Academic literature on the topic 'Graphene pn junction'

The electronic properties of single-layer, CVD-grown graphene were modulated by deep ultraviolet (DUV) light irradiation in different radiation environments. The graphene field-effect transistors (GFETs), exposed to DUV in air and pure O2, exhibited p-type doping behavior, whereas those exposed in vacuum and pure N2 gas showed n-type doping. The degree of doping increased with DUV exposure time. However, n-type doping by DUV in vacuum reached saturation after 60 min of DUV irradiation. The p-type doping by DUV in air was observed to be quite stable over a long period in a laboratory environment and at higher temperatures, with little change in charge carrier mobility. The p-doping in pure O2 showed ~15% de-doping over 4 months. The n-type doping in pure N2 exhibited a high doping effect but was highly unstable over time in a laboratory environment, with very marked de-doping towards a pristine condition. A lateral pn-junction of graphene was successfully implemented by controlling the radiation environment of the DUV. First, graphene was doped to n-type by DUV in vacuum. Then the n-type graphene was converted to p-type by exposure again to DUV in air. The n-type region of the pn-junction was protected from DUV by a thick double-coated PMMA layer. The photocurrent response as a function of Vg was investigated to study possible applications in optoelectronics.

L’optique quantique électronique, i.e. la réalisation de l’analogue électronique d’expériences d’optique quantique, constitue un champ de recherche récent, en plein développement, et offrant des perspectives intéressantes pour l’informatique quantique. Dans ce cadre, l’un des enjeux est la réalisation de bits quantiques en utilisant des états électroniques, ainsi que la formation d’états électroniques intriqués, éléments de bases pour réaliser des calculs quantiques plus élaborés. Les expériences menées jusqu’à présent dans des hétérostructures semi-conductrices de GaAs/AlGaAs ont mis en évidence la possibilité d’encoder l’information dans la charge ou le spin d’un électron, mais la décohérence importante de ces systèmes induit une grande fragilité de ces états quantiques, qui ne peuvent exister qu’en-dessous de 100mK et pour des tensions résiduelles inférieures à 40μV. Cette fragilité rend difficile la fabrication d’états intriqués, et est limitante pour le développement de calculs quantiques complexes. En 2005, la découverte d’un matériau novateur, le graphène, a ouvert de nouvelles perspectives avec la prédiction d’une cohérence de phase plus grande, et, d’autre part, l’existence en plus du spin d’un nouveau degré de liberté, la vallée, donnant accès à de nouvelles possibilités pour encoder l’information. Dans un premier temps, ce travail de thèse porte sur la manipulation cohérente de la vallée, nécessaire à la réalisation d’un bit quantique de vallée dans le graphène. Pour cela est utilisée, en régime Hall quantique, une jonction pn, formée à l’aide de grilles déposées sur un échantillon de graphène encapsulé dans du nitrure de Bohr. Afin d’obtenir un contrôle électrostatique sur la polarisation en vallée des électrons incidents, des grilles locales ont été déposées, à l’intersection de la jonction pn avec le bord physique du graphène. En alliant ce contrôle électrostatique à celui de la phase Aharanov-Bohm, il nous est possible de manipuler de manière cohérente la vallée d’un électron sur l’ensemble de la sphère de Bloch représentant la polarisation en vallée. Dans la suite, la cohérence des états quantiques formés est étudiée grâce à un interféromètre de Mach Zehnder, via l’observation de la dépendance des interférences en fonction de la tension appliquée sur les électrons incidents, et de la température du système. Les états quantiques obtenus sont exceptionnellement résistants, ils persistent au-delà de 1.5K et de 1mV, soit à des énergies près de 20 fois supérieures à celles observées dans le GaAs/AlGaAs.Puis, ce manuscrit décrit l’étude de la longueur de cohérence, correspondant à la distance sur laquelle un électron peut se propager en gardant sa cohérence de phase, ce qui n’avait encore jamais été mesuré dans le graphène. Pour ce faire, la dépendance des interférences vis-à-vis de la température a été mesurée sur trois jonctions pn de longueurs différentes. Une longueur de cohérence a ainsi été extraite pour les deux régimes de décohérence observés ; dont une record, pour le régime à basses températures, de plus de 374μm à 20mK. Pour finir, est investigué un des mécanismes causant la décohérence dans le système : les ondes de spin, se propageant lorsque le cœur du graphène est magnétique. Ainsi, au cours de ce projet, nous avons mis en évidence la possibilité d’encoder de l’information dans la vallée, ouvrant la voie vers un nouveau domaine : la vallée-tronique. D’autre part, la cohérence du système est exceptionnelle, permettant d’envisager la réalisation d’états intriqués grâce à une géométrie de double Mach Zehnder. Cela offre des perspectives prometteuses du point de vue de l’informatique quantique, mais aussi d’un point de vue fondamental avec la possibilité de démontrer pour la première fois, avec des fermions, la validité des prédictions de l’interprétation de Copenhague de la physique quantique dans le cadre du paradoxe EPR
Electron quantum optics, i.e. the realization of the electronic analogue of quantum optics experiments, represents a developing and recent research field, offering interesting perspectives for quantum computing. In this context, one of the main stakes is the achievement of quantum bits using electronic states, as well as the creation of entangled electronic states, which are the building blocks to achieve complex quantum computations. Up to now, the experiments carried out in semi-conducting GaAs/AlGaAs heterostructures exhibited the possibility to encode information in the charge or the spin of an electron, but strong decoherence in these systems implies a great weakness of these quantum states, which survives only below temperatures of 100mK and electrical biases of 40μV. This fragility makes it difficult to achieve entangled states and limits the development of complex quantum computations. In 2005, the discovery of a novel material, graphene, opened new prospects with on one hand the prediction of a larger phase coherence, and on the other hand the existence, in addition to the spin, of a new degree of freedom, named the valley, giving access to new possibilities to encode information. In a first part, this PhD work deals with the coherent manipulation of the valley, which is necessary to achieve a valley quantum bit in graphene. For this aim, we used, in the quantum Hall regime, a graphene pn junction, formed thanks to gates deposited on top of a stack composed of a graphene sheet encapsulated in Boron nitride crystals. In order to obtain an electrostatic control of the valley polarization of incoming electrons, we deposited local gates at the intersections between the pn junction and the graphene physical edge. Associating this electrostatic control to a tuning of the Aharanov-Bohm phase, we can coherently manipulate the valley of an electron over the whole states described by a valley Bloch sphere. In what follows, the coherence of the quantum states is investigated thanks to Mach Zehnder interferometry, by measuring the interferences dependence on the chemical potential of incoming electrons and on the temperature of the system. The quantum states formed are exceptionally steady, they persist up to 1.5K and 1mV, in other words at energies 20 times higher than what was observed in GaAs/AlGaAs.Then, the manuscript describes the study of the coherence length, i.e. the distance on which an electron can propagate while keeping its phase coherence, which has never been measured in the quantum Hall regime in graphene. To that end, the interferences dependence on the temperature was measured in three pn junctions of different lengths. By doing so, two coherence lengths, corresponding to two different regimes of decoherence, were extracted; in the regime occurring at low temperature, a record value of 374μm at 20mK was obtained.Finally, we investigated one of the mechanisms of decoherence in our system: spin waves, propagating in the graphene bulk when it is magnetized. During this project, we have shown the possibility to encode information in the valley and to manipulate coherently this degree of freedom, paving the way towards a new domain: the valleytronics. Furthermore, the coherence of the system is exceptional, enabling to envision the achievement of entangled electronic states by using a double Mach Zehnder interferometer geometry. This opens promising prospects for quantum computing, but also for fundamental purposes, with the possibility to demonstrate, for the first time with fermions, the validity of the Copenhagen interpretation of quantum physics within the EPR paradox framework

In this doctoral thesis the two dimensional material graphene has been studied in depth with particular respect to Zener tunnelling devices. From the hexagonal structure the Hamiltonian at a Dirac point was derived with the option of including an energy gap. This Hamiltonian was then used to obtain the tunnelling properties of various graphene nano-devices; the devices studied include Zener tunnelling potential barriers such as single and double graphene potential steps. A form of the Landauer formalism was obtained for graphene devices. Combined with the scattering properties of potential barriers the current and conductance was found for a wide range of graphene nano-devices. These results were then compared to recently obtained experimental results for graphene nano-ribbons, showing many similarities between nano-ribbons and infinite sheet graphene. The methods studied were then applied to materials which have been shown to possess three dimensional Dirac cones known as topological insulators. In the case of Cd3As2 the Dirac cone is asymmetrical with respect to the z-direction, the effect of this asymmetry has been discussed with comparison to the symmetrical case.

Long wavelength light contains the infrared and terahertz (THz) spetral range of the spectrum. This wavelength range spans approximately from 1 µm to 1 mm. Several applications can be explored in this spectral range such as thermal imaging, temperature monitoring, night vision, etc. Moreover, molecular vibrations resonate at these energies that are the fingerprints for compounds identification via molecular spectroscopy. Also, THz light has an important role in security since at these frequencies is possible to achieve a higher resolution for imaging compared to millimeter waves that are typically used in airports. Despite all these potential applications, long wavelength light technology still remains non-fully exploited. One of the reasons is due to the lack of competing instrumentation such as sources, modulators, detectors, sensors, etc. In particular, regarding the detectors, the commercially available technology present some issues such as working at room temperature, speed, sensitivity, dynamic range, broadband frequency operation, CMOS compatibility, size and compactness, etc. The extensive research during the last years on graphene and other 2D materials has opened new possibilities of novel light matter interactions that can unveil the next generation photodectectors and sensors, ascribed to the advantages respect to conventional semiconductors. In this thesis, we focus on developing novel photodetection platforms in the mid, longwave infrared and THz range based on graphene pn-junctions with integrated metallic nanostructures and hyperbolic 2D material. We have successfully integrated an antenna with a graphene pn-junction for highly sensitive and fast THz detection in this regime. This novel terahertz detector exploits efficiently the photothermoelectric (PTE) effect, based on a design that employs a dual-gated, dipolar antenna with a nanogap. We have demonstrated that this novel detector leads to an excellent performance, which fulfills a combination of figure-of-merits that is currently missing in the state-of-the-art detectors. We also overcame the main challenge of infrared photodetectors, which is to funnel the light into a small nanoscale active area and efficiently convert it into an electrical signal. We achieve this by efficient coupling of a plasmonic antenna to hyperbolic phonon-polaritons in hBN to highly concentrate mid-infrared light into a graphene pn-junction. We use a metallic bowtie antenna and H-shape resonant gates that besides concentrating the light into its nanogap, their plasmonic resonances spectrally overlap within the upper reststrahlen band (RB) of hBN (6-7 µm), thus launching efficiently these HPPs and guiding them with constructive interferences towards the photodetector active area. Additionally, by having two different antennas orientation, it allows us to have sensitive detection in two incident polarizations. Furthermore, we have shown mid and long-wave infrared photocurrent spectroscopy via electrical detection of graphene plasmons, hyperbolic phonon-polaritons and their hybridized modes. We combined in one single platform the efficiently excited polaritonic material that also acts as a detector itself. We identified peaks in the photocurrent spectra that evolves and blue shift by increasing the gate voltage, which are related to the polaritonic resonances. Finally, we investigated the electrical detection of molecular vibrations coupled to hyperbolic phonon polaritons in hBN. We detected this strong light-matter interaction via a graphene pn-junction placed at the vicinity of the molecules-hBN stack. The edges of the gap of the local gates launch efficiently the hBN HPPs that interact with the CBP molecular resonances that are spectrally located at the upper RB. We explored this interaction as a function of the thickness of the molecular layers, near and far field contribution, etc.
La luz de longitudes de onda largas consiste en el rango de infrarojo y terahercio (THz) del espectro. Este rango de longitud de ondas oscila entre los valores de 1 µm a 1 mm. En esta frecuencias, muchas aplicaciones pueden ser exploradas como por ejemplo las cámaras térmicas, monitorización de temperatura, visión nocturna, etc. Además, las vibraciones moleculares de muchos materiales oscilan en este rango de energías. Estas resonancias son utilizadas como huellas dactilares para la identificación de compuestos utilizando la espectroscopía molecular. También, la luz de terahercio juega un papel importante en el sector de seguridad. Esto se debe a que en estas frecuencias se puede conseguir una mayor resolución de imagen en comparación a las ondas milimétricas que son utilizadas mayoritariamente en los aeropuertos. A pesar de todo este potencial para diferentes sectores, la tecnología basada en luz de longitudes de onda largas sigue sin ser explotada del todo. Una de las razones es por la falta de equipos eficaces como por ejemplo las fuentes de luz, moduladores, detectores, sensores, etc. En particular, los detectores que se comercializan actualmente presentan limitaciones significativas como la temperatura de operación,velocidad, sensitividad, rango dinámico, ancho de banda de frecuencias, compatibilidad con CMOS, tamaño, etc. La investigación exhaustiva durante los últimos años en grafeno y otro materiales bidimensionales (2D) ha abierto nuevas posibilidades de nuevas interacciones entre materia y luz que podría contribuir para la nueva generación de fotodetectores y sensores debido a las ventajas de estos materiales respecto a los semiconductores convencionales. En esta tesis nos enfocaremos en el desarrollo de plataformas novedosos en fotodección en el infrarrojo medio, largo y en el rango de terahercio. Estas plataformas están basadas en junciones pn de grafeno integradas con nanoestructuras metálicas y materiales 2D hiperbólicos. Hemos integrado satisfactoriamente una antena con una junción pn de grafeno para una detección sensitividad alta y rápida de terahercio. Este fotodetector novedoso de terahercio utiliza eficientemente el efecto fototermoeléctrico, el cual esta basado en un diseño que emplea una antena con un nanogap que a su vez actúa como doble puerta. También hemos demostrado que este novedoso detector realiza un gran desempeño, consiguiendo una combinación de aspectos a destacar que actualmente no se encuentran en los detectores en literatura. Además. superamos el mayor desafío de los detectores de infrarrojo, el cual consiste en dirigir este tipo de luz en la nanoescala hacia el área activa del detector y convertirla en una señal eléctrica. Conseguimos esto mediante una acoplación eficiente de una antena plasmónica con los fonones polaritones hiperbólicos (HPPs) para concentrar altamente la luz infrarroja media a una junción pn de grafeno. Utilizamos una antenna "bowtie" metálica y unas puertas resonantes con forma de H que además de concentrar la luz en su nanogap, sus resonancias plasmónicas solapan espectralmente con la banda reststrahlen (RB) superior del hBN (6-7 µm). Esto induce a que se puedan excitar eficientemente los HPPs y se guian hacia la área activa del fotodetector mediante intereferencias constructivas. Más aún, hemos demostrado en la espectroscopía de fotocorriente en el infrarrojo medio y largo mediante la deteción eléctrica de polaritones 2D. Hemos combinado en una sola plataforma el material plasmónico que a su vez actúa como el fotodetector. Hemos identificado picos en el espectro de fotocorriente que evoluciona a medida que aumentamos el potencial de puerta, lo cual es una insignia de una resonancia polaritónica. Finalmente, investigamos la deteción eléctrica de vibraciones moleculares acopladas a HPPs en hBN. Hemos detectado esta fuerte interacción de luz y materia mediante una junción pn de grafeno que esta próxima a este
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