Formation and characterization of AlSb/CdS heterojunction رسالة ماجستير

Abstract

In this thesis, thin films of Aluminum Antimonide (AlSb) and Cadmium Sulfide (CdS) are prepared by thermal evaporation technique onto glass and Indium Tin Oxide (ITO) substrates under a vacuum pressure of 10-4 mbar. The films are structurally, optically and electrically characterized. It was observed that thin films of AlSb and CdS exhibit polycrystalline nature of cubic and hexagonal structures, respectively. The structural parameters including the lattice parameters, the crystallite size, the microstrains, the defect densities and stacking faults are determined. Stacking of CdS onto AlSb, resulted in increased defect density, increased microstrains and decreased crystallite sizes. The structural changes were attributed to the lattice mismatches and the bonding mechanisms. Optically, both of CdS and AlSb/CdS displayed direct optical gap transition with energy band gaps of 2.45 and 2.20 eV, respectively. In spite of it is metallic character, AlSb substrates successfully enhanced the light absorbability ( ) in the visible range of light showing a maximum of 14.8 at 2.21 eV. Analyses of the dielectric spectra have shown that AlSb can exhibit large dielectric constant values in the IR range of light. The dielectric constant of CdS decreased significantly when coated onto AlSb in the IR range. On the other hand, Drude-lorentz modeling of the imaginary part allowed determining the plasmon frequency, drift mobility, oscillator energy and relaxation time at femto second level. While AlSb displayed highest plasmon frequency, CdS showed higher mobility values. Interfacing of both layers resulted in moderate values of drift mobility and plasmon frequency. The values of plasmon frequency being in the gigahertz range nominate the studied films for 4G/5G technologies. Form electrical point of view, the impedance spectroscopy which was v studied in the spectral range of 10-1800 MHz have shown that ITO/AlSb/C (IAC) devices exhibit band pass filter characteristics above 1800 MHz and ITO/AlSb/CdS/C (IACC) show multiband stop filter characteristics above 1200 MHz. Analysis of AC conductivity indicated domination of correlated barrier hoping conduction in IAC devices and combined mechanism of CBH and quantum mechanical tunneling in IACC devices. While the capacitance spectra of IAC samples show decaying trend of variation with increasing frequency, IACC samples displayed three resonance peaks at 217 MHz, 829 MHz and 1098 MHz. To explore the origins of the AlSb formation nature, the structural and electrical characterizations were repeated for the remaining bulky melt of the AlSb source material. One interesting feature is that the tip of the bulk is composed of 43% AlSb and the bottom is composed of 32%. The bulky AlSb displayed band pass filter characteristics above 1600 MHz. The study has shown that the films and bulky samples can be effectively used in optoelectronic applications.