# Research of Si-ZnO Thin-Film Transistors Deposited by Atomic Layer Deposition

Authors: Chaeseon Hong; Minjae Kim; Jin-Gyu Lee; Qingyi Shao; Hong-Sub Lee; Hyung-Ho Park
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Si-ZnO thin-film transistors were fabricated using atomic layer deposition (ALD) then annealed in oxygen at 400°C for 1 hour and are demonstrated. Hexagonal wurtzite structure for ZnO was presented in all films, with a primary diffracted peak in the (100) direction regardless of the annealing treatment. First-principle density functional theory (DFT) calculations supported the observed crystalline characteristics. Electrical characteristics were improved with Si doping, increasing carrier concentration to3.51×1020cm−3 and reducing film resistivity to 0.28×10−1Ω.cm with well-matched optical bandgap widening from UV-vis results using Tauc plot: Burstein-Moss (BM) effects. After implementing the oxygen ambient annealing and Si doping, the device parameters to realize the low power/energy consumption were improved to an SS value of 0.67 V/decade; threshold voltage (Vth) of 0.68 V; and a field effect mobility (μFE) of 5.22 cm2/V·s, respectively. The improved device parameters were related to the reduced interface trap densities due to annealing passivated oxygen vacancies, and the BM effect induced percolation conduction with increased carrier concentration due to Si doping, respectively. Thus, incorporating Si-based ZnO materials by ALD offers a viable candidate for superior device characteristics for energy-saving appliances.


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## Body

## 1. Introduction

Metal oxide-based thin-film transistors (TFTs) for the display products have engendered considerable research interest recently due to their unique properties, including transparency in the visible range, excellent deposition uniformity onto large area substrates, and high field-effect mobility. Among them, amorphous In-Ga-Zn-O (a-IGZO) is widely used commercially to satisfy technical requirements for advanced display products [1, 2]. However, a-IGZO contains large quantities of indium (In) because it is the homologous compound (In2O3(ZnO)m) of each In2O3 and ZnO layered structure [3]. Consequently, In toxicity and poor abundance limit further developments for metal oxide materials including IGZO. Therefore, developing alternative to In is critical for the TFT industry. However, as the recent research trend on oxide materials has been focused on the characteristics centered on the typical 3A element-doped ZnO or IGZO, there are relatively few reports of new materials. In this regard, incorporating new metal elements into ZnO could still be an effective and easy-customizing solution.Many elements have been used to improve ZnO characteristics. In particular, the 3A elements Al, Ga, and In have been widely investigated, providing superior electrical and optical characteristics [4]. These typical 3A group elements serve as shallow donors, supplying one electron each [5]. From the point of a view of the introduction of a new element, silicon has also been shown to be an effective donor in the films fabricated using RF sputtering [6] and may be suitable as an effective dopant to improve various devices [7]. Since Si has atomic size of 1.17 Å, which is similar to or slightly less than Zn, using Si as ZnO film dopant could reduce lattice mismatch [8]. However, few previous studies considered Si-doped ZnO thin-film transistors; hence, further research is indicated.One of the most important challenges for optoelectronic applications, including light-emitting diodes (LEDs), photodiodes, and transparent TFTs, is low power consumption [9]. Improving subthreshold swing (SS), field-effect mobility (μFE), and threshold voltage (Vth) could reduce the required driving voltage and effectively reduce power consumption. In this regard, it is crucial to understand the relationship between the channel and insulators: interfacial trap density and EF (Fermi energy level) [10].Various deposition processes can be used to fabricate ZnO thin films. Atomic layer deposition (ALD) has attracted significant attention since these films exhibit high uniformity and conformality and precise thickness and roughness control. The ALD process for thin-film deposition involves precursor source and purging gas injected and deposited onto the substrate in an alternating sequence; hence, film properties can be affected by various processing parameters, including metal-organic precursor type, substrate temperature, and precycle conditioning [11, 12].Therefore, this study investigated Si doping influences on ALD-formed ZnO films and their device properties including SS,μFE, and Vth for TFTs with bottom gate structures. The devices were annealed under optimum conditions (400°C in oxygen for 1 hour) to further improve thin film characteristics and hence device properties.


## 2. Materials and Method

### 2.1. Material Deposition and Device Fabrication

We manufactured ZnO and Si-ZnO thin films on SiO2 (100 nm)/Si (p-type, 100) substrates and glass substrates (Corning, Fusion 1737 for display use) using a traveling-wave Lucida D100 ALD system with 150°C substrate temperature and 1 Torr working pressure (NCD Technology, Inc., Korea). Diethylzinc (DEZ: (C2H5)2Zn, Hansol Chemical Co., Ltd., Korea) and deionized (DI) water were used as Zn and O precursors, respectively, with tris (dimethylamino) silane (TDMAS: (Me2N)3SiH, Hansol Chemical Co., Ltd., Korea) as the Si doping precursor. DEZ was chilled to 10°C and supplied into the vacuum chamber at 20 sccm with high-purity N2 (99.999%) carrier gas. TDMAS was heated to 60°C and flowed at similar rate to DEZ to avoid any antecedent condensation.The cycle sequence for ALD growth to deposit ZnO and Si-ZnO thin films was as follows:(a)
ZnO thin films (FigureS1(a), Supplementary Information (SI)): DEZ pulse for 0.1 s, N2 purge for 10 s, H2O pulse for 0.1 s, and N2 purge for 10 s(b)
Si-ZnO thin films (FigureS1(b), (SI)): DEZ pulse for 0.1 s, N2 purge for 10 s, H2O pulse for 0.1 s, N2 purge for 10 s, TDMAS pulse for 0.1 s, N2 purge for 10 s, H2O pulse for 0.1 s, and N2 purge for 10 sTotal ALD cycles (200 cycles) for both ZnO and Si-ZnO thin films and Si-doping concentration were controlled by modulating the ratio between the Zn-O and Zn-O-Si-O cycles, e.g., Si-1/10 was achieved by20×1×band9×a. We fabricated Si-1/10, 1/20, 1/30, 1/40, and 1/50 Si-doped ZnO thin films.Figures1(a) and 1(b) show the TFT structure used in this study. Fabricated films for all conditions were postannealed after ALD for 1 hour at 400°C in oxygen atmosphere. Mask aligner (M-150, Pro Win Co., Ltd., Korea) was used to shape the source/drain electrodes and channel layer, and bottom-gate TFT devices were constructed with channel width W=100μm and length L=5μm. An e-beam evaporator was used to deposit the 4 nm thick Ti adhesion layer and 30 nm Au source/drain electrodes.Figure 1
Fabricated Si-doped ZnO thin-film transistors on highly doped p-type silicon wafer: (a) typical structure; (b) optical microscope image, top view; (c) ZnO thin-film thickness for as-deposited (as-dep.) and annealed samples; (d) Si-doped ZnO thin-film thickness for annealed samples using AFM; and ZnO and Si-ZnO crystalline characteristics for various Si content for (e) as-dep. and (f) annealed samples.
(a)(b)(c)(d)(e)(f)


### 2.2. Thin-Film and Device Characterization

Crystalline properties for the ZnO and Si-ZnO thin films were examined by X-ray diffraction (XRD; D/MAX-2000, Rigaku, Japan) with Cu Kα radiation source (λ=1.5418Å) operated at 40 kV and 30 mA. Film thickness was verified using ellipsometry (GAERTNER, L117 C, USA) and atomic force microscopy (AFM; XE100, Park Systems, Korea). Electrical characteristics (resistivity (ρ), carrier concentration (n), and Hall mobility (μH)) were determined using a Hall effect measuring device utilizing the van der Pauw technique (Ecopia, HMS-3000, Korea). Optical transmittance spectra 200–900 nm was acquired using an ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer (V-570, JASCO, Japan). Photoluminescence (PL) spectroscopy (LabRam Aramis, Horriba Jovin Yvon, Japan) was used to monitor the films at room temperature using a 325 nm laser source. X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo Scientific Inc., U.K.) using monochromated Al X-ray sources (Al kα line: 1486.6 eV) with 12 kV and 3 mA X-ray beam was used to analyze surface elemental composition for the films. A Keithley 2636B (USA) source meter was used to measure I–V characteristics for the constructed devices.


## 2.1. Material Deposition and Device Fabrication

We manufactured ZnO and Si-ZnO thin films on SiO2 (100 nm)/Si (p-type, 100) substrates and glass substrates (Corning, Fusion 1737 for display use) using a traveling-wave Lucida D100 ALD system with 150°C substrate temperature and 1 Torr working pressure (NCD Technology, Inc., Korea). Diethylzinc (DEZ: (C2H5)2Zn, Hansol Chemical Co., Ltd., Korea) and deionized (DI) water were used as Zn and O precursors, respectively, with tris (dimethylamino) silane (TDMAS: (Me2N)3SiH, Hansol Chemical Co., Ltd., Korea) as the Si doping precursor. DEZ was chilled to 10°C and supplied into the vacuum chamber at 20 sccm with high-purity N2 (99.999%) carrier gas. TDMAS was heated to 60°C and flowed at similar rate to DEZ to avoid any antecedent condensation.The cycle sequence for ALD growth to deposit ZnO and Si-ZnO thin films was as follows:(a)
ZnO thin films (FigureS1(a), Supplementary Information (SI)): DEZ pulse for 0.1 s, N2 purge for 10 s, H2O pulse for 0.1 s, and N2 purge for 10 s(b)
Si-ZnO thin films (FigureS1(b), (SI)): DEZ pulse for 0.1 s, N2 purge for 10 s, H2O pulse for 0.1 s, N2 purge for 10 s, TDMAS pulse for 0.1 s, N2 purge for 10 s, H2O pulse for 0.1 s, and N2 purge for 10 sTotal ALD cycles (200 cycles) for both ZnO and Si-ZnO thin films and Si-doping concentration were controlled by modulating the ratio between the Zn-O and Zn-O-Si-O cycles, e.g., Si-1/10 was achieved by20×1×band9×a. We fabricated Si-1/10, 1/20, 1/30, 1/40, and 1/50 Si-doped ZnO thin films.Figures1(a) and 1(b) show the TFT structure used in this study. Fabricated films for all conditions were postannealed after ALD for 1 hour at 400°C in oxygen atmosphere. Mask aligner (M-150, Pro Win Co., Ltd., Korea) was used to shape the source/drain electrodes and channel layer, and bottom-gate TFT devices were constructed with channel width W=100μm and length L=5μm. An e-beam evaporator was used to deposit the 4 nm thick Ti adhesion layer and 30 nm Au source/drain electrodes.Figure 1
Fabricated Si-doped ZnO thin-film transistors on highly doped p-type silicon wafer: (a) typical structure; (b) optical microscope image, top view; (c) ZnO thin-film thickness for as-deposited (as-dep.) and annealed samples; (d) Si-doped ZnO thin-film thickness for annealed samples using AFM; and ZnO and Si-ZnO crystalline characteristics for various Si content for (e) as-dep. and (f) annealed samples.
(a)(b)(c)(d)(e)(f)


## 2.2. Thin-Film and Device Characterization

Crystalline properties for the ZnO and Si-ZnO thin films were examined by X-ray diffraction (XRD; D/MAX-2000, Rigaku, Japan) with Cu Kα radiation source (λ=1.5418Å) operated at 40 kV and 30 mA. Film thickness was verified using ellipsometry (GAERTNER, L117 C, USA) and atomic force microscopy (AFM; XE100, Park Systems, Korea). Electrical characteristics (resistivity (ρ), carrier concentration (n), and Hall mobility (μH)) were determined using a Hall effect measuring device utilizing the van der Pauw technique (Ecopia, HMS-3000, Korea). Optical transmittance spectra 200–900 nm was acquired using an ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer (V-570, JASCO, Japan). Photoluminescence (PL) spectroscopy (LabRam Aramis, Horriba Jovin Yvon, Japan) was used to monitor the films at room temperature using a 325 nm laser source. X-ray photoelectron spectroscopy (XPS; K-alpha, Thermo Scientific Inc., U.K.) using monochromated Al X-ray sources (Al kα line: 1486.6 eV) with 12 kV and 3 mA X-ray beam was used to analyze surface elemental composition for the films. A Keithley 2636B (USA) source meter was used to measure I–V characteristics for the constructed devices.


## 3. Results and Discussion

### 3.1. Thickness and Crystalline Structure

Figure1(c) shows ellipsometry thickness for ZnO and Si-ZnO thin films deposited onto glass substrates using ALD with different cycle ratios before and after annealing. Averagethickness=37.25 and 37.16 nm for as-deposited (as-dep.) and annealed samples, respectively, with no significant changes due to Si doping or thermal treatment. Although ZnO film thickness was expected to reduce as more Si atoms were substituted into Zn sites, there is little difference in thickness between ZnO and Si-ZnO thin films because thickness change is primarily caused by ALD cycle ratio [13]. Figure 1(d) shows the annealed ZnO film thickness=37.58nm measured by AFM, which is similar to the ellipsometry results. Therefore, the overall average thin-film thickness for 200 ALD supercycles is 37.20±1.50nm.Figures1(e) and 1(f) show XRD results for ZnO and Si-ZnO films with as-dep. and annealed samples for Si-doping concentration, respectively. Hexagonal wurtzite structure was confirmed in all the films, with the highest peak at 31.92° (100) and others at 34.46° (002), 36.22° (101), and 56.73° (110). All diffraction peaks were consistent with reference code (JCPDS 96-210-7060). Generally, ZnO prefers the orientation for (002) planes since surface energy in that plane is lower than any other [14]. However, among the various ALD parameters, the deposition temperature has the largest effect on crystal orientation for ALD derived ZnO thin films [15]. Pung et al. showed that (100) and (002) oriented crystals were dominant in films at low temperatures (below 150°C), but the (100) orientation becomes dominant and (002) orientation reduces over 150–220°C, before switching back to (002) dominant at 220°C [16]. These orientation growths can be explained by a hindering process, proposed by Baji et al. [17]. Surface bonding sites are covered with -OH groups during chemisorption, with (100) sites 5.2 Å distant from one other, whereas (002) sites are 3.25 Å. DEZ has Zn-ethyl bond length=1.95Å. The DEZ molecule releases one ethyl group when chemisorbed onto the surface; therefore, the radius of the remaining specimen is little under 2 Å. Thus, the molecules have sufficient space to connect in the (100) direction without encountering steric barriers. In contrast, (002) direction sites have atomic spacing near the adsorbate size, and repulsion from surrounding molecules is likely to rearrange them into the next-closest packing configuration.The peak intensities increased after the annealing treatment, which aids atom migration to proper lattice locations and crystal formation [18]. However, the intensity is reduced in all conditioned samples as Si-doping concentration increased, implying that Si doping into Zn sites degraded thin-film ordered structure [19, 20]. Table 1 shows XRD parameters referring to d-spacing and cell volume.Table 1
Lattice parameters from XRD data for as-deposited (as-dep.) and annealed (anneal) samples.
ConditionAnnealing (400°C, 1 h)d (Å)Cell Vol. (Å3)(100)(002)ZnOAs-dep.2.8012.60147.1268Anneal2.8092.60247.404Si-1/50As-dep.2.8012.60147.111Anneal2.8082.60147.352Si-1/40As-dep.2.8012.60047.115Anneal2.8092.60247.419Si-1/30As-dep.2.8012.59847.066Anneal2.8062.59947.258Si-1/20As-dep.2.8022.60047.123Anneal2.8082.60047.334Si-1/10As-dep.2.8042.60047.187Anneal2.8062.59747.237Grain size was calculated from XRD results using the Scherrer equation [21]:
(1)D=KλβcosΘ,where β, K, and λ are peak FWHM, shapefactor=0.9, and Cu Kα wavelength, respectively, and interplanar distance d was calculated as (d)
(2)1d2=43∙h2+hk+k2a2+l2c2,where h, k, and l are Miller indices.The cell volume in Table1 is reduced with increasing Si-doping concentration in the annealed samples. As previously stated, this is related to the degradation of crystallinity caused by Si doping. That is to say, the electrons are donated as Si is substituted and ionized in the ZnO lattice, and in this process, the Si ion size decreases, and the total unit cell volume also decreases. This means that Si is stably doped into the thin film and successfully releases electrons.To clarify this discussion, we evaluated geometrical optimization using first-principles density functional theory (DFT) calculations with Cambridge Serial Total Energy Package (CASTEP). Supplementary Information(SI) provides relevant detailed DFT theory and calculation. Figures 2(a)–2(d) show ZnO and Si-ZnO 2×2×2 supercell structural changes with changing Si charge states. Si-doped ZnO structures exhibit shortened bonding length to the oxygen around SiZn compared with the original ZnZn; more charged states show more compressed structures, reducing d and cell volume. Therefore, we could expect that delocalized Si ions are incorporated in the ZnO thin film stably and can donate electrons to form various ion states. However, we cannot define which ion state in Si was more preferred, which will be discussed in Section 3.3. Table 2 shows optimal results for pure ZnO and Si-ZnO primitive cells with different charges, where finalized geometry parameters is well matched to the experimental results.Figure 2
Supercell structure2×2×2 ZnO with different Si (1 atom) charge states: (a) ZnO, (b) Si-ZnO with 0 charge, (c) Si-ZnO with +1 charge, and (d) Si-ZnO with +2 charge states; Zn, O, and Si atoms are represented by gray, red, and yellow spheres, respectively.
(a)(b)(c)(d)Table 2
Optimized parameters for primitive ZnO and Si-ZnO cells with different Si charge states.
ConditionLattice parameter (Å)Angle (deg.)d (Å)Cell volume (Å3)acAc(100)(002)ZnO3.2485.22890.000120.0002.8132.61447.772Si-ZnO (0) : Si2+3.2625.21090.000120.0002.8252.60547.999Si-ZnO (+1) : Si3+3.2145.12790.000120.0002.7842.56345.873Si-ZnO (+2) : Si4+3.1695.03990.000120.0002.7442.52043.815


### 3.2. Electrical Properties

Figure3 shows electrical properties from Hall effect measurements in the Van der Pauw geometry for ZnO and Si-ZnO thin films before and after annealing. N-type conductivity properties are cruised into all conditions of the films.Figure 3
Hall measurements for (a) as-dep. and (b) annealed ZnO and Si-ZnO films with different Si content.
(a)(b)Carrier concentration increased from0.02×1020 to 3.51×1020 and 0.010×1020 to 2.94×1020cm−1 as Si-doping concentration increased, whereas Hall mobility reduced from 6.99 to 2.18 and 11.36 to 4.99 cm2/V.s for as-deposited and annealed samples, respectively, due to increased charge carrier scattering probability. Carrier mobility behavior can be further described by investigation the direct relationship with carrier concentration.Hall mobility for metal oxide increases with carrier concentration is generally opposite to that for Si-based crystalline material. This conduction mechanism is known as percolation conduction and helps charge carriers to move in the conduction band by lowering the barrier height [22]. However, increasing mobility is ultimately restricted by another barrier, i.e., ionized impurity scattering [23]. Thus, reduced mobility despite increased carrier concentration from Si doping indicates these films have highly ionized scattering probability. Therefore, increased charge carrier concentration and internal defects, such as VO, could limit film mobility despite increase carrier concentration from Si doping. These internal defects are discussed in more details in the subsequent XPS section.Resistivity reduced from6.15×10−1 to 0.28×10−1 and 25.68×10−1 to 12.49×10−1Ω.cm for as-deposited and annealed samples, respectively.Electrical resistivity (i.e., conductivity) within thin films reduces in proportion to the product of mobility and carrier concentration:(3)1ρ=σ=neμ,where ρ, σ, n, e, and μ are the resistivity, conductivity, carrier concentration, electron charge, and Hall mobility, respectively.Electrical properties also improved after annealing, exhibiting reduced carrier concentration and increased mobility. Thermal energy delivered into the films during annealing helps atoms migrate to appropriate lattice positions, as discussed in the XRD section. Thus, electron donation properties are lost due to oxygen vacancies which are being filled with incoming oxygen [24].Therefore, thin film electrical properties were improved by both Si doping and oxygen annealing; hence, Si dopant was stably activated in annealed samples.


### 3.3. XPS Results

Figure4 shows XPS spectra to investigate additional phase formation, Si cation valence state, and O chemical states with increasing Si-doping concentration in ZnO and Si-ZnO films. Carbon (C) 1s at 284.5 eV was applied to compensate for charging effects in all spectra [25]. Figures 4(a) and 4(b) confirm Zn2p−bindingenergy=1021.08±0.1 and 1044.13±0.1eV for Zn 2p3/2 and Zn 2p1/2 peaks, respectively, which closely match with the previous study (1021.8±0.5 and 1045.0±0.5eV, respectively) [26]. Also, energy difference for spin-orbital splitting between Zn 2p3/2 and Zn 2p3/2 remained constant at 23.1 eV for both ZnO and Si-ZnO, confirming that Si was stably doped in ZnO without generating secondary phase artifacts, such as zinc silicates (e.g., ZnxSiyOz: Zn 2p3/2−1023.0±0.5eV) [27].Figure 4
X-ray photoelectron spectroscopy (XPS) spectra for ZnO and Si-ZnO films: (a) Zn 2p for as-dep., (b) Zn 2p for annealed, (c) Si 2p for as-dep., (d) Si 2p for annealed, (e) O 1s for as-dep., and (f) O 1s for annealed samples.
(a)(b)(c)(d)(e)(f)Figures4(c) and 4(d) show Si 2p XPS spectra for Si-ZnO thin films. Si 2p peak intensity increased as Si precursor pulse cycles increased. However, Si concentration is quite small compared with previous reports, remaining about 2 at% for Si-1/10, as shown in Table 3. This low doping concentration was attributed to low decomposition rate for Si-H species in the Si source precursor (TDMAS) due to low substrate temperature. Burton et al. confirmed that SiO2 growth rate using this precursor depends on the substrate temperature [28], suggesting that these Si-H species could be fully decomposed using higher reaction temperatures and/or more oxidant agents. This outcome was confirmed by comparing with Yuan results in Ref. [20], which achieved higher Si dopant concentration than the current study due to higher substrate temperature (300°C) and increased oxidant precursor (H2O2). Therefore, the low Si-doping concentration exhibited in this study was attributed to low substrate temperature (150°C) and using water. However, despite this, the current study confirmed that Si-doped thin films contained the specific Si quantity (i.e., the Si 2p peak around 102.1 eV); hence, Si ionization states were mainly Si3+.Table 3
Si concentration and areal ratio for of O 1s constituents OII/OI in ZnO and Si-ZnO in as-deposited (as-dep.) and annealed (anneal) samples.
GroupConditionCycle ratioSi (at%)OII/OIAs-dep.AnnealAs-dep.AnnealAs-depZnO——13.09%11.47%Si-ZnOSi-1/500.62%0.78%14.41%11.22%Si-1/401.22%1.16%16.41%9.32%Si-1/301.35%1.34%17.66%15.25%Si-1/201.40%1.32%21.30%12.15%Si-1/101.68%1.53%25.13%19.27%The Si 2p core level could be deconvolved into three subordinate chemical states corresponding to Si4+ (103.2 eV; SiO2), Si3+ (102.1 eV; Si2O3), and Si2+ (101.0 eV; SiO), respectively [29]. Since the highest intensity is the Si3+ state in this research, Si dopant mainly donates electrons into the conduction band in form of Si3+ rather than Si2+ or Si4+. Previous studies have also shown that Si mainly replaces Zn sites in Si3+ states and hence only contributes one electron to the conductivity [19, 30]. Therefore, the most defect equation for Si substitution into Zn site could be expressed as:
(4)Zn2+⟶Si3+SiZn2++e−.Figures4(e) and 4(f) show O 1s spectra for ZnO and Si-ZnO thin films before and after annealing. Chemical states for the O 1s peak could be categorized into three groups of O2- in Zn-O (OI: 530±0.3eV); oxygen vacancies, i.e., VO (OII: 531.0±0.3eV); and carbon contamination or hydrated oxides (OIII: 531.9±0.3eV), respectively [30]. Table 3 summarizes relative peak ratios for OII/OI.The ratio for OII/OI chemical states (i.e., VO ratio) for the annealed samples is lower than that of as-dep. samples. It means that VO began to be filled by oxygen atoms outside the films during annealing process. Previous studies have shown that higher oxygen ambient pressure reduces VO when ZnO thin films deposited using metal-organic chemical vapor deposition (MO-CVD) [31]. However, OII/OI ratio changed from 13.09 to 25.13% and 11.47 to 19.27% for as-dep. and annealed samples with increasing Si-doping concentration, respectively, which indicates that VO increased. This means that Si ions combine with O of the Zn-O lattice to form SiOx, thereby generating Vo around the Zn atoms. Previous study reported that SiO2 was created at the ZnO expense as SiOx and ZnO combination, which becomes oxygen-deficient in Si-ZnO thin films [32]. These results were attributed to free energy differences between SiOx and ZnO. Si atoms can be easily thermally oxidized to SiOx because the free energy for Si2O3 formation at ~300 K (Gf0 ≈ -304 kcal/mol) is much lower than for the ZnO layer (Gf0≈−161kcal/mol) [33, 34]. Therefore, Si2O3 is relatively more stable than zinc metal oxide [35]. Figure S2 (SI) shows overlapped normalized O 1s spectra for all conditions to further clarify chemical state for related Si-O compounds. Chemical states for Si-O compounds were investigated with SiO2 (SiI: 532.5 eV) and SiOx (SiII: 531.7 eV) [36]. OII, SiI, and SiII properties increased with increasing Si-doping concentration for both as-dep. and annealed samples. This result confirms that Si-ZnO thin films promoted VO generation at Zn-O bond expense and subsequently generated SiOx compounds. Since SiII intensity exceeds that for SiI, Si ionization states more dominant to Si3+ (SiOx) than Si4+ (SiO2). Thus, Si functions more as an electron donor than SiO2 insulator form. Therefore, defects for electron donation to oxygen vacancies after Si doping can be expressed as
(5)O2−⟶Si3+SiO2+VO+e−.Therefore, although Si dopant is widely acknowledged to be a multidonor, in this case, it mainly donates a single electron in the Si3+ form from Zn sites (SiZn). Consequently, oxygen vacancies (VO) are formed by consuming lattice oxygen in ZnO.


### 3.4. Optical Properties

FigureS3 (SI) shows UV-vis transmittance spectra for the Si-ZnO thin films before and after annealing. Average transmittance in the visiblerange400–800nm=79.89%, 80.21%, 80.46%, 80.38%, 81.61%, and 82.74% for ZnO, Si-1/50, 1/40, 1/30, 1/20, and 1/10 as-dep.; and 84.75%, 84.46%, 84.41%, 83.95%, 85.12%, and 86.14% for annealed samples, respectively, with the absorption edge between 300 and 400 nm. Previous studies have reported similar transmittance results, as summarized in Table S1 (SI) [37, 38]. These transmittances are sufficient for various commercial products such as LEDs, liquid crystal devices, touch screens, and optical windows for photovoltaic applications [4, 39, 40]. Although the spectra profile changed slightly with Si-doping level, no significant transmittance changes occurred. However, transmittance increased after annealing compared with as-dep. (average=80.88±1.08% and 84.81±0.76%, respectively), due to enhanced crystallinity, as shown in XRD analysis (Section 3.1) [41].Figures5(a) and 5(b) show the optical band gaps for as-dep. and annealed samples. The Tauc’s equation expresses the relationship between the absorption edge and the optical band gap as follows [42]:
(6)αhνn=Chν−Eg,where n=2 for direct band gap materials, C is a constant, hν is photon energy, and α is the absorption coefficient derived from the Beer-Lambert law [43]:
(7)α=1t∙ln1Tr,where t and Tr are thin-film thickness and transmittance, respectively. Thin-film direct band gap characteristics were calculated from the αhν2 wrt hv plot, extrapolating the linear region fit to the intercept.Figure 5
Tauc’s plots for (a) as-dep., and (b) annealed samples and photoluminescence (PL) spectra for (c) as-dep., and (d) annealed samples.
(a)(b)(c)(d)The interband absorption edge from 300 to 400 nm shifted to higher energy (blue shift) as Si-doping concentration increased, indicating that Si doping widened the optical bandgap in the films, which can be attributed to the BM effect [19, 20, 30, 44]. Through the BM effect, the conduction band minimum gradually filled as the Fermi level increased into the conduction band, preventing low-energy transitions and increasing the optical bandgap.FigureS4 (SI) shows that calculated optical bandgap monotonically increased from 3.173 to 3.174, 3.177, 3.198, 3.246, and 3.322 eV for Si-1/50, Si-1/40, Si-1/30, Si-1/20, and Si-1/10 as-dep. and 3.167, 3.169, 3.172, 3.173, 3.174, and 3.183 eV for annealed samples, respectively. Thus, the calculated optical bandgap and their increment rate in the annealed samples are lower than in the as-dep. samples. This phenomenon could be due to interactions between two mechanisms:
(1)
Decrease of band gap of thin film by the heat treatment(2)
Increase of band gap of thin films by the BM effect (i.e., Si doping)Generally, at a higher temperature, the crystallites in films may agglomerate, causing band gaps to shrink [45]. These band gap tailoring may be a result of quantum confinement effects, which are correlated with the relative grain size [46]. However, the BM effect from Si doping induces bandgap widening. The BM effect slightly increases the band gap with Si-doping concentration, whereas bandgap is reduced by annealing. Thus, optical bandgap behavior for these samples were mostly affected by quantum confinement rather than BM effects. Similar results have been reported previously: Sengupta et al. found the bandgap grew as Al-ZnO concentration increased for the same annealing temperature, but bandgap reduced with increasing heat treatment temperature at the same dopant concentration [47]. Irimpan et al. showed that the optical band for ZnO thin films reduced with increasing annealing temperature [48].


### 3.5. Photoluminescence

Figures5(c) and 5(d) show PL spectra for ZnO and Si-ZnO thin films before and after annealing. The films generally exhibit two significant peaks in the visible region: one high intensity PL peak at 382 nm (near band edge, NBE), and another broad emission band around 500 nm (deep level emission, DLE), due to VO and/or Zni in the ZnO crystal, respectively [49, 50].Annealed samples initially exhibit higher NBE peaks than as-dep. samples, since film crystalline quality influences NBE emission, implying that the crystallinity of the annealed films have improved [51]. However, the NBE peak reduced with increasing Si-doping concentration because thin film crystallinity reduced with increasing Si ions in the ZnO crystal lattice [20]. Also, the BM effect could explain the NBE shift toward shorter wavelengths [52]. These results are consistent with XRD and optical results, discussed in Sections 3.1 and 3.4, respectively.FiguresS5(a) and (b) (SI) show relative peak intensity IRel to help confirm DLE effects for Si doping and annealing [51, 53]:
(8)IRel=IDLE/INBE,where IDLE and INBE are DLE and NBE emission intensities, respectively. The relative peak strength close to the green emission range increased with Si-doping concentration, hence VO increased with increasing Si doping. However, the increase rate for VO in annealed samples was less than for as-dep. samples, which is confirming that the competition between increased VO due to Si doping and reduced VO due to oxygen annealing.


### 3.6. Device Properties

Figures1(a) and 1(b) show layered and top views for the fabricated TFT using a bottom-gate (BG) structure with heavily doped p-type Si substrates as a common-gate electrode and 100 nm thermally grown SiO2 as a gate insulator. TFT featured with W/Lratio=100/5 was used for characterization purposes. TFTs’ performance parameters were determined using the standard MOSFET equation [54]:
(9)Ids=W2LμsatCoxVgs−Vth2,where W, L, Cox, and Vgs are channel layer width and length, gate oxide capacitance per unit area, and gate-source voltage, respectively. Subthreshold swing (SS) was calculated as follows [55]:
(10)SS=dlogIdsdVgs−1,at the steepest point of the transfer slope. Table 4 shows extracted TFT performance parameters for different doping levels. The derived values from this study are comparable with previous study results [56–58].Table 4
Extracted device parameters from annealed ZnO and Si-ZnO samples with various Si concentrations for the current and previous studies.
ConditionVth [V]μFE [cm2/V·s]SS [V/decade]Ion/IoffDit_max [cm−2 eV−1]ZnO18.911.726.637.22E+052.42E+13Si-1/5015.611.364.011.39E+051.46E+13Si-1/4010.362.552.688.82E+059.66E+12Si-1/307.892.612.161.39E+067.74E+12Si-1/203.445.221.704.41E+056.05E+12Si-1/100.682.910.677.00E+052.25E+12Lee et. al0.800.700.281.66E+07—Liu et. al18.501.022.64.00E+07—Yang et. al3.405.100.603.20E+061.90E+12Figures6(a)–6(d) show ZnO TFT transfer curves using annealed samples, for gate voltage Vgs=−50 to +50 V with drain voltage Vds=0.5V. The device made by as-dep. samples (Figure 6(a)) showed a horizontal transfer curve of the metallic properties (i.e., no device characteristics) due to its relative highly conductivity in the channel layer allowing better electrons flow to traverse the source and drain. Ioff is related to channel layer conductivity [59]:
(11)Ioff=σWLVds∙tch,where σ is electrical conductivity for the channel layer and tch is the active layer thickness. Calculated conductivity from Hall measurements reduced from 1.63 S/cm for as-dep. sample to 0.39 S/cm for annealed sample after the annealing treatment. The reduced conductivity decreased Ioff, confirming the device’s characteristics. The final device achieved n-channel enhancement, as evidenced by the requirement for positive gate voltage to induce a conducting channel [60].Figure 6
Transfer curves for ZnO thin-film transistors with increasing thermal treatment time oxygen atmosphere: (a) as-dep., (b) 0.5 hour at 300°C, (c) 1 hour at 300°C, and (d) 1 hour at 400°C.
(a)(b)(c)(d)Table4 shows that the device threshold voltage (Vth) decreased with increasing Si doping from 18.91 to 0.68 V. The donated electron from Si tail states contributes to the device current with increasing Si-doping concentration [61]. Furthermore, the VO induced from Si doping generated more free electrons, which also contribute to the device current. The device’s field effect mobility (μFE) improved from 1.72 to 5.22 cm2/V·s until the Si-1/20 doping condition and decreased back to 2.91 cm2/V·s in the Si-1/10 condition. As already mentioned, it is well known that the charge mobility for metal oxide devices increases with increasing carrier concentration. As the EF of the thin film is uplifted into the conduction band due to the BM effect by Si doping, the energy barrier height existing in the conduction band is lowered, and the mobility of electrons in the band is increased without the scattering between the energy barriers, which can be attributed to the percolation conduction [62]. That is, the additional electrons from the subgap state as SiZn and VO2+ with increasing Si doping elevated the device current, and finally, the Vth decreased, and the μFE increased. However, as you can see in Si-1/10 condition, the increase in mobility is finally limited by ionized impurity scattering [23, 63].Figure7 shows transfer curves for TFTs fabricated from annealed ZnO and Si-ZnO (1/50, 1/40, 1/30, 1/20, and 1/10) films. The subthreshold slope becomes steeper with increasing Si doping, i.e., subthreshold swing (SS) reduced. The small swing value is important for the device in terms of power and energy saving, which means that the device can turn on faster [64]. The interfacial trap density (Dit) may be calculated using SS from the following equation [65]:
(12)Dit=SS∙logekBT/q−1Cq,where e is the natural constant, kB is Boltzmann’s constant, T is the absolute temperature, q is the electronic charge, and C is the insulator capacitance per unit area, respectively. The calculated Dit values were shown in Table 4. It can be seen that the interfacial trap density was effectively reduced. For further verification of reduction for interfacial trap density, the hysteresis of the transfer curves was measured under gate voltage sweeping from -50 V to 50 V back and forth for ZnO and Si-ZnO 1/10 cases, which is shown in Figure S6 (SI). The large difference of hysteresis around the subthreshold region in ZnO indicates the high trap density between the insulator and active layer, while it is suppressed in Si-ZnO 1/10, which means that the device drives faster and has an energy-saving effect [66].Figure 7
Transfer curves for ZnO and Si-ZnO thin-film transistors (TFTs) with increasing Si-doping concentrations: (a) ZnO, (b) Si-1/50, (c) Si-1/40, (d) Si-1/30, (e) Si-1/20, and (f) Si-1/10 samples.
(a)(b)(c)(d)(e)(f)Figure8 shows the energy band diagram with BM effect in the channel/dielectric interface for Si-ZnO TFTs. The density of states (gE) and Fermi-Dirac distribution (fFE) relationship can be expressed as follows [54]:
(13)NEgE=fFE=11+expECBE−EF/kBT,where NE, ECBE, EF, kB, and T are the distributed electron count, conduction band edge, Fermi energy, Boltzmann’s constant, and absolute temperature, respectively. Thus, carrier concentration is dependent on (ECBE-EF), where smaller difference improves concentration up to saturation (ECBE « EF). This effect is similar to BM effect derived from Si doping, i.e., since the Si tail state forms under the conduction band edge, continually increasing carrier concentration will promote EF into the conduction band at the gamma (Γ) position in the reciprocal lattice for direct semiconductors. Therefore, EF lies above the conduction band [67]. The device is accumulated with electrons when the bottom of the conduction band edge is filled with charge carriers. These accumulated electrons contribute to Ion and induce increased μFE, followed by negative Vth shift applying the gate voltage.Figure 8
Energy band diagram showing BM effect for Si-ZnO TFTs: (a) energy level distribution in density of states and band diagram for annealed (b) ZnO and (c) Si-ZnO samples.
(a)(b)(c)


## 3.1. Thickness and Crystalline Structure

Figure1(c) shows ellipsometry thickness for ZnO and Si-ZnO thin films deposited onto glass substrates using ALD with different cycle ratios before and after annealing. Averagethickness=37.25 and 37.16 nm for as-deposited (as-dep.) and annealed samples, respectively, with no significant changes due to Si doping or thermal treatment. Although ZnO film thickness was expected to reduce as more Si atoms were substituted into Zn sites, there is little difference in thickness between ZnO and Si-ZnO thin films because thickness change is primarily caused by ALD cycle ratio [13]. Figure 1(d) shows the annealed ZnO film thickness=37.58nm measured by AFM, which is similar to the ellipsometry results. Therefore, the overall average thin-film thickness for 200 ALD supercycles is 37.20±1.50nm.Figures1(e) and 1(f) show XRD results for ZnO and Si-ZnO films with as-dep. and annealed samples for Si-doping concentration, respectively. Hexagonal wurtzite structure was confirmed in all the films, with the highest peak at 31.92° (100) and others at 34.46° (002), 36.22° (101), and 56.73° (110). All diffraction peaks were consistent with reference code (JCPDS 96-210-7060). Generally, ZnO prefers the orientation for (002) planes since surface energy in that plane is lower than any other [14]. However, among the various ALD parameters, the deposition temperature has the largest effect on crystal orientation for ALD derived ZnO thin films [15]. Pung et al. showed that (100) and (002) oriented crystals were dominant in films at low temperatures (below 150°C), but the (100) orientation becomes dominant and (002) orientation reduces over 150–220°C, before switching back to (002) dominant at 220°C [16]. These orientation growths can be explained by a hindering process, proposed by Baji et al. [17]. Surface bonding sites are covered with -OH groups during chemisorption, with (100) sites 5.2 Å distant from one other, whereas (002) sites are 3.25 Å. DEZ has Zn-ethyl bond length=1.95Å. The DEZ molecule releases one ethyl group when chemisorbed onto the surface; therefore, the radius of the remaining specimen is little under 2 Å. Thus, the molecules have sufficient space to connect in the (100) direction without encountering steric barriers. In contrast, (002) direction sites have atomic spacing near the adsorbate size, and repulsion from surrounding molecules is likely to rearrange them into the next-closest packing configuration.The peak intensities increased after the annealing treatment, which aids atom migration to proper lattice locations and crystal formation [18]. However, the intensity is reduced in all conditioned samples as Si-doping concentration increased, implying that Si doping into Zn sites degraded thin-film ordered structure [19, 20]. Table 1 shows XRD parameters referring to d-spacing and cell volume.Table 1
Lattice parameters from XRD data for as-deposited (as-dep.) and annealed (anneal) samples.
ConditionAnnealing (400°C, 1 h)d (Å)Cell Vol. (Å3)(100)(002)ZnOAs-dep.2.8012.60147.1268Anneal2.8092.60247.404Si-1/50As-dep.2.8012.60147.111Anneal2.8082.60147.352Si-1/40As-dep.2.8012.60047.115Anneal2.8092.60247.419Si-1/30As-dep.2.8012.59847.066Anneal2.8062.59947.258Si-1/20As-dep.2.8022.60047.123Anneal2.8082.60047.334Si-1/10As-dep.2.8042.60047.187Anneal2.8062.59747.237Grain size was calculated from XRD results using the Scherrer equation [21]:
(1)D=KλβcosΘ,where β, K, and λ are peak FWHM, shapefactor=0.9, and Cu Kα wavelength, respectively, and interplanar distance d was calculated as (d)
(2)1d2=43∙h2+hk+k2a2+l2c2,where h, k, and l are Miller indices.The cell volume in Table1 is reduced with increasing Si-doping concentration in the annealed samples. As previously stated, this is related to the degradation of crystallinity caused by Si doping. That is to say, the electrons are donated as Si is substituted and ionized in the ZnO lattice, and in this process, the Si ion size decreases, and the total unit cell volume also decreases. This means that Si is stably doped into the thin film and successfully releases electrons.To clarify this discussion, we evaluated geometrical optimization using first-principles density functional theory (DFT) calculations with Cambridge Serial Total Energy Package (CASTEP). Supplementary Information(SI) provides relevant detailed DFT theory and calculation. Figures 2(a)–2(d) show ZnO and Si-ZnO 2×2×2 supercell structural changes with changing Si charge states. Si-doped ZnO structures exhibit shortened bonding length to the oxygen around SiZn compared with the original ZnZn; more charged states show more compressed structures, reducing d and cell volume. Therefore, we could expect that delocalized Si ions are incorporated in the ZnO thin film stably and can donate electrons to form various ion states. However, we cannot define which ion state in Si was more preferred, which will be discussed in Section 3.3. Table 2 shows optimal results for pure ZnO and Si-ZnO primitive cells with different charges, where finalized geometry parameters is well matched to the experimental results.Figure 2
Supercell structure2×2×2 ZnO with different Si (1 atom) charge states: (a) ZnO, (b) Si-ZnO with 0 charge, (c) Si-ZnO with +1 charge, and (d) Si-ZnO with +2 charge states; Zn, O, and Si atoms are represented by gray, red, and yellow spheres, respectively.
(a)(b)(c)(d)Table 2
Optimized parameters for primitive ZnO and Si-ZnO cells with different Si charge states.
ConditionLattice parameter (Å)Angle (deg.)d (Å)Cell volume (Å3)acAc(100)(002)ZnO3.2485.22890.000120.0002.8132.61447.772Si-ZnO (0) : Si2+3.2625.21090.000120.0002.8252.60547.999Si-ZnO (+1) : Si3+3.2145.12790.000120.0002.7842.56345.873Si-ZnO (+2) : Si4+3.1695.03990.000120.0002.7442.52043.815


## 3.2. Electrical Properties

Figure3 shows electrical properties from Hall effect measurements in the Van der Pauw geometry for ZnO and Si-ZnO thin films before and after annealing. N-type conductivity properties are cruised into all conditions of the films.Figure 3
Hall measurements for (a) as-dep. and (b) annealed ZnO and Si-ZnO films with different Si content.
(a)(b)Carrier concentration increased from0.02×1020 to 3.51×1020 and 0.010×1020 to 2.94×1020cm−1 as Si-doping concentration increased, whereas Hall mobility reduced from 6.99 to 2.18 and 11.36 to 4.99 cm2/V.s for as-deposited and annealed samples, respectively, due to increased charge carrier scattering probability. Carrier mobility behavior can be further described by investigation the direct relationship with carrier concentration.Hall mobility for metal oxide increases with carrier concentration is generally opposite to that for Si-based crystalline material. This conduction mechanism is known as percolation conduction and helps charge carriers to move in the conduction band by lowering the barrier height [22]. However, increasing mobility is ultimately restricted by another barrier, i.e., ionized impurity scattering [23]. Thus, reduced mobility despite increased carrier concentration from Si doping indicates these films have highly ionized scattering probability. Therefore, increased charge carrier concentration and internal defects, such as VO, could limit film mobility despite increase carrier concentration from Si doping. These internal defects are discussed in more details in the subsequent XPS section.Resistivity reduced from6.15×10−1 to 0.28×10−1 and 25.68×10−1 to 12.49×10−1Ω.cm for as-deposited and annealed samples, respectively.Electrical resistivity (i.e., conductivity) within thin films reduces in proportion to the product of mobility and carrier concentration:(3)1ρ=σ=neμ,where ρ, σ, n, e, and μ are the resistivity, conductivity, carrier concentration, electron charge, and Hall mobility, respectively.Electrical properties also improved after annealing, exhibiting reduced carrier concentration and increased mobility. Thermal energy delivered into the films during annealing helps atoms migrate to appropriate lattice positions, as discussed in the XRD section. Thus, electron donation properties are lost due to oxygen vacancies which are being filled with incoming oxygen [24].Therefore, thin film electrical properties were improved by both Si doping and oxygen annealing; hence, Si dopant was stably activated in annealed samples.


## 3.3. XPS Results

Figure4 shows XPS spectra to investigate additional phase formation, Si cation valence state, and O chemical states with increasing Si-doping concentration in ZnO and Si-ZnO films. Carbon (C) 1s at 284.5 eV was applied to compensate for charging effects in all spectra [25]. Figures 4(a) and 4(b) confirm Zn2p−bindingenergy=1021.08±0.1 and 1044.13±0.1eV for Zn 2p3/2 and Zn 2p1/2 peaks, respectively, which closely match with the previous study (1021.8±0.5 and 1045.0±0.5eV, respectively) [26]. Also, energy difference for spin-orbital splitting between Zn 2p3/2 and Zn 2p3/2 remained constant at 23.1 eV for both ZnO and Si-ZnO, confirming that Si was stably doped in ZnO without generating secondary phase artifacts, such as zinc silicates (e.g., ZnxSiyOz: Zn 2p3/2−1023.0±0.5eV) [27].Figure 4
X-ray photoelectron spectroscopy (XPS) spectra for ZnO and Si-ZnO films: (a) Zn 2p for as-dep., (b) Zn 2p for annealed, (c) Si 2p for as-dep., (d) Si 2p for annealed, (e) O 1s for as-dep., and (f) O 1s for annealed samples.
(a)(b)(c)(d)(e)(f)Figures4(c) and 4(d) show Si 2p XPS spectra for Si-ZnO thin films. Si 2p peak intensity increased as Si precursor pulse cycles increased. However, Si concentration is quite small compared with previous reports, remaining about 2 at% for Si-1/10, as shown in Table 3. This low doping concentration was attributed to low decomposition rate for Si-H species in the Si source precursor (TDMAS) due to low substrate temperature. Burton et al. confirmed that SiO2 growth rate using this precursor depends on the substrate temperature [28], suggesting that these Si-H species could be fully decomposed using higher reaction temperatures and/or more oxidant agents. This outcome was confirmed by comparing with Yuan results in Ref. [20], which achieved higher Si dopant concentration than the current study due to higher substrate temperature (300°C) and increased oxidant precursor (H2O2). Therefore, the low Si-doping concentration exhibited in this study was attributed to low substrate temperature (150°C) and using water. However, despite this, the current study confirmed that Si-doped thin films contained the specific Si quantity (i.e., the Si 2p peak around 102.1 eV); hence, Si ionization states were mainly Si3+.Table 3
Si concentration and areal ratio for of O 1s constituents OII/OI in ZnO and Si-ZnO in as-deposited (as-dep.) and annealed (anneal) samples.
GroupConditionCycle ratioSi (at%)OII/OIAs-dep.AnnealAs-dep.AnnealAs-depZnO——13.09%11.47%Si-ZnOSi-1/500.62%0.78%14.41%11.22%Si-1/401.22%1.16%16.41%9.32%Si-1/301.35%1.34%17.66%15.25%Si-1/201.40%1.32%21.30%12.15%Si-1/101.68%1.53%25.13%19.27%The Si 2p core level could be deconvolved into three subordinate chemical states corresponding to Si4+ (103.2 eV; SiO2), Si3+ (102.1 eV; Si2O3), and Si2+ (101.0 eV; SiO), respectively [29]. Since the highest intensity is the Si3+ state in this research, Si dopant mainly donates electrons into the conduction band in form of Si3+ rather than Si2+ or Si4+. Previous studies have also shown that Si mainly replaces Zn sites in Si3+ states and hence only contributes one electron to the conductivity [19, 30]. Therefore, the most defect equation for Si substitution into Zn site could be expressed as:
(4)Zn2+⟶Si3+SiZn2++e−.Figures4(e) and 4(f) show O 1s spectra for ZnO and Si-ZnO thin films before and after annealing. Chemical states for the O 1s peak could be categorized into three groups of O2- in Zn-O (OI: 530±0.3eV); oxygen vacancies, i.e., VO (OII: 531.0±0.3eV); and carbon contamination or hydrated oxides (OIII: 531.9±0.3eV), respectively [30]. Table 3 summarizes relative peak ratios for OII/OI.The ratio for OII/OI chemical states (i.e., VO ratio) for the annealed samples is lower than that of as-dep. samples. It means that VO began to be filled by oxygen atoms outside the films during annealing process. Previous studies have shown that higher oxygen ambient pressure reduces VO when ZnO thin films deposited using metal-organic chemical vapor deposition (MO-CVD) [31]. However, OII/OI ratio changed from 13.09 to 25.13% and 11.47 to 19.27% for as-dep. and annealed samples with increasing Si-doping concentration, respectively, which indicates that VO increased. This means that Si ions combine with O of the Zn-O lattice to form SiOx, thereby generating Vo around the Zn atoms. Previous study reported that SiO2 was created at the ZnO expense as SiOx and ZnO combination, which becomes oxygen-deficient in Si-ZnO thin films [32]. These results were attributed to free energy differences between SiOx and ZnO. Si atoms can be easily thermally oxidized to SiOx because the free energy for Si2O3 formation at ~300 K (Gf0 ≈ -304 kcal/mol) is much lower than for the ZnO layer (Gf0≈−161kcal/mol) [33, 34]. Therefore, Si2O3 is relatively more stable than zinc metal oxide [35]. Figure S2 (SI) shows overlapped normalized O 1s spectra for all conditions to further clarify chemical state for related Si-O compounds. Chemical states for Si-O compounds were investigated with SiO2 (SiI: 532.5 eV) and SiOx (SiII: 531.7 eV) [36]. OII, SiI, and SiII properties increased with increasing Si-doping concentration for both as-dep. and annealed samples. This result confirms that Si-ZnO thin films promoted VO generation at Zn-O bond expense and subsequently generated SiOx compounds. Since SiII intensity exceeds that for SiI, Si ionization states more dominant to Si3+ (SiOx) than Si4+ (SiO2). Thus, Si functions more as an electron donor than SiO2 insulator form. Therefore, defects for electron donation to oxygen vacancies after Si doping can be expressed as
(5)O2−⟶Si3+SiO2+VO+e−.Therefore, although Si dopant is widely acknowledged to be a multidonor, in this case, it mainly donates a single electron in the Si3+ form from Zn sites (SiZn). Consequently, oxygen vacancies (VO) are formed by consuming lattice oxygen in ZnO.


## 3.4. Optical Properties

FigureS3 (SI) shows UV-vis transmittance spectra for the Si-ZnO thin films before and after annealing. Average transmittance in the visiblerange400–800nm=79.89%, 80.21%, 80.46%, 80.38%, 81.61%, and 82.74% for ZnO, Si-1/50, 1/40, 1/30, 1/20, and 1/10 as-dep.; and 84.75%, 84.46%, 84.41%, 83.95%, 85.12%, and 86.14% for annealed samples, respectively, with the absorption edge between 300 and 400 nm. Previous studies have reported similar transmittance results, as summarized in Table S1 (SI) [37, 38]. These transmittances are sufficient for various commercial products such as LEDs, liquid crystal devices, touch screens, and optical windows for photovoltaic applications [4, 39, 40]. Although the spectra profile changed slightly with Si-doping level, no significant transmittance changes occurred. However, transmittance increased after annealing compared with as-dep. (average=80.88±1.08% and 84.81±0.76%, respectively), due to enhanced crystallinity, as shown in XRD analysis (Section 3.1) [41].Figures5(a) and 5(b) show the optical band gaps for as-dep. and annealed samples. The Tauc’s equation expresses the relationship between the absorption edge and the optical band gap as follows [42]:
(6)αhνn=Chν−Eg,where n=2 for direct band gap materials, C is a constant, hν is photon energy, and α is the absorption coefficient derived from the Beer-Lambert law [43]:
(7)α=1t∙ln1Tr,where t and Tr are thin-film thickness and transmittance, respectively. Thin-film direct band gap characteristics were calculated from the αhν2 wrt hv plot, extrapolating the linear region fit to the intercept.Figure 5
Tauc’s plots for (a) as-dep., and (b) annealed samples and photoluminescence (PL) spectra for (c) as-dep., and (d) annealed samples.
(a)(b)(c)(d)The interband absorption edge from 300 to 400 nm shifted to higher energy (blue shift) as Si-doping concentration increased, indicating that Si doping widened the optical bandgap in the films, which can be attributed to the BM effect [19, 20, 30, 44]. Through the BM effect, the conduction band minimum gradually filled as the Fermi level increased into the conduction band, preventing low-energy transitions and increasing the optical bandgap.FigureS4 (SI) shows that calculated optical bandgap monotonically increased from 3.173 to 3.174, 3.177, 3.198, 3.246, and 3.322 eV for Si-1/50, Si-1/40, Si-1/30, Si-1/20, and Si-1/10 as-dep. and 3.167, 3.169, 3.172, 3.173, 3.174, and 3.183 eV for annealed samples, respectively. Thus, the calculated optical bandgap and their increment rate in the annealed samples are lower than in the as-dep. samples. This phenomenon could be due to interactions between two mechanisms:
(1)
Decrease of band gap of thin film by the heat treatment(2)
Increase of band gap of thin films by the BM effect (i.e., Si doping)Generally, at a higher temperature, the crystallites in films may agglomerate, causing band gaps to shrink [45]. These band gap tailoring may be a result of quantum confinement effects, which are correlated with the relative grain size [46]. However, the BM effect from Si doping induces bandgap widening. The BM effect slightly increases the band gap with Si-doping concentration, whereas bandgap is reduced by annealing. Thus, optical bandgap behavior for these samples were mostly affected by quantum confinement rather than BM effects. Similar results have been reported previously: Sengupta et al. found the bandgap grew as Al-ZnO concentration increased for the same annealing temperature, but bandgap reduced with increasing heat treatment temperature at the same dopant concentration [47]. Irimpan et al. showed that the optical band for ZnO thin films reduced with increasing annealing temperature [48].


## 3.5. Photoluminescence

Figures5(c) and 5(d) show PL spectra for ZnO and Si-ZnO thin films before and after annealing. The films generally exhibit two significant peaks in the visible region: one high intensity PL peak at 382 nm (near band edge, NBE), and another broad emission band around 500 nm (deep level emission, DLE), due to VO and/or Zni in the ZnO crystal, respectively [49, 50].Annealed samples initially exhibit higher NBE peaks than as-dep. samples, since film crystalline quality influences NBE emission, implying that the crystallinity of the annealed films have improved [51]. However, the NBE peak reduced with increasing Si-doping concentration because thin film crystallinity reduced with increasing Si ions in the ZnO crystal lattice [20]. Also, the BM effect could explain the NBE shift toward shorter wavelengths [52]. These results are consistent with XRD and optical results, discussed in Sections 3.1 and 3.4, respectively.FiguresS5(a) and (b) (SI) show relative peak intensity IRel to help confirm DLE effects for Si doping and annealing [51, 53]:
(8)IRel=IDLE/INBE,where IDLE and INBE are DLE and NBE emission intensities, respectively. The relative peak strength close to the green emission range increased with Si-doping concentration, hence VO increased with increasing Si doping. However, the increase rate for VO in annealed samples was less than for as-dep. samples, which is confirming that the competition between increased VO due to Si doping and reduced VO due to oxygen annealing.


## 3.6. Device Properties

Figures1(a) and 1(b) show layered and top views for the fabricated TFT using a bottom-gate (BG) structure with heavily doped p-type Si substrates as a common-gate electrode and 100 nm thermally grown SiO2 as a gate insulator. TFT featured with W/Lratio=100/5 was used for characterization purposes. TFTs’ performance parameters were determined using the standard MOSFET equation [54]:
(9)Ids=W2LμsatCoxVgs−Vth2,where W, L, Cox, and Vgs are channel layer width and length, gate oxide capacitance per unit area, and gate-source voltage, respectively. Subthreshold swing (SS) was calculated as follows [55]:
(10)SS=dlogIdsdVgs−1,at the steepest point of the transfer slope. Table 4 shows extracted TFT performance parameters for different doping levels. The derived values from this study are comparable with previous study results [56–58].Table 4
Extracted device parameters from annealed ZnO and Si-ZnO samples with various Si concentrations for the current and previous studies.
ConditionVth [V]μFE [cm2/V·s]SS [V/decade]Ion/IoffDit_max [cm−2 eV−1]ZnO18.911.726.637.22E+052.42E+13Si-1/5015.611.364.011.39E+051.46E+13Si-1/4010.362.552.688.82E+059.66E+12Si-1/307.892.612.161.39E+067.74E+12Si-1/203.445.221.704.41E+056.05E+12Si-1/100.682.910.677.00E+052.25E+12Lee et. al0.800.700.281.66E+07—Liu et. al18.501.022.64.00E+07—Yang et. al3.405.100.603.20E+061.90E+12Figures6(a)–6(d) show ZnO TFT transfer curves using annealed samples, for gate voltage Vgs=−50 to +50 V with drain voltage Vds=0.5V. The device made by as-dep. samples (Figure 6(a)) showed a horizontal transfer curve of the metallic properties (i.e., no device characteristics) due to its relative highly conductivity in the channel layer allowing better electrons flow to traverse the source and drain. Ioff is related to channel layer conductivity [59]:
(11)Ioff=σWLVds∙tch,where σ is electrical conductivity for the channel layer and tch is the active layer thickness. Calculated conductivity from Hall measurements reduced from 1.63 S/cm for as-dep. sample to 0.39 S/cm for annealed sample after the annealing treatment. The reduced conductivity decreased Ioff, confirming the device’s characteristics. The final device achieved n-channel enhancement, as evidenced by the requirement for positive gate voltage to induce a conducting channel [60].Figure 6
Transfer curves for ZnO thin-film transistors with increasing thermal treatment time oxygen atmosphere: (a) as-dep., (b) 0.5 hour at 300°C, (c) 1 hour at 300°C, and (d) 1 hour at 400°C.
(a)(b)(c)(d)Table4 shows that the device threshold voltage (Vth) decreased with increasing Si doping from 18.91 to 0.68 V. The donated electron from Si tail states contributes to the device current with increasing Si-doping concentration [61]. Furthermore, the VO induced from Si doping generated more free electrons, which also contribute to the device current. The device’s field effect mobility (μFE) improved from 1.72 to 5.22 cm2/V·s until the Si-1/20 doping condition and decreased back to 2.91 cm2/V·s in the Si-1/10 condition. As already mentioned, it is well known that the charge mobility for metal oxide devices increases with increasing carrier concentration. As the EF of the thin film is uplifted into the conduction band due to the BM effect by Si doping, the energy barrier height existing in the conduction band is lowered, and the mobility of electrons in the band is increased without the scattering between the energy barriers, which can be attributed to the percolation conduction [62]. That is, the additional electrons from the subgap state as SiZn and VO2+ with increasing Si doping elevated the device current, and finally, the Vth decreased, and the μFE increased. However, as you can see in Si-1/10 condition, the increase in mobility is finally limited by ionized impurity scattering [23, 63].Figure7 shows transfer curves for TFTs fabricated from annealed ZnO and Si-ZnO (1/50, 1/40, 1/30, 1/20, and 1/10) films. The subthreshold slope becomes steeper with increasing Si doping, i.e., subthreshold swing (SS) reduced. The small swing value is important for the device in terms of power and energy saving, which means that the device can turn on faster [64]. The interfacial trap density (Dit) may be calculated using SS from the following equation [65]:
(12)Dit=SS∙logekBT/q−1Cq,where e is the natural constant, kB is Boltzmann’s constant, T is the absolute temperature, q is the electronic charge, and C is the insulator capacitance per unit area, respectively. The calculated Dit values were shown in Table 4. It can be seen that the interfacial trap density was effectively reduced. For further verification of reduction for interfacial trap density, the hysteresis of the transfer curves was measured under gate voltage sweeping from -50 V to 50 V back and forth for ZnO and Si-ZnO 1/10 cases, which is shown in Figure S6 (SI). The large difference of hysteresis around the subthreshold region in ZnO indicates the high trap density between the insulator and active layer, while it is suppressed in Si-ZnO 1/10, which means that the device drives faster and has an energy-saving effect [66].Figure 7
Transfer curves for ZnO and Si-ZnO thin-film transistors (TFTs) with increasing Si-doping concentrations: (a) ZnO, (b) Si-1/50, (c) Si-1/40, (d) Si-1/30, (e) Si-1/20, and (f) Si-1/10 samples.
(a)(b)(c)(d)(e)(f)Figure8 shows the energy band diagram with BM effect in the channel/dielectric interface for Si-ZnO TFTs. The density of states (gE) and Fermi-Dirac distribution (fFE) relationship can be expressed as follows [54]:
(13)NEgE=fFE=11+expECBE−EF/kBT,where NE, ECBE, EF, kB, and T are the distributed electron count, conduction band edge, Fermi energy, Boltzmann’s constant, and absolute temperature, respectively. Thus, carrier concentration is dependent on (ECBE-EF), where smaller difference improves concentration up to saturation (ECBE « EF). This effect is similar to BM effect derived from Si doping, i.e., since the Si tail state forms under the conduction band edge, continually increasing carrier concentration will promote EF into the conduction band at the gamma (Γ) position in the reciprocal lattice for direct semiconductors. Therefore, EF lies above the conduction band [67]. The device is accumulated with electrons when the bottom of the conduction band edge is filled with charge carriers. These accumulated electrons contribute to Ion and induce increased μFE, followed by negative Vth shift applying the gate voltage.Figure 8
Energy band diagram showing BM effect for Si-ZnO TFTs: (a) energy level distribution in density of states and band diagram for annealed (b) ZnO and (c) Si-ZnO samples.
(a)(b)(c)


## 4. Conclusion

This study investigated fabricated transistors employing ZnO and Si-ZnO thin films produced with ALD and doping influences after annealing. XRD scans confirmed that the ZnO films all contained hexagonal wurtzite structures, with no preferred orientation change for Si doping, and crystallinity improved with annealing. Si is a multivalence donor and hence can exist in various oxidation states, including Si4+, Si3+, and Si2+, but this study confirmed that it primarily occurs as Si3+, which donates one electron to the matrix. Si doping also generated VO in the thin film, resulting in donating one electron due to environmental thermal stress. These increased charge carrier concentrations induced the BM effect. Consequently, Si achieves superior electron donation efficiency compared with other dopants. However, optical band gap in annealed Si-doped ZnO is narrowed due to quantum confinement. This study confirmed that these band gap behaviors could be explained by interactions between bandgap narrowing from annealing and widening from BM effects. These results are strongly consistent with many previous Hall, XPS, UV-vis, and PL investigations. Optimized devices were operated as an enhancement mode, and the improved device parameters from the reduced interfacial trap density and the increased μFE by the percolation conduction would help to realize the low power and energy saving devices/applications. Advanced multivalence material research and commercialization, including Si and indium-free alternatives, are expected to result from the outcomes identified in this study.



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*Source: 7261186-2023-03-07.xml*