Управление потоками заряда и тепла в наноразмерных квазиодномерных проводниках тема диссертации и автореферата по ВАК РФ 01.04.07, кандидат наук Бубис Антон Владимирович

• Бубис Антон Владимирович
• кандидат наук
• 2022

Оглавление диссертации кандидат наук Бубис Антон Владимирович

Contents

1 Introduction

2 Experimental methods

2.1 Fabrication of HgTe QW based devices

2.1.1 Mesa etching

2.1.2 Contact metalization

2.1.3 Gate

2.2 Fabrication of InAs NW based devices

2.3 Shot noise measurements

2.3.1 Langevin approach for RF circuit

2.3.2 Calibration

2.3.3 Nonequilibrium noise measurement

2.3.4 Noise spectrum measurement

3 Chitosan-based lithography for delicate materials

3.1 Resist development principle

3.2 Optimization of resist performance

3.2.1 Sensitivity

3.2.2 Contrast

3.2.3 Resist residues and lift-off

3.3 Recipes

3.4 Organic semiconductors patterning

3.5 Electrical devices

3.6 Conclusion

4 Localization of helical edges in HgTe QW

4.1 Processing influence on 2DEG

4.2 Nonlocal transport

4.3 Localization in the absence of magnetic field

4.3.1 Low-temperature localization

4.3.2 Theoretical outlook

4.3.3 Magnetic field behavior

4.4 Shot noise

4.5 Conclusion

5 Heat transport in proximitized InAs nanowires

5.1 Theory

5.1.1 Semiclassical model

5.1.2 Electronic energy distributions

5.1.3 A suppression due to nonequilibrium EED

5.1.4 Shot noise

5.2 Shot noise measurement

5.3 Charge neutrality of transmitted quasiparticles

5.4 Conclusion

6 Summary and outlook

7 Acknowledgments

Введение диссертации (часть автореферата) на тему «Управление потоками заряда и тепла в наноразмерных квазиодномерных проводниках»

1 Introduction

Topic relevance One of the most promising research directions of modern condensed matter physics is the investigation of the topological aspects of electronic band theory. Unlike the classical Bloch theory of crystals, here, the interconnection of the spin and spatial degrees of freedom originating from strong spin-orbit interaction plays the central role. It turns out that this relationship takes place in entire classes of new materials, for example, in topological insulators.1,2 In these materials, tuning of the Fermi level in the bulk energy gap does not lead to vanishing of electrical conductivity due to the presence of gapless helical states. These states emerge on the boundary between topological and trivial insulators (surface in the three-dimensional case or edge in the two-dimensional case), and the spin and the momentum of such states are rigidly locked.

A fundamental property of the helical states is the complete backscattering suppression, which has a purely quantum origin and is also called topological protection. The most impressive manifestation of topologically protected states occurs in a one-dimensional case. They provide ballistic transfer of a spin-polarized electric current along the edge of a quantum spin Hall insulator.3 In addition to the edge states of the spin Hall insulator, helical states can also be realized in pure semiconductor nanowires with strong spin-orbit interaction placed into a high parallel to a nanowire magnetic field.4

The helical states are of great interest, particularly due to predictions for Majorana zero modes (MZMs) emergence valuable for fault-tolerant quantum computing. Among several proposals for MZM implementations in the real systems, proximitized semiconducting nanowires with strong spin-orbit coupling and topological insulator-superconductor hybrids got the most attention from the scientific community. Nowadays, still, there is no unambiguous observation of all signatures of Majorana zero modes in a single experiment. Thus, the experimental research in this area mainly focuses on the fabrication advances to obtain better devices (mostly growth techniques) and on the novel detection schemes for topological phase transition.

This work is dedicated to the most widespread materials used in Majorana research: topological insulators and proximitized nanowires. There was no goal to observe signatures of MZM in the experiment but to investigate the electronic transport in the helical edges and to implement a new measurement technique in the semiconductor-superconductor hybrid structures. A significant part of this work is dedicated to the development of fabrication techniques for delicate materials. The thesis is organized as follows:

• Chapter 2 is dedicated to description of fabrication and measurement used in this study. The implementation of lithographic methods for fabrication of HgTe quantum wells (QW) and InAs nanowires (NW) based devices is discussed in detail. Also shot noise and noise spectrum measurement procedure is described thoroughly.

• Chapter 3 is devoted to the advanced lithographic technique developed for delicate objects. It is proposed to utilize chitosan derivatives (CD) - bio-inspired and water-soluble polysaccharides as an e-beam and deep-ultraviolet resist. CD exhibits molecular scissoring under irradiation, similar to commonly used poly(methyl methacrylate) (PMMA). In

the proposed technology, the key feature is the chelation reaction, which allowed achieving clean enough development and, thus, lift-off capability while keeping the fabrication routine gentle and water-based.

• Chapter 4 is devoted to the first and well-studied 2D topological insulator, HgTe QW. Its bulk gap is quite large, « 30meV, so the temperature of 4.2 K (liquid He) is already sufficient to get rid of bulk conduction, provided the Fermi level is tuned to the bulk gap. Of great interest is the main ingredient of topological protection - time-reversal symmetry (TRS), which protects the helical edges from a single particle backscattering. If TRS is broken, e.g., by an external magnetic field, the helical edges localize due to disorder. This work demonstrates that these edges can localize at low enough temperature even without an external magnetic field. Theoretically, several scenarios are known for such behavior, and in this chapter, they are discussed in view of the experimental observations.

• In Chapter 5 the alternative scheme for detecting topological phase transition is discussed using the example of hybrid semiconductor-superconductor structures based on InAs nanowires. The idea is to measure the heat conductance of the proximitized segment of a nanowire, which acquires a universal value at the point of topological phase transition. Although studied devices cannot exhibit topological phase, they serve as a minimal toy model to study the heat transport in proximitized nanowires. Using the semiclassical approach, it is demonstrated that heat conductance in such devices can be obtained from nonlocal shot noise measurement, which is possible thanks to the charge-heat separation occurring at the normal metal-superconductor (NS) interface.

Aims and objectives

1. Development and tuning of fabrication routine for devices based on HgTe QWs, investigation of the different processing steps influence onto properties of a 2D electron gas. Measurement of the typical conductance against the magnetic field at the charge neutrality point to estimate Luttinger liquid constant, K.

2. Analytical derivation for the current noise spectral density in NSN-devices based on a diffusive nanowire with either grounded central superconducting terminal or floating superconducting island. Calculation of superconducting gap suppression in the floating island geometry due to nonequilibrium electronic energy distribution (EED). Fabrication of InAs NW based devices with central superconducting terminal and normal contacts on both sides.

3. Development of delicate lithographic technique and its basis - water-soluble resist. Investigation of developer properties influence on the development procedure, proof of chelation-dissolution mechanism. Tailoring of lithographic parameters to obtain 100 nm-wide lifted-off metal individual lines.

Key aspects to be defended

1. For the first time, localization of 2D TI edges of 8nm HgTe QW was demonstrated at millikelvin temperatures in the absence of an external magnetic field. The behavior of typical conductance at the charge neutrality point suggests Luttinger liquid constant K « 0.8-0.9, which on its own theoretically does not explain the observed localization. Current noise measurements revealed the Fano factor F « 0.5-0.7, greater than values for diffusive conductors even with strong electron-electron scattering. The latter can point to the disordered nature of the electronic transport through the edge and be the signature of upcoming localization, but still, the main scattering mechanism is unclear.

2. The expressions for local and nonlocal shot noise are obtained analytically for NSN diffusive devices with either grounded central superconducting terminal or floating superconducting island. It is demonstrated that the thermal conductance of the proximitized nanowire segment can be extracted from the nonlocal current noise slope dS/dV. In the layout with the floating superconducting island, the nonequilibrium EED suppresses the superconducting gap for the arbitrary asymmetry between normal nanowire segments, which manifests itself in two stable branches of A(V). Experimentally nonlocal shot noise is measured in InAs NSN devices, and its subgap values can be fitted with a single parameter - thermal conductance. The average charge of a transmitted through proximitized segment quasiparticle is estimated from above using both nonlocal shot noise and nonlocal conductance.

3. Genuine water-based lithographic technique was developed and employed to fabricate devices based on delicate materials. The main lithographic processing liquids are a chitosan derivative (resist), an aqueous solution of transition metal salt (developer), and a solution of a weak acid (remover). The chelation-dissolution competition allows achieving residue-free development and lift-off capability. The best obtained sensitivity is « 130 ^C/cm2 (at 50kV accelerating voltage), the narrowest individual metal line obtained via lift-off is 100nm-wide. This lithographic approach was successfully utilized to fabricate carbon nanotube, organic semiconductor, and porcine brain microtubule-based devices.

Approbation of the results This work results were reported on the following conferences: Interaction between Radiation and Quantum devices (November 2020, Moscow), XXV Symposium "Nanophysics and Nanoelectronics" (March 2021, Nizhny Novgorod). The papers [A1-A4] are published in peer-reviewed scientific journals and [A5] is uploaded to arXiv.

Заключение диссертации по теме «Физика конденсированного состояния», Бубис Антон Владимирович

5.4 Conclusion

In this section, the electronic transport in NSN devices based on diffusive nanowires is considered theoretically and experimentally. Following the semiclassical approach, the analytical expressions for shot noise were obtained in two geometries: the superconducting grounded reservoir and floating superconducting island. It was shown that both of these setups are capable of extraction of energy dependent thermal conductance, Gth(e), of nanowire segment beneath superconductor. Gth(e) is of great importance because it contains information about the proximity effect. The shot noise measurements were conducted in the grounded geometry, and the experimental data perfectly follow all theoretically obtained features.

6 Summary and outlook

The main conclusions of this thesis can be formulated as follows:

1. For the first time, localization of 2D TI edges of 8nm HgTe QW was demonstrated at millikelvin temperatures in the absence of an external magnetic field. The behavior of typical conductance at the charge neutrality point suggests Luttinger liquid constant K « 0.8-0.9, which on its own theoretically does not explain the observed localization. Current noise measurements revealed the Fano factor F « 0.5-0.7, greater than values for diffusive conductors even with strong electron-electron scattering. The latter can point to the disordered nature of the electronic transport through the edge and be the signature of upcoming localization, but still, the main scattering mechanism is unclear.

2. The expressions for local and nonlocal shot noise are obtained analytically for NSN diffusive devices with either grounded central superconducting terminal or floating superconducting island. It is demonstrated that the thermal conductance of the proximitized nanowire segment can be extracted from the nonlocal current noise slope dS/dV. In the layout with the floating superconducting island, the nonequilibrium EED suppresses the superconducting gap for the arbitrary asymmetry between normal nanowire segments, which manifests itself in two stable branches of A(V). Experimentally nonlocal shot noise is measured in InAs NSN devices, and its subgap values can be fitted with a single parameter - thermal conductance. The average charge of a transmitted through proximitized segment quasiparticle is estimated from above using both nonlocal shot noise and nonlocal conductance.

3. Genuine water-based lithographic technique was developed and employed to fabricate devices based on delicate materials. The main lithographic processing liquids are a chitosan derivative (resist), an aqueous solution of transition metal salt (developer), and a solution of a weak acid (remover). The chelation-dissolution competition allows achieving residue-free development and lift-off capability. The best obtained sensitivity is « 130 ^C/cm2 (at 50kV accelerating voltage), the narrowest individual metal line obtained via lift-off is 100nm-wide. This lithographic approach was successfully utilized to fabricate carbon nanotube, organic semiconductor, and porcine brain microtubule-based devices.

The fabrication of hybrid devices based on topological insulators is one of modern condensed matter research milestones. Numerous experiments demonstrate the absence of robust conductance quantization in the QSH effect, which motivated theoretical investigations of the possible origin for such behavior. Several of the considered scattering mechanisms were predicted to result in localization of the helical edge states without external magnetic field, which

was observed experimentally in 8 nm HgTe QW. Although the unambiguous reason for the observed localization was not found, the interaction of the helical states is most probably one of the prerequisites. There are several ways to investigate the interaction of the helical edges (Luttinger liquid parameter K). The first approach is to implement tunnel junction and extract exponent from power-law fits of Zsd-Vsd curves. The second one valuable for HgTe quantum wells is measurements of conductance and current correlations in corner junctions. Such experiments can shed light on the strength of e-e interaction in the edges and thus better describe the observed localization.

Semiconducting nanowire-superconductor hybrids deserved huge attention because of the proposed MZMs emergence in such systems. The research frontier in this area is split into two parts: the advances of nanowire synthesis and the novel methods for detecting topological phase transition. One of these methods is proposed earlier measurement of the thermal conductance of the proximitized nanowire segment. The application of shot noise allowed us to conduct such measurements on the model NSN diffusive InAs nanowire-based system. Also, the theoretical study demonstrates the possibility of utilizing different device layouts for thermal conductance extraction. Altogether this approach opens a new pathway for MZM detection, and it definitely deserves employment in the Majorana device.

The aforementioned materials HgTe QWs and some semiconducting nanowires (e.g., InSb or InAsSb) cannot withstand the common nanofabrication processing and require a low temperature of all the processes, including e-beam lithography. In the case of these materials, it is sufficient to decrease resist bake temperature to minimize damage during EBL. However, many emergent materials are easily damaged by the harsh processing liquids accompanied by conventional lithography. In this work, the genuine water-based resist is developed and employed for several prototype delicate materials. This approach can be implemented further to investigate the most delicate objects like biological and organic samples. Also, the proposed technology can be employed by the microfabrication industry because of its environmental friendliness.

Список литературы диссертационного исследования кандидат наук Бубис Антон Владимирович, 2022 год

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