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Title: Pulsed laser deposition of telluride thin films for photonics applications
Authors: Pang, Man-yee
Keywords: Hong Kong Polytechnic University -- Dissertations
Pulsed laser deposition
Semiconductors -- Materials
Thin films
Issue Date: 2009
Publisher: The Hong Kong Polytechnic University
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 ℃. 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 ℃ 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 ℃ 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.
Description: xvi, 146 leaves : ill. (some col.) ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577M AP 2009 Pang
Rights: All rights reserved.
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