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ПРЕДСТАВЛЕНИЕ НАУЧНОЙ РАБОТЫ
RADIATION EFFECTS IN ZnSe / (001) GaAs EPITAXIAL FILMS
Sharibaev Muratbai Boribayevich Karakalpak State University named after Berdakh, Nukus, Uzbekistan
E-mail: murat.sharibaev@mail. ru
Baymuratov Sharapatdin Jetkerbaebich, Tashkent University of Information Technologies, Nukus, Uzbekistan
Saparniyazova Gulbahar Kalievna Tashkent State Agrarian University, q Nukus, Uzbekistan
Аbstract. The optical and structural properties of undoped ZnSe epilayers grown by molecular beam epitaxy with thickness ranging from 0.5 to 2 mm on (001) GaAs subsrates were investigated by depth resolved optical and X-ray methods. It was shown that the epilayers with thickness above some value (>1 mm) contain three regions with different structural and optical quality. Two of them (near top surface and near interface ones) contain the higher defect density. The nature of luminescense line 446.1nm (4.2K) is discussed. Evidence is presented to show that it is not related to formation of free exciton. The radiation enhanced defect reactions mainly in top surface layer was observed.
Key words: photoluminescence, epitaxial film, temperature.
In recent years, ZnSe-based II-VI heterostructures have been intensive investigated as a promising candidates for light-emitting devices in the blue-green spectral range. However, their application is still limited by degradation processes [1]. It is known that degradation rate depends on dislocation density in buffer layer. So the buffer layer of h<hC thickness (hC - critical thickness for strain relaxation by misfit dislocations which is 0.15-0.2 mm for ZnSe/GaAs) is used as a rule. However the decrease of layer thickness results in the drop of thermal stability of heterostructures [2]. So the investigations of buffer layer characteristics in dependence of it's thickness in the region h>hC are to be interesting.
In present paper dependence of the structural and luminescence characteristics of ZnSe buffer layers both on their thickness and as a function of depth has been
ВЕСТНИК НАУКИ И ТВОРЧЕСТВА
investigated. As it will be shown the top region of thick (1.3^2 mm) epilayers contains the higher concentration of the extended defects and plays a noticeable role in radiation enhanced defect reaction.
Five series of undoped ZnSe layers with different thickness (0.5-2 mm) were grown by molecular beam epitaxy (MBE) on semiinsulating Cr-doped oriented 3° off (001) towards [110] GaAs substrates in a CATYN' machine equipped with conventional effusion cells for Zn and Se. The details for layers growth described in [3]. Table 1 shows the parameters of growth process and samples.
We used X-ray diffraction methods and X-ray topography combined with exciton and impurity luminescence spectroscopy to control the epilayer properties. Photoluminescence (PL) of ZnSe was excited by the mercury 200 W lamp (lexc=365 nm). PL spectra were recorded in the range from 442 nm to 1400 nm in temperature interval 4.2-77 K using the grating spectrometer MDR-23. Depth uniformity of ZnSe ELs was investigated with step etching [3]. For degradation process investigation the samples were subjected to the illumination by UV light of 500W mercury lamp.
An additional information about the epilayer quality was obtained from PL data. The PL spectra of the investigated ZnSe ELs at 4.2 K are shown on Figure 2. The spectra of thick samples (curve b,c) in the near band edge region consist of the narrow bands at lm1=442 nm, lm2=443.5 nm, lm3=446.8 nm and lm4=476.5 nm. The position of first two peaks corresponds to free-exciton transition IFX , and neutral donor-bound transition, I2 (D0, X). The I2 (D0, X) peak at 2 .796 eV is attributed usually to GaZn .
The free-exciton peak IFX for thicker samples is commonly a doublet like. These can be ascribed in light-hole (lh) and heavy-hole (hh) branches of the valence band split under the strain caused by lattice mismatch and differences in thermal expansion coefficients between ZnSe and GaAs cooling below the TG.
The band at 446.8 nm labeled as IV0, was attributed to extended defects, as well as the band Y0 at 476.5 nm. On the high energy side of IV0 peak the shoulder or the peak at 446 nm (IX) is often observed in our samples also (see Fig.4). This peak position is close to position of two-electron satellite (2EL) of (D0, X) transitions [4] and neutral acceptor bound exciton (connected with As or its complexes). Its possible nature will be considered further. In the spectral region between IV0 and Y0 the weak luminescence of different DA-pairs, namely DAP-1(456.5 nm), Q-DAPs (460-461 nm) and DAP-2 (462 nm), is observed [5].
The low value of ratio x»1 (x =I(D0,X)/I (FX) of donor bound exciton peak intensity to that of free-exciton) in our thick epilayers confirms their high optical quality. It should be noted that intensity of IV0 and Y0 relatively to IFX and and ratio I2/ IFX increases with epilayer thickness increase (Fig. 2, curve b,c).
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Fig. 1 4.2 K PL spectra of thick ZnSe film: a) unetched, b) etched down to 1.8 mm, c) etched down to 1.3 mm. The inset shows the peak heights of /2Ga and /V0 normalized by height of /FX as a function of depth in the ZnSe epilayer.
Energy (eV)
2.80 2.75 2.70 2.50 2.25 2.00 1.75
A3
to
OJ
440 450 460 500 600
Wavelength (nm)
700
Fig. 2 77 K PL spectra of 36 sample before (a) and after (b) UV irradiation, lEXC=365 nm
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For elucidation of the top imperfect layer contribution in degradation processes we investigated the influence of UV-radiation on it's PL characteristics. Fig. 2 shows the 77K photoluminescence spectra from ZnSe epilayers before (curve a) and after UV-irradiation (curve b). The near band edge emission from ZnSe EL shows the combination of band IFX at 444 nm and shoulder at 447 nm. These PL bands were identified as free exciton transition (FX) and a neutral acceptor bound exciton (A 0,X) one (due to VZn as an acceptor) correspondingly. In addition a weak structural donor-to-acceptor (ДА) band connected with Ga (450-480 nm) and the one connected with VZn-GaZn (620 nm) are observed. After UV irradiation within 3 hours at room temperature (ДА) band disappears completely. The intensity of the (A0,X) band as well as the IA band decreases as the time of treatment increases. By use of the step etching we show that UV light enhanced point defect reactions that include GaZn and VZn occur in the near top epilayer region mainly.
In conclusion, for the case of h>hC the increase of epilayer thickness up to 1.3 mm results in the increase of its structural quality and decrease on impurity concentration but further h increase leads to epilayer deterioration. Besides, the thick samples are depth inhomogeneous and consist of three region with different extended defect and impurity concentration: (i) near the interface region with high density of misfit dislocations and impurity concentration; (ii) the region with low extended defect and impurity concentration and (iii) near the top surface region with higher extended defect concentration. Experiment on step etching and temperature dependence of PL spectra allow us to conclude that IX line (l=446 nm) is connected with extended defects (dislocations).
References:
1. A. Ishibashi, J. Crystal Growth 159, 555 (1996).
2. G. Bacher, D. Tonnies, D. Eisert, A. Forchel, M.O. Mooller, M. Korn, B. Jobst, D. Hommel, G. Landwehr, J. Sollner, and M. Heuken, J. appl. Phys. 79, 4368 (1996).
3. J. Y. Leem, J. S. Son, C. R. Lee, C. S. Kim, Y. K. Cho, Hwack J. Lee, S. K. Noh, and I. H. Bae, Appl. Phys. Letters 71, 3257 (1997).
4. Sharibaev M. Determination by photoluminescence of extended defects in epitaxial ZnTe / GaAs films // Semiconductor physics and microelectronics. - 2019. - № 4. -pp. 45-48.
5. Шарибаев М. Исследование распределения точечных и микродефектов в эпитаксиальных слоях ZnSe/GaAs // Гелиотехника. - 2020. - № 1. - С. 234-235.
