| 1. |
- Beyer, Franziska, et al.
(författare)
-
Deep levels in hetero-epitaxial as-grown 3C-SiC
- 2010
-
Ingår i: AIP Conference Proceedings, Vol. 1292. - : AIP. - 9780735408470 ; , s. 63-66
-
Konferensbidrag (refereegranskat)abstract
- 3C-SiC grown hetero-epitaxially on 4H- or 6H-SiC using a standard or a chloride-based CVD process were electrically characterized using IV, CV and DLTS. The reverse leakage current of the Au-Schottky diodes was reduced to lower than 10-8 A at -2V by a thermal oxidation step using UV-light illumination at 200oC. The Schottky barrier height of the Ni and Au contacts were determined by IV measurement to be ØB = 0.575 eV and ØB = 0.593 eV, respectively, for a contact diameter of about 150 mm. One dominant DLTS peak was observed in the 3C-epilayers independently of the substrate at about EC0:60 eV which is attributed to W6-level in 3C-SiC. This deep level is thought to be related to an intrinsic defect.
|
|
| 2. |
- Beyer, Franziska, et al.
(författare)
-
Defects in low-energy electron-irradiated n-type 4H-SiC
- 2010
-
Ingår i: Physica Scripta, vol. T141. - : IOP Publishing. ; , s. 014006-
-
Konferensbidrag (refereegranskat)abstract
- The bistable M-center, previously observed in high-energy proton-implanted 4H-SiC, was detected in low-energy electron-irradiated 4H-SiC using deep-level transient spectroscopy (DLTS). Irradiation increased the DLTS signals of the intrinsic defects Z(1/2) and EH6/7 and introduced the frequently observed defects EH1 and EH3. After the M-center is annealed out at about 650K without bias and at about 575K with bias applied to the sample during the annealing process, a new bistable defect in the low temperature range of the DLTS spectrum, the EB-center, evolves. Since low-energy irradiation affects mainly the carbon atoms in SiC, the M-center and the newly discovered EB-center are most probably carbon-related intrinsic defects.
|
|
| 3. |
|
|
| 4. |
- Gogova, D., et al.
(författare)
-
Investigation of the structural and optical properties of free-standing GaN grown by HVPE
- 2005
-
Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 38:14, s. 2332-2337
-
Tidskriftsartikel (refereegranskat)abstract
- The potential of the high-growth rate hydride vapour phase epitaxy technique and laser lift-off for the fabrication of free-standing GaN substrates is explored. Structural and optical properties of 300 νm thick free-standing GaN have been investigated employing different analytical techniques. The x-ray diffraction (XRD) measurements prove good crystalline quality of the material grown. A comparatively low value of (3 ± 1) × 1016 cm-3 of Ga vacancy-related defects is inferred from positron annihilation spectroscopy data. Complete strain relaxation is observed on the Ga-polar face of the free-standing GaN by XRD and Raman spectroscopy measurements. The strain-free homoepitaxy will significantly reduce the defect density, and thus an improvement of the device performance and lifetime can be realized. © 2005 IOP Publishing Ltd.
|
|
| 5. |
- Gogova, Daniela, 1967-, et al.
(författare)
-
Strain-free bulk-like GaN grown by hydride-vapor-phase-epitaxy on two-step epitaxial lateral overgrown GaN template
- 2004
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 96:1, s. 799-806
-
Tidskriftsartikel (refereegranskat)abstract
- Crack-free bulk-like GaN with high crystalline quality has been obtained by hydride-vapor-phase-epitaxy (HVPE) growth on a two-step epitaxial lateral overgrown GaN template on sapphire. During the cooling down stage, the as-grown 270-μm-thick GaN layer was self-separated from the sapphire substrate. Plan-view transmission electron microscopy images show the dislocation density of the free-standing HVPE-GaN to be ∼2.5×107 cm−2 on the Ga-polar face. A low Ga vacancy related defect concentration of about 8×1015 cm−3 is extracted from positron annihilation spectroscopy data. The residual stress and the crystalline quality of the material are studied by two complementary techniques. Low-temperature photoluminescence spectra show the main neutral donor bound exciton line to be composed of a doublet structure at 3.4715 (3.4712) eV and 3.4721 (3.4718) eV for the Ga- (N-) polar face with the higher-energy component dominating. These line positions suggest virtually strain-free material on both surfaces with high crystalline quality as indicated by the small full width at half maximum values of the donor bound exciton lines. The E1(TO) phonon mode position measured at 558.52 cm−1 (Ga face) by infrared spectroscopic ellipsometry confirms the small residual stress in the material, which is hence well suited to act as a lattice-constant and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications. © 2004 American Institute of Physics.
|
|
| 6. |
- Hemmingsson, Carl, 1964-, et al.
(författare)
-
Deep-Level Defects in Electron-irradiated 4H SiC Epitaxial Layers
- 1997
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 81:9, s. 6155-6159
-
Tidskriftsartikel (refereegranskat)abstract
- Deep level defects in electron-irradiated 4H SiC epitaxial layers grown by chemical vapor deposition were studied using deep level transient spectroscopy. The measurements performed on electron-irradiated p+n junctions in the temperature range 100–750 K revealed several electron traps and one hole trap with thermal ionization energies ranging from 0.35 to 1.65 eV. Most of these defects were already observed at a dose of irradiation as low as ≈5×1013 cm-2. Dose dependence and annealing behavior of the defects were investigated. For two of these electron traps, the electron capture cross section was measured. From the temperature dependence studies, the capture cross section of these two defects are shown to be temperature independent. © 1997 American Institute of Physics.
|
|
| 7. |
- Hemmingsson, Carl, 1964-
(författare)
-
Deep Levels in Electron-Irradiated and As-Grown SiC Power Device Material
- 1998
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon Carbide (SiC) has several favorable physical properties for the fabrication of highpower, high-temperature and high-frequency devices. Devices in SiC can operate at high temperatures due to the wide band gap and the high thermal stability of the material. Heat produced by power losses are efficiently dissipated since the thermal conductivity of SiC is high(better than for copper). The high electrical breakdown field strength and the indirect band gap make it possible to design high voltage devices with low power losses. The high saturation drift velocity makes SiC attractive for high frequencies devices. In addition, SiC is chemically inert which allows the device to operate in very harsh environment.In order to control the carrier lifetime in SiC for bipolar device applications, it is necessary to intentionally introduce defects in the material. This can be done by diffusion of impurities or irradiation by high energetic particles such as electrons or ions. Electron irradiation of SiC introduces intrinsic defects such as vacancies, interstitials and antisites. Intrinsic defects are also introduced during the growth. For SiC, electron irradiation is an attractive technique since diffusion of impurities is difficult in SiC. Consequently, a detailed knowledge of the electronic properties of the introduced defects is necessary for utilizing the electron irradiation technique and to improve the carrier lifetime in unirradiated SiC.In the thesis, defects associated with intrinsic defects in electron irradiated and in as-grown 6H and 4H SiC have been studied. The electronic properties of the defects have been characterized using various forms of diode capacitance transient techniques. Paper I reports an investigation of electron irradiation induced defects in 4H SiC. Annealing behaviour and dose dependence for these defects were investigated. For two of these traps, the temperature dependence of the capture cross section was measured. Capture cross sections of electron irradiation induced defects in 6H SiC were investigated in paper II. The temperature dependence of the capture cross section for the so-called Ei, El and E2 centers was measured and it was shown that the electron capturing process to El and E2 is governed by a multiphonon capturing process. Further, it is shown that the El and E2 centers act as recombination channels.Many defects in semiconductors have a stable configuration. However, for some defects the configuration is altered when the charge state changes or if energy is supplied to the defect system. This gives rise to many interesting physical phenomena such as metastability and negative-U behavior of the defects. Both phenomena have been observed in SiC. In paper III observations of two complex metastable defects in 6H SiC are reported. These two defects always appear as an entity with similar binding energies and they are not possible to resolve by deeplevel transient spectroscopy (DLTS). However, one of the defect has three configurations labeled C1, C2 and C3, while the other one has four configurations, labeled C1, C2, C3 and C4. The common notations C1, C2, and C3 are used since their electronic behaviors are very similar. The configuration C4 is only observed after hole capture to the defect or after annealing with bias applied to the diode at temperatures above 610 K. The defects with three configurations and with four configurations differ by a factor of two in concentration. It indicates a correlation with the quasi-cubic (2) and hexagonal (1) sites of the 6H SiC lattice. Two of the configurations, the C2 and C3 were shown to be stable only when they were occupied by an electron. The electron capture rate was measured for these configurations and it was shown that the electron capturing process to C1 was responsible for the transformation process to these configurations. In paper IV a more detailed investigation of the configuration transition processes C1 ➔ C2 and C1 ➔ C3 is presented. An electron capturing and configuration transition model is presented and a transformation probability between the configurations has been defined and measured. In the introduction to this thesis additional photoionisation measurements are presented. The difference between the thermal position in the band gap and the optical ionization energy of C2 is large. It indicates that a large lattice relaxation occurs when the defect is transformed to C2. Configuration C3 was not possible to ionize with light. It suggests that a very large lattice relaxation occur.Electron irradiation of 6H and 4H SiC give rise to the so-called DLTS peaks E1/E2 and Z1, respectively. In paper V and VI it is demonstrated that both these peaks are associated to the two-electron emission from the acceptor levels of two negative-U centers. In both polytypes, two closely spaced negative-U centers are observed and it was suggested that the two centers may be associated to a defect residing at the cubic and hexagonal lattice sites, respectively. The electronic properties of the negative-U centers in 6H and 4H SiC are very similar. It indicates that the two negative-U defect systems in 4H and 6H SiC have a similar structure.
|
|
| 8. |
- Hemmingsson, Carl, 1964-, et al.
(författare)
-
Growth of bulk GaN in a vertical hydride vapour phase epitaxy reactor
- 2006
-
Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 40:4-6 SPEC. ISS., s. 205-213
-
Tidskriftsartikel (refereegranskat)abstract
- Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence. © 2006 Elsevier Ltd. All rights reserved.
|
|
| 9. |
- Kasic, A., et al.
(författare)
-
Characterization of crack-free relaxed GaN grown on 2″ sapphire
- 2005
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 98:7, s. 73525-
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the growth of high-quality and virtually strain-free bulklike GaN by hydride vapor-phase epitaxy in a vertical atmospheric-pressure reactor with a bottom-fed design. The 300‐μm-thick GaN layer was grown on a 2″ (0 0 0 1) sapphire substrate buffered with a ∼ 2‐μm-thick GaN layer grown by metal-organic chemical-vapor deposition. During the cool down process to room temperature, cracking was induced in the sapphire substrate, thereby allowing the bulklike GaN layer to relax without provoking cracking of itself. The crystalline quality and the residual strain in the 2″ GaN wafer were investigated by various characterization techniques. The lateral homogeneity of the wafer was monitored by low-temperature photoluminescence mapping. High-resolution x-ray diffraction and photoluminescence measurements proved the high crystalline quality of the material grown. The position of the main near-band-gap photoluminescence line and the phonon spectra obtained from infrared spectroscopic ellipsometry show consistently that the 2″ crack-free GaN is virtually strain-free over a diameter of approximately 4 cm.
|
|
| 10. |
|
|
| 11. |
- Monemar, Bo, 1942-, et al.
(författare)
-
Growth of thick GaN layers by hydride vapor phase epitaxy
- 2005
-
Ingår i: Journal of Ceramic Processing Research. - 1229-9162. ; 6:2, s. 153-162
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we describe recent experimental work on the growth of thick GaN layers (up to >300 μm) on sapphire with hydride vapour phase epitaxy (HVPE), the removal of the sapphire substrate by the laser liftoff technique, and the properties of these thick GaN layers. Two different growth setups were used, one horizontal and one vertical system. Specific conditions in the growth procedure, like gas flow pattern, growth rate and the use of buffer layers, strongly influence the properties of the grown layers. Important defect problems are cracking (both during and after growth), and the generation of dislocations and surface pits. A large bowing is also observed for thick layers, depending very much on the properties of the initially grown material. For growth of thick layers excessive parasitic growth of GaN upstream of the substrate has to be avoided. Laser liftoff is demonstrated to be a feasible process to remove the sapphire substrate, causing the GaN surface bowing to decrease and revert from convex to concave. The threading dislocation density of 300 μm thick GaN layers is found to be about 107 cm-2, rather independent of the type of buffer layer employed. It reduces further in thicker layers. The pit density varies with growth conditions, it can be reduced if the parasitic growth is avoided. The bowing is a serious problem, since the layers have to be polished to make the surface epi-ready. The XRD rocking curve widths measured seem to correlate with the bowing of the layers, a reduction by about a factor two is often observed when the substrate is removed. Optical characterisation like photoluminescence (PL) and ir spectroscopic ellipsometry (IRSE) is very useful to monitor strain in the layers, as well as impurities and point defects. Residual shallow donors are related to O and Si, shallow acceptors are mainly of intrinsic origin, i.e. complexes with the Ga vacancy.
|
|
| 12. |
- Monemar, Bo, et al.
(författare)
-
Mg related acceptors in GaN
- 2010
-
Ingår i: Phys. Status Solidi C 7. ; , s. 1850-
-
Konferensbidrag (refereegranskat)
|
|
| 13. |
|
|
| 14. |
|
|
| 15. |
- Nguyen, Son Tien, 1953-, et al.
(författare)
-
Electronic structure of deep defects in SiC
- 2004
-
Ingår i: Silicon Carbide: Recent Major Advances. - Berlin, Heidelberg : Springer Verlag. - 9783540404583 - 3540404589 ; , s. -899
-
Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Since the 1997 publication of Silicon Carbide - A Review of Fundamental Questions and Applications to Current Device Technology edited by Choyke, et al., there has been impressive progress in both the fundamental and developmental aspects of the SiC field. So there is a growing need to update the scientific community on the important events in research and development since then. The editors have again gathered an outstanding team of the world's leading SiC researchers and design engineers to write on the most recent developments in SiC. The book is divided into five main categories: theory, crystal growth, characterization, processing and devices. Every attempt has been made to make the articles as up-to-date as possible and assure the highest standards of accuracy. As was the case for earlier SiC books, many of the articles will be relevant a decade from now so that this book will take its place next to the earlier work as a permanent and essential reference volume.
|
|
| 16. |
- Pozina, Galia, 1966-, et al.
(författare)
-
Dynamics of bound excitons versus thickness in freestanding GaN wafers grown by halide vapor phase epitaxy
- 2007
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:22
-
Tidskriftsartikel (refereegranskat)abstract
- The bound exciton recombination properties in freestanding GaN layers of various thicknesses grown by halide vapor phase epitaxy have been characterized by time-resolved spectroscopy. Improvement of the donor bound exciton (D0 X) lifetime was observed with increasing GaN layer thickness up to ∼400 μm, while for thicker layers the recombination time of D0 X shows a tendency to saturate. The thickness-dependent behavior of the D0 X decay can be understood in terms of competition between two nonradiative mechanisms: one of which is connected to structural defects, and consequently more important for thinner layers, while for layers with thickness above 400 μm with low structural defect density the recombination time is limited by point defects such as impurities and vacancies. © 2007 American Institute of Physics.
|
|
| 17. |
|
|
| 18. |
- Pozina, Galia, 1966-, et al.
(författare)
-
Metastable behavior of the UV luminescence in Mg-doped GaN layers grown on quasibulk GaN templates
- 2007
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:22
-
Tidskriftsartikel (refereegranskat)abstract
- Metastability of near band gap UV emissions in Mg-doped GaN layers grown by metal-organic vapor phase epitaxy on thick GaN templates grown by halide vapor phase epitaxy has been studied by cathodoluminescence (CL). The CL spectrum changes its initial shape within a few minutes under electron irradiation. The acceptor bound exciton line vanishes while the emissions related to the stacking faults (SFs) of different geometry rise significantly. The increase of the defect-related luminescence is likely caused by recombination enhanced SF formation under electron irradiation. The CL spectrum transformation is permanent at low temperatures, however, the metastable process is reversible if samples are heated to room temperature. © 2007 American Institute of Physics.
|
|
| 19. |
- Pozina, Galia, 1966-, et al.
(författare)
-
Optical properties of metastable shallow acceptors in Mg-doped GaN layers grown by metal-organic vapor phase epitaxy
- 2010
-
Ingår i: AIP Conference Proceedings. - : AIP. - 0094-243X. - 9780735407367 ; , s. 110-111
-
Konferensbidrag (refereegranskat)abstract
- GaN layers doped by Mg show a metastable behavior of the near-band-gap luminescence caused by electron irradiation or UV excitation. At low temperatures < 30 K the changes in luminescence are permanent. Heating to room temperature recovers the initial low temperature spectrum shape completely. Two acceptors are involved in the recombination process as confirmed by transient PL. In as-grown samples a possible candidate for the metastable acceptor is C-N, while after annealing a second more stable acceptor related to Mg became active.
|
|
| 20. |
|
|
| 21. |
- Son, Nguyen Tien, et al.
(författare)
-
Radiation-induced defects in GaN
- 2010
-
Ingår i: Physica Scripta, Vol. T141. - : IOP Publishing. ; , s. 014015-
-
Konferensbidrag (refereegranskat)abstract
- Radiation-induced defects in n-type GaN irradiated by 2 MeV electrons at room temperature were studied by electron paramagnetic resonance (EPR). Four EPR spectra, labelled D1-D4, were observed in irradiated n-type GaN. The D1 spectrum is a broad line (similar to 10-12 mT in line width) with an isotropic g-value g similar to 2.03 and can be detected in all the studied samples in the temperature range of 4-300 K. The D2 centre has an electron spin S = 1/2 and shows a clear hyperfine structure due to interaction with three equivalent N-14. The g-values of the axial configuration are determined to be g(parallel to) = 2.001 and g(perpendicular to) = 1.999. On the basis of the observed hyperfine structure, formation conditions and annealing behaviour, the D2 defect was assigned to the gallium vacancy-oxygen pair in the negative charge state, (VGaON)(-).
|
|
