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Abstract: When studying the optical transmission of thin epitaxial lead selenide layers, the characterization of their parameters such as the index of refraction, and the reflection and absorption coefficients was performed. The forbidden gap width, mainly which increases as decreases the layer thickness, was determined by straightening of the absorption coefficients squared versus the photon energy. Along with the increase in the tangential lattice constant in strained layers, this fact confirms the realization of effective“negative” pressure in the layers (of biaxial stretching).
Key words: Optical characterization, transmission spectra, strained layer, forbidden gap, effective “negative” pressure.
1. Introduction ??
Fabrication of thin epitaxial layers of IV-VI semiconductors represents an interesting possibility for realization of effective “negative” pressure in the solid-state structure. Really, if a thin layer, for instance, of lead selenide has grown on the potassium chloride substrate with a larger lattice constant, it is stretched by the substrate, and the forbidden gap width increases. Hence, the realization of effective“negative” pressure can be supported by the increase in the lattice constant of the layer with increasing deformation and by the corresponding increase in the forbidden gap width. The strain relaxation occurs as the thickness of layers increases, and the residual deformations are more noticeable as the layer thickness decreases.
The study of lead selenide nanolayers, in which the abovementioned conditions are attained, is important for interesting applications including the creation of a quasi-dielectric state in doped narrow-gap IV-VI semiconductors with conservation of the character of band-band, direct electron transitions across the forbidden gap [1].
The present work deals with the investigation of the forbidden gap width of strained PbSe layers up to 180 nm in thickness, and their optical characterization by the transmission spectra and sequential determination of the index of refraction, and the reflection and absorption coefficients and their spectra.
2. Experiment
The epitaxial PbSe layers from 20 to 200 nm in thickness were grown on the KCl (100) substrates by“hot-wall” molecular epitaxy. The temperature of the source represented by polycrystalline PbSe was in the range from 450 to 510 °C, that of the KCl substrate, from 240 to 300 °C, the growth rate made up 0.4-2.0 nm/s [2].
The tangential lattice constants of the layers and their thickness were determined by the X-ray diffraction method using the reflections from crystal planes (200) and (400).
The transmission spectra were measured at T = 300 K in the range of wavelengths of 2.5-25 μm by double-beam prism-diffraction spectrophotometer SPECORD-75IR.
It should be noted that the tangential lattice constant does not always vary in inverse proportion to the thickness of layers. The deformation in the layers or the degree of strain relaxation in them depends on substrate properties to a large extent. Such a case is observed in sample layer SL-304. Despite the fact that, in this case, the layer thickness of 119 nm is significantly less than the thickness of layer SL-277 equal to 181 nm, the lattice constants of these layers differ just by 0.005?. As is seen from Figs. 1 and 2, the width of their forbidden gaps differs insignificantly as well. It should be noted that, as the layer thicknesses decrease, the absorption coefficient on free current carriers increases. A significant
appeared to be less than the ones obtained by processing of the spectrum with an invariable index of refraction. Such differences may be associated with two causes. First, the error at straightening of the absorption coefficient makes up 5-6 meV, second, the Fermi level decreases as the forbidden gap width increases.
References
[1] G. Gegiadze, O. Davarashvili, M. Enukashvili, N. Kekelidze, G. Darsavelidze, L. Gabrichidze, et al., New role of the inpurities in the inprovement of the characteristics of semiconductor lasers and photodetectors, Georgian Engineering News 2 (2005) 54-56.
[2] A. Pashaev, O. Davarashvili, V. Aliyev, G. Gegiadze, R. Gulyaev, M. Enukashvili, et al., Investigation of thin epitaxial layers of lead selenide, Georgian Chemical
Journal 9 (3) (2009) 201-203.
[3] Z. Akhvlediani, L. Bychkova, O. Davarashvili, M. Dzagania, M. Enukashvili, Investigation of the dispersion of refraction index and width of the forbidden gap of lead selenide by optical transmission spectrum, Bulletin of Nat. Geor. Acad. Sci. 36 (3) (2010) 316-322.
[4] Z. Akhvlediani, O. Davarashvili, A. Pashaev, M. Enukashvili, R. Gulyaev, On the contribution of point defects to the formation of supercritical nanostructures based on lead selenide, in: 16th International Conference on Radiation Effects in Insulators, Beijing, China, 2011, p. 216.
[5] L. Akhvlediani, L. Bychkova, O. Davarashvili, M. Enukashvili, M. Dzagania, Optical IR transmission spectra of thin epitaxial lead selenide layers, In: Book Abstracts of the International Conference on Functional Materials and Nanotechnologies, Riga, 2011, p. 272.
[6] E. Palik, D. Mitchell, I. Zemel, Magneto-optical studies of the band structure of PbS, Phys. Rev. 135 (1964) A763-A778.
[7] S. Prabahar, N. Suryanarayanan, K. Rajasekar, S. Srikanth, Lead selenide thin films from vacuum evaporation method-structural and optical properties, Chalcogenide Letters 6 (5) (2009) 203-211.
[8] I.N. Zemel, R.B. Jensen, Electrical and optical properties of epitaxial films of PbS, PbSe, PbTe and SnTe, Phys. Rev. A 140 (1) (1965) A330-A342.
[9] Y. Ukhanov, Optical properties of Semiconductors, Nauka, Moscow, 1977.
Key words: Optical characterization, transmission spectra, strained layer, forbidden gap, effective “negative” pressure.
1. Introduction ??
Fabrication of thin epitaxial layers of IV-VI semiconductors represents an interesting possibility for realization of effective “negative” pressure in the solid-state structure. Really, if a thin layer, for instance, of lead selenide has grown on the potassium chloride substrate with a larger lattice constant, it is stretched by the substrate, and the forbidden gap width increases. Hence, the realization of effective“negative” pressure can be supported by the increase in the lattice constant of the layer with increasing deformation and by the corresponding increase in the forbidden gap width. The strain relaxation occurs as the thickness of layers increases, and the residual deformations are more noticeable as the layer thickness decreases.
The study of lead selenide nanolayers, in which the abovementioned conditions are attained, is important for interesting applications including the creation of a quasi-dielectric state in doped narrow-gap IV-VI semiconductors with conservation of the character of band-band, direct electron transitions across the forbidden gap [1].
The present work deals with the investigation of the forbidden gap width of strained PbSe layers up to 180 nm in thickness, and their optical characterization by the transmission spectra and sequential determination of the index of refraction, and the reflection and absorption coefficients and their spectra.
2. Experiment
The epitaxial PbSe layers from 20 to 200 nm in thickness were grown on the KCl (100) substrates by“hot-wall” molecular epitaxy. The temperature of the source represented by polycrystalline PbSe was in the range from 450 to 510 °C, that of the KCl substrate, from 240 to 300 °C, the growth rate made up 0.4-2.0 nm/s [2].
The tangential lattice constants of the layers and their thickness were determined by the X-ray diffraction method using the reflections from crystal planes (200) and (400).
The transmission spectra were measured at T = 300 K in the range of wavelengths of 2.5-25 μm by double-beam prism-diffraction spectrophotometer SPECORD-75IR.
It should be noted that the tangential lattice constant does not always vary in inverse proportion to the thickness of layers. The deformation in the layers or the degree of strain relaxation in them depends on substrate properties to a large extent. Such a case is observed in sample layer SL-304. Despite the fact that, in this case, the layer thickness of 119 nm is significantly less than the thickness of layer SL-277 equal to 181 nm, the lattice constants of these layers differ just by 0.005?. As is seen from Figs. 1 and 2, the width of their forbidden gaps differs insignificantly as well. It should be noted that, as the layer thicknesses decrease, the absorption coefficient on free current carriers increases. A significant
appeared to be less than the ones obtained by processing of the spectrum with an invariable index of refraction. Such differences may be associated with two causes. First, the error at straightening of the absorption coefficient makes up 5-6 meV, second, the Fermi level decreases as the forbidden gap width increases.
References
[1] G. Gegiadze, O. Davarashvili, M. Enukashvili, N. Kekelidze, G. Darsavelidze, L. Gabrichidze, et al., New role of the inpurities in the inprovement of the characteristics of semiconductor lasers and photodetectors, Georgian Engineering News 2 (2005) 54-56.
[2] A. Pashaev, O. Davarashvili, V. Aliyev, G. Gegiadze, R. Gulyaev, M. Enukashvili, et al., Investigation of thin epitaxial layers of lead selenide, Georgian Chemical
Journal 9 (3) (2009) 201-203.
[3] Z. Akhvlediani, L. Bychkova, O. Davarashvili, M. Dzagania, M. Enukashvili, Investigation of the dispersion of refraction index and width of the forbidden gap of lead selenide by optical transmission spectrum, Bulletin of Nat. Geor. Acad. Sci. 36 (3) (2010) 316-322.
[4] Z. Akhvlediani, O. Davarashvili, A. Pashaev, M. Enukashvili, R. Gulyaev, On the contribution of point defects to the formation of supercritical nanostructures based on lead selenide, in: 16th International Conference on Radiation Effects in Insulators, Beijing, China, 2011, p. 216.
[5] L. Akhvlediani, L. Bychkova, O. Davarashvili, M. Enukashvili, M. Dzagania, Optical IR transmission spectra of thin epitaxial lead selenide layers, In: Book Abstracts of the International Conference on Functional Materials and Nanotechnologies, Riga, 2011, p. 272.
[6] E. Palik, D. Mitchell, I. Zemel, Magneto-optical studies of the band structure of PbS, Phys. Rev. 135 (1964) A763-A778.
[7] S. Prabahar, N. Suryanarayanan, K. Rajasekar, S. Srikanth, Lead selenide thin films from vacuum evaporation method-structural and optical properties, Chalcogenide Letters 6 (5) (2009) 203-211.
[8] I.N. Zemel, R.B. Jensen, Electrical and optical properties of epitaxial films of PbS, PbSe, PbTe and SnTe, Phys. Rev. A 140 (1) (1965) A330-A342.
[9] Y. Ukhanov, Optical properties of Semiconductors, Nauka, Moscow, 1977.