Multicomponent Oxynitride Thin Films: Precise Growth Control and

Subscriber access provided by CAL STATE UNIV BAKERSFIELD

Article

Multicomponent Oxynitride Thin Films: Precise Growth Control and Excited State Dynamics Wenrui Zhang, John L. Lyons, Jiajie Cen, Matthew Y. Sfeir, and Mingzhao Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00656 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Multicomponent Oxynitride Thin Films: Precise Growth Control and Excited State Dynamics Wenrui Zhang,1 John L. Lyons,2 Jiajie Cen,3 Matthew Y. Sfeir,1 Mingzhao Liu1* 1Center

for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973,

USA 2Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375, USA 3Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, USA ABSTRACT Multicomponent oxynitrides significantly broadens the library of material design and promise wide applications for optoelectronics and solar energy conversion. Controlled growth of multicomponent oxynitrides with defined stoichiometry, uniformity, morphology and scalability has been challenging. In this work, we demonstrate wafer-scale growth of uniform, crystalline GaN:ZnO thin films with tunable stoichiometry, digitally-controlled film thickness and atomically smooth surface, at a process temperature as low as room temperature. In comparison to the particle form that is commonly encountered, the GaN:ZnO thin films studied here bypass the limitations imposed by surface or size implications and reveal the intrinsic nature of optical absorption in this system. The GaN:ZnO thin films demonstrate strong visible light absorption at the composition of (GaN)0.67(ZnO)0.33, and enables the first quantitative determination of bulk absorption coefficients in this system. Based on transient absorption spectroscopy measurements, photocarriers of GaN:ZnO thin films, excited by an ultraviolet or a visible pump, present nearly identical relaxation dynamics, indicating a direct gap origin of the visible light absorption. This is consistent with complimentary steady-state optical absorption measurements and first-principle calculations. The ability to precisely control the growth of functional oxynitrides across the multicomponent scale is critical to advance materials modeling and design for various applications.

1 ACS Paragon Plus Environment

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

1. Introduction Semiconductor design and functionality tuning are highly sought after in a wide range of applications including transistors, magnetoelectrics, optoelectronics and solar energy conversion.15

The efficient use of sunlight requires semiconductors with appropriate band structures for visible

light absorption.6,7 Compared to transition metal oxides, oxynitrides present more favorable electronic structure to obtain narrow band gaps due to the N 2p and O 2p orbital hybridization.8 Current progress in developing visible light absorbers is mainly obtained by nitriding of metal oxides into oxynitrides, with most attention paid to single-cation oxynitrides such as TaON.9 It is more recently realized that metal cations, particularly the ones with d0 and d10 configurations, can also form valence bands with an energy level above O 2p state,8,10,11 which greatly expands the oxynitride family to multi-cation systems in the development of narrow gap semiconductors. While the multi-cation oxynitrides bring significant opportunities for novel materials design, the precise synthesis of multicomponent oxynitride systems remains a stubborn obstacle due to their chemical complexity. This brings structural or chemical variations that can compromise their functionalities and also complicate the analysis of the fundamental physics. For example, as a unique multicomponent oxynitride system, the (Ga1-xZnx)(N1-xOx) solid solution, abbreviated as GaN:ZnO, absorbs visible light from the combination of two wide-gap semiconductors that only absorb ultraviolet (UV) light.12 This band gap reduction phenomenon has generated intensive research interests and has been mostly reported in GaN:ZnO in the form of particles.13-17 However, these particles or nanocrystals may bring significant surface effect that causes inconsistency across different studies. Indeed, different synthesis methods give very different minimal Eg values (2.2 to 2.6 eV) and even identify opposite composition tuning trends.13-17


Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Compared to solution-based or chemical vapor phase synthesis methods, the pulsed laser deposition (PLD) technique is advantageous to transfer complex stoichiometry, provide uniform growth over large areas and digitally control the film thickness inherently through the pulse number. This makes PLD a facile method to grow multicomponent oxynitrides with precise control. For such demonstration, we present the first thin film growth of GaN:ZnO solid solutions using PLD in this study. This approach takes advantage of stoichiometric and energetic material transfer during the laser ablation process and thus enables convenient tuning of stoichiometry in GaN:ZnO thin films. The high-energy ablation plume brings the GaN:ZnO films wafer-scale spatial uniformity, decent crystallinity, and atomically smooth surface at a process temperature as low as room temperature. Furthermore, the successful development of such GaN:ZnO thin films overcomes the surface state or size effect limitation, and provides a unique perspective to quantify visible light absorption and excited state dynamics in this system. For the first time, the bulk absorption coefficients in GaN:ZnO are quantitatively determined through optical absorption measurements of this ideal thin film platform in a simple transmission geometry, which is quite challenging for dispersed GaN:ZnO particles. The transient absorption spectroscopy shares the same transmission geometry, and readily connect the photocarrier relaxation dynamics with steady-state optical absorption results in probing the fundamental origin of visible light absorption. 2. Results and Discussion 2.1 Structural and chemical characterization The GaN:ZnO solid solution thin films are grown on single-crystalline c-cut sapphire (α-Al2O3 (0001)) substrates by PLD (Figure 1a, b), with tunable stoichiometry, high uniformity, nanocrystalline quality, and atomically smooth surface. At optimized conditions, the laser ablation plume transfers stoichiometric materials in the form of high kinetic-energy adatoms/ions from the



Page 4 of 21

target to the substrate. By simply adjusting the target composition, we are able to tune the film composition fully from GaN-rich to ZnO-rich regimes. This is confirmed by energy-dispersive Xray (EDX) spectroscopy analysis (Figure S1 and Table S1). Besides, the high kinetic energy enables the growth of crystalline films even at room temperature, which is found to be critical to suppress nitrogen loss in these films. The superior quality of PLD-deposited GaN:ZnO solid solution films is demonstrated by structural and chemical characterizations. The cross-sectional transmission electron microscopy (TEM) image shows that the GaN:ZnO film is dense and uniform across its entire thickness (Figure 1c). According to the selected area electron diffraction (SAED) patterns, the GaN:ZnO film maintains the wurtzite structure and exhibit a crystalline nature (Figure 1d), despite the roomtemperature growth. The size of individual nanocrystalline domains ranges within 5 - 10 nm, as determined by the high-resolution TEM image (Figure 1e). The cross-sectional EDX mapping reveals a homogeneous distribution of the four elements in GaN:ZnO, which confirms the formation of solid solution from two parent phases (Figure 1f). Atomic force microscopy (AFM) confirms the extreme smoothness of the GaN:ZnO film (Figure 1g), with a root-mean-square (RMS) surface roughness (Ra) of merely 0.11 nm. The smooth growth of GaN:ZnO solid solutions can also be achieved on silicon substrates, although the surface roughness slightly increases (Ra = 0. 92 nm) (Figure S2). In this study, we mainly focus on the ones grown on sapphire substrates, which allows optical characterizations in a simple transmission geometry. High-resolution X-ray photoelectron spectroscopy (XPS) probes the valence states of individual elements in the solid solution films. The oxidation states of four component elements remain primarily at +2 (for Zn), +3 (Ga), -2 (O) and -3 (N) (Figure 2a). We further measure element-specific X-ray absorption near edge structure (XANES) to examine the local structure


Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


around the metal ions. The XANES spectra collected at Ga K-edge and Zn K-edge confirm the same wurtzite structure of the solid solution films as the single phase films (Figure 2b, c), where Ga and/or Zn cations are in tetrahedral coordination.18,19 The main absorption peak in the white line region corresponds to core 1s - 4p electron transitions. The peak shoulder at the lower energy side, as well as the neighboring peak at the higher energy side, correlates to a multiple scattering mechanism and indicates the degree of structural ordering.20,21 These two features barely appear in the XANES spectra of GaN and GaN:ZnO thin films, which suggests the lack of long-range order and is consistent with the solid solution formation. Compared to pristine ZnO or GaN, GaN:ZnO solid solutions present lower absorption edge energies for both Ga and Zn, in particular for Ga-rich solid solutions. This may be related to the formation of more oxygen vacancies, which reduces the mean oxidation state and shifts the absorption edge towards the lower energy side. 2.2 Band gap tuning and visible light absorption The most striking feature of GaN:ZnO solid solutions lies in the band gap reduction, which opens up numerous opportunities for novel materials design and great potential application for optoelectronics and solar energy conversion. Compared to GaN:ZnO particles studied previously, the thin film structure with high uniformness and smoothness in this study presents a unique platform to investigate the optical property with minimized interference of the surface and size effect. The as-synthesized GaN:ZnO films also allow a simple transmission geometry for optical characterization, since the underlying sapphire substrate is optically transparent in the wavelength range of interest. Importantly, the optical absorption of GaN:ZnO films deviates significantly from single phase films and depends strongly on the film composition. This visible light absorption is maximized at a composition around x = 0.33 ((GaN)0.67(ZnO)0.33) with an absorption onset at λ = 530 nm, which blue shifts continuously for higher or lower x (Figure 3a). Correspondingly, the



Page 6 of 21

(GaN)0.67(ZnO)0.33 film exhibits a yellowish color (Figure 3a, inset), in sharp contrast to the transparent substrate in the surrounding region. We further characterize the absorption coefficients (α) of the GaN:ZnO system, which has not be fully addressed in previous studies. With negligible diffuse scattering in these atomically smooth GaN:ZnO films, α is calculated by α = Aln10/t, where A is the absorbance and t is the film thickness. The (GaN)0.67(ZnO)0.33 film strongly absorbs visible light with a strength of α = 0.31 × 105 cm-1 at λ = 450 nm (Figure 3b). Further into the UV region, the solid solution film achieves absorption strength comparable to single-phase GaN or ZnO films, and α increases to 0.90 × 105 cm-1 at λ = 300 nm. To determine the optical band gaps in GaN:ZnO, Tauc plot method is employed (Figure S3). Based on the linear dependence of (αhv)2 on hv, we find that the visible light absorption in GaN:ZnO thin films corresponds to a direct band gap transition, with a minimal gap (Eg) of 2.65 ± 0.07 eV for the composition of (GaN)0.67(ZnO)0.33 (Figure 3b, inset). The slight distortion of the linear region in the Tauc plot is likely from subgap absorption that arise from disorder states.22 The Urbach tail effect, associated with disorder and subgap absorption, 23 is found to be minor and does not affect the Tauc plot analysis of the GaN:ZnO thin films (Figure S4). We note that the minimal Eg obtained from GaN:ZnO thin films is slightly larger than the ones (2.22.6 eV) from particles,13-17 which is likely attributed to the suppression of interference from surface or size effect. Temperature-dependence of band gap energy also implies the direct gap nature of GaN:ZnO as seen from the smaller electron-phonon interaction in GaN:ZnO than that in pristine ZnO. It is known that semiconductor band gap typically decreases with increasing temperature due to energy level shift coupled with lattice thermal expansion and affected electron-phonon interaction.24 This relation can be analyzed by a numerical model by O’Donnell and Chen: 𝐸𝑔(𝑇) = 𝐸𝑔(0) ― 𝑆〈ħ𝜔〉


Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[coth ( ) ― 1], where 𝐸 (𝑇) is the band gap at a given temperature T, 𝐸 (0) the band gap at 0 〈ħ𝜔〉 2𝑘𝑇

𝑔

𝑔

K, S a dimensionless constant measuring the electron-phonon coupling strength and 〈ħ𝜔〉 the mean phonon energy.25 This applies well to the temperature dependence of Eg for both (GaN)0.67(ZnO)0.33 and ZnO films in the temperature range of 80 K- 400 K (Figure 3c). The GaN:ZnO film exhibits smaller electron-phonon coupling strength (SGaN:ZnO = 1.43) and average phonon energy (〈ħ𝜔〉 GaN:ZnO = 20.19 meV) than those (SZnO = 2.71, 〈ħ𝜔〉 ZnO = 40.05 meV) of ZnO as a direct-gap semiconductor (Table S2). This reflects that under the similar structural environment, GaN:ZnO relies even less on the phonon-assisted optical transition than ZnO, so that the visible light absorption in this system should mainly arise from the direct-gap-dominated transition.26 To understand the origin of visible light absorption, first-principles calculations are performed using the hybrid density functional of Heyd, Scuseria, and Ernzerhof (HSE).27 We build special quasi-random structures (SQS) of the GaN:ZnO solid solutions and examine the electronic band structures. Computationally we use Vegard’s law to estimate the lattice parameters of GaN:ZnO solid solutions, and calculate band gaps on these alloy structures. The band gap reduction in GaN:ZnO is due to large bowing parameters, which leads to nearly parabolic behavior for the compositional dependence. A band gap minimum is obtained near the 50% ZnO composition, and the compositional dependence of the band gap is found small within 37.5%-62.5% ZnO. Similar behavior has been reported previously in a cluster expansion and Monte Carlo study.28 The band structure of the GaN:ZnO solid solutions exhibits a dispersive conduction band composed mostly of Ga and Zn 4s states, while the valence band is much heavier and is composed mainly of N 2p states (Figure 3d, inset). We find that the band gap is direct and occurs at Γ, as it does for bulk ZnO and GaN. Along with HSE calculations, one-shot G0W0 calculations are also performed to 7 ACS Paragon Plus Environment


Page 8 of 21

evaluate the compositional dependence of band gap, and the results are qualitatively consistent with experimental observations (Figure 3d and Figure S5). 2.3 Photocarrier relaxation dynamics To understand the relaxation dynamics of photoexcited states in GaN:ZnO films, we perform ultrafast broadband transient absorption (TA) measurements. The same transmission geometry is used so that the steady-state optical characterization results can be readily related to the TA spectroscopy. Another advantage of the thin film architecture is the minimization of surface states or size effect, which assists to probe the relaxation dynamics from intrinsic optical transitions. For pristine GaN or ZnO films, upon photoexcitation at 330 nm, optical bleaches only emerge around 350-360 nm (Figure 4a and Figure S6). Corresponding ground-state transient absorption spectra in these films are consistent with their steady-state absorption spectra (Figure 4d). When excited with the same pumping source, the (GaN)0.67(ZnO)0.33 film exhibits a much broader ground state bleach centered at 410 nm, which extends into the visible spectrum beyond 500 nm (Figure 4b). When switched to a visible pumping at 450 nm, a nearly identical transient bleach is observed from (GaN)0.67(ZnO)0.33 (Figure 4c). Therefore, these results reveal that the strong visible absorption in transient/steady-state spectra of GaN:ZnO arises from the interband transition rather than defect trap states, since the latter one would otherwise show stronger absorption with a longer excitation wavelength. The carrier relaxation dynamics of GaN:ZnO thin films are very similar to those of pristine GaN and ZnO films. To examine the relaxation kinetics of photoexcited carriers in these films, we first use a UV pumping at 330 nm and mainly focus on the early stage in a time scale up to 2.5 ns. Normalized kinetic traces of the primary transient bleach from (GaN)0.67(ZnO)0.33 can be fitted multi-exponentially with three decay time constants of 1.20 ps (τ1, comprising 50% of the total


Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decay amplitude), 13.4 ps (τ2, 31%) and 897 ps (τ3, 19%) (Figure 4e). Fitting details are provided in Table S3. The pristine GaN and ZnO films exhibit similar decay behavior and time constants except that the ZnO film shows slightly larger slow component (τ3-ZnO = 867 ps, 18%). Similar relaxation dynamics is observed in previous studies on pristine ZnO,29,30 where excited carrier relaxation/localization dominates the relaxation process with a lifetime of ~1-10 ps, interband recombination as a following process with a lifetime of ~20-50 ps and the deeply trapped carrier recombination for slower process of ~0.5-1 ns. We further excite the photocarriers in GaN:ZnO with a 450 nm laser pumping and find nearly identical relaxation dynamics to the ones with the 330 nm pumping (Figure S7). Therefore, the photocarriers excited at either ultraviolet or visible wavelengths share the same origin, which confirms that the visible light absorption in this system arise from the direct interband transition rather than defect trap states. 3. Conclusions We report precisely controlled growth of multicomponent oxynitride thin films by pulsed laser deposition. For the first time, nanocrystalline GaN:ZnO solid solution thin films are synthesized with highly tunable stoichiometry, wafer-scale uniformity, digitally controlled film thickness and atomically smooth surface at room temperature. The high-quality GaN:ZnO films in this study suppress the interference from surface or size defects and reflect mainly intrinsic optical properties of this system. Taking this advantage, we reveal the strong visible light absorption occurring at a composition near (GaN)0.67(ZnO)0.33, and quantitatively characterize its bulk absorption coefficients, a parameter not accessible before in nanostructures or particles. First-principle calculations reveal that the strong visible light absorption in this system shares the same origin of direct band gap transition at the Γ point with GaN or ZnO, but with much heavier N 2p states for the main valence band. Transient absorption spectroscopy reveals picosecond-scale relaxation



Page 10 of 21

dynamics in GaN:ZnO, which is very similar to single phases. Nearly identical relaxation behavior can be excited by either ultraviolet (330 nm) or visible (450 nm) pumping, suggesting the origin of direct band gap transition instead of defect trap states. The application of this solid solution system may go beyond visible light photocatalytic water splitting. For example, given the direct gap nature and band gap tunability in GaN:ZnO, they may be used to fabricate high-efficiency wavelength-tunable light emitters. As a facile and versatile approach, pulsed laser deposition appears to be applicable to complex multicomponent oxynitride systems, which is expected to drive novel materials design for optoelectronics and solar energy conversion. 4. Experimental Section Thin film growth. Thin films of GaN:ZnO solid solution and pristine GaN are grown on singlecrystalline double-side polished α-Al2O3 (0001) substrates at room temperature in 50 mTorr of nitrogen, by pulsed laser deposition using a KrF excimer laser (λ = 248 nm, laser fluence of 1.5 J/cm2, repetition rate of 10 Hz). Growth of pristine ZnO films is conducted under otherwise identical conditions, except for 50 mTorr of oxygen being used instead of nitrogen. The composition of GaN:ZnO thin films is controlled by adjusting the corresponding laser-ablated GaN:ZnO target composition. To prepare GaN:ZnO targets with different compositions, the mixture of high-purity Ga2O3 and ZnO powders in the designed ratios are pressed into disks and subsequently sintered in 900°C for 6 h under ammonia atmosphere of 1 atm with a flow rate of 5 sccm. The GaN:ZnO film thickness is set to be ~240-280 nm by controlling the laser shot number. Microstructural characterization. The film phase, crystallinity, composition and microstructure are investigated by X-ray diffraction (XRD, Rigaku Ultima III), scanning electron microscopy (SEM, Hitachi S-4800 and JEOL JSM-7600F) and transmission electron microscopy (TEM, JEOL 2100F). Atomic force microscopy (AFM, Park NX20, non-contact mode) is used to measure the


Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface morphology. X-ray photoemission spectroscopy (XPS) measurements were performed in an ultrahigh vacuum system using Mg Kα (hν = 1253.6 eV) as the excitation source. X-ray absorption spectroscopy (XAS) measurements were performed in fluorescence mode on beam line 8-ID ISS (Inner Shell Spectroscopy) at National Synchrotron Light Source II, Brookhaven National Laboratory. Optical and ultrafast spectroscopy. Steady-state optical absorption spectra are collected with a UV-Vis/NIR Spectrophotometer (PerkinElmer, Lambda 950). Both transmission and reflectance geometries have been examined, which give similar results with nearly identical absorption edges. The substrate absorption and scattering effects have been subtracted in the presented results. The variable temperature UV-Vis absorption measurement is performed with samples placed in a liquid nitrogen cryostat for variable temperature control. The room-temperature ellipsometry spectra are collected and analyzed using a variable angle spectroscopic ellipsometer (J.A. Woollam M-2000) at different incident angles (50°, 60°, 70°) over a spectral range of 250-1200 nm. For collection of broadband transient absorption spectra, we employ a typical pump-probe set-up with a Ti:Sapphire amplified laser system (1 kHz repetition rate) and optical parametric amplifier (OPA). The samples, placed in the transmission geometry, are pumped with ~100 fs laser pulse and probed with a broadband supercontinuum pulse at various time delays using dual spectrometers (signal and reference channels) equipped with fast Si based array detectors (Helios, Ultrafast Systems). DFT calculations. We employ hybrid density functional theory calculations based on the HSE06 functional28 with the projector-augmented wave technique31 as implemented in VASP32 to calculate the properties of GaN, ZnO, and their alloys. We first calculate the properties of the binary compounds, using 28% Hartree-Fock (HF) exchange for GaN and 36% for ZnO such that their band gaps (3.5 eV for GaN; 3.4 eV for ZnO) are in good agreement with experiment.32 A 600



Page 12 of 21

eV plane-wave cutoff and 6x6x4 Gamma-centered k-point meshes are used for these primitive cell calculations, and Ga 3d electrons are treated as valence. Lattice parameters (a = 3.14 Å and c = 5.16 Å for GaN; a = 3.22 Å and c = 5.18 Å for ZnO) are within 2% of experimental values.33 We then proceed to calculate the properties of the GaN:ZnO alloys using special quasirandom structures34 with 32-atom supercells. Alloy lattice parameters are interpolated from Vegard’s law using the HSE-calculated values of the binary components. We consider 7 alloy concentrations in steps of 12.5%, and use linearly interpolated amounts of HF exchange to calculate the electronic structure of each alloy. Using the HSE-calculated wavefunctions, we then perform G0W0 calculations35 on the SQS supercells using 960 bands and a 150 eV response function cutoff. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Energy-dispersive X-ray (EDX) spectroscopy for GaN:ZnO films, AFM image of (GaN)0.67(ZnO)0.33 film on a silicon (100) substrate, Tauc plot analysis for optical band gaps, Analysis of the Urbach energy from the UV-Vis absorption spectra, DFT calculation results of band gaps and band structures, Transient absorption plot for pure GaN thin film, TA spectra of (GaN)0.67(ZnO)0.33 excited at different wavelengths, EDX composition analysis of GaN:ZnO solid solutions, Analysis of temperature dependence of semiconductor band gap. Fitting details of transient absorption spectra results. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes 12 ACS Paragon Plus Environment

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing financial interest. ACKNOWLEDGMENTS This research used resources of the Center for Functional Nanomaterials and the 8-ID ISS (Inner Shell Spectroscopy) beamline at the National Synchrotron Light Source II. Both are U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Work by J.L.L. was supported by the Office of Naval Research Basic Research Program and made use of the computational resources of the DoD Major Shared Resource Center at AFRL. We thank Dr. K. Attenkofer and Dr. E. Stavitski on the help on XAS characterization and analysis. We thank Dr. M. Cotlet for support on variable temperature UV-Vis absorption measurements. We also thank S. Liang and L. Song for help on the nitration process of laser ablated targets. REFERENCES 1.

Bard, A. J. Design of Semiconductor Photoelectrochemical Systems for Solar Energy

Conversion. J Phys. Chem. 1982, 86, 172-177. 2.

Cao, L.; White, J. S.; Park, J.-S.; Schuller, J. A.; Clemens, B. M.; Brongersma, M. L.

Engineering Light Absorption in Semiconductor Nanowire Devices. Nat. Mater. 2009, 8, 643-647. 3.

Fortunato, E.; Barquinha, P.; Martins, R. Oxide Semiconductor Thin-Film Transistors: A

Review of Recent Advances. Adv. Mater. 2012, 24, 2945-2986. 4.

Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Semiconductor

Heterojunction Photocatalysts: Design, Construction, and Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234-5244. 5.

Zhang, W.; Ramesh, R.; MacManus-Driscoll, J. L.; Wang, H. Multifunctional, Self-

Assembled Oxide Nanocomposite Thin Films and Devices. MRS Bull. 2015, 40, 736-745. 6.

Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.;

Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.



7.

Page 14 of 21

Lee, D. K.; Lee, D.; Lumley, M. A.; Choi, K.-S. Progress on Ternary Oxide-Based

Photoanodes for Use in Photoelectrochemical Cells for Solar Water Splitting. Chem. Soc. Rev. 2019, 48, 2126-2157. 8.

Tsuyoshi, T.; Chengsi, P.; Kazunari, D. Recent Progress in Oxynitride Photocatalysts for

Visible-Light-Driven Water Splitting. Sci. Technol. Adv. Mater. 2015, 16, 033506. 9.

Abe, R.; Higashi, M.; Domen, K. Facile Fabrication of an Efficient Oxynitride Taon

Photoanode for Overall Water Splitting into H2 and O2 under Visible Light Irradiation. J. Am. Chem. Soc. 2010, 132, 11828-11829. 10.

Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem.

Soc. Rev. 2009, 38, 253-278. 11.

Pan, C.; Takata, T.; Nakabayashi, M.; Matsumoto, T.; Shibata, N.; Ikuhara, Y.; Domen, K.

A Complex Perovskite-Type Oxynitride: The First Photocatalyst for Water Splitting Operable at up to 600 nm. Angew. Chem. Int. Ed. 2015, 54, 2955-2959. 12.

Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN:ZnO

Solid Solution as a Photocatalyst for Visible-Light-Driven Overall Water Splitting. J. Am. Chem. Soc. 2005, 127, 8286-8287. 13.

Maeda, K.; Domen, K. Solid Solution of GaN and ZnO as a Stable Photocatalyst for

Overall Water Splitting under Visible Light. Chem. Mater. 2010, 22, 612-623. 14.

McDermott, E. J.; Kurmaev, E. Z.; Boyko, T. D.; Finkelstein, L. D.; Green, R. J.; Maeda,

K.; Domen, K.; Moewes, A. Structural and Band Gap Investigation of GaN:ZnO Heterojunction Solid Solution Photocatalyst Probed by Soft X-Ray Spectroscopy. J. Phys. Chem. C 2012, 116, 7694-7700. 15.

Chen, D. P.; Skrabalak, S. E. Synthesis of (Ga1–xZnx)(N1–xOx) with Enhanced Visible-Light

Absorption and Reduced Defects by Suppressing Zn Volatilization. Inorg. Chem. 2016, 55, 38223828. 16. xOx)

Lee, K.; Tienes, B. M.; Wilker, M. B.; Schnitzenbaumer, K. J.; Dukovic, G. (Ga1–xZnx)(N1– Nanocrystals: Visible Absorbers with Tunable Composition and Absorption Spectra. Nano

Lett. 2012, 12, 3268-3272. 17.

Wang, J.; Huang, B.; Wang, Z.; Wang, P.; Cheng, H.; Zheng, Z.; Qin, X.; Zhang, X.; Dai,

Y.; Whangbo, M.-H. Facile Synthesis of Zn-Rich (GaN)1-x(ZnO)x Solid Solutions Using Layered Double Hydroxides as Precursors. J. Mater. Chem. 2011, 21, 4562-4567. 14 ACS Paragon Plus Environment

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


18.

Bianconi, A.; Fritsch, E.; Calas, G.; Petiau, J. X-Ray-Absorption near-Edge Structure of

3d Transition Elements in Tetrahedral Coordination: The Effect of Bond-Length Variation. Phys. Rev. B 1985, 32, 4292-4295. 19.

Ram Boppana, V. B.; Doren, D. J.; Lobo, R. F. Analysis of Ga Coordination Environment

in Novel Spinel Zinc Gallium Oxy-Nitride Photocatalysts. J. Mater. Chem. 2010, 20, 9787-9797. 20.

Liu, T.; Xu, H.; Chin, W. S.; Yong, Z.; Wee, A. T. S. Local Structural Evolution of Co-

Doped ZnO Nanoparticles Upon Calcination Studied by in Situ Quick-Scan XAFS. J. Phys. Chem. C 2008, 112, 3489-3495. 21.

Chen, H.; Wen, W.; Wang, Q.; Hanson, J. C.; Muckerman, J. T.; Fujita, E.; Frenkel, A. I.;

Rodriguez, J. A. Preparation of (Ga1–xZnx)(N1–xOx) Photocatalysts from the Reaction of NH3 with Ga2O3/ZnO and ZnGa2O4: In Situ Time-Resolved XRD and XAFS Studies. J. Phys. Chem. C 2009, 113, 3650-3659. 22.

Sweenor, D. E.; O’Leary, S. K.; Foutz, B. E. On Defining the Optical Gap of an Amorphous

Semiconductor: An Empirical Calibration for the Case of Hydrogenated Amorphous Silicon. Solid State Commun. 1999, 110, 281-286. 23.

Urbach, F. The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic

Absorption of Solids. Phys. Rev. 1953, 92, 1324-1324. 24.

Lautenschlager, P.; Allen, P. B.; Cardona, M. Temperature Dependence of Band Gaps in

Si and Ge. Phys. Rev. B 1985, 31, 2163-2171. 25.

O’Donnell, K. P.; Chen, X. Temperature Dependence of Semiconductor Band Gaps. Appl.

Phys. Lett. 1991, 58, 2924-2926. 26.

Giustino, F. Electron-Phonon Interactions from First Principles. Rev. Mod. Phys. 2017, 89,

015003. 27.

Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb

Potential. J. Chem. Phys. 2003, 118, 8207-8215. 28.

Li, L.; Muckerman, J. T.; Hybertsen, M. S.; Allen, P. B. Phase Diagram, Structure, and

Electronic Properties of (Ga1–xZnx)(N1–xOx) Solid Solutions from DFT-Based Simulations. Phys. Rev. B 2011, 83, 134202. 29.

Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. Ultrafast Relaxation Dynamics of

Charge Carriers Relaxation in ZnO Nanocrystalline Thin Films. Chem. Phys. Lett. 2004, 387, 176181. 15 ACS Paragon Plus Environment


30.

Page 16 of 21

Zhang, W.; Yan, D.; Appavoo, K.; Cen, J.; Wu, Q.; Orlov, A.; Sfeir, M. Y.; Liu, M.

Unravelling Photocarrier Dynamics Beyond the Space Charge Region for Photoelectrochemical Water Splitting. Chem. Mater. 2017, 29, 4036-4043. 31.

Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979.

32.

Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave

Method. Phys. Rev. B 1999, 59, 1758-1775. 33.

Madelung, O., Semiconductors - Basic Data. 2nd revised ed ed.; Springer: Berlin, 1996.

34.

van de Walle, A.; Tiwary, P.; de Jong, M.; Olmsted, D. L.; Asta, M.; Dick, A.; Shin, D.;

Wang, Y.; Chen, L. Q.; Liu, Z. K. Efficient Stochastic Generation of Special Quasirandom Structures. Calphad 2013, 42, 13-18. 35.

Shishkin, M.; Kresse, G. Implementation and Performance of the Frequency-Dependent

Gw Method within the Paw Framework. Phys. Rev. B 2006, 74, 035101.


Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURES

Figure 1. (a) Schematic of thin film structure and (b) crystallographic structure of GaN:ZnO solid solutions. (c) Cross-sectional low-magnification TEM image of the (GaN)0.67(ZnO)0.33 film over the Al2O3 (0001) substrate. (d) Upper: SAED pattern of the GaN:ZnO film demonstrating the film nanocrystalline quality. Lower: SAED pattern of the underlying Al2O3 substrate for comparison. (e) High-resolution TEM image of the GaN:ZnO film showing nanocrystalline regions with different lattice orientations. Inset shows an enlarged nanocrystalline region with indexed lattice spacing according to the wurtzite structure d-spacing. (f) EDX maps and corresponding scanning TEM (STEM) image of the GaN:ZnO film showing highly uniform phase distribution. (g) Topology AFM image (scan area of 1 x 1 μm2) and corresponding surface height profile of the (GaN)0.67(ZnO)0.33 film demonstrating atomically smooth film surface.



Page 18 of 21

Figure 2. (a) High resolution XPS spectra of Zn 2p, Ga 2p, O 1s, N 1s core levels in a (GaN)0.67(ZnO)0.33 film. (b) Ga K-edge and (c) Zn K-edge XANES spectra of GaN:ZnO films with different film compositions. The orange arrow points to the main absorption peak in the white line region. The purple bars mark the absorption shoulder that arises from a multiple scattering process.


Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) Optical density as a functional of excitation wavelength/photon energy and film composition for GaN:ZnO solid solution thin films. Inset shows the optical image of the (GaN)0.67(ZnO)0.33 film, which exhibits strong visible light absorption representing the compositional region marked by the white dashed line. (b) Thin film absorption coefficient as a function of excitation wavelength for GaN, ZnO, and GaN:ZnO solid solution films with different compositions (ZnO% = 20%, 33%, 50%). Inset shows a linear extrapolation of Eg for (GaN)0.67(ZnO)0.33 from its UV-Visible absorption spectrum. (c) Eg as a function of temperature for (GaN)0.67(ZnO)0.33 and ZnO thin films from 80 K to 400 K. Numerical fitting is applied to obtain the electron-phonon coupling strength and average phonon energy in these semiconductors. (d) Eg as a function of the ZnO ratio in GaN:ZnO films. Inset shows the calculated band structure near the  point to illustrate the origin of visible light absorption in GaN:ZnO. 19 ACS Paragon Plus Environment


Page 20 of 21

Figure 4. Transient absorption plots for (a) ZnO, (b) (GaN)0.67(ZnO)0.33 thin films both pumped at 330 nm, and (c) (GaN)0.67(ZnO)0.33 thin film pumped at 450 nm. The transient absorption and kinetics spectra are extracted from the ground state absorption region. Time zero and chirp correction have been performed for all collected data. The region around the excitation wavelength of 450 nm has been subtracted due to the strong scattering of the pumping beam. (d) Wavelengthdependent transient absorption spectra and (e) normalized time-resolved decay traces for the ground state bleach for (GaN)0.67(ZnO)0.33, GaN and ZnO thin films.


Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic


Multicomponent Oxynitride Thin Films: Precise Growth Control and

Recommend Documents