PERCOLATION OF CARRIERS THROUGH LOW POTENTIAL CHANNELS IN THICK ALXGA1-XAS (X-LESS-THAN-0.35) BARRIERS

We study by photoluminescence excitation the heretofore unsolved puzzl e of a significant charge transfer over a thick (100 to 1500 Angstrom) AlxGa1-xAs barrier in GaAs/AlxGa1-xAs asymmetric double quantum wells , which the normally considered tunneling cannot account for. This phe nomenon is completely general, observed in all the samples grown under standard growth conditions (similar to 600 degrees C), that originate d from many different sources. The existence of such leakage is also c onfirmed by time-resolved photoluminescence experiments. However, when the alloy barrier is replaced by an equivalent GaAs/AlAs digital allo y, or by AlAs, the leak largely disappears. In addition, a GaAs barrie r separating two shallow InxGa1-xAs quantum wells permits only relativ ely small transfer. The leak has a weak dependence on the barrier thic kness at x=0.3, but is a very strong function of x around x=0.3. We ar gue that there is no way to explain all of the observed phenomena simu ltaneously other than by the existence of intrinsic structural inhomog eneities in the alloy. Essentially, there may exist low potential chan nels in the alloy barrier created by microscopic clustering of like mo lecules, through which percolationlike transport occurs. This picture is supported by a three-dimensional quantum-mechanical model calculati on. Our work pins down the dynamical implications of the partial order ing and clustering in AlxGa1-xAs and related semiconductor ternary all oys. The scope of this paper is exclusively for the transport over thi ck AlxGa1-xAs barriers (x<0.35) and does not include the relatively sm all but still non-negligible transport over thick homogeneous barriers such as GaAs, AlAs, and AlAs/GaAs digital alloys, and AlxGa1-xAs (x>0 .35). We contend that transport over these thick, homogeneous barriers may be owing to other mechanisms such as dipole-dipole transfer, phot on reabsorption, nonequilibrium distribution of carriers, and polarito n transport that are also unrelated to conventional tunneling.