Material Intermixing

The furnace crystallization of sublayers from the "low-temperature" material in a-Ge and a-Si based a-MLs, requires quite high annealing temperatures (>770 K for a-Ge:H [1] and >970 K for a-Si:H [23]). At these temperatures, undesired interdiffusion and alloying of the constituent materials may take place at ML interfaces. In certain studies on annealing of a-Ge:H/a-GeNx [101], a-Ge:H/ a-SiNx :H [103] and a-Ge/a-C [67] MLs alloying has not been observed. On the other hand, in a-Si:H/a-Ge:H MLs [111], the annealing at temperatures > 670 K leads to strong material intermixing, followed by the crystallization of the resulting silicon germanium alloy. Anisotropic Ge diffusion has been directly observed in SiGe/Si-strained superlattices [149] induced by nonuniform strain in SiGe/Si interface in the range 700-1000 °C. Additionly, it has been shown [150] that the Ge-Si interdiffusion increases by more than two orders of magnitude for stucked hut clusters and the degree of Si alloying in vertically aligned self-assembled islands increases with the number of stached layers. Interdiffusion has also been reported in annealed Si/SiC [112] MLs. A small region of mixed Ge-Si alloy at each interface has been observed in SiOx/GeOy MLs annealed 770 K [148].

Although thermal crystallization in chalcogenide a-MLs occurs at much lower temperatures than in a-Si- and a-Ge-based MLs, material intermixing has also been observed in MLs of various materials. The effect of annealing (at 340 K for 60 min) on photo and electrical conductivity, as well as on infrared absorption of As6Se94/Te, a-MLs has been studied in [70]. It has been observed that at the first step of the annealing process, a diffusion of Se atoms into Te sublayers occurred and As-Se/Se-Te a-ML formation took place. Further annealing leads to As-Se/As-Se-Te MLs formation. Systematic optical and electrical studies on photo-and thermo-induced changes of As2S3/Se, As2S3/SexTe1-x (0.1 < x < 0.5), and As6Se94/Se80Te20 a-MLs have indicated [80, 81] that the periodic structure of these MLs was changed by thermal treatment and laser beam exposure due to the interdiffusion. It should be pointed out that in this group of MLs, the corresponding photo-induced changes of their optical parameters (absorption, refraction) can be successfully used for optical recording. Raman scattering measurements have been carried out on as-deposited and annealed at 670 K multilayers of ZnSe/CdSe [151]. It has been shown that Znx Cd1-x Se interface regions were formed during sample preparation and a gradual change in the composition is characteristic for these regions. Raman spectra of ZnSe/CdSe MLs annealed at 670 K for 60 min are displayed in Figure 6. It can be seen that the intensity of the band at around 225 cm-1 (it is due to scattering from ZnxCd1-xSe) is rather high. This indicates that large ZnxCd1-xSe interface regions exist in annealed samples. The result has been explained assuming diffusion of Cd atoms from CdSe into ZnSe. It has been shown in this study that when the nominal thickness of CdSe layers is very small, taking advantage of material alloying at the interfaces, it is possible to produce ZnxCd1-XSe nanoparticles in a ZnSe matrix by means of thermal evaporation of CdSe and ZnSe in vacuum.

ZnSe/CdSe MLs

150 200 250 300 ¿If» 1 Raman shift (cmx) jf^X ■

150 200 250 300 ¿If» 1 Raman shift (cmx) jf^X ■

100 200 300

Raman shift (cm 1)

Figure 6. Raman scattering spectra of ZnSe/CdSe MLs with layer thicknesses indicated in the figure. The 647.1 nm line of a Kr+ laser was used for the excitation. Inset: Raman spectrum (solid line) fitted to one Lorentzian peaked at 212 cm-1 and two Gaussians at 180 cm-1 and 225 cm-1 (dashed lines).

Various results obtained under annealing of different amorphous MLs may be understood if one keeps in mind that several competitive factors can affect material intermixing and alloying at the ML interfaces. Diffusion coefficients of the atoms of each constituent material in the other one, and the possibility for alloying of both materials (i.e., the correlation between the magnitudes of the bond energies in the constituent materials and their alloy) could be among them. Hence, various approaches may be applied in order to avoid the layer intermixing. One of them is the selection of proper pairs of materials; the data reviewed imply that semiconductor/semiconductor oxide (or nitride) pairs are better than semiconductor/semiconductor ones. This suggestion is motivated by the results for nc-CdSe/SiOx [152], nc-Ge/a-GeNx [101], and nc-Ge/a-SiNx [103] MLs, in which no alloying has been observed and the considerable alloying reported for nc-CdSe/ZnSe, nc-CdSe/GeS2 [151, 153], nc-Ge/a-Si:H [111], etc. The second approach, which may be applied to avoid thermally induced sublayer intermixing, is the application of combined photo-thermal or "pure" photo-induced crystallization processes. As previously described [79, 86], a considerable diffusion of Se and Te takes place in chalcogenide materials upon long-time furnace annealing even at considerably low temperatures. In Se-based MLs, one may use the fact that "pure" amorphous selenium and, particularly, selenium-tellurium alloys exhibit strong photo thermally-induced crystallization at room temperature. Moreover, they crystallize under laser beam exposure at fairly low temperatures (<100 K), at which the thermally induced atomic diffusion is extremely slow. Thus, using photo crystallization at room and low temperatures, it is possible to make optical recording in amorphous chalco-genide MLs with minimum alloying at the ML interfaces. It has been shown [103] that crystallization of a-Ge in a-Ge:H/ a-SiNx :H MLs is a type of explosive process, caused by the high stress of the a-Ge:H, which is triggered by laser heating. In this situation, crystallization occurs for a very short time and no interface alloying has been detected. All given examples indicate that special attention should be paid to material intermixing, in particular, when furnace long-time thermal crystallization of a-MLs is applied, As for the short-time crystallization procedures (rapid thermal annealing and laser annealing), it seems that they do not deteriorate the interface quality and ML periodicity very much.

All silicon and germanium-based multilayers described in Section 3 are listed in Table 1 along with the techniques applied for their preparation and crystallization of the layers with the lower Tc. This information for chalcogenide-based MLs considered in Section 3 is summarized in Table 2.

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