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Sputtering of a hot ceramic target: Experiments with ZnO
Vacuum 168 (2019) 108854
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Sputtering of a hot ceramic target: Experiments with ZnO a,*
b
T a
Abubakar M. Ismailov , Vladimir A. Nikitenko , Murtuzali R. Rabadanov , Leyla L. Emiraslanovaa, Isa Sh. Alieva, Murtazali Kh. Rabadanova a b
Dagestan State University, Ul. Gadjieva 43a, Makhachkala, Dagestan, 367000, Russia Russian University of Transport, Ul. Obrazcova 9 (Building 9), Moscow, 127994, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetron sputtering Hot ceramic target Zinc oxide High deposition rate
The sputtering of hot metal targets is a rapidly developing field in the technology of magnetron sputtering systems, aimed at solving certain challenges inherent in advancing the potential of magnetron sputtering: how to increase growth rate, the solution of a number of specific problems of reactive sputtering, and the sputtering of magnetic targets. Here the results of the sputtering of hot ZnO ceramic targets are reported for the first time. It was established that during hot sputtering the morphology of the erosion zone of a ZnO target when insulated from the thermal activity of a magnetron undergoes substantial modification at power density of P > 25 W/ cm2. The hot ZnO target emits microparticles that play a major role in the process of film formation on the substrate. It was shown that the crystallization of ZnO involving microparticles in the process of magnetron sputtering of a hot target occurs through the liquid crystal mechanism. A maximum growth rate (15 nm/s) of (0001) ZnO/(0001) Al2O3 epitaxial films with high structural perfection and intense luminescence in the violet region of the spectrum was achieved (currently a record rate).
1. Introduction The magnetron sputtering method is one of the most commonly used in the production of thin films of various substances from the gaseous phase [1,2]. The sputtering of targets in magnetron sputtering systems (MSS) occurs through the action of intense ion bombardment, when more than 90% of the incoming power is converted into heat. The temperature of the uncooled target increases, significantly affecting the process of sputtering and of film formation on the substrate. At the present time the possibilities of the magnetron sputtering method are being significantly expanded in connection with the development of research into the sputtering of hot targets, which is being carried out in three directions. (1) The first direction involves magnetron sputtering systems with a liquid-phase target, in which metal used as a cathode is inserted in a crucible, which is thermally insulated from the cooling system, and heated to the melting temperature of the metal under the action of plasma ions [3–5]. It should be noted that the ‘self-sputtering mode’ during ‘hot’ sputtering is also possible for solid-phase targets with a high melting point [6–8]. The sputtering of a heated metal target leads to an increase in the rate of film deposition due to a combination of two cathode erosion mechanisms: sputtering and
*
evaporation. (2) The second direction of current research concerns the possibility of resolving problems associated with reactive magnetron sputtering (RMS) [9–14]. At high target metallic surface temperatures, there occurs the clearing of a dielectric (oxide or nitride) target material as a result of the sublimation of volatile components, which, in turn, significantly increases target sputtering and film growth rates, while also preventing arcing. (3) The third direction of current research involves target heating which presents one of the possibilities of solving problems in sputtering relatively thick magnetic targets of Ni, Co, Fe, and their alloys. When a ferromagnetic target warms up above the Curie temperature, a ferromagnetic-paramagnetic transition occurs and it no longer shields the magnetic field of the magnetron, thereby increasing the sputtering efficiency of the target [15,16]. To date, there appears to be no indication in the scientific literature of investigations in which there has been a deliberate decision not to cool nonmetallic (ceramic) targets with the specific intention of assessing the effects of high temperatures on sputtering processes and on the formation of films on a substrate. In our opinion this has been because under conditions of intense sputtering applied in order to increase the growth rate of films,
Corresponding author. E-mail addresses: [email protected] (A.M. Ismailov), [email protected] (V.A. Nikitenko).
https://doi.org/10.1016/j.vacuum.2019.108854 Received 29 December 2018; Received in revised form 30 July 2019; Accepted 3 August 2019 Available online 06 August 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
Vacuum 168 (2019) 108854
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perimeter of the target (thermocouple C). At the time the target was engaged in the hot spraying mode (for 1.5–2 h), a metal shutter was swung into a position between the thermocouples and the target. In order to measure the temperature at any desired point in the sputtering process, the discharge (plasma) was turned off, the shutter was retracted and the thermocouples gently lowered onto the hot target while simultaneously recording their signal using a B7-78/1 voltmeter (Russia). The target temperature corresponding to the thermocouple contact points A, B and C at the moment the discharge is turned off was determined by extrapolating the target cooling curves to the temperature axis corresponding to zero along the time axis. The study of the micromorphology of film surfaces was carried out on a SEM Leo-1450 (Zeiss, Germany) scanning electron microscope in the secondary electron mode. X-ray structural studies performed using an Empyrean series-2 diffractometer with a Cu-Kα source λ = 1.5406 Å (PANalytical, Netherlands). The structural perfection of the film was assessed by reflection high-energy electron diffraction (using an ‘Electronograph EU-75’, Russia). Photoluminescence spectra were measured on a standard laser spectrometer in the wavelength range of 200–1100 nm using a Y-fiberoptic system for transmitting and receiving photosignals. The photoluminescence spectra were excited by 337.1 nm long wave radiation emitted by a nitrogen laser. Laser pulse power density was measured with a universal single-channel energy/laser power meter (Model 11MLink, Detector 11QE25SP-H-MT, Standa Ltd, Lithuania) and was equal to 6.25 kW/cm2. Recording was carried out using an MS3504i imaging monochromator/spectrometer (SOL-Instruments, Belarus) and an HS101 (HR)-2048 × 122 CCD matrix camera (Hamamatsu, Japan) with a personal computer. The sequence of actions to obtain a sample to be used for research was as follows. The vacuum chamber was evacuated to a residual pressure of 10−4 Pa. Next, the substrate was heated to 650 °C, the working gas was injected to a pressure of 0.133 Pa, and a magnetron discharge was ignited at P = 7.4 W/cm2 on the shutter. For 50 min the power density was gradually increased to 53.5 W/cm2 (hot sputtering mode) while simultaneously increasing the substrate temperature to 900 °C. When the hot sputtering mode the process of film deposition on the substrate began by opening the shutter. The duration of the deposition process was 3 h, and the film thickness, as calculated from the deposition rate, was 162 μm.
researchers are confronted with problems of the heating of the nearsurface region of the target because of its poor thermal conductivity. Heating a target induces the formation on its surface of growths (nodules) in the form of mounds, cones or pyramids. They are destroyed by micro-arcs, as a result of which, along with the atomic components, nano- and microparticles are present in the flow of the substance deposited on the substrate, causing a deterioration in the quality of the films obtained [17–24]. One of the characteristic problems encountered has been that this phenomenon has presented a serious technological impediment to the industrial production of transparent conductive In1-xSnxO (indium tin oxide - ITO) films for use as transparent electrodes [18–22]. A similar problem of growth formation is also noted in the industrial technology for the production of zinc oxide films used as an alternative to ITO films [23]. There is also literature reporting the problem of growths with Si [17] and W–Ti [24] sputtering targets. Thus the formation of nodules on the surface of sputtered ceramic targets is an inevitable physical phenomenon. This has led us to ponder the following question: could the phenomenon actually be used to advantage? Our research indicates that the latter is indeed possible, if one pays attention to phenomena occurring on the surface of the substrate involving microparticles emitted from the target. Our current response to these questions is based on the provision of a high temperature of the growth surface, both by resistive heating of the substrate and by radiation from a hot target during which the particles entering the substrate melt. The formation of a film with the involvement of microparticles occurs through the liquid crystal mechanism. It is in the formulation of this task that our work is being carried out and is entering a new territory of investigation. 2. Experimental equipment and procedures The experiments reported here have been carried out using the VATT AMK-MI automated magnetron complex (FerriWatt, Russia) under ‘dry’ vacuum conditions using an oil-free scroll vacuum pump ISP-500C (ANEST IWATA Corporation, Japan) and a Cryo-Torr 8 cryogenic high-vacuum pump (CTI Cryogenics, USA). The working gas (oxygen) pressure was regulated using a RRG-10 device (Eltochpribor, Russia) and measured with a TELEVAC CC-10 wide-range vacuum meter (Fredericks, USA). The source of the current used in the magnetron was an APEL-M1.5PDC-650-2 (Applied Electronics, Russia) set to direct current (DC). The working gas pressure was ~0,133 Pa, gas flow rate ~8 l/h, while the power density on the target was P = 7.4–53.5 W/cm2. The target employed was a sintered ZnO ceramic disk (of 99.999% purity) with a diameter of 40 mm and a thickness of 2 mm Al2O3 (0001) plates with dimensions of 10 × 10 × 0.5 mm3 were used as a high-temperature substrate on which ZnO films were deposited through a sapphire mask measuring 5 × 5 mm2. The substrates were heated by a resistive heater (nichrome), the temperature being controlled using a chromelalumel thermocouple. The distance between the centers of the target and substrate was 3 cm, the angle between the flat surface of the target and substrate being 45°. Film thickness (h) was determined by calculating the crosssection of the image of the sample taken in a scanning electron microscope (SEM). The growth rate (deposition) (v) was estimated by deposition time (t) using the formula v = h/t. To measure the target temperature, a contact measurement method was employed (using a chromel-alumel thermocouple). For this, a mechanical device for conveniently registering temperature of a target within the vacuum chamber was constructed. It contains a unit to which 3 thermocouples with spring-tensioned holders were attached and which can be positioned above the target as desired. The distances between the thermocouples were selected so that they contacted the hot target at points according to the following disposition: in the center (thermocouple A), in the erosion zone (thermocouple B), and on the
3. Research results and assessment We previously reported in Ref. [25] on our work on the preparation of highly oriented ZnO films on amorphous substrates with high growth rates by the usual method of magnetron sputtering of a cold target at a constant current (a 5 mm thick target being placed on the cooled base of the magnetron without the application of pressure). It was shown that in order to achieve highly oriented growth it is necessary to choose an optimal substrate position in the magnetron sputtering system. The graph in Fig. 1 (curve 2) shows the dependence of the growth rate of highly oriented ZnO films on the discharge current density as established in Ref. [25]. The maximum achieved growth rate of 7 nm/s was limited by the maximum power dissipated by the target during stable combustion of the magnetron discharge (at power densities of P > 53.5 W/cm2 the process of destruction of the target takes place) rather than by a decrease in the structural perfection of the films. Mechanical stresses induced by a temperature gradient between the cold rear side and heated (sputtered) side of the 5 mm thick target caused the destruction of the target. At current densities of P > 34 W/cm2, the presence of microparticles was observed on the mirror smooth surface of the films. However, their appearance was only observed on samples sputtered for a duration of 50–60 min or more. In our opinion, this phenomenon occurs in the following way. As the SEM images taken of the target surface demonstrated, a characteristic 2
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Fig. 3. ZnO target cooling curves in areas A, B and C (see Fig. 2 (f)). Fig. 1. Dependence of the growth rate of ZnO films on discharge power density: 1– for a cold target soldered to a magnetron; 2– for a target mounted on a cooled base of the magnetron without pressure [25]; 3– for a hot target thermally insulated from a magnetron.
ZnO targets under conditions of high-current sputtering. At power densities of P > 53.5 W/cm2, a discharge transition to the arc mode was observed for a hot ZnO target (Fig. 2(d)). The arc (3A) moved along the annular zone of erosion, completing one revolution in 10–15 s, eventually leading to the destruction of the target. Operating beyond the transitional stage of the discharge to the arc mode produces undesirable side effects, therefore we worked at values of P ≤ 53.5 W/ см2, at which a stable hot sputtering mode is assured for a long time. As can be seen from Fig. 2(e) and (f), the circular ZnO target in our apparatus heats unevenly along its radius (as opposed to metallic targets [15]). The erosion zone (area B, Fig. 2(f)) heats up more strongly in comparison with the center (area A) and the perimeter (area C) of the target. The temperatures in these areas, determined by extrapolating the cooling curves (Fig. 3) at a discharge power of P = 53.5 W/cm2, are respectively TA = 679 °C, TB = 774 °C, TC = 547 °C. Between adjacent (neighboring) regions along the target radius, there is a significant temperature difference TB–TA = 95 °C, TB–TС = 227 °C. This feature of the hot spraying of ceramic targets often leads to their cracking during spraying or cooling - a significant disadvantage of this method. The analysis of SEM images taken of different areas of an erosion zone shows a significant difference in the surface micromorphology of cold and hot targets (Fig. 4). As can be seen in Fig. 4(b), the surface of the erosion zone (Fig. 4(a), area B) of a cold target after a 3-h sputtering at P = 7,4 W/cm2 is relatively flat and dense. This zone does not undergo significant changes even after more intensive sputtering at P = 34 W/cm2 (Fig. 4(c)). It thus may be assumed that the characteristic morphology of the target surface in region B is a form of relief development as a result of cascade sputtering [26]. However, in area A of the erosion zone, images revealed features somewhat resembling those of so-called dendritic growth [27] (Fig. 4(d)). Since this region is on the margins of the zone of intense sputtering, the origin of these elements of the relief may be attributed to growth from sputtering. Indeed, since such relief elements were not found on the target in area A, which were sputtered at low discharge current values (P = 7.4 W/cm2), it may be assumed that the material for growth in this zone comes from area B due to more intense sputtering of the target at high discharge current values. A substantial modification of the surface of an uncooled target occurs during its sputtering at P > 25 W/cm2. Fig. 4(e) shows the morphology of the erosion zone in area B of a hot target, formed during the sputtering process at P = 53.5 W/cm2. Here there is an already quite developed loose (flake-like) surface. As a consequence of intensive defect formation during ion bombardment, the diffusion coefficient of particles on the target surface will be significantly higher than with ordinary thermal diffusion. Ion bombardment also stimulates the evaporation of a substance (it may be called radiation-accelerated
morphology in the form of nodules gradually develops due to temperature increases in the erosion zone. The destruction of nodules by micro-arcs leads to the formation of a stream of microparticles directed toward the substrate. The above phenomena, which are characteristic of high-current magnetron sputtering, were taken as the motivation for the conducting of experiments in more detail into the mechanisms of hot sputtering of a ZnO target. In order to prevent fracturing of the ZnO target and to achieve a higher temperature, it was decided to reduce the target's thickness to 2 mm and to thermally insulate it as much as possible from the magnetron, while still maintaining the target-magnetron galvanic coupling. For thermal insulation, 3 plates (of dimensions 5 × 5x0.5 mm3) of polycore (Al2O3) were used, positioned separately in a triangular configuration between the cooled surface of the magnetron and the target. To ensure fully dependable ‘clean’ cold sputtering, a 2 mm thick ZnO target was soldered with indium to the water-cooled base of the magnetron. The photographs in Fig. 2 show the thermal modes of cold (in which the target is soldered to the magnetron - Fig. 2(a)) and hot (in which the target is thermally insulated from the magnetron - Fig. 2(b), (c) & (d))
Fig. 2. Photographs of ZnO targets under various sputtering regimes: (a) cold target, P = 34 W/cm2; (b) hot target, P = 34 W/cm2; (c) hot target, P = 53.5 W/cm2; (d) arc on the surface of a hot target, P = 63 W/cm2; (e) hot target, at the moment of discharge being turned off, P = 34 W/cm2; (f) hot target, at the moment of discharge being turned off, P = 53.5 W/cm2. 3
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Fig. 4. SEM images of ZnO targets (sputtering duration - 3 h; gas pressure – 0.133 Pa: (a) – section of a ZnO target with zones of SEM images indicated: A - lateral erosion zone, B - central erosion zone; (b) cold target (zone B, P = 7.4 W/cm2); (c) cold target (zone B, P = 34 W/cm2); (d) cold target (zone A, P = 34 W/cm2); (e) hot target (zone B, P = 53.5 W/ cm2); (f) hot target (zone A, P = 53.5 W/cm2).
sublimation). Thus, it may be assumed that the characteristic surface morphology in this area is a form of relief development resulting from cascade sputtering and radiation-accelerated processes of diffusion and evaporation. In area A of the hot target (Fig. 4(f)) relief developed of specific crystalline structures, some resembling whiskers. As can be seen in Fig. 2(c) and (f) the high temperature is concentrated in an annular zone of erosion (area B), while the rest of the target has a relatively low temperature. Thus we may assume that the area of erosion B is the zone of emission (evaporation) of the substance, while area A is the zone of deposition. Note that area A is also an area of less intensive sputtering compared to area B. Taking into account the above, we may conclude that the characteristic surface morphology in this region is a form of relief development resulting from two ‘competing’ physical processes: sputtering and crystal formation. From the graph in Fig. 1 (curves 1 and 3) it can be seen that as the power density increases to P = 34 W/cm2, the growth rate of ZnO epitaxial films for both a cold and hot target increases monotonically, although the growth rate for a hot target is several units higher than for a cold target. However, at power density higher than P > 34 W/cm2, the growth rate of ZnO films increases sharply for a hot target – to a threefold degree at P = 53.5 W/cm2. Here, obviously, two questions arise simultaneously: (1) what is the mechanism of the additional emission of a substance from the surface of a hot target, and (2) by what mechanism does the additional flux of the substance get absorbed by the growing surface while maintaining epitaxial growth? To answer the questions posed, we proceed from the fact that when a cold target is sputtered, its surface emits an atomic flow characteristic of a purely physical sputtering mechanism. This mechanism corresponds to a monotonic increase in the growth rate of films with an increase in the discharge power density and the absence of any particles on the film surface. The sharp increase in the growth rate in the case of the sputtering of a hot target when P > 34 mA/cm2 is reached can be explained by the triggering of a new physical mechanism for the additional emission of a substance from the target surface. Based on the analysis of a considerable number of SEM images of the surface of films investigated, it may be concluded that the additional component of the flow of matter directed to the substrate is the flow of microparticles emitted from the surface of a hot target. Direct evidence of this conclusion is the presence of microparticles on the growth surface. Figs. 5 and 6 show images of different parts of the same sample, providing sequential pictures of the dynamics of the process of film formation with the involvement of microparticles. The formation of such micro-particles directly on the growth surface or in the gap between the magnetron and the target substrate seems to be physically impossible. Fig. 5(a) and (b) shows the surface morphology of the ZnO epitaxial film at different scales of magnification, in the case of loose (sparse) filling of the growth surface with microparticles. The microparticles
Fig. 5. SEM images of the surface of a ZnO epitaxial film: 1 - solid microparticles, 2 – microparticles at the initial stage of melting, 3 - hexagonal islands, 4 - submerged tips of hexagonal microcrystals forming the structure of the growing film.
that have just been deposited onto the growth surface have no definite shape and are in the solid phase (Fig. 5(b), particles 1). Over time, they come into thermal equilibrium with the substrate and begin to melt (particles 2). In our opinion, the formation of hexagonal microparticles indicates that the crystallization mechanism proceeds in equilibrium conditions through the liquid phase the melting of the initial microparticles (Fig. 5(b), particles 3). Fig. 5(b) also shows the traces of the ends of the hexagons submerging in the lower layer (Fig. 5(b), particles 4), from which the completed sediment layer is formed. Apparently, at the stage of melting of the particles, their azimuthal orientation around the polar axis also occurs, sustaining autoepitaxy. Fig. 6(a) and (b) also confirm the mechanism of crystallization through the liquid phase with a denser accumulation of microparticles. There is a scatter of ‘liquid-like’ islands differing in size, each of which is probably formed from one or several initial microparticles in the solid phase. The dense packing of these islands is such that it almost suppresses the hexagonal facets of the crystals (see Fig. 6(b)). At a low 4
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Fig. 6. SEM images of: (a) initial phase of the melting of a two-dimensional dense array of ZnO micro-particles on the growth surface; (b) later stage of melting of micro-particles; (c) hexagonal ZnO nuclei on the growth surface.
particle density, the fusion of hexagonal islands proceeds along lateral equilibrium faces without deformation (Fig. 6(c)). Thus, there are sufficient grounds to assert that in our observations ZnO crystallization proceeds according to the liquid crystal mechanism, and not according to the vapor crystal mechanism characteristic of gaseous phase deposition methods, which include that of the magnetron sputtering method. The hexagonal form of the islands reflects the crystallographic individuality of ZnO, which is characterized by a hexagonal crystal lattice (structural type - Wurtzite). It can be seen from Figs. 5(b) and Fig. 6(c) that the polar axis of ZnO hexagons is perpendicular to the growing surface. On this basis, it can be preliminarily stated that the (0001) ZnO basal plane is parallel to the plane of the (0001) Al2O3 substrate. The xray diffraction pattern of the sample in Fig. 7 confirms the assumption made. The low peak width (0002) (FWHM = 0.0380) of the diffractogram also indicates the high structural perfection of the ZnO films obtained. The insert in Fig. 6 shows electron diffraction patterns from ZnO films at two different azimuthal angles, confirming the monocrystallinity of the obtained film. It should be noted that the characteristic crystallite size in ZnO films obtained by magnetron sputtering of cold targets varies in the nanometer range of 10–60 nm [28,29], while in our case the crystallite size (grain) in the ZnO film is determined by the size of the hexagonal particles - 1–3 μm (see Figs. 5(b) and Fig. 6(c)). Such a difference in crystallite sizes cannot but affect the physical properties of ZnO films. Fig. 8 shows the typical photoluminescence spectrum of the ZnO films under investigation at room temperature. According to Refs. [30–33], at the temperature of liquid nitrogen this spectrum decomposes into an equidistant series of bands A, A–LO, A–2LO, A–3LO, A–4LO (and so on), due to the radiative decay of free A-excitons with the participation of LO phonons. The temperature studies we carried out of the transformation of the obtained spectra in the temperature
Fig. 8. Photoluminescence spectrum of an epitaxial ZnO film when the sample is excited by a nitrogen laser (337.1 nm) with an excitation power density of 6.25 kW/cm2.
range of 80–300 K showed [31] that the narrow ultraviolet luminescence bands in Fig. 8 with a maximum at 382 and 392 nm are caused, respectively, by the dominant A–LO and A–2LO bands, and the equidistant structure itself gradually disappears as the temperature increases from 80 to 300 K. Meanwhile, the comparable intensities of the A–LO and A–2LO bands and their unique resolution in the spectrum at room temperature indicate a high degree of perfection of the crystal structure of the films obtained. Indeed, the large number of possible combinations of phonons in a two-phonon process (as a consequence of the law of conservation of momentum) considerably increases its probability [32–34]. As a result, in crystals of zinc oxide with a perfect crystalline structure, the intensity of two-phonon emission lines turns out to be comparable to the intensity of single-phonon A–LO lines [32]. In defective samples, the probability of an exciton interacting with one LO phonon ceases to depend on the exciton wave vector (since Froehlich scattering through states of alternating parity is allowed, due to the elastic scattering of excitons by defects), so the intensity of the (A–LO) band noticeably increases, and can many times exceed the intensity of the A–2LO band [32,34]. Another possible explanation for the presence in the luminescence spectrum of a ZnO film peak at 392 nm may be the appearance of radiation caused by the inelastic scattering of free excitons. However, this mechanism of radiative recombination of free excitons is usually manifested at high levels of excitation (hundreds – kW/cm2 [35]). With an increase in the excitation power density up to 1 MW/cm2, the radiation of an electron-hole plasma begins to be recorded, but the maximum of the corresponding band in the luminescence spectrum lies in the region of 400 nm [35]. It should be noted that, in any case, the presence of a structure in the ultraviolet luminescence spectrum in the samples under study at room temperature indicates their high quality.
Fig. 7. X-ray diffractogram of an epitaxial ZnO film. In the insert: 2 electron diffraction patterns taken at two different azimuth angles. 5
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The almost complete absence of visible luminescence of ZnO films indicates their purity (low content of impurities which form green, yellow-orange and red luminescence [36–39]). According to some researchers, the apparent luminescence of ZnO can also be associated with the intrinsic defects of the crystal structure [30,33]: in this case the weak visible luminescence of the ZnO films obtained also confirms their high quality.
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Russ. Acad. Sci. Phys. 79 (2) (2015) 160[32]. [33] P. Kuzmina, V.A. Nikitenko, Zinc Oxide. Production and Optical Properties, Moscow, Nauka, 1984. [34] С.Ю. Вербин, С.А. Пермогоров, А.Н. Резницкий и А.Н. Старухин, Физика твердого тела 19 (11) 3468 (1975), S.Y. Verbin, S.A. Permagorov, A.N. Rezitskiy, A.N. Starukhin, Solid State Phys. 19 (11) (1975) 3468. [35] D.M. Bagnall, Y. Chen, et al., ZnO Excitonic Lasers – the Future of Short Wavelength Emission, in: Onabe (Ed.), 2nd Intern. Symp. On Blue Laser and Light Emitting Diodes. Chiba. Japan, Ohmsha, 1998, pp. 536–539 Sept. 29 – Oct.2. [36] V.A. Nikitenko, Luminescence and EPR of zinc oxide (review), J. Appl. Spectrosc. 57 (5–6) (1992) 783. [37] Ya I. Alivov, M.V. Chukichev, V.A. Nikitenko, Green luminescence band of zinc oxide films copper-doped by thermal diffusion, Semiconductors 38 (1) (2004) 31. [38] V.A. Nikitenko, Optical and spectroscopy of point defects in ZnO, Proceedings of NATO International Workshop on Zinc Oxide as a Material for Micro- and Optoelectronic Applications, St. Petersburg, Russia, June 2004, pp. 23–25 Springer Verlag, Berlin, (2005) 69. [39] J.L. Lyons, A. Alkauskas, A. Janotti, C.G. Van de Walle, Deep donor state of the copper acceptor as a source of green luminescence in ZnO, Appl. Phys. Lett. 111 (2017) 042101.
4. Conclusion Thus, based on the above investigations, the following conclusions may be drawn. 1. The process of magnetron sputtering of a hot ZnO target was achieved. 2. It has been established that the surface of a ZnO target which is thermally insulated from a magnetron, undergoes substantial modification in the process of its being sputtering at power densities of P > 25 W/cm2. The surface of the central zone of erosion becomes loose (resembling flakes), and in areas (at the edge and center of the target) adjacent to it, the formation of characteristic crystalline structures is observed. 3. The hot target, along with the atomic flow, emits microparticles, which, falling on the growth surface, play the principal role in the process of the formation of ZnO epitaxial films. 4. Microparticles on the growth surface, melting and combining with each other, form hexagonal islands, which, in turn, form another continuous layer of epitaxial film above. The formation of the film occurs through the liquid crystal mechanism. 5. A maximum growth rate (15 nm/s) of (0001) ZnO/(0001)Al2O3 epitaxial films with high structural perfection and intense luminescence in the violet spectral region (382 nm) has been reached (a record experimental growth rate at this time). We believe that the fundamental physical and technological concepts underlying our work will serve as stimuli for other researchers to undertake further work in this newly opened territory of hot sputtering of ceramic targets. Wide-gap materials, such as GaN, SiC, AlN, Ga2O3, present particularly interesting potential. Acknowledgments This work was supported by the Russian Foundation for Basic Research, Russia [Project No. 16-02-00227a]. Professor Guy Petherbridge, Dagestan State University, is gratefully acknowledged for his collaboration in the technical presentation of this research. References [1] J.E. Greene, Review Article: Tracing the recorded history of thin-film sputter deposition: from the 1800s to 2017, J. Vac. Sci. Technol. A - Vacuum, Surfaces, and Films 35 (2017) 05C204. [2] G. Brauer, B. Szyszka, M. Vergohl, R. Bandorf, Magnetron sputtering – milestones of 30 years, Vacuum 84 (2010) 1354. [3] G.A. Bleykher, A.O. Borduleva, A.V. Yuryeva, V.P. Krivobokov, J. Lančok, J. Bulíř, J. Drahokoupil, L. Klimša, J. Kopeček, L. Fekete, R. Čtvrtlìk, J. Tomaštik, Features of copper coatings growth at high-rate deposition using magnetron sputtering systems with a liquid metal target, Surf. Coat. Technol. 324 (2017) 111. [4] A.V. Yuryeva, A.S. Shabunin, D.V. Korzhenko, O.S. Korneva, M.V. Nikolaev, Effect of material of the crucible on operation of magnetron sputtering system with liquidphase target, Vacuum 141 (2017) 135. [5] A.V. Kaziev, A.V. Tumarkin, K.A. Leonova, D.V. Kolodko, M.M. Kharkov, D.G. Ageychenkov, Discharge parameters and plasma characterization in a DC magnetron with liquid Cu target, Vacuum 156 (2018) 48. [6] E. Oks, A. Anders, A self-sputtering ion source: a new approach to quiescent metal ion beams, Rev. Sci. Instrum. 81 (2010) 02B306. [7] A. Vizir, A. Nikolaev, E. Oks, K. Savkin, M. Shandrikov, G. Yushkov, Boron ion beam generation using a self-sputtering planar magnetron, Rev. Sci. Instrum. 85 (2014) 02C302.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Sputtering of a hot ceramic target: Experiments with ZnO a,*
b
T a
Abubakar M. Ismailov , Vladimir A. Nikitenko , Murtuzali R. Rabadanov , Leyla L. Emiraslanovaa, Isa Sh. Alieva, Murtazali Kh. Rabadanova a b
Dagestan State University, Ul. Gadjieva 43a, Makhachkala, Dagestan, 367000, Russia Russian University of Transport, Ul. Obrazcova 9 (Building 9), Moscow, 127994, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetron sputtering Hot ceramic target Zinc oxide High deposition rate
The sputtering of hot metal targets is a rapidly developing field in the technology of magnetron sputtering systems, aimed at solving certain challenges inherent in advancing the potential of magnetron sputtering: how to increase growth rate, the solution of a number of specific problems of reactive sputtering, and the sputtering of magnetic targets. Here the results of the sputtering of hot ZnO ceramic targets are reported for the first time. It was established that during hot sputtering the morphology of the erosion zone of a ZnO target when insulated from the thermal activity of a magnetron undergoes substantial modification at power density of P > 25 W/ cm2. The hot ZnO target emits microparticles that play a major role in the process of film formation on the substrate. It was shown that the crystallization of ZnO involving microparticles in the process of magnetron sputtering of a hot target occurs through the liquid crystal mechanism. A maximum growth rate (15 nm/s) of (0001) ZnO/(0001) Al2O3 epitaxial films with high structural perfection and intense luminescence in the violet region of the spectrum was achieved (currently a record rate).
1. Introduction The magnetron sputtering method is one of the most commonly used in the production of thin films of various substances from the gaseous phase [1,2]. The sputtering of targets in magnetron sputtering systems (MSS) occurs through the action of intense ion bombardment, when more than 90% of the incoming power is converted into heat. The temperature of the uncooled target increases, significantly affecting the process of sputtering and of film formation on the substrate. At the present time the possibilities of the magnetron sputtering method are being significantly expanded in connection with the development of research into the sputtering of hot targets, which is being carried out in three directions. (1) The first direction involves magnetron sputtering systems with a liquid-phase target, in which metal used as a cathode is inserted in a crucible, which is thermally insulated from the cooling system, and heated to the melting temperature of the metal under the action of plasma ions [3–5]. It should be noted that the ‘self-sputtering mode’ during ‘hot’ sputtering is also possible for solid-phase targets with a high melting point [6–8]. The sputtering of a heated metal target leads to an increase in the rate of film deposition due to a combination of two cathode erosion mechanisms: sputtering and
*
evaporation. (2) The second direction of current research concerns the possibility of resolving problems associated with reactive magnetron sputtering (RMS) [9–14]. At high target metallic surface temperatures, there occurs the clearing of a dielectric (oxide or nitride) target material as a result of the sublimation of volatile components, which, in turn, significantly increases target sputtering and film growth rates, while also preventing arcing. (3) The third direction of current research involves target heating which presents one of the possibilities of solving problems in sputtering relatively thick magnetic targets of Ni, Co, Fe, and their alloys. When a ferromagnetic target warms up above the Curie temperature, a ferromagnetic-paramagnetic transition occurs and it no longer shields the magnetic field of the magnetron, thereby increasing the sputtering efficiency of the target [15,16]. To date, there appears to be no indication in the scientific literature of investigations in which there has been a deliberate decision not to cool nonmetallic (ceramic) targets with the specific intention of assessing the effects of high temperatures on sputtering processes and on the formation of films on a substrate. In our opinion this has been because under conditions of intense sputtering applied in order to increase the growth rate of films,
Corresponding author. E-mail addresses: [email protected] (A.M. Ismailov), [email protected] (V.A. Nikitenko).
https://doi.org/10.1016/j.vacuum.2019.108854 Received 29 December 2018; Received in revised form 30 July 2019; Accepted 3 August 2019 Available online 06 August 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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perimeter of the target (thermocouple C). At the time the target was engaged in the hot spraying mode (for 1.5–2 h), a metal shutter was swung into a position between the thermocouples and the target. In order to measure the temperature at any desired point in the sputtering process, the discharge (plasma) was turned off, the shutter was retracted and the thermocouples gently lowered onto the hot target while simultaneously recording their signal using a B7-78/1 voltmeter (Russia). The target temperature corresponding to the thermocouple contact points A, B and C at the moment the discharge is turned off was determined by extrapolating the target cooling curves to the temperature axis corresponding to zero along the time axis. The study of the micromorphology of film surfaces was carried out on a SEM Leo-1450 (Zeiss, Germany) scanning electron microscope in the secondary electron mode. X-ray structural studies performed using an Empyrean series-2 diffractometer with a Cu-Kα source λ = 1.5406 Å (PANalytical, Netherlands). The structural perfection of the film was assessed by reflection high-energy electron diffraction (using an ‘Electronograph EU-75’, Russia). Photoluminescence spectra were measured on a standard laser spectrometer in the wavelength range of 200–1100 nm using a Y-fiberoptic system for transmitting and receiving photosignals. The photoluminescence spectra were excited by 337.1 nm long wave radiation emitted by a nitrogen laser. Laser pulse power density was measured with a universal single-channel energy/laser power meter (Model 11MLink, Detector 11QE25SP-H-MT, Standa Ltd, Lithuania) and was equal to 6.25 kW/cm2. Recording was carried out using an MS3504i imaging monochromator/spectrometer (SOL-Instruments, Belarus) and an HS101 (HR)-2048 × 122 CCD matrix camera (Hamamatsu, Japan) with a personal computer. The sequence of actions to obtain a sample to be used for research was as follows. The vacuum chamber was evacuated to a residual pressure of 10−4 Pa. Next, the substrate was heated to 650 °C, the working gas was injected to a pressure of 0.133 Pa, and a magnetron discharge was ignited at P = 7.4 W/cm2 on the shutter. For 50 min the power density was gradually increased to 53.5 W/cm2 (hot sputtering mode) while simultaneously increasing the substrate temperature to 900 °C. When the hot sputtering mode the process of film deposition on the substrate began by opening the shutter. The duration of the deposition process was 3 h, and the film thickness, as calculated from the deposition rate, was 162 μm.
researchers are confronted with problems of the heating of the nearsurface region of the target because of its poor thermal conductivity. Heating a target induces the formation on its surface of growths (nodules) in the form of mounds, cones or pyramids. They are destroyed by micro-arcs, as a result of which, along with the atomic components, nano- and microparticles are present in the flow of the substance deposited on the substrate, causing a deterioration in the quality of the films obtained [17–24]. One of the characteristic problems encountered has been that this phenomenon has presented a serious technological impediment to the industrial production of transparent conductive In1-xSnxO (indium tin oxide - ITO) films for use as transparent electrodes [18–22]. A similar problem of growth formation is also noted in the industrial technology for the production of zinc oxide films used as an alternative to ITO films [23]. There is also literature reporting the problem of growths with Si [17] and W–Ti [24] sputtering targets. Thus the formation of nodules on the surface of sputtered ceramic targets is an inevitable physical phenomenon. This has led us to ponder the following question: could the phenomenon actually be used to advantage? Our research indicates that the latter is indeed possible, if one pays attention to phenomena occurring on the surface of the substrate involving microparticles emitted from the target. Our current response to these questions is based on the provision of a high temperature of the growth surface, both by resistive heating of the substrate and by radiation from a hot target during which the particles entering the substrate melt. The formation of a film with the involvement of microparticles occurs through the liquid crystal mechanism. It is in the formulation of this task that our work is being carried out and is entering a new territory of investigation. 2. Experimental equipment and procedures The experiments reported here have been carried out using the VATT AMK-MI automated magnetron complex (FerriWatt, Russia) under ‘dry’ vacuum conditions using an oil-free scroll vacuum pump ISP-500C (ANEST IWATA Corporation, Japan) and a Cryo-Torr 8 cryogenic high-vacuum pump (CTI Cryogenics, USA). The working gas (oxygen) pressure was regulated using a RRG-10 device (Eltochpribor, Russia) and measured with a TELEVAC CC-10 wide-range vacuum meter (Fredericks, USA). The source of the current used in the magnetron was an APEL-M1.5PDC-650-2 (Applied Electronics, Russia) set to direct current (DC). The working gas pressure was ~0,133 Pa, gas flow rate ~8 l/h, while the power density on the target was P = 7.4–53.5 W/cm2. The target employed was a sintered ZnO ceramic disk (of 99.999% purity) with a diameter of 40 mm and a thickness of 2 mm Al2O3 (0001) plates with dimensions of 10 × 10 × 0.5 mm3 were used as a high-temperature substrate on which ZnO films were deposited through a sapphire mask measuring 5 × 5 mm2. The substrates were heated by a resistive heater (nichrome), the temperature being controlled using a chromelalumel thermocouple. The distance between the centers of the target and substrate was 3 cm, the angle between the flat surface of the target and substrate being 45°. Film thickness (h) was determined by calculating the crosssection of the image of the sample taken in a scanning electron microscope (SEM). The growth rate (deposition) (v) was estimated by deposition time (t) using the formula v = h/t. To measure the target temperature, a contact measurement method was employed (using a chromel-alumel thermocouple). For this, a mechanical device for conveniently registering temperature of a target within the vacuum chamber was constructed. It contains a unit to which 3 thermocouples with spring-tensioned holders were attached and which can be positioned above the target as desired. The distances between the thermocouples were selected so that they contacted the hot target at points according to the following disposition: in the center (thermocouple A), in the erosion zone (thermocouple B), and on the
3. Research results and assessment We previously reported in Ref. [25] on our work on the preparation of highly oriented ZnO films on amorphous substrates with high growth rates by the usual method of magnetron sputtering of a cold target at a constant current (a 5 mm thick target being placed on the cooled base of the magnetron without the application of pressure). It was shown that in order to achieve highly oriented growth it is necessary to choose an optimal substrate position in the magnetron sputtering system. The graph in Fig. 1 (curve 2) shows the dependence of the growth rate of highly oriented ZnO films on the discharge current density as established in Ref. [25]. The maximum achieved growth rate of 7 nm/s was limited by the maximum power dissipated by the target during stable combustion of the magnetron discharge (at power densities of P > 53.5 W/cm2 the process of destruction of the target takes place) rather than by a decrease in the structural perfection of the films. Mechanical stresses induced by a temperature gradient between the cold rear side and heated (sputtered) side of the 5 mm thick target caused the destruction of the target. At current densities of P > 34 W/cm2, the presence of microparticles was observed on the mirror smooth surface of the films. However, their appearance was only observed on samples sputtered for a duration of 50–60 min or more. In our opinion, this phenomenon occurs in the following way. As the SEM images taken of the target surface demonstrated, a characteristic 2
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Fig. 3. ZnO target cooling curves in areas A, B and C (see Fig. 2 (f)). Fig. 1. Dependence of the growth rate of ZnO films on discharge power density: 1– for a cold target soldered to a magnetron; 2– for a target mounted on a cooled base of the magnetron without pressure [25]; 3– for a hot target thermally insulated from a magnetron.
ZnO targets under conditions of high-current sputtering. At power densities of P > 53.5 W/cm2, a discharge transition to the arc mode was observed for a hot ZnO target (Fig. 2(d)). The arc (3A) moved along the annular zone of erosion, completing one revolution in 10–15 s, eventually leading to the destruction of the target. Operating beyond the transitional stage of the discharge to the arc mode produces undesirable side effects, therefore we worked at values of P ≤ 53.5 W/ см2, at which a stable hot sputtering mode is assured for a long time. As can be seen from Fig. 2(e) and (f), the circular ZnO target in our apparatus heats unevenly along its radius (as opposed to metallic targets [15]). The erosion zone (area B, Fig. 2(f)) heats up more strongly in comparison with the center (area A) and the perimeter (area C) of the target. The temperatures in these areas, determined by extrapolating the cooling curves (Fig. 3) at a discharge power of P = 53.5 W/cm2, are respectively TA = 679 °C, TB = 774 °C, TC = 547 °C. Between adjacent (neighboring) regions along the target radius, there is a significant temperature difference TB–TA = 95 °C, TB–TС = 227 °C. This feature of the hot spraying of ceramic targets often leads to their cracking during spraying or cooling - a significant disadvantage of this method. The analysis of SEM images taken of different areas of an erosion zone shows a significant difference in the surface micromorphology of cold and hot targets (Fig. 4). As can be seen in Fig. 4(b), the surface of the erosion zone (Fig. 4(a), area B) of a cold target after a 3-h sputtering at P = 7,4 W/cm2 is relatively flat and dense. This zone does not undergo significant changes even after more intensive sputtering at P = 34 W/cm2 (Fig. 4(c)). It thus may be assumed that the characteristic morphology of the target surface in region B is a form of relief development as a result of cascade sputtering [26]. However, in area A of the erosion zone, images revealed features somewhat resembling those of so-called dendritic growth [27] (Fig. 4(d)). Since this region is on the margins of the zone of intense sputtering, the origin of these elements of the relief may be attributed to growth from sputtering. Indeed, since such relief elements were not found on the target in area A, which were sputtered at low discharge current values (P = 7.4 W/cm2), it may be assumed that the material for growth in this zone comes from area B due to more intense sputtering of the target at high discharge current values. A substantial modification of the surface of an uncooled target occurs during its sputtering at P > 25 W/cm2. Fig. 4(e) shows the morphology of the erosion zone in area B of a hot target, formed during the sputtering process at P = 53.5 W/cm2. Here there is an already quite developed loose (flake-like) surface. As a consequence of intensive defect formation during ion bombardment, the diffusion coefficient of particles on the target surface will be significantly higher than with ordinary thermal diffusion. Ion bombardment also stimulates the evaporation of a substance (it may be called radiation-accelerated
morphology in the form of nodules gradually develops due to temperature increases in the erosion zone. The destruction of nodules by micro-arcs leads to the formation of a stream of microparticles directed toward the substrate. The above phenomena, which are characteristic of high-current magnetron sputtering, were taken as the motivation for the conducting of experiments in more detail into the mechanisms of hot sputtering of a ZnO target. In order to prevent fracturing of the ZnO target and to achieve a higher temperature, it was decided to reduce the target's thickness to 2 mm and to thermally insulate it as much as possible from the magnetron, while still maintaining the target-magnetron galvanic coupling. For thermal insulation, 3 plates (of dimensions 5 × 5x0.5 mm3) of polycore (Al2O3) were used, positioned separately in a triangular configuration between the cooled surface of the magnetron and the target. To ensure fully dependable ‘clean’ cold sputtering, a 2 mm thick ZnO target was soldered with indium to the water-cooled base of the magnetron. The photographs in Fig. 2 show the thermal modes of cold (in which the target is soldered to the magnetron - Fig. 2(a)) and hot (in which the target is thermally insulated from the magnetron - Fig. 2(b), (c) & (d))
Fig. 2. Photographs of ZnO targets under various sputtering regimes: (a) cold target, P = 34 W/cm2; (b) hot target, P = 34 W/cm2; (c) hot target, P = 53.5 W/cm2; (d) arc on the surface of a hot target, P = 63 W/cm2; (e) hot target, at the moment of discharge being turned off, P = 34 W/cm2; (f) hot target, at the moment of discharge being turned off, P = 53.5 W/cm2. 3
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Fig. 4. SEM images of ZnO targets (sputtering duration - 3 h; gas pressure – 0.133 Pa: (a) – section of a ZnO target with zones of SEM images indicated: A - lateral erosion zone, B - central erosion zone; (b) cold target (zone B, P = 7.4 W/cm2); (c) cold target (zone B, P = 34 W/cm2); (d) cold target (zone A, P = 34 W/cm2); (e) hot target (zone B, P = 53.5 W/ cm2); (f) hot target (zone A, P = 53.5 W/cm2).
sublimation). Thus, it may be assumed that the characteristic surface morphology in this area is a form of relief development resulting from cascade sputtering and radiation-accelerated processes of diffusion and evaporation. In area A of the hot target (Fig. 4(f)) relief developed of specific crystalline structures, some resembling whiskers. As can be seen in Fig. 2(c) and (f) the high temperature is concentrated in an annular zone of erosion (area B), while the rest of the target has a relatively low temperature. Thus we may assume that the area of erosion B is the zone of emission (evaporation) of the substance, while area A is the zone of deposition. Note that area A is also an area of less intensive sputtering compared to area B. Taking into account the above, we may conclude that the characteristic surface morphology in this region is a form of relief development resulting from two ‘competing’ physical processes: sputtering and crystal formation. From the graph in Fig. 1 (curves 1 and 3) it can be seen that as the power density increases to P = 34 W/cm2, the growth rate of ZnO epitaxial films for both a cold and hot target increases monotonically, although the growth rate for a hot target is several units higher than for a cold target. However, at power density higher than P > 34 W/cm2, the growth rate of ZnO films increases sharply for a hot target – to a threefold degree at P = 53.5 W/cm2. Here, obviously, two questions arise simultaneously: (1) what is the mechanism of the additional emission of a substance from the surface of a hot target, and (2) by what mechanism does the additional flux of the substance get absorbed by the growing surface while maintaining epitaxial growth? To answer the questions posed, we proceed from the fact that when a cold target is sputtered, its surface emits an atomic flow characteristic of a purely physical sputtering mechanism. This mechanism corresponds to a monotonic increase in the growth rate of films with an increase in the discharge power density and the absence of any particles on the film surface. The sharp increase in the growth rate in the case of the sputtering of a hot target when P > 34 mA/cm2 is reached can be explained by the triggering of a new physical mechanism for the additional emission of a substance from the target surface. Based on the analysis of a considerable number of SEM images of the surface of films investigated, it may be concluded that the additional component of the flow of matter directed to the substrate is the flow of microparticles emitted from the surface of a hot target. Direct evidence of this conclusion is the presence of microparticles on the growth surface. Figs. 5 and 6 show images of different parts of the same sample, providing sequential pictures of the dynamics of the process of film formation with the involvement of microparticles. The formation of such micro-particles directly on the growth surface or in the gap between the magnetron and the target substrate seems to be physically impossible. Fig. 5(a) and (b) shows the surface morphology of the ZnO epitaxial film at different scales of magnification, in the case of loose (sparse) filling of the growth surface with microparticles. The microparticles
Fig. 5. SEM images of the surface of a ZnO epitaxial film: 1 - solid microparticles, 2 – microparticles at the initial stage of melting, 3 - hexagonal islands, 4 - submerged tips of hexagonal microcrystals forming the structure of the growing film.
that have just been deposited onto the growth surface have no definite shape and are in the solid phase (Fig. 5(b), particles 1). Over time, they come into thermal equilibrium with the substrate and begin to melt (particles 2). In our opinion, the formation of hexagonal microparticles indicates that the crystallization mechanism proceeds in equilibrium conditions through the liquid phase the melting of the initial microparticles (Fig. 5(b), particles 3). Fig. 5(b) also shows the traces of the ends of the hexagons submerging in the lower layer (Fig. 5(b), particles 4), from which the completed sediment layer is formed. Apparently, at the stage of melting of the particles, their azimuthal orientation around the polar axis also occurs, sustaining autoepitaxy. Fig. 6(a) and (b) also confirm the mechanism of crystallization through the liquid phase with a denser accumulation of microparticles. There is a scatter of ‘liquid-like’ islands differing in size, each of which is probably formed from one or several initial microparticles in the solid phase. The dense packing of these islands is such that it almost suppresses the hexagonal facets of the crystals (see Fig. 6(b)). At a low 4
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Fig. 6. SEM images of: (a) initial phase of the melting of a two-dimensional dense array of ZnO micro-particles on the growth surface; (b) later stage of melting of micro-particles; (c) hexagonal ZnO nuclei on the growth surface.
particle density, the fusion of hexagonal islands proceeds along lateral equilibrium faces without deformation (Fig. 6(c)). Thus, there are sufficient grounds to assert that in our observations ZnO crystallization proceeds according to the liquid crystal mechanism, and not according to the vapor crystal mechanism characteristic of gaseous phase deposition methods, which include that of the magnetron sputtering method. The hexagonal form of the islands reflects the crystallographic individuality of ZnO, which is characterized by a hexagonal crystal lattice (structural type - Wurtzite). It can be seen from Figs. 5(b) and Fig. 6(c) that the polar axis of ZnO hexagons is perpendicular to the growing surface. On this basis, it can be preliminarily stated that the (0001) ZnO basal plane is parallel to the plane of the (0001) Al2O3 substrate. The xray diffraction pattern of the sample in Fig. 7 confirms the assumption made. The low peak width (0002) (FWHM = 0.0380) of the diffractogram also indicates the high structural perfection of the ZnO films obtained. The insert in Fig. 6 shows electron diffraction patterns from ZnO films at two different azimuthal angles, confirming the monocrystallinity of the obtained film. It should be noted that the characteristic crystallite size in ZnO films obtained by magnetron sputtering of cold targets varies in the nanometer range of 10–60 nm [28,29], while in our case the crystallite size (grain) in the ZnO film is determined by the size of the hexagonal particles - 1–3 μm (see Figs. 5(b) and Fig. 6(c)). Such a difference in crystallite sizes cannot but affect the physical properties of ZnO films. Fig. 8 shows the typical photoluminescence spectrum of the ZnO films under investigation at room temperature. According to Refs. [30–33], at the temperature of liquid nitrogen this spectrum decomposes into an equidistant series of bands A, A–LO, A–2LO, A–3LO, A–4LO (and so on), due to the radiative decay of free A-excitons with the participation of LO phonons. The temperature studies we carried out of the transformation of the obtained spectra in the temperature
Fig. 8. Photoluminescence spectrum of an epitaxial ZnO film when the sample is excited by a nitrogen laser (337.1 nm) with an excitation power density of 6.25 kW/cm2.
range of 80–300 K showed [31] that the narrow ultraviolet luminescence bands in Fig. 8 with a maximum at 382 and 392 nm are caused, respectively, by the dominant A–LO and A–2LO bands, and the equidistant structure itself gradually disappears as the temperature increases from 80 to 300 K. Meanwhile, the comparable intensities of the A–LO and A–2LO bands and their unique resolution in the spectrum at room temperature indicate a high degree of perfection of the crystal structure of the films obtained. Indeed, the large number of possible combinations of phonons in a two-phonon process (as a consequence of the law of conservation of momentum) considerably increases its probability [32–34]. As a result, in crystals of zinc oxide with a perfect crystalline structure, the intensity of two-phonon emission lines turns out to be comparable to the intensity of single-phonon A–LO lines [32]. In defective samples, the probability of an exciton interacting with one LO phonon ceases to depend on the exciton wave vector (since Froehlich scattering through states of alternating parity is allowed, due to the elastic scattering of excitons by defects), so the intensity of the (A–LO) band noticeably increases, and can many times exceed the intensity of the A–2LO band [32,34]. Another possible explanation for the presence in the luminescence spectrum of a ZnO film peak at 392 nm may be the appearance of radiation caused by the inelastic scattering of free excitons. However, this mechanism of radiative recombination of free excitons is usually manifested at high levels of excitation (hundreds – kW/cm2 [35]). With an increase in the excitation power density up to 1 MW/cm2, the radiation of an electron-hole plasma begins to be recorded, but the maximum of the corresponding band in the luminescence spectrum lies in the region of 400 nm [35]. It should be noted that, in any case, the presence of a structure in the ultraviolet luminescence spectrum in the samples under study at room temperature indicates their high quality.
Fig. 7. X-ray diffractogram of an epitaxial ZnO film. In the insert: 2 electron diffraction patterns taken at two different azimuth angles. 5
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The almost complete absence of visible luminescence of ZnO films indicates their purity (low content of impurities which form green, yellow-orange and red luminescence [36–39]). According to some researchers, the apparent luminescence of ZnO can also be associated with the intrinsic defects of the crystal structure [30,33]: in this case the weak visible luminescence of the ZnO films obtained also confirms their high quality.
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4. Conclusion Thus, based on the above investigations, the following conclusions may be drawn. 1. The process of magnetron sputtering of a hot ZnO target was achieved. 2. It has been established that the surface of a ZnO target which is thermally insulated from a magnetron, undergoes substantial modification in the process of its being sputtering at power densities of P > 25 W/cm2. The surface of the central zone of erosion becomes loose (resembling flakes), and in areas (at the edge and center of the target) adjacent to it, the formation of characteristic crystalline structures is observed. 3. The hot target, along with the atomic flow, emits microparticles, which, falling on the growth surface, play the principal role in the process of the formation of ZnO epitaxial films. 4. Microparticles on the growth surface, melting and combining with each other, form hexagonal islands, which, in turn, form another continuous layer of epitaxial film above. The formation of the film occurs through the liquid crystal mechanism. 5. A maximum growth rate (15 nm/s) of (0001) ZnO/(0001)Al2O3 epitaxial films with high structural perfection and intense luminescence in the violet spectral region (382 nm) has been reached (a record experimental growth rate at this time). We believe that the fundamental physical and technological concepts underlying our work will serve as stimuli for other researchers to undertake further work in this newly opened territory of hot sputtering of ceramic targets. Wide-gap materials, such as GaN, SiC, AlN, Ga2O3, present particularly interesting potential. Acknowledgments This work was supported by the Russian Foundation for Basic Research, Russia [Project No. 16-02-00227a]. Professor Guy Petherbridge, Dagestan State University, is gratefully acknowledged for his collaboration in the technical presentation of this research. References [1] J.E. Greene, Review Article: Tracing the recorded history of thin-film sputter deposition: from the 1800s to 2017, J. Vac. Sci. Technol. A - Vacuum, Surfaces, and Films 35 (2017) 05C204. [2] G. Brauer, B. Szyszka, M. Vergohl, R. Bandorf, Magnetron sputtering – milestones of 30 years, Vacuum 84 (2010) 1354. [3] G.A. Bleykher, A.O. Borduleva, A.V. Yuryeva, V.P. Krivobokov, J. Lančok, J. Bulíř, J. Drahokoupil, L. Klimša, J. Kopeček, L. Fekete, R. Čtvrtlìk, J. Tomaštik, Features of copper coatings growth at high-rate deposition using magnetron sputtering systems with a liquid metal target, Surf. Coat. Technol. 324 (2017) 111. [4] A.V. Yuryeva, A.S. Shabunin, D.V. Korzhenko, O.S. Korneva, M.V. Nikolaev, Effect of material of the crucible on operation of magnetron sputtering system with liquidphase target, Vacuum 141 (2017) 135. [5] A.V. Kaziev, A.V. Tumarkin, K.A. Leonova, D.V. Kolodko, M.M. Kharkov, D.G. Ageychenkov, Discharge parameters and plasma characterization in a DC magnetron with liquid Cu target, Vacuum 156 (2018) 48. [6] E. Oks, A. Anders, A self-sputtering ion source: a new approach to quiescent metal ion beams, Rev. Sci. Instrum. 81 (2010) 02B306. [7] A. Vizir, A. Nikolaev, E. Oks, K. Savkin, M. Shandrikov, G. Yushkov, Boron ion beam generation using a self-sputtering planar magnetron, Rev. Sci. Instrum. 85 (2014) 02C302.
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