High-Quality LaVO3 Films as Solar Energy Conversion Material - ACS

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High quality LaVO films as solar energy conversion material Hai-Tian Zhang, Matthew J. Brahlek, Xiaoyu Ji, Shiming Lei, Jason Lapano, John W Freeland, Venkatraman Gopalan, and Roman Engel-Herbert ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16007 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

High quality LaVO3 films as solar energy conversion material Hai-Tian Zhang1, Matthew Brahlek1, Xiaoyu Ji1, Shiming Lei1, Jason Lapano1, John W. Freeland2 Venkatraman Gopalan1 and Roman Engel-Herbert1, * 1

Department of Materials Science and Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA 2

Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA.

Corresponding author: *E-mail: [email protected] Keywords: photovoltaic materials, Mott insulator, thin film, transitional metal oxide, physical vapor deposition.

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ABSTRACT Mott insulating oxides and their heterostructures have recently been identified as potential photovoltaic materials with favorable absorption properties and an intrinsic built-in electric field that can efficiently separate excited electron-hole pairs. At the same time, they are predicted to overcome the Shockley-Queisser limit due to strong electron-electron interaction present. Despite these premises a high concentration of defects commonly observed in Mott insulating films acting as recombination centers can derogate the photovoltaic conversion efficiency. Utilizing the self-regulated growth kinetics in hybrid molecular beam epitaxy this obstacle can be overcome. High quality, stoichiometric LaVO3 films were grown with defect densities of in-gap states up to two orders of magnitude lower compared to the films in literature, and a factor of three lower than LaVO3 bulk single crystals. Photoconductivity measurements revealed a significant photo-responsivity increase as high as 10-fold of stoichiometric LaVO3 films compared to their nonstoichiometric counterparts. This work marks a critical step towards the realization of high performance Mott insulator solar cells beyond conventional semiconductors.


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1. INTRODUCTION Oxides with strong electron-electron correlation exhibit exotic properties such as high temperature superconductivity, colossal magnetoresistance and metal-to-insulator transitions (MITs).1,2 The MIT of electronic phase change oxides can be triggered under external stimuli, attracting intensive research interest for low power logic applications such as transistors, high frequency switches and memory devices,3–5 and the enhanced carrier mass due to strong electron correlation allows the utilization of correlated metals as transparent conductors.6 In addition another class of strongly correlated oxides, perovskite Mott insulators with a general formula ABO3, have recently gained interest and were proposed as new photovoltaic materials.7 Unlike conventional semiconductors, Mott insulators are characterized by strong correlation among electrons with short electron-electron interaction times on the order of ~10 fs, much faster than the typical time scale of electron-phonon interaction (~1 ps).8 Hence, rather than thermalizing excess energy absorbed by photo-excited carriers it was theoretically predicted to excite more than one free carrier per photon,8,9 potentially overcoming the Schottky-Queisser limit.10 In the case of oxide heterostructures the polar discontinuity at interfaces can be used to set up an electrical field across the photo-absorbing Mott insulator layer to separate the photo-excited electron-hole pairs,7 rendering doping schemes obsolete and the development of oxide heterostructures for highly efficient solar cell designs favorable. In particular, the Mott insulator LaVO3 has a bandgap of 1.1~1.2 eV,11 which lies within the optimal range for solar cell applications.10



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However, for the implementation of ABO3 Mott insulators into photovoltaic devices the high concentration of intrinsic and extrinsic defects is of concern, which may act as recombination centers for photo-excited electron-hole pairs, leading to an inevitable reduction of solar cell efficiency.12 Some defects are easily introduced if the film has a non-stoichiometric A:B cation ratio, i.e. is deviating from an ideal La:V of 1:1.13,14 Therefore, it is critical to achieve a precise control over stoichiometry when growing LaVO3 thin films. Although tremendous progress has been made in the oxide thin film growth using modern deposition technique such as pulse laser deposition (PLD) and molecular beam epitaxy (MBE),15–18 precise A:B site ratio control remains challenging because of the large uncertainty in flux control and the low energy barrier to form nonstoichiometric defects.13,19,20 Here, we report the optical and electrical properties of high quality LaVO3 thin films grown by hybrid molecular beam epitaxy,21–25 whereby the La:V ratio was self-regulated within a growth window. This adsorption controlled growth mechanism was enabled by using a volatile metal organic precursor vanadium oxytriisopropoxide (VTIP) as the vanadium source.22,24,26,27 Structural, electrical and optical properties of the LaVO3 films were measured and related to film stoichiometry and growth conditions. While films grown within the growth window showed characteristic electronic transitions similar to those observed in bulk single crystals28 and predicted by first principle calculations,29,30 pronounced deviations were found for non-stoichiometric LaVO3 films grown outside the growth window, attributed to an increased defect concentration. Based on optical conductivity measurements, the defect density of gap states in stoichiometric films was estimated to be up to two orders of magnitude lower compared to nonstoichiometric and previously reported LaVO3 films,31,32 and a factor of three lower 4 ACS Paragon Plus Environment

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than the best reported bulk single crystal,28,33–35 further manifesting hybrid molecular beam epitaxy as promising synthesis route towards ‘electronic grade’ complex oxide thin films. Photoconductivity measurements on two-terminal devices were performed to relate film stoichiometry to carrier recombination rate, a direct measure of the defect concentration in the film. Stoichiometric LaVO3 films grown inside the growth window exhibited a three to tenfold increase in photocurrent compared to their La-rich and V-rich counterparts, attributed to a much lower carrier recombination rate. 2. EXPERIMENTAL SECTION 2.1 LaVO3 film growth: LaVO3 films were grown by hybrid molecular beam epitaxy on (001) (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrates. Elemental La (4N, Ames National Lab) was supplied by a

effusion

cell

and

V

was

supplied

through

a

metal

organic

precursor

vanadium

oxytriisopropoxide (VTIP) (trace metal impurity 4N, MULTIVALENT Laboratory).6,21–23 The substrates were sonicated in acetone and isopropyl alcohol bath and then baked in the load lock chamber (1×10-8 Torr) for 2 hours at 150 °C. Samples were subsequently transferred to the MBE growth reactor (DCA M600) and heated to the growth temperature of 850 °C (thermocouple reading). Prior to growth the LSAT was exposed to a remote oxygen plasma cleaning for 20 minutes (RF plasma power 250 W). The growth was performed in an oxygen background pressure of 1×10-7 Torr for 40 minutes. For the growth, the La flux was fixed at ΦLa=2.0×1013 cm-2⋅s-1 (calibrated by a quartz crystal microbalance) while the VTIP flux was varied from sample to sample by changing the gas inlet pressure pVTIP from 32.5 to 45 mTorr as determined by a capacitance manometer located in the VTIP



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gas inlet system).24

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The thickness of LaVO3 films was ~25 nm, confirmed by fitting the thickness

fringes in XRD measurements. 2.2 Structural characterization: A Phillips X’Pert Panalytical MRD Pro thin film diffractometer equipped with a duMond–Hart–Partels Ge (440) incident beam monochromator and Cu Kα1 radiation was used for XRD measurements. Atomic force microscope (AFM) experiments were carried out with a Bruker Dimension Icon atomic force microscope operated in a peak force tapping mode. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging was performed on the FEI Titan G2 60-300 kV equipped with an X-FEG operating at 80 keV. The sample was mechanically polished to 20 µm, followed by thinning to electron transparency using a Gatan PIPSII Ar-ion mill. 2.3 Electrical and optical characterization: Temperature dependent resistivity measurements were carried out in a physical property measurement system (PPMS) in Van-der-Pauw geometry. An M-2000U J. A. Woollam spectroscopic ellipsometer was used to monitor the optical properties of LaVO3 thin films and fit to an optical model, see Suppl. Note 1 for details. X-ray absorption spectroscopy (XAS) measurements were performed at beamline 4-ID-C of the Advanced Photon Source in the electron yield mode of acquisition at 300 K. A Fianium SC-450 supercontinuum laser combined with an acousto-optic tunable filter was used as the excitation source for the spectral photoconductivity measurements on the LaVO3 films. The laser beam diameter was 2 mm. The optical power illuminated on the LaVO3 films at each wavelength was measured by a Thorlabs S122C Ge photodiode detector. The photocurrent was measured under 1V bias using a Keithley 2635A 6 ACS Paragon Plus Environment

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sourcemeter in a two-probe geometry. The test structure was made by depositing 100 nm thick Pd electrode on LaVO3 films by electron beam evaporation.

3. RESULTS AND DISCUSSION A series of 25-nm-thick LaVO3 films was grown on insulating (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) substrates with a constant La flux of ΦLa= 2.0×1013 cm-2⋅s-1 at various VTIP gas inlet pressures pVTIP=32.5…45 mTorr). Figure 1a) shows 2θ-ω X-ray diffraction (XRD) scans around the LSAT 002 substrate peak. Thickness fringes were observed for all films attributed to multi-interference effects, indicating a smooth film surface and a sharp interface. A systematic change of the LaVO3 002 film peak position was found and the extracted out-of-plane lattice parameter of LaVO3 films (in the pseudo-cubic notation) is plotted as a function of pVTIP in Fig. 1b). For LaVO3 grown at pVTIP=34-38 mTorr, the out-of-plane lattice parameter remained constant and independent of growth condition marking the growth window of LaVO3, as previously reported using Rutherford backscattering spectrometry (RBS) for LaVO3 films grown on SrTiO3 substrates.22 For growth conditions outside the growth window the film lattice parameter was found smaller in agreement with previous studies.22,36 The out-of-plane film lattice parameter (~3.99 Å) of stoichiometric LaVO3 films within this ‘growth window’ was larger than the bulk value (3.93 Å) due to an in-plane compressive strain (-1.58%) imposed by the LSAT substrate. Reciprocal space mapping (RSM) of a stoichiometric film was performed around -103 peaks of the LSAT substrate for the film grown at pVTIP=36 mTorr, see Fig. 1c).

The in-plane lattice parameter of the LaVO3 film was found similar to that of the LSAT 7 ACS Paragon Plus Environment


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substrate indicating that the film was coherently strained. Since stoichiometric LaVO3 films have the largest lattice parameter, all films grown in the stoichiometric series are expected to be coherently strained as well. Thus the out-of-plane lattice parameter is a direct measure of the intrinsic lattice parameter. Atomic force microscopy (AFM) and scanning transmission electron microscopy (STEM) results of the films grown at pVTIP=36 mTorr are shown in Figs. 1d) and 1e). The film surface was atomically flat with a root mean square roughness (rms) value of 0.14 nm and step terrace morphology. An abrupt interface between the film and substrate was observed in STEM consistent with the thickness fringes observed in XRD. The LaVO3 film formed a good epitaxial relationship with the substrate and no defects were found in cross-sectional STEM images taken along the LaVO3 pseudo-cubic [010] zone axis. TEM scans over larger area were also performed on the La-rich, stoichiometric and V-rich films, see Fig. S1. No defect was observed in stoichiometric film as otherwise observed in the nonstoichiometric ones or previously reported films grown by pulsed laser deposition (PLD).37 The relationship between structural properties and growth conditions was found similar to the trends studied more in depth elsewhere.22 LaVO3 is a Mott-Hubbard type insulator in which the strong on-site Coulomb repulsion splits the vanadium t2g states into an occupied lower Hubbard band (LHB) and an empty upper Hubbard band (UHB), separated by the band gap.38 The Hubbard bands are located between lower lying states derived from oxygen 2p and higher lying vanadium eg bands. The density of states of LaVO3 around the Fermi level (EF) is schematically shown in Fig. 2a), which is based on previous bulk single crystal studies and first principle calculations.28,39,34 The three main transitions in the low energy region (< 6 eV) with 8 ACS Paragon Plus Environment

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relevance to photovoltaic applications are: transition I centered around 1.7 eV from the LHB to the unoccupied UHB; transition II centered around 2.2 eV from the LHB to vanadium eg band; and transition III centered around 5.3 eV from oxygen 2p to the UHB and the vanadium eg band. Figure 2b) shows the optical absorption spectrum of LaVO3 films grown at different pVTIP pressures. The peaks in the absorption spectra were labeled as I, II, and III according to their transition. All films had high absorption coefficients in the visible range of around ~mid 104 cm-1, which is comparable to that of common thin film solar cell absorber material, such as CuInGaSe2 (CIGS), CdTe and GaAs.40–42 For films grown within the growth window (pVTIP=34-38 mTorr) a pronounced two-peak feature with maxima around 1.7 and 2.2 eV and the onset of a strong absorption peak around 5 eV were observed, in good agreement with the expected absorption spectra from the density of states shown in Fig. 2a) and bulk single crystal data, see Fig. S2. For films grown under La-rich condition the double peak feature remained almost unchanged, while it transitioned into a single peak for films grown under V-rich conditions. More importantly, films grown under nonstoichiometric conditions revealed a dramatic increase in the optical absorption for below band gap excitation. The absorption coefficient at a photon energy of 1.10 eV, i.e. 0.02 eV below the band edge determined from photoconductivity measurements as discussed below, is shown in the inset of Fig. 2b). An almost two orders of magnitude increase of α was found independent whether the films were La-rich or V-rich. This increase of optical absorption for below band gap excitation was attributed to transitions involving defect states located within the band gap, indicating orders of magnitude higher defect concentration in the case of nonstoichiometric LaVO3 films. The growth window previously defined by structural analysis therefore directly translated into the physical 9 ACS Paragon Plus Environment


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properties of LaVO3 films. The absorption spectrum for films grown inside the growth window were virtually identical, exhibiting the lowest defect concentration independent of growth condition. To further explore how the local electronic structure of LaVO3 was affected by nonstoichiometry, X-ray absorption spectroscopy (XAS) measurements were performed on representative La-rich (pVTIP=32.5 mTorr), stoichiometric (pVTIP=36 mTorr) and V-rich films (pVTIP=40 and 45 mTorr). Figure 2c) shows the X-ray absorption spectra around the V L2,3 edge of LaVO3 films, which probe the electronic transition from occupied vanadium 2p core level to unoccupied vanadium 3d orbitals. In agreement with previous reports on RVO3 (R being an rare earth element),29,30,43 a double peak feature located at ~518 eV, labeled transitions A and B in Fig. 2c), and a relatively broad, single peak at ~524 eV (transition C) with pronounced intensity modulation on both sides were observed. These transitions were attributed to spin-orbit splitting of the vanadium 2p level into a four-fold degenerate 2p3/2 (518 eV) and a two-fold degenerate 2p1/2 (524 eV) state. In accordance with first principle calculations29,30 transition A and B were associated to excitations from the V 2p3/2 to the UHB derived from the V 3d t2g orbitals and V 3d eg orbitals, respectively. Two distinct characteristics were observed for V-rich films: 1) with increased VTIP flux, transition C shifted to higher energy by ~0.4 eV, indicating an increase in the valence state of vanadium.43 2) Relative intensity of transition A to transition B decreased as the film became vanadium rich [Fig. 2d)], suggesting fewer unoccupied electronic states in the UHB available for X-ray absorption possibly due to an electron donation effect from defect states introduced under V-rich conditions. The spectral features of La-rich and stoichiometric films were similar in both, the peak position of transition C and the intensity ratio of transition A and B. Temperature dependent resistivity 10 ACS Paragon Plus Environment

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measurement were performed and plotted in Fig. 2e). All films showed semiconducting behavior. While films grown under La-rich and stoichiometric conditions revealed a high room temperature resistivity of ~5 Ω⋅cm, the resistivity was reduced by orders of magnitude for films grown under V-rich conditions. Electrical transport measurements were consistent with XAS results and the electron donor scenario for V-rich films, causing a higher free carrier concentration. This scenario is further supported by Hall measurements where the room temperature free electron concentration increased from 2.4 ×1018 to 3.1×1020 cm-3 for stoichiometric LaVO3 compared to a V rich film grown with PVTIP=64.5 mTorr under a La flux of 2.5×1013 cm-2s-1. It was previously reported that stoichiometric LaVO3 single crystals show a marked kink in the resistivity measurement around 140 K due to orbital and spin ordering.44 However, for the stoichiometric LaVO3 thin films, the measurement limit was reached around 250 K due to their high sheet resistance. Defects in La-rich films are therefore expected to form deep trap levels because the room temperature resistivity did not change with respect to stoichiometric films. Considering that in a closed-packed perovskite structure, excess cations are unlikely to occupy interstitial sites, the following defect accommodation mechanisms are proposed for nonstoichiometric LaVO3 films: In case of V-rich films, the extra V ions with valence state higher than three (observed from XAS) occupy the vacant La3+ site, contributing free electrons to the conduction band. This scenario would be consistent with the reduced lattice parameter, since the radius of the vanadium ion is much smaller than the lanthanum ion, irrespective of its valence state. Although details of the point defect mechanism in La-rich LaVO3 films is not clear, STEM measurements revealed secondary phases precipitating in the perovskite matrix, see Fig. S3. While the L-V-O phase diagram did not contain any phase with a La:V ratio smaller than one but 11 ACS Paragon Plus Environment


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only La2O3 or LaVO4,45 the origin of the unwanted phase remained unclear and needs further investigation. To compare the quality of LaVO3 films grown in a self-regulated manner with results reported in literature, optical conductivity at 1.10 eV, just below the band edge, has been used as a measure for the defect concentration in the film. The optical conductivity σ is directly related to the absorption coefficient α by σ =

, where c is the velocity of light in vacuum, and vacuum permittivity.

Data for bulk crystals28,33–35 and films31,32 are compiled in Fig. 3a) along with the results obtained from the films grown by hybrid MBE. The best bulk single crystal had an optical conductivity at the band edge as low as ~7 Ω-1⋅cm-1, while the optical conductivity reported for thin films grown by PLD were around 30~300 Ω-1⋅cm-1. Films grown by hybrid MBE under La-rich (pink) and V-rich (green) conditions had similarly high optical conductivities around 50~300 Ω-1⋅cm-1. The high values were attributed to a high defect concentration resulting from a nonstoichiometric cation ratio in the film. On the other hand, stoichiometric films grown inside the growth window had a very low optical conductivity of only 2 Ω-1⋅cm-1 at the band edge, more than a factor of three lower than the best reported bulk single crystals and more than one order of magnitude lower than commonly reported values for LaVO3 thin films grown by PLD. Steady-state photoconductivity measurements were performed on La-rich, stoichiometric and V-rich LaVO3 films to expand defect analysis beyond optical excitation. Here defects act as recombination centers for photo-excited electron-hole pairs suppressing the measured photo current. The two-terminal test structure is schematically depicted in the inset of Fig. 3b). An electrode 12 ACS Paragon Plus Environment

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separation of 2 mm was chosen, much longer than the typical electron mean free path in perovskite oxides at room temperature.

46

Dark current measurements were performed to ensure that the contacts

were ohmic, see Fig. S4. The steady state condition ensured a balanced photoexcitation and carrier recombination rate. For a fixed photon flux a larger photocurrent was directly related to a longer carrier lifetime and thus a lower defect concentration. Since absorption coefficients for La-rich and V-rich films were slightly higher compared to films grown within the growth window [cf. Fig. 1b)], photo currents determined from nonstoichiometric samples were slightly overestimated. Figure 3b) shows the photo responsivity of LaVO3 films. An abrupt increase in the photo responsivity was found at 1.12±0.01 eV irrespective of the film’s stoichiometry, consistent with the absorption coefficient measurements on bulk crystals and thin films, marking the band gap of LaVO3.11,28,32 For stoichiometric films a much larger photo-responsivity was observed despite a smaller absorption coefficient, indicating a much longer carrier life time attributed to a lower recombination rate due to a lower defect concentration compared to nonstoichiometric LaVO3. By integrating the area below the photo-responsivity curve the photo current extraction efficiency of stoichiometric films was found 1000% (300%) higher to compared to V-rich (La-rich) films, respectively. The significantly increased photo-responsivity of stoichiometric LaVO3 films compared to their nonstoichiometric counterparts demonstrates the importance of stoichiometry control to minimize the defect mediated recombination rate for the promising high efficiency solar cell absorber material LaVO3. 4. CONCLUSIONS



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In conclusion, we have grown high quality LaVO3 thin films by employing a self-regulated growth mechanism available in hybrid molecular beam epitaxy. The structural, optical and electrical properties of LaVO3 thin films were systemically studied as a function of cation stoichiometry. While stoichiometric films grown inside the growth window were shown to have a high structural perfection and a record-low defect concentration even below those of LaVO3 bulk single crystal, a dramatic increase in defect concentration was observed for nonstoichiometric films grown outside the growth window. Orders of magnitude lower optical absorption for below band gap excitation and a much longer majority carrier lifetime, and thus photoconductivity efficiency, marks an important step towards realization of high efficiency solar cells using Mott insulators as effective absorber material. This technique can be extended to other materials systems to control the cation stoichiometry for potential high efficiency photovoltaic devices.25,47,48

ASSOCITAED CONTENS Support information The Supporting Information is available free of charge on the ACS Publications website at DOI: Further TEM characterization of the LaVO3 thin films, absorption coefficient comparison of the LaVO3 thin films with single crystals, spectroscopic ellipsometry fitting details and dark current measurements of the two terminal devices.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] 14 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS H.-T. Z., X. J., S. L., J. L., V. G. and R. E.-H acknowledge support from the National Science Foundation through the Penn State MRSEC program DMR-1420620. H.-T. Z. and R. E.-H. conceived the experiments, H.-T. Z. performed film growth, did XRD and AFM measurements and analyzed the data. Photoconductivity and spectroscopic ellipsometry measurements were performed by X. J. and S. L, respectively. J. L. prepared scanning transmission electron microscopy (STEM) samples and did STEM measurements. V. G. and R. E.-H. oversaw the project. M. B. was supported by the Department of Energy (Grant DE-SC0012375), helped with sample growth and measured electrical conductivity of the samples. J.W. F. led the XAS work at the Advanced Photon Source, Argonne and was supported by the U.S. Department of Energy, Office of Science under Grant No. DEAC02-06CH11357. We thank Dr. Bernd Kabius for help with the STEM experiments and Prof. Susan Trolier-McKinstry for allowing the use of spectroscopic ellipsometry equipment. All authors discussed the results. R. E.-H. and H.-T. Z. co-wrote the paper. We thank Lu Guo at University of Wisconsin–Madison and Lei Zhang at Penn State University for helpful discussions.

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Figure 1 (double column)

Figure 1. a) 2θ-ω X-ray diffraction (XRD) scans of LaVO3 films grown on LSAT (001) substrates with different La to VTIP flux ratios. The La flux was kept constant while the VTIP flux pVTIP was systemically varied. b) Out of plane lattice parameter a⊥ of LaVO3 films as a function of pVTIP. c) Reciprocal space map around -103 peaks of LSAT substrate and LaVO3 (pVTIP=36 mTorr). d) Atomic force microscopy (AFM) and e) High angle annular dark field scanning transmission electron microscopy (STEM) measurements taken along [100] of LaVO3 grown at pVTIP=36 mTorr.



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Figure 2 (double column)

Figure 2. a) Schematic of the electronic structure of LaVO3 around the Fermi level (EF). b) Absorption coefficient α of LaVO3 films grown at different pVTIP. The inset shows the value of α at 1.10 eV, 0.02 eV below band edge energy (1.12 eV, determined by photoconductivity measurements). c) X-ray absorption scans around the vanadium L2, 3 edge of LaVO3 films. d) A narrow range scan around the vanadium L3 edge of stoichiometric (pVTIP=36 mTorr) and V-rich films (pVTIP=45 mTorr). e) Temperature dependent resistivity of LaVO3 thin films grown at various pVTIP.


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Figure 3 (single column)

Figure 3. a) Comparison of optical conductivity below the band edge energy for LaVO3 bulk crystals (Refs. 28, 33-35) and thin films (Refs. 31, 32). Only LaVO3 films grown by pulse laser deposition (PLD) have been reported. b) Photoconductivity (photocurrent minus dark current) of La-rich, stoichiometric and V-rich LaVO3 films, grown at pVTIP of 32.5, 36 and 39 mTorr. A schematic of the device is shown in the inset.



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High-Quality LaVO3 Films as Solar Energy Conversion Material - ACS

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