Impact Of Ion Implantation On Quantum Dot Heterostructures And Devices

This book looks at the effects of ion implantation as an effective post-growth technique to improve the material properties, and ultimately, the device performance of In(Ga)As/GaAs quantum dot (QD) heterostructures. Over the past two decades, In(Ga)As/GaAs-based QD heterostructures have marked their superiority, particularly for application in lasers and photodetectors. Several in-situ and ex-situ techniques that improve material quality and device performance have already been reported. These techniques are necessary to maintain dot density and dot size uniformity in QD heterostructures and also to improve the material quality of heterostructures by removing defects from the system. While rapid thermal annealing, pulsed laser annealing and the hydrogen passivation technique have been popular as post-growth methods, ion implantation had not been explored largely as a post-growth method for improving the material properties of In(Ga)As/GaAs QD heterostructures. This work attempts to remedy this gap in the literature. The work also looks at introduction of a capping layer of quaternary alloy InAlGaAs over these In(Ga)As/GaAs QDs to achieve better QD characteristics. The contents of this volume will prove useful to researchers and professionals involved in the study of QDs and QD-based devices.

Ion implantation offers one of the best examples of a topic that starting from the basic research level has reached the high technology level within the framework of microelectronics. As the major or the unique procedure to selectively dope semiconductor materials for device fabrication, ion implantation takes advantage of the tremendous development of microelectronics and it evolves in a multidisciplinary frame. Physicists, chemists, materials sci entists, processing, device production, device design and ion beam engineers are all involved in this subject. The present monography deals with several aspects of ion implantation. The first chapter covers basic information on the physics of devices together with a brief description of the main trends in the field. The second chapter is devoted to ion im planters, including also high energy apparatus and a description of wafer charging and contaminants. Yield is a quite relevant is sue in the industrial surrounding and must be also discussed in the academic ambient. The slowing down of ions is treated in the third chapter both analytically and by numerical simulation meth ods. Channeling implants are described in some details in view of their relevance at the zero degree implants and of the available industrial parallel beam systems. Damage and its annealing are the key processes in ion implantation. Chapter four and five are dedicated to this extremely important subject.

Ion implantation is one of the promising areas of sciences and technologies. It has been observed as a continuously evolving technology. In this book, there is a detailed overview of the recent ion implantation research and innovation along with the existing ion implantation technological issues especially in microelectronics. The book also reviews the basic knowledge of the radiation-induced defects production during the ion implantation in case of a semiconductor structure for fabrication and development of the required perfect microelectronic devices. The improvement of the biocompatibility of biomaterials by ion implantation, which is a hot research topic, has been summarized in the book as well. Moreover, advanced materials characterization techniques are also covered in this book to evaluate the ion implantation impact on the materials.

This book describes the use of ion implantation to control the optical properties of solid state materials.

Silicon/silicon-germanium (Si/SiGe) heterostructures are useful as hosts for gated quantum dots. The quality of the as-grown Si/SiGe heterostructure has a large impact on the final quality of the quantum dot as a qubit host. For many years, quantum dots have been fab- ricated on strain-graded heterostructures. Commonly used strain-graded heterostructures inevitably develop plastic defects that lead to interface roughness, crosshatch, and mosaic tilt. All of these factors are sources of disorder in Si/SiGe quantum electronics. In this dissertation, I report the fabrication of Hall bars and gated quantum dots on heterostruc- tures grown on fully elastically relaxed SiGe nanomembranes, rather than strain-graded heterostructures. I report measurements of Hall bars demonstrating the creation of two- dimensional electron gases in these structures. I report the fabrication procedures used to create pairs of Hall bars and quantum dots on individual membranes. In addition, I explain a general process flow for the creation of Si/SiGe quantum devices. I focus especially on an ion-implantation technique I implemented for the fabrication of Hall bars and quantum dots in Si/SiGe heterostructures without modulation doping layers.

In nanostructures (NSs), to acquire a fundamental understanding of the electronic states by studying the optical properties is inherently complicated. A widely used simplification to this problem comes about by developing a model for a small scale representation of types of NSs and applying it to a hierarchy of fabrication methods. However, this methodology fails to account for structural differences incurred by the fabrication method that lead to differences in the optical properties. Proper modelling is realized by first considering the proper range of experimental parameters individually as inputs to a theoretical model and applying the correct parameters to the corresponding fabrication method. This thesis studies the connection between the structural and optical properties of NSs as a function of the fabrication method, using, principally, x-ray photoemission, Rutherford backscattering, photoluminesence, and Raman spectroscopy. Ion implanted Si and Ge quantum dots (QDs) in dielectric matrix were prepared to study the optical and structural properties, and compared against several other preparation methods. Ge QDs are known to exhibit a high concentration of defect states. The cause of these states was studied for QDs in a sapphire matrix and attributed to diffusion and desorption of Ge during annealing. Optical studies of Si QDs fabricated using an implantation mask revealed that state-filling and excitation transfer are important parameters in densely packed QD arrays. Structural analysis of Si QDs in silica revealed a well defined interface composed of Si$_2$O$_3$ and no stress was detected. Furthermore, the valence level was pinned at its bulk position possibly due to interface states. This information was used to refine our theoretical model of QDs and then compared with a range of crystalline and amorphous Si and Ge NSs. Stronger confinement effects were observed in amorphous Si and Ge NSs, possibly due to the nature of the interface or re-normalization of the effective mass as a function of NS size. These results establish a framework for proper parameter control in theoretical modelling.

In recent years, ion implantation has developed into the major doping technique for integrated circuits. Several series of conferences have dealt with the application of ion implantation to semiconductors and other materials (Thousand Oaks 1970, Garmisch-Partenkirchen 1971, Osaka 1974, Warwick 1975, Boulder 1976, Budapest 1978, and Albany 1980). Another series of conferences was devoted more to implantation equipment and tech­ niques (Salford 1977, Trento 1978, and Kingston 1980). In connection with the Third International Conference on Ion Implantation: Equipment and Tech­ niques, held at Queen's University,' Kingston, Ontario, Canada, July 8-11, 1980, a two-day instructional program was organized parallel to an implan­ tation conference for the first time. This implantation school concentra­ ted on aspects of implantation-equipment design. This book contains all lectures presented at the International Ion Implantation School organized in connection with the Fourth International Conference on Ion Implantation: Equipment and Techniques, held at the Convention Center, Berchtesgaden, Germany, September 13-17, 1982. In con­ trast to the first .school, the main emphasis in thiS school was placed on practical aspects of implanter operation and application. In three chap­ ters, various machine aspects of ion implantation (general concepts, ion sources, safety, calibration, dOSimetry), range distributions (stopping power, range profiles), and measuring techniques (electrical and nonelec­ tri ca 1 measu ri ng techni ques, annea 1 i ng) are di scussed. In the appendi x, a review of the state of the art in modern implantation equipment is given.

This thesis explores the understanding of the chemistry and physics of colloidal quantum dots for practical solar energy photoconversion. Solar cell devices that make use of PbS quantum dots generally rely on constant and unchanged optical properties such that band gap energies remain tuned within the device. The design and development of unique experiments to ascertain mechanisms of optical band gap shifts occurring in PbS quantum dot thin-films exposed to air are discussed. The systematic study of the absorption properties of PbS quantum dot films exposed to air, heat, and UV illumination as a function of quantum dot size has been described. A method to improve the air-stability of films with atomic layer deposition of alumina is demonstrated. Encapsulation of quantum dot films using a protective layer of alumina results in quantum dot solids that maintain tuned absorption for 1000 hours. This thesis focuses on the use of atomic force microscopy and electrical variants thereof to study the physical and electrical characteristics of quantum dot arrays. These types of studies have broad implications in understanding charge transport mechanisms and solar cell device operation, with a particular emphasis on quantum dot transistors and solar cells. Imaging the channel potential of a PbSe quantum dot thin-film in a transistor showed a uniform distribution of charge coinciding with the transistor current voltage characteristics. In a second study, solar cell device operation of ZnO/PbS heterojunction solar cells was investigated by scanning active cross-sections with Kelvin probe microscopy as a function of applied bias, illumination and device architecture. This technique directly provides operating potential and electric field profiles to characterize drift and diffusion currents occurring in the device. SKPM established a field-free region occurring in the quantum dot layer, indicative of diffusion-limited transport. These results provide the path to optimization of future architectures that may employ drift-based transport in the quantum dot layer for enhanced charge extraction and power conversion efficiency.