| 1. |
- Darakchieva, Vanya, et al.
(författare)
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Deformation potentials of the E-1(TO) mode in AlN
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:13, s. 2302-2304
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Tidskriftsartikel (refereegranskat)abstract
- The deformation potentials of the E-1(TO) mode in AlN are experimentally determined by combining infrared reflection spectroscopy and x-ray diffraction measurements and using a reported value of the Raman-stress factor for hydrostatically stressed bulk AlN. The deformation potentials are found to strongly depend on published stiffness constants of AlN. A comparison with earlier theoretically calculated values of the deformation potentials is made. (C) 2002 American Institute of Physics.
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| 2. |
- Darakchieva, Vanya, et al.
(författare)
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Generalized infrared ellipsometry study of thin epitaxial AlN layers with complex strain behavior
- 2003
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Konferensbidrag (refereegranskat)abstract
- The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated, and a sub-layer model of the degree of strain and related defects for all films is suggested. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sub-layer model has been successfully applied to the analysis of the ellipsometry data trough model calculations of the infrared dielectric function. © 2003 Elsevier B.V. All rights reserved.
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| 3. |
- Darakchieva, Vanya, et al.
(författare)
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Strain evolution in high temperature AlN buffer layers for HVPE-GaN growth
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 59-64
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Tidskriftsartikel (refereegranskat)abstract
- High temperature AlN buffer layers are deposited on a-plane sapphire by reactive magnetron sputtering. The effect of the buffer thickness on the AlN structural properties and surface morphology are studied in correlation with the subsequent hydride vapour phase epitaxy of GaN. A minimum degree of mosaicity and screw dislocation density is determined for a 50 nm thick AlN buffer. With increasing the AlN thickness, a strain relaxation occurs as a result of misfit dislocation generation and higher degree of mosaicity. A blue shift of the E-1(TO) frequency evaluated by means of infrared reflection spectroscopy is linearly correlated with an increase in biaxial compressive stress in the films through the IR stress factor k(E1)(b) = 2.57 +/- 0.26 cm(-1) GPa(-1).
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| 4. |
- Mopoung, Kunpot, et al.
(författare)
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Investigation of in situ annealing effect on the thermal stability and crystallinity of IrMn thin films by X-ray diffraction and electron energy loss spectroscopy
- 2022
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Ingår i: Thin Solid Films. - : ELSEVIER SCIENCE SA. - 0040-6090 .- 1879-2731. ; 762
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Tidskriftsartikel (refereegranskat)abstract
- The crystal reorganization of the IrMn thin film was observed by the in situ thermal annealing in the X-ray diffraction (XRD) and electron energy loss spectroscopy. From room temperature to 700 degrees C, the in situ annealing measurements identify the different Mn diffusion effects in three temperature ranges. First, between room temperature and 300 degrees C, the XRD profiles show the reorganization of Mn atoms from interstitials to IrMn lattice points. Second, between 300 and 400 degrees C, we observed the diffusion of Mn atoms from the IrMn thin film to the surface, causing the atomic Mn/Ir ratio to drop from 10 to 7. The MnO appeared on the film surface in this temperature range. Third, from 400 to 700 degrees C, the O content in the IrMn thin film increases to 8%, while the Mn/ Ir ratio continuously decreases from 7 to 5. The scanning transmission electron microscopy images also show that the crystal structure of IrMn thin film completely degrades to another structure.
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| 5. |
- Paskova, Tanja, et al.
(författare)
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Defect and stress relaxation in HVPE-GaN films using high temperature reactively sputtered AlN buffer
- 2001
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Ingår i: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 230:3-4, s. 381-386
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Tidskriftsartikel (refereegranskat)abstract
- The influence of high temperature buffer layers on the structural characteristics of GaN grown by hydride vapour phase epitaxy on sapphire was investigated. Strain relaxation as well as mismatch-induced defect reduction in thick GaN layers grown on AlN buffer was microscopically identified using cathodoluminescence and micro-Raman spectroscopy in cross-section of the films. The results were correlated with photoluminescence and Hall-effect data of layers with different thicknesses. These relaxation processes were suggested to account for the specific defect distribution in the buffers revealed by high-resolution X-ray diffraction and transmission electron microscopy. © 2001 Elsevier Science B.V. All rights reserved.
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| 6. |
- Paskova, Tanja, et al.
(författare)
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Defect reduction in HVPE growth of GaN and related optical spectra
- 2001
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 183:1, s. 197-203
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Tidskriftsartikel (refereegranskat)abstract
- GaN technology is still based on highly mismatched heteroepitaxial growth on foreign substrates, and therefore needs to overcome a high defect density and a high level of stress in the epitaxial layers. Various attempts have been made to reduce the defects and stress in thick GaN layers. We here report a reduction of the defect density in thick GaN layers grown by hydride vapour phase epitaxy, using regrowth on free-standing GaN films, as well as introducing an AlN buffer and AlN interlayer in the growth sequence. Special focus is put on the optical properties of the material.
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| 7. |
- Paskova, Tanja, et al.
(författare)
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Effect of Si doping of metalorganic chemical vapor deposition-GaN templates on the defect arrangement in hydride vapor phase epitaxy-GaN overgrown layers
- 2000
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Ingår i: Journal of Applied Physics. - 0021-8979 .- 1089-7550. ; 88:10, s. 5729-5732
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Tidskriftsartikel (refereegranskat)abstract
- Two different types of dislocation arrangements have been observed in hydride vapor-phase epitaxial GaN films grown on sapphire substrates using both undoped and Si-doped GaN templates grown by metalorganic chemical vapor deposition: (i) predominantly straight threading dislocations parallel to the [0001] direction in the layer grown on an undoped template, and (ii) a network of interacting dislocations of edge, screw, and mixed character in the layer grown on a Si-doped template. The two types of defect distribution result in essentially different surface morphologies, respectively: (i) low-angle grain boundaries formed by pure edge dislocations around spiral grown hillocks, and (ii) smooth surface intersected by randomly distributed dislocations. The Si doping of the GaN templates was found to enhance defect interaction in the templates and to enable a reduction of the dislocation density in the overgrown thick GaN films, although it does not lead to an improvement of the overall structural properties of the material. (C) 2000 American Institute of Physics. [S0021-8979(00)08422-X].
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| 8. |
- Paskova, Tanja, et al.
(författare)
-
Hydride vapour phase homoepitaxial growth of GaN on MOCVD-grown 'templates'
- 2000
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Konferensbidrag (refereegranskat)abstract
- We report on an improved quality of thick HVPE-GaN grown on MOCVD-GaN 'template' layers compared to the material grown directly on sapphire. The film-substrate interface revealed by cathodoluminescence measurements shows an absence of highly doped columnar structures which are typically present in thick HVPE-GaN films grown directly on sapphire. This improved structure results in a reduction of two orders of magnitude of the free carrier concentration from Hall measurements. It was found that the structure, morphology, electrical and optical properties of homoepitaxial thick GaN layers grown by HVPE were strongly influenced by the properties of the MOCVD-GaN 'template'. Additionally the effect of Si doping of the GaN buffer layers on the HVPE-GaN properties was analysed.
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| 9. |
- Paskova, Tanja, et al.
(författare)
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Polar and nonpolar GaN grown by HVPE : Preferable substrates for nitride-based emitting devices
- 2004
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 201:10, s. 2265-2270
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Tidskriftsartikel (refereegranskat)abstract
- We report on hydride vapor phase epitaxial growth of thick nonpolar GaN films on r-plane sapphire in comparison with polar GaN films on c-plane sapphire substrates with AlN buffer layers, aiming at developing of their quasi-substrate application. Both the thick films and the buffers were identified to have single crystalline structures. The microstructure of the films was studied by transmission electron microscopy. High resolution X-ray diffraction mapping and photoluminescence measurements were employed to characterize the strain present in both polar and nonpolar GaN films. In contrast to c-plane GaN, which is always characterised by isotropic in-plane properties, the a-plane GaN shows a strong in-plane anisotropy of the growth rate, morphology and strain distribution. Different defect, impurity and free carrier concentrations were observed in the polar and nonpolar material.
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| 10. |
- Rakpongsiri, Pornchai, et al.
(författare)
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Thermal Stress Evaluation of Tunneling Magnetoresistive Structures in Data Storage Devices
- 2018
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Ingår i: IEEE Magnetics Letters. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1949-307X .- 1949-3088. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The conventional read sensor technology used in hard disk drives is the tunneling magnetoresistive (TMR) device. The technology evolution of the TMR device toward higher signal-to-noise performance has been achieved through aggressive scaling of the thin film layered structures in the device to the point that thermal stability and thermally induced degradation have come to limit reliability. In this study, a thermal stress has been applied to TMR devices between 150-250 degrees C, and the resistance, amplitude, and asymmetry parameters, both before and after the thermal stress, were measured using a quasi-static test (QST). The results reveal a temperature dependence of the TMR device performance. The microstructure of the annealed devices was further studied using transmission electron microscopy and energy dispersive X-ray analysis, revealing structural defects that are related to the QST parametric changes. Both atomic misalignment of the MgO layer and Ir depletion are proposed as origins for the instability of the magnetic response in the device after thermal stress.
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| 11. |
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| 12. |
- Tungasmita, Sukkaneste, et al.
(författare)
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Enhanced quality of epitaxial AlN thin films on 6H-SiC by ultra-high-vacuum ion-assisted reactive dc magnetron sputter deposition
- 2000
-
Ingår i: Applied Physics Letters. - 0003-6951 .- 1077-3118. ; 76:2, s. 170-172
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial AlN thin films have been grown on 6H-SiC substrates by ultra-high-vacuum (UHV) ion-assisted reactive dc magnetron sputtering. The low-energy ion-assisted growth (E-i = 17-27 eV) results in an increasing surface mobility, promoting domain-boundary annihilation and epitaxial growth. Domain widths increased from 42 to 135 nm and strained-layer epitaxy was observed in this energy range. For E-i> 52 eV, an amorphous interfacial layer of AlN was formed on the SiC, which inhibited epitaxial growth. Using UHV condition and very pure nitrogen sputtering gas yielded reduced impurity levels in the films (O: 3.5 x 10(18) cm(-3)). Analysis techniques used in this study are in situ reflection high-energy electron diffraction, secondary-ion-mass spectroscopy, atomic-force microscopy, x-ray diffraction, and cross-section high-resolution electron microscopy. (C) 2000 American Institute of Physics. [S0003-6951(00)01802-7].
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| 13. |
- Tungasmita, Sukkaneste, 1972-
(författare)
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Epitaxial aluminum nitride thin films on 6H-silicon carbide, grown by magnetron sputter deposition
- 2000
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The research presented in this thesis is focused on epitaxial wurtzite-structure Aluminum Nitride (AlN) thin film synthesis, by ultra-high-vacuum (UHV) de magnetron sputter deposition, on Silicon Carbide (6H-SiC) substrates. The emphasis of the work has been put on controlling the growth and quality of the films to be able to use this material in electronic device applications.The quality of epitaxial AlN films is significantly improved by using low energy ion assistance (Ei = 17-27 eV), during growth. The ion-assisted growth results in an increased surface mobility, which promotes domain boundary annihilation and epitaxial growth. This results in lateral expansion of column width (100 nm-wide at film thickness above 100 nm). The film characterization results show a very good crystal quality as well as high purity material. The measured concentrations of O, C, and Si in the film are at 3.5x1018, l. 3x1018 and 3.5xl 018 cm-3, respectively, which are among the purest AlN material as has been reported. The appearance of near band edge CL emission (6.02 eV at 4K) is also an evidence ofa high quality material.
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| 14. |
- Tungasmita, Sukkaneste, et al.
(författare)
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Growth of epitaxial (SiC)(x)(AlN)(1-x) thin films on 6H-SiC by ion-assisted dual magnetron sputter deposition
- 2002
-
Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 389-3, s. 1481-1484
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Tidskriftsartikel (refereegranskat)abstract
- (SiC)(X)(AIN)(1-X) thin films have been grown epitaxially on vicinal 6H-SiC (0001) by low-energy ion assisted dual magnetron sputtering in UHV conditions. AES showed a decreasing Si and C content for an increasing magnetron power ratio, (P-Al/P-SiC). The epitaxial quality of the films was improved as the SiC fraction increased. Films containing less than 5% of Si and C show an evolution of domain width similar to the growth of pure AIN. HRXRD show a decreased c-axis lattice parameter for a film with composition of AINC(X) (0less than or equal toxless than or equal to0.1), indicating carbon substitution in AIN. CL spectra show defect-related peaks of similar to3.87 and similar to4.70 eV, corresponding to O and C impurities respectively as well as on un-identified peak at similar to3.40 eV.
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| 15. |
- Tungasmita, Sukkaneste, 1972-
(författare)
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Growth of Wide-Band Gap AlN and (SiC)x(AlN)1-x Thin Films by Reactive Magnetron Sputter Deposition
- 2001
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The research presented in this thesis is focused on thin film synthesis of epitaxial wurtzite structure aluminum nitride (AlN) and related alloy, (SiC)x(AlN)1-x,by ultra-high-vacuum (UHV) reactive magnetron sputter deposition, on silicon carbide (6H-SiC) substrates. The emphasis of the work is on controlling the growth and quality of the films to be able to use the materials in electronic device applications. The quality of epitaxial AlN films is significantly improved by usin glow-energy ion assistance (Ei = 17-27 eV), during growth. The ion-assisted growth gives an increased surface mobility, which promotes domain boundary annihilation and epitaxial growth. This results in lateral expansion of column width (100 nm-wide at film thickness above 100 nm). Structural characterization by high-resolution XRD and electron microscopy reveal a very good crystal quality. The measured concentrations of O, C, and Si impurities in the films are at 3.5x1018, 1.3x 10x18 andl 1.0 x1017 cm-3, respectively, which are among the purest AlN material that has been reported. The appearance of near band-edge CL emission (6.02 eV at 4K) is also an evidence of a high quality material. For metal-insulator-semiconductor devices (MIS) fabricated using AlN as the dielectric layer, electrical characterization by C-V measurement shows hysteresis with 1.2 V of flat-band voltage shift due to fixed charge and interface states, and a current leakage due to domain boundary formation in the AlN layer. Pulsed low-energy ion-assisted reactive magnetron sputtering was also applied to the growth of AlN in order to overcome the thickness limitation of DC ion-assisted growth due to a surface charging effect. AlN films with very large domain widths can thus be realized. The structural evolution resulting from increasing adatom mobility can be extended up to twice the thickness, compared to DC ionassisted deposition. The growth rate also increased by a factor of ∼4, compared to growth conditions with no ion assistance (Ei = 2 eV), and by a factor of ∼2 from the DC ion-assisted growth. Finally, solid solution (SiC)x(AlN)1-x thin films have beengrown epitaxially on 6H-SiC substrates. The composition of the films was controlled by varying the power of each magnetron during co-sputtering from A1 and SiC targets in a gas mixture of Ar and N2. Compositional investigation from AES showed a decreasing Si and C content for an increasing magnetron power ratio, (PA1/PSiC). The microstructure of the films was improved as the PA1/PSiC increases. Films grown at PA1/PSiC 3. 6 show structural evolution of domain width similar to the growth of pure AlN. High-resolution XRD shows a minimum in c-axis lattice parameter as PA1/PSiC =3.6. CL spectra show defect-related peaks corresponding to O and C impurities with some shifts due to structural defects and concentration of the impurities.
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| 16. |
- Tungasmita, Sukkaneste, et al.
(författare)
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Low-energy-ion-assisted reactive sputter deposition of epitaxial AlN thin films on 6H-SiC
- 2000
-
Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 338-3, s. 1519-1522
-
Tidskriftsartikel (refereegranskat)abstract
- Epitaxial wurzite-structure AIN thin films have been grown on 6H-SiC substrates by ultra-high-vacuum (UHV) low-energy-ion-assisted reactive de magnetron sputtering. The quality of epitaxial AIN films is significantly improved by low-energy ion assistance (E-i = 17-27 eV), during reactive magnetron sputter growth on vicinal (3.5 degrees) 6H-SiC. The ion-assisted growth results in an increased surface mobility, which promotes domain boundary annihilation and epitaxial growth. This results in lateral expansion of column width. Thus, AIN films with domains as large as 40 nm at the interface to 6H-SiC can be realized. At film thickness above 100 nm, the column width expands to 100 nm. The crystal quality of the films is very good with low background impurities (O: 3.5x10 (18)cm(-3)).
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| 17. |
- Tungasmita, Sukkaneste, et al.
(författare)
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Pulsed low-energy ion-assisted growth of epitaxial aluminum nitride layer on 6H-silicon carbide by reactive magnetron sputtering
- 2002
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 91:6, s. 3551-3555
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial aluminum nitride thin films have been grown on silicon carbide (6H-SiC) substrates by pulsed low-energy ion-assisted reactive magnetron sputter deposition (+5/-20 V of bias pulses), with ion-assisted energy (Ei)?22eV, under ultrahigh-vacuum conditions. Surface ion interactions during the negative bias pulse gave rise to enhanced surface mobility of adatoms with beneficial effects, which extended over the limit of ion repelling in the positive pulse as the film thickness increased. High-resolution electron microscopy shows that a large (>90 nm) AlN domain width can form on the substrate. Domain-boundary annihilation and domain suppression during film growth have been observed. The growth rate also increased by a factor of ~4 compared to growth conditions with no ion assistance (Ei=2eV) and by a factor of 2 from dc ion-assisted growth. This indicates that the supply of nitrogen is a limiting factor for AlN formation and that the reactivity of nitrogen is increased on the growing AlN film surface for pulse ion-assisted deposition. High-resolution x-ray diffraction shows a reduction in the full width at half maximum of the rocking curve from 1490 to 1180 arcsec when pulsed ions are used. The cathodoluminescence shows high intensity of near-band edge emissions at wavelengths of 206 (6.02 eV) and 212 nm (5.84 eV) at a measured temperature of 5 K, with relatively low defect and oxygen and carbon impurity related emission, which is indicative of a high quality electronic material. © 2002 American Institute of Physics.
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| 18. |
- Valcheva, E, et al.
(författare)
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Interface structure of hydride vapor phase epitaxial GaN grown with high-temperature reactively sputtered AlN buffer
- 2000
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Ingår i: Applied Physics Letters. - 0003-6951 .- 1077-3118. ; 76:14, s. 1860-1862
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Tidskriftsartikel (refereegranskat)abstract
- Thick hydride vapor phase epitaxy GaN layers have been grown on a-plane sapphire using high-temperature ion-assisted reactively sputtered AlN as a buffer layer. Transmission electron microscopy and atomic force microscopy were carried out to study the formation of the two interfaces sapphire/AlN and AlN/GaN, and their influence on the microstructure of both the buffer layer and the main GaN layer. It was demonstrated that the high-temperature reactively sputtered buffer layer provides a good alternative for hydride vapor phase epitaxy growth of GaN layers. In particular, the buffer promotes a specific interface ordering mechanism different from that observed on low-temperature buffers. (C) 2000 American Institute of Physics. [S0003-6951(00)00314-4].
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