Jessonet al. reply

Abstract

Jesson et al. Reply: The new structure (RS3) proposed by us in our Letter [1] is not identical to a structure RS2 (sometimes referred to as RH2) considered previously [2][3][4][5]. The phases have identical space groups (R3m) but different site occupancies as clearly indicated in our Letter [1]. LeGoues et al. [6] suggest that they did not discuss the difference between the phases because it is "small." However, we note that a 10% difference in site occupancy is readily measurable experimentally [1], so that the two phases are physically distinct. It is important to point out that LeGoues and co-workers have made no previous mention of any possible structure other than RS2 despite four papers [2-5] detailing their observations of the ordered phase. We note, in particular, that the theoretical work of Kelires and Tersoff [7] was applied by LeGoues et al. [2] to support their prior experimental determination of RSI [3] and not RS3. Furthermore, no experiments were undertaken to investigate the structural differences between RS2 and RS3, even though this is straightforward by x-ray diffraction as shown in our Letter [1], A major point of our Letter [l] is that ordering in Si x Geix alloys is linked to evolving surface morphology. In particular, we demonstrated that the average lateral size of the ordered domains is around 40 nm, of exactly the same dimensions as the islands seen to occur during growth. It is the locally vicinal surfaces presented by the islands which enable bilayer step flow to be established at unexpectedly low temperatures [8]. These regions, once established, are rather stable so that the ordered domains propagate up into the epilayer during growth. Thus, the surface morphology dictates the size of the domains.This explains the large domains observed in the Comment if it is appreciated that the surface morphology is unlikely to be flat as assumed by LeGoues et al. [6]. Rather, the observation of 0.5 fim domains implies the existence of surface mounds or "islands" of lateral dimensions 0.5 fim and height 90 A (assuming that bilayer step flow is stable for angles of one degree as shown in our Letter [1]). Unfortunately, the cross sectional transmission electron microscopy data presented by LeGoues et al. [6] lacks sufficient sensitivity to detect such features. The most likely explanation for the formation of a moundlike morphology lies in the inherent instability of the ideally flat (001) surface. Mounds of similar dimensions have recently been shown to occur for GaAs epitaxy [9], and a possible explanation was given in terms of a Schwoebel barrier. We believe, therefore, that ordering is again driven by surface morphology as in the case of coherent islanding as reported in our Letter [1], the only difference being the scale and physical origin of the island templates involved.Our ordering model, based on our earlier work on steps [10,11], is, therefore, fully consistent with all experimental data, including observations of interfacial ordering in ultrathin (Si w Ge rt ) p superlattices [12]. F...

Continuum modelling of semiconductor heteroepitaxy: an applied perspective

“…Then, for larger amplitudes, deviations occur and lead to an asymmetric profile with cusp-like structures separated by deep trenches, eventually diverging. This feature is well consistent with experimental observations [53,102,104,105,123]. An example is reported in Figure 3(c) for a surface morphology obtained by annealing a SiGe/Si film with low Ge content [124].…”

“…Their theory, named as ATG instability, can be considered as the most essential explanation of the dynamics of heteroepitaxy [53][54][55]. Despite its simplicity, the ATG model was successfully applied to the study of the film instability observed experimentally for a wide variety of materials, such as helium crystals [101], polymers [102] and semiconductors [53,[103][104][105]. Surface waviness is typically observed for low misfit systems (provided that dislocations are not present), while for larger misfits, three-dimensional structures are generally obtained [11,12,106,107] and their formation is affected by nucleation process [108].…”

“…Weakly, non-linear regimes have been explored by means of the approximation of the surface grooves with prescribed functions [104,109,110]. In the special case of cycloid-like profiles, an exact solution for the elastic field exists [111].…”

Initial stages of SiGe epitaxy on Si(001) studied by scanning tunneling microscopy

“…However, we believe that more detailed experiments, particularly those using scanning tunneling spectroscopy together with the STM, would eventually reveal a contrast between Si and Ge atoms. Further experiments are necessary to understand the mechanisms of ordering, segregation and composition fluctuations in Six_xGe x alloys which have been predicted [10] and observed by different methods on a coarse scale [3][4][5]11]. Different diffusion and sticking coefficients for Si and Ge atoms on the surface are expected to result in very interesting growth schemes and alloy atomic structures.…”

“…Very high electron mobilities and the fractional quantum Hall effect have been observed in the strained Si layers grown between relaxed SiGe layers [7][8][9]. Ordering of the Si and Ge atoms within the alloy has been predicted [10], observed by various groups [3][4][5] and it is still subject of controversy [11]. Since the SiGe alloy system is compatible with the existing silicon microfabrication technology, it represents a challenging alternative to III-V semiconductor based device technology and recently, high speed commercial applications have started to emerge from SiGe based devices [12].…”