Si Vertical Heterostructure

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Topological Crystalline Insulator SnTe/Si Vertical Heterostructure Photodetectors for High-performance Near-infrared Detection Hongbin Zhang, Baoyuan Man, and Qi Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01098 • Publication Date (Web): 11 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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ACS Applied Materials & Interfaces

Topological Crystalline Insulator SnTe/Si Vertical Heterostructure Photodetectors for High-performance Near-infrared Detection Hongbin Zhang1,*, Baoyuan Man1, Qi Zhang2,* 1. School of Physics and Electronics, Shandong Normal University, Jinan, Shandong 250014, P. R. China. 2. School for Radiological and Interdisciplinary Sciences (RAD-X) and Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection, Medical College of Soochow University, Suzhou 215123, China

ABSTRACT Due to the gapless surface state and narrow bulk band gap, the light absorption of topological crystalline insulators cover a broad spectrum ranging from terahertz to infrared, revealing promising applications in new generation optoelectronic devices. To date, the photodetectors based on topological insulators generally suffer from a large dark current and a weaker photocurrent especially under the near-infrared lights, which severely limits the practical application of devices. Owing to the lower excitation energy of infrared lights, realization of photodetection application of topological crystalline insulators in near-infrared region relies critically on the preparation and properties understanding of their heterostructures. Herein, we fabricate the high-quality topological crystalline insulator SnTe film/Si vertical heterostructure by a simple physical vapour deposition process. The resultant heterostructure exhibits excellent diode characteristic, enabling the construction of high-performance near-infrared photodetectors. The built-in electric field at SnTe/Si interface enhances the absorption efficiency of near-infrared lights, and greatly facilitates the separation of photogenerated carriers, making the device capable of operating as a self-driven photodetector. The as-grown SnTe film acts as the hole transport layer in heterostructure photodetectors, promoting the transport of holes to electrode and reducing electron-hole recombination effectively. These merits enable the SnTe/Si heterostructure photodetector to have a high responsivity of 2.36 A W-1, a high detectivity of 1.54 × 1014 Jones, and a large bandwidth of 104 Hz in near-infrared 1

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wavelength, which makes the detector have a promising market in novel device applications.

topological

KEYWORDS:

crystalline

insulator,

vertical

heterostructure,

near-infrared photodetector, high detectivity, broad bandwidth

1. INTRODUCTION The family of topological insulators has been extended by the introduction of topological crystalline insulators (TCIs), in which the mirror symmetry of the lattice ensures the existence of robust topological surface states rather than the time reversal symmetry 1-6. The first material predicted to be a TCI was SnTe with an ideal rocksalt structure, which supports an even number of Dirac cones for specific surface orientations

7-10

. The TCI SnTe has attracted much attention from both academic and

applied perspectives in recent years, and extensive efforts have already been devoted to investigate its novel physical properties. Among them, the static electronic properties of SnTe have been comprehensively imaged by angle-resolved photoemission spectroscopy

9,10

. The transport features originating from the surface

Dirac fermions were further confirmed via angle-dependent magneto-transport measurements11,12. Recently, various methods, including mechanical exfoliation, molecular beam epitaxial, chemical vapor deposition, and liquid solution synthesis, etc., have been developed to fabricate SnTe nanostructures, such as nanowires, nanoribbons, nanoplates, nanosheets and so on

13-17

. The earlier works have

established that the light absorption of gapless topological surface states allows to cover a broad spectra range, and ultrafast extraction of photo-generated electron-hole pairs is also allowed, due to its high carrier mobility 18-21. Therefore, according to the existence of gapless surface states and narrow bulk band gap, the TCI SnTe will also become a prime candidate for designing and fabricating of broad bandwidth photodetectors. The helicity-dependent photocurrent under circularly polarized light illumination has been observed on previously topological insulators in photoelectric transport experiments

22-24

. Besides, the photocurrent originating from the photoconductive

effect of topological insulators under non-polarized light illumination has also been reported

25-28

. However, these photoelectric effects are too weak for practical 2


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applications for near-infrared light detection, since the lack of a large band gap in topological insulators means small barriers for rapid carrier recombination. Therefore, to develop a TCI device with a large photocurrent response especially in the near-infrared waveband, effective separation and extraction of photo-generated carriers should be considered. According to previous studies, building photovoltaic devices based on topological insulator heterostructures becomes effective in promoting separation efficiency of photo-generated carriers

29,30

. The reported

heterostructure photodetectors could show a higher optical responsivity to the visible (Vis) lights, while the responsivity for near-infrared lights becomes negligible. On the one hand, the small light absorption ratio of topological insulators and the lower excitation energy of incident lights in near-infrared region are responsible for the lower photoelectric response. On the other hand, the previously discovered topological insulators are almost n-type, and a large number of background electrons may exist in the topological insulators, which usually cause an enhanced recombination with photo-generated holes in the heterostructure devices and consequently the decrease of near-infrared light response

29

. Fortunately, the

angle-resolved photoemission spectroscopy and magneto-transport experiments reveal that the chemical potential of SnTe crystals is often 0.5 eV below bulk band gap, and is typically p-type with a high concentration of holes, due to slight off stoichiometry 9-12

. Therefore, preparation of heterostructures via combining p-type TCI SnTe film

with n-type Si layer can introduce a robust Schottky barrier at the interface, which turns to be of crucial importance for fabricating a high-performance near-infrared photodetector. Herein, the high-quality TCI SnTe/Si vertical heterostructure was developed via growth of TCI SnTe film on Si substrate using a simple physical vapor deposition method. The resultant SnTe/Si heterostructure shows an obvious diode characteristic with a higher rectification ratio, and enables to fabricate a high-performance and self-driven photodetector with large responsivity, high detectivity, and broad inherent bandwidth. The built-in electric field inside the SnTe/Si heterostructure is very beneficial for the separation of photo-generated electron-hole pairs. The p-type charge carriers in TCI SnTe films can suppress the rapid recombination of photoexcited holes in transport process, and facilitate the transport of photo-generated holes to electrode, leading to a high light responsivity of near-infrared lights. Hence, the as-fabricate 3



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SnTe/Si heterostructure photodetectors are ideal for obtaining fast switching in near-infrared wavelengths, and very favorable for high-frequency applications of photoelectric detection. 2. RESULTS AND DISCUSSIONS The TCI SnTe films were grown by PVD process without any catalyst inside a horizontal tube furnace equipped with a 1-in.-diameter quartz tube, as shown by the schematic diagram of experimental setup in Figure 1(a). High-purity SnTe (99.999%) powder was used as the precursor for evaporation and placed at the hot center of the furnace during experiment. Before film growth, the quartz tube was pumped to a base pressure of 1.0×10-4 Torr and flushed with high-purity Ar gas repeatedly to eliminate oxygen contamination. In the process of SnTe film growth, the powder source was heated to 800 °C under low pressure (~1.0 Torr) with a constant Ar gas flow (~25 sccm). The clean and prepatterned SiO2/Si substrate was placed at the downstream of the quartz tube away from the hot center. The thickness of SnTe films could be tuned by adjusting the growth time. Afterwards, the SnTe films were naturally cooled to room temperature and then taken out for further characterizations. The surface morphology of the as-grown TCI SnTe films was firstly examined using the scanning electron microscopy (SEM) with a large area, as shown in Figure 1(b). The scale bar in the SEM image is 200 nm. We note that the film is formed with interconnected grains of SnTe, and the film surface is quite smooth in a large scale. The size of the crystalline grains is uniform, and become very regular as well, exhibiting the cubic crystal geometry. The atomic scheme structure of TCI SnTe films grown on top of Si layer was then evaluated by atomic force microscopy (AFM). The typical 3D and 2D AFM topographic images of as-grown SnTe films are shown in Figure 1(c) and (d), respectively. The result shows the SnTe films to have a uniform and smooth surface with a small roughness, and the height of the terraces is～0.4 nm, which is in accord with the periodicity of the rocksalt lattice of SnTe along the (111) direction. The SnTe films were further investigated by high resolution transmission electron microscopy (HRTEM), as illustrated in Figure 1(e) and (f) , respectively. The TEM images were observed from the cross section of the as-prepared SnTe films. This result reveals the as-grown SnTe films are crystalline, and the lattice fringes with a periodic spacing of 0.185nm correspond to the (111) plane of SnTe (Figure 1(e)). On the (111) plane of SnTe which is a TCI, there exist four Dirac cones centered at four 4


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time-reversal-invariant momenta in the surface Brillouin zone: one centered at Γ point and three at M points, which originate from the bulk-band inversion at the L

points12,31 Besides, the lattice fringes with a periodic spacing of 0.316nm correspond to the (100) plane of SnTe (Figure 1(f)), consistent with previous SnTe nanostructures as well

13-17

. Hence, the HRTEM images display that the rocksalt phase of the

as-prepared SnTe film with the lattice parameters of a = 0.631 nm is certainly experimentally accessible. The crystal structure and phase purity of the as-grown SnTe films were characterized by X-ray diffraction (XRD) for a wider angle range in Figure 1(g). Notably, only (111), (200), (220), (311), (400), (331), and (511) diffraction peaks are observed in the XRD pattern, which further confirms the α-phase SnTe is preserved. The film exhibits the cubic rock-salt crystal geometry belonging to the space group of Fm3m with no detectable impurities of other phases. X-ray photoemission spectroscopy (XPS) characterization was then performed on the as-prepared SnTe films. The XPS peaks of 3d5/2 and 3d3/2 core levels of Sn are clearly separated (Figure 1(h)) at 487 and 495.5 eV, respectively, and the 3d5/2 and 3d3/2 core levels of Te Figure 1(i)) at 573 and 583.4 eV, respectively. The XPS analysis also suggests an atomic ratio of 1: 1.06 is obtained for Sn/Te in the as-grown SnTe film, and a certain amount of Sn vacancies exist in the films, which can lead to p-type charge carriers in transport. These characterizations demonstrate collectively that high-quality SnTe films can be grown on Si layers by PVD technique, and these thin-film samples open new opportunities for experimentally exploring the physics of TCIs as well as for fabricating novel devices based on the unique nature of TCIs. Using the high-quality TCI SnTe films, the high performance SnTe/Si heterostructure photodetectors were fabricated, and their corresponding photoelectric response was also evaluated, as illustrated in Figure 2. Panel (a) in Figure 2 shows the schematic illustration of the fabricated SnTe film/Si vertical heterostructure photodetector and the corresponding experimental setup. To evaluate the photoresponse characteristics of the heterostructure device, several laser sources with different wavelengths of 1064nm, 1310nm, and 1550nm were adopted as the excitation lights. Besides, a solar simulator (NEWPORT, AM1.5) was used to detect the photovoltaic behavior of the device. Figure 2(b) depicts the schematic cross-section of the fabricated SnTe film/Si vertical heterostructure photodetectors. To fabricate the heterostructure photodetector as shown in Figure 2.(b), the SiO2 (300 5



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nm)/n-Si (resistivity 0.9-1.5 Ω cm, (100) plane) substrates were utilized. An opening window with an area of 0.09 cm2 was first defined on the clean SiO2/Si substrate through photolithography, and then, the SiO2 insulating layer in the opening window was removed away by a buffered oxide etching solution to expose the underlying n-type Si substrate. Afterward, the prepatterned SiO2/Si substrate was rinsed with deionized water and then dried by nitrogen gas stream. Subsequently, the SnTe film with a thickness of 60 nm was grown onto the as-prepared SiO2/Si substrate by PVD process. Then, gold (Au) top electrode (50 nm) was deposited onto the surface of SnTe layer via e-beam evaporation using a shadow mask as the ohmic contact. Indium-gallium (In-Ga) alloy was pasted onto the rear side of the Si substrate to form the bottom ohmic contact to Si wafer. Finally, electrical measurements of the fabricated SnTe/Si heterostructure photodetectors were performed on a semiconductor characterization system (Keithley 4200-SCS) combined with a probe station, and the high frequency response of the photodetectors was evaluated by a digital oscilloscope. The micromorphology of the cross section of SnTe/Si heterostructure device was observed by means of HRTEM, as shown in Figure 2.(c). The HRTEM image shows that a thin amorphous Sn-Te layer of ～2.0 nm is formed at the heterostructure interface during film growth, as reported previously 32,33. In principle, the existence of the thinner amorphous layer does not significantly influence the transport of photogenerated carriers, since the carriers can easily tunnel through the interface. In fact, the thin amorphous layer is beneficial to the device performance due to passivation of the surface defects on Si, and can help to reduce the rapid recombination of photoexcited carriers caused by the surface dangling bonds of Si layer

34

. Because the disturbance of the symmetry of Si crystal lattice at the wafer’s

surface usually results in nonsaturated dangling bonds, which become the electronically active defects at the Si surface. The dangling-bond-type defect has been identified to be responsible for the surface photoelectron recombination, severely limiting the performance of silicon devices. Thus, to keep the recombination losses of the photogenerated electron-hole pairs at minimal levels, a proper surface passivation scheme is usually needed at the Si interface, where recombination specific properties are determined by the overlying passivating material. According to previous reports, growing a thinner amorphous layer such as amorphous Si, Al2O3, SiO2, SiNx and so on, is beneficial for the passivation of the Si wafer

34,35

. The existence of positive or

6


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negative fixed charge in these amorphous layers can induce field-effect passivation, and hence, modify the dangling-bond-type defect on the Si surface. Similarly, a small fixed charge density can be present in our grown SnTe amorphous layer that can provide an excellent field-effect passivation by shielding holes from the Si surface, and thus the density of dangling bonds at Si surface can be reduced. Consequently, the surface recombination losses of the photon-generated carriers in the SnTe/Si heterostructure can be reduced, and the device performance can be improved due to the passivation behavior. The current-voltage characteristic of the SnTe/Si heterostructure device in dark was then studied via a semiconductor characterization system combined with a probe station at room temperature. The typical current density-voltage (J-V) characteristic curves of the heterostructure device were plotted in Figure 2.(d) in a semilogarithmic scale and linear scale, respectively. To fully evaluate the rectifying performance of the SnTe/Si

heterostructure,

Ratio(V ) =

I (+ V ) R(+ V ) = I (− V ) R(− V )

the

rectification

ratio

is

defined

as

36

:

where I(+V) and I(-V) are the measured currents under the applied positive and negative bias voltages, respectively, and R (+V) and R (-V) are the resistances calculated under the positive and negative bias voltages, respectively. During the analysis of the transport data, we fit linearly the I-V characteristic curve measured at dark in forward and reverse bias voltage directions, respectively. The rectification ratio of the SnTe/Si heterostructure is calculated using the ratio of the two slopes of linear fitting. Note that the heterostructure device exhibits an obvious diode characteristic with a rectification ratio of 2500, indicating that the as-grown TCI SnTe film can form a robust Schottky contact with n-type Si layer. For the as-fabricated SnTe/Si heterostructure, the measured dark current under negative bias voltages is in the range of 8.03×10-8 mA to 1.27×10-3 mA. Using the effective area of the heterostructure of 0.09 cm2, the density of the dark current is calculated in the range of 8.92×10-7 mA·cm-2 to 1.41×10-2 mA·cm-2 in the negative bias voltage region. While for the previously reported n-type topological insulator Bi2Se3/Si Schottky device, the density of the dark current under the negative bias voltages is mainly in the order of 10-3 mA·cm-2～100 mA·cm-2.

29

Thus, owing to the p-type carrier behavior of SnTe

films, the SnTe/Si heterostructure device shows a much lower dark current, and the 7



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reverse dark current density under the applied negative bias voltage was enhanced by 3～4 order of magnitudes, compared to that of previously reported Schottky detector. Via fabricating the SnTe/Si vertical heterostructure, we reduced the dark current at the negative bias voltages, in this case the detectivity (D*) of the photodetector can be greatly enhanced. A photodiode has the lowest dark current, thus leading to an improved detectivity, as well as maximized linearity and sensitivity. To assess the detection capability for near-infrared lights of the as-fabricated SnTe/Si heterostructure devices, the photoresponse characteristics of the device at optical telecommunication bands were explored. During measurements, three commercial 1064 nm,1310 nm,1550 nm laser with a normal incident light direction were used as the excitation light source. The typical photoresponse characteristics of the heterostructure photodetector irradiated by three monochromatic lights of varied light intensity were shown in Figure 2 (e)-(g), respectively. It’s worth to note that the photocurrent strongly depends on light intensity, and increases drastically with increasing light intensity at reverse bias direction, revealing the high sensitivity of the heterostructure device to the three near-infrared lights. It is found that the strongest photoresponse is achieved at about 1064 nm, and reduces gradually with increasing the light wavelength to 1550nm. The larger light response of the SnTe/Si heterostructure device at 1064 nm most likely derives from the stronger light absorption of SnTe film at this wavelength and a larger excitation energy of the incident light. For a heterostructure photodetector, it usually displays a rectifying behavior in the dark, while under the light illumination conditions, it can function at two modes, i.e., the photovoltaic mode and the photoconductive mode. Under the 1064 nm light illumination with lower intensities, the I-V curves of the SnTe/Si heterostructure photodetector usually exhibit a rectifying character, indicating that the photocurrent generation is mainly dominated by the photovoltaic effect. In this mode, the photogenerated electron-hole pairs are separated by the built-in electric field, which generates a considerable fill factor value. However, as the light intensity increases, the rectifying character of the I-V curves of the photodetector is found to be weakened, indicating that the built-in potential at the heterostructure interface is not strong enough. This result suggests that the electrical conductivity of the SnTe film increases due to the stronger absorption of the 1064 nm incident light, causing the photoconductive effect to be dominant on the photoresponse characteristic. Thus, in 8


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this photoconductive mode, the asymmetric I-V characteristic gradually turns to be symmetric, contributing a slow increase of the fill factor. Therefore, the value of the fill factor seems to be decreased with increase of light intensity, and the lower fill factor of the photodetector is most likely due to the higher resistive losses at the heterostructure 37,38. During photoelectric characterization of the SnTe/Si heterostructure photodetector, we found that the photocurrent response under the 1310nm and 1550nm light illumination is mainly located at the reverse bias voltage region, as shown in Figure 2(f) and (g). Although the photocurrent response increases all the time at the reverse voltage region as the light density increases, the change rate of the photocurrent is relative weaker, compared to the increase rate of the forward dark current with increasing the positive voltage. Therefore, we only measured the characteristic curves in the negative bias voltage region systematically. To reveal the device can be operated at zero bias voltage, the photovoltaic behavior of the SnTe/Si heterostructure device was further investigated, and the typical J-V characteristic curves of the device measured with and without light irradiation are depicted in Figure 2 (h). It presents clearly that the device exhibits evident photovoltaic behavior under the simulated solar irradiation due to the built-in potential at the SnTe/Si interface. Measurement results indicate that the SnTe/Si heterostructure device has a short-circuit current density of 3.886 mA cm-2, an open-circuit voltage (Voc) of 0.37 V and a fill factor of 0.482, yielding an energy conversion efficiency of 0.688%. This result implies that the heterostructure device can function as a self-driven photodetector operated at zero bias voltage, thus allowing low on chip power consumption. The p-type charge carriers of SnTe films greatly reduce the annihilation of electron-hole pairs in the heterostructure device at high irradiation intensities, and thus gives rise to a higher saturation value of Voc of 0.37V, which has been greatly enhanced with respect to that of previously reports

29

. The photocurrent switching behavior of the SnTe/Si

heterostructure photodetector was evaluated, and the corresponding temporal response of the photodetector measured at zero bias voltage under 1064nm illumination is shown in Figure 2(i). The result indicates that the device could be reversibly switched between high and low conduction states on light on/off, because the high mobility of carriers in SnTe films is ideal for obtaining fast switching and a high “on” current, while zero bandgap of surface states can induce a large “off” current. The light 9



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response of the device is highly stable in air, and remains identical after several tens of switching cycles. Additionally, due to the lower dark current at 0 V, the Iphoto/Idark ratio reached as high as 8×106, and exhibits the excellent stability and reproducibility even after storing in ambient condition for several months. The light intensity-dependent switching performance of the heterostructure photodetector was also analyzed under the telecommunication wavelengths of 1064nm, 1310 nm (O band) and 1550 nm (C band) at zero bias voltage, respectively. The responsivity (R) is an important figure of merit for a photodetector, which reflects its sensitivity to the incident light and is defined as R = Iphoto/P .39-45 Besides, the detectivity (D*) is another important parameter that quantitatively evaluates the capability of a diode-based detector to detect weak light, and this parameter D* can be calculated by using the following formula

46-51

: D∗ =

A1 2 R ,where Idark and A (2qI dark )1 2

represent the dark current and the detector area, respectively. In the equation for calculating the device detectivity (D*), the area parameter of A is defined as the area of Si opening window on the Si/SiO2 wafer of 0.09 cm2. During the photoelectrical measurement, the spot size of the incident light is adjusted to slightly smaller than the area of the Si opening window. Thus on the basis of the two equations and measurement data, the light responsivity (R) and detectivity (D*) of the SnTe/Si heterostructure photodetector were estimated as well. The error limit is usually less than 5.0%, and the uncertainty originates primarily from the sensitivity and resolution of the light power meter. Panel (a) in Figure 3 shows the typical photocurrent switching characteristic of the device measured under 1064nm light illumination with different light intensities. The extracted photocurrent as a function of light intensity is plotted in Panel (b) of Figure 3 in logarithmic scale, and the corresponding responsivity (R) and detectivity (D*) of the device to the 1064nm light illumination as a function of light intensities were depicted in Panel (c) of Figure 3 in a logarithmic scale. As displayed in Figure 3 (a), the device exhibits pronounced photoresponse at zero bias voltage and can still be effectively modulated by the pulsed light with different light intensities. Note that the extracted photocurrent increases linearly with increasing light intensity at a relatively lower light intensity of

Si Vertical Heterostructure

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