Distribution, number, and polarization control of site-controlled 1.55-µm InAs quantum dots on InP nanopyramids

Abstract

The work presented in this thesis concerns the growth, structural and optical properties of site-controlled 1.55-µm InAs quantum dots (QDs) on truncated InP nanopyramids grown by selective-area metalorganic vapor phase epitaxy (MOVPE). We report the impact of base size and shape on the evolution control of multifaceted InP (100) nanopyramids. The pyramid top surfaces are composed of a (100) center facet surrounded by high-index {103} and {115} facets. Their arrangement and (relative) size depend on the size and shape of the pyramid top area. For a certain shape only the (100) facet remains below a critical size of the top area. The arrangement and (relative) size of the top facets in turn are governed by the {110} and {111} side facets whose area (ratio) depends on the pyramid base size and shape. This self-consistently determines the ratio of the (100) top facet area and the sum of the {110} and {111} side facet areas as well as the height of the pyramids. Distribution control of 1.55-µm InAs QDs down to small numbers is achieved. The arrangement of the pyramid top facets is governed by the shape of the pyramid base and top surface area. The QDs preferentially nucleate on the high-index facets determining the position and distribution. The QD number is reduced with shrinking top surface size. Positioning of four, three, two, and single QDs is realized depending on the top surface shape and size. Sharp emission from a single QD is observed at 1.55 µm. The shape and polarization control of site-controlled multiple and single InAs QDs on InP pyramids is accomplished. The QD size increases with increasing growth temperature and the QDs strongly elongate. The QD elongation causes linear polarization of the photoluminescence. With reduced pyramid base/pyramid top area/QD number, the degree of polarization decreases, attributed to the symmetric pyramid top, reaching zero for single QDs grown at lower temperature. This control of the linear polarization is important for the realization of entangled photon sources operating in the 1.55-µm wavelength region. Position-controlled InAs QDs are integrated into planar InP structures by employing selective area growth of InP pyramids and regrowth. A smooth surface morphology is obtained at elevated regrowth temperature due to suppression of three-dimensional growth on the pyramids. The height differences are less than 30 nm after nominal 700 nm InP regrowth at 640 °C. Most important, the integrated QDs maintain good optical quality after regrowth for the realization of integrated nanophotonic devices and circuits operating at telecom wavelengths.