Physical characteristics of laser processed hydrogenated amorphous silicon

Hydrogenated amorphous silicon films subjected to KrF excimer laser irradiation with a profiled beam in air leads to the formation of microstructures. The main objective of this research was to perform a comprehensive study in understanding this material in three different aspects: thermal, electrical, and optical properties by experiment, SEM analysis and modelling. For the thermal interactions, analysis was carried out to investigate factors relating to the formation of the microstructures in a range of applied laser fluences from 93.8 to 443.8 mJ/cm2. The tallest microstructures were formed with average height from 1 to 3 ?m at laser fluence of 312.5 mJ/cm2. Investigation also include the effect of different applied laser fluence, different scanning schemes, the effect of the presence of 300 nm metal layer, and irradiation environment. Thermal modelling using COMSOL simulation software was used to simulate heat transfer during laser-material interaction and the results suggest a fair agreement with experimental findings. SEM and TEM reveal that the material formed was an oxynitride with embedded particles of crystalline silicon. In the electrical part, conductivity and field emission were the main tools to help elucidate the internal structure. Arrhenius plots acquired from conductivity measurements demonstrates a decrease in activation energy from 0.8957 eV from original sample to 0.3955 and 0.1727 eV for HE and LE sample respectively. Analysis also showed an agreement with Meyer Neldel rule for both samples. Observation made on the ratio of dark current to photogenerated current revealed the decrease from 59600 in original sample to 1.77 and 1.40 for HE and LE samples respectively. For the field emission properties, IE plots from samples were analysed using 170 ?m fixed gap structure, and lowest emission thresholds were achieved at 3 and 2.4 V/?m for HE and LE samples respectively. The results were fitted to a model of conducting particles in an insulating matrix offering a transport route to the surface. In the optical part, FTIR measurements were carried and analysis in IR absorbance profile within range of 550 to 2200 cm-1 demonstrates SiHx absorbance peaks at 640 cm-1, between 1980 to 2100 cm-1, and at 2095 cm-1. Hydrogen content was found to be decreased with the increase in applied laser fluence from 12 % in original sample to 4.2 and 1.5 % for HE and LE sample respectively. Measurements using UV-Vis between wavelengths of 200 to 1100 nm shows high absorbance up to 98% for laser process sample from 218.8 to 312.5 mJ/cm2. TR analysis demonstrated increasing absorbance properties at increasing incident angle. Raman spectroscopy showed an increase in the crystal fraction with laser fluence. The final analysis work in this thesis examines the material as a potential disordered photonic crystal and studies the propagation and localisation of light in ordered and disordered photonic crystal, modelled using COMSOL simulation software. This shows the transition from diffusive to localised propagation. A number of applications are suggested for this structured material. This is the first report of a new large area ‘black silicon’ material that has a number of interesting applications.

Name dc:type.qualificationname

Doctor of Philosophy

Level dc:type.qualificationlevel

Doctoral Thesis

Grantor dc:publisher.institution

University of Dundee

Year dc:date.issued

2012

Author dc:creator

• Halim, Mohd Mahadi

• Amorphous silicon
• Laser process

Language dc:language

eng

Identifier

oai:discovery.dundee.ac.uk:studenttheses/915dd8fc-abc4-4258-9789-6a91767712e5

OAI identifier oai:identifier

oai:discovery.dundee.ac.uk:studenttheses/915dd8fc-abc4-4258-9789-6a91767712e5