R. Asomoza

Effect of the substrate temperature and acidity of the spray solution on the physical properties of F-doped ZnO thin films deposited by chemical spray

F-doped ZnO thin films were prepared by using the spray pyrolysis technique. The dependence of the electrical, optical, structural and morphological properties on the substrate temperature and spray solution acidity was studied. Additionally, aging of the spray solution presents a clear effect on the resistivity of ZnO thin films. The best films obtained show a resistivity, mobility and carrier concentration of the order of 1.5 10 @2 O cm, 6 cm 2 /V s and 2 10 19 cm @3 , respectively. Wurtzite hexagonal structure, with a preferential growth along the [0 0 2] direction for all substrate temperatures and acidities used, was obtained. From scanning electron microscopy and atomic force microscopy analysis, it was determined that the grain size of the films decreases and its homogeneity increases when the acidity of the starting solution is increased. High optical transmittances, in the order of 90%, were obtained in all the cases. r 2002 Elsevier Science B.V. All rights reserved.

Growth of textured ZnO:In thin films by chemical spray deposition

Abstract Insium-dopod zinc thin films were grown using the chemical spray deposition technique and different doping compounds in the solution. The partucular, indium chloride, indium sulfate and indium acetate were used as indium sources. The films show a high degree of preferential crystalline orientation which depends mainly on the type of doping compound and the substrate temperature. The lowest value o the resistivity is 2 × 10 −3 ω cm and and the transmittance is on average higher than 90%. It was found that the texture of the films could be change quite easily making it possible to grow films for different applications.

SnO2 and SnO2:Pt thin films used as gas sensors

Abstract SnO 2 and SnO 2 :Pt thin films, prepared by chemical spray deposition, have been used as gas sensors. The electrical characterization results and their sensing properties in the presence of CO are reported. The films were grown by chemical spray deposition on glass substrates at different temperatures. The initial solution was obtained by diluting SnCl 4 ·5H 2 O in ethyl alcohol at 0.2 M. In the case of the doped films we have added PtCl 2 to the initial solution in the following proportions: 3, 5 and 8% at. The conductance of the films was calculated from the electrical resistance which was measured as a function of measuring temperature, thickness and Pt concentration in the solution. The values of the sensitivity for 3.8 torr of CO partial pressure are as high as 5 for SnO 2 and 4×10 2 for SnO 2 :Pt. Additionally we report results on surface topography and crystalline structure.

Si-doped AlxGa1−xN(0.56⩽×⩽1) layers grown by molecular beam epitaxy with ammonia

We describe experiments on Si doping in AlxGa1−xN grown by gas source molecular beam epitaxy with ammonia and silane. Growth conditions that minimize self-compensation were used to assure Si incorporation at a level of 2×1020cm−3 for the entire range of compositions investigated, from x=0.56 to 1.0. These conditions resulted in donor concentrations of ∼1×1019cm−3 up to x=0.85. Layers of AlxGa1−xN up to x=0.85 show good mobility and low resistivity. In these layers, the activation energy, Ea, of Si stays below ∼25meV and Si can be considered a shallow donor. For AlN content above x=0.85 the donor activation energy increases to Ea∼250meV in AlN. The change in donor activation energy correlates with increased incorporation of oxygen and carbon.

Mg and O codoping in p-type GaN and AlxGa1−xN (0<x<0.08)

We describe Mg and O codoping experiments in gas-source molecular-beam epitaxy of GaN and AlGaN that produce high levels of Mg incorporation and activation. In order to obtain the highest level of Mg incorporation the surface stoichiometry was optimized by adjusting the NH3/Ga and NH3/(Ga+Al) flux ratios. The lowest acceptor activation energy and the highest hole concentration, p=2×1018 cm−3, were measured in samples of p-GaN and p-AlxGa1−xN with well-defined Mg/O ratios determined by secondary ion mass spectrometry. Measurements of the temperature dependence of diffusion current in p–n junctions formed in Al0.08Ga0.92N and GaN show acceptor activation energy of 195±10 and 145±15 meV, respectively. Low activation energies are attributed to successful codoping.

Spinodal decomposition range of InxGa1−xNyAs1−y alloys

Spinodal decomposition range of InxGa1−xNyAs1−y quaternary alloys as the result of the strain and coherency strain energies, transformations of the bonds, and a lattice mismatch between the GaAs substrate and an alloy is described. The alloys are considered in the strictly regular approximation. The strain energy is presented in the valence-force field model. The spinodal decomposition temperatures of InxGa1−xNyAs1−y alloys are demonstrated up to 1000 °C. It is shown that nitrogen dramatically increases the temperature of the coherent spinodal.

Gas source molecular beam epitaxy of high quality AlxGa1−xN (0⩽x⩽1) on Si(111)

Layers of AlxGa1−xN, with 0⩽x⩽1, were grown on Si(111) substrates by gas source molecular beam epitaxy with ammonia. We show that the initial formation of the Si–N–Al interlayer between the Si substrate and the AlN layer, at a growth temperature of 1130–1190 K, results in very rapid transition to two-dimensional growth mode of AlN. The transition is essential for subsequent growth of high quality GaN, AlxGa1−xN, and AlGaN/GaN superlattices. The undoped GaN layers have a background electron concentration of (2–3)×1016 cm−3 and mobility up to (800±100) cm2/V s, for film thickness ∼2 μm. The lowest electron concentration in AlxGa1−xN, with 0.2<x<0.6, was ∼(2–3)×1016 cm−3 for 0.5–0.7-μm-thick film. Cathodoluminescence and optical reflectance spectroscopy were used to study optical properties of these AlxGa1−xN layers. We found that the band gap dependence on composition can be described as Eg(x)=3.42+1.21x+1.5x2. p–n junctions have been formed on crack-free layers of GaN with the use of Mg dopant. Light emitt...

Characteristics of ZnO:F thin films obtained by chemical spray. Effect of the molarity and the doping concentration

The effect of the zinc acetate molarity combined with the substrate temperatures of ZnO:F thin films deposited by spray pyrolysis, on their physical properties, was studied. The main interest was to optimize the resistivity and the optical transmittance of films deposited at different substrate temperatures. It is to be remarked the influence of the aging of the starting solution on the improvement of the electrical properties of the films. The lowest value of the resistivity was obtained with solutions of 0.2 M. In this case, the resistivity obtained for a film 325-nm thick was 5.5×10−2 Ωcm, decreasing to a value of 6.7×10−3 Ωcm for films 1400-nm thick. Mobility values were approximately 7 cm2/V-S. The transmittance in the visible is near 90% at 550 nm. All the films showed a preferred (002) crystalline orientation irrespective of the deposition conditions. Finally, the grain size increased as the molarity of the solution decreased.

Work function change caused by alkali ion sputtering

Abstract In the presented work we performed an experimental study of the work function decrease for a silicon sample caused by Cs+ ion bombardment. We varied the energy of primary Cs+ ions as well as the angle of incidence in order to reach a different concentration of implanted cesium ions and to find a dependence of the work function change on the cesium surface concentration. A “surface dipoles” model was developed. The model based on the electronegativity concept considers formation of Si–Cs dipoles with corresponding dipole moment. An electric field of these dipoles results in a decrease of the work function, whereas a partly ionic character of Si–Cs bond increases the surface binding energy of cesium and, as a consequence, its surface concentration. A good correlation between the model and the experimental data was found.

Chemical stability of doped ZnO thin films

We present the results of a chemical etching stability study carried out on ZnO thin films doped with several elements deposited by spray pyrolysis. Prior to the etching, a structural study was done by X-ray diffraction and the texture of the samples was obtained by scanning electron microscopy. The samples were etched employing a solution of dilute hydrochloric acid. The etching rates obtained for the different samples depend on the dopant element and our results confirm that films doped with Cr present the highest stability against chemical etching.