Dominik Köhl

The role of energetic ion bombardment during growth of TiO2 thin films by reactive sputtering

TiO2 thin films have been deposited by several different sputtering processes: (i) dc magnetron sputtering (dcMS) employing various geometrical conditions, (ii) ion-assisted dc magnetron sputtering where additional ion bombardment of the growing films was performed with an auxiliary ECR ion source and (iii) high power impulse magnetron sputtering (HiPIMS). Films have been investigated mainly by grazing incidence x-ray diffraction and atomic force microscopy. It is shown that the highly energetic oxygen ions inherent in reactive sputtering of metal oxides are the dominant energetic species governing structure formation of TiO2 films by their kinetic impact. The trajectories of these energetic oxygen ions strongly depend on the shape of the erosion trace and hence on the age of the target, which therefore has a strong influence on structure formation. Furthermore, in a HiPIMS discharge the role of this energetic oxygen ion bombardment is strongly intensified due to the increased target voltage and the lower deposition rate compared with a dcMS discharge. It is also demonstrated that films with pure rutile structure which are stable under a post-deposition thermal treatment can be deposited under intense energetic ion bombardment at low temperatures either by HiPIMS at high peak power densities or by ion-assisted dcMS.

Structural improvement of zinc oxide films produced by ion beam assisted reactive sputtering

Reactively sputtered zinc oxide thin films exhibit low crystalline order when deposited on unheated substrates. To improve the structural order, films are usually deposited onto heated substrates at temperatures of about 200–300 °C. Nevertheless, techniques that enable room temperature deposition of ZnO films with high structural quality would be advantageous. In this work ion bombardment from an auxiliary ion gun during film growth is employed to improve the crystalline quality. Xe+ ion bombardment under appropriate conditions leads to the growth of films with high crystalline order. Based on our structural investigations employing x-ray diffraction, atomic force microscopy and transmission electron microscopy, a growth model is proposed which explains the impact of ion bombardment on the structural evolution. We prove that it is especially the nucleation stage of the growth process which is susceptible to this ion bombardment.

Structure control of sputtered zinc oxide films by utilizing zinc oxide seed layers tailored by ion beam assisted sputtering

Reactively sputtered zinc oxide thin films typically exhibit a c-oriented (0 0 0 1) texture of low crystalline order when deposited on unheated substrates. The structural order can be significantly improved upon heating the substrates during deposition. Here it will be demonstrated that by utilizing c-textured seed layers, which are grown by an ion beam assisted sputtering (IBAS) process, films can be deposited at room temperature with significantly improved c-texture. These films are significantly less sensitive to detrimental oxygen ion bombardment. By tailoring the IBAS process, even seed layers with dominant a-texture can be produced. Subsequently thick ZnO films can be grown on appropriate seed layers which are pre-dominantly a-textured.

The uptake of oxygen by noble metal clusters on gas sensors

Abstract The uptake of oxygen by Pd clusters on gamma-alumina has been studied by means of XPS, XRD and TDS. Cluster size and the amount of surface and bulk oxygen are determined. A metallic surface covered with chemisorbed oxygen plays a key role in Pd for the detection of hydrogen in air at moderate temperatures. Bulk PdO is formed by oxidation at higher temperatures and the catalytic activity decreases. Simultaneously the sensitivity of the pellistor for hydrogen becomes very low. This behaviour is in contrast to the detection of methane, where the activity increases with the formation of bulk oxide. For comparison some results with Rh are included.

Ion beam assisted sputter deposition of ZnO for silicon thin-film solar cells

Ion beam assisted deposition (IBAD) is a promising technique for improving the material quality of ZnO-based thin films. The operation of an auxiliary Ar + ion source during deposition of ZnO : Ga thin films by dc magnetron sputtering led to an improvement in crystalline texture, especially at low temperatures due to momentum transfer from the ions to the growing film. Etching of IBAD-ZnO : Ga films in diluted HCl revealed crater-like surface structures with crater diameters of up to 600 nm. These structures are usually achieved after deposition at high substrate temperatures. This is an indication that the grain structure was remarkably changed by bombarding these films during deposition in terms of increasing the compactness of the ZnO : Ga films. Subsequent annealing procedures led to an improvement in the electrical and optical properties. Hydrogenated microcrystalline silicon (µc-Si : H) solar cells exhibited enhanced efficiency as compared to cells on other low-temperature sputtered reference ZnO films. This improvement was ascribed to light trapping by the modified etching behaviour of the IBAD-ZnO : Ga films as well as improved transparency after the vacuum annealing step.

Detection of phenylarsine in air

Abstract Phenylarsine (PhAsH2), a toxic gas used in epitaxial growth of GaAs, is detectable by a conductance change of a semiconducting thick film. The film consists of polycrystalline SnO2 without intentional doping on an alumina substrate. In a flow system at temperatures above 670 K, a reversible conductance increase on admixture of PhAsH2 to dry air is observed. At 890 K the conductance was found to increase linearly with phenylarsine concentration in the range from 0.5 to 20 ppm. Response and recovery time at 1.5 ppm amount to 5 and 9 s, respectively. The initial slope of conductance is also used as a sensor signal. In mass spectrometric investigations, H2O, C6H6 and C6H5As are found as decomposition products. XPS measurements show a deposit of arsenic on the SnO2 surface up to a temperature of 770 K. A decay reaction sequence of PhAsH2 on SnO2 is proposed, which explains the conductance increase by formation of oxygen vacancy donors during a water formation step.

An SnO2 sintered layer for phenylarsine detection

Abstract Phenylarsine (PhAsH2), a toxic gas used in the epitaxial growth of GaAs, is shown to be detectable by a semiconducting thick film. The film consists of polycrystalline SnO2 without intentional doping, sintered at 870 K on an alumina substrate. In a flow system at temperatures above 675 K a reversible conductance increase on introducing PhAsH2 to dry air is observed. At 890 K the conductance was found to increase linearly with phenylarsine concentration in the range 0.5–20 ppm. Response and recovery time at 1.5 ppm were 5s and 9s respectively. The initial slope of the conductance can be used as a sensor signal. The sensitivity to hydrogen, a gas also present in the epitaxial process, was also investigated. Kinetic data allow a donor adsorption-desorption process for phenylarsine to be excluded. A comparison with the results of hydrogen exposure shows that the initial time response to phenylarsine is probably governed by a dehydrogenation step.

Fast detection of phenylarsine, a comparison of sputtered and sintered SnO2 films

Abstract Phenylarsine (PhAsH2), a toxic gas used in epitaxial growth of GaAs, is detectable by a conductance change of an SnO2 film. Two types of films are prepared: a sintered film (9 μm) and a sputtered film (0.1 μm), both without intentional doping on alumina substrates. In a flow system at temperatures above 670 K a reversible conductance increase on admixture of 0.5 to 30 ppm PhAsH2 to dry air is observed. Saturation of the conductance is reached within 10 s for both film types at 890 K. At lower temperatures the sputtered film responds faster than the sintered film. Evaluation of the initial slope of conductance allows the phenylarsine concentration to be determined within 1 s. In thermal desorption spectroscopy (TDS) with a mass spectrometer, water, benzene, arsenic, a radical (C6H5As) and phenylarsine are found to leave the surface. Benzene formed on the surface seems to control the conductance response at low temperatures.