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Electronic Transport in Inas Quantum Devices

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Electronic Transport in Inas Quantum Devices Research Collection Doctoral Thesis Electronic Transport in InAs Quantum Devices Author(s): Mittag, Christopher Publication Date: 2020 Permanent Link: https://doi.org/10.3929/ethz-b-000408571 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Diss. ETH No. 26509 Electronic Transport in InAs Quantum Devices A thesis submitted to attain the degree of DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zurich) presented by Christopher Mittag M.Sc. in Physics, Technische Universit¨atM¨unchen born on 27.05.1990 citizen of Germany accepted on the recommendation of: Prof. Dr. K. Ensslin, examiner Prof. Dr. T. Ihn, co-examiner Prof. Dr. W. Wegscheider, co-examiner Prof. Dr. D. Loss, co-examiner 2020 Abstract We experimentally investigate electronic transport in two-dimensional electron gases embedded in a semiconductor heterostructure at cryogenic temperatures. The ob- jective of this thesis is the realization of nanostructures that enable the observation of quantum transport phenomena in quantum wells of the compound III-V semicon- ductor InAs. This material is characterized by a narrow band gap, a low effective electron mass, and a strong spin-orbit interaction. The latter feature has generated interest in InAs by providing a potential means of manipulating the spin degree of freedom in quantum dot based quantum computing schemes. Additionally, hybrid structures formed by combining spin-orbit materials such as InAs with superconduc- tors have garnered recent attention for realizing Majorana fermions in a solid state system. These hold the promise of providing a platform for fault tolerant topological quantum computation. A peculiarity of InAs is that, in contrast to most other semiconductors, it does not form a Schottky barrier when brought into contact with a metal. At its surface, the Fermi level is pinned in the conduction band, which causes band bending and electron accumulation. A consequence of this is the infeasibility of typical semicon- ductor microfabrication techniques based on etching, as the carriers accumulated at physical borders cause a parasitic edge conduction that shunts quantum devices formed by electrostatic gates. This has severely limited the availability of InAs two-dimensional electron gases for mesoscopic transport experiments. We study this edge conduction in the context of intending to eliminate it by em- ploying chemical passivation methods during fabrication. However, the application of these procedures did not significantly reverse edge conduction. As a result of this, we introduce a novel sample geometry in which multiple layers of gates electrostati- cally partition the electron gas, and contact these regions separately, thereby avoid- ing the need of etching steps. This geometry completely circumvents the parasitic edge conduction, thus overcoming the technological challenge of realizing quantum devices in InAs two-dimensional electron gases. Applying this technique, we study one-dimensional transport in a gate-defined quantum point contact that features a full pinch-off of the electron gas. We char- acterize its energy levels using finite bias spectroscopy and determine the electron g-factor in an applied magnetic field. Furthermore, the magnetoelectric subband structure and the influence of the coupling potential are analyzed. In an additional experiment, these fully controllable tunnel barriers enable the i formation of a zero-dimensional quantum dot in an InAs quantum well. We observe Coulomb blockade diamonds in the few electron regime and characterize one- and two-electron ground and excited states in a magnetic field. Singlet and triplet states show no avoided crossing, which hints at a vanishing spin-orbit interaction for this particular interplay of crystallographic and magnetic field orientations. Strong cou- pling of the quantum dot to the leads allows the observation of the Kondo effect, whose dependences on temperature, magnetic field, and bias voltage were found to be in line with theoretical predictions. The g-factor determined from the splitting of the Kondo resonance is in agreement with the value extracted from the excited state spectroscopy. Building on these results, we extend the system by coupling two quantum dots to demonstrate a double quantum dot in an InAs two-dimensional electron gas. By tuning the energy levels in each quantum dot individually, using their respective plunger gates, we map out the charge stability diagram. Finite bias triangles emerge at the transitions between regions of stable occupation when a bias voltage is applied between source and drain contacts. Singlet-triplet spin blockade due to the Pauli exclusion principle rectifies the current flow when both quantum dots are occupied by electrons of the same spin projection. At strong interdot coupling, the spin blockade is pronounced even at zero magnetic field, while at weak interdot coupling it is lifted for the resonant tunneling process. When a magnetic field is applied, this leakage current gives rise to a narrow resonance stemming from hyperfine coupling to the nuclear spins of the crystal. ii Zusammenfassung Wir pr¨asentieren experimentelle Untersuchungen des elektronischen Transports in zweidimensionalen Elektronengasen eingebettet in Halbleiterheterostrukturen bei tiefen Temperaturen. Das Ziel dieser Dissertation ist die Realisierung von Nano- strukturen welche es erm¨oglichen Quantentransportph¨anomene in Quantent¨opfen des III-V Verbindungshalbleiters InAs zu beobachten. Entscheidende Merkmale die- ses Materials sind eine kleine Bandlucke,¨ eine geringe effektive Elektronenmasse, sowie eine starke Spin-Bahn-Wechselwirkung. Besonders die letztere Eigenschaft hat Interesse an InAs hervorgerufen, da sie eine M¨oglichkeit der Manipulation des Spinfreiheitsgrads von Elektronen in Quantenpunkten bieten k¨onnte, worauf eine potentielle Implementation eines Quantencomputers basiert. Daruber¨ hinaus ha- ben Hybridstrukturen, welche gebildet werden indem man Materialien starker Spin- Bahn-Kopplung wie InAs mit Supraleitern kombiniert, jungst¨ Aufmerksamkeit er- langt um Majorana Fermionen in einem Festk¨orpersystem zu realisieren. Gem¨ass aktueller Theorien bilden diese Teilchen eine vielversprechende Plattform fur¨ eine fehlertolerante topologische Quanteninformationsverarbeitung. Eine Besonderheit von InAs im Gegensatz zu den meisten anderen Halbleitern ist, dass es keine Schottkybarriere in Kontakt mit Metallen bildet. Dies l¨asst sich auf das Ferminiveau zuruckf¨ uhren,¨ welches an der Oberfl¨ache im Leitungsband fest- gehalten ist, was eine Biegung des Leitungsbands und damit eine Anreicherung von Elektronen zur Folge hat. Als Konsequenz daraus sind typische Halbleitermikrofabri- kationstechnologien welche auf Atzverfahren¨ basieren ungeeignet fur¨ InAs, da die an der Oberfl¨ache der physischen Kanten angereicherten Ladungstr¨ager eine parasit¨are Randleitf¨ahigkeit verursachen, welche die mit elektrostatischen Kontrollelektroden gebildeten Nanostrukturen uberbr¨ uckt.¨ Infolgedessen ist die Nutzbarkeit von InAs Proben fur¨ mesoskopische Transportexperimente stark limitiert. Im Rahmen des Versuchs der Eliminierung der Randleitf¨ahigkeit durch chemi- sche Passivierungsmethoden w¨ahrend der Fabrikation wird diese untersucht. Kei- ne der getesteten Methoden konnte jedoch eine signifikante Aufhebung der Rand- leitf¨ahigkeit erzielen. Aufgrund dessen fuhren¨ wir eine neue Probengeometrie ein, in welcher das Elektronengas durch mehrere Schichten von Kontrollelektroden elektro- statisch aufgeteilt wird. Die unterschiedlichen Regionen werden separat kontaktiert, wodurch das Anwenden von Atzverfahren¨ redundant wird. Diese Geometrie erlaubt es die parasit¨are Randleitf¨ahigkeit komplett zu umgehen, wodurch die technologi- sche Herausforderung der Realisierung von Quantenapparaturen in zweidimensiona- iii len Elektronengasen von InAs bew¨altigt wird. Durch die Anwendung dieser Technik realisieren wir das vollst¨andige Abschnuren¨ des Elektronengases im eindimensionalen Elektronentransport durch einen von Kon- trollelektroden definierten Quantenpunktkontakt. Wir spektroskopieren dessen Ener- gieniveaus unter endlicher Vorspannung und determinieren den g-Faktor der Elek- tronen in einem Magnetfeld. Daruber¨ hinaus analysieren wir die magnetoelektrische Subband-Struktur sowie den Einfluss des Kopplungspotentials. In einem weiteren Experiment erm¨oglichen diese vollst¨andig abstimmbaren Tun- nelbarrieren die Formation einens nulldimensionalen Quantenpunkts in einem InAs Quantentopf. Im Regime weniger Elektronen beobachten wir Coulomb-Blockade- Diamanten und charakterisieren im Magnetfeld die Grund- und angeregten Zust¨ande bei Besetzung mit einem sowie zwei Elektronen. Zwischen den Uberg¨ ¨angen von Spin-Singlett und Spin-Triplett Zust¨anden wurde kein vermiedenes Kreuzen fest- gestellt. Dies k¨onnte auf ein Zusammenspiel der speziellen Orientierung des Ma- gnetfelds und der Kristallrichtung des Transports zuruckgef¨ uhrt¨ werden, welches die Spin-Bahn-Wechselwirkung verschwinden l¨asst. Eine starke Kopplung des Quanten- punkts an die Zuleitungen erm¨oglicht die Beobachtung des Kondo-Effekts. Dessen Abh¨angigkeiten von der Temperatur, dem Magnetfeld sowie der Vorspannung kor- respondieren mit theoretischen Erwartungen. Der aus der Aufspaltung der Kondo- Resonanz bestimmte g-Faktor stimmt mit dem aus der Spektroskopie des angeregten Ein-Elektronen-Zustands extrahierten
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