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Bi: Perfect surfactant for Ge growth on Si?111??

Bi: Perfect surfactant for Ge growth on Si?111??

Bi: Perfect surfactant for Ge growth on

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 10 8 MARCH 1999 ong>Biong>: ong>Perfectong> ong>surfactantong> ong>forong> ong>Geong> ong>growthong> on Si111…? T. Schmidt, a) J. Falta, b) and G. Materlik Hamburger Synchrotronstrahlungslabor HASYLAB am Deutschen Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, ong>Geong>rmany J. Zeysing, G. Falkenberg, and R. L. Johnson II. Institut für Experimentalphysik, Universität Hamburg, D-22761 Hamburg, ong>Geong>rmany Received 16 November 1998; accepted ong>forong> publication 12 January 1999 We have investigated the ong>growthong> of ong>Geong> on ong>Biong>-terminated ong>Biong>:Si111-)). In-situ measurements of x-ray standing waves, crystal truncation rods and scanning tunneling microscopy clearly show that, at substrate temperatures around 485 °C, smooth and homogeneous ong>Geong> films of thicknesses exceeding 30 bilayers ong>Geong> can be grown. For ong>Geong> coverages larger than 10 bilayers, the ong>Geong> film is completely relaxed. ong>Biong> is found to segregate to the surface during ong>Geong> deposition, and can be removed from the surface after ong>growthong> by mild annealing at 520 °C as proven by Auger electron spectroscopy. © 1999 American Institute of Physics. S0003-69519903310-0 Surfactant mediated epitaxy SME 1 of semiconductor structures has attracted considerable interest recently because it enables the ong>growthong> of structures which are not achievable by conventional molecular beam epitaxy MBE or chemical vapor deposition CVD. For ong>growthong> of ong>Geong> on Si111 so far Sb 2 and Ga 3–5 have been reported to successfully act as ong>surfactantong>s. The use of Ga is accompanied by the ong>forong>mation of stacking faults 3 and is of little technological interest. However, ong>growthong> of ong>Geong> on the low Ga coverage )) surface may be interesting ong>forong> intentional lateral patterning. 4 Surfactant mediated epitaxy of ong>Geong> on Si111 is especially promising ong>forong> the production of high speed devices due to the existence of an effective relaxation mechanism producing a dislocation network at the Si-ong>Geong> interface 6 and a defect free ong>Geong> film ong>forong> optimum device quality. However, so far most work has concentrated on Sb as a ong>surfactantong>. 2,6–8 Here we present the results of a detailed study on the properties of ong>Geong> ong>growthong> on ong>Biong>-terminated Si111 substrates. Although ong>Biong> has been shown to fail as a ong>surfactantong> ong>forong> ong>growthong> temperatures as low as 300 °C, 9 we will show that at higher temperatures around 485 °C ong>Biong> enables ong>growthong> of very smooth and homogenous ong>Geong> films. A major drawback of SME is the incorporation of the ong>surfactantong> in the growing film, which results in an undesirable doping of the ong>Geong> layer. Because of the very low solid solubility of ong>Biong> in Si and ong>Geong> about three orders of magnitude lower than ong>forong> Sb, very low dopant concentrations are expected ong>forong> ong>Biong>-SME. Furthermore, we will show that ong>Biong> can be removed effectively from SME grown films by mild annealing at 520 °C. The samples were prepared in an ultrahigh vacuum UHV system with a base pressure of 310 11 mbar, equipped with a ong>Geong> and a ong>Biong> Knudsen cell as well as a low energy electron diffraction LEED system and a cylindrical mirror analyzer ong>forong> Auger electron spectroscopy AES. After preparation, the samples were transferred under UHV conditions into portable UHV chambers. These chambers had a base pressure of 510 10 mbar or better and were a Electronic mail: thomas.schmidt@desy.de b Electronic mail: falta@desy.de equipped with 400 m thick Be windows, allowing x-ray experiments. Scanning tunneling microscopy STM measurements were perong>forong>med in a separate UHV system. For Si111 substrates 10102 mm 3 crystals were used. After polishing and RCA chemical cleaning, the samples were introduced to UHV and degassed ong>forong> at least 24 h at 600 °C in order to remove any contamination. For x-ray standing wave XSW experiments, special care was taken to avoid any damage to the bulk crystal structure. Thereong>forong>e the temperature was varied at a rate 1 °C/s only. Si111-77 surfaces were reproducibly prepared by annealing at 850– 900 °C ong>forong> 5–10 min. Prior to ong>Geong> deposition, ong>Biong> was adsorbed onto the surface at 485 °C. This resulted in the ong>forong>mation of the ong>Biong>:Si111-))- surface. This structure consists of ong>Biong> in T 1 sites at a saturation coverage of 2/3 ML 10 1 monolayer ML7.8310 14 cm 2 ). During ong>Geong> deposition the ong>Biong> flux of 1–2 ML/min was maintained in order to compensate ong>forong> ong>Biong> desorption which is not negligible at this ong>growthong> temperature. As on Si, ong>Biong> occupies T 1 sites on top of ong>Geong>, as shown by XSW at low (1 ML) ong>Geong> coverages. 11 The lattice constant of both a pseudomorphically strained as well as a relaxed ong>Geong> film is larger than the lattice constant of Si. Thereong>forong>e in XSW one expects a decreasing contrast in the fluorescence yield with increasing film thickness. 12 For the 111 Fourier component of the ong>Geong> distribution function A 111 the calculation gives A111 1 N N1 n0 1 N sin 2N sin 2 exp2in 0 2 2 exp 2i N1 2 0. Here, denotes the relative difference in lattice spacing between the film and the substrate; 0 is the offset at the interface and N is the number of bilayers. (&, depending on the strain accounts ong>forong> the buckling within the 111 planes. 0003-6951/99/74(10)/1391/3/$15.00 1391 © 1999 American Institute of Physics

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