Abstract
Properties of polycrystalline CdS layers, employed in formation of the CdS-Cu2S heterostructures, have been studied by combining contactless techniques of the time and spectrally resolved photoluminescence (TR-PL) spectroscopy and microwave-probed photoconductivity (MW-PC) transients. The confocal microscopy has been employed to correlate the homogeneity of photoluminescence and grain size in CdS layers. Three types of samples with crystallite grain size of <1 μm (the I-type) and of 2–10 μm of homogeneous (II-type) and inhomogeneous (III-type) grain distribution have been separated. The simultaneous record of MW-PC and TR-PL responses ensures the same sampling area on the layer under investigation, as both (MW-PC and TR-PL) signals are generated by the same UV laser excitation beam. Two PL bands peaked at 500 and 700 nm were revealed. It has been demonstrated that photoluminescence intensity strongly depends on the properties of the polycrystalline 15–26 μm thick CdS layers with equilibrium carrier density of about1.5×1013 cm−3, which serve as the substrates to form CdS-Cu2S junctions. The different carrier decay components were ascribed to different microareas with characteristic MW-PC and PL decay lifetimes of 2–10 ns, ascribed to microcrystallites with PL instantaneous decay lifetimes of 40–200 ns, and MW-PC decay lifetimes in the range of 100–1000 μs attributed to the inter-crystallite areas of CdS polycrystalline material.
Highlights
The CdS and Cu2S compounds are of great interest owing to their unique properties in variation of composition and their potential applications in numerous fields [1, 2], such as sensor elements in X-ray imaging devices [3], solar cells [4], and super ionic conductors, photodetectors, and photothermal converters [5,6,7,8,9]
The confocal microscopy images among the CdS polycrystalline layers of different types imply that the most homogeneous CdS layer is obtained for the II-type samples, deposited at 260∘C for 75 min
Value of free-carrier density of the n0 = 1.5 ⋅ 1013 cm−3 was estimated for junctions of II- and III-type, and it determines concentration of shallow impurities
Summary
The CdS and Cu2S compounds are of great interest owing to their unique properties in variation of composition and their potential applications in numerous fields [1, 2], such as sensor elements in X-ray imaging devices [3], solar cells [4], and super ionic conductors, photodetectors, and photothermal converters [5,6,7,8,9]. Intercrystallite volumes of disordered structure within the polycrystalline CdS material cause the effect of photoinduced modulation of junction potential barrier This determines a percolative carrier transport due Advances in Condensed Matter Physics to a system of disordered distribution of size and location of the microcrystals. The radiative and nonradiative recombination channels in crystallites have been examined by simultaneous sampling of the photoconductivity and photoluminescence transients attributed to the same local area within polycrystalline CdS layer. These investigations have been combined with confocal microscopy imaging (CMI) to correlate crystallite size and their density with distribution of the luminescence intensity
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