United States

Abstract Body: INTRODUCTION: Graphene – an atomically thin layer of carbon atoms arranged in a hexagonal lattice – has gained interest as a bioscaffold for tissue engineering for its exceptional mechanical, electrical, and thermal properties1-5. Graphene’s structure and properties– tightly coupled to synthesis and processing conditions4– are thought to influence biomolecular interactions at the graphene-cell interfaces. In this study, C2C12 cells, a multipotent mouse myoblast cell line, were grown on graphene bioscaffolds with specific structure–property–processing–performance (SP3) correlations. We found that SP3 correlations significantly influenced C2C12 differentiation, myotube formation, and gene expression, suggesting that graphene–cell interfaces can be engineered to control biomolecule structure and function in adherent cells. The experiments performed are the first to make a direct comparison on scaffold SP3 correlations and their impact on genetic expression of multipotent myoblast cells. METHODS: Chemical vapor deposition (CVD) grown graphene film, transferred to quartz glass slides, resulted with surface roughness (Ra) of 2.37 nm and graphene ink– produced by solvent assisted exfoliation– was printed using a Dimatix inkjet printer onto glass coverslips (Ra of 10.0 nm). Epitaxial graphene on SiC wafers, produced by Si sublimation (Istituto Italiano de Tecnologia, Pisa, Italy) (Ra of 2.24 nm). Prior to cell integration, cell attachment was facilitated with agarose gel applied to the edges of all graphene bioscaffolds and corresponding control bioscaffolds (quartz glass, glass coverslips and SiC wafers). C2C12 cells, maintained in a humidified incubator at 37°C and 5% CO2, were seeded onto the scaffolds at 1.3 x 104 cells/cm2 in Dulbecco’s Modified Eagle Medium (DMEM) with 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin (P/S). Cell growth and proliferation were monitored by transmitted light microscopy with images captured to study cell morphology. Gene expression patterns for each bioscaffold were analyzed for muscle myogenesis via qRT-PCR using a PCR array for muscle myogenesis and myopathy. RESULTS: Preliminary results show differences in cell morphology varied in a manner dependent upon the specific bioscaffold used. Myotube-like structures were evident in the control, CVD and epitaxial graphene (Fig.1A). Each of the three graphene bioscaffolds resulted in a unique overall genetic profile related to muscle myogenesis. However, four up-regulated genes were common to all biosaffolds when compared to their control. These genes are as follows: dystrophin (Dmd), myostatin (Mstn), leptin (Lep), pyruvate dehydrogenase kinase isoenzyme 4 (Pdk4) and Utrophin (Utm) (Fig. 1B). DISCUSSION: This preliminary work successfully shows that epitaxial, CVD, and printed graphene bioscaffold’s SP3 correlations impact the gene expression of C2C12 cells in culture. Each graphene bioscaffold presented with different and unique muscle gene expression profiles when compared to the control. The different morphologies of the cells as viewed in the transmitted light microscopy images confirms that the C2C12 cells grown on each bioscaffold, are uniquely different than the control. The following biomarkers: dystrophin, myostatin, leptin, pyruvate dehydrogenase kinase isoenzyme 4, and Utrophin were all upregulated in each of the graphene bioscaffolds. Epitaxial graphene shows the most genes upregulated that correlate to the muscle genotype followed by printed graphene and CVD graphene which showed the least number of upregulated genes relating to muscle (Fig. 1C). REFERENCES: 1. Nayak et al., ACS Nano., 2011. 2. Li et al., Sci Rep., 2013. 3. Krueger et al., ACS Biomaterials Science &amp; Engineering, 2016. 4. Novoselov et al., Nature, (2012): 192-200. 5. Nieto et al., Advanced Functional Materials, 2015. ACKNOWLEDGEMENTS: Support kindly provided by NIGMS under P20GM103408 and P20GM109095 and NSF under 0619793 and 0923535

EMC 2021

Differential Gene Expression in C2C12 Cells Due to Scaffold Structure-Property-Processing-Performance Correlations

bioscaffold

tissue engineering

graphene

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Abstract Body: INTRODUCTION: Graphene – an atomically thin layer of carbon atoms arranged in a hexagonal lattice – has gained interest as a bioscaffold for tissue engineering for its exceptional mechanical, electrical, and thermal properties1-5. Graphene’s structure and properties– tightly coupled to synthesis and processing conditions4– are thought to influence biomolecular interactions at the graphene-cell interfaces. In this study, C2C12 cells, a multipotent mouse myoblast cell line, were grown on graphene bioscaffolds with specific structure–property–processing–performance (SP3) correlations. We found that SP3 correlations significantly influenced C2C12 differentiation, myotube formation, and gene expression, suggesting that graphene–cell interfaces can be engineered to control biomolecule structure and function in adherent cells. The experiments performed are the first to make a direct comparison on scaffold SP3 correlations and their impact on genetic expression of multipotent myoblast cells. METHODS: Chemical vapor deposition (CVD) grown graphene film, transferred to quartz glass slides, resulted with surface roughness (Ra) of 2.37 nm and graphene ink– produced by solvent assisted exfoliation– was printed using a Dimatix inkjet printer onto glass coverslips (Ra of 10.0 nm). Epitaxial graphene on SiC wafers, produced by Si sublimation (Istituto Italiano de Tecnologia, Pisa, Italy) (Ra of 2.24 nm). Prior to cell integration, cell attachment was facilitated with agarose gel applied to the edges of all graphene bioscaffolds and corresponding control bioscaffolds (quartz glass, glass coverslips and SiC wafers). C2C12 cells, maintained in a humidified incubator at 37°C and 5% CO2, were seeded onto the scaffolds at 1.3 x 104 cells/cm2 in Dulbecco’s Modified Eagle Medium (DMEM) with 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin (P/S). Cell growth and proliferation were monitored by transmitted light microscopy with images captured to study cell morphology. Gene expression patterns for each bioscaffold were analyzed for muscle myogenesis via qRT-PCR using a PCR array for muscle myogenesis and myopathy. RESULTS: Preliminary results show differences in cell morphology varied in a manner dependent upon the specific bioscaffold used. Myotube-like structures were evident in the control, CVD and epitaxial graphene (Fig.1A). Each of the three graphene bioscaffolds resulted in a unique overall genetic profile related to muscle myogenesis. However, four up-regulated genes were common to all biosaffolds when compared to their control. These genes are as follows: dystrophin (Dmd), myostatin (Mstn), leptin (Lep), pyruvate dehydrogenase kinase isoenzyme 4 (Pdk4) and Utrophin (Utm) (Fig. 1B). DISCUSSION: This preliminary work successfully shows that epitaxial, CVD, and printed graphene bioscaffold’s SP3 correlations impact the gene expression of C2C12 cells in culture. Each graphene bioscaffold presented with different and unique muscle gene expression profiles when compared to the control. The different morphologies of the cells as viewed in the transmitted light microscopy images confirms that the C2C12 cells grown on each bioscaffold, are uniquely different than the control. The following biomarkers: dystrophin, myostatin, leptin, pyruvate dehydrogenase kinase isoenzyme 4, and Utrophin were all upregulated in each of the graphene bioscaffolds. Epitaxial graphene shows the most genes upregulated that correlate to the muscle genotype followed by printed graphene and CVD graphene which showed the least number of upregulated genes relating to muscle (Fig. 1C). REFERENCES: 1. Nayak et al., ACS Nano., 2011. 2. Li et al., Sci Rep., 2013. 3. Krueger et al., ACS Biomaterials Science & Engineering, 2016. 4. Novoselov et al., Nature, (2012): 192-200. 5. Nieto et al., Advanced Functional Materials, 2015. ACKNOWLEDGEMENTS: Support kindly provided by NIGMS under P20GM103408 and P20GM109095 and NSF under 0619793 and 0923535

technical paper

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Abstract Body: III-Nitride materials have long been promising candidates for the development of infrared optoelectronic devices. However, design of multiple quantum well (MQW) structures on conventional c-plane (0001)-oriented GaN substrates is hindered by the spontaneous and piezoelectric polarization fields associated with lack of inversion symmetry. Low-defect density free-standing m-plane (10-10) GaN substrates have enabled the growth of non-polar MQW structures without strong built-in polarization fields. We have previously demonstrated the potential of m-plane Al0.16Ga0.84N/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy (PAMBE) for terahertz devices [1]. To support higher energy near-infrared optical transitions, though, a larger conduction band offset (CBO) is required. This can be achieved through higher Al composition in the barriers. However, m-plane AlGaN layers with aluminum fractions above 0.2 grown under conventional gallium-rich PAMBE conditions exhibit severe material inhomogeneities [2]. Recently, we demonstrated that growth of m-plane AlGaN by indium surfactant assisted epitaxy (ISAE) at low temperature significantly reduces composition inhomogeneities [3]. Here, we present an in-depth comparison of the PAMBE growth of m-plane AlGaN with and without indium surfactant at 565°C. We investigated the compositions and growth rates of low temperature m-plane AlGaN layers grown by conventional gallium-rich epitaxy and by ISAE across a range of aluminum fluxes. It is expected that under nitrogen-limited growth conditions the growth rate would be independent of metal flux. We find the opposite to be true: as aluminum flux increases, the growth rate of AlGaN decreases rapidly. We attribute this phenomenon to nitrogen loss during PAMBE of m-plane AlGaN that increases super-linearly with increasing aluminum flux. These films also show a super-linear increase in aluminum content with increasing aluminum flux. Transmission Electron Microscopy (TEM) images indicate that these effects are correlated with high aluminum containing defects within the material. Our results indicate that ISAE mitigates this effect almost entirely. Films grown by ISAE with identical aluminum fluxes to the gallium-rich layers do not exhibit a significant decrease in growth rate with increasing aluminum flux, and their aluminum compositions increase linearly with aluminum flux. TEM images show the ISAE layers are thicker, and exhibit a higher degree of material uniformity up to 0.30 aluminum metal fraction. X-ray diffraction (XRD) measurements suggest minimal indium incorporation in the ISAE AlGaN layers. The CBO of MQWs can be further increased by using InGaN as the quantum well (QW) material. We have recently demonstrated the growth of high-quality m-plane InGaN with up to 0.16 indium metal fraction [4], enabling the growth of In0.055Al0.19Ga0.755N/In0.16Ga0.84N MQW structures using the ISAE technique [3]. We will also present results of structural and optical characterization of m-plane (In)Al0.3Ga0.7N/In0.16Ga0.84N MQW structures. Due to the anisotropic nature of strain in m-plane structures, AlGaN/InGaN structures cannot be perfectly strain-balanced. We will discuss the method used to control strain accumulation along the (0001) and (11-20) directions below the level for defect formation in both the InGaN QWs and AlGaN barriers. [1] C. Edmunds et al., Appl. Phys. Lett. 105, 021109 (2014). [2] M. Shirazi-HD et al., J. Appl. Phys. 123, 161581 (2018). [3] B. Dzuba et al., J. Appl. Phys. 128, 115701 (2020). [4] A. Senichev et al., APL Mater. 7, 121109 (2019).

Anomalous Super-Linear Growth Rate Reduction with Increasing Aluminum Flux in M-Plane AlGaN Grown by PAMBE

Abstract Body: A parametric analysis of growth conditions on the structural properties of N-Polar InGaN films was studied. Three series of N-polar InGaN films were grown on bulk GaN substrates by plasma-assisted molecular beam epitaxy (PAMBE) to study the effect of substrate temperature, Ga, and In fluxes. For all series, the surface morphology was measured by atomic force microscopy (AFM), and the composition and strain were determined by high resolution X-ray diffraction (HRXRD) and XRD reciprocal space mapping (RSM). All the samples in this study were 350 nm thick. First, a series of samples was grown at temperatures varying from 620 oC to 690 oC. The Ga flux for all samples in this series was kept at 3.8E-7 Torr and the In flux was increased by temperature to ensure metal-rich growth conditions. The surface morphology improved significantly by increasing the temperature, and the indium content was reduced as expected. The samples grown at 670 oC and 690 oC showed surprisingly smooth surface morphologies with surface roughness <0.7 nm which has never been reported before on such thick InGaN films. An In content of 16.5% and 13.6% was measured on samples grown at 670 oC and 690 oC, respectively. These temperatures are significantly higher than growth temperatures (<600 oC) typically used for the growth of InGaN films on Ga-polar GaN. In the next step, samples were grown with In fluxes of 3E-7 Torr and 4E-7 Torr. The Ga flux was kept at 3.8E-7 Torr and both samples were grown at a substrate temperature of 670 oC. Both samples showed step flow growth and low surface roughness despite the large difference in In flux. Nonetheless, increasing the In flux from 3E-7 Torr to 4E-7 Torr resulted in an improved surface morphology, and increased In composition and strain relaxation. These results indicate that the In flux can be tuned within an ample window to obtain specific sample properties. For the last series, three samples were grown at 660 oC and with In flux of 4E-7 Torr, while the Ga flux was varied from 3.2E-7 Torr to 3.6E-7 Torr. The surface morphology degraded with decreasing Ga flux. The Indium composition simultaneously increased due to the limited availability of Ga during growth. This indicates that the strain caused by increased In incorporation is relaxed by generating dislocations which in turn results in further increase in In incorporation as relaxation proceeds (the In pulling effect).

Growth of High Quality 350nm-Thick InGaN Films on N-Polar GaN Substrates by Plasma-Assisted Molecular Beam Epitaxy

Abstract Body: Carbon based electronics, also known as organic electronics are facilitating the development of emerging technologies due to their inherent bendability and potential for low cost manufacturing compared to brittle conventional silicon semiconductors. Furthermore, the chemical toolbox available to us enables the fine-tuning of the materials as a way to design and engineer the desired properties. This talk will focus on our groups recent advances in the development of high performing organic thin film transistors through the self-assembly of well-defined poly(ionic liquids) block copolymer based gating materails for n-type polymer based OTFTs. The use of these block copolymers over conventioanl dielectrics lead to drops in threshold voltages from 35V to under 1V while maininting high swtiching speeds and greater device currents. We aim to build structure property relationships between polymer architecture, thin film processing and device performance.

Low Voltage Operation of Flexible Organic Thin-Film Transistors Using Poly(Ionic Liquid) Block Copolymer Based Gating Materials

Abstract Body: This work presents the design, fabrication, and characterization of 3D thin-film metal electrodes for electrochemical bio-sensing (EBS) applications utilizing inkjet printing (IP) on shape memory polymer (SMP) substrates. Inkjet printing electrodes on SMP allows for 3D electrode formation in a single process step at low temperature and atmospheric pressure. In this study, a thermally contracting SMP has been utilized to print EBS electrode patterns using IP silver nanoparticle ink, extending previous work. The silver nanoparticle ink electrode geometry is sintered and shrunk simultaneously by placing it into an oven for 3 minutes. It is observed that the reduction factor of the processed EBS (sintered and shrunk) achieves up to 60% approximately and creates a complex 3D metal topology which is difficult to obtain using conventional electrode fabrication techniques. SMP is a polymer which remembers its initial shape and returns to that shape when heated above its glass transition temperature. Generally, at room temperature SMP stays at martensite phase and achieves austenite phase after heat application (in this study, 180 C). In order to observe the shrinking characteristics of the SMP film used for this study (MagicShrink), 18 different samples (1 cm by 1 cm) were shrunk at six different temperatures (140 C to 190 C) for 3 minutes. It is observed that, the shrinking mechanism is similar and stable for each sample. The reduction factor of the shrunk film is 0.6 for each sample. In this study, nanoparticle silver ink (ANP DGP-40-LT-15-C) is used to IP EBS pattern using Dimatix Materials Printer (Fujifilm, DMP-2831). This nanoparticle ink becomes conductive by curing it at 120 C ~150 C for 10 minutes. But it is perceived that the desired conductivity can be achieved only within 3 minutes at 140 C to 190 C with SMP (simultaneous curing and shrinking mechanisms). Resistivity measurements of the SMP fabricated electrodes have been performed to characterize the process impact on thin-film performance. Scanning electron microscopy is utilized to further characterize the 3D microstructure and topology created by IP on SMP fabrication process for EBS at different process temperatures. Further, electrochemical characterization of the 3D SMP electrodes are performed and compared to a conventional EBS 2D electrode. Fabricating EBS on SMP with IP technology have shown promising potential to improve electrochemical sensing. The inkjet on SMP process demonstrates significant 3D silver electrode using nanoparticle ink for an increased electrode surface area for EBS, which is difficult to fabricate using conventional techniques, with potential applications for other electrode technologies benefitting from increased device surface area. References [1] R. C. Roberts, J. Wu, N.Y. Hau, Y.H. Chang, S.P. Feng, D.C. Li, "Facile 3D Metal Electrode Fabrication for Energy Applications via Inkjet Printing and Shape Memory Polymer," Journal of Physics: Conference Series 557 (2014) 012006. [2] R. C. Roberts, S. Zeng, and N. C. Tien, "Design and Fabrication of Buckled Metal Strain Gauges using Shape Memory Polymer and inkjet Additive microfabrication," Technical Digest, The 14th IEEE Conference on Sensors (Sensors 2015), Busan, Korea, Nov. 1-4, 2015, pp. 1665-1668.

Inkjet Printed 3D Metal Electrodes on Shape Memory Polymer Towards Improved Electrochemical Bio-Sensing Performance

Abstract Body: Wearable electronics are increasingly important in healthcare applications for monitoring of the daily physical condition and diagnosis of an early stage of disorders. In particular, the wearable sensors on human skin can be utilized in real-time continuous monitoring systems because they provide comfortable wearing and consume less power. Pulse detection is one of the options and critical for high-risk groups of cardiovascular diseases such as angina, acute coronary syndrome, and myocardial infarction among the real-time monitoring of several physical parameters. Moreover, the pulse is easily detected from specific points on the human body such as carotid, temporal, fingertip, dorsal, and posterior tibial artery using skin-attachable sensor [Chen, et al. High durable, biocompatible and flexible piezoelectric pulse sensor using III-N thin film, Adv. Funct. Mater. 29, 1903162 (2019)]. Accurate sensing of the eyelid and eyeball movement is another new application. Brain disorders (e.g. Alzheimer's, Parkinson’s disease, and stroke) are accompanied by eye-related abnormal motions as early symptoms such as slow movements and the long interval between eye blinking. The diseases can be diagnosed in an early stage from the detection of unusual eye motions before they are seriously progressed. For both applications, the sensors should be light and small to attach on the human body, e.g. temple area. Recently developed thin-film piezoelectric skin-attachable sensor based on lead zirconate titanate (PZT) and ZnO inherently possess weakness for the wearable healthcare sensors. PZT contains lead, which is very harmful to the human body and environment by poisoning. A skin-attachable sensor should be containing toxic-free elements. ZnO sensor shows low sensitivity and resolution from the limitation of material, which demands additional signal processing for amplification of exact signal and minimization of the noise level. The benefit of non-toxicity is canceled by low sensitivity and extra work. As an alternative sensing element, III-N thin films are very promising to detect the physical motion of skin sensing due to their advantages of high sensitivity and durability, rapid response time, non-toxic nature on humans, lightweight, and low power consumption. Especially, single-crystalline gallium nitride (GaN) and aluminum-gallium nitride (AlGaN) piezoelectric thin films have been reported to show excellent output voltage, indicating outstanding sensitivity, rapid response time by a high electromechanical coupling factor, chemical and mechanical long-term stability, high thermal resistance, and excellent biocompatibility [Chen et al. Biocompatible and sustainable power supply for self-powered wearable and implantable electronics using flexible III-nitride thin-film-based piezoelectric generator, Nano Energy 57, 670 (2019)]. In this study, we developed and demonstrated skin-attachable GaN and AlGaN sensors for the detection of eye-lid motion to monitor the eye blinking and eyeball movements. Flexible III-N thin films were designed by layer transfer method from the rigid silicon substrate. They were analyzed by X-ray diffraction to reveal its single-crystalline quality without second phase or defects. The single sensor was applied to the temple area of the face for sensing of eye blinking and simple eyeball motion. Eye blinking is conducted with three different levels. Then, the multiple sensors were utilized with appropriate distance on the identical region to detect the transverse, longitudinal, diagonal, and rotational eyeball motions, which explain the abnormal eye movements. All the results from each sensor are distinguishable, which indicates that the sensors generated different values of output signals from each position with varied stimulates.

Skin-Attachable Physical Sensors Using Single-Crystalline III-N Piezoelectric Thin Film for Personal Health and Safety Monitoring

Abstract Body: Fabrication of a ZnTe (110) thin film is highly preferable for the application of terahertz wave detection devices utilizing the electro-optical (EO) effect. The preparation of the (110) oriented ZnTe (110) thin film has been tried on a sapphire substrate by molecular beam epitaxy. Sapphire (hetero material) is used as a non EO substrate, and ZnTe is a zincblende structure material and known to exhibit high electro-optical coefficients. However, it is difficult to prepare a ZnTe (110) thin film with high crystallinity. In previous works, the high quality ZnTe layer with the single domain structure of (111) was achieved on the S-plane sapphire substrate. It was also confirmed that high temperature treatments of m-plane sapphire substrates has resulted in nano-facet structures consisting of r- and S-planes. The calculation was performed and revealed that (110) oriented ZnTe layer on the sapphire substrate could be achieved by using a 17.66o off m-plane substrate, and by preferentially nucleating the (111) oriented domain on the S-plane of the nano-facet. The surface coverage of S-plane on the 17.66o off substrate was only about 31 %, which would hinder the preferential nucleation of ZnTe only on the S-plane. In this work, the nucleation process was varied, and the resulting film quality was characterized. The nucleation was initiated by a low temperature deposition of an amorphous layer, and its annealing to form nuclei only on the S-plane of the nano-facet. It was followed by the migration enhanced epitaxy (MEE) of ZnTe to develop the nuclei size. In this work, the MEE sequence was mostly focused, and the substrate temperature along with the MEE cycle were mainly adjusted. It was confirmed from the scanning electron microscopy (SEM) analysis that desorption from r-plane took place at lower temperature and nuclei on the S-plane nano-facet was preferentially remained when the annealing temperature was at around 420oC. When the annealing temperature was 400oC, the selectivity of the nuclei on the nano-facets were disappeared and the nuclei was uniformly observed on the substrate. The MEE was performed at 420oC for 200 cycles and then the substrate temperature was gradually lowered toward 340oC for the film growth. During this temperature lowering process, MEE was also introduced at 400oC with the cycle from 50 to 170. The theta-2theta scan of the film suggested that (110) oriented layer was formed on the substrate surface. Offset for the theta was not required to obtain the 220 diffraction signal. These results suggest that the nucleation of (111) oriented ZnTe preferentially took place on the S-plane of the nano-facet, and the (110) plane was formed parallel to the substrate surface. Although several kinds of additional domains were confirmed from the theta-2theta scan, the dominant domain was (110). When the MEE cycle at 400oC was increased to 170, the 220 X-ray diffraction peak became strong. It is probably associated with the size of nuclei developed during the MEE process. It was developed to much larger size by increasing the MEE cycle at 400 oC, which caused the improvement of the crystallinity. X-ray diffraction pole figure analysis was also performed. The (111) oriented domain formation on the S-plane nano-facet was clearly observed, and four addition domains were observed. Those additional domains were originated from twins formed on {111} planes. Zn molecular beam irradiation was also carried out during the last part of the annealing at 420oC. The pole figure image exhibited pronounced signals originated from the (111) oriented domain on the S-plane nano-facet when the Zn irradiation was employed. The Zn irradiation probably limited desorption of ZnTe from S-plane nano-facets, and increased the density of the nuclei remained on the S-plane nano-facet. This capping of the nuclei has resulted in the improvement of the film quality. This work was supported in part by Waseda University Specific Research Projects.

Control of Nucleation Layers for (110) Oriented ZnTe Thin Film Growth on Sapphire r, S-Plane Nano-Facet Substrates

Abstract Body: A molecular beam epitaxy (MBE) approach to selective area epitaxy (SAE) of III-V semiconductors could enable the seamless integration of metals, dielectrics, and high-quality crystalline semiconductors. This technique could advance novel optoelectronic structures, such as high-contrast photonics, site-controlled quantum emitters, stacked pixel detectors, and photonic integrated circuits. Using patterned dielectrics and metals to define crystal seeding regions, SAE can be performed to embed microstructures into crystalline semiconductors1. While SAE by metal organic chemical vapor deposition has been widely successful due to its high material deposition selectivity, a SAE MBE method could enable further advances through its high layer precision and access to non-equilibrium growth conditions1,2. SAE is difficult to achieve with conventional (continuous) MBE growth due to III-V polycrystalline nucleation on the mask, even at growth temperatures as high as 700°C. As a result, Allegretti et al. developed periodic supply epitaxy (PSE), a method to inhibit polycrystalline deposition by cycling group III deposition under a constant group V flux2,3. SAE with PSE was achieved using high growth temperatures and low growth rates3-4. This mitigates nucleation of poly-GaAs through a combination of increased Ga adatom diffusion to seeding windows and desorption of remaining Ga adatoms3-4, enabling high-quality planar encapsulation of the mask1. While an all-MBE approach has been demonstrated to achieve selective growth of features ~2µm in width and ~300nm in height, applications requiring larger features are limited by nucleation of polycrystalline semiconductor on the amorphous surface due to poor deposition selectivity and low adatom surface diffusion1,3. We present experimental results of GaAs growth on SiO2 films that demonstrate decreased poly-GaAs formation at higher temperatures and lower As4/Ga flux ratios, enabling SAE of larger patterned features. However, this improvement in selectivity has yet to be explained quantitatively. In order to expand the applicability of all-MBE SAE, we developed a PSE model that (1) identifies selective growth regimes to clarify their underlying mechanisms and (2) allows optimized SAE growth conditions to be determined for a given SiO2 pattern. A numerical 1D model was developed to describe PSE selectivity by fitting adsorption, desorption, and diffusion constants to GaAs growth on SiO2 films at 600°C with a growth rate of 0.7 Å/s and 65x As4/Ga ratio5-8. Two growth regimes have been identified: desorption- and diffusion-limited. The desorption-limited regime depends only on thermal desorption from the mask surface to achieve selectivity and as a result, is feature-independent. This model has identified the desorption-based selectivity limit to be an 18.7% PSE duty cycle and was demonstrated by observing no polycrystal formation after growth of 100nm GaAs with 10% PSE duty cycle on an SiO2 film. The remainder of the selective growth space is diffusion-limited and sufficient III adatom diffusion off the mask is required to achieve selectivity. Experiments to fully investigate the influence of growth temperature and V/III ratio on selectivity are underway and will be reported at the conference. [1] D. J. Ironside et al., ACS J. Crystal Growth and Design (2019). [2] A. M. Skipper et al., 2019 MRS Electronic Materials Conf. [3] F. E. Allegretti et al., Journal of Crystal Growth (1995). [4] S.C. Lee et al. Journal of Applied Physics (2002). [5] S. Shankar. Diffusion in 1D and 2D, MATLAB. Retrieved April 11, 2020. [6] Aseev et al. Nano. Lett. (2019). [7] S. C. Lee et al. Cryst. Growth Des. 2016. [8] E.M. Gibson et al. Appl. Phys. Lett. (1990). This research was partly done at the Texas Nanofabrication Facility supported by NSF grant NNCI-2025227 and was supported by Lockheed Martin and NSF through the UT CDCM: an NSF MRSEC under Cooperative Agreement No. DMR-1720595, as well as CCF-1838435 and DMR-1839175.

Experimentally-Calibrated Modeling of Molecular Beam Epitaxy Selective Area Regrowth

Abstract Body: The (In,Ga)N material system is attractive for photovoltaic applications as its optical absorption covers the solar spectrum. Thick InGaN films (hundreds of nm thick) with InN mole fraction larger than 10% are required for this purpose. However, the growth of high In-content InGaN films remains challenging due to 10% lattice mismatch between InN and GaN and the large difference between the thermal stability of GaN and InN. While the optimum growth temperature of GaN by plasma-assisted molecular beam epitaxy (PAMBE) is ~700 °C-730 °C, the growth temperature of InGaN is typically below 600 °C. Therefore, lower growth temperatures are required to grow InGaN films within the miscibility gap. InGaN films can be grown pseudo-morphically on GaN below a critical thickness. As the thickness increases beyond the critical thickness, the InGaN film relaxes. This relaxation may occur through different mechanisms depending on the growth technique and growth conditions (e.g., growth temperature). V-defect formation from threading dislocations is a common way of strain relaxation, especially in InGaN films grown via metal organic chemical vapor deposition, and has been observed in molecular beam epitaxy (MBE) grown InGaN films as well [1]. Pyramidal slip through the Matthews-Blakeslee mechanism and basal plane slip of misfit dislocation are other mechanisms through which strain relaxation occurs in InGaN films. Critical thickness reduces as InN mole fraction in InGaN increases. Bazioti et al [2] studied relaxation mechanism in InGaN films grown by PAMBE. They showed that at lower indium contents, strain relaxation dominantly occurred via formation of V-defects, with concurrent formation of an indium-rich interfacial zone. As indium content increased, this mechanism was replaced by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal plane stacking faults and threading dislocations in the rest of the film. In this talk, we will present growth of 700 nm-thick InGaN with an average InN mole fraction of ~8% coherently strained to GaN confirmed by XRD reciprocal space map scans. This is several times thicker than the critical thickness of In0.08Ga0.92N. Surprisingly, we did not observe generation of new threading dislocations in the InGaN films or misfit dislocations at the interface. Instead, we show that the strain is managed by self-formation of a superlattice structure. The superlattice was confirmed via scanning transmission electron microscopy and atom probe tomography. Photoluminescence (PL) measurements at room temperature revealed peak wavelength of ~450 nm, corresponding to In0.18Ga0.82N with a full width half max (FWHM) of ~36 nm. Reference: 1. C. Bazioti et al. Structure and strain variation in InGaN interlayers grown by PAMBE at low substrate temperatures Phys. status solidi 252 1155–62 (2015). 2. C. Bazioti et al. Defects, strain relaxation, and composition grading in high indium content InGaN epilayers grown by molecular beam epitaxy J. Appl. Phy. 118,155301 (2015).

Demonstration of Self-Assembled InGaN/GaN Superlattice on GaN Template Grown by Plasma-Assisted Molecular Beam Epitaxy

Abstract Body: The (In,Ga)N alloy system is attractive for optoelectronic applications as it enables direct bandgap tuning within a large range (0.7 eV–3.4 eV). While highly efficient GaN-based blue light emitting diodes (LEDs) have been commercially available for a decade, GaN red LEDs suffer from low efficiency. Red LEDs require high In content InGaN (>35%) QWs. Growth of high In-content InGaN on the GaN substrate is challenging due to high vapor pressure of InN. InN starts decomposing at 500 °C which is well below optimum GaN growth temperature of ~700 °C in molecular beam epitaxy (MBE). There is a large lattice mismatch between InN and GaN (∼10%). Therefore, there is a great deal of interest in developing relaxed InGaN buffer as pseudo-substrate for optoelectronics applications. Various techniques have been employed in the past to obtain relaxed InGaN buffer layers including graded InGaN transition film and growth of InGaN on micrometer-size tiles of porousified GaN. Growth on oxide substrates, such as ScAlMgO4 and ZnO have also been explored as an alternative. ZnO has the same crystal structure as (In,Ga)N (wurtzite) and is in-plane lattice-matched with In0.2Ga0.8N [1]. Recently, we demonstrated growth of high-quality ∼300 nm-thick (In,Ga)N films with the In content varying from 11% to 23% on ZnO [2]. For this purpose, we developed low-temperature atomically smooth GaN thin films (~2 monolayers (ML)) to suppress the interfacial reaction between nitrides and the ZnO substrate at elevated temperatures using metal-enhanced epitaxy (MEE). In this talk, we will present successful growth of high-quality 1µm-thick InGaN with In content ranging from 19.5% to 30.5% on ZnO substrate by plasma-assisted molecular beam epitaxy. PL analysis showed a relatively uniform InGaN composition on samples grown at 600 °C and 580 °C. Lowering the growth temperature to 560 °C led to clusters with higher In content. The uniformity of low temperature InGaN growth was improved by periodic desorption of In from the surface. Although, STEM image showed a high threading dislocation density generated at the substrate interface, TDD reduced significantly through annihilation of dislocations as InGaN thickness was increased. A self-assembled superlattice structure of the InGaN on ZnO was confirmed by scanning transmission electron microscopy and atom probe tomography. We will show the relaxation of InGaN on ZnO by XRD reciprocal space map measurement. Reference: 1. A. Kobayashi et al. Pulsed sputtering epitaxial growth on m-plane InGaN lattice matched to ZnO, Sci. Rep. 7 1-6 (2017). 2. K Khan, Growth of high quality (In,Ga)N films on O-face ZnO substrates by plasma-assisted molecular beam epitaxy, AIP Adv. 10 075120 (2020)

Investigation of 1µm-Thick InGaN Films Grown on O-face ZnO by Plasma-Assisted Molecular Beam Epitaxy

Abstract Body: Transition-metal dichalcogenides (TMDs) have attracted a lot of interest owing to their direct bandgap as they are reduced down to thickness of a monolayer (ML) [1]. Thus, they have potential for numerous applications in electronic and optoelectronic devices such as transistors, photodetection and sensing [2]. MoS2 is probably one of most investigated TMD material due to the ability to tune its band structure with applied strain or electric field [3, 4]. As a naturally occurring crystalline mineral, tuning the properties of MoS2 as a representative TMD material provides further insights into the effects of strain and doping on its properties before moving on to investigate rare and more complex TMD materials. The strain and doping in MoS2 can be modulated in various ways such as mechanical bending [5], use of different substrates [6] or by introducing a metal- MoS2 interface [7]. A metal-semiconductor interface provides an interesting platform to investigate the plasmon-exciton interactions and the charge transfer mechanism between the metal and the semiconductor. In this work, we create plasmonic hot spots between gold nanoparticles to modify the Raman and luminescence response of ML MoS2. Nominal 2, 4 and 7 nm of Au and 2nm of Al were evaporated on SiO2/Si (oxidized silicon). Au and Al do not wet well SiO2, resulting in the formation of nanoparticles. Au and Al nanoparticle films exhibit a plasmonic band centered around 550 nm and 300 nm respectively. Plasmonic hot spots are created upon excitation within the band of plasmonic response of the nanoparticles. Monolayers of MoS2 were mechanically exfoliated directly on the nanostructured metallic layers and on SiO2/Si as a reference sample. Raman spectroscopy and photoluminescence were performed to characterize the optical response of the MoS2 to the plasmonic hot spots and compared to the reference sample. By varying the thickness of the Au film from 2nm to 7nm, we observe a shift and splitting of the E12g Raman modes into E12g and E12g’. The A1g mode also red shifts. The shift and splitting are only observed when the plasmonic resonance is excited during the optical measurements, indicating a plasmonic hot spot effect. We also investigate the effect on the photoluminescence (PL). In comparison to the reference ML MoS2, the PL intensity from the ML MoS2 on Au is drastically reduced. As the thickness of the Au film increases, the PL emission from A and B excitonic peaks is quenched until the B-exciton is no longer seen for ML MoS2 on a 7nm Au film. This can be explained by the electron transfer from MoS2 to Au, which hinders the recombination of electron−hole pairs created by the photoexcitation. As the work function of MoS2 is lower than that of Au, the electrons in the excited state of MoS2 transfer to Au, leaving a hole behind, which causes p-doping in MoS2. The charge transfer phenomena, along with the strain effects on optoelectronic properties of MoS2 MLs will be presented and dicussed. This work shows the potential of combining plasmonics with 2D materials to modify their optical and phonon response. [1] Choi, W. et al. Mater. Today 20, 116–130 (2017). [2] Lorenz, T., Joswig, J.-O. & Seifert, G. 2D Mater. 1, 011007 (2014). [3] Castellanos-Gomez, A. et. Al. Nano Lett. 13, 5361–5366 (2013). [4] Chakraborty, B. et al. Phys. Rev. B 85, 161403 (2012). [5] Conley, H. J. et al. Nano Lett. 13, 3626–3630 (2013). [6] Sun, Y., Wang, R. & Liu, K. Appl. Phys. Rev. 4, 011301 (2017). [7] Moe, Y. A., Sun, Y., Ye, H., Liu, K. & Wang, R. ACS Appl. Mater. Interfaces 10, 40246–40254 (2018). Acknowledgement: This work is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 765075 (Project LIMQUET). Funding from Swiss National Science Foundation (project 196948) is also acknowledged.

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