ScN in the rock salt structure is a well-investigated material due to its desirable properties like the high hardness or large thermal conductivity. Recent computations by Adamski et al. [Appl. Phys. Lett. 115, 232103 (2019)] showed that ScN/GaN heterostructures exhibit an outstanding polarization gradient which would be beneficial for polarization induced electron gases. The pseudobinary semiconductor Sc Al N, when maintaining the cubic rock salt structure, could be beneficial for tailoring the polarization gradient using the Sc dependency of material properties. The structural properties of rs-Sc Al N are not fully discovered yet, thus in this work, DC-magnetron sputtered cubic rock salt Sc Al N thin films with were grown and analyzed on ScN(111)/Si(111). The epitaxial relation of ScN(111) thin films on the Si(111) substrate is determined to be ScN[110] Si[100]. Furthermore, concentration dependent properties like the lattice parameter of Sc Al N were measured [a(ScN) = 4.50 Å, a(Sc0.55Al0.45N) = 4.30 Å] and the stress within the layers was determined. The crystal quality was evaluated using -scans, revealing for Sc0.95Al0.05N. The diameters of the columns were determined by atomic force microscopy and scanning electron microscopy and they are range from 34 to 59 nm for . At , Sc Al N columns in the hexagonal wurtzite as well as cubic rock salt structure were detected. This information about the structural specifications of Sc Al N in the rock salt structure forms the basis for further investigations and experimental confirmation of the electric properties of ScN/GaN heterostructures or even a Sc Al N/GaN based approach for improved structures for high-electron-mobility transistors.
I. INTRODUCTION
A new approach for high-electron-mobility transistors (HEMTs) was proposed by Adamski et al.1, stating that ScN(111)/GaN(0001) heterostructures exhibit a giant polarization discontinuity at the interface. Therefore, a carrier sheet charge at the interface of could be achieved. This is especially compelling because cubic ScN(111) and hexagonal GaN(0001) can be grown with a lattice mismatch of merely 0.02%. If the stack of the heterostructure is reversed [GaN(0001)/Sc Al N(111)], the piezoelectric polarization of GaN only depends on the in-plane atomic distance dN-Sc of Sc Al N, which is dependent on the Sc content x. Polarization induced electron accumulation with a density up to was already observed for lattice-matched wz-Sc0.18Al0.82N(0001)/GaN(0001) heterostructures. This led to the formation of a two-dimensional electron gas (2DEG).2 In order to produce heterostructures of GaN(0001) and Sc Al N(111), sputter epitaxy is a desirable method due to it being a cost-efficient technique for industrial scale thin film production, besides other advantages like a large deposition area and compatibility with complementary metal-oxide-semiconductors (CMOSs),3 although the crystal quality and surface morphology still present a challenge.4 In order to use sputtered Sc Al N thin films for microelectromechanical devices, the crystal quality needs to be improved. When thin films show a comparable quality to thin films grown by molecular beam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE), they could also be used, e.g., for HEMT structures. In this work, an optimized pulsed DC-magnetron co-sputtering technique was used with a metal interlayer and NH3 as nitrogen source as described by Hörich et al.5 Along with low-cost Si(111) substrates, sputtering is a highly promising method for cost-efficient, heteroepitaxial Sc Al N thin films.6 Cubic Sc Al N has yet to be the focus of research and many of its properties have yet to be discovered, although the ternary semiconductor has numerous application possibilities due to enhanced properties compared to ScN and an often linear behavior dependent on the Sc concentration down to the alloy-induced phase transition to the wurtzite structure.7
So far, only wurtzitic Sc Al N was known for its prominent increase of the piezoelectric coefficient d33. It was revealed in 2009 that d33 can rise up to 28 pCN at C for a Sc concentration of 38%.8 For comparison, AlN only shows a value of under same measurement conditions.9 The binary compounds exhibit a hexagonal wurtzite (wz) structure for AlN (Strukturbericht designation B4, space group P63mc) and a cubic rock salt (rs) structure for ScN (Strukturbericht designation B1, space group Fm3m), respectively.10 The ternary compound Sc Al N undergoes a solid–solid phase transition with increasing Al content, unlike other group III-nitrides like Ga Al N or In Al N, which maintain the wurtzite structure for any alloy composition. An abrupt phase transition from hexagonal wurtzite to cubic rock salt structure is predicted to be within .7 The growth parameters and boundary conditions can be crucial in determining whether Sc Al N appears cubic or hexagonal, like the selection of the seed layer,11 the growth temperature,12 the strain,10 and the thickness of the film.13 For example, Sc Al N crystallites were stabilized in the rock salt structure within a TiN/Sc Al N superlattice with 14 or 15 by DC-magnetron co-sputtering. Going beyond thermodynamic stability criteria can cause undesired spinodal decomposition,9,16,17 abnormally oriented18 or misoriented grains or columns.19
Hitherto, epitaxial thin films of rs-Sc Al N were grown by magnetron sputter epitaxy20–22 on diverse substrates, but the structural properties of the films have not been analyzed in detail. For instance, there is insufficient information about column diameters or crystal quality of Sc Al N, but both properties are an important factor for material properties like thermal conductivity or electric conductivity. Electron mobility is decreased by dislocations and so is the device efficiency.23 This applies for dislocations as well as for other defects like grain boundaries. Therefore, samples with larger column diameters and, thus, a reduced number of grain boundaries are preferred for HEMT structures.7 Furthermore, the epitaxial relation of cubic Sc Al N to a ScN seed layer and to a Si(111) substrate was unknown. In this paper, the structural specifications of DC-magnetron sputtered rs-Sc Al N on Si(111) with a Sc content between 55% and 100% will be examined for thin film applications.
II. METHODS
A. Sample preparation
The series of samples investigated in this work was fabricated via DC-pulsed magnetron co-sputtering with power varying from 20 to 120 W on the Al target and a constant power on the Sc target with 100 W. p-Si(111) was used as a substrate, which was heated under H2 in the growth chamber in order to get an oxygen-free surface. A metallic interlayer was deposited on the substrate for 30 s with only 3 nm thickness. This is followed by few nm thick ScN grown with N2 as nitrogen supply. Then, the actual ScN buffer layer was grown with NH3 with around 100 nm thickness. On top, the Sc Al N thin film with around 250 nm thickness was grown with NH3. The ammonia flow was between 3 and 6 cm3/min. The Sc content was controlled by the power on the Al target and determined by x-ray fluorescence spectroscopy (XRF), see Table I. A base pressure of mbar and a working pressure of mbar was applied. Both targets have a high purity with 5N (Sc) and 6N5 (Al) and the gases have a purity of 9N. The growth temperature was kept constant at C.
Overview of sputtered ScxAl1−xN thin films and the correlation of the Sc content to the power P in W on the Al target. The Sc content was determined by XRF and correlates with the power applied to the Al target with steady power at the Sc target of 100 W.
x . | . | 1.00 . | 0.96 . | 0.92 . | 0.87 . | 0.85 . | 0.81 . | 0.74 . | 0.69 . | 0.59 . | 0.55 . |
---|---|---|---|---|---|---|---|---|---|---|---|
P in W | 20 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 |
x . | . | 1.00 . | 0.96 . | 0.92 . | 0.87 . | 0.85 . | 0.81 . | 0.74 . | 0.69 . | 0.59 . | 0.55 . |
---|---|---|---|---|---|---|---|---|---|---|---|
P in W | 20 | 40 | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 |
B. Sample characterization
III. RESULTS
A. Epitaxial relation of ScN(111) and Si(111)
-scans of reflection 004 of Si(111) and reflections 002 and 220 of ScN(111) show the epitaxial relation ScNmaj[110] Si[100] and ScNmin[100] Si[100]. ScN(111) builds contact twins with a rotation by , which is indicated by the peaks with smaller intensity. The majority of ScN(111) columns is rotated by with respect to the Si(111) substrate, the minority reveals a cube-on-cube epitaxy. These scans prove the in-plane orientation of the textured thin films.
-scans of reflection 004 of Si(111) and reflections 002 and 220 of ScN(111) show the epitaxial relation ScNmaj[110] Si[100] and ScNmin[100] Si[100]. ScN(111) builds contact twins with a rotation by , which is indicated by the peaks with smaller intensity. The majority of ScN(111) columns is rotated by with respect to the Si(111) substrate, the minority reveals a cube-on-cube epitaxy. These scans prove the in-plane orientation of the textured thin films.
(a) TEM image of the interface between the ScN(111) seed layer and the Si(111) substrate. (b) Epitaxial correlation between ScN(111) and Si(111) forming a coincidence lattice. One dangling bond appears every 5 Si atoms, matching to 6 N (Sc) atoms. (c) SAED-TEM diffraction pattern of the zone axis revealing 111 and reflections of ScN and Si, respectively. They are in-plane aligned in the same way; therefore, they are related ScNmin[100] Si[100].
(a) TEM image of the interface between the ScN(111) seed layer and the Si(111) substrate. (b) Epitaxial correlation between ScN(111) and Si(111) forming a coincidence lattice. One dangling bond appears every 5 Si atoms, matching to 6 N (Sc) atoms. (c) SAED-TEM diffraction pattern of the zone axis revealing 111 and reflections of ScN and Si, respectively. They are in-plane aligned in the same way; therefore, they are related ScNmin[100] Si[100].
B. ScN(111) seed layer
1. Structure of ScN(111)
The 100 nm thick ScN seed layers which were grown on Si(111) were investigated by XRD in order to extract the lattice parameters and . The -scan of ScN in Fig. 3 shows reflection 111 of ScN at as an example. Therefore, the ScN thin film is (111)-oriented, which means the plane (111) is parallel to the sample’s surface. For the whole series of samples, the average peak position of reflection 111 of ScN is . In order to calculate the mean lattice parameter of ScN(111) on Si(111), reflections 111, 222 [ ] and 333 [ ] were determined and the resulting mean lattice parameter is Å. In order to determine the in-plane lattice parameter, reflection 204 was determined with an asymmetric measurement geometry. Figure 3 shows reflection 204 at as an example. For the whole set of samples, the reflection appears at which results in an averaged lattice parameter Å. Both lattice parameters are according to published values by Moram et al. with Å.24 The crystal quality of the ScN thin film is still challenging, with a of a rocking curve (RC, -scan) of reflection 111. Burmistrova et al.26 proved the FWHM of a RC of the 111 reflection of ( ) nm thick ScN thin films to be ; therefore, the thin film quality from this work could still be improved upon. Figure 4 shows a cross section of a ScN thin film which depicts that the material is not monocrystalline but grows in columns which are in-plane oriented. With this SEM image, the column diameters can be roughly determined to be nm. The width of the column vary from 50 up to over 90 nm and exact values are presented in Sec. III E 2. In order to prove that the films are textured and to define the in-plane orientation of the columns, a pole figure is required. Figure 5 shows three reflections (200, 020, and 002) with high intensity at and , with a distance of , respectively. This threefold symmetry proves that the (111)-oriented columns are in-plane oriented. With a skew of in , three further reflections with lower intensity show up at . These observations indicate contact twinning of the columns, which means that the columns are rotated by to each other around . These results ensure the epitaxial relation which was determined in Sec. III A. Contact twins with a distance of were observed earlier for sputtered ScN(111) thin films on GaN(0001)25 as well as on MBE-grown ScN(111) thin films on GaN(0001)27 with no preferred in-plane orientation detectable in both cases. Moram et al.24 observed a threefold symmetry of 200, 020, and 002 reflections as well as reflections rotated by with lower intensity.
/2 -scans of rock salt ScN(111) reflection 111 (symmetric) at and 204 (asymmetric) at .
/2 -scans of rock salt ScN(111) reflection 111 (symmetric) at and 204 (asymmetric) at .
Cross-sectional SEM image of the samples. Monocrystalline Si(111) in dark gray on the bottom, then in bright gray columnar ScN(111) thin film with thickness nm and on top columnar Sc Al N(111) with nm. The diameters of the columns are nm.
Cross-sectional SEM image of the samples. Monocrystalline Si(111) in dark gray on the bottom, then in bright gray columnar ScN(111) thin film with thickness nm and on top columnar Sc Al N(111) with nm. The diameters of the columns are nm.
Pole figure ( – -scans) of rock salt ScN(111) reflections 200, 020, and 002. The rock salt (111)-oriented columns show a threefold symmetry of contact twins with a rotation by . The intensity values are adapted to relative units for better comparability.
Pole figure ( – -scans) of rock salt ScN(111) reflections 200, 020, and 002. The rock salt (111)-oriented columns show a threefold symmetry of contact twins with a rotation by . The intensity values are adapted to relative units for better comparability.
2. Stress state of ScN(111)
For the evaluation of stress state within the thin film, an unstrained lattice parameter is required. In this work, aScN = 4.5013 Å is used as the unstrained lattice parameter, as this value is confirmed by several publications.22,28–30 Therefore, the strain along can be calculated to be and the in-plane strain to be . The negative value indicates a compressive strain in the direction and the positive value means the lattice has a tensile strain in-plane, e.g., parallel to the surface. Using the elastic coefficients ( GPa, GPa, and GPa) simulated by Wu et al.,20 the stress within the ScN thin film is calculated to be GPa and GPa. The in-plane biaxial stress of ScN(111) thin films was determined earlier to be below 1 GPa but did not reveal any value.24 Nonetheless, the ScN(111) thin film is grown in a very low stress state on Si(111).
C. Interface ScxAl1−xN(111)/ScN(111)
The in-plane orientation of the columnar rs-Sc Al N(111) thin films can be proven using pole figures. Figure 6 shows a threefold rotation symmetry of the 200, 020, and 002 reflections at for rs-Sc0.59Al0.41N exemplarily. Each pole figure of the whole series of rs-Sc Al N thin films investigated in this work showed similar features. Comparable to the ScN seed layer, the Sc Al N thin films exhibit contact twins, which is visible by a shift of in of the reflections with lower intensity, maintaining strongly (111)-oriented columns. This is further proven by the fact that the 200, 020, and 002 reflections in Fig. 6 appear at the same and positions. Thin films of rs-Sc Al N with show the same in-plane orientation and comparable intensity differences between the contact twins as their ScN(111) seed layer. This relation is maintained over the whole thickness of 250 nm. Thus, the contact twins of the Sc Al N(111) thin film are prescribed by the ScN seed layer and originate at the interface to the Si(111) substrate. The in-plane orientation is maintained over the interface from ScN(111) to Sc Al N(111), keeping the same relation to Si(111) as described in the epitaxial relations [Eqs. (4) and (5)]. The properties of the 250 nm thick Sc Al N(111) layer are described in the following section.
Pole figure ( – -scans) of rs-Sc0.59Al0.41N(111) reflections 200, 020, and 002. The high-intensity reflections as well as the low-intensity reflections show a threefold symmetry and indicate the formation of contact twins with a rotation by in , respectively. The intensity values are adapted to relative units for better comparability.
Pole figure ( – -scans) of rs-Sc0.59Al0.41N(111) reflections 200, 020, and 002. The high-intensity reflections as well as the low-intensity reflections show a threefold symmetry and indicate the formation of contact twins with a rotation by in , respectively. The intensity values are adapted to relative units for better comparability.
D. ScxAl1−xN thin film
1. Structure of ScxAl1−xN
/2 -scans of sputtered rs-Sc Al N thin films on a Si(111) substrate. The ScN 200 reflection appears at .
/2 -scans of sputtered rs-Sc Al N thin films on a Si(111) substrate. The ScN 200 reflection appears at .
Lattice parameters (rs-Sc Al N reflection 111) and (rs-Sc Al N reflection 204) measured by XRD. A selection of experimentally determined lattice parameters c from wz-Sc Al N21 and rs-Sc Al N20,21 lattice parameters a are shown as full symbols to contextualize the measured data. The open symbols show simulated lattice parameters c17,36 and a.33 The straight line shows Vegard’s law between rs-AlN37 and rs-ScN.38
Lattice parameters (rs-Sc Al N reflection 111) and (rs-Sc Al N reflection 204) measured by XRD. A selection of experimentally determined lattice parameters c from wz-Sc Al N21 and rs-Sc Al N20,21 lattice parameters a are shown as full symbols to contextualize the measured data. The open symbols show simulated lattice parameters c17,36 and a.33 The straight line shows Vegard’s law between rs-AlN37 and rs-ScN.38
Lattice ratio of rs-Sc Al N thin films. Green triangles show the results of this work [rs-Sc Al N/Si(111)], and blue diamonds and squares show the results of Satoh et al.21 and Wu et al.,20 respectively.
Reciprocal space map of ScN and rs-Sc0.95Al0.05N reflection 204, respectively. The lattice parameter of Sc0.95Al0.05N [ Å] is slightly smaller than the one of ScN [ Å]. The reflections are aligned with respect to , proving the pseudomorph growth of the rs-Sc Al N thin films.
Reciprocal space map of ScN and rs-Sc0.95Al0.05N reflection 204, respectively. The lattice parameter of Sc0.95Al0.05N [ Å] is slightly smaller than the one of ScN [ Å]. The reflections are aligned with respect to , proving the pseudomorph growth of the rs-Sc Al N thin films.
2. Crystal quality of ScxAl1−xN
Many aspects are relevant when determining the crystal quality of a thin film semiconductor. In the following, -scans, so-called rocking curves, were measured by XRD and the full width at half maximum (FWHM) was determined. The FWHM of -scans is an indicator for the amount of crystal planes which are aligned in a way that they obey Bragg’s law at a certain angle. Tilted lattice planes, caused by misoriented grains, defects, or dislocations, interfere constructively at a slightly different angle and broaden the peak in the diffractogram. The FWHM of the -scans of the ScN seed layer reflection 111 measures the value and the FWHM of the rs-Sc Al N reflection 111 are given in Table II. The values are between for rs-Sc0.55Al0.45N and for rs-Sc0.95Al0.05N. This means that the crystal quality increases with increasing Sc content. This can be explained by the incorporation of the rather small Al atoms, compared to Sc atoms in the rs-ScN lattice, leading to inconsistent bond lengths and strain in the lattice. The bond length of the rs-lattice is increasing with increasing Sc content and, thus, the strain is increasing as well. These results reveal a higher crystal quality than ScN(111) sputtered on MgO(111) at C by Le Febvrier et al.28 with . These values of sputtered rs-Sc Al N thin films are even lower than sputtered wz-Sc Al N with of the 0002 reflection.12 The rs-Sc0.95Al0.05N 204 reflection shows a and for the thinner ScN seed layer (shown in Fig. 7). This is a remarkable crystal quality for sputtered rs-Sc Al N(111), which was grown with a stabilized growth process and using NH3 as nitrogen source.5 Of course, the crystal quality is not as good as ScN thin films grown by MBE [FWHM(ScN(111)) = ]39 or HVPE [FWHM(ScN(100)) = ].40
The FWHMω of the 111 reflection is given for rs-ScxAl1−xN(111) 250 nm thick films with 0.55 < x < 1.00.
x . | . | 0.96 . | 0.92 . | 0.87 . | 0.85 . | 0.81 . | 0.74 . | 0.69 . | 0.59 . | 0.55 . |
---|---|---|---|---|---|---|---|---|---|---|
FWHMω | 1.14 | 1.50 | 1.11 | 1.30 | 1.63 | 1.63 | 1.64 | 1.71 | 2.17 |
x . | . | 0.96 . | 0.92 . | 0.87 . | 0.85 . | 0.81 . | 0.74 . | 0.69 . | 0.59 . | 0.55 . |
---|---|---|---|---|---|---|---|---|---|---|
FWHMω | 1.14 | 1.50 | 1.11 | 1.30 | 1.63 | 1.63 | 1.64 | 1.71 | 2.17 |
3. Stress state of ScxAl1−xN
The stress and strain of Sc Al N(111) thin films for . The out-of-plane values are indicated by and the in-plane values by . The dashed line shows a quadratic fit of the data.
The stress and strain of Sc Al N(111) thin films for . The out-of-plane values are indicated by and the in-plane values by . The dashed line shows a quadratic fit of the data.
E. Surface properties of ScxAl1−xN
1. Surface morphology of ScxAl1−xN
Figure 12 depicts a SEM image of a rs-Sc0.87Al0.13N(111) thin film. The vast majority of column surfaces show a triangular shape with a threefold symmetry along , showing the facets. These columns are in-plane oriented, so they point to the top of the image, like the column highlighted in yellow. The minority of columns are pointing downward in the image, one of them highlighted with a red circle. They are in-plane oriented as well but build contact twins with a rotation of . Rotated domains were already examined for DC-magnetron sputtered ScN(111) on Si(111)24 or on GaN(0001).25 The amount of columns which point upward vs the amount of columns which point downward show the same relation as the reflections with higher intensity vs the reflections with lower intensity in the pole figure in Fig. 6. This image is exemplary for the whole set of samples with , whereas every sample showed triangularly shaped columns for most of the surface and few abnormally oriented grains (AOG). These AOG are -oriented and have a fourfold rotation symmetry, given by angles of the column edges. With diameters of 100–200 nm, AOGs are bigger than (111)-oriented Sc Al N columns. These abnormal oriented grains build rotation twins with an angle , which is inconsistent for each AOG. The Sc Al N(100) grains are fiber-textured, which means that they are oriented along [100] with respect to the growth direction, but there is no preferred in-plane orientation. Also, the Sc Al N(100) reflection cannot be detected by XRD. Sandu et al.18 reported AOGs in sputtered wz-Sc0.43Al0.57N(0001) thin films on Si(111) with larger diameters than the (0001)-oriented columns. Additionally, grains which are -oriented or in other arbitrary angles with respect to the direction are observable in the bottom right corner of Fig. 12, but there are insufficient grains to interfere constructively and be detectable in the x-ray diffractogram. Furthermore, two -oriented grains are observed to be penetration twins (Fig. 14), growing through each other. ScN(110) was only reported to grow epitaxially on MgO(110) and m-plane -Al2O3 so far.40
SEM image of rs-Sc0.87Al0.13N(111) showing threefold, triangular-shaped columns, forming in-plane oriented contact twins with a rotation angle of . Abnormal oriented grains (AOG) align along , or arbitrary angles.
SEM image of rs-Sc0.87Al0.13N(111) showing threefold, triangular-shaped columns, forming in-plane oriented contact twins with a rotation angle of . Abnormal oriented grains (AOG) align along , or arbitrary angles.
AFM images of rs-Sc0.95Al0.05N(111) [(a) and (b)] and rs-Sc0.55Al0.45N(111) [(c) and (d)], recorded in the AC mode. The height of the surface structure and the lock-in phase images are shown, respectively.
AFM images of rs-Sc0.95Al0.05N(111) [(a) and (b)] and rs-Sc0.55Al0.45N(111) [(c) and (d)], recorded in the AC mode. The height of the surface structure and the lock-in phase images are shown, respectively.
Mean column diameter of 250 nm thick rs-Sc Al N(111) films with (squares) and ScN(111) seed layers with 100 nm thickness (triangle). The data were extracted from AFM images recorded in the AC mode. Inset: (110)-oriented grains of rs-Sc0.95Al0.05N build penetration twins which are bigger than (111)-oriented columns. These abnormally oriented grains are bigger than 100 nm.
Mean column diameter of 250 nm thick rs-Sc Al N(111) films with (squares) and ScN(111) seed layers with 100 nm thickness (triangle). The data were extracted from AFM images recorded in the AC mode. Inset: (110)-oriented grains of rs-Sc0.95Al0.05N build penetration twins which are bigger than (111)-oriented columns. These abnormally oriented grains are bigger than 100 nm.
2. Column diameters
The column diameters on the surface of the rs-Sc Al N(111) thin films were extracted from AFM images which were recorded in the AC mode. Figure 13 shows height images and lock-in phase images of rs-Sc Al N(111) with [Figs. 13(a) and 13(b)] and [Figs. 13(c) and 13(d)] as an example. Due to the measurement mode, the triangular shape cannot be seen, but the dimensions of the roundly shaped columns can be determined in detail, especially in the lock-in phase images. This is advantageous compared to the extraction of the diameters from the SEM images. The diameters of the columns were extracted in an area of and the mean column diameters and their variations were calculated. The results for rs-Sc Al N(111) (squares) and ScN(111) (triangle) are given in Fig. 14. The range of column size varies a lot on each sample, nevertheless, an overall tendency of decreasing column diameters can be seen with increasing Al content from nm of pure ScN down to nm of rs-Sc0.55Al0.45N(111). These results were expected because the columns can grow larger when the thin film in less strained. As the strain increases with Al content, the column diameters decrease. These values exceed the previously reported values of rs-ScN with 60 nm size of twin domains38 and column diameters of ScN(111) with nm25 and even wz-Sc0.19Al0.81N(0001) with column diameters of 30–50 nm grown by MBE,41 although the column diameters differ by 30–60 nm on any reported samples. DC-magnetron co-sputtered wz-Sc0.43Al0.57N(0001) on Si(100) reveals larger column diameters than the samples measured in this work with nm.42 On the other hand, Casamento et al. showed wz-Sc0.18Al0.82N(0001) column diameters of nm.2 The reported values agree with the column diameters extracted from the SEM image (Fig. 12) and the intersection (Fig. 4). Considering the requirements of thin film properties for industrial applications, larger column diameters are favorable. For HEMT structures, for example, larger grains and, therefore, less grain boundaries as well as a reduced surface roughness reduce the probability of electron trapping.7 Therefore, rs-Sc Al N(111) thin films with higher Sc content would be preferred for microelectromechanical applications.
F. Phase transition
A metastable, pseudobinary alloy like Sc Al N can separate locally into Al- and Sc-rich regions by so-called spinodal decomposition. Another phenomenon to release the energy of the metastable compound is that it separates in two phases, namely, wurtzitic and rock salt which can co-exist with the same Sc content x. This can happen although the same boundary conditions like growth temperature, strain, etc., were applied during growth.12,41 It was reported that sputtered thin films with 90% Sc content grow as rs-Sc Al N(111) in Sc-rich areas and wz-Sc Al N(0001) domains in Al-rich areas.11 The SEM image in Fig. 15(a) shows two domains of the rs-Sc0.59Al0.41N thin film: triangularly shaped and arbitrary, round columns. The highlighted column in the center of the image shows a angle between crystal edges, proving the existence of a two-phase alloy by a wz-domain. This is caused by a locally lowered Sc content which could not be quantified due to the small region of occurring domains. The wz-domains appear with smaller grain diameters, whereas the triangular-shaped rs-domains reveal column diameters according to Fig. 14. The surface morphology changes immensely from the Sc0.55Al0.45N thin film compared to films with larger Sc contents, where only arbitrarily shaped grains are found in Fig. 15(b). For instance, the highlighted column shows a hexagonally shaped surface. Similar grain shapes but with only 10–20 nm diameters were observed in 120 nm thick, sputtered wz-Sc0.16Al0.84N(0001) thin films.14 The rs-Sc0.55Al0.45N 111 reflection in Fig. 7 has a broadened peak compared to the other thin films with lower Al contents. This is due to the decrease of crystalline uniformity of the film caused by the presence of more than one crystal phase. The reflection can be interpreted as wz-Sc0.55Al0.45N with lattice parameters Å (triangle in Fig. 7) and Å. Energy dispersive x-ray spectroscopy (EDXS) reveals a local Sc content of compared to the averaged Sc content of determined by x-ray fluorescence (XRF) spectroscopy. The resulting c/a ratio of 1.601 proves that this thin film contains wz- and rs-domains with varying Sc content. For samples with , such arbitrarily or hexagonally shaped columns are not found.
(a) SEM image of a Sc0.59Al0.41N thin film with rs-domains (triangular) and wz-domains (arbitrarily shaped), revealing angles between crystal edges. (b) Sc0.55Al0.45N thin films only showing arbitrary shaped grains with significantly smaller diameters than the rs-domains of other samples.
(a) SEM image of a Sc0.59Al0.41N thin film with rs-domains (triangular) and wz-domains (arbitrarily shaped), revealing angles between crystal edges. (b) Sc0.55Al0.45N thin films only showing arbitrary shaped grains with significantly smaller diameters than the rs-domains of other samples.
IV. CONCLUSION
In this work, DC-magnetron sputtered cubic rock salt Sc Al N thin films with and a thickness of 250 nm were grown on a ScN(111) seed layer on Si(111). The epitaxial relation of ScN(111) thin films on the Si(111) substrate was determined to be ScN[110] Si[100]. The lattice parameter [a(ScN) = 4.50 Å], crystal quality ), and the stress state ( GPa) of the ScN(111) seed layer were determined. The epitaxial relation between ScN(111) and rs-Sc Al N reveals a cube-on-cube epitaxy with rs-Sc Al N[100] ScN[100]. The lattice parameter of Sc Al N ranges from Å to Å. The crystal quality decreases from to . The stress state decreases from GPa to GPa. The surface morphology of the thin films was determined by SEM images and reveals triangularly shaped columns which build contact twins with rotation. Abnormally oriented Sc Al N(100) grains appear bigger [ nm] than (111)-oriented columns [ nm]. The diameters of the columns [ nm and nm] were determined depending on the Sc content by AFM and SEM. For , Sc Al N thin films show hexagonal columns in addition to triangularly shaped rock salt columns. At even lower Sc content with , only arbitrarily and hexagonally shaped columns are present at the surface. This information about the structural specifications of Sc Al N in the rock salt structure forms the basis for further investigations and experimental confirmation of the electric properties of Sc Al N/GaN based heterostructures.
ACKNOWLEDGMENTS
The authors would like to thank the Gips-Schüle Foundation, the Carl-Zeiss Foundation within the project SCHARF, and the German Science Foundation (DFG) who supported this work by the priority programme SPP 2312 (GaNius—Energy Efficient Power Electronics) within Projects Nos. 441885089 and 462722619 who made these scientific results possible.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
S. Mihalic: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (lead); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review & editing (equal). E. Wade: Formal analysis (equal); Investigation (equal); Validation (equal); Writing – review & editing (equal). C. Lüttich: Investigation (equal). F. Hörich: Investigation (equal). C. Sun: Investigation (equal); Writing – review & editing (supporting). Z. Fu: Investigation (equal). B. Christian: Data curation (equal); Validation (equal); Visualization (equal); Writing – review & editing (equal). A. Dadgar: Methodology (equal); Resources (equal); Supervision (supporting); Validation (equal); Writing – review & editing (equal). A. Strittmatter: Funding acquisition (equal); Resources (equal). O. Ambacher: Conceptualization (equal); Funding acquisition (equal); Resources (equal); Supervision (lead); Validation (equal).
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon reasonable request.