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DOE OSTI2020

Spin–orbit torque switching of a ferromagnet with picosecond electrical pulses

The development of approaches that can efficiently control the magnetization of magnetic materials is central to the creation of fast and low-power spintronic devices. Spin transfer torque can be used to electrically manipulate magnetic order in devices, but is typically limited to nanosecond timescales. Alternatively, spin–orbit torque can be employed, and switching with current pulses down to ~200 ps has been demonstrated. However, the upper limit to magnetization switching speed remains unestablished. Here, we show that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque. We probe the ultrafast magnetization dynamics due to spin–orbit torques with sub-picosecond resolution using the time-resolved magneto-optical Kerr effect (MOKE). We also estimate that the magnetization switching consumes less than 50 pJ in micrometre-sized devices.

42 ENGINEERING↗

DOE OSTI2020

Direct Evidence of Photoinduced Charge Transport Mechanism in 2D Conductive Metal Organic Frameworks

Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demon-strated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn2+ guest metal was also examined to uncover the contribution of through space pathway as the presence of the redox inactive 3d10 Zn2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (> 17 ns) are responsible for the long-term photoconductivity in Cu-THQ and transport through O-Cu-O motif with negligible contribution from interlayer ligand π-π stacking as incorporation of Zn2+ in Cu-THQ significantly reduced photoconductivi-ty. These unprecedent results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.

14 SOLAR ENERGY↗

DOE OSTI2021

Self-assembled multifunctional nanostructures for surface passivation and photon management in silicon photovoltaics

This work reports the fabrication and characterization of multifunctional, nanostructured passivation layers formed using a self-assembly process that provide both surface passivation and improved light trapping in crystalline silicon photovoltaic (PV) cells. Scalable block copolymer self-assembly and vapor phase infiltration processes are used to form arrays of aluminum oxide nanostructures (Al 2 O 3 ) on crystalline silicon without substrate etching. The Al 2 O 3 nanostructures are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and spectroscopic ellipsometry. Injection-level dependent photoconductance measurements are used to determine the effective carrier lifetime of the samples to confirm the nanostructures successfully passivate the Si surface. Finite element method simulations and reflectance measurement show that the nanostructures increase the internal rear reflectance of the PV cell by suppressing the parasitic optical losses in the metal contact. An optimized morphology of the structures is identified for their potential use in PV cells as multifunctional materials providing surface passivation, photon management, and carrier transport pathways.

14 SOLAR ENERGY↗

DOE OSTI2026

Strain-Driven Evolution of Structural and Carrier Recombination Dynamics in Germanium

Strain engineering in group-IV semiconductors, in particular germanium (Ge), offers a promising route for advancing next-generation optoelectronic device performance. High minority carrier lifetime serves as a direct indicator of superior material quality and device efficiency. Several factors govern the minority carrier lifetime in both strained Ge and strained GeSn, such as strain level, epilayer thickness, defects, surface roughness, and Sn alloying. In this study, the impact of biaxial strain on effective minority carrier lifetimes of tensile-strained Ge (0% ..epsilon..-Ge/Al?As, 0.78% ..epsilon..-Ge/In0.115?Ga0.885?As, and 0.93% ..epsilon..-Ge/In0.16?Ga0.84?As) and compressively strained GeSn (-0.53% c-Ge0.97?Sn0.03/Al?As) epilayers, grown by molecular beam epitaxy, was experimentally investigated via the microwave-reflection photoconductive decay (..mu..-PCD) technique as a simultaneous function of thickness and strain. The structural and strain-relaxation properties were analyzed by high-resolution x-ray diffraction and Raman spectroscopy, while surface morphology analysis using atomic force microscopy provided insights into the surface roughness of strained Ge-based epilayers. Defect characterization by cross-section transmission electron microscopy revealed superior crystalline quality of ..epsilon..-Ge in one epilayer, while twin boundary formation was observed in the other. We investigated the carrier recombination dynamics in epitaxially strained Ge by extracting the bulk recombination lifetime and surface recombination velocity using the thickness-dependent minority carrier lifetime. High effective minority carrier lifetimes of approximately 60-340 ns were achieved at high strain levels with minimal dependence on the thickness of the strained Ge-based epilayers by means of the ..mu..-PCD technique. Minority carrier lifetimes in Ge-based epilayers exhibit a quadratic dependence on the biaxial strain, ranging from -0.88% (compressive) to +1.6% (tensile). The analysis of the carrier recombination dynamics in strained Ge-based heterostructures offers a pathway to designing group-IV based optoelectronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Quantum Efficiency Gain in 2D Perovskite Photo and X‐Ray Detectors

Abstract The perovskite polycrystalline thin film detector fabricated by solution method is a promising low‐cost, scalable technology for radiation imaging, but its thin volume limits the sensing efficiency for high‐energy X‐ray photons. This work reports 2D perovskite thin film photo‐diodes with a detection gain when sensing visible and X‐ray photons. Detailed power and temperature‐dependent device characterizations reveal a charge multiplication effect to be responsible for the observed high gain. This is caused by a disparity in electron and hole transport, where electron transport is retarded by a trap/de‐trap process via shallow trap states, whereas the hole transport is fast enough to produce a photoconductive gain in satisfying the charge neutrality. The 2D perovskite made with butylamine spacers is also found to exhibit a larger efficiency gain than those made with phenylethylamines because of the higher likelihood of forming shallow traps in the former. The thin film diodes feature a high temporal response over 1 MHz due to the fast charge collection across a thin volume, and the discovery provides device physics mechanisms in connection to material structure‐function relationship for future optoelectronics design that can boost the efficiency of X‐ray sensing and dim light detection.

2D perovskites↗

DOE OSTI2024

Manipulating Photoconduction in Cu–Pyrene 1-D Coordination Nanosheets by Modulating Interlayer π–π Stacking

2D conductive metal organic frameworks (MOFs) have become promising candidates for applications in electronic and photoelectronic devices. However, the lack of a fundamental understanding of the charge transport mechanism and synthetic design to alter their conductivity limits their potential applications. In this work, we report the design and synthesis of two 1D Cu–pyrene coordination nanosheets, Cu–pyrene-4,5,9,10 tetrathiol (Cu-PTT) and Cu–2,7-di-tert-butylpyrene-4,5,9,10-tetrathiol (Cu– tBu PTT), as models of 2D conductive MOFs, where the introduction of two different functional groups (H and tert-butyl groups) at the 2 and 7 positions of the PTT ligands is used to tune the interlayer distance and π–π stacking of MOFs. Using optical pump terahertz probe spectroscopy, we show that both nanosheets exhibit appreciable photoconduction, but the photoconduction in Cu– tBu PTT is reduced with respect to that in Cu–PTT, which can be attributed to the larger interlayer distance in the former leading to the decoupling of the interlayer charge transport pathway. Furthermore, these results demonstrate a unique strategy to investigate the impact of out-of-plane extended conjugation on photoconduction in conductive conjugated polymers, MOFs, or coordination nanosheets through adjusting their interlayer spacing.

14 SOLAR ENERGY↗

DOE OSTI2021

Bulk Photovoltaic Effect Driven by Collective Excitations in a Correlated Insulator

We investigate the bulk photovoltaic effect, which rectifies light into electric current, in a collective quantum state with correlation driven electronic ferroelectricity. We show via explicit real-time dynamical calculations that the effect of the applied electric field on the electronic order parameter leads to a strong enhancement of the bulk photovoltaic effect relative to the values obtained in a conventional insulator. The enhancements include both resonant enhancements at sub-band-gap frequencies, arising from excitation of optically active collective modes, and broadband enhancements arising from nonresonant deformations of the electronic order. The deformable electronic order parameter produces an injection current contribution to the bulk photovoltaic effect that is entirely absent in a rigid-band approximation to a time-reversal symmetric material. Furthermore, our findings establish that correlation effects can lead to the bulk photovoltaic effect and demonstrate that the collective behavior of ordered states can yield large nonlinear optical responses.

14 SOLAR ENERGY↗

DOE OSTI2021

Observation of spatially resolved Rashba states on the surface of CH 3 NH 3 PbBr 3 single crystals

Hybrid organic-inorganic perovskites (HOIPs) are prime candidates for studying Rashba effects due to the heavy metal and halogen atoms in their crystal structure coupled with predicted inversion symmetry breaking. Nevertheless, observation of the Rashba effect in cubic CH 3 NH 3 PbBr 3 single crystals that possess bulk inversion symmetry is the subject of extensive debate due to the lack of conclusive experiments and theoretical explanations. In this work, we provide experimental evidence that Rashba state in cubic CH 3 NH 3 PbBr 3 single crystals at room temperature occurs exclusively on the crystal surface and depends on specific surface termination that results in local symmetry breaking. We demonstrate this using a suite of spatially resolved and depth-sensitive techniques, including circular photogalvanic effect, inverse spin Hall effect, and multiphoton microscopy, that are supported by first principle calculations. Our work suggests using surface Rashba states in these materials for spintronic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS2001

HgCdTe Photoconductive Mixers for 2-8 THz

Heterodyne spectroscopy has been taken to wavelengths as short as 63 micrometers with Schottky-diode mixers. Schottkys, however, are relatively insensitive compared to superconducting mixers such as the hot-electron microbolometer (HEB), which has an effective quantum efficiency of 3% at 120 micrometers (2.5 THz). Although HEB sensitivities are bound to improve, there will always be losses associated with antenna coupling of radiation into sub-micron size devices. Another approach to far infrared (FIR) mixer design is to use a photoconductive device which can be made much larger than a wavelength, and thus act as its own antenna. For example, HgCdTe photodiodes have been used as mixers in the lambda = 10 micrometers band for over 25 years, with sensitivities now only a factor of 2 from the quantum-noise-limit. HgCdTe can also be applied at FIR wavelengths, but surprisingly little work has been done to date. The exception is the pioneering work of Spears and Kostiuk and Spears, who developed HgCdTe photomixers for the 20-120 micrometer region. The spectral versatility of the HgCdTe alloy is well recognized for wavelengths as long as 8-20 micrometers. What is not so recognized, however, is that theoretically there is no long wavelength limit for appropriately composited HgCdTe. Although Spears successfully demonstrated a photoconductive response from HgCdTe at 120 micrometers, this initial effort was apparently never followed up, in part because of the difficulty of controlling the HgCdTe alloy composition with liquid-phase-epitaxy (LPE) techniques. With the availability of precise molecular-beam-epitaxy (MBE) since the early 1990's, it is now appropriate to reconsider HgCdTe for detector applications longward of lambda = 20 micrometers. We recently initiated an effort to fabricate detectors and mixers using II-VI materials for FIR wavelengths. Of particular interest are device structures called superlattices, which offer a number of advantages for high sensitivity direct detectors and very long wavelength heterodyne mixers.

Betz, A. L.↗

DOE OSTI2022

Nanostructures evolution assessment and spectroscopic properties modification induced by electronic energy loss in KTaO 3 crystal

Modifications of micro/nanostructure and photoelectric properties under swift ion irradiation (645 MeV Xe 35+ ) of KTaO 3 crystal with varying electronic energy losses (7.2–31.4 keV/nm) and ion velocities (0.18–5.00 MeV/u) have been studied by combining experimental and calculated approaches. The i-TS calculations combined with molecular dynamics simulations are compared with the experimental observations, revealing the inner track fine structures from individual spherical defects to continuous ion tracks with core–shell morphologies, and a quantitative relationship, including the melting (0.42 eV/atom), damage (0.75 eV/atom), and amorphous (1.71 eV/atom) thresholds, is established to successfully predict the inner disorder/amorphous proportions and track damage morphologies. The surface nanostructures of nanohillocks (isolated or partial overlaps) and nanopits (serious overlaps) directly depend on the combined action of the deposited potential and kinetic energies of incident ions, which induce local melting and sublimation in the near-surface region. Owing to the decreased recombination and the increased separation efficiency between electrons and holes, the higher photogenerated charge carrier mobility enhanced the photocurrent effect, further optimizing the photoconductivity performance in Xe 35+ -irradiated KTaO 3 . Therefore, controlled defect engineering using the ion irradiation technique, as an effective strategy, could design tailored nanostructure systems and regulate photoelectric properties, further promoting the development of novel technological applications.

36 MATERIALS SCIENCE↗

DOE OSTI2025

A self-supervised robotic system for autonomous contact-based spatial mapping of semiconductor properties

Integrating robotically driven contact-based material characterization techniques into self-driving laboratories can enhance measurement quality, reliability, and throughput. While deep learning models support robust autonomy, current methods lack reliable pixel-precision positioning and require extensive labeled data. To overcome these challenges, we propose an approach for building self-supervised autonomy into contact-based robotic systems that teach the robot to follow domain expert measurement principles at high throughputs. We demonstrate the performance of this approach by autonomously driving a 4-DOF robotic probe for 24 hours to characterize semiconductor photoconductivity at 3025 uniquely predicted poses across a gradient of drop-casted perovskite film compositions, achieving throughputs of more than 125 measurements per hour. Spatially mapping photoconductivity onto each drop-casted film reveals compositional trends and regions of inhomogeneity, valuable for identifying manufacturing defects. With this self-supervised neural network–driven robotic system, we enable high-precision and reliable automation of contact-based characterization techniques at high throughputs, thereby allowing measurement of previously inaccessible yet important semiconductor properties for self-driving laboratories.

Science & Technology - Other Topics↗

DOE OSTI2023

Broadband Photoconductive Sampling in Gallium Phosphide

Abstract Direct measurements of the electric field of light enable new observations of light–matter interactions. In the near‐infrared and visible spectral ranges, this typically relies on techniques that exploit nonlinearities in gases or solids, which limits their sensitivity. Here, a method for the detection of broadband near‐infrared fields spanning more than one octave from 110 to 220 THz based on linear absorption in a semiconductor is demonstrated. This technique, which avoids complex vacuum setups and works under ambient conditions, employs linear photoconductive sampling (LPS) in gallium phosphide. Simulations reveal that the response function of LPS is concerned with the intensity envelope of the gate field, in contrast to electro‐optic sampling, relaxing the stringent temporal requirements on the gate pulse.

36 MATERIALS SCIENCE↗

NASA NTRS1999

Corrugated Quantum Well Infrared Photodetectors and Arrays

Quantum well infrared photodetectors (QWIPs) have many advantages in infrared detection, mainly due to the mature Ill-V material technology. The employment of the corrugation structure further advances the technology by providing a simple, yet efficient light-coupling scheme. A C-QWIP enjoys the same flexibility as a detector with intrinsic normal incident absorption. In this paper, we will discuss the utilities of C-QWIPs in different applications, including two-color detection and polarization-sensitive detection. Besides practical applications, C-QWIPs are also useful in detector characterization. They can be used for measuring the absorption coefficient of light propagating parallel to the layers under bias and providing information on the energy resolved photoconductive gain. These two quantities have never been measured before. Based on the corrugation design, we have made several modifications that further improve the detector sensitivity without increasing its complexity. Other than the C-QWIP structure, we also continue searching for other sensitive detector architectures. In a quantum grid infrared photodetector, 3-dimensional electron confinement can be achieved, with which the detector is able to absorb light in all directions. At the same time, the photoconductive gain can also be improved. We further improve the design using a blazed structure. All the experimental results are supported by a rigorous electromagnetic modal transmission-line theory developed especially for these types of structures. Preliminary thermal imaging using C-QWIP FPAs validates the advantages of the present approach.

Choi, K. K.↗

DOE OSTI2023

Light-Induced Structural Dynamics and Charge Transport in Layered Halide Perovskite Thin Films

The dynamic nature of the metal halide perovskite lattice upon photoexcitation plays a vital role in their properties. Here we report an observation of light-induced structure dynamics in quasi-2D Ruddlesden–Popper phase perovskite thin films and its impact on the carrier transport properties. By a time-resolved X-ray scattering technique, we observe a rapid lattice expansion upon photoexcitation, followed by a slow relaxation over the course of 100 ns in the dark. Theoretical modeling suggests that the expansion originates from the lattice’s thermal fluctuations caused by photon energy deposition. Power dependent optical spectroscopy and photoconductivity indicate that high laser powers triggered a strong local structural disorder, which increased the charge dissociation activation energy that results in localized transport. Finally, our study investigates the impact of laser energy deposition on the lattices and the subsequent carrier transport properties, that are relevant to device operations.

2D perovskites↗

DOE OSTI2021

Photogating-driven enhanced responsivity in a few-layered ReSe 2 phototransistor

A wide variety of two-dimensional (2D) metal dichalcogenide compounds have recently attracted much research interest due to their very high photoresponsivities (R) making them excellent candidates for optoelectronic applications. High R in 2D photoconductors is associated with trap state dynamics leading to a photogating effect, which is often manifested by a fractional power dependence (γ) of the photocurrent (I ph ) at an effective illumination intensity (P eff ). Furthermore, we present photoconductivity studies as a function of gate voltages, over a wide temperature range (20 K to 300 K) of field-effect transistors fabricated using thin layers of mechanically exfoliated Rhenium Diselenide (ReSe 2 ). We obtain very high responsivities R ~ 16500 A/W and external quantum efficiency (EQE) ~ 3.2 × 106% (at 140 K, V g = 60 V and P eff = 0.2 nW). A strong correlation between R and γ was established by investigating the dependence of these two quantities at various gate voltages and over a wide range of temperatures. Such correlations indicate the importance of trap state mediated photogating and its role in promoting high photo-responsivities in these materials. We believe such correlations can offer valuable insights for the design and development of high-performance photoactive devices using 2D materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1987

II-VI and I-III-VI2 alloy crystals for photovoltaic applications

The results of the photoreflectance, photoconductivity, and photoluminescence studies on Mn(x)Cd(1-x)Te single crystals are discussed from the viewpoint of the utility of these compounds as gap components in multijunction solar cells. Attention is also given to recent results on CuInSe2 and on quaternary I-III-VI2 alloys based on CuInSe2. Recent Kelvin probe and X-ray photoemission spectroscopy studies of the surface properties of CuInSe2 and Mn(x)Cd(1-x)Te, respectively, are described. In addition, recent results on the radiation damage and annealing kinetics of Mn(x)Cd(1-x)Te are considered in the context of future work on the sputtering and ion beam deposition of thin films of this material.

Bachmann, K. J.↗

NASA NTRS1995

Photoelectrochemical etching of silicon carbide (SiC) and its characterization

Silicon carbide (SiC) is an attractive semiconductor material for high speed, high density, and high temperature device applications due to its wide bandgap (2.2-3.2 eV), high thermal conductivity, and high breakdown electric field (4 x 10(exp 6) V/cm). An instrumental process in the fabrication of semiconductor devices is the ability to etch in a highly controlled and selective manner for direct patterning techniques. A novel technique in etching using electrochemistry is described. This procedure involves the ultraviolet (UV) lamp-assisted photoelectrochemical etching of n-type 3C- and 6H-SiC to enhance the processing capability of device structures in SiC. While under UV illumination, the samples are anodically biased in an HF based aqueous solution since SiC has photoconductive properties. In order for this method to be effective, the UV light must be able to enhance the production of holes in the SiC during the etching process thus providing larger currents with light from the photocurrents generated than those currents with no light. Otherwise dark methods would be used as in the case of p-type 3C-SiC. Experiments have shown that the I/V characteristics of the SiC-electrolyte interface reveal a minimum etch voltage of 3 V and 4 V for n- and p-type 3C-SiC, respectively. Hence it is possible for etch-stops to occur. Etch rates calculated have been as high as 0.67 micrometer/min for p-type, 1.4 micrometer/min for n-type, and 1.1 micrometer/min for pn layer. On n-type 3C- SiC, an oxide formation is present where after etching a yellowish layer corresponds to a low Si/C ratio and a white layer corresponds to a high Si/C ratio. P-type 3C-SiC shows a grayish layer. Additionally, n-type 6H-SiC shows a brown layer with a minimum etch voltage of 3 V.

Collins, D. M.↗

DOE OSTI2022

Persistent opto-ferroelectric responses in molecular ferroelectrics

Persistent photoresponses require optical excitations to metastable states, which are rare of ionic origin due to the indirect photon-ion interaction. In this work, we explore the photoinduced metastable proton states in the proton-transfer type molecular ferroelectric croconic acid.We observe that, after the photoexcitation, the changes of structural and ferroelectric properties relax in ~10 3 s, indicating persistent photoresponses of ionic origin. In contrast, the photoconductivity relaxes within 1 s. The 10 3 s timescale suggests that the ionic metastable states result from proton transfer both along and out of the hydrogen bonds. Furthermore, this discovery unveils an ionic mechanism for the phototunability, which offers persistent opto-ferroelectric control for proton-transfer type molecular ferroelectrics.

36 MATERIALS SCIENCE↗