Effect of N[sub 2]O Plasma Treatment on the Performance of ZnO TFTs

Abstract

Postfabrication rapid thermal annealing ͑RTA͒ and subsequent nitrous oxide ͑N 2 O͒ plasma treatment improved the performance of zinc oxide ͑ZnO͒ thin-film transistors ͑TFTs͒ in terms of off current and on/off current ratio by almost 2 orders of magnitude. The off current of 2 ϫ 10 −8 A and on/off current ratio of 3 ϫ 10 3 obtained after RTA were improved to 10 −10 A and 10 5 , respectively, by the subsequent N 2 O plasma treatment. X-ray photoelectron spectroscopy analysis of the TFT samples showed that the RTAtreated ZnO surface had more oxygen vacancies as compared to as-deposited samples, and the oxygen vacancies at the surface of RTA-treated ZnO were reduced by subsequent N 2 O plasma treatment. The reduction of oxygen vacancies at the top region of the ZnO channel is the cause of better off current and on/off current ratio of the TFTs.Zinc oxide ͑ZnO͒, with a direct bandgap energy of 3.37 eV, is one of the semiconductor materials well suited for transparent thinfilm transistors ͑TFTs͒ in the application areas of active matrix liquid crystal displays ͑AMLCDs͒ and electronic papers. 1-10 However, there are some technical challenges to overcome to achieve manufacturability of ZnO-based TFTs. One of the problems facing ZnObased TFTs is high off current ͑leakage current͒ and hence low on/off current ratio. 5-9 Low off current as well as on/off current ratio higher than 10 6 are required for the ZnO TFTs to function as select transistors in AMLCDs. 10 In TFTs with bottom-gate configuration, the drain-to-source current flow through the undepleted channel region as well as the gate-leakage current contribute to the off current. To eliminate the undesired drain-to-source current under the off condition, thinner channel layers have been employed in TFTs. 2,4 However, the crystalline quality of the ZnO materials improves with thickness, so the thinner ZnO channel tends to have more defects, which affects the carrier mobility and subthreshold slope of devices. 1,6 In this work, we studied the effect of postfabrication rapid thermal annealing ͑RTA͒ and subsequent nitrous oxide ͑N 2 O͒ plasma treatment on the off current and on/off current ratio of the fabricated bottom-gated ZnO TFTs. We found that the TFTs subjected to N 2 O plasma treatment exhibit better off current and on/off current ratio. X-ray photoelectron spectroscopy ͑XPS͒ analysis was performed to examine the surface modification of ZnO channel layer due to RTA and N 2 O plasma treatment.Corning 1737 glass coated with 200 nm thick indium tin oxide ͑ITO͒ was used as starting substrates ͑sheet resistance = 4-8 ⍀/Ǣ, Delta Technologies limited, USA͒ for fabricating TFTs with bottomgate configuration, and the ITO acted as the gate electrode. The substrates were ultrasonically cleaned with acetone, methanol, and deionized water. ITO gate electrodes were defined by standard photolithography and wet-etching process using LCE-12k ͑ITO etchant, Cyantek Corporation͒ at 45°C. Next, a 200 nm thick silicon nitride gate-dielectric layer was deposited by plasma-enhanced chemi...

The Influence of Different Partial Pressure on the Fabrication of InGaO Ultraviolet Photodetectors

“…In order to differentiate the XPS measurements, the typical O 1s peak was divided into three peaks through Gaussian fitting, as shown in Figure 5. The deconvoluted results of the O 1s peaks exhibited a peak at around 529.6 eV (O I ), which is ascribed to the O 2 − ions among metal oxides; a second peak, around 531.1 eV (O II ), is usually attributed to the oxygen vacancies in the film; the third peak, located at about 531.7 eV, is related to weakly bound oxygen species on the film surface, such as CO 3 , OH − , or adsorbed O 2 [16]. The relative ratio of O II /O total are 36.4%, 35.2%, 33.8%, and 28.8% for pO 2 = 0%, 5%, 10%, and 20%, respectively.…”

Influence of positive bias stress on N2O plasma improved InGaZnO thin film transistor

“…The Ti/ Al/Ti ͑50/200/50 nm͒ source/drain electrodes were formed by sputtering and then patterned into the dimension of channel width/length ͑W / L͒ =5-30 m / 7 m. A 30 nm thick a-IGZO film was deposited by dc magnetron sputtering system at room temperature, using a target of In: Ga: Zn =1:1:1 in atomic ratio, a plasma discharge power of 300 W, and in an ambiance of gas mixture ratio of O 2 / Ar= 6.7% with a working pressure of 5 mTorr. 15 The excellent SS of 0.28 V/dec reveals only a few interface states were remained. Finally, all of devices were capped with a 200 nm SiO x layer by PECVD at 170°C, and sequentially annealed in an oven at 330°C for 2 h. The electrical properties of a-IGZO TFTs with L =7 m and W = 30 m were analyzed by using Agilent 4156-C system in dark, and characterized at 30°C in vacuum with a pressure of 10 −2 Torr to avoid the interference from environment.…”

Optimization of Zr-Doped ZnO Thin Films Prepared by Sol-Gel Method

“…Its unique electrical and optical properties have made it popular in piezoelectric transducers, surface acoustic wave (SAW) devices, laser diodes, photoconductive UV detectors, gas sensors, etc. Several reports [3][4][5] have indicated that ZnO TFTs exhibited a high off-current (I off ) and a low on-to-off current ratio because a transistor with an active channel layer made from undoped ZnO film has a high carrier density that causes the channel to conduct when an applied gate voltage is absent. It has been demonstrated that the electrical characteristics of ZnO films can be controlled by doping with ternary elements 6,7) or by heat treatment processes.…”