Aluminum Nitride Single Crystals Research Articles

Evolution of 2-inch AlN single crystal grown on SiC seed via PVT method

The crystallization of aluminum nitride (AlN) hetero-epitaxial seeding on silicon carbide (SiC) by physical vapor transport (PVT), was systematically studied using XRD, UV–Vis, PL, SIMS, Raman and stress simulation. The evolution of crystal quality, transmittance, purity and residual stress of 2-inch AlN crystal in the growth process are described. The crystal quality of AlN growth layer gradually improves with the crystal thickness, as reflected in the decrease of XRD full width at half maximum (FWHM) from higher than 300 arcsec to 85 arcsec, SIMS yielded Si concentrations decreases from 1020 cm−3 to 1018 cm−3, absorbance coefficient @320 nm from 56 cm−1 to 9 cm−1, and simulated residual stress from 270 MPa to 50 MPa, indicating a comprehensively improved quality by simply increasing the crystal thickness of AlN crystal on SiC seed. Furthermore, a 2-inch crack-free AlN single crystal with high quality was successfully fabricated by inducing a layered structure.

Growth and properties of tantalum carbide coatings on graphite by TRD technique

The tantalum carbide-coated graphite material exhibits remarkable chemical stability, corrosion resistance, and heat shock resistance, enabling it to demonstrate excellent stability during the growth of wide bandgap semiconductors. Its application significantly enhances the quality of third-generation semiconductor crystals. Nevertheless, the high cost of this material hinders its widespread utilization in the field of wide bandgap semiconductors. To address this challenge, this study employs the Thermo-reaction deposition and diffusion (TRD) technique to successfully fabricate a tantalum carbide coating on the surface of graphite. Through thermodynamic analysis, this article validate the thermodynamic feasibility of preparing tantalum carbide coatings using the TRD process. To achieve a dense and uniform TaC coating on the graphite surface, we systematically investigate the effects of reducing agent content, reaction temperature, and reaction time on the microstructure of the coating. Experimental research has found that a molar ratio of B4C to Ta2O5 of 1:0.64 optimizes the coating uniformity. Deviations from this ratio, either higher or lower, can lead to coating inhomogeneity or even failure in coating formation. Additionally, this article determine that a reaction temperature range of 1200–1400 °C, coupled with a reaction time not exceeding 6 h, represents the most suitable conditions for preparing TaC coatings on the graphite surface. Finally, to evaluate the practical performance of the coated graphite, this article simulate the corrosive environment encountered during the physical vapor transport method for growing aluminum nitride single crystals. The results demonstrate a significant improvement in the corrosion resistance of the coated graphite. This enhancement is primarily attributed to the formation of a dense TaC coating on the graphite surface, which exhibits remarkable stability at elevated temperatures.

Research on the Stability of Different Polar Surfaces in Aluminum Nitride Single Crystals

Wurtzite aluminum nitride (AlN) crystal has a non-centrosymmetric crystal structure with only a single axis of symmetry. In an AlN crystal, the electronegativity difference between the Al atom and N atom leads to a distortion of electron cloud distribution outside the nucleus and a spontaneous polarization (SP) along the c-axis direction. The N-polar surface along the directions of [000-1] has higher surface energy than the Al-polar surface along the directions of [0001]. Due to the different atomic arrangement, Al atoms on the Al-polar surface bond with O and OH− in the environment to generate Al2O3·xH2O, which prevents the reaction from occurring inside the crystal. After the Al2O3·xH2O dissolve in an alkaline environment, N atoms have three dangling bonds exposed on the surface, which can also protect OH− from destroying the internal Al-N bonds, so the Al-polar surface is more stable than the N-polar surface.

Growth of Spontaneous Nucleation AlN Crystals by Al-Base Alloy Evaporation in Nitrogen Atmosphere

Aluminum nitride (AlN) crystals with areas ranging from 1 mm2 to 2 mm2 were successfully grown through spontaneous nucleation at 1700 °C using a modified vapor transport method. In this approach, Cu–Al alloy served as the source of aluminum (Al), and nitrogen (N2) was employed as the nitrogen source. The morphology and crystalline quality of the AlN crystals were characterized by a stereo microscope, Raman spectrometer, photoluminescence (PL) and secondary-ion mass spectrometry (SIMS). Deposited on the graphite lid, the as-grown AlN crystals exhibited both rectangular and hexagonal shapes, identified as m-plane and c-plane AlN, respectively, based on Raman spectroscopy. The full width half maximum (FWHM) values of E2 (high) for the rectangular and hexagonal grains were measured to be 6.00 cm−1 and 6.06 cm−1, respectively, indicating high crystalline quality. However, PL and SIMS analysis indicated the presence of impurities associated with oxygen in the crystals. This paper elucidates the growth mechanism of the modified vapor transport method and highlights the role of the Cu–Al alloy in sustaining reactions at lower temperatures. The addition of copper (Cu) not only facilitates sustainable reactions, but also provides a novel perspective for the growth of AlN single crystals.

Optical activation of implanted lanthanoid ions in aluminum nitride semiconductors by high temperature annealing

Lanthanoid (Ln)-doped aluminum nitride (AlN) semiconductors are one candidate for optoelectric devices and single photon sources, although their optical properties are less understood. We clarify the room temperature optical properties of Ln ion implanted single crystal AlN semiconductors and their changes upon thermal annealing by up to 1700 °C. Photoluminescence (PL), cathodoluminescence (CL), and time-resolved PL for praseodymium (Pr), europium (Eu), and neodymium (Nd) ion implanted AlN are analyzed. Recovery of implantation induced damages and thermal diffusion of implanted Ln ions by thermal annealing are also investigated. Our systematic studies reveal that there is a trade-off between optical activation of implanted Ln ions due to recovery of implantation damage and deactivation (quenching) due to complex formation and aggregation of Ln ions. The PL intensity of implanted Pr ions increases with increasing annealing temperature in the case of high-dose implantation (above 1020 cm-3), whereas it rather decreases in the case of low-dose implantation (below 4 × 1019 cm-3). However, the PL intensity is significantly reduced after annealing at 1700 °C in both cases, indicating that the quenching factor is dominant in this temperature range.

High speed surface acoustic wave and laterally excited bulk wave resonator based on single-crystal non-polar AlN film

One approach to extend the acoustic applications of aluminum nitride (AlN) in the GHz frequency range is to take advantage of the piezoelectric performance and high acoustic velocity (∼11 350 m/s) along the c-axis of this material. In particular, in the case of high-frequency micro-electromechanical systems, it should be possible to simplify the construction of resonators by using a-plane AlN-based structures. In the work described in this Letter, a single-crystalline a-plane AlN layer on an r-plane sapphire substrate is obtained by combining sputtering and high-temperature annealing. Based on this non-polar AlN, a resonator with only planar interdigital transducer electrodes is fabricated. Experiments on this resonator reveal simultaneous excitation of an anisotropic Rayleigh surface acoustic wave (SAW) at 2.38 GHz and a laterally excited bulk acoustic wave (LBAW) at 4.00 GHz. It is found that the Rayleigh SAW exhibits outstanding performance, with a quality factor as high as 2458 and great stability under variations in temperature. The LBAW at 4.00 GHz is excited by pure planar interdigitated electrodes without the need for any cavity or bottom electrode structure, thus demonstrating a promising approach to the construction of high-frequency resonators with a relatively simple structure.

A solar-blind vacuum-ultraviolet photodetector based on free-standing lamellar aluminum nitride single crystal

High-quality aluminum nitride (AlN) crystals are the key material for the development of high-performance solid-state solar-blind vacuum-ultraviolet (VUV) photodetectors. However, the commonly used epitaxial method to grow AlN crystals would limit this development due to the existence of indispensable substrates. This study addresses this issue using free-standing lamellar AlN single crystals that are grown using the physical vapor transport method. The large lateral dimension of the crystal enables the construction of an Au-AlN-graphene van der Waals heterojunction, which can function as a vertical VUV photodetector with the graphene serving as the light window. The asymmetric junctions formed on the two sides of the crystal and the limited penetration of the VUV endow the device with a bias polarity-dependent photoresponse feature arising from different photoelectric processes. Furthermore, the device demonstrates a high responsivity of 5.77 A W−1 and a high specific detectivity of 1.71 × 1013 cm Hz1/2 W−1 under the illumination of a 193 nm laser. The high crystallinity of the AlN guarantees a high spectral selectivity of responsivity with a 193 nm/280 nm rejection ratio of 3 × 102. This work would inspire the development of wide-bandgap-semiconductor-based VUV photodetectors in terms of methodology and mechanism.

Atomic understanding of elastic-plastic deformation and crack evolution for single crystal AlN during nanoscratch

Single crystal aluminum nitride (AlN) as a new generation of ultra-wide band gap semiconductor materials is the preferred substrate material for high power, high RF devices, but single crystal AlN is hard and difficult to process. To obtain high-quality substrate requires comprehensive understanding of the material deformation and removal mechanism. In this paper, the molecular dynamics simulation method is used to scratch the AlN on the (0001) plane. Set up multiple sets of scratch depths to study the elastic-plastic deformation, material removal mechanism and crack evolution of single crystal AlN. The simulation results show that with the increase of scratch depth, the single crystal AlN workpiece undergoes elastic deformation to continuous plastic deformation and then to brittle fracture. The deformation mode in the elastic stage is phase-change, from wurtzite-structure to graphite-like (GL) phase. The plastic deformation of single crystal AlN is achieved by dislocations moving on the basal and pyramidal planes, and horizontally extended stacking faults. There is almost no subsurface damage below the stacking faults. When the scratch depth exceeds 6 nm, the material will undergo brittle fracture, and the fracture surface is the cleavage surface {10-10} of single crystal AlN.

Laser Doppler vibrometry for piezoelectric coefficient (d33) measurements in irradiated aluminum nitride

Sensors used for experiments in advanced reactors must survive under harsh environments. Few material systems can be used to construct sensors viable for extreme conditions. Aluminum nitride (AlN) is one such material because it has high thermal stability and radiation resistance, and sustains good piezoelectric and dielectric properties at high temperatures. In this work, the piezoelectric coefficient d 33 of the AlN single-crystal with thermal and irradiation damage was investigated using an indirect method with a laser Doppler vibrometer (LDV). Surface electrodes were deposited on the AlN samples, and the vibration response of the samples to an applied voltage was monitored using the LDV as a function of the excitation frequency. The d 33 estimation was based on the excitation voltage and the thickness-mode displacement extracted from the LDV measurements. Six AlN substrates were irradiated with 8 MeV Al 2＋ at three fluences (10 15 , 10 16 , and 10 17 ions/cm 2 ) and two temperatures (300 ° C and 500 ° C ). The d 33 for the six irradiated samples and one pristine sample were measured, and the measurement uncertainty was estimated based on five repeated tests. All samples were also measured by a commercial piezometer for comparison. The experimental results demonstrate that the piezoelectric coefficients obtained by LDV were about 0.8–1.16 pm/V lower than those obtained by the piezometer. With the compensation of the clamping effect, the corrected LDV values are similar to the piezometer results. Both show similar trends in all samples, which validates the feasibility of the proposed method for d 33 measurement. Based on the LDV results, the irradiated samples show a 12%–22% decrease in d 33 compared with the pristine samples. The samples irradiated under the same fluence at a higher temperature (500 °C) demonstrated a lower d 33 than those at 300 °C. The effect of the retro-reflective tape, sample temperature, and sample size on the d 33 measurement were also studied. • Laser vibrometry is used for piezoelectric coefficient (d 33 ) measurement of AlN. • AlN single crystals are irradiated with Al 2+ at three fluences and two temperatures. • AlN irradiated at a higher temperature shows a lower d 33 . • The d 33 of irradiated AlN at 10 17 and 500 °C is 22% lower than that of the pristine AlN. • The d 33 measured using laser vibrometry is validated by piezometer measurement.

(Invited) Development of 3-inch AlN Single Crystal Substrates

Aluminum nitride (AlN) holds great promise as a substrate for electronic devices such as high-power switches, high power density, high frequency and high operating temperature devices for RF. This promise is largely due to its superior properties, for instance its ultra-wide bandgap (UWBG) of 6.2 eV, high thermal conductivity (> 290 W/m*K), high melting point (> 2800°C), relatively good chemical resistance, similarity of its lattice structure and parameters to that of Gallium nitride (GaN), high hardness value of about 12 GPa (at 1 N using Vickers indenter), etc. Presently AlN substrates are successfully used for fabrication of ultraviolet (UV) light emitting diodes (LEDs) operating at wavelengths shorter than 280 nm, i.e., in the UVC region of the spectrum. The AlN-based UVC LEDs are employed in water disinfection, surface cleaning, air purification, and environmental sensing. Several factors including substrate availability, size, production capacity, quality, and cost influence the commercialization of the aluminum nitride both as an UWBG and as an opto-electronic substrate. In this work we present the present and future outlook for the AlN substrates made using Physical Vapor Transport (PVT) growth technique.The PVT crystal growth method of aluminum nitride is carried out in a Tungsten crucible that accommodate the AlN source material as well as the (0001) oriented AlN seed. The thermal gradient is provided by an rf-heating plus appropriate shielding and insulation. In this setup, the sublimed source material is transported to the Al-polar face of the seed where it incorporates into a single crystal. The AlN crystals are then oriented, sliced, and polished. The resulting 2-inch substrates are tested to meet certain specification needed for further epitaxial growth and LED processing. The two-inch AlN wafer specification as well as number of physical properties and purity were previously reported [1].In addition to the 2-inch AlN crystal growth there is an ongoing effort to increase the substrate diameter. Large-diameter (e.g. 100 mm) AlN substrates are a practical requirement benefiting high power electronic devices. Again, PVT growth method is used to expand the crystal size and diameter. However, several challenges have yet to be resolved en route to 100 mm diameter crystals. These challenges are associated with increased thermally induced stress, the need for better control of the axial/radial thermal gradients, vendor ability to provide materials, maintaining crystal quality during expansion, etc.Our PVT growth technique demonstrates high crystal growth yield which allows to contain relatively low production cost. Furthermore, we have established a high-capacity process capable to produce many thousands of 2-inch AlN substrates per year. Our current crystal growth process also demonstrated greater than 3-inch diameter single-crystal AlN substrates; properties of such wafers will be reported. Robert T Bondokov et al 2021 ECS Trans. 104 37

Investigation of the atomistic behavior in nanofinishing single-crystal aluminium nitride with hydroxyl radical ∙OH environment

The effect of depth of pressing and sliding velocity on chemical reaction, wear and atom removal mechanisms was investigated in nanofinishing single-crystal aluminum nitride with hydroxyl radical ∙OH environment. The simulation results show that aluminum nitride can be oxidized by hydroxyl radical ∙OH from the perspective of free energy change. Only chemical action, the decomposed atoms in the solution form bonds with the Al atoms on the aluminum nitride surface, and the bonds are expressed in the form of O-[Al]n, H-[Al]n and OH–[Al]n. However, no Al atoms were discovered to break away from the substrate. The potential energy of substrate and the number of Al-O bonds are positively correlated with the depth of pressing and sliding velocity, and the degree of chemical reaction can be promoted by strong mechanical behavior. The tangential force relies on interfacial Al-C and AlOC bonds, the more interfacial bonds between substrate and abrasive, the more severe wear of the substrate surface. The sliding of diamond abrasive leads to the amorphous evolution of the crystal structure, and the separation of atoms from the initial position produces defects, which destroys the stable covalent bond structure of aluminum nitride. As a result, the total number of dangling bonds (i.e. unsaturated bond) of surface layer atoms increases, and the greater the depth of pressing and velocity, the more obvious this phenomenon will be, and the more atoms will be removed. In this study, atomic removal behavior of aluminum nitride was systematically analyzed at atomic scale, which will provide theoretical guidance for ultra-precision and low-damage processing of aluminum nitride.

Dielectric and pyroelectric properties of AlN single-crystal layers grown by chloride-hydride epitaxy

This work presents the results of a comparative study of dielectric and pyroelectric properties of aluminum nitride (AlN) single-layer and multilayer samples with a thickness of 100 ÷ 200 μm grown in the [0001] direction by chloride-hydride epitaxy. A silicon wafer covered by silicon carbide nanolayer grown by solid-phase substitution (SiC/(111)Si) was used as a substrate for subsequent growth of AlN single crystalline layers. The samples were formed by layer-by-layer growth. It is shown that such method makes it possible to change the orientation of the polar hexagonal axis in AlN single crystals.

Research on the Stability of Different Polar Surfaces in Aluminium Nitride Single Crystals

Research on the Stability of Different Polar Surfaces in Aluminium Nitride Single Crystals

High-temperature diffusion of the acceptor impurity Be in AlN

The high-temperature diffusion of an acceptor impurity of beryllium (Be) into bulk single-crystal aluminum nitride (AlN) has been studied. It is shown that the introduction of Be leads to the appearance of green luminescence of AlN, which is stable at room temperature and is observed over the entire thickness of the sample. It was shown by the method of luminescence analysis that the Be diffusion process is most efficiently realized in the temperature range from 1800 to 2100oC and is characterized by extremely high diffusion coefficients D=10-7 cm2/s and 10-6 cm2/s, respectively. It is shown that a prolonged diffusion process (t≤1 hour) at a temperature of 2100oC leads to concentration quenching of the luminescence of near-surface AlN layers with a thickness of ~80 μm, which makes it possible to estimate the concentration of beryllium impurities in the near-surface layer on the order of ~1019 cm-3. Keywords: AlN PVT doping diffusion.

Высокотемпературная диффузия акцепторной примеси Ве в AlN

The high-temperature diffusion of an acceptor impurity of beryllium (Be) into bulk single-crystal aluminum nitride (AlN) has been studied. It is shown that the introduction of Be leads to the appearance of green luminescence of AlN, which is stable at room temperature and is observed over the entire thickness of the sample. It was shown by the method of luminescence analysis that the Be diffusion process is most efficiently realized in the temperature range from 1800°C to 2100°C and is characterized by extremely high diffusion coefficients D = 10-7 cm2/s and 10-6 cm2/s, respectively. It is shown that a prolonged diffusion process (t ≥ 1 hour) at a temperature of 2100°C leads to concentration quenching of the luminescence of near-surface AlN layers with a thickness of ≈ 80 μm, which makes it possible to estimate the concentration of beryllium impurities in the near-surface layer on the order of 10E19 cm-3.

The Effect of the Crucible on the Temperature Distribution for the Growth of a Large Size AlN Single Crystal.

The appropriate distribution of temperature in the growth system is critical for obtaining a large size high quality aluminum nitride (AlN) single crystal by the physical vapor transport (PVT) method. As the crystal size increases, the influence of the crucible on the temperature distribution inside the growth chamber becomes greater. In order to optimize the field of temperature and study the specific effects of various parts of the crucible on the large size AlN single crystal growth system, this study carried out a series of numerical simulations of the temperature field of two crucibles of different materials and put forward the concept of a composite crucible, which combines different materials in the crucible parts. Four composite crucible models were established with different proportions and positions of tantalum carbide (TaC) parts and graphite parts in the crucible. Calculations reveal that different parts of the crucible have different effects on the internal temperature distribution. The axial temperature gradient at the crystal was mainly governed by the crucible wall, whereas the temperature gradient was determined by the integrated effect of the crucible lid and the crucible wall in the radial direction. One type of composite crucible was chosen to minimize the thermal stress in grown AlN crystal, which is applicable to the growth of large sized AlN crystals in the future; it can also be used to grow AlN single crystals at present as well.

Two-Inch Aluminum Nitride (AIN) Single Crystal Growth for Commercial Applications

Aluminum Nitride (AlN) is an Ultra-wide Bandgap (UWBG) semiconductor that attracts significant interest for its potential application for high power switches, high power density, high frequency and high operating temperature for rf. Currently, AlN substrates are being used for ultraviolet (UV) light emitters at wavelengths shorter than 300nm including UVC Light Emitting Diodes (UVC LED) as well as laser diodes. Successful commercialization of the aluminum nitride as an UWBG substrate depends on several factors such as substrate availability, size, quality, and cost. Here we report on the current status of the bulk growth of AlN single-crystals as well as their subsequent processing for making substrates.The AlN crystals are grown using a proprietary physical vapor transport (PVT) technique where a sublimation-recondensation approach is utilized. This method shows very high potential in terms of growing large (> 2-in in diameter) bulk AlN crystals. However, some challenges have yet to be resolved en route to 100 mm diameter crystals. The crystal growth is carried out in a Tungsten crucible that contains the AlN seed and the source material. The AlN source material sublimes and decomposes into Aluminum atoms and Nitrogen molecules. These species are then transported to the seed where they form AlN and incorporate into the growing crystal along the c-axis. Aluminum nitride is used as a native seed utilizing the Al-polar face. Increasing the crystal dimensions demands careful tailoring of thermal gradients to lower the thermally induced stresses and achieve high crystalline quality. Currently, AlN single crystals with diameters > 2 inch are grown using the method described above and larger diameters are possible with appropriate furnace design. Large bulk crystals could be beneficial for production of non-polar and semi-polar substrates which are used to reduce the internal electric fields due to polarization.The AlN crystals are sliced into c-face oriented wafers and subsequently polished. Several substrate parameters are tracked such as the wafer bow, thickness variation, surface roughness, and sub-surface damage. The two-inch AlN wafers have a radius of curvature ≥ 20 m, RMS ≤ 5 Å, and zero sub-surface damage. The crystalline quality is measured by X-ray diffractometry (XRD) rocking curves, etch pit density, and photo-elastic birefringence pattern (cross-polar imaging). Narrow rocking curves for both symmetric and asymmetric reflections with full width at half maximum (FWHM) of 6 - 10 arcsec are measured across the whole the wafer area. The AlN wafers are transparent in the UV region. The graph shows a 52-point map of the absorption coefficient values 8 – 10 cm-1 at 265nm wavelength for a 2-in AlN substrate. The very low UV absorption is linked to the material purity measured by Secondary Ion Mass Spectrometry (SIMS) and Glow Discharge Mass Spectrometry (GDMS). Excellent material purity ensures achievement of high thermal conductivity, a crucial parameter for an UWBG substrate. We measure the thermal conductivity using a Xenon flash method conforming to the ASTM E1461-01 or ASTM E1461-13 standards. Using this method directly on 2-inch AlN substrate we obtain room temperature thermal conductivity 293 W/m*K at room temperature (RT) and with very high slope of about -1.6 for the temperature range from RT to 300°C suggesting minimal phonon scattering.AlN substrates made using the above described methods are used to fabricate high performance, long lifetime UVC LEDs for air purification, water disinfection, surface cleaning, and environmental sensing. High quality pseudomorphic epitaxial layers are grown using Metal-Organic Chemical Vapor Deposition (MOCVD) process to form the device structure. Commercially available WD product (Klaran™) achieve germicidal power outputs that exceed 80 mW with drive currents up to 700 mA at a nominal wavelength of 265 nm. The devices feature long lifetimes exceeding 6000 hours at L50B10 as well as a wide radiation pattern with a viewing angle of 130°. Figure 1

Prismatic Slip in AlN Crystals Grown By PVT

Aluminum nitride (AlN) single crystals are desired as substrates for nitride based electronic and optoelectronic applications[1]. A low density of dislocations and other structural defects in bulk AlN substrates is in demand to grow high-quality heteroepitaxial epilayers. Physical vapor transport (PVT) grown AlN crystals possess low density dislocations and other structural defects[2]. In AlN crystals grown by PVT, basal plane slip is the most frequently observed deformation mechanism. However, prismatic slip takes place as well in such crystals [3, 4].When the diameter of AlN wafers expands to 50 mm and larger, managing the thermal gradients in the PVT growth chamber becomes critical for reducing thermal stresses that cause deformation. Recent synchrotron X-ray topography [5] studies of 50 mm diameter AlN wafers showed that while most wafers contained few to no basal plane dislocations (BPDs), some wafers possessed a 6-fold BPD dislocation pattern [2] which was aligned along the <11-20> directions. This configuration indicated that prismatic slip had likely occurred. Screw typed prismatic dislocations cross slipped from the prismatic plane onto the basal plane, and underwent basal plane slip, leading to dislocation multiplication. To investigate the origins of prismatic slip, a radial thermal gradient model was established showing[6] the resolved shear stress across the entire area of the crystal boule during growth. The results from the model were compared with experimental observations to investigate the role of radial thermal gradients in the nucleation of prismatic slip in AlN during bulk growth. This insight will help guide efforts to minimize structural defects in PVT AlN.

Two-Inch Aluminum Nitride (AIN) Single Crystal Growth for Commercial Applications

Aluminum nitride (AlN) bulk single crystals were grown using physical vapor transport method. The resulting wafers were characterized according to the requirements for electronic device applications. Currently many thousands per year of such 2-inch wafers are used as substrates for UVC-LEDs emitting in the range 215 nm – 270 nm, which is the most efficient germicidal range for many pathogens. Highest crystallinity and the lowest published UV-transparency are confirmed by standard analytical techniques.

Common issues in the hetero-epitaxial seeding on SiC substrates in the sublimation growth of AlN crystals

Aluminium nitride (AlN) is a futuristic material for efficient next-generation high-power electronic and optoelectronic applications. Sublimation growth of AlN single crystals with hetero-epitaxial approach using silicon carbide substrates is one of the two prominent approaches emerged, since the pioneering crystal growth work from 1970s. Many groups working on this hetero-epitaxial seeding have abandoned AlN growth altogether due to lot of persistently encountered problems. In this article, we focus on most of the common problems encountered in this process such as macro- and micro-hole defects, cracks, 3D-nucleation, high dislocation density, and incorporation of unintentional impurity elements due to chemical decomposition of the substrate at very high temperatures. Possible ways to successfully solve some of these issues have been discussed. Other few remaining challenges, namely low-angle grain boundaries and deep UV optical absorption, are also presented in the later part of this work. Particular attention has been devoted in this work on the coloration of the crystals with respect to chemical composition. Wet chemical etching gives etch pit density (EPD) values in the order of 105 cm-2 for yellow-coloured samples, while greenish coloration deteriorates the structural properties with EPD values of at least one order more.