The early stages of growth of highly strained In. x Gai -^As on GaAs(lOO) have been investigated as a function of composition. The evolution of the film microstructure as determined by in situ STM and RHEED is from a two-dimensional rippled surface in the beginning stages of growth to a threedimensional island morphology. A growth mode is proposed whereby strain relaxation is initially achieved through the kinetically limited evolution of surface morphology. In contrast to traditional critical-thickness theories, significant strain relief is accommodated by a coherent island morphology. This study represents a new view for both the growth mode and initial strain relaxation in thin films.PACS numbers: 68.55. 61.14.Hg, 61.16.Di The growth of strained films has become a field of intense study in recent years. In particular, In x Gai-^As on GaAs is an ideal system for studying the dependence of the growth morphology on strain because the lattice constant of the overlayer film can be varied by 7.2% as the In concentration is changed from 0 to 1. Traditionally, the evolution of the film morphology has been viewed within the context of a critical overlayer thickness. van der Merwe examined the situation of a film under uniform strain and showed that at a straindependent thickness (i.e., "critical thickness") it is energetically favorable for an array of misfit dislocations to partially relax the overlayer. 1 Examination of experimental results for heteroepitaxial semiconductor growth revealed that generally the transition to a relaxed state occurred at a larger thickness than predicted by this theory. This has led to the inclusion of kinetic mechanisms in the theory and the possible metastability, after critical thickness, of the uniformly strained thin film. 2,3 Recently, a model has been proposed to explain the critical thickness t c and growth-mode transitions in highly strained In^Gaix As/GaAs(100). 4 In keeping with the traditional view, it is assumed that partial relaxation of the overlayer strain occurs through the nucleation of misfit dislocations. In particular, the experimental observation that the onset of the surface-lattice-constant relaxation (i.e., t c ) increases as either the growth temperature or In composition (i.e., strain) is reduced is explained on the basis of the kinetics of dislocation motion. In addition, it is proposed that the system undergoes a layer-to-island growth-mode transition as a result of cluster nucleation initiated by dislocations. There are several problems with this model. For low-strain films and/or low temperatures it fails to accurately predict the correct critical thickness. The data for Ino.25Gao.75As/ GaAs shown in Ref. 5 are in clear disagreement with this model. 5 For example, at a growth temperature of 450 °C a critical thickness of 340 A is observed experimentally, whereas the model predicts a thickness which is much larger. The most pronounced disagreement with the theory is seen in some recent data by Guha, Maduhkar, and Rajkumar, where transmission-electron-micr...

We have investigated the morphological evolution of strained fims during growth.Novel Monte Carlo studies, which incorporate linear elasticity, have been performed to simulate film growth with misfit. These studies demonstrate the onset of islanding for sufficiently large misfit. We present an analytic calculation whi9h shows that from the onset of deposition the films are energetically unstable to large-scale islanding. We argue that the kinetics ultimately determines the surface morphology. Dislocations are not necessary for surface lattice relaxation. Support for this picture is inferred from experimental results on a number of strained growth systems.

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We have investigated the molecular beam epitaxy growth of highly strained InGaAs on GaAs(100) as a function of the anion to cation flux ratio. Using reflection high energy electron diffraction the evolution of the film morphology is monitored and the surface lattice constant is measured. It is found that the cation to anion flux ratio dramatically affects the growth mode. Under arsenic-rich conditions, growth is characterized by a two-dimensional (2D) to three-dimensional (3D) morphological transformation. However, for cation-stabilized conditions, 3D islanding is completely suppressed, and 2D planar growth is observed. We associate these differences in the growth mode with corresponding changes in the surface tension of the overlayer. A high surface tension stabilizes 2D growth. An analysis which relates surface tension to a critical thickness for the onset of coherent island formation supports this view.