| 1. |
|
|
| 2. |
|
|
| 3. |
- Berg, Alexander, et al.
(author)
-
Growth parameter design for homogeneous material composition in ternary GaxIn1-xP nanowires.
- 2015
-
In: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 26:43
-
Journal article (peer-reviewed)abstract
- Ternary nanowires (NWs) often exhibit varying material composition along the NW growth axis because of different diffusion properties of the precursor molecules. This constitutes a problem for optoelectronic devices for which a homogeneous material composition is most often of importance. Especially, ternary GaInP NWs grown under a constant Ga-In precursor ratio typically show inhomogeneous material composition along the length of the NW due to the complexity of low temperature precursor pyrolysis and relative rates of growth species from gas phase diffusion and surface diffusion that contribute to synthesis of particle-assisted growth. Here, we present the results of a method to overcome this challenge by in situ tuning of the trimethylindium molar fraction during growth of ternary Zn-doped GaInP NWs. The NW material compositions were determined by use of x-ray diffraction, scanning transmission electron microscopy and energy dispersive x-ray spectroscopy and the optical properties by photoluminescence spectroscopy.
|
|
| 4. |
- Bi, Zhaoxia, et al.
(author)
-
High In-content InGaN nano-pyramids : Tuning crystal homogeneity by optimized nucleation of GaN seeds
- 2018
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:2
-
Journal article (peer-reviewed)abstract
- Uniform arrays of submicron hexagonal InGaN pyramids with high morphological and material homogeneity, reaching an indium composition of 20%, are presented in this work. The pyramids were grown by selective area metal-organic vapor phase epitaxy and nucleated from small openings in a SiN mask. The growth selectivity was accurately controlled with diffusion lengths of the gallium and indium species, more than 1 μm on the SiN surface. High material homogeneity of the pyramids was achieved by inserting a precisely formed GaN pyramidal seed prior to InGaN growth, leading to the growth of well-shaped InGaN pyramids delimited by six equivalent 10 1 ̄ 1 facets. Further analysis reveals a variation in the indium composition to be mediated by competing InGaN growth on two types of crystal planes, 10 1 ̄ 1 and (0001). Typically, the InGaN growth on 10 1 ̄ 1 planes is much slower than on the (0001) plane. The formation of the (0001) plane and the growth of InGaN on it were found to be dependent on the morphology of the GaN seeds. We propose growth of InGaN pyramids seeded by 10 1 ̄ 1-faceted GaN pyramids as a mean to avoid InGaN material grown on the otherwise formed (0001) plane, leading to a significant reduction of variations in the indium composition in the InGaN pyramids. The InGaN pyramids in this work can be used as a high-quality template for optoelectronic devices having indium-rich active layers, with a potential of reaching green, yellow, and red emissions for LEDs.
|
|
| 5. |
- Bi, Zhaoxia, et al.
(author)
-
InGaN Platelets : Synthesis and Applications toward Green and Red Light-Emitting Diodes
- 2019
-
In: Nano Letters. - : American Chemical Society. - 1530-6984 .- 1530-6992. ; 19:5, s. 2832-2839
-
Journal article (peer-reviewed)abstract
- In this work, we present a method to synthesize arrays of hexagonal InGaN submicrometer platelets with a top c-plane area having an extension of a few hundred nanometers by selective area metal-organic vapor-phase epitaxy. The InGaN platelets were made by in situ annealing of InGaN pyramids, whereby InGaN from the pyramid apex was thermally etched away, leaving a c-plane surface, while the inclined {101Ì1} planes of the pyramids were intact. The as-formed c-planes, which are rough with islands of a few tens of nanometers, can be flattened with InGaN regrowth, showing single bilayer steps and high-quality optical properties (full width at half-maximum of photoluminescence at room temperature: 107 meV for In 0.09 Ga 0.91 N and 151 meV for In 0.18 Ga 0.82 N). Such platelets offer surfaces having relaxed lattice constants, thus enabling shifting the quantum well emission from blue (as when grown on GaN) to green and red. For single InGaN quantum wells grown on the c-plane of such InGaN platelets, a sharp interface between the quantum well and the barriers was observed. The emission energy from the quantum well, grown under the same conditions, was shifted from 2.17 eV on In 0.09 Ga 0.91 N platelets to 1.95 eV on In 0.18 Ga 0.82 N platelets as a result of a thicker quantum well and a reduced indium pulling effect on In 0.18 Ga 0.82 N platelets. On the basis of this method, prototype light-emitting diodes were demonstrated with green emission on In 0.09 Ga 0.91 N platelets and red emission on In 0.18 Ga 0.82 N platelets.
|
|
| 6. |
- Bi, Zhaoxia, et al.
(author)
-
Realization of Ultrahigh Quality InGaN Platelets to be Used as Relaxed Templates for Red Micro-LEDs
- 2020
-
In: ACS Applied Materials and Interfaces. - : NLM (Medline). - 1944-8244 .- 1944-8252. ; 12:15, s. 17845-17851
-
Journal article (peer-reviewed)abstract
- In this work, arrays of predominantly relaxed InGaN platelets with indium contents of up to 18%, free from dislocations and offering a smooth top c-plane, are presented. The InGaN platelets are grown by metal-organic vapor phase epitaxy on a dome-like InGaN surface formed by chemical mechanical polishing of InGaN pyramids defined by 6 equivalent {101̅1} planes. The dome-like surface is flattened during growth, through the formation of bunched steps, which are terminated when reaching the inclined {101̅1} planes. The continued growth takes place on the flattened top c-plane with single bilayer surface steps initiated at the six corners between the c-plane and the inclined {101̅1} planes, leading to the formation of high-quality InGaN layers. The top c-plane of the as-formed InGaN platelets can be used as a high-quality template for red micro light-emitting diodes.
|
|
| 7. |
- Bi, Zhaoxia, et al.
(author)
-
Self-assembled InN quantum dots on side facets of GaN nanowires
- 2018
-
In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 123:16
-
Journal article (peer-reviewed)abstract
- Self-assembled, atomic diffusion controlled growth of InN quantum dots was realized on the side facets of dislocation-free and c-oriented GaN nanowires having a hexagonal cross-section. The nanowires were synthesized by selective area metal organic vapor phase epitaxy. A 3 Å thick InN wetting layer was observed after growth, on top of which the InN quantum dots formed, indicating self-assembly in the Stranski-Krastanow growth mode. We found that the InN quantum dots can be tuned to nucleate either preferentially at the edges between GaN nanowire side facets, or directly on the side facets by tuning the adatom migration by controlling the precursor supersaturation and growth temperature. Structural characterization by transmission electron microscopy and reciprocal space mapping show that the InN quantum dots are close to be fully relaxed (residual strain below 1%) and that the c-planes of the InN quantum dots are tilted with respect to the GaN core. The strain relaxes mainly by the formation of misfit dislocations, observed with a periodicity of 3.2 nm at the InN and GaN hetero-interface. The misfit dislocations introduce I1 type stacking faults (...ABABCBC...) in the InN quantum dots. Photoluminescence investigations of the InN quantum dots show that the emissions shift to higher energy with reduced quantum dot size, which we attribute to increased quantum confinement.
|
|
| 8. |
- Bjerke, Axel, et al.
(author)
-
On chemical interactions between an inclusion engineered stainless steel (316L) and (Ti,Al)N coated tools during turning
- 2023
-
In: Wear. - 0043-1648. ; 532-533
-
Journal article (peer-reviewed)abstract
- Non-metallic inclusions offer one of the most effective routes for improving the machinability of steels. However, the wear-reducing mechanisms activated by such inclusions are not fully understood. The interactions are notoriously difficult to predict due to the wide variety of steel grades, cutting conditions, and tool materials employed in industry. The interaction between PVD (Ti,Al)N coated cemented carbide tools, non-metallic inclusions, atmospheric oxygen, and the stainless steel 316L in a turning operation is therefore investigated here as a case study. The study includes turning experiments, nanometer resolution microscopy, and thermodynamic calculations. The paper explains how not only too high a contact pressures hinder the formation of protective deposits at the tool edge, but also how too low a contact pressure leads to excessive wear. A range of conditions specified in this paper must therefore be met for the two observed protective non-metallic inclusions Mg1Al2O4 and Al2Ca2Si1O7 to be preferentially deposited on a tool. Hence the coating wear is experimentally investigated, explained, and a thermodynamic calculation method for predicting the protective or degenerative potential of a deposit on the coating is presented.
|
|
| 9. |
- Bjerke, Axel, et al.
(author)
-
Onset of the degradation of CVD alpha-Al2O3 coating during turning of Ca-treated steels
- 2021
-
In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 477
-
Journal article (peer-reviewed)abstract
- The ability to control the shape, distribution and composition of non-metallic inclusions has had an important impact on many aspects of steel making. One such impact is on the machinability. Ca-treatments have shown to be able to reduce the abrasiveness of oxide inclusions, improve chip-breaking and lead to formation of deposits that reduce tool wear. However, machining Ca-treated steels with Al2O3 coated cemented carbide tools has not been as advantageous as expected. This study investigates the mechanisms behind the anomalous wear of Al2O3 coatings when turning soft Ca-treated steels. Longitudinal turning tests at a range of speeds (vc = 100-600 m/min) show rapid localized degradation of the Al2O3 coating limited to the sliding zone. Detailed analysis of the degradation mechanisms was performed using scanning and transmission electron microscopy. The results demonstrate a presence of chemical interactions between the alumina coating and non-metallic inclusions. The interaction resulted in the formation of mainly calcium aluminates and partly alumina-magnesia spinel. In-operando infrared thermography measurements indicate cutting temperatures of 850-1000 degrees C. Thermodynamic calculations give that CaO and MgO readily reacts with Al2O3, while the reaction with CaS requires presence of additional oxygen at these cutting conditions. Additional turning experiments investigate the influence of oxygen by controlling the cutting environment by adding oxygen (compressed air) or removing oxygen (supply argon). These additional tests show that the presence of additional oxygen has a limited impact on the possible Ca-Al2O3 interaction. This demonstrat a potential for further machinability improvements by controlling the chemical interaction between Ca and Mg based non-metallic inclusions and alumina coatings.
|
|
| 10. |
- Bjerke, Axel, et al.
(author)
-
Thermodynamic modeling framework for prediction of tool wear and tool protection phenomena in machining
- 2021
-
In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 484-485
-
Journal article (peer-reviewed)abstract
- Chemical, oxidational and diffusional interactions between the tool, chip and cutting environment are known tool wear mechanisms in machining. However, the interaction between tool, coating, workpiece, coolant and atmospheric oxygen can, under favorable conditions, lead to formation of reaction products that retard tool wear. A method with the ability to predict theses interactions, would therefore enable a better control over tool life in machining. An attempt to create such a modelling framework is developed in this study. This method can predict the phase composition and the driving force for degradation and the formation of protective interaction products in the cutting zone. This modeling approach is applicable across cutting processes in which chemical, diffusional and oxidational wear are dominant or present. This framework has been applied to investigate the interactions occurring in the cutting zone during turning of a medium alloyed low-carbon steel (Hybrid Steel (R) 55). A range of degradation events are predicted, as well as the formation of a protective corundum (Al,Fe,Cr)(2)O-3 or spinel (Al, Fe,Cr)(3)O-4 film due to an interaction between the Al-alloyed steel and the environment. Validation of the modeling was performed by studying tool wear and reaction products formed when machining with ceramics, PcBN and coated carbide tooling. Inserts are studied by the use of scanning and transmission electron microscopy, after cutting tests were performed. Additional tests were performed in different environments (dry, argon and coolant). The results confirmed the model predictions of oxidation and diffusion wear as well as the formation of an (Al,Fe,Cr)(3)O-4 tool protection layer. Thus, the proposed thermodynamic framework seem promising to serve as a predictive instrument for the correct pairing of existing tool and workpiece combinations and cutting parameters, or for tailoring respective material compositions for intentional formation of a tool protection layer. As well as guidance on how to apply present and future kinetic models when concurrent interaction mechanisms are present. Which lead to a reduction and minimization of costly experimental machining tests.
|
|
