  We investigated excitonic absorptions in GaAs/AlAs/GaAs single barrier
heterostructure using both photocapacitance and photocurrent spectroscopies at
room temperature. Photocapacitance spectra show well defined resonance peak of
indirect excitons formed around the Gamma-AlAs barrier. Unlike DC-photocurrent
spectra, frequency dependent photocapacitance spectra interestingly red shift,
sharpen up and then decrease with increasing tunneling at higher biases. Such
dissimilarities clearly point out that different exciton dynamics govern these
two spectral measurements. We also argue why such quantum confined dipoles of
indirect excitons can have thermodynamically finite probabilities to survive
even at room temperature. Finally, our observations demonstrate that
photocapacitance technique, which was seldom used to detect excitons in the
past, is useful for selective detection and experimental tuning of relatively
small numbers (~10^11/cm2) of photo-generated indirect excitons having large
effective dipole moments in this type of quasi-two dimensional
heterostructures.
