https://doi.org/10.15407/iopt.2019.54.119

Optoelectron. Semicond. Tech. 54, 119-125 (2019)

N.I. Karas, V.F. Onyshchenko

Relaxation of "slow" negative surface photo conductivity in structures of macroporous silicon in the visible spectral region

The relaxation of surface photoconductivity (PC) in the structures of macroporous silicon in the visible spectral range 0.38–0.62 μm is investigated. To do this, the following LEDs were used: ultraviolet, violet, blue, green, yellow, orange and red. In this case, the absorption coefficient varied in the range of 2.37∙10³–3∙10⁵ cm ^{– 1}, and the depth of absorption ranged from 0.03 μm to 4.22 μm. Over the entire wavelength range, a "slow" negative monopolar photoconductivity was observed, the relaxation time of which depended on the wavelength of illumination and varied in the range of 15.5 - 42 s. The minimum relaxation time (15.5 s) was recorded when illuminated with green light with a wavelength of 0.5215 microns. The source material for the formation of macroporous silicon structures was n-type silicon with orientation [100] and specific resistance of 4.5 ohm∙cm. Macropores with diameter D_p = 3.5 μm, depth h_p = 80 μm and concentration of macropores Np= 3.5∙10⁶ cm^-2 were formed using electrochemical etching. The reason for the negative monopolar photoconductivity is due to the capture of the main charge carries at the so-called “slow” surface levels. Sticking (as well as recombination) is carried out by trapping of the excited current carriers to local centers. Whether a recombination event takes place after a capture at a local center of a nonequilibrium carrier, or a current carrier sticks, it is determined by the probability characteristics of the local center, in particular by effective capture cross sections. Depending on surface conditions, surface bending zones Ys , sample temperature, method of excitation of nonequilibrium carriers, surface monopolar photoconductivity and their temporal relaxation can be determined by recombination, adhesion of minor or main (relative to volume) carriers, and simultaneously by several mechanisms. The relaxation of photoconductivity occurs due to the exchange of current carriers between the conduction band in the bulk of the semiconductor and local "slow" surface levels, which have distinctive features in kinetic characteristics for the three characteristic surface states - depletion, inversion and enrichment. The surface states of macroporous silicon structures in our experiment changed with a change in the wavelength of the illumination, which caused a change in the absorption coefficient, the absorption depth, and the surface bending zones Ys. As a result, the spectral dependence of the relaxation time of surface photoconductivity was observed.

Keywords: "slow" negative monopolar surface photoconductivity, photoconductivity relaxation, relaxation time.