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Title: Pulsed laser deposition of telluride thin films for photonics applications
Authors: Pang, Man-yee
Degree: M.Phil.
Issue Date: 2009
Abstract: The search for uncooled infrared detectors have aroused keen interest in very narrow bandgap semiconductor research in the range of about 0.1 to 1 eV. Mercury cadmium telluride (HgCdTe) has remained the most important semiconductor for middle to long wavelength infrared detection in the past three decades. Numerous studies have attempted to replace HgCdTe with alternative materials such as mercury-based alloys (HgZnTe, HgMnTe), PbSnTe and AlGaAs. We propose to use thin film of narrow bandgap semiconductor material, Bismuth Telluride (Bi2Te3), for infrared detection. Semiconducting Bi2Te3 has a band gap energy of about 0.15 eV at room temperature and is a possible material for middle wavelength IR (MWIR) detection. In this work, a novel heterojunction photovoltaic detector structure of Bi2Te3/GaN/Al2O3 was fabricated for MWIR detection. High quality n-GaN epitaxial layers grown on sapphire substrates by MOCVD technique were obtained from our GaN research group. We then deposited Bi2Te3 thin films onto these n-GaN/Al2O3 templates by pulsed laser deposition (PLD) technique. The wide bandgap semiconducting n-GaN layer and the Al2O3 substrate were used for visible-to-MWIR transmission in a back-side illuminated diode configuration. The PLD grown Bi2Te3 thin films (thickness 300-500 nm) were systematically studied by varying the deposition temperatures in the range from 150-350 oC. Surface morphology of the thin films was investigated by atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM). The average grain size growth with increasing temperatures and the crystals appear on the film grown at 300 oC is quite large with a few micrometers in size. Measurement results clearly exhibit the layered structure of Bi2Te3 with randomly oriented crystals. The films' chemical composition and crystalline quality are evaluated by x-ray photoelectron spectroscopy (XPS) and x-ray diffraction spectroscopy (XRD) respectively. It was found that the crystallinity of Bi2Te3 thin films was strongly dependent on the growth temperature. The chemical compositions of the as-deposited Bi2Te3 thin films were always bismuth rich. XPS and Raman characterizations of the bismuth rich-Bi2Te3 films reveal the co-existent of Bi2Te3 phase and segregation of elemental Bi. The influence of the composition on the structure of Bi-Te thin films was investigated. The electrical and optical responses of films with different structural quality were also examined. Current-voltage (I-V) characteristics allow us to determine the device parameters, such as series resistance (Rs), zero-bias resistance (R0) and ideality factors (n) of the junctions. Most detectors suffer from undesirable 1/f flicker noise under operation. Performance of sample fabricated under different conditions were examined by studying their low-frequency noise spectra measurement, which, in addition to being an important figure-of-merit for the detectors, has also shown to be an effective characterization tool for material defects. The experimental results suggest that, the noise level of the devices is correlated to the crystallinity of the BiiTes thin films. The spectral current responsivity of the Bi2Te3/GaN/Al2O3 heterojunction for photovoltaic detection of low energy photons in MWIR region was evaluated under back-side illumination. The correlation between crystallinity, diode parameters as determined from the I-V characterization and the responsivity measurements was analyzed. The experimental results reveal that the optical response of detectors grown at higher substrate temperature may improve from reduction of traps density in heterojunction. It is found that the Bi2Te3/GaN heterojunction grown at 300 oC presented the best performance among the other devices. Our results allow us to estimate the figure-of-merit of Bi2Te3/GaN heterojunction sensors for room temperature photovoltaic mid-infrared detection.
Subjects: Hong Kong Polytechnic University -- Dissertations.
Pulsed laser deposition.
Semiconductors -- Materials.
Thin films.
Pages: xvi, 146 leaves : ill. (some col.) ; 30 cm.
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