AlN Material Research Articles

Simulation of Piezoelectric Transducer Microphone Diaphragm Based on Different Materials

Piezoelectric microphone which utilizes MEMS technology is a type of transducer that converts an input acoustic signal into an output electrical signal. The characteristics of the microphone diaphragm such as the diaphragm design features and the type of piezoelectric materials used will affect the performance of the microphone in terms of sensitivity. It is hard to control the stress of the diaphragm used in the MEMS transducer microphone. A modification of the diaphragm is done in this project to reduce the residual stress of the piezoelectric transducer. In addition, finite element analysis namely structural, modal and harmonic were carried out using Ansys 15.0 to simulate the mechanical and dynamic behaviour of the microphone diaphragm. Two types of diaphragm structure were designed, namely square and circular, while three types of piezoelectric material which are AlN, PZT and ZnO were used as the diaphragm material. The structural analysis findings of the diaphragm subjected to 1 Pa pressure revealed that the circular diaphragm made of AlN material exhibited the highest stress, reaching 43.05 GPa, surpassing the stresses observed in the other two materials. On the contrary, the square diaphragm composed of PZT material demonstrated the lowest stress, with only 1.55 GPa. In terms of resonance frequency, the circular AlN diaphragm achieved the highest resonant frequency, reaching 449.84 kHz, whereas the square PZT diaphragm exhibited the lowest frequency at 200.25 kHz. In general, the circular diaphragm design consistently yielded higher first resonant frequencies compared to the square design.The results show that the circular diaphragm with AlN piezoelectric materials is the ideal diaphragm in the microphone because of the highest stress generated and the first resonant frequency. The stress is related to the sensitivity of a microphone while the high resonant frequency can lead to the better optimization of signal to noise ratio control.

Single-crystalline AlN/sapphire and composite electrode based ultra-high temperature surface acoustic wave devices

Single crystalline AlN material is very attractive for the development of high temperature electronic devices owing to its high temperature resistance. However, at ultrahigh temperatures, AlN film still will be oxidized and metal electrodes used for the devices will aggregate and vaporize. The deterioration of the material and metal electrodes eventually fails the AlN-based devices. Here, we report a single crystalline AlN film-based ultrahigh temperature surface acoustic wave (SAW) device. By using a thin alumina (Al2O3) protection layer on surface, oxidation of the AlN piezoelectric film was effectively inhibited at high temperatures. A composite electrode based on multilayer structure of Pt-Rh and Al2O3 was designed and fabricated, with the Al2O3 layer as the diffusion barrier layers to suppressed inter-diffusion, agglomeration and vaporization of the metals. The results showed that the developed AlN SAW devices can withstand high temperatures, and work well at temperatures up to 1400 °C.

NGHIÊN CỨU ẢNH HƯỞNG CỦA MẬT ĐỘ LÊN CẤU TRÚC VÀ ỨNG XỬ CƠ TÍNH CỦA VẬT LIỆU GỐM (AlN)0.9(Si3N4)0.1

Molecular dynamics (MD) simulations have been used to study three samples of (AlN)0.9(Si3N4)0.1 ceramic materials with density varying from 2.95 to 3.88 g.cm-3. The influence of density on microstructure, such as the change of structural units, bond lengths, the nearest common neighbors, and voids, was studied in detail. The analysis shows that these samples consist of regions of AlN and Si3N4 materials. The region of AlN material consists mostly of AlN crystals and small amount of AlN amorphous. The region of Si3N4 material has amorphous structure. Void analysis shows that there is the diffusion of Si atoms into the AlN amorphous region and conversely there is the diffusion of Al atoms into the Si3N4 amorphous region. The voids with large radius are localized in the amorphous structure region. The number of these voids decreases rapidly with the increase of density. This reduction improves significantly mechanical properties of samples.

Mechanism of defects and electrode structure on the performance of AlN-based metal semiconductor metal detectors

We used the metal-organic chemical vapor deposition(MOCVD) method to grow AlN material on a c-plane sapphire substrate and fabricate an AlN-based metal-semiconductor-metal (MSM) detector. Analyzing the influence mechanism of different dislocation densities in AlN materials and detector electrode structure on the detector performance, it was found that the lower the dislocations can effectively reduce the dark current of the detector under zero bias voltage, and help improve the performance of the detector. The study also found that when the finger spacing of the detector remained the same and the finger width increased, the efficiency of the detector decreased, while the response time of the detector increased, when the finger width of the detector electrodes remained unchanged and the finger spacing increased, the response time of the detector increased. Therefore, the electrode finger width and finger spacing must be compromised in the design of the electrode structure to improve the performance of the AlN-based MSM detector.

The mechanism of the enhanced intensity for polarization Coulomb field scattering in AlN/GaN heterostructure field effect transistors with submicron gate length

AlN/GaN heterostructure field-effect transistors (HFETs) was fabricated. Based on the polarization Coulomb field (PCF) scattering theory model, the additional polarization charges and mobility are obtained by iterative calculation method. By analyzing the results, it is found that (i) the electric field strength in the AlN barrier layer is large because it is ultrathin, and the piezoelectric tensor coefficient of AlN material is large; thus, the inverse piezoelectric effect is very strong. A large number of additional polarization charges are generated under the gate, which can generate a strong PCF scattering potential and enhance PCF scattering. What’s more, the AlN barrier layer cannot be infinitely strained by the gate bias with the converse piezoelectric effect, and the additional polarization charges will become saturated with the decrease in gate bias. (ii) The AlN barrier layer of AlN/GaN HFETs is ultrathin, the two-dimensional electron gas (2DEG) density under the gate is quickly decreased by the gate bias, and, with the decrease in 2DEG density, PCF scattering is enhanced.

Demonstration of Al0.85Ga0.15N Schottky barrier diode with > 3 kV breakdown voltage and the reverse leakage currents formation mechanism analysis

In this paper, high performance lateral Al0.85Ga0.15N Schottky barrier diodes (SBDs) with a field-plate structure and a heavily doped GaN cap layer in the Ohmic region are demonstrated. Employing Al0.85Ga0.15N rather than AlN for the channel layer effectively reconciles the tension between electron mobility and dopant activation caused by AlN material dilemma. The lateral Al0.85Ga0.15N SBD with an anode-cathode spacing (LAC) of 20 μm achieves a specific on-resistance (Ron,sp) of 0.19 Ω·cm2 and a breakdown voltage exceeding 3 kV, rendering Baliga's power figure of merit of more than 47 MW/cm2, which is the highest value among all AlN- or Al-rich-AlGaN (Al ratio > 70%)-based diodes. Reverse leakage current mechanisms of Al0.85Ga0.15N SBDs are dominated by a variable range of hopping and trap-assisted space charge-limited conduction before destructive breakdown occurs in the field-plate oxide, which are verified by analyzing the reverse leakage characteristics. Combined with the stable performance with temperature up to 275 °C, the Al0.85Ga0.15N SBD shows great promise for next--generation high power electronics applications after further resolving the Ohmic and electron mobility issues.

Void containing AlN layer grown on AlN nanorods fabricated by polarity selective epitaxy and etching method

Creating voids between thin films is a very effective method to improve thin film crystal quality. However, for AlN material systems, the AlN layer growth, including voids, is challenging because of the very high Al atom sticking coefficient. In this study, we demonstrated an AlN template with many voids grown on AlN nanorods made by polarity selective epitaxy and etching methods. We introduced a low V/III ratio and NH3 pulsed growth method to demonstrate high-quality coalesced AlN templates grown on AlN nanorods in a metal organic chemical vapor deposition reactor. The crystal quality and residual strain of AlN were enhanced by the void formations. It is expected that this growth method can contribute to the demonstration of high-performance deep UV LEDs and transistors.

Wafer-level experimental study of residual stress in AlN-based bimorph piezoelectric micromachined ultrasonic transducer

During the last decade, piezoelectric micromachined ultrasonic transducer (pMUT) have raised a great interest for various applications. In particular, AlN material has been used to build bimorph structures made of two AlN layers sandwiched between metallic electrodes. This kind of pMUT offer good performances for both sensing and actuating because of the multi-electrodes scheme of the design. Yet, this pMUT architecture is expected to be particularly sensitive to residual stress within the AlN layers. At the wafer level, stress dispersion is likely to cause important performance variations. In this paper, we characterize bimorph pMUT working in the 50–250 kHz range to investigate how much stress variations at the wafer level (8″) impact critical pMUT features such as resonant frequency, sensitivity and quality factor. Drive sensitivity up to 216 nm/V are obtained with a single electrode scheme. Two approaches are combined, respectively based on pMUT simulation and curvature measurement, to assess the contribution of AlN residual stress to the overall tension of the diaphragm, ranging from −300 to 900 N m−1. The resulting dispersion of resonant frequency is higher than 100%. We highlight how to obtain satisfactory yield.

Improvement of AlN Material Quality by High-Temperature Annealing Toward Power Diodes

AlN devices have the potential to outperform current GaN devices, especially in the high-voltage region, thanks to AlN’s larger critical electric field and thermal conductivity. In order for the AlN-based power diodes to be realized and performance to be demonstrated, high-quality AlN crystals would be the prerequisite. In this article, significantly improved AlN quality has been demonstrated through high-temperature treatment of AlN thin layers at high temperatures in the range of 1670–1730 °C, followed by regrowth of AlN layers on top. The produced AlN layer shows much improved crystalline quality when compared to AlN layer without thermal treatment. The thermal treatment of AlN nucleation layers recrystallizes the AlN materials and results in a significant reduction of the full-width at half-maximum (FWHM) values of X-ray rocking curves of AlN (002) and (102) planes. The annealing of AlN nucleation also produced much pronounced atomic step features and improves the transmittance for the nucleation layers. Regrowth of AlN templates on such annealed nucleation improves AlN crystalline quality further, producing high-quality templates for power diode applications.

A novel β‐Ga2O3 HEMT with fT of 166 GHz and X‐band POUT of 2.91 W/mm

AbstractIn this paper, a novel β‐Ga2O3 high electron mobility transistor (BGO‐HEMT) with record‐high intrinsic unity current gain cut‐off frequency (fT) of 166 GHz and RF output power (POUT) of 2.91 W/mm is demonstrated through 2D device simulations using an appropriate negative differential mobility model. The highly scaled proposed device uses 10 nm AlN barrier layer on 50 nm β‐Ga2O3 buffer with gate‐length (LG) of 50 nm and aspect‐ratio (gate length to barrier thickness) of 5 ensures significant gain in high‐frequency performance. The novel device design offers very low access and dynamic resistance due to highly doped n+ access regions, and a finite gap between ohmic contacts and barrier layer to mitigate source choking effect. The device's superior DC and RF performance is well supported by large two‐dimensional electron gas (2DEG) density (ns) of the order of 1013 cm−2 due to large band discontinuity in AlN/β‐Ga2O3 heterostructure and highly polarized AlN material. The device shows maximum drain current density (IDMAX) of ~11.5 A/mm and peak transconductance (gm) of 0.917 S/mm at VDS = 15 V and VGS = 0 and − 7 V respectively. Furthermore, the term 2π (fT × LG) for the device shows a value of 0.5 × 107 cm/s, very close to vsat of 1.5 × 107 cm/s in β‐Ga2O3. These promising results enhance the potential of β‐Ga2O3 for future high power RF and microwave applications.

Computer Simulation of Horizontal VPE Grown AlN

The three-dimensional growth chamber model was established by GAMBIT software, and the FLUENT software was used to simulate the Vapor Phase Epitaxy ( VPE ) growth AlN material. By changing parameters, results of simulation were obtained, in turn described the effect of uniformity and concentration of AlN. The study mainly focuses on the research of flow velocities of nitrogen and argon gas, the distance from the substrate to the inner tube mouth, the substrate front-back angular height difference, the inner tube diameter, and the substrate thickness. It is discovered through the research: above parameters play a very important role in the crystal growth of AlN film, and there are optimal parameter values. The structure of growth chamber is designed and optimized according to the simulation result. By using the simulation software, experimental costs can be greatly saved, which provides a theoretical basis for the optimal growth process of high quality AlN film, and has a certain guiding role for the actual growth process.

Sandwich method to grow high quality AlN by MOCVD

We report pulsed atomic layer epitaxy growth of a very high crystalline quality, thick (~2 µm) and crack-free AlN material on c-plane sapphire substrates via a sandwich method using metal organic chemical vapor deposition. This sandwich method involves the introduction of a relatively low temperature (1050 °C) 1500 nm thick AlN layer between two 250 nm thick AlN layers which are grown at higher temperature (1170 °C). The surface morphology and crystalline quality remarkably improve using this sandwich method. A 2 µm thick AlN layer was realized with 33 arcsec and 136 arcsec full width at half maximum values for symmetric and asymmetric reflections of ω-scan, respectively, and it has an atomic force microscopy root-mean-square surface roughness of ~0.71 nm for a 5 × 5 µm2 surface area.

Manufacturing of Reticulated Open‐Cellular Aluminum Nitride Ceramic Foams from Aqueous AlN Suspensions

Open‐celled aluminum nitride ceramic foams are prepared from aqueous suspensions of AlN powders passivated against hydrolysis by prior treatment with aluminum dihydrogenphosphate solutions. The sintering is performed with Y2O3 as sintering aid in a conventional tube furnace at 1 650 °C under N2. The mechanically stable cellular AlN material with a porosity of 91.8% is characterized with respect to its microstructure by SEM, confocal Raman microscopy and XRD/Rietveld analysis. The thermal conductivity is determined by TPS measurements and correlated to the porosity of the sample as well as the oxygen content of the AlN phase.

Protective capping and surface passivation of III-V nanowires by atomic layer deposition

Low temperature (∼200 °C) grown atomic layer deposition (ALD) films of AlN, TiN, Al2O3, GaN, and TiO2 were tested for protective capping and surface passivation of bottom-up grown III-V (GaAs and InP) nanowires (NWs), and top-down fabricated InP nanopillars. For as-grown GaAs NWs, only the AlN material passivated the GaAs surface as measured by photoluminescence (PL) at low temperatures (15K), and the best passivation was achieved with a few monolayer thick (2Å) film. For InP NWs, the best passivation (∼2x enhancement in room-temperature PL) was achieved with a capping of 2nm thick Al2O3. All other ALD capping layers resulted in a de-passivation effect and possible damage to the InP surface. Top-down fabricated InP nanopillars show similar passivation effects as InP NWs. In particular, capping with a 2 nm thick Al2O3 layer increased the carrier decay time from 251 ps (as-etched nanopillars) to about 525 ps. Tests after six months ageing reveal that the capped nanostructures retain their optical properties. Overall, capping of GaAs and InP NWs with high-k dielectrics AlN and Al2O3 provides moderate surface passivation as well as long term protection from oxidation and environmental attack.

Effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition

The effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition was investigated using atomic force microscopy, Raman spectroscopy, and deep ultra-violet photoluminescence (PL) with the excitation wavelength as short as ∼ 177 nm. Annealing experiments were carried out in either N2 or vacuum atmosphere with the annealing temperature ranging from 1200 °C to 1600 °C. It is found that surface roughness reduced and compressive strain increased with the annealing temperature increasing in both annealing atmospheres. As to optical properties, a band-edge emission peak at 6.036 eV and a very broad emission band peaking at about 4.7 eV were observed in the photoluminescence spectrum of the as-grown sample. After annealing, the intensity of the band-edge emission peak varied with the annealing temperature and atmosphere. It is also found that a much stronger emission band ranging from 2.5 eV to 4.2 eV is superimposed on the original spectra by annealing in either N2 or vacuum atmosphere. We attribute these deep-level emission peaks to the VAL—ON complex in the AlN material.

VIS/IR spectroscopy of thin AIN films grown by pulsed laser deposition at 400°C and 800°C and various N2 pressures

The optical properties of pulsed-laser-deposited (PLD) AlN films on silicon are reported in the wide visible and infrared spectral range from 5×104 cm−1 to 350 cm−1. The films were deposited at 400 °C or 800 °C and in vacuum or nitrogen gas ambient at pressures from 0.1 Pa to 10 Pa. The optical constants of the films were determined by modelling the spectroscopic ellipsometry data recorded in both the visible and infrared spectral ranges. The PLD AlN films deposited in vacuum have a bandgap energy of 6.2 eV in a good agreement with the crystalline AlN material, while an effective bandgap narrowing was observed for films deposited in N2 ambient. This effect was most pronounced in films deposited at 400°C, where the optical absorption below the fundamental edge was the largest due to a larger degree of disorder in the films. Fourier transform infrared spectroscopy was used to identify AlN phonon modes and stress in the films. The shift and broadening of the E1(TO) peak towards the higher wave numbers indicates that an increased compressive stress is developed in the deposited films as the nitrogen pressure is increased.

Simulation study on electrical characteristic of AlGaN/GaN high electron mobility transistors with AlN spacer layer

Two-dimensional electron gas (2DEG) property is crucial for the performance of GaN-based high electron mobility transistors (HEMTs). The 2DEG-related concentration and mobility can be improved as device’s performance booster. Electrical characteristics of AlGaN/AlN/GaN HEMT are numerically simulated and compared with conventional AlGaN/GaN HEMT. The main findings of this study indicate that 2DEG’s concentration level is increased when a spacer layer of AlN in the interface of AlGaN/AlN/GaN is inserted owing to large conduction band off set, high polarization field, and high barrier. Notably, when a thin spacer layer of AlN is introduced, the 2DEG’s distribution virtually shifts away from the interface which reduces the interface scattering. The scattering appearing in conventional AlGaN/GaN HEMT includes alloy and interface roughness scatterings. They are reduced in AlGaN/AlN/GaN HEMT due to binary nature of AlN material. A critical thickness of spacer layer for mobility is 0.5 nm and the maximum drain current and transconductance (Gm) are at 1.5 and 1.2 nm thickness of AlN spacer layer. Increasing thickness of AlN spacer layer deteriorates the ohmic resistance of source/drain contact and hence degrades the performance of device beyond 1.5-nm-thick AlN spacer layer.

Ultrathin HfON/SiO2 dual tunneling layer for improving the electrical properties of metal–oxide–nitride–oxide–silicon memory

A high-k gate stack structure with ultrathin HfON/SiO2 as dual tunneling layer (DTL), AlN as charge storage layer (CSL) and HfAlO as blocking layer (BL) is proposed to make a charge-trapping-type metal–oxide–nitride–oxide–silicon non-volatile memory device by employing in-situ sputtering method. The validity of the structure is examined and confirmed by transmission electron microscopy. The memory window, program/erase, endurance and retention properties are investigated and compared with similar gate stack structure with Si3N4/SiO2 as DTL, HfO2 as CSL and Al2O3 as BL. Results show that a large memory window of 3.55V at a program/erase (P/E) voltage of +8V/−15V, high P/E speed, and good endurance and retention characteristic can be achieved using the Au/ HfAlO/AlN/(HfON/SiO2)/Si gate stack structure. The main mechanisms lie in the enhanced electron injection through the ultrathin high-k HfON/SiO2 DTL with suitable band offset, high trapping efficiency of the high-k AlN material, and effective blocking role of the high-k HfAlO BL.

Evaluation of AlN material properties through vibration analysis of thin membranes

AbstractIn this work, the mechanical properties of thin piezoelectric AlN films along with the methods and instruments to obtain this information with sufficient accuracy via dynamic (vibration) and static analyses of thin membranes are reported. In addition, the impact of different damping mechanisms on the amplitude of forced oscillations have been considered in order to obtain the analytical expression relating the resonant amplitude of membrane to the ambient gas pressure. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An 8–18 GHz broadband high power amplifier

An 8–18 GHz broadband high power amplifier (HPA) with a hybrid integrated circuit (HIC) is designed and fabricated. This HPA is achieved with the use of a 4-fingered micro-strip Lange coupler in a GaAs MMIC process. In order to decrease electromagnetic interference, a multilayer AlN material with good heat dissipation is adopted as the carrier of the power amplifier. When the input power is 25 dBm, the saturated power of the continuous wave (CW) outputted by the power amplifier is more than 39 dBm within the frequency range of 8–13 GHz, while it is more than 38.6 dBm within other frequency ranges. We obtain the peak power output, 39.4 dBm, at the frequency of 11.9 GHz. In the whole frequency band, the power-added efficiency is more than 18%. When the input power is 18 dBm, the small signal gain is 15.7 ± 0.7 dB. The dimensions of the HPA are 25 × 15 × 1.5 mm3.