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Depth analysis of subgap electronic states in amorphous oxide semiconductor, a-In-Ga-Zn-O, studied by hard x-ray photoelectron spectroscopy
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10.1063/1.3560769
/content/aip/journal/jap/109/7/10.1063/1.3560769
http://aip.metastore.ingenta.com/content/aip/journal/jap/109/7/10.1063/1.3560769

Figures

Image of FIG. 1.
FIG. 1.

(Color online) Analysis of escape depth (δ0) using a-IGZO/SiO2/c-Si samples. (a) Si 1s and (b) O 1s HX-PES spectra as a function of a-IGZO film thickness. (c) Signal intensity as a function of film thickness. (Symbols) Experimental data obtained from (a) and (b). (Lines) Theoretical data following the relation for (solid line) Si 1s at 1844.5 eV and (dashed line) O 1s of the Si−O bond at ∼530 eV.

Image of FIG. 2.
FIG. 2.

(Color online) (a) HX-PES spectra of the bandgap region for the unannealed a-IGZO films with film thicknesses of 4.7–46.9 nm. (b) Binding energy shift of core levels (ΔEB.E. ) as a function of the film thickness. ΔEB.E . is defined as the energy shift with respect to the data for the thickest film (46.9 nm).

Image of FIG. 3.
FIG. 3.

(Color online) HX-PES spectra in (a) valence band region and (b) bandgap region for the unannealed, the dry annealed, and the wet annealed a-IGZO films. That of sc-IGZO film is also shown for comparison. The VBM energies are indicated by extrapolating the straight onset regions below VBM (dotted lines) to the signal baseline. Non-negligible tails extend from the VBM to midgap with the widths of ∼1.5 eV. Extra states are also observed just below E F (corresponding to the zero binding energy, near-CBM states).

Image of FIG. 4.
FIG. 4.

(Color online) HX-PES spectra at several effective escape depths (δ) for (a) unannealed, (b) dry annealed, and (c) wet annealed films. (d) Density of state of the near-VBM states as a function of δ. (Symbols) The density of states is obtained by deconvoluting the data in (a−c). The dashed lines show the fitting results. (e) Depth distributions of the near-VBM state densities extracted from the fitting analyses of (d).

Image of FIG. 5.
FIG. 5.

(Color online) O 1s core level spectra at several δ for (a) unannealed, (b) dry annealed, and (c) wet annealed films.

Image of FIG. 6.
FIG. 6.

(Color online) (a) Binding energy shift (ΔE B.E.) as a function of δ for the unannealed film with film thickness of 37.8 nm. The ΔE B.E. is defined as the energy shift with respect to the data at δ = 5.8 nm. (b) Schematic illustration of band structure deduced from (a).

Image of FIG. 7.
FIG. 7.

(Color online) Near-CBM states as a function of δ for (a) unannealed, (b) dry annealed, and (c) wet annealed films.

Image of FIG. 8.
FIG. 8.

(Color online) (a) Comparison of near-CBM states for a-IGZO films, sc-IGZO films, ZnO single crystal, and epi-ZnO film. (b,c) Carrier concentration dependence of near-CBM states for (b) unannealed a-IGZO and (c) sc-IGZO films.

Image of FIG. 9.
FIG. 9.

(Color online) TFT simulations based on the subgap DOS below EF that mimic the near-CBM states observed by HX-PES. (a,b) DOS models that consider (a) only occupied states, and (b) both occupied and unoccupied states. (c) Simulated TFT characteristics based on the subgap DOSs in (a,b). The measured source−drain current (I DS)-gate−source voltage (VGS) at drain−source voltage (V DS)=10 V is shown for comparison (Ref. 11).

Tables

Generic image for table
Table I.

Three types of a-IGZO thin films used for HX-PES measurements. The carrier densities were estimated from the measured electrical conductivities using the assumption that the electron mobility is 10 cm2(Vs)−1 (Ref. 2).

Generic image for table
Table II.

The VBM energy levels and state density of near-VBM and near-CBM states for unannealed, dry annealed, and wet annealed films at δ =0.5 and 5.8 nm.

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/content/aip/journal/jap/109/7/10.1063/1.3560769
2011-04-15
2014-04-25
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Depth analysis of subgap electronic states in amorphous oxide semiconductor, a-In-Ga-Zn-O, studied by hard x-ray photoelectron spectroscopy
http://aip.metastore.ingenta.com/content/aip/journal/jap/109/7/10.1063/1.3560769
10.1063/1.3560769
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