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Ultrafast processes in the semiconductors based on LT-GaAs doped by delta-layer of Si.

Scientific organization
Moscow Technological University (MIREA)
Academic degree
graduate student
Scientific discipline
Physics & Astronomy
Ultrafast processes in the semiconductors based on LT-GaAs doped by delta-layer of Si.
Не заполнено
optical spectroscopy, ultrafast, carriers, pump-probe, carried, femtosecond laser

GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs) appear one of the most perspective materials in the field of ultrafast optical communications. Its use in ultrafast optical commutators [1] will greatly speed up the data transfer, since the charge carriers in this material have greater mobility and shorter lifetime, in contrast to a simple structure based on gallium arsenide [2]. Also, this material is going to be used as a basis for terahertz antennas [2]. Such antennas are used in medicine, which will pass away to replace the X-rays as a terahertz safer.

To investigate the dynamics of charge carriers ultrafast optical spectroscopy technique of "pump-probe" is used. This technique is based on separation an ultrashort laser pulse into two: the weaker – the probe impulse, stronger - the pump pulse. These two pulses are retained in a special way in relation to each other by a delay line. As a source of laser radiation the tunable femtosecond Ti:sapphire laser with 800nm wavelength and pulse duration 100fs is used. The changes in the reflectance of the probe pulse as a function of time delay between the pump pulse and the probe are detected.

As the test samples was used LT-GaAs growth after annealing. Two types of samples were investigated: with delta doped silicon layer and without it. The samples was grown by molecular beam epitaxy methodic at relatively low temperature (300-400°C) on the (100) GaAs substrate. The width of the bandgap of GaAs in the amount is Eg≈1.4 eV [4].

Typical time dependencies were obtained for B1 and B2 structures on the intensity (figure 1). Figure 1 shows that after excitation of semiconductors by photon with 1.5 eV greatly increases the concentration of charge carriers by generating and then there is their recombination.

Figure 1. Dependencies of intensity refractive coefficient of probe pulse on the delay time between pulses for the sample Solid line is approximation within the model.

For quantitative analysis of observed approximation process was carried out on an example of the experimental data, previously developed to describe similar processes in direct-gap semiconductors [5-7].

The obtained relaxation times are shown in Table. Typically relaxation time τ1 for the B1 and B2 structures of the same order. However, for the B1 τ2 much more than B2. The same behavior of the relaxation time may be due to the fact that the concentration of defects in LT-GaAs after annealing is decreased, and the delta-doping of silicon creates reduces the lifetime of minority carriers [8,9].

Table 2. Relaxation times

Material number



τ1, ps



τ2, ps




Thus, the investigation of LT-GaAs semiconductors with the «pump-probe» femtosecond laser spectroscopy methodic was obtained. During the experiment, the surface of the sample when excited by femtosecond pulses were obtained depending on the response signals from time to time. Approximation of the characteristic relaxation times was obtained. It was shown that the delta-doping silicon layers reduces the lifetime of holes. This behavior is most likely due to a decrease in the concentration of acceptor levels in the structure, with a high donor concentration [9].



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