United States

Abstract Body: Dimensionality is the determining critical parameter in any material. Graphene and transition metal chalcogenides are two-dimensional (2D) layered nanomaterials which demonstrate wide array of physical and chemical properties when approaching few-layer thicknesses [1-2]. Large lateral size of these van der Waals nanomaterials with thinner mono- or few-layered flakes possess higher aspect ratios, which makes them suitable for electronic and optoelectronic applications. Among the most reliable techniques for exfoliation, sonication-assisted liquid-phase exfoliation (LPE) is considered to be a simple and cost-effective method for preparing graphene and its 2D inorganic counterparts at reasonable sizes and acceptable levels of defects. Although there were rapid advances in this field, improper sonication may result in unstable dispersions with wide flake sizes and thicknesses distributions, impurities and defects. Previous studies were mainly focused on the effects of solvent selection (predominantly, using high-boiling point solvents or surfactants), sonication time, and power input (usually, high power from probe/tip sonication can cause significant damage to the material). However, the effect of the sonication frequency is poorly understood and often ignored resulting in a low exfoliation efficiency. Here, we demonstrate that simple mild bath sonication at high frequency at relatively low powers contributes to the thickness, size and quality of the final exfoliated products. We show that monolayer graphene flakes can be directly exfoliated from graphite using only ethanol as a solvent at a reasonable sonication time by increasing the frequency of the bath sonicator from 37 kHz (which is usually used) to 80 kHz. The key mechanism involved is the generation of small-sized bubbles resulting from higher frequencies, which can easily penetrate between the interlayer spacing of the 2D materials to facilitate the delamination process with reduced damage to the material [3]. Detailed morphology studies by atomic force microscopy (AFM) indicate that the obtained product is dominated by large lateral size (10-15 μm) monolayer graphene flakes. High resolution TEM (HRTEM) show that the graphene flakes are of high-quality without any significant structural defects, although X-ray photoelectron spectroscopy (XPS) shows evidence of a small oxide population. Additionally, we show that this facile technique can be applied to other inorganic 2D nanomaterials such as digenite (Cu9S5) and silver sulfide (Ag2S). This simple and effective method facilitates graphene and transition metal chalcogenides’ processing for a wide range of applications. References [1] Coleman et.al, Science, 2011, 331, 568-571. [2] Novoselov et.al, PNAS, 2005, 102, 10451-10453. [3] Merouani et.al, Ultrason Sonochem., 2013, 20, 815-819.

EMC 2021

Higher Ultrasonic Frequency Liquid Phase Exfoliation as a More Efficient Method to Deposit Monolayer to Few-Layer Flakes of 2D Layered Materials

2d layered materials

exfoliation

graphene

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CAP Virtual Congress 2021

technical paper

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Abstract Body: Investigation of new group IV alloys to find a direct bandgap material that can emit and detect light efficiently has been pursued by Si photonics in the past decades. The integration of optoelectronics on the electronics chips enables high-speed data transfers through emitters and detectors that are compatible with Si technology 1. Although GeSn has been shown as a group IV that can achieve the above-mentioned goals by different groups 2, other alloys of group IV have not been yet fully investigated due to technical challenges such as insolubility, lattice mismatch, unavailability of proper precursor, etc. Theoretical calculations show that SixSn1-x alloys can also be a direct bandgap material like GeSn with the difference of having a larger bandgap due to high direct band edge of Si at 3.2 eV (Γ-valley)3. However, the theoretical study of this material is not well developed. The estimation of Sn concentration needed to achieve direct bandgap varies in the range of 40-60% while the experimentalist are expecting that to be in the 25% range.3 In this paper we have studied both material properties as well as electronic band structure using density functional theory (DFT) calculations. We have adopted generalized gradient approximation- Perdew–Burke–Ernzerhof (GGA-PBE) 4 with Vienna Ab-initio Simulation Package (VASP).5 The 16-atom cubic supercells of Si and Sn, as well as the unit cell, were used in this modeling in order to find the atomic configurations and energetics. The supercells were relaxed for all the atoms with a residual force of less than 5 × 10−5 eV/Å for each atom. A (4×4×4) Monkhorst−Pack mesh of k-points for a periodic 16-atom supercell was adopted to generate the Brillouin zone. We calculated the lattice constant and its formation energy for all compositions using the GGA–PBE exchange-correlation potential considering the structural relaxation of an isotropic stress for the supercell. The change in the lattice follows the Vegard’s law and a slight bowing (=0. 05 Å) is observed across the full range which is close to previously reported values 0.02-0.084 Å 3, 6 . The lattice constants of Si and α-Sn were estimated to be 5.47 and 6.65 Å, respectively. These values are in close agreement with previously reported values of 5.47Å for Si7 and 6.65–6.74 Å for α-Sn 8 using a similar modeling method of GGA–PBE. It is noteworthy that compared to other group IV binary alloys such as SiGe and GeSn, the proposed bowing parameter is slightly lower. The band structure of all compositions of SixSn1-x is calculated for both unit cell and supercell of 16 atoms. The electronic properties of the alloys are affected by the displacement of Si1-xSnx atoms; therefore, structural relaxation of the structures plays an important role in the band structure. These structures were relaxed to a high extent as stated above. Since the GGA-PBE method underestimates the bandgap of SixSn1-x alloys, different methods have been applied to bring the calculations closer to the real bandgap. In our calculations, we have included the valence band offset of SixSn1-x alloys. The valence band offset is attributed to the electronic interaction between the occupied and unoccupied orbitals of Si1-xSnx. Therefore, by considering an Sn dependent valence band offset as well as heavy, light, and split-off hole bands, the calculations are closer to the real bandgaps. More details of the material properties along with the electronic band structure calculations will be presented at the conference. References [1] A. Atabaki et al., Nature 556, 349 2018 [2] A. Mosleh et al. J Electron Mater 43, 938 2014 [3] J. Tolle et al., Appl. Phys. Lett. 89, 231924 2006 [4] J. P. Perdew et al. Physical Review B 46, 6671 1992 [5] G. Kresse et al. Physical Review B 54, 11169 1996 [6] P. Yakibchuk et al. J Physical Studies 23, 2703 2019 [7] SQ Wang et al. J Physics: Cond. Matt. 15, L197 2003 [8] R. Pandey et al. Appl. Phys. Lett. 75, 4127 1999

SiSn Electronic Band Structure Modelling Using Density Functional Theory for Optoelectronic Applications

Abstract Body: The next generation of solid-state power electronics will increasingly utilize the AlxGa1-xN system, benefitting from its large and direct band gap, high switching speed, and high FOM. Formation of selective-area p-type regions is critical for field management and remains a significant challenge for higher-voltage next-generation architectures. Activation of the p-type dopant Mg, especially in the case of ion-implanted AlxGa1-xN, requires temperatures high enough to cause significant diffusion of the dopant, potentially distorting the doped region shape. Microwave annealing has been shown to induce both dopant activation and diffusion, especially for rapid drive-in of B in Si1 Diffusion dynamics induced by microwave annealing can diverge from those suggested by the bulk thermal condition of the sample. This has been attributed to interaction of the microwave beam with ionized defects and dopants in the material, permitting higher diffusion of electrically charged (ionized) species in the lower-temperature bulk2. This makes understanding Mg redistribution in GaN due to microwave annealing in both implanted and in-situ doped material of critical importance to the continued development of selective-area doping. In order to better understand this diffusion and activation behavior, a study was conducted of spatially resolved Mg distribution in in-situ doped and implanted GaN grown by metalorganic chemical vapor deposition (MOCVD) and annealed by gyrotron microwave beam. Annealing conditions between 1160–1410 °C were performed for durations between 30–65 s in total using a quasi-gaussian beam intensity profile and a 2.5 cm beam raster across the sample surface. Low-temperature PL spectra were measured and used as a proxy for acceptor activation before and after annealing of doped GaN. At the locations of PL measurements on samples, depth profiles of [Mg] were taken using dynamic secondary ion mass spectroscopy (D-SIMS). Beyond concentrations of around 1016 cm-3, the intensity of PL bands is not linearly related to the density of acceptors in the material3, however PL spectra have been shown to indicate the presence of conductivity type, provided initial knowledge of the growth condition. The as-implanted [Mg] profile of all samples shows a profile of 1×1019 cm-3 to a depth of 250 nm with a quasi-Gaussian tail. The in-situ doped as-grown [Mg] of 7×1019 cm-3 remains constant from the surface to the p-GaN layer depth of ~120 nm before annealing, and the profile does not appreciably change after annealing. After annealing implanted samples above 1350 °C, diffusion of Mg dopant is observed to vary spatially across the sample, correlating approximately with the beam shape and raster. To understand potential correlation of such diffusion with PL bands, measurements on the same location as SIMS measurement was performed. PL measurements show the characteristic near-band edge (NBE) luminescence at 3.47 eV and Mg-acceptor-related UVL at 3.27 eV in all spectra. In the 2.2–2.4 eV range, either the VN-related GL2 band or the CN-ON-related YL band is dominant. Relative intensity of these bands varies across the sample area, however no correlation of PL activity with Mg diffusion is observed. We show that while PL activity reflects the presence of MgGa and other optically active defects within the laser excitation depth (~92 nm), (anneal-induced) diffusion has no significant impact on [Mg] within this depth, and therefore that diffused Mg deeper in the sample (200–800 nm) does not necessarily impact PL activity. This may be due in part to the upper limit of PL sensitivity to acceptor concentration3. This work was funded by a grant from ARPA-E PNDIODES program, grant #DE-FOA-001691 under the direction of Dr. Isik C. Kizilyalli. [1] Kohli, P., et al. (2002). J. of Electron. Mat., 31(3), 214-219. [2] Booske, J. H., Cooper, R. F., & Dobson, I. (1992). J. Mat. Research, 7(2), 495-501. [3] Reshchikov, M. A., et al. (2017). Scientific reports 7(1), 1-11.

Microwave-Induced Annealing, Its Impact on Mg Diffusion and Photoluminescence Activity in Implanted and In Situ Doped GaN

Abstract Body: Recently, carbon-based electrode, especially graphene, has been introduced to replace the noble metals as a low-cost alternative. In this work, we show that the turn on voltage of the electrochemical reaction can be tuned in a semiconductor-insulator-plasma etched graphene (SIEG) device, especially for hydrogen evolution reaction (HER). By using this structured device, the relative energy level can be shifted to modify the onset potential. Besides, O2 plasma etched graphene was introduced to increase the number of active sites for HER on the surface of the graphene. Figure 1a shows a schematic of the SIEG device. A plasma-etched graphene on an insulator–semiconductor is introduced here. Reactive ion etching (RIE) by oxygen flow method was used to dry etch the graphene to introduce the active reaction sites. Compared to pristine graphene (PG) in Raman Spectroscopy, the ID/IG ratio was increased from ~ 0 to 0.58 from PG to EG shown in Figure 1b, suggesting the disruption of the hexagonal configuration by oxygen plasma etching. The result implies the existence of a higher amount of defect sites due to the introduction of plasma-induced edge defects. Figure 2a shows the HER measurement result of SIEG device in both biased and unbiased conditions. It is noteworthy that the onset potential of HER can shift up to ~0.8 V and 90mA/cm2 current density could be achieved at -0.5V vs RHE at VGr-Si = 1.5V. Figure 2b shows the schematic of the mechanism of catalysis due to hot electrons. The high performance of the device comes from the hot electrons possessing high energy which requires a significantly smaller activation energy barrier to overcome compared to the thermal electrons. Figure 3 shows the mechanism of how hot electrons are introduced into the graphene from the silicon. The applied bias between graphene and silicon shifts the conduction band of silicon relative to the reduction potential of H+/H2 redox couple, injecting carriers from silicon through graphene, while driving the reaction before any scattering happens in the graphene. To clarify this effect of the applied bias, we analyzed the different current components from the system individually. Here we find out that the silicon current is the major current component for chemical reaction (Redox current) when graphene-silicon voltage is applied as shown in Figure 3b. This result shows that the hot electrons from silicon current directly drive the electrochemical reaction, without affecting the graphene current. Figure 4 shows the overlapping of HER current between SIPG and SIEG device. The result shows that the current density of SIEG device at same experimental condition is ~ 2x higher than that of SIPG device. This indicates that the newly introduced active site for the hydrogen ions breaking through the limiting factor of graphene surface area. Electrochemical surface area (ECSA) achieved by cyclic voltammetry measurement also indicates the enhanced electrochemical area of the plasma etched graphene. In conclusion, we demonstrate that a plasma etched graphene based SIEG device can be a promising electrocatalysts candidate. SIEG device is not confined to this certain structure, we could possibly get better results by replacing each component with other materials. Future experiments could explore other redox reactions for carbon dioxide reduction to useful materials, and other 2-D materials with a high density of active sites that transport electrons more efficiently and adsorb more protons on top of the surface to see if high efficiencies can be achieved at lower voltages.

Hot Electron Metal-Free Electrode for Hydrogen Evolution Reaction

Abstract Body: To alleviate the adverse impacts of utilizing fossil fuel reserves on the environment (global warming, greenhouse emissions), extensive research is being devoted to the development of alternative/renewable energy systems, which include: photovoltaics, fuel cells, batteries, etc. Among them, fuel cells are an excellent solution for energy conversion, where proton exchange membrane (PEM) fuel cells have a wide variety of applications. Some of these applications are portable systems, automotive, etc., where an important reaction occurring at the cathode is the oxygen reduction reaction (ORR). At the current stage in fuel cell technology, platinum (Pt)-based materials are the most practical catalysts1, which, undoubtedly, are the prime choice as the cathode catalyst for the electro-reduction of oxygen, but Pt-based catalysts are expensive. To reduce such costs in commercially viable fuel cells, extensive research is focused on developing alternative catalysts. Silver-based catalysts show a comparatively high thermodynamic stability and excellent catalytic activity over a wide pH range. The TMCs (O, S, Se, and especially Se-based chalcogenides) have attracted significant attention since selenium-based cluster compounds work. Selenium-based metal-rich TMCs catalysts exhibit good electrocatalytic activity, which is mainly attributed to selenium because it improves the electronic structure of the metal by preventing the electrochemical oxidation of metals in most transition metal chalcogenides, as seen for the cases of cobalt selenide and ruthenium selenide. Here, we present a high yield, industrially scalable, and the first-time report on the synthesis of silver selenide from silver foil and selenium powder using a two furnace atmospheric pressure-chemical vapor deposition (AP-CVD) system. We grew low temperature, LT-β-Ag2Se on Ag foil in the AP-CVD system using thermal annealing and leading to a hollow fern-like morphology material and its first-ever reported application as an ORR cathode catalyst. The exfoliated β-Ag2Se is characterized using XRD, EDS, HRTEM, AFM, and HRSEM to determine its stoichiometry and its hollow layered fern-like morphology. Differential scanning calorimetry (DSC) further confirms that the final product is the low-temperature phase β-Ag2Se. Electrochemical oxygen reduction reaction (ORR) of exfoliated β-Ag2Se exhibits sharp oxygen reduction current with an onset potential of 0.88 V/RHE under alkaline conditions, indicating the oxygen reduction property of β-Ag2Se. The large value of limiting current (3 mA cm-2) as well as small Tafel slope (68.5 mV dec-1) corroborates β-Ag2Se as a superior layered transition metal chalcogenide cathode material suitable for such energy-related applications (typically fuel cells and/or metal-air batteries). References (1) Ostroverkh, A.; Johánek, V.; Dubau, M.; Kúš, P.; Khalakhan, I.; Šmíd, B.; Fiala, R.; Václavu, M.; Ostroverkh, Y.; Matolín, V. Optimization of Ionomer-Free Ultra-Low Loading Pt Catalyst for Anode/Cathode of PEMFC via Magnetron Sputtering. Int. J. Hydrogen Energy 2019, 44 (35), 19344–19356. https://doi.org/10.1016/j.ijhydene.2018.12.206.

Facile and Scalable Ambient Pressure Chemical Vapor Deposition-Assisted Synthesis of Layered Silver Selenide (β-Ag2Se) on Ag Foil as a Possible Oxygen Reduction Catalyst in Alkaline Medium

Abstract Body: Photoelectrochemical (PEC) water splitting is a promising technology for using solar energy to convert water molecules into hydrogen gas. Si’s moderate bandgap, high charge mobility and diffusion length, and extensive technology base enable Si-based photoelectrodes to achieve very good PEC performance with the potential for very low cost. However, Si has poor stability in aqueous solutions, limiting long-term stability of Si photoelectrodes for PEC water oxidation or reduction. Metal-insulator-semiconductor (MIS) photoelectrodes have therefore been widely explored for highly efficient and stable PEC water splitting. However, the insulating oxide that protects the semiconductor layer in an MIS photoelectrode typically must be sufficiently thin – a few nm – to enable carrier transport from the semiconductor to the metal catalyst via tunneling, often degrading photoelectrode stability. In this work, we demonstrate a simple, low cost, and highly scalable method to fabricate MIS photoelectrodes with thick SiO2 insulating layers and localized electrical contacts between the Si absorber and metal catalyst structures. The fabricated photoelectrodes show high efficiency and outstanding stability in aqueous solutions, with photoanode onset potential as low as 0.7 V vs. RHE and saturation current density over 30 mA/cm2 under AM1.5G illumination. Both MIS photoanode and photocathode structures are being explored. In a 7 day stability test, constant photocurrent density was maintained with no apparent degradation in performance. These results demonstrate a low-cost, highly scalable approach for fabrication of efficient and very stable photoelectrodes that is suitable for large-scale commercial fabrication and readily adaptable to a variety of catalyst, insulator, and semiconductor materials.

Low-Cost, Scalable Fabrication of Stable, High Performance Si-Based Metal-Insulator-Semiconductor Photoelectrodes for Solar-Driven Water Splitting

Abstract Body: Ever-increasing energy storage demands for vehicle electrification, consumer electronics, and grid energy storage requires further innovations in their performance and safety. To this end, developing new cathode materials has been the major interest in the Li-ion battery community. While cycling, most conventional cathode materials undergo repeated volume changes during Li-ion insertion/extraction (i.e., cycling), which in turn generates stress to the active material particles [1]. As a result of the tensile stress, particles tend to crack and pulverize, leading to a loss of contact and capacity fading. To address the mechanical instability issue from cathode materials, Dr. Lee et al. [2] developed novel spinel-type Li2Co2-xAlxO4 (where x = 0 – 1.0) as a “zero-strain” cathode material; it has less than 0.02% of the volume change during charge/discharge processes. This low volume change was explained by a suppression of rock-salt to spinel phase transition due to the presence of Al cations located in 16c site in [Li2−yAly]16c[Co2−xAlx−yLiy]16dO4. Actually, operando synchrotron XRD analysis demonstrates no phase transition during cycling [2]. The zero-strain Li2Co2-xAlxO4 cathode can overcome the mechanical instability and offer an extended battery life. On the other hand, electrochemical stability of the Li2Co2-xAlxO4 – electrolyte interface is currently unknown. Although the Al-substitution is an effective approach to stabilize the bulk crystal phase, parasitic reactions occurring at cathode – electrolyte interfaces (CEIs) has been a major issue from different types of spinel materials; e.g., transition metal dissolutions from LiMn2O4 or LiNi0.5Mn1.5O4 [3]. Therefore, it is important to understand the CEI properties of the Li2Co2-xAlxO4 and improving the CEI stability further. In this regard, we examined the effect of LiNbO3 coating on the series of Li2Co2-xAlxO4 (x = 0 – 0.6). The LiNbO3 coating layer can offer a facile Li-ion conduction while passivating the CEI layers. The results show that the LiNbO3 coating enhances the cycle life and rate capabilities of the Li2Co2-xAlxO4 cathodes. Electrochemical impedance spectroscopy (EIS) combined with the distribution of relaxation times (DRT) technique has been studied to identify the improvement mechanisms of the LiNbO3. The aging mechanism of the spinel cathode will be demonstrated by various material characterization and electrochemical analysis. This study will be conducted for conventional Li-ion batteries, but the results will be applicable to all-solid-state batteries via future studies. Reference: [1] D.E. Demirocak, B. Bhushan, J. Power Sources, 280 (2015) 256–262. [2] E. Lee, B.J. Kwon, F. Dogan, Y. Ren, J.R. Croy, M.M. Thackeray, ACS Appl. Mater. Interfaces, 2 (2019) 6170-6175. [3] J.-H. Kim, N.P.W. Pieczonka, Y. Li, ChemPhysChem, 15 (2014) 1940-1954.

Surface-Modified Zero-Strain Spinel-Type Li2Co2-xAlxO4 (where x = 0, 0.3, and 0.60) Cathodes for Lithium-Ion Batteries

Abstract Body: Low work function (WF) materials are necessary for energy storage and conversion devices. Scientific literature is full of explorations made in this field with new and emerging technologies applied to different elements with the purpose of creating low work function materials in the bulk and low dimensional regime. Alkali metals and their alloys with tin, cobalt, indium, etc. have been researched very well in batteries and other energy devices. Alkali metals have been shown to effectively aid in reducing the WF of many metals including Pb, Indium, tin, etc. Particular focus has been laid on Sn/Li alloy system where a work function dip up to 1.25 eV has been shown for a stoichiometric ratio of Sn2Li5[1][2]. Of course, this has been demonstrated to occur in the bulk alloys. However, with the purpose of achieving a similar goal in low dimensions (thin films), we are demonstrating for the first time the formation of a thin film of low work function Sn/Li alloy, formed using physical vapour deposition technique (E-Beam and Thermal Evaporation) at Ultra High Vacuum (UHV) conditions. The experiment utilises the high-end NanoESCA facility at the University of Bristol, UK, where the in situ deposition and the characterisation using X – Ray photoemission spectroscopy (XPS), Ultra-Voilet photoemission spectroscopy (UPS), spot profile low energy electron diffraction (Spa-LEED) techniques is carried out.[3]. In the initial experiments, a single crystal Si substrate was heated at a temperature of 800 oC for the removal of contaminants and Sn was deposited using the E beam deposition while Li was deposited using thermal evaporation method on Si substrate. Si surface was found to be terminated with the oxygen atoms. This should be the reason for the further lowering of WF as oxygen has been found to improve the sticking coefficient between the terminating metal species and the substrate surface. In the beginning, the O terminated Si surface was seen to possess a WF of 4.65 eV and a characteristic 2×1 LEED surface pattern which was then lost due to the disordered Sn and Li layers on the surface. However, Sn 3d peak was seen to shift towards the lower binding energy (B.E.) characteristic of the formation of Sn/Li alloy[4]. The WF was seen to lower to 4.15 eV and the surface was seen to become ordered after going through a series of annealing steps i.e., at 200 oC and 300 oC for 1 hour each and then 350 oC for 2 hours. After the 350 oC anneal, the LEED pattern started emerging. However, the Sn 3d peak was shifted to higher B.E. again which demands further analysis in order to find the optimised stoichiometric ratio between S/Li for ultra-low work function demonstration. [1] K. P. Loh, X. N. Xie, S. W. Yang, J. S. Pan, and P. Wu, “A spectroscopic study of the negative electron affinity of cesium oxide-coated diamond (111) and theoretical calculation of the surface density-of-states on oxygenated diamond (111),” Diam. Relat. Mater., vol. 11, no. 7, pp. 1379–1384, Jul. 2002, doi: 10.1016/S0925-9635(02)00014-6. [2] B. B. Alchagirov, R. K. Arkhestov, and F. F. Dyshekova, “Electron work function in alloys with alkali metals,” Tech. Phys., vol. 57, no. 11, pp. 1541–1546, 2012, doi: 10.1134/S1063784212110023. [3] G. Wan, M. Cattelan, and N. A. Fox, “Electronic structure tunability of diamonds by surface functionalization,” J. Phys. Chem. C, 2019, doi: 10.1021/acs.jpcc.8b11232. [4] D.-T. Shieh, J. Yin, K. Yamamoto, M. Wada, S. Tanase, and T. Sakai, “Surface Characterization on Lithium Insertion/Deinsertion Process for Sputter-Deposited AgSn Thin-Film Electrodes by XPS,” J. Electrochem. Soc., vol. 153, no. 1, p. A106, Dec. 2006, doi: 10.1149/1.2133711.

Investigation of Lithiated Tin (Sn/Li Alloy) Thin Film as a Low Work Function Termination on Silicon/Diamond Substrates

Abstract Body: Zinc germanium nitride (ZnGeN2) is the II-IV-N2 analogue of GaN which has a band gap (Eg) very close to that of GaN (~3.4 eV) and a lattice mismatch of <0.1% with GaN [1]. Based on first principles calculations, the predicted valence band offset (VBO) of ZnGeN2 with GaN can be as high as 1.4 eV, [2] which provides great opportunities to design high efficiency InGaN/ZnGeN2 heterostructure quantum wells for visible light emitters [1]. However, there has been no report on the experimentally determined VBO at the ZnGeN2/GaN heterointerface, due to the lack of availability of high quality materials. In this work, we have experimentally determined the VBOs of ZnGeN2 and a 94% ZnGeN2 – 6% GaN alloy ((ZnGe)0.94Ga0.12N2) with GaN using Kraut’s method. The VBO between two materials A and B are determined using the core energy levels (ECL) in bulk A, bulk B and at the A/B heterointerface and the position of the valence band maxima (VBM) in bulk A and bulk B. X-ray photoemission spectroscopy (XPS) was used to measure the ECL and VBM values. To determine the VBO between ZnGeN2 and GaN, a GaN/c-sapphire template, a 16-nm-thick ZnGeN2 film and a 2-nm-thick ZnGeN2 layer grown on GaN template were used as the bulk GaN, bulk ZnGeN2 and ZnGeN2/GaN heterostructure, respectively. ZnGeN2 films were grown on GaN/c-sapphire templates via metalorganic chemical vapor deposition (MOCVD) [3,4]. By using the measured Zn 3d (Ge 3d) core levels from ZnGeN2 and the Ga 3d core levels from GaN, the VBO of ZnGeN2 with GaN was extracted to be 1.45±0.15 eV (1.65±0.15 eV). These values are comparable to the values predicted from first-principles calculations using explicit interface calculations [2]. Similarly, the VBO of (ZnGe)0.94Ga0.12N2 with GaN was determined. The samples used include a GaN/c-sapphire template as bulk GaN, a 20-nm-thick (ZnGe)0.94Ga0.12N2 films as bulk (ZnGe)0.94Ga0.12N2 and a 3-nm-thick (ZnGe)0.94Ga0.12N2 film as the (ZnGe)0.94Ga0.12N2/GaN heterostructure. These samples were grown on GaN/c-sapphire templates via MOCVD. The experimentally extracted VBO of (ZnGe)0.94Ga0.12N2 with GaN was 1.29±0.15 eV. Assuming a linear dependence of the VBO with composition x of (ZnGe)1-xGa2xN2, the predicted VBO of (ZnGe)0.94Ga0.12N2 from the theoretically calculated VBO of ZnGeN2 would be 1.32 eV. Thus, the experimentally measured VBO results agree well with the theoretically predicted values. The conduction band offsets were derived using the determined VBO values and the energy band gap Eg of the materials. The band gaps of ZnGeN2 and (ZnGe)0.94Ga0.12N2 were estimated from the inelastic energy loss features of N 1s peaks in the XPS spectra of 16-nm-thick ZnGeN2 and 20-nm-thick (ZnGe)0.94Ga0.12N2 samples. The Eg values determined by this method were 3.0±0.2 eV for ZnGeN2 and 3.1±0.2 eV for (ZnGe)0.94Ga0.12N2. The lower values of Eg as compared to the predicted band gap values (~3.4 eV) can be due to the presence of disorder in the cation sublattice. In conclusion, the valence band offsets of MOCVD-grown (ZnGe)1-xGa2xN2 with GaN, with x = 0 and 0.06 were determined experimentally using XPS. The measured VBO values are comparable to the predicted values from first-principles calculations. The results from this study will expand device designs based on pure III-nitrides to III-nitrides/II-IV-N2, which can potentially address key challenges in III-nitride based electronic and optoelectronic device technologies. This work is supported by the U.S. DOE (DOE SSL: DE-EE0008718) and by the NSF (DMREF: SusCHeM: 1533957). References 1. L. Han, K. Kash, and H. Zhao, J. Appl. Phys., 120, 103102 (2016). 2. A. P. Jaroenjittichai, S. Lyu, and W. R. L. Lambrecht, Phys. Rev. B., 96, 079907(E) (2017). 3. M. R. Karim, B. H. D. Jayatunga, M. Zhu, R. A Lalk, O. Licata, B. Mazumder, J. Hwang, K. Kash, H. Zhao, AIP Adv. 10, 065302 (2020). 4. B. H. D. Jayatunga, M. R. Karim, R. A. Lalk, O. Ohanaka, W. R. L. Lambrecht, H. Zhao, and K. Kash, Cryst. Growth Des., 20, 189 (2020).

Experimental Determination of Valence Band Offsets of ZnGeN2 and (ZnGe)0.94Ga0.12N2 with GaN

Abstract Body: ZnGeN2 and GaN are nearly lattice matched and each have band gaps of approximately 3.4 eV. The large band offset of 1.4 eV in the conduction band results in a type II band alignment that has great potential for novel device structures such as high-performance LED structures that emit wavelengths beyond green [1,2]. Recently published work included theoretical predictions revealed a slightly positive mixing energy, indicating a tendency toward phase separation [3], and the lowest energy configuration to be the octet-rule-preserving Pmn21 phase for ZnGeGa2N4. Other compositions can be rendered by random stacking of ZnGeN2 and GaN layers along the orthorhombic b axis in octet-rule-preserving (and hence lower energy) phases. [3] MOCVD growth of this alloy on c-, r-, and a-plane sapphire and c-GaN/sapphire substrates showed that films of 50-50 composition exhibit better surface morphologies when grown on r-sapphire substrates. A surprising trend of increasing Zn incorporation with increasing Ga was noted. The optimal growth temperature and pressure were determined to be 670 0C and 550 torr at which the highest growth rate, 3.46 µm/hr, was obtained for a film grown on r-sapphire. This film also yielded a wurtzite (110) 2θ-ω XRD diffraction peak at 2Θ = 57.700 with FWHM of 0.760 and the lowest RMS surface roughness over an area of 1 μm2 , ~ 10 nm measured by AFM. [4] The only other experimental work on this alloy reported to date employed a gas reduction nitridation method for synthesis of powders of different compositions, from pure ZnGeN2 to a 50-50 mixture, for photocatalytic applications [5]. Recent progress in growth and characterization is reported here. Atomic-resolution HAADF-STEM images show single-crystal atomic arrangements within 2-3 nm of the substrate interface. The lattice parameters a, b and c measured from these images are 6.429 ± 0.012 Å, 5.585 ± 0.079 Å, and 5.252 ± 0.085 Å, respectively. Introduction of a low-temperature ZnGeN2 buffer layer (480 oC at 600 torr) improved the surface morphology, resulted in higher Hall mobilities, and gave band edge photoluminescence at approximately 3.5 eV, close to the predicted value for the Pmn21 phase. [3] Evaluation of several growth experiments with respect to resulting surface morphology and x-ray rocking curve peak line widths show that the optimal growth conditions for ZnGeGa2N4 with low-temperature ZnGeN2 buffer layers differ significantly from the optimal growth conditions for ZnGeN2 without the low-temperature ZnGeN2 buffer layers. To obtain optimal, nearly stoichiometric ZnGeGa2N4 films, the precursor DEZn flow rate was reduced by nearly half of that used in the same system for optimal ZnGeN2 growth at the same temperature. This reduction in DEZn flow rate also allows for the achievement of higher growth temperatures for ZnGeGa2N4 than for ZnGeN2 while avoiding Zn depletion. Growth of a quaternary alloy with composition approximately 25:75% ZnGeN2 : GaN also resulted in a continuous film with similar surface morphology, and an rms surface roughness of 10.2 nm over an area of 1 μm2. This work is supported by the U.S. DOE (DOE SSL: DE-EE0008718) and by the NSF (DMREF: SusCHeM: 1533957). [1] L. Han, K. Kash, H. Zhao, J Appl Phys 120, 103102 (2016) [2] M. R. Karim, H. Zhao, J Appl Phys 124, 034303 (2018) [3] B.H.D. Jayatunga, S. Lyu, S. Kumar, K. Kash, W. R. L. Lambrecht, Phys Rev Mat 2 (2018) [4] B. H. D. Jayatunga, Md R. Karim, R. A. Lalk, O. Ohanaka, W. R. L. Lambrecht, H. Zhao, and K. Kash, Crystal Growth & Design 20, 189-196 (2020) [5] T. Suehiro, M. Tansho, T. Shimizu, J Phys Chem C 121, 27590 (2017)

Progress in MOCVD Growth and Characterization of ZnGeGa2N4 Films

Abstract Body: There is a significant scientific and technological interest in the epitaxial integration of ferromagnetic or antiferromagnetic materials with the topological insulators (TI). This allows the introduction of the ferromagnetic order into a topological insulator system and may lead to the development of novel spin electronic devices. It is anticipated that these heterostructures can be used as novel magnetoresistive random access memory (MRAM) devices with high charge-to-spin conversion efficiency. This implies that TI-based heterostructures can switch between 0 and 1 memory states much more efficiently, while consuming lower energy than the state-of-the-art magnetic tunneling junction-based heterostructures that use transition metals. The TI-based devices will be less susceptive to weak disorder and perturbations since writing process will occur via conventional writing field while electrical read out process will be topologically protected. A key requirement for an operational heterojunction is a successful coupling of a TI with the magnetic material, which is facilitated by “atomically-clean” and abrupt junction interface. This should involve low-temperature deposition of the magnetic layer on the TI surface. This eliminates usage of the standard magnetic materials growth techniques, such as molecular beam epitaxy (MBE) and pulsed laser deposition, as they require the substrate temperatures greater than 300-350 0C, above what TIs can survive. Here, we report on the innovative approach to grow a room-temperature antiferromagnetic semiconductor (MnTe) using atomic layer deposition (ALD) and to epitaxially integrate it with the topological insulator (Bi2Te3). MnTe films were deposited at <100> GaAs and <111> InP. While the MnTe at <100> GaAs were polycrystalline, epitaxial growth was achieved at closely lattice-matched <111> InP at temperatures as low as 120 0C. Bi2Te3 was grown by MBE at c-Al2O3, ex-situ transferred into the ALD system, and used as a template for MnTe growth. High-resolution X-ray diffraction studies demonstrated that epitaxial MnTe growth was achieved at 120 0C. Transmission electron microscopy showed that the MnTe / Bi2Te3 interface was atomically sharp. No amorphous and / or disordered regions were present at the interface within the imaged regions. Magnetotransport properties of the ALD-grown MnTe were also studied and will be reported here. In summary, ALD was successfully used for the low-temperature epitaxial integration of MnTe antiferromagnetic semiconductor with the Bi2Te3 topological insulator. This opens the path for the realization of near-room temperature energy-efficient spintronic devices.

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