Enhanced Photocharacteristics by Fermi Level Modulating in Sb2Te3/Bi2Se3 Topological Insulator p–n Junction

Abstract Topological insulators have recently received attention in optoelectronic devices because of their high mobility and broadband absorption resulting from their topological surface states. In particular, theoretical and experimental studies have emerged that can improve the spin generation efficiency in a topological insulator‐based p–n junction structure called a TPNJ, drawing attention in optospintronics. Recently, research on implementing the TPNJ structure is conducted; however, studies on the device characteristics of the TPNJ structure are still insufficient. In this study, the TPNJ structure is effectively implemented without intermixing by controlling the annealing temperature, and the photocharacteristics appearing in the TPNJ structure are investigated using a cross‐pattern that can compare the characteristics in a single device. Enhanced photo characteristics are observed for the TPNJ structure. An optical pump Terahertz probe and a physical property measurement system are used to confirm the cause of improved photoresponsivity. Consequently, the photocharacteristics are improved owing to the change in the absorption mechanism and surface transport channel caused by the Fermi level shift in the TPNJ structure.


Device fabrication
The characteristics of a single thin film and the junction structure could be compared under the same conditions according to the source-drain metal contact.A cross pattern was devised, as shown in Figure S1, to confirm only the junction effect at the interfaces, excluding external influences.The device was fabricated as follows: First, the SiO2 layer on the SiO2/Si substrate was etched using photolithography to deposit Bi2Se3 with an etching depth of 10 nm corresponding to approximately 10 QL (1 QL = 0.954 nm) Bi2Se3 thin film.The Bi2Se3layer was then grown on a SiO2/Si substrate, and a selenium capping layer was deposited to eliminate atmospheric effects.Photolithography and plasma etching were used to fabricate the Bi2Se3 channel, and a Sb2Te3 channel was grown on the Bi2Se3 channel using photolithography.The annealing temperature was optimized to prevent intermixing in the stacked structure and crystallize the Sb2Te3 layer above the Bi2Se3 layer.After completing the cross pattern, a SiO2 capping layer was deposited using sputtering to exclude oxidation.This process effectively prevented height differences during subsequent Sb2Te3 deposition, creating a high-quality topological insulator junction structure.

Interface analysis
The STEM analysis confirmed the formation of a well-defined interface.A comparison of the intensity in the line profile analysis enabled the distinction between the cases with and without mixed layers.In Figure S2a, where the interface is well separated, the boundary is clearly visible in the line profile, as shown in Figure S2c  Although it was confirmed that no intermixing layer exist at the interface, we confirmed changes in the chemical bonding state at the interface, influencing the junction effect through angle-resolved X-ray photoelectron spectroscopy (AR-XPS).AR-XPS measurements can confirm changes in the chemical bonding state at the Bi2Se3 and Sb2Te3 interfaces because the information according to the depth of the sample appeared depending on the angle at which the X-rays were incident on the sample.As the X-rays were incident vertically, deeper depth information appeared, and information on Bi2Se3 and Sb2Te3 was mixed.When the X-rays were incident closer to 0°, information on Sb2Te3, which is surface information, appeared dominantly.The chemical bonding states at the interfaces were confirmed through angledependent changes in the binding energy.The core-level XPS spectra of Sb2Te3/Bi2Se3 were obtained as a function of the incident angle from 0 to 85°, as shown in Figure S3a-d.The peak positions of all spectra were calibrated using the reference carbon peak at 284.8 eV; each peak was deconvoluted to observe the chemical bonding state clearly.The Bi core-level spectra of Bi2Se3/Sb2Te3 show the characteristic doublet representing Bi 4f5/2 and 4f7/2 orbitals, with peaks at 163.3 eV and 158 eV in Figure S3a, respectively, which is consistent with previously reported data.The Se core-level spectra also show doublet peaks of Se 3d3/2 and 3d5/2 at 54.25 and 53.4 eV, respectively, in Figure S3b.The Sb core-level (538.5 eV for Sb 3d3/2 and 529.2 eV for 3d5/2) and Te core-level spectra (583.1 eV for Te 3d3/2 and 572.7 eV for 3d5/2,) were also consistent with previous reports in Figure S3c-d.It was confirmed that no shift in the binding energy occurred due to the angle change or new chemical bonding.Therefore, the intermixing did not occur between the two materials at the interface, and the junction structure was effectively fabricated through heat treatment control.As shown in Figure S3e, the valence band maximum shifted by approximately 0.26 eV.That is, there was no change in the chemical bonding at the interfaces, but band bending, which improved electron-hole separation, occurred.However, when annealed at 180°, a binding-energy shift occurred in the Bi and Se atoms when using a low-angle incident beam, as shown in Figure S4.Additionally, the change in VBM decreased from 0.26 to 0.11 eV compared to when a clean interface was formed through annealing at 170°.This suppresses electron-hole separation and reduces the optoelectronic properties.The XRD, Raman, STEM, and AR-XPS results confirmed that a Sb2Te3/Bi2Se3 topological p-n junction structure was effectively formed without intermixing by controlling the annealing conditions.where Ψ is the digamma function, e is the electronic charge, and h is Planck's constant.The characteristic magnetic field is represented by , which is determined by the phase coherence length () and coefficient (⍺) that characterize the localization type.The applied magnetic field is denoted as H.In topological insulators, ⍺ is related to the surface channel and equals 0.5 when there is a single surface channel.Figure S7a shows that the ⍺ = 0.74 for Bi2Se3 as the top and bottom surface channels partially hybridize each other.In Sb2Te3 with 2 QLs deposited, the top surface channel of Sb2Te3 does not exist independently and hybridizes with the bottom surface channel of Bi2Se3.Since there is no topologically different surface channel at the interface between Bi2Se3/Sb2Te3, there is no surface channel.As Sb2Te3 thickness increases and an independent surface channel is formed on the top surface, ⍺ becomes 0.5; the overall ⍺ value becomes 1 due to the top and bottom surface channel of Bi2Se3, as shown in Figure S7d.

Photo-characteristics of Sb2Te3/Bi2Se3
The photocurrent results were analyzed as a function of the incident power, wavelength, and thickness.The photocurrent was found to increase with increasing power, but the dependence on the wavelength and thickness varied.As the thickness increased to 10 QLs, the photocurrent initially increased and then decreased.Furthermore, as the thickness increased, the difference in the photocurrent intensity as a function of power became more distinct, indicating a clear dependence on power.In addition, when fitted with a power law, the theta value increases, indicating an increase in the electron-hole separation.

Role of defect in TPNJ structure
Densitty functional theroy (DFT) calculations were performed on each model, namely Bi2Se3, Sb2Te3, and Bi2Se3/Sb2Te3, to assess the influence of defects.Our focus was on Se and Te vacancies while prioritsing energetically stable defects in the formation process.Prior studies (Superlattices and   Microstructures 2018, 120, 48-53, and PRL 2012, 108, 066809) were consulted as guides for our work.
A 2 × 2 × 1 supercells were constructed based on the optimized structures of 5 QLs Bi2Se3, 5 QLs Sb2Te3, and 5 QLs Sb2Te3/Bi2Se3 5 QLs, as well as 2 QLs Sb2Te3/Bi2Se3 5 QLs.Supercell calculations employed a 3 × 3 × 1 grid of k-points and 500 eV cut-off energy.Geometry optimizations were performed for supercells containing defects until 0.05 eV/Å condition was satisfied.Subsequently, electronic structures of the supercells were calculated, considering spin-orbit coupling (SOC).Band  The isosurface level has been fixed at 5x10 -12 e/Å 3 because variations in electron density can occur based on the isosurface level value.The band index information for each wavefunction is provided in Figure S14.
As a result, we identified deformed wavefunctions mainly located in the Bi2Se3 region due to defect states.This suggests that the defect state has little influence on the interaction between the wavefunction of Bi2Se3 and Sb2Te3.Furthermore, despite the reduction in wavefunction distribution due to defects, it is noteworthy that in thicker regions, similar to the initial analysis, the inter-state and Sb2Te3 wavefunctions remain separate and distinct.Therefore, even when considering the influence of defects, the absorption contributions of the surface states and channel separation continue to occur, indicating an enhancement in optical properties."S1 reveals that the reported responsivity in the previously studied TPNJ structures is approximately 0.146 A/W, which is lower than the achieved 0.34 A/W in our study. [29]Furthermore, the optical characteristics of the TPNJ structure exhibit improvements co individual topological insulators.In photodetectors, responsivity can vary significantly depending on factors such as light intensity, wavelength, voltage, and more. [49]Therefore, evaluating the significance of research based solely on performance metrics can be challenging.Nonetheless, our research remains significant as it allows for a specific and direct comparison of the characteristics of each channel within a single device.This enables us to quantitatively assess the improvement achieved in the Bi2Se3/Sb2Te3 structure compared to individual Bi2Se3 and Sb2Te3 materials.As a result, we can confirm that the responsivity has improved by more than six-fold in our study when compared to the individual materials.Our research primarily acknowledges the potential of the TPNJ structure, as well as serves as a pioneering investigation with the significance of understanding the contribution of topological surface states in TPNJ structures, which can find applications in the field of opto-spintronics.To delve more deeply into the information of not only the bulk but also the surfaces and interfaces, additional calculations of the band structure for each surface and interface have been conducted and incorporated as depicted in Figure S14a.This allows for a more detailed examination and separation of the band structures at the surfaces and interfaces.Based on the band structure calculations at the positions indicated in Figure S14a, the results are depicted in Figures S14b-d.The Fermi level is positioned within the surface bands on the bottom surface of Bi2Se3 and the top surface of Sb2Te3, respectively.Based on these results, it is evident that the increased absorption efficiency may be attributed to the absorption effects of the surface bands according to the Fermi level's position.

Figure S1 .
Figure S1.Schematic illustration of the device fabrication process for the Sb2Te3/Bi2Se3 topological insulator junction . However, an intermediate layer appears for S2b, where the interface is mixed, as shown in S2d.The intermediate layer consists of two layers: the first layer is the Se-rich SbSeTe, and the second layer is the Sb-rich SbSeTe.This was confirmed through intensity comparisons during line profile analysis.

Figure S2 .
Figure S2.STEM images and corresponding line profiles of the interfaces between Bi2Se3 and Sb2Te3 layers.The line profiles reveal a clear distinction between the layers with a welldefined interface (Figure S2a,c) and those with intermixing at the interface (Figure S2b,d).

Figure S3 .
Figure S3.Results of angle-resolved X-ray photoelectron spectroscopy in Sb2Te3/Bi2Se3 at 170 o C annealing temperature (a) Bi 4f, (b) Se 3d, (c) Sb 3d, (d) Te 3d, (e) change in valence band maximum (VBM) depending on the angle of incidence.The schematic shows the angle of incidence

Figure S13 .
Figure S13.The band structure of (a) Sb2Te3 2 QLs and (b) Sb2Te3 5 QLs on Bi2Se3 5 QLs with a defect state.The upper panels illustrate the spatial wavefunction distribution extracted from the band structure

Figure S14 .
Figure S14.Information regarding the band index of the wavefunctions.

Figure
Figure S16.(a) Schematic image of the calculated band structure for the specific region.The extracted band structures at the positions indicated in Figure S14 (a) for Bi2Se3 5QLs / Sb2Te3 5QLs.(b) Bi2Se3 surface, (c) Bi2Se3 interface, (d) Sb2Te3 interface, and (e) Sb2Te3 surface.