Yifan Wang, Xuanze Li, Pei Liu, Jing Xia,†, and Xiangmin Meng,†

1Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry,Chinese Academy of Sciences, Beijing 100190, China

2Centre of Material Science and Optoelectronic Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: Epitaxial high-crystallization film semiconductor heterostructures has been proved to be an effective method to prepare single-crystal films for different functional devices in modern microelectronics, electro-optics, and optoelectronics. With superior semiconducting properties, halide perovskite materials are rising as building blocks for heterostructures. Here, the conformal vapor phase epitaxy of CsPbBr3 on PbS single-crystal films is realized to form the CsPbBr3/PbS heterostructures via a two-stepvapordeposition process.ThestructuralcharacterizationrevealsthatPbS substratesandtheepilayer CsPbBr3have clearrelationships:CsPbBr3(110)// PbS(100),CsPbBr3[¯1 10]//PbS[001]andCsPbBr3[001] //PbS[010]. Theabsorption andphotoluminescence (PL) characteristics of CsPbBr3/PbS heterostructures show the broadband light absorption and efficient photogenerated carrier transfer. Photodetectors based on the heterostructures show superior photoresponsivity of 15 A/W, high detectivity of 2.65 × 1011 Jones, fast response speed of 96 ms and obvious rectification behavior. Our study offers a convenient method for establishing the high-quality CsPbBr3/PbS single-crystal film heterostructures and providing an effective way for their application in optoelectronic devices.

Key words: heteroepitaxial growth; CsPbBr3; PbS; single-crystal film; photodetector

1. Introduction

Semiconductor heterostructures designed by high crystallization materials expand the application for modern microelectronics and optoelectronics with interesting properties[1-4].Combining different materials to form heterostructures is crucial to manipulate the energy band in semiconductors[5-7],control the lasing dynamics[8,9]and bring the photogenerated carrier transfer in junction areas[10,11]. Such novel requirements vital for electro-optics would be otherwise impossible in a single material system. Over the past few decades, various methods including van der Waals (vdW) epitaxy and vapor phase epitaxy (VPE) have been used for heterostructure fabrications[12-16]. The VPE method, as a fancy and efficient technique, is widely used in IV-V, II-VI conventional semiconductor heterostructures. The epitaxial growth technology can effectively gain a high-purity phase with controlled morphology while maintaining materials' favorable intrinsic properties[17]. Promising heterostructure platforms manufactured by the VPE method realize applications for light-emitting diodes(LED)[18,19], heterostructure transistors[20,21], double heterostructure lasers[22], p-n heterojunction photodetectors[23]and so on. Recently, with the growing interest attracted to electronics and optoelectronics, the discovery of one class of promising materials as building blocks for heterojunction candidates becomes increasingly important.

All-inorganic cesium lead halide perovskite (CsPbX3, X =Cl, Br, I) has attracted significant exploration in optoelectronic applications, such as photodetectors[24,25], LEDs[26], laser devices and solar cells[27,28]. High-quality two-dimensional(2D) perovskite, especially CsPbBr3with better stability, has been demonstrated to be a promising material for creating heterostructures. Up to now, the perovskite-based heterostructures have been focused on abundant materials, such as CsPb-Br3on layered materials (MoS2, graphene, WS2)[29-32], CsPbBr3/TiO2nanocrystals[33]and CsPbBr3/SiO2nanoparticles[34]. Although widespread applications for CsPbBr3-based heterostructures have led to a new approach for band-gap engineering and novel devices, the defects that are caused by lattice mismatches will limit the lifetime of photogenerated electron-hole pairs. The lattice-matching epitaxial growth of CsPb-Br3heterostructures can reduce the defect density and improve the device performance. Recently, core-shell PbSe@CsPb-Br3wire heterostructures were realized by the chemical vapor deposition method[35]. The high-quality conformal CsPb-Br3shell growth on the PbSe core contributed to the high-performance photodetectors. Liuet al. fabricated CsPbBr3/PbS nanoplate heterostructures by vapor transport[36]. Photodetectors based on vertical and lateral heterostructures exhibit clear rectifying characteristics and broadband photodetection. However, epitaxial growth of perovskite heterostructures, especially CsPbBr3single-crystal thin film heterostructures, which is a prerequisite for the continuous improvement of electronic device performance, still remains a challenge so far.

Fig. 1. (Color online) Schematic diagram of the tube furnace setup for the growth of CsPbBr3/PbS heterostructures.

In this study, we report high-quality CsPbBr3/PbS singlecrystal film heterostructures through a two-step vapor phase epitaxy route, where orthorhombic CsPbBr3single-crystal films epitaxially grow on cubic PbS(100) thin films. X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) were used to characterize the crystal structures and epitaxial relationships of the as-prepared heterostructures. CsPbBr3/PbS heterostructures reveal broadband light absorption and wellphotogenerated electron transfer, which were confirmed by absorption and PL spectra. Owing to its unique optoelectronic properties, photodetectors based on the CsPbBr3/PbS heterostructures reveal superior responsivity, high detectivity,fast response speed and obvious rectification behavior.

2. Experimental section/methods

2.1. Growth and transfer for PbS single-crystal films

PbS single-crystal films were first grown on NaCl (100) crystals by the chemical vapor deposition method in a quartz tube furnace. In a typical growth, 30 mg PbCl2powder(99.95%, Alfa Aesar) and 0.5 g sulfur powder (99.95%, Alfa Aesar) were used as sources. Then, a freshly cleaved NaCl(100) crystal as the substrate was positioned on the PbCl2powder with the surface facing down. The PbCl2powder was placed in the center of the tube and the sulfur powder was located approximately 20 cm upstream from the heating center.Before heating, the tube was pumped down to expel the air and then purged with high-purity Ar gas. The heating center was then ramped to 580 °C with the heating rate of 30 °C/min and kept for 15 min. Finally, the tube furnace was cooled down to room temperature naturally. During the growth, a 50 sccm Ar gas was used as the carrier. Owing to the high solubility of alkali salts, epitaxial PbS films can be transferred onto SiO2/Si substrates and can maintain good integrity and crystallinity. Then the as-grown PbS films were used as the temple of CsPbBr3epitaxial growth.

2.2. Epitaxial growth of CsPbBr3/PbS single-crystal film heterostructures

The CsPbBr3/PbS single-crystal film heterostructures were synthesized on SiO2/Si substrates through a two-step growth process in a 1 inch diameter quartz heated by a high-temperature horizontal tube furnace. For CsPbBr3/PbS single-crystal film heterostructure growth, the CsPbBr3powder, which was synthesized via a solution-processed method[37], was used as the source to synthesize CsPbBr3films. And the CsPbBr3films were synthesized using a vapor phase epitaxy method, as illustrated in Fig. 1. In brief, 5 mg CsPbBr3powder was placed at the heating center of the furnace and the as-grown PbS films were positioned downstream at about 9 cm away from the CsPbBr3powder. The heating temperature was controlled at about 500 °C and was maintained at this temperature for 40 min. During the whole process, the quartz tube was kept at the pressure of about 10 Pa with a constant Ar flow of 60 sccm. The details can be found in our previous report[25].

2.3. Device fabrication and performance measurement

Morphologies of the sample were characterized using a Hitachi S4800 field emission scanning electron microscope.EBSD data collection was conducted in a JEOL 6500 SEM. XRD patterns were recorded via a Bruker D 8 Focus powder X-ray diffractometer using Cu Kα radiation (λ= 1.5418 Å). PL measurements were performed in a Nikon Eclipse Ti-U Inverted Microscope with a Princeton SP 2358 spectrometer and a 405 nm laser was adopted as the excitation source. Steadystate absorption spectra were obtained by a Cary 5000 Ultraviolet-visible-near infrared spectrophotometer with circularly polarized lamplight and controlled incident light using a polarizer plate.

Photodetectors based on CsPbBr3/PbS heterostructures were fabricated on the SiO2/Si substrates, which were pre-deposited Au electrodes. A copper grid shadow mask with a 200 mesh grid was covered on the surface of SiO2/Si substrates, followed by thermal evaporation of 100 nm Au as the electrodes. Then, the devices were annealed at 250 °C for 2 h in Ar atmosphere to reduce the contact resistance. Photoelectric characterizations were carried out using a Lakeshore probe station and a Keithley-4200 SCS semiconductor parameter analyzer with 450, 532, 785 and 1550 nm laser illumination under atmosphere environment.

3. Results and discussion

PbS has a NaCl-type structure with a unit cell thickness of 0.594 nm along the [001] direction, which is close to that of NaCl crystal (0.564 nm). From the small lattice mismatch(～5.3%), it can be well inferred that PbS can grow into a single-crystal film on the NaCl-type substrate. Fig. 2(a) displays that the as-synthesized PbS films have a smooth surface with little concave textures, suggesting the growth of the PbS films followed the Volmer-Weber mode. The XRD patterns of PbS films on NaCl (100) are shown in Fig. 2(b). The well-defined peaks located at 30.1° and 62.5° were readily assigned to (200) and (400) planes of cubic PbS by comparing with the standard JCPDS card of PDF#05-0592. The other two peaks related to NaCl can be readily assigned to NaCl (200)and (400) planes (JPCDS, PDF#05-0628). There are only four peaks in the XRD patterns, indicating that there is no other additional phase in PbS films as well as lattice-matching between epilayers and substrates.

Subsequently, the pre-synthesized PbS films were transferred to SiO2/Si substrates and used as the receiving substrates for the epitaxial growth of CsPbBr3/PbS film heterostructures. Representative cross-section and top-view scanning electron microscope (SEM) images of the CsPbBr3/PbS films are displayed in Figs. 3(a) and 3(b). It can be seen that the as-grown heterostructures have a clear interface and both the upper CsPbBr3and lower PbS films demonstrate uniformity and conformal characteristics. Some nucleation points were remained on the surface, indicating a typical Volmer-Weber growth process during the epitaxy of CsPbBr3films as well.XRD results provide further evidence for the crystallographic orientation and crystallinity of the as-synthesized heterostructures, as shown in Fig. 3(c). The peaks related to cubic PbS can be observed of the CsPbBr3/PbS heterostructures, proving excellent crystallinity of transferred PbS single-crystalline films. The epitaxial relationship can be determined as CsPbBr3(110) // PbS(100). In our previous work, the vapor phase epitaxial CsPbBr3films have been proved to be an orthorhombic structure at room temperature[25]. Therefore, the CsPbBr3films in this work showed diffraction peaks at 15.21°and 30.70°, which were compared to the (110) and (220)plane of orthorhombic CsPbBr3(ICSD #97851, space group Pbnm,a= 8.207 Å,b= 8.255 Å andc= 11.759 Å). Fig. 3(d) exhibits the magnified XRD results with three similar peaks. Besides PbS (200) and CsPbBr3(220), the peak at 30.38° was observed and verified to be the CsPbBr3(004) plane, which is reliable proof that the upper CsPbBr3films have orthorhombic structure as well. The peak located at 60.9° was from Si substrates. The absence of other distinct diffraction peaks indicates that no other crystalline phases have been formed.

Fig. 2. (Color online) (a) SEM image of PbS single-crystal films epitaxially grown on NaCl (100) substrates. (b) XRD patterns of the PbS films on NaCl (100) crystals (red) and reference data (JPCDS, PDF#05-0592, black).

Fig. 3. (Color online) (a, b) Cross-section and top-view SEM images of CsPbBr3 films epitaxially grown on PbS (100) films. (c) XRD patterns of the CsPbBr3/PbS heterostructures and standard cards of CsPbBr3 and PbS. (d) Magnified XRD pattern in (a).

Fig. 4. (Color online) (a, b) Experimental results of pole figures from EBSD results for CsPbBr3 and PbS. (c) EBSD orientation map of the CsPb-Br3 films epitaxial growth on PbS.

The orientation relation and crystal structure of CsPbBr3/PbS heterostructures were then characterized by EBSD. Figs.4(a) and 4(b) are pole figures of the epilayer CsPbBr3and PbS substrates. The sharp color contrast of the pole figures confirmed the high crystallinity of samples. Additionally, the measured pole figure of CsPbBr3in (001)-oriented fits well with the result of the PbS (001)-oriented pole figure, proving the orientation relationship is CsPbBr3(110) // PbS(100), CsPbBr3[¯ 110] //PbS[001] and CsPbBr3[001] // PbS[010], which is quite consistent with the XRD results. The EBSD orientation map, as displayed in Fig. 4(c), shows that the CsPbBr3film had a preferable growth direction along the [100] direction, representing a high crystallinity of the CsPbBr3films. Due to the presence of nucleation points remaining on the surface, some small pixels of different colors are shown on the EBSD map.

Fig. 5. (Color online) (a) UV-visible-IR absorption spectra and (b) PL spectra of pristine CsPbBr3 and CsPbBr3/PbS heterostructures.

Fig. 6. (Color online) Photodetector based on CsPbBr3/PbS film heterostructures. (a) I-V curves of the film photodetector in the dark and under 450 nm laser illumination with different light intensities. The inset is the schematic diagram of the device. (b) Light intensity dependence of the photocurrent at the bias of 5 V and corresponding fitting curve. (c, d) Time-resolved photoresponse and the enlarged portion of the device at the bias of 5 V and light intensity of 40 mW/cm2.

The spectra of UV-vis-infrared absorption and PL were used to study the absorbance and emission of the obtained single-crystal film heterostructures. Fig. 5(a) shows the absorption spectra of pure CsPbBr3samples and CsPbBr3/PbS heterostructures which were grown under the same conditions. Compared with CsPbBr3, the heterostructures exhibit broad-spectrum absorption from the visible to mid-infrared region,which makes it an appealing material for optoelectronics[36,38]. Fig. 5(b) shows the PL spectra for the pristine CsPb-Br3films and the CsPbBr3/PbS heterostructures, measured under the 405 nm laser excitation with the same intensity. It is evident that the PL results of heterostructures have strong quenching at the 524 nm wavelength, which is similar to the heterostructures with photogenerated carrier transfer processes[39]. This phenomenon also can be related to the transfer of energy from CsPbBr3to PbS, which hinders the recombination of photoexcited electron-hole pairs.

Fig. 7. (Color online) (a) Schematic diagram of the CsPbBr3/PbS p-n heterojunction photodetector under laser illumination. (b) Dark current and photocurrent of the CsPbBr3/PbS heterostructure detectors under a 532, 785 and 1550 nm light with a power density of 40 mW/cm2.

The photodetectors based on CsPbBr3/PbS single-crystal film heterostructures were directly fabricated on SiO2/Si substrates with pre-deposition Au electrodes. The inset of Fig. 6(a) shows the schematic model of the device. The as-fabricated devices were then annealed at 250 °C in an Ar atmosphere to improve contact between heterostructures and electrodes and reduce resistance. Fig. 6(a) exhibits the currentvoltage (I-V) curves of the photodetectors under 450 nm laser illumination with the light intensity varying from 0 to 40 mW/cm2. In contrast to the dark current, the photocurrentIph(Iph=Iilluminated-Idark) has a significant increase as the light intensity increases from 0.4 to 40 mW/cm2, demonstrating high detection sensitivity of the device.I-Vcurves show a linear increase of the current with the applied voltage ranging from -5 to 5 V, indicating good contact between the heterostructures and Au electrodes.

In previous studies, CsPbBr3shows p-type conductivity without intentional doping while PbS has n-type conductivity[41,42]. Therefore, the as-grown CsPbBr3/PbS film heterostructures are expected to form p-n heterojunctions. To investigate the optoelectronic properties of the CsPbBr3/PbS heterostructures, Au electrodes were redeposited on the surface of CsPbBr3and the bottom of PbS via the same method. The schematic model of the CsPbBr3/PbS p-n heterojunction photodetector is shown in Fig. 7(a). The 532, 785 and 1550 nm laser is used as incident light to illuminate the CsPbBr3/PbS p-n junction region. Fig. 7(b) shows theI-Vcurves of the photodetector under dark and light irradiation. The results show obvious rectification behavior from the visible to the infrared, indicating that a well-defined p-n junction is formed at the interface of the CsPbBr3/PbS heterojunction. In general, the narrow bandgap materials like PbS induce a high dark current[43]. The CsPbBr3/PbS devices have a dark current of value～10-9A, which is lower than that of single PbS photodetectors, attributed to the CsPbBr3as an electron barrier layer in the heterostructures[35,44].

4. Conclusion

In summary, CsPbBr3/PbS single-crystal film heterostructures have been achieved via vapor phase epitaxy. Both the as-synthesized PbS and CsPbBr3/PbS heterostructure films have uniform surface and excellent crystallinity. The epitaxial relationship between the CsPbBr3epilayers and PbS substrates investigated in detail using XRD and EBSD. The absorption and PL spectra have revealed wide light absorption and photogenerated electrons transfer optical properties of the heterojunctions. Photodetectors based on the CsPbBr3/PbS heterostructures exhibit good light absorption conversion efficiency, fast response speed of ～96 ms, high detectivity of～2.65×1011Jones, superior photoresponsivity of 15 A/W,good stability and repeatability. In addition, the devices based on CsPbBr3/PbS p-n heterojunctions show a clear rectification behavior, indicating the construct of functional lowdimensional architectures and a new generation of widespectrum optoelectronic devices.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant No. 11704389), Scientific Equipment Development Project and Youth Innovation Promotion Association Project of Chinese Academy of Sciences.