Deposition, Characterization, and Performance of Spinel InGaZnO₄

Abstract

Polycrystalline indium−gallium−zinc oxide (IGZO) in the spinel phase was obtained by physical vapor deposition (PVD), using reactive sputtering from an IGZO target with In/Ga/ Zn = 1:1:1 composition. The initial growth of spinel IGZO is investigated by X-ray diffraction measurements after annealing the film. Deposition of spinel IGZO initially starts as a mixed amorphous/c-axis-aligned crystalline (CAAC) film, after which a metastable spinel IGZO is formed. Using a template of polycrystalline spinel Ga 2 ZnO 4 , the growth of the spinel phase is immediately achieved and enables the electrical characterization of pure spinel IGZO channels in scaled thin-film field-effect transistors. The average effective channel field-effect mobility of spinel IGZO of 50 ± 10 cm 2 /(V s) is slightly higher than amorphous IGZO in the same devices. This is in line with a slightly lower effective electron mass, as is calculated with density functional theory. The calculated total energies and band gaps have similar values to CAAC-IGZO. This metastable nature identifies spinel IGZO as an intermediate phase before the onset of CAAC-IGZO formation during PVD. Spinel IGZO is an interesting alternative to amorphous IGZO (a-IGZO) and CAAC-IGZO because of potentially higher robustness to oxygen vacancy formation.

Oxygen Defect Stability in Amorphous, C-Axis Aligned, and Spinel IGZO

“…In the case studied here, however, doping is caused by oxygen defects, which could affect the band structure. We have seen, however, that for all phases discussed here, oxygen defect doping happens in an almost ideal way. , Upon removal of an oxygen atom, an additional state appears in the conduction band. This state is situated 1–1.5 eV above the minimum and donated two electrons to the bottom of the conduction band.…”

“…The structural models for the spinel phase are based on a 3 × 3 × 2 supercell of the primitive cell of the spinel phase . This supercell is 18.5 × 18.5 × 17.3 Å 3 in size and contains 72 formula units of IGZO.…”

“…The structure with the most symmetric distribution of indium and gallium sites also is the lowest in energy and is used in this study. Disorder in the indium and gallium sites, however, only causes marginal energy differences and changes in the electronic structure . Due to the high symmetry of the crystal phase, there are only two symmetry inequivalent oxygen sites.…”

“…The caac phase is well known in the literature, and recipes on how to grow it can be found in ref . The spinel phase is known to exist with a low concentration of indium at the gallium sites of spinel Ga 2 ZnO 4 . , Only recently, a fully stoichiometric, stable spinel IGZO was deposited using physical vapor deposition (PVD) and characterized at imec . A complete description of the fabrication methods and process condition of stoichiometric spinel IGZO is too involved at this place, and a full description will appear shortly (see ref ).…”

“…The spinel phase is known to exist with a low concentration of indium at the gallium sites of spinel Ga 2 ZnO 4 . , Only recently, a fully stoichiometric, stable spinel IGZO was deposited using physical vapor deposition (PVD) and characterized at imec . A complete description of the fabrication methods and process condition of stoichiometric spinel IGZO is too involved at this place, and a full description will appear shortly (see ref ). We use the outcome of our stability investigation to sketch guidelines to reduce transistor access contact resistances.…”