AlN Samples Research Articles

Deep Traps in AlN Hydride Vapor Phase Epitaxy Film on Low-Temperature AlN/Sapphire

Deep trap states were examined in c-plane, Al-polar AlN epilayers, deposited via metal organic chemical vapor deposition on 270 nm thick AlN buffer layers on sapphire substrates. Contacts were created using e-beam deposited Ni through a shadow mask, with I-V characteristics revealing a trapped limited current (TLC) regime and voltage-dependent hysteresis upon light-emitting diode illumination. Thermally stimulated current and photothermal ionization current spectroscopy measurements demonstrated a prominent trap activation energy of approximately 0.75 eV and additional trap energies of 0.6, 0.4, 0.25, 1.05, and 1.1 eV. The observed differences in photocurrent responses between forward and reverse biases suggest that forward bias induces electron trapping at deeper levels, influencing the TLC behavior. Comparisons with bulk n-type AlN crystals from previous studies show similarities in deep trap spectra, suggesting commonality in trap characteristics across different AlN samples.

Microstructure, Thermal and Mechanical Properties of AlN-MgO Composites Prepared by Spark Plasma Sintering

AlN-MgO composites with different compositions were prepared by spark plasma sintering, and the effects of their composition on their microstructure, thermal properties, and mechanical properties were systemically investigated. MgO compositions in the AlN-MgO composites were controlled from 20 to 80 wt%. The results indicated that a phase transition did not occur during the sintering process, and different solid solutions were formed within the MgO and AlN lattices. The AlN-MgO composites exhibited finer-grain microstructures than those of the sintered pure AlN and MgO samples. Transmission electron microscopy analysis showed that both oxygen-rich, low-density grain boundaries and clean boundaries with spinel phases were present in the composites. The sintered pure AlN sample exhibited the highest thermal conductivity (53.2 W/mK) and lowest coefficient of thermal expansion (4.47 × 10<sup>-6</sup> /K) at 100 °C. And, the sintered pure MgO sample exhibited moderate thermal conductivity (39.7 W/mK) and a high coefficient of thermal expansion (13.05 × 10<sup>-6</sup> /K). With increasing MgO contents in the AlN-MgO composites, however, the thermal conductivity of the AlN-MgO composites decreased, from 33.3 to 14.9 W/mK, while their coefficient of thermal expansion generally increased, from 6.49×10<sup>-6</sup> to 10.73×10<sup>-6</sup> /K with increasing MgO content. The composite with an MgO content of 60 wt% exhibited the best mechanical properties overall. Thus, the composition and microstructure of AlN-MgO composites have a determining effect on their thermal and mechanical properties.

Structural and Optical Properties of Aluminium Nitride Thin Films Fabricated Using Pulsed Laser Deposition and DC Magnetron Sputtering on Various Substrates

Abstract Aluminium nitride thin films were fabricated using pulsed laser deposition and DC magnetron sputtering. Different technological parameters and the effects of different substrates on the optical and structural parameters of AlN samples were studied. An X-ray diffraction study was performed for the layer deposited on the Si3N4 substrate. A high-energy electron diffraction study was also carried out for the layer deposited on a KCl substrate. Transmission spectra of layers on quartz, sapphire, and glass substrates were obtained. An evaluation of the optical band gap of the obtained layers was carried out (Eg form 3.81 to 5.81 eV) and the refractive index was calculated (2.58). The relative density of the film (N1TN-AlN sample) is 1.26 and was calculated using the Lorentz-Lorentz relationship. Layers of aluminium nitride show an amorphous character with a polycrystalline region. It was shown that the properties of AlN films strongly depend on the method, growth conditions, and substrate used.

Realization of Low Dislocation Density AlN on Patterned Sapphire Substrate by Hydride Vapor‐Phase Epitaxy for Deep Ultraviolet Light‐Emitting Diodes

Herein, the characteristics of AlN epilayers grown directly on hole‐type patterned sapphire substrate (HPSS) by hydride vapor‐phase epitaxy (HVPE) are reported. To investigate the effect of HPSS, the threading dislocation densities (TDDs) of AlN films grown simultaneously on HPSS and flat sapphire substrate (FSS) are analyzed by transmission electron microscopy. The corresponding TDD is measured to be 4.38 × 108 cm−2 for the AlN sample grown on HPSS, which is significantly lower than the value of 1.48 × 109 cm−2 on the FSS. The usability of the AlN/HPSS template for deep ultraviolet (DUV) light‐emitting diodes (LEDs) is proven by growth of AlGaN‐based LED structure emitting at 278 nm with single peak emission in a metal‐organic chemical vapor deposition reactor. The light output power of flip‐chip LED grown and fabricated on AlN/HPSS template is enhanced by a factor of 1.25 when compared with LED on AlN/FSS template at 350 mA injecting current. These results suggest that the high‐quality AlN template grown on properly designed HPSS by HVPE can make a significant contribution toward the realization of highly efficient nitride‐based DUV‐LEDs.

Strain hardening green cementitious composites reinforced with nanoparticles: Mechanical and microstructural properties and high temperature effect

This study investigates the effect of nano-particles on the composite's mechanical, microstructural, and high-temperature resistance by using TiO2 and Al2O3 nano-particles in different proportions in the engineered cementitious composites (ECC) composite. TiO2 and Al2O3 nanoparticles were used in three different ratios (0.5%, 1%, and 1.5%) in the ECC mixtures. Then, flexural strength, compressive strength, and ultrasonic pulse velocity (UPV) experiments were conducted on the prepared composites. The relationships between the concentration of TiO2 and Al2O3 nano-particles and the compressive strength were investigated. Scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) analyses were performed to investigate the microstructural contents of the prepared samples. In addition, the effects of high temperatures on the mechanical properties of nano-doped ECC samples were investigated at 200, 400, 600, and 800 °C. The content of 1% TiO2 (1 TiN) and 1.5% Al2O3 (1.5 AlN) nano-particles increased the compressive strength of the nano-doped-ECC samples by 19.6% and 20.2% compared to the nano-free control samples. Also, the increases in compressive strength of 1 TiN and 1.5 AlN samples exposed to 800 °C increased up to 26.24% and 34.56%, respectively.

Cathodoluminescence studies of point defects in aluminum nitride

There is near-ultraviolet and yellow light absorption in AlN crystal. The defect of this absorption hampers the transmittance of ultraviolet light, which, in turn, limits the application of aluminum nitride substrate in deep ultraviolet electronic devices. In this paper, the sources of absorption defects are determined through the first principle simulation method and the cathodoluminescence test of four samples with different growth modes on two substrates. The peak positions of the two AlN grown by metal–organic chemical vapor deposition were observed at 370 and 480 nm; two AlN samples grown by hydride vapor phase epitaxy only observed luminescence near 370 nm. Through the characterization of sample components by x-ray photoelectron spectroscopy, it is speculated that O impurity component and Al vacancy in AlN crystal form donor–acceptor pairs, which are the source of 370 nm luminescence; with the increase in O concentration, a composite defect with Al vacancy is formed, which is the source of 480 nm luminescence. This is very important for further research on eliminating impurity absorption in an AlN epitaxial layer.

Annealing of AlN guided by estimation on oxygen content

In this paper, an estimation method revealing relationship between residual oxygen content and Y2O3 additive amount used in AlN sintering was presented. It is revealed that AlN sample added with 3 wt% Y2O3 showed better performance than the others while the oxygen content is 0.765 wt% in the initial powders. Nevertheless, short-time sintering does not make AlN to obtain ultra-high performance consistent with the estimation. By two-step sintering regimes, the distribution of second phase in AlN ceramic was changed from continuous distribution to island-like distribution significantly. The bending strength increases first and then decreases, reaching a higher value of 418 MPa after 2-h annealing. The residual oxygen content continues to decrease as more liquid second phase precipitated with long-time annealing. After reaching a state with lower residual oxygen content than the estimation, high-performance AlN is prepared with thermal conductivity of 230 W/m·K and lower bending strength of 358 MPa.

Homoepitaxy Growth of High‐Quality AlN Film on MOCVD AlN Template by Hydride Vapor Phase Epitaxy

Herein, AlN film grown on metal‐organic chemical vapor deposition (MOCVD) AlN template by high‐temperature hydride vapor phase epitaxy (HVPE) is characterized. High‐resolution X‐ray diffraction results show that the crystal quality of the film is improved compared with that of the template. Atomic force microscopy shows that the root mean square roughness value also decreases. Cathodoluminescence spectra show that the main impurities in the epitaxial layer and template are oxygen atoms. Raman spectroscopy and geometric phase analysis show that HVPE AlN samples maintain a compressive stress internally, which is the key to inhibit film cracking. Transmission electron microscope characterization reveals that most of the dislocations at the interface inherit from MOCVD AlN template, incline in the subsequent growth, react with each other, and finally annihilate. The dislocation bending and reduction mechanism are demonstrated.

Cross-scale microstructure design of precursor-derived SiC-AlN nanoceramic composites hybrid with ex-situ ZrB2

ZrB2/SiC–AlN nanoceramic composites were densified at 1950 ​°C by hot pressing using an organic-precursor-derived SiC and commercially available AlN and ZrB2. A cross-scale microstructure was constructed by distributing the ZrB2 secondary phase (∼421 ​nm) within the SiC–AlN solid solution matrix. The substructure of the SiC–AlN matrix was agglomerated by nanograins with an average size of only 62 ​nm. ZrB2 connected around the majority of pores within the SiC–AlN matrix and contributed to the formation of numerous weak interfacial bonding, resulting in improved strength and toughness. The highest flexural strength and fracture toughness of 579 ​± ​52 ​MPa and 6.7 ​± ​0.1 ​MPa ​m1/2 were obtained from a 10 ​wt%-ZrB2/SiC–AlN sample, respectively. The high concentration of grain boundaries of the ZrB2/SiC–AlN nanoceramic composites resulted in heat insulation characteristic. The thermal diffusivity and conductivity were 3.6 ​mm2⋅s−1 and 14.3 ​W·(m·K)−1 at 1400 ​°C, respectively, while the electrical resistivity was 3.9×103 ​Ω·cm for the 10 ​wt%-ZrB2/SiC–AlN sample.

Impact of Defects for AlN Single Crystal Thin Film by Metal Nitride Vapor Phase Epitaxy.

Herein, the defect-related properties of an AlN sample prepared based on the optimal process parameters by metal nitride vapor phase epitaxy (MNVPE) were investigated. The FWHM values of the (0002)/(101̅2) planes of the sample by MNVPE are 397/422 arcsec; the advantages of similar FWHM values of (0002) and (101̅2) planes will have a huge advantage over other preparation methods such as MOCVD. From the cross-sectional TEM images of the AlN sample, it is found that the fusion of a large number of a + c type dislocations occur at the interface of the low temperature buffer layer and the epitaxial layer, which affects the growth mode of the epitaxial layer. The lower FHWM value of the E 2(high) peak of the Raman spectrum, the lower the point defect concentration, which made the sample gain higher energy defect emission bands in the PL spectra and higher transmittance in the UV-vis transmission spectrum.

High-quality AlN growth on flat sapphire at relatively low temperature by crystal island shape control method

In this study, we found that the two-step method for heteroepitaxial AlN on sapphire has some limitations. Growing the buffer layer for a too long time will make it challenging to obtain a flat film surface. So, three-step-growth was introduced and an intermediate layer was inserted. In this step, the surface morphology of samples is controlled by NH3 flow rate and the dislocation density is further reduced. Finally, we are able to obtain high-quality AlN samples grown by metalorganic chemical vapor deposition at 1120 ℃, which is a relatively low temperature for the AlN epitaxial layers. The X-ray diffraction ω-scan full width at half maximum values for (002) and (102) reflections are 172 and 145 arcsec, respectively.

Effects of substrate pretreatment and annealing processes on AlN thin films prepared by EVPE

Herein, a novel process of substrate pretreatment and high temperature annealing for elementary source vapor phase epitaxy (EVPE) is shown to be capable of optimizing the surface flatness of the AlN single crystal thin film, thus increasing its suitability for application in deep ultraviolet optoelectronic devices. Specifically, the increased surface smoothness/flatness of the so-prepared AlN samples are confirmed by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). In addition, the crystal quality, stress conditions and luminescence performance are significantly improved.

Analytical control for the content of small amounts of oxygen in aluminum nitride powders using a «Metavak-AK» analyzer

A method for prompt and reliable control of low oxygen content in aluminum nitride powder using a modern gas analyzer «Metavak-AK» operating in automatic mode and never used previously for this object is presented. Aluminum nitride was obtained under conditions of pilot production at ISMAN by the method of self-propagating high-temperature synthesis (SHS). A program for operation of a «Metavac-AK» analyzer has been developed for oxygen determination in AlN samples and optimal conditions of analysis were specified: reductive melting of a sample in a graphite crucible at a furnace temperature of 2700°C in the presence of a combined Sn/Ni flux in a helium flow. The total oxygen content in aluminum nitride powders and the relative standard deviation ranged within 1.01 – 1.18% and 1.75 – 9.87%, respectively.

Carbon‐containing droplet combustion–carbothermal synthesis of well‐distributed AlN nanometer powders

AbstractA novel three‐step technique was employed to synthesize the well‐distributed AlN nanopowders. First, the hollow spherical precursor particles with an average diameter of 2–5 μm, consisting of an amorphous structure mixture of Al2O3 and C, was prepared by carbon‐containing droplet combustion method by using glucose, urea, and aluminum nitrate as starting materials. The carbothermal reduction and nitridation (CRN) was carried out at 1500°C under N2 flow for 2 h and subsequently the CRN product was calcined at 700°C in air for 1 h to remove residual carbon and transform the CRN product to high‐purity AlN powders consisting of nanostructured hollow spheres. The formation mechanism of precursor and AlN hollow spheres was discussed in detail. The AlN powder exhibited well‐distributed spherical particles with a size of 30–50 nm and good sinterability. After additive‐free and pressureless sintering at 1800°C for 2 h, the relative density of the sintered AlN sample was measured to be 99.02%.

Influence of ceramic particles additions on the properties of Ca3Co4O9

Ca3Co4O9 + x wt% B4C, AlN, TiC, TiB2, or TiN (x = 0.0, 0.25, 0.50, and 0.75) samples were prepared by the conventional solid-state route. In all samples, only the Ca3Co4O9 phase was identified by powder XRD. Nevertheless, microstructural studies have shown that most of the additives have reacted with air and Ca3Co4O9 phase on their surfaces, producing new phases. Moreover, it seemed that grain sizes were, at least, slightly reduced. On the other hand, while nearly no modification of the Seebeck coefficient has been observed, independently of the added compound and proportion, electrical resistivity decreased in all cases, when compared to the pristine sample. Consequently, the power factor of samples with additions was higher than the one determined for the pure sample. Linear thermal expansion also decreased with these additives, pointing out to the formation of relatively strong grain boundaries which can improve the carrier mobility and decrease the thermal expansion. The lowest thermal expansion value has been measured in 0.25 wt% B4C samples, being only around 20% higher than that of Al2O3, which can help to reduce the differential thermal expansion in thermoelectric modules working at high temperatures; these results may be very interesting for applications prospects.Article highlightsAddition of different ceramic additives to Ca3Co4O9 phase increases electrical conductivity, and power factor.All additive ceramics react with oxygen and the Ca3Co4O9 phase modifying the properties of pristine compound.The decrease of thermal expansion coefficient will allow reducing the differential thermal expansion in modules.

Flexural strength, thermal and dielectric properties of AlN ceramics with BN nanoplatelets addition

The effects of the low-content addition of boron nitride nanoplatelets (BNp) on the microstructure, flexural strength, thermal and dielectric properties of AlN ceramics were studied. X-ray diffraction and microtopography indicated that the platelike BNp phases were detected in the grain boundary, decreasing the grain size of the AlN samples. Flexural strength increased with 0.5 wt% and 1 wt% BNp addition, and such increase was attributed to the smaller grain sizes and the two-dimensional structure bridging mechanism. Comparing the experimental thermal conductivity with the theoretical calculated values, no strong atomic interaction occurred between AlN and BNp, and thermal conductivity slightly decreased with the addition of a suitable amount of BNp. Moreover, the temperature dependence of dielectric spectrum of the samples exhibited a remarkable difference due to the additional polarisation mechanisms caused by the addition of BNp. Overall, suitable addition of BNp can improve flexural strength whilst maintaining the high thermal conductivity and excellent dielectric properties of AlN ceramics.

Investigation of thermal conductivity and dielectric properties of AlN-based ceramics with ZnO addition by impedance spectroscopy analysis

ABSTRACT The effect of ZnO on phase composition, microstructure, thermal and dielectric properties of AlN ceramics was studied. The X-ray diffraction indicated that ZnAl2O4 and ZnO phases were detected in samples. With 0.5 wt% ZnO added, the thermal conductivity of AlN sample increased to 193.7 W/m·K. Whereas, the thermal conductivity dropped sharply due to excess ZnO. Moreover, the addition of ZnO increased the permittivity and altered the relaxation mechanism of AlN ceramics. The fitting data and activation energy from impedance spectroscopy revealed the variation of defect mechanisms in samples. The addition of 0.5 wt% ZnO reduced the concentration of aluminum vacancy in AlN grains to improve thermal conductivity. Nevertheless, excess ZnO dissolved into the AlN lattice and created additional defects, causing the decrease in thermal conductivity and electrical resistivity for AlN ceramics. Overall, suitable addition of ZnO can effectively increase the thermal conductivity and alter the dielectric properties of AlN ceramics.

Influence of pulsed Al deposition on quality of Al-rich Al(Ga)N structures grown by molecular beam epitaxy

Surfaces of AlN epitaxial layers grown by molecular beam epitaxy (MBE) in continuous mode were analyzed by means of X-ray photoelectron spectroscopy, atomic force microscopy and reflection high-energy electron diffraction without breaking the ultra-high vacuum. Our studies show the presence of the Al-Al and N-N bonds on the AlN samples, as well as a rather rough 3D-like structure. Significant improvement of the surface quality was achieved by employing a low flux Al deposition in pulsed mode at the end of the MBE growth (still under a nitrogen background pressure). Subsequently, on the obtained AlN buffer layers, Al0.85Ga0.15N/GaN multiple quantum wells (MQWs) were grown to show the influence of the buffer layers quality on the entire structure properties. The photoluminescence studies indicate that the introduction of pulsed low flux Al deposition improves optical properties of the grown MQWs layers, i.e., the relative intensity between the QW emission and the low energy emission is clearly in favor of pulsed Al deposition.

High Quality AlN Layer Grown on Patterned Sapphire Substrates by Hydride Vapor-Phase Epitaxy: A Route for Cost Efficient AlN Templates for Deep Ultraviolet Light Devices

We report the characteristics of AlN epilayers grown directly on cylindrical-patterned sapphire substrates (CPSS) by hydride vapor-phase epitaxy (HVPE). To evaluate the effect of CPSS, we analyzed the threading dislocation densities (TDDs) of AlN films grown simultaneously on CPSS and flat sapphire substrate (FSS) by transmission electron microscopy (TEM). The corresponding TDD is measured to be 5.69 x 108 cm-2 for the AlN sample grown on the CPSS that is almost an order of magnitude lower than the value of 3.43 × 109 cm-2 on the FSS. The CPSS contributes to reduce the TDs originated from the AlN/sapphire interface via bending the TDs by lateral growth during the coalescence process. In addition, the reduction of direct interface area between AlN and sapphire by CPSS reduce the generation of TDs.

Enhanced mechanical and thermal properties of AlN ceramics via a chemical precipitation process

AbstractA uniform dispersion of sintering additives is crucial to improve the thermal and mechanical properties of AlN ceramics. In this study, the Y2O3‐coated AlN composite powder was successfully prepared by the chemical precipitation (CP) process, thereby improving the homogenization of Y2O3 in AlN green compacts. The precipitation coating behavior of Y2O3 precursor was investigated by FTIR and TG‐DSC, and the corresponding reaction equation was proposed. The results of TEM, XRD, and XPS for the CP processed AlN powder indicated that a uniform amorphous Y2O3 layer was fully wrapped on the surface of AlN powder. The microstructures and phases of the sintered AlN samples prepared via the CP and conventional ball‐milling (BM) processes, respectively, were compared. The CP process can result in decreasing oxygen content in AlN grains, facilitating the formation of the desirable isolated second phases, and strengthening the grain and grain boundary of AlN ceramic. As a result, the thermal conductivity, bending strength and fracture toughness of the CP processed AlN ceramic are 9.43%, 10.56%, and 18.50% higher than those of the BM processed sample, respectively, illustrating the CP process is a pretty effective way to simultaneously improve the thermal and mechanical properties of AlN ceramics.