| 1. |
- Cobet, C., et al.
(författare)
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Optical properties of SiC investigated by spectroscopic ellipsometry from 3.5 to 10 eV
- 2000
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 364:1, s. 111-113
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Tidskriftsartikel (refereegranskat)abstract
- In this work we present the dielectric function of hexagonal 4H- and 6H-SiC polytypes as well as the cubic 3C-SiC polytype in the energy range from 3.5 to 10 eV measured by spectroscopic ellipsometry. We operated with synchrotron radiation at the Berlin electron storage ring BESSY I. Additionally the samples were investigated by atomic force microscopy to correct the measured dielectric function for the influence of surface roughness. The experimental results are compared to theoretical calculations.
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| 2. |
- Cobet, C, et al.
(författare)
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Optical properties of wurtzite AlxGa1-xN (x < 0.1) parallel and perpendicular to the c axis
- 2001
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 64:16
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of the dielectric function components parallel and perpendicular to the optical axis of wurtzite GaN are presented. The results are obtained on a multilayer AlxGa1-xN / GaN film grown on gamma -LiAlO2 which exhibits a very small amount of Al (x<0.1) in the uppermost layer. We use the spectroscopic ellipsometry to determine (omega) around the interband critical points of GaN up to 9 eV. Since the optical axis of the films are parallel to the surface, a separation of the different epsilon components is possible. The line shape of the measured tensor components e(parallel to) and epsilon (perpendicular to) is in very good agreement with results of recently published band-structure calculations. By comparison with the calculations we assign structures in the dielectric function to critical points in the Brillouin zone. In addition, a comparison with similar spectra for CdS and CdSe is presented, together with a discussion of possible spin-orbit splitting effects.
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| 3. |
- Edwards, NV, et al.
(författare)
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Real-time assessment of overlayer removal on 4H-SiC surfaces : Techniques and relevance to contact formation
- 2000
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Ingår i: Materials Science Forum. - 0255-5476 .- 1662-9752. ; 338-3, s. 1033-1036
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Tidskriftsartikel (refereegranskat)abstract
- We applied real-time spectroscopic ellipsometric (SE) measurements to assess the removal of overlayer material from 4H-SiC Si- and C-face surfaces in order to investigate the final step of an otherwise standard RCA cleaning regimen commonly used to prepare SiC surfaces for contact formation. The selected treatments (buffered hydrofluoric acid (HF), concentrated HF, dilute HF and 5% HF in Methanol) removed 4 to 40 Angstrom of effective SiO2 overlayer thickness from these surfaces. We also found that the concentrated HF treatment yielded the best surface, i.e. the most abrupt bulk-to-ambient transition region.
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| 4. |
- Edwards, NV, et al.
(författare)
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Real-time assessment of selected surface preparation regimens for 4H-SiC surfaces using spectroscopic ellipsometry
- 2000
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Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 464:1, s. L703-L707
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Tidskriftsartikel (refereegranskat)abstract
- Spectroscopic ellipsometry (SE) was used to assess the removal of overlayer material from 4H-SiC (0001) and (0001) [Si- and C-face] surfaces in real time and, in particular, the critical final step of an otherwise standard RCA cleaning regimen commonly used to prepare SiC surfaces for contact formation. The treatments selected [buffered hydrofluoric acid (HF), concentrated HF, and dilute HF] removed 4-40 Angstrom of effective SiO2 overlayer thickness from these surfaces. The concentrated HF treatment yielded the best surface, i.e. that with the most abrupt transition region between bulk and surface and with the most oxide material removed. A fourth treatment regimen (sequential application of methanol, water, and 5% HF in methanol) was also developed for comparison with the full RCA clean. (C) 2000 Elsevier Science B.V. All rights reserved.
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| 5. |
- Edwards, N.V., et al.
(författare)
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Optical characterization of wide bandgap semiconductors
- 2000
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Ingår i: Thin Solid Films. - 0040-6090 .- 1879-2731. ; 364:1, s. 98-106
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Tidskriftsartikel (refereegranskat)abstract
- Our work primarily concerns the characterization of wide-gap III-V nitride semiconductors, nondestructively and at variable temperature, with spectroscopic ellipsometry (SE) and reflectometry in the spectral range from 1.5 to 6 eV. In the case of GaN, there are three main concerns associated with such data: (a) the quantification of the dispersion of the index of refraction with energy, (b) the removal of surface overlayers in real-time, and (c) the determination of the variation of valence bands with biaxial stress and the quantification of residual stress in thin films. The SE and reflectance capabilities provide (1) broadband spectra from 1.5 to 6 eV, which yield information about (a) below the bandgap and (b) above it, and (2) high resolution spectra (less than 1 meV at 3.4 eV) in the vicinity of the gap (3.3-3.6 eV), which enables (c). Here we will discuss issues concerning the relation of (c) to GaN material and growth parameters, though similar data for other wide bandgap materials will be discussed where relevant. Specifically, optimal heterostructure design for potential valence band engineering applications will be discussed in the context of trends in residual stress as a function of film thickness, growth temperature and substrate orientation for GaN/AlN/6H-SiC heterostructures. Standard heterostructures are mostly compressive for samples less than about 0.7 µm thick, are tensile up to about 2 µm and then abruptly become less tensile with stress values near 1 kbar thereafter. Additionally, these trends can be circumvented for moderately thick (approximately 2 µm) GaN layers (normally>2 kbar, tensile) by the introduction of a `buried interface' approach, namely, a strain mediating layer (SML) above the standard high-temperature AlN buffer layer designed to yield a range of compressive stresses from 0 to 2 kbar. The strain characteristics but also the growth rates of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML. This is achieved by in situ techniques during crystal growth without degrading the optical and structural properties of the deposited layer, as confirmed by XRD, SEM, PL, and AFM data taken on the overlying GaN layers. These results are interpreted in terms of coefficient of thermal expansion data for the layers and data concerning the planarization of GaN layers and growth behavior in non-(0001) directions.
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| 6. |
- Lindquist, O.P.A., et al.
(författare)
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Ordinary and extraordinary dielectric functions of 4H- and 6H-SiC from 3.5 to 9.0 eV
- 2001
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 78:18, s. 2715-2717
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Tidskriftsartikel (refereegranskat)abstract
- We report ordinary (??c axis) and extraordinary (??c axis) dielectric function data of 4H- and 6H-SiC from 3.5 to 9.0 eV. These data, which were obtained by with spectroscopic ellipsometry, are also compared to recently reported ab initio calculations. Critical point energies were found using real and reciprocal space analysis. © 2001 American Institute of Physics.
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