| 1. |
- Malmros, Anna, 1977, et al.
(författare)
-
Enhanced Mobility in InAlN/AlN/GaN HEMTs Using a GaN Interlayer
- 2019
-
Ingår i: IEEE Transactions on Electron Devices. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1557-9646 .- 0018-9383. ; 66:7, s. 2910-2915
-
Tidskriftsartikel (refereegranskat)abstract
- © 1963-2012 IEEE. An enhancement of the electron mobility ( \mu ) in InAlN/AlN/GaN heterostructures is demonstrated by the incorporation of a thin GaN interlayer (IL) between the InAlN and AlN. The introduction of a GaN IL increases \mu at room temperature (RT) from 1600 to 1930 cm2/Vs. The effect is further enhanced at cryogenic temperature (5 K), where the GaN IL sample exhibits a \mu of 16000 cm2/Vs, compared to 6900 cm2/Vs without IL. The results indicate the reduction of one or more scattering mechanisms normally present in InAlN/AlN/GaN heterostructures. We propose that the improvement in \mu is either due to the suppression of fluctuations in the quantum well subband energies or to reduced Coulomb scattering, both related to compositional variations in the InAlN. HEMTs fabricated on the GaN IL sample demonstrate larger improvement in dc-and high-frequency performance at 5 K; {f}-{\text {max}} increases by 25 GHz to 153 GHz, compared to an increase of 6 GHz to 133 GHz without IL. The difference in improvement was associated mainly with the drop in the access resistances.
|
|
| 2. |
- Tholander, Christopher, 1986-
(författare)
-
Piezoelectricity, Phase Stability, and Surface Diffusion in Multicomponent Nitrides
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The last hundred years have been full of scientific discoveries leading to technological advances, such as, computers, smart phones, etc. Most of the advances would not have been possible without new discoveries within the vast field of materials science. The specific area within materials science covered in this thesis is multicomponent nitride alloys, which are commonly used as thin films in industrial applications, e.g., as hard wear-resistant coatings for cutting-tools or as part of intricate electronic components in mobile telecommunication devices. The core of this thesis is towards the fundamental understanding of existing, and the discovery of new, nitride alloys using theoretical tools. Knowledge about the quantum mechanics of the alloys was gained using density functional theory, alloy theory, and thermodynamics investigating piezoelectricity, phase stability, and surface diffusion.The focus of the piezoelectricity research is on piezoelectric properties of both ordered and disordered nitrides. The exploration of disordered wurtzite nitrides revealed important aspects of the nitride alloying physics and the implications for their piezoelectric response, in addition to the discovery of interesting alloy candidates and their synthesis, e.g., YxIn1-xN. For the ordered nitrides, novel TMZnN2 (TM = Ti, Zr, Hf) structures with high piezoelectric responses have been predicted as stable.The focus of the piezoelectricity research is on piezoelectric properties of both ordered and disordered nitrides. The exploration of disordered wurtzite nitrides revealed important aspects of the nitride alloying physics and the implications for their piezoelectric response, in addition to the discovery of interesting alloy candidates and their synthesis, e.g., YxIn1-xN. For the ordered nitrides, novel TMZnN2 (TM = Ti, Zr, Hf) structures with high piezoelectric responses have been predicted as stable.The thermodynamic stability of novel alloys with interesting properties is investigated in order to determine if equilibrium or non-equilibrium synthesis is feasible. The studies consist of ternary phase diagrams of TM-Zn-N, mixing enthalpies for disordered YxAl1-xN and YxIn1-xN that can be used to predict possible synthesis routes and guide experiments. In addition, mixing enthalpies for strained ScxAl1-xN/InyAl1-yN superlattices show that the stability of certain phases and, therefore, the crystalline quality can be improved by modifying in-plane lattice parameters through higher indium content in the InAlN layers.Surface diffusion is studied because it is an important factor during thin film growth with, for example, physical vapor deposition. It is the main atomic transport mechanism and, thus, governs the structure development of thin films. Specifically, the research is focused on diffusion on the surfaces of disordered alloys, and in particular Ti, Al, and N adatom diffusion on TiN and TiAlN surfaces. The investigations revealed that Ti adatom mobilities are dramatically reduced in the presence of Al in the surface layer on the TiN and Ti0.5Al0.5N(0 0 1) surfaces, while Al adatoms are largely unaffected. Furthermore, the reverse effect is found on the TiN(1 1 1) surface, Al adatom migration is reduced while Ti adatom migration is unaffected. In addition, it is shown that neglecting the magnetic spin polarization of Ti adatoms will locally underestimate the binding energies and the diffusion path, e.g., underestimating the stability of TiN(0 0 1) bulk sites.
|
|
| 3. |
- Fashandi, Hossein, 1984-
(författare)
-
Novel Layered and 2D Materials for Functionality Enhancement of Contacts and Gas Sensors
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Chemical gas sensors are widely-used electronic devices for detecting or measuring the density levels of desired gas species. In this study, materials with established or potential applications for gas sensors are treated. For the case of high-temperature applications (≈ 600 °C), semiconductor-based gas sensors suffer from rapid oxidation of the metallic ohmic contacts, the same cause-of-failure as for the general case of high-temperature semiconductor electronics. 4H-SiC is an ideal semiconductor for high-temperature applications. Ti3SiC2 is a known ohmic contact to 4H-SiC with the known two-step synthesis process of post-annealing of pre-deposited Ti/Al multilayers or sputter-deposition of Ti3SiC2 films at > 900 °C. Here, sputter-deposition of Ti on 4H-SiC at > 900 °C is presented as a novel single-step method for the synthesis of Ti3SiC2 ohmic contacts, based on a concurrent reaction between sputter-deposited Ti and 4HSiC. Ti3SiC2, similar to any other known ohmic contact, degrade rapidly in high-temperature oxidizing ambient. To try to overcome this obstacle, noble metal diffusion into Ti3SiC2 has been s studied with the goal to retain ohmic properties of Ti3SiC2 and harnessing oxidation resistivity of noble metals. A novel exchange intercalation between Ti3SiC2 and Au is discovered which results in the almost complete exchange of Si with Au giving rise to novel Ti3AuC2 and Ti3Au2C2. Ti3IrC2 is also synthesized through exchange intercalation of Ir into Ti3Au2C2. All the aforementioned phases showed ohmic properties to 4H-SiC. This technique is also studied based on Ti2AlC and Ti3AlC2 resulting in the synthesis of novel Ti2Au2C and Ti3Au2C2, respectively. Using Ti3AuC2 and an Au/IrOx capping layer, an ohmic contact was manufactured, which maintained ohmic properties and showed no structural defects after 1000 h of aging at 600 °C air.Ti3SiC2 is a member of a large family of materials known as Mn+1AXn phases. While exchange reactions of Si (or Al) planes in Ti3SiC2 (Ti2AlC and Ti3AlC2) is presented here, a world-wide research already exists on chemical removal of the same atomic planes from different Mn+1AXn phases and the synthesis of Mn+1Xn sheets known as MXenes. I performed a theoretical study regarding simulation of electronic and structural properties of more than120 different possible MXene phases. The results show that some MXene phases, when terminated by particular gas species, turn into Dirac materials. That is, they possess massless Dirac fermions with different properties compared to graphene such as higher number of Dirac points at the Fermi level, giant spin orbit splitting, and preserved 2D-type electronic properties by extending the dimensionality. The general substantial change of the electronic properties of MXenes under different gas adsorption configurations stands out and can thus be harnessed for sensing applications.Growth of monolayer iron oxide on porous Pt sensing layers is another novel approach used in this study for applying the unique properties of 2D materials for gas sensors. A low temperature shift in CO oxidation characteristics is presented. The approach is similar to that previously reported using bulk single crystal Pt substrate, the latter being an unrealistic model for sensors and catalysts. Monolayer-coated Pt sensing layers were fabricated as the metal component of a metal oxide semiconductor (MOS) capacitor device, whereby the electrical response of the MOS device could be used to map out the catalytic properties of the sensing layer. The monolayer-coated Pt surface showed to be stable with retained improved catalytic properties for > 200 h. The MOS device measurements are here utilized as a handy method for in-situ monitoring of the surface chemical properties of the monolayer-coated Pt and the approach is highly functional for use and characterization of monolayer coatings of widely used sensingor catalytic layers.
|
|
| 4. |
- Luo, Jun, et al.
(författare)
-
Surface-energy triggered phase formation and epitaxy in nanometer-thick Ni1-xPtx silicide films
- 2010
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:3
-
Tidskriftsartikel (refereegranskat)abstract
- The formation of ultrathin silicide films of Ni1-xPtx at 450-850 degrees C is reported. Without Pt (x=0) and for t(Ni)< 4 nm, epitaxially aligned NiSi2-y films readily grow and exhibit extraordinary morphological stability up to 800 degrees C. For t(Ni)>= 4 nm, polycrystalline NiSi films form and agglomerate at lower temperatures for thinner films. Without Ni (x=1) and for t(Pt)=1-20 nm, the annealing behavior of the resulting PtSi films follows that for the NiSi films. The results for Ni1-xPtx of other compositions support the above observations. Surface energy is discussed as the cause responsible for the distinct behavior in phase formation and morphological stability.
|
|
| 5. |
- Alling, Björn, 1980-
(författare)
-
Configurational and Magnetic Interactions in Multicomponent Systems
- 2010
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is a theoretical study of configurational and magnetic interactions in multicomponent solids. These interactions are the projections onto the configurational and magnetic degrees of freedom of the underlying electronic quantum mechanical system, and can be used to model, explain and predict the properties of materials. For example, the interactions govern temperature induced configurational and magnetic order-disorder transitions in Heusler alloys and ternary nitrides.In particular three perspectives are studied. The first is how the interactions can be derived from first-principles calculations at relevant physical conditions. The second is their consequences, like the critical temperatures for disordering, obtained with e.g. Monte Carlo simulations. The third is their origin in terms of the underlying electronic structure of the materials.Intrinsic defects in the half-Heusler system NiMnSb are studied and it is found that low-energy defects do not destroy the important half-metallic property at low concentrations. Deliberate doping of NiMnSb with 3d-metals is considered and it is found that replacing some Ni with extra Mn or Cr creates new strong magnetic interactions which could be beneficial for applications at elevated temperature. A self-consistent scheme to include the effects of thermal expansion and one-electron excitations in the calculation of the magnetic critical temperature is introduced and applied to a study of Ni1−xCuxMnSb.A supercell implementation of the disordered local moments approach is suggested and benchmarked for the treatment of paramagnetic CrN as a disordered magnetic phase. It is found that the orthorhombic-to-cubic phase transition in this nitride can be understood as a first-order magnetic order-disorder transition. The ferromagnetism in Ti1−xCrxN solid solutions, an unusual property in nitrides, is explained in terms of a charge transfer induced change in the Cr-Cr magnetic interactions.Cubic Ti1−xAlxN solid solutions displays a complex and concentration dependent phase separation tendency. A unified cluster expansion method is presented that can be used to simulate the configurational thermodynamics of this system. It is shown that short range clustering do influence the free energy of mixing but only slightly change the isostructural phase diagram as compared to mean-field estimates.
|
|
| 6. |
- Alves Machado Filho, Manoel, et al.
(författare)
-
Self-Induced Core–Shell InAlN Nanorods: Formation and Stability Unraveled by Ab Initio Simulations
- 2023
-
Ingår i: ACS Nanoscience Au. - : American Chemical Society (ACS). - 2694-2496. ; 3:1, s. 84-93
-
Tidskriftsartikel (refereegranskat)abstract
- By addressing precursor prevalence and energetics using the DFT-based synthetic growth concept (SGC), the formation mechanism of self-induced InAlN core–shell nanorods (NRs) synthesized by reactive magnetron sputter epitaxy (MSE) is explored. The characteristics of In- and Al-containing precursor species are evaluated considering the thermal conditions at a typical NR growth temperature of around 700 °C. The cohesive and dissociation energies of In-containing precursors are consistently lower than those of their Al-containing counterparts, indicating that In-containing precursors are more weakly bonded and more prone to dissociation. Therefore, In-containing species are expected to exhibit lower abundance in the NR growth environment. At increased growth temperatures, the depletion of In-based precursors is even more pronounced. A distinctive imbalance in the incorporation of Al- and In-containing precursor species (namely, AlN/AlN+, AlN2/AlN2+, Al2N2/Al2N2+, and Al2/Al2+ vs InN/InN+, InN2/InN2+, In2N2/In2N2+, and In2/In2+) is found at the growing edge of the NR side surfaces, which correlates well with the experimentally obtained core–shell structure as well as with the distinctive In-rich core and vice versa for the Al-rich shell. The performed modeling indicates that the formation of the core–shell structure is substantially driven by the precursors’ abundance and their preferential bonding onto the growing edge of the nanoclusters/islands initiated by phase separation from the beginning of the NR growth. The cohesive energies and the band gaps of the NRs show decreasing trends with an increment in the In concentration of the NRs’ core and with an increment in the overall thickness (diameter) of the NRs. These results reveal the energy and electronic reasons behind the limited growth (up to ∼25% of In atoms of all metal atoms, i.e., InxAl1–xN, x ∼ 0.25) in the NR core and may be qualitatively perceived as a limiting factor for the thickness of the grown NRs (typically <50 nm).
|
|
| 7. |
- Bakhit, Babak, et al.
(författare)
-
Controlling the B/Ti ratio of TiBx thin films grown by high-power impulse magnetron sputtering
- 2018
-
Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : A V S AMER INST PHYSICS. - 0734-2101 .- 1520-8559. ; 36:3
-
Tidskriftsartikel (refereegranskat)abstract
- TiBx thin films grown from compound TiB2 targets by magnetron sputter deposition are typically highly over-stoichiometric, with x ranging from 3.5 to 2.4, due to differences in Ti and B preferential-ejection angles and gas-phase scattering during transport from the target to the substrate. Here, the authors demonstrate that stoichiometric TiB2 films can be obtained using highpower impulse magnetron sputtering (HiPIMS) operated in power-controlled mode. The B/Ti ratio x of films sputter-deposited in Ar is controllably varied from 2.08 to 1.83 by adjusting the length of HiPIMS pulses t(on) between 100 and 30 mu s, while maintaining average power and pulse frequency constant. This results in peak current densities J(T), peak ranging from 0.27 to 0.88 A/cm(2). Energy- and time-resolved mass spectrometry analyses of the ion fluxes incident at the substrate position show that the density of metal ions increases with decreasing t(on) due to a dramatic increase in J(T, peak) resulting in the strong gas rarefaction. With t(on)amp;lt;60 mu s (J(T),(peak)amp;gt; 0.4 A/cm(2)), film growth is increasingly controlled by ions incident at the substrate, rather than neutrals, as a result of the higher plasma dencity and, hence, electron-impact ionization probablity. Thus, since sputter- ejected Ti atoms have a higher probability of being ionized than B atoms, due to their lower first-ionization potential and larger ionization cross-section, the Ti concentration in as-deposited films increases with decreasing ton (increasing J(T,peak)) as ionized sputtered species are steered to the substrate by the plasma in order to maintain charge neutrality. Published by the AVS.
|
|
| 8. |
- Bangolla, Hemanth Kumar, et al.
(författare)
-
Composition-dependent photoconductivities in indium aluminium nitride nanorods grown by magnetron sputter epitaxy
- 2022
-
Ingår i: Nanoscale Advances. - : Royal Society of Chemistry. - 2516-0230. ; 4:22, s. 4886-4894
-
Tidskriftsartikel (refereegranskat)abstract
- Photoconduction (PC) properties were investigated for ternary indium aluminium nitride (InxAl1-xN) nanorods (NRs) with different indium compositions (x) from 0.35 to 0.68, as grown by direct-current reactive magnetron sputter epitaxy. Cross-sectional scanning transmission electron microscopy (STEM) reveals single-crystal quality of the vertically aligned InxAl1-xN NRs. Single-rod photodetector devices with good ohmic contacts were fabricated using the focused-ion-beam technique (FIB), where the In-rich In0.68Al0.32N NR exhibits an optimal photocurrent responsivity of 1400 A W-1 and photoconductive gain of 3300. A transition from a positive photoresponse to a negative photoresponse was observed, while increasing the In composition x from 0.35 to 0.57. The negative PC was further enhanced by increasing x to 0.68. A model based on the coexistence and competition of deep electron trap states and recombination centers was proposed to explain the interesting composition-dependent PC in these ternary III-nitride 1D nanostructures.
|
|
| 9. |
- Chang, Jui-Che, et al.
(författare)
-
Domain epitaxial growth of Ta3N5 film on c-plane sapphire substrate
- 2022
-
Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 443
-
Tidskriftsartikel (refereegranskat)abstract
- Tritantalum pentanitride (Ta3N5) semiconductor is a promising material for photoelectrolysis of water with high efficiency. Ta3N5 is a metastable phase in the complex system of TaN binary compounds. Growing stabilized single-crystal Ta3N5 films is correspondingly challenging. Here, we demonstrate the growth of a nearly single-crystal Ta3N5 film with epitaxial domains on c-plane sapphire substrate, Al2O3(0001), by magnetron sputter epitaxy. Introduction of a small amount ~2% of O2 into the reactive sputtering gas mixed with N2 and Ar facilitates the formation of a Ta3N5 phase in the film dominated by metallic TaN. In addition, we indicate that a single-phase polycrystalline Ta3N5 film can be obtained with the assistance of a Ta2O5 seed layer. With controlling thickness of the seed layer smaller than 10 nm and annealing at 1000 °C, a crystalline β phase Ta2O5 was formed, which promotes the domain epitaxial growth of Ta3N5 films on Al2O3(0001). The mechanism behind the stabilization of the orthorhombic Ta3N5 structure resides in its stacking with the ultrathin seed layer of orthorhombic β-Ta2O5, which is energetically beneficial and reduces the lattice mismatch with the substrate.
|
|
| 10. |
- Chang, Jui-Che, et al.
(författare)
-
HiPIMS-grown AlN buffer for threading dislocation reduction in DC-magnetron sputtered GaN epifilm on sapphire substrate
- 2023
-
Ingår i: Vacuum. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0042-207X .- 1879-2715. ; 217
-
Tidskriftsartikel (refereegranskat)abstract
- Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).
|
|
