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Medium-Entropy Engineering of Magnetism in Layered Antiferromagnet CuxNi2(1-x)CrxP2S6
Dinesh Upreti, Rabindra Basnet, M. M. Sharma, Santosh Karki Chhetri, Gokul Acharya, Md Rafique Un Nabi, Josh Sakon, Mourad Benamara, Mansour Mortazavi, Jin Hu
Abstract: view full publication here
Antiferromagnetic van der Waals-type M2P2X6 compounds provide a versatile material platform for studying 2D magnetism and relevant phenomena. Establishing ferromagnetism in 2D materials is technologically valuable. Though magnetism is generally tunable via a chemical way, it is challenging to induce ferromagnetism with isovalent chalcogen and bimetallic substitutions in M2P2X6. Here, we report co-substitution of Cu1+ and Cr3+ for Ni2+ in Ni2P2S6, creating CuxNi2(1-x)CrxP2S6 medium-entropy alloys spanning a full substitution range (x = 0 to 1). Such substitution strategy leads to a unique evolution in crystal structure and magnetic phases that are distinct from traditional isovalent bimetallic doping, with Cu and Cr co-substitution enhancing ferromagnetic correlations and generating a weak ferromagnetic phase in intermediate compositions. This aliovalent substitution strategy offers a universal approach for tuning layered magnetism in antiferromagnetic systems, which along with the potential for light-matter interaction and high-temperature ferroelectricity, can enable multifunctional device applications.
Ambipolar Transport in Polycrystalline GeSn Transistors for Complementary Metal-Oxide-Semiconductor Applications
Priyanka Petluru; Christopher R. Allemang; Shang Liu; Jifeng Liu; Tzu-Ming Lu
Abstract: view full publication here
Group-IV alloy GeSn is a promising material for electronic and optoelectronic applications due to its compatibility with both Si substrates and established Si fabrication processes. This study focuses on polycrystalline GeSn (10% Sn), which offers a cost-effective, large-area, and versatile alternative to epitaxial GeSn. We demonstrate ambipolar transport behavior in polycrystalline GeSn thin film transistors, achieving electron and hole field-effect mobilities reaching up to 0.05 cm 2 /Vs and 2.05 cm 2 /Vs, respectively. Through temperature-dependent analysis, we elucidate the underlying mechanism of this phenomenon, which we attribute to quantum tunneling between the Schottky barrier contact and the channel, as well as potential barriers between the grain boundaries of this polycrystalline film, thereby advancing the understanding of polycrystalline GeSn’s electrical properties. This work highlights the potential of ambipolar transport as a technique to employ towards the development of GeSn complementary metal-oxide-semiconductor field-effect transistors, promising to simplify and reduce the cost of GeSn manufacturing processes for edge computing and sensing applications.
Study of phase decoherence in GeSn (8%) through measurements of the weak antilocalization effect
Adelaide Bradicich, Priyanka Petluru, Shiva Davari, Haochen Zhao, Siddhant Gangwal, Chia-You Liu, Dragica Vasileska, Yuping Zeng, Hugh Churchill, Jiun-Yun Li, Michael P. Lilly, Tzu-Ming Lu
Abstract: view full publication here
Alloying germanium with tin offers a means to modulate germanium’s electronic structure, enabling a greater degree of control over quantum properties such as the retention of the phase or spin of the electron wave. However, the extent to which the presence of high dopant concentrations in GeSn alters these quantum behaviors is poorly understood. Here, we investigate the role of dopant concentrations on phase coherence through measurements of the weak antilocalization (WAL) effect at temperatures between 30 mK and 10 K in p-GeSn (8%) thin films, which were doped to a series of carrier densities on the order of 1012cm−2. Phase coherence and spin–orbit lengths were extracted from the magnetoconductivities using the 2D Hikami–Larkin–Nagaoka model. Phase coherence lengths peaked at 577, 593, and 737nm for the low-, mid-, and high-density samples, while upper limits on the spin–orbit lengths of less than 25nm were relatively independent of carrier density and temperature. The phase coherence lengths increased as the temperature decreased but changed only minimally with carrier density, contrary to common models of temperature-dependent inelastic scattering. Saturation of the phase coherence lengths occurred below 600mK. Based on these findings, intrinsically generated inelastic scattering mechanisms such as two-level systems or impurity band scattering likely contribute to phase decoherence in these alloys. Our results provide insight into the inelastic scattering mechanisms of GeSn, while suggesting a need for further investigation into phase decoherence mechanisms in doped group-IV alloys.
Type-II superconductivity at 9K in Pb–Bi alloy
N.K. Karn, Kapil Kumar, Naveen Kumar, Yogesh Kumar, M.M. Sharma , Jin Hu , V.P.S. Awana
Abstract: view full publication here
In the present work, we report the synthesis of Pb–Bi alloy with enhanced Tc of up to 9K, which is higher than that of Pb. The alloy is synthesized via a solid-state reaction route in the vacuum-encapsulated quartz tube at 700°C in an automated furnace. The synthesized sample is characterized by X-ray Diffraction(XRD) and Energy dispersive X-ray analysis(EDAX) for its phase purity and elemental composition. Rietveld refinement of XRD reveals that the end product is a majority hexagonal Pb7Bi3, with minor rhombohedral Bi. The electronic transport measurement shows metallic behavior with the Debye temperature of 108K and a superconductivity transition temperature (Tc) below 9K, which is the maximum to date for any reported Pb–Bi alloy, Pb or Bi at ambient pressure. Partial substitution of Bi at the Pb site may modify the free density of electronic states within the BCS model to attain the optimum Tc, which is higher by around 2K from the reported Tc of Pb. The superconductor phase diagram derived from magneto-transport measurements reveals that the synthesized alloy is a conventional superconductor with an upper critical field (Hc2) of 3.9 T, which lies well within the Pauli paramagnetic limit. The magnetization measurements carried out following ZFC(Zero Field Cool) protocols infer that the synthesized alloy is a bulk superconductor below 9K. The isothermal M-H(Magnetization vs. Field) measurements performed below Tc establish it as a type-II superconductor. The specific heat capacity measurements show that the Pb–Bi alloy is a strongly coupled bulk superconductor below around 9K with possibly two superconducting gaps.
Understanding and tuning magnetism in van der Waals-type metal thiophosphates
Rabindra Basnet, Jin Hu
Abstract: view full publication here
Over the past two decades, significant progress in two-dimensional (2D) materials has invigorated research in condensed matter and material physics in low dimensions. While traditionally studied in three-dimensional systems, magnetism has now been extended to the 2D realm. Recent breakthroughs in 2D magnetism have attracted substantial interest from the scientific community, owing to the stable magnetic order achievable in atomically thin layers of the van der Waals (vdW)-type layered magnetic materials. These advances offer an exciting platform for investigating related phenomena in low dimensions and hold promise for spintronic applications. Consequently, vdW magnetic materials with tunable magnetism have attracted significant attention. Specifically, antiferromagnetic metal thiophosphates MPX3 (M = transition metal, P = phosphorus, X = chalcogen) have been investigated extensively. These materials exhibit long-range magnetic order spanning from bulk to the 2D limit. The magnetism in MPX3 arises from localized moments associated with transition metal ions, making it tunable via substitutions and intercalations. In this review, we focus on such tuning by providing a comprehensive summary of various metal- and chalcogen-substitution and intercalation studies, along with the mechanism of magnetism modulation, and a perspective on the development of this emergent material family.
Correlated 4D-STEM and EDS for the classification of fine Beta-precipitates in aluminum alloy AA 6063-T6
L.M. Vogl, P. Schweizer, J. Donohue, A.M. Minor
Abstract: view full publication here
Tuning the properties of aluminum alloys AA 6063-T6 involves artificial aging to induce precipitate formation, particularly β’’ and β’ phases. Previous characterization challenges due to their similar appearance are addressed here by correlating 4D scanning transmission electron microscopy (4DSTEM) and energy-dispersive spectroscopy (EDS) mapping. This approach allows us to analyze the structure and composition of precipitates individually, overcoming limitations of conventional imaging and structural analysis techniques when the precipitates appear simultaneously, as is often the case. We present detailed characterizations of needle-shaped Beta precipitates, revealing distinct diffraction patterns (DPs) and compositional differences. The method’s applicability extends beyond aluminum alloys, offering a promising strategy for complex composite material characterization with multimodal scanning transmission electron microscopy (STEM) techniques.
Unraveling Interdiffusion Phenomena and the Role of Nanoscale Diffusion Barriers in the Copper–Gold System
Lilian M. Vogl, Peter Schweizer, Xavier Maeder, Ivo Utke, Andrew M. Minor, and Johann Michler
Abstract: view full publication here
Diffusion is one of the most fundamental concepts in materials science, playing a pivotal role in materials synthesis, forming, and degradation. Of particular importance is solid state interdiffusion of metals which defines the usable parameter space for material combinations in the form of alloys. This parameter space can be explored on the macroscopic scale by using diffusion couples. However, this method reaches its limit when going to low temperatures, small scales, and when testing ultrathin diffusion barriers. Therefore, this work transfers the principle of the diffusion couples to small scales by using core–shell nanowires and in situ heating. This allows us to delve into the interdiffusion dynamics of copper and gold, revealing the interplay between diffusion and the disorder–order phase transition. Our in situ TEM experiments in combination with chemical mapping reveal the interdiffusion coefficients of Cu and Au at low temperatures and highlight the impact of ordering processes on the diffusion behavior. The formation of ordered domains within the solid-solution is examined using high-resolution imaging and nanodiffraction including strain mapping. In addition, we examine the effectiveness of ultrathin Al2O3 barrier layers to control interdiffusion of the diffusion couple. Our findings indicate that a 5 nm thick layer serves as an efficient diffusion barrier. This research provides valuable insights into the interdiffusion behavior of Cu and Au on the nanoscale, offering potential applications in the development of miniaturized integrated circuits and nanodevices.
Modeling and Simulation of Electrostatics of Ge1-xSnx Layers Grown on Ge Substrates
Siddhant Gangwal, Shunda Chen, Tianshu Li, Tzu-Ming Lu, and Dragica Vasileska
Abstract: view full publication here
This work introduces a comprehensive simulation tool that provides a robust 1D Schrödinger – Poisson solver for modeling the electrostatics of heterostructures with an arbitrary number of layers, and non-uniform doping profiles along with the treatment of partial ionization of dopants at low temperatures. The effective masses are derived from the first-principles calculations. The solver is used to characterize three Ge 1-x Sn x /Ge heterostructures with non-uniform doping profiles and determine the subband structure at various temperatures. The simulation results of the sheet carrier densities show excellent agreement with the experimentally extracted data, thus demonstrating the capabilities of the solver.
Composition Quantification of SiGeSn Alloys Through Time-of-Flight Secondary Ion Mass Spectrometry: Calibration Methodologies and Validation With Atom Probe Tomography
Haochen Zhao, Shang Liu, Suho Park, Xu Feng, Zhaoquan Zeng, James Kolodzey, Shui-Qing Yu, Jifeng Liu, and Yuping Zeng
Abstract: view full publication here
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is a powerful technique for elemental compositional analysis and depth profiling of materials. However, it encounters the problem of matrix effects that hinder its application. In this work, we introduce a pioneering ToF-SIMS calibration method tailored for SixGeySnz ternary alloys. SixGe1-x and Ge1-zSnz binary alloys with known compositions are used as calibration reference samples. Through a systematic SIMS quantification study of SiGe and GeSn binary alloys, we unveil a linear correlation between secondary ion intensity ratio and composition ratio for both SiGe and GeSn binary alloys, effectively mitigating the matrix effects. Extracted relative sensitivity factor (RSF) value from SixGe1-x (0.07<x<0.83) and Ge1-zSnz (0.066<z<0.183) binary alloys are subsequently applied to those of SixGeySnz (0.011<x<0.113, 0.863<y<0.935 and 0.023<z<0.103) ternary alloys for elemental compositions quantification. These values are cross-checked by Atom Probe Tomography (APT) analysis, an indication of the great accuracy and reliability of as-developed ToF-SIMS calibration process. The proposed method and its reference sample selection strategy in this work provide a low-cost as well as simple-to-follow calibration route for SiGeSn composition analysis, thus driving the development of next-generation multifunctional SiGeSn-related semiconductor devices.
Transferable GeSn ribbon photodetectors for high-speed short-wave infrared photonic applications
Haochen Zhao; Suho Park; Guangyang Lin; Yuying Zhang; Tuofu Zhama; Chandan Samanta; Lorry Chang; Xiaofeng Zhu; Xu Feng; Kevin O. Díaz-Aponte ; Lin Cong; Yuping Zeng
Abstract: view full publication here
We experimentally demonstrate a low-cost transfer process of GeSn ribbons to insulating substrates for short-wave infrared (SWIR) sensing/imaging applications. By releasing the original compressive GeSn layer to nearly fully relaxed state GeSn ribbons, the room-temperature spectral response of the photodetector is further extended to 3.2 μm, which can cover the entire SWIR range. Compared with the as-grown GeSn reference photodetectors, the fabricated GeSn ribbon photodetectors have a fivefold improvement in the light-to-dark current ratio, which can improve the detectivity for high-performance photodetection. The transient performance of a GeSn ribbon photodetector is investigated with a rise time of about 40 μs, which exceeds the response time of most GeSn (Ge)-related devices. In addition, this transfer process can be applied on various substrates, making it a versatile technology that can be used for various applications ranging from optoelectronics to large-area electronics. These results provide insightful guidance for the development of low-cost and high-speed SWIR photodetectors based on Sn-containing group IV low-dimensional structures.
Magneto-transport study on Sn-rich Sn1−xGex thin films enabled by CdTe buffer layer
Rabindra Basnet; Dinesh Upreti; Tyler T. McCarthy; Zheng Ju; Allison M. McMinn; M. M. Sharma; Yong-Hang Zhang; Jin Hu
Abstract: view full publication here
α-Sn, generally known as gray tin, has attracted significant scientific interest due to its potential to host novel topological phases. Studying the transport properties of α-Sn thin films grown on the InSb substrate has been challenging, as the InSb substrate also significantly contributes to the transport properties. In this article, we report a novel approach to epitaxially grow α-Sn thin films on an InSb substrate with a resistive buffer layer of CdTe. Thin films of α−Sn1−xGex (x = 0, 0.025) alloy of 15 nm thickness have been grown using molecular beam epitaxy. The high quality of the samples has been determined through high-resolution x-ray diffraction. The CdTe buffer layer has high resistance and acts as an insulating virtual substrate, which significantly suppresses contribution from InSb. Magnetotransport measurements show clear Shubnikov–de Hass oscillations in α−Sn1−xGex (x = 0, 0.025) thin films. A change in oscillation frequency is observed upon alloying with Ge, implying a modification in the electronic structure and demonstrating the effectiveness of the CdTe buffer layer approach. This work provides a new approach that enables the electronic transport characterization of the α−Sn1−xGex alloy system, which enables the study of the topological quantum states using electronic transport and their device applications.
Unraveling the Highly Plastic Behavior of ALD-Aluminum Oxide Encapsulations by Small-Scale Tensile Testing
Lilian M. Vogl, Peter Schweizer, Andrew M. Minor, Johann Michler, Ivo Utke
Abstract: view full publication here
We present a study directly measuring the electron-beam-induced plasticity of amorphous Al2O3 coatings. Core–shell nanostructures are employed as small-scale model systems for two-dimensional coatings made by atomic layer deposition (ALD). Copper nanowires (NWs) are used as substrates for ALD deposition, representing a model system for interconnects commonly found in integrated circuits. Experiments are performed in situ in a transmission electron microscope (TEM) and further analyzed with electron energy loss spectroscopy (EELS). Our in situ TEM tensile experiments reveal the highly plastic behavior of the ALD shell, which withstands a maximum strain of 188%. Comparable samples under beam-off conditions show a brittle fracture, which underlines the effect of electron irradiation. The electron-beam-activated bond switching within the amorphous network enables compensation of the applied tensile strain, leading to viscous flow. By incorporating an intermediate nanocrystalline layer within the Al2O3 shell, the plasticity is suppressed and brittle fracture occurs. This work directly demonstrates the tuning of mechanical properties in amorphous ALD structures through electron irradiation.
Group IV topological quantum alloy and the role of short-range order: the case of Ge-rich Ge1–xPbx
Yunfan Liang, Shunda Chen, Xiaochen Jin, Damien West, Shui-Qing Yu, Tianshu Li & Shengbai Zhang
Abstract: view full publication here
Despite the explosion of interest in topological materials over the last decades, their applications remain limited due to challenges in growth and incorporation with today’s microelectronics. As a potential bridge to close this gap, we investigate the group-IV alloy Ge1–xPbx, in the Ge-rich condition using density functional theory and show that relatively low concentrations of Pb (~9.4%) can lead to a topological phase transition. Furthermore, the calculation of the Z2 invariant for both the random alloy and the alloy with short-range order (SRO) indicate that the topological phase of the material can be directly modified by the degree of SRO. These findings are understood in terms of local structural relaxation, which decreases the bandgap in the random alloy. However, in the SRO case, the mutual avoidance of Pb leads to minimal structural relaxation, alleviating strain. Our findings not only highlight the emerging importance of SRO in alloy properties but also indicate the possibility of constructing topological interfaces between materials of identical composition (and nominally identical structure). Moreover, they uncover a viable avenue toward the monolithic integration of quantum materials with today’s semiconductor industry.
Field-induced spin polarization in the lightly Cr-substituted layered antiferromagnet NiPS3
Rabindra Basnet, Dinesh Upreti, Taksh Patel, Santosh Karki Chhetri, Gokul Acharya, Md Rafique Un Nabi, M. M. Sharma, Josh Sakon, Mansour Mortazavi, and Jin Hu
Abstract: view full publication here
Tuning magnetic properties in layered magnets is an important route to realize novel phenomenon related to two-dimensional (2D) magnetism. Recently, tuning antiferromagnetic (AFM) properties through substitution and intercalation techniques has been widely studied in 𝑀P𝑋3 compounds. Interesting phenomena, such as diverse AFM structures and even the signatures of ferrimagnetism, have been reported. However, long-range ferromagnetic (FM) ordering has remained elusive. In this work, we explored the magnetic properties of the Cr-substituted NiPS3. We found that Cr substitution is extremely efficient in controlling spin orientation in NiPS3. Our study reveals a field-induced spin polarization in lightly (9%) Cr-substituted NiPS3, which is likely attributed to the attenuation of AFM interactions and magnetic anisotropy due to Cr doping. Our work provides a possible strategy to achieve FM phase in AFM 𝑀P𝑋3, which could be useful for investigating 2D magnetism as well as potential device applications.
Evolution of magnetism in the magnetic topological semimetal NdSb𝑥Te2−𝑥+𝛿
Santosh Karki Chhetri, Rabindra Basnet, Jian Wang, Krishna Pandey, Gokul Acharya, Md Rafique Un Nabi, Dinesh Upreti, Josh Sakon, Mansour Mortazavi, and Jin Hu
Abstract: view full publication here
Magnetic topological semimetals LnSbTe (Ln = lanthanide) have attracted intensive attention because of the presence of interplay between magnetism, topological, and electron correlations depending on the choices of magnetic Ln elements. Recently, varying Sb-Te composition has been found to effectively control the electronic and magnetic states in LnSbxTe2−x. With this motivation, we report the evolution of magnetic properties with Sb-Te substitution in NdSbxTe2−x+δ, (0⩽ x ⩽ 1). Our work reveals the interesting nonmonotonic change in magnetic ordering temperature with varying composition stoichiometry. In addition, reducing the Sb content x drives the reorientation of moments from in-plane (ab-plane) to out-of-plane (c-axis) direction that results in the distinct magnetic structures for two end compounds NdTe2 (x = 0) and NdSbTe (x = 1). Furthermore, the moment orientation in NdSbxTe2−x+δ is also found to be strongly tunable upon application of a weak magnetic field, leading to rich magnetic phases depending on the composition stoichiometry, temperature, and magnetic field. Such strong tuning of magnetism in this material establishes it as a promising platform for investigating tunable topological states and correlated topological physics.
Molecular beam epitaxy growth and characterization of GePb alloys
Tyler T. McCarthy, Allison M. McMinn, Xiaoyang Liu, Razine Hossain, Xin Qi, Zheng Ju, Mark Mangus, Shui-Qing Yu, Yong-Hang Zhang
Abstract: view full publication here
Pb based group-IV alloys such as GePb have been gaining interest as a potential alternative for infrared detectors, quantum materials, and high-speed electronic devices. Challenges remain in their growth due to the extremely low solid solubility of Pb in the Ge–Pb system. This paper reports molecular beam epitaxy growth of GePb alloy thin films on Ge(100) substrates. Effusion cells of Ge and Pb are used to control the flux ratio independently. The optimal substrate temperature is found to be near the thermocouple temperature of 300 °C based on the characterization of the grown films using high-resolution x-ray diffraction. A large change in the Ge:Pb beam equivalent pressure ratio from 10:1 to 1:1 results in only a minimal increase of the Pb composition from 0.74% to 2.84% as estimated from Raman spectroscopy and Rutherford backscattering spectrometry. Scanning electron microscopy images show a large volume of Pb islands on the surface that form into either long trapezoidal rods or uniform droplets, with increasing Pb flux and growth time the density of Pb islands increased.
Intricate short-range order in GeSn alloys revealed by atomistic simulations with highly accurate and efficient machine-learning potentials
Shunda Chen, Xiaochen Jin, Wanyu Zhao, and Tianshu Li
Abstract: view full publication here
GeSn alloys hold promise for silicon-compatible integrated applications in electronics, photonics, and topological quantum devices. However, understanding their intricate structures using density functional theory (DFT) calculations is hindered by spatiotemporal constraints. To overcome this limitation, we develop highly accurate and efficient machine-learning interatomic potentials based on a neuroevolution potential approach with farthest point sampling on a comprehensive DFT data set. The application of the developed machine-learning potential in large-scale atomistic simulations bridges the spatiotemporal gap between modeling and advanced characterizations, and facilitates the discovery of structural intricacies in GeSn alloys. Through extensive statistical sampling, we identify a type of short-range order (SRO) that is distinguished by both its structural signature and electronic band gap from the SRO structure previously predicted. Modeling based on a large simulation cell reveals the coexistence of nano SRO domains with various degrees of ordering, demonstrating a complex spatial heterogeneity of SRO structure. Our study not only reinforces the significance of fine-level structural information in alloys, but it also constitutes an effective framework for exploring SRO in a broad range of complex alloys based on highly accurate and effective machine-learning potentials.
Modeling Si/SiGe quantum dot variability induced by interface disorder reconstructed from multiperspective microscopy
Luis Fabián Peña, Justine C. Koepke, J. Houston Dycus, Andrew
Mounce, Andrew D. Baczewski, N. Tobias Jacobson, and Ezra Bussmann
Abstract: view full publication here
SiGe heteroepitaxial growth yields pristine host material for quantum dot qubits, but residual interface disorder can lead to qubit-to-qubit variability that might pose an obstacle to reliable SiGe-based quantum computing. By convolving data from scanning tunneling microscopy and high-angle annular dark field scanning transmission electron microscopy, we reconstruct 3D interfacial atomic structure and employ an atomistic multi-valley effective mass theory to quantify qubit spectral variability. The results indicate (1) appreciable valley splitting (VS) variability of ~50% owing to alloy disorder and (2) roughness-induced double-dot detuning bias energy variability of order 1–10 meV depending on well thickness. For measured intermixing, atomic steps have negligible influence on VS, and uncorrelated roughness causes spatially fluctuating energy biases in double-dot detunings potentially incorrectly attributed to charge disorder. Our approach yields atomic structure spanning orders of magnitude larger areas than post-growth microscopy or tomography alone, enabling more holistic predictions of disorder-induced qubit variability.
Composition and strain effects on Raman vibrational modes of GeSn alloys with Sn contents up to 31 % grown by low-temperature molecular beam epitaxy
Haochen Zhao, Guangyang Lin, Yuying Zhang, Suho Park, Ryan Hickey, Tuofu Zhama,
Peng Cui, Sagar Sourav, James Kolodzey, Yuping Zeng
Abstract: view full publication here
We study the behavior of Ge–Ge, Ge–Sn, and Sn–Sn vibrational modes in GeSn semiconductors with Raman Spectroscopy. Raman spectroscopy is a rapid, nanoscale spatial resolution and non-destructive approach to accurately determine the composition and strain information of GeSn, and it is thus crucial for the material investigation and device application of GeSn alloys. By using several excitation wavelengths at 532, 633 and 785 nm on a set of fully strained and fully relaxed Ge1-xSnx layers with the Sn composition in the range 2.3 % < xSn < 31 %, all modes are identified and their evolution as a function of strain and Sn content is determined. The Raman shifts of all vibrational modes are found to exhibit the same function versus the composition xSn and in-plane strain ε//, Δω = ωGeSn – ωGe = axSn + bε//, where a is the Sn composition factor and b is the strain shift factor. In addition, the Ge–Sn mode intensity increases with Sn content. It is discovered for the first time that the Sn composition determined from the plot of the intensity ratio of the Ge–Sn mode over the Ge–Ge mode as a function of Sn composition at 785 nm excitation agrees well with that the X-ray Diffraction (XRD) Reciprocal Space Mapping (RSM), offering a novel approach for determining Sn content by Raman spectroscopy.
Strain-Mediated Sn Incorporation and Segregation in Compositionally Graded Ge1−xSnx Epilayers Grown by MBE at Different Temperatures
Nirosh M. Eldose, Hryhorii Stanchu, Subhashis Das, Ilias Bikmukhametov, Chen Li, Satish Shetty, Yuriy I. Mazur, Shui-Qing Yu, and Gregory J. Salamo
Abstract: view full publication here
We investigated the process of Sn incorporation and surface segregation for compositionally graded Ge1−xSnx epilayers grown on high-quality Ge (001) substrates. The growth resulted in pseudomorphic GeSn layers with a ∼6% maximal Sn fraction at a constant substrate temperature. The maximal fraction of Sn was increased to 9.0% when the growth temperature was continuously lowered while increasing the Sn flux. The analysis of surface droplets and SIMS profiles of elemental composition give evidence of Sn rejection during the growth, potentially associated with a critical energy of elastic strain. The intentional reduction of the coherent strain by decreasing the Sn flux near the sample surface has been shown to trap a higher fraction of Sn in the Ge1−xSnx layer and lower surface segregation. Our results demonstrate that strain relief by misfit dislocations in the compositionally graded layer is inhibited, which leads to Sn segregation. Specifically, the compressive strain in the graded Ge1−xSnx epilayer is effectively “zero” near the interface with the Ge substrate and increases up to about −1.5 × 10−2 near the surface. Thus, although the nucleation of a dislocation may reduce the compressive strain for the top region of the epilayer, it is not beneficial for the bottom region.
The role of local atomic short-range order distribution in alloys: Why it matters in Si-Ge-Sn alloys
Xiaochen Jin, Shunda Chen, Christopher Lemkan, and Tianshu Li
Abstract: view full publication here
Short-range order (SRO) in alloys refers to deviations from a perfectly random distribution of atoms in lattice sites within a short distance. Conventionally, the degree of the deviations has been quantified using an average SRO parameter, but such a coarse-grained description does not reflect how the deviations occur at a finer level. Here we show the distribution of the local atomic SRO parameter, which describes the occurring frequency of a local structural motif, carries the crucial information for both structures and properties in Si-Ge-Sn alloy system. This is demonstrated through the fact that distinct SRO structures can exhibit the same average SRO parameter but
very different distributions and disparate electronic structures. By deliberately creating special structures that explicitly match the structural information at different levels, we show the distribution of local atomic SRO parameters contain critical structural features that are missing in the average SRO parameter but can substantially contribute to material’s properties. Our finding thus calls for the need for considering the finer structural details to effectively describe alloys’ structures and properties.
Local ordering in Ge/Ge–Sn semiconductor alloy core/shell nanowires revealed by extended x-ray absorption fine structure (EXAFS)
J Zach Lentz, JC Woicik, Matthew Bergschneider, Ryan Davis, Apurva Mehta, Kyeongjae Cho, Paul C McIntyre
Abstract: view full publication here
Short-range atomic order in semiconductor alloys is a relatively unexplored topic that may promote design of new materials with unexpected properties. Here, local atomic ordering is investigated in Ge–Sn alloys, a group-IV system that is attractive for its enhanced optoelectronic properties achievable via a direct gap for Sn concentrations exceeding ≈10 at. %. The substantial misfit strain imposed on Ge–Sn thin films during growth on bulk Si or Ge substrates can induce defect formation; however, misfit strain can be accommodated by growing Ge–Sn alloy films on Ge nanowires, which effectively act as elastically compliant substrates. In this work, Ge core/Ge1−xSnx (x ≈ 0.1) shell nanowires were characterized with extended x-ray absorption fine structure (EXAFS) to elucidate their local atomic environment. Simultaneous fitting of high-quality EXAFS data collected at both the Ge K-edge and the Sn K-edge reveals a large (≈ 40%) deficiency of Sn in the first coordination shell around a Sn atom relative to a random alloy, thereby providing the first direct experimental evidence of significant short-range order in this semiconductor alloy system. Comparison of path length data from the EXAFS measurements with density functional theory simulations provides alloy atomic structures consistent with this conclusion.
Improving the short-wave infrared response of strained GeSn/Ge multiple quantum wells by rapid thermal annealing
Haochen Zhao, Guangyang Lin, Chaoya Han, Ryan Hickey, Tuofu Zhama, Peng Cui,
Tienna Deroy, Xu Feng, Chaoying Ni, Yuping Zeng
Abstract: view full publication here
In this work, the evolution of structural, optical and optoelectronic properties of coherently strained Ge0.883Sn0.117/Ge multiple quantum wells (MQWs) grown by molecular beam epitaxy under rapid thermal annealing (RTA) is systematically investigated. The MQW structure remains fully-strained state with RTA at 400 ◦C or below and disrupts at higher annealing temperatures due to Sn segregation and interdiffusion of Ge and Sn atoms. The GeSn well layers exhibit the strongest absorption in 2.0–2.4 μm after annealing at 400 ◦C and become transparent above 1.8 μm after RTA at 600 ◦C or beyond due to serve Sn segregation. Owing to improved crystal quality after RTA at 400 ◦C, the dark current of the fabricated metal-semiconductor-metal photodetector is effectively lowered by more than two times. Additionally, the responsivities at 1.55 and 2.0 μm are improved by 4.15 and 3.78 folds, respectively, compared to those of the as-grown sample. The results can be an insightful guidance for the development of high-performance short-wave infrared photonic devices based on Sn-containing group-IV low-dimensional structures.