Stable and Metastable InGaAs/GaAs Island Shapes and Surfactantlike Suppression of the Wetting Transformation

Abstract

Contrasting behaviors are observed in InGaAs͞GaAs island formation during vapor phase epitaxy: variation of group V partial pressures gives different critical thicknesses for the onset of the StranskiKrastanow transformation, surface coverages, ratios between coherent and incoherent islands, and dissimilar morphologies upon annealing. The latter experiments show that small lens-shaped islands can be found in equilibrium if InGaAs surface energies are minimized, leading to the conclusion that AsH 3 can raise surface energies and act as an impurity-free "morphactant." [S0031-9007(98) The importance of Stranski-Krastanow (S-K) coherent island formation as a mechanism for strain relaxation has been established in both Ge͞Si [1] and InAs͞GaAs [2,3] heteroepitaxy. In the S-K or wetting transformation, growth is initially two dimensional, until the film reaches a strain dependent critical thickness. Deposition beyond this critical thickness results in island formation and subsequent increases in island densities until saturation. Interest in S-K growth has been rekindled by the first reports of these strained islands to make defect-free, self-assembled, InGaAs͞GaAs semiconductor quantum dots (QDs) [4][5][6]. Since then, a large number of studies have focused on strain relaxation by island formation. An improved understanding of the varying and often competing mechanisms that result in different morphologies during island nucleation will determine the successful utilization of these islands in zero-dimensional (0D) devices. Island shapes, aspect ratios, morphologies, and coherence (incoherence) all play a role into the electronic-optic-magnetic properties of selfforming semiconductor quantum dots.Recent reports show ripening [7] behavior during island formation in Ge͞Si and after annealing in InGaAs͞GaAs QDs. An in situ study of the evolution of island growth in Ge͞Si found an optimum range in uniformity [8] during the evolution to the stable dome-shaped configuration. Other reports show the coexistence of different types of islands [9] and a shape transition from small pyramid-shaped islands to dome-shaped islands [10]. In InGaAs͞GaAs QD formation, ripening has also been observed upon annealing, which can be partially suppressed by steps in miscut substrates [11]. Ripening in QDs would have obvious disadvantages for applications of island-based devices; therefore, determining if stable islands can be achieved is of both practical and fundamental interest.Studies using surfactants in the growth of Ge͞Si have produced striking results, from the total suppression of the S-K transformation [12,13], to different critical thicknesses [14] and island shapes [15]. It is plausible that similar surfactantlike effects might explain the present controversy surrounding the different island shapes reported in the InAs͞GaAs system.Kinetic barriers for island formation have enabled 2D growth of InGaAs at low temperature [16]. Here we report a partial suppression of the S-K transformation; however, our results can be explained by an of...

Strain relaxation by alloying effects in Ge islands grown on Si(001)

“…The reason for the size nonuniformity in strained island growth has been discussed elsewhere. 9,10,18 It was also found that small islands in both samples have a square shaped base with rounded corners, as has been observed by atomic force microscopy ͑AFM͒. 19 Cross-section TEM studies showed that, viewed along ͗110͘, islands in both samples have a similar height to basediameter aspect ratio ͑about 1:5͒, irrespective of the size of the islands.…”

Changes in luminescence emission induced by proton irradiation: InGaAs/GaAs quantum wells and quantum dots

“…10 After deposition of GaAs buffer layers at 650°C, the temperature was lowered to 550°C and nanometer sized InGaAs islands were grown by depositing ϳ5 ML of In 0.6 Ga 0.4 As. QW samples were obtained by stopping the growth of InGaAs before the onset of the StranskiKrastanow transformation, giving thin ͑1 nm͒ QWs.…”

Quantum coherence in semiconductor nanostructures for improved lasers and detectors.

“…6 As under different AsH 3 partial pressures and different growth temperatures. Lower growth temperatures have been identified previously as reducing surface segregation [11,12], but the role of AsH 3 on QD formation has only been speculated [16,17]. Conditions where surface segregation is suppressed and enhanced are identified.…”