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Realization of $\mathtt{B a Z r S}_{3}$ chalcogenide perovskite thin films for optoelectronics

Xiucheng Wei a, Haolei Hui a, Chuan Zhao a, Chenhua Deng b, Mengjiao Han c, Zhonghai $\mathrm{Yu}^{\mathrm{d}}$ , Aaron Sheng e, Pinku Roy f, Aiping Chen g, Junhao Lin c, David F. Watson e, Yi-Yang Sun h, Tim Thomay a, Sen Yang d, Quanxi Jia f, Shengbai Zhang i, Hao Zeng a,

a Department of Physics, University at Buffalo, The State University of New York, Buffalo, NY, 14260, USA
b Department of Chemistry, Taiyuan Normal University, Taiyuan, 030619, China
c Department of Physics, Southern University of Science & Technology, Shenzhen, 518055, China
d MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an, 710049, China
e Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, NY, 14260, USA
f Department of Materials Design and Innovation, University at Buffalo, The State University of New York, Buffalo, NY, 14260, USA
g Center for Integrated Nanotechnologies, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA
h State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 201899,
China

Department of Physics, Applied Physics & Astronomy, Rensselaer Polytechnic Institute, Troy, NY, 12180, USA

A R T I C L E I N F O

A B S T R A C T

Keywords:
Chalcogenide perovskite
Absorption coefficient
Hall effect
Carrier mobility
Defects

$\mathrm{BaZrS}{3}$ is a prototypical chalcogenide perovskite, an emerging class of unconventional semiconductor. Recent results on powder samples reveal that it is a material with a direct band gap of $1.7–1.8\ \mathrm{eV}$ , a very strong lightmatter interaction, and a high chemical stability. Due to the lack of quality thin films, however, many funda­ mental properties of chalcogenide perovskites remain unknown, hindering their applications in optoelectronics. Here we report the fabrication of $\mathrm{BaZrS}{3}$ thin films, by sulfurization of oxide films deposited by pulsed laser deposition. We show that these films are n-type with carrier densities in the range of $10^{19}–10^{20},\mathrm{cm}^{-3}$ . Depending on the processing temperature, the Hall mobility ranges from 2.1 to $13.7;\mathrm{cm}^{2}/\mathrm{Vs}$ . The absorption coefficient is $>$ $10^{5},\mathrm{cm}^{-1}$ at photon energy ${>}1.97,{\mathrm{eV}}$ . Temperature dependent conductivity measurements suggest shallow donor levels. By assuring that $\mathrm{\bfBaZrS_{3}}$ is a promising candidate, these results potentially unleash the family of chalco­ genide perovskites for optoelectronics such as photodetectors, photovoltaics, and light emitting diodes.

1. Introduction

Semiconductors can be considered as the backbone of modern soci­ ety. They have found broad applications in computer chips, power electronics, optical sensors, light emitting diodes (LEDs), solid-state la­ sers, and solar cells. Most conventional semiconductors are covalent materials with four-fold coordination of both the cations and anions. During last decade, however, the organic-inorganic halide perovskites have attracted considerable attention, as they rival the conventional semiconductors for photovoltaics in an unprecedented way [1–8]. These are ionic materials, which are characterized by a higher coordination maximizing the Coulomb attraction between cations and anions. The strong ionicity is believed to minimize the possibility of forming deep level anti-site defects responsible for non-radiative carrier recombina­ tion. Compared to conventional semiconductors, the halide perovskites have unusually low carrier concentration $({\sim}10^{13}\mathrm{cm}^{-3})$ and extremely long carrier lifetime (on the order of $1~\upmu s)$ [9]. The power conversion efficiency of solar cells made of halide perovskites has witnessed a stellar rate of increase, from an initial PCE of $3.8%$ in 2009 [10] to above $25%$ in 2019 [11].

Specifically, perovskites refer to a class of crystalline compounds adopting the generic chemical formula $\mathrm{ABX}{3}$ , where cation “B” has six nearest-neighbor anions. “X” and cation “A” sits in a cavity formed by eight corner-shared $\mathrm{BX}{6}$ octahedra. They demonstrate a rich spectrum of physical phenomena from 2D electron gas, ferroelectricity/piezoelec­ tricity, ferromagnetism, colossal magnetoresistance, multiferroicity, ionic conductivity, to superconductivity [12–17]. The most commonly studied perovskites are the complex metal-oxides, where X is oxygen. Due to their multifunctionality and highly tunable physical properties, the oxides are an extremely important class of materials for

technological applications.

Over the spectrum of covalency and iconicity, conventional semi­ conductors and oxide/halide perovskites are two extremes. Covalent bonding is directional, making the electronic and optical properties sensitive to bond distortions. In contrast, ionic bonding is often associ­ ated with a strong electron correlation, as the dielectric screening is reduced by the loss of shared valence electrons. Balancing ionicity with covalency therefore provides opportunities for discovering semi­ conductors with properties and performances unattainable in conven­ tional semiconductors. In this regard, it is quite surprising that only limited effort has been devoted to the development of materials inter­ mediate between covalency and ionicity.

Recently, chalcogenide perovskites have emerged as a novel class of semiconductor, where the anions are S and Se instead of O. They are more ionic than the conventional semiconductors but less so than either oxides or halides. Despite being synthesized more than a half century ago, these compounds have received little attention [18–22]. As a result, there is limited knowledge of their physical properties [23,24]. The situation changed only recently, after we theoretically screened [25] 18 $\mathrm{ABX}{3}$ chalcogenide materials, predicting exciting semiconductor prop­ erties for photovoltaics. For example, several of them were found to be direct bandgap semiconductors combining both a strong light absorp­ tion and a good carrier mobility, which is a rare trait for semiconductors and are hence particularly attractive for optoelectronic applications. Subsequent experimental efforts have succeeded in synthesizing several of the prototypical chalcogenide perovskites as well as related phases including $\mathtt{B a Z r S}{3}$ , $\mathrm{CaZrS_{3}}$ , $\mathrm{srTiS}{3}$ , $\mathrm{SrZrS}{3}$ [26], and $S\mathbf{r}\mathrm{HfS_{3}}$ [27,28]. In particular, we further confirmed that $\mathrm{BaZrS}{3}$ possesses a distorted perovskite structure with a $\mathrm{\sim}1.7$ eV [27] bandgap and strong light ab­ sorption, in good agreement with our theory. The material was found to be exceptionally stable against pressure [29], moisture and oxidation [27]. The structural and physical properties show little degradation four years after the synthesis (see Fig. S1 in the Supporting Information, SI). Besides, Niu et al. [26] synthesized and characterized $\mathrm{BaZrS}{3}$ and $\mathsf{S r Z r S}{3}$ . The latter has two different phases, with $\beta{-}S\mathrm{rZr}S{3}$ showing a bandgap of $2.13,{\mathrm{eV}}$ and green light emission. They further demonstrated a giant optical anisotropy in hexagonal ${\bf B a T i S}{3}$ crystal [30]. Meng et al. [31] studied the $\mathbf{BaZr}{1-\mathbf{x}}\mathbf{Ti}{\mathbf{x}}\mathbf{S}{3}$ alloy system, suggesting a limited con­ centration range before phase separation takes place. Optical and ther­ moelectric properties of $\mathrm{SrHfSe}{3}$ and $\mathrm{Sr}{1}{-}\mathrm{xSb}{\mathrm{x}}\mathrm{HfSe}{3}$ were also studied [32]. Intense green luminescence characteristics were found in both undoped and heavily $\scriptstyle\mathbf{n}-$ and $\mathfrak{p}$ -doped $\mathrm{SrHfS_{3}}$ [28]. On the theory front, Ruddlesden–Popper perovskite sulfides $\mathrm{A}{3}\mathrm{B}{2}\mathrm{S}{7}$ were proposed as a new family of ferroelectric photovoltaic materials in the visible [33]. Using machine learning, Agiorgousis et al. isolated ${\tt B a{2}A l N b S_{6}}$ , $\mathrm{{Ba_{2}G a N b S_{6}},}$ $\mathrm{Ca_{2}G a N b S_{6}}$ , $\mathrm{Sr}{2}\mathrm{InNb}\mathrm{S}{6}$ , and ${\bf B a}{2}{\bf S n H f S}{6}$ , out of 450 chalcogenide double perovskites, as the most promising photovoltaic materials [34]. Guided by computational screening of ternary sulfides, Kuhar et al. also iden­ tified and synthesized $\mathrm{LaYS}_{3}$ with a strong light absorption and photo­ luminescence as a promising candidate for photoelectrochemical water splitting [35].

While these studies reveal that chalcogenide perovskite and related compounds are indeed a unique family of optoelectronic materials with promises, a number of fundamental material properties such as the carrier type, concentration and mobility, optical absorption coefficient, and defect properties remain largely unexplored. This is due in large part to the lack of thin film samples, as most experiments have focused on powder or single crystal bulk samples. Lack of the thin film samples thus not only limits our basic understanding, but also becomes an obstacle against device applications.

In this paper, we report the first fabrication of $\mathrm{BaZrS}{3}$ thin films, by sulfurization of $\mathtt{B a Z r O3}$ precursor films deposited by pulsed laser deposition. We show that films fabricated by this method are n-type with carrier density in the range of $10^{19}–10^{20},\mathrm{cm}^{-3}$ . The Hall mobility ranges from 2.1 to $13.7\ \mathrm{cm}^{2}/\mathrm{Vs}$ depending on processing temperature. The optical absorption coefficient is greater than $10^{5},\mathrm{cm}^{-1}$ at photon energy greater than $1.97,\mathrm{,eV}.$ . Temperature dependent conductivity measure­ ments suggest shallow donor levels. Although further optimization is needed, our present results suggest that $\mathrm{BaZrS}{3}$ thin films are promising for optoelectronic applications.

2. Experimental

2.1. Synthesis of $B a Z r S_{3}$ thin films

$\mathtt{B a Z r O3}$ thin films with $100,\mathrm{nm}$ thickness were deposited on sapphire substrates via a Pulsed Laser Deposition (PLD) system at $800,^{\circ}\mathrm{C}$ Oxygen partial pressure of $2.0\ \mathrm{Pa}$ was introduced into the deposition chamber with a background vacuum level higher than $10^{-5}$ Pa. The laser fre­ quency and energy was set to be $5,\mathrm{Hz}$ and $250;\mathrm{mJ}$ , respectively. The asdeposited $\mathtt{B a Z r O}{3}$ films were then loaded into a $2^{\prime\prime}$ quartz tube furnace for sulfurization. The sulfurization procedure lasted for $4,\mathrm{h}$ in an $\mathrm{H}{2}/\mathrm{N}{2}$ atmosphere at 900, 950, 1000, and $1050\ ^{\circ}\mathrm{C}{:}$ respectively. Sulfurization was also done at $1050,^{\circ}\mathrm{C}$ for $2,\mathrm{{h}}$ for the samples used for photodetector measurements. $C S_{2}$ was introduced at $800\ ^{\circ}\mathrm{C}$ as the sulfur source, through $\mathrm{H}{2}/\mathrm{N}{2}$ gas bubbling at a flow rate of 20–25 standard cubic centimeters per minute (sccm).

A Rigaku Ultima IV X-ray diffraction (XRD) system with an opera­ tional X-ray tube power of 1.76 kW (40 kV, 44 mA) and Cu target source was used to acquire the X-ray diffraction pattern (XRD) for investigating the crystal structure. The XRD measurements were performed under theta/2 theta scanning mode and continuous scanning type with a step size of $0.02^{\circ}$ . A Renishaw inVia Raman Microscope was used to measure the room temperature Raman and Photoluminescence (PL) spectra with a $1200,\mathrm{\downarrow/mm}$ grating, $50\times$ objective lens, and $514,\textrm{n m}$ laser. Time resolved PL (TRPL) on the $\mathrm{BaZrS}_{3}$ film was done using an amplified ultrafast excitation laser (repetition rate $250,\mathrm{kHz}$ ) with a pulse duration of $<!200$ fs and a wavelength of $400\ \mathrm{nm}$ . TRPL was collected with a microscope objective with an NA of 0.2 and spectrally and temporally detected on a Hamamatsu streak camera with a time-resolution of $32,\mathrm{ps}$ A Labsphere RSA-HP-8453 reflectance spectroscopy accessory attached to Agilent 8453 ultra-violet/visible (UV–vis) spectroscopy system was used to obtain the absorption spectrum. Surface morphology and energy-dispersive X-ray elemental analysis were performed using a Focused Ion Beam-Scanning Electron Microscope (FIB-SEM) – Carl Zeiss AURIGA CrossBeam with an Oxford EDS system.

2.2. Atomic resolution imaging

Atomically resolved high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images were acquired on a FEI Titan Themis Cube with an X-FEG electron gun and a DCOR aberration corrector operating at $300~\mathrm{kV}$ . The semi convergence angle used was 30.1 mrad. The inner and outer collection angles for the STEM images (β1 and β2) were 54 and 143 mrad, respectively. The beam current was about $20,\mathrm{pA}$ for the ADF imaging. All imagings were performed at room temperature. The EDX elemental mapping was acquired using the SuperEDX system equipped with four detector configurations.

2.3. Device fabrication and optical and transport measurements

A two-terminal device was fabricated for photodetector measure­ ments. Au electrodes were deposited using an e-beam evaporator through a mask with a thickness of $50;\mathrm{nm}$ and a gap of $100,\upmu\mathrm{m}$ between the electrodes. A Keithley 2425 source/meter was used as a voltage source and a Keithley 6485 picoammeter was used as the current meter for I–V measurements under dark and illumination conditions. The illumination was provided by a diode laser at a wavelength of $532;\mathrm{nm}$ with a power of $46\mathrm{mW}$ and spot size of $5\mathrm{mm}$ . For Hall effect mea­ surements, the sample was cut into a Hall bar with dimension of $6,\mathrm{mm\times}$ $1\ \mathrm{mm}$ , and directly wired using silver paste. The transport data were acquired using a Quantum Design Physical Property Measurement

System (PPMS) interfaced to a Keithley 2425 source/meter and two Keithley 2182 voltmeters. The schematic drawings of the devices are shown in Fig. S2 of the Supporting Information.

3. Results and discussion

3.1. Structural characterizations

The XRD pattern of a $\mathtt{B a Z r S}{3}$ thin film sulfurized at $1050,^{\circ}\mathrm{C}$ is shown in Fig. 1(a). It can be seen that the film is polycrystalline with no pref­ erential orientation. The peaks can be matched to those of the standard file JCPDS 00-015-0327, showing that the sample possesses an ortho­ rhombic distorted perovskite structure with Pnma space group. The extremely intense peak is the (0001) peak from the sapphire substrate. In Fig. 1(b), the Raman spectrum of the $\mathrm{BaZrS}{3}$ film measured at room temperature is shown. Five broad peaks can be observed between 50 and $400\mathrm{cm}^{-1}$ , which are identified as $B_{1g}^{1},$ $A_{g}^{4}$ $\mathbb{1}{g}^{4},B{2g}^{6},B_{1g}^{4},$ and $B_{1g}^{5}$ modes. The peak positions match with the theoretical predictions [27,29] and published experimental reports [26,27]. Fig. 2(a) $-2(\mathrm{d})$ show the SEM images of $\mathrm{BaZrS}_{3}$ thin films sulfurized from $900^{\circ}\mathrm{C}$ to $1050\ ^{\circ}\mathrm{C}$ for $4;\mathrm{h}$ , respectively. It can be seen clearly that the films are polycrystalline, with the grain sizes increasing with increasing sulfurization temperature. As shown in Fig. 2(e), the grain size increases from $0.43\pm0.01{\upmu\mathrm{m}}$ at 900 $^{\circ}\mathrm{C}$ to $1.19\pm0.06{\upmu\mathrm{m}}$ at $1050\ ^{\circ}\mathrm{C}.$ . Such grain sizes are in the optimal range for certain optoelectronic applications such as photovoltaics. A typical EDX spectrum of the $\mathrm{BaZrS}{3}$ film synthesized at $1050,^{\circ}\mathrm{C}$ is shown in Fig. 2(f). The atomic ratio of Ba: Zr: S is found to be 1: 1.17: 2.91, close to the stoichiometric composition. The sulfur concentration ranges from 56.9 to $57.8%$ at 6 different locations measured on the same film, sug­ gesting a small inhomogeneity accompanied by a slight sulfur deficiency (Table S1 in Supporting Information). The sulfur deficiency is likely caused by sulfur vacancies [27], due to the high processing temperature. We notice that this is analogous to oxygen vacancies commonly observed in oxide perovskites [36–39]. At lower sulfurization temper­ atures, the S:Ba composition ratio decreases, as shown in Fig. S5 in SI. However, this should not be interpreted as increasing sulfur vacancies, but instead incomplete conversion of $\mathtt{B a Z r O}{3}$ [19]. Further in­ vestigations using aberration-corrected scanning transmission electron microscope (STEM) are shown in Fig. 2(g–i). The atomically resolved high-angle annular dark field (HAADF) image in Fig. 2(g) reveals a well-crystallized structure much resembles $\mathrm{ABO}{3}$ perovskites, consistent with the orthorhombic perovskite phase observed in the XRD mea­ surements. Meanwhile, individual S, $\mathbf{Z}\mathbf{r}$ , and Ba atomic column signals are clearly identified by the atom-by-atom EDX elemental mapping in another region of the crystal view along a different zone axis (Fig. 2(h)), as shown in Fig. 2(i). These atomic resolution characterizations indicate that each grain maintains well crystalline structures, demonstrating the high quality of the as-synthesized $\mathrm{BaZrS}{3}$ polycrystalline thin film.

3.2. Optical characterizations

The band gaps of $\mathrm{BaZrS}{3}$ with distorted perovskite structure have been theoretically calculated to be around 1.7–1.85 eV [25,27,31,40,41] and experimentally verified to be within the same range [26,27,31]. In Fig. 3(a), the UV–Vis absorption spectrum as a function of photon energy is plotted for the $\mathrm{BaZrS}{3}$ thin film synthesized at $1050;^{\circ}\mathrm{C}$ . The thin film geometry allows the extraction of the absorption coefficient α. It can be seen that $\upalpha$ rises rapidly in the range of $1.7–1.8,\mathrm{~eV}$ , and exceeds $10^{5}$ $\mathrm{cm}^{-1}$ at photon energy ${>}1.97$ eV. This confirms the strong light ab­ sorption of the $\mathrm{BaZrS}{3}$ . The band gap energy can be estimated from the Tauc plot in Fig. 3(b). The value is found to be $1.82,\mathrm{eV}$ slightly higher than our previously reported value for powder samples. The PL spectrum shows a broad peak centered at $1.81;{\mathrm{eV}}{\mathrm{.}}$ , with a width of ${\sim}200\ \mathrm{meV}$ , in good agreement with the absorption measurement. The time resolved PL is shown in Fig. 3(c). A bi-exponential fitting [42,43] is found to accu­ rately describes the data. Two time constants extracted are $\tau_{1}=40$ ns and $\tau_{2}=400$ ns, which suggest that the sample may be inhomogeneous, e.g., photo-carrier separation at domain boundaries could account for the slower recombination with a longer $\boldsymbol\uptau$ .

3.3. Transport measurements

To quantify the characteristics of carrier transport, Hall measure­ ments were performed on the four samples with sulfurization tempera­ tures ranging from 900 to $1050\ \ ^{\circ}\mathrm{C}.$ All samples showed n-type conductivity, suggesting that the dominant carriers are electrons. This is likely due to the sulfur vacancies, as each of them will contribute to 2 excess electrons. The carrier density obtained from the Hall effect measurements ranges from $1.06\times10^{\dot{1}9}$ to $4.7\times10^{20},\mathrm{cm}^{-3}$ , as shown in Fig. 4(a), suggesting substantially high doping levels. The conductivity is in the range of $3.52{-}588;;\mathrm{S/cm}$ . Combing these results, the Hall mobility can be calculated and plotted as a function of sulfurization temperature in Fig. 4(c). It can be seen that the Hall mobility increases monotonically with increasing sulfurization temperature. This is ex­ pected as higher processing temperature leads to larger grain size and higher crystallinity, both suppresses carrier scattering. The mobility at $1050,^{\circ}\mathrm{C}$ is found to be $13.7;\mathrm{cm}^{2}/\mathrm{Vs}$ . This value is comparable to that of halide perovskites, such as $\mathtt{M A P b I}_{3}$ [44]. The limiting factor seems to be the carrier concentration. We expect that with proper passivation and reduction of carrier density, this value can be improved by an order of magnitude.

Conductivity was measured as a function of temperature to elucidate the origin of the carriers. It can be seen that the conductivity increases with increasing temperature. Although the limited temperature and conductivity range makes differentiating transport mechanisms diffi­ cult, the conductivity vs. temperature can be best fitted by the Efros

Fig. 1. (a) An X-ray diffraction pattern of a $\mathrm{BaZrS_{3}}$ film sulfurized at $1050;^{\circ}\mathrm{C}.$ . The extremely intense peak comes from the (0001) peak of the sapphire substrate; (b) A Raman spectrum of the $\mathrm{BaZrS_{3}}$ film measured at 300K.

thin films sulfurized at temperatures thin films, obtained film sulfurized at The atomic ratio of Ba: Zr: S is $900\ ^{\circ}\mathrm{C},$ . The
Fig. 3. (a) The UV–vis absorption spectrum; (b) Red curve: the Tauc plot derived from the absorption spectrum; Black curve: the PL spectrum; (c) Time resolved PL spectrum of the $\mathrm{BaZrS}{3}$ thin film sulfurized at $1050,^{\circ}\mathrm{C}$ for $4,\mathrm{h}$ . Time constants $\tau{1}$ and $\tau_{2}$ are found to be 40 ns and $400~\mathrm{ns}$ , respectively from the bi-exponential fitting.

Shklovskii variable range hopping model, where $\begin{array}{r}{G=G_{0}\mathrm{exp}\bigg(-\sqrt{\frac{E}{k_{B}T}}\bigg)}\end{array}$ [45]. On the other hand, a fitting using Arrhenius Law shows a slightly larger deviation from experimental data (Fig. S7 in SI). An activation energy is extracted to be $1.7;\mathrm{meV}$ . This behavior may be understood as related to the average ionization energy of the donor levels to the con­ duction band edge, assuming a narrow distribution of density of states for such levels resulting from sulfur vacancies. Our results suggest that sulfur vacancies act as shallow defect levels that are readily ionized at room temperature. Further systematic studies are needed to identify and quantify the defects in $\mathrm{BaZrS}_{3}$ thin films, and pinpoint the carrier transport mechanisms.

3.4. Photodetector measurements

To investigate the suitability of the $\mathtt{B a Z r S}{3}$ films for optoelectronic applications, photodetector devices were fabricated using the samples sulfurized at $1050,^{\circ}\mathrm{C}.$ . The I–V curves in the dark and under illumination are plotted in Fig. 5. For the sample sulfurized at $1050\ ^{\circ}\mathrm{C}$ for $4;\mathrm{h}{\mathrm{}}$ the difference between the I–V curves in the dark and under illumination is small, due to the large carrier concentration of $2.7\times10^{20},\mathrm{cm}^{-3}$ (Fig. 5 (a)). Postulating that prolonged high temperature processing leads to sulfur vacancy formation, increasing the carrier concentration, we reduced the sulfurization time in an attempt to enhance the photoresponse. As can be seen from Fig. 5(b), reducing the sulfurization time to $2;\mathrm{{h}}$ significantly increases the sample resistivity and decreases the dark current by three orders of magnitude, suggesting that carrier density due to sulfur vacancy is greatly reduced. We are not able to perform Hall effect measurement to determine the carrier concentration on this sample due to its high resistance. Nevertheless, as seen in Fig. 5 (b), the photo-response is significantly enhanced, with an ON/OFF ratio of 20 at the bias voltage of $_{2\mathrm{V}}$ . These results indicate that for better photodetector performance, it is crucial to suppress the dark current by further reducing the carrier concentration. We suggest that p-type doping (e.g. by $\Upsilon$ or La) or sulfurization at high pressure can be considered to passivate the sulfur vacancy states.

Fig. 4. (a) Carrier density, (b) conductivity, and (c) Hall mobility of $\mathrm{BaZrS_{3}}$ thin films as a function of sulfurization temperature for samples sulfurized for $4;\mathrm{{h}}$ , obtained from the Hall effect measurements. (d) Conductivity plotted in logarithmic scale as a function of $T^{-1/2}$ for the $\mathrm{BaZrS_{3}}$ film sulfurized at $1050\ ^{\circ}\mathrm{C}$ for $4,\mathrm{h}$ .

Fig. 5. The I–V curves of the photodetector devices measured in the dark (black curves) and under illumination (red curves) for the $\mathrm{\bfBaZrS_{3}}$ films sulfurized at (a) $1050\ ^{\circ}\mathrm{C}$ for $4\mathrm{h}$ , and (b) $1050\ ^{\circ}\mathrm{C}$ for $2;\mathrm{{h}}$ .

4. Conclusion

In conclusion, we have fabricated $\mathrm{BaZrS}{3}$ chalcogenide perovskite thin films using sulfurization of oxide films deposited by PLD. The films show exceptionally strong light absorption with an absorption coeffi­ cient $>10^{5}\ \mathrm{cm}^{-1}$ at photon energy ${>}2\mathrm{eV}$ . The films are n-type with good carrier mobility of ${\sim}13.7\mathrm{cm}^{2}/\mathrm{Vs}$ . The films are defect tolerant with shallow donors possibly originating from sulfur vacancies. Com­ bined with its earth abundancy, high stability and non-toxicity, $\mathrm{BaZrS}{3}$ is a promising candidate for optoelectronics such as photodetectors, pho­ tovoltaics and light emitting diodes. As importantly, the findings here open the door for fabricating other high quality chalcogenide perovskite thin films for both fundamental studies and device applications.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

Work supported by US NSF CBET-1510121, CBET-1510948, MRI1229208, and DOE DE-EE0007364. Y. -Y.S. acknowledges support by NSFC under Grant 11774365. The work at Los Alamos National Labo­ ratory was supported by the NNSA’s Laboratory Directed Research and Development Program and was performed, in part, at the Center for Integrated Nanotechnologies (CINT), a Department of Energy Office of Science Nanoscale Science Research Center. QXJ is a user of CINT.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.

org/10.1016/j.nanoen.2019.104317.

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Xiucheng Wei received his B.S. degree in physics from Xi’an Jiaotong University, China. He is currently a graduate student in the department of physics at University at Buffalo. His research interests focus mainly on novel photovoltaic semi­ conducting materials and devices.

Haolei Hui received his bachelor’s degree in Xi’an Jiaotong University in 2018. He is currently a PhD student in department of physics at University at Buffalo. He is focusing on novel chalcogenide perovskite as photovoltaic materials.

Chuan Zhao received his PhD degree in Physics from Univer­ sity at Buffalo in 2019. He is currently working at Intel Cor­ poration, Dalian, China. His main research field is about 2D materials such as single-layer WSe2 and WS2.

Pinku Roy received his MTech. in Materials Science and En­ gineering from IIT Kanpur, India. He is currently a graduate student in Materials Design and Innovation department in University at Buffalo. His research focuses on X-ray diffraction, epitaxial thin film, electronic and magnetic materials.

Chenhua Deng received her Ph.D. degree from Shanxi Normal University in 2016. She is currently an associate professor at Taiyuan Normal University. Her current research interest in­ cludes semiconductor spintronic, magnetic recording media, chemistry and physics of nanoscale materials.

Aiping Chen is currently a staff scientist in Center for Inte­ grated Nanotechnologies, one of five Nanoscale Science Research Centers funded by U.S. DOE Office of Science. His research focuses on synthesizing quantum oxide hetero­ structures, tuning functionalities via microstructure, defect, strain and interface engineering, and exploring their applica­ tions in microelectronics.

Mengjiao Han received her Ph.D. in the field of materials sci­ ence and engineering from Institute of Metal Research, Uni­ versity of Chinese Academy of Sciences in 2019. Now she works as a postdoc at the Southern University of Science and Tech­ nology and focuses on the correlations between the atomic structure and the property of two-dimension (2D) materials.

Junhao Lin got his PhD in Physics from Vanderbilt University, USA, in 2015, and joined Southern University of Science and Technology (SuSTech) as an associated professor after 3 years postdoctoral training in National Advanced Institute of Science and Technology (AIST), Japan. His research interest includes analysis of complex defect structures in novel layered materials, real time in-situ observation of the dynamical processes in structural transition of materials under various environmental stimulations, and the phonon behavior of 2D materials as pro­ bed by monochromatic valence electron energy loss spectros­ copy (VEELS).

Zhonghai Yu received his bachelor of science degree from Taiyuan University of Technology in 2014. He is currently a doctoral student working on his PhD, focusing on Chalcogenide Perovskites solar cell at Xi’an Jiaotong University since 2015. He will be a visiting PhD student at the University at Buffalo.

David Watson received his Ph.D. in chemistry from Princeton University in 2001. He joined the University at Buffalo in 2004 after a postdoc at Johns Hopkins University. He is currently a professor and chair of the Department of Chemistry. His research involves synthesizing nanostructured materials and interfaces and characterizing their excited-state charge-transfer reactivity.

Aaron Sheng received his B.S in chemistry from University at Buffalo in 2015. He is currently a 5th year student in the Department of Chemistry at University at Buffalo in the Watson Lab. His research involves synthesis and characterizing of excited-state charge-transfer in vanadium oxide/quantum dot heterostructures.

Yi-Yang Sun received his Ph.D. in physics from National Uni­ versity of Singapore (NUS) in 2004. Since then, he has worked as a postdoc at NUS, National Renewable Energy Laboratory and Rensselaer Polytechnic Institute (RPI), USA. In 2010, he was appointed Research Assistant Professor and later Research Scientist at RPI. In 2017, he assumed a Professor position at Shanghai Institute of Ceramics, Chinese Academy of Sciences. He has been working on first-principles study of energy-related materials.

Tim Thomay received his Dr. rer. nat. in Physics from the University of Konstanz, Germany in 2009. He joined the Department of Physics at the University at Buffalo in 2019 after a 4-year postdoc at NIST Gaithersburg, MD and after joining the EE department at Buffalo as a researcher. His research interests are in the ultrafast dynamics of low dimensional solid-state structures and devices and low photon number quantum optics.

Shengbai Zhang received his Ph. D. in physics from University of California at Berkeley in 1989. He moved to Xerox PARC as a postdoc, before joining the National Renewable Energy Labo­ ratory in 1991. In 2008, he became the Senior Kodosky Constellation Chair and Professor in Physics at Rensselaer Polytechnic Institute. His expertise is first-principles theory, modeling, and calculation. Recent work involves chalcogenide perovskites for photovoltaic, ultrafast phase change memory materials, topological carbon networks, non-equilibrium growth, excited state dynamics, and unconventional twodimensional materials and excitonic insulators. He is a Fellow of the American Physical Society since 2001.

Sen Yang received his Ph.D. in materials physics from the Xi’an Jiaotong University (XJTU), China in 2005. He joined the Na­ tional Institute for Materials Science, Japan in 2005 as a JSPS (Japan Society for the Promotion of Science) post-doctor. In the year of 2010, he came back to XJTU and was promoted to full professor in 2013. His research interests are in magnetism and magnetic materials, smart materials, phase transition and so on.

Hao Zeng received his Ph.D. in physics from the University of Nebraska-Lincoln in 2001. He joined the Department of Physics at the University at Buffalo in 2004 after a 3-year postdoc at IBM T.J. Watson Research Center, and was promoted to full professor in 2014. His research interests are in nanoscale magnetism and magnetic materials, spintronics, 2D materials and unconventional semiconductor materials and devices.

Quanxi Jia is an Empire Innovation Professor and National Grid Professor of Materials Research at the University at Buffalo (UB). Prior to joining UB in 2016, he had worked at Los Alamos National Laboratory for 24 years, with the last two years serving as the co-Director and then Director of the Center for Integrated Nanotechnologies, a US Department of Energy Nanoscale Science Research Center operated jointly by Los Alamos and Sandia National Laboratories. His research focuses on nanostructured and multifunctional materials, with a particular effort on the synthesis and study of processingstructure–property relationships of epitaxial films for energy and electronic applications.