  We study the interlayer scattering mediated by long-range Coulomb interaction
between electrons (density n) and holes (p) in a double-layer system. The gated
device is made of InAs (e) and InGaSb (h) quantum wells separated by a AlSb
middle barrier such that the interlayer tunneling is negligibly small. By using
independent-layer contacts we measure the transport tensor \r{ho}_xx and
\r{ho}_xy that are solely from the InAs layer, while sweeping p in the InGaSb
layer. We found a strongly enhanced resistive scattering signal as the carrier
densities approach a total charge neutrality, n = p, which cannot be described
by the Fermi-liquid theory. Results of data analysis for density, temperature,
and magnetic field dependences are consistent with the emergence of excitonic
coupling between the two layers, stressing the dominance of Coulomb interaction
even in the presence of disorder.
