357. Conversion of an ultra-wide bandgap amorphous oxide insulator to a semiconductor

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Junghwan Kim, et al, NPG Asia Materials 9, e359 (2017)
https://doi.org/10.1038/am.2017.20

(1) Crystalline β-Ga2O3 is known as a transparent conducting oxide with an ultra-wide bandgap of ~4.9 eV, but amorphous a-Ga2Ox is just an electrical insulator because the combination of an ultra-wide bandgap and an amorphous structure has serious difficulties in attaining electronic conduction. Ultra-wide bandgap amorphous materials can attain electronic conduction only with difficulty because the ultra-wide gap leads to low electron density Ne and the amorphous structure leads to high electron trap density Dt.
(2) However, obtaining electron conduction in an amorphous material is much more difficul than obtaining electron conduction in a crystalline material. Only hydrogenated amorphous silicon and amorphous oxide semiconductors (AOSs) have been utilized as active layers in practical semiconductor devices.
(3) Only metal/oxide ion off-stoichiometry and hydrogen doping would be effective doping methods in AOSs, and consequently carrier control of the conventional AOSs has been conducted mainly by controlling oxygen partial pressure during deposition and thermal annealing (PO2) as well as by hydrogen doping.
(4) Electrical conductivities were obtained only if the film density was >5.2 gcm-3, where the electrical conductivity increased exponentially with the increase in the film density.
(5) A higher PO2 and a larger deposition rate are required to obtain a higher electronic conduction and a higher free electron density of PLD-deposited GaOx with a larger film density.
(6) The low density would come from incorporation of residual H2O and H-related molecules in the PLD chamber because a lower growth rate requires a longer deposition time and incorporates more impurity from the deposition atmosphere into the growing film.
(7) Both a-IGZO and a-GaOx have similar CBM structures made mainly of spherical metal s orbitals, which can form a highly dispersed conduction band with small electron effective masses even in a disordered amorphous structure.
(8) The keys to obtaining electronic conduction in a-GaOx are (i) a high film density and (ii) an appropriately high PO2 to suppress formation of electron traps.