Nuclear Instruments and Methods in Physics Research B 84 (1994) 218-221 North-Holland
Beam Interactions with Materials 6% Atoms
Formation of unstrained Si I_,Ge, layers by high-dose 74Ge ion implantation in SIMOX B. HolEnder, S. Mantl, W. Michelsen, S. Mesters, A. Hartmann and L. Vescan Inst.fiir Schicht- und Ionentechnik, Forschungszentrum Jiilich, Postfach 1913, D-51 70 Jiilich, Germany D. Gerthsen Inst. fiir Festkiirperjorschung, Forschungszentrum Jiilich, Postfach 1913, D-51 70 Jiilich, Germany
Single crystalline, strain relieved Si, mxGex layers were fabricated on buried, amorphous SiO, by high-dose 74Ge ion implantation into the Si surface layer of SIMOX structures followed by thermal treatment. Ion implantation was performed with energies between 80 and 160 keV and ion doses between 1.5 x 10” and 3 x 10” cm-‘. Rutherford backscattering spectrometry, He+ ion channeling and transmission electron microscopy were employed to characterize layer thickness, stoichiometry and crystalline quality of the newly formed Si, _,Ge, layers. The Ge concentration x of the resulting Si, _xGe, layer can be chosen by using an appropriate implantation dose. Depending on implantation’ dose and energy, homogeneous Si,_,Ge, layers with Ge concentrations between x = 0.13 and 0.29 were obtained. Strain measurements by ion channeling confirmed complete strain relaxation of the Si, _xGe, layer after annealing.
1. Introduction Si/Si,-,Ge, heterostructures have been successfully applied to a number of semiconductor devices. The electronic properties of this materials system can be tuned by changing the composition or the strain in the individual layers. Recently, fast Si-based heterojunction bipolar transistors have been fabricated using a strained Si,_,Ge, layer as a base [1,2]. The active layers must be grown pseudomorphically to avoid detrimental strain relaxation and the formation of crystal defects. Pseudomorphic growth of Si/Si,_,Ge, heterostructures on Si substrates can only lead to compressively strained Si,_,Ge, layers and unstrained Si layers. To fully exploit the range of electronic properties, e.g. for the fabrication of n-channel modulation doped structures, silicon layers under tensile strain are also necessary to obtain the proper band alignment . This requires the growth of relaxed Si,_,Ge, layers having a larger lattice parameter than Si, which serve as a virtual substrate. The basic problem of such layers is the formation of an array of misfit dislocations at the substrate interface, which usually launches a high density of threading dislocations extending to the surface of the buffer layer and leading to degradation of adjacent heterostructures. Recently, the crystalline quality has been considerably imprqved using thick, compositionally graded buffer layers [4,5]. In this paper, we investigate the formation of relaxed Si,-,Ge, on buried, amorphous SiO,. The
Si,_,Ge, layers were prepared by high dose 74Ge ion implantation into the Si surface layer of SIMOX (Separation by IMplantation of Oxygen) wafers and subsequent thermal treatment. In these structures, the buried, amorphous SiO, layer separates the lattice mismatched Si I_xGex layer from the Si substrate. The crystalline/ amorphous interface between Si 1_xGe, and SiO, should act as a free surface in terms of misfit dislocation production. In addition, this concept extends the SIMOX technology also to the SiGe material system. Rutherford backscattering spectrometry (RBS) and ion channeling were used to analyze layer thicknesses, stoichiometry, implantation profiles and crystallinity. RBS spectra were evaluated using the RUMP code . The microstructure was investigated by transmission electron microscopy @EM).
2. Experimental We used commercial, [loo] oriented 4 in. SIMOX wafers, n-type, lo-25 s2 cm . The Si surface layer had a thickness of about 2000 A. 74Ge+ ion implantation was performed with energies between 80 and 160 keV and beam currents of typically 10 FA over an area of 4 cm’. Elevated implantation temperatures between 350 and 450°C were chosen to avoid amorphization and to enhance defect annealing . RBS and channeling measurements were done with 1.4 MeV He+ ions
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0377-S
B. Hokinder et al. / Formation of unstrained Si,
and a scattering angle of 170”. Random spectra were collected while continuously rotating the sample around the surface normal to avoid channeling effect:. The implanted samples were protected with a 2000 A SiO, layer prior to rapid thermal annealing (RTA), which was stripped before RBS analysis. RTA was performed under flowing argon. Photoluminescence (PL) was measured at 4.2 K using a Fourier transform spectrometer with a liquid nitrogen cooled Ge p-i-n diode as a detector. The samples were excited by Ar+ laser light with a power of _ 5 W/cm’.
Fig. 1. Cross section
3. Results and discussion Fig. l(a) shows a cross section TEM micrograph (XTEM) of a SIMOX wafer after 100 keV Ge ion implantation with a dose of 3 x 1Or7 cme2 at 450°C. Before annealing, the Si surface layer exhibits an implantation-induced defect structure. After rapid thermal annealing at 1290°C (fig. l(b)) the resulting, homogenized Si,_,Ge, layer shows a sharp interface to the amorphous SiO, layer. The defect structure of the resulting Si,_,Ge, layer consisted mainly of stacking
of a SIMOX wafer after 100 keV 74Ge ion implantation (a) and after(b) rapid thermal annealing at 1290°C.
with a dose of 3 x 1017 cm-2
B. Hokinder et al. / Formation of unstrained Si, _ rGex layers
b 1 .o ,
Fig. 2. Random and aligned RBS spectra after 100 keV 74Ge ion implantation with a dose of 3 X 1017 cm-’ before (a) and after (b) rapid thermal annealing at 1290°C.
faults and dislocation loops, as judged from TEM investigations. The contrast in the upper part of the SiO, layer is presumably due to Ge precipitation during annealing, resulting from the long range tail of the Ge ion implantation. The size of these precipitates increases with decreasing distance to the interface. Fig. 2(a) shows random and [lOOI aligned RBS spectra of the sample in the as-implanted state. Surface backscattering energies for Si and Ge are marked. The Ge implantation peak and the corresponding decrease of the height of the Si signal are clearly visible. Because of sputtering, the Ge implantation profile extends up to the surface. Although the sample was heated up to 450°C during implantation, channeling is rather poor due to radiation damage. The corresponding spectra after rapid thermal annealing at 1290°C for 30 s are shown in fig. 2(b). The Ge depth profile is now completely homogeneous, restlting in a Si,,,Ge,, layer with a thickness of 1800 A. The SiO, layer acts as a perfect diffusion barrier and the Ge depth distribution shows a sharp cut-off at the SiO, interface. This is in contrast to direct ion implantation in Si(100) wafers, where the Ge distribution shows a pronounced tail after recrystallization . The channeling minimum yield xmin decreased to about 9% after RTA, indicating a reasonable crystalline quality of the alloy layer. No significant peak at the Ge low energy edge is visible. These results suggest that the dislocation structure in Si,_,Ge, on buried SiO, differs considerably from the heteroepitaxial growth of Si,_,Ge, directly on Si. PL measurements shown in fig. 3 exhibit a broad alloy peak and a sharp peak at 0.789 eV, which may be attributed to residual implantation damage [lO,ll]. No indication of the misfit dislocation related Dl and D2 lines has been observed. In fact, Dl and D2 lines, which originate from misfit dislocations at the interface
between Si,_,Ge, and Sl(100) and from dislocations injected into the Si substrate are not expected in these structures . The interface between Si,_,Ge, and the amorphous SiO, was investigated by high-resolution-TEM (HRTEM). Fig. 4 shows a HRTEM micrograph of this interface after 100 keV implantation with a dose of 1.7 x 101’ cm-’ and RTA at 1320°C for 30 s. layer had a thickness of 1800 The resulting Si,,,,,Ge,,,, A. The HRTEM investigations exhibit a sharp interface with a roughness of a few moaolayers over a length scale of the order of 1000 A. The residual tetragonal strain in the Si,_,Ge, layer has been determined by ion channeling. The results of these measurements show that the Si,_,Ge, layer on top of the amorphous SiO, is unstrained .
4. Conclusions Single crystalline, strain relieved Si, _xGex layers have been fabricated on amorphous SiO, by transformation of the Si surface layer of a SIMOX wafer into a
Fig. 3. PL spectrum after 100 keV 74Ge ion implantation with a dose of 3~ 1017 cm-’ and rapid thermal annealing at 129O”C,measured at T - 4.2 K.
B. Holliinder et al. / Formation of unstrained Si, _ .Ge,
Fig. 4. Cross section HRTEM micrograph of the interface between the Si, _$?ex layer and the amorphous SiO,.
Si, _xGe, alloy layer. The transformation was achieved into the Si surface by high dose 74Ge ion implantation layer and subsequent rapid thermal annealing. A 100 keV implantation with a dose of 3 X 101’ cm-* results layer with a thickness of 1800 A after in a %.&eo.Z~
RTA. The buried SiO, layer acts as a perfect diffusion barrier, therefore interdiffusion stops at the interface and thus excellent homogen~ation is achieved. The newly formed Si,_,Ge, layers show reasonable channeling with minimum yield xmin of about 9%. HRTEM investigations indicate that the interface between Si,_,Ge, and SiO, is sharp within a few monolayers on a length scale of some 1000 A.
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II. COMPOUND FORMATION