Intraband Absorption in Ge/Si Self-Assembled Quantum Dots
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P. Boucaud*, V. Le Thanh*, S. Sauvage*, T. Brunhes*, F. Fortuna**, D. Debarre*, D. Bouchier* * IEF, Universit6 Paris XI, UMR CNRS 8622, Bit. 220, 91405 Orsay, FRANCE, [email protected] ** CSNSM, Universit6 Paris XI, Bait. 108, 91405 Orsay, FRANCE ABSTRACT Mid-infrared intraband absorption in Ge/Si self-assembled quantum dots is reported. The self-assembled quantum dots are grown by ultra-high-vacuum chemical vapor deposition. The intraband absorption is observed using a photoinduced absorption technique. The mid-infrared absorption, which is in-plane polarized, is maximum around 300 meV. The absorption is attributed to a quantum dot hole transition between bound and continuum states. The absorption cross section is deduced from the saturation of the photoinduced intraband absorption. An inplane absorption cross section as large as 2 x 10-13 cm2 is measured for one dot plane. INTRODUCTION The intraband transitions which occur between quantum confined states in the conduction or in the valence band of semiconductor quantum dots have become a new research subject in the last years. Intraband transitions have been mostly observed in II1-V self-assembled quantum dots [1,2]. Most of the work reported to date was related to absorption although nonlinear optical measurements using intraband transitions has been recently reported [3]. The observation of mid-infrared absorption has opened the route to the development of new quantum
dot infrared photodetectors. Mid-infrared photoconductivity using InAs self-assembled quantum dots has been first reported [4,5] shortly followed by the characterization of quantum dot devices [6,7]. Si-based self-assembled quantum dots are also attracting a lot of interest due to their compatibility with Si-based signal processing. Intraband transitions in these nanostructures have only been observed very recently [8,9]. These first observations are likely to lead to the realization of new Si-based quantum dot infrared photodetectors. RESULTS Growth and structural characterizations of Ge/Si self-assembled quantum dots The self-assembled quantum dots were grown by ultra-high-vacuum chemical vapor deposition using silane and germane as gas precursors [10]. The samples, grown at 550' C, consist of 10 Ge layers separated by 22 nm thick Si barriers. Samples with different dot sizes were characterized. For infrared measurements, the samples were polished with 45' facets in a multipass waveguide geometry [2]. The self-assembled quantum dots are nominally undoped.
9 Mat. Res. Soc. Symp. Proc. Vol. 571 ©2000 Materials Research Society
A cross section electronic microscopy image of the 10 Ge layer sample is shown in figure 1. Due to the small thickness of the Si barrier, the quantum dots are vertically organized. As the number of layers increases, the quantum dot volume increases.
Figure 1. Transmission electronic microscopy image of the Ge/Si multilayer quantum dot sample. The average quantum dot base width is around 100 nm while the height varies between 4 and 7 nm. For the present growth conditio
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