Challenges for Group III Nitride Semiconductors for Solid State Lighting and Beyond

Resume : The scattering in the emission wavelength of InGaN/GaN MQW heterostructures is generally attributed to local variations in strain, thickness and In content. Here, we employed a combination of nanosphere lithography and reactive ion etching to produce tightly size-controlled nano-LEDs with a mean top diameter of 188.11±16.92 nm demonstrated to result in a high efficient and collimated light emission towards the surface normal. A statistically relevant study of the strain state in individual nano-LEDs was performed using non-resonant micro-Raman spectroscopy based on a clear splitting (nanorod and substrate) of the E2(high) phonon mode of GaN. We found a consistent strain relaxation with regard to the strain in the initial film and the nanostructuring procedure that was also confirmed by the blue shift of the CL emission. Complementary micro-Raman–CL spectroscopic measurements on the same, isolated nanorods covering the entire range of stress states showed that the same strain level does not necessarily result in the same QW emission wavelength for all nanorods. Band profile calculations agreed well with the optical transitions observed in the CL experiments, namely a mean emission at 441.09±5.82 nm, when fluctuations in the well thickness of 2–2.4 nm and in the In composition of 0.18 – 0.22 were considered with respect to the nominal values of 2.2 nm and 0.2, respectively. Our work establishes a powerful methodology for basic understanding of light emission from nano-emitters.

Resume : We have studied the bound electron and hole states in nitride superlattice heterostructures using a density functional theory. The systems are assumed to be differently strained in lateral directions, with a lattice vector changing evenly from one to the second a lattice constants of two compounds forming structure. The c lattice vector is allowed to relax fully. When considering heterostructures composed of a ternary compound, the a and c lattice vectors are taken from Vegard's law. The effects of these external influences as well as of changes in the geometry of the heterostructures on the localization of bound states are discussed in detail. Direct inspection of the obtained energy band profiles suggest that the localization of bound states differs significantly, due to differences in band offsets of valence and conduction bands, which are related not only to differences in strain conditions. When passing from binary to ternary compounds we have found unexpected almost zero valence band offset between well and a barrier, which could be easily changed from positive to negative values by small changes in strain conditions. These had strong influence on localization of bound valence states and accordingly strong influence on reduction of optical transition rates.

Resume : III-nitrides have recently emerged as promising materials for new intersubband (ISB) technologies with potential applications for fiber-optic communications and throughout the THz spectral range. ISB transitions in GaN/AlGaN quantum wells (QWs) can be tuned from 1.0 µm to 10 µm, and ISB absorption in the THz range (> 20 µm) has also been reported. For certain devices, it is interesting to replace QWs by laterally confined systems like quantum dots (QDs) to increase the intraband recovery times. In the case of GaN/AlN QDs, near-infrared TM-polarized intraband absorption has been observed, and is attributed to transitions from the ground state of the conduction band to the first excited electronic state confined along the growth axis. Furthermore, the lateral confinement in the QDs gives rise to additional mid-infrared transitions reacting to TE-polarized excitation. However, in the QD system the material choice and the nanostructure dimensions, which determine the operational wavelength, are limited by the elastic requirements for Stranski-Krastanov growth. In the case of nanowires (NWs), their large surface-to-volume ratio allows misfit strain to be elastically released, extending the viable active region size and composition beyond the limits of planar systems or QDs. In this work we present and discuss the intraband performance of these three kinds of nanostructures (QWs, QDs, NWs), studying the effect of their geometries and strain distribution on their optical properties.

Resume : An InGaN quantum dot (QD) embedded into a GaN nanowire (NW) is a promising system for realization of efficient quantum light emitters. Among those, single photon sources (SPSs) based on self-assembled (SA) GaN QDs and InGaN disks embedded into SA GaN NWs have been recently reported. We demonstrate here the emission of single photons from the QD-like states observed in the 20 nm thick InGaN disks embedded into GaN NWs ordered in a two-dimensional array. The structure was fabricated on (0001)GaN-on-sapphire templates using nanohole masks prepared by colloidal lithography. Scanning electron microscopy confirms the formation of NWs with pyramidal tops and highly uniform diameters and heights (200 and 500 nm, respectively), ordered in hexagonal matrices (with 270 nm pitch). Photoluminescence (PL) and transmission electron microscopy combined with cathodoluminescence reveal an intense emission band around 500 nm, originating from the apices of the InGaN sections. They further confirm that the luminescence originates from excitons confined in a strong electric field, with their initial lifetimes (~1 ns) nearly constant up to the room temperature. Micro-PL measurements reveal intense and narrow (< 500 μeV) QD-like emission lines. Photon correlation measurements performed on these emission centers using a Hanbury Brown and Twiss interferometer show pronounced antibunching. We find the g(2)(0) < 0.3, which is a clear signature of single photon emission.

Resume : The effects of doping on the minority charge carrier concentration in GaN remains a controversial issue, with many theoretical studies producing contradicting results. The standard method to model defects in crystals is the plane-wave supercell approach, which has disadvantages related to unwanted interaction between periodic images of defects. We present results of calculations based on a hybrid quantum mechanical/molecular mechanical embedded cluster approach to modelling defect formation associated with Group 2 dopants in GaN. As our approach does not employ periodic boundary conditions, provides access to the vacuum level, and allows the use of high level quantum chemistry approximations, accurate and unambiguous defect levels can be determined. From our calculations we find that a substantial amount of experimentally determined optical data can be attributed to the N vacancy in GaN, which is a charge-compensating defect for Group 2 dopant incorporation. Our calculated defect levels associated with the dopants are in excellent agreement with experiment where available. Furthermore, we definitively show that standard hybrid density functionals are inadequate for treating these defects and that double exchange is necessary for an accurate description.

Resume : In contrast to epitaxial films, single crystalline GaN nanowires (NWs) can be grown on dissimilar substrates because their high aspect ratio inhibits the propagation of dislocations along the NW axis. Consequently, GaN nanowires have attracted great interest for the monolithic integration of optoelectronic devices on silicon. However, the physical mechanisms governing the spontaneous nucleation and growth of NWs are not completely understood yet. In this work, we show that, in the absence of structural or morphological defects of the substrate, GaN NWs form spontaneously along the [000 1] direction. The in-situ investigation by line-of-sight quadrupole mass spectrometry of the desorbing Ga flux during NW formation allows us to elucidate the role of the growth parameters on both the nucleation and subsequent growth of GaN NWs. We demonstrate that a self-regulated process, which depends on the effective III/V flux ratio, determines the final NW radius. We also introduce an empirical model that provides a quantitative comprehensive description of the time evolution of the entire NW ensemble, where collective effects (shadowing and exchange of Ga atoms between adjacent NWs) must be taken into account. Furthermore, the present model also makes it possible to extract, from the time evolution of the desorbing Ga flux, relevant growth parameters such as the nucleation rate or the average incubation time that precedes the formation of GaN NWs.

Resume : The technology of III-V nitride based light emitting diodes (LEDs) is nowadays well established in the blue wavelength range with impressive performance, i.e. a maximum wall plug efficiency exceeding 50%. However, LED efficiency goes down at high current density and for longer wavelength range. There is therefore still a room for research to get rid of the efficiency droop, or at least to limit it, and to extend the operation wavelength toward longer wavelength, beyond the green, while keeping a reasonable efficiency. Long-wavelength InGaN quantum wells require high indium content, which in turn leads to materials degradation and strain issues. In addition, high In content InGaN alloys are subject to thermal degradation, which may occur during the growth of p-type layers. In this presentation, we will present the growth of Mg doped GaN layers at low-temperature (750°C) by molecular beam epitaxy (MBE). The electrical properties compare well with those obtained on state of the art metal organic vapour phase epitaxy grown layers. Successful implementation of MBE grown GaN p-type cladding layers in lasers and long-wavelength LEDs will be reported. In addition, we will show that the use of low temperatures limits the diffusion of doping species across the p-n junction interface allowing us to achieve tunnel junction with excellent characteristics.

Resume : Nowadays GaN light emitters are predominantly grown by MOVPE on sapphire substrates although LEDs on SiC, Si and even GaN show competitive performances. As the LED market is driven mostly by cost reduction GaN-on-silicon technology is perhaps the only current alternative to GaN-on-sapphire because of available substrate size and price. In the last decade the development of GaN on Si LEDs led to steadily increasing performances by solving numerous difficulties for the growth of GaN on Si. Of these difficulties cracking is the most severe which can be solved by several methods. AlGaN buffers, AlN/GaN superlattices, Al(Ga)N interlayers as well as selective growth are all viable pathways to control strain in LED structures and thereby prevent cracking during cooling after growth. However, these methods cannot be applied straightforward. For example, AlN/GaN superlattices and AlGaN interlayers for strain engineering introduce misfit dislocations but may also lead to additional threading dislocations. It has been observed that such interlayers have different impact on the following GaN layer in dependence of their specific position in the layer stack. We will demonstrate that the function of these layers depends on the strain state of the previous layer. Because of the high absorption coefficient of Si, LEDs have to be processed as thin film devices to achieve maximum light output. Here, high n-type doping is advised to enable efficient n-contact metallization and low operation voltage. We developed germanium doping for GaN layers with ND>10^20 cm-3 for this purpose. Ge doping has also the advantage over Si doping in avoiding edge type dislocation related tensile stress generation during growth. Our investigations show that Ge-doping behaves quite different to Si enabling much higher doping concentrations while maintaining a smooth GaN surface.

Resume : In this work we present a theoretical study of the effect of random alloy fluctuations in the quantum well (QW) of an InGaN/GaN LED on the spontaneous emission properties. The calculations are based on an empirical tight-binding (ETB) model, using an sp3d5s* parametrization. Strain is calculated with a valence force field (VFF), preconditioned with a continuous linear elasticity calculation. The devices we consider consist of a single 3 nm QW with varying In content, undoped GaN barriers and AlGaN EBL. We first seek a selfconsistent solution of the Schroedinger/drift-diffusion model at a realistic operating current, using a kp model. Then we calculate the first few electron and hole states using ETB, and from this the optical matrix elements. These calculations are repeated for a statistical ensemble of structures with uniform random In distributions in the QW for each mean In concentration. For a reasonable representation of the random alloy and the induced fluctuations of the particles' orbitals we use a periodic supercell in the plane of the QW of 6x6 nm^2. As a result we obtain statistical distributions of the transition energies and oscillator strengths for different In concentrations. We find on the one hand that the oscillator strength is correlated with the transition energy, and on the other hand that the mean oscillator strength of the random alloy calculations shows a stronger decrease with In concentration than that obtained using virtual crystal approximation.

Resume : We study MOCVD-grown green-emitting c-plane InGaN/GaN QW -structures of very high quality. They show no structural defects such as thickness fluctuations, phase separation or Indium clustering. TEM analyses confirm the excellent quality of our samples. Using temperature-dependent PL, we observe a drastically reduced internal quantum efficiency (IQE) with increasing thickness. Using photoluminescence and exciting only the QWs, we show that this the result of three effects: Firstly, the increasing internal electric fields due to the quantum-confined Stark effect (QCSE) separate electrons and holes along the growth direction reducing the radiative rates. Secondly, we see a loss of in-plane localization which combined with the longer carrier lifetime enhances carrier diffusion to non-radiative recombination centers in the thickest QWs (4.3~nm). Thidly, when using a high excitation power density, we observe excited states in the QWs, which emit in the blue to UV region and have very short lifetimes of less than 30ps. 8-band k.p calculations support our interpretation. These states comprise an additional non-radiative loss channel competing with Auger processes.

Resume : GaN is presently being explored as new material for power electronic devices, aiming at achieving superior performances to Si based technology. Although good to excellent performances have been demonstrated, current GaN devices still do not exploit the full performance potential of this material system, neither the reliability of the devices is fully adequate. I will review the latest developments in this field, including the role of Carbon doped GaN buffer layers for the working of the devices, in the addition the integration of GaN with diamond material systems to enable even higher power densities than presently possible.

Resume : Light-emitting diodes (LEDs) based on arrays of nanorod (NR) emitters have been widely re-searched in recent years. Advantages of such LEDs include increased internal quantum efficiency due to strain relaxation, low defect densities and increased light extraction efficiency. Another ad-vantage of NR-LEDs is the potential for highly directional light emission, a desirable attribute for etendue limited applications. Factors that affect this directionality include light waveguiding in the NRs and in any underlying GaN buffer layer from which light can be diffracted by highly ordered NR arrays. This paper presents the results of a study by simulation and measurement of the direc-tionality of light emission from NR-LEDs ICP etched from epitaxy containing either a single InGaN/GaN quantum well or a MQW oriented in the c-plane. It is shown how the diameter, height and sidewall angle of an NR, the thickness of any ITO contact layer and location of the dipoles in-fluence the far-field radiation pattern, notably the directionality of the emitted light. The emissive properties of single NRs were simulated by a modal expansion technique that accounts for reflection and diffraction at the nanorod ends, while those of nanorod arrays were studied using the finite-difference time-domain (FDTD) method. The predictions of both models were compared with detailed measured far-field angular emission patterns from green and blue light emitting NRs over a wide range of wavelengths, with excellent agreement achieved. The comparisons with results obtained by modal expansion reveal the dominant waveguide modes, which in turn depend on the wavelength of the emission and its radial and axial location within the NRs. The FDTD study revealed the diffractive behaviour.

Resume : There is a tremendous need for developing novel approaches to realize good quality thick InGaN epilayers to significantly improve the efficiency of concentrating photovoltaic systems. One possible solution for obtaining thicker and better InGaN is Nano Selective Area Growth (NSAG). Applying NSAG technology, we have grown successfully, 120 nm thick, perfectly selective InGaN nanorod and nanowire arrays over the dielectric mask using metal organic chemical vapor deposition. Uniform stripes and dots nanopatterns of SiO2 with 100 nm openings were realized using E-beam lithography. In the unmasked area InGaN is stymied with 3D growth, clusters of In and defects whereas the InGaN nanostructures in the patterened area are perfectly hexagonal, monocrystalline and homogenous in size, shape and composition exposing the smooth semipolar facets, which is sign of high quality nanostructures. The approximate indium composition and the strain state in the field were determined by the HRXRD analysis and the photoluminescence near band emission peak. Further structural properties were studied using cross sectional transmission electron microscope. The optical properties of these nanostructures were studied using the depth resolved cathodoluminescence spectroscopy, which showed considerable redshift in emission peaks confirming increase of In incorporation in the nanostructures. The interesting results of the structural and optical characterizations and possible growth mechanism of these InGaN

Resume : This work is focused both on InxGa1-xN single-layer and InxGa1-xN/GaN multilayered structures, with high Indium content (x>35%). In this study, InGaN/GaN films are epitaxially grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The microstructure of GaN and InxGa1-xN thin films are characterized by SEM/AFM/TEM [1] and optically by prism coupling/ ellipsometry for the refractive indices [2]. We focus here on the design and the fabrication of ultra-fast photodiodes operating in visible wavelength range. Different configurations for UTC photodiodes have been fabricated (sizes ranging from 2.5 to 50µm) and the technological processes optimized including dry etching processes for patterning GaN/InGaN layers. Ti/Al/Ni/Au and ITO materials for p and n type contacts are also investigated. [1] A. Gokarna, A. Gauthier-Brun, W. Liu, Y. Androussi, E. Dumont, E. Dogheche, J. H. Teng, S.J. Chua, D. Decoster, Applied Physics Letters ,Vol.96 , Issue: 19, May 2010. [2] A. Gauthier- Brun, J.H. Teng, E. Dogheche, Wei Liu, M Tonouchi A. Gokarna, S.J. Chua, D. Decoster, Properties of InxGa1-xN films in Terahertz range, Applied Physics Letter, 100, 071913 (2012)

Resume : Due to strong polarization properties of III-nitrides, GaN-based heterostructures, such as AlGaN/GaN heterostructure, InAlN/GaN heterostructure, etc, have a special two dimensional electron gas (2DEG) with high density and high mobility. However, since the 2DEG density is very high, a large quantity of electron can overflow from the triangle potential well of 2DEG interface under higher temperature. This overflow of 2DEG will lead to reduction of the total electron mobility [1], and the HEMT device cannot be pinched off. In this report, to confine the overflow of high-density 2DEG, many novel heterostructures, such as high Al-content AlGaN/GaN with deeper potential well [2], AlGaN/GaN/AlGaN and InAlN/GaN/AlGaN double heterostructures with a back-barrier[3,4], AlGaN/GaN/AlGaN/GaN and InAlN/GaN/InAlN/GaN hetero -structure with double channels[5], were studied. GaN-based devices with these structures exhibit excellent transport characteristics under high temperature, so they are more suitable for the applications in high temperature electronic devices. References: [1] Zhang Jinfeng, Wang Chong, Zhang Jincheng, Hao Yue. Chinses Physics, 15(5):1060, 2006. [2] Zhang ZhongFen, Zhang JinCheng, et al, Science in China-Series G: Physics Mechanics and Astronomy, 52(12):1879-1884, 2009. [3] Meng, Fanna, Zhang, Jincheng, Zhou, Hao, et al, Journal of Applied Physics, 112(2), 2012. [4] Juncai Ma, Jincheng Zhang, Junshuai Xue, Chinese Journal of Semiconductors, 33(1), 2012. [5] Xue, JunShuai, Zhang, JinCheng, et al, Journal of Applied Physics, 111(11), 2012.

Resume : Low power consumption, long-term stability and high specificity are some of the most sought after requirements for future gas sensors technologies. In this contribution, the potential of GaN-based devices to contribute to this field will be discussed. First, new optically-driven sensor device concepts featuring zero power consumption have shown that this is an open field for optically active materials[1]. Light emitters and absorbers active in the visible range, such as InGaN, are required to operate these new technologies in ambient light. Second, the traditional strategy to maximize the sensor response by choosing chemically and thermally unstable materials leads to well know long-term stability issues. Additionally, this choice produces vigorous and highly unspecific responses to gases. Following recent finding[2], an alternative strategy could be relying on highly stable gas-immune materials to build up the fundamental elements of the sensor platform in combination with highly specific functionalizations. The well-established microelectronic GaN processes would provide the necessary fine control over the electrical and optical properties of the device platform. The flexibility of molecular chemistry would deliver sufficient flexibility to tailor the specific response of the sensor almost at will. Finally, the first attempts to implement this strategy in new devices will be presented. [1]NanoEnergy 2013, 2, 514–522. [2]Adv.Funct.Mater. 2013, DOI: 10.1002/adfm.201301478

Resume : The detection of ultraviolet irradiation is important in environmental research and monitoring, flame detection and monitoring of industrial processes as UV curing of glues, adhesives or disinfection of drinking water by UV irradiation. Besides (Al,Ga)N semiconducting oxides present a material class that is very suited for realization of detectors operating in the UV-A, UV-B and even the UV-C spectral range. For most applications the determination of spectrally integrated UV radiation is not sufficient and a spectrally resolved detection of UV radiation is desired. Here, we demonstrate an approach towards monolithic wavelength-selective UV-A photo-detectors by using a continuous composition spread approach.

Resume : Boron containing III-nitrides are attractive system for deep-UV LEDs and LDs because of their wide bandgaps and flexible lattice. However the crystallinity and boron content have been limited due to large mismatch between BN and other nitrides. In this work, BAlN layers with boron composition from 1% to 5% were successfully grown on AlN template substrates by low-pressure organometallic vapor phase epitaxy. The samples were grown at 650˚C and then annealed at 1020˚C for recrystallization. Growth techniques such as temperature, growth time and TEB/III ratio in the gas phase were investigated. High quality BAlN layers were grown using flow-modulate epitaxy method that allows to enhance surface migration of boron atoms. 70 nm-thick layers show a good surface morphology. For the first time, clear XRD peak relating to 5% boron for this new material was observed, which suggests the formation of single-phase solid solution. Adding more boron to the AlN produced a shift in the peak positions to greater angles. Further results by TEM and optical characterizations will be presented. This new material is promising for deep-UV applications and gives more freedom for bandgap engineering of multi-structure devices.

Resume : For a detailed understanding of complex semiconductor heterostructures a systematic determination of the structural, chemical, and optical properties on a nanometer scale is essential. The combination of luminescence spectroscopy – in particular at liquid He temperatures - with the high spatial resolution of a scanning transmission electron microscope (STEM) (dx<1 nm at RT,dx <5 nm at 10K), as realized by the technique of low temperature scanning transmission electron microscopy cathodoluminescence spectroscopy (STEM-CL), provides a unique, extremely powerful tool for the optical nano-characterization. In this study we will present our STEM-CL results from an ordered array of InGaN/GaN nano rods (NR) grown in top-down fabrication on GaN/sapphire template: the GaN template was structured using standard photolithography and reactive ion etching. Subsequently, the NRs were overgrown by MBE with a GaN buffer followed by a thick InGaN layer. We observe the highest CL intensity in the upper part of each NR. Spectrally resolved CL measurements at 15 K reveal distinct luminescence contributions originating from the different InGaN sections. Highly resolved linescans of individual NRs exhibit a characteristic luminescence from the bottom InGaN followed by a middle InGaN region. In addition, the upper part of the NR layer shows a broad, strongly red-shifted luminescence giving a detailed understanding of the In composition with distinct In gradients in vertical and radial directions.

Resume : Axial (In,Ga)N/GaN nanowire (NW) heterostructures are considered as promising building blocks for the realization of novel high performance light-emitting diodes. Here we will report on the interplay between interface morphology, epitaxial strain and local chemical composition distribution in axial (In,Ga)/GaN NW heterostructures investigated by transmission electron microscopy (TEM). Two sets of NW samples are grown by plasma-assisted molecular beam epitaxy on Si(111) in a self-assembled way and on patterned GaN templates in an ordered way, respectively. Local electron energy-loss spectroscopy measurements show that both sets of NWs have a similar final indium concentration of 35%, but remarkably different interface profiles. The difference of interface profile is strongly linked to the interface morphology: i) In self-assembled NWs showing a large chemical interface width (~40 nm), the interface geometry exhibits a pencil-like shape, where 60°-type misfit dislocations are formed (i.e., with Burgers vector b=a/3[11-20]). ii) On the other hand, the ordered NWs containing flat boundaries and a small interface width of ~10 nm, are plastically relaxed by the formation of partial dislocations associated with stacking faults. Additionally, the residual strain distribution along the (In,Ga)N NWs is studied by high-resolution TEM and geometric phase analysis. The influence of interface morphology and chemical composition profile on local strain distribution and strain relaxation mechanism is further discussed by calculations based on finite-element method.

Resume : Three dimensional light emitting diodes (LEDs) with a shell geometry around a columnar GaN core are supposed to have substantial advantages over conventional planar LEDs. The active area along the sidewalls of the GaN pillars can substantially be increased by high aspect ratios. Due to the 3-dimensional shape, the electrical and optical characterization of such device structures is a substantial problem. Here we demonstrate that a nano-manipulator setup inside a scanning electron microscope can be used in combination with a cathodoluminescence (CL) system to characterize the electro-optical properties by directly contacting single facets of the 3D structure. Electron beam induced current (EBIC) images clearly prove that the pn-junction is completely wrapped around the core. By comparing spatially resolved CL and EBIC, the rate of charge carrier generation, trapping and separation in different regions is discussed. We will present electroluminescence (EL) spectra from single core-shell LEDs obtained at different injection currents, in both the region of MQW as well as defect related emission. A wavelength shift of the MQW emission by 60 nm is observed along the structure height for both excitation methods (CL and EL), indicating a gradient of the indium incorporation. In addition, metal contacts have been fabricated in order to get a defined contact area By evaluating the contact area and the EL spectra we gain an insight to the internal efficiency versus current density.

Resume : The rare-earth doped gallium nitride (GaN) materials have shown to be a promising candidate for LEDs, lasers and full-colour displays due to their ability to emit sharp and stable spectral lines. Based on these materials, efficient green and blue LEDs have been successfully developed. For realization of red emission Eu doping is considered. Here, we report on ultrafast optical spectroscopy investigations of state-of-the-art red-emitting GaN doped with trivalent europium (Eu3+) fabricated by organometallic vapor phase epitaxy. We employ experimental techniques of transient induced absorption (IA), using a conventional femtosecond pump-probe setup, and photoluminescence (PL) emission and excitation spectroscopy. In that way, carrier decay dynamics measured by IA is cross-correlated with photon generation responsible for PL. The spectrally and temporally resolved data allow for unique insights into the complex energy transfers in this system and their dynamics. In particular, we managed to separate carrier trapping from the build-up of the excited state population of Eu3+ ions, responsible for the red emission band. On this basis, an energy transfer model including dynamics of individual steps can be proposed for the first time. These findings provide fundamental information on the excitation and de-excitation mechanisms of Eu3+ ions in GaN and will serve for further optimization of this material towards lighting applications.

Resume : III-Nitride based microcavities (MC) are one of the most promising candidates for the realization of polariton lasers operating at room temperature since they exhibit highly stable excitons and large oscillator strength. These lasers operate without inversion in the strong coupling regime (SCR) where the squared strength of the light-matter coupling g has to overcome the mean value of the squared line widths of exciton and cavity mode. Since g is proportional to the square root of the number of quantum wells (QWs), the SCR can be reached by embedding a large number of QWs.We present the optical and the structural properties at the nanometer scale of a MOVPE grown MC structure comprising a large number of embedded InGaN QWs by cathodoluminescence in a scanning transmission electron microscope (STEM-CL). The sample consists of a 28-fold InGaN/GaN QWs embedded in a GaN λ cavity on top of an AlInN/GaN distributed Bragg reflector (DBR) grown on sapphire substrate. Direct comparison of the STEM images with simultaneously recorded CL mappings resolve the complete layer sequence. In particular, the DBR layer stack is proven to be laterally and vertically homogeneous with sharp GaN/AlInN interfaces. A dominant emission with a broad spectral range of the InGaN MQW can be observed. Spectrally resolved linescans across the active region exhibit a redshift from the bottom (425 nm) to the top (465 nm) visualizing strain relaxation, higher In incorporation, and/or increasing QW thickness.

Resume : Gallium Nitride (GaN) thin films have been prepared on sapphire by metal organic chemical vapor deposition (MOCVD) technique and a chemical etching method using KOH is used to develop nanoporous structures [1]. We present comparative investigations of porous and nonporous GaN layers. While the pores density is determined, we have investigated the microstructures in GaN films by using transmission electron microscopy (TEM). The refractive index dispersion has been evaluated through different techniques, ellipsometry and guided-wave prism coupling [2]. We have correlated the microstructure with the refractive index of the material. The aim of this research is to demonstrate that optical properties of GaN can be tuned by controlling the pores size and spacing. For a pores density of 20%, we report an index variation ∆n= -12*10-3. The control of the refractive index into GaN is therefore fundamental for the design of active and passive optical devices [1] S-H Gong, A. Stolz,G-H Myeong, E. Dogheche, A. Gokarna, S-W Ryu, D. Decoster, Y-H Cho, Optics letters, vol 36 no.21, pp4272-4274, nov 2011. [2] Y-H Lee, J-H Kang, S-W Ryu, Thin Solid Films, vol.540 150 (2013).

Resume : The nitride aluminium and indium alloy (InAlN), when lattice matched (LM) with GaN (i. e. In content at around 18%) is of great interest for opto- and micro-electronic applications. However, due to thermal mismatch between InN and AlN, the growth of good quality InAlN layers is still difficult and many workers have reported crystalline degradation versus thickness even at LM to GaN. The mechanisms governing this behaviour are not still clear, and many proposals are still under debate. In this work, we investigated InAlN layers, close to the LM composition, grown on GaN/Al2O3 by metal organic vapour phase epitaxy at different substrate temperatures and V/III ratios. Scanning transmission electron microscopy, X Ray diffraction, RBS and energy dispersive spectroscopies were used to determine the structure and composition. Our results show that the layers quality depends critically on the growth conditions. During our observations, structural disruption has been systematically observed versus thickness and depending on the growth conditions even completely polycrystalline layers were obtained. Therefore, it appears that obtaining good crystallinity and homogeneous composition layers probably needs a continuous layer evolution monitoring and corrections in the conditions during the growth. The influence and role of various parameters such as III/V ratios, growth temperature, sources control and overall pressure on the quality of the grown layers will be discussed.