found that highly pure Cu2O films were obtained at RO2 ∼ 0.002%. The increase in Tann improved the crystal quality of the Cu2O films, and the maximum Hall mobility of ∼26 cm2/(V s) was obtained at 700 °C. Bottom-gate Cu2O thin-film transistors (TFTs) using the optimum Cu2O channels exhibited clear p-type operation; however, the largest field effect mobility is as small as the order of 10−2 cm2/(V s), indicating the existence of high-density hole trap states. Several subgap states were observed by optical absorption spectra of films and photoresponse spectroscopy of the TFTs. X-ray reflectivity analysis detected a low-density dead layer at the Cu2O–SiO2 glass substrate interface, which would be attributed to Cu diffusion into the glass ...

Analyses of Surface and Interfacial Layers in Polycrystalline

Effects of low temperature (300^°C) annealing on Cu₂O films were investigated by analyzing the film stacking structures with photoemission spectroscopy, X-ray reflectivity spectroscopy and spectroscopic ellipsometory in relation to p-channel TFT characteristics and possible origins of trap states. The Hall mobility of optimum Cu₂O films was 2.1 cm²/(V·s); however, the bottom-gate Cu₂O TFT exhibited a much lower field effect mobility of the order of 10 ^-4 cm²/(V·s) and an on/off drain current ratio of 10 ³. This work detected a surface layer and an interface layer in the Cu₂O/ SiO₂ samples, i.e., the surface layer included the 2+ state of Cu ions that would form subgap hole trap states at the back channel region. In addition, the low-density layer at the Cu₂O-SiO₂ interface would produce extra interfacial trap states.

\hbox{Cu}_{2}\hbox{O}

Polycrystalline SnS thin films were fabricated by a H2S-free process combing pulsed laser deposition at room temperature and post-deposition thermal annealing in Ar. Thermal annealing improved the crystalline quality of the SnS films and the best films were obtained by 400 °C annealing. The obtained SnS films exhibited p-type conduction with the highest Hall mobility of 28 cm2/(V ⋅ s) and the carrier densities of 1.5 × 1015 – 1.8 × 1016 cm−3. The SnS TFT exhibited p-type operation with a field effect mobility and an on-off drain current ratio of 0.4 cm2/(V ⋅ s) and 20, respectively.

Thin-Film Transistors

We selected BaZn2As2 as a candidate for a high-mobility p-type amorphous semiconductor because of the valence band maximum formed mainly of widely spread As 4p orbitals. The hole mobility of amorphous BaZn2As2 films increased from 1 to 10 cm2 V−1 s−1 as the annealing temperature increased from 300 to 400 °C. 500 °C annealing started crystallizing the film with the hole mobility ∼20 cm2 V−1 s−1. The optical bandgaps of amorphous BaZn2As2 were 1.04–1.37 eV, which are much larger than that of the crystalline β-BaZn2As2 (0.23 eV). It is explained by the broken symmetry at the Ba site and the weakening of the As–As direct bonds, which is supported by 6 keV hard X-ray photoemission spectroscopy measurement.