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

• Low-loss EELS was used to acquire information about the strain state in group III-nitrides. Experimental and theoretical simulation results show that the bulk plasmon peak position varies near linearly with unit cell volume variations due to strain. A unit cell volume change of 1% results in a bulk plasmon peak shift of 0.159 eV, 0.168 eV, and 0.079 eV for AlN, GaN, and InN, respectively, according to simulations. The AlN peak shift was experimentally corroborated with a peak shift of 0.156 eV, where the applied strain caused a 1% volume change. It is also found that while the bulk plasmon energy can be used as a measure of the composition in a III-nitride alloy for relaxed structures, the presence of strain significantly affects such a measurement. The strain has a lower impact on the peak shift for Al(1-x)InxN (3% compositional error per 1 % volume change) and In(1-x)GaxN alloys compared to significant variations for Al(1-x)GaxN (16% compositional error for 1% volume change). Hence low-loss studies off III-nitrides, particularly for confined structures, must be undertaken with care and understanding.

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