Klaus Ellmer

Influence of discharge parameters on the layer properties of reactive magnetron sputtered ZnO:Al films

Abstract Aluminum-doped ZnO layers have been prepared by reactive d.c. magnetron sputtering from Zn:Al (2 wt.%) targets onto unheated substrates (Si, glass, glassy carbon). In dependence on the O2 partial pressure in the argon sputtering gas there exists a narrow process window around a p O2 (p Ar + p O2 ) ratio of 5–10% which yields transparent, low-resistance layers. The discharge voltage dependence on the oxygen partial pressure is a sensitive indicator for the oxidation state of the target surface and can be used for the regulation of the deposition process. Lower O2 partial pressures yield metallic-like, opaque, but highly resistant layers. Higher oxygen partial pressures lead to transparent but highly resistant ZnO layers. Layers of lowest resistivity (5 × 10−4 Ω cm) and highest optical transmission (90%) have a stoichiometric ratio Zn:O of 1.0 and exhibit the largest grains (≈40 nm) as has been measured by Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). By comparing the metallurgical Al content (using RBS) in the films with the carrier concentration (Hall and conductivity measurements) we obtain an overall electrical activation of aluminiu in the best case of about 60%. We found an exponential dependence of the specific resistance on the ZnO crystallite size which explains the strong dependence of the sheet resistance on the oxygen partial pressure.

Structural, optical and electrical properties of polycrystalline iron pyrite layers deposited by reactive d.c. magnetron sputtering

Thin iron disulphide pyrite (FeS2) films (5–100 nm) were prepared by reactive d.c. magnetron sputtering from an iron target in an Ar/H2S atmosphere. Optimum deposition was obtained with a sputtering power of 100 W in an 80% H2S gas mixture at a total pressure of 5 Pa. Iron pyrite films were successfully deposited on different glass substrates, at substrate temperatures up to 350°C. Higher temperatures led to sulphur loss of the films. X-ray diffraction measurements have shown that the films are polycrystalline with a grain size up to 50 nm. Rutherford backscattering analysis yielded an overall stoichiometry of FeS1.95 in the best films. From the optical transmission and reflection an optical absorption coefficient of 5×105 cm−1 for λ<560 nm was calculated. The films show p-type conductivity. A hole concentration upto 1×1020 cm−3 and a mobility up to 25 cm2V−1s−1 at room temperature was determined by Hall measurements. The conductivity type is also supported by UV photoelectron spectroscopy where the position of the Fermi level was found to be very close to the valence band edge.

Transient surface photovoltage of p-type Cu3BiS3

Thin films of Cu3BiS3 were prepared by coevaporation. Hall-effect, Seebeck-effect, and surface photovoltage measurements show that Cu3BiS3 is a p-type semiconductor with Hall-mobility, free carrier concentration, and thermo-electric power of 4 cm2/V s, 2×1016 cm−3, and 0.73 mV/K, respectively. The work function was determined by Kelvin probe force microscopy to be (4.37±0.04) eV before and (4.57±0.01) eV after deposition of a thin In2S3 layer. Transient surface photovoltage measurements at variable excitation wavelength showed the importance of defect states below the band gap for charge separation and the opportunity for surface defect passivation by a very thin In2S3 layer. The band bending at the Cu3BiS3/In2S3 interface was obtained. The role of grain boundaries for charge transport and charge separation is discussed.

Stoichiometry-, phase- and orientation-controlled growth of polycrystalline pyrite (FeS2) thin films by MOCVD

Abstract The growth process of polycrystalline pyrite thin films employing low pressure metalorganic chemical vapor deposition (LP-MOCVD) in a vertical cold wall reactor has been investigated. Iron pentacarbonyl (IPC) and t-butyldisulfide (TBDS) were utilized as precursors. Study of the growth rate as a function of temperature reveals a kinetically controlled growth process with an activation energy of 73 kJ / mol over the temperature range from 250 to 400°C. From 500 to 630°C, the growth rate is mainly mass transport limited. Decomposition of the films into pyrrhotite (Fe1 − xS) occurs at higher growth temperatures. The S Fe ratio in the films has been controlled from 1.23 up to 2.03 by changing the TBDS partial pressure. With increasing deposition temperature, the crystallites in the films show the tendency to grow [100]-oriented on amorphous substrates at a growth rate of 2.5 A / s. The grains show a preferential orientation in the [111] direction upon lowering the growth rate down to 0.3 A / s. Temperatures above 550°C are beneficial in enhancing the grain size in the columnar structured films up to 1.0 μm.

Preparation of textured and photoactive 2H-WS2 thin films by sulfurization of WO3

Abstract Photoactive thin films of tungsten disulfide have been prepared by sulfurization of WO3 layers. Tungsten trioxide films were deposited on heated quartz and glassy carbon substrates by gaseous reaction of W(CO)6 with oxygen. Subsequently, the oxide films were treated at 700 °C in a gaseous sulfur atmosphere either in a flowing system or in evacuated and sealed quartz ampoules. The films were investigated by X-ray diffractometry, scanning electron and scanning tunneling microscopy. The photoactivity was measured using time-resolved microwave conductivity. Films consisting of the 2H-WS2 platelets, highly textured and oriented with their c axis perpendicular to the substrate, were found. In addition, the phase relations in the phase triangle W-O-S were studied by thermochemical equilibrium calculations using the Gibbs free energy minimization technique. The inferred predominance area diagram log Ps2 vs. log pso2 confirms the high stability of WO2,9 and WS2 as observed under related experimental conditions. The effect of the nickel on the orientation of the 2H-WS2 crystallites obtained after sulfurization of the WO3 films can be explained by a flux on the basis of the binary Ni-S phase diagram or surfactant mediated growth effect.

Structural and photoelectrical properties of FeS2 (pyrite) thin films grown by MOCVD

The growth of FeS 2 (pyrite) thin films on natural FeS 2 and synthetic ZnS crystals by low pressure metalorganic chemical vapor deposition using iron pentacarbonyl (Fe(CO) 5 ) and di-tertiary butyldisulphide as precursors is reported. For the first time we are able to grow epitaxial films at 475°C on (100) FeS 2 crystals with high growth rates up to 0.7 μm/h and good structural properties. Despite the nearly perfect lattice matching to pyrite (lattice mismatch 0.1%) the growth on ZnS substrates is more complex due to the different crystal structures yielding always polycrystalline layers. However, the photoeffects of these films are much higher than on glass substrates as revealed by time resolved microwave conductivity (TRMC) measurements. The crystalline quality of the layers has been proved using X-ray diffraction and RBS channeling. The layer morphology was determined by scanning electron microscopy (SEM). Temperature dependent conductivity measurements give information about the electronic quality of the films.

Growth of FeS2 (pyrite) thin films on single crystalline substrates by low pressure metalorganic chemical vapour deposition

Abstract Thin films of pyrite have been prepared by low pressure metalorganic chemical vapour deposition (LP-MOCVD) from iron pentacarbonyl (Fe(CO) 5 ) and di-tert.-butyl disulphide (TBDS) on Si, GaP and ZnS substrates. It was found that stoichiometric pyrite films without marcasite inclusions could be prepared at deposition temperatures from 450 to 500°C. The films were always polycrystalline but showed a preferred orientation in the (111) direction at high temperatures (475°C). The grain sizes in the range 1–10 μm depend on the substrate orientation, the growth rate and the substrate temperature. The layer composition is stoichiometric (FeS 2 ) up to 500°C. The formation of pyrrhotite phases (Fe 1− x S) occurs at higher temperatures, when the sulphur partial pressure in the gas phase is too low to compete against the decomposition pressure of sulphur in the pyrite layer.

The impact of negative oxygen ion bombardment on electronic and structural properties of magnetron sputtered ZnO:Al films

In order to study the impact of negative oxygen ion bombardment on the electronic transport properties of ZnO:Al films, a systematic magnetron sputtering study from ceramic targets with excitation frequencies from DC to 27 MHz, accompanied by strongly varying discharge voltages, has been performed. Higher plasma excitation frequencies significantly improve the transport properties of ZnO:Al films. The effect of the bombardment of the films by energetic particles (negative oxygen ions) can be explained by the dynamic equilibrium between the formation of acceptor-like oxygen interstitials compensating the extrinsic donors and the self-annealing of the interstitial defects at higher deposition temperatures.

A comparative study of electronic and structural properties of polycrystalline and epitaxial magnetron-sputtered ZnO:Al and Zn1-xMgxO:Al Films—Origin of the grain barrier traps

Homoepitaxial and heteroepitaxial ZnO, ZnO:Al, and Zn1-xMgxO:Al films have been grown by magnetron sputtering from ceramic targets at substrate temperatures between 200 °C and 500 °C. We studied the relation between the electronic transport and structural properties for the epitaxially grown films and compared it to the properties of polycrystalline films by means of X-ray diffraction, transmission electron microscopy and optical reflectance and transmittance measurements. The results show that the epitaxial growth of ZnO:Al and Zn1-xMgxO:Al thin films, which has been observed for nearly all films prepared on single crystalline substrates, will not significantly improve the electronic transport properties in comparison to polycrystalline films unless the grain boundaries are eliminated completely. The grain boundary defect densities of about 3 × 1013 cm−2 are nearly independent on the structural quality of the different polycrystalline, hetero- and homoepitaxial films. This clearly proves that the grain b...

CRYSTALLIZATION OF LAYERED METAL-DICHALCOGENIDES FILMS ON AMORPHOUS SUBSTRATES

Anisotropic materials with layered structure, like MoS2 and WSe2, play an important role in a number of technologies. Some of these applications (lubrication, photovoltaics) require polycrystalline films oriented with their c axis perpendicular to the substrate surface (type‐II texture), which is the thermodynamically favorable texture. However, films with the substrate ∥c (type‐I texture) are usually obtained. We report that an ultrathin ( 2, or InSe disentangle the growth mode of the film from the underlying amorphous substrate, and hence, WSe2 films with a perfect type‐II texture and crystallites at least a few mm2 large are obtained at temperatures as low as 700 °C (van der Waals rheotaxy–vdWR). The mechanism for this growth mode is proposed.