Quantum Coherence at Low Temperatures in Mesoscopic Systems: Effect of Disorder

We study the disorder dependence of the phase coherence time of quasi one-dimensional wires and two-dimensional (2D) Hall bars fabricated from a high mobility GaAs/AlGaAs heterostructure. Using an original ion implantation technique, we can tune the intrinsic disorder felt by the 2D electron gas and continuously vary the system from the semi-ballistic regime to the localized one. In the diffusive regime, the phase coherence time follows a power law as a function of diffusion coefficient as expected in the Fermi liquid theory, without any sign of low temperature saturation. Surprisingly, in the semi-ballistic regime, it becomes independent of the diffusion coefficient. In the strongly localized regime we find a diverging phase coherence time with decreasing temperature, however, with a smaller exponent compared to the weakly localized regime.Comment: 21 pages, 30 figure

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Effet Kondo dans des boîtes quantiques couplées latéralement

The Kondo effect arises when a single magnetic impurity is tunnel coupled to a Fermi sea. The essence of this highly correlated electron state is found in the non perturbative nature of the exchange interaction between the local moment and the surrounding conduction electrons which leads to a non magnetic ground state at zero temperature. In the Kondo regime all the physical quantities of the system (impurity+electron reservoir) scale with a unique energy being the Kondo temperature TK. This universal character has been observed in metals containing a large number of magnetic impurities and in man made artificial magnets such as quantum dots. To be experimentally observable, the tunneling element connecting the local moment and the Fermi reservoir has to be large and the Kondo problem is often referred to as a strong coupling limit. Laterally defined quantum dots offer great possibilities to investigate further more the Kondo effect. In particular, constraining the Fermi sea to a finite region and studying how this influences the screening of the local moment appears to be a crucial question in the Kondo problem. We will present transport measurements through a double quantum dot where a small quantum dot acting as a magnetic impurity will be tunnel coupled to a large quantum dot being the finite size reservoir. Different experiments performed in the strong inter-dot coupling limit will confront us to the multi level nature of the large quantum dot where hybridization of both objects has to be carried over several energy levels. Furthermore, we will present data where evidences of transport mechanisms through the system are mediated by a Kondo mechanism through an hybridized level of the dots. At the double dot charge degeneracy point, a boosting of the Kondo temperature due to the reduction of the charging energy of the system, enables to reveal a Kondo singlet at finite temperature. By considering the different spin configurations possible in our system, we will discuss the competition between two possible singlet states one can stabilize in the system, a Kondo type singlet and a inter-dot singlet where multi level effects play an important role. We believe that the reduction of the exchange coupling between the two dots due to the small level spacing in the large dot explains the invariance of the phenomenon with respect to the occupation number of the large dot at our base electron temperature.L'effet Kondo naît du couplage tunnel entre une impureté magnétique et une mer de Fermi. L'essence de cet état de fortes corrélations électroniques trouve son origine dans la nature non perturbative du couplage d'échange entre le moment local et les électrons de conduction, et qui conduit à la formation d'un état fondamental non magnétique à température nulle. Dans le régime Kondo, toutes les propriétés physiques du système (impureté+réservoir d'électrons) s'expriment en fonction d'une unique échelle d'énergie, la température Kondo TK . Ce caractère universel a été observé dans des métaux contenant une grande quantité d'impuretés magnétiques ainsi que dans des impuretés artificielles comme les boîtes quantiques. Pour être mis en évidence expérimentalement, l'élément tunnel connectant le moment local et le réservoir de Fermi doit être large et l'on réfère souvent au problème Kondo comme à un problème de couplage fort. Les boîtes quantiques latérales offrent de grandes possibilités d' étudier plus en détail l'effet Kondo. En particulier, contraindre la mer de Fermi à une région finie de l'espace et comprendre comment cela influence l'écrantage du moment local, est une question cruciale du problème Kondo. Nous présenterons des mesures de transport à travers une double boîte quantique où une boîte quantique de petite taille jouant le rôle d'impureté magnétique sera couplée à une boîte quantique de grande taille jouant le rôle de réservoir fini. Différentes expériences effectuées dans le régime de couplage fort entre boîtes nous confronterons à la nature multi-niveaux de la grande boîte quantique et montrerons l'importance de considérer l'hybridation de multiples niveaux d'énergie. De plus, nous présenterons des données où le transport à travers le système est médié par des mécanismes de types Kondo impliquant un niveau hybridé entre boîtes. A la dégénérescence de charge des boîtes quantiques, une amplification de la température Kondo résultant de la réduction de l'énergie de charge du système, permet de révéler un singulet Kondo à température finie. En analysant les différentes configurations de spin possibles, nous discuterons la compétition entre deux singulets pouvant être stabilisés dans le système, un singulet de type Kondo et un singulet entre boîtes où les effets à multiples niveaux jouent un rôle important. Nous pensons que la réduction du couplage d'échange entre boîtes due au faible écart entre niveaux d'énergie dans la grande boite quantique explique l'invariance du phénomène observé en ce qui concerne l'occupation de cette même boîte, et ceci à la température électronique de base de notre expérience

Effet Kondo dans des boîtes quantiques couplées latéralement

The Kondo effect arises when a single magnetic impurity is tunnel coupled to a Fermi sea. The essence of this highly correlated electron state is found in the non perturbative nature of the exchange interaction between the local moment and the surrounding conduction electrons which leads to a non magnetic ground state at zero temperature. In the Kondo regime all the physical quantities of the system (impurity+electron reservoir) scale with a unique energy being the Kondo temperature TK. This universal character has been observed in metals containing a large number of magnetic impurities and in man made artificial magnets such as quantum dots. To be experimentally observable, the tunneling element connecting the local moment and the Fermi reservoir has to be large and the Kondo problem is often referred to as a strong coupling limit. Laterally defined quantum dots offer great possibilities to investigate further more the Kondo effect. In particular, constraining the Fermi sea to a finite region and studying how this influences the screening of the local moment appears to be a crucial question in the Kondo problem. We will present transport measurements through a double quantum dot where a small quantum dot acting as a magnetic impurity will be tunnel coupled to a large quantum dot being the finite size reservoir. Different experiments performed in the strong inter-dot coupling limit will confront us to the multi level nature of the large quantum dot where hybridization of both objects has to be carried over several energy levels. Furthermore, we will present data where evidences of transport mechanisms through the system are mediated by a Kondo mechanism through an hybridized level of the dots. At the double dot charge degeneracy point, a boosting of the Kondo temperature due to the reduction of the charging energy of the system, enables to reveal a Kondo singlet at finite temperature. By considering the different spin configurations possible in our system, we will discuss the competition between two possible singlet states one can stabilize in the system, a Kondo type singlet and a inter-dot singlet where multi level effects play an important role. We believe that the reduction of the exchange coupling between the two dots due to the small level spacing in the large dot explains the invariance of the phenomenon with respect to the occupation number of the large dot at our base electron temperature.L'effet Kondo naît du couplage tunnel entre une impureté magnétique et une mer de Fermi. L'essence de cet état de fortes corrélations électroniques trouve son origine dans la nature non perturbative du couplage d'échange entre le moment local et les électrons de conduction, et qui conduit à la formation d'un état fondamental non magnétique à température nulle. Dans le régime Kondo, toutes les propriétés physiques du système (impureté+réservoir d'électrons) s'expriment en fonction d'une unique échelle d'énergie, la température Kondo TK . Ce caractère universel a été observé dans des métaux contenant une grande quantité d'impuretés magnétiques ainsi que dans des impuretés artificielles comme les boîtes quantiques. Pour être mis en évidence expérimentalement, l'élément tunnel connectant le moment local et le réservoir de Fermi doit être large et l'on réfère souvent au problème Kondo comme à un problème de couplage fort. Les boîtes quantiques latérales offrent de grandes possibilités d' étudier plus en détail l'effet Kondo. En particulier, contraindre la mer de Fermi à une région finie de l'espace et comprendre comment cela influence l'écrantage du moment local, est une question cruciale du problème Kondo. Nous présenterons des mesures de transport à travers une double boîte quantique où une boîte quantique de petite taille jouant le rôle d'impureté magnétique sera couplée à une boîte quantique de grande taille jouant le rôle de réservoir fini. Différentes expériences effectuées dans le régime de couplage fort entre boîtes nous confronterons à la nature multi-niveaux de la grande boîte quantique et montrerons l'importance de considérer l'hybridation de multiples niveaux d'énergie. De plus, nous présenterons des données où le transport à travers le système est médié par des mécanismes de types Kondo impliquant un niveau hybridé entre boîtes. A la dégénérescence de charge des boîtes quantiques, une amplification de la température Kondo résultant de la réduction de l'énergie de charge du système, permet de révéler un singulet Kondo à température finie. En analysant les différentes configurations de spin possibles, nous discuterons la compétition entre deux singulets pouvant être stabilisés dans le système, un singulet de type Kondo et un singulet entre boîtes où les effets à multiples niveaux jouent un rôle important. Nous pensons que la réduction du couplage d'échange entre boîtes due au faible écart entre niveaux d'énergie dans la grande boite quantique explique l'invariance du phénomène observé en ce qui concerne l'occupation de cette même boîte, et ceci à la température électronique de base de notre expérience

Ion-assisted gate recess process induced damage in GaN channel of AlGaN/GaN Schottky barrier diodes studied by deep level transient spectroscopy

International audienceDeep traps in AlGaN/GaN Schottky barrier diodes have been investigated using deep level transient spectroscopy. It has been found that ion-assisted gate recess process leads to the formation of electron traps. The defects related to these traps are mainly located in the two-dimensional electron gas channel below the Schottky contact, meaning that the partial etching of the AlGaN layer produces damage on the top of the underlying GaN layer. The activation energies of the electron traps, extracted from the data, range between 0.28 and 0.41 eV. We believe that these centers are complexes linked with nitrogen vacancies which may behave as extended defects

Capping stability of Mg-implanted GaN layers grown on silicon

International audienceThe morphological stability during activation annealing of Mg‐implanted GaN layers (2 μm thick) grown on Si (111) is studied for several protective layers and fluencies in the 1013–1015 at. cm−2 range. We show that a thin capping, composed of a few nanometer thick AlN and SiNx stacks grown in situ just after GaN deposition, provides a good solution to retain flat morphology and no strain cracking up to 1 h annealing at 1100 °C in N2. These results are compared to thicker protective stackings with AlN layers of Si3N4 or SiO2 deposited after the implantation that withstand a thermal budget of up to 1 h at 1200 °C in N2. The efficiency of these different cap layers to limit GaN damage during high‐temperature annealing is studied as well as the impact of Mg implantation process on the cap resilience. The quality of the GaN sublayer is studied by low‐temperature photoluminescence to analyze structural/optical defects and Mg related complexes. X‐ray diffraction is performed to evaluate residual strains at the different process stages

Coherent tunneling in an AlGaN/AlN/GaN heterojunction captured through an analogy with a MOS contact

International audienceDue to their wide band gaps, III-N materials can exhibit behaviors ranging from the semiconductor class to the dielectric class. Through an analogy between a Metal/AlGaN/AlN/GaN diode and a MOS contact, we make use of this dual nature and show a direct path to capture the energy band diagram of the nitride system. We then apply transparency calculations to describe the forward conduction regime of a III-N heterojunction diode and demonstrate it realizes a tunnel diode, in contrast to its regular Schottky Barrier Diode designation. Thermionic emission is ruled out and instead, a coherent electron tunneling scenario allows to account for transport at room temperature and higher

The effect of AlN nucleation temperature on inverted pyramid defects in GaN layers grown on 200 mm silicon wafers

18th International Conference on Metal Organic Vapor Phase Epitaxy (ICMOVPE)JUL 10-15, 2016San Diego, CAInternational audienceWe have examined 200 mm GaN on silicon wafers, while varying the AlN nucleation temperature, and have found that higher temperatures result in a more convex bow on the wafers. In addition, by performing full wafer defect mapping, we have found that a higher nucleation temperature results in a higher density of inverted pyramid defects, which have previously been found to reduce the breakdown voltage of GaN on silicon layers. We have performed electrical measurements on a wafer with the lowest temperature AlN layer, which is still within our bow specification, and which therefore has the lowest density of inverted pyramid defects. This wafer showed the same leakage current density for both very small and very large test structures (2x10(-3) and 18.7 mm(2) respectively), with all but one of our large structures mAlNtAlNing a breakdown voltage greater than 700 V. This is a very promising result for high yield of devices on 200 mm GaN on silicon wafers

On the Understanding of Cathode Related Trapping Effects in GaN-on-Si Schottky Diodes

International audienceCathode related current collapse effect in GaN on Si Schottky barrier diodes is investigated in this paper. Capacitance and current relaxation measurements on diodes and gated-Van Der Pauw are associated with temperature dependent dynamic ${R_{\mathrm{ ON}}}$ transients analysis to identify the parasitic trapping locations in the devices. We show here that the main part of the current collapse at the cathode comes from a combination of electron trapping in the passivation layer and in a carbon related trap in the GaN buffer layers ( ${E_{A}} = \mathrm {E_{T}} - {E_{V}} \simeq 0.9 \,\mathrm {eV}$ ) that can be studied independently by using the appropriate stress configurations. These two parasitic effects can lead to long recovery time (>1 ks) after reverse bias stress