Spin-coated zinc oxide transparent transistors

B J Norris¹, J Anderson², J F Wager¹ and D A Keszler²

¹School of Electrical Engineering and Computer Science, 220 Owen Hall, Oregon State University, Corvallis, OR 97331-3211, USA
²Department of Chemistry, 153 Gilbert Hall, Oregon State University, Corvallis, OR 97331-4003, USA

Email: jfw@ece.orst.edu

Received 3 June 2003
Published 1 October 2003

Zinc oxide (ZnO) transparent thin-film transistors (TTFTs) are a very recent development (Carcia et al 2003, Hoffman et al 2003, Masuda et al 2003, Nishi et al 2003). To date, ZnO TTFT channel layers have been formed exclusively by vacuum deposition methods: ion-beam sputtering, rf sputtering, or pulsed laser deposition. The purpose of the work reported herein is to demonstrate the realization of a ZnO TTFT in which the channel layer is obtained via spin-coating deposition. Spin-coating deposition offers a simple and low-cost processing alternative to vacuum deposition. High quality ZnO thin films have been prepared by spin-coating, resulting in dense, oriented films (Ohya et al 1994, Nishio et al 1996, Ohyama et al 1996, 1997, Jimenez-Gonzalez et al 1998, Natsume and Sakata 2000, Lee et al 2002, Wessler et al 2002).

One possible application of ZnO TTFTs involves their use as transparent select-transistors in each pixel of an active-matrix liquid-crystal display (AMLCD). Other suitable applications may entail employing them as transparent alternatives to amorphous silicon or organic thin-film transistors (TFTs).

An important measure of TFT performance for these types of applications is the magnitude of the electron channel mobility. A higher mobility leads to a higher drive current and faster device operating speeds, which translates into more application possibilities (Wager 2003). Typical amorphous silicon TFT mobilities are 0.5-1 cm² V ^- 1 s ^- 1, with an estimated theoretical limit of ~10 cm² V ^- 1 s ^- 1 (Shur 1990). The largest organic TFT channel mobility reported to date is 2.7 cm² V ^- 1 s ^- 1 for pentacene, which is approaching the theoretical limit of 10 cm² V ^- 1 s ^- 1 for organic TFTs (Dimitrakopoulos and Mascaro 2001). In contrast, the largest ZnO TTFT channel mobility reported to date is 7 cm² V ^- 1 s ^- 1 (Nishi et al 2003). While the theoretical channel mobility limit for inorganic TFTs is not yet established, the recent demonstration of an engineered inorganic TTFT with a mobility of 80 cm² V ^- 1 s ^- 1 suggests that there is much room for future improvement (Nomura et al 2003).

A zinc nitrate precursor solution is prepared by mixing 3.6 parts by mass of 99.999% zinc nitrate hexahydrate from GFS chemicals with one part of 99.7% glycine from Alfa Aesar and 2.2 parts of 18.2 MΩ cm ^- 1 deionized water. The solution is then placed in a boiling water bath for 75 min. The solution decreases in volume by ~10 % during the heating cycle. Spin solutions are subsequently diluted with deionized water.

TTFTs are prepared on glass substrates, manufactured by the Nippon Electric Glass Company, coated with a 200 nm sputtered indium tin oxide (ITO) layer and a 220 nm atomic layer deposited superlattice of Al₂O₃ and TiO₂ (ATO). The ITO and ATO layers constitute the gate contact and insulator, respectively, of a bottom-gate TTFT. These ITO/ATO substrates are supplied by Planar Systems of Beaverton, OR. The zinc nitrate-based solution is spun onto the ITO/ATO substrate at 3000 rpm for 30 s, and then converted to zinc oxide by baking in air for 10 min at 600°C. A rapid thermal anneal is then performed in oxygen at a maximum temperature of 700°C to further improve film crystallinity. An optimal ZnO thickness is found to be approximately 30 nm. Zinc oxide islands measuring 400 µm × 300  µm are defined by photolithography. An HCl wet etch is used to remove unwanted zinc oxide. Source and drain regions of ITO, measuring 150 µm × 200 µm, are defined by photolithography. An ~100 nm ITO layer is then deposited via ion-beam sputtering and patterned by lift-off. After lift-off the TTFTs are rapid thermal annealed at 300°C in oxygen to improve the transparency of the ITO source and drain. Contact to the ITO gate is achieved by scratching through the ATO layer and soldering a thin indium metal layer into the scratches. The final device structure (figure 1) has a width-to-length ratio of 8.

Electrical characteristics are measured by using a Hewlett-Packard 4156B parameter analyser. Measurements are performed in the dark. Current-art spin-coated ZnO TTFTs exhibit more light sensitivity than ion-beam sputtered ZnO TTFTs. To accurately assess the dark current characteristics of these devices, an electrical pre-stress procedure is employed to minimize the effects of persistent photoconductivity, to reproducibly establish the initial state of the TTFT, and to stabilize electrical characteristics. The pre-stress procedure consists of (i) applying a constant gate voltage of 40 V and sweeping the drain voltage from 0 to 40 V and (ii) reducing the gate voltage to 0 V and again sweeping the drain voltage from 0 to 40 V.

Typical I_D-V_DS curves of a spin-coated ZnO TTFT are shown in figure 2. This TTFT requires application of a positive gate voltage, 10-20 V, to turn it on; thus, it exhibits n-channel, enhancement-mode behaviour. Except, for the V_DS  =  40 V curve, the slope of each I_D curve is flat, indicating `hard saturation', which is desirable for most circuit applications. I_D-V_GS curves (shown in figure 3) reveal a drain current on-to-off ratio of nearly 10⁷. A drain current on-to-off ratio of at least 10⁶ is required for AMLCD select-transistor applications.

The transistor stack is highly transparent to visible light as shown in figure 4. Optical measurements of the device stack in the channel region indicate a raw transmission of ~90% across the visible portion of the electromagnetic spectrum.

Further assessment of the I_D-V_DS characteristics of this device indicates an effective channel mobility of ~0.20 cm² V ^- 1 s ^- 1. This mobility is less than the 7 cm² V ^- 1 s ^- 1 channel mobility previously reported for pulsed laser deposited ZnO TFTs (Nishi et al 2003). It is likely that the lower mobility of the spin-coated TTFTs arises from a poorer crystallinity, a significant film porosity, larger impurity concentrations, or a combination thereof. Work is ongoing to improve mobility and to understand mobility trends with respect to process variations. Preliminary work indicates that the mobility varies with the ZnO thickness. Other researchers have shown the crystalline orientation of spin-coated ZnO layers to depend on film thickness (Ohyama et al 1997).

Spin-coated ZnO TTFTs fabricated to date show increased light sensitivity compared with ion-beam deposited ZnO TTFTs; i.e. leakage and saturation currents are larger when exposed to room light. This light sensitivity appears to be related to persistent photo-conductivity effects in ZnO, which are ascribed to chemisorption and desorption of oxygen at the ZnO/air interface (Takahashi et al 1994, Studenikin et al 2000). Scanning electron microscopy (as shown in figure 5) and atomic force microscopy analysis indicate that spin-coated ZnO thin film surfaces are rougher than those prepared by ion-beam sputtering. Thus, properties related to the film/air interface would be expected to be more pronounced with these rougher films. Therefore, we tentatively attribute differences in light sensitivity between ion-beam sputtered and spin-coated ZnO TTFTs to differences in surface roughness.

A TTFT is demonstrated in which the ZnO channel layer is synthesized by spin-coating deposition. The current performance of such devices is highlighted by an electron channel mobility as large as 0.20 cm² V ^- 1 s ^- 1 and a drain current on-to-off ratio of nearly 10⁷. Spin-coated ZnO TTFTs manufactured to date exhibit more light sensitivity than devices prepared by ion-beam sputtering; this is tentatively attributed to a larger degree of surface roughness of the spin-coated ZnO thin films. Continued development of the spin-coating and annealing procedures should lead to a reduction in light sensitivity and higher mobilities.

Acknowledgments

This work was funded by the US National Science Foundation under Grant No DMR-0071727 and by the Army Research Office under Contract No MURI E-18-667-G3.

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