arXiv subjects

M. Brooks Tellekamp

Publications and source records attributed to M. Brooks Tellekamp.

15 recordsLinked to original sources

arXiv2026

Enhanced thermal conductivity of (010) (AlxGa1-x)2O3 epitaxial films utilizing indium-catalyzed molecular beam epitaxy

(AlxGa1-x)2O3/beta-Ga2O3 transistors are an emerging candidate for high-power and high-frequency electronic devices. beta-Ga2O3 in particular is notably limited for anisotropic low thermal conductivity, which is further reduced in (AlxGa1-x)2O3 due to alloy and other defect-driven phonon scattering mechanisms. In this work we show that the thermal conductivity of (010) oriented (AlxGa1-x)2O3 thin films for 0.01 < x < 0.20, measured using time-domain thermoreflectance (TDTR), is limited by alloy scattering without observable adverse scattering by additional defects. This is enabled through the synthesis of the (AlxGa1-x)2O3 films using molecular beam epitaxy (MBE) on \b{eta}-Ga2O3 substrates, leveraging indium-catalyzed growth to suppress dislocation formation and phase separation to achieve single-phase pseudomorphic films up to x = 0.2. This growth process improved the thermal conductivity of (AlxGa1-x)2O3 by 2X as compared to previously reported values. For increasing Al composition (x), we observe a steady decline in (AlxGa1-x)2O3 thermal conductivity due to alloy scattering which is validated using virtual crystal approximation (VCA) model. We also show that the thermal boundary conductance across the Al/(AlxGa1-x)2O3 interface is reduced with increasing x, which we posit is due to the stiffening of the (AlxGa1-x)2O3 acoustic modes with increasing x by comparing experimental results with a diffuse mismatch model (DMM). Overall, these thermal characteristics provide valuable insights for designing heterostructures with optimized interfaces and composition.

cond-mat.mtrl-sci↗

arXiv2026

Face-to-face anneal temperature controls lattice parameter in Ta(C,N) virtual substrates for AlGaN power electronics

Tantalum carbide (TaC) thin film ''virtual'' substrates are a highly desirable material for $\text{Al}_{0.5}\text{Ga}_{0.5}\text{N}$ vertical power electronics devices due to lattice matching, thermal expansion matching, and metallic conductivity. However, the material has not been demonstrated to support $\text{Al}_{x}\text{Ga}_{1-x}\text{N}$ layers of variable composition $x$, limiting the range of device applications. We present a method to achieve tunable rock salt $\text{Ta}\text{C}_{x}\text{N}_{1-x}$ virtual substrates via a face-to-face annealing of TaC thin films in an $\text{N}_2$ atmosphere. The results suggest that annealing temperatures below $1600\,^{\circ}\text{C}$ promote partial uptake of nitrogen onto carbon and anion vacancy sites to form rock salt $\text{Ta(C,N)}$ with intermediate anion compositions. At temperatures $\ge 1600\,^{\circ}\text{C}$, nitrogen primarily occupies the anion sublattice and the crystalline quality and surface morphology simultaneously degrade coincident with the formation of secondary phases. This study demonstrates the growth and processing parameters necessary to make tunable lattice constant virtual substrates for $\text{Al}_{x}\text{Ga}_{1-x}\text{N}$ from $x = 0.5\text{-}1$, enabling vertically conducting power electronic devices with reduced defect density at high Al-content.

cond-mat.mtrl-sci↗

arXiv2026

Low resistance NiO/β-Ga_{2}O_{3} heterojunction diodes grown via molecular beam epitaxy

NiO is one of the most important p-type oxide contact materials used in many semiconductor technologies. However, current NiO growth methods can induce interfacial damage that diminishes device performance. Fine control of interfaces is especially important in implementing NiO heterojunction diodes and transistors based on ultra wide band gap (UWBG) semiconductors such as AlGaN and Ga_{2}O_{3} used for power electronic applications. Here, we report on how molecular beam epitaxy can be used to achieve low-defect, lightly doped NiO contact layers for a β-Ga_{2}O_{3} diodes. Although high-temperature growth does not measurably decrease the on-state resistance of the diode, increased growth rates up to 600 nm/hr lower on-state resistance in p-- NiO / β-Ga_{2}O_{3} heterojunction diodes without reducing film quality. At a NiO growth rate of 380 nm/hr, unoptimized diodes with 35 nm thick p-- NiO layers demonstrate a device-average specific on-state resistance of 1.46 Ω-cm^{2} and an ideality factor of 1.46. Individual devices grown at this condition show specific on-state resistance as low as 25 mΩ-cm2 with a rectification ratio of 2.7x106. Scanning transmission electron microscopy imaging reveals the (100) NiO/ (100) β-Ga_{2}O_{3} interface is coherent and atomically abrupt. These results open a new avenue to optimizing the NiO interface to produce robust, competitive kV-class power electronic devices based on β-Ga_{2}O_{3} and other UWBG semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

Revealing epitaxial relationships at Ga$_2$O$_3$ interfaces with p-type oxides

p-type oxide contact layers such as Cr$_2$O$_3$ and NiO are attracting increasing interest in pn-heterojunctions with n-type monoclinic $β$-Ga$_2$O$_3$ for high-power electronic devices and other extreme environment applications. However, scientific understanding of their epitaxial relationships remains incomplete. In this work we investigate the epitaxial relation of Cr$_2$O$_3$ and NiO layers to (001) and ($\bar{2}$01) out-of-plane oriented Ga$_2$O$_3$ substrates. Surprisingly, we find that, for the most commercially relevant (001)-orientation of the Ga$_2$O$_3$ substrate, the epitaxial relationships are Cr$_2$O$_3$ (0001) and NiO (111) $\parallel$ Ga$_2$O$_3$ (101), both at the non-intuitive $χ$ = 22.5 $^\circ$ angle with respect to the substrate normal. We explain this unusual discovery by the interfacial atomistic bonding dominated by oxygen sublattice equivalence of these Cr$_2$O$_3$ and NiO polar surface orientations to the tilted Ga$_2$O$_3$ (101), rather than Ga$_2$O$_3$ (001) substrate surface planes. Furthermore, we assign the in-plane orientation for Cr$_2$O$_3$ on ($\bar{2}$01)-oriented Ga$_2$O$_3$ as: Cr$_2$O$_3$ $[12\bar{3}0]$ $\parallel$ Ga$_2$O$_3$ $[010]$ with two in-plane rotational domains. Interface modeling confirms the in-plane orientation for Cr$_2$O$_3$/$(\bar{2}01)$ Ga$_2$O$_3$ and shows that strained O-terminated Ga$_2$O$_3$ $(\bar{2}01)$ surfaces have the lowest interfacial energy with Cr$_2$O$_3$ (0001). Beyond establishing the specific epitaxial relationships for Cr$_2$O$_3$ and NiO on Ga$_2$O$_3$, this work provides a systematic methodology for the unambiguous structural characterization of heterointerfaces involving materials with markedly different crystal symmetries.

cond-mat.mtrl-sci↗

arXiv2026

Thermally Desorbable InN Capping Layers for Nitride Surface Science

Group III-nitride thin films are essential for optoelectronic and power devices, where surface and interface quality critically influence performance. It is often necessary to transfer these films through atmosphere for processing or characterization steps, which can introduce significant surface contamination. Here we demonstrate a technique to protect the surface of III-N films during atmospheric exposure by capping with sacrificial InN layers. Using molecular beam epitaxy grown AlGaN films as our representative protected material, we take advantage of the lower decomposition and desorption temperatures of InN and metallic In to remove the protective cap layer in situ via thermal desorption without damaging the AlGaN film beneath. Angle-resolved photoemission spectroscopy (ARPES) measurements of the valence band dispersion in Al0.4Ga0.6N demonstrate that the film surface is recovered after InN decapping, highlighting the versatility of this process in allowing further surface-sensitive characterization of films that had been exposed to air.

cond-mat.mtrl-sci↗

arXiv2026

Revealing the Atomic Structure of NiO/Ga$_{2}$O$_{3}$ Interfaces

NiO/Ga$_{2}$O$_{3}$ heterojunctions have garnered significant attention for use in power electronics due to the ultrawide bandgap and wafer-scale availability of Ga$_{2}$O$_{3}$ and the controllable p-type doping of NiO. However, the structure of NiO/Ga$_{2}$O$_{3}$ interfaces remains underexplored, largely due to the complexity of the junction between their dissimilar cubic and monoclinic crystal structures. Here we investigate the atomistic structure of the NiO/Ga$_{2}$O$_{3}$ interface for (100), (-201), and (001) oriented Ga$_{2}$O$_{3}$ substrates using aberration-corrected scanning transmission electron microscopy (STEM) in combination with interface modeling and image simulations. We evaluate the abruptness and consistency of the interfaces and compare them to calculated interface models, proposing precise atomic structures and assessing potential structural variation arising from complexity of the monoclinic Ga$_{2}$O$_{3}$ crystal structure. Our interface analysis supports increased focus on (100) oriented Ga$_{2}$O$_{3}$ as a candidate for fabricating high quality, low defect density NiO/Ga$_{2}$O$_{3}$ heterojunction devices. Importantly, we consider the effects of specimen thickness and 3D-to-2D projection during the STEM imaging process to differentiate such effects from real crystal variations. This work provides insight into the effect of substrate orientation on NiO film and interface quality, creating a pathway to improving heterojunction properties. It further highlights important considerations for interpretation of stability and interlayer phase formation in these interfaces, which is crucial for their integration into reliable and robust power electronic devices.

cond-mat.mtrl-sci↗

arXiv2026

Fast Homoepitaxy on (100) \b{eta}-Ga2O3 Substrates with Large Grown-In Offcut

The choice of crystalline orientation and offcut angle is non-trivial for low-symmetry $β\text{-Ga}_2\text{O}_3$, where anisotropy impacts bulk and thin film synthesis, material properties, and power device fabrication and performance. Scalable (100)-oriented $β\text{-Ga}_2\text{O}_3$ wafers are desirable for electronic devices but are not typically used due to 10-30x slower growth rates compared to other orientations. Here we report molecular beam epitaxy (MBE) growth rates equal to the fast growth direction by using (100) $Ga_2O_3$ wafers with large grown-in offcuts. The offcuts (up to 13.4°) are directly grown by Edge-defined Film-fed Growth (EFG) of 2D ribbons with rotated seed crystals, avoiding material loss from crystal boule offcut methods while maintaining high crystalline quality. Chemical-mechanical polishing produces epitaxy-ready substrates, and step flow growth is observed across all offcut angles. We measure an unintentional n-type doping density of $2{\times}10^{15} cm^{-3}$, one of the lowest values reported for MBE-grown films. Planar Schottky barrier diodes on these epilayers without edge termination have an on/off ratio ~10$^5$ and an average breakdown field of 1.56 MV/cm, comparable to or exceeding similar devices fabricated on other orientations. Overall, these results illustrate the importance of both crystal face and offcut angle and validate the use of the scalable (100)-oriented $β\text{-Ga}_2\text{O}_3$ wafers.

cond-mat.mtrl-sci↗

arXiv2022

Designing low-cost TaC virtual substrates for $Al_xGa_{1-x}N$ epitaxy

$Al_xGa_{1-x}N$ is a critical ultra-wide bandgap material for optoelectronics, but the deposition of thick, high quality epitaxial layers has been hindered by a lack of lattice-matched substrates. Here we identify the (111) face of transition metal carbides as a suitable class of materials for substrates lattice matched to (0001) $Al_xGa_{1-x}N$ and demonstrate the growth of thin film TaC which has an effective hexagonal lattice constant matched to $Al_{0.45}Ga_{0.55}N$. We explore growth conditions for sputtered TaC on sapphire substrates and investigate the effects of sputter power, layer thickness and incident plasma angle on film structure and in- and out-of-plane strain. We then show critical improvements to film quality by annealing films in a face-to-face configuration at 1600 $^\circ$C, which significantly reduces full width at half max (FWHM) of in- and out-of-plane diffraction peaks and results in a step-and-terrace surface morphology. This work presents a path toward electrically conductive, lattice matched, thermally compatible substrates for $Al_xGa_{1-x}N$ heteroepitaxy, a critical step for vertical devices and other power electronics applications.

cond-mat.mtrl-sci↗

arXiv2020

Ternary Nitride Materials: Fundamentals and Emerging Device Applications

Interest in inorganic ternary nitride materials has grown rapidly over the past few decades, as their diversity of chemistries and structures make them appealing for a variety of applications. Due to synthetic challenges posed by the stability of N2, the number of predicted nitride compounds dwarfs those 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.

cond-mat.mtrl-sci↗

arXiv2020

Growth and Characterization of Homoepitaxial $β$-Ga$_2$O$_3$ Layers

$β$-Ga$_2$O$_3$ is a next-generation ultra wide bandgap semiconductor (E$_g$ = 4.8 eV to 4.9 eV) that can be homoepitaxially grown on commercial substrates, enabling next-generation power electronic devices among other important applications. Analyzing the quality of deposited homoepitaxial layers used in such devices is challenging, in part due to the large probing depth in traditional x-ray diffraction (XRD) and also due to the surface-sensitive nature of atomic force microscopy (AFM). Here, a combination of evanescent grazing-incidence skew asymmetric XRD and AFM are investigated as an approach to effectively characterize the quality of homoepitaxial $β$-Ga$_2$O$_3$ layers grown by molecular beam epitaxy at a variety of Ga/O flux ratios. Accounting for both structure and morphology, optimal films are achieved at a Ga/O ratio of $\sim$1.15, a conclusion that would not be possible to achieve by either XRD or AFM methods alone. Finally, fabricated Schottky barrier diodes with thicker homoepitaxial layers are characterized by $J-V$ and $C-V$ measurements, revealing an unintentional doping density of 4.3 $\times$ 10$^{16}$ cm$^{-3}$ - 2 $\times$ 10$^{17}$ cm$^{-3}$ in the epilayer. These results demonstrate the importance of complementary measurement methods for improving the quality of the $β$-Ga$_2$O$_3$ homoepitaxial layers used in power electronic and other devices.

physics.app-ph↗

arXiv2020

Heteroepitaxial integration of ZnGeN2 on GaN buffers using molecular beam epitaxy

Recently theorized hybrid II-IV-N{_2} / III-N heterostructures, based on current commercialized (In,Ga)N devices, are predicted to significantly advance the design space of highly efficient optoelectronics in the visible spectrum, yet there are few epitaxial studies of II-IV-N{_2} materials. In this work, we present heteroepitaxial ZnGeN{_2} grown on GaN buffers and AlN templates. We demonstrate that a GaN nucleating surface is crucial for increasing the ZnGeN{_2} crystallization rate to combat Zn desorption, extending the stoichiometric growth window from 215 {\degree}C on AlN to 500 {\degree}C on GaN buffers. Structural characterization reveals well crystallized films with threading dislocations extending from the GaN buffer. These films have a critical thickness for relaxation of 20 nm - 25 nm as determined by reflection high energy electron diffraction (RHEED) and cross-sectional scanning electron microscopy (SEM). The films exhibit a cation-disordered wurtzite structure, with lattice constants a = 3.216 Å {\pm} 0.004 Å and c = 5.215 Å {\pm} 0.005 Å determined by RHEED and X-ray diffraction (XRD). This work demonstrates a significant step towards the development of hybrid ZnGeN{_2}-GaN integrated devices.

cond-mat.mtrl-sci↗

arXiv2019

Molecular Beam Epitaxy Growth of High Crystalline Quality LiNbO$_{3}$

Lithium niobate is a multi-functional material with wide reaching applications in acoustics, optics, and electronics. Commercial applications for lithium niobate require high crystalline quality currently limited to bulk and ion sliced material. Thin film lithium niobate is an attractive option for a variety of integrated devices, but the research effort has been stagnant due to poor material quality. Both lattice matched and mismatched lithium niobate are grown by molecular beam epitaxy (MBE) and studied to understand the role of substrate and temperature on nucleation conditions and material quality. Growth on sapphire produces partially coalesced columnar grains with atomically flat plateaus and no twin planes. A symmetric rocking curve shows a narrow linewidth with a full width at half-maximum (FWHM) of 8.6 arcsec (0.0024°) which is comparable to the 5.8 arcsec rocking curve FWHM of the substrate, while the film asymmetric rocking curve is 510 arcsec FWHM. These values indicate that the individual grains are relatively free of long-range disorder detectable by x-ray diffraction (XRD) with minimal measurable tilt and twist and represents the highest structural quality epitaxial material grown on lattice mismatched sapphire without twin planes. Lithium niobate is also grown on lithium tantalate producing high quality coalesced material without twin planes and with a symmetric rocking curve of 193 arcsec, which is nearly equal to the substrate rocking curve of 194 arcsec. The surface morphology of lithium niobate on lithium tantalate is shown to be atomically flat by atomic force microscopy (AFM).

physics.app-ph↗

arXiv2019

Molecular Beam Epitaxy of lithium niobium oxide multifunctional materials

The role of stoichiometry and growth temperature in the preferential nucleation of material phases in the Li-Nb-O family are explored yielding an empirical growth phase diagram. It is shown that while single parameter variation often produces multi-phase films, combining substrate temperature control with the previously published lithium flux limited growth allows the repeatable growth of high quality single crystalline films of many different oxide phases. Higher temperatures (800-1050 °C) than normally used in MBE were necessary to achieve high quality materials. At these temperatures the desorption of surface species is shown to play an important role in film composition. Using this method single phase films of NbO, NbO$_{2}$, LiNbO$_{2}$, Li$_{3}$NbO$_{4}$, LiNbO$_{3}$, and LiNb$_{3}$O$_{8}$ have been achieved in the same growth system, all on c-plane sapphire. Finally, the future of these films in functional oxide heterostructures is briefly discussed.

physics.app-ph↗

arXiv2019

Scalable Memdiodes Exhibiting Rectification and Hysteresis for Neuromorphic Computing

Metal-Nb$_{2}$O$_{5-x}$-metal memdiodes exhibiting rectification, hysteresis, and capacitance are demonstrated for applications in neuromorphic circuitry. These devices do not require any post-fabrication treatments such as filament creation by electroforming that would impede circuit scalability. Instead these devices operate due to Poole-Frenkel defect controlled transport where the high defect density is inherent to the Nb$_{2}$O$_{5-x}$ deposition rather than post-fabrication treatments. Temperature dependent measurements reveal that the dominant trap energy is 0.22 eV suggesting it results from the oxygen deficiencies in the amorphous Nb$_{2}$O$_{5-x}$. Rectification occurs due to a transition from thermionic emission to tunneling current and is present even in thick devices (> 100 nm) due to charge trapping which controls the tunneling distance. The turn-on voltage is linearly proportional to the Schottky barrier height and, in contrast to traditional metal-insulator-metal diodes, is logarithmically proportional to the device thickness. Hysteresis in the I-V curve occurs due to the current limited filling of traps.

physics.app-ph↗

arXiv2018

Diffuson-driven Ultralow Thermal Conductivity in Amorphous Nb2O5 Thin Films

Niobium pentoxide (Nb2O5) has been extensively reported for applications of electrochemical energy storage, memristors, solar cells, light emitting diodes (LEDs), and electrochromic devices. The thermal properties of Nb2O5 play a critical role in device performance of these applications. However, very few studies on the thermal properties of Nb2O5 have been reported and a fundamental understanding of heat transport in Nb2O5 is still lacking. The present work closes this gap and provides the first study of thermal conductivity of amorphous Nb2O5 thin films. Ultralow thermal conductivity is observed without any size effect in films as thin as 48 nm, which indicates that propagons contribute negligibly to the thermal conductivity and that the thermal transport is dominated by diffusons. Density-function-theory (DFT) simulations combined with a diffuson-mediated minimum-thermal-conductivity model confirms this finding. Additionally, the measured thermal conductivity is lower than the amorphous limit (Cahill model), which proves that the diffuson model works better than the Cahill model to describe the thermal conduction mechanism in the amorphous Nb2O5 thin films. Additionally, the thermal conductivity does not change significantly with oxygen vacancy concentration. This stable and low thermal conductivity facilitates excellent performance for applications such as memristors.

cond-mat.mes-hall↗