The GeSn Alloy and its Optoelectronic Properties: A Critical Review of the Current Understanding

GeSn is nowadays recognized as a promising candidate to enable monolithic on-chip Si photonics operating in the near-infrared (NIR) and short-wave infrared (SWIR) wavelengths. The addition of Sn to the Ge lattice induces a red-shift in the material bandgap, extending the absorption cut-off wavelength towards the infrared. In addition, above 7-9 at.% Sn, the GeSn alloy acquires a direct bandgap, enabling its use as active material in SWIR light-emitting devices. Ge-rich GeSn alloys have been demonstrated in a plethora of optoelectronic devices including photodetectors, lasers and light emitting diodes (LEDs). Furthermore, the high theoretical mobility of GeSn motivated research for GeSn high-mobility field-effect transistors (FETs), while the possibility of monolithic integration on Si platforms has also pushed the investigation of GeSn for on-chip thermoelectric applications. However, despite more than 15 years of intensive research in the field, there exists no commercial device to date based on GeSn. In fact, there are numerous challenges hindering the rise of this material for the next-generation (opto)electronics. Here, we give a concise review of the historical achievements in GeSn research and the withstanding challenges. This is followed by a detailed description of the GeSn physical properties relevant for its use in optoelectronic devices. We conclude with a discussion of the open questions in the field.Comment: Draft versio

Epitaxial Growth of Si-Ge-Sn Alloys for Optoelectronic Device Application

Microelectronics industry has experienced a tremendous change over the last few decades and has shown that Moore’s law has been followed by doubling the number of transistors on the chip every 18 months. However, continuous scaling down of the transistors size is reaching the physical limits and data transfer through metal interconnects will not be able to catch up with the increasing data processing speed in the future. Therefore, optical data transfer between chips and on-chip has been widely investigated. Silicon based optoelectronics has received phenomenal attention since Si has been the core material on which microelectronic industry has been built. However, due to the indirect bandgap nature of Si, its optical characteristics fall short compared to similar III-IV semiconductors. The efforts in III-V incorporation on Si substrate have not been successful due to the incompatibility of the growth with complementary metal oxide semiconductor processing. Germanium has been studied in order to develop a Si compatible technology and it has been shown that a direct bandgap material is achievable by alloying Sn in Ge. Further investigations on Si-Ge-Sn material system showed its viability as a technology that can be used for fabrication of Si-compatible light source and detectors. The work presented in this dissertation is focused on the low temperature growth of Si-Ge-Sn alloys. High quality crystalline homoepitaxial silicon films were deposited at 250 °C using a plasma-enhanced chemical vapor deposition (PECVD) system. Strain-relaxed Ge and SiGe films were also grown on Si substrate at 350-550 °C in a reduced pressure CVD system. Commercial precursors of silane and germane were used to grow the films at different chamber pressures. Germanium-tin and silicon-germanium-tin alloys were grown by a cold-wall chemical vapor deposition system at low temperatures (300-450 °C) directly on Si substrates. Two different delivery systems were adopted for the delivery of stannic chloride and deuterated stannane as Sn precursors along with silane and germane. Crystallinity and growth quality of the films were investigated through material characterization methods including X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Elemental characterization of the films was done using Rutherford backscattering measurement and energy-dispersive X-ray spectroscopy. Moreover, optical characterizations were performed using Raman spectroscopy and photoluminescence on the samples to investigate Sn incorporation in the films. Additionally, compressively strained

Isothermal Heteroepitaxy of Ge₁–ₓSnₓ Structures for Electronic and Photonic Applications

Epitaxy of semiconductor-based quantum well structures is a challenging task since it requires precise control of the deposition at the submonolayer scale. In the case of Ge1–xSnx alloys, the growth is particularly demanding since the lattice strain and the process temperature greatly impact the composition of the epitaxial layers. In this paper, the realization of high-quality pseudomorphic Ge1–xSnx layers with Sn content ranging from 6 at. % up to 15 at. % using isothermal processes in an industry-compatible reduced-pressure chemical vapor deposition reactor is presented. The epitaxy of Ge1–xSnx layers has been optimized for a standard process offering a high Sn concentration at a large process window. By varying the N2 carrier gas flow, isothermal heterostructure designs suitable for quantum transport and spintronic devices are obtained

GeSn Devices for Short-Wave Infrared Optoelectronics

The electronics industry has a large silicon infrastructure for the manufacture of complementary-metal oxide semiconductor (CMOS) based electronics. The increasing density of Si based circuits has set a pace that is now pushing the physical limits of connectivity between devices over conventional wire based links. This has driven the increasing interest in Si based optoelectronics and to use the groundwork already established by the electronics industry for lower cost optical communications. The greatest limitation to this effort has been the incorporation of a Si based laser, which requires integration of a direct bandgap material within this CMOS process. The Ge1-xSnx alloy is one material of interest for this field of Si photonics due to its compatibility on Si CMOS circuits and its direct bandgap for increasing Sn content. The past decade of material development in this field has led to Ge1-xSnx films grown on Si with direct bandgaps. The work in this dissertation set out to develop Ge1-xSnx based optoelectronics operating in the short-wave infrared (SWIR) region. The fabrication methodology of Ge1-xSnx active photonic components such as microdisk resonators, photoconductors, and avalanche photodiodes were developed. A simple, one-mask fabrication method was developed to create Ge1-xSnx microdisk resonators on Si, which could serve as a platform for the first on-Si CMOS laser. A study of the noise levels, effective carrier lifetime, and specific detectivity was conducted for the first time on any Ge1-xSnx detector. A systematic study of detectors with Sn content ranging from 0.9 to 10.0% were fabricated and measured for their responsivity and spectral response in the SWIR. A record high responsivity of 1.63 A/W was measured at the 1.55 μm wavelength for a 10% Sn photoconductor at reduced temperature. A long-wavelength cut-off for this device was measured out to 2.4 μm. Avalanche photodiodes were also developed and tested for devices with Ge1-xSnx absorption regions. The low noise operation and high responsivity of these detectors yield a detectivity that is comparable with commercially available detectors. This work established the baseline performance for this technology and demonstrates this material can be used for Si based optoelectronics

Kinetic Control of Morphology and Composition in Ge/GeSn Core/Shell Nanowires

The growth of Sn-rich group-IV semiconductors at the nanoscale provides new paths for understanding the fundamental properties of metastable GeSn alloys. Here, we demonstrate the effect of the growth conditions on the morphology and composition of Ge/GeSn core/shell nanowires by correlating the experimental observations with a theoretical interpretation based on a multi-scale approach. We show that the cross-sectional morphology of Ge/GeSn core/shell nanowires changes from hexagonal to dodecagonal upon increasing the supply of the Sn precursor. This transformation strongly influences the Sn distribution as a higher Sn content is measured under the {112} growth front. Ab-initio DFT calculations provide an atomic-scale explanation by showing that Sn incorporation is favored at the {112} surfaces, where the Ge bonds are tensile-strained. A phase-field continuum model was developed to reproduce the morphological transformation and the Sn distribution within the wire, shedding light on the complex growth mechanism and unveiling the relation between segregation and faceting. The tunability of the photoluminescence emission with the change in composition and morphology of the GeSn shell highlights the potential of the core/shell nanowire system for opto-electronic devices operating at mid-infrared wavelengths

Germanium-tin-silicon epitaxial structures grown on silicon by reduced pressure chemical vapour deposition

Crystalline germanium-tin (GeSn) binary alloys have been subject to a significant research effort in recent years. This research effort is motivated by the myriad of potential applications that GeSn alloys offer. Crystalline epitaxial layers of GeSn and silicon-germanium-tin (SiGeSn) have been grown onto Si(001) substrates on a relaxed Ge buffer using reduced pressure CVD and commercially available precursors. X-ray diffraction, transmission electron microscopy, atomic force microscopy, secondary ion mass spectrometry and Raman spectroscopy were used to determine layer composition, layer thickness, crystallinity, degree of strain relaxation, surface features and roughness of the samples investigated in this work. The epilayers produced have been both fully strained to their growth platform and partially relaxed. The Sn fraction of the alloy layers varied from 1 to 12 at. % Sn. Using N2 as the carrier gas during growth is observed to inhibit Ge1-xSnx growth. Off-axis substrates are determined to hinder the production of crystalline layers of GeSn. In-situ material characterization of GeSn layers during thermal treatment has identified the existence of a critical temperature for higher Sn fraction layers, beyond which the material quality degrades rapidly. This critical temperature is dependent on the layer composition, layer thickness, layer strain state and annealing environment. Layers of germanium-tin-oxide are produced by thermal oxidation and shown to have similar oxide formation rates to pure Ge. The low thermal budget limit for the high Sn fraction alloys has driven research into forming Ohmic metal contacts on GeSn layers with processes limited to low temperatures. Gold is determined to be the optimum electrical contact material

GeSn semiconductor for micro-nanoelectronic applications

Within the last few years the steady electronic evolution lead the semiconductor world to study innovative device architectures and new materials able to replace Si platforms. In this scenario Ge1-xSnx alloy attracts the interest of the scientific community due to its ability to tune the material bandgap as a function of Sn content and its extreme compatibility with Si processing. Although the enhanced optical properties of Ge1-xSnx are evident, the augmented electrical properties such as the higher electron and holes mobility are also beneficial for metal oxide semiconductor. Therefore the alloy is expected to be a potential solution to integrate both electrical and optical devices. On one hand, several theoretical and experimental works depict the Ge1-xSnx alloy as a novel and fascinating solution to replace Si; on the other hand the material novelty forces us to enhance the knowledge of its fundamental physical and chemical properties, re-adapting the processing steps necessary to develop electronic and optical devices. In this dissertation a comprehensive study on Ge1-xSnx has been undertaken and discussed analysing a wide range of topics. The first chapter provides a detailed theoretical study on the electronic properties of the GeSn performed using first principle methods; subsequently the data obtained have been inserted into a TCAD software in order to create and calibrate a library used to simulate electrical devices. It is important to note, that at the beginning of this PhD GeSn was not an available material in the Synopsys device software, and thus it had to be defined from scratch As a next point, since the ever decreasing device size push toward the definition of Ohmic contacts, different stanogermanide films have been thoroughly analysed using various metals (Ni, Pt and Ti) annealed with two distinct methodologies (Rapid Thermal Annealing and Laser Thermal Annealing). Subsequently, considering the material limitation such as the limited thermal budget and the Sn segregation, an exhaustive study on the material doping has been firstly discussed theoretically and after experimentally characterized using both classical ion implantation and layer deposition techniques. The different building blocks of Field Effect Transistors have been investigated and tuned individually with the aim to develop FET devices with bottom up approach. Then, Field Effect Transistor devices using GeSn NWs grown by a VLS methodology with Sn composition ranging from (0.03-0.09 at.%) have been developed and extensively characterized with the state of the art present in literature. Finally the analysis of highly selective etch recipes lead to the development of sub-nm device configuration such as Gate-All-Around (GAA) structure obtained using classical top down lithography approach. The innovative structure was electrically characterized highlighting the possibility to obtain decananometer device architecture with this innovative alloy. Lastly thesis summary and final outlooks were reported with the aim to outline the thesis contribution and the future material investigations

Investigation of Optical and Structural Properties of GeSn Heterostructures

Silicon (Si)-based optoelectronics have gained traction due to its primed versatility at developing light-based technologies. Si, however, features indirect bandgap characteristics and suffers relegated optical properties compared to its III-V counterparts. III-Vs have also been hybridized to Si platforms but the resulting technologies are expensive and incompatible with standard complementary-metal-oxide-semiconductor processes. Germanium (Ge), on the other hand, have been engineered to behave like direct bandgap material through tensile strain interventions but are well short of attaining extensive wavelength coverage. To create a competitive material that evades these challenges, transitional amounts of Sn can be incorporated into Ge matrix to form direct bandgap GeSn alloys that have led to the increasing possibility of engineering a suite of low-cost, light emission sources that applies to a wide range of infrared photonics and optoelectronics systems. Hence, the importance of studying the structural and optical properties of these GeSn heterostructures cannot be overemphasized. The first part of this dissertation investigates the structural and optical properties of SiGeSn/GeSn/SiGeSn quantum wells (QWs) where the photoluminescence (PL) behaviors of thick (22 nm in well) and thin (9 nm in well) GeSn QW samples are compared. Using PL results from two excitation lasers (532 nm and 1550 nm lasers) as well as studying their respective optical transitions, the result reveals that the thicker well sample shows i) a more direct bandgap outcome in addition to a much lower ground energy Г valley; ii) a higher carrier density within the well, and iii) an increased barrier height coupled with improved carrier confinement. All of these resulted in a significantly enhanced emission that allows for the first-ever estimation of GeSn QWs quantum efficiency (QE) while also suggesting a path towards efficient mid-infrared devices. To further improve the carrier confinement while also reducing the carrier leakage in the thicker well design, a SiGeSn/GeSn/GeSn/SiGeSn separate confinement heterostructure (SCH) is introduced. The sample is characterized and the optical properties are compared with the previously reported 9 nm and 22 nm well non-SCH samples. Based on the optical transition analysis, the SCH QW also shows significantly higher carrier confinement compared to reference samples. In addition to these studies, an attempt is made to investigate advanced quantum well structures through an all-inclusive structural and optical study of SiGeSn/GeSn/SiGeSn multi-quantum wells (MQWs). The resulting analysis shows evidence of intermixing diffusion during growth. The second part of this work provides insights into the behavior of annealed GeSn bulk samples near the indirect-to-direct transition point. The study attempts to provide connections between the strain, composition, and defect densities before and after annealing. The result reveals the impact of annealing on a sample may either i) lower the strain giving rise to an increased PL while reducing the energy separation or ii) introduce misfit dislocation/ surface roughness leading to an affected or decreased PL. Finally, this work also explores the low-temperature capability of our in-house plasma-enhanced ultra-high vacuum chemical vapor deposition system through the growth of Si-on-Ge epitaxy and pressure-dependent growth of GeSn bulk heterostructures

Activated and Metallic Conduction in p-DType Modulation-Doped Ge-Sn Devices

Ge_{1-x}Sn_{x} quantum wells can be incorporated into Si-Ge-based structures with low-carrier effective masses, high mobilities, and the possibility of direct band-gap devices with x ∼ 0.1. However, the electrical properties of p-type Ge_{1-x} Sn_{x} devices are dominated by a thermally activated mobility and metallic behavior. At 30 mK the transport measurements indicate localization with a mobility of 380 cm^{2}/Vs, which is thermally activated with a temperature-independent carrier density of 4x 10^{11} cm^{-2}. This weakly disordered system with conductivity, sigma ~ epsilon^{2}/h, where e is the fundamental charge and h is Planck’s constant, is a result of negatively charged “Sn-vacancy” complex states in the barrier layers that act as hole traps. A measured hole effective mass of 0.090±0.005m_{e} from the Shubnikov-de Haas effect, where m_{e} is the free electron mass shows that the valence band is heavy hole dominated and is similar to p-type Ge with the compressive strain playing the role of quenching the spin-orbit coupling and shifting the unoccupied light-hole states to higher hole energies. The Ge_{1-x} Sn_{x} devices have a high quantum mobility of approximately 36 000 cm^{2}/Vs that is not thermally activated. The ratio of transport-to-quantum mobility of approximately 0.01 in Ge_{1-x} Sn_{x} devices is unusual and points to several competing scattering mechanisms in the different experimental regimes

Mid-infrared light emission > 3 µm wavelength from tensile strained GeSn microdisks

GeSn alloys with Sn contents of 8.4 % and 10.7 % are grown pseudomorphically on Ge buffers on Si (001) substrates. The alloys as-grown are compressively strained, and therefore indirect bandgap. Undercut GeSn on Ge microdisk structures are fabricated and strained by silicon nitride stressor layers, which leads to tensile strain in the alloys, and direct bandgap photoluminescence in the 3–5 µm gas sensing window of the electromagnetic spectrum. The use of pseudomorphic layers and external stress mitigates the need for plastic deformation to obtain direct bandgap alloys. It is demonstrated, that the optically pumped light emission overlaps with the methane absorption lines, suggesting that GeSn alloys are well suited for mid-infrared integrated gas sensors on Si chips