Effect of strain on low-loss electron energy loss spectra of group-III nitrides

Abstract

Thin films of AlN experiencing different strain states were investigated with a scanning transmission electron microscope (STEM) by low-loss electron energy loss spectroscopy (EELS). The results conclude that the low-loss properties and in particular, the plasmon peak position is shifted as a direct consequence of the inherent strain of the sample. The results reveal that strain, even minor levels, can be measured by STEM-EELS. These results were further corroborated by full potential calculations and expanded to include the similar III nitrides GaN and InN. It is found that a unit-cell volume change of 1% results in a bulk plasmon peak shift of 0.159, 0.168, and 0.079 eV for AlN, GaN, and InN, respectively, according to simulations. The AlN peak shift was experimentally corroborated with a corresponding peak shift of 0.156 eV. The unit-cell volume is used here since it is found that regardless of in-and out-of-plane lattice augmentation, the low-loss properties appear near identical for constant volume. These results have an impact on the interpretation of the plasmon energy and its applicability for determining and separating stress and composition. It is found that while the bulk plasmon energy can be used as a measure of the composition in a group-III nitride alloy for relaxed structures, the presence of strain significantly affects such a measurement. The strain is found to have a lower impact on the peak shift for Al 1-x In x N (∼3% compositional error per 1% volume change) and In 1-x Ga x N alloys compared to significant variations for Al 1-x Ga x N (16% compositional error for 1% volume change). Hence a key understanding in low-loss studies of III nitrides is that strain and composition are coupled and affect one another.

Surface Plasmon Resonances and Local Field Enhancement in Aluminum Nanoparticles Embedded in Silicon Nitride

“…higher) values in AlN-rich (resp. Si 3 N 4 -rich) regions in accordance with the AlN and Si 3 N 4 bulk plasmon energies 63,64.…”

Quantitative parameters for the examination of InGaN QW multilayers by low-loss EELS

“…Note that while the ZLP carries little chemical information, the plasmon peak is directly related to the composition of the material and has been exploited in group-III nitride compound semiconductors. [12][13][14][15][16] Fig. 1a shows a typical low-loss EELS spectrum from In x Ga 1Àx N, which includes the mentioned features.…”

“…The complete low-loss EELS region of In x Ga 1Àx N features a number of interband transitions in the vicinity of the plasmon peak, 8,12,23,24 prominently from the Ga 3d band (more below). Additional effects exist; for instance, the plasmon peak in EELS is known to shift in response to deformation in strained structures, 13 and, plasmon broadening in response to strain and the presence of interfaces and defects has been reported as well. 16,25 These measurements indicate an enhancement of the de-excitation processes through structural and chemical inhomogeneities which is yet to be fully understood.…”

Characterization of InGaN/GaN quantum well growth using monochromated valence electron energy loss spectroscopy

“…23 To grow strain-free In x Ga 1Àx N on GaN strain must first be relaxed by a number of misfit dislocations or other more notable structural defects. This is not the case in the investigated structures as a perfectly pseudomorphic growth was already concluded.…”