Conductance anomalies in quantum point contacts

2009-07-25T22:00:00Z

Conductance anomalies in quantum point contacts Frucci, Giulia 2009-07-26 190-193 We present a study of the conductance of quantum point contacts fabricated in AlGaN/GaN and Si/SiGe heterostructures. The investigated devices differ for typology (split gates and etched devices, respectively) and for the resulting potential profiles. We observe conductance quantization in multiple of 2e2/h units with superimposed anomalous plateaus and/or structures suggesting that correlation effects should be included in the description of our 1D systems.

Quantum transport in low-dimensional Si/SiGe and AlGaN/GaN systems

2010-01-19T23:00:00Z

Quantum transport in low-dimensional Si/SiGe and AlGaN/GaN systems Frucci, Giulia 2010-01-20 In recent years the study of electronic properties of low dimensional mesoscopic systems has attracted considerable interest. One of the reasons for this is the opportunity it gives of investigating a wide range of new effects related to ballistic transport and phase coherence. Another reason can be found in the possibility it gives of fabricating nanostructures both for microelectronics and for possible applications in quantum computing and spintronics in general. The object of this thesis is the investigation of quantum transport in Si/SiGe- and AlGaN/GaN -based Quantum Point Contacts (QPCs). In particular, we focus on ballistic transport eﬀects going beyond the oneelectron Landauer picture expected for a system of non-interacting electrons. Si-based nanostructures are one of the most important material systems for applications in spintronics and quantum information due to the weak spinorbit coupling and to the presence of nuclear zero spin isotopes, which make electron spin coherence time extremely long. However, silicon has a near degeneracy of orbital states in the conduction band, arising from multiple valley minima, which can enhance decoherence rates and make qubit operation in quantum computing more complicated. It has been shown that quantum conﬁnement in nanostructures provides some amount of control over the valley splitting [1]. In this work, we have investigated quantum transport properties of strongly-conﬁned Shottky-gated constrictions, made starting from Si-based 2DEG and focusing on the conductance behaviour of nanostructures with various geometries. Measurements have been made as a function of the gate voltage, the source-drain bias and the magnetic ﬁeld. Our results reveal a complex framework due to the occurrence of deviations from the ideal quantized conductance behaviour. For instance, these can be due to backscat1 tering from impurities or transmission resonances, produced by multiple reﬂections, for the presence of an abrupt geometry of the conﬁning potential. However our ﬁndings have revealed a zero-ﬁeld energy valley splitting in our etched-nanostructures, due to the strong conﬁnement generated by physical etching of the 2DEG heterostructures. In practice, in diﬀerent devices we found a valley splitting energy of the order of ∼ 1meV that is comparable to values reported in literature. In the past ten years, due to developments in the ﬁeld of AlGaN/GaN heterostructures, research has focused also on GaN -based 2DEG. The latter is in fact among the most promising materials for the study of properties related to ballistic transport and it is interesting from a technological point of view. GaN -based 2DEGs are a novel system in which the large band oﬀset and the strong piezoelectric eﬀect in this material system have been shown to generate an intrinsic high sheet density two-dimensional electron gas, ns ∼ 1013 cm−2 in our sample, with enhanced electron mobility [2, 3]. In addition, the relatively heavy mass of electrons makes GaN 2DEGs a convenient system for studying spin-polarized and electron-electron correlation eﬀects. The strong spontaneous and piezoelectric polarization charge gives these systems a strong asymmetric electric ﬁeld at the interface, which can also enhance the spin-orbit interaction, thus providing a spin-splitting energy of the conduction band states at zero-external ﬁeld [4]. In this thesis we focused on the study of the electrical properties of an AlGaN/GaN 2DEG-system, exploiting both classical and quantum Hall effect. In our investigation, new interesting problems came out from the analysis of both Shubnikov-de Haas and low-ﬁeld measurements: the occupancy of a second energy level of the 2DEG, the occurrence of a zero-ﬁeld spinsplitting due to spin-orbit interaction and the occurrence of the key-feature of weak antilocalization [5], namely positive magnetoresistance. Electron quantum transport of mesoscopic devices on GaN -based heterostructures was also investigated. For these systems we measured the conductance as a function of the gate voltage and the magnetic ﬁeld. In addition, we investigated the eﬀect of deliberately introducing an asymmetry in the conﬁning potential. We have obtained an interesting and rich framework in which we speculate the possibility of a zero-ﬁeld spin-polarization as being due to the eﬀect of asymmetry of the conﬁning potential and the presence of a spin- 2 orbit coupling [6]. References [1] S. Goswami, K. A. Slinker, M. Friesen, L. M. McGuire, J. L. Truitt, C. Tahan, L. J. Klein, J. O. Chu, P. M. Mooney, D. W. van der Wiede, R. Joynt, S. N. Coppersmith, and M. A. Eriksson, Nature Physics 3 (2007), 41. [2] O. Ambacher, B. Foutz, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, A. J. Sierakowski, W. J. Schaﬀ, L. F. Eastman, R. Dimitrov, A. Mitchell, and M. Stutzmann, J. Appl. Phys. 87 (2000), 334. [3] A. D. Bykhovski, R. Gaska, and M. S. Shur, Appl. Phys. Lett. 72 (1998), 3577. [4] S. Schmult, M. J. Manfra, A. Punnoose, A. M. Sergent, K. W. Baldwin, and R. J. Molnar, Phys. Rev. B 74 (2006), 03302. [5] A. E. Belyaev, V. G. Raicheva, A. M. Kurakin, N. Klein, and S. A. Vitusevich, Phys. Rev. B 77 (2008), 035311. [6] P. Debray, S. M. Rahman, J. Wan, R. S. Newrock, M. Cahay, A. T. Ngo, S. E. Ulloa, S. T. Herbert, M. Muhammad, and M. Johnson, Nature Nanotechnology advance online publication (2009),

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Conductance anomalies in quantum point contacts

Quantum transport in low-dimensional Si/SiGe and AlGaN/GaN systems