U.W.Pohl

InGaAs/GaAs Quantum Dots for 1.3 µm Applications

U. W. Pohl*, A. Schliwa. I. Kaiander, T. Germann, A. Strittmatter, and D. Bimberg

Institut für Festkörperphysik EW5-1, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany

* Email: pohl@physik.tu-berlin.de

Zero-dimensional charge carrier localization in the active region of a semiconductor laser was predicted two decades ago to lead to improved device performance. Self-organized growth of quantum dots (QDs) has since then evolved into the decisive method for defect-free QD fabrication to realize such localization, and the first QD injection laser demonstrated the basic validity of previous predictions [1]. Much effort was subsequently spend to extend the emission wavelength of In(Ga)As QDs in GaAs matrix to the datacom range at 1.3 µm. The basic approach aims at decreasing the energy of the dot’s electronic ground state by lowering the hydrostatic strain exerted on the dot by the matrix. Model calculations proved that strain release induced by a thin InGaAs layer with a lower In content on top of the QDs significantly decreases the ground state energy [2]. Combination of such strain-reducing layer (SRL) with a similar, additional layer underneath the dots leads to the dot-in-a-well (DWELL) approach which was successfully applied using molecular beam epitaxy to fabricate GaAs-based QD devices emitting at 1.3 µm. QD devices reaching this target have been fabricated only very recently using metalorganic vapor phase epitaxy (MOVPE) with its scaling ability for mass production. In the presentation concepts for growing InGaAs dots for 1.3 µm emission are discussed and encouraging latest results are presented. We used tertiarybutylarsine as a favorable arsenic precursor [3] and an individual adjustment of growth parameters within the stack of active QDs in a laser [4]. PL is used as a monitor to identify critical growth parameters. Data indicate a crucial role of the V/III ratio applied during growth [5]. InGaAs/GaAs QDs with a strain-reducing layer grown using a low V/III ratio show a robust thermal stability. The good performance is promising to open a way for 1.3 µm device fabrication using metalorganic vapor phase epitaxy.

Video Content Length 15:36 Copyright: © 2008 Pohl et al

References

[1] N. Kirstaedter et al., Electron.Lett. 30, 1416 (1994).
[2] F. Guffarth, R. Heitz, A. Schliwa, O. Stier, N.N. Ledentsov, A. R. Kovsh, V. M. Ustinov, and D.Bimberg, Phys. Rev. B 64, 085305 (2001).
[3] R. Sellin, I. Kaiander, D. Ouyang, T. Kettler, U. W. Pohl, D. Bimberg, N. D. Zakharov, P. Werner, Appl. Phys. Lett. 82, 841 (2003).
[4] A. Strittmatter, T. D. Germann, Th. Kettler, K. Posilovic, U. W. Pohl, D. Bimberg, Appl. Phys. Lett. 88, 262104 (2006).
[5] A. Strittmatter, T. Germann, K. Posilovic, Th. Kettler, U. W. Pohl, D. Bimberg, MRS Fall Meeting, Boston, USA 2006.

Citation:

U. W. Pohl, A. Schliwa. I. Kaiander, T. Germann, A. Strittmatter, and D. Dimberg,
OAtube Nanotechnology 1, 905 (2008). http://www.oatube.org/2008/09/uwpohl.html

J.H.Lee

Localized Fabrication of Self-Assembled Quantum Structures on photolithographically patterned surfaces with the surface modulation of only 35nm

J. H. Lee*, Zh. M. Wang, B. L. Liang, W. T. Black, Vas P. Kunets, Yu I. Mazur, and G. J. Salamo
Institute of Nanoscale Science and Engineering University of Arkansas, Fayetteville, AR 72701, USA
* Email: jxl14@uark.edu

Semiconductor quantum nanostructures with two and three dimensional confinement have received significant attention due to their unique physical, optical and electronic properties [1-5], which have led to many device applications [6-8]. For some device applications, localized fabrication of quantum structures is of necessary. To realize localization of quantum nanostructures, photolithographically patterned surface can provide a successful route for the nucleation to generate tailored quantum nanostructures and the ensembles of quantum nanostructures. Therefore, the use of nano-scale patterns to guide the formation of nano- and quantum-structures has attracted considerable attentions [9-12]. To date, most investigations to generate ordered arrays of quantum nanostructures have been demonstrated on deeply patterned substrates that are on the order of a few hundred nanometers to microns in depth.

In this work, we present localized formations of several quantum structures including self-assembled InAs quantum dots (QDs) and GaAs quantum wires on photolithographically nano-patterned GaAs (100) surfaces using molecular-beam epitaxy (MBE). In distinction from the former works [9-12], the presented results were demonstrated on nano-scale shallow patterns of only 35nm, which can potentially provide flexibility on quantum-structures based device fabrication.

Video Content Length 25:23 Copyright: © 2008 Lee et al

References

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Citation

J. H. Lee, Zh. M. Wang, B. L. Liang, W. T. Black, Vas P. Kunets, Yu I. Mazur, and G. J. Salamo,
OAtube Nanotechnology 1, 904 (2008). http://www.oatube.org/2008/09/jhlee.html

A.Othonos

Surface-related states in oxidized silicon nanocrystals enhance carrier relaxation and inhibit Auger recombination
 
Andreas Othonos^1* , Emmanouil Lioudakis^1  A. G. Nassiopoulou^2 

^1 Department of Physics, Research Center of Ultrafast Science, University of Cyprus P.O. Box 20537, 1678, Nicosia, Cyprus
^2 IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 153 10 Athens, Greece 
*Email: othonos@ucy.ac.cy

Silicon is considered as the key material of today’s integrated circuit technology; however, one of the major drawbacks of this semiconductor is its inability to efficiently emit light. The observation of efficient photoluminescence a few years ago from porous silicon [1-4] and silicon nanocrystals [5] has provided hope for Si-based optoelectronics and has stirred research interest in the area of Si nanostructures as a potential candidate for silicon based emission devices [6-9]. It is well known that semiconductor nanocrystals (NCs) exhibit interesting size dependent properties, mainly due to the large fraction of surface atoms to the total number of atoms in the NC and quantum size effects that may allow tuning of the light emission peak from such nanostructures. Although there have been different forms of Si nanocrystals manufactured, Si-NCs embedded in a amorphous SiO2 matrix [10, 11] have gained considerable interest due to their PL stability with time for light emission applications and their nanoelectronics applications. Since the demonstration of this type of Si-NCs there has been a significant research interest in their photoluminescence properties, with little emphasis on the ultrafast carrier dynamics [12].

In this work we have studied femtosecond carrier dynamics in oxidized silicon NCs and the role that surface-related states play to the various relaxation mechanisms over a broad range of photon excitation energy corresponding to energy levels below and above the direct bandgap of the formed NCs [13]. Transient photoinduced absorption techniques [14] have been employed to investigate the effects of surface-related states on the relaxation dynamics of photogenerated carriers in 2.8 nm oxidized silicon NCs. Independent of the excitation photon energy, non-degenerate measurements reveal several distinct relaxation regions corresponding to relaxation of photoexcited carriers from the initial excited states, the lowest indirect states and the surface-related states. Furthermore, degenerate and non-degenerate measurements at difference excitation fluences reveal a linear dependence of the maximum of the photoinduced absorption signal and an identical decay suggesting that Auger recombination does not play a significant role in these nanostructures even for fluence generating up to 20 carriers/NC.

Video Content Length 20:00 Copyright: © 2008 Othonos et al

Reference
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[11] J  Heitmann, F. Muller, M. Zacharias and U Gosele, Advanced Materials 17 (7) 795 (2005).
[12] E. Lioudakis, A.G. Nassiopoulou, A. Othonos, Appl. Phys. Lett. 90, 171103 (2007).
[13] A. Othonos, E. Lioudakis, A.G. Nassiopoulou, accepted for publication in Nanoscale Research Letters.
[14] A. Othonos,  J. Appl. Phys. 83, 1789 (1998).

Citation

A. Othonos, E. Lioudakis, and A. G. Nassiopoulou,
OAtube Nanotechnology 1, 903 (2008). http://www.oatube.org/2008/09/aothonos.html

W.Wu

Fabrication of large area periodic nanostructures using Nanosphere Photolithography

Wei Wu, Dibyendu Dey, Alex Katsnelson, Omer G. Memis, Hooman Mohseni*
EECS Department, Northwestern University, Evanston, IL, USA
* Email:hmohseni@ece.northwestern.edu

Large area periodic nanostructures exhibit unique optical and electronic properties and have found many applications, such as photonic band-gap materials [1], high dense data storage [2], and photonic devices [3]. To fabricate these periodic nanostructures, conventional photolithography methods cannot easily reach the resolution required. High-resolution methods such as e-beam lithography and focal ion beam milling are too slow to reach a large area because of their inherent serial property. Nano-imprint methods are fast to be applied, but it needs to use the mold, which requires the same resolutions as the patterns. So, it also benefits from the development of fast, economic and high throughput fabrication methods with a high resolution. We have developed a maskless photolithography method—Nanosphere Photolithography (NSP)—to produce a large area of periodic nanopatterns in photoresist utilizing the silica micro-spheres to focus UV light [4][5]. Here we will extend the idea to fabricate large areas of periodic metallic nanostructures using the NSP method. We produced a large area periodic uniform nanohole array perforated in different metallic films, such as gold and aluminum. The diameters of these nanoholes are much smaller than the wavelength of UV light used and they are very uniformly distributed. The method introduced here inherently has both the advantages of photolithography and self-assembled methods. Besides, it also generates very uniform repetitive nanopatterns because the focused beam waist is almost unchanged with different sphere sizes.

Video Content Length 12:33 Copyright: © 2008 Wu et al

References
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[3] A. G. Brolo, E. Arctander, R. Gordon, B. Leathem and K. L. Kavanagh, Nano Lett. 4, 2015 (2004) .
[4] W. Wu, O. G. Memis, A. Katsnelson and H. Mohseni, Nanotechnology, 18, 485302 (2007).
[5] W. Wu, D. Dey, O. G. Memis, A. Katsnelson and H. Mohseni, Nanoscale Res. Lett. 3, 123 (2008).

Citation
W. Wu, D. Dey, A. Katsnelson, O. G. Memis, and H. Mohseni,
OAtube Nanotechnology 1, 902 (2008). http://www.oatube.org/2008/09/wwu.html

J.I.Climente

Antibonding hole ground state in artificial molecules

J.I.Climente^1,^2 *, M. Korkusinski^3 , M.F. Doty^4, M. Scheibner^4, A.S. Bracker^4, G. Goldoni^2, D. Gammon^4, and P. Hawrylak^3
^1 Departament de Química Física i Analítica, Universitat Jaume I, Castellon, Spain
^2 National Research Center S3, CNR-INFM, Modena, Italy
^3 Institute of Microstructural Sciences, National Research Council, Ottawa, Canada
^4 Naval Research Laboratory, Washington, USA
* Email: climente@unimo.it

Resonant tunneling of carriers between vertically coupled quantum dots enables the formation of hybridized, molecular-like orbitals which are important in many quantum dot-based devices, including those aiming at optically-controlled quantum information storage.[1] The differences in size and composition of quantum dots is overcome by the application of the vertical electric field, which brings the two quantum dot levels into resonance and induces either electron or hole tunneling.[2] The tunneling of electrons is now well understood[1-4], it leads to the formation of bonding molecular ground states in analogy to natural diatomic molecules. However, tunneling of holes does not have a counterpart in diatomic molecules and is less understood. In fact, previous atomistic calculations suggested a reversal of bonding and antibonding hole molecular ground states as the interdot barrier distance increases.[5-7]

In this work, we present theory and experimental observation of the formation of the antibonding hole molecular ground state. Using a 4-band k·p approximation, the hole states are described as Luttinger spinors[8], which contain all the relevant symmetries. It is shown that the strong spin-orbit interaction in the valence band breaks the parity in the growth direction, mixing bonding and antibonding heavy- and light-hole components of the spinor. This mixing destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal. Molecular ground states are then found to have up to ~95% antibonding character. These conclusions are reproduced by numerical, atomistic multi-million-atom calculations using a sp^3d^5s* tight-binding model applied to the realistic self-assembled InGaAs/GaAs double quantum dot structures, including strain, structural asymmetries and vertical electric fields. The results are in qualitative agreement with the k·p theory and predict a bonding-to-antibonding ground state reversal at interdot distances of d»2 nm. Clear experimental evidence of this peculiar hole behavior is found in magneto-photoluminescence experiments of double dots. The character of the hole molecular orbitals is identified from the electric field dependence of the Zeeman splitting of the neutral exciton when resonant hole tunneling is induced[9]. Comparison of samples with different inter-dot separation shows the bonding-to-antibonding ground state reversal in agreement with theory [10].

Video Content Length 25:36 Copyright: © 2008 Climente et al.

References

[1] M. Bayer et al., Science 291, 451 (2001).
[2] E.A. Stinaff et al., Science 311, 636 (2006).
[3] A.S. Bracker et al., Appl. Phys. Lett. 89, 233110 (2006).
[4] H.J. Krenner et al., Phys. Rev. Lett. 94, 057402 (2005); G. Ortner et al. ibid 94, 157401 (2005).
[5] W. Jaskolski, Acta Phys.Pol. A 106, 193 (2004); Phys. Rev. B 74, 195339 (2006).
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[7] M. Korkusinski et al., Proceedings of the 27th Int. Conf. Phys. Semicond., 685 (2005).
[8] J.M. Luttinger, and W. Kohn, Phys. Rev. 97, 869 (1955); L.Rego et al . Phys. Rev. B 55, 15694 (1997).
[9] M.F. Doty et al. Phys. Rev. Lett. 97, 197202 (2006).
[10] M. F. Doty et at. ArXiv:0804.3097 (2008).

Citation

J.I. Climente, M. Korkusinski, M.F. Doty, M. Scheibner, A.S. Bracker, G. Goldoni, D. Gammon, and P. Hawrylak,
OAtube Nanotechnology 1, 901 (2008). http://www.oatube.org/2008/09/jiclimente.html