Fabien Massabuau | University of Cambridge (original) (raw)

Papers by Fabien Massabuau

We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocat... more 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 InGaNbased 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 article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Applied Physics and may be found at http://dx.doi.org/10.1063/1.4973278. Copyright 2017. This article is distributed under a Creative Commons Attribution (CC BY) License.

physica status solidi (c), 2016

Applied Physics Letters, 2015

Two-band superlinear electroluminescence in GaSb based nanoheterostructures with AlSb/InAs1−x Sbx... more Two-band superlinear electroluminescence in GaSb based nanoheterostructures with AlSb/InAs1−x Sbx/AlSb deep quantum well Raman and photoluminescence characterization of focused ion beam patterned InGaN/GaN multi-quantum-wells nanopillar array

physica status solidi (b), 2014

Journal of Physics D-Applied Physics, 2014

ABSTRACT Optoelectronic devices based on the III-nitride system exhibit remarkably good optical e... more ABSTRACT 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.

physica status solidi (c), 2012

Abstract InAlN has many potential applications in photonics and electronics, but the growth rate ... more Abstract InAlN has many potential applications in photonics and electronics, but the growth rate of high quality material with smooth surfaces remains low, at∼ 4 nm/min, preventing its wider use in thicker structures such as distributed Bragg reflectors, although this is not a ...

physica status solidi (c), 2014

High resolution transmission electron microscopy has been employed to investigate the impact of t... more High resolution transmission electron microscopy has been employed to investigate the impact of the GaN barrier growth technique on the composition profile of InGaN quantum wells
(QWs). We show that the profiles deviate from their nominal
configuration due to the presence of an indium tail at the upper
interface of the QW. This indium tail, thought to be associated
with a segregation effect from the indium surfactant layer, has
been shown to strongly depend on the growth method. The
effect of this tail has been investigated using a self-consistent
Schrödinger–Poisson simulation. For the simulated conditions,
a graded upper interface has been found to result in a decreased electron-hole wavefunction overlap of up to 31% compared to a QW with a rectangular profile, possibly leading to a decrease in radiative-recombination rate. Therefore, in order to maximize the efficiency of a QW structure, it is important to grow the active region using a growth method which leads to QW interfaces which are as abrupt as possible. The results of this experiment find applications in every study where the emission properties of a device are correlated to a particular active region design.

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated... more The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence
emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a
model that the “green gap” problem might relate to localized strain relaxation occurring through defects.

We have studied the carrier recombination dynamics in an InGaN/GaN multiple quantum well structur... more We have studied the carrier recombination dynamics in an InGaN/GaN multiple quantum well structure as a function of emission energy and excitation density between temperatures of 10 K and 100 K. Under relatively low levels of excitation, the photoluminescence (PL) intensity and decay time of emission on the high energy side of the luminescence spectrum decrease strongly between 10 K and 50 K. In contrast, for emission detected on the low energy side of the spectrum, the PL intensity and decay time increase over the same temperature range. These results are consistent with a thermally activated carrier redistribution process in which the (temperature dependent) average timescale for carrier transfer into or out of a localised state depends on the energy of the given state. Thus, the transfer time out of shallow, weakly localised states is considerably shorter than the arrival time into more deeply localised states. This picture is consistent with carriers hopping between localisation sites in an uncorrelated disorder potential where the density of localised states decreases with increasing localisation depth, e.g., a exponential or Gaussian distribution resulting from random alloy disorder. Under significantly higher levels of excitation, the increased occupation fraction of the localised states results in a greater average separation distance between unoccupied localised states, causing a suppression of the spectral and dynamic signatures of the hopping transfer of carriers. V C 2014 AIP Publishing LLC. [http://dx.

J. Cryst. Growth 386, 88 , Jan 2014

The impact of the miscut of a (0001) c-plane substrate on the structural and optical properties o... more The impact of the miscut of a (0001) c-plane substrate on the structural and optical properties of InGaN/GaN quantum wells grown by metal-organic vapour phase epitaxy using a two-temperature method has been investigated. The two-temperature growth method involves exposure of the uncapped InGaN quantum well to a temperature ramp in an ammonia atmosphere before growth of the GaN barrier at a higher temperature. The resulting quantum well, consists of interlinking InGaN strips containing gaps which may impede carrier diffusion to dislocations. By increasing the substrate misorientation from 0° to 0.5° we show that the density of InGaN strips increases while the strip width reduces. Our data show that the PL efficiency increases with miscut and that the peak efficiency occurs at a lower excitation power density.

Photoluminescence and electroluminescence measurements on InGaN/GaN quantum well (QW) structures ... more Photoluminescence and electroluminescence measurements on InGaN/GaN quantum well (QW) structures and light emitting diodes suggest that QWs with gross fluctuations in width (formed when, during growth, the InGaN is exposed unprotected to high temperatures) give higher room temperature quantum efficiencies than continuous QWs. The efficiency does not depend on the growth temperature of the GaN barriers. Temperature-dependent electroluminescence measurements suggest that the higher efficiency results from higher activation energies for defect-related non-radiative recombination in QW samples with gaps. At high currents the maximum quantum efficiency is similar for all samples, indicating the droop term is not dependent on QW morphology.

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) with cathodoluminescence (CL... more Atomic force microscopy (AFM) and scanning electron microscopy (SEM) with cathodoluminescence (CL) were performed on exactly the same defects in a blue-emitting InGaN/GaN multiple quantum well (QW) sample enabling the direct correlation of the morphology of an individual defect with its emission properties. The defects in question are observed in AFM and SEM as a trench partially or fully enclosing a region of the QW having altered emission properties. Their sub-surface structure has previously been shown to consist of a basal plane stacking fault (BSF) in the plane of the QW stack, and a stacking mismatch boundary (SMB) which opens up into a trench at the sample surface. In CL, the material enclosed by the trench may emit more or less intensely than the surrounding material, but always exhibits a redshift relative to the surrounding material. A strong correlation exists between the width of the trench and both the redshift and the intensity ratio, with the widest trenches surrounding regions which exhibit the brightest and most redshifted emission. Based on studies of the evolution of the trench width with the number of QWs from four additional MQW samples, we conclude that in order for a trench defect to emit intense, strongly redshifted light, the BSF must be formed in the early stages of the growth of the QW stack. The data suggest that the SMB may act as a non-radiative recombination center.

In a wide variety of InGaN/GaN quantum well (QW) structures, defects are observed which consist o... more In a wide variety of InGaN/GaN quantum well (QW) structures, defects are observed which consist of a trench partially or fully enclosing a region of the QW having altered emission properties. For various different defect morphologies, cathodoluminescence studies suggest that the emission is redshifted in the enclosed region. Based on transmission electron microscopy and atomic force microscopy data, we suggest that the sub-surface structure of the trench defect consists of a basal plane stacking fault bounded by a stacking mismatch boundary which terminates at the apex of a V-shaped trench.

Dislocations in undoped GaN move in response to the in-plane tensile stress present during film g... more Dislocations in undoped GaN move in response to the in-plane tensile stress present during film growth. Dislocation movement during growth relieves tensile stress, produces arrays of a-type dislocations and reduces the overall dislocation density, with preferential reduction of (aþc)-type dislocations. However, Si-doping limits dislocation movement, limiting the relief of the tensile stress that develops during growth and limiting dislocation reduction, probably due to the formation of Si impurity atmospheres at dislocations. Consequently, Si-doped films are under relatively greater tensile stress compared to undoped GaN films grown under similar conditions. Alternative dopants could be chosen to reduce tensile stress development, such as Ge.

physica status solidi (c), 2016

Applied Physics Letters, 2015

physica status solidi (b), 2014

Journal of Physics D-Applied Physics, 2014

physica status solidi (c), 2012

physica status solidi (c), 2014

J. Cryst. Growth 386, 88 , Jan 2014