Chemistry, Physics and Technology of Surface

ISSN 2518-1238 (Online), ISSN 2079-1704 (Print)

An experimental study has beencarried out on the spectral dependence of the relaxation time of surface monopolar photoconductivity in the structures of macroporous silicon, with strong surface absorption of light, while the absorption coefficient varied in the range 2.37∙10³–3∙10⁵ cm^–1, and the absorption depth from 0.03 μm to 4.22 μm. Visible LEDs 0.38–0.62 µm were used for the studies: ultraviolet, violet, blue, green, yellow, orange and red. Structures of macroporous silicon were formed on n-type silicon with orientation [100] and specific resistivity of 4.5 Ω∙cm using photoelectrochemical etching. The macropores had a diameter of D_p = 3.5 μm, a depth of h_p = 80 μm and a concentration of N_p = 3.5∙10⁶ cm^–2. In the structures of macroporous silicon, a "slow" relaxation of monopolar photoconductivity was experimentally observed. It has been found that the relaxation time of monopolar photoconductivity is dependent on the wavelength of the illumination and varieds in the range of 15.5–42 s in the visible range of 0.38–0.62 μm. Moreover, the dependence of the photoconductivity relaxation time on the illumination wavelength had a minimum relaxation time of 15.5 s when illuminated by green light with a wavelength of 0.5215 μm. The reason for the dependence of the relaxation time of the monopolar photoconductivity on the wavelength is the competition of the processes of recombination and trapping of the main charge carriers on the so-called "slow" surface levels. The dominance of the recombination or trapping process is influenced by the characteristics of local centers, the surface conditions, the surface bending of the Y_s zones, which determines the surface monopolar photoconductivity and its relaxation. In the long-wavelength region of the spectrum, the decrease in the photoconductivity relaxation time from 42 s to 15.5 s is explained by the dominance of the trapping process of the main charge carriers at "slow" surface levels, which are localized within 1 nm of the silicon surface in the SiO_x transition layer. In the short-wavelength region of the spectrum, the slow increase in the photoconductivity relaxation time from 15.5 to 23.7 s is explained by the fact that the trapping of the main charge carriers in the "slow" surface states, which are localized in SiO₂ and have a longer photoconductivity relaxation time, begins to influence the photoconductivity relaxation time. The level of sticking gradually turns into the level of slow recombination with a large absorption coefficient, reaching values of 3∙10⁵cm^–1. The photoconductivity relaxation associated with surface levels changes with a change in the wavelength of illumination, which causes a change in the absorption coefficient, absorption depth, and surface bending of the Y_s zones. Due to this, a spectral dependence was observed of the relaxation time of surface photoconductivity in the structure of macroporous silicon.

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