Temperature-dependent Raman investigation of rolled up InGaAs/GaAs microtubes
© Rodriguez et al.; licensee Springer. 2012
Received: 20 July 2012
Accepted: 12 October 2012
Published: 26 October 2012
Large arrays of multifunctional rolled-up semiconductors can be mass-produced with precisely controlled size and composition, making them of great technological interest for micro- and nano-scale device fabrication. The microtube behavior at different temperatures is a key factor towards further engineering their functionality, as well as for characterizing strain, defects, and temperature-dependent properties of the structures. For this purpose, we probe optical phonons of GaAs/InGaAs rolled-up microtubes using Raman spectroscopy on defect-rich (faulty) and defect-free microtubes. The microtubes are fabricated by selectively etching an AlAs sacrificial layer in order to release the strained InGaAs/GaAs bilayer, all grown by molecular beam epitaxy. Pristine microtubes show homogeneity of the GaAs and InGaAs peak positions and intensities along the tube, which indicates a defect-free rolling up process, while for a cone-like microtube, a downward shift of the GaAs LO phonon peak along the cone is observed. Formation of other type of defects, including partially unfolded microtubes, can also be related to a high Raman intensity of the TO phonon in GaAs. We argue that the appearance of the TO phonon mode is a consequence of further relaxation of the selection rules due to the defects on the tubes, which makes this phonon useful for failure detection/prediction in such rolled up systems. In order to systematically characterize the temperature stability of the rolled up microtubes, Raman spectra were acquired as a function of sample temperature up to 300°C. The reversibility of the changes in the Raman spectra of the tubes within this temperature range is demonstrated.
Self-positioning nanostructures with controlled composition and size compatible with mass-production fabrication techniques have been studied for almost two decades, giving rise to the so-called smart tubes in exciting forms such as nanojets[1, 2]. The versatility in controlling the size and composition of the smart tubes has made them attractive candidates for applications ranging from spintronics to designing novel substrates for cell adhesion. However, apart from the work of Deneke et al., not much attention has been devoted to the questions concerning how GaAs/InGaAs smart tubes collapse and the role of the TO phonon on identifying defective tubes. In the present work, we also investigate the temperature stability of these rolled up structures by measuring the LO phonon modes of GaAs and InGaAs in situ while heating the structures up to 300°C.
Results and discussion
Strain made visible by Raman spectroscopy imaging
Acquiring Raman spectra across a sample in a point-wise manner allows spatial sample heterogeneities arising from variations in physico-chemical properties. These can be made visible by selecting relevant Raman shifts and plotting their intensities as color-coded pictures (Figures2b,c,d). Individual mapping of the unstrained LO GaAs (InGaAs) mode intensity allows a clear visualization of the substrate (the faulty region of the tube shown in Figure2b). In this section, we aim at identifying common signatures in faulty rolled up tubes. These tubes are those lacking the uniform cylindrical symmetry of perfect roll-ups, and their analysis might give clues about the origin of the structural failure of the tubes as well as their early detection using Raman spectroscopy.
TO phonons: the signature of faulty structures
The tube presented in Figure2b is analyzed in terms of the peak position of the LO mode (Figure4a) and the Raman intensity of the TO mode (Figure4b). Notice in the map of the LO peak position that, contrary to the case of the cone discussed above, the regions with lowest LO Raman shifts do not match with the regions where the TO mode is observed. So far, the reasons for this observation are unclear. Nevertheless, the TO peak intensity is remarkably high in regions of the tube that show high structural disorder. This reinforces the hypothesis of the link of this mode with collapse of the rolled up structure.
Heating up: are the tubes stable?
Finally, the effect of temperature was studied on a uniform tube in order to calibrate the temperature-related shifts and to investigate whether or not the structures are stable at temperatures up to 300°C. To the best of our knowledge, the only similar study reported on the temperature stability of rolled up structures was made in Deneke et al. and Songmuang et al., although in those reports, the sample temperature and peak positions were not determined independently as is the case in the present work.
These observations show that defect-free tubes remain stable at relatively high temperatures (which exceed the temperature range for standard electronic applications of maximum 85°C for the industrial grade) due to the good heat dissipation of the membrane-like tubes.
Raman spectroscopy with mapping capabilities becomes an interesting tool for the assessment of heterogeneities and fractures in faulty InGaAs/GaAs semiconductor rolled up tubes. The appearance of the TO mode of the GaAs tube is suggested as an indication of defects in these structures due to a further relaxation of the selection rules in regions with high defect concentration, although this requires statement/hypothesis further analysis. Raman spectroscopy could then be used as a failure diagnostic tool in structural characterization. Thermal stability of the structures was verified for temperatures up to 300°C. A linear red-shift in the Raman spectra of the structures has been observed which can be attributed to thermal expansion without involving any damage or degradation of the tubes.
The work was supported by DFG project ZA146/22-1 ‘Raman investigations of In(Ga)As/Al(Ga)As self-assembled quantum dot structures: from ensembles to single quantum dots’, by DFG Research Unit 1713 ‘Sensoric Micro- and Nanosystems’, by the Bundesministerium für Bildung und Forschung, and Project ‘Kompetenznetzwerk für Nanosystemintegration – Anwendung von Nanotechnologien für energieeffiziente Sensorsysteme (nanett)’, project number 03IS2011. Thanks to Alexander Villabona for the support in setting up the Raman imaging experiments.
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