Introduction

Topological insulators (TI) are one of the main classes of electronic material finding applications in next-generation high-speed devices1,2. The two members of this family, i.e., Bi2Se3 and Bi2Te3 have been studied extensively for their preparation, properties evaluation, and applications in bulk and film forms3,4. The surface states of these materials are topologically protected while bulk is insulating5,6. Efforts were made to effectively enhance the contribution of topological surface states towards conductance by reducing the thickness of the deposited film7,8. The decrease in film thickness causes the symmetry breaking along the z-axis due to loss in infinite crystal periodicity9. The symmetry breaking is also dependent on fabrication techniques (even in thick samples) and interaction with substrates (interfaces). It was observed by Wiesner et al.10, that the substrate interfaces also cause symmetry breaking due to interaction between Bi2Te3 and substrates. This makes the surface properties different from that of the from bulk even in samples with thickness ̴150 nm11,12. Studies have been reported for the Bi2Se3 based heterostructure for surface modulation due to interface effect. There have been reports where Bi2Se3 heterostructure are explored as potential photodetector application13,14,15,16.

The surface electronic states in Bi2Se3 are gapless and are characterized by a linear dispersion known as Dirac states. Angle-resolved photoemission spectroscopy (ARPES) measurements identify two Dirac cones, consist of occupied (SS1) and unoccupied (SS2) surface states, which are separated by an energy gap of ~1.8 eV17. The existence of such inter-band transition at the surface provides a way to distinguish optical transitions. The surface excitation of the charge carriers has different charge transfer dynamics from the bulk that is interesting to explore18. Several spectroscopy methods have been reported to study ultrafast dynamics of generated charge carriers in TI materials. Among them, more studied are transient reflectivity (TR) spectroscopy, TrARPES, SHG, etc. Our group has performed study on topological material based device optimization for transport and interface dynamics in Si/Bi2Te3 and Ge/Bi2Te319,20,21. Work related to the study of the transport property in p-Si/n-Bi2Te2Se heterojunction under illumination has also been recently reported by us22. TR spectroscopy was performed on Bi2Se3 nanoflakes at 10 K and 300 K by Jnawali et al.23 to study the thermalization and recombination dynamics over a wide energy range using 810 nm (1.5 eV) pump wavelength. This pump energy is directly interacting with the second surface state (SS2) at higher energy. Glinka et al.24 studied the ultrafast carrier dynamics by transient absorption (TA) spectroscopy of two-photon excited electron (~3.4 eV) in Bi2Se3 film using 730 nm (~1.7 eV) pump wavelength and probing in the visible region. They concluded that the carrier relaxation dynamics are highly influenced by the position of the Dirac point. When the energy gap between the Dirac cones is higher than phonon and resonance defect energy, the relaxation dynamics of the charge carrier are forced to be very slow. They also suggested for mid-IR probe range to study the ultrafast carrier relaxation dynamics in the lowest most Dirac surface states.

High quality Bi2Se3 films of low thickness are more commonly prepared by molecular beam epitaxy (MBE) and suffer with high cost and time consuming fabrication process25. In the present work we have fabricated the polycrystalline Bi2Se3 film of ~150 nm over n-GaN substrate using cost effective thermal deposition technique. GaN is a direct wide band gap semiconductor material with band gap of 3.4 eV15 having wurtzite structure [hexagonal, space group P63 mc (186)] matches well with crystal structure as of Bi2Se3 (rhombohedral, space group R-3m (166)) forming well aligned morphology. In this work, we have studied the GaN/Bi2Se3 interface contributing to the modulation of topological surface states in the n-GaN/p-Bi2Se3 topological heterojunction. To explain the bulk and heterojunction photo-generated carrier dynamics and charge transfer (CT) states formation, we have performed ultrafast femtosecond transient absorption spectroscopy study on Bi2Se3 films over a glass and GaN/Bi2Se3 heterojunction using 410 nm (3.024 eV) pump wavelength and probed in both visible and NIR region. The photoelectric effect of n-GaN/p-Bi2Se3 heterojunction was studied under the presence of optical light with a varying wavelength. The CT states were further investigated and quantified by the capacitance-voltage (C-V) measurements. The time constant and behavior were probed by conductance/radial frequency- radial frequency (Gp/ω−ω) plot. The study gives a significant insight and deeper understanding of CT states that are responsible for the topological surface states existence in thick Bi2Se3 topological insulator film in n-GaN/p-Bi2Se3 heterojunction.

Results and discussion

Comparative X-ray diffractogram of Glass/Bi2Se3 and n-GaN/Bi2Se3 with bulk Bi2Se3 was studied for the phases of the film grown Bi2Se3 TI film (refer. Supplementary Fig. 2). XRD peaks of films confirmed the polycrystalline rhombohedral lattice phase formation. Crystallinity of Bi2Se3 film was enhanced after annealing and peaks are shifted towards a lower angle compared to bulk. Glass/Bi2Se3 shows a higher shift compared to n-GaN/Bi2Se3 films due to substrate strain effect and strain coefficient was calculated to be 0.022 and 0.00408 over glass and n-GaN respectively. GaN substrate imposes a lesser local strain than glass due to close crystalline structure to Bi2Se3. The Hall measurement confirmed the charge carrier in the Bi2Se3 TI deposited film on a glass substrate to be p-type with charge carrier density ~3.19 × 1019/cm3 and mobility ~6.16 cm2/V-s respectively (refer. Supplementary Fig. 3). The values obtained are in good agreement with earlier reported values for the same26. Thickness of the film over glass was ~150 nm. Same conditions were used to deposit film over n-GaN substrates. Energy dispersive X-ray spectroscopy (EDS) confirmed the stoichiometry and SEM image confirmed the uniformity of films (refer. Supplementary Fig. 4a, b. AFM images of Glass/Bi2Se3 and n-GaN/Bi2Se3 are shown (refer. Supplementary Fig. 5). Surface topography of GaN/Bi2Se3 film and GaN/Bi2Se3 film had the root mean square roughness (RMS) of 10 nm and 14 nm respectively (refer. Supplementary Fig. 5).

Raman spectroscopy study

To study the phonon vibrations and electron-phonon interaction in Glass/Bi2Se3 and n-GaN/Bi2Se3, Raman spectroscopy was performed. As shown in Fig. 1, Raman active bands are visible in both the films. Bi2Se3 consists of 5 atoms per unit cell i.e., 15 zone center phonon branches (3 acoustic and 12 optical modes) in phonon dispersion relation. In 12 optical mode phonons, 4 are Raman active and 4 are IR active. Raman spectra show distinct optical phonon modes (A11g, E2g, A21g,) as summarized in Table 1. Raman shift and intensity are helpful to identify the quality of the interface. A slight shift of ~ 2-3 cm−1 towards lower frequency is observed in the heterostructure. As also inferred in XRD (refer Supplementary Fig. 2), GaN substrate is making a substantial effect on Bi2Se3 bond forces resulting in shift towards lower frequency. The peaks are Lorentz fit and data are summarized in Table 1. As can be seen, the full-width half maxima (FWHM) are highly increased in n-GaN/Bi2Se3 films for all the peaks. As also seen in MoS2/WS2 heterostructure, the frequency decreases by 2 cm−1 due to slightly increased interlayer distance and weaken interlayer coupling27. Raman data of GaN/Bi2Se3 fits well with five peaks (Table 1). Surface phonon mode (SPM) in n-GaN/Bi2Se3 heterojunction is observed at ~95 cm−1 along with the Raman forbidden A21u mode at ~162 cm−1. The appearance of A21u mode signifies the symmetry breaking of Bi2Se3 TI due to interface effect. Similar observation was also reported by Wiesner et al. for Bi2Te3/GaAs where A21u mode appeared in micro Raman analysis due to substrate effect10. The symmetry breaking is caused by charge transfer (CT) from n-GaN to p-Bi2Se3. There is effective electron-acoustic phonon coupling in n-GaN/Bi2Se3 heterojunction leading to spin like oscillations of electrons on the surface due to symmetry breaking by substrate at room temperature (refer. Supplementary Figs. 6 and 7)28,29. The ratio of I(A21g)/I(E2g) is increased in n-GaN/Bi2Se3 heterojunction as compared to Glass/Bi2Se3 film suggesting weaker interaction between the layer caused by the effective CT from n-GaN to Bi2Se3 which is further confirmed by transient absorption spectroscopy (Table 1).

Fig. 1: The Raman spectroscopic results with Lorentz peak fitting.
figure 1

a Glass/Bi2Se3 heterojunction, b n-GaN/Bi2Se3 heterojunction in the surface phonon mode (SPM).

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Table 1 Summarizes the vibrational modes obtained by the Raman spectroscopy of Glass/Bi2Se3 and n-GaN/Bi2Se3.
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Transient absorption spectroscopy study

Transient absorption profiles were recorded using 410 nm pump wavelength with a power of 0.2 mW probed in both visible and NIR regions. Probing in the visible region demonstrates charge carrier dynamics in higher energy levels i.e., bulk states (BS) and higher surface state (SS2), whereas, the NIR region probes the charge carrier dynamics in the lower surface states (SS1). Pump is selected below the GaN bandgap and higher than the Bi2Se3 bandgap energy. Figure 2 shows the TA spectra of GaN substrate, Glass/Bi2Se3 and GaN/Bi2Se3 in visible and NIR regions. GaN shows strong ground state bleaching (GSB) at 365 nm for the direct band gap transitions. At higher wavelengths oscillatory signals were observed due to corrugated structure of GaN substrate. Table 2 and Fig. 3 depict the position of transitions, lifetimes and paths followed on excitation and relaxation.

Fig. 2: The transient absorption (TA) spectra results with the amplitude variation (ΔA) vs. wavelength (nm) plot in the visible range(450–800 nm).
figure 2

a GaN template, b Glass/Bi2Se3 heterojunction, c n-GaN/Bi2Se3 at different time delays. TA spectra with amplitude variation (ΔA) vs. wavelength (nm) plot in the near infrared (NIR) range (800–1600 nm) at different time delays is shown for c GaN template, d Glass/Bi2Se3 heterojunction and e n-GaN/Bi2Se3 heterojunction.

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Table 2 Parameters derived from the Glass/Bi2Se3 and n-GaN/Bi2Se3 transient absorption (TA) spectra and global fitting for lifetime estimation.
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Fig. 3: The energy band structure for the transition of the spin-oriented charge carriers between the different conducting states such as SS1 to SS2, CB1 to SS2, VB1 to SS1, and SS1 to CB2, respectively.
figure 3

a Glass/Bi2Se3, b n-GaN/p-Bi2Se3 heterojunction. The SS1, SS2 here represents the two surface states; CB1, CB2 are the first and the second conduction band; VB1, VB2 are the first, and the second valence band. CT here represents the charge transfer state which is present in the n-GaN/p-Bi2Se3 heterojunction.

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On excitation with 410 nm pump wavelength in Glass/Bi2Se3 film, a ground state bleaching (GSB)24,30 appears after 1.28 ps delay in the region of 650–800 nm with maxima ~760 nm (731 fs) for a very short time (parenthesis shows lifetime) for transition 1 (Fig. 2b). GSB is replaced with a transient absorption in the region of 500 nm to 650 nm with maxima ~565 nm for transition 2 (Fig. 3). TA fits with two exponential decay model where the long-lived species shows lifetime of τ1 = 287 ps. Transient species also appears after 3 ps with maxima ~765 nm for transition 3. In NIR region, broad GSB appears after 1.8 ps delay time with maxima at ~1200 nm with a lifetime of ~5.2 ps (Fig. 2e) for transition 4. TA also appears at 840 nm after ~7 ps of excitation with a lifetime of 862 ps for transition 5.

In the n-GaN/Bi2Se3 heterojunction in contrast to Glass/Bi2Se3 film, transient absorption appears immediately after 1 ps delay time with ground state bleaching (GSB) at 764 nm (1.71 ps) (Fig. 2c). The transient species appears at 630 nm and 760 nm, and lifetimes are shown in Table 2. Broad GSB band can be seen with maxima ~1245 nm in NIR region (Fig. 2f) for transition 4. Charge transfer band appears at 630 nm with a very long lifetime of ~7 ns shown as transition 2 in Fig. 3b. GSB appears due to transition of charge carriers from VB to higher CB in both Glass/Bi2Se3 films and n-GaN/Bi2Se3 heterojunction on pumping with 3.06 eV radiation. However, the lifetime of these transitions is highly different in Bi2Se3 film and heterojunctions. In Bi2Se3 film, this transition relaxes ultrafast while in heterojunction charge carrier in the excited state are stabilized due to interface effect of Bi2Se3 and GaN forming p-n junctions. At the junction electron-hole pair generation is also observed in I-V characteristics of the device at 0 V in dark which is discussed in section 2.3. The excited charge carriers stay longer and may involve in other transitions before relaxing back. The long-lived transient species in heterojunction is due to effective charge transfer from GaN to Bi2Se3 which further supports the conclusions made by Raman study.

Electrical measurement (I–V) on n-GaN/p-Bi2Se3 heterojunction device under dark and illumination conditions

To examine the behavior of the n-GaN/Bi2Se3 heterojunction, a schematic in the form of device is constructed (refer. Supplementary Fig. 8a). To examine the real behavior, contacts on either side of the junction have to be ohmic in nature. Aluminum (Al) contact having lesser work function (WAl = 4.28 eV) was used as electrode to achieve ohmic contacts with n-GaN semiconductor and low band gap Bi2Se3 TI film26,31,32. Ohmic nature of Al-metal contact was further ascertained by measuring I–V characteristics on either side n-GaN/Bi2Se3 heterojunction (refer. Supplementary Fig. 9a, b). The I–V results showed rectifying nature with a cut-in voltage of ~0.34 V (Fig. 4a, b) due to depletion region formation at the n-GaN and p-Bi2Se3 interface (refer. Supplementary Fig. 8b). The p-n diode I-V characteristics across the n-GaN/Bi2Se3 heterojunction under the dark conditions is fitted with the ideal diode equation33 to find various parameters such as ideality factor (n), series resistance and maximum rectification ratio (RR) (refer. Supplementary Section 3 and 4)19,20,34. The ideality factor (n) and series resistance (RS) was evaluated to be 12.6 and 787 Ω. Other diode parameters are the reverse saturation current (IR) = 2.22 µA (at −1 V) and the forward current (IF) = 621 µA (at +1 V) under dark conditions have good rectifying capacity with maximum rectification ratio \(\left( {{\mathrm{RR}}} \right) = \left( {{{I}}_{{{\mathrm{F}}}}/{{I}}_{{{\mathrm{R}}}}} \right)_{{{{\mathrm{at}}}} \pm 1\,{{{\mathrm{V}}}}}\) ≈ 280. The high ideality factor (n=12.6) in comparison to ideal value (≈1) is due to enhanced recombination in depletion region across the heterojunction or due to edge recombination35,36 or imperfections in the form of interface states that hints towards CT between GaN and Bi2Se3 as we discussed in previous sections. The experimental results of the n-GaN/p-Bi2Se3 heterojunction diode under dark were further fitted using the 2-diode simulation model37,38,39, to extract various diode parameters and found close to the experimental values (refer. Supplementary Section 4 and Fig. 10).

Fig. 4: The current−voltage (I−V), capacitance−voltage (C−V) and conductance/radial frequency (Gp/ω) characteristics are shown for the n-GaN/p-Bi2Se3 heterojunction.
figure 4

a The I−V plot, b semi-log (I−V) plot under dark and illumination conditions with wavelength varied from 300 to 1000 nm at a constant incident light power of 1.96 μW, c C−V plot under dark conditions at constant frequency of 50 and 100 kHz with driving voltage from −1 V to +3 V, inset shows the 1/C2−V plot at 100 kHz frequency with x-intercept showing built-in voltage (Vbi), d Gp/ω−ω plot for voltage biasing from +1 V to +3 V under dark conditions, e Gp/ω−ω plot at λ = 450 nm, power =1.96 μW, f Gp/ω−ω plot at λ = 650 nm, and power =1.96 μW.

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The photoresponse of n-GaN/p-Bi2Se3 heterojunction diode was studied under the presence of light by varying the wavelength from 300 nm to 1000 nm at a constant incident light power of 1.96 µW (Fig. 4a, b). The power was kept low avoiding any possible damage to the surface of the Bi2Se3 TI film. The wavelength (λ) of the light was kept in the range from 400 nm to 1000 nm considering the energy bandgap of the two material i.e., the n-GaN (3.4 eV)15 and p-Bi2Se3 (0.3 eV)19,20,34. There can be a hindrance in our results due to the thermoelectric potential which is not desired. This problem is overcome by nullifying the potential across the electrodes by adjusting the incident spot of light without the applied bias40. Under the optical illumination of the light of wavelength (λ) = 300–400 nm an increase in the saturation current (IR) is observed from the I–V results (Fig. 4b). There is an enhancement in the forward current (IF) due to the absorption of the light in the n-GaN film having a band gap (Eg ≈ 3.4 eV) which corresponds to the wavelength (λ) ≤ 365 nm given by the expression \({{{\mathrm{E}}}}_{{{\mathrm{g}}}} = hc/\lambda = 1.24 \,\times 10^3/\lambda\) (nm). Here, the Planck’s constant (h) = 6.6 × 10−34 J-s, the velocity of light (c) = 3.00 × 108 m/s and Eg is the bandgap in eV, where, 1 eV = 1.6 × 10−19 J. This enhancement in current is also prevalent for a higher range of wavelength till 600 nm wavelength of light (Fig. 4b). However, the saturation or reverse current did not show much increment beyond the 400 nm wavelength, as the charge carrier generated is comparatively lesser due to the least absorption in the n-GaN TI film. From the optical results, we observed that at higher wavelength (λ) ≥ 700 nm of light, current shows a decreasing trend beyond even the dark characteristics which is not the normal trend. This trend is also consistent with higher range of wavelengths of light within the instrument limit. This effect can be correlated with the interfacial effect of the n-GaN/p-Bi2Se3 heterojunction which has been seen due to the formation of CT state (Fig. 3b) in the ultrafast study of the charge carrier relaxation. To get deep understanding we need to perform the energy band modeling for the interface behavior of n-GaN/p-Bi2Se3 heterojunction device.

Energy band modeling of the n-GaN/p-Bi2Se3 heterojunction under dark and illumination

Figure 5 shows the energy band alignment of the n-GaN/p-Bi2Se3 heterojunction at thermal equilibrium and forward bias under the dark and the illuminated conditions. Ec, EV, Ef are the notations used for the respective conduction band, valence band, and Fermi energy level with subscript 1 & 2 denoting n-GaN and p-Bi2Se3 material respectively. Figure 5a shows the heterojunction formation with the alignment of the Fermi level and the creation of the space charge region with a built-in potential. ∆Ec, ∆EV are the valence and the conduction band offset which will form a barrier to the conduction of the electrons from p-Bi2Se3 to n-GaN and holes from n-GaN to p-Bi2Se3. The theoretical value of ∆Ec, ∆EV are calculated to be 1.2 eV and 1.9 eV respectively (refer. Supplementary Section 4.2). Under this high barrier of the band offset the only possible conduction is in forward bias as shown through the schematic of the band alignment due to the application of a positive potential to p-Bi2Se3 with respect to the n-GaN. Under the thermal equilibrium, there is a small current due to the diffusion of the excess carrier (Fig. 5a) in the form of electrons from n-GaN to p-Bi2Se3 and due to the holes from p-Bi2Se3 to n-GaN and the formation of the space charge region which balance out the further flow of the charge carriers. This current in the forward biasing is due to the injection of the minority carriers across the junction under the influence of the biasing potential34,41. The forward bias shifts the n-GaN potential and energy bands upwards with respect to p-Bi2Se3 and makes it easier for the charge carriers which are electrons and holes (Fig. 5b). The high barrier of the offsets ∆Ec, ∆EV (1.2 eV and 1.9 eV) did not allow the current in the reverse direction so a very minute negligible current flows in the reverse biasing at room temperature under dark conditions which explain the rectifying behavior of the n-GaN/p-Bi2Se3 heterojunction41,42.

Fig. 5: The schematic of energy band diagram and modelling highlighting the alignment of the band structure of the n-GaN/p-Bi2Se3 heterojunction and depletion region (in red).
figure 5

a Thermal equilibrium under the dark condition with a built-in voltage and across the junction due to the diffusion of charge carriers, b forward bias with the splitting of the Fermi level of the respective material of the heterojunction, c forward bias under the influence of light of wavelength (λ) with the generation of the charge carriers and enhancement of current, d the surface influence of the Bi2Se3 film with the transition of the spin-oriented charge carriers between the SS1 to SS2, CB1 to SS2, and VB1 to SS1, respectively. Ie1, Ih1 represents the photon-induced current due to electrons and the holes at SS1 and VB1. Ie2, Ih2 represent the photon-induced current due to electrons in SS2 and holes in SS1 and CB1.

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Figure 5c shows the forward-biased n-GaN/p-Bi2Se3 heterojunction in presence of the light with wavelength (λ). As shown in the schematic of energy bands (Fig. 5c) the incident optical light will generate the charge carrier with the generation of the charge carriers in the form of electrons at the conduction band and holes in the valence band. This generation of charge carriers by the optical light will enhance the current in the n-GaN/p-Bi2Se3 heterojunction device as observed from its I–V characteristics (Fig. 4b). The incident photon with wavelength (λ) ≤ 365 nm will be absorbed by the n-GaN having bandgap (Eg ≈ 3.4 eV)15. Therefore, for the generated charge carriers in n-GaN there is an observed change in the saturation current (IR) in reverse bias for 300–400 nm of wavelength (Fig. 4b). There is an increase in the forward current in comparison to the dark conditions due to an increase in charge carrier in the n-GaN/p-Bi2Se3 heterojunction as the incident wavelength was varied from 300–600 nm. For wavelength (λ) ≥ 700 nm the forward current showed suppression beyond the dark forward current (Fig. 4b). This phenomenon has been attributed to the interface phenomenon of the n-GaN/p-Bi2Se3 heterojunction. To study and quantify this interface phenomenon the capacitance-voltage (C−V) measurement for the n-GaN/p-Bi2Se3 heterojunction devices were done.

The C−V characteristic of heterojunction is shown in Fig. 4c. No major hysteresis is found when swept with voltage from −1 V to +3 V. A shift towards the positive x-axis in the C−V characteristics of the n-GaN/p-Bi2Se3 heterojunction is observed revealing the presence of interface states trapped across the junction43. The capacitance is being observed from the C−V characteristics to be enhanced at lower frequency of measurement due to trap states which was able to follow the range of alternating current (AC) frequency and enhances the capacitance value across the heterojunction44. The capacitance effect on the biasing becomes less pronounced as we move to high biasing (V > 2 V). The carrier concentration of p-Bi2Se3 film (ND) and the built-in potential (Vbi) across the junction can be evaluated from the slope and the x-intercept of the 1/C2−V plot45 of n-GaN/p-Bi2Se3 heterojunction (inset of Fig. 4c) using Eq. 1 as: -

$$\frac{1}{{C^2}} = \frac{2}{{q\varepsilon A^2N_D}}\left( {V_{bi} - V} \right)$$
(1)

The electric permittivity (ε) of p-Bi2Se3 TI film is given as ε = 58.25ε037 and permittivity of free space (ε0) = 8.85 × 10−12 F/m, the area of the device (A) = 0.0049 cm2. The value of ND and Vbi was calculated as 3.37 × 1016 cm−3 and 0.5 V from the 1/C2−V plot45 and using Eq. 1. The area of our device is lesser than what we have assumed (i.e, \(700 \,*\, 700 \,{\rm{um}}^2\)) as the diameter of the Aluminum (Al) contact is small (300 um) and the fringing current cannot be normalized with that area as the effective area is lesser that the 0.0049 cm2. It is the reason why the extracted carrier density in Bi2Se3 was calculated to be on the lower side (3.37 × 1016 cm−3) than what we got from the Hall measurement (ND = 1.9 × 1018 cm−3). A more accurate effective area can be derived using the numerical simulation for the calculation of the effective area of our device. This discrepancy in the number of charge carriers in p-Bi2Se3 film is due to interface defects or barrier in homogeneities46 which can be observed by the positive shift of the C−V plot (Fig. 4c).

To get deeper understanding of the behavior and interface density trap44, frequency (f) dependence conductance (Gp) measurement was done. Figure 4d–f, show Gp/ω vs. radial frequency (ω = 2πf) plot for heterojunction device under dark and illumination showing almost linear decreasing relation with minimum trap density as highlighted from the minima peak in Fig. 4d. The interface traps were not evident in the dark conditions especially at a higher voltage where the trap density was observed to be minimum. At higher frequency (ω) the trap density has the minimum effect on the frequency as the trap states are not able to follow the high AC frequency44. The interface peak traps show negligible dependence on the biasing of the heterojunction under the influence of light. The interface traps here, however, showed a little shift towards lower frequency as we move towards the higher wavelength range. At the higher wavelength, the absorption is mostly due to p-Bi2Se3 TI film. This trap follows the AC signal of lower frequency44,47. The peaks however have a minimum effect on the biasing of n-GaN/p-Bi2Se3 heterojunction. The value of these interface states (Nss) and the time constant (τ) can be calculated from the Gp/ω−ω plot44 using the Eq. 2 as: -

$$\frac{{{{{\mathrm{G}}}}_p}}{\omega } = \frac{{qN_{SS}}}{{2\omega \tau }}\ln [ {1 + \left( {\omega \tau } \right)^2} ]$$
(2)

The density of state (NSS) can be calculated from the maxima of the first derivative of Eq. 2 which is: -

$$N_{SS} = \frac{{2.5}}{q}\left( {\frac{{G_p}}{\omega }} \right)_{\max }$$
(3)

The values of interface state (NSS) are calculated using Eq. 3 and the time constant (τ) of the interface traps are evaluated by fitting the data (refer. Supplementary Fig. 11). All the evaluated parameters are listed in Table 3 below. As observed from the Table 3, the density of interface states (NSS) was of the order of 108–109 eV−1 cm−2 and the time constant (τ) ~0.5–10 μs. The time constant (τ) of the interface traps under the dark is found to be almost double for n-GaN/p-Bi2Se3 heterojunction as compared with the presence of light. This interface phenomenon of n-GaN/p-Bi2Se3 heterojunction has a charge trapped state at ~2 eV (Fig. 5d), as observed in the band diagram of the top surface of the Bi2Se3 film, corresponding to the wavelength (λ) = 620 nm of the light as also observed in TA study. Therefore, the wavelength of the light beyond 620 nm i.e., 700–1000 nm shows a suppressed current in comparison to the dark forward current (Fig. 3).

Table 3 The calculated value of the density of states (NSS) and the time constant (τ) for the n-GaN/p-Bi2Se3 under the presence of light.
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It was not usual for a current to have suppressed forward current beyond a particular wavelength as observed for the n-GaN/p-Bi2Se3 heterojunction. This result was consistent for an even range of wavelength beyond 700 nm within the instrument limit. This anomaly in the behavior of the n-GaN/p-Bi2Se3 heterojunction is tried to be modeled in the form of the top Dirac cone surface states (Fig. 5d) and its transport properties is studied. Under the presence of the optical light and forward biasing across the n-GaN/p-Bi2Se3 heterojunction the spin-polarized charge carriers in n-GaN inject into the neighboring p-Bi2Se3 (Fig. 5d). As the holes have stronger relaxation48 and lower mobility49 in comparison to the electrons, therefore, the current is mainly due to the spin injection of the photo-induced electrons. Under the excitation of the n-GaN/p-Bi2Se3 heterojunction with wavelength (λ) ≤ 700 nm, transition from the first surface states (SS1) to the second surface states (SS2), the transition from the first conduction band (CB1) to the second surface state (SS2), and that from the first valence band (VB1) to the SS1, VB1 to VB2 and charge transfer from CB of GaN to CB of Bi2Se3 forming CT state are possible48. The Dirac point of SS2 is located above the Dirac point of SS148,50,51,52,53,54,55. The spin-polarized electrons in n-GaN will inject into p-Bi2Se3 film, and injected spin-up electrons will recombine with holes in the VB1. Thereby reducing the holes contribution from the total current particularly at the bottom of the film due to n-GaN substrate. Since, the current we observe is due to combination of current of top and bottom surface current denoted here by Ie1 and Ie2. The suppression of current at bottom surface will enhance the contribution of surface of p-Bi2Se3 film in total current49. Therefore, a larger current here is observed for wavelength (λ) up to 700 nm. As we move towards a higher wavelength from 700–1000 nm there will be lesser injected electrons from the n-GaN to the p-Bi2Se3 as compared to the wavelength (λ) ≤ 700 nm. The other factor is the visible interface CT state which is been shown in p-Bi2Se3 film energy structure having energy = 2 eV which corresponds to the wavelength (λ) of 620 nm. Therefore, the charge carriers particularly the electrons cross the barrier of 2 eV when being incident with the wavelength (λ) ≤ 600 nm of the photon light. However, for wavelength (λ) ≥ 700 nm the CT will trap the electrons and reduces the charge carriers and the total flow of the current in the n-GaN/p-Bi2Se3 heterojunction device circuit.

To understand the behavior of the photogenerated carrier dynamics, we have performed ultrafast femtosecond transient absorption spectroscopy of Bi2Se3 topological insulator film grown on glass substrates and n-GaN substrate. Ground state bleach due to transition of the charge carriers from valence to conduction band and transient absorptions are recorded for both the Bi2Se3 film and GaN/Bi2Se3 heterojunction on pumping with 3.06 eV radiation probed in visible and NIR regions. Substantial effect on lifetime of these transitions is observed in heterojunction structure. In Bi2Se3 films, these transitions relax ultrafast while, in GaN-heterojunction charge carriers in the excited state are stabilized due to interface effect of GaN/Bi2Se3 forming p-n junctions. A clear charge-transfer state formation is observed in TA spectra. The generated electron-hole pair is observed in the I–V results at 0 V, which stayed longer and involve in other transitions before relaxing back. The long-lived transient species in heterojunction is due to effective charge transfer from GaN to Bi2Se3 and well supported by the Raman study. Low frequency shift in GaN/Bi2Se3 heterostructure implicates GaN effect on the bonding force of Bi2Se3 due to electron-phonon coupling. The increased I(A21g)/I(E2g) in GaAs/Bi2Se3 heterojunction confirmed the weaker interaction between the layers. A deeper understanding was established by studying the electrical parameters of n-GaN/p-Bi2Se3 heterojunction diode under dark and illuminated conditions. The capacitance-voltage measurement of the n-GaN/p-Bi2Se3 diode quantized the number of the interface traps and their time constant and given an understanding of their behavior. The study is significant from the perspective of understanding the modulation of surface states at room temperature in GaN-topological insulator-based interface device showing their potential towards next-generation electronic and optical devices.

Methods

Experimental details

The n-GaN film of thickness ~3.5 μm grown over a c-plane sapphire (S) having a thickness of ~0.33 mm was employed as a substrate to fabricate Bi2Se3 TI heterojunctions. For n-GaN/p-Bi2Se3 heterojunction fabrication, substrate was loaded in thermal coating unit (Scientific & Analytical Instruments). A powdered Bi2Se3 precursor of a purity ~99.98% (on the metal basis) was used to grow a thin TI film on n-GaN substrates. Bi2Se3 TI film of same thickness was also grown on glass substrate for reference. Thickness was controlled by Quartz crystal monitor in vacuum chamber and also confirmed by ellipsometer (J. A. Wollam co. Inc., United States). The grown Bi2Se3 TI film was annealing temperature of 200 °C for 2 h in the presence of the Argon (Ar) gas. The crystal planes of Bi2Se3 TI films were identified by X-ray diffractometer (XRD) (Rigaku Miniflex, model no. BD68000014-01). Microstructure and morphology study were done by scanning electron microscopy (SEM-EDS) (Model No. EVO 15, ZEISS EVO Series) and atomic force microscopy (AFM) (Agilent Technologies, Model: 5500). Raman spectroscopy was used to study the active modes. The Raman characterization was done with 534 nm laser light with the and scattering was with the lowest Raman shift of 10 cm−1. Pump-probe spectroscopy was used to probe the transient dynamics of charge carriers of the Bi2Se3 TI film grown on glass and n-GaN substrates. The Titanium (Ti): Sapphire laser amplifier with specifications: 35 fs, 4 mJ/pulse,1 kHz, 800 nm) was used with two fragments of light. 410 nm pump wavelength was used and probe was varied from visible (VB) to near-infrared (NIR) range (400–1600 nm) at 7 fs time delay with pump. The pump power was kept low at 35 µW to make it a non-destructive process and preserving the film from any possible damage. n-GaN/Bi2Se3 heterojunctions device was realized by growing metallic contacts across the junction. Aluminum wire with a purity of ~99.999 % was used to grow metallic contacts by thermal evaporation technique under a high vacuum (10–6 Torr). The metal contacts were grown through the metallic shadow mask with dimensions: diameters ~300 µm and inter distance ~600 µm. The metal contact across the n-GaN/Bi2Se3 heterojunction of thickness ~150 nm was confirmed by the ellipsometer. The nearest contacts across the n-GaN/Bi2Se3 heterojunction were used as 2-probes for electrical characterization (Keithley 4200SCS). A tunable light source (NewPort TLS 300XU) with varying wavelengths (300–1000 nm) was used to illuminate light across these nearest contacts across the n-GaN/Bi2Se3 heterojunction. Semi-Probe station was used for all the current–voltage (I−V), capacitance–voltage (C−V), and capacitance–frequency (C−f) and optical characterization through two 4, 225 RPM/SMU. The details fabrication process of the n-GaN/p-Bi2Se3 heterojunction diode is discussed in details in the Supplementary material (Section 1) and also with the fabrication flow process (Supplementary Fig. 1). Details can also be found in our previous works19,20,22,34.