H. Fritzsche

Characterized of glow-discharge deposited a-Si:H

Abstract The study of hydrogenated amorphous silicon, a-Si:H has become an active and large subfield of growing interest in noncrystalline semiconductors. This paper reviews recent successes as well as difficulties encountered in interpreting some fundamental physical properties of a-Si:H prepared by glow-discharge deposition. This material appears to contain structural and compositional heterogeneities which depend on the preparation conditions and affect its mechanical and electronic properties. These are related to the concentration and bonding configurations of hydrogen. Determinations of the density and distribution of localized gap states by means of field effect and capacitance measurements are discussed as well as evidence for and against the presence of surface states. Optical absorption near the absorption edge is described as are conductivity measurements. The latter reveal many interesting puzzles. Staebler and Wronski discovered that the dark conductivity and the photoconductivity can be changed significantly by strong exposure to light. Moreover, space charge layers adjacent to the substrate interface and near the free surface can strongly influence the electronic properties of a-Si:H films, especially those having low densities of gap states. The pre-exponential factor of the conductivity increases with its activation energy in accordance with the Meyer-Neldel rule regardless of whether the changes in dark conductivity are produced by doping, light exposure or space charge layers. Attempts are made to identify causes for the scatter of data on some film properties reported by various laboratories. Our present understanding of the recombination processes involved in photoluminescence and photoconductivity is presented. We conclude that a-Si:H films of superior quality can be expected when the preparation conditions become more fully understood.

Effect of annealing on the optical properties of plasma deposited amorphous hydrogenated silicon

Abstract Amorphous silicon films prepared by radiofrequency plasma decomposition of silane contain between 10 and 25 at % hydrogen depending on the potential and temperature of the substrate and plasma parameters. The way hydrogen is bonded in these films has been determined from the infrared stretching, bending, and rocking or wagging modes of the Si-H complexes. Monohydrides SiH, dihydrides SiH2 and short chains of polysilane ( SiH 2 ) n an be identified. The potential of the substrate with respect to the plasma and to a lesser degree the substrate temperature determine the prevalent Si-H complex. At substrate potentials close to that of the plasma ( SiH 2 )n complexes are favored whereas isolated SiH and SiH2 groups are found in films prepared on strongly negative potential substrates. The films can be dehydrogenated by annealing. Substrate potential and temperature influence strongly the refractive index and the onset of interband optical transitions. The optical gap of these films lies between 1.6 and 1.85 eV which is appreciably higher than the values 1.2–1.5 eV of sputtered and evaporated amorphous Si films. As the hydrogen is driven out by annealing the optical gap decreases to 1.6–1.7 eV. At photon energies below the optical gap one observes a preparation sensitive absorption tail between α=10 2 and10 3 cm −1 which cannot be removed by annealing. The films crystallize between 700 and 780°C with a crystallization energy of (2±0.3) kcal/mol.

Effect of adsorbed gases on the conductance of amorphous films of semiconducting silicon‐hydrogen alloys

The conductance of amorphous Si films produced by glow discharge of SiH4 is found to be very sensitive to various adsorbates such as water, ammonia, and dimethyl ether. Films exposed to air and light must first be heated to 150 °C in vacuum to remove adsorbed moisture and the reversible photoelectronic effect discovered by Staebler and Wronski. When exposed to adsorbates, the conductance of undoped films (about 0.36 μm thick) decreases by several orders of magnitude; the conductance of lightly phosphorus‐doped films first increases rapidly and then slowly decreases to a saturation value which lies above the conductance of the annealed films. Heating in vacuum at 150 °C restores the original values. The results indicate that the conductance changes produced by light and by adsorbates are two separate processes, the latter being a surface effect.

Hydrogen content and density of plasma‐deposited amorphous silicon‐hydrogen

The hydrogen concentration and density of amorphous semiconducting films prepared by glow‐discharge decomposition of silane have been measured as a function of deposition temperature. An inductively coupled as well as a capacitively coupled plasma‐decomposition system was used. For samples prepared by the capacitively coupled system, the hydrogen content decreased from 26 to 8 at.% and the density increased from 1.9 to 2.27 g/cm3 as the substrate temperature was increased from 25 to 450 °C.

Photoreduction and oxidation of as‐deposited microcrystalline indium oxide

Indium oxide (InOx) films with a thickness of 100–500 nm were prepared by dc magnetron sputtering onto Corning 7059 glass substrates. The as‐deposited films were microcrystalline as revealed by x‐ray diffraction analysis. The conductivity of the as‐deposited films was of the order of 10−3–10−2 Ω−1 cm−1 and increased up to the order of 101–102 Ω−1 cm−1 by exposure to ultraviolet light hν≥3.5 eV in vacuum. By subsequently exposing the same films to an oxidizing atmosphere they reverted to the insulating state. Photoreduction and oxidation have been shown to change the conductivity properties of one and the same film in a fully reversible manner. In this article, we demonstrate that the large reversible conductivity changes produced by UV photoreduction and oxidation are not limited to amorphous InOx but are equally observed in microcrystalline InOx.

Photoinduced metastable surface effects in boron‐doped hydrogenated amorphous silicon films

We observed metastable light‐induced conductance changes in p‐type hydrogenated amorphous silicon films which are different from those discovered by Staebler and Wronski. In contrast to the large metastable decrease in the dark conductance after prolonged exposure to band‐gap light (Staebler–Wronski effect) we observed an increase in the dark conductance by more than a factor of 30 at room temperature. This light‐induced increase can be reversed by annealing. The anomalous conductance increase is associated with the presence of a surface oxide layer because it disappears after etching the sample. We find that the enhanced conductance is due to an increase of a hole accumulation layer in the sample caused by a light‐induced metastable change in the surface oxide.

Porosity and oxidation of amorphous silicon films prepared by evaporation, sputtering and plasma-deposition☆

Abstract The amount of water adsorbed by amorphous silicon films upon exposure to normal humidity levels has been measured with a quartz-crystal microbalance. Films electron-beam evaporated on room temperature substrates absorb about 11 mol% H 2 O on the internal surfaces of an interconnected void structure. We estimate that nearly 15% of the silicon atoms lie at internal void surfaces. The water causes internal oxidation and cannot be removed by drying. It produces an increase in resistance and a shift of the optical absorption edge to higher photon energies. Hydrogenated amorphous silicon prepared by sputtering at 200°C or by rf plasma deposition at 25 and 150°C, on the other hand, is not porous. Exposure to 60% relative humidity at 300 K produces a 5 A thick oxide layer in about 20 h. Water adsorption strongly affects the conduction in the space charge layer near the surface of plasma-deposited films but has little or no effect on the conductivity of sputtered films. Long term relaxation effects are observed in the conductivity of freshly plasma-deposited amorphous silicon-hydrogen films prepared at room temperature. Based on the very low hydrogen content found in chemical-vapor-deposited (CVD) amorphous silicon we conclude that the CVD films are impervious to water.

Dependence of hydrogen evolution from a-Si: H on boron doping and substrate potential

Abstract The dependence of the hydrogen effusion rate f h on temperature T reflects the two-phase compositional inhomogenous observed by proton NMR spectroscopy in plasma-deposited hydrogenated amorphous Si. We studied h(T) of 1 μm thich a-Si: H films prepared in an rf diode plasma reactor at 260°C from a SiH 4 / Ar = 0.1 mixture doped with 10 −7 2 H 6 /SiH 4 −4 . We find that h(T) depends on the self-bias potential V s of the substrate with respect to the plasma. For V s V , h(T) peaks at two temperature decreases from 570 to 350°C and the higher one from 620 to 450°C with increasing boron concentration. Near B 2 H 6 / SiH 4 =5×10 −5 one finds changes in film morphology because of boron-induced weakening of hydrogen-bonding configurations. We associate the low and the high temperature hydrogen evolution peaks with the hydrogen concentrated and hydrogen dilute phases of the material. We conclude that boron doping and ion bombardment during growth strongly affect the film structure and the hydrogen bonding and hence fundamental properties of the amorphous film. Since V s is more negative for cathode films than for anode films, their properties are different.

Effect of the surface condition on the conductance of hydrogenated amorphous silicon

We observed that the conductance of undoped hydrogenated amorphous silicon films (a‐Si:H) is dominated by an electron accumulation layer at their surface. With increasing oxide thickness the band bending diminishes and then converts to hole accumulation. The surface changes from n type to p type as the result of light exposure. The surface space‐charge layer was tested and changed by adsorbate layers of water and oxygen and layers of evaporated Se. It is found that erroneous conclusions will be drawn about the values of the conductivity activation energy, the prefactor, and the Meyer–Neldel relation between these parameters unless the conduction in the surface space‐charge layer is eliminated or accounted for.

Thickness and refractive‐index changes associated with photodarkening in evaporated As2S3 films

Employing a sensitive variable angle interference technique the changes in thickness Δl/l and in refractive index Δn/n accompanying the photo‐induced shift of the optical absorption edge toward longer wavelengths were determined in evaporated As2S3 films. The irreversible heat or light‐induced structural change was found to be associated with a decrease in thickness Δl/l=0.011±0.001 and an increase in refractive index Δn/n=0.045±0.005. Changes of l and n associated with the reversible photodarkening effect were found to be less than 0.2 and 0.5 percent, respectively.