Indium Segregation and Enrichment in CoherentInxGa1−xAs/GaAsQua

Abstract

Significant differences in the image features of In x Ga 12x As quantum dots (QDs) grown on (001) and vicinal (001) GaAs were seen in [001] on-zone bright-field transmission electron microscope images. Simulated images were obtained by modeling the strain field distribution of the QDs with finite element analysis and then using this model in dynamical electron diffraction contrast simulations. Comparison of the experimental images and the simulated images shows that (i) In segregation exists in the QDs and (ii) the average In content of the QDs is higher than the average In content of the film. [S0031-9007(99)09460-0] PACS numbers: 81.05.Ea, 61.14. Lj, 61.16.Bg, 68.35.Dv The composition, shape, and size of quantum dots (QDs) are essential parameters in determining their optoelectronic properties [1]. Many techniques have been used to study these parameters [2,3]. Among them, transmission electron microscopy (TEM) has been one of the most frequently used. It is well known that, under the usual dynamical two-beam or on-zone axis multibeam imaging conditions, the TEM diffraction contrast image arises largely from the strain field, which is sensitive to the composition, shape, size, and elastic constants of both the substrate and the QD. Consequently, it should be possible to obtain information about the composition, shape, and size of QDs from such images. However, to correctly interpret diffraction contrast images, image simulations are essential [2].Obtaining the strain field distribution within and around the QDs is necessary for image simulations. Finite element analysis (FEA) methods have been widely and successfully used to study the strain field of nanometer-size semiconductor QDs and applied to TEM investigations. For example, Christiansen et al. [4] used FEA to obtain the displacement field of Ge(Si) islands grown on Si. Using convergent beam electron diffraction, their study showed that the displacement field obtained in this way is correct. Rosenauer et al. [3] determined the composition profile of In x Ga 12x As QDs using high resolution TEM and FEA. Benabbas et al.[5] applied FEA to generate strain fields of InAs͞GaAs QDs, and they calculated two-beam dynamical electron diffraction contrast images to study their shape.In this study, we use three dimensional anisotropic FEA to generate strain field distributions of In x Ga 12x As QDs grown on (001) GaAs. We then use these strain field distributions in multibeam dynamical electron diffraction calculations to produce contrast simulations of [001] on-zone bright-field TEM images. An earlier study [6] showed that the [001] on-zone bright-field imaging can be used to estimate the size of the unburied QDs. Here, we show that indium composition plays a role in defining the boundary of the TEM image and in determining several of the image features. Both electron diffraction patterns and comparison of the simulated and experimental images show that the composition of the dots is different from the composition of the thin film alloy from which the QDs formed....

Adatom condensation and quantum dot sizes in InGaAs/GaAs (001)

“…This can be investigated further with the growth of binary InAs islands which would also eliminate possible effects from indium enrichment in ternary alloys. 8,9 In conclusion, we have shown that the sizes of surface and capped InGaAs QDs differ, and that this discrepancy increases with decreasing island coverage. Differences in island coverage were obtained with simultaneous growths on different GaAs ͑100͒ vicinal surfaces and with structures deposited at growth rates varying with position.…”

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

“…Although many investigations have concentrated on the shape and size 6,7 and evolution 8,9,10 during the QD growth, relatively less attention has been paid to the composition. 11,12,13 In the classical Stranski-Krastanow 14 ͑S-K͒ mode of coherent island formation, one material with a different lattice parameter and low interfacial energy is initially deposited on a substrate surface, layer by layer, forming a ''wetting layer''. When the wetting layer reaches a critical thickness ͓usually three to five monolayers for pure Ge on Si͑001͒, 3,15 island growth starts to partially release the mismatch strain energy between the epitaxial layer and the substrate.…”

Carrier capture and relaxation in Stranski-KrastanowInxGa1−xAs/Lobo1, Perret2, Morris3 et al. 2000Phys. Rev. BSelf Cite22040View full textAdd to dashboardCite

“…Indium segregation and enrichment of InGaAs/GaAs͑100͒ QD's has been found to result in QD's with average indium compositions 30%-45% higher than that of the deposited film. 6 Stress-induced indium enrichment of quantum dots formed from In x Ga 1Ϫx As films has been predicted to reduce the nucleation barrier for islanding, with a resultant island composition that depends critically on the island shape and fractional elastic relaxation of the island relative to the planar layer. 7 Indium enrichment effects have not been determined in InGaAs/GaAs(311)B QD's, but may well be reduced relative to ͑100͒ QD's, which would result in a higher gallium content in (311)B QD's compared to ͑100͒ QD's of similar diameter.…”