Characterization of electronic states in molecular beam epitaxy grown GaN by optical admittance spectroscopy: Comparison of different nitrogen plasmasources

Optical transitions between the bands and electronic states in n-type GaN l ayers grown by molecular beam epitaxy on sapphire substrates using an elect ron cyclotron resonance (ECR) or a radio frequency (rf) nitrogen plasma sou rce were investigated by means of optical admittance spectroscopy. The spec tra of all layers similarly consist of a band gap region, a blue and a yell ow band, and several defect-to-band transitions. However, in rf grown layer s distinct transitions are separable, whereas ECR grown samples reveal broa d bands, originating from potential fluctuations due to structural inhomoge neities induced by the ECR source. A defect at 0.82 eV is found characteris tic for all ECR samples. (C) 1999 American Institute of Physics. [S0003-695 1(99)02014-8].