ERA NanoSci - Coupling of Single Quantum Dots to Two-Dimensional Systems
Theoretische Chemie: Elektronenstruktur, Dynamik, Simulation
Zusammenfassung der Projektergebnisse
Coupling of nano-objects is of broad-ranging interest: it allows the study of fundamental quantum-mechanical effects, which can then be exploited to build devices with new functionality for information storage and processing. For instance, two self-assembled quantum dots (QDs) coupled via tunneling in a vertical stack form an artificial molecule. Such molecules can, on one hand, give new insight into quantum-mechanical coupling. On the other hand, two entangled quantum-bit (qubit) states are formed, which are a basis for quantum computation. Similarly, the coupling of self-assembled QDs to a two-dimensional electron gas (2DEG) provides detailed insight into the physics of the interaction between a zero-dimensional nanoobject and a two-dimensional system (2D system). This interaction has been studied within this project called “Coupling of single quantum dots to two-dimensional systems” (QD2D) with a special focus on GaSb QDs. Sb-based QDs in type-II heterostructures have strong hole confinement yielding a potential retention time of many years at room temperature, enabling the analysis of the influence of charged QDs on a 2D system up to 300 K. In the mid-long term perspective, the results of this project will be important for future generations of memories: knowledge of the interaction of a 2D system with just a single QD as an ultimate limit of charged-based memories (in particular Flash). The scientific inspiration behind the proposed project was to combine studies of these Sb-based QDs with the concept of a split-gate structure. The final goal within the project was to control the chargestate of a single QD and study the interfacing to a 2DHG in transport measurements and show first memory device operations. Four leading groups with complementary expertise in the field of molecular beam epitaxy growth of Sb-based nanostructures (Manus Hayne – Lancaster University), characterization with scanning tunneling measurements (Paul Koenraad – TU Eindhoven), transport measurements (Martin Geller – Uni Duisburg-Essen) and devices based on self-assembled QDs (Dieter Bimberg – TU Berlin) formed a European consortium to meet the challenges. To summarize the progress towards the initial objectives (i) to grow high-quality GaSb QD structures with >10 s storage time at 300 K, (ii) to obtain images of wave functions in Sbbased QDs, (iii) to get charge state detection of a single QD by coupling to a two-dimensional conducting channel and (iv) to realize a fast read-out (< 10 ns) of a charge state at room temperature: University of Lancaster established growth of high-quality GaSb QD and quantum ring (QR) structures that were measured at TU Berlin using deep level transient spectroscopy (DLTS). The highest ever observed maximum activation energy of 670 meV for GaSb QDs with a capture cross section of 10-12 cm2 yielded a room temperature storage time of ~10 ms. An important outcome of the QD2D project was the finding that besides the localization energy the capture cross section is a key property of the QDs to alter the storage time which cannot be neglected. An original expected hole storage time of more than 10 s had to be lowered by three orders of magnitude to a more reasonable value of 10 ms due to the unexpectedly large capture cross-section for GaSb QDs; this value has been achieved experimentally. Samples grown at the University Lancaster were also investigated at TU Eindhoven with cross-sectional scanning tunneling microscopy (X-STM), where images of the Sbcomposition and wave-functions in Sb-based QDs were obtained. To demonstrate the charge state detection by conductance measurements of the 2DHG, TU Berlin prepared different modulation doped field effect transistors (MODFETs) with embedded InAs QDs. For high temperature operation, a GaAs-MODFET with embedded InAs QDs and a 90 %-AlGaAs barrier was prepared. To investigate the temperature characteristic of the MODFET structures, hysteresis curves at different temperatures and sweep times were measured. For the Al0.9Ga0.1As-MODFET a clear hysteresis opening is visible up to a temperature of 290 K. This operation almost at room-temperature showed the feasibility of the conductance measurement as a read-out method for a future memory device based on self-assembled QDs. In addition to the work on high-temperature conductance measurements at TU Berlin, Uni Duisburg-Essen scaled down the MODFET device structure with a different approach to the original idea stated in the research proposal and reached a gate area of 0.5x2.5 µm2, such that only 100 QDs were active. This device showed single charge state detection at 4 K were sub-ensembles or even single QD charging peaks are visible. Hence, the scaling process was successful up to a factor of more than 10 5. However, single charge-detection of single self-assembled QDs is still missing and is planned for the near future. Finally, significant progress has been made in the direction of a fast read-out (<10 ns) of a charge state. A single shot read-out by measurement of the change in conductance of the two-dimensional channel was successful with a read-out time of 3 ns at a temperature of 10 K. Future work will concentrate on measurements on GaSb QDs at higher temperature with the original aim of single dot read-out at room temperature.
Projektbezogene Publikationen (Auswahl)
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Using a two-dimensional electron gas to study nonequilibrium tunneling dynamics and charge storage in self-assembled quantum dots Appl. Phys. Lett. 95, 022113 (2009)
B. Marquardt, M. Geller, A. Lorke, Dirk Reuter, A. D. Wieck
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Using a two-dimensional electron gas to study nonequilibrium tunneling dynamics and charge storage in self-assembled quantum dots Appl. Phys. Lett. 95, 022113 (2009).
B. Marquardt, M. Geller, A. Lorke, Dirk Reuter, A. D. Wieck
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A Two-Dimensional Electron Gas as a Sensitive Detector for Time-Resolved Tunneling Measurements on Self-Assembled Quantum Dots Nanoscale Res. Lett. 5, 829 (2010)
M. Geller, B. Marquardt, A. Lorke, Dirk Reuter, A. D. Wieck
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GaSb quantum dot morphology for different growth temperatures and the dissolution effect of the GaAs capping layer Journal of Physics D: Applied Physics 43, 065402 (2010)
M. Ahmad Kamarudin, M. Hayne, Q. D. Zhuang, O. Kolosov, T. Nuytten, V.V. Moshchalkov and F. Dinelli
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The influence of charged InAs quantum dots on the conductance of a two-dimensional electron gas: Mobility vs. carrier concentration Appl. Phys. Lett. 99, 223510 (2011)
Bastian Marquardt, Andreas Beckel, Axel Lorke, Andreas D. Wieck, Dirk Reuter, and Martin Geller
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The influence of charged InAs quantum dots on the conductance of a two-dimensional electron gas: Mobility vs. carrier concentration Appl. Phys. Lett. 99, 223510 (2011)
Bastian Marquardt, Andreas Beckel, Axel Lorke, Andreas D. Wieck, Dirk Reuter, and Martin Geller
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Time-resolved high-temperature detection with single charge resolution of holes tunneling into many-particle quantum dot states Phys. Rev. B 84, 075309 (2011).
T. Nowozin, A. Marent, G. Hönig, A. Schliwa, D. Bimberg, A. Beckel, B. Marquardt, A. Lorke, and M. Geller
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Transport spectroscopy of non-equilibrium many-particle spin states in self-assembled quantum dots. Nature Commun. 2, 209 (2011)
B. Marquardt, M. Geller, B. Baxevanis, D. Pfannkuche, A. D. Wieck, D. Reuter & A. Lorke
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Tuning the properties of exciton complexes in self-assembled GaSb/GaAs quantum rings .Phys. Rev. B 83, 115311 (2011)
M. Ahmad Kamarudin, M. Hayne, R. J. Young, Q. D. Zhuang, T. Ben and S. I. Molina
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GaSb/Gas quantum dot formation and demolition studied with cross-sectional scanning tunnelling microscopy. Appl. Phys. Lett. 100, 142116 (2012)
E.P. Smakman, J.K. Garleff, R.J. Young, M. Hayne, P. Rambabu and P.M. Koenraad
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Linking structural and electronic properties of high-purity self-assembled GaSb/GaAs quantum dots Phys. Rev. B 86, 035305 (2012)
T. Nowozin, A. Marent, L. Bonato, A. Schliwa, D. Bimberg, E.P. Smakman, J. Garleff, P.M. Koenraad, R.J. Young, and M. Hayne
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Optical observation of single-carrier charging in type-II quantum ring ensembles. Appl. Phys. Lett. 100, 082104 (2012)
R. J. Young, E. P. Smakman, A. M. Sanchez, P. Koenraad, P. Hodgson, and M. Hayne
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Time-resolved detection of many-particle hole states in InAs/GaAs quantum dots using a two-dimensional hole gas up to 77 K Phys. stat. sol. C 9, No. 2, 243 (2012)
T. Nowozin, A. Marent, D. Bimberg, A. Beckel, B. Marquardt, A. Lorke, and M. Geller
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„Memory“, US Patent Application 12/970,744 (16.12.2010) and “Speicherzelle auf Basis von Nanostrukturen aus Verbindungshalbleitern“ PCT Application PCT/EP2011/072181 (8.12.2011), US-Patent 8,331,142 B2 granted on Dec. 11, 2012
Marent, M. Geller, T. Nowozin, D. Bimberg
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High-accuracy scanning-probe-microscopy analysis of nanoscale semiconductor layers using beam-exit Ar-ion polishing Applied Materials and Interfaces 5, 3241 (2013).
A. J. Robson, I. Grishin, R.J. Young, A.M. Sanchez, O.V. Kolosov and M. Hayne
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Long-wavelength photoluminescence from stacked layers of high quality type-II GaSb/GaAs quantum rings Crystal Growth and Design 13, 1226 (2013)
P.J. Carrington, R.J. Young, P.D. Hodgson, A.M. Sanchez, M. Hayne and A. Krier
