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Thin-Film Transistor Fabricated in Single-Crystalline Transparent Oxide Semiconductor

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Science  23 May 2003:
Vol. 300, Issue 5623, pp. 1269-1272
DOI: 10.1126/science.1083212

Abstract

We report the fabrication of transparent field-effect transistors using a single-crystalline thin-film transparent oxide semiconductor, InGaO3(ZnO)5, as an electron channel and amorphous hafnium oxide as a gate insulator. The device exhibits an on-to-off current ratio of ∼106 and a field-effect mobility of ∼80 square centimeters per volt per second at room temperature, with operation insensitive to visible light irradiation. The result provides a step toward the realization of transparent electronics for next-generation optoelectronics.

Transparent electronic circuits (1) are expected to serve as the basis for new optoelectronic devices. A key device for realizing transparent circuits is the transparent field-effect transistor (TFET). TFETs have been developed on the basis of compound wide–band gap semiconductors such as GaN (2) and SiC (3). These exhibit good performance [e.g., a field-effect mobility of ∼140 cm2 V-1 s-1] and durability in high-temperature and high-power operation. Oxide semiconductors present an alternative opportunity for discovering new transparent electronics applications with added functionality, because oxides display many properties in their magnetic and electronic behavior that originate from a variety of crystal structure and constituent elements.

Transparent conductive oxides such as indium-tin oxide (ITO) and ZnO have found applications as electrical interconnections and as window electrodes in flat panel displays and solar cells. The discovery of a p-type transparent oxide semiconductor (TOS), CuAlO2 (4), and the development of key device components fabricated from TOSs, such as pn-junction rectifiers (5, 6) and ultraviolet light–emitting diodes (7), have led to the emergence of TOSs as viable materials for further development of transparent electronics.

However, the TFETs fabricated to date using conventional TOSs such as SnO2 and ZnO (8, 9) exhibit poor performance. For instance, their on-to-off current ratios and field-effect mobilities are on the order of 103 and as low as 10 cm2 V-1 s-1, respectively, and the device exhibits “normally-on” characteristics. Although a polycrystalline ZnO TFET has previously been found to have “normally-off ” characteristics with an on-to-off current ratio of ∼107, its field-effect mobility is only 3 cm2 V-1 s-1. Grain-boundary potential barriers are thought to limit the performance (10). The large off-current and the normally-on characteristics may originate from the fact that these conventional TOSs contain many carriers in the as-prepared state (as the result of a somewhat large nonstoichiometry in the chemical composition), making it difficult to control the carrier density down to less than 1017 cm-3 without counterdoping of acceptors. Hence, it is imperative to choose a material that can control the carrier concentration down to the intrinsic level and to develop a method to grow a high-quality single-crystalline thin film.

We report the fabrication and performance of TFETs that use a single-crystalline film of a TOS, InGaO3(ZnO)5, for the active channel layer. This material has advantages over conventional TOSs, including easy growth of a high-quality single-crystalline film and good controllability of carrier concentration. The use of a gate insulator with a high dielectric constant (“high-k dielectric”), amorphous HfO2, was found to improve the FET performance.

The structure of InGaO3(ZnO)5 is characterized by its layered superlattice structure (Fig. 1A)—in which InO2 layers and GaO(ZnO)5+ blocks are alternately stacked along the 〈 0001 〉 axis (11, 12)—and is thought to be the origin of its superior electronic properties. The layers are similar to those of ITO and Ga-doped ZnO, in which carrier doping is controlled by the amount of Ga. However, the Ga3+ ion incorporated in the GaO(ZnO)5+ block does not generate carriers in InGaO3(ZnO)5 because the Ga3+ ion does not substitute the Zn2+ tetrahedral sites only, but also takes trigonal-bipyramidal coordination sites, which keeps the local electroneutrality. Moreover, the In2O3 layer may work as a blocking barrier for oxygen outdiffusion, and thereby may suppress the formation of oxygen vacancy; this idea is supported by our transmission electron microscope observation that mass transport proceeds much faster in the GaO(ZnO)5+ blocks than across the InO2 and GaO(ZnO)5+ layers. It is therefore easier to maintain the material in stoichiometry and control the carrier concentration down to the intrinsic level in a single crystal.

Fig. 1.

Structure of InGaO3(ZnO)5. (A) Schematic of the crystal structure. A HRTEM lattice image is shown for comparison. The InO2 layer (In3+ ion locates at an octahedral site coordinated by oxygens) and the GaO+(ZnO)5 block (Ga3+ and Zn2+ ions share trigonal-bipyramidal and tetrahedral sites) are alternately stacked along the 〉 0001 〉 direction at a period of 1.9 nm (d0003). (B and C) Cross-sectional HRTEM images of a InGaO3(ZnO)5 thin film grown on YSZ(111) by reactive solid-phase epitaxy. Periodic stacking of the InO2 layer and the GaO+(ZnO)5 block is clearly visible, which is also confirmed in the electron diffraction image [(C), inset]. Single-crystalline film is formed over the entire observation area. The topmost layer of the film is the InO2 layer.

However, because of the complex structure and composition, it is difficult to obtain single-crystalline films of such oxides with the use of a conventional vapor-phase growth technique alone. Complex oxides, in general, require high temperatures to grow in single-crystalline phase, and some chemical components may evaporate at lower temperatures in the vacuum deposition chamber. We have developed a reactive solid-phase epitaxy (R-SPE) technique that can be used to grow single-crystalline TOS thin films with an atomically flat surface (13, 14). The selection of the film formation technique is not crucial as long as a suitable template layer is formed. We have shown that this concept is applicable to a variety of materials (15).

Pulsed laser deposition (PLD) (16, 17) was used to deposit a 2-nm-thick ZnO epitaxial layer at 700°C on a (111) single-crystal yttria–stabilized zirconia (YSZ) substrate as the template, followed by a 120-nm-thick InGaO3(ZnO)5 layer at room temperature. The resulting bilayer structure was covered with a YSZ plate to suppress the evaporation of film components and was then subjected to thermal annealing at 1400°C for 30 min in an atmospheric electric furnace, resulting in the growth of its single-crystalline phase (18).

Cross-sectional high-resolution transmission electron microscopy (HRTEM) images of the InGaO3(ZnO)5 film (Fig. 1B) showed the distinct layered lattice structure composed of the periodic stacking of the InO2 layers and GaO(ZnO)5+ blocks. The film-substrate interface is atomically flat without a reaction layer despite the high-temperature annealing, as confirmed by TEM energy-dispersive x-ray spectrum. The lattice mismatch is relaxed in a few atomic layers at the film-substrate interface, and field-emission scanning microscopic observation revealed no defect structure such as grain boundary and dislocation over the entire area.

Figure 2 shows an atomic force microscope image of a single-crystalline InGaO3(ZnO)5 thin film as fabricated, showing an atomically flat terraces-and-steps structure. The step height (1.9 nm) corresponds to the separation between adjacent InO2 layers in the InGaO3(ZnO)5 crystal. The topmost layer is made of InO2, as observed by HRTEM. The film conductivity is less than ∼10-5 S cm-1. The carrier concentration is estimated to be ∼1013 cm-3, as derived from an electron mobility value of ∼80 cm2 V-1 s-1 (which is obtained as a field-effect mobility).

Fig. 2.

AFM image of the InGaO3(ZnO)5 film. Atomically flat terraces and steps are observed. The step height is 1.9 nm, corresponding to the space between the adjacent InO2 layers. The upper figure shows a cross section measured along the line A-B.

We fabricated top-gate TFETs with the use of a single-crystalline film grown on a 10 mm by 10 mm YSZ chip (Fig. 3A). The source, drain, gate contacts, and gate insulator were defined by standard photolithography and lift-off techniques. An 80-nm-thick amorphous HfO2 (a-HfO2) layer was used for the gate insulator, and ITO (10% Sn) was used for source, drain, and gate electrodes. The ITO and a-HfO2 layers were deposited by PLD at room temperature. The dielectric constant of a-HfO2 films was measured to be ∼18, which is a reasonable value compared with those reported previously (19). The channel length and gate width were 50 μm and 200 μm, respectively, corresponding to a width-to-length ratio of 4:1 (Fig. 3B). The chip is optically transparent in the whole visible-light region (Fig. 3B, inset). The optical transmittance is >80% in the wave-length range between 390 nm and 3200 nm [including the effects of the YSZ(111) substrate], which indicates that transmission losses due to the film and the TFETs are negligible. The reproducibility of the device characteristics was confirmed by measuring more than 100 fabricated TFETs.

Fig. 3.

(A) Illustration of the TFET device structure. The InGaO3(ZnO)5 channel layer and the a-HfO2 gate insulator layer are 120 nm and 80 nm in thickness, respectively. Channel length and gate width are 50 μm and 200 μm, respectively. (B) Optical transmission spectrum of a TFET chip. Those of YSZ substrates with and without an InGaO3(ZnO)5 film are given for comparison, showing that the TFET is fully transparent to visible light. Inset: Photograph of a TFET chip placed on a background text (left) and a magnified photograph of a TFET device (right). The light illumination condition was tuned to make the TFET device structure visible.

Typical TFET characteristics (Fig. 4) show that source-to-drain (IDS) current increases markedly as source-to-drain voltage (VDS) increases at a positive gate bias VGS (Fig. 4A), hence the channel is n-type and electron carriers are generated by positive VGS. A large IDS (>1 mA) is obtained at VGS = 10 V and VDS = 15 V. IDS exhibits a clear pinch-off and current saturation, which indicates that the operation of this TFET conforms to the standard field-effect transistor theory and that the Fermi level in the channel is fully controlled by the gate and drain bias. A field-effect mobility μeff ∼ 80 cm2 V-1 s-1 is obtained both from the transconductance value and from the saturation current. The large μeff value obtained is thought to result from high-quality single-crystalline InGaO3(ZnO)5 thin film and the improved channel-insulator interface. Note that TFETs using amorphous aluminum oxide for the gate insulator gave at most a μeff value of only 2 cm2 V-1 s-1, indicating that the choice of a gate insulator material is also an important factor for achieving good performance (20).

Fig. 4.

Typical TFET characteristics fabricated in a single-crystalline InGaO3(ZnO)5 film. (A) Output characteristics; (B) transfer characteristics. The TFET operates in the enhanced mode with a threshold voltage of ∼3 V. A field-effect mobility of ∼80 cm2 V-1 s-1 and an on-to-off current ratio of ∼106 are obtained. The gate leak current is orders of magnitude less than the source-to-drain current, which guarantees that the FET characteristics are not affected by the gate leak.

The off-current is very low, on the order of 10-9 A, and an on-to-off current ratio of ∼106 is obtained (Fig. 4B). The threshold gate voltage is ∼3 V, showing that the TFET operates in the enhancement mode. These characteristics are much improved over those reported for TOS TFETs fabricated using SnO2 (9).

We examined the photoresponse against the light illumination from a commercially available 30-W fluorescent tube. A photoresponse of the off-current weaker than the dark level (<10-9 A) was observed under typical room illumination conditions (∼1.6 W m-2). At six times this intensity of illumination (10 W m-2), the off-current increased only to 3 × 10-9 A.

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