Non-polar and polar nitrides quantum dots

People :
Edith Bellet-Amalric, Bruno Daudin, Henri Mariette
Benoît AmstattJohann Coraux, Sébastien Founta
 
Overview and results :
Study of the growth and of structural and optical properties of GaN quantum dots (QDs) obeys several motivations, from both basic and applied research points of view. On one hand, the high density of structural defects in nitride epilayers, which results from the lack of adapted substrates and from the lattice mismatch between members of the family, makes the fabrication of defect-free self-assembled QDs particularly attractive. On the other hand, the strong band offset between GaN and AlN makes these QDs suitable candidates for optical devices operating at room temperature. Concerning opto-electrical applications, however, the strong internal electric field present in [0001] oriented heterostructures is detrimental to their optical efficiency and has motivated an interest in non-polar (i.e. [1-100] and [11-20]) GaN QDs.
A special effort has been devoted to the control of the density of GaN QDs of all crystallographic orientations in order to make possible single dot spectroscopy measurements.
 
The general scientific strategy has been to control the growth mode for fabricating self-assembled GaN QDs of every orientation and to characterize them structurally by combining four techniques, namely high resolution electron microscopy (HRTEM), grazing incidence anomalous Xrays diffraction and diffraction anomalous fine structure (DAFS), medium energy ion scattering (MEIS) and Raman spectroscopy.
With the goal of eventually realizing coupled QDs systems, the capping of them by AlN has been studied in details. By combining AFM and TEM experiments, it has been found (see figure 1) that following initial wetting of GaN QDs with AlN up to a coverage of about 4 monolayers (MLs), preferential growth of AlN is observed in-between dots, till a complete smoothing of the surface. No GaN/AlN interdiffusion was found.
 
As a consequence of this study, the conditions for realizing short period superlattices have been determined. More generally, the vertical correlation of stacked GaN QDs planes has been studied by Xrays diffraction and Raman spectroscopy experiments. Xrays diffraction studies were performed at the ESRF using synchrotron radiation. They have included in-situ studies of GaN QD growth and capping with AlN. The technique used was anomalous Xrays diffraction which allows one to discriminate between different chemical species diffracting at the same position. It is concluded that vertical correlation mediated by the AlN barrier deformation exerted by GaN dots is effective for an AlN spacer smaller than about 9 nm.
 
Nonpolar GaN QDs
 
Growth of nonpolar [11-20] GaN on [11-20] has been studied. Contrary to the case of [0001] orientation, it has been established that growth in Ga-rich conditions results in rough material whereas N-rich conditions allow one to obtain a smooth surface. Next, growth of nonpolar [11-20] GaN QDs on [11-20] AlN/SiC has been studied in details. Remarkably such dots are found to be aligned along [1-100] direction (figure 2a). By combining TEM, Scanning Electron Microscopy (SEM), AFM and RHEED, it was possible to eventually propose a shape for [11-20] GaN QDs (figure 2b). It was furthermore possible to relate the assymetry of dots to the orientation of [0001] direction, i.e. to the in-plane polarity of the material.
 
Alternately, growth of [1-100] GaN QDs and QWires has also been performed on [1-100] AlN/SiC. Similarly to the case of [11-20] material, it has been found that [1-100] AlN exhibits a marked morphological anisotropy. Depending on AlN strain state, it has been found that QWires (on strained AlN) or QDs (on relaxed AlN) could be grown. The critical thickness for plastic relaxation of AlN buffer layer was found to be about 300 nm. Figure 3 shows AFM images of [1-100] nanostructures illustrating the effect of AlN buffer layer thickness/strain state. In addition, it is shown that the shape of nanostructures also depends on the amount of GaN deposited, dots bing progressively evolving towards wiresfor GaN amount changing from 5 to 20 MLs.
As a general result of these studies, it has been demonstrated that GaN QDs formation far from depending of lattice mismatch only is also strongly dependent on surface and interface energy. The non-interdiffusion of GaN/AlN system makes it a model system for such studies. Finally, the growth of [0001] GaN QDs on AlGaN with Al content varying between 100 and 34 % has been achieved and has revealed the combined effect of lattice mismatch and interface energy on dot morphology.
 
Future :
The mastering of GaN QDs which has been achieved opens now the way to specific optical experiments. On one hand, single dot spectroscopy is now possible for every kind of QDs and QWires and should be made. On the other hand, insertion of QDs in devices such as microcavities, micropillars or light emitting diodes is now realistic and will be considered.
 
In collaboration with :
Hubert Renevier1,Vincent Favre-Nicolin1, D. Jalabert2
 
(1) Laboratoire de Nanostructures et Rayonnement Synchrotron
(2) Laboratoire SINAPS
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