| 1. |
- Gluba, L., et al.
(författare)
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On the nature of the Mn-related states in the band structure of (Ga,Mn)As alloys via probing the E-1 and E-1 + Delta(1) optical transitions
- 2014
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 105:3
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Tidskriftsartikel (refereegranskat)abstract
- The dilute (Ga,Mn)As became a model ferromagnetic semiconductor, however there is still a disagreement on the source of its magnetism. In this paper, we verify the ellipsometric results and compare them with more precise photoreflectance method, which gives an important insight into the interaction of the Mn-related states with the ones of GaAs valence band. No spectral shifts observed for the E-1 and E-1 + Delta(1) interband transitions in highly doped and annealed (Ga,Mn)As epitaxial layers indicate that the coupling between a detached Mn impurity band and the valence band does not occur. Our findings are supported by the characterizations of the (Ga,Mn)As epitaxial layers with the high resolution transmission electron microscopy and magnetization measurements. (c) 2014 AIP Publishing LLC.
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| 2. |
- Tengborn, Elisabeth, 1978, et al.
(författare)
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Effect of the misorientation of the 4H-SiC substrate on the open volume defects in GaN grown by metal-organic chemical vapor deposition
- 2006
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Ingår i: Applied Physics Letters. ; 89:091905, s. 3-
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Tidskriftsartikel (refereegranskat)abstract
- Positron annihilation spectroscopy has been used to study GaN grown by metal-organic chemical vapor deposition on misoriented 4H-SiC substrates. Two kinds of vacancy defects are observed: Ga vacancies and larger vacancy clusters in all the studied layers. In addition to vacancies, positrons annihilate at shallow traps that are likely to be dislocations. The results show that the vacancy concentration increases and the shallow positron trap concentration decreases with the increasing substrate misorientation.
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| 3. |
- Antipov, S., et al.
(författare)
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Improved bandwidth of a 2THz hot-electron bolometer heterodyne mixer fabricated on sapphire with a GaN buffer layer
- 2019
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Ingår i: Superconductor Science and Technology. - : IOP Publishing. - 0953-2048 .- 1361-6668. ; 32:7
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Tidskriftsartikel (refereegranskat)abstract
- We report on the signal-to-noise and gain bandwidth of a niobium nitride (NbN) hot-electron bolometer (HEB) mixer at 2 THz fabricated on a sapphire substrate with a GaN buffer layer. Two mixers with different DC properties and geometrical dimensions were studied and they demonstrated very close bandwidth performance. The signal-to-noise bandwidth is increased to 8 GHz in comparison to the previous results, obtained without a buffer-layer. The data were taken in a quasi-optical system with the use of the signal-to-noise method, which is close to the signal levels used in actual astrophysical observations. We find an increase of the gain bandwidth to 5 GHz. The results indicate that prior results obtained on a substrate of crystalline GaN can also be obtained on a conventional sapphire substrate with a few micron MOCVD-deposited GaN buffer-layer.
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| 4. |
- Desmaris, Vincent, 1977, et al.
(författare)
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Surface Engineering for high efficiency GaN HEMTs
- 2011
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Ingår i: Proceeedings of the 7th international Workshop on Semiconductor Surface Passivation, 11-15 Sept. 2011, Krakow, Poland.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- High Electron Mobility Transistors (HEMT) based on III-Nitrides have demonstrated excellent microwave performance as power transistors, owing to the large bandgaps of the materials they are as made of, as well as the high band offset and high electron mobility observed in the AlGaN/GaN heterojunctions. Nevertheless, the microwave performance of GaN HEMTs is still largely determined by their surface condition due to the inherent high polarization fields present in their epistructure. Such performance degradation mechanisms are often referred as Gate-lag.In this paper, different surface engineering approaches for mitigating negative surface influence on the device performance are investigated. Different passivation materials, dielectric deposition methods, device fabrication technique and epistructure (GaN Cap layers) device (recess) have been studied for the optimization of GaN device performances. As a result, optimized structures have demonstrated about 6 W/mm and 10 W/mm of CW output power density for GaN HEMTs grown on sapphire and SiC respectively.
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| 5. |
- Krause, Sascha, 1989, et al.
(författare)
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Ambient temperature growth of mono- and polycrystalline NbN nanofilms and their rigorous composition and surface analysis
- 2015
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Ingår i: European Conference on Applied Superconductivity.
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Konferensbidrag (refereegranskat)abstract
- This paper reflects on the rigorous investigation of high-quality 5nm thin NbN films which were deposited by means of reactive DC magnetron sputtering at ambient temperatures. Monocrystalline NbN films have been epitaxially grown onto hexagonal GaN buffer-layers (0002) and showing a distinct, low defect interface as confirmed from HRTEM. The critical temperature (Tc) of those films reached 10.4K. Furthermore, a poly-crystalline structure was observed on films grown onto Si (100) substrates, exhibiting a Tc of 8.1K albeit a narrow transition from the normal to the superconducting state. The deposition at ambient temperatures offers major advantages from a processing point of view and motivates the in-depth characterization and comparison of present films with high quality films grown at elevated temperatures. X-ray photoelectron spectroscopy and reflected electron energy loss spectroscopy verified that the composition of NbN did not differ irrespectively of applied substrate heating. Moreover, the native oxide layer at the surface of NbN has been identified as NbO2 and thus is in contrast to the Nb2O5, usually being formed at the surface of Nb when exposed to air. These findings are of great significance since it was proven the possibility of growing epitaxial NbN onto GaN buffer layer in the absence of high temperatures hence paving the way to employ NbN in more advanced fabrication processes involving a higher degree of complexity. Particularly low-noise THz receiver could benefit from the eased integration of e.g IF circuitry or general multi-layer structures which take advantage of lift-off techniques.
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| 6. |
- Ochalski, T.J., et al.
(författare)
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Optical study of AlGaN/GaN based HEMT structures
- 2005
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Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - 1610-1634 .- 1610-1642. ; 2, s. 2791-2794
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Tidskriftsartikel (refereegranskat)
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| 7. |
- Ochalski, TJ, et al.
(författare)
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Optical study of AlGaN/GaN based HEMT structures grown on sapphire and SiC
- 2005
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 202:7, s. 1300-1307
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Tidskriftsartikel (refereegranskat)abstract
- A detailed photoluminescence (PL), time-resolved photoluminescence (TRPL), and photoreflectance (PR) analysis of AlGaN/GaN heterostructures grown on different substrates: sapphire and silicon carbide (SiC) is presented in this paper. The properties of high electron mobility transistors (HEMTs) based on AlGaN/GaN structures are strongly dependent on the quality of the AlxGa1-xN top layer. We have examined a number of samples, grown on sapphire, in which the thicknesses of the Al0.3Ga0.7N layers vary from 18 to 75 nm. Room temperature PL under pulsed 266 nm excitation allowed for determination of the AI content in the examined thin AlGaN layers. Time-resolved PL measured at 1.6 K showed huge difference in the emission dynamics for different Al0.3Ga0.7N layer thicknesses. We observed an enormous increase of the emission decay time above the critical thickness of the AlGaN layer. PR spectra (associated with the GaN main layer) measured on AlGaN/GaN systems are discussed in terms of the thickness of the capping layer. The PR modulated by a high power 266 nm pulsed laser measured on a transistor structure exhibited an additional feature placed between signals related to the GaN and AlGaN layers, respectively. Such a transition is possible to monitor only for the structures of the best quality and is accordingly observable only on samples grown on SiC. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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