Quantum Confinement Effect in Amorphous In–Ga–Zn–O Heterojunction Channels for Thin-Film Transistors

Abstract

Electrical and carrier transport properties in In-Ga-Zn-O thin-film transistors (IGZO TFTs) with a heterojunction channel were investigated. For the heterojunction IGZO channel, a high-In composition IGZO layer (IGZO-high-In) was deposited on a typical compositions IGZO layer (IGZO-111). From the optical properties and photoelectron yield spectroscopy measurements, the heterojunction channel was expected to have the type-II energy band diagram which possesses a conduction band offset (∆E c ) of~0.4 eV. A depth profile of background charge density indicated that a steep ∆E c is formed even in the amorphous IGZO heterojunction interface deposited by sputtering. A field effect mobility (µ FE ) of bottom gate structured IGZO TFTs with the heterojunction channel (hetero-IGZO TFTs) improved to~20 cm 2 V −1 s −1 , although a channel/gate insulator interface was formed by an IGZO−111 (µ FE =~12 cm 2 V −1 s −1 ). Device simulation analysis revealed that the improvement of µ FE in the hetero-IGZO TFTs was originated by a quantum confinement effect for electrons at the heterojunction interface owing to a formation of steep ∆E c . Thus, we believe that heterojunction IGZO channel is an effective method to improve electrical properties of the TFTs.

Heterojunction Channels in Oxide Semiconductors for Visible‐Blind Nonvolatile Optoelectronic Memories

“…Results of an enhanced field‐effect mobility and lower subthreshold swings have been reported in previous works. [ 34–37 ] However, in contrast, this work adopts a smaller band gap layer at the front channel to achieve an opto‐electronic memory function; these operation methods will be verified in later discussions. Therefore, when applying voltage to the gate and drain terminals, carriers will conduct only at the In‐rich front layer since the conduction band offset restricts carriers from flowing to the Zn‐rich layer.…”

“…[ 33 ] When increasing the indium ratio, the band gap of the InGaZnO film decreases, and a conduction band barrier is formed from the front to back channel. The heterojunction channel is thus achieved and has been reported in previous works by the tuning of different metal‐ratios [ 34,35 ] or oxygen content in InGaZnO. [ 36,37 ] It is a mature method to improving the characteristics of a‐InGaZnO TFTs.…”

High‐Performance Coplanar Dual‐Channel a‐InGaZnO/a‐InZnO Semiconductor Thin‐Film Transistors with High Field‐Effect Mobility

“…From carrier concentration profile, carrier movement through two conduction paths separated by ∆ E C in dual‐channel TFT architecture better supports the conduction mechanism similar to the 2DEG system at heterointerface. [ 38 ] Besides, a‐IZO/a‐IGZO/GI TFT exhibits the V Th close to 0 V and to support the electron confinement effect similar to 2DEG formation. [ 34,37,38 ] The formation and distribution of the 2DEG system in coplanar dual‐channel TFTs need further investigation.…”

“…[ 38 ] Besides, a‐IZO/a‐IGZO/GI TFT exhibits the V Th close to 0 V and to support the electron confinement effect similar to 2DEG formation. [ 34,37,38 ] The formation and distribution of the 2DEG system in coplanar dual‐channel TFTs need further investigation. Accounting for the advantage of ∆ E C on surface band bending, dual‐channel conduction takes place in a‐IZO/a‐IGZO/GI TFT with considerably large carrier transport separated by the ∆ E C barrier which might also help to avoid carrier scattering with large I D , excellent μ FE of 49.5 cm 2 V −1 s −1 , and reduced I OFF of ≈10 –13 A under V DS = 1.0 V in our dual‐channel MOS TFTs.…”

“…also studied the quantum confinement effect in a‐InGaZnO heterojunction TFT to explain twofold increased mobility. [ 38 ] Therefore, a precise process control such as optimum channel thickness, carrier densities of individual channels, proper energy band alignment, additional doping, postfabrication annealing, and numerical simulation are important to understand the excellent electrical characteristics with coplanar TFT architecture. However, to the best of our knowledge, previous reports could not touch the coplanar structure using dual‐layer TFT architecture.…”