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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.

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.

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.

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