AlN Samples Research Articles

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers.

Highly Oriented Growth of AlxSc1-X N Ferroelectric Film on W Bottom Electrodes

Ferroelectric materials have been utilized for non-volatile memory applications [1]. Beside extensive works on ferroelectric HfO2 thin film [2, 3], recent works on nitride-based materials have proven that AlxSc1-xN (ASN) also shows ferroelectricity with a box-like characteristic [4]. A high remnant polarization of over 100 µC/cm2 is attractive for future non-volatile memory applications. However, the leakage current is still high for ASN thin films. Thus, a better-oriented growth method or an insulator is needed. The purpose of this study is to evaluate the impact of adding an AlN seed layer which also acts as an insulator for ASN thin film.ASN films, AlN films, and ASN films with 3nm AlN seed layer, three samples were fabricated in this research. A W layer was deposited on an n+Si substrate. Then, 50-nm-thick films of ASN and AlN films were deposited by DC reactive sputtering, respectively. For ASN films with a 3nm AlN seed layer, a W layer was deposited on an n+Si substrate. Then, 3-nm-thick AlN films were deposited by DC reactive sputtering. In the end, 50-nm-thick ASN films were deposited by DC reactive sputtering. All fabrication processes were kept constant at 400oC. XRD and XRC measurements were performed on these three samples. The source of the x-ray was CuKa with a divergence angle of 0.04o. The out-of-plane and in-plane XRD patterns of these three samples are shown in fig. 1(a) and (b), respectively. In out of plane XRD patterns, the (002) peak for the ASN sample is at 35.52 o, yielding parameter c = 0.505nm, which is slightly larger than the reported value of c = 0.4989 ± 0.0005nm [5]. On the other hand, in in-plane XRD patterns, the (100) peak for the AlN sample is at 33.07 o, yielding parameter a = 0.313nm, which is in perfect agreement with the reported value for pure AlN [6]. The (100) peak for the ASN sample is at 31.64 o, yielding parameter a = 0.327nm, which the composition of Sc x= 0.205 by following the reported Gaussian fitting [5]. However, the results didn't show any improvement in the oriented growth of adding the AlN seed layer. Further analyses such as JV, CV, and PV measurements are needed. XRC results of the AlScN (002) plane are shown in fig. 1(c). The results revealed a full-width half-maximum (FWHM) of 7.28o, 15.90o, and 16.17 for ASN, AlN, and ASN/AlN samples, respectively, indicating the oriented growth for these films. Moreover, the seven times higher intensity obtained with the ASN sample suggests a highly oriented film to (00l) plane.In conclusion, XRD and XRC results for ASN films, AlN films, and ASN films with a 3nm AlN seed layer were performed. The results for AlN films met the reported value for pure AlN. ASN grew highly oriented when directly deposited on W electrodes. Further measurements such as JV, CV, and PV are needed.

Experimental observation of high intrinsic thermal conductivity of AlN

AlN is an ultra-wide bandgap semiconductor which has been developed for applications including power electronics and optoelectronics. Thermal management of these applications is the key for stable device performance and allowing for long lifetimes. AlN, with its potentially high thermal conductivity, can play an important role serving as a dielectric layer, growth substrate, and heat spreader to improve device performance. However, the intrinsic high thermal conductivity of bulk AlN predicted by theoretical calculations has not been experimentally observed because of the difficulty in producing materials with low vacancy and impurity levels, and other associated defect complexes in AlN which can decrease the thermal conductivity. This work reports the growth of thick AlN layers by MOCVD with an air-pocketed AlN layer and the first experimental observation of intrinsic thermal conductivity from 130 K to 480 K that matches density-function-theory calculations for single crystal AlN, producing some of the highest values ever measured. Detailed material characterizations confirm the high quality of these AlN samples with one or two orders of magnitude lower impurity concentrations than seen in commercially available bulk AlN. Measurements of these commercially available bulk AlN substrates from 80 K to 480 K demonstrated a lower thermal conductivity, as expected. A theoretical thermal model is built to interpret the measured temperature dependent thermal conductivity. Our work demonstrates that it is possible to obtain theoretically high values of thermal conductivity in AlN and such films may impact the thermal management and reliability of future electronic and optoelectronics devices.

Preparation and characterization of AlN seeds for homogeneous growth

Large size AlN bulk crystal has been grown on SiC heterogeneous seed by physical vapor transport (PVT). The properties of AlN wafer were characterized by high resolution X-ray diffraction (HRXRD), Raman spectroscopy, etched method and atomic force microscope (AFM). Growth mechanism of AlN crystal grown on heterogeneous SiC seeds was proposed. Crystallization quality of AlN samples were improved with the growth process, which is associated with the growth mechanism. AlN single wafer has excellent crystallization quality, which is indicated by HRXRD showing the (0002), ( ) XRD FWHM of 76.3, 52.5 arcsec, respectively. The surface of the AlN wafer is measured by AFM with a roughness of 0.15 nm, which is a promising seed for AlN homogeneous growth.

Effect of hot-pressing sintering on thermal and electrical properties of AlN ceramics with impedance spectroscopy and dielectric relaxations analysis

The effects of hot-pressing sintering on the phase composition, microstructure, thermal and electrical properties of AlN ceramics with CeO2–CeF3 additives were studied. During hot-pressing sintering, high pressure reduced the grain boundary phase CeAlO3 and decreased the concentration of oxygen in AlN ceramics. The hot-pressing sintered AlN samples had a much higher thermal conductivity of 191.9 W/m·K than pressureless sintered ones because of the great reduction of grain boundary phases and oxygen impurities in AlN ceramic. As the carbon content in hot-pressing sintered sample was very high, carbon contamination led to the decrease in electrical resistivity and changes in polarization mechanisms for AlN ceramics. The relaxation peak in the dielectric temperature spectrum with an activation energy of 0.64 eV for hot-pressing sintered samples was caused by electrons from free carbon at low temperature. Overall, hot-pressing sintering can effectively increase the thermal conductivity and change the electrical properties of AlN ceramics.

Effect of oxidation on flexural strength and thermal properties of AlN ceramics with residual stress and impedance spectroscopy analysis of defects and impurities

Abstract The effect of oxidation in air on the phase composition, microstructure, flexural strength and thermal property of AlN ceramics was investigated. Oxidation was found to produce a continuous oxide layer on the surface of AlN samples. The flexural strength and thermal conductivity of AlN ceramics significantly improved after oxidation at 1000 °C and 1100 °C. Residual stress in the AlN ceramic matrix was enhanced by oxidation. The enhanced residual compressive stresses inhibited crack propagation and reduced interfacial thermal resistance, thereby improving the flexural strength and thermal conductivity of AlN ceramics. Electrochemical impedance spectroscopy was further used to analyze the defects in AlN ceramics. The increase in fitting grain resistance revealed a decrease in aluminum vacancy concentration in oxidized AlN sample, which resulted in high thermal conductivity. Therefore, oxidation at a certain temperature is pretty effective to obtain excellent performance for AlN ceramics.

Thermally stimulated luminescence properties of Eu-doped AlN ceramic

We synthesized non- and Eu-doped AlN ceramics by the spark plasma sintering (SPS) method and characterized their photoluminsecence (PL) and thermally stimutrated luminescence (TSL) properties. In the PL properties, the non-doped AlN samples showed a broad emission peaking around 400 nm. In contract, the Eu-doped AlN samples showed two broad emission bands around 400 and 520 nm, and two sharp emission peaks at 610 and 700 nm. The origins of the emission peaks at 400, 520, 610 and 700 nm were due to complexes composed of Al vacancy and O impurity, 5d-4f transitons of Eu2+, 5D0→7F2 and 5D0→7F4 transitions of Eu3+, respectively. Regarding the TSL properties, the TSL intensities of the 0.01 and 0.1% Eu-doped AlN samples were higher than that of the non-doped AlN sample. It was confirmed that the 0.1% Eu-doped AlN showed a good linearity from 0.01 to 1000 mGy.

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb2O3 and YbF3 additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb2O3–YbF3. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb2O3 and YbF3 additives sintered using two-step regimes.

Effects of Y2O3 and yttrium aluminates as sintering additives on the thermal conductivity of AlN ceramic substrates

AlN ceramic substrates are prepared by tape casting and presureless sintering at 1850 °C for 10 h, with 8 wt% Y2O3 and yttrium aluminates of Y3Al5O12 (YAG), YAlO3 (YAP), and Y4Al2O9 (YAM) as sintering additives. The effects of the sintering additives on the phase assemblage, microstructure, thermal conductivity, and bending strength of the sintered AlN ceramics are discussed. With Y2O3, YAG, YAP, and YAM as sintering additives, the secondary phases in the sintered AlN ceramics are YAM, YAG, YAP + YAG, and YAM + YAP, and the average sizes of AlN grains are 5.5, 3.6, 4.4, and 4.9 µm, respectively. In the order of sintering additives with decreasing Y2O3/Al2O3 ratio, viz. Y2O3 > YAM > YAP > YAG, both the thermal conductivity and bending strength of the sintered AlN ceramics decrease. With Y2O3 as the sintering additive, the sintered AlN sample shows the maximum thermal conductivity of 205 W/(m K) and bending strength of 295 MPa.

The influence of point defects on the thermal conductivity of AlN crystals

The average bulk thermal conductivity of free-standing physical vapor transport and hydride vapor phase epitaxy single crystal AlN samples with different impurity concentrations is analyzed using the 3ω method in the temperature range of 30–325 K. AlN wafers grown by physical vapor transport show significant variation in thermal conductivity at room temperature with values ranging between 268 W/m K and 339 W/m K. AlN crystals grown by hydride vapor phase epitaxy yield values between 298 W/m K and 341 W/m K at room temperature, suggesting that the same fundamental mechanisms limit the thermal conductivity of AlN grown by both techniques. All samples in this work show phonon resonance behavior resulting from incorporated point defects. Samples shown by optical analysis to contain carbon-silicon complexes exhibit higher thermal conductivity above 100 K. Phonon scattering by point defects is determined to be the main limiting factor for thermal conductivity of AlN within the investigated temperature range.

Exploring possibilities of band gap measurement with off-axis EELS in TEM

A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inelastic electron scattering probabilities demonstrate the beneficial influence of the Bessel aperture in this setup even for high accelerating voltages. The importance of selecting the optimal experimental convergence and collection angles is highlighted. The effect of the created off-axis acquisition conditions on the selection of the transitions from valence to conduction bands is discussed in detail on a simplified isotropic two band model. This opens the opportunity for deliberately selecting certain transitions by carefully tuning the microscope parameters.The suggested approach is experimentally demonstrated and provides good signal to noise ratio and interpretable band gap signals on reference samples of diamond, GaN and AlN while offering spatial resolution in the nm range.

Advanced Mechanical Properties of a Powder Metallurgy Ti-Al-N Alloy Doped with Ultrahigh Nitrogen Concentration

Titanium and its alloys are recognized for their attractive properties. However, high-performance Ti alloys are often alloyed with rare or noble-metal elements. In the present study, Ti alloys doped with only ubiquitous elements were produced via powder metallurgy. The experimental results showed that pure Ti with 1.5 wt.% AlN incorporated exhibited excellent tensile properties, superior to similarly extruded Ti-6Al-4V. Further analysis revealed that its remarkably advanced strength could primarily be attributed to nitrogen solid-solution strengthening, accounting for nearly 80% of the strength increase of the material. In addition, despite the ultrahigh nitrogen concentration up to 0.809 wt.%, the Ti-1.5AlN sample showed elongation to failure of ~ 10%. This result exceeds the well-known limitation for nitrogen (over 0.45 wt.%) that causes embrittlement of Ti alloys.

High efficiency ultraviolet GaN-based vertical light emitting diodes on 6-inch sapphire substrate using ex-situ sputtered AlN nucleation layer.

We demonstrated the growth of crack-free high-quality GaN-based UV vertical LEDs (VLEDs) (λ = 365 nm) on 6-inch sapphire substrates by using an ex-situ sputtered AlN nucleation layer (NL) and compared their performance with that of UV VLEDs with an in situ low temperature (LT) AlGaN NL. The X-ray diffraction (XRD) results showed that the ex-situ AlN sample contained lower densities of screw-type and edge-type threading dislocations than the in situ AlGaN NL sample. The micro-Raman results revealed that the ex-situ AlN sample was under more compressive stress than the in situ AlGaN sample. As the current was increased, the electroluminescence peaks of both of the samples blue-shifted, reached a minimum wavelength at 1000 mA, and then slightly red-shifted. Packaged VLEDs with the ex-situ AlN NL yielded 6.5% higher light output power at 500 mA than that with the in situ AlGaN NL. The maximum EQEs of the VLED with the in situ AlGaN and ex-situ AlN NLs were 43.7% and 48.2%, respectively. Based on the XRD and Raman results, the improved light output power of the ex-situ AlN sample is attributed to the lower density of TDs.

Effect of Hydrogen Implantation on the Mechanical Properties of AlN throughout Ion-Induced Splitting

The ability to transfer bulk quality III-N thin layers onto foreign platforms is a powerful strategy to enable high-efficiency and low-cost optoelectronic devices. Ion-cut using sub-surface defect engineering has been an effective process to split and transfer a variety of semiconductors. With this perspective, hydrogen-implanted AlN samples were annealed in air at temperatures ranging from 300°C to 600°C for 5 min to study the influence of pre-layer splitting treatments on the nanomechanical properties. There is a clear dependence of the hardness on implanted hydrogen implantation fluence. We observe that the as-implanted hardness increased from 18 GPa for the virgin reference sample to ∼25 GPa for the highest fluence of 3 × 1017 H cm−2 prior to annealing. In the case of reference single crystalline Si samples, a significant drop in the hardness and elastic modulus is observed in the H implantation-induced damage zone subsequent to thermal annealing , while for crystalline epitaxial AlN samples with 0.5 × 1017 and 2.0 × 1017 H implant fluences, the hardness increases and peaks until the thermal annealing temperature reaches 350°C and subsequently begins to drop thereafter for higher annealing temperatures. However, for the 1.0 × 1017 H implantation fluence the hardness continues to increase with increasing thermal annealing temperature.

Defect-related photoluminescence emission from annealed ZnO films deposited on AlN substrates

ZnO film deposited on AlN shows excellent blue emission. Photoluminescence (PL) emission is further enhanced by annealed treatment. The corresponding emission mechanism is discussed. VAl-ON are the dominant form of VAl in as grown AlN samples. When the energy of the incident photons is just enough to pump the electrons up to the VAl-ON energy level, a mass of electrons can be directly trapped by the VAl-ON defect centers, which will induce effective transitions from these defect energy level to the top of the valence band, and then transitions to Zn vacancies levels in ZnO due to similar lattice constants between ZnO and AlN. The energy interval between the VAl-ON in AlN and the Zn vacancies defect states in ZnO is about 2.96eV, which is well consistent with the energy of the blue peak at 420nm (2.96eV).

Improved structural quality of AlN grown on sapphire by 3D/2D alternation growth

Three dimensional (3D) and two dimensional (2D) alternation growth was used to grow AlN epitaxial layers on sapphire substrates. AlN samples grown using this technique have higher crystalline quality and lower dislocation density than samples grown using only 3D or 2D growth modes as witnessed by the high-resolution X-ray diffraction. Smooth atomic terraces with root mean square roughness of 0.107nm were observed using atomic force microscopy (AFM) when the 3D and 2D AlN were 75nm and 425nm, respectively. This sample possesses single crystallographic orientation along the c-axis identified by Raman spectroscopy. Furthermore, the 3D/2D alternating growth mode modulates internal stress in AlN epitaxial layer by adjusting 2D AlN thickness, and the mechanism was studied in detail.

Coating formation at laser irradiation of a dusty gas medium

A free vertical jet of microparticles in a gas is illuminated by a horizontal focused laser beam. The products of the interaction of particles with radiation are deposited on a heated substrate and form a coating. The substrate is a cathode or an anode at the application of an auxiliary discharge. The parameters of the jet have been calculated and the threshold intensity of laser radiation needed for the evaporation of particles has been estimated. The radiation spectra of the dusty gas medium have been measured and the temperature of particles has been determined. Samples of AlN, Ti, and Ti/AlN coating on steel St.3 have been obtained.

Room temperature luminescence and ferromagnetism of AlN:Fe

AlN:Fe polycrystalline powders were synthesized by a modified solid state reaction (MSSR) method. Powder X-ray diffraction and transmission electron microscopy results reveal the single phase nature of the doped samples. In the doped AlN samples, Fe is in Fe2+ state. Room temperature ferromagnetic behavior is observed in AlN:Fe samples. Two photoluminescence peaks located at about 592 nm (2.09 eV) and 598 nm (2.07 eV) are observed in AlN:Fe samples. Our results suggest that AlN:Fe is a potential material for applications in spintronics and high power laser devices.

AlON phase formation in hot-pressing sintering Al2O3/AlN composites and their oxidation behavior

Samples with various ratios of AlN/Al2O3 were fabricated via vacuum hot-pressing at 1700 °C under an applied pressure of 30 MPa for 90 min using Y2O3 as a sintering aid. The densification process, microstructural development, and formation of AlON phase from Al2O3 and AlN were investigated. Al2O3 additive can improve the physical and mechanical properties, and purification of the grain boundaries. AlON are almost formed completely when the content of Al2O3 is 80 vol%. The effects of composition, microstructure, and exposure time at 1400 °C on the oxidation behavior of AlN samples with different amounts of Al2O3 were investigated. The sample with 80 vol% alumina has the best oxidation resistance.

Effect of the Si content on structure and mechanical properties in Al1-xSixN materials

Molecular dynamics simulations of Al1-xSixN samples with different Si content were carried out to investigate their structures, void distributions and mechanical properties. Such effects on other microscopic characteristics, such as bond-length distance, bond-angle and coordination number distribution have been observed. Based on the common neighbor analysis, we found that Si3N4 sample has amorphous state and AlN sample has major amorphous phase mixed with crystalline AlN. The Al1-xSixN samples show the structures of crystalline and amorphous AlN segregated with amorphous Si3N4. The crystalline phase contains major fcc-AlN and minor hcp-AlN. The AlN region is compressed by amorphous Si3N4 region and becomes more denser with increasing Si content. The mostly big voids locate in the Si3N4 regions. The volume of voids increases but the radii of voids decrease with increasing Si content. The strengthening enhancement is observed in the Al0.5Si0.5N sample with denser phases and the lowest ratio nhcp-AlN/nfcc-AlN.