Effect of Mg Doping on the Electrical Performance of a Sol-Gel-Processed SnO₂ Thin-Film Transistor

Abstract

Sol-gel-processed Mg-doped SnO₂ thin-film transistors (TFTs) were successfully fabricated. The effect of Mg concentration on the structural, chemical, and optical properties of thin films and the corresponding TFT devices was investigated. The results indicated that an optimal Mg concentration yielded an improved negative bias stability and increased optical band gap, resulting in transparent devices. Furthermore, the optimal device performance was obtained with 0.5 wt% Mg. The fabricated 0.5 wt% Mg-doped SnO₂ TFT was characterized by a field effect mobility, a subthreshold swing, and I_on/I_off ratio of 4.23 cm²/Vs, 1.37 V/decade, and ~1 × 10⁷, respectively. The added Mg suppressed oxygen-vacancy formation, thereby improving the bias stability. This work may pave the way for the development of alkaline-earth-metal-doped SnO₂-based thin-film devices.

1. Introduction

Oxide semiconductor-based thin-film transistors (TFTs) have attracted considerable attention due to their potential application in next-generation displays, including transparent displays, transparent sensors, and electrochromic windows [1,2,3,4]. To date, indium-based metal-oxide systems, such as InGaZnO, ZrInZnO, InGaO, and InZnO, have been used for these transistors, which exhibit high mobility. Indium-based metal-oxide TFTs can be fabricated through a low-temperature process, unlike conventional polysilicon or amorphous silicon-based thin-film transistors [5,6,7,8]. Unfortunately, In is a costly rare metal found in only a few mining sites worldwide [9]. Thus, finding a new metal oxide system consisting of non-rare metals is critical for high-performance transparent novel devices. Tin dioxide (SnO₂) is easily crystallized due to its low melting temperatures and has a high intrinsic mobility, and large band gap (3.6 eV), and is therefore considered a promising candidate for replacing indium-based oxide semiconductors. In addition, the electronic configuration of Sn (1s²2s²p6³s²p⁶d¹⁰5s²p²) is similar to that of In (1s²2s²p⁶3s²p⁶d¹⁰4s²p⁶d¹⁰5s²p¹) [10]. Owing to these attributes, TFTs fabricated with SnO₂ are characterized by excellent electrical properties [11,12,13]. Unfortunately, metal-oxide-based TFTs are prone to electrical instability due to various factors, such as ambient light, external bias voltage, and environmental gases. The instability originates from charge trap sites with oxygen vacancies or other defect sites [14,15,16]. To overcome this drawback, an oxygen vacancy suppressor can be added to the material. Oxygen vacancies can act as carriers as well as defects, thereby causing instability.

Metal-oxide-based devices are usually fabricated with vacuum-based equipment, which hinders the low-cost fabrication of large area devices [17,18,19]. As a result, simple techniques that can be performed without complex and expensive vacuum systems have been introduced. The sol-gel process is relatively simple and is therefore considered a promising deposition process. The composition ratio of each component comprising the solution can be easily controlled. In addition, the liquid phase precursors can be applied as inks for spin coating, inkjet printing, and gravure printing, without the use of expensive vacuum and pumping systems [12,13,20]. In the work reported here, we describe the first-ever Mg-doped SnO₂ TFTs fabricated by means of a sol-gel method. Added Mg was used as a carrier suppressor for reducing the density of oxygen defects. The electrical and optical properties of Mg-doped SnO₂ thin films were investigated. The 0.5 wt% Mg-doped SnO₂ TFTs exhibited satisfactory stability and a field effect mobility of 4.5 cm²/Vs with good negative bias stress (NBS) properties. Although alkaline earth-metal-doped SnO₂ thin films have been previously studied, published reports on the electrical characteristics of Mg-doped SnO₂ TFTs are rare. This work may pave the way for the development of alkaline earth-metal-doped SnO₂-based thin-film devices.

2. Materials and Methods

All reagents in this study including the tin (II) chloride dehydrate (SnCl₂∙2H₂O) and the magnesium nitrate hexahydrate (Mg(NO₃)₂∙6H₂O) were purchased from Sigma Aldrich. The SnCl₂∙2H₂O and Mg(NO₃)₂∙6H₂O were dissolved in ethanol to a concentration of 0.03 M. Two different concentrations (0.5 and 1 wt%) of Mg(NO₃)₂∙6H₂O solutions were dissolved in a prepared SnO₂ precursor. We fabricated the TFTs with an inverted coplanar structure. A heavily doped p-type silicon was used as the substrate and a gate electrode with a thermally grown 100-nm-thick silicon dioxide layer was used as the gate dielectric. The substrates were cleaned using a UV/O₃ cleaner for 10 min. The 50-nm-thick Au source/drain electrodes were deposited by an e-beam evaporator and patterned via the lift-off method. SnO₂ with/without Mg semiconductor layers was deposited by means of a spin-coating process performed at 3000 rpm for 50 s. The spin-coated films were pre-annealed at 150 °C for 10 m and post-annealed for 1 h at 500 °C using a hot plate. The coated films were patterned using mechanical elimination for full isolation that prevents fringing effects and a gate leakage current. The phase and structural properties of the films were investigated by means of X-ray reflectivity (XRR; PANalytical Empyrean) and grazing incidence X-ray diffraction (GIXRD; Philips X’pert Pro) measurements. A small incident angle (0.3°) and fixed CuKα radiation (λ = 154.06 pm) were employed. Furthermore, X-ray photoelectron spectroscopy (XPS; ULVAC-PHI) was used to evaluate changes in the chemical composition of the films. A monochromatic AlKα (1488 eV) source was used as the X-ray source. The base pressure of the chamber was maintained at 4 × 10⁻⁷ Pa, and the pass energy was 40 eV. The optical properties needed for calculation of the optical band gap were obtained through ultraviolet-visible spectroscopy (UV-Vis; LAMBDA 265) measurements. The electrical characteristics of the fabricated TFTs at room temperature in air were obtained using an Agilent 4155 semiconductor parameter analyzer and a probe station.

3. Results

Figure 1a shows the GIXRD spectra of solution-processed SnO₂ films associated with different Mg doping concentrations (0–1 wt%). All films are polycrystalline phase SnO₂ with a tetragonal structure, consistent with pure SnO₂ thin films (JCPDS card number: 41-1445). The peaks of other phases, such as MgO and Sn_xMg_yO, are absent from the patterns. The diffraction peaks at 26.61°, 33.89°, 37.95°, and 51.78° correspond to the (110), (101), (200), and (211) crystal planes, respectively, of the SnO₂ tetragonal structure. The full width at half-maximum (FWHM) of a GIXRD peak is closely correlated with the crystallite size of a specific crystal orientation. The GIXRD results indicate that the FWHM of the SnO₂ (110) plane is smaller than the FWHM of the other planes and, hence, SnO₂ crystallites grow mainly in the (110) plane. Figure 1a confirms that the intensity of each peak increases with increasing Mg doping concentration.

The ionic radii of Sn⁴⁺ (0.69 Å) and Mg²⁺ (0.72 Å) are almost identical and, hence, the lattice parameters are unaffected by the Mg doping concentration. The crystallite sizes of the SnO₂ films are estimated from Scherrer’s formula, D = 0.9λ/(βcos θ), where D is the crystallite size, λ is the Cu Κα wavelength (1.54 Å), β is the FWHM of the peak, and θ is the peak position. The SnO₂ crystallite size slightly increases after Mg doping [21]. Crystallite sizes of 7.68, 7.83, and 8.36 nm for 0.0, 0.5, and 1 wt% doping, respectively, are calculated at the (110) plane. The density of SnO₂ films doped at concentrations ranging from 0 to 1 wt% was determined by means of XRR analysis (see Figure 1b). The film density for total external reflection is determined by the critical angle, i.e., the density increases with increasing values of this angle. Critical angles of 0.261°, 0.286°, and 0.278° as well as film densities (determined via Buyden’s method) of 3.70, 4.43, and 4.18 g/cm³ are obtained for doping concentrations of 0.0, 0.5, and 1 wt%, respectively. That is, the largest density is obtained for the 0.5 wt% Mg-doped SnO₂ film (see

Table 1. Critical angle, Film density, and Crystalline size of solution-processed SnO₂ films doped with 0–1 wt% Mg.

	Critical Angle (Degree)	Density (g/cm3)	Crystallite size (nm)
0.0 wt%	0.261	3.70	7.68
0.5 wt%	0.286	4.43	7.83
1.0 wt%	0.278	4.18	8.36

for the measured structural properties of the SnO₂ thin films).

Figure 2 shows the O 1s peak of XPS spectra corresponding to 0–1 wt% Mg doping concentrations. The binding energy (BE) of each element was calibrated based on the C 1s peak at 284.6 eV. We analyzed the O 1s peaks, which consisted of three sub-peaks associated with lattice oxygen (O_L: O_lattice), oxygen vacancies (O_V: O_vacancy), and hydroxide impurities (-OH). The O_V and O_L peaks vary with increasing Mg doping concentration, with the corresponding intensities decreasing slightly from 30.45% to 24.38% and increasing from 59.53% to 62.98%, respectively. The -OH peak changes, but these changes appear to be somewhat random. Dopants acting as carrier suppressors are characterized by several properties. For example, the electronegativity of the dopants should differ significantly from the electronegativity of oxygen (3.44). A relatively low standard electrode potential (SEP) results in increasing the bond strength of dopant-oxygen bonds. The bond strength of metal-oxygen is also crucial for enhancing the stability of TFTs by controlling oxygen vacancies [22]. Other important properties of the dopant include the ionic radius, effective nuclear charge, and Lewis acid strength (L) [23]. Mg dopants have lower electronegativity and SEP (1.31, −2.37 V) than their Sn hosts (1.96, −0.14 V). Thus, the addition of Mg dopants as oxygen binders leads to the suppression of oxygen vacancies to secure good bias properties because they are more strongly combined with oxygen.

Figure 3 shows the optical band gap measured via UV-VIS. In addition, Figure 3b,c show the XPS-measured valence band (VB) spectra and the schematic energy band diagram of Mg-doped SnO₂ with 0–1 wt% Mg doping concentrations. The optical band gap is widened from 3.85 eV to 3.95 eV with increasing Mg doping concentration, as shown in Figure 3a. As the concentration increases, the optical band gap increases, consistent with the results of a previous study. Figure 3b shows that the VB offset between the VB maximum value and the Fermi level decreases from 2.70 eV to 2.54 eV with increasing Mg doping concentration. Based on these results, we confirmed that the conduction band (CB) offset between the CM minimum value and the Fermi level increases from 1.15 eV to 1.41 eV and the carrier concentration decreases depending on the doping concentration [24]. The band gap of defect-containing SnO₂ is smaller than that of defect-free SnO₂ due to the increase in VB and narrowing of the band gap induced by the homogeneous oxygen vacancies. As the Mg doping concentration increases, the density of oxygen vacancies can be reduced, leading to VB reduction and band gap widening [25].

Figure 4 shows the output curves measured in air after fabrication of the SnO₂ TFTs at different Mg doping concentrations. The drain current versus drain voltage (I_D-V_D) curves of the TFTs were measured at different gate voltages (V_G) ranging from –30 V to +30 V with an increment of 10 V. The curves shown in Figure 4a–c correspond to typical n-type semiconductors with normally on state electrical properties. The on-current is reduced by the changing carrier concentration at increasing Mg doping concentrations. At a low V_D, the I_D-V_D plots are non-linear, owing to Schottky barrier formation induced by the large work function difference between the n-type SnO₂ and the Au source/drain electrodes [26].

Figure 4d shows the corresponding transfer characteristics of the SnO₂ TFTs with respect to Mg doping concentration. The drain current at a drain voltage of +30 V is measured as a function of the gate voltage, ranging from −30 V to +30 V. The saturation mobility (μ) and the subthreshold swing (SS) are calculated from the following equations for a typical transistor:

$$I_D=\mu C_i\frac{W}{2L}{\left(V_g-V_{th}\right)}^2$$

(1)

$$SS={\left(\frac{\delta log{I}_D}{\delta V_G}\right)}^{-1}$$

(2)

where, C_i is the capacitance per unit area of the oxide gate insulator, W and L are the channel width and length, respectively, V_th is the threshold voltage, V_G is the gate voltage, and I_D is the drain current. The V_th was obtained from the intercept of the fitting line used, when we extract the mobility in the saturation regime from Equation (1). The extracted parameters for the SnO₂ TFTs with different Mg concentrations are plotted in Figure 5.

The extracted saturation mobilities of the 0, 0.5, and 1 wt% Mg-doped transistors were 13.84, 4.23, and 0.67 cm²/Vs, respectively. Degradation of the saturation mobility is revealed by the O 1s peak of the XPS spectra and energy band diagrams of the extracted UV-vis and XPS spectra, as shown in Figure 2 and Figure 3. The density of oxygen vacancies decreases with Mg dopants combined with oxygen. The suppression of oxygen vacancies leads to a decline in the carrier concentration. Moreover, we verified that the CB offset of the energy band diagram broadens from 1.15 eV to 1.41 eV with increasing Mg-doping concentration. Consequently, the carrier concentration decreases considerably. The decrease in the saturation mobility is also correlated with the crystalline size of each SnO₂ TFT. Generally, the number of grain boundary increases with decreasing crystalline size. Many grain boundaries play a role in surface scattering, thereby resulting in the deterioration of saturation mobility [27]. However, a different trend is observed in this study. The main carrier transport mechanism operating inside metal-oxide semiconductors is the percolation conduction mechanism that can be improved by filling the trap state at high carrier concentrations [28]. The field effect mobility and on-current are proportional to the carrier concentration. The assumption is that the effect of saturation mobility reduction induced by carrier-concentration reduction is greater than the effect of the reduction induced by surface scattering. The on/off current ratios are all >10⁵ and the extracted SSs for 0, 0.5, and 1 wt% doping are 2.16, 1.37, and 1.92 V/decade, respectively. The difference in SS is dominant, owing to the influence of the trap density in each film. Figure 6 shows the transfer curves of the respective TFTs under a negative bias stress (NBS). The NBS measurement of the non-passivated SnO₂ TFTs in the air is evaluated for 1000 s at a gate voltage of −30 V and a drain voltage of +5 V. In the NBS condition, the V_th of the TFTs for Mg doping concentrations of 0, 0.5, and 1 wt% shifts −32.13 V, −6.99 V, and −9.85 V, respectively, in the negative direction The V_th shift is lowest for the 0.5 wt% Mg-doped SnO₂ TFTs. According to the above-mentioned XPS results, the Mg dopants have lower electronegativity and SEP than their Sn hosts. Therefore, the defect-related oxygen vacancies in the SnO₂ thin-film are reduced and the lattice bonding of metals is increased, resulting in decreased field effect mobility and improved bias stability.

Figure 7 shows a schematic of the degradation mechanism for TFTs under NBS. The negatively shifted V_th originates from the trapping of positive charges at dielectric/semiconductor interfaces. Furthermore, trapping of the positive charges results from the ionization of the oxygen vacancies (V_O→ V_O²⁺+2e^-) inside semiconductor layers and V_O²⁺ immigration to the interface after NBS. The introduction of Mg leads to a decrease in the density of oxygen vacancies in the semiconductor through the formation of Mg-V_O pairs. Negative shifts of V_th and degraded SS imply defect-state generation or redistribution. These defect states may exist in the deep levels.

In addition, the negative shift of V_th in a non-passivated SnO₂ TFT is strongly influenced by the moisture absorption mechanism. The film qualities, such as the density and porosity, can determine the sensitivity to absorption molecules. In the above results, the decrease in the V_th shift is attributed to multiple effects resulting from the decrease in oxygen vacancies. This decrease results from changes in the density of the SnO₂ thin-film due to the added Mg dopants. The 0.5 wt% Mg-doped SnO₂ thin-film, with its high density and densely packed structures with relatively small contact areas, prevented the adsorption of water molecules and yielded improvement in the NBS.

4. Conclusions

In the present work, Mg-doped SnO₂ TFTs are successfully fabricated by means of a sol-gel process. The effect of Mg concentration on the structural, chemical, and optical properties of thin films and the corresponding TFT devices is investigated. Controlling the oxygen vacancies and density of the films with added Mg, yields the optimal device performance at 0.5 wt% Mg. The fabricated 0.5 wt% Mg-doped SnO₂ TFT is characterized by a field effect mobility, SS, and I_on/I_off ratio of 4.23 cm²/Vs, 1.37 V/decade, and ~1 × 10⁷, respectively. The added Mg suppresses oxygen-vacancy formation and improves bias stability. This work may pave the way for the development of alkaline-earth-metal-doped SnO₂-based thin-film devices.