The GeSn Alloy and its Optoelectronic Properties: A Critical Review of the Current Understanding

GeSn is nowadays recognized as a promising candidate to enable monolithic on-chip Si photonics operating in the near-infrared (NIR) and short-wave infrared (SWIR) wavelengths. The addition of Sn to the Ge lattice induces a red-shift in the material bandgap, extending the absorption cut-off wavelength towards the infrared. In addition, above 7-9 at.% Sn, the GeSn alloy acquires a direct bandgap, enabling its use as active material in SWIR light-emitting devices. Ge-rich GeSn alloys have been demonstrated in a plethora of optoelectronic devices including photodetectors, lasers and light emitting diodes (LEDs). Furthermore, the high theoretical mobility of GeSn motivated research for GeSn high-mobility field-effect transistors (FETs), while the possibility of monolithic integration on Si platforms has also pushed the investigation of GeSn for on-chip thermoelectric applications. However, despite more than 15 years of intensive research in the field, there exists no commercial device to date based on GeSn. In fact, there are numerous challenges hindering the rise of this material for the next-generation (opto)electronics. Here, we give a concise review of the historical achievements in GeSn research and the withstanding challenges. This is followed by a detailed description of the GeSn physical properties relevant for its use in optoelectronic devices. We conclude with a discussion of the open questions in the field.Comment: Draft versio

Fundamental limits in the external quantum efficiency of single nanowire solar cells

The fundamental limits for the measurement of the efficiency of single nanowire solar cell devices are presented. We evaluate the effect of the substrate, light polarization, and existence of Mie resonances in the absorption of the solar spectrum for nanowires with diameters from 10 to 300 nm. We find that the efficiency measured under such configuration can be underestimated between a factor 1.6 and 7.0 for GaAs nanowires and between 6.7 and 15.9 for silicon nanowires. These results constitute a reference for understanding the limits in the measurement of single nanowire devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3672168

Phonon confinement and plasmon-phonon interaction in nanowire based quantum wells

Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon quantization consistent with 2.4 nm thick quantum wells is observed, in agreement with cross-section transmission electron microscopy measurements and photoluminescence experiments. The creation of a high density plasma within the quantized structures is demonstrated by the observation of coupled plasmon-phonon modes. The density of the plasma and thereby the plasmon-phonon interaction is controlled with the excitation power. This work represents a base for further studies on confined high density charge systems in nanowires

Light Generation and Harvesting in a Van der Waals Heterostructure

Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors (FETs). Their high mechanical flexibility, stability and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of two-dimensional (2D) semiconductors and silicon.Comment: Submitted versio

Vapor Phase Growth of Semiconductor Nanowires: Key Developments and Open Questions

Nanowires are filamentary crystals with a tailored diameter that can be obtained using a plethora of different synthesis techniques. In this review, we focus on the vapor phase, highlighting the most influential achievements along with a historical perspective. Starting with the discovery of VLS, we feature the variety of structures and materials that can be synthesized in the nanowire form. We then move on to establish distinct features such as the three-dimensional heterostructure/doping design and polytypism. We summarize the status quo of the growth mechanisms, recently confirmed by in situ electron microscopy experiments and defining common ground between the different synthesis techniques. We then propose a selection of remaining defects, starting from what we know and going toward what is still to be learned. We believe this review will serve as a reference for neophytes but also as an insight for expertsin an effort to bring open questions under a new light