Organic-inorganic hybrid halide perovskites have taken the center stage in photovoltaics research over the past decade [1][2][3][4], since Kojima et al. first introduced their application as a visible-light sensitizer in 2009 [5]. Many studies have further established their outstanding performance for optoelectronic applications, such as photodetectors [6,7], light emitting diodes (LEDs) [8,9], optical sensors [10,11], and especially solar cells [12,13]. However, the best performing halide perovskites for solar cells often contain the toxic element Pb [5,14]. Furthermore, hybrid perovskites show inherent instability against moisture, heat, light illumination, and electric field. These disadvantages have severely impeded their commercial applications. A lot of effort has been devoted to enhancing the stability of organic perovskites, such as A-site cation engineering (e.g. replacing MA + by FA + and CS + ) [15][16][17] and dimension reduction (e.g. using the two-dimensional Ruddlensden-Popper phase) [18][19][20].
To address the toxicity and stability limitations of hybrid perovskites, inorganic chalcogenide perovskites have been proposed as an alternative family of perovskite semiconductors for optoelectronics. In 2015, Sun et al. theoretically screened 18 ABX3 chalcogenide perovskites for photovoltaics, with A as Ca/Sr/Ba, B as Ti/Zr/Hf, and X as S/Se [21]. Several ABX3 chalcogenide perovskites were identified to have suitable band gaps and absorption properties for photovoltaics. As a prototypical chalcogenide perovskite, BaZrS3 possesses a direct band gap of 1.8 eV, and strong near edge absorption [22][23][24][25][26]. Furthermore, BaZrS3 has been shown to possess a stable perovskite structure against high pressure, moisture, and heat [23,27]. However, little is known about its carrier transport properties as the majority of publications are on bulk powder or single crystals. We first reported the synthesis of BaZrS3 thin films by high temperature (~1000 ℃) sulfurization of BaZrO3 films deposited via pulsed laser deposition (PLD) [28]. These films show n-type conductivity due to sulfur vacancies, with a carrier mobility of around 10 cm 2 /Vs. Comparotto et al. fabricated BaZrS3 thin films by cosputtering of BaS and Zr targets, followed by a thermal treatment at temperatures varying from 650 to 1000 ℃. It was reported that 900 ℃ was required for good crystallization. However, no transport properties were reported [29]. Gupta et al. synthesized BaZrS3 thin films by sulfurization of BaZrO3 films made by chemical solution deposition at 1050 ℃ [30]. Such high processing temperatures are incompatible with device fabrication, and could also lead to unintended high concentration of electron doping due to sulfur vacancy defects. Thus, lowering the processing temperature of chalcogenide perovskite thin films is a crucial step for establishing this family of materials as a viable competitor to hybrid organic perovskites for practical applications.
In this paper we report the synthesis of BaZrS3 thin films at temperatures as low as 500 ℃, which is comparable to the processing temperature of copper indium gallium selenide (CIGS) solar cells [31,32]. This is achieved by changing the chemical reaction pathway from sulfurization of oxide perovskites to crystallization of pulsed laser deposited amorphous BaZrSx films. The films processed at low temperatures show stoichiometric single-phase orthorhombic perovskite structure with smooth surfaces. Elimination of excess sulfur vacancies leads to reduced carrier concentration to ~4×10 10 cm -3 . As a demonstration for the potential use of the BaZrS3 thin films towards various electronic and optoelectronic applications, BaZrS3 thin-film photodetectors and field effect transistors (FET) were fabricated. The photodetector measurements show photocurrent close to 2 orders of magnitude greater than dark current for the films annealed at 650 ℃. The FETs with ionic liquid (IL) gating show ambipolar behavior, with an on/off ratio of 6.510 3 at gate voltage of 1 V and carrier mobilities estimated to be 16.8 cm 2 /Vs and 2.6 cm 2 /Vs for electrons and holes, respectively. Our study paves the way for the integration of chalcogenide perovskite thin films in optoelectronic devices.
Preparation of the BaZrS3 PLD target: BaZrS3 powder was synthesized from BaZrO3 powder following published procedure [24]. The pre-synthesized BaZrS3 powder was cold-pressed into a pellet with a 25 T hydraulic press using a circular mold of 20 mm in diameter. Before pressing, a small amount of polyvinyl alcohol (PVA) was added to enhance the adhesion and stability of the pellet. The BaZrS3 pellet was sintered at 1050 ℃ in Ar atmosphere for 2 hours, and used as target for PLD.
A 1 cm×1 cm (0001) sapphire wafer was loaded into the ultrahigh vacuum (UHV) deposition chamber of the PLD system equipped with a KrF pulsed excimer laser (λ=248 nm), at a base pressure of 1×10 -8 torr. The substrate holder was kept at temperatures of 500-700 ℃ during the deposition, with a laser repetition rate of 5 Hz and laser fluence of 5 J/cm 2 . During the deposition, the substrate and target were both rotated at 30 rpm to ensure homogeneity of the films. The as-grown amorphous BaZrSx thin films were cooled down to room temperature in the UHV chamber and then transferred to an MTI two-zone tube furnace in a 2-inch diameter quartz tube for annealing under CS2 carried by Ar gas flow at 12 standard cubic centimeters. The heating profile and a sketch of the setup are shown in the supporting information (SI) Fig S1 . The as-deposited thin films were placed at the center of zone 2 downstream, with temperatures ranging from 500 to 900 ℃ and corresponding total annealing time ranging from 2 to 8 hours. The temperature of zone 1 was kept at 900 ℃ for complete decomposition of CS2, as the decomposition rate of CS2 is highly temperature dependent [33].
The X-ray Diffraction (XRD) θ-2θ scans were performed using an X'pert Pro X-ray diffractometer operating at 1.6 kW (Cu Kα radiation). Raman spectra were obtained from a HORIBA Raman spectrometer working under 532 nm laser excitation. The scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) analysis were acquired from a Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) -Carl Zeiss AURIGA CrossBeam with an Oxford EDS system. The Bruker AXS-Innova atomic force microscope (AFM) was used to obtain the surface roughness and thickness profiles. The absorption spectra were collected from a Cary series UV-Vis-NIR spectrophotometer measured from 400 nm to 800 nm.
The photodetector devices were fabricated by depositing the source and drain electrodes by electron beam evaporation of a 30 nm thick Au film through a shadow mask, with a deposition rate of 0.4 Å/s, using an AJA Dual ATC-Orion UHV system with a base pressure of 1×10 -8 torr. The photodetector has a channel width of 1000 μm and a gap of 50 μm. The I-V curves were measured by a Keithley 4200-SCS Semiconductor Parameter Analyzer, in the dark and under illumination with an excitation wavelength of 532 nm and light intensity of 0.14 W/cm 2 . The FET devices were fabricated by the same technique through a T-shaped mask. The FET has a channel length of 50 μm and a width of 1000 μm. The side gate is separated from the source and drain electrodes by a 150 μm gap. A drop of the IL Diethylmethyl(2-methoxyethyl)ammoniumbis(trifluoromethylsulfonyl)imide (DEME-TFSI), about 20 µL, was dispensed to cover the active area of the BaZrS3 FET. The device was baked at 120 ℃ for 12 hours in a N2 glove box to remove any moisture in the IL. The drain-source current IDS as a function of drain-source voltage VDS under different gate voltages VG and draingate capacitance CDG vs drain-gate voltage VG were measured by a Keithley 4200-SCS Semiconductor Parameter Analyzer.
Conventional semiconductors and oxide/halide perovskites represent two ends of the spectrum in terms of the covalency-iconicity duality. Chalcogenide perovskites clearly belong to the ionic side of the spectrum, in stark contrast to conventional four-fold coordinated semiconductors. As a crude quantification of the effective ionicity/covalency, one may calculate the Pauling electronegativity ( 𝜒) difference per anion [34]. For BaZrS3, ∆𝜒 = [3𝜒(𝑆)-𝜒(Ba)-𝜒(Zr)] 3 = 1.84 eV. Using the same methodology, ∆𝜒 is found to be 1.43 eV for AlN and 0.37 eV for GaAs. To put it into perspective, BaZrS3 has a moderate band gap of 1.8 eV, close to that of GaAs; but its effective ionicity is even higher than that of wide gap AlN with a band gap of 6 eV. Possessing a moderate band gap with an exceptionally large ionicity puts BaZrS3 and other chalcogenide perovskites in a unique category of defect tolerant semiconductors that may offer new opportunities for electronic and optoelectronic applications.
The high processing temperature of BaZrS3 thin films in earlier studies was mainly attributed to the chemical reaction pathways with high activation barriers. As both group IIA and group IVB elements in perovskites have low electronegativity, their oxides are very stable and difficult to be converted into chalcogenides. In the approach of co-sputtering of BaS/Zr [29], the high processing temperature above 900 C may also be related to the difficulty of the chemical reaction between BaS and Zr, as Zr is a refractory metal with a high melting point of 1855 C. The residual oxides during the handling may further exacerbate the formation of BaZrS3 using sulfur. Therefore, to reduce the processing temperature, a change of the reaction pathway is necessary. PLD has been used to deposit a wide range of compounds including oxides, nitrides and carbides [35][36][37]. PLD is also known to be able to keep the stoichiometry of the target material under optimal conditions [38]. Postulating that by PLD deposition of a stoichiometric BaZrS3 compound target under optimal conditions, BaZrS3 perovskite thin films can be obtained under milder conditions. We found that the stoichiometry of the Ba:Zr cations of the as-deposited films is dependent on laser fluence of the PLD system. This effect can be understood as primarily due to the large difference in the cohesive energy of Ba vs. Zr (1.9 eV/atom vs. 6.25 eV/atom). Therefore, low laser fluence is expected to result in Ba-rich films, while very high laser fluence may result in Ba deficiency due to reverse sputtering. Under a narrow window of fluence of close to 5 J/cm 2 , a 1:1 ratio of Ba:Zr was obtained, as shown in Fig S2(a), However, it was found that the asdeposited films were amorphous, even with an in-situ substrate temperature as high as 700 C (Fig S3). It was further determined that these films are highly sulfur deficient, with a composition of BaZrS2. To obtain stoichiometric, single phase BaZrS3 in the orthorhombic perovskite structure, thermal annealing of the as-deposited amorphous BaZrS2.4 films in CS2 at various temperatures ranging from 500 ℃ to 900 ℃ were performed in a two-zone tube furnace. All of the BaZrS3 films after annealing appeared dark red in color (see optical images in Fig 1(b)), except the one annealed at 500 C, indicating strong visible light absorption. The 500 C sample does show lighter color, suggesting incomplete crystallization. The sulfur concentration as measured by EDX is annealing temperature dependent, as seen in Fig S4 . While as-deposited BaZrS2.4 films are sulfur deficient, the composition of the films progressively approaches perfect stoichiometry of Ba:Zr:S=1:1:3 with decreasing annealing temperature. To investigate the annealing temperature dependent structural evolution of the BaZrS3 films, XRD θ-2θ scans were performed for films annealed at 900 ℃, 850 ℃, 650 ℃, 550 ℃, and 500 ℃, respectively, as shown in Fig 1(a). The peaks labeled with Miller indices indicate standard peak positions of BaZrS3 with a Pnma structure, retrieved from the PDF card JCPDS 00-015-0327. It is found that all discernible peaks for the samples annealed from 500 to 900 C match well with the reference. No additional peaks were observed, suggesting that the obtained films were of a single phase. Some diffraction peaks with low intensities were not clearly observed from films annealed at 550 ℃ and 500 ℃, as a result of low crystallinity and small grain sizes at such low temperatures. As can be seen from Fig 1(b), the full width at half maximum (FWHM) of the (121) diffraction peak decreases from 0.62 to 0.45 with increasing annealing temperatures from 550 ℃ to 900 ℃. The crystallite sizes as calculated from Scherrer's formula using the FWHM of the (121) diffraction peak decreases monotonically with decreasing annealing temperature. The lateral grain sizes, as imaged by AFM and SEM (shown in Fig 2(a)-(h) and Fig S5) also decreases monotonically from ~110 nm at 900 ℃ to 35 nm at 550 ℃. It is interesting to note that the lateral grain sizes are several times larger than the vertical grain sizes. Understanding the detailed crystallization mechanisms in the future will help to obtain films with enhanced grain size and crystallinity.
The surface roughness of a semiconductor thin films plays an important role in determining the interface quality, carrier transport in junctions, and thus device performance [39,40]. It is expected that a lower annealing temperature with smaller crystallite size should also possess lower surface roughness. The AFM top-view images in The room temperature Raman spectra of BaZrS3 films annealed at 900 ℃, 850 ℃, 650 ℃, and 550 ℃ are shown in Fig 3(a). Several peaks can be identified in the range of 50 cm -1 to 500 cm - 1 and six of them are assigned to be 𝐵 1𝑔 1 , 𝐴 𝑔 4 , 𝐵 2𝑔 6 , 𝐵 1𝑔 4 , 𝐵 1𝑔 5 and 𝐵 3𝑔 5 vibrational modes calculated theoretically for BaZrS3 with the Pnma structure [27]. The Raman spectra measured at low temperature (100K) are provided in Supporting Information Fig. S6. In addition, we measured the Raman spectra at higher wavenumbers between 1000 cm -1 and 2000 cm-1 for all the films to investigate the possibility of carbon contamination resulting from CS2 decomposition. As can be seen from Fig. 3(b), for the film annealed at 900 C, two peaks at 1330 and 1620 cm -1 were observed, which can be attributed to the D and G bands of carbon materials such as graphite and nano-diamonds. While these modes are barely observable for the sample annealed at 850 C, they are absent in the films annealed at and below 650 C. This suggests that carbon contamination has been eliminated by low temperature processing. The presence of carbon also explains the color difference of BaZrS3 processed at different temperatures: while the films processed at 1050 C show black color (not shown), the films processed at 900 C and below are dark red.
BaZrS3 thin films annealed at 650 C and 900 C were selected for UV-vis absorption measurements to examine their optical properties. The thicknesses of these films are about 280 nm, as measured by AFM (Fig 4(a)). The absorption spectra are shown in Fig 4 (b). It can be seen that their absorption coefficient α above the band gap are ~ 5×10 4 and ~ 6×10 4 cm -1 , respectively. These values are somewhat smaller than that of BaZrS3 films processed at higher temperatures [28]. A linear fitting near the band edge of the Tauc plot results in a band gap value of 1.98 and 1.86 eV, respectively, for the films annealed at 650 C and 900 C, respectively, as shown in Fig 4(c). These values are slightly higher than the band gap in the films processed at 1050 C. Several factors could contribute to these differences: 1. The presence of a minute amount of amorphous oxides/oxysulfides, possibly introduced in the target processing, could not be completely eliminated in low temperature processing. The wide gap oxides/oxysulfides may lead to an overestimate of the band gap, as well as lower the absorption coefficient. Careful examination of XRD patterns of samples processed at different temperatures (Fig. 1(a)) reveals no oxide peaks or shift of the peaks compared to the BaZrS3 standard, within the instrument limit. This rules out the presence of measurable amount of crystalline oxides or oxysulfides; 2. The presence of carbon contamination in previous high temperature processed samples may lead to overestimate of the absorption coefficient and underestimate of band gap, as carbon absorbs in a broad range of wavelengths; 3. We observed a small but noticeable shift of the Raman peaks to higher wavenumbers at higher processing temperatures (Fig. S6). While presence of oxysulfides could lead to shift of Raman modes to higher frequencies, this is inconsistent with the fact that this shift is for samples processed at higher temperatures and therefore conversion to the BaZrS3 phase is more complete. We suggest that a slight difference in structural distortion (octahedron rotation) at different processing temperatures is responsible for the observed Raman shift, which could contribute to subtle differences in electronic structures and band gaps [41].
In summary, lower temperature growth of BaZrS3 results in stoichiometric films with minimized concentration of sulfur vacancies, eliminated carbon contamination, smaller crystallite sizes and lower surface roughness. These are expected to strongly affect the carrier transport characteristics and device performance. To investigate the influence of film microstructure on carrier transport, we fabricated and measured photodetector and FET devices using the BaZrS3 films annealed at different temperatures as active semiconductors.
The I-V curves of the photodetector devices were measured in the dark and under illumination, for films annealed at temperatures of 850 ℃, 650 ℃, and 550 ℃, respectively, as shown in . At 850 ℃, the I-V curves show little difference under dark and illumination. This suggests that carriers available for the transport are essentially intrinsic to the material, and the contribution from the photogenerated carriers to the transport is negligible. The high dark current can be attributed to the high concentration of sulfur vacancies, as each sulfur vacancy can donate two electrons. Using the sulfur vacancy concentration measured by EDX (Fig S4 ), we estimate the carrier density to be 1.710 21 cm -3 . For the film annealed at 650 ℃, the measured photocurrent is 80 times greater than the dark current, as can be seen in Fig 5(b). The film annealed at 550 ℃ shows the highest photocurrent of 17 nA at 10 V. However, the on/off ratio is only 20 times due to the relatively higher dark current. The relatively higher dark current could be caused by the lower crystallinity, where a number of defects may be present and contribute to excess carriers, as shown by our unpublished theoretical study [42]. The photo responsivity is calculated to be 0.08 mA/W at a bias voltage of 5V for the films annealed at 550 ℃. The moderate photo response is primarily due to the relatively small grain sizes, which leads to significant carrier scattering and limits the carrier mobility. In , respectively. The total specific capacitance of the IL is determined by the capacitance of the electrical double layer (EDL), and can reach as high as ~20 μF/cm 2 [43], which makes it possible to generate high electric field of ~ 10 MV/cm at low gate voltages [44], comparing to commonly used metal oxide dielectric such as SiO2, Al2O3 or HfO2. At a VDS of 1 V, IDS is measured to be 310 -11 A without gating, which suggests very small intrinsic carrier density. The intrinsic carrier type is found to be n-type, which is consistent with our previous work [28]. The intrinsic carrier concentration is found to be 4.4×10 10 cm -3 . These results further confirm that carrier concentration is suppressed by reducing the density of sulfur vacancies in BaZrS3 films processed at low temperatures. With increasing VG to 1V, IDS reaches to 210 -7 A, resulting in an ON/OFF ratio of 6.510 3 . Similar but smaller IDS can be observed for negative VG, as shown in Fig 6(b), suggesting ambipolar behavior in our FETs. We further estimate the electron and hole mobilities to be 16.8 cm 2 /Vs and 2.6 cm 2 /Vs, respectively, from the saturated IDS vs. VG data. The capacitance of the IL layer used in the calculation was obtained from the C-V measurements. The carrier mobility is primarily limited by the small grain sizes which contribute to substantial carrier scattering. Future work should focus on improving the grain size of BaZrS3 films processed at low temperatures to improve the performance of electronic and optoelectronic devices.
In conclusion, we have obtained polycrystalline perovskite BaZrS3 thin films, by low temperature crystallization of PLD deposited amorphous BaZrSx films in CS2. The films with good crystallinity and a surface roughness smaller than 0.6 nm can be obtained at processing temperatures as low as 550 ℃. The low processing temperatures result in the elimination of both excessive carriers due to vacancy formation and carbon contamination. Prototype photodetector devices fabricated with such films show good visible light sensitivity, and their wavelength dependent photo response confirms a band gap of ~ 1.85 eV. The FET indicates ambipolarity of the charge carriers, with electron and hole mobilities of 16.8 cm 2 /Vs and 2.6 cm 2 /Vs, respectively. Our findings will expedite the progress towards applications of BaZrS3 and other chalcogenide perovskites for electronics and optoelectronics.