Research Article www.acsami.org
Integrating AlN with GdN Thin Films in an in Situ CVD Process: Influence on the Oxidation and Crystallinity of GdN Stefan Cwik,† Sebastian M. J. Beer,† Stefanie Hoffmann,† Michael Krasnopolski,† Detlef Rogalla,‡ Hans-Werner Becker,‡ Daniel Peeters,† Andreas Ney,§ and Anjana Devi*,† †
Inorganic Materials Chemistry and ‡RUBION, Ruhr-University Bochum, 44801 Bochum, Germany Institute of Semiconductor & Solid State Physics, Johannes Kepler University, 4040 Linz, Austria
§
S Supporting Information *
ABSTRACT: The application potential of rare earth nitride (REN) materials has been limited due to their high sensitivity to air and moisture leading to facile oxidation upon exposure to ambient conditions. For the growth of device quality films, physical vapor deposition methods, such as molecular beam epitaxy, have been established in the past. In this regard, aluminum nitride (AlN) has been employed as a capping layer to protect the functional gadolinium nitride (GdN) from interaction with the atmosphere. In addition, an AlN buffer was employed between a silicon substrate and GdN serving as a seeding layer for epitaxial growth. In pursuit to grow highquality GdN thin films by chemical vapor deposition (CVD), this successful concept is transferred to an in situ CVD process. Thereby, AlN thin films are included step-wise in the stack starting with Si/GdN/AlN structures to realize long-term stability of the oxophilic GdN layer. As a second strategy, a Si/AlN/ GdN/AlN stacked structure was grown, where the additional buffer layer serves as the seeding layer to promote crystalline GdN growth. In addition, chemical interaction between GdN and the Si substrate can be prevented by spatial segregation. The stacked structures grown for the first time with a continuous CVD process were subjected to a detailed investigation in terms of structure, morphology, and composition, revealing an improved GdN purity with respect to earlier grown CVD thin films. Employing thin AlN buffer layers, the crystallinity of the GdN films on Si(100) could additionally be significantly enhanced. Finally, the magnetic properties of the fabricated stacks were evaluated by performing superconducting quantum interference device measurements, both of the as-deposited films and after exposure to ambient conditions, suggesting superparamagnetism of ferromagnetic GdN grains. The consistency of the magnetic properties precludes oxidation of the REN material due to the amorphous AlN capping layer. KEYWORDS: chemical vapor deposition, magnetic materials, rare earth, nitrides, thin films, magnetic materials
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INTRODUCTION
derived from the challenging fabrication of the pure GdN, which is hindered by its high oxophilicity. As a result, the growth of stoichiometric layers of the aforementioned material without the incorporation of oxygen is challenging. Furthermore, the functional properties are strongly influenced by stress and strain in the crystal lattice,11,23 nitrogen vacancies,6,12,24 and oxygen impurities.25,26 Besides theoretical studies of REN, the experimental reports are mostly based on physical vapor deposition (PVD) techniques,18 like molecular beam epitaxy (MBE),5,10,16,17,19,20,27−29 magnetron sputtering,23,30−33 and pulsed laser deposition.15,16,34,35 More recently, RENs could additionally be fabricated with chemical vapor deposition (CVD)36−39 and plasma-enhanced atomic layer deposition
Rare earth nitrides (RENs) such as gadolinium nitride (GdN) and dysprosium nitride (DyN) are promising for spintronic applications owing to their intrinsic semiconducting and magnetic characteristics. For their potential applications in next-generation logic devices, both the spin of the carriers and their charge to register binary states are utilized. In general, given the immense potential of RENs for spintronic-based applications, there is a growing need for developing high-quality REN-based materials. This poses a significant challenge owing to the difficulties in growing well-ordered materials with stoichiometric RE/N ratios and low levels of oxygen. Crystalline, stoichiometric GdN is a ferromagnetic1−6 material with a reported Curie temperature between 60 and 70 K1,3,6−22 and a magnetic moment of 7 μB per Gd3+ ion.13,14,17 The optical band gap of this semiconductor6,15 is reported to be 0.9 eV in the ferromagnetic phase.22,23 The discussion about the magnetic1−4 and electrical7,24 properties of this RE pnictide is © XXXX American Chemical Society
Received: June 8, 2017 Accepted: July 20, 2017
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DOI: 10.1021/acsami.7b08221 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces (PEALD).26 It is obvious that the future applicability of these materials depends on the development of methods for producing high-quality materials and device structures. However, CVD and atomic layer deposition (ALD) of RENs are relatively unexplored, although these techniques have the distinct advantage in terms of large-area deposition, conformal coverage, and scalability. CVD or ALD relies on the accessibility of appropriate precursors, which have to be designed for the specific process. We were the first ones to report upon a CVD process for GdN via a single-source precursor approach,36 which was achieved using all-nitrogen coordinated metalorganic precursors. In this case, the RE trisguanidinates were employed as precursors, which led to the successful deposition of GdN and DyN layers.36,37 In a follow-up work, the RE trisamidinates were investigated as an alternative class of precursors, and in this case the presence of an additional nitrogen source, namely ammonia (NH3), was needed to obtain the right stoichiometry between the RE and N elements.37,38 Although both classes of precursor yielded REN layers, the inclusion of oxygen could not be avoided. Owing to the fast degradation of RENs when exposed to moisture or oxygen, the material needs to be protected from interaction with the atmosphere. In our first attempts to prevent oxidation, metallic copper serving as a capping layer was deposited on the REN, but the oxidation could only be slowed down.37 For long-term stability, the protective thin film has to completely and uniformly cover the REN, which could not be achieved with copper, which has a strong tendency to form crystalline films with polycrystalline grains. In addition, the capping layer has to be chemically inert toward the REN and preferentially amorphous to prevent grain formation, because grain boundaries favor the diffusion of air through the layer. In this context, other alternative capping layers are sought after, and one suitable candidate is aluminum nitride (AlN)17,23,40 in its amorphous form, which could fulfill these requirements, which is beside GaN5,6,21,22 state of the art. Approaches employing metallic capping layers like W,12 Cr,12 Cu,36−38 TaN,26 NbN41,42 and the insulator yttria stabilized zirconia (YSZ)15 and MgF26 were not followed up. The selfprotective nature of AlN against oxidation in particular renders it a highly capable material. After the oxidation of the AlN surface, the respective oxide (Al2O3) serves as a gas diffusion barrier for oxidation and consequently prevents further oxidation. To avoid exposure to the atmosphere and oxidation of the samples, they have to be prepared in one continuous process in a reactor employing a two-bubbler system serving as the respective metal sources. The high feasibility of [Gd(DPAMD)3] and [Al2(NMe2)6] as precursors for the respective metal nitrides in the presence of ammonia has been proven in the past.37,38,43−46 For spintronic applications, the magnetic properties of GdN have to be optimized, which strongly depend on the crystallinity of the material. Therefore, an additional seeding/ buffer layer can be employed to enhance the crystallization with minimized stress and strain between GdN and the underlying material to enhance the functional properties as demonstrated for AlN/GdN/AlN stacks with MBE.16,17,19 This PVD approach was adapted in this work for a continuous CVD process. The preferential orientation of the rock salt GdN crystallites is defined by the orientation of the underlying hexagonal AlN plane. A (0001) wurtzite AlN lattice induces (111) face-centered cubic (fcc)-GdN growth due to the
relatively low lattice mismatch of 13% of these planes.16 Nevertheless, the (100) AlN plane (lattice parameters: 4.9792 Å × 3.1114 Å)47 favors GdN growth in the (100) direction (lattice parameter: 4.999 Å).48 Our approach to tackle the issues related to preventing oxidation and enhancing crystallinity of REN was by building stacked layers of AlN and GdN via CVD, wherein AlN serves the purpose of capping and buffer. A similar multilayer approach (Si/AlN/GdN/AlN) has been reported for two different PVD techniques, namely MBE16,17,19 and reactive sputtering,23,31 proving the high suitability of AlN as a buffer and capping layer. Natali et al. could show that the application of an AlN capping layer allows the fabrication of epitaxial GdN with MBE and, additionally, rules out silicide formation by isolating the RE material from the silicon substrate.17 These promising results motivated us to transfer this concept to CVD, which enables a significant scale up in comparison to the ultrahigh vacuum technique, MBE. In addition, CVD in contrast to MBE allows a continuous process for the fabrication of the entire stack avoiding additional predeposition cleaning steps necessary for multilayer MBE samples.18 The in situ CVD-grown stacked structures were subjected to a detailed investigation in terms of structure and morphology by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The composition profile of the stacks was studied employing complementary tools such as X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), and nuclear reaction analysis (NRA). The deposited thin film stacks (Si/GdN/AlN and Si/AlN/GdN/AlN) were also investigated with respect to their magnetic characteristics employing superconducting quantum interference device (SQUID) measurements. Because long-term stability and hence performance of the capping layer is of particular interest, the samples were again analyzed by SQUID following exposure to ambient conditions after 12 weeks. The highly encouraging results in terms of seeding and buffer for REN layers are described herewith.
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EXPERIMENTAL SECTION
Thin-Film Deposition. The metalorganic precursors, tris(N,N′diisopropyl-acetamidinato)gadolinium(III) [Gd(DPAMD)3] and hexakis(dimethylamido)dialuminium(III) [Al2(NMe2)6], employed for the deposition of GdN and AlN, respectively, were synthesized following previously reported methods in the literature.49,50,37 Film depositions via CVD were performed in a custom-built horizontal cold-wall reactor. Before deposition, 1 cm2 Si(100) substrates were cleaned using a sequence of sonication in 2-propanol and water to remove surface contaminations. The vaporizer temperatures were set to 120 and 80 °C for the Gd and Al source, respectively. Nitrogen (5.0) was used as a carrier gas, and all depositions were performed in the presence of dry ammonia. GdN layers were grown at 800 °C with flow rates of 50 sccm for both gases, whereas the deposition temperature for AlN films was set to 400 °C (capping layer) and 500 °C (buffer layer) with flow rates of 25 sccm. All multilayer structures were prepared in a continuous CVD process to avoid interaction of the oxophilic layers with the atmosphere. Thin-Film Characterization. XRD measurements were performed with a Bruker AXS D8 Advance diffractometer with Cu Kα radiation (1.5418 Å). Surface morphology was determined by SEM with an FEI instrument (FEI ESEM DualBeam Quanta 3D FEG). Composition of the thin films was analyzed with RBS analysis coupled with NRA and XPS. For RBS measurements, a 2.0 MeV 4He+ beam with an intensity of 20−40 nA (tilt angle 7°) from the dynamitron tandem laboratory of RUBION in Bochum was used. The backscattered particles were measured at an angle of 160° by a Si detector with a resolution of 16 B
DOI: 10.1021/acsami.7b08221 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces keV. NRA measurements were carried out with a 2H+ beam (1 MeV). The samples were tilted at an angle of 7°. The emitted protons were detected at an angle of 135° with respect to the beam axis. The detector covered a solid angle of 23 msrad and was shielded by a 6 μm Ni foil to eliminate elastically scattered deuterons. Typical beam currents on the samples were close to 40 nA in an area of ∼1 mm in diameter, whereas the collected beam charge for a sample was 12 μC. The RBS/NRA data were analyzed by means of the SIMNRA program.51 The XPS measurements were performed using a VersaProbe spectrometer from Physical Electronics operating with monoclinic Al Kα radiation. The spectra were calibrated with respect to the 4f7/2 core level of a clean gold sample at 83.8 eV. Depth profiling of the thin film was realized by in situ Ar+ bombardment for 1 min each at 1 kV bombardment for 1 min each at 1 kV (ion current density of 0.126 μA/mm). Integral magnetic measurements were performed using a commercial SQUID magnetometer (Quantum Design MPMS XL5) applying the magnetic field in the film plane. Magnetism/hysteresis (M(H)) curves were recorded at 300 and 2 K, respectively. M(T) curves were measured while warming from 2 to 300 K in a field of 10 mT, after the sample was either cooled from 300 to 2 K in a 5 T field (field-heated (FH)) or after demagnetizing the sample in an oscillatory field at 300 K (zero-field cooled (ZFC)). Subsequently, an M(T) curve was measured while cooling down to 2 K in 10 mT (field-cooled (FC)). The diamagnetic contribution was derived from the M(H) behavior at high magnetic fields at 300 K and was subtracted from all magnetization data. The SQUID measurements were carried out at identical conditions after exposure to ambient conditions (air, ∼20 °C, 1 bar) for 12 weeks.
Strategy 1: Capping Layer to Prevent Oxidation. AlN as the capping layer to prevent GdN interaction with the atmosphere XRD was employed to investigate the morphology of the Si/GdN/AlN stack (black plot in Figure 1). All reflexes
Figure 1. Representative X-ray diffractograms of Si(100)/GdN(150 nm)/AlN(200 nm) (black) and Si(100)/AlN(50 nm)/GdN(100 nm)/AlN(50 nm) (red) stacks with thicknesses as stated in brackets. The indices correspond to fcc-GdN (JCPDS: 015-0888).48
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RESULTS AND DISCUSSION As discussed in the introduction, RENs react readily with the water/oxygen present in the ambient atmosphere, and a capping layer is absolutely essential for any ex situ studies of film properties.9 CVD is a proven tool for the growth of epilayers especially for III−V semiconductors (GaN, InN, and AlN), and unlike most of the III−Vs that bond covalently, the RENs are largely ionic and crystallize in the NaCl structure. Hence, standard III−V nitride growth techniques on Si may not be directly transferable to RENs. Nevertheless, growth on Si is desirable, because of the huge interest in integrating spintronic devices with conventional Si-based electronics. Epitaxial growth of REN on silicon is hindered by a lattice mismatch of 13%16 but more importantly the reactivity of Gd with Si, which could lead to a metallic GdSix layer, which prevents the epitaxial growth of GdN on silicon.52,53 Furthermore, the presence of native silicon oxide should lead to the formation of RE2O3 at the interface as observed for deposition on oxygen-containing substrates like YSZ.16 Hence, our strategy was to grow GdN films on an AlN buffer layer grown in situ by CVD to obtain enhanced crystallinity. From our previous studies, we demonstrated the first CVD process for GdN, wherein polycrystalline layers were obtained on Si substrates.36−38 To enhance the crystallinity of the GdN and prevent postoxidation, two different strategies were adopted. Multilayers were fabricated on Si substrates in two different configurations. To prevent oxidation, Si/GdN/AlN stacks, and to reduce the lattice mismatch between Si and GdN, an intermediate AlN as a buffer layer, were introduced in the following sequence: Si/AlN/GdN/AlN. In both the cases, CVD routes were employed for depositing AlN as the capping and buffer layer. The CVD process parameters were optimized to obtain homogeneous and stoichiometric layers of GdN and AlN on Si substrates, and the salient features of the data obtained from the film analysis are discussed.
present in the diffractogram can be attributed to the fcc (Fm3m) GdN phase (JCPDS: 015-0888).48 The material preferentially crystallizes in the (200) plane. The coexistence of the (111) reflex with a fifth of the (200) intensity suggests polycrystalline GdN growth on the bare silicon substrate. The absence of an AlN reflex in the diffractogram of the Si/ GdN/AlN stack implies the amorphous behavior of the capping layer. In previous depositions of AlN on Si(100) at the selected temperature (400 °C), an XRD-amorphous nature was determined, whereas an increase of the processing temperature by 100 °C leads to crystallization in the hexagonal wurtzite (spacegroup: P63mc) structure (JCPDS: 025-1133)47 with the (100) reflex as the preferred orientation (Figure S1). The influence of the additional AlN buffer layer, grown at 500 °C, on the crystallization behavior of GdN will be discussed in the following section (Strategy 2: Buffer Layer To Enhance Crystallization). Using SEM analysis, the microstructure of the AlN capping layers was investigated. For the successful protection of the GdN from oxidation due to exposure to the atmosphere, the protective layer has to be dense. In addition, the formation of crystalline grains should be avoided because oxygen can diffuse through the grain boundaries. The SEM micrograph (Figures 2 and S2) reveals a dense, uniform, and pore-free structure of the AlN capping. Furthermore, the amorphous nature of the AlN material (Figure S1) hinders grain formation. The film thickness of 200 nm evaluated by the SEM analysis is sufficient to take advantage of the self-protective nature of AlN against oxidation of the bulk material. It should be noted that both layers form a well-defined interface inhibiting diffusion of the materials. While the AlN thin film shows a featureless appearance, crystalline grains of the GdN are clearly visible as described in the literature.21,23,36−38 The smooth and dense nature of the capping layer is comparable to the sputtered AlN C
DOI: 10.1021/acsami.7b08221 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 2. SEM image of Si(100)/GdN/AlN stack.
capping23 and is superior to the metallic capping layers consisting of crystalline grains that allow slow oxygen diffusion. RBS in combination with NRA is a very powerful analytical tool for thin-film composition analysis. The strength of RBS is the detection of elements with higher atomic numbers (here, Al, Si, and Gd), whereas NRA is well suited for lower atomic number elements (here, C, N, and O). The interdependent data obtained from both methods are combined in a common fitting procedure. However, the precise description of multilayer systems, as presented here, is highly challenging because of a poor depth resolution of NRA. Moreover, the nuclear reaction probability is a function of the kinetic ion energy, which decreases with the sample depth. Therefore, the composition of the AlN layers was set to the same stoichiometry as evaluated for the preliminary deposits of AlN thin films on Si(100) using identical conditions (Figure S3 and Table S1). This constraint of the buffer and capping layer facilitated the composition determination of the GdN film of the multilayer stack. The resulting description of the GdN thin film reveals a metal to nitrogen ratio close to the stoichiometric value. Because the carbon content within the Si/GdN/AlN stack is mostly attributed to the capping layer with a given atomic concentration of 4.2%, the GdN consequently contains negligible carbon. The oxygen content within the stack can only partially be associated to the AlN capping layer. Consequently, some oxygen content corresponds to the GdN layer (GdNO0.4). The high oxophilicity of RE elements in combination with the native silicon oxide on the substrate and water impurities in the reactive gas ammonia can explain this finding. Owing to the native oxide, the presence of partially oxidized GdN or GdSiOx at the interface is expected, followed predominantly by the formation of GdN yielding in a Gd/N ratio of 1:1. Because the RBS fitting summarizes the entire Gdbased material, a high oxygen content is estimated for the entire layer. Shielding of the native oxide from GdN by the buffer layer is discussed in Strategy 2: Buffer Layer To Enhance Crystallization. The strength of RBS beyond composition analysis is well pronounced here, because beside the abundance of the elements in the layers, the spectrum (black plot in Figure 3) additionally reveals depth information. The shift of the Gd peak toward lower energy and hence channel number is in agreement with the capping layer thickness. In the region between channels 410 and 385, no Gd is detected ruling out diffusion of Gd ions into the overlying film, the AlN capping layer or to the sample surface. Furthermore, the RE peak shape without considerable tailing effects suggests a well-defined interface, which is consistent with the SEM observations. The
Figure 3. Representative RBS spectra of Si(100)/GdN/AlN (black) and Si(100)/AlN/GdN/AlN (red) stacks.
same is true for the Al peak, which is clearly separated from the Si continuum due to the Si edge shift caused by the Gd film in between. Strategy 2: Buffer Layer To Enhance Crystallization. For the fabrication of the AlN buffer layer, a deposition temperature of 500 °C was selected. Being the optimum compromise in terms of composition and morphology, it resulted in a slightly crystalline AlN material and an oxygen content below 5 atom %. The (100) reflex of the hexagonal AlN phase appears to be broad and shows a reduced intensity. At higher temperatures, the thin films become more crystalline, but unwanted oxygen-containing impurities increase as well (Figures S1, S3, and Table S1). Even less pronounced crystallinity of the underlying seeding layer is expected to enhance the crystalline growth of the REN thin film. The diffractogram of the Si/AlN/GdN/AlN stack (red plot in Figure 1) clearly reveals an improved crystallization behavior of GdN, which can be concluded from the strong rise in intensity of the (200) reflex. In addition, the preferential crystallization in the (200) plane is enhanced by the buffer layer. This is obvious from the drastic change in the intensity ratio of the (200) to (111) reflexes, which increased from 5 to 53. Notably, the grain size (37−38 nm) of the (200) crystallites is not affected by the introduction of the AlN buffer layer as estimated by Scherrer equation.54 It should be noted that the AlN (100) reflex is not visible in the Si/AlN/GdN/AlN stack, which can be explained by the low AlN film thickness and the very low intensity with respect to the RE material. The crystallization on the (100) oriented buffer layer could be enhanced with respect to reported CVD processes, which led to the formation of polycrystalline GdN thin films,36,37,39 wherein the main (200) reflex coexists with (111) and (220). Whereas the PEALD processes yielded in an amorphous material,26 all thermal CVD processes with ammonia as the coreactant resulted in crystalline GdN. In the present study, oriented GdN thin films were obtained, which are in agreement with the crystallization behavior described by Krasnopolski et al.38 The ultimate goal to achieve epitaxial growth as achieved with PVD techniques,15,23 especially MBE,5,16,17,20,21,55 remains our next challenge in terms of CVD process optimization. Low buffer layer thickness makes visualization with SEM challenging, indicating the existence of crystalline AlN grains D
DOI: 10.1021/acsami.7b08221 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces speculative. The well-defined interfaces between GdN and the surrounding AlN films can be seen in the micrographs (Figure 4). The growth behavior of the capping layer is obviously not
Figure 4. SEM micrograph of the Si(100)/AlN/GdN/AlN stack.
affected by the additional fabrication step, which can be concluded from the identical appearance of the amorphous protective layer. The composition analysis of the three-layer stack confirms the formation of the AlN/GdN/AlN layer structure with a low level of contaminations, which cannot be unambiguously assigned to a specific interface or layer with this analytical tool as mentioned previously. As described earlier for the Si/ GdN/AlN, the well-defined layer structure could be confirmed (red plot in Figure 3). The Al signal shape of the additional buffer layer (channels 210−230) results from the partial overlay with the silicon continuum. In comparison with the films prepared without the additional buffer layer, the oxygen content in the functional GdN thin film could be considerably decreased (Table 1), whereas the carbon content is consistently very low. Apparently, the buffer layer delivers additional protection from oxidation by shielding the REN film from the silicon substrate surface where oxygen-containing species, like hydroxides and native oxide, are located, which is further investigated by XPS depth profiling. The native oxide should lead to an increased oxygen content in the RE material,26 as observed for the growth on YSZ substrates, where a RE oxide was formed at the interface.16 To estimate the composition in the different layers, an XPS depth profile of Si/AlN/GdN/AlN stack was measured, verifying the formation of the respective nitride materials (Figures 5 and S6). The spectrum of the first few nanometers of the stack (as introduced) shows typically appearing carbon and oxygen surface contamination, which can be explained by exposure to the atmosphere during the sample transfer. These contaminations disappear after the first Ar+ erosion. The AlN capping layer with very low carbon (
