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Section 14. Physics
Polvonov Bakhtiyor Zaylobidinovich, Ph D., Head of the Department of Physics of Fergana Polytechnic Institute E-mail: r_rasulov51@mail.ru Nasirov Mardon, Researcher of Fergana Polytechnic Institute Mirzayev Valijon, Researcher of Fergana Polytechnic Institute Raziqov Jurabek, Researcher of Fergana State University
SPECTRUM OF THE SHORT CIRCUIT PHOTO CURRENT OF CdTe, CdTe: In PHOTOLOLATIC FILMS DEPENDING ON THE TEMPERATURE
Abstract: It is found out that the generation of the abnormal photo voltage in the alloyed angle-sprayed thin CdTe: In films is caused by the light of the spectrum sphere of self absorption and mixed as well. The spectrum of short- circuited photo current was analyzed in accordance of the temperature. Keywords:
In [1], the possibility of generating an anomalously photovoltage (APV order 102 ^103 V/sm) in skew-deposited films CdTe: Ag under light excitation from the region of not only intrinsic but also impurity (hv < Eg) absorption was investigated. It is shown that the impurity anomalous photovoltaic effect (APVE) is due to the quantum deep-level-zone transition in barrier regions upon photoexcitation and the subsequent spatial separation of free carriers of the same sign and residual impurity charge under the influence of the internal electric field of asymmetric micro potential barriers of the grain boundary. The purpose of this work is to research the possible impurity contributions to APV by analyzing the short-circuit current spectra IshC as a function of temperature in the CdTe and CdTe: In films with the APV property.
The doped layer of CdTe: In was grown by thermal evaporation in vacuum 10-4 10-5mm.Hg by preparing CdTe and In from separate crucibles. The initial mass In was 3 ^ 7 % by mass CdTe. The temperature of the glass substrate was varied within 200 ^ 500 K. Freshly prepared polycrystalline samples CdTe: In with a thickness of d « 0.8 ^ 1.5 mkm and an area of 5x20mm2 (condensation rate vK « 1.5^2.0 nm/s, deposition angle 30 ^ 60°) turned out to be lower resistance and relatively weakly expressed APV properties (VAPV = 50 ^ 100 V .After optimal heat treatment (HT) in
vacuum, the resistance of the samples increased by a factor of 2-3, and at the same time, at room temperature, they generated the maximum photovoltage to values of (2 + 4) • 103 V, i.e. an order of magnitude greater than that of specially un-doped samples CdTe (where VAPV = 200 ^ 600 V), and the short circuit photocurrent increased by more than two orders of magnitude and reached IShC «10-8 A [2].
In fig. 1 shows typical short-circuit current spectra IShC (v) recorded at room temperature (Figure a) and liquid nitrogen temperature (b) for undoped CdTe (curve 1), freshly prepared (curve 2) and annealed (curve 3) CdTe : In samples. As can be seen from the comparison of the curves of the figures, the spectral sensitivities of IShC different film samples differ sharply, and depend significantly on temperature.
At T = 300 K, the maxima of the spectra with an accuracy of ±0.05eV coincide and correspond with the same accuracy to the edge of the intrinsic absorption (Eg = 1.51 eV [3]). The long-wavelength tail of spectra IShC noticeably extends to photon energy hv = 0.9 eV and is due to impurity absorption of light in the barrier regions of the crystallites, causing impurity APV. The maximum value of IShC increases significantly in doped samples CdTe : In (curves 2 and 3 in fig. 1, a). Absorption of light in the quasi-neutral regions of the grains is responsible for the short-wavelength decrease of
SPECTRUM OF THE SHORT CIRCUIT PHOTO CURRENT OF CdTe, CdTe: InPHOTOLOLATIC FILMS DEPENDING ON THE TEMPERATURE
the spectra, leading to bulk photoconductivity of the shunt layer and, thus, to a drop in the spectral value of the photovoltage VAPV (v) = IshC (v) • Rpl (v) . The integrated short-circuit current in the doped films exceeds by two or three orders of magnitude compared to pure samples CdTe. This is achieved in the case of unannealed CdTe: In films to a greater extent due to photoconductivity (where VAPV is only 60 ^ 100 V/Sm), and for heat-treated films - mainly due to AP effect (in which VAPV reaches values 3 • 103 V / Sm).
From the point of view of the analysis of the formation iShc ( mechanism of APV, the comparison of spectra IShC for the its doublet structure appears (curve 2 in fig. 1, b)).
three samples considered above, taken at a temperature of T = 77 K liquid nitrogen, turned out to be very productive (fig. 1b). In accordance with the results of[4], for films CdTe , the integrally IK3 practically does not change with temperature in the T = (77 300) K interval studied. However, in the case of doped samples CdTe: In, as the temperature decreases from 300 K to 77 K, both the integral value IShC and its spectral maximum decrease by more than an order of magnitude. There is a significant long-wavelength shift of the I^c (v) spectrum for the freshly prepared film CdTe: In and
a) b)
Figure 1. The short-circuit current IShC spectrum of the CdTe: In films (curve 1), (2 - fresh-prepared and 3 - after thermal annealing) with the property at temperatures T = 300K (a ) and T = 77K (b)
The latter is due to two contributions of the same order to the integral APV, which are caused by the impurity and intrinsic absorption ( Eg = 1.59eV [3]) oflight, and it should be especially noted that the "impurity" contribution prevails over the "own" contribution (see curve 2 at hv < 1.6 eV ). However, we note that the "own" contribution APV in the annealed film CdTe : In also, as in the unalloyed film CdTe, exceeds the "impurity" contribution (curves 1 and 3), while the doublet structure of the spectrum with opposite shoulders than in curve 2 is retained. The short-wavelength shift of the maxima of spectral curves 1 and 3 in (Fig. 2) is associated with an increase in the band gap CdTe with a decrease in temperature to Т = 77К ([3]). Thus, in skew-deposited films CdTe : In, generation APV is caused by light from the spectral region of both its own and impurity absorption, and the "impurity" contribution, depending on the technological conditions, can exceed its own and even change its sign.
To identify specific impurity levels involved in the generation ofAPV, the long-wavelength tails of the Ira (v) spectra were analyzed in more detail; in fig. 1 by curves 1-3, by studying the photon capture cross section [1]. Comparing the experimental and theoretical spectral curves (Fig. 2), we determined the following deep levels of local centers at T = 77 K: Е1 = Ес-(1.43±0.02)eV , Е2 = Еc-(1.05±0.02)eV , Е3 = Ес-(1.31 ± 0.02)eV, Е4 = Еc-(1.18 ± 0.02)eV and Е5 = Еc -(0.85 ± 0.02)eV. As can be seen from (Fig. 2), a, in the undoped sample CdTe, three levels are photovoltaic: Е1, Е2 and Е3, which are created by single, double-charged cadmium vacancies and excess tellurium [5], respectively. In the freshly prepared film CdTe: In, the level Е1 is not detected (Fig. 2b), the reason for which is apparently the saturation of the charge state V— and the formation of donor - acceptor pairs (I+Va) with the energy of the electron transfer to the conduction band equal to 1.18 eV
a) b) c)
Figure 2. Short-circuit current spectra (bullet points) and theoretical spectra of photon capture cross-section (light points, crosses, triangles, squares) of CdTe films (a), CdTe : In (b- freshly prepared, c-after thermal
annealing) at T = 77 K.
During thermal annealing doped film self-compensation of donors In and acceptors V— occurs [6], in the spectrum IK3 a level E5 (Fig. 2 c) appears, which is most likely associated with the formation of pairs (I+Va ) in the near-surface barrier region of the crystalline grains. This is accompanied by a sharp increase in light resistance and VAPV more than an order of magnitude. The contribution of level E5 to APV is due to the absorption of light in the barrier region, causing an electron to transfer from the level of E5 to the conduction band. Since the degree of filling with electrons of this level decreases exponentially as it approaches the surface of the
grain, the contribution of centers E5 to the impurity APV is insignificant compared to the contributions of local centers E2, E3 and E4 (see Fig. 2 c).
In conclusion, we once again note that the doping of CdTe films with indium in indium qualitatively changes its photovoltaic properties: the maximum value of VAPV increases by an order of magnitude, and IShC increases by more than two orders of magnitude; a sharp temperature dependence of IShC and its spectrum is detected; the impurity contribution to the integral value of VAPV becomes almost of the same order as compared to the contribution due to the intrinsic absorption of light.
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