Enhanced electrical properties of amorphous In-Sn-Zn oxides through heterostructuring with Bi2Se3 topological insulators

Amorphous indium tin zinc oxide (a-ITZO)/Bi2Se3 nanoplatelets (NPs) were fabricated using a two-step procedure. First, Bi2Se3 NPs were synthesized through thermal chemical vapor deposition at 600 °C on a glass substrate, and then a-ITZO was deposited on the surface of the Bi2Se3 NPs via magnetron sputtering at room-temperature. The crystal structures of the a-ITZO/Bi2Se3 NPs were determined via X-ray diffraction spectroscopy and high-resolution transmission electron microscopy. The elemental vibration modes and binding energies were measured using Raman spectroscopy and X-ray photoelectron spectroscopy. The morphologies were examined using field-emission scanning electron microscopy. The electrical properties of the a-ITZO/Bi2Se3 NPs were evaluated using Hall effect measurements. The bulk carrier concentration of a-ITZO was not affected by the heterostructure with Bi2Se3. In the case of the Bi2Se3 heterostructure, the carrier mobility and conductivity of a-ITZO were increased by 263.6% and 281.4%, respectively, whereas the resistivity of a-ITZO was reduced by 73.57%. This indicates that Bi2Se3 significantly improves the electrical properties of a-ITZO through its heterostructure, expanding its potential applications in electronic and thermoelectric devices.

In addition to the aforementioned AOS materials, amorphous indium tin zinc oxide (a-ITZO) has attracted considerable attention owing to its advantageous characteristics, including large carrier mobility and high carrier concentrations 12,13 .The primary factor contributing to the high electron carrier mobility within the conduction-band minimum of a-ITZO is the increasing overlap area between the orbitals of In 5 s and Sn 5 s.These orbitals possess strong divergence, high symmetry, and an electronic configuration similar to that of (n-1)d 10 n 0 (n ≥ 4) [14][15][16] .In addition to the improved carrier mobility, the carrier concentration of a-ITZO is increased.This is achieved by substituting lattice In 3+ ions with Zn 2+ and Sn 4+ ions, which form acceptor defects of Zn 2+ In 3+ ′ and donor defects of Sn 4+ In 3+ • .The presence of Zn 2+ induces lattice distortion through the Jahn-Teller effect, leading to the formation of oxygen vacancies ( V 0 O 2−
Rhombohedral bismuth selenide (Bi 2 Se 3 ) is a direct n-type topological insulator with a narrow band-gap of 0.35 eV 32 .The bulk structure of Bi 2 Se 3 consists of five stacked atomic layers, i.e., Se 1 − Bi 1 − Se ′ 1 − Bi 1 − Se 1 , and is referred to as a quintuple layer (QL) 33 .Within the QLs, covalent bonds between Se and Bi dominate, whereas van der Waals (vdW) forces govern the bonding between QLs 34 .Bi 2 Se 3 is a unique material because of its insulating bulk state and gapless conducting surface state, which are attributed to the spin-orbital coupling (SOC) and time-reversal symmetry (TRS) 35,36 .These properties prevent the surface backscattering effect caused by non-magnetic impurities, resulting in efficient electron transport at the surface 37,38 .Bi 2 Se 3 exhibits a high electron carrier mobility of up to 600 cm 2 V −1 s −139 .Consequently, Bi 2 Se 3 , with its gapless conducting surface state, has several notable features, including (a) photon-like and spin-polarized electrons, (b) a low power dissipation rate, and (c) the quantum spin Hall effect 34,[39][40][41] .
In this study, Bi 2 Se 3 NPs were fabricated on a glass substrate via thermal chemical vapor deposition (CVD), followed by the deposition of a-ITZO via magnetron sputtering.Subsequently, the electrical properties of the ITZO/Bi 2 Se 3 NPs, including the bulk carrier concentration, carrier mobility, resistivity, and conductivity were analyzed via Hall effect measurements at room-temperature.

Surface morphologies
Figure 3a-d show FESEM images of Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs before and after annealing at 250 °C.They exhibit a hexagonal shape, as observed in the HRTEM images of Fig. 2c,d.The average thicknesses of these NPs were estimated using the ImageJ software, and the results indicated that each Bi 2 Se 3 NPs had a thicknesses of approximately 71.8 nm (divided by 20 pieces) and 86.8 nm (divided by 20 pieces) before and after annealing at 250 °C, respectively.The thickness of each QL was approximately 0.955 nm 46 .Thus, the average number of QLs was 83 for each pristine Bi 2 Se 3 NPs.On average, each ITZO/Bi 2 Se 3 NPs had thicknesses of 247.3 and 232.5 nm before and after annealing at 250 °C, respectively.These results explain why the ITZO/Bi 2 Se 3 NPs shown in Fig. 2e is not transparent.The thickness of the covered ITZO thin films was estimated to be approximately 160 nm.Photographs of the Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs before annealing at 250 °C are presented in the insets of Fig. 3a,b.Their colors differed significantly; the Bi 2 Se 3 NPs were grey, whereas the ITZO/Bi 2 Se 3 NPs were yellowish-green.The cross-sectional SEM images in the insets of Fig. 3c,d indicate the total deposition thicknesses of the Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs after annealing at 250 °C, which were approximately 2.2 and 2.5 μm, respectively.The SEM-EDS results for the Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs are presented in Fig. S2a,b, which confirm the existence of Bi, Se, In, Sn, and Zn.

Vibration modes
Figure 4a,b show the Raman spectra of the ITZO thin film and Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs before and after annealing at 250 °C.The Raman spectrum of ITZO before and after annealing at 250 °C exhibited a broad peak in the range of 300-700 cm −1 , which was centered at approximately 563.01 and 587.78 cm −1 , respectively.The Bi 2 Se 3 NPs exhibited three significant vibration modes of E 2 g , A 2 1g , and Se-Se bonds 47,48 before annealing at 250 °C at wavenumbers of 126.69, 170.51, and 248.49cm −1 , respectively, as shown in Fig. S3a.In addition, the ITZO/ Bi 2 Se 3 NPs exhibited two significant modes of E 2 g and A 2 1g at 128.99 and 170.64 cm −1 before annealing at 250 °C, as shown in Fig. S3b, while the Se-Se mode was suppressed.After annealing at 250 °C, the Bi 2 Se 3 NPs exhibited the same vibration modes ( E 2 g , A 2 1g , and Se-Se at 122.37, 165.90, and 244.67 cm −1 , respectively, as shown in Fig. S3c) as before the annealing at 250 °C, as did the ITZO/Bi 2 Se 3 NPs ( E 2 g and A 2 1g at 124.66 and 166.67 cm −1 , respectively, as shown in Fig. S3d).
Bi 2 Se 3 has a layered crystal structure, as shown in the inset of Fig. 4a 49 , where each layer comprises five monoatomic layers, i.e., Se 1 − Bi 1 − Se ′ 1 − Bi 1 − Se 1 ; therefore, it is called a quintuple layer (QL).Covalent bonds dominate the bonding within the QL, and the vdW force connects the QLs 32 .E 2 g is a Raman active mode, i.e., the in-plane symmetric bending mode associated with the shearing of the upper/lower Se 1 − Bi 1 bond in the opposite vibration direction.A 2 1g is a Raman active mode similar to E 2 g and represents the out-of-plane symmetric stretching of the upper/lower Se 1 − Bi 1 bond in the opposite vibration direction 34,47 .The Se-Se vibration mode is assigned to the in-plane vibration of the topmost hexagonal network of Se atoms in Bi 2 Se 3 layered structures 48,50 .Therefore, the Se-Se vibration mode was observed in the Bi 2 Se 3 NPs at 248.49 and 244.67 cm −1 before and after  www.nature.com/scientificreports/annealing at 250 °C, respectively.The Se-Se vibration mode of the Bi 2 Se 3 NPs was suppressed after the NPs were covered with the ITZO thin film, as shown in Fig. 4a (before annealing) and b (after annealing), implying that the topmost Se 1 atoms in the Bi 2 Se 3 layered structure bonded with the ITZO thin film, suppressing the in-plane Se-Se vibration.These results confirmed that ITZO/Bi 2 Se 3 NPs were successfully fabricated.

Binding energies
Figure 5a-d show the X-ray photoelectron spectra (XPS) of the ITZO thin film for the In 3d, Sn 3d, Zn 2p, and O 1 s orbitals, respectively.In Fig. 5a, the spectrum is split into two peaks of In 3d 5/2 and In 3d 3/2 at 443.63 and 451.18 eV with an energy difference of 7.55 eV, indicating that In mainly existed in a trivalent form (In 3+ ) in the In 2 O 3 lattice 51 .In Fig. 5b, the spectrum for the Sn 3d orbital is split to 485.29 and 493.74 eV peaks, which correspond to Sn 3d 5/2 and Sn 3d 3/2 , respectively, indicating the presence of tetravalent Sn (Sn 4+ ) in the SnO 2 lattice 52 .The broad peak centered at 496.72 eV near Sn 3d 3/2 is related to the Sn-loss signal (Sn loss ) 52 , implying that the ITZO thin film had a high conductivity 53 .In Fig. 5c, the splitting of spin-orbit doublets of Zn 2p 3/2 and Zn 2p 1/2 is observed at energy of 1021.08 and 1044.25 eV, which is assigned to the divalent zinc (Zn 2+ ) in the ZnO lattice 54 .In Fig. 5d, the O1s orbital is deconvoluted into two peaks at approximately 528.89 and 530.73 eV.The former is related to the oxygen bonded with the metal forming the metal-oxygen (M-O) bonds in the metallic oxides, whereas the latter is attributed to the chemisorbed oxygen (O chemi : O 2 − , O − etc.) on the nanostructure surface 52,54,55 .These results confirmed that Sn 4+ and Zn 2+ replaced In 3+ in the lattice.
Figure 6a-c show the XPS spectra of the Bi 2 Se 3 NPs.In Fig. 6a, four peaks are observed at 158.12, 163.43, 159.68, and 164.06 eV, respectively.The first two peaks are attributed to the Bi 4f 7/2 and Bi 4f 5/2 orbitals of Bi 3+ in the Bi 2 Se 3 lattice, whereas the last two peaks are assigned to the Bi 4f 7/2 and Bi 4f 5/2 orbitals of Bi 3+ in the Bi 2 O 3 lattice 34 .In Fig. 6b, a broad peak is deconvoluted into peaks at 53.51 and 54.18 eV, respectively.The former peak is related to the divalent Se (Se 2-) of Se 3d 5/2 in the Bi 2 Se 3 lattice, whereas the latter is ascribed to Se 3d 3/256 .In Fig. 6c, O 1 s peaks are observed at 530.23 and 532.52 eV.The former peak is related to the oxygen binding energy of the metal-oxygen bonds in the metal-oxide lattice, and the latter peak corresponds to the chemisorbed oxygen on the nanostructure surface.
Figure 7 presents the In 3d (Fig. 7a), Sn 3d (Fig. 7b), Zn 2p (Fig. 7c), O 1 s (Fig. 7d), Bi 4f. (Fig. 7e), and Se 3d (Fig. 7f) XPS spectra of the ITZO/Bi 2 Se 3 NPs.As shown in Fig. 7a, the In 3d spectrum is split into two peaks at approximately 443.85 and 451.41 eV, implying the splitting of In 3+ into the In 3d 5/2 and In 3d 3/2 orbitals in the In 2 O 3 phase.The Sn 3d orbital presented in Fig. 7b shows three peaks at 485.62, 493.99, and 496.91 eV.The first two peaks are attributed to Sn 3d 5/2 and Sn 3d 3/2 , indicating the presence of Sn 3+ in the SnO 2 lattice.The last peak is assigned to Sn loss , which is easily detected in highly conductive materials such as ITZO.The Zn 2p peak is observed two peaks located at 1021.04 and 1044.13 eV, as shown in Fig. 7c.The former peak was ascribed to Zn 2p 3/2 , and the latter peak corresponded to Zn 2p 1/2 , implying the presence of Zn 2+ in the ZnO lattice.The O 1 s peak was deconvoluted into two peaks at 529.20 and 530.22 eV, which were related to the metal-oxygen bonds in the metal-oxide lattices and the chemisorbed oxygen on the nanostructure surface, respectively, as shown in Fig. 7d.The signal intensities of Bi 4f. and Se 3d were reduced owing to the ITZO covering, as shown in Fig. 7e,f.Relevant Bi 2 Se 3 lattice peaks of Bi 4f 7/2 (159.11eV), Bi 4f 5/2 (163.58 eV), and Se 3d 5/2 (53.67 eV) were still detected.In addition, the related Bi 2 O 3 peaks of Bi 4f 7/2 at 160.45 eV and Bi 4f 5/2 at 165.56 eV were observed, even when the Bi 2 Se 3 NPs were covered with the ITZO thin film.The samples were stored in an ambient environment; therefore,

Proposed mechanism for the enhanced carrier mobility in ITZO/Bi 2 Se 3 NPs
Bi 2 Se 3 is identified as an n-type semiconductor with a narrow band-gap of 0.35 eV, and its Fermi level resides within its conduction band 32,57 .On the other hand, ITZO is characterized as an n-type semiconductor with a broad band-gap of 3.40 eV 58 .The band structure at the interface undergoes bending upon the formation of the heterostructure between ITZO and Bi 2 Se 3 .The band diagram of the ITZO/Bi 2 Se 3 NPs is depicted in Fig. 9, with Fig. 9a illustrating the band diagram before the contact of ITZO and Bi 2 Se 3 in a thermal equilibrium state.Here, E VAC represents the vacuum level, E C is the conduction band level, E F is the Fermi energy level, and E V is the valence band level.Figure 9b shows the band bending after the contact of ITZO and Bi 2 Se 3 in a thermal equilibrium state, leading to the migration of electrons (e -) and holes (h + ) from ITZO to Bi 2 Se 3 .The primary carrier (e -) concentrations progressively increase from the ITZO surface to the interface between ITZO and Bi 2 Se 3 due to the migration of electrons from ITZO to Bi 2 Se 3 .Additionally, defects such as dislocations and impurities at the ITZO/Bi 2 Se 3 interface could contribute to decreased carrier concentrations at the ITZO surface of the ITZO/ Bi 2 Se 3 NPs 59 .The carrier mobility, determined through Hall effect measurements, can be estimated using the formula 60 , µ H = σ en H , where µ H is the carrier mobility, e is the free electron charge, n H is the carrier concentra- tion.Therefore, σ and n H directly affect the µ H .The higher σ corresponds to a the larger µ H , while a lower n H results in a larger µ H .As discussed above, the σ in ITZO/Bi 2 Se 3 is significantly higher than that in pure ITZO.Moreover, the extrinsic properties following the contact of ITZO with Bi 2 Se 3 lead to a decrease in the concentration of primary charged carriers (e -) from the ITZO/Bi 2 Se 3 interface to the ITZO surface.Simultaneously, the minor charged carriers (h + ) in ITZO migrate to Bi 2 Se 3 , indicating that e -in ITZO/Bi 2 Se 3 NPs exhibits a longer lifetime than that in pure ITZO due to the low recombination rate between e -and h + .Therefore, the synergetic effect after the contact between ITZO and Bi 2 Se 3 enhances the electrical properties of the ITZO/Bi 2 Se 3 NPs.

Conclusions
ITZO/Bi 2 Se 3 NPs were synthesized on a pure glass substrate through a thermal CVD at 600°C and magnetron sputtering at room-temperature.XRD and HRTEM analyses confirmed the formation of crystalline Bi 2 Se 3 and disordered ITZO phases.FESEM images indicated that the average thicknesses of the Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs were approximately 79.3 and 239.9 nm, respectively.The Raman spectra indicated that the ITZO coverage suppressed the Se-Se vibration mode of the Bi 2 Se 3 NPs.XPS measurements revealed the elemental binding energies of the ITZO and Bi 2 Se 3 lattices, confirming that ITZO covered the Bi 2 Se 3 NPs, which was consistent with the HRTEM image of the ITZO/Bi 2 Se 3 NPs.Hall effect measurements of the electrical properties revealed that the bulk carrier concentration of ITZO did not affect through the heterostructure with Bi 2 Se 3 .However, the formation of the heterostructure with Bi 2 Se 3 increased the carrier mobility and conductivity of ITZO by 263.6% and 281.4%, respectively, and reduced the resistivity of ITZO by 73.57%.These results indicate that the electrical properties of ITZO can be significantly improved through the formation of a heterostructure with Bi 2 Se 3 owing to its gapless surface state, expanding the potential applications in electronic and thermoelectric devices of ITZO/Bi 2 Se 3 heterostructures.

Experimental Fabrication of ITZO thin films
The ITZO thin films were deposited on a glass substrate (20 × 20 × 7 mm 3 ) via magnetron sputtering under a base pressure of 6.0 × 10 −6 Torr.The substrate-target distance was 17 cm.The 3-inch ITZO target was composed of 85 wt% In 2 O 3 , 15 wt% SnO 2 , and 5 wt% ZnO.The deposition was performed at a power of 200 W for 750 s, with the introduction of Ar gas at 25 sccm under a working pressure of 3.0 × 10 −3 Torr.

Figure 3 Figure 1 .
Figure 2a shows a low-magnitude TEM image of a 270-nm-thick ITZO thin film deposited on a pure glass substrate after annealing at 250 °C.The inset presents the HRTEM-selected-area electron diffraction (SAED) pattern of ITZO, indicating an amorphous structure.The SAED pattern exhibits two significant rings with inner and outer d-spacings of 0.283 and 0.165 nm, respectively.The ITZO used in this study was composed of 85 wt% In 2 O 3 , 10 wt% SnO 2 , and 5 wt% ZnO, indicating that the In 2 O 3 was the host material.Sn 4+ and Zn 2+ ions prefer to replace In 3+ ions at the b-and d-sites in the In 2 O 3 lattice 11 .Hence, the d ITZO -spacings of 0.283 and 0.165 nm were related to the In 2 O 3 (321) and In 2 O 3 (611) planes (JCPDS 71-2195), respectively.The former result is agreed with the XRD results as shown in Fig. 1. Figure 2b shows an HRTEM image of ITZO indicating a disordered lattice with estimated d ITZO -spacings of 0.111 and 0.109 nm, which correspond to the In 2 O 3 (833) and In 2 O 3 (248)

Figure 3 .
Figure 3. FESEM images of the Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs of (a,b) before, and (c,d) after annealing at 250 °C, respectively.The insets in (a,b) show the photographs of the Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs before annealing at 250 °C, respectively.The insets in (c,d) are the cross-section images of Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs after annealing at 250 °C.

Figure 4 . 47 .
Figure 4. Raman spectra of ITZO thin film, and Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs (a) before and (b) after annealing at 250 °C.The inset shows the typical layered structure of Bi 2 Se 3 47 .

Figure 8 .
Figure 8. Bulk carrier concentration, resistivity, carrier mobility, and Conductivity of the ITZO thin film, and Bi 2 Se 3 and ITZO/Bi 2 Se 3 NPs after annealing at 250 °C.