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,1 Shunda Chen,1 Christopher Lemkan,1 and Tianshu Li1, ∗
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.