| 1. |
- Persson, P. O.Å., et al.
(författare)
-
Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers
- 2023
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 122:15
-
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.
|
|
| 2. |
- Gogova, D., et al.
(författare)
-
High crystalline quality homoepitaxial Si-doped β-Ga2O3(010) layers with reduced structural anisotropy grown by hot-wall MOCVD
- 2024
-
Ingår i: Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films. - 0734-2101. ; 42:2
-
Tidskriftsartikel (refereegranskat)abstract
- A new growth approach, based on the hot-wall metalorganic chemical vapor deposition concept, is developed for high-quality homoepitaxial growth of Si-doped single-crystalline β -Ga 2 O 3 layers on (010)-oriented native substrates. Substrate annealing in argon atmosphere for 1 min at temperatures below 600 ° C is proposed for the formation of epi-ready surfaces as a cost-effective alternative to the traditionally employed annealing process in oxygen-containing atmosphere with a time duration of 1 h at about 1000 ° C. It is shown that the on-axis rocking curve widths exhibit anisotropic dependence on the azimuth angle with minima for in-plane direction parallel to the [001] and maximum for the [100] for both substrate and layer. The homoepitaxial layers are demonstrated to have excellent structural properties with a β -Ga 2 O 3 (020) rocking curve full-widths at half-maximum as low as 11 arc sec, which is lower than the corresponding one for the substrates (19 arc sec), even for highly Si-doped (low 10 19 cm − 3 range) layers. Furthermore, the structural anisotropy in the layer is substantially reduced with respect to the substrate. Very smooth surface morphology of the epilayers with a root mean square roughness value of 0.6 nm over a 5 × 5 μ m 2 area is achieved along with a high electron mobility of 69 cm 2 V − 1 s − 1 at a free carrier concentration n = 1.9 × 10 19 cm − 3 . These values compare well with state-of-the-art parameters reported in the literature for β -Ga 2 O 3 (010) homoepitaxial layers with respective Si doping levels. Thermal conductivity of 17.4 W m − 1 K − 1 is determined along the [010] direction for the homoepitaxial layers at 300 K, which approaches the respective value of bulk crystal (20.6 W m − 1 K − 1 ). This result is explained by a weak boundary effect and a low dislocation density in the homoepitaxial layers.
|
|
| 3. |
- Paskova, T., et al.
(författare)
-
Strain-free low-defect-density bulk GaN with nonpolar orientations
- 2006
-
Ingår i: MRS Proceedings. - : Springer Science and Business Media LLC. - 1946-4274.
-
Konferensbidrag (refereegranskat)abstract
- Bulk GaN sliced in bars along (11-20) and (1-100) planes from a boule grown in the [0001] direction by HVPE was confirmed as strain free material with a low dislocation density by using several characterization techniques. The high-structural quality of the material allows photoluminescence studies of free excitons, principal donor bound excitons and their twoelectron satellites with regard to the optical selection rules. Raman scattering study of the bulk GaN with nonpolar orientations allows a direct access to the active phonon modes and a direct determination of their strain-free positions.
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Bartos, I., et al.
(författare)
-
Electron band bending of polar, semipolar and non-polar GaN surfaces
- 2016
-
Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 119:10, s. 105303-
-
Tidskriftsartikel (refereegranskat)abstract
- The magnitudes of the surface band bending have been determined by X-ray photoelectron spectroscopy for polar, semipolar, and non-polar surfaces of wurtzite GaN crystals. All surfaces have been prepared from crystalline GaN samples grown by the hydride-vapour phase epitaxy and separated from sapphire substrates. The Ga 3d core level peak shifts have been used for band bending determination. Small band bending magnitudes and also relatively small difference between the band bendings of the surfaces with opposite polarity have been found. These results point to the presence of electron surface states of different amounts and types on surfaces of different polarity and confirm the important role of the electron surface states in compensation of the bound surface polarity charges in wurtzite GaN crystals. (C) 2016 AIP Publishing LLC.
|
|
| 7. |
|
|
| 8. |
- Darakchieva, Vanya, 1971-, et al.
(författare)
-
Anisotropic strain and phonon deformation potentials in GaN
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195217-
-
Tidskriftsartikel (refereegranskat)abstract
- We report optical phonon frequency studies in anisotropically strained c -plane- and a -plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E1 (TO), E1 (LO), and E2 phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials c E1 (TO), c E1 (LO), and c E2 are determined. A distinct correlation between anisotropic strain and the A1 (TO) and E1 (LO) frequencies of a -plane GaN films reveals the a A1 (TO), b A1 (TO), a E1 (LO), and b E1 (LO) phonon deformation potentials. The a A1 (TO) and b A1 (TO) are found to be in very good agreement with previous results from Raman experiments. Our a A1 (TO) and a E1 (LO) phonon deformation potentials agree well with recently reported theoretical estimations, while b A1 (TO) and b E1 (LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented. © 2007 The American Physical Society.
|
|
| 9. |
- Darakchieva, Vanya, et al.
(författare)
-
Effect of anisotropic strain on phonons in a-plane and c-plane GaN layers
- 2007
-
Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 300:1, s. 233-238
-
Tidskriftsartikel (refereegranskat)abstract
- We have studied phonons in two types of anisotropically strained GaN films: c-plane GaN films grown on a-plane sapphire and a-plane GaN films grown on r-plane sapphire. The anisotropic strain in the films is determined by high-resolution X-ray diffraction (HRXRD) in different measuring geometries and the phonon parameters have been assessed by generalized infrared spectroscopic ellipsometry (GIRSE). The effect of strain anisotropy on GaN phonon frequencies is presented and the phonon deformation potentials aA1 (TO), bA1 (TO), cE1 (TO) and cE1 (LO) are determined. © 2006 Elsevier B.V. All rights reserved.
|
|
| 10. |
- Delgado Carrascon, Rosalia, 1987-, et al.
(författare)
-
Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN : Understanding Nucleation and Design of Growth Strategies
- 2022
-
Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7483 .- 1528-7505. ; 22:12, s. 7021-7030
-
Tidskriftsartikel (refereegranskat)abstract
- Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 x 1015 cm-3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.
|
|
