Effect of extended defects on AlGaN QDs for electron-pumped UV-emitters

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like defects starting at the first AlN buffer/superlattice interface and propagating vertically. These defects are associated with a dislocation that produces strong shear strain, which favors the formation of 30{\deg} faceted pits. The cone-like structures present Ga enrichment at the boundary facets and larger QDs within the defect. The bimodality is attributed to the differing dot size/composition within the defects and at the defect boundaries, which is confirmed by the correlation of microscopy results and Schr\"odinger-Poisson calculations

Modification of porous SiOCH by first contact with water vapor after plasma process

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Cathodoluminescence spectroscopy of plasmonic patch antennas: towards lower order and higher energies

International audienceWe report on the cathodoluminescence characterization of Au, Al and a Au/Al bimetal circular plasmonic patch antennas, with disk diameter ranging from 150 to 900 nm. It allows us access to monomode operation of the antennas down to the fundamental dipolar mode, in contrast to previous studies on similar systems. Moreover we show that we can can shift the operation range of the antennas towards the blue spectral range by using Al. Our experimental results are compared to a semi-analytical model that provides qualitative insight on the mode structure sustained by the antennas

Interaction of humidity with plasma-damaged porous low-k

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Kelvin probe force microscopy under variable illumination: a novel technique to unveil charge carrier dynamics in (Al/In/Ga)N

International audienceHerein, we show a novel technique based on Kelvin probe force microscopy (KPFM) under variable illumination in order to study the charge carrier dynamics and charge trapping by two case studies: n.i.d. GaN on silicon and GaN/InGaN MQW µLEDs. KPFM measures the contact potential difference (CPD) between a probing tip and the sample with nanometric resolution. By measuring the CPD in darkness and under illumination, we can map the surface photovoltage (SPV), defined as the change in the surface potential induced by the reorganization of the photo-generated carriers. The SPV, therefore, can shed light onto the different processes of charge transport, recombination and trapping. Although KPFM under illumination is a well-established method to measure surface photovoltage and charge carrier lifetime in organic and inorganic materials, the novelty of our approach consists in measuring the SPV while increasing the light intensity. This measurement method allows us not only to observe the SPV decay once the light is turned off, but to follow the SPV generation as a function of the laser power as well. Furthermore, KPFM is performed simultaneously with AFM (atomic force microscopy), allowing for the correlation of the spatial distribution of CPD and SPV with the surface's topography. In addition, we performed the KPFM protocol at low temperature in order to observe the effect of the freezing out of the defect levels. Regarding the GaN on Si, although illuminating with a laser with sub-band gap energy (405 nm), SPV was induced, implying electron-hole pair generation through defect levels. Furthermore, the SPV presented very long decay time after turning off the laser (around 70 s), compared to the carrier lifetimes reported in literature, on the order of ns, suggesting that this decay does not correspond only to recombination but also to de-trapping from defect energy levels. The evolution of the SPV curve as a function of laser power and time allowed us to propose a mechanism for the charge trapping dynamics during and after illumination. Finally, lower SPV and higher time decay constants were detected around dislocation pits. However, no correlation was found between SPV and dislocations after submitting the sample to a thermal treatment, suggesting a modification of the surface states, possibly through water desorption. The low temperature measurements showed a modification in the SPV curves, in a consistent manner with our proposed trapping/de-trapping mechanism. In the case of the GaN/InGan MQW µLEDs, the SPV originates from the electron-hole pair generation in the QWs and its decay after illumination was also on the order of seconds. However, in contrast to the previous case, it could be better described by a double exponential, indicating the presence of at least two different dynamics on the de-trapping process with same sign. Additionally, there was no SPV variation near the border of the mesas, contrary to cathodoluminescence data, which showed lower light emission up to 3 µm from the border. Therefore, this well-known µLED efficiency drawback is not caused by the charge trapping processes detected through our KPFM protocol but by a different mechanism. Finally, the SPV decreases with temperature, consistent with the freezing out of the defect energy levels. This work, done at CEA's NanoCharacterization PlatForm (PFNC), was supported by the "Recherches Technologiques de Base" programme, the French Agency for National Research (ANR) via Carnot funding, as well as by PowerElec project, which has received funding from the EMPIR programme

In situ cleaning/passivation of surfaces for contact technology on III-V materials

International audienceIn this work we introduce the use of physical plasmas (e.g. Ar-and He-based plasmas) in order to study the in situ cleaning (prior to metal deposition) of InGaAs layers dedicated to the realisation of self-aligned contacts. For the characterisation of cleaning efficiency, we performed surface analyses like X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy in attenuated total reflection mode. The first results described in this work are encouraging. We have found efficient processes for removing totally or partially III-V native oxides

Study of Oxidation Mechanisms in Ge-rich GeSbTe: Structural Evolution in Temperature and N Doping Impact

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Non-radiative recombination centres in InGaN/GaN nanowires revealed by statistical analysis of cathodoluminescence intensity maps and electron microscopy

International audienceAbstract The methodology of statistical analysis of cathodoluminescence intensity mappings on ensembles of several hundreds of InGaN/GaN nanowires used to quantify non-radiative recombination centres was validated on InGaN/GaN nanowires exhibiting spatially homogeneous cathodoluminescence at the scale of single nanowires. Cathodoluminescence intensity variations obeying Poisson's statistics were assigned to the presence of randomly incorporated point defects acting as non-radiative recombination centres. Additionally, another type of non-radiative recombination centres, namely extended defects leading to spatially inhomogeneous cathodoluminescence intensity at the scale of single InGaN/GaN nanowires are revealed by high resolution scanning transmission electron microscopy, geometrical phase analysis and two-beam diffraction conditions techniques. Such defects are responsible for deviations from Poissons’s statistics, allowing one to achieve a rapid evaluation of the crystallographic and optical properties of several hundreds of nanowires in a single cathodoluminescence intensity mapping experiment

Kelvin probe force microscopy under variable illumination: a novel technique to unveil charge carrier dynamics in GaN: T. Frontiers of in-situ materials characterization - from new instrumentation and methods to imagingaided materials design

International audienceKelvin probe force microscopy (KPFM) permits to map the contact potential difference (CPD) between a probing tip and a sample with nanometric resolution. By measuring the CPD in darkness and under illumination, we can map the surface photovoltage (SPV), which can shed light onto the different processes of charge transport, recombination and trapping in the material. Herein, we use a characterization method based on KPFM under variable illumination by showing two case studies: n.i.d. GaN-on-Si and GaN/InGaN MQW mesa structures. The KPFM protocol consists on measuring the SPV as a function of the laser power, which allows us to follow the SPV during the generation and recombination of charge carriers and relate it to the topography. These measurements were complemented by cathodoluminescence (CL), which counts with nanometric resolution too.For both studied samples, we measured a long SPV decay time after turning off the laser (around 70 s), compared to the carrier lifetimes reported in bibliography, on the order of ns or ps, suggesting that this decay does not correspond only to recombination but also to de-trapping from defect energy levels. Regarding the n.i.d. GaN on Si sample, its SPV curve showed two different components, independently of the tip position: one with positive sign, attributed to the transfer of holes to the surface due to the upwards band bending, and a negative signal, pinned on the trapping of electrons on defect levels. This evolution of the SPV curve as a function of laser power and time allowed us to propose a model for the charge trapping dynamics during and after illumination. Finally, lower SPV and higher time decay constants were measured around dislocation pits, indicating that the charge trapping mechanisms detected by our KPFM protocol could be the origin of the lower radiative recombination rate on dislocations, measured by CL. In the case of the GaN/InGan MQW mesas, the SPV decay after illumination could be better described by a double exponential, indicating the presence of at least two different dynamics on the de-trapping process with same sign. Additionally, there was no SPV decrease near the border of the mesas, contrary to the light emission intensity maps obtained by CL, meaning that the border effect is not caused by the charge trapping processes detected through our KPFM protocol but by a different mechanism. Finally, we mapped the light emission efficiency of the top p GaN layer and, unlike the MQW light emission, it did not decrease near the border of the mesas, indicating that the border effect is not originated nor influenced by this layer.This work, done at CEA's NanoCharacterization PlatForm (PFNC), was supported by the "Recherches Technologiques de Base" programme, the French Agency for National Research (ANR) via Carnot funding, as well as by PowerElec project, which has received funding from the EMPIR programme

Kelvin probe force microscopy under variable illumination: a novel technique to unveil charge carrier dynamics in GaN

International audienceGaN is a widely used material for optical and power devices. However, the performance of GaN-based devices is often reduced by charge trapping processes due to defect levels in the band gap. In this paper, we studied a GaN on silicon sample via Kelvin probe force microscopy (KPFM) under variable illumination. This novel methodology allows the surface photovoltage (SPV) to be followed on the GaN surface as a function of time, light intensity and topography with nanometric resolution. The measured SPV decay after turning off the light is too slow (on the order of seconds to days) to correspond to electron-hole recombination, suggesting that it is due to charge trapping. Two processes of opposite sign contributing to the SPV generation and decay were observed, indicating two different charge trapping mechanisms. In this study, we propose a model that accounts for the observed SPV behavior at different measurement temperatures. Furthermore, the contribution to the electronic behavior of possible contamination surface states is discussed