The effects of Si-doped prelayers on the optical properties of InGaN/GaN single quantum well structures

In this paper, we report on the effects of including Si-doped (In)GaN prelayers on the low temperature optical properties of a blue-light emitting InGaN/GaN single quantum well. We observed a large blue shift of the photoluminescence peak emission energy and significant increases in the radiative recombination rate for the quantum well structures that incorporated Si-doped prelayers. Simulations of the variation of the conduction and valence band energies show that a strong modification of the band profile occurs for the quantum wells on Si-doped prelayers due to an increase in strength of the surface polarization field. The enhanced surface polarization field opposes the built-in field across the quantum well and thus reduces this built-in electric field. This reduction of the electric field across the quantum well reduces the Quantum Confined Stark Effect and is responsible for the observed blue shift and the change in the recombination dynamics.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/I012591/1 and EP/ H011676/1.This is the accepted manuscript version of the article. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/105/9/10.1063/1.4894834

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 (90 meV) 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 (10 meV) 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

A comparison of the optical properties of InGaN/GaN multiple quantum well structures grown with and without Si-doped InGaN prelayers

In this paper, we report on a detailed spectroscopic study of the optical properties of InGaN/GaN multiple quantum well structures, both with and without a Si-doped InGaN prelayer. In photoluminescence and photoluminescence excitation spectroscopy, a 2nd emission band, occurring at a higher energy, was identified in the spectrum of the multiple quantum well structure containing the InGaN prelayer, originating from the first quantum well in the stack. Band structure calculations revealed that a reduction in the resultant electric field occurred in the quantum well immediately adjacent to the InGaN prelayer, therefore leading to a reduction in the strength of the quantum confined Stark effect in this quantum well. The partial suppression of the quantum confined Stark effect in this quantum well led to a modified (higher) emission energy and increased radiative recombination rate. Therefore, we ascribed the origin of the high energy emission band to recombination from the 1st quantum well in the structure. Study of the temperature dependent recombination dynamics of both samples showed that the decay time measured across the spectrum was strongly influenced by the 1st quantum well in the stack (in the sample containing the prelayer) leading to a shorter average room temperature lifetime in this sample. The room temperature internal quantum efficiency of the prelayer containing sample was found to be higher than the reference sample (36% compared to 25%) which was thus attributed to the faster radiative recombination rate of the 1st quantum well providing a recombination pathway that is more competitive with non-radiative recombination processes.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/I012591/1 and EP/ H011676/1.This is the final version of the article. It first appeared from AIP Publishing via http://dx.doi.org/10.1063/1.494132

Effect of QW growth temperature on the optical properties of blue and green InGaN/GaN QW structures

In this paper we report on the impact that the quantum well growth temperature has on the internal quantum efficiency and carrier recombination dynamics of two sets of InGaN/GaN multiple quantum well samples, designed to emit at 460 and 530 nm, in which the indium content of the quantum wells within each sample set was maintained. Measurements of the internal quantum efficiency of each sample set showed a systematic variation, with quantum wells grown at a higher temperature exhibiting higher internal quantum efficiency and this variation was preserved at all excitation power densities. By investigating the carrier dynamics at both 10 K and 300 K we were able to attribute this change in internal quantum efficiency to a decrease in the non-radiative recombination rate as the QW growth temperature was increased which we attribute to a decrease in incorporation of the point defects.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/I012591/1 and EP/H011676/1.This is the final version of the article. It first appeared from Wiley via https://doi.org/10.1002/pssc.20151018

Effect of electron blocking layers on the conduction and valence band profiles of InGaN/GaN LEDs

In this paper we investigate the effect of including an electron blocking layer between the quantum well active region and the p-type layers of a light emitting diode has on the conduction and valence band profile of a light emitting diode. Two light emitting diode structures with nominally identical quantum well active regions one containing an electron blocking layer and one without were grown for the purposes of this investigation. The conduction and valence band profiles for both structures were then calculated using a commercially available Schrödinger-Poisson calculator, and a modification to the electric field across the QWs observed. The results of these calculations were then compared to photoluminescence and photoluminescence time decay measurements. The modification in electric field across the quantum wells of the structures resulted in slower radiative recombination in the sample containing an electron blocking layers. The sample containing an electron blocking layer was also found to exhibit a lower internal quantum efficiency, which we attribute to the observed slower radiative recombination lifetime making radiative recombination less competitive.This work was carried out with the financial support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/I012591/1 and EP/H011676/1.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/pssc.20151018

The Importance of Imaging Assessment Before Endovascular Repair of Thoracic Aorta

AbstractIndications for and experience with placement of endovascular stent grafts in the thoracic aorta are still evolving. Recent advances in imaging technologies have drastically boosted the role of pre-procedural imaging. The accepted diagnostic gold standard, digital subtraction angiography, is now being challenged by the state-of-the-art computed tomography angiography (CTA), magnetic resonance angiography (MRA) and trans-oesophageal echocardiography (TEE). Among these, technological advancements of multidetector computed tomography (MDCT) have propelled it to being the default modality used, optimising the balance between spatial and temporal resolutions and invasiveness. MDCT angiography allows the comprehensive evaluation of thoracic lesions in terms of morphological features and extent, presence of thrombus, relationship with adjacent structures and branches as well as signs of impending or acute rupture, and is routinely used in these settings.In this article, we review the current state-of-the-art radiological imaging for thoracic endovascular aneurysm repair (TEVAR), especially focusing on the role of MDCT angiography. After analysing the technical aspects for optimised imaging protocols for thoracic aortic diseases, we discuss pre-procedural determinants of candidacy, and how to formulate interventional plans based on cross-sectional imaging

Directly correlated microscopy of trench defects in InGaN quantum wells

Directly correlated measurements of the surface morphology, light emission and subsurface structure and composition were carried out on the exact same nanoscale trench defects in InGaN quantum well (QW) structures. Multiple scanning probe, scanning electron and transmission electron microscopy techniques were used to explain the origin of their unusual emission behaviour and the relationship between surface morphology and cathodoluminescence (CL) redshift. Trench defects comprise of an open trench partially or fully enclosing material in InGaN QWs with different CL emission properties to their surroundings. The CL redshift was shown to typically vary with the width of the trench and the prominence of the material enclosed by the trench above its surroundings. Three defects, encompassing typical and atypical features, were prepared into lamellae for transmission electron microscopy (TEM). A cross marker technique was used in the focused ion beam-scanning electron microscope (FIB-SEM) to centre the previously characterised defects in each lamella for further analysis. The defects with wider trenches and strong redshifts in CL emission had their initiating basal-plane stacking fault (BSF) towards the bottom of the QW stack, while the BSF formed near the top of the QW stack for a defect with a narrow trench and minimal redshift. The raised-centre, prominent defect showed a slight increase in QW thickness moving up the QW stack while QW widths in the level-centred defect remained broadly constant. The indium content of the enclosed QWs increased above the BSF positions up to a maximum, with an increase of approximately 4% relative to the surroundings seen for one defect examined. Gross fluctuations in QW width (GWWFs) were present in the surrounding material in this sample but were not seen in QWs enclosed by the defect volumes. These GWWFs have been linked with indium loss from surface step edges two or more monolayers high, and many surface step edges appear pinned by the open trenches, suggesting another reason for the higher indium content seen in QWs enclosed by trench defects

Carrier localization in the vicinity of dislocations in InGaN

We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaN-based devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.This project is funded in part by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 279361 (MACONS). The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483-ESTEEM2 (Integrated Infrastructure InitiativeI3). F.M. would also like to acknowledge the financial support from EPSRC Doctoral Prize Awards and Cambridge Philosophical Society. M.H. would like to acknowledge support from the Lindemann Fellowship

Microscopy of defects in semiconductors

In this chapter, the authors discuss microscopy techniques that can be useful in addressing defects in semiconductors. They focus on three main families: scanning probe microscopy, scanning electron microscopy and transmission electron microscopy. They first address the basic principles of the selected microscopy techniques In discussions of image formation, they elucidate the mechanisms by which defects are typically imaged in each technique. Then, in the latter part of the chapter, they describe some key examples of the application of microscopy to semiconductor materials, addressing both point and extended defects and both two-dimensional (2D) and three-dimensional (3D) materials

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.ER