Effects of an Aluminum Contact on the Carrier Mobility and Threshold Voltage of Zinc Tin Oxide Transparent Thin Film Transistors

Abstract

We fabricated amorphous zinc tin oxide (ZTO) transparent thin-film transistors (TTFTs). The effects of Al electrode on the mobility and threshold voltage of the ZTO TTFTs were investigated. It was found that the aluminum (Al)-ZTO contact decreased the mobility and increased the threshold voltage. Traps, originating from $AlO_x$, were assumed to be the cause of degradation. An indium tin oxide film was inserted between Al and ZTO as a buffer layer, forming an ohmic contact, which was revealed to improve the performance of ZTO TTFTs.

1. Introduction

Considerable improvement in the performance of transparent thin-film transistors (TTFTs) has been achieved using amorphous oxide semiconductors (AOSs) as active layers in TTFTs [1-3]. Amorphous films have a smoother surface than polycrystalline films. Furthermore, posttransition metal cations contained in AOSs generate extraneous conduction paths in the crystal structure via the overlap of spherically spread metal ns orbitals (n is the principal quantum number), which can yield high electron mobility in AOS TTFTs [4].

Thus far, the most widely studied topics related to AOS TTFTs have been mobility improvement [5, 6] and bias stability [7, 8]. It was assumed that the gate dielectric-AOS interface influences both the carrier mobility and the bias stability. However, the contact properties between the AOS and the source/drain electrodes are also important factors in the advancement of AOS TTFTs.

A variety of metals such as Al [9], In [10], and Au/Ti [11] have been used as source/drain electrodes for AOS TTFTs. Among them, Al is the most desirable electrode because of its extremely low resistivity, good adhesion, and weldability. On the other hand, it is also known that Al is easily oxidized when exposed to air. Because of the high oxygen affinity of Al, an AlOx layer will be formed at the Al-AOS interface as AOSs contain a considerable amount of oxygen, which might reduce the quality of AOS TTFTs.

In this study, we fabricated TTFTs using amorphous zinc tin oxide as an active layer. The zinc tin oxide (ZTO) film was thought to be an effective active layer for TTFTs because of its wide energy band gap and high resistivity [12-14]. We have reported that ZTO TTFTs employing In as source/drain electrodes exhibit a mobility greater than 18.0 cm2/Vs [15]. Indium is known to be ohmic when combined with zinc-base oxide [10]. Although indium is known to be ohmic with zinc-base oxide, it has limited practical applications, because of its ease of scratching and low melting point. Alternatively, we adopted Al as the source / drain electrode for ZTO TTFTs. Mobility deterioration in ZTO TTFTs with Al electrodes was observed. Thus, we deposited a transparent indium tin oxide (ITO) conductor between Al electrodes and ZTO as a buffer layer. We could improve the mobility of ZTO TTFTs successfully by the ITO buffer layer.

 

2. Experimental Procedure

Heavily doped n-type Si wafers, acting as gate electrodes, were thermally oxidized. An Al2O3 film was sequentially grown by atomic layer deposition to form a gate dielectric layer. The thicknesses of SiO2 and Al2O3 were 120 nm and 30 nm, respectively. A 40-nm-thick ZTO film was deposited on the wafer using RF magnetron sputtering. A 3-in zinc tin oxide (Zn : Sn = 3:2) target, purchased from Aldrich, was sputtered in oxygen-mixed argon [O2/(Ar + O2) = 40%] at 0.66 Pa and 70 W. The substrate temperature was maintained at 300 °C during ZTO deposition. Next, the ZTO-coated wafer was rapidly annealed at 400 °C for 5 min. A section of the dielectric film was etched to expose the surface of the heavily doped Si wafer. Finally, 600-nm-thick In or 300-nm-thick Al was deposited on the ZTO films as the source, drain, and gate electrodes of the ZTO TTFTs. Alternatively, 20-nm-thick ITO was deposited on the ZTO films before Al deposition to act as a buffer layer. RF magnetron sputtering was carried out at 0.66 Pa and 100 W to deposit ITO. The channel length and width defined by the stainless steel masks were 400 and 500 μm, respectively.

The crystallographic properties of ZTO films were studied by X-ray diffraction with CuKα (λ = 1.5406 Å) radiation, where 2θ = 20°–60°. The drain current (ID) as a function of both the drain-source voltage (VDS) and the gate-source voltage (VGS) was measured in the dark using a semiconductor parameter analyzer (Keithley 4200). We derived the threshold voltage (VTH) using a linear extrapolation of the ID 1/2-VGS curve [16] and derived the mobility (μSAT) using the slope of the ID 1/2-VGS curve [17].

 

3. Results and Discussion

We examined the crystalline structure of ZTO used as an active layer. Fig. 1 shows the XRD results for 250-nmthick ZTO films, both as-deposited and annealed at 400 °C for 30 min. Both the ZTO films showed a broad peak that appears to be a combination of a broad ZnO (002) peak and a SnO2 (110) peak, confirming that the ZTO films were in an amorphous state.

Fig. 1.XRD spectra of ZTO films: (a) as-deposited and (b) annealed at 400 °C.

It is well known that a high contact resistance between the source/drain and the channel layer reduces the voltage drop across the channel, which aggravates the device performance of TFTs [18-20]. First, we metalized ZTO TTFTs with In to create source/drain electrodes [ZTO TTFT(In)]. Fig. 2 shows the output characteristics of the ZTO TTFT(In). The typical output characteristics of TFTs with a good saturation were obtained. Second, ZTO TTFTs with Al source/drain electrodes [ZTO TTFT(Al)] were fabricated so that their characteristics could be compared with those of ZTO TTFT(In). The transfer characteristics [log (ID)-VGS and ID 1/2-VGS] of ZTO TTFT(Al) and ZTO TTFT(In) at a drain-source voltage (VDS) = 15 V are shown in Fig. 3. Mobility μSAT, threshold voltage VTH, and on/off current ratio (ION/IOFF) of ZTO TTFT(In) were 17.4 cm2/Vs, 2.5 V, and >107, respectively. A decrease in μSAT and an increase in VTH were found for ZTO TTFT(Al), relative to ZTO TTFT(In). The μSAT, VTH, and ION/IOFF of ZTO TTFT(Al) were 12.7 cm2/Vs, 5.0 V, and >107, respectively. Many research groups [18-20] have reported that the current retarding factors existed at the metal electrode- AOS active layer interface adversely affects the properties of TTFTs. A potential barrier, which restricts electron injection from the metal electrode into the active layer, can be formed at the metal-semiconductor interface. The potential barrier height (фB) is determined by the difference between the work function potential of the metal (фm) and χ of the semiconductor, where qχ is the electron affinity. When фm > χ in an n-type semiconductor, a rectifying contact is created. The electron affinity of ZnO [21] and SnO2 [22] has been reported to be dependent on the material conditions: electron density, crystal structure, surface state, etc. Because electron affinity measurement is beyond the scope of our research, we cannot estimate the фB variation with metal contact. From the mobility degradation in ZTO TTFT(Al), however, it is clear that current-retarding factors are created at the Al-ZTO contact. If the Al-ZTO contact is a rectifying contact, either the source-channel junction or the channel-drain junction will be reverse biased. Electrons are transported across a metalsemiconductor junction under reverse bias in the following ways [23-25]: thermal generation (IGE) in a space charge layer, field emission (IFE) or thermionic field emission (ITFE) through the potential barrier, and thermionic emission (ITE) over the potential barrier.

Fig. 2.Output characteristics [ID versus VDS] of ZTO TTFT(In) using In as the source/drain electrode with W/L = 500 μm/400 μm.

Fig. 3.Transfer characteristics [log ID versus VGS and ID 1/2 versus VGS] of ZTO TTFT(In) and ZTO TTFT(Al).

The thermionic emission current ITE of the Schottky barrier is negligible in the reverse bias mode. Accordingly, IGE, IFE, or ITFE is the dominant current at the reverse-biased metal-semiconductor contact. The trap-assisted thermal generation current IGE is proportional to (1 + VR/Vbi)1/2 [23], where VR and Vbi are the voltage drop across the Schottky barrier and the built-in potential of the Schottky contact, respectively. The field emission current and thermionic field emission current are known to be exponentially proportional to VR such that IFE ∝ VR 2exp(-K1/qVR) [24] and ITFE ∝ exp(qVR/K2) [25], where K1 and K2 are the characteristic energies of the field emission current and thermionic field emission current, respectively. The drainsource voltage VDS is divided between the two Schottky contacts and the channel. The source-channel contact is reverse biased, and the drain-channel contact is forward biased. Thus, VR in the current relations can be replaced by VDS if VDS < (VGS – VTH). Fig. 4 shows ID versus VDS 1/2 curves for the ZTO TTFT(Al). The linearity of the curve indicates that the thermal generation current IGE is dominant in the Al-ZTO contact. No evidences of the field emissions were observed through the curve fittings of the output characteristics of the ZTO TTFT(Al). To further study the Al-ZTO interface, we intentionally annealed the ZTO TTFT(Al) at 400 °C for 5 min in air. We assumed that annealing creates more traps at the Al-ZTO interface by generating more incomplete Al-oxygen bonds. The transfer characteristic of the annealed ZTO TTFT(Al) is shown in Fig. 5. The threshold voltage VTH of the annealed ZTO TFT(Al) shifted toward a negative direction . It is known that the neutralized positive charges by trapping electrons in the gate dielectric shift VTH toward a positive direction [26].

Fig. 4.ID versus VDS 1/2 of ZTO TTFT(Al). The linearity means that the thermal generation current is dominant when VDS > (VGS – VTH).

Fig. 5.Transfer characteristic of the ZTO TTFT(Al) annealed at 400 °C for 5 min. Transfer characteristic of the as-prepared ZTO TTFT(Al) are presented for comparison.

We think that some of the neutralized positive charges detrap electrons during the annealing process, and the revived positive charges may be the cause of the negative VTH shift of the annealed ZTO TFT(Al). It was found that the VTH of the annealed ZTO TFT(Al) increased by repeating measurements. The mobility μSAT of the annealed ZTO TFT(Al) decreased to 2.5 cm2/Vs. The output characteristics of the ZTO TTFT(Al) before and after annealing are compared in Fig. 6. A significant difference in the trend of the curve is observed, which means that changes in the conduction mechanism at the Al-ZTO interface occurred by the annealing process. The output characteristic curves of the annealed ZTO TTFT(Al) show the general form of parabola, i.e. ID∝VDS2. This squarelaw dependence was found at the space-charge-limited current (ISCL) flow from source to drain after punchthrough [27]. Richman [27] induced ISCL resulting from the injection of electron from the source as

where, εs is the permittivity of ZTO, μn is the electron mobility, A is the area of the depletion region, V is the voltage applied at the depletion region, and L is the length of the depletion region. The transformed curves [ID 1/2 versus VDS] of the annealed ZTO TTFT(Al) are shown in Fig. 7.

The linearity found in Fig. 7 indicates that ISCL is a dominant constituent of ID when VDS < (VGS – VTH). It was reported that the square-law dependence of VDS on ID is evident where the injected carrier density is greater than the background concentration of thermally generated carriers in the depletion region [27]. The increased traps at the Al/ZTO interface by the annealing process capture electrons in the channel, which expands the depletion region into the channel. It is considered that the electrons captured by the traps are easily injected into the depletion region, which is the source for ISCL. The space-chargelimited current through the depletion region at the reversebiased source-channel interface dominates ID. The length of the depletion region L was estimated from Eq. (1) by substituting the measured ID and VDS [Fig. 7] for ISCL and V. The field-effect mobility μn of the annealed ZTO TTFT(Al) (0.3 cm2/Vsec) was extracted from Fig. 5. Fig. 8 shows the calculated L as a function of VGS. The L is found to be decreased with VGS, which is due to the increased channel conductivity [28]. The obtained L is shown to be 0.8 ~2.2 μm which is too thick to be tunneled. The depletion length at a p+-n junction is inversely proportional to the donor density (ND) [29]. The L of 0.8 ~ 2.2 μm corresponds to ND ≈ 1.9 x 1015 ~ 0.7 x 1015 cm-3 when being estimated from the p+-n junction. On the basis of the above observation, we concluded that the Al-ZTO contact is rectifying, and the AlOx layer exists at the Al-ZTO interface. The incomplete bonds in the AlOx layer act as traps, i.e., scattering centers, which are the cause of the mobility degradation in the ZTO TTFT(Al). The traps are negatively charged by electron occupation, which increases the potential barrier height at the Al-ZTO contact, resulting in a high threshold voltage comparing with the In-ZTO contact.

Fig. 6.Output characteristic of the ZTO TTFT(Al) annealed at 400 °C for 5 min. The inset shows the output characteristic of the as-prepared ZTO TTFT(Al).

Fig. 7.The transformed curve [ID 1/2 versus VDS] of the ZTO TTFT(Al) annealed at 400 °C for 5 min.

Fig. 8.The calculated length of the depletion region as a function of VGS.

The potential barrier width at the metal-semiconductor contact is proportional to the carrier density of the semiconductor. Commonly, the active region under the source/drain electrodes is heavily doped prior to metal deposition, so as to eliminate the contact resistance. The degenerated semiconductor narrows the width of the potential barrier, and electrons penetrate the thin potential barrier. This tunneling effect neutralizes the potential barrier that obstructs the flow of electrons between the channel and the source/drain electrodes. High-resistivity AOS films are used to create the active layer of the TTFT to suppress the leakage current through the channel. It is difficult to dope the AOS films for source/drain contact. Accordingly, we coated a highly conductive ITO film as a buffer layer between the Al electrodes and the ZTO film so as to reduce the source/drain contact resistance to ID. The schematic diagram and transfer characteristic of the TTFT adopting ITO as a buffer layer [ZTO TTFT(ITO)] are shown in Fig. 9(a) and (b), respectively. The measured μSAT, VTH, and ION/IOFF of the ZTO TTFT(ITO) were 20.3 cm2/Vs, 4.0 V, and >107, respectively. It was found that the ITO buffer layer effectively eliminated the contact resistance at the Al-ZTO contact, which confirms that the highly conductive ITO film plays the role of the degenerated layer between electrodes and semiconductor films narrowing the width of the potential [30]. The annealing at 400 °C for 5 min degraded μSAT of the ZTO TTFT(ITO) to 15.2 cm2/Vs which was recovered to its original value a day later.

Fig. 9.(a) Schematic diagram and (b) Transfer characteristics [log ID versus VGS and ID1/2 versus VGS] of ZTO TTFT(ITO). Indium tin oxide was inserted between Al and ZTO as a buffer layer to improve the contact properties.

 

4. Conclusion

We fabricated ZTO TTFTs using Al and In as source/drain electrodes. A decrease in mobility and an increase in threshold voltage were observed when Al was employed as an electrode, compared to the case when In was used as an electrode. The traps created by the incomplete bonds at the Al-ZTO contact can reduce the carrier mobility of the ZTO TTFT(Al), and the negatively charged traps (caused by electron occupation) can increase the potential barrier height at the Al-ZTO contact, resulting in a high threshold voltage. We inserted a highly conductive ITO film as a buffer layer between the Al electrodes and the ZTO film. This buffer layer ITO enhanced the mobility to 20.3 cm2/Vs and lowered the threshold voltage to 4.0 V.