CREST Center for Dynamic Multiscale and Multimodal Brain Mapping over the Lifespan [D-MAP]

Igniting Curiosity through Inclusive Education and Communication in Environmental Nanotechnology

Excellence in Cardiovascular Sciences Program Promotes Interest in Research and Biomedical Careers

This research aims to make a substantial contribution to the field of photonics by synergistically exploring the utilization of innovative multiferroic materials while pushing the boundaries of photonic device design. Drawing upon a robust foundation of preliminary data from prior modeling endeavors, this research seeks to realize a practical optical switch through the intricate interplay between multiferroic materials and both electric and magnetic fields. The proposed Anisotropic Photonic Crystal Slab promises to achieve complete photonic band gaps across all polarizations, unlocking a realm of extraordinary phenomena including self-collimation, negative refraction, and zero-index refraction. 

The architecture of the photonic device is designed using the CrystalWave numerical simulation suite, which empowers calculations employing the 3D Finite-Difference Time-Domain (FDTD) method across both spatial and frequency domains, alongside the Plane Wave Expansion method. Advanced equipment such as the Direct Laser Writer and E-Beam Lithography will be employed to manufacture prototypes. 

The current phase of the investigation has yielded several promising photonic structures, with a select few demonstrating favorable attributes. In collaboration with students from the Department of Material Science and Engineering, various types of multiferroic nanoparticles suitable for this project have been fabricated. Presently, ongoing efforts involve the exploration of the alteration of dielectric constants within diverse polymer-multiferroic nanoparticle blends under the influence of electromagnetic fields. 

The proposed research stands poised to drive substantial progress in the realm of Photonics, delivering noteworthy contributions to the evolution of pioneering devices for optical computing while potentially extending its sphere of influence to other scientific domains. These include the advancement of sophisticated photonic solar sails and biosensors.

Exploring the synergy of light and matter interactions: Multiferroic photonic crystals for novel photonics applications

Molecular electronics are being developed as materials for light-harvesters and -emitters, sensors, and quantum technology. The themes for optimizing these systems are miniaturization and improved precision, which increasingly moves these materials into the quantum-mechanical regime. Natural systems like photosynthetic pigment-protein complexes have shown that vast improvements to functional bottlenecks like energy-transport can be achieved, if the structural and energetic properties are finely controlled. In that spirit, we are developing modular molecular networks whose structures are controlled by DNA scaffolds. This approach controls the network’s structural, energetic, and dynamic properties. However, this increased control presents new needs and challenges. We must know what the ideal parameters are for optimal function. In turn, that optimization requires a better understanding of the molecular-scale characteristics of the systems. Another challenge is that these characteristics often include many degrees of freedom, especially for large chemical networks, so an approach must be developed to treat large amounts of data efficiently, yet thoroughly. Here, we describe approaches to address these challenges. By combining machine-learning tools with experimental spectroscopies and computational models (Figure 1), we can simultaneously understand and map the sample characteristics on the molecular scale, the energy transport characteristics on the mesoscale, and the material performance on the bulk scale. We will discuss how we have used these tools to predicted optimal system configurations for energy-transport, and how they reveal the interchromophore effects in chromophore arrays.

Understanding Electronic Energy Transport in DNA-Scaffolded Molecular Networks Using Machine-Learning Methods

In this work, we report on the use of Triethyl Gallium for ‘in-situ Ga etching’ of β-Ga2O3 inside an MOCVD reactor. We demonstrate a maximum etch rate of ~8.5 μm/hr at a substrate temperature of 1000 °C and Ga flux of 140.47 μmol/min. In addition, low surface roughness of 2.9 nm was also obtained at a Ga flux of 6.05 μmol/min and substrate temperature of 800 °C. The ultra-wide band gap semiconductor β-Ga2O3 (band gap=4.7 eV, breakdown field=8 MV/cm) has made rapid strides in terms of substrates, material quality and device performance over the last decade. Ga2O3 devices with performance exceeding that of SiC has already been demonstrated [ref]. Several promising device structures in β-Ga2O3 require the use of 3-D structures like fins and trenches. For fabrication of such structures in β-Ga2O3, dry etching using BCl3/Ar has been widely used as the standard process. However, a key challenge when using dry etching is the induced subsurface damage. This has been shown to be significant in Ga2O3 resulting in charge depletion, reduction in channel mobility and increased sidewall roughness. In addition to dry etching, wet etching using HF, H3PO4 and KOH has also been reported. However, wet etching is found to mostly result in angled sidewalls which are not ideal. A promising etch technique for Ga2O3 is the use of suboxide formation upon exposure to Ga (Ga (s)+ Ga2O3 (s) Ga2O (g)) at sufficiently high temperatures. Suboxide based etching of β-Ga2O3 performed in an MBE chamber was previously shown to result in low damage etching of Ga2O3 along with vertical sidewalls for fins and trenches [1]. In this work, we studied ‘in-situ Ga etching’ performed inside an MOCVD reactor using Triethyl Gallium (TEGa) precursor. At sufficiently high substrate temperature, TEGa undergoes cracking resulting in the deposition of Ga on the substrate surface while the remaining hydro-carbon group (CxHy) get pumped out of the chamber. The deposited Ga reacts with Ga2O3 forming volatile suboxide, resulting in etching. We studied the effects of various parameters like TEGa flow rate, substrate temperature (Tsub) and substrate orientation ((001) and (010)) on etch characteristics like etch rate and surface roughness. Substrate temperatures ranged from 650 °C to 1000 °C to determine the effect on etch rate and surface roughness. Pressure was held constant at 15 Torr throughout the experiments. Samples with a constant etch depth of ~300 nm was used to measure surface roughness using AFM. Gallium flux spanned from 3.63 μmol/min to 140.47 μmol/min to determine its effect on the previously stated etch characteristics. The etch rate was found to increase with substrate temperature from 1.8 m/hr at Tsub=700 to 3.7 m/hr at Tsub=1000 . The surface roughness of etched surfaces also decreased with increase in Tsub for both (010) and (001) Ga2O3 surfaces. As expected, etch rate was found to monotonically increase with TEGa flow rates both at Tsub=800 and Tsub=1000 . At a substrate temperature of 1000 and TEGa molar flow of 140.5 μmol/min, etch rates exceeding 8.5 m/hr was achieved. However, RMS roughness ((010) β-Ga2O3 surface) was also found to increase at high TEGa flow rates. Lowest surface roughness was obtained at low TEGa flow rates (<12 μmol/min) and Tsub=800 with the lowest obtained RMS roughness of ~2.8 nm. Patterned etching using SiO2 hard mask was also demonstrated with a total etch depth of 2 m. In conclusion, we demonstrate MOCVD-based ‘in-situ Ga etching’ using TEGa as the Ga source. Etch rates as high as 8.5 μm/hr was obtained at a substrate temperature of 1000 and TEGa flow rate of 140.47 μmol/min. In addition, low surface roughness <3 nm and patterned etching using SiO2 as the hard mask is also demonstrated. This work was supported by faculty startup fund from Arizona State University. References: [1] Kalarickal, N. K., Applied Physics Letters, 119(12), 123503.

Demonstration of MOCVD Based 'in situ Ga Etching’ of β-Ga2O3 Using TEGa

Polycrystalline diamond has excellent material properties such as high carrier mobility, excellent thermal and mechanical properties. Although polycrystalline diamond has been widely used as a surface protection layer, its practical use in electronic applications has been limited due to an unfavorable synthesis conditions. Polycrystalline diamond nanomembrane (PCD NM) is a different format of polycrystalline diamond film that is a free-standing form. The PCD NM is also a transferable diamond film that has a wide range of freedom in their thicknesses (a few tens of nm to hundreds of microns), size (micron-size to a full wafer scale), doping type/concentration, therefore, PCD NMs can be heterogeneously integrated with other foreign substrates or platform easily. In this talk, we will present our recent development of transferrable PCD NMs, including the NM synthesis and manipulation processes and their electrical properties. Fig.1(a) shows a schematic illustration of the creation of PCD NMs from the bulk PCD substrate. A 3um thick boron doped PCD thin-film was originally grown on a 2-inch SiO2/Si substrate via microwave plasma assisted chemical vapor deposition (MPACVD) growth reactors. In order to achieve a high-quality PCD thin-film, the pre-seeding process with diamond powder was performed prior to the growth. Once the PCD growth was completed, an array of etching holes was formed to access the underlying SiO2 layer, thus enabling a selective removal of the SiO2 layer using hydrofluoric acid. Then, the top PCD thin-film was separated from the substrate and formed a PCD NM. This PCD NM can be manipulated using various micro-transfer-printing techniques. Fig. 1(b)-(d) show the optical and scanning electron microscopy (SEM) images of free-standing PCD NMs after the process step (v) in Fig. 1(a). Fig. 1(e) presents transfer-printed PCD NM onto PET substrate using a micro-transfer-printing method. The free-standing nature of PCD NM allows us to access both top and bottom surfaces. Fig.2 shows a statistical distribution of grain sizes of the top (Fig. 2(a)) and bottom (Fig. 2(b)). Interestingly, the average grain size of the top and bottom sides of PCD NM were measured to be ~1.2um and 0.17um, respectively. This is due to a growth mechanism of the PCD thin-film (seed-to-crystalization), therefore, this result gives a possibility to engineer the size of the PCD grain larger. As shown in Fig.3, changes in a structural property of PCD thin-film and NM were investigated using Raman spectroscopy with the following three surfaces: (i) top surface of as-grown PCD on SiO2/Si sample, (ii) top surface of PCD NM, (iii) bottom surface of PCD NM. Strong sp3 peaks at 1332 cm-1 were obsevered from all three conditions. Noticebly, a relative intensity of carbon-boron vibrational modes at 1220 cm-1 is higher on the top surface than the bottom surface, suggesting that boron incorporation in the carbon lattice was poor in the beginning. To further investigate the doping quality in PCD NM, a transmission line method (TLM) method was employed. Total six TLM ohmic metal (Ti/Au=30nm/150nm) pads were deposited using an electron-beam evaporator. Fig.4 indicates I-V curves measured from TLM pads, showing a good linear I-V characteristic. Contact resistance and sheet resistance values were calculated from Resistance-Distance curve in Fig. 4(b), yielding 27 ohm and 37 ohm/sq, respectively. In summary, high-quality PCD NMs were successfully created and their manipulation methods were demonstrated. Various material properties of top and bottom surfaces of PCD NMs can be investigated and reveals differences in grain size, surface morphology and doping. PCD NMs can readily be implemented in various ways in electronics to improve the performance or to create new applications. For example, PCD NMs can be used to demonstrate 3D-stackable electronics, highly reliable bioelectronics, less-corrosive electrodes, heat dissipator and flexible electronics

Synthesis of Free-Standing Polycrystalline Diamond Nanomembrane

Two-dimensional transition metal dichalcogenides (2D TMDs) have been extensively studied in recent years due to their remarkable electronic, optical, and mechanical properties. Their ability to withstand high levels of strain without fracture makes 2D TMDs attractive candidate materials for devices such as quantum emitters and wearable energy harvesters. This same ability, however, can have pronounced effects on device performance when unintentional sources of strain cause property changes via deformation potentials and electromechanical coupling. One such source of strain is nanobubbles, which are ubiquitous in mechanically exfoliated 2D samples. When using piezoresponse force microscopy (PFM) to measure the flexoelectric effect in 2D TMDs, we observed that nanobubbles showed greatly enhanced electromechanical coupling compared to flat regions, beyond the difference expected due to the absence of substrate pinning. Measurements of flexoelectric moduli in ostensibly flat monolayers may therefore yield overestimates due to the inclusion of nanobubbles in the analyzed area. Our PFM measurements show consistent reduction in the calculated effective flexoelectric coefficient when the data are filtered to exclude nanobubbles, resulting in a more accurate value for unstrained material. We have also used PFM to characterize the detailed influence of strain, via nanobubbles in exfoliated MoS2 monolayers, on electromechanical behavior in monolayer TMDs, focusing in particular on strain-dependent enhancement of out-of-plane electromechanical responses. Small bubbles with diameters under 100 nm consistently produce enhanced piezoresponse compared to flat regions. Large bubbles with diameters on the order of hundreds of nanometers exhibit an unexpected PFM profile. Rather than increasing monotonically from perimeter to apex, the piezoresponse amplitude of large bubbles reaches its peak near the perimeter and subsequently decreases toward a local minimum at the bubble center. Combined with the calculation of the strain tensor by representing nanobubbles as thin plates and numerically solving the biharmonic von Karman partial differential equation, these results reveal a correlation between electromechanical response and strain derivatives. We posit that this correlation occurs due to their mutual connection to the local symmetry of the crystal. Within the bubble, the crystal’s mirror symmetry is broken, allowing one or more of the out-of-plane piezoelectric tensor elements to become nonzero. The enhanced electromechanical response associated with inhomogeneous strain is likely to have significant implications for a broad range of TMD-based 2D device structures due to the ubiquity of localized strain and the presence of band-edge energy shifts and electrostatic bound charges in those same features.

Effects of Strain and Local Topography on Electromechanical Coupling in Monolayer Transition Metal Dichalcogenides

1. Introduction By integrating the 32 graphene FET with antibody modified channel, the new corona virus was detected with 100~1000 times higher sensitivity than the conventional antigen detection kits. The detection time is ~10~20 minutes, which is ~10 times shorter than the PCR system. By using the graphene FET with and without the antibody modification for detecting the signal of virus and for the reference signal, the problems of the signal drift and the physical adsorption of virus were solved. Also by adapting the micro-TAS and automated washing system, the restriction of the Debye length was solved and the impurity such as a saliva could be washed away and the pure signal only from the virus which bind to antibody could be detected. 2. Experimental Process The channel length and width of the fabricated graphene FETs are 10mm and 100mm, respectively, and 32 graphene FET’s are integrated. The 13FET’s in the half of left hand side are modified by antibody through PBASE, and the 13FET’s in the half of right hand side are without antibody modification and used as reference FET. Onto this integrated graphene FET array, the micro total analysis system (m-TAS) made by PDMS was set to introduce the phosphate-buffered saline (PBS) solution from the right hand side inlet and through the graphene FET channel, and to drain from the left hand side outlet. PBS solution was introduced from the reservoir by the computer controlled micropump. In order to control the Debye length, the different concentration of PBS solution should be used for the introduction of virus and for the detection of the charge of virus. 3. Results & Discussions The time dependence of the average of the Dirac point of 13 graphene FET’s with and without the antibody modification was measured using the 0.01xPBS and its Debye length is 7nm. There is the difference I n the Dirac point between with and without the antibody modification, which is attributed by the positive charge of the antibody modified on the graphene surface. 2 time of 1xPBS was introduced into the system without virus to confirm the no influence by the exchange of the high concentration of PBS. Then, the new corona virus with 1x108FFU/mL in high concentration of 1xPBS solution was introduced to the graphene FET. Virus combines well to the antibody in the high concentration of 1xPBS solution. The Debye length at 1xPBS solution is as short as 0.7nm, and the charge of virus could not be detected by the graphene FET as the height of the antibody is as large as ~10nm. Again, the low concentration of 0.01xPBS was introduced to extend the Debye length. Then, the negative charge of the virus was detected and the Dirac point increased positively. The detail of this Dirac point shift was analyzed. Dirac point of the 13FETs with antibody modification shift as large as ~20mV , while that without antibody as small as ~3.5mV after the introduction of virus. The large shift of Dirac point with antibody FET is attributed to three reasons as (1)charge of virus which combine to antibody, (2)charge of the physical adsorbed virus on the graphene, and (3)Drift characteristic of the total measurement system. While the small shift of Dirac point without antibody FET is attributed to two reasons as (2) and (3). The difference of these value, i.e., ~16.5mV(20mV – 3.5mV) is due to the net charge of virus which combined to only antibody. Using the SEM, the new corona virus on the graphene FET channel could be counted. About 10 times larger number of virus binds to graphene surface with antibody modification than without antibody modification. The number of the virus on the channel is linearly proportional to the Dirac point shift. 3. Conclusions Using the integrated graphene FET array and automated washing systems, the new corona virus was detected with the 100~1000times higher sensitivity than the conventional antigen test.

Integrated Graphene FET Array for High Sensitive Detection of New Corona Virus with Automated Solution Exchange System

Gallium nitride (GaN) MOSCAPs and MOSFETs are often investigated for high-power applications ranging from electric vehicles to photovoltaic invertors and smart electric grids. The specific material properties of GaN, including its wide bandgap, higher obtainable breakdown voltages, and higher electron mobilities, enable novel advantages not attainable using traditional silicon power devices. Among the many challenges reported for vertical GaN MOSFETs, optimization of the semiconductor/dielectric interface is crucial to improve important parameters such as threshold voltage, leakage currents, and interface trap densities (DIT). Previous work on GaAs has shown the importance of GaxOy mitigation through sequenced TMA purging for improved interfaces but has not elaborated on the impact of this on electrical output characteristics. For GaN, to date little work has been published that analyzes the impact of minimized interfacial GaxOy from optimized ALD techniques and its effect on electrical performance in MOS devices. Minimizing GaxOy formation at the semiconductor-dielectric interface is necessary to obtain optimal performance in GaN MOS structures. The fundamental understanding of the impact of improved oxide deposition techniques for GaN MOSFET gates is critical to accurately describe device performance. Relative improvements to dielectric leakage, DIT distributions, and capacitance-voltage (C-V) trends may be analyzed to understand how each is affected through the elimination of native oxide concentrations at the interface of GaN MOSFET gates. This study analyzes changes in electrical performance of Al2O3 dielectrics fabricated using atomic layer deposition (ALD) techniques on GaN epilayers grown on bulk GaN substrates (GaN-on-GaN). Traditional ALD deposition techniques for aluminum oxide (Al2O3) utilize reactions of trimethyl aluminum (TMA) and water to form dielectric films on various substrates. Previous studies have shown that gallium oxide (GaxOy) formation on gallium arsenide (GaAs) can be alleviated by cycling TMA purges during ALD deposition to control oxidation states and mitigate Ga-O and As-O bonding. For GaN, improvements to gate interfaces of high-k dielectrics such as Al2O3 are crucial to ensure optimal switching and electrical response of complex vertical GaN MOSFETs. Leakage currents and high densities of interfacial traps have been shown to reduce device performance in previous studies. This work shows improved C-V responses of ALD Al2O3 films using quasi-static capacitance measurements. Optimized films were fabricated by the mitigation of GaxOy at the semiconductor/dielectric interface. Films grown on substrates that did not receive a TMA dose prior to the H2O precursor for ALD growth of Al2O3 films show a significantly higher amount of native oxide between the semiconductor and gate dielectric. The relative percentage of native oxide found at the interface is quantified via x-ray photoelectron spectroscopy (XPS) and shows a significant reduction following TMA purges. This improved ALD technique for Al2O3 deposition also shows a reduction of mid-gap trap states and hysteresis observed during C-V testing which further indicates improved interface quality and electrical response of the tested devices. Relative improvement to film quality is dependent upon improved gate dielectric deposition techniques which diminish GaxOy concentration at the semiconductor/dielectric interface. The impact of gate interfaces on GaN MOS devices is directly linked to calculated DIT, leakage currents, and other important electrical characteristics which must be optimized on simple device structures prior to implementation in complex architectures such as vertical GaN MOSFETs.

Improving Electrical Performance of GaN-on-GaN MOS Devices Via Optimized Atomic Layer Deposition of Al2O3 Gate Dielectrics

The need for higher power RF and energy conversion electronics has motivated the search for new device heterojunctions beyond what can be grown using conventional techniques. Instead, we can conceive of new material layer combinations that result in higher performance transport characteristics, e.g., channel conductivity, breakdown voltage, and heat dissipation. However, realizing a high-quality bonded interface has several challenges including the production and processing of low-defect thin material layers and reducing the interface trap density. Here, we have fabricated bonded n--GaN/p+-Si diodes to investigate the dependence of interface quality on surface pretreatments and post-bond annealing. Si-doped GaN films were grown on BN-sapphire templates [1] before being exfoliated using stressed Ni-spalling [2]. The GaN films were bonded top (Ga-polar)-side down onto (100) P-doped Si. IV and CV measurements were performed to characterize the transport across and the charge at the interface. Conducting scanning probe measurements were performed on the GaN surface to characterize the bond areal uniformity. These results are generally applicable to the optimization of interface properties for novel bonded junctions. [1] M. Snure, G. Siegel, D. C. Look, and Q. Paduano, J. Cryst. Growth 464, 168 (2017). [2] E. W. Blanton, M. J. Motala, T. A. Prusnick, A. Hilton, J. L. Brown, A. Bhattacharyya, S. Krishnamoorthy, K. Leedy, N. R. Glavin, and M. Snure, Small 17, 2102668 (2021).

Investigation of Bonded GaN-Si p-n Junction Interface Properties

Recently, stretchable strain sensor is considered as the promising technology for various application, such as wearable electronics, healthcare monitoring, and robotic skin/arms, due to its mechanical stretchability and multi-axial sensing capabilities. The recent research for stretchable strain sensors have made tremendous efforts to significantly improve key deterministic parameters, including sensitivity, durability, sensing range, and response time. Although various high-performance strain sensors have been demonstrated using a range of mechanistic device structures, most conventional strain sensors are only capable of detecting uniaxial strain and lack the ability to measure directional variable motions. In order to overcome this challenge, novel sensor devices with geometrically engineered architectures, composed of multidimensionally stacked channels or cross-shaped detecting electrodes have been developed. These architectures were developed to address the challenge of achieving multidirectional strain sensing, by employing isotropic conducting pathways. While these advancements are significant, the complicated and intermixed pathways in the channel region can cause unsatisfactory linearity and make it difficult to determine the strain direction under complex stimulation environments. In addition, it is worth noting that many recent studies have utilized machine learning techniques, such as Neural network to classify data obtained by strain sensor. However, in order for Neural network to be applied effectively in industry, optimization processes such as model lightweighting are necessary. Here, we propose a simply designed array of wearable strain sensor capable of detecting and interpreting the direction and intensity of strain through a neural network algorithm trained through cognitive learning. The wearable strain sensor arrays comprises a heterogeneous surface architecture consisting of a stretchable substrate made of polydimethylsiloxane (PDMS), strain-insensitive rigid electrode regions made of polyethylene terephthalate (PET) film, and a strain-sensing channel region composed of multi-walled carbon nanotube (MWCNT) with carbon black (CB) nanofiber dispersed in a PDMS matrix. We utilized a mechanism based on the change in the number of conducting pathways between MWCNT and CB depending on the direction of strain. The use of a heterogeneous device architecture can reduce the negative effects of electrode strain on electrical interruptions, resulting in more reliable strain sensing characteristics. This wearable strain sensor arrays exhibited high sensitivity with a gauge factor of approximately 8, a wide strain sensing range of 0 to 35%, and excellent reliability after 3,000 stretching cycles. To detect strain from any direction, Strain sensor array was made using three sensors arranged at 45-degree tilted angles. Furthermore, to achieve high-accuracy detection of both the intensity and direction of strain, a machine learning algorithm known as a multi-output neural network was employed. A neural network was constructed for strain sensing, which showed an accuracy of over 95% for both strain intensity and direction. In addition, optimization of the strain sensing system’s neural network was done by adjusting the number of sensors and input data dimension to achieve a lightweight model. By employing this method, not only data obtained from the wearable strain sensor arrays be efficiently handled, but also the strain intensity and direction can be accurately classified.

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CREST Center for Dynamic Multiscale and Multimodal Brain Mapping over the Lifespan [D-MAP]

Igniting Curiosity through Inclusive Education and Communication in Environmental Nanotechnology

Excellence in Cardiovascular Sciences Program Promotes Interest in Research and Biomedical Careers

Exploring the synergy of light and matter interactions: Multiferroic photonic crystals for novel photonics applications

Understanding Electronic Energy Transport in DNA-Scaffolded Molecular Networks Using Machine-Learning Methods

Demonstration of MOCVD Based 'in situ Ga Etching’ of β-Ga2O3 Using TEGa

Synthesis of Free-Standing Polycrystalline Diamond Nanomembrane

Effects of Strain and Local Topography on Electromechanical Coupling in Monolayer Transition Metal Dichalcogenides

Integrated Graphene FET Array for High Sensitive Detection of New Corona Virus with Automated Solution Exchange System

Improving Electrical Performance of GaN-on-GaN MOS Devices Via Optimized Atomic Layer Deposition of Al2O3 Gate Dielectrics

Investigation of Bonded GaN-Si p-n Junction Interface Properties

Carbon Based Omnidirectional Wearable Strain Sensor Arrays with Optimized Multi-Output Neural Networks

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