United States

Abstract Body: In this work, we investigate the properties of spatially indirect excitons (IXs) confined in pairs of atomically think GaN wells, separated by polar AlN barriers. Atomically thin GaN is a promising material for realizing strongly bound excitons because of its extreme quantum confinement effect. Also, the spontaneous polarization fields in nitride heterostructures allow IXs to form in atomically thin GaN quantum wells even without external electric fields. We performed first-principles calculations based on density functional theory and many-body perturbation theory to investigate the properties of IXs and DXs in pairs of atomically thin GaN quantum wells separated by polar AlN layers with varying thickness. Our calculation shows that the overlap of the electron and hole wavefunctions, the degree of electron-hole interaction, and the character (IX or DX) of the lowest-energy exciton can be controlled by changing the thickness and the resulting electrical polarization of the separating AlN barrier. We demonstrate that room-temperature stable IXs with radiative decay rates several orders of magnitude lower than DXs can be realized in these atomically thin polar nitride heterostructures for potential excitonic applications at room temperature based on a commercial semiconductor platform. For realization of IXs in atomically-thin GaN quantum wells, molecular beam epitaxial growth was performed on an n-type Si substrate using a Veeco GEN II system with radio frequency plasma-assisted nitrogen source. The epitaxy includes ~100 nm long GaN nanowires which serves as a template for the subsequent epitaxy of ~150 nm long AlN nanowires. Then, monolayer GaN quantum well/AlN barrier/monolayer GaN quantum well heterostructure was grown on top of AlN nanowires. The thickness of AlN barrier was varied by changing the growth duration. Figure 1 shows the photoluminescence spectra of two samples with 1 ML and 7 MLs thick AlN barriers incorporated, respectively. The emission peak around 5.93 eV can be attributed to the excitonic emission from AlN. DX emission dominates in the spectrum of sample with 7 MLs thick AlN barrier. In comparison, there’s a clear shoulder emerging on the lower energy side observed in the sample with 1 ML thick AlN barrier, which can be assigned to indirect exciton (IX) emission. The work is supported by the University of Michigan College of Engineering Blue Sky Research Program. W.L. was partially supported by the Kwanjeong Educational Foundation Scholarship. Computational resources were provided by the DOE NERSC facility.

EMC 2021

Deep Ultraviolet Luminescence and Charge-Transfer Excitons in Atomically Thin GaN Quantum Wells

atomically-thin gan

deep-ultraviolet luminescence

indirect excitons

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Abstract Body: Two-dimensional transition metal carbides and nitrides, so-called MXenes, have quickly attracted intensive attention and have found quite promising applications in various fields, such as water desalination, energy storage, catalysis, etc. These cutting-edge applications require the production of high quality MXene nanosheets with a high concentration. However, this has proven to be quite challenging. Here in this talk, I will describe the synthesis of high-quality MXene nanosheets, followed by the oxidation stability mechanism of MXene aqueous solutions, which is quite important for the long shelf life and is the prerequisite for the industrial application of MXenes.1 Then I will show the importance of the viscous inks in the efficient fabrication of high-performance transparent conductive films/coatings,2 scalable production of micro-supercapacitors assisted by the additive-manufacturing techniques (Figure 1).3 The ink viscous nature also allows the MXene nanosheets to perform as a conductive binder for Si nanoparticles, resulting in high mass-loading MXene/Si anodes with an ultrahigh areal capacity (Figure 2), which is 5 times higher than that of commercial anode.4 In addition, I will show the formulation of organic MXene inks for scalable additive-free printing of energy storage devices, as well as the formulation of composite ink for Lithium-sulfur batteries (Figure 3).5 We therefore believe the high-quality MXene inks hold a bright future for the next-generation energy storage devices at a low cost.

High-Quality MXene Inks for Energy Storage

Abstract Body: Sustainable energy storage devices demand new materials for energy storage systems, which alleviate their cost and increase longevity. A prominent energy storage technology for modern applications is Lithium-ion batteries. In this regard, two-dimensional (2D) materials attracted substantial and considerable attention due to their unique properties and abundant potential in various applications, including batteries1. Controllable synthesis of 2D materials with high quality and better efficiency is essential for their large scale applications, such as their functions as electrode material in batteries. The methods for the synthesis of two-dimensional materials include exfoliation, solvothermal, and chemical vapor deposition (CVD) routes, among which CVD offers better quality, efficiency, and consistency. In this perspective, the family of 2D transition metal di-chalcogenides (TMDCs) gained significant attention for their ability to store metal ions such as Li, Na, and Mg. TMDCs of group 6 transition metals (MX2 where M = Mo, W, and X = S, Se, Te) have a lamellar structure (space group P63/mmc) similar to graphite but with larger interlayer spacing. Tungsten di-chalcogenides are significant here because of the larger size of W that provides a further alteration of the 2D structure and has been successfully utilized for various applications such as photodetectors, field-effect transistors (FETs), etc. The typical group 6 TMDC lattice structure show W atoms confined in a trigonal prismatic coordination sphere neighbored to Se atoms. Because of its very high density (~9.32 g cm−3), WSe2 also has a high volumetric capacity, thereby proving it as a prospective LiB electrode material2. Our work focuses on a facile synthesis of 2H-WSe2 on W foil in the ambient-pressure chemical vapor deposition system using only Argon as carrier gas during the reaction. Extensive characterization of the bulk and exfoliated material confirm that the as-synthesized 2H-WSe2 is layered (i.e., 2D). XRD (X-Ray Diffraction) confirms the phase. At the same time, HRSEM (High-Resolution Scanning Electron Microscopy), HRTEM (High-Resolution Scanning Electron Microscopy), and AFM (Atomic Force Microscopy) clarify the morphology of the material. FIB-SEM (Focused-Ion Beam Scanning Electron Microscopy) estimates the depth of the 2H-WSe2 formation on W foil around 5-8 μm and Raman/UV-Vis measurements prove the quality of the exfoliated 2H-WSe2. The redox processes of Lithium-ion Battery (LiB) studies show an increase in capacity until 500 cycles. On prolonged cycling, the discharge capacity till the 50th cycle at 250 mA/g, the material shows a stable capacity of 550 mAh/g. These observations indicate exfoliated 2H-WSe2 has promising applications as LiB electrode material. References (1) Xia, H.; Xu, Q.; Zhang, J. Recent Progress on Two-Dimensional Nanoflake Ensembles for Energy Storage Applications. Nano-Micro Lett. 2018, 10 (4), 1–30. https://doi.org/10.1007/s40820-018-0219-z. (2) Share, K.; Lewis, J.; Oakes, L.; Carter, R. E.; Cohn, A. P.; Pint, C. L. Tungsten Diselenide (WSe2) as a High Capacity, Low Overpotential Conversion Electrode for Sodium-Ion Batteries. RSC Adv. 2015, 5 (123), 101262–101267. https://doi.org/10.1039/C5RA19717A.

Scalable Synthesis of Few-Layered 2D Tungsten Diselenide (2H-WSe2) Nanosheets Directly Grown on Tungsten (W) Foil Using Ambient-Pressure Chemical Vapor Deposition for Reversible Li-Ion Storage

Abstract Body: The formulation of inks based on 2D materials could enhance the performance of and provide new functional behaviors to printed electronics while retaining scalable manufacturing processes. Recent advances in solution-based exfoliation have demonstrated that the electronic properties of single semiconducting 2D nanosheets can be preserved during ink processing, but there remain several challenges to the production of thin-film transistors based on printed 2D materials. In addition to minimizing inter-sheet resistivity and preserving bulk-like mobilities, the local electronic properties and microstructure of the composite material must be optimized to minimize the resistance of overlapping nanosheets (i.e. junction resistance). To rationally guide the development of 2D inks and related processing, we explored the design principles by integrating device modeling with electrical and scanning probe characterization on transistors consisting of partially overlapping 2D channels (Figure 1a). We examined the limiting factors to the junction resistance by benchmarking a gate-dependent resistor network model with experimentally measured electrical characteristics for model MoS2/MoS2 homojunctions and utilized the model to quantify performance trade-offs determined by nanosheet thickness, percent overlap, sheet resistance and inter-sheet resistivity. Kelvin probe force microscopy (KPFM) was used to measure the surface potential profiles in transistor channels as a function of applied bias conditions and morphological characteristics such as nanosheet thickness. Detailed analysis of the abrupt potential drop measured at the conformal edge provides evidence that the on-state transistor performance of overlapping nanosheets is limited by the local sheet resistance (Figure 1b), which is likely due to weakly screened trapped charges at the edge state. This suggests that passivation and/or functionalization schemes targeting the edges of nanosheets could greatly improve transistor performance. Moreover, we found that screening of the upper portions of overlapping nanosheets produces current crowding at junctions between the edge of one nanosheet and the basal plane of another. Reducing the nanosheet thickness (i.e. the number of layers) increases the carrier concentration of the top nanosheet, thus distributing the current over longer distance and facilitating efficient inter-sheet charge transfer (Figure 2a), which increases the effective mobility and optimum percent overlap (Figure 2b). This provides further motivation to develop exfoliation schemes that preserve intrinsic monolayer properties.

Towards the Rational Design of Printed 2D Materials

Abstract Body: Tungsten ditelluride (WTe2) is a layered, type-II Weyl semimetal transition metal dichalcogenide (TMD) typically observed in a distorted 1T (1T’) phase with an orthorhombic crystal structure comprised of planes of distorted triangular lattices of tungsten atoms sandwiched by tellurium atoms. The distortion pushes tungsten atoms closer together in the x-axis than the y or z-axis, generating quasi-one-dimensional chains of these atoms, leading to strongly anisotropic electronic properties throughout the material. It has been shown to have extraordinary physical properties, such as a high magnetoresistance, anisotropic ultra-low thermal conductivity and metal-insulator transition and it also exhibits interesting quantum phenomena such as the quantum spin-Hall effect and pressure-driven superconductivity. These unique properties make WTe2 an exciting candidate for emerging applications, including phase change memory electrodes, magnetic field sensors, biosensors, microelectromechanical systems, hard disk drives and quantum computing. While bulk crystals of the material have been available for many years, to date, thin films of WTe2 have only recently been synthesized using techniques such as molecular beam epitaxy and powder source chemical vapor deposition (CVD) that utilize Te powder and W-feedstock. These techniques, however, have a number of challenges including tellurium dimer formation, that can severely hinder growth with high dissociation energies, and the use of salt assisted growth promotors during CVD growth which could contaminate future device fabrication. Metalorganic CVD (MOCVD) growth is of interest as it enables precise delivery of precursors to the substrate at moderate growth pressures (100-700 Torr), while proper precursor selection mitigates the Te dimer formation, but has not yet been studied in detail. In this study, we investigate the use of MOCVD for the growth of WTe2 on c-plane sapphire substrates. The studies were carried out in a vertical cold wall MOCVD reactor using tungsten hexacarbonyl (W(CO)6) and diethyltelluride (DETe) as precursors for W and Te, respectively, in a H2 carrier gas. Initial studies, carried out at 100 Torr reactor pressure using a W(CO)6 flow rate of 1.3 x10-4 sccm and Te/W ratio of ~8000 : 1, demonstrate the growth of centimeter scale WTe2 as confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Additional studies to elucidate the growth mechanism and properties of MOCVD grown WTe2 were then undertook. The growth rate was found to decrease with increasing temperature over the range of 350 - 600oC. Similarly, higher growth rates were observed at lower growth pressures when examined over a range of 50 - 300 Torr. The effect of precursor flow rates was also investigated and demonstrated that the DETe flow rate had little effect on growth rate while changes in W(CO)6 flow rate can be used to directly control growth rate of films. Peaks at ~1340 cm-1 and ~1600 cm-1 were present in the Raman spectra of some films indicating the presence of carbon in the layers resulting from the DETe source. Higher carbon content was observed at higher growth temperatures and growth pressures. This suggests that our growths are becoming limited by parasitic prereactions of our precursors in the gas phase. When precursors react in the gas phase before reaching the substrate surface, they form carbon containing species which deposit, thus limiting the available reactants to interact on the surface and form WTe2. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis further indicate that films are hexagonal and exhibit the layered 1T’ structure. XPS studies over a period of a few weeks reveals that films do not readily oxidize and are quite stable.

MOCVD Growth of Tungsten Ditelluride Thin Films

Abstract Body: We recently demonstrated a non-volatile, two-dimensional (2D) crystal random access memory (RAM) using a monolayer electrolyte1 consisting of cobalt crown ether phthalocyanine (CoCrPc) and lithium salt. Each CoCrPc molecule includes four crown ethers that each solvate one lithium ion and form an ordered array on 2D crystals including graphene, WSe2 and MoS2. The electrolyte is deposited by drop casting from solvent followed by annealing in argon2. The key mechanism of the nonvolatile memory is the toggling of lithium ions back and forth through the crown ether cavity by an applied electrical field. The positioning of the ions near the semiconductor surface results in electric double layer (EDL) gating which creates a low resistance (ON) state; when ions are moved away from the semiconductor surface, a high resistance (OFF) state is created3. Our published work primarily focused on the combination of 15-crown 5-ether (15C5) CoCrPc and LiClO4 where the salt is deposited by drop-casting in ethanol. However, this salt-solvent combination increased the number of features on the semiconductor surface resembling aggregates of CoCrPc and/or lithium salt. Literature indicates additional combinations of lithium salts (such as LiCl and LiI) and solvents that promote the complexation between 15C5 and Li+. Therefore, in this study we explore additional salt-solvent pairs to decrease the size and density of the aggregates or eliminate them entirely. The quality of the deposition can be characterized by root mean square roughness (Rq), skewness (Rsk), and kurtosis (Rku) using atomic force microscope (AFM) topological scans. In this study, WSe2 flakes were exfoliated on p-type Si (90 nm SiO2) and a monolayer of 15C5 CoCrPc was deposited on the flake surfaces. To isolate the impact of anion identity on the monolayer quality, we kept the deposition solvent constant as ethanol while varying the lithium anions: LiClO4, LiCl, and LiI. To isolate the impact of solvent identity, we kept the salt, LiClO4, constant and varied the solvent from ethanol to acetone and acetonitrile (note that acetone and acetonitrile were chosen for their reported tendency to promote ion-(15C5) crown complexation4.) The monolayer quality is evaluated using AFM in an Argon-filled glovebox with water and oxygen concentration < 0.6 ppm. Among the three salts, LiCl in ethanol gives the most homogenous monolayer with a Rq,LiCl = 0.25 ± 0.01 nm. Specifically, with ethanol as the solvent, LiCl gives a monolayer that is 2.4 and 6.2 x less rough than LiClO4 and LiI, respectively. The skewness of LiCl (Rsk,LiCl = 0.15 ± 0.01) is close to zero indicating that the surface topology is relatively flat, which is also consistent with the small surface roughness of 2.5 angstrom. Among the three solvents, we observed that acetone gives the best LiClO4 deposition with the lowest Rq (0.42 ± 0.05 nm), which is ~1.5 and 3 x lower than ethanol and acetonitrile, respectively. Also, the skewness (Rsk,acetone = 0.72 ± 0.24) and the kurtosis (Rku,acetone = 5.23 ± 0.31) were approximately 1.5 times lower than the other two solvents. These data were taken by averaging over six, 200 x 200 nm locations on each flake. The data indicate LiCl and acetone as the most promising salt-solvent combination; these results will be reported in the talk. This work is supported by the NSF under Grant # NSF-DMR-EPM CAREER: 1847808. (1) Liang, et. al., Nano Lett. 2019, 19 (12), 8911–8919. (2) Lu, et.al., J. Phys. Chem. C 2015, 119 (38), 21992–22000. (3) Xu, et. al., ACS Nano 2017, 11 (6), 5453–5464. (4) Izatt, et. al., Chem. Rev. 1985, 85 (4), 271–339.

Impact of Salt and Solvent Identity on the Deposition of a Cobalt Crown Ether Phthalocyanine Monolayer Electrolyte

Abstract Body: Metal oxide semiconductors such as indium gallium zinc oxide (IGZO) and indium zinc oxide (IZO) have received significant attention over the past few decades due to their desirable electrical and optical properties. Sol-gel techniques are commonly employed to deposit these materials because of the simplicity of the synthesis and the ability to easily tune the film composition. These sol-gel based oxide semiconductors often demonstrate high field-effect mobility even in the amorphous phase, but temperatures of at least 300 C are typically required to decompose the sol-gel and form high-quality semiconductor film. The desire to create high-quality flexible transistors on low-temperature plastic substrates has driven an investigation into alternative thermal processing methods for oxide semiconductors. One such option being explored is pulsed-light processing based on flashlamps, commonly known as photonic curing. Photonic curing offers advantages over conventional thermal processing methods such as ultra-short processing time and compatibility with low-temperature substrates. However, previous work on photonically cured thin-film transistors (TFTs) often results in significant heating of the entire substrate rather than just the thin film at the surface. In this talk, we will discuss our recent work using photonic curing to rapidly and effectively convert metal oxide sol-gels to realize high-quality TFTs. Sol-gel indium zinc oxide (IZO) based TFTs are photonically cured with efficient molybdenum gate absorbers using a xenon flashlamp (NovaCentrix PulseForge 1300) using intense white light delivering radiant energy up to 6 Jcm-2. Simulations indicate that the IZO film reaches a peak temperature of ~590 C while the back of the glass substrate stays near room temperature. This process yields TFTs with a field effect mobility above 20 cm2V-1s-1 and an IOn/IOff ratio approaching 108, which exceeds the performance of samples annealed at 500 C for 1 hr. We will discuss the requirements and design guidelines for photonic curing metal oxide semiconductors for high-performance TFT applications, focusing on the importance of effective gate absorbers and optimized pulse designs to efficiently and effectively cure the sol-gel films.

Photonic Curing of Solution-Processed Indium Zinc Oxide Thin-Film Transistors

Abstract Body: Spin-orbit torque (SOT) driven deterministic control of the magnetization state of a magnet with perpendicular magnetic anisotropy (PMA) is key to next-generation spintronic applications including non-volatile, ultrafast, and energy-efficient data storage devices1-3. But field-free deterministic switching of perpendicular magnetization remains a challenge because it requires an out-of-plane anti-damping torque, which is not allowed in conventional spin source materials such as heavy metals (HM)1-3 and topological insulators4 due to system symmetry. The exploitation of low-crystal symmetries in emergent quantum materials offers a unique approach to achieve spin-orbit torque with unconventional form5,6. Here, we report the first experimental realization of field-free deterministic magnetic switching of a perpendicularly polarized van der Waals (vdW) magnet employing an out-of-plane anti-damping SOT generated in layered WTe2 which is a low-crystal symmetry quantum material. The numerical simulations confirm that out-of-plane antidamping torque in WTe2 is responsible for the observed magnetization switching in the perpendicular direction. 1. Miron, I. M. et al. Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature 476, 189-193, (2011). 2. Liu, L. et al. Spin-Torque Switching with the Giant Spin Hall Effect of Tantalum. Science 336, 555-558, (2012). 3. Liu, L. et al. Current-Induced Switching of Perpendicularly Magnetized Magnetic Layers Using Spin Torque from the Spin Hall Effect. Physical Review Letters 109, 096602, (2012). 4. Mellnik, A. R. et al. Spin-transfer torque generated by a topological insulator. Nature 511, 449-451, (2014). 5. MacNeill, D. et al. Control of spin-orbit torques through crystal symmetry in WTe2/ferromagnet bilayers. Nature Physics 13, 300-305, (2017). 6. Xue, F. et al. Unconventional spin-orbit torque in transition metal dichalcogenide--ferromagnet bilayers from first-principles calculations. Physical Review B 102, 014401, (2020).

Deterministic Switching of a Perpendicularly Polarized Magnet Using Unconventional Spin-Orbit Torques in WTe2

Abstract Body: The oxide compounds containing iron are widely studied because of many interesting phenomena including charge disproportionation, spin polarization, and the magentoresistive effect. CaFe2O4 crystallizes in the orthorombic Pnma structure which consists of distorted FeO6 distorted octahedra through edge- and corner-sharing with intercalated Ca atoms. We have recently grown a large single crystal of CaFe2O4 by the optical floating-zone method. The two transitions T1~240 K and T2~200 K are observed in our magnetic susceptibility measurements. Our neutron powder diffraction reveals two competing magnetic phases Γ6 and Γ3 at low temperatures which are distinguished by their c-axis stacking of ferromagnetic b-axis stripes. Γ6-type magnetic phase develops below 240 K that starts to reduce with the emergence of Γ3-type magnetic phase below 200 K. The detailed magnetoresistance measurements in different crystal orientations will also be discussed in the presentation.

Neutron Diffraction and Transport Properties of Antiferromagnet CaFe2O4

Abstract Body: Vanadium tetracyanoethylene, V(TCNE)2, is a room temperature ferrimagnetic semiconductorwith a Tc ~ 600 K, which has very low loss ferromagnetic resonance and spin-wave propagation.[1,2,3,4,5] Previous firstprinciples calculations of the electronic structure have indicated an indirect band gap at 0.8 eV, substantiallylarger than the 0.5 eV from experimental inference from the conductivity activation energy. [6,7] The study of Ref.1 used a local-orbital calculation with B3LYP hybrid functional. Here we explore the electronic structure using aplane-wave code VASP and XC functional Heyd-Scuseria-Ernzerhof (HSE06).[8,9,10] We confirm that the structure ofVTCNE has a triclinic unit cell with each V atom surrounded by 6 organic ligands, as found in Ref. 1. However,in contrast to the previous study we find a direct band gap of 0.4 eV, instead of an indirect band gap, located ata different k-point. This band gap better agrees with the experiment. We also studied magnetic anisotropy, theoptical properties, magnetic dynamics, and magnetoelastic properties. We acknowledge funding from NSF Grant No. DMR-1808742. [1] H. Yu, M. Harberts, R. Adur, Y. Lu, P. C. Hammel, E. Johnston-Halperin, and A. J. Epstein, Appl. Phys. Lett. 105, 012407 (2014). [2] N. Zhu, X. Zhang, I. H. Froning, M. E. Flatté, E. Johnston-Halperin, and H. X. Tang, Appl. Phys. Lett. 109, 082402 (2016). [3] H. Liu, C. Zhang, H. Malissa, M. Groesbeck, M. Kavand, R. McLaughlin, S. Jamali, J. Hao, D. Sun, R. A. Davidson, L. Wojcik, J. S. Miller, C. Boehme, and X. V. Vardeny, Nat. Mater. 17, 308–312 (2018). [4] A. Franson, N. Zhu, S. Kurfman, M. Chilcote, D. R. Candido, K. S. Buchanan, M. E. Flatté, H. X. Tang, and E. Johnston-Halperin, APL Materials 7, 121113 (2019). [5] J. M. Manriquez, G. T. Yee, R. S. McLean, A. J. Epstein, and J. S. Miller, Science 252, 1415 (1991). [6] G. C. De Fusco, L. Pisani, B. Montanari, and N. M. Harrison, Phys. Rev. B 79, 085201 (2009). [7] V. N. Prigodin, P. R. Nandyala, K. I. Pokhodnya, J. S. Miller, and A. J. Epstein, Adv. Mater. 14, 1230 (2002). [8] G. Kresse and J. Hafner, Phys. Rev. B 47, 558 (1993); ibid. 49, 14 251 (1994). [9] G. Kresse and J. Furthmüller, Comput. Mat. Sci. 6, 15 (1996). [10] G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11 169 (1996).

First Principles Calculation of the Electronic Structure of V(TCNE)2

Abstract Body: In recent years, AlGaN nanowires grown by molecular beam epitaxy (MBE) have emerged as an appealing platform for semiconductor deep ultraviolet (UV) light-emitting devices [1, 2]. Although the progress, a better understanding of some of the fundamental physical properties of such nanowires is necessary. For example, the internal quantum efficiency (IQE) of AlGaN nanowires is often investigated by the temperature-dependent photoluminescence (PL) technique that assumes a 100% IQE at low temperatures. This assumption is, however, not always valid. This contrasts with the comprehensive understanding of IQE of AlGaN quantum wells and epilayers. Moreover, the efficiency droop of the deep UV emitting AlGaN nanowires is much less studied. In this regard, we analyzed the room-temperature IQE of AlGaN nanowires emitting at ~260 nm, being considered as the most efficient wavelength for disinfection, through a theoretical analysis on the room-temperature excitation dependent PL. A peak IQE of ~55% was derived. Moreover, a carrier delocalization mechanism was pinpointed as a droop mechanism. In this work, the AlGaN nanowires were grown on Si (111) substrates by MBE in the nitrogen-rich condition at a relatively low substrate temperature. Detailed growth conditions can be found in Ref. [3]. The power-dependent PL experiments were performed in a setup consisting of a 213 nm laser and a deep UV spectrometer. We further used a theoretical model that considers the major recombination channels in semiconductors, i.e., An as the Shockley-Read Hall (SRH) nonradiative recombination, Bn2 as the bimolecular radiative recombination, and Cn3 as the higher-order nonradiative recombination, where n is the carrier density and A, B, C are the respective recombination coefficients, to analyze the excitation power dependent integrated PL intensity (IPL). In this framework and assuming at steady state, the generation rate G follows G = An+Bn2+Cn3. Further expressing IPL = γBn2, where γ is an experimental parameter, one can have, G = A(IPL)1/2/(Bγ)1/2 + IPL/γ + C(IPL)3/2/(Bγ)3/2 (1) Here, G can be experimentally determined by G = P(1-RF)α/(Aspothv), where P is the peak excitation power, RF is the Fresnel reflection, α is the absorption coefficient, Aspot is the laser spot size, and hv is the excitation photon energy. The experimentally determined G – IPL dependence can be further fitted to Eq. (1), allowing one to determine the parameter γ; and the IQE is further calculated by IQE = IPL/γG. The scattered symbols and the solid line in Fig. 1 show the experimentally determined G – IPL dependence and the fitting curve according to Eq. (1), respectively. It is seen that a good fitting is obtained. The calculated IQE as a function of G is shown in the inset of Fig. 1. It is seen that a peak IQE of ~55% is reached, followed by a droop. To further analyze the droop mechanism, the excitation dependent PL peak energy and full-width-at-half-maximum (FWHM) were analyzed. Shown in Fig. 2, a blueshift of the PL peak energy is observed accompanied by an overall increasing trend of the FWHM, suggesting the role of the carrier delocalization in the efficiency droop. Current studies are on the Al content dependent efficiency droop behavior and will be reported. References: [1] S. Zhao et al., "AlGaN Nanowires: Path to Electrically Injected Semiconductor Deep Ultraviolet Lasers," IEEE Journal of Quantum Electronics 54, 1 (2018). [2] S. Zhao et al., "AlGaN Nanowires for Ultraviolet Light-Emitting: Recent Progress, Challenges, and Prospects," Micromachines 11, 125 (2020). [3] Y. Zhong et al., "Molecular beam epitaxial growth and optical characterization of AlGaN nanowires with reduced substrate temperature," AIP Advances 10, 025022 (2020).

Deep Ultraviolet Luminescence and Charge-Transfer Excitons in Atomically Thin GaN Quantum Wells

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Deep Ultraviolet Luminescence and Charge-Transfer Excitons in Atomically Thin GaN Quantum Wells

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