Title of Thesis
Preparation and Characterization of Advanced
Materials for Electronic and Opto-Electronic Devices
Centre of Excellence in Solid State Physics /
Punjab University, Lahore
|Number of Pages|
|Keywords (Extracted from title, table of contents and
abstract of thesis)|
Preparation, Characterization, Advanced,
Materials, Electronic, Opto-Electronic, Devices, spintronic,
applications, phase transformation
The research work reported in this thesis involved preparation and characterization of advance materials for electronic and optoelectronic devices / fields. These materials, once optimized through characterization, were then used to fabricate devices for photovoltaic and spintronic applications. These materials and devices were fabricated by the use of physical techniques involving evaporation and sputtering.
For the photovoltaic applications copper indium gallium selenide (CIGS) was used as the absorber layer whereas cadmium sulphide (CdS) was used to form the p-n junction. CIGS was deposited by sequential elemental layer (SEL) deposition technique. SEL deposition involves post-deposition heating step for compound formation. CIGS was heat-treated in the temperature range 150 – 1250°C in order to study and optimize the phase transformation. Grain growth in relation with the microstrain was also studied and correlated to the phase formation: 3 distinct regions. RT - 250°C, 250 - 750°C, and 950 - 1250°C, are reported regarding the structural properties. CdS was deposited by the standard thermal evaporation technique. and was found to be of acceptable quality for device fabrication. A novel transparent conducting oxide, ZnAI204, (ZAO) was prepared by thermal evaporation and was optimized for the application on top of the CdS laver. A large variation in band gap (2.3 - 4.5 eV) with varying oxygen content is reported for the first time. The individual films were characterized for their optical, electrical and structural properties in detail for in-depth analyses before their application to the device fabrication. The device, AI/ZnAl204/CdS/CIGS/Mo/Glass was characterized for its external and internal parameters. The internal parameters were mainly calculated by fitting the current-voltage data to a one-diode model. There was a reasonable fit of one-diode model to the experimental data except for the region around maximum power point. This region was then explored analytically by computing the value of fill factor.
This analysis led to the reasons of poor performance (efficiency of 8.85%) of the devices.
Double barrier magnetic tunnel junctions (DBMTJ) were formed with the help of sputtering technique in order to study the magneto-transport properties of materials for application in the field of spintronics. These DBMTJs were fabricated in the form of continuous thin films and granular structures. A high quality granular DBMTJ with C075FE25 (0.4-1.5 nm) as inner layer surrounded by amorphous Co60Fe20B20 magnetic electrodes was fabricated and investigated room temperature and low temperatures. The TMR
values of 18, 22, 25, 29, 31. 34 at RT. 200K, 100K, 77K, 30K & 4.2K
respectively have been obtained. The optimized granular junctions
were found to be formed when the central layer, i.e. Co75Fe25. had a thickness of 0.8 nm. The TMR value (34%) for the optimized granular junction is the highest reported so far in the literature. These results also show that Co75Fe25 granular DBMTJs are promising candidates to observe the SET phenomenon in spin electronic devices.
Sol-gel technique was also used to prepare various materials related to application in the field of electronics. The technique was established by initially preparing thin films of alumina. Synthesis of the pre-cursors was performed with the locally available raw materials. Once the expertise was developed, advance materials such as thin films barium titanate and nano-wires of cobalt-platinum were prepared by the sol-gel method. Tetragonal BaTiO3 films were achieved at a reaction temperature of as low as 300°C (lowest reported so far for such materials). Dielectric constant values of 270-380 were observed in the frequency range 30 Hz – 300 MHz. Leakage current measurements in the Cu/BaTiO3/Cu capacitors showed conductivity of as low as 2.0x10-15 S/cm. Further. nanowires have been formed by the sol-gel method using anodic aluminum oxide templates. The AAO templates developed pores of 30 – 50 nm diameter. CoPt nanowires were formed in these templates using electrochemically induced sol-gel method. These nano-wires were characterized in detail for their potential application in the field of spintronics.