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DOE OSTI2023

Unlocking AlN Piezoelectric Performance with Earth‐Abundant Dopants

Abstract The increasing demand for high‐performance piezoelectric materials and toxicity and thermal stability issues of the widely used lead zirconate titanates (PZT) have spurred a search for better alternatives in electronic devices. In comparison to PZT, group III nitrides such as aluminum nitride (AlN), are only weakly piezoelectric, but doping AlN with scandium (Sc) improves the piezoelectric response by nearly 500%. Relative to PZT, doped‐AlN piezoelectric materials are advantageous because they are far more compatible with complementary metal–oxide–semiconductor (CMOS) materials, and they maintain both piezoelectric and thermodynamic stability up to very high temperatures. Unfortunately, rare‐earth metals are notoriously expensive, and fabricating stable films with rare‐earth dopants is also challenging, limiting their use in industrial applications. In this work, ab initio calculations are combined with targeted fabrication and experimentation to identify alternative earth‐abundant dopants for AlN from the periodic table d‐block. Amongst the 23 elements screened, it is found that group IVB metals, titanium, zirconium, and hafnium induce large piezoelectric enhancements comparable to Sc. This improvement is traced to shifts in the atomic sublattice structure and changes in the local charge states. In demonstrating a highly accessible and affordable path for technological adaptation of AlN‐based piezoelectrics, this work provides the foundation for sustainable, next‐generation electronics.

Startt, Jacob↗

DOE OSTI2020

Characterization of Amplification Properties of the Superconducting-Ferromagnetic Transistor

Here we report on the measurement results of the superconducting-ferromagnetic transistors (SFTs). The devices were made at Northwestern University and Hypres (SeeQC), Inc. (Nevirkovets et al., 2014; 2015). SFT is a multiterminal device with the SIS'FIFS (or SFIFS'IS) structure (where S, I, and F denote a superconductor, an insulator, and a ferromagnetic material, respectively) exploiting intense quasiparticle injection in order to modify the nonlinear I-V curve of a superconducting tunnel junction. SFT is capable of providing voltage, current, and power amplification while having good input/output isolation. We characterized the devices using different measurement techniques. We measured S parameters of the single- and double-acceptor devices at frequencies up to 5 MHz. Importantly, we confirmed that the isolation between the input and output of the device is quite good. However, the techniques typically employed to characterize semiconductor devices do not allow for revealing the full potential of our low-resistive SFT devices, especially those having two acceptors. In the latter case, we also tested the devices using the battery-powered current sources with floating grounds. Analyzing double-acceptor I-V curves recorded at different levels of injection currents, for an optimal load, we deduced a small-signal voltage gain of 33 and a power gain of 2.4. We suggest that further improvement of the SFT device parameters is possible in optimized devices, so that the device potentially may serve as a preamplifier for readout of output signals of cryogenic detectors and be useful as an element of other superconductor-based circuits. In addition, we used scanning transmission electron microscopy to identify some problems in the fabrication of the devices without any planarization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Computationally Guided Discovery of Axis-Dependent Conduction Polarity in NaSnAs Crystals

Most electronic materials exhibit a single dominant charge carrier type, either holes or electrons, along all crystallographic directions. However, there are a small number of compounds, mostly metals, that exhibit simultaneous p-type and n-type conduction behavior along different crystallographic directions. We demonstrate that the experimental discovery of semiconductors with this axis-dependent conduction polarity can be facilitated by identifying a large anisotropy of either the electron or hole effective masses (m*) or both, providing the electron and hole masses dominate along different crystallographic directions. We calculated the layered semiconductor NaSnAs to have a lower electron m* in-plane than the cross-plane and a very large hole m* in-plane and small hole m* cross-plane. We established the growth of >3 mm-sized NaSnAs crystals via Sn flux and confirmed the band gap to be 0.65 eV, in agreement with theory. NaSnAs exhibits p-type thermopowers cross-plane and n-type thermopowers in-plane, confirming that the large anisotropy in the effective mass at the band edges is an excellent indicator for axis-dependent conduction polarity. Altogether, this work shows that the discovery of semiconductors with such a phenomenon can be accelerated by computationally evaluating the anisotropic curvatures of the band edges, paving the way for their future discovery and application.

30 DIRECT ENERGY CONVERSION↗

DOE OSTI2020

Analysis of GaAs Solar Cells Grown on 50 mm Wafers at 700 degrees C by Dynamic Hydride Vapor Phase Epitaxy

We grew and analyzed GaAs photovoltaics devices grown by dynamic hydride vapor phase epitaxy (D-HVPE) on 50 mm GaAs substrates. These were grown at 700 degrees C in a kinetically-limited growth regime. These devices exhibited nearly identical performance to our previous single-junction GaAs devices (>25% efficiency) that were grown at 650 degrees C in a transport-limited growth regime. We determined that the relatively low non-uniformity in device efficiency was not related to variations in structural characteristics, such as GaAs base thickness or Ga x In 1-x P composition or thickness, but rather uniformity of the absorber doping. These results show that growth of high quality material is possible in multiple growth regimes by D-HVPE.

14 SOLAR ENERGY↗

DOE OSTI2024

Photon management in silicon photovoltaic cells: A critical review

With the practical efficiency of the silicon photovoltaic (PV) cell approaching its theoretical limit, pushing conversion efficiencies even higher now relies on reducing every type of power loss that can occur within the device. Limiting optical losses is therefore critical and requires effective management of incident photons in terms of how they interact with the device. Ultimately, photon management within a PV cell means engineering the device and constituent materials to maximize photon absorption within the active semiconductor and therefore reduce the number of photons lost through other means, most notably reflection and parasitic absorption. There have been great advancements in the front and the rear side photon management techniques in recent years. This review aims to discuss these advancements and compare the various approaches, not only in terms of increases in photogenerated current, but also their compatibility with different PV cell architectures and potential trade-offs, like increased surface recombination or scalability for high-volume manufacturing. In this review, a comprehensive discussion of a wide variety of the front and the rear side photon management structures are presented with suggestions to improve the already achieved performance further. Here, this review is unique because it not only presents the recent development in photon management techniques, but also offers through analysis of these techniques and pathways to improve further.

14 SOLAR ENERGY↗

DOE OSTI2022

Formation of tubular conduction channel in a SiGe(P)/Si core/shell nanowire heterostructure

Realizing a tubular conduction channel within a one-dimensional core–shell nanowire (NW) enables better understanding of quantum phenomena and exploration of electronic device applications. Herein, we report the growth of a SiGe(P)/Si core/shell NW heterostructure using a chemical vapor deposition coupled with vapor–liquid–solid growth mechanism. The entire NW heterostructure behaves as a p-type semiconductor, which demonstrates that the high-density carriers are confined within the 4 nm-thick Si shell and form a tubular conduction channel. These findings are confirmed by both calculations and the gate-dependent current–voltage ( I d – V g ) characteristics. Atomic resolution microscopic analyses suggest a coherent epitaxial core/shell interface where strain is released by forming dislocations along the axial direction of the NW heterostructure. Additional surface passivation achieved via growing a SiGe(P)/Si/SiGe core/multishell NW heterostructure suggests potential strategies to enhance the tubular carrier density, which could be further modified by improving multishell crystallinity and structural design.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Non-Equilibrium Synthesis of Semiconductor Alloys for Energy Applications

This project explores the synthesis under equilibrium/non-equilibrium conditions of semiconductor alloys critical to SSL to impose top-down influence on the formation of atomic-scale microstructures. A plasma-enhanced MOCVD (PE-MOCVD) technique is employed, in which plasma generated within the low-pressure reactor environment introduces additional energy that is delivered to the gas-phase reactants and growth surface, allowing relatively low substrate temperatures to be maintained, rapidly switching the crystal-growth environment into a non-equilibrium state. The project coordinates on-going work at National Renewable Energy Laboratory (NREL) that addresses order/disorder Al x In 1-x P (AlInP) confinement structures, which are predicted to surpass the power-conversion efficiency of current materials used for red/amber LEDs as components of so-called color-mixed LEDs, which integrate multiple LEDs in a single device for improved color rendering. Ordering, phase separation, and strain relaxation are central to this technology. During growth, each spontaneous process could be suppressed or enhanced on demand by application of plasma as an additional handle. This technique could have broader applications to other energy technologies, such as photovoltaics.

14 SOLAR ENERGY↗

DOE OSTI2025

Weyl semimetal phases and intrinsic spin-Hall conductivity in SbAs ordered alloys

Here, using density functional theory calculations, we investigated possible Weyl semimetal (WSM) phases in antimony arsenide ordered alloys Sb 1-x ⁢As x (x=0, 1/6, 1/3, 1/2, 2/3, 5/6, 1). We find WSM phases for all As compositions of Sb 1-x ⁢As x with broken inversion symmetry, in contrast to Bi 1-x⁢ Sb x where only compositions x=1/2 and 5/6 were predicted to exhibit WSM phases. The WSM phases in Sb 1-x ⁢As x are characterized by the presence of 12 Weyl points, located within 55 meV from the Fermi level in the case of x = 1/2. The robust spin-orbit coupling strength and Berry curvature in these alloys produce large spin-Hall conductivity in the range of 176–602 ($\hslash$/e)(S/cm), comparable to that in the BiSb alloys. Finally, Sb 0.5 ⁢As 0.5 is predicted to be almost lattice matched to GaAs(111), with the Fermi level within the gap of the semiconductor, facilitating growth and characterization, and thus, offering promising integration with conventional semiconductors.

36 MATERIALS SCIENCE↗

NASA NTRS1999

Multi-Dimensional Quantum Tunneling and Transport Using the Density-Gradient Model

We show that quantum effects are likely to significantly degrade the performance of MOSFETs (metal oxide semiconductor field effect transistor) as these devices are scaled below 100 nm channel length and 2 nm oxide thickness over the next decade. A general and computationally efficient electronic device model including quantum effects would allow us to monitor and mitigate these effects. Full quantum models are too expensive in multi-dimensions. Using a general but efficient PDE solver called PROPHET, we implemented the density-gradient (DG) quantum correction to the industry-dominant classical drift-diffusion (DD) model. The DG model efficiently includes quantum carrier profile smoothing and tunneling in multi-dimensions and for any electronic device structure. We show that the DG model reduces DD model error from as much as 50% down to a few percent in comparison to thin oxide MOS capacitance measurements. We also show the first DG simulations of gate oxide tunneling and transverse current flow in ultra-scaled MOSFETs. The advantages of rapid model implementation using the PDE solver approach will be demonstrated, as well as the applicability of the DG model to any electronic device structure.

Biegel, Bryan A.↗

DOE OSTI2024

High conductivity coherently strained quantum well XHEMT heterostructures on AlN substrates with delta doping

Polarization-induced two-dimensional electron gases (2DEGs) in AlN/GaN/AlN quantum well high-electron-mobility transistors on ultrawide bandgap AlN substrates offer a promising route to advance microwave and power electronics with nitride semiconductors. The electron mobility in thin GaN quantum wells embedded in AlN is limited by high internal electric field and the presence of undesired polarization-induced two-dimensional hole gases (2DHGs). To enhance the electron mobility in such heterostructures on AlN, previous efforts have resorted to thick, relaxed GaN channels with dislocations. In this work, we introduce n-type compensation δ-doping in a coherently strained single-crystal (Xtal) AlN/GaN/AlN heterostructure to counter the 2DHG formation at the GaN/AlN interface, and simultaneously lower the internal electric field in the well. This approach yields a δ-doped XHEMT structure with a high 2DEG density of ∼3.2×1013 cm−2 and a room temperature (RT) mobility of ∼855 cm2/Vs, resulting in the lowest RT sheet resistance 226.7 Ω/□ reported to date in coherently strained AlN/GaN/AlN HEMT heterostructures on the AlN platform.

Physics↗

NASA NTRS1994

Chemical beam epitaxy for high efficiency photovoltaic devices

InP-based multijunction tandem solar cells show great promise for the conversion efficiency (eta) and high radiation resistance. InP and its related ternary and quanternary compound semiconductors such as InGaAs and InGaAsP offer desirable combinations for energy bandgap values which are very suitable for multijunction tandem solar cell applications. The monolithically integrated InP/In(0.53)Ga(0.47)As tandem solar cells are expected to reach efficiencies above 30 percent. Wanlass, et.al., have reported AMO efficiencies as high as 20.1% for two terminal cells fabricated using atmospheric-pressure metalorganic vapor phase epitaxy (APMOVPE). The main limitations in their technique are first related to the degradation of the intercell ohmic contact (IOC), in this case the In(0.53)Ga(0.47)As tunnel junction during the growth of the top InP subcell structure, and second to the current matching, often limited by the In(0.53)Ga(0.47)As bottom subcell. Chemical beam epitaxy (CBE) has been shown to allow the growth of high quality materials with reproducible complex compositional and doping profiles. The main advantage of CBE compared to metalorganic chemical vapor deposition (MOCVD), the most popular technique for InP-based photovoltaic device fabrication, is the ability to grow high purity epilayers at much lower temperatures (450 C - 530 C). In a recent report it was shown that cost-wise CBE is a breakthrough technology for photovoltaic (PV) solar energy progress in the energy conversion efficiency of InP-based solar cells fabricated using chemical beam epitaxy. This communication summarizes our recent results on PV devices and demonstrates the strength of this new technology.

Bensaoula, A.↗

DOE OSTI2020

Structural, magnetic, and electronic evolution of the spin-ladder system BaFe 2 S 3-x Se x with isoelectronic substitution

We reWe report experimental studies of a series of BaF e 2 S 3 - x Se x ( 0 ≤ $x$ ≤ 3 ) single crystals and powder specimens using x-ray diffraction, neutron-diffraction, muon-spin-relaxation, and electrical transport measurements. A structural transformation from $C$ $mcm$ ( BaFe 2 Se 3 ) to $P$ $nma$ ( BaFe 2 Se 3 ) was identified around $x$ = 0.7 - 1 . Neutron-diffraction measurements on the samples with $x$ = 0.2, 0.4, and 0.7 reveal that the Néel temperature of the stripe antiferromagnetic order is gradually suppressed from ~120 to 85 K, while the magnitude of the ordered Fe 2+ moments shows very little variation. Similarly, the block antiferromagnetic order in BaFe 2 Se 3 remains robust for 1.5 ≤ $x$ ≤ 3 with negligible variation in the ordered moment and a slight decrease of the Néel temperature from 250 K ( $x$ = 3 ) to 225 K ( $x$ = 1.5 ). The sample with $x$ = 1 near the $C$ $mcm$ and $P$ $nma$ border shows coexisting, two-dimensional, short-range stripe- and block-type antiferromagnetic correlations. The system remains insulating for all $x$ , but the thermal activation gap shows an abrupt increase when traversing the boundary from the $C$ $mcm$ stripe phase to the $P$ $nma$ block phase. The results demonstrate that the crystal structure, magnetic order, and electronic properties are strongly coupled in the BaFe 2 S 3-x Se x system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1996

Preparation and Thermoelectric Properties of the Skutterudite-Related Phase Ru(0.5)Pd(0.5)Sb3

A new skutterudite phase Ru(0.5)Pd(0.5)Sb3 was prepared. This new phase adds to a large number of already known materials with the skutterudite structure which have shown good potential for thermoelectric applications. Single phase, polycrystalline samples were prepared and characterized by x-ray analysis, electron probe microanalysis, density, sound velocity, thermal-expansion coefficient, and differential thermal analysis measurements. Ru(0.5)Pd(0.5)Sb3 has a cubic lattice, space group Im3 (T(sup 5, sub h)), with a = 9.298 A and decomposes at about 920 K. The Seebeck coefficient, the electrical resistivity, the Hall effect, and the thermal conductivity were measured on hot-pressed samples over a wide range of temperatures. Preliminary results show that Ru(0.5)Pd(0.5)Sb3 behaves as a heavily doped semiconductor with an estimated band gap of about 0.6 eV. The lattice thermal conductivity of Ru(0.5)Pd(0.5)Sb3 is substantially lower than that of the binary isostructural compounds CoSb3 and IrSb3. The unusually low thermal conductivity might be explained by additional hole and charge transfer phonon scattering in this material. The potential of this material for thermoelectric applications is discussed.

Caillat, T.↗

NASA NTRS2019

Technology Development for the Modification of High Aspect Ratio Geometries for Thermal and Environmental Control

A key technology development driver in environmental control systems and next generation optics are discussed utilizing thin film development borrowed from the semiconductor industry. The optical and physical properties of spacecraft radiator coatings are dictated by orbital environmental conditions. For example, coatings must adequately dissipate charge buildup when orbital conditions, such as polar, geostationary or gravity neutral, result in surface charging. Current dissipation techniques include depositing a layer of ITO (indium tin oxide) on the radiator surface in a high temperature process. The application of these enhanced coatings must be such that the properties in question are tailored to mission-specific requirements. The multi-billion-dollar semiconductor industry has adopted Atomic Layer Deposition (ALD) for self-assembly and atomic-scale placement. ALD is a cost-effective nanoadditive-manufacturing technique that allows for the conformal coating of substrates with atomic control in a benign temperature and pressure environment. By using ALD, modification of these coatings can be accomplished during coating application preprocessing. The preprocessing is rendered directly on the coating dry pigment before binding. Through the introduction of paired precursor gases, thin films can be deposited on a myriad of substrates ranging from glass, polymers, aerogels, metals, powders, and other high aspect-ratio micro- and nano- structures. By providing atomic-level control, where single layers of atoms can be deposited, the fabrication of metal transparent films, precise nano-laminates, and coatings of nano-channels and pores is achievable. A method has been demonstrated for the ALD of In2O3 and films on a variety of substrates from Si(100) wafers, glass slides, and on Z93P pigments resulting in a direct spaceflight application. Results will be presented that verify the chemical composition of ALD pigments and charge dissipation properties when the pigment goes through its binding and coating process and we present early results of ALD for carbon nanotube formation and encapsulation.

Dwivedi, Vivek H.↗

NASA NTRS2019

Application and Development of Atomic Layer Deposition Techniques to Improve Thermo-Optical Coatings for Spacecraft Thermal Control and Advanced Optical Instruments

Dwivedi, Vivek H.↗

DOE OSTI2020

Cathodoluminescence of Ultrathin Twisted Ge 1–x Sn x S van der Waals Nanoribbon Waveguides

Ultrathin van der Waals semiconductors have shown extraordinary optoelectronic and photonic properties. Propagating photonic modes make layered crystal waveguides attractive for photonic circuitry and for studying hybrid light-matter states. Accessing guided modes by conventional optics is challenging due to limited spatial resolution and poor out-of-plane far field coupling. Scanning near-field optical microscopy (SNOM) can overcome these issues and can characterize waveguide modes down to tens of nm resolution, albeit for planar samples or nanostructures with moderate height variations. Electron microscopy provides atomic-scale localization also for more complex geometries, and recent advances extended the accessible excitations from interband transitions to phonons. In this work, bottom-up synthesized layered semiconductor (Ge 1-x Sn x S) nanoribbons with axial twist and deep sub-wavelength thickness are demonstrated as a platform for realizing waveguide modes, and cathodoluminescence (CL) spectroscopy is introduced as a tool to characterize them. Combined experiments and simulations show the excitation of guided modes by the electron beam and their efficient detection via photons emitted in the ribbon plane, which enables measuring key properties such as the evanescent field into the vacuum cladding with nanometer resolution. The results identify van der Waals waveguides operating in the infrared and highlight an electron microscopy-based approach for probing complex-shaped nanophotonic structures.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Shape Control of Colloidal Semiconductor Nanocrystals through Thermodynamically Driven Aggregative Growth

The optoelectronic properties of colloidal semiconductor nanocrystals (NCs) can be manipulated by changing their geometric shapes. The precise synthetic control over particle morphologies, however, has remained elusive. Conventional growth techniques rely on the kinetic assembly of atomic units, where supersaturation and precipitation processes can lead to a broad distribution of particle shapes. In this paper, we demonstrate that replacing atomic precursors with small-size nanocrystals as building blocks for larger colloids offers an easier, more predictive control over nanoparticle shape evolution. The reported growth strategy is illustrated via shape-selective syntheses of CdSe and CdS NC cubes, spheres, rods, as well as unprecedented “donut” and ring-like structures. Growth of desired particle morphologies was achieved by choosing an appropriate reaction solvent that minimizes the surface free energy through a unique shape formation. This so-called "aggregative growth mechanism" is explained using a thermodynamic model for interacting viscous colloids. We expect that this novel growth strategy will offer a viable platform for shape–selective synthesis of many inorganic colloids, providing pathways to optimizable optoelectronic materials.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Intersubband transitions in nonpolar and semipolar III-nitrides: Materials, devices, and applications

In the last two decades, the third-generation wide bandgap semiconductor III-nitrides have revolutionized a myriad of electronic and photonic devices and applications, including power electronics, extreme-environment electronics, RF amplifiers, and optoelectronics such as light-emitting diodes and laser diodes. Recently, III-nitride heterostructures (e.g., AlGaN/GaN) based intersubband transition (ISBT) has garnered considerable research interest for infrared (IR), terahertz (THz), and ultrafast optoelectronics (e.g., photodetectors and quantum cascade lasers) due to its large conduction band offset, large optical phonon energy, and promising room-temperature operation. This paper presents a comprehensive review on the recent progress and challenges of III-nitrides based ISBT from the perspectives of materials, structures, devices, and applications, with a focus on nonpolar and semipolar III-nitrides. Various device structures have been demonstrated for III-nitrides based ISBT, including quantum wells, dots, and wires, among which AlGaN/GaN quantum wells are the most widely used. The effects of device parameters, crystal orientations, and doping on the ISBT properties of AlGaN/GaN quantum wells are discussed. Although the room-temperature operation is still elusive, theoretical and experimental studies show that nonpolar and semipolar III-nitrides based ISBT exhibits tunable ISBT wavelength from far-IR to THz spectral range with higher efficiency compared with polar c-plane ISBT. This review can serve as a gateway to and an important reference for the recent progress and challenges of III-nitrides based ISBT and its potential applications in sensing, communication, ultrafast optoelectronics, and integrated photonics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗