P-type Oxide Research Articles

Highly sensitive and room temperature detection of ultra-low concentrations of O3 using self-powered sensing elements of Cu2O nanocubes.

The fundamental development of the design of novel self-powered ozone sensing elements, operating at room temperature, based on p-type metal oxides paves the way to a new class of low cost, highly promising gas sensing devices. In this work, p-type Cu2O nanocubes were synthesized by a simple solution-based method and tested as a self-powered ozone sensing element, at room temperature (25 °C) for the first time. Highly crystalline Cu2O nanocubes with 30 nm size were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Self-powered sensing elements of Cu2O nanocubes have been successfully fabricated by deposition of Cu2O nanocubes on interdigitated electrodes (IDEs) consisting of two connection tracks with 500 digits and a gap of 5 μm in order to investigate their response to ozone at room temperature. The experimental results showed that the use of nanocubes as sensing elements was suitable for detecting ultra-low concentrations of O3 down to 10 ppb at room temperature with very high sensitivity (28%) and a very low response/recovery time. The reversible sensing process of the relatively weak binding of O3 species by trapping sites on Cu2O facets with increased oxygen content was studied by using density functional theory (DFT) calculations.

Double p-SiOx layers to improve the efficiency of p–i–n a-SiGe:H thin film solar cells

In this work, p–i–n hydrogenated amorphous silicon germanium (a-SiGe:H) thin film solar cells were fabricated by using double p-type silicon oxide (p-SiOx) layers, and the power conversion efficiency (PCE) was increased from 7.79 to 9.10%. The optoelectronic properties and functions of p-SiOx layer in a-SiGe:H cells were measured and discussed. A window layer (~ 20 nm) was deposited and characterized to determine the optimal single p-SiOx layer parameters. After that, an ultra-thin contact layer (~ 4 nm) of p-SiOx was deposited in front of the window layer. Through comparative analyses between single and double p-SiOx layers, the performance of the device has greatly improved in open-circuit voltage (Voc), fill factor (FF) and short-circuit current density (Jsc). The optimization of interface contact between top transparent conductive oxide (TCO) and p-layer effectively improves the device efficiency. Finally, a-SiGe:H solar cell with high Voc = 750 mV, FF = 68.36% and Jsc = 17.75 mA/cm2 were fabricated successfully. A high efficiency of 9.10% has been achieved by double p-SiOx layers for a-SiGe:H thin film solar cell.

Point defect concentrations in surface layers of binary oxides

The dependence of point defect concentrations in the surface layer (the outer most layer of the bulk) of binary oxides, on oxygen partial pressure, P(O2), and acceptor concentration, Ab, is evaluated. With one calibration point the defect concentration, at constant temperature, under any P(O2) and Ab can be determined. The method of calculation is demonstrated on binary oxides which exhibit oxygen vacancies, electrons and holes. A relation between the dependence on P(O2) and Ab of concentrations of point defects in the surface layer, and the dependence of the concentrations of the corresponding defects deep in the neutral bulk, is presented. Two types of surface layers are distinguished according to their electron states. One that forms a band which is partially populated by electrons, leading to a two dimensional metallic surface and Fermi level pinning (FLP) and a second, insulating one and no FLP. Four characteristic cases are identified: FLP or the absence of FLP in the surface layer combined with the presence or absence of significant chemisorption. It is shown that the dependence on P(O2) and Ab of the defect concentrations in the surface layer may be the same as that of the corresponding defects deep in the neutral bulk. The difference is only in the magnitude. The conditions for that to take place are an insulating surface and low concentration of chemisorbed particles. The latter conditions are quite common, as the surface of oxides is mostly not metallic, oxygen chemisorption is low at elevated temperature and for acceptor doped, p-type oxides, chemisorption is also low at low temperature. Different relations are derived under other conditions. In particular, for metallic surface, the oxygen vacancy concentration in the surface layer follows only one dependence, P(O2)−1/2, irrespective of the dependence on P(O2) and Ab of the bulk vacancy concentration and irrespective of chemisorption. The electrical potential distribution is derived for the four cases.

Active {1 1 1}-faceted ultra-thin NiO single-crystalline porous nanosheets supported highly dispersed Pt nanoparticles for synergetic enhancement of gas sensing and photocatalytic performance

Proper morphology, surface and interface structure designing are required to obtain efficient gas sensing and photocatalytic materials. In the present work, ultra-thin NiO single-crystalline porous nanosheets with dominant {1 1 1} crystal facets (denoted as SP-NiO) and hierarchical NiO porous microspheres (denoted as HP-NiO) supported highly dispersed Pt nanoparticles with controllable sizes were designed and synthesized. Their gas sensing and photocatalytic performance were investigated. It was found that both the formaldehyde sensing and methyl orange photocatalytic degradation performance were greatly enhanced by decorating Pt nanoparticles on SP-NiO, while Pt nanoparticles decoration contributed little to the improved photocatalytic performance of HP-NiO. The results indicated that surface structure of the NiO support could also produce significant impact on the activity of Pt/NiO heterojunctions. Moreover, Pt decorated SP-NiO with stable structure showed a marked long-term stability with negligible attenuation of gas sensitivity (less than 5%) for 45 days, while Pt decorated HP-NiO exhibited obvious attenuation of gas sensitivity (more than 30%) due to the structural collapse. The work not only offers promising materials for gas sensing and photocatalytic application, but also brings new dawn for the designing of efficient p-type metal oxides gas sensing and photocatalytic materials through the synergistic effect of single-crystalline porous structures modulation, crystal facets engineering and facet-selective deposition of highly-dispersed Pt nanoparticles.

Nickel Doping on Cobalt Oxide Thin Film Using by Sputtering Process-a Route for Surface Modification for p-type Metal Oxide Gas Sensors

This study proposes a route for surface modification for p-type cobalt oxide-based gas sensors. We deposit a thin layer of Ni on the Co oxide film by sputtering process and annealed at 350 °C for 15 min in air, which changes a typical sputtered film surface into one interlaced with a high density of hemispherical nanoparticles. Our in-depth materials characterization using transmission electron microscopy discloses that the microstructure evolution is the result of an extensive inter-diffusion of Co and Ni, and that the nanoparticles are nickel oxide dissolving some Co. Sensor performance measurement unfolds that the surface modification results in a significant sensitivity enhancement, nearly 200% increase for toluene (at 250 °C) and CO (at 200 °C) gases in comparison with the undoped samples.

Effect of Annealing Temperature of Cugao2 thin Films by Using RF Magnetron Sputtering Technique on Optical and Structural Properties

Cu-based conductive oxide such as CuGaO2 is seen to be a promising transparent p-type oxide material. The study of p-type semiconductor CuGaO2 thin films have been carried out to investigate the effects of different parameters in providing the optimum result in achieving good optical transparency and conductivity of the thin film. The CuGaO2 thin films were fabricated on quartz substrate via the Radio Frequency (RF) magnetron sputtering technique with varying substrate temperatures and different annealing temperatures. The p-type thin films were deposited at a temperature ranging from room temperature, 100°C, 200°C and 300°C. The samples were also annealed varying from temperature of 500°C, 600°C, 700°C and 800°C. The fabricated sample were characterized using X-ray diffraction (XRD), UV-Visible spectroscopy, and atomic force microscope (afm). XRD showed a peak at 2θ = 36.10° (012). The optical transparency values achieved from UV-Vis spectrometer were seen to be approximately 80% and the bandgaps were found to be in the range of 3.34-3.43 eV which is in line with the bandgap value from the research on CuGaO2 thin films. From the afm, the mean surface roughness increases with increasing temperature and this is due to the increment of grain size. The highest grain size was observed at substrate temperature of 200°C.

Preparing p-type ZnO by micro-oxidation of PVD-ZnN

In this study, a micro-oxidation process was proposed to prepare p-type conductive zinc oxide (ZnO) films. The zinc nitride (ZnN) was used as the precursor and oxidized slightly. During the micro-oxidation, a hydrogen peroxide (H2O2) was used to oxidize ZnN to a nitrogen-contained ZnO material (ZnO:N) at room temperature. Since ZnN film was oxidized slightly, there were N atoms residing within the processed film. The presence of nitrogen (N) atoms in the processed film made the completed ZnO:N being p-type conductive. This p-type conduction was attributed to that these N atoms form “No” states in ZnO:N film, which act as acceptors. The carrier concentration and resistivity of the optimized oxidized ZnN film were 1.36 × 1014 cm−3 and 1.31 × 103 Ω cm, respectively. A good rectified current–voltage characteristic with a turn-on voltage of 2.76 V was achieved for the optimized ZnO/ZnO:N junction.

Enhanced sensing performance to toluene and xylene by constructing NiGa2O4-NiO heterostructures

The selective detection of methyl benzene (e.g. toluene and xylene) using oxide semiconductor-based gas/vapor sensors is highly desirable but limited by the low chemical reactivity of the benzene compounds. Exploiting p-type semiconductor oxides (e.g., NiO) which provide distinctive catalytic activities for methyl benzene detection is an essential approach. However, the intrinsic responses of p-type semiconductor oxides are often low. In this paper, high-performance toluene and xylene detection has been realized by a sensor based on novel p-p heterojunction NiGa2O4-NiO nanospheres. Construction of optimized p-p heterojunctions, which resulting from the rationally controlling of the NiGa2O4 content in NiGa2O4-NiO, leads to significantly promoted sensing properties for toluene and xylene detection. It is found that the sensor based on 50% NiGa2O4-NiO exhibits the best sensing performances. Its highest response (Rg/Ra = 12.7 to toluene; Rg/Ra = 16.3 to xylene) is almost 10 times higher than that of the pure NiO (Rg/Ra < 2) to 100 ppm toluene and xylene at 230 °C. More importantly, the sensor exhibits superior selectively for detection of the methyl benzene even against more reactive interfering gases/vapors, such as formaldehyde and ethanol. The p-p oxide heterojunction suggests a promising sensing material for toluene and xylene detection with high response, excellent selectivity, good stability and rapid response/recover time.

Enhanced carrier concentration of Fe doped delafossite CuAlO2 by double-effect: Divalent metal ions doping and excess oxygen

Bulk CuAlO2 compound has been widely studied as a p-type metal oxide semiconductor material due to the simplicity in its synthesis and use of inexpensive raw materials. The Fe doping in CuAlO2 has been demonstrated to enhance the electrical conductivity. An in-depth analysis of the effect of Fe doping in CuAlO2 on the carrier concentration improvement was revealed. Delafossite CuAl1−xFexO2 powders with x = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.30 were synthesized by a solid-state reaction method. The X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) studies were used to measure the atomic-ion concentrations and the oxidation state of each element. The variations of carrier concentration corresponded with a ratio of Cu2+/Cu1+ in CuAlO2. We found that the increase of Cu2+/Cu1+ was caused by double effects through divalent metal ions (Fe2+) doping and excess oxygen (O2−δ) in CuAlO2. The maximum carrier concentration of 8.09 × 1017 cm−3 was obtained for the CuAlO2 sample at Fe content of 10 at.%.

Carrier Generation in p-Type Wide-Gap Oxide: SnNb2O6 Foordite

Wide-gap oxides with their valence band maximum (VBM) composed of s orbitals are essential for realizing practical p-type transparent oxide semiconductors. We prepared a new p-type wide-gap oxide, SnNb2O6 foordite, with its VBM composed of Sn 5s orbitals. To discuss carrier generation, we prepared both p-type and n-type SnNb2O6 by controlling the annealing conditions. The carrier mobility and density were 3.8 × 10–1 cm2 V–1 s–1 and 3.7 × 1018 cm–3, respectively, for the p-type sample and 9.9 cm2 V–1 s–1 and 7.5 × 1015 cm–3, respectively, for the n-type sample. The crystal structure of SnNb2O6 foordite consists of two types of alternating layers, Sn and Nb2O6 octahedra, where three nonequivalent oxygen sites exist. Six oxygens in the chemical formula of SnNb2O6 are distributed at the three sites in pairs, where the oxygens in three nonequivalent sites were named O1–O3. Hole and electron carriers were considered to be generated by Sn4+-on-Nb5+ substitutional defects (SnNb′) and oxygen vacancies of O1 and O2...

Significant Enhancement of Hydrogen-Sensing Properties of ZnO Nanofibers through NiO Loading.

Metal oxide p-n heterojunction nanofibers (NFs) are among the most promising approaches to enhancing the efficiency of gas sensors. In this paper, we report the preparation of a series of p-NiO-loaded n-ZnO NFs, namely (1−x)ZnO-xNiO (x = 0.03, 0.05, 0.7, 0.1, and 0.15 wt%), for hydrogen gas sensing experiments. Samples were prepared through the electrospinning technique followed by a calcination process. The sensing experiments showed that the sample with 0.05 wt% NiO loading resulted in the highest sensing performance at an optimal sensing temperature of 200 °C. The sensing mechanism is discussed in detail and contributions of the p-n heterojunctions, metallization of ZnO and catalytic effect of NiO on the sensing enhancements of an optimized gas sensor are analyzed. This study demonstrates the possibility of fabricating high-performance H2 sensors through the optimization of p-type metal oxide loading on the surfaces of n-type metal oxides.

Silver-doped tin oxide for electrical property enhancement in p-type channel thin film transistor

Though tin oxide (SnO) is one of the strong candidates for p-type oxide semiconductor thin film transistors (TFTs), poor properties and a narrow window of deposition condition are obstacles for implementation. An Ag-doped Sn thin film was studied for the improvement of electrical performance of TFTs. SnO and Ag-doped SnO thin films were deposited by direct current magnetron sputtering as the channel for thin film transistors. The concentration of Ag doping was controlled by the amount of Ag pellets in the sputtering target preparation. A change in oxide states was observed in an Ag-doped SnO film, which can lead to the reduction of the Sn4+ oxide phase. Ag-doped SnO showed improvement of carrier mobility and concentration and resulted in improved electrical characteristics.

On the Possibility of Valence Control of Aluminum Oxide for Electronics Applications

Aluminum oxide, including crystalline Al2O3 and amorphous AlOx, is usually regarded as an insulator. If its conductivity is controlled by donor and acceptor doping, it can be used for various electronics applications as a wide-gap semiconductor or a transparent conductive oxide. In search for shallow donors and acceptors, the energy levels introduced by various impurities are calculated for corundum Al2O3 by the first-principles calculation based on the density-functional theory. Density of states is calculated for a supercell consisting of 120 atoms including one impurity atoms. According to the results, group VII elements (F, Cl, Br, I) can act as a shallow donor, and group II elements (Mg, Zn, Ca) can act as a shallow acceptor. Thus, there is a possibility that both n- and p-type aluminum oxides are fabricated by proper doping and applied for electronics.

GaN metal-oxide-semiconductor devices with ZrO2 as dielectric layers

P-type GaN metal oxide semiconductors (MOS) with ZrO2 dielectrics have been fabricated. Here, H2O and O3 were selected as oxidizers for ZrO2 growth by atomic layer deposition. The MOS devices with O3 oxidant demonstrated a smaller flat band voltage than that with H2O oxidant. A Ga2O3 interlayer was formed between GaN and O3-grown ZrO2, which can effectively decrease interfacial state density to 1.5 × 1011 cm−2. Meanwhile, the innovation of AlZnO (AZO)/Ag nanowires (AgNWs)/AlZnO composite electrodes can further decrease the flat band voltage to about a half compared to that with traditional Cr/Au electrodes and superior electrical performance was achieved.

Electroless Deposition of p-type Cuprous Oxide Thin Film for Solar Application

Solar cells hold a promising prospect in the foreseeable future. Presently cuprous oxides (Cu2O), due to their excellent photovoltaic properties, are being considered as p-type semiconductor materials for solar cell. This work aims at electroless deposition of p-type Cu2O thin film on copper substrate and studying its various properties through characterizations. A bath containing varying amounts of CuCl2.2H20, CoCl2.6H20, NaH2PO2.6H20 and KNaTar.4H2O (Potassium sodium tartrate tetrahydrate) was used for the process. NaOH pellets were used in order to get the desired pH level ranging from 11 to 14. The bath temperature was varied between 30°C to 70°C. The deposited films were characterized by Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS), X-ray diffractometer (XRD) and UV spectroscopy. It was observed that thin films were deposited at different conditions and uniform deposition of Cu2O thin film was visible at pH 14. The coating thickness was found to be in the range of 0.48 to 4.6 μm. The bandgap for the deposited Cu2O was found to be in the range of 1.3-1.4 eV. Some variation in results at different pH were observed and the reasons were identified.

Synthesis, characterization and photo-response of p-type cupric oxide thin films prepared by sol-gel technique

Cupric oxide (CuO) has been deposited onto glass and n-Si (1 0 0) substrate by sol-gel technique. pH of the sol and annealing temperature during film growth were found to be the key parameters for depositing the above film. Microstructural, optical and bonding environmental studies were performed on the above films. The grain sizes in these polycrystalline films varied between ∼50 and 100 nm. Films were preferentially oriented in (−1 1 1) direction. Electrical and galvanomagnetic measurements indicated films to be predominantly p-type with carrier concentration value ∼3.47 × 1015/cm3 and mobility values ∼4.475 cm2/Vs. A typical p-CuO/n-Si hetero-junction structure has been successfully fabricated with open circuit voltage (VOC) ∼359 mV and short-circuits current density (JSC) ∼4.65 mA/cm2, fill factor (FF) ∼0.38 and efficiency (η) ∼1.05%.

Controllable construction of multishelled p-type cuprous oxide with enhanced formaldehyde sensing

Hollow metal oxide semiconductor (MOS) materials with controllable shells have attracted increasing attention because of their interesting properties and potential applications in sensors, catalysis, biology, etc. Cuprous oxide (Cu2O), which is a typical p-type semiconductor material, has four kinds of nanostructures (i.e., single-, double-, triple-, and quadruple-shelled spheres) and was successfully synthesized by the simple regulation of hexadecyl trimethyl ammonium bromide (CTAB) concentration. All as-obtained samples were at the nanometer level, and the hollow layers were also located between the two shells of the Cu2O nanostructures. The structural evolution and formation mechanism of the core-in-hollow multishelled nanostructure were also studied in this work. Moreover, the gas sensing performance of four kinds of materials was measured. The performance of the quadruple-shelled Cu2O-based formaldehyde (HCHO) sensor was greater than that of other sensors. The results indicated that the well-defined multishelled structure may significantly enhance HCHO detection by facilitating the gas adsorption quantity and transport rate.

Electrochemical deposition of Cu-doped p-type iron oxide thin films

Fe2O3 and γ-FeOOH, major constituents of iron rust, are n-type semiconductors with a bandgap of 2.0 ∼ 2.6 eV. In this work, we deposited Cu-doped p-type iron oxide by the electrochemical deposition method. p-type iron oxide is expected to be applied for photo-cathodes in electrochemistry and also for pn junction solar cells. The films were deposited from an aqueous solution containing FeSO4 and CuSO4 and annealed in air at 400 °C. The conduction type was converted into p-type when the ratio of copper to iron in the film was about 10% or more. Annealing in air improved film adhesion to the substrate and enhanced p-type conductivity.

Oxygen Vacancy Defects Boosted High Performance p-Type Delafossite CuCrO2 Gas Sensors

p-type ternary oxides can be extensively explored as alternative sensing channels to binary oxides with diverse structural and compositional versatilities. Seeking a novel approach to magnify their sensitivities toward gas molecules, e.g., volatile organic compounds (VOCs), will definitely expand their applications in the frontier area of healthcare and air-quality monitoring. In this work, delafossite CuCrO2 (CCO) nanoparticles with different grain sizes have been utilized as p-type ternary oxide sensors. It was found that singly ionized oxygen vacancies (Vo•) defects, compared with the grain size of CCO nanoparticles, play an important role in enhancing the charge exchange at the VOCs molecules/CCO interface. In addition to suppressing the hole concentration of the sensor channel, the unpaired electron trapped in Vo• provides an active site for chemisorptions of environmental oxygen and VOCs molecules. The synergetic effect is responsible for the observed increase of sensitivity. Furthermore, the sensitive (Vo• defect-rich) CCO sensor exhibits good reproducibility and stability under a moderate operation temperature (<325 °C). Our work highlights that Vo• defects, created via either in situ synthesis or postannealing treatment, could be explored to rationally boost the performance of p-type ternary oxide sensors.

P-type conductivity in hydrogenated radio frequency sputtered tin oxide thin films

P-type oxide semiconductors are being intensively investigated for their potential use in optoelectronic and microelectronic applications, such as active electronics and transparent transistors. In this work, the p-type conductivity of tin oxide thin films was examined. The films were deposited using radio frequency magnetron sputtering in an atmosphere containing a mixture of hydrogen and argon. The structural, chemical, optical, and electrical properties of the films were analyzed as the hydrogen-to-argon concentration was varied from 0% to 20%. The films were polycrystalline except those deposited under the highest hydrogen concentration which became amorphous. Chemical analysis of the films revealed a substantial concentration of oxygen vacancies. Optically, the films were transparent and their band gaps decreased as the hydrogen concentration increased. The pure tin oxide films had n-type conductivity, which was attributed to oxygen vacancies. However, the conductivity was converted to p-type as the hydrogen concentration increased. This was attributed to the formation of hydrogen shallow acceptor states, which was supported by the observation of a red-shift of the band gap. The optimum hydrogen concentration for p-type conductivity was found to be 10%. The p-type character of the films was demonstrated by fabricating a hetero p-n junction based on SnO2/Si, which exhibited the expected non-linear diode rectification behavior.