Innovation on human-machine interfacing technologies is critical for the development of smart, multifunctional and efficient electronic/optoelectronic systems. The effect of piezotronics is a newly started field of study, which utilizes piezoelectric polarization that is mechanically induced inside a piezoelectric semiconductor to regulate electron transport across electronic contact interfaces. With the concept coined in 2006, many efforts have been contributed to studying the underlying physical mechanism of this effect as well as demonstrating various applications based on single nanowire piezotronic devices. This thesis selects ZnO as the material foundation and was started by firstly studying flexible, controllable and scalable synthesis methods for ZnO nanowires array and thin film. By replacing the use of random, individual nanowires with these materials, novel piezotronic and piezophototronic devices were designed, fabricated and tested to achieve the function of strain sensing, tactile imaging, piezo-enhanced photodetection and solar energy harvesting. The adoption of nanowires array and thin film materials over single nanowires leads to significant advantages in terms of scalable fabrication, industrial compatibility and broader functionality. By consistently going down this route, we believe that the field of piezotronics will eventually make revolutionary impact on MEMS, optoelectronics, multifunctional sensor networks, human-machine interfacing and so on.

Zinc oxide is a semiconducting material that has received lot of attention due to its numerous proeprties such as wide direct band gap, piezoelectricity, and numerous low cost and robust methods of synthesizing nanomaterials. Its piezoelectric properties have been harnessed for use in energy production through nanogenerators, and to tune carrier transport, birthing a field known as piezotronics. However, one weakness of ZnO is that it is notoriously difficult to dope p-type. Antimony was investigated as a p-type dopant for ZnO, and found to have a stability of up to 3 years, which is completely unprecedented in the literature. Furthermore, a variety of zinc oxide structures ranging from ultra-long nanowires to thin films were produced and their piezotronic properties were demonstrated. By making p-n homojunctions using doped and undoped ZnO, enhanced nanogenerators were produced which could see application in gesture recognition. As a proof of concept, a simple photodetector was also derived from a core-shell nanowire structure. Finally, the ability to integrate this material with other semiconductors was demonstrated by growing a heterojunction with silicon nanowires, and investigating its electrical properties. All this work together lays the foundation for a fundamentally new material that could see application in future electronics, optoelectronics, and human-machine interfacing.

Materials have always had a large impact on society over the different ages. Piezoelectric materials are the often ‘invisible’ materials which find widespread use, unknown to the general public by large. Mobile electronics, automotive systems, medical and industrial systems are few of the key areas where ‘piezoelectricity’ is indispensable. The parking sensor of our car uses the effect and even the echo to image an unborn baby in a womb requires the exploitation of the piezoelectric effect. The work presented in this thesis investigates the piezoelectric effect in semiconductors, namely in III V, III N and II VI materials to have a better understanding and design potential applications in light emitting diodes (LEDs) and other electronic devices. The current work focuses on the non-linear behaviour in the strain of the piezo effect, which is manifested by the generation of electric field under crystal deformation. Previous works have already confirmed the reports of the existence of non-linear piezoelectric effects in zincblende III V semiconductors. Here, the same semiempirical approach using Density Functional Theory has been utilized to investigate the strain dependent elastic and dielectric properties of wurtzite III N materials. While we report the strong non-linear strain induced piezoelectric behaviour with second order coefficients, all spontaneous polarization terms are substantially smaller than the previously proposed values. We show that, unlike existing models, our calculated piezoelectric coefficients and nonlinear model provide a close match to the internal piezoelectric fields of quantum well and superlattice structures. Also, pressure dependence of the piezoelectric field in InGaN based LEDs predicts a significant improvement of the spontaneous emission rate can be achieved as a result of a reduction of the internal field. The LED devices using the proposed structures including a metamorphic layer under the active region of the device are expected to increase their light output power by up to 10%. We also explored the impact of the non-linear piezo effect in nanowires and present a further theoretical computational study of single photon sources optimization in InGaN based wurtzite single quantum dots. We observed the light emission can be made by those single photon sources covering the entire visible spectrum through suitable change in the alloy composition.

Group-III-nitrides have been intensively investigated for optoelectronics and power electronics and are uniquely suitable for energy-related applications, such as solar hydrogen generation and nanogenerators. Compared to planar group-III-nitrides, their nanostructures offer a high surface-to-volume ratio, increased light absorption cross-section, and improved carrier transportation behavior. This thesis focuses on molecular-beam-epitaxy-grown group-III-nitrides, specifically nanowires and membranes, and applications in renewable energy harvesting and conversion. A Mo2C-decorated (In,Ga)N nanowire-based photocathode was demonstrated for nitrogen fixation. The conventional Haber-Bosch method demands high reaction pressure and temperature while releasing a considerable amount of greenhouse gas. The proposed photoelectrocatalytic method can utilize solar energy to generate ammonia without carbon emissions. The proposed photocathodes can achieve maximum faradaic efficiency of 12 %, ammonia yield of 8.9 µg/h/cm2, and excellent stability for over 12 hrs. Moreover, group-III-nitrides were fabricated into a freestanding membrane through a novel method combining electrochemical porosification and controlled spalling. The novel method is reproducible and scalable, which can significantly reduce the consumption of sacrificial substrates compared to existing nitride membrane exfoliation techniques, thus promising a scalable platform. The as-fabricated GaN membranes were demonstrated for photoelectrocatalytic methylene blue degradation. Through laboratory tests and rooftop field tests, we proved the feasibility of our wafer-scale GaN membranes in achieving a dye degradation efficiency of 92%, a total organic carbon removal rate of 50.2%, and extraordinary stability for ~ 50 hours under solar illumination. The membrane can also degrade ~87% of MB under visible-light illumination. Furthermore, the (Al,Ga)N membranes were fabricated into flexible transparent piezoelectric devices. The devices can sense compression pressure and bending strain while giving a comparable compression sensitivity to other thin film piezotronics devices of ~ 2.41 mV/kPa and 42.36 pA/kPa, a maximum bending gauge factor of ~ 1271, and an output power density of ~ 5.38 nW/cm2. The sensors can withstand over 35000 cycles of operation and can be utilized for sensing and harvesting mechanical energies from human motions and environmental signals. This research utilized nanowires and membrane-based group-III-nitrides for different photoelectrocatalytic reactions and piezotronics devices, from material preparation and characterizations, and demonstrated practical devices for clean energy-related applications.

Novel technology which can provide new solutions and enable augmented capabilities to CMOS based technology is highly desired. Piezotronic nanodevices and integrated systems exhibit potential in achieving these application goals. By combining laser interference lithography and low temperature hydrothermal method, an effective approach for ordered growth of vertically aligned ZnO NWs array with high-throughput and low-cost at wafer-scale has been developed, without using catalyst and with a superior control over orientation, location/density and morphology of as-synthesized ZnO NWs. Beyond the materials synthesis, by utilizing the gating effect produced by the piezopotential in a ZnO NW under externally applied deformation, strain-gated transistors (SGTs) and universal logic operations such as NAND, NOR, XOR gates have been demonstrated for performing piezotronic logic operations for the first time. In addition, the first piezoelectrically-modulated resistive switching device based on piezotronic ZnO NWs has also been presented, through which the write/read access of the memory cell is programmed via electromechanical modulation and the logic levels of the strain applied on the memory cell can be recorded and read out for the first time. Furthermore, the first and by far the largest 3D array integration of vertical NW piezotronic transistors circuitry as active pixel-addressable pressure-sensor matrix for tactile imaging has been demonstrated, paving innovative routes towards industrial-scale integration of NW piezotronic devices for sensing, micro/nano-systems and human-electronics interfacing. The presented concepts and results in this thesis exhibit the potential for implementing novel nanoelectromechanical devices and integrating with MEMS/NEMS technology to achieve augmented functionalities to state-of-the-art CMOS technology such as active interfacing between machines and human/ambient as well as micro/nano-systems capable of intelligent and self-sufficient multi-dimensional operations.

Ce projet de thèse aborde la question de l’intégration hétérogène de nanofils interconnectés sur des puces microélectroniques à destination de dispositifs de type MEMS et NEMS. Ces dispositifs visent à adresser la problématique globale qu’est le « More than Moore », c’est-à-dire la transformation des filières CMOS classiques pour permettre le développement de nouveaux micro et nano-composants intégrés.En particulier, ces dernières années, une variété de dispositifs à base de nanomatériaux ont vu le jour, conférant à des dispositifs de type micro-actionneurs et micro-capteurs de nouvelles fonctionnalités et/ou des performances accrues, e.g. en termes de résolution, sensibilité, sélectivité. Nous nous intéresserons ici à un certain type de nanostructures, les nanofils d’oxyde de zinc (ZnO), qui ont surtout été utilisés pour concevoir des dispositifs dont le principe de fonctionnement exploite l’effet piézoélectrique, souvent astucieusement combiné avec leurs propriétés semiconductrices. En effet, sous l’effet d’une contrainte mécanique ou d’un déplacement, les nanofils piézoélectriques génèrent un potentiel électrique (piézopotentiel). Si, en outre, les nanofils sont semiconducteurs, le piézopotentiel peut être utilisé pour contrôler un courant externe en fonction de la contrainte mécanique imposée au nanofil (effet piézotronique). L’avantage d’utiliser des nanostructures unidimensionnelles réside dans la modularité de leurs propriétés mécaniques et piézoélectriques en comparaison avec le matériau massif. Par ailleurs, leur intégration est aujourd’hui possible par des voies de croissance compatibles avec les procédés microélectroniques (CMOS/MEMS). Toutes ces considérations rendent possibles la conception de dispositifs très haute performance combinant la faible dimension des éléments fonctionnels (et donc une forte densité d’intégration synonyme de haute résolution spatiale) et leur sensibilité à des phénomènes d’échelle nanoscopique.Dans ce projet de thèse, on adoptera une vision très technologique de la conception de capteurs matriciels à base de nanofils piézoélectriques verticaux en ZnO. S’appuyant sur la prédiction des performances théoriques et la levée des verrous technologiques associés à la conception et la fabrication du capteur, cette étude s’attache à fournir des prototypes faisant la preuve de concept de ces dispositifs haute performance. Dans un premier temps, la réflexion s’articule autour de modèles multi-physiques par éléments finis (FEM) de la réponse piézoélectrique d’un seul nanofil en flexion, modèle que nous avons fait évoluer vers des pixels complets représentatifs d’un nanofil interconnecté dans une matrice. Sur la base de ces considérations, nous avons imaginé des moyens de caractérisation de la réponse piézoélectrique d’un fil, puis d’un pixel. Le banc de caractérisation mis en place a mis en évidence la complexité d’une mesure piézoélectrique systématique, calibrée et décorrélée des éléments environnants du pixel. Des solutions technologiques adéquates ont pu être imaginées et mises en œuvre à travers la réalisation de pixels élémentaires caractérisables et dont la réponse piézoélectrique peut être prédite théoriquement.Cette réalisation a fait appel à un développement en plusieurs étapes, incluant la croissance par voie chimique des nanofils en ZnO, puis la conception de la matrice d’électrodes contactant individuellement les nanofils. La première se découpe en deux étapes : d’abord le choix d’une couche de germination favorisant la croissance sur puce silicium et compatible avec les procédés de salle blanche ; ensuite le développement d’un procédé de croissance permettant la localisation des nanofils au sein d’une matrice d’électrodes. La seconde moitié du travail de fabrication a consisté à définir et à optimiser l’empilement technologique respectant toutes les considérations abordées jusqu’alors, et à définir les procédés technologiques aboutissant à la fabrication de la matrice finale. / This thesis project deals with the question of heterogeneous integration of interconnected nanowires on microelectronics chips in a view to MEMS and NEMS type devices. These devices aim to address the global problematic of “More than Moore”, that is the transformation of classical CMOS microelectronics processes to enable the development of new integrated micro and nanocomponents.In particular, over the past few years, a variety of nanomaterial-based devices have arisen, revealing micro-actuators and micro-sensors with new functionalities and/or improved performances, e.g. in terms of resolution, sensitivity, selectivity. Here we will focus on a certain type of nanostructures, Zinc Oxide (ZnO) nanowires, which have mostly been used so far to design devices whose working principle exploits the piezoelectric effect, often judiciously combined with their semiconducting properties. Indeed, when submitted to a mechanical constraint or displacement, piezoelectric nanowires generate an electrical potential (piezopotential). If, in addition to this, nanowires are also semiconducting, the piezopotential can be exploited to control an external current as a function of the mechanical constraint imposed to the nanowire (piezotronic effect). The advantage of using one-dimensional nanostructures lies into the modularity of both their mechanical and piezoelectric properties, in comparison with the bulk material. Moreover, their integration is now possible thanks to growth processes compatible with microelectronic processes (CMOS/MEMS). All these considerations make it possible to design very high performance devices combining the very small dimension of their functional unit elements (hence a high integration density which implies a high spatial resolution) and their sensitivity to nanoscale phenomena.In this project, we will adopt a very technology-oriented vision of the design of vertically-aligned ZnO-piezoelectric-nanowire matrix-type sensors. Relying on theoretical performance predictions and technological choices to solve device design and fabrication issues, this study aims to produce proof-of-concept prototypes of these high performance devices. First of all, the design process is elaborated based on finite element multiphysics models (FEM) of the piezoelectric response of a single bent nanowire, which we upgraded towards complete pixels, representative of an interconnected nanowire within a matrix. Following these considerations, we have imagined means of characterization of the piezoelectric response of a wire, then of a pixel. The implemented characterization experiment highlighted the complexity of carrying out a systematic, calibrated piezoelectric measurement, decorrelated from the environment of the pixel. Adequate technological solutions could then be implemented through the fabrication of elementary pixels suitable for characterization and whose piezoelectric response could be predictively modeled.This technological part of the work encompassed several development stages, including the chemical growth of ZnO nanowires and the design of the electrode matrix contacting the nanowires individually. The former splits into two steps: first choosing a clean-room compatible seed layer which will favor growth on a Silicon chip; secondly developing a selective growth process enabling the localization of nanowires within a predefined matrix of electrodes. The second part of the fabrication work focused on defining and optimizing the technological stack with respect to all the above mentioned considerations, and implementing the technological processes yielding the final targeted matrix.