Direct bandgap group IV materials may thus represent a pathway towards the monolithic integration of Si-photonic circuitry and CMOS technology.Although a group IV direct bandgap material has not been demonstrated yet, silicon photonics using CMOS-compatible processes has made great progress through the development of Si-based waveguides 12 , photodetectors 13 and modulators 14 . The thus emerging technology is rapidly expanding the landscape of photonics applications towards tele-and data communication as well as sensing from the infrared to the mid infrared wavelength range 15-17 . Today's light sources of such systems are lasers made from direct bandgap group III-V materials operated off-or on-chip which requires fibre coupling or heterogeneous integration, for example by wafer bonding 3 , contact printing 4,5 or direct growth 6,7 , respectively. Hence, a laser source made of a direct bandgap group IV material would further boost lab-on-a-chip and trace gas sensing 15 as well as optical interconnects 18 by enabling monolithic integration. In this context, Ge plays a prominent role since the conduction band minimum at the -point of the Brillouin-zone (referred to as -valley) is 3 located only approx. 140 meV above the fourfold degenerate indirect L-valley. To compensate for this energy difference and thus form a laser gain medium, heavy n-type doping of slightly tensile strained Ge has been proposed 19 . Later, laser action has been reported for optically 20 and electrically pumped Ge 21 doped to approx. 1 and 4×10 19 cm -3 , respectively. However, pump-probe measurements of similarly doped and strained material did not show evidence for net gain 22 , and in spite of numerous attempts, researchers failed to substantiate above results up to today. Other investigated concepts concern the engineering of the Ge band structure towards a direct bandgap semiconductor using micromechanicallystressed Ge nanomembranes 9 or silicon nitride (Si 3 N 4 ) stressor layers 23 . Very recently, Süess et al. 10 presented a stressor-free technique which enables the introduction of more than 5.7 % 24 uniaxial tensile strain in Ge µ-bridges via selective wet under-etching of a pre-stressedlayer. An alternative technique in order to achieve direct bandgap material is to incorporate Sn atoms into a Ge lattice, which primarily reduces the gap at the -point. At a sufficiently high fraction of Sn, the energy of the -valley decreases below that of the L-valley. This indirect-to-direct transition for relaxed GeSn binaries has been predicted to occur at about 20 % Sn by Jenkins et al. 25 , but more recent calculations indicate much lower required Sn concentrations in the range of 6.5-11.0 % 26,27 . A major challenge for the realization of such GeSn alloys is the low (< 1 %) equilibrium solubility of Sn in Ge 28 and the large lattice mismatch of about 15 % between Ge and -Sn. For GeSn grown on Ge substrates, this mismatch induces biaxial compressive strain causing a shift of the  and L-valley crossover towards higher Sn concentrations ...

The strong correlation between advancing the performance of Si microelectronics and their demand of low power consumption requires new ways of data communication. Photonic circuits on Si are already highly developed except for an eligible on-chip laser source integrated monolithically. The recent demonstration of an optically pumped waveguide laser made from the Si-congruent GeSn alloy, monolithical laser integration has taken a big step forward on the way to an all-inclusive nanophotonic platform in CMOS. We present group IV microdisk lasers with significant improvements in lasing temperature and lasing threshold compared to the previously reported nonundercut Fabry−Perot type lasers. Lasing is observed up to 130 K with optical excitation density threshold of 220 kW/cm 2 at 50 K. Additionally the influence of strain relaxation on the band structure of undercut resonators is discussed and allows the proof of laser emission for a just direct Ge 0.915 Sn 0.085 alloy where Γ and L valleys have the same energies. Moreover, the observed cavity modes are identified and modeled.

No abstract

Abstract:In this letter, we propose a heterostructure design for tunnel field effect transistors with two low direct bandgap group IV compounds, GeSn and highly tensely strained Ge in combination with ternary SiGeSn alloy. Electronic band calculations show that strained Ge, used as channel, grown on Ge 1-x Sn x (x>9%) buffer, as source, becomes a direct bandgap which significantly increases the tunneling probability. The SiGeSn ternaries are well suitable as drain since they offer a large indirect bandgap. The growth of such heterostructures with the desired band alignment is presented. The crystalline quality of the (Si)Ge(Sn) layers is similar to state-of-the-art SiGe layers.

A comprehensive study of optical transitions in direct bandgap Ge 0.875 Sn 0.125 group IV alloys via photoluminescence measurements as a function of temperature, compressive strain and excitation power is performed. The analysis of the integrated emission intensities reveals a strain-dependent indirect-to-direct bandgap transition, in good agreement with band structure calculations based on 8 band k•p and deformation potential method. We have observed and quantified  valley -heavy hole and  valley -light hole transitions at low pumping power and low temperatures in order to verify the splitting of the valence band due to strain. We will demonstrate that the intensity evolution of these transitions supports the conclusion about the fundamental direct bandgap in compressively strained GeSn alloys. The presented investigation, thus, demonstrates that direct bandgap group IV alloys can be directly grown on Ge-buffered Si(001) substrates despite their residual compressive strain.2 KEYWORDS: direct bandgap, photoluminescence, germanium tin, group IV, compressive strain TOC GRAPHIC 3 Group IV semiconductors are known for their excellent electronic transport properties but limited optical applicability due to their indirect bandgap nature, turning them into inefficient light emitters. However, the pioneering work of R.Soref and C.H. Perry 1 and, later He and Atwater 2 as well as subsequent theoretical studies [3][4][5] indicated that alloying two group IV elements, i.e. semiconducting Ge and semimetallic -Sn, should result in a group IV semiconductor which could be tuned from a fundamental indirect to a direct bandgap material by increasing the substitutional Sn concentration in the Ge lattice. Although it was unanimously accepted that the -valley of the conduction band can be decreased below the Lvalley, theoretical estimates of the required Sn content for this transition as well as the impact of strain on the transition are widely spread. 6,7 This prospect has driven large efforts to grow device-grade GeSn epilayers, [8][9][10][11] prove their fundamental direct bandgap and finally to demonstrate photonic functionality. Recently, advances in Chemical Vapor Deposition (CVD)of GeSn binaries with high Sn contents of up to 14% has been reported 9,10,12-15 which enabled not only the proof of the direct bandgap nature but also the unambiguous demonstration of laser action at 2.3 µm under optical pumping. 16 Hence, direct bandgap GeSn alloys are CMOS-compatible IV-IV semiconductors with novel optical and electrical properties that are similar to those of III-V and II-VI compounds, used today in optoelectronic applications, i.e. The growth temperature (350°C), the total pressure and the partial pressures of the source gases were kept constant, resulting in the growth of GeSn alloys with a Sn content of 12.5 ± 0.5%. 11,15 Due to the lattice mismatch between the GeSn film and Ge virtual substrate, the GeSn layers were biaxially compressively strained. The films are fully strained for thicknesses below the critical t...