PHOTOLUMINESCENCE INVESTIGATION OF A DEGENERATE 2-DIMENSIONAL ELECTRON-GAS IN GAN ALGAN HETEROJUNCTION/

We report on photoluminescence (PL) experiments performed on a n-type modulation doped GaN/Al0.15Ga0.85N heterojunction grown by low pressur e metal organic chemical vapor deposition (MOCVD) technique. In the te mperature range 7.4-80 K, we have investigated (i) the photoluminescen ce spectra from the degenerate 2D electron gas confined near the GaN/A lGaN interface, (ii) the impurity and band edge photoluminescence of t he 3D undoped region of the GaN layer. The experiments were performed in the following manner: we excited the sample at 333.6 nm in two ways , on the AlGaN barrier (2D channel side) and on the substrate side. Th e comparison between the corresponding spectra and the temperature dep endence of lines-intensity allowed us to identify without ambiguity th e 2D and 3D character of the different lines. We find a very broad lin e which could come from the first E-1 subband transitions and at a hig her energy two lines with which intensities increase with temperature. We attributed those lines to excitonic transitions associated to the second subband. Between E-1 and these 2D excitonic lines we observed a very intense line with which intensity decreased rapidly when tempera ture increased. The intensity of this line behave typically like the o ne of a transition at the Fermi energy of the 2D degenerate electron g as (Fermi edge singularity). (C) 1997 Elsevier Science S.A.