Rozprawy doktorskie na temat „Lateral heterostructures”

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 73-75).
Quantum dots in Si/SiGe have long spin decoherence times, due to the low density of nuclear spins and weak coupling between nuclear and electronic spins. Because of this, they are excellent candidates for use as solid state qubits. The initial approach towards creating controllable Si/SiGe quantum dots was to fabricate them in delta doped heterostructures. We provide evidence that the delta doping layer in these heterostructures provides a parallel conduction path, which prevents one from creating controllable quantum dots. Instead, it may be more favorable to supply electrons in the 2DEG through capactive gating, instead of a delta doping layer. We therefore discuss efforts to fabricate Si/SiGe quantum dots from undoped heterostructures and the difficulties encountered. A new method for fabricating ohmics in undoped heterostructures is discussed. We also discuss parallel conduction which occurs in the Si cap layer of these undoped heterostructures, which appears to be a major obstacle towards achieving workable devices in undoped Si/SiGe heterostructures.
by Andrew P. Lai.
S.M.

Based on first principles Density Functional Theory calculations, we have investigated for possible paths for engineering electronic band structure of Transition Metal Dichalco- genides (TMDs). We have considered two approaches which have shown to be promising for engineering electronic bands of TMDs: substitutional chemical doping and heterostruc- turing. All the calculations are done using first principles Density Functional Theory as it is implemented in Quantum Espresso package. Two possible substitutional doping meth- ods for MoS2 are considered in our calculations; cation doping where Mo is substituted by metal atoms and anion doping where Nitrogen and halogen group atoms take the posi- tion of S-sites. We observe the n-type characteristics for halogen group doping and p-type characteristics for Nitrogen group doping at S site. Similarly, we observe these bipolar characteristics when substituted by the transition metal elements (4d elements in the peri- odic table) at Mo site. Our results on doping monolayer MoS2 are in agreement with those results obtained by Dolui et al. for similar systems. Our work is extended to explore the effect of substitutional doping in bilayer MoS2. We observe the promising bipolar char- acteristics on doping while the magnitude of the band gap decreases upon the controlled S-site doping with F and As. In the second part, we considered two types of heterostructuring; Van der Waals heterostructures, and lateral heterostructures. In Van der Waals heterostructures, a direct band gap is observed with a physical separation of charges into two layers from orbital isosurface plots. We present a brief overview of the folding of energy bands in supercell approach. Using heterogeneous supercell approach, we studied the electronic properties of a mixed system of MoS2 -WS2 . The separation of the charges into the two sections shows that our MoS2 -WS2 in-plane heterostructure shows a potential for a pn junction. These systematic studies of the doped and heterostructures of TMDs can be useful for device applications.

Atomically thin two-dimensional materials and their hybrids represent an elegant approach to designing and synthesizing functional nanomaterials and are expected to find applications across a broad range of new technologies. This project explored scalable synthesis of various two-dimensional layered materials and their hybrid counterparts on silicon carbide, an industrially relevant device substrate. It demonstrates the integration of graphene, hexagonal boron nitride and transition metal dichalcogenide layers which were characterised by high resolution scanning probe microscopy and electron spectroscopy. The procedures developed in this work are expected to facilitate a route towards large-scale synthesis of novel nanoscale devices directly on-chip.

Le graphène (Gr) et le nitrure de bore hexagonal (hBN) ont un paramètre de réseau similaire (décalage de ~1,5 %) et des propriétés différentes : le Gr est un métal connu pour sa conductivité élevée et le hBN est un isolant à grand écart (~6eV) avec une forte émission d'UV. En raison de ces deux remarques, ils sont des candidats parfaits pour être empilés côte à côte dans une hétérostructure latérale au lieu d'être empilés l'un sur l'autre dans une hétérostructure verticale plus courante. Dans cette thèse, je m'intéresserai à la modélisation des propriétés électroniques et optiques des hétérostructures latérales composées de nanorubans successifs de graphène et de nitrure de bore (AGBN). Cependant, lors de la synthèse de ce type d'hétérostructures, des défauts, tels que des rugosités ou des défauts non-hexagonaux, peuvent apparaître à l'interface et affecter les propriétés des AGBN.Dans la première partie de la thèse, je combinerai des techniques ab-initio telles que la théorie de la fonctionnelle de la densité (DFT) et un modèle perturbatif de liaison étroite (TB) pour étudier la sensibilité opposée et complémentaire de la largeur de la fente des nanorubans armchair Gr et hBN isolés (AGNR et ABNNR) en fonction de différents stimuli.Dans les parties suivantes, je présenterai la structure électronique de l'AGBN réalisée avec la DFT et le spectre optique calculé par GW et l'équation de Bethe-Salpeter (BSE). Je passerai des caractéristiques générales, comme la structure de bande, à l'explication détaillée du rôle de chaque matériau et du confinement caractéristique de l'exciton dans la partie Gr des hétérostructures.Parallèlement à cette étude, je vais paramétrer un modèle TB semi-empirique et fixer ses limites de validité pour décrire le spectre d'absorption de l'AGBN dans l'approximation des particules indépendantes. Par conséquent, je dois établir une correspondance entre les pics excitoniques sur les spectres d'absorption BSE et les transitions dans les spectres IP, ce qui nous permettra d'estimer les effets excitoniques à partir des spectres TB IP. En particulier, cette approche sera utilisée dans la dernière partie de la thèse pour caractériser l'impact d'une faible rugosité à l'interface ou de défauts non-hexagonaux comme les Stone-Wales ou les divacances
Graphene (Gr) and hexagonal boron nitride (hBN) have a similar lattice parameter (~1.5% mismatch) and different properties , Gr is a metal known by its high conductivity and hBN is a large gap insulator ~6eV) with a strong UV emission. Due to these two remarks, they are perfect candidates to be stacked side-by-side in a lateral heterostructures instead of one of the top of the other in a more common vertical heterostructure. In this thesis I will be interested at modelling the electronic and optical properties of lateral heterostructures composed of successive armchair graphene and boron nitride nanoribbons (AGBN). However, during the synthesis of this kind of heterostructures defects, such as roughness or non-hexagonal defect, may appear at the interface affecting to the properties of AGBN.In the first part of the thesis, will combine ab-initio techniques such a density functional theory (DFT) and a perturbative tight-binding (TB) modem to study the opposite and complementary sensitivity of the gapwidth of isolated Gr and hBN armchair nanoribbons (AGNR and ABNNR) upon different stimuli.In the next parts I will present the electronic structure of AGBN carry out with DFT and optical spec-trum calculated by GW and the Bethe-Salpeter equation (BSE). I will revise from the general features, like the band structure, to explaining in detail the role of each material and the characteristic confining of the exciton in the Gr part of the heterostructures.Parallel to this study, I will parametrise a semi-empirical TB model and set its limits of validity to de-scribe the absorption spectrum of AGBN in the independent-particle approximation. Therefore, I have to set a correspondence between excitonic peaks on the BSE absorption spectra and transitions in IP spec-tra will allow us to estimate excitonic effects from the TB IP spectra. In particular, this approach will be used in the last part of the thesis to finally characterise the impact of weak roughness at the interface or non-hexagonal defects like Stone-Wales or divacancies

Nos travaux recents ont montre l'interet des superreseaux de semiconducteurs pour etudier le mouvement des electrons dans une structure periodique en presence de champ electrique. En effet, contrairement aux materiaux massifs, les superreseaux ont des parametres structuraux (periode, largeur de bande electronique) qui permettent une observation des phenomenes prevus theoriquement en appliquant des champs electriques relativement faibles: les etats propres devienne localises (echelle de wannier-stark), ce qui se manifeste par un decalage vers le bleu du seuil d'absorption du superreseau. L'adjonction d'un champ magnetique perpendiculairement au plan des couches permet de quantifier le mouvement dans ce plan sans influer sur le mouvement le long de l'axe de croissance. Ainsi suivant les champs appliques, le superreseau peut se comporter comme une structure 3d, 2d, 1d ou 0d. Les spectres d'absorption ont permis de mettre en evidence les modifications de la densite d'etat, et aussi l'importance des effets excitoniques dans les transitions optiques. Pour mesurer directement les effets du champ electrique, nous avons egalement etudie la possibilite d'observer la luminescence intrabande entre niveaux de wannier, ce qui peut etre interprete comme une preuve directe de l'existence d'oscillateurs de bloch. Enfin, nous presentons une etude de nouvelles structures unidimensionnelles et sans dimension. Les progres de la technologie permettent de fabriquer des lignes quantiques et des boites quantiques de largeur inferieure a 1000 a et les resultats d'experiences d'optique presentes dans cette these montrent clairement des effets dus au confinement lateral dans ces structures

碩士
國立交通大學
電子物理系所
105
Semiconductor heterostructures based on two-dimensional transition metal dichalcogenides (TMDs) have attracted great interests recently. These two-dimensional heterostructures were given hopes for the future flexible optoelectronic devices due to their unique structural and optical properties. In these heterostructrues, the outer material usually exhibits serious strain variation caused by large lattice mismatch, i.e., ~1.59% variation has been observed in the MoS2-WSe2 system. In this work, the spatial inhomogeneity in lateral MoS2-WSe2 and WSe2-MoSe2 heterostructures were investigated by spatial-resolved photoluminescence (PL), showing strong correlated PL intensity and energy. The PL correlation is found to be caused by carrier occupation between different valleys, which can be described by the Boltzmann distribution. After analyzing by model, the band parameters including energy difference and ratio of deformation potential for involved valleys can be determined. Finally, we demonstrated the strain-free MoS2 and WSe2 monolayers are direct and indirect semiconductors, respectively. Our work presented here provides not only the important band parameters but also information on the lateral heterostructures. Particularly, the local strain variation could result in not only the small change of band gap energy but also the great change of direct/indirect gap, which could largely impact the device properties for future planar TMD heterostructures.

abstract: This dissertation presents research findings on the three materials systems: lateral Si nanowires (SiNW), In₂Se₃/Bi₂Se₃ heterostructures and graphene. The first part of the thesis was focused on the growth and characterization of lateral SiNW. Lateral here refers to wires growing along the plane of substrate; vertical NW on the other hand grow out of the plane of substrate. It was found, using the Au-seeded vapor – liquid – solid technique, that epitaxial single-crystal SiNW can be grown laterally along Si(111) substrates that have been miscut toward [11− 2]. The ratio of lateral-to-vertical NW was found to increase as the miscut angle increased and as disilane pressure and substrate temperature decreased. Based on this observation, growth parameters were identified whereby all of the deposited Au seeds formed lateral NW. Furthermore, the nanofaceted substrate guided the growth via a mechanism that involved pinning of the trijunction at the liquid/solid interface of the growing nanowire. Next, the growth of selenide heterostructures was explored. Specifically, molecular beam epitaxy was utilized to grow In₂Se₃ and Bi₂Se₃ films on h-BN, highly oriented pyrolytic graphite and Si(111) substrates. Growth optimizations of In₂Se₃ and Bi₂Se₃ films were carried out by systematically varying the growth parameters. While the growth of these films was demonstrated on h-BN and HOPG surface, the majority of the effort was focused on growth on Si(111). Atomically flat terraces that extended laterally for several hundred nm, which were separated by single quintuple layer high steps characterized surface of the best In₂Se₃ films grown on Si(111). These In₂Se₃ films were suitable for subsequent high quality epitaxy of Bi₂Se₃ . The last part of this dissertation was focused on a recently initiated and ongoing study of graphene growth on liquid metal surfaces. The initial part of the study comprised a successful modification of an existing growth system to accommodate graphene synthesis and process development for reproducible graphene growth. Graphene was grown on Cu, Au and AuCu alloys at varioua conditions. Preliminary results showed triangular features on the liquid part of the Cu metal surface. For Au, and AuCu alloys, hexagonal features were noticed both on the solid and liquid parts.
Dissertation/Thesis
Doctoral Dissertation Materials Science and Engineering 2014

博士
國立中興大學
物理學系所
106
In this study, we investigate the properties of the depletion region in MoTe2 nano-flake and MoTe2/SnS2 heterostructure devices by scanning photocurrent microscopy (SPCM). We fabricated the multi-layer MoTe2 transistor devices and MoTe2/SnS2 vertical heterostructure transistor devices by the standard mechanically exfoliation method from MoTe2 and SnS2 flakes. The MoTe2 bulk material is obtained from natural mineral, and SnS2 flakes are grown by CVD method. The thickness of the MoTe2 and SnS2 are of the order of 10 nm. The 10-nm-Ti/90-nm-Au metal contacts are defined by e-beam lithography and lift off process. The sample is scanned by a focused 633-nm laser with a 1.5 um diameter on the focused plane and the drain-to-source photocurrent is collected via a low-noise current preamplifier. The DC transport measurements of the MoTe2 transistor shows an ON/OFF ratio about 103 and the field-effect mobility about 9.4 cm2/Vs at VSD = 0.5 V. From the bias dependence of the photocurrent peak obtained from the SPCM line scan data, we extract the contact barrier height of metal/MoTe2 to be about 100 meV. The areal mapping of the photocurrent shows a very large fluctuation near the junction between the metal and MoTe2, indicating that the barrier height is not uniform along the junction. The Schottky barrier fluctuations in metal/MoTe2 junction are identified to be caused by a non-uniform phase transition from a 2H semiconductor phase to a 1T’ metallic phase due to long-term laser beam irradiation. Here we have used I-V transport data, SPCM measurements, second-harmonic-generation (SHG) mapping, and AFM topography to reach the conclusion. The tunnel field effect transistors based on multilayer MoTe2/SnS2 vertical van der Waals heterosturcture devices have been investigated. The device with a MoTe2 layer stacked on a SnS2 layer shows an ON/OFF ratio over 104 and the subthreshold swing is about 1.5 V/dec at VDS = 1 V in ambient air, which is equivalent to about 56 mV/dec. if the dielectric is scaled down to 10 nm equivalent oxide thickness. The energy barrier extracted from the temperature dependence of I-V characteristics at VGS = 30 V is about 250 meV. We use SPCM with a 1 uW 633nm laser to investigate the heterostructure devices. The photocurrent is most significant along the boundary of MoTe2 and MoTe2/SnS2 heterostructure and mainly on the MoTe2 side. This indicates that there exists a depletion region near this boundary. We also conclude that the MoTe2/SnS2 junction should possess a type III band alignment. Besides, we compare the transport and photocurrent results of devices with different stacked sequences, MoTe2/SnS2/SiO2 and SnS2/MoTe2/SiO2. We find that the band alignment between the bare MoTe2 and the heterostructure part is different for these two samples. This could be caused by different charge trapping effect of the layer materials on the substrate. Finally, we report the detection the micro-crack in a multilayer MoTe2/SnS2 heterostructure thin film field effect transistors (MoTe2/SnS2 FETs) by SPCM. The crack with a width less than 50 nm is confirmed by the image of Transmission Electron Microscope (TEM). Near the crack inside the MoTe2/SnS2 heterostrucure, we observed an abnormal photocurrent signal that is due to the fracture in the SnS2 layer.