1 Introduction

Wurtzite aluminium nitride (AlN) is a semiconductor material, important for the development of ultraviolet (UV) light sources and UV photodetectors, and for using as a buffer layer or a native substrate for nitride-based electronic devices due to its wide band gap (6.089 eV at low temperatures) [1]. Appliance of these devices, for example, as high-efficiency small and compact deep UV light-emitting diodes (LEDs) and laser diodes (LDs) for reinstating the low-efficiency and highly toxic mercury lamps and gas lasers used in water/air purification and disinfection or for high-resolution photolithography is based on AlN and high-aluminium (Al)-content aluminium gallium nitride (AlGaN). Enabling such far-reaching applications with a realization of eco-friendly technology based on AlN and their alloys is an immense benefit and is a must. For nontoxic solid-state lighting applications, the potentials of low-dimensional materials like quantum dots, nanocrystals have additionally attracted extensive attention [2, 3]. The results of UV LEDs based on AlN or AlGaN quantum dots have also been published by other groups [4, 5]. It was shown that the performance of AlGaN-based UV LEDs could be increased, especially due to the improved quality of AlGaN layers with high Al content [6]. The efficiency might further be increased using AlN single crystalline wafer as a substrate, since epitaxial AlGaN layers grown on such lattice-matched substrates by metalorganic chemical vapour deposition (MOCVD) method have significantly lower dislocation densities.

Crack‐free AlGaN layers up to 2 μm thickness were grown on sapphire substrates by hydride vapour phase epitaxy (HVPE), and also 40-µm-thick crack-free AlGaN layers on SiC substrates were reported [7]. For high-Al-content AlxGa1–xN (x ∼ 0.7–0.8) layers grown on sapphire, high‐resolution X‐ray diffraction rocking curve (XRD-RC) widths ranging from 250 to 650 arcsec and from 1400 to 1900 arcsec were reported for the (00.2) symmetric and (10.2) asymmetric reflections, respectively. These layers contain high densities of screw dislocations (6 x 108 cm–2) and edge dislocations (2 x 109 cm–2). Similarly, AlxGa1–xN (x = 0.54) films grown on sapphire substrate by low-pressure MOCVD have also been reported [34]. The concentration of silicon impurity, quantitatively derived from Si 2p peak, is about 1.7 at%, and this value well correlates with the EPMA measurement data for Si. So, it can be inferred that even good quality, yellow-coloured AlN single crystals are being contaminated by certain amount of silicon from the substrate. The peak for Al appears for the electron configuration 2s at around 120 eV and 2p at around 75 eV. Nitrogen could be detected as 1s electron configuration with a peak at approximately 395 eV.

3.3 Crack formation in the crystals and polycrystalline rim

Different thermal expansion coefficients between AlN, SiC substrate (mismatch ~ 1%) and also TaC seed holder cause AlN crystal to become more stressed when it is cooled down to room temperature, after the completion of the growth process. Hence, the total thermal mismatch between these three materials is the reason for crack formation in the grown AlN crystal. As an example, the photograph of one of the 2-mm-thick AlN crystals is shown in Fig. 3a, and the cracks are clearly seen on the surface of the crystal. The density of cracks increases when the single crystalline part is in contact with the surrounding AlN polycrystalline rim. The formation of AlN polycrystalline rim may be due to the fast evaporation of the SiC substrate peripheries/edges, wherein the substrate is rapidly and fully decomposing. Consequently, the crystal growth occurs on polycrystalline TaC seed holder in the edges, and hence, the polycrystalline AlN rim is formed (Fig. 3b) surrounding the single crystalline area. As AlN possesses anisotropic thermal expansion, it is understandable that the contact of the single crystal part with that of AlN poly rim will produce additional cracks in the single crystal. It leads to the problems in producing large-size AlN wafers for device processing and hence the cracks should be avoided. In homo-epitaxial growth, it is conventional that additional heating elements are inserted near the seed holder’s edge to avoid such polycrystalline rim formation, but this may not be applicable in this case of hetero-epitaxial growth, since this will enhance the decomposition of SiC even faster.

Fig. 3
figure 3

Photographs of AlN single crystals. a Surface of a crystal exhibiting crack formation; b polycrystalline rim grown on TaC holder, surrounding the single crystalline part the crystal.

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3.4 Multi-source nucleation and high mosaicity

When on-axis (0° off-oriented) substrates are used for the growth experiments, 3D multi-source nucleation has been observed in the crystals even in the as-grown surfaces, as shown in Fig. 4a. The absence of steps and kink positions in these types of substrates leads to nucleation of grains (3D islands formation) at many different places of the surface of the seed, which might have started as 2D nuclei in the initial stages. Growth on an on-axis substrate offers large, atomically smooth surfaces and implies the presence of a kinetic barrier for adatoms and suppresses the diffusion length of adatoms. Hence, the layer coverage on the whole surface is via the coalition of these differently nucleated grains, which governs the origin of tilt and/or twist angle between them. This misorientation angle hinders the formation of smooth and perfect (0001) crystallographic plane or otherwise called c-plane. But the off-oriented substrate offers closely spaced steps and the growth proceeds with step flow mode as investigated using atomic force microscopy (AFM) and as shown in Fig. 4b. In this case, there are lesser chances for the formation of 3D nuclei. However, after the growth has been completed, the surface of some thicker AlN crystals grown on 2° off-oriented SiC substrates exhibit individual crystallites of different sizes, which looks similar to macro-steps as presented in Fig. 4c. This type of crystallites/features might be formed during cooling down process after the growth, by the recondensation of remaining species still existing in the vapour phase, at temperatures somewhat lower than the actual growth temperature.

Fig. 4
figure 4

a Photograph of AlN single crystal grown on on-axis SiC showing high density of multi-nucleation; b AFM picture showing a step flow growth of AlN on misoriented SiC substrates; c laser scanning micrograph (magnification: 20x) of a crystal surface grown on 2° off-oriented SiC; d X-ray diffraction rocking curve measured on one of the worst crystals (case study), exhibiting high mosaicity

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In the crystals that consist of 3D nucleated grains, occurrence of low-angle grain boundaries was more probable that increases the mosaicity of the crystal, as seen by XRD measurements. The XRD rocking curve measurements taken on one such a bad crystal (this is the worst crystal grown and only taken for illustration to show a higher mosaicity) grown on on-axis SiC substrate exhibited a rocking curve (peak) with a full width at half maximum (FWHM) value of approximately 0.2° (i.e. 720 arcsec, Fig. 4d). This too high value of FWHM (mosaicity) is unsuitable for device quality substrates and further impedes the usefulness of these crystals for any device applications. Such a high mosaicity is repeatedly reported for AlN crystals grown on SiC substrates [33, 34]. But each single grain is of high quality, and when measured by HRXRD, they show a sharp rocking curve with a FWHM of 70–100 arcsec. It should also be mentioned that all the individual grains are always (0001)-oriented. It is remarkably a good value when compared to the recent literature values of AlN/SiC samples, where the FWHM values of >100 arcsec are reported [35]. This multi-source nucleation could be reduced when off-oriented SiC wafers (between 2° and 4° off) are used as substrates [36] with controlled temperature gradient during growth. In general, the decrease in vapour supersaturation and increase in substrate misorientation angle result in the transformation of a surface with 3D (or 2D) nuclei to that with steps [37]. The off-oriented substrates provide step edges and kink positions for easy attachment of atoms resulting in the step flow growth or layer by layer growth. Further, in PVT growth, higher growth temperatures (> 2000 °C) are beneficial for increasing the surface kinetics (i.e. hel** the atoms to properly arrange themselves at the surface). But in AlN/SiC hetero-epitaxy, the growth temperature cannot exceed 2000 °C that will result in more thermal etching/evaporation of SiC substrate. Hence, in this case, off-oriented substrates are more beneficial. However, the growth on highly off-oriented substrates ends up with step bunching.

3.5 High etch pit density

Though the lattice constant values favour the selection of hexagonal SiC as a substrate over the other materials, still the small difference in the values between SiC and AlN is good enough to generate stress and misfit edge dislocations at the hetero-epitaxial interface. These edge type dislocations propagate through the crystal length and are detrimental to the lifetime of the devices. But the thick epitaxial layers/bulk crystals, as like in our case, may have the opportunity to offset the lattice mismatch induced stress, with growing thickness due to relaxation, and the quality of the crystal is getting better as the length increases. There were also screw-type dislocations and mixed dislocations found in the crystal, exhibited by wet chemical defect-selective etching. The wet chemical etching allows a first, simple determination of defect structures and density. It was observed that the impurity content of the samples influences the etching time and temperatures of etching. For the etch pit density analyses of the grown AlN samples, the optimum etching condition was found to be 120–150 seconds at 350 °C. Most of the etch pits are hexagonally shaped and are related to threading dislocations. Longer duration of etching resulted in over-etching of the samples and also merging of the etch pits was seen.

Although the middle part of the crystal, depicted in Fig. 2a, looks yellowish in colour, a slice cut from that part exhibits different coloration at the centre (yellow) and at the edges (green). This indicates more impurity incorporation in the perimeter of the crystal. In such cases, high variation in the etch pit density values was observed in the sample. The etch pit density at the yellow regions of such slice is in the order of 105 cm-2 wherein the dislocations are uniformly distributed (see Fig. 5a). But at the green regions of the same sample, the etch pit density is observed to be at least one order high. The EPD values of different coloured crystals are presented in Table 1. It reveals that the green coloration deteriorates the structural quality of AlN. Carbon and silicon present in the growth environment act as additional sources of local stresses by incorporating into the growing crystal. Some areas, where the macro-holes were formed, the etching was predominant. In Fig. 5b, the orientation of the hole defect is visible, and in this case, the etch pits surrounding the hole defect roughly mirror the shape of this hexagonal hole. Further, the differently oriented lines of the etch pits (for, for example, oriented in the directions [100] and [010] as marked in the figure) meet at 120° angles. As discussed in section 3.1, the hole defect in Fig. 5b reflects the hexagonal symmetry of the crystal and the dislocations in its close proximity are most likely a consequence of stress deformation traces around the hole and therefore adapt the shape in a macroscopic scale. The lowest etch pit density values of the order of 105 cm-2 obtained in the good portions of the crystal are bit high for real device applications, as compared to homo-epitaxial grown AlN substrate–103 cm-2 [6] (for, for example, vertical device structures where the light is extracted through the substrate).

Fig. 5
figure 5

Optical micrograph of the wet chemically etched surface of the slice cut from the middle part of the crystal of figure-1a, a at the centre yellow region with defect density of the order of 105 cm-2; b at the green-coloured edge where a macro-hole was observed. The etch pits surrounding the macro-hole lie on the lines which mirrors the shape of the hexagonal hole

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4 Improved crystal growth and properties

With the continuous efforts for the past few years, it was possible to successfully address some of the issues [38, 39]. For example, (i) the growth rate of 50 µm/h from the usual growth rates of 20–30 μm could be achieved by optimizing the growth parameters (especially increasing the temperature gradient, ΔT) in such a way to enhance the mass transport without increasing the decomposition of the SiC substrate (i.e. temperature of growth). Two 1-inch-diameter AlN crystals obtained using the optimized crystal growth conditions, and having improved properties are shown in Fig. 6a and b. Very smooth and uniformly stepped surface morphology could be observed in these crystals. AlN crystals grown in this study (either free-standing wafers or AlN/SiC templates) have better structural properties in comparison with the other reported results of hetero-epitaxial growth [35]. The detailed description of these results could be found elsewhere [36]; (ii) the wafers produced from hetero-epitaxially grown crystals may be used as native seeds for growing high-purity bulk crystals in tungsten crucible set-up with tungsten heating elements (i.e. carbon-free environment), in which these impurities can be reduced to a greater extent, for producing deep UV transparent AlN samples. It is common understanding that the major impurities in bulk AlN crystals are O, C and Si and the key issue is their controllability. Recently, high deep UV transparent AlN crystal grown under optimized crystal growth conditions was reported [12]. A 60 mm dia. wafer with 98% usable area exhibiting excellent UV transparency in the range 4.43–4.77 eV (260–280 nm) with absorption coefficients of 14–21 cm-1 was demonstrated. As long as a reasonable concentration of silicon exists, a high deep UV transparency could be achievable even though relatively high C and O impurities are present in the AlN crystals. Despite the growth issues stated in this article, for the development of large-area native seeds of AlN, hetero-epitaxial growth approach on SiC looks very promising [12, 40] when compared to other approaches, namely self-nucleation and subsequent homo-epitaxial seeding, where the lateral diameter enlargement and iterative growth runs are cumbersome and time-consuming [41]. However, the availability of high-quality large-area (> 4 inch) SiC substrates as seeds with low defect density and with a very few (better zero) micro-pipes is crucial for hetero-epitaxy to grow large diameter AlN single crystals.

Fig. 6
figure 6

Photograph of good AlN single crystals grown on off-oriented SiC substrate with optimized process parameters showing a highly uniform step flow growth, b very smooth surface without any cracks.

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5 Persisting issues

5.1 Lower growth rate

Decomposition of the SiC substrate as well as the propagation of micro-pipes can be reduced to some extent, by performing the growth experiments at relatively lower temperatures. However, once when the growth temperature is reduced in the experiment (i.e. lower than 1850 °C), the obtained growth rate is 50 µm/h due to the fact that the growth rate is exponentially decreasing with decreasing temperature. As a result of this, it takes much longer time to grow a fairly thick crystal as compared to homo-epitaxy, in which the growth rate is at least 4 times more. The growth rate is determined mainly by the surface kinetics thus in turn by the crystal growth temperature. The typical temperature of AlN growth on SiC substrates is 200–300 °C lower than that of the growth on native AlN substrates, where high temperatures could be applied, and a growth rate of 200 μm/h could easily be attained. A growth rate of 200–250 μm/h is very attractive and expected for industrial production.

5.2 Unintentional impurities

In AlN, as shown above, the unintentional impurities incorporated during the growth process some way influence the morphology, the chemical content and the coloration of the crystals. However, these impurities do not have much adverse effect on the structural properties. On the other hand, the optical and electrical properties are strongly affected. Particularly for DUV light sources, the optical transparency of the AlN substrate in the wavelength region of 210 – 280 nm is an important requirement for the vertical device structures, where the light extraction through the substrate is necessary. But the unintentional impurities exhibit optical transition bands below the band gap energy, close to blue and UV regions. It is reported that these below-band-gap absorption bands are attributed to the presence of Al vacancies (VAl), substitutional impurities (carbon, oxygen) and their complexes [42]. In addition to the crucible material (mostly TaC) and the purity of the source, the SiC substrate as well determines the extensiveness of the concentration level of these impurities. The origin for a very strong below-band-gap absorption is assigned to the high concentration of C impurities [46]. But, no DUV transparency (> 80%) with homogeneous absorption property in the entire range of 210–280 nm is so far reported for AlN crystals grown along [0001] direction. The hetero-epitaxially grown crystals show even high mosaicity represented by a broader HRXRD FWHM peaks [47] and a high EPD value of 105 cm-2 revealed by preferential chemical etching. This obstructs the real utilization of AlN as a substrate material for the fabrication of DUV light sources.

6 Conclusions

In the present work, an insight into the common encountered difficulties in growing AlN on SiC substrates by the hetero-epitaxial growth approach by the PVT method has been given. Unlike other approaches and methods, this does not require any additional steps of growing a AlN buffer or patterning on the substrate to reduce dislocations, propagation of micro-pipes defects and also, to avoid cracks due to lattice and thermal expansion coefficient mismatches of substrate material and AlN. Many issues related to this approach, namely micro-hole defect, crystal coloration, crystal cracking, 3D multi-source nucleation, high defect density and low growth rate, have been dealt in detail. Possible suggestions are described for overcoming most of these issues, if not all, like process modification and controlling the initial stages of growth. AlN crystals exhibit different colours such as brown, green, golden yellow or light yellow depending on the growth set-up materials/environments and their corresponding impurity incorporation into the growing crystal. The discussions and results are supported by the various data from different analytical methods. This crystal coloration influences the structural property of the crystal as assessed by wet chemical etching and EPMA. By applying suitable modifications, viz. temperature gradient, controlling the SiC substrate decomposition, a successful growth of bulk AlN single crystals of slightly bigger than 1 inch diameter has been attained using the hetero-epitaxial approach. X-ray rocking curves of the good samples usually show a narrow FWHM value, and the EPDs are also only in the order of 105 cm-2. The hetero-epitaxial approach using SiC substrates has an advantage of achieving wide-area AlN substrates (> 4 inch) within a reasonable time frame. Large diameter SiC wafers (about 6 inch) are readily available with relatively moderate cost. As the cost of the SiC wafers may go down further in the next years, this approach would even then be more attractive if the problems like mosaicity, comparatively high defect density are solved. We believe that a possibility of 3–4-inch AlN wafers for device community is imminent in future and further scaling up beyond that size would also be feasible.