Electron Transport in Nanowire Quantum Devices
Abstract: This thesis focuses on electron transport in semiconductor InAs/InP and InSb nanowire quantum devices. However, first the temperature dependence of classical charge transport in InSb nanowire field-effect transistors, FETs, is characterized, using InAs nanowire FETs as a reference. We find that the InSb FETs, in the same way as InAs FETs, are n-type with good current saturation and low voltage operation. The off-current for the InSb FET shows a strong temperature dependence, which we attribute to a lowering of the barrier due to an increased band-to-band tunneling in the drain part of the channel. Next, we demonstrate multiple tunnel junction, MTJ, memories, fabricated from InAs/InP heterostructure nanowires. Two types of devices were considered. We demonstrate storage of single electrons at a temperature of 4.2 K, using a second InAs/InP heterostructure nanowire single-electron transistor as a detector. We also present temperature dependence and write-speed measurements on a many-electron MTJ nanowire memory, using a lithographically defined storage node and a nanowire FET detector. The MTJ memory operates at temperatures up to around 150 K and has write-times down to at least 15 ns. The main part of the thesis concerns low temperature electron transport in InSb nanowire devices. We report on magnetotransport measurements in InSb quantum dots. Here, we show that the quantum levels of InSb quantum dots have giant, level-dependent g factors, with absolute values up to ? 70, the largest value ever reported for semiconductor quantum dots. The presence of giant g factors indicates that considerable contributions from the orbital motion of electrons are preserved in the measured InSb nanowire quantum dots, while the level-to-level fluctuations arise from spin-orbit interaction. We have deduced a value of ?so = 280 ?eV for the strength of spin-orbit interaction from an avoided level crossing between the ground state and first excited state of an InSb nanowire quantum dot with a fixed number of electrons. We also demonstrate InSb quantum dots, with a strong coupling between localized electrons inside the quantum dots, and non-localized electrons in the metal leads. Here, we report on the spin-1/2 as well as the even electron-number Kondo effect. We report specifically on the effect of spin-orbit coupling on even electron-number Kondo effect. Using the even electron-number Kondo effect or the onset of inelastic cotunneling, we characterize the spin-orbit strength in a full magnetotransport spectrum of over 30 level crossings. In this spectrum we also measure the quantum level g factors and the quantum level spacing and relate this to the measured spin-orbit coupling strengths. In addition, due to the strongly level dependent g factors, we are able to characterize a degeneracy of two quantum levels of equal spin, in the strong coupling regime. Here, we find a strong suppression of the cotunneling background at the level degeneracy. This we attribute to destructive interference of two spin-correlated conduction paths. We also demonstrate measurements on a hybrid Ti/Al-InSb-Ti/Al, superconductor-semiconductor-superconductor nanowire device. Here, we demonstrate a conductance fluctuation-dependent, induced supercurrent through the InSb nanowire, at temperatures below ? 400 mK, with a critical current up to 7 nA. We report on multiple Andreev reflections between the two superconductor-semiconductor interfaces. We investigate the magnetic and temperature-dependent properties of both the induced supercurrent as well as the multiple Andreev reflections. Finally, we demonstrate a radio-frequency, single-electron transistor, RF-SET, fabricated from suspended InAs/InP double-barrier heterostructure nanowires. The charge sensitivity was measured to be 2.5 ?erms/?Hz at T = 4.2 K, which is similar to the best values reported for conventional Al/AlOx RF-SETs. At low frequencies this device showed a typical 1/f noise behavior, with a level extrapolated to 300 ?erms/?Hz at 10 Hz.
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