A transmission electron microscopy ͑TEM͒ investigation of the morphology of In x Ga 1Ϫx As quantum dots grown on a GaAs͑001͒ substrate has been carried out. The size and the shape of the quantum dots have been determined using bright-field images of cross-section TEM specimens and ͓001͔ on-zone bright-field images with imaging simulation from plan-view TEM specimens. The results suggest that the coherent quantum dots are lens shaped with base diameters of 25-40 nm and aspect ratios of height to diameter of 1:6 -1:4.
͓S0163-1829͑99͒00920-0͔Since Esaki and Tsu 1 proposed the idea of semiconductor heterostructures and successfully grew Al x Ga 1Ϫx As/GaAs superlattices nearly three decades ago, quantum semiconductor structures have received increasing attention due to their potential applications in electronic and optoelectronic devices and circuits. 2,3 In recent years, fabrication of lowdimensional quantum semiconductor structures, such as quantum wires and quantum dots, has become possible with the development of modern epitaxial techniques. Compared with bulk or quantum well systems, these low-dimensional quantum semiconductor structures have unique and superior optical properties for optoelectronic devices. For quantum dots, carriers are confined three dimensionally, leading to different optoelectronic properties from those in bulk materials, quantum wells, and quantum wires. Since the shape and size of quantum dots are critical parameters in determining their optoelectronic properties, 4-7 determination of these parameters is important. Several techniques have been used to estimate these parameters, such as atomic force microscopy ͑AFM͒, 8-14 scanning tunneling microscopy, 15-17 reflection high-energy electron diffraction, 18,19 and transmission electron microscopy ͑TEM͒. 20-24 Different shapes of quantum dots such as lens-shaped, 9,10,23 cone-shaped, 14,17 pyramids with different facets, 11,12,[18][19][20][21] and truncated pyramids 22 have been reported using the above techniques. Differences in the predicted values for quantum dot ground state and excited state emission, and in intersublevel energies will be obtained depending on what shapes and aspect ratios are assumed in the calculation. Calculations for both pyramid-shaped 25,26 and lens-shaped In x Ga 1Ϫx As/GaAs quantum dots 27 have been reported; however, an exact experimental determination of the shape of these islands is at present controversial. AFM has been the most commonly used technique for the shape and size study of uncapped quantum dots. However, the convolution of the AFM tip with the dot structure limits its ability to resolve the shape of very small quantum dots, 28,29 and especially of the dots with facets. Although TEM techniques have been used to study the shape and size of the dots, reliable information has not been obtained. In this study, a comprehensive TEM investigation of the shape and the size of uncapped In x Ga 1Ϫx As quantum dots grown on GaAs͑001͒ is carried out.The In 0.6 Ga 0.4 As quantum dots were grown on a GaAs͑001͒ ...