The intrinsic stress of the Stranski-Krastanov system Ge͞Si(001) was investigated in the range 700-1050 K. Characteristic stress features indicate that the relief of the misfit strain proceeds mainly in two steps: (i) by the formation of 3D islands on top of the Ge wetting layer and (ii) via misfit dislocations in larger 3D islands and upon their percolation. The temperature dependence of strain relief by 3D islands as well as their nucleation and growth behavior support a kinetic pathway for 3D islanding.[S0031-9007 (98)05549-5] PACS numbers: 68.55.Jk, 68.60.Bs, 81.15.HiThe growth of Ge on Si substrates has attracted considerable interest in the past because of their importance for the fabrication of optoelectronic devices (based, e.g., on strained-layer superlattices or uniform quantum-dot arrays) as well as of Si-based high speed transistors in GeSi alloys. On Si(001) film growth proceeds by SK (StranskiKrastanov) mode, i.e., 3D islands nucleate on top of a several ML (monolayer) thick wetting layer. At temperatures below 800 K the initial 3D islands preferentially are small square pyramids with ͕105͖ facets [1-5], which due to their regular shapes were named as "hut" clusters [1]. At higher temperatures larger ("macroscopic") islands with more complex facet structures are formed [1,6,7]. Interestingly, up to lateral dimensions as large as 100 nm the 3D islands of Ge grow coherent on the substrate, i.e., dislocation free [6,8]. Analogous growth behavior has also been observed in other strained systems (e.g., [9]).By now it is still discussed controversially, why the growth of Ge͞Si(001) as well as of SK systems, in general, suddenly switches from 2D to 3D. The prevailing explanation is based primarily on energetic arguments [10]. Because of the larger unit cell dimensions of Ge a pseudomorphic layer on Si (001) is compressed by about 4.1%, which according to bulk elasticity gives rise to misfit stress of 26.0 GPa. In the pseudomorphic growth stage the elastic energy therefore increases linearly with thickness and destabilizes the film. Compressive stress of 25.65 GPa indeed was observed in the wetting layer by Schell-Sorokin and Tromp [11] at 770 K. As predicted by the thermodynamic models and confirmed recently by x-ray diffraction [12], the 3D islands partially relax the misfit strain because upper layers are free to adjust their interatomic distances to the respective equilibrium values. According to these models the driving force for 3D islanding is the reduction of the elastic energy which compensates the additional cost of free surface energy [10]. In addition to the thermodynamic explanation there is increasing evidence for kinetic processes to be involved in the process of 3D islanding. For instance, Mo et al. [1] speculated that the hut clusters represent a metastable intermediate state for the formation of macroscopic islands. Various studies emphasize the importance of the misfit strain field, which may influence nucleation and growth of the 3D islands [1,13] and suppress their coalescence [14] or f...
i3ber die Temperaturabh/ingigkeit der spektralen lichtelektrischen Elektronenemission halbleitender Wismutfilme* Won R. SUHRMANN, G. WEDLER und E.-A. DIERK Mit 8 Figuren im Text (Eingegangen arn g. Juli 1958) Es wird die spektrale photoelektrische Empfindlichkeit dfinner im Hochvakuum aufgedampfter Filme aus reinem und aus mit Tellur dotiertem Wismut im ungeordneten und geordneten Zustand gemessen. --Die Auswertung der nach Fow-LER aufgetragenen Empfindlichkeitskurven ergibt folgendes : Das Austrittspotential q3(Bi)9oo der ungeordneten Filme ist 4,223V, das der geordneten q)(Bi)293o= 4,288 V im Mittel. Die geordneten Filme weisen bei 90 ~ K zwei Austrittspotentime ~1(Bi)9oo=4,245 und #2(Bi)90o= 4,370 V auf. q)l wird einer Emission aus dem Leitungsband, ~2 aus dem Valenzband zugeschrieben. --Mit zwei bis drei Atomprozent Tellur dotierte Bi-Filme zeigeI1 im Gegensatz zu den reinen Filmen einen metallischen Widerstands-Temperaturkoeffizienten. Bei ihnen finder man nut eine Emission aus dem Leitungsband. Die entsprechenden Austrittspotentiale sind: q~(Bi, Te)90o des ungeordneten Films 4,316 V, q~(Bi, Te)9oo des geordneten 4,802 V, q} (Bi, Te)293o = 4,849 V. Die Potentialwerte werden im Zusammenhang mit dem B/~ndermodell diskutiert.
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