AlN Thin Films Research Articles

Higher-Order Overtone Thickness-Shear Vibrations of Multilayered Thin-Film Acoustic Wave Resonators and Angular Rate Sensing

We propose a new structure for piezoelectric gyroscopes. It is made from multilayered thin films of AlN or ZnO with alternating c-axes along the film thickness. It is shown theoretically that when such a film is electrically driven into higher-order overtone thickness-shear vibration in one of the two in-plane directions of the film and is rotating about the film normal, the Coriolis force due to the rotation causes a higher-order overtone thicknessshear vibration in a perpendicular direction with an electrical output that can be used to measure the angular rate of the rotation. Different from existing thickness-shear mode piezoelectric gyroscopes which are based on the fundamental or the second overtone thickness-shear mode, the proposed gyroscope operates with higher-order overtone thickness-shear modes with higher frequencies and hence potentially higher sensitivity. Because of the overtone modes, the Coriolis force acting on the gyroscope forms a self-equilibrated system and does not transmit a net force or torque to the mounting structure. This implies higher device quality factor and better performance.

Surface Topography and Optical Properties of Thin AlN Films Produced on GaAs (100) Substrate by Reactive Ion-Plasma Sputtering

A study of the surface topography and optical characteristics of thin AlN films used as passivating and antireflection coatings deposited on n-GaAs (100) substrates by reactive ion-plasma sputtering is reported. It was found that the process conditions affect the structure and the optical characteristics of the films, which makes it possible to obtain coatings with prescribed parameters. An analysis of the results furnished by ellipsometry and atomic-force microscopy of the surface shows that the refractive index of the films is correlated with the surface structure.

Crystal growth of a MnS buffer layer for non-polar AlN on Si (100) deposited by radio frequency magnetron sputtering

The growth conditions of MnS thin film on a Si (100) substrate deposited by the RF-magnetron sputtering method were investigated. The MnS is a buffer layer for the epitaxial growth of non-polar AlN thin film on the Si (100) substrate. The 4°-off-Si (100) substrate and the insertion of MnS film grown at room temperature (RT-MnS) improved the crystallinity and the surface roughness of the MnS film. In particular, the 20 nm thick RT-MnS showed a reduction of surface roughness of the MnS layer deposited at 550 °C. The root mean square value of the MnS layer was 0.23 nm, which is in the same range as that of the Si substrate. X-ray photoelectron spectroscopy measurements revealed that RT-MnS insertion with a thickness over 10 nm reduced the sulfur vacancy formation in the MnS film deposited on RT-MnS, and the MnS was thermally stable at the growth temperature of AlN.

All-solid-state VUV frequency comb at 160 nm using high-harmonic generation in nonlinear femtosecond enhancement cavity.

We realized a solid-state-based vacuum ultraviolet frequency comb by harmonics generation in an external enhancement cavity. Optical conversions were so far reported by only using gaseous media. We present a theory that allows the most suited solid generation medium to be selected for specific target harmonics by adapting the material's bandgap. We experimentally use a thin AlN film grown on a sapphire substrate to realize a compact frequency comb high-harmonic source in the Deep Ultraviolet (DUV) / Vacuum Ultraviolet (VUV) spectral range. By extending our earlier VUV source [Opt. Express26, 21900 (2018)] with the enhancement cavity, a sub-Watt level Ti:sapphire femtosecond frequency comb is enhanced to 24 W stored average power, its 3rd, 5th, and 7th harmonics are generated, and the targeted 5th harmonic's power at 160 nm increased by two orders of magnitude. The emerging nonlinear effects in the solid medium, together with suitable intra-cavity dispersion management, support optimal enhancement and stable locking. To demonstrate the realized frequency comb's spectroscopic ability, we report on the beat measurement between the 3rd harmonic beam and a 266 nm CW laser reaching about 1 MHz accuracy.

Influence of residual stress on performance of AlN thin film based piezoelectric MEMS accelerometer structure

The presence of residual stress is inevitable and major constraint for MEMS devices as they induce deformation, fracture, fatigue and micro structural changes in the structure. This paper presents the influence of residual stresses (up to − 2100 MPa) on the performance of AlN based piezoelectric MEMS accelerometer structure. The MEMS structure consists of an AlN coated silicon proof mass (800 µm × 800 µm × 300 µm) suspended by four beams (400 µm × 200 µm × 10 µm). Under the influence of residual stress (− 2100 MPa), the first three modal frequencies (13.54 kHz, 20.95 kHz and 20.95 kHz) of the accelerometer structure are reduced to 5.19 kHz, 10.17 kHz and 10.17 kHz respectively. The location of proof mass deflection and maximum von-Mises stress also get altered. Hence the structure sensitivity is also modified from 3.64 × 10−2/g (at no residual stress) to 2.54 × 10−4/g (at − 2100 MPa). The damping factor of the structure also gets modified from 3.96 to 10.38 due to the residual stress. Thus, the accelerometer frequency response (3 dB bandwidth) is found to be diminished from 1.81 to 0.26 kHz due to the presence of residual stress.

A Geometrical Study on the Roof Tile-Shaped Modes in AlN-Based Piezoelectric Microcantilevers as Viscosity⁻Density Sensors.

Cantilever resonators based on the roof tile-shaped modes have recently demonstrated their suitability for liquid media monitoring applications. The early studies have shown that certain combinations of dimensions and order of the mode can maximize the Q-factor, what might suggest a competition between two mechanisms of losses with different geometrical dependence. To provide more insight, a comprehensive study of the Q-factor and the resonant frequency of these modes in microcantilever resonators with lengths and widths between 250 and 3000 µm and thicknesses between 10 and 60 µm is presented. These modes can be efficiently excited by a thin piezoelectric AlN film and a properly designed top electrode layout. The electrical and optical characterization of the resonators are performed in liquid media and then their performance is evaluated in terms of quality factor and resonant frequency. A quality factor as high as 140 was measured in isopropanol for a 1000 × 900 × 10 µm3 cantilever oscillating in the 11th order roof tile-shaped mode at 4 MHz; density and viscosity resolutions of 10−6 g/mL and 10−4 mPa·s, respectively are estimated for a geometrically optimized cantilever resonating below 1 MHz.

The composition and interfacial properties of annealed AlN films deposited on 4H-SiC by atomic layer deposition

We have investigated and compared the influence of annealing temperature on composition and interfacial properties of AlN thin films deposited on 4H-SiC substrates by atomic layer deposition using trimethylaluminum (TMA) and ammonia as precursors at 400 °C. Those as-deposited films were annealed by rapid thermal annealing at 700 °C, 800 °C, 900 °C and 1000 °C for 1 min in N2 atmosphere, respectively. As the annealing temperature increased, the thickness of AlN films decreased and the refractive index increased due to the densification and compositional changes in the films. The roughness of films decreased as the films became denser after annealing. In addition, the deposited AlN film is not amorphous structure and the intensity of the (0002) diffraction of the AlN films enhanced in the annealed films. N-O-Al bond broke down and the intensity of Al-N bond increased with the increase of annealing temperature. Moreover, the interface between AlN and SiC was mainly a transition layer containing Al-O-Si, and the intensity of Al-O-Si was enhanced after annealing at 1000 °C. And the annealing process reduced the negative charge and the density of interface trap of the AlN annealed film in the MIS capacitor. These results provide a significant reference for AlN as a gate dielectric for SiC-based MOS devices in the future.

Synaptic Learning and Forgetting Behavior in Ag/AlN/Al Memristor With ${\text{O}_{2}}$ Annealing Effect

The learning and forgetting behavior of AlN thin film with O 2 annealing effect memristor had been investigated in this study. Such synaptic behavior could be observed after AlN thin film executed O 2 annealing process. The oxygen-induced AlN thin film exhibited memristive effect in resistance with positive and negative current-voltage (I-V) measurement loops which had never been observed in AlN thin film without annealing effect. The O 2 annealing process could decrease the number of oxygen vacancies (V+o ) in interface between top electrode and active layer. A typical I-V measurement is shown in this study. If continuous positive voltage pulses applied, the device conductance would increase gradually which is called learning behavior of device. If negative voltage pulse applied, the device conductance would decrease gradually which is called forgetting behavior of device. The memristor conductance was thought to be closely associated with synaptic weights, the strength of synaptic connections. The learning and forgetting behavior of memristor is corresponding to LTP and LTD process in synaptic system. The ion diffusion played an important role in memristive properties which could form one or more local conductive filaments (CFs) in active layer. If the paths were gradually formed or broke, this would cause the memristor conductance changed gradually with measured voltage. The retention time and memristor conductance should depend on stimuli pulse shape, number and interval time.

AlN epitaxy on SiC by low-temperature atomic layer deposition via layer-by-layer, in situ atomic layer annealing.

AlN thin films were epitaxially grown on a 4H-SiC substrate via atomic layer deposition (ALD) along with atomic layer annealing (ALA). By applying the layer-by-layer, in situ ALA treatment using helium/argon plasma in each ALD cycle, the as-deposited film gets crystallization energy from the plasma, which results in significant enhancement of the crystal quality to achieve a highly crystalline AlN epitaxial layer at a deposition temperature as low as 300 °C. In a nanoscale AlN epitaxial layer with a thickness of ∼30 nm, X-ray diffraction reveals a low full-width-at-half-maximum of the AlN (0002) peak of only 176.4 arcsec. Atomic force microscopy, high-resolution transmission electron microscopy, and Fourier diffractograms indicate a smooth surface and high-quality hetero-epitaxial growth of a nanoscale AlN layer on 4H-SiC. This research demonstrates the impact of the ALA treatment on the evolution of ALD techniques from conventional thin film deposition to low-temperature atomic layer epitaxy.

Investigation of structure of AlN thin films using Fourier-transform infrared spectroscopy

This study focuses on structural imperfections caused by hydrogen impurities in AlN thin films obtained using atomic layer deposition method (ALD). Currently, there is a severe lack of studies regarding the presence of hydrogen in the bulk of AlN films. Fourier-transform infrared spectroscopy (FTIR) is one of the few methods that allow detection bonds of light elements, in particular - hydrogen. Hydrogen is known to be a frequent contaminant in AlN films grown by ALD method, it may form different bonds with nitrogen, e.g. amino (–NH2) or imide (–NH) groups, which impair the quality of the resulting film. Which is why, it is important to investigate the phenomenon of hydrogen as well as to search for the suitable methods to eliminate or at least reduce its quantity. In this work several samples have been prepared using different precursors, substrates and deposition parameters and characterized using FTIR and additional techniques such as AFM, XPS and EDS to provide a comparative and comprehensive analysis of topography, morphology and chemical composition of AlN thin films.

Ионно-лучевое осаждение тонких пленок AlN на Al-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

Thin AlN films were obtained by the ion-beam deposition method on sapphire substrates. Studies were carried out using scanning electron microscopy, Raman scattering and optical spectroscopy. Dependences of influence of the ion-beam deposition parameters (composition of gas mixture, energy of the ion beam) on morphology, structure and optical properties of the AlN films on sapphire were revealed.

Топография поверхности и оптические характеристики тонких пленок AlN на подложке GaAs (100), полученных методом реактивного ионно-плазменного распыления

AbstractA study of the surface topography and optical characteristics of thin AlN films used as passivating and antireflection coatings deposited on n -GaAs (100) substrates by reactive ion-plasma sputtering is reported. It was found that the process conditions affect the structure and the optical characteristics of the films, which makes it possible to obtain coatings with prescribed parameters. An analysis of the results furnished by ellipsometry and atomic-force microscopy of the surface shows that the refractive index of the films is correlated with the surface structure.

Preparation of high-quality stress-free (001) aluminum nitride thin film using a dual Kaufman ion-beam source setup

We proposed and demonstrated a preparation method of (001) preferentially oriented stress-free AlN piezoelectric thin films. The AlN thin films were deposited by a reactive sputtering technique at substrate temperatures up to 330 °C using a dual Kaufman ion-beam source setup. We deposited the AlN on Si (100), Si (111), amorphous SiO2, and a (001) preferentially oriented Ti thin film and compared their crystallographic, optical, and piezoelectric properties. The AlN thin films deposited on the (001) preferentially oriented Ti thin films have the highest crystallographic quality. The stress-free AlN reached a high value of the piezoelectric coefficient d33 = (7.33 ± 0.08) pC·N−1. The properties of the AlN thin film prepared at such low temperatures are suitable for numerous microelectromechanical systems, piezoelectric sensors, and actuators monolithically integrated with complementary metal-oxide-semiconductor signal-processing circuits.

Direct van der Waals Epitaxy of Crack-Free AlN Thin Film on Epitaxial WS₂.

Van der Waals epitaxy (vdWE) has drawn continuous attention, as it is unlimited by lattice-mismatch between epitaxial layers and substrates. Previous reports on the vdWE of III-nitride thin film were mainly based on two-dimensional (2D) materials by plasma pretreatment or pre-doping of other hexagonal materials. However, it is still a huge challenge for single-crystalline thin film on 2D materials without any other extra treatment or interlayer. Here, we grew high-quality single-crystalline AlN thin film on sapphire substrate with an intrinsic WS2 overlayer (WS2/sapphire) by metal-organic chemical vapor deposition, which had surface roughness and defect density similar to that grown on conventional sapphire substrates. Moreover, an AlGaN-based deep ultraviolet light emitting diode structure on WS2/sapphire was demonstrated. The electroluminescence (EL) performance exhibited strong emissions with a single peak at 283 nm. The wavelength of the single peak only showed a faint peak-position shift with increasing current to 80 mA, which further indicated the high quality and low stress of the AlN thin film. This work provides a promising solution for further deep-ultraviolet (DUV) light emitting electrodes (LEDs) development on 2D materials, as well as other unconventional substrates.

Structural, Thermal and Luminescence Properties of AlN:Tm Thin Films Deposited on Silicon Substrate and Optical Fiber

Thin films of AlN:Tm are deposited on a Si(111) and Si(100) substrates and optical fiber by rf magnetron sputtering method. 200–400 nm thick films are deposited at various temperatures to observe the effect of temperature on their structure and morphology. Liquid nitrogen is used to deposit films at low temperature of –196°C. An electric heater is used to grow films at 250–700°C range. The deposited films are analyzed for their structure and thermal stability using X-ray diffraction and thermogravimetric analysis. Films deposited at liquid nitrogen temperature are amorphous and those deposited at high temperature are crystalline. Average grain size, strain and interatomic distance of the crystalline films are calculated. The grain size and inter-atomic distance increase from 36.54 to 45.12 nm and from 0.2368 to 0.25 nm respectively. Photoluminescence of the films excited by 783 nm crystal laser shows an intense infrared emission from Tm+3 at 802 nm as a result of 3H4 → 3H6 transition.

The effect of AlGaAs/GaAs laser heterostructure ionic surface cleaning on the structure and properties of AlN films deposited by the method of reactive ion-plasma sputtering

The paper presents studies of the structural and optical properties of AlN thin films grown on an AlGaAs/GaAs semiconductor laser heterostructure with the use of preliminary ionic surface cleaning and without it. After cleavage of the heterostructure, a natural surface oxide forms on the facet. In the paper, the ionic etching regime providing the removal of the surface oxide layer without significant defects in the semiconductor heterostructure is determined.

Lamb Wave Focusing Transducer for Efficient Coupling to Wavelength-Scale Structures in Thin Piezoelectric Films

This paper describes the theoretical and experimental investigation of interdigitated transducers capable of producing focused acoustical beams in thin film piezoelectric materials. A mathematical formalism describing focused acoustical beams, lamb beams, is presented and related to their optical counterparts in two- and three-dimensions. A novel Fourier domain transducer design methodology is developed and utilized to produce near diffraction limited focused beams within a thin film AlN membrane. The properties of the acoustic beam formed by the transducer were studied by means of Doppler vibrometry implemented with a scanning confocal balanced homodyne interferometer. The Fourier domain modal analysis confirmed that 83% of the acoustical power was delivered to the targeted focused beam which was constituted from the lowest order symmetric mode, while 1% was delivered unintentionally to the beam formed from the anti-symmetric mode, and the remaining power was isotropically scattered. The transmission properties of the acoustic beams as they interact with devices with wavelength scale features were also studied, demonstrating minimal insertion loss for devices in which a subwavelength and pinhole apertures were included. [2018-0059]

Characterization of Er-doped AlN films prepared by RF magnetron sputtering

Er-doped AlN thin film was deposited on sapphire substrates (0001) by RF magnetron sputtering at different sputtering times. The crystalline structure, surface morphology and electrical properties of the thin films have been investigated. The XRD patterns and the SEM sectional diagram indicate that Er-doped AlN thin films present the preferred orientation of C axis. The crystalline quality of the films rises first and then decreases with the increase of sputtering time and reaches best at 90 min. Piezoelectric coefficient d33 indicates maximum value of 9.53 pm/V. Correspondingly, the best surface morphology of thin film was obtained at 90 min and the surface roughness reached a minimum of 2.012 nm. In addition, the change of resistance is same with change of the crystalline quality and the resistivity reached a maximum of 4.36 ∗ 1012 Ω·cm at 90 min.

Properties of AlN Thin Films on p-Ge Deposited by Thermal Atomic Layer Deposition

The electrical and interfacial properties of atomic layer deposited AlN on p-Ge were investigated. A very large capacitance–voltage (C–V) hysteresis at 1 MHz was observed, resulted from the presence of high density of slow traps that is associated with the GeO volatilization. Nonetheless, well-behaved C–V characteristics with minimal frequency dispersion and the absence of bumps near the flat-band voltage were obtained except 1 MHz, implying that the AlN passivation occurred near the AlN/Ge interface. Parallel conductance values showed that both slow and fast states were located within a narrow energy range. X-ray photoelectron spectroscopy (XPS) spectra revealed that large amount of oxygen atoms are present near the AlN/Ge interface. Above the AlN critical thickness, the strong Al–Al bond in the Al 2p spectra was observed, which could be explained by the strain relaxation due to the lattice mismatch between AlN and Ge.

Effects of Cr doping on the mechanical properties of AlN films grown by the co-sputtering technique

We report the mechanical properties of Cr-doped AlN thin films synthesized by reactive magnetron co-sputtering technique with different Cr concentrations (0, 2, 4, 6 and 11 at%). Surface chemical analysis and crystal structure studies of Cr doped AlN films are carried out using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXRD) techniques. All these films are crystallized with wurtzite structure and grown predominantly along a-axis orientation in the absence of secondary phases belonging to Cr. The surface morphology and RMS roughness are measured by atomic force microscope (AFM). The residual stress of these films exhibit a tensile behavior and it decreases with Cr concentration as calculated by sin2ψ technique. Indentation hardness of these films is measured by nanoindentation technique, which is ranged from 17.5 to 23.0 GPa. However, indentation modulus is enhanced with Cr concentration due to strong p-d hybridization between Cr and N atoms. Indentation impression of these films exhibit a sink-in behavior and gives an evidence of high elastic recovery without radial cracks.