Читать книгу All sciences. №9, 2023. International Scientific Journal - Екатерина Александровна Селивёрстова, Ibratjon Xatamovich Aliyev, Екатерина Александровна Мулярчик (Буча) - Страница 7

For a qualitative description of the physical nature of the transfer phenomenon occurring in the CdTe – SiO2 – Si (semiconductor – oxide – semiconductor, i.e. POP) structure when a voltage is applied to it, consider a model in which a stationary current consists of a stream of electrons tunneling from the conduction band of a semiconductor into a deep level located in the oxide (and including the trap at the interface). Since the thickness of the silicon oxide in the structure under consideration is 0.4 microns, according to our estimates, the first contribution to the total flow is insignificant (less than 25%).

Tunneling of current carriers from the CdTe film into deep levels of silicon oxide leads to a change in the filling of the surface state. The latter, depending on the magnitude of the built-in charge, modifies the potential relief of the structure. So that the photogeneration rate will depend on the magnitude of the built-in charge, i.e. on the magnitude of the corona discharge potential in the structure. This means that the magnitude of the photo-EMF will be determined by the degree of asymmetry of the potential relief.

For a qualitative description of the physical nature of the kinetic phenomenon in the structure of semiconductor CdTe – oxide semiconductor SiO2 – semiconductor Si, a model based on the theory of a TIR (metal-dielectric-semiconductor) transistor can be considered. In this case, we mean that in a thick (0.4 µm) oxide layer, the main mechanism of current flow is determined by the Fowler – Nordheim model [5] and the corresponding current is denoted as

where i is the emission current density, E is the electric field strength, φ is the output operation, functions a and b depend on the geometry and operation of the output, for example, the degree of asymmetry, height, and width of the potential barrier. The current carrier flow should occur: a) due to the increasing (due to the Poole-Frenkel effect) thermionic emission through the potential barrier (jFN) of electrons with an increase in the magnitude of the corona discharge potential, b) due to the autoelectronic emission of current carriers trapped in the semiconductor oxide into the CdTe (jFN) conduction band. Since the contributions to the total current from the above currents are different in magnitude, the continuity of the current is disrupted at the interface. Thus, the excess (nonequilibrium) current carriers that appear in this case lead to the accumulation of charge at the interface. This leads to a redistribution of the internal electric field, which is essential in the formation of a potential barrier relief.

When the surface corona discharge potential is turned on at the boundary of CdTe films and the dielectric layer, charge carriers (electrons and holes) are tunneled from the semiconductor layer into the deep levels of the dielectric. Charge carriers in the film and at the interface, depending on the magnitude of the built-in charge, change the potential relief, therefore, when this layer is photoexcited, they will be generated under the influence of the built-in charge, changes the distribution of current carriers generated on the surface in such a way that draws them into an area that is accessible only to weakly absorbed electromagnetic radiation. Therefore, photo EMF also occurs during long-wave excitation. The asymmetry of the barriers is such that weakly absorbed radiation generates a photo EMF of the reverse sign compared to strongly absorbed radiation. Then, under the influence of a volumetric charge, the inversion of the sign of the photo EMF will mix the short-wave region, and the photosensitivity increases in the region of the electromagnetic radiation spectrum under study.

It should be noted that during corona discharge, the activation energies of the deep level (0.7 eV) change significantly depending on the potential of the corona discharge (see Figure 2 in the box). 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 assume that this change occurs due to the Pool – Frenkel effect [5], then the mixing (delta-E) level can be estimated using the formula

where, is the dielectric constant of CdTe, is the charge of the electron. Then, according to our estimates, the electric field strength in the vicinity of the defect is 105 V/s, which is quite reliable.

The situation arising in a CdTe film under the action of an embedded field corresponds to the model developed for a polysilicon field effect transistor [6]. The model considered in this paper is similar to the model [6], if identified with the control electrode of a field-effect transistor. Therefore, the numerical calculations of the potential distribution in a polycrystalline semiconductor are quite applicable for the embedded charge of a CdTe film. From the calculation results, the effect of an external field on the polycrystalline structure follows that a weak field only deforms the distribution of carriers, while a strong field leads to a decrease in the value of intercrystalline barriers due to the unification of the volume of the crystallite. These results show that the built – in field can lead to a decrease in the height of the barrier in the film (at U <10 V), and even to its disappearance (at U> 60 V) (on one of its surfaces), and then the remaining potential barrier becomes predominant, in the other – its opposite near-surface region.