Antimonide Heterostructure Nanowires - Growth, Physics and Devices

Abstract in UndeterminedThis thesis investigates the growth and application of antimonide heterostructure nanowires for low-power electronics. In the first part of the thesis, GaSb, InSb and InAsSb nanowire growth is presented, and the distinguishing features of the growth are described. It is found that the presence of Sb results in more than 50 at. % group-III concentration in the Au seed particle on top of the nanowires. It is further concluded that the effective V/III ratio inside the seed particle is reduced compared to the outside. This enables the suppression of radial growth with remaining high axial growth rate. Furthermore, the low effective V/III ratio may affect the crystal structure formation, which is pure Zinc-blende in all investigated Sb-based nanowires. The strong segregating properties of Sb results in a strong Sb memory effect, and a difficulty to nucleate Sb-based nanowires directly on substrates.The second part of the thesis deals with the growth and application of GaSb/InAs(Sb) nanowires for tunnel device applications. The GaSb/InAs(Sb) nanowire heterojunction has a defect-free crystal structure with an extremely abrupt heterojunction due to an inherent delay before the initiation of InAs(Sb) growth. The Sb carry-over from the GaSb growth step into the InAs growth leads to a high Sb background in the InAs(Sb) segment. The diameter of the heterojunction can be reduced below 30 nm by an in-situ annealing treatment, in which material is selectively etched from the region near the heterojunction.The performance of GaSb/InAs(Sb) tunnel diodes is modeled and measured on fabricated single nanowire devices. The diodes exhibit peak current levels of 67 kA/cm^{2} , peak-to-valley current ratio between 2 and 3 at room temperature and a tunnel current at V_{D} = -0.5 V of 1.7 MA/cm^{2} . The expected performance of GaSb/InAs(Sb) tunnel field-effect transistors is discussed and preliminary measurement data on top-gated devices with 300 nm gate length is also presented

Advantage of Binary Stochastic synapses for hardware Spiking Neural Networks with realistic memristors

Hardware implementing spiking neural networks (SNNs) has the potential to provide transformative gains in energy efficiency and throughput for energy-restricted machine-learning tasks. This is enabled by large arrays of memristive synapse devices that can be realized by various emerging memory technologies. But in practice, the performance of such hardware is limited by non-ideal features of the memristor devices such as nonlinear and asymmetric state updates, limited bit-resolution, limited cycling endurance and device noise. Here we investigate how stochastic switching in binary synapses can provide advantages compared with realistic analog memristors when using unsupervised training of SNNs via spike timing-dependent plasticity. We find that the performance of binary stochastic SNNs is similar to or even better than analog deterministic SNNs when one considers memristors with realistic bit-resolution as well in situations with considerable cycle-to-cycle noise. Furthermore, binary stochastic SNNs require many fewer weight updates to train, leading to superior utilization of the limited endurance in realistic memristive devices

Impact of Temperature-Induced Oxide Defects on HfxZr1−xO2 Ferroelectric Tunnel Junction Memristor Performance

In this work, we evaluate the importance of the postmetallization annealing (PMA) temperature on the performance of HfxZr1−xO2-based ferroelectric tunnel junctions (FTJs). Our results indicate a significant difference in tunneling electroresistance (TER) ratio and endurance, depending on the PMA temperature despite negligible variations in remanent polarization. We conclude that the minimization of conductive oxide defect states is central to achieve high performance. Through carefully optimized PMA conditions, we demonstrate FTJs with a TER = 3 and low mean cycle-to-cycle variation of < 1.5% combined with at least 16 separable conductance states providing a 4-bit resolution analog FTJ

Historien om en kanal: Klimaforandringer og vandpolitik i det peruanske højland

Klimaforandringerne øger presset på vandressourcerne i det peruanske Andes. Artiklens forfatter følger i tid og rum en vandingskanal i provinsen Recuay i Perus højland. Historisk følges kanalen før, under og efter jordbrugsreformerne. Med inspiration fra Norman Longs Encounters at the Interface (2002) er det muligt at lokalisere et antal interfaces langs kanalen. Gennem en analyse af interfaces og kanalens forløb bliver det muligt at konkludere, at vandmanglen er et resultat af et samspil mellem økologiske og sociale relationer. Til slut fremføres det, at samtidig med at klimaforandringerne øger presset på vandressourcerne, skabes der også nye muligheder for dialog mellem den lokale befolkning og eksperterne. Søgeord: klimaforandring, vand, lokalpolitik, Andes, Peru English: The Story of a Canal. Climate Change and the Politics of Water in the Peruvian HighlandClimate change increases pressure upon water resources in the Peruvian Andes, and the issue of water scarcity may become even more pertinent. The exploration of the problematic of water is about the relationship between water and social organization, and the impact of climate change upon that relationship. “The story of a canal” traces the course of an irrigation canal in the Peruvian highland province of Recuay in time and space. The history of the canal is followed through different phases of before, under and after the agrarian reform up until the establishment of the present day peasant communities, thus inserting the course of the canal into historical land tenure dynamics. Following the analytical framework set up by Norman Long in Encounters at the Interface (2001), a number of interfaces of interaction is located along the course. Through an analysis of the interfaces and the course of the canal it is argued that water scarcity is the result of a complex interplay between ecological and social relations. Finally it is argued, that while climate change increases pressure upon water resources, it also can produce new openings through renewed dialogue between the local population and experts. Keywords: Climate change, water, local politics, Andes, Peru