CC BY
12
https://doi.org/10.29013/ESR-21-1.2-30-34
Mamatov Olmosbek Maxammatdovidovich, senior teacher of Fergana Polytechnic Institute E-mail: [email protected] Yuldashev Nosirjon Khaydarovich, professor of Fergana Polytechnic Institute
FEATURE OF TECHNOLOGY FOR OBTAINING BY THERMOVACUUM EVAPORATION METHOD OF FILM n-CdS / p-CdTe HETEROSTRUCTURES WITH PHOTOVOLTAIC AND PHOTORESISTIVE PROPERTIES
Abstract. An original thermal vacuum technology is proposed for obtaining an n-CdS / p-CdTe film heterostructure with anomalous photovoltaic and photoresistive properties. It is shown that when a CdS photoresistor is illuminated by illumination from the region of its own absorption (hv > 2.5 eV), the photoelectric properties of the active CdTe layer are significantly modulated. This undoubtedly opens up new applied possibilities of this structure in the field of photonics and film optoelectronics.
Keywords: thermal vacuum technology, n-CdS/p-CdTe film heterostructure, anomalous photovoltaic effect, photoresistor, electron micrograph.
Introduction. The anomalous photovoltaic structural inhomogeneities, such as intrinsic lattice de-
(APV) effect is closely related to the structure-sensitive fects, porosity, grain boundaries, the presence of an
properties of semiconductors and manifests itself in interface layer between grains and surface levels have
the form of generation of anomalously high photo- not been adequately considered. In [4-6], with the
voltages (AHPV, of the order of 10-104 V/cm), as a participation of the authors of this article, an original
rule, in specially co-deposited thin films with a thick- method for the technology ofobtaining thin polycrys-
ness d < 1 pm on a dielectric substrate under excitation talline CdTe films with the APV property was devel-
with light from the spectral region of own and impu- oped. First, polycrystalline CdTe layers were grown on
rity absorption [1-3] (see also the bibliographies a glass substrate by thermal vacuum co-deposition in
shown in them). To date, considerable experimental a quasi-closed volume, generating AHPV at T = 300 K
material has been accumulated in the literature on the with a maximum value VAHPV ~ (600 ^ 700) V / cm at
study of the APV properties of CdTe films. Thus, the an intensity of L »104 Ix of illumination of an incan-
influence oftechnological parameters on the change in descent lamp. Then some of these samples were doped
the polarity of the photo-EMF, on the angular, spectral, with In impurities by the method of ion implantation,
lux-volt, temperature and other characteristics of thin as a result of which their VAHPV dropped to
layers with a stoichiometric composition, and with a 150-300 V/cm, and after thermal treatment of these
disturbed stoichiometry, as well as the possibility of films in vacuum at T » 600 K for 10-20 min, VAHPV
their application in optoelectronics has been studied increased stronglyand reachedvalues of ~ 3000 V / cm. in detail. However, until now, the processes of AHPV Based on the analysis of literature data [1-12], a
formation and destruction depending on specific tempting idea naturally arises to create an optoelec-
tronic device from a film heterostructure, operating based on the simultaneous use of photoconductivity and APV effect in thin polycrystalline films. For this purpose, cadmium chalcogenides are suitable semiconductor materials. Thus, polycrystalline film structures made of cadmium chalcogenides are distinguished by high photosensitivity due to the presence of specific photosensitive local centers and special electronic properties of the grain boundary surface. The photoconductivity of CdS, CdTe films, and APV properties of an obliquely sprayed CdTe film are studied in detail. Recently, there has been a growing interest in the study of the CdS/CdTe film heterostructure [7-12] as an efficient solar cell. An integral film heterosystem consisting of a highly sensitive photoresist CdS layer on a transparent conducting substrate and a CdTe film in the form of an upper sandwich layer with the APV property is ofgreat interest from both the physical and the applied point ofview for optoelectronics and photonics. Obviously, under a certain optimal condition of the technology for obtaining such an active CdS/CdTe film heterostructure by modulating the CdS photoconductivity with the help of additional illumination, it will be possible to carry out targeted optical control of the APV parameters of the CdTe film, which makes it possible to use it as an optical modulator, phototransistor, photoswitch, memory elements, and etc.
The purpose of this work is to develop an original technology for obtaining a film heterostructure n - Cds / p - CdTe with photoresistive and anomalous photovoltaic properties by the thermal vacuum evaporation method for further study of its electrical, photoelectric, and optical characteristics, as well as the fundamental possibility of manufacturing new optoelectronic devices on its basis, in particular film efficient solar cell.
Technology for obtaining the n - CdS / p - CdTe film heterostructure with anomalous photovoltaic and photoresistive properties. The main task of the technology was that, firstly, in the vacuum on a clean transparent substrate by the method of thermal evaporation, it was necessary to grow a thin polycrystalline
layer of n - CdS(d » 0.3 - 0.5 mm) with sufficiently high photoconductivity, and its dark resistance was significantly greater than the resistance of the CdTe layer (d < 1 mm). Second, without changing the degree of vacuum, a p-CdTe layer with a noticeable APV property should be deposited on the surface of a freshly prepared n-CdS layer. Preliminary studies have shown that the known methods of film production [1-6] turned out to be unsuitable for obtaining a sharp n - CdS / p - CdTe heterostructure with the required photoresistive and APV properties. A CdTe layer grown in this way on a photoconductive CdS layer sometimes did not exhibit noticeable APV properties. It was found that, in this case, the determining varying factors for fabricating an integrated n - CdS / p - CdTe heterosystem with the required photosensitive parameters are the relative resistance, the direction of crystal grain growth, and the thicknesses of the CdS and CdTe layers.
After repeated careful experiments, it was still possible to form an n - CdS / p - CdTe heterostructure (Fig. 1) with the above properties in a single technological cycle by the method of a kind of sequential deposition of powdered CdS and CdTe with the brand "for semiconductors" on transparent glass substrates (1) in a vacuum with a residual gas pressure of p »10-2 -10-3 Pa. First, at a substrate temperature of 573-623 K at an angle a1 = 40 - 600 relative to the normal to the substrate surface, a CdS (2) layer with an area of 20 x 5 mm2 and a thickness of 0.2-0.4 pm was deposited through special masks. Then it was sensed by subsequent thermal annealing in vacuum at: T = 773-823 K for 10-15 minutes. The dark resistance of the CdS film with electronic conductivity reached values of R »1012 W, and the multiplicity of changes in resistance under the influence of illumination reached K » 102 -103 rel. units. According to electron micrographs of the transverse cleavage (Fig. 2) and the surface, the grown CdS layer had a columnar structure without pores, the sizes of crystal grains along the substrate surface were found to be of the order of dcr » 0.3 - 0.5mm.
CdTe
CdS
Figure 1. Schematic view of the n-CdS/p-CdTe film heterostructure on a transparent glass substrate (1): 2 - CdS photoresistor, 3 -photovoltaic layer CdTe, 4 - current collector ohmic contacts
In the second stage of growing the structure, without destroying the achieved vacuum, a p-CdTe layer (3 in Fig. 1) with a thickness dfl = 0.5 - 0.8 mm was deposited directly onto the surface of the photoresistive n-CdS film at a rate of 1.5 - 2.0 A / c at an angle a =-(50 - 300) at a substrate temperature T = 423-573 K. Therefore, according to the conditions of this technology, the directions of the molecular beams of CdS and CdTe towards the substrate diverged by an angle of « 900. Failure to observe this angle, as well as an increase in T and d^, led to a drop in the maximum value of VHPV generated by the CdTe layer under incandescent lamp illumination. A
noticeable generation of photovoltage occurred only in those cases when the dark resistance of the n-CdS layer significantly exceeded the light resistance of the APV of the p-CdTe layer. The upper ohmic contacts (4 in Fig. 1) were formed by deposition of Ag in the form of strips through the masks. The active area of the n — CdS / p — CdTe heterostructure was 50-80 mm2. The stoichiometric composition of the films was achieved by preliminary preparation of the charge with a closed curtain between the substrate and the boat, controlled by the subsequent processing of the X-ray diffraction analysis spectra on a DRON-3 setup.
Vji
^ '' №
b
T A * '
: * I f 1
5 fy; %
\ r*
Figure 2. Electron micrographs of a transverse cleavage of the n-CdS/p-CdTe film heterostructure from the side of the CdTe film surface, obtained at a substrate temperature Ts = 523 K(a) and Ts = 573 K(b). 1 - glass substrate, 2 -layer CdS, 3 - layer CdTe, 4 - surface CdTe
The results of electron microscopic studies also show that annealing of the films leads to a change in the crystallite size. From the micrographs of the surface and transverse cleavage of the sensed CdTe film in the n - CdS / p - CdTe heterostructure presented in (Fig. 2 a, b), obtained at the substrate temperature T = 523 K and 573 K, it can be seen that an increase
s '
in Ts leads to an increase in the grain size due to other grains, straightening borders and smoothing the corners of the joint. The value of the grain size for CdTe films obtained at T = 523K is about
s
1.0 -1.5 mm, and at Ts 573K -1.5 - 2.0 mm. It is also seen from the figures that the columnar structure extends in some cases to the entire thickness of the film. However, it should be noted that although the transverse structure visually observed from fracto-grams is monolithic, in fact, the average value of the coherent scattering regions is much smaller, as established by the results of X-ray structural studies.
In the n - CdS / p - CdTe film heterostructure fabricated according to the technology described above, the main working element is the p-CdTe pho-todetector. At room temperature, with frontal stationary illumination with light with a wavelength in the range of 600 <1< 800 nm and an intensity of
L » 105 • Ix, it generated a photovoltage VAHPV ~ 200V. With an additional rear (i.e. from the side of the substrate) illumination in the spectral region of l £ 500 nm already at La » 1 •Ix, the photovoltage VAPV drops noticeably, and at La » 105 • h, it practically disappears. Note that anotherisotypic n - CdS / p - CdTe:In heterostructure with indium ohmic contacts possessed a similar APV property with photoresistive modulation. In this case, the APV layer of CdTe: In was obtained by additional heat treatment in the vacuum or in the air with the presence of CdCl2 molecules, which leads to an increase in the crystallite size and an increase in the proportion of cubic modification.
In conclusion, it should be noted that the original technology proposed here allows the obtained n - CdS / p - CdTe heterostructure to operate in the transverse photoconductivity mode, which is important for creating a thin-film optical modulator, phototransistor, photoswitch, etc. In a film solar cell, the n - CdS / p - CdTe heterostructure operates in the longitudinal photoconductivity mode, which requires the inclusion of an additional technological process for manufacturing transparent ohmic contacts, which will be dedicated to separate work.
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