Aqueous-solution-driven HfGdOx gate dielectrics for low-voltage-operated α-InGaZnO transistors and inverter circuits

doi:10.1016/j.jmst.2020.03.007

Abstract

Abstract:

In this work, a non-toxic and environmentally friendly aqueous-solution-based method has been adopted to prepare gadolinium-doped hafnium oxide (HfO₂) gate dielectric thin films. By adjusting the gadolinium (Gd) doping concentration, the oxygen vacancy content, band offset, interface trap density, and dielectric constant of HfGdO_x (HGO) thin films have been optimized. Results have confirmed that HGO thin films with Gd doping ratio of 15 at.% have demonstrated appropriate dielectric constant of 27.1 and lower leakage current density of 5.8 × 10 ^-9 A cm ^-2. Amorphous indium-gallium-zinc oxide (α-IGZO) thin film transistors (TFTs) based on HGO thin film (Gd: 15 at.%) as gate dielectric layer have exhibited excellent electrical performance, such as larger saturated carrier mobility (μ_sat) of 20.1 cm ² V ^-1 S ^-1, high on/off current ratio (I_on/I_off) of ~10 ⁸, smaller sub-threshold swing (SS) of 0.07 V decade ^-1, and a negligible threshold voltage shift (△V_TH) of 0.08 V under positive bias stress (PBS) for 7200 s. To confirm its potential application in logic circuit, a resistor-loaded inverter based on HGO/α-IGZO TFTs has been constructed. A high voltage gain of 19.8 and stable full swing characteristics have been detected. As a result, it can be concluded that aqueous-solution-driven HGO dielectrics have potential application in high resolution flat panel displays and ultra-large-scale integrated logic circuits.

Key words: Aqueous-solution-driven, Low-voltage-operating, HfGdOx gate dielectrics, Rare earth element doping, α-IGZO TFTs

Yongchun Zhang, Gang He, Wenhao Wang, Bing Yang, Chong Zhang, Yufeng Xia. Aqueous-solution-driven HfGdO_x gate dielectrics for low-voltage-operated α-InGaZnO transistors and inverter circuits[J]. J. Mater. Sci. Technol., 2020, 50: 1-12.

Figures/Tables 16

Fig. 1. Thermogravimetric analysis of HGO powder with 15% Gd doping ratio.

Fig. 2. X-ray diffraction spectra of HGO films with various Gd doping ratios.

Table 1 Content of elements in HGO films with different Gd doping ratios, and comparison of theoretical and practical content of oxygen (Definition: The content of Hf is defined as 1 to calculate the content of Gd and O. The theoretical content of O is calculated from the actual content of Hf and Gd according to the molecular formula of HfO2, Gd2O3.).

Gd atom ratio (Gd/Gd+Hf) (at.%)	Hf	Gd	Actual content of oxygen	Theoretical content of oxygen (HfO₂, Gd₂O₃)
5.7	1	0.06	1.99	2.09
13.9	1	0.16	2.27	2.24
30.8	1	0.44	2.72	2.66

Fig. 3. (a-c) Deconvolution of O 1s XPS spectra for HGO thin films with various Gd doping ratios. (d) Area percentage of OI peak, OII peak and OIII peak in O1 s peaks of HGO films with various Gd doping ratios.

Fig. 4. Hf 4f and Gd 4d XPS spectra of HGO thin ?lms with 30%(a, d), 15%(b, e), 5%(c, f), Gd doping ratios.

Fig. 5. Optical transmission spectra of HGO films with various Gd doping ratios. The insert is the determination of band gap of HGO thin films.

Fig. 6. VBM spectra of HGO thin films with various Gd doping ratios. The inset shows the schematic band diagram of HGO thin films on Si substrates.

Fig. 7. (a) C-V characteristics of HGO films with various Gd doping ratios. (b) Leakage current curves of HGO films with various Gd doping ratios.

Table 2 Parameters extracted from C-V curves.

Gd doping ratio (at.%)	C(pF)	K	ΔVfb (V)	N_bt (cm^-2)
0	291	21.9	0.21	1.21 × 10¹²
5	234	17.7	0.072	3.35 × 10¹¹
15	359	27.1	0.042	3.00 × 10¹¹
30	300	22.6	0.084	4.9 × 10¹¹

Fig. 8. (a-d) Summarized output curves of HGO/α-IGZO TFTs fabricated with different Gd doping ratios. (e-h) Transfer characteristic curves of HGO/α-IGZO TFT devices fabricated with different Gd doping ratios.

Table 3 Performance parameters of TFT devices fabricated by HGO thin films with different Gd doping ratios.

Gd doping ratio (at.%)	μsat (cm² V^-1 S^-1)	I_ON/I_OFF	SS (V decade^-1)	V_TH (V)	Interfacial trap density, N_it (cm^-2 eV^-1)
0	11.0	1.4 × 10⁴	0.27	0.7	2.63 × 10¹³
5	15.3	7.0 × 10⁵	0.11	0.58	9.3 × 10¹²
15	20.1	1 × 10⁸	0.07	0.89	5.1 × 10¹²
30	14.0	4.6 × 10⁶	0.09	0.53	5.6 × 10¹²

Fig. 9. (a-c) PBS test curves of HGO/α-IGZO TFT devices based on HGO gate dielectrics different Gd doping ratios. (d) Threshold voltage as a function of time. The inset shows the time dependence of ΔVTH in the IGZO/HGO TFTs under the bias stress of 1.5 V. (e) Mechanism analysis of threshold voltage shift in PBS test. (f) Schematic diagram of conduction band offset at the interface between active layer and gate dielectric layer with different Gd doping ratio.

Fig. 10. (a) Static voltage transfer characteristics (VTCs) of the inverter with various supply voltages (VDD). (b) Voltage gain of inverters under various supply voltages. (c) The dynamic response behavior curve of resistor-loaded inverter. (d) Circuit schematic diagram of inverter.

Table 4 Reported voltage gain comparison of inverters.

Year	Channel	Gate Dielectrics	Gain
2014 [59]	graphene	ion gel	1.9
2014 [60]	IZO	sodium alginate	13.9
2016 [65]	ZnO/ SnO	HfO₂	<12
2017 [66]	Rubrene/MoS₂	SiO₂	2.3
2017 [67]	MoS₂/SWCNT	SiO₂	15
2017 [68]	H-diamond	TiO₂/Al₂O₃	12.7
2017 [69]	CuSCN	Al₂O₃	3.4
2018 [60]	ZnSiSnO/SWCNT	Al₂O₃	41.5
This work	IGZO	HGO	19.8

Table 5 Characteristic parameters of resistor-loaded inverter at different VDD.

V_DD (V)	V_OH (V)	V_IH (V)	V_OL (V)	V_IL (V)	Gain (V/V)	NM_H (V)	NM_L (V)	Voltage swing (%)
0.5	0.43	0.48	0.19	0.32	1.8	-0.05	0.13	48
1	0.93	0.65	0.18	0.33	3.9	0.28	0.15	75
1.5	1.43	0.72	0.21	0.33	6.2	0.71	0.12	81
2	1.93	0.8	0.2	0.34	8.4	1.13	0.14	87
2.5	2.43	0.84	0.22	0.34	10.5	1.59	0.12	88
3	2.93	0.88	0.23	0.35	12.5	2.05	0.12	90
3.5	3.43	0.91	0.24	0.35	14.3	2.52	0.11	91
4	3.94	0.95	0.24	0.35	16.1	2.99	0.11	93
4.5	4.43	0.97	0.26	0.36	17.9	3.46	0.10	93
5	4.93	1	0.27	0.36	19.8	3.93	0.09	93

Fig. 11. Output current curve of inverter under different frequency input signals.

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