III-nitride heteroepitaxial thin films and heterostructures suffer from the presence of large densities of structural defects, which are detrimental for the development of efficient devices. These defects result from differences between III-nitride films and foreign substrates (structure, chemistry, lattice parameters). Transmission electron microscopy (TEM) is the technique of choice for studying such crystalline defects. The understanding of the origin and behavior of structural defects may allow their tailoring and the development of low defect density materials.

III-nitrides are classically grown along the polar c-direction. In this case, internal electric fields play a major role in the properties of heterostructures. In order to eliminate or at least to reduce the influence of internal electric fields, growth along alternative directions with the c-direction in the growth plane (nonpolar) or inclined to it (semipolar) have been developed. Heteroepitaxial nonpolar and semipolar films contain large densities of basal plane stacking faults (BSF) and related partial dislocations together with prismatic stacking faults.

In this presentation, results of TEM studies of semipolar GaN films deposited on patterned substrates will be presented. Different TEM techniques from diffraction contrast classical imaging, high resolution TEM and scanning TEM to analytical techniques as energy dispersive X-ray spectroscopy (EDS) have been employed to obtain a complete view of semipolar GaN microstructures. The understanding of the nucleation and propagation of defects allowed us to develop several growth processes resulting in a drastic reduction of the defect density:

• A 3-step growth process for (11-22) GaN deposited on patterned r-sapphire leads to high quality GaN films with dislocation densities as low as 7×10⁷ cm^-2 and BSF densities below 10² cm^-1 (figure 1)^1,2.
• A method based on the introduction of Si at an intermediate stage of the growth (before coalescence of nucleation islands) allows, in the case of (10-11) GaN on patterned (001) 7° off-axis Si, blocking the propagation of dislocations (figure 2). A 4nm thick Si-rich (5% Si from EDS analysis) layer is revealed by HRSTEM (figure 3). This layer has the wurtzite structure of the surrounding GaN and does not introduce significant strain (as revealed by GPA analysis).
• Selective growth on deeply grooved sapphire substrate results in GaN (11-22) bands with a dislocation density in the mid 10⁶ cm^-2 on 100µm-wide regions compatible with the fabrication of laser diodes.

Besides presenting results on the improvement of material quality through innovative growth processes, this presentation emphasizes the importance of TEM studies for the developments of heteroepitaxial semiconductors structures.

The authors acknowledge the support from GANEX (ANR-11- LABX-0014).GANEX belongs to the public funded “Investissements d’Avenir” program managed by the French ANR agency.

¹ F. Tendille, P.De Mierry, P. Vennéguès, S. Chenot, M. Teisseire, J. Cryst. Growth 404 (2014) 177

² P. Vennéguès, F. Tendille and P. De Mierry, J. Phys. D: Appl. Phys. 48 (2015) 325103

Figures:

Large field plan-view multibeam image of a (11-22) GaN layer deposited on patterned r-sapphire using a 3-step growth method. Examples of basal plane stacking faults (BSF) and threading dislocations (TD) are highlighted.

Bright field STEM cross-section image (zone axis [11-20]) of a (10-11) GaN layer deposited on (001) 7° off-axis patterned silicon. The blocking of TDs is clearly observed.

Cross-section HAADF image (zone axis [11-20]) of the Si-rich defect blocking layer shown in figure 2. The high resolution STEM experiments were performed on the Nanocharacterisation platform at MINATEC.

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