| 1. |
- Håkanson, Ulf, et al.
(författare)
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Electric field effects in single semiconductor quantum dots observed by scanning tunneling luminescence
- 2003
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Ingår i: Journal of Vacuum Science and Technology B. - : American Vacuum Society. - 1520-8567. ; 21:6, s. 2344-2347
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy (STM) and scanning tunneling luminescence were used to correlate the topography with the emission spectra from individual self-assembled, InP quantum dots (QDs). We have investigated in detail how the electric field induced by the STM tip affects the emission from the QDs. This was done when exciting a QD, by altering the bias for constant current, by altering the current for constant bias, or by changing the tip position. An increased bias (increased electric field) leads to Stark shift of the QD emission, whereas a larger tunneling current results in state filling of the emission. Furthermore, when exciting the QD, the position of the STM tip is shown to have large effects on the QD luminescence.
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| 2. |
- Håkanson, Ulf, et al.
(författare)
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Luminescence polarization of ordered GaInP/InP islands
- 2003
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:4, s. 627-629
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Tidskriftsartikel (refereegranskat)abstract
- The luminescence polarization properties of GaInP islands have been investigated. The islands, which form during overgrowth of InP quantum dots, were studied using scanning tunneling luminescence (STL) and photoluminescence (PL). STL from these islands shows emission at an energy below the main emission peak of the bulk GaInP. The linear PL polarization anisotropy was measured at low temperature, for which the emission from the islands shows high polarization anisotropy. The intensity maximum for the emission occurs for light polarized parallel to the elongation of the islands. The observed linear PL polarization anisotropy indicates the presence of highly ordered domains of GaInP in the islands. (C) 2003 American Institute of Physics.
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| 3. |
- Håkanson, Ulf, et al.
(författare)
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Nano-aperture fabrication for single quantum dot spectroscopy
- 2003
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 14:6, s. 675-679
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Tidskriftsartikel (refereegranskat)abstract
- We present a simple and controllable method for fabricating nano-apertures in a metal film using polystyrene nano-spheres as masks during the metal evaporation. We show how the processing conditions used during deposition of the spheres such as spin velocity, nano-sphere concentration and a reduction of the surface tension interplay and control the distribution of spheres. The fabrication method is ideal for luminescence studies by isolating individual nanometre-sized objects, which is exemplified by photoluminescence spectroscopy of single self-assembled Stranski-Krastanow quantum dots.
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| 4. |
- Håkanson, Ulf, et al.
(författare)
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Photon mapping of quantum dots using a scanning tunneling microscope
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 81:23, s. 4443-4445
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Tidskriftsartikel (refereegranskat)abstract
- Scanning tunneling microscopy (STM) and scanning tunneling luminescence (STL) have been used to investigate the geometric and optical properties of individual self-assembled InP quantum dots overgrown with a thin layer of GaInP. STL spectra and monochromatic photon maps were used to correlate the surface topography with the optical properties of single quantum dots. We find a spatial resolution of about 10 nm in the photon maps. Theoretical emission spectra were calculated by six-band k.p theory using a realistic shape of the dot as well as of the cap layer. The calculated emission spectrum of a single dot is in good agreement with the experimental findings. (C) 2002 American Institute of Physics.
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| 5. |
- Håkanson, Ulf, et al.
(författare)
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Photon mapping of single quantum dots by scanning tunneling microscopy induced luminescence spectroscopy
- 2002
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Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Konferensbidrag (refereegranskat)abstract
- Scanning tunneling microscopy induced luminescence (STML) has been used to investigate individual self-assembled InP quantum dots overgrown with GaInP. We will present results correlating the surface morphology with the optical properties of single dots. In particular, the strain induced energy-shift of the dot emission with increasing cap layer thickness and its relation to the overgrowth will be discussed. Effects of the dots on the properties of the overgrown GaInP will also be treated. STML spectra and monochromatic photon maps are compared with results from photoluminescence and transmission electron microscopy measurements. Furthermore, a comparison with theoretical calculations is made
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| 6. |
- Håkanson, Ulf, et al.
(författare)
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Quantum-dot-induced ordering in GaxIn1-xP/InP islands
- 2002
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 66:23
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Tidskriftsartikel (refereegranskat)abstract
- Thin layers of GaxIn1-xP grown on top of self-assembled InP quantum dots has been studied using transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and low-temperature scanning tunneling luminescence (STL). STM reveals that the overgrowth is highly uneven, in which elongated GaxIn1-xP islands covering the dots are formed. TEM and high-spatial-resolution STL show that the quantum dots locally induce domains with higher degree of ordering in the islands. The luminescence from these domains is observed as a strong GaxIn1-xP peak at an energy below the emission from the GaxIn1-xP barrier material.
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| 7. |
- Håkanson, Ulf
(författare)
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Scanning Tunneling Microscopy Induced Luminescence Studies of Semiconductor Nanostructures
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis treats scanning tunneling luminescence (STL) investigations of semiconductor nanostructures. The STL technique combines scanning tunneling microscopy (STM) with detection of photons, induced by the tunneling electrons. The high spatial resolution in STM and the local excitation allow for optical investigations on the nanometer-scale. The work concerns design and implementation of an optical detection system used for STL studies of single InP quantum dots (QDs) overgrown with thin layers of GaInP. Constant current imaging together with STL spectra and monochromatic photon mapping were used to correlate the surface topography with the optical properties of the QDs. It was found that the QDs act as seeds for the GaInP overgrowth, where elongated GaInP islands are formed. The emission from single QDs was observed to be shifted towards higher energies with increasing cap layer thickness, which by multi-band k.p theory was determined to be induced by strain. The geometry of the overgrowth was realistically modelled in the calculations, using data from STM and transmission electron microscopy. Theoretical emission energies were also calculated, which are in good agreement with the experimental results. Studies of the GaInP islands showed that the InP QDs locally induce domains in the islands with high degree of ordering in the GaInP. The emission from these domains was found to occur at an energy below the emission from the GaInP barrier material. High polarization anisotropy for the island luminescence was observed by photoluminescence measurements, in which maximum emission intensity was detected for light polarized parallel to the elongation of the islands.
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| 8. |
- Håkanson, Ulf, et al.
(författare)
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Single InP/GaInP quantum dots studied by scanning tunneling microscopy and scanning tunneling microscopy induced luminescence
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:3, s. 494-496
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the optical and structural properties of single, self-assembled InP quantum dots (QDs) overgrown with nominally 5 nm of GaInP, using an ultrahigh-vacuum scanning tunneling microscope (STM) operating at low temperatures. The STM is combined with an optical detection system, which allows us to detect the emission from individual quantum dots with high spatial resolution. We find that the InP QDs act as nucleation points for the GaInP overgrowth, where the strain induced by the overlayer give rise to a QD emission around 1.46 eV. (C) 2002 American Institute of Physics.
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| 9. |
- Håkanson, Ulf, et al.
(författare)
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Ultrahigh vacuum scanning probe investigations of metal induced void formation in SiO2/Si(111))
- 2002
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Ingår i: JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B. - : American Vacuum Society. - 1071-1023 .- 1520-8567. ; 20:1, s. 226-229
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Konferensbidrag (refereegranskat)abstract
- Using scanning electron microscopy guidance, atomically resolved scanning tunneling microscopy investigations inside openings formed during the decomposition of a thick SiO2 film on a Si(111) surface have been performed. We report, for the first time, the presence of Ni atoms inside self-formed openings. The SiO2 decomposition as a function of annealing temperature and time was studied. By comparing self-formed and lithographically designed openings in the SiO2 layer we have shown that Ni contamination of the SiO2/Si(111) has a profound effect on the formation of self-induced lateral oxide openings. (C) 2002 American Vacuum Society.
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| 10. |
- Johansson, Mikael, et al.
(författare)
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Correlation between overgrowth morphology and optical properties of single self-assembled InP quantum dots
- 2003
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 68:12
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the early stages of GaInP overgrowth on InP quantum dots (QD's) experimentally and theoretically. A direct correlation between the surface morphology and the optical properties of individual InP QD's is made using scanning tunneling microscopy (STM) and scanning tunneling luminescence. The geometric structure of the islands is further investigated using cross-sectional transmission electron microscopy (TEM). The overgrowth occurs in three stages; initially the InP QD's act as seeding points for the overgrowth, where the GaInP grows laterally from the side facets of the QD. The growth occurs preferentially in the [110] direction and elongated GaInP/InP islands are formed. As the overgrowth continues the islands increase laterally in size and GaInP also starts to grow between the islands, but not covering the top of the InP QD's. The growth of GaInP on top of the QD's commences once the islands have begun to coalesce. Using a model based on the STM and TEM results the electronic structures of the QD's have been calculated by eight-band k.p theory. The calculations are in good agreement with the experimental results. Our findings unravel the details of the strain induced energy shift of the QD luminescence previously reported [Pistol , Appl. Phys. Lett. 67, 1438 (1995)].
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| 11. |
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