Experimental and calculated EELS from GaN alloys and quantum wells

Full text: Light emitting diodes (LEDs) based on GaN and its alloys with InN and AlN are beginning to find wide application, even as conventional light sources. Devices are normally fabricated as quantum well structures and there is a need for analytical tools that can study composition and electronic structure with high spatial resolution. Electron energy loss spectroscopy (EELS) is one such tool and this work concentrates on how the fine structure in the EEL spectrum can give us information about the optical properties and electronic structure. The fine structure in the ionisation edges in the EEL spectrum (sometimes called the electron energy loss near edge structure (ELNES)) measures the unoccupied density of states (DOS). The N-K ELNES changes significantly in InGaN and AlGaN when compared to pure GaN. Similar changes are also seen in the InGaN quantum wells used in LEDs and the magnitude of these changes correlates with the In concentration Band structure calculations, to obtain the expected unoccupied DOS and ELNES, have been performed. When the effects of the core-hole are included, the agreement between experimental and calculated spectra is excellent. The electronic structure of GaN is anisotropic and the ELNES reflects this by varying as the specimen orientation is changed. The ionisation edges probe transitions from core electron states into the unoccupied states (the conduction band) whereas the low-loss region of the spectrum probes transitions between the valence band and the conduction band. Therefore, the low loss region of the EEL spectrum contains information about the optical properties of the material. For example, the band-gap can be measured directly and large changes in the low-loss region of the spectrum have been observed in InGaN alloys. However, the spatial resolution when studying this area of the spectrum is very much increased (to 5-10 nm) due to delocalisation effects. Thus, measuring changes in the bandgap in a quantum well is very difficult. Finally, and perhaps surprisingly, large shifts in the energy of the plasmon peak are observed with In concentration. Plasmon peaks are due to collective oscillations of the valence electrons are often described very well by free electron theory. This is not the case in GaN as the band of 3d electrons, at about 20 eV below the Fermi level, has a large influence on the plasmon energy. Copyright (2002) Australian Society for Electron Microscopy Inc