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Hydrogen (H) plays critical roles in the electrical properties of semiconductor materials and devices. In this work, we report multiple states and roles of H in SnS by H plasma treatment and density functional theory (DFT) calculations. The as-deposited SnS films include impurity H at 2.3 × 1019 cm−3, four orders of magnitude larger than the hole density. The DFT calculations reveal that H exists in multiple states at the equilibrium mainly at the interstitial and the Sn-substitutional sites, which have formation enthalpies lower than those for the intrinsic defects. These H states work as donors and acceptors, respectively, and strongly pin the Fermi level in the p-type region. The native p-type conduction in the actual SnS semiconductors is caused mainly by the H-on-Sn (HSn) acceptors, rather than the previously reported Sn vacancies (VSn) for pure SnS. It is also confirmed that even stronger H doping with larger H chemical potentials cannot convert SnS to an n-type conductor because it reduces SnS to Sn metal.

Effects of low temperature $({\hbox{300}}^{\circ}\hbox{C})$ annealing on $\hbox{Cu}_{2}\hbox{O}$ 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 $\hbox{Cu}_{2}\hbox{O}$ films was ${\hbox{2.1}}~\hbox{cm}^{2}/(\hbox{V}{\cdot}\hbox{s})$ ; however, the bottom-gate $\hbox{Cu}_{2}\hbox{O}$ TFT exhibited a much lower field effect mobility of the order of 10 $^{-4}~\hbox{cm}^{2}/(\hbox{V}{\cdot}\hbox{s})$ and an on/off drain current ratio of 10 $^{3}$ . This work detected a surface layer and an interface layer in the $\hbox{Cu}_{2}\hbox{O}/~\hbox{SiO}_{2}$ 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 $\hbox{Cu}_{2}\hbox{O}{\hbox{--}}\hbox{SiO}_{2}$ interface would produce extra interfacial trap states.

Effects of Pb doping on the microstructural, optical, and hole transport properties of Sn1−xfPbxfO films fabricated by pulsed laser deposition were investigated. It was found that Pb was not solved in bulk ceramic samples, while solved up to xf = 0.035 in the thin films. The Pb doping enlarged the optical bandgap, decreased the hole concentration, and reduced the Hall mobility (μHall, from 1.95 to 0.68 cm 2 V −1 s −1 up to xf = 0.035) as xf increased. The field-effect mobility in thin-film transistors also decreased with xf (μlin, from 0.34 to 0.0024 cm2 V−1 s−1), but the deterioration of μlin was significantly larger than that of μHall. It is speculated that the deterioration of μHall would be due to the increase in grain boundary potential barriers. The deterioration of μlin, low on-state current, and high off-state current are explained semiquantitatively by a simple hole trap model. Similar explanations would be applied also to previous reports on Sb / Y doped-SnO. Location of the highest occupied states of Pb 2+ relative to that of Sn 2+ (i.e., Pb 6s 2 and Sn 5s 2 , respectively) clarified by DFTcalculations provides an explantion for the observed result.

Wurtzite structure ZnO films (3×3 mm2) with a partial-area Cu doping were successfully prepared using a micro-area Ar+-ion beam (~ 380 μm in diameter) and a simultaneous Cu supply at room temperature. A Cu2O phase was formed in the ZnO films by Cu doping. The partially Cu-doped ZnO films exhibited room-temperature ferromagnetism (RTFM) with a saturation magnetization of 1.6×10−5 emu and a coercive field of 40 Oe. Since Zn, Cu, and their compounds are not ferromagnetic, the observed RTFMis attributed to the intrinsic property of Cu-doped ZnO films. As confirmed by the low temperature photoluminescence (PL) spectra, no serious optical damage was recognized in the region without Ar+-ion irradiation. Thus, it was believed that the micro-area Ar+-ion irradiation with a simultaneous Cu supply was promising to integrate the magnetic and optical properties of ZnO-based materials.

Wurtzite structure ZnO films doped with ~2 at% Cu were deposited at substrate temperatures ( T s ) from 350 to 600 °C by helicon magnetron sputtering. All the films exhibited room-temperature (RT) ferromagnetism (FM) and the maximum saturation magnetization ( M s ) was 1.2 emu/cm 3 (~0.15 μ B /Cu). Cu ions were mainly in a divalent state as identified by X-ray photoelectron spectroscopy. FM tended to increase with decreasing T s , and vacuum annealing enhanced the M s . These results suggested that oxygen vacancies and/or zinc interstitials might contribute to the ferromagnetic performance. Thus, the observed FM was explained in terms of the defect related mechanism.

Wurtzite structure ZnO films doped with 0.5 and 1.7 at% Cu were deposited by helicon magnetron sputtering. The prepared films exhibited room-temperature (RT) ferromagnetism (FM). Maximum RT saturation magnetization of 2 emu/cm3 (∼0.3μB/Cu) was observed for ZnO film with 1.7 at% Cu. Cu ions were in a bivalent state as identified by X-ray photoelectron spectroscopy (XPS). In photoluminescence spectra, the green emission peak increased and redshifted due to the incorporation of Cu or defects induced by Cu incorporation. Since Cu and Cu-related oxides are not RT ferromagnetic, and no trace of ferromagnetic contamination was detected in XPS results, the observed FM is considered to be an intrinsic property of Cu-doped ZnO films. The FM was thought to originate from defect-related mechanisms. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using erbium acetate as an Er source, transparent ZnO films doped with Er were fabricated by sol–gel method through spin-coating on quartz substrates. The as-prepared ZnO:Er films were annealed in air at 400 °C for 6 h, at 600 °C, 800 °C and 1000 °C for 2 h, respectively. The ZnO:Er films exhibit wurtzite structure with preferential orientation of the (0 0 2) plane. The film that annealed at 800 °C shows the highest degree of preferential orientation. Coalescence of small grains result in very large and irregularly shaped grains for the film annealed at 1000 °C. And the films annealed at other temperature all have rather uniform spherical grains with diameter ranging from 30 nm to 60 nm. The estimated optical band gap of Er-doped ZnO films annealed at different temperatures is 3.28 eV which is almost independent of annealing temperature and quite similar to that of undoped ZnO film. Photoluminescence peak at about 1535 nm originating from intra-4f shell transition in Er3+ ions and a shoulder peak at about 1500 nm originating from states split are observed for all the prepared ZnO:Er films. The 1535 nm emission intensity increases with post-annealing temperature ranging from 400 °C to 800 °C, thereafter decreases. The decrease may attribute to the worse crystal quality and Er atom diffusion towards the surface of the film. Due to the active oxygen movement during post-annealing process, the most active bonding state of Er–O is obtained for the films annealed at 800 °C, which is probably similar to a pseudo-octahedron with C4v point structure reported in literature.

N-doped TiO2 powders with high photocatalytic activity under visible-light irradiation have been prepared by calcining anatase nanocrystalline TiO2 powders and guanidine hydrochloride at 350 °C, 450 °C, 550 °C and 650 °C with the holding time ranged from 1.5 h to 3 h, respectively. The guanidine hydrochloride content was varied from 1 to 6 M for the experiments. XRD patterns indicate that all the powders are anatase crystalline phase. The specific surface area is 25~45 m2/g measured by BET method. XPS results show that N atoms were incorporated into the lattice of TiO2 and the amount of N doped in TiO2 is reached up to 8.26 at%. UV-Vis absorption spectra show that the absorption edge of the synthesized N-doped TiO2 powders, calcined at 350 °C for 2.5 h, has red-shifted well into visible region up to 700 nm. Degradation of methylene blue (MB) aqueous solution has been investigated by UV-Vis light spectrophotometer. The 400-mL 10 mg/L aqueous solution of methylene blue could be degraded completely with 4 g N-doped TiO2 within 4.5 h of visible light irradiation.

Nanosize anatase TiO2 powders have been synthesized by hydrothermal synthesis by using technical grade TiOSO4 as precursor and urea as precipitating agent. The initial pressure of the reaction system was set at 6 MPa. Stirring speed was fixed at 300r/min. The reaction system reacted at the temperature ranged from 110 to 150°C for holding 2hrs to 8hrs and the concentration of the precursor was ranged from 0.25M to1.5M. XRD patterns show that the synthesized powders are in the form of anatase phase. Calculated grain size is ranged from 6.7 to 8.9nm by Scherrer method from the line broadening of the (101) diffraction peak of anatase. The specific surface area of the powders synthesized under different conditions is ranged from 124 to 240m2/g. The grain size of the powders increases with the increase of the reaction temperature, holding time and precursor concentration, respectively. The specific surface area decreases with the increase of reaction temperature and holding time, and does not obviously change with the change of precursor concentration when the concentration of the precursor is less than 1M. However, when the concentration is higher than 1M, the specific surface area will decrease quickly with the increase of the precursor concentration. XRD and DSC-TG analysis shows that the synthesized anatase TiO2 will begin to transform to rutile TiO2 at about 840°C. When heated to 1000°C for holding 1h, the anatase powders will transform to rutile completely.

A new functional photocatalytic paint was prepared by adding N-doped nanocrystalline anatase TiO2 powders into self-engineered oxidation resistance latex paint system. The property of the UV-Vis light absorbance of the paint was characterized. The effects of the amount, types, and the introducing methods of the TiO2 in the paints on the behavior of the UV absorption, photocatalytic properties of the paint were systematically investigated. UV-Vis absorption spectra reveal that the ultraviolet light can strongly be absorbed by the paints. The absorption edge of the N-doped nanocrystalline anatase powders modified paint has red-shifted to visible light region. The colony counting method is used to study its sterilization performance under visible light irradiation. The sterilization experiments show that the rate of sterilizing staphylococcal bacteria by the N-doped TiO2 modified paint can exceed more than 99% after the paint has been irradiated by the visible light for 2 to 4 hours.

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.

Beta-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that beta-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of beta-BaZn2As2 (previously-reported value ~0.2 eV) is one order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of beta-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the tem-perature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemi-cal 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 non-bonding Ba 5dx2-y2 orbitals form unexpectedly deep conduction band minimum (CBM) in beta-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between beta-BaZn2As2 and LaZnAsO.

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.

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.

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.

Cu-doped ZnO and AlN films both exhibiting room-temperature (RT) ferromagnetism (FM) were deposited by helicon magnetron sputtering. Magnetic anisotropy was observed for all the films; saturated magnetization was larger under the applied field perpendicular to the sample surface than the in-plane field, which indicates intrinsic ferromagnetism. The easy axis of magnetization was along the [0 0 0 2] direction. Electron spin resonance (ESR) analysis showed that spins tended to align in a perpendicular direction to the surface, which might account for the observed magnetic anisotropy.

BaZn2As2 is expected for a good p-type semiconductor and has two crystalline phases of an orthorhombic alpha phase and a higher-symmetry tetragonal beta phase. Here, we report high-quality epitaxial films of the tetragonal beta-BaZn2As2 were grown on single-crystal MgO (001) substrates by a reactive solid-phase epitaxy technique. Out-of-plane and in-plane epitaxial relationships between the film and the substrate were BaZn2As2 (00l)//MgO (001) and BaZn2As2 [200]//MgO [200], respectively. The full-widths at half maximum were 0.082o for a 008 out-of-plane rocking curve and 0.342o for a 200 in-plane rocking curve. A step-and-terrace structure was observed by atomic force microscopy. The band gap of beta-BaZn2As2 was evaluated to be around 0.2 eV, which is much smaller than that of a family compound LaZnOAs (1.5 eV). Density functional theory calculation using the Heyd-Scuseria-Ernzerhof hybrid functionals supports the small band gap., 6 pages, 8 figures, Thin Solid Films, in press

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.

This review article summarizes briefly some important achievements of our recent research on the synthesis, optical properties and novel applications of nano-structured ZnO such as nanorods and honeycomb shaped 3-D (dimension) nano random-walls. A chemical reaction/vapor transportation deposition technique was employed to fabricate those structures on ZnO/SiO2/Si substrate without any catalyst and additive in a simple tube furnace to aim the low-cost and high qualified samples. Random laser action with strong coherent feedback at the wavelength between 375 nm-395 nm has been firstly observed under 355 nm optical excitation with threshold pumping intensity 0.38 MW/cm2 in ZnO 3-D random-wall nanostructure.

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 ...

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.

The valence band structure of well-ordered Fe3O4(0 0 1) films was investigated in situ by angle-resolved photoemission spectroscopy (ARPES) at room temperature (RT) and 90 K. A band gap of about 70 meV below the Fermi level (E F) can be observed at 90 K, suggesting a ‘metal–insulator’ transition. With temperature reduced from RT to 90 K, O 2p-derived features shown in ARPES spectra change substantially. Meanwhile, the feature near E F of the Fe3O4(0 0 1) film at 90 K becomes about 0.1 eV deeper than that at RT. The results are compared with recent theoretical studies.

Structural and magnetic properties, and the valence band structure of pure and Co-doped (up to 33%) Fe3O4(100) films were investigated. Reconstruction of the Fe3O4(100) surface is found to be blocked by Co doping. Doped Co ions in Fe3O4 are in a charge state of 2 + and substitute the Fe2+ in the B site of Fe3O4. All the films exhibit room temperature ferromagnetism. Co doping changes the coercivity and reduces saturation magnetization. The density of states near the Fermi level is reduced by Co doping due to the decrease of Fe2+ in the B site, which might responsible for the decrease in conductivity and magnetoresistance of Co-doped Fe3O4. The Verwey transition in the range of 100–120 K is observed for the pure Fe3O4 film, while no transition could be detected for Co-doped Fe3O4 films.

We have investigated the effects of Er concentration, post-annealing time and temperature through a sol–gel preparation method on the structure and 1.54-µm-related photoluminescence (PL) of ZnO:Er thin films. The results illustrated that the 1.54 µm emission was greatly influenced by the local structure of Er–O complex and ZnO host. The active oxygen movement during annealing process resulted in the formation of optical active center of Er ions, which probably attributed to the formation of a similar pseudo-octahedron with C 4v structure around Er. The preferential orientation of ZnO host had more effect on the 1.54 µm PL intensity than the crystallinity of ZnO host. Therefore, the optimum annealing condition was about 800 °C/2 h and the appropriate concentration was about 0.05 at. % Er. A low-cost and fast formation of highly efficient Er centers in ZnO host for strong luminescence at near-infared region should be benefit for both fundamental research and also applications of light-emitting devices.

We report structural and magnetization properties of nickel-doped zinc oxide thin films prepared on Si substrates by spray pyrolysis technique. Grazing angle X-ray diffraction measurements confirm the wurtzite structure of Zn1-x Ni x O films and no secondary phase was found for ZnO thin films doped with 5% Ni or less. Hysteresis measurements demonstrate that the Ni-doped ZnO films are magnetically anisotropic and they exhibit intrinsic ferromagnetic behavior at room temperature. Higher magnetization in 3 kOe is observed for a field applied perpendicular to the sample surface compared with the in-plane direction.

AlN films doped with 3.2–8.2 at. % Cu were deposited by helicon magnetron sputtering. The films exhibited ferromagnetism with a Curie temperature above 360 K. The observed magnetic anisotropy and exclusion of ferromagnetic contamination indicated that the ferromagnetism was the intrinsic property of Cu-doped AlN films. Room-temperature (RT) saturation magnetization (Ms) tended to decrease with increasing Cu concentration and the maximum RT Ms obtained was about 8.5 emu/cm3 (0.6 μB/Cu). Vacuum annealing increased the RT Ms values and meanwhile reduced ratio of N/(Al+Cu), which suggested that Al interstitial and N vacancy might contribute to the observed ferromagnetic behavior.