Coefficient Of Thermal Expansion Mismatch Research Articles

INTERFACE MODIFICATION OF SUBSTRATE FOR ENHANCING ADHESIVE STRENGTH OF CVD DIAMOND COATING BY SOLID PARTICLES

The strong adhesive strength is essential for the widespread applications of diamond coating in the mechanical field. In this paper, a novel method combining conventional chemical vapor deposition (CVD) diamond technique with solid particles was proposed to modify the interface material properties of substrate to improve the adhesive strength of the diamond coating. Prior to deposition, wet dispersion was adopted to uniformly distribute solid particles on the substrate, with which the diamond coatings with the WC, TiC and BN particles embedded at the interface were fabricated on Co-cemented tungsten carbide (WC-Co) substrate. In addition, the pure diamond coating was also fabricated for comparison. The as-deposited diamond coatings were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. The results indicated that the diamond coating with TiC exhibited the highest residual stress, which may be due to the larger lattice mismatch between TiC and diamond as compared to WC and BN. The indentation test suggested that WC and BN particles were favorable for the improvement of the adhesive strength and the crack resistance of diamond coating, while this phenomenon was not observed in the diamond coating with TiC particle. This might be caused by the large lattice parameter, high lattice mismatch, and high thermal expansion coefficient mismatch of TiC as compared with diamond. Furthermore, the lattice parameter of additional particles might be an important factor to determine whether or not the adhesive strength of diamond coating can be enhanced, because it determined the compressive stress or tensile stress generated inside diamond coating. Hence, interface modification of substrate by dispersing the solid particles with low lattice parameter or similar material properties of diamond may be an effective and convivence approach to improve the adhesive strength of the diamond coating.

Effect of the IMC layer geometry on a solder joint thermomechanical behavior

PurposeIn a printed circuit board assembly (PCBA), the coefficient of thermal expansion (CTE) mismatch between the solder joint materials has a detrimental impact on reliability. The mechanical stresses caused by the thermal changes of the assembly lead to fatigue and sometimes the failure of the solder joints. The purpose of this study is to propose a novel pad design to obtain an interrupted solder/substrate interface, to improve the PCBA reliability.Design/methodology/approachAn interruption in the continuous intermetallic compound (IMC) layer of a solder joint was implemented, by the deposition of a silicone film in the pad, changing its geometry. That change allows a redistribution of stresses in the most ductile zone of the solder joint, the solder. The stress concentration at the solder/substrate interface is reduced, as well as the general state of stress at the solder joint.FindingsA new way was developed to reduce the stress on the solder joints, caused by thermal variations, because of the different components CTEs mismatch. This new method consists of interrupting the IMC layers of the solder joint, strategically, redirecting the usual stresses to a more ductile area of the joint, the solder. This is an innovative method that allows increase the lifetime of PCBAs and the equipments.Originality/valueIn this study, a new pad design concept for higher solder joint reliability was developed to reduce the shear stress in the solder joints because of the CTE mismatch between all the solder joint components.

Effect of Grain Structure and Ni/Au-UBM Layer on Electromigration-Induced Failure Mechanism in Sn-3.0Ag-0.5Cu Solder Joints

The development of advanced electronic devices leads to highly miniaturized interconnect circuits (ICs), which significantly increases the electromigration (EM) phenomenon of solder and circuits due to higher current density. The electromigration of solder joints under high current density has become a severe reliability concern in terms of microelectronic product reliability. The microstructure of the solder plays an important role in the electromigration induced degradation. In this study, Sn-3.0Ag-0.5Cu solder bumps with Ni/Au under bump metallization (UBM) layer were fabricated and electromigration acceleration tests were conducted under current density of 1.4 × 104 A/cm2 and 120 °C to investigate the effect of grain structure and Ni/Au-UBM layer on EM-induced failure. Grain structures of solder bumps were determined by utilizing the Electron Backscatter Diffraction (EBSD) technique, and single-crystal solder, single-crystal dominated solder, and polycrystalline solder are observed in different test samples. According to the Scanning Electron Microscope (SEM) images, it is observed that the Ni/Au-UBM layer of the Cu pad can inhibit atom diffusion between solder bump and Cu pad, which reduces the consumption of Cu pad but causes a large void and crack at the interface. The EM lifetime of single crystal solder bumps is lower than that of polycrystalline solder bumps when the c-axis of single crystal solder bumps is perpendicular to the electron flow direction. Additionally, the single crystal structure will increase the brittleness of the solder bump, and cracks are easily generated and expanded under the stress caused by the mismatch of thermal expansion coefficients between the solder bump and Ni/Au-UBM layer near Cu pad. Polycrystalline solder bumps with a higher misorientation angle (15–55°) have a higher atom diffusion rate, which will result in the acceleration of the EM-induced failure.

TiN-SiO2 double layer composite coating with enhanced oxidation resistance and reusability in anti-coking applications

The passivation coating has been the main solution to the coking problem in the field of active cooling technology and ethylene cracking furnace. The need for repeated use and essential decoking process poses a great challenge to the single-layer coating. Herein, a new TiN/SiO2 double-layer coating suitable for internal channels has been prepared and characterized, whose results are compared with single-layer TiN coating. The oxidation resistance was revealed by HT-XRD and TGA. The reusability was analyzed from surface roughness, bonding force and coking inhibition performance in the repeated use of the coking-decoking cycle (RP-3 supercritical pyrolysis, on-line decoking technology). Results reveal the synergistic effect of the inner and outer layers of TiN/SiO2 coatings. The outer amorphous SiO2 with high crystallization temperature plays the role of isolating O and C atoms from penetrating, and the inner TiN acts to reduce the mismatch of thermal expansion coefficient between substrate and SiO2. Therefore, excellent oxidation resistance, with a bearable temperature of 1000 °C, can be obtained for the TiN/SiO2 coatings. In addition, during the coking-decoking cycles, the roughness of TiN/SiO2 coatings has been maintained at a low level; the coating has been tightly adhered to the substrate throughout the process; and the coking resistance has not been weakened with an average anti-coking ratio of 93%, higher than that of TiN (31–67%).

Thermal shock behavior of LaMgAl11O19/Yb2Si2O7/Si thermal/environmental barrier coatings with LaMgAl11O19-LiAlSiO4 transition layer

Thermal/environmental barrier coatings (T/EBCs) could be used to protect SiC-based ceramic matrix composites (CMCs) from chemical attack and lower the substrate temperature for improving their lifetime. To mitigate the mismatch of thermal expansion coefficient (CTE) between LaMgAl11O19 (LMA) TBC and Yb2Si2O7 (YbDS) EBC, a transition layer formed by positive thermal-expansion LMA and negative thermal-expansion LiAlSiO4 (LAS) was introduced. Thermo-mechanical performance of LMA-LAS composite coating was studied. LMA-LAS composite coating exhibited an intermediate CTE and an excellent fracture toughness, revealing that LMA-LAS coating was an acceptable transition layer. Thermal shock behavior of LMA/YbDS/Si without and with LMA-LAS transition layer were compared. The thermal cycling lifetime of T/EBC was improved by about 20% with LMA-LAS intermediate layer. The final failure of T/EBC coating system was mainly associated with thermal mismatch and thermally grown oxide (TGO) formation.

Investigation of the Specification Degradation Mechanism of CMOS Power Amplifier under Thermal Shock Test.

To investigate the critical specifications of a power amplifier (PA) under rapidly changing temperature conditions, we fabricated and tested a 0.3–1.1 GHz complementary metal–oxide–semiconductor (CMOS) PA under thermal shock tests. The results show that high- and low-temperature shocks can accelerate the degradation of critical specifications of PAs and that the critical specifications of PAs degrade with the increasing number of shocks. The main reason for the degradation of critical specifications of PAs with increasing thermal shock tests is the mismatch of thermal expansion coefficients of printed circuit boards (PCB) with FR-4 as a substrate. The results of this paper can provide a reference for the development of temperature-robust PAs design guidelines for the implementation of temperature-robust PAs using low-cost silicon technology.

Oxygen annealing induced crystallization and cracking of pulsed laser deposited Ga2O3 films

Amorphous gallium oxide (Ga 2 O 3 ) films had been prepared on c-sapphire substrate through utilizing pulsed laser deposition at 400 °C and then annealed at various temperatures in an oxygen circumstance. The influence of annealing temperature from 500 to 1000 °C on texture, optical characters, chemical valence state, and surface topography of Ga 2 O 3 films was elaborated systematically. The amorphous film begins to crystallize at 500 °C and the crystallization has been continuously enhanced with the increasing annealing temperature. The optimization annealing temperature is obtained at 700 °C for Ga 2 O 3 thin films with the ratio of 90.4% Ga 3+ valence state and 88.6% lattice oxygen which owns a relatively good crystal quality and flat-continuous surface. However, when the temperature goes up to 800 °C and above, some cracks appeared in the film, which could be contributed to the high temperature enhanced Al diffusion into the film as well as the thermal expansion coefficient mismatch between the Ga 2 O 3 films and substrate. The efforts were directed to understand the annealing behavior and thus to derive enhanced ability for manipulation with the specific crystal structure and phase while optimizing the conditions to obtain desirable properties for the integration of Ga2O3 films in electronic device applications. • The influence of annealing temperature in a wide range from 500 to 1000 °C on the properties of pulsed laser deposited amorphous Ga2O3 film at 400 °C on sapphire substrate was elaborated systematically. • Two kinds of annealing mechanisms were clarified in different temperature range, namely thermal enhanced crystallization (≤700 °C) and high temperature enhanced Al diffusion (800 °C–1000 °C). • Some cracks appeared in the film after high temperature annealing, which could be mostly contributed to the high temperature enhanced Al diffusion into the film. • The optimization annealing temperature is obtained at 700 °C for Ga 2 O 3 thin films with the ratio of 90.4% Ga 3+ valence state and 88.6% lattice oxygen which owns a relatively good crystal quality and flat-continuous surface.

Prediction of residual stresses within dissimilar Al/steel friction stir lap welds using an Eulerian-based modeling approach

Friction stir welding (FSW) was applied to fabricate dissimilar welds between 5052 aluminum alloy and DP590 dual-phase steel with a lap configuration. The interfacial formation and microstructure were investigated experimentally, and in-process thermomechanical conditions with emphasis on resultant residual stress distribution were numerically studied through a fully coupled three-dimensional (3D) Eulerian-based finite element modeling. The Al/steel thermal-mechanical interaction driven by the rotational pin penetrating the steel produced the serrated bonding interface. The brittle intermetallic layer was formed in the interface, showing distribution discontinuity along the welding direction. The simulation results showed that increasing rotation velocity resulted in higher welding temperature and cooling rate, leading to the increment in the residual stress level of the weld zone. Near interfacial residual stresses exhibited an M-shape in the Al alloy while it showed W-shape in the steel, which originated from the thermomechanical processing condition and the effect of mismatch of thermal expansion coefficient. Increasing rotation velocity decreased the near interfacial compressive stress in the steel side. Numerically predicted residual stresses were in acceptable agreement with those measured using the X-ray diffusion measurement with the cosα method, validating prediction reasonability. Hopefully, the current simulation work will provide a new alternative modeling strategy for predicting residual stress fields within the same or dissimilar FSW.

Fabrication and accelerated long-term stability test of asymmetrical hollow fiber-supported thin film oxygen separation membrane

Asymmetrical hollow fiber-supported thin film membrane may provide microstructural advantages for air separation and oxygen production. The fabrication of such a membrane is usually very difficult, particularly the sintering behaviors and thermal expansion coefficient (TEC) mismatch among multiple layers. This directly affects the reliability and long-term stability of the membrane. In this research, the sintering behaviors of a set of simple oxides are systematically studied and ZnO is identified as a material component for composite substrate La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)-ZnO. The LSCF-ZnO ratios are further optimized by trading off several factors, e.g., matching the sintering behaviors and TECs of the LSCF-ZnO composite with those of LSCF thin film separation layer while enhancing the substrate strength. With the identified LSCF-ZnO ratio, the LSCF-ZnO composite hollow fiber substrate precursor is formed through slurry spinning process with porous microstructure being formed via phase inversion process. The thin film LSCF dense separation layer is then dip-coated on the substrate followed by co-sintering process. A thin, porous PrBaCo(Fe0.6Zr0.2Y0.2)O5+δ (PBCFZY) catalyst layer is fabricated on the dense LSCF separation layer with dip-coating and sintering process, forming an asymmetrical membrane device LSCF-ZnO/LSCF/PBCFZY. Oxygen permeation test of the device is systematically conducted, and the fundamental mechanisms are analyzed. An accelerated long-term stability test of the membrane is also conducted (∼550h, 46 thermal cycling loads), demonstrating excellent robustness and durability. The device is characterized and analyzed before and after the test. By replacing a considerable amount of high cost LSCF with low cost ZnO in the substrate, it not only enhances the stability but also reduces the capital cost of the membrane for practical applications.

Releasing the residual stress of Cf/SiC-GH3536 joint by designing an Ag-Cu-Ti + Sc2(WO4)3 composite filler metal

• The work is the first one to introduce Sc 2 (WO 4 ) 3 into brazing process. • The Sc 2 (WO 4 ) 3 maintains its negative thermal expansion effect in Ag-Cu-Ti system. • The residual stress releasing mechanism is revealed by finite element analysis method. • The shear strength improves 2.6 times by introducing Sc 2 (WO 4 ) 3 reinforcing phase. Due to native character of thermal expansion coefficient (CTE) mismatch between C f /SiC and GH3536, achieving high strength joint was a huge challenge for C f /SiC-GH3536 joints. Herein, a composite filler metal of Ag-Cu-Ti + Sc 2 (WO 4 ) 3 was developed to join C f /SiC and GH3536. This work introduced Sc 2 (WO 4 ) 3 to Ag-Cu-Ti system as a negative thermal expansion (NTE) reinforcing phase to release joint residual stress. Sc 2 (WO 4 ) 3 was evenly distributed in the brazing seam and reacted with Ti to form Ti 3 O 5 reaction layer. The results of finite element analysis showed that the residual stress of the joints was effectively released by introducing Sc 2 (WO 4 ) 3 reinforcing phase, and the mises stress was decreased from 447 to 401 MPa. The maximum shear strength of the C f /SiC-GH3536 joint brazed with Ag-Cu-Ti + 6 vol% Sc 2 (WO 4 ) 3 filler alloys was 64 MPa, which was about 2.6 times higher than that of Ag-Cu-Ti alloys. The results of this study provide a promising strategy for the introduction of new Sc 2 (WO 4 ) 3 reinforcing phase in Ag-Cu-Ti system, and improve the reliability and feasibility of composite brazing alloy in brazing filed.

Improving the flexural strength of porcelain by residual stress in anorthite coating

Flexural strength is an important parameter of domestic ceramics to meet the criteria for mechanized washing, filling, and sealing process in the automatic production line. In this work, the anorthite coating was prepared using calcite, silica, and alumina as raw materials. Taking the temperature with the highest matrix strength as the optimal temperature, the influences of chemical formulation on the phase composition and the coefficient of thermal expansion (CTE) of coatings were investigated. The enhancement effect of the coating with different formulations and surface number and thickness was compared. As a result, the flexural strength of the ordinary domestic ceramic body was improved due to the residual compressive stress in the coating caused by a mismatch of CTE between the coating and the matrix. At the optimal sintering temperature (1280°C), the coating with 20 mass% alumina addition has the best strengthening effect on the body, in which the thickness of the anorthite coating is approximately 50 µm and the flexural strength has increased by 64.6%, from 88 ± 4 to 144 ± 6 MPa.

Discrete element method to simulate interface delamination and fracture of plasma-sprayed thermal barrier coatings

This contribution deals with a discrete element method (DEM) framework to simulate and investigate the mechanisms leading to the failure of plasma-sprayed thermal barrier coating (TBC) systems. A hybrid lattice-particle approach is proposed to determine residual stress fields induced by the coefficient of thermal expansion mismatch during a cooling-down phase. Besides, this is combined with a mixed-mode cohesive zone model to simulate interface delamination, and the removed discrete element failure criterion to model crack initiation and propagation in TBC system. The context of a unit cell model with a perfectly sinusoidal interface profile is first investigated to highlight the suitability of the proposed DEM-based approach in terms of stress fields and failure process. The case of a real microstructure reproduced by the image processing is then discussed. This underlines the effect of porosity and surface asperities on the failure mechanisms. Results exhibit the potential of the proposed DEM approach to model complex cracks phenomena occurring in TBC systems under thermal loading.

The Influence of Thermal Treatments on Anchor Effect in NMT Products.

The anchor effect in nanomolding technology (NMT) refers to the effect that polymer nanorods in nanopores on metal surfaces act as anchors to firmly bond the outside polymer components onto the metal surface. In this work, the influences of thermal treatments on the anchor effect are studied at microscopic level from the perspective of interfacial interaction by a model system (poly(n-butyl methacrylate) (PBMA) and alumina nanopore composite). The differential scanning calorimeter and fluorescence results indicate that the formation of a dense polymer layer in close contact with the pore walls after proper thermal treatments is the key for a strong interfacial interaction. Such polymer layers were formed in NMT products composed of PBMA and aluminum after slow cooling or annealing, with an up to eighteen-fold improvement of the interfacial bonding strength. The polymer chains near the nanopore walls eliminate the thermal stress induced by the mismatch of thermal expansion coefficients through relaxation over time and remain in close proximity with the pore walls during the cooling process of nanomolding. The above dynamic behaviors of the polymer chains ensure the formation of stable interfacial interaction, and then lead to the formation of the anchor effect.

Performance of B4C/CeB6 composites fabricated via hot pressing under different processes

AbstractIn this work, CeO2 sintering additive reinforced B4C ceramic composites were prepared by hot‐pressing reaction sintering under different processes of low temperature–long holding time (1980°C, 30 MPa, 3 h, 4 wt% CeO2) and high temperature–short holding time (2050°C, 30 MPa, .5 h, 4 wt% and 6 wt% CeO2). The effect of sintering process and CeO2 content on the microstructure and mechanical properties of B4C‐CeB6 composites were investigated. The existed impurities in the obtained composites were also analyzed. Results show that CeO2 is an active sintering additive. CeB6 is formed by the reaction between CeO2, B4C and C in sintering process. The densification of B4C ceramics is enhanced, and the grains can be refined by the formed CeB6, which promotes the strength. The thermal expansion coefficient mismatch, crack deflection, and fracture mode change caused by the in situ formed CeB6 improve the toughness. The process of low temperature–long holding time is more suitable for playing the role of CeO2 additive in sintering of B4C, under which condition the relative density, flexural strength, fracture toughness, and hardness reach 99%, 417 MPa, 5.32 MPa·m1/2, and 30.66 GPa, respectively. The impurities in the composites are the kinds of Ti‐contained, C‐O‐Mg‐Ca‐contained, C‐O‐Ca‐S‐contained, and Si‐contained impurities.

Mechanical, electrical properties and microstructures of hot-pressed B4C-WB2 composites

In this paper, dense B4C-WB2 composites were fabricated at 1950 °C using B4C and WB2 as raw materials via a hot press method. The phase composition, microstructures and mechanical properties of the B4C-WB2 composites with different B4C volume fraction were studied. The obtained 68.7 vol%B4C-WB2 composites demonstrated good comprehensive properties with high flexural strength of 696 MPa, superior hardness of 34.8 GPa, and acceptable fracture toughness of 3.3 MPa m1/2. The high flexural strength mainly resulted from the pinning effect of preferentially oriented strip-shape WB2 grains and clean grain interfaces between B4C and WB2 phases. The toughening mechanism of the B4C-WB2 composites was associated with the interfacial residual stress induced by the mismatch of thermal expansion coefficient. In addition, the B4C-WB2 composites demonstrated good electrical conductivity (3.3 × 105 S/m) with a low density of 5.589 g/cm3, making them of potential interest for cutting tools and armor protection applications.

Board Level Reliability of Automotive Grade WLCSP for Radar Applications

Wafer-Level Chip Scale Packages (WLCSPs) are becoming commonplace in the industry due to their small form factor. Applications include industrial and automotive which demand high reliability performance. Additionally, WLCSPs may be superior in some implementations to other package options for RF performance in the mmWave spectrum, which is desired for automotive radar application. But board level reliability can be a challenge for some WLCSP package due to CTE mismatch between Si and PCB. Variety of factors including PCB materials, sphere alloys, and board level underfills can influence the board level reliability of WLCSP packages. In this study the industry’s first auto grade 1 capable large WLCSP package. (~ 72 mm2 body size, 18x15 BGA array, 0.5 mm pitch) is presented. Board level underfill application was utilized to achieve automotive grade board level reliability. Underfills are typically selected based on thermomechanical properties of unaged materials. An understanding of the evolution of underfill material properties under thermal aging is important for selecting a stable material capable of meeting the reliability requirements.
 This study evaluates board level underfills and edge bond materials in the form of stand-alone samples and applied to a large daisy-chain WLCSP. The underfilled daisy-chain WLCSPs and the stand-alone samples are placed in a ‑40/125C air cycling chamber (1 cycle/hour). Glass transition temperature (Tg), elastic modulus (E), and coefficient of thermal expansion (CTE) are measured using Dynamic Mechanical Analysis (DMA) and Thermomechanical Analysis (TMA) on the stand-alone samples at various intervals to monitor the evolution of material properties. Simultaneously, the underfilled daisy chain WLCSPs are monitored electrically using an event detector. The combination of material property measurements and cycles to electrical failure can be used to correlate underfill material properties and WLCSP board-level reliability. The results of this study can provide material property guidance for underfill selection.

Compact fiber-optic Fabry-Perot cavity based on sandwich structure adopting direct bonding of quartz glass.

A compact fiber-optic Fabry-Perot (F-P) cavity for a sensor is designed based on a sandwich structure, adopting direct bonding of quartz glass. The reflective F-P cavity is manufactured by a fiber optic with a quartz glass ferrule and the sandwich structure with an air cavity, which is achieved by direct bonding of quartz glass. This fabrication process includes plasma surface activation, hydrophilic pre-bonding, high-temperature annealing, and dicing. The cross section of the bonding interface tested by a scanning electron microscope indicates that the sandwich structure is well bonded, and the air cavity is not deformed. Experiments show that the quality factor of the F-P cavity is 2711. Tensile strength testing shows that the bond strength exceeds 35 MPa. The advantage of direct bonding of quartz glass is that high consistency and mass production of the cavity can be realized. Moreover, the cavity is free of problems caused by the mismatch of thermal expansion coefficients between different materials. Therefore, the F-P cavity can be made into a sensor, which is promising in detecting air pressure, acoustic and high temperature.

Degradation and lifetime of self-healing thermal barrier coatings containing MoSi2 as self-healing particles in thermo-cycling testing

Yttria-stabilized zirconia (YSZ) is the state-of-the-art top coat material for thermal barrier coatings (TBCs) applied on highly loaded gas turbine parts. During operation at high temperatures, stresses are induced by the thermal expansion coefficient mismatch between the ceramic TBC and the metallic substrate. As a consequence cracks can grow, propagate and finally lead to a spallation of the top coat. Using atmospheric plasma spraying (APS), so-called self-healing MoSi2 particles can be incorporated into the YSZ matrix to mitigate the propagation of cracks leading to a lifetime gain and possibly higher temperature capability of the TBC. In the present work, the healing process is realized by the oxidation of the self-healing particles, which introduces a volume expansion by a formation of reaction products, which can seal the cracks. The self-healing particles were introduced within the first 150 μm of the YSZ coating matrix immediately on top of the bond coat. The degradation and lifetime of such systems were studied in furnace cycling and in burner rig tests, in which a temperature gradient through the sample was applied. The lifetime of the self-healing coatings was then compared to the lifetime of an YSZ coating without self-healing particles. In burner rig tests a clear lifetime extension of the self-healing TBCs was observed. The origin of this different behavior was investigated by microstructural analysis in scanning electron microscopy. A further insight into the failure mechanisms was gained by the analysis of a self-healing TBC cycled in a furnace cycling test only for about 55% of its expected lifetime.

The aging behavior, microstructure and mechanical properties of AlN/AZ91 composite

Microstructure evolution and mechanical properties of the aging treated AlN/AZ91 composites were systematically investigated by optical microscopy (OM), high resolution scanning electron microscopy (HRSEM) with an energy dispersive spectrum (EDS), and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The results show that the higher fracture elongation (14 ± 1%) and ultimate tensile strength (275 ± 6 MPa) were simultaneously obtained in the peak-aged AlN/AZ91 composites. Comparied with AZ91 matrix alloy, the strength was increased by about 44% and the elongation was approximately five times higher, which mainly attributed to the precipitation of nano-sized γ-Mg17Al12 phase and the activation of non-basal slip systems induced by in-situ AlN particles at room temperature. However, the in-situ formation of AlN reinforcements consumed part of Al element in the matrix alloy, which resulted into the volume fraction decreasing of γ-Mg17Al12 precipitates, and then the age hardening and strengthening efficiency were reduced in the AlN/AZ91 composites. On the other hand, the mismatch of thermal expansion coefficient between AlN particles and AZ91 matrix generated high density dislocations around AlN particles, which promoted the precipitation of γ-Mg17Al12 phase, and then the peak aging time and temperature were decreased.

Realizing crack-free high-aluminum-mole-fraction AlGaN on patterned GaN beyond the critical layer thickness

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction AlxGa1−xN on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of AlxGa1−xN superlattices with an average Al molar fraction 0.11 < x¯ < 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the AlxGa1−xN superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa1−xN/GaN quantum-well laser heterostructure designed for operation at ∼370 nm.