Composition fluctuations in InGaN analyzed by transmission electron microscopy

A series of GaN/In(x)Gal(1-x)N/GaN quantum well structures was investigated by transmission electron microscopy (TEM) and photoluminescence spectrosco py (PL). The structures were grown by metal organic chemical vapor depositi on on Al2O3(0001) substrates varying only the substrate temperature between 800 and 860 degrees C for the InGaN growth under otherwise unaltered growt h conditions. A strong shift of the PL emission wavelength from 390 to 480 nm is observed. Composition analyses were carried out by measuring local la ttice parameters, which are directly related to the local In concentration, from high-resolution TEM lattice finge images. An increase of the average In concentration from 9% to 18% is found as the growth temperature is lower ed. A laterally inhomogeneous In concentration is observed in all samples w ith inclusions of high In content embedded in a quantum well with lower In concentration indicating that phase separation occurs even at a low average In concentration of 9%.