Structural and strain anisotropies of N-polar GaN epilayers on offcut sapphire substrates

Abstract

The dependence of the structure and strain upon the crystal direction (anisotropies) of N-polar GaN films grown on offcut c-plane sapphire substrates was examined by varying the degree of substrate offcut and the V/III ratio. On offcut substrates, the broadening of GaN (0002¯) rocking curves was found to depend upon the x-ray incidence azimuth, whereby the maxima and minima of the broadening, respectively, occurred parallel and perpendicular to the offcut direction. In contrast, no such structural anisotropies were observed for samples grown on the on-axis substrates. Mosaic tilt was suggested as the determining factor for the structural anisotropy, which was further aided by the different island coalescence rates in the two orthogonal directions during the initial nucleation stage. In addition, the off-axis samples exhibited larger in-plane strains occurring in the direction perpendicular to the substrate offcut than in other directions, which was significantly different from the uniform in-plane strain found in the on-axis sample. The in-plane strain anisotropy was attributed to preferential strain relaxation along the substrate offcut direction via the formation of geometrical partial misfit dislocations along the orthogonal direction. Moreover, the structural (strain) anisotropy decreased (increased) as the substrate offcut increased, and both anisotropies were affected only marginally by the V/III ratio.

Lateral‐Polarity Structure of AlGaN Quantum Wells: A Promising Approach to Enhancing the Ultraviolet Luminescence

“…The best‐performing UV LEDs have been grown on the III‐polar (0001) structure so far. [1a] Nevertheless, the usage of N‐polar (000) nitride films has also gained attentions recently due to their distinct advantages such as reduced efficiency droop in the application of LEDs, enhanced electrical field distribution in p‐i‐n diodes, and susceptibility to wet etching, which can be used for nanostructure fabrication …”

Annihilation of Nanoscale Inversion Domains in Nitrogen-Polar AlN under High-Temperature Annealing

“…To further verify the atomic structure of the AlN IDs, a HRTEM image along the [11][12][13][14][15][16][17][18][19][20] axis pinpointing a thin stripe is shown in Figure 2c. A sharp interface perpendicular to the sample surface is clearly observed with a small tilt angle around 1.7°.…”

“…Based on the already reported bonding configuration near IDB, we herein propose a model to describe the atom movement. Figure 5 illustrates a schematic atom structure showing the IDB region along the [11][12][13][14][15][16][17][18][19][20] direction. The IDB region is denoted by the vertical dashed line, N-polar is located on the left-hand side, and Al-polar is located on the right-hand side.…”

“…However, N-polar III-nitrides present some unique characteristics. For example, the quantum-confined stark effect in N-polar LEDs can be greatly alleviated due to reverse polarization-induced electric field, enhancing the overlap of electron and hole wavefunctions and promoting the radiative recombination efficiency . Despite the benefits of N-polar III-nitrides mentioned above, the development of N-polar III-nitride devices is still in the embryonic stage.…”

“…For example, the quantum-confined stark effect in N-polar LEDs can be greatly alleviated due to reverse polarization-induced electric field, enhancing the overlap of electron and hole wavefunctions and promoting the radiative recombination efficiency. 11 Despite the benefits of N-polar III-nitrides mentioned above, the development of N-polar III-nitride devices is still in the embryonic stage. One of the bottlenecks that prevents their commercial applications is the appearance of oppositely poled domains or inversion domains (IDs) in N-polar thin films, which has been commonly attributed to insufficient nitridation or carbon precipitation prior to AlN growth.…”