U. Zastrow

Efforts to improve carrier mobility in radio frequency sputtered aluminum doped zinc oxide films

This study addresses the electrical and optical properties of radio frequency magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films. The main focus was on the improvement in carrier mobility μ to achieve simultaneously high transparency for visible and particularly near-infrared light and low resistivity. The influence of Al concentration in the target, film thickness, sputter power, deposition pressure, and substrate temperature on material properties was investigated. The structural, compositional, electrical and optical properties were studied using x-ray diffraction, secondary ion mass spectrometry (SIMS), room temperature Hall effect measurements and spectral photometry, respectively. All ZnO:Al films were polycrystalline and preferentially oriented along [002]. The grain size along the direction of growth increased with higher Al doping and with increasing film thickness. The SIMS measurements revealed that the Al concentration in the film was nearly the same as in the target. Carrier concent...

Hydrogen stability in amorphous germanium films

Abstract The thermal stability of hydrogen in hydrogenated amorphous germanium films has been studied by hydrogen evolution and by deuterium and hydrogen interdiffusion experiments. Similar to observations on hydrogenated amorphous silicon, the hydrogen stability in amorphous germanium is found to depend strongly on the film microstructure and on the position of the Fermi level.

Influence of base pressure and atmospheric contaminants on a-Si:H solar cell properties

The influence of atmospheric contaminants oxygen and nitrogen on the performance of thin-film hydrogenated amorphous silicon (a-Si:H) solar cells grown by plasma-enhanced chemical vapor deposition at 13.56 MHz was systematically investigated. The question is addressed as to what degree of high base pressures (up to 10−4 Torr) are compatible with the preparation of good quality amorphous silicon based solar cells. The data show that for the intrinsic a-Si:H absorber layer exists critical oxygen and nitrogen contamination levels (about 2×1019 atoms/cm3 and 4×1018 atoms/cm3, respectively). These levels define the minimum impurity concentration that causes a deterioration in solar cell performance. This critical concentration is found to depend little on the applied deposition regime. By enhancing, for example, the flow of process gases, a higher base pressure (and leak rate) can be tolerated before reaching the critical contamination level. The electrical properties of the corresponding films show that incre...

Solubility and diffusion of hydrogen in hydrogenated crystalline and amorphous silicon

Hydrogen diffusion and solubility effects in hydrogenated crystalline and amorphous silicon prepared by hydrogen implantation with approximately equal hydrogen concentrations are compared. Hydrogen diffusion is found to be limited by hydrogen solubility in both cases. Between 300 and 450°C, an agreement of the hydrogen diffusion coefficients for the two Si:H materials approximately within a factor of ten is observed. With increasing hydrogen concentration, hydrogen diffusion increases for amorphous Si:H but decreases for crystalline Si:H. This latter decrease is attributed to a hydrogen related microstructure which appears in crystalline Si:H at about a factor of ten smaller hydrogen concentration than in amorphous Si:H.

Diffusion and effusion of hydrogen in microcrystalline silicon

The diffusion and effusion of hydrogen in hydrogenated microcrystalline silicon films deposited in an electron cyclotron resonance reactor were studied for various deposition temperatures T{sub s}. For deposition temperatures below 250 C, hydrogen effusion is found to be dominated by desorption of hydrogen from internal surfaces followed by rapid out-diffusion of H{sub 2}. Higher substrate temperatures result in an increased hydrogen stability suggesting the growth of a more compact material. For this latter type of samples, a hydrogen diffusion coefficient similar as in compact plasma-grown a-Si:H films is found despite a different predominant bonding of hydrogen according to infrared absorption.

Comparative study of hydrogen stability in hydrogenated amorphous and crystalline silicon

The dependence of hydrogen effusion and diffusion on hydrogen concentration was studied for crystalline and amorphous Si:H samples which were hydrogenated by hydrogen ion implantation. The results are compared to data for a-Si:H and μc-Si:H films grown with various H concentrations. Although general trends of the H concentration dependence of H stability are similar, there are differences between c-Si:H and a-Si:H. These differences involve larger hydrogen-generated microstructural effects in c-Si:H and a hydrogen diffusion coefficient in compact material increasing in a-Si:H and decreasing in c-Si:H with increasing H concentration. A different flexibility of the atomic network in amorphous and crystalline Si could be the origin. Microcrystalline Si:H films show a similar concentration dependence of H diffusion as a-Si:H.

Structural and optoelectronic properties of microcrystalline silicon germanium

Abstract The structural and optoelectronic properties of μc-Si 1− x Ge x :H alloys prepared by plasma enhanced chemical vapor deposition (PECVD) were investigated as a function of the deposition parameters germane concentration and hydrogen dilution. With decreasing hydrogen dilution we found a transition from microcrystalline to amorphous growth, which shifts to higher hydrogen dilution if the germanium concentration is enhanced. Additionally, we observed different built-in coefficients of germanium in the amorphous and crystalline phase. We correlate these structural properties with the optoelectronic properties. While the dark conductivity is only weakly affected by the germanium concentration, it decreases strongly upon a decrease of hydrogen dilution. Although the optical absorption increases with rising germanium the photoconductivity decreases indicating an increase of the defect density.

Hydrogen Diffusion in Zinc Oxide Thin Films

Hydrogen diffusion in zinc oxide thin films was studied by secondary ion mass spectrometry (SIMS) measurements, investigating the spreading of implanted deuterium profiles by annealing. By effusion measurements of implanted rare gases He and Ne the microstructure of the material was characterized. While for material prepared by low pressure chemical vapour deposition an interconnected void structure and a predominant diffusion of molecular hydrogen was found, sputter-deposited ZnO films showed a more compact structure and long range diffu- sion of atomic hydrogen. Hydrogen diffusion energies of 1.8 - 2 eV, i.e. higher than reported in literature were found. The results are discussed in terms of a H diffusion model analogous to the model applied for hydrogen diffusion in hydrogenated amorphous and microcrystalline silicon.

Hydrogen diffusion in the a-Si-Ge alloy system

Hydrogen diffusion in the a-Si-Ge alloy system was studied by SIMS profiling of deuterium and hydrogen interdiffusion and by hydrogen evolution. Hydrogen diffusion coefficients close to that of amorphous germanium are obtained throughout the alloy system. It is concluded that hydrogen diffuses predominantly via weak or broken Ge-Ge bonds.

Helium effusion, diffusion and precipitation as a probe of microstructure in hydrogenated amorphous silicon

The effusion of implanted helium was studied for doped and undoped radio frequency plasma-deposited a-Si:H films as well as for Si:H films deposited by other techniques. The results suggest that the observed helium diffusion and precipitation effects give information on the material microstructure.