AlN Layer Research Articles

Single-structure 3-axis Lorentz force magnetometer based on an AlN-on-Si MEMS resonator

This work presents a single-structure 3-axis Lorentz force magnetometer (LFM) based on an AlN-on-Si MEMS resonator. The operation of the proposed LFM relies on the flexible manipulation of applied excitation currents in different directions and frequencies, enabling the effective actuation of two mechanical vibration modes in a single device for magnetic field measurements in three axes. Specifically, the excited out-of-plane drum-like mode at 277 kHz is used for measuring the x- and y-axis magnetic fields, while the in-plane square-extensional mode at 5.4 MHz is used for measuring the z-axis magnetic field. The different configurations of applied excitation currents ensure good cross-interference immunity among the three axes. Compared to conventional capacitive LFMs, the proposed piezoelectric LFM utilizes strong electromechanical coupling from the AlN layer, which allows it to operate at ambient pressure with a high sensitivity. To understand and analyze the measured results, a novel equivalent circuit model for the proposed LFM is also reported in this work, which serves to separate the effect of Lorentz force from the unwanted capacitive feedthrough. The demonstrated 3-axis LFM exhibits measured magnetic responsivities of 1.74 ppm/mT, 1.83 ppm/mT and 6.75 ppm/mT in the x-, y- and z-axes, respectively, which are comparable to their capacitive counterparts.

Reststrahlen band infrared damping, microwave transparent AlN/polymeric film filters

The low-temperature (t ≈ 20–30 °C) technology of hybrid helicon-arc ion-plasma deposition to grow the amorphous AlN layers on polymeric substrates for getting the IR blocking filters was used. Flexible substrates based on Mylar, Polyethylene Terephthalate/BaSO4 filling and Teflon polymeric films were applied to grow the AlN layers having the Reststrahlen Band at λ∼11–17 μm for filters, which are damping the IR radiation in this spectral region. The Polyvinyl chloride polymeric films without coatings and the Mylar/AlN structures were used to make composed damping filters for spectral range λ∼7.5–25 μm. The transparency of such composed filters in the spectral range λ ∼7.5–25 μm can suppress about 96–98 % of radiation from the objects being at t ∼310 K. In this spectral range, the objects radiate ∼75 % of the total radiated power from a black body in the full EM band spectra. An explanation of the behavior's peculiarities of the spectral dependences of the IR reflectance of polymeric films/AlN structures with using the classical dielectric function model was proposed. The transparency of all polymeric/AlN structures in the microwave frequency range (ν = 60–270 GHz) gains T ≥ 80–90 %.

Diffusion Nitride Surface Layers on Aluminum Substrates Produced by Hybrid Method Using Gas Nitriding

While gas nitriding of steel is currently used in industry, nitriding of aluminum alloys remains an open challenge. The main obstacle is aluminum’s high susceptibility to passivation. The oxide film provides an effective barrier to nitrogen diffusion. Attempts to overcome this problem have mainly focused on glow discharge nitriding using cathode sputtering of an oxide layer. The produced AlN layers exhibit no diffusion zone and show limited performance properties. In this work, the effect of hybrid treatment aimed at producing diffusion layers of nitrides other than AlN on aluminum alloys was investigated on the model system of iron nitride–aluminum substrate. Hybrid treatment combines an electrochemical process involving the removal of the aluminum oxide layer from the substrate, its subsequent iron plating, and a further gas nitriding in high-purity ammonia. The obtained results prove that the hybrid treatment allows the production, at 530 °C/10 h, of diffusion layers of Fe3N iron nitrides on aluminum substrates with a nitrogen diffusion zone range in aluminum of ca. 12 µm. In alloys containing magnesium, its unfavorable effect on the nitrogen diffusion and the functional properties of the layers was observed. An interesting direction for further research is hybrid treatment of precipitation-hardened alloys without magnesium.

Van der Waals epitaxial AlGaN/GaN growth on hexagonal BN via two-dimensional N-induced dislocation slip

We demonstrate the epitaxial growth of nitride heterojunction structures on two-dimensional materials. The growth behavior of Al atoms on hexagonal boron nitride (h-BN) involves preferential adsorption and diffusion along the N-top position, considerably improving AlN formation in the early stage of nucleation. Preferential N adsorption enables h-BN to provide a natural N surface for the substrate. Moreover, the natural N surface provided by h-BN can induce three-dimensional island-like order merging of the upper AlN layer, promoting dislocation slip annihilation. The dislocations annihilation depth of GaN films was shortened to 870 nm, resulting in an order of magnitude decrease in the density of screw dislocations to 5.50 × 107 cm−2 and considerable improvement in the performance of HEMT devices on SiC. Therefore, h-BN films can be used as two-dimensional insertion layers to obtain high-quality van der Waals epitaxial nitride films in GaN-based HEMT devices.

Spatially correlated stress-photoluminescence evolution in GaN/AlN multi-quantum wells

In the manufacture of semiconductor devices, cracking of heterostructures has been recognized as a major obstacle for their post-growth processing. In this work, we explore cracked GaN/AlN multi-quantum wells (MQWs) to study the influence of pressure on the recombination energy of the photoluminescence (PL) from the polar GaN QWs. We grow GaN/AlN MQWs on a GaN(0001)/sapphire template, which provides 2.4% tensile strain for epitaxial AlN. This strain relaxes through the generation and propagation of cracks, resulting in a final inhomogeneous distribution of stress throughout the film. The crack-induced strain variation investigated by micro-Raman spectroscopy and X-ray diffraction mapping revealed a correlation between the spacing of the cracks and the amount of strain between them. We have developed a 2D model that allows us to calculate the spatial variation of the in-plane strain in the GaN and AlN layers. The measured values of compressive in-plane strain in the GaN QWs vary from -0.4 % away from cracks, to -0.7 % near cracks. PL from the GaN QWs exhibits a clear correlation to the varying strain resulting in an energy shift of ∼ 140 meV. As a result, we can experimentally calculate a pressure coefficient of PL energy of ∼ -60.4 meV/GPa for the ∼ 7 nm thick polar GaN QWs. This agrees well with the previously predicted theoretical results by Kaminska et al. in 2016 [DOI: 10.1063/1.4962282], which were demonstrated to break down for such wide QWs. We will discuss this difference with respect to the reduction in both the expected point defects and extended defects resulting from not doping and growth on a GaN template, respectively. As a result, our work indicates that cracks can be utilized for investigating some fundamental material properties related to strain effects.

High performance of Ag/AlN/Si Schottky diode: Study of the DC sputtering power effect on its electrical properties

The DC sputtering power (DCSP) effect on the chemical composition, structural and electrical properties of Ag/AlN/Si Schottky diode was studied. The AlN films grown on Si substrate were examined by X-ray diffraction (XRD), Fourier Transformed Infra-Red (FTIR), and Raman spectroscopy. The analyzes by XRD, FTIR and Raman show stability at the level of the structure and the qualitative chemical composition of the AlN layer for DCSP of 200 W and 400 W. However, the quantity of chemical compounds on the surface of AlN is affected by this variation. From Current-Voltage (I-V) measurements, the ideality factor (n), barrier height (φb), and series resistance (Rs) were extracted by adopting Cheung functions. Diodes with an AlN interlayer elaborated at 400 W have good rectifying behavior with n, Rs and φb of 3.11, 1930 Ω, and 0.93 eV, respectively. The enhancement of the Ag/AlN/Si diode performance was attributed to the reduction of undesirable impurities in the Ag/AlN interface as well as the morphological improvement of the AlN layer. Capacitance-Voltage (C-V) measurements showed that carrier donor concentrations are strongly affected by DC sputtering power. The elaborate structures may provide new materials with electrical properties having a wide range of applications in electronics and photovoltaic field.

Nonvolatile memory operations using intersubband transitions in GaN/AlN resonant tunneling diodes grown on Si(111) substrates

Nonvolatile memory using intersubband transitions and quantum-well electron accumulation in GaN/AlN resonant tunneling diodes (RTDs) is a promising candidate for high-speed nonvolatile memory operating on a picosecond timescale. This memory has been fabricated on sapphire(0001) substrates to date because of the high affinity between the nitride materials and the substrate. However, the fabrication of this memory on Si(111) substrates is attractive to realize hybrid integration with Si devices and nonvolatile memory and three-dimensional integration such as chip-on-wafer and wafer-on-wafer. In this study, GaN/AlN RTDs are fabricated on a Si(111) substrate using metal-organic vapor phase epitaxy. The large strain caused by the differences in the thermal expansion coefficients and lattice constants between the Si(111) substrate and nitride materials are suppressed by a growth technique based on the insertion of low-temperature-grown AlGaN and thin AlN layers. The GaN/AlN RTDs fabricated on Si(111) substrates show clear GaN/AlN heterointerfaces and a high ON/OFF ratio of >220, which are equivalent to those for devices fabricated on sapphire(0001) substrates. However, the nonvolatile memory characteristics fluctuate by repeated write/erase memory operations. Evaluation of the ON/OFF switching time and endurance characteristics indicates that the instability of the nonvolatile memory characteristics is caused by electron leakage through deep levels in the quantum-well structure. Possible methods for suppressing this are discussed with an aim of realizing high-speed and high-endurance nonvolatile memory.

Surface Morphology of AlN Layers Grown on a Nano-Structured SiNx/Si(100) Template

Surface Morphology of AlN Layers Grown on a Nano-Structured SiNx/Si(100) Template

High‐Quality Nonpolar a‐Plane AlGaN Film Grown on Si‐Doped AlN Template by Metal Organic Chemical Vapor Deposition

High crystalline quality and flat a‐plane aluminum gallium nitride (AlGaN) films are obtained on Si‐doped AlN templates with a moderate silane (SiH4) flow rate by metal‐organic chemical vapor deposition (MOCVD). The effects of the SiH4 flow rate on the surface morphology, crystalline quality, stress state, and optical property of a‐plane AlN templates and AlGaN films are comprehensively investigated. As the SiH4 flow rate increases from 0 to 7.0 nmol min−1, the full width at half maximum of X‐ray rocking curve values along [0001]/[1–100] directions for the AlGaN films are monotonically reduced to 1124/1143 arcsec. Meanwhile, the surface root mean square roughness value is decreased to 0.88 nm. These achievements are attributed to the suppression of the anisotropy degrees and in‐plane stress of AlN templates. In addition, an excess SiH4 flow rate leads to deteriorated surface morphologies and increased basal plane stacking fault (BPSF) densities for both AlN and AlGaN films. This work suggests that doping AlN layers with an appropriate SiH4 flow rate is a promising route to obtain high‐quality a‐plane AlGaN films for efficient nonpolar deep ultraviolet (DUV) devices.

Wafer-Scale Carbon Nanotubes Diodes Based on Dielectric-Induced Electrostatic Doping.

Diodes based on p-n junctions are fundamental building blocks for numerous circuits, including rectifiers, photovoltaic cells, light-emitting diodes (LEDs), and photodetectors. However, conventional doping techniques to form p- or n-type semiconductors introduce impurities that lead to Coulomb scattering. When it comes to low-dimensional materials, controllable and stable doping is challenging due to the feature of atomic thickness. Here, by selectively depositing dielectric layers of Y2O3 and AlN, direct formation of wafer-scale carbon-nanotube (CNT) diodes are demonstrated with high yield and spatial controllability. It is found that the oxygen interstitials in Y2O3, and the oxygen vacancy together with Al-Al bond in AlN/Y2O3 electrostatically modulate the intrinsic CNTs channel, which leads to p- and n-type conductance, respectively. These CNTs diodes exhibit a high rectification ratio (>104) and gate-tunable rectification behavior. Based on these results, we demonstrate the applicability of the diodes in electrostatic discharge (ESD) protection and photodetection.

Structural and electrical investigation of Al/Ti/TiN/Au based N-face n-GaN contact stack

In this work, the structure of Ti/Al/TiN/Au contact layer stack on the N-face of a single-crystal n-GaN substrate is studied after heat treatment at 750 °C. Since TiN is widely regarded as a diffusion barrier in the stack, the formed structures with three different initial TiN thicknesses (15, 60 and 90 nm) in the contact layers are investigated in detail. The primary tool used for the structural investigations was a (scanning) transmission electron microscope ((S)TEM). In all three samples a low resistivity ohmic contact was formed. However, the TiN layer has not completely blocked the diffusion for any of the samples and both Al and Au diffused through the TiN layer at the temperature of 750 °C. As a result of the heat treatment, a complex AlN/TiN/Au2Al/TiN/Au + Al2.67O4 stack was formed on the surface of the GaN substrate. The measurements have not shown the often reported Ti–Al alloy phase in the samples. The formation of the AlN and TiN layers can be explained by the separation of N from GaN creating N vacancies in the GaN substrate which could help the formation of the ohmic behavior.

Characterization of Thin AlN/Ag/AlN-Reflector Stacks on Glass Substrates for MEMS Applications

Thin metal layers such as silver (Ag) are being utilized for various optical and plasmonic applications as well as for electrical purposes, e.g., as transparent electrodes in display devices or solar cells. This paper focuses on optical MEMS applications such as the Fabry–Pérot interferometer (FPI). Within such filters, reflector materials such as distributed Bragg reflectors (DBRs) or subwavelength gratings (SWGs) have been widely used so far, whereas metallic thin films (MTFs) were limited in application due to their comparatively higher absorption. In this paper, thin sputtered Ag layers with thicknesses of 20, 40 and 60 nm on glass substrates have been investigated, and it is shown that the absorption is very low in the visible spectral range (VIS) and increases only in near-infrared (NIR) with increasing wavelength. Thus, we consider Ag-thin layers to be an interesting reflector material at least for the VIS range, which can be easily fabricated and integrated. However, Ag is not inert and stable when exposed to the atmosphere. Hence, it needs a passivation material. For this purpose, AlN has been chosen in this contribution, which can be deposited by sputtering as well. In this contribution, we have chosen thin AlN layers for this purpose, which can also be deposited by sputtering. Thus, various AlN/Ag/AlN-reflector stacks were created and patterned by lift-off technology preferably. The fabricated reflectors were characterized with respect to adhesion, stress, cohesion, homogeneity, and most importantly, their optical properties. It was found that the thickness of the AlN can be used to adjust the reflectance–transmittance ratio in the VIS range, and influences the adsorption in the NIR range as well. Based on the measured values of the reflectors with 40 nm Ag, an exemplary transmission filter characteristics has been predicted for a wavelength range from 400 to 800 nm. Both the maximum transmittance and the full width at half maximum (FWHM) can be tuned by variation of the AlN thickness from 20 to 60 nm.

GaN/AlN bilayers for integrated photonics

III-Nitride semiconductors offer a versatile platform for integrated photonic circuits operating from the ultra-violet to the near-infrared spectral range. Either pure AlN or pure GaN waveguiding layers have usually been investigated so far. In this work, we report on the study of GaN/AlN bilayers epitaxially-grown on a sapphire substrate for photonic circuits. Quality factors up to 410,000 are demonstrated with microring resonators in the near-infrared spectral range. We emphasize the peculiar advantages of these bilayers for nonlinear photonics: GaN offers a larger nonlinear susceptibility as compared to AlN. More importantly, both materials exhibit nonlinear susceptibilities with opposite signs that can be advantageous for nonlinear conversion. Thick epitaxial III-nitride bilayers are associated with the occurrence of cracks in the epi-layers and multimode waveguide propagation. We show that the multimode character can lead to peculiar resonance line shapes with the capacity to control full transmission and reflection by phase engineering.

Fabrication of a piezoelectric micromachined ultrasonic transducer (PMUT) with dual heterogeneous piezoelectric thin film stacking

In this study, a piezoelectric micromachined ultrasonic transducer (pMUT) was developed using dual heterogeneous piezoelectric thin films stacked on top of each other. One piezoelectric layer is specialized for the actuation, while the other is specialized for the reception of the ultrasonic waves. This combined use of two materials promises to realize a pMUT transceiver array with an excellent transmitting and receiving performance and a high fill factor. Taking fabrication feasibility into consideration, AlN/Pb(Mg1/3, Nb2/3)O3-PbTiO3 (PMN-PT) and Pb(Zr, Ti)O3 (PZT)/AlN pMUTs were selected as two candidates for prototyping as the dual-layer pMUTs. The driving tests were performed by actuation of each piezoelectric layer and a resonance frequencies around 265 kHz and 203 kHz were confirmed for AlN/PMN-PT and PZT/AlN pMUT, respectively. The diaphragm of AlN/PMN-PT pMUT has a displacement sensitivity of 3538 nm V−1 and 306 nm V−1 when actuating PMN-PT layer and AlN layer at resonance frequency, respectively. While the diaphragm of PZT/AlN pMUT has a displacement sensitivity of 1036 nm V−1 and 744 nm V−1 when actuating the PZT layer and the AlN layer at the resonance frequency, respectively.

Improving the structural performance of low-temperature sputtered AlN on silicon substrate

Preparing high-quality AlN films at low temperatures is always highly demanded in plenty of application-fields, despite the high temperature is always necessary to enable the AlN crystallization. Therefore, improving the structural properties at low temperature is still challenging in the field. In the present work, a metal organic chemical vapor deposition (MOCVD) grown AlN nucleation layer is employed to improve the crystallinity and morphology of sputtered AlN on 6-inch Si (111) substrate at temperatures even as low as 100 °C where it is not principally possible to achieve crystallization of sputtered AlN. When compared with the as-sputtered AlN prepared even at 650 °C, it is found that the full width at half maximum of AlN (002) x-ray rocking curves is intensively decreased to 0.96° from 1.61°, and such a reduction indicates that the screw dislocation density is decreased from 7.31 × 1010 down to 2.14 × 1010 cm−2. In addition to the crystallinity, the morphology is also obviously improved that the root-mean-square roughness is reduced from 4.99 nm down to 0.83 nm in a scanned area of 3 × 3 μm2 through introducing AlN nucleation. Therefore, such a combination highlights the contribution of AlN nucleation grown by MOCVD in improving the structural properties of low-temperature sputtered AlN layer which is compatible with the manufacturing of Si integrated circuits.

Origin of the parasitic luminescence of 235 nm UVC LEDs grown on different AlN templates

AlN layers annealed at high temperatures offer low threading dislocation densities of mid 108 cm−2 and are therefore increasingly used as base layers in ultraviolet (UV) light emitting diode (LED) heterostructure growth. These LEDs, just like those grown on conventional metalorganic vapor phase epitaxy (MOVPE) AlN templates, often suffer from long-wavelength parasitic luminescence. In this work, luminescence properties of far-UVC LED heterostructures grown on MOVPE-AlN/sapphire templates and high-temperature annealed AlN/sapphire templates are compared. To investigate the origin of parasitic emission with high spatial resolution, cross section scanning transmission electron microscopy was combined with cathodoluminescence measurements. As a result, the main origin of the parasitic luminescence band centered at 3.5 eV (354 nm) for the heterostructure grown on annealed AlN is assigned to point defects related to oxygen in the AlN template layer. The defect band centered at 3.0 eV (413 nm) for the heterostructure grown on MOVPE-AlN was found to be related to self-compensating VAl-Si point defect complexes in the n-AlGaN layer and oxygen incorporation close to the AlN/sapphire interface. The results also suggest that the type of AlN template determines the kind of parasitic luminescence from the n-AlGaN layer.

Femtosecond Laser Slicing of Single Crystal AlN Layer Grown by Hydride Vapor Phase Epitaxy on an AlN Substrate Generated by Physical Vapor Transport

Femtosecond Laser Slicing of Single Crystal AlN Layer Grown by Hydride Vapor Phase Epitaxy on an AlN Substrate Generated by Physical Vapor Transport

Magnetic sputtering preparation of hybrid AlN/SiCN thin films with ultrahigh piezoresistivity

Pressure sensors for high temperatures are extensively applied in a variety of industries processes including aerospace, energy conversion, petrochemicals, automotive and oil wells, real-time pressure detection during equipment operation is a critical concern. Silicon is widely used for piezoresistive sensing with good sensitivity, however, harsh environments with temperatures above 150 °C will lead to plastic deformation as well as current leakage. In this work, AlN/SiCN hybrid thin films systems with ultra-high piezoresistive performance are proposed and enabled by coupling with piezoelectric effect. The gauge factors of the AlN/SiCN sensing elements are from 3260 to 21428 upon different stresses, which are significantly greater than those of other piezoresistive ceramics. The piezoresistive performance of SiCN upon compressive stress is significantly enhanced by incorporation piezoelectric effect of AlN. Moreover, the interfacial strengths of the AlN/SiCN thin films are larger than those of single-layer SiCN, demonstrating that incorporation of AlN layers also improves interfacial strength of SiCN films. This work provides a crucial basis for designing novel piezoresistive sensors.

Boosting phonon transport across AlN/SiC interface by fast annealing amorphous layers

The high increase in interface density has become the main bottleneck for heat dissipation in gallium nitride/aluminum nitride (AlN)/silicon carbide (SiC) based nanodevices. In this paper, the interfacial thermal conductance (ITC) of AlN/SiC interface is investigated by non-equilibrium molecular dynamics simulation. It is found that introducing amorphous layers at AlN/SiC interface will result in an enhancement of its ITC by 2.32 times. Three different amorphous layers are investigated and can be achieved by fast thermal annealing. Among them, the amorphous SiC layers work best, and the amorphous AlN layers work worst. Further spectral analysis reveals that the enhancement of ITC comes from the strengthening of interfacial inelastic phonon processes, which boosts the transport of modes at a wide frequency range. What is more, as the thickness of amorphous layers increases, the enhancement of ITC weakens. This research provides a highly operational strategy to enhance ITC and enriches our understanding of inelastic phonon process at interface.

Modeling of light transmission in multilayer epitaxial AlN/GaN structures for biomedical pyrosensors

The spectral-pulse diversity of modern lasers currently used for subcutaneous surgeries provides ample opportunities for the realization of rather complex modes of radiation exposure and allows the simultaneous performance of diagnostic, therapeutic and surgical procedures; this in turn leads to an urgent need for appropriate spectrally indifferent sensors oriented for in vivo operation. In the present work, we propose the use of weakly absorbing epitaxial GaN/AlN/GaN structures as a basis for the fabrication of pass-through pyrometric sensors embedded in a subcutaneous light guide. The use of such sensors in vivo will allow the local study of the absorption or scattering of intracavitary tissues in a wide spectral range, as well as the real-time control of the pulse structure of the laser beam. For such structures, a mathematical model of light propagation taking into account absorption and heat generation was constructed and a computational algorithm for obtaining post-pulse temperature distributions and for calculating the pyrocoefficient at different values of thickness and donor concentration of absorbing GaN layers was developed in the MATLAB environment. A criterion for the efficiency of the sensor performance was proposed, based on which the optimal ratios between the thickness values of AlN and GaN layers for different values of the absorption coefficient αGaN were obtained. Key words: subcutaneous operations, laser therapy, laser surgery, pyroeffect, pyrosensors, aluminum nitride, epitaxial technologies, multilayer GaN/AlN structures.