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Compositional analysis of Si/SiGe quantum dots using STEM and EDX
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Surface and Semiconductor Physics . Linköping University, The Institute of Technology.
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2006 (English)In: Volume 6129 - Quantum Dots, Particles, and Nanoclusters III, Proceedings of SPIE, 2006Conference paper (Refereed)
Abstract [en]

Ge islands fabricated on Si(100) by molecular beam epitaxy at different growth temperatures, were studied using crosssectional scanning transmission electron microscopy and energy-dispersive X-ray spectrometry combined with electron energy loss spectrometry experiments. The island size, shape, strain, and material composition define the dot-related optical transition energies, but they are all strongly dependent on the growth temperature. We have performed quantitative investigations of the material composition of Ge/Si(001) quantum dots. The samples were grown at temperatures ranging from 430 to 730 °C, with one buried and one uncapped layer of Ge islands separated by 140 nm intrinsic Si. The measurements showed a Ge concentration very close to 100 % in the islands of samples grown at 430 °C. With a growth temperature of 530 °C, a ~20 % reduction of the Ge fraction was observed, which is due to intermixing of Si and Ge. This is consistent with our previous photoluminescence results, which revealed a significant blue shift of the Ge dot-related emission peak in this growth temperature range. The Ge concentration decreases more slowly when the growth temperature is increased above 600 °C, which can be explained by geometrical arguments. The longer distance between the interface and the core of these larger sized dome-shaped islands implies that less Si atoms reach the dot center. In general, the uncapped Ge dots have similar widths as the embedded islands, but the height is almost exclusively larger. Furthermore, the Ge concentration is slightly lower for the overgrown dots.

Place, publisher, year, edition, pages
Keyword [en]
SiGe, quantum dots, composition, intermixing, growth temperature
National Category
Physical Sciences
URN: urn:nbn:se:liu:diva-12572DOI: 10.1117/12.647318OAI: diva2:1725
Available from: 2008-09-15 Created: 2008-09-15 Last updated: 2009-03-02
In thesis
1. Si-based structures for light emission and detection
Open this publication in new window or tab >>Si-based structures for light emission and detection
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Efforts to improve the optical performance of the indirect bandgap semiconductor silicon (Si) has been a major subject of research in the field of Si photonics due to the promising applications of Si based light emitters and detectors for optical communication. With that motivation three different Si based material systems were investigated; Si:Er/O layered structures, SiGe quantum dots and SiSn nano structures, all grown using the technique of molecular beam epitaxy (MBE). The main focus of this work has been on Si:Er/O layers, which lead to fabrication of Si-based light emitting diodes (LED) emitting at 1.54 mm wavelength. The work on SiGe structures lead to the fabrication of near-infrared light detectors, whereas the SiSn structures have not shown any strong optical character.

Studies include epitaxial growth, structural characterization, device processing, electrical and optical characterizations. Material characterization of Si:Er/O structures using analytical electron microscopy (AEM) revealed interesting results with identification of two different type of microstructures in these layers depending on the Er and O concentrations. Several Si:Er/O LEDs were fabricated with different Er and O concentrations and the optical characteristics were investigated in order to find the best doping levels of Er and O for efficient light emission. The electroluminescence measurements revealed a strong 1.54 mm emission from these devices due to the intra 4f shell transition of Er3+ from the excited state (4I13/2) to the ground state (4I15/2). Si:Er/O waveguide LEDs have also been grown on SOI wafers using the optimized structure parameters obtained from mode confinement simulations as well as the microstructure investigations. The Si:Er/O waveguide LEDs are aimed at fabricating a planar Si cavity with Bragg mirrors on both sides to obtain light amplification and realise an electrically pumped Si laser. A focused ion beam (FIB) instrument was used to fabricate the Bragg mirrors but initial attempts did not result in light amplification in our Si:Er/O waveguide cavities.

SiGe quantum dots are well-known quantum structures which are formed in a selfassembled fashion from Si/SiGe layer structures with a variety of shapes, sizes and compositions depending mainly on parameters like growth temperature and layer thicknesses. Optical properties of SiGe quantum structures have been studied while there has been little knowledge about their composition. A detailed compositional investigation of different SiGe dots on a nanometer scale was performed using AEM. The results showed a large degree of interdiffusion in large quantum dots, which was consistent with the optical properties of these dots. Using a multiple stack of Ge quantum dots and SiGe quantum wells, MOSFET type photodetectors working at 1.3 – 1.55 mm wavelength have also been fabricated and characterized.

Research on the SiSn system was mainly motivated by the possibility to obtain a direct bandgap transition in Si based material as it was predicted theoretically and experimentally observed in the related GeSn material system by other researchers. Structural and optical characterizations of SiSn nano structures were performed. Although the same SiSn nano structures exhibit a weak signature of optical absorption, low temperature photoluminescence measurements did not reveal any emission peaks related to the SiSn dots.

Abstract [sv]

Kisel är den helt dominerande halvledaren för tillverkning av de integrerade kretsar som utgör grunden för vårt nuvarande IT-samhälle. Samtidigt har det inte varit möjligt att använda kisel-komponenter inom det stora optoelektronikområdet, exempelvis i sändare och mottagare för fiberoptisk kommunikation. På grund av de stora fördelar det skulle innebära att kunna integrera optoelektronik med kiselbaserade komponenter på samma halvledarskiva så har det under de senaste åren varit mycket forskning kring möjligheterna att modifiera kisel så att det kan användas inom optoelektronik.

I denna avhandling har tre olika materialkombinationer studerats för att undersöka om kiselbaserade material kan användas för emission eller detektion av optiska signaler. Den största delen av arbetet handlar om att tillverka, studera och utnyttja strukturer med låga halter av erbium och syre som kan fungera som lysdioder i det infraröda området med just den våglängd, 1,54 μm, som används i de flesta fall av fiberoptisk kommunikation. Genom att studera hur ljusintensiteten beror på erbium- och syrekoncentrationerna samt den erhållna mikrostrukturen i materialet så har tillverkningen kunnat optimerats för högsta intensitet. Vidare har multilagerstrukturer utvecklats så att ljusemissionen sker ifrån en s.k. vågledare. Målsättningen har varit att skapa möjligheter för att erhålla s.k. stimulerad emission vilket skulle resultera i en kiselbaserad laser.

En annan del av avhandlingen berör tillverkning, karakterisering och användning av s.k. germanium kvantprickar som kan erhållas på kiselytor om man belägger ytan med ett antal atomlager av germanium. Dessa öar kan sedan begravas av kisel och får intressanta optiska egenskaper. Studier har gjorts av hur stor interdiffusionen är av kisel och germanium i kvantprickarna vilket påverkar vilka ljusvåglängder som kan absorberas eller emitteras av kvantprickarna. Med hjälp av sådana kvantprickar har detektorer för våglängder 1,3-1,5 μm tillverkats och karakteriserats.

Då även tenn/kisel kvantprickar har rapporterats ge användbar absorption av vissa våglängder har en studie genomförts av tillverkning och karakterisering av material som innehåller sådana kvantprickar.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2008. 106 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1213
National Category
Physical Sciences
urn:nbn:se:liu:diva-12575 (URN)978-91-7393-798-6 (ISBN)
Public defence
2008-09-25, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Available from: 2008-09-15 Created: 2008-09-15 Last updated: 2009-05-04Bibliographically approved

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