TEM studies of nanostructures

Catherine Bougerol, R. André, H. Boukari, B. Daudin, H. Mariette, S. Tatarenko, T. Aichele, B. Amstatt, S. Founta, L. Maingault, R. Najjar, I.C. Robin
Overview and results
The goal of our electron microscopy work has been to obtain structural informations at the atomic scale on different nanostructures of II-VI (selenides and tellurides) and III-V semiconductors (quantum wells (QWs) quantum dots (QDs) nanowires (NWs)), grown in the NPSC team for most of them, and in close relation with the optical properties studies. Two main approaches will be illustrated in the report :
- studies of the nanostructures as a function of the growth parameters (such as temperature or elements’ fluxes), and of their evolution during the successive steps of the growth process
- studies of the morphology, the defects and the strain states of the nanostructures.
High Resolution Electron Microscopy has been the main method of investigation. However, complementary studies have been undertaken in some cases by chemical imaging techniques such as EFTEM and Z-contrast imaging. As often as possible, quantitative analyses have been carried out, in order to get informations on the strain state of the nanostructures or on the fluctuations of the composition for instance.
Most of the work has been done on the microscopes of the SP2M-LEMMA group. Furthermore, a close and fruitful collaboration has been established with Prof. G. van Tendeloo (EMAT, Antwerpen, Belgium) through a “Partenariat Hubert Curien – Tournesol” program.
Influence of the temperature of encapsulation on CdSe-ZnSe QDs
It is known that CdSe QDs can be obtained by desorption of amorphous Se deposited on 3 MLs of strained CdSe. We have then studied the influence of the deposition temperature of a capping ZnSe barrier. After encapsulation at 280°C, surprisingly, we did not observe any reminiscence of CdSe islands pointing to the surface. On the contrary, dark-field images reveal a sharp CdSe/ZnSe interface on the upper-barrier side, but large fluctuations of the CdSe layer thickness towards the lower barrier (Top left). The contrast on high resolution images (Bottom left) indicates a non homogenous distribution of Cd in the island, with a maximum at the outskirt. However, it is known that in a ZnCdSe ternary alloy of sphallerite structure, a contrast inversion occurs at about 41% of Cd. Z-contrast imaging (Top right) confirms that CdSe forms flat islands about 50 nm long and 8nm high, originating from the wetting layer and pointing to the substrate, with a Cd-poor core. From off-axis high resolution image analyses carried out with both the DALI software and the Phase Method it appears that the core contains 30-40% Zn and the shell consists of almost pure CdSe. On the other hand, when the encapsulation occurs at 240°C, QDs are formed (Bottom right),about 1nm high and laying on a 1.5nm wetting layer. There is no shell effect and the orientation of the dots is the same than before deposition of the capping layer.
ZnSe/CdSe nanowires
Recently, ZnSe/CdSe nanowires have been grown by molecular beam epitaxy (MBE) in the vapour-liquid-solid (VLS) growth mode assisted by gold catalysts. Size, shape and crystal structure are found to strongly depend on the growth conditions. For Zn rich growth conditions, nano-needles are formed, about 1 µm long, with a diameter varying between 80nm at the bottom and 5nm at the top (Top left). The NW grows with the wurtzite type structure, along [11-20] (Top right). For Se rich growth conditions, up to 400°C temperature, NWs of regular diameter are obtained (20-50nm) (Bottom left). They also grow in the wurzite type structure, but along the [0001] axis. A low growth temperature (300°C) leads to the formation of many stacking faults corresponding to the cubic zinc blende structure are observed (Bottom right). On the contrary, higher growth temperature, 450°C, seems to stabilize the cubic phase.
The next step will be the localization of CdSe small regions in the defect-free region close to the top, in order to create QDs.
Strain Analysis of Non-polar GaN Quantum Nanostructures
In order to reduce the spontaneous polarization appearing when the heterostructures are grown along the c-axis of the wurtzite-type structure, GaN quantum nanostructures have been grown along two non-polar directions of the wurtzite structure, namely [11-20] and [1-100]. Because of the dependence of the band gap value, and therefore optical properties, on the strain state of the nanostructures, we have undertaken a detailed and quantitative analysis of their structure and of their deformation state. The strain profile of the GaN dots and of the AlN layers separating successive GaN dots planes has been deduced by using the Geometrical Phase Method. For QDs grown along [11-20], a relaxation of the strain from the bottom to the top of the plane is observed, as expected (Top right). The QDs grown along [1-100] are less high than the previous ones and present a strongly anisotropic morphology (bottom).
In the next years, atomic scale structural studies of semiconducting nanostructures will be continued,
 with probably an increasing part devoted to nanowires, due to development of growth activities in that field (III-V nanowires grown by MBE or MOCVD). On the other hand, the new possibilities offered by the recently installed Titan microscope, should enable us to enlarge and combine different methods of investigation, in close collaboration with the LEMMA group. For instance, we plan to reinforce our work on the chemical imaging by high resolution STEM to enhance the chemical effects and reduce the strain contrast. We also would like to use tomography for visualizing the morphology of quantum dots in non conventional growth orientations.

In collaboration with :

B. Van Daele, J. Verbeeck, G. van Tendeloo
EMAT, Antwerpen University, Belgium
E. Margapoti and S. Mahapatra
Würzburg University, Germany

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