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Presenter: Peng Wu
Supervisor:

Date: Fri, December 14, 2018
Time: 12:00:00 - 13:30:00
Place: EOW 230

ABSTRACT

Abstract:

Direct-gap semiconductors with a bandgap energy (Eg) of ~1.4 eV are desirable for use in both photovoltaic and photocatalytic energy conversion.  However, the variety of binary semiconductors with Eg ≈ 1.4 is limited to several compounds such as GaAs, InP, and CdTe.  As for semiconductor alloys, Eg of InxGa1-xN is adjustable to 1.4 eV by varying indium content.  However, these compounds consist of rare or toxic elements, and moreover crystal growth of them generally needs high temperature. These facts make it difficult to produce cost-effective solar cells based on these semiconductors on large-area less expensive substrates like glass. Hence, earth-abundant direct-gap semiconductors with Eg = 1.4 eV, which can be grown at low temperature, is eagerly anticipated. 

The II3V2 semiconductors are a little-explored class of semiconductor materials which can be composed of environmentally-benign, earth-abundant elements, and have bandgaps in the visible and near infrared part of the spectrum.  The crystal structure of these materials is relatively complex consisting of three interpenetrating fcc lattices, with one of the lattices half occupied. We have directed our attention to Zn3N2 and Mg3N2 as an earth-abundant and nontoxic semiconductor. Epitaxial single crystals zinc nitride and magnesium nitride thin films were grown on (110) sapphire and (200) MgO substrates by plasma-assisted molecular beam epitaxy with nitrogen gas. The optical absorption coefficient was calculated from the transmittance spectrum; the optical band gap of the Mg3N2 and Zn3N2 thin films were found to be 1.3 eV and 2.5 eV, respectively.  Accordingly, Eg of Zn3N2-Mg3N2 alloy (Zn3-3xMg3xN2) can be adjusted to 1.4 eV by varying the Mg content (x). Furthermore, the electron transport measurement shows that the single crystal zinc nitride has an electron mobility as high as 395 cm2 /Vs.  This contrasts sharply with InN, GaN, and InxGa1-xN: electron mobilities in those polycrystalline films are one or more orders of magnitude smaller than those in Zn3N2 films. Therefore, Zn3N2 can be an excellent photovoltaic absorber, if the bandgap is adjusted to ~1.4 eV. The bandgap tuning of Zn3N2 is a challenging issue to render it as a photovoltaic absorber.

 In this seminar, I propose a novel nitride semiconductor alloy system, Zn3-3xMg3xN2, of which bandgap is adjustable to ~1.4 eV.