Speaker
Prof.
Igor Sokolov
(Deputy head of laboratory)
Description
GaAs detectors for registration of X-radiation and gamma radiation are found
applications in medicine and for the tasks of high-energy physics. Since the
registered signals are small enough the main problem of such devices can be
formulated as follows - realization of maximal detector volume keeping constant low
noise level. The capacity and return current should be minimized. The is achieved
via (i) fabrication of GaAs layers with low amount of defects and low carrier
concentration, and (ii) structure optimization and creation of contacts with low
resistance.
The non-steady-state photoelectromotive force effect was used for characterization
of transport parameters of ultra-pure GaAs thin films grown on semi-insulating
gallium arsenide wafers using gas-phase epitaxy technique. Such structures are used
for fabrication GaAs detectors for registration of X-radiation and gamma radiation.
The mechanism responsible for the effect can be described as follows. Illumination
of a photoconductive sample by an interference pattern formed by two coherent light
beams produces a nonuniform excitation of free carriers. Diffusion of the
photoexcited carriers towards the dark regions leads to charge redistribution
between traps. A space charge field grating arises; this grating is spatially
shifted by 90 degrees relative to the optical interference pattern and
photoconductivity distribution. Small vibrations of the light along the grating
vector excites an alternating current through the short-circuited crystal because of
the time-dependent phase shift between oscillating spatially-periodic free carriers
and fixed space charge field distributions.
The structures were fabricated at A.F. Ioffe Physico-Technical Institute (laboratory
of Yu.V. Zhilyaev). For our experiments we choose the GaAs sample with the layer
thickness of 400 microns. The experiments can be carried out only in the geometry of
Michelson interferometer at the illumination wavelength of 532 nm. The modulation
frequency was 1 kHz and the light power of the signal and reference beam on the
sample’s surface was about 20 mW, the inter-electrode distance was 2,5 mm. The
dependence of the signal photocurrent amplitude versus spatial frequency of the
interference pattern was measured. It is important to point out that the signal
peaks for the spatial frequency of the interference pattern K equal to the inverse
diffusion length of photocarriers. The value of diffusion length of the as grown
structure was estimated to be 40 microns.
Author
Prof.
Igor Sokolov
(Deputy head of laboratory)