B. Pantchev

Hydrogen bonding and structural order in hydrogenated amorphous silicon prepared with hydrogen-diluted silane

A study of the structural development of hydrogenated amorphous silicon (a-Si : H) during plasma-enhanced chemical vapour deposition with hydrogen-diluted silane has been carried out with focus on the variations in the hydrogen bonding configuration and in the amorphous silicon network with increasing film thickness. In addition, the hypothesis of a high fraction of non-bonded (molecular) hydrogen in a-Si : H has been tested. The total hydrogen concentration and its silicon-bonded fraction have been estimated by means of nuclear reaction analysis and infrared spectroscopy, respectively. It has been shown that the presumable molecular hydrogen is not detectable within the limits of the measurement accuracy of the methods used. The hydrogen concentration is uniformly distributed along the growth direction, and the infrared absorption modes at 2000 and 2100 cm−1 are not affected by increasing the film thickness. Raman spectroscopy has been used to follow the variations in the structure of the silicon network. The increase in the film thickness leads to an improved ordering of the amorphous network on the short and medium range scale for films deposited at low substrate temperatures. In films deposited at high substrate temperatures, the tendency of structural improvement has been detected only on the medium range scale.

Effect of film thickness on hydrogenated amorphous silicon grown with hydrogen diluted silane

Thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition with 10% SiH4 in hydrogen have been studied concerning the effect of film thickness on the hydrogen concentration, interconnected void network and mechanical stress. The hydrogen concentration was determined by nuclear reaction analysis. The interconnected void network was studied by the method of ion exchange in glass substrate. The films were prepared at a substrate temperature in the range of 150–270 °C. The results show that at the substrate temperature of 150 °C the film starts to grow with an extensive void network, and its structural improvement with thickness is manifested by an increase of the film density. In contrast, at 270 °C the film starts to grow with a dense structure, and its improvement is manifested by an increase of the intrinsic compressive stress. The hydrogen concentration does not depend on the film thickness at any substrate temperature.

Depth distributions of hydrogen and intrinsic stress in a-Si:H films prepared from hydrogen-diluted silane

The thickness dependencies and depth distributions of hydrogen and intrinsic mechanical stress are studied for a-Si:H films prepared with 10% silane in hydrogen. Nuclear reaction analysis has been used to establish the total concentration of the incorporated hydrogen. It has been shown that the hydrogen distribution in the films is uniform and does not depend on the film thickness. On the contrary, the intrinsic stress depends on the film thickness and has a nonuniform depth distribution, as the stress increase linearly in the direction from the substrate/film interface to the film surface. The obtained results are discussed in view of the hydrogen-related processes and structural improvement of the silicon network during the film growth.

The effect of structural disorder on mechanical stress in a-Si:H films

The effect of ion implantation on mechanical stress in a-Si:H films was studied with the aim of separating the contributions that the hydrogen content and structural defects make to the intrinsic compressive stress. The a-Si:H films were prepared by plasma-enhanced chemical vapour deposition. Silicon ions with an energy of 160 keV were implanted and the implantation-induced structural damage was studied by means of Raman backscattering spectroscopy. The stress in the films was compressive and its value correlated with the short and intermediate range orders. The results have shown that the value of compressive stress in the material could be lowered by changing the structural order of the silicon network without changing the hydrogen content.

Field‐assisted ion exchange in glass: The effect of masking films

The two‐step ion exchange process for the fabrication of optical waveguide structures in glass is studied. The first step is field‐assisted ion exchange through the structure forming ‘‘windows’’ in the masking films. It is established that, in the case of metal masking films, the exchange ability of glass in the region under the mask decreases significantly. The second ion exchange in these regions is even totally blocked when soda‐lime glass is used. The mechanism and the application of this effect are briefly considered.

Effect of post-hydrogenation on the structural properties of amorphous silicon network

Post-hydrogenation of magnetron sputtered amorphous silicon films has been carried out with the aim to study the effect of hydrogen interaction with amorphous silicon network on its short and medium range order. Raman spectroscopy has been used to study the variations in the amorphous structure. Nuclear reaction analysis (NRA) has been used to determine the total amount and depth distribution of the penetrated hydrogen atoms. The concentration of the silicon-bonded hydrogen and the bonding configurations have been established by means of infrared (IR) transmission measurements. The values of hydrogen concentration evaluated by NRA and IR spectroscopy coincide within the measurement accuracy, suggesting that the hydrogen diffusion proceeds via interaction with the host silicon atoms. This interaction is accompanied by a rearrangement of the strained Si-Si bonds which leads to an improvement of the amorphous network.

SHORT-RANGE ORDER AND MICROSTRUCTURE IN HYDROGENATED AMORPHOUS SILICON

An experimental study has been made on the relationship between short‐range order and microstructure in hydrogenated amorphous silicon films. The properties of the material have been varied by applying rf power of different magnitudes. The change in the short‐range order has been characterized by Raman scattering measurements. Microstructure has been determined by means of field assisted ion exchange technique. The observed correlation between the two structural length scales suggests that the presence of dihydride groups in these materials is a key factor for the release of the silicon network strain.

Electron irradiation of a-Si :H films prepared from hydrogen-diluted silane

The effect of 18 MeV electron irradiation on the optoelectronic and structural properties of a-Si:H films has been studied. It has been established that the irradiation leads to a strong decrease of photo- and dark conductivities, causes a change in the state distribution of the valence band-tail, as well as in the recombination mechanism of the photoexcited carriers. The structural properties of the films have been studied by means of Raman spectroscopy. The observed change in the short and medium-range order of the amorphous silicon network suggests that the high-energy electron irradiation induces structural defects, as well.

Nanoindentation-induced pile-up in hydrogenated amorphous silicon

Nanoindentation-induced material extrusion around the nanoindent (pile-up) leads to an overestimation of elastic modulus, E, and nanohardness, H, when the test results are evaluated using the Oliver and Pharr method. Factors affecting the pile-up during testing are residual stresses in film and ratio of film and substrate mechanical properties. Nanoindentation of hydrogenated amorphous silicon (a-Si:H) films has been carried out with the aim to study the effect of residual compressive stress on the pile-up in this material. To distinguish the contribution of compressive stress to the appearance of pile-up ion implantation has been used as a tool, which reduces the compressive stress in a-Si:H. Scanning probe microscope has been used for the imaging of the indent and evaluation of the pile-up. The values of E and H have been obtained from the experimental load-displacement curves using depth profiling with Berkovich tip, which has created negligible pile-up. A sharper cube corner tip has been used to study the pile-up. It has been established that pile-up is determined by the material plasticity, when the compressive stress is below 200 MPa. The contribution of mechanical stress to the pile-up is essential for the stress as high, as about 500 MPa.

Hydrogen solubility limit in hydrogenated amorphous silicon

Hydrogen solubility has been studied in hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapour deposition with hydrogen-diluted silane. Post-hydrogenation experiments have been carried out using hydrogen plasma and hydrogen ion implantation. Thermal annealing and silicon ion implantation have been used to change the defect density in the amorphous silicon network. Hydrogen concentration has been established by means of nuclear reaction analysis and infrared spectroscopy. It has been shown that the hydrogen solubility in a-Si:H is strongly related to the density of the hydrogen trapping sites in the silicon network and the value of the solubility limit is determined by the material structure and consequently by the a-Si:H preparation conditions. The ratio between the hydrogen concentration and its solubility limit has been discussed in the context of the light-induced degradation of a-Si:H.