| 1. |
- Persson, P. O.Å., et al.
(författare)
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Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers
- 2023
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 122:15
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Tidskriftsartikel (refereegranskat)abstract
- Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1-xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼9 × 10 12 cm-2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.
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| 2. |
- Gogova, Daniela, et al.
(författare)
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Epitaxial growth of β -Ga 2 O 3 by hot-wall MOCVD
- 2022
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226. ; 12:5, s. 055022-055022
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Tidskriftsartikel (refereegranskat)abstract
- The hot-wall metalorganic chemical vapor deposition (MOCVD) concept, previously shown to enable superior material quality and highperformance devices based on wide bandgap semiconductors, such as Ga(Al)N and SiC, has been applied to the epitaxial growth ofβ-Ga2O3. Epitaxial β-Ga2O3 layers at high growth rates (above 1 μm/h), at low reagent flows, and at reduced growth temperatures(740 ○C) are demonstrated. A high crystalline quality epitaxial material on a c-plane sapphire substrate is attained as corroborated by a combination of x-ray diffraction, high-resolution scanning transmission electron microscopy, and spectroscopic ellipsometry measurements. Thehot-wall MOCVD process is transferred to homoepitaxy, and single-crystalline homoepitaxial β-Ga2O3 layers are demonstrated with a 201 ¯rocking curve width of 118 arc sec, which is comparable to those of the edge-defined film-fed grown (201) ¯ β-Ga2O3 substrates, indicative ofsimilar dislocation densities for epilayers and substrates. Hence, hot-wall MOCVD is proposed as a prospective growth method to be furtherexplored for the fabrication of β-Ga2O3
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| 3. |
- Knight, Sean Robert, et al.
(författare)
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Room temperature two-dimensional electron gas scattering time, effective mass, and mobility parameters in AlxGa1−xN/GaN heterostructures (0.07 ≤ x ≤ 0.42)
- 2023
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Ingår i: Journal of Applied Physics. - : American Institute of Physics Inc.. - 0021-8979 .- 1089-7550. ; 134:18
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Tidskriftsartikel (refereegranskat)abstract
- Al xGa 1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the Al xGa 1−xN barrier layers between x=0.07 and x=0.42. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x<0.42, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3 m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer.
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| 4. |
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| 5. |
- Lattemann, Martina, et al.
(författare)
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Investigation of high power impulse magnetron sputtering pretreated interfaces for adhesion enhancement of hard coatings on steel
- 2006
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 200:22-23, s. 6495-6499
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Tidskriftsartikel (refereegranskat)abstract
- In order to improve the adhesion of hard coatings such as CrN, a surface pretreatment by the novel high power impulse magnetron sputtering (HIPIMS) technique followed by reactive unbalanced d.c. magnetron sputtering deposition was performed using a Cr target. The HIPIMS plasma comprising a high metal ion-to-neutral ratio consisting of single- and double-charged metal species identified by mass spectrometry increased the metal ion flux to the substrate. When applying a negative substrate bias Ub the adhesion was enhanced due to sputter cleaning of the surface and metal ion intermixing in the interface region. This intermixing, resulting in a gradual change of the composition, is considered to enhance the adhesion of the hard coatings on steel substrates. The pretreatment was carried out in an inert gas atmosphere at a pressure of pAr = 1 mTorr, the duration was varied between 25 and 75 min, whereas the negative substrate bias was varied between 400 V and 1200 V. The adhesion was found to depend on the substrate bias as well as on the target power and, for low substrate bias, on the duration of the pretreatment. For CrN the critical load of failure determined by scratch test could be increased in comparison to the values reported for specimens pretreated by conventional Ar etching. The influence of the target peak voltage, the substrate bias as well as pretreatment time on the constitution and morphology of the interface after the pretreatment is discussed applying analytical transmission electron microscopy.
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| 6. |
- Persson, P. O. Å., et al.
(författare)
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Ti2Al(O,N) formation by solid state reaction between substoichiometric TiN thin films and Al2O3(0001) substrates
- 2011
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Ingår i: Thin Solid Films. - : Elsevier. - 0040-6090 .- 1879-2731. ; 519:8, s. 2421-2425
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Tidskriftsartikel (refereegranskat)abstract
- Titanium nitride TiNx (0.1 ≤ x ≤ 1) thin films were deposited onto Al2O3(0001) substrates using reactive magnetron sputtering at substrate temperatures (Ts) ranging from 800 ºC to 1000 ºC and N2 partial pressures (pN2) between 0.1 and 1.0 mTorr. It is found that Al and O from the substrates diffuse into the substoichiometric TiNx films during deposition. Solid state reactions between the film and substrate result in the formation of Ti2O and Ti3Al domains at low N2 partial pressures, while for increasing pN2, the Ti2AlN MAX phase nucleates and grows together with TiNx. Depositions at increasingly stoichiometric conditions result in a decreasing incorporation of the substrate species into the growing film. Eventually, a stoichiometric deposition gives a stable TiN(111) || Al2O3(0001) structure without the incorporation of substrate species. Growth at Ts 1000 ºC yields Ti2AlN(0001), leading to a reduced incorporation of substrate species compared to films grown at 900 ºC, but contains also Ti2AlN(101ɸ3) grains. Finally, the Ti2AlN domains incorporate O, likely on the N site, such that a MAX phase oxynitride Ti2Al(O,N) is formed. The results were obtained by a combination of structural methods, including X-ray diffraction (XRD) and (scanning) transmission electron microscopy ((S)TEM), together with spectroscopy methods, which comprise elastic recoil detection analysis (ERDA), energy dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS).
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| 7. |
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