Advanced source and drain contact engineering for low parasitic series resistance

Master'sMASTER OF ENGINEERIN

Fabrication and Characterisation of Nitride DBRs and Nitride Membranes by Electrochemical Etching Techniques

A Distributed Bragg Reflector (DBR) is an important component for semiconductor microcavities and optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), resonant cavity light-emitting diodes (RCLEDs). In the past thirty years, epitaxially grown GaAs-based DBRs have made great achievements of the application of III-V VCSELs in communications and mobile applications. At the same time, III-nitrides have demonstrated excellent performance in solid-state lighting and advanced optoelectronic devices due to the wide bandgap and unique properties. In recent years, GaN-based semiconductors have made great progress in the application of blue VCSELs. However, the absence of high-performance DBRs is a challenge for developing higher-power GaN-based VCSELs. Currently, the typical epitaxial GaN-based DBRs are limited by a long growth period, low optical performance, and poor quality of growth. Therefore, this project proposes a method to fabricate nanoporous (NP)/GaN-based DBR by electrochemical etching (EC), which are grown using metalorganic vapour-phase epitaxy (MOVPE). The heavily silicon doped GaN layer is transformed into an NP structure by selective etching, resulting in a higher refractive index contrast in each periodic layer. Moreover, a lateral etching method is proposed to further improve the EC etching of DBRs. This method can confine the etching in each sacrificial layer and make the etching aperture directions highly uniform. The corresponding characterizations have been carried out to explore the mechanisms of different etching methods, by optical microscopy, scanning electron microscopy (SEM) and reflectance measurements. It further confirms that the laterally etched NP GaN-based DBRs exhibit a higher reflectivity and wider stopband. The GaN sacrificial layers required for the EC etching are typically heavily silicon doped (>1019cm-3), resulting in a rough surface and saturated conductivity. On the other hand, the heavily silicon doped AlGaN with a low Al content (≤5%) exhibits an atomically flat surface and an enhanced electrical conductivity. Therefore, in this work, we introduced multiple pairs of heavily doped n++-Al0.01Ga0.99N/GaN to replace the widely used multiple pairs of heavily doped n++-GaN/GaN to fabricate lattice-matched NP DBRs by EC etching. Consequently, the epitaxially grown n++-Al0.01Ga0.99N/GaN-based DBR demonstrates a smoother surface than the n++-GaN/GaN-based DBR. Moreover, the NP-Al0.01Ga0.99N/GaN-based DBR exhibits higher reflectivity and wider stopband after lateral EC etching compared to the NP-GaN/GaN-based DBR. This method has been successfully applied to fabrication of high-performance DBR structures with the wavelength range from blue to deep yellow by modifying the epitaxial growth conditions. Furthermore, it is found that a very thin Al-Si diffusion layer is formed at the interface between an AlN buffer layer and a silicon substrate when growing the low-temperature AlN buffer layer on the n-doped silicon substrate by MOVPE. The diffusion layer exhibits high conductivity and can be EC-etched and polished as a sacrificial layer. Therefore, this method is proposed for stripping large-area GaN membranes by EC etching. A sample with AlN/AlGaN/GaN layers is first epitaxially grown by MOVPE on an n-doped (111) silicon substrate, and then bonded upside-down to a new glass host substrate and EC etched. Finally, lift-off of a large size GaN-based membrane has been realized with an area of 2.625cm2 and a crack-free and nanoscale smooth surface. Compared to other lift-off methods such as laser lift-off (LLO), chemical lift-off (CLO), and mechanical release techniques, this method does not involve bulky and expensive equipment, which can be used to fabricate high-performance III-nitride devices on the membrane at low cost in the future

Semiconductor Infrared Devices and Applications

Infrared (IR) technologies—from Herschel’s initial experiment in the 1800s to thermal detector development in the 1900s, followed by defense-focused developments using HgCdTe—have now incorporated a myriad of novel materials for a wide variety of applications in numerous high-impact fields. These include astronomy applications; composition identifications; toxic gas and explosive detection; medical diagnostics; and industrial, commercial, imaging, and security applications. Various types of semiconductor-based (including quantum well, dot, ring, wire, dot in well, hetero and/or homo junction, Type II super lattice, and Schottky) IR (photon) detectors, based on various materials (type IV, III-V, and II-VI), have been developed to satisfy these needs. Currently, room temperature detectors operating over a wide wavelength range from near IR to terahertz are available in various forms, including focal plane array cameras. Recent advances include performance enhancements by using surface Plasmon and ultrafast, high-sensitivity 2D materials for infrared sensing. Specialized detectors with features such as multiband, selectable wavelength, polarization sensitive, high operating temperature, and high performance (including but not limited to very low dark currents) are also being developed. This Special Issue highlights advances in these various types of infrared detectors based on various material systems

Bolometers

Infrared Detectors and technologies are very important for a wide range of applications, not only for Military but also for various civilian applications. Comparatively fast bolometers can provide large quantities of low cost devices opening up a new era in infrared technologies. This book deals with various aspects of bolometer developments. It covers bolometer material aspects, different types of bolometers, performance limitations, applications and future trends. The chapters in this book will be useful for senior researchers as well as beginning graduate students

Annual report 2007 // Institute of Ion Beam Physics and Materials Research

[no abstract available

Advanced contact engineering for silicon, germanium and germanium-tin devices

Ph.DDOCTOR OF PHILOSOPH

Fabrication, characterization, and modeling of silicon multi-gate devices

Ph.DDOCTOR OF PHILOSOPH

Charakterisierung funktionaler Nanomaterialien für biomagnetische Sensoren und Atemanalyse

The presented thesis is covering materials aspects for the development of magnetoelectric sensors for biomagnetic sensing and solid state sensors for breath monitoring. The electrophysiological signals of the human body and especially their irregularities provide extremely valuable information about the heart, brain or nerve malfunction in medical diagnostics. Similar and even more detailed information is contained in the generated biomagnetic fields which measurement offers improved diagnostics and treatment of the patients. A new type of room temperature operable magnetoelectric composite sensors is developed in the framework of the CRC1261 Magnetoelectric Sensors: From Composite Materials to Biomagnetic Diagnostics. This thesis focuses on the individual materials structure-property relations and their combination in magnetoelectric composite sensors studied by electron beam based techniques, at lengths scales ranging from micrometers to atomic resolution. The first part of this thesis highlights selected studies on the structural and analytic aspects of single phase materials and their composites using TEM as the primary method of investigation. With respect to the piezoelectric phase, alternatives to AlN have been thoroughly investigated to seek for improvement of specific sensor approaches. In this context, the alloying of Sc into the AlN matrix has been demonstrated to yield high quality films with improved piezoelectric and unprecedented ferroelectric properties grown under the control of deposition parameters. Lead-free titanate films with large piezo-coefficients at the verge of the morphotropic phase boundary as alternative to PZT films have been investigated in terms of crystal symmetry, defect structure and domains of cation ordering. New morphologies of ZnO and GaN semiconductors envisioned for a piezotronic-based sensor approach were subject of in-depth defect and analytical studies describing intrinsic defects and lattice strains upon deposition as well as hollow composite structures. When the dimensions of a materials are reduced, novel exciting properties such as in-plane piezoelectricity can arise in planar transition-metal dichalcogenides. Here, the turbostratic disorder in a few-layered MoSe2 film has been investigated by nanobeam electron diffraction and Fast Fourier Transformations. From the perspective of magnetic materials, the atomic structure of magnetostrictive multilayers of FeCo/TiN showing stability up to elevated temperatures has been analyzed in detail regarding the crystallographic relationship of heteroepitaxy in multilayer composites exhibiting individual layer thicknesses below 1 nm. Further, magnetic hard layers have been investigated in the context of exchange spring concepts and ME composites based on shape memory alloy substrates have been studied regarding structural changes implied by different annealing processes. The second part of this thesis introduces materials aspects and sensor studies on gas detection in the clinical context of breath analysis. The detection of specific vapors in the human breath is of medical relevance, since certain species can be enriched depending on the conditions and processes within the human body. Hence, they can be regarded as biomarkers for the patients condition of health. The selection of suitable materials and the gas measurement working principle are considered and selected studies on solid state sensors with different surface functionalization or targeted application on basis of ZnO or CuO-oxide and Fe-oxide species are presented