Polymer Doping Enables a Two‐Dimensional Electron Gas for High‐Performance Homojunction Oxide Thin‐Film Transistors

Abstract

deposition (PLD), and radiofrequency sputtering. [2,[11][12][13] Nevertheless, solutionprocessing of MO precursors holds significant promise for lower cost production and compatibility with flexible substrates. [2,9,14,15] In this regard, indium oxide (In 2 O 3 ) is one of the most investigated solution-processed oxide semiconductors, [7,[16][17][18][19][20][21] however, the carrier density of pristine In 2 O 3 is difficult to control and In 2 O 3 films are usually polycrystalline, limiting their performance uniformity over large areas as well as mechanical flexibility. [22] To address both of these issues, metals such as Zn and/or Ga are added to In 2 O 3 , enabling the growth of amorphous MO thin films with enhanced electronic properties and ultimately affording far more stable TFT characteristics. [22][23][24][25][26] Recently, we reported a new strategy for high performance, near-zero threshold voltage (V Th ), bias stress-stable, and amorphous In 2 O 3 films and TFTs by simply doping the In 2 O 3 with an electrically insulating polymer such as poly(4-vinylphenol) (PVP) or polyethylenimine (PEI) to afford amorphous MO:polymer blend semiconducting nanocomposites. [2,17,22] The attraction of electron-rich PEI versus PVP is that the TFT High-performance solution-processed metal oxide (MO) thin-film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In 2 O 3 ) and polyethylenimine (PEI)-doped In 2 O 3 (In 2 O 3 :x% PEI, x = 0.5-4.0 wt%) as the channel layer. A two-dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In 2 O 3 and PEI-In 2 O 3 via work function tuning of the In 2 O 3 :x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In 2 O 3 ) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm 2 V −1 s −1 on a 300 nm SiO 2 gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In 2 O 3 materials, which achieve a maximum mobility of ≈4 cm 2 V −1 s −1 . Furthermore, a mobility as high as 30 cm 2 V −1 s −1 is achieved on a high-k ZrO 2 dielectric in the homojunction devices. This is the first demonstration of 2DEG-based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material. Thin-Film TransistorsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.As a consequence of their outstanding charge transport properties and excellent optical transparency, metal oxide thin-film transistors (MOTFTs) have been heavily investigated for applications in state-of-the-art flat panel display technologies and next-generation electronics. [1][2][3][4][5][6][7][8][9][10] To date, several growth techniques have been utilized to fabricate MO films for TFTs including atomic layer deposition (ALD), pulsed-laser

Printable Semiconductors for Backplane TFTs of Flexible OLED Displays

“…The markedly improved electrical performance originated from the presence of free electrons confined on the heterointerface plane, which were induced by the large conduction band offset between ZnO and In 2 O 3 . The 2DEG was further realized via polymer‐doped oxide TFTs by fabricating a homojunction of In 2 O 3 and polyethylenimine (PEI)‐doped In 2 O 3 . The resultant TFTs achieved decent μ FE of 10 cm 2 V −1 s −1 on a 300 nm SiO 2 gate dielectric.…”

Recent Advances of Solution-Processed Heterojunction Oxide Thin-Film Transistors

“…By adjusting the thickness of AlInO and the doping amount of Al, AlInO (30%)/In 2 O 3 heterostructure TFTs with a high mobility of 40 cm 2 /Vs, a threshold slope of 0.7 V/dec, and an on/off ratio of 10 7 could be realized [58]. [56]. The 2D electron gas was achieved by creating a band offset between In2O3 and In2O3:PEI via work function tuning of the PEI-doping ratio.…”

“…When the doping amount of Li was 20%, the mobility of In 2 O 3 /Li-ZnO heterojunction TFTs reached the maximum value of 11.4 cm 2 /Vs and the on/off current ratio of ~10 5 . Chen et al demonstrated high performance In 2 O 3 /In 2 O 3 :polyethylenimine (PEI) heterostructure TFTs [ 56 ]. The 2D electron gas was achieved by creating a band offset between In 2 O 3 and In 2 O 3 :PEI via work function tuning of the PEI-doping ratio.…”

Molecule Charge Transfer Doping for p‐Channel Solution‐Processed Copper Oxide Transistors

“…Similarly, the mobility enhancement in n‐type oxide TFTs by n‐doping was recently reported. [ 14 ] It is worthy to note that the F 4 TCNQ layers delivered insulating behavior with low conduction current of several tens of picoamperes (Figure S3, Supporting Information). This confirms that the performance enhancement of F 4 TCNQ‐inserted Cu x O TFTs originated from charge transfer p‐type doping rather than from the conductance of F 4 TCNQ layers.…”

“…Unlike conventional atomic substitution doping, the doping process does not induce any defects or impurities into the host lattice, thereby eliminating undesired carrier scattering while reserving the fundamental transport properties. [ 11 ] Originally, MCTD was mainly used with organic semiconductors [ 12,13 ] and recently has been extended to n‐type metal oxides [ 14,15 ] and low‐dimensional nanomaterials (2D semiconductors, [ 16 ] carbon nanotubes, [ 17,18 ] and nanocrystals [ 19 ] ). For p‐type MCTD doping, the dopants should have a higher electron affinity than that of the host materials; commonly used candidates are MoO 3 , NO 2 , and tetrafluoro‐tetracyanoquinodimethane (F 4 TCNQ).…”