HAL Id: jpa-00226723
https://hal.archives-ouvertes.fr/jpa-00226723
Submitted on 1 Jan 1987
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
CHARACTERISATION OF NANOMETER-SCALE EPITAXIAL STRUCTURES BY
GRAZING-INCIDENCE X-RAY SCATTERING
T. Ryan, C. Lucas, P. Hatton, S. Bates
To cite this version:
T. Ryan, C. Lucas, P. Hatton, S. Bates. CHARACTERISATION OF NANOMETER-SCALE EPI- TAXIAL STRUCTURES BY GRAZING-INCIDENCE X-RAY SCATTERING. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-109-C5-111. �10.1051/jphyscol:1987519�. �jpa-00226723�
JOURNAL DE PHYSIQUE
Colloque C5, supplkment au noll, Tome 48, novembre 1987
CHARACTERISATION OF NANOMETER-SCALE EPITAXIAL STRUCTURES BY GRAZING- INCIDENCE X-RAY SCATTERING
T.W. RYAN, C. LUCAS', P.D. HATTON* and S. BATES'
Philips I and E, Lelyweg 1, NL-7602 EA Almelo. The Netherlands '~epartment of Physics, University of Edinburgh,
GB-Edinburgh EH9 3JZ, (Scotland) Great-Britain Abstract
By the use of grazing incidence scattering geometries and a triple- crystal x-ray diffractometer, x-ray scattering can be used to obtain structural information on ultra-thin heteroepitaxial layers.
In this paper the experimental techniques are briefly outlined and illustrated by measurements from a 200 1 thick AlInAs layer grown on an InP substrate.
INTRODUCTION
X-ray rocking curve analysis is a powerful and non-destructive tool for the characterisation of heteroepitaxial layer structures ( 1 ) . Information can be obtained on lattice parameter, structure factor and crystal quality with depth. In the conventional, symmetrical scattering geometry the penetration depth of the x-ray beam is of the order of 10 microns and the technique lacks sensitivity for single layers of less than 0.1 micron. However, it is possible to enhance the sensitivity of an x-ray scattering measurement to extremely thin surface layers by the adoption of grazing incidence scattering geometries, to limit the penetration depth, and by the use of a diffracted-beam analyser crystal to help resolve very weak scattering features from the instrumental background noise. In this paper we describe how a combination of grazing incidence diffraction and specular reflectivity measurements can be used to obtain structural information on epitaxial structures less than l00A in thickness using a conventional laboratory x-ray source.
METHOD
In two recent publications we have described the use of grazing incidence x-ray scattering methods to study very thin semiconductor structures (2,3). For structures grown on InP or GaAs substrates the (044) .Brag9 reflection satisfies the conditions for a highly- assyrnetric, glancing-incidence scattering measurement. The incidence angle is approximately 3 O for InP and 5 O for GaAs, limiting the penetration depth and enhancing the relative contribution of thin surface layers by up to a factor of 10 over the symmetricd inci- dence case. The use of the triple-crystal spectrometer provides additional gains in sensitivity. In contrast to the, more usual, double crystal instrument the triple crystal spectrometer has very good resolution in two dimensions in reciprocal (diffraction) space.
It is possible, with this resolution, to map out the distribution of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987519
JOURNAL DE PHYSIQUE
scattered intensity around a reciprocal lattice point and to distinguish the very weak scattering arising from a thin layer structure. For thin layers (<1000A), the interaction with the incident x-ray beam is very weak and so interpretation of the measured scattering can be carried out using a simple Kinematical model (4).
The second technique, specular x-ray reflectivity, probes the variation in refractive index of the sample material with depth, and so is sensitive to the presence of amorphous, as well as crystal- line, layers. We have recently described the application of this technique, using a triple crystal x-ray spectrometer, to the study of oxide layers on silicon (3). Despite its long history as a method of thin film investigation, it has received comparatively little attention as a characterisation tool for epitaxial layer structures.
RESULTS
The experiments were carried out using the triple crystal x-ray spectrometer at Edinburgh University. The x-ray source was a rotating anode generator, operating at 3kW. A Ge (111 ) crystal was used to monochromate and collimate a C u k , incident beam and the diffracted beam was collimated by an identical Ge crystal. The sample crystal was grown by MBE and was, nominally a 200E thick AlInAs layer on an InP substrate, with a thin ( < 100f) GaAs capping layer.
Finite lattice effects in heteroepitaxial layer structures give rise to a rod of scattering in reciprocal space around bulk reciprocal lattice points (2). This rod of diffuse scattering lies in a direction normal to the plane of the layers. The triple crystal spectrometer allows one to distinguish this scattering from the backgound. The intensity distribution of such a diffuse scattering rod around the InP (044) Bragg reflection in our sample is shown in figure 1. This scan bears some correspondence to a conventional angular rocking curve but note, the horizontal axis is given in reciprocal lattice units, a*, where a*= and (a) is the lattice constant perpendicular to the layer. a
Apart from the obvious substrate Bragg peak (044), a weak, broad peak can be seen. This is the AlInAs layer peak. From the position of the peak we can calculate the lattice parameter mismatch,
% =
-0.0045, between the layer and the substrate. The breadth of the peak gives an indication of the layer thickness. But note that, even over the intensity range shown in this scan, five orders of magnitu- de, the layer peak is a weak, diffuse feature.
Figure 2 shows the measured specular reflectivity as a function of incidence angle, 8. It is immediately obvious that this measurement contains a wealth of information. The fringes, which are visible out to tenth order, result from interference between the partially reflected beams from the surface and the internal interface(s). We have simulated this data, based upon an extension of the model described previously and have determined the AlInAs layer thickness to be 211(2) the de sity difference to be
lower electron density,
=-0.11(1). A surface 50(3)8 in thickness, with a slightly
Y
'9
=-0.17(3), is responsible for the visible modulation of the fringe intensities. The measured density difference of the epilayer, b p =-0.11, corres onds to a composition of A10.57In0.43As. The compgition of the 5 0 9 surface layer is not clear. The lattice mismatch between InP and GaAs is, however, very large and it is possible that this layer is composed of partially oxidised, polycrystalline GaAs.Figure 1. X-ray scattering intensity Figure 2. The intensity of the along the direction (OOl), the specularly reflected x-ray hcam surface normal, around the InP (0,4,4) as a function of incidence angle, 8.
Bragg peak.
CONCLUSIONS
Using a laboratory x-ray source and a triple-crystal x-ray spectro- meter, we have shown that it i possible to obtain structural infor- mation on epitaxial layers 2001, or less, in thickness. The diffrac- tion and specular reflectivity measurements provide comlementary information. The x-ray techniques are non-destructive and the inter- pretation of the results is quite straightforward.
ACKNOWLEGDEMENTS
This work was carried out in collaboration with Dr. B.K. Tanner and Mr. S. Myles of Durham University, U.K.
REFERENCES
1. Bartels W.J. and Nijman W (1978). J. Crystal Growth 44 518-525
2. Ryan TW, Hatton PD, Bates S, Watt M, Sotomayor-Torres C, ClaxtOn PA and Roberts J S (1987) Semicond. Sci Technol. 2 241-243
3. Cowley RA and Ryan TW (1987) J. Phys. D: Appl. pkys. 20 61-68 4. Bates S, Hatton P.D. and Ryan T.W. to be published J. Phys. C:
Solid State Phys.
