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We present comprehensive modeling of a SiGeSn multi-quantum well laser that has been previously experimentally shown to feature an order of magnitude reduction in the optical pump threshold compared to bulk lasers. We combine experimental material data obtained over the last few years with k · p theory to adapt transport, optical gain, and optical loss models to this material system (drift-diffusion, thermionic emission, gain calculations, free carrier absorption, and intervalence band absorption). Good consistency is obtained with experimental data, and the main mechanisms limiting the laser performance are discussed. In particular, modeling results indicate a low non-radiative lifetime, in the 100 ps range for the investigated material stack, and lower than expected Γ-L energy separation and/or carrier confinement to play a dominant role in the device properties. Moreover, they further indicate that this laser emits in transverse magnetic polarization at higher temperatures due to lower intervalence band absorption losses. To the best of our knowledge, this is the first comprehensive modeling of experimentally realized SiGeSn lasers, taking the wealth of experimental material data accumulated over the past years into account. The methods described in this paper pave the way to predictive modeling of new (Si)GeSn laser device concepts.

International audience; In recent years much effort has been made to increase the Sn content in GeSn alloys in order to increase direct bandgap charge carrier recombination and, therefore, to reach room temperature lasing. While being successful for the former, the increase of Sn content is detrimental, leading to increased defect concentrations and a lower thermal budget regarding processing. In this work we demonstrate strong photoluminescence enhancement in low Sn content Ge 0.94 Sn 0.06 layers by implementing tensile strain. Fitting of the calculated photoluminescence spectra to reproduce our experimental results indicates a strain of ~1.45%, induced via an SiN x stressor layer, which is strong enough to transform the investigated layer into a direct bandgap semiconductor. Moreover, theoretical calculations, using the 8-band k·p model, show the advantages of using low Sn content tensile strained GeSn layers in respect to gain and lasing temperature. We show that low Sn content GeSn alloys have a strong potential to enable efficient room temperature lasers on electronic-photonic integrated circuits. Si photonics is entering the market for short haul interconnects to construct energy efficient data centers and high performance computers. It offers high bandwidths and thus high speed data communication. In this regard, the conventional chip design is replaced or extended by optical devices like waveguides, modulators, detectors and lasers, in which information is processed via photons. For the latter, GeSn lasers have emerged as a viable solution for Si-compatible integrated group-IV laser sources 1,2

GeSn is discussed as solution to realize the dream of a group IV light source integrated on a Si chip. Sn added into a Ge lattice decreases the conduction band energies leading to a direct bandgap semiconductor band structure. However, the compressive strain increases the direct band energy imposing a large Sn content in the GeSn bulk. In spite of many difficulties regarding the growth of epitaxial GeSn alloys on Si, several hundred nm thick GeSn layers with various Sn concentrations up to 15% could be realized and used as gain material for lasers. Nowadays research concentrates on increasing the Sn content towards 20 at% as well as structural layout. The challenge here is the decreasing quality at high Sn contents and the isolation of the active layer from the mists formed at the interface with Ge/Si which increase the laser threshold. In this direction we discuss the influence on lasing and threshold of MQW SiGeSn/GeSn heterostructures with different quantum well thicknesses. Other solution proposed is the change of intrinsic strain type from compressive into tensile by introducing Si3N4 stressors and also GeSn on Insulator technology. These methods are well known in CMOS technology and can be applied to very low Sn content GeSn alloys. The discussion on the best way to reach room temperature laser is addressed both theoretical and experimental.

9th Advanced Research Workshop on Future Trends in Microelectronics (FTM)Location: Sardinia, ITALYDate: JUN 10-16, 2018; International audience; The recent rise of GeSn-based optically pumped lasers have multiplied the efforts to fabricate a fully CMOS compatible and group IV-based light emitter. Their integration with Si-based electronics may yield heavily reduced power consumption in integrated circuits and pave the way towards new sensing or medical applications. Here, we discuss the epitaxy of group IV GeSn and SiGeSn semiconductors and show their suitability for light emitting applications. Double and multi quantum well heterostructures are evaluated, whereby the latter enables an inherently easier control over the formation of deleterious misfit defects. Consequently, microdisk lasers fabricated from those show greatly enhanced light emission and reduced lasing thresholds. The use of in-situ p-i-n doping schemes allow the formation of light emitting diodes, resulting in electrically-enabled light emission already at room temperature.

GeSn/SiGeSn multiple quantum well structures have been grown by means of low temperature reactive gas source epitaxy on Ge pseudosubstrates. The MQW structures exhibit type I band alignment. Optically pumped μ-disc laser exhibit a threshold of 40kW/cm2 to obtain optically pumped lasing.

International audience; 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.

We report on the direct observation of lattice relaxation and Sn segregation of GeSn/Ge/Si heterostructures under annealing. We investigated strained and partially relaxed epi-layers with Sn content in the 5 at. %-12 at. % range. In relaxed samples, we observe a further strain relaxation followed by a sudden Sn segregation, resulting in the separation of a β-Sn phase. In pseudomorphic samples, a slower segregation process progressively leads to the accumulation of Sn at the surface only. The different behaviors are explained by the role of dislocations in the Sn diffusion process. The positive impact of annealing on optical emission is also discussed.

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 and SiGeSn are promising materials for the fabrication of a group IV laser source offering a number of design options from bulk to heterostructures and quantum wells. Here, we investigate GeSn/SiGeSn multi quantum wells using the optically pumped laser effect. Three complex heterostructures were grown on top of 200 nm thick strain-relaxed Ge0.9Sn0.1 buffers. The lasing is investigated in terms of threshold and maximal lasing operation temperature by comparing multiple quantum well to double heterostructure samples. Pumping under two different wavelengths of 1064 and 1550 nm yields comparable lasing thresholds. The design with multi quantum wells reduces the lasing threshold to 40 ± 5 kW/cm2 at 20 K, almost 10 times lower than for bulk structures. Moreover, 20 K higher maximal lasing temperatures were found for lower energy pumping of 1550 nm.

SiGeSn ternaries are grown on Ge-buffered Si wafers incorporating Si or Sn contents of up to 15 at%. The ternaries exhibit layer thicknesses up to 600 nm, while maintaining a high crystalline quality. Tuning of stoichiometry and strain, as shown by means of absorption measurements, allows bandgap engineering in the short-wave infrared range of up to about 2.6 µm. Temperature-dependent photoluminescence experiments indicate ternaries near the indirect-to-direct bandgap transition, proving their potential for ternary-based light emitters in the aforementioned optical range. The ternaries' layer relaxation is also monitored to explore their use as strain-relaxed buffers, since they are of interest not only for light emitting diodes investigated in this paper but also for many other optoelectronic and electronic applications. In particular, the authors have epitaxially grown a GeSn/SiGeSn multiquantum well heterostructure, which employs SiGeSn as barrier material to efficiently confine carriers in GeSn wells. Strong room temperature light emission from fabricated light emitting diodes proves the high potential of this heterostructure approach.

We present optically pumped lasing from group IV GeSn/SiGeSn heterostructures. A comparison between double heterostructure and multi-quantum-well microdisk cavities reveals advantages of the multi-well design. Strongly reduced lasing thresholds compared to values from bulk devices are observed.

We demonstrate epitaxial growth of direct bandgap group IV GeSn/SiGeSn double heterostructures and multi quantum wells. While both designs offer high structural quality and strong light emission, multi quantum wells benefit from a smaller number of defects at the active region.

We have investigated the formation and optoelectronic properties of strain relaxed Ge1−x−y Si x Sn y /Ge1−x Sn x /Ge1−x−y Si x Sn y double heterostructures on ion-implanted Ge substrates. The strain relaxation of Ge1−x−y Si x Sn y and Ge1−x Sn x epitaxial layers was achieved using an ion-implanted Ge substrate. The maximal degree of strain relaxation (DSR) of the Ge1−x Sn x layers was evaluated to be 46%. In addition, we obtained a sharp and strong peak in the photoluminescence (PL) spectra from the sample with a DSR of 46%, while no strong peak was detected from a sample with a smaller DSR (22%). From the theoretical calculation of the energy band structure and the measurement temperature dependence of the PL intensity, the sharp and strong peak can be explained by the transition from an indirect to direct bandgap semiconductor due to the increase of the DSR and a concomitant increase of the Γ-valley electron population. Moreover, the PL intensity increases by the improvement of the crystallinity by a post deposition annealing process.

Energy-efficient integrated circuits for on-chip or chip-to-chip data transfer via photons could be tackled by monolithically grown group IV photonic devices. The major goal here is the realization of fully integrated group IV room temperature electrically driven lasers. An approach beyond the already demonstrated optically-pumped lasers would be the introduction of GeSn/(Si)Ge(Sn) heterostructures and exploitation of quantum mechanical effects by reducing the dimensionality, which affects the density of states. In this contribution we present epitaxial growth, processing and characterization of GeSn/(Si)Ge(Sn) heterostructures, ranging from GeSn/Ge multi quantum wells (MQWs) to GeSn quantum dots (QDs) embedded in a Ge matrix. Light emitting diodes (LEDs) were fabricated based on the MQW structure and structurally analyzed via TEM, XRD and RBS. Moreover, EL measurements were performed to investigate quantum confinement effects in the wells. The GeSn QDs were formed via Sn diffusion /segregation upon thermal annealing of GeSn single quantum wells (SQW) embedded in Ge layers. The evaluation of the experimental results is supported by band structure calculations of GeSn/(Si)Ge(Sn) heterostructures to investigate their applicability for photonic devices.

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.

The optoelectronic properties of Ge1−x−y Si x Sn y /Ge1−x Sn x /Ge1−x−y Si x Sn y double heterostructures pseudomorphically grown on a Ge substrate were investigated. The photoluminescence (PL) intensity of the sample with Ge0.66Si0.23Sn0.11 cladding layers is three times larger compared to PL from structure with a Ge cladding layer, which can be attributed to higher energy band offsets at both conduction and valence band edges at the Ge0.91Sn0.09/Ge0.66Si0.23Sn0.11 interface. The PL spectrum of the sample with the Ge0.66Si0.23Sn0.11 cladding layer at room temperature can be deconvoluted into four components, and the origins of these components can be assigned to direct and indirect transitions by measuring the temperature dependence of each component's intensity. In addition, we examined the formation and characterization of strain-relaxed Ge1−x−y Si x Sn y /Ge1−x Sn x /Ge1−x−y Si x Sn y double heterostructures to relieve the compressive strain in the Ge1−x Sn x layer. Stacking faults were observed in the Ge1−x Sn x and Ge1−x−y Si x Sn y layers. The PL peak intensity of the strain-relaxed Ge1−x Sn x layer decreases by a factor of 1/20 compared to the PL peak intensity of the double heterostructure pseudomorphically grown on a Ge(001) substrate. In addition, PL intensity can be increased by post-deposition annealing owing to decreasing defects.

Silicon (Si)-based group IV materials form the backbone of our digital society, being found in every smartphone and tablet. They, however, lack the possibility to efficiently emit and detect light, because of their indirect bandgap. A truly Si-compatible direct bandgap material allows merging of state-of-the-art CMOS technology on-chip with photonic components to tackle bottlenecks of modern electronics by radically reducing the required power consumption or increasing the available data bandwidth [1]. Alloying the group IV elements germanium (Ge) and tin (Sn) has turned out to be a viable solution, as a direct bandgap semiconductor – even suitable for lasing – is formed for relaxed epilayers with Sn incorporation above ∼9 at.% [2]. The value of the bandgap can further be controlled by adding Si into GeSn, which can be exploited for carrier confinement inside heterostructures [3]. In this contribution, we discuss the epitaxy of various Sn-based group IV alloys. Different heterostructure designs combine direct bandgap active layers, ternary barriers and a GeSn buffer technology. In Figure 1a, a TEM micrograph of a SiGeSn/GeSn/SiGeSn double heterostructure (DHS) is presented, overlapped with the elemental distribution taken by energy-dispersive X-Ray spectroscopy (EDX). A Sn incorporation of roughly 14.5 at.% ensures a direct bandgap inside the active region. Another approach facilitates direct bandgap GeSn multi quantum wells (MQW), sandwiched between SiGeSn barriers. Atom Probe Tomography (APT) allows to obtain precise elemental distribution within this complex heterostructure stack, as is demonstrated in Figure 1b. Techniques such as temperature-dependent photoluminescence are used to compare the optical properties of the grown heterostructures. Moreover, the impact of quantum confinement on MQW emission is revealed, backed up by bandstructure calculations. Optically pumped microdisk resonators are formed to demonstrate lasing from different types of heterostructures, as shown in Figure 1c. As depicted in the inset, the superiority of MQW structures over DHSs (and bulk GeSn layers) is demonstrated, leading to an order of magnitude reduced lasing thresholds at cryogenic temperatures. In the next step of integrated light emitters, carriers need to be injected electrically, rather than optically. We will show first experiments on epitaxially grown p-i-n heterostructure diodes. In Figure 1d, incorporation of phosphorus and boron for the formation of n- and p-doped carrier injection layers is shown by SIMS. We will further point towards possible improvements that may pave the way towards future energy efficient opto-electronic integrated circuits (OEICs). [1] L. Vivien, Computer technology: Silicon chips lighten up, Nature. 528 (2015) 483–484. [2] S. Wirths, R. Geiger et al., Lasing in direct-bandgap GeSn alloy grown on Si, Nat. Photonics. 9 (2015) 88–92. [3] N. von den Driesch et al., Advanced GeSn/SiGeSn Group IV Heterostructure Lasers, Adv. Sci. (2018). doi:10.1002/advs.201700955. Figure 1