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Growth-temperature-dependent band alignment in Si/Ge quantum dots from photoluminescence spectroscopy
Linköping University, Department of Physics, Chemistry and Biology, Materials Science . 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, Materials Science . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
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2006 (English)In: Physical Review B, ISSN 1098-0121, Vol. 73, no 19, 195319-1--195319-7 p.Article in journal (Refereed) Published
Abstract [en]

The present work is a photoluminescence study of Si-embedded Stranski-Krastanov Ge quantum dots. The value of the conduction band offset is a result of the magnitude of the tensile strain in the Si surrounding the compressive strained Ge dot. Due to the increased Si/Ge intermixing and reduced strain in the Si barrier, a reduction of the conduction band offset is observed at increased growth temperatures. The optical properties as derived from photoluminescence spectroscopy are correlated with structural properties obtained as a function of the growth temperature. High growth temperatures result in large Ge dots with low density due to the pronounced surface diffusion and Si/Ge intermixing. As confirmed by photoluminescence, the band gap of the Ge dots increases with increased growth temperature due to the higher degree of Si/Ge intermixing. The band alignment is of type II in these structures, but the occurrence of both spatially indirect and spatially direct transitions are confirmed in temperature-dependent photoluminescence measurements with varied excitation power conditions. An increasing temperature results in a gradual transition from the spatially indirect to the spatially direct recombination in the type-II band lineup, due to higher oscillator strength for the spatially direct transition combined with a higher population factor at higher temperatures.

Place, publisher, year, edition, pages
2006. Vol. 73, no 19, 195319-1--195319-7 p.
Keyword [en]
germanium, silicon, elemental semiconductors, semiconductor quantum dots, semiconductor growth, photoluminescence, conduction bands, internal stresses, surface diffusion, chemical interdiffusion, electron-hole recombination, oscillator strengths
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-13795DOI: 10.1103/PhysRevB.73.195319OAI: oai:DiVA.org:liu-13795DiVA: diva2:21559
Note
Original Publication: Mats Larsson, Anders Elfving, Wei-Xin Ni, Göran V. Hansson and Per-Olof Holtz, Growth-temperature-dependent band alignment in Si/Ge quantum dots from photoluminescence spectroscopy, 2006, Physical Review B, (73), 195319. http://dx.doi.org/10.1103/PhysRevB.73.195319 Copyright: American Physical Society http://www.aps.org/ Available from: 2009-01-15 Created: 2009-01-15 Last updated: 2009-03-04Bibliographically approved
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

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Larsson, MatsElfving, AndersNi, Wei-XinHansson, Göran V.Holtz, Per-Olof

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