Stress and Relief of Misfit Strain of Ge/Si(001)

Abstract

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...

Alternative mechanism for misfit dislocation generation during high-temperature Ge(Si)/Si (001) island growth

“…Based on the fact that the highest stress in an island occurs at the corner where the island meets the substrate, it has been suggested that strain-relieving perfect misfit dislocations are generated at the island edge when the island reaches its critical size. [3][4][5][6] Their generation and morphology have been considered theoretically. [7][8][9] In addition, partial misfit dislocations have been identified in islands by scanning tunnelling microscopy 10 and transmission electron microscopy ͑TEM͒ 11-13 and a recent theoretical study 14 has concluded that partial misfit dislocations are energetically preferred to perfect dislocations in some regions of quantum dots.…”

Alloying of self-organized semiconductor 3D islands

“…Further pathways to strain relaxation emerge when the constraint of perfectly coherent heterostructures is dropped. In this case, the formation of point-and particularly line-like defects of the lattice, as vacancies or interstitials and dislocations, can be very efficient ways of releasing the accumulated strain energy [47][48][49]. Here we shall however confine our discussion to the case of strict epitaxial coherence.…”

Ultra thin films of nanocrystalline Ge studied by AFM and interference enhanced Raman scattering

“…3 Depending on the surface orientation of Si and substrate temperature (T s ), the critical thickness and the shape of the islands are found to vary. The critical thickness is ~ 3-7 monolayers (ML) for Ge on Si(001) 4 whereas it increases up to 10 ML for (015) surface. 5 The 3D islands can be of different shapes like triangular on Si(111) 6 , rectangular on Si(001) (at T s = 700°C) 4 and hut clusters on Si(015).…”

“…The critical thickness is ~ 3-7 monolayers (ML) for Ge on Si(001) 4 whereas it increases up to 10 ML for (015) surface. 5 The 3D islands can be of different shapes like triangular on Si(111) 6 , rectangular on Si(001) (at T s = 700°C) 4 and hut clusters on Si(015). 5 Bimodal distribution of the islands were also observed for the Ge film prepared on Si(001) at a T s of 600°C.…”

“…1 The strain in the initial layers of Ge have been measured by AFM force measurements on Si(111) 6 and on Si(100). 4 Waltz et al 6 have observed compressive stress ~ 6 GPa for Ge film of 0⋅8 nm thickness. TEM studies have also been performed to study the initial growth of Ge films on Si.…”