Fabien Massabuau

Cathodoluminescence hyperspectral imaging of trench-like defects in InGaN/GaN quantum well structures

Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (≈90xa0meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (≈10xa0meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes.

Dislocations in AlGaN: Core Structure, Atom Segregation, and Optical Properties

We conducted a comprehensive investigation of dislocations in Al0.46Ga0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In0.09Ga0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts.

Characterisation of InGaN by Photoconductive Atomic Force Microscopy

Nanoscale structure has a large effect on the optoelectronic properties of InGaN, a material vital for energy saving technologies such as light emitting diodes. Photoconductive atomic force microscopy (PC-AFM) provides a new way to investigate this effect. In this study, PC-AFM was used to characterise four thick (∼130 nm) InxGa1−xN films with x = 5%, 9%, 12%, and 15%. Lower photocurrent was observed on elevated ridges around defects (such as V-pits) in the films with x≤12%. Current-voltage curve analysis using the PC-AFM setup showed that this was due to a higher turn-on voltage on these ridges compared to surrounding material. To further understand this phenomenon, V-pit cross sections from the 9% and 15% films were characterised using transmission electron microscopy in combination with energy dispersive X-ray spectroscopy. This identified a subsurface indium-deficient region surrounding the V-pit in the lower indium content film, which was not present in the 15% sample. Although this cannot directly explain the impact of ridges on turn-on voltage, it is likely to be related. Overall, the data presented here demonstrate the potential of PC-AFM in the field of III-nitride semiconductors.

Effects of a Si-doped InGaN Underlayer on the Optical Properties of InGaN/GaN Quantum Well Structures with Different Numbers of Quantum Wells

In this paper we report on the optical properties of a series of InGaN polar quantum well structures where the number of wells was 1, 3, 5, 7, 10 and 15 and which were grown with the inclusion of an InGaN Si-doped underlayer. When the number of quantum wells is low then the room temperature internal quantum efficiency can be dominated by thermionic emission from the wells. This can occur because the radiative recombination rate in InGaN polar quantum wells can be low due to the built-in electric field across the quantum well which allows the thermionic emission process to compete effectively at room temperature limiting the internal quantum efficiency. In the structures that we discuss here, the radiative recombination rate is increased due to the effects of the Si-doped underlayer which reduces the electric field across the quantum wells. This results in the effect of thermionic emission being largely eliminated to such an extent that the internal quantum efficiency at room temperature is independent of the number of quantum wells.

Alloy fluctuations at dislocations in III-nitrides: identification and impact on optical properties

We investigated alloy fluctuations at dislocations in III-Nitride alloys (InGaN and AlGaN). We found that in both alloys, atom segregation (In segregation in InGaN and Ga segregation in AlGaN) occurs in the tensile part of dislocations with an edge component. In InGaN, In atom segregation leads to an enhanced formation of In-N chains and atomic condensates which act as carrier localization centers. This feature results in a bright spot at the position of the dislocation in the CL images, suggesting that non-radiative recombination at dislocations is impaired. On the other hand, Ga atom segregation at dislocations in AlGaN does not seem to noticeably affect the intensity recorded by CL at the dislocation. This study sheds light on why InGaN-based devices are more resilient to dislocations than AlGaN-based devices. An interesting approach to hinder non-radiative recombination at dislocations may therefore be to dope AlGaN with In.

Research data supporting "Carrier Localization in the Vicinity of Dislocations in InGaN"

FIG. 1. AFM (a), SEM (b), panchromatic CL (c), and ADF-STEM (d) performed on the same micrometre-scale area. To guide the eye, a few dislocations are indicated by arrows in each picture. (e) High-resolution (HR) STEM image of the dislocation indicated by a square in (a)-(d), enabling the identification of the core structure (here dissociated 7/4/8/5-atom ring), and (f) geometric phase analysis (GPA) showing the e_xx strain component of the dislocation core region. n nFIG. 2. Schematic showing the electron probe in the SEM-CL scanning across a V-pit. The scale of the schematic, although indicative, is representative of the experimental conditions in which the experiment was conducted. Distance to nearest neighbor dependence of the intensity ratio (a)(c) and energy shift (b)(d) measured at the center (a)(b) and facet (c)(d) of the V-pits. n nFIG. 3. (a) Histogram of the number of In-N chains as a function of the number of indium atoms in the chains, located within a 10 A radius centered on the dislocation, in the case of a random distribution of indium (i.e. initial configuration of the simulation) or segregation of indium (i.e. equilibrium configuration of the simulation). Abstract representation of the data in (a), in the case of a random distribution (b) or segregation (c) of indium atoms. n nFIG. 4. ADF-STEM image of the clustered dislocations 26 (a) and 87 (b). The white strain-related contrast between the neighboring dislocations is indicated by an arrow. Aberration-corrected HAADF-STEM image of the core of dislocation 26 (dissociated 7/4/8/4/9-atom ring)(c) and 87 (undissociated double 5/6-atom ring)(d). An ABSF-filter (Average Background Subtraction Filter) has been applied to (c) and (d) in order to remove noise from the images. n nFIG. 5. 16K CL integrated intensity (a)(c) and peak emission energy (b)(d) maps of isolated n n(a)(b) and clustered (c)(d) dislocations. To guide the eye, the position of the bright spots, directly observable in (a) and (c), is indicated by circles in all the maps. To emphasize the relative variations in intensity and energy between isolated and clustered configurations, a common color scale is used in (a) and (c) and in (b) and (d).