Influence of low-temperature buffer layers on the optical waveguiding properties of Al_(x)Ga_(1-x)N alloys

摘要

The influence of buffer layers on the optical waveguide properties of nitride related semiconductors films grown on sapphire by metalorganic vapor phase epitaxy has been investigated. Using the prism coupling technique, we measured the ordinary (n_(TE)) and extraordinary (n_(TM)) refractive indices of Al_(x)Ga_(1-x)N/AlN and Al_(x)Ga_(1-x)N/GaN heterostructures, where AlN, GaN are low temperature buffer layers. We investigated the relationship between the refractive indices of Al_(x)Ga_(1-x)N alloys and the Al content (x) at a wavelength of 632.8 nm: n_(TE)=2.3240-0.33x. This investigation demonstrates optical waveguiding in gallium nitride materials. It is shown that the indices of thick Al_(x)Ga_(1-x)N films are independent of the buffer layer (AlN, GaN). However, the optical propagation losses are highly sensitive to the crystalline quality of the layers, particularly to the density of dislocations in the film. The best optical result of 1.2 dB/cm is obtained when the GaN buffer layer was employed, in comparison to 1.8 dB/cm with the AlN buffer layer. We interpret these results in relation with the transmission electron microscopy analysis.