Periodic Reporting for period 4 - HEMs-DAM (Hybrid Epitaxial Materials for Novel Quantum State Detection and Manipulation)
Período documentado: 2022-02-01 hasta 2022-07-31
The potential for exponential speed up for solving certain problems using a quantum computer renders unparalleled advantage compared to classical computation and gives hope for disruptive technology impact in fields such as chemistry, materials, finance, cryptography and artificial intelligence.
Recent advent in topological superconductivity has initiated research efforts towards realization of a system to create and manipulate a protected quantum state, with prospects of efficient fault tolerant quantum computation. Such quantum states are predicted to emerge at the ends of 1D topological materials and follow non-abelian exchange statistics. As a result, the encoded information becomes protected against decoherence – also known as topological protection.
Such 1D topological superconductors do not exist in nature; however, it is suggested that the topological modes can be realized in synthetic condensed matter platforms. Hybrid semiconductor-superconductor nanowires are prime candidates for such states if they possess certain fundamental properties and fulfill several quality requirements. Developing such material was a main objective of this project.
The robustness of the topology protection relies on the uniformity, which is affected by any disorder. Such disorder may for example lead to local Andreev bound states that mimic signatures of a topological gap and thus can lead to misinterpretations. Therefore, hybrid nanowire systems require (i) a pristine transition/interface region between semi-conductor and superconductor (and possible magnetic components) and (ii) high quality crystalline structure of the semiconductor and superconductor bulk components.
Summarizing, our research efforts were directed towards disorder free hybrid epitaxial crystals, for which we iteratively targeted three areas:
1) synthesizing hybrid materials using crystal methods, such as Vapor Liquid Solid (VLS) epitaxy and implement new methodologies viz. selective area growth (SAG) for new scalable material combinations
2) subsequent structural and electrical characterization of the hybrid-interface using various techniques to delineate the impact of non-uniformities on the electronic properties
3) performing theoretical modeling and simulations to understand and improve the crystal growth dynamics as well as electronic and structural properties, thus closing the development loop
Recent advent in topological superconductivity has initiated research efforts towards realization of a system to create and manipulate a protected quantum state, with prospects of efficient fault tolerant quantum computation. Such quantum states are predicted to emerge at the ends of 1D topological materials and follow non-abelian exchange statistics. As a result, the encoded information becomes protected against decoherence – also known as topological protection.
Such 1D topological superconductors do not exist in nature; however, it is suggested that the topological modes can be realized in synthetic condensed matter platforms. Hybrid semiconductor-superconductor nanowires are prime candidates for such states if they possess certain fundamental properties and fulfill several quality requirements. Developing such material was a main objective of this project.
The robustness of the topology protection relies on the uniformity, which is affected by any disorder. Such disorder may for example lead to local Andreev bound states that mimic signatures of a topological gap and thus can lead to misinterpretations. Therefore, hybrid nanowire systems require (i) a pristine transition/interface region between semi-conductor and superconductor (and possible magnetic components) and (ii) high quality crystalline structure of the semiconductor and superconductor bulk components.
Summarizing, our research efforts were directed towards disorder free hybrid epitaxial crystals, for which we iteratively targeted three areas:
1) synthesizing hybrid materials using crystal methods, such as Vapor Liquid Solid (VLS) epitaxy and implement new methodologies viz. selective area growth (SAG) for new scalable material combinations
2) subsequent structural and electrical characterization of the hybrid-interface using various techniques to delineate the impact of non-uniformities on the electronic properties
3) performing theoretical modeling and simulations to understand and improve the crystal growth dynamics as well as electronic and structural properties, thus closing the development loop
An overview of the results can be divided into 3 material platform categories exploring:
1. single hybrid nanowires grown using VLS growth approach
2. hybrid nanowires grown using novel scalable SAG approach
3. basic interface properties in planar samples (layer by layer)
1) Vapor Liquid Solid (VLS) epitaxy is a useful platform for exploring new material combinations without compromising on quality. Typically, an ‘ex-situ’ lithography step followed by etching is required to pattern the superconductor. With conventional ex-situ/top-down processing it is difficult to avoid etching residues and damaged surfaces, which deteriorates the device’s performance. In our research we contrived a new VLS methodology to create a gate tunable Josephson junction ‘in-situ’ via shadowing (Fig. 1b) which can be sued with a variety of semiconductors e.g. InAs, InSb and InAsxSb1-x and superconductor combinations (Al, Pb, Sn). The research has resulted in more than 25 research papers in high impact journals, and several collaborations with leading measurement groups globally.
2) Selective Area Growth. Despite the popularity of VLS nanowires among researchers looking for Majorana Fermions, this growth platform is not suitable for complex design needed for qubit operations and large-scale manufacturability. Therefore, it became our objective to prioritize a scalable approach, and we turned to a selective area growth (SAG) method wherein nanowires are grown selectively inside lithographically defined trenches. We have progressed considerably and achieved huge success in forming scalable planar networks grown by Molecular Beam Epitaxy, a growth process that traditionally has not been considered as suitable for selective area growth.
The hallmark of the low-disorder transport in the semiconductor nanowires with induced superconductivity is attainment of the ballistic transport regime. Thus, for quantifying the disorder in afore-mentioned nanowires, carrier mobilities (peak transconductance) are measured at low temperatures using field effect devices (Fig. 2 c,d) and Hall-bar devices (Fig. 2 e,f).
As we progressed, we realized that the quality of platform does not meet the requirements of forming clear evidence of such quantum states. So, focus changed to improve the growth process and resulted in 6 publications and many more promising experiments are on the way. As an example, we have successfully grown SAG nanowires (Figure 2b), achieved dislocation free interfaces and significantly improved purity.
3) Single hybrid interfaces. The electronic properties of hetero-interfaces like band-offset, energy level positions of the interface states are of paramount importance for engineering the hybrid nanowire quantum device, where proximity coupling of a semiconductor to a s-wave superconductor induces topological superconductivity in the former. Therefore, we focused our research efforts for determining the band alignment of the semiconductor/superconductor epitaxial interface. The first material system investigated by us was planar InAs/Al. We successfully used angle-resolved photoelectron spectroscopy (ARPES), photon energy dependent core-level spectroscopy, and self-consistent electronic structure calculations to determine the band alignment of the epitaxial interface. Many material combinations (including ferromagnetic components) were grown and analyzed and have given new insight to the electronic structure of some of the most promising structures for topological quantum computing.
1. single hybrid nanowires grown using VLS growth approach
2. hybrid nanowires grown using novel scalable SAG approach
3. basic interface properties in planar samples (layer by layer)
1) Vapor Liquid Solid (VLS) epitaxy is a useful platform for exploring new material combinations without compromising on quality. Typically, an ‘ex-situ’ lithography step followed by etching is required to pattern the superconductor. With conventional ex-situ/top-down processing it is difficult to avoid etching residues and damaged surfaces, which deteriorates the device’s performance. In our research we contrived a new VLS methodology to create a gate tunable Josephson junction ‘in-situ’ via shadowing (Fig. 1b) which can be sued with a variety of semiconductors e.g. InAs, InSb and InAsxSb1-x and superconductor combinations (Al, Pb, Sn). The research has resulted in more than 25 research papers in high impact journals, and several collaborations with leading measurement groups globally.
2) Selective Area Growth. Despite the popularity of VLS nanowires among researchers looking for Majorana Fermions, this growth platform is not suitable for complex design needed for qubit operations and large-scale manufacturability. Therefore, it became our objective to prioritize a scalable approach, and we turned to a selective area growth (SAG) method wherein nanowires are grown selectively inside lithographically defined trenches. We have progressed considerably and achieved huge success in forming scalable planar networks grown by Molecular Beam Epitaxy, a growth process that traditionally has not been considered as suitable for selective area growth.
The hallmark of the low-disorder transport in the semiconductor nanowires with induced superconductivity is attainment of the ballistic transport regime. Thus, for quantifying the disorder in afore-mentioned nanowires, carrier mobilities (peak transconductance) are measured at low temperatures using field effect devices (Fig. 2 c,d) and Hall-bar devices (Fig. 2 e,f).
As we progressed, we realized that the quality of platform does not meet the requirements of forming clear evidence of such quantum states. So, focus changed to improve the growth process and resulted in 6 publications and many more promising experiments are on the way. As an example, we have successfully grown SAG nanowires (Figure 2b), achieved dislocation free interfaces and significantly improved purity.
3) Single hybrid interfaces. The electronic properties of hetero-interfaces like band-offset, energy level positions of the interface states are of paramount importance for engineering the hybrid nanowire quantum device, where proximity coupling of a semiconductor to a s-wave superconductor induces topological superconductivity in the former. Therefore, we focused our research efforts for determining the band alignment of the semiconductor/superconductor epitaxial interface. The first material system investigated by us was planar InAs/Al. We successfully used angle-resolved photoelectron spectroscopy (ARPES), photon energy dependent core-level spectroscopy, and self-consistent electronic structure calculations to determine the band alignment of the epitaxial interface. Many material combinations (including ferromagnetic components) were grown and analyzed and have given new insight to the electronic structure of some of the most promising structures for topological quantum computing.
The progress beyond the state of the art is enumerated below (for details, refer to the previous question)
1. Extraction of the band-offset at the hybrid interfaces, using a novel methodology employing photo emission spectroscopy and core level analysis
2. Establishment of selective area growth (SAG) approach for epitaxial growth of nanowires. The scalability of this approach and development of nanowire networks, renders the VLS nanowire process irrelevant for scalable solutions
3. Successful development of a method using ARPES, for the direct observation of the electronic structure of SAG nanowires ensemble
4. Assessment of the hybrid interface quality based on transport measurements, using junction-less field effect devices
5. Development of an alternate approach comprising of ferro-magnetic insulators to induce Zeeman splitting via magnetic exchange coupling. We successfully demonstrated growth of fully coherent and misfit dislocation free, semiconductor (InAs)– ferromagnetic insulator (EuS) – superconductor (Al) nanowires
6. Crystal growth kinetic model for quantitative control of selective area growth
7. Novel SAG nanowire growth directions, including novel crystal shapes
8. Multiplexing characterization methods to significantly speed up material quality optimization process
Complex device design using the developed SAG platform including semiconductor, superconductor and possibly ferromagnetic phases will enable novel measurement in the years to come for topological protected states.
1. Extraction of the band-offset at the hybrid interfaces, using a novel methodology employing photo emission spectroscopy and core level analysis
2. Establishment of selective area growth (SAG) approach for epitaxial growth of nanowires. The scalability of this approach and development of nanowire networks, renders the VLS nanowire process irrelevant for scalable solutions
3. Successful development of a method using ARPES, for the direct observation of the electronic structure of SAG nanowires ensemble
4. Assessment of the hybrid interface quality based on transport measurements, using junction-less field effect devices
5. Development of an alternate approach comprising of ferro-magnetic insulators to induce Zeeman splitting via magnetic exchange coupling. We successfully demonstrated growth of fully coherent and misfit dislocation free, semiconductor (InAs)– ferromagnetic insulator (EuS) – superconductor (Al) nanowires
6. Crystal growth kinetic model for quantitative control of selective area growth
7. Novel SAG nanowire growth directions, including novel crystal shapes
8. Multiplexing characterization methods to significantly speed up material quality optimization process
Complex device design using the developed SAG platform including semiconductor, superconductor and possibly ferromagnetic phases will enable novel measurement in the years to come for topological protected states.