Korrelation struktureller und optischer Eigenschaften einzelner Halbleiter-Nanodrähte mit Kern-Mantel-Heterostruktur
Zusammenfassung der Projektergebnisse
The central objective of the project has been to correlate the structural and optical properties of GaAs/(In,Ga)As core-shell nanowires (NWs) by establishing nanofocused x-ray diffraction (XRD) and luminescence spectroscopy measurements on one and the same single, free-standing NW. Our joint efforts with complementary expertise have been very successful, and we have so far published 13 articles in peer-reviewed journals. More manuscripts are in preparation. In order to analyze one and the same single, free-standing NW subsequently by different techniques, we developed suitable procedures to grow NWs at pre-defined positions. NWs were synthesized by molecular beam epitaxy (MBE) in the self-assisted vapor-liquid-solid mode utilizing Ga droplets on Si(111) substrates with a SiO2 mask pre-patterned by electron beam lithography. Growth parameters needed for a high yield of vertical NWs growing at the openings in the mask result, however, in fairly thick NWs whose diameter increases towards the top. Such NWs do not favor the formation of a homogenous (In,Ga)As shell. Hence, we devised a two-step growth approach to fabricate thin NWs with homogeneous diameter. As part of this study, we modeled the diameter evolution during growth in a comprehensive way. For the growth of (In,Ga)As shell quantum wells (QWs), the Ga droplets were consumed and growth conditions were adjusted. Very surprisingly, we discovered that the arrangement of the cells in the growth chamber decisively influences the QW luminescence properties. Bright emission is obtained only if the group-III elements and As impinge on different sidewall facets. In this configuration, substrate rotation leads to effectively pulsed growth. The luminescence of group-III arsenide NWs is limited by non-radiative recombination at the surface. Consequently, simple GaAs NWs are typically passivated with an (Al,Ga)As shell. We investigated for GaAs/(In,Ga)As core-shell NWs how different types of outer shell affect their photoluminescence. We found that an AlAs outer shell enhances room temperature emission because thermionic escape of charge carriers from the QW is suppressed. However, in order to maintain efficient luminescence at low temperatures, a GaAs spacer layer is needed between QW and AlAs. For a detailed spectroscopic analysis, narrow emission is needed, but (In,Ga)As shell QWs emit light in a fairly broad spectral range, and at low temperatures carrier localization is significant. We obtained much narrower emission spectra by developing an annealing procedure involving the deposition of a SiNx cap shell for protection. We based our structural analysis of GaAs/(In,Ga)As core-shell NWs on methods of X-ray nanodiffraction (nXRD). To this end, we used suitable setups provided at 3rd generation synchrotron radiation facilities and developed the methodology to access the structure parameters in directions parallel and perpendicular to the NW growth axis. For the first time, we were able to measure the complete strain state of single NWs modelling the measured reciprocal space maps by means of finite element method (FEM) simulations. We discovered small variations in structural parameters comparing various single NWs grown on the same substrate under the same growth conditions. This supports our statement made in the proposal that structure to property correlation requires measurements by the two different probes on one and the same NW. We established this approach successfully by correlating the particular crystal phase structure of individual core-shell-shell NWs with the spatial distribution of the cathodoluminescence (CL) signal taken from the same NWs. It turns out that the maximum CL intensity is measured in the NW region with mixed phase composition. In order to access also small variations in strain we applied the method of coherent diffraction imaging and X-ray ptychography for strain analysis of single NWs. This modern technique makes use of the high degree of coherence of the synchrotron beam provided by 3rd generation facilities. High sensitivity to strain is achieved by phase retrieval of the complex diffraction amplitude. For the first time, the method was applied to core-shell-shell NWs. We discovered an additional threefold symmetry superimposed on the shape function of the major zinc-blende phase nanowires whereas a hexagonal symmetry is found at wurtzite phase segments. At the same time, the use of nanobeam setups at 3rd generation synchrotron facilities comes with the risk of inducing radiation damage to nanoobjects. In order to assess this risk, we systematically studied the impact on single NWs irradiated by a X-ray nanobeam for long time. We discovered that severe radiation damage (up to melting of the NWs) can be induced if the radiation dose exceeds a certain threshold. These findings have to be considered for future nXRD experiments.
Projektbezogene Publikationen (Auswahl)
- Anomalous Strain Relaxation in Core–Shell Nanowire Heterostructures via Simultaneous Coherent and Incoherent Growth, Nano Lett. 17, 136 (2017)
R.B. Lewis, L. Nicolai, H. Küpers, M. Ramsteiner, A. Trampert, and L. Geelhaar
(Siehe online unter https://doi.org/10.1021/acs.nanolett.6b03681) - Characterization of individual stacking faults in a wurtzite GaAs nanowire by nanobeam X-ray diffraction, J. Synchrotron Radiat. 24, 981 (2017)
A. Davtyan, S. Lehmann, D. Kriegner, R.R. Zamani, K.A. Dick, D. Bahrami, A. Al- Hassan, S.J. Leake, U. Pietsch, and V. Holý
(Siehe online unter https://doi.org/10.1107/S1600577517009584) - Self-Assembly of InAs Nanostructures on the Sidewalls of GaAs Nanowires Directed by a Bi Surfactant, Nano Lett. 17, 4255 (2017)
R.B. Lewis, P. Corfdir, J. Herranz, H. Küpers, U. Jahn, O. Brandt, and L. Geelhaar
(Siehe online unter https://doi.org/10.1021/acs.nanolett.7b01185) - Surface preparation and patterning by nano imprint lithography for the selective area growth of GaAs nanowires on Si(111), Semicond. Sci. Technol. 32, 115003 (2017)
H. Küpers, A. Tahraoui, R.B. Lewis, S. Rauwerdink, M. Matalla, O. Krüger, F. Bastiman, H. Riechert, and L. Geelhaar
(Siehe online unter https://doi.org/10.1088/1361-6641/aa8c15) - Threefold rotational symmetry in hexagonally shaped core–shell (In,Ga)As/GaAs nanowires revealed by coherent X-ray diffraction imaging, J. Appl. Crystallogr. 50, 673 (2017)
A. Davtyan, T. Krause, D. Kriegner, A. Al-Hassan, D. Bahrami, S.M. Mostafavi Kashani, R.B. Lewis, H. Küpers, A. Tahraoui, L. Geelhaar, M. Hanke, S.J. Leake, O. Loffeld, and U. Pietsch
(Siehe online unter https://doi.org/10.1107/S1600576717004149) - Coherent X-ray diffraction imaging meets ptychography to study core-shell-shell nanowires, MRS Adv. 3, 2317 (2018)
A. Davtyan, V. Favre-Nicolin, R.B. Lewis, H. Küpers, L. Geelhaar, D. Kriegner, D. Bahrami, A. Al-Hassan, G. Chahine, O. Loffeld, and U. Pietsch
(Siehe online unter https://doi.org/10.1557/adv.2018.466) - Complete structural and strain analysis of single GaAs/(In,Ga)As/GaAs core–shell–shell nanowires by means of in-plane and outof-plane X-ray nanodiffraction, J. Appl. Crystallogr. 51, 1387 (2018)
A. Al Hassan, A. Davtyan, H. Küpers, R.B. Lewis, D. Bahrami, F. Bertram, G. Bussone, C. Richter, L. Geelhaar, and U. Pietsch
(Siehe online unter https://doi.org/10.1107/S1600576718011287) - Determination of indium content of GaAs/(In,Ga)As/(GaAs) core-shell(-shell) nanowires by x-ray diffraction and nano xray fluorescence, Phys. Rev. Mater. 2, 014604 (2018)
A. Al Hassan, R.B. Lewis, H. Küpers, W.-H. Lin, D. Bahrami, T. Krause, D. Salomon, A. Tahraoui, M. Hanke, L. Geelhaar, and U. Pietsch
(Siehe online unter https://doi.org/10.1103/PhysRevMaterials.2.014604) - Diameter evolution of selective area grown Ga-assisted GaAs nanowires, Nano Res. 11, 2885 (2018)
H. Küpers, R.B. Lewis, A. Tahraoui, M. Matalla, O. Krüger, F. Bastiman, H. Riechert, and L. Geelhaar
(Siehe online unter https://doi.org/10.1007/s12274-018-1984-1) - Nanowires Bending over Backward from Strain Partitioning in Asymmetric Core–Shell Heterostructures, Nano Lett. 18, 2343 (2018)
R.B. Lewis, P. Corfdir, H. Küpers, T. Flissikowski, O. Brandt, and L. Geelhaar
(Siehe online unter https://doi.org/10.1021/acs.nanolett.7b05221) - Impact of Outer Shell Structure and Localization Effects on Charge Carrier Dynamics in GaAs/(In,Ga)As Nanowire Core–Shell Quantum Wells, Phys. Status Solidi – Rapid Res. Lett. 13, 1800527 (2019)
H. Küpers, P. Corfdir, R.B. Lewis, T. Flissikowski, A. Tahraoui, H.T. Grahn, O. Brandt, and L. Geelhaar
(Siehe online unter https://doi.org/10.1002/pssr.201800527)
