Amorphous structure and electrical performance of low-temperature annealed amorphous indium zinc oxide transparent thin film transistors
Introduction
Electronics based on amorphous metal oxide semiconductor (AOS) thin film transistors (TFTs) offer an alternative to amorphous Si-based devices. These materials are optically transparent and have important application to organic light emitting-based diodes which, as current-driven devices, require a higher on/off current ratio than field-driven liquid crystal display devices. Amorphous oxides with the (n − 1)d10ns0 (n ≥ 4) electronic configuration of heavy-metal cations such as zinc tin oxide [1], zinc-rich indium oxide [2], indium gallium oxide [3], indium-rich zinc oxide (a-IZO) [4], [5], [6], [7], [8] and indium gallium zinc oxide (a-IGZO) [9], [10], [11] have been demonstrated by a number of groups. Amorphous oxide channel TFTs have higher field effect mobility than amorphous Si-based devices and, therefore, should yield faster TFT switching speeds and deliver higher on-current [8], [12].
Of the AOS materials, In2O3-10wt.%ZnO (IZO) provides the highest reported Hall electron mobility, and allows controlled growth of both conducting (degenerate doping of 5×1020/cm3) and semiconducting (1015/cm3) materials from the same IZO sputter target by control of the oxygen content in the sputter gas during deposition [4], [5], [13]. In the fabrication of a-IZO channel devices, it is difficult to achieve low enough carrier concentrations in the channel [4], [5], [14] and this can lead to high TFT off-state currents. Consequently, most reports of IZO-based TFT devices use very thin [4], [5] (10–30 nm is typical) channel layers while others add Ga as a third cation species (IGZO) [14], [15] to suppress native defect-based carrier formation. The addition of a third cation species increases carrier scattering and significantly decreases channel carrier mobility.
We have reported compositionally homogeneous semiconducting IZO channel/IZO metallization structured TFTs. In this configuration, both the semiconducting channel and source/drain metallization layers are deposited from the same IZO target by adjusting the process oxygen during deposition to tune the IZO carrier density between ~ 1016/cm3 for the channel to ~ 5 × 1020/cm3 for the metallization layer. As homojunctions, these compositionally homogeneous IZO/IZO interfaces have a well-aligned band structure and, consequently, low contact resistance is expected [13].
Amorphous oxide-based IZO TFTs have been fabricated using only room temperature processing [4], [7], [8], [9]; however, post-annealing has been shown to improve field effect mobility [11], [16], electrical stability [17] and contact resistance between channel and source/drain metallization [13], [18], [19]. Nomura et al.[17], for example, reported post-deposition annealing at 400 °C to improve IGZO TFT stability but they also found that thermal annealing results in up to ~−7 V threshold voltage shift. According to Kikuchi et al. [20], both pulsed-laser deposition (PLD) and sputter processed IGZO TFTs show negative threshold voltage shift up to ~−48 V after dry and wet annealing at 150–400 °C which they attribute to the decrease of deep trap density. Barquinha et al.[7] studying IZO TFTs reported a large negative threshold voltage shift of ~−13 V after annealing at 125 °C but a smaller shift of ~−6 V after 250 °C annealing. The cause of threshold voltage shift and instability are not yet established in these oxide materials.
Section snippets
Experiment
The structure of IZO films and the performance of IZO based TFTs were evaluated before and after annealing at 100 and 200 °C. The amorphous/crystalline structure of the film was determined using glancing incident angle x-ray diffraction (GIAXRD, Siemens D5000) and transmission electron microscopy (TEM, JEOL JEM-2010) while test-TFTs were fabricated and characterized to determine field effect mobility, threshold voltage, and sub-threshold slope. Transmission line measurements (TLM) were made on
Results
Sputter deposition of IZO thin films onto unheated substrates results in the formation of an amorphous phase that is much more stable [4], [5] with respect to crystallization than either pure amorphous Indium oxide or ITO. In the amorphous state, the octahedral coordination unit of InO6 that forms the structural basis of the 80-atom crystalline bixbyite unit cell is preserved but medium and long range order are absent. Fig. 2(a) shows GIAXRD of as-deposited and annealed 100-nm-thick IZO films
Discussion
The change in IZO resistivity with annealing at 200 °C from 8.2 to 0.33 Ωcm amounts to a 20-fold increase in carrier density. This increase in carrier density is unexpected since annealing in air should consume oxygen vacancies (which contribute 2 free electrons) in the IZO and thereby should decrease the total carrier density. However, at the low anneal temperature used in this work (200 °C) the kinetics of diffusion of oxygen into the film (or vacancies to the surface of the film) is likely to
Conclusions
We have investigated the effect of low-temperature annealing in air on the electrical performance of room temperature processed amorphous IZO TFTs. No change in the amorphous structure of the annealed film was detectable by GIAXRD or HRTEM/SAD analysis. A negative 15 V shift in the TFT threshold voltage was observed with annealing at 200 °C which can be attributed to an increase in carrier density in the channel and secondarily to changes in the density of interface states. The increase in
Acknowledgements
The authors gratefully acknowledge the financial support of the National Science Foundation (NSF) Award No. DMR-0804915 and Fuji Film Corporation.
References (26)
- et al.
Solid-State Electron.
(2006) - et al.
Thin Solid Films
(2008) - et al.
Thin Solid Films
(2007) J. Non-Cryst. Solids
(2006)- et al.
Thin Solid Films
(2008) - et al.
Thin Solid Films
(2010) - et al.
Solid-State Electron.
(2005) - et al.
Solid-State Electron.
(2005) - et al.
Thin Solid Films
(2006) - et al.
Appl. Phys. Lett.
(2005)
J. Appl. Phys.
Appl. Phys. Lett.
Appl. Phys. Lett.
Cited by (34)
Origin of an unintended increase in carrier density of ternary cation-based amorphous oxide semiconductors
2021, Applied Surface ScienceCitation Excerpt :All of these characteristics are expected to be a key enabler for realizing the next generation ultra-high-definition displays. Post-process annealing is widely employed in AOS-based TFT fabrication since annealing has been shown to improve field effect mobility [13–14] and channel/metallization contacts [11,15] as well as reduce trap density [16], which often leads to unstable device performance or unfavorable hysteresis in their transistor characterstics [16–17]. It has been reported that post-annealing at temperatures ranging from 100 °C to 400 °C in air increased TFT field effect mobility from 5-10 to 15 cm2/vs for ternary cation AOS of IGZO channel and 10–20 to 25–30 cm2/vs for binary cation, IZO channel.
Rapid thermal annealing effect of transparent ITO source and drain electrode for transparent thin film transistors
2021, Ceramics InternationalCitation Excerpt :Metal oxide materials, a main component of transparent TFTs, have been widely used and investigated as semiconductor channels, insulators, and electrodes in industry and academia due to their novel nature, ease of large-area processing, the tunability of their electrical properties, high optical transparency, and so on [4–6]. Specifically, multicomponent oxide semiconductors such as zinc oxide (ZnO), indium zinc oxide (IZO), zinc tin oxide (ZTO), indium zinc tin oxide (IZTO), and indium gallium zinc oxide (IGZO) have been actively researched as promising transparent channel materials that can replace opaque Si channels in high-performance transparent TFTs [7–16]. As transparent active channels, multicomponent oxide semiconductors show high mobility, low off-state current, good uniformity, and low processing temperature.
Temporal and voltage stress stability of high performance indium-zinc-oxide thin film transistors
2017, Solid-State ElectronicsCitation Excerpt :It is well established [17,24] that native-defect doping by doubly-charged oxygen vacancies is the dominant source of carriers in IZO. The carrier density in IZO can, therefore, be controlled by tuning the oxygen partial pressure during the IZO deposition process [17], or by annealing in an oxygen ambient, or via a solid-state reaction [24,25]. Typically, in order to fabricate functional TFTs that can be depleted by acceptably low gate voltages, the oxygen vacancy (and hence the carrier density) of deposited IZO must be controlled at the 1017–1018 cm−3 level.
Channel scaling and field-effect mobility extraction in amorphous InZnO thin film transistors
2017, Solid-State ElectronicsCitation Excerpt :To date, many researchers have extensively contributed to the development of high-performance and stable AOS TFTs. These efforts include studies of thermal [12] and bias stress [13,14] stability, the elucidation of doping mechanisms [15,16], threshold voltage stability [12–14,17], the investigation/improvement of channel/metallization contact properties [2,18,19], and amorphous phase stability [12,20]. Therefore, some AOS materials such as InGaZnO (IGZO) are now being deployed in high performance and flexible active-matrix liquid crystal displays and active-matrix organic light emitting diode technologies [10].
Performance enhancement of amorphous indium-zinc-oxide thin film transistors by microwave annealing
2015, Applied Surface ScienceCitation Excerpt :These incorporation ions such as Ar+ and H+ work as carrier scattering centers and degrade the electrical performance [9]. Therefore, posting conventional thermal annealing above 200 °C for 1 h or longer is required to improve the electrical performance of a-IZO TFTs device [10,11]. However, this conventional thermal annealing technology causes the threshold voltage shift to large negative, resulting in the stability and reliability issues for practical applications.