United States

Abstract Body: The rapid increase in data quantity and processing necessitate novel approaches to computing. Digital computing based on von Neumann architectures may not be optimal for data-intensive operations like machine learning due to the high energy costs of moving data between processor and memory. Neuromorphic computing that combines logic and memory functionality into a single analogue nonvolatile memory element is a promising energy-efficient solution that decreases the energy consumption by orders of magnitude. Filament-based phase-change memory and resistive random access memory are the most well-investigated approaches to neuromorphic computing, but these memories are stochastic due to the unreliability of the nanosized filaments used to store information. In this work, we develop the “bulk” resistive random access memory that stores information using the oxygen vacancy concentration in the 3D bulk. This cell contains a solid electrolyte sandwiched between two mixed ionic and electronic conductors (MIECs). The ion-conducting solid electrolyte blocks electrons and prevents the positive feedback that results in filament formation. Instead, the oxygen vacancy ions are uniformly distributed in the absence of filaments and thermal gradients. To switch the cell, electrochemical voltage pulses (+/-1.5V, 2 μs) shuttle vacancies between the MIECs; the concentration of the vacancies in one MIEC stores the analogue resistive state. This enables exceptionally reliable, linear, and deterministic switching among hundreds of analogue information states, while utilizing CMOS-compatible materials with long endurance and information retention. Atomic force microscopy and physical modeling confirms the absence of filaments. Three types of solid electrolytes are utilized: a thick model single-crystal yttrium-stabilized zirconia (YSZ), a thin-film YSZ grown by sputtering, and ultra-thin-film electrolytes of alumina, hafnia, and zirconia grown by atomic layer deposition. The oxygen vacancy ionic conductivities of these materials play a crucial role in the switching and retention times. The broad classes of oxygen conductors provide significant opportunities for future materials research towards next-generation analogue nonvolatile memory. Reference: “Filament-free bulk resistive memory enables deterministic analogue switching,” Adv Mater, 32, 2003984 (2020)

EMC 2021

Deterministic Analogue Resistive Memory for Neuromorphic Computing Based on Transition Metal Oxides


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The energy cost of storing and moving data is enormous, and is growing at such a rapid pace that within a few years the power demand may overwhelm some developed nations. Non-volatile memory (NVM) and neuromorphic computing are enabling critical advances in this area, but there are many scientific and technological challenges. This conference will cover the materials challenges of different memory technologies, from the fundamental processes and mechanisms, to industry perspectives on materials specifications and manufacturing.

Materials Challenges for Memory

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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Abstract Body: Background. β-Ga2O3 has been actively studied for power electronics, RF devices, etc. Among various device topologies, fin-structures are particularly attractive since they enable the RESURF effect, which is key in designing high-field electronics and enhancement-mode transistor operation1,2. Fin-structures have been successfully employed in demonstrating record-setting devices in β-Ga2O33,4. However, we also observed in these devices that a “waist” tends to develop in the middle of a fin thus current flow can be severely pinched by “narrowed waist”. Here, we report what has been learned to date on fin-waist formation during ICP reactive ion etching (RIE). Experiments. The fin/trench arrays were fabricated on single-crystal β-Ga2O3 substrates with 2 orientations: (010) and (001). Hard masks of Pt were patterned by EBL for fin/trench widths less than 1 μm and optical lithography for wider ones. Definitions of dimensional parameters for fin, trench, fin waist and waist narrowing are illustrated in Fig.1. The dry etch parameters were fixed at 350 W ICP power, 20 W RIE power, 5 mTorr, 20C with 35 sccm BCl3 and 5 sccm Ar. Prior to SEM, the etched samples were soaked in HCl and/or HF to remove etch damage/residue. This dry+wet etch condition has rendered consistently rectangular-shaped single fins on (001) β-Ga2O3 with a fin height/width aspect ratio (AR) up to 9 and used in our reported devices3. Fin-waist dependence on the fin orientation and AR of fins/trenches. Arrays of fins were etched on (001) β-Ga2O3 with fin orientation varied. The fin width was varied from 0.15 to 8 μm, the trench width was varied from 1 to 8 μm, and the fin/trench depth was varied from 1.1 to 2 μm, which corresponds to a fin AR of 0.2-9 and a trench AR of 0.5-1.3. Upon examining cross-sections of these fins/trenches under SEM (Figs. 2-4), we discover following trends: 1) all outermost sidewalls of fin arrays and single fins appear to be vertical, while inner sidewalls all show waist; 2) there is no noticeable dependence on fin orientation; 3) waist narrowing increases from 50±10 nm for a trench AR of 0.5 to 200±10 nm for a trench AR up to 1.3, while appears nearly independent of fin width; and 4) wet etch worsens waist narrowing. Fin-waist dependence on the substrate orientation. Similar trench etching experiments were applied to (010) substrates, given their popular use in device development. Example cross-section SEM images are shown in Fig. 5. Though the ranges of fin/trench geometric parameters are slightly different from the ones explored on the (001) β-Ga2O3, the trends observed are largely consistent across different substrate orientations. Discussions. To advance fin-based device performance, it is essential to increase filling factor of fins with respect to entire area occupied by devices on wafer; the preferred trench AR is >1. Under the present ICP-RIE etch condition, waist narrowing is minimal for a low trench AR; but low trench AR implies a low filling factor of current-carrying fins. Waist narrowing is tolerable for fat fins, but the resultant fins can suffer from severe waist narrowing for large-AR trenches and narrow fins (Fig.6) thus detrimental to realizing smooth current flow in devices. Based on our observations, we hypothesize that in trenches with a high AR, a combination of poor sidewall passivation and ion/electron redistribution can result in appreciable lateral etching thus forming waist in fins or bowing of sidewalls. Similar effects were widely observed in other material systems, and improved passivation of sidewalls and neutralization of charges on sidewalls are common tactics to suppress bowing. Ongoing studies are undertaken to curb this undesirable waist narrowing. 1Hu, et al, EDL, 39, 869(2018). 2Li, et al, TED, 67, 3954(2020). 3Li, et al, IEDM, 270(2019). 4Li, et al, EDL, 41, 107(2020). * This work is in part supported by AFOSR Center of Excellence: ACCESS (FA9550-18-1-0529). CNF uses are partly supported by NSF NNCI-2025233.

Fin-Waist Formation in β-Ga2O3 Fin Arrays Defined by ICP-RIE

Abstract Body: β-Ga2O3 is an emerging ultra-wide bandgap (UWBG) semiconductor that holds promise to enable advances in power, RF and opto-electronic devices. With the introduction of β-(Al,Ga)2O3, UWBG heterostructures are possible that increase the range of viable devices. In fact, two-dimensional electron gases (2DEG) in modulation-doped β-(Al,Ga)2O3/Ga2O3heterojunctions [1, 2] have already been reported. However, while there are numerous studies on defect spectroscopies and electrical properties of β-Ga2O3, comparatively little information on electrically active defects and their impact on material properties in β-(Al,Ga)2O3 is available. An understanding of traps in β-(Al,Ga)2O3 will be important to identify and mitigate sources for device degradation, as well as to optimize epitaxial growth. Presented here are deep level transient and optical spectroscopy (DLTS/DLOS) results obtained from β-(Al0.1Ga0.9)2O3 epitaxial layers grown by metal organic chemical vapor deposition (MOCVD). These are discussed in the context of our prior β-Ga2O3 results, to provide a baseline from which trends and conclusions can be provided. MOCVD was used to grow b-(Al0.1Ga0.9)2O3 layers in an Agnitron Agilis R&D system, using TEGa, TMAl and O2 precursors on β-Ga2O3 (010) Sn-doped EFG-grown substrates with a growth temperature of 880 °C. To enable useful comparisons with our β-Ga2O3 results, an Al cation mole fraction of 0.10 was targeted for our initial studies, which should yield a bandgap of approximately 5.0 eV. Ni Schottky diodes were fabricated on the sample using conventional photolithography methods. Capacitance-voltage, current-voltage and internal photoemission measurements (IPE) revealed the Schottky diodes were suitable for defect spectroscopy measurements and characterize the Ni/b-(Al0.1Ga0.9)2O3 Schottky properties themselves. Comparisons are made to previously characterized Schottky diodes fabricated on a β-Ga2O3 epitaxial film grown by MOCVD in the same reactor under the same growth conditions [3]. The net doping concentration in the initial layers under study was measured by CV to be 1.8 x 1018 cm-3. Good Schottky behavior was observed, even at this high doping value, with strong rectification, ideality factor of ~1.5, and IPE-determined Schottky barrier heights of ~1.4 V. Preliminary deep level defect characterization using admittance spectroscopy (AS) along with DLTS and DLOS revealed the presence of five trap states within the b-(Al0.1Ga0.9)2O3 bandgap. A state with an activation energy of ~EC-0.2 eV as observed by AS; three trap levels were revealed by DLTS with approximate trap energies of ~EC-0.3 eV, ~EC-0.4 eV and EC-0.8 eV; and two trap levels were observed in the steady-state photo-capacitance (SSPC) spectra having onset energies of ~EC-2.0 eV and ~EC-4.6 eV. Overall, a similar energy distribution of trap states is observed compared with MOCVD grown b-Ga2O3 is suggestive of similar physical sources of traps between these the two materials, accounting for the small bandgap differences. A notable difference is the EC-0.8 eV trap, which is absent in the MOCVD b-Ga2O3, but matches the signature in b-Ga2O3 grown by other techniques suggesting it may be related to Fe [4]. Much more work is needed to confirm the meaning of these observations, which is ongoing. However, total trap concentrations are markedly different, being ~ 100x larger in the b-(Al0.1Ga0.9)2O3 epitaxial films, which is indicative of the very early stage of β-(Al,Ga)2O3 epitaxial growth development. Complete discussion of the nature of the traps seen in b-(Al0.1Ga0.9)2O3 and an expanded range of alloy compositions and growth conditions will be presented, including further comparisons to prior data obtained from high quality b-Ga2O3[3]. [1] N. Kalarickal, et al, J. Appl. Phys. 127, 215706 (2020) [2] Y. Zhang, et al, Appl. Phys. Lett. 112, 233503 (2018) [3] H. Ghadi, et al, Appl. Phys. Lett. 117, 172106 (2020) [4] J. F. McGlone, et al, Electron Dev. Lett. 39, 1042 (2018)

Quantitative Defect Characterization of MOCVD-Grown β-(Al,Ga)2O3 and Comparison with β-Ga2O3

Abstract Body: Ultra-wide-bandgap semiconductors (UWBG's) are a very promising material for the next-generation power semiconductors devices due to the rapid developments and requirements of highly efficient power electronics. The beta phase of Ga2O3 (β-Ga2O3) is considered in a family of UWBG semiconductors, which provides a new opportunity in myriad applications in electronics, optoelectronics, and photonics, with a superior performance matrix than conventional WBG materials. It exhibits some remarkable properties such as ultra-wide bandgap (>4.8 eV) at room temperature, thermally and chemically stability, high carrier mobility (>300 cm2/v.s), critical electric breakdown filed (~8 MV/cm), highest electron-Baliga’s Figure of Merit (BFoM). All of these properties could lead β-Ga2O3, a potential candidate for high-frequency devices, deep-ultraviolet optoelectronics, and high-power applications. However, the monoclinic and anisotropic crystal structure of β-Ga2O3 (a= 12.214 Å, b= 3.037 Å, and c=5.798 Å) exhibits larger lattice constant on the a-axis other than b-axes and c-axes. These benefits could open up a new direction of creating a nanoscale freestanding form of sub-micron thick β-Ga2O3 flake (typically known as nanomembranes, NMs), Recently several research groups have demonstrated creating NMs using the “Scotch tape” method. Thus, β-Ga2O3 provides a new opportunity to realize the use of UWBG’s in flexible devices that have the advantages of high breakdown electric field, transparency and solar blind photodetection while maintaining good mechanical stability. In fact, our group demonstrated the first flexible electronics (Schottky barrier diode and Solar blind photodetector) based on β-Ga2O3 NMs. However, structural-property relationship investigation of β-Ga2O3 NMs under mechanical strain conditions has not been well studied yet. Here in this presentation, we will first discuss the formation of nanogaps β-Ga2O3 NMs under uniaxial strain conditions, and the impact of randomly generated nanogaps on the electrical and optical properties using the β-Ga2O3 NMs fabricated on a plastic substrate. The scanning electron microscope (SEM) and the atomic force microscope (AFM) was used to identify the strain-induced nano-cracks, which also provided direct evidence of layer delamination and fracture in the β-Ga2O3 NMs. It was noticed that the electrical properties of β-Ga2O3 NM, such as the sheet resistance and capacitance were changed under uniaxial strain conditions. In order to investigate the change in sheet resistance and capacitance, a transmission line measurement (TLM) pattern was used. The results uncovered a significant degradation in the electrical properties and uneven distribution of the charges in the β-Ga2O3 NMs. The sheet resistance increased, and the total capacitance decreased due to substantial degradation of β-Ga2O3 NM’s under bending conditions. The optical properties of flexible β-Ga2O3 NMs were also investigated as a solar-blind photodetector. Interestingly, the degraded performance in β-Ga2O3 NM was recovered by employing a water vapor treatment. The presence of OH bonds in β-Ga2O3 NM confirmed by X-ray photoelectron spectroscopy (XPS) indicated that a water vapor treatment chemically bonds the nano-cracks effectively. Hence it was possible to recover up to 90 % of the original electrical properties. In addition, the water vapor treatment prevented further performance degradation in β-Ga2O3 NM from multiple bending cycles, thus providing good reliability. This result provides a viable route in realizing high-performance flexible devices, which is one of the crucial components in the upcoming internet of things era.

Nanogaps Formation and Recovery Process of Flexible β-Ga2O3 Nanomembranes Under Uniaxial Strain

Abstract Body: Light–matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis, energy harvesting and photodetection, among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with this hot-electron injection remains unclear. Plasmon-assisted photocatalytic efficiencies can improve when intermediate insulation layers are used to inhibit the charge transfer or when off-resonance excitations are employed, which suggests that additional energy transport and thermal effects could play an explicit role even if the charge transfer is inhibited. This provides an additional interfacial mechanism for the catalytic and plasmonic enhancement at interfaces that moves beyond the traditionally assumed physical charge injection. In this work, we report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, after the optical excitation of a metal/semiconductor heterostructure. We explicitly demonstrate that in cases of strong non-equilibrium, a novel energy transduction mechanism arises at the metal/semiconductor interface. We find that hot electrons in the metal contact transfer their energy to pre-existing free electrons in the semiconductor, without an equivalent spatiotemporal transfer of charge. Further, we demonstrate that this ballistic thermal injection mechanism can be utilized as a unique means to modulate plasmonic interactions. These experimental results are well-supported by both rigorous multilayer optical modelling and first-principle ab initio calculations.

Long-Lived Modulation of Plasmonic Absorption by Ballistic Thermal Injection

Abstract Body: Surface plasmon polariton (SPP), which result from the strong coupling of bound electromagnetic (EM) waves and collective charge oscillations, enable subwavelength manipulation of light and matter interaction. Conventional noble metals-based SPP excited in the visible and near-Infrared spectra ranges suffer from large energy losses and cannot be dynamically tuned. Thanks to its 2D nature, graphene has emerged as a promising alternative plasmonic material with well- confined SPPs in the mid-infrared (MIR) and terahertz (THz) ranges and remarkable tunability. On the other hand, polar dielectric materials support low optical losses and sub-wavelength mode in the MIR and THz regions via surface phonon polariton (SPhP)mode' stimulation within the Reststrahlen band, a narrow spectral range between transverse optical (TO) and longitudinal optical (LO) frequencies. Due to the excellent polaritonic responses in both graphene and silicon carbide in the MIR and THz regions, devices combining the two materials are inferred to greatly advance photonics and THz technologies. We have recently theoretically demonstrated the strong confinement and large propagation figures of merit (FOM) for hybrid surface plasmon and surface phonon polariton in the epitaxial graphene on silicon carbide. The ability to confine such MIR light into small volume through excitation of SPP and SPhP has many implications for optoelectronic technologies such as the MIR photodetection[2], sensing[3], and solar cells[4]. Epitaxial graphene(EG) grown on silicon carbide(SiC) on silicon(Si) has been investigated as a suitable platform for graphene growth on semiconductors to meet the prospective technologies [5]. Despite this platform being very advantageous, experimental and theoretical works demonstrating the potentials properties/applications of EG on SiC on Si are still missing. Here we combine electromagnetic simulations based on finite elements method (FEM) with attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy to indicate improved MIR absorption, E field enhancement, and tunability for graphene-coated SiC nanowires (NWs). As anticipated, low absorption was confirmed from both simulations and measurements on bare SiC NWs, further supported by the calculated weak electric field due to the thin size of SiC NWs being incapable of absorbing sufficient light. On the other hand, enhanced absorption for the SiC NWs was measured by ATR-FTIR after a conformal graphitization of NWs[6]. Our previous study of this material system revealed a low-density oxide layer formed between graphene and SiC NWs[6]. This oxide layer between EG and SiC NW has been implemented in the FEM model, leading to field enhancement of incident EM field and strongly confined in the thin oxide. This substantial absorption and field enhancement originates from the coupling between the incident MIR photon and the plasmon/ phonon in graphene and SiC, respectively, with the oxide layer acting as a coupling medium. Furthermore, we demonstrate the modes' tunability whereby tuning the graphene's Fermi energy, the modes' resonance frequencies can be shifted to ~ 100 cm-1. Lastly, we realize substantial field enhancement, which is achieved by carefully tuning the graphene's Fermi energy and adjusting the spectra position of mode 1(mode on the left of LO frequency) closer to TO frequency. These results hold promise for various photonic applications such as perfect absorber to improve solar cell, MIR photodetectors, and other tunable photonics. [1]P. Rufangura, et al., J. Phys. Mater., vol. 3, no. 3, p. 032005, 2020. [2]T. Low and P. Avouris, ACS nano, vol. 8, no. 2, pp. 1086-1101, 2014. [3]A. G. Brolo, Nat. Photon., vol. 6, no. 11, p. 709, 2012. [4]H. A. Atwater and A. Polman, Nat. Mater., vol. 9, no. 3, p. 205, 2010. [5] N. Mishra et al., Phys. Status Solidi (a), vol. 213, no. 9, pp. 2277-2289, 2016. [6]N. Mishra et al., J. Appl. Phys., vol. 126, no. 6, p. 065304, 2019.

Enhanced Absorption with Core/Shell Silicon Carbide/Graphene Nanowires for Tunable Mid-Infrared Nanophotonics

Abstract Body: Phonon polaritons are collective oscillations resulting from the coupling of photons with optical phonons in polar materials and are supported within a material-specific spectral region called the reststrahlen band, which is bounded by the transverse and longitudinal optical phonons. In this region, the material behaves optically like a metal; it is highly reflective and has a negative real part of the permittivity. When polar materials are nanostructured, phonon polaritons can enable a variety of near-field optical effects such as sub-diffraction light confinement. Polar materials that support phonon polaritons can also have anisotropic optical properties, such that the optical constants depend on the propagation direction and polarization of the incoming light. When the principal components of the permittivity tensor have opposite signs, the material is referred to as hyperbolic. Hyperbolic materials are somewhat unusual as they behave optically like a dielectric and metal along different crystal axes, and this anisotropy of the metal-like response has important implications for the physics of its supported modes. Here, we report on the first experimental observation of hyperbolic phonon polaritons (HPs) in calcite nanopillar arrays, demonstrate the aspect ratio dependence of the HP resonance frequencies, discuss fabrication challenges, observe a new, possibly higher order mode as the pitch is reduced, and compare our results to both numerical simulations and an analytical model. We also found that the simulated electric field distributions for the HP modes are not localized to the pillars but extend appreciably into the calcite substrate. In addition, we have fabricated nanohole arrays in calcite, which also support HPs. However, the HP resonances appear to exhibit a slight aspect ratio dependence but opposite in trend to what was observed for the nanopillars. Additionally, the nanohole arrays have fewer HP modes and these modes are blueshifted in comparison to the nanopillars. We are currently using finite difference time domain simulations to characterize the HP modes supported in the nanohole arrays and to understand the differences between these modes and the HP modes confined within the nanopillars. Calcite is an ideal low-loss material for studying HPs that could find applications in mid-IR nanophotonic devices, and so these results are an important step toward creating a library of materials with the appropriate phonon properties to span the infrared.

Hyperbolic Phonon Polaritons in Calcite Nanopillar and Nanohole Arrays for Nanoscale Infrared Confinement

Abstract Body: Topological photonic or plasmonic systems have recently gained significant interest for their unique optical properties and potential applications in unidirectional waveguides, field enhancement, and lasing from topologically protected states. One of the simpler models available to investigate topological systems is a 1-D diatomic staggered chain of period Δ, the Su-Schrieffer-Heeger (SSH) model where adjacent resonant particles are spaced by βΔ/2 & (2-β)Δ/2, (β/2 ∈ (0,1)). In the commonly studied quasi-static (QS) limit, radiative damping is typically neglected and only the nearest neighbor effects are considered. Such an approach is reasonably accurate when the size of an individual particle is much smaller than either of the inter-particle spacings. However, as we approach experimental sizes, where the particle radii are on the order of the interparticle gap, radiative damping and long-range effects can become too significant to neglect[1]. In this work we investigate sub-wavelength resonators leveraging localized surface phonon resonances (LSPhR), taking into account radiative damping and long-range effects. We design, fabricate and characterize topological phononic chains consisting of aluminum nitride (AlN) pillars on a molybdenum (Mo) ground plane which support edge modes in the long-wave infrared regime. AlN has a negative permittivity in the spectral range bridging AlN longitudinal optical (LO) and transverse optical (TO) phonon energies (AlN Reststrahlen band). In this spectral range, AlN can support propagating and/or localized surface phonon modes at the interface between AlN and a dielectric medium. In the QS approach, an infinite staggered chain of particles is assumed, with periodic boundary conditions. Using the nearest neighbor model, one solves the 2×2 Bloch Hamiltonian matrix to obtain the dispersion relation for the chain. The edge mode, however, cannot be observed for such a case (infinite chain assumption). In order to do so, we must apply an open boundary condition, where each particle has a unique polarization due to its interaction with all the other particles in the chain, including those at the end of the chain. Therefore, for an N-particles chain, we obtain an N×N matrix. The solution for this system gives the dispersion relation, or the allowed frequencies of the finite chain for a given Δ, r & β. One observes that set of allowed frequencies, and the phononic band gap, are the same for β & (2 – β), however, presence of the edge modes is only observed (in the band gap) for β >1. The fabricated sample consists of 1.2μm thick AlN over 200nm of Mo on a silicon substrate. The topological chain of AlN pillars of radius, r = 1.5μm, Δ = 20μm is etched using a standard photo-lithography & ICP RIE process. Similarly, an isolated 3μm wide strip of Mo is etched under the AlN pillars on the edge of the sample, to be used for thermal excitation of the topological chain. The reflectance spectrum of the pillars is measured using a Bruker V80v Fourier transform infrared (FTIR) spectrometer. The collected spectra are compared to RCWA-simulated reflection spectra, as well as reflection from a bulk AlN sample. While the bulk sample shows the absorption feature at λ ≈ 11.2μm corresponding to the AlN Berreman mode[2], the pillar spectra show coupling to LSPhR modes at λ ≈ 12.1μm, in excellent agreement with the plasma frequency calculated for the pillars and our numerical simulations. Future work will look to collect the edge mode emission from one end of chain by thermal excitation of the chain's opposite end. Authors AK and DW gratefully acknowledge Air Force award #FA8075-18-D-0002 via Macauley-Brown Inc subcontract. Authors MSA and JWA are thankful for the funding support through AFOSR Lab Task 20RWCOR090 and the AFRL Munition Directorate’s Chief Scientist Office. 1. S. R. Pocock et al. ...V. Giannini, ACS Photonics, 5(6):2271–2279, 2018. 2. L. Nordin et al. ... D. Wasserman.Applied Physics Letters, 111(9):091105, 2017.

Long-Wave Infrared Topological Phonon Chain

Abstract Body: The rocksalt-structure semiconductor ScN is a relatively unexplored group IIIB transition metal nitride with outstanding physical properties that, combined with its III-nitride counterparts, has the potential to dramatically expand the range of their electrical, optical, and thermoelectric device applications. Recent works have focused on the degenerate nature of ScN by substitutional impurities ON and FN, which shift the direct (X–X) gap transition to higher energies via the Burstein–Moss effect. Here, we used cathodoluminescence spectroscopy (CLS) to observe optical signatures of both the midgap VN precursor to ON doping as well as signatures above the direct (X–X) bandgap corresponding to band-to-band transitions from four separate conduction bands near the Γ point with the valence band minimum, in agreement with calculated band structure diagrams. Thin film ScN grown by reactive magnetron sputtering displays mild degenerate doping by substitutional oxygen as indicated by elevated (X–X) transition energies and the presence of Sc–O bonding determined via x-ray photoelectron spectroscopy (XPS), while ScN grown by physical vapor transport exhibited the intrinsic, non-degenerate (X–X) bandgap predicted by theory. CLS reveals a sharp, sub-bandgap emission at 1.26 eV for sputter grown ScN on GaN, which we attribute to nitrogen vacancies (VN) based on surface sensitive CLS and XPS chemical trends. This finding is in strong agreement with theoretical calculations for VN predicting the formation of a defect energy level within the gap. Since O substitution into VN defects creates the shallow donor level responsible for degenerate doping, optical identification of this doping precursor can help understand the growth and processing that influence ScN doping. L.J.B., M.S.H., and S.S. acknowledge the support from the National Science Foundation, Grant No. DMR 18-00130. A.H. and J.H.E. acknowledge the support for ScN crystal growth from the National Science Foundation under Grant No. DMR-1508172. This research was also supported by the Air Force Office of Scientific Research through Project No. FA9550-RY17COR490 (J.S.C. and A.N.R.).

Cathodoluminescence and X-Ray Photoelectron Spectroscopy of ScN: Dopant, Defects and Band Structure

Abstract Body: The AlN wurtzite structure retains piezoelectricity to at least 1000oC and AlN films can be grown using a wide range of techniques, making AlN an attractive high temperature piezoelectric material for harsh environment microwave acoustic sensor applications. Several theoretical predictions and experimental thin film growth studies reported in the literature have indicated that the addition of ScN into the AlN wurtzite lattice to form a ScxAl1-xN alloy can increase the value of the piezoelectric coupling coefficient, d33. The most common method that has been used for ScxAl1-xN film synthesis is RF magnetron sputtering from a fixed target composition which allows uniform film growth over large wafer areas. In this work, a plasma-assisted epitaxial growth method has been used to synthesize highly oriented (0002) ScxAl1-xN films on pre-nitrided c-sapphire substrates over a range of stoichiometries from x = 7 at.% to 55 at.% by co-evaporating Sc and Al high purity metal sources in the presence of a nitrogen plasma generated by an RF plasma source within a highly controlled ultra-high vacuum environment. This plasma-assisted epitaxy approach allows ScxAl1-xN films to be produced with the desired highly oriented (0002) grains needed for piezoelectric activity using precisely controlled Sc, Al, and N-plasma fluxes, and the method easily allows synthesis of many ScxAl1-xN film compositions. ScxAl1-xN film stoichiometries are dictated by the Sc and Al arrival rates on the 930oC growth surface during the plasma epitaxy process and were verified by in situ X-ray photoelectron spectroscopy analysis immediately after growth. X-ray diffraction pole figures and in situ RHEED indicate that the desired highly oriented ScAlN (0002) grains epitaxially nucleate on prenitrided c-sapphire substrates for Sc concentrations below 20 at.%. At Sc concentrations above 20 at.% the (0002) film texture becomes less pronounced, and above ~45 at.% the ScAlN (0002) wurtzite structure becomes unstable and non-wurtzite phases are produced. The piezoelectric activity of the ScxAl1-xN films was evaluated by measuring the response of one-port surface acoustic wave resonators (SAWRs) that were fabricated on top of the ScxAl1-xN epitaxial films with Pt-Ni interdigitated transducer electrodes patterned by a photolithographic lift-off process. To successfully fabricate the SAWRs, a 10nm thick PE-CVD SixNy layer was needed to prevent etching of the ScAlN film by the photoresist developer. The degree of piezoelectric coupling was inferred by measuring the reflection coefficient (|S11|) response of one-port resonators and calculating the respective conductance and susceptance values for the SAWR devices. In addition to the quality of (0002) epitaxy and grain mosaic spread, other ScAlN film characteristics affect the amount of piezoelectric coupling including the Sc concentration, oxygen impurity content, and other grain distributions and defects within the films, as evidenced by high resolution x-ray diffraction, RHEED, and x-ray photoelectron spectroscopy data.

Plasma-Assisted Epitaxy of Piezoelectric ScxAl1-xN Films for Use in Harsh Environment Microwave Acoustic Sensors

Abstract Body: Alloys made of III-nitrides such as GaN, AlN, InN have been and continue to be of large technological and economic importance. They have found use in devices such as light-emitting diodes and High Electron Mobility Transistors. However, lattice mismatches limit the performance of III-nitride-based devices. One proposed solution that has received interest in recent years is the incorporation of ScN to enhance optical and electronic properties. In particular, scandium aluminum nitride (ScxAl1-xN) has been demonstrated to exhibit spontaneous polarization, high piezoelectricity, a wide bandgap, and an excellent lattice match to GaN. These properties have been demonstrated to be dependent on the Sc content and growth parameters such as temperature, pressure, and sputtering mode. ScxAl1-xN-based devices are expected to find use in applications such as filters and resonators, microelectromechanical systems and transistors, among others. This work reports the characterization of ScxAl1-xN films deposited on oxidized silicon substrates by a pulsed DC magnetron sputtering technique. The Sc and Al sources used were 99.999% pure Sc and 99.9995% pure Al targets. Sample composition and crystalline orientation were investigated using X-ray photoelectron spectroscopy and X-ray diffraction. Investigation on surface roughness was conducted using atomic force microscopy. Our results contribute to the understanding and development of ScxAl1-xN synthesis and characterization for the next generation of electronic devices and sensors.

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