GaN-based optoelectronic structures have been successfully applied in the fabrication of blue light emitting diodes (LEDs) and laser diodes [1]. The fabrication of high-efficient green LEDs remains challenging which is referred to as the “green gap”. In GaN-based green LEDs, the active region consists of In_xGa_1-xN quantum wells (QWs) and GaN barrier layers (BLs) where the emitting wavelength is controlled by the In content (x).  However, In incorporation often results in inferior crystal quality due to the miscibility gap and lattice mismatch when the In content is increased [2]. Although conventional c-plane In_xGa_1-xN /GaN QWs have demonstrated the highest crystal quality,  the quantum efficiency remains low as a result of the polarization field along the c-axis which tilts the conduction and valence bands thus reducing the recombination probability (i.e., quantum confined Stark effect, QCSE). Semipolar In_xGa_1-xN/GaN structures, on the other hand, offer a good compromise between reduced QCSE and acceptable crystal quality [3].

In this study, we investigated the crystal quality of two sets of In_xGa_1-xN/GaN structures grown on (11-22) and (10-11)-semipolar planes. The samples were grown on sapphire by using metalorganic vapor phase epitaxy (MOVPE). The weak-beam dark-field technique (WBDF) was applied to investigate the distribution of the threading dislocations (TDs). Figure 1 shows the WBDF images of a (11-22)-semipolar sample. The GaN growth starts on the c-plane-like facets by MOVPE, forming triangular shaped stripes. The stripes subsequently coalesce forming a closed {11-22}-semipolar surface. TDs originate from the inclined c-plane-like GaN/sapphire interface and propagate in c-direction and then bend 90° to the a-directions. Most of the dislocations accumulate around the coalescence area and penetrate the QWs. The bending phenomenon is related to the triangular shape of the original GaN stripes and the SiN_x interlayers deposited during the growth of GaN.

By using high-resolution TEM, we characterized the local structures of the QWs. Figure 2 shows the experimental HRTEM images of a (10-11)-semipolar sample acquired from the active region. Structural factors, such as the thickness of the QWs, the interfacial sharpness between the QWs and the BLs and the stacking faults formed with in the active region, can be analyzed with atomic resolution. Stacking faults are observed and confirmed to be the I₂ intrinsic type. [4]

References

[1] Shuji Nakamura and Gerhard Fasol. The Blue Laser Diode-GaN Based Light Emitter and Lasers. Springer Berlin Heidelberg, 2000.

[2] F Scholz. Semipolar GaN grown on foreign substrates: a review. Semiconductor Science and Technology, 27(2):024002, 2012.

[3] J. S. Speck and S. F. Chichibu. Nonpolar and semipolar group III nitride-based materials. MRS Bulletin, 34:304312, 5 2009.

[4] We gratefully acknowledge financial support by the DFG (KA1295/22-1) and technical support by Sabine Grözinger in cross-section TEM sample preparation. 

Figures:

Figure 1 (a) WBDF image of a (11-22)-sample along [1-100] zone-axis with g =11-20. GaN was grown on prestructured {10-12} r-plane sapphire substrate with c-plane-like side walls. TDs originate from the inclined interface and propagate in c-direction and then bend 90° to the a-directions. Most of the dislocations accumulate around the coalescence area and penetrate the QWs. (b) and (c) are the detail images of the marked areas in (a). WBDF imaging was conducted on a Philips CM20 microscope with the acceleration voltage of 200kV.

Figure 2 (a) [11-20] HRTEM image showing five InxGa1-xN/GaN QWs of (10-11)-semipolar sample. The stripes with dark contrast are the QWs. The upper interfaces are not as sharp as the lower. Stacking faults are marked by the white arrows. They are formed at one of the lower InxGa1-xN/GaN interfaces and penetrate till the surface. (b) HRTEM image of the stacking faults in QW area (rotated to make c-direction goes up). They are confirmed to be I2 intrinsic type. HRTEM imaging was conducted on a FEI Titan 80-300 operated at 300 kV.

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EMC Abstracts - https://emc-proceedings.com/abstract/analysis-of-semipolar-inxga1-xngan-heterostructures-by-wbdf-and-hrtem-imaging/