4. Optical Properties

Potential fluctuation plays an important role in determining the optical properties of alloy semiconductors. Although theoretical calculations do not show an unstable phase segregation in Al _xGa _1-xN alloys,92 optical studies of Al _xGa _1-xN alloys with high Al contents have observed the S-shaped PL shift and Stokes shift,93 ^, 94 which can be readily explained by alloy potential fluctuation in a fashion similar to InGaN/GaN MQWs.95 ^, 96

Chung et al.97 presented an optical study of alloy potential fluctuation in Al _xGa _1-xN using a combination of optical characterization tools en-compassing PL, optical absorption (OA), photo-current )PC), and persistent photoconductivity (PCC). The band edge peak positions of the Al _xGa _1-xN epilayers measured by PL for x=0.08, 0.15, and 0.33 are plotted in Figure 24 as a function of temperature. Al _0.08Ga _0.92N follows the typical temperature behavior of the energy band-gap shrinkage described by Varshni's equation:

Figure 24: T-dependent PL spectra of Al _xGa _1-xN epilayers with various Al compositions. [After ref. 97 ]

However, Al _0.33Ga _0.67N shows the "S-shaped" emission peak shift, i.e., decrease-increase-decrease, behavior with increasing temperature. It clearly shows that the increase of Al content is accompanied by the aggravation of the S-shaped behavior of PL peak energy. The depth of localized states obtained from PPC decay kinetics for Al _0.33Ga _0.67N is 152 meV that is somehow related to...