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Tellekamp, M. Brooks

Publications and source records attributed to Tellekamp, M. Brooks.

14 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Epitaxial (Al x Ga 1-x-y In y ) 2 O 3 Alloys Lattice Matched to Monoclinic Ga 2 O 3 Substrates

We have epitaxially stabilized a series of monoclinic (Al x Ga 1-x-y In y ) 2 O 3 alloys by careful choice of molecular beam epitaxy growth conditions, which balance alloy growth with suboxide desorption. The films are pseudomorphic to (010) β-Ga 2 O 3 substrates at thicknesses up to 150 nm with compositions ranging from (Al 0.01 Ga 0.83 In 0.16 ) 2 O 3 to (Al 0.24 Ga 0.75 In 0.03 ) 2 O 3 . The absorption edge shifts from approximately 4.62-5.14 eV with coincidently increasing Al and decreasing In mole fractions. J-V measurements reveal an increase in resistivity over four orders of magnitude with a maximum value of 4.2 x 10 5 Ω-cm for (Al 0.17 Ga 0.76 In 0.07 ) 2 O 3 , which has nearly identical lattice parameters (both in-plane and out-of-plane) to the underlying β-Ga 2 O 3 . Scanning transmission electron microscopy of this sample reveals a mostly uniform and single crystalline film, though we identify areas of non-uniform In incorporation and some γ-phase inclusions. This work demonstrates the feasibility of thick layers lattice-matched to β-Ga 2 O 3 with increased bandgap compared to phase-separation limited (Al,Ga) 2 O 3 . These alloys can enable higher bandgap epitaxial dielectrics and high sheet charge density transistors by increasing the conduction band offset with respect to β-Ga 2 O 3 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

NiGa 2 O 4 interfacial layers in NiO/Ga 2 O 3 heterojunction diodes at high temperature

NiO/Ga 2 O 3 heterojunction diodes have attracted attention for high-power applications, but their high temperature performance and reliability remain underexplored. Here, we report the time evolution of the electrical properties in the widely studied p-NiO/n-Ga 2 O 3 heterojunction diodes and formation of NiGa 2 O 4 interfacial layers at high temperatures. Results of our thermal cycling experiment show an initial leakage current increase which stabilizes after sustained thermal load, due to reactions at the NiO–Ga 2 O 3 interface. High-resolution TEM microstructure analysis of the devices after thermal cycling indicates that the NiO–Ga 2 O 3 interface forms a ternary compound at high temperatures, and thermodynamic calculations suggest the formation of the spinel NiGa 2 O 4 layer between NiO and Ga 2 O 3 . First-principles defect calculations find that NiGa 2 O 4 shows low p-type intrinsic doping and hence can serve to limit electric field crowding at the interface. Vertical NiO/ Ga 2 O 3 diodes with intentionally grown 5 nm thin spinel-type NiGa 2 O 4 interfacial layers show an excellent device ON/OFF ratio of >10 10 (± 3 V), V ON of ~1.9 V, and increased breakdown voltage of ~1.2 kV for an initial unoptimized 300 lm diameter device. These p–n heterojunction diodes are promising for high-voltage, high temperature applications.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2024

Modeling the Impacts of Material Properties on Oscillatory Neuron Behavior

In this study, neuromorphic computing, which mimics the functions of biological brains, offers improvements in both latency and energy efficiency over typical von Neumann computing architectures. Spiking neural networks can be especially power-efficient because they encode information temporally and can use more sparse electrical inputs. Here, we study the design of volatile memristors (variable resistors with memory) for neuronal devices, with particular consideration toward the feasibility of all-on-chip oscillation using built-in capacitance. We use circuit simulations to model the behavior of oscillator neurons with a range of realistic material properties. We find that energy inputs increase with insulating-phase resistivity, thermal conductivity, and device aspect ratio. However, we also find that the minimum capacitance needed for oscillation decreases with increasing insulating-phase resistivity, which opposes the constraints for power efficiency. Based on published data on NbO 2 , VO 2 , and EuNiO 3 , we find that existing materials can be engineered for all-on-chip spiking using their parasitic capacitance.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Comparing the influence of cation order and composition in simulated Zn(Sn, Ge)N 2 on structure, elastic moduli, and polarization for solid state lighting

Alloying and site ordering play complementary roles in dictating a material’s properties. However, deconvolving the impacts of these separate phenomena can be challenging. In this work, we simulate structures of Zn(Sn,Ge)N 2 with varied Sn content and site ordering to determine the impacts of order and composition on structural and electronic properties. We assess the formation enthalpy, lattice parameters, elastic constants, spontaneous polarization, and piezoelectric coefficients. In mostly disordered structures (order parameters ranging from 0.2 to 0.4), the formation enthalpy exhibits local extrema as a function of the order parameter, deviating from the more linear trends seen in both fully disordered and fully ordered systems. This anomalous deviation from the otherwise linear trend in formation enthalpy with order manifests in each of the other properties calculated. This range of order parameters of interest may be caused by a transition in the ordering of the quaternary material similar to phase changes seen in ternary compounds but stretched over a region inclduing 20% of the order parameter range. Most parameters calculated are more sensitive to order than to composition in the limited composition range tested; however, the lattice parameter c, piezoelectric coefficient e 33 , and elastic moduli C 12 , C 13 , and C 23 are more sensitive to composition. Of the properties compared, the piezoelectric coefficients are influenced most significantly by changes in both the composition and order parameter. Lattice parameters undergo the smallest changes with order and composition, but these small differences appear to impart large trends in the other properties. Better understanding the effects of disorder and group IV alloying in Zn(Sn,Ge)N 2 allows for more accurate modeling of characteristics of this material system for solid state lighting and other applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Gallium Oxide Heterojunction Diodes for 400 °C High‐Temperature Applications

β ‐Ga 2 O 3 ‐based semiconductor devices are expected to have significantly improved high‐power and high‐temperature performance due to its ultrawide bandgap of close to 5 eV. However, the high‐temperature operation of these ultrawide‐bandgap devices is usually limited by the relatively low 1–2 eV built‐in potential at the Schottky barrier with most high‐work‐function metals. Herein, heterojunction p ‐NiO/n‐ β ‐Ga 2 O 3 diodes fabrication and optimization for high‐temperature device applications are reported, demonstrating a current rectification ratio ( I ON / I OFF ) of more than 10 6 at 410 °C. The NiO heterojunction diode can achieve higher turn‐on ( V ON ) voltage and lower reverse leakage current compared to the Ni‐based Schottky diode fabricated on the same single‐crystal β ‐Ga 2 O 3 substrate, despite charge transport dominated by interfacial recombination. Electrical characterization and device modeling show that these advantages are due to a higher built‐in potential and additional band offset. These results suggest that heterojunction p–n diodes based on β ‐Ga 2 O 3 can significantly improve high‐temperature electronic device and sensor performance.

Sohel, Shahadat H.↗
DOE OSTI2022

Composition Dependent Electrochemical Properties of Earth-Abundant Ternary Nitride Anodes

Growing energy storage demands on lithium-ion batteries necessitate exploration of new electrochemical materials as next-generation battery electrode materials. In this work, we investigate the previously unexplored electrochemical properties of earth-abundant and tunable Zn 1-x Sn 1+x N 2 (x = -0.4 to x = 0.4) thin films, which show high electrical conductivity and high gravimetric capacity for Li insertion. Enhanced cycling performance is achieved compared to previously published end-members Zn 3 N 2 and Sn 3 N 4 , showing decreased irreversible loss and increased total capacity and cycle stability. The average reversible capacity observed is >1050 mAh/g for all compositions and 1220 mAh/g for Zn-poor (x = 0.2) films. Extremely Zn-rich films (x = -0.4) show improved adhesion; however, Zn-rich films undergo a phase transformation on the first cycle. Zn-poor and stoichiometric films do not exhibit significant phase transformations which often plague nitride materials and show no required overpotential at the 0.5 V plateau. Cation composition x is explored as a mechanism for tuning relevant mechanical and electrochemical properties, such as capacity, overpotential, phase transformation, electrical conductivity, and adhesion. The lithiation/delithiation experiments confirm the reversible electrochemical reactions. Without any binding additives, the as-deposited electrodes delaminate resulting in fast capacity degradation. We demonstrate the mechanical nature of this degradation through decreased electrode thinning, resulting in cells with improved cycling stability due to increased mechanical stability. Combining composition and electrochemical analysis, this work demonstrates for the first time composition dependent electrochemical properties for the ternary Zn 1-x Sn 1+x N 2 and proposes earth-abundant ternary nitride anodes for increased reversible capacity and cycling stability.

25 ENERGY STORAGE↗
DOE OSTI2021

Ternary Nitride Materials: Fundamentals and Emerging Device Applications

Interest in inorganic ternary nitride materials has grown rapidly over the past few decades, as their diverse chemistries and structures make them appealing for a variety of applications. Due to synthetic challenges posed by the stability of N 2 , the number of predicted nitride compounds dwarfs the number that have been synthesized, offering a breadth of opportunity for exploration. This review summarizes the fundamental properties and structural chemistry of ternary nitrides, leveraging metastability and the impact of nitrogen chemical potential. A discussion of prevalent defects, both detrimental and beneficial, is followed by a survey of synthesis techniques and their interplay with metastability. Throughout the review, we highlight applications (such as solid-state lighting, electrochemical energy storage, and electronic devices) in which ternary nitrides show particular promise.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Epitaxy of LiNbO 3 : Historical Challenges and Recent Success

High-quality epitaxial growth of thin film lithium niobate (LiNbO 3 ) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In this paper, we provide a short summary of the primary challenges of lithium niobate epitaxy followed by a brief historical review of lithium niobate epitaxy for prevalent epitaxial techniques. Available figures of merit for crystalline quality and optical transmission losses are given for each growth method. The highest crystalline quality lithium niobate thin film was recently grown by halide-based molecular beam epitaxy and is comparable to bulk lithium niobate crystals. However, these high-quality crystals are grown at slow rates that limit many practical applications. Given the many challenges that lithium niobate epitaxy imposes and the wide variety of methods that have unsuccessfully attempted to surmount these barriers, new approaches to lithium niobate epitaxy are required to meet the need for simultaneously high crystalline quality and sufficient thickness for devices not currently practical by existing techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Utilizing Site Disorder in the Development of New Energy-Relevant Semiconductors

We report controlling site disorder in ternary and multinary compounds enables tuning optical and electronic properties at fixed lattice constants and stoichiometries, moving beyond many of the challenges facing binary alloy systems. Here, we consider possible enhancements to energy-related applications through the integration of disorder-tunable materials in devices such as light-emitting diodes, photonics, photovoltaics, photocatalytic materials, batteries, and thermoelectrics. However, challenges remain in controlling and characterizing disorder. Focusing primarily on II–IV–V 2 materials, we identify three metrics for experimentally characterizing cation site disorder. Complementary to these experiments, we discuss simulation methods to understand disordered materials. Nonidealities, such as off-stoichiometry and oxygen incorporation, can occur while synthesizing metastable disordered materials. While nonidealities may seem undesirable, we describe how if harnessed they could provide another knob for tuning disorder and subsequently properties. To illustrate the effects of disorder on device-relevant properties, we provide case examples of disordered materials and their potential in device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Using resonant energy X-ray diffraction to extract chemical order parameters in ternary semiconductors

II–IV–V 2 materials, ternary analogs to III–V materials, are emerging for their potential applications in devices such as LEDs and solar cells. Controlling cation ordering in II–IV–V 2 materials offers the potential to tune properties at nearly fixed compositions and lattice parameters. While tuning properties at a fixed lattice constant through ordering has the potential to be a powerful tool used in device fabrication, cation ordering also creates challenges with characterization and quantification of ordering. In this work, we investigate two different methods to quantify cation ordering in ZnGeP 2 thin films: a stretching parameter calculated from lattice constants , and an order parameter determined from the cation site occupancies ( S ). We use high resolution X-ray diffraction (HRXRD) to determine and resonant energy X-ray diffraction (REXD) to extract S . REXD is critical to distinguish between elements with similar Z -number ( e.g. Zn and Ge). We found that samples with a corresponding to the ordered chalcopyrite structure had only partially ordered S values. The optical absorption onset for these films occurred at lower energy than expected for fully ordered ZnGeP 2 , indicating that S is a more accurate descriptor of cation order than the stretching parameter. Since disorder is complex and can occur on many length scales, metrics for quantifying disorder should be chosen that most accurately reflect the physical properties of interest.

36 MATERIALS SCIENCE↗