Daniela Stange

High- k gate stacks on low bandgap tensile strained Ge and GeSn alloys for field-effect transistors

We present the epitaxial growth of Ge and Ge0.94Sn0.06 layers with 1.4% and 0.4% tensile strain, respectively, by reduced pressure chemical vapor deposition on relaxed GeSn buffers and the formation of high-k/metal gate stacks thereon. Annealing experiments reveal that process temperatures are limited to 350 °C to avoid Sn diffusion. Particular emphasis is placed on the electrical characterization of various high-k dielectrics, as 5 nm Al2O3, 5 nm HfO2, or 1 nmAl2O3/4 nm HfO2, on strained Ge and strained Ge0.94Sn0.06. Experimental capacitance-voltage characteristics are presented and the effect of the small bandgap, like strong response of minority carriers at applied field, are discussed via simulations.

Negative differential resistance in direct bandgap GeSn p-i-n structures

Certain GeSn alloys are group IV direct bandgap semiconductors with prospects for electrical and optoelectronical applications. In this letter, we report on the temperature dependence of the electrical characteristics of high Sn-content Ge0.89Sn0.11 p-i-n diodes. NiGeSn contacts were used to minimize the access resistance and ensure compatibility with silicon technology. The major emphasis is placed on the negative differential resistance in which peak to valley current ratios up to 2.3 were obtained. TCAD simulations were performed to identify the origin of the various current contributions, providing evidence for direct band to band tunneling and trap assisted tunneling.

Short-wave infrared LEDs from GeSn/SiGeSn multiple quantum wells

Group IV photonics is on its way to be integrated with electronic circuits, making information transfer and processing faster and more energy efficient. Light sources, a critical component of photonic integrated circuits, are still in development. Here, we compare multi-quantum-well (MQW) light-emitting diodes (LEDs) with Ge0.915Sn0.085 wells and Si0.1Ge0.8Sn0.1 barriers to a reference Ge0.915Sn0.085 homojunction LED. Material properties as well as band structure calculations are discussed, followed by optical investigations. Electroluminescence spectra acquired at various temperatures indicate effective carrier confinement for electrons and holes in the GeSn quantum wells and confirm the excellent performance of GeSn/SiGeSn MQW light emitters.

Advanced GeSn/SiGeSn Group IV Heterostructure Lasers

Abstract Growth and characterization of advanced group IV semiconductor materials with CMOS‐compatible applications are demonstrated, both in photonics. The investigated GeSn/SiGeSn heterostructures combine direct bandgap GeSn active layers with indirect gap ternary SiGeSn claddings, a design proven its worth already decades ago in the III–V material system. Different types of double heterostructures and multi‐quantum wells (MQWs) are epitaxially grown with varying well thicknesses and barriers. The retaining high material quality of those complex structures is probed by advanced characterization methods, such as atom probe tomography and dark‐field electron holography to extract composition parameters and strain, used further for band structure calculations. Special emphasis is put on the impact of carrier confinement and quantization effects, evaluated by photoluminescence and validated by theoretical calculations. As shown, particularly MQW heterostructures promise the highest potential for efficient next generation complementary metal‐oxide‐semiconductor (CMOS)‐compatible group IV lasers.

Direct bandgap GeSn light emitting diodes for short-wave infrared applications grown on Si

The experimental demonstration of fundamental direct bandgap, group IV GeSn alloys has constituted an important step towards realization of the last missing ingredient for electronic-photonic integrated circuits, i.e. the efficient group IV laser source. In this contribution, we present electroluminescence studies of reduced-pressure CVD grown, direct bandgap GeSn light emitting diodes (LEDs) with Sn contents up to 11 at.%. Besides homojunction GeSn LEDs, complex heterojunction structures, such as GeSn/Ge multi quantum wells (MQWs) have been studied. Structural and compositional investigations confirm high crystalline quality, abrupt interfaces and tailored strain of the grown structures. While also being suitable for light absorption applications, all devices show light emission in a narrow short-wave infrared (SWIR) range. Temperature dependent electroluminescence (EL) clearly indicates a fundamentally direct bandgap in the 11 at.% Sn sample, with room temperature emission at around 0.55 eV (2.25 µm). We have, however, identified some limitations of the GeSn/Ge MQW approach regarding emission efficiency, which can be overcome by introducing SiGeSn ternary alloys as quantum confinement barriers.

Quantum confinement effects in GeSn/SiGeSn heterostructure lasers

The development of a light source on Si, which can be integrated in photonic circuits together with CMOS electronics, is an outstanding goal in the field of Silicon photonics. This could e.g. help to overcome bandwidth limitations and losses of copper interconnects as the number of high-speed transistors on a chip increases. Here, we discuss direct bandgap group IV materials, GeSn/SiGeSn heterostructures and resulting quantum confinement effects for laser implementation. After material characterization, optical properties, including lasing, are probed via photoluminescence spectrometry. The quantum confinement effect in GeSn wells of different thicknesses is investigated. Theoretical calculations show strong quantum confinement to be undesirable past a certain level, as the very different effective masses of r and L electrons lead to a decrease of the L-to Γ-valley energy difference. A main limiting factor for lasing devices turns out to be the defective region at the interface to the Ge substrate due to the high lattice mismatch to GeSn. The use of buffer technology and subsequent pseudomorphic growth of multi-quantum-wells structures offers confinement of carriers in the active material, far from the misfit dislocations region. Performance is strongly boosted, as a reduction of lasing thresholds from 300 kW/cm2 for bulk devices to below 45 kW/cm2 in multi-quantum-well lasers is observed at low temperatures, with the reduction in threshold far outpacing the reduction in active gain material volume.

GeSn lasers for monolithic integration on Si

Lasing under optical pumping is shown in suspended GeSn microdisks fabricated on a Ge virtual substrate with a lasing threshold below 1 mW at 20K.

GeSn for nanoelectronic and optical applications

Progress of Si based devices and technology is in strong interdependence with the development of new materials. Presently the list of group IV semiconductors is being extended beyond Ge to the semimetal Sn. Investigation in group IV semiconductors has been extended above strain engineering and Ge to the next element in group IV, the semimetal Sn. Significant efforts have been directed toward the epitaxial growth of GeSn binary and SiGeSn ternary alloys. In this work we discuss the growth and material properties of GeSn alloys and their application possibilities in photonic and nanoelectronic devices.

Si-Ge-Sn heterostructures: Growth and applications

This study presents the synthesis of a SiGeSn buffer layer by reduced pressure CVD method. Band structure calculation and transmission electron microscopy are used for material characterization. The use of this heterostructure for MOSFET and MOS capacitor fabrication is also addressed.

Investigation of carrier confinement in direct bandgap GeSn/SiGeSn 2D and 0D heterostructures

Since the first demonstration of lasing in direct bandgap GeSn semiconductors, the research efforts for the realization of electrically pumped group IV lasers monolithically integrated on Si have significantly intensified. This led to epitaxial studies of GeSn/SiGeSn hetero- and nanostructures, where charge carrier confinement strongly improves the radiative emission properties. Based on recent experimental literature data, in this report we discuss the advantages of GeSn/SiGeSn multi quantum well and quantum dot structures, aiming to propose a roadmap for group IV epitaxy. Calculations based on 8-band k∙p and effective mass method have been performed to determine band discontinuities, the energy difference between Γ- and L-valley conduction band edges, and optical properties such as material gain and optical cross section. The effects of these parameters are systematically analyzed for an experimentally achievable range of Sn (10 to 20 at.%) and Si (1 to 10 at.%) contents, as well as strain values (−1 to 1%). We show that charge carriers can be efficiently confined in the active region of optical devices for experimentally acceptable Sn contents in both multi quantum well and quantum dot configurations.