Control of Threshold Voltage in ZnO/Al₂O₃ Thin-Film Transistors through Al₂O₃ Growth Temperature

Abstract

Ultra-thin ZnO thin-film transistors with a channel thickness of <10 nm have disadvantages of a high threshold voltage and a low carrier mobility due to a low carrier concentration. Although these issues can be addressed by utilizing the strong reducing power of tri-methyl-aluminum, a method is required to control parameters such as the threshold voltage. Therefore, we fabricated a ZnO/Al₂O₃ thin-film transistor with a thickness of 6 nm and adjusted the threshold voltage and carrier mobility through the modulation of carrier generation by varying the growth temperature of Al₂O₃. As the growth temperature of Al₂O₃ increased, oxygen vacancies generated at the hetero–oxide interface increased, supplying a free carrier into the channel and causing the threshold voltage to shift in the negative direction. The optimized device, a ZnO/Al₂O₃ thin-film transistor with a growth temperature of 140 °C, exhibited a μ_sat of 12.26 cm²/V∙s, V_th of 8.16 V, SS of 0.65 V/decade, and I_ON/OFF of 3.98 × 10⁶. X-ray photoelectron spectroscopy was performed to analyze the properties of ZnO/Al₂O₃ thin films.

1. Introduction

Over the last few years, amorphous oxide thin-film transistors (TFTs) are extensively studied due to their numerous advantages such as transparency in visible light, uniformity, high electron mobility and low processing temperature suitable for flexible substrates [1,2,3]. Because of these advantages, amorphous oxide TFTs are expected to be utilized in the emerging next generation displays, such as Augmented Reality (AR) and Virtual Reality (VR) displays. These next generation displays demand resolutions in the order of several thousand pixels per inch (ppi), far surpassing the 100–500 ppi resolution found in traditional devices like smartphones and TVs. Therefore, to meet these requirements, scaling of TFTs is essential. However, in oxide semiconductors, the total number of free electrons is proportional to the thickness of the channel. Therefore, during scaling, as the channel becomes thinner, the number of free electrons within the channel decreases, resulting in TFTs having high threshold voltages and low mobility. Due to these issues, the amorphous oxide semiconductor material ZnO, commonly employed as the channel layer material for TFTs, faces challenges in being utilized at thicknesses below 10 nm [4,5,6,7].

To supply free electrons in an ultra-thin ZnO channel, a high reductive agent such as a tri-methyl-aluminum (TMA) precursor can be utilized. The TMA reduces the bottom layer and forms oxygen vacancies which are a major source of free electrons in oxide TFT on the bottom layer [8,9,10,11,12,13]. The oxygen vacancies, generated by TMA, are concentrated at the interface between ZnO and Al₂O₃ and act as electron donors, facilitating the provision of free electrons even in thin ZnO channel layers with a thickness of less than 10 nm.

The threshold voltage (V_th) adjustment is crucial as it influences the power consumption of TFT devices. Adjusting V_th can be realized by either altering the channel thickness or controlling the oxygen vacancy concentration within the channel [14,15,16,17]. In the case of the ZnO/Al₂O₃ channel, which are supplied with free electrons through the reducing power of TMA, V_th control was achieved by adjusting the thickness of either the upper layer (Al₂O₃) or the bottom layer (ZnO). However, adjusting the thickness of the upper layer or the bottom layer may conflict with scaling the device. Therefore, a new approach to adjust V_th is required.

Therefore, in this study, we aimed to adjust the V_th in ultra-thin ZnO/Al₂O₃ TFTs through the growth temperature of Al₂O₃. Specifically, we adjusted the oxygen vacancies through the growth temperature of Al₂O₃ as a method for controlling the threshold voltage applicable to ultra-thin ZnO/Al₂O₃ TFTs. Through X-ray photoelectron spectroscopy (XPS) analysis, it was confirmed that an increase in the growth temperature of Al₂O₃ from 110 °C to 150 °C resulted in a higher concentration of oxygen vacancies generated at the channel due to the reduction power of TMA. Consequently, the increase in the growth temperature of Al₂O₃ led to a generation of more free carriers in ZnO/ Al₂O₃ TFT, resulting in a negative shift in V_th.

2. Materials and Methods

Inverted-staggered type ZnO and ZnO/Al₂O₃ TFTs were fabricated on Si/SiO₂ wafers used as the substrate. Heavily doped p-type Si is acting as the gate electrode, and a 100nm thick thermally grown SiO₂ is acting as a gate dielectric. The ZnO and Al₂O₃ thin films were deposited using diethylzinc (DEZ), TMA, and H₂O as a Zn precursor, an Al precursor, and an oxidant, respectively. In ZnO deposition, one cycle of ALD consisted of exposure to DEZ (0.3 s), a 20 s purge, exposure to H₂O (0.3 s), and a 20 s purge. In Al₂O₃ deposition, one cycle of ALD consisted of exposure to TMA (0.3s), a 20 s purge, exposure to H₂O (0.3 s), and a 20 s purge. N₂ gas flowing at a rate of 60sccm was used to deliver the precursors into the reaction chamber. The working pressure in the chamber was 200 mTorr. The deposition rates of ZnO and Al₂O₃ were 2 Å/cycle and 1 Å/cycle, respectively.

First, in order to confirm the effect of the Al₂O₃ layer thickness, the ZnO films with Al₂O₃ layer thicknesses of 0 nm and the ZnO/Al₂O₃ films with Al₂O₃ layer thicknesses of 1 nm and 2 nm were deposited as the channel layer of TFTs. The total thickness of the ZnO and ZnO/Al₂O₃ channel layers was fixed at 6 nm, and the growth temperature of the film was 110 °C. To investigate the effect of Al₂O₃ growth temperature, ZnO/Al₂O₃ TFTs with a 1 nm thick Al₂O₃ layer were fabricated. The total thickness of the ZnO/Al₂O₃ channel layers was fixed at 6nm, and the growth temperature of ZnO was fixed at 110 °C, and the growth temperature of Al₂O₃ was increased from 110 °C to 150 °C in increments of 10 °C. In addition, to investigate the influence according to the growth temperature of ZnO, growth temperature of Al₂O₃ was fixed at 140 °C, and the growth temperature of ZnO was increased to 110 °C, 130 °C, 140 °C, and 150 °C. An additional annealing process was not performed to prevent Al diffusion. Next, 100 nm thick Al was deposited using a shadow mask by a thermal evaporator to serve as the source/drain (S/D) electrodes. The fabricated TFTs had a channel width (W) and length (L) of 1000 μm and 100 μm, respectively. Figure 1 shows the cross-sectional schematic diagram of the fabricated inverted-staggered type (a) ZnO and (b) ZnO/Al₂O₃ TFTs.

The oxygen binding states in the ZnO/Al₂O₃ layers were analyzed by XPS in order to confirm the effect according to the growth temperature of Al₂O₃. The electrical characteristics of the fabricated TFTs were measured using an EL423 semiconductor parameter analyzer from Elecs Co.

3. Results and Discussion

Figure 2 presents the transfer characteristic curves of ZnO/Al₂O₃ TFTs according to the thickness of Al₂O₃. In the case of a ZnO/Al₂O₃ TFT with an Al₂O₃ thickness of 0nm (ZnO TFT), there is an insufficiency of free electrons in the channel. Consequently, ZnO TFT with a channel thickness of 6 nm exhibits a significantly high V_th exceeding 40 V and a low saturation mobility of 0.0088 cm²/V∙s. However, in the cases of ZnO (5 nm)/Al₂O₃ (1 nm) TFT and ZnO (4 nm)/Al₂O₃ (2 nm) TFT, the V_th shifted towards the negative compared to the ZnO TFT, and there was an improvement in saturation mobility and an increase in the on current. This indicates that when the Al₂O₃ is deposited onto the ZnO layer, oxygen vacancies are generated in the ZnO layer due to the reducing power of TMA, supplying free electrons into the channel. Furthermore, as the Al₂O₃ thickness increased from 1nm to 2 nm, the V_th shifted in the negative direction from 22.24 V to 9.60 V, and the saturation mobility increased from 1.54 cm²/V∙s to 2.82 cm²/V∙s. The negative shift in Vth and improvement in saturation mobility with increasing thickness of Al₂O₃ is attributed to the increased generation of free electrons in the ZnO layer. It aligns with the results of other studies that suggest an increase due to carrier confinement through the Al₂O₃ barrier [9,18,19].

The electrical parameters, such as saturation mobility (μ_sat), V_th, SS, and on–off current ratio (I_ON/OFF) of ZnO/Al₂O₃ TFTs according to the thickness of Al₂O₃, are presented in

Table 1. Electrical parameters of ZnO/Al₂O₃ TFTs according to the thickness of Al₂O₃.

	μsat [cm2/V∙s]	Vth [V]	SS [V/dec]	ION/OFF
ZnO 6 nm	0.0088	-	-	-
ZnO (5 nm)/Al2O3 (1 nm)	1.54	22.24	0.55	7.24 × 105
ZnO (4 nm)/Al2O3 (2 nm)	2.82	9.60	0.97	1.95 × 106

.

Figure 3a shows the transfer characteristic curves of ZnO/Al₂O₃ TFTs with Al₂O₃ growth temperatures of 110 °C, 120 °C, 130 °C, 140 °C, and 150 °C. The V_th of ZnO/Al₂O₃ TFT decreases from 22.24 V to 2.68 V as the growth temperature of Al₂O₃ increases from 110 °C to 150 °C, accompanied by an increase in the on current. Typically, as the carrier concentration in the channel increases, the V_th shifts in the negative direction, and the on current increases. Therefore, it can be inferred that as the growth temperature of Al₂O₃ increases from 110 °C to 150 °C, the carrier density in the channel increases. Since the growth temperature of ZnO was fixed at 110 °C, the increase in carrier concentration with the increased growth temperature of Al₂O₃ is attributed to the oxygen vacancies generated by the reducing power of TMA. Furthermore, as the growth temperature of Al₂O₃ increases, the free electrons in the channel increase due to the reducing power of TMA, resulting in an increase in the saturation mobility from 1.54 cm²/V∙s to 7.37 cm²/V∙s.

As the growth temperature of Al₂O₃ increases, the V_th shifts decrease. When the growth temperature of Al₂O₃ increases from 110 °C to 120 °C, the threshold voltage shifts from 22.24 V to 15.36 V. However, when the growth temperature of Al₂O₃ increases from 130 °C to 140 °C, the threshold voltage shifts from 8.48 V to 6.72 V. This indicates that as the growth temperature of Al₂O₃ increases, the generation of oxygen vacancies and free electrons in the channel gradually decreases due to the reducing power of TMA. Moreover, as the growth temperature of Al₂O₃ increases, both the on current and off current increase. Therefore, except for 110 °C and 120 °C where there is a significant difference in the increase in the on current compared to the increase in the off current, the on–off ratio maintains a constant value.

Another notable feature is the degradation of SS when the growth temperature of Al₂O₃ is 150 °C. The oxygen vacancies generated by TMA serve both as a source of free electrons and as a trap. Therefore, the increase in the oxygen vacancies generated at the interface region between the ZnO and the Al₂O₃ leads to an increase in the total trap density, resulting in the degradation of SS [20]. A summary of the electrical parameters from the transfer curves of the TFTs is shown in Figure 3b.

The transfer characteristic curves of ZnO/Al₂O₃ TFTs according to the growth temperature of ZnO are shown in Figure 4. Generally, in ZnO TFTs with a channel thickness of over 20 nm, as the growth temperature increases, the carrier concentration in the channel increases and the V_th decreases. However, in 6 nm ZnO/Al₂O₃ TFTs, despite the increase in growth temperature of ZnO from 110 °C to 150 °C, the V_th is maintained near 7 V. Furthermore, regardless of the growth temperature of ZnO, the on current maintains a constant value, resulting in the I_ON/OFF being maintained at around 4.00 × 10⁶. This indicates that the origin of the dominant free electrons in the channel is not ZnO itself, but rather the oxygen vacancies in the channel generated by the reducing power of TMA. Moreover, it signifies that the generation of free electrons resulting from the oxygen vacancy created by the reducing power of TMA is independent of the growth temperature of the bottom ZnO layer.

On the contrary, as the growth temperature of ZnO increases from 110 °C to 150 °C, the saturation mobility improves. At a low temperature, the residual OH- could be present in the ZnO films because the ALD reactions could be incompletely processed. These OH- act as a trap and could decrease the saturation mobility [21]. The electrical parameters of ZnO/Al₂O₃ TFTs according to the growth temperature of ZnO are presented in

Table 2. Electrical parameters of ZnO/Al₂O₃ TFTs according to the growth temperature of ZnO.

	μsat (cm2/V∙s)	Vth (V)	SS (V/dec)	ION/OFF
110 °C	7.37	6.72	0.60	6.61 × 106
130 °C	10.93	6.88	0.66	4.07 × 106
140 °C	12.26	8.16	0.65	3.98 × 106
150 °C	13.97	7.20	0.96	1.38 × 106

.

To investigate the effect of the growth temperature of Al₂O₃ on the performance of ZnO/Al₂O₃ TFTs, we performed XPS analysis. Figure 5 shows the deconvolution of the O 1s peak in ZnO/Al₂O₃ films with different growth temperatures of Al₂O₃: (a) 110 °C, (b) 130 °C, and (c) 150 °C. The O 1s peak spectra were deconvoluted into three different energy level peaks: the low binding energy peak (O_L, 530.2 ± 0.1 eV), which refers to the metal oxide (M-O) lattice species; the middle binding energy peak (O_M, 531.4 ± 0.1 eV), which corresponds to the oxygen-deficient regions (V_o); and the high energy peak (O_H, 532.4 ± 0.1 eV), which signifies the metal hydroxyl group (M-OH) lattice species. V_o of ZnO and V_o of Al₂O₃ have not been distinguished. Spectra were calibrated to the C 1s core level peak (284.6 eV) of the carbon. The ratio of the O_L to the total O 1s peak (O_L/O_Total) in relation to the growth temperature of the Al₂O₃ is 49.36% at 110 °C, 49.00% at 130 °C, and 44.66% at 150 °C. The ratio of the O_M to the total O 1s peak (O_M/O_Total) in relation to the growth temperature of the Al₂O₃ is 30.65% at 110 °C, 32.69% at 130 °C, and 36.86% at 150 °C. The ratio of the O_H to the total O 1s peak (O_H/O_Total) in relation to the growth temperature of the Al₂O₃ is 19.99% at 110 °C, 18.31% at 130 °C, and 18.48% at 150 °C. As the growth temperature of the Al₂O₃ increases, the O_M/O_Total rises. A high O_M/O_Total ratio indicates a higher concentration of oxygen vacancies, implying that, with the increase in the growth temperature of the Al₂O₃ layer, the concentration of oxygen vacancies within ZnO has increased, leading to a greater generation of free electrons. Therefore, the increased growth temperature of the Al₂O₃ resulted in a higher electron concentration in the ZnO/Al₂O₃ channel, causing the V_th to shift in the negative direction and an increase in the on current. Furthermore, as the growth temperature of Al₂O₃ increased from 110 °C to 130 °C, the O_H/O_Total decreased from 19.99% to 18.31%, while the O_M/O_Total increased from 30.65% to 32.69%. However, as the growth temperature of Al₂O₃ increased from 130 °C to 150 °C, the O_L/O_Total decreased from 49.00% to 44.66%, while the O_M/O_Total increased from 32.69% to 36.86%, and the O_H/O_Total remained constant.

4. Conclusions

We investigated the electrical performance of ZnO/Al₂O₃ TFTs with varying Al₂O₃ growth temperatures. As the Al₂O₃ growth temperature increased, we confirmed the increase in the O_M/O_total ratio due to the reducing power of TMA through XPS analysis. The increasing oxygen vacancies with the growth temperature of Al₂O₃ signify an increase in electron concentration within the channel, resulting in a negative shift in V_th and an enhancement in saturation mobility. Additionally, generation of free electrons, resulting from the oxygen vacancies created by the reducing power of TMA, is independent of the growth temperature of the bottom ZnO layer. As a result, even with an increase in the growth temperature of ZnO, V_th maintains a constant value. Therefore, controlling the growth temperature of Al₂O₃ can serve as a method to adjust the V_th of ZnO/Al₂O₃ TFTs. The optimal performance was achieved with an Al₂O₃ growth temperature of 140 °C and the ZnO/Al₂O₃ TFT showed μ_sat of 12.26 cm²/V∙s, V_th of 8.16 V, SS of 0.65 V/decade, and I_ON/OFF of 3.98 × 10⁶.