The increasing demand for new opto-electronics devices such as solar cells, laser diodes (LD), and high-brightness light-emitting diodes (HBLED), combined with the economic necessity to achieve lower energy consumption levels and higher device yields, is motivating researchers to develop new materials. The semiconductor industry is actively looking for alternatives to silicon, for example, to address new niche market applications in power devices. Constant efforts employed to reduce production costs are leading manufacturers to grow GaN on silicon substrate, creating new technical challenges, especially regarding the control of defect density on wafer. For all these reasons many studies are being initiated to improve understanding of the fundamental physical properties and behavior of compound semiconductor materials used in quantum wells, quantum dots and nanowire-like structures. Cathodoluminescence (CL) is a spectroscopy method that can generate reliable, quantitative, and stable data for research as well as prepare a basis for quality control during productio

Corfdir, P.

Ganière, J.-D

Gougeon, O.

Sonderegger, S.

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34 www.microscopy-today.com  •  2013 Januarydoi:10.1017/S1551929512000983Quantitative Cathodoluminescence Opens  New Areas of Investigation in Semiconductor  Research and ProductionP. Corfdir,1 O. Gougeon,2 * S. Sonderegger,2 and J.-D. Ganière11Ecole Polytechnique Fédérale de Lausanne, ICMP, 1015 Lausanne, Switzerland2Attolight AG, EPFL Innovation Square, PSE D, 1015 Lausanne, Switzerland*olivier.gougeon@attolight.comIntroductionThe increasing demand for new opto-electronics devices such as solar cells, laser diodes (LD), and high-brightness light-emitting diodes (HBLED), combined with the economic necessity to achieve lower energy consumption levels and higher device yields, is motivating researchers to develop new materials. The semiconductor industry is actively looking for alternatives to silicon, for example, to address new niche market applications in power devices. Constant efforts employed to reduce production costs are leading manufacturers to grow GaN on silicon substrate, creating new technical challenges, especially regarding the control of defect density on wafer. For all these reasons many studies are being initiated to improve understanding of the fundamental physical properties and behavior of compound semiconductor materials used in quantum wells, quantum dots and nanowire-like structures. Cathodoluminescence (CL) is a spectroscopy method that can generate reliable, quantitative, and stable data for research as well as prepare a basis for quality control during production.CL refers to the light emitted by a material under electron irradiation. The most common example of this phenomenon is a television using a cathode ray tube. When combined with a scanning electron microscope (SEM), CL is a powerful spectroscopy technique with analytical spatial resolution down to the nanometer scale [1]. The multimode imaging capabilities of the SEM enable correlation of optical properties (via CL) with surface morphology at the nanometer scale [2]. For example, CL can reveal defects that are invisible to other detection methods [3, 4] and is also capable of probing a single selected nanostructure. Generated by high-energy electrons, CL is often the method of choice to study wide bandgap semiconductors such as diamond, boron nitride, or aluminium gallium nitride. In addition, the innovative CL system of Attolight can be upgraded to a time-resolved mode that yields time resolutions below 10 ps allowing the study of carrier dynamics in nanostructures. This technique has been used extensively [5–10] to study gallium nitride based nanostructures. The behavior of charge carriers in structures can be important for evaluating elementary components in quantum computers [11, 12], optical quantum gates [13], or single photon sources [14–16] for quantum cryptography [17]. Conventionally, CL is a challenging technique generally carried out using unwieldy laboratory-assembled imaging devices. A SEM is required along with a CL add-on device consisting of a light-collection mirror and a detection system. These components must be assembled on site, which can result in cumbersome and unstable alignment plus interfacing difficulties. This is particularly the case with cryogenic positioning stages. The collection mirror (parabolic or elliptic) has a very limited field of view (several micrometers). Thus, when the sample is moved slightly, most of the signal is lost and realignment is required. In addition, when light is not emitted exactly at the focal point of the mirror, the collection efficiency is reduced, resulting in limited sensitivity.The Attolight company makes CL available in a simplified system that integrates CL with both light and electron microscopy. It features a hyperspectral imaging mode that reveals ultra-trace impurities and crystallographic defects not visible using other imaging modalities. At production sites, it is expected to become a major tool for detecting defective materials at an early stage of the manufacturing process thereby generating significant savings.Methods and MaterialsThe Attolight system is an independent spectroscopy system in which an optical microscope is embedded within the electromagnetic objective lens of a SEM, matching both fields of view. The optical microscope visually positions the Figure 1: ZnO nano-structures. The acquisition of a CL micrograph (left) does not affect the detection of secondary electrons (right). Copyright 2012 Attolight AG.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929512000983Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 017 at 20:57:20, subject to the Cambridge Core terms of use, available athttps:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929512000983Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 20:57:20, subject to the Cambridge Core terms of use, available at36 www.microscopy-today.com  •  2013 JanuaryDetermination of Threading Dislocation Density in a GaN TemplateThe Attolight hybrid CL system is particularly efficient for characterizing defects in semiconductors. As an example, we display in the left-hand side of Figure 3 a secondary electron micrograph of the surface of a defective GaN template, taken with Attolight’s proprietary SEM. The image reveals particles at the surface, but no threading dislocation (TD) is observed. However, CL can detect these TDs. When an electron beam is focused onto a sample, it generates charge carriers, which recombine either radiatively (light emission) or non- radiatively (heat emission). The Attolight CL embedded optics inside the column of the SEM are optimized to collect the emitted light, the latter being detected by a photo- multiplier tube (UV-visible). On the CL micrograph in Figure 3 (right), TDs appear as dark spots because these crystal defects act as efficient non-radiative recombination centers. This polychromatic CL micrograph reveals a high density of dark spots, which correspond to TDs. Simply counting the number of dark spots per unit area allows determination of the TD density (TDD), which in the present case is of the order of 3 × 107/cm2.Acquisition of both SEM and CL images allows for precise determination of the local density of TDs emerging at the surface of a GaN sample. Depending on the acceleration voltage, which is typically set between 3 and 10 kV, the impinging electron beam generates charge carriers in a volume with a spatial extension between 10 nm and 1 μm. Threading dislocations therefore can be potentially detected in a specimen down to a depth of 1 μm. Future developments can be envisaged to automate the measurement of TDD through image processing for large-scale manufacturing control purposes.Local Lifetime Measurements in ZnO Nano BeltsTime-resolved cathodoluminescence (TRCL) can provide reliable evidence of high crystalline quality in isolated ZnO nano-objects [18]. Figure 4 shows continuous CL experiments carried out on a ZnO nano belt. Hyperspectral CL mapping shows that the CL intensity fluctuates along the length of the structure, with a dramatic reduction of the signal where the wire is slightly bent. In addition, when scanning along the axis of the nano belt, the emission peak wavelength shifts by approximately 2 nm revealing local strain fluctuations. Photoluminescence experiments could not provide such a nanometer-scale char- acterization of ZnO nano belt emission properties. The spatial resolution of photo- luminescence, limited typically to few microns, is therefore insufficient to rate the quality of single nanostructures. Unfor- tunately, continuous-wave CL alone does not reveal the respective variations of charge carrier non-radiative and radiative recom- bination lifetimes at the origin of the emission intensity fluctuations observed Quantitative Cathodoluminescencesystem so that no other optical alignment is required. This innovative design is opti- mized to achieve superior CL imaging without compro-mising SEM performance: secondary electrons are not obstructed by the collection optics (Figure 1). Integrated light optics allows consistent high-efficiency light collec- tion over the entire field of view, and, thanks to a numerical aperture of 0.72 (f/0.5), the average collection efficiency over the 300 μm field of view is up to 100 times larger than for a conventional CL apparatus (Figure 2). Quantitative measurements are not possible with the traditional parabolic mirror approach because the field of view is limited to a few microns and the optical image is plagued by blur and vignetting. However, with the Attolight design, quantitative CL measurements can be performed and data can be compared from one sample to another. In addition, the light optical system is achromatic from the ultraviolet to the infrared spectral range, allowing a wide range of applications. For advanced applications requiring a deep understanding of the properties of a material or to boost the CL emission efficiency, an optional cryogenic nano-positioning stage can control the specimen temperature from 15 to 300 K. This facilitates the measurement of weak luminescent samples, providing low drift and low vibration. A proprietary mechanical design with 6 degrees of freedom ensures arbitrary sample positioning. The system also features, without degrading the spatial resolution, a time-resolved option for lifetime and charge-carrier dynamics measurements in which picosecond electron pulses are generated to excite the specimen.Figure 2: Attolight optical micro- scope features constant resolution and photon collection efficiency over a field of view of 300 µm. Copyright 2012 Attolight AG.Figure 3: Left: SEM image taken on a polished GaN layer. Right: cathodoluminescence image showing threading dislocations in the GaN layer; dislocations appear as dark spots because of non-radiative recombination in their vicinity. On the secondary electron image (left) taken of the same region, no threading dislocations can be identified. Copyright 2012 Attolight AG.https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929512000983Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 20:57:20, subject to the Cambridge Core terms of use, available at372013 January  •  www.microscopy-today.commethod to probe the dynamics of charge carriers, where photoluminescence appears to be ineffective because of the lack of spatial resolution.Although most of our experience is so far in semiconductor applications, we think that our method can also open new possibilities in geology and biology. The fact that the system Quantitative Cathodoluminescencein Figure 4. Still, this knowledge is indispensable to under- stand what causes local intensity drops along the nano-belt axis.Time-resolved measurements have therefore been carried out to investigate the spatial dependence of charge carriers radiative and non-radiative lifetimes. The general equations below (Equations 1 and 2) give the time evolution of the CL intensity IX(t): IX(t) ∝  τr  exp[-t/(  τr  +  τnr  )-1] (1) IX(0) ∝  τr (2)where τr and τnr are the charge carriers radiative and non-radiative lifetimes, respectively. Equation 2 clearly shows that at t = 0 (that is, at the beginning of the decay) the CL intensity is inversely proportional to the radiative lifetime, τr. Measurements at different locations on the nano belt show that the CL intensity at t = 0 stays constant (Figure 5), leading to the conclusion that τr does not vary along the nano belt. In other words, the CL intensity drop observed in Figure 4 is only due to a local decrease of the non-radiative lifetime. In agreement, the CL decay taken where the nano belt is bent (green curve in Fig. 5) is the one exhibiting the fastest decay.By measuring the variation in CL decay time along the nano-belt axis (Figure 6), and assuming that the internal quantum efficiency  η is given by η =   τr  +  τnr   (3)it is possible to trace the evolution of η along the nano belt axis and to characterize locally the crystalline quality of the material.ConclusionThe new CL method presented here is a powerful tool able to provide a deeper understanding into the quality of semiconductor devices. TRCL has proven to be an efficient Figure 6: Top: Time-integrated CL mapping of a ZnO nano belt at 30 K. The signal has been spectrally integrated between 3.34 and 3.40 eV. Below: TRCL reveals non-radiative lifetime variations along the length of the structure. Where the structure is bent, we can clearly observe a steep decrease of the non-radiative recombination time, leading to a reduction of the CL emission intensity. From Equation 3, it is then possible to extract the internal radiative efficiency along the length of the nano belt (shown in red).Figure 4: Continuous CL on a ZnO nano belt at T = 30 K. A drastic drop in CL intensity is observed where the nano belt is slightly bent. Compared to the extremities of the nanostructure, the emission is redshifted when the excitation is delivered in the vicinity of the region where CL intensity drops.Figure 5: TRCL on a ZnO nano belt. The curves correspond to two different excitation spots along the nano-belt axis. The constant emission intensity at zero delay shows that the charge carrier radiative lifetime is constant along the nanostructure.1 1 11τnr https:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929512000983Downloaded from https:/www.cambridge.org/c re. University of B sel Library, on 30 May 2017 at 20:57:20, subject to the Cambridge Core terms of use, available at38 www.microscopy-today.com  •  2013 January [9] S Sonderegger, E Feltin, M Merano, A Crottini, JF Carlin, R Sachot, B Deveaud, N Grandjean, and JD Ganière, Appl Phys Lett 89 (2006) 232109–11. [10] M Merano, S Sonderegger, A Crottini, S Collin, P Renucci, E Pelucchi, A Malko, MH Baier, E Kapon, B Deveaud and JD Ganière, Nature 438 (2005) 479–82. [11] D Loss and DP DiVincenzo, Phys Rev A 57 (1998)120–26. [12] A Imamoglu, Physica E 16 (2003) 47–50. [13] X Li, Y Wu, D Steel, TH Stievater, DS Katzer, D Park,  C Piermarocchi, and LJ Sham, Science 301 (2003) 809–11. [14] O Benson, C Santori, M Pelton, and Y Yamamoto, Phys Rev Lett 84 (2000) 2513–16. [15] JM Gerard and B Gayral, J Lightwave Technol 17 (1999) 2089–95. [16] MH Baier, E Pelucchi, E Kapon, S Varoutsis, M Gallart, I Robert-Philip, and I Abram, Appl Phys Lett 84 (2004) 648–50. [17] N Gisin, GG Ribordy, W Tittel, and H Zbinden, Review of Modern Physics 74 (2002) 145–95. [18] P Corfdir, M Abid, A Mouti, PA Stadelmann, E Papa, JP Ansermet, JD Ganière, and B Deveaud-Plédran, Biexciton emission and crystalline quality of ZnO nano-objects, Nanotechnology 22 (2011) 285710.Quantitative Cathodoluminescencecan measure low signals, with better resolution, over a large field of view, all without any alignment, will certainly open new areas of investigations.References [1] BG Yacobi and DB Holt, Cathodoluminescence Microscopy of Inorganic Solids, Springer, New York,  1990. [2] L Reimer, Scanning Electron Microscopy, Springer, New York, 1998, ch. 1. [3] N Pauc, MR Phillips, V Aimez, and D Drouin, Appl Phys Lett 89 (2006) 161905–07. [4] SJ Rosner, EC Carr, MJ Ludowise, G Girolami, and  HI Erikson, Appl Phys Lett 70 (1997) 420–22. [5] P Corfdir, D Simeonov, E Feltin, JF Carlin, P Lefebvre, N Grandjean, B Deveaud-Plédran, and JD Ganière, Phys Status Solidi C 8(4) (2011) 1394–97. [6] P Corfdir, P Lefebvre, L Balet, S Sonderegger, A Dussaigne, T Zhu, D Martin, JD Ganière, N Grandjean, and  B Deveaud-Plédran, J Appl Phys 107 (2010) 043524–28. [7] P Corfdir, J Ristić, P Lefebvre, T Zhu, D Martin,  A Dussaigne, JD Ganière, N Grandjean, and B Deveaud-Plédran, Appl Phys Lett 94 (2009) 201115–17. [8] N Grandjean, E Feltin, R Butté, JF Carlin, S Sonderegger, B Deveaud, and JD Ganière, Philos Mag 87(13) 2067–75.New to the Advances in Microscopy and Microanalysis book series!www.cambridge.org/us800.872.7423Scanning Electron Microscopy for the Life SciencesHeide SchattenUniversity of Missouri, ColumbiaUS$120.00: Hb: 978-0-521-19599-7: 312 ppRecent developments in scanning electron microscopy (SEM) have resulted in a wealth of new applications for cell and molecular biology, as well as related biological disciplines. It is now possible to analyze macro molecular complexes within their three-dimensional cellular microenvironment in near native states at high resolution, and to identify specifi c molecu les and their structural and molecular interactions. New approach es include cryo-SEM applications and environmental SEM (ESEM), staining techniques and processing ap plications combining embedding and resin-extraction for imaging with high resolution SEM, and advances in immuno-labeling.  With chapters written by experts, this guide gives an overview of SEM and sample processing for SEM, and highlights several advances in cell and molecular biology that greatly benefi ted from using conventional, cryo, immuno, and high-resolution SEM.About the seriesThe Press currently publishes the Microscopy and Microanalysis (MAM) journal in conjunction with the MSA, which reaches 4,000 microscopists and is affi liated with 12 international microscopy societies. The series would be a natural development from this journal, and will take a broad view of the discipline, covering topics from instrumentation to imaging, methodology and analysis across physical science, materials science, biology and medicine. Books commissioned for the series will range from advanced undergraduate textbooks through to research and practitioner oriented monographs for researchers. The series aims to produce a coherent source of material, encouraging the communication and exchange of ideas across these divergent fi elds, ensuring that the series appeals to a broad community in the physical and life sciences.Forthcoming titles in this series:Microscopic Nanocharacterization of Materials by Michael IsaacsonEnergy Filtered Electron Microscopy and Electron Spectroscopy by Richard LeapmanDynamic and Transmission Electron Microscopy by Nigel Browning, Thomas LaGrange, Bryan Reed, Hennng Stahlberg, Bradley Siwickhttps:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929512000983Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 20:57:20, subject to the Cambridge Core terms of use, available athttps:/www.cambridge.org/core/terms. https://doi.org/10.1017/S1551929512000983Downloaded from https:/www.cambridge.org/core. 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Quantitative Cathodoluminescence Opens New Areas of Investigation in Semiconductor Research and Production

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