Optical and Etching Studies of Native Aluminum Oxide Layers for Use in Microcavity Photonic Devices

Optical communication and computing systems are required to meet future information transfer and processing needs. Microcavity devices serve as an enabling technology to implement and integrate optoelectronic systems. It is important to understand the optical and mechanical properties of materials utilize within microcavity devices. Only then is it possible to accurately model and analyze structures. Microcavity structures incorporating a high aluminum content AlGaAs layers are designed, grown, processed, and measured. The processing of these devices includes the conversion of high aluminum-content AlGaAs layers to native aluminum oxide (AlO) layers through the process of thermal oxidation. This selective conversion of microcavity layers provides for the necessary electrical and optical confinement required to produce a plethora of microphotonic devices. The optical properties of hydrolyzed AlO layers within a monolithic microcavity structure are experimentally determined. Also examined is the induced AlO layer stress, a result of volumetric shrinkage. Additional mechanical properties of GaAs/AlAs multilayer Fabry-Perot etalon structures are explored through the process of chemical etching. A suitable chemical solution to selectively etch converted AlO layers within a microcavity structure is developed. This research provides the foundation for future III-V MEMS technology development

Monolithic integration for nonlinear optical frequency conversion in semiconductor waveguides

This thesis presents an investigation into the feasibility of tunable, monolithically integrated, nonlinear optical frequency conversion sources which work under the principles of an optical parametric oscillator (OPO). The room-temperature continuous wave (CW) operation of these devices produces narrow line-width, near- and mid-infrared wavelengths, primarily used in chemical sensing applications. The devices detailed here, based on the GaAs–AlGaAs superlattice material system, benefit from post growth, ion implantation induced, quantum well intermixing, to achieve 1st order phase matching. The experiments, which have been performed to optimize the second-order nonlinear processes in our GaAs–AlGaAs superlattice waveguides, have demonstrated improved conversion efficiencies when compared to the performance achieved previously in similar superlattice nonlinear waveguides. We have achieved pulsed type-I phase matched second harmonic generation (SHG) with powers up to 3.65 μW (average pulse power), CW type-I phase matched SHG up to 1.6 μW for the first time, and pulsed type-II phase matched SHG up to 2 μW (average pulse power), again for the first time. Moreover, we have been able to achieve both CW type-I and CW type-II phase matched difference frequency generation, which converts C-band wavelengths into L- and U-band wavelengths, over at least a 20 nm conversion bandwidth. These results have been made possible through the systematic optimization of processes developed to fabricate nonlinear optical waveguides. Fabrication processes have also been developed to facilitate the incorporation of on-chip lasers and optical routing components, required to achieve a fully integrated OPO and nonlinear optical frequency converter. The optical routing in these devices has been demonstrated using a frequency selective multi-mode interference (MMI) coupler. The superlattice laser material has been designed by optimizing the material structure and employing different growth technologies. Room-temperature CW laser action has been achieved in 100 nm thick, superlattice core, half-ring lasers. The laser excitation is measured at 801 nm, and the internal power of the on-chip pump is estimated to be in excess of 200 mW in a full-ring, after accounting for optical routing, linear, bending and nonlinear losses. We have been able to conclude that our designed OPO and frequency converter is just feasible with the performance achieved in different components

Micro- and Nanotechnology of Wide Bandgap Semiconductors

Owing to their unique characteristics, direct wide bandgap energy, large breakdown field, and excellent electron transport properties, including operation at high temperature environments and low sensitivity to ionizing radiation, gallium nitride (GaN) and related group III-nitride heterostructures proved to be enabling materials for advanced optoelectronic and electronic devices and systems. Today, they are widely used in high performing short wavelength light emitting diodes (LEDs) and laser diodes (LDs), high performing radar, wireless telecommunications, as well ‘green’ power electronics. Impressive progress in GaN technology over the last 25 years has been driven by a continuously growing need for more advanced systems, and still new challenges arise and need to be solved. Actually, lighting industry, RF defene industry, and 5G mmWave telecommunication systems are driving forces for further intense research in order to reach full potential of GaN-based semiconductors. In the literature, there is a number of review papers and publications reporting technology progress and indicating future trends. In this Special Issue of Electronics, eight papers are published, the majority of them focusing materials and process technology of GaN-based devices fabricated on native GaN substrates. The specific topics include: GaN single crystalline substrates for electronic devices by ammonothermal and HVPE methods, Selective – Area Metalorganic Vapour – Phase Epitaxy of GaN and AlGaN/GaN hetereostructures for HEMTs, Advances in Ion Implantation of GaN and Related Materials including high pressure processing (lattice reconstruction) of ion implanted GaN (Mg and Be) and III-Nitride Nanowires for electronic and optoelectronic devices

Resonant tunnelling diodes for THz communications

Resonant tunnelling diodes realised in the InGaAs/AlAs compound semiconductor system lattice-matched to InP substrates represent one of the fastest electronic solid-state devices, with demonstrated oscillation capability in excess of 2 THz. Current state-of-the-art offers a poor DC-to-RF conversion efficiency. This thesis discusses the structural issues limiting the device performance and offers structural design optimums based on quantum transport modelling. These structures are viewed in the context of epitaxial growth limitations and their extrinsic oscillator performance. An advanced non-destructive characterisation scheme based on low-temperature photoluminescence spectroscopy and high-resolution TEM is proposed to verify the epitaxial perfection of the proposed structure, followed by recommendations to improve the statistical process control, and eventually yield of these very high-current density mesoscopic devices. This work concludes with an outward look towards other compound semiconductor systems, advanced layer structures, and antenna designs

MOVPE growth and characterisation of ZnO properties for optoelectronic applications

ZnO, Epitaxy, Metal organic vapor phase epitaxy, MOCVD, CVD, Semiconductor, Optoelectronics, X-ray diffraction, Cathodoluminescence, MicroelectronicsMagdeburg, Univ., Fak. für Naturwiss., Diss., 2007von Nikolay OleynikZsfassung in dt. Sprach

GaAs-Based Distributed Feedback Lasers Based on GaAs-InGaP Regrowth Technology

This thesis describes the conceptualisation and realisation of GaAs-based self-aligned stripe (SAS) distributed feedback lasers (DFB) based on GaAs-InGaP regrowth technology, and its incorporation into the development of master oscillator power amplifier (MOPA) photonic integrated circuit (PIC). GaAs-based SAS DFB lasers operate via a single longitudinal mode and provide a robust, portable and low cost solution to enable a broad range of potential applications. Compared to other waveguides, e.g. ridge waveguide, SAS structures enable narrower active regions and demonstrate better characteristics with a lower sensitivity to temperature. In my designs, InGaP/GaAs buried gratings are formed utilising an Al-free grating sequence GaAs-InGaP-GaAs, whilst the SAS waveguides are realised via a stripe-etched n-doped InGaP optoelectronic confinement layer, where no AlGaAs is exposed during the fabrication process. Chapter 1 goes through the development of DFB lasers over almost 5 decades since its birth in 1970s, followed by discussion of the gap between present GaAs-based PIC technologies and their commercialisation. After, Chapter 2 introduces the experimental methodology involved in the research activities conducted: fundamental principles of DFB lasers and the 4-stage research process. The following 3 chapters describe the 3 main projects in this research. Chapter 3 begins with the design of 2×, 4× and 6× InGaAs QWs narrow ridge DFB lasers in, and then moved onto the conceptualisation and realisation of 2× and 4× InGaAs QWs SAS DFB lasers in Chapter 4. This SAS-DFB technology was then applied to the development of monolithically integrated 4× InGaAs QWs MOPA PIC in Chapter 5. In Chapter 6, I outline some future work to be conducted for further achievement. An optimised design of SAS-DFB-MOPA is first discussed. I then present some preparatory works for two other potential future directions: widely tunable GaAs-based sampled grating distributed Bragg reflector laser (SG-DBR) and high power ~1180nm In(Ga)As/GaAs DWELL (dot-in-a-well) SAS-DFB-MOPA

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

Specular and diffuse X-ray scattering studies of surfaces and interfaces

The behaviour of thin film semiconducting and magnetic devices depends upon the chemical and physical status of the as-grown structure. Since the dimensions of many devices can be in the Angstrom and nanometre region, characterisation techniques capable of measuring chemical and physical parameters in this regime are necessary if an understanding of the effect of specimen structure on observed properties is to be achieved. This thesis uses high resolution x-ray scattering techniques to characterise sub-micron layered structures of semiconducting and magnetic materials. Double crystal diffraction is routinely employed in the semiconductor industry for the on line inspection of sample quality. While material parameters such as sample perfection and layer composition may be rapidly deduced, the non-destructive measurement of layer thickness is more difficult (particularly for multilayered samples) and lengthy simulation procedures are often necessary to extract the thickness information from a double crystal diffraction profile. However, for semiconductor structures which act as Bragg case interferometers, oscillations (known as thickness fringes) appear in the diffracted profile. The period of these fringes can be directly related to layer thickness. Attempts to Fourier transform diffraction data, in order to automatically extract the frequency" of thickness fringes, have previously been only partially successful. It is shown that the relatively weak intensity of the thickness fringes and the presence of the substrate peak in the analysed diffraction data, drastically reduce the quality of the subsequent Fourier transform. A procedure for the manipulation of diffraction data is suggested, where an "average” envelope is fitted to the thickness fringes and used to normalise the data. The application of an auto-correlation is shown to further increase the quality of the Fourier transform of the normalised data. The application of Fourier transform techniques to the routine analysis of double crystal diffraction data is discussedA novel technique for the measurement of absolute lattice parameters of single crystals is presented, which is capable of determining lattice constants with an absolute accuracy of around 2 parts in 10(^5). The technique requires only the use of a conventional triple crystal diffractometer with motorised 20 circle movement and the provision for a fine, precise rocking motion of the analyser. To demonstrate the technique, exemplary measurements on GaAs and InAs crystals are presented. Triple crystal diffi-action analysis has been performed on three material systems of current technological interest; the Hg(_1-x)Mn(_x)Te on GaAs, the Cd(_1-x)Hg(_x)Te on CdTe/Cd(_1-x)Zn(_x)Te and the low temperature grown GaAs systems. Studies on the Hg(_1-x)Mn(_x)Te on GaAs system reveal that the principal contribution to the rocking curve widths of layers grown using the direct alloy growth (DAG) method, arise from the tilt (i.e., mosaicity) of layer sub-grains. This finding is confirmed by double crystal topography which shows that the layers are highly mosaic with a typical grain size of (130±5)µm. Topographic studies of Hg(_1-x)Mn(_x)Te on GaAs, grown using the interdiffused multilayer process (IMP), show that sample quality is significantly improved with single crystal material being produced using this growth method. Triple crystal diffraction studies of the Cd(_1-x)Hg(_x)Te on CdTe/Cd(_0.96)Zn(_0.04)Te systems reveal several findings. These are that the main contribution to rocking curve widths is from lattice tilts and that the tilt distribution increases as the layer thickness decreases. Further, the quality of the Cd(_0.96)Zn(_0.04)Te substrate analysed is superior to that of the CdTe and that Cd(_1-x)Hg(_x)Te layers grown on Cd(_0.96)Zn(_0.04)Te substrates are generally of a higher quality than those grown on CdTe. Triple crystal analysis of MBE and ALE grown GaAs films, deposited at low growth temperatures, show that, at equivalent temperatures, superior quality films are grown by the ALE technique. Narrow lattice dilation and tilt distributions are reported for GaAs films grown at temperatures as low as 300ºC by the ALE method. While diffraction techniques are highly suitable for the study of relatively perfect crystalline material, they are not appropriate to the analysis of heavily dislocated or even amorphous specimens. This is not the case for the Grazing Incidence X-Ray Reflectivity (GIXR) technique, whose sensitivity is not dependent upon sample structure. The GIXR technique is currently attracting increasing interest following the development of commercial instruments. In this thesis, GIXR has been used to probe the layer thickness and interfacial roughness of a series of magnetic multilayer samples and Si/Si(_x)Ge(_1-x) superlattices. The technique is shown to be capable of measuring layer thickness to an accuracy of one monolayer. Modelling of specular GIXR data for the Si/Si(_x)Ge(_1-x) superlattices has shown that the magnitude of interfacial roughness is different for the two types of interface within the high Ge content superlattice samples, the Si(_x)Ge(_1-x)→Si interface possessing a long range sinusoidal roughness of (0.9±0.3)nm, in addition to die short range roughness of (0.5±0.2)nm present at all interfaces. By collecting the diffuse scatter from a GIXR experiment, conformal, or correlated, roughness has been observed in both the multilayer and superlattice samples