Amorphous Tin Oxide Applied to Solution Processed Thin-Film Transistors

Abstract

The limited choice of materials for large area electronics limits the expansion of applications. Polycrystalline silicon (poly-Si) and indium gallium zinc oxide (IGZO) lead to thin-film transistors (TFTs) with high field-effect mobilities (>10 cm²/Vs) and high current ON/OFF ratios (I_On/I_Off > ~10⁷). But they both require vacuum processing that needs high investments and maintenance costs. Also, IGZO is prone to the scarcity and price of Ga and In. Other oxide semiconductors require the use of at least two cations (commonly chosen among Ga, Sn, Zn, and In) in order to obtain the amorphous phase. To solve these problems, we demonstrated an amorphous oxide material made using one earth-abundant metal: amorphous tin oxide (a-SnO_x). Through XPS, AFM, optical analysis, and Hall effect, we determined that a-SnO_x is a transparent n-type oxide semiconductor, where the SnO₂ phase is predominant over the SnO phase. Used as the active material in TFTs having a bottom-gate, top-contact structure, a high field-effect mobility of ~100 cm²/Vs and an I_On/I_Off ratio of ~10⁸ were achieved. The stability under 1 h of negative positive gate bias stress revealed a Vth shift smaller than 1 V.

Keywords: amorphous oxide semiconductor; hafnium oxide; solution process; thin-film transistor; tin oxide.

Figure 1

Physical properties of a-SnO_x thin films annealed at 300 °C. (a) XRD pattern of a 40 nm thick SnO_x. (b) High-resolution TEM image of SnO_x used in a thin-film transistor (TFT) channel region; the inset shows the local fast Fourier transform (FFT) pattern. (c) Transmittance of a-SnO_x thin film and (d) extraction of a-SnO_x optical bandgap using a Tauc plot. The orange line is the extrapolation from the curve and the intersection with the abscissa axis gives the optical bandgap; the value is indicated in the blue box. (e) An AFM image of the a-SnO_x surface. (f) The XRR measurement of the a-SnO_x. In (a,f) arb. Stands for arbitrary.

Figure 1

Physical properties of a-SnO_x thin films annealed at 300 °C. (a) XRD pattern of a 40 nm thick SnO_x. (b) High-resolution TEM image of SnO_x used in a thin-film transistor (TFT) channel region; the inset shows the local fast Fourier transform (FFT) pattern. (c) Transmittance of a-SnO_x thin film and (d) extraction of a-SnO_x optical bandgap using a Tauc plot. The orange line is the extrapolation from the curve and the intersection with the abscissa axis gives the optical bandgap; the value is indicated in the blue box. (e) An AFM image of the a-SnO_x surface. (f) The XRR measurement of the a-SnO_x. In (a,f) arb. Stands for arbitrary.

Figure 2

Composition analysis of a-SnO_x. (a) O1s spectrum and decomposed components of a-SnO_x thin film. The blue part represents oxygen in SnO₂, the red one oxygen in SnO. (b) Deconvolution of the Sn3d 5/2 peak. The blue part represents Sn as in SnO₂, the red part Sn as in SnO. (c) Sn4d peak analysis. The two doublets related to SnO₂ are in blue, the doublets related to SnO are in red. (d) Valence band analysis of a-SnO_x. (e) TOF-SIMS profile of a-SnO_x. In blue, red, and black are shown CsSi+, CsC+, and CsSn+ depth profile, respectively. In the above charts, Arb. stands for arbitrary.

Figure 2

Composition analysis of a-SnO_x. (a) O1s spectrum and decomposed components of a-SnO_x thin film. The blue part represents oxygen in SnO₂, the red one oxygen in SnO. (b) Deconvolution of the Sn3d 5/2 peak. The blue part represents Sn as in SnO₂, the red part Sn as in SnO. (c) Sn4d peak analysis. The two doublets related to SnO₂ are in blue, the doublets related to SnO are in red. (d) Valence band analysis of a-SnO_x. (e) TOF-SIMS profile of a-SnO_x. In blue, red, and black are shown CsSi+, CsC+, and CsSn+ depth profile, respectively. In the above charts, Arb. stands for arbitrary.

Figure 3

The a-SnO_x TFT structure: (a) a schematic diagram of the bottom gate and top contact a-SnO_x TFT. (b) An illustration of an a-SnO_x TFT. (c) TEM image of an a-SnO_x TFT taken near the source/drain region.

Figure 4

Electrical characterization of a-SnO_x TFT. Typical (a) transfer and (b) output of a TFT using HfO₂ as the gate insulator. For the transfer curve, V_DS = 0.1 and 1 V are shown in dark lines with symbols. The green line is the gate leakage. For the output curves, V_DS was varied from 0 to 1 V, and V_GS was set from 0–1 V.

Figure 5

a-SnOx TFTs under various stress during 3600 s. The evolution of the transfer curve of a-SnO_x TFT under (a) positive gate bias stress and (b) negative bias stress. After 50, 100, 1000, and 3600 s, a transfer curve was measured (at V_DS = 0.1 V). One color is shown per measurement.