Xray characterization of Si δdoping in GaAsL. Hart, M. R. Fahy, R. C. Newman, and P. F. Fewster Citation: Applied Physics Letters 62, 2218 (1993); doi: 10.1063/1.109421 View online: http://dx.doi.org/10.1063/1.109421 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/62/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Si δ-doped GaAs structure with different dopant distribution models J. Appl. Phys. 91, 2118 (2002); 10.1063/1.1424051 Effects of As flux on Si δdoped GaAs J. Vac. Sci. Technol. B 11, 905 (1993); 10.1116/1.586735 Nonlinear dependencies of Si diffusion in δdoped GaAs Appl. Phys. Lett. 59, 1452 (1991); 10.1063/1.105285 Photoreflectance measurements on Si δdoped GaAs samples grown by molecularbeam epitaxy J. Appl. Phys. 67, 4149 (1990); 10.1063/1.344976 Incorporation of Si in δdoped GaAs studied by local vibrational mode spectroscopy Appl. Phys. Lett. 55, 978 (1989); 10.1063/1.101695

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X-ray characterization of Si S-doping in CaAs L. Hart, M. R. Fahy, and R. C. Newman Interdisciplinary Research Centre. for Semiconductor Materials, Imperial College, London SW7 2BZ, United Kingdom

P. F. Fewster Philips Research Laboratories, Cross Oak Lane, Redhill RHI 5HA, United Kingdom

(Received 1 October 1992; accepted for publication 10 February 1993)

High-resolution triple-axis x-ray diffractometry has been used to examine the structural properties of a S-doped superlattice of sixty periods, each consisting of half a monolayer of Si and 500 A of GaAs, grown by molecular beam epitaxy (MBE) at 400 “C under an arsenic flux. The measurements indicated that the superlattice was of high structural quality. Using dynamical simulation, it was demonstrated that the period variation was equal to 3%, while the silicon spreading was no greater than 2 monolayers. It was possible to extract this information because of the high-resolution diffractometer which produced the theoretical line shape and wide dynamic range. Using a simple model, it was deduced that virtually all Si atoms were located on Ga lattice sites.

There has recently been considerable interest in the epitaxial growth of thin layers of Si on GaAs and the sub- sequent overgrowth of the Si by GaAs,*?’ when significant Si surface segregation and diffusion could occur, depending on the growth conditions. Although secondary ion mass spectroscopy (SIMS) can be used to measure the concen- tration of Si in &doped layers, it has a limited sensitivity to the spreading of layers. As an alternative, high-resolution x-ray diffractometry can be used to obtain this information, together with variations in the superlattice period.3 Double-crystal x-ray diffractometry has been used to char- acterize thin layers4 and superlattice,? of Si in GaAs grown at 450 “C.

Recently, we have grown Si b-layers by molecular beam epitaxy on (001) GaAs with a substrate temperature of 400 “C to inhibit diffusion and subsurface segregation of the Si.6 Reflection high energy electron diffraction (RHEED) measurements implied that the Si atoms occu- pied surface Ga sites during deposition for layers up to half a monolayer thick.7 Subsequent measurements of the site occupancy made by localized vibrational mode (LVM) infrared absorption spectroscopy revealed only SiGa donors (Si on Ga sites);8 preliminary Raman scattering measure- ments gave the same result.’ These results are different from those reported in Refs. 4 and 5, but in those works, a higher growth temperature of 450°C was used and the arsenic flux was discontinued during Si deposition, increas- ing the possibility that As surface sites were occupied.

We now report the use of high resolution x-ray dif- fractometry combined with dynamical simulation to deter- mine the degree of structural perfection of a Si S-doped GaAs superlattice grown at 400 “C!. In particular, it is neo essary to separate the effects of superlattice period varia- tion from spreading of S-layers in the growth direction. Information of the atomic site distribution is also required and can be obtained by using quasiforbidden reflections such as the 002.

The sample was grown by MBE in a VG V80 system on a (001) GaAs wafer bonded to a molybdenum block

using iridium. A GaAs buffer layer of 1000 A was grown at a substrate temperature of 580 “C. A superlattice structure with 60 periods of half a monolayer of Si and 500 A GaAs was then grown at a substrate temperature of 400 “C!. The silicon S-doped layers were grown by switching from Ga to Si, while maintaining the arsenic flux. The sample was ro- tated throughout growth. Subsequent SIMS measurements showed that the Si S-layers were half a monolayer (the area1 density of Si atoms was 3.4X lOI cmw2).s Prelimi- nary transmission electron microscopy studies showed that the Si S-layers were pseudomorphic with the GaAs, with very few threading or misfit dislocations at the interfaces. (The dislocation densities were below the resolution limit of lo5 cm-z.)

.A high-resolution triple-axis x-ray diffractometer” with a 4-reflection Ge 220 monochromator and a Ge 111 analyzer was used to map the diffracted intensity in recip- rocal space around the 004 and 002 Bragg peaks, using Cu Kal radiation. The 0-2~’ scans (o and 20’ are the angels of the sample and analyzer with respect to the inci- dent x-ray beam) lo perpendicular to the diffraction planes were summed because of spreading of the peaks in w par- allel to the diffraction planes, due to local tilting. The ex- perimental data were then compared with a number of simulated diffraction profiles, calculated using x-ray dy- namical diffraction theory. *’

Periodic superlattice structures produce a series of in- terference peaks in the diffraction profile, with a spacing inversely proportional to the superlattice period. Varia- tions in the period cause broadening of these satellite peaks without affecting the integrated intensities, while interface spreading changes the intensities, particularly for the high- order satellites. This can be understood since the x-ray diffraction profile is simply related to the Fourier trans- form of the periodic structure. If the interfaces are abrupt, the modulation is rectangular, resulting in a large number of satellite peaks, whereas if there is Si diffusion or segre- gation to form, in the limit, a sine wave modulation, only the first-order satellite peaks will appear. The satellite peak

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12 32.5

Angle (degrees)

FIG. 1. Experimental data and simulated (heavy line) 004 diffraction FIG. 2. Experimental data and simulated (heavy line) 002 diffraction profile for 60 per iods of a monolayer S&,sG~,sAs and 512 A GaAs, profile for 60 per iods of a monolayer (Sio,sG%,s) (Vo.lzsAk,s75) and 512 A including a per iod variation of 3 % (high-angle satellites not shown). GaAs, including a per iod variation of 3%.

intensities depend on the modu lation of both the lattice parameter and the structure factor. Modu lation of the structure factor also produces an asymmetry in the inten- sities about the zero-order satellite.

A superposit ion of the experimental and simulated 004 diffraction profiles is shown in F ig. 1. Satellite peaks were observed out to the eighteenth order (only the low-angle peaks are shown). The satellite peak intensities are asym- metric about the zero-order peak, with higher intensities on the low-angle side. The central doublet consists of the zero-order interference peak from the 60 period superlat- tice, and the GaAs substrate peak, which is at a lower angle (larger lattice parameter). The relative positions of these two peaks depend on the difference between the av- erage lattice parameter of the-superlattice and that of the substrate, allowing the total amount of silicon and the av- erage thickness of each Si S-layer to be determined.

The simulated diffraction profile was calculated from a mode l with 60 periods of a mono layer (2.7 A) S&Gao,sAs and 5 12 A GaAs. The lattice parameter of Si,,G%,,As was determined by Vegard’s law from the lattice parameters of GaAs and SiAs, which was calculated from the covalent radii of Si and As.‘” The measured lattice parameters for III-V compounds such as GaAs were found to be around 0.3% higher than values calculated from covalent radii, so the value for SiAs was increased to 5.445 A. The simula- tion includes a period thickness fluctuation of 3%, which broadens the satellite peaks to fit the measured data. Sim- ulations also showed that there was no spreading of the Si S-layers beyond two mono layers in the growth direction (67% of the Si in- the S-layer and 33% in the next Ga plane). This spreading affected only the highest-order sat- ellite intensities and could only be measured because of the high signal to background ratio obtained. It can be seen in F ig. 1 that there is excellent agreement between the exper- imental and simulated diffraction profiles.

In the S&G%.sAs mode l, Si replaces Ga on half the group III sites in the S-layer and As atoms occupy all the group V sites. The mode l is consistent with LVM infrared absorpt ion measurements from samples with lower Si con- centrations, which revealed Si only on the Ga sites;8 Ra- man scattering measurements from samples similar to the one used here, again showed only Sio, impurities, al though

2219 Appl. Phys. L&t., Vol. 62, No. 18, 3 May IQ93

15.6

q 15.8 16 16.2 16.4

Angle (degrees)

the strength of the scattering was 10w.~ Since the 004 sat- ellite intensities depend predominantly on lattice parame- ter modu lation, and the covalent radii of Si and As are similar, an alternative mode l could be (Sio,sGao,s) (SissAso,s). However, mode ls with additional Si atoms on 0.1-0.5 of the As sites gave increasingly inferior fits, with very slightly higher satellite intensities than in the experi- mental data. These mode ls would also be inconsistent with the SIMS measurements and Sio,-Si,, pairs should have been detected in the LVM spectra.* A simulation using GaSiesAso.s produced a very different profile, with negligi- ble peak splitting and much lower satellite intensities than in the experimental data, confirming that the Si was not on As sites.

Experimental and simulated data for the 002 reflection are shown in F ig. 2. The experimental data were collected without using the analyzer crystal because of the very weak satellite intensities. F ive orders of satellite peaks could be seen on the high-angle side and one on the low-angle side. The 002 satellite intensities depend predominantly on the structure factor modu lation and give information on the atomic site distribution, thus imposing further constraints on the mode l which are not required to explain the 004 case. A simulation calculated using the original mode l re- sulted in too great an asymmetry of the satellite peaks (by a factor of -2), with higher intensities on the high-angle side than in the experimental data. The mode l was mod i- fied to give the simulation shown by decreasing the scat- tering factor on the arsenic sites, keeping the lattice param- eter constant so that the 004 profile remained unchanged. This could be explained by the presence of arsenic vacan- cies ( V,,), al though this would seem unlikely considering the arsenic overpressure during growth. Another physical mode l, proposed in our previous paper,s involves displace- ment of alternate Si atoms in the DX-configuration to ac- count for the loss of carriers, al though it is not clear how to accommodate this in the x-ray mode l. W e conclude that a unique mode l cannot be specified and further measure- ments are required to elucidate the structure of the Si &layers.

In summary, it has been shown that high-resolution x-ray diffractometry can be used to determine the degree of structural perfection of a Si S-doped GaAs superlattice

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grown at 400 “C. It was found that Si occupied half the Ga sites in the S-layer, with spreading no greater than two monolayers. It has been possible to extract this information because of the high-resolution triple-axis diffractometer used in this study, which produces experimental data with line shapes close to those expected theoretically, and a wide dynamic range.

The authors would like to thank C. Roberts (IRC) for preparation of the GaAs substrates, N. L. Andrew (PRL) for help with the x-ray measurements and J. Wagner (Fraunhofer Institut, Freiburg ) for communications of un- published results. The Science and Engineering Research Council UK, Imperial College and the Research Develop- ment Corporation of Japan under the auspices of “Atomic Arrangement: Design and Control for New Materials” Joint Research Programme, are gratefully acknowledged for their financial support.

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Published without author corrections

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