Alloying in Ge„Si… Õ Si„001… self-assembled islands during their growth and capping: XPS and AFM study M. De Seta, G. Capellini,* and F. Evangelisti Dipartimento di Fisica “E. Amaldi,” Università Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy Received 7 August 2007; revised manuscript received 21 December 2007; published 29 January 2008 In this paper, we present a study on the Ge composition and shape evolution of self-assembled Ge / Si001 islands during the island growth and the subsequent Si capping at 750 ° C. By combining atomic force micro- scope images and x-ray photoemission spectroscopy data, we quantitatively determine the Ge distribution in the wetting layer and in the islands, separately. We found that in as-grown sample, the wetting layer is substantially Si-richer than the islands, its average composition being independent of the growth rate. Upon capping, the islands proceed to a reverse Stranski-Krastanov shape evolution, with a progressive Si enrichment of both the wetting layer and the islands. We demonstrate that this evolution occurs at constant island volume. The observed behavior indicates the suppression of the lateral diffusion of both Ge and Si atoms from the wetting layer to the surface of the enlarging islands, and vice versa. DOI: 10.1103/PhysRevB.77.045431 PACS numbers: 68.65.Hb, 81.70.Jb, 81.07.Ta, 68.37.Ps I. INTRODUCTION Strained Ge / Si001 self-assembled islands have been in- vestigated extensively by many research groups in recent years due to their potential applications in optoelectronic devices. 1,2 The optoelectronic properties of such systems are strongly related to the island composition and shape. These features are, in turn, influenced by the intermixing of Si and Ge atoms occurring both during the Ge island growth 3,4 and the island capping with a silicon layer. 5–8 As a matter of fact, intermixing is a competitive mechanism reducing the strain in heteroepitaxy and must, therefore, be included in each comprehensive model describing island growth or post- growth processes, such as island annealing or capping. 9–12 Recently, theoretical studies have indicated intermixing be- tween surface and subsurface layers to be the key factor con- trolling the transition from planar growth to island formation. 13 Intermixing was shown to influence drastically the shape and size of self-assembled islands during the growth and postgrowth annealing. The relative prevalence of thermodynamic and kinetic effects is still under debate, and the effects of deposition temperature, growth rate, and the presence of carrier gases on the alloying have been the sub- ject of a wealth of studies. 14 Although it has also been estab- lished that the island shape and composition change dramati- cally as a function of Si flux in a large range of deposition temperatures and growth rates, the evolution of GeSi island shape and composition under Si capping is still under debate. Atomic force microscopy AFM and/or scanning tunneling microscopy images indicated that this interdiffusion was ac- complished by island flattening and enlargement, in quanti- tative agreement with the strain relaxation due to the lower Ge content measured in the island during the capping pro- cess. In these works, constant or slightly increasing island volume combined with increasing coverage of the silicon cap layer has been measured by AFM. On the other hand, in a recent work, Lang et al. 15 ob- served in real time, by means of in situ cross-section trans- mission electron microscopy, shrinkage of the Ge island dur- ing Si capping at a temperature in the 550–600 °C range. They attributed this phenomenon to the diffusion of Ge at- oms from islands into the wetting layer WL. Although they did not provide evidence of a WL dilution during the cap- ping, they suggested that the Ge flux is promoted by the dropping of the Ge concentration in the WL below the criti- cal value for island formation. 13 Katsaros et al., 16 by com- bining wet etching and AFM measurements, have shown that after being capped at 580 °C, the volume of the Ge buried islands with a Ge content higher than 20% is reduced by more than half, while the island shape remains unchanged at 350 and 450 ° C. Comparing island shape and morphology of the surface during the capping process, they also conclude that depending on the growth conditions, the surface mor- phology of the capped island layer can be radically different from the morphology of the buried islands. Similar conclu- sions were previously reported by two groups: 7,17 the pres- ence of three-dimensional 3D structures on the capping layer surface during the growth of the silicon capping layer at high rate and low temperature was attributed to the kineti- cally limited conformal growth of a silicon layer above the islands. It is clear that the main problem to overcome in order to understand the island evolution during the capping process is the simultaneous monitoring of the actual island shape and of the Ge content, both in the islands and in the WL. In this work, we characterize the samples both by AFM and x-ray photoemission spectroscopy XPS. In particular, the Ge2p photocurrent signal that depends on the Ge atomic distribu- tion within the first 0–1 nm below the free surface is mea- sured, which allows us to check whether the surface mor- phology measured by AFM is that of “bare” intermixed islands or if it corresponds to the development of Si layers on top of the intermixed islands. By combining AFM and XPS measurements involving the more bulklike Ge3d photo- current signal, 3,6,18 we quantify SiGe intermixing in the ep- ilayer, taking into account that the average composition of the WL and the islands can be different. For this purpose, we deposited two Ge / Si001 island samples at different growth rates so as to vary the surface coverage of the islands and, consequently, the contribution of the island-to-WL ratio to PHYSICAL REVIEW B 77, 045431 2008 1098-0121/2008/774/0454319 ©2008 The American Physical Society 045431-1