Optical on wafer measurement of Ge content of virtual SiGe-substrates Michael Oehme * , Matthias Bauer Institut fuer Halbleitertechnik, Universitaet Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany Abstract We developed a procedure for a fast and non-destructive determination of the Ge-content in virtual Si 12x Ge x -substrates in the range from x  0:2±1. The virtual substrates were grown in our MBE-system. The re¯ectivity of SiGe substrates depends on the Ge content x and on the crosshatch surface corrugations. The extremes in relative re¯ectance values are rather insensitive to surface corrugations contrary to the absolute values which depend signi®cantly on the surface morphology. The re¯ectance of ®lms was measured ex situ with a commercial measuring system (NanoSpec). This system measures relatively to a measurement standard wafer (Silicon-wafer) over wavelength range from 370 to 800 nm. The re¯ection graphs have maxima in the range from 410 to 590 nm depending on the Ge-content. Results obtained by these measurements with the NanoSpec measuring system were in a good agreement with SIMS-analysis and XRD data. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Si-MBE; Virtual substrate; SiGe; Ge-content; Optical re¯ectivity 1. Introduction SiGe/Si heterostructures have gained considerable atten- tion for both electronic and optoelectronic applications due their compatibility with existing Si technology [1±3]. The growth of high quality Si 12x Ge x epilayers on Si by MBE is of great interest both for device application and fundamental research. The crystal structure of SiGe layers on Si substrates is de®ned essentially by the lattice mismatch at the interface. The strain in these lattice-mismatched struc- tures can be adjusted by intermediate relaxed buffer layers with the proper lattice constant. Relaxed SiGe ®lms are favourable as intermediate lattice spacing buffers adjusting the lattice constants of the following SiGe active device layer. This combination of a substrate and a fully or partly relaxed buffer can be considered as a virtual substrate [4]. In this work we grew virtual SiGe substrates and measured ex situ, the optical re¯ectance spectra of the relaxed SiGe buffers for the determination of the Ge frac- tion. 2. Experimental details The thin relaxed SiGe layers for virtual substrates with a well de®ned Ge content were grown on (100) Si p(B) substrates by MBE. The experimental equipment and sample preparation are described in detail in Ref. [5]. The Ge fraction was varied between 0.2 and 1 and the thickness of the SiGe ®lms varied in the range of 200±300 nm. For the immediate veri®cation of SiGe layer composition in virtual substrates, we have developed a non-destructive rapid method based on the optical characterization. This method allows on wafer ex situ determination of Ge content in the layers by rather simple measurements of re¯ectivity using a commercially available NanoSpec instrument of Nanometrics. The NanoSpec measures surface re¯ectance relative to a measurement standard wafer (Silicon wafer) over wavelength range from 370 to 800 nm. Since the re¯ec- tance of SiGe alloys is a function of the Ge content x, it can thus be used as an aid to determine x. 3. Results and discussion The relationship between the re¯ectivity R, the refraction index n and the extinction coef®cient k is given for absorb- ing layers by: R  n 2 1   2 1k 2   = n 1 1   2 1k 2   Initially, on the basis of literature data extracted from measurements [6] the dependencies of the absolute re¯ec- tivity on the wavelength for various SiGe compositions were calculated. Relative re¯ectivities of various SiGe alloys (relating to Si) were calculated and plotted in Fig. 1. The relative re¯ectivity values obtain maxima in the visi- ble spectra regime. The wavelength of the maximum posi- Thin Solid Films 336 (1998) 347±349 0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-6090(98)01252-8 * Corresponding author.