GeSn epitaxial heterostructures are emerging as prominent candidates for the monolithic integration of light sources on Si substrates. Here we propose a judicious explanation for their temperature-dependent photoluminescence (PL) that is based upon the so far disregarded optical activity of dislocations. By working at the onset of plastic relaxation, which occurs whenever the epilayer releases the strain accumulated during growth on the lattice mismatched substrate, we demonstrate that dislocation nucleation can be explicitly seen in the PL data. Notably, our findings point out that a monotonous thermal PL quenching can be observed in coherent films, in spite of the indirect nature of the GeSn bandgap. Our investigation, therefore, contributes to a deeper understanding of the recombination dynamics in this intriguing group IV alloy and offers insights into crucial phenomena shaping the light emission efficiency.
Controlling the growth kinetics from the vapor phase has been a powerful paradigm enabling a variety of metastable epitaxial semiconductors such as Sn-containing group IV semiconductors (Si)GeSn. In addition to its importance for emerging photonic and optoelectronic applications, this class of materials is also a rich platform to highlight the interplay between kinetics and thermodynamic driving forces during growth of strained, nonequilibrium alloys. Indeed, these alloys are inherently strained and supersaturated in Sn and thus can suffer instabilities that are still to be fully elucidated. I…
Understanding the nature and behavior of vacancy-like defects in epitaxial GeSn metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as …
The thermodynamic stability of germanium tin $\mathrm{Ge_{1-x}Sn_x}$ alloys is investigated across the composition range $0 \le x \le 1$ by applying density functional theory (DFT) together with the cluster expansion formalism (CE). It is known that GeSn alloys are immiscible and that non-equilibrium growth techniques are required to produce metastable films and nanostructures. Insight into the driving forces behind component segregation is gained by investigating the equilibrium thermodynamics of GeSn systems. The alloy free energy of mixing is computed by combining enthalpies from CE with en…
By independently engineering strain and composition, this work demonstrates and investigates direct band gap emission in the mid-infrared range from GeSn layers grown on silicon. We extend the room-temperature emission wavelength above ~4.0 μm upon post-growth strain relaxation in layers with uniform Sn content of 17 at.%. The fundamental mechanisms governing the optical emission are discussed based on temperature-dependent photoluminescence, absorption measurements, and theoretical simulations. Regardless of strain and composition, these analyses confirm that single-peak emission is always ob…
Magnetic field dependent polarization selective photoluminescence(PL) study has been carried out at 1.5~K on Gd-doped GaN epitaxial layers grown on c-SiC substrates by molecular beam epitaxy technique. It has been found that the incorporation of Gd in GaN leads to the generation of three types of donor like defects that result in neutral donor bound excitonic features in low temperature PL. The study reveals that the rate of spin-flip scattering for all the three excitonic features becomes almost $B$-independent suggesting that these signals must be stemming from defects which are ferromagneti…
We report on the precise determination of both the band gap E$_\text{g}$, and the characteristic energy $U$ of the band tail of localized defect states, for monocrystalline Ag$_2$ZnSnSe$_4$. Both photoluminescence excitation and time-resolved photoluminescence studies lead to E$_\text{g} = 1223\pm3$ meV, and $U = 20\pm3$ meV, at 6 K. The interest of the methodology developed here is to account quantitatively for the time-resolved photoluminescence and photoluminescence excitation spectra by only considering standard textbook density of states, and state filling effects. Such an approach is dif…
Ge-on-Si structures in-situ doped with phosphorus or arsenic via metal organic chemical vapor deposition (MOCVD) were investigated. Surface roughness, strain, threading dislocation desnity, Si-Ge interdiffusion, dopant diffusion, and photoluminescence were characterized to study the impacts of defect annealing and Si substrate offcut effects on the Ge film quality and most importantly, the light emission properties. All samples have a smooth surface (roughness < 1.5 nm), and the Ge films have a small tensile strain of 0.2%. As-grown P and As-doped Ge films have threading dislocaiton densities …
In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, UV', blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy (MOVPE) induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increa…
We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown MoS$_2$ on graphene/Ir(111). This observation is explained in terms of a weak graphene-MoS$_2$ interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS$_2$ with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS$_2$ and a different thermal expansion of graphene and Mo…
We report on photoluminescence in the 3-7 $μ$m mid-wave infrared (MWIR) range from sub-100 nm strained thin films of rocksalt PbSe(001) grown on GaAs(001) substrates by molecular beam epitaxy. These bare films, grown epitaxially at temperatures below 400 °C, luminesce brightly at room temperature and have minority carrier lifetimes as long as 172 ns. The relatively long lifetimes in PbSe thin films are achievable despite threading dislocation densities exceeding $10^9$ $cm^{-2}$ arising from island growth on the nearly 8% lattice- and crystal-structure-mismatched GaAs substrate. Using quasi-co…
Strain engineering has been a ubiquitous paradigm to tailor the electronic band structure and harness the associated new or enhanced fundamental properties in semiconductors. In this regard, semiconductor membranes emerged as a versatile class of nanoscale materials to control lattice strain and engineer complex heterostructures leading to the development of a variety of innovative applications. Herein we exploit this quasi-two-dimensional platform to tune simultaneously the lattice parameter and bandgap energy in group IV GeSn semiconductor alloys. As Sn content is increased to reach a direct…
Magnetic field induced polarization enhancement in the photoluminescence of MBE-grown WSe$_2$ layers
Institute of Experimental Physics, Faculty of Physics, University of Warsaw（华沙大学物理学院实验物理研究所）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（物质材料结构研究中心）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（电子与光学材料研究中心）
CVD growth is capable of producing multiple single crystal islands of atomically thin TMDs over large area substrates, with potential control of their morphology, lateral size, and epitaxial alignment to substrates with hexagonal symmetry. Subsequent merging of epitaxial domains can lead to single-crystal monolayer sheets - a step towards scalable production of high quality TMDs. For CVD growth to be effectively used for such production it is necessary to be able to rapidly assess the quality of material across entire large area substrates. To date characterisation has been limited to sub 0.1 …
Computationally guided high-throughput synthesis is used to explore the Zn-V-N phase space, resulting in the synthesis of a novel ternary nitride Zn$_2$VN$_3$. Following a combinatorial PVD screening, we isolate the phase and synthesize polycrystalline Zn$_2$VN$_3$ thin films with wurtzite structure on conventional borosilicate glass substrates. In addition, we demonstrate that cation-disordered, but phase-pure (002)-textured, Zn$_2$VN$_3$ thin films can be grown using epitaxial stabilization on α-Al2O3 (0001) substrates at remarkably low growth temperatures well below 200 °C. The structural p…
Single-photon superradiance can emerge when a collection of identical emitters are spatially separated by distances much less than the wavelength of the light they emit, and is characterized by the formation of a superradiant state that spontaneously emits light with a rate that scales linearly with the number of emitters. This collective phenomena has only been demonstrated in a few nanomaterial systems, all requiring temperatures below 10K. Here, we rationally design a single colloidal nanomaterial that hosts multiple (nearly) identical emitters that are impervious to the fluctuations which …
Ultra-high-vacuum cluster tool for epitaxial synthesis and optical spectroscopy of reactive 2D materials
Walter Schottky Institute and TUM School of Natural Sciences, Technical University of Munich（慕尼黑工业大学沃尔特·肖特基研究所与自然科学学院）；Cavendish Laboratory, University of Cambridge（剑桥大学卡文迪许实验室）
Light emission in atomically thin heterostructures is known to depend on the type of materials, number and stacking sequence of the constituent layers. Here we show that the thickness of a two-dimensional substrate can be crucial in modulating the light emission. We study the layer-dependent charge transfer in vertical heterostructures built from monolayer tungsten disulphide (WS2) on one- and two-layer epitaxial graphene, unravelling the effect that the interlayer electronic coupling has on the excitonic properties of such heterostructures. We bring evidence that the excitonic properties of W…
Heteroepitaxial Growth of PbSe on InP Substrates via Lattice-Matched III-V Buffer Layers
CREOL, The College of Optics and Photonics, University of Central Florida（中佛罗里达大学光学与光子学院）；Department of Materials Science and Engineering, University of Central Florida（中佛罗里达大学材料科学与工程系）
