Implications for (In,Ga)N incorporation

The investigation by electron tomography revealed morphological details which occur due to specific growth conditions. The rough surface at the apex of the nanocolumn is formed

during the deposition of the GaN cap at 625^◦C under nitrogen-rich condition (growth de-tails see [53]). Tarsa et al. [228] demonstrated the roughening of planar GaN layers that are homoepitaxially grown on (0001) surfaces by plasma-assisted MBE. They have applied comparable growth parameters, i.e. 650^◦C and a beam equivalent pressure of nitrogen that has been twelve times higher than the one of Ga.

The nanofaceting which is observed on the pyramidal side facets, is assumed to be com-posed of low-index facets. These are energetically favourable over vicinal high-index facets.

Theoretical calculations by Mutombo and Romanyuk [229] demonstrated that a mixture of {1¯101} and {1¯100} facets is energetically preferred over the semi-polar GaN {20¯21} facets.

This case is experimentally confirmed by HAADF STEM observations presented in a work of Zhaoet al.[230].

(a) (b) (c)

100 nm 500 nm [1120] 100 nm

[0001]

(a) (c)

Figure 5.8.The HAADF STEM images give an overview on the nanocolumn morphologies. The different ratio between pyramidal and basal facets characterizes the enlarged areas in (a) and (c) along with their overall different size.

Figure 5.8 broadens the view onto the nanocolumn morphology near the apex. The HAADF STEM micrographs are taken from a lamella that comprises several nanocolumns.

Two extreme examples of variable shapes are captured at a higher magnification. They represent the generally observed differences. There are nanocolumns with a more or less pronounced pyramidal habit. Those ones with the hardly developed pyramidal facets are shorter than the others. Albertet al.[231] have elaborated on the control of the nanocolumn tip shape. They have fabricated GaN nanocolumns by MBE applying SAG on GaN (0001) templates. The array of 3D objects has appeared homogeneous in contrast to the presented case which is depicted in figures A.3(a) and (b). Beside the different template orientations, the substrate patterning has been carried out with electron beam lithography instead of the colloidal approach applied for the here investigated sample. Hence, morphological hetero-geneities could originate from variations in the patterning. For instance, variable hole diam-eters induce the different nanocolumn sizes. Once the objects passed the mask, the strongly anisotropic material supply in MBE promotes heterogeneities. Then, shadowing affects the morphology of the outer shape as well as of the (In,Ga)N insertions. This effect is described for example by Grandal et al.[153]. A further aspect is considered because nanocolumns are roughly classified in two types. The occurrence of inversion domains or the formation of columns with opposite polarity, respectively, could explain different sizes, too. The growth velocity of GaN along the [0001] and the [000¯1] direction is different [232]. The promotion of heterogeneities due to anisotropic material supply remains.

It is shown that the insertion of a core-shell like (In,Ga)N layer on inclined GaN columns grown on the semi-polar GaN (11¯22) substrate is principally feasible. The incorporation of indium and the (In,Ga)N growth rate are highest on the basal plane. The consideration of figure 5.8 shows that a more pronounced pyramidal habit reduces the size of the insertion on the (0001) plane. Consequently, a control of the apex shape is desirable in order to sup-press the growth along the polar [0001] axis (cf. section A.2 and [233–235]). Albertet al.

[231] have manipulated the apex shape of SAG grown GaN columns on (0001) substrates by growth temperature and the ratio of Ga and nitrogen supply. The presented tomogram reveals a mixture of prismatic and pyramidal facets around the tip of the GaN core. Hence, the geometry of the inclined nanocolumns requires further consideration of the growth pa-rameters to control the apex shape. Furthermore, the (In,Ga)N insertion on the basal plane is shown to laterally expand during growth (cf. figure 5.7). It has to be considered whether a (0001) facet evolves during (In,Ga)N growth on a purely pyramidal tip.

The revealed domination of the (01¯10) and the (1¯100) facets (cf. figure 5.3) opens a further possibility to enhance the prevalence of non-polar surfaces. The coalescence of nanocolumns is expected after continued growth resulting in a closed GaN surface. This surface is assumed to be predominantly composed of micrometre-sized {1¯100} facets.

200 nm

(a) (b) (c)

[1100]

[0001]

[1120]

3 eV → 10% In 2.4 eV → 26% In

100 nm

Figure 5.9.(a) The isosurface represents the morphology of the investigated nanocolumn. (b) The outer shape appear as semi-transparent isosurface. The opaque green and blue isosurfaces correspond to (In,Ga)N insertions with a higher and lower indium concentration, respectively. (c) The SEM image is superimposed with the schematically presented cathodoluminescence emission of a nanocolumn according to the results presented in the publication of Bengoechea-Encaboet al.[53]. All images show the nanocolumns in the same viewing direction.

Finally, the presented results confirm and extend the ideas on the indium incorporation from the work of Bengoechea-Encabo et al.[53]. The tomographic data proves the differ-ent indium concdiffer-entrations on the basal and prismatic facets. The correlation is illustrated in figure 5.9. The isosurface of the nanocolumn and a montage of three different isosur-faces are shown next to an SEM image. The opaque grey isosurface reflects the nanocolumn morphology in 5.9(a). It reappears semi-transparently in figure 5.9(b). The green and blue opaque parts represent the (In,Ga)N insertions on the basal plane and on two of the prismatic planes. The blue isosurface is cropped below the green one. The schematic presentation of CL results as adapted from the respective publication [53] is superimposed on the SEM im-age in figure 5.9(c). In addition to this principal correlation, a core-shell like morphology of the (In,Ga)N insertion is unveiled. Pronounced pyramidal {1¯10`} facets exhibit an (In,Ga)N

layer, too. They are expected to cause a separate photoluminescence peak according to the work of Albert et al. [231]. Furthermore, the tomogram reveals layer thickness variations between the differently developed facets of the complex nanocolumn morphology.