Wolfgang Hanrieder

Stability of semiconducting gallium oxide thin films

Abstract In bulk, gallium oxide is a semiconducting oxygen-sensitive material at temperatures of over 500 °C. With the aid of a high frequency sputter process, films could be produced with thicknesses in the micrometre range using a Ga2O3 ceramics target. The stoichiometry and purity of these films could be demonstrated with Rutherford backscattering spectroscopy and X-ray fluorescence. Scanning electron microscopy investigations showed that, at temperatures of over 1100 °C for 50 h, crystallites approximately 200 nm in size are formed. The d.c. conductivity of these films was measured for the first time. At high temperatures, semiconducting properties similar to those of the bulk material were demonstrated. The conductivity of the Ga2O3 films is dependent on the oxygen content of the surrounding atmosphere. The exponent of the σ vs. PO2−n curve was n⩽ 1 4 . Auger spectroscopy was used to show that, at temperatures around 1000 °C, interdiffusion of the Al3+ and the Ga3+ ions takes place on substrates containing aluminium, thus destroying the conductivity of the film. Therefore Al2O3 ceramic substrates which are standard in thick film technology are unsuitable for stable oxygen-sensitive Ga2O3 thin film devices. Alternatives are shown.

Scanning tunnelling microscopy on insulating Ga2O3 thin-film ceramics

Scanning tunnelling microscopy images of stoichiometric and thus electrically insulating Ga2O3 thin films are presented for the first time. These ceramic films were deposited by RF sputtering. Characterisation was by means of RBS (stoichiometry) and X-ray fluorescence (contaminants). The scanning tunnelling microscopy results are consistent with those of high-resolution scanning electron microscopy. For scanning electron microscopy the specimens had to be covered by a thin metallisation. This was not necessary for the scanning tunnelling microscopy images. The presented results show that scanning tunnelling microscopy is suitable for the detailed study of insulating thin-film ceramics. This is a prerequisite for future studies of ultra-thin metallisation on thin film ceramics (supported catalysts) with superior vertical resolution.

Scanning tunnelling microscopy on oxygen-sensitive Ga2O3 thin-film ceramics

Abstract Scanning tunnelling microscopy (STM), which yields lateral and vertical resolution in the sub-nm range, was applied to thin films of stoichiometric, oxygen-sensitive Ga2O3. In a next step a combined STM and Auger electron spectroscopy study was performed for ultra-thin platinum metallizations of these Ga2O3 ceramic carriers. The drastically enhanced activity and its degradation at temperatures around 1000 K of this real supported catalyst with respect to CO oxidation rates is demonstrated by means of a quadrupole mass spectrometer in varying O2/CO atmospheres. For the first time STM images and images of the local barrier height of these ultra-thin Pt layers are shown before and after degradation of the catalytic activity. Special care has been taken to prepare tunnelling tips of extremely low curvature radii and of sufficiently high mechanical stability to enable realistic imaging of these rough surfaces to be performed. General guidelines for tip-shape preparation result from a finite-element simulation of d.c. etched tungsten tips.

Causes of fast degradation of thin-film lambda probes in motor vehicle exhaust and initial measures to improve long-term stability

Abstract Oxygen sensors for cylinder-selective measurement of the lambda value in motor vehicle engines are exposed to extreme environmental conditions. At the mounting position close to the cylinder outlet valve, temperatures up to 1000 °C prevail, as well as vibrations > 50 g and powerful turbulent flows that are loaded with particles from the engine wear and with contaminants from engine oil and fuel. Thin-film oxygen sensors made from metal oxides, which function in a completely satisfactory and stable way in extensive laboratory experiments, already show significant signs of degradation during engine operation after a relatively short time (10 h). This paper discusses, for the first time in detail, experimental results on the degradation of thin-film oxygen sensors in motor vehicle engines and its concrete causes. For this purpose, comprehensive studies with surface and microanalysis methods (Auger electron spectroscopy, X-ray fluorescence analysis, X-ray photoemission spectroscopy, electron microprobe and optical microscopy) have been performed and correlated with changes in the sensor characteristics. The results show that both doping of the sensor material by contaminants from oil and fuel and deposition of particles from engine wear on the sensor surface give rise to changes in the sensor characteristic. An improved sensor housing reduces the particle deposition and thus contributes to a significant increase in the long-term stability of the fast lambda probe.

Photocapacitance characteristics of amorphous‐silicon Schottky diode sensor arrays and their changes due to the Staebler–Wronski effect

The capacitancemeasurements on a‐Si:H Schottky diodes reported in the literature refer to large‐area (some 10‐mm2) specimens and were carried out in the dark at frequencies typically around 10 Hz. The main interest in these publications is the determination of the density of states in the band gap. For linear photodetector array applications, however, interest centers around the voltage and frequency characteristics of the capacitance of small‐area back‐to‐back Schottky diodes under illumination and at frequencies typically around 1 kHz. But photoinduced changes in the capacitance are also of significance with a view to possible read‐out circuits and their long‐term stability. The capacitance was therefore measured over the frequency range 0.2–20 kHz and over the voltage range −5 to +5 V. Whereas the dark capacitance exhibited no changes due to photoinduced degradation, significant differences between degraded and nondegraded states were observed in the voltage and frequency characteristics of the photocapacitance. During degradation under high bias voltage (−5 V), the maximum of the capacitance shifted from −0.1 to +0.4 V indiumtin oxide bias voltage and the capacitance showed a distinct increase. The photoinduced changes observed for the photocapacitance characteristics could be minimized by ensuring that the elements adjacent to the sensor were at equipotential with the sensor. Full reversibility of the photoinduced changes due to thermal annealing could be verified. The changes of the C‐V and C‐ω characteristics can be explained by the occurence of the well‐known Staebler–Wronski effect.

Photoinduced degradation of reverse‐biased small‐area a‐Si:H Schottky barriers

Photoinduced changes (Staebler–Wronski effect) in the photocurrent and dark current of a reverse‐biased small‐area Schottky barrier made from amorphous silicon are investigated. The well‐known, small reductions in the photocurrent are contrasted with the large increases in the dark bias current. This behavior of the dark bias current is a new phenomenon and cannot be explained on the basis of a bulk mechanism. The indications are that even under high bias voltages recombination processes take place close to the interface and give rise to thermally reversible changes of the gap density‐of‐states. Special attention is paid to the bias voltage and wavelength dependence of the degradation effect. The results are discussed in qualitative terms.