rus
Issue #6/2024
A.K.Abduev, A.K.Akhmedov, E.K.Murliev, A.S.Asvarov
STABILITY OF PROPERTIES AND DEGRADATION MECHANISMS OF MULTILAYER TRANSPARENT CONDUCTIVE STRUCTURES DURING THEIR "DAMP HEAT" TESTING
STABILITY OF PROPERTIES AND DEGRADATION MECHANISMS OF MULTILAYER TRANSPARENT CONDUCTIVE STRUCTURES DURING THEIR "DAMP HEAT" TESTING
INTRODUCTION
Transparent conducting layers based on wide-gap oxide materials characterised by high electrical conductivity and high optical transmittance in the visible range of the spectrum have been widely used in production of transparent electrodes in various optoelectronic applications [1-3]. However, the novel and more stringent requirements for layer functional performance, cost and environmental friendliness of materials and technologies used in the intensively developing flexible transparent electronics industry have intensified research aimed at finding new promising materials and technological solutions in various fields [4, 5]. The transparent electrodes design for flexible devices has travelled a long way from the use of homogeneous oxide layers [6] to three-layer oxide/metal/oxide type structures [7] and finally to the search for new architectures based on superlattices and modulated doped structures [8].
Three-layer ITO/Ag/ITO structures deposited on polymer carriers without forced heating have become widespread due to their high conductivity and relatively small thickness determining the minimum bending radius, which is very critical for flexible electronics devices [9, 10]. At the same time, the prospect of using multilayer periodic structures and structures with modulated doping as transparent electrodes is in possibility of reducing the scattering on impurity ions [11].
The commercialisation of new developments in the field of transparent electronics functional layers requires extensive testing of their performance stability against established standards. One of the most important tests in the transparent electronics functional layer industry is the so-called damp heat test, which involves 1000 hours of testing at temperature of 85 °C and a relative humidity of 85%. Such tests make it possible to establish the factors affecting layer characteristics stability, mechanisms of their degradation, and, based on the obtained data, to develop recommendations for their protection.
In the present work, comparative studies of surface resistance stability and optical transmittance of three-layer ITO/Ag/ITO, GZO/Al/GZO, and thin-film periodic IZO (In2O3 + ZnO (10 weight %)) structures with thickness-modulated oxygen content have been carried out during 1000-hour tests at 85 °C and 85% relative humidity. The degradation mechanisms of the structures depending on composition and properties of single layers have been analysed.
RESEARCH METHODS
All tested thin-film structures were deposited on glass and silicon substrates by high-frequency magnetron sputtering of targets of appropriate composition on a drum-type machine equipped with two diametrically arranged sputtering magnetrons. Three-layer ITO/Ag/ITO and GZO/Al/GZO structures were obtained by sequential low-temperature (T≤50 °C) deposition of the layers in pure (99.999%) argon atmosphere. Thin-film periodic structures of high conductivity with modulated oxygen content of IZO were prepared by the technique given in [12].
The block diagram of the test bench for 1000-hour wet heat test at 85 °C and 85% relative humidity is given in [13].
SII-3 (Standards Information Index, Russia) was used to measure surface resistivity of the conductive structures. Transmission electron microscopy (TEM) data on the thickness, microstructure and composition of the synthesised structures were obtained using a TEM Tecnai Osiris FEI electron microscope (USA). Optical transmittance data of the samples were obtained using a UV-3600 Shimadzu spectrophotometer (Japan).
RESULTS
Fig.1 shows a photograph of the tested structures appearance before and after testing. It can be seen that the periodic IZO structure shows no visible changes, while the other structures clearly show signs of degradation, mainly at the edges of the samples.
Table 1 shows data on surface resistivity changes and optical transmittance of the studied thin-film structures during 1000-hour testing. The minimum increase in surface resistivity (4.26%) is observed in the periodic IZO structure with modulated oxygen content along the thickness, while the optical transmittance in it remains practically unchanged. At the same time, the resistivity increase in the ITO/Ag/ITO and GZO/Al/GZO three-layer structures is significantly higher at 33 and 22%, respectively.
Fig.2 shows comparative TEM-micrographs of the initial structures and maps of oxygen distribution in them before testing. It can be seen that the oxide layers in the three-layer structures have a similar block morphology, while the metal layers have significant differences. The Ag layer (Fig.2, a) shows clearer interface boundaries with oxide layers and more complete nucleation coalescence with the formation of a continuous layer with respect to the Al layer (Fig.2, b). At the same time, the metallic aluminium layer (Fig.2, b1), unlike the silver layer (Fig.2, a1) contains a significant amount of oxygen not only in the interface region, but also in the layer volume.
The cross section of the initial IZO sample with thickness-modulated oxygen content has a dense homogeneous structure (Fig.2, c). The integral distribution of oxygen in this structure is also quite homogeneous (Fig.2, c1). The amplitude of oxygen modulation in the original structure was about 0.4% and its pitch was about 6 nm. Both of these parameters are beyond the threshold resolution of the energy dispersive microanalysis method used, so we can only observe integral distribution in the corresponding map (Fig.2, c1).
DISCUSSION
Joint analysis of the obtained results suggests the following mechanisms of structure degradation.
In three-layer ITO/Ag/ITO structures, diffusion of oxygen and water vapour occurs mainly along the metal-oxide interfaces, with partial delamination of the structure, which reduces its optical transmission and accelerates its further degradation and destruction. Darkening of the edge regions of the structure can also be associated with the intake of sulphur compound vapours from the atmosphere into the interlayer region with subsequent formation of silver sulphide film in the interfaces area.
In GZO/Al/GZO structures, the initial aluminium layer is already partially oxidized at grain boundaries and in the area of interlayer interfaces, so resistance of the initial GZO/Al/GZO structure is significantly higher than that of the ITO/Ag/ITO structure. We believe that due to high chemical activity of aluminium, its oxidation during three-layer structure formation occurs in several processes: oxidation in the gas phase by residual oxygen in the chamber, oxidation from the surface by oxygen of the sputtered GZO target during deposition of the upper oxide layer, and oxidation at the interfaces due to interaction with adjacent oxide layers. The oxide layer on the boundaries of aluminium grains reduces their electrical percolation, preventing excitation of planar surface plasmons at the metal-oxide boundaries and, consequently, reducing optical transmittance of the structure [14]. At the same time, the dense oxide layer on the boundaries of aluminium grains during the tests prevents further diffusion of oxygen and water vapour, slowing down the structure degradation.
The IZO thin-film periodic structure with thickness-modulated oxygen content showed better stability of surface resistance and optical transmittance. The oxygen modulation effect in this structure is about 13% [12]. The relative resistance growth after 1000-hour testing was about 4%, which indicates high stability of the modulated oxygen state. We believe that high density and homogeneity of the periodic structure of IZO prevent rapid diffusion of atmospheric oxygen and water vapour through the pores and grain boundaries, and the main mechanism of oxygen diffusion in the structure under the test conditions remains a much slower vacancy mechanism.
CONCLUSIONS
The studies have shown that the degradation mechanism of the multilayer structure depends on a number of factors: microstructure, density and chemical resistance of single layers, mutual adhesion of adjacent layers and differences in their temperature coefficients of expansion, and the interlayer interfaces.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Transparent conducting layers based on wide-gap oxide materials characterised by high electrical conductivity and high optical transmittance in the visible range of the spectrum have been widely used in production of transparent electrodes in various optoelectronic applications [1-3]. However, the novel and more stringent requirements for layer functional performance, cost and environmental friendliness of materials and technologies used in the intensively developing flexible transparent electronics industry have intensified research aimed at finding new promising materials and technological solutions in various fields [4, 5]. The transparent electrodes design for flexible devices has travelled a long way from the use of homogeneous oxide layers [6] to three-layer oxide/metal/oxide type structures [7] and finally to the search for new architectures based on superlattices and modulated doped structures [8].
Three-layer ITO/Ag/ITO structures deposited on polymer carriers without forced heating have become widespread due to their high conductivity and relatively small thickness determining the minimum bending radius, which is very critical for flexible electronics devices [9, 10]. At the same time, the prospect of using multilayer periodic structures and structures with modulated doping as transparent electrodes is in possibility of reducing the scattering on impurity ions [11].
The commercialisation of new developments in the field of transparent electronics functional layers requires extensive testing of their performance stability against established standards. One of the most important tests in the transparent electronics functional layer industry is the so-called damp heat test, which involves 1000 hours of testing at temperature of 85 °C and a relative humidity of 85%. Such tests make it possible to establish the factors affecting layer characteristics stability, mechanisms of their degradation, and, based on the obtained data, to develop recommendations for their protection.
In the present work, comparative studies of surface resistance stability and optical transmittance of three-layer ITO/Ag/ITO, GZO/Al/GZO, and thin-film periodic IZO (In2O3 + ZnO (10 weight %)) structures with thickness-modulated oxygen content have been carried out during 1000-hour tests at 85 °C and 85% relative humidity. The degradation mechanisms of the structures depending on composition and properties of single layers have been analysed.
RESEARCH METHODS
All tested thin-film structures were deposited on glass and silicon substrates by high-frequency magnetron sputtering of targets of appropriate composition on a drum-type machine equipped with two diametrically arranged sputtering magnetrons. Three-layer ITO/Ag/ITO and GZO/Al/GZO structures were obtained by sequential low-temperature (T≤50 °C) deposition of the layers in pure (99.999%) argon atmosphere. Thin-film periodic structures of high conductivity with modulated oxygen content of IZO were prepared by the technique given in [12].
The block diagram of the test bench for 1000-hour wet heat test at 85 °C and 85% relative humidity is given in [13].
SII-3 (Standards Information Index, Russia) was used to measure surface resistivity of the conductive structures. Transmission electron microscopy (TEM) data on the thickness, microstructure and composition of the synthesised structures were obtained using a TEM Tecnai Osiris FEI electron microscope (USA). Optical transmittance data of the samples were obtained using a UV-3600 Shimadzu spectrophotometer (Japan).
RESULTS
Fig.1 shows a photograph of the tested structures appearance before and after testing. It can be seen that the periodic IZO structure shows no visible changes, while the other structures clearly show signs of degradation, mainly at the edges of the samples.
Table 1 shows data on surface resistivity changes and optical transmittance of the studied thin-film structures during 1000-hour testing. The minimum increase in surface resistivity (4.26%) is observed in the periodic IZO structure with modulated oxygen content along the thickness, while the optical transmittance in it remains practically unchanged. At the same time, the resistivity increase in the ITO/Ag/ITO and GZO/Al/GZO three-layer structures is significantly higher at 33 and 22%, respectively.
Fig.2 shows comparative TEM-micrographs of the initial structures and maps of oxygen distribution in them before testing. It can be seen that the oxide layers in the three-layer structures have a similar block morphology, while the metal layers have significant differences. The Ag layer (Fig.2, a) shows clearer interface boundaries with oxide layers and more complete nucleation coalescence with the formation of a continuous layer with respect to the Al layer (Fig.2, b). At the same time, the metallic aluminium layer (Fig.2, b1), unlike the silver layer (Fig.2, a1) contains a significant amount of oxygen not only in the interface region, but also in the layer volume.
The cross section of the initial IZO sample with thickness-modulated oxygen content has a dense homogeneous structure (Fig.2, c). The integral distribution of oxygen in this structure is also quite homogeneous (Fig.2, c1). The amplitude of oxygen modulation in the original structure was about 0.4% and its pitch was about 6 nm. Both of these parameters are beyond the threshold resolution of the energy dispersive microanalysis method used, so we can only observe integral distribution in the corresponding map (Fig.2, c1).
DISCUSSION
Joint analysis of the obtained results suggests the following mechanisms of structure degradation.
In three-layer ITO/Ag/ITO structures, diffusion of oxygen and water vapour occurs mainly along the metal-oxide interfaces, with partial delamination of the structure, which reduces its optical transmission and accelerates its further degradation and destruction. Darkening of the edge regions of the structure can also be associated with the intake of sulphur compound vapours from the atmosphere into the interlayer region with subsequent formation of silver sulphide film in the interfaces area.
In GZO/Al/GZO structures, the initial aluminium layer is already partially oxidized at grain boundaries and in the area of interlayer interfaces, so resistance of the initial GZO/Al/GZO structure is significantly higher than that of the ITO/Ag/ITO structure. We believe that due to high chemical activity of aluminium, its oxidation during three-layer structure formation occurs in several processes: oxidation in the gas phase by residual oxygen in the chamber, oxidation from the surface by oxygen of the sputtered GZO target during deposition of the upper oxide layer, and oxidation at the interfaces due to interaction with adjacent oxide layers. The oxide layer on the boundaries of aluminium grains reduces their electrical percolation, preventing excitation of planar surface plasmons at the metal-oxide boundaries and, consequently, reducing optical transmittance of the structure [14]. At the same time, the dense oxide layer on the boundaries of aluminium grains during the tests prevents further diffusion of oxygen and water vapour, slowing down the structure degradation.
The IZO thin-film periodic structure with thickness-modulated oxygen content showed better stability of surface resistance and optical transmittance. The oxygen modulation effect in this structure is about 13% [12]. The relative resistance growth after 1000-hour testing was about 4%, which indicates high stability of the modulated oxygen state. We believe that high density and homogeneity of the periodic structure of IZO prevent rapid diffusion of atmospheric oxygen and water vapour through the pores and grain boundaries, and the main mechanism of oxygen diffusion in the structure under the test conditions remains a much slower vacancy mechanism.
CONCLUSIONS
The studies have shown that the degradation mechanism of the multilayer structure depends on a number of factors: microstructure, density and chemical resistance of single layers, mutual adhesion of adjacent layers and differences in their temperature coefficients of expansion, and the interlayer interfaces.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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