Heteroepitaxy of Ni-Based Alloys on Diamond - Crystal Growth

Article pubs.acs.org/crystal

Heteroepitaxy of Ni-Based Alloys on Diamond Roman A. Khmelnitsky,† Stanislav A. Evlashin,*,†,‡ Victor P. Martovitsky,† Pavel V. Pastchenko,‡ Sarkis A. Dagesian,§ Alexandr A. Alekseev,∥ Nikolay V. Suetin,‡ and Alexey A. Gippius† †

P.N. Lebedev Physical Institute, Russian Academy of Sciences, 53 Leninsky Prospect, 119991 Moscow, Russia D.V. Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, 1(2) Leninskie Gory, 119991 Moscow, Russia § Department of Physics, M.V. Lomonosov Moscow State University, 1(2) Leninskie Gory, 119991 Moscow, Russia ∥ Technological Institute for Superhard and Novel Carbon Materials, 7a Centralnaya Street, 142190 Troitsk, Moscow, Russia ‡

ABSTRACT: Growth of perfect heteroepitaxial metal layers on diamond can extend applications of diamond in microelectronics and open new areas for its use in devices with unique electrical properties for operations at elevated temperatures and in harsh environments. This work implements the heteroepitaxy of Ni-based alloys on single-crystal diamond and demonstrates a significant improvement of heteroepitaxial quality. The crystal lattices of the Ni−Cu, Ni−Cu−Cr, and Ni−W alloys used for heteroepitaxy have parameters close to those of diamond, a = 3.567 Å. Heteroepitaxy was achieved on all diamond faces; the best results were obtained on the {100} faces. According to X-ray diffraction data, the lattice parameters of the heteroepitaxial films and diamond in the epitaxial plane differ by no more than 0.5%. The heteroepitaxial quality of the films is characterized by values of 0.3−0.4° of the full widths at half-maximum of the X-ray rocking curve. Structural perfection of the NiCu epitaxial layers becomes better when their lattice parameters match with the diamond ones at the growth temperature. Owing to a good epitaxial matching, epitaxial crystallites of the Ni-based alloys demonstrate good wetting of diamond, despite the absence of chemical adsorption and a metal carbide layer.

1. INTRODUCTION The recent decades have witnessed gradual improvement in quality of synthetic diamond grown by the CVD technique. Especially promising is the use of diamond in high-power, hightemperature microwave electronics.1−3 Many specialists are focusing their efforts on the fabrication of the Schottky contact as a basic element of diamond electronics.4 One of the necessary conditions for any electric contact on diamond is the good adhesion of metal, but this is difficult to implement due to the high chemical inertness of diamond. This is a common problem in metallization of diamond, which occurs in many areas of diamond applications, especially in the tool-making industry, where it is important that diamonds are firmly held in a tool. Until now, the only way of solving this problem was to use carbide-forming metals such as Ti, Cr, Mo, and W, which form an adhesively stable carbide layer at the interface of diamond.5 The layer is formed by a mixture of metal carbide crystals of different compositions, sizes, and orientations.6 For example, low-resistance contacts are fabricated by evaporating or sputtering sequential layers of Ti, Cr, and Mo onto the semiconducting diamond and then annealing at temperatures higher than 500 °C.7 Generally, there is a strong correlation between carbide formation and the transition from Schottky to ohmic for metal−diamond contact.4 It is only by precise choice of the carbide-forming metal, with a thickness of several atomic © 2016 American Chemical Society

layers at the metal−diamond interface, that an adhesively strong Schottky contact can be produced.8,9 Formation of carbides on this interface is often undesirable. The heteroepitaxy of metals on diamond could be an alternative solution. There are several metals which have a facecentered cubic (FCC) lattice with a parameter close to that of diamond (a = 3.567 Å), including iridium with a = 3.839 Å, which is 7.65% higher than in diamond. This served as the basis for works on the heteroepitaxy of diamond on Ir.10 There is evidence of good heteroepitaxy of Ir on diamond,11 so we implemented the heteroepitaxy of Ir on diamond and investigated its properties.12 Iridium does not form carbides. At the same time, heteroepitaxial Ir forms a mechanically durable, thermally resistant, and chemically stable strongly adhesive film on diamond. The film consists of nanocrystallites. Ir crystallites in the heteroepitaxial film are, in a complicated way, elastoplastically stressed due to a large mismatch of the iridium and diamond lattice parameters. Other metals whose FCC lattice parameters are close to those of diamond are Ni (a = 3.524 Å, 1.2% smaller) and Cu (a = 3.615 Å, 1.35% larger). The heteroepitaxy on diamond using these metals was Received: October 28, 2015 Revised: January 12, 2016 Published: January 28, 2016 1420

DOI: 10.1021/acs.cgd.5b01520 Cryst. Growth Des. 2016, 16, 1420−1427

Crystal Growth & Design

Article

demonstrated in refs 13 and 14. However, the metal deposition process proved not optimal: structural perfection was not high, and adhesion to diamond was very weak. To grow epitaxial films of Ni and Cu on diamond, we used the magnetron sputtering process.15 Heteroepitaxial films of Ni and Cu deposited on diamond plates are homogeneous and visually smooth, and they demonstrate sufficiently high crystal perfection. They consist of separate crystallites 20−400 nm in size, oriented along the crystallographic directions of the diamond substrate, with a pronounced faceting by {100} and {111} faces. Adhesion of Ni and Cu films to diamond proved weak.15 The quality of Schottky contacts produced in this way is not very good;8,14 it is difficult to use them in practice. Evidently, this was the main reason why the earlier works were suspended.13,14 Further development of the idea of metal-on-diamond heteroepitaxy involves using metal alloys that have the FCC lattices parameters as close as possible to those of the diamond lattice. This would make it possible to deposit perfect epitaxial films with unique properties on any diamond crystal surface. Such alloys should not form carbides on the diamond surface at epitaxy temperatures. These coatings are supposed to have high mechanical durability, thermal resistance, and chemical stability. We analyzed a large number of binary alloys, which conform to these conditions, and their phase diagrams. Of all possible candidates, nickel proves to be the only metal with its FCC lattice parameter smaller than that of diamond. All the other suitable metals had an increased lattice parameter in the alloy. This is how we selected nickel as an alloy base for heteroepitaxy on diamond. In this work we selected three alloys for the heteroepitaxy on diamond: 1. Ni−Cu. The temperature range for diamond substrates which provides for the best quality of both Ni and Cu heteroepitaxial films was found to be 300−500 °C.15 This makes it possible to assume that the heteroepitaxy of a lattice-parameter-matched Ni−Cu alloy on diamond can be obtained. The solubility of C in Ni and Cu at these temperatures is negligible.16 It is essential that Cu does not form carbides altogether, and Ni does at temperatures higher than 850 °C.8 It is remarkable that Ni and Cu form a continuous series of solid solutions [ref 17, pp 85−95]. Many mechanical and chemical properties of Ni−Cu alloys are much better than those of pure Ni and Cu [ref 18, pp 6−10]. 2. Ni−Cu−Cr. A previous study established a weak adhesion of Ni and Cu on diamond.15 To improve the adhesion of a Ni−Cu alloy, addition of a minor amount of highly soluble Cr to it was proposed. Chromium is a carbide-forming metal, but on diamond it forms carbides only at temperatures higher than 1000 °C [ref 16, p 525], which is much higher than the heteroepitaxy temperatures. A minor addition of Cr to low-temperature alloys is known to increase sharply the wetting of diamond by an alloy.19 This is due to the formation of strong chemical bonds between Cr and C on the diamond surface. 3. Ni−W. At a fraction of W less than 11 at.% (31 wt%), an alloy with Ni has an FCC lattice [ref 16, p 1244]. With increasing W content, the lattice parameter of the alloy increases, which makes it possible to choose the proportion of W in the alloy at which the lattice parameter is equal to that of diamond. Ni−W alloys

possess such a high mechanical durability and chemical resistance that they have found use in chemical technologies for operations in aggressive and hot mediums. Using the lattice parameter data for some widely used Ni alloys, one can choose the composition of an alloy of exact epitaxial match with diamond. However, in the case of alloys, there is some uncertainty related to different thermal expansion coefficients (TECs) of metals and diamond. The linear TEC of diamond is 1.1 × 10−6 deg−1.20 For the metals and alloys that we use, it is much higher. In such Ni−Cu alloys as constantan and copel, it is 16 and 14 × 10−6 deg−1, respectively. Deposition of metals on diamond is performed at an elevated temperature. The heteroepitaxy temperature range for Ni-based alloys is approximately 300−500 °C.15 Under such heating conditions, the lattice parameter of diamond increases insignificantly, whereas that of alloys rises greatly. Thus, at 500 °C, the lattice parameter of a Ni−Cu alloy increases by approximately 0.8%. This is much, from the viewpoint of the requirements for qualitative epitaxy. Under these conditions, two approaches are possible: 1. To deposit on diamond an alloy of such a composition which is epitaxial at room temperature, i.e., has the lattice parameter a = 3.567 Å. But then, at the deposition temperature, the parameters will be mismatched due to different TEC values, which will inevitably affect the crystal perfection of the alloy, its microstructure, and its properties. 2. To deposit on diamond an alloy of such a composition in which the mismatch of the lattice parameters at the deposition temperature is minimal. This should provide for quality epitaxy. However, as it cools down to room temperature, the heteroepitaxial film of metal will experience a tensile stress, which can remain elastic or can partially relax due to forming dislocations, i.e., a worsening crystal perfection of the epitaxial film. This latter scenario is typical of metals. The second approach is preferable for qualitative heteroepitaxy. Unfortunately, optimal temperatures for the heteroepitaxy of alloys on diamond are not known; determining them is the main aim of this research. This means that it is impossible to fabricate beforehand targets for the magnetron sputtering of alloys with a minimal lattice parameter mismatch at an optimal deposition temperature. It is for this reason that we decided to begin with the first approach. In this case, the compositions of the alloys were as follows: Ni−Cu: 48 wt% Ni + 52 wt% Cu Ni−Cu−Cr: 48 wt% Ni + 51 wt% Cu + 1 wt% Cr Ni−W: 86 wt% Ni + 14 wt% W The aim of this study was to work out a process for depositing heteroepitaxial films matched by the lattice parameter of alloys with that of diamond and to investigate their crystal perfection, morphology, and properties. Perfect heteroepitaxy of Ni-based alloys might realize the solid adhesion of diamond-plating metallization without forming an intermediate layer of carbides. The use of carbide-forming metals in diamond metallization is often undesirable. The formation of a carbide layer on the surface of diamond increases the thermal resistance and adversely affects the surface layer of diamond, which is the basis of thin-film semiconductors and optical devices. The resulting layer of metal carbides can be 1421



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Figure 1. (a) Rocking curves of a Ni−Cu(001) sample on the (004) reflection in a logarithmic scale, recorded before (upper curve) and after (lower curve) the rotation of the sample by 180° around normal to the growth surface; on the inset picture, a rocking curve for a Ni−Cu layer on the (002) reflection of this sample in a linear scale. (b) Scan curves of the third analyzing crystal on the (200) reflection maximum recorded in grazing diffraction and on the (002) reflection maximum, the middle vertical line corresponds to the angular position for the forbidden (200) reflection of diamond. both standard and grazing diffraction geometry with triple Ge(220) analyzer. In order to determine a layer’s structural parameters, we used both (2θ−ω)-scans and 2θ-scan curves, the latter ones have lower curve widths than the former because the diffraction picture in reciprocal space from thin epitaxial layers (70−150 nm) consists of spots elongated perpendicularly to the growth surface. But the main reason for using 2θ-scan curves was misorientation on 3−7° of the most natural diamond substrates from the crystallographic surface (001) or (101). The presence of dislocations at the interface and/or elastic deformation of the layer lattice results in the fact that maxima peaks of the substrate and the layer do not belong to the same (2θ−ω)-scan curve in the triple-crystal geometry. The best way to determine the lattice parameters of such layers is to obtain a twodimensional reciprocal space area by recording the set of either the (2θ−ω)-scans with variable offset or the 2θ-scans with increasing values of ω.24 The low intensity of the peak layer in the triple-crystal geometry requires more time to complete the two-dimensional area, so we used a rapid method for finding the maximum by recording 2θscans at the maximum layer rocking curve and at the angle deviation approximately 0.1 of the full width at half-maximum (fwhm) on both sides of the maximum.

removed from the diamond surface only by way of mechanical polishing.

2. EXPERIMENTAL SECTION Natural diamond (type Ia) polished (001), (101), and (111) plates of about 4 × 4 mm2 were used as substrates for the deposition of Nibased alloys. Misorientation angle of diamond substrate (001), (101) was 1−7° in the azimuthal direction [110], [010], respectively. The plane of a plate usually deviates from the crystallographic plane by several degrees. Specificity of the wafer fabrication of single crystals diamond as the hardest material in the world is that most of the substrates are obtained misoriented by several degrees from low-index crystallographic planes. The misorientation of the substrate contributes to a more uniform embedding of components with different atomic or ionic radius and was specifically used in some systems.21,22 Diamond belongs to the cubic crystal system and should therefore exhibit isotropic optical properties. However, it has long been documented that diamonds often exhibit strain induced birefringence arising from dislocations, inclusions, and structural defects.23 Plates with small internal stresses were selected using the birefringence technique. The following procedure was used to eliminate defects of mechanical treatment in the subsurface layer. Samples were annealed in a vacuum at 1500 °C and then cleaned by etching in a mixture of H2SO4 and K2Cr2O7 at 180 °C and boiled in distilled water. The mean-square roughness of each plate surface was less than 1 nm, as measured by optical topography. The smooth plates are obtained as a result of a fine polishing. Immediately prior to metal deposition the diamond substrates were annealed in vacuum at 600 °C. Metals were deposited onto diamond substrates by DC magnetron sputtering. The purity of the metal targets was better than 99.9%. The deposition was performed in Ar (99.999) atmosphere. Discharge power was 5.7 W/cm2, operating pressure 5 × 10−2 Torr, distance from target to substrate was 10 cm. The lattice parameters of Ni-based alloy targets for magnetron sputtering were determined by X-ray diffraction. They differed from the value of a = 3.567 Å by no more than 0.1%. The magnetron sputtering setup used in these experiments allowed metals to be deposited at a rate (V) up to 3 nm/s and substrate temperature (T) up to 800 °C.15 Metal films were formed on diamond by sputtering of targets in an Ar atmosphere at a constant current (purity of argon, 99.999%). The morphology of metal films on diamond was studied by scanning electron microscopy (SEM) on Zeiss Supra 40 and JEOL JSM7001F instruments with energy 10 keV. The structural properties of deposits were studied by X-ray diffraction on the PANalytical X’Pert Pro MRD Extended Diffractometer using Cu Kα1 radiation and a primary four-crystal Ge(220) monochromator combined with X-ray mirror (30 MImp/sec, divergence 12 arc seconds) or hybrid monochromator (400 MImp/sec, divergence 25 arc seconds). In order to determine a film’s lattice parameters, we used

3. RESULTS AND DISCUSSION The rocking curves of samples and the third analyzing crystal scans from Ni-based alloy films on diamond were measured on various symmetric and asymmetric reflections. When the lattice parameter of an investigated film was close to the parameter of the substrate, for reliable lattice parameter determination we recorded the (002) or (222) reflections forbidden for the diamond structure but allowed for the FCC structure of nickel alloys. Strictly speaking, a weak (222) reflection from diamond is in fact observed, but its intensity is by several orders of magnitude lower than in the (111) reflection. The intensity of the (222) reflection from diamond is comparable with the intensities of reflections from 150−200 nm thick epitaxial layers, and the occurrence of a narrow peak of the substrates does not prevent the analysis of a broader peak of a layer, even if they are close to each other. X-ray diffraction studies confirmed that all orientations of diamond substrates yield a heteroepitaxial growth of metal alloys. This follows from an approximate parallelism of the symmetric reflections (001)C− (001)Me on (001)C plates; (101)C−(101)Me on (101)C plates; (111)C−(111)Me on (111)C plates, as well as asymmetric reflections (113)C−(113)Me, for all three orientations. Here, C is diamond, and Me is a metal film on diamond. The main 1422



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Figure 2. Third analyzing crystal scans for the (101) Ni−W epitaxial layer (a) for the (202) symmetrical reflection and (b) for the (222) asymmetric reflection. The intensities of substrate peaks (shown by thin lines) are divided by 750 (a) and 50 (b).

criteria of crystal perfection of heteroepitaxial films are fwhm of the rocking curves of samples and of the scan curves of the third analyzing crystal. We present the results of X-ray diffraction studies for the best samples with heteroepitaxial films based on Ni alloys on the (001), (101), and (111) diamond surfaces. Figure 1a shows the (004) rocking curves for the Ni−Cu(001) sample in a logarithmic scale. The diamond surface is misoriented from the (001) plane by ΔωC = 5.777° approximately along the azimuthal [110] direction, whereas the growth surface of the Ni−Cu layer is misoriented from the (001) plane by an angle ΔωNiCu = 5.272°. Thus, the (001) planes of the layer and substrate are misoriented between each other by an angle Δφ = −0.505°. Therefore, at a small angle of incidence of the X-ray on the sample, ω = θB − ΔωC, where θB is the value of the Bragg angle of the substrate on the (004) reflection, the difference in interplanar distances of the layer and substrate, and the misorientation shift the peak of the layer toward larger angles. For this reason, the peak of the layer is far from the peak of the substrate. In contrast, after the rotation of a sample by 180° around the normal to the growth surface at a large angle of incidence, ω = θB + ΔωC, these two factors act in opposite directions, and the peak of the layer is superimposed onto the more intense peak of the substrate. The exact values of interplanar distances were determined using the third analyzing crystal calibrated by the (004) reflection of silicon. For the layer of Ni−Cu the value of the lattice parameter, a⊥(004) = 3.5554 Å, is smaller than that of diamond, a = 3.56717 Å. The inset of Figure 1a also presents a rocking curve of the layer in a linear scale on the (002) reflection, in which the intensity of the peak of the layer is by an order of magnitude greater than that on the (004) reflection, and there is no peak from the substrate. The fwhm of the reflection is only 0.33°. The value of the layer parameter for this reflection is a⊥(002) = 3.5483 Å, which is noticeably smaller than the value of a⊥(004) = 3.5554 Å. In contrast, the uncertainty of measuring the angular position of the films for the third analyzing crystal scan, Δ(2θ) = ± 0.001°, corresponds to the error in parameter Δ(a⊥(002)) = ± 0.0001 Å. This means that there are defects in the epitaxial layer. With the increase of diffraction angle, the intensity from more defective areas goes down, and the value of the layer lattice parameter becomes closer to the parameter of the substrate. Shifting and broadening reflections, with simultaneously increasing diffuse scattering for large reflections’ indexes, in partially ordered metallic alloys were investigated in the classical book by Guinier.25 This effect differs from the well-known broadening of reflections due to both small crystal sizes and microstrains.

A change in the value of the lattice parameter for the epitaxial layer with the increase of diffraction angle does not enable us to measure correctly the value of a|| when using the (113) asymmetric reflection, because the diffraction angle for it (θB(113) = 45.94°) is larger than for the (002) reflection (θB(002) = 25.73°) but smaller than for the (004) reflection (θB(004) = 60.27°). For this reason, a correct method of determining the lattice parameter of the layer along the interface is to record the (200) layer reflection in grazing diffraction geometry, in which a sample is tilted to an angle ψ around the horizontal axis lying in the plane of the sample, up to the going out of the (200) reflecting plane into the vertical position. In practice, this is done in several stages due to a small intensity of this reflection and a large inclination angle. First, the strong reflection (220) of diamond is found to make certain of the right direction of slope of the sample, i.e., the direction in which the maximal intensity of the reflection is observed at an angle ψ(220) = 45° − ΔωC (ΔωC ≈ 5.777°). The strong peak (400) of diamond is then found, for which ψ(400) = 90° − ΔωC; on it, the rocking curve of the sample and the third analyzing crystal scan are recorded to make certain of the same angles 2θ on the (004) and (400) reflections. Only after those procedures, the (200) reflection of the epitaxial layer at an angle ψ(200) = 90° − ΔωNiCu (ΔωNiCu ≈ 5.272°) is found, and the rocking curve of the sample and the third analyzing crystal scan are recorded. Figure 1b presents the scan curves of the third analyzing crystal on the (002) and (200) reflections of the Ni−Cu(001) epitaxial layer. On the interface, the layer has a greater value of the lattice parameter than that of diamond, a||(200) = 3.5856 Å. We also determined a||(400) = 3.5832 Å. Thus, with the increase of diffraction angle the value of the lattice parameter in the epitaxy plane also approaches that of diamond, but to a lower degree than in the perpendicular direction. Using the two found values, a⊥(002) and a||(200), one can determine the relaxed value of the lattice parameter for the layer (arelax) via the coefficients of elastic stiffness of the alloys according to26 c11 arelax(001) = (a⊥ − a||) + a|| c11 + 2c12 arelax(110) =

c11 + 0.5C (a⊥ − a||) + a|| c11 + 2c12

arelax(111) =

c11 + 2C /3 (a⊥ − a||) + a|| c11 + 2c12

where C = 2c44 − c11 + c12, and c11, c12, and c44 are the elastic stiffness coefficients of the alloys (hereinafter in GPa) calculated proportionally to the atomic fractions of each 1423



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Table 1. Structural Characteristics of Heteroepitaxial Films on Diamond, Based on X-ray Diffraction Dataa epitaxial film and orientation of the substrate

substrate misorientation, deg, azimuth

Ni−Cu−Cr(001) Ni−Cu−Cr(111) Ni−W(001) Ni−W(101)

5.4, [110] 0.0 3.2, [110] 5.0, [010]

290 340 480 480

0.5 0.15 0.15 0.15

0.99 0.68 0.429 0.364

Ni−W(111) Ni−Cu(101) Ni−Cu(111) Ni−Cu(001)

0.0 7.0, [010] 0.0 4.9, [110]

480 380 380 380

0.15 0.3 0.3 0.3

0.420 0.350 0.36 0.330

T, °C V, nm/s

fwhm, deg

reflection, a⊥, Å

reflection, a||, Å

arelax, Å

(002), 3.5354 (222), 3.5587 (002), 3.5338 (202)1, 3.5444 (202)2, 3.5596 (222), 3.5481 (202), 3.5614 (222), 3.5648 (002), 3.5483

(113), 3.5896 (22̅2̅), 3.5786 (113), 3.5499 (222)1, 3.5546 (222)2, 3.6101 (022), 3.5669 (222), 3.5728 (22̅2̅), 3.5798 (200), 3.5856

3.5660 3.5664 3.5424 3.5486 3.5802 3.5550 3.5663 3.5706 3.5694

a

T, deposition temperature; V, growth rate; fwhm, full width at half-maximum of rocking curves; a⊥, a||, values of the lattice parameter; arelax, relaxed value of the lattice parameter.

Figure 3. SEM images of heteroepitaxial films on diamond. (a) Ni−Cu on a (001) substrate. (b) Ni−Cu on a (101) substrate. (c) Ni−Cu−Cr on a (001) substrate. (d) Ni−Cu−Cr on a (111) substrate. (e) Ni−W on a (001) substrate. (f) Ni−W on a (101) substrate.

a⊥1(202) = 3.5444 Å, a||1(222) = 3.5546 Å, and arelax1 = 3.5486 Å. We used the values c11 = 259.2, c12 = 149.8, and c44 = 126.2, proceeding from the values for pure Ni and W.26 For the other peak, a⊥2(202) = 3.5596 Å, a||2(222) = 3.6101 Å, and arelax2 = 3.5802 Å. This suggests that the grains with two different concentrations of tungsten are formed in the epitaxial layer during the growth on a (101) substrate, so that grains with a larger lattice parameter have higher level of tetragonal deformation, which is consistent with the increase of rigidity of the Ni−W alloy with the increase of tungsten concentration. For the Ni−W(001) sample, all reflections have a single rocking curve with the mean values of parameter a = 3.5338 Å from the reflection (002) and a = 3.5499 Å from the reflection (113), thus arelax = 3.5424 Å was obtained. The lattice parameter of a Ni−W target, from which the layers were sputtered, is atarget = 3.5506 Å. This means that Ni is sputtered from the target faster than W because of the difference of the atomic weights of Ni and W. However, after several tens sputtering procedures the former polycrystalline grains in the target sputtering area were transformed to [100] texture, and the film value arelax approached the value atarget. The lattice of the epitaxial layer on the Ni−W(111) sample undergoes a rhombohedral deformation during cooling down to room temperature, as the result all three angles of the unit cell become greater than 90°. We have recorderd the (222) symmetric reflection (2θ(222) = 97.537°, a⊥ = 3.5481 Å) and the (2−2−2) asymmetric reflection inclined by an angle ψ = 70.53° to the (111) plane (2θ(2−2−2) = 96.928°) in order to calculate the rhombohedral lattice parameters aR = 3.5608 Å, αR = 90.40° and a|| = 3.5669 Å, arelax = 3.5550 Å as well. The angular difference between the equivalent reflections (222) and (2−2−

metal in the alloy. The data for pure Ni and Cu are taken from ref 27. For the epitaxial layers of Ni−Cu, values c11 = 205.9, c12 = 133.8, and c44 = 99.1 were used. The calculated value arelax1 = 3.5694 Å on the (002) and (200) reflections for the Ni− Cu(001) sample is very close to the lattice parameter we obtained for diamond, a = 3.56738 Å. If, however, the relaxed value of the lattice parameter is calculated for the (004) and (400) reflections, the obtained value is slightly larger, arelax2 = 3.5711 Å. At a growth temperature of 380 °C, the lattice parameter of a Ni−Cu layer increases to arelax 380 = 3.5880 Å, which accounts for the linear TEC = 14.5 × 10−6 deg−1. Note that the latter value virtually coincides with the value of a||(200) = 3.5856 Å measured at 20 °C. We suppose that, at the growth temperature, the entire mismatch between the lattice parameters of the layer and substrate in the Ni−Cu(001) sample is released by the plastic relaxation, with misfit dislocations leading to a misorientation Δφ = −0.505° between the (001) planes of the layer and substrate. During the cooling to room temperature, the lattice parameter of the layer on the interface almost does not change, which leads to a tetragonal distortion of the layer lattice in the perpendicular direction. This is indicative of a high degree of adhesion of the epitaxial layer to diamond, which does not let the layer be compressed in the plane of the interface. Figure 2 represents the third analyzing crystal scans of the Ni−W(101) sample for the symmetric (202) reflection (a) and for the asymmetric (222) reflection (b). Two (202) peaks from the layer are at larger angles (Figure 2a) in relation to the peak of the substrate (2θB(202) = 75.291°), whereas in Figure 2b the substrate (222) peak (2θB(222) = 96.842°) is between the two layer peaks. For one peak of Ni−W we found the values of 1424



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Figure 4. SEM images of Ni−Cu heteroepitaxial films. (a) An islet-structured thin film on the (332) diamond face. (b) A thicker film at the stage of crystallite islets merger on the (112) face. (c) An almost solid 200 nm thick film on the (112) face.

2) for the cubic lattice gives the αR value for the rhombohedral distorted lattice, whereas the absolute angle values of these reflections determine the aR value. Comparison of the relaxed values of the lattice parameters for Ni−W layers grown on substrates of various orientations enables a conclusion that, under the conditions of compression stresses at the growth temperature of 480 °C, tungsten has a tendency of anisotropic distribution along various crystallographic directions, which is especially well seen in (101) layers with coexisting grains of various tungsten concentrations. The main results for some of the most perfect epitaxial layers on diamond are given in Table 1. Examples of the morphology of 200 nm thick Ni-alloy heteroepitaxial films on diamond are given in Figure 3. The morphology and X-ray diffraction studies of heteroepitaxial films of Ni-based alloys on diamond allow us to make several conclusions: 1. Heteroepitaxy is achieved on all crystal faces of diamond. This is very important, as even an impeccably polished diamond surface has scratches, indents, cavities, and steps, on which, nevertheless, heteroepitaxy is implementable as long as there is no non-diamond material on the surface. 2. For all the alloys, the greatest perfection of films is achieved on the {100} crystal faces of diamond (Figure 3a,c,e). Already at a small film thickness, separate crystallites merge to form a smooth solid film, which subsequently is developed by the layer growth mechanism. 3. On the other faces, a solid film demonstrates a relief formed by the {100}, {111}, and, to a lesser degree, {110} growth faces (Figure 3b,f). The most perfect heteroepitaxy on diamond was produced with the Ni−Cu alloy. In qualitative tests, the Ni−W alloy expectedly demonstrated high adhesion to diamond. The Ni− Cu−Cr alloy fell short of expectations. Adhesion of this alloy to diamond proved better than of Ni−Cu but worse than of Ni− W. However, a minor (1%) addition of Cr negatively affects the quality of the heteroepitaxial film. Chromium impurity spoils the growth steps of metal crystallites, especially on the {111} faces, which causes twinning, secondary crystallization, and formation of non-epitaxial crystallites on the surface of a growing film. This deteriorates the structural perfection of the Ni−Cu−Cr film. We investigated the electrical properties of Ni-based heteroepitaxial contacts to diamond. A pair of contacts were deposited on the surface of the single crystal film (thickness of 3.6 μm) of semiconducting diamond (concentration of boron acceptor is (6 ± 0.2) × 1017 cm−3). It was found that the current−voltage characteristics of such contacts are ohmic in the range from 10 mV to 10 V. To study the crystallographic regularities of heteroepitaxial film growth, we prepared a sample faceted with the (001),

(113), (112), (233), (322), (221), (331), and (110) crystallographic faces from single-crystal diamond. A heteroepitaxial film of the Ni−Cu alloy was deposited on the sample at a temperature of 380 °C through a mask shaped as a slit. Herewith, due to the effect of the shadow, the thickness of the film varied from 0 to 200 nm from the rim to the center of the film. The morphology of the heteroepitaxial films on different faces was studied by means of SEM. Different growth stages of Ni−Cu heteroepitaxial films can be followed in Figure 4, which illustrates the morphology of films of various thicknesses at some faces of diamond. Analysis of the morphology of thin and thick heteroepitaxial films on different diamond faces allows us to make the following conclusions: 1. Growth of heteroepitaxial films on diamond begins from the formation of separate roundish crystallite islets (Figure 4a) whose size increases with film growth from several nanometers to hundreds of nanometers. 2. Separate crystallite islets wet the diamond surface because they are very flat and their wetting angles are noticeably smaller than 90° (Figure 4a,b). 3. As they grow, crystallite islets acquire faceting, mainly with the {111} and {100}, as well as {110}, faces (Figure 4b). 4. After the merger of crystallite islets, the layer growth of the film is observed on the basis of the development of layers along planes of the types {100} and {111} (Figure 4c). The latter observation explains the perfection of heteroepitaxial films on the {100} diamond faces (Figure 3a,c,e) and a pronounced faceting of crystallites on the {111} (Figure 3d) and {110} (Figure 3b,f) diamond faces. This mechanism of crystallite growth is characteristic of metals with the FCC crystal lattice. The main disadvantage of the heteroepitaxy of metals on diamond is the islet character of the film. This is explained by the fact that in the process of diffusion on the diamond surface it is energetically more profitable for metal atoms to interact one with another than with carbon atoms, especially when no “metal atom−carbon atom” chemical bond is formed. But the improvement of heteroepitaxial quality has already resulted in a decrease of the angle of wetting the diamond surface by crystallite islets. These islets grow mainly along the surface of diamond, and the film becomes continuous already at small thicknesses. Thus, the entire diamond surface proves coated with metal, which is especially important for electric contacts. From the viewpoint of heteroepitaxial film quality the most advantageous is the {100} diamond face, along which the layer growth of metal proceeds. The heteroepitaxial films of Ni-based alloys on diamond demonstrate a ferromagnetic character and should exhibit interesting anisotropic magnetic properties.28 1425



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Figure 5. Curves (ω−2θ) scanning for reflections (004) and (400) (a), and (ω−2θ) scanning in triple-crystal geometry for reflections (002) and (200) (b) of the sample with an epitaxial layer Ni0.7Cu0.3, the lattice parameter of which coincides with the diamond lattice parameter at the growth temperature. The solid vertical line in (b) shows the angular position of the diamond forbidden reflection (002).

layer. Layer thickness was estimated as 70 ± 5 nm using fwhm values of reflections (002), (204), and (−204) forbidden for the diamond lattice. The last two reflections were inclined at 26.57° in opposite directions from the [001] direction, so we used the mean fwhm values between them.

The structural perfection of epitaxial layers NiCu increases noticeably when the lattice parameters of the epitaxial layer and the diamond substrate coincide at a growth temperature. Figure 5a shows curves (ω−2θ) scanning with wide slit at the detector on symmetrical reflection (004) and reflection (400) recorded in the grazing diffraction with sample rotation on ψ = 85 about horizontal axis coinciding with the growth surface. The reflection (004) was recorded in such an azimuth horizontal direction that there is no misorientation of the (001) reflecting plane of the substrate and the layer. The presence of the diamond reference peak in each rocking curve clearly shows that the lattice of the epitaxial layer is elastically strained. Moreover, in the growth direction the layer lattice parameter is significantly less than that of diamond, whereas in the interface it is slightly larger that of the substrate. Determination of the layer interplanar distance was obtained using the (2θ−ω) scanning on the reflections (002) and (200) (Figure 5b). When comparing the curves in Figure 5b with the curves in Figure 1b, a few differences can be observed. For 2θ(200) = 51.088° the inplane layer lattice parameter the value is equal to a|| = 3.5728 Å and becomes much closer to the diamond parameter, whereas for 2θ(002) = 51.748° the value is equal to a⊥ = 3.5303 Å and moves even farther from it. That means the reducing of a plastic relaxation percent when the layer lattice parameter comes nearer to the diamond lattice parameter at growth temperature. Due to increasing of the nickel concentration in the alloy up to 70% we used the values of c11 = 223.1, c12 = 139.5, c44 = 109.9, and TEC = 14 × 10−6 deg−1 for parameter calculation. The relaxed parameter value is arelax = 3.5539 Å, which at a room temperature is appreciably smaller than that of diamond. At the growth temperature of 380 °C the lattice parameter of diamond is a C 380 = 3.5686 Å, and the layer lattice parameter is aNiCu 380 = 3.5718 Å, which is close to the value a|| that we obtained. The elastic deformation of the lattice layer is ε|| = 100%(a|| − arelax)/arelax = 0.55%. The elastic deformation for epitaxial layer NiW can be even higher due to both increase of rigidity of the alloy and the possible pinning of dislocation near tungsten atoms. It is well-known that epitaxial layers of semiconductor materials grown at low temperatures sometimes maintain big values of lattice deformation in comparison with one predicted by equilibrium theory.29 Lattice deformation of 2.14% was observed in epitaxial layers of alloy NiAl,30 having a biaxial modulus value (200 GPa) close to that of alloy Ni0.7Cu0.3 (188 GPa). The second feature of the curve (2θ−ω) scanning can be observed in broadening of the reflection (002) as compared to the reflection (200) due to the small thickness of the epitaxial

4. CONCLUSION This work implemented for the first time the heteroepitaxy of nickel-based (Ni−Cu, Ni−Cu−Cr, and Ni−W) alloys on the surface of diamond and showed a significant increase in quality of heteroepitaxial films as compared to the heteroepitaxy of pure Ni and Cu on diamond. Layers with the lattice parameters close to those of diamond (at room temperature) were grown on all diamond crystal faces. Growth of heteroepitaxial films on diamond begins with the formation of crystallite islets, which wet the diamond surface and develop layer-wise along the {100} and {111} planes. Due to the difference in the thermal expansion coefficients of the alloys and diamond, the heteroepitaxial films are stressed, but, herewith, their lattice parameters and those of diamond differ by no more than 0.5% in the epitaxial plane. The heteroepitaxial quality of films is characterized by the rocking curves’ fwhm values of 0.3−0.4°. The greatest film perfection is achieved on the (001) diamond faces for Ni0.7Cu0.3 alloy, with lattice parameter close to that of diamond at the growth temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the project 16-12-10438 of the Russian Scientific Foundation. REFERENCES

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Heteroepitaxy of Ni-Based Alloys on Diamond - Crystal Growth

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