All sciences. №9, 2023. International Scientific Journal - страница 11

It should be noted that during corona discharge, the activation energy of the deep level (0.7eV) changes significantly depending on the potential of the corona discharge. This change is due to the influence of the optical ionization energy of the deep level located in the region of the volume charge near the SiO2 layer (this is indicated by experimental results). If we consider that this change occurs due to the Poole – Frenkel effect [64; p.52], then the mixing of the level can be estimated by the formula

where, is the dielectric constant of CdTe, e is the electron charge. Then, according to our estimates, the electric field strength in the vicinity of the defect reaches 103 V/cm.

To verify and analyze the above, the CdTe layer was separated from the SiO2 surface and installed on the sapphire surface. After that, contactless registration of transient decay processes for excess carriers was carried out using the microwave probe photoconductivity (WPC) method [2]. The parameters of deep traps and the state of their filling are determined by the photoionization of the captured media. Photoionization took place under the action of laser pulses varying in spectrum.

The cross section in the Lukovsky model is expressed in the following form

where B is the multiplicative coefficient [3]. For photons with energy hv, the change in a (hv) absorption coefficient at

it will also be proportional to the density of the captured media. The density of photo-emission carriers

at a fixed surface density F (hv), the incident photons are controlled by the MW probe. At the same time, the density of Nd traps can be independently estimated from the spectra of the absorption coefficient a (hv). The n/N fill factor can be controlled by combined measurements of the peak value of the MW-PC or a (hv) signal depending on F|hv, and saturation of these characteristics indicates complete photoionization of Nd traps.

Depending on the excitation wavelength, transient processes of contact photoconductivity were additionally measured. These measurements were carried out by exciting the interelectrode gap of the photoresistor and registering the photocurrent on a 50 ohm load resistor. The photocurrent and MW response signals were recorded using a Tektronix 1 GHz TDS-5104 oscilloscope.

Figure 2.a shows the recorded MW-PC transients at a relatively low (I1/I2=0.2) Excitation densities for the separated CdTe layer. From Fig. 2.a, it can be concluded that with an increase in the excitation wavelength, the shape of the MW-PC transient process changes from two components to one exponential. For a very thin sample with bare surfaces, this means the manifestation of surface recombination [2].

This phenomenon is caused by a decrease in the amplitude A1 of the main attenuation mode with an effective excitation depth, which is the inverse of the absorption coefficient α [2]. The absorption coefficient increases sharply with the energy of the excitation photon at the absorption edge of the CdTe material in the wavelength range of about 800—1000 nm [4]. At the same time, the dependence of the absorption coefficient on the layer thickness inherent in CdTe nanostructures [5] can be ignored, since the layer thickness of 1 µm is sufficient to have the properties of a bulk material.

The change in carrier density over time t along the plate depth coordinate x is described by the ratio