The amplitude of terahertz radiation (THz) from a series of oxide films on GaAs was measured by time resolved THz
emission system. The barrier heights and the densities of the interfacial states are determined from the PR intensity as a
function of the pump power density. The oxide-GaAs structures fabricated by in situ molecular beam epitaxy exhibit low
interfacial state densities in the range of 1011 cm-2. It is found that the amplitude of THz radiation from Al2O3-,
Ga2O3-,
and Ga2O3(Gd2O3)-GaAs structures are increases with interfacial electric field. The reason is that the electric field is
lower than the "critical electric field", the amplitude is proportional to the product of the electric field and the number of
photo-excited carriers. However, as the field higher than the critical electric field, sample of air-GaAs structure, the lower
THz amplitude was obtained due to the maximum drift velocity declines slightly as the field increases.
Room-temperature contactless electroreflectance (CER) was used to investigate the optical properties of a
N,N'-didecylperylene-3,4,9,10-tetracarboxylic diimide
(PTCDI-C10H21) thin-film sandwiched between indium tin oxide
and aluminum electrodes (Al/PTCDI/ITO/glass substrate) under vacuum conditions. The electromodulated optical
responses of the Al/PTCDI/ITO/glass structures were characterized by various alternating current biases. The optical
transitions of PTCDI were perturbed by energy shifts of electronic states due to the Stark effect induced by the
modulated electric field. The modulated CER spectrum of PTCDI is strongly enhanced by performing first derivatives on
the absorption spectrum of PTCDI. The CER spectrum involves fundamental transitions, doping states, and Davydov
splitting. Moreover, the intensity of the field-induced transition peak of PTCDI increases with increasing CER
modulation voltage. The transition energies between the lowest unoccupied molecular orbital and highest occupied
molecular orbital of the Al/PTCDI/ITO system is obtained from the peak positions in the CER spectrum.
Room temperature photoreflectance (PR) was used to investigate the energy gaps transition, the surface state densities
and the surface barrier height of InxAlyGa1-x-yAs, in a series of epitaxial surface intrinsic-n+ structures with different Al
concentration. Features of Franz-Keldysh oscillations originating from the built-in electric field in the intrinsic top layer
were observed. Based on the thermionic emission theory and current-transport theory, the surface state density can be
determined from the square of maximum electric field as a function of various pump beam flux intensities.
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