Abstract:
This work first represents a detailed high-resolution X-ray reflectivity (XRR) and rocking curve (XRC) analysis of a spontaneous superlattice structure in MOCVD grown AlxGa1-xAs epilayer on (001) GaAs. A model-independent Distorted Wave Born Approximation formalism was applied to the XRR results to obtain the compositional information as a function of depth. A kinematical model was then used successfully to fit the sharp peaks observed in the XRC using the characteristic length and compositional parameters obtained from the XRR simulation. The thermal stability of such superlattice was analyzed with the help of X-ray rocking scans at elevated temperatures. The ordered (spontaneous) structure irreversibly transformed to the disordered one retaining its single-crystalline nature at 900o C. For photo detection, the (Au-Ge) alloy was deposited on the as-grown and the disordered sample to realize an (MSM) photodetector (400-900 nm). Furthermore, a comparative analysis of the two devices was also discussed. In the next part, a symmetric coplanar MSM structure with Nickel as metal contacts was fabricated on quaternary (AlxGa1-x) yIn1-yP epitaxial film on (100) SIGaAs for visible photodetection. The crystallinity and alloy composition (y=0.66 and x=0.67) of the grown material was determined by high-resolution X-ray rocking curve and photoluminescence experiment. The photo-response of the device was studied in the wavelength range of 300-1000 nm which exhibited a peak at 610 nm. The photodetector showed a fast rise time of 91 μs however, the decay characteristics exhibited two channels with decay times of 83 μs and 282 μs, respectively. In our final work, X-ray, SEM, and photoluminescence analysis were employed to understand the band-edge related excitonic lasing features from the vertically oriented ZnO nanowires grown by CVD on sapphire. The nanowires exhibited high degree of crystallinity, strong preferred orientation, and relative inplane alignment having 6-fold hexagonal symmetry of the Wurtzite phase. A 325 nm laser source was employed to optically pump the nanowires. The nanowires showed the onset of lasing below 150 K being prominent at 3.4 K. The threshold pump power was found to be 14.5 mW at 3.4 K. Such nanowires are suitable for lasing in the ultraviolet region of the electromagnetic spectrum below a characteristic temperature of 150 K at a low threshold optical pumping power.
