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Spatially direct and indirect transitions observed for Si/Ge quantum dots
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
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2003 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 82, no 26, 4785-4787 p.Article in journal (Refereed) Published
Abstract [en]

The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.

Place, publisher, year, edition, pages
2003. Vol. 82, no 26, 4785-4787 p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13794DOI: 10.1063/1.1587259OAI: oai:DiVA.org:liu-13794DiVA: diva2:21558
Available from: 2006-02-27 Created: 2006-02-27 Last updated: 2017-12-13
In thesis
1. Near-infrared photodetectors based on Si/SiGe nanostructures
Open this publication in new window or tab >>Near-infrared photodetectors based on Si/SiGe nanostructures
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Two types of photodetectors containing Ge/Si quantum dots have been fabricated based on materials grown by molecular beam epitaxy and characterized with several experimental techniques. The aim was to study new device architectures with the implementation of Ge nanostructures, in order to obtain high detection efficiency in the near infrared range at room temperature.

Heterojunction bipolar phototransistors were fabricated with 10 Ge dot layers in the base-collector (b-c) junction. With the illumination of near infrared radiation at 1.31 to 1.55 µm, the incident light would excite the carriers. The applied field across the b-c junction caused hole transport into the base, leading to a reduced potential barrier between the emitter-base (e-b) junction. Subsequently, this resulted in enhanced injection of electrons across the base into the collector, i.e., forming an amplified photo-induced current. We have therefore obtained significantly enhanced photo-response for the Ge-dot based phototransistors, compared to corresponding quantum dot p-i-n photodiodes. Responsivity values up to 470 mA/W were measured at 1.31 µm using waveguide geometry, and ∼2.5 A/W at 850 nm, while the dark current was as low as 0.01 mA/cm2 at –2 V.

Metal-oxide field-effect phototransistors were also studied. These lateral detectors were processed with three terminals for source, drain and gate contacts. The Ge quantum dot layers were sandwiched between pseudomorphically grown SiGe quantum wells. The detector devices were processed using a multi-finger comb structure with an isolated gate contact on top of each finger and patterned metal contacts on the side edges for source and drain. It was found that the photo-responsivity was increased by a factor of more than 20 when a proper gate bias was applied. With VG above threshold, the measured response was 350 and >30 mA/W at 1.31 and 1.55 µm, respectively.

Properties of Si/Si1-xGex nanostructures were examined, in order to facilitate proper design of the above mentioned transistor types of photodetectors. The carrier recombination processes were characterized by photoluminescence measurements, and the results revealed a gradual change from spatially indirect to direct transitions in type II Si1-xGex islands with increased measurement temperature. Energy dispersive X-ray spectrometry of buried Ge islands produced at different temperatures indicated a gradual decrease of the Ge concentration with temperature, which was due to the enhanced intermixing of Si and Ge atoms. At a deposition temperature of 730°C the Ge concentration was as low as around 40 %.

Finally, the thermal stability of the Si/SiGe(110) material system, which is a promising candidate for future CMOS technology due to its high carrier mobility, was investigated by high resolution X-ray diffraction reciprocal space mapping. Anisotropic strain relaxation was observed with maximum in-plane lattice mismatch in the [001] direction.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi, 2006
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1003
Keyword
SiGe, Ge dots, nanostructures, molecular beam epitaxy, photodetector
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-5909 (URN)91-85497-24-X (ISBN)
Public defence
2006-02-27, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
On the day of the defence date the status of article IV was Manuscript and the title was "A three-terminal Ge dot/SiGe quantum well MOSFET photodetector for near infrared light detection"; the status of article VI was Submitted and the title was "Band alignment studies in Si/Ge quantum dots based on optical and structural investigations"; the status of article VII was Manuscript and the title was "Thermal stability of SiGe/Si(110) investigated by high-resolution X-ray diffraction reciprocal space mapping".Available from: 2006-02-27 Created: 2006-02-27 Last updated: 2009-02-18
2. Spectroscopy of semiconductor quantum dots
Open this publication in new window or tab >>Spectroscopy of semiconductor quantum dots
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum dots in the Si/Ge and InAs/GaAs materials systems are examined by means of photoluminescence. The spectroscopic study of Si/Ge quantum dots has demonstrated two different radiative recombination channels in the type II band alignment: The spatially direct transition inside the dot and the spatially indirect transition across the dot interface. Increased sample temperature results in a gradual transfer from the spatially indirect to the spatially direct recombination due to higher oscillator strength combined with the increased electron population inside the dot. In contrast to the spatially direct transition, the spatially indirect transition is shown to be sensitive to the carrier density due to the band bending at the Si/Ge interface. Due to an increased Si/Ge intermixing and hence reduced strain in the Si barrier, a reduction of the conduction band offset at increased growth temperatures is observed utilizing the different recombination channels as probes. The optical properties as derived from photoluminescence are correlated with the structural properties obtained by atomic force microscopy. Furthermore, by applying an electric field across the Si/Ge quantum dot structure, a reversed quantum confined Stark effect is demonstrated for the spatially indirect transition. By switching between the two different field directions, unique information on the growth related asymmetric strain profile derived at the through self-assembly of the quantum dots can be gained since corresponding information can not be obtained for type I systems.

The studies of the InAs/GaAs quantum dots show that external electric and magnetic fields alter the in-plane carrier transport to the dots. The results obtained from the micro-photoluminescence exciton spectra of a single dot demonstrate a redistribution of the excitonic lines when a lateral electric field is applied. This fact exhibits an effective charge reconfiguration of the dot from a purely negative charge state to a neutral state, demonstrating that the number of electrons and holes are controlled by the electric field. The model proposed to explain the charge redistribution is based on an effective hole localization at the potential fluctuations of the wetting layer at low temperatures and low fields. Furthermore, it is demonstrated that the quantum dot photoluminescence signal is considerably increased (up to a factor of 4) depending on the magnitude of the external electric field. The experimental results also show that the internal field is altered by an additional infrared illumination of the sample. An applied magnetic field perpendicular to the quantum dot layer at low temperatures is found to enhance the carrier localization in the wetting layer and accordingly reduce the quantum dot photoluminescence intensity. At higher temperatures (>100K), an enhanced photoluminescence intensity is instead observed due to increased capture, localization, and recombination rate of the carriers in the quantum dots.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2005. 65 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 976
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-31204 (URN)16952 (Local ID)91-85457-48-5 (ISBN)16952 (Archive number)16952 (OAI)
Public defence
2005-11-18, Hörsal Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2012-12-11

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Larsson, MatsElfving, AndersHoltz, Per-OlofHansson, GöranNi, Wei-Xin

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