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Optoelectronic Investigation of Single CdS Nanosheets and Single GaP/GaAs Nanowire Heterostructures

Kumar, Parveen
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1378112845

Abstract Details

, PhD, University of Cincinnati, Arts and Sciences: Physics.
Photocurrent from photoexcitation of charged carriers in CdS nanosheets (NSs) and GaP/GaAs heterostructured nanowires (NWs) has been studied. Devices were fabricated based on single CdS NS and GaP/GaAs NW and their optoelectronic properties fully characterized using various light sources and at different wavelengths. The CdS NSs are grown using a gold catalyst-assisted vapor phase transport growth method at 800°C for 20min. Metal-semiconductor-metal nanodevices are made with both Schottky and Ohmic contacts using photolithography followed by Ti/Al (20nm/200nm) metal evaporation and lift-off. Ohmic contacts are formed by Ar+ bombardment before the metal deposition to create donor sulfur vacancies which increases the electron concentration. Scanning photocurrent microscopy (SPCM) is used to obtain spatial imaging of the photocurrent. An Ar+ laser with emission at 488nm was used for above band-gap excitation, whereas a Ti:S mode-locked laser with emission ranging 700nm to 1000nm is used for sub band-gap excitation. Spatial imaging of the photocurrent shows that the photosensitive regions are localized at the reverse biased contact for Schottky type contacts and uniformly distributed throughout the nanosheet for Ohmic contacts. A polarization analysis shows that the photocurrent is maximized for laser excitation polarized perpendicular to the c-axis of the NS. Photocurrent spectra excited above the CdS band-gap using a filtered monochromatic white light source at low temperatures reveal optical transitions between the A, B, C valence bands and the conduction band of the CdS NS at energies of 2.552eV, 2.569eV and 2.635eV respectively, which are in agreement with the accepted values for these bands. At room temperature these resonance peaks shift by ~0.053eV towards lower energies because of temperature dependence of the band-gap. The photocurrent increases linearly with power for above gap excitation. Photocurrents excited by a below-gap laser pulse increase nonlinearly (quadratic) with laser power, consistent with two-photon absorption (TPA). A nonlinear coefficient of β = 2cm/GW is obtained in agreement with bulk measurements of β. The GaP-GaAs heterostructured nanowires are grown by Metal-Organic Chemical Vapor Deposition (MOCVD) on Si (111) substrate using Au nano-particles as catalysts. The prepared NWs are typically ~10µm long with ~60nm circular diameter. A lattice mismatch between GaAs and GaP of 3.7% is expected with the GaAs section under compression and the GaP under tension. This introduced stress/strain can be used to modify the energy band gap in semiconductors, an example of strain engineering. Photolithography followed by Ti/Al (20nm/200nm) metal evaporation and lift-off was utilized to fabricate Ohmic type contact pads separated by ~3-5 µm across a single NW. Spatial imaging of the photocurrent distinguishes the GaP and GaAs regions in these NWs. The peak photocurrent is observed near the GaP-GaAs interface. A broadband coherent white light supercontinuum generated by mode lock Ti:S laser tuned at 800nm and nonlinear fiber is used to obtain photocurrent as a function of wavelength (500nm to1100nm). Distinct spectroscopic signatures corresponding to transitions from the valence to Γ, X and L valley points in the Brillouin zone for strained GaAs near the interface are observed which are consistent with Raman measurements.
Leigh Smith, Ph.D. (Committee Chair)
Howard Everett Jackson, Ph.D. (Committee Member)
Kay Kinoshita, Ph.D. (Committee Member)
Frank Pinski, Ph.D. (Committee Member)
Jan Yarrison-Rice, Ph.D. (Committee Member)
170 p.
Physics
Photocurrent; Nanowire; Two Photon Absorption; Optoelectronic; CdS; GaAs-GaP

Recommended Citations

Citations

  • Kumar, P. (n.d.). Optoelectronic Investigation of Single CdS Nanosheets and Single GaP/GaAs Nanowire Heterostructures [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1378112845

    APA Style (7th edition)

  • Kumar, Parveen. Optoelectronic Investigation of Single CdS Nanosheets and Single GaP/GaAs Nanowire Heterostructures. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1378112845.

    MLA Style (8th edition)

  • Kumar, Parveen. "Optoelectronic Investigation of Single CdS Nanosheets and Single GaP/GaAs Nanowire Heterostructures." Doctoral dissertation, University of Cincinnati. Accessed APRIL 26, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1378112845

    Chicago Manual of Style (17th edition)

ucin1378112845
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