Si-based Germanium Tin Photodetectors for Short-Wave and Mid-Wave Infrared Detections

The demand of light-weight and inexpensive imaging system working in the infrared range keeps increasing for the last decade, especially for civil applications. Although several group IV materials such as silicon and germanium are used to realize detectors in the visible and near infrared region, they are not the efficient approach for imaging system in the short-wave infrared detection range and beyond due to bandgap limit. On the other hand, this market is heavily relied upon mature technology from III-V and II-VI elements over years, which are costly to growth and incompatible with available Si complementary metal-oxide-semiconductor (CMOS) foundries. This limits the fabrication of large scale focal plan arrays detectors in this detection range. Therefore, a material system that meets the necessary requirements has long been in demand. The Ge1-xSnx material system has been introduced as a potential solution for low-cost high-performance photodetector for short-wave infrared towards mid-infrared detections due to its compatibility with Si CMOS process and wide detection range by incorporating more Sn in the alloy. Since then, immense growth efforts have been made to improve the material quality reaching device-quality using commercial chemical vapor deposition (CVD) reactors or molecular beam epitaxy (MBE) chambers. This dissertation will develop Si-based GeSn photodetectors technology to realize low-cost high-performance focal plane arrays detectors working in the SWIR towards MIR. It began with the development of fabrication process of single element GeSn photoconductor and photodiode, followed by systematic characterization and analysis of detectors’ figures of merits to provide a more optimized structure. A peak responsivity of 20 A/W (photoconductor) and 0.34 A/W (photodiode) at 2 µm were achieved. An external quantum efficiency of 20 % was reported for the first time. The highest value of specific detectivity D* is only 3-4 times less than commercially available Extended-InGaAs detector. Surface passivation technique was also pursued to reduce surface leakage current. Finally, infrared imaging capability was demonstrated using single pixel detector. The study involves a wide range of Sn composition from 10 to 22 %

Growth of Group IV and III-V Semiconductor Materials for Silicon Photonics: Buffer Layer and Light Source Development

High data transmission speeds, high levels of integration, and low manufacturing costs have established Si photonics as a crucial technology for next-generation data interconnects and communications systems. It involves a variety of components including light emitters, photodetectors, amplifiers, waveguides, modulators, and more. Because of its indirect bandgap, silicon is unable to serve as an efficient light source on a chip, hence this has been one of the formidable challenges. Within the framework of the monolithic approach, this thesis presents the study of two essential aspects of this challenge, the optimisation of buffer layers and development of light sources, by incorporating and improving different systems of Group IV thin films and III-V quantum dots (QDs) semiconductor materials. The monolithic approach focuses on the direct epitaxial growth of highly efficient light sources, usually by the epitaxy of III-V semiconductors lasers on a single Si chip. However, because of the material dissimilarities between III-V materials and Si, during the heteroepitaxy, a high density of crystalline defects such as threading dislocations (TDs), thermal cracks and anti-phase domains are introduced, severely impeding the performance and yield of the laser. For instance, TDs act as non-radiative recombination centres, while thermal cracks cause issues with the efficient evanescent coupling of the emitted light with Si waveguide. To address these defects, typically complex buffer growth techniques with micron-scale thickness are employed. The research in this thesis is divided into two parts, namely buffer layer optimisation and light source development. Each part outlines alternative strategies for overcoming the above-mentioned hurdles for monolithic growth. The first part highlights the optimisation of buffer layer growth to reduce threading dislocations for the monolithic integration of high-performance direct-bandgap III-V and group IV light sources on Si. The growth optimisation of low defect-density Ge buffer layers epitaxially grown on Si was first investigated. Defect elimination in Ge buffers with doped and undoped seed layers of increasing total thickness is studied under a variety of growth regimes, doping techniques, and annealing processes. This study demonstrates that a 500 nm thin Ge achieves the same defect level (1.3 × 108 cm -2) as 2.2 μm GaAs grown on Si, which greatly increases the thickness budget for the subsequent dislocation filter layers (DFLs) and laser structure growth before the formation of thermal cracks. Meanwhile, a low threading dislocation density of 3.3 × 107 cm -2 is obtained for 1 μm Ge grown on Si. The second part places emphasis on the development of light sources in the near-infrared wavelength range for Si photonics. 1) The development of GeSn, an emerging direct bandgap light source for Si photonics, is shown, which has wide bandgap tuneability and full compatibility with Si complementary metal-oxide semiconductor (CMOS). Growing the high Sn composition of GeSn required for efficient light generation is challenging and its growth generally severely affected by large surface roughness and Sn segregation. In this work, first, ex-situ rapid thermal annealing for the grown GeSn layer is investigated, showing that by proper annealing the strain can be relaxed by 90% without intriguing Sn segregation. This method shows its potential for both material growth and device fabrication. Besides, strain compensated layer and in-situ annealing techniques have been developed. Significantly improved surface quality has been confirmed by in-situ reflection high-energy electron diffraction (RHEED) observations and atomic force microscopy (AFM) images. Transmission electron microscopy (TEM) results reveal the high crystal quality of the multiple quantum wells (MQWs) grown on such buffer layers. 2) The final section details the development of InAs/InP QDs emitting near the strategic 1.55 μm, the lowest optical fibre loss window. The InAs/InP QDs growth is prone to inhomogeneous quantum dash morphologies which broaden the photoluminescence (PL) spectra and degrade the carrier confinement. Research has been conducted on growth parameters and techniques including deposition thickness, growth temperature and Indium-flush technique is applied to improve the uniformity of the dots, and narrow room temperature PL linewidths of 47.9 meV and 50.9 meV have been achieved for single-layer and five-layer quantum dot samples, respectively. The structures enable the fabrication of small footprint microdisk lasers with lasing thresholds as low as 30 μW

High-Sn-content GeSn Alloy towards Room-temperature Mid Infrared Laser

Si photonics is a rapidly expanding technology that integrates photonic circuits onto a Si substrate. The integration of Si electronics and photonics has been a successful technology for a wide range of applications. Group-IV alloy GeSn has drawn great attentions as a complementary metal–oxide–semiconductor compatible optoelectronic material for Si photonics. The devices based on GeSn alloy could be monolithically integrated into well-established and high-yield Si integrated circuits, which is favorable for chip-scale Si photonics featuring smaller size, lower cost, and higher reliability. The relaxed GeSn with high material quality and high Sn composition is highly desirable to cover mid-infrared wavelength. A systematic study of GeSn strain relaxation mechanism and its effects on Sn incorporation during the epitaxy via chemical vapor deposition was conducted. It was discovered that Sn incorporation into Ge lattice sites is limited by high compressive strain rather than historically acknowledged chemical reaction dynamics, which was also confirmed by Gibbs free energy calculation. Following the discovered growth mechanism, a world-record Sn content of 22.3% was achieved. Even higher Sn content could be obtained if further continuous growth with the same recipe is conducted. The GeSn laser with higher Sn content is highly desired to cover longer wavelength in mid-infrared. This work demonstrated optically pumped edge-emitting GeSn lasers under two different pumping lasers with 1064 and 1950 nm wavelengths. The device structure featured Sn compositional graded with the maximum Sn content of 22.3%. Under the 1950 nm pumping laser, the GeSn laser achieved the world-record near room temperature lasing (270 K). The corresponding lasing wavelength has been extended up to 3442 nm, an unprecedented GeSn lasing wavelength so far in the world. The GeSn/GeSn/GeSn single and double quantum wells were also investigated to further improve laser performance. The unintentional Ge interlayer between barrier and well region of QW structure was removed by introducing the GeSn with variable Sn content as the buffer layer. As a result, the QW structure was demonstrated as the true type-I and direct bandgap structure, which is advantageous for the optoelectronic devices

Growth and Characterization of Silicon-Germanium-Tin Semiconductors for Future Nanophotonics Devices

The bright future of silicon (Si) photonics has attracted research interest worldwide. The ultimate goal of this growing field is to develop a group IV based Si foundries that integrate Si-photonics with the current complementary metal–oxide–semiconductor (CMOS) on a single chip for mid-infrared optoelectronics and high speed devices. Even though group IV was used in light detection, such as photoconductors, it is still cannot compete with III-V semiconductors for light generation. This is because most of the group IV elements, such as Si and germanium (Ge), are indirect bandgap materials. Nevertheless, Ge and Si attracted industry attention because they are cheap to be used with low cost and high volume manufacturing. Thus, enhancing their light efficiency is highly desired. A key solution to improve the light efficiency of Ge is by growing tensile strained Ge-on-Si and SixGe1-x-ySny (Sn: tin) alloys. In this dissertation, Si-Ge-Sn material system was grown using chemical vapor deposition technique and further characterized by advanced optical and material techniques. Ge-on-Si was grown at low growth temperatures by using plasma enhancement in order to achieve growth conditions compatible with CMOS technology with high quality Ge layers. First, a single step Ge layer was grown at low temperatures (T 450°C). The material and optical characterization of the single step reveal low material and optical qualities. Second, a two-step Ge-on-Si was grown (T 525°C) to improve the quality. The results show low threading dislocation density on the order of 107 cm-2 with roughness values on the order of several nm. Optical characterization reveal optical quality close to a Ge buffer grown by a traditional high temperature method. In addition, bulk and quantum well SixGe1-x-ySny alloys were grown. The results indicate that lattice matched bulk SiGeSn/Ge can be grown with high optical and material qualities using low cost commercial precursors. In addition, band structure and optical analysis results from a single Ge0.865Sn0.135 quantum well with Si0.04Ge0.895Sn0.065 double barriers on a relaxed Ge0.918Sn0.08 buffer indicate a type-I band alignment with direct bandgap emission. Moreover, SiGeSn barriers improved the optical confinement as compared to GeSn barriers

Silicon Nanodevices

This book is a collection of scientific articles which brings research in Si nanodevices, device processing, and materials. The content is oriented to optoelectronics with a core in electronics and photonics. The issue of current technology developments in the nanodevices towards 3D integration and an emerging of the electronics and photonics as an ultimate goal in nanotechnology in the future is presented. The book contains a few review articles to update the knowledge in Si-based devices and followed by processing of advanced nano-scale transistors. Furthermore, material growth and manufacturing of several types of devices are presented. The subjects are carefully chosen to critically cover the scientific issues for scientists and doctoral students