4:00pm - 4:30pmInvited
Highly Spatially Resolved STEM-CL Analysis of 3D GaN Nanostructures
Otto von Guericke University Magdeburg, Germany
For a comprehensive understanding of complex semiconductor nano-heterostructures and the physics of devices based on them, a systematic determination and correlation of the structural, chemical, electronic, and optical properties on a true nanometer scale is essential. Luminescence techniques belong to the most sensitive, non-destructive methods of semiconductor research. The combination of luminescence spectroscopy - in particular at liquid He temperatures - with the high spatial resolution of a scanning transmission electron microscopy (STEM) as realized by the technique of low temperature scanning transmission electron microscopy cathodoluminescence microscopy (STEM-CL), provides a unique and extremely powerful tool for the optical nano-characterization of semiconductors.
Typical results, which will be presented, include the nm-scale analysis of 3 dimensional GaN based nanostructures visualizing the enormous capability of STEM-CL characterization:
We give the direct evidence of quantum dot (QD) emission from self-organized GaN islands within AlN, nucleated in close proximity of threading dislocations (TDs). These islands result from GaN quantum well layer growth on AlN/sapphire templates. Ultra-narrow emission line widths down to 440 µeV of individual islands reveal the QD like behavior. The single photon emission of the QD luminescence is verified by a clear anti-bunching observed in Hanbury-Brown-Twiss photoluminescence experiments at 8 K yielding a time resolution limited experimental value of g2(0) = 0.42. We investigated the inner optical and structural properties of the GaN islands. Spectrally different sharp QD lines were emitted from different island regions, revealing the formation of several spatially separated QD states in an island. In addition to the reasonably assumed complex strain field due to the high number of TDs, directly verified thickness fluctuations in an island lead to clustered QDs within a single island.
4:30pm - 5:00pmInvited
High-Resolution Quantitative Cathodoluminescence (CL) for Material Science and Nanophotonics
Attolight AG, Switzerland
High spatial resolution spectroscopic information may be acquired by using an electron beam in a modern scanning electron microscope (SEM), exploiting a phenomenon called cathodoluminescence (CL). CL can be used to perform non-destructive analysis of a broad range of materials comprising insulators, semiconductors and metals. This approach offers several advantages over usual optical spectroscopy techniques. The multimode imaging capabilities of the SEM enable the correlation of optical properties (via CL) with surface morphology (secondary electron – SE – mode) at the nanometer scale. In semiconductors and insulators, the CL spectrum gives local information on the electronic bandgap and defect states. In metals and nanostructured materials, CL is sensitive to the local density of optical states (LDOS) and allows direct probing of nanophotonic devices.
We will show how high resolution hyperspectral CL microscopy is routinely used to perform defect and homogeneity metrology as well as failure analysis in semiconducting materials. Examples on optoelectronic and solar cells devices will be highlighted. In addition, we will give examples of CL imaging of nanophotonic structures used as single photon sources, or for lasing and sensing applications. Finally, we will show how the introduction of pulsed electron excitation and time resolved detection of the CL signal (TRCL) allows carrier dynamic probing at the nanoscale.
5:00pm - 5:30pmInvited
A TEM-based Platform for Nanoscale Imaging and Optical Spectroscopy
Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore
This paper will discuss Scanning TEM (STEM)-based imaging and spectroscopy on semiconductor heterostructures and on plasmonic nanostructures. Our nano-optical platform can measure from the same sample location extinction spectra (using monochromated EELS [1-4]) as well as photoemission spectra (using cathodoluminescence [5, 6]).
Measurements are performed with an electron beam with a diameter of about 1 nm which can be placed with sub-nanometer precision at any sample location of interest. Despite this very high precision, the resolution of the nano-optical measurements are more delocalized, leading typically to a resolution of ~10 nm for measurements in the visible . This nevertheless allows us to perform deep sub-wavelength optical measurements. And interestingly, the (S)TEM images that are taken are not affected by the delocalization effect, as they are formed by elastic rather than inelastic scattering.
The talk will discuss collaborative efforts to integrate new wide-bandgap materials with plasmonic nanostructures. The aim of this effort is to get far-field access to dark plasmon modes, which are optical states currently inaccessible for device applications.
The National Research Foundation (NRF) is kindly acknowledged for supporting this research under the NRF ANR Joint Grant Call (Award No. NRF2016-NRF-ANR002). This work results from close collaborations with the groups of Joel Yang (SUTD, Singapore), Erik Dujardin (CEMES, France), Tripathy Sudhiranjan (IMRE Singapore), Antonio Fernández-Domínguez (UAM, Spain), Wu Lin & Bai Ping (IHPC, Singapore).
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 J Nelayah et al., Nature Phys. 3, 348 – 353 (2007).
 M Bosman et al., Nanotechnology 18, 165505 (2007).
 A Teulle et al. Nature Materials 14, 87-94 (2015).
 T Coenen et al. Phys Rev B 93, 195429 (2016).
 M Kociak, LF Zagonel Ultramicroscopy 174 50-69 (2017).
 M Bosman et al. Appl. Phys. Lett. 95, 101110 (2009).
5:30pm - 6:00pmInvited
Optoelectronic Device Parameters of Wide Bandgap Semiconductors Determined by Spectroscopic Ellispometry
University of Delhi, India
Transparent conducting oxide and nitride semiconductors are characterized by a remarkable combination of a high optical transparency and a good electrical conductivity in the visible region, which can be simultaneously realised by appropriate material engineering involving either the introduction of metal dopants or a slight non-stoichiometry in the intrinsic material. The variety of electronic properties achieved by such engineering renders them suitable for a range of devices including solar cells, UV detectors, flat panel displays, and room temperature UV optoelectronic devices such as light emitting devices and lasers.
A comprehensive non-invasive characterization of the material parameters offers the advantage of directly translating the characterized film into a device. We demonstrate spectroscopic ellipsometry as a one-step technique for the simultaneous determination of both the electrical and optical properties. This is done by investigating the complex dielectric function (ε (E)= ε1 (E)+ iε2 (E)) over a wide spectral range in the UV-VIS-NIR range(0.5-6 eV). The measured ellipsometric angles, are directly related to the electronic and optical properties and the use of theoretical models for the dielectricfunction gives the optical constants, film thickness, surface roughness and bandgap. Bandgap changes, induced by material modifications such as nonstoichiometry and metal doping, is also quantified. Additionally, the technique gives estimates of the carrier concentration and mobility. It has the ability to determine these values for very thin films, which is not possible by conventional Hall measurements. The measured parameters are correlated to the stoichiometry and the density of the films quantified by Rutherford backscattering spectroscopy.
 H. Fuziwara, Ellipsometry Principles and Applications (John Wiley & Sons Ltd., England, 2007).
 C. Singh, S.Nozaki, and ShyamaRath, J. Appl. Phys. 118 (2015) DOI: 10.1063/1.4935629
. Chaman Singh and ShyamaRath, J. Appl. Phys. 113 (2013) art. no. 163104
6:00pm - 6:15pmOral
Thickness-dependent Tunable Optoelectronic Properties of n-MoO3/p-Si Heterojunction
1SUNAG Laboratory, Institute of Physics, Bhubaneswar, India; 2Homi Bhabha National Institute, India; 3Department of Condensed Matter Physics, Weizmann Institute of Science, Israel; 4Department of Physics, Alagappa University, India
In this work, we present the tunable optoelectronic properties of a heterojunction consisting of wide band-gap (3.3 to 4.2 eV) n-MoO3 and p-Si for different thicknesses of MoO3 (10, 20, and 30 nm). The films were prepared by radio-frequency magnetron sputtering technique at room temperature. X-ray diffraction study reveals the amorphous nature of the as-grown MoO3 thin films. In addition, the average specular reflectance (%R) of the conformally grown MoO3 films turns out to be ~42% (in the wavelength range of 300 to 800 nm), albeit the band gap decreases (from 3.95 to 3.89 eV) with increasing film thickness. MoO3/Si heterojunctions exhibit diode-like rectifying I-V behavior measured under both dark and illuminated conditions. Interestingly, all the MoO3/Si heterojunctions prevent the flow of holes but allow the flow of electrons. Moreover, The 30 nm-thick film exhibits a poor photoresponse in terms of generation of photocurrent ~30 mA, whereas the 10 nm-thick film shows a remarkable enhancement in the photocurrent up to ~73 mA. The responsitivity and sensitivity are also higher for the latter film. These results are explained in terms of thickness-dependent change in the optical and electrical properties of the MoO3 films. The observed results are very important for the constructions of oxide-based solar cell and photodiodes based on MoO3/Si heterostructures.