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STUDY OF THE CONTROL PROPERTIES OF POLYCRYSTALLINE STRUCTURES BASED ON SILICON AND CADMIUM TELLURIDE
UDC 544.22
Salim Madrahimovich Otajonov
Doctor of Physical and Mathematical Sciences, Professor of the Department of «Professional Education» of the Faculty of Physics and Technology of Fergana State University
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
Botirov Qodir Abdullayevich
Lecturer of the Department of «Professional Education» of the Faculty of Physics and Technology of Ferghana State University
Ferghana State University, Ferghana, Uzbekistan
Annotation. In this paper, the photoelectric properties of CdTe – SiO2 – Si heterostructures are investigated. For the first time, the possibility of controlling the spectrum of short – circuit current and photo-EMF using an integrated charge in a dielectric (SiO2) has been demonstrated. It was found that with an increase in the corona discharge potential, the spectra will mix into the short – wavelength regions of the spectrum from 0.93 to 1.5 eV, while the activation energy of the deep level of 0.73 eV changes significantly and this change occurs due to the Poole-Frenkel effect. It is found that the electric field strength in the vicinity of the defect is ? = 105 V/cm.
Keywords: photoconductivity, PHOTOEMF, spectral distribution of photosensitivity, short-circuit current, asymmetry of barriers, surface photo-EMF, deep levels, impurity photoconductivity, corona discharge.
Аннотация. В данной работе исследованы фотоэлектрические свойства гетероструктур на основе CdTe – SiO
– Si. Впервые продемонстрирована возможность управления спектра тока короткого замыкания и фото – ЭДС при помощи встроенного заряда в диэлектрике (SiO
). Установлено, что с увеличением потенциала коронного разряда спектры смешается в коротковолновые области спектра от 0,93 до 1,5 эВ, при этом существенно изменяется энергии активации глубокого уровня 0,73 эВ и это изменение возникает за счёт эффекта Пула – Френкеля. Найдено, напряжённость электрического поля в окрестности дефекта ? = 10
В/см.
Ключевые слова: фотопроводимость, фото-ЭДС, спектральное распределение фоточувствительности, ток короткого замыкания, асимметрия барьеров, поверхностная фото-ЭДС, глубокие уровни, примесная фотопроводимость, коронный разряд.
Introduction
The development of micro – nano electronics and new technological possibilities for the manufacture of complex semiconductor structures stimulate further study of new optical and photovoltaic phenomena in active film elements.
Currently, oxides and nitrides of semiconductors and semiconductor films grown on their surfaces are widely used in the manufacture of multichannel photovoltaic converters and other active elements of microelectronics circuits, and in particular, optoelectronics. In this case, it is possible to obtain high-quality and dielectric layers of semiconductors with deep levels. At the same time, it is easier and cheaper to use polycrystalline films sprayed on amorphous substrates rather than epitaxial ones.
CdTe semiconductor films are an important material for the creation of photodetector devices based on heterostructures operating in the near (up to 3 microns) and far (8—14 microns) The IR range. It is of interest to obtain heterostructures based on photosensitive layers with different types of conductivity. A promising p-type material doped with silver and copper, which give an acceptor level in the forbidden zone with a long lifetime of non-main charge carriers [1—14].
The aim of the work is to study new photovoltaic properties of active CdTe thin films and heterostructures in a system with SiO2-Si under conditions of specific external influences.
The results of experimental studies of the photovoltaic properties of textures from sprayed layers of CdTe – SiO2 – Si, etc., allow the development of new devices based on polycrystalline films with controlled properties.
Below we investigate the photosensitivity of the CdTe – SiO2 – Si structure, which can be used, for example, as a metal – silicon nitride oxide semiconductor (MNP) – a transistor with a polarized dielectric [1,2], which allows electrical rewriting of information.
Experimental results
Polycrystalline (grain sizes are 0.05—0.1 microns) CdTe films were obtained on the surface of SiO2 – Si. CdTe and Ag and Cu impurities evaporated in a vacuum of 10—5 mmHg from separate evaporators onto the heated oxidized surface of Si. The relative arrangement of the layers of the CdTe – SiO2 – Si structure and the ohmic contacts to them is schematically shown in Fig.1. In such a structure, photosensitivity is controlled by external influences, such as an electric field or corona discharge, which change the built-in field in the dielectric. In this case, we have a «reverse» field – effect transistor of the CdTe – SiO2 – Si type, when the control charge is located under the semiconductor layer, and its surface remains open.
Fig.1 The relative position of the layers of the CdTe – SiO2 – Si structure. 1,2 – contacts; 3 – filtering contacts.
Currently, electrification using a corona discharge is the main method of sensitizing photovoltaic layers in industrial electrography [3].
An experimental setup was used for corona electrification of the studied structures, the block diagram of which is presented in [4]. Electrification occurs due to deposition of positive or negative ions in a corona discharge on the surface of the layer. Corona discharge occurs if the voltage between the metallized surface of the Al layer and the electrode exceeds 6 kV, when the field embedded in the structure reached 100 V. The spectra of the short – circuit current charged in this way in the CdTe – SiO2 – Si structure were studied depending on the magnitude of the external corona discharge and showed that in the static mode a shift of the spectra to the short-wave region is observed (Fig.2). It turned out that in such a structure, the photosensitivity of the layer can be controlled by the action of an external corona discharge potential (using the «field effect» method), which, as it turns out below, induce embedded electric charges in the dielectric.
In Fig.2. The spectral dependences of the short-circuit current (Icz) of the CdTe layer for various values of the corona discharge intensity, which were carried out by contact (2) and electric probe contact (3) to the surface of the CdTe semiconductor, are presented. 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 h?= 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 100 V, the inversion position of the short-circuit current sign will mix into the short-wave region of the spectrum. In this case, the maximum photosensitivity of the Icz will be mixed into the short-wavelength region of the spectrum 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 70 angstems (curve 3).
Fig.2. Spectral dependences of the Icz for the CdTe-SiO2-Si structure on the magnitude of the corona discharge potential: jcr = 0 V (curve 1), 40 V (2),70 V (3). The inset shows the photosensitivity spectra of the impurity region of light absorption on a logarithmic scale.
Discussion of the results
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, ? 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.
Conclusion
Summing up the analysis of the results, it is shown that the spectral photosensitivity of the CdTe layer by short – circuit current and photo EMF can be controlled by the induced built – in electric charge of the dielectric created by the external corona discharge potential in the CdTe (film) – SiO2 (dielectric) – Si (semiconductor) heterostructure.
This opens up new possibilities for the creation of semiconductor devices sensitive to electromagnetic radiation, 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 robots (the visual organ of a robot, where color vision is needed), for devices and information recording systems.
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