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In this work, we discuss carrier confinement in double heterostructures and multiple quantum well GeSn/(Si)Ge(Sn) heterostructures, aiming for efficient light emitting devices. Several heterostructures are ep-itaxially grown and characterized in detail concerning their structural and optical properties. Theoretical band structure calculations are used to support exper-imental data and provide predictions for optimized heterostructures.

Dissertation, RWTH Aachen University, 2017; Jülich : Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / key technologies ; Band/volume 168, Band/volume 168, 1 Online-Ressource (xx, 165, XII Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2017, Within the last few years single crystalline GeSn semiconductor alloys aroused significant scientific interest, especially since 2015, when GeSn with sufficiently high Sn content and crystalline quality was demonstrated as fundamentally direct bandgap group IV semiconductor. While enhanced optical properties are evident for direct bandgap materials compared to the fundamentally indirect Ge and Si group IV semiconductors, also enhanced electrical properties like increased carrier mobilities and enhanced band-to-band tunneling are expected for direct bandgap GeSn which are beneficial for metal-oxide-semiconductor transistors and tunnel field-effect transistors, respectively. The novel GeSn semiconductor alloys thereby manifests a fascinating emerging material system allowing a wide scope to study its fundamental physical, electrical, optical and chemical properties. On the other hand the novelty of the material system demands the re-development or modification and verification of all steps necessary to build GeSn based semiconductor devices. A comprehensive study is presented, focusing on the electrical properties of GeSn, their dependence on Sn content and possible applications in novel electronic devices. The building blocks of field-effect transistors are studied individually. GeSn surface composition and manipulation are investigated via X-ray photoemission spectroscopy to study pre-high-k deposition cleaning and highly selective Ge/GeSn etching processes. NiGeSn alloys for the use as electrical contacts of GeSn devices are structurally and electrically characterized using X-ray diffraction, transmission electron microscopy and temperature dependent current voltage measurements, respectively. Schottky barrier height, sheet resistance and specific contact resistivity are extracted. The modification of the NiGeSn/GeSn Schottky barrier height via dopant segregation is demonstrated for the first time. Schottky-barrier heights as low as 0.06 eV are observed. As a next module metal-oxide-semiconductor capacitors are comprehensively studied. High-k/GeSn interface trap densities are extracted for a wide range Sn contents. The focus is placed on the effect of the electronic band structure of GeSn on the capacitance voltage characteristics. Fundamental trends demonstrating the correlation of Sn-induced bandgap shrinkage and minority carrier response are observed. Furthermore a maximum capacitance of approx. 3 µF/cm² is achieved. As a step towards GeSn based tunnel field-effect transistors, Esaki diodes (tunnel diodes) are fabricated and electrically characterized. Negative differential resistance with a peak-to-valley current ratio of 2.3 is observed as an experimental proof of band-to-band tunneling. Enhanced band-to-band tunneling rates are observed in Ge0.89Sn0.11 p-i-n diodes compared to Ge taking advantage of the low and direct bandgap. These studies lead to the realization of vertical heterojunction Ge0.93Sn0.07/Ge tunnel field-effect transistors. An extensive analysis is provided identifying the various contributions to the overall transistor current, particularly band-to-band tunneling and trap-assisted tunneling. Finally, Hall measurements are presented, showing enhanced electron mobilities in direct bandgap GeSn as compared to Ge. With up to 4600 cm²/Vs this marks the highest bulk electron mobilities at the respective doping level of 2.9∙10^17 1/cm³ in a group IV semiconductor so far., Published by Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag, Jülich

The ongoing growth of consumer electronics market, as well as the demand for even more complex data and telecommunication systems require energy-efficient integrated circuits and data links [1]. One possibility to tackle this challenge is to replace electrons with photons for low-power on-chip and/or chip-to-chip data transfer [2]. Here, monolithically integrated, Si-based photonic devices would be the most convenient solution due to the accessibility to low-cost Si CMOS fabrication. Concerning group IV based photonic integrated circuits (PIC), one major disadvantage is their indirect bandgap nature, causing them to be inefficient light emitters. Very recently, the demonstration of a direct bandgap group IV laser based on GeSn [3] represents a breakthrough in the field of group IV photonics and opened the path towards fully integrated electronic and photonic circuitry [4]. In this contribution, we present theoretical studies as well as the growth and thorough structural characterization of GeSn/(Si)Ge(Sn) quantum-well and quantum dot structures that are suitable for light emitting devices (i.e. LEDs). The heterostructures were grown using a 200 mm industrial compatible AIXTRON RPCVD reactor with showerhead technology employing growth temperatures between 350 °C and 425 °C. Material properties like the Sn concentration, crystalline quality and strain were analyzed by RBS, TEM, XRD and SIMS.

The presence of large densities of electrically active defects is still an unsolved issue for future high-mobility/high-k CMOS device technologies. This relates to degraded device performance and reliability.Regrettably, conventional admittance-based characterization techniques often fail when applied to non-Si based devices. Among others, enhanced generation of minority carriers and much longer defect time constants make their results inaccurate. Rather than of seeking to adapt commonly-used techniques, we instead aim at direct measuring the semiconductor surface potential by means of the Saturation surface PhotoVoltage (SPV) technique. This approach allows for a DIT estimation which is not limited by the trap response time or hindered by minority carrier generation. Moreover, the DIT can be estimated over the whole bandgap regardless of sample doping type.We here report several case studies in support of the proposed approach. We will also show that SPV can be applied for the characterization of multi-layered Ge and III-V devices incorporating high-k insulators. ispartof: ECS Journal of Solid State Science and Technology vol:5 issue:4 pages:P3031-P3036 status: published

The observation of lasing action in GeSn opens a new path for monolithic integration of electronics and photonics based on Si. In this context, the incorporation of Si into GeSn and epitaxial growth of GeSn/(Si)Ge(Sn) heterostructures are essential for electrically pumped optoelectronic devices. We will discuss the electronic band structure calculations for a variety of Group IV heterostructures, including e.g. SiGeSn claddings and GeSn active layers with type I band alignment suitable for light emitting diodes and electrically pumped laser diodes. Here, Si and Sn concentrations up to 14 at.% in the cladding layer offer an effective carrier confinement with band offsets of a few hundred meV with respect to the active layer.Epitaxial growth of direct GeSn and indirect SiGeSn layers using an industry compatible AIXTRON reduced pressure CVD reactor with showerhead technology will be presented. Si2H6, Ge2H6 and SnCl4 were employed as precursors, while B2H6 and PH3 allow in-situ doping of the grown layers. Single epilayers and multi quantum well structures have been analyzed using RBS, TEM and X-Ray diffraction showing excellent layer morphology and high Sn substitutionality. Optical characterization was performed via photoluminescence and electroluminescence on both layers and pin diodes. Room temperature electroluminescence is observed for small current densities of about 50 Acm-². This demonstrates the potential of Group IV heterostructure for optoelectronics.