  We present a reliable flip-chip technique for dual-side processing of thin
(<1 micron) high-mobility GaAs/AlGaAs epitaxial layers. The technique allows
the fabrication of small (micron-scale with standard UV photolithography)
patterned back gates and dual-gate structures on the thin GaAs/AlGaAs films
with good alignment accuracy using only frontside alignment. The technique
preserves the high-mobility (>10^6 cm^2 /V-s at 2 K) and most (>95%) of the
charge density of the 2-dimensional electron gas (2DEG) systems, and allows
linear control of the charge density with small (< 1 V) electrostatic gate
bias. Our technique is motivated by a novel THz quantum-well detector based on
intersubband transitions in a single, wide GaAs/AlGaAs quantum well, in which a
symmetric, well-aligned dual-gate structure (with a typical gate dimension of
~5 micron by 5 micron) is required for accurate and precise tuning of the THz
detection frequency. Using our Indium-Bond-And-Stop-Etch (IBASE) technique, we
realize such dual-gate structure on 660-nm thick GaAs/AlGaAs epitaxial layers
that contain a modulation-doped, 40-nm wide, single square quantum well. By
independently controlling the charge density and the DC electric field set
between the gates, we demonstrate robust tuning of the intersubband absorption
behavior of the 40-nm quantum well near 3.44 THz at 30 K.
