Optics: measuring and testing – By dispersed light spectroscopy – With raman type light scattering
Reexamination Certificate
2002-09-18
2004-06-01
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With raman type light scattering
C438S016000
Reexamination Certificate
active
06744501
ABSTRACT:
The invention relates to a method for analyzing Si—Ge alloys, with which a Raman spectrum of a sample is recorded and Raman frequencies and Raman intensities of the Si—Si mode and the Si—Ge mode of the alloy layer are evaluated.
Components on a Si—Ge basis, such as, for example, infrared photodetectors, field effect transistors and photonic mixing detectors, are used, in particular, in optoelectronics. These components are optimized with respect to their properties, such as quantum efficiency at low dissipation power and low noise, essentially via the optimization of the Si—Ge alloy layers present. Optimization means in this respect that an alloy layer grows with a predetermined Ge content, for example, as buffer layer in as relaxed a manner as possible, i.e., is strained as little as possible or grows in a suitable and relaxed way.
It is known from the article “Raman scattering analysis of relaxed GexSi
1-x
alloy layers” of P. M. Mooney et al., Appl. Phys. Lett. 62 (17), 2069 (1993) for the portion of Ge in an alloy layer to be ascertainable via the ratio of the integrated intensities of the Si—Si mode to the Si—Ge mode.
The article “Measurements of alloy composition and strain in thin Ge
x
Si
1-x
layers” of J. C. Tsang et al., J. Appl. Phys. 75 (12), 8098, 1994 describes, in particular, in conjunction with
FIG. 6
therein a method as to how the specified intensities can be ascertained.
The Si—Si mode is attributable to phonon excitations on account of Si—Si oscillation movements, the Si—Ge mode to Si—Ge oscillation movements. In this respect, these are LO/TO phonons at k=0 in the crystalline SiGe.
Proceeding on this basis, the object underlying the invention is to improve the method of analysis specified at the outset such that any strain and any Ge portion in an alloy layer can be ascertained in a simple and as exact a manner as possible.
This object is accomplished in accordance with the invention, with the method specified at the outset, in that one or more spectrum contributions lying outside the Si—Ge modes and the Si—Si modes are evaluated as oscillation modes (i.e. vibration modes).
Intermediate modes and/or additional modes in Si cover layers or Si intermediate layers are not, therefore, considered as background in accordance with the invention but rather as a specific spectrum contribution. In accordance with the invention, the Raman spectrum is, when fitted, composed of a plurality of mode lines, namely, in particular, of the Si—Si mode, the Si—Ge mode, the intermediate modes and the cover layer modes and/or intermediate layer modes. As a result, on the other hand, the line profiles relevant for determining the strain via the shift in the Raman frequency and the line profiles required for determining the Ge concentration can be read selectively from the spectrum. Therefore, an optimized peak profile analysis may be carried out by means of the inventive method of analysis in order to obtain profile and position of the Si—Si mode and Si—Ge mode with minimum error.
Furthermore, the influence of lattice dislocations, cover layers and intermediate layers may be ascertained explicitly in order to obtain in this way an exact profile of the Si—Si mode and Si—Ge mode.
Complex Si—Ge alloy layers, which comprise a Si cover layer in addition to a Si—Ge alloy layer or corresponding layer sequences and/or one or more inserted Si intermediate layers which can, in particular, also be strained, may, in particular, be analyzed in accordance with the invention. The Si—Si mode of a cover layer or intermediate layer has a different frequency position to the Si—Si mode of a Si—Ge alloy layer. As a result of the inventive procedure, the profile and the peak position of the Si—Si mode and the Si—Ge mode may be determined with great precision even with the presence of such cover layers or intermediate layers in order to, on the other hand, be able to carry out a concentration analysis of Ge and relaxation determination.
As a result of the inventive method of analysis, a Raman spectrum can, in particular, be evaluated very quickly, i.e., the corresponding results of analysis are available very quickly. As a result, it is possible, on the other hand, to carry out measurements at short time intervals. A layer which has been produced may, in particular, be analyzed instantaneously during a coating process. As a result, it is, again, possible to influence the coating process accordingly in order to obtain an optimized overgrowth of layers on a substrate.
The intermediate modes are, in particular, local Si—Si oscillation modes. The above-mentioned Si—Si mode is brought about by way of Si—Si movement in the Si—Ge alloy. The above-mentioned Si—Ge mode is brought about by way of the Si—Ge movement in the Si—Ge alloy. These are bulk modes, wherein bulk modes are used in this case without any particular designation. The intermediate modes are, in particular, local Si—Si modes which result due to compositional dislocations. The phonon structure is modified by deviations from the perfect crystal lattice or due to defects. They have, in this case, an addition, such as, for example, “local” mode. Cover layers and intermediate layers (in particular, consisting of Si) also have a phonon structure which differs from that of a Si—Ge alloy layer.
The Si—Si mode and the Si—Ge mode are fitted, in particular, by way of an asymmetric curve. An intermediate mode is, on the other hand, fitted by way of a symmetric curve.
A reliable and rapid analysis of a Si—Ge alloy layer may be achieved when a fit spectrum which consists of a plurality of individual fit curves is fitted to the measured spectrum. In this respect, each individual fit curve is, in particular, a symmetric curve and, in addition, it is favorable when each individual fit curve is a Gauss-Lorentz curve. A Gauss-Lorentz curve thereby consists of the product of a Lorentz curve and a Gaussian curve.
It has proven to be advantageous when the Si—Si mode is fitted by way of three individual fit curves while the Si—Ge mode is fitted by way of two individual fit curves.
It has, furthermore, proven to be advantageous when an intermediate mode is fitted by way of a single fit curve.
As a result of such fits a fit spectrum is obtained which has minimal errors, for example, ascertained via a &khgr;
2
test, in relation to the measured spectrum. The parts, in particular, of the Si—Si mode and the Si—Ge mode of the alloy may, in particular, be separated out from such a spectrum in order to be able to carry out a rapid and reliable evaluation.
Furthermore, a background is deducted from the measured spectrum in order to eliminate parts of the spectrum not caused by Raman scattering (and, in particular, parts of the spectrum caused by Rayleigh scattering).
A concentration x of Ge in the Si—Ge alloy is determined in accordance with the formula
I
(
S
i
-
S
i
)
I
(
S
i
-
G
e
)
=
A
1
-
x
2
x
,
wherein I(Si—Si) is the integrated intensity of the Si—Si mode, I(Si—Ge) is the integrated intensity of the Si—Ge mode and A is a parameter dependent on the Raman spectroscopy device. This formula is also designated as Mooney formula. The Ge content in a Si—Ge alloy layer may be determined by means of this formula from measured parameters, namely the profiles of the corresponding modes, and also optimized accordingly. The profile of the relevant modes may, on the other hand, be determined by the inventive method of analysis in a reliable and exact manner, namely at short time intervals. As a result, an “in situ determination” of the Ge concentration is possible. The degree of relaxation within a Si—Ge layer may, on the other hand, be determined from the Ge concentration ascertained and the ascertainment of the strain via the ascertainment of the shift in frequency of the Si—Si mode.
The parameter A may be determined from comparative measurements, such as SIMS, XRD or EDX. As a result, a specified Raman spectroscopy device may, again, be calibrated in order to facilitate the direct deter
Deutsches Zentrum fuer Luft-und Raumfahrt e.V.
Evans F. L.
Geisel Kara
Lipsitz Barry R.
McAllister Douglas M.
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