Читать книгу All sciences. №9, 2023. International Scientific Journal - Екатерина Александровна Селивёрстова, Ibratjon Xatamovich Aliyev, Екатерина Александровна Мулярчик (Буча) - Страница 11

UDC 544.22

Alimov Nodir Esonalievich

Doctor of Philosophy in Physical and Mathematical Sciences, Lecturer at the Department of Physics, Faculty of Physics and Technology, Ferghana State University
Ferghana State University, Ferghana, Uzbekistan

E-mail: alimov.nodir.esonaliyevich@gmail.com

Annotation. In this article, the rates of surface recombination in polycrystalline CdTe films obtained on oxidized substrates are studied, and the results of the action of corona discharge into the CdTe-SiO2—Si-Al structure are presented. In the static mode, a shift of the short-circuit current spectra to the short-wave region was observed. To analyze the displacement of the short-circuit current spectra, the microwave probe photoconductivity (MW-PC) method was used and contactless registration of transient decay processes for redundant carriers was performed. From the data obtained, it was found that the rate of surface recombination was estimated at 19 ns. It was determined that filling of surface traps in CdTe leads to a decrease in the effect of surface recombination.

Keywords: semicrystalline structures, surface recombination rate, polycrystalline films, spectrum shift.

Аннотация. В данной статье изучено скорости поверхностной рекомбинации в поликристаллических пленках CdTe полученных на окисленных подложках, Приведены результаты действия коронного разряда в структуру CdTe-SiO₂—Si-Al. в статическом режиме наблюдался смещение спектров тока короткого замыкания в коротко волновую область. Для анализа смещения спектров тока короткого замыкания использован метод микроволновой зондовой фотопроводимости (MW-PC) и проведена бесконтактная регистрация переходных процессов распада для избыточных носителей. Из полученных данных установлено что скорость поверхностной рекомбинации была оценена 19 нс. Определено, что заполнение поверхностных ловушек в CdTe приводит к уменьшению воздействия поверхностной рекомбинации.

Ключевые слова: полукристаллические структуры, скорость поверхностной рекомбинации, поликристаллические плёнки, смещение спектров.

CdTe semiconductor films are an important material for the creation of photodetector devices based on its heterostructures operating in the near (up to 3 microns) and far (8—14 microns) The IR range. This paper presents studies of the heterostructure obtained from growing CdTe on the surface of SiO2 – Si. This CdTe – SiO2 – Si heterostructure is interesting because using the built-in charge in the SiO2 layer, it is possible to control the PHOTOEMF and the short-circuit current spectrum.

Polycrystalline CdTe films with a grain size of 0.05—0.1 microns were grown on a heated SiO2-Si surface in a vacuum of 105 mmHg. The photosensitivity of the resulting structure is controlled by the action of an electric field or corona discharge, which change the built-in field in the SiO2 layer. To enhance the effect, an Al layer is applied to the Si surface, and we get a «reverse» CdTe—SiO2—Si—Al type field effect transistor, where a control charge is located under the semiconductor layer, and its surface remains open.

When the voltage between the Al layer and the electrode exceeds 6 kV, a corona discharge occurs, while the embedded field inside the structure reaches 100V. At the same time, at the boundary of the CdTe and SiO2 layers, 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. This changes the distribution of current carriers generated on the surface in such a way that it draws them into an area that is accessible only to weakly absorbed electromagnetic radiation. Therefore, photoedics also occurs with long-wave excitation. Due to the asymmetry of the barriers, weak absorbed radiation also generates photoedcs of the reverse sign. Then, under the influence of the volume charge, the inversion of the photoedc sign will mix the short-wave region, and the photosensitivity increases in the region of the electromagnetic radiation spectrum we are studying.

As stated above, when studying the effect of a corona discharge on the CdTe-SiO2—Si-Al structure, it showed that the short-circuit current spectra, depending on the magnitude of the external corona discharge in static mode, their displacement into the short-wave region was observed (Fig.1).

Figure 1 shows the spectral dependences of the short-circuit current (Icz) of the CdTe layer for various values of corona discharge intensity, which were carried out by contact (2) and electric probe contact (3) to the surface of the CdTe semiconductor. It can be seen that in the absence of external influences in the Icz (v) spectra, an inversion of the Icz sign is observed in the vicinity of the light quantum energy value equal to hv= 1.21eV (curve 1) the inclusion of the surface corona discharge potential between the CdTe layer and silicon leads to a significant change in the spectral sensitivity of the short-circuit current (Icz). When the surface potential changes within its value from 0 to 100V, the inversion position of the short-circuit current sign will mix into the short-wave region of the spectrum. In this case, the maximum photo sensitivity of the Icz will be mixed into the short-wavelength region of the spectrum and in the range from 0.93 eV to 1.5 eV. The position of the maximum value of the Icz increases by more than 1000 times at = 70V (curve 3) [63; – p.22—25]. For a qualitative description of the physical nature of the transfer phenomenon occurring in the CdTe-SiO2-Si-Al structure (semiconductor – oxide – semiconductor, i.e. When a voltage is applied to it, consider a model in which a stationary current is a flow of electrons tunneling from the conduction band of a semiconductor into a deep level located in an oxide (including into a trap at the interface). Since the thickness of the silicon oxide in the structure under consideration is 0.4 microns, we estimate that the first contribution and the total flux are insignificant (less than 25%).

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

where is the concentration of excess carrier pairs introduced by light, is the attenuation amplitude attributed to the spatial frequency m of the decay mode, is the lifetime of volumetric recombination, is the coefficient of ambipolar diffusion of carriers, is the rate of surface recombination.

Fig. 2. (a) Change in the excitation wavelength (at low excitation values I1/I2=0.2) – (b) the excitation density (at 700 nm) of MW-PC transient-dependent processes recorded at T = 300 K in a CdTe film mounted on a sapphire.

From the amplitude of the transients, it follows that with homogeneous excitation, the recombination processes will be linear (Fig. 2.b), while the decay of carriers in the initial phase indicates a non-exponential attenuation process in inhomogeneous excitation (Fig. 2.a).

Inherent in a number of transients determined by surface recombination and obtained for different excitation depths (Fig. 2.a) is an almost constant effective lifetime in the phase of asymptotic attenuation (considered as the same attenuation slope in Fig. 2.a), the value of 19 ns was determined [2].

An increase in the excitation intensity leads to an increase in the relative amplitude of the asymptotic attenuation component with inhomogeneous excitation of 700nm (Fig. 2.b). This hints at the rate of surface recombination, depending on the concentration of excess carriers.

This result is easy to understand if we assume that the filling of most surface traps is saturated. In experiments, when the highest excitation densities were used, it led to a decrease in the rate of surface recombination to values of 5 · 106 cm/s.

Summing up the analysis of the results, it is shown that the spectral photosensitivity of the CdTe layer in terms of short – circuit current and photo EMF can be controlled by the induced built – in electric charge of the dielectric created by the external potential of the corona discharge and the heterostructure of CdTe (film) – SiO2 (dielectric) – Si (semiconductor) – Al (aluminum).

This opens up new possibilities for the creation of semiconductor devices sensitive to electromagnetic radiation, which are also used in optoelectronics as a photosensitive device with a spectral characteristic in a wide sensitivity range. This effect is also associated with fundamentally new capabilities of semiconductor devices with variable spectral characteristics and matching it with an emitter, which is important for devices and information recording systems.

Literature

[1] Gaubas E., Čeponis T., Vaitkus J.V., Pulsed Capacitance Technique for Evaluation of Barrier Structures, LAMBERT Academic Publishing, Saarbrucken-Berlin, 2013 (ISBN:978-3-659-50518-8).

[2] Gaubas E., Simoen E., Vanhellemont J., Review – carrier lifetime spectroscopy for defect characterisation in semiconductor materials and devices, ECS J. Solid State Sci. Technol. 5 (2016) p.3108—3137.

[3] Lucovsky G., On the photoionization of deep impurity centers in semiconductors, Solid State Commun. 3 (1965) p.299—302.