Intrinsic degradation mechanism of nearly lattice-matched InAlN layers grown on GaN substrates

Thanks to its high refractive index contrast, band gap and polarization mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an attractive solution for applications such as distributed Bragg reflectors, ultraviolet light-emitting diodes, or high electron mobility transistors. In order to study the structural degradation mechanism of InAlN layers with increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN substrates with a low density of threading dislocations, for In compositions of 13.5% (layers under tensile strain), and 19.7% (layers under compressive strain). In both cases, a surface morphology with hillocks is initially observed, followed by the appearance of V-defects. We propose that those hillocks arise due to kinetic roughening, and that V-defects subsequently appear beyond a critical hillock size. It is seen that the critical thickness for the appearance of V-defects increases together with the surface diffusion length either by increasing the temperature or the In flux because of a surfactant effect. In thick InAlN layers, a better (worse) In incorporation occurring on the concave (convex) shape surfaces of the V-defects is observed leading to a top phase-separated InAlN layer lying on the initial homogeneous InAlN layer after V-defects coalescence. It is suggested that similar mechanisms could be responsible for the degradation of thick InGaN layers

MOCVD growth mechanisms of ZnO nanorods

ZnO is a promising material for the fabrication of light emitting devices. One approach to achieve this goal is to use ZnO nanorods because of their expected high crystalline and optical quality. Catalyst free growth of nanorods by metalorganic chemical vapour deposition (MOCVD) was carried out on (0001) sapphire substrates. Arrays of well-aligned, vertical nanorods were obtained with uniform lengths and diameters. A thin wetting layer in epitaxy with the sapphire substrate is formed first, followed by pyramids and nanorods. The nucleation of nanorods occurs either directly at the interface, or later on top of some of the pyramids, suggesting various nucleation mechanisms. It is shown that crystal polarity plays a critical role in the growth mechanism with nanorods of Zn polarity and their surrounding pyramids with O polarity. A growth mechanism is proposed to explain that most threading dislocations lie in the wetting layer, with only a few in the pyramids and none in the nanorods

Strain relaxation by dislocation glide in ZnO/ZnMgO core-shell nanowires

4 pagesInternational audiencePlastic relaxation of the misfit stress in core-shell semi-conducting nanowires can lead to structural defects, detrimental to applications. Core-shell Zn{0.7}Mg{0.3}O/ZnO quantum well heterostructures were deposited on ZnO nanowires. Strain along the a and c axes of the wurtzite structure is relaxed through the glide of dislocation half-loops from the free surfaces, within pyramidal and prismatic planes. Some half-loops are closed up in the barriers to accommodate the misfit at two consecutive interfaces of the quantum well stack. Dislocations are also observed within the nanowire core: contrary to two-dimensional structures, both the core and the shell can be plastically relaxed

Core-shell multi-quantum wells in ZnO / ZnMgO nanowires with high optical efficiency at room temperature

International audienceNanowire-based light-emitting devices require multi-quantum well heterostructures with high room temperature optical efficiencies. We demonstrate that such efficiencies can be attained through the use of ZnO/Zn(1-x)MgxO core shell quantum well heterostructures grown by metal organic vapour phase epitaxy. Varying the barrier Mg concentration from x=0.15 to x=0.3 leads to the formation of misfit induced dislocations in the multi quantum wells. Correlatively, temperature dependant photoluminescence reveals that the radial well luminescence intensity decreases much less rapidly with increasing temperature for the lower Mg concentration. Indeed, about 54% of the 10K intensity is retained at room temperature with x=0.15, against 2% with x=0.30. Those results open the way to the realization of high optical efficiency nanowire-based light emitting diodes

Compared growth mechanisms of Zn-polar ZnO nanowires on O-polar ZnO and on sapphire

Controlling the growth of zinc oxide nanowires is necessary to optimize the performances of nanowire-based devices such as photovoltaic solar cells, nano-generators, or light-emitting diodes. In this view, we investigate the nucleation and growth mechanisms of ZnO nanowires grown by metalorganic vapor phase epitaxy either on O-polar ZnO or on sapphire substrates. Whatever the substrate, ZnO nanowires are Zn-polar, as demonstrated by convergent beam electron diffraction. For growth on O-polar ZnO substrate, the nanowires are found to sit on O-polar pyramids. As growth proceeds, the inversion domain boundary moves up in order to remain at the top of the O-polar pyramids. For growth on sapphire substrates, the nanowires may also originate from the sapphire / ZnO interface. The presence of atomic steps and the non-polar character of sapphire could be the cause of the Zn-polar crystal nucleation on sapphire, whereas it is proposed that the segregation of aluminum impurities could account for the nucleation of inverted domains for growth on O-polar ZnO

Homoepitaxial growth of catalyst-free GaN wires on N-polar substrates

3 pagesInternational audienceThe shape of c-oriented GaN nanostructures is found to be directly related to the crystal polarity. As evidenced by convergent beam electron diffraction applied to GaN nanostructures grown by metal-organic vapor phase epitaxy on c-sapphire substrates: wires grown on nitridated sapphire have the N-polarity ([000math]) whereas pyramidal crystals have Ga-polarity ([0001]). In the case of homoepitaxy, the GaN wires can be directly selected using N-polar GaN freestanding substrates and exhibit good optical properties. A schematic representation of the kinetic Wulff's plot points out the effect of surface polarity

Formation and annealing of dislocation loops induced by nitrogen implantation of ZnO

Although zinc oxide is a promising material for the fabrication of short wavelength optoelectronic devices, p-type doping is a step that remains challenging for the realization of diodes. Out of equilibrium methods such as ion implantation are expected to dope ZnO successfully provided that the non-radiative defects introduced by implantation can be annealed out. In this study, ZnO substrates are implanted with nitrogen ions, and the extended defects induced by implantation are studied by transmission electron microscopy and X-ray diffraction (XRD), before and after annealing at 900^{\circ}C. Before annealing, these defects are identified to be dislocation loops lying either in basal planes in high N concentration regions, or in prismatic planes in low N concentration regions, together with linear dislocations. An uniaxial deformation of 0.4% along the c axis, caused by the predominant basal loops, is measured by XRD in the implanted layer. After annealing, prismatic loops disappear while the density of basal loops decreases and their diameter increases. Moreover, dislocation loops disappear completely from the sub-surface region. XRD measurements show a residual deformation of only 0.05% in the implanted and annealed layer. The fact that basal loops are favoured against prismatic ones at high N concentration or high temperature is attributed to a lower stacking fault energy in these conditions. The coalescence of loops and their disappearance in the sub-surface region are ascribed to point defect diffusion. Finally, the electrical and optical properties of nitrogen-implanted ZnO are correlated with the observed structural features.Comment: 8 page

Growth mechanism and crystalline defects in ZnO nanowire structures for LEDs

Les nanofils de ZnO à puits quantiques et le dopage p par implantation ionique d'azote sont étudiés pour la fabrication de LED ultra-violettes. Des pyramides de polarité O et des nanofils de polarité Zn sur substrats de saphir et ZnO sont élaborés. La croissance organisée de nanofils sur ZnO de polarité Zn est démontrée. De même, des pyramides ou des nanofils de GaN sont obtenus sur GaN de polarité Ga ou N. Sur saphir, l'élimination des dislocations dans les pyramides sous-jacentes aux nanofils est analysée. Les nanofils sans défauts structuraux permettent l'élaboration de puits quantiques coeur-coquille ZnO/Zn(1-x)MgxO. La relaxation plastique dans les nanofils est étudiée, puis la composition en Mg est optimisée pour l'éviter et atteindre un rendement quantique interne de 54%. Concernant l'implantation, les défauts sont identifiés avant et après recuit. Ils disparaissent en surface, d'où une guérison facilitée des nanofils. Un matériau guéri avec des accepteurs activés n'est pas obtenu.Quantum well ZnO nanowires and p-type doping by nitrogen ion implantation are studied to make ultraviolet light-emitting diodes. O-polar pyramids and Zn-polar nanowires on sapphire and ZnO substrates are grown. Organized growth of nanowires on a masked Zn-polar ZnO is demonstrated. Similarly, GaN pyramids and nanowires are grown on Ga and N-polar GaN respectively. On sapphire, the dislocation elimination in the underlying pyramids is analyzed. Nanowires with no structural defects allow the growth of ZnO / Zn (1-x) Mg x O core-shell quantum wells. Plastic relaxation is studied, and the Mg composition is optimized to avoid it and attain an internal quantum efficiency as high as 54%. Concerning ion implantation, the defects are identified before and after annealing. They disappear in the near-surface, which lead to an easier recovery of nanowires compared to bulk ZnO. However, a recovered material with activated acceptors is not obtained

Mécanismes de croissance et défauts cristallins dans les structures à nanofils de ZnO pour les LED

Quantum well ZnO nanowires and p-type doping by nitrogen ion implantation are studied to make ultraviolet light-emitting diodes. O-polar pyramids and Zn-polar nanowires on sapphire and ZnO substrates are grown. Organized growth of nanowires on a masked Zn-polar ZnO is demonstrated. Similarly, GaN pyramids and nanowires are grown on Ga and N-polar GaN respectively. On sapphire, the dislocation elimination in the underlying pyramids is analyzed. Nanowires with no structural defects allow the growth of ZnO / Zn (1-x) Mg x O core-shell quantum wells. Plastic relaxation is studied, and the Mg composition is optimized to avoid it and attain an internal quantum efficiency as high as 54%. Concerning ion implantation, the defects are identified before and after annealing. They disappear in the near-surface, which lead to an easier recovery of nanowires compared to bulk ZnO. However, a recovered material with activated acceptors is not obtained.Les nanofils de ZnO à puits quantiques et le dopage p par implantation ionique d'azote sont étudiés pour la fabrication de LED ultra-violettes. Des pyramides de polarité O et des nanofils de polarité Zn sur substrats de saphir et ZnO sont élaborés. La croissance organisée de nanofils sur ZnO de polarité Zn est démontrée. De même, des pyramides ou des nanofils de GaN sont obtenus sur GaN de polarité Ga ou N. Sur saphir, l'élimination des dislocations dans les pyramides sous-jacentes aux nanofils est analysée. Les nanofils sans défauts structuraux permettent l'élaboration de puits quantiques coeur-coquille ZnO/Zn(1-x)MgxO. La relaxation plastique dans les nanofils est étudiée, puis la composition en Mg est optimisée pour l'éviter et atteindre un rendement quantique interne de 54%. Concernant l'implantation, les défauts sont identifiés avant et après recuit. Ils disparaissent en surface, d'où une guérison facilitée des nanofils. Un matériau guéri avec des accepteurs activés n'est pas obtenu

Mécanismes de croissance et défauts cristallins dans les structures à nanofils de ZnO pour les LED

Les nanofils de ZnO à puits quantiques et le dopage p par implantation ionique d'azote sont étudiés pour la fabrication de LED ultra-violettes. Des pyramides de polarité O et des nanofils de polarité Zn sur substrats de saphir et ZnO sont élaborés. La croissance organisée de nanofils sur ZnO de polarité Zn est démontrée. De même, des pyramides ou des nanofils de GaN sont obtenus sur GaN de polarité Ga ou N. Sur saphir, l'élimination des dislocations dans les pyramides sous-jacentes aux nanofils est analysée. Les nanofils sans défauts structuraux permettent l'élaboration de puits quantiques coeur-coquille ZnO/Zn(1-x)MgxO. La relaxation plastique dans les nanofils est étudiée, puis la composition en Mg est optimisée pour l'éviter et atteindre un rendement quantique interne de 54%. Concernant l'implantation, les défauts sont identifiés avant et après recuit. Ils disparaissent en surface, d'où une guérison facilitée des nanofils. Un matériau guéri avec des accepteurs activés n'est pas obtenu.Quantum well ZnO nanowires and p-type doping by nitrogen ion implantation are studied to make ultraviolet light-emitting diodes. O-polar pyramids and Zn-polar nanowires on sapphire and ZnO substrates are grown. Organized growth of nanowires on a masked Zn-polar ZnO is demonstrated. Similarly, GaN pyramids and nanowires are grown on Ga and N-polar GaN respectively. On sapphire, the dislocation elimination in the underlying pyramids is analyzed. Nanowires with no structural defects allow the growth of ZnO / Zn (1-x) Mg x O core-shell quantum wells. Plastic relaxation is studied, and the Mg composition is optimized to avoid it and attain an internal quantum efficiency as high as 54%. Concerning ion implantation, the defects are identified before and after annealing. They disappear in the near-surface, which lead to an easier recovery of nanowires compared to bulk ZnO. However, a recovered material with activated acceptors is not obtained.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF