Anders Petersson

Study of the two‐dimensional–three‐dimensional growth mode transition in metalorganic vapor phase epitaxy of GaInP/InP quantum‐sized structures

Ga0.5In0.5P/InP quantum‐sized structures, grown by metalorganic vapor phase epitaxy, have been optically characterized by photoluminescence, cathodoluminescence, and photoluminescence excitation spectroscopy. Additional structural information has been obtained by atomic force microscopy. We find that the two‐dimensional layer‐by‐layer growth mode is limited to the growth of 1‐ML‐thick and, in part, 2‐ML‐thick quantum wells. The transition towards three‐dimensional Stranski–Krastanow island growth occurs before the second monolayer of InP is completed. To further study the dynamics of the island formation, growth interruptions were introduced between the InP deposition and the subsequent growth of the upper GaInP barrier. The two types of coherent islands show a quantum confinement in vertical direction, corresponding to about 2‐ and 3‐ML‐thick and about 9‐ and 10‐ML‐thick InP strained quantum wells.

Alignment of InP Stranski–Krastanow dots by growth on patterned GaAs/GaInP surfaces

Stranski–Krastanow islands of InP nucleate in straight rows when deposited by metalorganic vapor phase epitaxy on linearly patterned GaInP/GaAs surfaces. The patterns were produced by overgrowth of lithographically defined W stripes 30° off from [110] on a GaAs(001) surface. Depending on the geometry of the grown GaAs/GaInP mesa stripes the islands were found to align either on top of the ridges, at the sidewall near to the mesa edge or at the bottom of the trenches. The highest density of almost equidistant coherent islands observed in some of the rows is in the order of 10 islands/μm, corresponding to a surface area density of 1010 InP islands/cm2. The maximum density of randomly distributed islands in the unpatterned area otherwise is only 2×109 islands/cm2. The results show a successful combination of overgrowth of conducting metal stripes and lateral geometrical positioning of dots in one in situ growth step.

Cathodoluminescence spectroscopy and imaging of individual GaN dots

Uncapped GaN dots on AlGaN barrier material, grown by metal organic chemical vapor deposition on 6H–SiC substrates, were studied. Cathodoluminescence (CL) microscopy and scanning electron microscopy (SEM) were used to investigate both luminescence and structure of individual GaN dots. The correlation between the luminescence and the actual position of self-assembled dots was demonstrated. The position of a dot was established with high resolution SEM and a CL image was used to display the corresponding luminescence. The spectrum from a single dot was obtained by positioning the electron beam on one particular dot. The luminescence from dots with a lateral size of 100 nm and a height of 40 nm was determined to be 3.47 eV.

Nano-Optical Studies of Individual Nanostructures

Optical techniques play a significant role in studies of nano-structures. The electronic structures of quantum dots, for example, vary with the geometric sizes in an ensemble, resulting in broadened spectral lines. Recently, different forms of local spectroscopic techniques have been applied to investigate such inhomogeneously broadened emission lines. In this paper we report on three methods for local spectroscopy : cathodo-luminescence, luminescence induced by a scanning tunnel microscope and microphotoluminescence. Each of these techniques is shown to have the capacity to investigate single quantum dots, with linewidths in the range 40-1000 μev. Besides demonstrating the possibility of imaging and spectroscopically studing individual dots, we also demonstrate the possibility of investigating single impurity atoms, in imaging as well as in emission spectroscopy modes.

Maskless selective growth of InGaAs/InP quantum wires on (100) GaAs

A new fabrication process to create InGaAs/InP quantum wires on (100) GaAs substrates is demonstrated. The process is based on the selectivity of the growth of InP on lines created by focused ion beam bombardment, together with the selectivity of the growth of InGaAs on the InP wires. Intense photoluminescene is observed from the wires and the emission shows clear polarization parallel and perpendicular to the wires. Cathodoluminescene images confirm that the luminescence originates from the wires.

Manipulating InAs island sizes with chemical beam epitaxy growth on GaAs patterns

We demonstrate the manipulation of InAs island size through growth on patterned GaAs substrates. Using chemical beam epitaxy, we deposited islands on patterns consisting of concentric circular trenches. One sample was left uncapped for atomic force microscopy, and another was capped with GaAs for micro-photoluminescence. Luminescence images taken at particular energies show that at different orientations along the circular arcs, the islands have different luminescence energies and thus different sizes. In correlating the island distributions found by atomic force microscopy images with the luminescence, we conclude that the InAs islands that form in high-density one-dimensional chains are smaller than those that are not in chains.

Compositional variation of AlGaN epitaxial films on 6H-SiC substrates determined by cathodoluminescence.

High quality epitaxial films of A(l)xGa(1-x)N, grown on SiC substrates, were investigated using spatially resolved cathodoluminescence (CL), scanning electron microscopy, and atomic force microscopy. A variation in the observed peak energy position of the CL was related to alloy fluctuations. CL was used to reveal relative alloy fluctuations of approximately 1% on a sub-micrometer scale, with a precision difficult to surpass with other available techniques. By correlating data from the different techniques, a model was derived. The main feature of it is an alloy fluctuation on the micrometer scale, seeded during the initial growth and extending through the epitaxial film. These alloy fluctuations seems to be related to terrace steps ( approximate to5 nm in height), formed preferentially at scratches on the SiC surface. This investigation indicates that the initial growth of epitaxial films is critical and structures formed at the beginning of the growth tend to persist throughout the growth. Further, a strain gradient from the SiC interface extending towards the surface, was observed.

Characterization of bulk Cu0.85In1.05Se2 by photoluminescence and cathodoluminescence

Abstract Melt grown crystals in the CuInSe system show the phenomenon of phase segregation into a nearly stoichiometric CuInSe 2 compound with chalcopyrite structure and a strongly copper deficient compound with an ordered defect chalcopyrite (ODC) structure. The photoluminescence of such a two phase Bridgman-grown CuInSe crystal with the selected composition Cu 0.85 In 1.05 Se 2 has two broad emission bands: a transition A around 0.93 eV and a transition B around 1.10 eV, the latter being above the bandgap energy of CuInSe 2 . Both transitions A and B exhibit donor-acceptor pair type recombination behaviour. Spatially resolved cathodoluminescence shows that the two emission energies come from different regions of the crystal and have the form of micrometer-sized stripes. We conclude that the two transitions A and B originate from the two different phases which penetrate the crystal in growth direction as micrometer-sized lamellae [M. Hornung et al., J. Crystal Growth 154 (1995) 315]. From the emission energies, transition A at 0.93 eV can be attributed to the CuInSe 2 compound whereas transition B at 1.10 eV belongs to the ODC phase.

Luminescence spectroscopy on individual nanostructures and impurity atoms using STM and SEM

In the exploration of the nano-world of semiconductors there is a strong focus on low-dimensional structures and ultra-small devices. Two fundamental problems, which challenge progress in this field are: (1) large ensembles of nano-objects, like Quantum Dots (QDs), do not have identical geometrical shapes and electronic properties, and, (2) the properties of a low-dimensional structure can be dominated by a few impurity atoms, whereas the properties of a macroscopic structures is determined by the quasi-continuous background of dopant impurities. To allow QDs and discrete impurities to be studied, novel experimental techniques are required. In this paper the authors describe how local luminescence has been excited from single QDs using electrons injected from a Scanning Electron Microscope (SEM), from the tip of a Scanning Tunneling Microscope (STM) or using highly focused photons for excitation. They present images of QDs as well as characteristic spectra of individual QDs. They finally show how the local character of the excitation enable them to excite and image individual impurities in low-dimensional structures, including the measurement of characteristic emission spectra from a single impurity atom in GaAs.