J. Herion

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.

Fermi energy dependence of surface desorption and diffusion of hydrogen in a-Si:H☆

Abstract Using hydrogen evolution and SIMS profiling, the surface desorption energy ΔG and the diffusion energy ED of hydrogen (deuterium) in a-Si:H was determined as a function of the Fermi level EF. Both ΔG and ED show a maximum for EF somewhat above midgap and decrease towards the valence and conduction bands. The effects are explained by a Fermi energy dependent SiH bond rupture energy.

SIMS depth profile analysis for investigations of the lithium-diffusion in hydrogenated amorphous silicon

SummarySIMS depth profiling, using an oxygen primary beam at close to normal incidence, was applied to study lithium diffusion in thin films of hydrogenated amorphous silicon (a-Si:H) as well as in layered structures of doped and undoped silicon-based alloys. A strongly increasing decay length of the lithium profiles was observed for increasing primary beam energies and is attributed to the Li accumulation at the SiOx/Si interface. The stability of the lithium incorporation in a-Si:H is found to depend on the presence of charged acceptors or defects and of doping gradient related electrical fields.

Surface and bulk density of states of a-Si:H determined by CPM and total-yield photoelectron spectroscopy

The density of states (DOS) of a-Si:H and the influence of light soaking were studied with the constant photocurrent method (CPM) and total-yield photoelectron spectroscopy for films prepared at substrate temperatures between 100°C and 400°C. While by the first method information about the bulk is obtained the other yields the DOS in the surface-near region. At the surface we observed a relative to the bulk enhanced density of states as well as an enhanced influence of light soaking. In addition changes of the DOS of fresh films occur upon annealing at 190°C which can be significantly larger than the light induced effects.

Hydrogen Stability in Hydrogenated Amorphous Silicon-Based Alloys

The thermal stability of hydrogen in amorphous silicon-based alloy films was studied by deuterium/hydrogen interdiffusion and hydrogen effusion experiments. Depending on the film structure, hydrogen stability is limited by hydrogen surface desorption or hydrogen diffusion. The hydrogen surface desorption energy is found to decrease with rising germanium content and to increase with rising nitrogen and carbon content. At T = 400°C, hydrogen diffusion is found to proceed in the germanium subnetwork for a-SiGe alloys and in the silicon subnetwork for a-SiN and a-SiC alloys.

Formation of platinum silicide on hydrogenated amorphous silicon

Abstract The formation of platinum silicide at the interface between a-Si:H (and a-Si alloys) and a surface platinum layer is found to depend strongly on the hydrogen content and the structure of the amorphous films. The formation process is diffusion-limited with a diffusion coefficient comparable to c-Si for hydrogen-poor a-Si:H films as well as for films with microstructural effects. The considerably smaller diffusion coefficient observed for device-grade a-Si:H films is attributed to hydrogen accumulation at the silicide-Pt interface.

Surface photovoltage in a-Si:H

Abstract The surface photovoltage (SPV) was measured as a function of temperature and light intensity for 2.0 eV and 2.5 eV photons for clean, oxidized and ligth soaked a-Si:H films. The undoped samples were transferred by a vacuum lock from the plasma deposition system to the analysis chamber and investigated by the kelvin technique. The results suggest that the SPV for 2.5 eV photons is mainly determined by the Dember voltage whereas for 2.0 eV photons by band bending reduction due to (weak) minority carrier recombination via surface states.

Surface studies on as-deposited a-Si:H films

Abstract The surface photovoltage (SPV), the contact potential difference (CPD) and the conductivity were measured as a function of temperature (170 K – 470 K) for undoped glow-discharge a-Si:H films. The samples have been transferred from the preparation system to the surface analysis chamber by an UHV lock. The investigations concern the as-deposited, the oxidized and the sputter-cleaned state of the samples as well as the state after bombardment with hydrogen ions and annealing. Most remarkable results are the strongly negative SPV at low temperatures, the decrease of the CPD below 280 K for all treatments except sputtering, the irreversible decrease of the conductivity after oxidation, and the occurrence of hopping conductivity after sputtering.

The prefactor of the DC conductivity for amorphous silicon determined by combined contact potential difference and conductivity measurements

Abstract The contact potential difference (CPD) between clean a-Si:H surfaces and a reference electrode was measured along with the conductivity σ as a function of temperature. The temperature derivatives of CPD and log σ for highly doped n- and p-type films at a given temperature yield the prefactor of the dc conductivity.

Diffusion of deuterium into silicon single crystals: An investigation with SIMS

ZusammenfassungSilicium-Einkristalle wurden bei verschiedenen Temperaturen zwischen 25° C und 350° C in einer Deuteriumentladung getempert und anschließend mittels der Sekundärionen-Massenspektrometrie (SIMS) untersucht. Ausgehend von der Oberfläche nimmt das Deuteriumsignal exponentiell mit der Tiefe ab (Abklinglänge ∼ 30 nm bei 9 keV O2+-Beschuß). Das Verschwinden dieses Oberflächenpeaks nach einer Behandlung mit HF und die Abhängigkeit der Abklinglänge von der Beschußenergie zeigen, daß ein Ionenstrahl-induzierter Effekt vorliegt. Untersuchungen Bor-dotierter Kristalle lassen auf die Existenz einer Oberflächenbarriere, wahrscheinlich die Oxidschicht, für die Eindiffusion von Deuterium schließen. Die Permeation von Deuterium durch diese Barriere beträgt ∼ 2×1011 Atome/cm2 s bei 350°C und nimmt mit abnehmender Temperatur stark ab.SummarySilicon single crystals were annealed at various temperatures between 25 and 350° C in a deuterium discharge and were subsequently analyzed with Secondary Ion Mass Spectrometry (SIMS). Beginning at the surface, the deuterium signal decays exponentially to the interior (decay length ∼ 30 nm under bombardment with 9-keV O2+ J ions). The influence of the bombarding energy on the decay length and the disappearance of the deuterium signal after a treatment with HF show that the deuterium profile is due to an ion beam-induced effect. It is concluded from investigations of boron-doped crystals (both implanted and diffused B) that a surface barrier exists for the indiffusion of deuterium, probably the native oxide layer. The permeation rate of deuterium through this barrier is ∼ 2×1011 atoms/cm2s at 350° C and decreases strongly with decreasing temperature.