ZnO-based materials show promise in energy harvesting applications, such as piezoelectric, photovoltaic and thermoelectric. In this work, ZnO-based vertical Schottky barrier solar cells were fabricated by MOCVD de- position of ZnO thin films on ITO back ohmic contact, while Ag served as the top Schottky contact. Various rapid thermal annealing conditions were studied to modify the carrier density and crystal quality. Greater than 200 nm thick ZnO films formed polycrystalline crystal structure, and were used to demonstrate Schottky solar cells. I-V characterizations of the devices showed photovoltaic performance, but but need further development. This is the first demonstration of vertical Schottky barrier solar cell based on wide bandgap ZnO film. Thin film and bulk ZnO grown by MOCVD or melt growth were also investigated in regards to their room- temperature thermoelectric properties. The Seebeck coefficient of bulk ZnO was found to be much larger than that of thin film ZnO at room temperature due to the higher crystal quality in bulk materials. The Seebeck coefficients decrease while the carrier concentration increases due to the crystal defects caused by the charge carriers. The co-doped bulk Zn0:96Ga0:02Al0:02O showed enhanced power factors, lower thermal conductivities and promising ZT values in the whole temperature range (300-1300 K).
The wide-bandgap semiconductor ZnO has gained major interest in research community for its unique properties and wide range of applications. In this review article, we present synthesis techniques and a few emerging applications for ZnO. Common techniques for growing ZnO films are discussed briefly, and a detailed discussion of MOCVD growth of ZnO is provided citing previous experimental reports on this technique by our group and others. A few important and distinctive uses of ZnO are discussed for various applications focusing on the current limitations of ZnO to realize its feasibility in these applications.
Wide band gap dilute magnetic semiconductors have recently been of interest due to theoretical predictions of room
temperature ferromagnetism in these materials. In this work Ga1-xGdxN thin films were grown by Metalorganic
Chemical Vapor Deposition. These films were found to be ferromagnetic at room temperature and electrically
conducting. However, only GaN:Gd layers and devices grown with a TMHD3Gd precursor that contained oxygen
showed strong ferromagnetism, while materials grown with an oxygen-free Cp3Gd precursor did not show ferromagnetic behavior. This experimental observation was consistent with first-principles calculations based on density functional theory calculations that we completed that showed the ferromagnetism was mediated by interstitial oxygen. The results confirmed the first successful realization of Ga1-xGdxN-based spin-polarized LED with 14.6% degree of polarization at 5000 Gauss is obtained.
The dependency of the structural and optoelectronic properties of InN thin films grown by high-pressure chemical
vapor deposition technique on the group V/III molar precursor ratio has been studied. X-ray diffraction, Raman
spectroscopy, and IR reflectance spectroscopy have been utilized to study local- and long-range structural ordering as
well as optoelectronic properties of the InN epilayers grown on crystalline sapphire substrates. The investigated InN
epilayers were grown with group V/III molar precursor ratio varying from 900 to 3600, while all other growth
parameters were kept constant. For a group V/III precursor ratio of 2400, the full width-half maximum of the Raman
E2(high) mode and XRD (0002) Bragg reflex exhibit minimums of 7.53 cm⁻¹ and 210 arcsec, respectively, with
maximized grain size and reduced in-plane strain effect. FTIR data analysis reveals a growth rate of 120 nm/hr, a carrier
mobility of 1020 cm²V⁻¹s⁻¹, and a free carrier concentration of 1.7×1018 cm⁻³ for a V/III ratio of 2400. The Raman
analysis indicate that non-polar E2(high) mode position remains unaffected from a changing V/III ratio; whereas, polar
A1(LO) mode position significantly changes with changing V/III ratio. Optical analysis also suggests that LO-phonon
correlates with free carrier concentration (ne) and TO-phonon correlates with free carrier mobility (μ) in the InN
epilayers.
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