Aluminum Nitride Single Crystals Research Articles

A comparison of the thermal decomposition mechanism of wurtzite AlN and zinc blende AlN

The sublimation route is one of the primary and most significant methods for the synthesis of an aluminum nitride (AlN) single crystal. Its long synthesis time and high reaction temperature, however, limit the production of its commercial product. In this work, we applied HSC Chemistry 6 software, ab initio molecular dynamics, and X-ray diffraction to investigate the thermal decomposition of AlN. We calculated the decomposition temperatures of AlN under vacuum and simulated the decomposition mechanism of AlN by the ab initio molecular dynamics method. According to the thermodynamic calculations, the decomposition temperature of AlN decreased following a decrease in the system pressure. The ab initio molecular dynamics results indicated that wurtzite-type AlN (w-AlN) was decomposed by the layer-by-layer mechanism and followed a decomposition reaction equation of AlN → Al(g) + 0.29N2(g) + 0.42N(g), which originated from the inequality sp3 hybridization. The zinc-type AlN (z-AlN) decomposed from the surface to interior of the structure because of the equality of the sp3 hybridization, and the z-AlN decomposition reaction equation followed AlN → Al(g) + 0.5N2(g). The AlN decomposition experiments further verified that Al(g) was the product of the wurtzite-type AlN thermal decomposition. This work can provide valuable information for the preparation of the AlN single crystal.

Octave-spanning visible supercontinuum generation from an aluminum nitride single crystal pumped by a 355 nm nanosecond pulse

Supercontinuum generation (SC) of more than one octave spectrum spanning covering from 400 nm to 820 nm was achieved by pumping a piece of aluminum nitride (AIN) single crystal using a nanosecond 355 nm ultraviolet laser. The AlN with a thickness of ∼0.8 mm was grown by an optimized physical vapor transport technique and polished with solidification technology. Compared to previously reported ones, the achieved visible SC exhibited the broadest spectrum spanning from bulk materials pumped by a nanosecond pulse laser. The visible supercontinuum in AlN presents new opportunities for bulk material-based white light SC and may find more potential applications beyond typical applications in integrated semiconductive photoelectronic devices.

Thermal Analysis of High-Power Flip-Chip-Bonded Photodiodes

The performance of high-power photodiodes flip-chip bonded on polycrystalline aluminum nitride (AlN), single-crystal AlN, and diamond submounts are compared. The thermal boundary conductance (inverse of the thermal boundary resistance) between submount/titanium interfaces was measured and found to be the primary impedance to heat dissipation. Thermal profiles of the flip-chip-bonded devices were simulated to project the power density at failure, which is found to be inversely proportional to the diameter of the photodiodes.

PVT growth of AlN single crystals with the diameter from nano- to centi-meter level

Physical vapor transport (PVT) is the most successful and widely used approach for bulk aluminum nitride (AlN) single crystals. During the process of PVT growing AlN crystals, crucible materials, the growth setup, and the growth parameters (e.g., temperature distribution, growth pressure) are crucial. This work proposes a detailed study on the PVT growth of single AlN crystals with sizes ranging from nanometers to centimeters. AlN crystals with different sizes are grown by spontaneous nucleation. Furthermore, it discusses and contrasts the growth conditions and mechanisms of AlN crystals with different sizes. The structural and optical properties of the AlN crystals are also involved.

Optical nonlinear and electro‐optical coefficients in bulk aluminium nitride single crystals

Bulk aluminium nitride single crystals grown by physical vapour transport were characterised for their nonlinear optical properties. The two independent nonzero optical nonlinear coefficients were determined by the Maker Fringe method, with a LiNbO3 plate as a reference. At a wavelength of 1030 nm, the measured coefficient |d33| = (4.3 ± 0.3) pm V−1 and |d31| = |d33|/(45 ± 5). Michelson interferometer was used to measure two unclamped electro‐optic coefficients to give r33 = (1.16 ± 0.13) pm V−1 and r13 = (0.11 ± 0.02) pm V−1 at a wavelength of 633 nm. By calculating the dispersion relation for d33, we show that AlN is a very promising material for quasi phase‐matched frequency conversion into UV.

Investigation of AlN thin film growth on MgO(111) substrates using low temperature helicon sputtering system

An aluminum nitride (AlN) thin film has been grown on annealed magnesium oxide (MgO) (111)-plane substrate using low temperature helicon sputtering system. Both AlN films on as-received and annealed MgO(111) substrate are single crystalline with AlN[0001] || MgO[111]. X-ray rocking curve shows that AlN film on annealed MgO exhibits superior crystalline quality, which means more suitable for AlN crystal growth. Two different growths were found for the deposition of AlN on annealed MgO. It is believed that the partially recovered substrate surface caused by annealing process provides atomic smooth surface terraces with small lattice mismatch for AlN crystal to grow in 2D mode, enhance grain size, and thus reduce the dislocation density. This is the first time demonstrated for the growth mechanism of single crystal AlN thin film prepared on MgO(111) by sputtering system.

Environmentally friendly method to grow wide-bandgap semiconductor aluminum nitride crystals: Elementary source vapor phase epitaxy.

Aluminum nitride (AlN) has attracted increasing interest as an optoelectronic material in the deep ultraviolet spectral range due to its wide bandgap of 6.0 eV (207 nm wavelength) at room temperature. Because AlN bulk single crystals are ideal device substrates for such applications, the crystal growth of bulky AlN has been extensively studied. Two growth methods seem especially promising: hydride vapor phase epitaxy (HVPE) and sublimation. However, the former requires hazardous gases such as hydrochloric acid and ammonia, while the latter needs extremely high growth temperatures around 2000 °C. Herein we propose a novel vapor-phase-epitaxy-based growth method for AlN that does not use toxic materials; the source precursors are elementary aluminum and nitrogen gas. To prepare our AlN, we constructed a new growth apparatus, which realizes growth of AlN single crystals at a rate of ~18 μm/h at 1550 °C using argon as the source transfer via the simple reaction Al + 1/2N2 → AlN. This growth rate is comparable to that by HVPE, and the growth temperature is much lower than that in sublimation. Thus, this study opens up a novel route to achieve environmentally friendly growth of AlN.

Optical properties of aluminum nitride single crystals in the THz region

We report on measurements of the refractive indices and the absorption in bulk single crystals of aluminum nitride, in the region from 1 to 8 THz. The birefringence is approximately 0.2 and is larger than in the optical frequency range. Both indices exhibit normal dispersion with no pronounced absorption resonances. Optical power loss coefficients are approximately 2 cm−1 and 4 cm−1 and the estimated static dielectric constants are 7.84 and 9.22, for the ordinary and extraordinary polarization, respectively.

Helical growth of aluminum nitride: new insights into its growth habit from nanostructures to single crystals.

By understanding the growth mechanism of nanomaterials, the morphological features of nanostructures can be rationally controlled, thereby achieving the desired physical properties for specific applications. Herein, the growth habits of aluminum nitride (AlN) nanostructures and single crystals synthesized by an ultrahigh-temperature, catalyst-free, physical vapor transport process were investigated by transmission electron microscopy. The detailed structural characterizations strongly suggested that the growth of AlN nanostructures including AlN nanowires and nanohelixes follow a sequential and periodic rotation in the growth direction, which is independent of the size and shape of the material. Based on these experimental observations, an helical growth mechanism that may originate from the coeffect of the polar-surface and dislocation-driven growth is proposed, which offers a new insight into the related growth kinetics of low-dimensional AlN structures and will enable the rational design and synthesis of novel AlN nanostructures. Further, with the increase of temperature, the growth process of AlN grains followed the helical growth model.

Fabrication and characterisation of non-polar M-plane AlN crystals and LEDs

Aluminium nitride crystals and devices grown along the polar c-axis direction are affected by the strong spontaneous and piezoelectric polarisation fields. It is necessary to grow non-polar plane aluminium nitride crystals in order to solve the problem. The M-plane surface-oriented aluminium nitride single crystals were grown by physical vapour transport on a resistance heating furnace, which was designed by ourselves and included two heaters on sublimation region and crystalline region, respectively. The M-plane aluminium nitride light-emitting diode was further fabricated and characterised. The current–voltage characteristic showed a Schottky diode behaviour. A white light emission from the biased diode can be clearly seen by naked eyes in a dark place at room temperature. The ultra-wide electroluminescent spectrum covered from deep UV (200 nm) to infrared (1100 nm) and centred at 496 nm (2·5 eV). At last, the emission mechanism was discussed.

Microfluidic Injector Simulation With FSAW Sensor for 3-D Integration

This paper presents a possible creation of the optimized liquid sensors for the inkjet nozzles. The proposed focused surface acoustic wave (FSAW) device utilizing aluminum nitride (AlN) single crystal as the piezoelectric substrate is based on the pressure variation due to the continuous droplet ejector. The design, specification, and numerical simulation results are described. Comparisons between the output response of the conventional and concentric structures indicate a more efficient operation of the multiple-segment focused interdigital transducer (FIDT) structure. According to the angular spectrum of the plane wave theory, the amplitude field of FIDTs is calculated through that of straight interdigital transducers. The 3-D integrated model of the FSAW device has a number of advantages, such as the enhancement of the surface displacement amplitudes and an easier fabrication. It is able to detect the breakup appearance of the liquid in the droplet formation process. For the piezoelectric substrate AlN, it is compatible with the CMOS fabrication technology, leading to an inexpensive and reliable system. Moreover, for the proposed FIDTs with multiple straight segments, the acoustic energy is more optimized and focused near the center of the inkjet nozzle. The droplet generation process begins at an output voltage of roughly 0.074 V within 0.25 μs, and the background level of the attenuation of both the mechanical and electrical energy.

Sample size induced brittle-to-ductile transition of single-crystal aluminum nitride

Ceramics are known to be mechanically hard, chemically inert and electrically insulating for many important applications. However, they usually suffer from brittleness and have moderate strength that strongly depends on their microscopic structure. In this study, we report a size induced brittle-to-ductile transition in single-crystal aluminum nitride (AlN). When the specimen diameters are smaller than ∼3–4μm, AlN micropillars show metal-like plastic flow under room-temperature uniaxial compression. The unprecedented plastic strain of ∼5–10% together with the ultrahigh strength of ∼6.7GPa has never been achieved before. Transmission electron microscopy demonstrates that dislocations play a dominant role in the plasticity of the micro-sized AlN.

Achieving high thermal conductivity from AlN films deposited by high-power impulse magnetron sputtering

We report on thermal conductivity measurements of aluminum nitride (AlN) films using the fast pulsed photo-thermal technique. The films were deposited by high-power impulse magnetron sputtering with different thicknesses ranging from 1000 to 8000 nm on (1 0 0) oriented silicon substrates. The films were characterized by x-ray diffraction (XRD), Raman spectroscopy, profilometry, scanning electron microscopy and atomic force microscopy. The XRD measurements showed that AlN films were textured along the (0 0 2) direction. Moreover, x-ray rocking curve measurements indicated that the crystalline quality of AlN was improved with the increase in film thickness. The thermal conductivities of the samples were found to rapidly increase when the film thickness increased up to 3300 nm and then showed a tendency to remain constant. A thermal boundary resistance as low as 8 × 10−9 W−1 K m2 and a thermal conductivity as high as 250 ± 50 W K−1 m−1 were obtained for the AlN films, at room temperature. This high thermal conductivity value is close to that of an AlN single crystal and highlights the potential of these films as a dielectric material for thermal management.

Role of SiC substrate polarity on the growth and properties of bulk AlN single crystals

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and \((000\overline{1} )\) C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on \((000\overline{1} )\) C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum ≈380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (<2 wt%).

AlN 단결정 성장에 관한 연구

최근 관심이 높아지고 있는 GaN, SiC 단결정과 함께 자외선 LED 및 전력 반도체 용 기판 소재로서 응용성이 높은 질화갈륨(AlN, Aluminum Nitride) 단결정을 성장하였다. 아직 상용화된 AlN 기판은 없지만, 단결정 성장에 대한 연구가 이루어지고 있다. 본 연구에서는 국내 최초로 AlN 단결정의 성장 결과 직경 약 8 mm의 결정을 성장하였다. 성장된 단결정은 광학현미경으로 관찰하였으며, DCXRD를 통하여 결정성을 평가한 결과를 보고하고자 한다. Recently, it has been interested much that AlN (Aluminum Nitride) crystals can be applied to UV LEDs and high power devices as like GaN and SiC crystals. The reports about commercial grade of AlN wafers in the world have been absent, however several results for growth of large size of AlN single crystals have been reported from abroad. In this report, the result of AlN single crystals of a diameter of about 8 mm grown are reported. Optical microscopic characterization was applied to observe the form of the crystals and the crystal quality was evaluated by FWHM measurement by DCXRD rocking curve analysis.

Quantitative Annular Dark Field Electron Microscopy Using Single Electron Signals

One of the difficulties in analyzing atomic resolution electron microscope images is that the sample thickness is usually unknown or has to be fitted from parameters that are not precisely known. An accurate measure of thickness, ideally on a column-by-column basis, parameter free, and with single atom accuracy, would be of great value for many applications, such as matching to simulations. Here we propose such a quantification method for annular dark field scanning transmission electron microscopy by using the single electron intensity level of the detector. This method has the advantage that we can routinely quantify annular dark field images operating at both low and high beam currents, and under high dynamic range conditions, which is useful for the quantification of ultra-thin or light-element materials. To facilitate atom counting at the atomic scale we use the mean intensity in an annular dark field image averaged over a primitive cell, with no free parameters to be fitted. To illustrate the potential of our method, we demonstrate counting the number of Al (or N) atoms in a wurtzite-type aluminum nitride single crystal at each primitive cell over the range of 3-99 atoms.

Nucleation and Chemical Vapor Deposition Growth of Polycrystalline Diamond on Aluminum Nitride: Role of Surface Termination and Polarity

We have investigated the growth and atomic interface structures of diamond on aluminum nitride (AlN). The two-step chemical vapor deposition technique is used to control diamond nucleation density, crystal size, and AlN surface orientation and polarity. Highly uniform diamond layers with a nucleation density in the range of 105–1011 cm–2 and a grain size of 0.1–5 μm are fabricated. Crystallographically abrupt interfaces between polycrystalline diamond and single-crystal AlN(0001) layers have been observed via high-resolution transmission electron microscopy and electron energy-loss spectroscopy. A majority of the diamond crystals have been found to have the diamond(111)/AlN(0001) interface relationship. Atomistic models of the bonding mechanism at the heterointerface are used to elucidate experimental observations and the role of hydrogen plasma on the growth of diamond on AlN. Nonpolar and semipolar AlN surfaces have been found to have higher resistance to process plasma and led to better crystallinity o...

Longitudinal-Type Leaky Surface Acoustic Wave on LiNbO3 with High-Velocity Thin Film

The loss reduction of a longitudinal-type leaky surface acoustic wave (LLSAW) by loading with a dielectric thin film with a higher velocity than the substrate is proposed. An aluminum nitride (AlN) thin film was adopted as a high-velocity thin film, and the propagation properties of an LLSAW on an X36°Y-LiNbO3 (LN) substrate were investigated. First, the elastic constants c11 and c44 of an amorphous AlN (a-AlN) thin film deposited by RF magnetron sputtering were determined from the measured phase velocities of two SAW modes with mutually perpendicular particle motion, and they were 78 and 96% of those of a single-crystal AlN thin film. Next, from the theoretical calculation for the LLSAW on X36°Y-LN using the determined constants, it was found that the LLSAW attenuation can be reduced to zero by loading with an a-AlN thin film. Then, the propagation properties of the LLSAW on X36°Y-LN were measured by using an interdigital transducer pair. It was found that the losses due to bulk wave radiation can be reduced by loading with an a-AlN thin film.

The impact of film thickness and substrate surface roughness on the thermal resistance of aluminum nitride nucleation layers

Thickness dependent thermal conductivity measurements were made on aluminum nitride (AlN) thin films grown by two methods on the (0001) surfaces of silicon carbide (SiC) and sapphire substrates with differing surface roughness. We find that the AlN itself makes a small contribution to the overall thermal resistance. Instead, the thermal boundary resistance (TBR) of 5.1 ± 2.8 m2K/GW between the AlN and substrate is equivalent to 240 nm of highly dislocated AlN or 1450 nm of single crystal AlN. An order-of-magnitude larger TBR was measured between AlN films and SiC substrates with increased surface roughness (1.2 nm vs. 0.2 nm RMS). Atomic resolution TEM images reveal near-interface planar defects in the AlN films grown on the rough SiC that we hypothesize are the source of increased TBR.

Bulk AlN single crystal growth on foreign substrate and preparation of free-standing native seeds

Physical vapour transport growth of aluminium nitride (AlN) single crystals on silicon carbide (SiC) substrates has been optimised and crack-free, large-area, free-standing (0001) AlN wafers were prepared from the grown template crystals. 28 mm diameter single crystals without any polycrystalline surroundings were obtained. Different off-oriented substrates give rise to different growth modes. Sharp and symmetric line shapes of X-ray diffraction (XRD) rocking curves and high intense Raman phonon mode peaks prove high structural quality and homogeneity of the grown crystals. Full width at half maximum of the rocking curves is 72 arcsec for symmetric 00.2 reflection and 200 arcsec for asymmetric 10.3 reflection, representing a low screw as well as edge type threading dislocations. Wet chemical etching results also confirm the above XRD results and the estimated etch pit density is as low as 2–5 × 105 cm−2. The growth surfaces of all the crystals show only Al-polarity as inferred by the etching analysis. The concentration of silicon and carbon impurities incorporated from the SiC substrate decreases with the growth length of the AlN crystals. These impurities might play a decisive role in determining the optical properties of the crystal and be responsible for the absence of near-bandgap excitonic luminescence. Confocal Raman spectra show only the phonon modes allowed by the selection rules for the measured symmetry. The observed E2(high) phonon mode frequency very closely matches the reported stress-free phonon frequency of AlN. This work demonstrates that AlN templates prepared on SiC as a foreign substrate can be used as native seeds for the growth of further homo-epitaxial layers and crystalline boules.