High-Responsivity, High-Detectivity, Ultrafast Topological Insulator Bi2Se3/Silicon Heterostructure Broadband Photodetectors Hongbin Zhang, Xiujuan Zhang,* Chang Liu, Shuit-Tong Lee, and Jiansheng Jie* Institute of Functional Nano & Soft Materials (FUNSOM), Collaborative Innovation Center of Suzhou Nano Science and Technology, Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, P. R. China S Supporting Information *
ABSTRACT: As an exotic state of quantum matter, topological insulators have promising applications in newgeneration electronic and optoelectronic devices. The realization of these applications relies critically on the preparation and properties understanding of high-quality topological insulators, which however are mainly fabricated by high-cost methods like molecular beam epitaxy. We here report the successful preparation of high-quality topological insulator Bi2Se3/Si heterostructure having an atomically abrupt interface by van der Waals epitaxy growth of Bi2Se3 films on Si wafer. A simple, low-cost physical vapor deposition (PVD) method was employed to achieve the growth of the Bi2Se3 films. The Bi2Se3/Si heterostructure exhibited excellent diode characteristics with a pronounced photoresponse under light illumination. The built-in potential at the Bi2Se3/Si interface greatly facilitated the separation and transport of photogenerated carriers, enabling the photodetector to have a high light responsivity of 24.28 A W−1, a high detectivity of 4.39 × 1012 Jones (Jones = cm Hz1/2 W−1), and a fast response speed of aproximately microseconds. These device parameters represent the highest values for topological insulator-based photodetectors. Additionally, the photodetector possessed broadband detection ranging from ultraviolet to optical telecommunication wavelengths. Given the simple device architecture and compatibility with silicon technology, the topological insulator Bi2Se3/Si heterostructure holds great promise for high-performance electronic and optoelectronic applications. KEYWORDS: topological insulator, silicon, heterostructure photodetector, high responsivity, fast photoresponse
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real-time visualization of the transient carrier transitions between the electronic band structures of topological insulators under the excitation of ultrafast light pulses.11−13 The experimental results have proved that optical excitation can lead to transient carrier population at both the empty bulk conduction band and unoccupied surface state, which may be responsible for the photoconductivity in topological insulators. The fast dynamical response of electrons under optical excitation is a general property of topological insulators (but usually not in conventional metals), which makes topological insulators an outstanding candidate for the new-generation ultrafast optoelectronic devices.13 Recently, helicity-dependent photocurrents in topological insulators under circularly polarized light illumination have been investigated, while light
opological insulators, an exotic state of quantum matter characterized by an insulating bulk gap and metallic Dirac surface states, have attracted extensive attention in recent years due to their unique physical properties and promising applications for new-generation electronic and optoelectronic devices.1−4 Among the various topological insulators, Bi2Se3 stands out because of its simple surface state structure with a well-defined single Dirac cone at the momentum zero point in k-space and a bulk bandgap of 0.3 eV. The helical nature of the topological surface state suppresses electron backscattering and enables excellent transport properties with a high carrier mobility, while the robust topological protection makes Bi2Se3 highly resistant to the ambient environment, retaining excellent electrical conductivity under a high density of defects and dislocations.5−7 Topological insulators are predicted to be a promising candidate material for high-performance photodetectors due to light absorption over a broad spectral range.8−10 Experimental studies using time- and angle-resolved photoelectron spectroscopy have provided direct © 2016 American Chemical Society
Received: January 12, 2016 Accepted: April 26, 2016 Published: April 26, 2016 5113
DOI: 10.1021/acsnano.6b00272 ACS Nano 2016, 10, 5113−5122
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Bi2Se3 film were characterized by X-ray diffraction (XRD) in Figure 1a. The film exhibits the rhombohedral crystal geometry
heating induced photothermoelectric currents have been detected in Bi2Se3 materials.14−16 However, those two photoelectric effects are too weak for practical applications for light detection. Time- and angle-resolved photoemission measurements have elucidated that a transient metastable electron population can form at the higher-lying bulk states of Bi2Se3 within 2.0 ps under optical excitation; however, the transient population relaxes quickly to the lower energy states via inter- and intraband phonon-mediated scatterings, and an exponential decay of the excited carriers gives a population lifetime of 6 ps.11−13 Therefore, the photoconductivity is hard to observe (except when the special measurement configurations or polarized light sources were used, refs14−16), since the lack of a large band gap in topological insulators means no or small barriers for rapid carrier recombination. Alternatively, preparation of heterostructures by combining topological insulator Bi2Se3 with crystalline silicon can introduce a robust Schottky barrier at the interface, which offers the possibility of original device designs based on sub-bandgap internal photoemission from Bi2Se3 into silicon.17,18 The built-in potential at the Bi2Se3/Si interface can effectively separate the photogenerated electron−hole pairs in Bi2Se3 and concurrently suppress the rapid recombination of photoexcited carriers, leading to a remarkable photocurrent in the external circuit. Furthermore, as silicon is one of the most important substrates for optoelectronic applications, integration of the topological insulator Bi2Se3 with the mature Si technology is of particular interest for device applications.19−21 So far, the high-quality Bi2Se3 single-crystalline films used to study the topological insulator properties are mainly prepared by molecular beam epitaxy (MBE). However, the MBE methods generally suffer from high cost and low growth rate. Due mainly to surface tension and lattice misfit, Bi2Se3 films grown on Si substrate via the MBE technique are usually accompanied by a disordered surface.22−24 Bi2Se3 has a layered structure consisting of five atomic layers Se(1)-Bi-Se(2)-BiSe(1), where the bonding within each quintuple layer (QL) is via strong covalent bonding and that between the QLs is via weak van der Waals force; thus, the van der Waals epitaxial growth of Bi2Se3 is particularly suitable on Si substrate.24,25 Herein, epitaxial growth of Bi2Se3 topological insulator film on Si substrate with an atomically abrupt interface was achieved via a simple physical vapor deposition (PVD) method. The resultant Bi2Se3/Si heterostructure showed pronounced photovoltaic response under light illumination, making it capable of operating as a self-driven photodetector for broadband light detection from ultraviolet (UV) to optical communication wavelengths with excellent stability and reproducibility. Owing to the high crystal quality of the topological insulator film as well as the strong built-in potential at interface, the heterostructure photodetector exhibited excellent performance in terms of large responsivity, high detectivity, and fast response speed, which represented the best parameters achieved for topological insulator-based photodetectors thus far. This result demonstrates the great potential of topological insulator/silicon heterostructures for high-performance optoelectronic device applications.
Figure 1. Characterizations of the topological insulator Bi2Se3/Si heterostructure. (a) Representative XRD pattern of the Bi2Se3 film on Si substrate. (b) Typical Raman spectrum of the Bi2Se3 films. (c and d) The core-level spectra of (c) Bi 4f and (d) Se 3d in the Bi2Se3 films. (e) Corresponding AFM image of the as-grown Bi2Se3 film. Inset presents height analyses of the film surface. (f) Crosssectional HRTEM image of the Bi2Se3/Si heterostructure. Inset shows the corresponding SAED pattern of the Bi2Se3 film.
belonging to the space group of R3m ̅ /D53d (JCPDs No. 892008) with no detectable impurities of other phases. Notably, only (0 0 n) (n = 3, 6, 9, 12, 15, 18) diffraction peaks are observed in the XRD pattern, indicating that the as-prepared Bi2Se3 film has a preferred orientation along the c-axis direction. The typical Raman spectrum of Bi2Se3 film in Figure 1b shows three pronounced characteristic peaks in the low wavenumber region; the peak at ∼142 cm−1 can be assigned to the in-plane E2g vibration, while the other two peaks at ∼83 and ∼185 cm−1 to the out-of plane A11g and A21g vibrations, respectively.26 X-ray photoemission spectroscopy (XPS) characterization was also performed on the Bi2Se3 films. The XPS peaks of 4f5/2 and 4f7/2 core levels of Bi are clearly separated (Figure 1c) at 163.5 and 158.2 eV, respectively, and the 3d3/2 and 3d5/2 core levels of Se (Figure 1d) at 54.5 and 53.7 eV, respectively. Atomic force microscopy (AFM) shows the Bi2Se3 film to have a uniform and smooth surface with a small roughness of ∼1.0 nm (inset in Figure 1e). The Bi2Se3 film/Si interface was investigated by highresolution transmission electron microscopy (HRTEM), as shown in Figure 1f and Figure S1 (Supporting Information). The Bi2Se3 film displays a layer-by-layer structure along the [001] zone axis with a layer thickness of 0.955 nm, corresponding to the QL of Se-Bi-Se-Bi-Se. Since the bonding
RESULTS AND DISCUSSION Layered topological insulator Bi2Se3 films were grown on Si(100) substrate by thermal evaporation via van der Waals epitaxy. The crystal structure and phase purity of the as-grown 5114
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Figure 2. Optoelectronic characteristics of the Bi2Se3/Si heterostructure. (a) Schematic illustration of the heterostructure device with Au and In−Ga as top and bottom electrodes, respectively. (b) J−V characteristic curve of the Bi2Se3/Si heterostructure measured at room temperature in the dark. (c) Photovoltaic behavior of the Bi2Se3/Si heterostructure measured under simulated solar illumination. (d) J−V characteristics of the heterostructure measured under 808 nm NIR light illumination with different light intensities. (e) Spectral response of the device measured in a spectral range from 300 to 1100 nm. (f) UV−vis-NIR absorption spectra of the Bi2Se3 topological insulator films with different thicknesses on quartz substrates.
within each QL of Bi2Se3 is via the strong covalent bond, while that between different QLs is via weak van der Waals interaction, the film growth of QL-by-QL is natural. The HRTEM image shows that a thin amorphous SiO2 layer of ∼1.0 nm is formed at the heterostructure interface from Si oxidation during film deposition. The existence of the ultrathin oxide layer has little influence on the film growth and the carrier transportation. In principle, lattice matching with the substrate is not required for van der Waals epitaxy because of the absence of surface dangling bonds for the layered materials. Also, the ∼1 nm SiO2 layer does not significantly influence the transport of carriers, since the carriers can easily tunnel through the heterostructure interface. In fact, the thin SiO2 layer might be beneficial to the device performance due to passivation of the surface defects on Si. Significantly, the HRTEM image also unveils a full epitaxial structure of the Bi2Se3 film down to the first QL without any deformation, yielding an atomically sharp interface between the Bi2Se3 film and Si substrate. Moreover, the selected-area electron diffraction (SAED) pattern of the film could be indexed to a 6-fold symmetry [001] zone axis pattern (inset in Figure 1f), which is consistent with the layered structure along the z-axis. The sharp diffraction spots in the SAED pattern confirm the single-crystalline nature of the Bi2Se3 thin film, and can be well fitted with the orthorhombic structure of Bi2Se3 according to lattice parameters of a = 0.414 nm and c = 2.864 nm. The above characterizations collectively demonstrate that high-quality epitaxial Bi2Se3 film was grown on Si by PVD. We note that the growth mechanism of the Bi2Se3 film is different from that of Bi2Te3 nanoplates in a previous report, in which a Te seed layer was predeposited for the epitaxial growth of Bi2Te3 crystals on Si substrate.27 In this work, a Se seed layer was not observed at the interface; therefore, the Bi2Se3 film is more likely grown according to the conventional nucleation and growth mechanism. Figure 2a shows the schematic diagram of the Bi2Se3 film/Si heterostructure photodetector. A SiO2 window was prepre-
pared on the n-Si substrate to define the effective area of the photodetector (0.03 cm2), and Au and In−Ga alloy were used as the top and bottom electrodes, respectively. Figure 2b plots the typical current density−voltage (J−V) characteristic curve of the Bi2Se3/Si heterostructure device in the dark at room temperature. Note that the device shows obvious diode characteristics with a rectification ratio of 50, indicating that the Bi2Se3 film can form a robust Schottky contact with n-type Si. The current density through a Schottky contact is generally represented by the well-known Richardson-Dushman thermionic emission theory given as follows at forward bias.28−31 ⎛ qV ⎞⎡ ⎛ qV ⎞⎤ ⎟⎢1 − exp⎜ − ⎟⎥ J = Js exp⎜ ⎝ nkT ⎠⎣ ⎝ kT ⎠⎦
(1)
where q is the electron charge, V is the forward-bias voltage, k is the Boltzmann constant, T is the absolute temperature, n is the ideality factor, J is the real current density, and Js is the apparent saturation current density. In the J−V measurement, the experimental value of the apparent saturation current Js can be extracted from the extrapolated straight line intercept of the forward-bias ln J versus V plot at V = 0 V (see Figure S2, Supporting Information). Further, the saturation current Js is given by the following equation:28−31 ⎛ qΦ ⎞ Js = A*T 2 exp⎜ − b0 ⎟ ⎝ kT ⎠
(2)
where Φb0 is the zero-bias barrier height and A* = 4πqm*k2/h3 is the effective Richardson constant with m* being the effective mass of electron. The theoretical value of A* is 112 A cm−2 K−2 for n-type Si. On the basis of eq 2, the Schottky barrier height was deduced to be 0.65 eV for the Bi 2 Se 3 film/Si heterostructure. Typical J−V characteristics of the Bi2Se3/Si heterostructure measured with and without light irradiation are depicted in Figure 2c. It shows clearly that under the simulated solar 5115
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Figure 3. Photocurrent switching properties of the Bi2Se3/Si heterostructure photodetector. (a) Schematic diagram of the setup for studying the time response of the photodetector. (b) Photocurrent switching performance of the photodetector measured under 808 nm light illumination at zero bias voltage. The current ON/OFF ratio is as high as 1.55 × 105. (c) Photocurrent switching behaviors obtained under different reverse bias voltages. (d) Photocurrent density as a function of light intensities. Inset shows the linear fitting of the photocurrent density versus light intensity in logarithmic plot. (e) Light intensity-dependent photoresponsivity of the photodetector at −1.0 V and zero bias voltage, respectively, in semi-log plots. (f and g) Photocurrent switching performance of the photodetector under 1310 and 1550 nm light illumination with different light intensities, respectively.
pulsed incident light was generated by modulating the 808 nm laser by a mechanical light chopper. The temporal response of the photodetector measured at zero bias voltage in Figure 3b shows that the device could be reversibly switched between high and low conduction states on light on/off. The light response of the device remains identical after several tens of switching cycles, showing the excellent stability and reproducibility of the device. Further, the heterostructure photodetector is highly stable in air, showing negligible changes even after storing in ambient condition for half a year (see Figure S5, Supporting Information). In addition, due to the low dark current of the heterostructure device at 0 V, the Jphoto/Jdark ratio reached as high as 1.55 × 105. The bias-dependent switching characteristics of the photodetector were investigated by applying different bias voltages of 0, −0.25, −0.75, and −1.0 V to the device, as shown in Figure 3c. The photocurrent increases remarkably as the bias voltage increases. The photocurrent density at 0 V is 22.08 mA cm−2, and increased to 243.01 mA cm−2 at −1 V, revealing an approximate linear relationship of the photocurrent with bias voltage (see Figure 5116
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Figure 4. Transient behaviors and speed characteristics of the Bi2Se3/Si heterostructure photodetector. (a and b) Representative transient open-circuit voltage response of the photodetector under pulsed 808 nm light irradiation with the frequencies of (a) 50 and (b) 4000 Hz. (c) Relative balance (Vmax − Vmin)/Vmax × 100% versus the light frequency plot in the range of 50−4000 Hz. (d) Transient response of the photodetector measured under the higher frequency of 0.06 MHz. (e and f) Characteristic response times at (e) rise edge and (f) fall edge.
and is defined as R = Jphoto/P.35,36 On the basis of this equation, the responsivity R of the Bi2Se3/Si heterostructure photodetector was estimated as a function of 808 nm intensity in a semilogarithmic scale at bias voltages of −1.0 and 0 V as depicted in Figure 3e. Note that R value is as high as 24.28 A W−1 at −1.0 V under a relatively low light intensity of 60 μW cm−2, and remains at about 2.60 A W−1 even at 0 V. Significantly, the R value of the Bi2Se3/Si heterostructure photodetector is much larger than that of the commercial Sibased p−n or Schottky junction photodetectors, which usually have a smaller value of ∼0.8 A W−1.19,20 This may be attributed to the strong NIR light absorption of the Bi2Se3 film. On the other hand, R tends to decrease at high light intensity, which may be attributed to the increased carrier recombination with increasing light intensity. In addition, as a predominant parameter to evaluate a photodetector, photoconductive gain (G) defines the number of photoexcited carriers generated per absorbed photon and is usually expressed as 38−41
S6, Supporting Information). This result offers the possibility of tuning the photoresponse of the device by applying an appropriate bias voltage. Figure 3d plots the photocurrent density versus light intensity (P) at zero bias voltage (the photocurrent switching performance at different light intensities is shown in Figure S7, Supporting Information). The photocurrent increases steeply with increasing light intensity at a relatively lower light intensity of
